RIFT: Routing in Fat Trees
draft-ietf-rift-rift-12
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Tony Przygienda
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Alankar Sharma
,
Pascal Thubert
,
Bruno Rijsman
,
Dmitry Afanasiev
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2021-05-25
(latest revision 2020-05-26)
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draft-przygienda-rift
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Submitted to IESG for Publication
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Zheng Zhang
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RIFT Working Group A. Przygienda, Ed.
Internet-Draft Juniper
Intended status: Standards Track A. Sharma
Expires: November 27, 2020 Comcast
P. Thubert
Cisco
Bruno. Rijsman
Individual
Dmitry. Afanasiev
Yandex
May 26, 2020
RIFT: Routing in Fat Trees
draft-ietf-rift-rift-12
Abstract
This document defines a specialized, dynamic routing protocol for
Clos and fat-tree network topologies optimized towards minimization
of configuration and operational complexity. The protocol
o deals with no configuration, fully automated construction of fat-
tree topologies based on detection of links,
o minimizes the amount of routing state held at each level,
o automatically prunes and load balances topology flooding exchanges
over a sufficient subset of links,
o supports automatic disaggregation of prefixes on link and node
failures to prevent black-holing and suboptimal routing,
o allows traffic steering and re-routing policies,
o allows loop-free non-ECMP forwarding,
o automatically re-balances traffic towards the spines based on
bandwidth available and finally
o provides mechanisms to synchronize a limited key-value data-store
that can be used after protocol convergence to e.g. bootstrap
higher levels of functionality on nodes.
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Status of This Memo
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Table of Contents
1. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 8
3. Reference Frame . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 13
4. RIFT: Routing in Fat Trees . . . . . . . . . . . . . . . . . 15
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1.1. Properties . . . . . . . . . . . . . . . . . . . . . 16
4.1.2. Generalized Topology View . . . . . . . . . . . . . . 17
4.1.2.1. Terminology . . . . . . . . . . . . . . . . . . . 17
4.1.2.2. Clos as Crossed Crossbars . . . . . . . . . . . . 18
4.1.3. Fallen Leaf Problem . . . . . . . . . . . . . . . . . 28
4.1.4. Discovering Fallen Leaves . . . . . . . . . . . . . . 30
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4.1.5. Addressing the Fallen Leaves Problem . . . . . . . . 31
4.2. Specification . . . . . . . . . . . . . . . . . . . . . . 32
4.2.1. Transport . . . . . . . . . . . . . . . . . . . . . . 33
4.2.2. Link (Neighbor) Discovery (LIE Exchange) . . . . . . 33
4.2.2.1. LIE FSM . . . . . . . . . . . . . . . . . . . . . 36
4.2.3. Topology Exchange (TIE Exchange) . . . . . . . . . . 46
4.2.3.1. Topology Information Elements . . . . . . . . . . 46
4.2.3.2. South- and Northbound Representation . . . . . . 46
4.2.3.3. Flooding . . . . . . . . . . . . . . . . . . . . 49
4.2.3.4. TIE Flooding Scopes . . . . . . . . . . . . . . . 56
4.2.3.5. 'Flood Only Node TIEs' Bit . . . . . . . . . . . 59
4.2.3.6. Initial and Periodic Database Synchronization . . 60
4.2.3.7. Purging and Roll-Overs . . . . . . . . . . . . . 60
4.2.3.8. Southbound Default Route Origination . . . . . . 61
4.2.3.9. Northbound TIE Flooding Reduction . . . . . . . . 62
4.2.3.10. Special Considerations . . . . . . . . . . . . . 67
4.2.4. Reachability Computation . . . . . . . . . . . . . . 67
4.2.4.1. Northbound SPF . . . . . . . . . . . . . . . . . 67
4.2.4.2. Southbound SPF . . . . . . . . . . . . . . . . . 68
4.2.4.3. East-West Forwarding Within a non-ToF Level . . . 69
4.2.4.4. East-West Links Within ToF Level . . . . . . . . 69
4.2.5. Automatic Disaggregation on Link & Node Failures . . 69
4.2.5.1. Positive, Non-transitive Disaggregation . . . . . 69
4.2.5.2. Negative, Transitive Disaggregation for Fallen
Leaves . . . . . . . . . . . . . . . . . . . . . 73
4.2.6. Attaching Prefixes . . . . . . . . . . . . . . . . . 75
4.2.7. Optional Zero Touch Provisioning (ZTP) . . . . . . . 84
4.2.7.1. Terminology . . . . . . . . . . . . . . . . . . . 85
4.2.7.2. Automatic SystemID Selection . . . . . . . . . . 86
4.2.7.3. Generic Fabric Example . . . . . . . . . . . . . 87
4.2.7.4. Level Determination Procedure . . . . . . . . . . 88
4.2.7.5. ZTP FSM . . . . . . . . . . . . . . . . . . . . . 89
4.2.7.6. Resulting Topologies . . . . . . . . . . . . . . 95
4.2.8. Stability Considerations . . . . . . . . . . . . . . 97
4.3. Further Mechanisms . . . . . . . . . . . . . . . . . . . 98
4.3.1. Overload Bit . . . . . . . . . . . . . . . . . . . . 98
4.3.2. Optimized Route Computation on Leaves . . . . . . . . 98
4.3.3. Mobility . . . . . . . . . . . . . . . . . . . . . . 98
4.3.3.1. Clock Comparison . . . . . . . . . . . . . . . . 100
4.3.3.2. Interaction between Time Stamps and Sequence
Counters . . . . . . . . . . . . . . . . . . . . 100
4.3.3.3. Anycast vs. Unicast . . . . . . . . . . . . . . . 101
4.3.3.4. Overlays and Signaling . . . . . . . . . . . . . 101
4.3.4. Key/Value Store . . . . . . . . . . . . . . . . . . . 101
4.3.4.1. Southbound . . . . . . . . . . . . . . . . . . . 101
4.3.4.2. Northbound . . . . . . . . . . . . . . . . . . . 102
4.3.5. Interactions with BFD . . . . . . . . . . . . . . . . 102
4.3.6. Fabric Bandwidth Balancing . . . . . . . . . . . . . 103
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4.3.6.1. Northbound Direction . . . . . . . . . . . . . . 103
4.3.6.2. Southbound Direction . . . . . . . . . . . . . . 106
4.3.7. Label Binding . . . . . . . . . . . . . . . . . . . . 106
4.3.8. Leaf to Leaf Procedures . . . . . . . . . . . . . . . 106
4.3.9. Address Family and Multi Topology Considerations . . 106
4.3.10. Reachability of Internal Nodes in the Fabric . . . . 107
4.3.11. One-Hop Healing of Levels with East-West Links . . . 107
4.4. Security . . . . . . . . . . . . . . . . . . . . . . . . 107
4.4.1. Security Model . . . . . . . . . . . . . . . . . . . 107
4.4.2. Security Mechanisms . . . . . . . . . . . . . . . . . 109
4.4.3. Security Envelope . . . . . . . . . . . . . . . . . . 110
4.4.4. Weak Nonces . . . . . . . . . . . . . . . . . . . . . 113
4.4.5. Lifetime . . . . . . . . . . . . . . . . . . . . . . 114
4.4.6. Key Management . . . . . . . . . . . . . . . . . . . 114
4.4.7. Security Association Changes . . . . . . . . . . . . 114
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.1. Normal Operation . . . . . . . . . . . . . . . . . . . . 115
5.2. Leaf Link Failure . . . . . . . . . . . . . . . . . . . . 117
5.3. Partitioned Fabric . . . . . . . . . . . . . . . . . . . 118
5.4. Northbound Partitioned Router and Optional East-West
Links . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6. Implementation and Operation: Further Details . . . . . . . . 120
6.1. Considerations for Leaf-Only Implementation . . . . . . . 120
6.2. Considerations for Spine Implementation . . . . . . . . . 121
6.3. Adaptations to Other Proposed Data Center Topologies . . 121
6.4. Originating Non-Default Route Southbound . . . . . . . . 122
7. Security Considerations . . . . . . . . . . . . . . . . . . . 122
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.2. ZTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.3. Lifetime . . . . . . . . . . . . . . . . . . . . . . . . 123
7.4. Packet Number . . . . . . . . . . . . . . . . . . . . . . 123
7.5. Outer Fingerprint Attacks . . . . . . . . . . . . . . . . 123
7.6. TIE Origin Fingerprint DoS Attacks . . . . . . . . . . . 123
7.7. Host Implementations . . . . . . . . . . . . . . . . . . 124
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 124
8.1. Requested Multicast and Port Numbers . . . . . . . . . . 124
8.2. Requested Registries with Suggested Values . . . . . . . 125
8.2.1. Registry RIFT_v4/common/AddressFamilyType . . . . . . 125
8.2.1.1. Requested Entries . . . . . . . . . . . . . . . . 125
8.2.2. Registry RIFT_v4/common/HierarchyIndications . . . . 125
8.2.2.1. Requested Entries . . . . . . . . . . . . . . . . 125
8.2.3. Registry RIFT_v4/common/IEEE802_1ASTimeStampType . . 125
8.2.3.1. Requested Entries . . . . . . . . . . . . . . . . 126
8.2.4. Registry RIFT_v4/common/IPAddressType . . . . . . . . 126
8.2.4.1. Requested Entries . . . . . . . . . . . . . . . . 126
8.2.5. Registry RIFT_v4/common/IPPrefixType . . . . . . . . 126
8.2.5.1. Requested Entries . . . . . . . . . . . . . . . . 126
8.2.6. Registry RIFT_v4/common/IPv4PrefixType . . . . . . . 126
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8.2.6.1. Requested Entries . . . . . . . . . . . . . . . . 126
8.2.7. Registry RIFT_v4/common/IPv6PrefixType . . . . . . . 126
8.2.7.1. Requested Entries . . . . . . . . . . . . . . . . 127
8.2.8. Registry RIFT_v4/common/PrefixSequenceType . . . . . 127
8.2.8.1. Requested Entries . . . . . . . . . . . . . . . . 127
8.2.9. Registry RIFT_v4/common/RouteType . . . . . . . . . . 127
8.2.9.1. Requested Entries . . . . . . . . . . . . . . . . 127
8.2.10. Registry RIFT_v4/common/TIETypeType . . . . . . . . . 128
8.2.10.1. Requested Entries . . . . . . . . . . . . . . . 128
8.2.11. Registry RIFT_v4/common/TieDirectionType . . . . . . 128
8.2.11.1. Requested Entries . . . . . . . . . . . . . . . 128
8.2.12. Registry RIFT_v4/encoding/Community . . . . . . . . . 128
8.2.12.1. Requested Entries . . . . . . . . . . . . . . . 128
8.2.13. Registry RIFT_v4/encoding/KeyValueTIEElement . . . . 129
8.2.13.1. Requested Entries . . . . . . . . . . . . . . . 129
8.2.14. Registry RIFT_v4/encoding/LIEPacket . . . . . . . . . 129
8.2.14.1. Requested Entries . . . . . . . . . . . . . . . 129
8.2.15. Registry RIFT_v4/encoding/LinkCapabilities . . . . . 130
8.2.15.1. Requested Entries . . . . . . . . . . . . . . . 130
8.2.16. Registry RIFT_v4/encoding/LinkIDPair . . . . . . . . 130
8.2.16.1. Requested Entries . . . . . . . . . . . . . . . 131
8.2.17. Registry RIFT_v4/encoding/Neighbor . . . . . . . . . 131
8.2.17.1. Requested Entries . . . . . . . . . . . . . . . 131
8.2.18. Registry RIFT_v4/encoding/NodeCapabilities . . . . . 131
8.2.18.1. Requested Entries . . . . . . . . . . . . . . . 132
8.2.19. Registry RIFT_v4/encoding/NodeFlags . . . . . . . . . 132
8.2.19.1. Requested Entries . . . . . . . . . . . . . . . 132
8.2.20. Registry RIFT_v4/encoding/NodeNeighborsTIEElement . . 132
8.2.20.1. Requested Entries . . . . . . . . . . . . . . . 132
8.2.21. Registry RIFT_v4/encoding/NodeTIEElement . . . . . . 132
8.2.21.1. Requested Entries . . . . . . . . . . . . . . . 133
8.2.22. Registry RIFT_v4/encoding/PacketContent . . . . . . . 133
8.2.22.1. Requested Entries . . . . . . . . . . . . . . . 133
8.2.23. Registry RIFT_v4/encoding/PacketHeader . . . . . . . 133
8.2.23.1. Requested Entries . . . . . . . . . . . . . . . 134
8.2.24. Registry RIFT_v4/encoding/PrefixAttributes . . . . . 134
8.2.24.1. Requested Entries . . . . . . . . . . . . . . . 134
8.2.25. Registry RIFT_v4/encoding/PrefixTIEElement . . . . . 134
8.2.25.1. Requested Entries . . . . . . . . . . . . . . . 135
8.2.26. Registry RIFT_v4/encoding/ProtocolPacket . . . . . . 135
8.2.26.1. Requested Entries . . . . . . . . . . . . . . . 135
8.2.27. Registry RIFT_v4/encoding/TIDEPacket . . . . . . . . 135
8.2.27.1. Requested Entries . . . . . . . . . . . . . . . 135
8.2.28. Registry RIFT_v4/encoding/TIEElement . . . . . . . . 135
8.2.28.1. Requested Entries . . . . . . . . . . . . . . . 136
8.2.29. Registry RIFT_v4/encoding/TIEHeader . . . . . . . . . 136
8.2.29.1. Requested Entries . . . . . . . . . . . . . . . 137
8.2.30. Registry RIFT_v4/encoding/TIEHeaderWithLifeTime . . . 137
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8.2.30.1. Requested Entries . . . . . . . . . . . . . . . 137
8.2.31. Registry RIFT_v4/encoding/TIEID . . . . . . . . . . . 137
8.2.31.1. Requested Entries . . . . . . . . . . . . . . . 138
8.2.32. Registry RIFT_v4/encoding/TIEPacket . . . . . . . . . 138
8.2.32.1. Requested Entries . . . . . . . . . . . . . . . 138
8.2.33. Registry RIFT_v4/encoding/TIREPacket . . . . . . . . 138
8.2.33.1. Requested Entries . . . . . . . . . . . . . . . 138
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 138
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 139
10.1. Normative References . . . . . . . . . . . . . . . . . . 139
10.2. Informative References . . . . . . . . . . . . . . . . . 141
Appendix A. Sequence Number Binary Arithmetic . . . . . . . . . 143
Appendix B. Information Elements Schema . . . . . . . . . . . . 144
B.1. common.thrift . . . . . . . . . . . . . . . . . . . . . . 146
B.2. encoding.thrift . . . . . . . . . . . . . . . . . . . . . 152
Appendix C. Constants . . . . . . . . . . . . . . . . . . . . . 161
C.1. Configurable Protocol Constants . . . . . . . . . . . . . 161
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 163
1. Authors
This work is a product of a list of individuals which are all to be
considered major contributors independent of the fact whether their
name made it to the limited boilerplate author's list or not.
Tony Przygienda, Ed. | Alankar Sharma | Pascal Thubert
Juniper Networks | Comcast | Cisco
Bruno Rijsman | Ilya Vershkov | Dmitry Afanasiev
Individual | Mellanox | Yandex
Don Fedyk | Alia Atlas | John Drake
Individual | Individual | Juniper
Table 1: RIFT Authors
2. Introduction
Clos [CLOS] and Fat-Tree [FATTREE] topologies have gained prominence
in today's networking, primarily as result of the paradigm shift
towards a centralized data-center based architecture that is poised
to deliver a majority of computation and storage services in the
future. Today's current routing protocols were geared towards a
network with an irregular topology and low degree of connectivity
originally but given they were the only available options,
consequently several attempts to apply those protocols to Clos have
been made. Most successfully BGP [RFC4271] [RFC7938] has been
extended to this purpose, not as much due to its inherent suitability
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but rather because the perceived capability to easily modify BGP and
the immanent difficulties with link-state [DIJKSTRA] based protocols
to optimize topology exchange and converge quickly in large scale
densely meshed topologies. The incumbent protocols precondition
normally extensive configuration or provisioning during bring up and
re-dimensioning. This tends to be viable only for a set of
organizations with according networking operation skills and budgets.
For many IP fabric builders a desirable protocol would be one that
auto-configures itself and deals with failures and mis-configurations
with a minimum of human intervention only. Such a solution would
allow local IP fabric bandwidth to be consumed in a 'standard
component' fashion, i.e. provision it much faster and operate it at
much lower costs than today, much like compute or storage is consumed
already.
In looking at the problem through the lens of data center
requirements, RIFT addresses challenges in IP fabric routing not
through an incremental modification of either a link-state
(distributed computation) or distance-vector (diffused computation)
but rather a mixture of both, colloquially best described as "link-
state towards the spine" and "distance vector towards the leaves".
In other words, "bottom" levels are flooding their link-state
information in the "northern" direction while each node generates
under normal conditions a "default route" and floods it in the
"southern" direction. This type of protocol allows naturally for
highly desirable aggregation. Alas, such aggregation could blackhole
traffic in cases of misconfiguration or while failures are being
resolved or even cause partial network partitioning and this has to
be addressed by some adequate mechanism. The approach RIFT takes is
described in Section 4.2.5 and is basically based on automatic,
sufficient disaggregation of prefixes in case of link and node
failures.
For the visually oriented reader, Figure 1 presents a first level
simplified view of the resulting information and routes on a RIFT
fabric. The top of the fabric is holding in its link-state database
the nodes below it and the routes to them. In the second row of the
database table we indicate that partial information of other nodes in
the same level is available as well. The details of how this is
achieved will be postponed for the moment. When we look at the
"bottom" of the fabric, the leaves, we see that the topology is
basically empty and they only hold a load balanced default route to
the next level under normal conditions.
The balance of this document details a dedicated IP fabric routing
protocol, fills in the specification details and ultimately includes
resulting security considerations.
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. [A,B,C,D]
. [E]
. +-----+ +-----+
. | E | | F | A/32 @ [C,D]
. +-+-+-+ +-+-+-+ B/32 @ [C,D]
. | | | | C/32 @ C
. | | +-----+ | D/32 @ D
. | | | |
. | +------+ |
. | | | |
. [A,B] +-+---+ | | +---+-+ [A,B]
. [D] | C +--+ +-+ D | [C]
. +-+-+-+ +-+-+-+
. 0/0 @ [E,F] | | | | 0/0 @ [E,F]
. A/32 @ A | | +-----+ | A/32 @ A
. B/32 @ B | | | | B/32 @ B
. | +------+ |
. | | | |
. +-+---+ | | +---+-+
. | A +--+ +-+ B |
. 0/0 @ [C,D] +-----+ +-----+ 0/0 @ [C,D]
Figure 1: RIFT Information Distribution
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 8174 [RFC8174].
3. Reference Frame
3.1. Terminology
This section presents the terminology used in this document. It is
assumed that the reader is thoroughly familiar with the terms and
concepts used in OSPF [RFC2328] and IS-IS [ISO10589-Second-Edition],
[ISO10589] as well as the according graph theoretical concepts of
shortest path first (SPF) [DIJKSTRA] computation and DAGs.
Crossbar: Physical arrangement of ports in a switching matrix
without implying any further scheduling or buffering disciplines.
Clos/Fat Tree: This document uses the terms Clos and Fat Tree
interchangeably whereas it always refers to a folded spine-and-
leaf topology with possibly multiple Points of Delivery (PoDs) and
one or multiple Top of Fabric (ToF) planes. Several modifications
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such as leaf-2-leaf shortcuts and multiple level shortcuts are
possible and described further in the document.
Directed Acyclic Graph (DAG): A finite directed graph with no
directed cycles (loops). If links in Clos are considered as
either being all directed towards the top or vice versa, each of
such two graphs is a DAG.
Folded Spine-and-Leaf: In case Clos fabric input and output stages
are analogous, the fabric can be "folded" to build a "superspine"
or top which we will call Top of Fabric (ToF) in this document.
Level: Clos and Fat Tree networks are topologically partially
ordered graphs and 'level' denotes the set of nodes at the same
height in such a network, where the bottom level (leaf) is the
level with lowest value. A node has links to nodes one level down
and/or one level up. Under some circumstances, a node may have
links to nodes at the same level. As footnote: Clos terminology
uses often the concept of "stage" but due to the folded nature of
the Fat Tree we do not use it to prevent misunderstandings.
Superspine vs. Aggregation and Spine vs. Edge/Leaf:
Traditional level names in 5-stages folded Clos for Level 2, 1 and
0 respectively. We normalize this language to talk about top-of-
fabric (ToF), top-of-pod (ToP) and leaves.
Zero Touch Provisioning (ZTP): Optional RIFT mechanism which allows
to derive node levels automatically based on minimum configuration
(only ToF property has to be provisioned on according nodes).
Point of Delivery (PoD): A self-contained vertical slice or subset
of a Clos or Fat Tree network containing normally only level 0 and
level 1 nodes. A node in a PoD communicates with nodes in other
PoDs via the Top-of-Fabric. We number PoDs to distinguish them
and use PoD #0 to denote "undefined" PoD.
Top of PoD (ToP): The set of nodes that provide intra-PoD
communication and have northbound adjacencies outside of the PoD,
i.e. are at the "top" of the PoD.
Top of Fabric (ToF): The set of nodes that provide inter-PoD
communication and have no northbound adjacencies, i.e. are at the
"very top" of the fabric. ToF nodes do not belong to any PoD and
are assigned "undefined" PoD value to indicate the equivalent of
"any" PoD.
Spine: Any nodes north of leaves and south of top-of-fabric nodes.
Multiple layers of spines in a PoD are possible.
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Leaf: A node without southbound adjacencies. Its level is 0 (except
cases where it is deriving its level via ZTP and is running
without LEAF_ONLY which will be explained in Section 4.2.7).
Top-of-fabric Plane or Partition: In large fabrics top-of-fabric
switches may not have enough ports to aggregate all switches south
of them and with that, the ToF is 'split' into multiple
independent planes. Introduction and Section 4.1.2 explains the
concept in more detail. A plane is subset of ToF nodes that see
each other through south reflection or E-W links.
Radix: A radix of a switch is basically number of switching ports it
provides. It's sometimes called fanout as well.
North Radix: Ports cabled northbound to higher level nodes.
South Radix: Ports cabled southbound to lower level nodes.
South/Southbound and North/Northbound (Direction):
When describing protocol elements and procedures, we will be using
in different situations the directionality of the compass. I.e.,
'south' or 'southbound' mean moving towards the bottom of the Clos
or Fat Tree network and 'north' and 'northbound' mean moving
towards the top of the Clos or Fat Tree network.
Northbound Link: A link to a node one level up or in other words,
one level further north.
Southbound Link: A link to a node one level down or in other words,
one level further south.
East-West Link: A link between two nodes at the same level. East-
West links are normally not part of Clos or "fat-tree" topologies.
Leaf shortcuts (L2L): East-West links at leaf level will need to be
differentiated from East-West links at other levels.
Routing on the host (RotH): Modern data center architecture variant
where servers/leaves are multi-homed and consecutively participate
in routing.
Northbound representation: Subset of topology information flooded
towards higher levels of the fabric.
Southbound representation: Subset of topology information sent
towards a lower level.
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South Reflection: Often abbreviated just as "reflection" it defines
a mechanism where South Node TIEs are "reflected" from the level
south back up north to allow nodes in the same level without E-W
links to "see" each other's node TIEs.
TIE: This is an acronym for a "Topology Information Element". TIEs
are exchanged between RIFT nodes to describe parts of a network
such as links and address prefixes, in a fashion similar to ISIS
LSPs or OSPF LSAs. A TIE has always a direction and a type. We
will talk about North TIEs (sometimes abbreviated as N-TIEs) when
talking about TIEs in the northbound representation and South-TIEs
(sometimes abbreviated as S-TIEs) for the southbound equivalent.
TIEs have different types such as node and prefix TIEs.
Node TIE: This stands as acronym for a "Node Topology Information
Element" that contains all adjacencies the node discovered and
information about node itself. Node TIE should NOT be confused
with a North TIE since "node" defines the type of TIE rather than
its direction.
Prefix TIE: This is an acronym for a "Prefix Topology Information
Element" and it contains all prefixes directly attached to this
node in case of a North TIE and in case of South TIE the necessary
default routes the node advertises southbound.
Key Value TIE: A South TIE that is carrying a set of key value pairs
[DYNAMO]. It can be used to distribute information in the
southbound direction within the protocol.
TIDE: Topology Information Description Element, equivalent to CSNP
in ISIS.
TIRE: Topology Information Request Element, equivalent to PSNP in
ISIS. It can both confirm received and request missing TIEs.
De-aggregation/Disaggregation: Process in which a node decides to
advertise more specific prefixes Southwards, either positively to
attract the corresponding traffic, or negatively to repel it.
Disaggregation is performed to prevent black-holing and suboptimal
routing to the more specific prefixes.
LIE: This is an acronym for a "Link Information Element", largely
equivalent to HELLOs in IGPs and exchanged over all the links
between systems running RIFT to form three way adjacencies.
Flood Repeater (FR): A node can designate one or more northbound
neighbor nodes to be flood repeaters. The flood repeaters are
responsible for flooding northbound TIEs further north. They are
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similar to MPR in OSLR. The document sometimes calls them flood
leaders as well.
Bandwidth Adjusted Distance (BAD): Each RIFT node can calculate the
amount of northbound bandwidth available towards a node compared
to other nodes at the same level and can modify the route distance
accordingly to allow for the lower level to adjust their load
balancing towards spines.
Overloaded: Applies to a node advertising `overload` attribute as
set. The semantics closely follow the meaning of the same
attribute in [ISO10589-Second-Edition].
Interface: A layer 3 entity over which RIFT control packets are
exchanged.
Three-Way Adjacency: RIFT tries to form a unique adjacency over an
interface and exchange local configuration and necessary ZTP
information. An adjacency is only advertised in node TIEs and
used for computations after it achieved three-way state, i.e. both
routers reflected each other in LIEs including relevant security
information. LIEs before three-way state is reached may carry ZTP
related information already.
Bi-directional Adjacency: Bidirectional adjacency is an adjacency
where nodes of both sides of the adjacency advertised it in the
node TIEs with the correct levels and system IDs. Bi-
directionality is used to check in different algorithms whether
the link should be included.
Neighbor: Once a three-way adjacency has been formed a neighborship
relationship contains the neighbor's properties. Multiple
adjacencies can be formed to a remote node via parallel interfaces
but such adjacencies are NOT sharing a neighbor structure. Saying
"neighbor" is thus equivalent to saying "a three-way adjacency".
Cost: The term signifies the weighted distance between two
neighbors.
Distance: Sum of costs (bound by infinite distance) between two
nodes.
Shortest-Path First (SPF): A well-known graph algorithm attributed
to Dijkstra that establishes a tree of shortest paths from a
source to destinations on the graph. We use SPF acronym due to
its familiarity as general term for the node reachability
calculations RIFT can employ to ultimately calculate routes of
which Dijkstra algorithm is one.
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North SPF (N-SPF): A reachability calculation that is progressing
northbound, as example SPF that is using South Node TIEs only.
Normally it progresses a single hop only and installs default
routes.
South SPF (S-SPF): A reachability calculation that is progressing
southbound, as example SPF that is using North Node TIEs only.
Security Envelope RIFT packets are flooded within an authenticated
security envelope that allows to protect the integrity of
information a node accepts.
3.2. Topology
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^ N +--------+ +--------+
Level 2 | |ToF 21| |ToF 22|
E <-*-> W ++-+--+-++ ++-+--+-++
| | | | | | | | |
S v P111/2 P121/2 | | | |
^ ^ ^ ^ | | | |
| | | | | | | |
+--------------+ | +-----------+ | | | +---------------+
| | | | | | | |
South +-----------------------------+ | | ^
| | | | | | | All TIEs
0/0 0/0 0/0 +-----------------------------+ |
v v v | | | | |
| | +-+ +<-0/0----------+ | |
| | | | | | | |
+-+----++ optional +-+----++ ++----+-+ ++-----++
Level 1 | | E/W link | | | | | |
|Spin111+----------+Spin112| |Spin121| |Spin122|
+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
| | | South | | | |
| +---0/0--->-----+ 0/0 | +----------------+ |
0/0 | | | | | | |
| +---<-0/0-----+ | v | +--------------+ | |
v | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +---+-+-+
Level 0 | | (L2L) | | | | | |
|Leaf111+~~~~~~~~~~+Leaf112| |Leaf121| |Leaf122|
+-+-----+ +-+---+-+ +--+--+-+ +-+-----+
+ + \ / + +
Prefix111 Prefix112 \ / Prefix121 Prefix122
multi-homed
Prefix
+---------- PoD 1 ---------+ +---------- PoD 2 ---------+
Figure 2: A Three Level Spine-and-Leaf Topology
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.+--------+ +--------+ +--------+ +--------+
.|ToF A1| |ToF B1| |ToF B2| |ToF A2|
.++-+-----+ ++-+-----+ ++-+-----+ ++-+-----+
. | | | | | | | |
. | | | | | +---------------+
. | | | | | | | |
. | | | +-------------------------+ |
. | | | | | | | |
. | +-----------------------+ | | | |
. | | | | | | | |
. | | +---------+ | +---------+ | |
. | | | | | | | |
. | +---------------------------------+ | |
. | | | | | | | |
.++-+-----+ ++-+-----+ +--+-+---+ +----+-+-+
.|Spine111| |Spine112| |Spine121| |Spine122|
.+-+---+--+ ++----+--+ +-+---+--+ ++---+---+
. | | | | | | | |
. | +--------+ | | +--------+ |
. | | | | | | | |
. | -------+ | | | +------+ | |
. | | | | | | | |
.+-+---+-+ +--+--+-+ +-+---+-+ +---+-+-+
.|Leaf111| |Leaf112| |Leaf121| |Leaf122|
.+-------+ +-------+ +-------+ +-------+
Figure 3: Topology with Multiple Planes
We will use topology in Figure 2 (called commonly a fat tree/network
in modern IP fabric considerations [VAHDAT08] as homonym to the
original definition of the term [FATTREE]) in all further
considerations. This figure depicts a generic "single plane fat-
tree" and the concepts explained using three levels apply by
induction to further levels and higher degrees of connectivity.
Further, this document will deal also with designs that provide only
sparser connectivity and "partitioned spines" as shown in Figure 3
and explained further in Section 4.1.2.
4. RIFT: Routing in Fat Trees
We present here a detailed outline of a protocol optimized for
Routing in Fat Trees (RIFT) that in most abstract terms has many
properties of a modified link-state protocol
[RFC2328][ISO10589-Second-Edition] when distributing information
northbound and distance vector [RFC4271] protocol when distributing
information southbound. While this is an unusual combination, it
does quite naturally exhibit the desirable properties we seek.
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4.1. Overview
4.1.1. Properties
The most singular property of RIFT is that it floods flat link-state
information northbound only so that each level obtains the full
topology of levels south of it. Link-State information is, with some
exceptions, never flooded East-West or back South again. Exceptions
like south reflection is explained in detail in Section 4.2.5.1 and
east-west flooding at ToF level in multi-plane fabrics is outlined in
Section 4.1.2. In southbound direction, the protocol operates like a
"fully summarizing, unidirectional" path vector protocol or rather a
distance vector with implicit split horizon. Routing information,
normally just the default route, propagates one hop south and is 're-
advertised' by nodes at next lower level. However, RIFT uses
flooding in the southern direction as well to avoid the overhead of
building an update per adjacency. We omit describing the East-West
direction for the moment.
Those information flow constraints create not only an anisotropic
protocol (i.e. the information is not distributed "evenly" or
"clumped" but summarized along the N-S gradient) but also a "smooth"
information propagation where nodes do not receive the same
information from multiple directions at the same time. Normally,
accepting the same reachability on any link, without understanding
its topological significance, forces tie-breaking on some kind of
distance metric. And such tie-breaking leads ultimately in hop-by-
hop forwarding to shortest paths only. In contrast to that, RIFT,
under normal conditions, does not need to tie-break same reachability
information from multiple directions. Its computation principles
(south forwarding direction is always preferred) leads to valley-free
forwarding behavior. And since valley free routing is loop-free, it
can use all feasible paths which is another highly desirable property
if available bandwidth should be utilized to the maximum extent
possible.
To account for the "northern" and the "southern" information split
the link state database is partitioned accordingly into "north
representation" and "south representation" TIEs. In simplest terms
the North TIEs contain a link state topology description of lower
levels and and South TIEs carry simply default routes towards the
level above. This oversimplified view will be refined gradually in
following sections while introducing protocol procedures and state
machines at the same time.
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4.1.2. Generalized Topology View
This section will shed some light on the topologies RIFT addresses,
including multi plane fabrics and their implications. Readers that
are only interested in single plane designs, i.e. all top-of-fabric
nodes being topologically equal and initially connected to all the
switches at the level below them, can skip the rest of Section 4.1.2
and resulting Section 4.2.5.2 as well.
It is quite difficult to visualize multi plane design, which are
effectively multi-dimensional switching matrices. To cope with that,
we will introduce a methodology allowing us to depict the
connectivity in two-dimensional pictures. Further, we will leverage
the fact that we are dealing basically with stacked crossbar fabrics
where ports align "on top of each other" in a regular fashion.
A word of caution to the reader; at this point it should be observed
that the language used to describe Clos variations, especially in
multi-plane designs, varies widely between sources. This description
follows the terminology introduced in Section 3.1. It is unavoidable
to have it present to be able to follow the rest of this section
correctly.
4.1.2.1. Terminology
This section describes the terminology and acronyms used in the rest
of the text.
P: Denotes the number of PoDs in a topology.
S: Denotes the number of ToF nodes in a topology.
K: Denotes the number of ports in radix of a switch pointing north or
south. Further, K_LEAF denotes number of ports pointing south,
i.e. towards leaves, and K_TOP for number of ports pointing north
towards a higher spine level. To simplify the visual aids,
notations and further considerations, K will be mostly set to
Radix/2.
ToF Plane: Set of ToFs that are aware of each other by means of
south reflection. We number planes by capital letters, e.g.
plane A.
N: Denote the number of independent ToF planes in a topology.
R: Denotes a redundancy factor, i.e. number of connections a spine
has towards a ToF plane. In single plane design K_TOP is equal to
R.
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Fallen Leaf: A fallen leaf in a plane Z is a switch that lost all
connectivity northbound to Z.
4.1.2.2. Clos as Crossed Crossbars
The typical topology for which RIFT is defined is built of P number
of PoDs and connected together by S number of ToF nodes. A PoD node
has K number of ports (also called Radix). We consider half of them
(K=Radix/2) as connecting host devices from the south, and the other
half connecting to interleaved PoD Top-Level switches to the north.
Ratio K can be chosen differently without loss of generality when
port speeds differ or the fabric is oversubscribed but K=R/2 allows
for more readable representation whereby there are as many ports
facing north as south on any intermediate node. We represent a node
hence in a schematic fashion with ports "sticking out" to its north
and south rather than by the usual real-world front faceplate designs
of the day.
Figure 4 provides a view of a leaf node as seen from the north, i.e.
showing ports that connect northbound. For lack of a better symbol,
we have chosen to use the "o" as ASCII visualisation of a single
port. In this example, K_LEAF has 6 ports. Observe that the number
of PoDs is not related to Radix unless the ToF Nodes are constrained
to be the same as the PoD nodes in a particular deployment.
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Top view
+---+
| |
| o | e.g., Radix = 12, K_LEAF = 6
| |
| o |
| | -------------------------
| o ------- Physical Port (Ethernet) ----+
| | ------------------------- |
| o | |
| | |
| o | |
| | |
| o | |
| | |
+---+ |
|| || || || || || ||
+----+ +------------------------------------------------+
| | | |
+----+ +------------------------------------------------+
|| || || || || || ||
Side views
Figure 4: A Leaf Node, K_LEAF=6
The Radix of a PoD's top node may be different than that of the leaf
node. Though, more often than not, a same type of node is used for
both, effectively forming a square (K*K). In general case, we could
have switches with K_TOP southern ports on nodes at the top of the
PoD which are not necessarily the same as K_LEAF. For instance, in
the representations below, we pick a 6 port K_LEAF and a 8 port
K_TOP. In order to form a crossbar, we need K_TOP Leaf Nodes as
illustrated in Figure 5.
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+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| | | | | | | | | | | | | | | |
| o | | o | | o | | o | | o | | o | | o | | o |
| | | | | | | | | | | | | | | |
| o | | o | | o | | o | | o | | o | | o | | o |
| | | | | | | | | | | | | | | |
| o | | o | | o | | o | | o | | o | | o | | o |
| | | | | | | | | | | | | | | |
| o | | o | | o | | o | | o | | o | | o | | o |
| | | | | | | | | | | | | | | |
| o | | o | | o | | o | | o | | o | | o | | o |
| | | | | | | | | | | | | | | |
| o | | o | | o | | o | | o | | o | | o | | o |
| | | | | | | | | | | | | | | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
Figure 5: Southern View of a PoD, K_TOP=8
As further visualized in Figure 6 the K_TOP Leaf Nodes are fully
interconnected with the K_LEAF PoD-top nodes, providing connectivity
that can be represented as a crossbar when "looked at" from the
north. The result is that, in the absence of a failure, a packet
entering the PoD from the north on any port can be routed to any port
in the south of the PoD and vice versa. And that is precisely why it
makes sense to talk about a "switching matrix".
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E<-*->W
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| | | | | | | | | | | | | | | |
+--------------------------------------------------------+
| o o o o o o o o |
+--------------------------------------------------------+
+--------------------------------------------------------+
| o o o o o o o o |
+--------------------------------------------------------+
+--------------------------------------------------------+
| o o o o o o o o |
+--------------------------------------------------------+
+--------------------------------------------------------+
| o o o o o o o o |
+--------------------------------------------------------+
+--------------------------------------------------------+
| o o o o o o o o |<-+
+--------------------------------------------------------+ |
+--------------------------------------------------------+ |
| o o o o o o o o | |
+--------------------------------------------------------+ |
| | | | | | | | | | | | | | | | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ |
^ |
| |
| ---------- --------------------- |
+----- Leaf Node PoD top Node (Spine) --+
---------- ---------------------
Figure 6: Northern View of a PoD's Spines, K_TOP=8
Side views of this PoD is illustrated in Figure 7 and Figure 8.
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Connecting to Spine
|| || || || || || || ||
+----------------------------------------------------------------+ N
| PoD top Node seen sideways | ^
+----------------------------------------------------------------+ |
|| || || || || || || || *
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+ |
| | | | | | | | | | | | | | | | v
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+ S
|| || || || || || || ||
Connecting to Client nodes
Figure 7: Side View of a PoD, K_TOP=8, K_LEAF=6
Connecting to Spine
|| || || || || ||
+----+ +----+ +----+ +----+ +----+ +----+ N
| | | | | | | | | | | PoD top Nodes ^
+----+ +----+ +----+ +----+ +----+ +----+ |
|| || || || || || *
+------------------------------------------------+ |
| Leaf seen sideways | v
+------------------------------------------------+ S
|| || || || || ||
Connecting to Client nodes
Figure 8: Other Side View of a PoD, K_TOP=8, K_LEAF=6, 90o turn in
E-W Plane
As next step, let us observe that a resulting PoD can be abstracted
as a bigger node with a number K of K_POD= K_TOP * K_LEAF, and the
design can recurse.
It will be critical at this point that, before progressing further,
the concept and the picture of "crossed crossbars" is clear. Else,
the following considerations might be difficult to comprehend.
To continue, the PoDs are interconnected with each other through a
Top-of-Fabric (ToF) node at the very top or the north edge of the
fabric. The resulting ToF is NOT partitioned if, and only if (IIF),
every PoD top level node (spine) is connected to every ToF Node.
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This topology is also referred to as a single plane configuration and
is quite popular due to its simplicity. In order to reach a 1:1
connectivity ratio between the ToF and the leaves, it results that
there are K_TOP ToF nodes, because each port of a ToP node connects
to a different ToF node, and K_LEAF ToP nodes for the same reason.
Consequently, it will take (P * K_LEAF) ports on a ToF node to
connect to each of the K_LEAF ToP nodes of the P PoDs, as shown in
Figure 9.
[ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] <-----+
| | | | | | | | |
[=================================] | -----------
| | | | | | | | +----- Top-of-Fabric
[ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] +----- Node -------+
| ----------- |
| v
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ <-----+ +-+
| | | | | | | | | | | | | | | | | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] ------------------------- | |
[ |o| |o| |o| |o| |o| |o| |o| |o<--- Physical Port (Ethernet) | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] ------------------------- | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | |
| | | | | | | | | | | | | | | | | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] -------------- | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] <--- PoD top level | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] node (Spine) ---+ | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] -------------- | | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | | |
| | | | | | | | | | | | | | | | -+ +- +-+ v | |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | | --| |--[ ]--| |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | ----- | --| |--[ ]--| |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] +--- PoD ---+ --| |--[ ]--| |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | ----- | --| |--[ ]--| |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | | --| |--[ ]--| |
[ |o| |o| |o| |o| |o| |o| |o| |o| ] | | --| |--[ ]--| |
| | | | | | | | | | | | | | | | -+ +- +-+ | |
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
Figure 9: Fabric Spines and TOFs in Single Plane Design, 3 PoDs
The top view can be collapsed into a third dimension where the hidden
depth index is representing the PoD number. We can then show one PoD
as a class of PoDs and hence save one dimension in our
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representation. The Spine Node expands in the depth and the vertical
dimensions, whereas the PoD top level Nodes are constrained, in
horizontal dimension. A port in the 2-D representation represents
effectively the class of all the ports at the same position in all
the PoDs that are projected in its position along the depth axis.
This is shown in Figure 10.
/ / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / / ]
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ ]]
| | | | | | | | | | | | | | | | ] ---------------------------
[ |o| |o| |o| |o| |o| |o| |o| |o| ] <-- PoD top level node (Spine)
[ |o| |o| |o| |o| |o| |o| |o| |o| ] ---------------------------
[ |o| |o| |o| |o| |o| |o| |o| |o| ]]]]
[ |o| |o| |o| |o| |o| |o| |o| |o| ]]] ^^
[ |o| |o| |o| |o| |o| |o| |o| |o| ]] // PoDs
[ |o| |o| |o| |o| |o| |o| |o| |o| ] // (in depth)
| |/| |/| |/| |/| |/| |/| |/| |/ //
+-+ +-+ +-+/+-+/+-+ +-+ +-+ +-+ //
^
| ----------------
+----- Top-of-Fabric Node
----------------
Figure 10: Collapsed Northern View of a Fabric for Any Number of PoDs
As simple as single plane deployment is it introduces a limit due to
the bound on the available radix of the ToF nodes that has to be at
least P * K_LEAF. Nevertheless, we will see that a distinct
advantage of a connected or non-partitioned Top-of-Fabric is that all
failures can be resolved by simple, non-transitive, positive
disaggregation (i.e. nodes advertising more specific prefixes with
the default to the level below them that is however not propagated
further down the fabric) as described in Section 4.2.5.1 . In other
words; non-partitioned ToF nodes can always reach nodes below or
withdraw the routes from PoDs they cannot reach unambiguously. And
with this, positive disaggregation can heal all failures and still
allow all the ToF nodes to see each other via south reflection.
Disaggregation will be explained in further detail in Section 4.2.5.
In order to scale beyond the "single plane limit", the Top-of-Fabric
can be partitioned by a N number of identically wired planes where N
is an integer divider of K_LEAF. The 1:1 ratio and the desired
symmetry are still served, this time with (K_TOP * N) ToF nodes, each
of (P * K_LEAF / N) ports. N=1 represents a non-partitioned Spine
and N=K_LEAF is a maximally partitioned Spine. Further, if R is any
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integer divisor of K_LEAF, then N=K_LEAF/R is a feasible number of
planes and R a redundancy factor. If proves convenient for
deployments to use a radix for the leaf nodes that is a power of 2 so
they can pick a number of planes that is a lower power of 2. The
example in Figure 11 splits the Spine in 2 planes with a redundancy
factor R=3, meaning that there are 3 non-intersecting paths between
any leaf node and any ToF node. A ToF node must have, in this case,
at least 3*P ports, and be directly connected to 3 of the 6 PoD-ToP
nodes (spines) in each PoD.
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
+-| |--| |--| |--| |--| |--| |--| |--| |-+
| | o | | o | | o | | o | | o | | o | | o | | o | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+
+-| |--| |--| |--| |--| |--| |--| |--| |-+
| | o | | o | | o | | o | | o | | o | | o | | o | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+
+-| |--| |--| |--| |--| |--| |--| |--| |-+
| | o | | o | | o | | o | | o | | o | | o | | o | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
Plane 1
----------- . ------------ . ------------ . ------------ . --------
Plane 2
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
+-| |--| |--| |--| |--| |--| |--| |--| |-+
| | o | | o | | o | | o | | o | | o | | o | | o | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+
+-| |--| |--| |--| |--| |--| |--| |--| |-+
| | o | | o | | o | | o | | o | | o | | o | | o | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+
+-| |--| |--| |--| |--| |--| |--| |--| |-+
| | o | | o | | o | | o | | o | | o | | o | | o | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
^
|
| ----------------
+----- Top-of-Fabric node
"across" depth
----------------
Figure 11: Northern View of a Multi-Plane ToF Level, K_LEAF=6, N=2
At the extreme end of the spectrum it is even possible to fully
partition the spine with N = K_LEAF and R=1, while maintaining
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connectivity between each leaf node and each Top-of-Fabric node. In
that case the ToF node connects to a single Port per PoD, so it
appears as a single port in the projected view represented in
Figure 12. The number of ports required on the Spine Node is more or
equal to P, the number of PoDs.
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Plane 1
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ -+
+-| |--| |--| |--| |--| |--| |--| |--| |-+ |
| | o | | o | | o | | o | | o | | o | | o | | o | | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ |
----------- . ------------------- . ------------ . -------- |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ |
| | o | | o | | o | | o | | o | | o | | o | | o | | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ |
----------- . ------------ . ---- . ------------ . -------- |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ |
| | o | | o | | o | | o | | o | | o | | o | | o | | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ |
----------- . ------------ . ------------------- . -------- +<-+
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ | |
| | o | | o | | o | | o | | o | | o | | o | | o | | | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ | |
----------- . ------------ . ------------ . ---- . -------- | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ | |
| | o | | o | | o | | o | | o | | o | | o | | o | | | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ | |
----------- . ------------ . ------------ . --------------- | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ | |
| | o | | o | | o | | o | | o | | o | | o | | o | | | |
+-| |--| |--| |--| |--| |--| |--| |--| |-+ | |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ -+ |
Plane 6 ^ |
| |
| ---------------- ------------- |
+----- ToF Node Class of PoDs ---+
---------------- -------------
Figure 12: Northern View of a Maximally Partitioned ToF Level, R=1
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4.1.3. Fallen Leaf Problem
As mentioned earlier, RIFT exhibits an anisotropic behavior tailored
for fabrics with a North / South orientation and a high level of
interleaving paths. A non-partitioned fabric makes a total loss of
connectivity between a Top-of-Fabric node at the north and a leaf
node at the south a very rare but yet possible occasion that is fully
healed by positive disaggregation as described in Section 4.2.5.1.
In large fabrics or fabrics built from switches with low radix, the
ToF ends often being partitioned in planes which makes the occurrence
of having a given leaf being only reachable from a subset of the ToF
nodes more likely to happen. This makes some further considerations
necessary.
We define a "Fallen Leaf" as a leaf that can be reached by only a
subset, but not all, of Top-of-Fabric nodes due to missing
connectivity. If R is the redundancy factor, then it takes at least
R breakages to reach a "Fallen Leaf" situation.
In a maximally partitioned fabric, the redundancy factor is R= 1, so
any breakage in the fabric may cause one or more fallen leaves.
However, not all cases require disaggregation. The following cases
do not require particular action in such scenario:
If a southern link on a node goes down, then connectivity through
that node is lost for all nodes south of it. There is no need to
disaggregate since the connectivity to this node is lost for all
spine nodes in a same fashion.
If a ToF Node goes down, then northern traffic towards it is
routed via alternate ToF nodes in the same plane and there is no
need to disaggregate routes.
In a general manner, the mechanism of non-transitive positive
disaggregation is sufficient when the disaggregating ToF nodes
collectively connect to all the ToP nodes in the broken plane. This
happens in the following case:
If the breakage is the last northern link from a ToP node to a ToF
node going down, then the fallen leaf problem affects only The ToF
node, and the connectivity to all the nodes in the PoD is lost
from that ToF node. This can be observed by other ToF nodes
within the plane where the ToP node is located and positively
disaggregated within that plane.
On the other hand, there is a need to disaggregate the routes to
Fallen Leaves in a transitive fashion, all the way to the other
leaves in the following cases:
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o If the breakage is the last northern link from a leaf node within
a plane (there is only one such link in a maximally partitioned
fabric) that goes down, then connectivity to all unicast prefixes
attached to the leaf node is lost within the plane where the link
is located. Southern Reflection by a leaf node, e.g., between ToP
nodes, if the PoD has only 2 levels, happens in between planes,
allowing the ToP nodes to detect the problem within the PoD where
it occurs and positively disaggregate. The breakage can be
observed by the ToF nodes in the same plane through the North
flooding of TIEs from the ToP nodes. The ToF nodes however need
to be aware of all the affected prefixes for the negative,
possibly transitive disaggregation to be fully effective (i.e. a
node advertising in control plane that it cannot reach a certain
more specific prefix than default whereas such disaggregation must
in extreme condition propagate further down southbound). The
problem can also be observed by the ToF nodes in the other planes
through the flooding of North TIEs from the affected leaf nodes,
together with non-node North TIEs which indicate the affected
prefixes. To be effective in that case, the positive
disaggregation must reach down to the nodes that make the plane
selection, which are typically the ingress leaf nodes. The
information is not useful for routing in the intermediate levels.
o If the breakage is a ToP node in a maximally partitioned fabric -
in which case it is the only ToP node serving the plane in that
PoD - goes down, then the connectivity to all the nodes in the PoD
is lost within the plane where the ToP node is located.
Consequently, all leaves of the PoD fall in this plane. Since the
Southern Reflection between the ToF nodes happens only within a
plane, ToF nodes in other planes cannot discover fallen leaves in
a different plane. They also cannot determine beyond their local
plane whether a leaf node that was initially reachable has become
unreachable. As the breakage can be observed by the ToF nodes in
the plane where the breakage happened, the ToF nodes in the plane
need to be aware of all the affected prefixes for the negative
disaggregation to be fully effective. The problem can also be
observed by the ToF nodes in the other planes through the flooding
of North TIEs from the affected leaf nodes, if there are only 3
levels and the ToP nodes are directly connected to the leaf nodes,
and then again it can only be effective it is propagated
transitively to the leaf, and useless above that level.
For the sake of easy comprehension let us roll the abstractions back
into a simple example and observe that in Figure 3 the loss of link
Spine 122 to Leaf 122 will make Leaf 122 a fallen leaf for Top-of-
Fabric plane B. Worse, if the cabling was never present in first
place, plane B will not even be able to know that such a fallen leaf
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exists. Hence partitioning without further treatment results in two
grave problems:
o Leaf 111 trying to route to Leaf 122 MUST choose Spine 111 in
plane A as its next hop since plane B will inevitably blackhole
the packet when forwarding using default routes or do excessive
bow tying. This information must be in its routing table.
o Any kind of "flooding" or distance vector trying to deal with the
problem by distributing host routes will be able to converge only
using paths through leaves. The flooding of information on Leaf
122 would have to go up to Top-of-Fabric A and then "loopback"
over other leaves to ToF B leading in extreme cases to traffic for
Leaf 122 when presented to plane B taking an "inverted fabric"
path where leaves start to serve as TOFs, at least for the
duration of a protocol's convergence.
4.1.4. Discovering Fallen Leaves
As illustrated later, and without further proof, the way to deal with
fallen leaves in multi-plane designs, when aggregation is used, is
that RIFT requires all the ToF nodes to share the same north topology
database. This happens naturally in single plane design by the means
of northbound flooding and south reflection but needs additional
considerations in multi-plane fabrics. To satisfy this RIFT, in
multi-plane designs, relies at the ToF level on ring interconnection
of switches in multiple planes. Other solutions are possible but
they either need more cabling or end up having much longer flooding
paths and/or single points of failure.
In detail, by reserving two ports on each Top-of-Fabric node it is
possible to connect them together by interplane bi-directional rings
as illustrated in Figure 13. The rings will be used to exchange full
north topology information between planes. All ToFs having same
north topology allows by the means of transitive, negative
disaggregation described in Section 4.2.5.2 to efficiently fix any
possible fallen leaf scenario. Somewhat as a side-effect, the
exchange of information fulfills the ask to present full view of the
fabric topology at the Top-of-Fabric level, without the need to
collate it from multiple points by additional complexity of
technologies like [RFC7752].
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+---+ +---+ +---+ +---+ +---+ +---+ +--------+
| | | | | | | | | | | | | |
| | | | | | | |
+-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ |
+-| |--| |--| |--| |--| |--| |--| |-+ |
| | o | | o | | o | | o | | o | | o | | o | | | Plane A
+-| |--| |--| |--| |--| |--| |--| |-+ |
+-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ |
| | | | | | | |
+-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ |
+-| |--| |--| |--| |--| |--| |--| |-+ |
| | o | | o | | o | | o | | o | | o | | o | | | Plane B
+-| |--| |--| |--| |--| |--| |--| |-+ |
+-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ |
| | | | | | | |
... |
| | | | | | | |
+-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ |
+-| |--| |--| |--| |--| |--| |--| |-+ |
| | o | | o | | o | | o | | o | | o | | o | | | Plane X
+-| |--| |--| |--| |--| |--| |--| |-+ |
+-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ +-o-+ |
| | | | | | | |
| | | | | | | | | | | | | |
+---+ +---+ +---+ +---+ +---+ +---+ +--------+
Rings 1 2 3 4 5 6 7
Figure 13: Connecting Top-of-Fabric Nodes Across Planes by Rings
4.1.5. Addressing the Fallen Leaves Problem
One consequence of the "Fallen Leaf" problem is that some prefixes
attached to the fallen leaf become unreachable from some of the ToF
nodes. RIFT proposes two methods to address this issue, the positive
and the negative disaggregation. Both methods flood South TIEs to
advertise the impacted prefix(es).
When used for the operation of disaggregation, a positive South TIE,
as usual, indicates reachability to a prefix of given length and all
addresses subsumed by it. In contrast, a negative route
advertisement indicates that the origin cannot route to the
advertised prefix.
The positive disaggregation is originated by a router that can still
reach the advertised prefix, and the operation is not transitive. In
other words, the receiver does not generate its own flooding south as
a consequence of receiving positive disaggregation advertisements
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from a higher level node. The effect of a positive disaggregation is
that the traffic to the impacted prefix will follow the longest match
and will be limited to the northbound routers that advertised the
more specific route.
In contrast, the negative disaggregation can be transitive, and is
propagated south when all the possible routes have been advertised as
negative exceptions. A negative route advertisement is only
actionable when the negative prefix is aggregated by a positive route
advertisement for a shorter prefix. In such case, the negative
advertisement "punches out a hole" in the positive route in the
routing table, making the positive prefix reachable through the
originator with the special consideration of the negative prefix
removing certain next hop neighbors.
When the ToF is not partitioned, the collective southern flooding of
the positive disaggregation by the ToF nodes that can still reach the
impacted prefix is in general enough to cover all the switches at the
next level south, typically the ToP nodes. If all those switches are
aware of the disaggregation, they collectively create a ceiling that
intercepts all the traffic north and forwards it to the ToF nodes
that advertised the more specific route. In that case, the positive
disaggregation alone is sufficient to solve the fallen leaf problem.
On the other hand, when the fabric is partitioned in planes, the
positive disaggregation from ToF nodes in different planes do not
reach the ToP switches in the affected plane and cannot solve the
fallen leaves problem. In other words, a breakage in a plane can
only be solved in that plane. Also, the selection of the plane for a
packet typically occurs at the leaf level and the disaggregation must
be transitive and reach all the leaves. In that case, the negative
disaggregation is necessary. The details on the RIFT approach to
deal with fallen leaves in an optimal way are specified in
Section 4.2.5.2.
4.2. Specification
This section specifies the protocol in a normative fashion by either
prescriptive procedures or behavior defined by Finite State Machines
(FSM).
Some FSM figures are provided as [DOT] description due to limitations
of ASCII art.
"On Entry" actions on FSM state are performed every time and right
before the according state is entered, i.e. after any transitions
from previous state.
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"On Exit" actions are performed every time and immediately when a
state is exited, i.e. before any transitions towards target state are
performed.
Any attempt to transition from a state towards another on reception
of an event where no action is specified MUST be considered an
unrecoverable error.
The FSMs and procedures are normative in the sense that an
implementation MUST implement them either literally or an
implementation MUST exhibit externally observable behavior that is
identical to the execution of the specified FSMs.
Where a FSM representation is inconvenient, i.e. the amount of
procedures and kept state exceeds the amount of transitions, we defer
to a more procedural description on data structures.
4.2.1. Transport
All packet formats are defined in Thrift [thrift] models in
Appendix B.
The serialized model is carried in an envelope within a UDP frame
that provides security and allows validation/modification of several
important fields without de-serialization for performance and
security reasons.
4.2.2. Link (Neighbor) Discovery (LIE Exchange)
RIFT LIE exchange auto-discovers neighbors, negotiates ZTP parameters
and discovers miscablings. It uses a three-way handshake mechanism
which is a cleaned up version of [RFC5303]. Observe that for easier
comprehension the terminology of one/two and three-way states does
NOT align with OSPF or ISIS FSMs albeit they use roughly same
mechanisms. The formation progresses under normal conditions from
one-way to two-way and then three-way state at which point it is
ready to exchange TIEs per Section 4.2.3.
LIE exchange happens over well-known administratively locally scoped
and configured or otherwise well-known IPv4 multicast address
[RFC2365] and/or link-local multicast scope [RFC4291] for IPv6
[RFC8200] using a configured or otherwise a well-known destination
UDP port defined in Appendix C.1. LIEs SHOULD be sent with an IPv4
Time to Live (TTL) / IPv6 Hop Limit (HL) of 1 to prevent RIFT
information reaching beyond a single L3 next-hop in the topology.
LIEs SHOULD be sent with network control precedence.
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Originating port of the LIE has no further significance other than
identifying the origination point. LIEs are exchanged over all links
running RIFT.
An implementation MAY listen and send LIEs on IPv4 and/or IPv6
multicast addresses. A node MUST NOT originate LIEs on an address
family if it does not process received LIEs on that family. LIEs on
same link are considered part of the same negotiation independent of
the address family they arrive on. Observe further that the LIE
source address may not identify the peer uniquely in unnumbered or
link-local address cases so the response transmission MUST occur over
the same interface the LIEs have been received on. A node MAY use
any of the adjacency's source addresses it saw in LIEs on the
specific interface during adjacency formation to send TIEs. That
implies that an implementation MUST be ready to accept TIEs on all
addresses it used as source of LIE frames.
A three-way adjacency over any address family implies support for
IPv4 forwarding if the `v4_forwarding_capable` flag is set to true
and a node can use [RFC5549] type of forwarding in such a situation.
It is expected that the whole fabric supports the same type of
forwarding of address families on all the links. Operation of a
fabric where only some of the links are supporting forwarding on an
address family and others do not is outside the scope of this
specification.
The protocol does NOT support selective disabling of address
families, disabling v4 forwarding capability or any local address
changes in three-way state, i.e. if a link has entered three-way IPv4
and/or IPv6 with a neighbor on an adjacency and it wants to stop
supporting one of the families or change any of its local addresses
or stop v4 forwarding, it has to tear down and rebuild the adjacency.
It also has to remove any information it stored about the adjacency
such as LIE source addresses seen.
Unless ZTP as described in Section 4.2.7 is used, each node is
provisioned with the level at which it is operating. It MAY be also
provisioned with its PoD. If any of those values is undefined, then
accordingly a default level and/or an "undefined" PoD are assumed.
This means that leaves do not need to be configured at all if initial
configuration values are all left at "undefined" value. Nodes above
ToP MUST remain at "any" PoD value which has the same value as
"undefined" PoD. This information is propagated in the LIEs
exchanged.
Further definitions of leaf flags are found in Section 4.2.7 given
they have implications in terms of level and adjacency forming here.
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A node tries to form a three-way adjacency if and only if
1. the node is in the same PoD or either the node or the neighbor
advertises "undefined/any" PoD membership (PoD# = 0) AND
2. the neighboring node is running the same MAJOR schema version AND
3. the neighbor is not member of some PoD while the node has a
northbound adjacency already joining another PoD AND
4. the neighboring node uses a valid System ID AND
5. the neighboring node uses a different System ID than the node
itself
6. the advertised MTUs match on both sides AND
7. both nodes advertise defined level values AND
8. [
i) the node is at level 0 and has no three way adjacencies
already to nodes at Highest Adjacency Three-Way level (HAT as
defined later in Section 4.2.7.1) with level different than
the adjacent node OR
ii) the node is not at level 0 and the neighboring node is at
level 0 OR
iii) both nodes are at level 0 AND both indicate support for
Section 4.3.8 OR
iv) neither node is at level 0 and the neighboring node is at
most one level away
].
The rules checking PoD numbering MAY be optionally disregarded by a
node if PoD detection is undesirable or has to be ignored. This will
not affect the correctness of the protocol except preventing
detection of certain miscabling cases.
A node configured with "undefined" PoD membership MUST, after
building first northbound three way adjacencies to a node being in a
defined PoD, advertise that PoD as part of its LIEs. In case that
adjacency is lost, from all available northbound three way
adjacencies the node with the highest System ID and defined PoD is
chosen. That way the northmost defined PoD value (normally the ToP
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nodes) can diffuse southbound towards the leaves "forcing" the PoD
value on any node with "undefined" PoD.
LIEs arriving with IPv4 Time to Live (TTL) / IPv6 Hop Limit (HL)
larger than 1 MUST be ignored.
A node SHOULD NOT send out LIEs without defined level in the header
but in certain scenarios it may be beneficial for trouble-shooting
purposes.
4.2.2.1. LIE FSM
This section specifies the precise, normative LIE FSM and can be
omitted unless the reader is pursuing an implementation of the
protocol.
Initial state is `OneWay`.
Event `MultipleNeighbors` occurs normally when more than two nodes
see each other on the same link or a remote node is quickly
reconfigured or rebooted without regressing to `OneWay` first. Each
occurrence of the event SHOULD generate a clear, according
notification to help operational deployments.
The machine sends LIEs on several transitions to accelerate adjacency
bring-up without waiting for the timer tic.
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Enter
|
V
+-----------+
| OneWay |<----+
| | | HALChanged [StoreHAL]
| Entry: | | HALSChanged [StoreHALS]
| [CleanUp] | | HATChanged [StoreHAT]
| | | HoldTimerExpired [-]
| | | InstanceNameMismatch [-]
| | | LevelChanged [UpdateLevel, PUSH SendLie]
| | | LieReceived [ProcessLIE]
| | | MTUMismatch [-]
| | | NeighborAddressAdded [-]
| | | NeighborChangedAddress [-]
| | | NeighborChangedLevel [-]
| | | NeighborChangedMinorFields [-]
| | | NeighborDroppedReflection [-]
| | | PODMismatch [-]
| | | SendLIE [SendLIE]
| | | TimerTick [PUSH SendLIE]
| | | UnacceptableHeader
| | | UpdateZTPOffer [SendOfferToZTPFSM]
| |-----+
| |
| |<--------------------- (ThreeWay)
| |--------------------->
| | ValidReflection [-]
| |
| |---------------------> (Multiple
| | MultipleNeighbors Neighbors
+-----------+ [StartMulNeighTimer] Wait)
^ |
| |
| | NewNeighbor [PUSH SendLIE]
| V
(TwoWay)
LIE FSM
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(OneWay)
| ^
| | HoldTimeExpired [-]
| | InstanceNameMismatch [-]
| | LevelChanged [StoreLevel]
| | MTUMismatch [-]
| | NeighborChangedAddress [-]
| | NeighborChangedLevel [-]
| | PODMismatch [-]
| | UnacceptableHeader [-]
V |
+-----------+
| TwoWay |<----+
| | | HALChanged [StoreHAL]
| | | HALSChanged [StoreHALS]
| | | HATChanged [StoreHAT]
| | | LevelChanged [StoreLevel]
| | | LIERcvd [ProcessLIE]
| | | SendLIE [SendLIE]
| | | TimerTick [PUSH SendLIE,
| | | IF HoldTimer expired
| | | PUSH HoldTimerExpired]
| | | UpdateZTPOffer [SendOfferToZTPFSM]
| |-----+
| |
| |<----------------------
| |----------------------> (Multiple
| | NewNeighbor Neighbors
| | [StartMulNeighTimer] Wait)
| | MultipleNeighbors
+-----------+ [StartMulNeighTimer]
^ |
| | ValidReflection [-]
| V
(ThreeWay)
LIE FSM (continued)
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(TwoWay) (OneWay)
^ | ^
| | | HoldTimerExpired [-]
| | | InstanceNameMismatch [-]
| | | LevelChanged [UpdateLevel]
| | | MTUMismatch [-]
| | | NeighborChangedAddress [-]
| | | NeighborChangedLevel [-]
NeighborDropped- | | | PODMismatch [-]
Reflection [-] | | | UnacceptableHeader [-]
| V |
+-----------+ |
| ThreeWay |-----+
| |
| |<----+
| | | HALChanged [StoreHAL]
| | | HALSChanged [StoreHALS]
| | | HATChanged [StoreHAT]
| | | LieReceived [ProcessLIE]
| | | SendLIE [SendLIE]
| | | TimerTick [PUSH SendLie,
| | | IF HoldTimer expired
| | | PUSH HoldTimerExpired]
| | | UpdateZTPOffer [SendOfferToZTPFSM]
| | | ValidReflection [-]
| |-----+
| |----------------------> (Multiple
| | MultipleNeighbors Neighbors
+-----------+ [StartMulNeighTimer] Wait)
LIE FSM (continued)
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(TwoWay) (ThreeWay)
| |
V V
+------------+
| Multiple |<----+
| Neighbors | | HALChanged [StoreHAL]
| Wait | | HALSChanged [StoreHALS]
| | | HATChanged [StoreHAT]
| | | MultipleNeighbors
| | | [StartMultipleNeighborsTimer]
| | | TimerTick [IF MulNeighTimer expired
| | | PUSH MultipleNeighborsDone]
| | | UpdateZTPOffer [SendOfferToZTP]
| |-----+
| |
| |<---------------------------
| |---------------------------> (OneWay)
| | LevelChanged [StoreLevel]
+------------+ MultipleNeighborsDone [-]
LIE FSM (continued)
Events
o TimerTick: one second timer tic
o LevelChanged: node's level has been changed by ZTP or
configuration
o HALChanged: best HAL computed by ZTP has changed
o HATChanged: HAT computed by ZTP has changed
o HALSChanged: set of HAL offering systems computed by ZTP has
changed
o LieRcvd: received LIE
o NewNeighbor: new neighbor parsed
o ValidReflection: received own reflection from neighbor
o NeighborDroppedReflection: lost previous own reflection from
neighbor
o NeighborChangedLevel: neighbor changed advertised level
o NeighborChangedAddress: neighbor changed IP address
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o UnacceptableHeader: unacceptable header seen
o MTUMismatch: MTU mismatched
o InstanceNameMismatch: Instance mismatched
o PODMismatch: Unacceptable PoD seen
o HoldtimeExpired: adjacency hold down expired
o MultipleNeighbors: more than one neighbor seen on interface
o MultipleNeighborsDone: cooldown for multiple neighbors expired
o SendLie: send a LIE out
o UpdateZTPOffer: update this node's ZTP offer
Actions
on MultipleNeighbors in OneWay finishes in MultipleNeighborsWait:
start multiple neighbors timer as 4 * DEFAULT_LIE_HOLDTIME
on NeighborDroppedReflection in ThreeWay finishes in TwoWay: no
action
on NeighborDroppedReflection in OneWay finishes in OneWay: no
action
on PODMismatch in TwoWay finishes in OneWay: no action
on NewNeighbor in TwoWay finishes in MultipleNeighborsWait: PUSH
SendLie event
on LieRcvd in OneWay finishes in OneWay: PROCESS_LIE
on UnacceptableHeader in ThreeWay finishes in OneWay: no action
on UpdateZTPOffer in TwoWay finishes in TwoWay: send offer to ZTP
FSM
on NeighborChangedAddress in ThreeWay finishes in OneWay: no
action
on HALChanged in MultipleNeighborsWait finishes in
MultipleNeighborsWait: store new HAL
on NeighborChangedAddress in TwoWay finishes in OneWay: no action
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on MultipleNeighbors in TwoWay finishes in MultipleNeighborsWait:
start multiple neighbors timer as 4 * DEFAULT_LIE_HOLDTIME
on LevelChanged in ThreeWay finishes in OneWay: update level with
event value
on LieRcvd in ThreeWay finishes in ThreeWay: PROCESS_LIE
on ValidReflection in OneWay finishes in ThreeWay: no action
on NeighborChangedLevel in TwoWay finishes in OneWay: no action
on MultipleNeighbors in ThreeWay finishes in
MultipleNeighborsWait: start multiple neighbors timer as 4 *
DEFAULT_LIE_HOLDTIME
on InstanceNameMismatch in OneWay finishes in OneWay: no action
on NewNeighbor in OneWay finishes in TwoWay: PUSH SendLie event
on UpdateZTPOffer in OneWay finishes in OneWay: send offer to ZTP
FSM
on UpdateZTPOffer in ThreeWay finishes in ThreeWay: send offer to
ZTP FSM
on MTUMismatch in ThreeWay finishes in OneWay: no action
on TimerTick in OneWay finishes in OneWay: PUSH SendLie event
on SendLie in TwoWay finishes in TwoWay: SEND_LIE
on ValidReflection in ThreeWay finishes in ThreeWay: no action
on InstanceNameMismatch in TwoWay finishes in OneWay: no action
on HoldtimeExpired in OneWay finishes in OneWay: no action
on TimerTick in ThreeWay finishes in ThreeWay: PUSH SendLie event,
if holdtime expired PUSH HoldtimeExpired event
on HALChanged in TwoWay finishes in TwoWay: store new HAL
on HoldtimeExpired in ThreeWay finishes in OneWay: no action
on HALSChanged in TwoWay finishes in TwoWay: store HALS
on HALSChanged in ThreeWay finishes in ThreeWay: store HALS
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on ValidReflection in TwoWay finishes in ThreeWay: no action
on MultipleNeighborsDone in MultipleNeighborsWait finishes in
OneWay: no action
on NeighborAddressAdded in OneWay finishes in OneWay: no action
on TimerTick in MultipleNeighborsWait finishes in
MultipleNeighborsWait: decrement MultipleNeighbors timer, if
expired PUSH MultipleNeighborsDone
on MTUMismatch in OneWay finishes in OneWay: no action
on MultipleNeighbors in MultipleNeighborsWait finishes in
MultipleNeighborsWait: start multiple neighbors timer as 4 *
DEFAULT_LIE_HOLDTIME
on LieRcvd in TwoWay finishes in TwoWay: PROCESS_LIE
on HATChanged in MultipleNeighborsWait finishes in
MultipleNeighborsWait: store HAT
on HoldtimeExpired in TwoWay finishes in OneWay: no action
on NeighborChangedLevel in ThreeWay finishes in OneWay: no action
on LevelChanged in OneWay finishes in OneWay: update level with
event value, PUSH SendLie event
on SendLie in OneWay finishes in OneWay: SEND_LIE
on HATChanged in OneWay finishes in OneWay: store HAT
on LevelChanged in TwoWay finishes in TwoWay: update level with
event value
on HATChanged in TwoWay finishes in TwoWay: store HAT
on PODMismatch in ThreeWay finishes in OneWay: no action
on LevelChanged in MultipleNeighborsWait finishes in OneWay:
update level with event value
on UnacceptableHeader in TwoWay finishes in OneWay: no action
on NeighborChangedLevel in OneWay finishes in OneWay: no action
on InstanceNameMismatch in ThreeWay finishes in OneWay: no action
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on HATChanged in ThreeWay finishes in ThreeWay: store HAT
on HALChanged in OneWay finishes in OneWay: store new HAL
on UnacceptableHeader in OneWay finishes in OneWay: no action
on HALChanged in ThreeWay finishes in ThreeWay: store new HAL
on UpdateZTPOffer in MultipleNeighborsWait finishes in
MultipleNeighborsWait: send offer to ZTP FSM
on NeighborChangedMinorFields in OneWay finishes in OneWay: no
action
on NeighborChangedAddress in OneWay finishes in OneWay: no action
on MTUMismatch in TwoWay finishes in OneWay: no action
on PODMismatch in OneWay finishes in OneWay: no action
on SendLie in ThreeWay finishes in ThreeWay: SEND_LIE
on TimerTick in TwoWay finishes in TwoWay: PUSH SendLie event, if
holdtime expired PUSH HoldtimeExpired event
on HALSChanged in OneWay finishes in OneWay: store HALS
on HALSChanged in MultipleNeighborsWait finishes in
MultipleNeighborsWait: store HALS
on Entry into OneWay: CLEANUP
Following words are used for well known procedures:
1. PUSH Event: pushes an event to be executed by the FSM upon exit
of this action
2. CLEANUP: neighbor MUST be reset to unknown
3. SEND_LIE: create a new LIE packet
1. reflecting the neighbor if known and valid and
2. setting the necessary `not_a_ztp_offer` variable if level was
derived from last known neighbor on this interface and
3. setting `you_are_flood_repeater` to computed value
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4. PROCESS_LIE:
1. if lie has wrong major version OR our own system ID or
invalid system ID then CLEANUP else
2. if lie has non matching MTUs then CLEANUP, PUSH
UpdateZTPOffer, PUSH MTUMismatch else
3. if PoD rules do not allow adjacency forming then CLEANUP,
PUSH PODMismatch, PUSH MTUMismatch else
4. if lie has undefined level OR my level is undefined OR this
node is leaf and remote level lower than HAT OR (lie's level
is not leaf AND its difference is more than one from my
level) then CLEANUP, PUSH UpdateZTPOffer, PUSH
UnacceptableHeader else
5. PUSH UpdateZTPOffer, construct temporary new neighbor
structure with values from lie, if no current neighbor exists
then set neighbor to new neighbor, PUSH NewNeighbor event,
CHECK_THREE_WAY else
1. if current neighbor system ID differs from lie's system
ID then PUSH MultipleNeighbors else
2. if current neighbor stored level differs from lie's level
then PUSH NeighborChangedLevel else
3. if current neighbor stored IPv4/v6 address differs from
lie's address then PUSH NeighborChangedAddress else
4. if any of neighbor's flood address port, name, local
linkid changed then PUSH NeighborChangedMinorFields and
5. CHECK_THREE_WAY
5. CHECK_THREE_WAY: if current state is one-way do nothing else
1. if lie packet does not contain neighbor then if current state
is three-way then PUSH NeighborDroppedReflection else
2. if packet reflects this system's ID and local port and state
is three-way then PUSH event ValidReflection else PUSH event
MultipleNeighbors
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4.2.3. Topology Exchange (TIE Exchange)
4.2.3.1. Topology Information Elements
Topology and reachability information in RIFT is conveyed by the
means of TIEs which have good amount of commonalities with LSAs in
OSPF.
The TIE exchange mechanism uses the port indicated by each node in
the LIE exchange and the interface on which the adjacency has been
formed as destination. It SHOULD use TTL of 1 as well and set inter-
network control precedence on according packets.
TIEs contain sequence numbers, lifetimes and a type. Each type has
ample identifying number space and information is spread across
possibly many TIEs of a certain type by the means of a hash function
that a node or deployment can individually determine. One extreme
design choice is a prefix per TIE which leads to more BGP-like
behavior where small increments are only advertised on route changes
vs. deploying with dense prefix packing into few TIEs leading to more
traditional IGP trade-off with fewer TIEs. An implementation may
even rehash prefix to TIE mapping at any time at the cost of
significant amount of re-advertisements of TIEs.
More information about the TIE structure can be found in the schema
in Appendix B.
4.2.3.2. South- and Northbound Representation
A central concept of RIFT is that each node represents itself
differently depending on the direction in which it is advertising
information. More precisely, a spine node represents two different
databases over its adjacencies depending whether it advertises TIEs
to the north or to the south/sideways. We call those differing TIE
databases either south- or northbound (South TIEs and North TIEs)
depending on the direction of distribution.
The North TIEs hold all of the node's adjacencies and local prefixes
while the South TIEs hold only all of the node's adjacencies, the
default prefix with necessary disaggregated prefixes and local
prefixes. We will explain this in detail further in Section 4.2.5.
The TIE types are mostly symmetric in both directions and Table 2
provides a quick reference to main TIE types including direction and
their function.
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+--------------------+----------------------------------------------+
| TIE-Type | Content |
+--------------------+----------------------------------------------+
| Node North TIE | node properties and adjacencies |
+--------------------+----------------------------------------------+
| Node South TIE | same content as node North TIE |
+--------------------+----------------------------------------------+
| Prefix North TIE | contains nodes' directly reachable prefixes |
+--------------------+----------------------------------------------+
| Prefix South TIE | contains originated defaults and directly |
| | reachable prefixes |
+--------------------+----------------------------------------------+
| Positive | contains disaggregated prefixes |
| Disaggregation | |
| South TIE | |
+--------------------+----------------------------------------------+
| Negative | contains special, negatively disaggregated |
| Disaggregation | prefixes to support multi-plane designs |
| South TIE | |
+--------------------+----------------------------------------------+
| External Prefix | contains external prefixes |
| North TIE | |
+--------------------+----------------------------------------------+
| Key-Value North | contains nodes northbound KVs |
| TIE | |
+--------------------+----------------------------------------------+
| Key-Value South | contains nodes southbound KVs |
| TIE | |
+--------------------+----------------------------------------------+
Table 2: TIE Types
As an example illustrating a databases holding both representations,
consider the topology in Figure 2 with the optional link between
spine 111 and spine 112 (so that the flooding on an East-West link
can be shown). This example assumes unnumbered interfaces. First,
here are the TIEs generated by some nodes. For simplicity, the key
value elements which may be included in their South TIEs or North
TIEs are not shown.
ToF 21 South TIEs:
Node South TIE:
NodeElement(level=2, neighbors((Spine 111, level 1, cost 1),
(Spine 112, level 1, cost 1), (Spine 121, level 1, cost 1),
(Spine 122, level 1, cost 1)))
Prefix South TIE:
SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))
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Spine 111 South TIEs:
Node South TIE:
NodeElement(level=1, neighbors((ToF 21, level 2, cost 1,
links(...)),
(ToF 22, level 2, cost 1, links(...)),
(Spine 112, level 1, cost 1, links(...)),
(Leaf111, level 0, cost 1, links(...)),
(Leaf112, level 0, cost 1, links(...))))
Prefix South TIE:
SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))
Spine 111 North TIEs:
Node North TIE:
NodeElement(level=1,
neighbors((ToF 21, level 2, cost 1, links(...)),
(ToF 22, level 2, cost 1, links(...)),
(Spine 112, level 1, cost 1, links(...)),
(Leaf111, level 0, cost 1, links(...)),
(Leaf112, level 0, cost 1, links(...))))
Prefix North TIE:
NorthPrefixesElement(prefixes(Spine 111.loopback)
Spine 121 South TIEs:
Node South TIE:
NodeElement(level=1, neighbors((ToF 21,level 2,cost 1),
(ToF 22, level 2, cost 1), (Leaf121, level 0, cost 1),
(Leaf122, level 0, cost 1)))
Prefix South TIE:
SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))
Spine 121 North TIEs:
Node North TIE:
NodeElement(level=1,
neighbors((ToF 21, level 2, cost 1, links(...)),
(ToF 22, level 2, cost 1, links(...)),
(Leaf121, level 0, cost 1, links(...)),
(Leaf122, level 0, cost 1, links(...))))
Prefix North TIE:
NorthPrefixesElement(prefixes(Spine 121.loopback)
Leaf112 North TIEs:
Node North TIE:
NodeElement(level=0,
neighbors((Spine 111, level 1, cost 1, links(...)),
(Spine 112, level 1, cost 1, links(...))))
Prefix North TIE:
NorthPrefixesElement(prefixes(Leaf112.loopback, Prefix112,
Prefix_MH))
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Figure 14: Example TIES Generated in a 2 Level Spine-and-Leaf
Topology
It may be here not necessarily obvious why the node South TIEs
contain all the adjacencies of the according node. This will be
necessary for algorithms given in Section 4.2.3.9 and Section 4.3.6.
4.2.3.3. Flooding
The mechanism used to distribute TIEs is the well-known (albeit
modified in several respects to take advantage of fat tree topology)
flooding mechanism used by today's link-state protocols. Although
flooding is initially more demanding to implement it avoids many
problems with update style used in diffused computation such as
distance vector protocols. Since flooding tends to present an
unscalable burden in large, densely meshed topologies (fat trees
being unfortunately such a topology) we provide as solution close to
optimal global flood reduction and load balancing optimization in
Section 4.2.3.9.
As described before, TIEs themselves are transported over UDP with
the ports indicated in the LIE exchanges and using the destination
address on which the LIE adjacency has been formed. For unnumbered
IPv4 interfaces same considerations apply as in equivalent OSPF case.
4.2.3.3.1. Normative Flooding Procedures
On reception of a TIE with an undefined level value in the packet
header the node SHOULD issue a warning and indiscriminately discard
the packet.
This section specifies the precise, normative flooding mechanism and
can be omitted unless the reader is pursuing an implementation of the
protocol.
Flooding Procedures are described in terms of a flooding state of an
adjacency and resulting operations on it driven by packet arrivals.
The FSM itself has basically just a single state and is not well
suited to represent the behavior. An implementation MUST behave on
the wire in the same way as the provided normative procedures of this
paragraph.
RIFT does not specify any kind of flood rate limiting since such
specifications always assume particular points in available
technology speeds and feeds and those points are shifting at faster
and faster rate (speed of light holding for the moment). The encoded
packets provide hints to react accordingly to losses or overruns.
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Flooding of all according topology exchange elements SHOULD be
performed at highest feasible rate whereas the rate of transmission
MUST be throttled by reacting to adequate features of the system such
as e.g. queue lengths or congestion indications in the protocol
packets.
A node SHOULD NOT send out any topology information elements if the
adjacency is not in a "three-way" state. No further tightening of
this rule is possible due to possible link buffering and re-ordering
of LIEs and TIEs/TIDEs/TIREs.
A node MUST drop any received TIEs/TIDEs/TIREs unless it is in three-
way state.
TIDEs and TIREs MUST NOT be re-flooded the way TIEs of other nodes
MUST be always generated by the node itself and cross only to the
neighboring node.
4.2.3.3.1.1. FloodState Structure per Adjacency
The structure contains conceptually the following elements. The word
collection or queue indicates a set of elements that can be iterated:
TIES_TX: Collection containing all the TIEs to transmit on the
adjacency.
TIES_ACK: Collection containing all the TIEs that have to be
acknowledged on the adjacency.
TIES_REQ: Collection containing all the TIE headers that have to be
requested on the adjacency.
TIES_RTX: Collection containing all TIEs that need retransmission
with the according time to retransmit.
Following words are used for well known procedures operating on this
structure:
TIE Describes either a full RIFT TIE or accordingly just the
`TIEHeader` or `TIEID`. The according meaning is unambiguously
contained in the context of the algorithm.
is_flood_reduced(TIE): returns whether a TIE can be flood reduced or
not.
is_tide_entry_filtered(TIE): returns whether a header should be
propagated in TIDE according to flooding scopes.
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is_request_filtered(TIE): returns whether a TIE request should be
propagated to neighbor or not according to flooding scopes.
is_flood_filtered(TIE): returns whether a TIE requested be flooded
to neighbor or not according to flooding scopes.
try_to_transmit_tie(TIE):
A. if not is_flood_filtered(TIE) then
1. remove TIE from TIES_RTX if present
2. if TIE" with same key on TIES_ACK then
a. if TIE" same or newer than TIE do nothing else
b. remove TIE" from TIES_ACK and add TIE to TIES_TX
3. else insert TIE into TIES_TX
ack_tie(TIE): remove TIE from all collections and then insert TIE
into TIES_ACK.
tie_been_acked(TIE): remove TIE from all collections.
remove_from_all_queues(TIE): same as `tie_been_acked`.
request_tie(TIE): if not is_request_filtered(TIE) then
remove_from_all_queues(TIE) and add to TIES_REQ.
move_to_rtx_list(TIE): remove TIE from TIES_TX and then add to
TIES_RTX using TIE retransmission interval.
clear_requests(TIEs): remove all TIEs from TIES_REQ.
bump_own_tie(TIE): for self-originated TIE originate an empty or re-
generate with version number higher then the one in TIE.
The collection SHOULD be served with following priorities if the
system cannot process all the collections in real time:
Elements on TIES_ACK should be processed with highest priority
TIES_TX
TIES_REQ and TIES_RTX
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4.2.3.3.1.2. TIDEs
`TIEID` and `TIEHeader` space forms a strict total order (modulo
incomparable sequence numbers in the very unlikely event that can
occur if a TIE is "stuck" in a part of a network while the originator
reboots and reissues TIEs many times to the point its sequence# rolls
over and forms incomparable distance to the "stuck" copy) which
implies that a comparison relation is possible between two elements.
With that it is implicitly possible to compare TIEs, TIEHeaders and
TIEIDs to each other whereas the shortest viable key is always
implied.
When generating and sending TIDEs an implementation SHOULD ensure
that enough bandwidth is left to send elements of Floodstate
structure.
4.2.3.3.1.2.1. TIDE Generation
As given by timer constant, periodically generate TIDEs by:
NEXT_TIDE_ID: ID of next TIE to be sent in TIDE.
TIDE_START: Begin of TIDE packet range.
a. NEXT_TIDE_ID = MIN_TIEID
b. while NEXT_TIDE_ID not equal to MAX_TIEID do
1. TIDE_START = NEXT_TIDE_ID
2. HEADERS = At most TIRDEs_PER_PKT headers in TIEDB starting at
NEXT_TIDE_ID or higher that SHOULD be filtered by
is_tide_entry_filtered and MUST either have a lifetime left >
0 or have no content
3. if HEADERS is empty then START = MIN_TIEID else START = first
element in HEADERS
4. if HEADERS' size less than TIRDEs_PER_PKT then END =
MAX_TIEID else END = last element in HEADERS
5. send sorted HEADERS as TIDE setting START and END as its
range
6. NEXT_TIDE_ID = END
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The constant `TIRDEs_PER_PKT` SHOULD be generated and used by the
implementation to limit the amount of TIE headers per TIDE so the
sent TIDE PDU does not exceed interface MTU.
TIDE PDUs SHOULD be spaced on sending to prevent packet drops.
4.2.3.3.1.2.2. TIDE Processing
On reception of TIDEs the following processing is performed:
TXKEYS: Collection of TIE Headers to be send after processing of
the packet
REQKEYS: Collection of TIEIDs to be requested after processing of
the packet
CLEARKEYS: Collection of TIEIDs to be removed from flood state
queues
LASTPROCESSED: Last processed TIEID in TIDE
DBTIE: TIE in the LSDB if found
a. LASTPROCESSED = TIDE.start_range
b. for every HEADER in TIDE do
1. DBTIE = find HEADER in current LSDB
2. if HEADER < LASTPROCESSED then report error and reset
adjacency and return
3. put all TIEs in LSDB where (TIE.HEADER > LASTPROCESSED and
TIE.HEADER < HEADER) into TXKEYS
4. LASTPROCESSED = HEADER
5. if DBTIE not found then
I) if originator is this node then bump_own_tie
II) else put HEADER into REQKEYS
6. if DBTIE.HEADER < HEADER then
I) if originator is this node then bump_own_tie else
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i. if this is a North TIE header from a northbound
neighbor then override DBTIE in LSDB with HEADER
ii. else put HEADER into REQKEYS
7. if DBTIE.HEADER > HEADER then put DBTIE.HEADER into TXKEYS
8. if DBTIE.HEADER = HEADER then
I) if DBTIE has content already then put DBTIE.HEADER
into CLEARKEYS
II) else put HEADER into REQKEYS
c. put all TIEs in LSDB where (TIE.HEADER > LASTPROCESSED and
TIE.HEADER <= TIDE.end_range) into TXKEYS
d. for all TIEs in TXKEYS try_to_transmit_tie(TIE)
e. for all TIEs in REQKEYS request_tie(TIE)
f. for all TIEs in CLEARKEYS remove_from_all_queues(TIE)
4.2.3.3.1.3. TIREs
4.2.3.3.1.3.1. TIRE Generation
Elements from both TIES_REQ and TIES_ACK MUST be collected and sent
out as fast as feasible as TIREs. When sending TIREs with elements
from TIES_REQ the `lifetime` field MUST be set to 0 to force
reflooding from the neighbor even if the TIEs seem to be same.
4.2.3.3.1.3.2. TIRE Processing
On reception of TIREs the following processing is performed:
TXKEYS: Collection of TIE Headers to be send after processing of
the packet
REQKEYS: Collection of TIEIDs to be requested after processing of
the packet
ACKKEYS: Collection of TIEIDs that have been acked
DBTIE: TIE in the LSDB if found
a. for every HEADER in TIRE do
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1. DBTIE = find HEADER in current LSDB
2. if DBTIE not found then do nothing
3. if DBTIE.HEADER < HEADER then put HEADER into REQKEYS
4. if DBTIE.HEADER > HEADER then put DBTIE.HEADER into TXKEYS
5. if DBTIE.HEADER = HEADER then put DBTIE.HEADER into ACKKEYS
b. for all TIEs in TXKEYS try_to_transmit_tie(TIE)
c. for all TIEs in REQKEYS request_tie(TIE)
d. for all TIEs in ACKKEYS tie_been_acked(TIE)
4.2.3.3.1.4. TIEs Processing on Flood State Adjacency
On reception of TIEs the following processing is performed:
ACKTIE: TIE to acknowledge
TXTIE: TIE to transmit
DBTIE: TIE in the LSDB if found
a. DBTIE = find TIE in current LSDB
b. if DBTIE not found then
1. if originator is this node then bump_own_tie with a short
remaining lifetime
2. else insert TIE into LSDB and ACKTIE = TIE
else
1. if DBTIE.HEADER = TIE.HEADER then
i. if DBTIE has content already then ACKTIE = TIE
ii. else process like the "DBTIE.HEADER < TIE.HEADER" case
2. if DBTIE.HEADER < TIE.HEADER then
i. if originator is this node then bump_own_tie
ii. else insert TIE into LSDB and ACKTIE = TIE
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3. if DBTIE.HEADER > TIE.HEADER then
i. if DBTIE has content already then TXTIE = DBTIE
ii. else ACKTIE = DBTIE
c. if TXTIE is set then try_to_transmit_tie(TXTIE)
d. if ACKTIE is set then ack_tie(TIE)
4.2.3.3.1.5. TIEs Processing When LSDB Received Newer Version on Other
Adjacencies
The Link State Database can be considered to be a switchboard that
does not need any flooding procedures but can be given new versions
of TIEs by a peer. Consecutively, a peer receives from the LSDB
newer versions of TIEs received by other peers and processes them
(without any filtering) just like receiving TIEs from its remote
peer. This publisher model can be implemented in many ways.
4.2.3.3.1.6. Sending TIEs
On a periodic basis all TIEs with lifetime left > 0 MUST be sent out
on the adjacency, removed from TIES_TX list and requeued onto
TIES_RTX list.
4.2.3.4. TIE Flooding Scopes
In a somewhat analogous fashion to link-local, area and domain
flooding scopes, RIFT defines several complex "flooding scopes"
depending on the direction and type of TIE propagated.
Every North TIE is flooded northbound, providing a node at a given
level with the complete topology of the Clos or Fat Tree network that
is reachable southwards of it, including all specific prefixes. This
means that a packet received from a node at the same or lower level
whose destination is covered by one of those specific prefixes will
be routed directly towards the node advertising that prefix rather
than sending the packet to a node at a higher level.
A node's Node South TIEs, consisting of all node's adjacencies and
prefix South TIEs limited to those related to default IP prefix and
disaggregated prefixes, are flooded southbound in order to allow the
nodes one level down to see connectivity of the higher level as well
as reachability to the rest of the fabric. In order to allow an E-W
disconnected node in a given level to receive the South TIEs of other
nodes at its level, every *NODE* South TIE is "reflected" northbound
to level from which it was received. It should be noted that East-
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West links are included in South TIE flooding (except at ToF level);
those TIEs need to be flooded to satisfy algorithms in Section 4.2.4.
In that way nodes at same level can learn about each other without a
lower level, e.g. in case of leaf level. The precise, normative
flooding scopes are given in Table 3. Those rules govern as well
what SHOULD be included in TIDEs on the adjacency. Again, East-West
flooding scopes are identical to South flooding scopes except in case
of ToF East-West links (rings) which are basically performing
northbound flooding.
Node South TIE "south reflection" allows to support positive
disaggregation on failures describes in Section 4.2.5 and flooding
reduction in Section 4.2.3.9.
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+-----------+---------------------+----------------+-----------------+
| Type / | South | North | East-West |
| Direction | | | |
+-----------+---------------------+----------------+-----------------+
| node | flood if level of | flood if level | flood only if |
| South TIE | originator is equal | of originator | this node |
| | to this node | is higher than | is not ToF |
| | | this node | |
+-----------+---------------------+----------------+-----------------+
| non-node | flood self- | flood only if | flood only if |
| South TIE | originated only | neighbor is | self-originated |
| | | originator of | and this node |
| | | TIE | is not ToF |
+-----------+---------------------+----------------+-----------------+
| all North | never flood | flood always | flood only if |
| TIEs | | | this node is |
| | | | ToF |
+-----------+---------------------+----------------+-----------------+
| TIDE | include at least | include at | if this node is |
| | all non-self | least all node | ToF then |
| | originated North | South TIEs and | include all |
| | TIE headers and | all South TIEs | North TIEs, |
| | self-originated | originated by | otherwise only |
| | South TIE headers | peer and | self-originated |
| | and | all North TIEs | TIEs |
| | node South TIEs of | | |
| | nodes at same | | |
| | level | | |
+-----------+---------------------+----------------+-----------------+
| TIRE as | request all North | request all | if this node is |
| Request | TIEs and all peer's | South TIEs | ToF then apply |
| | self-originated | | North scope |
| | TIEs and | | rules, |
| | all node South TIEs | | otherwise South |
| | | | scope rules |
+-----------+---------------------+----------------+-----------------+
| TIRE as | Ack all received | Ack all | Ack all |
| Ack | TIEs | received TIEs | received TIEs |
+-----------+---------------------+----------------+-----------------+
Table 3: Normative Flooding Scopes
If the TIDE includes additional TIE headers beside the ones
specified, the receiving neighbor must apply according filter to the
received TIDE strictly and MUST NOT request the extra TIE headers
that were not allowed by the flooding scope rules in its direction.
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As an example to illustrate these rules, consider using the topology
in Figure 2, with the optional link between spine 111 and spine 112,
and the associated TIEs given in Figure 14. The flooding from
particular nodes of the TIEs is given in Table 4.
+-----------+----------+--------------------------------------------+
| Router | Neighbor | TIEs |
| floods to | | |
+-----------+----------+--------------------------------------------+
| Leaf111 | Spine | Leaf111 North TIEs, Spine 111 node South |
| | 112 | TIE |
| Leaf111 | Spine | Leaf111 North TIEs, Spine 112 node South |
| | 111 | TIE |
| | | |
| Spine 111 | Leaf111 | Spine 111 South TIEs |
| Spine 111 | Leaf112 | Spine 111 South TIEs |
| Spine 111 | Spine | Spine 111 South TIEs |
| | 112 | |
| Spine 111 | ToF 21 | Spine 111 North TIEs, Leaf111 |
| | | North TIEs, Leaf112 North TIEs, ToF 22 |
| | | node South TIE |
| Spine 111 | ToF 22 | Spine 111 North TIEs, Leaf111 |
| | | North TIEs, Leaf112 North TIEs, ToF 21 |
| | | node South TIE |
| | | |
| ... | ... | ... |
| ToF 21 | Spine | ToF 21 South TIEs |
| | 111 | |
| ToF 21 | Spine | ToF 21 South TIEs |
| | 112 | |
| ToF 21 | Spine | ToF 21 South TIEs |
| | 121 | |
| ToF 21 | Spine | ToF 21 South TIEs |
| | 122 | |
| ... | ... | ... |
+-----------+----------+--------------------------------------------+
Table 4: Flooding some TIEs from example topology
4.2.3.5. 'Flood Only Node TIEs' Bit
RIFT includes an optional ECN (Explicit Congestion Notification)
mechanism to prevent "flooding inrush" on restart or bring-up with
many southbound neighbors. A node MAY set on its LIEs the according
bit to indicate to the neighbor that it should temporarily flood node
TIEs only to it. It SHOULD only set it in the southbound direction.
The receiving node SHOULD accommodate the request to lessen the
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flooding load on the affected node if south of the sender and SHOULD
ignore the bit if northbound.
Obviously this mechanism is most useful in southbound direction. The
distribution of node TIEs guarantees correct behavior of algorithms
like disaggregation or default route origination. Furthermore
though, the use of this bit presents an inherent trade-off between
processing load and convergence speed since suppressing flooding of
northbound prefixes from neighbors will lead to blackholes.
4.2.3.6. Initial and Periodic Database Synchronization
The initial exchange of RIFT is modeled after ISIS with TIDE being
equivalent to CSNP and TIRE playing the role of PSNP. The content of
TIDEs and TIREs is governed by Table 3.
4.2.3.7. Purging and Roll-Overs
When a node exits the network, if "unpurged", residual stale TIEs may
exist in the network until their lifetimes expire (which in case of
RIFT is by default a rather long period to prevent ongoing re-
origination of TIEs in very large topologies). RIFT does however not
have a "purging mechanism" in the traditional sense based on sending
specialized "purge" packets. In other routing protocols such
mechanism has proven to be complex and fragile based on many years of
experience. RIFT simply issues a new, empty version of the TIE with
a short lifetime and relies on each node to age out and delete such
TIE copy independently. Abundant amounts of memory are available
today even on low-end platforms and hence keeping those relatively
short-lived extra copies for a while is acceptable. The information
will age out and in the meantime all computations will deliver
correct results if a node leaves the network due to the new
information distributed by its adjacent nodes breaking bi-directional
connectivity checks in different computations.
Once a RIFT node issues a TIE with an ID, it SHOULD preserve the ID
as long as feasible (also when the protocol restarts), even if the
TIE looses all content. The re-advertisement of empty TIE fulfills
the purpose of purging any information advertised in previous
versions. The originator is free to not re-originate the according
empty TIE again or originate an empty TIE with relatively short
lifetime to prevent large number of long-lived empty stubs polluting
the network. Each node MUST timeout and clean up the according empty
TIEs independently.
Upon restart a node MUST, as any link-state implementation, be
prepared to receive TIEs with its own system ID and supersede them
with equivalent, newly generated, empty TIEs with a higher sequence
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number. As above, the lifetime can be relatively short since it only
needs to exceed the necessary propagation and processing delay by all
the nodes that are within the TIE's flooding scope.
TIE sequence numbers are rolled over using the method described in
Appendix A. First sequence number of any spontaneously originated
TIE (i.e. not originated to override a detected older copy in the
network) MUST be a reasonably unpredictable random number in the
interval [0, 2^30-1] which will prevent otherwise identical TIE
headers to remain "stuck" in the network with content different from
TIE originated after reboot. In traditional link-state protocols
this is delegated to a 16-bit checksum on packet content. RIFT
avoids this design due to the CPU burden presented by computation of
such checksums and additional complications tied to the fact that the
checksum must be "patched" into the packet after the computation, a
difficult proposition in binary hand-crafted formats already and
highly incompatible with model-based, serialized formats. The
sequence number space is hence consciously chosen to be 64-bits wide
to make the occurrence of a TIE with same sequence number but
different content as much or even more unlikely than the checksum
method. To emulate the "checksum behavior" an implementation could
e.g. choose to compute 64-bit checksum over the packet content and
use that as first sequence number after reboot.
4.2.3.8. Southbound Default Route Origination
Under certain conditions nodes issue a default route in their South
Prefix TIEs with costs as computed in Section 4.3.6.1.
A node X that
1. is NOT overloaded AND
2. has southbound or East-West adjacencies
originates in its south prefix TIE such a default route IIF
1. all other nodes at X's' level are overloaded OR
2. all other nodes at X's' level have NO northbound adjacencies OR
3. X has computed reachability to a default route during N-SPF.
The term "all other nodes at X's' level" describes obviously just the
nodes at the same level in the PoD with a viable lower level
(otherwise the node South TIEs cannot be reflected and the nodes in
e.g. PoD 1 and PoD 2 are "invisible" to each other).
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A node originating a southbound default route MUST install a default
discard route if it did not compute a default route during N-SPF.
4.2.3.9. Northbound TIE Flooding Reduction
Section 1.4 of the Optimized Link State Routing Protocol [RFC3626]
(OLSR) introduces the concept of a "multipoint relay" (MPR) that
minimize the overhead of flooding messages in the network by reducing
redundant retransmissions in the same region.
A similar technique is applied to RIFT to control northbound
flooding. Important observations first:
1. a node MUST flood self-originated North TIEs to all the reachable
nodes at the level above which we call the node's "parents";
2. it is typically not necessary that all parents reflood the North
TIEs to achieve a complete flooding of all the reachable nodes
two levels above which we choose to call the node's
"grandparents";
3. to control the volume of its flooding two hops North and yet keep
it robust enough, it is advantageous for a node to select a
subset of its parents as "Flood Repeaters" (FRs), which combined
together deliver two or more copies of its flooding to all of its
parents, i.e. the originating node's grandparents;
4. nodes at the same level do NOT have to agree on a specific
algorithm to select the FRs, but overall load balancing should be
achieved so that different nodes at the same level should tend to
select different parents as FRs;
5. there are usually many solutions to the problem of finding a set
of FRs for a given node; the problem of finding the minimal set
is (similar to) a NP-Complete problem and a globally optimal set
may not be the minimal one if load-balancing with other nodes is
an important consideration;
6. it is expected that there will be often sets of equivalent nodes
at a level L, defined as having a common set of parents at L+1.
Applying this observation at both L and L+1, an algorithm may
attempt to split the larger problem in a sum of smaller separate
problems;
7. it is another expectation that there will be from time to time a
broken link between a parent and a grandparent, and in that case
the parent is probably a poor FR due to its lower reliability.
An algorithm may attempt to eliminate parents with broken
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northbound adjacencies first in order to reduce the number of
FRs. Albeit it could be argued that relying on higher fanout FRs
will slow flooding due to higher replication load reliability of
FR's links seems to be a more pressing concern.
In a fully connected Clos Network, this means that a node selects one
arbitrary parent as FR and then a second one for redundancy. The
computation can be kept relatively simple and completely distributed
without any need for synchronization amongst nodes. In a "PoD"
structure, where the Level L+2 is partitioned in silos of equivalent
grandparents that are only reachable from respective parents, this
means treating each silo as a fully connected Clos Network and solve
the problem within the silo.
In terms of signaling, a node has enough information to select its
set of FRs; this information is derived from the node's parents' Node
South TIEs, which indicate the parent's reachable northbound
adjacencies to its own parents, i.e. the node's grandparents. A node
may send a LIE to a northbound neighbor with the optional boolean
field `you_are_flood_repeater` set to false, to indicate that the
northbound neighbor is not a flood repeater for the node that sent
the LIE. In that case the northbound neighbor SHOULD NOT reflood
northbound TIEs received from the node that sent the LIE. If the
`you_are_flood_repeater` is absent or if `you_are_flood_repeater` is
set to true, then the northbound neighbor is a flood repeater for the
node that sent the LIE and MUST reflood northbound TIEs received from
that node.
This specification proposes a simple default algorithm that SHOULD be
implemented and used by default on every RIFT node.
o let |NA(Node) be the set of Northbound adjacencies of node Node
and CN(Node) be the cardinality of |NA(Node);
o let |SA(Node) be the set of Southbound adjacencies of node Node
and CS(Node) be the cardinality of |SA(Node);
o let |P(Node) be the set of node Node's parents;
o let |G(Node) be the set of node Node's grandparents. Observe
that |G(Node) = |P(|P(Node));
o let N be the child node at level L computing a set of FR;
o let P be a node at level L+1 and a parent node of N, i.e. bi-
directionally reachable over adjacency A(N, P);
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o let G be a grandparent node of N, reachable transitively via a
parent P over adjacencies ADJ(N, P) and ADJ(P, G). Observe that N
does not have enough information to check bidirectional
reachability of ADJ(P, G);
o let R be a redundancy constant integer; a value of 2 or higher for
R is RECOMMENDED;
o let S be a similarity constant integer; a value in range 0 .. 2
for S is RECOMMENDED, the value of 1 SHOULD be used. Two
cardinalities are considered as equivalent if their absolute
difference is less than or equal to S, i.e. |a-b|<=S.
o let RND be a 64-bit random number generated by the system once on
startup.
The algorithm consists of the following steps:
1. Derive a 64-bits number by XOR'ing 'N's system ID with RND.
2. Derive a 16-bits pseudo-random unsigned integer PR(N) from the
resulting 64-bits number by splitting it in 16-bits-long words
W1, W2, W3, W4 (where W1 are the least significant 16 bits of the
64-bits number, and W4 are the most significant 16 bits) and then
XOR'ing the circularly shifted resulting words together:
A. (W1<<1) xor (W2<<2) xor (W3<<3) xor (W4<<4);
where << is the circular shift operator.
3. Sort the parents by decreasing number of northbound adjacencies
(using decreasing system id of the parent as tie-breaker):
sort |P(N) by decreasing CN(P), for all P in |P(N), as ordered
array |A(N)
4. Partition |A(N) in subarrays |A_k(N) of parents with equivalent
cardinality of northbound adjacencies (in other words with
equivalent number of grandparents they can reach):
A. set k=0; // k is the ID of the subarrray
B. set i=0;
C. while i < CN(N) do
i) set j=i;
ii) while i < CN(N) and CN(|A(N)[j]) - CN(|A(N)[i]) <= S
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a. place |A(N)[i] in |A_k(N) // abstract action,
maybe noop
b. set i=i+1;
iii) /* At this point j is the index in |A(N) of the first
member of |A_k(N) and (i-j) is C_k(N) defined as the
cardinality of |A_k(N) */
set k=k+1;
/* At this point k is the total number of subarrays, initialized
for the shuffling operation below */
5. shuffle individually each subarrays |A_k(N) of cardinality C_k(N)
within |A(N) using the Durstenfeld variation of Fisher-Yates
algorithm that depends on N's System ID:
A. while k > 0 do
i) for i from C_k(N)-1 to 1 decrementing by 1 do
a. set j to PR(N) modulo i;
b. exchange |A_k[j] and |A_k[i];
ii) set k=k-1;
6. For each grandparent G, initialize a counter c(G) with the number
of its south-bound adjacencies to elected flood repeaters (which
is initially zero):
A. for each G in |G(N) set c(G) = 0;
7. Finally keep as FRs only parents that are needed to maintain the
number of adjacencies between the FRs and any grandparent G equal
or above the redundancy constant R:
A. for each P in reshuffled |A(N);
i) if there exists an adjacency ADJ(P, G) in |NA(P) such
that c(G) < R then
a. place P in FR set;
b. for all adjacencies ADJ(P, G') in |NA(P) increment
c(G')
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B. If any c(G) is still < R, it was not possible to elect a set
of FRs that covers all grandparents with redundancy R
Additional rules for flooding reduction:
1. The algorithm MUST be re-evaluated by a node on every change of
local adjacencies or reception of a parent South TIE with changed
adjacencies. A node MAY apply a hysteresis to prevent excessive
amount of computation during periods of network instability just
like in case of reachability computation.
2. Upon a change of the flood repeater set, a node SHOULD send out
LIEs that grant flood repeater status to newly promoted nodes
before it sends LIEs that revoke the status to the nodes that
have been newly demoted. This is done to prevent transient
behavior where the full coverage of grandparents is not
guaranteed. Such a condition is sometimes unavoidable in case of
lost LIEs but it will correct itself though at possible transient
hit in flooding propagation speeds.
3. A node MUST always flood its self-originated TIEs.
4. A node receiving a TIE originated by a node for which it is not a
flood repeater SHOULD NOT reflood such TIEs to its neighbors
except for rules in Paragraph 6.
5. The indication of flood reduction capability MUST be carried in
the node TIEs and MAY be used to optimize the algorithm to
account for nodes that will flood regardless.
6. A node generates TIDEs as usual but when receiving TIREs or TIDEs
resulting in requests for a TIE of which the newest received copy
came on an adjacency where the node was not flood repeater it
SHOULD ignore such requests on first and only first request.
Normally, the nodes that received the TIEs as flooding repeaters
should satisfy the requesting node and with that no further TIREs
for such TIEs will be generated. Otherwise, the next set of
TIDEs and TIREs MUST lead to flooding independent of the flood
repeater status. This solves a very difficult incast problem on
nodes restarting with a very wide fanout, especially northbound.
To retrieve the full database they often end up processing many
in-rushing copies whereas this approach load-balances the
incoming database between adjacent nodes and flood repeaters
should guarantee that two copies are sent by different nodes to
ensure against any losses.
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4.2.3.10. Special Considerations
First, due to the distributed, asynchronous nature of ZTP, it can
create temporary convergence anomalies where nodes at higher levels
of the fabric temporarily see themselves lower than they belong.
Since flooding can begin before ZTP is "finished" and in fact must do
so given there is no global termination criteria, information may end
up in wrong layers. A special clause when changing level takes care
of that.
More difficult is a condition where a node (e.g. a leaf) floods a TIE
north towards its grandparent, then its parent reboots, in fact
partitioning the grandparent from leaf directly and then the leaf
itself reboots. That can leave the grandparent holding the "primary
copy" of the leaf's TIE. Normally this condition is resolved easily
by the leaf re-originating its TIE with a higher sequence number than
it sees in northbound TIEs, here however, when the parent comes back
it won't be able to obtain leaf's North TIE from the grandparent
easily and with that the leaf may not issue the TIE with a higher
sequence number that can reach the grandparent for a long time.
Flooding procedures are extended to deal with the problem by the
means of special clauses that override the database of a lower level
with headers of newer TIEs seen in TIDEs coming from the north.
4.2.4. Reachability Computation
A node has three possible sources of relevant information for
reachability computation. A node knows the full topology south of it
from the received North Node TIEs or alternately north of it from the
South Node TIEs. A node has the set of prefixes with their
associated distances and bandwidths from corresponding prefix TIEs.
To compute prefix reachability, a node runs conceptually a northbound
and a southbound SPF. We call that N-SPF and S-SPF denoting the
direction in which the computation front is progressing.
Since neither computation can "loop", it is possible to compute non-
equal-cost or even k-shortest paths [EPPSTEIN] and "saturate" the
fabric to the extent desired but we use simple, familiar SPF
algorithms and concepts here as example due to their prevalence in
today's routing.
4.2.4.1. Northbound SPF
N-SPF MUST use exclusively northbound and East-West adjacencies in
the computing node's node North TIEs (since if the node is a leaf it
may not have generated a node South TIE) when starting SPF. Observe
that N-SPF is really just a one hop variety since Node South TIEs are
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not re-flooded southbound beyond a single level (or East-West) and
with that the computation cannot progress beyond adjacent nodes.
Once progressing, we are using the next higher level's node South
TIEs to find according adjacencies to verify backlink connectivity.
Just as in case of IS-IS or OSPF, two unidirectional links MUST be
associated together to confirm bidirectional connectivity.
Particular care MUST be paid that the Node TIEs do not only contain
the correct system IDs but matching levels as well.
Default route found when crossing an E-W link SHOULD be used IIF
1. the node itself does NOT have any northbound adjacencies AND
2. the adjacent node has one or more northbound adjacencies
This rule forms a "one-hop default route split-horizon" and prevents
looping over default routes while allowing for "one-hop protection"
of nodes that lost all northbound adjacencies except at Top-of-Fabric
where the links are used exclusively to flood topology information in
multi-plane designs.
Other south prefixes found when crossing E-W link MAY be used IIF
1. no north neighbors are advertising same or supersuming non-
default prefix AND
2. the node does not originate a non-default supersuming prefix
itself.
i.e. the E-W link can be used as a gateway of last resort for a
specific prefix only. Using south prefixes across E-W link can be
beneficial e.g. on automatic de-aggregation in pathological fabric
partitioning scenarios.
A detailed example can be found in Section 5.4.
4.2.4.2. Southbound SPF
S-SPF MUST use exclusively the southbound adjacencies in the node
South TIEs, i.e. progresses towards nodes at lower levels. Observe
that E-W adjacencies are NEVER used in the computation. This
enforces the requirement that a packet traversing in a southbound
direction must never change its direction.
S-SPF MUST use northbound adjacencies in node North TIEs to verify
backlink connectivity by checking for presence of the link beside
correct SystemID and level.
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4.2.4.3. East-West Forwarding Within a non-ToF Level
Using south prefixes over horizontal links MAY occur if the N-SPF
includes East-West adjacencies in computation. It can protect
against pathological fabric partitioning cases that leave only paths
to destinations that would necessitate multiple changes of forwarding
direction between north and south.
4.2.4.4. East-West Links Within ToF Level
E-W ToF links behave in terms of flooding scopes defined in
Section 4.2.3.4 like northbound links and MUST be used exclusively
for control plane information flooding. Even though a ToF node could
be tempted to use those links during southbound SPF and carry traffic
over them this MUST NOT be attempted since it may lead in, e.g.
anycast cases to routing loops. An implementation MAY try to resolve
the looping problem by following on the ring strictly tie-broken
shortest-paths only but the details are outside this specification.
And even then, the problem of proper capacity provisioning of such
links when they become traffic-bearing in case of failures is vexing.
4.2.5. Automatic Disaggregation on Link & Node Failures
4.2.5.1. Positive, Non-transitive Disaggregation
Under normal circumstances, node's South TIEs contain just the
adjacencies and a default route. However, if a node detects that its
default IP prefix covers one or more prefixes that are reachable
through it but not through one or more other nodes at the same level,
then it MUST explicitly advertise those prefixes in an South TIE.
Otherwise, some percentage of the northbound traffic for those
prefixes would be sent to nodes without according reachability,
causing it to be black-holed. Even when not black-holing, the
resulting forwarding could 'backhaul' packets through the higher
level spines, clearly an undesirable condition affecting the blocking
probabilities of the fabric.
We refer to the process of advertising additional prefixes southbound
as 'positive de-aggregation' or 'positive dis-aggregation'. Such
dis-aggregation is non-transitive, i.e. its' effects are always
contained to a single level of the fabric only. Naturally, multiple
node or link failures can lead to several independent instances of
positive dis-aggregation necessary to prevent looping or bow-tying
the fabric.
A node determines the set of prefixes needing de-aggregation using
the following steps:
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1. A DAG computation in the southern direction is performed first,
i.e. the North TIEs are used to find all of prefixes it can reach
and the set of next-hops in the lower level for each of them.
Such a computation can be easily performed on a fat tree by e.g.
setting all link costs in the southern direction to 1 and all
northern directions to infinity. We term set of those
prefixes |R, and for each prefix, r, in |R, we define its set of
next-hops to be |H(r).
2. The node uses reflected South TIEs to find all nodes at the same
level in the same PoD and the set of southbound adjacencies for
each. The set of nodes at the same level is termed |N and for
each node, n, in |N, we define its set of southbound adjacencies
to be |A(n).
3. For a given r, if the intersection of |H(r) and |A(n), for any n,
is null then that prefix r must be explicitly advertised by the
node in an South TIE.
4. Identical set of de-aggregated prefixes is flooded on each of the
node's southbound adjacencies. In accordance with the normal
flooding rules for an South TIE, a node at the lower level that
receives this South TIE SHOULD NOT propagate it south-bound or
reflect the disaggregated prefixes back over its adjacencies to
nodes at the level from which it was received.
To summarize the above in simplest terms: if a node detects that its
default route encompasses prefixes for which one of the other nodes
in its level has no possible next-hops in the level below, it has to
disaggregate it to prevent black-holing or suboptimal routing through
such nodes. Hence a node X needs to determine if it can reach a
different set of south neighbors than other nodes at the same level,
which are connected to it via at least one common south neighbor. If
it can, then prefix disaggregation may be required. If it can't,
then no prefix disaggregation is needed. An example of
disaggregation is provided in Section 5.3.
A possible algorithm is described last:
1. Create partial_neighbors = (empty), a set of neighbors with
partial connectivity to the node X's level from X's perspective.
Each entry in the set is a south neighbor of X and a list of
nodes of X.level that can't reach that neighbor.
2. A node X determines its set of southbound neighbors
X.south_neighbors.
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3. For each South TIE originated from a node Y that X has which is
at X.level, if Y.south_neighbors is not the same as
X.south_neighbors but the nodes share at least one southern
neighbor, for each neighbor N in X.south_neighbors but not in
Y.south_neighbors, add (N, (Y)) to partial_neighbors if N isn't
there or add Y to the list for N.
4. If partial_neighbors is empty, then node X does not disaggregate
any prefixes. If node X is advertising disaggregated prefixes in
its South TIE, X SHOULD remove them and re-advertise its
according South TIEs.
A node X computes reachability to all nodes below it based upon the
received North TIEs first. This results in a set of routes, each
categorized by (prefix, path_distance, next-hop set). Alternately,
for clarity in the following procedure, these can be organized by
next-hop set as ( (next-hops), {(prefix, path_distance)}). If
partial_neighbors isn't empty, then the following procedure describes
how to identify prefixes to disaggregate.
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disaggregated_prefixes = { empty }
nodes_same_level = { empty }
for each South TIE
if (South TIE.level == X.level and
X shares at least one S-neighbor with X)
add South TIE.originator to nodes_same_level
end if
end for
for each next-hop-set NHS
isolated_nodes = nodes_same_level
for each NH in NHS
if NH in partial_neighbors
isolated_nodes =
intersection(isolated_nodes,
partial_neighbors[NH].nodes)
end if
end for
if isolated_nodes is not empty
for each prefix using NHS
add (prefix, distance) to disaggregated_prefixes
end for
end if
end for
copy disaggregated_prefixes to X's South TIE
if X's South TIE is different
schedule South TIE for flooding
end if
Figure 15: Computation of Disaggregated Prefixes
Each disaggregated prefix is sent with the according path_distance.
This allows a node to send the same South TIE to each south neighbor.
The south neighbor which is connected to that prefix will thus have a
shorter path.
Finally, to summarize the less obvious points partially omitted in
the algorithms to keep them more tractable:
1. all neighbor relationships MUST perform backlink checks.
2. overload bits as introduced in Section 4.3.1 have to be respected
during the computation.
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3. all the lower level nodes are flooded the same disaggregated
prefixes since we don't want to build an South TIE per node and
complicate things unnecessarily. The lower level node that can
compute a southbound route to the prefix will prefer it to the
disaggregated route anyway based on route preference rules.
4. positively disaggregated prefixes do NOT have to propagate to
lower levels. With that the disturbance in terms of new flooding
is contained to a single level experiencing failures.
5. disaggregated Prefix South TIEs are not "reflected" by the lower
level, i.e. nodes within same level do NOT need to be aware
which node computed the need for disaggregation.
6. The fabric is still supporting maximum load balancing properties
while not trying to send traffic northbound unless necessary.
In case positive disaggregation is triggered and due to the very
stable but un-synchronized nature of the algorithm the nodes may
issue the necessary disaggregated prefixes at different points in
time. This can lead for a short time to an "incast" behavior where
the first advertising router based on the nature of longest prefix
match will attract all the traffic. An implementation MAY hence
choose different strategies to address this behavior if needed.
To close this section it is worth to observe that in a single plane
ToF this disaggregation prevents blackholing up to (K_LEAF * P) link
failures in terms of Section 4.1.2 or in other terms, it takes at
minimum that many link failures to partition the ToF into multiple
planes.
4.2.5.2. Negative, Transitive Disaggregation for Fallen Leaves
As explained in Section 4.1.3 failures in multi-plane Top-of-Fabric
or more than (K_LEAF * P) links failing in single plane design can
generate fallen leaves. Such scenario cannot be addressed by
positive disaggregation only and needs a further mechanism.
4.2.5.2.1. Cabling of Multiple Top-of-Fabric Planes
Let us return in this section to designs with multiple planes as
shown in Figure 3. Figure 16 highlights how the ToF is cabled in
case of two planes by the means of dual-rings to distribute all the
North TIEs within both planes. For people familiar with traditional
link-state routing protocols ToF level can be considered equivalent
to area 0 in OSPF or level-2 in ISIS which need to be "connected" as
well for the protocol to operate correctly.
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. ++==========++ ++==========++
. II II II II
.+----++--+ +----++--+ +----++--+ +----++--+
.|ToF A1| |ToF B1| |ToF B2| |ToF A2|
.++-+-++--+ ++-+-++--+ ++-+-++--+ ++-+-++--+
. | | II | | II | | II | | II
. | | ++==========++ | | ++==========++
. | | | | | | | |
.
. ~~~ Highlighted ToF of the previous multi-plane figure ~~
Figure 16: Topologically Connected Planes
As described in Section 4.1.3 failures in multi-plane fabrics can
lead to blackholes which normal positive disaggregation cannot fix.
The mechanism of negative, transitive disaggregation incorporated in
RIFT provides the according solution.
4.2.5.2.2. Transitive Advertisement of Negative Disaggregates
A ToF node that discovers that it cannot reach a fallen leaf
disaggregates all the prefixes of such leaves. It uses for that
purpose negative prefix South TIEs that are, as usual, flooded
southwards with the scope defined in Section 4.2.3.4.
Transitively, a node explicitly loses connectivity to a prefix when
none of its children advertises it and when the prefix is negatively
disaggregated by all of its parents. When that happens, the node
originates the negative prefix further down south. Since the
mechanism applies recursively south the negative prefix may propagate
transitively all the way down to the leaf. This is necessary since
leaves connected to multiple planes by means of disjoint paths may
have to choose the correct plane already at the very bottom of the
fabric to make sure that they don't send traffic towards another leaf
using a plane where it is "fallen" at which in point a blackhole is
unavoidable.
When the connectivity is restored, a node that disaggregated a prefix
withdraws the negative disaggregation by the usual mechanism of re-
advertising TIEs omitting the negative prefix.
4.2.5.2.3. Computation of Negative Disaggregates
The document omitted so far the description of the computation
necessary to generate the correct set of negative prefixes. Negative
prefixes can in fact be advertised due to two different triggers. We
describe them consecutively.
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The first origination reason is a computation that uses all the node
North TIEs to build the set of all reachable nodes by reachability
computation over the complete graph and including ToF links. The
computation uses the node itself as root. This is compared with the
result of the normal southbound SPF as described in Section 4.2.4.2.
The difference are the fallen leaves and all their attached prefixes
are advertised as negative prefixes southbound if the node does not
see the prefix being reachable within southbound SPF.
The second mechanism hinges on the understanding how the negative
prefixes are used within the computation as described in Figure 17.
When attaching the negative prefixes at certain point in time the
negative prefix may find itself with all the viable nodes from the
shorter match nexthop being pruned. In other words, all its
northbound neighbors provided a negative prefix advertisement. This
is the trigger to advertise this negative prefix transitively south
and normally caused by the node being in a plane where the prefix
belongs to a fabric leaf that has "fallen" in this plane. Obviously,
when one of the northbound switches withdraws its negative
advertisement, the node has to withdraw its transitively provided
negative prefix as well.
4.2.6. Attaching Prefixes
After SPF is run, it is necessary to attach the resulting
reachability information in form of prefixes. For S-SPF, prefixes
from an North TIE are attached to the originating node with that
node's next-hop set and a distance equal to the prefix's cost plus
the node's minimized path distance. The RIFT route database, a set
of (prefix, prefix-type, attributes, path_distance, next-hop set),
accumulates these results.
In case of N-SPF prefixes from each South TIE need to also be added
to the RIFT route database. The N-SPF is really just a stub so the
computing node needs simply to determine, for each prefix in an South
TIE that originated from adjacent node, what next-hops to use to
reach that node. Since there may be parallel links, the next-hops to
use can be a set; presence of the computing node in the associated
Node South TIE is sufficient to verify that at least one link has
bidirectional connectivity. The set of minimum cost next-hops from
the computing node X to the originating adjacent node is determined.
Each prefix has its cost adjusted before being added into the RIFT
route database. The cost of the prefix is set to the cost received
plus the cost of the minimum distance next-hop to that neighbor while
taking into account its attributes such as mobility per
Section 4.3.3. Then each prefix can be added into the RIFT route
database with the next-hop set; ties are broken based upon type first
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and then distance and further on `PrefixAttributes` and only the best
combination is used for forwarding. RIFT route preferences are
normalized by the according Thrift [thrift] model type.
An example implementation for node X follows:
for each South TIE
if South TIE.level > X.level
next_hop_set = set of minimum cost links to the
South TIE.originator
next_hop_cost = minimum cost link to
South TIE.originator
end if
for each prefix P in the South TIE
P.cost = P.cost + next_hop_cost
if P not in route_database:
add (P, P.cost, P.type,
P.attributes, next_hop_set) to route_database
end if
if (P in route_database):
if route_database[P].cost > P.cost or
route_database[P].type > P.type:
update route_database[P] with (P, P.type, P.cost,
P.attributes,
next_hop_set)
else if route_database[P].cost == P.cost and
route_database[P].type == P.type:
update route_database[P] with (P, P.type,
P.cost, P.attributes,
merge(next_hop_set, route_database[P].next_hop_set))
else
// Not preferred route so ignore
end if
end if
end for
end for
Figure 17: Adding Routes from South TIE Positive and Negative
Prefixes
After the positive prefixes are attached and tie-broken, negative
prefixes are attached and used in case of northbound computation,
ideally from the shortest length to the longest. The nexthop
adjacencies for a negative prefix are inherited from the longest
positive prefix that aggregates it, and subsequently adjacencies to
nodes that advertised negative for this prefix are removed.
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The rule of inheritance MUST be maintained when the nexthop list for
a prefix is modified, as the modification may affect the entries for
matching negative prefixes of immediate longer prefix length. For
instance, if a nexthop is added, then by inheritance it must be added
to all the negative routes of immediate longer prefixes length unless
it is pruned due to a negative advertisement for the same next hop.
Similarly, if a nexthop is deleted for a given prefix, then it is
deleted for all the immediately aggregated negative routes. This
will recurse in the case of nested negative prefix aggregations.
The rule of inheritance must also be maintained when a new prefix of
intermediate length is inserted, or when the immediately aggregating
prefix is deleted from the routing table, making an even shorter
aggregating prefix the one from which the negative routes now inherit
their adjacencies. As the aggregating prefix changes, all the
negative routes must be recomputed, and then again the process may
recurse in case of nested negative prefix aggregations.
Although these operations can be computationally expensive, the
overall load on devices in the network is low because these
computations are not run very often, as positive route advertisements
are always preferred over negative ones. This prevents recursion in
most cases because positive reachability information never inherits
next hops.
To make the negative disaggregation less abstract and provide an
example let us consider a ToP node T1 with 4 ToF parents S1..S4 as
represented in Figure 18:
+----+ +----+ +----+ +----+ N
| S1 | | S1 | | S1 | | S1 | ^
+----+ +----+ +----+ +----+ W< + >E
| | | | v
|+--------+ | | S
||+-----------------+ |
|||+----------------+---------+
||||
+----+
| T1 |
+----+
Figure 18: A ToP Node with 4 Parents
If all ToF nodes can reach all the prefixes in the network; with
RIFT, they will normally advertise a default route south. An
abstract Routing Information Base (RIB), more commonly known as a
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routing table, stores all types of maintained routes including the
negative ones and "tie-breaks" for the best one, whereas an abstract
Forwarding table (FIB) retains only the ultimately computed
"positive" routing instructions. In T1, those tables would look as
illustrated in Figure 19:
+---------+
| Default |
+---------+
|
| +--------+
+---> | Via S1 |
| +--------+
|
| +--------+
+---> | Via S2 |
| +--------+
|
| +--------+
+---> | Via S3 |
| +---------+
|
| +--------+
+---> | Via S4 |
+--------+
Figure 19: Abstract RIB
In case T1 receives a negative advertisement for prefix 2001:db8::/32
from S1 a negative route is stored in the RIB (indicated by a ~
sign), while the more specific routes to the complementing ToF nodes
are installed in FIB. RIB and FIB in T1 now look as illustrated in
Figure 20 and Figure 21, respectively:
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+---------+ +-----------------+
| Default | <-------------- | ~2001:db8::/32 |
+---------+ +-----------------+
| |
| +--------+ | +--------+
+---> | Via S1 | +---> | Via S1 |
| +--------+ +--------+
|
| +--------+
+---> | Via S2 |
| +--------+
|
| +--------+
+---> | Via S3 |
| +---------+
|
| +--------+
+---> | Via S4 |
+--------+
Figure 20: Abstract RIB after Negative 2001:db8::/32 from S1
The negative 2001:db8::/32 prefix entry inherits from ::/0, so the
positive more specific routes are the complements to S1 in the set of
next-hops for the default route. That entry is composed of S2, S3,
and S4, or, in other words, it uses all entries the the default route
with a "hole punched" for S1 into them. These are the next hops that
are still available to reach 2001:db8::/32, now that S1 advertised
that it will not forward 2001:db8::/32 anymore. Ultimately, those
resulting next-hops are installed in FIB for the more specific route
to 2001:db8::/32 as illustrated below:
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+---------+ +---------------+
| Default | | 2001:db8::/32 |
+---------+ +---------------+
| |
| +--------+ |
+---> | Via S1 | |
| +--------+ |
| |
| +--------+ | +--------+
+---> | Via S2 | +---> | Via S2 |
| +--------+ | +--------+
| |
| +--------+ | +--------+
+---> | Via S3 | +---> | Via S3 |
| +--------+ | +--------+
| |
| +--------+ | +--------+
+---> | Via S4 | +---> | Via S4 |
+--------+ +--------+
Figure 21: Abstract FIB after Negative 2001:db8::/32 from S1
To illustrate matters further let us consider T1 receiving a negative
advertisement for prefix 2001:db8:1::/48 from S2, which is stored in
RIB again. After the update, the RIB in T1 is illustrated in
Figure 22:
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+---------+ +----------------+ +------------------+
| Default | <----- | ~2001:db8::/32 | <------ | ~2001:db8:1::/48 |
+---------+ +----------------+ +------------------+
| | |
| +--------+ | +--------+ |
+---> | Via S1 | +---> | Via S1 | |
| +--------+ +--------+ |
| |
| +--------+ | +--------+
+---> | Via S2 | +---> | Via S2 |
| +--------+ +--------+
|
| +--------+
+---> | Via S3 |
| +---------+
|
| +--------+
+---> | Via S4 |
+--------+
Figure 22: Abstract RIB after Negative 2001:db8:1::/48 from S2
Negative 2001:db8:1::/48 inherits from 2001:db8::/32 now, so the
positive more specific routes are the complements to S2 in the set of
next hops for 2001:db8::/32, which are S3 and S4, or, in other words,
all entries of the parent with the negative holes "punched in" again.
After the update, the FIB in T1 shows as illustrated in Figure 23:
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+---------+ +---------------+ +-----------------+
| Default | | 2001:db8::/32 | | 2001:db8:1::/48 |
+---------+ +---------------+ +-----------------+
| | |
| +--------+ | |
+---> | Via S1 | | |
| +--------+ | |
| | |
| +--------+ | +--------+ |
+---> | Via S2 | +---> | Via S2 | |
| +--------+ | +--------+ |
| | |
| +--------+ | +--------+ | +--------+
+---> | Via S3 | +---> | Via S3 | +---> | Via S3 |
| +--------+ | +--------+ | +--------+
| | |
| +--------+ | +--------+ | +--------+
+---> | Via S4 | +---> | Via S4 | +---> | Via S4 |
+--------+ +--------+ +--------+
Figure 23: Abstract FIB after Negative 2001:db8:1::/48 from S2
Further, let us say that S3 stops advertising its service as default
gateway. The entry is removed from RIB as usual. In order to update
the FIB, it is necessary to eliminate the FIB entry for the default
route, as well as all the FIB entries that were created for negative
routes pointing to the RIB entry being removed (::/0). This is done
recursively for 2001:db8::/32 and then for, 2001:db8:1::/48. The
related FIB entries via S3 are removed, as illustrated in Figure 24.
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+---------+ +---------------+ +-----------------+
| Default | | 2001:db8::/32 | | 2001:db8:1::/48 |
+---------+ +---------------+ +-----------------+
| | |
| +--------+ | |
+---> | Via S1 | | |
| +--------+ | |
| | |
| +--------+ | +--------+ |
+---> | Via S2 | +---> | Via S2 | |
| +--------+ | +--------+ |
| | |
| | |
| | |
| | |
| | |
| +--------+ | +--------+ | +--------+
+---> | Via S4 | +---> | Via S4 | +---> | Via S4 |
+--------+ +--------+ +--------+
Figure 24: Abstract FIB after Loss of S3
Say that at that time, S4 would also disaggregate prefix
2001:db8:1::/48. This would mean that the FIB entry for
2001:db8:1::/48 becomes a discard route, and that would be the signal
for T1 to disaggregate prefix 2001:db8:1::/48 negatively in a
transitive fashion with its own children.
Finally, let us look at the case where S3 becomes available again as
a default gateway, and a negative advertisement is received from S4
about prefix 2001:db8:2::/48 as opposed to 2001:db8:1::/48. Again, a
negative route is stored in the RIB, and the more specific route to
the complementing ToF nodes are installed in FIB. Since
2001:db8:2::/48 inherits from 2001:db8::/32, the positive FIB routes
are chosen by removing S4 from S2, S3, S4. The abstract FIB in T1
now shows as illustrated in Figure 25:
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+-----------------+
| 2001:db8:2::/48 |
+-----------------+
|
+---------+ +---------------+ +-----------------+
| Default | | 2001:db8::/32 | | 2001:db8:1::/48 |
+---------+ +---------------+ +-----------------+
| | | |
| +--------+ | | | +--------+
+---> | Via S1 | | | +---> | Via S2 |
| +--------+ | | | +--------+
| | | |
| +--------+ | +--------+ | | +--------+
+---> | Via S2 | +---> | Via S2 | | +---> | Via S3 |
| +--------+ | +--------+ | +--------+
| | |
| +--------+ | +--------+ | +--------+
+---> | Via S3 | +---> | Via S3 | +---> | Via S3 |
| +--------+ | +--------+ | +--------+
| | |
| +--------+ | +--------+ | +--------+
+---> | Via S4 | +---> | Via S4 | +---> | Via S4 |
+--------+ +--------+ +--------+
Figure 25: Abstract FIB after Negative 2001:db8:2::/48 from S4
4.2.7. Optional Zero Touch Provisioning (ZTP)
Each RIFT node can operate in zero touch provisioning (ZTP) mode,
i.e. it has no configuration (unless it is a ToF or it is configured
to operate in the overall topology as leaf and/or support leaf-2-leaf
procedures) and it will fully configure itself after being attached
to the topology. Configured nodes and nodes operating in ZTP can be
mixed and will form a valid topology if achievable.
The derivation of the level of each node happens based on offers
received from its neighbors whereas each node (with possibly
exceptions of configured leaves) tries to attach at the highest
possible point in the fabric. This guarantees that even if the
diffusion front reaches a node from "below" faster than from "above",
it will greedily abandon already negotiated level derived from nodes
topologically below it and properly peers with nodes above.
The fabric is very consciously numbered from the top to allow for
PoDs of different heights and minimize number of provisioning
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necessary, in this case just a TOP_OF_FABRIC flag on every node at
the top of the fabric.
This section describes the necessary concepts and procedures for ZTP
operation.
4.2.7.1. Terminology
The interdependencies between the different flags and the configured
level can be somewhat vexing at first and it may take multiple reads
of the glossary to comprehend them.
Automatic Level Derivation: Procedures which allow nodes without
level configured to derive it automatically. Only applied if
CONFIGURED_LEVEL is undefined.
UNDEFINED_LEVEL: A "null" value that indicates that the level has
not been determined and has not been configured. Schemas normally
indicate that by a missing optional value without an available
defined default.
LEAF_ONLY: An optional configuration flag that can be configured on
a node to make sure it never leaves the "bottom of the hierarchy".
TOP_OF_FABRIC flag and CONFIGURED_LEVEL cannot be defined at the
same time as this flag. It implies CONFIGURED_LEVEL value of 0.
TOP_OF_FABRIC flag: Configuration flag that MUST be provided to all
Top-of-Fabric nodes. LEAF_FLAG and CONFIGURED_LEVEL cannot be
defined at the same time as this flag. It implies a
CONFIGURED_LEVEL value. In fact, it is basically a shortcut for
configuring same level at all Top-of-Fabric nodes which is
unavoidable since an initial 'seed' is needed for other ZTP nodes
to derive their level in the topology. The flag plays an
important role in fabrics with multiple planes to enable
successful negative disaggregation (Section 4.2.5.2).
CONFIGURED_LEVEL: A level value provided manually. When this is
defined (i.e. it is not an UNDEFINED_LEVEL) the node is not
participating in ZTP. TOP_OF_FABRIC flag is ignored when this
value is defined. LEAF_ONLY can be set only if this value is
undefined or set to 0.
DERIVED_LEVEL: Level value computed via automatic level derivation
when CONFIGURED_LEVEL is equal to UNDEFINED_LEVEL.
LEAF_2_LEAF: An optional flag that can be configured on a node to
make sure it supports procedures defined in Section 4.3.8. In a
strict sense it is a capability that implies LEAF_ONLY and the
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according restrictions. TOP_OF_FABRIC flag is ignored when set at
the same time as this flag.
LEVEL_VALUE: In ZTP case the original definition of "level" in
Section 3.1 is both extended and relaxed. First, level is defined
now as LEVEL_VALUE and is the first defined value of
CONFIGURED_LEVEL followed by DERIVED_LEVEL. Second, it is
possible for nodes to be more than one level apart to form
adjacencies if any of the nodes is at least LEAF_ONLY.
Valid Offered Level (VOL): A neighbor's level received on a valid
LIE (i.e. passing all checks for adjacency formation while
disregarding all clauses involving level values) persisting for
the duration of the holdtime interval on the LIE. Observe that
offers from nodes offering level value of 0 do not constitute VOLs
(since no valid DERIVED_LEVEL can be obtained from those and
consequently `not_a_ztp_offer` MUST be ignored). Offers from LIEs
with `not_a_ztp_offer` being true are not VOLs either. If a node
maintains parallel adjacencies to the neighbor, VOL on each
adjacency is considered as equivalent, i.e. the newest VOL from
any such adjacency updates the VOL received from the same node.
Highest Available Level (HAL): Highest defined level value seen from
all VOLs received.
Highest Available Level Systems (HALS): Set of nodes offering HAL
VOLs.
Highest Adjacency Three Way (HAT): Highest neighbor level of all the
formed three way adjacencies for the node.
4.2.7.2. Automatic SystemID Selection
RIFT nodes require a 64 bit SystemID which SHOULD be derived as
EUI-64 MA-L derive according to [EUI64]. The organizationally
governed portion of this ID (24 bits) can be used to generate
multiple IDs if required to indicate more than one RIFT instance."
As matter of operational concern, the router MUST ensure that such
identifier is not changing very frequently (or at least not without
sending all its TIEs with fairly short lifetimes) since otherwise the
network may be left with large amounts of stale TIEs in other nodes
(though this is not necessarily a serious problem if the procedures
described in Section 7 are implemented).
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4.2.7.3. Generic Fabric Example
ZTP forces us to think about miscabled or unusually cabled fabric and
how such a topology can be forced into a "lattice" structure which a
fabric represents (with further restrictions). Let us consider a
necessary and sufficient physical cabling in Figure 26. We assume
all nodes being in the same PoD.
. +---+
. | A | s = TOP_OF_FABRIC
. | s | l = LEAF_ONLY
. ++-++ l2l = LEAF_2_LEAF
. | |
. +--+ +--+
. | |
. +--++ ++--+
. | E | | F |
. | +-+ | +-----------+
. ++--+ | ++-++ |
. | | | | |
. | +-------+ | |
. | | | | |
. | | +----+ | |
. | | | | |
. ++-++ ++-++ |
. | I +-----+ J | |
. | | | +-+ |
. ++-++ +--++ | |
. | | | | |
. +---------+ | +------+ |
. | | | | |
. +-----------------+ | |
. | | | | |
. ++-++ ++-++ |
. | X +-----+ Y +-+
. |l2l| | l |
. +---+ +---+
Figure 26: Generic ZTP Cabling Considerations
First, we must anchor the "top" of the cabling and that's what the
TOP_OF_FABRIC flag at node A is for. Then things look smooth until
we have to decide whether node Y is at the same level as I, J (and as
consequence, X is south of it) or at the same level as X. This is
unresolvable here until we "nail down the bottom" of the topology.
To achieve that we choose to use in this example the leaf flags in X
and Y. In case where Y would not have a leaf flag it will try to
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elect highest level offered and end up being in same level as I and
J.
4.2.7.4. Level Determination Procedure
A node starting up with UNDEFINED_VALUE (i.e. without a
CONFIGURED_LEVEL or any leaf or TOP_OF_FABRIC flag) MUST follow those
additional procedures:
1. It advertises its LEVEL_VALUE on all LIEs (observe that this can
be UNDEFINED_LEVEL which in terms of the schema is simply an
omitted optional value).
2. It computes HAL as numerically highest available level in all
VOLs.
3. It chooses then MAX(HAL-1,0) as its DERIVED_LEVEL. The node then
starts to advertise this derived level.
4. A node that lost all adjacencies with HAL value MUST hold down
computation of new DERIVED_LEVEL for a short period of time
unless it has no VOLs from southbound adjacencies. After the
holddown expired, it MUST discard all received offers, recompute
DERIVED_LEVEL and announce it to all neighbors.
5. A node MUST reset any adjacency that has changed the level it is
offering and is in three-way state.
6. A node that changed its defined level value MUST readvertise its
own TIEs (since the new `PacketHeader` will contain a different
level than before). Sequence number of each TIE MUST be
increased.
7. After a level has been derived the node MUST set the
`not_a_ztp_offer` on LIEs towards all systems offering a VOL for
HAL.
8. A node that changed its level SHOULD flush from its link state
database TIEs of all other nodes, otherwise stale information may
persist on "direction reversal", i.e. nodes that seemed south
are now north or east-west. This will not prevent the correct
operation of the protocol but could be slightly confusing
operationally.
A node starting with LEVEL_VALUE being 0 (i.e. it assumes a leaf
function by being configured with the appropriate flags or has a
CONFIGURED_LEVEL of 0) MUST follow those additional procedures:
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1. It computes HAT per procedures above but does NOT use it to
compute DERIVED_LEVEL. HAT is used to limit adjacency formation
per Section 4.2.2.
It MAY also follow modified procedures:
1. It may pick a different strategy to choose VOL, e.g. use the VOL
value with highest number of VOLs. Such strategies are only
possible since the node always remains "at the bottom of the
fabric" while another layer could "invert" the fabric by picking
its preferred VOL in a different fashion than always trying to
achieve the highest viable level.
4.2.7.5. ZTP FSM
This section specifies the precise, normative ZTP FSM and can be
omitted unless the reader is pursuing an implementation of the
protocol.
Initial state is ComputeBestOffer.
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Enter
|
v
+------------------+
| ComputeBestOffer |
| |<----+
| Entry: | | BetterHAL [LEVEL_COMPUTE]
| [LEVEL_COMPUTE] | | BetterHAT [LEVEL_COMPUTE]
| | | ChangeLocalConfiguredLevel [StoreConfigLevel,
| | | LEVEL_COMPUTE]
| | | ChangeLocalHierarchyIndications
| | | [StoreLeafFlags,
| | | LEVEL_COMPUTE]
| | | LostHAT [LEVEL_COMPUTE]
| | | NeighborOffer [IF NoLevelOffered
| | | THEN REMOVE_OFFER
| | | ELSE IF OfferedLevel > Leaf
| | | THEN UPDATE_OFFER
| | | ELSE REMOVE_OFFER
| | | ShortTic [RemoveExpiredOffers]
| |-----+
| |
| |<---------------------
| |---------------------> (UpdatingClients)
| | ComputationDone [-]
+------------------+
^ |
| | LostHAL [IF AnySouthBoundAdjacenciesPresent
| | THEN UpdateHoldDownTimerToNormalValue
| | ELSE FireHoldDownTimerImmediately]
| V
(HoldingDown)
ZTP FSM FSM
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(ComputeBestOffer)
| ^
| | ChangeLocalConfiguredLevel [StoreConfiguredLevel]
| | ChangeLocalHierarchyIndications [StoreLeafFlags]
| | HoldDownExpired [PURGE_OFFERS]
V |
+------------------+
| HoldingDown |
| |<----+
| | | BetterHAL [-]
| | | BetterHAT [-]
| | | ComputationDone [-]
| | | LostHAL [-]
| | | LostHat [-]
| | | NeighborOffer [IF NoLevelOffered
| | | THEN REMOVE_OFFER
| | | ELSE IF OfferedLevel > Leaf
| | | THEN UPDATE_OFFER
| | | ELSE REMOVE_OFFER
| | | ShortTic [RemoveExpiredOffers,
| | | IF HoldDownTimer expired
| | | THEN PUSH HoldDownExpired]
| |-----+
+------------------+
^
|
(UpdatingClients)
ZTP FSM FSM (continued)
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(ComputeBestOffer)
| ^
| | BetterHAL [-]
| | BetterHAT [-]
| | LostHAT [-]
| | ChangeLocalHierarchyIndications [StoreLeafFlags]
| | ChangeLocalConfiguredLevel [StoreConfigLevel]
V |
+------------------+
| UpdatingClients |
| |<----+
| Entry: | |
| [UpdateAllLIE- | | NeighborOffer [IF NoLevelOffered
| FSMsWith- | | THEN REMOVE_OFFER
| Computation- | | ELSE IF OfferedLevel > Leaf
| Results] | | THEN UPDATE_OFFER
| | | ELSE REMOVE_OFFER
| | | ShortTic [RemoveExpiredOffers]
| |-----+
+------------------+
|
| LostHAL [IF AnySouthBoundAdjacenciesPresent
| THEN UpdateHoldDownTimerToNormalValue
| ELSE FireHoldDownTimerImmediately]
V
(HoldingDown)
ZTP FSM FSM (continued)
Events
o ChangeLocalHierarchyIndications: node locally configured with new
leaf flags
o ChangeLocalConfiguredLevel: node locally configured with a defined
level
o NeighborOffer: a new neighbor offer with optional level and
neighbor state
o BetterHAL: better HAL computed internally
o BetterHAT: better HAT computed internally
o LostHAL: lost last HAL in computation
o LostHAT: lost HAT in computation
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o ComputationDone: computation performed
o HoldDownExpired: holddown expired
o ShortTic: one second timer tick, to be ignored if transition does
not exist
Actions
on ShortTic in HoldingDown finishes in HoldingDown: remove expired
offers and if holddown timer expired PUSH_EVENT HoldDownExpired
on ShortTic in ComputeBestOffer finishes in ComputeBestOffer:
remove expired offers
on HoldDownExpired in HoldingDown finishes in ComputeBestOffer:
PURGE_OFFERS
on ChangeLocalConfiguredLevel in HoldingDown finishes in
ComputeBestOffer: store configured level
on ShortTic in UpdatingClients finishes in UpdatingClients: remove
expired offers
on BetterHAT in ComputeBestOffer finishes in ComputeBestOffer:
LEVEL_COMPUTE
on BetterHAL in HoldingDown finishes in HoldingDown: no action
on ChangeLocalHierarchyIndications in HoldingDown finishes in
ComputeBestOffer: store leaf flags
on BetterHAT in UpdatingClients finishes in ComputeBestOffer: no
action
on BetterHAL in UpdatingClients finishes in ComputeBestOffer: no
action
on ChangeLocalHierarchyIndications in UpdatingClients finishes in
ComputeBestOffer: store leaf flags
on LostHAL in HoldingDown finishes in HoldingDown:
on LostHAT in ComputeBestOffer finishes in ComputeBestOffer:
LEVEL_COMPUTE
on LostHAT in HoldingDown finishes in HoldingDown: no action
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on BetterHAT in HoldingDown finishes in HoldingDown: no action
on NeighborOffer in UpdatingClients finishes in UpdatingClients:
if no level offered then REMOVE_OFFER
else
if offered level > leaf then UPDATE_OFFER
else REMOVE_OFFER
on LostHAL in ComputeBestOffer finishes in HoldingDown: if any
southbound adjacencies present then update holddown timer to
normal duration else fire holddown timer immediately
on LostHAL in UpdatingClients finishes in HoldingDown: if any
southbound adjacencies present then update holddown timer to
normal duration else fire holddown timer immediately
on ComputationDone in ComputeBestOffer finishes in
UpdatingClients: no action
on LostHAT in UpdatingClients finishes in ComputeBestOffer: no
action
on ComputationDone in HoldingDown finishes in HoldingDown:
on ChangeLocalConfiguredLevel in ComputeBestOffer finishes in
ComputeBestOffer: store configured level and LEVEL_COMPUTE
on ChangeLocalConfiguredLevel in UpdatingClients finishes in
ComputeBestOffer: store configured level
on NeighborOffer in ComputeBestOffer finishes in ComputeBestOffer:
if no level offered then REMOVE_OFFER
else
if offered level > leaf then UPDATE_OFFER
else REMOVE_OFFER
on NeighborOffer in HoldingDown finishes in HoldingDown:
if no level offered then REMOVE_OFFER
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else
if offered level > leaf then UPDATE_OFFER
else REMOVE_OFFER
on ChangeLocalHierarchyIndications in ComputeBestOffer finishes in
ComputeBestOffer: store leaf flags and LEVEL_COMPUTE
on BetterHAL in ComputeBestOffer finishes in ComputeBestOffer:
LEVEL_COMPUTE
on Entry into UpdatingClients: update all LIE FSMs with
computation results
on Entry into ComputeBestOffer: LEVEL_COMPUTE
Following words are used for well known procedures:
1. PUSH Event: pushes an event to be executed by the FSM upon exit
of this action
2. COMPARE_OFFERS: checks whether based on current offers and held
last results the events BetterHAL/LostHAL/BetterHAT/LostHAT are
necessary and returns them
3. UPDATE_OFFER: store current offer with adjacency holdtime as
lifetime and COMPARE_OFFERS, then PUSH according events
4. LEVEL_COMPUTE: compute best offered or configured level and HAL/
HAT, if anything changed PUSH ComputationDone
5. REMOVE_OFFER: remove the according offer and COMPARE_OFFERS, PUSH
according events
6. PURGE_OFFERS: REMOVE_OFFER for all held offers, COMPARE OFFERS,
PUSH according events
4.2.7.6. Resulting Topologies
The procedures defined in Section 4.2.7.4 will lead to the RIFT
topology and levels depicted in Figure 27.
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. +---+
. | As|
. | 24|
. ++-++
. | |
. +--+ +--+
. | |
. +--++ ++--+
. | E | | F |
. | 23+-+ | 23+-----------+
. ++--+ | ++-++ |
. | | | | |
. | +-------+ | |
. | | | | |
. | | +----+ | |
. | | | | |
. ++-++ ++-++ |
. | I +-----+ J | |
. | 22| | 22| |
. ++--+ +--++ |
. | | |
. +---------+ | |
. | | |
. ++-++ +---+ |
. | X | | Y +-+
. | 0 | | 0 |
. +---+ +---+
Figure 27: Generic ZTP Topology Autoconfigured
In case we imagine the LEAF_ONLY restriction on Y is removed the
outcome would be very different however and result in Figure 28.
This demonstrates basically that auto configuration makes miscabling
detection hard and with that can lead to undesirable effects in cases
where leaves are not "nailed" by the accordingly configured flags and
arbitrarily cabled.
A node MAY analyze the outstanding level offers on its interfaces and
generate warnings when its internal ruleset flags a possible
miscabling. As an example, when a node's sees ZTP level offers that
differ by more than one level from its chosen level (with proper
accounting for leaf's being at level 0) this can indicate miscabling.
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. +---+
. | As|
. | 24|
. ++-++
. | |
. +--+ +--+
. | |
. +--++ ++--+
. | E | | F |
. | 23+-+ | 23+-------+
. ++--+ | ++-++ |
. | | | | |
. | +-------+ | |
. | | | | |
. | | +----+ | |
. | | | | |
. ++-++ ++-++ +-+-+
. | I +-----+ J +-----+ Y |
. | 22| | 22| | 22|
. ++-++ +--++ ++-++
. | | | | |
. | +-----------------+ |
. | | |
. +---------+ | |
. | | |
. ++-++ |
. | X +--------+
. | 0 |
. +---+
Figure 28: Generic ZTP Topology Autoconfigured
4.2.8. Stability Considerations
The autoconfiguration mechanism computes a global maximum of levels
by diffusion. The achieved equilibrium can be disturbed massively by
all nodes with highest level either leaving or entering the domain
(with some finer distinctions not explained further). It is
therefore recommended that each node is multi-homed towards nodes
with respective HAL offerings. Fortunately, this is the natural
state of things for the topology variants considered in RIFT.
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4.3. Further Mechanisms
4.3.1. Overload Bit
The overload bit MUST be respected by all necessary SPF computations.
A node with the overload bit set SHOULD advertise all locally hosted
prefixes both northbound and southbound, all other southbound
prefixes SHOULD NOT be advertised.
Leaf nodes SHOULD set the overload bit on all originated Node TIEs.
If spine nodes were to forward traffic not intended for the local
node, the leaf node would not be able to prevent routing/forwarding
loops as it does not have the necessary topology information to do
so.
4.3.2. Optimized Route Computation on Leaves
Leaf nodes only have visibility to directly connected nodes and
therefore are not required to run "full" SPF computations. Instead,
prefixes from neighboring nodes can be gathered to run a "partial"
SPF computation in order to build the routing table.
Leaf nodes SHOULD only hold their own N-TIEs, and in cases of L2L
implementations, the N-TIEs of their East/West neighbors. Leaf nodes
MUST hold all S-TIEs from their neighbors.
Normally, a full network graph is created based on local N-TIEs and
remote S-TIEs that it receives from neighbors, at which time,
necessary SPF computations are performed. Instead, leaf nodes can
simply compute the minimum cost and next-hop set of each leaf
neighbor by examining its local adjacencies. Associated N-TIEs are
used to determine bi-directionality and derive the next-hop set.
Cost is then derived from the minimum cost of the local adjacency to
the neighbor and the prefix cost.
Leaf nodes would then attach necessary prefixes as described in
Section 4.2.6.
4.3.3. Mobility
The RIFT control plane MUST maintain the real time status of every
prefix, to which port it is attached, and to which leaf node that
port belongs. This is still true in cases of IP mobility where the
point of attachment may change several times a second.
There are two classic approaches to explicitly maintain this
information:
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timestamp: With this method, the infrastructure SHOULD record the
precise time at which the movement is observed. One key advantage
of this technique is that it has no dependency on the mobile
device. One drawback is that the infrastructure MUST be precisely
synchronized in order to be able to compare timestamps as the
points of attachment change. This could be accomplished by
utilizing Precision Time Protocol (PTP) IEEE Std. 1588
[IEEEstd1588] or 802.1AS [IEEEstd8021AS] which is designed for
bridged LANs. Both the precision of the synchronization protocol
and the resolution of the timestamp must beat the highest possible
roaming time on the fabric. Another drawback is that the presence
of a mobile device may only be observed asynchronously, such as
when it starts using an IP protocol like ARP [RFC0826], IPv6
Neighbor Discovery [RFC4861], IPv6 Stateless Address Configuration
[RFC4862], DHCP [RFC2131], or DHCPv6 [RFC8415].
sequence counter: With this method, a mobile device notifies its
point of attachment on arrival with a sequence counter that is
incremented upon each movement. On the positive side, this method
does not have a dependency on a precise sense of time, since the
sequence of movements is kept in order by the mobile device. The
disadvantage of this approach is the lack of support for protocols
that may be used by the mobile device to register its presence to
the leaf node with the capability to provide a sequence counter.
Well-known issues with sequence counters such as wrapping and
comparison rules MUST be addressed properly. Sequence numbers
MUST be compared by a single homogenous source to make operation
feasible. Sequence number comparison from multiple heterogeneous
sources would be extremely difficult to implement.
RIFT supports a hybrid approach by using an optional
'PrefixSequenceType' attribute (that we also call a 'monotonic
clock') that consists of a timestamp and optional sequence number
field. When this attribute is present (observe that per data schema
the attribute itself is optional but in case it is included the
'timestamp' field is required):
o The leaf node MAY advertise a timestamp of the latest sighting of
a prefix, e.g., by snooping IP protocols or the node using the
time at which it advertised the prefix. RIFT transports the
timestamp within the desired prefix North TIEs as 802.1AS
timestamp.
o RIFT MAY interoperate with "Registration Extensions for 6LoWPAN
Neighbor Discovery" [RFC8505], which provides a method for
registering a prefix with a sequence number called a Transaction
ID (TID). In such cases, RIFT SHOULD transport the derived TID
without modification.
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o RIFT also defines an abstract negative clock (ASNC) (also called
an 'undefined' clock). ASNC MUST be considered older than any
other defined clock. By default, when a node receives a prefix
North TIE that does not contain a 'PrefixSequenceType' attribute,
it MUST interpret the absence as ASNC.
o Any prefix present on the fabric in multiple nodes that has the
`same` clock is considered as anycast.
o RIFT specification assumes that all nodes are being synchronized
to at least 200 milliseconds of precision. This is achievable
through the use of NTP [RFC5905]. An implementation MAY provide a
way to reconfigure a domain to a different value, we call this
variable MAXIMUM_CLOCK_DELTA.
4.3.3.1. Clock Comparison
All monotonic clock values MUST be compared to each other using the
following rules:
1. ASNC is older than any other value except ASNC AND
2. Clock with timestamp differing by more than MAXIMUM_CLOCK_DELTA
are comparable by using the timestamps only AND
3. Clocks with timestamps differing by less than MAXIMUM_CLOCK_DELTA
are comparable by using their TIDs only AND
4. An undefined TID is always older than any other TID AND
5. TIDs are compared using rules of [RFC8505].
4.3.3.2. Interaction between Time Stamps and Sequence Counters
For attachment changes that occur less frequently (e.g. once per
second), the timestamp that the RIFT infrastructure captures should
be enough to determine the most current discovery. If the point of
attachment changes faster than the maximum drift of the time stamping
mechanism (i.e. MAXIMUM_CLOCK_DELTA), then a sequence number SHOULD
be used to enable necessary precision to determine currency.
The sequence counter in [RFC8505] is encoded as one octet and wraps
around using Appendix A.
Within the resolution of MAXIMUM_CLOCK_DELTA, sequence counter values
captured during 2 sequential iterations of the same timestamp SHOULD
be comparable. This means that with default values, a node may move
up to 127 times in a 200 millisecond period and the clocks will
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remain comparable. This allows the RIFT infrastructure to explicitly
assert the most up-to-date advertisement.
4.3.3.3. Anycast vs. Unicast
A unicast prefix can be attached to at most one leaf, whereas an
anycast prefix may be reachable via more than one leaf.
If a monotonic clock attribute is provided on the prefix, then the
prefix with the `newest` clock value is strictly preferred. An
anycast prefix does not carry a clock or all clock attributes MUST be
the same under the rules of Section 4.3.3.1.
Observe that it is important that in mobility events the leaf is re-
flooding as quickly as possible the absence of the prefix that moved
away.
Observe further that without support for [RFC8505] movements on the
fabric within intervals smaller than 100msec will be seen as anycast.
4.3.3.4. Overlays and Signaling
RIFT is agnostic to any overlay technologies and their associated
control and transports that run on top of it (e.g. VXLAN). It is
expected that leaf nodes and possibly Top-of-Fabric nodes can perform
necessary data plane encapsulation.
In the context of mobility, overlays provide another possible
solution to avoid injecting mobile prefixes into the fabric as well
as improving scalability of the deployment. It makes sense to
consider overlays for mobility solutions in IP fabrics. As an
example, a mobility protocol such as LISP [RFC6830] may inform the
ingress leaf of the location of the egress leaf in real time.
Another possibility is to consider that mobility as an underlay
service and support it in RIFT to an extent. The load on the fabric
augments with the amount of mobility obviously since a move forces
flooding and computation on all nodes in the scope of the move so
tunneling from leaf to the Top-of-Fabric may be desired to speed up
convergence times.
4.3.4. Key/Value Store
4.3.4.1. Southbound
RIFT supports the southbound distribution of key-value pairs that can
be used to distribute information to facilitate higher levels of
functionality (e.g. distribution of configuration information). KV
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South TIEs may arrive from multiple nodes and therefore MUST execute
the following tie-breaking rules for each key:
1. Only KV TIEs received from nodes to which a bi-directional
adjacency exists MUST be considered.
2. For each valid KV South TIEs that contains the same key, the
value within the South TIE with the highest level will be
preferred. If the levels are identical, the highest originating
system ID will be preferred. In the case of overlapping keys in
the winning South TIE, the behavior is undefined.
Consider that if a node goes down, nodes south of it will lose
associated adjacencies causing them to disregard corresponding KVs.
New KV South TIEs are advertised to prevent stale information being
used by nodes that are farther south. KV advertisements southbound
are not a result of independent computation by every node over the
same set of South TIEs, but a diffused computation.
4.3.4.2. Northbound
Certain use cases necessitate distribution of essential KV
information that is generated by the leaves in the northbound
direction. Such information is flooded in KV North TIEs. Since the
originator of the KV North TIEs is preserved during flooding,
overlapping keys MAY be used. However, to avoid further protocol
complexity, the same tie-breaking rules as used in southbound
distribution SHOULD be used.
4.3.5. Interactions with BFD
RIFT MAY incorporate BFD [RFC5881] to react quickly to link failures.
In such case following procedures are introduced:
After RIFT three-way hello adjacency convergence a BFD session MAY
be formed automatically between the RIFT endpoints without further
configuration using the exchanged discriminators. The capability
of the remote side to support BFD is carried in the LIEs.
In case established BFD session goes Down after it was Up, RIFT
adjacency SHOULD be re-initialized and subsequently started from
Init after it sees a consecutive BFD Up.
In case of parallel links between nodes each link MAY run its own
independent BFD session or they MAY share a session.
If link identifiers or BFD capabilities change, both the LIE and
any BFD sessions SHOULD be brought down and back up again. In
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case only the advertised capabilities change, the node MAY choose
to persist the BFD session.
Multiple RIFT instances MAY choose to share a single BFD session,
in such cases the behavior for which discriminators are used is
undefined. However, RIFT MAY advertise the same link ID for the
same interface in multiple instances to "share" discriminators.
BFD TTL follows [RFC5082].
4.3.6. Fabric Bandwidth Balancing
A well understood problem in fabrics is that in case of link
failures, it would be ideal to rebalance how much traffic is sent to
switches in the next level based on available ingress and egress
bandwidth.
RIFT supports a very light weight mechanism that can deal with the
problem in an approximate way based on the fact that RIFT is loop-
free.
4.3.6.1. Northbound Direction
Every RIFT node SHOULD compute the amount of northbound bandwidth
available through neighbors at higher level and modify distance
received on default route from this neighbor. Default routes with
differing distances SHOULD be used to support weighted ECMP
forwarding. We call such a distance Bandwidth Adjusted Distance or
BAD. This is best illustrated by a simple example.
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. 100 x 100 100 MBits
. | x | |
. +-+---+-+ +-+---+-+
. | | | |
. |Spin111| |Spin112|
. +-+---+++ ++----+++
. |x || || ||
. || |+---------------+ ||
. || +---------------+| ||
. || || || ||
. || || || ||
. -----All Links 10 MBit-------
. || || || ||
. || || || ||
. || +------------+| || ||
. || |+------------+ || ||
. |x || || ||
. +-+---+++ +--++-+++
. | | | |
. |Leaf111| |Leaf112|
. +-------+ +-------+
Figure 29: Balancing Bandwidth
Figure 29 depicts an example topology where links between leaf and
spine nodes are 10 MBit/s and links from spine nodes northbound are
100 MBit/s. Consider a parallel link failure between Leaf 111 and
Spine 111 and as a result, Leaf 111 wants to forward more traffic
toward Spine 112. Additionally, we consider an uplink failure on
Spine 111.
The local modification of the received default route distance from
upper level is achieved by running a relatively simple algorithm
where the bandwidth is weighted exponentially, while the distance on
the default route represents a multiplier for the bandwidth weight
for easy operational adjustments.
On a node, L, use Node TIEs to compute from each non-overloaded
northbound neighbor N to compute 3 values:
L_N_u: as sum of the bandwidth available to N
N_u: as sum of the uplink bandwidth available on N
T_N_u: as sum of L_N_u * OVERSUBSCRIPTION_CONSTANT + N_u
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For all T_N_u determine the according M_N_u as
log_2(next_power_2(T_N_u)) and determine MAX_M_N_u as maximum value
of all such M_N_u values.
For each advertised default route from a node N modify the advertised
distance D to BAD = D * (1 + MAX_M_N_u - M_N_u) and use BAD instead
of distance D to weight balance default forwarding towards N.
For the example above, a simple table of values will help in
understanding of the concept. We assume that all default route
distances are advertised with D=1 and that OVERSUBSCRIPTION_CONSTANT
= 1.
+---------+-----------+-------+-------+-----+
| Node | N | T_N_u | M_N_u | BAD |
+---------+-----------+-------+-------+-----+
| Leaf111 | Spine 111 | 110 | 7 | 2 |
+---------+-----------+-------+-------+-----+
| Leaf111 | Spine 112 | 220 | 8 | 1 |
+---------+-----------+-------+-------+-----+
| Leaf112 | Spine 111 | 120 | 7 | 2 |
+---------+-----------+-------+-------+-----+
| Leaf112 | Spine 112 | 220 | 8 | 1 |
+---------+-----------+-------+-------+-----+
Table 5: BAD Computation
If a calculation produces a result exceeding the range of the type,
e.g. bandwidth, the result is set to the highest possible value for
that type.
BAD SHOULD be only computed for default routes. A node MAY compute
and use BAD for any disaggregated prefixes or other RIFT routes. A
node MAY use a different algorithm to weight northbound traffic based
on bandwidth. If a different algorithm is used, its successful
behavior MUST NOT depend on uniformity of algorithm or
synchronization of BAD computations across the fabric. E.g. it is
conceivable that leaves could use real time link loads gathered by
analytics to change the amount of traffic assigned to each default
route next hop.
Furthermore, a change in available bandwidth will only affect, at
most, two levels down in the fabric, i.e. the blast radius of
bandwidth adjustments is constrained no matter the fabric's height.
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4.3.6.2. Southbound Direction
Due to its loop free nature, during South SPF, a node MAY account for
maximum available bandwidth on nodes in lower levels and modify the
amount of traffic offered to the next level's southbound nodes. It
is worth considering that such computations may be more effective if
standardized, but do not have to be. As long as a packet continues
to flow southbound, it will take some viable, loop-free path to reach
its destination.
4.3.7. Label Binding
A node MAY advertise in its LIEs, a locally significant, downstream
assigned, interface specific label. One use of such a label is a
hop-by-hop encapsulation allowing forwarding planes to be easily
distinguished among multiple RIFT instances.
4.3.8. Leaf to Leaf Procedures
RIFT implementations SHOULD support special East-West adjacencies
between leaf nodes. Leaf nodes supporting these procedures MUST:
advertise the LEAF_2_LEAF flag in its node capabilities AND
set the overload bit on all leaf's node TIEs AND
flood only a node's own north and south TIEs over E-W leaf
adjacencies AND
always use E-W leaf adjacency in all SPF computations AND
install a discard route for any advertised aggregate routes in a
leaf?s TIE AND
never form southbound adjacencies.
This will allow the E-W leaf nodes to exchange traffic strictly for
the prefixes advertised in each other's north prefix TIEs (since the
southbound computation will find the reverse direction in the other
node's TIE and install its north prefixes).
4.3.9. Address Family and Multi Topology Considerations
Multi-Topology (MT)[RFC5120] and Multi-Instance (MI)[RFC8202]
concepts are used today in link-state routing protocols to support
several domains on the same physical topology. RIFT supports this
capability by carrying transport ports in the LIE protocol exchanges.
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Multiplexing of LIEs can be achieved by either choosing varying
multicast addresses or ports on the same address.
BFD interactions in Section 4.3.5 are implementation dependent when
multiple RIFT instances run on the same link.
4.3.10. Reachability of Internal Nodes in the Fabric
RIFT does not require that nodes have reachable addresses in the
fabric, though it is clearly desirable for operational purposes.
Under normal operating conditions this can be easily achieved by
injecting the node's loopback address into North and South Prefix
TIEs or other implementation specific mechanisms.
Special considerations arise when a node loses all northbound
adjacencies, but is not at the top of the fabric. These are outside
the scope of this document and could be discussed in a separate
document.
4.3.11. One-Hop Healing of Levels with East-West Links
Based on the rules defined in Section 4.2.4, Section 4.2.3.8 and
given presence of E-W links, RIFT can provide a one-hop protection
for nodes that lost all their northbound links. This can also be
applied to multi-plane designs where complex link set failures occur
at the Top-of-Fabric when links are exclusively used for flooding
topology information. Section 5.4 outlines this behavior.
4.4. Security
4.4.1. Security Model
An inherent property of any security and ZTP architecture is the
resulting trade-off in regard to integrity verification of the
information distributed through the fabric vs. provisioning and auto-
configuration requirements. At a minimum the security of an
established adjacency should be ensured. The stricter the security
model the more provisioning must take over the role of ZTP.
RIFT supports the following security models to allow for flexible
control by the operator.
o The most security conscious operators may choose to have control
over which ports interconnect between a given pair of nodes, we
call this the "Port-Association Model" (PAM). This is achievable
by configuring each pair of directly connected ports with a
designated shared key or public/private key pair.
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o In physically secure data center locations, operators may choose
to control connectivity between entire nodes, we call this the
"Node-Association Model" (NAM). A benefit of this model is that
it allows for simplified port sparing.
o In the most relaxed environments, an operator may only choose to
control which nodes join a particular fabric. We call this the
"Fabric-Association Model" (FAM). This is achievable by using a
single shared secret across the entire fabric. Such flexibility
makes sense when we consider servers as leaf devices, which are
replaced more often than network nodes. In addition, this model
allows for simplified node sparing.
o These models may be mixed throughout the fabric depending upon
security requirements at various levels of the fabric and
willingness to accept increased provisioning complexity.
In order to support the cases mentioned above, RIFT implementations
supports, through operator control, mechanisms that allow for:
a. specification of the appropriate level in the fabric,
b. discovery and reporting of missing connections,
c. discovery and reporting of unexpected connections while
preventing them from forming insecure adjacencies.
Operators may only choose to configure the level of each node, but
not explicitly configure which connections are allowed. In this
case, RIFT will only allow adjacencies to establish between nodes
that are in adjacent levels. Operators with the lowest security
requirements may not use any configuration to specify which
connections are allowed. Nodes in such fabrics could rely fully on
ZTP and only established adjacencies between nodes in adjacent
levels. Figure 30 illustrates inherent tradeoffs between the
different security models.
Some level of link quality verification may be required prior to an
adjacency being used for forwarding. For example, an implementation
may require that a BFD session comes up before advertising the
adjacency.
For the cases outlined above, RIFT has two approaches to enforce that
a local port is connected to the correct port on the correct remote
node. One approach is to piggy-back on RIFT's authentication
mechanism. Assuming the provisioning model (e.g. the YANG model) is
flexible enough, operators can choose to provision a unique
authentication key for:
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a. each pair of ports in "port-association model" or
b. each pair of switches in "node-association model" or
c. each pair of levels or
d. the entire fabric in "fabric-association model".
The other approach is to rely on the system-id, port-id and level
fields in the LIE message to validate an adjacency against the
expected cabling topology, and optionally introduce some new rules in
the FSM to allow the adjacency to come up if the expectations are
met.
^ /\ |
/|\ / \ |
| / \ |
| / PAM \ |
Increasing / \ Increasing
Integrity +----------+ Flexibility
& / NAM \ &
Increasing +--------------+ Less
Provisioning / FAM \ Configuration
| +------------------+ |
| / Level Provisioning \ |
| +----------------------+ \|/
| / Zero Configuration \ v
+--------------------------+
Figure 30: Security Model
4.4.2. Security Mechanisms
RIFT Security goals are to ensure:
1. authentication
2. message integrity
3. the prevention of replay attacks
4. low processing overhead
5. efficient messaging
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Message confidentiality is a non-goal.
The model in the previous section allows a range of security key
types that are analogous to the various security association models.
PAM and NAM allow security associations at the port or node level
using symmetric or asymmetric keys that are pre-installed. FAM
argues for security associations to be applied only at a group level
or to be refined once the topology has been established. RIFT does
not specify how security keys are installed or updated, though it
does specify how the key can be used to achieve security goals.
The protocol has provisions for "weak" nonces to prevent replay
attacks and includes authentication mechanisms comparable to
[RFC5709] and [RFC7987].
4.4.3. Security Envelope
RIFT MUST be carried in a mandatory secure envelope illustrated in
Figure 31. Any value in the packet following a security fingerprint
MUST be used only after the appropriate fingerprint has been
validated.
Local configuration MAY allow for the envelope's integrity checks to
be skipped.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
UDP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | RIFT destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer Security Envelope Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIFT MAGIC | Packet Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | RIFT Major | Outer Key ID | Fingerprint |
| | Version | | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Fingerprint covers all following content ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Weak Nonce Local | Weak Nonce Remote |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remaining TIE Lifetime (all 1s in case of LIE) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TIE Origin Security Envelope Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TIE Origin Key ID | Fingerprint |
| | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Fingerprint covers all following content ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Serialized RIFT Model Object
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Serialized RIFT Model Object ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31: Security Envelope
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RIFT MAGIC: 16 bits. Constant value of 0xA1F7 that allows to
classify RIFT packets independent of used UDP port.
Packet Number: 16 bits. An optional, per packet type monotonically
growing number rolling over using sequence number arithmetic
defined in Appendix A. A node SHOULD correctly set the number on
subsequent packets or otherwise MUST set the value to
`undefined_packet_number` as provided in the schema. This number
can be used to detect losses and misordering in flooding for
either operational purposes or in implementation to adjust
flooding behavior to current link or buffer quality. This number
MUST NOT be used to discard or validate the correctness of
packets.
RIFT Major Version: 8 bits. It allows to check whether protocol
versions are compatible, i.e. if the serialized object can be
decoded at all. An implementation MUST drop packets with
unexpected values and MAY report a problem.
Outer Key ID: 8 bits to allow key rollovers. This implies key type
and algorithm. Value 0 means that no valid fingerprint was
computed. This key ID scope is local to the nodes on both ends of
the adjacency.
TIE Origin Key ID: 24 bits. This implies key type and used
algorithm. Value 0 means that no valid fingerprint was computed.
This key ID scope is global to the RIFT instance since it implies
the originator of the TIE so the contained object does not have to
be de-serialized to obtain it.
Length of Fingerprint: 8 bits. Length in 32-bit multiples of the
following fingerprint (not including lifetime or weak nonces). It
allows the structure to be navigated when an unknown key type is
present. To clarify, a common corner case when this value is set
to 0 is when it signifies an empty (0 bytes long) security
fingerprint.
Security Fingerprint: 32 bits * Length of Fingerprint. This is a
signature that is computed over all data following after it. If
the significant bits of fingerprint are fewer than the 32 bits
padded length than the significant bits MUST be left aligned and
remaining bits on the right padded with 0s. When using PKI the
Security fingerprint originating node uses its private key to
create the signature. The original packet can then be verified
provided the public key is shared and current.
Remaining TIE Lifetime: 32 bits. In case of anything but TIEs this
field MUST be set to all ones and Origin Security Envelope Header
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MUST NOT be present in the packet. For TIEs this field represents
the remaining lifetime of the TIE and Origin Security Envelope
Header MUST be present in the packet. The value in the serialized
model object MUST be ignored.
Weak Nonce Local: 16 bits. Local Weak Nonce of the adjacency as
advertised in LIEs.
Weak Nonce Remote: 16 bits. Remote Weak Nonce of the adjacency as
received in LIEs.
TIE Origin Security Envelope Header: It MUST be present if and only
if the Remaining TIE Lifetime field is NOT all ones. It carries
through the originators key ID and according fingerprint of the
object to protect TIE from modification during flooding. This
ensures origin validation and integrity (but does not provide
validation of a chain of trust).
Observe that due to the schema migration rules per Appendix B the
contained model can be always decoded if the major version matches
and the envelope integrity has been validated. Consequently,
description of the TIE is available to flood it properly including
unknown TIE types.
4.4.4. Weak Nonces
The protocol uses two 16 bit nonces to salt generated signatures. We
use the term "nonce" a bit loosely since RIFT nonces are not being
changed in every packet as common in cryptography. For efficiency
purposes they are changed at a high enough frequency to dwarf
practical replay attack attempts. Therefore, we call them "weak"
nonces.
Any implementation including RIFT security MUST generate and wrap
around local nonces properly. When a nonce increment leads to
`undefined_nonce` value, the value MUST be incremented again
immediately. All implementation MUST reflect the neighbor's nonces.
An implementation SHOULD increment a chosen nonce on every LIE FSM
transition that ends up in a different state from the previous and
MUST increment its nonce at least every 5 minutes (such
considerations allow for efficient implementations without opening a
significant security risk). When flooding TIEs, the implementation
MUST use recent (i.e. within allowed difference) nonces reflected in
the LIE exchange. The schema specifies the maximum allowable nonce
value difference on a packet compared to reflected nonces in the
LIEs. Any packet received with nonces deviating more than the
allowed delta MUST be discarded without further computation of
signatures to prevent computation load attacks.
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In cases where a secure implementation does not receive signatures or
receives undefined nonces from a neighbor (indicating that it does
not support or verify signatures), it is a matter of local policy as
to how those packets are treated. A secure implementation MAY refuse
forming an adjacency with an implementation that is not advertising
signatures or valid nonces, or it MAY continue signing local packets
while accepting a neighbor's packets without further security
validation.
As a necessary exception, an implementation MUST advertise the remote
nonce value as `undefined_nonce` when the FSM is not in two-way or
three-way state and accept an `undefined_nonce` for its local nonce
value on packets in any other state than three-way.
As optional optimization, an implementation MAY send one LIE with
previously negotiated neighbor's nonce to try to speed up a
neighbor's transition from three-way to one-way and MUST revert to
sending `undefined_nonce` after that.
4.4.5. Lifetime
Protecting flooding lifetime may lead to an excessive number of
security fingerprint computations and to avoid this the application
generating the fingerprints for advertised TIEs, MAY round the value
down to the next `rounddown_lifetime_interval`. Such an optimization
in the presence of security hashes over advancing weak nonces, may
not be feasible.
4.4.6. Key Management
As outlined in Section Section 7, either a private shared key or a
public/private key pair is used to authenticate the adjacency. Both
the key distribution and key synchronization methods are out of scope
for this document. Both nodes in the adjacency MUST share the same
keys, key type, and algorithm for a given key ID. Mismatched keys
will not inter-operate as their security envelopes will be
unverifiable.
Key roll-over while the adjacency is active MAY be supported. The
specific mechanism is well documented in [RFC6518].
4.4.7. Security Association Changes
There in no mechanism to convert a security envelope for the same key
ID from one algorithm to another once the envelope is operational.
The recommended procedure to change to a new algorithm is to take the
adjacency down, make the necessary changes, and bring the adjacency
back up. Obviously, an implementation MAY choose to stop verifying
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security envelope for the duration of algorithm change to keep the
adjacency up but since this introduces a security vulnerability
window, such roll-over SHOULD NOT be recommended.
5. Examples
5.1. Normal Operation
^ N +--------+ +--------+
Level 2 | |ToF 21| |ToF 22|
E <-*-> W ++-+--+-++ ++-+--+-++
| | | | | | | | |
S v P111/2 |P121/2 | | | |
^ ^ ^ ^ | | | |
| | | | | | | |
+--------------+ | +-----------+ | | | +---------------+
| | | | | | | |
South +-----------------------------+ | | ^
| | | | | | | All TIEs
0/0 0/0 0/0 +-----------------------------+ |
v v v | | | | |
| | +-+ +<-0/0----------+ | |
| | | | | | | |
+-+----++ +-+----++ ++----+-+ ++-----++
Level 1 | | | | | | | |
|Spin111| |Spin112| |Spin121| |Spin122|
+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
| | | South | | | |
| +---0/0--->-----+ 0/0 | +----------------+ |
0/0 | | | | | | |
| +---<-0/0-----+ | v | +--------------+ | |
v | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +---+-+-+
Level 0 | | | | | | | |
|Leaf111| |Leaf112| |Leaf121| |Leaf122|
+-+-----+ +-+---+-+ +--+--+-+ +-+-----+
+ + \ / + +
Prefix111 Prefix112 \ / Prefix121 Prefix122
multi-homed
Prefix
+---------- PoD 1 ---------+ +---------- PoD 2 ---------+
Figure 32: Normal Case Topology
This section describes RIFT deployment in example topology given in
Figure 32 without any node or link failures. We disregard flooding
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reduction for simplicity's sake and compress the node names in some
cases to fit them into the picture better.
First, the following bi-directional adjacencies will be established:
1. ToF 21 (PoD 0) to Spine 111, Spine 112, Spine 121, and Spine 122
2. ToF 22 (PoD 0) to Spine 111, Spine 112, Spine 121, and Spine 122
3. Spine 111 to Leaf 111, Leaf 112
4. Spine 112 to Leaf 111, Leaf 112
5. Spine 121 to Leaf 121, Leaf 122
6. Spine 122 to Leaf 121, Leaf 122
Leaf 111 and Leaf 112 originate N-TIEs for Prefix 111 and Prefix 112
(respectively) to both Spine 111 and Spine 112 (Leaf 112 also
originates an N-TIE for the multi-homed prefix). Spine 111 and Spine
112 will then originate their own N-TIEs, as well as flood the N-TIEs
received from Leaf 111 and Leaf 112 to both ToF 21 and ToF 22.
Similarly, Leaf 121 and Leaf 122 originate North TIEs for Prefix 121
and Prefix 122 (respectively) to Spine 121 and Spine 122 (Leaf 121
also originates an North TIE for the multi-homed prefix). Spine 121
and Spine 122 will then originate their own North TIEs, as well as
flood the North TIEs received from Leaf 121 and Leaf 122 to both ToF
21 and ToF 22.
Spines hold only North TIEs of level 0 for their PoD, while leaves
only hold their own North TIEs while at this point, both ToF 21 and
ToF 22 (as well as any northbound connected controllers) would have
the complete network topology.
ToF 21 and ToF 22 would then originate and flood South TIEs
containing any established adjacencies and a default IP route to all
spines. Spine 111, Spine 112, Spine 121, and Spine 122 will reflect
all Node South TIEs received from ToF 21 to ToF 22, and all Node
South TIEs from ToF 22 to ToF 21. South TIEs will not be re-
propagated southbound.
South TIEs containing a default IP route are then originated by both
Spine 111 and Spine 112 toward Leaf 111 and Leaf 112. Similarly,
South TIEs containing a default IP route are originated by Spine 121
and Spine 122 toward Leaf 121 and Leaf 122.
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At this point IP connectivity across maximum number of viable paths
has been established for all leaves, with routing information
constrained to only the minimum amount that allows for normal
operation and redundancy.
5.2. Leaf Link Failure
. | | | |
.+-+---+-+ +-+---+-+
.| | | |
.|Spin111| |Spin112|
.+-+---+-+ ++----+-+
. | | | |
. | +---------------+ X
. | | | X Failure
. | +-------------+ | X
. | | | |
.+-+---+-+ +--+--+-+
.| | | |
.|Leaf111| |Leaf112|
.+-------+ +-------+
. + +
. Prefix111 Prefix112
Figure 33: Single Leaf Link Failure
In the event of a link failure between Spine 112 and Leaf 112, both
nodes will originate new Node TIEs that contain their connected
adjacencies, except for the one that just failed. Leaf 112 will send
a Node North TIE to Spine 111. Spine 112 will send a Node North TIE
to ToF 21 and ToF 22 as well as a new Node South TIE to Leaf 111 that
will be reflected to Spine 111. Necessary SPF recomputation will
occur, resulting in Spine 112 no longer being in the forwarding path
for Prefix 112.
Spine 111 will also disaggregate Prefix 112 by sending new Prefix
South TIE to Leaf 111 and Leaf 112. Though we cover disaggregation
in more detail in the following section, it is worth mentioning ini
this example as it further illustrates RIFT's blackhole mitigation
mechanism. Consider that Leaf 111 has yet to receive the more
specific (disaggregated) route from Spine 111. In such a scenario,
traffic from Leaf 111 toward Prefix 112 may still use Spine 112's
default route, causing it to traverse ToF 21 and ToF 22 back down via
Spine 111. While this behavior is suboptimal, it is transient in
nature and preferred to black-holing traffic.
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5.3. Partitioned Fabric
+--------+ +--------+
Level 2 |ToF 21| |ToF 22|
++-+--+-++ ++-+--+-++
| | | | | | | |
| | | | | | | 0/0
| | | | | | | |
| | | | | | | |
+--------------+ | +--- XXXXXX + | | | +---------------+
| | | | | | | |
| +-----------------------------+ | | |
0/0 | | | | | | |
| 0/0 0/0 +- XXXXXXXXXXXXXXXXXXXXXXXXX -+ |
| 1.1/16 | | | | | |
| | +-+ +-0/0-----------+ | |
| | | 1.1./16 | | | |
+-+----++ +-+-----+ ++-----0/0 ++----0/0
Level 1 | | | | | 1.1/16 | 1.1/16
|Spin111| |Spin112| |Spin121| |Spin122|
+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
| | | | | | | |
| +---------------+ | | +----------------+ |
| | | | | | | |
| +-------------+ | | | +--------------+ | |
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +---+-+-+
Level 3 | | | | | | | |
|Leaf111| |Leaf112| |Leaf121| |Leaf122|
+-+-----+ ++------+ +-----+-+ +-+-----+
+ + + +
Prefix111 Prefix112 Prefix121 Prefix122
1.1/16
Figure 34: Fabric Partition
Figure 34 shows one of more catastrophic scenarios where ToF 21 is
completely severed from access to Prefix 121 due to a double link
failure. If only default routes existed, this would result in 50% of
traffic from Leaf 111 and Leaf 112 toward Prefix 121 being black-
holed.
The mechanism to resolve this scenario hinges on ToF 21's South TIEs
being reflected from Spine 111 and Spine 112 to ToF 22. Once ToF 22
sees that Prefix 121 cannot be reached from ToF 21, it will begin to
disaggregate Prefix 121 by advertising a more specific route (1.1/16)
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along with the default IP prefix route to all spines (ToF 21 still
only sends a default route). The result is Spine 111 and Spine112
using the more specific route to Prefix 121 via ToF 22. All other
prefixes continue to use the default IP prefix route toward both ToF
21 and ToF 22.
The more specific route for Prefix 121 being advertised by ToF 22
does not need to be propagated further south to the leaves, as they
do not benefit from this information. Spine 111 and Spine 112 are
only required to reflect the new South Node TIEs received from ToF 22
to ToF 21. In short, only the relevant nodes received the relevant
updates, thereby restricting the failure to only the partitioned
level rather than burdening the whole fabric with the flooding and
recomputation of the new topology information.
To finish our example, the following table shows sets computed by ToF
22 using notation introduced in Section 4.2.5:
|R = Prefix 111, Prefix 112, Prefix 121, Prefix 122
|H (for r=Prefix 111) = Spine 111, Spine 112
|H (for r=Prefix 112) = Spine 111, Spine 112
|H (for r=Prefix 121) = Spine 121, Spine 122
|H (for r=Prefix 122) = Spine 121, Spine 122
|A (for ToF 21) = Spine 111, Spine 112
With that and |H (for r=Prefix 121) and |H (for r=Prefix 122) being
disjoint from |A (for ToF 21), ToF 22 will originate an South TIE
with Prefix 121 and Prefix 122, which will be flooded to all spines.
5.4. Northbound Partitioned Router and Optional East-West Links
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. + + +
. X N1 | N2 | N3
. X | |
.+--+----+ +--+----+ +--+-----+
.| |0/0> <0/0| |0/0> <0/0| |
.| A01 +----------+ A02 +----------+ A03 | Level 1
.++-+-+--+ ++--+--++ +---+-+-++
. | | | | | | | | |
. | | +----------------------------------+ | | |
. | | | | | | | | |
. | +-------------+ | | | +--------------+ |
. | | | | | | | | |
. | +----------------+ | +-----------------+ |
. | | | | | | | | |
. | | +------------------------------------+ | |
. | | | | | | | | |
.++-+-+--+ | +---+---+ | +-+---+-++
.| | +-+ +-+ | |
.| L01 | | L02 | | L03 | Level 0
.+-------+ +-------+ +--------+
Figure 35: North Partitioned Router
Figure 35 shows a part of a fabric where level 1 is horizontally
connected and A01 lost its only northbound adjacency. Based on N-SPF
rules in Section 4.2.4.1 A01 will compute northbound reachability by
using the link A01 to A02. A02 however, will NOT use this link
during N-SPF. The result is A01 utilizing the horizontal link for
default route advertisement and unidirectional routing.
Furthermore, if A02 also loses its only northbound adjacency (N2),
the situation evolves. A01 will no longer have northbound
reachability while it sees A03's northbound adjacencies in South Node
TIEs reflected by nodes south of it. As a result, A01 will no longer
advertise its default route in accordance with Section 4.2.3.8.
6. Implementation and Operation: Further Details
6.1. Considerations for Leaf-Only Implementation
RIFT can and is intended to be stretched to the lowest level in the
IP fabric to integrate ToRs or even servers. Since those entities
would run as leaves only, it is worth to observe that a leaf only
version is significantly simpler to implement and requires much less
resources:
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1. Leaf nodes only need to maintain a multipath default route under
normal circumstances. However, in cases of catastrophic
partitioning, leaf nodes SHOULD be capable of accommodating all
the leaf routes in its own PoD to prevent black-holing.
2. Leaf nodes hold only their own North TIEs and South TIEs of Level
1 nodes they are connected to.
3. Leaf nodes do not have to support any type of de-aggregation
computation or propagation.
4. Leaf nodes are not required to support overload bit.
5. Leaf nodes do not need to originate S-TIEs unless optional leaf-
2-leaf features are desired.
6.2. Considerations for Spine Implementation
Spine nodes will never act as Top of Fabric, and are therefore not
required to run a full RIFT implementation. Specifically, spines do
not need to perform negative disaggregation computation other than
respecting northbound disaggregation advertised from the north.
6.3. Adaptations to Other Proposed Data Center Topologies
. +-----+ +-----+
. | | | |
.+-+ S0 | | S1 |
.| ++---++ ++---++
.| | | | |
.| | +------------+ |
.| | | +------------+ |
.| | | | |
.| ++-+--+ +--+-++
.| | | | |
.| | A0 | | A1 |
.| +-+--++ ++---++
.| | | | |
.| | +------------+ |
.| | +-----------+ | |
.| | | | |
.| +-+-+-+ +--+-++
.+-+ | | |
. | L0 | | L1 |
. +-----+ +-----+
Figure 36: Level Shortcut
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RIFT is not strictly limited to Clos topologies. The protocol only
requires a sense of "compass rose directionality" either achieved
through configuration or derivation of levels. So, conceptually,
leaf-2-leaf links and even shortcuts between levels could be
included. Figure 36 depicts an example of a shortcut between levels.
In this example, sub-optimal routing will occur when traffic is sent
from L0 to L1 via S0's default route and back down through A0 or A1.
In order to ensure that only default routes from A0 or A1 are used,
all leaves would be required to install each others routes.
While various technical and operational challenges may require the
use of such modifications, discussion of those topics are outside the
scope of this document.
6.4. Originating Non-Default Route Southbound
An implementation MAY choose to originate more specific prefixes (P')
southbound instead of only the default route (as described in
Section 4.2.3.8). In such a scenario, all addresses carried within
the RIFT domain MUST be contained within P'.
7. Security Considerations
7.1. General
One can consider attack vectors where a router may reboot many times
while changing its system ID and pollute the network with many stale
TIEs or TIEs are sent with very long lifetimes and not cleaned up
when the routes vanish. Those attack vectors are not unique to RIFT.
Given large memory footprints available today those attacks should be
relatively benign. Otherwise a node SHOULD implement a strategy of
discarding contents of all TIEs that were not present in the SPF tree
over a certain, configurable period of time. Since the protocol,
like all modern link-state protocols, is self-stabilizing and will
advertise the presence of such TIEs to its neighbors, they can be re-
requested again if a computation finds that it sees an adjacency
formed towards the system ID of the discarded TIEs.
7.2. ZTP
Section 4.2.7 presents many attack vectors in untrusted environments,
starting with nodes that oscillate their level offers to the
possibility of nodes offering a three-way adjacency with the highest
possible level value and a very long holdtime trying to put itself
"on top of the lattice" thereby allowing it to gain access to the
whole southbound topology. Session authentication mechanisms are
necessary in environments where this is possible and RIFT provides
the security envelope to ensure this if so desired.
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7.3. Lifetime
Traditional IGP protocols are vulnerable to lifetime modification and
replay attacks that can be somewhat mitigated by using techniques
like [RFC7987]. RIFT removes this attack vector by protecting the
lifetime behind a signature computed over it and additional nonce
combination which makes even the replay attack window very small and
for practical purposes irrelevant since lifetime cannot be
artificially shortened by the attacker.
7.4. Packet Number
Optional packet number is carried in the security envelope without
any encryption protection and is hence vulnerable to replay and
modification attacks. Contrary to nonces this number must change on
every packet and would present a very high cryptographic load if
signed. The attack vector packet number present is relatively
benign. Changing the packet number by a man-in-the-middle attack
will only affect operational validation tools and possibly some
performance optimizations on flooding. It is expected that an
implementation detecting too many "fake losses" or "misorderings" due
to the attack on the packet number would simply suppress its further
processing.
7.5. Outer Fingerprint Attacks
A node can try to inject LIE packets observing a conversation on the
wire by using the outer key ID albeit it cannot generate valid hashes
in case it changes the integrity of the message so the only possible
attack is DoS due to excessive LIE validation.
A node can try to replay previous LIEs with changed state that it
recorded but the attack is hard to replicate since the nonce
combination must match the ongoing exchange and is then limited to a
single flap only since both nodes will advance their nonces in case
the adjacency state changed. Even in the most unlikely case the
attack length is limited due to both sides periodically increasing
their nonces.
7.6. TIE Origin Fingerprint DoS Attacks
A compromised node can attempt to generate "fake TIEs" using other
nodes' TIE origin key identifiers. Albeit the ultimate validation of
the origin fingerprint will fail in such scenarios and not progress
further than immediately peering nodes, the resulting denial of
service attack seems unavoidable since the TIE origin key id is only
protected by the, here assumed to be compromised, node.
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7.7. Host Implementations
It can be reasonably expected that with the proliferation of RotH
servers, rather than dedicated networking devices, will represent a
significant amount of RIFT devices. Given their normally far wider
software envelope and access granted to them, such servers are also
far more likely to be compromised and present an attack vector on the
protocol. Hijacking of prefixes to attract traffic is a trust
problem and cannot be easily addressed within the protocol if the
trust model is breached, i.e. the server presents valid credentials
to form an adjacency and issue TIEs. In an even more devious way,
the servers can present DoS (or even DDos) vectors of issuing too
many LIE packets, flood large amounts of North TIEs and attempt
similar resource overrun attacks. A prudent implementation forming
adjacencies to leaves should implement according thresholds
mechanisms and raise warnings when e.g. a leaf is advertising an
excess number of TIEs or prefixes. Additionally, such implementation
could refuse any topology information except the node's own TIEs and
authenticated, reflected South Node TIEs at own level.
To isolate possible attack vectors on the leaf to the largest
possible extent a dedicated leaf-only implementation could run
without any configuration by hard-coding a well-known adjacency key
(which can be always rolled-over by the means of e.g. well-known key-
value distributed from top of the fabric), leaf level value and
always setting overload bit. All other values can be derived by
automatic means as described earlier in the protocol specification.
8. IANA Considerations
This specification requests multicast address assignments and
standard port numbers. Additionally registries for the schema are
requested and suggested values provided that reflect the numbers
allocated in the given schema.
8.1. Requested Multicast and Port Numbers
This document requests allocation in the 'IPv4 Multicast Address
Space' registry the suggested value of 224.0.0.120 as
'ALL_V4_RIFT_ROUTERS' and in the 'IPv6 Multicast Address Space'
registry the suggested value of FF02::A1F7 as 'ALL_V6_RIFT_ROUTERS'.
This document requests allocation in the 'Service Name and Transport
Protocol Port Number Registry' the allocation of a suggested value of
914 on udp for 'RIFT_LIES_PORT' and suggested value of 915 for
'RIFT_TIES_PORT'.
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8.2. Requested Registries with Suggested Values
This section requests registries that help govern the schema via
usual IANA registry procedures. A top level 'RIFT' registry should
hold the according registries requested in following sections with
their pre-defined values. IANA is requested to store the schema
version introducing the allocated value as well as, optionally, its
description when present. This will allow to assign different values
to an entry depending on schema version. Alternately, IANA is
requested to consider a root RIFT/3 registry to store RIFT schema
major version 3 values and may be requested in the future to create a
RIFT/4 registry under that. In any case, IANA is requested to store
the schema version in the entries since that will allow to
distinguish between minor versions in the same major schema version.
All values not suggested as to be considered `Unassigned`. The range
of every registry is a 16-bit integer. Allocation of new values is
always performed via `Expert Review` action.
8.2.1. Registry RIFT_v4/common/AddressFamilyType
Address family type.
8.2.1.1. Requested Entries
Name Value Schema Version Description
Illegal 0 4.1
AddressFamilyMinValue 1 4.1
IPv4 2 4.1
IPv6 3 4.1
AddressFamilyMaxValue 4 4.1
8.2.2. Registry RIFT_v4/common/HierarchyIndications
Flags indicating node configuration in case of ZTP.
8.2.2.1. Requested Entries
Name Value Schema Version Description
leaf_only 0 4.1
leaf_only_and_leaf_2_leaf_procedures 1 4.1
top_of_fabric 2 4.1
8.2.3. Registry RIFT_v4/common/IEEE802_1ASTimeStampType
Timestamp per IEEE 802.1AS, all values MUST be interpreted in
implementation as unsigned.
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8.2.3.1. Requested Entries
Name Value Schema Version Description
AS_sec 1 4.1
AS_nsec 2 4.1
8.2.4. Registry RIFT_v4/common/IPAddressType
IP address type.
8.2.4.1. Requested Entries
Name Value Schema Version Description
ipv4address 1 4.1 Content is IPv4
ipv6address 2 4.1 Content is IPv6
8.2.5. Registry RIFT_v4/common/IPPrefixType
Prefix advertisement.
@note: for interface addresses the protocol can propagate the address
part beyond the subnet mask and on reachability computation that has
to be normalized. The non-significant bits can be used for
operational purposes.
8.2.5.1. Requested Entries
Name Value Schema Version Description
ipv4prefix 1 4.1
ipv6prefix 2 4.1
8.2.6. Registry RIFT_v4/common/IPv4PrefixType
IPv4 prefix type.
8.2.6.1. Requested Entries
Name Value Schema Version Description
address 1 4.1
prefixlen 2 4.1
8.2.7. Registry RIFT_v4/common/IPv6PrefixType
IPv6 prefix type.
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8.2.7.1. Requested Entries
Name Value Schema Version Description
address 1 4.1
prefixlen 2 4.1
8.2.8. Registry RIFT_v4/common/PrefixSequenceType
Sequence of a prefix in case of move.
8.2.8.1. Requested Entries
Name Value Schema Description
Version
timestamp 1 4.1
transactionid 2 4.1 Transaction ID set by client in e.g.
in 6LoWPAN.
8.2.9. Registry RIFT_v4/common/RouteType
RIFT route types.
@note: route types which MUST be ordered on their preference PGP
prefixes are most preferred attracting traffic north (towards spine)
and then south normal prefixes are attracting traffic south (towards
leafs), i.e. prefix in NORTH PREFIX TIE is preferred over SOUTH
PREFIX TIE.
@note: The only purpose of those values is to introduce an ordering
whereas an implementation can choose internally any other values as
long the ordering is preserved
8.2.9.1. Requested Entries
Name Value Schema Version Description
Illegal 0 4.1
RouteTypeMinValue 1 4.1
Discard 2 4.1
LocalPrefix 3 4.1
SouthPGPPrefix 4 4.1
NorthPGPPrefix 5 4.1
NorthPrefix 6 4.1
NorthExternalPrefix 7 4.1
SouthPrefix 8 4.1
SouthExternalPrefix 9 4.1
NegativeSouthPrefix 10 4.1
RouteTypeMaxValue 11 4.1
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8.2.10. Registry RIFT_v4/common/TIETypeType
Type of TIE.
This enum indicates what TIE type the TIE is carrying. In case the
value is not known to the receiver, the TIE MUST be re-flooded. This
allows for future extensions of the protocol within the same major
schema with types opaque to some nodes UNLESS the flooding scope is
not the same as prefix TIE, then a major version revision MUST be
performed.
8.2.10.1. Requested Entries
Name Value Schema Description
Version
Illegal 0 4.1
TIETypeMinValue 1 4.1
NodeTIEType 2 4.1
PrefixTIEType 3 4.1
PositiveDisaggregationPrefixTIEType 4 4.1
NegativeDisaggregationPrefixTIEType 5 4.1
PGPrefixTIEType 6 4.1
KeyValueTIEType 7 4.1
ExternalPrefixTIEType 8 4.1
PositiveExternalDisaggregationPrefixTIEType 9 4.1
TIETypeMaxValue 10 4.1
8.2.11. Registry RIFT_v4/common/TieDirectionType
Direction of TIEs.
8.2.11.1. Requested Entries
Name Value Schema Version Description
Illegal 0 4.1
South 1 4.1
North 2 4.1
DirectionMaxValue 3 4.1
8.2.12. Registry RIFT_v4/encoding/Community
Prefix community.
8.2.12.1. Requested Entries
Name Value Schema Version Description
top 1 4.1 Higher order bits
bottom 2 4.1 Lower order bits
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8.2.13. Registry RIFT_v4/encoding/KeyValueTIEElement
Generic key value pairs.
8.2.13.1. Requested Entries
Name Value Schema Version Description
keyvalues 1 4.1
8.2.14. Registry RIFT_v4/encoding/LIEPacket
RIFT LIE Packet.
@note: this node's level is already included on the packet header
8.2.14.1. Requested Entries
Name Value Schema Description
Version
name 1 4.1 Node or adjacency name.
local_id 2 4.1 Local link ID.
flood_port 3 4.1 UDP port to which we can
receive flooded TIEs.
link_mtu_size 4 4.1 Layer 3 MTU, used to
discover to mismatch.
link_bandwidth 5 4.1 Local link bandwidth on
the interface.
neighbor 6 4.1 Reflects the neighbor once
received to provide
3-way connectivity.
pod 7 4.1 Node's PoD.
node_capabilities 10 4.1 Node capabilities shown in
LIE. The capabilities
MUST match the capabilities
shown in the Node TIEs,
otherwise the behavior
is unspecified. A node
detecting the mismatch
SHOULD generate according
error.
link_capabilities 11 4.1 Capabilities of this link.
holdtime 12 4.1 Required holdtime of the
adjacency, i.e. how much
time MUST expire
without LIE for the
adjacency to drop.
label 13 4.1 Unsolicited, downstream
assigned locally
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significant label value
for the adjacency.
not_a_ztp_offer 21 4.1 Indicates that the level
on the LIE MUST NOT be used
to derive a ZTP level by
the receiving node.
you_are_flood_repeater 22 4.1 Indicates to northbound
neighbor that it should
be reflooding this node's
N-TIEs to achieve flood
reduction and balancing
for northbound flooding. To
be ignored if received
from a northbound
adjacency.
you_are_sending_too_quickly 23 4.1 Can be optionally set to
indicate to neighbor that
packet losses are seen
on reception based on
packet numbers or the rate
is too high. The
receiver SHOULD temporarily
slow down flooding
rates.
instance_name 24 4.1 Instance name in case
multiple RIFT instances
running on same
interface.
8.2.15. Registry RIFT_v4/encoding/LinkCapabilities
Link capabilities.
8.2.15.1. Requested Entries
Name Value Schema Description
Version
bfd 1 4.1 Indicates that the link is
supporting BFD.
v4_forwarding_capable 2 4.1 Indicates whether the interface
will support v4 forwarding.
8.2.16. Registry RIFT_v4/encoding/LinkIDPair
LinkID pair describes one of parallel links between two nodes.
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8.2.16.1. Requested Entries
Name Value Schema Description
Version
local_id 1 4.1 Node-wide unique value for
the local link.
remote_id 2 4.1 Received remote link ID for
this link.
platform_interface_index 10 4.1 Describes the local
interface index of the link.
platform_interface_name 11 4.1 Describes the local
interface name.
trusted_outer_security_key 12 4.1 Indication whether the link
is secured, i.e. protected
by outer key, absence of
this element means no
indication, undefined
outer key means not secured.
bfd_up 13 4.1 Indication whether the link
is protected by established
BFD session.
address_families 14 4.1 Optional indication which
address families are up on
the interface
8.2.17. Registry RIFT_v4/encoding/Neighbor
Neighbor structure.
8.2.17.1. Requested Entries
Name Value Schema Version Description
originator 1 4.1 System ID of the originator.
remote_id 2 4.1 ID of remote side of the link.
8.2.18. Registry RIFT_v4/encoding/NodeCapabilities
Capabilities the node supports.
@note: The schema may add to this field future capabilities to
indicate whether it will support interpretation of future schema
extensions on the same major revision. Such fields MUST be optional
and have an implicit or explicit false default value. If a future
capability changes route selection or generates blackholes if some
nodes are not supporting it then a major version increment is
unavoidable.
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8.2.18.1. Requested Entries
Name Value Schema Description
Version
protocol_minor_version 1 4.1 Must advertise supported minor
version dialect that way.
flood_reduction 2 4.1 Can this node participate in
flood reduction.
hierarchy_indications 3 4.1 Does this node restrict itself
to be top-of-fabric or leaf
only (in ZTP) and does it
support leaf-2-leaf
procedures.
8.2.19. Registry RIFT_v4/encoding/NodeFlags
Indication flags of the node.
8.2.19.1. Requested Entries
Name Value Schema Description
Version
overload 1 4.1 Indicates that node is in overload, do not
transit traffic through it.
8.2.20. Registry RIFT_v4/encoding/NodeNeighborsTIEElement
neighbor of a node
8.2.20.1. Requested Entries
Name Value Schema Description
Version
level 1 4.1 level of neighbor
cost 3 4.1 Cost to neighbor.
link_ids 4 4.1 can carry description of multiple parallel
links in a TIE
bandwidth 5 4.1 total bandwith to neighbor, this will be
normally sum of the bandwidths of all the
parallel links.
8.2.21. Registry RIFT_v4/encoding/NodeTIEElement
Description of a node.
It may occur multiple times in different TIEs but if either
capabilities values do not match or
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flags values do not match or
neighbors repeat with different values
the behavior is undefined and a warning SHOULD be generated.
Neighbors can be distributed across multiple TIEs however if the sets
are disjoint. Miscablings SHOULD be repeated in every node TIE,
otherwise the behavior is undefined.
@note: Observe that absence of fields implies defined defaults.
8.2.21.1. Requested Entries
Name Value Schema Description
Version
level 1 4.1 Level of the node.
neighbors 2 4.1 Node's neighbors. If neighbor systemID
repeats in other node TIEs of same
node the behavior is undefined.
capabilities 3 4.1 Capabilities of the node.
flags 4 4.1 Flags of the node.
name 5 4.1 Optional node name for easier
operations.
pod 6 4.1 PoD to which the node belongs.
startup_time 7 4.1 optional startup time of the node
miscabled_links 10 4.1 If any local links are miscabled, the
indication is flooded.
8.2.22. Registry RIFT_v4/encoding/PacketContent
Content of a RIFT packet.
8.2.22.1. Requested Entries
Name Value Schema Version Description
lie 1 4.1
tide 2 4.1
tire 3 4.1
tie 4 4.1
8.2.23. Registry RIFT_v4/encoding/PacketHeader
Common RIFT packet header.
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8.2.23.1. Requested Entries
Name Value Schema Description
Version
major_version 1 4.1 Major version of protocol.
minor_version 2 4.1 Minor version of protocol.
sender 3 4.1 Node sending the packet, in case of
LIE/TIRE/TIDE also the originator of
it.
level 4 4.1 Level of the node sending the packet,
required on everything except LIEs.
Lack of presence on LIEs indicates
UNDEFINED_LEVEL and is used in ZTP
procedures.
8.2.24. Registry RIFT_v4/encoding/PrefixAttributes
Attributes of a prefix.
8.2.24.1. Requested Entries
Name Value Schema Description
Version
metric 2 4.1 Distance of the prefix.
tags 3 4.1 Generic unordered set of route tags,
can be redistributed to other
protocols or use within the context
of real time analytics.
monotonic_clock 4 4.1 Monotonic clock for mobile
addresses.
loopback 6 4.1 Indicates if the interface is a node
loopback.
directly_attached 7 4.1 Indicates that the prefix is
directly attached, i.e. should be
routed to even if the node is in
overload.
from_link 10 4.1 In case of locally originated
prefixes, i.e. interface
addresses this can describe which
link the address belongs to.
8.2.25. Registry RIFT_v4/encoding/PrefixTIEElement
TIE carrying prefixes
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8.2.25.1. Requested Entries
Name Value Schema Description
Version
prefixes 1 4.1 Prefixes with the associated attributes.
If the same prefix repeats in multiple TIEs of
same node behavior is unspecified.
8.2.26. Registry RIFT_v4/encoding/ProtocolPacket
RIFT packet structure.
8.2.26.1. Requested Entries
Name Value Schema Version Description
header 1 4.1
content 2 4.1
8.2.27. Registry RIFT_v4/encoding/TIDEPacket
TIDE with sorted TIE headers, if headers are unsorted, behavior is
undefined.
8.2.27.1. Requested Entries
Name Value Schema Version Description
start_range 1 4.1 First TIE header in the tide
packet.
end_range 2 4.1 Last TIE header in the tide packet.
headers 3 4.1 _Sorted_ list of headers.
8.2.28. Registry RIFT_v4/encoding/TIEElement
Single element in a TIE.
Schema enum `common.TIETypeType` in TIEID indicates which elements
MUST be present in the TIEElement. In case of mismatch the
unexpected elements MUST be ignored. In case of lack of expected
element the TIE an error MUST be reported and the TIE MUST be
ignored.
This type can be extended with new optional elements for new
`common.TIETypeType` values without breaking the major but if it is
necessary to understand whether all nodes support the new type a node
capability must be added as well.
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8.2.28.1. Requested Entries
Name Valu Schem Description
e a Ver
sion
node 1 4.1 Used in case of enum comm
on.TIETypeType.NodeTIEType
.
prefixes 2 4.1 Used in case of enum comm
on.TIETypeType.PrefixTIETy
pe.
positive_disaggregation_prefixe 3 4.1 Positive prefixes (always
s southbound). It MUST
NOT be advertised within a
North TIE and ignored
otherwise.
negative_disaggregation_prefixe 5 4.1 Transitive, negative
s prefixes (always
southbound) which MUST
be aggregated and
propagated according
to the specification
southwards towards lower
levels to heal
pathological upper level
partitioning, otherwise
blackholes may occur in
multiplane fabrics. It
MUST NOT be advertised
within a North TIE.
external_prefixes 6 4.1 Externally reimported
prefixes.
positive_external_disaggregatio 7 4.1 Positive external
n_prefixes disaggregated prefixes
(always southbound).
It MUST NOT be advertised
within a North TIE and
ignored otherwise.
keyvalues 9 4.1 Key-Value store elements.
8.2.29. Registry RIFT_v4/encoding/TIEHeader
Header of a TIE.
@note: TIEID space is a total order achieved by comparing the
elements in sequence defined and comparing each value as an unsigned
integer of according length.
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@note: After sequence number the lifetime received on the envelope
must be used for comparison before further fields.
@note: `origination_time` and `origination_lifetime` are normally
disregarded for comparison purposes and carried purely for debugging/
security purposes if present. They may be used for comparison of
last resort to differentiate otherwise equal ties
8.2.29.1. Requested Entries
Name Value Schema Description
Version
tieid 2 4.1 ID of the tie.
seq_nr 3 4.1 Sequence number of the tie.
origination_time 10 4.1 Absolute timestamp when the TIE
was generated. This can be used on
fabrics with synchronized
clock to prevent lifetime
modification attacks.
origination_lifetime 12 4.1 Original lifetime when the TIE
was generated. This can be used on
fabrics with synchronized
clock to prevent lifetime
modification attacks.
8.2.30. Registry RIFT_v4/encoding/TIEHeaderWithLifeTime
Header of a TIE as described in TIRE/TIDE.
8.2.30.1. Requested Entries
Name Value Schema Description
Version
header 1 4.1
remaining_lifetime 2 4.1 Remaining lifetime that expires
down to 0 just like in ISIS.
TIEs with lifetimes differing by
less than `lifetime_diff2ignore`
MUST be considered EQUAL.
8.2.31. Registry RIFT_v4/encoding/TIEID
ID of a TIE.
@note: TIEID space is a total order achieved by comparing the
elements in sequence defined and comparing each value as an unsigned
integer of according length.
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8.2.31.1. Requested Entries
Name Value Schema Version Description
direction 1 4.1 direction of TIE
originator 2 4.1 indicates originator of the TIE
tietype 3 4.1 type of the tie
tie_nr 4 4.1 number of the tie
8.2.32. Registry RIFT_v4/encoding/TIEPacket
TIE packet
8.2.32.1. Requested Entries
Name Value Schema Version Description
header 1 4.1
element 2 4.1
8.2.33. Registry RIFT_v4/encoding/TIREPacket
TIRE packet
8.2.33.1. Requested Entries
Name Value Schema Version Description
headers 1 4.1
9. Acknowledgments
A new routing protocol in its complexity is not a product of a parent
but of a village as the author list shows already. However, many
more people provided input, fine-combed the specification based on
their experience in design, implementation or application of
protocols in IP fabrics. This section will make an inadequate
attempt in recording their contribution.
Many thanks to Naiming Shen for some of the early discussions around
the topic of using IGPs for routing in topologies related to Clos.
Russ White to be especially acknowledged for the key conversation on
epistemology that allowed to tie current asynchronous distributed
systems theory results to a modern protocol design presented in this
scope. Adrian Farrel, Joel Halpern, Jeffrey Zhang, Krzysztof
Szarkowicz, Nagendra Kumar, Melchior Aelmans, Kaushal Tank, Will
Jones, Moin Ahmed, Sandy Zhang and Jordan Head (in no particular
order) provided thoughtful comments that improved the readability of
the document and found good amount of corners where the light failed
to shine. Kris Price was first to mention single router, single arm
default considerations. Jeff Tantsura helped out with some initial
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thoughts on BFD interactions while Jeff Haas corrected several
misconceptions about BFD's finer points and helped to improve the
security section around leaf considerations. Artur Makutunowicz
pointed out many possible improvements and acted as sounding board in
regard to modern protocol implementation techniques RIFT is
exploring. Barak Gafni formalized first time clearly the problem of
partitioned spine and fallen leaves on a (clean) napkin in Singapore
that led to the very important part of the specification centered
around multiple Top-of-Fabric planes and negative disaggregation.
Igor Gashinsky and others shared many thoughts on problems
encountered in design and operation of large-scale data center
fabrics. Xu Benchong found a delicate error in the flooding
procedures and a schema datatype size mismatch.
Last but not least, Alvaro Retana guided the undertaking by asking
many necessary procedural and technical questions which did not only
improve the content but did also lay out the track towards
publication.
10. References
10.1. Normative References
[EUI64] IEEE, "Guidelines for Use of Extended Unique Identifier
(EUI), Organizationally Unique Identifier (OUI), and
Company ID (CID)", IEEE EUI,
<http://standards.ieee.org/develop/regauth/tut/eui.pdf>.
[ISO10589]
ISO "International Organization for Standardization",
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473), ISO/IEC
10589:2002, Second Edition.", Nov 2002.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
DOI 10.17487/RFC1982, August 1996,
<https://www.rfc-editor.org/info/rfc1982>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, DOI 10.17487/RFC2365, July 1998,
<https://www.rfc-editor.org/info/rfc2365>.
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[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<https://www.rfc-editor.org/info/rfc5082>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way
Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
DOI 10.17487/RFC5303, October 2008,
<https://www.rfc-editor.org/info/rfc5303>.
[RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
Layer Reachability Information with an IPv6 Next Hop",
RFC 5549, DOI 10.17487/RFC5549, May 2009,
<https://www.rfc-editor.org/info/rfc5549>.
[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, DOI 10.17487/RFC5709, October
2009, <https://www.rfc-editor.org/info/rfc5709>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://www.rfc-editor.org/info/rfc5881>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
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[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
DOI 10.17487/RFC6830, January 2013,
<https://www.rfc-editor.org/info/rfc6830>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC7987] Ginsberg, L., Wells, P., Decraene, B., Przygienda, T., and
H. Gredler, "IS-IS Minimum Remaining Lifetime", RFC 7987,
DOI 10.17487/RFC7987, October 2016,
<https://www.rfc-editor.org/info/rfc7987>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8202] Ginsberg, L., Previdi, S., and W. Henderickx, "IS-IS
Multi-Instance", RFC 8202, DOI 10.17487/RFC8202, June
2017, <https://www.rfc-editor.org/info/rfc8202>.
[RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Neighbor
Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
<https://www.rfc-editor.org/info/rfc8505>.
[thrift] Apache Software Foundation, "Thrift Interface Description
Language", <https://thrift.apache.org/docs/idl>.
10.2. Informative References
[CLOS] Yuan, X., "On Nonblocking Folded-Clos Networks in Computer
Communication Environments", IEEE International Parallel &
Distributed Processing Symposium, 2011.
[DIJKSTRA]
Dijkstra, E., "A Note on Two Problems in Connexion with
Graphs", Journal Numer. Math. , 1959.
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[DOT] Ellson, J. and L. Koutsofios, "Graphviz: open source graph
drawing tools", Springer-Verlag , 2001.
[DYNAMO] De Candia et al., G., "Dynamo: amazon's highly available
key-value store", ACM SIGOPS symposium on Operating
systems principles (SOSP '07), 2007.
[EPPSTEIN]
Eppstein, D., "Finding the k-Shortest Paths", 1997.
[FATTREE] Leiserson, C., "Fat-Trees: Universal Networks for
Hardware-Efficient Supercomputing", 1985.
[IEEEstd1588]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
IEEE Standard 1588,
<https://ieeexplore.ieee.org/document/4579760/>.
[IEEEstd8021AS]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Timing and Synchronization for Time-Sensitive
Applications in Bridged Local Area Networks",
IEEE Standard 802.1AS,
<https://ieeexplore.ieee.org/document/5741898/>.
[ISO10589-Second-Edition]
International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", Nov 2002.
[RFC0826] Plummer, D., "An Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37,
RFC 826, DOI 10.17487/RFC0826, November 1982,
<https://www.rfc-editor.org/info/rfc826>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC3626] Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
State Routing Protocol (OLSR)", RFC 3626,
DOI 10.17487/RFC3626, October 2003,
<https://www.rfc-editor.org/info/rfc3626>.
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[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines", RFC 6518,
DOI 10.17487/RFC6518, February 2012,
<https://www.rfc-editor.org/info/rfc6518>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[VAHDAT08]
Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
Commodity Data Center Network Architecture", SIGCOMM ,
2008.
[Wikipedia]
Wikipedia,
"https://en.wikipedia.org/wiki/Serial_number_arithmetic",
2016.
Appendix A. Sequence Number Binary Arithmetic
The only reasonably reference to a cleaner than [RFC1982] sequence
number solution is given in [Wikipedia]. It basically converts the
problem into two complement's arithmetic. Assuming a straight two
complement's subtractions on the bit-width of the sequence number the
according >: and =: relations are defined as:
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U_1, U_2 are 12-bits aligned unsigned version number
D_f is ( U_1 - U_2 ) interpreted as two complement signed 12-bits
D_b is ( U_2 - U_1 ) interpreted as two complement signed 12-bits
U_1 >: U_2 IIF D_f > 0 AND D_b < 0
U_1 =: U_2 IIF D_f = 0
The >: relationship is anti-symmetric but not transitive. Observe
that this leaves >: of the numbers having maximum two complement
distance, e.g. ( 0 and 0x800 ) undefined in our 12-bits case since
D_f and D_b are both -0x7ff.
A simple example of the relationship in case of 3-bit arithmetic
follows as table indicating D_f/D_b values and then the relationship
of U_1 to U_2:
U2 / U1 0 1 2 3 4 5 6 7
0 +/+ +/- +/- +/- -/- -/+ -/+ -/+
1 -/+ +/+ +/- +/- +/- -/- -/+ -/+
2 -/+ -/+ +/+ +/- +/- +/- -/- -/+
3 -/+ -/+ -/+ +/+ +/- +/- +/- -/-
4 -/- -/+ -/+ -/+ +/+ +/- +/- +/-
5 +/- -/- -/+ -/+ -/+ +/+ +/- +/-
6 +/- +/- -/- -/+ -/+ -/+ +/+ +/-
7 +/- +/- +/- -/- -/+ -/+ -/+ +/+
U2 / U1 0 1 2 3 4 5 6 7
0 = > > > ? < < <
1 < = > > > ? < <
2 < < = > > > ? <
3 < < < = > > > ?
4 ? < < < = > > >
5 > ? < < < = > >
6 > > ? < < < = >
7 > > > ? < < < =
Appendix B. Information Elements Schema
This section introduces the schema for information elements. The IDL
is Thrift [thrift].
On schema changes that
1. change field numbers or
2. add new *required* fields or
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3. remove any fields or
4. change lists into sets, unions into structures or
5. change multiplicity of fields or
6. changes name of any field or type or
7. change data types of any field or
8. adds, changes or removes a default value of any *existing* field
or
9. removes or changes any defined constant or constant value or
10. changes any enumeration type except extending
`common.TIETypeType` (use of enumeration types is generally
discouraged)
major version of the schema MUST increase. All other changes MUST
increase minor version within the same major.
The above set of rules guarantees that every decoder can process
serialized content generated by a higher minor version of the schema
and with that the protocol can progress without a 'fork-lift'.
Additionally, based on the propagated minor version in encoded
content and added optional node capabilities new TIE types or even
de-facto mandatory fields can be introduced without progressing the
major version albeit only nodes supporting such new extensions would
decode them. Given the model is encoded at the source and never re-
encoded flooding through nodes not understanding any new extensions
will preserve the according fields.
Content serialized using a major version X is NOT expected to be
decodable by any implementation using decoder for a model with a
major version lower than X.
Observe especially that introducing an optional field does not cause
a major version increase even if the fields inside the structure are
optional with defaults.
All signed integer as forced by Thrift [thrift] support must be cast
for internal purposes to equivalent unsigned values without
discarding the signedness bit. An implementation SHOULD try to avoid
using the signedness bit when generating values.
The schema is normative.
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B.1. common.thrift
/**
Thrift file with common definitions for RIFT
*/
/** @note MUST be interpreted in implementation as unsigned 64 bits.
* The implementation SHOULD NOT use the MSB.
*/
typedef i64 SystemIDType
typedef i32 IPv4Address
/** this has to be long enough to accomodate prefix */
typedef binary IPv6Address
/** @note MUST be interpreted in implementation as unsigned */
typedef i16 UDPPortType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32 TIENrType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32 MTUSizeType
/** @note MUST be interpreted in implementation as unsigned
rolling over number */
typedef i64 SeqNrType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32 LifeTimeInSecType
/** @note MUST be interpreted in implementation as unsigned */
typedef i8 LevelType
/** optional, recommended monotonically increasing number
_per packet type per adjacency_
that can be used to detect losses/misordering/restarts.
@note MUST be interpreted in implementation as unsigned
rolling over number */
typedef i16 PacketNumberType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32 PodType
/** @note MUST be interpreted in implementation as unsigned.
This is carried in the
security envelope and MUST fit into 8 bits. */
typedef i8 VersionType
/** @note MUST be interpreted in implementation as unsigned */
typedef i16 MinorVersionType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32 MetricType
/** @note MUST be interpreted in implementation as unsigned
and unstructured */
typedef i64 RouteTagType
/** @note MUST be interpreted in implementation as unstructured
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label value */
typedef i32 LabelType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32 BandwithInMegaBitsType
/** @note Key Value key ID type */
typedef string KeyIDType
/** node local, unique identification for a link (interface/tunnel
* etc. Basically anything RIFT runs on). This is kept
* at 32 bits so it aligns with BFD [RFC5880] discriminator size.
*/
typedef i32 LinkIDType
typedef string KeyNameType
typedef i8 PrefixLenType
/** timestamp in seconds since the epoch */
typedef i64 TimestampInSecsType
/** security nonce.
@note MUST be interpreted in implementation as rolling
over unsigned value */
typedef i16 NonceType
/** LIE FSM holdtime type */
typedef i16 TimeIntervalInSecType
/** Transaction ID type for prefix mobility as specified by RFC6550,
value MUST be interpreted in implementation as unsigned */
typedef i8 PrefixTransactionIDType
/** Timestamp per IEEE 802.1AS, all values MUST be interpreted in
implementation as unsigned. */
struct IEEE802_1ASTimeStampType {
1: required i64 AS_sec;
2: optional i32 AS_nsec;
}
/** generic counter type */
typedef i64 CounterType
/** Platform Interface Index type, i.e. index of interface on hardware,
can be used e.g. with RFC5837 */
typedef i32 PlatformInterfaceIndex
/** Flags indicating node configuration in case of ZTP.
*/
enum HierarchyIndications {
/** forces level to `leaf_level` and enables according procedures */
leaf_only = 0,
/** forces level to `leaf_level` and enables according procedures */
leaf_only_and_leaf_2_leaf_procedures = 1,
/** forces level to `top_of_fabric` and enables according
procedures */
top_of_fabric = 2,
}
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const PacketNumberType undefined_packet_number = 0
/** This MUST be used when node is configured as top of fabric in ZTP.
This is kept reasonably low to alow for fast ZTP convergence on
failures. */
const LevelType top_of_fabric_level = 24
/** default bandwidth on a link */
const BandwithInMegaBitsType default_bandwidth = 100
/** fixed leaf level when ZTP is not used */
const LevelType leaf_level = 0
const LevelType default_level = leaf_level
const PodType default_pod = 0
const LinkIDType undefined_linkid = 0
/** default distance used */
const MetricType default_distance = 1
/** any distance larger than this will be considered infinity */
const MetricType infinite_distance = 0x7FFFFFFF
/** represents invalid distance */
const MetricType invalid_distance = 0
const bool overload_default = false
const bool flood_reduction_default = true
/** default LIE FSM holddown time */
const TimeIntervalInSecType default_lie_holdtime = 3
/** default ZTP FSM holddown time */
const TimeIntervalInSecType default_ztp_holdtime = 1
/** by default LIE levels are ZTP offers */
const bool default_not_a_ztp_offer = false
/** by default everyone is repeating flooding */
const bool default_you_are_flood_repeater = true
/** 0 is illegal for SystemID */
const SystemIDType IllegalSystemID = 0
/** empty set of nodes */
const set<SystemIDType> empty_set_of_nodeids = {}
/** default lifetime of TIE is one week */
const LifeTimeInSecType default_lifetime = 604800
/** default lifetime when TIEs are purged is 5 minutes */
const LifeTimeInSecType purge_lifetime = 300
/** round down interval when TIEs are sent with security hashes
to prevent excessive computation. **/
const LifeTimeInSecType rounddown_lifetime_interval = 60
/** any `TieHeader` that has a smaller lifetime difference
than this constant is equal (if other fields equal). This
constant MUST be larger than `purge_lifetime` to avoid
retransmissions */
const LifeTimeInSecType lifetime_diff2ignore = 400
/** default UDP port to run LIEs on */
const UDPPortType default_lie_udp_port = 914
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/** default UDP port to receive TIEs on, that can be peer specific */
const UDPPortType default_tie_udp_flood_port = 915
/** default MTU link size to use */
const MTUSizeType default_mtu_size = 1400
/** default link being BFD capable */
const bool bfd_default = true
/** undefined nonce, equivalent to missing nonce */
const NonceType undefined_nonce = 0;
/** outer security key id, MUST be interpreted as in implementation
as unsigned */
typedef i8 OuterSecurityKeyID
/** security key id, MUST be interpreted as in implementation
as unsigned */
typedef i32 TIESecurityKeyID
/** undefined key */
const TIESecurityKeyID undefined_securitykey_id = 0;
/** Maximum delta (negative or positive) that a mirrored nonce can
deviate from local value to be considered valid. If nonces are
changed every minute on both sides this opens statistically
a `maximum_valid_nonce_delta` minutes window of identical LIEs,
TIE, TI(x)E replays.
The interval cannot be too small since LIE FSM may change
states fairly quickly during ZTP without sending LIEs*/
const i16 maximum_valid_nonce_delta = 5;
/** Direction of TIEs. */
enum TieDirectionType {
Illegal = 0,
South = 1,
North = 2,
DirectionMaxValue = 3,
}
/** Address family type. */
enum AddressFamilyType {
Illegal = 0,
AddressFamilyMinValue = 1,
IPv4 = 2,
IPv6 = 3,
AddressFamilyMaxValue = 4,
}
/** IPv4 prefix type. */
struct IPv4PrefixType {
1: required IPv4Address address;
2: required PrefixLenType prefixlen;
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}
/** IPv6 prefix type. */
struct IPv6PrefixType {
1: required IPv6Address address;
2: required PrefixLenType prefixlen;
}
/** IP address type. */
union IPAddressType {
/** Content is IPv4 */
1: optional IPv4Address ipv4address;
/** Content is IPv6 */
2: optional IPv6Address ipv6address;
}
/** Prefix advertisement.
@note: for interface
addresses the protocol can propagate the address part beyond
the subnet mask and on reachability computation that has to
be normalized. The non-significant bits can be used
for operational purposes.
*/
union IPPrefixType {
1: optional IPv4PrefixType ipv4prefix;
2: optional IPv6PrefixType ipv6prefix;
}
/** Sequence of a prefix in case of move.
*/
struct PrefixSequenceType {
1: required IEEE802_1ASTimeStampType timestamp;
/** Transaction ID set by client in e.g. in 6LoWPAN. */
2: optional PrefixTransactionIDType transactionid;
}
/** Type of TIE.
This enum indicates what TIE type the TIE is carrying.
In case the value is not known to the receiver,
the TIE MUST be re-flooded. This allows for
future extensions of the protocol within the same major schema
with types opaque to some nodes UNLESS the flooding scope is not
the same as prefix TIE, then a major version revision MUST
be performed.
*/
enum TIETypeType {
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Illegal = 0,
TIETypeMinValue = 1,
/** first legal value */
NodeTIEType = 2,
PrefixTIEType = 3,
PositiveDisaggregationPrefixTIEType = 4,
NegativeDisaggregationPrefixTIEType = 5,
PGPrefixTIEType = 6,
KeyValueTIEType = 7,
ExternalPrefixTIEType = 8,
PositiveExternalDisaggregationPrefixTIEType = 9,
TIETypeMaxValue = 10,
}
/** RIFT route types.
@note: route types which MUST be ordered on their preference
PGP prefixes are most preferred attracting
traffic north (towards spine) and then south
normal prefixes are attracting traffic south
(towards leafs), i.e. prefix in NORTH PREFIX TIE
is preferred over SOUTH PREFIX TIE.
@note: The only purpose of those values is to introduce an
ordering whereas an implementation can choose internally
any other values as long the ordering is preserved
*/
enum RouteType {
Illegal = 0,
RouteTypeMinValue = 1,
/** First legal value. */
/** Discard routes are most preferred */
Discard = 2,
/** Local prefixes are directly attached prefixes on the
* system such as e.g. interface routes.
*/
LocalPrefix = 3,
/** Advertised in S-TIEs */
SouthPGPPrefix = 4,
/** Advertised in N-TIEs */
NorthPGPPrefix = 5,
/** Advertised in N-TIEs */
NorthPrefix = 6,
/** Externally imported north */
NorthExternalPrefix = 7,
/** Advertised in S-TIEs, either normal prefix or positive
disaggregation */
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SouthPrefix = 8,
/** Externally imported south */
SouthExternalPrefix = 9,
/** Negative, transitive prefixes are least preferred */
NegativeSouthPrefix = 10,
RouteTypeMaxValue = 11,
}
B.2. encoding.thrift
/**
Thrift file for packet encodings for RIFT
*/
include "common.thrift"
namespace rs models
namespace py encoding
/** Represents protocol encoding schema major version */
const common.VersionType protocol_major_version = 4
/** Represents protocol encoding schema minor version */
const common.MinorVersionType protocol_minor_version = 1
/** Common RIFT packet header. */
struct PacketHeader {
/** Major version of protocol. */
1: required common.VersionType major_version =
protocol_major_version;
/** Minor version of protocol. */
2: required common.MinorVersionType minor_version =
protocol_minor_version;
/** Node sending the packet, in case of LIE/TIRE/TIDE
also the originator of it. */
3: required common.SystemIDType sender;
/** Level of the node sending the packet, required on everything
except LIEs. Lack of presence on LIEs indicates UNDEFINED_LEVEL
and is used in ZTP procedures.
*/
4: optional common.LevelType level;
}
/** Prefix community. */
struct Community {
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/** Higher order bits */
1: required i32 top;
/** Lower order bits */
2: required i32 bottom;
}
/** Neighbor structure. */
struct Neighbor {
/** System ID of the originator. */
1: required common.SystemIDType originator;
/** ID of remote side of the link. */
2: required common.LinkIDType remote_id;
}
/** Capabilities the node supports.
@note: The schema may add to this
field future capabilities to indicate whether it will support
interpretation of future schema extensions on the same major
revision. Such fields MUST be optional and have an implicit or
explicit false default value. If a future capability changes route
selection or generates blackholes if some nodes are not supporting
it then a major version increment is unavoidable.
*/
struct NodeCapabilities {
/** Must advertise supported minor version dialect that way. */
1: required common.MinorVersionType protocol_minor_version =
protocol_minor_version;
/** Can this node participate in flood reduction. */
2: optional bool flood_reduction =
common.flood_reduction_default;
/** Does this node restrict itself to be top-of-fabric or
leaf only (in ZTP) and does it support leaf-2-leaf
procedures. */
3: optional common.HierarchyIndications hierarchy_indications;
}
/** Link capabilities. */
struct LinkCapabilities {
/** Indicates that the link is supporting BFD. */
1: optional bool bfd =
common.bfd_default;
/** Indicates whether the interface will support v4 forwarding.
@note: This MUST be set to true when LIEs from a v4 address are
sent and MAY be set to true in LIEs on v6 address. If v4
and v6 LIEs indicate contradicting information the
behavior is unspecified. */
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2: optional bool v4_forwarding_capable =
true;
}
/** RIFT LIE Packet.
@note: this node's level is already included on the packet header
*/
struct LIEPacket {
/** Node or adjacency name. */
1: optional string name;
/** Local link ID. */
2: required common.LinkIDType local_id;
/** UDP port to which we can receive flooded TIEs. */
3: required common.UDPPortType flood_port =
common.default_tie_udp_flood_port;
/** Layer 3 MTU, used to discover to mismatch. */
4: optional common.MTUSizeType link_mtu_size =
common.default_mtu_size;
/** Local link bandwidth on the interface. */
5: optional common.BandwithInMegaBitsType
link_bandwidth = common.default_bandwidth;
/** Reflects the neighbor once received to provide
3-way connectivity. */
6: optional Neighbor neighbor;
/** Node's PoD. */
7: optional common.PodType pod =
common.default_pod;
/** Node capabilities shown in LIE. The capabilities
MUST match the capabilities shown in the Node TIEs, otherwise
the behavior is unspecified. A node detecting the mismatch
SHOULD generate according error. */
10: required NodeCapabilities node_capabilities;
/** Capabilities of this link. */
11: optional LinkCapabilities link_capabilities;
/** Required holdtime of the adjacency, i.e. how much time
MUST expire without LIE for the adjacency to drop. */
12: required common.TimeIntervalInSecType
holdtime = common.default_lie_holdtime;
/** Unsolicited, downstream assigned locally significant label
value for the adjacency. */
13: optional common.LabelType label;
/** Indicates that the level on the LIE MUST NOT be used
to derive a ZTP level by the receiving node. */
21: optional bool not_a_ztp_offer =
common.default_not_a_ztp_offer;
/** Indicates to northbound neighbor that it should
be reflooding this node's N-TIEs to achieve flood reduction and
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balancing for northbound flooding. To be ignored if received
from a northbound adjacency. */
22: optional bool you_are_flood_repeater =
common.default_you_are_flood_repeater;
/** Can be optionally set to indicate to neighbor that packet losses
are seen on reception based on packet numbers or the rate is
too high. The receiver SHOULD temporarily slow down
flooding rates.
*/
23: optional bool you_are_sending_too_quickly =
false;
/** Instance name in case multiple RIFT instances running on same
interface. */
24: optional string instance_name;
}
/** LinkID pair describes one of parallel links between two nodes. */
struct LinkIDPair {
/** Node-wide unique value for the local link. */
1: required common.LinkIDType local_id;
/** Received remote link ID for this link. */
2: required common.LinkIDType remote_id;
/** Describes the local interface index of the link. */
10: optional common.PlatformInterfaceIndex platform_interface_index;
/** Describes the local interface name. */
11: optional string platform_interface_name;
/** Indication whether the link is secured, i.e. protected by
outer key, absence of this element means no indication,
undefined outer key means not secured. */
12: optional common.OuterSecurityKeyID
trusted_outer_security_key;
/** Indication whether the link is protected by established
BFD session. */
13: optional bool bfd_up;
/** Optional indication which address families are up on the
interface */
14: optional set<common.AddressFamilyType> address_families;
}
/** ID of a TIE.
@note: TIEID space is a total order achieved by comparing
the elements in sequence defined and comparing each
value as an unsigned integer of according length.
*/
struct TIEID {
/** direction of TIE */
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1: required common.TieDirectionType direction;
/** indicates originator of the TIE */
2: required common.SystemIDType originator;
/** type of the tie */
3: required common.TIETypeType tietype;
/** number of the tie */
4: required common.TIENrType tie_nr;
}
/** Header of a TIE.
@note: TIEID space is a total order achieved by comparing
the elements in sequence defined and comparing each
value as an unsigned integer of according length.
@note: After sequence number the lifetime received on the envelope
must be used for comparison before further fields.
@note: `origination_time` and `origination_lifetime` are
normally disregarded for comparison purposes and carried
purely for debugging/security purposes if present.
They may be used for comparison of last resort to
differentiate otherwise equal ties
*/
struct TIEHeader {
/** ID of the tie. */
2: required TIEID tieid;
/** Sequence number of the tie. */
3: required common.SeqNrType seq_nr;
/** Absolute timestamp when the TIE
was generated. This can be used on fabrics with
synchronized clock to prevent lifetime modification attacks. */
10: optional common.IEEE802_1ASTimeStampType origination_time;
/** Original lifetime when the TIE
was generated. This can be used on fabrics with
synchronized clock to prevent lifetime modification attacks. */
12: optional common.LifeTimeInSecType origination_lifetime;
}
/** Header of a TIE as described in TIRE/TIDE.
*/
struct TIEHeaderWithLifeTime {
1: required TIEHeader header;
/** Remaining lifetime that expires down to 0 just like in ISIS.
TIEs with lifetimes differing by less than
`lifetime_diff2ignore` MUST be considered EQUAL. */
2: required common.LifeTimeInSecType remaining_lifetime;
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}
/** TIDE with sorted TIE headers, if headers are unsorted, behavior
is undefined. */
struct TIDEPacket {
/** First TIE header in the tide packet. */
1: required TIEID start_range;
/** Last TIE header in the tide packet. */
2: required TIEID end_range;
/** _Sorted_ list of headers. */
3: required list<TIEHeaderWithLifeTime> headers;
}
/** TIRE packet */
struct TIREPacket {
1: required set<TIEHeaderWithLifeTime> headers;
}
/** neighbor of a node */
struct NodeNeighborsTIEElement {
/** level of neighbor */
1: required common.LevelType level;
/** Cost to neighbor.
@note: All parallel links to same node
incur same cost, in case the neighbor has multiple
parallel links at different cost, the largest distance
(highest numerical value) MUST be advertised.
@note: any neighbor with cost <= 0 MUST be ignored
in computations */
3: optional common.MetricType cost
= common.default_distance;
/** can carry description of multiple parallel links in a TIE */
4: optional set<LinkIDPair> link_ids;
/** total bandwith to neighbor, this will be normally sum of the
bandwidths of all the parallel links. */
5: optional common.BandwithInMegaBitsType
bandwidth = common.default_bandwidth;
}
/** Indication flags of the node. */
struct NodeFlags {
/** Indicates that node is in overload, do not transit traffic
through it. */
1: optional bool overload = common.overload_default;
}
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/** Description of a node.
It may occur multiple times in different TIEs but if either
<list>
<t>capabilities values do not match or</t>
<t>flags values do not match or</t>
<t>neighbors repeat with different values</t>
</list>
the behavior is undefined and a warning SHOULD be generated.
Neighbors can be distributed across multiple TIEs however if
the sets are disjoint. Miscablings SHOULD be repeated in every
node TIE, otherwise the behavior is undefined.
@note: Observe that absence of fields implies defined defaults.
*/
struct NodeTIEElement {
/** Level of the node. */
1: required common.LevelType level;
/** Node's neighbors. If neighbor systemID repeats in other
node TIEs of same node the behavior is undefined. */
2: required map<common.SystemIDType,
NodeNeighborsTIEElement> neighbors;
/** Capabilities of the node. */
3: required NodeCapabilities capabilities;
/** Flags of the node. */
4: optional NodeFlags flags;
/** Optional node name for easier operations. */
5: optional string name;
/** PoD to which the node belongs. */
6: optional common.PodType pod;
/** optional startup time of the node */
7: optional common.TimestampInSecsType startup_time;
/** If any local links are miscabled, the indication is flooded. */
10: optional set<common.LinkIDType> miscabled_links;
}
/** Attributes of a prefix. */
struct PrefixAttributes {
/** Distance of the prefix. */
2: required common.MetricType metric
= common.default_distance;
/** Generic unordered set of route tags, can be redistributed
to other protocols or use within the context of real time
analytics. */
3: optional set<common.RouteTagType> tags;
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/** Monotonic clock for mobile addresses. */
4: optional common.PrefixSequenceType monotonic_clock;
/** Indicates if the interface is a node loopback. */
6: optional bool loopback = false;
/** Indicates that the prefix is directly attached, i.e. should be
routed to even if the node is in overload. */
7: optional bool directly_attached = true;
/** In case of locally originated prefixes, i.e. interface
addresses this can describe which link the address
belongs to. */
10: optional common.LinkIDType from_link;
}
/** TIE carrying prefixes */
struct PrefixTIEElement {
/** Prefixes with the associated attributes.
If the same prefix repeats in multiple TIEs of same node
behavior is unspecified. */
1: required map<common.IPPrefixType, PrefixAttributes> prefixes;
}
/** Generic key value pairs. */
struct KeyValueTIEElement {
/** @note: if the same key repeats in multiple TIEs of same node
or with different values, behavior is unspecified */
1: required map<common.KeyIDType,string> keyvalues;
}
/** Single element in a TIE.
Schema enum `common.TIETypeType`
in TIEID indicates which elements MUST be present
in the TIEElement. In case of mismatch the unexpected
elements MUST be ignored. In case of lack of expected
element the TIE an error MUST be reported and the TIE
MUST be ignored.
This type can be extended with new optional elements
for new `common.TIETypeType` values without breaking
the major but if it is necessary to understand whether
all nodes support the new type a node capability must
be added as well.
*/
union TIEElement {
/** Used in case of enum common.TIETypeType.NodeTIEType. */
1: optional NodeTIEElement node;
/** Used in case of enum common.TIETypeType.PrefixTIEType. */
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2: optional PrefixTIEElement prefixes;
/** Positive prefixes (always southbound).
It MUST NOT be advertised within a North TIE and
ignored otherwise.
*/
3: optional PrefixTIEElement positive_disaggregation_prefixes;
/** Transitive, negative prefixes (always southbound) which
MUST be aggregated and propagated
according to the specification
southwards towards lower levels to heal
pathological upper level partitioning, otherwise
blackholes may occur in multiplane fabrics.
It MUST NOT be advertised within a North TIE.
*/
5: optional PrefixTIEElement negative_disaggregation_prefixes;
/** Externally reimported prefixes. */
6: optional PrefixTIEElement external_prefixes;
/** Positive external disaggregated prefixes (always southbound).
It MUST NOT be advertised within a North TIE and
ignored otherwise.
*/
7: optional PrefixTIEElement
positive_external_disaggregation_prefixes;
/** Key-Value store elements. */
9: optional KeyValueTIEElement keyvalues;
}
/** TIE packet */
struct TIEPacket {
1: required TIEHeader header;
2: required TIEElement element;
}
/** Content of a RIFT packet. */
union PacketContent {
1: optional LIEPacket lie;
2: optional TIDEPacket tide;
3: optional TIREPacket tire;
4: optional TIEPacket tie;
}
/** RIFT packet structure. */
struct ProtocolPacket {
1: required PacketHeader header;
2: required PacketContent content;
}
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Appendix C. Constants
C.1. Configurable Protocol Constants
This section gathers constants that are provided in the schema files
and in the document.
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+----------------+--------------+-----------------------------------+
| | Type | Value |
+----------------+--------------+-----------------------------------+
| LIE IPv4 | Default | 224.0.0.120 or all-rift-routers |
| Multicast | Value, | to be assigned in IPv4 |
| Address | Configurable | Multicast Address Space Registry |
| | | in Local Network Control Block |
+----------------+--------------+-----------------------------------+
| LIE IPv6 | Default | FF02::A1F7 or all-rift-routers to |
| Multicast | Value, | be assigned in IPv6 Multicast |
| Address | Configurable | Address Assignments |
+----------------+--------------+-----------------------------------+
| LIE | Default | 914 |
| Destination | Value, | |
| Port | Configurable | |
+----------------+--------------+-----------------------------------+
| Level value | Constant | 24 |
| for | | |
| TOP_OF_FABRIC | | |
| flag | | |
+----------------+--------------+-----------------------------------+
| Default LIE | Default | 3 seconds |
| Holdtime | Value, | |
| | Configurable | |
+----------------+--------------+-----------------------------------+
| TIE | Default | 1 second |
| Retransmission | Value | |
| Interval | | |
+----------------+--------------+-----------------------------------+
| TIDE | Default | 5 seconds |
| Generation | Value, | |
| Interval | Configurable | |
+----------------+--------------+-----------------------------------+
| MIN_TIEID | Constant | TIE Key with minimal values: |
| signifies | | TIEID(originator=0, |
| start of TIDEs | | tietype=TIETypeMinValue, |
| | | tie_nr=0, direction=South) |
+----------------+--------------+-----------------------------------+
| MAX_TIEID | Constant | TIE Key with maximal values: |
| signifies end | | TIEID(originator=MAX_UINT64, |
| of TIDEs | | tietype=TIETypeMaxValue, |
| | | tie_nr=MAX_UINT64, |
| | | direction=North) |
+----------------+--------------+-----------------------------------+
Table 6: all_constants
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Authors' Addresses
Tony Przygienda (editor)
Juniper
1137 Innovation Way
Sunnyvale, CA
USA
Email: prz@juniper.net
Alankar Sharma
Comcast
1800 Bishops Gate Blvd
Mount Laurel, NJ 08054
US
Email: Alankar_Sharma@comcast.com
Pascal Thubert
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Bruno Rijsman
Individual
Email: brunorijsman@gmail.com
Dmitry Afanasiev
Yandex
Email: fl0w@yandex-team.ru
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