Network Working Group P. Psenak, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track S. Hegde, Ed.
Expires: August 20, 2018 Juniper Networks, Inc.
C. Filsfils
Cisco Systems, Inc.
A. Gulko
Thomson Reuters
February 16, 2018
ISIS Segment Routing Flexible Algorithm
draft-hegdeppsenak-isis-sr-flex-algo-02.txt
Abstract
IGP protocols traditionally compute best paths over the network based
on the IGP metric assigned to the links. Many network deployments
use RSVP-TE based or Segment Routing based Traffic Engineering to
enforce traffic over a path that is computed using different metrics
or constraints than the shortest IGP path. Various mechanisms are
used to steer the traffic towards such traffic engineered paths.
This document proposes a solution that allows IGPs themselves to
compute constraint based paths over the network without the use of
the above mentioned traffic engineering technologies.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 20, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 3
3. Flexible Algorithm Advertisement . . . . . . . . . . . . . . 3
4. Flexible Algorithm Definition Advertisement . . . . . . . . . 4
4.1. Flexible Algorithm Definition Sub-TLV . . . . . . . . . . 4
4.2. Flexible Algorithm Exclude Admin Group Sub-TLV . . . . . 7
4.3. Flexible Algorithm Include Admin Group Sub-TLVs . . . . . 7
5. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 8
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8.1. Sub TLVs for Type 242 . . . . . . . . . . . . . . . . . . 10
8.2. New Sub-Sub-TLV registry . . . . . . . . . . . . . . . . 10
8.2.1. Flexible Algorithm Definition TLV Metric Registry . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
IGP computed path based on the shortest IGP metric must often be
replaced by traffic engineered path due to the traffic requirements
which are not reflected in the IGP metric. Some networks engineer
the IGP metric assignments in a way that the IGP Metric reflects the
link bandwidth or delay. If, for example, the IGP metric is
reflecting the bandwidth on the link and the application traffic is
delay sensitive, the best IGP path may not reflect the best path from
such application's perspective.
To overcome such IGP limitation, various sorts of traffic engineering
has been deployed, including RSVP-TE or SR-TE, in which case the TE
component is responsible for computing the path based on additional
metrics and/or constraints. Such paths need to be installed in the
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forwarding and replace the original paths computed by IGPs. Tunnels
are often used to represent the engineered paths and mechanisms like
one described in [RFC3906] are used to replace the native IGP paths
with such tunnel paths.
Segment Routing (SR) allows a flexible definition of end-to-end paths
within IGP topologies by encoding paths as sequences of topological
sub-paths, called segments. It also defines an algorithm that
defines how the paths are computed. It also provides a way to
associate Prefix-SID with an algorithm. This allows IGPs to compute
paths based on various algorithms and cause traffic to be forwarded
on such paths using the algorithm specific segments.
This document describes the IS-IS extension to support Segment
Routing Flexible Algorithm on an MPLS data-plane.
1.1. Requirements notation
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 [RFC2119].
2. Flexible Algorithm
Many possible constraints may be used to compute a path over a
network. Some networks are deployed as multiple planes. A simple
form of constraint may be to use a particular plane. A more
sophisticated form of constraint can include some extended metric as
described in [RFC7810]. Constraints which restrict paths to links
with specific affinities or avoid links with specific affinities are
also possible. Combinations of these are also possible.
To provide maximum flexibility we do not want to provide a strict
mapping between the set of constraints and the algorithm that is
associated with it. We want the mapping between the algorithm value
and it's meaning to be flexible and defined by the user. As far as
all routers in the domain have the common understanding what the
particular algorithm value represents, the computation for such
algorithm is consistent and traffic is not subject to any looping.
Because the meaning of the algorithm is not defined by any standard,
but is defined by the user, we call it Flex-Algorithm.
3. Flexible Algorithm Advertisement
[I-D.ietf-isis-segment-routing-extensions] defines an SR-Algorithm.
This algorithm defines how the best path is computed by the IGP.
Routers advertise the support for the algorithm as a node capability.
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Prefix SIDs are also advertised with an algorithm value and as such
are tightly coupled with the algorithm.
Existing advertisement of the SR-Algorithm is used for the Flex-
Algorithm advertisements as defined in
[I-D.ietf-isis-segment-routing-extensions].
SR-Algorithm is a one octet value. We propose to split the range of
values as follows:
0-127 - standardised values assigned by IANA
128-255 - user defined values.
4. Flexible Algorithm Definition Advertisement
To guarantee the loop free forwarding for paths computed for a
particular Flex-Algorithm, all routers in the flooding scope of the
algorithm definition MUST agree on the definition of the Flex-
Algorithm.
4.1. Flexible Algorithm Definition Sub-TLV
Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used to
advertise the definition of the Flex-Algorithm.
FAD Sub-TLV is advertised as Sub-TLV of the IS-IS Router Capability
TLV-242 that is defined in [RFC7981].
When the definition of the Flex Algorithm is advertised, it is
applicable to all topologies supported on the receiving node.
FAD Sub-TLV has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Algorithm | Metric Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alg. Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD1
Length: variable, dependent on the included Sub-TLVs
Algorithm: Flex-Algorithm number. Value between 128 and 255
inclusive.
Metric Type: Type of metric to be used during the calculation.
Following values are defined:
0: IGP Metric
1: Min Unidirectional Link Delay as defined in [RFC7810].
2: TE default metric as defined in [RFC5305].
Algorithm Type: Single octet identifying the algorithm type used
to compute paths for the Flex-Algoritm. Values are defined in
"IGP Algorithm Types" registry defined under "Interior Gateway
Protocol (IGP) Parameters" IANA registries.
Priority: Single octet that specifies the priority of the
advertisement.
Sub-TLVs - optional sub-TLVs.
When the router is configured with the local definition of the Flex-
Algorithm, the router MUST advertise its local definition in the FAD
Sub-TLV. If the local definition of the Flex-Algorithm is not
advertised, the inconsistency in the configuration of the Flex-
Algorithm on various nodes cannot be detected and traffic routed
based on a Flex-Algorithm path may loop permanently.
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Every router, that is configured to support a particular Flex-
Algorithm, MUST select the Flex-Algorithm definition based on the
following rules:
From the received advertisements of the FAD, select the one(s)
with the highest priority.
If there are multiple advertisements of the FAD with the same
highest priority, select the one that is originated from the
router with the highest Router ID. Router ID is required to be
advertised in every Router Capability TLV [RFC7981].
If the router has a local definition of the Flex-Algorithm,
compare it with the received FAD advertisements using the same
rules as have been used to pick the best FAD advertisement, e.g.,
priority and Router ID.
A router that is not configured to support a particular Flex-
Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex-
Algorithm.
Having a deterministic way that always produces a valid Flex-
Algorithm definition avoids conflicts and maximizes the availability
of the forwarding for the traffic that is using the Flex-Algorithm
paths.
Any change in the Flex-Algorithm definition may result in temporary
disruption of traffic that is forwarded based on such Flex-Algorithm
paths. The impact is similar to any other event that requires
network wide convergence
The FAD Sub-TLV of the IS-IS Router Capability TLV-242 MUST be
propagated throughout the level. It MAY be advertised across level
boundaries, if the S-flag in the Router Capability TLV is set. The
S-Flag SHOULD not be set by default unless local configuration policy
on the originating router indicates domain wide flooding.
Flex-Algorithm definition is topology independent. A node which
advertises support for a given Flex-Algorithm may support that Flex-
Algorithm on any subset of the topologies it supports. Enabling of a
supported Flex-Algorithm on a given topology is a matter of local
configuration. For a given topology, if out of the set of nodes
supporting that topology AND advertising support for a given Flex-
Algorithm only a subset of the nodes actually compute/install Flex-
Algorithm specific paths in the forwarding plane for that topology,
some traffic intended for such topology/Flex-Algorithm could be
dropped if forwarded to a node on which the Flex-Algorithm is not
enabled on that topology.
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4.2. Flexible Algorithm Exclude Admin Group Sub-TLV
The Flexible-Algorithm definition can specify 'colors' that are used
by the operator to exclude links during the Flex-Algorithm path
computation.
Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is a
Sub-TLV of the FAD Sub-TLV. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
FAEAG Sub-TLV SHOULD only appear once in FAD Sub-TLV. If it appears
more then once, FAD Sub-TLV MUST be ignored by the receiver.
4.3. Flexible Algorithm Include Admin Group Sub-TLVs
The Flexible-Algorithm definition can specify 'colors' that are used
by the operator to include links during the Flex-Algorithm path
computation.
The format of the include Sub-TLVs is identical to the format of the
FAEAG Sub-TLV in Section 4.2.
Two forms of inclusion are available - include-any and include-all.
Flexible Algorithm Include-Any Admin Group Sub-TLV - Type 2.
Flexible Algorithm Include-All Admin Group Sub-TLV - Type 3.
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Flexible Algorithm Include Admin Group Sub-TLVs SHOULD only appear
once in FAD Sub-TLV. If any of these Sub-TLVs appear more then once,
FAD Sub-TLV MUST be ignored by the receiver.
5. Calculation of Flexible Algorithm Paths
A router may compute path for multiple Flex-Algorithms.
A router MUST be configured to support Flex-Algorithm K before it can
compute any path for Flex-Algorithm K.
A router MUST either be configured with a local definition of Flex-
Algorithm K or receive the definition via the FAD Sub-TLV, as
described in Section 4.1, before it can compute any path for Flex-
Algorithm K.
When computing the path for Flex-Algorithm K, all nodes that do not
advertise support for Flex-Algorithm K in SR-Algorithm Sub-TLV
([I-D.ietf-isis-segment-routing-extensions]), MUST be pruned from the
topology.
When computing the path for Flex-Algorithm K, the metric that is part
of the Flex-Algorithm definition (Section 4.1) MUST be used.
Various link include or exclude rules can be part of the Flex-
Algorithm definition. These rules use Extended Administrative Groups
(EAG) as defined in [RFC7308]. [RFC7308] uses term 'colors' as a
shorthand to refer to particular bits with an EAG. Link
advertisement CAN also include EAG, which describe which color is set
on the link.
Link advertisement CAN also include Administrative Group (AG) TLV
([RFC5305]). The coexistence of EAG and AG is described in the
section 2.3.1 of [RFC7308].
Rules, in the order as specified below, MUST be used to prune link
from the topology during the Flex-Algorithm computation.
For all links in the topology:
1. Check if any exclude rule is part of the Flex-Algorithm
definition. If such exclude rule exists, check if any color that
is part of the exclude rule is also set on the link. If such a
color exist, the link MUST be pruned from the computation.
2. Check if any include-any rule is part of the Flex-Algorithm
definition. if such include-any rule exists, check if any color
that is part of the include-any rule is also set on the link. If
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such color does not exist, the link MUST be pruned from the
computation.
3. Check if any include-all rule is part of the Flex-Algorithm
definition. If such include-all rule exists, check if all colors
that are part of the include-all rule are also set on the link.
If not all such colors are set on the link, the link MUST be
pruned from the computation.
4. If the Flex-Algorithm definition uses other than IGP metric
(Section 4.1), and such metric is not advertised for the
particular link in a topology for which the computation is done,
such link MUST be pruned from the computation. A metric of value
0 MUST NOT be assumed in such case.
Flex-Algorithm K path MUST be installed in the MPLS forwarding plane
using the MPLS label that corresponds to the Prefix-SID that was
advertised for algorithm K. If the Prefix SID for algorithm K is not
known, the Flex-Algorithm K path to such prefix MUST NOT be installed
in the MPLS forwarding plane.
Loop Free Alternate (LFA) paths for Flex-Algorithm K path MUST be
computed using the same constraints as the calculation of the primary
paths for Flex-Algorithm K. LFA path MUST only use Prefix-SIDs
advertised specifically for algorithm K to enforce the traffic over
such path. LFA path MUST NOT use Adjacency-SID that belong to the
link that has been pruned from the computation.
If LFA protection is being used to protect Flex-Algorithm K paths,
all routers in the area SHOULD advertise at least one Flex-Algorithm
K specific Prefix-SID. These Prefix-SIDs are used to enforce traffic
over the LFA computed backup path.
Flex-Algorithm paths MAY be used by other applications, that do not
utilize MPLS forwarding plane. It is outside of the sope of this
specification, how these application learn and use the Flex-Algorithm
specific paths.
Any Shortest Path Tree calculation is limited to a single area. Same
applies to Flex-Algorithm calculations. Given that the computing
router may not have the visibility to the topology of remote areas,
the Flex-Algorithm K path to an inter-area prefix will only be
computed for the local area. The egress L1/L2 router will be
selected based on the best path for the Flex-Algorithm K in the local
area and such egress L1/L2 router will be responsible to compute the
best Flex-Algorithm K path over the next area. This may produce end-
to-end path, which is not the best from the Flex-Algorithm K
perspective. If the best end-to-end path for Flex-Algorithm K needs
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to be used for inter-area destinations, paths for such destinations
need to be computed by the entity that has the topological
information about all areas.
6. Backward Compatibility
This extension brings no new backward compatibility issues.
7. Security Considerations
This extension adds no new security considerations.
8. IANA Considerations
This documents request allocation for the following ISIS TLVs and
subTLVs.
8.1. Sub TLVs for Type 242
This document makes the following registrations in the "sub-TLVs for
TLV 242" registry.
Type: TBD1 (suggested value 24).
Description: Flexible Algorithm Definition Sub-TLV.
Reference: This document (Section 4.1).
8.2. New Sub-Sub-TLV registry
This document creates the following Sub-TLV Registry:
Registry: Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Registration Procedure: Expert review
Reference: This document (Section 4.1)
This document resisters following Sub-TLVs in the "Sub-TLVs for
Flexible Algorithm Definition Sub-TLV" registry:
Type: 1
Description: Flexible Algorithm Exclude Admin Group Sub-TLV
Reference: This document (Section 4.2).
Type: 2
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Description: Flexible Algorithm Include-Any Admin Group Sub-TLV
Reference: This document (Section 4.3).
Type: 3
Description: Flexible Algorithm Include-All Admin Group Sub-TLV
Reference: This document (Section 4.3).
8.2.1. Flexible Algorithm Definition TLV Metric Registry
This document creates the following Registry:
Registry: Flexible Algorithm Definition TLV Metric Registry
Registration Procedure: Expert review
Reference: This document (Section 4.1)
This document registers following values in the "Flexible
Algorithm Definition TLV Metric Registry":
Type: TBD, suggested value 0
Description: IGP metric
Reference: This document (Section 4.1)
Type: TBD, suggested value 1
Description: Min Unidirectional Link Delay [RFC7810]
Reference: This document (Section 4.1)
Type: TBD, suggested value 2
Description: TE Default Metric [RFC5305]
Reference: This document (Section 4.1)
9. Acknowledgments
This draft, among other things, is also addressing the problem that
the [I-D.gulkohegde-routing-planes-using-sr] was trying to solve.
All authors of that draft agreed to join this draft.
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Thanks to Les Ginsberg and Ketan Talaulikar for review and useful
comments.
Thanks to Cengiz Halit for his review and feedback during initial
phase of the solution definition.
10. References
10.1. Normative References
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-15 (work in progress), December
2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
Traffic Engineering (MPLS-TE)", RFC 7308,
DOI 10.17487/RFC7308, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.
[RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
RFC 7810, DOI 10.17487/RFC7810, May 2016,
<https://www.rfc-editor.org/info/rfc7810>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
10.2. Informative References
[I-D.gulkohegde-routing-planes-using-sr]
Hegde, S. and a. arkadiy.gulko@thomsonreuters.com,
"Separating Routing Planes using Segment Routing", draft-
gulkohegde-routing-planes-using-sr-00 (work in progress),
March 2017.
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[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Apollo Business Center
Mlynske nivy 43
Bratislava, 82109
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde (editor)
Juniper Networks, Inc.
Embassy Business Park
Bangalore, KA, 560093
India
Email: shraddha@juniper.net
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Arkadiy Gulko
Thomson Reuters
Email: arkadiy.gulko@thomsonreuters.com
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