IGP Flexible Algorithm
draft-ietf-lsr-flex-algo-16
Network Working Group P. Psenak, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track S. Hegde
Expires: November 27, 2021 Juniper Networks, Inc.
C. Filsfils
K. Talaulikar
Cisco Systems, Inc.
A. Gulko
Edward Jones
May 26, 2021
IGP Flexible Algorithm
draft-ietf-lsr-flex-algo-16
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
steer traffic over a path that is computed using different metrics or
constraints than the shortest IGP path. This document proposes a
solution that allows IGPs themselves to compute constraint-based
paths over the network. This document also specifies a way of using
Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets
along the constraint-based paths.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 27, 2021.
Psenak, et al. Expires November 27, 2021 [Page 1]
Internet-Draft IGP Flexible Algorithm May 2021
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 5
5. Flexible Algorithm Definition Advertisement . . . . . . . . . 6
5.1. ISIS Flexible Algorithm Definition Sub-TLV . . . . . . . 6
5.2. OSPF Flexible Algorithm Definition TLV . . . . . . . . . 8
5.3. Common Handling of Flexible Algorithm Definition TLV . . 9
6. Sub-TLVs of ISIS FAD Sub-TLV . . . . . . . . . . . . . . . . 10
6.1. ISIS Flexible Algorithm Exclude Admin Group Sub-TLV . . . 10
6.2. ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV . 11
6.3. ISIS Flexible Algorithm Include-All Admin Group Sub-TLV . 12
6.4. ISIS Flexible Algorithm Definition Flags Sub-TLV . . . . 12
6.5. ISIS Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 13
7. Sub-TLVs of OSPF FAD TLV . . . . . . . . . . . . . . . . . . 14
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . . 14
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV . 14
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV . 15
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV . . . . 15
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 16
8. ISIS Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . . 17
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . . 18
10. OSPF Flexible Algorithm ASBR Reachability Advertisement . . . 19
10.1. OSPFv2 Extended Inter-Area ASBR LSA . . . . . . . . . . 19
10.1.1. OSPFv2 Extended Inter-Area ASBR TLV . . . . . . . . 21
10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV . . . . . . 22
11. Advertisement of Node Participation in a Flex-Algorithm . . . 24
11.1. Advertisement of Node Participation for Segment Routing 24
11.2. Advertisement of Node Participation for Other
Applications . . . . . . . . . . . . . . . . . . . . . . 24
12. Advertisement of Link Attributes for Flex-Algorithm . . . . . 24
Psenak, et al. Expires November 27, 2021 [Page 2]
Internet-Draft IGP Flexible Algorithm May 2021
13. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 25
13.1. Multi-area and Multi-domain Considerations . . . . . . . 27
14. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . . 29
14.1. Segment Routing MPLS Forwarding for Flex-Algorithm . . . 30
14.2. SRv6 Forwarding for Flex-Algorithm . . . . . . . . . . . 30
14.3. Other Applications' Forwarding for Flex-Algorithm . . . 31
15. Operational Considerations . . . . . . . . . . . . . . . . . 31
15.1. Inter-area Considerations . . . . . . . . . . . . . . . 31
15.2. Usage of SRLG Exclude Rule with Flex-Algorithm . . . . . 32
15.3. Max-metric consideration . . . . . . . . . . . . . . . . 33
16. Backward Compatibility . . . . . . . . . . . . . . . . . . . 33
17. Security Considerations . . . . . . . . . . . . . . . . . . . 33
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
18.1. IGP IANA Considerations . . . . . . . . . . . . . . . . 34
18.1.1. IGP Algorithm Types Registry . . . . . . . . . . . . 34
18.1.2. IGP Metric-Type Registry . . . . . . . . . . . . . . 34
18.2. Flexible Algorithm Definition Flags Registry . . . . . . 35
18.3. ISIS IANA Considerations . . . . . . . . . . . . . . . . 35
18.3.1. Sub TLVs for Type 242 . . . . . . . . . . . . . . . 35
18.3.2. Sub TLVs for for TLVs 135, 235, 236, and 237 . . . . 35
18.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-
TLV . . . . . . . . . . . . . . . . . . . . . . . . 35
18.4. OSPF IANA Considerations . . . . . . . . . . . . . . . . 36
18.4.1. OSPF Router Information (RI) TLVs Registry . . . . . 36
18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs . . . . . . . . 37
18.4.3. OSPFv3 Extended-LSA Sub-TLVs . . . . . . . . . . . . 37
18.4.4. OSPF Flex-Algorithm Prefix Metric Bits . . . . . . . 37
18.4.5. OSPF Opaque LSA Option Types . . . . . . . . . . . . 37
18.4.6. OSPFv2 Externded Inter-Area ASBR TLVs . . . . . . . 38
18.4.7. OSPFv2 Inter-Area ASBR Sub-TLVs . . . . . . . . . . 38
18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLV
Registry . . . . . . . . . . . . . . . . . . . . . . 38
18.4.9. Link Attribute Applications Registry . . . . . . . . 40
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
20.1. Normative References . . . . . . . . . . . . . . . . . . 40
20.2. Informative References . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
An IGP-computed path based on the shortest IGP metric is often be
replaced by a traffic-engineered path due to the traffic requirements
which are not reflected by 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
Psenak, et al. Expires November 27, 2021 [Page 3]
Internet-Draft IGP Flexible Algorithm May 2021
delay sensitive, the best IGP path may not reflect the best path from
such an application's perspective.
To overcome this limitation, various sorts of traffic engineering
have been deployed, including RSVP-TE and SR-TE, in which case the TE
component is responsible for computing paths based on additional
metrics and/or constraints. Such paths need to be installed in the
forwarding tables in addition to, or as a replacement for, 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.
This document specifies a set of extensions to ISIS, OSPFv2, and
OSPFv3 that enable a router to advertise TLVs that identify (a)
calculation-type, (b) specify a metric-type, and (c) describe a set
of constraints on the topology, that are to be used to compute the
best paths along the constrained topology. A given combination of
calculation-type, metric-type, and constraints is known as a
"Flexible Algorithm Definition". A router that sends such a set of
TLVs also assigns a Flex-Algorithm value to the specified combination
of calculation-type, metric-type, and constraints.
This document also specifies a way for a router to use IGPs to
associate one or more SR Prefix-SIDs or SRv6 locators with a
particular Flex-Algorithm. Each such Prefix-SID or SRv6 locator then
represents a path that is computed according to the identified Flex-
Algorithm.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
This section defines terms that are often used in this document.
Flexible Algorithm Definition (FAD) - the set consisting of (a)
calculation-type, (b) metric-type, and (c) a set of constraints.
Flexible Algorithm - a numeric identifier in the range 128-255 that
is associated via configuration with the Flexible-Algorithm
Definition.
Psenak, et al. Expires November 27, 2021 [Page 4]
Internet-Draft IGP Flexible Algorithm May 2021
Local Flexible Algorithm Definition - Flexible Algorithm Definition
defined locally on the node.
Remote Flexible Algorithm Definition - Flexible Algorithm Definition
received from other nodes via IGP flooding.
Flexible Algorithm Participation - per application configuration
state that expresses whether the node is participating in a
particular Flexible Algorithm.
IGP Algorithm - value from the the "IGP Algorithm Types" registry
defined under "Interior Gateway Protocol (IGP) Parameters" IANA
registries. IGP Algorithms represents the triplet (Calculation Type,
Metric, Constraints), where the second and third elements of the
triple MAY be unspecified.
ABR - Area Border Router. In ISIS terminology it is also known as
L1/L2 router.
ASBR - Autonomous System Border Router.
4. 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 [RFC8570]. 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 want to provide a mechanism that
allows a router to (a) identify a particular calculation-type, (b)
metric-type, (c) describe a particular set of constraints, and (d)
assign a numeric identifier, referred to as Flex-Algorithm, to the
combination of that calculation-type, metric-type, and those
constraints. We want the mapping between the Flex-Algorithm and its
meaning to be flexible and defined by the user. As long as all
routers in the domain have a common understanding as to what a
particular Flex-Algorithm represents, the resulting routing
computation is consistent and traffic is not subject to any looping.
The set consisting of (a) calculation-type, (b) metric-type, and (c)
a set of constraints is referred to as a Flexible-Algorithm
Definition.
Psenak, et al. Expires November 27, 2021 [Page 5]
Internet-Draft IGP Flexible Algorithm May 2021
Flexible-Algorithm is a numeric identifier in the range 128-255 that
is associated via configuratin with the Flexible-Algorithm
Definition.
IANA "IGP Algorithm Types" registry defines the set of values for IGP
Algorithms. We propose to allocate the following values for Flex-
Algorithms from this registry:
128-255 - Flex-Algorithms
5. Flexible Algorithm Definition Advertisement
To guarantee the loop-free forwarding for paths computed for a
particular Flex-Algorithm, all routers that (a) are configured to
participate in a particular Flex-Algorithm, and (b) are in the same
Flex-Algorithm definition advertisement scope MUST agree on the
definition of the Flex-Algorithm.
5.1. ISIS Flexible Algorithm Definition Sub-TLV
The ISIS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used
to advertise the definition of the Flex-Algorithm.
The ISIS FAD Sub-TLV is advertised as a Sub-TLV of the ISIS Router
Capability TLV-242 that is defined in [RFC7981].
ISIS FAD Sub-TLV 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 |Flex-Algorithm | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 26
Length: variable, dependent on the included Sub-TLVs
Psenak, et al. Expires November 27, 2021 [Page 6]
Internet-Draft IGP Flexible Algorithm May 2021
Flex-Algorithm: Single octet 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 [RFC8570],
section 4.2, encoded as application specific link attribute as
specified in [RFC8919] and Section 12 of this document.
2: Traffic Engineering Default Metric as defined in [RFC5305],
section 3.7, encoded as application specific link attribute as
specified in [RFC8919] and Section 12 of this document.
Calc-Type: value from 0 to 127 inclusive from the "IGP Algorithm
Types" registry defined under "Interior Gateway Protocol (IGP)
Parameters" IANA registries. IGP algorithms in the range of 0-127
have a defined triplet (Calculation Type, Metric, Constraints).
When used to specify the Calc-Type in the FAD Sub-TLV, only the
Calculation Type defined for the specified IGP Algorithm is used.
The Metric/Constraints MUST NOT be inherited. If the required
calculation type is Shortest Path First, the value 0 SHOULD appear
in this field.
Priority: Value between 0 and 255 inclusive that specifies the
priority of the advertisement.
Sub-TLVs - optional sub-TLVs.
The ISIS FAD Sub-TLV MAY be advertised in an LSP of any number, but a
router MUST NOT advertise more than one ISIS FAD Sub-TLV for a given
Flexible-Algorithm. A router receiving multiple ISIS FAD Sub-TLVs
for a given Flexible-Algorithm from the same originator SHOULD select
the first advertisement in the lowest numbered LSP.
The ISIS FAD Sub-TLV has an area scope. The Router Capability TLV in
which the FAD Sub-TLV is present MUST have the S-bit clear.
ISIS L1/L2 router MAY be configured to re-generate the winning FAD
from level 2, without any modification to it, to level 1 area. The
re-generation of the FAD Sub-TLV from level 2 to level 1 is
determined by the L1/L2 router, not by the originator of the FAD
advertisement in the level 2. In such case, the re-generated FAD
Sub-TLV will be advertised in the level 1 Router Capability TLV
originated by the L1/L2 router.
Psenak, et al. Expires November 27, 2021 [Page 7]
Internet-Draft IGP Flexible Algorithm May 2021
L1/L2 router MUST NOT re-generate any FAD Sub-TLV from level 1 to
level 2.
5.2. OSPF Flexible Algorithm Definition TLV
OSPF FAD TLV is advertised as a top-level TLV of the RI LSA that is
defined in [RFC7770].
OSPF FAD TLV 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Metric-Type | Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 16
Length: variable, dependent on the included Sub-TLVs
Flex-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 [RFC7471],
section 4.2, encoded as application specific link attribute as
specified in [RFC8920] and Section 12 of this document.
2: Traffic Engineering metric as defined in [RFC3630], section
2.5.5, encoded as application specific link attribute as
specified in [RFC8920] and Section 12 of this document.
Calc-Type: as described in Section 5.1
Psenak, et al. Expires November 27, 2021 [Page 8]
Internet-Draft IGP Flexible Algorithm May 2021
Priority: as described in Section 5.1
Sub-TLVs - optional sub-TLVs.
When multiple OSPF FAD TLVs, for the same Flexible-Algorithm, are
received from a given router, the receiver MUST use the first
occurrence of the TLV in the Router Information LSA. If the OSPF FAD
TLV, for the same Flex-Algorithm, appears in multiple Router
Information LSAs that have different flooding scopes, the OSPF FAD
TLV in the Router Information LSA with the area-scoped flooding scope
MUST be used. If the OSPF FAD TLV, for the same algorithm, appears
in multiple Router Information LSAs that have the same flooding
scope, the OSPF FAD TLV in the Router Information (RI) LSA with the
numerically smallest Instance ID MUST be used and subsequent
instances of the OSPF FAD TLV MUST be ignored.
The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED. The
Autonomous System flooding scope SHOULD NOT be used by default unless
local configuration policy on the originating router indicates domain
wide flooding.
5.3. Common Handling of Flexible Algorithm Definition TLV
This section describes the protocol-independent handling of the FAD
TLV (OSPF) or FAD Sub-TLV (ISIS). We will refer to it as FAD TLV in
this section, even though in case of ISIS it is a Sub-TLV.
The value of the Flex-Algorithm MUST be between 128 and 255
inclusive. If it is not, the FAD TLV MUST be ignored.
Only a subset of the routers participating in the particular Flex-
Algorithm need to advertise the definition of the Flex-Algorithm.
Every router, that is configured to participate in a particular Flex-
Algorithm, MUST select the Flex-Algorithm definition based on the
following ordered rules. This allows for the consistent Flex-
Algorithm definition selection in cases where different routers
advertise different definitions for a given Flex-Algorithm:
1. From the advertisements of the FAD in the area (including both
locally generated advertisements and received advertisements)
select the one(s) with the highest priority value.
2. 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 System-ID, in the case of ISIS, or Router
Psenak, et al. Expires November 27, 2021 [Page 9]
Internet-Draft IGP Flexible Algorithm May 2021
ID, in the case of OSPFv2 and OSPFv3. For ISIS, the System-ID is
described in [ISO10589]. For OSPFv2 and OSPFv3, standard Router
ID is described in [RFC2328] and [RFC5340] respectively.
A router that is not configured to participate in a particular Flex-
Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex-
Algorithm.
A router that is not participating in a particular Flex-Algorithm is
allowed to advertise FAD for such Flex-Algorithm. Receiving routers
MUST consider FAD advertisement regardless of the Flex-Algorithm
participation of the FAD originator.
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.
If a node is configured to participate in a particular Flexible-
Algorithm, but the selected Flex-Algorithm definition includes
calculation-type, metric-type, constraint, flag, or Sub-TLV that is
not supported by the node, it MUST stop participating in such
Flexible-Algorithm. That implies that it MUST NOT announce
participation for such Flexible-Algorithm as specified in Section 11
and it MUST remove any forwarding state associated with it.
Flex-Algorithm definition is topology independent. It applies to all
topologies that a router participates in.
6. Sub-TLVs of ISIS FAD Sub-TLV
6.1. ISIS 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.
The ISIS Flexible Algorithm Exclude Admin Group Sub-TLV is used to
advertise the exclude rule that is used during the Flex-Algorithm
path calculation as specified in Section 13.
The ISIS Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-
TLV) is a Sub-TLV of the ISIS FAD Sub-TLV. It has the following
format:
Psenak, et al. Expires November 27, 2021 [Page 10]
Internet-Draft IGP Flexible Algorithm May 2021
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].
The ISIS FAEAG Sub-TLV MUST NOT appear more than once in an ISIS FAD
Sub-TLV. If it appears more than once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
6.2. ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV
The Flexible Algorithm definition can specify 'colors' that are used
by the operator to include links during the Flex-Algorithm path
computation.
The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV is used
to advertise include-any rule that is used during the Flex-Algorithm
path calculation as specified in Section 13.
The format of the ISIS Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the FAEAG Sub-TLV in
Section 6.1.
The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
2.
The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more than once in an ISIS FAD Sub-TLV. If it appears more
than once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.
Psenak, et al. Expires November 27, 2021 [Page 11]
Internet-Draft IGP Flexible Algorithm May 2021
6.3. ISIS Flexible Algorithm Include-All Admin Group Sub-TLV
The Flexible Algorithm definition can specify 'colors' that are used
by the operator to include link during the Flex-Algorithm path
computation.
The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV is used
to advertise include-all rule that is used during the Flex-Algorithm
path calculation as specified in Section 13.
The format of the ISIS Flexible Algorithm Include-All Admin Group
Sub-TLV is identical to the format of the FAEAG Sub-TLV in
Section 6.1.
The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV Type is
3.
The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more than once in an ISIS FAD Sub-TLV. If it appears more
than once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.
6.4. ISIS Flexible Algorithm Definition Flags Sub-TLV
The ISIS Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
is a Sub-TLV of the ISIS 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 4
Length: variable, non-zero number of octets of the Flags field
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
Psenak, et al. Expires November 27, 2021 [Page 12]
Internet-Draft IGP Flexible Algorithm May 2021
M-flag: when set, the Flex-Algorithm specific prefix metric
MUST be used for inter-area and external prefix calculation.
This flag is not applicable to prefixes advertised as SRv6
locators.
Bits are defined/sent starting with Bit 0 defined above. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order so as to minimize the number of bits that will
need to be transmitted.
Undefined bits MUST be transmitted as 0.
Bits that are NOT transmitted MUST be treated as if they are set to 0
on receipt.
The ISIS FADF Sub-TLV MUST NOT appear more than once in an ISIS FAD
Sub-TLV. If it appears more than once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
If the ISIS FADF Sub-TLV is not present inside the ISIS FAD Sub-TLV,
all the bits are assumed to be set to 0.
6.5. ISIS Flexible Algorithm Exclude SRLG Sub-TLV
The Flexible Algorithm definition can specify Shared Risk Link Groups
(SRLGs) that the operator wants to exclude during the Flex-Algorithm
path computation.
The ISIS Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG) is used to
advertise the exclude rule that is used during the Flex-Algorithm
path calculation as specified in Section 13.
The ISIS FAESRLG Sub-TLV is a Sub-TLV of the ISIS 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Value |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5
Psenak, et al. Expires November 27, 2021 [Page 13]
Internet-Draft IGP Flexible Algorithm May 2021
Length: variable, dependent on number of SRLG values. MUST be a
multiple of 4 octets.
Shared Risk Link Group Value: SRLG value as defined in [RFC5307].
The ISIS FAESRLG Sub-TLV MUST NOT appear more than once in an ISIS
FAD Sub-TLV. If it appears more than once, the ISIS FAD Sub-TLV MUST
be ignored by the receiver.
7. Sub-TLVs of OSPF FAD TLV
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV
The Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is
a Sub-TLV of the OSPF FAD TLV. It's usage is described in
Section 6.1. 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].
The OSPF FAEAG Sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV
The usage of this Sub-TLVs is described in Section 6.2.
The format of the OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
Section 7.1.
Psenak, et al. Expires November 27, 2021 [Page 14]
Internet-Draft IGP Flexible Algorithm May 2021
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
2.
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more than once in an OSPF FAD TLV. If it appears more than
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV
The usage of this Sub-TLVs is described in Section 6.3.
The format of the OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
Section 7.1.
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
3.
The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more than once in an OSPF FAD TLV. If it appears more than
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV
The OSPF Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
is a Sub-TLV of the OSPF FAD 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 4
Length: variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
Flags:
Psenak, et al. Expires November 27, 2021 [Page 15]
Internet-Draft IGP Flexible Algorithm May 2021
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
M-flag: when set, the Flex-Algorithm specific prefix and ASBR
metric MUST be used for inter-area and external prefix
calculation. This flag is not applicable to prefixes
advertised as SRv6 locators.
Bits are defined/sent starting with Bit 0 defined above. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order so as to minimize the number of bits that will
need to be transmitted.
Undefined bits MUST be transmitted as 0.
Bits that are NOT transmitted MUST be treated as if they are set to 0
on receipt.
The OSPF FADF Sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
If the OSPF FADF Sub-TLV is not present inside the OSPF FAD TLV, all
the bits are assumed to be set to 0.
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV
The OSPF Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG Sub-TLV) is
a Sub-TLV of the OSPF FAD TLV. Its usage is described in
Section 6.5. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Value |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5
Length: variable, dependent on the number of SRLGs. MUST be a
multiple of 4 octets.
Psenak, et al. Expires November 27, 2021 [Page 16]
Internet-Draft IGP Flexible Algorithm May 2021
Shared Risk Link Group Value: SRLG value as defined in [RFC4203].
The OSPF FAESRLG Sub-TLV MUST NOT appear more than once in an OSPF
FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be
ignored by the receiver.
8. ISIS Flexible Algorithm Prefix Metric Sub-TLV
The ISIS Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the
advertisement of a Flex-Algorithm specific prefix metric associated
with a given prefix advertisement.
The ISIS FAPM Sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237 and
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 |Flex-Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 6
Length: 5 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Metric: 4 octets of metric information
The ISIS FAPM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
If a prefix is advertised with a Flex-Algorithm prefix metric larger
then MAX_PATH_METRIC as defined in [RFC5305] this prefix MUST NOT be
considered during the Flexible-Algorithm computation.
The usage of the Flex-Algorithm prefix metric is described in
Section 13.
The ISIS FAPM Sub-TLV MUST NOT be advertised as a sub-TLV of the ISIS
SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions]. The ISIS SRv6
Locator TLV includes the Algorithm and Metric fields which MUST be
used instead. If the FAPM Sub-TLV is present as a sub-TLV of the
Psenak, et al. Expires November 27, 2021 [Page 17]
Internet-Draft IGP Flexible Algorithm May 2021
ISIS SRv6 Locator TLV in the received LSP, such FAPM Sub-TLV MUST be
ignored.
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV
The OSPF Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the
advertisement of a Flex-Algorithm specific prefix metric associated
with a given prefix advertisement.
The OSPF Flex-Algorithm Prefix Metric (FAPM) Sub-TLV is a Sub-TLV of
the:
- OSPFv2 Extended Prefix TLV [RFC7684]
- Following OSPFv3 TLVs as defined in [RFC8362]:
Inter-Area Prefix TLV
External Prefix TLV
OSPF FAPM Sub-TLV 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3 for OSPFv2, 26 for OSPFv3
Length: 8 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Flags: single octet value
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E| |
+-+-+-+-+-+-+-+-+
Psenak, et al. Expires November 27, 2021 [Page 18]
Internet-Draft IGP Flexible Algorithm May 2021
E bit : position 0: The type of external metric. If bit is
set, the metric specified is a Type 2 external metric. This
bit is applicable only to OSPF External and NSSA external
prefixes. This is semantically the same as E bit in section
A.4.5 of [RFC2328] and section A.4.7 of [RFC5340] for OSPFv2
and OSPFv3 respectively.
Bits 1 through 7: MUST be cleared by sender and ignored by
receiver.
Reserved: Must be set to 0, ignored at reception.
Metric: 4 octets of metric information
The OSPF FAPM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
The usage of the Flex-Algorithm prefix metric is described in
Section 13.
10. OSPF Flexible Algorithm ASBR Reachability Advertisement
An OSPF ABR advertises the reachability of ASBRs in its attached
areas to enable routers within those areas to perform route
calculations for external prefixes advertised by the ASBRs. OSPF
extensions for advertisement of Flex-Algorithm specific reachability
and metric for ASBRs is similarly required for Flex-Algorithm
external prefix computations as described further in Section 13.1.
10.1. OSPFv2 Extended Inter-Area ASBR LSA
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA is an OSPF Opaque
LSA [RFC5250] that is used to advertise additional attributes related
to the reachability of the OSPFv2 ASBR that is external to the area
yet internal to the OSPF domain. Semantically, the OSPFv2 EIA-ASBR
LSA is equivalent to the fixed format Type 4 Summary LSA [RFC2328].
Unlike the Type 4 Summary LSA, the LSID of the EIA-ASBR LSA does not
carry the ASBR Router-ID - the ASBR Router-ID is carried in the body
of the LSA. OSPFv2 EIA-ASBR LSA is advertised by an OSPFv2 ABR and
its flooding is defined to be area-scoped only.
An OSPFv2 ABR generates the EIA-ASBR LSA for an ASBR when it is
advertising the Type-4 Summary LSA for it and has the need for
advertising additional attributes for that ASBR beyond what is
conveyed in the fixed format Type-4 Summary LSA. An OSPFv2 ABR MUST
NOT advertise the EIA-ASBR LSA for an ASBR for which it is not
Psenak, et al. Expires November 27, 2021 [Page 19]
Internet-Draft IGP Flexible Algorithm May 2021
advertising the Type 4 Summary LSA. This ensures that the ABR does
not generate the EIA-ASBR LSA for an ASBR to which it does not have
reachability in the base OSPFv2 topology calculation. The OSPFv2 ABR
SHOULD NOT advertise the EIA-ASBR LSA for an ASBR when it does not
have additional attributes to advertise for that ASBR.
The OSPFv2 EIA-ASBR LSA 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
The Opaque Type used by the OSPFv2 EIA-ASBR LSA is TBD (suggested
value 11). The Opaque Type is used to differentiate the various
types of OSPFv2 Opaque LSAs and is described in Section 3 of
[RFC5250]. The LS Type MUST be 10, indicating that the Opaque LSA
flooding scope is area-local [RFC5250]. The LSA Length field
[RFC2328] represents the total length (in octets) of the Opaque LSA,
including the LSA header and all TLVs (including padding).
The Opaque ID field is an arbitrary value used to maintain multiple
OSPFv2 EIA-ASBR LSAs. For OSPFv2 EIA-ASBR LSAs, the Opaque ID has no
semantic significance other than to differentiate OSPFv2 EIA-ASBR
LSAs originated by the same OSPFv2 ABR. If multiple OSPFv2 EIA-ASBR
LSAs specify the same ASBR, the attributes from the Opaque LSA with
the lowest Opaque ID SHOULD be used.
The format of the TLVs within the body of the OSPFv2 EIA-ASBR LSA is
the same as the format used by the Traffic Engineering Extensions to
OSPFv2 [RFC3630]. The variable TLV section consists of one or more
nested TLV tuples. Nested TLVs are also referred to as sub- TLVs.
The Length field defines the length of the value portion in octets
(thus, a TLV with no value portion would have a length of 0). The
TLV is padded to 4-octet alignment; padding is not included in the
Psenak, et al. Expires November 27, 2021 [Page 20]
Internet-Draft IGP Flexible Algorithm May 2021
Length field (so a 3-octet value would have a length of 3, but the
total size of the TLV would be 8 octets). Nested TLVs are also
32-bit aligned. For example, a 1-byte value would have the Length
field set to 1, and 3 octets of padding would be added to the end of
the value portion of the TLV. The padding is composed of zeros.
10.1.1. OSPFv2 Extended Inter-Area ASBR TLV
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) TLV is a top-level TLV
of the OSPFv2 EIA-ASBR LSA and is used to advertise additional
attributes associated with the reachability of an ASBR.
The OSPFv2 EIA-ASBR TLV 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASBR Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable
ASBR Router ID: four octets carrying the OSPF Router ID of the
ASBR whose information is being carried.
Sub-TLVs : variable
Only a single OSPFv2 EIA-ASBR TLV MUST be advertised in each OSPFv2
EIA-ASBR LSA and the receiver MUST ignore all instances of this TLV
other than the first one in an LSA.
OSPFv2 EIA-ASBR TLV MUST be present inside an OSPFv2 EIA-ASBR LSA
with at least a single sub-TLV included, otherwise the OSPFv2 EIA-
ASBR LSA MUST be ignored by the receiver.
Psenak, et al. Expires November 27, 2021 [Page 21]
Internet-Draft IGP Flexible Algorithm May 2021
10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV
The OSPF Flexible Algorithm ASBR Metric (FAAM) Sub-TLV supports the
advertisement of a Flex-Algorithm specific metric associated with a
given ASBR reachability advertisement by an ABR.
The OSPF Flex-Algorithm ASBR Metric (FAAM) Sub-TLV is a Sub-TLV of
the:
- OSPFv2 Extended Inter-Area ASBR TLV as defined in Section 10.1.1
- OSPFv3 Inter-Area-Router TLV defined in [RFC8362]
OSPF FAAM Sub-TLV 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1 for OSPFv2, TBD (suggested value 30) for OSPFv3
Length: 8 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Reserved: Must be set to 0, ignored at reception.
Metric: 4 octets of metric information
The OSPF FAAM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
The advertisement of the ASBR reachability using the OSPF FAAM Sub-
TLV inside the OSPFv2 EIA-ASBR LSA follows the section 12.4.3 of
[RFC2328] and inside the OSPFv3 E-Inter-Area-Router LSA follows the
section 4.8.5 of [RFC5340]. The reachability of the ASBR is
evaluated in the context of the specific Flex-Algorithm.
Psenak, et al. Expires November 27, 2021 [Page 22]
Internet-Draft IGP Flexible Algorithm May 2021
The FAAM computed by the ABR will be equal to the metric to reach the
ASBR for a given Flex-Algorithm in a source area or the cumulative
metric via other ABR(s) when the ASBR is in a remote area. This is
similar in nature to how the metric is set when the ASBR reachability
metric is computed in the default algorithm for the metric in the
OSPFv2 Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA.
An OSPF ABR MUST NOT include the OSPF FAAM Sub-TLV with a specific
Flex-Algorithm in its reachability advertisement for an ASBR between
areas unless that ASBR is reachable for it in the context of that
specific Flex-Algorithm.
An OSPF ABR MUST include the OSPF FAAM Sub-TLVs as part of the ASBR
reachability advertisement between areas for the Flex-Algorithm for
which the winning FAD includes the M-flag and the ASBR is reachable
in the context of that specific Flex-Algorithm.
OSPF routers MUST use the OSPF FAAM Sub-TLV to calculate the
reachability of the ASBRs if the winning FAD for the specific Flex-
Algorithm includes the M-flag. OSPF routers MUST NOT use the OSPF
FAAM Sub-TLV to calculate the reachability of the ASBRs for the
specific Flex-Algorithm if the winning FAD for such Flex-Algorithm
does not include the M-flag. Instead, the OSPFv2 Type 4 Summary LSAs
or the OSPFv3 Inter-Area-Router-LSAs MUST be used instead as
specified in section 16.2 of [RFC2328] and section 4.8.5 of [RFC5340]
for OSPFv2 and OSPFv3 respectively.
The processing of the new or changed OSPF FAAM Sub-TLV triggers the
processing of the External routes similar to what is described in
section 16.5 of the [RFC2328] for OSPFv2 and section 4.8.5 of
[RFC5340] for OSPFv3 for the specific Flex-Algorithm. The External
and NSSA External route calculation should be limited to Flex-
Algorithm(s) for which the winning FAD(s) includes the M-flag.
Processing of the OSPF FAAM Sub-TLV does not require the existence of
the equivalent OSPFv2 Type 4 Summary LSA or the OSPFv3 Inter-Area-
Router-LSA that is advertised by the same ABR inside the area. When
the OSPFv2 EIA-ASBR LSA or the OSPFv3 E-Inter-Area-Router-LSA are
advertised along with the OSPF FAAM Sub-TLV by the ABR for a specific
ASBR, it is expected that the same ABR would advertise the
reachability of the same ASBR in the equivalent base LSAs - i.e., the
OSPFv2 Type 4 Summary LSA or the OSPFv3 Inter-Area-Router-LSA. The
presence of the base LSA is not mandatory for the usage of the
extended LSA with the OSPF FAAM Sub-TLV. This means that the order
in which these LSAs are received is not significant.
Psenak, et al. Expires November 27, 2021 [Page 23]
Internet-Draft IGP Flexible Algorithm May 2021
11. Advertisement of Node Participation in a Flex-Algorithm
When a router is configured to support a particular Flex-Algorithm,
we say it is participating in that Flex-Algorithm.
Paths computed for a specific Flex-Algorithm MAY be used by various
applications, each potentially using its own specific data plane for
forwarding traffic over such paths. To guarantee the presence of the
application specific forwarding state associated with a particular
Flex-Algorithm, a router MUST advertise its participation for a
particular Flex-Algorithm for each application specifically.
11.1. Advertisement of Node Participation for Segment Routing
[RFC8667], [RFC8665], and [RFC8666] (IGP Segment Routing extensions)
describe how the SR-Algorithm is used to compute the IGP best path.
Routers advertise the support for the SR-Algorithm as a node
capability as described in the above mentioned IGP Segment Routing
extensions. To advertise participation for a particular Flex-
Algorithm for Segment Routing, including both SR MPLS and SRv6, the
Flex-Algorithm value MUST be advertised in the SR-Algorithm TLV
(OSPF) or sub-TLV (ISIS).
Segment Routing Flex-Algorithm participation advertisement is
topology independent. When a router advertises participation in an
SR-Algorithm, the participation applies to all topologies in which
the advertising node participates.
11.2. Advertisement of Node Participation for Other Applications
This section describes considerations related to how other
applications can advertise their participation in a specific Flex-
Algorithm.
Application-specific Flex-Algorithm participation advertisements MAY
be topology specific or MAY be topology independent, depending on the
application itself.
Application-specific advertisement for Flex-Algorithm participation
MUST be defined for each application and is outside of the scope of
this document.
12. Advertisement of Link Attributes for Flex-Algorithm
Various link attributes may be used during the Flex-Algorithm path
calculation. For example, include or exclude rules based on link
Psenak, et al. Expires November 27, 2021 [Page 24]
Internet-Draft IGP Flexible Algorithm May 2021
affinities can be part of the Flex-Algorithm definition as defined in
Section 6 and Section 7.
Link attribute advertisements that are to be used during Flex-
Algorithm calculation MUST use the Application-Specific Link
Attribute (ASLA) advertisements defined in [RFC8919] or [RFC8920],
unless, in the case of IS-IS, the L-Flag is set in the ASLA
advertisement. If the L-Flag is set, as defined in [RFC8919]
Section 4.2 subject to the constraints discussed in Section 6 of the
[[RFC8919], then legacy advertisements are to be used instead.
The mandatory use of ASLA advertisements applies to link attributes
specifically mentioned in this document (Min Unidirectional Link
Delay, TE Default Metric, Administrative Group, Extended
Administrative Group and Shared Risk Link Group) and any other link
attributes that may be used in support of Flex-Algorithm in the
future.
A new Application Identifier Bit is defined to indicate that the ASLA
advertisement is associated with the Flex-Algorithm application.
This bit is set in the Standard Application Bit Mask (SABM) defined
in [RFC8919] or [RFC8920]:
Bit-3: Flexible Algorithm (X-bit)
ASLA Admin Group Advertisements to be used by the Flexible Algorithm
Application MAY use either the Administrative Group or Extended
Administrative Group encodings. If the Administrative Group encoding
is used, then the first 32 bits of the corresponding FAD sub-TLVs are
mapped to the link attribute advertisements as specified in RFC 7308.
13. Calculation of Flexible Algorithm Paths
A router MUST be configured to participate in a given Flex-Algorithm
K and MUST select the FAD based on the rules defined in Section 5.3
before it can compute any path for that Flex-Algorithm.
As described in Section 11, participation for any particular Flex-
Algorithm MUST be advertised on a per-application basis. Calculation
of the paths for any particular Flex-Algorithm MUST be application
specific.
The way applications handle nodes that do not participate in
Flexible-Algorithm is application specific. If the application only
wants to consider participating nodes during the Flex-Algorithm
calculation, then when computing paths for a given Flex-Algorithm,
all nodes that do not advertise participation for that Flex-Algorithm
in their application-specific advertisements MUST be pruned from the
Psenak, et al. Expires November 27, 2021 [Page 25]
Internet-Draft IGP Flexible Algorithm May 2021
topology. Segment Routing, including both SR MPLS and SRv6, are
applications that MUST use such pruning when computing Flex-Algorithm
paths.
When computing the path for a given Flex-Algorithm, the metric-type
that is part of the Flex-Algorithm definition (Section 5) MUST be
used.
When computing the path for a given Flex-Algorithm, the calculation-
type that is part of the Flex-Algorithm definition (Section 5) MUST
be used.
Various link include or exclude rules can be part of the Flex-
Algorithm definition. To refer to a particular bit within an AG or
EAG we use the term 'color'.
Rules, in the order as specified below, MUST be used to prune links
from the topology during the Flex-Algorithm computation.
For all links in the topology:
1. Check if any exclude AG 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 is set, the link MUST be pruned from the computation.
2. Check if any exclude SRLG rule is part of the Flex-Algorithm
definition. If such exclude rule exists, check if the link is
part of any SRLG that is also part of the SRLG exclude rule. If
the link is part of such SRLG, the link MUST be pruned from the
computation.
3. Check if any include-any AG 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
no such color is set, the link MUST be pruned from the
computation.
4. Check if any include-all AG 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 all such colors are not set on the link, the link MUST be
pruned from the computation.
5. If the Flex-Algorithm definition uses other than IGP metric
(Section 5), and such metric is not advertised for the particular
link in a topology for which the computation is done, such link
Psenak, et al. Expires November 27, 2021 [Page 26]
Internet-Draft IGP Flexible Algorithm May 2021
MUST be pruned from the computation. A metric of value 0 MUST NOT
be assumed in such case.
13.1. Multi-area and Multi-domain Considerations
Any IGP Shortest Path Tree calculation is limited to a single area.
This applies to Flex-Algorithm calculations as well. Given that the
computing router does not have visibility of the topology of the next
areas or domain, the Flex-Algorithm specific path to an inter-area or
inter-domain prefix will be computed for the local area only. The
egress L1/L2 router (ABR in OSPF), or ASBR for inter-domain case,
will be selected based on the best path for the given Flex-Algorithm
in the local area and such egress ABR or ASBR router will be
responsible to compute the best Flex-Algorithm specific path over the
next area or domain. This may produce an end-to-end path, which is
sub-optimal based on Flex-Algorithm constraints. In cases where the
ABR or ASBR has no reachability to a prefix for a given Flex-
Algorithm in the next area or domain, the traffic may be dropped by
the ABR/ASBR.
To allow the optimal end-to-end path for an inter-area or inter-
domain prefix for any Flex-Algorithm to be computed, the FAPM has
been defined in Section 8 and Section 9. For external route
calculation for prefixes originated by ASBRs in remote areas in OSPF,
the FAAM has been defined in Section 10.2 for the ABR to indicate its
ASBR reachability along with the metric for the specific Flex-
Algorithm.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, an ABR or ASBR MUST include the FAPM (Section 8,
Section 9) when advertising the prefix, that is reachable in a given
Flex-Algorithm, between areas or domains. Such metric will be equal
to the metric to reach the prefix for that Flex-Algorithm in its
source area or domain. This is similar in nature to how the metric
is set when prefixes are advertised between areas or domains for the
default algorithm. When a prefix is unreachable in its source area
or domain in a specific Flex-Algorithm, then an ABR or ASBR MUST NOT
include the FAPM for that Flex-Algorithm when advertising the prefix
between areas or domains.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, the FAPM MUST be used during the calculation of
prefix reachability for the inter-area and external prefixes. If the
FAPM for the Flex-Algorithm is not advertised with the inter-area or
external prefix reachability advertisement, the prefix MUST be
considered as unreachable for that Flex-Algorithm. Similarly in the
case of OSPF, for ASBRs in remote areas, if the FAAM is not
advertised by the local ABR(s), the ASBR MUST be considered as
Psenak, et al. Expires November 27, 2021 [Page 27]
Internet-Draft IGP Flexible Algorithm May 2021
unreachable for that Flex-Algorithm and the external prefix
advertisements from such an ASBR are not considered for that Flex-
Algorithm.
Flex-Algorithm prefix metrics and the OSPF Flex-Algorithm ASBR
metrics MUST NOT be used during the Flex-Algorithm computation unless
the FAD selected based on the rules defined in Section 5.3 includes
the M-Flag, as described in (Section 6.4 or Section 7.4).
In the case of OSPF, when calculating external routes in a Flex-
Algorithm (with FAD selected includes the M-Flag) where the
advertising ASBR is in a remote area, the metric will be the sum of
the following:
o the FAPM for that Flex-Algorithm advertised with the external
route by the ASBR
o the metric to reach the ASBR for that Flex-Algorithm from the
local ABR i.e., the FAAM for that Flex-Algorithm advertised by the
ABR in the local area for that ASBR
o the Flex-Algorithm specific metric to reach the local ABR
This is similar in nature to how the metric is calculated for routes
learned from remote ASBRs in the default algorithm using the OSPFv2
Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA.
If the FAD selected based on the rules defined in Section 5.3 does
not includes the M-flag, then the IGP metrics associated with the
prefix reachability advertisements used by the base ISIS and OSPF
protocol MUST be used for the Flex-Algorithm route computation.
Similarly, in the case of external route calculations in OSPF, the
ASBR reachability is determined based on the base OSPFv2 Type 4
Summary LSA and the OSFPv3 Inter-Area-Router LSA.
It is NOT RECOMMENDED to use the Flex-Algorithm for inter-area or
inter-domain prefix reachability without the M-flag set. The reason
is that without the explicit Flex-Algorithm Prefix Metric
advertisement (and the Flex-Algorithm ASBR metric advertisement in
the case of OSPF external route calculation), it is not possible to
conclude whether the ABR or ASBR has reachability to the inter-area
or inter-domain prefix for a given Flex-Algorithm in the next area or
domain. Sending the Flex-Algoritm traffic for such prefix towards
the ABR or ASBR may result in traffic looping or black-holing.
During the route computation, it is possible for the Flex-Algorithm
specific metric to exceed the maximum value that can be stored in an
unsigned 32-bit variable. In such scenarios, the value MUST be
Psenak, et al. Expires November 27, 2021 [Page 28]
Internet-Draft IGP Flexible Algorithm May 2021
considered to be of value 4,294,967,295 during the computation and
advertised as such.
The FAPM MUST NOT be advertised with ISIS L1 or L2 intra-area, OSPFv2
intra-area, or OSPFv3 intra-area routes. If the FAPM is advertised
for these route-types, it MUST be ignored during the prefix
reachability calculation.
The M-flag in FAD is not applicable to prefixes advertised as SRv6
locators. The ISIS SRv6 Locator TLV
[I-D.ietf-lsr-isis-srv6-extensions] includes the Algorithm and Metric
fields. When the SRv6 Locator is advertised between areas or
domains, the metric field in the Locator TLV of ISIS MUST be used
irrespective of the M-flag in the FAD advertisement.
OSPF external and NSSA external prefix advertisements MAY include a
non-zero forwarding address in the prefix advertisements in the base
protocol. In such a scenario, the Flex-Algorithm specific
reachability of the external prefix is determined by Flex-Algorithm
specific reachability of the forwarding address.
In OSPF, the procedures for translation of NSSA external prefix
advertisements into external prefix advertisements performed by an
NSSA ABR [RFC3101] remain unchanged for Flex-Algorithm. An NSSA
translator MUST include the OSPF FAPM Sub-TLVs for all Flex-
Algorithms that are in the original NSSA external prefix
advertisement from the NSSA ASBR in the translated external prefix
advertisement generated by it regardless of its participation in
those Flex-Algorithms or its having reachability to the NSSA ASBR in
those Flex-Algorithms.
An area could become partitioned from the perspective of the Flex-
Algorithm due to the constraints and/or metric being used for it,
while maintaining the continuity in the algorithm 0. When that
happens, some destinations inside that area could become unreachable
in that Flex-Algorithm. These destinations will not be able to use
an inter-area path. This is the consequence of the fact that the
inter-area prefix reachability advertisement would not be available
for these intra-area destinations within the area. It is RECOMMENDED
to avoid such partitioning by providing enough redundancy inside the
area for each Flex-Algorithm being used.
14. Flex-Algorithm and Forwarding Plane
This section describes how Flex-Algorithm paths are used in
forwarding.
Psenak, et al. Expires November 27, 2021 [Page 29]
Internet-Draft IGP Flexible Algorithm May 2021
14.1. Segment Routing MPLS Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SR MPLS
forwarding.
Prefix SID advertisements include an SR-Algorithm value and, as such,
are associated with the specified SR-Algorithm. Prefix-SIDs are also
associated with a specific topology which is inherited from the
associated prefix reachability advertisement. When the algorithm
value advertised is a Flex-Algorithm value, the Prefix SID is
associated with paths calculated using that Flex-Algorithm in the
associated topology.
A Flex-Algorithm path MUST be installed in the MPLS forwarding plane
using the MPLS label that corresponds to the Prefix-SID that was
advertised for that Flex-algorithm. If the Prefix SID for a given
Flex-algorithm is not known, the Flex-Algorithm specific path cannot
be installed in the MPLS forwarding plane.
Traffic that is supposed to be routed via Flex-Algorithm specific
paths, MUST be dropped when there are no such paths available.
Loop Free Alternate (LFA) paths for a given Flex-Algorithm MUST be
computed using the same constraints as the calculation of the primary
paths for that Flex-Algorithm. LFA paths MUST only use Prefix-SIDs
advertised specifically for the given algorithm. LFA paths MUST NOT
use an Adjacency-SID that belongs to a link that has been pruned from
the Flex-Algorithm computation.
If LFA protection is being used to protect a given Flex-Algorithm
paths, all routers in the area participating in the given Flex-
Algorithm SHOULD advertise at least one Flex-Algorithm specific Node-
SID. These Node-SIDs are used to steer traffic over the LFA computed
backup path.
14.2. SRv6 Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SRv6
forwarding.
In SRv6 a node is provisioned with topology/algorithm specific
locators for each of the topology/algorithm pairs supported by that
node. Each locator is an aggregate prefix for all SIDs provisioned
on that node which have the matching topology/algorithm.
The SRv6 locator advertisement in ISIS
[I-D.ietf-lsr-isis-srv6-extensions] includes the MTID value that
associates the locator with a specific topology. SRv6 locator
Psenak, et al. Expires November 27, 2021 [Page 30]
Internet-Draft IGP Flexible Algorithm May 2021
advertisements also includes an Algorithm value that explicitly
associates the locator with a specific algorithm. When the algorithm
value advertised with a locator represents a Flex-Algorithm, the
paths to the locator prefix MUST be calculated using the specified
Flex-Algorithm in the associated topology.
Forwarding entries for the locator prefixes advertised in ISIS MUST
be installed in the forwarding plane of the receiving SRv6 capable
routers when the associated topology/algorithm is participating in
them. Forwarding entries for locators associated with Flex-
Algorithms in which the node is not participating MUST NOT be
installed in the forwarding plane.
When the locator is associated with a Flex-Algorithm, LFA paths to
the locator prefix MUST be calculated using such Flex-Algorithm in
the associated topology, to guarantee that they follow the same
constraints as the calculation of the primary paths. LFA paths MUST
only use SRv6 SIDs advertised specifically for the given Flex-
Algorithm.
If LFA protection is being used to protect locators associated with a
given Flex-Algorithm, all routers in the area participating in the
given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm
specific locator and END SID per node and one END.X SID for every
link that has not been pruned from such Flex-Algorithm computation.
These locators and SIDs are used to steer traffic over the LFA-
computed backup path.
14.3. Other Applications' Forwarding for Flex-Algorithm
Any application that wants to use Flex-Algorithm specific forwarding
needs to install some form of Flex-Algorithm specific forwarding
entries.
Application-specific forwarding for Flex-Algorithm MUST be defined
for each application and is outside of the scope of this document.
15. Operational Considerations
15.1. Inter-area Considerations
The scope of the FA computation is an area, so is the scope of the
FAD. In ISIS, the Router Capability TLV in which the FAD Sub-TLV is
advertised MUST have the S-bit clear, which prevents it to be flooded
outside of the level in which it was originated. Even though in OSPF
the FAD Sub-TLV can be flooded in an RI LSA that has AS flooding
scope, the FAD selection is performed for each individual area in
which it is being used.
Psenak, et al. Expires November 27, 2021 [Page 31]
Internet-Draft IGP Flexible Algorithm May 2021
There is no requirement for the FAD for a particular Flex-Algorithm
to be identical in all areas in the network. For example, traffic
for the same Flex-Algorithm may be optimized for minimal delay (e.g.,
using delay metric) in one area or level, while being optimized for
available bandwidth (e.g., using IGP metric) in another area or
level.
As described in Section 5.1, ISIS allows the re-generation of the
winning FAD from level 2, without any modification to it, into a
level 1 area. This allows the operator to configure the FAD in one
or multiple routers in the level 2, without the need to repeat the
same task in each level 1 area, if the intent is to have the same FAD
for the particular Flex-Algorithm across all levels. This can
similarly be achieved in OSPF by using the AS flooding scope of the
RI LSA in which the FAD Sub-TLV for the particular Flex-Algoritm is
advertised.
Re-generation of FAD from a level 1 area to the level 2 area is not
supported in ISIS, so if the intent is to regenerate the FAD between
ISIS levels, the FAD MUST be defined on router(s) that are in level
2. In OSPF, the FAD definition can be done in any area and be
propagated to all routers in the OSPF routing domain by using the AS
flooding scope of the RI LSA.
15.2. Usage of SRLG Exclude Rule with Flex-Algorithm
There are two different ways in which SRLG information can be used
with Flex-Algorithm:
In a context of a single Flex-Algorithm, it can be used for
computation of backup paths, as described in
[I-D.ietf-rtgwg-segment-routing-ti-lfa]. This usage does not
require association of any specific SRLG constraint with the given
Flex-Algorithm definition.
In the context of multiple Flex-Algorithms, it can be used for
creating disjoint sets of paths by pruning the links belonging to
a specific SRLG from the topology on which a specific Flex-
Algorithm computes its paths. This usage:
Facilitates the usage of already deployed SRLG configurations
for setup of disjoint paths between two or more Flex-
Algorithms.
Requires explicit association of a given Flex-Algorithm with a
specific set of SRLG constraints as defined in Section 6.5 and
Section 7.5.
Psenak, et al. Expires November 27, 2021 [Page 32]
Internet-Draft IGP Flexible Algorithm May 2021
The two usages mentioned above are orthogonal.
15.3. Max-metric consideration
Both ISIS and OSPF have a mechanism to set the IGP metric on a link
to a value that would make the link either non-reachable or to serve
as the link of last resort. Similar functionality would be needed
for the Min Unidirectional Link Delay and TE metric, as these can be
used to compute Flex-Algorithm paths.
The link can be made un-reachable for all Flex-Algorithms that use
Min Unidirectional Link Delay as metric, as described in Section 5.1,
by removing the Flex-Algorithm ASLA Min Unidirectional Link Delay
advertisement for the link. The link can be made the link of last
resort by setting the delay value in the Flex-Algorithm ASLA delay
advertisement for the link to the value of 16,777,215 (2^24 - 1).
The link can be made un-reachable for all Flex-Algorithms that use TE
metric, as described in Section 5.1, by removing the Flex-Algorithm
ASLA TE metric advertisement for the link. The link can be made the
link of last resort by setting the TE metric value in the Flex-
Algorithm ASLA delay advertisement for the link to the value of (2^24
- 1) in ISIS and (2^32 - 1) in OSPF.
16. Backward Compatibility
This extension brings no new backward compatibility issues. ISIS,
OSPFv2 and OSPFv3 all have well defined handling of unrecognized TLVs
and sub-TLVs that allows the introduction of the new extensions,
similar to those defined here, without introducing any
interoperability issues.
17. Security Considerations
This draft adds two new ways to disrupt IGP networks:
An attacker can hijack a particular Flex-Algorithm by advertising
a FAD with a priority of 255 (or any priority higher than that of
the legitimate nodes).
An attacker could make it look like a router supports a particular
Flex-Algorithm when it actually doesn't, or vice versa.
Both of these attacks can be addressed by the existing security
extensions as described in [RFC5304] and [RFC5310] for ISIS, in
[RFC2328] and [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552]
for OSPFv3.
Psenak, et al. Expires November 27, 2021 [Page 33]
Internet-Draft IGP Flexible Algorithm May 2021
18. IANA Considerations
18.1. IGP IANA Considerations
18.1.1. IGP Algorithm Types Registry
This document makes the following registrations in the "IGP Algorithm
Types" registry:
Type: 128-255.
Description: Flexible Algorithms.
Reference: This document (Section 4).
18.1.2. IGP Metric-Type Registry
IANA is requested to set up a registry called "IGP Metric-Type
Registry" under an "Interior Gateway Protocol (IGP) Parameters" IANA
registries. The registration policy for this registry is "Standards
Action" ([RFC8126] and [RFC7120]).
Values in this registry come from the range 0-255.
This document registers following values in the "IGP Metric-Type
Registry":
Type: 0
Description: IGP metric
Reference: This document (Section 5.1)
Type: 1
Description: Min Unidirectional Link Delay as defined in
[RFC8570], section 4.2, and [RFC7471], section 4.2.
Reference: This document (Section 5.1)
Type: 2
Description: Traffic Engineering Default Metric as defined in
[RFC5305], section 3.7, and Traffic engineering metric as defined
in [RFC3630], section 2.5.5
Reference: This document (Section 5.1)
Psenak, et al. Expires November 27, 2021 [Page 34]
Internet-Draft IGP Flexible Algorithm May 2021
18.2. Flexible Algorithm Definition Flags Registry
IANA is requested to set up a registry called "ISIS Flexible
Algorithm Definition Flags Registry" under an "Interior Gateway
Protocol (IGP) Parameters" IANA registries. The registration policy
for this registry is "Standards Action" ([RFC8126] and [RFC7120]).
This document defines the following single bit in Flexible Algorithm
Definition Flags registry:
Bit # Name
----- ------------------------------
0 Prefix Metric Flag (M-flag)
Reference: This document (Section 6.4, Section 7.4).
18.3. ISIS IANA Considerations
18.3.1. Sub TLVs for Type 242
This document makes the following registrations in the "sub-TLVs for
TLV 242" registry.
Type: 26.
Description: Flexible Algorithm Definition.
Reference: This document (Section 5.1).
18.3.2. Sub TLVs for for TLVs 135, 235, 236, and 237
This document makes the following registrations in the "Sub-TLVs for
for TLVs 135, 235, 236, and 237" registry.
Type: 6
Description: Flexible Algorithm Prefix Metric.
Reference: This document (Section 8).
18.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
This document creates the following Sub-Sub-TLV Registry:
Registry: Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Registration Procedure: Expert review
Psenak, et al. Expires November 27, 2021 [Page 35]
Internet-Draft IGP Flexible Algorithm May 2021
Reference: This document (Section 5.1)
This document defines the following Sub-Sub-TLVs in the "Sub-Sub-TLVs
for Flexible Algorithm Definition Sub-TLV" registry:
Type: 1
Description: Flexible Algorithm Exclude Admin Group
Reference: This document (Section 6.1).
Type: 2
Description: Flexible Algorithm Include-Any Admin Group
Reference: This document (Section 6.2).
Type: 3
Description: Flexible Algorithm Include-All Admin Group
Reference: This document (Section 6.3).
Type: 4
Description: Flexible Algorithm Definition Flags
Reference: This document (Section 6.4).
Type: 5
Description: Flexible Algorithm Exclude SRLG
Reference: This document (Section 6.5).
18.4. OSPF IANA Considerations
18.4.1. OSPF Router Information (RI) TLVs Registry
This specification updates the OSPF Router Information (RI) TLVs
Registry.
Type: 16
Description: Flexible Algorithm Definition TLV.
Reference: This document (Section 5.2).
Psenak, et al. Expires November 27, 2021 [Page 36]
Internet-Draft IGP Flexible Algorithm May 2021
18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs
This document makes the following registrations in the "OSPFv2
Extended Prefix TLV Sub-TLVs" registry.
Type: 3
Description: Flexible Algorithm Prefix Metric.
Reference: This document (Section 9).
18.4.3. OSPFv3 Extended-LSA Sub-TLVs
This document makes the following registrations in the "OSPFv3
Extended-LSA Sub-TLVs" registry.
Type: 26
Description: Flexible Algorithm Prefix Metric.
Reference: This document (Section 9).
Type: TBD (suggested value 30)
Description: OSPF Flexible Algorithm ASBR Metric Sub-TLV
Reference: This document (Section 10.2).
18.4.4. OSPF Flex-Algorithm Prefix Metric Bits
This specification requests creation of "OSPF Flex-Algorithm Prefix
Metric Bits" registry under the OSPF Parameters Registry with the
following initial values.
Bit Number: 0
Description: E bit - External Type
Reference: this document.
The bits 1-7 are unassigned and the registration procedure to be
followed for this registry is IETF Review.
18.4.5. OSPF Opaque LSA Option Types
This document makes the following registrations in the "OSPF Opaque
LSA Option Types" registry.
Psenak, et al. Expires November 27, 2021 [Page 37]
Internet-Draft IGP Flexible Algorithm May 2021
Value: TBD (suggested value 11)
Description: OSPFv2 Extended Inter-Area ASBR LSA
Reference: This document (Section 10.1).
18.4.6. OSPFv2 Externded Inter-Area ASBR TLVs
This specification requests creation of "OSPFv2 Extended Inter-Area
ASBR TLVs" registry under the OSPFv2 Parameters Registry with the
following initial values.
Value: 1
Description : Extended Inter-Area ASBR TLV
Reference: this document
The values 2 to 32767 are unassigned, values 32768 to 33023 are
reserved for experimental use while the values 0 and 33024 to 65535
are reserved. The registration procedure to be followed for this
registry is IETF Review or IESG Approval.
18.4.7. OSPFv2 Inter-Area ASBR Sub-TLVs
This specification requests creation of "OSPFv2 Extended Inter-Area
ASBR Sub-TLVs" registry under the OSPFv2 Parameters Registry with the
following initial values.
Value: 1
Description : OSPF Flexible Algorithm ASBR Metric Sub-TLV
Reference: this document
The values 2 to 32767 are unassigned, values 32768 to 33023 are
reserved for experimental use while the values 0 and 33024 to 65535
are reserved. The registration procedure to be followed for this
registry is IETF Review or IESG Approval.
18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
This document creates the following registry:
Registry: OSPF Flexible Algorithm Definition TLV sub-TLV
Registration Procedure: Expert review
Psenak, et al. Expires November 27, 2021 [Page 38]
Internet-Draft IGP Flexible Algorithm May 2021
Reference: This document (Section 5.2)
The "OSPF Flexible Algorithm Definition TLV sub-TLV" registry will
define sub-TLVs at any level of nesting for the Flexible Algorithm
TLV and should be added to the "Open Shortest Path First (OSPF)
Parameters" registries group. New values can be allocated via IETF
Review or IESG Approval.
This document registers following Sub-TLVs in the "TLVs for Flexible
Algorithm Definition TLV" registry:
Type: 1
Description: Flexible Algorithm Exclude Admin Group
Reference: This document (Section 7.1).
Type: 2
Description: Flexible Algorithm Include-Any Admin Group
Reference: This document (Section 7.2).
Type: 3
Description: Flexible Algorithm Include-All Admin Group
Reference: This document (Section 7.3).
Type: 4
Description: Flexible Algorithm Definition Flags
Reference: This document (Section 7.4).
Type: 5
Description: Flexible Algorithm Exclude SRLG
Reference: This document (Section 7.5).
Types in the range 32768-33023 are for experimental use; these will
not be registered with IANA, and MUST NOT be mentioned by RFCs.
Types in the range 33024-65535 are not to be assigned at this time.
Before any assignments can be made in the 33024-65535 range, there
MUST be an IETF specification that specifies IANA Considerations that
covers the range being assigned.
Psenak, et al. Expires November 27, 2021 [Page 39]
Internet-Draft IGP Flexible Algorithm May 2021
18.4.9. Link Attribute Applications Registry
This document registers following bit in the Link Attribute
Applications Registry:
Bit-3
Description: Flexible Algorithm (X-bit)
Reference: This document (Section 12).
19. Acknowledgements
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.
Thanks to Eric Rosen, Tony Przygienda, William Britto A J, Gunter Van
De Velde, Dirk Goethals, Manju Sivaji and, Baalajee S for their
detailed review and excellent comments.
Thanks to Cengiz Halit for his review and feedback during initial
phase of the solution definition.
Thanks to Kenji Kumaki for his comments.
Thanks to Acee Lindem for editorial comments.
20. References
20.1. Normative References
[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extension to Support Segment Routing over
IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-14
(work in progress), April 2021.
[ISO10589]
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.
Psenak, et al. Expires November 27, 2021 [Page 40]
Internet-Draft IGP Flexible Algorithm May 2021
[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>.
[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
[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>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[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>.
[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>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
Psenak, et al. Expires November 27, 2021 [Page 41]
Internet-Draft IGP Flexible Algorithm May 2021
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
December 2019, <https://www.rfc-editor.org/info/rfc8666>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[RFC8919] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
RFC 8919, DOI 10.17487/RFC8919, October 2020,
<https://www.rfc-editor.org/info/rfc8919>.
[RFC8920] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020,
<https://www.rfc-editor.org/info/rfc8920>.
20.2. Informative References
[I-D.gulkohegde-routing-planes-using-sr]
Hegde, S. and A. Gulko, "Separating Routing Planes using
Segment Routing", draft-gulkohegde-routing-planes-using-
sr-00 (work in progress), March 2017.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", draft-ietf-rtgwg-segment-
routing-ti-lfa-06 (work in progress), February 2021.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 3101, DOI 10.17487/RFC3101, January 2003,
<https://www.rfc-editor.org/info/rfc3101>.
Psenak, et al. Expires November 27, 2021 [Page 42]
Internet-Draft IGP Flexible Algorithm May 2021
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[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>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[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>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
Psenak, et al. Expires November 27, 2021 [Page 43]
Internet-Draft IGP Flexible Algorithm May 2021
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Apollo Business Center
Mlynske nivy 43
Bratislava, 82109
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde
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
Ketan Talaulikar
Cisco Systems, Inc.
S.No. 154/6, Phase I, Hinjawadi
PUNE, MAHARASHTRA 411 057
India
Email: ketant@cisco.com
Psenak, et al. Expires November 27, 2021 [Page 44]
Internet-Draft IGP Flexible Algorithm May 2021
Arkadiy Gulko
Edward Jones
Email: arkadiy.gulko@edwardjones.com
Psenak, et al. Expires November 27, 2021 [Page 45]