RFC 9351 PROPOSED STANDARD ROUTING · BGP-LS · SEGMENT ROUTING
BGP-LS Flex-Algo: Border Gateway Protocol - Link State (BGP-LS) Extensions for Flexible Algorithm Advertisement
New BGP-LS TLVs that let controllers learn how routers define their Flexible Algorithms.
RFC EDITOR
Talaulikar, K., Ed., Psenak, P., Zandi, S., and G. Dawra, "Border Gateway Protocol - Link State (BGP-LS) Extensions for Flexible Algorithm Advertisement", RFC 9351, DOI 10.17487/RFC9351, February 2023, <https://www.rfc-editor.org/info/rfc9351>.
BIBTEX
@misc{rfc9351,
series = {Request for Comments},
number = 9351,
howpublished = {RFC 9351},
publisher = {RFC Editor},
doi = {10.17487/RFC9351},
url = {https://www.rfc-editor.org/info/rfc9351},
author = {K. Talaulikar and P. Psenak and S. Zandi and G. Dawra},
title = {{Border Gateway Protocol - Link State (BGP-LS) Extensions for Flexible Algorithm Advertisement}},
pagetotal = 14,
year = 2023,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) K. Talaulikar, Ed.
Request for Comments: 9351 P. Psenak
Category: Standards Track Cisco Systems
ISSN: 2070-1721 S. Zandi
G. Dawra
LinkedIn
February 2023
Border Gateway Protocol - Link State (BGP-LS) Extensions for Flexible
Algorithm Advertisement
Abstract
Flexible Algorithm is a solution that allows some routing protocols
(e.g., OSPF and IS-IS) to compute paths over a network based on user-
defined (and hence, flexible) constraints and metrics. The
computation is performed by routers participating in the specific
network in a distributed manner using a Flexible Algorithm Definition
(FAD). This definition is provisioned on one or more routers and
propagated through the network by OSPF and IS-IS flooding.
Border Gateway Protocol - Link State (BGP-LS) enables the collection
of various topology information from the network. This document
defines extensions to the BGP-LS address family to advertise the FAD
as a part of the topology information from the network.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9351.
Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. Overview of BGP-LS Extensions for Flexible Algorithm
3. Flexible Algorithm Definition TLV
3.1. Flexible Algorithm Exclude-Any Affinity Sub-TLV
3.2. Flexible Algorithm Include-Any Affinity Sub-TLV
3.3. Flexible Algorithm Include-All Affinity Sub-TLV
3.4. Flexible Algorithm Definition Flags Sub-TLV
3.5. Flexible Algorithm Exclude SRLG Sub-TLV
3.6. Flexible Algorithm Unsupported Sub-TLV
4. Flexible Algorithm Prefix Metric TLV
5. IANA Considerations
6. Manageability Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The classical IGP (e.g., OSPF and IS-IS) computation of best paths
over the network is based on the IGP metric assigned to the links in
the network. Many network deployments use solutions based on RSVP-TE
[RFC3209] or Segment Routing (SR) Policy [RFC8402] to enforce traffic
over a path that is computed using different metrics or constraints
than the shortest IGP path. [RFC9350] defines the Flexible Algorithm
solution that allows IGPs themselves to compute constraint-based
paths over the network.
Flexible Algorithm is called so because it allows a user the
flexibility to define:
* the type of calculation to be used (e.g., shortest path),
* the metric type to be used (e.g., IGP metric or TE metric), and
* the set of constraints to be used (e.g., inclusion or exclusion of
certain links using affinities).
The operations of the IGP Flexible Algorithm solution are described
in detail in [RFC9350].
The BGP-LS extensions for SR are defined in [RFC9085] and
[IDR-BGPLS-SRV6-EXT] for SR-MPLS and Segment Routing over IPv6
(SRv6), respectively. They include the extensions for advertisement
of SR information including various types of Segment Identifiers
(SIDs) as below:
* SR Algorithm TLV to indicate the participation of a node in a
Flexible Algorithm computation
* Prefix-SID TLV to indicate the association of the Prefix-SIDs to a
specific Flexible Algorithm for SR-MPLS forwarding
* SRv6 Locator TLV to indicate the Locator for a specific Flexible
Algorithm for SRv6 forwarding
This document defines extensions to BGP-LS for the advertisement of
the Flexible Algorithm Definition (FAD) information to enable
learning of the mapping of the Flexible Algorithm number to its
definition in each area/domain of the underlying IGP. This
definition indicates the type of computation used and the constraints
for a given Flexible Algorithm. This information can then be used
for setting up SR Policy paths end to end across domains by using the
appropriate Flexible-Algorithm-specific SIDs in its segment list
[RFC9256]. For example, picking the Flexible Algorithm Prefix-SID
(in case of SR-MPLS) or End SID (in case of SRv6) of Area Border
Routers (ABRs) or Autonomous System Border Routers (ASBRs)
corresponding to a definition that optimizes on the delay metric
enables the building of an end-to-end low-latency path across IGP
domains with minimal SIDs in the SID list.
1.1. 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.
2. Overview of BGP-LS Extensions for Flexible Algorithm
BGP-LS [RFC7752] specifies the Node Network Layer Reachability
Information (NLRI) for the advertisement of nodes, along with their
attributes using the BGP-LS Attribute; the Link NLRI for the
advertisement of links, along with their attributes using the BGP-LS
Attribute; and the Prefix NLRI for the advertisement of prefixes,
along with their attributes using the BGP-LS Attribute.
The FADs advertised by a node are considered as a node-level
attribute and advertised as specified in Section 3.
Various link attributes, like affinities and Shared Risk Link Group
(SRLG), that are used during the Flexible Algorithm route
calculations in IS-IS and OSPF are advertised in those protocols
using the Application-Specific Link Attribute (ASLA) advertisements,
as described in [RFC8919], [RFC8920], and [RFC9350]. The BGP-LS
extensions for ASLA advertisements are specified in [RFC9294].
The Flexible Algorithm Prefix Metric (FAPM) is considered as a prefix
attribute and advertised as specified in Section 4.
3. Flexible Algorithm Definition TLV
This document defines a new optional BGP-LS Attribute TLV associated
with the Node NLRI called the "Flexible Algorithm Definition TLV"
("FAD TLV" for short), and its format is as follows:
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 Algo | Metric-Type | Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLVs ... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Flexible Algorithm Definition TLV
where:
Type: 1039
Length: The total length of the value field (including any sub-
TLVs) in octets. The length value MUST be 4 or larger.
Flexible Algorithm (Flex Algo): Single octet value carrying the
Flexible Algorithm number between 128 and 255 inclusive, as
defined in [RFC9350].
Metric-Type: Single octet value carrying the metric type, as
defined in [RFC9350].
Calc-Type: Single octet value carrying the calculation type, as
defined in [RFC9350].
Priority: Single octet value carrying the priority of the FAD
advertisement, as defined in [RFC9350].
sub-TLVs: Zero or more sub-TLVs may be included, as described
further in this section.
The FAD TLV that is advertised in the BGP-LS Attribute along with the
Node NLRI of a node is derived from the following IGP protocol-
specific advertisements:
* in the case of IS-IS, from the IS-IS Flexible Algorithm Definition
sub-TLV in [RFC9350]
* in the case of OSPFv2/OSPFv3, from the OSPF Flexible Algorithm
Definition TLV in [RFC9350]
The BGP-LS Attribute associated with a Node NLRI may include one or
more FAD TLVs corresponding to the FAD for each algorithm that the
particular node is advertising.
The following subsections define sub-TLVs of the FAD TLV.
3.1. Flexible Algorithm Exclude-Any Affinity Sub-TLV
The Flexible Algorithm Exclude-Any Affinity sub-TLV is an optional
sub-TLV that is used to carry the affinity constraints associated
with the FAD and enable the exclusion of links carrying any of the
specified affinities from the computation of the specific algorithm,
as described in [RFC9350]. The affinity is expressed in terms of the
Extended Admin Group (EAG), as defined in [RFC7308].
The 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Exclude-Any EAG (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Flexible Algorithm Exclude-Any Affinity Sub-TLV
where:
Type: 1040
Length: The total length of the value field in octets dependent
on the size of the EAG. It MUST be a non-zero value and a
multiple of 4.
Exclude-Any EAG: The EAG value, as defined in [RFC9350].
The information in the Flexible Algorithm Exclude-Any Affinity sub-
TLV is derived from the IS-IS and OSPF protocol-specific Flexible
Algorithm Exclude Admin Group sub-TLV, as defined in [RFC9350].
3.2. Flexible Algorithm Include-Any Affinity Sub-TLV
The Flexible Algorithm Include-Any Affinity sub-TLV is an optional
sub-TLV that is used to carry the affinity constraints associated
with the FAD and enable the inclusion of links carrying any of the
specified affinities in the computation of the specific algorithm, as
described in [RFC9350]. The affinity is expressed in terms of the
EAG, as defined in [RFC7308].
The 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Include-Any EAG (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Flexible Algorithm Include-Any Affinity Sub-TLV
where:
Type: 1041
Length: The total length of the value field in octets dependent
on the size of the EAG. It MUST be a non-zero value and a
multiple of 4.
Include-Any EAG: The EAG value, as defined in [RFC9350].
The information in the Flexible Algorithm Include-Any Affinity sub-
TLV is derived from the IS-IS and OSPF protocol-specific Flexible
Algorithm Include-Any Admin Group sub-TLV, as defined in [RFC9350].
3.3. Flexible Algorithm Include-All Affinity Sub-TLV
The Flexible Algorithm Include-All Affinity sub-TLV is an optional
sub-TLV that is used to carry the affinity constraints associated
with the FAD and enable the inclusion of links carrying all of the
specified affinities in the computation of the specific algorithm, as
described in [RFC9350]. The affinity is expressed in terms of the
EAG, as defined in [RFC7308].
The 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Include-All EAG (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Flexible Algorithm Include-All Affinity Sub-TLV
where:
Type: 1042
Length: The total length of the value field in octets dependent
on the size of the EAG. It MUST be a non-zero value and a
multiple of 4.
Include-All EAG: The EAG value, as defined in [RFC9350].
The information in the Flexible Algorithm Include-All Affinity sub-
TLV is derived from the IS-IS and OSPF protocol-specific Flexible
Algorithm Include-All Admin Group sub-TLV, as defined in [RFC9350].
3.4. Flexible Algorithm Definition Flags Sub-TLV
The Flexible Algorithm Definition Flags sub-TLV is an optional sub-
TLV that is used to carry the flags associated with the FAD that are
used in the computation of the specific algorithm, as described in
[RFC9350].
The 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Flexible Algorithm Definition Flags Sub-TLV
where:
Type: 1043
Length: The total length of the value field in octets dependent
on the size of the flags. It MUST be a non-zero value and a
multiple of 4.
Flags: The bitmask used to represent the flags for the FAD, as
defined in [RFC9350].
The information in the Flexible Algorithm Definition Flags sub-TLV is
derived from the IS-IS and OSPF protocol-specific Flexible Algorithm
Definition Flags sub-TLV, as defined in [RFC9350].
3.5. Flexible Algorithm Exclude SRLG Sub-TLV
The Flexible Algorithm Exclude SRLG sub-TLV is an optional sub-TLV
that is used to carry the Shared Risk Link Group (SRLG) information
associated with the FAD and enable the exclusion of links that are
associated with any of the specified SRLG in the computation of the
specific algorithm, as described in [RFC9350]. The SRLGs associated
with a link are carried in the BGP-LS Shared Risk Link Group (TLV
1096) [RFC7752].
The 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Values (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Flexible Algorithm Exclude SRLG Sub-TLV
where:
Type: 1045
Length: The total length of the value field in octets dependent
on the number of SRLG values carried. It MUST be a non-zero
value and a multiple of 4.
Shared Risk Link Group Values: One or more SRLG values, each with
a size of 4 octets, as defined in [RFC9350].
The information in the Flexible Algorithm Exclude SRLG sub-TLV is
derived from the IS-IS and OSPF protocol-specific Flexible Algorithm
Exclude SRLG sub-TLV, as defined in [RFC9350].
3.6. Flexible Algorithm Unsupported Sub-TLV
The OSPF and IS-IS signaling for FAD allows for extensions via new
sub-TLVs under the respective IGP's Flexible Algorithm Definition
TLV. As specified in Section 5.3 of [RFC9350], it is important that
the entire FAD be understood by anyone using it for computation
purposes. Therefore, the FAD is different from most other protocol
extensions, where the skipping or ignoring of unsupported sub-TLV
information does not affect the base behavior.
The Flexible Algorithm Unsupported sub-TLV is an optional sub-TLV
that is used to indicate the presence of unsupported FAD sub-TLVs.
The need for this sub-TLV arises when the BGP-LS implementation on
the advertising node does not support one or more of the FAD sub-TLVs
present in the IGP advertisement.
The 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol-ID | sub-TLV types (variable) ... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Flexible Algorithm Unsupported Sub-TLV
where:
Type: 1046
Length: The total length of the value field in octets (including
any included sub-TLV types).
Protocol-ID: Indicates the BGP-LS Protocol-ID of the protocol
from which the FAD is being advertised via BGP-LS. The values
are from the IANA "BGP-LS Protocol-IDs" subregistry under the
"Border Gateway Protocol - Link State (BGP-LS) Parameters"
registry <https://www.iana.org/assignments/bgp-ls-parameters/>.
sub-TLV types: Zero or more sub-TLV types that are not supported
by the node originating the BGP-LS advertisement. The size of
each sub-TLV type depends on the protocol indicated by the
Protocol-ID field. For example, for IS-IS, each sub-TLV type
would be 1 octet in size, while for OSPF, each sub-TLV type
would be 2 octets in size.
The node originating the advertisement MUST include the Flexible
Algorithm Unsupported sub-TLV when it comes across an unsupported
sub-TLV in the corresponding FAD in the IS-IS and OSPF advertisement.
When advertising the Flexible Algorithm Unsupported sub-TLV, the
protocol-specific sub-TLV types that are not supported SHOULD be
included. This information serves as a diagnostic aid.
The discussion on the use of the FAD information by the consumers of
the BGP-LS information is beyond the scope of this document.
However, it is RECOMMENDED that the choice of the node used for
originating the IGP topology information into BGP-LS be made such
that the advertising node supports all the FAD extensions in use in
its part of the network. This avoids the scenario where an
incomplete FAD gets advertised via BGP-LS.
4. Flexible Algorithm Prefix Metric TLV
This document defines a new optional BGP-LS Attribute TLV associated
with the Prefix NLRI called the "Flexible Algorithm Prefix Metric TLV
("FAPM TLV" for short), and its format is as follows:
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 Algo | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Flexible Algorithm Prefix Metric TLV
where:
Type: 1044
Length: 8 octets
Flexible Algorithm (Flex Algo): Single octet value carrying the
Flexible Algorithm number between 128 and 255 inclusive, as
defined in [RFC9350].
Flags: Single octet value and only applicable for OSPF, as
defined in [RFC9350]. The value MUST be set to 0 for IS-IS.
Reserved: 2-octet value that MUST be set to 0 by the originator
and MUST be ignored by the receiver.
Metric: 4-octet field to carry the metric information.
The FAPM TLV that is advertised in the BGP-LS Attribute along with
the Prefix NLRI from a node is derived from the following IGP
protocol-specific advertisements:
* in the case of IS-IS, from the IS-IS Flexible Algorithm Prefix
Metric sub-TLV in [RFC9350]
* in the case of OSPFv2/OSPFv3, from the OSPF Flexible Algorithm
Prefix Metric sub-TLV in [RFC9350]
The BGP-LS Attribute associated with a Prefix NLRI may include one or
more FAPM TLVs corresponding to the Flexible Algorithm Prefix Metric
for each algorithm associated with that particular prefix.
5. IANA Considerations
IANA has allocated code points in the "BGP-LS Node Descriptor, Link
Descriptor, Prefix Descriptor, and Attribute TLVs" registry
<https://www.iana.org/assignments/bgp-ls-parameters> based on the
table below for the TLVs/sub-TLVs introduced by this document.
+================+=========================================+
| TLV Code Point | Description |
+================+=========================================+
| 1039 | Flexible Algorithm Definition |
+----------------+-----------------------------------------+
| 1040 | Flexible Algorithm Exclude-Any Affinity |
+----------------+-----------------------------------------+
| 1041 | Flexible Algorithm Include-Any Affinity |
+----------------+-----------------------------------------+
| 1042 | Flexible Algorithm Include-All Affinity |
+----------------+-----------------------------------------+
| 1043 | Flexible Algorithm Definition Flags |
+----------------+-----------------------------------------+
| 1044 | Flexible Algorithm Prefix Metric |
+----------------+-----------------------------------------+
| 1045 | Flexible Algorithm Exclude SRLG |
+----------------+-----------------------------------------+
| 1046 | Flexible Algorithm Unsupported |
+----------------+-----------------------------------------+
Table 1: Flexible Algorithm Code Points
6. Manageability Considerations
The new protocol extensions introduced in this document augment the
existing IGP topology information that can be distributed via
[RFC7752]. Procedures and protocol extensions defined in this
document do not affect the BGP protocol operations and management
other than what is discussed in the "Manageability Considerations"
section of [RFC7752]. Specifically, the malformed NLRIs attribute
tests in the "Fault Management" section of [RFC7752] now encompass
the new TLVs for the BGP-LS NLRI in this document.
The extensions specified in this document do not specify any new
configuration or monitoring aspects in BGP or BGP-LS. The
specification of BGP models is an ongoing work based on
[IDR-BGP-MODEL].
7. Security Considerations
Security considerations for acquiring and distributing BGP-LS
information are discussed in [RFC7752].
The TLVs introduced in this document are used to propagate the IGP
Flexible Algorithm extensions defined in [RFC9350]. It is assumed
that the IGP instances originating these TLVs will support all the
required security (as described in [RFC9350]) for Flexible Algorithm
deployment.
This document specifies extensions for the advertisement of node and
prefix-related Flexible Algorithm information. Tampering with this
Flexible-Algorithm-related information may affect applications using
it, including impacting route calculation and programming. As the
advertisements defined in this document are related to a specific
Flexible Algorithm topology, the impact of tampering is similarly
limited in scope.
8. References
8.1. Normative References
[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>.
[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>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[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>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.
8.2. Informative References
[IDR-BGP-MODEL]
Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
YANG Model for Service Provider Networks", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-model-15, 13
October 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-bgp-model-15>.
[IDR-BGPLS-SRV6-EXT]
Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
Bernier, D., and B. Decraene, "BGP Link State Extensions
for SRv6", Work in Progress, Internet-Draft, draft-ietf-
idr-bgpls-srv6-ext-13, 14 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
bgpls-srv6-ext-13>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[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>.
[RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085,
DOI 10.17487/RFC9085, August 2021,
<https://www.rfc-editor.org/info/rfc9085>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
[RFC9294] Talaulikar, K., Ed., Psenak, P., and J. Tantsura,
"Application-Specific Link Attributes Advertisement Using
the Border Gateway Protocol - Link State (BGP-LS)",
RFC 9294, DOI 10.17487/RFC9294, August 2022,
<https://www.rfc-editor.org/info/rfc9294>.
Acknowledgements
The authors would like to thank Les Ginsberg, Amalesh Maity,
Y. F. Siu, Vijay Gurbani, and Donald Eastlake 3rd for their reviews
and contributions to this work. The authors would like to thank Jie
Dong for his shepherd review. The authors would like to thank Alvaro
Retana for his detailed AD review and suggestions for improving this
document.
Authors' Addresses
Ketan Talaulikar (editor)
Cisco Systems
India
Email: ketant.ietf@gmail.com
Peter Psenak
Cisco Systems
Slovakia
Email: ppsenak@cisco.com
Shawn Zandi
LinkedIn
United States of America
Email: szandi@linkedin.com
Gaurav Dawra
LinkedIn
United States of America
Email: gdawra.ietf@gmail.com
AI ELI5
On this page
PROBLEMFlexible Algorithm lets OSPF and IS-IS compute paths using user-defined metrics and constraints, but the algorithm definitions live inside the IGP. Anything outside the IGP, such as a controller using BGP-LS, could not learn them.
MECHANISMThe RFC defines a Flexible Algorithm Definition TLV with six sub-TLVs for the Node NLRI, and a Flexible Algorithm Prefix Metric TLV for the Prefix NLRI, each derived from the matching IGP advertisement in RFC 9350.
WHY IT MATTERSBGP-LS consumers can map a Flexible Algorithm number to its definition in each IGP area or domain, which helps set up end-to-end SR Policy paths across domains.
§1Why BGP-LS Needs Flex-Algo Data
Classical IGP (OSPF and IS-IS) path computation is based on the IGP metric on links. Many networks use RSVP-TE (RFC 3209) or Segment Routing (SR) Policy (RFC 8402) to push traffic over paths computed with other metrics or constraints. RFC 9350 defines Flexible Algorithm, which lets the IGPs themselves compute constraint-based paths.
Flexible Algorithm lets a user choose the type of calculation (such as shortest path), the metric type (such as IGP metric or TE metric) and the constraints (such as including or excluding links by affinity).
BGP-LS already carries some SR information, including the SR Algorithm TLV, the Prefix-SID TLV and the SRv6 Locator TLV (defined in RFC 9085 for SR-MPLS and in an Internet-Draft for SRv6). What was missing is the Flexible Algorithm Definition (FAD) itself. This document adds it so a consumer can learn the mapping from algorithm number to definition in each area or domain.
With that, an SR Policy (RFC 9256) can be built end to end across domains. For example, choosing the Flex-Algo Prefix-SID or End SID of border routers for a delay-optimizing definition builds a low-latency path across IGP domains with few SIDs in the list.
Routers in each neighborhood agree on special travel rules, like 'fastest road only'. This RFC gives an outside planner a way to read those rules so it can plan trips across neighborhoods.
§2Where the New TLVs Attach
BGP-LS (RFC 7752) has Node, Link and Prefix NLRI, each with a BGP-LS Attribute. This document adds data to two of them. The FADs a node advertises are a node-level attribute and go with the Node NLRI. The Flexible Algorithm Prefix Metric (FAPM) is a prefix attribute and goes with the Prefix NLRI.
Link attributes used in Flex-Algo calculation, such as affinities and Shared Risk Link Groups (SRLG), are already advertised in the IGPs using Application-Specific Link Attribute (ASLA) advertisements. Their BGP-LS extensions are in RFC 9294, so this document does not repeat them.
All the new TLVs are optional and derived from the matching IS-IS or OSPF advertisements in RFC 9350.
The rules about a whole router go on the router's card, and the extra cost for reaching a place goes on that place's card.
§3The Flexible Algorithm Definition TLV
The FAD TLV (type 1039) is an optional BGP-LS Attribute TLV on the Node NLRI. Its length covers the value field including sub-TLVs and MUST be 4 or larger. After Type and Length come four single-octet fields: the Flex Algo number (between 128 and 255 inclusive), Metric-Type, Calc-Type and Priority, all as defined in RFC 9350. Zero or more sub-TLVs follow.
In IS-IS it is derived from the Flexible Algorithm Definition sub-TLV, and in OSPFv2 or OSPFv3 from the OSPF Flexible Algorithm Definition TLV. A node's attribute may hold several FAD TLVs, one per algorithm it advertises.
Six sub-TLVs are defined:
- Exclude-Any Affinity (1040): exclude links with any listed affinity.
- Include-Any Affinity (1041): include links with any listed affinity.
- Include-All Affinity (1042): include links with all listed affinities.
- Definition Flags (1043): a bitmask of FAD flags.
- Exclude SRLG (1045): exclude links in any listed SRLG.
- Unsupported (1046): lists sub-TLV types the originating node does not understand.
The three affinity sub-TLVs use the Extended Admin Group (RFC 7308) encoding. Their lengths, and those of Flags and SRLG, MUST be non-zero and a multiple of 4. SRLG values are 4 octets each.
The main card says which rule set this is. Extra stickers say which roads to avoid, which roads are required, and a warning sticker if the sender could not read some of the rules.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 1039 | 1039. |
LENGTH | Length | 2 bytes | Length of the value field including sub-TLVs; 4 or larger. | |
FLEX-ALGO | Flex Algo | 1 byte | Algorithm number, 128 to 255. | |
METRIC-TYPE | Metric-Type | 1 byte | Metric type as defined in RFC 9350. | |
CALC-TYPE | Calc-Type | 1 byte | Calculation type as defined in RFC 9350. | |
PRIORITY | Priority | 1 byte | Priority of the FAD advertisement. |
§4The Unsupported Sub-TLV
FAD is different from most extensions, where skipping an unknown sub-TLV is harmless. RFC 9350 says everyone using a FAD for computation must understand all of it. If the BGP-LS node cannot handle some sub-TLV in the IGP advertisement, it would otherwise pass on an incomplete definition.
The Flexible Algorithm Unsupported sub-TLV carries a Protocol-ID (the BGP-LS protocol the FAD came from) followed by the sub-TLV types that are not supported. A type is 1 octet for IS-IS and 2 octets for OSPF.
The originating node MUST include this sub-TLV when it meets an unsupported sub-TLV in the IGP FAD, and SHOULD list the unsupported types as a diagnostic aid. It is RECOMMENDED to originate IGP topology into BGP-LS from a node that supports all FAD extensions in use in its part of the network, to avoid an incomplete FAD being advertised. How consumers use FAD information is out of scope.
If the messenger cannot read part of a rule, it must say so on the note, so nobody thinks they have the whole rulebook when they do not.
§5Prefix Metric TLV and Code Points
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 1044 | 1044. |
LENGTH | Length | 2 bytes | 8 | 8 octets. |
FLEX-ALGO | Flex Algo | 1 byte | Algorithm number, 128 to 255. | |
FLAGS | Flags | 1 byte | OSPF only; 0 for IS-IS. | |
RESERVED | Reserved | 2 bytes | Set to 0 by sender, ignored by receiver. | |
METRIC | Metric | 4 bytes | The metric information. |
The Flexible Algorithm Prefix Metric TLV (FAPM, type 1044) attaches to the Prefix NLRI and has length 8 octets. Its fields are the Flex Algo number (128 to 255), Flags (single octet, only for OSPF, MUST be 0 for IS-IS), a 2-octet Reserved field (set to 0 by the sender, ignored by the receiver) and a 4-octet Metric. It comes from the IS-IS or OSPF Flexible Algorithm Prefix Metric sub-TLV in RFC 9350, and a prefix may carry one per algorithm.
IANA allocated code points 1039 through 1046 in the BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs registry, as the table in the viz lists.
Each place can have its own extra cost for each rule set, written as a number on its card.
- 1039Flexible Algorithm Definition — Node attribute TLV.
- 1040Exclude-Any Affinity — Sub-TLV.
- 1041Include-Any Affinity — Sub-TLV.
- 1042Include-All Affinity — Sub-TLV.
- 1043Definition Flags — Sub-TLV.
- 1044Prefix Metric — Prefix attribute TLV.
- 1045Exclude SRLG — Sub-TLV.
- 1046Unsupported — Sub-TLV.
§6Manageability and Security
The extensions add to the IGP topology information already distributed through RFC 7752 and do not change BGP operation. The malformed-attribute tests in the Fault Management section of RFC 7752 now cover the new TLVs. No new configuration or monitoring is defined.
For security, RFC 7752 covers acquiring and distributing BGP-LS information, and the IGP instances originating the data are assumed to provide the security RFC 9350 requires. Tampering with Flex-Algo information could affect applications using it, including route calculation and programming, but because the data relates to a specific Flex-Algo topology, the impact is similarly limited in scope.
Messing with these notes could confuse the planner, but only for the one rule set the notes belong to.
§7References
[RFC9350]RFC 9350Psenak, P., Ed., et al., "IGP Flexible Algorithm", RFC 9350, February 2023.February 2023[RFC7752]RFC 7752Gredler, H., Ed., et al., "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, March 2016.March 2016[RFC9085]RFC 9085Previdi, S., et al., "Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing", RFC 9085, August 2021.August 2021[RFC9294]RFC 9294Talaulikar, K., Ed., et al., "Application-Specific Link Attributes Advertisement Using the Border Gateway Protocol - Link State (BGP-LS)", RFC 9294, August 2022.August 2022[RFC7308]RFC 7308Osborne, E., "Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)", RFC 7308, July 2014.July 2014[RFC9256]RFC 9256Filsfils, C., et al., "Segment Routing Policy Architecture", RFC 9256, July 2022.July 2022[RFC8402]RFC 8402Filsfils, C., Ed., et al., "Segment Routing Architecture", RFC 8402, July 2018.July 2018