< draft-ietf-lsvr-bgp-spf-11.txt   draft-ietf-lsvr-bgp-spf-12.txt >
Network Working Group K. Patel Network Working Group K. Patel
Internet-Draft Arrcus, Inc. Internet-Draft Arrcus, Inc.
Intended status: Standards Track A. Lindem Intended status: Standards Track A. Lindem
Expires: February 4, 2021 Cisco Systems Expires: July 30, 2021 Cisco Systems
S. Zandi S. Zandi
LinkedIn LinkedIn
W. Henderickx W. Henderickx
Nokia Nokia
August 3, 2020 January 26, 2021
Shortest Path Routing Extensions for BGP Protocol BGP Link-State Shortest Path First (SPF) Routing
draft-ietf-lsvr-bgp-spf-11 draft-ietf-lsvr-bgp-spf-12
Abstract Abstract
Many Massively Scaled Data Centers (MSDCs) have converged on Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on operational simplicity have led many of these MSDCs to converge on
BGP as their single routing protocol for both their fabric routing BGP as their single routing protocol for both their fabric routing
and their Data Center Interconnect (DCI) routing. This document and their Data Center Interconnect (DCI) routing. This document
describes a solution which leverages BGP Link-State distribution and describes extensions to BGP to use BGP Link-State distribution and
the Shortest Path First (SPF) algorithm similar to Internal Gateway the Shortest Path First (SPF) algorithm used by Internal Gateway
Protocols (IGPs) such as OSPF. Protocols (IGPs) such as OSPF. In doing this, it allows BGP to be
efficiently used as both the underlay protocol and the overlay
protocol in MSDCs.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 4, 2021. This Internet-Draft will expire on July 30, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.2. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4
2. BGP Peering Models . . . . . . . . . . . . . . . . . . . . . 5 1.3. Document Overview . . . . . . . . . . . . . . . . . . . . 6
2.1. BGP Single-Hop Peering on Network Node Connections . . . 5 1.4. Requirements Language . . . . . . . . . . . . . . . . . . 6
2.2. BGP Peering Between Directly Connected Network Nodes . . 5 2. Base BGP Protocol Relationship . . . . . . . . . . . . . . . 6
2.3. BGP Peering in Route-Reflector or Controller Topology . . 6 3. BGP Link-State (BGP-LS) Relationship . . . . . . . . . . . . 7
3. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . . . 6 4. BGP Peering Models . . . . . . . . . . . . . . . . . . . . . 8
4. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . . . 6 4.1. BGP Single-Hop Peering on Network Node Connections . . . 8
4.1. Node NLRI Usage . . . . . . . . . . . . . . . . . . . . . 7 4.2. BGP Peering Between Directly-Connected Nodes . . . . . . 8
4.1.1. Node NLRI Attribute SPF Capability TLV . . . . . . . 7 4.3. BGP Peering in Route-Reflector or Controller Topology . . 9
4.1.2. BGP-LS Node NLRI Attribute SPF Status TLV . . . . . . 8 5. BGP Shortest Path Routing (SPF) Protocol Extensions . . . . . 9
4.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . . . 8 5.1. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . 9
4.2.1. BGP-LS Link NLRI Attribute Prefix-Length TLVs . . . . 9 5.1.1. BGP-LS-SPF NLRI TLVs . . . . . . . . . . . . . . . . 9
4.2.2. BGP-LS Link NLRI Attribute SPF Status TLV . . . . . . 9 5.1.2. BGP-LS Attribute . . . . . . . . . . . . . . . . . . 10
4.3. Prefix NLRI Usage . . . . . . . . . . . . . . . . . . . . 10 5.2. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . 11
4.3.1. BGP-LS Prefix NLRI Attribute SPF Status TLV . . . . . 10 5.2.1. Node NLRI Usage . . . . . . . . . . . . . . . . . . . 11
4.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . . . 10 5.2.1.1. Node NLRI Attribute SPF Capability TLV . . . . . 11
5. Decision Process with SPF Algorithm . . . . . . . . . . . . . 11 5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV . . 12
5.1. Phase-1 BGP NLRI Selection . . . . . . . . . . . . . . . 12 5.2.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . 13
5.2. Dual Stack Support . . . . . . . . . . . . . . . . . . . 13 5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs 14
5.3. SPF Calculation based on BGP-LS NLRI . . . . . . . . . . 13 5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV . . 14
5.4. NEXT_HOP Manipulation . . . . . . . . . . . . . . . . . . 16 5.2.3. IPv4/IPv6 Prefix NLRI Usage . . . . . . . . . . . . . 15
5.5. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 16 5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV . 16
5.6. NLRI Advertisement and Convergence . . . . . . . . . . . 17 5.2.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . 16
5.6.1. Link/Prefix Failure Convergence . . . . . . . . . . . 17 5.3. NEXT_HOP Manipulation . . . . . . . . . . . . . . . . . . 17
5.6.2. Node Failure Convergence . . . . . . . . . . . . . . 17 6. Decision Process with SPF Algorithm . . . . . . . . . . . . . 18
5.7. Error Handling . . . . . . . . . . . . . . . . . . . . . 18 6.1. BGP NLRI Selection . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 6.1.1. BGP Self-Originated NLRI . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18 6.2. Dual Stack Support . . . . . . . . . . . . . . . . . . . 20
8. Management Considerations . . . . . . . . . . . . . . . . . . 18 6.3. SPF Calculation based on BGP-LS-SPF NLRI . . . . . . . . 20
8.1. Configuration . . . . . . . . . . . . . . . . . . . . . . 18 6.4. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 25
8.2. Operational Data . . . . . . . . . . . . . . . . . . . . 19 6.5. NLRI Advertisement . . . . . . . . . . . . . . . . . . . 25
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 19 6.5.1. Link/Prefix Failure Convergence . . . . . . . . . . . 25
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 6.5.2. Node Failure Convergence . . . . . . . . . . . . . . 26
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20 7. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 26
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 7.1. Processing of BGP-LS-SPF TLVs . . . . . . . . . . . . . . 26
12.1. Normative References . . . . . . . . . . . . . . . . . . 20 7.2. Processing of BGP-LS-SPF NLRIs . . . . . . . . . . . . . 27
12.2. Information References . . . . . . . . . . . . . . . . . 21 7.3. Processing of BGP-LS Attribute . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10. Management Considerations . . . . . . . . . . . . . . . . . . 31
10.1. Configuration . . . . . . . . . . . . . . . . . . . . . 31
10.1.1. Link Metric Configuration . . . . . . . . . . . . . 31
10.1.2. backoff-config . . . . . . . . . . . . . . . . . . . 31
10.2. Operational Data . . . . . . . . . . . . . . . . . . . . 31
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 32
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
14.1. Normative References . . . . . . . . . . . . . . . . . . 33
14.2. Informational References . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction 1. Introduction
Many Massively Scaled Data Centers (MSDCs) have converged on Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on operational simplicity have led many of these MSDCs to converge on
BGP [RFC4271] as their single routing protocol for both their fabric BGP [RFC4271] as their single routing protocol for both their fabric
routing and their Data Center Interconnect (DCI) routing. routing and their Data Center Interconnect (DCI) routing [RFC7938].
Requirements and procedures for using BGP are described in [RFC7938].
This document describes an alternative solution which leverages BGP- This document describes an alternative solution which leverages BGP-
LS [RFC7752] and the Shortest Path First algorithm similar to LS [RFC7752] and the Shortest Path First algorithm used by Internal
Internal Gateway Protocols (IGPs) such as OSPF [RFC2328]. Gateway Protocols (IGPs) such as OSPF [RFC2328].
[RFC4271] defines the Decision Process that is used to select routes This document leverages both the BGP protocol [RFC4271] and the BGP-
for subsequent advertisement by applying the policies in the local LS [RFC7752] protocols. The relationship, as well as the scope of
Policy Information Base (PIB) to the routes stored in its Adj-RIBs- changes are described respectively in Section 2 and Section 3. The
In. The output of the Decision Process is the set of routes that are modifications to [RFC4271] for BGP SPF described herein only apply to
announced by a BGP speaker to its peers. These selected routes are IPv4 and IPv6 as underlay unicast Subsequent Address Families
stored by a BGP speaker in the speaker's Adj-RIBs-Out according to Identifiers (SAFIs). Operations for any other BGP SAFIs are outside
policy. the scope of this document.
[RFC7752] describes a mechanism by which link-state and TE This solution avails the benefits of both BGP and SPF-based IGPs.
information can be collected from networks and shared with external These include TCP based flow-control, no periodic link-state refresh,
components using BGP. This is achieved by defining NLRI advertised and completely incremental NLRI advertisement. These advantages can
within the BGP-LS/BGP-LS-SPF AFI/SAFI. The BGP-LS extensions defined reduce the overhead in MSDCs where there is a high degree of Equal
in [RFC7752] makes use of the Decision Process defined in [RFC4271]. Cost Multi-Path (ECMPs) and the topology is very stable.
Additionally, using an SPF-based computation can support fast
convergence and the computation of Loop-Free Alternatives (LFAs).
The SPF LFA extensions defined in [RFC5286] can be similarly applied
to BGP SPF calculations. However, the details are a matter of
implementation detail. Furthermore, a BGP-based solution lends
itself to multiple peering models including those incorporating
route-reflectors [RFC4456] or controllers.
This document augments [RFC7752] by replacing its use of the existing 1.1. Terminology
Decision Process. Rather than reusing the BGP-LS SAFI, the BGP-LS-
SPF SAFI is introduced to insure backward compatibility. The Phase 1
and 2 decision functions of the Decision Process are replaced with
the Shortest Path First (SPF) algorithm also known as the Dijkstra
algorithm. The Phase 3 decision function is also simplified since it
is no longer dependent on the previous phases. This solution avails
the benefits of both BGP and SPF-based IGPs. These include TCP based
flow-control, no periodic link-state refresh, and completely
incremental NLRI advertisement. These advantages can reduce the
overhead in MSDCs where there is a high degree of Equal Cost Multi-
Path (ECMPs) and the topology is very stable. Additionally, using an
SPF-based computation can support fast convergence and the
computation of Loop-Free Alternatives (LFAs) [RFC5286] in the event
of link failures. Furthermore, a BGP based solution lends itself to
multiple peering models including those incorporating route-
reflectors [RFC4456] or controllers.
Support for Multiple Topology Routing (MTR) as described in [RFC4915] This specification reuses terms defined in section 1.1 of [RFC4271]
is an area for further study dependent on deployment requirements. including BGP speaker, NLRI, and Route.
1.1. BGP Shortest Path First (SPF) Motivation Additionally, this document introduces the following terms:
BGP SPF Routing Domain: A set of BGP routers that are under a single
administrative domain and exchange link-state information using
the BGP-LS-SPF SAFI and compute routes using BGP SPF as described
herein.
BGP-LS-SPF NLRI: This refers to BGP-LS Network Layer Reachability
Information (NLRI) that is being advertised in the BGP-LS-SPF SAFI
(Section 5.1) and is being used for BGP SPF route computation.
Dijkstra Algorithm: An algorithm for computing the shortest path
from a given node in a graph to every other node in the graph. At
each iteration of the algorithm, there is a list of candidate
vertices. Paths from the root to these vertices have been found,
but not necessarily the shortest ones. However, the paths to the
candidate vertex that is closest to the root are guaranteed to be
shortest; this vertex is added to the shortest-path tree, removed
from the candidate list, and its adjacent vertices are examined
for possible addition to/modification of the candidate list. The
algorithm then iterates again. It terminates when the candidate
list becomes empty. [RFC2328]
1.2. BGP Shortest Path First (SPF) Motivation
Given that [RFC7938] already describes how BGP could be used as the Given that [RFC7938] already describes how BGP could be used as the
sole routing protocol in an MSDC, one might question the motivation sole routing protocol in an MSDC, one might question the motivation
for defining an alternate BGP deployment model when a mature solution for defining an alternate BGP deployment model when a mature solution
exists. For both alternatives, BGP offers the operational benefits exists. For both alternatives, BGP offers the operational benefits
of a single routing protocol. However, BGP SPF offers some unique of a single routing protocol as opposed to the combination of an IGP
advantages above and beyond standard BGP distance-vector routing. for the underlay and BGP as an overlay. However, BGP SPF offers some
unique advantages above and beyond standard BGP distance-vector
routing. With BGP SPF, the standard hop-by-hop peering model is
relaxed.
A primary advantage is that all BGP speakers in the BGP SPF routing A primary advantage is that all BGP-LS-SPF speakers in the BGP SPF
domain will have a complete view of the topology. This will allow routing domain will have a complete view of the topology. This will
support for ECMP, IP fast-reroute (e.g., Loop-Free Alternatives), allow support for ECMP, IP fast-reroute (e.g., Loop-Free
Shared Risk Link Groups (SRLGs), and other routing enhancements Alternatives), Shared Risk Link Groups (SRLGs), and other routing
without advertisement of addition BGP paths or other extensions. In enhancements without advertisement of additional BGP paths [RFC7911]
short, the advantages of an IGP such as OSPF [RFC2328] are availed in or other extensions. In short, the advantages of an IGP such as OSPF
BGP. [RFC2328] are availed in BGP.
With the simplified BGP decision process as defined in Section 5.1, With the simplified BGP decision process as defined in Section 6,
NLRI changes can be disseminated throughout the BGP routing domain NLRI changes can be disseminated throughout the BGP routing domain
much more rapidly (equivalent to IGPs with the proper much more rapidly (equivalent to IGPs with the proper
implementation). implementation). The added advantage of BGP using TCP for reliable
transport leverages TCP's inherent flow-control and guaranteed in-
order delivery.
Another primary advantage is a potential reduction in NLRI Another primary advantage is a potential reduction in NLRI
advertisement. With standard BGP distance-vector routing, a single advertisement. With standard BGP distance-vector routing, a single
link failure may impact 100s or 1000s prefixes and result in the link failure may impact 100s or 1000s prefixes and result in the
withdrawal or re-advertisement of the attendant NLRI. With BGP SPF, withdrawal or re-advertisement of the attendant NLRI. With BGP SPF,
only the BGP speakers corresponding to the link NLRI need withdraw only the BGP speakers corresponding to the link NLRI need to withdraw
the corresponding BGP-LS Link NLRI. This advantage will contribute the corresponding BGP-LS-SPF Link NLRI. Additionally, the changed
to both faster convergence and better scaling. NLRI will be advertised immediately as opposed to normal BGP where it
is only advertised after the best route selection. These advantages
will afford NLRI dissemination throughout the BGP SPF routing domain
with efficiencies similar to link-state protocols.
With controller and route-reflector peering models, BGP SPF With controller and route-reflector peering models, BGP SPF
advertisement and distributed computation require a minimal number of advertisement and distributed computation require a minimal number of
sessions and copies of the NLRI since only the latest version of the sessions and copies of the NLRI since only the latest version of the
NLRI from the originator is required. Given that verification of the NLRI from the originator is required. Given that verification of the
adjacencies is done outside of BGP (see Section 2), each BGP speaker adjacencies is done outside of BGP (see Section 4), each BGP speaker
will only need as many sessions and copies of the NLRI as required will only need as many sessions and copies of the NLRI as required
for redundancy (e.g., one for the SPF computation and another for for redundancy (see Section 4). Additionally, a controller could
backup). Functions such as Optimized Route Reflection (ORR) are inject topology that is learned outside the BGP SPF routing domain.
supported without extension by virtue of the primary advantages.
Additionally, a controller could inject topology that is learned
outside the BGP routing domain.
Given that controllers are already consuming BGP-LS NLRI [RFC7752], Given that controllers are already consuming BGP-LS NLRI [RFC7752],
reusing for the BGP-LS SPF leverages the existing controller this functionality can be reused for BGP-LS-SPF NLRI.
implementations.
Another potential advantage of BGP SPF is that both IPv6 and IPv4 can Another potential advantage of BGP SPF is that both IPv6 and IPv4 can
be supported in the same address family using the same topology. both be supported using the BGP-LS-SPF SAFI with the same BGP-LS-SPF
Although not described in this version of the document, multi- NLRIs. In many MSDC fabrics, the IPv4 and IPv6 topologies are
topology extensions can be used to support separate IPv4, IPv6, congruent. Although beyond the scope of this document, multi-
topology extensions could be used to support separate IPv4, IPv6,
unicast, and multicast topologies while sharing the same NLRI. unicast, and multicast topologies while sharing the same NLRI.
Finally, the BGP SPF topology can be used as an underlay for other Finally, the BGP SPF topology can be used as an underlay for other
BGP address families (using the existing model) and realize all the BGP SAFIs (using the existing model) and realize all the above
above advantages. A simplified peering model using IPv6 link-local advantages.
addresses as next-hops can be deployed similar to [RFC5549].
1.2. Requirements Language 1.3. Document Overview
The document begins with sections defining the precise relationship
that BGP SPF has with both the base BGP protocol [RFC4271]
(Section 2) and the BGP Link-State (BGP-LS) extensions [RFC7752]
(Section 3). This is required to dispel the notion that BGP SPF is
an independent protocol. The BGP peering models, as well as the
their respective trade-offs are then discussed in Section 4. The
remaining sections, which make up the bulk of the document, define
the protocol enhancements necessary to support BGP SPF. The BGP-LS
extensions to support BGP SPF are defined in Section 5. The
replacement of the base BGP decision process with the SPF computation
is specified in Section 6. Finally, BGP SPF error handling is
defined in Section 7
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. BGP Peering Models 2. Base BGP Protocol Relationship
Depending on the requirements, scaling, and capabilities of the BGP With the exception of the decision process, the BGP SPF extensions
speakers, various peering models are supported. The only requirement leverage the BGP protocol [RFC4271] without change. This includes
is that all BGP speakers in the BGP SPF routing domain receive link- the BGP protocol Finite State Machine, BGP messages and their
state NLRI on a timely basis, run an SPF calculation, and update encodings, processing of BGP messages, BGP attributes and path
their data plane appropriately. The content of the Link NLRI is attributes, BGP NLRI encodings, and any error handling defined in the
described in Section 4.2. [RFC4271] and [RFC7606].
2.1. BGP Single-Hop Peering on Network Node Connections Due to the changes to the decision process, there are mechanisms and
encodings that are no longer applicable. While not necessarily
required for computation, the ORIGIN, AS_PATH, MULTI_EXIT_DISC,
LOCAL_PREF, and NEXT_HOP path attributes are mandatory and will be
validated. The ATOMIC_AGGEGATE, and AGGREGATOR are not applicable
within the context of BGP SPF and SHOULD NOT be advertised. However,
if they are advertised, they will be accepted, validated, and
propagated consistent with the BGP protocol.
The simplest peering model is the one described in section 5.2.1 of Section 9 of [RFC4271] defines the decision process that is used to
[RFC7938]. In this model, EBGP single-hop sessions are established select routes for subsequent advertisement by applying the policies
over direct point-to-point links interconnecting the SPF domain in the local Policy Information Base (PIB) to the routes stored in
nodes. For the purposes of BGP SPF, Link NLRI is only advertised if its Adj-RIBs-In. The output of the Decision Process is the set of
a single-hop BGP session has been established and the Link-State/SPF routes that are announced by a BGP speaker to its peers. These
address family capability has been exchanged [RFC4790] on the selected routes are stored by a BGP speaker in the speaker's Adj-
corresponding session. If the session goes down, the corresponding RIBs-Out according to policy.
Link NLRI will be withdrawn. Topologically, this would be equivalent
to the peering model in [RFC7938] where there is a BGP session on
every link in the data center switch fabric.
2.2. BGP Peering Between Directly Connected Network Nodes The BGP SPF extension fundamentally changes the decision process, as
described herein, to be more like a link-state protocol (e.g., OSPF
[RFC2328]). Specifically:
In this model, BGP speakers peer with all directly connected network 1. BGP advertisements are readvertised to neighbors immediately
nodes but the sessions may be multi-hop and the direct connection without waiting or dependence on the route computation as
discovery and liveliness detection for those connections are specified in phase 3 of the base BGP decision process. Multiple
independent of the BGP protocol. How this is accomplished is outside peering models are supported as specified in Section 4.
the scope of this document. Consequently, there will be a single
session even if there are multiple direct connections between BGP
speakers. For the purposes of BGP SPF, Link NLRI is advertised as
long as a BGP session has been established, the Link-State/SPF
address family capability has been exchanged [RFC4790] and the
corresponding link is considered is up and considered operational.
This is much like the previous peering model only peering is on a
single loopback address and the switch fabric links can be
unnumbered. However, there will be the same number of sessions as
with the previous peering model unless there are parallel links
between switches in the fabric.
2.3. BGP Peering in Route-Reflector or Controller Topology 2. Determining the degree of preference for BGP routes for the SPF
calculation as described in phase 1 of the base BGP decision
process is replaced with the mechanisms in Section 6.1.
In this model, BGP speakers peer solely with one or more Route 3. Phase 2 of the base BGP protocol decision process is replaced
with the Shortest Path First (SPF) algorithm, also known as the
Dijkstra algorithm Section 1.1.
3. BGP Link-State (BGP-LS) Relationship
[RFC7752] describes a mechanism by which link-state and TE
information can be collected from networks and shared with external
entities using BGP. This is achieved by defining NLRI advertised
using the BGP-LS AFI. The BGP-LS extensions defined in [RFC7752]
make use of the decision process defined in [RFC4271]. This document
reuses NLRI and TLVs defined in [RFC7752]. Rather than reusing the
BGP-LS SAFI, the BGP-LS-SPF SAFI Section 5.1 is introduced to insure
backward compatibility for the BGP-LS SAFI usage.
The BGP SPF extensions reuse the Node, Link, and Prefix NLRI defined
in [RFC7752]. The usage of the BGP-LS NLRI, metric attributes, and
attribute extensions is described in Section 5.2.1. The usage of
others BGP-LS attributes is not precluded and is, in fact, expected.
However, the details are beyond the scope of this document and will
be specified in future documents.
Support for Multiple Topology Routing (MTR) similar to the OSPF MTR
computation described in [RFC4915] is beyond the scope of this
document. Consequently, the usage of the Multi-Topology TLV as
described in section 3.2.1.5 of [RFC7752] is not specified.
The rules for setting the NLRI next-hop path attribute for the BGP-
LS-SPF SAFI will follow the BGP-LS SAFI as specified in section 3.4
of [RFC7752].
4. BGP Peering Models
Depending on the topology, scaling, capabilities of the BGP-LS-SPF
speakers, and redundancy requirements, various peering models are
supported. The only requirements are that all BGP SPF speakers in
the BGP SPF routing domain exchange BGP-LS-SPF NLRI, run an SPF
calculation, and update their routing table appropriately.
4.1. BGP Single-Hop Peering on Network Node Connections
The simplest peering model is the one where EBGP single-hop sessions
are established over direct point-to-point links interconnecting the
nodes in the BGP SPF routing domain. Once the single-hop BGP session
has been established and the BGP-LS-SPF AFI/SAFI capability has been
exchanged [RFC4760] for the corresponding session, then the link is
considered up from a BGP SPF perspective and the corresponding BGP-
LS-SPF Link NLRI is advertised. If the session goes down, the
corresponding Link NLRI will be withdrawn. Topologically, this would
be equivalent to the peering model in [RFC7938] where there is a BGP
session on every link in the data center switch fabric. The content
of the Link NLRI is described in Section 5.2.2.
4.2. BGP Peering Between Directly-Connected Nodes
In this model, BGP-LS-SPF speakers peer with all directly-connected
nodes but the sessions may be between loopback addresses (i.e., two-
hop sessions) and the direct connection discovery and liveliness
detection for the interconnecting links are independent of the BGP
protocol. the scope of this document. For example, liveliness
detection could be done using the BFD protocol [RFC5880]. Precisely
how discovery and liveliness detection is accomplished is outside the
scope of this document. Consequently, there will be a single BGP
session even if there are multiple direct connections between BGP-LS-
SPF speakers. BGP-LS-SPF Link NLRI is advertised as long as a BGP
session has been established, the BGP-LS-SPF AFI/SAFI capability has
been exchanged [RFC4760], and the link is operational as determined
using liveliness detection mechanisms outside the scope of this
document. This is much like the previous peering model only peering
is between loopback addresses and the interconnecting links can be
unnumbered. However, since there are BGP sessions between every
directly-connected node in the BGP SPF routing domain, there is only
a reduction in BGP sessions when there are parallel links between
nodes.
4.3. BGP Peering in Route-Reflector or Controller Topology
In this model, BGP-LS-SPF speakers peer solely with one or more Route
Reflectors [RFC4456] or controllers. As in the previous model, Reflectors [RFC4456] or controllers. As in the previous model,
direct connection discovery and liveliness detection for those direct connection discovery and liveliness detection for those links
connections are done outside the BGP protocol. More specifically, in the BGP SPF routing domain are done outside of the BGP protocol.
the Liveliness detection is done using BFD protocol described in BGP-LS-SPF Link NLRI is advertised as long as the corresponding link
[RFC5880]. For the purposes of BGP SPF, Link NLRI is advertised as is considered up as per the chosen liveness detection mechanism.
long as the corresponding link is up and considered operational.
This peering model, known as sparse peering, allows for many fewer This peering model, known as sparse peering, allows for fewer BGP
BGP sessions and, consequently, instances of the same NLRI received sessions and, consequently, fewer instances of the same NLRI received
from multiple peers. It is discussed in greater detail in from multiple peers. Normally, the route-reflectors or controller
BGP sessions would be on directly-connected links to avoid dependence
on another routing protocol for session connectivity. However,
multi-hop peering is not precluded. The number of BGP sessions is
dependent on the redundancy requirements and the stability of the BGP
sessions. This is discussed in greater detail in
[I-D.ietf-lsvr-applicability]. [I-D.ietf-lsvr-applicability].
3. BGP-LS Shortest Path Routing (SPF) SAFI 5. BGP Shortest Path Routing (SPF) Protocol Extensions
In order to replace the Phase 1 and 2 decision functions of the 5.1. BGP-LS Shortest Path Routing (SPF) SAFI
existing Decision Process with an SPF-based Decision Process and
streamline the Phase 3 decision functions in a backward compatible
manner, this draft introduces the BGP-LS-SFP SAFI for BGP-LS SPF
operation. The BGP-LS-SPF (AFI 16388 / SAFI TBD1) [RFC4790] is
allocated by IANA as specified in the Section 6. A BGP speaker using
the BGP-LS SPF extensions described herein MUST exchange the AFI/SAFI
using Multiprotocol Extensions Capability Code [RFC4760] with other
BGP speakers in the SPF routing domain.
4. Extensions to BGP-LS In order to replace the existing BGP decision process with an SPF-
based decision process in a backward compatible manner by not
impacting the BGP-LS SAFI, this document introduces the BGP-LS-SPF
SAFI. The BGP-LS-SPF (AFI 16388 / SAFI 80) [RFC4760] is allocated by
IANA as specified in the Section 8. In order for two BGP-LS-SPF
speakers to exchange BGP SPF NLRI, they MUST exchange the
Multiprotocol Extensions Capability [RFC5492] [RFC4760] to ensure
that they are both capable of properly processing such NLRI. This is
done with AFI 16388 / SAFI 80 for BGP-LS-SPF advertised within the
BGP SPF Routing Domain. The BGP-LS-SPF SAFI is used to carry IPv4
and IPv6 prefix information in a format facilitating an SPF-based
decision process.
5.1.1. BGP-LS-SPF NLRI TLVs
The NLRI format of BGP-LS-SPF SAFI uses exactly same format as the
BGP-LS AFI [RFC7752]. In other words, all the TLVs used in BGP-LS
AFI are applicable and used for the BGP-LS-SPF SAFI. These TLVs
within BGP-LS-SPF NLRI advertise information that describes links,
nodes, and prefixes comprising IGP link-state information.
In order to compare the NLRI efficiently, it is REQUIRED that all the
TLVs within the given NLRI must be ordered in ascending order by the
TLV type. For multiple TLVs of same type within a single NLRI, it is
REQUIRED that these TLVs are ordered in ascending order by the TLV
value field. Comparison of the value fields is performed by treating
the entire value field as a hexadecimal string. NLRIs having TLVs
which do not follow the ordering rules MUST be considered as
malformed and discarded with appropriate error logging.
[RFC7752] defines certain NLRI TLVs as a mandatory TLVs. These TLVs
are considered mandatory for the BGP-LS-SPF SAFI as well. All the
other TLVs are considered as an optional TLVs.
5.1.2. BGP-LS Attribute
The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly same format
of the BGP-LS AFI [RFC7752]. In other words, all the TLVs used in
BGP-LS attribute of the BGP-LS AFI are applicable and used for the
BGP-LS attribute of the BGP-LS-SPF SAFI. This attribute is an
optional, non-transitive BGP attribute that is used to carry link,
node, and prefix properties and attributes. The BGP-LS attribute is
a set of TLVs.
The BGP-LS attribute may potentially grow large in size depending on
the amount of link-state information associated with a single Link-
State NLRI. The BGP specification [RFC4271] mandates a maximum BGP
message size of 4096 octets. It is RECOMMENDED that an
implementation support [RFC8654] in order to accommodate larger size
of information within the BGP-LS Attribute. BGP-LS-SPF speakers MUST
ensure that they limit the TLVs included in the BGP-LS Attribute to
ensure that a BGP update message for a single Link-State NLRI does
not cross the maximum limit for a BGP message. The determination of
the types of TLVs to be included by the BGP-LS-SPF speaker
originating the attribute is outside the scope of this document.
When a BGP-LS-SPF speaker finds that it is exceeding the maximum BGP
message size due to addition or update of some other BGP Attribute
(e.g., AS_PATH), it MUST consider the BGP-LS Attribute to be
malformed and the attribute discard handling of [RFC7606] applies.
In order to compare the BGP-LS attribute efficiently, it is REQUIRED
that all the TLVs within the given attribute must be ordered in
ascending order by the TLV type. For multiple TLVs of same type
within a single attribute, it is REQUIRED that these TLVs are ordered
in ascending order by the TLV value field. Comparison of the value
fields is performed by treating the entire value field as a
hexadecimal string. Attributes having TLVs which do not follow the
ordering rules MUST NOT be considered as malformed.
All TLVs within the BGP-LS Attribute are considered optional unless
specified otherwise.
5.2. Extensions to BGP-LS
[RFC7752] describes a mechanism by which link-state and TE [RFC7752] describes a mechanism by which link-state and TE
information can be collected from networks and shared with external information can be collected from IGPs and shared with external
components using BGP protocol. It describes both the definition of components using the BGP protocol. It describes both the definition
BGP-LS NLRI that describes links, nodes, and prefixes comprising IGP of the BGP-LS-SPF NLRI that advertise links, nodes, and prefixes
link-state information and the definition of a BGP path attribute comprising IGP link-state information and the definition of a BGP
(BGP-LS attribute) that carries link, node, and prefix properties and path attribute (BGP-LS attribute) that carries link, node, and prefix
attributes, such as the link and prefix metric or auxiliary Router- properties and attributes, such as the link and prefix metric or
IDs of nodes, etc. auxiliary Router-IDs of nodes, etc. This document extends the usage
of BGP-LS NLRI for the purpose of BGP SPF calculation via
advertisement in the BGP-LS-SPF SAFI.
The BGP protocol will be used in the Protocol-ID field specified in The protocol identifier specified in the Protocol-ID field [RFC7752]
table 1 of [I-D.ietf-idr-bgpls-segment-routing-epe]. The local and will represent the origin of the advertised NLRI. For Node NLRI and
remote node descriptors for all NLRI will be the BGP Router-ID (TLV Link NLRI, this MUST be the direct protocol (4). Node or Link NLRI
516) and either the AS Number (TLV 512) [RFC7752] or the BGP with a Protocol-ID other than direct will be considered malformed.
Confederation Member (TLV 517) [RFC8402]. However, if the BGP For Prefix NLRI, the specified Protocol-ID MUST be the origin of the
Router-ID is known to be unique within the BGP Routing domain, it can prefix. The local and remote node descriptors for all NLRI MUST
be used as the sole descriptor. include the BGP Identifier (TLV 516) and the AS Number (TLV 512)
[RFC7752]. The BGP Confederation Member (TLV 517) [RFC7752] is not
appliable and SHOULD not be included. If TLV 517 is included, it
will be ignored.
4.1. Node NLRI Usage 5.2.1. Node NLRI Usage
The BGP Node NLRI will be advertised unconditionally by all routers The Node NLRI MUST be advertised unconditionally by all routers in
in the BGP SPF routing domain. the BGP SPF routing domain.
4.1.1. Node NLRI Attribute SPF Capability TLV 5.2.1.1. Node NLRI Attribute SPF Capability TLV
The SPF capability is a new Node Attribute TLV that will be added to The SPF capability is an additional Node Attribute TLV. This
those defined in table 7 of [RFC7752]. The new attribute TLV will attribute TLV MUST be included with the BGP-LS-SPF SAFI and SHOULD
only be applicable when BGP is specified in the Node NLRI Protocol ID NOT be used for other SAFIs. The TLV type 1180 will be assigned by
field. The TBD TLV type will be defined by IANA. The new Node IANA. The Node Attribute TLV will contain a single-octet SPF
Attribute TLV will contain a single-octet SPF algorithm as defined in algorithm as defined in [RFC8665].
[RFC8402].
0 1 2 3 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 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 | | Type (1180) | Length - (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Algorithm | | SPF Algorithm |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
The SPF Algorithm may take the following values: The SPF algorithm inherits the values from the IGP Algorithm Types
registry [RFC8665]. Algorithm 0, (Shortest Path Algorithm (SPF)
0 - Normal Shortest Path First (SPF) algorithm based on link based on link metric, is supported and described in Section 6.3.
metric. This is the standard shortest path algorithm as Support for other algorithm types is beyond the scope of this
computed by the IGP protocol. Consistent with the deployed specification.
practice for link-state protocols, Algorithm 0 permits any
node to overwrite the SPF path with a different path based on
its local policy.
1 - Strict Shortest Path First (SPF) algorithm based on link
metric. The algorithm is identical to Algorithm 0 but Algorithm
1 requires that all nodes along the path will honor the SPF
routing decision. Local policy at the node claiming support for
Algorithm 1 MUST NOT alter the SPF paths computed by Algorithm 1.
Note that usage of Strict Shortest Path First (SPF) algorithm is
defined in the IGP algorithm registry but usage is restricted to
[I-D.ietf-idr-bgpls-segment-routing-epe]. Hence, its usage for BGP-
LS SPF is out of scope.
When computing the SPF for a given BGP routing domain, only BGP nodes When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability attribute will be included the advertising the SPF capability TLV with same SPF algorithm will be
Shortest Path Tree (SPT). included in the Shortest Path Tree (SPT). An implementation MAY
optionally log detection of a BGP node that has either not advertised
the SPF capability TLV or is advertising the SPF capability TLV with
an algorithm type other than 0.
4.1.2. BGP-LS Node NLRI Attribute SPF Status TLV 5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV to BGP-LS Node NLRI is defined to indicate the A BGP-LS Attribute TLV of the BGP-LS-SPF Node NLRI is defined to
status of the node with respect to the BGP SPF calculation. This indicate the status of the node with respect to the BGP SPF
will be used to rapidly take a node out of service or to indicate the calculation. This will be used to rapidly take a node out of service
node is not to be used for transit (i.e., non-local) traffic. If the Section 6.5.2 or to indicate the node is not to be used for transit
SPF Status TLV is not included with the Node NLRI, the node is (i.e., non-local) traffic Section 6.3. If the SPF Status TLV is not
considered to be up and is available for transit traffic. included with the Node NLRI, the node is considered to be up and is
available for transit traffic. The SPF status is acted upon with the
execution of the next SPF calculation Section 6.3. A single TLV type
will be shared by the BGP-LS-SPF Node, Link, and Prefix NLRI. The
TLV type 1184 will be assigned by IANA.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD Type | Length | | Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status | | SPF Status |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved BGP Status Values: 0 - Reserved
1 - Node Unreachable with respect to BGP SPF 1 - Node Unreachable with respect to BGP SPF
2 - Node does not support transit with respect 2 - Node does not support transit with respect
to BGP SPF to BGP SPF
3-254 - Undefined 3-254 - Undefined
255 - Reserved 255 - Reserved
4.2. Link NLRI Usage If the SPF Status TLV is received and the corresponding Node NLRI has
not been received, then the SPF Status TLV is ignored and not used in
SPF computation but is still announced to other BGP speakers. An
implementation MAY log an error for further analysis. If a BGP
speaker received the Node NLRI but the SPF Status TLV is not
received, then any previously received information is considered as
implicitly withdrawn and the update is propagated to other BGP
speakers. A BGP speaker receiving a BGP Update containing a SPF
Status TLV in the BGP-LS attribute [RFC7752] with a value that is
outside the range of defined values SHOULD be processed and announced
to other BGP speakers. However, a BGP speaker MUST not use the
Status TLV in its SPF computation. An implementation MAY log this
condition for further analysis.
5.2.2. Link NLRI Usage
The criteria for advertisement of Link NLRI are discussed in The criteria for advertisement of Link NLRI are discussed in
Section 2. Section 4.
Link NLRI is advertised with local and remote node descriptors as Link NLRI is advertised with unique local and remote node descriptors
described above and unique link identifiers dependent on the dependent on the IP addressing. For IPv4 links, the link's local
addressing. For IPv4 links, the links local IPv4 (TLV 259) and IPv4 (TLV 259) and remote IPv4 (TLV 260) addresses will be used. For
remote IPv4 (TLV 260) addresses will be used. For IPv6 links, the IPv6 links, the local IPv6 (TLV 261) and remote IPv6 (TLV 262)
local IPv6 (TLV 261) and remote IPv6 (TLV 262) addresses will be addresses will be used. For unnumbered links, the link local/remote
used. For unnumbered links, the link local/remote identifiers (TLV identifiers (TLV 258) will be used. For links supporting having both
258) will be used. For links supporting having both IPv4 and IPv6 IPv4 and IPv6 addresses, both sets of descriptors MAY be included in
addresses, both sets of descriptors may be included in the same Link the same Link NLRI. The link identifiers are described in table 5 of
NLRI. The link identifiers are described in table 5 of [RFC7752]. [RFC7752].
The link IGP metric attribute TLV (TLV 1095) as well as any others For a link to be used in Shortest Path Tree (SPT) for a given address
required for non-SPF purposes SHOULD be advertised. The metric value family, i.e., IPv4 or IPv6, both routers connecting the link MUST
in this TLV is variable length dependent on specific protocol usage have an address in the same subnet for that address family. However,
(refer to section 3.3.2.4 in [RFC7752]). For simplicity, the BGP-LS an IPv4 or IPv6 prefix associated with the link MAY be installed
SPF metric length will be 4 octets. Algorithms such as setting the without the corresponding address on the other side of link.
metric inversely to the link speed as done in the OSPF MIB [RFC4750]
MAY be supported. However, this is beyond the scope of this The link IGP metric attribute TLV (TLV 1095) MUST be advertised. If
a BGP speaker receives a Link NLRI without an IGP metric attribute
TLV, then it SHOULD consider the received NLRI as a malformed and the
receiving BGP speaker MUST handle such malformed NLRI as 'Treat-as-
withdraw' [RFC7606]. The BGP SPF metric length is 4 octets. Like
OSPF [RFC2328], a cost is associated with the output side of each
router interface. This cost is configurable by the system
administrator. The lower the cost, the more likely the interface is
to be used to forward data traffic. One possible default for metric
would be to give each interface a cost of 1 making it effectively a
hop count. Algorithms such as setting the metric inversely to the
link speed as supported in the OSPF MIB [RFC4750] MAY be supported.
However, this is beyond the scope of this document. Refer to
Section 10.1.1 for operational guidance.
The usage of other link attribute TLVs is beyond the scope of this
document. document.
4.2.1. BGP-LS Link NLRI Attribute Prefix-Length TLVs 5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs
Two BGP-LS Attribute TLVs to BGP-LS Link NLRI are defined to Two BGP-LS Attribute TLVs of the BGP-LS-SPF Link NLRI are defined to
advertise the prefix length associated with the IPv4 and IPv6 link advertise the prefix length associated with the IPv4 and IPv6 link
prefixes. The prefix length is used for the optional installation of prefixes derived from the link descriptor addresses. The prefix
prefixes corresponding to Link NLRI as defined in Section 5.3. length is used for the optional installation of prefixes
corresponding to Link NLRI as defined in Section 6.3.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD IPv4 or IPv6 Type | Length | |IPv4 (1182) or IPv6 Type (1183)| Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix-Length | | Prefix-Length |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Prefix-length - A one-octet length restricted to 1-32 for IPv4 Prefix-length - A one-octet length restricted to 1-32 for IPv4
Link NLRI endpoint prefixes and 1-128 for IPv6 Link NLRI endpoint prefixes and 1-128 for IPv6
Link NLRI endpoint prefixes. Link NLRI endpoint prefixes.
4.2.2. BGP-LS Link NLRI Attribute SPF Status TLV The Prefix-Length TLV is only relevant to Link NLRIs. The Prefix-
Length TLVs MUST be discarded as an error and not passed to other BGP
peers as specified in [RFC7606] when received with any NLRIs other
than Link NRLIs. An implementation MAY log an error for further
analysis.
A BGP-LS Attribute TLV to BGP-LS Link NLRI is defined to indicate the The maximum prefix-length for IPv4 Prefix-Length TLV is 32 bits. A
status of the link with respect to the BGP SPF calculation. This prefix-length field indicating a larger value than 32 bits MUST be
will be used to expedite convergence for link failures as discussed discarded as an error and the received TLV is not passed to other BGP
in Section 5.6.1. If the SPF Status TLV is not included with the peers as specified in [RFC7606]. The corresponding Link NLRI is
Link NLRI, the link is considered up and available. considered as malformed and MUST be handled as 'Treat-as-withdraw'.
An implementation MAY log an error for further analysis.
The maximum prefix-length for IPv6 Prefix-Length Type is 128 bits. A
prefix-length field indicating a larger value than 128 bits MUST be
discarded as an error and the received TLV is not passed to other BGP
peers as specified in [RFC7606]. The corresponding Link NLRI is
considered as malformed and MUST be handled as 'Treat-as-withdraw'.
An implementation MAY log an error for further analysis.
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF Link NLRI is defined to
indicate the status of the link with respect to the BGP SPF
calculation. This will be used to expedite convergence for link
failures as discussed in Section 6.5.1. If the SPF Status TLV is not
included with the Link NLRI, the link is considered up and available.
The SPF status is acted upon with the execution of the next SPF
calculation Section 6.3. A single TLV type will be shared by the
Node, Link, and Prefix NLRI. The TLV type 1184 will be assigned by
IANA.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD Type | Length | | Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status | | SPF Status |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved BGP Status Values: 0 - Reserved
1 - Link Unreachable with respect to BGP SPF 1 - Link Unreachable with respect to BGP SPF
2-254 - Undefined 2-254 - Undefined
255 - Reserved 255 - Reserved
4.3. Prefix NLRI Usage If the SPF Status TLV is received and the corresponding Link NLRI has
not been received, then the SPF Status TLV is ignored and not used in
SPF computation but is still announced to other BGP speakers. An
implementation MAY log an error for further analysis. If a BGP
speaker received the Link NLRI but the SPF Status TLV is not
received, then any previously received information is considered as
implicitly withdrawn and the update is propagated to other BGP
speakers. A BGP speaker receiving a BGP Update containing an SPF
Status TLV in the BGP-LS attribute [RFC7752] with a value that is
outside the range of defined values SHOULD be processed and announced
to other BGP speakers. However, a BGP speaker MUST not use the
Status TLV in its SPF computation. An implementation MAY log this
information for further analysis.
Prefix NLRI is advertised with a local node descriptor as described 5.2.3. IPv4/IPv6 Prefix NLRI Usage
above and the prefix and length used as the descriptors (TLV 265) as
described in [RFC7752]. The prefix metric attribute TLV (TLV 1155)
as well as any others required for non-SPF purposes SHOULD be
advertised. For loopback prefixes, the metric should be 0. For non-
loopback prefixes, the setting of the metric is a local matter and
beyond the scope of this document.
4.3.1. BGP-LS Prefix NLRI Attribute SPF Status TLV IPv4/IPv6 Prefix NLRI is advertised with a Local Node Descriptor and
the prefix and length. The Prefix Descriptors field includes the IP
Reachability Information TLV (TLV 265) as described in [RFC7752].
The prefix metric attribute TLV (TLV 1155) MUST be advertised. The
IGP Route Tag TLV (TLV 1153) MAY be advertised. The usage of other
attribute TLVs is beyond the scope of this document. For loopback
prefixes, the metric should be 0. For non-loopback prefixes, the
setting of the metric is a local matter and beyond the scope of this
document.
A BGP-LS Attribute TLV to BGP-LS Prefix NLRI is defined to indicate 5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV
the status of the prefix with respect to the BGP SPF calculation.
This will be used to expedite convergence for prefix unreachability A BGP-LS Attribute TLV to BGP-LS-SPF Prefix NLRI is defined to
as discussed in Section 5.6.1. If the SPF Status TLV is not included indicate the status of the prefix with respect to the BGP SPF
with the Prefix NLRI, the prefix is considered reachable. calculation. This will be used to expedite convergence for prefix
unreachability as discussed in Section 6.5.1. If the SPF Status TLV
is not included with the Prefix NLRI, the prefix is considered
reachable. A single TLV type will be shared by the Node, Link, and
Prefix NLRI. The TLV type 1184 will be assigned by IANA.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD Type | Length | | Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status | | SPF Status |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved BGP Status Values: 0 - Reserved
1 - Prefix down with respect to SPF 1 - Prefix Unreachable with respect to SPF
2-254 - Undefined 2-254 - Undefined
255 - Reserved 255 - Reserved
4.4. BGP-LS Attribute Sequence-Number TLV If the SPF Status TLV is received and the corresponding Prefix NLRI
has not been received, then the SPF Status TLV is ignored and not
used in SPF computation but is still announced to other BGP speakers.
An implementation MAY log an error for further analysis. If a BGP
speaker received the Prefix NLRI but the SPF Status TLV is not
received, then any previously received information is considered as
implicitly withdrawn and the update is propagated to other BGP
speakers. A BGP speaker receiving a BGP Update containing an SPF
Status TLV in the BGP-LS attribute [RFC7752] with a value that is
outside the range of defined values SHOULD be processed and announced
to other BGP speakers. However, a BGP speaker MUST not use the
Status TLV in its SPF computation. An implementation MAY log this
information for further analysis.
A new BGP-LS Attribute TLV to BGP-LS NLRI types is defined to assure 5.2.4. BGP-LS Attribute Sequence-Number TLV
the most recent version of a given NLRI is used in the SPF
computation. The TBD TLV type will be defined by IANA. The new BGP- A BGP-LS Attribute TLV of the BGP-LS-SPF NLRI types is defined to
LS Attribute TLV will contain an 8-octet sequence number. The usage assure the most recent version of a given NLRI is used in the SPF
of the Sequence Number TLV is described in Section 5.1. computation. The Sequence-Number TLV is mandatory for BGP-LS-SPF
NLRI. The TLV type 1181 has been assigned by IANA. The BGP-LS
Attribute TLV will contain an 8-octet sequence number. The usage of
the Sequence Number TLV is described in Section 6.1.
0 1 2 3 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 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 | | Type (1181) | Length (8 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (High-Order 32 Bits) | | Sequence Number (High-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Low-Order 32 Bits) | | Sequence Number (Low-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number Sequence Number
The 64-bit strictly increasing sequence number is incremented for The 64-bit strictly-increasing sequence number MUST be incremented
every version of BGP-LS NLRI originated. BGP speakers implementing for every self-originated version of BGP-LS-SPF NLRI. BGP speakers
this specification MUST use available mechanisms to preserve the implementing this specification MUST use available mechanisms to
sequence number's strictly increasing property for the deployed life preserve the sequence number's strictly increasing property for the
of the BGP speaker (including cold restarts). One mechanism for deployed life of the BGP speaker (including cold restarts). One
accomplishing this would be to use the high-order 32 bits of the mechanism for accomplishing this would be to use the high-order 32
sequence number as a wrap/boot count that is incremented anytime the bits of the sequence number as a wrap/boot count that is incremented
BGP router loses its sequence number state or the low-order 32 bits any time the BGP router loses its sequence number state or the low-
wrap. order 32 bits wrap.
When incrementing the sequence number for each self-originated NLRI, When incrementing the sequence number for each self-originated NLRI,
the sequence number should be treated as an unsigned 64-bit value. the sequence number should be treated as an unsigned 64-bit value.
If the lower-order 32-bit value wraps, the higher-order 32-bit value If the lower-order 32-bit value wraps, the higher-order 32-bit value
should be incremented and saved in non-volatile storage. If by some should be incremented and saved in non-volatile storage. If by some
chance the BGP Speaker is deployed long enough that there is a chance the BGP-LS-SPF speaker is deployed long enough that there is a
possibility that the 64-bit sequence number may wrap or a BGP Speaker possibility that the 64-bit sequence number may wrap or a BGP-LS-SPF
completely loses its sequence number state (e.g., the BGP speaker speaker completely loses its sequence number state (e.g., the BGP
hardware is replaced or experiences a cold-start), the phase 1 speaker hardware is replaced or experiences a cold-start), the BGP
decision function (see Section 5.1) rules will insure convergence, NLRI selection rules (see Section 6.1) will insure convergence,
albeit, not immediately. albeit not immediately.
5. Decision Process with SPF Algorithm The Sequence-Number TLV is mandatory for BGP-LS-SPF NLRI. If the
Sequence-Number TLV is not received then the corresponding Link NLRI
is considered as malformed and MUST be handled as 'Treat-as-
withdraw'. An implementation MAY log an error for further analysis.
5.3. NEXT_HOP Manipulation
All BGP peers that support SPF extensions would locally compute the
Loc-RIB Next-Hop as a result of the SPF process. Consequently, the
Next-Hop is always ignored on receipt. The Next-Hop address MUST be
encoded as described in [RFC4760]. BGP speakers MUST interpret the
Next-Hop address of MP_REACH_NLRI attribute as an IPv4 address
whenever the length of the Next-Hop address is 4 octets, and as a
IPv6 address whenever the length of the Next-Hop address is 16
octets.
[RFC4760] modifies the rules of NEXT_HOP attribute whenever the
multiprotocol extensions for BGP-4 are enabled. BGP speakers MUST
set the NEXT_HOP attribute according to the rules specified in
[RFC4760] as the BGP-LS-SPF routing information is carried within the
multiprotocol extensions for BGP-4.
6. Decision Process with SPF Algorithm
The Decision Process described in [RFC4271] takes place in three The Decision Process described in [RFC4271] takes place in three
distinct phases. The Phase 1 decision function of the Decision distinct phases. The Phase 1 decision function of the Decision
Process is responsible for calculating the degree of preference for Process is responsible for calculating the degree of preference for
each route received from a BGP speaker's peer. The Phase 2 decision each route received from a BGP speaker's peer. The Phase 2 decision
function is invoked on completion of the Phase 1 decision function function is invoked on completion of the Phase 1 decision function
and is responsible for choosing the best route out of all those and is responsible for choosing the best route out of all those
available for each distinct destination, and for installing each available for each distinct destination, and for installing each
chosen route into the Loc-RIB. The combination of the Phase 1 and 2 chosen route into the Loc-RIB. The combination of the Phase 1 and 2
decision functions is characterized as a Path Vector algorithm. decision functions is characterized as a Path Vector algorithm.
The SPF based Decision process replaces the BGP best-path Decision The SPF based Decision process replaces the BGP Decision process
process described in [RFC4271]. This process starts with selecting described in [RFC4271]. This process starts with selecting only
only those Node NLRI whose SPF capability TLV matches with the local those Node NLRI whose SPF capability TLV matches with the local BGP-
BGP speaker's SPF capability TLV value. Since Link-State NLRI always LS-SPF speaker's SPF capability TLV value. Since Link-State NLRI
contains the local descriptor [RFC7752], it will only be originated always contains the local node descriptor Section 5.2.1, each NLRI is
by a single BGP speaker in the BGP routing domain. These selected uniquely originated by a single BGP-LS-SPF speaker in the BGP SPF
Node NLRI and their Link/Prefix NLRI are used to build a directed routing domain (the BGP node matching the NLRI's Node Descriptors).
graph during the SPF computation. The best paths for BGP prefixes Instances of the same NLRI originated by multiple BGP speakers would
are installed as a result of the SPF process. be indicative of a configuration error or a masquerading attack
(Section 9). These selected Node NLRI and their Link/Prefix NLRI are
used to build a directed graph during the SPF computation as
described below. The best routes for BGP prefixes are installed in
the RIB as a result of the SPF process.
When BGP-LS-SPF NLRI is received, all that is required is to When BGP-LS-SPF NLRI is received, all that is required is to
determine whether it is the best-path by examining the Node-ID and determine whether it is the most recent by examining the Node-ID and
sequence number as described in Section 5.1. If the received best- sequence number as described in Section 6.1. If the received NLRI
path NLRI had changed, it will be advertised to other BGP-LS-SPF has changed, it will be advertised to other BGP-LS-SPF peers. If the
peers. If the attributes have changed (other than the sequence attributes have changed (other than the sequence number), a BGP SPF
number), a BGP SPF calculation will be scheduled. However, a changed calculation will be triggered. However, a changed NLRI MAY be
NLRI MAY be advertised to other peers almost immediately and advertised immediately to other peers and prior to any SPF
propagation of changes can approach IGP convergence times. To calculation. Note that the BGP MinRouteAdvertisementIntervalTimer
accomplish this, the MinRouteAdvertisementIntervalTimer and and MinASOriginationIntervalTimer [RFC4271] timers are not applicable
MinASOriginationIntervalTimer [RFC4271] are not applicable to the to the BGP-LS-SPF SAFI. The scheduling of the SPF calculation, as
BGP-LS-SPF SAFI. Rather, SPF calculations SHOULD be triggered and described in Section 6.3, is an implementation issue. Scheduling MAY
dampened consistent with the SPF back-off algorithm specified in be dampened consistent with the SPF back-off algorithm specified in
[RFC8405]. [RFC8405].
The Phase 3 decision function of the Decision Process [RFC4271] is The Phase 3 decision function of the Decision Process [RFC4271] is
also simplified since under normal SPF operation, a BGP speaker would also simplified since under normal SPF operation, a BGP speaker MUST
advertise the NLRI selected for the SPF to all BGP peers with the advertise the changed NLRIs to all BGP peers with the BGP-LS-SPF AFI/
BGP-LS/BGP-LS-SPF AFI/SAFI. Application of policy would not be SAFI and install the changed routes in the Global RIB. The only
prevented however its usage to best-path process would be limited as exception are unchanged NLRIs or stale NLRIs, i.e., NLRI received
the SPF relies solely on link metrics. with a less recent (numerically smaller) sequence number.
5.1. Phase-1 BGP NLRI Selection 6.1. BGP NLRI Selection
The rules for NLRI selection are greatly simplified from [RFC4271]. The rules for all BGP-LS-SPF NLRIs selection for phase 1 of the BGP
decision process, section 9.1.1 [RFC4271], no longer apply.
1. If the NLRI is received from the BGP speaker originating the NLRI 1. Routes originated by directly connected BGP SPF peers are
(as determined by the comparing BGP Router ID in the NLRI Node preferred. This condition can be determined by comparing the BGP
identifiers with the BGP speaker Router ID), then it is preferred Identifiers in the received Local Node Descriptor and OPEN
over the same NLRI from non-originators. This rule will assure message. This rule will assure that stale NLRI is updated even
that stale NLRI is updated even if a BGP-LS router loses its if a BGP-LS router loses its sequence number state due to a cold-
sequence number state due to a cold-start. start.
2. If the Sequence-Number TLV is present in the BGP-LS Attribute, 2. The NLRI with the most recent Sequence Number TLV, i.e., highest
then the NLRI with the most recent, i.e., highest sequence number sequence number is selected.
is selected. BGP-LS NLRI with a Sequence-Number TLV will be
considered more recent than NLRI without a BGP-LS Attribute or a
BGP-LS Attribute that doesn't include the Sequence-Number TLV.
3. The final tie-breaker is the NLRI from the BGP Speaker with the 3. The route received from the BGP SPF speaker with the numerically
numerically largest BGP Router ID. larger BGP Identifier is preferred.
When a BGP speaker completely loses its sequence number state, i.e., When a BGP SPF speaker completely loses its sequence number state,
due to a cold start, or in the unlikely possibility that that i.e., due to a cold start, or in the unlikely possibility that that
sequence number wraps, the BGP routing domain will still converge. 64-bit sequence number wraps, the BGP routing domain will still
This is due to the fact that BGP speakers adjacent to the router will converge. This is due to the fact that BGP speakers adjacent to the
always accept self-originated NLRI from the associated speaker as router will always accept self-originated NLRI from the associated
more recent (rule # 1). When BGP speaker reestablishes a connection speaker as more recent (rule # 1). When a BGP speaker reestablishes
with its peers, any existing session will be taken down and stale a connection with its peers, any existing session will be taken down
NLRI will be replaced by the new NLRI and stale NLRI will be and stale NLRI will be replaced. The adjacent BGP speaker will
discarded independent of whether or not BGP graceful restart is update their NLRI advertisements, hop by hop, until the BGP routing
deployed, [RFC4724]. The adjacent BGP speaker will update their NLRI domain has converged.
advertisements in turn until the BGP routing domain has converged.
The modified SPF Decision Process performs an SPF calculation rooted The modified SPF Decision Process performs an SPF calculation rooted
at the BGP speaker using the metrics from Link and Prefix NLRI at the BGP speaker using the metrics from the Link Attribute IGP
Attribute TLVs [RFC7752]. As a result, any attributes that would Metric TLV (1095) and the Prefix Attribute Prefix Metric TLV (1155)
influence the Decision process defined in [RFC4271] like ORIGIN, [RFC7752]. As a result, any other BGP attributes that would
MULTI_EXIT_DISC, and LOCAL_PREF attributes are ignored by the SPF influence the BGP decision process defined in [RFC4271] including
algorithm. Furthermore, the NEXT_HOP attribute value is preserved ORIGIN, MULTI_EXIT_DISC, and LOCAL_PREF attributes are ignored by the
but otherwise ignored during the SPF or best-path. SPF algorithm. Furthermore, the NEXT_HOP attribute value is
preserved but otherwise ignored during the SPF computation for BGP-
LS-SPF NLRIs. The AS_PATH and AS4_PATH [RFC6793] attributes are
preserved and used for loop detection [RFC4271]. They are ignored
during the SPF computation for BGP-LS-SPF NRLIs.
5.2. Dual Stack Support 6.1.1. BGP Self-Originated NLRI
Node, Link, or Prefix NLRI with Node Descriptors matching the local
BGP speaker are considered self-originated. When self-originated
NLRI is received and it doesn't match the local node's NLRI content
(including sequence number), special processing is required.
o If a self-originated NLRI is received and the sequence number is
more recent (i.e., greater than the local node's sequence number
for the NLRI), the NLRI sequence number will be advanced to one
greater than the received sequence number and the NLRI will be
readvertised to all peers.
o If self-originated NLRI is received and the sequence number is the
same as the local node's sequence number but the attributes
differ, the NLRI sequence number will be advanced to one greater
than the received sequence number and the NLRI will be
readvertised to all peers.
o If self-originated Link or Prefix NLRI is received and the Link or
Prefix NLRI is no longer being advertised by the local node, the
NLRI will be withdrawn.
The above actions are performed immediately when the first instance
of a newer self-originated NLRI is received. In this case, the newer
instance is considered to be a stale instance that was advertised by
the local node prior to a restart where the NLRI state is lost.
However, if subsequent newer self-originated NLRI is received for the
same Node, Link, or Prefix NLRI, the readvertisement or withdrawal is
delayed by 5 seconds since it is likely being advertised by a
misconfigured or rogue BGP-LS-SPF speaker Section 9.
6.2. Dual Stack Support
The SPF-based decision process operates on Node, Link, and Prefix The SPF-based decision process operates on Node, Link, and Prefix
NLRIs that support both IPv4 and IPv6 addresses. Whether to run a NLRIs that support both IPv4 and IPv6 addresses. Whether to run a
single SPF instance or multiple SPF instances for separate AFs is a single SPF computation or multiple SPF computations for separate AFs
matter of a local implementation. Normally, IPv4 next-hops are is an implementation matter. Normally, IPv4 next-hops are calculated
calculated for IPv4 prefixes and IPv6 next-hops are calculated for for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
IPv6 prefixes. However, an interesting use-case is deployment of prefixes.
[RFC5549] where IPv6 next-hops are calculated for both IPv4 and IPv6
prefixes. As stated in Section 1, support for Multiple Topology
Routing (MTR) is an area for future study.
5.3. SPF Calculation based on BGP-LS NLRI 6.3. SPF Calculation based on BGP-LS-SPF NLRI
This section details the BGP-LS SPF local routing information base This section details the BGP-LS-SPF local routing information base
(RIB) calculation. The router will use BGP-LS Node, Link, and Prefix (RIB) calculation. The router will use BGP-LS-SPF Node, Link, and
NLRI to populate the local RIB using the following algorithm. This Prefix NLRI to compute routes using the following algorithm. This
calculation yields the set of intra-area routes associated with the calculation yields the set of routes associated with the BGP-LS
BGP-LS domain. A router calculates the shortest-path tree using domain. A router calculates the shortest-path tree using itself as
itself as the root. Variations and optimizations of the algorithm the root. Optimizations to the BGP-LS-SPF algorithm are possible but
are valid as long as it yields the same set of routes. The algorithm MUST yield the same set of routes. The algorithm below supports
below supports Equal Cost Multi-Path (ECMP) routes. Weighted Unequal Equal Cost Multi-Path (ECMP) routes. Weighted Unequal Cost Multi-
Cost Multi-Path are out of scope. The organization of this section Path routes are out of scope. The organization of this section owes
owes heavily to section 16 of [RFC2328]. heavily to section 16 of [RFC2328].
The following abstract data structures are defined in order to The following abstract data structures are defined in order to
specify the algorithm. specify the algorithm.
o Local Route Information Base (RIB) - This is abstract contains o Local Route Information Base (LOC-RIB) - This routing table
reachability information (i.e., next hops) for all prefixes (both contains reachability information (i.e., next hops) for all
IPv4 and IPv6) as well as the Node NLRI reachability. prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF node
Implementations may choose to implement this as separate RIBs for reachability. Implementations may choose to implement this with
each address family and/or Node NLRI. separate RIBs for each address family and/or Prefix versus Node
reachability. It is synonymous with the Loc-RIB specified in
[RFC4271].
o Link State NLRI Database (LSNDB) - Database of BGP-LS NLRI that o Global Routing Information Base (GLOBAL-RIB) - This is Routing
facilitates access to all Node, Link, and Prefix NLRI as well as Information Base (RIB) containing the current routes that are
all the Link and Prefix NLRI corresponding to a given Node NLRI. installed in the router's forwarding plane. This is commonly
Other optimization, such as, resolving bi-directional connectivity referred to in networking parlance as "the RIB".
associations between Link NLRI are possible but of scope of this
document.
o Candidate List - This is a list of candidate Node NLRI with the o Link State NLRI Database (LSNDB) - Database of BGP-LS-SPF NLRI
lowest cost Node NLRI at the front of the list. It is typically that facilitates access to all Node, Link, and Prefix NLRI.
implemented as a heap but other concrete data structures have also
been used. o Candidate List (CAN-LIST) - This is a list of candidate Node
NLRIs. The list is sorted by the cost to reach the Node NLRI with
the Node NLRI with the lowest reachability cost at the head of the
list. This facilitates execution of the Dijkstra algorithm
Section 1.1 where the shortest paths between the local node and
other nodes in graph area computed. The CAN-LIST is typically
implemented as a heap but other data structures have been used.
The algorithm is comprised of the steps below: The algorithm is comprised of the steps below:
1. The current local RIB is invalidated. The local RIB is rebuilt 1. The current LOC-RIB is invalidated, and the CAN-LIST is
during the course of the SPF computation. The existing routing initialized to empty. The LOC-RIB is rebuilt during the course
entries are preserved for comparison to determine changes that of the SPF computation. The existing routing entries are
need to be installed in the global RIB. preserved for comparison to determine changes that need to be
made to the GLOBAL-RIB in step 6.
2. The computing router's Node NLRI is installed in the local RIB 2. The computing router's Node NLRI is updated in the LOC-RIB with a
with a cost of 0 and as the sole entry in the candidate list. cost of 0 and the Node NLRI is also added to the CAN-LIST. The
next-hop list is set to the internal loopback next-hop.
3. The Node NLRI with the lowest cost is removed from the candidate 3. The Node NLRI with the lowest cost is removed from the candidate
list for processing. If the BGP-LS Node attribute includes an list for processing. If the BGP-LS Node attribute includes an
SPF Status TLV (Section 4.1.2) indicating the node is SPF Status TLV (Section 5.2.1.2) indicating the node is
unreachable, the Node NLRI is ignored and the next lowest cost unreachable, the Node NLRI is ignored and the next lowest cost
Node NLRI is selected from candidate list. The Node Node NLRI is selected from candidate list. The Node
corresponding to this NLRI will be referred to as the Current corresponding to this NLRI will be referred to as the Current-
Node. If the candidate list is empty, the SPF calculation has Node. If the candidate list is empty, the SPF calculation has
completed and the algorithm proceeds to step 6. completed and the algorithm proceeds to step 6.
4. All the Prefix NLRI with the same Node Identifiers as the Current 4. All the Prefix NLRI with the same Node Identifiers as the
Node will be considered for installation. The cost for each Current-Node will be considered for installation. The next-
prefix is the metric advertised in the Prefix NLRI added to the hop(s) for these Prefix NLRI are inherited from the Current-Node.
cost to reach the Current Node. The cost for each prefix is the metric advertised in the Prefix
Attribute Prefix Metric TLV (1155) added to the cost to reach the
Current-Node. The following will be done for each Prefix NLRI
(referred to as the Current-Prefix):
* If the BGP-LS Prefix attribute includes an SPF Status TLV * If the BGP-LS Prefix attribute includes an SPF Status TLV
indicating the prefix is unreachable, the BGP-LS Prefix NLRI indicating the prefix is unreachable, the Current-Prefix is
is considered unreachable and the next BGP-LS Prefix NLRI is considered unreachable and the next Prefix NLRI is examined in
examined. Step 4.
* If the prefix is in the local RIB and the cost is greater than * If the Current-Prefix's corresponding prefix is in the LOC-RIB
the Current route's metric, the Prefix NLRI does not and the cost is less than the Current-Prefix's metric, the
contribute to the route and is ignored. Current-Prefix does not contribute to the route and the next
Prefix NLRI is examined in Step 4.
* If the prefix is in the local RIB and the cost is less than * If the Current-Prefix's corresponding prefix is not in the
the current route's metric, the Prefix is installed with the LOC-RIB, the prefix is installed with the Current-Node's next-
Current Node's next-hops replacing the local RIB route's next- hops installed as the LOC-RIB route's next-hops and the metric
hops and the metric being updated. being updated. If the IGP Route Tag TLV (1153) is included in
the Current-Prefix's NLRI Attribute, the tag(s) are installed
in the current LOC-RIB route's tag(s).
* If the prefix is in the local RIB and the cost is same as the * If the Current-Prefix's corresponding prefix is in the LOC-RIB
current route's metric, the Prefix is installed with the and the cost is less than the current route's metric, the
Current Node's next-hops being merged with local RIB route's prefix is installed with the Current-Node's next-hops
next-hops. replacing the LOC-RIB route's next-hops and the metric being
updated and any route tags removed. If the IGP Route Tag TLV
(1153) is included in the Current-Prefix's NLRI Attribute, the
tag(s) are installed in the current LOC-RIB route's tag(s).
5. All the Link NLRI with the same Node Identifiers as the Current * If the Current-Prefix's corresponding prefix is in the LOC-RIB
and the cost is the same as the current route's metric, the
Current-Node's next-hops will be merged with LOC-RIB route's
next-hops. If the IGP Route Tag TLV (1153) is included in the
Current-Prefix's NLRI Attribute, the tag(s) are merged into
the LOC-RIB route's current tags.
5. All the Link NLRI with the same Node Identifiers as the Current-
Node will be considered for installation. Each link will be Node will be considered for installation. Each link will be
examined and will be referred to in the following text as the examined and will be referred to in the following text as the
Current Link. The cost of the Current Link is the advertised Current-Link. The cost of the Current-Link is the advertised IGP
metric in the Link NLRI added to the cost to reach the Current Metric TLV (1095) from the Link NLRI BGP-LS attribute added to
Node. the cost to reach the Current-Node. If the Current-Node is for
the local BGP Router, the next-hop for the link will be a direct
next-hop pointing to the corresponding local interface. For any
other Current-Node, the next-hop(s) for the Current-Link will be
inherited from the Current-Node. The following will be done for
each link:
* Optionally, the prefix(es) associated with the Current Link A. The prefix(es) associated with the Current-Link are installed
are installed into the local RIB using the same rules as were into the LOC-RIB using the same rules as were used for Prefix
used for Prefix NLRI in the previous steps. NLRI in the previous steps. Optionally, in deployments where
BGP-SPF routers have limited routing table capacity,
installation of these subnets can be suppressed. Suppression
will have an operational impact as the IPv4/IPv6 link
endpoint addresses will not be reachable and tools such as
traceroute will display addresses that are not reachable.
* If the current Node NLRI attributes includes the SPF status B. If the Current-Node NLRI attributes includes the SPF status
TLV (Section 4.1.2) and the status indicates that the Node TLV (Section 5.2.1.2) and the status indicates that the Node
doesn't support transit, the next link for the current node is doesn't support transit, the next link for the Current-Node
processed. is processed in Step 5.
* The Current Link's endpoint Node NLRI is accessed (i.e., the C. If the Current-Link's NLRI attribute includes an SPF Status
Node NLRI with the same Node identifiers as the Link TLV indicating the link is down, the BGP-LS-SPF Link NLRI is
endpoint). If it exists, it will be referred to as the considered down and the next link for the Current-Node is
Endpoint Node NLRI and the algorithm will proceed as follows: examined in Step 5.
+ If the BGP-LS Link NLRI attribute includes an SPF Status D. The Current-Link's Remote Node NLRI is accessed (i.e., the
TLV indicating the link is down, the BGP-LS Link NLRI is Node NLRI with the same Node identifiers as the Current-
considered down and the next BGP-LS Link NLRI is examined. Link's Remote Node Descriptors). If it exists, it will be
referred to as the Remote-Node and the algorithm will proceed
as follows:
+ All the Link NLRI corresponding the Endpoint Node NLRI will + If the Remote-Node's NLRI attribute includes an SPF Status
be searched for a back-link NLRI pointing to the current TLV indicating the node is unreachable, the next link for
node. Both the Node identifiers and the Link endpoint the Current-Node is examined in Step 5.
identifiers in the Endpoint Node's Link NLRI must match for
a match. If there is no corresponding Link NLRI
corresponding to the Endpoint Node NLRI, the Endpoint Node
NLRI fails the bi-directional connectivity test and is not
processed further.
+ If the Endpoint Node NLRI is not on the candidate list, it + All the Link NLRI corresponding the Remote-Node will be
is inserted based on the link cost and BGP Identifier (the searched for a Link NLRI pointing to the Current-Node.
latter being used as a tie-breaker). Each Link NLRI is examined for Remote Node Descriptors
matching the Current-Node and Link Descriptors matching
the Current-Link (e.g., sharing a common IPv4 or IPv6
subnet). If both these conditions are satisfied for one
of the Remote-Node's links, the bi-directional
connectivity check succeeds and the Remote-Node may be
processed further. The Remote-Node's Link NLRI providing
bi-directional connectivity will be referred to as the
Remote-Link. If no Remote-Link is found, the next link
for the Current-Node is examined in Step 5.
+ If the Endpoint Node NLRI is already on the candidate list + If the Remote-Link NLRI attribute includes an SPF Status
with a lower cost, it need not be inserted again. TLV indicating the link is down, the Remote-Link NLRI is
considered down and the next link for the Current-Node is
examined in Step 5.
+ If the Endpoint Node NLRI is already on the candidate list + If the Remote-Node is not on the CAN-LIST, it is inserted
with a higher cost, it must be removed and reinserted with based on the cost. The Remote Node's cost is the cost of
a lower cost. Current-Node added the Current-Link's IGP Metric TLV
(1095). The next-hop(s) for the Remote-Node are inherited
from the Current-Link.
* Return to step 3 to process the next lowest cost Node NLRI on + If the Remote-Node NLRI is already on the CAN-LIST with a
the candidate list. higher cost, it must be removed and reinserted with the
Remote-Node cost based on the Current-Link (as calculated
in the previous step). The next-hop(s) for the Remote-
Node are inherited from the Current-Link.
6. The local RIB is examined and changes (adds, deletes, + If the Remote-Node NLRI is already on the CAN-LIST with
modifications) are installed into the global RIB. the same cost, it need not be reinserted on the CAN-LIST.
However, the Current-Link's next-hop(s) must be merged
into the current set of next-hops for the Remote-Node.
5.4. NEXT_HOP Manipulation + If the Remote-Node NLRI is already on the CAN-LIST with a
lower cost, it need not be reinserted on the CAN-LIST.
A BGP speaker that supports SPF extensions MAY interact with peers E. Return to step 3 to process the next lowest cost Node NLRI on
that don't support SPF extensions. If the BGP-LS address family is the CAN-LIST.
advertised to a peer not supporting the SPF extensions described
herein, then the BGP speaker MUST conform to the NEXT_HOP rules
specified in [RFC4271] when announcing the Link-State address family
routes to those peers.
All BGP peers that support SPF extensions would locally compute the 6. The LOC-RIB is examined and changes (adds, deletes,
Loc-RIB next-hops as a result of the SPF process. Consequently, the modifications) are installed into the GLOBAL-RIB. For each route
NEXT_HOP attribute is always ignored on receipt. However, BGP in the LOC-RIB:
speakers SHOULD set the NEXT_HOP address according to the NEXT_HOP
attribute rules specified in [RFC4271].
5.5. IPv4/IPv6 Unicast Address Family Interaction * If the route was added during the current BGP SPF computation,
install the route into the GLOBAL-RIB.
While the BGP-LS SPF address family and the IPv4/IPv6 unicast address * If the route modified during the current BGP SPF computation
families install routes into the same device routing tables, they (e.g., metric, tags, or next-hops), update the route in the
GLOBAL-RIB.
* If the route was not installed during the current BGP SPF
computation, remove the route from both the GLOBAL-RIB and the
LOC-RIB.
6.4. IPv4/IPv6 Unicast Address Family Interaction
While the BGP-LS-SPF address family and the IPv4/IPv6 unicast address
families MAY install routes into the same device routing tables, they
will operate independently much the same as OSPF and IS-IS would will operate independently much the same as OSPF and IS-IS would
operate today (i.e., "Ships-in-the-Night" mode). There will be no operate today (i.e., "Ships-in-the-Night" mode). There is no
implicit route redistribution between the BGP address families. implicit route redistribution between the BGP address families.
However, implementation specific redistribution mechanisms SHOULD be
made available with the restriction that redistribution of BGP-LS SPF
routes into the IPv4 address family applies only to IPv4 routes and
redistribution of BGP-LS SPF route into the IPv6 address family
applies only to IPv6 routes.
Given the fact that SPF algorithms are based on the assumption that It is RECOMMENDED that BGP-LS-SPF IPv4/IPv6 route computation and
all routers in the routing domain calculate the precisely the same installation be given scheduling priority by default over other BGP
SPF tree and install the same set of routes, it is RECOMMENDED that address families as these address families are considered as underlay
BGP-LS SPF IPv4/IPv6 routes be given priority by default when SAFIs. Similarly, it is RECOMMENDED that the route preference or
installed into their respective RIBs. In common implementations the administrative distance give active route installation preference to
prioritization is governed by route preference or administrative BGP-LS-SPF IPv4/IPv6 routes over BGP routes from other AFI/SAFIs.
distance with lower being more preferred. However, this preference MAY be overridden by an operator-configured
policy.
5.6. NLRI Advertisement and Convergence 6.5. NLRI Advertisement
5.6.1. Link/Prefix Failure Convergence 6.5.1. Link/Prefix Failure Convergence
A local failure will prevent a link from being used in the SPF A local failure will prevent a link from being used in the SPF
calculation due to the IGP bi-directional connectivity requirement. calculation due to the IGP bi-directional connectivity requirement.
Consequently, local link failures should always be given priority Consequently, local link failures SHOULD always be given priority
over updates (e.g., withdrawing all routes learned on a session) in over updates (e.g., withdrawing all routes learned on a session) in
order to ensure the highest priority propagation and optimal order to ensure the highest priority propagation and optimal
convergence. convergence.
An IGP such as OSPF [RFC2328] will stop using the link as soon as the An IGP such as OSPF [RFC2328] will stop using the link as soon as the
Router-LSA for one side of the link is received. With normal BGP Router-LSA for one side of the link is received. With a BGP
advertisement, the link would continue to be used until the last copy advertisement, the link would continue to be used until the last copy
of the BGP-LS Link NLRI is withdrawn. In order to avoid this delay, of the BGP-LS-SPF Link NLRI is withdrawn. In order to avoid this
the originator of the Link NLRI will advertise a more recent version delay, the originator of the Link NLRI SHOULD advertise a more recent
of the BGP-LS Link NLRI including the SPF Status TLV Section 4.2.2 version with an increased Sequence Number TLV for the BGP-LS-SPF Link
indicating the link is down with respect to BGP SPF. After some NLRI including the SPF Status TLV (Section 5.2.2.2) indicating the
configurable period of time, e.g., 2-3 seconds, the BGP-LS Link NLRI link is down with respect to BGP SPF. After some configurable period
can be withdrawn with no consequence. If the link becomes available of time, which is an implementation dependent, e.g., 2-3 seconds, the
in that period, the originator of the BGP-LS LINK NLRI will simply BGP-LS-SPF Link NLRI can be withdrawn with no consequence. If the
advertise a more recent version of the BGP-LS Link NLRI without the link becomes available in that period, the originator of the BGP-LS-
SPF Status TLV in the BGP-LS Link Attributes. SPF LINK NLRI will simply advertise a more recent version of the BGP-
LS-SPF Link NLRI without the SPF Status TLV in the BGP-LS Link
Attributes.
Similarly, when a prefix becomes unreachable, a more recent version Similarly, when a prefix becomes unreachable, a more recent version
of the BGP-LS Prefix NLRI will be advertised with the SPF Status TLV of the BGP-LS-SPF Prefix NLRI will be advertised with the SPF Status
Section 4.3.1 indicating the prefix is unreachable in the BGP-LS TLV (Section 5.2.3.1) indicating the prefix is unreachable in the
Prefix Attributes and the prefix will be considered unreachable with BGP-LS Prefix Attributes and the prefix will be considered
respect to BGP SPF. After some configurable period of time, e.g., unreachable with respect to BGP SPF. After some configurable period
2-3 seconds, the BGP-LS Prefix NLRI can be withdrawn with no of time, which is implementation dependent, e.g., 2-3 seconds, the
consequence. If the prefix becomes reachable in that period, the BGP-LS-SPF Prefix NLRI can be withdrawn with no consequence. If the
originator of the BGP-LS Prefix NLRI will simply advertise a more prefix becomes reachable in that period, the originator of the BGP-
recent version of the BGP-LS Prefix NLRI without the SPF Status TLV LS-SPF Prefix NLRI will simply advertise a more recent version of the
in the BGP-LS Prefix Attributes. BGP-LS-SPF Prefix NLRI without the SPF Status TLV in the BGP-LS
Prefix Attributes.
5.6.2. Node Failure Convergence 6.5.2. Node Failure Convergence
With BGP without graceful restart [RFC4724], all the NLRI advertised With BGP without graceful restart [RFC4724], all the NLRI advertised
by node are implicitly withdrawn when a session failure is detected. by a node are implicitly withdrawn when a session failure is
If fast failure detection such as BFD is utilized, and the node is on detected. If fast failure detection such as BFD is utilized, and the
the fastest converging path, the most recent versions of BGP-LS NLRI node is on the fastest converging path, the most recent versions of
may be withdrawn while these versions are in-flight on longer paths. BGP-LS-SPF NLRI may be withdrawn. This will result into an older
This will result the older version of the NLRI being used until the version of the NLRI being used until the new versions arrive and,
new versions arrive and, potentially, unnecessary route flaps. potentially, unnecessary route flaps. Therefore, BGP-LS-SPF NLRI
Therefore, BGP-LS SPF NLRI SHOULD always be retained before being SHOULD always be retained before being implicitly withdrawn for a
implicitly withdrawn for a brief configurable interval, e.g., 2-3 configurable implementation-dependent interval, e.g., 2-3 seconds.
seconds. This will not delay convergence since the adjacent nodes This will not delay convergence since the adjacent nodes will detect
will detect the link failure and advertise a more recent NLRI the link failure and advertise a more recent NLRI indicating the link
indicating the link is down with respect to BGP SPF Section 5.6.1 and is down with respect to BGP SPF (Section 6.5.1) and the BGP SPF
the BGP-SPF calculation will failure the bi-directional connectivity calculation will fail the bi-directional connectivity check
check. Section 6.3.
5.7. Error Handling 7. Error Handling
This section describes the Error Handling actions, as described in
[RFC7606], that are specific to SAFI BGP-LS-SPF BGP Update message
processing.
7.1. Processing of BGP-LS-SPF TLVs
When a BGP speaker receives a BGP Update containing a malformed Node
NLRI SPF Status TLV in the BGP-LS Attribute [RFC7752], it MUST ignore
the received TLV and MUST NOT pass it to other BGP peers as specified
in [RFC7606]. When discarding an associated Node NLRI with a
malformed TLV, a BGP speaker SHOULD log an error for further
analysis.
When a BGP speaker receives a BGP Update containing a malformed Link
NLRI SPF Status TLV in the BGP-LS Attribute [RFC7752], it MUST ignore
the received TLV and MUST NOT pass it to other BGP peers as specified
in [RFC7606]. When discarding an associated Link NLRI with a
malformed TLV, a BGP speaker SHOULD log an error for further
analysis.
When a BGP speaker receives a BGP Update containing a malformed
Prefix NLRI SPF Status TLV in the BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in [RFC7606]. When discarding an associated Prefix NLRI
with a malformed TLV, a BGP speaker SHOULD log an error for further
analysis.
When a BGP speaker receives a BGP Update containing a malformed SPF When a BGP speaker receives a BGP Update containing a malformed SPF
Capability TLV in the Node NLRI BGP-LS Attribute [RFC7752], it MUST Capability TLV in the Node NLRI BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and the Node NLRI and not pass it to other ignore the received TLV and the Node NLRI and MUST NOT pass it to
BGP peers as specified in [RFC7606]. When discarding a Node NLRI other BGP peers as specified in [RFC7606]. When discarding a Node
with malformed TLV, a BGP speaker SHOULD log an error for further NLRI with a malformed TLV, a BGP speaker SHOULD log an error for
analysis. further analysis.
6. IANA Considerations When a BGP speaker receives a BGP Update containing a malformed IPv4
Prefix-Length TLV in the Link NLRI BGP-LS Attribute [RFC7752], it
MUST ignore the received TLV and the Node NLRI and MUST NOT pass it
to other BGP peers as specified in [RFC7606]. The corresponding Link
NLRI is considered as malformed and MUST be handled as 'Treat-as-
withdraw'. An implementation MAY log an error for further analysis.
This document defines an AFI/SAFI for BGP-LS SPF operation and When a BGP speaker receives a BGP Update containing a malformed IPv6
requests IANA to assign the BGP-LS/BGP-LS-SPF (AFI 16388 / SAFI TBD1) Prefix-Length TLV in the Link NLRI BGP-LS Attribute [RFC7752], it
as described in [RFC4760]. MUST ignore the received TLV and the Node NLRI and MUST NOT pass it
to other BGP peers as specified in [RFC7606]. The corresponding Link
NLRI is considered as malformed and MUST be handled as 'Treat-as-
withdraw'. An implementation MAY log an error for further analysis.
This document also defines five attribute TLVs for BGP-LS NLRI. We 7.2. Processing of BGP-LS-SPF NLRIs
request IANA to assign types for the SPF capability TLV, Sequence
A Link-State NLRI MUST NOT be considered as malformed or invalid
based on the inclusion/exclusion of TLVs or contents of the TLV
fields (i.e., semantic errors), as described in Section 5.1 and
Section 5.1.1.
A BGP-LS-SPF Speaker MUST perform the following syntactic validation
of the BGP-LS-SPF NLRI to determine if it is malformed.
1. Does the sum of all TLVs found in the BGP MP_REACH_NLRI attribute
correspond to the BGP MP_REACH_NLRI length?
2. Does the sum of all TLVs found in the BGP MP_UNREACH_NLRI
attribute correspond to the BGP MP_UNREACH_NLRI length?
3. Does the sum of all TLVs found in a BGP-LS-SPF NLRI correspond to
the Total NLRI Length field of all its Descriptors?
4. When an NLRI TLV is recognized, is the length of the TLV and its
sub-TLVs valid?
5. Has the syntactic correctness of the NLRI fields been verified as
per [RFC7606]?
6. Has the rule regarding ordering of TLVs been followed as
described in Section 5.1.1?
When the error determined allows for the router to skip the malformed
NLRI(s) and continue processing of the rest of the update message
(e.g., when the TLV ordering rule is violated), then it MUST handle
such malformed NLRIs as 'Treat-as-withdraw'. In other cases, where
the error in the NLRI encoding results in the inability to process
the BGP update message (e.g., length related encoding errors), then
the router SHOULD handle such malformed NLRIs as 'AFI/SAFI disable'
when other AFI/SAFI besides BGP-LS are being advertised over the same
session. Alternately, the router MUST perform 'session reset' when
the session is only being used for BGP-LS-SPF or when its 'AFI/SAFI
disable' action is not possible.
7.3. Processing of BGP-LS Attribute
A BGP-LS Attribute MUST NOT be considered as malformed or invalid
based on the inclusion/exclusion of TLVs or contents of the TLV
fields (i.e., semantic errors), as described in Section 5.1 and
Section 5.1.1.
A BGP-LS-SPF Speaker MUST perform the following syntactic validation
of the BGP-LS Attribute to determine if it is malformed.
1. Does the sum of all TLVs found in the BGP-LS-SPF Attribute
correspond to the BGP-LS Attribute length?
2. Has the syntactic correctness of the Attributes (including BGP-LS
Attribute) been verified as per [RFC7606]?
3. Is the length of each TLV and, when the TLV is recognized then,
its sub-TLVs in the BGP-LS Attribute valid?
When the detected error allows for the router to skip the malformed
BGP-LS Attribute and continue processing of the rest of the update
message (e.g., when the BGP-LS Attribute length and the total Path
Attribute Length are correct but some TLV/sub-TLV length within the
BGP-LS Attribute is invalid), then it MUST handle such malformed BGP-
LS Attribute as 'Attribute Discard'. In other cases, when the error
in the BGP-LS Attribute encoding results in the inability to process
the BGP update message, then the handling is the same as described
above for malformed NLRI.
Note that the 'Attribute Discard' action results in the loss of all
TLVs in the BGP-LS Attribute and not the removal of a specific
malformed TLV. The removal of specific malformed TLVs may give a
wrong indication to a BGP-LS-SPF speaker that the specific
information is being deleted or is not available.
When a BGP-LS-SPF speaker receives an update message with Link-State
NLRI(s) in the MP_REACH_NLRI but without the BGP-LS-SPF Attribute, it
is most likely an indication that a BGP-LS-SPF speaker preceding it
has performed the 'Attribute Discard' fault handling. An
implementation SHOULD preserve and propagate the Link-State NLRIs in
such an update message so that the BGP-LS-SPF speaker can detect the
loss of link-state information for that object and not assume its
deletion/withdrawal. This also makes it possible for a network
operator to trace back to the BGP-LS-SPF speaker which actually
detected a problem with the BGP-LS Attribute.
An implementation SHOULD log an error for further analysis for
problems detected during syntax validation.
When a BGP speaker receives a BGP Update containing a malformed IGP
metric TLV in the Link NLRI BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and the Link NLRI and MUST NOT pass it to
other BGP peers as specified in [RFC7606]. When discarding a Link
NLRI with a malformed TLV, a BGP speaker SHOULD log an error for
further analysis.
8. IANA Considerations
This document defines the use of SAFI (80) for BGP SPF operation
Section 5.1, and requests IANA to assign the value from the First
Come First Serve (FCFS) range in the Subsequent Address Family
Identifiers (SAFI) Parameters registry.
This document also defines five attribute TLVs of BGP-LS-SPF NLRI.
We request IANA to assign types for the SPF capability TLV, Sequence
Number TLV, IPv4 Link Prefix-Length TLV, IPv6 Link Prefix-Length TLV, Number TLV, IPv4 Link Prefix-Length TLV, IPv6 Link Prefix-Length TLV,
and SPF Status TLV from the "BGP-LS Node Descriptor, Link Descriptor, and SPF Status TLV from the "BGP-LS Node Descriptor, Link Descriptor,
Prefix Descriptor, and Attribute TLVs" Registry. Prefix Descriptor, and Attribute TLVs" Registry.
7. Security Considerations +-------------------------+-----------------+--------------------+
| Attribute TLV | Suggested Value | NLRI Applicability |
+-------------------------+-----------------+--------------------+
| SPF Capability | 1180 | Node |
| SPF Status | 1184 | Node, Link, Prefix |
| IPv4 Link Prefix Length | 1182 | Link |
| IPv6 Link Prefix Length | 1183 | Link |
| Sequence Number | 1181 | Node, Link, Prefix |
+-------------------------+-----------------+--------------------+
This extension to BGP does not change the underlying security issues Table 1: NLRI Attribute TLVs
inherent in the existing [RFC4271], [RFC4724], and [RFC7752].
8. Management Considerations 9. Security Considerations
This section includes unique management considerations for the BGP-LS This document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This
SPF address family. document does not change the underlying security issues inherent in
the BGP protocol [RFC4271]. The Security Considerations discussed in
[RFC4271] apply to the BGP SPF functionality as well. The analysis
of the security issues for BGP mentioned in [RFC4272] and [RFC6952]
also applies to this document. The analysis of Generic Threats to
Routing Protocols done in [RFC4593] is also worth noting. As the
modifications described in this document for BGP SPF apply to IPv4
Unicast and IPv6 Unicast as undelay SAFIs in a single BGP SPF Routing
Domain, the BGP security solutions described in [RFC6811] and
[RFC8205] are somewhat constricted as they are meant to apply for
inter-domain BGP where multiple BGP Routing Domains are typically
involved. The BGP-LS-SPF SAFI NLRI described in this document are
typically advertised between EBGP or IBGP speakers under a single
administrative domain.
8.1. Configuration In the context of the BGP peering associated with this document, a
BGP speaker MUST NOT accept updates from a peer that is not within
any administrative control of an operator. That is, a participating
BGP speaker SHOULD be aware of the nature of its peering
relationships. Such protection can be achieved by manual
configuration of peers at the BGP speaker.
In addition to configuration of the BGP-LS SPF address family, In order to mitigate the risk of peering with BGP speakers
implementations SHOULD support the configuration of the masquerading as legitimate authorized BGP speakers, it is recommended
INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY, TIME_TO_LEARN, that the TCP Authentication Option (TCP-AO) [RFC5925] be used to
and HOLDDOWN_INTERVAL as documented in [RFC8405]. authenticate BGP sessions. If an authorized BGP peer is compromised,
that BGP peer could advertise modified Node, Link, or Prefix NLRI
will result in misrouting, repeating origination of NLRI, and/or
excessive SPF calculations. When a BGP speaker detects that its
self-originated NLRI is being originated by another BGP speaker, an
appropriate error should be logged so that the operator can take
corrective action.
8.2. Operational Data 10. Management Considerations
This section includes unique management considerations for the BGP-
LS-SPF address family.
10.1. Configuration
All routers in BGP SPF Routing Domain are under a single
administrative domain allowing for consistent configuration.
10.1.1. Link Metric Configuration
Within a BGP SPF Routing Domain, the IGP metrics for all advertised
links SHOULD be configured or defaulted consistently. For example,
if a default metric is used for one router's links, then a similar
metric should be used for all router's links. Similarly, if the link
cost is derived from using the inverse of the link bandwidth on one
router, then this SHOULD be done for all routers and the same
reference bandwidth should be used to derive the inversely
proportional metric. Failure to do so will not result in correct
routing based on link metric.
10.1.2. backoff-config
In addition to configuration of the BGP-LS-SPF address family,
implementations SHOULD support the "Shortest Path First (SPF) Back-
Off Delay Algorithm for Link-State IGPs" [RFC8405]. If supported,
configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST be
supported [RFC8405]. Section 6 of [RFC8405] recommends consistent
configuration of these values throughout the IGP routing domain and
this also applies to the BGP SPF Routing Domain.
10.2. Operational Data
In order to troubleshoot SPF issues, implementations SHOULD support In order to troubleshoot SPF issues, implementations SHOULD support
an SPF log including entries for previous SPF computations, Each SPF an SPF log including entries for previous SPF computations. Each SPF
log entry would include the BGP-LS NLRI SPF triggering the SPF, SPF log entry would include the BGP-LS-SPF NLRI SPF triggering the SPF,
scheduled time, SPF start time, SPF end time, and SPF type if SPF scheduled time, SPF start time, SPF end time, and SPF type if
different types of SPF are supported. Since the size of the log will different types of SPF are supported. Since the size of the log will
be finite, implementations SHOULD also maintain counters for the be finite, implementations SHOULD also maintain counters for the
total number of SPF computations of each type and the total number of total number of SPF computations and the total number of SPF
SPF triggering events. Additionally, to troubleshoot SPF scheduling triggering events. Additionally, to troubleshoot SPF scheduling and
and back-off [RFC8405], the current SPF back-off state, remaining back-off [RFC8405], the current SPF back-off state, remaining time-
time-to-learn, remaining holddown, last trigger event time, last SPF to-learn, remaining holddown, last trigger event time, last SPF time,
time, and next SPF time should be available. and next SPF time should be available.
9. Implementation Status 11. Implementation Status
Note RFC Editor: Please remove this section and the associated Note RFC Editor: Please remove this section and the associated
references prior to publication. references prior to publication.
This section records the status of known implementations of the This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942]. Internet-Draft and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may features. Readers are advised to note that other implementations may
exist. exist.
According to RFC 7942, "this will allow reviewers and working groups According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature. and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as It is up to the individual working groups to use this information as
they see fit". they see fit".
The BGP-LS SPF implementatation status is documented in The BGP-LS-SPF implementation status is documented in
[I-D.psarkar-lsvr-bgp-spf-impl]. [I-D.psarkar-lsvr-bgp-spf-impl].
10. Acknowledgements 12. Acknowledgements
The authors would like to thank Sue Hares, Jorge Rabadan, Boris The authors would like to thank Sue Hares, Jorge Rabadan, Boris
Hassanov, Dan Frost, Matt Anderson, Fred Baker, and Lukas Krattiger Hassanov, Dan Frost, Matt Anderson, Fred Baker, and Lukas Krattiger
for their review and comments. Thanks to Pushpasis Sarkar for for their review and comments. Thanks to Pushpasis Sarkar for
discussions on preventing a BGP SPF Router from being used for non- discussions on preventing a BGP SPF Router from being used for non-
local traffic (i.e., transit traffic). local traffic (i.e., transit traffic).
The authors extend special thanks to Eric Rosen for fruitful The authors extend special thanks to Eric Rosen for fruitful
discussions on BGP-LS SPF convergence as compared to IGPs. discussions on BGP-LS-SPF convergence as compared to IGPs.
11. Contributors 13. Contributors
In addition to the authors listed on the front page, the following In addition to the authors listed on the front page, the following
co-authors have contributed to the document. co-authors have contributed to the document.
Derek Yeung Derek Yeung
Arrcus, Inc. Arrcus, Inc.
derek@arrcus.com derek@arrcus.com
Gunter Van De Velde Gunter Van De Velde
Nokia Nokia
skipping to change at page 20, line 35 skipping to change at page 33, line 25
abhay@arrcus.com abhay@arrcus.com
Venu Venugopal Venu Venugopal
Cisco Systems Cisco Systems
venuv@cisco.com venuv@cisco.com
Chaitanya Yadlapalli Chaitanya Yadlapalli
AT&T AT&T
cy098d@att.com cy098d@att.com
12. References 14. References
12.1. Normative References
[I-D.ietf-idr-bgpls-segment-routing-epe] 14.1. Normative References
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "BGP-LS extensions for Segment Routing
BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
segment-routing-epe-19 (work in progress), May 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006, DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>. <https://www.rfc-editor.org/info/rfc4271>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, DOI 10.17487/RFC4593,
October 2006, <https://www.rfc-editor.org/info/rfc4593>.
[RFC4750] Joyal, D., Ed., Galecki, P., Ed., Giacalone, S., Ed.,
Coltun, R., and F. Baker, "OSPF Version 2 Management
Information Base", RFC 4750, DOI 10.17487/RFC4750,
December 2006, <https://www.rfc-editor.org/info/rfc4750>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K. [RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages", Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015, RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>. <https://www.rfc-editor.org/info/rfc7606>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752, Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016, DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>. <https://www.rfc-editor.org/info/rfc7752>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., [RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Decraene, B., Litkowski, S., and R. Shakir, "Segment Specification", RFC 8205, DOI 10.17487/RFC8205, September
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 2017, <https://www.rfc-editor.org/info/rfc8205>.
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A., [RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF) Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405, Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018, DOI 10.17487/RFC8405, June 2018,
<https://www.rfc-editor.org/info/rfc8405>. <https://www.rfc-editor.org/info/rfc8405>.
12.2. Information References [RFC8654] Bush, R., Patel, K., and D. Ward, "Extended Message
Support for BGP", RFC 8654, DOI 10.17487/RFC8654, October
2019, <https://www.rfc-editor.org/info/rfc8654>.
[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>.
14.2. Informational References
[I-D.ietf-lsvr-applicability] [I-D.ietf-lsvr-applicability]
Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and
Applicability of Link State Vector Routing in Data Applicability of Link State Vector Routing in Data
Centers", draft-ietf-lsvr-applicability-05 (work in Centers", draft-ietf-lsvr-applicability-05 (work in
progress), March 2020. progress), March 2020.
[I-D.psarkar-lsvr-bgp-spf-impl] [I-D.psarkar-lsvr-bgp-spf-impl]
Sarkar, P., Patel, K., Pallagatti, S., and s. Sarkar, P., Patel, K., Pallagatti, S., and s.
sajibasil@gmail.com, "BGP Shortest Path Routing Extension sajibasil@gmail.com, "BGP Shortest Path Routing Extension
skipping to change at page 22, line 15 skipping to change at page 35, line 43
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>. <https://www.rfc-editor.org/info/rfc4456>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y. [RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724, Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007, DOI 10.17487/RFC4724, January 2007,
<https://www.rfc-editor.org/info/rfc4724>. <https://www.rfc-editor.org/info/rfc4724>.
[RFC4750] Joyal, D., Ed., Galecki, P., Ed., Giacalone, S., Ed.,
Coltun, R., and F. Baker, "OSPF Version 2 Management
Information Base", RFC 4750, DOI 10.17487/RFC4750,
December 2006, <https://www.rfc-editor.org/info/rfc4750>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790,
DOI 10.17487/RFC4790, March 2007,
<https://www.rfc-editor.org/info/rfc4790>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007, RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>. <https://www.rfc-editor.org/info/rfc4915>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286, IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008, DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>. <https://www.rfc-editor.org/info/rfc5286>.
[RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
Layer Reachability Information with an IPv6 Next Hop",
RFC 5549, DOI 10.17487/RFC5549, May 2009,
<https://www.rfc-editor.org/info/rfc5549>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>. <https://www.rfc-editor.org/info/rfc5880>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of [RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938, BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016, DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>. <https://www.rfc-editor.org/info/rfc7938>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running [RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205, Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016, RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>. <https://www.rfc-editor.org/info/rfc7942>.
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