wdiff draft-ietf-lsvr-bgp-spf-11.txt draft-ietf-lsvr-bgp-spf-12.txt

Network Working Group K. Patel
Internet-Draft Arrcus, Inc.
Intended status: Standards Track A. Lindem
Expires: February 4, July 30, 2021 Cisco Systems
S. Zandi
LinkedIn
W. Henderickx
Nokia
August 3, 2020
January 26, 2021

BGP Link-State Shortest Path First (SPF) Routing Extensions for BGP Protocol
draft-ietf-lsvr-bgp-spf-11
draft-ietf-lsvr-bgp-spf-12

Abstract

Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on
BGP as their single routing protocol for both their fabric routing
and their Data Center Interconnect (DCI) routing. This document
describes a solution which leverages extensions to BGP to use BGP Link-State distribution and
the Shortest Path First (SPF) algorithm similar to used by Internal Gateway
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

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on February 4, July 30, 2021.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4
1.2.
1.3. Document Overview . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 5 6
2. Base BGP Protocol Relationship . . . . . . . . . . . . . . . 6
3. BGP Link-State (BGP-LS) Relationship . . . . . . . . . . . . 7
4. BGP Peering Models . . . . . . . . . . . . . . . . . . . . . 5
2.1. 8
4.1. BGP Single-Hop Peering on Network Node Connections . . . 5
2.2. 8
4.2. BGP Peering Between Directly Connected Network Directly-Connected Nodes . . 5
2.3. . . . . 8
4.3. BGP Peering in Route-Reflector or Controller Topology . . 6
3. 9
5. BGP Shortest Path Routing (SPF) Protocol Extensions . . . . . 9
5.1. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . 9
5.1.1. BGP-LS-SPF NLRI TLVs . . 6
4. Extensions to . . . . . . . . . . . . . . 9
5.1.2. BGP-LS Attribute . . . . . . . . . . . . . . . . . . 10
5.2. Extensions to BGP-LS . . . . . 6
4.1. . . . . . . . . . . . . . 11
5.2.1. Node NLRI Usage . . . . . . . . . . . . . . . . . . . . . 7
4.1.1. 11
5.2.1.1. Node NLRI Attribute SPF Capability TLV . . . . . . . 7
4.1.2. BGP-LS 11
5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV . . . . . . 8
4.2. 12
5.2.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. BGP-LS 13
5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs . . . . 9
4.2.2. BGP-LS 14
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV . . 14
5.2.3. IPv4/IPv6 Prefix NLRI Usage . . . . 9
4.3. Prefix NLRI Usage . . . . . . . . . 15
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV . 16
5.2.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . 16
5.3. NEXT_HOP Manipulation . . 10
4.3.1. BGP-LS Prefix NLRI Attribute SPF Status TLV . . . . . 10
4.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . . . 10
5. . 17
6. Decision Process with SPF Algorithm . . . . . . . . . . . . . 11
5.1. Phase-1 18
6.1. BGP NLRI Selection . . . . . . . . . . . . . . . 12
5.2. Dual Stack Support . . . . . . 19
6.1.1. BGP Self-Originated NLRI . . . . . . . . . . . . . 13
5.3. SPF Calculation based on BGP-LS NLRI . 20
6.2. Dual Stack Support . . . . . . . . . 13
5.4. NEXT_HOP Manipulation . . . . . . . . . . 20
6.3. SPF Calculation based on BGP-LS-SPF NLRI . . . . . . . . 16
5.5. 20
6.4. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 16
5.6. 25
6.5. NLRI Advertisement and Convergence . . . . . . . . . . . 17
5.6.1. . . . . . . . . 25
6.5.1. Link/Prefix Failure Convergence . . . . . . . . . . . 17
5.6.2. 25
6.5.2. Node Failure Convergence . . . . . . . . . . . . . . 17
5.7. 26
7. Error Handling . . . . . . . . . . . . . . . . . . . . . 18
6. . . 26
7.1. Processing of BGP-LS-SPF TLVs . . . . . . . . . . . . . . 26
7.2. Processing of BGP-LS-SPF NLRIs . . . . . . . . . . . . . 27
7.3. Processing of BGP-LS Attribute . . . . . . . . . . . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. 30
10. Management Considerations . . . . . . . . . . . . . . . . . . 18
8.1. 31
10.1. Configuration . . . . . . . . . . . . . . . . . . . . . 31
10.1.1. Link Metric Configuration . . 18
8.2. . . . . . . . . . . . 31
10.1.2. backoff-config . . . . . . . . . . . . . . . . . . . 31
10.2. Operational Data . . . . . . . . . . . . . . . . . . . . 19
9. 31
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 19
10. 32
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
11. 32
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
12. 32
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. 33
14.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Information 33
14.2. Informational References . . . . . . . . . . . . . . . . . 21 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 36

1. Introduction

Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on
BGP [RFC4271] as their single routing protocol for both their fabric
routing and their Data Center Interconnect (DCI) routing.
Requirements and procedures for using BGP are described in routing [RFC7938].
This document describes an alternative solution which leverages BGP-
LS [RFC7752] and the Shortest Path First algorithm similar to used by Internal
Gateway Protocols (IGPs) such as OSPF [RFC2328].

[RFC4271] defines the Decision Process that is used to select routes
for subsequent advertisement by applying the policies in

This document leverages both the local
Policy Information Base (PIB) to BGP protocol [RFC4271] and the routes stored in its Adj-RIBs-
In. BGP-
LS [RFC7752] protocols. The output of the Decision Process is relationship, as well as the set scope of routes that are
announced by a BGP speaker to its peers. These selected routes
changes are
stored by a BGP speaker described respectively in the speaker's Adj-RIBs-Out according to
policy.

[RFC7752] describes a mechanism by which link-state and TE
information can be collected from networks Section 2 and shared with external
components using BGP. This is achieved by defining NLRI advertised
within the BGP-LS/BGP-LS-SPF AFI/SAFI. Section 3. The BGP-LS extensions defined
in [RFC7752] makes use of the Decision Process defined in [RFC4271].

This document augments [RFC7752] by replacing its use of the existing
Decision Process. Rather than reusing the BGP-LS SAFI, the BGP-LS-
modifications to [RFC4271] for BGP SPF SAFI is introduced described herein only apply to insure backward compatibility. The Phase 1
IPv4 and 2 decision functions of the Decision Process are replaced with
the Shortest Path First (SPF) algorithm also known IPv6 as underlay unicast Subsequent Address Families
Identifiers (SAFIs). Operations for any other BGP SAFIs are outside
the Dijkstra
algorithm. The Phase 3 decision function is also simplified since it
is no longer dependent on the previous phases. scope of this document.

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 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] (LFAs).
The SPF LFA extensions defined in [RFC5286] can be similarly applied
to BGP SPF calculations. However, the event details are a matter of link failures.
implementation detail. Furthermore, a BGP based BGP-based solution lends
itself to multiple peering models including those incorporating route-
reflectors
route-reflectors [RFC4456] or controllers.

Support for Multiple Topology

1.1. Terminology

This specification reuses terms defined in section 1.1 of [RFC4271]
including BGP speaker, NLRI, and Route.

Additionally, this document introduces the following terms:

BGP SPF Routing (MTR) 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 [RFC4915] the BGP-LS-SPF SAFI
(Section 5.1) and is an area being used for further study dependent on deployment requirements.

1.1. 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
sole routing protocol in an MSDC, one might question the motivation
for defining an alternate BGP deployment model when a mature solution
exists. For both alternatives, BGP offers the operational benefits
of a single routing protocol. protocol as opposed to the combination of an IGP
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 BGP-LS-SPF speakers in the BGP SPF
routing domain will have a complete view of the topology. This will
allow support for ECMP, IP fast-reroute (e.g., Loop-Free
Alternatives), Shared Risk Link Groups (SRLGs), and other routing
enhancements without advertisement of addition additional BGP paths [RFC7911]
or other extensions. In short, the advantages of an IGP such as OSPF
[RFC2328] are availed in BGP.

With the simplified BGP decision process as defined in Section 5.1, 6,
NLRI changes can be disseminated throughout the BGP routing domain
much more rapidly (equivalent to IGPs with the proper
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
advertisement. With standard BGP distance-vector routing, a single
link failure may impact 100s or 1000s prefixes and result in the
withdrawal or re-advertisement of the attendant NLRI. With BGP SPF,
only the BGP speakers corresponding to the link NLRI need to withdraw
the corresponding BGP-LS BGP-LS-SPF Link NLRI. This advantage Additionally, the changed
NLRI will contribute be advertised immediately as opposed to both faster convergence and better scaling. 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
advertisement and distributed computation require a minimal number of
sessions and copies of the NLRI since only the latest version of the
NLRI from the originator is required. Given that verification of the
adjacencies is done outside of BGP (see Section 2), 4), each BGP speaker
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
backup). Functions such as Optimized Route Reflection (ORR) are
supported without extension by virtue of the primary advantages. (see Section 4). Additionally, a controller could
inject topology that is learned outside the BGP SPF routing domain.

Given that controllers are already consuming BGP-LS NLRI [RFC7752],
reusing
this functionality can be reused for the BGP-LS SPF leverages the existing controller
implementations. BGP-LS-SPF NLRI.

Another potential advantage of BGP SPF is that both IPv6 and IPv4 can
both be supported in the same address family using the BGP-LS-SPF SAFI with the same topology. BGP-LS-SPF
NLRIs. In many MSDC fabrics, the IPv4 and IPv6 topologies are
congruent. Although not described in this version of beyond the scope of this document, multi-
topology extensions can could be used to support separate IPv4, IPv6,
unicast, and multicast topologies while sharing the same NLRI.

Finally, the BGP SPF topology can be used as an underlay for other
BGP address families SAFIs (using the existing model) and realize all the above
advantages. A simplified

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 model using IPv6 link-local
addresses models, as next-hops can be deployed similar 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 [RFC5549].

1.2. 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Base BGP Protocol Relationship

With the exception of the decision process, the BGP SPF extensions
leverage the BGP protocol [RFC4271] without change. This includes
the BGP protocol Finite State Machine, BGP messages and their
encodings, processing of BGP messages, BGP attributes and path
attributes, BGP NLRI encodings, and any error handling defined in the
[RFC4271] and [RFC7606].

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.

Section 9 of [RFC4271] defines the decision process that is used to
select routes for subsequent advertisement by applying the policies
in the local Policy Information Base (PIB) to the routes stored in
its Adj-RIBs-In. The output of the Decision Process is the set of
routes that are announced by a BGP speaker to its peers. These
selected routes are stored by a BGP speaker in the speaker's Adj-
RIBs-Out according to policy.

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:

1. BGP advertisements are readvertised to neighbors immediately
without waiting or dependence on the route computation as
specified in phase 3 of the base BGP decision process. Multiple
peering models are supported as specified in Section 4.

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.

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 requirements, topology, scaling, and capabilities of the BGP BGP-LS-SPF
speakers, and redundancy requirements, various peering models are
supported. The only requirement
is requirements are that all BGP SPF speakers in
the BGP SPF routing domain receive link-
state NLRI on a timely basis, exchange BGP-LS-SPF NLRI, run an SPF
calculation, and update their data plane routing table appropriately. The content of the Link NLRI is
described in Section 4.2.

2.1.

4.1. BGP Single-Hop Peering on Network Node Connections

The simplest peering model is the one described in section 5.2.1 of
[RFC7938]. In this model, where EBGP single-hop sessions
are established over direct point-to-point links interconnecting the SPF domain
nodes. For
nodes in the purposes of BGP SPF, Link NLRI is only advertised if
a SPF routing domain. Once the single-hop BGP session
has been established and the Link-State/SPF
address family BGP-LS-SPF AFI/SAFI capability has been
exchanged [RFC4790] on [RFC4760] for the corresponding session. 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.

2.2. The content
of the Link NLRI is described in Section 5.2.2.

4.2. BGP Peering Between Directly Connected Network Directly-Connected Nodes

In this model, BGP BGP-LS-SPF speakers peer with all directly connected network directly-connected
nodes but the sessions may be multi-hop between loopback addresses (i.e., two-
hop sessions) and the direct connection discovery and liveliness
detection for those connections the interconnecting links are independent of the BGP
protocol. How 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 BGP-LS-
SPF speakers. For the purposes of BGP SPF, BGP-LS-SPF Link NLRI is advertised as long as a BGP
session has been established, the Link-State/SPF
address family BGP-LS-SPF AFI/SAFI capability has
been exchanged [RFC4790] [RFC4760], and the
corresponding link is considered is up and considered operational. operational as determined
using liveliness detection mechanisms outside the scope of this
document. This is much like the previous peering model only peering
is on a
single between loopback address addresses and the switch fabric interconnecting links can be
unnumbered. However, since there will be the same number of are BGP sessions as
with between every
directly-connected node in the previous peering model unless BGP SPF routing domain, there is only
a reduction in BGP sessions when there are parallel links between switches in the fabric.

2.3.
nodes.

4.3. BGP Peering in Route-Reflector or Controller Topology

In this model, BGP BGP-LS-SPF speakers peer solely with one or more Route
Reflectors [RFC4456] or controllers. As in the previous model,
direct connection discovery and liveliness detection for those
connections links
in the BGP SPF routing domain are done outside of the BGP protocol. More specifically,
the Liveliness detection is done using BFD protocol described in
[RFC5880]. For the purposes of BGP SPF,
BGP-LS-SPF Link NLRI is advertised as long as the corresponding link
is up and considered operational. up as per the chosen liveness detection mechanism.

This peering model, known as sparse peering, allows for many fewer BGP
sessions and, consequently, fewer instances of the same NLRI received
from multiple peers. It 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].

5. BGP Shortest Path Routing (SPF) Protocol Extensions

5.1. BGP-LS Shortest Path Routing (SPF) SAFI

In order to replace the Phase 1 and 2 decision functions of the existing Decision Process BGP decision process with an SPF-based Decision Process and
streamline the Phase 3 SPF-
based decision functions process in a backward compatible
manner, manner by not
impacting the BGP-LS SAFI, this draft document introduces the BGP-LS-SFP SAFI for BGP-LS SPF
operation. BGP-LS-SPF
SAFI. The BGP-LS-SPF (AFI 16388 / SAFI TBD1) [RFC4790] 80) [RFC4760] is allocated by
IANA as specified in the Section 6. A 8. In order for two BGP-LS-SPF
speakers to exchange BGP speaker using
the BGP-LS SPF extensions described herein NLRI, they MUST exchange the AFI/SAFI
using
Multiprotocol Extensions Capability Code [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 SPF routing domain.

4. 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
information can be collected from networks IGPs and shared with external
components using the BGP protocol. It describes both the definition
of
BGP-LS the BGP-LS-SPF NLRI that describes advertise links, nodes, and prefixes
comprising IGP link-state information and the definition of a BGP
path attribute (BGP-LS attribute) that carries link, node, and prefix
properties and attributes, such as the link and prefix metric or
auxiliary Router-
IDs Router-IDs of nodes, etc.

The 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 protocol will be used identifier specified in the Protocol-ID field [RFC7752]
will represent the origin of the advertised NLRI. For Node NLRI and
Link NLRI, this MUST be the direct protocol (4). Node or Link NLRI
with a Protocol-ID other than direct will be considered malformed.
For Prefix NLRI, the specified in
table 1 Protocol-ID MUST be the origin of [I-D.ietf-idr-bgpls-segment-routing-epe]. the
prefix. The local and remote node descriptors for all NLRI will be MUST
include the BGP Router-ID Identifier (TLV 516) and either the AS Number (TLV 512) [RFC7752] or the
[RFC7752]. The BGP Confederation Member (TLV 517) [RFC8402]. However, if the BGP
Router-ID [RFC7752] is known to not
appliable and SHOULD not be unique within the BGP Routing domain, included. If TLV 517 is included, it can
will be used as the sole descriptor.

4.1. ignored.

5.2.1. Node NLRI Usage

The BGP Node NLRI will MUST be advertised unconditionally by all routers in
the BGP SPF routing domain.

4.1.1.

5.2.1.1. Node NLRI Attribute SPF Capability TLV

The SPF capability is a new an additional Node Attribute TLV that will be added to
those defined in table 7 of [RFC7752]. The new TLV. This
attribute TLV will
only MUST be applicable when BGP is specified in included with the Node NLRI Protocol ID
field. BGP-LS-SPF SAFI and SHOULD
NOT be used for other SAFIs. The TBD TLV type 1180 will be defined assigned by
IANA. The new Node Attribute TLV will contain a single-octet SPF
algorithm as defined in
[RFC8402]. [RFC8665].

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1180) | Length - (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Algorithm |
+-+-+-+-+-+-+-+-+

The SPF Algorithm may take the following values:

0 - Normal Shortest Path First (SPF) algorithm based on link
metric. This is inherits the standard shortest path algorithm as
computed by values from the IGP protocol. Consistent with the deployed
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 Types
registry [RFC8665]. Algorithm 0, (Shortest Path First Algorithm (SPF) algorithm
based on link
metric. The algorithm metric, 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 supported and described in Section 6.3.
Support for
Algorithm 1 MUST NOT alter the SPF paths computed by Algorithm 1.

Note that usage of Strict Shortest Path First (SPF) other algorithm types is
defined in beyond 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 scope of scope. this
specification.

When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability attribute TLV with same SPF algorithm will be
included in the Shortest Path Tree (SPT).

4.1.2. BGP-LS 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.

5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV

A BGP-LS Attribute TLV to BGP-LS of the BGP-LS-SPF Node NLRI is defined to
indicate the status of the node with respect to the BGP SPF
calculation. This will be used to rapidly take a node out of service
Section 6.5.2 or to indicate the node is not to be used for transit
(i.e., non-local) traffic. traffic Section 6.3. If the SPF Status TLV is not
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 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 (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+

BGP Status Values: 0 - Reserved
1 - Node Unreachable with respect to BGP SPF
2 - Node does not support transit with respect
to BGP SPF
3-254 - Undefined
255 - Reserved

4.2.

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
Section 2. 4.

Link NLRI is advertised with unique local and remote node descriptors as
described above and unique link identifiers
dependent on the IP addressing. For IPv4 links, the links link's local
IPv4 (TLV 259) and remote IPv4 (TLV 260) addresses will be used. For
IPv6 links, the local IPv6 (TLV 261) and remote IPv6 (TLV 262)
addresses will be used. For unnumbered links, the link local/remote
identifiers (TLV 258) will be used. For links supporting having both
IPv4 and IPv6 addresses, both sets of descriptors may MAY be included in
the same Link NLRI. The link identifiers are described in table 5 of
[RFC7752].

For a link to be used in Shortest Path Tree (SPT) for a given address
family, i.e., IPv4 or IPv6, both routers connecting the link MUST
have an address in the same subnet for that address family. However,
an IPv4 or IPv6 prefix associated with the link MAY be installed
without the corresponding address on the other side of link.

The link IGP metric attribute TLV (TLV 1095) as well as any others
required for non-SPF purposes SHOULD MUST be advertised. The If
a BGP speaker receives a Link NLRI without an IGP metric value
in this TLV is variable length dependent on specific protocol usage
(refer to section 3.3.2.4 in [RFC7752]). For simplicity, attribute
TLV, then it SHOULD consider the BGP-LS 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 will be 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 done supported in the OSPF MIB [RFC4750] MAY be supported.
However, this is beyond the scope of this document.

4.2.1. BGP-LS Refer to
Section 10.1.1 for operational guidance.

The usage of other link attribute TLVs is beyond the scope of this
document.

5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs

Two BGP-LS Attribute TLVs to BGP-LS of the BGP-LS-SPF Link NLRI are defined to
advertise the prefix length associated with the IPv4 and IPv6 link
prefixes.
prefixes derived from the link descriptor addresses. The prefix
length is used for the optional installation of prefixes
corresponding to Link NLRI as defined in Section 5.3. 6.3.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD IPv4
|IPv4 (1182) or IPv6 Type | (1183)| Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix-Length |
+-+-+-+-+-+-+-+-+

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.

4.2.2. BGP-LS

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.

The maximum prefix-length for IPv4 Prefix-Length TLV is 32 bits. A
prefix-length field indicating a larger value than 32 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.

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 to BGP-LS 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 5.6.1. 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.

BGP Status Values: 0 - Reserved
1 - Link Unreachable with respect to BGP SPF
2-254 - Undefined
255 - Reserved

4.3.

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.

5.2.3. IPv4/IPv6 Prefix NLRI Usage

IPv4/IPv6 Prefix NLRI is advertised with a local node descriptor as described
above Local Node Descriptor and
the prefix and length used as length. The Prefix Descriptors field includes the descriptors IP
Reachability Information TLV (TLV 265) as described in [RFC7752].
The prefix metric attribute TLV (TLV 1155)
as well as any others required for non-SPF purposes SHOULD 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 non-loopback prefixes, the
setting of the metric is a local matter and beyond the scope of this
document.

4.3.1. BGP-LS

5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV

A BGP-LS Attribute TLV to BGP-LS BGP-LS-SPF Prefix NLRI is defined to
indicate the status of the prefix with respect to the BGP SPF
calculation. This will be used to expedite convergence for prefix
unreachability as discussed in Section 5.6.1. 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.

BGP Status Values: 0 - Reserved
1 - Prefix down Unreachable with respect to SPF
2-254 - Undefined
255 - Reserved

4.4.

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.

5.2.4. BGP-LS Attribute Sequence-Number TLV

A new BGP-LS Attribute TLV to BGP-LS of the BGP-LS-SPF NLRI types is defined to
assure the most recent version of a given NLRI is used in the SPF
computation. The TBD Sequence-Number TLV is mandatory for BGP-LS-SPF
NLRI. The TLV type will be defined 1181 has been assigned by IANA. The new BGP-
LS BGP-LS
Attribute TLV will contain an 8-octet sequence number. The usage of
the Sequence Number TLV is described in Section 5.1. 6.1.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1181) | Length (8 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (High-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Low-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sequence Number

The 64-bit strictly increasing strictly-increasing sequence number is MUST be incremented
for every self-originated version of BGP-LS NLRI originated. BGP-LS-SPF NLRI. BGP speakers
implementing this specification MUST use available mechanisms to
preserve the sequence number's strictly increasing property for the
deployed life of the BGP speaker (including cold restarts). One
mechanism for accomplishing this would be to use the high-order 32
bits of the sequence number as a wrap/boot count that is incremented anytime
any time the BGP router loses its sequence number state or the low-order low-
order 32 bits wrap.

When incrementing the sequence number for each self-originated NLRI,
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
should be incremented and saved in non-volatile storage. If by some
chance the BGP Speaker 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 BGP-LS-SPF
speaker completely loses its sequence number state (e.g., the BGP
speaker hardware is replaced or experiences a cold-start), the phase 1
decision function BGP
NLRI selection rules (see Section 5.1) rules 6.1) will insure convergence,
albeit,
albeit not immediately.

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
distinct phases. The Phase 1 decision function of the Decision
Process is responsible for calculating the degree of preference for
each route received from a BGP speaker's peer. The Phase 2 decision
function is invoked on completion of the Phase 1 decision function
and is responsible for choosing the best route out of all those
available for each distinct destination, and for installing each
chosen route into the Loc-RIB. The combination of the Phase 1 and 2
decision functions is characterized as a Path Vector algorithm.

The SPF based Decision process replaces the BGP best-path Decision process
described in [RFC4271]. This process starts with selecting only
those Node NLRI whose SPF capability TLV matches with the local
BGP BGP-
LS-SPF speaker's SPF capability TLV value. Since Link-State NLRI
always contains the local node descriptor [RFC7752], it will only be Section 5.2.1, each NLRI is
uniquely originated by a single BGP BGP-LS-SPF speaker in the BGP SPF
routing domain. domain (the BGP node matching the NLRI's Node Descriptors).
Instances of the same NLRI originated by multiple BGP speakers would
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. computation as
described below. The best paths 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
determine whether it is the best-path most recent by examining the Node-ID and
sequence number as described in Section 5.1. 6.1. If the received best-
path NLRI had
has changed, it will be advertised to other BGP-LS-SPF peers. If the
attributes have changed (other than the sequence number), a BGP SPF
calculation will be scheduled. triggered. However, a changed NLRI MAY be
advertised immediately to other peers almost immediately and
propagation of changes can approach IGP convergence times. To
accomplish this, prior to any SPF
calculation. Note that the BGP MinRouteAdvertisementIntervalTimer
and MinASOriginationIntervalTimer [RFC4271] timers are not applicable
to the BGP-LS-SPF SAFI. Rather, The scheduling of the SPF calculations SHOULD calculation, as
described in Section 6.3, is an implementation issue. Scheduling MAY
be triggered and dampened consistent with the SPF back-off algorithm specified in
[RFC8405].

The Phase 3 decision function of the Decision Process [RFC4271] is
also simplified since under normal SPF operation, a BGP speaker would MUST
advertise the NLRI selected for the SPF changed NLRIs to all BGP peers with the
BGP-LS/BGP-LS-SPF AFI/SAFI. Application of policy would not be
prevented however its usage to best-path process would be limited as BGP-LS-SPF AFI/
SAFI and install the SPF relies solely on link metrics.

5.1. Phase-1 changed routes in the Global RIB. The only
exception are unchanged NLRIs or stale NLRIs, i.e., NLRI received
with a less recent (numerically smaller) sequence number.

6.1. BGP NLRI Selection

The rules for NLRI all BGP-LS-SPF NLRIs selection are greatly simplified from [RFC4271].

1. If the NLRI is received from for phase 1 of the BGP speaker originating the NLRI
(as
decision process, section 9.1.1 [RFC4271], no longer apply.

1. Routes originated by directly connected BGP SPF peers are
preferred. This condition can be determined by the comparing the BGP Router ID
Identifiers in the NLRI received Local Node
identifiers with the BGP speaker Router ID), then it is preferred
over the same NLRI from non-originators. Descriptor and OPEN
message. This rule will assure that stale NLRI is updated even
if a BGP-LS router loses its sequence number state due to a cold-start. cold-
start.

2. If the Sequence-Number TLV is present in the BGP-LS Attribute,
then the The NLRI with the most recent, recent Sequence Number TLV, i.e., highest
sequence number 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 route received from the BGP Speaker SPF speaker with the numerically largest
larger BGP Router ID. Identifier is preferred.

When a BGP SPF speaker completely loses its sequence number state,
i.e., due to a cold start, or in the unlikely possibility that that
64-bit sequence number wraps, the BGP routing domain will still
converge. This is due to the fact that BGP speakers adjacent to the
router will always accept self-originated NLRI from the associated
speaker as more recent (rule # 1). When a BGP speaker reestablishes
a connection with its peers, any existing session will be taken down
and stale NLRI will be replaced by the new NLRI and stale NLRI will be
discarded independent of whether or not BGP graceful restart is
deployed, [RFC4724]. replaced. The adjacent BGP speaker will
update their NLRI
advertisements in turn advertisements, hop by hop, until the BGP routing
domain has converged.

The modified SPF Decision Process performs an SPF calculation rooted
at the BGP speaker using the metrics from the Link Attribute IGP
Metric TLV (1095) and the Prefix NLRI Attribute TLVs Prefix Metric TLV (1155)
[RFC7752]. As a result, any other BGP attributes that would
influence the Decision BGP decision process defined in [RFC4271] like including
ORIGIN, MULTI_EXIT_DISC, and LOCAL_PREF attributes are ignored by the
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.

6.1.1. BGP Self-Originated NLRI

Node, Link, or best-path.

5.2. 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
NLRIs that support both IPv4 and IPv6 addresses. Whether to run a
single SPF instance computation or multiple SPF instances computations for separate AFs
is a
matter of a local implementation. an implementation matter. Normally, IPv4 next-hops are calculated
for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
prefixes. However, an interesting use-case is deployment of
[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.

6.3. SPF Calculation based on BGP-LS BGP-LS-SPF NLRI

This section details the BGP-LS SPF BGP-LS-SPF local routing information base
(RIB) calculation. The router will use BGP-LS BGP-LS-SPF Node, Link, and
Prefix NLRI to populate the local RIB compute routes using the following algorithm. This
calculation yields the set of intra-area routes associated with the BGP-LS
domain. A router calculates the shortest-path tree using itself as
the root. Variations and optimizations of Optimizations to the BGP-LS-SPF algorithm are valid as long as it yields possible but
MUST yield the same set of routes. The algorithm below supports
Equal Cost Multi-Path (ECMP) routes. Weighted Unequal Cost Multi-Path Multi-
Path routes are out of scope. The organization of this section owes
heavily to section 16 of [RFC2328].

The following abstract data structures are defined in order to
specify the algorithm.

o Local Route Information Base (RIB) (LOC-RIB) - This is abstract routing table
contains reachability information (i.e., next hops) for all
prefixes (both IPv4 and IPv6) as well as the Node NLRI BGP-LS-SPF node
reachability. Implementations may choose to implement this as with
separate RIBs for each address family and/or Prefix versus Node NLRI.
reachability. It is synonymous with the Loc-RIB specified in
[RFC4271].

o Global Routing Information Base (GLOBAL-RIB) - This is Routing
Information Base (RIB) containing the current routes that are
installed in the router's forwarding plane. This is commonly
referred to in networking parlance as "the RIB".

o Link State NLRI Database (LSNDB) - Database of BGP-LS BGP-LS-SPF NLRI
that facilitates access to all Node, Link, and Prefix NLRI as well as
all the Link and Prefix NLRI corresponding to a given Node NLRI.
Other optimization, such as, resolving bi-directional connectivity
associations between Link NLRI are possible but of scope of this
document.

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
lowest cost Node NLRI with the lowest reachability cost at the front head of the
list. It 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 concrete data structures have also been used.

The algorithm is comprised of the steps below:

1. The current local RIB LOC-RIB is invalidated, and the CAN-LIST is invalidated.
initialized to empty. The local RIB LOC-RIB is rebuilt during the course
of the SPF computation. The existing routing entries are
preserved for comparison to determine changes that need to be installed in
made to the global RIB. GLOBAL-RIB in step 6.

2. The computing router's Node NLRI is installed updated in the local RIB LOC-RIB with a
cost of 0 and as the sole entry in Node NLRI is also added to the candidate list. 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
list for processing. If the BGP-LS Node attribute includes an
SPF Status TLV (Section 4.1.2) 5.2.1.2) indicating the node is
unreachable, the Node NLRI is ignored and the next lowest cost
Node NLRI is selected from candidate list. The Node
corresponding to this NLRI will be referred to as the Current Current-
Node. If the candidate list is empty, the SPF calculation has
completed and the algorithm proceeds to step 6.

4. All the Prefix NLRI with the same Node Identifiers as the Current
Node
Current-Node will be considered for installation. The next-
hop(s) for these Prefix NLRI are inherited from the Current-Node.
The cost for each prefix is the metric advertised in the Prefix NLRI
Attribute Prefix Metric TLV (1155) added to the cost to reach the Current Node.
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
indicating the prefix is unreachable, the BGP-LS Prefix NLRI Current-Prefix is
considered unreachable and the next BGP-LS Prefix NLRI is
examined. examined in
Step 4.

* If the Current-Prefix's corresponding prefix is in the local RIB LOC-RIB
and the cost is greater less than the Current route's Current-Prefix's metric, the Prefix NLRI
Current-Prefix does not contribute to the route and the next
Prefix NLRI is ignored. examined in Step 4.

* If the Current-Prefix's corresponding prefix is not in the local RIB
LOC-RIB, the prefix is installed with the Current-Node's next-
hops installed as the LOC-RIB route's next-hops and the metric
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 Current-Prefix's corresponding prefix is in the LOC-RIB
and the cost is less than the current route's metric, the Prefix
prefix is installed with the
Current Node's Current-Node's next-hops
replacing the local RIB LOC-RIB route's next-
hops next-hops and the metric being updated.
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).

* If the Current-Prefix's corresponding prefix is in the local RIB LOC-RIB
and the cost is the same as the current route's metric, the Prefix is installed with the
Current Node's
Current-Node's next-hops being will be merged with local RIB 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 Current-
Node will be considered for installation. Each link will be
examined and will be referred to in the following text as the
Current Link.
Current-Link. The cost of the Current Link Current-Link is the advertised
metric in IGP
Metric TLV (1095) from the Link NLRI BGP-LS attribute added to
the cost to reach the Current
Node.

* Optionally, 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:

A. The prefix(es) associated with the Current Link Current-Link are installed
into the local RIB LOC-RIB using the same rules as were used for Prefix
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.

B. If the current Node Current-Node NLRI attributes includes the SPF status
TLV (Section 4.1.2) 5.2.1.2) and the status indicates that the Node
doesn't support transit, the next link for the current node Current-Node
is
processed.

* processed in Step 5.

C. If the Current-Link's NLRI attribute includes an SPF Status
TLV indicating the link is down, the BGP-LS-SPF Link NLRI is
considered down and the next link for the Current-Node is
examined in Step 5.

D. The Current Link's endpoint Current-Link's Remote Node NLRI is accessed (i.e., the
Node NLRI with the same Node identifiers as the Link
endpoint). Current-
Link's Remote Node Descriptors). If it exists, it will be
referred to as the
Endpoint Node NLRI Remote-Node and the algorithm will proceed
as follows:

+ If the BGP-LS Link Remote-Node's NLRI attribute includes an SPF Status
TLV indicating the link is down, the BGP-LS Link NLRI node is
considered down and unreachable, the next BGP-LS Link NLRI link for
the Current-Node is examined. examined in Step 5.

+ All the Link NLRI corresponding the Endpoint Node NLRI Remote-Node will be
searched for a back-link Link NLRI pointing to the current
node. Both the Current-Node.
Each Link NLRI is examined for Remote Node identifiers and Descriptors
matching the Current-Node and Link endpoint
identifiers in Descriptors matching
the Endpoint Node's Link NLRI must match for Current-Link (e.g., sharing a match. common IPv4 or IPv6
subnet). If there is no corresponding 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
corresponding providing
bi-directional connectivity will be referred to as the Endpoint Node NLRI,
Remote-Link. If no Remote-Link is found, the Endpoint Node next link
for the Current-Node is examined in Step 5.

+ If the Remote-Link NLRI fails attribute includes an SPF Status
TLV indicating the bi-directional connectivity test link is down, the Remote-Link NLRI is
considered down and the next link for the Current-Node is not
processed further.
examined in Step 5.

+ If the Endpoint Node NLRI Remote-Node is not on the candidate list, CAN-LIST, it is inserted
based on the link cost. The Remote Node's cost and BGP Identifier (the
latter being used as a tie-breaker). is the cost of
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.

+ If the Endpoint Node Remote-Node NLRI is already on the candidate list CAN-LIST with a lower
higher cost, it need not must be inserted again. 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.

+ If the Endpoint Node Remote-Node NLRI is already on the candidate list CAN-LIST with a higher
the same cost, it must need not be removed and 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.

+ If the Remote-Node NLRI is already on the CAN-LIST with a
lower cost.

* cost, it need not be reinserted on the CAN-LIST.

E. Return to step 3 to process the next lowest cost Node NLRI on
the candidate list. CAN-LIST.

6. The local RIB LOC-RIB is examined and changes (adds, deletes,
modifications) are installed into the global RIB.

5.4. NEXT_HOP Manipulation

A BGP speaker that supports SPF extensions MAY interact with peers
that don't support SPF extensions. GLOBAL-RIB. For each route
in the LOC-RIB:

* If the BGP-LS address family is
advertised to a peer not supporting route was added during the current BGP SPF extensions described
herein, then computation,
install the BGP speaker MUST conform to route into the NEXT_HOP rules
specified in [RFC4271] when announcing GLOBAL-RIB.

* If the Link-State address family
routes to those peers.

All route modified during the current BGP peers that support SPF extensions would locally compute computation
(e.g., metric, tags, or next-hops), update the
Loc-RIB next-hops as a result of route in the SPF process. Consequently,
GLOBAL-RIB.

* If the
NEXT_HOP attribute is always ignored on receipt. However, route was not installed during the current BGP
speakers SHOULD set SPF
computation, remove the NEXT_HOP address according to route from both the NEXT_HOP
attribute rules specified in [RFC4271].

5.5. GLOBAL-RIB and the
LOC-RIB.

6.4. IPv4/IPv6 Unicast Address Family Interaction

While the BGP-LS SPF 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
operate today (i.e., "Ships-in-the-Night" mode). There will be is no
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
all routers in the routing domain calculate the precisely the same
SPF tree and install the same set of routes, it

It is RECOMMENDED that
BGP-LS SPF BGP-LS-SPF IPv4/IPv6 routes route computation and
installation be given scheduling priority by default when
installed into their respective RIBs. In common implementations the
prioritization over other BGP
address families as these address families are considered as underlay
SAFIs. Similarly, it is governed by RECOMMENDED that the route preference or
administrative distance with lower being more preferred.

5.6. give active route installation preference to
BGP-LS-SPF IPv4/IPv6 routes over BGP routes from other AFI/SAFIs.
However, this preference MAY be overridden by an operator-configured
policy.

6.5. NLRI Advertisement and Convergence

5.6.1.

6.5.1. Link/Prefix Failure Convergence

A local failure will prevent a link from being used in the SPF
calculation due to the IGP bi-directional connectivity requirement.
Consequently, local link failures should SHOULD always be given priority
over updates (e.g., withdrawing all routes learned on a session) in
order to ensure the highest priority propagation and optimal
convergence.

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 a BGP
advertisement, the link would continue to be used until the last copy
of the BGP-LS BGP-LS-SPF Link NLRI is withdrawn. In order to avoid this
delay, the originator of the Link NLRI will SHOULD advertise a more recent
version
of with an increased Sequence Number TLV for the BGP-LS BGP-LS-SPF Link
NLRI including the SPF Status TLV Section 4.2.2 (Section 5.2.2.2) indicating the
link is down with respect to BGP SPF. After some configurable period
of time, which is an implementation dependent, e.g., 2-3 seconds, the BGP-LS
BGP-LS-SPF Link NLRI can be withdrawn with no consequence. If the
link becomes available in that period, the originator of the BGP-LS BGP-LS-
SPF LINK NLRI will simply advertise a more recent version of the BGP-LS 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
of the BGP-LS BGP-LS-SPF Prefix NLRI will be advertised with the SPF Status
TLV
Section 4.3.1 (Section 5.2.3.1) indicating the prefix is unreachable in the
BGP-LS Prefix Attributes and the prefix will be considered
unreachable with respect to BGP SPF. After some configurable period
of time, which is implementation dependent, e.g., 2-3 seconds, the BGP-LS
BGP-LS-SPF Prefix NLRI can be withdrawn with no consequence. If the
prefix becomes reachable in that period, the originator of the BGP-LS BGP-
LS-SPF Prefix NLRI will simply advertise a more recent version of the BGP-LS
BGP-LS-SPF Prefix NLRI without the SPF Status TLV in the BGP-LS
Prefix Attributes.

5.6.2.

6.5.2. Node Failure Convergence

With BGP without graceful restart [RFC4724], all the NLRI advertised
by a node are implicitly withdrawn when a session failure is
detected. If fast failure detection such as BFD is utilized, and the
node is on the fastest converging path, the most recent versions of BGP-LS
BGP-LS-SPF NLRI may be withdrawn while these versions are in-flight on longer paths. withdrawn. This will result the into an older
version of the NLRI being used until the new versions arrive and,
potentially, unnecessary route flaps. Therefore, BGP-LS SPF BGP-LS-SPF NLRI
SHOULD always be retained before being implicitly withdrawn for a brief
configurable implementation-dependent interval, e.g., 2-3 seconds.
This will not delay convergence since the adjacent nodes will detect
the link failure and advertise a more recent NLRI indicating the link
is down with respect to BGP SPF Section 5.6.1 (Section 6.5.1) and the BGP-SPF BGP SPF
calculation will failure fail the bi-directional connectivity
check.

5.7. check
Section 6.3.

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
Capability TLV in the Node NLRI BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and the Node NLRI and not MUST NOT pass it to
other BGP peers as specified in [RFC7606]. When discarding a 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 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.

When a BGP speaker receives a BGP Update containing a malformed IPv6
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.

7.2. Processing of BGP-LS-SPF NLRIs

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 an AFI/SAFI the use of SAFI (80) for BGP-LS BGP SPF operation
Section 5.1, and requests IANA to assign the BGP-LS/BGP-LS-SPF (AFI 16388 / SAFI TBD1)
as described value from the First
Come First Serve (FCFS) range in [RFC4760]. the Subsequent Address Family
Identifiers (SAFI) Parameters registry.

This document also defines five attribute TLVs for BGP-LS 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,
and SPF Status TLV from the "BGP-LS Node Descriptor, Link Descriptor,
Prefix Descriptor, and Attribute TLVs" Registry.

Table 1: NLRI Attribute TLVs

9. Security Considerations

This extension to document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This
document does not change the underlying security issues inherent in
the existing [RFC4271], [RFC4724], 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 [RFC7752].

8. [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.

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 order to mitigate the risk of peering with BGP speakers
masquerading as legitimate authorized BGP speakers, it is recommended
that the TCP Authentication Option (TCP-AO) [RFC5925] be used to
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.

10. Management Considerations

This section includes unique management considerations for the BGP-LS
SPF BGP-
LS-SPF address family.

8.1.

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 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 as documented in MUST be
supported [RFC8405].

8.2. 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
an SPF log including entries for previous SPF computations, computations. Each SPF
log entry would include the BGP-LS BGP-LS-SPF NLRI SPF triggering the SPF,
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
be finite, implementations SHOULD also maintain counters for the
total number of SPF computations of each type and the total number of SPF
triggering events. Additionally, to troubleshoot SPF scheduling and
back-off [RFC8405], the current SPF back-off state, remaining
time-to-learn, time-
to-learn, remaining holddown, last trigger event time, last SPF time,
and next SPF time should be available.

11. Implementation Status

Note RFC Editor: Please remove this section and the associated
references prior to publication.

This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft,
Internet-Draft and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.

According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".

The BGP-LS SPF implementatation BGP-LS-SPF implementation status is documented in
[I-D.psarkar-lsvr-bgp-spf-impl].

10.

12. Acknowledgements

The authors would like to thank Sue Hares, Jorge Rabadan, Boris
Hassanov, Dan Frost, Matt Anderson, Fred Baker, and Lukas Krattiger
for their review and comments. Thanks to Pushpasis Sarkar for
discussions on preventing a BGP SPF Router from being used for non-
local traffic (i.e., transit traffic).

The authors extend special thanks to Eric Rosen for fruitful
discussions on BGP-LS SPF BGP-LS-SPF convergence as compared to IGPs.

11.

13. Contributors

In addition to the authors listed on the front page, the following
co-authors have contributed to the document.

Derek Yeung
Arrcus, Inc.
derek@arrcus.com

Gunter Van De Velde
Nokia
gunter.van_de_velde@nokia.com

Abhay Roy
Arrcus, Inc.
abhay@arrcus.com

Venu Venugopal
Cisco Systems
venuv@cisco.com

Chaitanya Yadlapalli
AT&T
cy098d@att.com

12.

14. References

12.1.

14.1. Normative References

[I-D.ietf-idr-bgpls-segment-routing-epe]
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
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.

[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.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.

[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S.,

[RFC8205] Lepinski, M., Ed. and R. Shakir, "Segment
Routing Architecture", K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8402, 8205, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>. 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.

[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018,
<https://www.rfc-editor.org/info/rfc8405>.

12.2. Information

[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]
Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and
Applicability of Link State Vector Routing in Data
Centers", draft-ietf-lsvr-applicability-05 (work in
progress), March 2020.

[I-D.psarkar-lsvr-bgp-spf-impl]
Sarkar, P., Patel, K., Pallagatti, S., and s.
sajibasil@gmail.com, "BGP Shortest Path Routing Extension
Implementation Report", draft-psarkar-lsvr-bgp-spf-impl-00
(work in progress), June 2020.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.

[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007,
<https://www.rfc-editor.org/info/rfc4724>.

[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.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.

[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<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
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<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
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.

[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.

Authors' Addresses

Keyur Patel
Arrcus, Inc.

Email: keyur@arrcus.com

Acee Lindem
Cisco Systems
301 Midenhall Way
Cary, NC 27513
USA

Email: acee@cisco.com
Shawn Zandi
LinkedIn
222 2nd Street
San Francisco, CA 94105
USA

Email: szandi@linkedin.com

Wim Henderickx
Nokia
Antwerp
Belgium

Email: wim.henderickx@nokia.com