draft-ietf-lsvr-bgp-spf-14.xml

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<rfc category="std" docName="draft-ietf-lsvr-bgp-spf-14"
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  <!-- ***** FRONT MATTER ***** -->

  <front>

    <title abbrev="BGP Link-State SPF Routing">
    BGP Link-State Shortest Path First (SPF) Routing</title>

    <!-- add 'role="editor"' below for the editors if appropriate -->

    <!-- Another author who claims to be an editor -->

    <author fullname="Keyur Patel" initials="K"
            surname="Patel">

      <organization>Arrcus, Inc.</organization>

      <address>
        <email>keyur@arrcus.com</email>
      </address>
    </author>

    <author fullname="Acee Lindem" initials="A"
            surname="Lindem">

      <organization>Cisco Systems</organization>

      <address>
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          <street>301 Midenhall Way</street>

          <city>Cary</city>

          <region>NC</region>

          <code>27513</code>

          <country>USA</country>
        </postal>

        <email>acee@cisco.com</email>

      </address>
    </author>

    <author fullname="Shawn Zandi" initials="S"
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      <organization>LinkedIn</organization>

      <address>
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          <city>San Francisco</city>

          <region>CA</region>

          <code>94105</code>

          <country>USA</country>
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        <email>szandi@linkedin.com</email>

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    <author fullname="Wim Henderickx" initials="W"
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          <city>Antwerp</city>

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        <email>wim.henderickx@nokia.com</email>

      </address>
    </author>

    <date/>

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    <area>General</area>

    <workgroup>Network Working Group</workgroup>

    <keyword>IDR</keyword>

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<abstract>
<t>
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 extensions to BGP to use BGP
Link-State distribution and the Shortest Path First (SPF) algorithm 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.
</t>
</abstract>

  </front>

  <middle>

<section anchor="introduction" title="Introduction">
<t>
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 <xref target="RFC4271"/>
as their single routing protocol for both their fabric routing and
their Data Center Interconnect (DCI) routing <xref target="RFC7938"/>.
This document describes an alternative solution which leverages
BGP-LS <xref target="RFC7752"/> and the Shortest Path First algorithm used by
Internal Gateway Protocols (IGPs) such as OSPF <xref target="RFC2328"/>.
</t>
<t>This document leverages both the BGP protocol <xref target="RFC4271"/> and
the BGP-LS <xref target="RFC7752"/> protocols. The relationship, as well as
the scope of changes are described respectively in <xref target="BGP-base"/>
and <xref target="BGP-LS"/>. The modifications to <xref target="RFC4271"/>
for BGP SPF described herein only apply to IPv4 and IPv6 as underlay unicast
Subsequent Address Families Identifiers (SAFIs).
Operations for any other BGP SAFIs are outside the scope of this document.
</t>
<t>
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). The SPF LFA extensions defined
in <xref target="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 <xref target="RFC4456"/>
or controllers.
</t>
<section anchor="terms" title="Terminology">
  <t>This specification reuses terms defined in section 1.1 of <xref target="RFC4271"/>
  including BGP speaker, NLRI, and Route.</t>
  <t>Additionally, this document introduces the following terms:
  <list style="hanging">
    <t hangText="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.</t>
    <t hangText="BGP-LS-SPF NLRI:"> This refers to BGP-LS Network Layer Reachability
    Information (NLRI) that is being advertised in the BGP-LS-SPF SAFI (<xref target="SAFI"/>)
    and is being used for BGP SPF route computation.</t>
    <t hangText="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. <xref target="RFC2328"/></t>
  </list></t>
</section>
<section title="BGP Shortest Path First (SPF) Motivation">
<t>
Given that <xref target="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 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.
</t>
<t>
A primary advantage is that all BGP 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 additional
BGP paths <xref target="RFC7911"/>  or other extensions. In short, the advantages
of an IGP such as OSPF <xref target="RFC2328"/> are availed in BGP.
</t>
<t>
  With the simplified BGP decision process as defined in
  <xref target="bgp-decision"/>, 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.
</t>
<t>
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 SPF speakers corresponding to
the link NLRI need to withdraw the corresponding BGP-LS-SPF Link NLRI. Additionally,
the changed 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.
</t>
<t>
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 <xref target="peering-models"/>), each BGP SPF speaker will
only need as many sessions and copies of the NLRI as required for
redundancy (see <xref target="peering-models"/>).
Additionally, a controller could inject topology that
is learned outside the BGP SPF routing domain.
</t>
<t>
Given that controllers are already consuming BGP-LS NLRI
<xref target="RFC7752"/>, this functionality can be reused for BGP-LS-SPF NLRI.
</t>
<t>
Another advantage of BGP SPF is that both IPv6 and IPv4 can
be supported using the BGP-LS-SPF SAFI with the same BGP-LS-SPF NLRIs. In many
MSDC fabrics, the IPv4 and IPv6 topologies are congruent, refer to
<xref target="Link-NLRI"/> and <xref target="Prefix-NLRI"/>.
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.
</t>
<t>
Finally, the BGP SPF topology can be used as an underlay for other BGP
SAFIs (using the existing model) and realize all the above
advantages.
</t>
</section>
<section title="Document Overview">
<t>
  The document begins with sections defining the precise relationship that BGP SPF has
  with both the base BGP protocol <xref target="RFC4271"/>  (<xref target="BGP-base"/>) and the
  BGP Link-State (BGP-LS)  extensions <xref target="RFC7752"/>
  (<xref target="BGP-LS"/>). 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
  <xref target="peering-models"/>. 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 <xref target="protocol-extend"/>. The replacement of the base BGP decision process
  with the SPF computation is specified in <xref target="bgp-decision"/>. Finally, BGP SPF error
  handling is defined in <xref target="error-handling"/>
</t>
</section>
      <section title="Requirements Language">
       <t>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 <xref target="RFC2119"/> <xref target="RFC8174"/>
        when, and only when, they appear in all capitals, as shown here.</t>
       </section>
</section> <!-- for Introductions section -->


<section anchor="BGP-base" title="Base BGP Protocol Relationship">
  <t>
    With the exception of the decision process, the BGP SPF extensions leverage the BGP
    protocol <xref target="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 <xref target="RFC4271"/> and
    <xref target="RFC7606"/>.
  </t>
  <t>
    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.
  </t>
  <t>
    Section 9 of <xref target="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.
  </t>
  <t>
    The BGP SPF extension fundamentally changes the decision process, as described
    herein, to be more like a link-state protocol
    (e.g., OSPF <xref target="RFC2328"/>).  Specifically:
    <list style="numbers">
      <t>
	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
	<xref target="peering-models"/>.
      </t>
      <t>
	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 <xref target="Phase-1"/>.
      </t>
      <t>
	Phase 2 of the base BGP protocol decision process is replaced with the
	Shortest Path First (SPF) algorithm, also known as the Dijkstra algorithm
        <xref target="terms"/>.
      </t>
    </list>
  </t>
</section> <!-- for BGP relationship section -->

<section anchor="BGP-LS" title="BGP Link-State (BGP-LS) Relationship">
<t>
 <xref target="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
 <xref target="RFC7752"/> make use of the decision process defined in
 <xref target="RFC4271"/>.  This document reuses NLRI and TLVs defined in <xref target="RFC7752"/>.
 Rather than reusing the BGP-LS SAFI, the BGP-LS-SPF SAFI
 <xref target="SAFI"/> is introduced to insure backward compatibility for the BGP-LS SAFI usage.
</t>
<t>
  The BGP SPF extensions reuse the Node, Link, and Prefix NLRI defined
  in <xref target="RFC7752"/>.  The usage of the BGP-LS
  NLRI, attributes, and attribute extensions is described in
  <xref target="NLRI-Use"/>. 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.
</t>
<t>
  Support for Multiple Topology Routing (MTR) similar to the OSPF MTR computation described in
  <xref target="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 <xref target="RFC7752"/> is not specified.
</t>
<t>
  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 <xref target="RFC7752"/>.
</t>
</section> <!-- for BGP-LS relationship section -->

<section anchor="peering-models" title="BGP Peering Models">
<t>
  Depending on the topology, scaling, capabilities of the BGP 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.
</t>
<section title="BGP Single-Hop Peering on Network Node Connections">
<t>
  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
  <xref target="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 <xref target="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 <xref target="Link-NLRI"/>.
</t>
</section>
<section title="BGP Peering Between Directly-Connected Nodes">
  <t>
    In this model, BGP 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.
    For example, liveliness detection could be
    done using the BFD protocol <xref target="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 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 <xref target="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.
  </t>
</section>
<section title="BGP Peering in Route-Reflector or Controller Topology">
<t>
  In this model, BGP SPF speakers peer solely with one or more Route Reflectors
  <xref target="RFC4456"/> or controllers. As in the previous model, direct
  connection discovery and liveliness detection for those links in the BGP
  SPF routing domain are done outside of the BGP protocol.
  BGP-LS-SPF Link NLRI is advertised as long as the corresponding link is
  considered up as per the chosen liveness detection mechanism.
</t>
<t>
  This peering model, known as sparse peering, allows for fewer BGP sessions
  and, consequently, fewer instances of the same NLRI received 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 <xref target="I-D.ietf-lsvr-applicability"/>.
</t>
</section>
</section>

<section anchor="protocol-extend" title="BGP Shortest Path Routing (SPF) Protocol Extensions">
  <section anchor="SAFI" title="BGP-LS Shortest Path Routing (SPF) SAFI">
    <t>
      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) <xref target="RFC4760"/> is allocated by IANA
      as specified in the <xref target="IANA"/>.
      In order for two BGP SPF speakers to exchange BGP SPF NLRI, they MUST exchange
      the Multiprotocol Extensions Capability <xref target="RFC5492"/> <xref target="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.
    </t>

    <section anchor="BGP-LS-TLV" title="BGP-LS-SPF NLRI TLVs">
      <t>
	The NLRI format of BGP-LS-SPF SAFI uses exactly same format as the BGP-LS AFI
	<xref target="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.
      </t>

      <t>
	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.
      </t>

      <t>
	<xref target="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.
      </t>

      <t>
        Given that there is a single BGP-LS Attribute for all the BGP-LS-SPF NLRI in a
        BGP Update, Section 3.3, <xref target="RFC7752"/>, a BGP Update will
        normally contain a single BGP-LS-SPF NLRI since advertising multiple NLRI
        would imply identical attributes.
      </t>
    </section>

    <section title="BGP-LS Attribute">
      <t>
	The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly same format of the BGP-LS AFI
	<xref target="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.
      </t>

      <t>
	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 <xref target="RFC4271"/> mandates a maximum BGP
	message size of 4096 octets.  It is RECOMMENDED that an
	implementation support <xref target="RFC8654"/> in order to accommodate larger size
	of information within the BGP-LS Attribute.  BGP 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 SPF speaker
	originating the attribute is outside the scope of this document.
	When a BGP 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
	<xref target="RFC7606"/> applies.
      </t>

      <t>
	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.
      </t>

      <t>
	All TLVs within the BGP-LS Attribute are considered optional unless specified otherwise.
      </t>

    </section>
  </section>

<section anchor="NLRI-Use" title="Extensions to BGP-LS">
<t>
  <xref target="RFC7752"/> describes a mechanism by which link-state and TE
  information can be collected from IGPs and shared with external components
  using the BGP protocol. It describes both the definition of the BGP-LS NLRI
  that 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 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.
</t>

<t>
  The protocol identifier specified in the Protocol-ID field <xref target="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 Protocol-ID
  MUST be the origin of the prefix. The local and remote node descriptors for all NLRI MUST
  include the BGP Identifier (TLV 516) and the AS Number (TLV 512)
  <xref target="RFC7752"/>. The BGP Confederation Member (TLV 517)
  <xref target="RFC7752"/> is not appliable and SHOULD not be included. If TLV 517
  is included, it will be ignored.
</t>
<section title="Node NLRI Usage">
<t>
The Node NLRI MUST be advertised unconditionally by all routers in
the BGP SPF routing domain.
</t>
<section anchor="node-spf-cap-tlv" title="BGP-LS-SPF Node NLRI Attribute SPF Capability TLV">
<t>
The SPF capability is an additional Node Attribute TLV.
This attribute TLV MUST be included with the
BGP-LS-SPF SAFI and SHOULD NOT be used for other SAFIs.
The TLV type 1180 will be assigned by IANA. The Node
Attribute TLV will contain a single-octet SPF algorithm as defined
in <xref target="RFC8665"/>.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
    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 |
   +-+-+-+-+-+-+-+-+

]]></artwork>
</figure>
</t>
<t>The SPF algorithm inherits the values from the IGP Algorithm Types
registry <xref target="RFC8665"/>. Algorithm 0, (Shortest Path Algorithm (SPF)
based on link metric, is supported and described in <xref target="BGP-SPF"/>.
Support for other algorithm types is beyond the scope of this specification.
</t>
<t>
When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability TLV with same SPF algorithm
will be included in the Shortest Path Tree (SPT) <xref target="BGP-SPF"/>.
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.
</t>
</section>
<section anchor="node-status-tlv" title="BGP-LS-SPF Node NLRI Attribute SPF Status TLV">
<t>
A BGP-LS Attribute TLV 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 <xref target="node-failure"/> or to indicate the node is not to be
used for transit (i.e., non-local) traffic <xref target="BGP-SPF"/>. 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 <xref target="BGP-SPF"/>.
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.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
    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 (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

]]></artwork>
</figure>
</t>
<t>
  The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF Status TLV,
  and Prefix Attribute SPF Status TLV use the same TLV Type (1184). This implies
  that a BGP Update cannot contain multiple NLRI with differing status.
  If the BGP-LS-SPF Status TLV is advertised and the advertised value is not defined
  for all NLRI included in the BGP update, then the SPF Status TLV is ignored
  and not used in SPF computation but is still announced to other BGP SPF speakers.
  An implementation MAY log an error for further analysis.
</t>
<t>
If a BGP SPF 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 SPF speakers.
A BGP SPF speaker receiving a BGP Update containing
a SPF Status TLV in the BGP-LS attribute <xref target="RFC7752"/> with a value
that is outside the range of defined values SHOULD be processed and announced to other
BGP SPF speakers. However, a BGP SPF speaker MUST NOT use the Status TLV in its SPF computation.
An implementation MAY log this condition for further analysis.
</t>
</section>
</section>
<section anchor="Link-NLRI" title="Link NLRI Usage">
  <t>
    The criteria for advertisement of Link NLRI are discussed in
    <xref target="peering-models"/>.
</t>
<t>
  Link NLRI is advertised with unique local and remote node descriptors
  dependent on the IP addressing. For IPv4 links, the
  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 be included in the same Link NLRI. The link identifiers are
  described in table 5 of <xref target="RFC7752"/>.
</t>
<t>
  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.
</t>
<t>
  The link IGP metric attribute TLV (TLV 1095) MUST be advertised. If a BGP SPF 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 SPF speaker MUST handle such
  malformed NLRI as 'Treat-as-withdraw' <xref target="RFC7606"/>.
  The BGP SPF metric length is 4 octets.
  Like OSPF <xref target="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 <xref target="RFC4750"/> MAY be supported. However, this is beyond the scope
  of this document. Refer to <xref target="link-metric-config"/> for
  operational guidance.
</t>
<t>
  The usage of other link attribute TLVs is beyond the scope of this document.
</t>
<section title="BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs">
<t>
  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 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 <xref target="BGP-SPF"/>.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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.
]]></artwork>
</figure>
</t>
<t>
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 <xref target="RFC7606"/> when received with any NLRIs other
than Link NRLIs. An implementation MAY log an error for further analysis.
</t>
<t>
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 <xref target="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.
</t>
<t>
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 <xref target="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.
</t>
</section>
<section anchor="link-status-tlv" title="BGP-LS-SPF Link NLRI Attribute SPF Status TLV">
<t>
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 <xref target="failure-converge"/>. 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 <xref target="BGP-SPF"/>.
A single TLV type will be shared by the Node, Link, and Prefix NLRI.
The TLV type 1184 will be assigned by IANA.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
    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 (1184)                 |      Length (1 Octet)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SPF Status    |
   +-+-+-+-+-+-+-+-+

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

]]></artwork>
</figure>
</t>
<t>
  The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF Status TLV,
  and Prefix Attribute SPF Status TLV use the same TLV Type (1184). This implies
  that a BGP Update cannot contain multiple NLRI with differing status.
  If the BGP-LS-SPF Status TLV is advertised and the advertised value is not defined
  for all NLRI included in the BGP update, then the SPF Status TLV is ignored
  and not used in SPF computation but is still announced to other BGP SPF speakers.
  An implementation MAY log an error for further analysis.
</t>
<t>
  If a BGP SPF 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 SPF speakers.
  A BGP SPF speaker receiving a BGP Update containing
  an SPF Status TLV in the BGP-LS attribute <xref target="RFC7752"/> with a value
  that is outside the range of defined values SHOULD be processed and announced to other
  BGP SPF speakers. However, a BGP SPF speaker MUST NOT use the Status TLV in its SPF computation.
  An implementation MAY log this information for further analysis.
</t>

</section>
</section>
<section anchor="Prefix-NLRI" title="IPv4/IPv6 Prefix NLRI Usage">
<t>
  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 <xref target="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.
</t>
<section anchor="prefix-status-tlv" title="BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV">
  <t>
    A BGP-LS Attribute TLV to 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 <xref target="failure-converge"/>.
    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.
  </t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
    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 (1184)                 |      Length (1 Octet)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SPF Status    |
   +-+-+-+-+-+-+-+-+

    BGP Status Values: 0 - Reserved
                       1 - Prefix Unreachable with respect to SPF
                   2-254 - Undefined
                     255 - Reserved
]]></artwork>
</figure>
</t>
<t>
  The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF Status TLV,
  and Prefix Attribute SPF Status TLV use the same TLV Type (1184). This implies
  that a BGP Update cannot contain multiple NLRI with differing status.
  If the BGP-LS-SPF Status TLV is advertised and the advertised value is not defined
  for all NLRI included in the BGP update, then the SPF Status TLV is ignored
  and not used in SPF computation but is still announced to other BGP SPF speakers.
  An implementation MAY log an error for further analysis.
</t>
<t>
  If a BGP SPF 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 SPF speakers.
  A BGP SPF speaker receiving a BGP Update containing
  an SPF Status TLV in the BGP-LS attribute <xref target="RFC7752"/> with a value
  that is outside the range of defined values SHOULD be processed and announced to other
  BGP SPF speakers. However, a BGP SPF speaker MUST NOT use the Status TLV in its
  SPF computation. An implementation MAY log this information for further analysis.
</t>
</section>
</section>
<section title="BGP-LS Attribute Sequence-Number TLV">
  <t>
    A BGP-LS Attribute TLV 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 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 <xref target="Phase-1"/>.
  </t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
    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)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</t>
<t>
  Sequence Number <vspace blankLines="1" />
  The 64-bit strictly-increasing sequence number MUST be incremented for every
  self-originated version of BGP-LS-SPF NLRI. BGP SPF speakers implementing this specification
  MUST use available mechanisms to preserve the sequence number's strictly increasing property
  for the deployed life of the BGP SPF 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 any time the BGP router
  loses its sequence number state or the low-order 32 bits wrap.
</t>
<t>
  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 a BGP SPF speaker completely
  loses its sequence number state (e.g., the BGP SPF speaker hardware
  is replaced or experiences a cold-start), the BGP NLRI selection rules
  (see <xref target="Phase-1"/>) will insure convergence, albeit not immediately.
</t>

<t>
  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.
</t>
</section>
</section>
<section anchor="NEXT-HOP" title="NEXT_HOP Manipulation">
<t>
  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 <xref target="RFC4760"/>.
  BGP SPF 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.
</t>
<t>
  <xref target="RFC4760"/> modifies the rules of NEXT_HOP attribute whenever the
  multiprotocol extensions for BGP-4 are enabled. BGP SPF speakers MUST set the
  NEXT_HOP attribute according to the rules specified in <xref target="RFC4760"/> as
  the BGP-LS-SPF routing information is carried within the multiprotocol extensions for
  BGP-4.
</t>
</section>
</section>

<section anchor="bgp-decision" title="Decision Process with SPF Algorithm">
  <t>
    The Decision Process described in <xref target="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 SPF 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.
  </t>
  <t>
    The SPF based Decision process replaces the BGP Decision process described in
    <xref target="RFC4271"/>. This process starts with selecting only those Node NLRI whose
    SPF capability TLV matches with the local BGP SPF speaker's SPF capability TLV value.
    Since Link-State NLRI always contains the local node descriptor
    <xref target="NLRI-Use"/>, each NLRI is uniquely originated by a single
    BGP SPF speaker in the BGP SPF routing domain (the BGP node matching the NLRI's Node
    Descriptors). Instances of the same NLRI originated by multiple BGP SPF speakers would be
    indicative of a configuration error or a masquerading attack (<xref target="Security"/>).
    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.
  </t>
  <t>
    When BGP-LS-SPF NLRI is received, all that is required is to determine
    whether it is the most recent by examining the Node-ID and sequence number as described
    in <xref target="Phase-1"/>. If the received NLRI 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 triggered. However, a changed NLRI MAY be advertised immediately
    to other peers and prior to any SPF calculation. Note that the BGP
    MinRouteAdvertisementIntervalTimer and
    MinASOriginationIntervalTimer <xref target="RFC4271"/> timers are not applicable
    to the BGP-LS-SPF SAFI. The scheduling of the SPF calculation, as described in
    <xref target="BGP-SPF"/>, is an implementation issue. Scheduling MAY be dampened
    consistent with the SPF back-off algorithm
    specified in <xref target="RFC8405"/>.
  </t>
  <t>
    The Phase 3 decision function
    of the Decision Process <xref target="RFC4271"/> is also simplified since under
    normal SPF operation, a BGP SPF speaker MUST advertise the changed NLRIs
    to all BGP peers with the BGP-LS-SPF AFI/SAFI and install the 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.
  </t>

<section anchor="Phase-1" title="BGP NLRI Selection">
<t>
  The rules for all BGP-LS-SPF NLRIs selection for phase 1 of the BGP
  decision process, section 9.1.1 <xref target="RFC4271"/>, no longer apply.
<list style="numbers">
<t>
  Routes originated by directly connected BGP SPF peers are preferred.
  This condition can be determined by comparing the BGP Identifiers in
  the received Local Node 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.
</t>
<t>
  The NLRI with the most recent Sequence Number TLV, i.e., highest sequence number is selected.
</t>
<t>
   The route received from the BGP SPF speaker with the numerically larger BGP
   Identifier is preferred.
</t>
</list>
</t>
<t>
When a BGP SPF speaker completely loses its sequence number state, i.e., due to a cold start, or
in the unlikely possibility that 64-bit sequence number wraps, the BGP routing domain will
still converge. This is due to the fact that BGP SPF speakers adjacent to the router will
always accept self-originated NLRI from the associated speaker as more recent (rule # 1). When a
BGP SPF speaker reestablishes a connection with its peers, any existing session will be taken
down and stale NLRI will be replaced. The adjacent BGP SPF speaker will update their NLRI
advertisements, hop by hop, until the BGP routing domain has converged.
</t>
<t>
The modified SPF Decision Process performs an SPF calculation rooted at the BGP SPF
speaker using the metrics from the Link Attribute IGP Metric TLV (1095) and
the Prefix Attribute Prefix Metric TLV (1155) <xref target="RFC7752"/>.
As a result, any other BGP attributes that
would influence the BGP decision process defined in <xref target="RFC4271"/> including
ORIGIN, MULTI_EXIT_DISC, and
LOCAL_PREF attributes are ignored by the SPF algorithm. The NEXT_HOP attribute
is discussed in <xref target="NEXT-HOP"/>.
The AS_PATH and AS4_PATH <xref target="RFC6793"/> attributes
are preserved and used for loop detection <xref target="RFC4271"/>. They are ignored
during the SPF computation for BGP-LS-SPF NRLIs.
</t>
<section anchor="Self-Origin" title="BGP Self-Originated NLRI">
  <t>
    Node, Link, or Prefix NLRI with Node Descriptors matching the local BGP SPF 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.
    <list style="symbols">
      <t>
        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.
      </t>
      <t>
        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.
      </t>
      <t>
        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.
      </t>
    </list>
  </t>
  <t>
    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 SPF speaker <xref target="Security"/>.
  </t>
</section>
</section>

<section anchor="dual-stack" title="Dual Stack Support">
<t>
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 computation or multiple
SPF computations for separate AFs is an implementation matter. Normally, IPv4
next-hops are calculated for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
prefixes.
</t>
</section>

<section anchor="BGP-SPF" title="SPF Calculation based on BGP-LS-SPF NLRI">
<t>
  This section details the BGP-LS-SPF local routing information base (RIB) calculation.
  The router will use BGP-LS-SPF Node, Link, and Prefix NLRI to compute routes using the
  following algorithm. This calculation yields the set of routes associated
  with the BGP SPF Routing Domain.  A router calculates the shortest-path tree using itself
  as the root. Optimizations to the BGP-LS-SPF algorithm are possible but MUST yield
  the same set of routes. The algorithm below supports Equal Cost Multi-Path (ECMP)
  routes. Weighted Unequal Cost Multi-Path routes are out of scope. The organization
  of this section owes heavily to section 16 of <xref target="RFC2328"/>.
</t>
<t>
The following abstract data structures are defined in order to specify the algorithm.
<list style="symbols">
<t>
  Local Route Information Base (LOC-RIB) - This routing table contains reachability information
  (i.e., next hops) for all prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF node
  reachability. Implementations may choose to implement this with separate RIBs for each
  address family and/or Prefix versus Node reachability. It is synonymous with the Loc-RIB
  specified in <xref target="RFC4271"/>.
</t>
<t>
  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".
</t>
<t>
  Link State NLRI Database (LSNDB) - Database of BGP-LS-SPF NLRI that facilitates access to
  all Node, Link, and Prefix NLRI.
</t>
<t>
  Candidate List (CAN-LIST) - This is a list of candidate Node NLRIs used during the BGP SPF
  calculation <xref target="BGP-SPF"/>. 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 <xref target="terms"/>
  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.
</t>
</list></t>
<t>The algorithm is comprised of the steps below:
<list style="numbers">
  <t>
    The current LOC-RIB is invalidated, and the CAN-LIST is initialized to empty.
    The 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 made to the GLOBAL-RIB in
    step 6.
</t>
<t>
  The computing router's Node NLRI is updated in the LOC-RIB with a 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.
</t>
<t>
  The Node NLRI with the lowest cost is removed from the candidate list for processing.
  If the BGP-LS Node attribute doesn't include an SPF Capability TLV
  (<xref target="node-spf-cap-tlv"/>,
  the Node NLRI is ignored and the next lowest cost  Node NLRI is selected
  from candidate list. The
  If the BGP-LS Node attribute includes an SPF Status TLV (<xref target="node-spf-cap-tlv"/>)
  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-Node. If the candidate
  list is empty, the SPF calculation has completed and the algorithm proceeds to step 6.
</t>
<t>
  All the Prefix NLRI with the same Node Identifiers as the 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 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):
  <list style="symbols">
    <t>
      If the BGP-LS Prefix attribute includes an SPF Status TLV indicating the prefix is
      unreachable, the Current-Prefix is considered unreachable and the next Prefix
      NLRI is examined in Step 4.
    </t>
    <t>
      If the Current-Prefix's corresponding prefix is in the LOC-RIB and the
      LOC-RIB cost is less than the Current-Prefix's metric,
      the Current-Prefix does not contribute to the route and the next Prefix NLRI is
      examined in Step 4.
    </t>
    <t>
      If the Current-Prefix's corresponding prefix is not in the 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).
    </t>
    <t>
      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 is installed with the Current-Node's 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).
    </t>
    <t>
      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.
    </t>
   </list></t>
<t>
  All the Link NLRI with the same Node Identifiers as the 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. The cost of the Current-Link is the advertised IGP Metric TLV (1095)
  from the Link NLRI BGP-LS attribute added to 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:
  <list style="letters">
    <t>
      The prefix(es) associated with the Current-Link are installed into the
      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.
    </t>
    <t>
      If the Current-Node NLRI attributes includes the SPF status TLV
      (<xref target="node-status-tlv"/>) and the status
      indicates that the Node doesn't support transit, the next link for the Current-Node is
      processed in Step 5.
    </t>
    <t>
      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.
    </t>
    <t>
      The Current-Link's Remote Node NLRI is accessed (i.e., the Node NLRI
      with the same Node identifiers as the Current-Link's Remote Node Descriptors). If it exists,
      it will be referred to as the Remote-Node and the algorithm will proceed as follows:
      <list style="symbols">
        <t>
          If the Remote-Node's NLRI attribute includes an SPF Status TLV indicating the node is
          unreachable, the next link for the Current-Node is examined in Step 5.
        </t>
        <t>
          All the Link NLRI corresponding the Remote-Node will be searched for a Link
          NLRI pointing to the Current-Node. 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.
        </t>
        <t>
          If the Remote-Link NLRI attribute includes an SPF Status 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.
        </t>
        <t>
          If the Remote-Node is not on the CAN-LIST, it is inserted based
          on the cost. The Remote Node's cost 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.
        </t>
        <t>
          If the Remote-Node NLRI is already on the CAN-LIST with a 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.
        </t>
        <t>
          If the Remote-Node NLRI is already on the CAN-LIST with 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.
        </t>
        <t>
          If the Remote-Node NLRI is already on the CAN-LIST with a lower cost, it need
          not be reinserted on the CAN-LIST.
        </t>
  </list></t>
  <t>
    Return to step 3 to process the next lowest cost Node NLRI on the CAN-LIST.
  </t>
</list></t>
<t>
The LOC-RIB is examined and changes (adds, deletes, modifications) are installed into
the GLOBAL-RIB. For each route in the LOC-RIB:
<list style="symbols">
  <t>
    If the route was added during the current BGP SPF computation, install the route into
    the GLOBAL-RIB.
  </t>
  <t>
    If the route modified during the current BGP SPF computation (e.g., metric, tags,
    or next-hops), update the route in the GLOBAL-RIB.
  </t>
  <t>
    If the route was not installed during the current BGP SPF computation, remove the route
    from both the GLOBAL-RIB and the LOC-RIB.
  </t>
</list></t>
</list></t>
</section>

<section title="IPv4/IPv6 Unicast Address Family Interaction">
  <t>
    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 operate today (i.e., "Ships-in-the-Night" mode). There is no implicit
    route redistribution between the BGP address families. 
  </t>
  <t>
    It is RECOMMENDED that BGP-LS-SPF IPv4/IPv6 route computation and
    installation be given scheduling priority by default over other BGP address families
    as these address families are considered as underlay SAFIs. Similarly, it is RECOMMENDED that
    the route preference or administrative distance 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.
  </t>
</section>

<section anchor="NLRI-Advertise" title="NLRI Advertisement">
<section anchor="failure-converge" title="Link/Prefix Failure Convergence">
  <t>
    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 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.
  </t>
  <t>
    An IGP such as OSPF <xref target="RFC2328"/> will stop using the link as soon as
    the 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 of the BGP-LS-SPF Link NLRI
    is withdrawn. In order to avoid this delay, the originator of the Link NLRI SHOULD
    advertise a more recent version with an increased Sequence Number TLV for
    the BGP-LS-SPF Link NLRI including the SPF Status TLV
    (<xref target="link-status-tlv"/>) indicating the link
    is down with respect to BGP SPF.
    The configurable LinkStatusDownAdvertise timer
    controls the interval that the BGP-LS-LINK NLRI is advertised with SPF Status indicating
    the link is down prior to withdrawal.
    If the link becomes available
    in that period, the originator of the BGP-LS-SPF LINK NLRI SHOULD advertise a more recent
    version of the BGP-LS-SPF Link NLRI without the SPF Status TLV in the BGP-LS Link Attributes.
    The suggested default value for the LinkStatusDownAdvertise timer is 2 seconds.
  </t>
  <t>
    Similarly, when a prefix becomes unreachable, a more recent version of the BGP-LS-SPF
    Prefix NLRI SHOULD be advertised with the SPF Status TLV (<xref target="prefix-status-tlv"/>)
    indicating the prefix is unreachable in the BGP-LS Prefix Attributes and the prefix will be
    considered unreachable with respect to BGP SPF.
    The configurable PrefixStatusDownAdvertise timer
    controls the interval that the BGP-LS-Prefix NLRI is advertised with SPF Status indicating
    the prefix is unreachable prior to withdrawal.
    If the prefix becomes reachable in that period, the originator of the BGP-LS-SPF Prefix NLRI
    SHOULD advertise a more recent version of the BGP-LS-SPF Prefix NLRI without the
    SPF Status TLV in the BGP-LS Prefix Attributes.
    The suggested default value for the PrefixStatusDownAdvertise timer is 2 seconds.
  </t>
</section>
<section anchor="node-failure" title="Node Failure Convergence">
  <t>
    With BGP without graceful restart <xref target="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-SPF NLRI may be
    withdrawn. This will result into an older version of the NLRI being used until the
    new versions arrive and, potentially, unnecessary route flaps.
    For the BGP-LS-SPF SAFI, NLRI SHOULD NOT be implicitly withdrawn immediately
    to prevent such unnecessary route flaps.
    The configurable NLRIImplicitWithdrawalDelay timer
    controls the interval that NLRI is retained prior to implicit withdrawal after a BGP
    SPF speaker has transitioned out of Established state.
    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 (<xref target="failure-converge"/>) and the BGP SPF calculation will fail the
    bi-directional connectivity check <xref target="BGP-SPF"/>.
    The suggested default value for the NLRIImplicitWithdrawalDelay timer is 2 seconds.
  </t>
</section>
</section>
</section>

<section anchor="error-handling" title="Error Handling">
<t>
This section describes the Error Handling actions, as described in
<xref target="RFC7606"/>, that are specific to SAFI BGP-LS-SPF BGP Update
message processing.
</t>

<section anchor="new-TLVs" title="Processing of BGP-LS-SPF TLVs">
<t>
When a BGP SPF speaker receives a BGP Update containing a malformed Node NLRI
SPF Status TLV in the BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in <xref target="RFC7606"/>.
When discarding an associated Node NLRI with a malformed TLV,
a BGP SPF speaker SHOULD log an error for further analysis.
</t>

<t>
When a BGP SPF speaker receives a BGP Update containing a malformed Link NLRI
SPF Status TLV in the BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in <xref target="RFC7606"/>.
When discarding an associated Link NLRI with a malformed TLV,
a BGP SPF speaker SHOULD log an error for further analysis.
</t>

<t>
When a BGP SPF speaker receives a BGP Update containing a malformed Prefix NLRI
SPF Status TLV in the BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in <xref target="RFC7606"/>.
When discarding an associated Prefix NLRI with a malformed TLV,
a BGP SPF speaker SHOULD log an error for further analysis.
</t>

<t>
When a BGP SPF speaker receives a BGP Update containing a malformed SPF Capability TLV
in the Node NLRI BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and the Node NLRI and MUST NOT pass it to other
BGP peers as specified in <xref target="RFC7606"/>.
When discarding a Node NLRI with a malformed TLV, a BGP SPF speaker SHOULD log an error for
further analysis.
</t>

<t>
When a BGP SPF speaker receives a BGP Update containing a malformed IPv4 Prefix-Length TLV
in the Link NLRI BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and the Node NLRI and MUST NOT pass it to other
BGP peers as specified in <xref target="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.
</t>

<t>
When a BGP SPF speaker receives a BGP Update containing a malformed IPv6 Prefix-Length TLV
in the Link NLRI BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and the Node NLRI and MUST NOT pass it to other BGP peers as
specified in <xref target="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.
</t>
</section>

<section anchor="bgpspf-nlri" title="Processing of BGP-LS-SPF NLRIs">
<t>
  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 <xref target="SAFI"/>
  and <xref target="BGP-LS-TLV"/>.
</t>

<t>
   A BGP-LS-SPF Speaker MUST perform the following syntactic validation of
   the BGP-LS-SPF NLRI to determine if it is malformed.
   <list style="numbers">
     <t>
       Does the sum of all TLVs found in the BGP MP_REACH_NLRI attribute
       correspond to the BGP MP_REACH_NLRI length?
     </t>

   <t>
     Does the sum of all TLVs found in the BGP MP_UNREACH_NLRI
     attribute correspond to the BGP MP_UNREACH_NLRI length?
   </t>

   <t>
     Does the sum of all TLVs found in a BGP-LS-SPF NLRI correspond to
      the Total NLRI Length field of all its Descriptors?
   </t>

   <t>
     When an NLRI TLV is recognized, is the length of the TLV and
     its sub-TLVs valid?
   </t>

   <t>
     Has the syntactic correctness of the NLRI fields been verified as
     per <xref target="RFC7606"/>?
   </t>
   <t>
     Has the rule regarding ordering of TLVs been followed as described
     in <xref target="BGP-LS-TLV"/>?
   </t>
   </list>
</t>
<t>
  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.
</t>
</section>

<section anchor="bgpspf-attribute" title="Processing of BGP-LS Attribute">
<t>
   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 <xref target="SAFI"/>
   and <xref target="BGP-LS-TLV"/>.
</t>

<t>
   A BGP-LS-SPF Speaker MUST perform the following syntactic validation of
   the BGP-LS Attribute to determine if it is malformed.
<list style="numbers">
   <t>  Does the sum of all TLVs found in the BGP-LS-SPF Attribute correspond
      to the BGP-LS Attribute length?
   </t>
   <t>  Has the syntactic correctness of the Attributes (including BGP-LS
      Attribute) been verified as per <xref target="RFC7606"/>?
   </t>
   <t>  Is the length of each TLV and, when the TLV is recognized then,
      its sub-TLVs in the BGP-LS Attribute valid?
   </t>
</list>
</t>

<t>
   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.
</t>

<t>
   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 SPF speaker that the specific information
   is being deleted or is not available.
</t>

<t>
   When a BGP 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 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 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 SPF speaker which actually detected a problem with the BGP-LS
   Attribute.
</t>

<t>
   An implementation SHOULD log an error for further analysis for problems
   detected during syntax validation.
</t>
<t>
  When a BGP SPF speaker receives a BGP Update containing a malformed IGP metric TLV
  in the Link NLRI BGP-LS Attribute <xref target="RFC7752"/>,
  it MUST ignore the received TLV and the Link NLRI and MUST NOT pass it to other
  BGP peers as specified in <xref target="RFC7606"/>.
  When discarding a Link NLRI with a malformed TLV, a BGP SPF speaker SHOULD log
  an error for further analysis.
</t>
</section>
</section>

    <section anchor="IANA" title="IANA Considerations">
<t>
    This document defines the use of SAFI (80) for BGP SPF operation
    <xref target="SAFI"/>, and requests IANA to assign the value from the
    First Come First Serve (FCFS) range in the
    Subsequent Address Family Identifiers (SAFI) Parameters registry.
</t>
<t>
  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,
  and SPF Status TLV
  from the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor,
  and Attribute TLVs" Registry.
</t>
        <texttable anchor="tab.iana-attr" title="NLRI Attribute TLVs">
          <ttcol>Attribute TLV</ttcol>
          <ttcol>Suggested Value</ttcol>
          <ttcol>NLRI Applicability</ttcol>
          <c>SPF Capability</c><c>1180</c><c>Node</c>
          <c>SPF Status</c><c>1184</c><c>Node, Link, Prefix</c>
          <c>IPv4 Link Prefix Length</c><c>1182</c><c>Link</c>
          <c>IPv6 Link Prefix Length</c><c>1183</c><c>Link</c>
          <c>Sequence Number</c><c>1181</c><c>Node, Link, Prefix</c>
        </texttable>

</section>

<section anchor="Security" title="Security Considerations">
  <t>
    This document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This document
    does not change the underlying security issues inherent in the BGP protocol
    <xref target="RFC4271"/>. The Security Considerations
    discussed in <xref target="RFC4271"/> apply to the BGP SPF functionality as well.
    The analysis of the security issues for BGP mentioned
    in <xref target="RFC4272"/> and <xref target="RFC6952"/> also applies to this document.
    The analysis of Generic Threats to Routing Protocols done in <xref target="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 <xref target="RFC6811"/> and <xref target="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.
  </t>
  <t>
    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.
  </t>
  <t>
    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) <xref target="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.
  </t>
</section>

<section anchor="Management" title="Management Considerations">
  <t>
    This section includes unique management considerations for the BGP-LS-SPF address family.
  </t>
  <section anchor="Config" title="Configuration">
    <t>
      All routers in BGP SPF Routing Domain are under a single administrative domain
      allowing for consistent configuration.
    </t>
    <section anchor="link-metric-config" title="Link Metric Configuration">
      <t>
	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.
      </t>
    </section>
    <section anchor="spf-backoff-config" title="backoff-config">
      <t>
	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"
	<xref target="RFC8405"/>. If supported, configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
	LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST be supported <xref target="RFC8405"/>.
	Section 6 of <xref target="RFC8405"/> recommends consistent configuration of these values
	throughout the IGP routing domain and this also applies to the BGP SPF Routing Domain.
      </t>
    </section>
  </section>
  <section anchor="Operation" title="Operational Data">
    <t>
      In order to troubleshoot SPF issues, implementations SHOULD support an SPF log including
      entries for previous SPF computations. Each SPF log entry would include the 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 and the
      total number of SPF triggering events. Additionally, to troubleshoot SPF scheduling and
      back-off <xref target="RFC8405"/>, the current SPF back-off state, remaining time-to-learn,
      remaining holddown, last trigger event time, last SPF time, and next SPF time should be
      available.
    </t>
</section>
</section>

<section anchor="implementation" title="Implementation Status">
  <t>Note RFC Editor: Please remove this section and the associated references
  prior to publication.</t>
  <t>This section records the status of known implementations of the
     protocol defined by this specification at the time of posting of
     this Internet-Draft and is based on a proposal described in
     <xref target="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.</t>

     <t>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".</t>

     <t>The BGP-LS-SPF implementation status is documented in
        <xref target="I-D.psarkar-lsvr-bgp-spf-impl"/>.</t>

</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>
The authors would like to thank Sue Hares, Jorge Rabadan, Boris Hassanov, Dan Frost,
Matt Anderson, Fred Baker, Lukas Krattiger, Yingzhen Qu 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).
</t>
<t>
The authors extend special thanks to Eric Rosen for fruitful discussions on
BGP-LS-SPF convergence as compared to IGPs.
</t>
</section>

<section anchor="Contributors" title="Contributors">
<t>
In addition to the authors listed on the front page, the following
co-authors have contributed to the document.
</t>

<figure align="center">
<artwork align="left"><![CDATA[
  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
]]></artwork>
</figure>

</section>


  </middle>

  <!--  *****BACK MATTER ***** -->

  <back>
    <!-- References split into informative and normative -->

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    <references title="Normative References">

      &RFC2119;

      &RFC4271;

      &RFC4272;

      &RFC4593;

      &RFC4750;

      &RFC4760;

      &RFC5492;

      &RFC5925;

      &RFC6793;

      &RFC6811;

      &RFC7606;

      &RFC7752;

      &RFC8174;

      &RFC8205;

      &RFC8665;

      &RFC8405;

      &RFC8654;

    </references>

    <references title="Informational References">

      &RFC2328;

      &RFC4456;

      &RFC4724;

      &RFC4915;

      &RFC5286;

      &RFC5880;

      &RFC6952;

      &RFC7911;

      &RFC7938;

      &RFC7942;

      &I-D.ietf-lsvr-applicability;

      &I-D.psarkar-lsvr-bgp-spf-impl;

    </references>

  </back>
</rfc>