draft-ietf-lsvr-bgp-spf-00.xml

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<rfc category="std" docName="draft-ietf-lsvr-bgp-spf-00.txt"
 ipr="pre5378Trust200902">
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  <!-- ***** FRONT MATTER ***** -->

  <front>

    <title abbrev="BGP Protocol SPF Extensions">
    Shortest Path Routing Extensions for BGP Protocol </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>
        <postal>
          <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"
            surname="Zandi">

      <organization>Linkedin</organization>

      <address>
        <postal>
          <street>222 2nd Street</street>

          <city>San Francisco</city>

          <region>CA</region>

          <code>94105</code>

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

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

      </address>
    </author>

    <author fullname="Wim Henderickx" initials="W"
            surname="Henderickx">

      <organization>Nokia</organization>

      <address>
        <postal>
          <street></street>

          <city>Antwerp</city>

          <region></region>

          <code></code>

          <country>Belgium</country>
        </postal>

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

      </address>
    </author>

    <date/>

    <!-- Meta-data Declarations -->

    <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 lead 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 BGP 
Link-State distribution and the Shortest Path First algorithm similar to 
Internal Gateway Protocols (IGPs) such as OSPF.
</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 lead 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. Requirements and procedures
for using BGP are described in <xref target="RFC7938"/>.
This document describes an alternative solution which leverages 
BGP-LS <xref target="RFC7752"/> and the Shortest Path First algorithm similar
to Internal Gateway Protocols (IGPs) such as OSPF <xref target="RFC2328"/>. 
</t>
<t>
<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>
<xref target="RFC7752"/> describes a mechanism by which link-state and TE information can
be collected from networks and shared with external components using BGP.
This is achieved by defining NLRIs carried within BGP-LS AFI and BGP-LS
SAFIs. The BGP-LS extensions defined in <xref target="RFC7752"/> makes use of the 
Decision Process defined in <xref target="RFC4271"/>. 
</t>

<t>
This document augments <xref target="RFC7752"/> by replacing its use of the existing 
Decision Process. The BGP-LS-SPF and BGP-LS-SPF-VPN AFI/SAFI are introduced to 
insure backward compatibility. The Phase 1 and 2 decision functions of the
Decision Process are replaced with the Shortest Path Algorithm (SPF) 
also known as the Dijkstra Algorithm. The Phase 3 decision function is also 
simplified since it is no longer dependent on the previous phases. 
This solution avails the benefits of both BGP and SPF-based IGPs.
These include TCP based flow-control, no periodic link-state refresh, and 
completely incremental NLRI advertisement. These advantages can reduce the 
overhead in MSDCs where there is a high degree of Equal Cost Multi-Path 
(ECMPs) and the topology is very stable. 
Additionally, using a SPF-based computation can support fast convergence and 
the computation of Loop-Free Alternatives (LFAs) <xref target="RFC5286"/> in the 
event of link failures. 
Furthermore, a BGP based solution lends itself to multiple peering models
including those incorporating route-reflectors <xref target="RFC4456"/> 
or controllers. 
</t>
<t> 
Support for Multiple Topology Routing (MTR) as described in 
<xref target="RFC4915"/> is an area for further study dependent on deployment 
requirements. 
</t>
<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. However, BGP SPF offers some unique advantages above 
and beyond standard BGP distance-vector routing.
</t>
<t>
A primary advantage is that all BGP speakers in the BGP SPF routing domain
will have a complete view of the topology. This will allow support of ECMP, 
IP fast-reroute (e.g., Loop-Free Alternatives), Shared Risk Link Groups 
(SRLGs), and other routing enhancements without advertisement of addition 
BGP paths 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="Phase-1"/>, 
NLRI changes can be disseminated throughout the BGP routing domain much
more rapidly (equivalent to IGPs with the proper implementation).
</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 speakers corresponding to 
the link NLRI need withdraw the corresponding BGP-LS Link NLRI. This 
advantage will contribute to both faster convergence and better scaling. 
</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 verion 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 speaker will 
only need as many sessions and copies of the NLRI as required for
redundancy (e.g., one for SPF computation and another for backup). 
Functions such as Optimized Route Reflection (ORR) 
are supported without extension by virture of the primary advantages. 
Additionally, a controller could inject topology that 
is learned outside the BGP routing domain.
</t>
<t>
Given that controllers are already consuming BGP-LS NLRIs 
<xref target="RFC7752"/>, reusing for the BGP-LS SPF leverages the 
existing controller implementations. 
</t>
<t> 
Another potential advantage of BGP SPF is that both IPv6 and IPv4 can be 
supported in the same address family using the same topology. Although not
described in this version of the document, multi-topology extensions can
be used to support separate IPv4, IPv6, unicast, and multicast topologies
while sharing the same NLRIs. 
</t> 
<t>
Finally, the BGP SPF topology can be used as an underlay for other BGP 
address families (using the existing model) and realize all the above
advantages. A simplified peering model using IPv6 link-local addresses
as next-hops can be deployed similar to  <xref target="RFC5549"/>.
</t>
</section>

      <section title="Requirements Language">
        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
        document are to be interpreted as described in <xref
        target="RFC2119">RFC 2119</xref>.</t>
      </section>
</section> <!-- for Introductions section -->


<section anchor="peering-models" title="BGP Peering Models">
<t>
Depending on the requirements, scaling, and capabilities of the BGP speakers, various
peering models are supported. The only requirement is that all BGP speakers in the
BGP SPF routing domain receive link-state NLRIs on a timely basis, run an SPF calculation, 
and update their data plane appropriately. The content of the Link NLRI is described 
in <xref target="Link-NLRI"/>.
</t>
<section title="BGP Single-Hop Peering on Network Node Connections">
<t>
The simplest peering model is the one described in section 5.2.1 of 
<xref target="RFC7938"/>. In this model,
EBGP single-hop sessions are established over direct point-to-point links
interconnecting the network nodes. For the purposes of BGP SPF, Link NLRI is only 
advertised if a single-hop BGP session has been established and the Link-State/SPF
adddress family capability has been exchanged <xref target="RFC4790"/> on the
corresponding session. 
If the session goes down, the NLRI will be withdrawn. 
</t>
</section>
<section title="BGP Peering Between Directly Connected Network Nodes">
<t>
In this model, BGP speakers peer with all directly connected
network nodes but the sessions may be multi-hop and the direct connection
discovery and liveliness detection for those connections are 
independent of the BGP protocol. How this is accomplished is outside
the scope of this document. 
Consequently, there will be a single session even if there are multiple 
direct connections between BGP speakers.
For the purposes of BGP SPF, Link NLRI is advertised as long as 
a BGP session has been established, the Link-State/SPF address family 
capability has been exchanged <xref target="RFC4790"/> and
the corresponding link is up and considered operational. 
</t>
</section>
<section title="BGP Peering in Route-Reflector or Controller Topology">
<t>
In this model, BGP 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 connections are 
done outside the BGP protocol. More specifically, the Liveliness detection is
done using BFD protocol described in <xref target="RFC5880"/>. For the 
purposes of BGP SPF, Link NLRI is 
advertised as long as the corresponding link is up and considered operational. 
</t>
</section>
</section>

<section title="BGP-LS Shortest Path Routing (SPF) SAFI">
<t>
  In order to replace the Phase 1 and 2 decision functions of the 
  existing Decision Process with an SPF-based Decision Process and streamline 
  the Phase 3 decision functions in a backward compatible manner, this draft 
  introduces a couple AFI/SAFIs for BGP LS SPF operation.
  The BGP-LS-SPF (AF 16388 / SAFI TBD1) and BGP-LS-SPF-VPN 
  (AFI 16388 / SAFI TBD2) <xref target="RFC4790"/> are allocated by IANA
  as specified in the <xref target="IANA"/>. A BGP speaker wanting to run BGP LS
  SPF operation must exchange the AFI/SAFI using Multiprotocol Extensions
  Capabilty Code, as defined in <xref target="RFC4760"/>.
</t>
</section>

<section title="Extensions to BGP-LS">
<t>
  <xref target="RFC7752"/> describes a mechanism by which link-state and TE
  information can be collected from networks and shared with external components
  using BGP protocol. It contains two parts: definition of a new BGP NLRI
  that describes links, nodes, and prefixes comprising IGP link-state
  information and definition of a new 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.
</t>

<t>
The BGP protocol will be used in the Protocol-ID field specified in
table 1 of <xref target="I-D.ietf-idr-bgpls-segment-routing-epe"/>. 
The local and remote node descriptors for all NLRIs will be the BGP Router-ID (TLV 516) 
and either the AS Number (TLV 512) <xref target="RFC7752"/> or the BGP Confederation
Member (TLV 517) <xref target="I-D.ietf-idr-bgpls-segment-routing-epe"/>. 
However, if the BGP Router-ID is known to be unique within the BGP Routing domain,
it can be used as the sole descriptor. 
</t>
<section title="Node NLRI Usage and Modifications">
<t>
The SPF capability is a new Node Attribute TLV that will be added
to those defined in table 7 of <xref target="RFC7752"/>. The
new attribute TLV will only be applicable when BGP is specified 
in the Node NLRI Protocol ID field. 
The TBD
TLV type will be defined by IANA. The new Node
Attribute TLV will contain a single octet SPF algorithm field: 
</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             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SPF Algorithm |
   +-+-+-+-+-+-+-+-+

 The SPF Algorithm may take the following values:

   1 - Normal SPF 
   2 - Strict SPF 

]]></artwork>
</figure>
</t>
<t> 
When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability attribute will be included the Shortest
Path Tree (SPT). 
</t>
</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 local and remote node descriptors as described 
  above and unique link identifiers dependent on the addressing. For IPv4 links, the 
  links 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>
The link IGP metric attribute TLV (TLV 1095) as well as any others required for non-SPF 
purposes SHOULD be advertised.
Algorithms such as setting the metric inversely to the link speed as done in the 
OSPF MIB <xref target="RFC4750"/> may be supported. However, this is beyond the scope 
of this document. 
</t>
</section>
<section title="Prefix NLRI Usage">
<t>
Prefix NLRI is advertised with a local descriptor as described above and the prefix and
length used as the descriptors (TLV 265) as described in <xref target="RFC7752"/>. 
The prefix metric attribute TLV (TLV 1155) as well as any others required for non-SPF 
purposes SHOULD be advertised. For loopback prefixes, the metric should be 0. For non-loopback,
the setting of the metric is beyond the scope of this document. 
</t>
</section>
<section title="BGP-LS Attribute Sequence-Number TLV">
<t>
A BGP-LS Attribute TLV type is added to BGP-LS NLRI types to assure the most
recent version of a given NLRI is used in the SPF computation. 
The TBD TLV type will be defined by IANA. The new BGP-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             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                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 is incremented for every 
version of BGP-LS NLRI originated. 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 the BGP Router 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&nbhy;bit value should
be incremented and saved in non-volatile storage. If by some chance the BGP Speaker is 
deployed long enough that there is a possibility that the 64-bit sequence number may wrap 
or a BGP Speaker completely loses its sequence number state (e.g, the BGP speaker hardware
is replaced), the phase 1 decision function (see <xref target="Phase-1"/>) rules should 
insure convergance, albeit, not immediately.
</t>
</section>
</section> 



<section 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 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 
also known as a Path vector algorithm.
</t>
<t>
The SPF based Decision process replaces the BGP Bestpath Decision process described in 
<xref target="RFC4271"/>. This process starts with selecting only those Node NLRIs whose 
SPF capability TLV matches with the local BGP speaker's SPF capability TLV value. 
Since Link-State NLRI always contains the local descriptor 
<xref target="RFC7752"/>, it will only 
be originated by a single BGP speaker in the BGP routing domain. 
These selected Node NLRIs and their Link/Prefix NLRIs are used to build a directed
graph during the SPF computation. The best paths for BGP prefixes
are installed 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 best-path by examining the Node-ID and sequence number as described
in <xref target="Phase-1"/>. If the received best-path NLRI had 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. However, a changed best-path can be 
advertised to other peer immediately and propagation of changes can approach 
IGP convergence times with appropriately tuned MinRouteAdvertisementIntervalTimer. 
</t>
<t>
The Phase 3 decision function 
of the Decision Process <xref target="RFC4271"/> is also simplified since under
normal SPF operation, a BGP speaker would advertise the NLRI
selected for the SPF to all BGP peers with the BGP-LS/BGP-SPF AFI/SAFI. 
Application of policy would not be prevented however its usage to bestpath process
would be limited as the SPF relies solely on link metrics.  
</t>

<section anchor="Phase-1" title="Phase-1 BGP NLRI Selection">
<t>
The rules for NLRI selection are greatly simplified from <xref target="RFC4271"/>.
<list style="numbers">
<t>
If the NLRI is received from the BGP speaker originating the NLRI (as determined by the
comparing BGP Router ID in the NLRI Node identifiers with the BGP speaker Router ID), 
then it is preferred over the same NLRI from non-originators. 
</t>
<t>
If the Sequence-Number TLV is present in the BGP-LS Attribute, then the NLRI with the
most recent, 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 or a BGP-LS Attribute that
doesn't include the Sequence-Number TLV. 
</t>
<t>The final tie-breaker is the NLRI from the BGP Speaker with the numerically largest
BGP Router ID.
</t>
</list> 
</t>
<t>
The modified Decision Process with SPF algorithm uses the metric from Link and Prefix NLRI
Attribute TLVs <xref target="RFC7752"/>. As a result, any attributes that would influence the 
Decision process defined in <xref target="RFC4271"/> like 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 or best-path. 
</t>
</section>

<section 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 instance or multiple
SPF instances for separate AFs is a matter of a local implementation. 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 <xref target="RFC5549"/> where 
IPv6 link-local next-hops are calculated for both IPv4 and IPv6 prefixes. As stated in 
<xref target="introduction"/>, support for Multiple Topology Routing (MTR) is an area 
for future study.
</t>
</section>

<section title="NEXT_HOP Manipulation">
<t>
A BGP speaker that supports SPF extensions MAY interact with peers that don't support
SPF extensions. If the BGP Link-State address family is advertised to a peer not
supporting the SPF extensions described herein, then the BGP speaker 
MUST conform to the NEXT_HOP rules mentioned in <xref target="RFC4271"/> when announcing 
the Link-State address family routes to those peers.
</t>

<t>
All BGP peers that support SPF extensions would locally compute the NEXT_HOP values as
result of the SPF process. As a result, the NEXT_HOP attribute is always ignored on 
receipt. However BGP speakers should set the NEXT_HOP address according to the
NEXT_HOP attribute rules mentioned in <xref target="RFC4271"/>.
</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 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 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.
</t>
<t> 
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
routers, it is RECOMMENDED that BGP-LS SPF IPv4/IPv6 routes be given priority by default
when installed into their respective RIBs. In common implementations the prioritization
is governed by route preference or administrative distance with lower being more preferred.
</t>
</section>


<section anchor="NLRI-Advertise" title="NLRI Advertisement and 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
failues should always be given priority over updates (e.g., withdrawing all 
routes learned on a session) in order to ensure the highest priority progation 
and optimal convergence.
</t>
<t>Delaying the withdrawal of non-local routes is an area for further study as 
more IGP-like mechanisms would be required to prevent usage of stale NLRI.</t>
</section> 

<section title="Error Handling">
<t>
When a BGP 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 not pass it to other BGP peers as
specified in <xref target="RFC7606"/>.
When discarding a Node NLRI with malformed TLV, a BGP speaker SHOULD log an error for 
further analysis. 
</t>

</section>
</section>

    <section anchor="IANA" title="IANA Considerations">
<t>
  This document defines a couple AFI/SAFIs for BGP LS SPF operation and
  requests IANA to assign the BGP-LS-SPF AFI 16388 / SAFI TBD1 and the 
  BGP-LS-SPF-VPN  AFI 16388 / SAFI TBD2 as described in 
  <xref target="RFC4750"/>.
</t>
<t>
  This document also defines two attribute TLV for BGP LS NLRI.
  We request IANA to assign TLVs for the SPF capability and 
  the Sequence Number from the "BGP-LS Node Descriptor, Link Descriptor, 
  Prefix Descriptor, and   Attribute TLVs" Registry. 
  Additionally, IANA is requested to create a 
  new registry for "BGP-LS SPF Capability Algorithms" for the value
  of the algorithm both in the BGP-LS Node Attribute TLV and the 
  BGP SPF Capability. The initial assignments are:  
  <figure title="BGP-LS SPF Capability Algorithms">
        <artwork>
        +-------------+-----------------------------------+
        | Value(s)    | Assignment Policy                 |
        +-------------+-----------------------------------+
        | 0           | Reserved (not to be assigned)     |
        |             |                                   |
        | 1           | SPF                               |
        |             |                                   |
        | 2           | Strict SPF                        |
        |             |                                   |
        | 3-254       | Unassigned (IETF Review)          |
        |             |                                   |
        | 255         | Reserved (not to be assigned)     |
        +-------------+-----------------------------------+
     </artwork>
    </figure>
</t>
</section>

<section anchor="Security" title="Security Considerations">
<t>
This extension to BGP does not change the underlying security issues
inherent in the existing <xref target="RFC4724"/> and <xref
target="RFC4271"/>.
</t>

<section anchor="Acknowledgements" title="Acknowledgements">
<t>
The authors would like to thank .... for the review and comments.
</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
  Cisco Systems
  akr@cisco.com

  Venu Venugopal
  Cisco Systems
  venuv@cisco.com
]]></artwork>
</figure>

</section>


    </section>
  </middle>

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

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

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    with a value containing a set of directories to search.  These can be
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    <references title="Normative References">

      &RFC2119;

      &RFC4271;

      &RFC7606;

      &RFC7752;

      &RFC7938;

      &I-D.ietf-idr-bgpls-segment-routing-epe;

    </references>

    <references title="Information References">

      &RFC2328;

      &RFC4456;

      &RFC4750;

      &RFC4760;

      &RFC4790;

      &RFC4724;

      &RFC4915;

      &RFC5286;

      &RFC5549;

      &RFC5880;
    </references>

    <!-- Change Log

v00 2008-10-01  KP    Initial version
    -->
  </back>
</rfc>