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RFC 7752

Proposed STD
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North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP

Part 1 of 3, p. 1 to 19
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Internet Engineering Task Force (IETF)                   H. Gredler, Ed.
Request for Comments: 7752                        Individual Contributor
Category: Standards Track                                      J. Medved
ISSN: 2070-1721                                               S. Previdi
                                                     Cisco Systems, Inc.
                                                               A. Farrel
                                                  Juniper Networks, Inc.
                                                                  S. Ray
                                                              March 2016


  North-Bound Distribution of Link-State and Traffic Engineering (TE)
                         Information Using BGP

Abstract

   In a number of environments, a component external to a network is
   called upon to perform computations based on the network topology and
   current state of the connections within the network, including
   Traffic Engineering (TE) information.  This is information typically
   distributed by IGP routing protocols within the network.

   This document describes a mechanism by which link-state and TE
   information can be collected from networks and shared with external
   components using the BGP routing protocol.  This is achieved using a
   new BGP Network Layer Reachability Information (NLRI) encoding
   format.  The mechanism is applicable to physical and virtual IGP
   links.  The mechanism described is subject to policy control.

   Applications of this technique include Application-Layer Traffic
   Optimization (ALTO) servers and Path Computation Elements (PCEs).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7752.

Page 2 
Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................5
   2. Motivation and Applicability ....................................5
      2.1. MPLS-TE with PCE ...........................................5
      2.2. ALTO Server Network API ....................................6
   3. Carrying Link-State Information in BGP ..........................7
      3.1. TLV Format .................................................8
      3.2. The Link-State NLRI ........................................8
           3.2.1. Node Descriptors ...................................12
           3.2.2. Link Descriptors ...................................16
           3.2.3. Prefix Descriptors .................................18
      3.3. The BGP-LS Attribute ......................................19
           3.3.1. Node Attribute TLVs ................................20
           3.3.2. Link Attribute TLVs ................................23
           3.3.3. Prefix Attribute TLVs ..............................28
      3.4. BGP Next-Hop Information ..................................31
      3.5. Inter-AS Links ............................................32
      3.6. Router-ID Anchoring Example: ISO Pseudonode ...............32
      3.7. Router-ID Anchoring Example: OSPF Pseudonode ..............33
      3.8. Router-ID Anchoring Example: OSPFv2 to IS-IS Migration ....34
   4. Link to Path Aggregation .......................................34
      4.1. Example: No Link Aggregation ..............................35
      4.2. Example: ASBR to ASBR Path Aggregation ....................35
      4.3. Example: Multi-AS Path Aggregation ........................36
   5. IANA Considerations ............................................36
      5.1. Guidance for Designated Experts ...........................37
   6. Manageability Considerations ...................................38
      6.1. Operational Considerations ................................38
           6.1.1. Operations .........................................38
           6.1.2. Installation and Initial Setup .....................38
           6.1.3. Migration Path .....................................38

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           6.1.4. Requirements on Other Protocols and
                  Functional Components ..............................38
           6.1.5. Impact on Network Operation ........................38
           6.1.6. Verifying Correct Operation ........................39
      6.2. Management Considerations .................................39
           6.2.1. Management Information .............................39
           6.2.2. Fault Management ...................................39
           6.2.3. Configuration Management ...........................40
           6.2.4. Accounting Management ..............................40
           6.2.5. Performance Management .............................40
           6.2.6. Security Management ................................41
   7. TLV/Sub-TLV Code Points Summary ................................41
   8. Security Considerations ........................................42
   9. References .....................................................43
      9.1. Normative References ......................................43
      9.2. Informative References ....................................45
   Acknowledgements ..................................................47
   Contributors ......................................................47
   Authors' Addresses ................................................48

1.  Introduction

   The contents of a Link-State Database (LSDB) or of an IGP's Traffic
   Engineering Database (TED) describe only the links and nodes within
   an IGP area.  Some applications, such as end-to-end Traffic
   Engineering (TE), would benefit from visibility outside one area or
   Autonomous System (AS) in order to make better decisions.

   The IETF has defined the Path Computation Element (PCE) [RFC4655] as
   a mechanism for achieving the computation of end-to-end TE paths that
   cross the visibility of more than one TED or that require CPU-
   intensive or coordinated computations.  The IETF has also defined the
   ALTO server [RFC5693] as an entity that generates an abstracted
   network topology and provides it to network-aware applications.

   Both a PCE and an ALTO server need to gather information about the
   topologies and capabilities of the network in order to be able to
   fulfill their function.

   This document describes a mechanism by which link-state and TE
   information can be collected from networks and shared with external
   components using the BGP routing protocol [RFC4271].  This is
   achieved using a new BGP Network Layer Reachability Information
   (NLRI) encoding format.  The mechanism is applicable to physical and
   virtual links.  The mechanism described is subject to policy control.

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   A router maintains one or more databases for storing link-state
   information about nodes and links in any given area.  Link attributes
   stored in these databases include: local/remote IP addresses, local/
   remote interface identifiers, link metric and TE metric, link
   bandwidth, reservable bandwidth, per Class-of-Service (CoS) class
   reservation state, preemption, and Shared Risk Link Groups (SRLGs).
   The router's BGP process can retrieve topology from these LSDBs and
   distribute it to a consumer, either directly or via a peer BGP
   speaker (typically a dedicated Route Reflector), using the encoding
   specified in this document.

   The collection of link-state and TE information and its distribution
   to consumers is shown in the following figure.

                           +-----------+
                           | Consumer  |
                           +-----------+
                                 ^
                                 |
                           +-----------+
                           |    BGP    |               +-----------+
                           |  Speaker  |               | Consumer  |
                           +-----------+               +-----------+
                             ^   ^   ^                       ^
                             |   |   |                       |
             +---------------+   |   +-------------------+   |
             |                   |                       |   |
       +-----------+       +-----------+             +-----------+
       |    BGP    |       |    BGP    |             |    BGP    |
       |  Speaker  |       |  Speaker  |    . . .    |  Speaker  |
       +-----------+       +-----------+             +-----------+
             ^                   ^                         ^
             |                   |                         |
            IGP                 IGP                       IGP

           Figure 1: Collection of Link-State and TE Information

   A BGP speaker may apply configurable policy to the information that
   it distributes.  Thus, it may distribute the real physical topology
   from the LSDB or the TED.  Alternatively, it may create an abstracted
   topology, where virtual, aggregated nodes are connected by virtual
   paths.  Aggregated nodes can be created, for example, out of multiple
   routers in a Point of Presence (POP).  Abstracted topology can also
   be a mix of physical and virtual nodes and physical and virtual
   links.  Furthermore, the BGP speaker can apply policy to determine
   when information is updated to the consumer so that there is a

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   reduction of information flow from the network to the consumers.
   Mechanisms through which topologies can be aggregated or virtualized
   are outside the scope of this document

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].

2.  Motivation and Applicability

   This section describes use cases from which the requirements can be
   derived.

2.1.  MPLS-TE with PCE

   As described in [RFC4655], a PCE can be used to compute MPLS-TE paths
   within a "domain" (such as an IGP area) or across multiple domains
   (such as a multi-area AS or multiple ASes).

   o  Within a single area, the PCE offers enhanced computational power
      that may not be available on individual routers, sophisticated
      policy control and algorithms, and coordination of computation
      across the whole area.

   o  If a router wants to compute a MPLS-TE path across IGP areas, then
      its own TED lacks visibility of the complete topology.  That means
      that the router cannot determine the end-to-end path and cannot
      even select the right exit router (Area Border Router (ABR)) for
      an optimal path.  This is an issue for large-scale networks that
      need to segment their core networks into distinct areas but still
      want to take advantage of MPLS-TE.

   Previous solutions used per-domain path computation [RFC5152].  The
   source router could only compute the path for the first area because
   the router only has full topological visibility for the first area
   along the path, but not for subsequent areas.  Per-domain path
   computation uses a technique called "loose-hop-expansion" [RFC3209]
   and selects the exit ABR and other ABRs or AS Border Routers (ASBRs)
   using the IGP-computed shortest path topology for the remainder of
   the path.  This may lead to sub-optimal paths, makes alternate/back-
   up path computation hard, and might result in no TE path being found
   when one really does exist.

   The PCE presents a computation server that may have visibility into
   more than one IGP area or AS, or may cooperate with other PCEs to
   perform distributed path computation.  The PCE obviously needs access

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   to the TED for the area(s) it serves, but [RFC4655] does not describe
   how this is achieved.  Many implementations make the PCE a passive
   participant in the IGP so that it can learn the latest state of the
   network, but this may be sub-optimal when the network is subject to a
   high degree of churn or when the PCE is responsible for multiple
   areas.

   The following figure shows how a PCE can get its TED information
   using the mechanism described in this document.

                +----------+                           +---------+
                |  -----   |                           |   BGP   |
                | | TED |<-+-------------------------->| Speaker |
                |  -----   |   TED synchronization     |         |
                |    |     |        mechanism:         +---------+
                |    |     | BGP with Link-State NLRI
                |    v     |
                |  -----   |
                | | PCE |  |
                |  -----   |
                +----------+
                     ^
                     | Request/
                     | Response
                     v
       Service  +----------+   Signaling  +----------+
       Request  | Head-End |   Protocol   | Adjacent |
       -------->|  Node    |<------------>|   Node   |
                +----------+              +----------+

     Figure 2: External PCE Node Using a TED Synchronization Mechanism

   The mechanism in this document allows the necessary TED information
   to be collected from the IGP within the network, filtered according
   to configurable policy, and distributed to the PCE as necessary.

2.2.  ALTO Server Network API

   An ALTO server [RFC5693] is an entity that generates an abstracted
   network topology and provides it to network-aware applications over a
   web-service-based API.  Example applications are peer-to-peer (P2P)
   clients or trackers, or Content Distribution Networks (CDNs).  The
   abstracted network topology comes in the form of two maps: a Network
   Map that specifies allocation of prefixes to Partition Identifiers
   (PIDs), and a Cost Map that specifies the cost between PIDs listed in
   the Network Map.  For more details, see [RFC7285].

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   ALTO abstract network topologies can be auto-generated from the
   physical topology of the underlying network.  The generation would
   typically be based on policies and rules set by the operator.  Both
   prefix and TE data are required: prefix data is required to generate
   ALTO Network Maps, and TE (topology) data is required to generate
   ALTO Cost Maps.  Prefix data is carried and originated in BGP, and TE
   data is originated and carried in an IGP.  The mechanism defined in
   this document provides a single interface through which an ALTO
   server can retrieve all the necessary prefix and network topology
   data from the underlying network.  Note that an ALTO server can use
   other mechanisms to get network data, for example, peering with
   multiple IGP and BGP speakers.

   The following figure shows how an ALTO server can get network
   topology information from the underlying network using the mechanism
   described in this document.

     +--------+
     | Client |<--+
     +--------+   |
                  |    ALTO    +--------+     BGP with    +---------+
     +--------+   |  Protocol  |  ALTO  | Link-State NLRI |   BGP   |
     | Client |<--+------------| Server |<----------------| Speaker |
     +--------+   |            |        |                 |         |
                  |            +--------+                 +---------+
     +--------+   |
     | Client |<--+
     +--------+

         Figure 3: ALTO Server Using Network Topology Information

3.  Carrying Link-State Information in BGP

   This specification 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.

   It is desirable to keep the dependencies on the protocol source of
   this attribute to a minimum and represent any content in an IGP-
   neutral way, such that applications that want to learn about a link-
   state topology do not need to know about any OSPF or IS-IS protocol
   specifics.

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3.1.  TLV Format

   Information in the new Link-State NLRIs and attributes is encoded in
   Type/Length/Value triplets.  The TLV format is shown in Figure 4.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                        Value (variable)                     //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 4: TLV Format

   The Length field defines the length of the value portion in octets
   (thus, a TLV with no value portion would have a length of zero).  The
   TLV is not padded to 4-octet alignment.  Unrecognized types MUST be
   preserved and propagated.  In order to compare NLRIs with unknown
   TLVs, all TLVs MUST be ordered in ascending order by TLV Type.  If
   there are more TLVs of the same type, then the TLVs MUST be ordered
   in ascending order of the TLV value within the TLVs with the same
   type by treating the entire Value field as an opaque hexadecimal
   string and comparing leftmost octets first, regardless of the length
   of the string.  All TLVs that are not specified as mandatory are
   considered optional.

3.2.  The Link-State NLRI

   The MP_REACH_NLRI and MP_UNREACH_NLRI attributes are BGP's containers
   for carrying opaque information.  Each Link-State NLRI describes
   either a node, a link, or a prefix.

   All non-VPN link, node, and prefix information SHALL be encoded using
   AFI 16388 / SAFI 71.  VPN link, node, and prefix information SHALL be
   encoded using AFI 16388 / SAFI 72.

   In order for two BGP speakers to exchange Link-State NLRI, they MUST
   use BGP Capabilities Advertisement to ensure that they are both
   capable of properly processing such NLRI.  This is done as specified
   in [RFC4760], by using capability code 1 (multi-protocol BGP), with
   AFI 16388 / SAFI 71 for BGP-LS, and AFI 16388 / SAFI 72 for
   BGP-LS-VPN.

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   The format of the Link-State NLRI is shown in the following figures.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            NLRI Type          |     Total NLRI Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                  Link-State NLRI (variable)                 //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 5: Link-State AFI 16388 / SAFI 71 NLRI Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            NLRI Type          |     Total NLRI Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                       Route Distinguisher                     +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                  Link-State NLRI (variable)                 //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 6: Link-State VPN AFI 16388 / SAFI 72 NLRI Format

   The Total NLRI Length field contains the cumulative length, in
   octets, of the rest of the NLRI, not including the NLRI Type field or
   itself.  For VPN applications, it also includes the length of the
   Route Distinguisher.

                   +------+---------------------------+
                   | Type | NLRI Type                 |
                   +------+---------------------------+
                   |  1   | Node NLRI                 |
                   |  2   | Link NLRI                 |
                   |  3   | IPv4 Topology Prefix NLRI |
                   |  4   | IPv6 Topology Prefix NLRI |
                   +------+---------------------------+

                            Table 1: NLRI Types

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   Route Distinguishers are defined and discussed in [RFC4364].

   The Node NLRI (NLRI Type = 1) is shown in the following figure.

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Local Node Descriptors (variable)            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 7: The Node NLRI Format

   The Link NLRI (NLRI Type = 2) is shown in the following figure.

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Local Node Descriptors (variable)             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Remote Node Descriptors (variable)            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                  Link Descriptors (variable)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 8: The Link NLRI Format

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   The IPv4 and IPv6 Prefix NLRIs (NLRI Type = 3 and Type = 4) use the
   same format, as shown in the following figure.

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //              Local Node Descriptors (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Prefix Descriptors (variable)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 9: The IPv4/IPv6 Topology Prefix NLRI Format

   The Protocol-ID field can contain one of the following values:

            +-------------+----------------------------------+
            | Protocol-ID | NLRI information source protocol |
            +-------------+----------------------------------+
            |      1      | IS-IS Level 1                    |
            |      2      | IS-IS Level 2                    |
            |      3      | OSPFv2                           |
            |      4      | Direct                           |
            |      5      | Static configuration             |
            |      6      | OSPFv3                           |
            +-------------+----------------------------------+

                       Table 2: Protocol Identifiers

   The 'Direct' and 'Static configuration' protocol types SHOULD be used
   when BGP-LS is sourcing local information.  For all information
   derived from other protocols, the corresponding Protocol-ID MUST be
   used.  If BGP-LS has direct access to interface information and wants
   to advertise a local link, then the Protocol-ID 'Direct' SHOULD be
   used.  For modeling virtual links, such as described in Section 4,
   the Protocol-ID 'Static configuration' SHOULD be used.

   Both OSPF and IS-IS MAY run multiple routing protocol instances over
   the same link.  See [RFC6822] and [RFC6549].  These instances define
   independent "routing universes".  The 64-bit Identifier field is used
   to identify the routing universe where the NLRI belongs.  The NLRIs
   representing link-state objects (nodes, links, or prefixes) from the
   same routing universe MUST have the same 'Identifier' value.  NLRIs

Top      ToC       Page 12 
   with different 'Identifier' values MUST be considered to be from
   different routing universes.  Table 3 lists the 'Identifier' values
   that are defined as well-known in this document.

             +------------+----------------------------------+
             | Identifier | Routing Universe                 |
             +------------+----------------------------------+
             |     0      | Default Layer 3 Routing topology |
             +------------+----------------------------------+

                 Table 3: Well-Known Instance Identifiers

   If a given protocol does not support multiple routing universes, then
   it SHOULD set the Identifier field according to Table 3.  However, an
   implementation MAY make the 'Identifier' configurable for a given
   protocol.

   Each Node Descriptor and Link Descriptor consists of one or more
   TLVs, as described in the following sections.

3.2.1.  Node Descriptors

   Each link is anchored by a pair of Router-IDs that are used by the
   underlying IGP, namely, a 48-bit ISO System-ID for IS-IS and a 32-bit
   Router-ID for OSPFv2 and OSPFv3.  An IGP may use one or more
   additional auxiliary Router-IDs, mainly for Traffic Engineering
   purposes.  For example, IS-IS may have one or more IPv4 and IPv6 TE
   Router-IDs [RFC5305] [RFC6119].  These auxiliary Router-IDs MUST be
   included in the link attribute described in Section 3.3.2.

   It is desirable that the Router-ID assignments inside the Node
   Descriptor are globally unique.  However, there may be Router-ID
   spaces (e.g., ISO) where no global registry exists, or worse, Router-
   IDs have been allocated following the private-IP allocation described
   in RFC 1918 [RFC1918].  BGP-LS uses the Autonomous System (AS) Number
   and BGP-LS Identifier (see Section 3.2.1.4) to disambiguate the
   Router-IDs, as described in Section 3.2.1.1.

3.2.1.1.  Globally Unique Node/Link/Prefix Identifiers

   One problem that needs to be addressed is the ability to identify an
   IGP node globally (by "globally", we mean within the BGP-LS database
   collected by all BGP-LS speakers that talk to each other).  This can
   be expressed through the following two requirements:

   (A)  The same node MUST NOT be represented by two keys (otherwise,
        one node will look like two nodes).

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   (B)  Two different nodes MUST NOT be represented by the same key
        (otherwise, two nodes will look like one node).

   We define an "IGP domain" to be the set of nodes (hence, by extension
   links and prefixes) within which each node has a unique IGP
   representation by using the combination of Area-ID, Router-ID,
   Protocol-ID, Multi-Topology ID, and Instance-ID.  The problem is that
   BGP may receive node/link/prefix information from multiple
   independent "IGP domains", and we need to distinguish between them.
   Moreover, we can't assume there is always one and only one IGP domain
   per AS.  During IGP transitions, it may happen that two redundant
   IGPs are in place.

   In Section 3.2.1.4, a set of sub-TLVs is described, which allows
   specification of a flexible key for any given node/link information
   such that global uniqueness of the NLRI is ensured.

3.2.1.2.  Local Node Descriptors

   The Local Node Descriptors TLV contains Node Descriptors for the node
   anchoring the local end of the link.  This is a mandatory TLV in all
   three types of NLRIs (node, link, and prefix).  The length of this
   TLV is variable.  The value contains one or more Node Descriptor
   Sub-TLVs defined in Section 3.2.1.4.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //              Node Descriptor Sub-TLVs (variable)            //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 10: Local Node Descriptors TLV Format

3.2.1.3.  Remote Node Descriptors

   The Remote Node Descriptors TLV contains Node Descriptors for the
   node anchoring the remote end of the link.  This is a mandatory TLV
   for Link NLRIs.  The length of this TLV is variable.  The value
   contains one or more Node Descriptor Sub-TLVs defined in
   Section 3.2.1.4.

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      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //              Node Descriptor Sub-TLVs (variable)            //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 11: Remote Node Descriptors TLV Format

3.2.1.4.  Node Descriptor Sub-TLVs

   The Node Descriptor Sub-TLV type code points and lengths are listed
   in the following table:

           +--------------------+-------------------+----------+
           | Sub-TLV Code Point | Description       |   Length |
           +--------------------+-------------------+----------+
           |        512         | Autonomous System |        4 |
           |        513         | BGP-LS Identifier |        4 |
           |        514         | OSPF Area-ID      |        4 |
           |        515         | IGP Router-ID     | Variable |
           +--------------------+-------------------+----------+

                     Table 4: Node Descriptor Sub-TLVs

   The sub-TLV values in Node Descriptor TLVs are defined as follows:

   Autonomous System:  Opaque value (32-bit AS Number)

   BGP-LS Identifier:  Opaque value (32-bit ID).  In conjunction with
      Autonomous System Number (ASN), uniquely identifies the BGP-LS
      domain.  The combination of ASN and BGP-LS ID MUST be globally
      unique.  All BGP-LS speakers within an IGP flooding-set (set of
      IGP nodes within which an LSP/LSA is flooded) MUST use the same
      ASN, BGP-LS ID tuple.  If an IGP domain consists of multiple
      flooding-sets, then all BGP-LS speakers within the IGP domain
      SHOULD use the same ASN, BGP-LS ID tuple.

   Area-ID:  Used to identify the 32-bit area to which the NLRI belongs.
      The Area Identifier allows different NLRIs of the same router to
      be discriminated.

   IGP Router-ID:  Opaque value.  This is a mandatory TLV.  For an IS-IS
      non-pseudonode, this contains a 6-octet ISO Node-ID (ISO system-
      ID).  For an IS-IS pseudonode corresponding to a LAN, this

Top      ToC       Page 15 
      contains the 6-octet ISO Node-ID of the Designated Intermediate
      System (DIS) followed by a 1-octet, nonzero PSN identifier (7
      octets in total).  For an OSPFv2 or OSPFv3 non-pseudonode, this
      contains the 4-octet Router-ID.  For an OSPFv2 pseudonode
      representing a LAN, this contains the 4-octet Router-ID of the
      Designated Router (DR) followed by the 4-octet IPv4 address of the
      DR's interface to the LAN (8 octets in total).  Similarly, for an
      OSPFv3 pseudonode, this contains the 4-octet Router-ID of the DR
      followed by the 4-octet interface identifier of the DR's interface
      to the LAN (8 octets in total).  The TLV size in combination with
      the protocol identifier enables the decoder to determine the type
      of the node.

      There can be at most one instance of each sub-TLV type present in
      any Node Descriptor.  The sub-TLVs within a Node Descriptor MUST
      be arranged in ascending order by sub-TLV type.  This needs to be
      done in order to compare NLRIs, even when an implementation
      encounters an unknown sub-TLV.  Using stable sorting, an
      implementation can do binary comparison of NLRIs and hence allow
      incremental deployment of new key sub-TLVs.

3.2.1.5.  Multi-Topology ID

   The Multi-Topology ID (MT-ID) TLV carries one or more IS-IS or OSPF
   Multi-Topology IDs for a link, node, or prefix.

   Semantics of the IS-IS MT-ID are defined in Section 7.2 of RFC 5120
   [RFC5120].  Semantics of the OSPF MT-ID are defined in Section 3.7 of
   RFC 4915 [RFC4915].  If the value in the MT-ID TLV is derived from
   OSPF, then the upper 9 bits MUST be set to 0.  Bits R are reserved
   and SHOULD be set to 0 when originated and ignored on receipt.

   The format of the MT-ID TLV is shown in the following figure.

      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=2*n           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R R R R|  Multi-Topology ID 1  |             ....             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //             ....             |R R R R|  Multi-Topology ID n  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 12: Multi-Topology ID TLV Format

   where Type is 263, Length is 2*n, and n is the number of MT-IDs
   carried in the TLV.

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   The MT-ID TLV MAY be present in a Link Descriptor, a Prefix
   Descriptor, or the BGP-LS attribute of a Node NLRI.  In a Link or
   Prefix Descriptor, only a single MT-ID TLV containing the MT-ID of
   the topology where the link or the prefix is reachable is allowed.
   In case one wants to advertise multiple topologies for a given Link
   Descriptor or Prefix Descriptor, multiple NLRIs need to be generated
   where each NLRI contains an unique MT-ID.  In the BGP-LS attribute of
   a Node NLRI, one MT-ID TLV containing the array of MT-IDs of all
   topologies where the node is reachable is allowed.

3.2.2.  Link Descriptors

   The Link Descriptor field is a set of Type/Length/Value (TLV)
   triplets.  The format of each TLV is shown in Section 3.1.  The Link
   Descriptor TLVs uniquely identify a link among multiple parallel
   links between a pair of anchor routers.  A link described by the Link
   Descriptor TLVs actually is a "half-link", a unidirectional
   representation of a logical link.  In order to fully describe a
   single logical link, two originating routers advertise a half-link
   each, i.e., two Link NLRIs are advertised for a given point-to-point
   link.

   The format and semantics of the Value fields in most Link Descriptor
   TLVs correspond to the format and semantics of Value fields in IS-IS
   Extended IS Reachability sub-TLVs, defined in [RFC5305], [RFC5307],
   and [RFC6119].  Although the encodings for Link Descriptor TLVs were
   originally defined for IS-IS, the TLVs can carry data sourced by
   either IS-IS or OSPF.

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   The following TLVs are valid as Link Descriptors in the Link NLRI:

   +-----------+---------------------+--------------+------------------+
   |  TLV Code | Description         |  IS-IS TLV   | Reference        |
   |   Point   |                     |   /Sub-TLV   | (RFC/Section)    |
   +-----------+---------------------+--------------+------------------+
   |    258    | Link Local/Remote   |     22/4     | [RFC5307]/1.1    |
   |           | Identifiers         |              |                  |
   |    259    | IPv4 interface      |     22/6     | [RFC5305]/3.2    |
   |           | address             |              |                  |
   |    260    | IPv4 neighbor       |     22/8     | [RFC5305]/3.3    |
   |           | address             |              |                  |
   |    261    | IPv6 interface      |    22/12     | [RFC6119]/4.2    |
   |           | address             |              |                  |
   |    262    | IPv6 neighbor       |    22/13     | [RFC6119]/4.3    |
   |           | address             |              |                  |
   |    263    | Multi-Topology      |     ---      | Section 3.2.1.5  |
   |           | Identifier          |              |                  |
   +-----------+---------------------+--------------+------------------+

                       Table 5: Link Descriptor TLVs

   The information about a link present in the LSA/LSP originated by the
   local node of the link determines the set of TLVs in the Link
   Descriptor of the link.

      If interface and neighbor addresses, either IPv4 or IPv6, are
      present, then the IP address TLVs are included in the Link
      Descriptor but not the link local/remote Identifier TLV.  The link
      local/remote identifiers MAY be included in the link attribute.

      If interface and neighbor addresses are not present and the link
      local/remote identifiers are present, then the link local/remote
      Identifier TLV is included in the Link Descriptor.

      The Multi-Topology Identifier TLV is included in Link Descriptor
      if that information is present.

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3.2.3.  Prefix Descriptors

   The Prefix Descriptor field is a set of Type/Length/Value (TLV)
   triplets.  Prefix Descriptor TLVs uniquely identify an IPv4 or IPv6
   prefix originated by a node.  The following TLVs are valid as Prefix
   Descriptors in the IPv4/IPv6 Prefix NLRI:

   +-------------+---------------------+----------+--------------------+
   |   TLV Code  | Description         |  Length  | Reference          |
   |    Point    |                     |          | (RFC/Section)      |
   +-------------+---------------------+----------+--------------------+
   |     263     | Multi-Topology      | variable | Section 3.2.1.5    |
   |             | Identifier          |          |                    |
   |     264     | OSPF Route Type     |    1     | Section 3.2.3.1    |
   |     265     | IP Reachability     | variable | Section 3.2.3.2    |
   |             | Information         |          |                    |
   +-------------+---------------------+----------+--------------------+

                      Table 6: Prefix Descriptor TLVs

3.2.3.1.  OSPF Route Type

   The OSPF Route Type TLV is an optional TLV that MAY be present in
   Prefix NLRIs.  It is used to identify the OSPF route type of the
   prefix.  It is used when an OSPF prefix is advertised in the OSPF
   domain with multiple route types.  The Route Type TLV allows the
   discrimination of these advertisements.  The format of the OSPF Route
   Type TLV is shown in the following figure.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Route Type   |
     +-+-+-+-+-+-+-+-+

                   Figure 13: OSPF Route Type TLV Format

   where the Type and Length fields of the TLV are defined in Table 6.
   The OSPF Route Type field values are defined in the OSPF protocol and
   can be one of the following:

   o  Intra-Area (0x1)

   o  Inter-Area (0x2)

   o  External 1 (0x3)

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   o  External 2 (0x4)

   o  NSSA 1 (0x5)

   o  NSSA 2 (0x6)

3.2.3.2.  IP Reachability Information

   The IP Reachability Information TLV is a mandatory TLV that contains
   one IP address prefix (IPv4 or IPv6) originally advertised in the IGP
   topology.  Its purpose is to glue a particular BGP service NLRI by
   virtue of its BGP next hop to a given node in the LSDB.  A router
   SHOULD advertise an IP Prefix NLRI for each of its BGP next hops.
   The format of the IP Reachability Information TLV is shown in the
   following figure:

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Prefix Length | IP Prefix (variable)                         //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 14: IP Reachability Information TLV Format

   The Type and Length fields of the TLV are defined in Table 6.  The
   following two fields determine the reachability information of the
   address family.  The Prefix Length field contains the length of the
   prefix in bits.  The IP Prefix field contains the most significant
   octets of the prefix, i.e., 1 octet for prefix length 1 up to 8, 2
   octets for prefix length 9 to 16, 3 octets for prefix length 17 up to
   24, 4 octets for prefix length 25 up to 32, etc.



(page 19 continued on part 2)

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