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Versions: (draft-psenak-ospf-segment-routing-extensions) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21

Open Shortest Path First IGP                              P. Psenak, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             C. Filsfils
Expires: May 1, 2017                                 Cisco Systems, Inc.
                                                              H. Gredler
                                                            RtBrick Inc.
                                                               R. Shakir
                                                            Google, Inc.
                                                           W. Henderickx
                                                                   Nokia
                                                             J. Tantsura
                                                              Individual
                                                        October 28, 2016


                  OSPF Extensions for Segment Routing
             draft-ietf-ospf-segment-routing-extensions-10

Abstract

   Segment Routing (SR) allows a flexible definition of end-to-end paths
   within IGP topologies by encoding paths as sequences of topological
   sub-paths, called "segments".  These segments are advertised by the
   link-state routing protocols (IS-IS and OSPF).

   This draft describes the OSPF extensions required for Segment
   Routing.

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].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on May 1, 2017.

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
   2.  Segment Routing Identifiers . . . . . . . . . . . . . . . . .   3
     2.1.  SID/Label Sub-TLV . . . . . . . . . . . . . . . . . . . .   4
   3.  Segment Routing Capabilities  . . . . . . . . . . . . . . . .   4
     3.1.  SR-Algorithm TLV  . . . . . . . . . . . . . . . . . . . .   4
     3.2.  SID/Label Range TLV . . . . . . . . . . . . . . . . . . .   6
     3.3.  SR Local Block Sub-TLV  . . . . . . . . . . . . . . . . .   8
     3.4.  SRMS Preference Sub-TLV . . . . . . . . . . . . . . . . .   9
   4.  OSPF Extended Prefix Range TLV  . . . . . . . . . . . . . . .  10
   5.  Prefix SID Sub-TLV  . . . . . . . . . . . . . . . . . . . . .  12
   6.  SID/Label Binding Sub-TLV . . . . . . . . . . . . . . . . . .  16
     6.1.  ERO Metric Sub-TLV  . . . . . . . . . . . . . . . . . . .  18
     6.2.  ERO Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . .  18
       6.2.1.  IPv4 ERO Sub-TLV  . . . . . . . . . . . . . . . . . .  19
       6.2.2.  Unnumbered Interface ID ERO Sub-TLV . . . . . . . . .  19
       6.2.3.  IPv4 Backup ERO Sub-TLV . . . . . . . . . . . . . . .  21
       6.2.4.  Unnumbered Interface ID Backup ERO Sub-TLV  . . . . .  21
   7.  Adjacency Segment Identifier (Adj-SID)  . . . . . . . . . . .  23
     7.1.  Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . . . .  23
     7.2.  LAN Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . .  24
   8.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .  26
     8.1.  Intra-area Segment routing in OSPFv2  . . . . . . . . . .  26
     8.2.  Inter-area Segment routing in OSPFv2  . . . . . . . . . .  27
     8.3.  SID for External Prefixes . . . . . . . . . . . . . . . .  28
     8.4.  Advertisement of Adj-SID  . . . . . . . . . . . . . . . .  28
       8.4.1.  Advertisement of Adj-SID on Point-to-Point Links  . .  28
       8.4.2.  Adjacency SID on Broadcast or NBMA Interfaces . . . .  28
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     9.1.  OSPF OSPF Router Information (RI) TLVs Registry . . . . .  29



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     9.2.  OSPF Extended Prefix LSA TLV Registry . . . . . . . . . .  29
     9.3.  OSPF Extended Prefix LSA Sub-TLV Registry . . . . . . . .  29
     9.4.  OSPF Extended Link LSA Sub-TLV Registry . . . . . . . . .  30
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  30
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  31
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  31
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     14.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   Segment Routing (SR) allows a flexible definition of end-to-end paths
   within IGP topologies by encoding paths as sequences of topological
   sub-paths, called "segments".  These segments are advertised by the
   link-state routing protocols (IS-IS and OSPF).  Prefix segments
   represent an ECMP-aware shortest-path to a prefix (or a node), as per
   the state of the IGP topology.  Adjacency segments represent a hop
   over a specific adjacency between two nodes in the IGP.  A prefix
   segment is typically a multi-hop path while an adjacency segment, in
   most cases, is a one-hop path.  SR's control-plane can be applied to
   both IPv6 and MPLS data-planes, and does not require any additional
   signalling (other than IGP extensions).  The IPv6 data plane is out
   of the scope of this specification - it is not applicable to OSPFv2
   which only supports the IPv4 address-family.  For example, when used
   in MPLS networks, SR paths do not require any LDP or RSVP-TE
   signalling.  However, SR can interoperate in the presence of LSPs
   established with RSVP or LDP.

   This draft describes the OSPF extensions required for Segment
   Routing.

   Segment Routing architecture is described in
   [I-D.ietf-spring-segment-routing].

   Segment Routing use cases are described in
   [I-D.filsfils-spring-segment-routing-use-cases].

2.  Segment Routing Identifiers

   Segment Routing defines various types of Segment Identifiers (SIDs):
   Prefix-SID, Adjacency-SID, LAN Adjacency SID and Binding SID.

   For the purpose of the advertisements of various SID values, new
   Opaque LSAs [RFC5250] are defined in [RFC7684].  These LSAs are
   generic containers that can be used to advertise any additional



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   attributes associated with a prefix or link.  These Opaque LSAs are
   complementary to the existing LSAs and are not aimed to replace any
   of the existing LSAs.

2.1.  SID/Label Sub-TLV

   The SID/Label Sub-TLV appears in multiple TLVs or Sub-TLVs defined
   later in this document.  It is used to advertise the SID or label
   associated with a prefix or adjacency.  The SID/Label TLV has
   following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SID/Label (variable)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 1

      Length: variable, 3 or 4 octet

      SID/Label: If length is set to 3, then the 20 rightmost bits
      represent a label.  If length is set to 4, then the value
      represents a 32-bit SID.

      The receiving router MUST ignore SID/Label Sub-TLV if the length
      is other then 3 or 4.

3.  Segment Routing Capabilities

   Segment Routing requires some additional router capabilities to be
   advertised to other routers in the area.

   These SR capabilities are advertised in the Router Information Opaque
   LSA (defined in [RFC7770]).

3.1.  SR-Algorithm TLV

   The SR-Algorithm TLV is a top-level TLV of the Router Information
   Opaque LSA (defined in [RFC7770]).

   The SR-Algorithm Sub-TLV is optional.  It MAY only be advertised once
   in the Router Information Opaque LSA.  If the SID/Label Range TLV, as
   defined in Section 3.2, is advertised, then the SR-Algorithm TLV MUST



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   also be advertised.  If the SR-Algorithm TLV is not advertised by the
   node, such node is considered as not being segment routing capable.

   An SR Router may use various algorithms when calculating reachability
   to OSPF routers or prefixes in an OSPF area.  Examples of these
   algorithms are metric based Shortest Path First (SPF), various
   flavors of Constrained SPF, etc.  The SR-Algorithm TLV allows a
   router to advertise the algorithms currently used by the router to
   other routers in an OSPF area.  The SR-Algorithm TLV has following
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Algorithm 1 | Algorithm...  |   Algorithm n |               |
   +-                                                             -+
   |                                                               |
   +                                                               +

   where:

      Type: TBD, suggested value 8

      Length: variable

      Algorithm: Single octet identifying the algorithm.  The following
      values are defined by this document:

         0: Shortest Path First (SPF) algorithm based on link metric.
         This is the standard shortest path algorithm as computed by the
         OSPF protocol.  Consistent with the deployed practice for link-
         state protocols, Algorithm 0 permits any node to overwrite the
         SPF path with a different path based on its local policy.  If
         the SR-Algorithm Sub-TLV is advertised, Algorithm 0 MUST be
         included.

         1: Strict Shortest Path First (SPF) algorithm based on link
         metric.  The algorithm is identical to Algorithm 0 but
         Algorithm 1 requires that all nodes along the path will honor
         the SPF routing decision.  Local policy at the node claiming
         support for Algorithm 1 MUST NOT alter the SPF paths computed
         by Algorithm 1.

   When multiple SR-Algorithm sub-TLVs are received from a given router
   the receiver SHOULD use the first occurrence of the sub-TLV in the
   Router Information LSA.  If the SR-Algorithm sub-TLV appears in



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   multiple Router Information LSAs that have different flooding scopes,
   the SR-Algorithm sub-TLV in the Router Information LSA with the
   lowest flooding scope SHOULD be used.  If the SR-Algorithm sub-TLV
   appears in multiple Router Information LSAs that have the same
   flooding scope, the SR-Algorithm sub-TLV in the Router Information
   LSA with the numerically smallest Instance ID SHOULD be used and
   subsequent instances of the SR-Algorithm sub-TLV SHOULD be ignored.

   The RI LSA can be advertised at any of the defined opaque flooding
   scopes (link, area, or Autonomous System (AS)).  For the purpose of
   SR-Algorithm TLV advertisement, area scope flooding is required.

3.2.  SID/Label Range TLV

   The SID/Label Range TLV is a top-level TLV of the Router Information
   Opaque LSA (defined in [RFC7770]).

   The SID/Label Range TLV MAY appear multiple times and has the
   following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Range Size                 |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sub-TLVs (variable)                    |
   +-                                                             -+
   |                                                               |
   +                                                               +

   where:

      Type: TBD, suggested value 9

      Length: variable

      Range Size: 3 octets of the SID/label range

   Initially, the only supported Sub-TLV is the SID/Label TLV as defined
   in Section 2.1.  The SID/Label advertised in the SID/Label TLV
   represents the first SID/Label in the advertised range.

   Multiple occurrences of the SID/Label Range TLV MAY be advertised, in
   order to advertise multiple ranges.  In such case:





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   o  The originating router MUST encode each range into a different
      SID/Label Range TLV.

   o  The originating router decides the order in which the set of SID/
      Label Range TLVs are advertised inside the Router Information
      Opaque LSA.  The originating router MUST ensure the order is the
      same after a graceful restart (using checkpointing, non-volatile
      storage or any other mechanism) in order to assure the SID/label
      range and SID index correspondence is preserved across graceful
      restarts.

   o  The receiving router must adhere to the order in which the ranges
      are advertised when calculating a SID/label from a SID index.

   The following example illustrates the advertisement of multiple
   ranges:

      The originating router advertises following ranges:
         Range 1: [100, 199]
         Range 2: [1000, 1099]
         Range 3: [500, 599]

      The receiving routers concatenate the ranges and build the Segment
      Routing Global Block (SRGB) as follows:

      SRGB = [100, 199]
             [1000, 1099]
             [500, 599]

      The indexes span multiple ranges:

         index=0 means label 100
         ...
         index 99 means label 199
         index 100 means label 1000
         index 199 means label 1099
         ...
         index 200 means label 500
         ...

   The RI LSA can be advertised at any of the defined flooding scopes
   (link, area, or autonomous system (AS)).  For the purpose of SID/
   Label Range TLV advertisement, area scope flooding is required.








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3.3.  SR Local Block Sub-TLV

   The SR Local Block (SRLB) Sub-TLV contains the range of labels the
   node has reserved for local SIDs.  Local SIDs are used, e.g., for
   Adjacency-SIDs, and may also be allocated by other components than
   OSPF protocol.  As an example, an application or a controller may
   instruct the router to allocate a specific local SID.  Therefore, in
   order for such applications or controllers to know what are the local
   SIDs available in the router, it is required that the router
   advertises its SRLB.  The SRLB Sub-TLV is used for that purpose.

   The SR Local Block (SRLB) Sub-TLV is a top-level TLV of the Router
   Information Opaque LSA (defined in [RFC7770]).

   The SR Local Block Sub-TLV MAY only be advertised once in the Router
   Information Opaque LSA and has the following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Range Size                 |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sub-TLVs (variable)                    |
   +-                                                             -+
   |                                                               |
   +                                                               +

   where:

      Type: TBD, suggested value 12

      Length: variable

      Range Size: 3 octets of the SID/label range.  MUST be higher then
      0.

   Initially, the only supported Sub-TLV is the SID/Label TLV as defined
   in Section 2.1.  The SID/Label advertised in the SID/Label TLV
   represents the first SID/Label in the advertised range.

   When multiple SRLB sub-TLVs are received from a given router the
   receiver SHOULD use the first occurrence of the sub-TLV in the Router
   Information LSA.  If the SRLB sub-TLV appears in multiple Router
   Information LSAs that have different flooding scopes, the SRLB sub-
   TLV in the Router Information LSA with the lowest flooding scope
   SHOULD be used.  If the SRLB sub-TLV appears in multiple Router



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   Information LSAs that have the same flooding scope, the SRLB sub-TLV
   in the Router Information LSA with the numerically smallest Instance
   ID SHOULD be used and subsequent instances of the SRLB sub-TLV SHOULD
   be ignored.

   Each time a SID from the SRLB is allocated, it SHOULD also be
   reported to all components (e.g.: controller or applications) in
   order for these components to have an up-to-date view of the current
   SRLB allocation.  This is required to avoid collision between
   allocation instructions.

   Within the context of OSPF, the reporting of local SIDs is done
   through OSPF Sub-TLVs such as the Adjacency-SID (Section 7).
   However, the reporting of allocated local SIDs may also be done
   through other means and protocols which mechanisms are outside the
   scope of this document.

   A router advertising the SRLB TLV may also have other label ranges,
   outside of the SRLB, used for its local allocation purposes which are
   NOT advertised in the SRLB.  For example, it is possible that an
   Adjacency-SID is allocated using a local label that is not part of
   the SRLB.

   The RI LSA can be advertised at any of the defined flooding scopes
   (link, area, or autonomous system (AS)).  For the purpose of SR Local
   Block Sub-TLV TLV advertisement, area scope flooding is required.

3.4.  SRMS Preference Sub-TLV

   The Segment Routing Mapping Server (SRMS) Preference sub-TLV is used
   to advertise a preference associated with the node that acts as a SR
   Mapping Server.  SRMS preference is defined in
   [I-D.ietf-spring-conflict-resolution].

   The SRMS Preference Sub-TLV is a top-level TLV of the Router
   Information Opaque LSA (defined in [RFC7770]).

   The SRMS Preference Sub-TLV MAY only be advertised once in the Router
   Information Opaque LSA and has the following format:












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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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Preference    |                 Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 13

      Length: 4 octets

      Preference: 1 octet.  SRMS preference value from 0 to 255.

   When multiple SRMS Preference sub-TLVs are received from a given
   router the receiver SHOULD use the first occurrence of the sub-TLV in
   the Router Information LSA.  If the SRMS Preference sub-TLV appears
   in multiple Router Information LSAs that have different flooding
   scopes, the SRLB sub-TLV in the Router Information LSA with the
   lowest flooding scope SHOULD be used.  If the SRMS Preference sub-TLV
   appears in multiple Router Information LSAs that have the same
   flooding scope, the SRMS Preference sub-TLV in the Router Information
   LSA with the numerically smallest Instance ID SHOULD be used and
   subsequent instances of the SRMS Preference sub-TLV SHOULD be
   ignored.

   The RI LSA can be advertised at any of the defined flooding scopes
   (link, area, or autonomous system (AS)).  For the purpose of the SRMS
   Preference Sub-TLV advertisement, AS scope flooding is required.  If
   the SRMS advertisements from the SRMS server are only used inside the
   area to which the SRMS server is attached, area scope flooding may be
   used.

4.  OSPF Extended Prefix Range TLV

   In some cases it is useful to advertise attributes for a range of
   prefixes.  The Segment Routing Mapping Server, which is described in
   [I-D.filsfils-spring-segment-routing-ldp-interop], is an example
   where we need a single advertisement to advertise SIDs for multiple
   prefixes from a contiguous address range.

   The OSPF Extended Prefix Range TLV, which is a new top level TLV of
   the Extended Prefix LSA described in [RFC7684] is defined for this
   purpose.





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   Multiple OSPF Extended Prefix Range TLVs MAY be advertised in each
   OSPF Extended Prefix Opaque LSA, but all prefix ranges included in a
   single OSPF Extended Prefix Opaque LSA MUST have the same flooding
   scope.  The OSPF Extended Prefix Range TLV has the following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Prefix Length |     AF        |         Range Size            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |                Reserved                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Address Prefix (variable)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sub-TLVs (variable)                      |
   +-                                                             -+
   |                                                               |

   where:

      Type: TBD, suggested value 2.

      Length: Variable

      Prefix length: Length of the prefix

      AF: 0 - IPv4 unicast

      Range size: Represents the number of prefixes that are covered by
      the advertisement.  The Range Size MUST NOT exceed the number of
      prefixes that could be satisfied by the prefix length without
      including the IPv4 multicast address range (224.0.0.0/3).

      Flags: Single octet field.  The following flags are defined:


     0  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |IA|  |  |  |  |  |  |  |
   +--+--+--+--+--+--+--+--+

   where:

         IA-Flag: Inter-Area flag.  If set, advertisement is of inter-
         area type.  The ABR that is advertising the OSPF Extended
         Prefix Range TLV between areas MUST set this bit.



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         This bit is used to prevent redundant flooding of Prefix Range
         TLVs between areas as follows:

            An ABR always prefers intra-area Prefix Range advertisements
            over inter-area advertisements.

            An ABR does not consider inter-area Prefix Range
            advertisements coming from non-backbone areas.

            An ABR only propagates an inter-area Prefix Range
            advertisement from the backbone area to connected non-
            backbone areas if the advertisement is considered to be the
            best one.

      Address Prefix: The prefix, encoded as an even multiple of 32-bit
      words, padded with zero bits as necessary.  This encoding consumes
      ((PrefixLength + 31) / 32) 32-bit words.  The Address Prefix
      represents the first prefix in the prefix range.

5.  Prefix SID Sub-TLV

   The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV
   described in [RFC7684] and the OSPF Extended Prefix Range TLV
   described in Section 4.  It MAY appear more than once in the parent
   TLV and has the following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Flags    |   Reserved    |      MT-ID    |    Algorithm  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     SID/Index/Label (variable)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 2.

      Length: Variable

      Flags: Single octet field.  The following flags are defined:








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     0  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |  |NP|M |E |V |L |  |  |
   +--+--+--+--+--+--+--+--+

   where:

         NP-Flag: No-PHP flag.  If set, then the penultimate hop MUST
         NOT pop the Prefix-SID before delivering packets to the node
         that advertised the Prefix-SID.

         M-Flag: Mapping Server Flag.  If set, the SID was advertised by
         a Segment Routing Mapping Server as described in
         [I-D.filsfils-spring-segment-routing-ldp-interop].

         E-Flag: Explicit-Null Flag.  If set, any upstream neighbor of
         the Prefix-SID originator MUST replace the Prefix-SID with the
         Explicit-NULL label (0 for IPv4) before forwarding the packet.

         V-Flag: Value/Index Flag.  If set, then the Prefix-SID carries
         an absolute value.  If not set, then the Prefix-SID carries an
         index.

         L-Flag: Local/Global Flag.  If set, then the value/index
         carried by the Prefix-SID has local significance.  If not set,
         then the value/index carried by this Sub-TLV has global
         significance.

         Other bits: Reserved.  These MUST be zero when sent and are
         ignored when received.

      MT-ID: Multi-Topology ID (as defined in [RFC4915]).

      Algorithm: Single octet identifying the algorithm the Prefix-SID
      is associated with as defined in Section 3.1.

      A router receiving a Prefix-SID from a remote node and with an
      algorithm value that such remote node has not advertised in the
      SR-Algorithm sub-TLV (Section 3.1) MUST ignore the Prefix-SID sub-
      TLV.

      SID/Index/Label: According to the V and L flags, it contains
      either:

         A 32-bit index defining the offset in the SID/Label space
         advertised by this router.





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         A 24-bit label where the 20 rightmost bits are used for
         encoding the label value.

   If multiple Prefix-SIDs are advertised for the same prefix, the
   receiving router MUST use the first encoded SID and MAY use the
   subsequent SIDs.

   When propagating Prefix-SIDs between areas, if multiple prefix-SIDs
   are advertised for a prefix, an implementation SHOULD preserve the
   original order when advertising prefix-SIDs to other areas.  This
   allows implementations that only support a single Prefix-SID to have
   a consistent view across areas.

   When calculating the outgoing label for the prefix, the router MUST
   take into account the E and P flags advertised by the next-hop router
   if that router advertised the SID for the prefix.  This MUST be done
   regardless of whether the next-hop router contributes to the best
   path to the prefix.

   The NP-Flag (No-PHP) MUST be set for Prefix-SIDs allocated to inter-
   area prefixes that are originated by the ABR based on intra-area or
   inter-area reachability between areas.  When the inter-area prefix is
   generated based on a prefix which is directly attached to the ABR,
   the NP-Flag SHOULD NOT be set.

   The NP-Flag (No-PHP) MUST be be set for the Prefix-SIDs allocated to
   redistributed prefixes, unless the redistributed prefix is directly
   attached to the ASBR, in which case the NP-flag SHOULD NOT be set.

   If the NP-Flag is not set, then any upstream neighbor of the Prefix-
   SID originator MUST pop the Prefix-SID.  This is equivalent to the
   penultimate hop popping mechanism used in the MPLS dataplane.  In
   such case, MPLS EXP bits of the Prefix-SID are not preserved for the
   final destination (the Prefix-SID being removed).  If the NP-flag is
   not set then the received E-flag is ignored.

   If the NP-flag is set then:

      If the E-flag is not set, then any upstream neighbor of the
      Prefix-SID originator MUST keep the Prefix-SID on top of the
      stack.  This is useful when the originator of the Prefix-SID must
      stitch the incoming packet into a continuing MPLS LSP to the final
      destination.  This could occur at an Area Border Router (prefix
      propagation from one area to another) or at an AS Boundary Router
      (prefix propagation from one domain to another).

      If the E-flag is set, then any upstream neighbor of the Prefix-SID
      originator MUST replace the Prefix-SID with an Explicit-NULL



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      label.  This is useful, e.g., when the originator of the Prefix-
      SID is the final destination for the related prefix and the
      originator wishes to receive the packet with the original EXP
      bits.

   When the M-Flag is set, the NP-flag and the E-flag MUST be ignored at
   reception.

   As the Mapping Server does not specify the originator of a prefix
   advertisement, it is not possible to determine PHP behavior solely
   based on the Mapping Server advertisement.  However, PHP behavior may
   safely be done in following cases:

      The Prefix is intra-area type and the downstream neighbor is the
      originator of the prefix.

      The Prefix is inter-area type and downstream neighbor is an ABR,
      which is advertising the prefix reachability and is also
      generating the Extended Prefix TLV with the A-flag set for this
      prefix as described in section 2.1 of [RFC7684].

      The Prefix is external type and downstream neighbor is an ASBR,
      which is advertising the prefix reachability and is also
      generating the Extended Prefix TLV with the A-flag set for this
      prefix as described in section 2.1 of [RFC7684].

   When a Prefix-SID is advertised in an Extended Prefix Range TLV, then
   the value advertised in Prefix SID Sub-TLV is interpreted as a
   starting SID value.

   Example 1: If the following router addresses (loopback addresses)
   need to be mapped into the corresponding Prefix SID indexes:

             Router-A: 192.0.2.1/32, Prefix-SID: Index 1
             Router-B: 192.0.2.2/32, Prefix-SID: Index 2
             Router-C: 192.0.2.3/32, Prefix-SID: Index 3
             Router-D: 192.0.2.4/32, Prefix-SID: Index 4

   then the Prefix field in the Extended Prefix Range TLV would be set
   to 192.0.2.1, Prefix Length would be set to 32, Range Size would be
   set to 4, and the Index value in the Prefix-SID Sub-TLV would be set
   to 1.

   Example 2: If the following prefixes need to be mapped into the
   corresponding Prefix-SID indexes:






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             10.1.1/24, Prefix-SID: Index 51
             10.1.2/24, Prefix-SID: Index 52
             10.1.3/24, Prefix-SID: Index 53
             10.1.4/24, Prefix-SID: Index 54
             10.1.5/24, Prefix-SID: Index 55
             10.1.6/24, Prefix-SID: Index 56
             10.1.7/24, Prefix-SID: Index 57

   then the Prefix field in the Extended Prefix Range TLV would be set
   to 10.1.1.0, Prefix Length would be set to 24, Range Size would be 7,
   and the Index value in the Prefix-SID Sub-TLV would be set to 51.

6.  SID/Label Binding Sub-TLV

   The SID/Label Binding Sub-TLV is used to advertise a SID/Label
   mapping for a path to the prefix.

   The SID/Label Binding Sub-TLV MAY be originated by any router in an
   OSPF domain.  The router may advertise a SID/Label binding to a FEC
   along with at least a single 'nexthop style' anchor.  The protocol
   supports more than one 'nexthop style' anchor to be attached to a
   SID/Label binding, which results in a simple path description
   language.  In analogy to RSVP, the terminology for this is called an
   'Explicit Route Object' (ERO).  Since ERO style path notation allows
   anchoring SID/label bindings to both link and node IP addresses, any
   Label Switched Path (LSP) can be described.  Additionally, SID/Label
   Bindings from external protocols can be easily re-advertised.

   The SID/Label Binding Sub-TLV may be used for advertising SID/Label
   Bindings and their associated Primary and Backup paths.  In a single
   TLV, a primary ERO Path, backup ERO Path, or both can be advertised.
   If a router wants to advertise multiple parallel paths, then it can
   generate several TLVs for the same Prefix/FEC.  Each occurrence of a
   Binding TLV for a given FEC Prefix will add a new path.

   The SID/Label Binding Sub-TLV is a Sub-TLV of the OSPF Extended
   Prefix TLV described in [RFC7684] and the OSPF Extended Prefix Range
   TLV described in Section 4.  Multiple SID/Label Binding TLVs can be
   present in their parent TLV.  The SID/Label Binding Sub-TLV has
   following format:











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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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |    Reserved   |    MT-ID      |    Weight     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sub-TLVs (variable)                      |
   +-                                                             -+
   |                                                               |

   where:

      Type: TBD, suggested value 3

      Length: Variable

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |M|             |
   +-+-+-+-+-+-+-+-+

      where:

         M-bit - When the bit is set, the binding represents a mirroring
         context as defined in
         [I-D.minto-rsvp-lsp-egress-fast-protection].

      MT-ID: Multi-Topology ID (as defined in [RFC4915]).

      Weight: Weight used for load-balancing purposes.  The use of the
      weight is defined in [I-D.ietf-spring-segment-routing].

   The SID/Label Binding Sub-TLV supports the following Sub-TLVs:

      SID/Label Sub-TLV as described in Section 2.1.  This Sub-TLV MUST
      appear in the SID/Label Binding Sub-TLV and it SHOULD only appear
      once.  If the SID/Label Sub-TLV is not included in the SID/Label
      Binding Sub-TLV, the SID/Label Binding Sub-TLV MUST be ignored.
      If the SID/Label Sub-TLV appears in the SID/Label Binding Sub-TLV
      more than once, instances other than the first will be ignored and
      the condition SHOULD be logged for possible action by the network
      operator.

      ERO Metric Sub-TLV as defined in Section 6.1.




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      ERO Sub-TLVs as defined in Section 6.2.

6.1.  ERO Metric Sub-TLV

   The ERO Metric Sub-TLV is a Sub-TLV of the SID/Label Binding TLV.

   The ERO Metric Sub-TLV advertises the cost of an ERO path.  It is
   used to compare the cost of a given source/destination path.  A
   router SHOULD advertise the ERO Metric Sub-TLV in an advertised ERO
   TLV.  The cost of the ERO Metric Sub-TLV SHOULD be set to the
   cumulative IGP or TE path cost of the advertised ERO.  Since
   manipulation of the Metric field may attract or repel traffic to and
   from the advertised segment, it MAY be manually overridden.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Metric (4 octets)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         ERO Metric Sub-TLV format

   where:

      Type: TBD, suggested value 8

      Length: Always 4

      Metric: A 4-octet metric representing the aggregate IGP or TE path
      cost.

6.2.  ERO Sub-TLVs

   All ERO information represents an ordered set which describes the
   segments of a path.  The first ERO Sub-TLV describes the first
   segment of a path.  Similiarly, the last ERO Sub-TLV describes the
   segment closest to the egress point.  If a router extends or stitches
   a path, it MUST prepend the new segment's path information to the ERO
   list.  This applies equally to advertised backup EROs.

   All ERO Sub-TLVs must immediately follow the SID/Label Sub-TLV.

   All Backup ERO Sub-TLVs must immediately follow the last ERO Sub-TLV.






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6.2.1.  IPv4 ERO Sub-TLV

   The IPv4 ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.

   The IPv4 ERO Sub-TLV describes a path segment using IPv4 Address
   style encoding.  Its semantics have been borrowed from [RFC3209].

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     IPv4 Address (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          IPv4 ERO Sub-TLV format

   where:

      Type: TBD, suggested value 4

      Length: 8 octets

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |L|             |
   +-+-+-+-+-+-+-+-+

      where:

         L-bit - If the L-bit is set, then the segment path is
         designated as 'loose'.  Otherwise, the segment path is
         designated as 'strict'.  The terms 'loose' and 'strict' are
         defined for RSVP subobjects in [RFC3209].

      IPv4 Address - The address of the explicit route hop.

6.2.2.  Unnumbered Interface ID ERO Sub-TLV

   The Unnumbered Interface ID ERO Sub-TLV is a Sub-TLV of the SID/Label
   Binding Sub-TLV.

   The appearance and semantics of the 'Unnumbered Interface ID' have
   been borrowed from [RFC3477].



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   The Unnumbered Interface-ID ERO Sub-TLV describes a path segment that
   includes an unnumbered interface.  Unnumbered interfaces are
   referenced using the interface index.  Interface indices are assigned
   local to the router and therefore are not unique within a domain.
   All elements in an ERO path need to be unique within a domain and
   hence need to be disambiguated using a domain unique Router-ID.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Router ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Interface ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

                Unnumbered Interface ID ERO Sub-TLV format

      Type: TBD, suggested value 5

      Length: 12 octets

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |L|             |
   +-+-+-+-+-+-+-+-+

   where:

         L-bit - If the L-bit is set, then the segment path is
         designated as 'loose'.  Otherwise, the segment path is
         designated as 'strict'.  The terms 'loose' and 'strict' are
         defined for RSVP subobjects in [RFC3209]

      Router-ID: Router-ID of the next-hop.

      Interface ID: The identifier assigned to the link by the router
      specified by the Router-ID.






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6.2.3.  IPv4 Backup ERO Sub-TLV

   IPv4 Prefix Backup ERO Sub-TLV is a Sub-TLV of the SID/Label Binding
   Sub-TLV.

   The IPv4 Backup ERO Sub-TLV describes a path segment using IPv4
   Address style of encoding.  Its semantics have been borrowed from
   [RFC3209].

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     IPv4 Address (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      IPv4 Backup ERO Sub-TLV format

   where:

      Type: TBD, suggested value 6

      Length: 8 octets

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |L|             |
   +-+-+-+-+-+-+-+-+

      where:

         L-bit - If the L-bit is set, then the segment path is
         designated as 'loose'.  Otherwise, the segment path is
         designated as 'strict'.  The terms 'loose' and 'strict' are
         defined for RSVP subobjects in [RFC3209]

      IPv4 Address - The address of the explicit route hop.

6.2.4.  Unnumbered Interface ID Backup ERO Sub-TLV

   The Unnumbered Interface ID Backup ERO Sub-TLV is a Sub-TLV of the
   SID/Label Binding Sub-TLV.




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   The appearance and semantics of the 'Unnumbered Interface ID' have
   been borrowed from [RFC3477].

   The Unnumbered Interface-ID Backup ERO Sub-TLV describes a path
   segment that includes an unnumbered interface.  Unnumbered interfaces
   are referenced using the interface index.  Interface indices are
   assigned local to the router and are therefore not unique within a
   domain.  All elements in an ERO path need to be unique within a
   domain and hence need to be disambiguated with specification of the
   domain unique Router-ID.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Router ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Interface ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Unnumbered Interface ID Backup ERO Sub-TLV format

   where:

      Type: TBD, suggested value 7

      Length: 12 octets

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |L|             |
   +-+-+-+-+-+-+-+-+

      where:

         L-bit - If the L-bit is set, then the segment path is
         designated as 'loose'.  Otherwise, the segment path is
         designated as 'strict'.

      Router-ID: Router-ID of the next-hop.

      Interface ID: The identifier assigned to the link by the router
      specified by the Router-ID.



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7.  Adjacency Segment Identifier (Adj-SID)

   An Adjacency Segment Identifier (Adj-SID) represents a router
   adjacency in Segment Routing.

7.1.  Adj-SID Sub-TLV

   Adj-SID is an optional Sub-TLV of the Extended Link TLV defined in
   [RFC7684].  It MAY appear multiple times in the Extended Link TLV.
   Examples where more than one Adj-SID may be used per neighbor are
   described in section 4 of
   [I-D.filsfils-spring-segment-routing-use-cases].  The Adj-SID Sub-TLV
   has the following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |    Reserved   |   MT-ID       |  Weight       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   SID/Label/Index (variable)                  |
   +---------------------------------------------------------------+

   where:

      Type: TBD, suggested value 2.

      Length: Variable.

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |B|V|L|G|       |
   +-+-+-+-+-+-+-+-+

   where:

         B-Flag: Backup Flag.  If set, the Adj-SID refers to an
         adjacency that is eligible for protection (e.g.: using IPFRR or
         MPLS-FRR) as described in section 3.5 of
         [I-D.ietf-spring-segment-routing].

         The V-Flag: Value/Index Flag.  If set, then the Adj-SID carries
         an absolute value.  If not set, then the Adj-SID carries an
         index.




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         The L-Flag: Local/Global Flag.  If set, then the value/index
         carried by the Adj-SID has local significance.  If not set,
         then the value/index carried by this Sub-TLV has global
         significance.

         The G-Flag: Group Flag.  When set, the G-Flag indicates that
         the Adj-SID refers to a group of adjacencies (and therefore MAY
         be assigned to other adjacencies as well).

         Other bits: Reserved.  These MUST be zero when sent and are
         ignored when received.

      MT-ID: Multi-Topology ID (as defined in [RFC4915].

      Weight: Weight used for load-balancing purposes.  The use of the
      weight is defined in [I-D.ietf-spring-segment-routing].

      SID/Index/Label: According to the V and L flags, it contains
      either:

         A 32-bit index defining the offset in the SID/Label space
         advertised by this router.

         A 24-bit label where the 20 rightmost bits are used for
         encoding the label value.

   An SR capable router MAY allocate an Adj-SID for each of its
   adjacencies and set the B-Flag when the adjacency is eligible for
   protection by an FRR mechanism (IP or MPLS) as described in section
   3.5 of [I-D.ietf-spring-segment-routing].

7.2.  LAN Adj-SID Sub-TLV

   LAN Adj-SID is an optional Sub-TLV of the Extended Link TLV defined
   in [RFC7684].  It MAY appear multiple times in the Extended-Link TLV.
   It is used to advertise a SID/Label for an adjacency to a non-DR
   router on a broadcast, NBMA, or hybrid [RFC6845] network.














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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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |    Reserved   |     MT-ID     |    Weight     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Neighbor ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    SID/Label/Index (variable)                 |
   +---------------------------------------------------------------+

   where:

      Type: TBD, suggested value 3.

      Length: Variable.

      Flags: Single octet field containing the following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |B|V|L|G|       |
   +-+-+-+-+-+-+-+-+

   where:

         B-Flag: Backup-flag.  If set, the LAN-Adj-SID refers to an
         adjacency that is eligible for protection (e.g.: using IPFRR or
         MPLS-FRR) as described in section 3.5 of
         [I-D.ietf-spring-segment-routing].

         The V-Flag: Value/Index Flag.  If set, then the Prefix-SID
         carries an absolute value.  If not set, then the Prefix-SID
         carries an index.

         The L-Flag: Local/Global Flag.  If set, then the value/index
         carried by the Prefix-SID has local significance.  If not set,
         then the value/index carried by this Sub-TLV has global
         significance.

         The G-Flag: Group Flag.  When set, the G-Flag indicates that
         the LAN-Adj-SID refers to a group of adjacencies (and therefore
         MAY be assigned to other adjacencies as well).

         Other bits: Reserved.  These MUST be zero when sent and are
         ignored when received.




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      MT-ID: Multi-Topology ID (as defined in [RFC4915].

      Weight: Weight used for load-balancing purposes.  The use of the
      weight is defined in [I-D.ietf-spring-segment-routing].

      Neighbor ID: The Router ID of the neighbor for which the LAN-Adj-
      SID is advertised.

      SID/Index/Label: According to the V and L flags, it contains
      either:

         A 32-bit index defining the offset in the SID/Label space
         advertised by this router.

         A 24-bit label where the 20 rightmost bits are used for
         encoding the label value.

8.  Elements of Procedure

8.1.  Intra-area Segment routing in OSPFv2

   An OSPFv2 router that supports segment routing MAY advertise Prefix-
   SIDs for any prefix to which it is advertising reachability (e.g., a
   loopback IP address as described in Section 5).

   If multiple routers advertise a Prefix-SID for the same prefix, then
   the Prefix-SID MUST be the same.  This is required in order to allow
   traffic load-balancing when multiple equal cost paths to the
   destination exist in the OSPFv2 routing domain.

   Prefix-SID can also be advertised by the SR Mapping Servers (as
   described in [I-D.filsfils-spring-segment-routing-ldp-interop]).  The
   Mapping Server advertises Prefix-SIDs for remote prefixes that exist
   in the OSPFv2 routing domain.  Multiple Mapping Servers can advertise
   Prefix-SIDs for the same prefix, in which case the same Prefix-SID
   MUST be advertised by all of them.  The flooding scope of the OSPF
   Extended Prefix Opaque LSA that is generated by the SR Mapping Server
   could be either area scoped or AS scoped and is determined based on
   the configuration of the SR Mapping Server.

   The SR Mapping Server MUST use OSPF Extended Prefix Range TLV when
   advertising SIDs for prefixes.  Prefixes of different route-types can
   be combined in a single OSPF Extended Prefix Range TLV advertised by
   the SR Mapping Server.

   Area-scoped OSPF Extended Prefix Range TLV are propagated between
   areas.  Similar to propagation of prefixes between areas, an ABR only
   propagates the OSPF Extended Prefix Range TLV that it considers to be



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   the best from the set it received.  The rules used to pick the best
   OSPF Extended Prefix Range TLV are described in Section 4.

   When propagating an OSPF Extended Prefix Range TLV between areas,
   ABRs MUST set the IA-Flag, that is used to prevent redundant flooding
   of the OSPF Extended Prefix Range TLV between areas as described in
   Section 4.

8.2.  Inter-area Segment routing in OSPFv2

   In order to support SR in a multi-area environment, OSPFv2 must
   propagate Prefix-SID information between areas.  The following
   procedure is used to propagate Prefix SIDs between areas.

   When an OSPF ABR advertises a Type-3 Summary LSA from an intra-area
   prefix to all its connected areas, it will also originate an Extended
   Prefix Opaque LSA, as described in [RFC7684].  The flooding scope of
   the Extended Prefix Opaque LSA type will be set to area-scope.  The
   route-type in the OSPF Extended Prefix TLV is set to inter-area.  The
   Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID
   value will be set as follows:

      The ABR will look at its best path to the prefix in the source
      area and find the advertising router associated with the best path
      to that prefix.

      The ABR will then determine if such router advertised a Prefix-SID
      for the prefix and use it when advertising the Prefix-SID to other
      connected areas.

      If no Prefix-SID was advertised for the prefix in the source area
      by the router that contributes to the best path to the prefix, the
      originating ABR will use the Prefix-SID advertised by any other
      router when propagating the Prefix-SID for the prefix to other
      areas.

   When an OSPF ABR advertises Type-3 Summary LSAs from an inter-area
   route to all its connected areas, it will also originate an Extended
   Prefix Opaque LSA, as described in [RFC7684].  The flooding scope of
   the Extended Prefix Opaque LSA type will be set to area-scope.  The
   route-type in OSPF Extended Prefix TLV is set to inter-area.  The
   Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID
   will be set as follows:

      The ABR will look at its best path to the prefix in the backbone
      area and find the advertising router associated with the best path
      to that prefix.




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      The ABR will then determine if such router advertised a Prefix-SID
      for the prefix and use it when advertising the Prefix-SID to other
      connected areas.

      If no Prefix-SID was advertised for the prefix in the backbone
      area by the ABR that contributes to the best path to the prefix,
      the originating ABR will use the Prefix-SID advertised by any
      other router when propagating the Prefix-SID for the prefix to
      other areas.

8.3.  SID for External Prefixes

   Type-5 LSAs are flooded domain wide.  When an ASBR, which supports
   SR, generates Type-5 LSAs, it should also originate Extended Prefix
   Opaque LSAs, as described in [RFC7684].  The flooding scope of the
   Extended Prefix Opaque LSA type is set to AS-scope.  The route-type
   in the OSPF Extended Prefix TLV is set to external.  The Prefix-SID
   Sub-TLV is included in this LSA and the Prefix-SID value will be set
   to the SID that has been reserved for that prefix.

   When an NSSA ABR translates Type-7 LSAs into Type-5 LSAs, it should
   also advertise the Prefix-SID for the prefix.  The NSSA ABR
   determines its best path to the prefix advertised in the translated
   Type-7 LSA and finds the advertising router associated with that
   path.  If the advertising router has advertised a Prefix-SID for the
   prefix, then the NSSA ABR uses it when advertising the Prefix-SID for
   the Type-5 prefix.  Otherwise, the Prefix-SID advertised by any other
   router will be used.

8.4.  Advertisement of Adj-SID

   The Adjacency Segment Routing Identifier (Adj-SID) is advertised
   using the Adj-SID Sub-TLV as described in Section 7.

8.4.1.  Advertisement of Adj-SID on Point-to-Point Links

   An Adj-SID MAY be advertised for any adjacency on a P2P link that is
   in neighbor state 2-Way or higher.  If the adjacency on a P2P link
   transitions from the FULL state, then the Adj-SID for that adjacency
   MAY be removed from the area.  If the adjacency transitions to a
   state lower then 2-Way, then the Adj-SID advertisement MUST be
   removed from the area.

8.4.2.  Adjacency SID on Broadcast or NBMA Interfaces

   Broadcast, NBMA or or hybrid [RFC6845] networks in OSPF are
   represented by a star topology where the Designated Router (DR) is
   the central point to which all other routers on the broadcast, NBMA,



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   or hybrid network connect.  As a result, routers on the broadcast,
   NBMA, or hybrid network advertise only their adjacency to the DR.
   Routers that do not act as DR do not form or advertise adjacencies
   with each other.  They do, however, maintain 2-Way adjacency state
   with each other and are directly reachable.

   When Segment Routing is used, each router on the broadcast or NBMA
   network MAY advertise the Adj-SID for its adjacency to the DR using
   the Adj-SID Sub-TLV as described in Section 7.1.

   SR capable routers MAY also advertise an LAN-Adj-SID for other
   neighbors (e.g., BDR, DR-OTHER) on the broadcast, NBMA or hybrid
   network using the LAN-ADJ-SID Sub-TLV as described in Section 7.2.

9.  IANA Considerations

   This specification updates several existing OSPF registries.

9.1.  OSPF OSPF Router Information (RI) TLVs Registry

   o 8 (IANA Preallocated) - SR-Algorithm TLV

   o 9 (IANA Preallocated) - SID/Label Range TLV

   o 12 - SR Local Block Sub-TLV

   o 13 - SRMS Preference Sub-TLV

9.2.  OSPF Extended Prefix LSA TLV Registry

   Following values are allocated:

   o 2 - OSPF Extended Prefix Range TLV

9.3.  OSPF Extended Prefix LSA Sub-TLV Registry

   Following values are allocated:

   o 1 - SID/Label Sub-TLV

   o 2 - Prefix SID Sub-TLV

   o 3 - SID/Label Binding Sub-TLV

   o 4 - IPv4 ERO Sub-TLV

   o 5 - Unnumbered Interface ID ERO Sub-TLV




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   o 6 - IPv4 Backup ERO Sub-TLV

   o 7 - Unnumbered Interface ID Backup ERO Sub-TLV

   o 8 - ERO Metric Sub-TLV

9.4.  OSPF Extended Link LSA Sub-TLV Registry

   Following initial values are allocated:

   o 1 - SID/Label Sub-TLV

   o 2 - Adj-SID Sub-TLV

   o 3 - LAN Adj-SID/Label Sub-TLV

10.  Implementation Status

   An implementation survey with seven questions related to the
   implementer's support of OSPFv2 Segment Routing was sent to the OSPF
   WG list and several known implementers.  This section contains
   responses from two implementers who completed the survey.  No
   external means were used to verify the accuracy of the information
   submitted by the respondents.  The respondents are considered experts
   on the products they reported on.  Additionally, responses were
   omitted from implementers who indicated that they have not
   implemented the function yet.

   Responses from Nokia (former Alcatel-Lucent):

   Link to a web page describing the implementation:
   https://infoproducts.alcatel-lucent.com/cgi-bin/dbaccessfilename.cgi/
   3HE10799AAAATQZZA01_V1_7450%20ESS%207750%20SR%20and%207950%20XRS%20Un
   icast%20Routing%20Protocols%20Guide%20R14.0.R1.pdf

   The implementation's level of maturity: Production.

   Coverage: We have implemented all sections and have support for the
   latest draft.

   Licensing: Part of the software package that needs to be purchased.

   Implementation experience: Great spec.  We also performed inter-
   operability testing with Cisco's OSPF Segment Routing implementation.

   Contact information: wim.henderickx@nokia.com

   Responses from Cisco Systems:



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   Link to a web page describing the implementation:

   www.segment-routing.net/home/tutorial

   The implementation's level of maturity: Production.

   Coverage: All sections, except the section 6 (SID/Label Binding Sub-
   TLV) have been implemented according to the latest draft.

   Licensing: Part of a commercial software package.

   Implementation experience: Many aspects of the draft are result of
   the actual implementation experience, as the draft evolved from its
   initial version to the current one.  Interoperability testing with
   Alcatel-Lucent was performed, which confirmed the draft's ability to
   serve as a reference for the implementors.

   Contact information: ppsenak@cisco.com

   Responses from Juniper:

   The implementation's name and/or a link to a web page describing the
   implementation:

   Feature name is OSPF SPRING

   The implementation's level of maturity: To be released in 16.2
   (second half of 2016)

   Coverage: All sections implemented except Sections 4, and 6.

   Licensing: JUNOS Licensing needed.

   Implementation experience: NA

   Contact information: shraddha@juniper.net

11.  Security Considerations

   Implementations must assure that malformed TLV and Sub-TLV
   permutations do not result in errors which cause hard OSPF failures.

12.  Contributors

   The following people gave a substantial contribution to the content
   of this document: Acee Lindem, Ahmed Bashandy, Martin Horneffer,
   Bruno Decraene, Stephane Litkowski, Igor Milojevic, Rob Shakir and
   Saku Ytti.



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13.  Acknowledgements

   We would like to thank Anton Smirnov for his contribution.

   Many thanks to Yakov Rekhter, John Drake and Shraddha Hedge for their
   contribution on earlier incarnations of the "Binding / MPLS Label
   TLV" in [I-D.gredler-ospf-label-advertisement].

   Thanks to Acee Lindem for the detail review of the draft,
   corrections, as well as discussion about details of the encoding.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

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

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <http://www.rfc-editor.org/info/rfc3209>.

   [RFC3477]  Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003,
              <http://www.rfc-editor.org/info/rfc3477>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <http://www.rfc-editor.org/info/rfc3630>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <http://www.rfc-editor.org/info/rfc4915>.

   [RFC5250]  Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
              OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
              July 2008, <http://www.rfc-editor.org/info/rfc5250>.




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   [RFC6845]  Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
              and Point-to-Multipoint Interface Type", RFC 6845,
              DOI 10.17487/RFC6845, January 2013,
              <http://www.rfc-editor.org/info/rfc6845>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <http://www.rfc-editor.org/info/rfc7684>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <http://www.rfc-editor.org/info/rfc7770>.

14.2.  Informative References

   [I-D.filsfils-spring-segment-routing-ldp-interop]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing interoperability with LDP", draft-
              filsfils-spring-segment-routing-ldp-interop-02 (work in
              progress), September 2014.

   [I-D.filsfils-spring-segment-routing-use-cases]
              Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
              Crabbe, "Segment Routing Use Cases", draft-filsfils-
              spring-segment-routing-use-cases-01 (work in progress),
              October 2014.

   [I-D.gredler-ospf-label-advertisement]
              Gredler, H., Amante, S., Scholl, T., and L. Jalil,
              "Advertising MPLS labels in OSPF", draft-gredler-ospf-
              label-advertisement-03 (work in progress), May 2013.

   [I-D.ietf-spring-conflict-resolution]
              Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka,
              "Segment Routing Conflict Resolution", draft-ietf-spring-
              conflict-resolution-01 (work in progress), June 2016.









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   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
              and E. Crabbe, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-00 (work in progress), December
              2014.

   [I-D.minto-rsvp-lsp-egress-fast-protection]
              Jeganathan, J., Gredler, H., and Y. Shen, "RSVP-TE LSP
              egress fast-protection", draft-minto-rsvp-lsp-egress-fast-
              protection-03 (work in progress), November 2013.

Authors' Addresses

   Peter Psenak (editor)
   Cisco Systems, Inc.
   Apollo Business Center
   Mlynske nivy 43
   Bratislava  821 09
   Slovakia

   Email: ppsenak@cisco.com


   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: sprevidi@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Hannes Gredler
   RtBrick Inc.

   Email: hannes@rtbrick.com






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   Rob Shakir
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: robjs@google.com


   Wim Henderickx
   Nokia
   Copernicuslaan 50
   Antwerp  2018
   BE

   Email: wim.henderickx@nokia.com


   Jeff Tantsura
   Individual
   US

   Email: jefftant.ietf@gmail.com




























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