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Versions: 00 01 02 03 04

Open Shortest Path First IGP                              P. Psenak, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             C. Filsfils
Expires: August 5, 2015                              Cisco Systems, Inc.
                                                              H. Gredler
                                                  Juniper Networks, Inc.
                                                               R. Shakir
                                                         British Telecom
                                                           W. Henderickx
                                                          Alcatel-Lucent
                                                             J. Tantsura
                                                                Ericsson
                                                        February 1, 2015


                  OSPF Extensions for Segment Routing
             draft-ietf-ospf-segment-routing-extensions-04

Abstract

   Segment Routing (SR) allows for 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 August 5, 2015.

Copyright Notice

   Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . .   5
   4.  OSPF Extended Prefix Range TLV  . . . . . . . . . . . . . . .   7
   5.  Prefix SID Sub-TLV  . . . . . . . . . . . . . . . . . . . . .   9
   6.  SID/Label Binding Sub-TLV . . . . . . . . . . . . . . . . . .  12
     6.1.  ERO Metric Sub-TLV  . . . . . . . . . . . . . . . . . . .  14
     6.2.  ERO Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . .  14
       6.2.1.  IPv4 ERO Sub-TLV  . . . . . . . . . . . . . . . . . .  15
       6.2.2.  Unnumbered Interface ID ERO Sub-TLV . . . . . . . . .  16
       6.2.3.  IPv4 Backup ERO Sub-TLV . . . . . . . . . . . . . . .  17
       6.2.4.  Unnumbered Interface ID Backup ERO Sub-TLV  . . . . .  18
   7.  Adjacency Segment Identifier (Adj-SID)  . . . . . . . . . . .  19
     7.1.  Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  LAN Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . .  20
   8.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Intra-area Segment routing in OSPFv2  . . . . . . . . . .  22
     8.2.  Inter-area Segment routing in OSPFv2  . . . . . . . . . .  23
     8.3.  SID for External Prefixes . . . . . . . . . . . . . . . .  24
     8.4.  Advertisement of Adj-SID  . . . . . . . . . . . . . . . .  24
       8.4.1.  Advertisement of Adj-SID on Point-to-Point Links  . .  24
       8.4.2.  Adjacency SID on Broadcast or NBMA Interfaces . . . .  25
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  OSPF OSPF Router Information (RI) TLVs Registry . . . . .  25
     9.2.  OSPF Extended Prefix LSA TLV Registry . . . . . . . . . .  25
     9.3.  OSPF Extended Prefix LSA Sub-TLV Registry . . . . . . . .  25



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     9.4.  OSPF Extended Link LSA Sub-TLV Registry . . . . . . . . .  26
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  26
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     13.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   Segment Routing (SR) allows for 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).  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
   [I-D.ietf-ospf-prefix-link-attr].  These new LSAs are defined as
   generic containers that can be used to advertise any additional
   attributes associated with a prefix or link.  These new Opaque LSAs
   are complementary to the existing LSAs and are not aimed to replace
   any of the existing LSAs.




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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 bytes

      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 [RFC4970]).

3.1.  SR-Algorithm TLV

   The SR-Algorithm TLV is a top-level TLV of the Router Information
   Opaque LSA (defined in [RFC4970]).

   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 SR-Algorithm TLV MUST
   also be advertised.

   An SR Router may use various algorithms when calculating reachability
   to OSPF routers or prefixes in an OSPF area.  Examples of these



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   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 that the router is currently using
   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
      value is defined by this document:

         0: IGP metric based Shortest Path Tree (SPT)

   The RI LSA can be advertised at any of the defined opaque flooding
   scopes (link, area, or Autonomous System (AS)).  For the purpose of
   the SR-Algorithm TLV propagation, 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 [RFC4970]).

   The SID/Label Range TLV MAY appear multiple times 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    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 occurrence of the SID/Label Range TLV MAY be advertised, in
   order to advertise multiple ranges.  In such case:

   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 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:








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   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) is 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 purposes of the SR-
   Capability TLV propagation, area scope flooding is required.

4.  OSPF Extended Prefix Range TLV

   In some cases it is useful to advertise attributes for the range of
   prefixes.  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.

   OSPF Extended Prefix Range TLV, which is a new top level TLV of the
   Extended Prefix LSA described in [I-D.ietf-ospf-prefix-link-attr] is
   defined for this purpose.

   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:








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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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 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: 1 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.  ABR that is advertising the OSPF Extended Prefix
         Range TLV between areas MUST set this bit.

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





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

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

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

      Address Prefix: the prefix, encoded as an even multiple of 32-bit
      words, padded with zeroed 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 [I-D.ietf-ospf-prefix-link-attr] 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: 1 octet field.  The following flags are defined:


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

   where:




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

         M-Flag: Mapping Server Flag.  If set, the SID is advertised
         from the Segment Routing Mapping Server functionality 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 a
         Prefix-SID having an Explicit-NULL value (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: one octet identifying the algorithm the Prefix-SID is
      associated with as defined in Section 3.1.

      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.

   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.



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   When calculating the outgoing label for the prefix, the router MUST
   take into account E and P flags advertised by the next-hop router, if
   next-hop 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 on the 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 the prefix which is directly attached to
   the ABR, NP-Flag SHOULD NOT be set

   The NP-Flag (No-PHP) MUST be be set on the Prefix-SIDs allocated to
   redistributed prefixes, unless the redistributed prefix is directly
   attached to 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
   clear 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 inter-area border router
      (prefix propagation from one area to another) or at an inter-
      domain border 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 a Prefix-SID having an
      Explicit-NULL value.  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 M-Flag is set, NP-flag MUST be set and E-bit MUST NOT be set.

   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.





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   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:

             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 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 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



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   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 [I-D.ietf-ospf-prefix-link-attr] 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:

    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: 1 octet field of following flags:

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

      where:

         M-bit - When the bit is set the binding represents the
         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 TLV supports the following Sub-TLVs:






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      SID/Label Sub-TLV as described in Section 2.1.  This Sub-TLV MUST
      appear in the SID/Label Binding Sub-TLV and it MUST only appear
      once.

      ERO Metric Sub-TLV as defined in Section 6.1.

      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.




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   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.

6.2.1.  IPv4 ERO Sub-TLV

   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 bytes

      Flags: 1 octet field of 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'.

      IPv4 Address - the address of the explicit route hop.








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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].

   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 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 bytes

      Flags: 1 octet field of 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'.




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

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

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 bytes

      Flags: 1 octet field of 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'.

      IPv4 Address - the address of the explicit route hop.





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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.

   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 bytes

      Flags: 1 octet field of 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'.



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

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

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
   [I-D.ietf-ospf-prefix-link-attr].  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. 1 octet field of following flags:

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

   where:

         B-Flag: Backup Flag.  If set, the Adj-SID refers to an
         adjacency being protected (e.g.: using IPFRR or MPLS-FRR) as
         described in section 3.1 of
         [I-D.filsfils-spring-segment-routing-use-cases].



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         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.

         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 S-Flag.  Set Flag.  When set, the S-Flag indicates that the
         Adj-SID refers to a set 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 protected by an
   FRR mechanism (IP or MPLS) as described in section 3.1 of
   [I-D.filsfils-spring-segment-routing-use-cases].

7.2.  LAN Adj-SID Sub-TLV

   LAN Adj-SID is an optional Sub-TLV of the Extended Link TLV defined
   in [I-D.ietf-ospf-prefix-link-attr].  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 node on a broadcast or NBMA 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. 1 octet field of following flags:

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

   where:

         B-Flag: Backup-flag: set if the LAN-Adj-SID refer to an
         adjacency being protected (e.g.: using IPFRR or MPLS-FRR) as
         described in section 3.1 of
         [I-D.filsfils-spring-segment-routing-use-cases].

         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 S-Flag.  Set Flag.  When set, the S-Flag indicates that the
         Adj-SID refers to a set 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].

      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 network.

   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.

   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, ABR only
   propagates the OSPF Extended Prefix Range TLV that it considers to be
   the best from the set it received.  The rules used to pick the best
   OSPF Extended Prefix Range TLV is described in Section 4.




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   When propagating OSPF Extended Prefix Range TLV between areas, ABR
   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.

   If the Prefix-SID that is advertised in Prefix SID Sub-TLV is also
   covered by the OSPF Extended Prefix Range TLV, the Prefix-SID
   advertised in Prefix SID Sub-TLV MUST be preferred.

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 in order 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 [I-D.ietf-ospf-prefix-link-attr].
   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 (e.g.: a Prefix-SID coming from an SR Mapping Server as
      defined in [I-D.filsfils-spring-segment-routing-ldp-interop]) 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 [I-D.ietf-ospf-prefix-link-attr].
   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:






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      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 ABR that contributes to the best path to the prefix, the
      originating ABR will use the Prefix-SID advertised by any other
      router (e.g.: a Prefix-SID coming from an SR Mapping Server as
      defined in [I-D.filsfils-spring-segment-routing-ldp-interop]) 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 an Extended
   Prefix Opaque LSAs, as described in [I-D.ietf-ospf-prefix-link-attr].
   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 (e.g.: a Prefix-SID coming from an SR Mapping
   Server as defined in
   [I-D.filsfils-spring-segment-routing-ldp-interop]).

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



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   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 or NBMA 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 or NBMA network connect.  As a result,
   routers on the broadcast or NBMA 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
   Adj-SID Sub-TLV as described in Section 7.1.

   SR capable routers MAY also advertise an Adj-SID for other neighbors
   (e.g.  BDR, DR-OTHER) on the broadcast or NBMA 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

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



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   o 5 - Unnumbered Interface ID ERO Sub-TLV

   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.  Security Considerations

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

11.  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.

12.  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.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [RFC3477]  Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", RFC 3477, January 2003.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630, September
              2003.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC
              4915, June 2007.

   [RFC4970]  Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R., and S.
              Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 4970, July 2007.

   [RFC5250]  Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
              OSPF Opaque LSA Option", RFC 5250, July 2008.

13.2.  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.




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Internet-Draft     OSPF Extensions for Segment Routing     February 2015


   [I-D.ietf-ospf-prefix-link-attr]
              Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", draft-ietf-ospf-prefix-link-attr-02 (work
              in progress), December 2014.

   [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






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Internet-Draft     OSPF Extensions for Segment Routing     February 2015


   Hannes Gredler
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: hannes@juniper.net


   Rob Shakir
   British Telecom
   London
   UK

   Email: rob.shakir@bt.com


   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerp  2018
   BE

   Email: wim.henderickx@alcatel-lucent.com


   Jeff Tantsura
   Ericsson
   300 Holger Way
   San Jose, CA  95134
   US

   Email: Jeff.Tantsura@ericsson.com


















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