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Versions: (draft-psenak-ospf-segment-routing-ospfv3-extension) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 RFC 8666

Open Shortest Path First IGP                              P. Psenak, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             C. Filsfils
Expires: February 20, 2015                           Cisco Systems, Inc.
                                                              H. Gredler
                                                  Juniper Networks, Inc.
                                                               R. Shakir
                                                         British Telecom
                                                           W. Henderickx
                                                          Alcatel-Lucent
                                                             J. Tantsura
                                                                Ericsson
                                                         August 19, 2014


                 OSPFv3 Extensions for Segment Routing
          draft-ietf-ospf-ospfv3-segment-routing-extensions-00

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 OSPFv3 extensions that are 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 February 20, 2015.

Copyright Notice

   Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . .   3
   3.  Segment Routing Capabilities  . . . . . . . . . . . . . . . .   4
     3.1.  SR-Algorithm TLV  . . . . . . . . . . . . . . . . . . . .   4
     3.2.  SID/Label Range TLV . . . . . . . . . . . . . . . . . . .   5
   4.  OSPFv3 Extended Prefix Range TLV  . . . . . . . . . . . . . .   7
   5.  Prefix SID Sub-TLV  . . . . . . . . . . . . . . . . . . . . .   8
   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.  IPv6 ERO Sub-TLV  . . . . . . . . . . . . . . . . . .  16
       6.2.3.  Unnumbered Interface ID ERO Sub-TLV . . . . . . . . .  17
       6.2.4.  IPv4 Backup ERO Sub-TLV . . . . . . . . . . . . . . .  18
       6.2.5.  IPv6 Backup ERO Sub-TLV . . . . . . . . . . . . . . .  19
       6.2.6.  Unnumbered Interface ID Backup ERO Sub-TLV  . . . . .  20
   7.  Adjacency Segment Identifier (Adj-SID)  . . . . . . . . . . .  21
     7.1.  Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . . . .  21
     7.2.  LAN Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . .  23
   8.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Intra-area Segment routing in OSPFv3  . . . . . . . . . .  24
     8.2.  Inter-area Segment routing in OSPFv3  . . . . . . . . . .  25
     8.3.  SID for External Prefixes . . . . . . . . . . . . . . . .  26
     8.4.  Advertisement of Adj-SID  . . . . . . . . . . . . . . . .  26
       8.4.1.  Advertisement of Adj-SID on Point-to-Point Links  . .  27
       8.4.2.  Adjacency SID on Broadcast or NBMA Interfaces . . . .  27
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  OSPF Router Information (RI) TLVs Registry  . . . . . . .  27



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     9.2.  OSPFv3 Extend-LSA TLV Registry  . . . . . . . . . . . . .  27
     9.3.  OSPFv3 Extend-LSA Sub-TLV registry  . . . . . . . . . . .  27
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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 of the 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 signaling (other than the regular
   IGP).  For example, when used in MPLS networks, SR paths do not
   require any LDP or RSVP-TE signaling.  Still, SR can interoperate in
   the presence of LSPs established with RSVP or LDP.

   This draft describes the OSPFv3 extensions required for segment
   routing.

   Segment Routing architecture is described in
   [I-D.filsfils-rtgwg-segment-routing].

   Segment Routing use cases are described in
   [I-D.filsfils-rtgwg-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.

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:





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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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      SID/Label (variable)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 3

      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 the SID/Label Sub-TLV if the
      length is other then 3 or 4.

3.  Segment Routing Capabilities

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

   These SR capabilities are advertised in OSPFv3 Router Information LSA
   (defined in [RFC4970]).

3.1.  SR-Algorithm TLV

   The SR-Algorithm TLV is a TLV of the OSPFv3 Router Information LSA
   (defined in [RFC4970]).

   The SR-Algorithm TLV is optional.  It MAY only be advertised once in
   the OSPFv3 Router Information LSA.  If the SID/Label Range TLV, as
   defined in Section 3.2, is advertised, then the SR-Algorithm TLV MUST
   also be advertised.

   An OSPFv3 router may use various algorithms when calculating
   reachability to other nodes in area or to prefixes attached to these
   nodes.  Examples of these algorithms are metric based Shortest Path
   First (SPF), various sorts 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 area.  The SR-Algorithm TLV
   has following structure:





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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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Algorithm 1  |  Algorithm... |  Algorithm n |                |
   +-                                                             -+
   |                                                               |
   +                                                               +

   where:

      Type: TBD, suggested value 8

      Length: variable

      Algorithm: Single octet identifying the algorithm.  The following
      value has been defined:

         0: IGP metric based SPT.

   The RI LSA can be advertised at any of the defined 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 TLV of the OSPFv3 Router Information LSA
   (defined in [RFC4970]).

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

   where:

      Type: TBD, suggested value 9



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      Length: variable

      Range Size: 3 octets of 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 in the OSPFv3 Router Information
      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 the SID index.

   o  A router not supporting multiple occurrences of the SID/Label
      Range TLV MUST use first advertised SID/Label Range TLV.

   The following example illustrates the advertisement of multiple
   ranges:






















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   The originating router advertises the 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 purpose of the SR-
   Capability TLV propagation, area scope flooding is required.

4.  OSPFv3 Extended Prefix Range TLV

   In some cases it is useful to advertise attributes for a range of
   prefixes.  Segment Routing Mapping Server, which is described in
   [I-D.filsfils-rtgwg-segment-routing], is an example where we need a
   single advertisement to advertise SIDs for multiple prefixes from a
   contiguous address range.  The OSPFv3 Extended Prefix Range TLV is
   defined for this purpose.

   The OSPFv3 Extended Prefix Range TLV is a new top level TLV of the
   following LSAs defined in [I-D.ietf-ospf-ospfv3-lsa-extend]:

      E-Intra-Area-Prefix-LSA

      E-Inter-Area-Prefix-LSA

      E-AS-External-LSA

      E-Type-7-LSA





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   Multiple OSPFv3 Extended Prefix Range TLVs MAY be advertised in these
   extended LSAs.  The OSPFv3 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Address Prefix (variable)                 |
   |                           ...                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sub-TLVs (variable)                      |
   +-                                                             -+
   |                                                               |

   where:

      Type: TBD, suggested value 9.

      Length: variable

      Prefix length: length of the prefix

      AF: 0 - IPv6 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 addresses from other than the IPv6 unicast address
      class.

      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 following OSPFv3 TLVs as
   defined in [I-D.ietf-ospf-ospfv3-lsa-extend] and in Section 4:

      Intra-Area Prefix TLV

      Inter-Area Prefix TLV




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      External Prefix TLV

      OSPFv3 Extended Prefix Range TLV

   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       |  Algorithm  |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       SID/Index/Label (variable)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 4.

      Length: variable

      Flags: 1 octet field.  The following flags are defined:

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

   where:

         N-Flag: Node-SID flag.  If set, then the Prefix-SID refers to
         the router identified by the prefix.  Typically, the N-Flag is
         set to Prefix-SIDs corresponding to a router loopback address.
         The N-Flag is set when the Prefix-SID is a Node-SID as
         described in [I-D.filsfils-rtgwg-segment-routing].

         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-rtgwg-segment-routing-use-cases].





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         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.

         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.

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

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

      SID/Index/Label: label or index value depending on the V-bit
      setting.

         Examples:

            A 32 bit global index defining the offset in the SID/Label
            space advertised by this router - in this case the V and L
            flags MUST NOT be set.

            A 24 bit local label where the 20 rightmost bits are used
            for encoding the label value - in this case the V and L
            flags MUST be set.

   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 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.



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   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,
   NP-Flag SHOULD NOT be set

   The NP-Flag (No-PHP) MUST 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.

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

             Router-A: 192::1/128, Prefix-SID: Index 1
             Router-B: 192::2/128, Prefix-SID: Index 2
             Router-C: 192::3/128, Prefix-SID: Index 3
             Router-D: 192::4/128, Prefix-SID: Index 4



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   then the Address Prefix field in the OSPFv3 Extended Prefix Range TLV
   is set to 192::1, Prefix Length would be set to 128, 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::0/120,   Prefix-SID: Index 51
             10:1:1::100/120, Prefix-SID: Index 52
             10:1:1::200/120, Prefix-SID: Index 53
             10:1:1::300/120, Prefix-SID: Index 54
             10:1:1::400/120, Prefix-SID: Index 55
             10:1:1::500/120, Prefix-SID: Index 56
             10:1:1::600/120, Prefix-SID: Index 57

   then the Address Prefix field in the OSPFv3 Extended Prefix Range TLV
   is set to 10:1:1::0, Prefix Length would be set to 120, Range Size
   would be set to 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 SID/Label mapping
   for a path to the prefix.

   The SID/Label Binding TLV MAY be originated by any router in an
   OSPFv3 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 into 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.  Furthermore, SID/Label
   Bindings from external protocols can also be re-advertised.

   The SID/Label Binding TLV may be used for advertising SID/Label
   Bindings and their associated Primary and Backup paths.  In one
   single TLV, either a primary ERO Path, backup ERO Path, or both are
   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.

   SID/Label Binding Sub-TLV is a Sub-TLV of the following OSPFv3 TLVs,
   as defined in [I-D.ietf-ospf-ospfv3-lsa-extend] and in Section 4:




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      Intra-Area Prefix TLV

      Inter-Area Prefix TLV

      External Prefix TLV

      OSPFv3 Extended Prefix Range TLV

   Multiple SID/Label Binding Sub-TLVs can be present in these TLVs.
   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       |   Weight    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Sub-TLVs (variable)                     |
   +-                                                             -+
   |                                                               |

   where:

      Type: TBD, suggested value 7

      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].

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

   SID/Label Binding Sub-TLV currently supports 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 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 ERO Sub-TLV format

   where:

      Type: TBD, suggested value 9

      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.  IPv6 ERO Sub-TLV

   IPv6 ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.

   The IPv6 ERO Sub-TLV (Type TBA) describes a path segment using IPv6
   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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-                                                             -+
      |                                                               |
      +-                         IPv6 Address                        -+
      |                                                               |
      +-                                                             -+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          IPv6 ERO Sub-TLV format

   where:

      Type: TBD, suggested value 10

      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'.

      IPv6 Address - the address of the explicit route hop.





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6.2.3.  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
   spans over 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 11

      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.

6.2.4.  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 12

      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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6.2.5.  IPv6 Backup ERO Sub-TLV

   The IPv6 ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.

   The IPv6 Backup ERO Sub-TLV describes a Backup path segment using
   IPv6 Address style of encoding.  Its appearance and semantics have
   been borrowed from [RFC3209].

   The 'L' bit in the Flags is a one-bit attribute.  If the L bit is
   set, then the value of the attribute is 'loose.'  Otherwise, the
   value of the attribute is 'strict.'

       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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-                                                             -+
      |                                                               |
      +-                         IPv6 Address                        -+
      |                                                               |
      +-                                                             -+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      IPv6 Backup ERO Sub-TLV format

   where:

      Type: TBD, suggested value 13

      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'.



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      IPv6 Address - the address of the explicit route hop.

6.2.6.  Unnumbered Interface ID Backup ERO Sub-TLV

   The Unnumbered Interface ID Backup 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 spans over 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 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 14

      Length: 12 bytes

      Flags: 1 octet field of following flags:

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

      where:





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         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: 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

   The extended OSPFv3 LSAs, as defined in
   [I-D.ietf-ospf-ospfv3-lsa-extend], are used to advertise prefix SID
   in OSPFv3

   The Adj-SID Sub-TLV is an optional Sub-TLV of the Router-Link TLV as
   defined in [I-D.ietf-ospf-ospfv3-lsa-extend].  It MAY appear multiple
   times in Router-Link TLV.  Examples where more than one Adj-SID may
   be used per neighbor are described in
   [I-D.filsfils-rtgwg-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         |     Weight    |             Reserved          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   SID/Label/Index (variable)                  |
   +---------------------------------------------------------------+

   where:

      Type: TBD, suggested value 5.

      Length: variable.

      Flags. 1 octet field of following flags:







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    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 [I-D.filsfils-rtgwg-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.

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

      SID/Index/Label: label or index value depending on the V-bit
      setting.

         Examples:

            A 32 bit global index defining the offset in the SID/Label
            space advertised by this router - in this case the V and L
            flags MUST NOT be set.

            A 24 bit local label where the 20 rightmost bits are used
            for encoding the label value - in this case the V and L
            flags MUST be set.

            16 octet IPv6 address - in this case the V-flag MUST be set.
            The L-flag MUST be set for link-local IPv6 address and MUST
            NOT be set for IPv6 global unicast address.





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   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 a
   FRR mechanism (IP or MPLS) as described in
   [I-D.filsfils-rtgwg-segment-routing-use-cases].

7.2.  LAN Adj-SID Sub-TLV

   The LAN Adj-SID is an optional Sub-TLV of the Router-Link TLV.  It
   MAY appear multiple times in the Router-Link TLV.  It is used to
   advertise a SID/Label for an adjacency to a non-DR neighbor on a
   broadcast or NBMA network.

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

   where:

      Type: TBD, suggested value 6.

      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 [I-D.filsfils-rtgwg-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.





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         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 subTLV 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.

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

      SID/Index/Label: label or index value depending on the V-bit
      setting.

         Examples:

            A 32 bit global index defining the offset in the SID/Label
            space advertised by this router - in this case the V and L
            flags MUST NOT be set.

            A 24 bit local label where the 20 rightmost bits are used
            for encoding the label value - in this case the V and L
            flags MUST be set.

            16 octet IPv6 address - in this case the V-flag MUST be set.
            The L-flag MUST be set for link-local IPv6 address and MUST
            NOT be set for IPv6 global unicast address.

8.  Elements of Procedure

8.1.  Intra-area Segment routing in OSPFv3

   An OSPFv3 router that supports segment routing MAY advertise Prefix-
   SIDs for any prefix that it is advertising reachability for (e.g.,
   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.

   The Prefix-SID can also be advertised by the SR Mapping Servers (as
   described in [I-D.filsfils-rtgwg-segment-routing-use-cases]).  The
   Mapping Server advertises Prefix-SID for remote prefixes that exist



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   in the network.  Multiple Mapping Servers can advertise Prefix-SID
   for the same prefix, in which case the same Prefix-SID MUST be
   advertised by all of them.  The SR Mapping Server could use either
   area scope or autonomous system flooding scope when advertising
   Prefix SID for prefixes, based on the configuration of the SR Mapping
   Server.  Depending on the flooding scope used, the SR Mapping Server
   chooses the LSA that will be used.  If the area flooding scope is
   needed, E-Intra-Area-Prefix-LSA ([I-D.ietf-ospf-ospfv3-lsa-extend])
   is used.  If autonomous system flooding scope is needed, E-AS-
   External-LSA ([I-D.ietf-ospf-ospfv3-lsa-extend]) is used.

   When a Prefix-SID is advertised by the Mapping Server, which is
   indicated by the M-flag in the Prefix-SID Sub-TLV (Section 5), the
   route type as implied by the LSA type is ignored and the Prefix-SID
   is bound to the corresponding prefix independent of the route type.

   Advertisement of the Prefix-SID by the Mapping Server using Inter-
   Area Prefix TLV, External-Prefix TLV or Intra-Area-Prefix TLV
   ([I-D.ietf-ospf-ospfv3-lsa-extend]) does not itself contribute to the
   prefix reachability.  The NU-bit MUST be set in the PrefixOptions
   field of the LSA which is used by the Mapping Server to advertise SID
   or SID range, which prevents the advertisement to contribute to
   prefix reachability.

8.2.  Inter-area Segment routing in OSPFv3

   In order to support SR in a multi-area environment, OSPFv3 must
   propagate Prefix-SID information between areas.  The following
   procedure is used in order to propagate Prefix SIDs between areas.

   When an OSPFv3 ABR advertises a Inter-Area-Prefix-LSA from an intra-
   area prefix to all its connected areas, it will also include Prefix-
   SID Sub-TLV, as described in Section 5.  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 out 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




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      defined in [I-D.filsfils-rtgwg-segment-routing-use-cases]) when
      propagating Prefix-SID for the prefix to other areas.

   When an OSPFv3 ABR advertises Inter-Area-Prefix-LSA LSAs from an
   inter-area route to all its connected areas it will also include
   Prefix-SID Sub-TLV, as described in Section 5.  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 out the advertising router associated with the best
      path to that prefix.

      The ABR will then look 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-rtgwg-segment-routing-use-cases]) when
      propagating Prefix-SID for the prefix to other areas.

8.3.  SID for External Prefixes

   AS-External-LSAs are flooded domain wide.  When an ASBR, which
   supports SR, generates E-AS-External-LSA, it should also include
   Prefix-SID Sub-TLV, as described in Section 5.  The Prefix-SID value
   will be set to the SID that has been reserved for that prefix.

   When an NSSA ASBR translates an E-NSSA-LSA into an E-AS-External-LSA,
   it should also advertise the Prefix-SID for the prefix.  The NSSA ABR
   determines its best path to the prefix advertised in the translated
   E-NSSA-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 in
   the E-AS-External-LSA.  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-rtgwg-segment-routing-use-cases]).

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.






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

   An Adj-SID MAY be advertised for any adjacency on p2p link that is in
   a 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 or NBMA networks in OSPFv3 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
   a 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 Router Information (RI) TLVs Registry

   o 8 (IANA Preallocated) - SR-Algorithm TLV

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

9.2.  OSPFv3 Extend-LSA TLV Registry

   Following values are allocated:

   o suggested value 9 - OSPF Extended Prefix Range TLV

9.3.  OSPFv3 Extend-LSA Sub-TLV registry

   o suggested value 3 - SID/Label Sub-TLV

   o suggested value 4 - Prefix SID Sub-TLV



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   o suggested value 5 - Adj-SID Sub-TLV

   o suggested value 6 - LAN Adj-SID Sub-TLV

   o suggested value 7 - SID/Label Binding Sub-TLV

   o suggested value 8 - ERO Metric Sub-TLV

   o suggested value 9 - IPv4 ERO Sub-TLV

   o suggested value 10 - IPv6 ERO Sub-TLV

   o suggested value 11 - Unnumbered Interface ID ERO Sub-TLV

   o suggested value 12 - IPv4 Backup ERO Sub-TLV

   o suggested value 13 - IPv6 Backup ERO Sub-TLV

   o suggested value 14 - Unnumbered Interface ID Backup ERO Sub-TLV

10.  Security Considerations

   Implementations must assure that malformed permutations of the newly
   defined sub-TLvs do not result in errors which cause hard OSPFv3
   failures.

11.  Acknowledgements

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

   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].

12.  References

12.1.  Normative References

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

   [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.




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

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

12.2.  Informative References

   [I-D.filsfils-rtgwg-segment-routing]
              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 Architecture", draft-filsfils-rtgwg-
              segment-routing-01 (work in progress), October 2013.

   [I-D.filsfils-rtgwg-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-rtgwg-
              segment-routing-use-cases-02 (work in progress), October
              2013.

   [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-ospf-ospfv3-lsa-extend]
              Lindem, A., Mirtorabi, S., Roy, A., and F. Baker, "OSPFv3
              LSA Extendibility", draft-ietf-ospf-ospfv3-lsa-extend-03
              (work in progress), May 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










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   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
   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









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   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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