OSPFv3 Extensions for Segment Routing
draft-ietf-ospf-ospfv3-segment-routing-extensions-10

Open Shortest Path First IGP                              P. Psenak, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             C. Filsfils
Expires: March 9, 2018                               Cisco Systems, Inc.
                                                              H. Gredler
                                                            RtBrick Inc.
                                                               R. Shakir
                                                            Google, Inc.
                                                           W. Henderickx
                                                                   Nokia
                                                             J. Tantsura
                                                              Individual
                                                       September 5, 2017


                 OSPFv3 Extensions for Segment Routing
          draft-ietf-ospf-ospfv3-segment-routing-extensions-10

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 https://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 March 9, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (https://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
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   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 . . . . . . . . . . . . . . . . . . .   6
     3.3.  SR Local Block Sub-TLV  . . . . . . . . . . . . . . . . .   7
     3.4.  SRMS Preference Sub-TLV . . . . . . . . . . . . . . . . .   9
     3.5.  SR-Forwarding Capabilities  . . . . . . . . . . . . . . .  10
   4.  OSPFv3 Extended Prefix Range TLV  . . . . . . . . . . . . . .  10
   5.  Prefix SID Sub-TLV  . . . . . . . . . . . . . . . . . . . . .  12
   6.  SID/Label Binding Sub-TLV . . . . . . . . . . . . . . . . . .  16
     6.1.  ERO Metric Sub-TLV  . . . . . . . . . . . . . . . . . . .  18
     6.2.  ERO Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . .  19
       6.2.1.  IPv4 ERO Sub-TLV  . . . . . . . . . . . . . . . . . .  19
       6.2.2.  IPv6 ERO Sub-TLV  . . . . . . . . . . . . . . . . . .  20
       6.2.3.  Unnumbered Interface ID ERO Sub-TLV . . . . . . . . .  21
       6.2.4.  IPv4 Backup ERO Sub-TLV . . . . . . . . . . . . . . .  22
       6.2.5.  IPv6 Backup ERO Sub-TLV . . . . . . . . . . . . . . .  23
       6.2.6.  Unnumbered Interface ID Backup ERO Sub-TLV  . . . . .  24
   7.  Adjacency Segment Identifier (Adj-SID)  . . . . . . . . . . .  25
     7.1.  Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . . . .  25
     7.2.  LAN Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . .  27
   8.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .  29
     8.1.  Intra-area Segment routing in OSPFv3  . . . . . . . . . .  29
     8.2.  Inter-area Segment routing in OSPFv3  . . . . . . . . . .  30
     8.3.  SID for External Prefixes . . . . . . . . . . . . . . . .  31
     8.4.  Advertisement of Adj-SID  . . . . . . . . . . . . . . . .  32
       8.4.1.  Advertisement of Adj-SID on Point-to-Point Links  . .  32



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       8.4.2.  Adjacency SID on Broadcast or NBMA Interfaces . . . .  32
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
     9.1.  OSPF Router Information (RI) TLVs Registry  . . . . . . .  32
     9.2.  OSPFv3 Extend-LSA TLV Registry  . . . . . . . . . . . . .  33
     9.3.  OSPFv3 Extend-LSA Sub-TLV registry  . . . . . . . . . . .  33
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  33
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  33
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     12.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

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.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.

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




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

   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



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   currently using to other routers in an area.  The SR-Algorithm TLV
   has following structure:

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

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

   When multiple SR-Algorithm sub-TLVs are received from a given router
   the receiver SHOULD use the first occurrence of the sub-TLV in the
   OSPFv3 Router Information LSA.  If the SR-Algorithm sub-TLV appears
   in multiple OSPFv3 Router Information LSAs that have different
   flooding scopes, the SR-Algorithm sub-TLV in the OSPFv3 Router
   Information LSA with the lowest flooding scope SHOULD be used.  If
   the SR-Algorithm sub-TLV appears in multiple OSPFv3 Router
   Information LSAs that have the same flooding scope, the SR-Algorithm
   sub-TLV in the OSPFv3 Router Information LSA with the numerically
   smallest Instance ID SHOULD be used and subsequent instances of the
   SR-Algorithm sub-TLV SHOULD be ignored.



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   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

      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.



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   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:

   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 SID/
   Label Range TLV propagation, area scope flooding is required.

3.3.  SR Local Block Sub-TLV

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





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   The SR Local Block (SRLB) Sub-TLV is a top-level TLV of the OSPFv3
   Router Information Opaque LSA (defined in [RFC7770]).

   The SR Local Block Sub-TLV MAY appear multiple times in the OSPFv3
   Router Information Opaque LSA and has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Range Size                 |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sub-TLVs (variable)                    |
   +-                                                             -+
   |                                                               |
   +                                                               +

   where:

      Type: TBD, suggested value 12

      Length: variable

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

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

   When multiple SRLB sub-TLVs are received from a given router the
   behavior of the receiving system is undefined.

   The originating router MUST NOT advertise overlapping ranges.

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

   Within the context of OSPFv3, the reporting of local SIDs is done
   through OSPF Sub-TLVs such as the Adjacency-SID (Section 7).
   However, the reporting of allocated local SIDs may also be done
   through other means and protocols which mechanisms are outside the
   scope of this document.




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

   The OSPFv3 RI LSA can be advertised at any of the defined flooding
   scopes (link, area, or autonomous system (AS)).  For the purpose of
   SR Local Block Sub-TLV TLV advertisement, area scope flooding is
   required.

3.4.  SRMS Preference Sub-TLV

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

   The SRMS Preference Sub-TLV is a top-level TLV of the OSPFv3 Router
   Information Opaque LSA (defined in [RFC7770]).

   The SRMS Preference Sub-TLV MAY only be advertised once in the OSPFv3
   Router Information Opaque LSA and has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Preference    |                 Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 13

      Length: 4 octets

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

   When multiple SRMS Preference sub-TLVs are received from a given
   router the receiver SHOULD use the first occurrence of the sub-TLV in
   the OSPFv3 Router Information LSA.  If the SRMS Preference sub-TLV
   appears in multiple OSPFv3 Router Information LSAs that have
   different flooding scopes, the SRLB sub-TLV in the OSPFv3 Router
   Information LSA with the lowest flooding scope SHOULD be used.  If
   the SRMS Preference sub-TLV appears in multiple OSPFv3 Router
   Information LSAs that have the same flooding scope, the SRMS



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   Preference sub-TLV in the OSPFv3 Router Information LSA with the
   numerically smallest Instance ID SHOULD be used and subsequent
   instances of the SRMS Preference sub-TLV SHOULD be ignored.

   The OSPFv3 RI LSA can be advertised at any of the defined flooding
   scopes (link, area, or autonomous system (AS)).  For the purpose of
   the SRMS Preference Sub-TLV advertisement, AS scope flooding is
   required.  If the SRMS advertisements from the SRMS server are only
   used inside the area to which the SRMS server is attached, area scope
   flooding may be used.

3.5.  SR-Forwarding Capabilities

   OSPFv3 router supporting Segment Routing needs to advertise its SR
   data-plane capabilities.  Data-plane capabilities are advertised in
   OSPF Router Informational Capabilities TLV, which is defined in
   section 2.3 of RFC 4970 [RFC4970].

   Two new bits are allocated in the OSPF Router Informational
   Capability Bits as follows:

      Bit-6 - MPLS IPv6 flag.  If set, then the router is capable of
      processing SR MPLS encapsulated IPv6 packets on all interfaces.

      Bit-7 - If set, then the router is capable of processing the IPv6
      Segment Routing Header on all interfaces as defined in
      [I-D.previdi-6man-segment-routing-header].

   For the purpose of the SR-Forwarding Capabilities 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-spring-segment-routing-ldp-interop], is an example
   where we need a single advertisement to advertise SIDs for multiple
   prefixes from a contiguous address range.  The 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



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      E-Type-7-LSA

   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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Flags      |                 Reserved                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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.

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


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

   where:




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         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:

            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 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

      External Prefix TLV

      OSPFv3 Extended Prefix Range TLV

   It MAY appear more than once in the parent TLV 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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     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  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |  |NP|M |E |V |L |  |  |
   +--+--+--+--+--+--+--+--+

   where:

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

         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.




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         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.

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

      SID/Index/Label: 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.

   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






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   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 and E-flag MUST be ignored at reception.

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

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

      Prefix is of inter-area type and downstream neighbor is an ABR,
      which is advertising the prefix reachability and is setting LA-bit
      in the Prefix Options as described in section 3.1 of
      [I-D.ietf-ospf-ospfv3-lsa-extend].

      Prefix is of external type and downstream neighbor is an ASBR,
      which is advertising the prefix reachability and is setting LA-bit
      in the Prefix Options as described in section 3.1 of
      [I-D.ietf-ospf-ospfv3-lsa-extend].



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   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

   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 Sub-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.



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   The SID/Label Binding Sub-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:

      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:




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         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 section 3.5.1 of
      [I-D.ietf-spring-segment-routing].

   SID/Label Binding Sub-TLV currently supports following Sub-TLVs:

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



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6.2.  ERO Sub-TLVs

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

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

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

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:





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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'.

      IPv4 Address - the address of the explicit route hop.

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:





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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'.

      IPv6 Address - the address of the explicit route hop.

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:









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

   where:

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

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

      Interface ID: 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:







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    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.

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



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      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.

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



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      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'.

      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 section 4 of
   [I-D.filsfils-spring-segment-routing-use-cases].  The Adj-SID Sub-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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Flags         |     Weight    |             Reserved          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   SID/Label/Index (variable)                  |
   +---------------------------------------------------------------+

   where:

      Type: TBD, suggested value 5.

      Length: variable.

      Flags. 1 octet field of following flags:

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

   where:

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

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

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

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

         P-Flag.  Persistent flag.  When set, the P-Flag indicates that
         the Adj-SID is persistently allocated, i.e., the Adj-SID value
         remains consistent across router restart and/or interface flap.





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         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 section 3.5.1 of
      [I-D.ietf-spring-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 NOT be set if the IPv6 address is globally
            unique.

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

   An SR capable router MAY allocate more than one Adj-SID to an
   adjacency

   An SR capable router MAY allocate the same Adj-SID to different
   adjacencies

   When the P-flag is not set, the Adj-SID MAY be persistent.  When the
   P-flag is set, the Adj-SID MUST be persistent.

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.







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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     |     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|G|P|     |
   +-+-+-+-+-+-+-+-+

   where:

         B-Flag: Backup-flag: set if the LAN-Adj-SID refer to an
         adjacency that is eligible for protection (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 LAN Adj-SID
         carries an absolute value.  If not set, then the LAN Adj-SID
         carries an index.

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

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

         P-Flag.  Persistent flag.  When set, the P-Flag indicates that
         the Adj-SID is persistently allocated, i.e., the Adj-SID value
         remains consistent across router restart and/or interface flap.



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         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 section 3.5.1 of
      [I-D.ietf-spring-segment-routing].

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

      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 NOT be set if the IPv6 address is globally
            unique.

      When the P-flag is not set, the Adj-SID MAY be persistent.  When
      the P-flag is set, the Adj-SID MUST be persistent.

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-spring-segment-routing-ldp-interop]).  The
   Mapping Server advertises Prefix-SID for remote prefixes that exist
   in the network.  Multiple Mapping Servers can advertise Prefix-SID
   for the same prefix, in which case the same Prefix-SID MUST be



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   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.

   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.

   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.

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:



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      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 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 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.





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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 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

   o 12 - SR Local Block Sub-TLV

   o 13 - SRMS Preference Sub-TLV






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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

   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 definition of the "Binding / MPLS Label TLV".




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12.  References

12.1.  Normative References

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

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

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

   [RFC4970]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 4970, DOI 10.17487/RFC4970, July
              2007, <https://www.rfc-editor.org/info/rfc4970>.

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

12.2.  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.





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   [I-D.ietf-ospf-ospfv3-lsa-extend]
              Lindem, A., Roy, A., Goethals, D., Vallem, V., and F.
              Baker, "OSPFv3 LSA Extendibility", draft-ietf-ospf-ospfv3-
              lsa-extend-14 (work in progress), April 2017.

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

   [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-01 (work in progress), February
              2015.

   [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.

   [I-D.previdi-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
              J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment
              Routing Header (SRH)", draft-previdi-6man-segment-routing-
              header-08 (work in progress), October 2015.

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




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   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Hannes Gredler
   RtBrick Inc.
   Austria

   Email: hannes@rtbrick.com


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

   Email: robjs@google.com


   Wim Henderickx
   Nokia
   Copernicuslaan 50
   Antwerp  2018
   BE

   Email: wim.henderickx@nokia.com


   Jeff Tantsura
   Individual
   US

   Email: jefftant.ietf@gmail.com













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