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Versions: (draft-psenak-ospf-segment-routing-extensions) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Open Shortest Path First IGP                              P. Psenak, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             C. Filsfils
Expires: January 4, 2015                             Cisco Systems, Inc.
                                                              H. Gredler
                                                  Juniper Networks, Inc.
                                                               R. Shakir
                                                         British Telecom
                                                           W. Henderickx
                                                          Alcatel-Lucent
                                                             J. Tantsura
                                                                Ericsson
                                                            July 3, 2014


                  OSPF Extensions for Segment Routing
             draft-ietf-ospf-segment-routing-extensions-01

Abstract

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

   This draft describes the OSPF extensions required for Segment
   Routing.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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

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

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



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   This Internet-Draft will expire on January 4, 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 . . . . . . . . . . . . . . . . . . . .   4
   3.  Segment Routing Capabilities  . . . . . . . . . . . . . . . .   4
     3.1.  SR-Algorithm TLV  . . . . . . . . . . . . . . . . . . . .   4
     3.2.  SID/Label Range TLV . . . . . . . . . . . . . . . . . . .   5
   4.  OSPFv2 Extended Prefix Opaque LSA . . . . . . . . . . . . . .   7
     4.1.  OSPF Extended Prefix TLV  . . . . . . . . . . . . . . . .   8
     4.2.  Prefix SID Sub-TLV  . . . . . . . . . . . . . . . . . . .   9
     4.3.  SID/Label Binding sub-TLV . . . . . . . . . . . . . . . .  13
       4.3.1.  ERO Metric sub-TLV  . . . . . . . . . . . . . . . . .  15
       4.3.2.  ERO sub-TLVs  . . . . . . . . . . . . . . . . . . . .  15
   5.  Adjacency Segment Identifier (Adj-SID)  . . . . . . . . . . .  20
     5.1.  OSPFv2 Extended Link Opaque LSA . . . . . . . . . . . . .  20
     5.2.  OSPFv2 Extended Link TLV  . . . . . . . . . . . . . . . .  21
     5.3.  Adj-SID sub-TLV . . . . . . . . . . . . . . . . . . . . .  21
     5.4.  LAN Adj-SID Sub-TLV . . . . . . . . . . . . . . . . . . .  23
   6.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .  24
     6.1.  Intra-area Segment routing in OSPFv2  . . . . . . . . . .  24
     6.2.  Inter-area Segment routing in OSPFv2  . . . . . . . . . .  25
     6.3.  SID for External Prefixes . . . . . . . . . . . . . . . .  26
     6.4.  Advertisement of Adj-SID  . . . . . . . . . . . . . . . .  26
       6.4.1.  Advertisement of Adj-SID on Point-to-Point Links  . .  26
       6.4.2.  Adjacency SID on Broadcast or NBMA Interfaces . . . .  27
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     7.1.  OSPF Extend Prefix LSA TLV Registry . . . . . . . . . . .  27
     7.2.  OSPF Extend Prefix LSA sub-TLV Registry . . . . . . . . .  28
     7.3.  OSPF Extend Link LSA TLV Registry . . . . . . . . . . . .  29
     7.4.  OSPF Extend Link LSA sub-TLV Registry . . . . . . . . . .  29



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   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  30
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     11.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

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

   This draft describes the OSPF extensions required for Segment
   Routing.

   Segment Routing architecture is described in
   [I-D.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.

   For the purpose of the advertisements of various SID values, new
   Opaque LSAs (defined in [RFC5250]) are defined.  These new LSAs are
   defined as generic containers that can be used to advertise any
   additional attributes associated with the prefix or link.  These new
   Opaque LSAs are complementary to the existing LSAs and are not aimed
   to replace any of the existing LSAs.






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2.1.  SID/Label sub-TLV

   SID/Label sub-TLV appears in multiple TLVs or sub-TLVs defined later
   in this document.  It is used to advertise SID or label associated
   with the prefix or adjacency.  SID/Label TLV has following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SID/Label (variable)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 1

      Length: variable, 3 or 4 bytes

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

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

3.  Segment Routing Capabilities

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

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

3.1.  SR-Algorithm TLV

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

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

   As SR Router may use various algorithms when calculating reachability
   to OSPF routers or prefixes in an OSPF area.  Examples of these
   algorithms are metric based Shortest Path First (SPF), various



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   flavors of Constrained SPF, etc.  The SR-Algorithm TLV allows a
   router to advertise the algorithms that the router is currently using
   to other routers in an OSPF area.  The SR-Algorithm TLV has following
   format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Algorithm 1 | Algorithm...  |   Algorithm n |               |
   +-                                                             -+
   |                                                               |
   +                                                               +

   where:

      Type: TBD, suggested value 8

      Length: variable

      Algorithm: Single octet identifying the algorithm.  The following
      value is defined by this document:

         0: IGP metric based Shortest Path Tree (SPT)

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

3.2.  SID/Label Range TLV

   The SID/Label Range TLV is a top-level TLV of Router Information
   Opaque LSA (defined in [RFC4970]).

   The SID/Label Range TLV MAY appear multiple times and has the
   following format:














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

   where:

      Type: TBD, suggested value 9

      Length: variable

      Range Size: 3 octet of the SID/label range

   Currently the only supported Sub-TLV is the SID/Label TLV as defined
   in Section 2.1.  The SID/Label advertised in 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 in which order the set of SID/Label
      Range TLVs are advertised inside Router Information Opaque LSA.
      The originating router MUST ensure the order is same after a
      graceful restart (using checkpointing, non-volatile storage or any
      other mechanism) in order to SID/label range and SID index
      correspondence is preserved across graceful restarts.

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

   The following example illustrates the advertisement of multiple
   ranges:









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

      The receiving routers concatenate the ranges and build the SRGB
      is as follows:

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

      The indexes span multiple ranges:

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

   The RI LSA can be advertised at any of the defined flooding scopes
   (link, area, or autonomous system (AS)).  For the purposes of the SR-
   Capability TLV propagation, area scope flooding is required.

4.  OSPFv2 Extended Prefix Opaque LSA

   A new Opaque LSA (defined in [RFC5250]) is defined in OSPFv2 in order
   to advertise additional prefix attributes: OSPFv2 Extended Prefix
   Opaque LSA.

   Multiple OSPFv2 Extended Prefix Opaque LSAs can be advertised by an
   OSPFv2 router.  The flooding scope of the OSPFv2 Extended Prefix
   Opaque LSA depends on the scope of the advertised prefixes and is
   under the control of the advertising router.  In some cases (e.g.,
   mapping server deployment), the LSA flooding scope may be greater
   than the scope of the corresponding prefixes.

   The format of the OSPFv2 Extended Prefix Opaque LSA is as follows:










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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            LS age             |     Options   |  9, 10, or 11 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opaque type  |                  Instance                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Advertising Router                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     LS sequence number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         LS checksum           |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                            TLVs                             -+
   |                             ...                               |

   The opaque type used by OSPFv2 Extended Prefix Opaque LSA is 7.

   The format of the TLVs within the body of the LSA is the same as the
   format used by the Traffic Engineering Extensions to OSPF defined in
   [RFC3630].  The LSA payload consists of one or more nested
   Type/Length/Value (TLV) triplets.  The format of each TLV is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Value...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Length field defines the length of the value portion in octets.
   The TLV is padded to 4-octet alignment; padding is not included in
   the length field.  Nested TLVs are also 32-bit aligned.  Unrecognized
   types are ignored.

4.1.  OSPF Extended Prefix TLV

   The OSPF Extended Prefix TLV is used in order to advertise additional
   attributes associated with the prefix.  Multiple OSPF Extended Prefix
   TLVs MAY be advertised in each OSPFv2 Extended Prefix Opaque LSA but
   all prefixes included in a single OSPFv2 Extended Prefix Opaque LSA
   MUST have the same flooding scope.  The OSPF Extended Prefix 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Route Type   | Prefix Length |     AF        |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Address Prefix (variable)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sub-TLVs (variable)                      |
   +-                                                             -+
   |                                                               |

   where:

      Type: TBD, suggested value 1.

      Length: variable

      Route type: type of the OSPF route.  Supported types are:

                   0 - unspecified
                   1 - intra-area
                   3 - inter-area
                   5 - external
                   7 - NSSA external

      If the route type is 0 (unspecified), the information inside the
      OSPF External Prefix TLV applies to the prefix regardless of
      prefix's route-type.  This is useful when some prefix specific
      attributes are advertised by some external entity, which is not
      aware of the route-type associated with the prefix.

      Prefix length: length of the prefix

      AF: 0 - IPv4 unicast

      Address Prefix: the prefix itself 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 default
      route is represented by a prefix of length 0.

4.2.  Prefix SID Sub-TLV

   The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV.
   It MAY appear more than once and has following format:





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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Flags    |   Reserved    |      MT-ID    |    Algorithm  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Range Size           |            Reserved           +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     SID/Index/Label (variable)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 2.

      Length: variable

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


     0  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |N |NP|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 on Prefix-SIDs attached 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].

         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.





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         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 PrefixSID has local significance.  If not set,
         then the value/index carried by this subTLV has global
         significance.

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

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

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

      Range size: this field provides the ability to specify a range of
      addresses and their associated Prefix SIDs.  It represents a
      compression scheme to distribute a continuous Prefix and their
      continuous, corresponding SID/Label Block.  If a single SID is
      advertised then the Range Size field MUST be set to one.  For
      range advertisements > 1, Range Size represents the number of
      addresses that need to be mapped into a Prefix-SID.

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

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

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

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

   When propagating Prefix-SIDs between areas, if multiple prefix-SIDs
   are advertised for a prefix, an implementation SHOULD preserve the
   original ordering, when advertising prefix-SIDs to other areas.  This
   allows implementations that only use 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 next-hop router contributing to the best path to the
   prefix or not.



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

   NP-flag (no-PHP) MUST NOT 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 to the
   ultimate hop (the Prefix-SID being removed).  If the NP-flag is clear
   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 PrefixSID 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.

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

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

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



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

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

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

4.3.  SID/Label Binding sub-TLV

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

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

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

   The SID/Label Binding sub-TLV is a sub-TLV of the OSPF Extended
   Prefix TLV.  Multiple SID/Label Binding TLVs can be present in OSPF
   Extended Prefix TLV.  The SID/Label Binding sub-TLV has following
   format:









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

   where:

      Type: TBD, suggested value 3

      Length: variable

      Flags: 1 octet field of following flags:

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

      where:

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

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

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

      Range Size: usage is the same as described in Section 4.2.

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

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

      ERO Metric sub-TLV as defined in Section 4.3.1.

      ERO sub-TLVs as defined in Section 4.3.2.



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4.3.1.  ERO Metric sub-TLV

   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.  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: 4 bytes

      Metric: 4 bytes

4.3.2.  ERO sub-TLVs

   All 'ERO' information represents an ordered set which describes the
   segments of a path.  The last ERO sub-TLV describes the segment
   closest to the egress point, contrary the first ERO sub-TLV describes
   the first segment of a path.  If a router extends or stitches a path
   it MUST prepend the new segments path information to the ERO list.

   The above similarly applies to backup EROs.

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

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








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4.3.2.1.  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 4

      Length: 8 bytes

      Flags: 1 octet field of following flags:

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

      where:

         L-bit - If the L bit is set, then the value of the attribute is
         'loose.'  Otherwise, the value of the attribute is 'strict.'

      IPv4 Address - the address of the explicit route hop.

4.3.2.2.  Unnumbered Interface ID ERO sub-TLV

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

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





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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Router ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Interface ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

                Unnumbered Interface ID ERO sub-TLV format

      Type: TBD, suggested value 5

      Length: 12 bytes

      Flags: 1 octet field of following flags:

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

   where:

         L-bit - If the L bit is set, then the value of the attribute is
         'loose.'  Otherwise, the value of the attribute is '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.








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4.3.2.3.  IPv4 Backup ERO sub-TLV

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     IPv4 Address (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      IPv4 Backup ERO sub-TLV format

   where:

      Type: TBD, suggested value 6

      Length: 8 bytes

      Flags: 1 octet field of following flags:

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

      where:

         L-bit - If the L bit is set, then the value of the attribute is
         'loose.'  Otherwise, the value of the attribute is 'strict.'

      IPv4 Address - the address of the explicit route hop.

4.3.2.4.  Unnumbered Interface ID Backup ERO sub-TLV

   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].



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   The Unnumbered Interface-ID ERO sub-TLV describes a path segment that
   includes an unnumbered interface.  Unnumbered interfaces are
   referenced using the interface index.  Interface indices are assigned
   local to the router and therefore 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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Unnumbered Interface ID Backup ERO sub-TLV format

   where:

      Type: TBD, suggested value 7

      Length: 12 bytes

      Flags: 1 octet field of following flags:

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

      where:

         L-bit - If the L bit is set, then the value of the attribute is
         'loose.'  Otherwise, the value of the attribute is '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.








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5.  Adjacency Segment Identifier (Adj-SID)

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

5.1.  OSPFv2 Extended Link Opaque LSA

   A new Opaque LSA (defined in [RFC5250] is defined in OSPFv2 in order
   to advertise additional link attributes: the OSPFv2 Extended Link
   Opaque LSA.

   The OSPFv2 Extended Link Opaque LSA has an area flooding scope.
   Multiple OSPFv2 Extended Link Opaque LSAs can be advertised by a
   single router in an area.

   The format of the OSPFv2 Extended Link Opaque LSA is as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            LS age             |     Options   |       10      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opaque type  |                   Instance                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Advertising Router                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     LS sequence number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         LS checksum           |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                            TLVs                             -+
   |                             ...                               |

   Opaque type used by OSPFv2 Extended Link Opaque LSA is 8.

   The format of the TLVs within the body of LSA is the same as the
   format used by the Traffic Engineering Extensions to OSPF defined in
   [RFC3630].  The LSA payload consists of one or more nested
   Type/Length/Value (TLV) triplets.  The format of each TLV is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Value...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The Length field defines the length of the value portion in octets.
   The TLV is padded to 4-octet alignment; padding is not included in
   the length field.  Nested TLVs are also 32-bit aligned.  Unrecognized
   types are ignored.

5.2.  OSPFv2 Extended Link TLV

   OSPFv2 Extended Link TLV is used in order to advertise various
   attributes of the link.  It describes a single link and is
   constructed of a set of Sub-TLVs.  There are no ordering requirements
   for the Sub-TLVs.  Only one Extended Link TLV SHALL be advertised in
   each Extended Link Opaque LSA, allowing for fine granularity changes
   in the topology.

   The Extended Link 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Link-Type   |                Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Link ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Link Data                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sub-TLVs (variable)                      |
   +-                                                             -+
   |                                                               |

   where:

      Type is 1.

      Length is variable.

      Link-Type: as defined in section A.4.2 of [RFC2328].

      Link-ID: as defined in section A.4.2 of [RFC2328].

      Link Data: as defined in section A.4.2 of [RFC2328].

5.3.  Adj-SID sub-TLV

   Adj-SID is an optional sub-TLV of the Extended Link TLV.  It MAY
   appear multiple times in the Extended Link TLV.  Examples where more
   than one Adj-SID may be used per neighbor are described in



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

   where:

      Type: TBD, suggested value 2.

      Length: variable.

      Flags. 1 octet field of following flags:

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

   where:

         B-Flag: Backup-flag: set if 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 PrefixSID 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: MUST be zero when originated and ignored when
         received.




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

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

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

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

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

   An SR capable router MAY allocate an Adj-SID for each of its
   adjacencies and set the B-Flag when the adjacency is protected by an
   FRR mechanism (IP or MPLS) as described in
   [I-D.filsfils-rtgwg-segment-routing-use-cases].

5.4.  LAN Adj-SID Sub-TLV

   LAN Adj-SID is an optional sub-TLV of the Extended Link TLV.  It MAY
   appear multiple times in Extended Link TLV.  It is used to advertise
   SID/Label for adjacency to non-DR node on 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     |    Reserved   |     MT-ID     |    Weight     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Neighbor ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    SID/Label/Index (variable)                 |
   +---------------------------------------------------------------+

   where:

      Type: TBD, suggested value 3.

      Length: variable.

      Flags. 1 octet field of following flags:







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    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.

         The L-Flag: Local/Global Flag.  If set, then the value/index
         carried by the PrefixSID 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: MUST be zero when originated and ignored when
         received.

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

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

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

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

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

6.  Elements of Procedure

6.1.  Intra-area Segment routing in OSPFv2

   The OSPFv2 node that supports segment routing MAY advertise Prefix-
   SIDs for any prefix to which it is advertising reachability (e.g., a
   loopback IP address as described in Section 4.2).



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

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

6.2.  Inter-area Segment routing in OSPFv2

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

   When an OSPF ABR advertises a Type-3 Summary LSA from an intra-area
   prefix to all its connected areas, it will also originate an Extended
   Prefix Opaque LSA, as described in Section 4.  The flooding scope of
   the Extended Prefix Opaque LSA type will be set to area-scope.  The
   route-type in OSPF Extended Prefix TLV is set to inter-area.  The
   Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID
   value will be set as follows:

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

      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,
      then the 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 the Prefix-SID for the prefix to other areas.

   When an OSPF ABR advertises Type-3 Summary LSAs from an inter-area
   route to all its connected areas it will also originate an Extended
   Prefix Opaque LSA, as described in Section 4.  The flooding scope of
   the Extended Prefix Opaque LSA type will be set to area-scope.  The
   route-type in OSPF Extended Prefix TLV is set to inter-area.  The
   Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID
   will be set as follows:




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

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

      If no Prefix-SID was advertised for the prefix in the source area
      by the ABR that contributes to the best path to the prefix, the
      originating ABR will use the Prefix-SID advertised by any other
      router (e.g.: a Prefix-SID coming from an SR Mapping Server as
      defined in [I-D.filsfils-rtgwg-segment-routing-use-cases]) when
      propagating the Prefix-SID for the prefix to other areas.

6.3.  SID for External Prefixes

   Type-5 LSAs are flooded domain wide.  When an ASBR, which supports
   SR, generates Type-5 LSAs, it should also originate an Extended
   Prefix Opaque LSAs, as described in Section 4.  The flooding scope of
   the Extended Prefix Opaque LSA type is set to AS-scope.  The route-
   type in OSPF Extended Prefix TLV is set to external.  The Prefix-SID
   sub-TLV is included in this LSA and the Prefix-SID value will be set
   to the SID that has been reserved for that prefix.

   When a NSSA ASBR translates Type-7 LSAs into Type-5 LSAs, it should
   also advertise the Prefix-SID for the prefix.  The NSSA ABR
   determines its best path to the prefix advertised in the translated
   Type-7 LSA and finds the advertising router associated with such
   path.  If such advertising router has advertised a Prefix-SID, for
   the prefix, then the NSSA ASBR uses it when advertising the Prefix-
   SID for the Type-5 prefix.  Otherwise, the Prefix-SID advertised by
   any other router will be used (e.g.: a Prefix-SID coming from an SR
   Mapping Server as defined in
   [I-D.filsfils-rtgwg-segment-routing-use-cases]).

6.4.  Advertisement of Adj-SID

   The Adjacency Segment Routing Identifier (Adj-SID) is advertised
   using the Adj-SID Sub-TLV as described in Section 5.

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

   Adj-SID MAY be advertised for any adjacency on a p2p link that is in
   neighbor state 2-Way or higher.  If the adjacency on a p2p link
   transitions from the FULL state, then the Adj-SID for that adjacency
   MAY be removed from the area.  If the adjacency transitions to a




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   state lower then 2-Way, then the Adj-SID MUST be removed from the
   area.

6.4.2.  Adjacency SID on Broadcast or NBMA Interfaces

   Broadcast or NBMA networks in OSPF are represented by a star topology
   where the Designated Router (DR) is the central point all other
   routers on the broadcast or NBMA network connect to.  As a result,
   routers on the broadcast or NBMA network advertise only their
   adjacency to DR and BDR.  Routers that are neither DR nor BDR do not
   form and do not advertise adjacencies between them.  They, however,
   maintain a 2-Way adjacency state between them.

   When Segment Routing is used, each router on the broadcast or NBMA
   network MAY advertise the Adj-SID for its adjacency to DR using Adj-
   SID Sub-TLV as described in Section 5.3.

   SR capable router MAY also advertise Adj-SID for other neighbors
   (e.g.  BDR, DR-OTHER) on broadcast or NBMA network using the LAN ADJ-
   SID Sub-TLV as described in section 5.1.1.2.  Section 5.4.

7.  IANA Considerations

   This specification updates two existing OSPF registries.

   Opaque Link-State Advertisements (LSA) Option Types:

   o suggested value 7 - OSPFv2 Extended Prefix Opaque LSA

   o suggested value 8 - OSPFv2 Extended Link Opaque LSA

   OSPF Router Information (RI) TLVs:

   o suggested value 8 - SR-Algorithm TLV

   o suggested value 9 - SID/Label Range TLV

   This specification also creates four new registries:

   - OSPF Extended Prefix LSA TLVs and sub-TLVs

   - OSPF Extended Link LSA TLVs and sub-TLVs

7.1.  OSPF Extend Prefix LSA TLV Registry

   The OSPF Extend Prefix LSA TLV registry will define top-level TLVs
   for Extended Prefix LSAs and should be placed in the existing OSPF




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   IANA registry.  New values can be allocated via IETF Consensus or
   IESG Approval.

   Following initial values are allocated:

   o 0 - Reserved

   o 1 - OSPF Extended Prefix TLV

   Types in the range 32768-32023 are for experimental use; these will
   not be registered with IANA, and MUST NOT be mentioned by RFCs.

   Types in the range 32023-65535 are not to be assigned at this time.
   Before any assignments can be made in this range, there MUST be a
   Standards Track RFC that specifies IANA Considerations that covers
   the range being assigned.

7.2.  OSPF Extend Prefix LSA sub-TLV Registry

   The OSPF Extended Prefix sub-TLV registry will define will define
   sub-TLVs at any level of nesting for Extended Prefix LSAs and should
   be placed in the existing OSPF IANA registry.  New values can be
   allocated via IETF Consensus or IESG Approval.

   Following initial values are allocated:

   o 0 - Reserved

   o 1 - SID/Label sub-TLV

   o 2 - Prefix SID sub-TLV

   o 3 - SID/Label Binding sub-TLV

   o 4 - IPv4 ERO sub-TLV

   o 5 - Unnumbered Interface ID ERO sub-TLV

   o 6 - IPv4 Backup ERO sub-TLV

   o 7 - Unnumbered Interface ID Backup ERO sub-TLV

   o 8 - ERO Metric sub-TLV

   Types in the range 32768-32023 are for experimental use; these will
   not be registered with IANA, and MUST NOT be mentioned by RFCs.





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   Types in the range 32023-65535 are not to be assigned at this time.
   Before any assignments can be made in this range, there MUST be a
   Standards Track RFC that specifies IANA Considerations that covers
   the range being assigned.

7.3.  OSPF Extend Link LSA TLV Registry

   The OSPF Extend Link LSA TLV registry will define top-level TLVs for
   Extended Link LSAs and should be placed in the existing OSPF IANA
   registry.  New values can be allocated via IETF Consensus or IESG
   Approval.

   Following initial values are allocated:

   o 0 - Reserved

   o 1 - OSPFv2 Extended Link TLV

   Types in the range 32768-32023 are for experimental use; these will
   not be registered with IANA, and MUST NOT be mentioned by RFCs.

   Types in the range 32023-65535 are not to be assigned at this time.
   Before any assignments can be made in this range, there MUST be a
   Standards Track RFC that specifies IANA Considerations that covers
   the range being assigned.

7.4.  OSPF Extend Link LSA sub-TLV Registry

   The OSPF Extended Link LSA sub-TLV registry will define will define
   sub-TLVs at any level of nesting for Extended Link LSAs and should be
   placed in the existing OSPF IANA registry.  New values can be
   allocated via IETF Consensus or IESG Approval.

   Following initial values are allocated:

   o 1 - SID/Label sub-TLV

   o 2 - Adj-SID sub-TLV

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

   Types in the range 32768-32023 are for experimental use; these will
   not be registered with IANA, and MUST NOT be mentioned by RFCs.

   Types in the range 32023-65535 are not to be assigned at this time.
   Before any assignments can be made in this range, there MUST be a
   Standards Track RFC that specifies IANA Considerations that covers
   the range being assigned.



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8.  Security Considerations

   In general, new LSAs defined in this document are subject to the same
   security concerns as those described in [RFC2328].  Additionally,
   implementations must assure that malformed TLV and Sub-TLV
   permutations do not result in errors which cause hard OSPF failures.

9.  Contributors

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

10.  Acknowledgements

   We would like to thank Anton Smirnov for his contribution.

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

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

11.  References

11.1.  Normative References

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

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

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

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

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






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

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

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

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

Authors' Addresses

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

   Email: ppsenak@cisco.com



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   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


   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerp  2018
   BE

   Email: wim.henderickx@alcatel-lucent.com










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

   Email: Jeff.Tantsura@ericsson.com












































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