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Versions: 00 01 02 03 04 05 draft-ietf-ospf-segment-routing-extensions

Network Working Group                                     P. Psenak, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             C. Filsfils
Expires: December 30, 2013                           Cisco Systems, Inc.
                                                           June 28, 2013


                  OSPF Extensions for Segment Routing
            draft-psenak-ospf-segment-routing-extensions-00

Abstract

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

   This draft describes the necessary OSPF extensions that need to be
   introduced 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."

   This Internet-Draft will expire on December 30, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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.  OSPFv2 Segment Routing Capability TLV  . . . . . . . . . . . .  3
     2.1.  SID Range Sub-TLV  . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Segment Routing Mirroring Context Sub-TLV (SRMC) . . . . .  5
   3.  Segment Routing Identifiers  . . . . . . . . . . . . . . . . .  6
     3.1.  OSPFv2 Extended Prefix Opaque LSA type . . . . . . . . . .  7
       3.1.1.  OSPF Extended Prefix TLV . . . . . . . . . . . . . . .  8
       3.1.2.  Prefix SID Sub-TLV . . . . . . . . . . . . . . . . . .  8
     3.2.  Adjacency Segment Identifier (Adj-SID) . . . . . . . . . . 10
       3.2.1.  OSPFv2 Extended Link Opaque LSA  . . . . . . . . . . . 10
       3.2.2.  OSPFv2 Extended Link TLV . . . . . . . . . . . . . . . 11
       3.2.3.  Adj-SID sub-TLV  . . . . . . . . . . . . . . . . . . . 12
       3.2.4.  Explicit Path sub-TLV  . . . . . . . . . . . . . . . . 14
   4.  Elements of Procedure  . . . . . . . . . . . . . . . . . . . . 15
     4.1.  Intra-area Segment routing in OSPFv2 . . . . . . . . . . . 15
     4.2.  Inter-area Segment routing in OSPFv2 . . . . . . . . . . . 16
     4.3.  SID for External Prefixes  . . . . . . . . . . . . . . . . 17
     4.4.  Advertisement of Adj-SID . . . . . . . . . . . . . . . . . 17
       4.4.1.  Advertisement of Adj-SID on Point-to-Point Links . . . 17
       4.4.2.  Adjacency SID on Broadcast Interfaces  . . . . . . . . 17
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   6.  Manageability Considerations . . . . . . . . . . . . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19










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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).  Two
   types of segments are defined, Prefix segments and Adjacency
   segments.  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
   do 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 necessary OSPF extensions that need to be
   introduced for Segment Routing.

   Segment Routing architecture is described in
   [draft-filsfils-rtgwg-segment-routing-00].

   Segment Routing use cases are described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].


2.  OSPFv2 Segment Routing Capability TLV

   Segment Routing requires each router to advertise various
   capabilities associated with Segment Routing throughout the
   autonomous system.

   For the purposes of Segment Routing, a new TLV is defined in Router
   Information Opaque LSA (defined in [RFC4970]: the Segment Routing
   Capability TLV (SR-Cap TLV).

   If the SR-Cap TLV appear in Router Information Opaque LSA, it MUST
   appear only 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               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       SR Capability Flags                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                       Sub-TLVs (variable)                   -+
   |                             ...                               |

   where:

      Type: TBA.

      Length: A 16-bit field that indicates the length of the value
      portion in octets.  Set to 12.

      SR Capabilities Flags: 2 octets field of following flags:
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|X|                                                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

         R-flag: SR Control-Plane flag.  If set, then the advertising
         router is capable of SR control-plane.

         X-Flag: Index flag.  If set, then each SID with the G-Flag set
         that is advertised by this router represents an index in the
         SID space also advertised by this router (using the encodings
         defined below).  Indexed SID values are described in
         [draft-filsfils-rtgwg-segment-routing-00].  If the X-Flag is
         not set, then SID values 0-63 are reserved and MUST NOT be used
         by the SR control plane for either node or adjacency segments.
         Any SID which is received with a value 0-63 and the X-Flag
         unset MUST be ignored and the router SHOULD log an error.

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

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

   Within the SR-Cap TLV, new Sub-TLVs are defined:




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      SID Range Sub-TLV

      Segment Routing Mirroring Context Sub-TLV (SRMC)

2.1.  SID Range Sub-TLV

   The SID Range Sub-TLV allows a router to advertise its SID space to
   other routers in the area.

   The SID Range Sub-TLV is a Sub-TLV of the SR-Cap-TLV.  It MUST appear
   only once 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        First SID Value                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Last  SID Value                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      First SID Value and Last SID Value represent the local SID range
      of the router.  The semantic and procedures of these values are
      described in [draft-filsfils-rtgwg-segment-routing-use-cases-00].

2.2.  Segment Routing Mirroring Context Sub-TLV (SRMC)

   Segment Routing allows the use of a context SID that determines a
   specific use case or applicability.  Use cases are described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].  One of such use
   cases is the service mirroring, for which the new sub-TLV is defined.

   Another example of mirroring use case is described in
   [I-D.minto-rsvp-lsp-egress-fast-protection].

   The SRMC Sub-TLV is an optional sub-TLV of SR-Cap-TLV.  It MAY appear
   in SR-Cap-TLV 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    |          SRMC Flags           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Mirrored Address (4 octets)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Context-SID (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBA.

      Length: 2 octets.

      SRMC Flags: 2 octets field of flags.  Following flags are defined:
    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

         R-flag: Segment Routing Identifier.  If set, the Mirrored
         Address contains a SID.

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

      Mirrored Address: IPv4 address or SID value representing the
      mirrored element.

      Context-SID: 32 bit value of Segment Identifier for the Mirroring
      Service.


3.  Segment Routing Identifiers

   Segment Routing defines two types of Segment Identifiers: Prefix-SID
   and Adjacency-SID.

   For the purpose of the Prefix-SID and Adjacency-SID advertisement new
   Opaque LSAs (defined in [RFC5250]) are defined.  These new LSAs are
   defined as generic containers that can be used in order 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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3.1.  OSPFv2 Extended Prefix Opaque LSA type

   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 a
   single router.  Flooding scope of the OSPFv2 Extended Prefix Opaque
   LSA depends on the content inside the LSA and is in control of the
   originating router.

   The format of the OSPFv2 Extended Prefix 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   |  9, 10, or 11 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Opaque type  |                  Instance                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Advertising Router                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     LS sequence number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         LS checksum           |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                            TLVs                             -+
   |                             ...                               |

   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.






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3.1.1.  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 carried in each OSPFv2 Extended Prefix Opaque LSA,
   however all prefixes included in the single OSPFv2 Extended Prefix
   Opaque LSA MUST have the same flooding scope.  The structure of the
   OSPF Extended Prefix TLV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Route Type   | Prefix Length |     AF        |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Address Prefix (variable)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sub-TLVs (variable)                      |
   +-                                                             -+
   |                                                               |

   Type is TBA.

   Length is variable

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

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

   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.

3.1.2.  Prefix SID Sub-TLV

   The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV.
   It may MAY appear more than once for the same prefix 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             |    MT-ID        |   Reserved  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Prefix SID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type is TBA.

      Length: A 16-bit field that indicates the length of the value
      portion in octets.  Set to 8.

      Flags: 2 octets field.  The following flags are defined:
    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |N|P|G|M|                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 [draft-filsfils-rtgwg-segment-routing-00].

         P-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.

         G-Flag: Global flag.  When set, the SID value has global
         significance which means the SID has been allocated from the
         Segment Routing Global Block (SRGB) as described in
         [draft-filsfils-rtgwg-segment-routing-00].  When unset, the SID
         value has local (within the router) significance which means
         the SID has NOT been allocated from the SRGB.  When the Prefix-
         SID Sub-TLV carries an IGP Prefix SID, then the G-flag MUST be
         set.







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         M-Flag: Mapping Server Flag.  If set, the SID is advertised
         from the Segment Routing Mapping Server functionality as
         described in
         [draft-filsfils-rtgwg-segment-routing-use-cases-00].

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

   Prefix Segment Identifier (SID): 32 bits of Prefix-SID.

   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.

   The No-PHP flag MUST be set on the Prefix-SIDs allocated to inter-
   area prefixes that are originated by the router based on intra-area
   reachability.

3.2.  Adjacency Segment Identifier (Adj-SID)

   An Adjacency Segment Identifier (Adj-SID) represents a router
   adjacency in Segment Routing.  At the current stage of Segment
   Routing architecture it is assumed that the Adj-SID value has local
   significance (to the router).  The Adj-SID value is encoded as a 32
   bit number.

3.2.1.  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:















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

   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...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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.

3.2.2.  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 carried in
   each Extended Link Opaque LSA, allowing for fine granularity changes
   in the topology.

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

   where:

      Type is TBA.

      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].

3.2.3.  Adj-SID sub-TLV

   Adj-SID is an optional Sub-TLV of the Extended Link TLV.  It MAY
   appear multiple times in Extended Link TLV.  Examples where more than
   one Adj-SID may be used per neighbor are described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].  The structure
   of the Adj-SID Sub-TLV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   MT-ID         | Reserved    |            Flags              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Adjacency SID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Neighbor ID   (optional)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Adjacency SID (optional)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Neighbor ID   (optional)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Adjacency SID (optional)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type is TBA.

      Length is variable.

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

      Flags: 2 octets field of following flags:
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|G|S|T|                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

         F-Flag: FA flag.  If set, then Adj-SID refers to a Forwarding
         Adjacency.

         G-Flag: Global flag.  When set, the SID value has global
         significance which means the SID has been allocated from the
         Segment Routing Global Block (SRGB) as described in
         [draft-filsfils-rtgwg-segment-routing-00].  When unset, the SID
         value has local (within the router) significance which means
         the SID has NOT been allocated from the SRGB.  When the Adj-SID
         Sub-TLV carries the IGP Adjacency SID, then the G-Flag MUST be
         unset.



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         S-Flag: Secondary SID.  If set the SID represents the secondary
         SID.  One case where secondary SID may be useful is the
         parallel adjacency case as described in
         [draft-filsfils-rtgwg-segment-routing-00].

         T-Flag: Protected-flag: set if the Adj-SID refer to an
         adjacency being protected (e.g.: using IPFRR or MPLS-FRR) as
         described in
         [draft-filsfils-rtgwg-segment-routing-use-cases-00].

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

      Adj-SID: the 32 bits of Adjacency Segment Identifier.

      Neighbor ID: the Neighbor ID on broadcast segment to which the SID
      belongs.  Only used if more then one SID is advertised in the Adj-
      SID Sub-TLV.

   The first Adjacency SID value in TLV is mandatory and advertises the
   Adjacency Segment Identifier for the neighbor that is identified by
   the Extended Link TLV itself.  Subsequent Neighbor ID/Adjacency SID
   tuples are optional and represent Adj-SIDs for other neighbors on the
   broadcast segment.

   A SR capable router SHOULD allocate an Adj-SID for each of its
   adjacencies and set the T-Flag when the adjacency is protected by a
   FRR mechanism (IP or MPLS) as described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].

3.2.4.  Explicit Path sub-TLV

   Explicit Path Sub-TLV is a Sub-TLV of the Extended Link TLV.  The
   structure of the Explicit Path Sub-TLV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type            |              Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Flags      |    MT-ID      |      Reserved               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Explicit Route Hop #1                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Explicit Route Hop #...                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:




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      Type is TBA.

      Length is variable.

      Flags: 16-bit flag field where following flags are defined:
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |R|             |
   +-+-+-+-+-+-+-+-+

   where:

         R-Flag: Segment Routing flag.  When set, the content of the
         Explicit Route Object contains SIDs.

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

      Explicit Route Hop: the address of the explicit route hop.  IPv4
      and SR hops are encoded using a 32 bit value.


4.  Elements of Procedure

4.1.  Intra-area Segment routing in OSPFv2

   The OSPFv2 node that supports segment routing MAY advertise Prefix-
   SIDs for any prefix that is advertising reachability to (e.g.
   loopback IP address) as described in Section 3.1.  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
   functionality (as described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00]) that advertise
   Prefix-SID for remote prefixes which exist in the network.  Multiple
   SR Mapping Servers can advertise Prefix-SID for the same prefix, in
   which case the same Prefix-SID MUST be advertised by all of them.
   Flooding scope of the OSPF Extended Prefix Opaque LSA that is
   generated by the SR Mapping Server could be either area scope or
   autonomous system scope and is decided based on the configuration of
   the SR Mapping Server.

   Prefix-SID advertisements do not contribute to the prefix
   reachability.





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4.2.  Inter-area Segment routing in OSPFv2

   In order to support SR in a multi-area environment, OSPFv2 must
   propagate SR 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
   route to all its connected areas, it will also originate an Extended
   Prefix Opaque LSA, as described in Section 3.1.  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 it's best path to the prefix in the source
      area and find out 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 [draft-filsfils-rtgwg-segment-routing-use-cases-00])
      when propagating 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 3.1.  The flooding scope
   of the Extended Prefix Opaque LSA type will be set to area-scope.
   The route-type in OSPF Extended Prefix TLV is set to inter-area.  The
   Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID
   will be set as follows:

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

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

      If no Prefix-SID was advertised for the prefix in the source area
      by the ABR that contributes to the best path to the prefix, the
      originating ABR will use the Prefix-SID advertised by any other
      router (e.g.: a Prefix-SID coming from an SR Mapping Server as
      defined in [draft-filsfils-rtgwg-segment-routing-use-cases-00])
      when propagating Prefix-SID for the prefix to other areas.




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4.3.  SID for External Prefixes

   Type-5 LSAs are flooded domain wide.  When an ASBR, which supports
   SR, generates Type-5 LSAs, it should also originate Extended Prefix
   Opaque LSAs, as described in Section 3.1.  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.  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
   [draft-filsfils-rtgwg-segment-routing-use-cases-00]).

4.4.  Advertisement of Adj-SID

   The Adjacency Segment Routing Identifier (Adj-SID) is advertised
   using the Adj-SID Sub-TLV as described in Section 3.2.

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

   The Adj-SID MUST be advertised for any adjacency on a p2p link that
   is in a FULL state.  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 MUST be removed from
   the area.

4.4.2.  Adjacency SID on Broadcast Interfaces

   Broadcast networks in OSPF are represented by a star topology where
   the Designated Router (DR) is the central point all other routers on
   the broadcast network connect to.  As a result, the routers on the
   broadcast 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
   state between them.

   When Segment Routing is used, each router in the broadcast segment
   must advertise the Adj-SID for each of its neighbors.  To satisfy



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   this requirement Adj-SID on broadcast networks are advertised as
   follows:

   If the router is acting as DR on a broadcast segment, the Adj-SID
   MUST be advertised for each adjacency in a FULL state and MAY be
   advertised for any adjacency in a state 2-way or higher.  When the DR
   advertises the Adj-SID Sub-TLV for the broadcast link, the first Adj-
   SID in the Adj-SID sub-TLV corresponds to the adjacency of DR to
   itself and as such MUST be set to zero and ignored.

   If the router is acting as BDR or as DR-Other, then the Adj-SID MUST
   be advertised for each adjacency in a state 2-Way or higher.


5.  IANA Considerations

   TBD


6.  Manageability Considerations

   TBD


7.  Security Considerations

   TBD


8.  Acknowledgements

   We would like to thank Anton Smirnov for his contribution to the
   content of this document.


9.  References

9.1.  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.

   [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.

9.2.  Informative References

   [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-02 (work in
              progress), April 2013.

   [draft-filsfils-rtgwg-segment-routing-00]
              Previdi, S. and C. Filsfils, "Segment Routing", May 2013.

   [draft-filsfils-rtgwg-segment-routing-use-cases-00]
              Filsfils, C., "Segment Routing Use Cases", May 2013.


Authors' Addresses

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

   Email: ppsenak@cisco.com


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

   Email: sprevidi@cisco.com






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   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels,
   Belgium

   Email: cfilsfil@cisco.com













































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