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Versions: (draft-previdi-isis-segment-routing-extensions) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19

IS-IS for IP Internets                                   S. Previdi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                             A. Bashandy
Expires: April 28, 2015                              Cisco Systems, Inc.
                                                              H. Gredler
                                                  Juniper Networks, Inc.
                                                            S. Litkowski
                                                             B. Decraene
                                                                  Orange
                                                             J. Tantsura
                                                                Ericsson
                                                        October 25, 2014


                  IS-IS Extensions for Segment Routing
             draft-ietf-isis-segment-routing-extensions-03

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 IS-IS 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."




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   This Internet-Draft will expire on April 28, 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.  Prefix Segment Identifier (Prefix-SID Sub-TLV)  . . . . .   4
       2.1.1.  Flags . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.1.2.  Prefix-SID Propagation  . . . . . . . . . . . . . . .   8
     2.2.  Adjacency Segment Identifier  . . . . . . . . . . . . . .   8
       2.2.1.  Adjacency Segment Identifier (Adj-SID) Sub-TLV  . . .   9
       2.2.2.  Adjacency Segment Identifiers in LANs . . . . . . . .  11
     2.3.  SID/Label Sub-TLV . . . . . . . . . . . . . . . . . . . .  13
     2.4.  SID/Label Binding TLV . . . . . . . . . . . . . . . . . .  13
       2.4.1.  Flags . . . . . . . . . . . . . . . . . . . . . . . .  15
       2.4.2.  Weight  . . . . . . . . . . . . . . . . . . . . . . .  16
       2.4.3.  Range . . . . . . . . . . . . . . . . . . . . . . . .  16
       2.4.4.  Prefix Length, Prefix . . . . . . . . . . . . . . . .  17
       2.4.5.  Mapping Server Prefix-SID . . . . . . . . . . . . . .  18
       2.4.6.  SID/Label Sub-TLV . . . . . . . . . . . . . . . . . .  18
       2.4.7.  ERO Metric sub-TLV  . . . . . . . . . . . . . . . . .  18
       2.4.8.  IPv4 ERO subTLV . . . . . . . . . . . . . . . . . . .  19
       2.4.9.  IPv6 ERO subTLV . . . . . . . . . . . . . . . . . . .  19
       2.4.10. Unnumbered Interface ID ERO subTLV  . . . . . . . . .  20
       2.4.11. IPv4 Backup ERO subTLV  . . . . . . . . . . . . . . .  21
       2.4.12. IPv6 Backup ERO subTLV  . . . . . . . . . . . . . . .  21
       2.4.13. Unnumbered Interface ID Backup ERO subTLV . . . . . .  22
       2.4.14. Prefix ERO and Prefix Backup ERO subTLV path
               semantics . . . . . . . . . . . . . . . . . . . . . .  23
   3.  Router Capabilities . . . . . . . . . . . . . . . . . . . . .  23
     3.1.  SR-Capabilities Sub-TLV . . . . . . . . . . . . . . . . .  23
     3.2.  SR-Algorithm Sub-TLV  . . . . . . . . . . . . . . . . . .  26
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27



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     4.1.  Sub TLVs for Type 22,23,222 and 223 . . . . . . . . . . .  27
     4.2.  Sub TLVs for Type 135,235,236 and 237 . . . . . . . . . .  27
     4.3.  Sub TLVs for Type 242 . . . . . . . . . . . . . . . . . .  28
     4.4.  New TLV Codepoint and Sub-TLV registry  . . . . . . . . .  28
   5.  Manageability Considerations  . . . . . . . . . . . . . . . .  30
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  30
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

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, 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 IS-IS extensions that need to be
   introduced for Segment Routing.

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

   Segment Routing use cases are described in
   [I-D.filsfils-spring-segment-routing-use-cases].

2.  Segment Routing Identifiers

   Segment Routing architecture ([I-D.filsfils-spring-segment-routing])
   defines different types of Segment Identifiers (SID).  This document
   defines the IS-IS encodings for the IGP-Prefix-SID, the IGP-
   Adjacency-SID, the IGP-LAN-Adjacency-SID and the Binding-SID.





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2.1.  Prefix Segment Identifier (Prefix-SID Sub-TLV)

   A new IS-IS sub-TLV is defined: the Prefix Segment Identifier sub-TLV
   (Prefix-SID sub-TLV).

   The Prefix-SID sub-TLV carries the Segment Routing IGP-Prefix-SID as
   defined in [I-D.filsfils-spring-segment-routing].  The 'Prefix SID'
   MUST be unique within a given IGP domain (when the L-flag is not
   set).  The 'Prefix SID' MUST carry an index (when the V-flag is not
   set) that determines the actual SID/label value inside the set of all
   advertised SID/label ranges of a given router.  A receiving router
   uses the index to determine the actual SID/label value in order to
   construct forwarding state to a particular destination router.

   In many use-cases a 'stable transport' IP Address is overloaded as an
   identifier of a given node.  Because the IP Prefixes may be re-
   advertised into other levels there may be some ambiguity (e.g.
   Originating router vs. L1L2 router) for which node a particular IP
   prefix serves as identifier.  The Prefix-SID sub-TLV contains the
   necessary flags to disambiguate IP Prefix to node mappings.
   Furthermore if a given node has several 'stable transport' IP
   addresses there are flags to differentiate those among other IP
   Prefixes advertised from a given node.

   A Prefix-SID sub-TLV is associated to a prefix advertised by a node
   and MAY be present in any of the following TLVs:

      TLV-135 (IPv4) defined in [RFC5305].

      TLV-235 (MT-IPv4) defined in [RFC5120].

      TLV-236 (IPv6) defined in [RFC5308].

      TLV-237 (MT-IPv6) defined in [RFC5120].

      Binding-TLV defined in Section 2.4.

   When the IP Reachability TLV is propagated across level boundaries,
   the Prefix-SID sub-TLV SHOULD be kept.

   The Prefix-SID sub-TLV has the following format:










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

   where:

      Type: TBD, suggested value 3

      Length: variable.

      Flags: 1 octet field of following flags:

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

      where:

         R-Flag: Re-advertisement flag.  If set, then the prefix to
         which this Prefix-SID is attached, has been propagated by the
         router either from another level (i.e.: from level-1 to level-2
         or the opposite) or from redistribution (e.g.: from another
         protocol).

         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-spring-segment-routing].

         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.

         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 and 2 for
         IPv6) before forwarding the packet.

         V-Flag: Value flag.  If set, then the Prefix-SID carries a
         value (instead of an index).  By default the flag is UNSET.





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         L-Flag: Local Flag.  If set, then the value/index carried by
         the Prefix-SID has local significance.  By default the flag is
         UNSET.

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

      Algorithm: the router may use various algorithms when calculating
      reachability to other nodes or to prefixes attached to these
      nodes.  Algorithms identifiers are defined in Section 3.2.
      Examples of these algorithms are metric based Shortest Path First
      (SPF), various sorts of Constrained SPF, etc.  The algorithm field
      of the Prefix-SID contains the identifier of the algorithm the
      router has used in order to compute the reachability of the prefix
      the Prefix-SID is associated to.

      At origination, the Prefix-SID algorithm field MUST be set to 0 on
      all Prefix-SID of prefixes computed using SPF algorithm (Shortest
      Path First).  On reception of the Prefix-SID sub-TLV, any non-zero
      algorithm value MUST match what advertised in the SR-Algorithm
      sub-TLV (Section 3.2).

      A router receiving a Prefix-SID from a remote node and with an
      algorithm value that such remote node has not advertised in the
      SR-Algorithm sub-TLV (Section 3.2) MUST ignore the Prefix-SID sub-
      TLV.

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

      *  A 4 octet index defining the offset in the SID/Label space
         advertised by this router using the encodings defined in
         Section 3.1.  In this case the V and L flags MUST be unset.

      *  A 3 octet local label where the 20 rightmost bits are used for
         encoding the label value.  In this case the V and L flags MUST
         be set.

2.1.1.  Flags

2.1.1.1.  R and N Flags

   The R-Flag MUST be set for prefixes that are not local to the router
   and either:

      advertised because of propagation (Level-1 into Level-2);

      advertised because of leaking (Level-2 into Level-1);



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      advertised because of redistribution (e.g.: from another
      protocol).

   In the case where a Level-1-2 router has local interface addresses
   configured in one level, it may also propagate these addresses into
   the other level.  In such case, the Level-1-2 router MUST NOT set the
   R bit.  The R-bit MUST be set only for prefixes that are not local to
   the router and advertised by the router because of propagation and/or
   leaking.

   The N-Flag is used in order to define a Node-SID.  A router MAY set
   the N-Flag only if all of the following conditions are met:

      The prefix to which the Prefix-SID is attached is local to the
      router.  I.e.: the prefix is configured on one of the local
      interfaces. (e.g.: 'stable transport' loopback).

      The prefix to which the Prefix-SID is attached MUST have a Prefix
      length of either /32 (IPv4) or /128 (IPv6).

   The router MUST ignore the N-Flag on a received Prefix-SID if the
   prefix has a Prefix length different than /32 (IPv4) or /128 (IPv6).

2.1.1.2.  E and P Flags

   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.

   When propagating (either from Level-1 to Level-2 or vice versa) a
   reachability advertisement originated by another IS-IS speaker, the
   router MUST set the P-flag and MUST clear the E-flag of the related
   Prefix-SIDs.

   The following behavior is associated with the settings of the E and P
   flags:

   o  If the P-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 which
      improves performance of the ultimate hop.  MPLS EXP bits of the
      Prefix-SID are not preserved to the ultimate hop (the Prefix-SID
      being removed).  If the P-flag is unset the received E-flag is
      ignored.

   o  If the P-flag is set then:



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      *  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, e.g., 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.

2.1.2.  Prefix-SID Propagation

   The Prefix-SID sub-TLV MUST be preserved when the IP Reachability TLV
   gets propagated across level boundaries.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels MUST set the R-flag.

   If the Prefix-SID contains a global index (L and V flags unset) and
   it is propagated as such (with L and V flags unset), the value of the
   index MUST be preserved when propagated between levels.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels MAY change the setting of the L and V flags in case a local
   label value is encoded in the Prefix-SID instead of the received
   value.

2.2.  Adjacency Segment Identifier

   A new IS-IS sub-TLV is defined: the Adjacency Segment Identifier sub-
   TLV (Adj-SID sub-TLV).

   The Adj-SID sub-TLV is an optional sub-TLV carrying the Segment
   Routing IGP-Adjacency-SID as defined in
   [I-D.filsfils-spring-segment-routing] with flags and fields that may
   be used, in future extensions of Segment Routing, for carrying other
   types of SIDs.

   IS-IS adjacencies are advertised using one of the IS-Neighbor TLVs
   below:

      TLV-22 [RFC5305]



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      TLV-222 [RFC5120]

      TLV-23 [RFC5311]

      TLV-223 [RFC5311]

      TLV-141 [RFC5316]

   Multiple Adj-SID sub-TLVs MAY be associated with a single IS-
   neighbor.  Examples where more than one Adj-SID may be used per IS-
   neighbor are described in
   [I-D.filsfils-spring-segment-routing-use-cases].

2.2.1.  Adjacency Segment Identifier (Adj-SID) Sub-TLV

   The following format is defined for the Adj-SID sub-TLV:

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

   where:

      Type: TBD, suggested value 31

      Length: variable.

      Flags: 1 octet field of following flags:

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

      where:

         F-Flag: Address-Family flag.  If unset, then the Adj-SID refers
         to an adjacency with outgoing IPv4 encapsulation.  If set then
         the Adj-SID refers to an adjacency with outgoing IPv6
         encapsulation.

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



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         V-Flag: Value flag.  If set, then the Adj-SID carries a value.
         By default the flag is SET.

         L-Flag: Local Flag.  If set, then the value/index carried by
         the Adj-SID has local significance.  By default the flag is
         SET.

         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.

      Weight: 1 octet.  The value represents the weight of the Adj-SID
      for the purpose of load balancing.  The use of the weight is
      defined in [I-D.filsfils-spring-segment-routing].

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

      *  A 3 octet local label where the 20 rightmost bits are used for
         encoding the label value.  In this case the V and L flags MUST
         be set.

      *  A 4 octet index defining the offset in the SID/Label space
         advertised by this router using the encodings defined in
         Section 3.1.  In this case V and L flags MUST be unset.

      *  A 16 octet IPv6 address.  In this case the V flag MUST be set.
         The L flag MUST be unset if the IPv6 address is globally
         unique.

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

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

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

      Examples of use of the Adj-SID sub-TLV are described in
      [I-D.filsfils-spring-segment-routing]. and
      [I-D.previdi-6man-segment-routing-header].




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      The F-flag is used in order for the router to advertise the
      outgoing encapsulation of the adjacency the Adj-SID is attached
      to.  Use cases of the use of the F-flag are described in
      [I-D.filsfils-spring-segment-routing-use-cases].

2.2.2.  Adjacency Segment Identifiers in LANs

   In LAN subnetworks, the Designated Intermediate System (DIS) is
   elected and originates the Pseudonode-LSP (PN-LSP) including all
   neighbors of the DIS.

   When Segment Routing is used, each router in the LAN MAY advertise
   the Adj-SID of each of its neighbors.  Since, on LANs, each router
   only advertises one adjacency to the DIS (and doesn't advertise any
   other adjacency), each router advertises the set of Adj-SIDs (for
   each of its neighbors) inside a newly defined sub-TLV part of the TLV
   advertising the adjacency to the DIS (e.g.: TLV-22).

   The following new sub-TLV is defined: LAN-Adj-SID (Type: TBD,
   suggested value 32) containing the set of Adj-SIDs the router
   assigned to each of its LAN neighbors.

   The format of the LAN-Adj-SID 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    |    Weight     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     System-ID (6 octets)                      |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   where:

      Type: TBD, suggested value 32

      Length: variable.

      Flags: 1 octet field of following flags:




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

      where F, B, V, L and S flags are defined in Section 2.2.1.  Other
      bits: MUST be zero when originated and ignored when received.

      Weight: 1 octet.  The value represents the weight of the Adj-SID
      for the purpose of load balancing.  The use of the weight is
      defined in [I-D.filsfils-spring-segment-routing].

      System-ID: 6 octets of IS-IS System-ID of length "ID Length" as
      defined in [ISO10589].

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

      *  A 3 octet local label where the 20 rightmost bits are used for
         encoding the label value.  In this case the V and L flags MUST
         be set.

      *  A 4 octet index defining the offset in the SID/Label space
         advertised by this router using the encodings defined in
         Section 3.1.  In this case V and L flags MUST be unset.

      *  A 16 octet IPv6 address.  In this case the V flag MUST be set.
         The L flag MUST be unset if the IPv6 address is globally
         unique.

   Multiple LAN-Adj-SID sub-TLVs MAY be encoded.

   In case one TLV-22/23/222/223 (reporting the adjacency to the DIS)
   can't contain the whole set of LAN-Adj-SID sub-TLVs, multiple
   advertisements of the adjacency to the DIS MUST be used, MUST have
   the same metric and SHOULD be inserted within the same LSP fragment.

   Each router within the level, by receiving the DIS PN LSP as well as
   the non-PN LSP of each router in the LAN, is capable of
   reconstructing the LAN topology as well as the set of Adj-SID each
   router uses for each of its neighbors.

   A label is encoded in 3 octets (in the 20 rightmost bits).

   An index is encoded in 4 octets.

   An ipv6 address SID is encoded in 16 octets (IPv6 Adj-SID is defined
   in [I-D.previdi-6man-segment-routing-header]).



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2.3.  SID/Label Sub-TLV

   The SID/Label sub-TLV is present in the following sub-TLVs defined in
   this document:

      Binding TLV Section 2.4.

      SR Capability sub-TLV Section 3.1.

   The SID/Label sub-TLV contains a SID or a MPLS Label.  The SID/Label
   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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          SID/Label (variable)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 1

      Length: variable

      SID/Label: if length is set to 3 then the 20 rightmost bits
      represent a MPLS label.

2.4.  SID/Label Binding TLV

   The SID/Label Binding TLV MAY be originated by any router in an IS-IS
   domain.  There are multiple uses of the SID/Label Binding TLV:

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

   o  The SID/Label Binding TLV may be used for advertising SID/Label
      Bindings and their associated Primary and Backup paths.  In one
      single TLV either a primary ERO Path, a backup ERO Path or both
      are advertised.  If a router wants to advertise multiple parallel



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      paths then it can generate several TLVs for the same Prefix/FEC.
      Each occurrence of a Binding TLV with respect with a given FEC
      Prefix has accumulating and not canceling semantics.  Due the
      space constraints in the 8-Bit IS-IS TLVs an originating router
      MAY encode a primary ERO path in one SID/Label Binding TLV and the
      backup ERO path in a second SID/Label Binding TLV.  Note that the
      FEC Prefix and SID/Label sub-TLV MUST be identical in both TLVs.

   o  The SID/Label Binding TLV may also be used in order to advertise
      prefixes to SID/Label mappings.  This functionality is called the
      'Mapping Server' and it's used when, in a heterogeneous network,
      not all nodes are capable of advertising their own SIDs/Labels.
      When the SID/Label Binding TLV is used by the Mapping Server in
      order to advertise prefix to SID/label mappings, the index/label
      MUST include the Prefix-SID SubTLV (Section 2.1).

   The SID/Label Binding TLV has Type TBD (suggested value 149), and has
   the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |     Length    |     Flags     |     Weight    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Range              | Prefix Length |  FEC Prefix   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //           FEC Prefix (continued, variable)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    SubTLVs (variable)                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: SID/Label Binding TLV format

   o  Type: TBD, suggested value 149

   o  Length: variable.

   o  1 octet of flags

   o  1 octet of Weight

   o  2 octets of Range

   o  1 octet of Prefix Length

   o  0-16 octets of FEC Prefix

   o  sub-TLVs, where each sub-TLV consists of a sequence of:



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      *  1 octet of sub-TLV type

      *  1 octet of length of the value field of the sub-TLV

      *  0-243 octets of value

2.4.1.  Flags

   Flags: 1 octet field of following flags:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |F|M|S|D|A|     |
   +-+-+-+-+-+-+-+-+

   where:

      F-Flag: Address Family flag.  If unset, then the Prefix FEC
      carries an IPv4 Prefix.  If set then the Prefix FEC carries an
      IPv6 Prefix.

      M-Flag: Mirror Context flag.  Set if the advertised SID/path
      corresponds to a mirrored context.  The use of the M flag is
      described in [I-D.filsfils-spring-segment-routing].

      S-Flag: If set, the SID/Label Binding TLV SHOULD be flooded across
      the entire routing domain.  If the S flag is not set, the SID/
      Label Binding TLV MUST NOT be leaked between levels.  This bit
      MUST NOT be altered during the TLV leaking.

      D-Flag: when the SID/Label Binding TLV is leaked from level-2 to
      level-1, the D bit MUST be set.  Otherwise, this bit MUST be
      clear.  SID/Label Binding TLVs with the D bit set MUST NOT be
      leaked from level-1 to level-2.  This is to prevent TLV looping
      across levels.

      A-Flag: Attached flag.  The originator of the SID/Label Binding
      TLV MAY set the A bit in order to signal that the prefixes and
      SIDs advertised in the SID/Label Binding TLV are directly
      connected to their originators.  The mechanisms through which the
      originator of the SID/Label Binding TLV can figure out if a prefix
      is attached or not are outside the scope of this document (e.g.:
      through explicit configuration).

      An implementation MAY decide not to honor the S-flag in order not
      to leak Binding TLV's between levels (for policy reasons).  In all
      cases, the D flag MUST always be set by any router leaking the
      Binding TLV from level-2 into level-1 and MUST be checked when



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      propagating the Binding TLV from level-1 into level-2.  If the D
      flag is set, the Binding TLV MUST NOT be propagated into level-2.

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

2.4.2.  Weight

   Weight: 1 octet: The value represents the weight of the path for the
   purpose of load balancing.  The use of the weight is defined in
   [I-D.filsfils-spring-segment-routing].

2.4.3.  Range

   The 'Range' field provides the ability to specify a range of
   addresses and their associated Prefix SIDs.  This functionality is
   called "Mapping Server".  It is essentially a compression scheme to
   distribute a continuous Prefix and their continuous, corresponding
   SID/Label Block.  If a single SID is advertised then the range field
   MUST be set to one.  For range advertisements > 1, the number of
   addresses that need to be mapped into a Prefix-SID and the starting
   value of the Prefix-SID range.

   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

      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    |0|0|           |     Weight    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Range = 4          |       /32     |      192      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       .0      |        .2     |       .1      |Prefix-SID Type|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | sub-TLV Length|     Flags     |   Algorithm   |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                             1 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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




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   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

      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    |0|0|           |     Weight    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Range = 7          |       /24     |      10       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       .1      |        .1     |Prefix-SID Type| sub-TLV Length|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Flags      | Algorithm     |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           51  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   It is not expected that a network operator will be able to keep fully
   continuous FEC Prefix / SID/Index mappings.  In order to support
   noncontinuous mapping ranges an implementation MAY generate several
   instances of Binding TLVs.

   For example if a router wants to advertise the following ranges:

      Range 16: { 192.168.1.1-15, Index 1-15 }

      Range 6: { 192.168.1.22-27, Index 22-27 }

      Range 41: { 192.168.1.44-84, Index 80-120 }

   A router would need to advertise three instances of the Binding TLV.

2.4.4.  Prefix Length, Prefix

   The 'FEC Prefix' represents the Forwarding equivalence class at the
   tail-end of the advertised path.  The 'FEC Prefix' does not need to
   correspond to a routable prefix of the originating node.

   The 'Prefix Length' field contains the length of the prefix in bits.
   Only the most significant octets of the Prefix FEC are encoded.  I.e.
   1 octet for FEC prefix length 1 up to 8, 2 octets for FEC prefix
   length 9 to 16, 3 octets for FEC prefix length 17 up to 24 and 4




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   octets for FEC prefix length 25 up to 32, ...., 16 octets for FEC
   prefix length 113 up to 128.

2.4.5.  Mapping Server Prefix-SID

   The Prefix-SID sub-TLV (suggested value 3) is defined in Section 2.1
   and contains the SID/index/label value associated with the prefix and
   range.  The Prefix-SID SubTLV MUST be used when the SID/Label Binding
   TLV is used by the Mapping Server (i.e.: advertising one or a range
   of prefixes and their associated SIDs/Labels).

   A node receiving a MS entry for a prefix MUST check the existence of
   such prefix in its link-state database prior to consider and use the
   associated SID.

2.4.5.1.  Prefix-SID Flags

   The Prefix-SID flags are defined in Section 2.1.  The Mapping Server
   MAY advertise a mapping with the N flag set when the prefix being
   mapped is known in the link-state topology with a mask length of 32
   and when the prefix represents a node.  The mechanisms through which
   the operator defines that a prefix represents a node are outside the
   scope of this document (typically it will be through configuration).

   The other flags defined in Section 2.1 are not used by the Mapping
   Server and MUST be ignored at reception.

2.4.5.2.  Prefix-SID Algorithm

   The algorithm field contains the identifier of the algorithm the
   router MUST use in order to compute reachability to the range of
   prefixes.  Use of the algorithm field is described in Section 2.1.

2.4.6.  SID/Label Sub-TLV

   The SID/Label sub-TLV (Type: TBD, suggested value 1) contains the
   SID/Label value as defined in Section 2.3.  It MAY be present in the
   SID/Label Binding TLV.

2.4.7.  ERO Metric sub-TLV

   ERO Metric sub-TLV (Type: TBD, suggested value 10) is a sub-TLV of
   the SID/Label Binding TLV.

   The ERO Metric sub-TLV carries the cost of an ERO path.  It is used
   to compare the cost of a given source/destination path.  A router MAY
   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



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   ERO.  Since manipulation of the Metric field may attract or distract
   traffic from and to 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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Metric (continued)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         ERO Metric sub-TLV format

   where:

      Type: TBD, suggested value 10

      Length: 4

      Metric: 4 bytes

2.4.8.  IPv4 ERO subTLV

   The IPv4 ERO subTLV (Type: TBD, suggested value 11) describes a path
   segment using IPv4 address style of encoding.  Its semantics have
   been borrowed from [RFC3209].

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Length    |L|   Reserved  |  IPv4 address |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          IPv4 address  (continued)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: IPv4 ERO subTLV format

2.4.9.  IPv6 ERO subTLV

   The IPv6 ERO subTLV (Type: TBD, suggested value 12) describes a path
   segment using IPv6 Address style of encoding.  Its semantics have
   been borrowed from [RFC3209].




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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Length    |L|   Reserved  |  IPv6 address |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: IPv6 ERO subTLV format

2.4.10.  Unnumbered Interface ID ERO subTLV

   The appearance and semantics of the 'Unnumbered Interface ID' have
   been borrowed from Section 4 [RFC3477].

   The Unnumbered Interface-ID ERO subTLV (Type: TBD, suggested value
   13) describes a path segment that spans over an unnumbered interface.
   Unnumbered interfaces are referenced using the interface index.
   Interface indices are assigned local to the router and therefore not
   unique within a domain.  All elements in an ERO path need to be
   unique within a domain and hence need to be disambiguated using a
   domain unique Router-ID.

   The 'Router-ID' field contains the router ID of the router which has
   assigned the 'Interface ID' field.  Its purpose is to disambiguate
   the 'Interface ID' field from other routers in the domain.

   IS-IS supports two Router-ID formats:

   o  (TLV 134, 32-Bit format) [RFC5305]

   o  (TLV 140, 128-Bit format) [RFC6119]

   The actual Router-ID format gets derived from the 'Length' field.

   o  For 32-Bit Router-ID width the subTLV length is set to 8 octets.

   o  For 128-Bit Router-ID width the subTLV length is set to 20 octets.



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   The 'Interface ID' is the identifier assigned to the link by the
   router specified by the router ID.

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |     Length    |L|   Reserved  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                   Router ID (32 or 128 bits)                //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Interface ID (32 bits)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: Unnumbered Interface ID ERO subTLV format

2.4.11.  IPv4 Backup ERO subTLV

   The IPv4 Backup ERO subTLV (Type: TBD, suggested value 14) describes
   a Backup path segment using IPv4 Address style of encoding.  Its
   appearance and semantics have been borrowed from [RFC3209].

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Length    |L|   Reserved  |  IPv4 address |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          IPv4 address  (continued)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 5: IPv4 Backup ERO subTLV format

2.4.12.  IPv6 Backup ERO subTLV

   The IPv6 Backup ERO subTLV (Type: TBD, suggested value 15) describes
   a Backup path segment using IPv6 Address style of encoding.  Its
   appearance and semantics have been borrowed from [RFC3209].

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



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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    |L|   Reserved  |  IPv6 address |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 Address (continued)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 6: IPv6 Backup ERO subTLV format

2.4.13.  Unnumbered Interface ID Backup ERO subTLV

   The appearance and semantics of the 'Unnumbered Interface ID' have
   been borrowed from Section 4 [RFC3477].

   The Unnumbered Interface-ID Backup ERO subTLV (Type: TBD, suggested
   value 16) describes a Backup LSP path segment that spans over an
   unnumbered interface.  Unnumbered interfaces are referenced using the
   interface index.  Interface indices are assigned local to the router
   and therefore not unique within a domain.  All elements in an ERO
   path need to be unique within a domain and hence need to be
   disambiguated using a domain unique Router-ID.

   The 'Router-ID' field contains the router ID of the router which has
   assigned the 'Interface ID' field.  Its purpose is to disambiguate
   the 'Interface ID' field from other routers in the domain.

   IS-IS supports two Router-ID formats:

   o  (TLV 134, 32-Bit format) [RFC5305]

   o  (TLV 140, 128-Bit format) [RFC6119]

   The actual Router-ID format gets derived from the 'Length' field.

   o  For 32-Bit Router-ID width the subTLV length is set to 8 octets.

   o  For 128-Bit Router-ID width the subTLV length is set to 20 octets.

   The 'Interface ID' is the identifier assigned to the link by the
   router specified by the router ID.




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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |     Length    |L|   Reserved  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                   Router ID (32 or 128 bits)                //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Interface ID (32 bits)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 7: Unnumbered Interface ID Backup ERO subTLV format

2.4.14.  Prefix ERO and Prefix Backup ERO subTLV path semantics

   All 'ERO' and 'Backup ERO' information represents an ordered set
   which describes the segments of a path.  The last ERO subTLV
   describes the segment closest to the egress point of the path.
   Contrary the first ERO subTLV describes the first segment of a path.
   If a router extends or stitches a label switched path it MUST prepend
   the new segments path information to the ERO list.  The same ordering
   applies for the Backup ERO labels.  An implementation SHOULD first
   encode all primary path EROs followed by the bypass EROs.

3.  Router Capabilities

3.1.  SR-Capabilities Sub-TLV

   Segment Routing requires each router to advertise its SR data-plane
   capability and the range of MPLS label values it uses for Segment
   Routing in the case where global SIDs are allocated (i.e.: global
   indexes).  Data-plane capabilities and label ranges are advertised
   using the newly defined SR-Capabilities sub-TLV inserted into the IS-
   IS Router Capability TLV-242 that is defined in [RFC4971].

   The Router Capability TLV specifies flags that control its
   advertisement.  The SR Capabilities sub-TLV MUST be propagated
   throughout the level and SHOULD NOT be advertised across level
   boundaries.  Therefore Router Capability TLV distribution flags
   SHOULD be set accordingly, i.e.: the S flag MUST be unset.

   The SR Capabilities sub-TLV (Type: TBD, suggested value 2) MAY appear
   multiple times inside the Router Capability TLV 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      |    Range      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Range (cont.)       | SID/Label sub-TLV (variable)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: TBD, suggested value 2

      Length: variable.

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

      0
      0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |I|V|           |
      +-+-+-+-+-+-+-+-+

      where:

         I-Flag: IPv4 flag.  If set, then the router is capable of
         outgoing IPv4 encapsulation on all interfaces.

         V-Flag: IPv6 flag.  If set, then the router is capable of
         outgoing IPv6 encapsulation on all interfaces.

      Range: 3 octets value defining the number of values of the range
      from the starting value defined in the SID/Label sub-TLV.

      SID/Label sub-TLV: SID/Label value as defined in Section 2.3.

   Multiple occurrence of the SR-Capabilities sub-TLV MAY be advertised,
   in order to advertise multiple ranges.  In such case:

   o  Only the Flags in the first occurrence of the sub-TLV are to be
      taken into account.

   o  The originating router MUST encode ranges each into a different
      SR-Capability sub-TLV and all SR-Capability TLVs MUST be encoded
      within the same LSP fragment.

   o  The order of the ranges (i.e.: SR-Capability sub-TLVs) in the LSP
      fragment is decided by the originating router and hence the
      receiving routers MUST NOT re-order the received ranges.  This is



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      required for avoiding label churn when for example a numerical
      lower Segment/Label Block gets added to an already advertised
      Segment/Label Block.

   o  An originator router SHOULD add newly configured block at the end,
      and SHOULD NOT change the order of previously advertised block.
      Indeed, changing the order to block being used will create label
      churn in the FIB and blackhole / misdirect some traffic during the
      IGP convergence.  In particular, if a range, which is not the
      last, is extended it's preferable to add a new range rather than
      extending the previously advertised range.

   o  The originating router decides the order of the set of originated
      SR-Capability sub-TLV.  In all cases, 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 guarantee the same order before and after GR.

   o  The originator router MUST NOT advertising overlapping range.

   o  A router not supporting multiple occurrences of the SR-Capability
      sub-TLV MUST take into consideration the first occurrence in the
      received set.

   Here follows an example of advertisement of multiple ranges:

      The originating router advertises following ranges:
         SR-Cap: range: 100, SID value: 100
         SR-Cap: range: 100, SID value: 1000
         SR-Cap: range: 100, SID value: 500

      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
         ...



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3.2.  SR-Algorithm Sub-TLV

   The router may use various algorithms when calculating reachability
   to other nodes or to prefixes attached to these nodes.  Examples of
   these algorithms are metric based Shortest Path First (SPF), various
   sorts of Constrained SPF, etc.  The SR-Algorithm sub-TLV (Type: TBD,
   suggested value 19) allows the router to advertise the algorithms
   that the router is currently using.  The following value has been
   defined:

      0: Shortest Path First (SPF) algorithm based on link metric.

   The SR-Algorithm sub-TLV is inserted into the IS-IS Router Capability
   TLV-242 that is defined in [RFC4971].

   The Router Capability TLV specifies flags that control its
   advertisement.  The SR-Algorithm MUST be propagated throughout the
   level and need not to be advertised across level boundaries.
   Therefore Router Capability TLV distribution flags MUST be set
   accordingly, i.e.: the S flag MUST be unset.

   The SR-Algorithm sub-TLV is optional, it MAY only appear a single
   time inside the Router Capability TLV.

   When the originating router does not advertise the SR-Algorithm sub-
   TLV, then all the Prefix-SID advertised by the router MUST have
   algorithm field set to 0.  Any receiving router MUST assume SPF
   algorithm (i.e.: Shortest Path First).

   The SR-Algorithm sub-TLV has following format:

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

   where:

      Type: TBD, suggested value 19

      Length: variable.

      Algorithm: 1 octet of algorithm Section 2.1





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4.  IANA Considerations

   This documents request allocation for the following TLVs and subTLVs.

4.1.  Sub TLVs for Type 22,23,222 and 223

   This document makes the following registrations in the "sub-TLVs for
   TLV 22, 23, 222 and 223" registry.



         Type: TBD (suggested value 31)

         Description: Adjacency Segment Identifier

         TLV 22: yes

         TLV 23: yes

         TLV 222: yes

         TLV 223: yes

         Reference: This document (Section 2.2.1)



         Type: TBD (suggested value 32)

         Description: LAN Adjacency Segment Identifier

         TLV 22: yes

         TLV 23: yes

         TLV 222: yes

         TLV 223: yes

         Reference: This document (Section 2.2.2)

4.2.  Sub TLVs for Type 135,235,236 and 237

   This document makes the following registrations in the "sub-TLVs for
   TLV 135,235,236 and 237" registry.

      Type: TBD (suggested value 3)




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      Description: Prefix Segment Identifier

      TLV 135: yes

      TLV 235: yes

      TLV 236: yes

      TLV 237: yes

      Reference: This document (Section 2.1)

4.3.  Sub TLVs for Type 242

   This document makes the following registrations in the "sub-TLVs for
   TLV 242" registry.



         Type: TBD (suggested value 2)

         Description: Segment Routing Capability

         Reference: This document (Section 3.1)



         Type: TBD (suggested value 19)

         Description: Segment Routing Algorithm

         Reference: This document (Section 3.2)

4.4.  New TLV Codepoint and Sub-TLV registry

   This document registers the following TLV:

      Type: TBD (suggested value 149)

      name: Segment Identifier / Label Binding

      IIH: no

      LSP: yes

      SNP: no

      Purge: no



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      Reference: This document (Section 2.4)

   This document creates the following sub-TLV Registry:

      Registry: sub-TLVs for TLV 149

      Registration Procedure: Expert review

      Reference: This document (Section 2.4)



         Type: TBD, suggested value 1

         Description: SID/Label

         Reference: This document (Section 2.3)

         Type: TBD, suggested value 3

         Description: Prefix-SID

         Reference: This document (Section 2.1)



         Type: TBD, suggested value 10

         Description: ERO Metric

         Reference: This document (Section 2.4.7)



         Type: TBD, suggested value 11

         Description: IPv4 ERO

         Reference: This document (Section 2.4.8)



         Type: TBD, suggested value 12

         Description: IPv6 ERO

         Reference: This document (Section 2.4.9)




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         Type: TBD, suggested value 13

         Description: Unnumbered Interface-ID ERO

         Reference: This document (Section 2.4.10)



         Type: TBD, suggested value 14

         Description: IPv4 Backup ERO

         Reference: This document (Section 2.4.11)



         Type: TBD, suggested value 15

         Description: IPv6 Backup ERO

         Reference: This document (Section 2.4.12)



         Type: TBD, suggested value 16

         Description: Unnumbered Interface-ID Backup ERO

         Reference: This document (Section 2.4.13)

5.  Manageability Considerations

   TBD

6.  Security Considerations

   TBD

7.  Contributors

   The following people gave a substantial contribution to the content
   of this document: Martin Horneffer, Igor Milojevic, Rob Shakir, Saku
   Ytti, Wim Henderickx, Les Ginsberg, Steven Luong and Jesper Skriver.








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8.  Acknowledgements

   We would like to thank Dave Ward, Dan Frost, Stewart Bryant and
   Pierre Francois for their contribution to the content of this
   document.

   Many thanks to Yakov Rekhter and Ina Minei for their contribution on
   earlier incarnations of the "Binding / MPLS Label TLV".

9.  References

9.1.  Normative References

   [ISO10589]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/IEC
              10589:2002, Second Edition, Nov 2002.

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

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

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

   [RFC4971]  Vasseur, JP., Shen, N., and R. Aggarwal, "Intermediate
              System to Intermediate System (IS-IS) Extensions for
              Advertising Router Information", RFC 4971, July 2007.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
              2008.






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   [RFC5311]  McPherson, D., Ginsberg, L., Previdi, S., and M. Shand,
              "Simplified Extension of Link State PDU (LSP) Space for
              IS-IS", RFC 5311, February 2009.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, December 2008.

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, February 2011.

9.2.  Informative References

   [I-D.filsfils-spring-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-spring-
              segment-routing-04 (work in progress), July 2014.

   [I-D.filsfils-spring-segment-routing-use-cases]
              Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
              Crabbe, "Segment Routing Use Cases", draft-filsfils-
              spring-segment-routing-use-cases-01 (work in progress),
              October 2014.

   [I-D.previdi-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., and I. Leung, "IPv6
              Segment Routing Header (SRH)", draft-previdi-6man-segment-
              routing-header-02 (work in progress), July 2014.

Authors' Addresses

   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
   BE

   Email: cfilsfil@cisco.com


   Ahmed Bashandy
   Cisco Systems, Inc.
   170, West Tasman Drive
   San Jose, CA  95134
   US

   Email: bashandy@cisco.com


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

   Email: hannes@juniper.net


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com


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

   Email: Jeff.Tantsura@ericsson.com




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