IS-IS Extensions for Segment Routing
draft-ietf-isis-segment-routing-extensions-23

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Replaces draft-previdi-isis-segment-routing-extensions
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IS-IS for IP Internets                                   S. Previdi, Ed.
Internet-Draft                                                    Huawei
Intended status: Standards Track                        L. Ginsberg, Ed.
Expires: September 30, 2019                                  C. Filsfils
                                                     Cisco Systems, Inc.
                                                             A. Bashandy
                                                              Individual
                                                              H. Gredler
                                                            RtBrick Inc.
                                                             B. Decraene
                                                                  Orange
                                                          March 29, 2019

                  IS-IS Extensions for Segment Routing
             draft-ietf-isis-segment-routing-extensions-23

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 operating on an MPLS data-plane.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   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 September 30, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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 . . . . . . . .  10
     2.3.  SID/Label Sub-TLV . . . . . . . . . . . . . . . . . . . .  12
     2.4.  SID/Label Binding TLV . . . . . . . . . . . . . . . . . .  13
       2.4.1.  Flags . . . . . . . . . . . . . . . . . . . . . . . .  14
       2.4.2.  Range . . . . . . . . . . . . . . . . . . . . . . . .  15
       2.4.3.  Prefix Length, Prefix . . . . . . . . . . . . . . . .  15
       2.4.4.  Mapping Server Prefix-SID . . . . . . . . . . . . . .  15
       2.4.5.  SID/Label Sub-TLV . . . . . . . . . . . . . . . . . .  16
       2.4.6.  Example Encodings . . . . . . . . . . . . . . . . . .  16
     2.5.  Multi-Topology SID/Label Binding TLV  . . . . . . . . . .  18
   3.  Router Capabilities . . . . . . . . . . . . . . . . . . . . .  19
     3.1.  SR-Capabilities Sub-TLV . . . . . . . . . . . . . . . . .  19
     3.2.  SR-Algorithm Sub-TLV  . . . . . . . . . . . . . . . . . .  22
     3.3.  SR Local Block Sub-TLV  . . . . . . . . . . . . . . . . .  23
     3.4.  SRMS Preference Sub-TLV . . . . . . . . . . . . . . . . .  25
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     4.1.  Sub TLVs for Type 22,23,25,141,222, and 223 . . . . . . .  25
     4.2.  Sub TLVs for Type 135,235,236 and 237 . . . . . . . . . .  26
     4.3.  Sub TLVs for Type 242 . . . . . . . . . . . . . . . . . .  27

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     4.4.  New TLV Codepoint and Sub-TLV registry  . . . . . . . . .  28
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   Segment Routing (SR) allows for a flexible definition of end-to-end
   paths within IGP topologies by encoding paths as sequences of
   topological sub-paths, called "segments".  These segments are
   advertised by the link-state routing protocols (IS-IS and OSPF).
   Prefix segments represent an ECMP-aware shortest-path to a prefix (or
   a node), as per the state of the IGP topology.  Adjacency segments
   represent a hop over a specific adjacency between two nodes in the
   IGP.  A prefix segment is typically a multi-hop path while an
   adjacency segment, in most of the cases, is a one-hop path.  SR's
   control-plane can be applied to both IPv6 and MPLS data-planes, and
   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.

   There are additional segment types, e.g., Binding SID defined in
   [RFC8402].  This draft also defines an advertisement for one type of
   Binding SID: the Mirror Context segment.

   This draft describes the necessary IS-IS extensions that need to be
   introduced for Segment Routing operating on an MPLS data-plane.

   The Segment Routing architecture is described in [RFC8402].

   Segment Routing use cases are described in [RFC7855].

2.  Segment Routing Identifiers

   The Segment Routing architecture [RFC8402] defines different types of
   Segment Identifiers (SID).  This document defines the IS-IS encodings
   for the IGP-Prefix Segment, the IGP-Adjacency Segment, the IGP-LAN-
   Adjacency Segment and the Binding Segment.

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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 [RFC8402].  The 'Prefix SID' MUST be unique within a given
   IGP domain (when the L-flag is not set).

   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 (Extended IPv4 reachability) defined in [RFC5305].

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

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

      TLV-237 (Multitopology IPv6 IP Reachability) defined in [RFC5120].

      Binding-TLV and Multi-Topology Binding-TLV defined in Section 2.4
      and Section 2.5 respectively.

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

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

   where:

      Type: 3

      Length: 5 or 6 depending on the size of the SID (described below)

      Flags: 1 octet field of following flags:

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

      where:

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         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
         [RFC8402].

         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.

         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 uses to compute the reachability of the prefix to which the
      Prefix-SID is associated.

      At origination, the Prefix-SID algorithm field MUST be set to 0 or
      to any value 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.

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      SID/Index/Label as defined in Section 2.1.1.1.

   When the Prefix SID is an index (the V-flag is not set) the value is
   used to determine the actual label value inside the set of all
   advertised label ranges of a given router.  This allows a receiving
   router to construct forwarding state to a particular destination
   router.

   In many use-cases a 'stable transport' address is overloaded as an
   identifier of a given node.  Because 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 Prefix to node mappings.  Furthermore if a given node
   has several 'stable transport' addresses there are flags to
   differentiate those among other Prefixes advertised from a given
   node.

2.1.1.  Flags

2.1.1.1.  V and L Flags

   The V-flag indicates whether the SID/Index/Label field is a value or
   an index.

   The L-Flag indicates whether the value/index in the SID/Index/Label
   field has local or global significance.

   The following settings for V and L flags are valid:

   V-flag is set to 0 and L-flag is set to 0: The SID/Index/Label field
   is a 4 octet index defining the offset in the SID/Label space
   advertised by this router using the encodings defined in Section 3.1.

   V-flag is set to 1 and L-flag is set to 1: The SID/Index/Label field
   is a 3 octet local label where the 20 rightmost bits are used for
   encoding the label value.

   All other combinations of V-flag and L-flag are invalid and any SID
   advertisement received with an invalid setting for V and L flags MUST
   be ignored.

2.1.1.2.  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);

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

      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 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., a 'stable transport' loopback).

      The prefix to which the Prefix-SID is attached has 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).

   The Prefix Attributes Flags sub-TLV [RFC7794] also defines the N and
   R flags and with the same semantics of the equivalent flags defined
   in this document.  Whenever the Prefix Attributes Flags sub-TLV is
   present for a given prefix the values of the N and R flags advertised
   in that sub-TLV MUST be used and the values in a corresponding Prefix
   SID sub-TLV (if present) MUST be ignored.

2.1.1.3.  E and P Flags

   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:

      *  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

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         Prefix-SID must stitch the incoming packet into a continuing
         MPLS LSP to the final destination.  This could occur at an
         inter-area border router (prefix propagation from one area to
         another) or at an inter-domain border router (prefix
         propagation from one domain to another).

      *  If the E-flag is set then any upstream neighbor of the Prefix-
         SID originator MUST replace the PrefixSID with a Prefix-SID
         having an Explicit-NULL value.  This is useful, e.g., when the
         originator of the Prefix-SID is the final destination for the
         related prefix and the originator wishes to receive the packet
         with the original EXP bits.

   When 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.

2.1.2.  Prefix-SID Propagation

   The Prefix-SID sub-TLV MUST be included when the associated Prefix
   Reachability TLV is propagated across level boundaries.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels maintains the content (flags and SID) except as noted in
   Section 2.1.1.2 and Section 2.1.1.3.

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 [RFC8402] 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 (Extended IS reachability)[RFC5305]

      TLV-222 (Multitopology IS)[RFC5120]

      TLV-23 (IS Neighbor Attribute)[RFC5311]

      TLV-223 (Multitopology IS Neighbor Attribute)[RFC5311]

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      TLV-141 (inter-AS reachability information)[RFC5316]

   Multiple Adj-SID sub-TLVs MAY be associated with a single IS-
   neighbor.

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: 31

      Length: 5 or 6 depending on size of the SID

      Flags: 1 octet field of following flags:

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

      where:

         F-Flag: Address-Family flag.  If unset, then the Adj-SID is
         used when forwarding IPv4 encapsulated traffic to the neighbor.
         If set then the Adj-SID is used when forwarding IPv6
         encapsulated traffic to the neighbor.

         B-Flag: Backup flag.  If set, the Adj-SID is eligible for
         protection (e.g.: using IPFRR or MPLS-FRR) as described in
         [RFC8402].

         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.

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         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).

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

         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 [RFC8402].

      SID/Index/Label as defined in Section 2.1.1.1.

      An SR capable router MAY allocate an Adj-SID for each of its
      adjacencies

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

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

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

      Examples of use of the Adj-SID sub-TLV are described in [RFC8402].

      The F-flag is used in order for the router to advertise the
      outgoing encapsulation of the adjacency the Adj-SID is attached
      to.

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).

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   The following new sub-TLV is defined: LAN-Adj-SID 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     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Neighbor System-ID (ID length octets)        |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   where:

      Type: 32

      Length: variable.

      Flags: 1 octet field of following flags:

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

      where F, B, V, L, S and P 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 [RFC8402].

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

      SID/Index/Label as defined in Section 2.1.1.1.

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

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   Note that this sub-TLV MUST NOT appear in TLV 141.

   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 and all
   advertisements MUST have the same metric.

   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.

2.3.  SID/Label Sub-TLV

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

   SR-Capabilities Sub-TLV (Section 3.1)

   SR Local Block Sub-TLV (Section 3.3)

   SID/Label Binding TLV (Section 2.4)

   Multi-Topology SID/Label Binding TLV (Section 2.5)

   Note that the code point used in all of the above cases is the SID/
   Label Sub-TLV code point specified in the new "sub-TLVs for TLV 149
   and 150" registry created by this document.

   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: 1

      Length: 3 or 4

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      SID/Label: if length is set to 3 then the 20 rightmost bits
      represent a MPLS label.  If length is set to 4 then the valu eis a
      32 bit index

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.

   The SID/Label Binding TLV may be used to advertise prefixes to SID/
   Label mappings.  This functionality is called the Segment Routing
   Mapping Server (SRMS).  The behavior of the SRMS is defined in
   [I-D.ietf-spring-segment-routing-ldp-interop].

   The SID/Label Binding TLV may also be used to advertise a Mirror SID
   to advertise the ability to process traffic originally destined to
   another IGP node.  This behavior is defined in [RFC8402].

   The SID/Label Binding TLV has the following format:

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

                  Figure 1: SID/Label Binding TLV format

   o  Type: 149

   o  Length: variable.

   o  1 octet of flags

   o  1 octet of RESERVED

   o  2 octets of Range

   o  1 octet of Prefix Length

   o  0-16 octets of Prefix

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

      *  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 carries an
      IPv4 Prefix.  If set then the Prefix carries an IPv6 Prefix.

      M-Flag: Mirror Context flag.  Set if the advertised SID
      corresponds to a mirrored context.  The use of a mirrored context
      is described in [RFC8402].

      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-Flag MUST be set.  Otherwise, this flag MUST be
      clear.  SID/Label Binding TLVs with the D-Flag 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).  If the Binding TLV is leaked to
      other areas/levels the A-flag MUST be cleared.

      An implementation may decide not to honor the S-flag in order not
      to leak Binding TLV's between levels (for policy reasons).

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

2.4.2.  Range

   The 'Range' field provides the ability to specify a range of
   addresses and their associated Prefix SIDs.  This advertisement
   supports the SRMS functionality.  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 range field MUST be set to the number of addresses that need to
   be mapped into a Prefix-SID.  In either case the prefix is the first
   address to which a SID is to be assigned.

2.4.3.  Prefix Length, Prefix

   The 'Prefix' represents the Forwarding equivalence class at the tail-
   end of the advertised path.  The '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 are encoded (i.e., 1
   octet for prefix length 1 up to 8, 2 octets for prefix length 9 to
   16, 3 octets for prefix length 17 up to 24 and 4 octets for prefix
   length 25 up to 32, ...., 16 octets for prefix length 113 up to 128).

2.4.4.  Mapping Server Prefix-SID

   The Prefix-SID sub-TLV is defined in Section 2.1 and contains the
   SID/index/label value associated with the prefix and range.  The
   Prefix-SID Sub-TLV MUST be present in the SID/Label Binding TLV when
   the M-flag is clear.  The Prefix-SID Sub-TLV MUST NOT be present when
   the M-flag is set.

2.4.4.1.  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
   (IPv4) or 128 (IPv6) 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.

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2.4.4.2.  PHP Behavior when using Mapping Server Advertisements

   As the mapping server does not specify the originator of a prefix
   advertisement it is not possible to determine PHP behavior solely
   based on the Mapping Server Advertisement.  However, if additional
   information is available PHP behavior may safely be done.  The
   required information consists of:

   o  A prefix reachability advertisement for the prefix has been
      received which includes the Prefix Attribute Flags sub-TLV
      [RFC7794].

   o  X and R flags are both set to 0 in the Prefix Attribute Flags sub-
      TLV.

   In the absence of an Prefix Attribute Flags sub-TLV [RFC7794] the A
   flag in the binding TLV indicates that the originator of a prefix
   reachability advertisement is directly connected to the prefix and
   thus PHP MUST be done by the neighbors of the router originating the
   prefix reachability advertisement.  Note that A-flag is only valid in
   the original area in which the Binding TLV is advertised.

2.4.4.3.  Prefix-SID Algorithm

   The algorithm field contains the identifier of the algorithm
   associated with the SIDs for the prefix(es) in the range.  Use of the
   algorithm field is described in Section 2.1.

2.4.5.  SID/Label Sub-TLV

   The SID/Label sub-TLV (Type: 1) contains the SID/Label value as
   defined in Section 2.3.  It MUST be present in the SID/Label Binding
   TLV when the M-flag is set in the Flags field of the parent TLV.

2.4.6.  Example Encodings

   Example 1: if the following IPv4 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

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

   Example-2: If the following IPv4 prefixes need to be mapped into the
   corresponding Prefix-SID indexes:

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

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

   Example-3: If the following IPv6 prefixes need to be mapped into the
   corresponding Prefix-SID indexes:

   2001:DB8:1/48, Prefix-SID: Index 151
   2001:DB8:2/48, Prefix-SID: Index 152
   2001:DB8:3/48, Prefix-SID: Index 153
   2001:DB8:4/48, Prefix-SID: Index 154

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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    |1|0|0|0|0|     |     RESERVED  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Range = 4          |        48     |     0x20      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       0x01    |       0xD     |       0xB8    |    0x0        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    0x1        |Prefix-SID Type| sub-TLV Length|  Flags        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Algorithm     |                 0                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       151     |
     +-+-+-+-+-+-+-+-+

   It is not expected that a network operator will be able to keep fully
   continuous 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: { 192.0.2.1-15, Index 1-15 }

      Range 6: { 192.0.2.22-27, Index 22-27 }

      Range 41: { 192.0.2.44-84, Index 80-120 }

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

2.5.  Multi-Topology SID/Label Binding TLV

   The Multi-Topology SID/Label Binding TLV allows the support of M-ISIS
   as defined in [RFC5120].  The Multi-Topology SID/Label Binding TLV
   has the same format as the SID/Label Binding TLV defined in
   Section 2.4 with the difference consisting of a Multitopology
   Identifier (MTID) as defined here below:

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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    |             MTID              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Flags     |     RESERVED  |            Range              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Prefix Length |            Prefix (variable)                 //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Sub-TLVs (variable)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 2: Multi-Topology SID/Label Binding TLV format

   where:

      Type: 150

      Length: variable

      MTID is the multitopology identifier defined as:

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | RESVD |         MTID          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         RESVD: reserved bits.  MUST be reset on transmission and
         ignored on receive.

         MTID: a 12-bit field containing the non-zero ID of the topology
         being announced.  The TLV MUST be ignored if the ID is zero.
         This is to ensure the consistent view of the standard unicast
         topology.

      The other fields and Sub-TLVs are defined in Section 2.4.

3.  Router Capabilities

   This section defines sub-TLVs which are inserted into the IS-IS
   Router Capability TLV-242 that is defined in [RFC7981].

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

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   indexes).  Data-plane capabilities and label ranges are advertised
   using the newly defined SR-Capabilities sub-TLV.

   The Router Capability TLV specifies flags that control its
   advertisement.  The SR Capabilities sub-TLV MUST be propagated
   throughout the level and MUST NOT be advertised across level
   boundaries.  Therefore Router Capability TLV distribution flags are
   set accordingly, i.e., the S flag in the Router Capability TLV
   [RFC7981] MUST be unset.

   The SR Capabilities sub-TLV has following format:

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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Range                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID/Label Sub-TLV (variable)                 //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 2

      Length: variable.

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

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

      where:

         I-Flag: MPLS IPv4 flag.  If set, then the router is capable of
         processing SR MPLS encapsulated IPv4 packets on all interfaces.

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

      One or more SRGB Descriptor entries, each of which have the
      following format:

         Range: 3 octets.

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         SID/Label sub-TLV (as defined in Section 2.3).

   SID/Label sub-TLV contains the first value of the SRGB while the
   range contains the number of SRGB elements.  The range value MUST be
   higher than 0.

   The SR-Capabilities sub-TLV MAY be advertised in an LSP of any number
   but a router MUST NOT advertise more than one SR-Capabilities sub-
   TLV.  A router receiving multiple SR-Capabilities sub-TLVs from the
   same originator SHOULD select the first advertisement in the lowest
   numbered LSP.

   When multiple SRGB Descriptors are advertised the entries define an
   ordered set of ranges on which a SID index is to be applied.  For
   this reason changing the order in which the descriptors are
   advertised will have a disruptive effect on forwarding.

   When a router adds a new SRGB Descriptor to an existing SR-
   Capabilities sub-TLV the new Descriptor SHOULD add the newly
   configured block at the end of the sub-TLV and SHOULD NOT change the
   order of previously advertised blocks.  Changing the order of the
   advertised descriptors 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.

   The originating router MUST ensure the order is unchanged after a
   graceful restart (using checkpointing, non-volatile storage or any
   other mechanism).

   The originating router MUST NOT advertise overlapping ranges.

   When a router receives multiple overlapping ranges, it MUST conform
   to the procedures defined in [I-D.ietf-spring-segment-routing-mpls].

   Here follows an example of advertisement of multiple ranges:

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      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 in the received
      order and build the SRGB 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
         ...

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 allows the
   router to advertise the algorithms that the router is currently
   using.  Algorithm values are defined in the "IGP Algorithm Type"
   registry defined in [I-D.ietf-ospf-segment-routing-extensions].  The
   following values have been defined:

      0: Shortest Path First (SPF) algorithm based on link metric.  This
      is the well-known shortest path algorithm as computed by the IS-IS
      Decision process.  Consistent with the deployed practice for link-
      state protocols, algorithm 0 permits any node to overwrite the SPF
      path with a different path based on local policy.

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

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

   The SR-Algorithm sub-TLV is optional.  It MUST NOT be advertsied more
   than once at a given level.  A router receiving multiple SR-Algorithm
   sub-TLVs from the same originator SHOULD select the first
   advertisement in the lowest numbered LSP.

   When the originating router does not advertise the SR-Algorithm sub-
   TLV, this implies that the only algorithm supported by routers
   supporting the extensions defined in this document is Algorithm 0.

   When the originating router does advertise the SR-Algorithm sub-TLV,
   then algorithm 0 MUST be present while non-zero algorithms MAY be
   present.

   The SR-Algorithm 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    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Algorithm 1   |  Algorithm 2  | Algorithm ... |  Algorithm n  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: 19

      Length: variable.

      Algorithm: 1 octet of algorithm

3.3.  SR Local Block Sub-TLV

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

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   The SRLB Sub-TLV is used for that purpose 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    |    Flags      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Range                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID/Label Sub-TLV (variable)                 //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 22

      Length: variable.

      Flags: 1 octet of flags.  None are defined at this stage.

      One or more SRLB Descriptor entries, each of which have the
      following format:

         Range: 3 octets.

         SID/Label sub-TLV (as defined in Section 2.3).

   SID/Label sub-TLV contains the first value of the SRLB while the
   range contains the number of SRLB elements.  The range value MUST be
   higher than 0.

   The SRLB sub-TLV MAY be advertised in an LSP of any number but a
   router MUST NOT advertise more than one SRLB sub-TLV.  A router
   receiving multiple SRLB sub-TLVs, from the same originator, SHOULD
   select the first advertisement in the lowest numbered LSP.

   The originating router MUST NOT advertise overlapping ranges.

   It is important to note that each time a SID from the SRLB is
   allocated, it should also be reported to all components (e.g.:
   controller or applications) in order for these components to have an
   up-to-date view of the current SRLB allocation and in order to avoid
   collision between allocation instructions.

   Within the context of IS-IS, the reporting of local SIDs is done
   through IS-IS Sub-TLVs such as the Adjacency-SID.  However, the
   reporting of allocated local SIDs may also be done through other

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   means and protocols which mechanisms are outside the scope of this
   document.

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

3.4.  SRMS Preference Sub-TLV

   The Segment Routing Mapping Server (SRMS) Preference sub-TLV is used
   in order to associate a preference with SRMS advertisements from a
   particular source.

   The SRMS Preference 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     | Preference    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 24

      Length: 1.

      Preference: 1 octet.  Unsigned 8 bit SRMS preference.

   The SRMS Preference sub-TLV MAY be advertised in an LSP of any number
   but a router MUST NOT advertise more than one SRMS Preference sub-
   TLV.  A router receiving multiple SRMS Preference sub-TLVs, from the
   same originator, SHOULD select the first advertisement in the lowest
   numbered LSP.

   The use of the SRMS Preference during the SID selection process is
   described in [I-D.ietf-spring-segment-routing-ldp-interop]

4.  IANA Considerations

   This documents request allocation for the following TLVs and Sub-
   TLVs.

4.1.  Sub TLVs for Type 22,23,25,141,222, and 223

   This document makes the following registrations in the "sub-TLVs for
   TLV 22, 23, 25, 141, 222 and 223" registry.

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         Type: 31

         Description: Adjacency Segment Identifier

         TLV 22: yes

         TLV 23: yes

         TLV 25: no

         TLV 141: yes

         TLV 222: yes

         TLV 223: yes

         Reference: This document (Section 2.2.1)

         Type: 32

         Description: LAN Adjacency Segment Identifier

         TLV 22: yes

         TLV 23: yes

         TLV 25: no

         TLV 141: 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: 3

      Description: Prefix Segment Identifier

      TLV 135: yes

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      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: 2

         Description: Segment Routing Capability

         Reference: This document (Section 3.1)

         Type: 19

         Description: Segment Routing Algorithm

         Reference: This document (Section 3.2)

         Type: 22

         Description: Segment Routing Local Block (SRLB)

         Reference: This document (Section 3.3)

         Type: 24

         Description: Segment Routing Mapping Server Preference (SRMS
         Preference)

         Reference: This document (Section 3.4)

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4.4.  New TLV Codepoint and Sub-TLV registry

   This document registers the following TLV:

      Type: 149

      name: Segment Identifier / Label Binding

      IIH: no

      LSP: yes

      SNP: no

      Purge: no

      Reference: This document (Section 2.4)

      Type: 150

      name: Multi-Topology Segment Identifier / Label Binding

      IIH: no

      LSP: yes

      SNP: no

      Purge: no

      Reference: This document (Section 2.5)

   This document creates the following sub-TLV Registry:

      Registry: sub-TLVs for TLV 149 and 150

      Registration Procedure: Expert review

      Reference: This document (Section 2.4)

         Type: 1

         Description: SID/Label

         Reference: This document (Section 2.4.5)

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         Type: 3

         Description: Prefix-SID

         Reference: This document (Section 2.1)

5.  Security Considerations

   With the use of the extensions defined in this document, IS-IS
   carries information which will be used to program the MPLS data plane
   [RFC3031].  In general, the same types of attacks that can be carried
   out on the IP/IPv6 control plane can be carried out on the MPLS
   control plane resulting in traffic being misrouted in the respective
   data planes.  However, the latter may be more difficult to detect and
   isolate.  Existing security extensions as described in [RFC5304] and
   [RFC5310] apply to these segment routing extensions.

6.  Acknowledgements

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

7.  Contributors

   The following people gave a substantial contribution to the content
   of this document and should be considered as co-authors:

   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant@gmail.com

   Peter Psenak
   Cisco Systems Inc.
   US

   Email: ppsenak@cisco.com

   Martin Horneffer
   Deutsche Telekom
   DE

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   Email: Martin.Horneffer@telekom.de

   Wim Henderickx
   Nokia
   BE

   Email: wim.henderickx@nokia.com

   Edward Crabbe
   Oracle
   US

   Email: edward.crabbe@oracle.com

   Rob Shakir
   Google
   UK

   Email: robjs@google.com

   Igor Milojevic
   Individual
   RS

   Email: milojevicigor@gmail.com

   Saku Ytti
   TDC
   FI

   Email: saku@ytti.fi

   Steven Luong
   Cisco Systems Inc.
   US

   Email: sluong@cisco.com

8.  References

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8.1.  Normative References

   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-27 (work in progress), December 2018.

   [I-D.ietf-spring-segment-routing-ldp-interop]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B., and
              S. Litkowski, "Segment Routing interworking with LDP",
              draft-ietf-spring-segment-routing-ldp-interop-15 (work in
              progress), September 2018.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-19
              (work in progress), March 2019.

   [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,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

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   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

8.2.  Informative References

   [RFC5311]  McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
              Shand, "Simplified Extension of Link State PDU (LSP) Space
              for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
              <https://www.rfc-editor.org/info/rfc5311>.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
              December 2008, <https://www.rfc-editor.org/info/rfc5316>.

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   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
              2016, <https://www.rfc-editor.org/info/rfc7855>.

Authors' Addresses

   Stefano Previdi (editor)
   Huawei
   IT

   Email: stefano@previdi.net

   Les Ginsberg (editor)
   Cisco Systems, Inc.
   IT

   Email: ginsberg@cisco.com

   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com

   Ahmed Bashandy
   Individual

   Email: abashandy.ietf@gmail.com

   Hannes Gredler
   RtBrick Inc.

   Email: hannes@rtbrick.com

   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com

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