Internet Engineering Task Force (IETF)                   S. Previdi, Ed.
Request for Comments: 8667                           Huawei Technologies
Category: Standards Track                               L. Ginsberg, Ed.
ISSN: 2070-1721                                              C. Filsfils
                                                     Cisco Systems, Inc.
                                                             A. Bashandy
                                                                  Arrcus
                                                              H. Gredler
                                                            RtBrick Inc.
                                                             B. Decraene
                                                                  Orange
                                                          September
                                                            October 2019

                  IS-IS Extensions for Segment Routing

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8667.

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
     1.1.  Requirements Language
   2.  Segment Routing Identifiers
     2.1.  Prefix Segment Identifier (Prefix-SID) Sub-TLV
       2.1.1.  Flags
       2.1.2.  Prefix-SID Propagation
     2.2.  Adjacency Segment Identifier
       2.2.1.  Adjacency Segment Identifier (Adj-SID) Sub-TLV
       2.2.2.  Adjacency Segment Identifiers in LANs
     2.3.  SID/Label Sub-TLV
     2.4.  SID/Label Binding TLV
       2.4.1.  Flags
       2.4.2.  Range
       2.4.3.  Prefix Length, Prefix
       2.4.4.  Mapping Server Prefix-SID
       2.4.5.  SID/Label Sub-TLV
       2.4.6.  Example Encodings
     2.5.  Multi-Topology SID/Label Binding TLV
   3.  Router Capabilities
     3.1.  SR-Capabilities Sub-TLV
     3.2.  SR-Algorithm Sub-TLV
     3.3.  SR Local Block Sub-TLV
     3.4.  SRMS Preference Sub-TLV
   4.  IANA Considerations
     4.1.  Sub-TLVs for Types 22, 23, 25, 141, 222, and 223
     4.2.  Sub-TLVs for Types 135, 235, 236, and 237
     4.3.  Sub-TLVs for Type 242
     4.4.  New TLV Codepoint and Sub-TLV Registry
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

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

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

   This document describes the 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].

1.1.  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.

2.  Segment Routing Identifiers

   The Segment Routing architecture [RFC8402] defines different types of
   Segment Identifiers (SIDs).  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.

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

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

      The Binding TLV and Multi-Topology Binding TLV are defined in
      Sections 2.4 and 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 the 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 from either another
                        level (i.e., from Level-1 to Level-2 or the
                        opposite) or 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 that
                        are 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 (No Penultimate Hop-Popping 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 that
                        has 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.  Algorithm 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 (see
      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 (see Section 3.2) MUST ignore the Prefix-SID
      sub-TLV.

      SID/Index/Label as defined in Section 2.1.1.1.

   When the Prefix SID is an index (and 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 the 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 the 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:

      The V-flag and L-flag are 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.

      The V-flag and L-flag are 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 the 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 are advertised because of:

      propagation (Level-1 into Level-2);

      leaking (Level-2 into Level-1); or

      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 Attribute 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 Attribute 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:

   *  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 data
      plane, 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.

   *  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
         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 interdomain border router (prefix propagation
         from one domain to another).

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

   When propagating (from either Level-1 to Level-2 or Level-2 to Level-
   1) 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
   Sections 2.1.1.2 and 2.1.1.3.

2.2.  Adjacency Segment Identifier

   A new IS-IS sub-TLV is defined: the Adjacency Segment Identifier
   (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 (MT-ISN) [RFC5120]

      TLV-23 (IS Neighbor Attribute) [RFC5311]

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

      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 the 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 IP Fast Reroute
                        (IPFRR) or MPLS Fast Reroute (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.

               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 Adj-SID sub-TLV use 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 that is a part
   of the TLV advertising the adjacency to the DIS (e.g., TLV-22).

   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 the following flags:

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

               where the 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:  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.

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

   In case TLV-22, TLV-23, TLV-222, or TLV-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-SIDs 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 an 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

      SID/Label:  If the length is set to 3, then the 20 rightmost bits
                  represent an MPLS label.  If the length is set to 4,
                  then the value is 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
   [RFC8661].

   The SID/Label Binding TLV may also be used to advertise a Mirror SID
   indicating the ability of a node 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

   *

   where:

      Type:       149

   *

      Length:     Variable

   *

      Flags:      1 octet of flags

   *

      RESERVED:   1 octet of RESERVED (SHOULD be transmitted as 0 and MUST be
                  ignored on receipt)

   *

      Range:      2 octets of Range

   *

      Prefix Length:  1 octet of Prefix Length

   *

      Prefix:     0-16 octets of prefix

   *

      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 the 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 to
      not leak Binding TLVs between levels (for policy reasons).

      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 up
   16, 3 octets for prefix length 17 up to 24, 4 octets for prefix
   length 25 up to 32, ...., and 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.

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:

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

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

   In the absence of a 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; thus,
   PHP MUST be done by the neighbors of the router originating the
   prefix reachability advertisement.  Note that the 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, then:

      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|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, then:

      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, then:

      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

      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    |       0x0d    |       0xb8    |     0x00      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    0x01       |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

   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 Multi-
   Topology IS-IS (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) (MT ID) as defined here below:

      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             MT ID             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     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

      MT ID 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         MT ID         |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

         MTID:

         MT ID:   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 that are inserted into the IS-IS Router
   Capability 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
   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 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      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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 Segment Routing Global Block (SRGB) Descriptor
      entries, each of which have the following format:

         Range:   3 octets

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

   The 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 black
   hole / misdirect some traffic during the IGP convergence.  In
   particular, if a range that 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 [RFC8660].

   Here follows an example of the advertisement of multiple ranges:

         The originating router advertises the 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

         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 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
   [RFC8665].  The following values have been defined:

      0:  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 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.

   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 advertised 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, it implies that Algorithm algorithm 0 is the only algorithm supported by
   the routers that support the extensions defined in this document.

   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 components other 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 local SIDs
   are available in the router, it is required that the router
   advertises its SRLB.

   The SRLB sub-TLV is used for this 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  MPLS label as defined in Section 2.3) 2.3

   The 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.

   When a router receives multiple overlapping ranges, it MUST conform
   to the procedures defined in [RFC8660].

   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 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
   means and protocols that are outside the scope of this document.

   A router advertising the SRLB sub-TLV may also have other label
   ranges, outside the SRLB, for its local allocation purposes that 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 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 the following format:

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

      Type:        24

      Length:      1

      Preference:  1 octet and 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 [RFC8661].

4.  IANA Considerations

   Per this document, IANA has allocated the following TLVs and sub-
   TLVs.

4.1.  Sub-TLVs for Types 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.

   +------+--------------------+----+----+----+-----+-----+-----+
   | Type | Description        | 22 | 23 | 25 | 141 | 222 | 223 |
   +======+====================+====+====+====+=====+=====+=====+
   |  31  | Adjacency Segment  | y  | y  | n  |  y  |  y  |  y  |
   |      | Identifier         |    |    |    |     |     |     |
   +------+--------------------+----+----+----+-----+-----+-----+
   |  32  | LAN Adjacency      | y  | y  | n  |  y  |  y  |  y  |
   |      | Segment Identifier |    |    |    |     |     |     |
   +------+--------------------+----+----+----+-----+-----+-----+

                              Table 1

4.2.  Sub-TLVs for Types 135, 235, 236, and 237

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

   +------+---------------------------+-----+------+-----+-----+
   | Type | Description               | 135 | 2235 | 236 | 237 |
   +======+===========================+=====+======+=====+=====+
   |  3   | Prefix Segment Identifier |  y  |  y   |  y  |  y  |
   +------+---------------------------+-----+------+-----+-----+

                              Table 2

4.3.  Sub-TLVs for Type 242

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

   +------+------------------------------------+
   | Type | Description                        |
   +======+====================================+
   |  2   | Segment Routing Capability         |
   +------+------------------------------------+
   |  19  | Segment Routing Algorithm          |
   +------+------------------------------------+
   |  22  | Segment Routing Local Block (SRLB) |
   +------+------------------------------------+
   |  24  | Segment Routing Mapping Server     |
   |      | Preference (SRMS Preference)       |
   +------+------------------------------------+

                      Table 3

4.4.  New TLV Codepoint and Sub-TLV Registry

   This document registers the following TLV:

   +-------+----------------------------+-----+-----+-----+-------+
   | Value | Name                       | IIH | LSP | SNP | Purge |
   +=======+============================+=====+=====+=====+=======+
   |  149  | Segment Identifier/Label   |  n  |  y  |  n  |   n   |
   |       | Binding                    |     |     |     |       |
   +-------+----------------------------+-----+-----+-----+-------+
   |  150  | Multi-Topology Segment     |  n  |  y  |  n  |   n   |
   |       | Identifier / Label Binding |     |     |     |       |
   +-------+----------------------------+-----+-----+-----+-------+

                               Table 4

   This document creates the following sub-TLV Registry:

   Name:  Sub-TLVs for TLVs 149 and 150

   Registration Procedure:  Expert Review [RFC8126]

   +-------+-------------+
   |  Type | Description |
   +=======+=============+
   |   0   |   Reserved  |
   +-------+-------------+
   |   1   |  SID/Label  |
   +-------+-------------+
   |   2   |  Unassigned |
   +-------+-------------+
   |   3   |  Prefix SID |
   +-------+-------------+
   | 4-255 |  Unassigned |
   +-------+-------------+

           Table 5

5.  Security Considerations

   With the use of the extensions defined in this document, IS-IS
   carries information that will be used to program the MPLS data plane
   [RFC3031].  In general, the same type 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.  References

6.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- 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,
              November 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>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, September 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8661]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., and S. Litkowski, "Segment Routing
              Interworking with LDP", RFC 8661, DOI 10.17487/RFC8661,
              September 2019, <https://www.rfc-editor.org/info/rfc8661>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, September 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

6.2.  Informative References

   [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>.

   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

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.

Contributors

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

   Stephane Litkowski
   Orange
   France
   Email: stephane.litkowski@orange.com

   Jeff Tantsura
   Apstra, Inc.
   Email: jefftant@gmail.com

   Peter Psenak
   Cisco Systems Inc.
   United States of America
   Email: ppsenak@cisco.com

   Martin Horneffer
   Deutsche Telekom
   Germany
   Email: Martin.Horneffer@telekom.de

   Wim Henderickx
   Nokia
   Belgium
   Email: wim.henderickx@nokia.com

   Edward Crabbe
   Oracle
   United States of America
   Email: edward.crabbe@oracle.com

   Rob Shakir
   Google
   United Kingdom
   Email: robjs@google.com

   Igor Milojevic
   Individual
   Serbia
   Email: milojevicigor@gmail.com

   Saku Ytti
   TDC
   Finland
   Email: saku@ytti.fi

   Steven Luong
   Cisco Systems, Inc.
   United States of America
   Email: sluong@cisco.com

Authors' Addresses

   Stefano Previdi (editor)
   Huawei Technologies
   Italy

   Email: stefano@previdi.net

   Les Ginsberg (editor)
   Cisco Systems, Inc.
   United States of America

   Email: ginsberg@cisco.com

   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com

   Ahmed Bashandy
   Arrcus

   Email: abashandy.ietf@gmail.com

   Hannes Gredler
   RtBrick Inc.

   Email: hannes@rtbrick.com

   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com