



Internet Engineering Task Force (IETF)                    P. Psenak, Ed.
Request for Comments: 9352                                   C. Filsfils
Updates: 7370                                                A. Bashandy
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                              B. Decraene
                                                                  Orange
                                                                   Z. Hu
                                                     Huawei Technologies
                                                           February 2023


  IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane

Abstract

   The Segment Routing (SR) architecture allows a flexible definition of
   the end-to-end path by encoding it as a sequence of topological
   elements called "segments".  It can be implemented over the MPLS or
   the IPv6 data plane.  This document describes the IS-IS extensions
   required to support SR over the IPv6 data plane.

   This document updates RFC 7370 by modifying an existing registry.

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/rfc9352.

Copyright Notice

   Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  SRv6 Capabilities Sub-TLV
   3.  Advertising Supported Algorithms
   4.  Advertising Maximum SRv6 SID Depths
     4.1.  Maximum Segments Left MSD Type
     4.2.  Maximum End Pop MSD Type
     4.3.  Maximum H.Encaps MSD Type
     4.4.  Maximum End D MSD Type
   5.  SRv6 SIDs and Reachability
   6.  Advertising Anycast Property
   7.  Advertising Locators and End SIDs
     7.1.  SRv6 Locator TLV Format
     7.2.  SRv6 End SID Sub-TLV
   8.  Advertising SRv6 Adjacency SIDs
     8.1.  SRv6 End.X SID Sub-TLV
     8.2.  SRv6 LAN End.X SID Sub-TLV
   9.  SRv6 SID Structure Sub-Sub-TLV
   10. Advertising Endpoint Behaviors
   11. IANA Considerations
     11.1.  SRv6 Locator TLV
       11.1.1.  SRv6 End SID Sub-TLV
       11.1.2.  IS-IS Sub-TLVs for TLVs Advertising Prefix
               Reachability Registry
     11.2.  SRv6 Capabilities Sub-TLV
     11.3.  IS-IS Sub-Sub-TLVs for the SRv6 Capabilities Sub-TLV
             Registry
     11.4.  SRv6 End.X SID and SRv6 LAN End.X SID Sub-TLVs
     11.5.  MSD Types
     11.6.  IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs Registry
     11.7.  Prefix Attribute Flags Sub-TLV
     11.8.  IS-IS SRv6 Capabilities Sub-TLV Flags Registry
     11.9.  IS-IS SRv6 Locator TLV Flags Registry
     11.10. IS-IS SRv6 End SID Sub-TLV Flags Registry
     11.11. IS-IS SRv6 Adjacency SID Sub-TLVs Flags Registry
   12. Security Considerations
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   With Segment Routing (SR) [RFC8402], a node steers a packet through
   an ordered list of instructions, which are called segments.

   Segments are identified through Segment Identifiers (SIDs).

   SR can be directly instantiated on the IPv6 data plane through the
   use of the Segment Routing Header (SRH) defined in [RFC8754].  SRv6
   refers to this SR instantiation on the IPv6 data plane.

   The network programming paradigm [RFC8986] is central to SRv6.  It
   describes how any behavior can be bound to a SID and how any network
   program can be expressed as a combination of SIDs.

   This document specifies IS-IS extensions that allow the IS-IS
   protocol to encode some of these SIDs and their behaviors.

   Familiarity with the network programming paradigm [RFC8986] is
   necessary to understand the extensions specified in this document.

   The new SRv6 Locator top-level TLV announces SRv6 Locators -- a form
   of summary address for the set of topology-/algorithm-specific SIDs
   instantiated at the node.

   The SRv6 Capabilities sub-TLV announces the ability to support SRv6.

   Several new sub-TLVs are defined to advertise various SRv6 Maximum
   SID Depths (MSDs).

   The SRv6 End SID sub-TLV, the SRv6 End.X SID sub-TLV, and the SRv6
   LAN End.X SID sub-TLV are used to advertise which SIDs are
   instantiated at a node and what Endpoint behavior is bound to each
   instantiated SID.

   This document updates [RFC7370] by modifying an existing registry
   (Section 11.1.2).

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.  SRv6 Capabilities Sub-TLV

   A node indicates that it supports the SR Segment Endpoint Node
   functionality as specified in [RFC8754] by advertising a new SRv6
   Capabilities sub-TLV of the Router Capability TLV [RFC7981].

   The SRv6 Capabilities sub-TLV may contain optional sub-sub-TLVs.  No
   sub-sub-TLVs are currently defined.

   The SRv6 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              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   optional sub-sub-TLVs...

   Type:  25.  Single octet, as defined in Section 9 of [ISO10589].

   Length:  Single octet, as defined in Section 9 of [ISO10589].  The
      length value is 2 + length of sub-sub-TLVs.

   Flags:  2 octets.  The following flags are defined:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | |O|       Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      where:
         O-flag:  If set, the router supports use of the O-bit in the
            SRH, as defined in [RFC9259].

            The remaining bits, including bit 0, are reserved for future
            use.  They MUST be set to zero on transmission and MUST be
            ignored on receipt.

3.  Advertising Supported Algorithms

   An SRv6-capable router indicates one or more supported algorithms by
   advertising the Segment Routing Algorithm sub-TLV, as defined in
   [RFC8667].

4.  Advertising Maximum SRv6 SID Depths

   [RFC8491] defines the means to advertise node-/link-specific values
   for MSDs of various types.  Node MSDs are advertised in a sub-TLV of
   the Router Capability TLV [RFC7981].  Link MSDs are advertised in a
   sub-TLV of TLVs 22, 23, 25, 141, 222, and 223.

   This document defines the relevant SRv6 MSDs and requests MSD type
   assignments in the "IGP MSD-Types" registry created by [RFC8491].

4.1.  Maximum Segments Left MSD Type

   The Maximum Segments Left MSD Type signals the maximum value of the
   "Segments Left" field [RFC8754] in the SRH of a received packet
   before applying the Endpoint behavior associated with a SID.

      SRH Max Segments Left Type: 41

      If no value is advertised, the supported value is 0.

4.2.  Maximum End Pop MSD Type

   The Maximum End Pop MSD Type signals the maximum number of SIDs in
   the SRH to which the router can apply "Penultimate Segment Pop (PSP)
   of the SRH" or "Ultimate Segment Pop (USP) of the SRH" behavior, as
   defined in "Flavors" (Section 4.16 of [RFC8986]).

      SRH Max End Pop Type: 42

      If the advertised value is zero or no value is advertised, then
      the router cannot apply PSP or USP flavors.

4.3.  Maximum H.Encaps MSD Type

   The Maximum H.Encaps MSD Type signals the maximum number of SIDs that
   can be added to the segment list of an SRH as part of the "H.Encaps"
   behavior, as defined in [RFC8986].

      SRH Max H.encaps Type: 44

      If the advertised value is zero or no value is advertised, then
      the headend can apply an SR Policy that only contains one segment
      without inserting any SRH header.

      A non-zero SRH Max H.encaps MSD indicates that the headend can
      insert an SRH up to the advertised number of SIDs.

4.4.  Maximum End D MSD Type

   The Maximum End D MSD Type specifies the maximum number of SIDs
   present in an SRH when performing decapsulation.  As specified in
   [RFC8986], the permitted SID types include, but are not limited to,
   End.DX6, End.DT4, End.DT46, End with USD, and End.X with USD.

      SRH Max End D Type: 45

      If the advertised value is zero or no value is advertised, then
      the router cannot apply any behavior that results in decapsulation
      and forwarding of the inner packet if the outer IPv6 header
      contains an SRH.

5.  SRv6 SIDs and Reachability

   As discussed in [RFC8986], an SRv6 Segment Identifier (SID) is 128
   bits and consists of locator, function, and argument parts.

   A node is provisioned with topology-/algorithm-specific locators for
   each of the topology/algorithm pairs supported by that node.  Each
   locator is a covering prefix for all SIDs provisioned on that node
   that have the matching topology/algorithm.

   Locators MUST be advertised in the SRv6 Locator TLV (see
   Section 7.1).  Forwarding entries for the locators advertised in the
   SRv6 Locator TLV MUST be installed in the forwarding plane of
   receiving SRv6-capable routers when the associated topology/algorithm
   is supported by the receiving node.  The processing of the prefix
   advertised in the SRv6 Locator TLV, the calculation of its
   reachability, and the installation in the forwarding plane follows
   the process defined for the Prefix Reachability TLV 236 [RFC5308] or
   TLV 237 [RFC5120].

   Locators associated with algorithms 0 and 1 (for all supported
   topologies) SHOULD also be advertised in a Prefix Reachability TLV
   (236 or 237) so that legacy routers (i.e., routers that do not
   support SRv6) will install a forwarding entry for algorithms 0 and 1
   SRv6 traffic.

   In cases where the same prefix with the same prefix length, Multi-
   Topology Identifier (MTID), and algorithm is received in both a
   Prefix Reachability TLV and an SRv6 Locator TLV, the Prefix
   Reachability advertisement MUST be preferred when installing entries
   in the forwarding plane.  This is to prevent inconsistent forwarding
   entries between SRv6-capable and SRv6-incapable routers.  Such
   preference of Prefix Reachability advertisement does not have any
   impact on the rest of the data advertised in the SRv6 Locator TLV.

   Locators associated with Flexible Algorithms (see Section 4 of
   [RFC9350]) SHOULD NOT be advertised in Prefix Reachability TLVs (236
   or 237).  Advertising the Flexible Algorithm locator in a regular
   Prefix Reachability TLV (236 or 237) would make the forwarding for it
   follow the algorithm 0 path.

   SRv6 SIDs are advertised as sub-TLVs in the SRv6 Locator TLV, except
   for SRv6 SIDs that are associated with a specific neighbor/link and
   are therefore advertised as sub-TLVs in TLVs 22, 23, 25, 141, 222,
   and 223.

   SRv6 SIDs received from other nodes are not directly routable and
   MUST NOT be installed in the forwarding plane.  Reachability to SRv6
   SIDs depends upon the existence of a covering locator.

   Adherence to the rules defined in this section will ensure that SRv6
   SIDs associated with a supported topology/algorithm pair will be
   forwarded correctly, while SRv6 SIDs associated with an unsupported
   topology/algorithm pair will be dropped.  NOTE: The drop behavior
   depends on the absence of a default/summary route covering a given
   locator.

   In order for forwarding to work correctly, the locator associated
   with SRv6 SID advertisements must be the longest match prefix
   installed in the forwarding plane for those SIDs.  In order to ensure
   correct forwarding, network operators should take steps to make sure
   that this requirement is not compromised.  For example, the following
   situations should be avoided:

   *  Another locator associated with a different topology/algorithm is
      the longest match.

   *  Another prefix advertisement (i.e., from TLV 236 or 237) is the
      longest match.

6.  Advertising Anycast Property

   Both prefixes and SRv6 Locators may be configured as anycast; as
   such, the same value can be advertised by multiple routers.  It is
   useful for other routers to know that the advertisement is for an
   anycast identifier.

   A new flag in the Prefix Attribute Flags sub-TLV [RFC7794] is defined
   to advertise the anycast property:

   Bit #:  4
   Name:  Anycast Flag (A-flag)

   When the prefix/SRv6 Locator is configured as anycast, the A-flag
   SHOULD be set.  Otherwise, this flag MUST be clear.

   The A-flag MUST be preserved when the advertisement is leaked between
   levels.

   The A-flag and the N-flag MUST NOT both be set.  If both the N-flag
   and the A-flag are set in the prefix/SRv6 Locator advertisement, the
   receiving routers MUST ignore the N-flag.

   The same prefix/SRv6 Locator can be advertised by multiple routers.
   If at least one of them sets the A-flag in its advertisement, the
   prefix/SRv6 Locator SHOULD be considered as anycast.

   A prefix/SRv6 Locator that is advertised by a single node and without
   an A-flag MUST be considered node specific.

   All the nodes advertising the same anycast locator MUST instantiate
   the exact same set of SIDs under that anycast locator.  Failure to do
   so may result in traffic being dropped or misrouted.

   The Prefix Attribute Flags sub-TLV can be carried in the SRv6 Locator
   TLV as well as the Prefix Reachability TLVs.  When a router
   originates both the Prefix Reachability TLV and the SRv6 Locator TLV
   for a given prefix, it SHOULD advertise the Prefix Attribute Flags
   sub-TLV, if used, in both TLVs and use the same flags.  However,
   unlike TLVs 236 [RFC5308] and 237 [RFC5120], the X-flag in the Prefix
   Attributes Flags sub-TLV is valid when sent in the SRv6 Locator TLV.
   When included in the Locator TLV, the state of the X-flag in the
   Prefix Attributes Flags sub-TLV MUST match the setting of the
   embedded "X-bit" in any advertisement for the same prefix in TLVs 236
   [RFC5308] and 237 [RFC5120].  In case of any inconsistency between
   the Prefix Attribute Flags advertised in the Locator TLV and in the
   Prefix Reachability TLV, the ones advertised in the Prefix
   Reachability TLV MUST be preferred.

7.  Advertising Locators and End SIDs

   The SRv6 Locator TLV is introduced to advertise SRv6 Locators and End
   SIDs associated with each locator.

   This new TLV shares the sub-TLV space defined for TLVs 135, 235, 236,
   and 237.

7.1.  SRv6 Locator TLV Format

   The SRv6 Locator 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    |R|R|R|R|    MTID               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Locator Entries . . .                       |

   Type:  27.  Single octet, as defined in Section 9 of [ISO10589].

   Length:  Single octet, as defined in Section 9 of [ISO10589].  The
      length value is variable.

   R Bits:  Reserved for future use.  They MUST be set to zero on
      transmission and MUST be ignored on receipt.

   MTID:  Multi-Topology Identifier, as defined in [RFC5120].  Note that
      the value 0 is legal.

   The SRv6 Locator TLV is followed by one or more locator entries of
   the form:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Metric                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |  Algorithm    |  Loc Size     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //              Locator (continued, variable)                  //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Sub-TLV-len  |         Sub-TLVs (variable) . . .             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Metric:  4 octets, as described in Section 4 of [RFC5305].

   Flags:  1 octet.  The following flags are defined:

       0
       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |D|    Reserved |
      +-+-+-+-+-+-+-+-+

         D-flag: "up/down bit" as described in Section 4.1 of [RFC5305].

         The remaining bits are reserved for future use.  They MUST be
         set to zero on transmission and MUST be ignored on receipt.

   Algorithm:  1 octet, as defined in the "IGP Algorithm Types" registry
      [RFC8665].

   Loc-Size:  1 octet.  Number of bits in the SRv6 Locator field, which
      MUST be from the range (1-128).  The entire TLV MUST be ignored if
      the Loc-Size is outside this range.

   Locator:  1-16 octets.  This field encodes the advertised SRv6
      Locator.  The SRv6 Locator is encoded in the minimal number of
      octets for the given number of bits.  Trailing bits MUST be set to
      zero and ignored when received.

   Sub-TLV-length:  1 octet.  Number of octets used by sub-TLVs.

   Optional Sub-TLVs:  Supported sub-TLVs are specified in
      Section 11.1.2.  Any sub-TLV that is not allowed in the SRv6
      Locator TLV MUST be ignored.

   The Prefix Attribute Flags sub-TLV [RFC7794] SHOULD be included in
   the Locator TLV.

   The Prefix Attribute Flags sub-TLV MUST be included in the Locator
   TLV when it is leaked upwards in the hierarchy or originated as a
   result of the redistribution from another protocol or another IS-IS
   instance.  If the Prefix Attribute Flags sub-TLV is not included in
   these cases, receivers will be unable to determine the correct source
   of the advertisement.  The receivers will be unable to detect the
   violation.

7.2.  SRv6 End SID Sub-TLV

   The SRv6 End SID sub-TLV is introduced to advertise SRv6 SIDs with
   Endpoint behaviors that do not require a particular neighbor in order
   to be correctly applied.  SRv6 SIDs associated with a neighbor are
   advertised using the sub-TLVs defined in Section 8.

   Supported behavior values, together with parent TLVs in which they
   are advertised, are specified in Section 10 of this document.  Please
   note that not all behaviors defined in [RFC8986] are defined in this
   document, e.g., End.T is not.

   This new sub-TLV is advertised in the SRv6 Locator TLV defined in the
   previous section.  SRv6 End SIDs inherit the topology/algorithm from
   the parent locator.

   The SRv6 End 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      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Endpoint Behavior       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (128 bits) . . .                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-TLV-len|         Sub-sub-TLVs (variable) . . .         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:  5.  Single octet, as defined in Section 9 of [ISO10589].

   Length:  Single octet, as defined in Section 9 of [ISO10589].  The
      length value is variable.

   Flags:  1 octet.  No flags are currently defined.  All bits are
      reserved for future use.  They MUST be set to zero on transmission
      and MUST be ignored on receipt.

   Endpoint Behavior:  2 octets, as defined in [RFC8986].  Supported
      behavior values for this sub-TLV are defined in Section 10 of this
      document.  Unsupported or unrecognized behavior values are ignored
      by the receiver.

   SID:  16 octets.  This field encodes the advertised SRv6 SID.

   Sub-sub-TLV-length:  1 octet.  Number of octets used by sub-sub-TLVs.

   Optional Sub-sub-TLVs:  Supported sub-sub-TLVs are specified in
      Section 11.6.  Any sub-sub-TLV that is not allowed in the SRv6 End
      SID sub-TLV MUST be ignored.

   The SRv6 End SID MUST be allocated from its associated locator.  SRv6
   End SIDs that are not allocated from the associated locator MUST be
   ignored.

   Multiple SRv6 End SIDs MAY be associated with the same locator.  In
   cases where the number of SRv6 End SID sub-TLVs exceeds the capacity
   of a single TLV, multiple Locator TLVs for the same locator MAY be
   advertised.  For a given MTID/Locator, the algorithm MUST be the same
   in all TLVs.  If this restriction is not met, all TLVs for that MTID/
   Locator MUST be ignored.

8.  Advertising SRv6 Adjacency SIDs

   Certain SRv6 Endpoint behaviors [RFC8986] are associated with a
   particular adjacency.

   This document defines two new sub-TLVs of TLVs 22, 23, 25, 141, 222,
   and 223 -- namely "SRv6 End.X SID sub-TLVs" and "SRv6 LAN End.X SID
   sub-TLVs".

   IS-IS neighbor advertisements are topology specific but not algorithm
   specific.  SIDs advertised in SRv6 End.X SID and SRv6 LAN End.X SID
   sub-TLVs therefore inherit the topology from the associated neighbor
   advertisement, but the algorithm is specified in the individual SID.

   All SIDs advertised in SRv6 End.X SID and SRv6 LAN End.X SID sub-TLVs
   MUST be a subnet of a Locator with matching topology and algorithm
   that are advertised by the same node in an SRv6 Locator TLV.  SIDs
   that do not meet this requirement MUST be ignored.  This ensures that
   the node advertising these SIDs is also advertising its corresponding
   Locator with the algorithm that will be used for computing paths
   destined to the SID.

8.1.  SRv6 End.X SID Sub-TLV

   This sub-TLV is used to advertise an SRv6 SID associated with a
   point-to-point adjacency.  Multiple SRv6 End.X SID sub-TLVs MAY be
   associated with the same adjacency.

   The SRv6 End.X 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   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Weight      |        Endpoint Behavior      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (128 bits) . . .                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-tlv-len|         Sub-sub-TLVs (variable) . . .         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type: 43.  Single octet, as defined in Section 9 of [ISO10589].

   Length:  Single octet, as defined in Section 9 of [ISO10589].  The
      length value is variable.

   Flags:  1 octet.

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |B|S|P|Reserved |
      +-+-+-+-+-+-+-+-+

      where:
         B-Flag:  Backup flag.  If set, the SID is eligible for
            protection, e.g., using IP Fast Reroute (IPFRR) [RFC5286],
            as described in [RFC8355].

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

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

   Algorithm:  1 octet, as defined in the "IGP Algorithm Types" registry
      [RFC8665].

   Weight:  1 octet.  The value represents the weight of the SID for the
      purpose of load balancing.  The use of the weight is defined in
      [RFC8402].

   Endpoint Behavior:  2 octets, as defined in [RFC8986].  Supported
      behavior values for this sub-TLV are defined in Section 10 of this
      document.  Unsupported or unrecognized behavior values are ignored
      by the receiver.

   SID:  16 octets.  This field encodes the advertised SRv6 SID.

   Sub-sub-TLV-length:  1 octet.  Number of octets used by sub-sub-
      TLVs.

   Optional Sub-sub-TLVs:  Supported sub-sub-TLVs are specified in
      Section 11.6.  Any sub-sub-TLV that is not allowed in SRv6 End.X
      SID sub-TLV MUST be ignored.

   Note that multiple TLVs for the same neighbor may be required in
   order to advertise all the SRv6 SIDs associated with that neighbor.

8.2.  SRv6 LAN End.X SID Sub-TLV

   This sub-TLV is used to advertise an SRv6 SID associated with a LAN
   adjacency.  Since the parent TLV is advertising an adjacency to the
   Designated Intermediate System (DIS) for the LAN, it is necessary to
   include the System-ID of the physical neighbor on the LAN with which
   the SRv6 SID is associated.  Given that many neighbors may exist on a
   given LAN, multiple SRv6 LAN END.X SID sub-TLVs may be associated
   with the same LAN.  Note that multiple TLVs for the same DIS neighbor
   may be required in order to advertise all the SRv6 SIDs associated
   with that neighbor.

   The SRv6 LAN End.X 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    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |             Neighbor System-ID (ID length octets)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |   Algorithm   |   Weight      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Endpoint Behavior       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (128 bits) . . .                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-TLV-len|      Sub-sub-TLVs (variable) . . .            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:  44.  Single octet, as defined in Section 9 of [ISO10589].

   Length:  Single octet, as defined in Section 9 of [ISO10589].  The
      length value is variable.

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

   Flags:  1 octet.

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |B|S|P|Reserved |
      +-+-+-+-+-+-+-+-+

      The B-, S-, and P-flags are as described in Section 8.1.  Reserved
      bits MUST be zero when originated and MUST be ignored when
      received.

   Algorithm:  1 octet, as defined in the "IGP Algorithm Types" registry
      [RFC8665].

   Weight:  1 octet.  The value represents the weight of the SID for the
      purpose of load balancing.  The use of the weight is defined in
      [RFC8402].

   Endpoint Behavior:  2 octets, as defined in [RFC8986].  Supported
      behavior values for this sub-TLV are defined in Section 10 of this
      document.  Unsupported or unrecognized behavior values are ignored
      by the receiver.

   SID:  16 octets.  This field encodes the advertised SRv6 SID.

   Sub-sub-TLV-length:  1 octet.  Number of octets used by sub-sub-
      TLVs.

   Optional Sub-sub-TLVs:  Supported sub-sub-TLVs are specified in
      Section 11.6.  Any sub-sub-TLV that is not allowed in SRv6 LAN
      End.X SID sub-TLV MUST be ignored.

   Note that multiple TLVs for the same neighbor, on the same LAN, may
   be required in order to advertise all the SRv6 SIDs associated with
   that neighbor.

9.  SRv6 SID Structure Sub-Sub-TLV

   The SRv6 SID Structure sub-sub-TLV is an optional sub-sub-TLV of:

   *  SRv6 End SID sub-TLV (Section 7.2)

   *  SRv6 End.X SID sub-TLV (Section 8.1)

   *  SRv6 LAN End.X SID sub-TLV (Section 8.2)

   The SRv6 SID Structure sub-sub-TLV is used to advertise the structure
   of the SRv6 SID, as defined in [RFC8986].  It 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     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    LB Length  |  LN Length    | Fun. Length   |  Arg. Length  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:
      Type:  1.  Single octet, as defined in Section 9 of [ISO10589].

      Length:  Single octet, as defined in Section 9 of [ISO10589].  The
         length value is 4 octets.

      LB Length:  1 octet.  SRv6 SID Locator Block length in bits.

      LN Length:  1 octet.  SRv6 SID Locator Node length in bits.

      Fun. Length:  1 octet.  SRv6 SID Function length in bits.

      Arg. Length:  1 octet.  SRv6 SID Arguments length in bits.

   The IS-IS SRv6 SID Structure sub-sub-TLV MUST NOT appear more than
   once in its parent sub-TLV.  If it appears more than once in its
   parent sub-TLV, the parent sub-TLV MUST be ignored by the receiver.

   The sum of all four sizes advertised in the IS-IS SRv6 SID Structure
   sub-sub-TLV MUST be less than or equal to 128 bits.  If the sum of
   all four sizes advertised in the IS-IS SRv6 SID Structure sub-sub-TLV
   is larger than 128 bits, the parent sub-TLV MUST be ignored by the
   receiver.

   The SRv6 SID Structure sub-sub-TLV is intended for informational use
   by the control and management planes.  It MUST NOT be used at a
   transit node (as defined in [RFC8754]) for forwarding packets.  As an
   example, this information could be used for the following:

   *  validation of SRv6 SIDs being instantiated in the network and
      advertised via IS-IS.  These can be learned by controllers via
      Border Gateway Protocol - Link State (BGP-LS) and then be
      monitored for conformance to the SRv6 SID allocation scheme chosen
      by the operator, as described in Section 3.2 of [RFC8986].

   *  verification and automation for securing the SRv6 domain by
      provisioning filtering rules at SR domain boundaries, as described
      in Section 5 of [RFC8754].

   The details of these potential applications are outside the scope of
   this document.

10.  Advertising Endpoint Behaviors

   Endpoint behaviors are defined in [RFC8986].  The codepoints for the
   Endpoint behaviors are defined in the "SRv6 Endpoint Behaviors"
   registry defined in [RFC8986].  If a behavior is advertised, it MUST
   only be advertised in the TLV(s) marked with "Y" in the table below
   and MUST NOT be advertised in the TLV(s) marked with "N" in the table
   below.

    +===================+===================+=====+=======+===========+
    | Endpoint Behavior | Endpoint Behavior | End | End.X | Lan End.X |
    |                   | Codepoint         | SID | SID   | SID       |
    +===================+===================+=====+=======+===========+
    | End (PSP, USP,    | 1-4, 28-31        | Y   | N     | N         |
    | USD)              |                   |     |       |           |
    +-------------------+-------------------+-----+-------+-----------+
    | End.X (PSP, USP,  | 5-8, 32-35        | N   | Y     | Y         |
    | USD)              |                   |     |       |           |
    +-------------------+-------------------+-----+-------+-----------+
    | End.DX6           | 16                | N   | Y     | Y         |
    +-------------------+-------------------+-----+-------+-----------+
    | End.DX4           | 17                | N   | Y     | Y         |
    +-------------------+-------------------+-----+-------+-----------+
    | End.DT6           | 18                | Y   | N     | N         |
    +-------------------+-------------------+-----+-------+-----------+
    | End.DT4           | 19                | Y   | N     | N         |
    +-------------------+-------------------+-----+-------+-----------+
    | End.DT46          | 20                | Y   | N     | N         |
    +-------------------+-------------------+-----+-------+-----------+

                        Table 1: Endpoint Behaviors

11.  IANA Considerations

   This document requests allocation for the following TLVs, sub-TLVs,
   and sub-sub-TLVs by updating the existing registries and defining new
   registries under the "IS-IS TLV Codepoints" grouping.

11.1.  SRv6 Locator TLV

   The SRv6 Locator TLV shares sub-TLV space with TLVs advertising
   prefix reachability.  IANA has updated the "IS-IS Sub-TLVs for TLVs
   Advertising Prefix Reachability" registry initially defined in
   [RFC7370] by adding this document as a reference and updating the
   description of that registry to include the SRv6 Locator TLV (27).

   This document makes the following registration in the "IS-IS Top-
   Level TLV Codepoints" registry:

            +=======+==============+=====+=====+=====+=======+
            | Value | Name         | IIH | LSP | SNP | Purge |
            +=======+==============+=====+=====+=====+=======+
            | 27    | SRv6 Locator | n   | y   | n   | n     |
            +-------+--------------+-----+-----+-----+-------+

             Table 2: IS-IS Top-Level TLV Codepoints Registry

11.1.1.  SRv6 End SID Sub-TLV

   This document makes the following registration:

    +======+==============+====+=====+=====+=====+=====+=============+
    | Type | Description  | 27 | 135 | 235 | 236 | 237 | Reference   |
    +======+==============+====+=====+=====+=====+=====+=============+
    | 5    | SRv6 End SID | y  | n   | n   | n   | n   | RFC 9352,   |
    |      |              |    |     |     |     |     | Section 7.2 |
    +------+--------------+----+-----+-----+-----+-----+-------------+

           Table 3: IS-IS Sub-TLVs for TLVs Advertising Prefix
                          Reachability Registry

11.1.2.  IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability
         Registry

   IANA has updated the "IS-IS Sub-TLVs for TLVs Advertising Prefix
   Reachability" registry to include a column for the SRv6 Locator TLV
   (27) as shown below:

       +======+=======================+====+=====+=====+=====+=====+
       | Type | Description           | 27 | 135 | 235 | 236 | 237 |
       +======+=======================+====+=====+=====+=====+=====+
       | 1    | 32-bit Administrative | y  | y   | y   | y   | y   |
       |      | Tag Sub-TLV           |    |     |     |     |     |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 2    | 64-bit Administrative | y  | y   | y   | y   | y   |
       |      | Tag Sub-TLV           |    |     |     |     |     |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 3    | Prefix Segment        | n  | y   | y   | y   | y   |
       |      | Identifier            |    |     |     |     |     |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 4    | Prefix Attribute      | y  | y   | y   | y   | y   |
       |      | Flags                 |    |     |     |     |     |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 6    | Flexible Algorithm    | n  | y   | y   | y   | y   |
       |      | Prefix Metric (FAPM)  |    |     |     |     |     |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 11   | IPv4 Source Router ID | y  | y   | y   | y   | y   |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 12   | IPv6 Source Router ID | y  | y   | y   | y   | y   |
       +------+-----------------------+----+-----+-----+-----+-----+
       | 32   | BIER Info             | n  | y   | y   | y   | y   |
       +------+-----------------------+----+-----+-----+-----+-----+

            Table 4: IS-IS Sub-TLVs for TLVs Advertising Prefix
                           Reachability Registry

11.2.  SRv6 Capabilities Sub-TLV

   This document makes the following registration in the "IS-IS Sub-TLVs
   for IS-IS Router CAPABILITY TLV" registry:

            +=======+===================+=====================+
            | Value | Description       | Reference           |
            +=======+===================+=====================+
            | 25    | SRv6 Capabilities | RFC 9352, Section 2 |
            +-------+-------------------+---------------------+

                  Table 5: IS-IS Sub-TLVs for IS-IS Router
                          CAPABILITY TLV Registry

11.3.  IS-IS Sub-Sub-TLVs for the SRv6 Capabilities Sub-TLV Registry

   IANA has created the "IS-IS Sub-Sub-TLVs for SRv6 Capabilities Sub-
   TLV" registry under the "IS-IS TLV Codepoints" grouping for the
   assignment of sub-TLV types for the SRv6 Capabilities sub-TLV
   specified in this document (Section 2).  This registry defines sub-
   sub-TLVs for the SRv6 Capabilities sub-TLV (25) advertised in the IS-
   IS Router CAPABILITY TLV (242).

   The registration procedure is "Expert Review", as defined in
   [RFC8126].  Guidance for the designated experts is provided in
   [RFC7370].  No sub-sub-TLVs are defined by this document, except for
   the reserved type 0.

                    +=======+=============+===========+
                    | Value | Description | Reference |
                    +=======+=============+===========+
                    | 0     | Reserved    | RFC 9532  |
                    +-------+-------------+-----------+
                    | 1-255 | Unassigned  |           |
                    +-------+-------------+-----------+

                      Table 6: IS-IS Sub-Sub-TLVs for
                         SRv6 Capabilities Sub-TLV
                                  Registry

11.4.  SRv6 End.X SID and SRv6 LAN End.X SID Sub-TLVs

   This document makes the following registrations in the "IS-IS Sub-
   TLVs for TLVs Advertising Neighbor Information" registry:

   +======+=============+====+====+====+=====+=====+=====+=============+
   | Type | Description | 22 | 23 | 25 | 141 | 222 | 223 | Reference   |
   +======+=============+====+====+====+=====+=====+=====+=============+
   | 43   | SRv6 End.X  | y  | y  | y  | y   | y   | y   | RFC 9352,   |
   |      | SID         |    |    |    |     |     |     | Section     |
   |      |             |    |    |    |     |     |     | 8.1         |
   +------+-------------+----+----+----+-----+-----+-----+-------------+
   | 44   | SRv6 LAN    | y  | y  | y  | y   | y   | y   | RFC 9352,   |
   |      | End.X SID   |    |    |    |     |     |     | Section     |
   |      |             |    |    |    |     |     |     | 8.2         |
   +------+-------------+----+----+----+-----+-----+-----+-------------+

     Table 7: IS-IS Sub-TLVs for TLVs Advertising Neighbor Information
                                  Registry

11.5.  MSD Types

   This document makes the following registrations in the "IGP MSD-
   Types" registry:

                 +=======+==================+===========+
                 | Value | Name             | Reference |
                 +=======+==================+===========+
                 | 41    | SRH Max SL       | RFC 9352  |
                 +-------+------------------+-----------+
                 | 42    | SRH Max End Pop  | RFC 9352  |
                 +-------+------------------+-----------+
                 | 44    | SRH Max H.encaps | RFC 9352  |
                 +-------+------------------+-----------+
                 | 45    | SRH Max End D    | RFC 9352  |
                 +-------+------------------+-----------+

                          Table 8: IGP MSD-Types

11.6.  IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs Registry

   IANA has created the "IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs"
   registry under the "IS-IS TLV Codepoints" grouping to assign sub-TLV
   types for the SID sub-TLVs specified in this document (Sections 7.2,
   8.1, and 8.2).

   This registry defines sub-sub-TLVs for SRv6 SID sub-TLVs.  This
   includes the following sub-TLVs:

   *  SRv6 End SID (5) (Advertised in SRv6 Locator TLV (27))

   *  SRv6 End.X SID (43) (Advertised in TLVs advertising neighbor
      information)

   *  SRv6 LAN End.X SID (44) (Advertised in TLVs advertising neighbor
      information)

   The registration procedure is "Expert Review", as defined in
   [RFC8126].  Guidance for the designated experts is provided in
   [RFC7370].  The following assignments are made by this document:

         +=======+====================+===+====+====+===========+
         | Type  | Description        | 5 | 43 | 44 | Reference |
         +=======+====================+===+====+====+===========+
         | 0     | Reserved           |   |    |    | RFC 9352  |
         +-------+--------------------+---+----+----+-----------+
         | 1     | SRv6 SID Structure | y | y  | y  | RFC 9352  |
         +-------+--------------------+---+----+----+-----------+
         | 2-255 | Unassigned         |   |    |    |           |
         +-------+--------------------+---+----+----+-----------+

            Table 9: IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs
                                 Registry

11.7.  Prefix Attribute Flags Sub-TLV

   This document adds a new bit in the "IS-IS Bit Values for Prefix
   Attribute Flags Sub-TLV" registry:

          +=======+=======================+=====================+
          | Bit # | Name                  | Reference           |
          +=======+=======================+=====================+
          | 4     | Anycast Flag (A-flag) | RFC 9352, Section 6 |
          +-------+-----------------------+---------------------+

              Table 10: IS-IS Bit Values for Prefix Attribute
                           Flags Sub-TLV Registry

11.8.  IS-IS SRv6 Capabilities Sub-TLV Flags Registry

   IANA has created the "IS-IS SRv6 Capabilities Sub-TLV Flags" registry
   under the "IS-IS TLV Codepoints" grouping to assign bits 0 to 15 in
   the Flags field of the IS-IS SRv6 Capabilities sub-TLV specified in
   this document (Section 2).  This registry defines bit values
   advertised in the Flags field of the SRv6 Capabilities sub-TLV (25).
   This sub-TLV is advertised in the IS-IS Router CAPABILITY TLV (242).

   The registration procedure is "Expert Review", as defined in
   [RFC8126].  Guidance for the designated experts is provided in
   [RFC7370].  The following assignments are made by this document:

               +======+=============+=====================+
               | Type | Description | Reference           |
               +======+=============+=====================+
               | 0    | Unassigned  |                     |
               +------+-------------+---------------------+
               | 1    | O-flag      | RFC 9352, Section 2 |
               +------+-------------+---------------------+
               | 2-15 | Unassigned  |                     |
               +------+-------------+---------------------+

                  Table 11: IS-IS SRv6 Capabilities Sub-
                            TLV Flags Registry

11.9.  IS-IS SRv6 Locator TLV Flags Registry

   IANA has created the "IS-IS SRv6 Locator TLV Flags" registry under
   the "IS-IS TLV Codepoints" grouping to assign bits 0 to 7 in the
   Flags field of the SRv6 Locator TLV specified in this document
   (Section 7.1).  This registry defines bit values advertised in the
   Flags field of the SRv6 Locator TLV (27).

   The registration procedure is "Expert Review", as defined in
   [RFC8126].  Guidance for the designated experts is provided in
   [RFC7370].  The following assignments are made by this document:

              +=======+=============+=======================+
              | Value | Description | Reference             |
              +=======+=============+=======================+
              | 0     | D-flag      | RFC 9352, Section 7.1 |
              +-------+-------------+-----------------------+
              | 1-7   | Unassigned  |                       |
              +-------+-------------+-----------------------+

              Table 12: IS-IS SRv6 Locator TLV Flags Registry

11.10.  IS-IS SRv6 End SID Sub-TLV Flags Registry

   IANA has created the "IS-IS SRv6 End SID Sub-TLV Flags" registry
   under the "IS-IS TLV Codepoints" grouping to assign bits 0 to 7 in
   the Flags field of the IS-IS SRv6 End SID sub-TLV specified in this
   document (Section 7.2).  This registry defines bit values advertised
   in the Flags field of the SRv6 End SID sub-TLV (5), which is
   advertised in the SRv6 Locator TLV (27).

   The registration procedure is "Expert Review", as defined in
   [RFC8126].  Guidance for the designated experts is provided in
   [RFC7370].  No assignments are made by this document.

                          +=======+=============+
                          | Value | Description |
                          +=======+=============+
                          | 0-7   | Unassigned  |
                          +-------+-------------+

                            Table 13: IS-IS SRv6
                           End SID Sub-TLV Flags
                                  Registry

11.11.  IS-IS SRv6 Adjacency SID Sub-TLVs Flags Registry

   IANA has created the "IS-IS SRv6 Adjacency SID Sub-TLVs Flags"
   registry under the "IS-IS TLV Codepoints" grouping to assign bits 0
   to 7 in the Flags field of the IS-IS SRv6 End.X SID and LAN End.X SID
   sub-TLVs (Sections 8.1 and 8.2).

   This registry defines bit values advertised in the Flags field of
   SRv6 SID sub-TLVs associated with adjacencies.  These sub-TLVs are
   advertised in TLVs advertising neighbor information.  The list of
   sub-TLVs includes:

   *  SRv6 End.X SID (43)

   *  SRv6 LAN End.X SID (44)

   The registration procedure is "Expert Review", as defined in
   [RFC8126].  Guidance for the designated experts is provided in
   [RFC7370].  The following assignments are made by this document:

              +=======+=============+=======================+
              | Value | Description | Reference             |
              +=======+=============+=======================+
              | 0     | B-flag      | RFC 9352, Section 8.1 |
              +-------+-------------+-----------------------+
              | 1     | S-flag      | RFC 9352, Section 8.1 |
              +-------+-------------+-----------------------+
              | 2     | P-flag      | RFC 9352, Section 8.1 |
              +-------+-------------+-----------------------+
              | 3-7   | Unassigned  |                       |
              +-------+-------------+-----------------------+

                Table 14: IS-IS SRv6 Adjacency SID Sub-TLVs
                               Flags Registry

12.  Security Considerations

   Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
   and [RFC5310].  While IS-IS is deployed under a single administrative
   domain, there can be deployments where potential attackers have
   access to one or more networks in the IS-IS routing domain.  In these
   deployments, the stronger authentication mechanisms defined in the
   aforementioned documents SHOULD be used.

   This document describes the IS-IS extensions required to support SR
   over an IPv6 data plane.  The security considerations for SR are
   discussed in [RFC8402].  [RFC8986] defines the SRv6 Network
   Programming concept and specifies the main SR behaviors to enable the
   creation of interoperable overlays; the security considerations from
   that document apply too.

   The advertisement for an incorrect MSD value may have negative
   consequences; see [RFC8491] for additional considerations.

   Security concerns associated with the setting of the O-flag are
   described in [RFC9259].

   Security concerns associated with the usage of Flexible Algorithms
   are described in [RFC9350]).

13.  References

13.1.  Normative References

   [ISO10589] ISO, "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)", Second Edition, ISO/IEC 10589:2002, 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>.

   [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>.

   [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>.

   [RFC7370]  Ginsberg, L., "Updates to the IS-IS TLV Codepoints
              Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
              <https://www.rfc-editor.org/info/rfc7370>.

   [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>.

   [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>.

   [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>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [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, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

   [RFC9259]  Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
              Chen, "Operations, Administration, and Maintenance (OAM)
              in Segment Routing over IPv6 (SRv6)", RFC 9259,
              DOI 10.17487/RFC9259, June 2022,
              <https://www.rfc-editor.org/info/rfc9259>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/rfc/rfc9350>.

13.2.  Informative References

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [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>.

   [RFC8355]  Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R.
              Shakir, "Resiliency Use Cases in Source Packet Routing in
              Networking (SPRING) Networks", RFC 8355,
              DOI 10.17487/RFC8355, March 2018,
              <https://www.rfc-editor.org/info/rfc8355>.

Acknowledgements

   Thanks to Christian Hopps for his review comments and shepherd work.

   Thanks to Alvaro Retana and John Scudder for AD review and comments.

Contributors

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

   Stefano Previdi
   Huawei Technologies
   Email: stefano@previdi.net


   Paul Wells
   Cisco Systems
   Saint Paul, Minnesota
   United States of America
   Email: pauwells@cisco.com


   Daniel Voyer
   Email: daniel.voyer@bell.ca


   Satoru Matsushima
   Email: satoru.matsushima@g.softbank.co.jp


   Bart Peirens
   Email: bart.peirens@proximus.com


   Hani Elmalky
   Email: hani.elmalky@ericsson.com


   Prem Jonnalagadda
   Email: prem@barefootnetworks.com


   Milad Sharif
   Email: msharif@barefootnetworks.com


   Robert Hanzl
   Cisco Systems
   Millenium Plaza Building, V Celnici 10, Prague 1
   Prague
   Czech Republic
   Email: rhanzl@cisco.com


   Ketan Talaulikar
   Cisco Systems, Inc.
   Email: ketant@cisco.com


Authors' Addresses

   Peter Psenak (editor)
   Cisco Systems
   Pribinova Street 10
   81109 Bratislava
   Slovakia
   Email: ppsenak@cisco.com


   Clarence Filsfils
   Cisco Systems
   Brussels
   Belgium
   Email: cfilsfil@cisco.com


   Ahmed Bashandy
   Cisco Systems
   Milpitas,
   United States of America
   Email: bashandy@cisco.com


   Bruno Decraene
   Orange
   Chatillon
   France
   Email: bruno.decraene@orange.com


   Zhibo Hu
   Huawei Technologies
   Email: huzhibo@huawei.com
