Network Working Group                                          W. Cheng
Internet Draft                                                 W. Jiang
Intended status: Standards Track                           China Mobile
Expires: February 05, 2026                                       C. Lin
                                                                M. Chen
                                                   New H3C Technologies
                                                                L. Gong
                                                           China Mobile
                                                                  Y.Liu
                                                        ZTE Corporation
                                                        August 04, 2025



           IS-IS Extension to Advertise SRv6 SIDs using SID Block
                  draft-cheng-lsr-isis-srv6-sid-block-04


Abstract

   This document proposes a simplified method to advertise SRv6 SIDs in
   IS-IS. The SRv6 SID Block is composed of a number of continuous SIDs
   within the address range of a Locator. When a SID is assigned from
   the SID Block, it is described by an index based on the SID Block,
   instead of the whole 128-bit IPv6 address.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This Internet-Draft will expire on February 05, 2026.



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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Table of Contents


   1. Introduction...................................................2
      1.1. Requirements Language.....................................4
   2. SRv6 SID Block sub-TLV.........................................4
   3. SRv6 End.X SID Index sub-TLV...................................6
   4. SRv6 LAN End.X SID Index sub-TLV...............................7
   5. SRv6 End SID Index sub-TLV.....................................8
   6. Computations of SID Index......................................9
   7. Rule-based SID Block for Large-Scale End.X SID Advertisement..10
   8. Advertising SRv6 SID Offset for Flex-Algorithm................13
      8.1. Algorithm Offset for SRv6 Locator........................13
      8.2. Algorithm Offset for SRv6 SID Block......................15
   9. Backward Compatibility........................................16
   10. Security Considerations......................................16
   11. IANA Considerations..........................................17
   12. References...................................................17
      12.1. Normative References....................................17
      12.2. Informative References..................................17
   13. Acknowledgments..............................................18
   Contributors.....................................................18
   Authors' Addresses...............................................18



1. Introduction

   The Segment Routing (SR) allows for a flexible definition of end-to-
   end paths by encoding paths as sequences of topological sub-paths,
   called "segments". As defined in [RFC8402] and [RFC8986], an SRv6
   Segment Identifier (SID) is an IPv6 address explicitly associated


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   with the segment and consists of Locator, Function and Argument
   parts.

   [RFC9352] defines the SRv6 End SID sub-TLV, the SRv6 End.X SID sub-
   TLV, and the SRv6 LAN End.X SID sub-TLV in IS-IS.

   The SRv6 End SID sub-TLV is used to advertise an SRv6 SID with
   Endpoint behaviors which do not require a particular neighbor. The
   SRv6 End SID sub-TLV is used to advertise an SRv6 SID associated
   with a point to point adjacency. The SRv6 LAN End.X SID sub-TLV sub-
   TLV is used to advertise an SRv6 SID associated with a LAN
   adjacency. Each of these sub-TLVs contains a complete 128-bit SID
   and the sub-TLV length is quite long.

   Multiple SRv6 End.X SIDs can be associated with the same point to
   point adjacency or the same physical LAN neighbor. Each SID is
   advertised in a single SRv6 End.X SID sub-TLV or SRv6 LAN End.X SID
   sub-TLV. These SIDs are possibly associated to the same Locator,
   therefore the main differences among the sub-TLVs may be a few bits
   in the Function part of SID and the Endpoint Behavior value
   indicating different flavors.

   The number of End.X SIDs has a positive correlation with the number
   of neighbors. Assume that, each neighbor is assigned with End.X
   SIDs, and each End.X behavior has several different flavors, such as
   PSP, USP, USD, no PSP/USP/USD, etc. Then, the number of End.X SIDs
   will be at least the number of neighbors multiplied by the number of
   flavors.

   If Flexible-Algorithm is applied on SRv6 forwarding plane as defined
   in [RFC9350], a node generally advertises a Flex-Algorithm specific
   locator for each Flex-Algorithm it participates in and also
   advertises associated SRv6 END.X SIDs for every link that has not
   been pruned from the Flex-Algorithm computation.

   There may be other scenarios in which the number of End.X SIDs is
   quite large. Under these circumstances, advertising SRv6 SIDs needs
   considerable amounts of IS-IS LSPs and may be a burden of IS-IS
   nodes.

   This document proposes a simplified method to advertise SRv6 SIDs in
   IS-IS. A number of continuous SIDs are assigned within the address
   range of a Locator, which is defined as the SRv6 SID Block. When a
   SID is assigned from the block, it is described by an index based on
   the block, instead of the whole 128-bit IPv6 address.




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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 SID Block sub-TLV

   An SRv6 SID Block is composed of a number of continuous SIDs within
   the address range of a Locator. When any SID assigned in an SRv6 SID
   Block is advertised in IS-IS, the new sub-TLVs defined in this
   document SHOULD be used, instead of the SRv6 End SID sub-TLV, the
   SRv6 End.X SID sub-TLV, and the SRv6 LAN End.X SID sub-TLV defined
   in [RFC9352].

   The SRv6 SID Block sub-TLV is defined in this document to advertise
   an SRv6 SID Block. This new sub-TLV is carried in the SRv6 Locator
   TLV defined in [RFC9352].

   The SRv6 SID Block sub-TLV has the following format:


























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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |     Offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (128 bits) . . .                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (128 bits) . . .                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-tlv-len|        Sub-sub-TLVs (variable) . . .          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Block-ID: 1 octet. Identity of the advertised SRv6 SID Block. It
      MUST be unique in a node.

   o Offset: 1 octet. Number of shifting bits for the index of SIDs in
      this block.

   o Start SID: 16 octets. The smallest SID in this block.

   o End SID: 16 octets. The largest SID in this block.

   o Sub-sub-tlv-len: 1 octet. Number of octets used by sub-sub-TLVs.

   o Optional Sub-sub-TLVs.

   The address range of an SRv6 SID Block, indicated by the Start SID
   and the End SID, must be covered by the associated Locator. Multiple


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   SRv6 SID Blocks can be associated to the same Locator, and their
   address ranges should not overlap.

3. SRv6 End.X SID Index sub-TLV

   [RFC9352] defines SRv6 End.X SID sub-TLV to advertise an SRv6 SID
   associated with a point to point adjacency. If such SID is assigned
   in an SRv6 SID Block, it must be advertised by SRv6 End.X SID Index
   sub-TLV instead.

   The SRv6 End.X SID Index 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |   Index Num   |  Index Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index 0 (variable)     |       Endpoint Behavior 0     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index 1 (variable)     |       Endpoint Behavior 1     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index Num-1 (variable) |       Endpoint Behavior Num-1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Flags: 1 octet. Same as the Flags field in SRv6 End.X SID sub-
      TLV.

   o Algorithm: 1 octet. Same as the Algorithm field in SRv6 End.X SID
      sub-TLV.

   o Weight: 1 octet. Same as the Weight field in SRv6 End.X SID sub-
      TLV.

   o Block-ID: 1 octet. Indicating the associated SRv6 SID Block in
      which the SIDs are assigned.

   o Index Num: 1 octet. Number of the following indexes.


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   o Index Length: 1 octet. Number of octets used by a single index.

   o Index N: variable. Index of a SID in the SRv6 SID Block.

   o Endpoint Behavior N: 2 octets. Behavior of the SID indicated by
      the preceding Index N field. Same as the Endpoint Behavior field
      in SRv6 End.X SID sub-TLV.

   The SRv6 End.X SID Index sub-TLV can carry multiple SIDs which are
   assigned in an SRv6 SID Block identified by the Block-ID. These SIDs
   are indicated by the indexes in the SRv6 SID Block, instead of the
   whole 128-bit IPv6 addresses. They share the same Flags, Algorithm
   and Weight, but their Endpoint Behavior can be different.

   The conversion from 128-bit SID into index, or vice versa, is
   described in Section 6.

4. SRv6 LAN End.X SID Index sub-TLV

   The SRv6 LAN End.X SID Index sub-TLV is used to advertise SRv6 SIDs
   associated with a LAN adjacency, as substitution of the SRv6 LAN
   End.X SID sub-TLV for SID Block. The main difference from the SRv6
   End.X SID Index sub-TLV is that the SRv6 LAN End.X SID Index sub-TLV
   has to include the System ID of the physical neighbor on the LAN
   with which the SRv6 SIDs are associated.

   The SRv6 LAN End.X SID Index sub-TLV has the following format:





















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |             Neighbor System-ID (ID length octets)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |   Algorithm   |     Weight    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |   Index Num   |  Index Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index 0 (variable)     |       Endpoint Behavior 0     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index 1 (variable)     |       Endpoint Behavior 1     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index Num-1 (variable) |       Endpoint Behavior Num-1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Neighbor System-ID: IS-IS System-ID of length "ID Length" as
      defined in [ISO10589]. Same as the Neighbor System-ID field in
      SRv6 LAN End.X SID sub-TLV.

   o Flags, Algorithm, Weight, Block-ID, Index Num, Index Length,
      Index N, Endpoint Behavior N: Same with the corresponding fields
      in SRv6 End.X SID Index sub-TLV.

5. SRv6 End SID Index sub-TLV

   [RFC9352] defines SRv6 End SID sub-TLV to advertise SRv6 End SID. If
   such SID is assigned in an SRv6 SID Block, it must be advertised by
   SRv6 End SID Index sub-TLV instead.











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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type       |     Length    |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |   Index Num   |  Index Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index 0 (variable)     |       Endpoint Behavior 0     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index 1 (variable)     |       Endpoint Behavior 1     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Index Num-1 (variable) |       Endpoint Behavior Num-1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Flags: 1 octet. Same as the Flags field in SRv6 End SID sub-TLV.

   o Block-ID: 1 octet. Indicating the associated SRv6 SID Block in
      which the SIDs are assigned.

   o Index Num: 1 octet. Number of the following indexes.

   o Index Length: 1 octet. Number of octets used by a single index.

   o Index N: variable. Index of a SID in the SRv6 SID Block.

   o Endpoint Behavior N: 2 octets. Behavior of the SID indicating by
      the preceding Index N field. Used as the same as the Endpoint
      Behavior field in SRv6 End SID sub-TLV.

6. Computations of SID Index

   When a SID is assigned from a SRv6 SID Block, its index can be
   computed in the following way:

   Index = (SID - Start SID) right shift Offset bits

   When a node receives any of the above-mentioned SID Index sub-TLVs,
   along with the SRv6 SID Block sub-TLV, it can convert the index to
   SID in the following way:

   SID = (Index left shift Offset bits) + Start SID


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7. Rule-based SID Block for Large-Scale End.X SID Advertisement

   In this section, the rule-based SID Block is proposed to achieve
   greater compressibility for End.X SID advertisements in massive
   End.X SIDs scenarios.

   Three new sub-TLVs are defined: the Rule-based End.X SID Block sub-
   TLV, the Rule-based End.X SID Index sub-TLV or the Rule-based LAN
   End.X SID Index sub-TLV.

   The Rule-based End.X SID Block sub-TLV advertises a SID Block, along
   with the rule for assigning a series of End.X SIDs with different
   flavors in the SID Block. The Rule-based End.X SID Index sub-TLV or
   the Rule-based LAN End.X SID Index sub-TLV advertises a start index
   in the SID Block, indicating that End.X SIDs with different flavors
   are assigned following the start index: <Index 0 (start index),
   Endpoint Behavior 0>, <Index 1 (start index + 1), Endpoint Behavior
   1>, ..., <Index N (start index + N), Endpoint Behavior N>. N is
   equal to (Behavior Num - 1).

   The Rule-based End.X SID Block sub-TLV has the following format:



























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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |     Offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (128 bits) . . .                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (128 bits) . . .                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End SID (cont . . .)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |   Algorithm   |     Weight    | Behavior Num  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Endpoint Behavior 0      |        Endpoint Behavior 1    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~              ...              |   Endpoint Behavior (Num-1)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-tlv-len|        Sub-sub-TLVs (variable) . . .          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Block-ID: 1 octet. Identity of the advertised SRv6 SID Block. It
      MUST be unique in a node.

   o Offset: 1 octet. Number of shifting bits for the index of SIDs in
      this block.

   o Start SID: 16 octets. The smallest SID in this block.

   o End SID: 16 octets. The largest SID in this block.


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   o Flags: 1 octet. Same as the Flags field in SRv6 End.X SID sub-TLV
      or SRv6 LAN End.X SID sub-TLV. SIDs assigned in the same rule-
      based SID Block share the same Flags.

   o Algorithm: 1 octet. Same as the Algorithm field in SRv6 End.X SID
      sub-TLV or SRv6 LAN End.X SID sub-TLV. SIDs assigned in the same
      rule-based SID Block share the same Algorithm.

   o Weight: 1 octet. Same as the Weight field in SRv6 End.X SID sub-
      TLV or SRv6 LAN End.X SID sub-TLV. SIDs assigned in the same
      rule-based SID Block share the same Weight.

   o Behavior Num: 1 octet. Number of End.X SIDs with different
      flavors for the same neighbor.

   o Endpoint Behavior N: 2 octets. Behavior of the Nth End.X SID.
      Endpoint Behavior 0 to (Num-1) specify a series of End.X SIDs
      with different flavors.

   o Sub-sub-tlv-len: 1 octet. Number of octets used by sub-sub-TLVs.

   o Optional Sub-sub-TLVs.

   The Rule-based End.X SID Index 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |  Index Length |    Start Index  (variable)    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Block-ID: 1 octet. Indicating the associated Rule-based SID Block
      in which the SIDs are assigned.

   o Index Length: 1 octet. Number of octets used by the following
      Start Index filed.

   o Start Index: variable. Index of the first SID in the Rule-based
      SID Block.

   The Rule-based LAN End.X SID Index sub-TLV has the following format:


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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |             Neighbor System-ID (ID length octets)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Block-ID   |  Index Length |    Start Index  (variable)    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Neighbor System-ID: IS-IS System-ID of length "ID Length" as
      defined in [ISO10589]. Same as the Neighbor System-ID field in
      SRv6 LAN End.X SID sub-TLV.

   o Block-ID: 1 octet. Indicating the associated Rule-based SID Block
      in which the SIDs are assigned.

   o Index Length: 1 octet. Number of octets used by the following
      Start Index filed.

   o Start Index: variable. Index of the first SID in the Rule-based
      SID Block.

8. Advertising SRv6 SID Offset for Flex-Algorithm

   [I-D.draft-chan-lsr-igp-adv-offset] describes a method to advertise
   Flex-Algorithm specific offset on the MPLS-SR data plane so that the
   Adjacency-SIDs and Prefix SIDs for Flex-Algorithm and Virtual Flex-
   Algorithm could be automatically derived based on Algorithm 0.

   Similar mechanism can also be used for the SRv6 data plane, and may
   be further combined with the SRv6 SID Block to achieve better
   efficiencies of SRv6 SID advertisements for Flex-Algorithm.

8.1. Algorithm Offset for SRv6 Locator

   The SRv6 Locator Algorithm Offset sub-TLV advertises the algorithm
   offset for the SRv6 End SID, the SRv6 End.X SID and the SRv6 LAN
   End.X SID, which belong to a Locator associated with a Flex-
   Algorithm or Virtual Flex-Algorithm. It is carried as a sub-TLV in
   the SRv6 Locator TLV.

   The SRv6 Locator Algorithm Offset sub-TLV has the following format:


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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Virtual Flex-Algorithm                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Base Loc Size |           Base Locator (variable)            //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Offset Size  |           SID Offset (variable)              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Virtual Flex-Algorithm: 4 octets. Value 0 means that it is not
      VFA. Value >=256 means VFA identification number, and the base
      topology is denoted by Algorithm Value 128 to 255 are invalid.
      See [I-D.draft-chan-lsr-igp-adv-offset].

   o Base Loc Size: 1 octet. Number of bits in the Base Locator field,
      which MUST be from the range (1-128).

   o Base Locator: Indicating the base SRv6 Locator of Algorithm 0.
      Encoded in the minimal number of octets for the given number of
      bits.

   o Offset Size: 1 octet. Number of bits in the SID Offset field,
      which MUST be from the range (1-128).

   o SID Offset: variable. The offset of SID value between the Flex-
      Algorithm (or VFA) and Algorithm 0. Encoded in the minimal number
      of octets for the given number of bits.

   The SRv6 Locator of a Flex-Algorithm or Virtual Flex-Algorithm is
   associated with a base SRv6 Locator of Algorithm 0. When an SRv6 SID
   is assigned in that base SRv6 Locator of Algorithm 0 and advertised
   by SRv6 End.X SID sub-TLV, SRv6 LAN End.X SID sub-TLV, or SRv6 End
   SID sub-TLV, a corresponding SRv6 SID of that Flex-Algorithm or
   Virtual Flex-Algorithm is also assigned to the same point-to-point
   adjacency, LAN adjacency, or node.

   The SID value is sum of (SID of Algorithm 0 - Base Locator + FA/VFA
   Locator + SID Offset).




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8.2. Algorithm Offset for SRv6 SID Block

   The SRv6 SID Block Algorithm Offset sub-sub-TLV advertises the
   algorithm offset for an SRv6 SID Block which belongs to a Locator
   associated with a Flex-Algorithm or Virtual Flex-Algorithm. It is
   carried as a sub-sub-TLV in SRv6 SID Block sub-TLV.

   The SRv6 SID Block Algorithm Offset sub-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    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Virtual Flex-Algorithm                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Base Block-ID |   Offset Size |    Index Offset (variable)   //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: variable.

   o Virtual Flex-Algorithm: 4 octets. Value 0 means that it is not
      VFA. Value >=256 means VFA identification number, and the base
      topology is denoted by Algorithm Value 128 to 255 are invalid.
      See [I-D.draft-chan-lsr-igp-adv-offset].

   o Base Block-ID: 1 octet. Indicating the base SRv6 SID Block of
      Algorithm 0.

   o Offset Size: 1 octet. Number of bits in the Index Offset field,
      which MUST be from the range (1-128).

   o Index Offset: variable. The offset of index value between the
      Flex-Algorithm (or VFA) and Algorithm 0. Encoded in the minimal
      number of octets for the given number of bits.

   The SRv6 SID Block of a Flex-Algorithm or Virtual Flex-Algorithm is
   associated with a base SRv6 SID Block of Algorithm 0. When an SRv6
   SID is assigned in that base SRv6 SID Block of Algorithm 0 and
   advertised by SRv6 End.X SID Index sub-TLV in Section 3, SRv6 LAN
   End.X SID Index sub-TLV in Section 4, or SRv6 End SID Index sub-TLV
   in Section 5, a corresponding SRv6 SID of that Flex-Algorithm or
   Virtual Flex-Algorithm is also assigned to the same point-to-point
   adjacency, LAN adjacency, or node.


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   The SID value is sum of (Block Start SID + Index in Base Block +
   Index Offset).

9. Backward Compatibility

   To achieve backward compatibility, routers supporting the feature
   described in this document MUST advertise an IS-IS Router Capability
   TLV-242 that includes the following SRv6 SID Block 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    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Upon detecting the presence of a reachable TLV-242 without an SRv6
   SID Block Sub-TLV, all routers MUST advertise SRv6 SIDs using the
   SRv6 End SID sub-TLV, the SRv6 End.X SID sub-TLV, and the SRv6 LAN
   End.X SID sub-TLV defined in [RFC9352]. Besides, an implementations
   MAY only advertise SRv6 SIDs in the new way when it is clearly
   configured to do so.

   In order to realize the goal of reducing SRv6 SID advertisements, it
   is recommended that the operator should make sure that all routers
   in the same network have been upgraded to support the simplified
   advertisement method defined in this document.

10. Security Considerations

   TBD


















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11. IANA Considerations

   Type  Description
   ----  ---------------------------------
   TBD   SRv6 SID Block sub-TLV
   TBD   SRv6 End.X SID Index sub-TLV
   TBD   SRv6 LAN End.X SID Index sub-TLV
   TBD   SRv6 End SID Index sub-TLV
   TBD   Rule-based End.X SID Block sub-TLV
   TBD   Rule-based End.X SID Index sub-TLV
   TBD   Rule-based LAN End.X SID Index sub-TLV
   TBD   SRv6 SID Block Algorithm Offset sub-sub-TLV

12. References

12.1. Normative References

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

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [RFC9352] Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
             and Z. Hu, "IS-IS Extensions to Support Segment Routing
             over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
             February 2023, <https://www.rfc-editor.org/info/rfc9352>.

   [I-D.draft-chan-lsr-igp-adv-offset] Louis, C. and S. Krzysztof, "IGP
             extensions for Advertising Offset for Flex-Algorithm",
             draft-chan-lsr-igp-adv-offset-02 (work in progress), March
             2023.

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

12.2. Informative References

   [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>.




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   [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>.

   [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/info/rfc9350>.

13. Acknowledgments

   The authors would like to thank the following for their valuable
   contributions of this document:

   TBD

Contributors

   Louis Chan
   Juniper Networks Group (Hong Kong) Limited
   Hong Kong
   China

   Email: louisc@juniper.net

Authors' Addresses

   Weiqiang Cheng
   China Mobile
   China

   Email: chengweiqiang@chinamobile.com


   Wenying Jiang
   China Mobile
   China

   Email: jiangwenying@chinamobile.com








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   Changwang Lin
   New H3C Technologies
   China

   Email: linchangwang.04414@h3c.com


   Mengxiao Chen
   New H3C Technologies
   China

   Email: chen.mengxiao@h3c.com


   Liyan Gong
   China Mobile
   China

   Email: gongliyan@chinamobile.com


   Yao Liu
   ZTE Corporation
   China

   Email: liu.yao71@zte.com.cn






















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