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Compressed SRv6 Segment List Encoding
draft-ietf-spring-srv6-srh-compression-16

The information below is for an old version of the document.
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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Weiqiang Cheng , Clarence Filsfils , Zhenbin Li , Bruno Decraene , Francois Clad
Last updated 2024-04-24 (Latest revision 2024-04-08)
Replaces draft-filsfilscheng-spring-srv6-srh-compression
RFC stream Internet Engineering Task Force (IETF)
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Send notices to pcamaril@cisco.com
draft-ietf-spring-srv6-srh-compression-16
SPRING                                                     W. Cheng, Ed.
Internet-Draft                                              China Mobile
Intended status: Standards Track                             C. Filsfils
Expires: 26 October 2024                             Cisco Systems, Inc.
                                                                   Z. Li
                                                     Huawei Technologies
                                                             B. Decraene
                                                                  Orange
                                                            F. Clad, Ed.
                                                     Cisco Systems, Inc.
                                                           24 April 2024

                 Compressed SRv6 Segment List Encoding
               draft-ietf-spring-srv6-srh-compression-16

Abstract

   Segment Routing over IPv6 (SRv6) is the instantiation of Segment
   Routing (SR) on the IPv6 dataplane.  This document specifies new
   flavors for the SR segment endpoint behaviors defined in RFC 8986,
   which enable the compression of an SRv6 SID list.  Such compression
   significantly reduces the size of the SRv6 encapsulation needed to
   steer packets over long segment lists.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 26 October 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   3.  Basic Concepts  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  SR Segment Endpoint Flavors . . . . . . . . . . . . . . . . .   6
     4.1.  NEXT-C-SID Flavor . . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  End with NEXT-C-SID . . . . . . . . . . . . . . . . .   9
       4.1.2.  End.X with NEXT-C-SID . . . . . . . . . . . . . . . .  10
       4.1.3.  End.T with NEXT-C-SID . . . . . . . . . . . . . . . .  11
       4.1.4.  End.B6.Encaps with NEXT-C-SID . . . . . . . . . . . .  11
       4.1.5.  End.B6.Encaps.Red with NEXT-C-SID . . . . . . . . . .  12
       4.1.6.  End.BM with NEXT-C-SID  . . . . . . . . . . . . . . .  12
       4.1.7.  Combination with PSP, USP and USD flavors . . . . . .  13
     4.2.  REPLACE-C-SID Flavor  . . . . . . . . . . . . . . . . . .  13
       4.2.1.  End with REPLACE-C-SID  . . . . . . . . . . . . . . .  15
       4.2.2.  End.X with REPLACE-C-SID  . . . . . . . . . . . . . .  17
       4.2.3.  End.T with REPLACE-C-SID  . . . . . . . . . . . . . .  17
       4.2.4.  End.B6.Encaps with REPLACE-C-SID  . . . . . . . . . .  18
       4.2.5.  End.B6.Encaps.Red with REPLACE-C-SID  . . . . . . . .  18
       4.2.6.  End.BM with REPLACE-C-SID . . . . . . . . . . . . . .  19
       4.2.7.  End.DX and End.DT with REPLACE-C-SID  . . . . . . . .  19
       4.2.8.  Combination with PSP, USP, and USD flavors  . . . . .  20
   5.  C-SID Allocation  . . . . . . . . . . . . . . . . . . . . . .  20
     5.1.  Global C-SID  . . . . . . . . . . . . . . . . . . . . . .  21
     5.2.  Local C-SID . . . . . . . . . . . . . . . . . . . . . . .  21
     5.3.  GIB/LIB Usage . . . . . . . . . . . . . . . . . . . . . .  21
     5.4.  Recommended Installation of C-SIDs in FIB . . . . . . . .  22
   6.  SR Source Node  . . . . . . . . . . . . . . . . . . . . . . .  24
     6.1.  SID Validation for Compression  . . . . . . . . . . . . .  24
     6.2.  Segment List Compression  . . . . . . . . . . . . . . . .  24
     6.3.  Rules for segment lists containing NEXT-C-SID flavor
           SIDs  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     6.4.  Rules for segment lists containing REPLACE-C-SID flavor
           SIDs  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     6.5.  Upper-Layer Checksums . . . . . . . . . . . . . . . . . .  29
   7.  Inter-Domain Compression  . . . . . . . . . . . . . . . . . .  30
     7.1.  End.PS: Prefix Swap . . . . . . . . . . . . . . . . . . .  30

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       7.1.1.  End.PS with NEXT-C-SID  . . . . . . . . . . . . . . .  31
       7.1.2.  End.PS with REPLACE-C-SID . . . . . . . . . . . . . .  31
     7.2.  End.XPS: L3 Cross-Connect and Prefix Swap . . . . . . . .  31
       7.2.1.  End.XPS with NEXT-C-SID . . . . . . . . . . . . . . .  32
       7.2.2.  End.XPS with REPLACE-C-SID  . . . . . . . . . . . . .  32
   8.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .  32
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  34
     9.1.  Pinging a SID . . . . . . . . . . . . . . . . . . . . . .  34
     9.2.  ICMP Error Processing . . . . . . . . . . . . . . . . . .  35
     9.3.  Upper Layer Checksum Considerations . . . . . . . . . . .  36
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  36
     10.1.  Cisco Systems  . . . . . . . . . . . . . . . . . . . . .  37
     10.2.  Huawei Technologies  . . . . . . . . . . . . . . . . . .  37
     10.3.  Nokia  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.4.  Arrcus . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.5.  Juniper Networks . . . . . . . . . . . . . . . . . . . .  39
     10.6.  Marvell  . . . . . . . . . . . . . . . . . . . . . . . .  39
     10.7.  Broadcom . . . . . . . . . . . . . . . . . . . . . . . .  39
     10.8.  ZTE Corporation  . . . . . . . . . . . . . . . . . . . .  40
     10.9.  New H3C Technologies . . . . . . . . . . . . . . . . . .  40
     10.10. Ruijie Network . . . . . . . . . . . . . . . . . . . . .  41
     10.11. Ciena  . . . . . . . . . . . . . . . . . . . . . . . . .  41
     10.12. Centec . . . . . . . . . . . . . . . . . . . . . . . . .  41
     10.13. Open Source  . . . . . . . . . . . . . . . . . . . . . .  42
     10.14. Interoperability Reports . . . . . . . . . . . . . . . .  42
       10.14.1.  Bell Canada / Ciena 2023  . . . . . . . . . . . . .  42
       10.14.2.  EANTC 2023  . . . . . . . . . . . . . . . . . . . .  42
       10.14.3.  China Mobile 2020 . . . . . . . . . . . . . . . . .  43
   11. Applicability to other SR Segment Endpoint Behaviors  . . . .  44
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  44
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
     13.1.  SRv6 Endpoint Behaviors  . . . . . . . . . . . . . . . .  46
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     15.2.  Informative References . . . . . . . . . . . . . . . . .  50
   Appendix A.  Complete pseudocodes . . . . . . . . . . . . . . . .  53
     A.1.  End with NEXT-C-SID . . . . . . . . . . . . . . . . . . .  53
     A.2.  End.X with NEXT-C-SID . . . . . . . . . . . . . . . . . .  55
     A.3.  End.T with NEXT-C-SID . . . . . . . . . . . . . . . . . .  57
     A.4.  End.B6.Encaps with NEXT-C-SID . . . . . . . . . . . . . .  59
     A.5.  End.BM with NEXT-C-SID  . . . . . . . . . . . . . . . . .  61
     A.6.  End with REPLACE-C-SID  . . . . . . . . . . . . . . . . .  63
     A.7.  End.X with REPLACE-C-SID  . . . . . . . . . . . . . . . .  65
     A.8.  End.T with REPLACE-C-SID  . . . . . . . . . . . . . . . .  67
     A.9.  End.B6.Encaps with REPLACE-C-SID  . . . . . . . . . . . .  69
     A.10. End.BM with REPLACE-C-SID . . . . . . . . . . . . . . . .  70
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  72

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  73

1.  Introduction

   The Segment Routing (SR) architecture [RFC8402] describes two data
   plane instantiations of SR: SR over MPLS (SR-MPLS) and SR over IPv6
   (SRv6).

   SRv6 Network Programming [RFC8986] defines a framework to build a
   network program with topological and service segments (also referred
   to by their Segment Identifier (SID)) carried in a Segment Routing
   Header (SRH) [RFC8754].

   Some SRv6 applications such as strict path traffic engineering may
   require long segment lists.  Compressing the encoding of these long
   segment lists in the packet header can significantly reduce the
   header size.  This document specifies new flavors to the SR segment
   endpoint behaviors defined in [RFC8986] that enable a compressed
   encoding of the SRv6 segment list.

   The flavors defined in this document leverage the SRv6 data plane
   defined in [RFC8754] and [RFC8986], and are compatible with the SRv6
   control plane extensions for IS-IS [RFC9352], OSPF [RFC9513], and BGP
   [RFC9252].

2.  Terminology

   This document leverages the terms defined in [RFC8402], [RFC8754],
   and [RFC8986].  The reader is assumed to be familiar with this
   terminology.

   This document introduces the following new terms:

   *  Locator-Block: The most significant bits of a SID locator that
      represent the SRv6 SID block.  The Locator-Block is referred to as
      "B" in Section 3.1 of [RFC8986].

   *  Locator-Node: The least significant bits of a SID locator that
      identify the SR segment endpoint node instantiating the SID.  The
      Locator-Node is referred to as "N" in Section 3.1 of [RFC8986].

   *  Compressed-SID (C-SID): A compressed encoding of a SID.  The C-SID
      includes the Locator-Node and Function bits of the SID being
      compressed.

   *  C-SID container: A 128-bit container holding a list of one or more
      C-SIDs.

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   *  C-SID sequence: A group of one or more consecutive SID list
      entries carrying the common Locator-Block and at least one C-SID
      container.

   *  Uncompressed SID sequence: A group of one or more consecutive
      uncompressed SIDs in a SID list.

   *  Compressed SID list: A segment list encoding that reduces the
      packet header length thanks to one or more C-SID sequences.  A
      compressed SID list also contains zero, one, or more uncompressed
      SID sequences.

   *  Global Identifiers Block (GIB): The pool of C-SID values available
      for global allocation.

   *  Local Identifiers Block (LIB): The pool of C-SID values available
      for local allocation.

   In this document, the length of each constituent part of a SID is
   referred to as follows.

   *  LBL is the Locator-Block length of the SID.

   *  LNL is the Locator-Node length of the SID.

   *  FL is the Function length of the SID.

   *  AL is the Argument length of the SID.

   In addition, LNFL is the sum of the Locator-Node length and the
   Function length of the SID.  It is also referred to as the C-SID
   length.

2.1.  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.

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3.  Basic Concepts

   In an SR domain, all SRv6 SIDs instantiated from the same Locator-
   Block share the same most significant bits.  In addition, when the
   combined length of the SRv6 SID Locator, Function, and Argument is
   smaller than 128 bits, the least significant bits of the SID are
   padded with zeros.  The compressed segment list encoding seeks to
   decrease the packet header length by avoiding the repetition of the
   same Locator-Block and reducing the use of padding bits.

   The compressed segment list encoding is fully compatible with and
   builds upon the mechanisms specified in [RFC8754] and [RFC8986].  The
   compressed encoding leverages a SID list compression logic on the SR
   source node (Section 6) in combination with new flavors of the base
   SRv6 segment endpoint behaviors that process the compressed SID list
   (Section 4).

   A segment list can be encoded in the packet header using any
   combination of compressed and uncompressed sequences.  The C-SID
   sequences leverage the flavors defined in this document, while the
   uncompressed sequences use behaviors and flavors defined in other
   documents, such as [RFC8986].  An SR source node constructs and
   compresses the SID list depending on the SIDs instantiated on each SR
   segment endpoint node that the packet should traverse, as well as its
   own compression capabilities.

   The compressed segment list encoding works with any Locator-Block
   allocation.  For example, each routing domain within the SR domain
   can be allocated a /48 Locator-Block from a global IPv6 block
   available to the operator, or from a prefix allocated to SRv6 SIDs as
   discussed in Section 5 of [I-D.ietf-6man-sids].

4.  SR Segment Endpoint Flavors

   This section defines two SR segment endpoint flavors, NEXT-C-SID and
   REPLACE-C-SID, for the End, End.X, End.T, End.B6.Encaps,
   End.B6.Encaps.Red, and End.BM behaviors of [RFC8986].  This section
   also defines a REPLACE-C-SID flavor for the End.DX6, End.DX4,
   End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M
   behaviors of [RFC8986].  A counterpart NEXT-C-SID flavor is not
   defined for these SIDs because they can be included within a C-SID
   sequence that uses the NEXT-C-SID flavor without any modification of
   the procedure defined in [RFC8986].  Future documents may extend the
   applicability of the NEXT-C-SID and REPLACE-C-SID flavors to other SR
   segment endpoint behaviors (see Section 11).

   The use of these flavors, either individually or in combination,
   enables the compressed segment list encoding.

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   The NEXT-C-SID flavor and the REPLACE-C-SID flavor both leverage the
   SID Argument to determine the next SID to be processed, but employ
   different SID list compression schemes.  With the NEXT-C-SID flavor,
   each C-SID container is a fully formed SRv6 SID with the common
   Locator-Block for all the C-SIDs in the C-SID container, a Locator-
   Node and Function that are those of the first C-SID, and an Argument
   carrying the subsequent C-SIDs.  With the REPLACE-C-SID flavor, only
   the first element in a C-SID sequence is a fully formed SRv6 SID.  It
   has the common Locator-Block for all the C-SIDs in the C-SID
   sequence, and a Locator-Node and Function that are those of the first
   C-SID.  The remaining elements in the C-SID sequence are C-SID
   containers carrying the subsequent C-SIDs without the Locator-Block.

   SRv6 is intended for use in a variety of networks that require
   different prefix lengths and SID numbering spaces.  Each of the two
   flavors introduced in this document comes with its own
   recommendations for Locator-Block and C-SID length, as specified in
   Section 4.1 and Section 4.2.  These flavors are best suited for
   different environments, depending on the requirements of the network.
   For instance, larger C-SID lengths may be more suitable for networks
   requiring ample SID numbering space, while smaller C-SID lengths are
   better for compression efficiency.  The two compression flavors allow
   the compressed segment list encoding to adapt to a range of
   requirements, with support for multiple compression levels.  Network
   operators can choose the flavor that best suits their use case,
   deployment design, and network scale.

   The SIDs of both flavors can co-exist in the same SR domain, on the
   same SR segment endpoint node, and even in the same segment list.
   However, it is RECOMMENDED, for ease of operation, that a single
   compressed encoding flavor be used in a given routing domain.  In a
   multi-domain deployment, different flavors may be used in different
   routing domains of the SR domain.

   In the remainder of this document, the term "a SID of this document"
   refers to any End, End.X, End.T, End.B6.Encaps, End.B6.Encaps.Red, or
   End.BM SID with the NEXT-C-SID or the REPLACE-C-SID flavor, and with
   any combination of Penultimate Segment Pop (PSP), Ultimate Segment
   Pop (USP), and Ultimate Segment Decapsulation (USD) flavor, or any
   End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V,
   End.DT2U, or End.DT2M with the REPLACE-C-SID flavor.  All the SIDs
   introduced in this document are listed in Table 1.

   In the remainder of this document, the terms "NEXT-C-SID flavor SID"
   and "REPLACE-C-SID flavor SID" refer to any SID of this document with
   the NEXT-C-SID flavor and with the REPLACE-C-SID flavor,
   respectively.

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4.1.  NEXT-C-SID Flavor

   A C-SID sequence using the NEXT-C-SID flavor comprises one or more
   C-SID containers.  Each C-SID container is a fully formed 128-bit SID
   structured as shown in Figure 1.  It carries a Locator-Block followed
   by a series of C-SIDs.  The Locator-Node and Function of the C-SID
   container are those of the first C-SID, and its Argument is the
   contiguous series of subsequent C-SIDs.  The second C-SID is encoded
   in the most significant bits of the C-SID container Argument, the
   third C-SID is encoded in the bits of the Argument that immediately
   follow the second C-SID, and so on.  When all C-SIDs have the same
   length, a C-SID container can carry up to K C-SIDs, where K is
   computed as floor((128-LBL)/LNFL) (floor(x) is the greatest integer
   less than or equal to x [GKP94]).  Each C-SID container for NEXT-
   C-SID is independent, such that contiguous C-SID containers in a
   C-SID sequence can be considered as separate C-SID sequences.

   When a C-SID sequence comprises at least two C-SIDs, the last C-SID
   in the sequence is not required to have the NEXT-C-SID flavor.  It
   can be bound to any behavior and flavor(s), including the REPLACE-
   C-SID flavor, as long as the updated destination address resulting
   from the processing of the previous C-SID in the sequence is a valid
   form for that last SID.  Line S12 of the first pseudocode in
   Section 6.2 provides sufficient conditions to ensure this property.

   +------------------------------------------------------------------+
   |     Locator-Block      |Loc-Node|            Argument            |
   |                        |Function|                                |
   +------------------------------------------------------------------+
    <-------- LBL ---------> < LNFL > <------------- AL ------------->

        Figure 1: Structure of a NEXT-C-SID flavor SID (scaled for a
     48-bit Locator- Block, 16-bit combined Locator-Node and Function,
                            and 64-bit Argument)

   An implementation MUST support a 32-bit Locator-Block length (LBL)
   and a 16-bit C-SID length (LNFL) for NEXT-C-SID flavor SIDs, and may
   support any other Locator-Block and C-SID length.

   A deployment should use a consistent Locator-Block length and C-SID
   length for all SIDs of the SR domain.  Heterogeneous lengths, while
   possible, may impact the compression efficiency.

   The Argument length (AL) for NEXT-C-SID flavor SIDs is equal to 128-
   LBL-LNFL.

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   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as a SID with the NEXT-C-SID flavor, the SR segment
   endpoint node applies the procedure specified in the following
   subsection that corresponds to the SID behavior.  If the SID also has
   the PSP, USP, or USD flavor, the procedure is modified as described
   in Section 4.1.7.

   An SR segment endpoint node instantiating a SID of this document with
   the NEXT-C-SID flavor MUST accept any Argument value for that SID.

   At high level, for any SID with the NEXT-C-SID flavor, the SR segment
   endpoint node determines the next SID of the SID list as follows.  If
   the Argument value of the active SID is non-zero, the SR segment
   endpoint node constructs the next SID from the active SID by copying
   the entire SID Argument value to the bits that immediately follow the
   Locator-Block, thus overwriting the active SID Locator-Node and
   Function with those of the next C-SID, and filling the least
   significant LNFL bits of the Argument with zeros.  Otherwise (if the
   Argument value is 0), the SR segment endpoint node copies the next
   128-bit Segment List entry from the SRH to the Destination Address
   field of the IPv6 header.

4.1.1.  End with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End SID with the NEXT-C-SID flavor, the procedure
   described in Section 4.1 of [RFC8986] is executed with the following
   modifications.

   The below pseudocode is inserted between lines S01 and S02 of the SRH
   processing in Section 4.1 of [RFC8986].  In addition, this pseudocode
   is executed before processing any extension header that is not an
   SRH, a Hop-by-Hop header or a Destination Option header, or before
   processing the upper-layer header, whichever comes first.

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.   Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   N09. }

      |  Notes:
      |  
      |     *  DA.Argument identifies the value contained in the bits
      |        [(LBL+LNFL)..127] in the Destination Address of the IPv6
      |        header.
      |  
      |     *  The value in the Segments Left field of the SRH is not
      |        modified when DA.Argument in the received packet has a
      |        non-zero value.

   A rendering of the complete pseudocode is provided in Appendix A.1.

4.1.2.  End.X with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.X SID with the NEXT-C-SID flavor, the
   procedure described in Section 4.2 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.1.1 of this document is modified by
   replacing line N08 as shown below.

   N08.   Submit the packet to the IPv6 module for transmission to the
            new destination via a member of J.

      |  Note: the variable J is defined in Section 4.2 of [RFC8986].

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   The resulting pseudocode is inserted between lines S01 and S02 of the
   SRH processing in Section 4.1 of [RFC8986] after applying the
   modification described in Section 4.2 of [RFC8986].  In addition,
   this pseudocode is executed before processing any extension header
   that is not an SRH, a Hop-by-Hop header or a Destination Option
   header, or before processing the upper-layer header, whichever comes
   first.

   A rendering of the complete pseudocode is provided in Appendix A.2.

4.1.3.  End.T with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.T SID with the NEXT-C-SID flavor, the
   procedure described in Section 4.3 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.1.1 of this document is modified by
   replacing line N08 as shown below.

   N08.1.   Set the packet's associated FIB table to T.
   N08.2.   Submit the packet to the egress IPv6 FIB lookup for
              transmission to the new destination.

      |  Note: the variable T is defined in Section 4.3 of [RFC8986].

   The resulting pseudocode is inserted between lines S01 and S02 of the
   SRH processing in Section 4.1 of [RFC8986] after applying the
   modification described in Section 4.3 of [RFC8986].  In addition,
   this pseudocode is executed before processing any extension header
   that is not an SRH, a Hop-by-Hop header or a Destination Option
   header, or before processing the upper-layer header, whichever comes
   first.

   A rendering of the complete pseudocode is provided in Appendix A.3.

4.1.4.  End.B6.Encaps with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.B6.Encaps SID with the NEXT-C-SID flavor, the
   procedure described in Section 4.13 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.1.1 of this document is modified by
   replacing line N08 as shown below.

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   N08.1.   Push a new IPv6 header with its own SRH containing B.
   N08.2.   Set the outer IPv6 SA to A.
   N08.3.   Set the outer IPv6 DA to the first SID of B.
   N08.4.   Set the outer Payload Length, Traffic Class, Flow Label,
              Hop Limit, and Next Header fields.
   N08.5.   Submit the packet to the egress IPv6 FIB lookup for
              transmission to the next destination.

      |  Note: the variables A and B, as well as the values of the
      |  Payload Length, Traffic Class, Flow Label, Hop Limit, and Next
      |  Header are defined in Section 4.13 of [RFC8986].

   The resulting pseudocode is inserted between lines S01 and S02 of the
   SRH processing in Section 4.13 of [RFC8986].  In addition, this
   pseudocode is executed before processing any extension header that is
   not an SRH, a Hop-by-Hop header or a Destination Option header, or
   before processing the upper-layer header, whichever comes first.

   A rendering of the complete pseudocode is provided in Appendix A.4.

   Similar to the base End.B6.Encaps SID defined in Section 4.13 of
   [RFC8986], the NEXT-C-SID flavor variant updates the Destination
   Address field of the inner IPv6 header to the next SID in the
   original segment list before encapsulating the packet with the
   segment list of SR Policy B.  At the endpoint of SR Policy B, the
   encapsulation is removed and the inner packet is forwarded towards
   the exposed destination address, which already contains the next SID
   in the original segment list.

4.1.5.  End.B6.Encaps.Red with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.B6.Encaps.Red SID with the NEXT-C-SID flavor,
   the procedure described in Section 4.14 of [RFC8986] is executed with
   the same modifications as in Section 4.1.4 of this document.

4.1.6.  End.BM with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.BM SID with the NEXT-C-SID flavor, the
   procedure described in Section 4.15 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.1.1 of this document is modified by
   replacing line N08 as shown below.

   N08.1.   Push the MPLS label stack for B.
   N08.2.   Submit the packet to the MPLS engine for transmission.

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      |  Note: the variable B is defined in Section 4.15 of [RFC8986].

   The resulting pseudocode is inserted between lines S01 and S02 of the
   SRH processing in Section 4.15 of [RFC8986].  In addition, this
   pseudocode is executed before processing any extension header that is
   not an SRH, a Hop-by-Hop header or a Destination Option header, or
   before processing the upper-layer header, whichever comes first.

   A rendering of the complete pseudocode is provided in Appendix A.5.

4.1.7.  Combination with PSP, USP and USD flavors

   PSP: The PSP flavor defined in Section 4.16.1 of [RFC8986] is
   unchanged when combined with the NEXT-C-SID flavor.

   USP: The USP flavor defined in Section 4.16.2 of [RFC8986] is
   unchanged when combined with the NEXT-C-SID flavor.

   USD: The USP flavor defined in Section 4.16.3 of [RFC8986] is
   unchanged when combined with the NEXT-C-SID flavor.

4.2.  REPLACE-C-SID Flavor

   A C-SID sequence using the REPLACE-C-SID flavor starts with a C-SID
   container in fully formed 128-bit SID format.  The Locator-Block of
   this SID is the common Locator-Block for all the C-SIDs in the C-SID
   sequence, its Locator-Node and Function are those of the first C-SID,
   and its Argument carries the index of the current C-SID in the
   current C-SID container.  The Argument value is initially 0.  When
   more segments are present in the segment list, the C-SID sequence
   continues with one or more C-SID containers in packed format carrying
   the subsequent C-SIDs in the sequence.  Each container in packed
   format is a 128-bit Segment List entry split into K "positions" of
   LNFL bits, where K is computed as floor(128/LNFL).  If LNFL does not
   divide into 128 perfectly, a zero pad is added in the least
   significant bits of the C-SID container to fill the bits left over.
   The second C-SID in the C-SID sequence is encoded in the least
   significant bit position of the first C-SID container in packed
   format (position K-1), the third C-SID is encoded in position K-2,
   and so on.

   The last C-SID in the C-SID sequence is not required to have the
   REPLACE-C-SID flavor.  It can be bound to any behavior and flavor(s),
   including the NEXT-C-SID flavor, as long as it meets the conditions
   defined in Section 6.

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   The structure of a SID with the REPLACE-C-SID flavor is shown in
   Figure 2.  The same structure is also that of the C-SID container for
   REPLACE-C-SID in fully formed 128-bit SID format.

   +-------------------------------------------------------------------+
   |     Locator-Block      |  Locator-Node  |        Argument         |
   |                        |   + Function   |                         |
   +-------------------------------------------------------------------+
    <-------- LBL ---------> <---- LNFL ----> <--------- AL ---------->

      Figure 2: Structure of a REPLACE-C-SID flavor SID (scaled for a
     48-bit Locator- Block, 32-bit combined Locator-Node and Function,
                            and 48-bit Argument)

   The structure of a C-SID container for REPLACE-C-SID in packed format
   is shown in Figure 3.

   +-------------------------------------------------------------------+
   |  Fourth C-SID  |  Third C-SID   |  Second C-SID  |  First C-SID   |
   |  (position 0)  |  (position 1)  |  (position 2)  |  (position 3)  |
   +-------------------------------------------------------------------+
    <---- LNFL ----> <---- LNFL ----> <---- LNFL ----> <---- LNFL ---->

      Figure 3: Structure of a C-SID container for REPLACE-C-SID using
                       a 32-bit C-SID length (K = 4)

   The REPLACE-C-SID flavor SIDs support any Locator-Block length (LBL),
   depending on the needs of the operator, as long as it does not exceed
   128-LNFL-ceiling(log_2(128/LNFL)) (ceiling(x) is the least integer
   greater than or equal to x [GKP94]), so that enough bits remain
   available for the C-SID and Argument.  A Locator-Block length of 48,
   56, 64, 72, or 80 bits is recommended for easier reading in
   operation.

   This document defines the REPLACE-C-SID flavor for 16-bit and 32-bit
   C-SID lengths (LNFL).  An implementation MUST support a 32-bit C-SID
   length for REPLACE-C-SID flavor SIDs.

   A deployment should use a consistent Locator-Block length and C-SID
   length for all SIDs of the SR domain.  Heterogeneous C-SID lengths,
   while possible, may impact the compression efficiency.

   The Argument length (AL) for REPLACE-C-SID flavor SIDs is equal to
   128-LBL-LNFL.  The index value is encoded in the least significant X
   bits of the Argument, where X is computed as ceiling(log_2(128/
   LNFL)).

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   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as a SID with the REPLACE-C-SID flavor, the SR segment
   endpoint node applies the procedure specified in the following
   subsection that corresponds to the SID behavior.  If the SID also has
   the PSP, USP, or USD flavor, the procedure is modified as described
   in Section 4.2.8.

   At high level, at the start of a C-SID sequence using the REPLACE-
   C-SID flavor, the first C-SID container in fully formed 128-bit SID
   format is copied to the Destination Address of the IPv6 header.
   Then, for any SID with the REPLACE-C-SID flavor, the SR segment
   endpoint node determines the next SID of the SID list as follows.
   When an SRH is present, the SR segment endpoint node decrements the
   index value in the Argument of the active SID if the index value is
   not 0 or, if it is 0, decrements the Segments Left value in the SRH
   and sets the index value in the Argument of the active SID to K-1.
   The updated index value indicates the position of the next C-SID
   within the C-SID container in packed format at the "Segment List"
   index "Segments Left" in the SRH.  The SR segment endpoint node then
   constructs the next SID by copying this next C-SID to the bits that
   immediately follow the Locator-Block in the Destination Address field
   of the IPv6 header, thus overwriting the active SID Locator-Node and
   Function with those of the next C-SID.  If no SRH is present, the SR
   segment endpoint node ignores the index value in the SID Argument
   (except End.DT2M, see Section 4.2.7) and processes the upper-layer
   header as per [RFC8986].  The C-SID sequence ends with a last C-SID
   in the last C-SID container that does not have the REPLACE-C-SID
   flavor, or with the special C-SID value 0, or when reaching the end
   of the segment list, whichever comes first.

4.2.1.  End with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End SID with the REPLACE-C-SID flavor, the SRH
   processing described in Section 4.1 of [RFC8986] is executed with the
   following modifications.

   Line S02 of SRH processing in Section 4.1 of [RFC8986] is replaced as
   follows.

   S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
              Segment List[0][DA.Arg.Index-1] == 0)) {

   Lines S09 to S15 are replaced by the following pseudo code.

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   R01. If (DA.Arg.Index != 0) {
   R02.   If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   R03.     Send an ICMP Parameter Problem to the Source Address,
              Code 0 (Erroneous header field encountered),
              Pointer set to the Segments Left field,
              interrupt packet processing and discard the packet.
   R04.   }
   R05.   Decrement DA.Arg.Index by 1.
   R06.   If (Segment List[Segments Left][DA.Arg.Index] == 0) {
   R07.     Decrement Segments Left by 1.
   R08.     Decrement IPv6 Hop Limit by 1.
   R09.     Update IPv6 DA with Segment List[Segments Left]
   R10.     Submit the packet to the egress IPv6 FIB lookup for
             transmission to the new destination.
   R11.   }
   R12. } Else {
   R13.   If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   R14.     Send an ICMP Parameter Problem to the Source Address,
              Code 0 (Erroneous header field encountered),
              Pointer set to the Segments Left field,
              interrupt packet processing and discard the packet.
   R15.   }
   R16.   Decrement Segments Left by 1.
   R17.   Set DA.Arg.Index to (floor(128/LNFL) - 1).
   R18. }
   R19. Decrement IPv6 Hop Limit by 1.
   R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits
          [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
          header.
   R21. Submit the packet to the egress IPv6 FIB lookup for
          transmission to the new destination.

      |  Notes:
      |  
      |     *  DA.Arg.Index identifies the value contained in the bits
      |        [(128-ceiling(log_2(128/LNFL)))..127] in the Destination
      |        Address of the IPv6 header.
      |  
      |     *  Segment List[Segments Left][DA.Arg.Index] identifies the
      |        value contained in the bits
      |        [DA.Arg.Index*LNFL..(DA.Arg.Index+1)*LNFL-1] in the SRH
      |        Segment List entry at index Segments Left.

   The upper-layer header processing described in Section 4.1.1 of
   [RFC8986] is unchanged.

   A rendering of the complete pseudocode is provided in Appendix A.6.

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4.2.2.  End.X with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.X SID with the REPLACE-C-SID flavor, the
   procedure described in Section 4.2 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.2.1 of this document is modified by
   replacing lines R10 and R21 as shown below.

   R10. Submit the packet to the IPv6 module for transmission to the
          new destination via a member of J.

   R21. Submit the packet to the IPv6 module for transmission to the
          new destination via a member of J.

      |  Note: the variable J is defined in Section 4.2 of [RFC8986].

   The SRH processing in Section 4.2 of [RFC8986] is replaced with the
   resulting pseudocode.  The upper-layer header processing is
   unchanged.

   A rendering of the complete pseudocode is provided in Appendix A.7.

4.2.3.  End.T with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.T SID with the REPLACE-C-SID flavor, the
   procedure described in Section 4.3 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.2.1 of this document is modified by
   replacing lines R10 and R21 as shown below.

   R10.1. Set the packet's associated FIB table to T.
   R10.2. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.

   R21.1. Set the packet's associated FIB table to T.
   R21.2. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.

      |  Note: the variable T is defined in Section 4.3 of [RFC8986].

   The SRH processing in Section 4.3 of [RFC8986] is replaced with the
   resulting pseudocode.  The upper-layer header processing is
   unchanged.

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   A rendering of the complete pseudocode is provided in Appendix A.8.

4.2.4.  End.B6.Encaps with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.B6.Encaps SID with the REPLACE-C-SID flavor,
   the procedure described in Section 4.13 of [RFC8986] is executed with
   the following modifications.

   The pseudocode in Section 4.2.1 of this document is modified by
   replacing lines R10 and R21 as shown below.

   R10.1. Push a new IPv6 header with its own SRH containing B.
   R10.2. Set the outer IPv6 SA to A.
   R10.3. Set the outer IPv6 DA to the first SID of B.
   R10.4. Set the outer Payload Length, Traffic Class, Flow Label,
            Hop Limit, and Next Header fields.
   R10.5. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.

   R21.1. Push a new IPv6 header with its own SRH containing B.
   R21.2. Set the outer IPv6 SA to A.
   R21.3. Set the outer IPv6 DA to the first SID of B.
   R21.4. Set the outer Payload Length, Traffic Class, Flow Label,
            Hop Limit, and Next Header fields.
   R21.5. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.

      |  Note: the variables A and B, as well as the values of the
      |  Payload Length, Traffic Class, Flow Label, Hop Limit, and Next
      |  Header are defined in Section 4.13 of [RFC8986].

   The SRH processing in Section 4.13 of [RFC8986] is replaced with the
   resulting pseudocode.  The upper-layer header processing is
   unchanged.

   A rendering of the complete pseudocode is provided in Appendix A.9.

4.2.5.  End.B6.Encaps.Red with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.B6.Encaps.Red SID with the REPLACE-C-SID
   flavor, the procedure described in Section 4.14 of [RFC8986] is
   executed with the same modifications as in Section 4.2.4 of this
   document.

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4.2.6.  End.BM with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.BM SID with the REPLACE-C-SID flavor, the
   procedure described in Section 4.15 of [RFC8986] is executed with the
   following modifications.

   The pseudocode in Section 4.2.1 of this document is modified by
   replacing lines R10 and R21 as shown below.

   R10.1. Push the MPLS label stack for B.
   R10.2. Submit the packet to the MPLS engine for transmission.

   R21.1. Push the MPLS label stack for B.
   R21.2. Submit the packet to the MPLS engine for transmission.

      |  Note: the variable B is defined in Section 4.15 of [RFC8986].

   The SRH processing in Section 4.15 of [RFC8986] is replaced with the
   resulting pseudocode.  The upper-layer header processing is
   unchanged.

   A rendering of the complete pseudocode is provided in Appendix A.10.

4.2.7.  End.DX and End.DT with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.DX6, End.DX4, End.DT6, End.DT4, End.DT46,
   End.DX2, End.DX2V, or End.DT2U SID with the REPLACE-C-SID flavor, the
   corresponding procedure described in Sections 4.4 through 4.11 of
   [RFC8986] is executed.

   These SIDs differ from those defined in [RFC8986] by the presence of
   an Argument as part of the SID structure.  The Argument value is
   ignored by the SR segment endpoint node.

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.DT2M SID with the REPLACE-C-SID flavor, the
   procedure described in Section 4.12 of [RFC8986] is executed with the
   following modification.

   For any End.DT2M SID with the REPLACE-C-SID flavor, the value of
   Arg.FE2 is 16-bit long.  The SR segment endpoint node obtains the
   value Arg.FE2 from the 16 most significant bits of DA.Argument if
   DA.Arg.Index is zero, or from the 16 least significant bits of the
   next position in the current C-SID container (Segment List[Segments
   Left][DA.Arg.Index-1]) otherwise (DA.Arg.Index is non-zero).

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4.2.8.  Combination with PSP, USP, and USD flavors

   PSP: When combined with the REPLACE-C-SID flavor, the additional PSP
   flavor instructions defined in Section 4.16.1.2 of [RFC8986] are
   inserted after lines R09 and R20 of the pseudocode in Section 4.2.1,
   and the first line of the inserted instructions after R20 is modified
   as follows.

   R20.1.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
                Segment List[0][DA.Arg.Index-1] == 0)) {

      |  Note: Segment List[Segments Left][DA.Arg.Index-1] identifies
      |  the value contained in the bits [(DA.Arg.Index-
      |  1)*LNFL..DA.Arg.Index*LNFL-1] in the SRH Segment List entry at
      |  index Segments Left.

   USP: When combined with the REPLACE-C-SID flavor, the line S03 of the
   pseudocode in Section 4.2.1 are substituted by the USP flavor
   instructions S03.1 to S03.4 defined in Section 4.16.2 of [RFC8986].
   Note that S03 is shown in the complete pseudocode in Appendix A.6.

   USD: The USD flavor defined in Section 4.16.3 of [RFC8986] is
   unchanged when combined with the REPLACE-C-SID flavor.

5.  C-SID Allocation

   The C-SID value of 0 is reserved.  It is used to indicate the end of
   a C-SID container.

   In order to efficiently manage the C-SID numbering space, a
   deployment may divide it into two non-overlapping sub-spaces: a
   Global Identifiers Block (GIB) and a Local Identifiers Block (LIB).

   The C-SID values that are allocated from the GIB have a global
   semantic within the Locator-Block, while those that are allocated
   from the LIB have a local semantic on an SR segment endpoint node and
   within the scope of the Locator-Block.

   The concept of LIB is applicable to SRv6 and specifically to its
   NEXT-C-SID and REPLACE-C-SID flavors.  The shorter the C-SID, the
   more benefit the LIB brings.

   The opportunity to use these sub-spaces, their size, and their C-SID
   allocation policy depends on the C-SID length relative to the size of
   the network (e.g., number of nodes, links, service routes).  Some
   guidelines for a typical deployment scenario are provided in the
   below subsections.

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5.1.  Global C-SID

   A global C-SID is a C-SID allocated from the GIB.

   A global C-SID identifies a segment defined at the Locator-Block
   level.  The tuple (Locator-Block, C-SID) identifies the same segment
   across all nodes of the SR domain.  A typical example is a prefix
   segment bound to the End behavior.

   A node can have multiple global C-SIDs under the same Locator-Block
   (e.g., one per IGP flexible algorithm ([RFC9350])).  Multiple nodes
   may share the same global C-SID (e.g., anycast).

5.2.  Local C-SID

   A local C-SID is a C-SID allocated from the LIB.

   A local C-SID identifies a segment defined at the node level and
   within the scope of a particular Locator-Block.  The tuple (Locator-
   Block, C-SID) identifies a different segment on each node of the SR
   domain.  A typical example is a non-routed Adjacency segment bound to
   the End.X behavior.

   Let N1 and N2 be two different physical nodes of the SR domain and I
   a local C-SID value, N1 may allocate value I to SID S1 and N2 may
   allocate the same value I to SID S2.

5.3.  GIB/LIB Usage

   GIB and LIB usage is a local implementation and/or configuration
   decision, however, some guidelines for determining usage for specific
   SID behaviors and recommendations are provided.

   The GIB number space is shared among all SR segment endpoint nodes
   using SRv6 locators under a Locator-Block space.  The more SIDs
   assigned from this space, per node, the faster it is exhausted.
   Therefore its use is prioritized for global segments, such as SIDs
   that identify a node.

   The LIB number space is unique per node.  Each node is able to fully
   utilize the entire LIB number space without consideration of
   assignments at other nodes.  Therefore its use is prioritized for
   local segments, such as SIDs that identify services (of which there
   may be many) at nodes, cross-connects, or adjacencies.

   While a longer C-SID length permits more flexibility in which SID
   behaviors may be assigned from the GIB, it also reduces the
   compression efficiency.

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   Given the previous Locator-Block and C-SID length recommendations,
   the following GIB/LIB usage is recommended:

   *  NEXT-C-SID:

      -  GIB: End

      -  LIB: End.X, End.T, End.DT4/6/46/2U/2M, End.DX4/6/2/2V
         (including large-scale pseudowire), End.B6.Encaps,
         End.B6.Encaps.Red, End.BM

   *  REPLACE-C-SID:

      -  GIB: End, End.X, End.T, End.DT4/6/46/2U/2M, End.DX4/6/2/2V,
         End.B6.Encaps, End.B6.Encaps.Red, End.BM

      -  LIB: End.DX2/2V for large-scale pseudowire

   Any other allocation is possible but may lead to a suboptimal use of
   the C-SID numbering space.

5.4.  Recommended Installation of C-SIDs in FIB

   Section 4.3 of [RFC8754] defines how an SR segment endpoint node
   identifies a locally instantiated SRv6 SID.  To ensure that any valid
   argument value is accepted, an SR segment endpoint node instantiating
   a NEXT-C-SID or REPLACE-C-SID flavor SID should install a
   corresponding FIB entry that matches only the Locator and Function
   parts of the SID (i.e., with a prefix length of LBL + LNL + FL).

   In addition, an SR segment endpoint node instantiating NEXT-C-SID
   flavor SIDs from both GIB and LIB may install combined "Global +
   Local" FIB entries to match a sequence of global and local C-SIDs in
   a single longest prefix match (LPM) lookup.

   For example, let us consider an SR segment endpoint node 10
   instantiating the following two NEXT-C-SID flavor SIDs according to
   the C-SID length, Locator-Block length, and GIB/LIB recommendations
   in this section.

   *  2001:db8:b1:10:: bound to the End behavior with the NEXT-C-SID
      flavor is instantiated from GIB with

      -  Locator-Block length (LBL) = 48 (Locator-Block value
         0x20010db800b1),

      -  Locator-Node length (LNL) = 16 (Locator-Node value 0x0010),

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      -  Function length (FL) = 0, and

      -  Argument length (AL) = 64.

   *  2001:db8:b1:f123:: bound to the End.X behavior for its local IGP
      adjacency 123 with the NEXT-C-SID flavor is instantiated from LIB
      with

      -  Locator-Block length (LBL) = 48 (Locator-Block value
         0x20010db800b1),

      -  Locator-Node length (LNL) = 0,

      -  Function length (FL) = 16 (Function value 0xf123), and

      -  Argument length (AL) = 64.

   For SID 2001:db8:b1:10::, Node 10 would install the FIB entry
   2001:db8:b1:10::/64 bound the End SID with the NEXT-C-SID flavor.

   For SID 2001:db8:b1:f123::, Node 10 would install the FIB entry
   2001:db8:b1:f123::/64 bound the End.X SID for adjacency 123 with the
   NEXT-C-SID flavor.

   In addition, Node 10 may also install the combined FIB entry
   2001:db8:b1:10:f123::/80 bound the End.X SID for adjacency 123 with
   the NEXT-C-SID flavor.

   As another example, let us consider an SR segment endpoint node 20
   instantiating the following two REPLACE-C-SID flavor SIDs according
   to the C-SID length, Locator-Block length, and GIB/LIB
   recommendations in this section.

   *  2001:db8:b2:20:1:: from GIB with Locator-Block length (LBL) = 48,
      Locator-Node length (LNL) = 16, Function length (FL) = 16,
      Argument length (AL) = 48, and bound to the End behavior with the
      REPLACE-C-SID flavor.

   *  2001:db8:b2:20:123:: from GIB with Locator-Block length (LBL) =
      48, Locator-Node length (LNL) = 16, Function length (FL) = 16,
      Argument length (AL) = 48, and bound to the End.X behavior for its
      local IGP adjacency 123 with the REPLACE-C-SID flavor.

   For SID 2001:db8:b2:20:1::, Node 20 would install the FIB entry
   2001:db8:b2:20:1::/80 bound the End SID with the REPLACE-C-SID
   flavor.

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   For SID 2001:db8:b2:20:123::, Node 20 would install the FIB entry
   2001:db8:b2:20:123::/80 bound the End.X SID for adjacency 123 with
   the REPLACE-C-SID flavor.

6.  SR Source Node

   An SR source node may learn from a control plane protocol (see
   Section 8) or local configuration the SIDs that it can use in a
   segment list, along with their respective SR segment endpoint
   behavior, flavors, structure, and any other relevant attribute (e.g.,
   the set of L3 adjacencies associated with an End.X SID).

6.1.  SID Validation for Compression

   As part of the compression process or as a preliminary step, the SR
   source node MUST validate the SID structure of each SID of this
   document in the segment list.  The SR source node does so regardless
   of whether the segment list is explicitly configured, locally
   computed, or advertised by a controller (e.g., via BGP
   [I-D.ietf-idr-sr-policy-safi] or PCEP
   [I-D.ietf-pce-segment-routing-ipv6]).

   A SID structure is valid for compression if it meets all the
   following conditions.

   *  The Locator-Block length is not 0.

   *  The sum of the Locator-Node length and Function length is not 0.

   *  The Argument length is equal to 128-LBL-LNL-FL.

   When compressing a SID list, the SR source node MUST treat an invalid
   SID structure as unknown.  A SID with an unknown SID structure is
   incompressible.

   Section 8 discusses how the SIDs of this document and their structure
   can be advertised to the SR source node through various control plane
   protocols.  The SID structure may also be learned through
   configuration or other management protocols.  The details of such
   mechanisms are outside the scope of this document.

6.2.  Segment List Compression

   An SR source node MAY compress a SID list when it includes NEXT-C-SID
   and/or REPLACE-C-SID flavor SIDs in order to reduce the packet header
   length.

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   It is out of the scope of this document to describe the mechanism
   through which an uncompressed SID list is derived.  As a general
   guidance for implementation or future specification, such a mechanism
   should aim to select the combination of SIDs that would result in the
   shortest compressed SID list.  For example, by selecting a C-SID
   flavor SID over an equivalent non-C-SID flavor SID or by consistently
   selecting SIDs of the same C-SID flavor within each routing domain.

   The SID list that the SR source node pushes onto the packet MUST
   comply with the rules in Section 6.3 and Section 6.4 and express the
   same list of segments as the original SID list.  If these rules are
   not followed, the packet may get dropped or misrouted.

   If an SR source node chooses to compress the SID list, one method is
   described below for illustrative purposes.  Any other method
   producing a compressed SID list of equal or shorter length than the
   uncompressed SID list is compliant.

   This method walks the uncompressed SID list and compresses each
   series of consecutive NEXT-C-SID flavor SIDs and each series of
   consecutive REPLACE-C-SID flavor SIDs.

   *  When the compression method encounters a series of one or more
      consecutive compressible NEXT-C-SID flavor SIDs, it compresses the
      series as follows.  A SID with the NEXT-C-SID flavor is
      compressible if its structure is known to the SR source node and
      its Argument value is 0.

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   S01. Initialize a C-SID container equal to the first SID in the
          series, and initialize the remaining capacity of the C-SID
          container to the AL of that SID
   S02. For each subsequent SID in the series {
   S03.   If the current SID Locator-Block matches that of the C-SID
            container and the current SID LNFL is lower than or equal to
            the remaining capacity of the C-SID container {
   S04.     Copy the current SID Locator-Node and Function to the most
              significant remaining Argument bits of the C-SID container
              and decrement the remaining capacity by LNFL
   S05.   } Else {
   S06.     Push the C-SID container onto the compressed SID list
   S07.     Initialize a new C-SID container equal to the current SID in
              the series, and initialize the remaining capacity of the
              C-SID container to the AL of that SID
   S08.   } // End If
   S09. } // End For
   S10. If at least one SID remains in the uncompressed SID list
          (following the series of compressible NEXT-C-SID flavor SIDs){
   S11.   Set S to the next SID in the uncompressed SID list
   S12.   If S is advertised with a SID structure, and the Locator-Block
            of S matches that of the C-SID container, and the sum of the
            Locator-Node, Function, and Argument length of S is lower
            than or equal to the remaining capacity of the C-SID
            container {
   S13.     Copy the Locator-Node, Function, and Argument of S to the
              most significant remaining Argument bits of the C-SID
              container
   S14.   } // End If
   S15. } // End If
   S16. Push the C-SID container onto the compressed SID list

   *  When the compression method encounters a series of REPLACE-C-SID
      flavor SIDs of the same C-SID length in the uncompressed SID list,
      it compresses the series as per the following high-level pseudo
      code.  A compression checking function ComCheck(F, S) is defined
      to check if two SIDs F and S share the same SID structure and
      Locator-Block value, and if S has either no Argument or an
      Argument with value 0.  If the check passes, then ComCheck(F,S)
      returns true.

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   S01. Initialize the first C-SID container in full SID format equal to
          the first SID in the series
   S02. Initialize the second C-SID container in packed format if there
          are more than one SIDs, and initialize the remaining capacity
          of the C-SID container to 128 bits
   S03. For each subsequent SID in the uncompressed SID list {
   S04.   Set S to the current SID in the uncompressed SID list
   S05.   If ComCheck(First SID, S) {
   S06.     If the LNFL of S is lower than or equal to
              the remaining capacity of the C-SID container {
   S07.       Copy the Locator-Node and Function of S to the least
                significant remaining bits of the C-SID container
                and decrement the remaining capacity by LNFL  // Note
   S08.     } Else {
   S09.       Push the C-SID container onto the compressed SID list
   S10.       Initialize a new C-SID container in packed format with all
                bits set to 0
   S11        Copy the Locator-Node and Function of S to the least
                significant remaining bits of the C-SID container
                and decrement the remaining capacity by LNFL  // Note
   S12.     }
   S13.     If S is not a REPLACE-C-SID flavor SID, then break
   S14.   } Else {
   S15.     Break
   S16.   } // End If
   S17. } // End For
   S18. Push the C-SID container (if it is not empty) onto the
          compressed SID list

      |  Note: When the last C-SID is an End.DT2M SID with the REPLACE-
      |  C-SID flavor, if there is 0 or at least two C-SID positions
      |  left in the current C-SID container, the C-SID is encoded as
      |  described above and the value of the Arg.FE2 argument is placed
      |  in the 16 least significant bits of the next C-SID position.
      |  Otherwise (if there is only one C-SID position left in the
      |  current C-SID container), the current C-SID container is pushed
      |  onto the SID list (the value of the C-SID position 0 remains
      |  zero) and the End.DT2M SID with the REPLACE-C-SID flavor is
      |  encoded in full SID format with the value of the Arg.FE2
      |  argument in the 16 most significant bits of the SID Argument.

   *  In all remaining cases (i.e., when the compression method
      encounters a SID in the uncompressed SID list that is not handled
      by any of the previous subroutines), it pushes this SID as is onto
      the compressed SID list.

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   Regardless of how a compressed SID list is produced, the SR source
   node writes it in the IPv6 packet as described in Sections 4.1 and
   4.1.1 of [RFC8754].  The text is reproduced below for reference.

   |  A source node steers a packet into an SR Policy.  If the SR Policy
   |  results in a Segment List containing a single segment, and there
   |  is no need to add information to the SRH flag or add TLV; the DA
   |  is set to the single Segment List entry, and the SRH MAY be
   |  omitted.
   |  
   |  When needed, the SRH is created as follows:
   |  
   |  The Next Header and Hdr Ext Len fields are set as specified in
   |  [RFC8200].
   |  
   |  The Routing Type field is set to 4.
   |  
   |  The DA of the packet is set with the value of the first segment.
   |  
   |  The first element of the SRH Segment List is the ultimate segment.
   |  The second element is the penultimate segment, and so on.
   |  
   |  The Segments Left field is set to n-1, where n is the number of
   |  elements in the SR Policy.
   |  
   |  The Last Entry field is set to n-1, where n is the number of
   |  elements in the SR Policy.
   |  
   |  TLVs (including HMAC) may be set according to their specification.
   |  
   |  The packet is forwarded toward the packet's Destination Address
   |  (the first segment).
   |  
   |  When a source does not require the entire SID list to be preserved
   |  in the SRH, a reduced SRH may be used.
   |  
   |  A reduced SRH does not contain the first segment of the related SR
   |  Policy (the first segment is the one already in the DA of the IPv6
   |  header), and the Last Entry field is set to n-2, where n is the
   |  number of elements in the SR Policy.

6.3.  Rules for segment lists containing NEXT-C-SID flavor SIDs

   1.  If a Destination Option header would follow an SRH with a segment
       list of more than one segment compressed as a single NEXT-C-SID
       container, the SR source node MUST NOT omit the SRH.

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   2.  When the last Segment List entry (index 0) in the SRH is a C-SID
       container representing more than one segment, the PSP operation
       is performed at the segment preceding the first segment of this
       C-SID container in the segment list.  If the PSP behavior should
       instead be performed at the penultimate segment along the path,
       the SR source node MUST NOT compress the ultimate SID of the SID
       list into a C-SID container.

   3.  If a Destination Option header would follow an SRH with a last
       Segment List entry being a NEXT-C-SID container representing more
       than one segment, the SR source node MUST ensure that the PSP
       operation is not performed before the penultimate SR segment
       endpoint node along the path.

6.4.  Rules for segment lists containing REPLACE-C-SID flavor SIDs

   1.  All SIDs compressed in a REPLACE-C-SID sequence MUST share the
       same Locator-Block and the same compression scheme.

   2.  All SIDs except the last one in a C-SID sequence for REPLACE-
       C-SID MUST have the REPLACE-C-SID flavor.  If the last C-SID
       container is fully filled (i.e., the last C-SID is at position 0
       in the C-SID container) and the last SID in the C-SID sequence is
       not the last segment in the segment list, the last SID in the
       C-SID sequence MUST NOT have the REPLACE-C-SID flavor.

   3.  When a REPLACE-C-SID flavor C-SID is present as the last SID in a
       container that is not the last Segment List entry (index 0) in
       the SRH, the next element in the SID list MUST be a REPLACE-C-SID
       container in packed format carrying at least one C-SID.

   The SR source node determines the compression scheme of REPLACE-C-SID
   flavor SIDs as follows.

   When receiving a SID advertisement for a REPLACE-C-SID flavor SID
   with LNL=16, FL=0, AL=128-LBL-NL-FL, and the value of the Argument is
   all 0, the SR source node marks both the SID and its locator as using
   16-bit compression.  All other SIDs allocated from this locator with
   LNL=16, FL=16, AL=128-LBL-NL-FL, and the value of the Argument is all
   0 are also marked as using 16-bit compression.  When receiving a SID
   advertisement for a REPLACE-C-SID flavor SID with LNFL=32, AL=128-
   LBL-NL-FL, and the value of the Argument is all 0, the SR source node
   marks both the SID and its locator as using 32-bit compression.

6.5.  Upper-Layer Checksums

   The Destination Address used in the IPv6 pseudo-header (Section 8.1
   of [RFC8200]) is that of the ultimate destination.

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   At the SR source node, that address will be the Destination Address
   as it is expected to be received by the ultimate destination.  When
   the last element in the compressed SID list is a C-SID container,
   this address can be obtained from the last element in the
   uncompressed SID list or by repeatedly applying the segment behavior
   as described in Section 9.2.  This applies regardless of whether an
   SRH is present in the IPv6 packet or omitted.

   At the ultimate destination(s), that address will be in the
   Destination Address field of the IPv6 header.

7.  Inter-Domain Compression

   Some SRv6 traffic may need to cross multiple routing domains, such as
   different Autonomous Systems (ASes) or different routing areas within
   an SR domain.  Different routing domains may use different addressing
   schema and Locator-Blocks.

   A property of a C-SID sequence is that all C-SIDs in the sequence
   share the same Locator-Block.  Therefore, a segment list that spans
   across multiple routing domains using different Locator-Blocks may
   need a separate C-SID sequence for each domain.

   This section defines an OPTIONAL solution to improve the efficiency
   of C-SID compression in multi-domain environments by enabling a C-SID
   sequence to combine C-SIDs having different Locator-Blocks.

   The solution leverages two new SR segment endpoint behaviors,
   "Endpoint with SRv6 Prefix Swap" ("End.PS" for short) and "Endpoint
   with L3 cross-connect and SRv6 Prefix Swap" ("End.XPS" for short),
   that enable modifying the Locator-Block for the next C-SID in the
   C-SID sequence at the routing domain boundary.

7.1.  End.PS: Prefix Swap

   The End.PS behavior is a variant of the End behavior that modifies
   the Locator-Block of the active C-SID sequence.  This document
   defines the End.PS behavior with the NEXT-C-SID flavor and the End.PS
   behavior with the REPLACE-C-SID flavor.

   An End.PS SID is used to transition to a new Locator-Block when the
   routing domain boundary is on the SR segment endpoint node.

   Each instance of an End.PS SID is associated with a target Locator-
   Block B2/m, where B2 is an IPv6 address prefix and m is the
   associated prefix length.  The target Locator-Block is a local
   property of the End.PS SID on the SR segment endpoint node.

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      |  Note: a local SID property is an attribute associated with the
      |  SID when it is instantiated on the SR segment endpoint node.
      |  When the SR segment endpoint node identifies the destination
      |  address of a received packet as a locally instantiated SID, it
      |  also retrieves any local property associated with this SID.
      |  Other examples of local SID properties include the set of L3
      |  adjacencies of an End.X SID (Section 4.2 of [RFC8986]) and the
      |  lookup table of an End.DT6 SID (Section 4.6 of [RFC8986]).

   The means by which an SR source node learns the target Locator-Block
   associated with an End.PS SID are outside the scope of this document.
   As examples, it could be learnt via configuration or signaled by a
   controller.

7.1.1.  End.PS with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.PS SID with the NEXT-C-SID flavor and
   associated with the target Locator-Block B2/m, the SR segment
   endpoint node applies the procedure specified in Section 4.1.1 with
   the lines N05 to N06 replaced as follows.

   N05.1. Initialize an IPv6 address A equal to B2.
   N05.2. Copy DA.Argument into the bits [m..(m+AL-1)] of A.
   N06.   Copy A to the Destination Address of the IPv6 header.

7.1.2.  End.PS with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.PS SID with the REPLACE-C-SID flavor and
   associated with the target Locator-Block B2/m, the SR segment
   endpoint node applies the procedure specified in Section 4.2.1 with
   the line R20 replaced as follows.

   R20.1. Initialize an IPv6 address A equal to B2.
   R20.2. Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [m..m+LNFL-1] of the Destination Address of the IPv6 header.
   R20.3. Copy A to the Destination Address of the IPv6 header.

7.2.  End.XPS: L3 Cross-Connect and Prefix Swap

   The End.XPS behavior is a variant of the End.X behavior that modifies
   the Locator-Block of the active C-SID sequence.  This document
   defines the End.XPS behavior with the NEXT-C-SID flavor and the
   End.XPS behavior with the REPLACE-C-SID flavor.

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   An End.XPS SID is used to transition to a new Locator-Block when the
   routing domain boundary is on a link adjacent to the SR segment
   endpoint node.

   Each instance of an End.XPS SID is associated with a target Locator-
   Block B2/m and a set, J, of one or more L3 adjacencies.  The target
   Locator-Block and set of adjacencies are local properties of the
   End.XPS SID on the SR segment endpoint node.

   The means by which an SR source node learns the target Locator-Block
   associated with an End.XPS SID are outside the scope of this
   document.  As examples, it could be learnt via configuration or
   signaled by a controller.

7.2.1.  End.XPS with NEXT-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.XPS SID with the NEXT-C-SID flavor and
   associated with the target Locator-Block B2/m, the SR segment
   endpoint node applies the procedure specified in Section 4.1.2 with
   the lines N05 to N06 (of the pseudocode in Section 4.1.1) replaced as
   follows.

   N05.1. Initialize an IPv6 address A equal to B2.
   N05.2. Copy DA.Argument into the bits [m..(m+AL-1)] of A.
   N06.   Copy A to the Destination Address of the IPv6 header.

7.2.2.  End.XPS with REPLACE-C-SID

   When processing an IPv6 packet that matches a FIB entry locally
   instantiated as an End.XPS SID with the REPLACE-C-SID flavor and
   associated with the target Locator-Block B2/m, the SR segment
   endpoint node applies the procedure specified in Section 4.2.2 with
   the line R20 (of the pseudocode in Section 4.2.1) replaced as
   follows.

   R20.1. Initialize an IPv6 address A equal to B2.
   R20.2. Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [m..m+LNFL-1] of the Destination Address of the IPv6 header.
   R20.3. Copy A to the Destination Address of the IPv6 header.

8.  Control Plane

   This document does not require any new extensions to routing
   protocols.

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   Section 8 of [RFC8986] provides an overview of the control plane
   protocols used for signaling of the SRv6 SIDs introduced by that
   document.  The SRv6 SIDs introduced by this document are advertised
   using the same SRv6 extensions for various routing protocols, such as

   *  IS-IS [RFC9352]

   *  OSPFv3 [RFC9513]

   *  BGP [RFC9252], [RFC9514], [I-D.ietf-idr-sr-policy-safi]

   *  BGP-LS [I-D.ietf-idr-bgp-ls-sr-policy]

   *  PCEP [I-D.ietf-pce-segment-routing-ipv6]

   The SR segment endpoint node MUST set the SID Argument bits to 0 when
   advertising a locally instantiated SID of this document in the
   routing protocol (e.g., IS-IS [RFC9352], OSPF [RFC9513], or BGP-LS
   [RFC9514]).

   Signaling the SRv6 SID Structure is REQUIRED for all the SIDs
   introduced in this document.  It is used by an SR source node to
   compress a SID list as described in Section 6.  The node initiating
   the SID advertisement MUST set the length values in the SRv6 SID
   Structure to match the format of the SID on the SR segment endpoint
   node.  For example, for a SID of this document instantiated from a
   /48 SRv6 SID block and a /64 Locator, and having a 16-bit Function,
   the SRv6 SID Structure advertisement carries the following values.

   *  Locator-Block length: 48

   *  Locator-Node length: 16

   *  Function length: 16

   *  Argument length: 48 (= 128-48-16-16)

   A local C-SID may be advertised in the control plane individually
   and/or in combination with a global C-SID instantiated on the same SR
   segment endpoint node, with the End behavior, and the same Locator-
   Block and flavor as the local C-SID.  A combined global and local
   C-SID is advertised as follows.

   *  The SID Locator-Block is that shared by the global and local
      C-SIDs

   *  The SID Locator-Node is that of global C-SID

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   *  The SID Function is that of the local C-SID

   *  The SID Argument length is equal to 128-LBL-LNL-FL and the SID
      Argument value is 0

   *  All other attributes of the SID (e.g., endpoint behavior or
      algorithm) are those of the local C-SID

   The local C-SID combined advertisement is needed in particular for
   control plane protocols mandating that the SID is a subnet of a
   locator advertised in the same protocol (e.g., Section 8 of [RFC9352]
   and Section 9 of [RFC9513] for advertising Adjacency SIDs in IS-IS
   and OSPFv3, respectively).

   For a segment list computed by a controller and signaled to an SR
   source node (e.g., via BGP [I-D.ietf-idr-sr-policy-safi] or PCEP
   [I-D.ietf-pce-segment-routing-ipv6]), the controller provides the
   ordered segment list comprising the uncompressed SIDs, with their
   respective behavior and structure, to the SR source node.  The SR
   source node may then compress the SID list as described in Section 6.

   When a node that does not support this specification receives an
   advertisement of a SID of this document, it handles it as described
   in the corresponding control plane specification (e.g., Sections 7.2,
   8.1, and 8.2 of [RFC9352], Sections 8, 9.1, and 9.2 of [RFC9513], and
   Section 3.1 of [RFC9252]).

9.  Operational Considerations

9.1.  Pinging a SID

   An SR source node may ping an SRv6 SID by sending an ICMPv6 echo
   request packet destined to the SRv6 SID, with or without intermediate
   segments.  This operation is illustrated in Appendix A.1.2 of
   [RFC9259].

   When pinging a SID of this document, with or without intermediate
   segments, the SR source node MUST construct the IPv6 packet as
   described in Section 6, including computing the ICMPv6 checksum as
   described in Section 6.5.

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   In particular, when pinging a SID of this document without
   intermediate segments, the SR source node places the SID with
   Argument value 0 in the destination address of the ICMPv6 echo
   request and computes the ICMPv6 checksum using this SID as the
   destination address in the IPv6 pseudo-header.  The Argument value 0
   allows the SID SR segment endpoint node (Section 4) to identify
   itself as the ultimate destination of the packet and process the
   ICMPv6 payload.

   An operator originating ICMP echo requests to a NEXT-C-SID or
   REPLACE-C-SID flavor SID using the simplified SR source node
   processing described in the previous paragraph should ensure that the
   SID Argument is 0.

9.2.  ICMP Error Processing

   When an IPv6 node encounters an error while processing a packet, it
   may report that error by sending an IPv6 error message to the packet
   source with an enclosed copy of the invoking packet.  For the source
   of an invoking packet to process the ICMP error message, the ultimate
   destination address of the IPv6 header may be required.

   Section 5.4 of [RFC8754] defines the logic that an SR source node
   follows to determine the ultimate destination of an invoking packet
   containing an SRH.

   For an SR source node that supports the compressed segment list
   encoding defined in this document, the logic to determine the
   ultimate destination is generalized as follows.

   *  If the destination address of the invoking IPv6 packet matches a
      known SRv6 SID, modify the invoking IPv6 packet by applying the
      SID behavior associated with the matched SRv6 SID;

   *  Repeat until the application of the SID behavior would result in
      the processing of the upper-layer header.

   The destination address of the resulting IPv6 packet may be used as
   the ultimate destination of the invoking IPv6 packet.

   Since the SR source node that needs to determine the ultimate
   destination is the same node that originally built the SID list in
   the invoking packet, it is able to perform this operation for all the
   SIDs in the packet.

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9.3.  Upper Layer Checksum Considerations

   Upper layer checksums are computed by the originator of an IPv6
   packet and verified by the ultimate destination(s) as it processes
   the upper layer protocol.

   As specified in Section 6.5, SR source nodes originating TCP/UDP
   packets ensure that the upper layer checksum is correctly calculated
   based on the ultimate destination of the session, which may be
   different from the address placed in the IPv6 destination address.
   Such SR source nodes leveraging TCP/UDP offload engines may require
   enhancements to convey the ultimate destination address.  These
   implementation enhancements are outside the scope of this document.

   Middleboxes such as packet sniffers, if deployed inside the SR
   domain, may fail to verify the upper layer checksum of transit SRv6
   traffic.  Making these middleboxes SRv6 aware in general or C-SID
   aware in particular is out of the scope of this document.

10.  Implementation Status

   This section is to be removed before publishing as an RFC.

   RFC-Editor: Please clean up the references cited by this section
   before publication.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   This section is provided in compliance with the SPRING working group
   policies ([SPRING-WG-POLICIES]).

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10.1.  Cisco Systems

   Cisco Systems reported the following implementations of the SR
   segment endpoint node NEXT-C-SID flavor (Section 4.1) and the SR
   source node efficient SID list encoding (Section 6) for NEXT-C-SID
   flavor SIDs.  These are used as part of its SRv6 TI-LFA, micro-loop
   avoidance, and traffic engineering functionalities.

   *  Cisco NCS 540 Series routers running IOS XR 7.3.x or above
      [IMPL-CISCO-NCS540]

   *  Cisco NCS 560 Series routers running IOS XR 7.6.x or above
      [IMPL-CISCO-NCS560]

   *  Cisco NCS 5500 Series routers running IOS XR 7.3.x or above
      [IMPL-CISCO-NCS5500]

   *  Cisco NCS 5700 Series routers running IOS XR 7.5.x or above
      [IMPL-CISCO-NCS5700]

   *  Cisco 8000 Series routers running IOS XR 7.5.x or above
      [IMPL-CISCO-8000]

   *  Cisco ASR 9000 Series routers running IOS XR 7.5.x or above
      [IMPL-CISCO-ASR9000]

   At the time of this report, all the implementations listed above are
   in production and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   NEXT-C-SID flavor.

   This report was last updated on January 11, 2023.

10.2.  Huawei Technologies

   Huawei Technologies reported the following implementations of the SR
   segment endpoint node REPLACE-C-SID flavor (Section 4.2).  These are
   used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic
   engineering functionalities.

   *  Huawei ATN8XX,ATN910C,ATN980B routers running VRPV800R021C00 or
      above.

   *  Huawei CX600-M2 routers running VRPV800R021C00 or above.

   *  Huawei NE40E,ME60-X1X2,ME60-X3X8X16 routers running VRPV800R021C00
      or above.

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   *  Huawei NE5000E,NE9000 routers running VRPV800R021C00 or above.

   *  Huawei NCE-IP Controller running V1R21C00 or above.

   At the time of this report, all the implementations listed above are
   in production and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   REPLACE-C-SID flavor.

   This report was last updated on January 11, 2023.

10.3.  Nokia

   Nokia reported the following implementations ([IMPL-NOKIA-20.10]) of
   the SR segment endpoint node NEXT-C-SID flavor (Section 4.1).  These
   are used as part of its shortest path forwarding (in algorithm 0 and
   Flex-Algo), remote and TI-LFA repair tunnel, and Traffic Engineering
   functionalities.

   *  Nokia 7950 XRS 20/20e routers running SROS Release 22.10 or above

   *  Nokia 7750 SR-12e routers running SROS Release 22.10 or above

   *  Nokia 7750 SR-7/12 routers running SROS Release 22.10 or above

   *  Nokia 7750 SR-7s/14s routers running SROS Release 22.10 or above

   *  Nokia 7750 SR-1/1s/2s routers running SROS Release 22.10 or above

   At the time of this report, all the implementations listed above are
   in production and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   NEXT-C-SID flavor.

   This report was last updated on February 3, 2023.

10.4.  Arrcus

   Arrcus reported the following implementations of the SR segment
   endpoint node NEXT-C-SID flavor (Section 4.1).  These are used as
   part of its SRv6 shortest path forwarding (in algorithm 0 and Flex-
   Algo), TI-LFA, micro-loop avoidance and Traffic Engineering
   functionalities.

   *  Arrcus running on Ufi Space routers S9510-28DC, S9710-76D,
      S9600-30DX and S9700-23D with ArcOS v5.2.1 or above

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   *  Arrcus running n Ufi Space routers S9600-72XC and S9700-53DX with
      ArcOS v5.1.1D or above

   *  Arrcus running on Quanta router IXA and IXAE with ArcOS v5.1.1D or
      above

   At the time of this report, all the implementations listed above are
   in production and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   NEXT-C-SID flavor.

   This report was last updated on March 11, 2023.

10.5.  Juniper Networks

   Juniper Networks reported the following implementations of the SR
   segment endpoint node NEXT-C-SID flavor (Section 4.1).  These are
   used as part of its SRv6 shortest path forwarding (in algorithm 0 and
   Flex-Algo), TI-LFA, micro-loop avoidance, and Traffic Engineering
   functionalities.

   Juniper release 23.3 onwards supports this functionality.

   At the time of this report, all the implementations listed above are
   in development and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   NEXT-C-SID flavor.

   This report was last updated on May 30, 2023.

10.6.  Marvell

   Marvell reported support in the Marvell Prestera Packet Processor for
   the SR segment endpoint node NEXT-C-SID flavor (Section 4.1) and
   REPLACE-C-SID flavor (Section 4.2).

   This report was last updated on February 15, 2023.

10.7.  Broadcom

   Broadcom reported the following implementations of the SR segment
   endpoint node NEXT-C-SID flavor (Section 4.1) and REPLACE-C-SID
   flavor (Section 4.2).  These are used as part of its SRv6 TI-LFA,
   micro-loop avoidance, and traffic engineering functionalities.  All
   implementation of the following list is in general availability for
   customers using BCM SDK 6.5.26 or above.

   *  88850 (Jericho2c+) series

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   *  88690 (Jericho2) series

   *  88800 (Jericho2c) series

   *  88480 (Qunran2a) series

   *  88280 (Qunran2u) series

   *  88295 (Qunran2n) series

   *  88830 (Jericho2x) series

   At the time of this report, all the implementations listed above are
   in production and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   NEXT-C-SID and REPLACE-C-SID flavors.

   For 78900 (Tomahawk) series-related support, please contact the
   Broadcom team.

   This report was last updated on February 21, 2023.

10.8.  ZTE Corporation

   ZTE Corporation reported the following implementations of the SR
   segment endpoint node REPLACE-C-SID flavor (Section 4.2).  These are
   used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic
   engineering functionalities.

   *  ZTE M6000-18S(BRAS), M6000-8S Plus(BRAS) routers running
      V5.00.10.09 or above.

   *  ZTE M6000-18S(SR), M6000-8S Plus(SR) routers running V5.00.10.80
      or above.

   *  ZTE T8000-18 routers running V5.00.10.07 or above.

   This report was last updated on March 29, 2023.

10.9.  New H3C Technologies

   New H3C Technologies reported the following implementations of the SR
   segment endpoint node REPLACE-C-SID flavor (Section 4.2).  These are
   used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic
   engineering functionalities.

   *  H3C CR16000-F, SR8800-X routers running Version 7.1.075 or above.

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   *  H3C CR18000, CR19000 routers running Version 7.1.071 or above.

   This report was last updated on March 29, 2023.

10.10.  Ruijie Network

   Ruijie Network reported the following implementations of the SR
   segment endpoint node REPLACE-C-SID flavor (Section 4.2).  These are
   used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic
   engineering functionalities.

   *  RUIJIE RG-N8018-R, RG-N8010-R routers running N8000-R_RGOS
      12.8(3)B0801 or above.

   This report was last updated on March 29, 2023.

10.11.  Ciena

   Ciena reported the following implementations of the SR segment
   endpoint node NEXT-C-SID flavor (Section 4.1).  These are used as
   part of its shortest path forwarding (in algorithm 0 and Flex-Algo),
   remote and TI-LFA repair tunnel, and Traffic Engineering
   functionalities.

   The following platforms support implementation of the above.

   *  Ciena 5162, 5164, 5166, 5168 routers running SAOS 10.10 or above

   *  Ciena 8110, 8112, 8190 routers running SAOS 10.10 or above

   At the time of this report, all the implementations listed above are
   in production and follow the specification in the latest version of
   this document, including all the "MUST" and "SHOULD" clauses for the
   NEXT-C-SID flavor.

   This report was last updated on February 6, 2024.

10.12.  Centec

   Centec reported the following implementations of the SR segment
   endpoint node REPLACE-C-SID flavor (Section 4.2).  These are used as
   part of its SRv6 TI-LFA, micro-loop avoidance, and traffic
   engineering functionalities.  All implementation of the following
   list is in general availability for customers using Centec SDK 5.6.8
   or above.

   *  CTC7132 (TsingMa) Series

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   *  CTC8180 (TsingMa.MX) Series

   This report was last updated on February 14, 2024.

10.13.  Open Source

   The authors found the following open source implementations of the SR
   segment endpoint node NEXT-C-SID flavor (Section 4.1).

   *  The Linux kernel, version 6.1 [IMPL-OSS-LINUX]

   *  The Software for Open Networking in the Cloud (SONiC), version
      202212 [IMPL-OSS-SONIC], and Switch Abstraction Interface (SAI),
      version 1.9.0 [IMPL-OSS-SAI]

   *  The Vector Packet Processor (VPP), version 20.05 [IMPL-OSS-VPP]

   *  A generic P4 implementation [IMPL-OSS-P4]

   The authors found the following open source implementations of the SR
   segment endpoint node REPLACE-C-SID flavor (Section 4.2).

   *  ONOS and P4 Programmable Switch based [IMPL-OSS-ONOS]

   *  Open SRv6 Project [IMPL-OSS-OPEN-SRV6]

   This section was last updated on January 11, 2023.

10.14.  Interoperability Reports

10.14.1.  Bell Canada / Ciena 2023

   Bell Canada is currently evaluating interoperability between Ciena
   and Cisco implementations of the NEXT-C-SID flavor defined in this
   document.  Further information will be added to this section when the
   evaluation is complete.

10.14.2.  EANTC 2023

   In April 2023, the European Advanced Networking Test Center (EANTC)
   successfully validated multiple implementations of SRv6 NEXT-C-SID
   flavor (a.k.a., SRv6 uSID) [EANTC-23].

   The participating vendors included Arista, Arrcus, Cisco, Huawei,
   Juniper, Keysight, Nokia, and Spirent.

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10.14.3.  China Mobile 2020

   In November 2020, China Mobile successfully validated multiple
   interoperable implementations of the NEXT-C-SID and REPLACE-C-SID
   flavors defined in this document.

   This testing covered two different implementations of the SRv6
   endpoint flavors defined in this document:

   *  Hardware implementation in Cisco ASR 9000 running IOS XR

   *  Software implementation in Cisco IOS XRv9000 virtual appliance

   *  Hardware implementation in Huawei NE40E and NE5000E running VRP

   The interoperability testing consisted of a packet flow sent by an SR
   source node N0 via an SR traffic engineering policy with a segment
   list <S1, S2, S3, S4, S5, S6, S7>, where S1..S7 are SIDs instantiated
   on SR segment endpoint nodes N1..N7, respectively.

   N0 --- N1 --- N2 --- N3 --- N4 --- N5 --- N6 --- N7
         (S1)   (S2)   (S3)   (S4)   (S5)   (S6)   (S7)

   *  N0 is a generic packet generator.

   *  N1, N2, and N3 are Huawei routers.

   *  N4, N5, and N6 are Cisco routers.

   *  N7 is a generic traffic generator acting as a packet receiver.

   The SR source node N0 steers the packets onto the SR policy by
   setting the IPv6 destination address and creating an SRH (as
   described in Section 4.1 of [RFC8754]) using a compressed segment
   list encoding.  The length of the compressed segment list encoding
   varies for each scenario.

   All SR segment endpoint nodes execute a variant of the End behavior:
   regular End behavior (as defined in Section 4.1 of [RFC8986]), End
   behavior with NEXT-C-SID flavor, and End behavior with REPLACE-C-SID
   flavor.  The variant being used at each SR segment endpoint node
   varies for each scenario.

   The interoperability was validated for the following scenarios:

   *Scenario 1:*

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   *  S1 and S2 are associated with the End behavior with the REPLACE-
      C-SID flavor

   *  S3 is associated with the regular End behavior (no flavor)

   *  S4, S5, and S6 are associated with the End behavior with the NEXT-
      C-SID flavor

   *  The SR source node imposes a compressed segment list encoding of 3
      SIDs.

   *Scenario 2:*

   *  S1, S2..., S6 are associated with the End behavior with the NEXT-
      C-SID flavor

   *  The SR source node imposes a compressed segment list encoding of 2
      SIDs.

   *Scenario 3:*

   *  S1, S2..., S6 are associated with the End behavior with the
      REPLACE-C-SID flavor

   *  The SR source node imposes a compressed segment list encoding of 3
      SIDs.

11.  Applicability to other SR Segment Endpoint Behaviors

   Future documents may extend the applicability of the NEXT-C-SID and
   REPLACE-C-SID flavors to other SR segment endpoint behaviors.

   For an SR segment endpoint behavior that can be used before the last
   position of a segment list, a C-SID flavor is defined by reproducing
   the same logic as described in Section 4.1 and Section 4.2 of this
   document to determine the next SID in the SID list.

12.  Security Considerations

   Section 8 of [RFC8402] discusses the security considerations for
   Segment Routing.

   Section 5 of [RFC8754] describes the intra-SR-domain deployment model
   and how to secure it.  Section 7 of [RFC8754] describes the threats
   applicable to SRv6 and how to mitigate them.

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   Section 9 of [RFC8986] discusses the security considerations
   applicable to the SRv6 network programming framework, as well as the
   SR source node and SR segment endpoint node behaviors that it
   defines.

   This document introduces two new flavors for some of the SR segment
   endpoint behaviors defined in [RFC8986] and a method by which an SR
   source node may leverage the SIDs of these flavors to produce a
   compressed segment list encoding.

   An SR source node constructs an IPv6 packet with a compressed segment
   list encoding as defined in Sections 3.1 and 4.1 of [RFC8754] and
   Section 5 of [RFC8986].  The paths that an SR source node may enforce
   using a compressed segment list encoding are the same, from a
   topology and service perspective, as those that an SR source node
   could enforce using the SIDs of [RFC8986].

   An SR segment endpoint node processes an IPv6 packet matching a
   locally instantiated SID as defined in [RFC8986], with the pseudocode
   modifications in Section 4 of this document.  These modifications
   change how the SR segment endpoint node determines the next SID in
   the packet, but not the semantic of either the active or the next
   SID.  For example, an adjacency segment instantiated with the End.X
   behavior remains an adjacency segment regardless of whether it uses
   the unflavored End.X behavior defined in Section 4.2 of [RFC8986] or
   a C-SID flavor of that behavior.  This document does not introduce
   any new SID semantic.

   Any other transit node processes the packet as described in
   Section 4.2 of [RFC8754].

   This document defines a new method of encoding the SIDs inside a SID
   list at the SR source node and decoding them at the SR segment
   endpoint node, but it does not change how the SID list itself is
   encoded in the IPv6 packet nor the semantic of any segment that it
   comprises.  Therefore, this document is subject to the same security
   considerations that are discussed in [RFC8402], [RFC8754], and
   [RFC8986].

13.  IANA Considerations

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13.1.  SRv6 Endpoint Behaviors

   This I-D. requests the IANA to update the reference of the following
   registrations from the "SRv6 Endpoint Behaviors" registry under the
   top-level "Segment Routing" registry-group
   (https://www.iana.org/assignments/segment-routing/) with the RFC
   number of this document once it is published, and transfer change
   control to the IETF.

      +=======+=========================================+===========+
      | Value | Description                             | Reference |
      +=======+=========================================+===========+
      | 43    | End with NEXT-CSID                      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 44    | End with NEXT-CSID & PSP                | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 45    | End with NEXT-CSID & USP                | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 46    | End with NEXT-CSID, PSP & USP           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 47    | End with NEXT-CSID & USD                | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 48    | End with NEXT-CSID, PSP & USD           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 49    | End with NEXT-CSID, USP & USD           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 50    | End with NEXT-CSID, PSP, USP & USD      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 52    | End.X with NEXT-CSID                    | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 53    | End.X with NEXT-CSID & PSP              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 54    | End.X with NEXT-CSID & USP              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 55    | End.X with NEXT-CSID, PSP & USP         | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 56    | End.X with NEXT-CSID & USD              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 57    | End.X with NEXT-CSID, PSP & USD         | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 58    | End.X with NEXT-CSID, USP & USD         | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 59    | End.X with NEXT-CSID, PSP, USP & USD    | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 85    | End.T with NEXT-CSID                    | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 86    | End.T with NEXT-CSID & PSP              | This I-D. |
      +-------+-----------------------------------------+-----------+

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      | 87    | End.T with NEXT-CSID & USP              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 88    | End.T with NEXT-CSID, PSP & USP         | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 89    | End.T with NEXT-CSID & USD              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 90    | End.T with NEXT-CSID, PSP & USD         | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 91    | End.T with NEXT-CSID, USP & USD         | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 92    | End.T with NEXT-CSID, PSP, USP & USD    | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 93    | End.B6.Encaps with NEXT-CSID            | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 94    | End.B6.Encaps.Red with NEXT-CSID        | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 95    | End.BM with NEXT-CSID                   | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 96    | End.PS with NEXT-CSID                   | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 97    | End.XPS with NEXT-CSID                  | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 101   | End with REPLACE-CSID                   | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 102   | End with REPLACE-CSID & PSP             | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 103   | End with REPLACE-CSID & USP             | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 104   | End with REPLACE-CSID, PSP & USP        | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 105   | End.X with REPLACE-CSID                 | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 106   | End.X with REPLACE-CSID & PSP           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 107   | End.X with REPLACE-CSID & USP           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 108   | End.X with REPLACE-CSID, PSP & USP      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 109   | End.T with REPLACE-CSID                 | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 110   | End.T with REPLACE-CSID & PSP           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 111   | End.T with REPLACE-CSID & USP           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 112   | End.T with REPLACE-CSID, PSP & USP      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 114   | End.B6.Encaps with REPLACE-CSID         | This I-D. |
      +-------+-----------------------------------------+-----------+

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      | 115   | End.BM with REPLACE-CSID                | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 116   | End.DX6 with REPLACE-CSID               | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 117   | End.DX4 with REPLACE-CSID               | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 118   | End.DT6 with REPLACE-CSID               | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 119   | End.DT4 with REPLACE-CSID               | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 120   | End.DT46 with REPLACE-CSID              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 121   | End.DX2 with REPLACE-CSID               | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 122   | End.DX2V with REPLACE-CSID              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 123   | End.DT2U with REPLACE-CSID              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 124   | End.DT2M with REPLACE-CSID              | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 127   | End.B6.Encaps.Red with REPLACE-CSID     | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 128   | End with REPLACE-CSID & USD             | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 129   | End with REPLACE-CSID, PSP & USD        | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 130   | End with REPLACE-CSID, USP & USD        | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 131   | End with REPLACE-CSID, PSP, USP & USD   | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 132   | End.X with REPLACE-CSID & USD           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 133   | End.X with REPLACE-CSID, PSP & USD      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 134   | End.X with REPLACE-CSID, USP & USD      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 135   | End.X with REPLACE-CSID, PSP, USP & USD | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 136   | End.T with REPLACE-CSID & USD           | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 137   | End.T with REPLACE-CSID, PSP & USD      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 138   | End.T with REPLACE-CSID, USP & USD      | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 139   | End.T with REPLACE-CSID, PSP, USP & USD | This I-D. |
      +-------+-----------------------------------------+-----------+
      | 140   | End.PS with REPLACE-CSID                | This I-D. |
      +-------+-----------------------------------------+-----------+

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      | 141   | End.XPS with REPLACE-CSID               | This I-D. |
      +-------+-----------------------------------------+-----------+

                         Table 1: Registration List

14.  Acknowledgements

   The authors would like to thank Kamran Raza, Xing Jiang, YuanChao Su,
   Han Li, Yisong Liu, Martin Vigoureux, Joel Halpern, and Tal Mizrahi
   for their insightful feedback and suggestions.

   The authors would also like to thank Andrew Alston, Linda Dunbar,
   Adrian Farrel, Boris Hassanov, and Alvaro Retana for their thorough
   review of this document.

15.  References

15.1.  Normative References

   [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>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

   [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>.

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15.2.  Informative References

   [EANTC-23] European Advanced Networking Test Center (EANTC), "Multi-
              Vendor MPLS SDN Interoperability Test Report", 18 April
              2023,
              <https://eantc.de/fileadmin/eantc/downloads/events/2023/
              EANTC-InteropTest2023-TestReport.pdf>.

   [GKP94]    Graham, R., Knuth, D., and O. Patashnik, "Concrete
              Mathematics: A Foundation for Computer Science",
              ISBN 9780201558029, 1994.

   [I-D.ietf-6man-sids]
              Krishnan, S., "SRv6 Segment Identifiers in the IPv6
              Addressing Architecture", Work in Progress, Internet-
              Draft, draft-ietf-6man-sids-06, 15 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              sids-06>.

   [I-D.ietf-idr-bgp-ls-sr-policy]
              Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J.
              Tantsura, "Advertisement of Segment Routing Policies using
              BGP Link-State", Work in Progress, Internet-Draft, draft-
              ietf-idr-bgp-ls-sr-policy-04, 20 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
              ls-sr-policy-04>.

   [I-D.ietf-idr-sr-policy-safi]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
              D. Jain, "Advertising Segment Routing Policies in BGP",
              Work in Progress, Internet-Draft, draft-ietf-idr-sr-
              policy-safi-02, 16 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
              policy-safi-02>.

   [I-D.ietf-pce-segment-routing-ipv6]
              Li, C., Kaladharan, P., Sivabalan, S., Koldychev, M., and
              Y. Zhu, "Path Computation Element Communication Protocol
              (PCEP) Extensions for IPv6 Segment Routing", Work in
              Progress, Internet-Draft, draft-ietf-pce-segment-routing-
              ipv6-25, 4 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              segment-routing-ipv6-25>.

   [IMPL-CISCO-8000]
              Cisco Systems, "Segment Routing Configuration Guide for
              Cisco 8000 Series Routers", 4 November 2022,
              <https://www.cisco.com/c/en/us/td/docs/iosxr/cisco8000/

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              segment-routing/75x/b-segment-routing-cg-cisco8000-75x/
              configuring-segment-routing-over-ipv6-srv6-micro-
              sids.html>.

   [IMPL-CISCO-ASR9000]
              Cisco Systems, "Segment Routing Configuration Guide for
              Cisco ASR 9000 Series Routers", 6 November 2022,
              <https://www.cisco.com/c/en/us/td/docs/routers/asr9000/
              software/asr9k-r7-5/segment-routing/configuration/guide/b-
              segment-routing-cg-asr9000-75x/configure-srv6-micro-
              sid.html>.

   [IMPL-CISCO-NCS540]
              Cisco Systems, "Segment Routing Configuration Guide for
              Cisco NCS 540 Series Routers", 2 November 2022,
              <https://www.cisco.com/c/en/us/td/docs/iosxr/ncs5xx/
              segment-routing/73x/b-segment-routing-cg-73x-ncs540/
              configure-srv6.html>.

   [IMPL-CISCO-NCS5500]
              Cisco Systems, "Segment Routing Configuration Guide for
              Cisco NCS 5500 Series Routers", 6 November 2022,
              <https://www.cisco.com/c/en/us/td/docs/iosxr/ncs5500/
              segment-routing/73x/b-segment-routing-cg-ncs5500-73x/
              configure-srv6-micro-sid.html>.

   [IMPL-CISCO-NCS560]
              Cisco Systems, "Segment Routing Configuration Guide for
              Cisco NCS 560 Series Routers", 14 October 2022,
              <https://www.cisco.com/c/en/us/td/docs/iosxr/ncs560/
              segment-routing/76x/b-segment-routing-cg-76x-ncs560/m-
              configure-srv6-usid-ncs5xx.html>.

   [IMPL-CISCO-NCS5700]
              Cisco Systems, "Segment Routing Configuration Guide for
              Cisco NCS 5700 Series Routers", 6 November 2022,
              <https://www.cisco.com/c/en/us/td/docs/iosxr/ncs5500/
              segment-routing/75x/b-segment-routing-cg-ncs5500-75x/
              configure-srv6-micro-sid.html>.

   [IMPL-NOKIA-20.10]
              Nokia, "Segment Routing and PCE User Guide", December
              2022, <https://documentation.nokia.com/sr/22-
              10/books/Segment%20Routing%20and%20PCE%20User%20Guide/
              segment-rout-with-ipv6-data-plane-srv6.html>.

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   [IMPL-OSS-LINUX]
              Abeni, P., "Add NEXT-C-SID support for SRv6 End behavior",
              20 September 2022,
              <https://git.kernel.org/pub/scm/linux/kernel/git/netdev/
              net-next.git/
              commit/?id=cec9d59e89362809f17f2d854faf52966216da13>.

   [IMPL-OSS-ONOS]
              Open Networking Foundation, "Stratum CMCC G-SRv6 Project",
              24 March 2021,
              <https://wiki.opennetworking.org/display/COM/
              Stratum+CMCC+G-SRv6+Project>.

   [IMPL-OSS-OPEN-SRV6]
              "Open SRv6 Project", n.d.,
              <http://opensrv6.org.cn/en/srv6-2/>.

   [IMPL-OSS-P4]
              Salsano, S. and A. Tulumello, "SRv6 uSID (micro SID)
              implementation on P4", 3 January 2021,
              <https://github.com/netgroup/p4-srv6-usid>.

   [IMPL-OSS-SAI]
              Agrawal, A., "Added new behaviors to support uSID
              instruction", 8 June 2021,
              <https://github.com/opencomputeproject/SAI/pull/1231/
              commits/02e58d95ad966ca9efc24eb9e0c0fa10b21de2a4>.

   [IMPL-OSS-SONIC]
              Shah, S. and R. Sudarshan, "SONiC uSID", 21 August 2022,
              <https://github.com/sonic-net/SONiC/blob/master/doc/srv6/
              SRv6_uSID.md>.

   [IMPL-OSS-VPP]
              FD.io, "Srv6 cli reference", n.d., <https://s3-
              docs.fd.io/vpp/23.02/cli-reference/clis/
              clicmd_src_vnet_srv6.html>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC9252]  Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
              B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
              Based on Segment Routing over IPv6 (SRv6)", RFC 9252,
              DOI 10.17487/RFC9252, July 2022,
              <https://www.rfc-editor.org/info/rfc9252>.

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   [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/info/rfc9350>.

   [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>.

   [RFC9513]  Li, Z., Hu, Z., Talaulikar, K., Ed., and P. Psenak,
              "OSPFv3 Extensions for Segment Routing over IPv6 (SRv6)",
              RFC 9513, DOI 10.17487/RFC9513, December 2023,
              <https://www.rfc-editor.org/info/rfc9513>.

   [RFC9514]  Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
              Bernier, D., and B. Decraene, "Border Gateway Protocol -
              Link State (BGP-LS) Extensions for Segment Routing over
              IPv6 (SRv6)", RFC 9514, DOI 10.17487/RFC9514, December
              2023, <https://www.rfc-editor.org/info/rfc9514>.

   [SPRING-WG-POLICIES]
              SPRING Working Group Chairs, "SPRING Working Group
              Policies", 14 October 2022,
              <https://wiki.ietf.org/en/group/spring/WG_Policies>.

Appendix A.  Complete pseudocodes

   The content of this section is purely informative rendering of the
   pseudocodes of [RFC8986] with the modifications in this document.
   This rendering may not be used as a reference.

A.1.  End with NEXT-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End SID with the NEXT-C-SID flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address
              with Code 0 (Hop limit exceeded in transit),
              interrupt packet processing, and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.   Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   N09. }
   S02. If (Segments Left == 0) {
   S03.   Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header.
   S04. }
   S05. If (IPv6 Hop Limit <= 1) {
   S06.   Send an ICMP Time Exceeded message to the Source Address
            with Code 0 (Hop limit exceeded in transit),
            interrupt packet processing, and discard the packet.
   S07. }
   S08. max_LE = (Hdr Ext Len / 2) - 1
   S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.   Send an ICMP Parameter Problem to the Source Address
            with Code 0 (Erroneous header field encountered)
            and Pointer set to the Segments Left field,
            interrupt packet processing, and discard the packet.
   S11. }
   S12. Decrement IPv6 Hop Limit by 1.
   S13. Decrement Segments Left by 1.
   S14. Update IPv6 DA with Segment List[Segments Left].
   S15. Submit the packet to the egress IPv6 FIB lookup for
          transmission to the new destination.

   Before processing the Upper-Layer header or any IPv6 extension header
   other than Hop-by-Hop or Destination Option of a packet matching a
   FIB entry locally instantiated as an End SID with the NEXT-C-SID
   flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.   Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   N09. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End SID with the NEXT-C-SID flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
            with Code 4 (SR Upper-layer Header Error)
            and Pointer set to the offset of the Upper-Layer header,
            interrupt packet processing, and discard the packet.
   S05. }

A.2.  End.X with NEXT-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.X SID with the NEXT-C-SID flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address
              with Code 0 (Hop limit exceeded in transit),
              interrupt packet processing, and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.   Submit the packet to the IPv6 module for transmission to the
            new destination via a member of J.
   N09. }
   S02. If (Segments Left == 0) {
   S03.   Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header.
   S04. }
   S05. If (IPv6 Hop Limit <= 1) {
   S06.   Send an ICMP Time Exceeded message to the Source Address
            with Code 0 (Hop limit exceeded in transit),
            interrupt packet processing, and discard the packet.
   S07. }
   S08. max_LE = (Hdr Ext Len / 2) - 1
   S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.   Send an ICMP Parameter Problem to the Source Address
            with Code 0 (Erroneous header field encountered)
            and Pointer set to the Segments Left field,
            interrupt packet processing, and discard the packet.
   S11. }
   S12. Decrement IPv6 Hop Limit by 1.
   S13. Decrement Segments Left by 1.
   S14. Update IPv6 DA with Segment List[Segments Left].
   S15. Submit the packet to the IPv6 module for transmission
          to the new destination via a member of J.

   Before processing the Upper-Layer header or any IPv6 extension header
   other than Hop-by-Hop or Destination Option of a packet matching a
   FIB entry locally instantiated as an End.X SID with the NEXT-C-SID
   flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.   Submit the packet to the IPv6 module for transmission to the
            new destination via a member of J.
   N09. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.X SID with the NEXT-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
            with Code 4 (SR Upper-layer Header Error)
            and Pointer set to the offset of the Upper-Layer header,
            interrupt packet processing, and discard the packet.
   S05. }

A.3.  End.T with NEXT-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.T SID with the NEXT-C-SID flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address
              with Code 0 (Hop limit exceeded in transit),
              interrupt packet processing, and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.1. Set the packet's associated FIB table to T.
   N08.2. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.
   N09. }
   S02. If (Segments Left == 0) {
   S03.   Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header.
   S04. }
   S05. If (IPv6 Hop Limit <= 1) {
   S06.   Send an ICMP Time Exceeded message to the Source Address
            with Code 0 (Hop limit exceeded in transit),
            interrupt packet processing, and discard the packet.
   S07. }
   S08. max_LE = (Hdr Ext Len / 2) - 1
   S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.   Send an ICMP Parameter Problem to the Source Address
            with Code 0 (Erroneous header field encountered)
            and Pointer set to the Segments Left field,
            interrupt packet processing, and discard the packet.
   S11. }
   S12. Decrement IPv6 Hop Limit by 1.
   S13. Decrement Segments Left by 1.
   S14. Update IPv6 DA with Segment List[Segments Left].
   S15.1. Set the packet's associated FIB table to T.
   S15.2. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.

   Before processing the Upper-Layer header or any IPv6 extension header
   other than Hop-by-Hop or Destination Option of a packet matching a
   FIB entry locally instantiated as an End.T SID with the NEXT-C-SID
   flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.1. Set the packet's associated FIB table to T.
   N08.2. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.
   N09. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.T SID with the NEXT-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
            with Code 4 (SR Upper-layer Header Error)
            and Pointer set to the offset of the Upper-Layer header,
            interrupt packet processing, and discard the packet.
   S05. }

A.4.  End.B6.Encaps with NEXT-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.B6.Encaps SID with the NEXT-C-SID flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.1. Push a new IPv6 header with its own SRH containing B.
   N08.2. Set the outer IPv6 SA to A.
   N08.3. Set the outer IPv6 DA to the first SID of B.
   N08.4. Set the outer Payload Length, Traffic Class, Flow Label,
            Hop Limit, and Next Header fields.
   N08.5. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   N09. }
   S02. If (Segments Left == 0) {
   S03.   Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header.
   S04. }
   S05. If (IPv6 Hop Limit <= 1) {
   S06.   Send an ICMP Time Exceeded message to the Source Address
            with Code 0 (Hop limit exceeded in transit),
            interrupt packet processing, and discard the packet.
   S07. }
   S08. max_LE = (Hdr Ext Len / 2) - 1
   S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.   Send an ICMP Parameter Problem to the Source Address
            with Code 0 (Erroneous header field encountered)
            and Pointer set to the Segments Left field,
            interrupt packet processing, and discard the packet.
   S11. }
   S12. Decrement IPv6 Hop Limit by 1.
   S13. Decrement Segments Left by 1.
   S14. Update IPv6 DA with Segment List[Segments Left].
   S15. Push a new IPv6 header with its own SRH containing B.
   S16. Set the outer IPv6 SA to A.
   S17. Set the outer IPv6 DA to the first SID of B.
   S18. Set the outer Payload Length, Traffic Class, Flow Label,
          Hop Limit, and Next Header fields.
   S19. Submit the packet to the egress IPv6 FIB lookup for
          transmission to the new destination.

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   Before processing the Upper-Layer header or any IPv6 extension header
   other than Hop-by-Hop or Destination Option of a packet matching a
   FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-
   C-SID flavor:

   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.1. Push a new IPv6 header with its own SRH containing B.
   N08.2. Set the outer IPv6 SA to A.
   N08.3. Set the outer IPv6 DA to the first SID of B.
   N08.4. Set the outer Payload Length, Traffic Class, Flow Label,
            Hop Limit, and Next Header fields.
   N08.5. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   N09. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.B6.Encaps SID with the NEXT-
   C-SID flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
            with Code 4 (SR Upper-layer Header Error)
            and Pointer set to the offset of the Upper-Layer header,
            interrupt packet processing, and discard the packet.
   S05. }

A.5.  End.BM with NEXT-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.BM SID with the NEXT-C-SID flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.1. Push the MPLS label stack for B.
   N08.2. Submit the packet to the MPLS engine for transmission.
   N09. }
   S02. If (Segments Left == 0) {
   S03.   Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header.
   S04. }
   S05. If (IPv6 Hop Limit <= 1) {
   S06.   Send an ICMP Time Exceeded message to the Source Address
            with Code 0 (Hop limit exceeded in transit),
            interrupt packet processing, and discard the packet.
   S07. }
   S08. max_LE = (Hdr Ext Len / 2) - 1
   S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.   Send an ICMP Parameter Problem to the Source Address
            with Code 0 (Erroneous header field encountered)
            and Pointer set to the Segments Left field,
            interrupt packet processing, and discard the packet.
   S11. }
   S12. Decrement IPv6 Hop Limit by 1.
   S13. Decrement Segments Left by 1.
   S14. Update IPv6 DA with Segment List[Segments Left].
   S15. Push the MPLS label stack for B.
   S16. Submit the packet to the MPLS engine for transmission.

   Before processing the Upper-Layer header or any IPv6 extension header
   other than Hop-by-Hop or Destination Option of a packet matching a
   FIB entry locally instantiated as an End.BM SID with the NEXT-C-SID
   flavor:

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   N01. If (DA.Argument != 0) {
   N02.   If (IPv6 Hop Limit <= 1) {
   N03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   N04.   }
   N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
            Destination Address.
   N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
   N07.   Decrement IPv6 Hop Limit by 1.
   N08.1. Push the MPLS label stack for B.
   N08.2. Submit the packet to the MPLS engine for transmission.
   N09. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.BM SID with the NEXT-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
            with Code 4 (SR Upper-layer Header Error)
            and Pointer set to the offset of the Upper-Layer header,
            interrupt packet processing, and discard the packet.
   S05. }

A.6.  End with REPLACE-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End SID with the REPLACE-C-SID flavor:

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   S01. When an SRH is processed {
   S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
              Segment List[0][DA.Arg.Index-1] == 0)) {
   S03.     Stop processing the SRH, and proceed to process the next
              header in the packet, whose type is identified by
              the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   R01.   If (DA.Arg.Index != 0) {
   R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   R03.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R04.     }
   R05.     Decrement DA.Arg.Index by 1.
   R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
   R07.       Decrement Segments Left by 1.
   R08.       Decrement IPv6 Hop Limit by 1.
   R09.       Update IPv6 DA with Segment List[Segments Left]
   R10.       Submit the packet to the egress IPv6 FIB lookup for
               transmission to the new destination.
   R11.     }
   R12.   } Else {
   R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   R14.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R15.     }
   R16.     Decrement Segments Left by 1.
   R17.     Set DA.Arg.Index to (128/LNFL - 1).
   R18.   }
   R19.   Decrement IPv6 Hop Limit by 1.
   R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
            header.
   R21.   Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.
   S16. }

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   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End SID with the REPLACE-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
             with Code 4 (SR Upper-layer Header Error)
             and Pointer set to the offset of the Upper-Layer header,
             interrupt packet processing, and discard the packet.
   S05. }

A.7.  End.X with REPLACE-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.X SID with the REPLACE-C-SID flavor:

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   S01. When an SRH is processed {
   S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
              Segment List[0][DA.Arg.Index-1] == 0)) {
   S03.     Stop processing the SRH, and proceed to process the next
              header in the packet, whose type is identified by
              the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   R01.   If (DA.Arg.Index != 0) {
   R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   R03.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R04.     }
   R05.     Decrement DA.Arg.Index by 1.
   R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
   R07.       Decrement Segments Left by 1.
   R08.       Decrement IPv6 Hop Limit by 1.
   R09.       Update IPv6 DA with Segment List[Segments Left]
   R10.       Submit the packet to the IPv6 module for transmission to
                the new destination via a member of J.
   R11.     }
   R12.   } Else {
   R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   R14.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R15.     }
   R16.     Decrement Segments Left by 1.
   R17.     Set DA.Arg.Index to (128/LNFL - 1).
   R18.   }
   R19.   Decrement IPv6 Hop Limit by 1.
   R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
            header.
   R21.   Submit the packet to the IPv6 module for transmission to the
            new destination via a member of J.
   S16. }

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   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.X SID with the REPLACE-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
             with Code 4 (SR Upper-layer Header Error)
             and Pointer set to the offset of the Upper-Layer header,
             interrupt packet processing, and discard the packet.
   S05. }

A.8.  End.T with REPLACE-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.T SID with the REPLACE-C-SID flavor:

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   S01. When an SRH is processed {
   S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
              Segment List[0][DA.Arg.Index-1] == 0)) {
   S03.     Stop processing the SRH, and proceed to process the next
              header in the packet, whose type is identified by
              the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   R01.   If (DA.Arg.Index != 0) {
   R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   R03.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R04.     }
   R05.     Decrement DA.Arg.Index by 1.
   R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
   R07.       Decrement Segments Left by 1.
   R08.       Decrement IPv6 Hop Limit by 1.
   R09.       Update IPv6 DA with Segment List[Segments Left]
   R10.1.     Set the packet's associated FIB table to T.
   R10.2.     Submit the packet to the egress IPv6 FIB lookup for
                transmission to the new destination.
   R11.     }
   R12.   } Else {
   R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   R14.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R15.     }
   R16.     Decrement Segments Left by 1.
   R17.     Set DA.Arg.Index to (128/LNFL - 1).
   R18.   }
   R19.   Decrement IPv6 Hop Limit by 1.
   R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
            header.
   R21.1. Set the packet's associated FIB table to T.
   R21.2. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.
   S16. }

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   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.T SID with the REPLACE-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
             with Code 4 (SR Upper-layer Header Error)
             and Pointer set to the offset of the Upper-Layer header,
             interrupt packet processing, and discard the packet.
   S05. }

A.9.  End.B6.Encaps with REPLACE-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.B6.Encaps SID with the REPLACE-C-SID flavor:

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
              Segment List[0][DA.Arg.Index-1] == 0)) {
   S03.     Stop processing the SRH, and proceed to process the next
              header in the packet, whose type is identified by
              the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   R01.   If (DA.Arg.Index != 0) {
   R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   R03.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R04.     }
   R05.     Decrement DA.Arg.Index by 1.
   R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
   R07.       Decrement Segments Left by 1.
   R08.       Decrement IPv6 Hop Limit by 1.
   R09.       Update IPv6 DA with Segment List[Segments Left]
   R10.1.     Push a new IPv6 header with its own SRH containing B.
   R10.2.     Set the outer IPv6 SA to A.
   R10.3.     Set the outer IPv6 DA to the first SID of B.
   R10.4.     Set the outer Payload Length, Traffic Class, Flow Label,
                Hop Limit, and Next Header fields.

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   R10.5.     Submit the packet to the egress IPv6 FIB lookup for
                transmission to the next destination.
   R11.     }
   R12.   } Else {
   R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   R14.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R15.     }
   R16.     Decrement Segments Left by 1.
   R17.     Set DA.Arg.Index to (128/LNFL - 1).
   R18.   }
   R19.   Decrement IPv6 Hop Limit by 1.
   R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
            header.
   R21.1. Push a new IPv6 header with its own SRH containing B.
   R21.2. Set the outer IPv6 SA to A.
   R21.3. Set the outer IPv6 DA to the first SID of B.
   R21.4. Set the outer Payload Length, Traffic Class, Flow Label,
            Hop Limit, and Next Header fields.
   R21.5. Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   S16. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.B6.Encaps SID with the REPLACE-
   C-SID flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
             with Code 4 (SR Upper-layer Header Error)
             and Pointer set to the offset of the Upper-Layer header,
             interrupt packet processing, and discard the packet.
   S05. }

A.10.  End.BM with REPLACE-C-SID

   When processing the SRH of a packet matching a FIB entry locally
   instantiated as an End.BM SID with the REPLACE-C-SID flavor:

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   S01. When an SRH is processed {
   S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
              Segment List[0][DA.Arg.Index-1] == 0)) {
   S03.     Stop processing the SRH, and proceed to process the next
              header in the packet, whose type is identified by
              the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   R01.   If (DA.Arg.Index != 0) {
   R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   R03.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R04.     }
   R05.     Decrement DA.Arg.Index by 1.
   R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
   R07.       Decrement Segments Left by 1.
   R08.       Decrement IPv6 Hop Limit by 1.
   R09.       Update IPv6 DA with Segment List[Segments Left]
   R10.1.     Push the MPLS label stack for B.
   R10.2.     Submit the packet to the MPLS engine for transmission.
   R11.     }
   R12.   } Else {
   R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   R14.       Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing and discard the packet.
   R15.     }
   R16.     Decrement Segments Left by 1.
   R17.     Set DA.Arg.Index to (128/LNFL - 1).
   R18.   }
   R19.   Decrement IPv6 Hop Limit by 1.
   R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
            [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
            header.
   R21.1. Push the MPLS label stack for B.
   R21.2. Submit the packet to the MPLS engine for transmission.
   S16. }

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   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.BM SID with the REPLACE-C-SID
   flavor:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
             with Code 4 (SR Upper-layer Header Error)
             and Pointer set to the offset of the Upper-Layer header,
             interrupt packet processing, and discard the packet.
   S05. }

Contributors

   Liu Aihua
   ZTE Corporation
   China
   Email: liu.aihua@zte.com.cn

   Dennis Cai
   Alibaba
   United States of America
   Email: d.cai@alibaba-inc.com

   Darren Dukes
   Cisco Systems, Inc.
   Canada
   Email: ddukes@cisco.com

   James N Guichard
   Futurewei Technologies Ltd.
   United States of America
   Email: james.n.guichard@futurewei.com

   Cheng Li
   Huawei Technologies
   China
   Email: c.l@huawei.com

   Robert Raszuk
   NTT Network Innovations
   United States of America

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   Email: robert@raszuk.net

   Ketan Talaulikar
   Cisco Systems, Inc.
   India
   Email: ketant.ietf@gmail.com

   Daniel Voyer
   Bell Canada
   Canada
   Email: daniel.voyer@bell.ca

   Shay Zadok
   Broadcom
   Israel
   Email: shay.zadok@broadcom.com

Authors' Addresses

   Weiqiang Cheng (editor)
   China Mobile
   China
   Email: chengweiqiang@chinamobile.com

   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium
   Email: cf@cisco.com

   Zhenbin Li
   Huawei Technologies
   China
   Email: lizhenbin@huawei.com

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

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   Francois Clad (editor)
   Cisco Systems, Inc.
   France
   Email: fclad.ietf@gmail.com

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