\
SPRING                                                     W. Cheng, Ed.
Internet-Draft                                              China Mobile
Intended status: Standards Track                             C. Filsfils
Expires: 22 September 2022                           Cisco Systems, Inc.
                                                                   Z. Li
                                                     Huawei Technologies
                                                             B. Decraene
                                                                  Orange
                                                                  D. Cai
                                                                 Alibaba
                                                                D. Voyer
                                                             Bell Canada
                                                            F. Clad, Ed.
                                                     Cisco Systems, Inc.
                                                                S. Zadok
                                                                Broadcom
                                                             J. Guichard
                                             Futurewei Technologies Ltd.
                                                                L. Aihua
                                                         ZTE Corporation
                                                               R. Raszuk
                                                 NTT Network Innovations
                                                                   C. Li
                                                     Huawei Technologies
                                                           21 March 2022


              Compressed SRv6 Segment List Encoding in SRH
               draft-ietf-spring-srv6-srh-compression-01

Abstract

   This document specifies new flavors for the SR endpoint behaviors
   defined in RFC 8986, which enable a compressed SRv6 Segment-List
   encoding in the Segment Routing Header (SRH).

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/.






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   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 22 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  Basic Concepts  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SR Endpoint Flavors . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  NEXT-C-SID Flavor . . . . . . . . . . . . . . . . . . . .   5
       4.1.1.  End with NEXT-C-SID . . . . . . . . . . . . . . . . .   6
       4.1.2.  End.X with NEXT-C-SID . . . . . . . . . . . . . . . .   7
       4.1.3.  Combination with PSP, USP and USD flavors . . . . . .   7
     4.2.  REPLACE-C-SID Flavor  . . . . . . . . . . . . . . . . . .   7
       4.2.1.  End with REPLACE-C-SID  . . . . . . . . . . . . . . .   8
       4.2.2.  End.X with REPLACE-C-SID  . . . . . . . . . . . . . .   9
       4.2.3.  Combination with PSP, USP and USD flavors . . . . . .   9
     4.3.  Combined NEXT-and-REPLACE-C-SID Flavor  . . . . . . . . .   9
   5.  GIB, LIB, global C-SID and local C-SID  . . . . . . . . . . .  11
     5.1.  Global C-SID  . . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Local C-SID . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  C-SID and Block Length  . . . . . . . . . . . . . . . . . . .  12
     6.1.  C-SID Length  . . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  Block Length  . . . . . . . . . . . . . . . . . . . . . .  12
     6.3.  GIB/LIB Usage . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Efficient SID-list Encoding . . . . . . . . . . . . . . . . .  13
   8.  Inter Routing Domains with the End.XPS behavior . . . . . . .  13
   9.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .  15
   10. Illustrations . . . . . . . . . . . . . . . . . . . . . . . .  15



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   11. Interoperability Status . . . . . . . . . . . . . . . . . . .  15
   12. Deployment Model  . . . . . . . . . . . . . . . . . . . . . .  16
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     15.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Segment Routing architecture is defined in [RFC8402].

   SRv6 Network Programming [RFC8986] defines a framework to build a
   network program with topological and service segments carried in a
   Segment Routing header (SRH) [RFC8754].

   This document specifies new flavors to the SR endpoint behaviors
   defined in Section 4 of [RFC8986].  These flavors enable a compressed
   encoding of the SRv6 Segment-List in the SRH and therefore address
   the requirements described in
   [I-D.srcompdt-spring-compression-requirement].

   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
   [I-D.ietf-lsr-isis-srv6-extensions], OSPF
   [I-D.ietf-lsr-ospfv3-srv6-extensions], and BGP
   [I-D.ietf-bess-srv6-services].

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:

   *  Compressed-SID (C-SID): A C-SID is a short encoding of a SID in
      SRv6 packet that does not include the SID block bits (locator
      block).

   *  Compressed-SID container (C-SID container): An entry of the SRH
      Segment-List field (128 bits) that contains a sequence of C-SIDs.






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   *  Compressed-SID sequence (C-SID sequence): A group of one or more
      C-SID containers in a segment list that share the same SRv6 SID
      block.

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

   *  Compressed Segment List encoding: A segment list encoding that
      reduces the packet header length thanks to one or more C-SID
      sequences.  A compressed Segment List encoding may also contain
      any number of uncompressed SID sequences.

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.

3.  Basic Concepts

   In an SRv6 domain, the SIDs are allocated from a particular IPv6
   prefix: the SRv6 SID block.  Therefore, all SRv6 SIDs instantiated
   from the same SRv6 SID block share the same most significant bits.
   These common bits are named Locator-Block in [RFC8986].  Furthermore,
   when the combined length of the SRv6 SID Locator, Function and
   Argument is smaller than 128 bits, the trailing bits are set to zero.

   When a sequence of consecutive SIDs in a Segment List shares a common
   Locator-Block, a compressed SRv6 Segment-List encoding can optimize
   the packet header length by avoiding the repetition of the Locator-
   Block and trailing bits with each individual SID.

   The compressed Segment List encoding is fully compliant with the
   specifications in [RFC8402], [RFC8754] and [RFC8986].  Efficient
   encoding is achieved by combining a compressed Segment List encoding
   logic on the SR policy headend with new flavors of the base SRv6
   endpoint behaviors that decode this compressed encoding.

   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 Policy headend constructs and
   compresses the SID-list depending on the capabilities of each SR
   endpoint node that the packet should traverse, as well as its own
   compression capabilities.



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   It is expected that compressed encoding flavors be available on
   devices with limited packet manipulation capabilities, such as legacy
   ASICs.

   The compressed Segment List encoding supports any SRv6 SID Block
   allocation.  While other options are supported and may provide higher
   efficiency, each routing domain can be allocated a /48 prefix from a
   global IPv6 block (see Section 6.2).

4.  SR Endpoint Flavors

   This section defines several options to achieve compressed Segment
   List encoding, in the form of two new flavors for the END, END.X and
   END.T behaviors of [RFC8986].  These flavors could also be combined
   with behaviors defined in other documents.

   The compressed encoding can be achieved by leveraging any of these SR
   endpoint flavors.  The NEXT-C-SID flavor and the REPLACE-C-SID flavor
   expose the same high-level behavior in their use of the SID argument
   to determine the next segment to be processed, but they have
   different low-level characteristics that can make one more or less
   efficient than the other for a particular SRv6 deployment.  The NEXT-
   and-REPLACE-C-SID flavor is the combination of the NEXT-C-SID flavor
   and the REPLACE-C-SID flavor.  It provides the best efficiency in
   terms of encapsulation size at the cost of increased complexity.

   It is recommended for ease of operation that a single compressed
   encoding flavor be used in a given SRv6 domain.  However, in a multi-
   domain deployment, different flavors can be used in different
   domains.

   All three flavors leverage the following variables:

   *  Variable B is the Locator Block length of the SID.

   *  Variable NF is the sum of the Locator Node and the Function
      lengths of the SID.  It is also referred to as C-SID length.

   *  Variable A is the Argument length of the SID.

4.1.  NEXT-C-SID Flavor

   A SID instantiated with the NEXT-C-SID flavor takes an argument that
   carries the remaining C-SIDs in the current C-SID container.

   The length A of the argument is equal to 128-B-NF and should be a
   multiple of NF.




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   +------------------------------------------------------------------+
   |     Locator-Block      |Loc-Node|            Argument            |
   |                        |Function|                                |
   +------------------------------------------------------------------+
    <--------- B ----------> <- NF -> <------------- A -------------->

      Figure 1: Example of a NEXT-C-SID flavored SID structure using a
       48-bit block, 16-bit combined locator and function, and 64-bit
                                  argument

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], and a second time before line
   S01 of the upper-layer header processing in Section 4.1.1 of
   [RFC8986].

   S01. If (DA.Argument != 0) {
   S02.   If (IPv6 Hop Limit <= 1) {
   S03.     Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing and discard the packet.
   S04.   }
   S05.   Copy the value of DA.Argument into the bits [B..(B+A-1)]
            of the Destination Address.
   S06.   Set the bits [(B+A)..127] of the Destination Address to
            zero.
   S07.   Decrement Hop Limit by 1.
   S08.   Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
   S09. }

   Notes:

   *  DA.Argument identifies the bits [(B+NF)..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.







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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
   same modifications as in Section 4.1.1 of this document, except for
   line S08 that is replaced as follows.

   S08.   Submit the packet to the IPv6 module for transmission to the
            new destination via a member of J.

4.1.3.  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 USD flavor is unchanged when combined with the NEXT-C-SID
   flavor.  The pseudocodes defined in Section 4.1.1 and Section 4.1.2
   of this document are inserted at the beginning of the modified upper-
   layer header processing defined in Section 4.16.3 of [RFC8986] for
   End and End.X, respectively.

4.2.  REPLACE-C-SID Flavor

   A SID instantiated with the REPLACE-C-SID flavor takes an argument
   that indicates the index of the next C-SID in the appropriate
   container.

   The length A of the argument should be at least ceil(log_2(128/NF)).

   All SIDs that are part of a C-SID sequence using the REPLACE-C-SID
   flavor have the same C-SID length NF.

   +-------------------------------------------------------------------+
   |     Locator-Block      |  Locator-Node  |Argument|       0        |
   |                        |   + Function   |        |                |
   +-------------------------------------------------------------------+
    <--------- B ----------> <----- NF -----> <- A -->

     Figure 2: Example of a REPLACE-C-SID flavored SID structure using
      a 48-bit block, 32-bit combined locator and function, and 16-bit
                                  argument






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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 replaced as
   follows.

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0 and DA.Argument == 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
   S09.   If (DA.Argument != 0) {
   S10.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
   S11.        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.
   S11.     }
   S12.     Decrement DA.Argument by 1.
   S13.   } Else {
   S14.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
   S15.        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.
   S11.     }
   S12.     Decrement Segments Left by 1.
   S13.     Set DA.Argument to (128/NF - 1).
   S14.   }
   S15.   Decrement IPv6 Hop Limit by 1
   S16.   Write Segment List[Segments Left][DA.Argument] into the bits
            [B..B+NF-1] of the Destination Address of the IPv6 header.
   S17.   Write DA.Argument into the bits [B+NF..B+NF+A-1] of the
            Destination Address of the IPv6 header.
   S18.   Submit the packet to the egress IPv6 FIB lookup for
             transmission to the new destination.
   S19. }

   Notes:




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   *  DA.Argument identifies the bits [(B+NF)..(B+NF+A-1)] in the
      Destination Address of the IPv6 header.

   *  Segment List[Segments Left][DA.Argument] identifies the bits
      [DA.Argument*NF..(DA.Argument+1)*NF-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.

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
   same modifications as in Section 4.2.1 of this document, except for
   line S18 that is replaced as follows.

   S18.   Submit the packet to the IPv6 module for transmission to the
            new destination via a member of J.

4.2.3.  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 line S17 of the pseudocode in Section 4.2.1, and the
   first line of the inserted instructions is modified as follows.

   S17.1.   If (Segments Left == 0 and DA.Argument == 0) {

   USP: When combined with the REPLACE-C-SID flavor, the lines S02-S04
   of the pseudocode in Section 4.2.1 are substituted by the USP flavor
   instructions defined in Section 4.16.2 of [RFC8986], with the
   following modification.

   S02.   If (Segments Left == 0 and DA.Argument == 0) {

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

4.3.  Combined NEXT-and-REPLACE-C-SID Flavor

   A SID instantiated with the NEXT-and-REPLACE-C-SID flavor takes a
   two-parts argument comprising, Arg.Next and Arg.Index, and encoded in
   the SID in this order.

   The length A_I of Arg.Index should be at least ceil(log_2(128/NF)).




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   The length A_N of Arg.Next is equal to 128-B-NF-A_I and must be a
   multiple of NF.

   The total SID argument length A is the sum of A_I and A_N.

   The NEXT-and-REPLACE-C-SID flavor also leverages an additional
   variable, C_DA, that is equal to (1 + (A_N/NF)) and represents the
   number of C-SID's that can be encoded in the IPv6 Destination
   Address.

   All SIDs that are part of a C-SID sequence using the NEXT-and-
   REPLACE-C-SID flavor must have the same C-SID length NF.
   Furthermore, this NF must be a divisor of 128.

   +-------------------------------------------------------------------+
   |     Locator-Block      |Loc-Node|        Arg.Next        |  Arg.  |
   |                        |Function|                        | Index  |
   +-------------------------------------------------------------------+
    <--------- B ----------> <- NF -> <-------- A_N ---------> <- A_I ->

         Figure 3: Example of a NEXT-and-REPLACE-C-SID flavored SID
        structure using a 48-bit block, 16-bit combined locator and
               function, 48-bit Arg.Next and 16-bit Arg.Index

   Pseudo-code:


























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 1.   If (DA.Arg.Next != 0) {
 2.     Copy DA.Arg.Next into the bits [B..(B+A_N-1)] of the
          Destination Address of the IPv6 header.
 3.     Set the bits [(B+A_N)..(B+NF+A_N-1)] of the Destination Address
          of the IPv6 header to zero.
 4.   } Else If (DA.Arg.Index >= C_DA) {
 5.     Decrement DA.Arg.Index by C_DA.
 6.     Copy C_DA*NF bits from Segment List[Segments Left][DA.Arg.Index]
          into the bits [B..B+C_DA*NF-1] of the Destination Address of
          the IPv6 header.
 7.   } Else If (Segments Left != 0) {
 8.     Decrement Segments Left by 1.
 9.     Set DA.Arg.Index to ((DA.Arg.Index - C_DA) % (128/NF)).
10.     Copy C_DA*NF bits from Segment List[Segments Left][DA.Arg.Index]
          into the bits [B..B+C_DA*NF-1] of the Destination Address of
          the IPv6 header.
11.   } Else {
12.     Copy DA.Arg.Index*NF bits from Segment List[0][0] into the bits
          [B..B+DA.Arg.Index*NF-1] of the Destination Address of the
          IPv6 header.
13.     Set the bits [B+DA.Arg.Index*NF..B+NF+A_N-1] of the Destination
          Address of the IPv6 header to zero.
14.     Set DA.Arg.Index to 0.
15.   }

   Notes:

   *  DA.Arg.Next identifies the bits [(B+NF)..(B+NF+A_N-1)] in the
      Destination Address of the IPv6 header.

   *  DA.Arg.Index identifies the bits [(B+NF+A_N)..(B+NF+A_N+A_I-1)] in
      the Destination Address of the IPv6 header.

   *  Segment List[Segments Left][DA.Arg.Index] identifies the bits
      [DA.Arg.Index*NF..(DA.Arg.Index+1)*NF-1] in the SRH Segment List
      entry at index Segments Left.

5.  GIB, LIB, global C-SID and local C-SID

   GIB: The set of IDs available for global C-SID allocation.

   LIB: The set of IDs available for local C-SID allocation.

5.1.  Global C-SID

   A C-SID from the GIB.





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   A Global C-SID typically identifies a shortest-path to a node in the
   SRv6 domain.  An IP route is advertised by the parent node to each of
   its global C-SID's, under the associated C-SID block.  The parent
   node executes a variant of the END behavior.

   A node can have multiple global C-SID's under the same C-SID blocks
   (e.g. one per IGP flexible algorithm).  Multiple nodes may share the
   same global C-SID (anycast).

5.2.  Local C-SID

   A C-SID from the LIB.

   A local C-SID may identify a cross-connect to a direct neighbor over
   a specific interface or a VPN context.

   No IP route is advertised by a parent node for its local C-SID's.

   If N1 and N2 are two different physical nodes of the SRv6 domain and
   I is a local C-SID value, then N1 and N2 may bind two different
   behaviors to I.

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

   The allocation of C-SID's from the GIB and LIB depends on the C-SID
   length (see Section 6.3).

6.  C-SID and Block Length

6.1.  C-SID Length

   The NEXT-C-SID flavor supports both 16- and 32-bit C-SID lengths.  A
   C-SID length of 16-bit is recommended.

   The REPLACE-C-SID flavor supports both 16- and 32-bit C-SID lengths.
   A C-SID length of 32-bit is recommended.

6.2.  Block Length

   The recommended SRv6 SID block sizes for the NEXT-C-SID flavor are
   16, 32 or 48 bits.  The smaller the block, the higher the compression
   efficiency.

   The recommended SRv6 SID block size for the REPLACE-C-SID flavor can
   be 48, 56, 64, 72 or 80 bits, depending on the needs of the operator.




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6.3.  GIB/LIB Usage

   The previous block and C-SID length recommendations, call for the
   following GIB/LIB usage:

   *  NEXT-C-SID:

      -  GIB: END.NEXT-C-SID

      -  LIB: END.X.NEXT-C-SID, END.DX.NEXT-C-SID, END.DT.NEXT-C-SID

      -  LIB: END.DX.NEXT-C-SID for large-scale PW support

   *  REPLACE-C-SID:

      -  GIB: END.REPLACE-C-SID, END.X.REPLACE-C-SID, END.DX.REPLACE-
         C-SID, END.DT.REPLACE-C-SID

      -  LIB: END.DX.REPLACE-C-SID for large-scale PW support

7.  Efficient SID-list Encoding

   The compressed SID-list encoding logic is a local behavior of the SR
   Policy headend node and hence out of the scope of this document.

8.  Inter Routing Domains with the End.XPS behavior

   The End.XPS behavior described in this section is OPTIONAL.

   Some SRv6 traffic may need to cross multiple routing domains, such as
   different Autonomous Systems (ASes) or different routing areas.
   Different routing domains may use different addressing schema and
   SRv6 SID blocks.

   This section defines an optional solution and SID behavior allowing
   for the use of different SRv6 SID blocks between routing domains.

   The solution requires a new SID behavior, called "Endpoint with
   cross-connect to an array of layer-3 adjacencies and SRv6 Prefix
   Swap" (End.XPS for short) allowing for this transition of SRv6 SID
   block between two routing domains.

   End.XPS is a variant of End.X, performing both "End.X Layer-3 Cross-
   Connect" and the translation of the SRv6 SID block between the two
   routing domains.






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   The processing takes as an additional parameter the prefix B2/m
   corresponding the SRv6 SID block in the second domain.  This
   parameter is a property of the (received) SID and is given as a
   result of the lookup on the IPv6 destination address which identifies
   the SRv6 SID and its properties.

   The End.XPS behavior is compatible with the NEXT-C-SID, REPLACE-
   C-SID, and NEXT-and-REPLACE-C-SID flavors described in this document.

   When a router R receives a packet whose IPv6 DA matches a local
   End.XPS SID with the NEXT-C-SID flavor, that is associated with a set
   J of one or more Layer-3 adjacencies and the SRv6 SID block B2/m of
   the neighbor routing domain, R processes the packet as follows.

    1.   If (DA.Argument != 0) {
    2.     Write B2 into the most significant bits of the Destination
             Address of the IPv6 header.
    3.     Write DA.Argument into the bits [m..(m+A-1)] of the
             Destination Address of the IPv6 header.
    4.     Set the bits [(m+A)..127] of the Destination Address
             of the IPv6 header to zero.
    5.   } Else {
    6.     Decrement Segments Left by 1.
    7.     Copy Segment List[Segments Left] from the SRH to the
             Destination Address of the IPv6 header.
    8.   }
    9.   Submit the packet to the IPv6 module for transmission to the
           new destination via a member of J.

   When a router R receives a packet whose IPv6 DA matches a local
   End.XPS SID with the REPLACE-C-SID flavor, that is associated with a
   set J of one or more Layer-3 adjacencies and the SRv6 SID block B2/m
   of the neighbor routing domain, R processes the packet as follows.


















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    1.   If (DA.Argument != 0) {
    2.     Decrement DA.Argument by 1.
    3.   } Else {
    4.     Decrement Segments Left by 1.
    5.     Set DA.Argument to (128/NF - 1).
    6.   }
    7.   Write B2 into the most significant bits of the Destination
           Address of the IPv6 header.
    8.   Write Segment List[Segments Left][DA.Argument] into the bits
           [m..m+NF-1] of the Destination Address of the IPv6 header.
    9.   Write DA.Argument into the bits [m+NF..m+NF+A-1] of the
           Destination Address of the IPv6 header.
   10.   Set the bits [(m+NF+A)..127] of the Destination Address
           of the IPv6 header to zero.
   11.   Submit the packet to the IPv6 module for transmission to the
           new destination via a member of J.

   Note: the way the SRv6 SID Block B2 of the next routing domain is
   known is out of scope of this document.  As examples, it could be
   learnt via configuration, or using a signaling protocol either with
   the peer domain or with a central controller (e.g.  PCE).

   When End.XPS SID behavior is used, the restriction on the C-SID
   length for the REPLACE-C-SID and the NEXT-and-REPLACE-C-SID flavors
   is relaxed and becomes: all SID the are part of a C-SID sequence
   *within a domain* MUST have the same SID length NF.

9.  Control Plane

   This document does not require any control plane modification.

10.  Illustrations

   Illustrations for the functionalities defined in this document are
   provided in [I-D.clad-spring-srv6-srh-compression-illus].

11.  Interoperability Status

   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



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   *  Hardware implementation in Huawei NE40E and NE5000E running VRP

   The interoperability was validated for the following scenario:

   *  Packet forwarding through a traffic engineering segment list
      combining, in the same SRH ([RFC8754]), SRv6 SIDs bound to an
      endpoint behavior with the NEXT-C-SID flavor and SRv6 SIDs bound
      to an endpoint behavior with the REPLACE-C-SID flavor.

   Further interoperability testing is ongoing and will be reported in
   this document as the work progresses.

12.  Deployment Model

   Section 5 of [RFC8754] defines the intra-SR-domain deployment model
   and associated security procedures.

   The same deployment model apply to the SIDs defined in this document.

13.  Security Considerations

   The security requirements and mechanisms described in [RFC8402] and
   [RFC8754] also apply to this document.

   This document does not introduce any new security consideration.

14.  Acknowledgements

   The authors would like to thank Kamran Raza, Xing Jiang, YuanChao Su,
   Han Li and Yisong Liu.

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>.

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



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   [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>.

15.2.  Informative References

   [EMAIL1]   "SPRING chairs email on the adoption of draft-
              filsfilscheng-spring-srv6-srh-compression-02", October
              2021, <https://mailarchive.ietf.org/arch/msg/spring/
              VjVIxo7fZFhsIHJ5wFQXIBvvtNM/>.

   [EMAIL2]   "SPRING chairs email on working group process", February
              2022, <https://mailarchive.ietf.org/arch/msg/spring/
              vCc9Ckvwu5HA-RCleV712dsA5OA/>.

   [I-D.clad-spring-srv6-srh-compression-illus]
              Clad, F. and D. Dukes, "Illustrations for Compressed SRv6
              Segment List Encoding in SRH", Work in Progress, Internet-
              Draft, draft-clad-spring-srv6-srh-compression-illus-00, 15
              October 2021, <https://www.ietf.org/archive/id/draft-clad-
              spring-srv6-srh-compression-illus-00.txt>.

   [I-D.ietf-bess-srv6-services]
              Dawra, G., Filsfils, C., Talaulikar, K., Raszuk, R.,
              Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based
              Overlay Services", Work in Progress, Internet-Draft,
              draft-ietf-bess-srv6-services-13, 19 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-bess-srv6-
              services-13.txt>.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Segment Routing over
              IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
              ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
              extensions-18.txt>.

   [I-D.ietf-lsr-ospfv3-srv6-extensions]
              Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
              "OSPFv3 Extensions for SRv6", Work in Progress, Internet-



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              Draft, draft-ietf-lsr-ospfv3-srv6-extensions-03, 19
              November 2021, <https://www.ietf.org/archive/id/draft-
              ietf-lsr-ospfv3-srv6-extensions-03.txt>.

   [I-D.srcompdt-spring-compression-requirement]
              Cheng, W., Xie, C., Bonica, R., Dukes, D., Li, C., Shaofu,
              P., and W. Henderickx, "Compressed SRv6 SID List
              Requirements", Work in Progress, Internet-Draft, draft-
              srcompdt-spring-compression-requirement-07, 11 July 2021,
              <https://www.ietf.org/archive/id/draft-srcompdt-spring-
              compression-requirement-07.txt>.

Appendix A.  Open Issues

   This section was added as requested by the SPRING chair in [EMAIL1].

   Issues raised during and after the adoption call for this draft are
   tracked in an issue tracker.  The remainder of this section
   identifies the most significant open issues, from the adoption call,
   for the working group to keep track of.

   As a reminder to those reading this section, this document is a work
   in progress, and subject to change by the working group.  As noted at
   the front of this document, "It is inappropriate to use Internet-
   Drafts as reference material"

   *  Given that the working group has said that it wants to standardize
      one data plane solution, and given that the document contains
      multiple SRv6 EndPoint behaviors that some WG members have stated
      are multiple data plane solutions, the working group will address
      whether this is valid and coherent with its one data plane
      solution objective.

   *  As reminded in the conclusion of the adoption call, this document
      is subject to the policy announced by the SPRING chairs in
      [EMAIL2].  In particular, this means that this document can not go
      to WG last call until 6man completes handling of an Internet Draft
      that deals with the relationship of C-SIDs to RFC 4291.  It is
      hoped and expected that said resolution will be a WG last call and
      document approval in 6man of a document providing for the way that
      C-SIDs use the IPv6 destination address field.

Authors' Addresses

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



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   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


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


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


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


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


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





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


   Robert Raszuk
   NTT Network Innovations
   United States of America
   Email: robert@raszuk.net


   Cheng Li
   Huawei Technologies
   China
   Email: chengli13@huawei.com



































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