Compressed SRv6 Segment List Encoding in SRH
draft-filsfilscheng-spring-srv6-srh-compression-00

SPRING                                                     W. Cheng, Ed.
Internet-Draft                                              China Mobile
Intended status: Standards Track                             C. Filsfils
Expires: January 7, 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
                                                           July 06, 2021

              Compressed SRv6 Segment List Encoding in SRH
           draft-filsfilscheng-spring-srv6-srh-compression-00

Abstract

   This document defines a compressed SRv6 Segment List Encoding in the
   SRH.  This solution does not require any SRH data plane change nor
   any SRv6 control plane change.  This solution leverages the SRv6
   Network Programming model.

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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Basic Concepts  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SR Endpoint Flavors . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  NEXT-C-SID Flavor . . . . . . . . . . . . . . . . . . . .   5
     4.2.  REPLACE-C-SID Flavor  . . . . . . . . . . . . . . . . . .   6
     4.3.  Combined NEXT-and-REPLACE-C-SID Flavor  . . . . . . . . .   7
   5.  GIB, LIB, global C-SID and local C-SID  . . . . . . . . . . .   8
     5.1.  Global C-SID  . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Local C-SID . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  C-SID and Block Length  . . . . . . . . . . . . . . . . . . .   9
     6.1.  C-SID Length  . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Block Length  . . . . . . . . . . . . . . . . . . . . . .   9
     6.3.  GIB/LIB Usage . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Efficient SID-list Encoding . . . . . . . . . . . . . . . . .  10
   8.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Illustrations . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. Interoperability Status . . . . . . . . . . . . . . . . . . .  10
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     13.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Inter Routing Domains with the End.XPS behavior  . .  12

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   Appendix B.  Larger C-SID length by combining k C-SIDs to
                identify a behavior  . . . . . . . . . . . . . . . .  14
     B.1.  Combining k C-SIDs in a NEXT-C-SID sequence . . . . . . .  14
     B.2.  Combining k C-SIDs in a REPLACE-C-SID sequence  . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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 adds new flavors to the SR endpoint behaviors defined
   in [RFC8986].  These flavors enable a compressed encoding of the SRv6
   Segment-List in the SRH and therefore address the requirements
   described in [I-D.cheng-spring-shorter-srv6-sid-requirement].

   The flavors defined in this document leverage the SRH data plane
   without any change and do not require any SRv6 control plane change.

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:

   o  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).

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

   o  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.

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

   o  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.

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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.  No SRv6 SRH
   data plane change nor control plane extension is required.

   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.

   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

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   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 that 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:

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

   o  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.

   o  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.

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

   Pseudo-code:

    1.   If (DA.Argument != 0) {
    2.     Copy DA.Argument into the bits [B..(B+A-1)] of the
             Destination Address of the IPv6 header.
    3.     Set the bits [(B+A)..127] of the Destination Address
             of the IPv6 header to zero.
    4.   } Else {
    5.     Decrement Segments Left by 1.
    6.     Copy Segment List[Segments Left] from the SRH to the
             Destination Address of the IPv6 header.
    7.   }

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   Note: "DA.Argument" identifies the bits "[(B+NF)..127]" in the
   Destination Address of the IPv6 header.

   The NEXT-C-SID flavor has been previously documented in
   [I-D.filsfils-spring-net-pgm-extension-srv6-usid] under the name
   "SHIFT" flavor.  In that context, a C-SID and a C-SID-sequence are
   respectively named a Micro-Segment (uSID) and a Micro-Program.

4.2.  REPLACE-C-SID Flavor

   A SID instantiated with the REPLACE-C-SID flavor takes an argument,
   which is used to determine the index of the next C-SID in the
   appropriate container.

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

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

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

   Pseudo-code:

    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.   Copy Segment List[Segments Left][DA.Argument] into the bits
           [B..B+NF-1] of the Destination Address of the IPv6 header.

   Notes:

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

   o  "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 REPLACE-C-SID flavor has been previously documented in
   [I-D.cl-spring-generalized-srv6-for-cmpr] under the name
   "COC(Continue of Compression)" flavor.  In that context, a C-SID and

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   a C-SID-sequence are respectively named a G-SID and G-SRv6
   compression sub-path.

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 is equal to ceil(log_2(128/NF)).

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

   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:

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

   o  "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.

   o  "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 compressed Segment List encoding supports any SRv6 SID Block
   allocation either from GUA or LUA space.

   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.

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   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.

6.3.  GIB/LIB Usage

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

   o  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

   o  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.  Control Plane

   This document does not require any control plane modification.

9.  Illustrations

   Illustrations will be provided in a separate document.

10.  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:

   o  Hardware implementation in Cisco ASR 9000 running IOS XR

   o  Software implementation in Cisco IOS XRv9000 virtual appliance

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   o  Hardware implementation in Huawei NE40E and NE5000E running VRP

   The interoperability was validated for the following scenario:

   o  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.

11.  Security Considerations

   TBD

12.  Acknowledgements

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

13.  References

13.1.  Normative References

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

   [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>.

13.2.  Informative References

   [I-D.cheng-spring-shorter-srv6-sid-requirement]
              Cheng, W., Chongfeng, Pang, R., Li, Z., Chen, R., Lijun,
              Duan, X., Mirsk, G., Dukes, D., and S. Zadok, "Shorter
              SRv6 SID Requirements", draft-cheng-spring-shorter-srv6-
              sid-requirement-02 (work in progress), July 2020.

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   [I-D.cl-spring-generalized-srv6-for-cmpr]
              Cheng, W., Li, Z., Li, C., Clad, F., Liu, A., Xie, C.,
              Liu, Y., and S. Zadok, "Generalized SRv6 Network
              Programming for SRv6 Compression", draft-cl-spring-
              generalized-srv6-for-cmpr-03 (work in progress), April
              2021.

   [I-D.filsfils-spring-net-pgm-extension-srv6-usid]
              Filsfils, C., Garvia, P. C., Cai, D., Voyer, D., Meilik,
              I., Patel, K., Henderickx, W., Jonnalagadda, P., Melman,
              D., Liu, Y., and J. Guichard, "Network Programming
              extension: SRv6 uSID instruction", draft-filsfils-spring-
              net-pgm-extension-srv6-usid-10 (work in progress), March
              2021.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

Appendix A.  Inter Routing Domains with the End.XPS behavior

   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.

   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.

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   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.

    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).

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   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.

Appendix B.  Larger C-SID length by combining k C-SIDs to identify a
             behavior

   When an SR endpoint node needs more bits than allowed by the C-SID
   length (NF) to identify a behavior, it MAY combine two (resp. k)
   C-SIDs to effectively benefit from a C-SID of length "2*NF" (resp.
   "k*NF").  This is a local behavior of this SR endpoint node that does
   not require any specific encoding in the SRH.  The C-SID sequence
   only needs to have at least two (resp. k) remaining C-SIDs, instead
   of a single one.

   When two C-SIDs are combined, the first C-SID may be seen as having
   the role of a "Context SID" identifying a context-specific SID space
   or table, with the second SID being looked up in this context-
   specific table.  This is similar to the Context-Specific label space
   defined in the section 3 of [RFC5331].

B.1.  Combining k C-SIDs in a NEXT-C-SID sequence

   As part of a NEXT-C-SID sequence, k C-SIDs encoded in the same C-SID
   container can be seamlessly combined together by installing a more
   specific entry in FIB.  This entry would match the concatenation of
   the SRv6 SID block and the k C-SIDs to be combined.

   For example, the 16-bit C-SIDs 1111, 2222 and 3333 in the SRv6 SID
   block 2001:db8::/48 can be combined as one 48-bit C-SID on router R
   by installing a FIB entry for the IPv6 prefix
   2001:db8:0:1111:2222:3333::/96.

   This capability does not require any change to the existing SRv6 SID
   behaviors.

B.2.  Combining k C-SIDs in a REPLACE-C-SID sequence

   As part of a REPLACE-C-SID sequence, k C-SIDs can be combined
   together using the COMBINE-k-C-SID flavor.  A SID instantiated with
   the COMBINE-k-C-SID flavor takes a SID argument that indicates the
   index of the next C-SID in the current C-SID container, and a local
   parameter k >= 2.  The parameter k is a property of the C-SID and is
   given as a result of the FIB lookup that identifies the IPv6
   Destination Address as an SRv6 SID.

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   This section describes the COMBINE-k-C-SID flavor applied to the
   End.DX2 SID behavior, but the flavor is equally applicable to others
   SID behaviors.

   When a router R receives a packet whose IPv6 DA matches a local
   End.DX2 with a COMBINE-k-C-SID flavor, a local parameter k and a SID
   argument DA.Argument, R processes the packet as per Section 4.9 of
   [RFC8986], with the following modifications.

   Line S02 of the SRH processing behavior

   S02. If (Segments Left != 0) {

   is replaced with

   S02. If (Segments Left + floor((DA.Argument - k + 1) * NF/128) != 0){

   and lines S02-S03 of the upper-layer header processing behavior

   S02. Remove the outer IPv6 Header with all its extension headers
   S03. Forward the Ethernet frame to the OIF I

   are replaced with

   S02.a. Read the k-1 next C-SIDs located at positions Segment
            List[Segments Left + floor((DA.Argument - k + 1) * NF/128)]
            [(DA.Argument - k + 1) % (128/NF)] to Segment Left[Segments
            Left + floor((DA.Argument - 1) * NF/128)][(DA.Argument - 1)
            % (128/NF)] in the SRH.
   S02.b. Remove the outer IPv6 Header with all its extension headers
   S03.   Forward the Ethernet frame to the OIF identified by the k-1
            next C-SIDs, in the context of the matched IPv6 Destination
            Address.

Authors' Addresses

   Weiqiang Cheng (editor)
   China Mobile
   China

   Email: chengweiqiang@chinamobile.com

   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com

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   Zhenbin Li
   Huawei Technologies
   China

   Email: lizhenbin@huawei.com

   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com

   Dennis Cai
   Alibaba
   USA

   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.
   USA

   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
   USA

   Email: robert@raszuk.net

   Cheng Li
   Huawei Technologies
   China

   Email: chengli13@huawei.com

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