Compressed SRv6 Segment List Encoding in SRH
draft-filsfilscheng-spring-srv6-srh-comp-sl-enc-00

SPRING                                                     W. Cheng, Ed.
Internet-Draft                                              China Mobile
Intended status: Standards Track                             C. Filsfils
Expires: November 20, 2020                           Cisco Systems, Inc.
                                                                   Z. Li
                                                     Huawei Technologies
                                                                  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
                                                            May 19, 2020

              Compressed SRv6 Segment List Encoding in SRH
           draft-filsfilscheng-spring-srv6-srh-comp-sl-enc-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
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   This Internet-Draft will expire on November 20, 2020.

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Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Basic Concepts  . . . . . . . . . . . . . . . . . . . . . . .   3
   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  . . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  GIB/LIB Usage . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Efficient SID-list Encoding . . . . . . . . . . . . . . . . .  11
   8.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Illustrations . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Problems with REPLACE-C-SID flavor and 16-bit C-SIDs  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The Segment Routing architecture is defined in [RFC8402].

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   SRv6 Network Programming [I-D.ietf-spring-srv6-network-programming]
   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 [I-D.ietf-spring-srv6-network-programming].  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
   [I-D.ietf-spring-srv6-network-programming].  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.

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
   [I-D.ietf-spring-srv6-network-programming].  Furthermore, when the

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   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
   [I-D.ietf-spring-srv6-network-programming].  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 [I-D.ietf-spring-srv6-network-programming].  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 [I-D.ietf-spring-srv6-network-programming].  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 that the other for a particular SRv6 deployment.  The NEXT-

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   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)..(B+NF+A-1)] 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.   }

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

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   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
   [draft-cl-spring-generalized-srv6-for-cmpr] under the name
   "COC(Continue of Compression)" flavor.  In that context, a C-SID and
   a C-SID-sequence are respectively named a G-SID and G-SRv6
   compression sub-path.

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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+F+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 allows

   o  to place multiple C-SID's in the DA

   o  to mix C-SID's of different sizes in the DA/any container

   o  to process multiple C-SID's at once (with one single FIB lookup)

   For these reasons, the NEXT-C-SID flavor benefits from a lower C-SID
   length granularity and 16 bits is recommended.

   The REPLACE-C-SID flavor does not allow

   o  to place multiple C-SID's in the DA

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   o  to mix C-SID's of different sizes in the DA/any container

   o  to process multiple C-SID's at once (with one single FIB lookup)

   For these reasons, the REPLACE-C-SID flavor must "replace" more bits
   when updating the DA.  A longer C-SID length is needed, and 32 bits
   is recommended.

   Note: Appendix A describes the problems that arise if REPLACE-C-SID
   is used with 16-bit C-SID length

   In summary:

   o  NEXT-C-SID: 16-bit C-SID length

   o  REPLACE-C-SID: 32-bit C-SID length

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.

   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

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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.  Security Considerations

   TBD

11.  Acknowledgements

   TBD

12.  References

12.1.  Normative References

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-15 (work in
              progress), March 2020.

   [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>.

12.2.  Informative References

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   [draft-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-00 (work in progress) , May
              2020.

   [I-D.cheng-spring-shorter-srv6-sid-requirement]
              Cheng, W., Xie, C., Pang, R., Li, Z., Chen, R., Lijun, L.,
              Duan, X., and G. Mirsky, "Shorter SRv6 SID Requirements",
              draft-cheng-spring-shorter-srv6-sid-requirement-01 (work
              in progress), March 2020.

   [I-D.filsfils-spring-net-pgm-extension-srv6-usid]
              Filsfils, C., Camarillo, P., Cai, D., Voyer, D., Meilik,
              I., Patel, K., Henderickx, W., Jonnalagadda, P., Melman,
              D., and Y. Liu, "Network Programming extension: SRv6 uSID
              instruction", draft-filsfils-spring-net-pgm-extension-
              srv6-usid-05 (work in progress), May 2020.

Appendix A.  Problems with REPLACE-C-SID flavor and 16-bit C-SIDs

   In this section, we show the problems that would arise if REPLACE-
   C-SID is used with C-SID length of 16bits.

   The use of 16-bit C-SIDs requires to allocate END.X.REPLACE-C-SID,
   END.DT.REPLACE-C-SID and END.DX.REPLACE-C-SID SIDs from the LIB.

   In the case of an END.REPLACE-C-SID SID followed by an END.X.REPLACE-
   C-SID SID instantiated on the same node, this would require to:

   o  Lookup the END.REPLACE-C-SID SID in the DA

   o  Replace active C-SID with the next one in SRH

   o  Lookup the END.X.REPLACE-C-SID SID in the DA

   o  Replace active C-SID with the next one in SRH

   o  Forward the packet via X-connect

   In the case of an END.REPLACE-C-SID SID following by an
   END.DT.REPLACE-C-SID SID instantiated on the same node, this would
   require to:

   o  Lookup the END.REPLACE-C-SID SID in the DA

   o  Replace active C-SID with the next one in SRH

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   o  Lookup the END.DT.REPLACE-C-SID SID in the DA

   o  Decapsulate the packet

   o  Lookup the inner DA in the appropriate table

   o  Forward the packet

   This double or triple lookup is a major inefficiency if one would
   want to deploy the REPLACE-C-SID flavor with 16-bit C-SIDs.  It
   really mandates the use of 32-bit C-SIDs, such that END.X.REPLACE-
   C-SID, END.DT.REPLACE-C-SID and END.DX.REPLACE-C-SID SIDs can be
   allocated from the GIB and it is not required to prefix them with an
   END.REPLACE-C-SID SID.

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

   Dennis Cai
   Alibaba
   USA

   Email: d.cai@alibaba-inc.com

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   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

   Liu Aihua
   ZTE Corporation
   China

   Email: liu.aihua@zte.com.cn

Cheng, et al.           Expires November 20, 2020              [Page 14]