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SPRING Working Group                                            Yao. Liu
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: December 30, 2020                                 June 28, 2020


                  SRv6 Compressed Path Recover Method
                 draft-pl-spring-compr-path-recover-00

Abstract

   This document describes a path information recovery method for
   compressed SRv6 segment lists.

Status of This Memo

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

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   This Internet-Draft will expire on December 30, 2020.

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Segment List Recover TLV  . . . . . . . . . . . . . . . . . .   3
   4.  Illustration  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Shifting Method . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Stitching Method  . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Mapping Method  . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node by inserting an ordered list of instructions, called
   segments.Segment Routing can be deployed on the MPLS data plane by
   encoding 32-bits SIDs in MPLS label stack [RFC8660].  It can also be
   deployed on the IPv6 data plane by encoding a list of 128-bits SRv6
   SIDs in IPv6 Segment Routing Extension Header (SRH)[RFC8754].

   Several proposals are introduced to reduce the overhead for both the
   traffic volume and the network processor in SRv6.  The main ideas of
   them are basically to use a shorter ID associated with the complete
   128 bit SID.  The SRH does not carry complete path information after
   compression, however, the transit and egress nodes may need to know
   the complete SID information along the path in some scenarios such as
   OAM.

   This document describes a path information recovery method for
   compressed SRv6 segment lists.

2.  Overview

   Several proposals are introduced to reduce the overhead for both the
   traffic volume and the network processor in SRv6.  The main ideas of
   them are basically to use a shorter ID associated with the complete
   128 bit SID.  The method to establish such association could be
   mapping,stitching, shifting, or translation.

   Existing compression schemes use the 128 bit space as the carrier of
   compressing SIDs.  In the compressed SRH, the meaning of a 128 bit
   segment may be:



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   a) an uncompressed SID

   b) a unified prefix, followed by several short SIDs, and possibly
   existing end marker or padding, which is generally set to value 0

   c) short SIDs of the same type and length, possibly followed by end
   marker or padding, which is generally set to value 0

   So after receiving the SRv6 packets, the node or application cannot
   obtain the complete path information according to segment list in SRH
   any more.

3.  Segment List Recover TLV

   This document defines an SRH TLV called Segment List Recover TLV,
   where,

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |    Length     |            List ID            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              List Recover Info (optional)                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 1: Segment List Recover TLV

   List ID: 16 bits, identification of a complete path with local
   significance.

   List Recover Info: variable length.  It provides auxiliary
   information to recover the complete path information from the segment
   list in the received SRH.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|     Index     |    Range      |     BL        |  ST   | count |      Basic Compressed Description Unit
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
~                       ... ...                                 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



              Figure 2: List Recover Info detailed structure





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   List Recover Info contains one or more Basic Compressed Description
   Unit, where:

   Index: 8-bit index, number of 128 bit unit remaining from the current
   carrier, which is a bit like segments left([RFC8200], Section 4.4).

   Index refers to the start of (consecutive) carrier(s) that contains
   the compression information.  If a segment list contains both
   compressed carrier and complete SID, the index will skip the complete
   SID.

   Range: 8 bit.It indicates the number of consecutive 128-bit carrier
   in the same format, starting from the carrier directed by the index.

   In this way, if multiple consecutive carriers use the same format,
   only one Basic Compressed Description Unit needs to be carried.

   When range is set to 1, it indicates that the basic unit carries
   informational of only one 128 bit carrier.

   BL: It indicates the prefix length.  When set to 0, it means that the
   128 bit carrier does not carry prefix information but only contains
   short SIDs.

   ST:short SID type, 4 bits with following values:

   0001: 32-bits SID

   0010: 16-bits SID

   Existing proposals mainly focus on the 16-bits and 32-bits short
   SIDs, other values may be defined in the future.

   count: 4 bits.It indicates the number of short SIDs in the 128 bit
   carrier.

   When the headend of the SR policy sends the SRv6 packet, it can
   contain the TLV used to describe the above compression details in the
   SRH.Upon receiving the above SRv6 packets, the transit node or egress
   node can perform segment list restoration according to the TLV
   carried in the SRH.

   The restored SID list information can be used as an important
   reference for constructing the reverse path, or used for displaying
   to the user or for other OAM purposes.

   This document provides two path restoration options:




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   a) The node locally maintains the mapping between the list ID and the
   path information.  Upon receiving the packet, the node identifies the
   path corresponding to the SRH according to the list ID carried in the
   SRH TLV.  List Recover Info is not needed in this case.

   b) According to the information carried in the list recover info,
   combined with the segment list in the SRH, the node recovers the
   complete path information.

   It should be noticed that option b can't be used in reduced SRH.

4.  Illustration

   This section describes how to restore a complete path utilizing a
   Segment List Recover TLV.

   The current chapter focuses on option B combined with some
   representative schemes.  Option A will be discussed in the later
   version.

4.1.  Shifting Method

   [I-D.filsfils-spring-net-pgm-extension-srv6-usid] extends SRv6
   Network Programming with a new type of SRv6 SID behavior.

   A uSID carrier is a 128 bit SRv6 SID of format <uSID-Block><Active-
   uSID><Next-uSID>...<Last-uSID><End-of-Carrier>...<End-of-Carrier>.

   So the segment list may be encoded as figure 3 after compression.

   It is consist of six 128-bit units.  SID-1 is an uncompressed 128-bit
   SID and is the first segment to be processed in the segment list.



















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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|       Block(64)               |   uSIDA(32)   |   uSIDB(32)   |   index=0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|       Block(64)               |   uSIDA(32)   |   uSIDB(32)   |   index=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|  Block(32)    |   uSID7(32)   |   uSID8(32)   |   uSID9(32)   |   index=2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|  Block(32)    |   uSID4(32)   |   uSID5(32)   |   uSID6(32)   |   index=3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|  Block(32)    |   uSID1(32)   |   uSID2(32)   |   uSID3(32)   |   index=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|                      SID-1(128 bit)                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -



                 Figure 3: segment list encoding example A

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |            List ID=1          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|   Index=4     |    Range=3    |     BL=32     |ST=0001|count=3|      Basic Compressed Description Unit 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|   Index=1     |    Range=2    |     BL=64     |ST=0001|count=2|      Basic Compressed Description Unit 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -



           Figure 4: Segment List Recover TLV encoding example A

   Figure 4 shows the corresponding Segment List Recover TLV encoding of
   the compressed segment list.

   Basic Compressed Description Unit 1 has following components, where,

   Index=4: It indicates that the compression information starting from
   the 128-bit unit with index 4 is described in this Basic Compressed
   Description Unit.

   Range=3: It indicates that there are 3 128-bit unit of the same
   structure described in this Basic Compressed Description Unit

   BL=32: It indicates that the block length is 32 bit.

   ST=0001: It indicates that the length of each short SID is 32 bit.




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   count=3: It indicates that this carrier contains 3 short SIDs.

   The meaning of Basic Compressed Description Unit2 is similar, so it
   will not be explained in details.

4.2.  Stitching Method

   The compression method of stitching is to extract the public prefix
   information in the SIDs, the remaining part is short SID and stored
   in SRH.  The prefix and short SID are spliced into a complete 128-bit
   address.

   [I-D.decraene-spring-srv6-vlsid] and
   [I-D.li-spring-compressed-srv6-np] use the stitching method.
   Stitching is also one of the compression modes supported by
   [I-D.mirsky-6man-unified-id-sr].

   Figure 5 shows a possible SRH encoding in stitching method.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|                  uncompressed SID(128 bit)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|  uSID1(32)    |   uSID2(32)   |   uSID3(32)   |   uSID4(32)   |   index=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|  uSID1(32)    |   uSID2(32)   |   uSID3(32)   |   uSID4(32)   |   index=2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|                 Block(96)                     |   uSID1(32)   |   index=3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -




                 Figure 5: segment list encoding example B

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |            List ID=1          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|   Index=3     |    Range=1    |     BL=96     |ST=0001|count=1|      Basic Compressed Description Unit 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
|   Index=2     |    Range=2    |     BL=0      |ST=0001|count=4|      Basic Compressed Description Unit 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -



           Figure 6: Segment List Recover TLV encoding example B




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   When restoring the path, the node will save the prefix used in the
   current carrier, and stich the prefix, each short SID and possible
   padding into a complete 128-bit address.  If BL in the next Basic
   Compressed Description Unit is not 0, it indicates that there's a new
   prefix, and the node will replace the previous prefix with the new
   prefix, and continue the processing.  If BL is 0 as in unit 2 showed
   in figure 6, the previous prefix will still be used.

   SRH may also be used carry short SIDs without any prefix information.
   In this case, when the node recovers segment list, it may obtain the
   prefix from the source IP of the IPv6 header.

4.3.  Mapping Method

   In the mapping method, the short SID is no longer part of the
   complete 128-bit address, unlike shifting or stitching.

   Stitching is supported in [I-D.mirsky-6man-unified-id-sr].

   For the compression proposal using the mapping mode, option A can be
   used to restore the segment list.

   The node need to maintains the mapping between the list ID and the
   path information.  Upon receiving the packet, the node identifies the
   path corresponding to the SRH according to the list ID carried in the
   SRH TLV.  The list ID can be assigned by a centralized controller or
   by a node locally.

   This will be further discussed in a future version of this document.

5.  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 vulnerabilities.

6.  IANA Considerations

   TBD

7.  References

7.1.  Normative References








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   [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., Liu, Y., and J. Guichard, "Network Programming
              extension: SRv6 uSID instruction", draft-filsfils-spring-
              net-pgm-extension-srv6-usid-07 (work in progress), May
              2020.

   [I-D.mirsky-6man-unified-id-sr]
              Cheng, W., Mirsky, G., Peng, S., Aihua, L., Wan, X., and
              C. Wei, "Unified Identifier in IPv6 Segment Routing
              Networks", draft-mirsky-6man-unified-id-sr-06 (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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [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>.

7.2.  Informative References

   [I-D.decraene-spring-srv6-vlsid]
              Decraene, B., Raszuk, R., Li, Z., and C. Li, "SRv6 vSID:
              Network Programming extension for variable length SIDs",
              draft-decraene-spring-srv6-vlsid-03 (work in progress),
              March 2020.

   [I-D.li-spring-compressed-srv6-np]
              Li, Z., Li, C., Xie, C., LEE, K., Tian, H., Zhao, F.,
              Guichard, J., Cong, L., and S. Peng, "Compressed SRv6
              Network Programming", draft-li-spring-compressed-
              srv6-np-02 (work in progress), February 2020.








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Authors' Addresses

   Liu Yao
   ZTE Corporation
   No. 50 Software Ave, Yuhuatai Distinct
   Nanjing
   China

   Email: liu.yao71@zte.com.cn


   Peng Shaofu
   ZTE Corporation
   No. 50 Software Ave, Yuhuatai Distinct
   Nanjing
   China

   Email: peng.shaofu@zte.com.cn

































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