Truncated SID Inter Domain Considerations
draft-peng-spring-truncated-sid-inter-domain-01

Document Type Active Internet-Draft (individual)
Authors Liu Yao  , Shaofu Peng  , Greg Mirsky 
Last updated 2021-02-01
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SPRING Working Group                                              Y. Liu
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: August 5, 2021                                        G. Mirsky
                                                               ZTE Corp.
                                                        February 1, 2021

               Truncated SID Inter Domain Considerations
            draft-peng-spring-truncated-sid-inter-domain-01

Abstract

   This document introduces a method for interworking between domains of
   Segment Routing in IPv6 network that use different levels of Segment
   Identifier's compression.

Status of This Memo

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  Interworking Between Different Domains  . . . . . . . . . . .   3
   4.  Control Plane Extensions  . . . . . . . . . . . . . . . . . .   5
     4.1.  Extensions to BGP-LS  . . . . . . . . . . . . . . . . . .   5
     4.2.  Extensions to IGP . . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Extensions to IS-IS . . . . . . . . . . . . . . . . .   5
       4.2.2.  Extensions to OSPFv3  . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  BGP-LS SRv6 SID Swapped Block Sub-TLV . . . . . . . . . .   7
     6.2.  IS-IS SRv6 SID Swapped Block Sub-TLV  . . . . . . . . . .   7
     6.3.  OSPFv3 SRv6 SID Swapped Block Sub-TLV . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Segment Routing [RFC8402] leverages the source routing paradigm.  An
   ingress node steers a packet through an ordered list of instructions,
   called segments.  [I-D.ietf-spring-srv6-network-programming] proposes
   the Segment Routing in IPv6 (SRv6) Network Programming to specify a
   packet processing program by encoding a sequence of instructions in
   the IPv6 packet's extension header.
   [I-D.ietf-spring-srv6-network-programming] defined a Segment
   Identifier (SID) as consisting of LOC:FUNCT:ARG, where a locator
   (LOC) is encoded in the L most significant bits of the SID, followed
   by F bits of function (FUNCT) and A bits of arguments (ARG).  A
   locator may be represented as BLOCK:NODE where BLOCK is the SRv6 SID
   block (IPv6 prefix allocated for SRv6 SIDs by the operator) and NODE
   is the identifier of the node instantiating the SID.  Note that "SID
   Locator Block" and "BLOCK" are used interchangeably throughout the
   document.

   As demonstrated in several proposals, such as
   [I-D.cl-spring-generalized-srv6-for-cmpr] and
   [I-D.mirsky-6man-unified-id-sr], SIDs could be allocated by SRv6
   nodes from within the same SRv6 SID Locator Block, and referring to
   the idea described in [RFC6554] SRH only needs to store the
   difference between SIDs, such as NODE:FUNCT:ARGS, the non-BLOCK part,
   and does not need to contain the SRv6 SID Locator Block information.
   In this case, some or all SIDs encoded in the SRH are the non-BLOCK
   part of the 128-bit SIDs.

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   An SRv6 path could traverse domains that use different block prefixes
   and achieving different compressed SIDs lengths.  This document
   provides a method of higher compression efficiency for interworking
   between domains.  It introduces a new SID flavor and defines the
   required control plane extensions.

2.  Conventions used in this document

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.  Interworking Between Different Domains

   The interworking of different domains is illustrated in Figure 1.

        .................................  .......................
        :                               :  :                     :
      +----+     +----+     +----+     +----+     +----+     +----+
      + S  +-----+ A  +-----+ B  +-----+ABR +-----+ C  +-----+ D  +
      +----+     +----+     +----+     +----+     +----+     +----+
        :                               :  :                     :
        .......... domain 1..............  .......domain2.........

                  Figure 1: Interworking between domains

   An end-to-end SRv6 path from A to D passes through two domains.  The
   block prefix of Domain 1 is BLOCK1, and the compressed SID's length
   is 32 bits.  The block prefix of Domain 2 is BLOCK2, and the
   compressed SID's length is 16 bits.

   The following notation is used in the document, 128bits-BLOCK1-SID-
   A-32 means that this is a 128-bit SID of node A whose block is BLOCK1
   and itself can be compressed to 32 bits if needed.  The original
   segment list before the compression is <128bits-BLOCK1-SID-A-32,
   128bits-BLOCK1-SID-B-32, 128bits-BLOCK1-SID-ABR-32, 128bits-BLOCK2-
   SID-C-16, 128bits-BLOCK2-SID-D-16>.

   This segment list is selected/computed by a Path Computation Element
   (PCE) system or a controller and the details are for further
   discussion.

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   A new SID flavor is defined in this document to make the entire
   segment list more compressible.

   Block Swapping (BS) Flavor: it indicates that SRv6 SID Locator Block
   will be swapped.  And the information about the new block with
   block's length is maintained in the local SID entry.

   So in ABR, a 128bits-BLOCK1-ABR-32 SID with BS flavor is allocated.
   It is within 128-bit SID with block1 and has BS Flavor, and the local
   SID entry will swap BLOCK1 to BLOCK2.  Thus, the next short 16 bit
   SID can be stitched with BLOCK2 to generate a full 128 bit SID.  With
   the help of the BS flavor, the corresponding compressed segment list
   could be <32bits-SID-A,32bits-SID-B,32bits-SID-ABR(BS), 16bits-SID-C,
   16bits-SID-D>.

   32bits-SID-A means this is a 32-bit compressed SID of node A.

   32bits-SID-ABR(BS) means this is a 32-bit compressed SID of node ABR
   with BS flavor.

   At the headend S, the packet sent to node A is (SA,DA=BLOCK1+32bits-
   SID-A)(16bits-SID-D, 16bits-SID-C, 32bits-SID-ABR, 32bits-SID-B,
   32bits-SID-A; SL=3), i.e., only one single 128 segment entry is
   needed in SRH.  Note that SL=3 means there are three 32-bits segment
   items within SRH remaining to be visited.

   The packet sent from node A to node B is (SA,DA=BLOCK1+32bits-SID-
   B)(16bits-SID-D, 16bits-SID-C, 32bits-SID-ABR, 32bits-SID-B, 32bits-
   SID-A; SL=2).

   The packet sent from B to ABR is (SA,DA=BLOCK1+32bits-SID-
   ABR)(16bits-SID-D, 16bits-SID-C, 32bits-SID-ABR, 32bits-SID-B,
   32bits-SID-A; SL=1).

   When the packet arrives at ABR, it matches the local SID entry.  The
   endpoint behavior of that entry is END with BS Flavor, and the new
   block with its block length is provided in the local SID entry.  The
   next short 16 bit SID can be retrieved from SRH and be stitched with
   the new block to generate a full 128 bit SID to be copied to the DA
   field.  Thus, the packet sent from ABR to C could be
   (SA,DA=BLOCK2+16bits-SID-C)(16bits-SID-D, 16bits-SID-C, 32bits-SID-
   ABR, 32bits-SID-B, 32bits-SID-A; SL=1).  Note that SL=1 means there
   are one 16-bits segment items within SRH remaining to be visited.

   Generally, multiple BS-flavored SIDs may be allocated on the boundary
   node.  For example, SID-12 with BS flavor indicates that SRv6 SID
   Locator Block switches from BLOCK1 to BLOCK2, and SID-13 indicates
   that SRv6 SID Locator Block switches from BLOCK1 to block3.

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   For the inter-AS scenario, END.X SID allocated for SR-EPE or a direct
   link can also have BS flavor to indicate the new block information of
   the next AS.

4.  Control Plane Extensions

4.1.  Extensions to BGP-LS

   This document introduces an SRv6 SID Swapped Block Sub-TLV.  It is an
   optional TLV for use in the BGP-LS Attribute for an SRv6 SID NLRI and
   as a sub-TLV of the SRv6 End.X, IS-IS SRv6 LAN End.X and OSPFv3 SRv6
   LAN End.X TLVs [I-D.ietf-idr-bgpls-srv6-ext].  This TLV is validate
   only when the SID has a BS flavor.

   The TLV has the following format:

        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               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Variable Swapped Block                        //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: BGP-LS SRv6 SID Swapped Block Sub-TLV

   Type: TBA

   Length: 2 octet field, indicate the length of Swapped Block.

   Swapped Block: This field encodes the advertised swapped block prefix
   information.

4.2.  Extensions to IGP

   Different nodes in the same IGP domain may have different block
   prefixes.  So IGP extensions are also needed in this scenario.

4.2.1.  Extensions to IS-IS

   SID Swapped Block Sub-Sub-TLV is introduced in this document.  It is
   an optional Sub-Sub-TLV of SRv6 End SID Sub-TLV, SRv6 End.X SID Sub-
   TLV and SRv6 LAN End.X SID Sub-TLV
   [I-D.ietf-lsr-isis-srv6-extensions].

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       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     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //               Variable Swapped Block                        //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3: ISIS SRv6 SID Swapped Block Sub-Sub-TLV

   Type: TBA, 1 octets.

   Length: 1 octet field, indicate the length of Swapped Block.

   Swapped Block: This field encodes the advertised swapped block prefix
   information.

4.2.2.  Extensions to OSPFv3

   SID Swapped Block Sub-TLV is introduced in this document.  It is used
   as an optional Sub-TLV of SRv6 End SID Sub-TLV, SRv6 End.X SID Sub-
   TLV, SRv6 LAN End.X SID Sub-TLV
   [I-D.ietf-lsr-ospfv3-srv6-extensions].

        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               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Variable Swapped Block                        //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: OSPFv3 SRv6 SID Swapped Block Sub-Sub-TLV

   Type: TBA, 2 octets.

   Length: 2 octet field, indicate the length of Swapped Block.

   Swapped Block: This field encodes the advertised swapped block prefix
   information.

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5.  Security Considerations

   TBD

6.  IANA Considerations

6.1.  BGP-LS SRv6 SID Swapped Block Sub-TLV

   This document requests IANA to assign a new code point in the "BGP-LS
   Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
   TLVs" sub-registry under the "Border Gateway Protocol - Link State
   (BGP-LS) Parameters" registry.

   +-----------+---------------------+--------------+------------------+
   |  TLV Code | Description         |  IS-IS TLV/  | Reference        |
   |   Point   |                     |   Sub-TLV    |                  |
   +-----------+---------------------+--------------+------------------+
   |    TBA1   | SRv6 SID Swapped    |     ---      | This document    |
   |           | Block               |              |                  |
   +-----------+---------------------+--------------+------------------+

6.2.  IS-IS SRv6 SID Swapped Block Sub-TLV

   This document requests IANA to assign a new code point in the "Sub-
   Sub-TLVs for SID Sub-TLVs" registry under the IS-IS registry.
   Referring to section "12.5.  Sub-Sub-TLVs for SID Sub-TLVs" of
   [I-D.ietf-lsr-isis-srv6-extensions], the registration procedure of
   "Sub-Sub-TLVs for SID Sub-TLVs" is still "Expert Review" as defined
   in [RFC8126].

   +----------+----------------------------------------+---------------+
   | TLV Code |             Description                | Reference     |
   |  Point   |                                        |               |
   +----------+----------------------------------------+---------------+
   |  TBA2    |   SRv6 SID Swapped Block               | this document |
   +----------+----------------------------------------+---------------+

6.3.  OSPFv3 SRv6 SID Swapped Block Sub-TLV

   This document requests IANA to assign a new code point in the "OSPFv3
   Extended-LSA Sub-TLVs" registry under the "Open Shortest Path First
   v3 (OSPFv3) Parameters" registry.

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   +----------+----------------------------------------+---------------+
   | TLV Code |             Description                | Reference     |
   |  Point   |                                        |               |
   +----------+----------------------------------------+---------------+
   |  TBA3    |   SRv6 SID Swapped Block               | this document |
   +----------+----------------------------------------+---------------+

7.  References

7.1.  Normative References

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              daniel.bernier@bell.ca, d., and B. Decraene, "BGP Link
              State Extensions for SRv6", draft-ietf-idr-bgpls-
              srv6-ext-05 (work in progress), November 2020.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-11
              (work in progress), October 2020.

   [I-D.ietf-lsr-ospfv3-srv6-extensions]
              Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
              "OSPFv3 Extensions for SRv6", draft-ietf-lsr-
              ospfv3-srv6-extensions-01 (work in progress), August 2020.

   [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-28 (work in
              progress), December 2020.

   [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>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <https://www.rfc-editor.org/info/rfc6554>.

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   [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>.

7.2.  Informative References

   [I-D.cl-spring-generalized-srv6-for-cmpr]
              Cheng, W., Li, Z., Li, C., Clad, F., Aihua, L., Xie, C.,
              Liu, Y., and S. Zadok, "Generalized SRv6 Network
              Programming for SRv6 Compression", draft-cl-spring-
              generalized-srv6-for-cmpr-02 (work in progress), November
              2020.

   [I-D.mirsky-6man-unified-id-sr]
              Cheng, W., Mirsky, G., Peng, S., Aihua, L., and G. Mishra,
              "Unified Identifier in IPv6 Segment Routing Networks",
              draft-mirsky-6man-unified-id-sr-08 (work in progress),
              January 2021.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Authors' Addresses

   Liu Yao
   ZTE Corporation

   Email: liu.yao71@zte.com.cn

   Peng Shaofu
   ZTE Corporation

   Email: peng.shaofu@zte.com.cn

   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com

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