BGP Extensions for Unified SID in TE Policy
draft-liu-idr-segment-routing-te-policy-complement-00

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Authors Liu Yao  , Shaofu Peng 
Last updated 2020-03-07
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IDR Working Group                                               Yao. Liu
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: September 8, 2020                                 March 7, 2020

              BGP Extensions for Unified SID in TE Policy
         draft-liu-idr-segment-routing-te-policy-complement-00

Abstract

   This document defines extensions to BGP in order to advertise Unified
   SIDs in SR-TE policies.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR policy with Unified SID  . . . . . . . . . . . . . . . . .   2
     2.1.  BGP Extensions  . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Head-end Processing . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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-segment-routing-policy] details the concepts of SR
   Policy and steering into an SR Policy.

   [I-D.ietf-idr-segment-routing-te-policy] specifies the way to use BGP
   to distribute one or more of the candidate paths of an SR Policy to
   the headend of that policy.

   With increasing requirements for a shortened identifier in a segment
   routing network with the IPv6 data plane,
   [I-D.mirsky-6man-unified-id-sr] proposed an extension of SRH that
   enables the use of a shorter segment identifier, such as 32-bits
   Label format SID or 32-bits IP address format SID.

   This document defines extensions to BGP in order to advertise Unified
   SIDs in SR-TE policies.

   Firstly, we focus on how to carry 32-bits IP address format U-SID,
   other type of U-SID will be considered in future version.

2.  SR policy with Unified SID

   As discussed in [I-D.ietf-spring-srv6-network-programming], the node
   with the SRv6 capability will maintain its local SID table.  A Local
   SID is generally composed of two parts, that is, LOC:FUNCT, or may
   carry arguments at the same time, that is, LOC:FUNCT:ARGS.

   FUNCT indicates the local function of the packet on the node that
   generates the LOC.ARGS may contain information related to traffic and
   services, or any other information required for executing the

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   function.LOC indicates locator.  In most cases, other nodes in the
   network can forward packets to the node that generates this LOC
   according to the corresponding routing table entries.

   The controller plane protocol can also use B:N to represent an LOC,
   where B is SRv6 SID Locator Block and N to represent node N.  In
   other words, the structure of a complete SID is B:N:FUNCT:ARGS.

   [I-D.ietf-lsr-isis-srv6-extensions] defines the extension of ISIS to
   support SRv6, and each node can announce the SID assigned by itself.
   In particular, SRv6 SID Structure Sub-Sub-TLV is defined and the
   specific structure of the corresponding SID is provided, including
   the length of SRv6 SID Locator Block, the length of SRv6 SID Locator
   Node, the length of SRv6 SID Function, and the length of SRv6 SID
   Arguments.

   Similarly, [I-D.ietf-bess-srv6-services] also provide the SID
   structure information for L3VPN or EVPN service related SID.

   Thus, it can be seen that the existing control plane protocol reveals
   a very intuitive method to reduce the size of SRH.  That is , under
   the specific address planning(the SIDs allocated by all SRv6 nodes
   are in the same SRv6 SID Locator Block), SRH only needs to store the
   difference between SIDs (N:FUNCT:ARGS), and does not need to contain
   the SRv6 SID Locator Block information.  In a 128-bit classic SRv6
   SID, the highest part is SRv6 SID Locator Block, and the following 32
   bits are composed of SRv6 SID Locator Node, SRv6 SID Function and
   SRv6 SID Arguments, and the rest bits are zeros.

   As for how to obtain the SRv6 SID Locator Block information during
   packet forwarding, there are two cases:

   1)For the head-end node, when the node sends a packet along the
   segment list to the first segment, it already knows the 128-bit
   classical SID before truncaturing.  The head node copies it directly
   to the DA of IPv6 Header, but the SRH carries the 32-bit truncatured
   SIDs.

   2)For the transit node, it can obtain the SRv6 SID Locator Block
   information from the DA of the received IPv6 packet.

2.1.  BGP Extensions

   This document defines a new one-bit flag field in the segment-list
   sub-TLV [I-D.ietf-idr-segment-routing-te-policy] RESERVED field,
   where,

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   T: Truncatured-Flag, when set, it indicates the presence of 32-bits
   IP address format U-SID(s) in the SR path

       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            |T|   RESERVED  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                           sub-TLVs                          //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: T-Flag in Segment List sub-TLV

   In this document, the Flags field of each segment sub-TLV(type B/I/J/
   K) [I-D.ietf-idr-segment-routing-te-policy] is extended to indicate
   the block length (BL) and non-block length (NBL) of a 128-bit SID.

   Figure 2 uses the type B segment sub-TLV as an example to illustrate
   the extended LT field.  Other types of segment sub-TLV are similar.

       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      | LT  | Flags     |   RESERVED  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                       SRv6 SID (16 octets)                  //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: Length Type Field in Segment sub-TLV

   LT: Length Type, 3-bit field with the following values:

   000 unknown

   001 BL=96bits, NBL=32bits,

   010 BL=64bits, NBL=32bits,

   011 BL=32bits, NBL=32bits,

   Other values are reserved for future use.

   It should be noted that NBL represents the length of the Node:Func
   that is immediately followed the block.

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2.2.  Head-end Processing

   Take the length of the short SID as 32 bits as an example.

   On the head-end node, if the SR-TE tunnel has enabled the SRv6 SID
   compression, and the compression mode is to use 32-bits IPv4 address
   U-SID , then it analyzes whether these SIDs are in the same block and
   whether the length of Node:Func does not exceed 32 bits based on the
   NBL length corresponding to each SID contained in the segment List
   received from the controller.

   If the above conditions are met, the head-end node uses a 32-bit
   short SID optimization SID List for SRH encapsulation.

   Note that it can also be the responsibility of the controller to
   check if there could use IPv4 address U-SID for the entire SID list,
   especially for the inter-domain case.  In this case the headend can
   simply follow the decision of the controller.

3.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the security considerations discussed in
   [I-D.ietf-idr-segment-routing-te-policy].

4.  IANA Considerations

   TBD

5.  References

5.1.  Normative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
              Rosen, E., Jain, D., and S. Lin, "Advertising Segment
              Routing Policies in BGP", draft-ietf-idr-segment-routing-
              te-policy-08 (work in progress), November 2019.

   [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-05
              (work in progress), February 2020.

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   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-06 (work in progress),
              December 2019.

   [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-10 (work in
              progress), February 2020.

   [I-D.mirsky-6man-unified-id-sr]
              Cheng, W., Mirsky, G., Peng, S., Aihua, L., Wan, X., Wei,
              C., and S. Shay, "Unified Identifier in IPv6 Segment
              Routing Networks", draft-mirsky-6man-unified-id-sr-05
              (work in progress), February 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>.

5.2.  Informative References

   [I-D.ietf-bess-srv6-services]
              Dawra, G., Filsfils, C., Raszuk, R., Decraene, B., Zhuang,
              S., and J. Rabadan, "SRv6 BGP based Overlay services",
              draft-ietf-bess-srv6-services-01 (work in progress),
              November 2019.

Authors' Addresses

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

   Email: liu.yao71@zte.com.cn

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

   Email: peng.shaofu@zte.com.cn

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