BGP Extensions for Unified SID in TE Policy
draft-liu-idr-segment-routing-te-policy-complement-04

Versions: 00 01 02 03 04                                                
IDR Working Group                                               Yao. Liu
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: September 28, 2020                               March 27, 2020


              BGP Extensions for Unified SID in TE Policy
         draft-liu-idr-segment-routing-te-policy-complement-01

Abstract

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

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   This Internet-Draft will expire on September 28, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR policy with Unified SID  . . . . . . . . . . . . . . . . .   2
     2.1.  BGP Extensions  . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Controller Processing . . . . . . . . . . . . . . . . . .   5
     2.3.  Head-end Processing . . . . . . . . . . . . . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 (such as 16-bits) 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.





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   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
   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 maybe three 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 truncating.  The head node copies it directly to
   the DA of IPv6 Header, but the SRH carries the 32-bit truncatured
   SIDs.

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

   3)For the inter-domain border node, it can obtain the new SRv6 SID
   Locator Block information from the local SID entry.





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2.1.  BGP Extensions

   This document defines two flags in the segment-list sub-TLV
   [I-D.ietf-idr-segment-routing-te-policy] RESERVED field, where,

   T-Flag: Truncatured-Flag, one bit, when set, it indicates there are
   truncated piece information of classical IPv6 SID in the SR path.

   FSU-Flag: First SID UET flag, two bits, it indicates the UET type of
   the first SID, in other words, indicates the UET domain constructed
   by the headend and the first segment node.

       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|FSU|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,
   that is a simple representation of SID structure information.

   Figure 2 uses the type B segment sub-TLV as an example to illustrate
   the extended SSI 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      |V|A| SSI |UET| |   RESERVED    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                       SRv6 SID (16 octets)                  //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: Length Type Field in Segment sub-TLV

   Where

   SSI: SID Structure Indication, 3-bit field with the following values:

   000 unknown

   001 BL=96bits, NBL=32bits,




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   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:ARGs that is immediately followed the block.

   UET: U-SID Encapsulation Type, 2-bit field, it indicates the UET type
   of the next SID, in other words, indicates the UET domain constructed
   by the current segment node and the next segment node.  The value
   refers to [I-D.mirsky-6man-unified-id-sr].

2.2.  Controller Processing

   Controller can collect UET capability information of all nodes, see
   [I-D.mirsky-6man-unified-id-sr], each node can support one or more
   than one UET capabilities.  In general, a border node that belongs to
   multiple UET domain will support multiple UET capabilities, while
   other nodes can only support a single UET capability.

   Controller can also collect SID per UET of all nodes.  If a node
   support an UET capability, it will also allocate related SIDs for
   this UET Flavor.

   When controller computed an SR path, it can check the UET capability
   of each segment node within the segment list, to outline which UET
   domains the SR path crosses.  For example, from Headend H to endpoint
   E, a segment list <X1, X2, X3, B, Y1, Y2, Y3, E> may cross two UET
   domains, the node H, X1, X2, X3, B all support UET-1, and the node B,
   Y1, Y2, Y3, E all support UET-2.  In this case, the FSU-flag will be
   set to UET-1, it indicates the UET domian which the first SID X1
   belongs to.  At the same time, the controller will select UET related
   SID for each segment according to the UET domain which the segment
   node belongs to, i.e., the UET Flag of SID X1, X2, X3 will be set to
   UET-1, and the UET Flag of SID B, Y1, Y2, Y3, E will be se to UET-2.
   Note that in this case, SID B with UET-2 Flavor, but not UET-1
   Flavor, is inserted in ths list for the purpose of seamless splicing.

   Then, controller need to check the structure information of each
   selected SID, to ensure they can safely construct an SID list with
   UET information.  For example, the structure information of SID X1
   (with UET-1 Flavor), SID X2 (with UET-1 Flavor), SID X3 (with UET-1
   Flavor), SID B (with UET-2 Flavor), MUST support to get UET-1
   (because the UET of prev SID is UET-1) related truncated piece
   information (Node:Func:ARGS) from the original IPv6 SID.  Similarly,
   the structure information of SID Y1 (with UET-2 Flavor), SID Y2 (with



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   UET-2 Flavor), SID Y3 (with UET-2 Flavor), SID E (with UET-2 Flavor),
   MUST support to get UET-2 (because the UET of prev SID is UET-2)
   related truncated piece information from the original IPv6 SID.

   There maybe another segment list example, <B, Y1, Y2, Y3, E> also
   cross two UET domains, that is, the node H, B all support UET-1, and
   the node B, Y1, Y2, Y3, E all support UET-2.  In this case, the FSU-
   flag will be also set to UET-1, it indicates the UET domian which the
   first SID B belongs to.  At the same time, the controller will select
   UET related SID for each segment according to the UET domain which
   the segment node belongs to, i.e., the UET Flag of SID B, Y1, Y2, Y3,
   E will be se to UET-2.  Note that in this case, SID B with UET-2
   Flavor, but not UET-1 Flavor, is inserted in ths list for the purpose
   of seamless splicing.  Then, the controller check the structure
   information of each selected SID to ensure they can safely construct
   an SID list with UET information.  That is, the structure information
   of SID B (with UET-2 Flavor), MUST support to get UET-1 (because the
   UET of prev SID is UET-1) related truncated piece information from
   the original IPv6 SID.  Similarly, the structure information of SID
   Y1 (with UET-2 Flavor), SID Y2 (with UET-2 Flavor), SID Y3 (with
   UET-2 Flavor), SID E (with UET-2 Flavor), MUST support to get UET-2
   (because the UET of prev SID is UET-2) related truncated piece
   information from the original IPv6 SID.

   If a SID can not support to get UET related truncated piece according
   to the UET of prev SID, the controller MUST select another prev SID
   with UET-0 flavor.

2.3.  Head-end Processing

   When headend received the SR policy, for each segment list of the
   candidate path, if the Truncatured-Flag is set, the segment list
   could try to be optimized to an SID list that contains short U-SIDs.

   For each original SID within the received SID list, it will be
   optimized to an U-SID according to the UET of prev SID.  For example,
   for the above segment list <X1, X2, X3, B, Y1, Y2, Y3, E>, the
   original SID X1 could be optimized to U-SID X1 according the UET of
   prev SID (in fact, it is FSU), get the truncated prev-UET related
   piece from the original SID with the help of its SSI field.
   Similarly, the original SID B could be optimized to U-SID B according
   the UET of prev SID X3, get the truncated prev-UET related piece from
   the original SID with the help of its SSI field.

   The SRH will contain the optimized U-SIDs, and the initial SRH.UET
   will be set as FSU.  Other procedures refer to
   [I-D.mirsky-6man-unified-id-sr].




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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-07
              (work in progress), March 2020.

   [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-14 (work in
              progress), March 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>.



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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-02 (work in progress),
              February 2020.

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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