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

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Authors Liu Yao  , Shaofu Peng 
Last updated 2020-11-23
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IDR Working Group                                               Yao. Liu
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: May 27, 2021                                  November 23, 2020

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

Abstract

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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 27, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR policy with Unified SID  . . . . . . . . . . . . . . . . .   2
     2.1.  Advertisement of SID Attribute  . . . . . . . . . . . . .   4
       2.1.1.  Option 1: Advertising SID Attribute within existing
               sub-TLVs  . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Option 2: Introducing a new U-segment list sub-TLV  .   7
     2.2.  Controller Processing . . . . . . . . . . . . . . . . . .   7
     2.3.  Headend Processing  . . . . . . . . . . . . . . . . . . .   8
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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

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   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
   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.

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

2.1.  Advertisement of SID Attribute

   The U-SID solution defined in [I-D.mirsky-6man-unified-id-sr] reply
   two attributes of SID, they are: SID structure attribute and Endpoint
   Behavior attribute.  However,
   [I-D.ietf-idr-segment-routing-te-policy] does not provide these
   information now.  This document discusses two options to supplement
   these information.

2.1.1.  Option 1: Advertising SID Attribute within existing sub-TLVs

   In this section, a new sub-sub-TLV is introduced in each segment sub-
   TLV(type B/I/J/K) [I-D.ietf-idr-segment-routing-te-policy] to offer
   the SID structure information.

   Since the new compression information-related sub-sub-TLV is included
   in segment List sub-TLV, the meaning of the whole segment list will
   be changed, that is, the headend cannot regard this segment list as a
   classic segment list to process and encapsulate the classic 128 bit
   SRH.  Therefore, the controller must know the compression capability
   supported by the head node when delivering SR policy to the it.

   There are two ways to do this.

   Opntion 1, negotiate of compression capacity through BGP session.
   The controller only sends the Segment List Sub-TLV with compression
   information to the BGP neighbors with compression capability.

   It is necessary to consider the scenario with a route reflector.  The
   BGP session is not directly established between the controller and
   the head node.  One or more RT Extended Community can be carried in
   the SR policy UPDATE announcement message to contain the specific
   head node Router-ID information.

   If the controller learns that the head node has the compression
   capability by some means (such as collecting through BGP-LS), but the
   RR does not have the abilities , then the controller can still choose
   to send to the RR according to the actual destination node notified
   by UPDATE.

   If the reflector does not recognize the newly added sub-TLV / sub-
   sub-TLV compression information, it is necessary to decide whether to
   unconditionally transmit it to the head node according to the
   positive bit in the top-level TLV (that is, the Tunnel Encapsulation
   Attribute).

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   If the reflector recognizes the newly added sub-TLV / sub-sub-TLV, it
   is necessary to check whether the headend has compression capability.
   If not, RR will not reflect the Segment List Sub-TLV containing
   compressed information to the head node.

   Opntion 2, the controller collects the compression capability of the
   head node through BGP-LS.  If the head node has compression
   capability, the controller can deliver an segment list Sub-TLV
   containing compression information to the head node.  Otherwise, only
   an Segment List sub-TLV containing 128-bit SIDs can be delivered.

   If there is a RR, it only needs to decide whether to transmit it
   unconditionally to the head node according to transitive bit in the
   top-level TLV (that is, Tunnel Encapsulation Attribute).

   The first method is too complicated, so option 2 is recommended.

   Figure 2 uses the type B segment sub-TLV as an example, other types
   of segment sub-TLV are similar.

   The SRv6 SID Structure Sub-Sub-TLV has the following format:

              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |sub-Type=STRUCT|   Length      |     Count     |   RESERVED    |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |  BL of SID 1  |  TL of SID 1  |  BL of SID 2  |  TL of SID 1  |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |                            ... ...                            |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |            ... ...            |  BL of SID N  |  TL of SID N  |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 1: SRv6 SID Structure Sub-Sub-TLV format

   where,

   Count: 1 octet, the count of segments.The value of count must be
   consistent with the number of Segment Sub-TLV contained in segment
   list sub-TLV; otherwise, the whole segment list sub-TLV must be
   ignored.

   BL: block length of classical 128 bit SID in bits, value: 1~ 128.  If
   the corresponding SID is an MPLS label, BL is 0.

   TL: truncated length of the compressed SID in bits, value: 1~ 128.
   If a 128 bit SID is compressed to 32 bits, TL is 32.  If a 128 bit
   SID is not compressed, TL is 128. the TL of a 32-bit MPLS label is
   32.

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   As above, if the headend does not recognize the Segment Truncated
   sub-TLV, the entire Segment List sub-TLV must be ignored.

   A new flag is introduced in Segment Sub-TLV,

        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|     |UET| |   RESERVED    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //                       SRv6 SID (16 octets)                  //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 2: UET-Flavor Flag in Segment sub-TLV

   where,

   UET: U-SID Encapsulation Type Flag, 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.
   It could be the following values:

      0: the UET domain following the current segment node is UET-128
      domain, that means the next SID does not need compression and
      remains 128 bits.

      1: the UET domain following the current segment node is UET-32
      domain, that means the next SID needs to be compressed to a 32-bit
      IP address.

      2: the UET domain following the current segment node is UET-
      32-MPLS domain, that means the next SID needs to be compressed to
      a 32-bit MPLS Label.

      3: the UET domain following the current segment node is UET-16
      domain, that means the next SID needs to be compressed to a 16-bit
      IP address.

   Currently, if only the 32-bit IP address compression mode is
   considered, the UET-Flavor value is 0 or 1.

   Two factors should be considered for the value of UET-Flavor: the
   compression domain type composed of this segment node and the next
   segment node, and the structure of the next selected SID can be
   compressed in the way indicated by UET-Flavor.

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2.1.2.  Option 2: Introducing a new U-segment list sub-TLV

   In order to solve the forward compatibility problem more
   conveniently, a new U-Segment List Sub-TLV is defined, which can
   contain compressed information.

   Similarly, the controller decides whether to send U-Segment List Sub-
   TLV with compression information or Segment List Sub-TLV without
   compression information to the head node based on whether the head
   node has the compression capability.

   The specific extension:

   1) Add a UET-Flavor Flag to the existing Segment Sub-TLV.

   2) In U-Segment List Sub-TLV, define Segment Truncated sub-TLV to
   describe the compression result.

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

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   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
   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.  Headend Processing

   When the headend receives the SR policy, it obtains the compressed
   information of each SID according to the TL field in the Segment
   Truncated sub-TLV.  The headend should identify the UET-Flavor of
   each SID, which can be verified with the compression result, that is,
   the UET-Flavor of a certain SID must be consistent with the
   compression result of the next SID, otherwise the entire Segment List
   sub-TLV must be ignored .

   In particular, the UET-Flavor of the last SID can be used as a clear
   basis to decide what compression method should be adopted for the
   overlay SID, such as the VPN service.

   Optionally, the headend can use reduced SRH that exclude the first
   SID, to further reduce the cost of SRH.

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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-11 (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-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-09 (work in progress),
              November 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-24 (work in
              progress), October 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-07 (work in progress),
              July 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., Talaulikar, K., Raszuk, R.,
              Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based
              Overlay services", draft-ietf-bess-srv6-services-05 (work
              in progress), November 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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