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Versions: 00 03

IDR Working Group                                               Yao. Liu
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: November 12, 2020                                  May 11, 2020


              BGP Extensions for Unified SID in TE Policy
         draft-liu-idr-segment-routing-te-policy-complement-03

Abstract

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

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 November 12, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR policy with Unified SID  . . . . . . . . . . . . . . . . .   2
   3.  SR policy with U-SID Information Encoding . . . . . . . . . .   4
     3.1.  Option 1: Advertising SID Attribute within existing
           Segment List sub-TLV  . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Indicating the UET Flavor of the First SID  . . . . .   5
       3.1.2.  Indicating the UET Flavor of the Other SIDs . . . . .   6
       3.1.3.  Indicating the SID Structure and Truncated
               Information . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Option 2: Introducing a new U-Segment List sub-TLV  . . .   8
   4.  Controller Processing . . . . . . . . . . . . . . . . . . . .   9
   5.  Head-end Processing . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 flow 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 shorter 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.

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 advertise 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 straightforward method to reduce the size of SRH.  That is, under
   the specific address planning, i.e., 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
   classical 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
      classicalal SID before truncating.  The headend 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 or current active SID 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, which is
      installed for a SID with explicit Block Switch configuration.

3.  SR policy with U-SID Information Encoding

   The U-SID solution defined in [I-D.mirsky-6man-unified-id-sr] depend
   on 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.

3.1.  Option 1: Advertising SID Attribute within existing Segment List
      sub-TLV

   In this option, new flags are introduced in each Segment Sub-TLV(type
   B/I/J/K) [I-D.ietf-idr-segment-routing-te-policy] to provide UET
   flavor information, and new flags and sub-TLVs are introduced in the
   existing Segment List sub-TLV to provide SID structure and truncated
   information.

   Since the above new compressed information 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 classical
   segment list to process and encapsulate the classical 128 bit SIDs in
   SRH.  Therefore, the controller must know the SRv6 SRH compression
   capability supported by the headend before advertising an SR policy
   to the headend.

   There are two ways for the controller to get the SRv6 SRH compression
   capability of headend:

   o Method 1: negotiation of SRv6 SRH compression capability during BGP
   session.

   The controller only sends the Segment List sub-TLV including U-SID
   compression information to the BGP speakers who have SRv6 SRH
   compression capability.  However, it is necessary to consider the
   scenario with a route reflector, in this case, the BGP session is not
   directly established between the controller and the headend.  One or
   more RT Extended Community can be carried in the SR policy UPDATE
   message to contain the specific headend Router-ID information.

   If the controller learns that the headend has the SRv6 SRH
   compression capability by other means (such as collecting through
   BGP-LS), but the RR has not, the controller can still choose to send
   the UPDATE message including U-SID compression information to the RR



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   according to the actual destination headend included in UPDATE
   message.

   If the reflector does not recognize the new added sub-TLV / sub-sub-
   TLV compression information introduced in Tunnel Encapsulation
   Attribute, it can simply transmit to the headend according to the
   tranmit bit set in Tunnel Encapsulation Attribute.

   If the reflector recognizes the new added sub-TLV / sub-sub-TLV, it
   is necessary to check whether the headend has SRv6 SRH compression
   capability.  If not, RR will not reflect the Segment List sub-TLV
   containing compressed information to the headend.

   o Method 2: the controller collects the SRv6 SRH compression
   capability of the headend through BGP-LS.

   If the headend has the SRv6 SRH compression capability, the
   controller can advertise a Segment List sub-TLV containing U-SID
   compression information to the headend.  Otherwise, only a Segment
   List sub-TLV containing classicalal 128-bit SIDs can be advertised.

   [I-D.chen-lsr-igp-shorter-srv6-extensions] has defined U-capability
   of an SRv6 node, the U-capability is just the he SRv6 SRH compression
   capability.  An SRv6 node with U-capability indicates that it
   supports the encapsulate and decapsulate the U-SID, that is to say,
   the SID list composed of multiple classical 128 bit SIDs can be
   compressed into an U-SID list containing multiple shorter U-SIDs,
   which is encapsulated in SRH, or the shorter U-SID can be obtained
   from SRH and restored to the classical 128 bit SID.

   The first method will introduce more complex processing to BGP, this
   docoment suggest the seccond one.  The U-capability is independent
   with SBI which is selected to advertise SR policy, such as PCEP, BGP,
   etc.

3.1.1.  Indicating the UET Flavor of the First SID

   A new flag is introduced in the RESERVED field of Segment List sub-
   TLV [I-D.ietf-idr-segment-routing-te-policy] to indicate the UET
   Flavor of the first SID, as presented in Figure 1.











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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            |FSU|  RESERVED |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                           sub-TLVs                          //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 1: FSU Flag in Segment List sub-TLV

   FSU-Flag: First SID UET flag, two bits, it indicates how to compress
   the first SID.  It could be the following values:

      0: the first SID does not need compression and remains 128 bits.

      1: the first SID needs to be compressed to a 32-bit IP address.

      2: the first SID needs to be compressed to a 32-bit MPLS Label.

      3: the first SID needs to be compressed to a 16-bit IP address.

   The value set in FSU-Flag field need consider two factors: the UET
   domain constructed by the headend and the first segment node, and
   whether the structure information of the first SID support to be
   compressed according the FSU-Flag.

   Optionally, no matter how FSU-Flag field is set, the headend can use
   reduced SRH that exclude the first SID, to further reduce the cost of
   SRH.

3.1.2.  Indicating the UET Flavor of the Other SIDs

   A new flag is introduced in the Flag field of Segment Sub-TLV
   [I-D.ietf-idr-segment-routing-te-policy] to indicate the UET Flavor
   of each SID (except the first one), as presented in Figure 2.

       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 Flag in Segment sub-TLV





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   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.

   The value set in UET-Flag field need consider two factors: the UET
   domain constructed by the current segment node and the next segment
   node, and whether the structure information of the next SID support
   to be compressed according the UET-Flag.

3.1.3.  Indicating the SID Structure and Truncated Information

   A new SRv6 Segment Truncated sub-TLV is introduced in Segment List
   sub-TLV to provide each SRv6 SID structure and truncated information,
   as presented in Figure 3.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |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 3: SRv6 Segment Truncated sub-TLV format

   where,




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   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 classicalal 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.
   For example, for a 128 bit SID which is compressed to 32 bits, TL is
   32; for a 128 bit SID which is not compressed, TL is 128; for a
   32-bit MPLS label SID, TL is 32.  If TL is less than 128, BL plus TL
   must also be less than or equal to 128.

   If the headend does not recognize the Segment Truncated sub-TLV, the
   entire Segment List sub-TLV MUST be ignored.

3.2.  Option 2: Introducing a new U-Segment List sub-TLV

   For more easy compatibility with older headend devices, a new
   U-Segment List sub-TLV could be defined, which can contain SRv6 SID
   compressed information as defined in Option 1.

   The controller can send U-Segment List sub-TLV with SRv6 SID
   compression information to the headend if the headend has
   U-capability, or Segment List sub-TLV without compression information
   to the headend if the headend has not U-capability.

   The U-Segment List sub-TLV has the same format as Segment List sub-
   TLV, but of different type values.

       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 =TBD   |             Length            |FSU|  RESERVED |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                           sub-TLVs                          //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4: U-Segment List sub-TLV format

   Other extensions are same as Option 1:

      Introduce FSU Flag in U-Segment List sub-TLV to indicate the UET
      Flavor of the first SID.

      Introduce UET Flag in Segment Sub-TLV to indicate the UET Flavor
      of other SIDs.



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      Introduce SRv6 Segment Truncated sub-TLV in U-Segment List sub-TLV
      to indicate the structure and truncated information of each SID.

4.  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 SHOULD 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
   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,



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   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.

5.  Head-end 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 verify if the compression
   result is correct, that is, the UET-Flavor of a certain SID must be
   consistent with the compression result, i.e., TL, of the next SID,
   otherwise the entire Segment List sub-TLV must be ignored.
   Especialy, TL of the first SID could be verified by FSU.

   In particular, the UET-Flavor of the last SID can be used as a clear
   indication to decide which compression type 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.

6.  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].

7.  IANA Considerations

   TBD








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8.  References

8.1.  Normative References

   [I-D.chen-lsr-igp-shorter-srv6-extensions]
              Chen, R. and S. Peng, "IGP Extensions for Shorter SRv6
              SID", draft-chen-lsr-igp-shorter-srv6-extensions-02 (work
              in progress), May 2020.

   [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-08
              (work in progress), April 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-07 (work in progress),
              May 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-15 (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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8.2.  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

























Liu & Peng              Expires November 12, 2020              [Page 12]


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