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Network                                                      C. Weiqiang
Internet-Draft                                              China Mobile
Intended status: Standards Track                               G. Mirsky
Expires: September 15, 2020                                    ZTE Corp.
                                                               P. Shaofu
                                                                L. Aihua
                                                         ZTE Corporation
                                                              W. Xiaolan
                                            New H3C Technologies Co. Ltd
                                                                  C. Wei
                                                                  Centec
                                                                S. Zadok
                                                                Broadcom
                                                          March 14, 2020


          Unified Identifier in IPv6 Segment Routing Networks
                   draft-mirsky-6man-unified-id-sr-06

Abstract

   Segment Routing architecture leverages the paradigm of source
   routing.  It can be realized in a network data plane by prepending
   the packet with a list of instructions, a.k.a. segments.  A segment
   can be encoded as a Multi-Protocol Label Switching (MPLS) label, IPv4
   address, or IPv6 address.  Segment Routing can be applied in MPLS
   data plane by encoding segments in the MPLS label stack.  It also can
   be applied to IPv6 data plane by encoding a list of segment
   identifiers in IPv6 Segment Routing Extension Header (SRH).  This
   document extends the use of the SRH to unified segment identifiers
   encoded, for example, as MPLS label or IPv4 address, to compress the
   SRH, and support more detailed network programming and interworking
   between SR-MPLS and SRv6 domains.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on September 15, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   4
   2.  Segment Routing Extension Header: Benefits and Challenges . .   4
   3.  Unified SIDs in IPv6 Segment Routing Extension Header . . . .   4
   4.  The Use Case of Unified Segment Identifier  . . . . . . . . .   6
     4.1.  Interworking Between SR-MPLS and SRv6 Using U-SID . . . .   6
   5.  Operations with Unified Segment Identifier  . . . . . . . . .   9
     5.1.  Procedures of SR-MPLS over IP . . . . . . . . . . . . . .  10
     5.2.  Packet Forwarding . . . . . . . . . . . . . . . . . . . .  10
   6.  Interworking Between Different UET domain Considerations  . .  12
     6.1.  Interworking Using Binding SID  . . . . . . . . . . . . .  12
     6.2.  Interworking Using Mixing U-SID . . . . . . . . . . . . .  12
       6.2.1.  UET capability Advertisement  . . . . . . . . . . . .  13
       6.2.2.  SRv6 SID Allocated per UET  . . . . . . . . . . . . .  13
       6.2.3.  Packets Forwarding Procedures . . . . . . . . . . . .  15
       6.2.4.  SRH with mixing elements Pseudo-code  . . . . . . . .  18
   7.  Control Plane in Support of Unified SID . . . . . . . . . . .  20
   8.  U-SID supporting SRv6 programming . . . . . . . . . . . . . .  21
   9.  Implementation Considerations . . . . . . . . . . . . . . . .  21
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23






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1.  Introduction

   Segment Routing architecture [RFC8402] leverages the paradigm of
   source routing.  It can be realized in a network data plane by
   prepending the packet with a list of instructions, a.k.a. segment
   identifiers (SIDs).  A segment can be encoded as a Multi-Protocol
   Label Switching (MPLS) label, IPv4 address, or IPv6 address.  Segment
   Routing can be applied in MPLS data plane by encoding 20-bits SIDs in
   MPLS label stack [RFC8660].  It also can be applied to IPv6 data
   plane by encoding a list of 128-bits SIDs in IPv6 Segment Routing
   Extension Header (SRH) [I-D.ietf-6man-segment-routing-header].

   This document extends the use of the SRH
   [I-D.ietf-6man-segment-routing-header] to unified identifiers encoded
   as MPLS label or IPv4 address to support more detailed network
   programming and interworking between SR-MPLS and SRv6 domains.

1.1.  Conventions used in this document

1.1.1.  Terminology

   SR: Segment Routing

   SRH: Segment Routing Extension Header

   MPLS: Multiprotocol Label Switching

   SR-MPLS: Segment Routing using MPLS data plane

   SID: Segment Identifier

   IGP: Interior Gateway Protocol

   DA: Destination Address

   ILM: Incoming Label Map

   FEC: Forwarding Equivalence Class

   FTN: FEC-to-NHLFE map

   OAM: Operation, Administration and Maintenance

   TE: Traffic Engineering

   SRv6: Segment Routing in IPv6

   U-SID: Unified Segment Identifier



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   PSP: Penultimate Segment Popping

   FIB: Forwarding Information Base

1.1.2.  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.

2.  Segment Routing Extension Header: Benefits and Challenges

   Many functions related to Operation, Administration and Maintenance
   (OAM) require identification of the SR tunnel ingress and the path,
   constructed by segments, between the ingress and the egress SR nodes.
   Combination of IPv6 encapsulation [RFC8200] and SRH
   [I-D.ietf-6man-segment-routing-header], referred to as SRv6, comply
   with these requirements while it is challenging when applying SR in
   MPLS networks, also referred to as SR-MPLS.

   On the other hand, the size of IPv6 SID presents a scaling challenge
   to use topological instructions that define strict explicit traffic-
   engineered (TE) path or support network programming in combination
   with service-based instructions.  At the same time, that is where SR-
   MPLS approach provides better results due to smaller SID length.  It
   can be used to compress the SRv6 header size when a smaller namespace
   of available SIDs is sufficient for addressing the particular
   network.

   SR-MPLS is broadly used in metro networks.  With the gradual
   deployment of SRv6 in the core networks, supporting interworking
   between SR-MPLS and SRv6 becomes the necessity for operators.  It is
   operationally more efficient and straightforward if SRv6 can use the
   same size SIDs as in SR-MPLS.  The SRH can be extended to define the
   same as in SR-MPLS SID length to support the unified segment
   identifier (U-SID).  As a result, end-to-end SR tunnel may use U-SIDs
   across SR-MPLS and SRv6 domains.

3.  Unified SIDs in IPv6 Segment Routing Extension Header

   SRH format has been defined in Section 3 of
   [I-D.ietf-6man-segment-routing-header] 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Next Header   |  Hdr Ext Len  | Routing Type  | Segments Left |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Last Entry   |     Flags     |              Tag              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |            Segment List[0] (128 bits IPv6 address)            |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                                                               |
                                     ...
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |            Segment List[n] (128 bits IPv6 address)            |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //                                                             //
       //         Optional Type Length Value objects (variable)       //
       //                                                             //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                           Figure 1: SRH format

   This document defines a new field Size in the SRH Flags field as a
   two-bits field, termed as UET (U-SID Encapsulation Type), with the
   following values:

      0b00 - 128-bits SID, an IPv6 address.

      0b01 - 32-bits SID.  In some environments, the context could be of
      IPv4 address, while in some other cases, it could represent an
      index of list or range of IPv4/IPv6 addresses.  Another
      interpretation of 32-bits SID could be as a complementary element
      of an IPv4/IPv6 prefix.  The setting of the interpretation might
      be done through the control plane based signaling and is outside
      the scope of this document.  If this SID represents a
      complementary part of an IPv4/IPv6 prefix, the original IP address
      can be re-constructed by using, for example, mapping, stitching,
      shifting or translating operation.  Specification of such a
      mechanism is outside the scope of this document.



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      0b10 - 32-bits SID, which includes an MPLS label in the leftmost
      20-bits as displayed in Figure 2.  Information in the Context
      field could be interpreted as a flavor of a particular network
      programming behavior.  Specification of the network programming
      using this type of U-SID is outside the scope of this document.
      [Ed.note.  Replace with a reference to the U-SID network
      programming document.]

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                MPLS Label             |        Context        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: Format of Unified SID with MPLS Label

      0b11 - Reserved for future use.

   This document also introduce a compatible operation on Segment Left
   field, also termed as SRH.SL.  That is, if SRH.UET Flag is 0b00,
   SRH.SL represents the count of 128bits-SID items in SRH, if SRH.UET
   Flag is 0b01 or 0b10, SRH.SL represents the count of 32bits-SID items
   in SRH.

4.  The Use Case of Unified Segment Identifier

   U-SID can be used for interworking between SR-MPLS and SRv6 domains.
   SR-MPLS is often used in a metro network, for example, in the
   backhaul metro network of CMCC.  If the core network uses SRv6, for
   example, the core network of the same operator, U-SID can be used in
   the SRv6 domain to interwork with SR-MPLS in the metro network to
   form an end-to-end tunnel.

4.1.  Interworking Between SR-MPLS and SRv6 Using U-SID

   SR-MPLS uses SR SIDs as MPLS label in MPLS stack, and the SIDs are
   32-bits long.  SRv6 uses SR SIDs as IPv6 extension header in SRH, and
   the SIDs are 128-bits long.

   The U-SID uses the same 32-bits long SIDs in MPLS stack and SRH.
   Thus, four 32-bits long U-SIDs can be placed in the space of a single
   128-bits long header.  The encapsulation is illustrated in Figure 3.









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           +---------+          +----------------------------------+
           |         |          |           IPv6 header            |
           | Ethernet|          +----------------------------------+
           |         |          |           SRH                    |
           +---------+          +----------------------------------+
           |  USID1  |          | USID1  | USID2  | ...   | USID4  |
           +---------+          +----------------------------------+
           |  USID2  |          | USID5  |...     | USIDn | Null   |
           +---------+          +----------------------------------+
           | ...     |          +           Payload                |
           +---------+          +----------------------------------+
           |  USIDn  |
           +---------+
           | Payload |
           +---------+

                Figure 3: 32-bits long U-SIDs Encapsulation

   The SR-MPLS and SRv6 interworking is illustrated in Figure 4.  An
   end-to-end SR tunnel from A to F crosses the SR-MPLS and SRv6
   domains.  The SR-MPLS domain could be using IPv4 or IPv6 address
   family.  The SRv6 border nodes (E/G) receive SR-MPLS packets and
   forward them into the SRv6 domain using an SR-MPLS Binding SID
   [RFC8660].



























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           +-----+           +-----+           +-----+           +-----+
           |  A  +-----------+  B  +-----------+  E  +-----------+  F  |
           +-----+           +--+--+           +--+--+           +--+--+
              |    SR-MPLS      |                 |     SRv6        |
              |                 |                 |                 |
           +-----+           +--+--+           +--+--+           +--+--+
           |  C  |-----------|  D  +-----------+  G  +-----------+  H  |
           +-----+           +-----+           +-----+           +-----+

                                                      +--------------+
                                                      |   Eth(E->G)  |
              +--------------+                        +--------------+
              |   Eth(A->B)  |                        |IPv6 DA:G.intf|
              +--------------+    +--------------+    +--------------+
              |   USID(B)    |    |   Eth(B->E)  |    |SRH           |
              +--------------+    +--------------+    |NH:MPLS   SL:2|
              |   USID(E1)   |    |   USID(E1)   |    |USID(ADJ E->G)|
              +--------------+    +--------------+    |USID(ADJ G->H)|
              |   USID(E2)   |    |   USID(E2)   |    |USID(ADJ H->F)|
              +--------------+    +--------------+    +--------------+
              |Label(service)|    |Label(service)|    |Label(service)|
              +--------------+    +--------------+    +--------------+
              |    Payload   | -> |    Payload   | -> |    Payload   |
              +--------------+    +--------------+    +--------------+


                  Figure 4: SR-MPLS and SRv6 interworking

   The SRv6 edge node E assigns two SIDs, e.g., E1 and E2, E1 is an SR-
   MPLS Node-SID, E2 is an SR-MPLS Binding-SID, which represents an SRv6
   policy (from E to F, via segment list E-G-H-F) with U-SID
   encapsulation.  At the headend A, the end-to-end segment list could
   be B-E1-E2.  Figure 6 demonstrates an example of the packet
   forwarding, where U-SID is an MPLS label.

   The reverse interworking is illustrated in Figure 5.  An end-to-end
   SR tunnel from F to A crosses the SRv6 and SR-MPLS domains.  The SRv6
   border nodes (E/G) receive SRv6 packets and forward them into the SR-
   MPLS domain using an SR-MPLS Binding SID or normal Prefix/Adjacency
   SID.











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           +-----+           +-----+           +-----+           +-----+
           |  A  +-----------+  B  +-----------+  E  +-----------+  F  |
           +-----+           +--+--+           +--+--+           +--+--+
              |    SR-MPLS      |                 |     SRv6        |
              |                 |                 |                 |
           +-----+           +--+--+           +--+--+           +--+--+
           |  C  |-----------|  D  +-----------+  G  +-----------+  H  |
           +-----+           +-----+           +-----+           +-----+

                                                      +--------------+
                                                      |   Eth(F->H)  |
                                                      +--------------+
                                                      |IPv6 DA:H.intf|
                                                      +--------------+
                                                      |SRH           |
                                                      |NH:MPLS   SL:2|
                                                      |USID(ADJ F->H)|
                                  +--------------+    |USID(ADJ H->G)|
                                  |   Eth(E->B)  |    |USID(ADJ G->E)|
              +--------------+    +--------------+    +--------------+
              |   Eth(B->A)  |    |   USID(B)    |    |   USID(B)    |
              +--------------+    +--------------+    +--------------+
              |   USID(A)    |    |   USID(A)    |    |   USID(A)    |
              +--------------+    +--------------+    +--------------+
              |Label(service)|    |Label(service)|    |Label(service)|
              +--------------+    +--------------+    +--------------+
              |    Payload   | <- |    Payload   | <- |    Payload   |
              +--------------+    +--------------+    +--------------+


              Figure 5: SR-MPLS and SRv6 reverse interworking

   The SRv6 edge node F assigns an SR-MPLS Binding-SID F2, which
   represents an SRv6 policy (from F to E, via segment list F-H-G-E)
   with U-SID encapsulation.  At the headend F, the end-to-end segment
   list could be F2-B-A.

5.  Operations with Unified Segment Identifier

   When SRH is used to include 32-bits long U-SIDs, the ingress and
   transit nodes of an SR tunnel act as described in Section 5.1 and
   Section 5.2 of [I-D.ietf-6man-segment-routing-header] respectively.

   If U-SID is used to support interworking between SR-MPLS and SRv6
   domains, it is beneficial that U-SID type matches to an MPLS label.
   In that case, an ILM (Incoming Label Map) entry can be used to map a
   U-SID to an IPv6 address.  As a result, it is not necessary to
   introduce a new type of index-based mapping table.  For ILM entry of



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   Adjacency-SID, the mapping result copied to DA (Destination Address)
   is the remote interface IPv6 address, for ILM entry of Node-SID, the
   mapping result that is copied into DA is a remote node loopback IPv6
   address.

   Operations on an MPLS label of U-SID type are the same as those
   defined in [RFC8663].  However, SR-MPLS over SRH has the following
   advantages compared with SR-MPLS over UDP:

   o  SRH is flexible to extend flags or sub-TLVs for service
      requirements, but UDP not.

   o  Labels in SRH can meet 8 bytes alignment requirements as per
      [RFC8200], but UDP not.

   o  The source address of the SR policy is not discarded, but UDP not.

5.1.  Procedures of SR-MPLS over IP

   Procedures of SR-MPLS over IP of [RFC8663] described how to construct
   an adjusted SR-MPLS FTN (FEC-to-NHLFE map) and ILM entry towards a
   prefix-SID when next-hops are IP-only routers.  The action of FTN and
   ILM entry will steer the packet along an outer tunnel to the
   destination node that has originated the FEC (Forwarding Equivalence
   Class).  UDP header is removed and put again at the each segment
   endpoint.  However, for SR-MPLS over SRH in this document we don't
   try to depend on that adjusted FIB (Forwarding Information Base)
   entry, because there are not any actions needed to get from the FIB
   entry, a traditional ILM entry (maybe without out-label because of
   IP-only next-hop) is enough to get the FEC information, i.e., to map
   a U-SID to an IPv6 address and copy to DA.  Note that an
   implementation can get both FEC and next-hop/interface forwarding
   information from the ILM entry, to avoid extra FIB lookup.  An SRv6
   policy chosen to encapsulate U-SID list within SRH is determined at
   the ingress node of this SRv6 policy, SRH is preserved along the SR
   to egress, though PSP (Penultimate Segment Popping) may be used, that
   is different from SR-MPLS over IP/UDP method [RFC8663], so the source
   address (i.e., the ingress of the SRv6 policy) is not discarded.

5.2.  Packet Forwarding

   U-SID based packet forwarding is similar to the processing described
   in [RFC8663].  But it differs from that in FIB action and segment
   list processing.  For completeness, we repeat the description of
   [RFC8663] with modification as follows.






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      +-----+       +-----+       +-----+        +-----+        +-----+
      |  A  +-------+  B  +-------+  C  +--------+  D  +--------+  H  |
      +-----+       +--+--+       +--+--+        +--+--+        +-----+
                       |             |              |
                       |             |              |
                    +--+--+       +--+--+        +--+--+
                    |  E  +-------+  F  +--------+  G  |
                    +-----+       +-----+        +-----+

           +--------+           +--------+            +--------+
           |IP(A->E)|           |IP(A->G)|            |IP(A->G)|
           +--------+           +--------+            +--------+
           |SRH     |           |SRH     |            |SRH     |(or PSP)
           |  SL:2  |           |  SL:1  |            |  SL:0  |
           |  L(E)  |           |  L(E)  |            |  L(E)  |
           |  L(G)  |           |  L(G)  |            |  L(G)  |
           |  L(H)  |           |  L(H)  |            |  L(H)  |
           +--------+           +--------+            +--------+
           | Packet |   --->    | Packet |      --->  | Packet |
           +--------+           +--------+            +--------+


                    Figure 6: Packet Forwarding Example

   In the example shown in Figure 6, assume that routers A, E, G, and H
   are U-SID capable (i.e., both SR-MPLS and SRv6 capable ) while the
   remaining routers (B, C, D, and F) are only capable of forwarding IP
   packets.  Routers A, E, G, and H advertise their Segment Routing
   related information via IS-IS or OSPF.

   Now assume that router A (the Domain ingress) wants to send a packet
   to router H (the Domain egress) via an SRv6 policy with the explicit
   path {E->G->H}. Router A will impose an MPLS label stack within SRH
   on the packet that corresponds to that explicit path.  Router A
   searches ILM entry by the top label (that indicated router E), get
   the FEC information and next-hop/interface forwarding information, a
   loopback IPv6 address of E, and then copy to DA and sends the packet.
   The value of SRH.SL is 2.

   When the IPv6 packet arrives at router E, router E picks the next
   segment (label) within SRH based on the SRH.SL value of 2, searches
   ILM entry by the next label, get the FEC information and next-hop/
   interface forwarding information, a loopback IPv6 address of G, and
   then copy to DA and sends the packet.  The value of SRH.SL is 1.

   When the IPv6 packet arrives at router G, router G gets the next
   segment (label) within SRH based on the SRH.SL value of 1, looks up
   ILM entry by the next label, gets the FEC information and next-hop/



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   interface forwarding information, a loopback IPv6 address of H, and
   then copies it to IP DA and transmits the packet.  Because the value
   of SRH.SL is 0, the SRH can be removed if the behavior flavor
   codepoint of next segment (label) is set to PSP.

6.  Interworking Between Different UET domain Considerations

6.1.  Interworking Using Binding SID

   In Figure 4 and Figure 5, We have seen a simple interworking solution
   based on Binding SID.

   This document RECOMMOND this method for interworking, because Binding
   SID is very simple to hide the difference U-SID type of different
   domain.  Especially, a headend only with classical SRv6 SRH
   encapsulation capability, i.e., no capability to put multiple short
   U-SIDs to a single 128bits item, will not need to upgrade.

   Altough Binding SID that is allocated for the specific SR policy
   instance will bring more states on some domain boder nodes, the SR
   policy instance itself maybe pre-exist due to other requirements.
   The SR policy is created within each UET domain which is upgraded
   separately.

   In order to do interwork, as mentioned before, an MPLS Binding SID
   could be allocated for an SRv6 policy, used to hide the details of
   UET-0b00 domain (classical SRv6) for a traditional MPLS Label stack.
   Similarly, an SRv6 Binding SID could be allocated for an SR-MPLS
   policy, used to hide the details of UET-0b10 domain for a traditional
   SRv6 SRH.  And, an SRv6 Binding SID allocated for an SRv6 policy
   which enable UET-0b01 compression style, will hide the details of
   UET-0b01 domain for a traditional SRv6 SRH.  There maybe other
   combinations, and not list one by one.

   Note that in some cases, Binding SID will cause multiple SRH to be
   inserted in IPv6 header.

6.2.  Interworking Using Mixing U-SID

   U-SRH can also provide an alternate interworking scheme to support an
   end-to-end SR tunnel or policy using mixing type of U-SIDs, if more
   headend nodes have be upgraded to support encapsulating mixing U-SID
   in SRH.  For example, an SID list could contain some 128bits-SIDs,
   some 32bits-SIDs, some 32bits-Labels.  For this purpose, we need
   explicitly define U-SID types, in other words, the type is just UET.

   The interworking of different UET domain is illustrated in Figure 7.
   An end-to-end SR tunnel or policy from S to D with segment list <X,



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   ABR1, Y, ABR2, Z, D>, crosses the UET-0b00 domain, UET-0b01 domain
   and UET-0b10 domain.  Note that any order of UET domains are also
   possible and similar with the following illustration.


     .....................   ..................    ...................
        :                     : :                  :  :                   :
  +----+     +----+     +----+     +----+     +----+     +----+     +----+
  | S  +-----+ X  +-----+ABR1+-----+ Y  +-----+ABR2+-----+ Y  +-----+ D  |
  +----+     +----+     +----+     +----+     +----+     +----+     +----+
    :                     : :                  :  :                   :
         ......UET(0b00)......   .....UET(0b01)....    .....UET(0b10).....


             Figure 7: Interworking Between Different UET SID

6.2.1.  UET capability Advertisement

   In SRv6 network, each node can configure its UET capability, and
   advertise it to other nodes.  Controller can also collect UET
   capability information of all nodes.  Typical UET capability is:

      UET-0: Support classical 128bits SRv6 SID.

      UET-1: Support shorter 32bits IPv4 address or number.

      UET-2: Support shorter 32bits MPLS label.

      UET-3: Support shorter 16bits number.

   Each node can support one or more than one UET capabilitys.  Refer to
   Figure 7, node S/X/ABR1 can configure to support UET-0 capability,
   node ABR1/Y/ABR2 can configure to support UET-1 capability, and node
   ABR2/Y/D can configure to support UET-2 capability.

6.2.2.  SRv6 SID Allocated per UET

   An SRv6 SID is allocated per UET capability.  In control plane, an
   SRv6 SID is always 128bits, however:

      An SRv6 SID that has UET-0 attribute means it is allocated for
      traditional 128bits encapsulation purpose, that means in SRH the
      next SID is also a 128bits SID.

      An SRv6 SID that has UET-1 attribute means it is allocated for
      32bits IPv4 encapsulation purpose, that means in SRH the next SID
      is a 32bits IPv4 address.




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      An SRv6 SID that has UET-2 attribute means it is allocated for
      32bits MPLS encapsulation purpose, that means in SRH the next SID
      is a 32bits MPLS Label.

      An SRv6 SID that has UET-3 attribute means it is allocated for
      16bits number encapsulation purpose, that means in SRH the next
      SID is a 16bits number.

   For the local SID entry of each SRv6 SID allocated per UET
   capability, it will explicitly give the UET attribute information.

   Each node allocate its SRv6 SID per UET capability, and advertise it
   to other nodes with additional UET-Flavor.  Controller can also
   collect these SIDs used for E2E SID List programming.

   In order to save label resources, MPLS label is not allocated per
   UET.  The UET information can be directly inserted in the context
   field of the label item in SRH.

   For example, refer to Figure 7, each node allocate the following SRv6
   SID per UET.

      Node S: 128bits-END-SID-S for UET-0.

      NOde X: 128bits-END-SID-X for UET-0.

      NOde ABR1: 128bits-END-SID-ABR1 for UET-0, and 128bits-END-SID-
      ABR1' for UET-1.

      NOde Y: 128bits-END-SID-Y for UET-1.

      NOde ABR2: 128bits-END-SID-ABR2 for UET-1, and 128bits-END-SID-
      ABR2' for UET-2.

      NOde Z: 32bits-PREFIX-SID-Z.  Note that MPLS Label allocation is
      independent with UET.

      NOde D: 32bits-PREFIX-SID-D.  Note that MPLS Label allocation is
      independent with UET.

   Note that the above SRv6 SID itself is always a 128bits IPv6 address,
   no relationship with its UET attribute.  The UET attribute indicate
   the next SID type, i.e., 128bits classical SID, 32bits IPv4 address,
   or 32bits MPLS Label, etc.







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6.2.3.  Packets Forwarding Procedures

   Controller compute an E2E segment list <X, ABR1, Y, ABR2, Z, D>.

   Controller knows that ABR1 and ABR2 are the border nodes between
   different UET domain for the above segment list.

   So, the SID list informed to headend is:

      No.1 SID: 128bits-END-SID-X (with UET-0 Flag, and BL|NBL info)

      No.2 SID: 128bits-END-SID-ABR1' (with UET-1 Flag, and BL|NBL info)

      No.3 SID: 128bits-END-SID-Y (with UET-1 Flag, and BL|NBL info)

      No.4 SID: 128bits-END-SID-ABR2' (with UET-2 Flag, and BL|NBL info)

      No.5 SID: 32bits-PREFIX-SID-Z, (with UET-2 Flag)

      No.6 SID: 32bits-PREFIX-SID-D, (with UET-0 Flag)

   Note: BL is Block Length, NBL is non-Block Length.

   The headend analysis how to get the compressed SID List.

   The No.1 SID, 128bits-END-SID-X, has UET-0 Flag, so keep 128bits.

   The No.1 SID, 128bits-END-SID-X, has UET-0 Flag, that means the next
   SID, 128bits-END-SID-ABR1', need also keep 128bits.

   The No.2 SID, 128bits-END-SID-ABR1' has UET-1 Flag, that means the
   next SID, 128bits-END-SID-Y, need to be compressed as 32bits IPv4
   address.

   The No.3 SID, 128bits-END-SID-Y, has UET-1 Flag, that means the next
   SID, 128bits-END-SID-ABR2', need to be compressed as 32bits IPv4
   address.

   The No.4 SID, 128bits-END-SID-ABR2', has UET-2 Flag, that means the
   next SID, 32bits-PREFIX-SID-Z, need keep 32bits.

   The No.5 SID, 32bits-PREFIX-SID-Z, has UET-2 Flag, that means the
   next SID, 32bits-PREFIX-SID-D, need keep 32bits.

   The No.6 SID, 32bits-PREFIX-SID-D, has UET-0 Flag, that means the
   next SID, maybe a VPN service SRv6 SID, need keep 128bits.

   So, the headend can get the following compressed SID List:



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      128bits-END-SID-X for UET-0

      128bits-END-SID-ABR1' for UET-1

      32bits of 128bits-END-SID-Y for UET-1

      32bits of 128bits-END-SID-ABR2' for UET-2

      32bits-PREFIX-SID-Z (with UET-2 in context.field)

      32bits-PREFIX-SID-D (with UET-0 in context.field)

   At headend, the encapsulated SRH could be:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Next Header   |  Hdr Ext Len  | Routing Type  | Segments Left |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Last Entry   |   Flags |UET| |              Tag              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~             128bits VPN-SID                                   ~ [0]
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
    |             32bits-PREFIX-SID-D (with UET-0 in context.field) |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             32bits-PREFIX-SID-Z (with UET-2 in context.field) |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ [1]
    |             32bits of 128bits-END-SID-ABR2' for UET-2         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             32bits of 128bits-END-SID-Y for UET-1             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
    ~             128bits-END-SID-ABR1' for UET-1                   ~ [2]
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
    ~             128bits-END-SID-X for UET-0                       ~ [3]
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                 Figure 8: SRH including Different UET SID

   The initial SRH.SL is set to 4, that is the count of 128bits based
   SIDs in SRH, and the initial SRH.UET is set to 0b00, that represent
   the first UET domain.




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   During the process of packets passing through multiple UET domains,
   if SRH.UET change from 0b00 to 0b01 or 0b10, SRH.SL will quadruple,
   i.e., SRH.SL = SRH.SL * 4, that is the count of 32bits based SIDs in
   SRH.  When SRH.UET changed from 0b01 or 0b10 to 0b00, SRH.SL will
   revert to its original size, i.e., SRH.SL = SRH.SL / 4, that is the
   count of 128bits based SIDs in SRH.

   Refer to Figure 7, next we will describe the process of packets
   passing through each UET domain.

   At the headend S, when packets sent to the No.1 segment node X, it
   will decrement SRH.SL by 1, get the first 128bits SID from
   SRH.List[], 128bits-END-SID-X for UET-0, copy to DA, and lookup FIB
   to send packets.  At this time, SRH.SL is 3, SRH.UET is 0b00.

   At the No.1 segment node X, the local SID matched by DA has UET-0
   attribute, that is, SRH.UET has no change.  It will continue to
   decrement SRH.SL by 1, get the next 128bits SID from SRH.List[],
   128bits-END-SID-ABR1' for UET-1, copy to DA, and lookup FIB to send
   packets.  At this time, SRH.SL is 2, SRH.UET is 0b00.

   At the No.2 segment node ABR1, the local SID matched by DA has UET-1
   attribute, that is, SRH.UET has changed from UET-0 to UET-1.  It will
   firstly let SRH.SL * 4, then decrement SRH.SL by 1, get the next
   32bits SID from SRH.List[], 32bits of 128bits-END-SID-Y for UET-1,
   convert it to a complete IPv6 SID, copy to DA, and lookup FIB to send
   packets.  At this time, SRH.SL is 7, SRH.UET is 0b01.

   At the No.3 segment node Y, the local SID matched by DA has UET-1
   attribute, that is, SRH.UET has no change.  It will continue to
   decrement SRH.SL by 1, get the next 32bits SID from SRH.List[],
   32bits of 128bits-END-SID-ABR2' for UET-2, convert it to a complete
   IPv6 SID, copy to DA, and lookup FIB to send packets.  At this time,
   SRH.SL is 6, SRH.UET is 0b01.

   At the No.4 segment node ABR2, the local SID matched by DA has UET-2
   attribute, that is, SRH.UET has changed from UET-1 to UET-2.  Because
   the size of SID has no change, it will continue to decrement SRH.SL
   by 1, get the next 32bits SID from SRH.List[], 32bits-PREFIX-SID-Z
   (with UET-2 in context.field), map it to a complete IPv6 prefix FEC
   by ILM entry, copy to DA, and lookup FIB (or directly get forwarding
   information from ILM entry) to send packets.  Note that the UET
   information in context.field need update to SRH.UET again and see if
   it changes, no change at this time, so there is no additional
   processing.  At this time, SRH.SL is 5, SRH.UET is 0b02.

   At the No.5 segment node Z, the normal address route entry matched by
   DA has not any further UET attribute, that is, SRH.UET has no change.



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   It will continue to decrement SRH.SL by 1, get the next 32bits SID
   from SRH.List[], 32bits-PREFIX-SID-D (with UET-0 in context.field),
   map it to a complete IPv6 prefix FEC by ILM entry, copy to DA, and
   lookup FIB (or directly get forwarding information from ILM entry) to
   send packets.  Note that the UET information in context.field need
   update to SRH.UET again and see if it changes, it is changed from
   UET-2 to UET-0, so SRH.SL will be revert to its original size, i.e.,
   let SRH.SL / 4.  At this time, SRH.SL is 1, SRH.UET is 0b00.

   At the No.6 segment node D, the normal address route entry matched by
   DA has not any further UET attribute, that is, SRH.UET has no change.
   It will continue to decrement SRH.SL by 1, get the next 128bits SID
   from SRH.List[], 128bits VPN-SID, and follow the rest process
   described in [I-D.ietf-spring-srv6-network-programming].

6.2.4.  SRH with mixing elements Pseudo-code

   Processing of SRH with mixing elements is demonstrated in the
   following pseudo-code:

   Headend sending packet:






























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S01. set initial SRH.UET, respond to the UET type of the first SID;
S02. set initial SRH.SL, it is the count of 128bits-based SID;
S03. if (SRH.UET == 0b00) {
S04.   SRH.SL --;
S05.   Get SRH.List[SRH.SL], 128bits, copy to IPv6 Header DA; Or, headend
       know the first SID before SRH encapsulation, just copy it to DA.
S06.   FIB lookup according to DA, and forward packet;
S07. }
S08. else if (SRH.UET == 0b01) {
S09.   SRH.SL =  SRH.SL * 4;
S10.   SRH.SL --;
S11.   Get SRH.List[SRH.SL], 32bits, convert to 128bits SRv6 SID, copy to
       IPv6 Header DA; Or, headend know the first SID before SRH
           encapsulation, just copy it to DA;
S12.   FIB lookup according to DA, and forward packet;
S13. }
S14. else if (SRH.UET == 0b10) {
S15.   SRH.SL =  SRH.SL * 4;
S16.   SRH.SL --;
S17.   Get SRH.List[SRH.SL], 32bits, lookup ILM entry and map it to 128
       IPv6 address, copy it to IPv6 Header DA; Or, headend know the first
       SID before SRH encapsulation, just copy it to DA;
S18.   FIB lookup according to DA, or, directly get forwarding information
       from ILM entry, and forward packet;
S19. }


   Transit/Egress receive packets:























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S01. If DA matched local SID entry, copy the UET attr of local SID entry
     to SRH.UET, check when SRH.UET changed from 0b00 to 0b01 or 0b10,
         SRH.SL*4, when from 0b01 or 0b10 to 0b00, SRH.SL / 4; Else If DA
         matched normal address route entry, SRH.UET no update;
S02. if (SRH.SL == 0) {
S03.   process the inner payload;
S04. }
S05. else {
S06.   if (SRH.UET == 0b00) {
S07.     SRH.SL -- ;
S08.     Get SRH.List[SRH.SL], 128bits, copy it to IPv6 Header DA;
S09.     FIB lookup according to DA, and forward packet;
S10.   }
S11.   else if (SRH.UET == 0b01) {
S12.     SRH.SL -- ;
S13.     Get SRH.List[SRH.SL], 32bits, convert to 128bits SRv6 SID, copy
         to IPv6 Header DA;
S14.     FIB lookup according to DA, and forward packet;
S15.   }
S16.   else if (SRH.UET == 0b10) {
S17.     SRH.SL --
S18.     Get SRH.List[SRH.SL], 32bits, lookup ILM entry, map it to 128bits
         IPv6 address, copy it to IPv6 Header DA;
S19.     Get UET info from SRH.List[SRH.SL] Context Field, copy it to
         SRH.UET. Check if  SRH.UET changed from 0b10 to 0b00, SRH.SL / 4;
S20.     FIB lookup according to DA, or, directly get forwarding
         information from ILM entry, and forward packet;
S21.   }
S22. }


7.  Control Plane in Support of Unified SID

   The introduction of the Unified Identifier may rely on the existing
   SR extensions to the routing protocols.  But some enhancements in the
   control plane are still required.  This section references to the
   existing protocols and identifies necessary extensions.

   Each node in the SRv6 domain need advertise its U-SID Encapsulation
   capability, this information can be carried within SRv6-Capabilities
   sub-TLV defined in [I-D.ietf-lsr-isis-srv6-extensions] and SRv6
   Capabilities TLV defined in [I-D.ietf-lsr-ospfv3-srv6-extensions].
   It need also allocate SRv6 SID (Topology type and Service Function
   type) per UET and advertise to other nodes, the advertisement of SRv6
   END SID, END.X SID, LAN END.X SID defined in
   [I-D.ietf-lsr-isis-srv6-extensions] and
   [I-D.ietf-lsr-ospfv3-srv6-extensions] need to be extended to carry
   UET-Flavor information.  These information can be collected and sent



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   to central controller through BGP-LS.  Then, controller can send the
   computed segment list to the headend through BGP or PCEP, and each
   segment will include an explicit UET information.  All these will be
   defined in other documents.

   The SR-MPLS extensions to Interior Gateway Protocols (IGP), IS-IS
   [RFC8667], OSPF [RFC8665], and OSPFv3 [RFC8666], defined how 20-bits
   and 32-bits SIDs advertised and bound to SR objects and/or
   instructions.  Extensions to BGP Link-state address family
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] enabled propagation of
   segment information of variable length via BGP.  The existed SR-MPLS
   extensions can be used to get MPLS U-SID mapping FIB entry, and it
   can coexist with SRv6 extensions to the same IGP/BGP-LS instance.
   For simplicity purpose, tihs document suggest to use the existed
   mature SR-MPLS control plane and FIB entry to server the MPLS U-SID
   advertisement and mapping entry.  However, it is possible to based on
   SRv6 related TLVs/sub-TLVs to advertise the MPLS U-SID, and that will
   be discussed in another document.

8.  U-SID supporting SRv6 programming

   U-SID can support SRv6 programming defined by
   [I-D.ietf-spring-srv6-network-programming].  The details will be
   described in another document.

9.  Implementation Considerations

   The Unified SID solution has been already implemented and tested by
   two companies:

   o  Centec has conducted its PoC, and the report is available at
      https://cloud.tencent.com/developer/article/1540023.

   o  Broadcom, in its lab, also conducted PoC testing of the U-SID
      solution.

10.  IANA Considerations

   IANA is requested to allocate from the Segment Routing Header Flags
   registry the two-bits long field referred to as Size.

11.  Security Considerations

   This specification inherits all security considerations of [RFC8402]
   and [I-D.ietf-6man-segment-routing-header].






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12.  Acknowledgements

   TBD

13.  Normative References

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-26 (work in
              progress), October 2019.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
              (work in progress), June 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-06
              (work in progress), March 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-00 (work in progress), February
              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-13 (work in
              progress), March 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>.

   [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>.







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   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8663]  Xu, X., Bryant, S., Farrel, A., Hassan, S., Henderickx,
              W., and Z. Li, "MPLS Segment Routing over IP", RFC 8663,
              DOI 10.17487/RFC8663, December 2019,
              <https://www.rfc-editor.org/info/rfc8663>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8666]  Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
              for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
              December 2019, <https://www.rfc-editor.org/info/rfc8666>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

Authors' Addresses

   Cheng Weiqiang
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com






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   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com


   Peng Shaofu
   ZTE Corporation
   No.50 Software Avenue, Yuhuatai District
   Nanjing
   China

   Email: peng.shaofu@zte.com.cn


   Liu Aihua
   ZTE Corporation
   Zhongxing Industrial Park, Nanshan District
   Shenzhen
   China

   Email: liu.aihua@zte.com.cn


   Wan Xiaolan
   New H3C Technologies Co. Ltd
   No.8, Yongjia Road, Haidian District
   Beijing
   China

   Email: wxlan@h3c.com


   Cheng Wei
   Centec
   Building B, No.5 Xing Han Street, Suzhou Industrial Park
   Suzhou
   China

   Email: Chengw@centecnetworks.com


   S.Zadok
   Broadcom
   Israel

   Email: shay.zadok@broadcom.com




Weiqiang, et al.       Expires September 15, 2020              [Page 24]


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