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Versions: 00

SPRING Working Group                                               Z. Li
Internet-Draft                                                     C. Li
Intended status: Standards Track                     Huawei Technologies
Expires: August 14, 2020                                        W. Cheng
                                                            China Mobile
                                                                  C. Xie
                                                                   C. Li
                                                           China Telecom
                                                                 H. Tian
                                                                 F. Zhao
                                                                   CAICT
                                                       February 11, 2020


                   Generalized Segment Routing Header
                    draft-lc-6man-generalized-srh-00

Abstract

   Generalized SRv6 network programming defines the enhanced mechanisms
   of SRv6 to encode SRv6 SIDs, Compressed SIDs and even the MPLS labels
   or IPv4 tunnel information in a single SRH.  This type of SRH is
   called Generalized SRH (G-SRH), which can reduce the overhead of SRv6
   and also provide more flexibility for network programming.  This
   document defines the encapsulation and packet processing of G-SRH.

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."

   This Internet-Draft will expire on August 14, 2020.

Copyright Notice

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




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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  Generalized Segment Identifier  . . . . . . . . . . . . . . .   4
     3.1.  SRv6 G-SID  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Compression G-SID . . . . . . . . . . . . . . . . . . . .   4
     3.3.  MPLS G-SID  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  IPv4 G-SID  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Generalized Segment Routing Header (G-SRH)  . . . . . . . . .   7
     4.1.  G-SRH for SRv6 Compression  . . . . . . . . . . . . . . .   8
     4.2.  G-SRH for Crossing SR-MPLS Domains  . . . . . . . . . . .  10
     4.3.  G-SRH for Crossing IPv4 Domains . . . . . . . . . . . . .  11
   5.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Crossing SRv6 Compression Domains . . . . . . . . . . . .  14
     5.2.  Crossing SR-MPLS Domains  . . . . . . . . . . . . . . . .  17
       5.2.1.  END.M . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.3.  Crossing IPv4 Domains . . . . . . . . . . . . . . . . . .  17
       5.3.1.  END.4 . . . . . . . . . . . . . . . . . . . . . . . .  17
       5.3.2.  END.B4  . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node by inserting an ordered list of instructions, called segments.

   When segment routing is deployed on the IPv6 data plane, it is called
   SRv6 [I-D.ietf-6man-segment-routing-header].  For support of SR, a



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   new routing header called Segment Routing Header (SRH), which
   contains a list of SIDs and other information, has been defined in
   [I-D.ietf-6man-segment-routing-header].  In use cases like Traffic
   Engineering, an ordered SID List with multiple SIDs is inserted into
   the SRH to steer packets along an explicit path.

   The overhead of SIDs (16 bytes per SID) in SRH proposes challenges
   for packet processing and payload efficiency.
   [I-D.li-spring-compressed-srv6-np] proposes a mechanism for reducing
   the overhead of SRv6 by using the Compressed SID (C-SID) in SRH.
   However it requires all the middle segment endpoint nodes to support
   compression.

   [I-D.cl-spring-generalized-srv6-np] proposes Generalized Segment
   Routing over IPv6 (G-SRv6) Networking Programming, which supports to
   encode multiple types of Segments in an SRH, called Generalized
   SRH(G-SRH).  These segments can be called Generalized Segment, and
   the ID can be Generalized Segment Identifier(G-SID), which may
   include an SRv6 SID(128 bits), C-SIDs, MPLS labels, or IPv4 tunnel
   information.  Therefore, G-SRv6 does not require all the nodes along
   the path to be compression capable, which supports incremental
   deployment, and brings benefits to the networks.  Moreover it
   provides more flexibilities for network programming.  This document
   defines the encapsulation and packet processing of Generalized SRH.

2.  Terminology

   This document makes use of the terms defined in
   [I-D.ietf-6man-segment-routing-header], [RFC8402] and [RFC8200], and
   the reader is assumed to be familiar with that terminology.  This
   document introduces the following new terms:

   C-SRH: Compressed Segment Routing Header

   C-SID: Compressed Segment Identifier

   G-SRH: Generalized Segment Routing Header

   G-SID: Generalized Segment Identifier.

2.1.  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.




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3.  Generalized Segment Identifier

   A G-SID is a 128 bits value, and it can be used for carry multiple
   types of Segment ID or segment information, so may contain:

   o  an SRv6 SID

   o  a compression G-SID

   o  an MPLS G-SID

   o  an IPv4 G-SID

3.1.  SRv6 G-SID

   SRv6 SID is a type of G-SID.  A G-SID contains a single SRv6 SID, and
   does not change anything of SRv6 SID.

3.2.  Compression G-SID

   As per [I-D.li-spring-compressed-srv6-np], a C-SID is a sub-set of a
   compressable SRv6 SID, which can be used for routing the packets in
   compressed SRv6 network programming.  The format of compressable SID
   and C-SID is shown below.  The argument part is specified by use
   cases, and it is zero by default.  It is shared by the compressable
   SRv6 SIDs in a C-SRv6 sub-path.

    0         Variable Length            32 bits                128 bits
    +--------------------------------------------------------------+
    |             Common Prefix   |  C-SID         | Argument      |
    +--------------------------------------------------------------+
    |<-------- Locator ---------------->|<-FuncID->|<--Argument -->|
                                  |<--->|
                                     |
                          Different part of Locator

             Figure 1. 32 bits C-SID in Compressable SRv6 SID


   A compression G-SID shown in Figure 2 may contain several C-SIDs and
   optional padding. when C-SID is a 32 bits value, a compression G-SID
   can consist of 4 C-SIDs.  If the length of C-SIDs in a G-SID is less
   than 128 bits, then padding is required.








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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         C-SID 0                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         C-SID 1                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         C-SID 2                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         C-SID 3                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   (a)


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Padding                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Padding                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         C-SID 0                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         C-SID 1                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   (b)

                        Figure 2. Compression G-SID


3.3.  MPLS G-SID

   An MPLS G-SID shown in Figure 3 can contain 4 MPLS Label
   Encapsulations, if number of MPLS Label Encapsulations is less than
   4, then padding is required in G-SID to align with 128 bits.
















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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Label 0                     |  TC |S|     TTL       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Label 1                     |  TC |S|     TTL       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Label 2                     |  TC |S|     TTL       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Label 3                     |  TC |S|     TTL       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   (a)


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Padding                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Padding                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Label 0                     |  TC |1|     TTL       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Label 1                     |  TC |S|     TTL       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   (b)

                        Figure 3. MPLS G-SID


3.4.  IPv4 G-SID

   An IPv4 G-SID shown in Figure 4 can contain 128 bits information of
   IPv4 tunnel.  The format of the IPv4 G-SID is shown below.

















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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  |               Tunnel Parameters                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       IPv4 Src Address                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       IPv4 Dest Address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Tunnel Parameters                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4. IPv4 G-SID


4.  Generalized Segment Routing Header (G-SRH)

   Generalized Segment Routing Header (G-SRH) is an enhancement of the
   SRH, and it supports to encode different types of G-SIDs in a single
   SRH.  The format of G-SRH is shown as below.

        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   |     Flag  |CL |              Tag              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |        Generalized Segment List[0] (128 bits value)           |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
                                  ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |        Generalized Segment List[n] (128 bits value)           |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //                                                             //
       //         Optional Type Length Value objects (variable)       //
       //                                                             //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5. Generalized Segment Routing Header




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   Where:

   o  Next Header, Hdr Ext Len, Routing Type, Segments Left, Last Entry
      and Tag are the same as those defined in
      [I-D.ietf-6man-segment-routing-header].

   o  CL (C-SID Left): 2-bit unsigned integer to indicate the index of
      C-SID in the Compression G-SID.  There can be four 32-bit C-SIDs
      within a Compression G-SID.

   o  Segment List[n]: 128-bit G-SID representing the nth Generalized
      Segment in the Segment List.  In G-SRH, the Generalized Segment
      can be an SRv6 SID, Compression G-SID, MPLS G-SID, or IPv4 G-SID,
      etc.

   o  Type Length Value (TLV): It is the same as that defined in
      [I-D.ietf-6man-segment-routing-header].

4.1.  G-SRH for SRv6 Compression

   When an SRv6 path travels normal SRv6 domains and compressed SRv6
   domains, the SRv6 SID and Compression G-SID(containing C-SIDs) can be
   encoded in a single G-SRH.  For easier understanding, this document
   assumes that the compressable consists of 96 bits common prefix and
   32 bits C-SID, while actually the length of common prefix can be
   configured as less than 96 to fit the address planning of the
   networks.  The encoding can be shown as follows.

     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   |    Flag   |CL |               Tag             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |        Generalized Segment List[0] (128 bits value)           |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                            ...                              .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    |                      Optional Padding                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    Compression
    |                          C-SID [0]                            |      G-SID 0
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                           ...                                 .        ...



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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          C-SID 3                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    |                          C-SID 0                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          C-SID 1                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    Compression
    |                          C-SID 2                              |      G-SID k
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          C-SID 3                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    |                       Common Prefix                           |
    |                                                               |    Compressable
    |                                                               |      SRv6 SID
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         C-SID 0                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         ---
    |                                                               |
    |        Generalized Segment List[n1] (128 bits value)          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
                               ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |        Generalized Segment List[n] (128 bits SRv6 SID)        |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6. G-SRH for SRv6 Compression


   Where:

   o  Common Prefix: 96-bit value for the common prefix shared by a
      group of Compressable SRv6 SIDs.  Operators are free to configure
      the length and the value of the common prefix based on the address
      planning of their networking.  In a Compressable SRv6 SID, when
      the length of Common prefix is less than 96, and the length of the
      C-SID is less than 32 bits, the the rest bits can be used as
      arguments.




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   o  C-SID: 32-bit compressed SID of a Compressable SRv6 SID.

   o  Padding: Must be zero.  When the length of C-SIDs within G-SID is
      less than 128 bits within a compression G-SID, then padding is
      needed.

4.2.  G-SRH for Crossing SR-MPLS Domains

   The G-SRH support to encode the MPLS labels and SRv6 SID or
   Compression G-SID within a G-SRH.  The G-SRv6 path for crossing SR-
   MPLS domains can be encoded in the G-SRH as follows:

     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   |     Flag  |CL |               Tag             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |         Generalized Segment List[0] (128 bits value)          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                              ...                              .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       ---
    |                      Optional Padding                         |      MPLS
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     G-SID 0
    |           Label 0                     |  TC |S|     TTL       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                              ...                              .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Label 3                     |  TC |S|     TTL       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       ---
    |           Label 0                     |  TC |S|     TTL       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Label 1                     |  TC |S|     TTL       |       MPLS
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      G-SID n
    |           Label 2                     |  TC |S|     TTL       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Label 3                     |  TC |S|     TTL       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    |                                                               |
    |         Generalized Segment List[n2] (128 bits value)         |       End.M
    |                                                               |      SRv6 SID
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---



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    |                                                               |
                               ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |         Generalized Segment List[n2] (128 bits value)         |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 7. G-SRH for Crossing SR-MPLS Domains


   Where:

   o  Label: 32-bit MPLS label.

   o  TC: 3-bit value for traffic class of MPLS label encapsulation.

   o  S: 1-bit flag to indicate the bottom of MPLS label stack.  It can
      be used for indicating the end of the MPLS label stack.

   o  TTL: 8-bit value for TTL of MPLS label encapsulation.

   o  Padding: When the number of MPLS labels is less than 4 in an MPLS
      G-SID, then padding is required to align with 128 bits.

4.3.  G-SRH for Crossing IPv4 Domains

   The G-SRv6 path for crossing IPv4 domains can be encoded in the G-SRH
   as follows:

















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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   |     Flag   |CL|              Tag              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |         Generalized Segment List[0] (128 bits value)          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
                               ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |         Generalized Segment List[n3] (128 bits value)         |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    | Type  |               Tunnel Parameters                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       IPv4 Src Address                        |       IPv4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       G-SID
    |                       IPv4 Dest Address                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Tunnel Parameters                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    |                                                               |
    |         Generalized Segment List[n1] (128 bits vlaue)         |       End.4
    |                                                               |      SRv6 SID
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        ---
    |                                                               |
                               ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |        Generalized Segment List[n2] (128 bits value)       |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 8. G-SRH for Crossing IPv4 Domains




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   Where:

   o  Type: 4-bit value to indicate the types of IPv4 tunnel.

   o  IPv4 Src Address: 32-bit value for the source address of the IPv4
      tunnel.

   o  IPv4 Dest Address: 32-bit value for the destination address of the
      IPv4 tunnel.

   o  Tunnel Parameters: tunnel parameters of the IPv4 tunnel except the
      source address and destination address.

   This document defines the encoding for IPv4 UDP tunnel and VxLAN
   tunnel.

   The IPv4 UDP tunnel information encapsulated in the G-SRH is as
   follows:

        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  |                   Reserved                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       IPv4 Src Address                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       IPv4 Dest Address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Src Port          |        Dest Port              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 9. G-SID for IPv4 UDP Tunnel


   Where,

   o  Reserved: It is the reserved field and MUST be set as 0.

   o  Src Port: 16-bit value for the source port of the IPv4 UDP tunnel.

   o  Dest Port: 16-bit value for the destination port of the IPv4 UDP
      tunnel.

   The IPv4 VXLAN tunnel information encapsulated in the G-SRH is as
   follows:






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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  | Resv  |                  VN ID                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       IPv4 Src Address                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       IPv4 Dest Address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Src Port          |        Dest Port              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 10. G-SID for IPv4 VXLAN Tunnel


   Where,

   o  Resv: It is the reserved field and MUST be set as 0.

   o  VN ID: 24-bit value for the VN ID of the IPv4 VXLAN tunnel.

   o  Src Port: 16-bit value for the source port of the IPv4 VXLAN
      tunnel.

   o  Dest Port: 16-bit value for the destination port of the IPv4 VXLAN
      tunnel.

   The other types of IPv4 tunnel information will be described in the
   future.

5.  Packet Processing

   This section describes the packet processing of G-SRH.

5.1.  Crossing SRv6 Compression Domains

   There are several cases when encoding the SRv6 SID and Compressable
   SRv6 SID in a single G-SRH.

   o  SRv6 path

   o  Compressed SRv6 path

   o  SRv6 path -> Compressed SRv6 path

   o  Compressed SRv6 path -> SRv6 path

   o  Combination of the above options.



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   The ingress node of G-SRv6 encodes the G-SID list in G-SRH based on
   the type of path and the order of SIDs.

   If the beginning of the G-SRv6 path is a Compressed SRv6 sub-path,
   the CL MUST be initiated accordingly.

   If the first Compressable SRv6 SID is inserted in the G-SRH, then the
   CL MUST be set as 0.  If the first Compressable SRv6 SID is reduced,
   the the CL MUST set to 3, which points to the first C-SID in the
   first G-SID.

   If the beginning of the G-SRv6 path is a SRv6 sub-path, and there is
   any compressable SRv6 SID in the list, the CL MUST be set to 0.

   If the G-SRv6 path is a SRv6 path , then the CL SHOULD be set to 0,
   and it MUST be ignored in G-SRH processing.

   When a compression G-SID is encoded in the G-SRH, new actions should
   be defined for processing it.  When the SL >0, the pseudo code is
   shown below:

  01. If ipv6 DA is a normal local SRv6 SID,                    //Ref1
  02.     SL- -;  DA = SRH [SL];
  03.     Process the packet based on the type of SID
  04.     Forward the packet to the new DA;
  05. if ipv6 DA is a local Compressable SRv6 SID                //Ref2
  06.     If CL = 0
  07.          SL- -; CL = 3;
  08.          DA[CP..CP+31] = SRH[SL][CL] ;
  09.     Else
  10.          CL --;
  11.          DA[CP..CP+31] = SRH[SL][CL]
  12.     Process the packet based on the type of SID
  13.     Forward the packet to the new DA;
  14. if ipv6 DA is a local Compressable SRv6 SID with EOC flavor //Ref3
  15.     SL --
  16.     CL = 0
  17.     DA = SRH[SL]
  18.     Process the packet based on the type of SID
  19.     Forward the packet to the new DA
  20. else
  21.     other G-SIDs processing;


   o  Ref1: nothing is changed when the IPv6 DA is an SRv6 SID, the SID
      will be processed as per
      [I-D.ietf-spring-srv6-network-programming], so G-SRH is compatible
      with SRH.



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   o  Ref2: when the IPv6 DA is a local compressable SRv6 SID, the node
      should process it based on the type of SID and update the IPv6 DA
      based on the value of CL.  If the CL is 0, the next C-SID should
      be selected from the first C-SID in the next compression G-SID and
      updated to the IPv6 DA.  If the CL is greater than 0, then the
      next C-SID in this G-SID should be updated to the IPv6 DA.

   o  Ref3: If the IPv6 DA is a local Compressable SRv6 SID with EOC
      flavor, means this is the last C-SID of the C-SIDs list of this
      compression SRv6 sub-path.  The next G-SID should be updated to
      the IPv6 DA, and the CL is set to 0 in order to help the process
      of the next Compression SRv6 SID.

   When SL = 0, the pseudo code is shown below.

  01. If ipv6 DA is a local SRv6 SID                             //Ref1
  02.     SRv6 processing based on the type of SID
  03. If ipv6 DA is a local compressable SID                     //Ref2
  04.     If CL = 0
  05.         SRv6 processing based on the type of SID
  06.     Else
  07.         CL --;
  08.         DA[CP..CP+31] = SRH[SL][CL];
  09.         Process the packet based on the type of SID
  09.         Forward the packet to the new DA;
  10. If ipv6 DA is a local compressable SID with EOC Flavor     //Ref 3
  11.     Process the packet based on the type of SID
  12. Else
  13.     Other G-SIDs processing;



   o  Ref1: nothing is changed when the IPv6 DA is an SRv6 SID, the SID
      will be processed as per
      [I-D.ietf-spring-srv6-network-programming], so G-SRH is compatible
      with SRH.

   o  Ref2: when the IPv6 DA is a local compressable SRv6 SID, the node
      should process it based on the type of SID and update the IPv6 DA
      based on the value of CL.  If the CL is 0, meaning this C-SID is
      the last segment, the node should process the SID and the G-SID
      processing should be finished.  If the CL is greater than 0, the
      next C-SID in the G-SID should be updated to the IPv6 DA.

   o  Ref3: If the IPv6 DA is a local Compressable SRv6 SID with EOC
      flavor, meaning this is the last C-SID in the C-SIDs list of this
      compression SRv6 sub-path, the node should process the SID and the
      G-SID processing should be finished.



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5.2.  Crossing SR-MPLS Domains

   In some scenarios such as SD-WAN, an SRv6 packet may cross MPLS
   networking.  Usually, the packet can be steered into an SR-MPLS
   policy based on END.BM SID
   [I-D.ietf-spring-srv6-network-programming].  For reducing
   configuration in the middle nodes, this document proposes END.M (End
   function with SR-MPLS path instantiation) to indicate the SR-MPLS/
   MPLS path by the MPLS label stack carried after it.

5.2.1.  END.M

   An End.M SID indicates the beginning of the SR-MPLS SID list/MPLS
   Label stack, and the S-bit in MPLS label indicates the end of the
   MPLS label stack.

   When a node processes an END.M SID, it copies the SR-MPLS SID list/
   MPLS labels to the outer MPLS header, and updates the SL pointing to
   the next G-SID after these MPLS G-SIDs, set the IPv6 DA as the
   G-SRH[SL], and then forwards the packet based on the active MPLS
   label.

   The pesudo code is shown below:

    01. Copy the MPLS labels to MPLS header                //Ref1
    02. Update the SL point to next G-SID after the MPLS G-SID
    03. Set the IPv6 DA as the SRH[SL]
    04. Forward the packet based on the active MPLS label

   Ref1.  The Node will copy the MPLS labels within the MPLS G-SID to
   the outer MPLS header.  The end of the MPLS labels is indicated by
   the S-bit in the MPLS label.

   The MPLS G-SID MUST NOT be the last G-SID in the G-SID list.

5.3.  Crossing IPv4 Domains

   In some scenarios such as SD-WAN, an SRv6 packet may cross IPv4
   networking.  Usually, the packet will be transported by an IPv6 over
   IPv4 tunnel.  This document proposes END.4 (End with IPv4 tunnel
   instantiation) and END.B4 (End binding to an IPv4 tunnel) behaviors
   for indicating the IPv4 tunnel.

5.3.1.  END.4

   The End.4 indicates the instantiation of an IPv4 Tunnel, and the
   parameter of this IPv4 Tunnel is carried by the IPv4 G-SID.  An End.4
   SID is never the last segment in a G-SID list.



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   When a node processes a END.4 SID, it will encapsulate an outer IPv4
   tunnel header according to the tunnel information in the associated
   IPv4 G-SID, and set the SL point the next G-SID after the IPv4
   G-SIDs, update the inner IPv6 DA by G-SRH[SL], and then forward the
   packet based on the outer IPv4 DA.

   The pesudo code is shown below:

 01. Encapsulate the outer IPv4 tunnel header according to the
     information in IPv4 G-SID(IPv4 DA, SA, etc.)                     //Ref1
 02. Update the SL point to next G-SID after the IPv4 G-SID
 03. Set the inner IPv6 DA as the SRH[SL]
 04. Forward the packet based on the outer IPv4 DA.

   Ref1: When encapsulating the outer IPv4 tunnel header, the source
   address and destination address and tunnel related parameters are
   learned from the IPv4 G-SID.

   The IPv4 G-SID MUST NOT be the last G-SID in the G-SID list.

5.3.2.  END.B4

   This is a variation of the End behavior, and it will be bound to an
   IPv4 tunnel initiated at endpoint node, while the parameters of this
   tunnel are not carried by SRH but maintained by node.

   The parameters of the IPv4 tunnel associated to the END.B4 SID can be
   configured and advertised in SID instantiation.  This is out of scope
   of this document, and will be described in other documents.

   An End.B4 SID is never the last segment in a SID list.

   When a node processes an END.B4 SID, it will encapsulate an outer
   IPv4 tunnel header based on the parameters binding to the End.B4 SID,
   and set the SL point the next G-SID, update the inner IPv6 DA by
   G-SRH[SL], and then forward the packet based on the outer IPv4 DA.

   The pseudo code is shown below.

  01. Encapsulate the outer IPv4 tunnel header according to
      the parameters binding to the End.B4 SID                    //Ref1
  02. Update the SL point to next G-SID after the IPv4 G-SID
  03. Set the inner IPv6 DA as the SRH[SL]
  04. Forward the packet based on the outer IPv4 DA.

   Ref1: When encapsulating the outer IPv4 tunnel header, the source
   address and destination address and tunnel related parameters are




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   learned from the parameters binding to the SID, the information is
   maintained at the node.

6.  IANA Considerations

   TBD

7.  Security Considerations

   TBD

8.  Contributors

   Zhibo Hu
   Huawei Technologies
   huzhibo@huawei.com

   Yang Xia
   Huawei Technologies
   yolanda.xia@huawei.com


9.  Acknowledgements

10.  References

10.1.  Normative References

   [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>.

   [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>.

   [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.




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   [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>.

   [I-D.li-spring-compressed-srv6-np]
              Li, Z., Li, C., Xie, C., Voyer, D., LEE, K., Tian, H.,
              Zhao, F., Guichard, J., Cong, L., and S. Peng, "Compressed
              SRv6 Network Programming", draft-li-spring-compressed-
              srv6-np-01 (work in progress), January 2020.

10.2.  Informative References

   [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-09 (work in
              progress), February 2020.

   [I-D.filsfils-spring-srv6-net-pgm-illustration]
              Filsfils, C., Camarillo, P., Li, Z., Matsushima, S.,
              Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and
              J. Leddy, "Illustrations for SRv6 Network Programming",
              draft-filsfils-spring-srv6-net-pgm-illustration-01 (work
              in progress), August 2019.

Authors' Addresses

   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: lizhenbin@huawei.com


   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: chengli13@huawei.com







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   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com


Chongfeng Xie
China Telecom
China Telecom Information Science&Technology Innovation park, Beiqijia Town,Changping District
Beijing
China

Email: xiechf.bri@chinatelecom.cn


Cong Li
China Telecom
China Telecom Information Science&Technology Innovation park, Beiqijia Town,Changping District
Beijing
China

Email: licong.bri@chinatelecom.cn


   Hui Tian
   CAICT
   Beijing
   China

   Email: tianhui@caict.ac.cn


   Feng Zhao
   CAICT
   Beijing
   China

   Email: zhaofeng@caict.ac.cn













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