Network                                                      C. Weiqiang
Internet-Draft                                              China Mobile
Intended status: Standards Track                               G. Mirsky
Expires: May 6, 2020                                           ZTE Corp.
                                                               P. Shaofu
                                                                L. Aihua
                                                         ZTE Corporation
                                                              W. Xiaolan
                                            New H3C Technologies Co. Ltd
                                                                  C. Wei
                                                                  Centec
                                                                S. Zadok
                                                                Broadcom
                                                        November 3, 2019


          Unified Identifier in IPv6 Segment Routing Networks
                   draft-mirsky-6man-unified-id-sr-04

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 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 identifiers encoded as MPLS
   label or IPv4 address, to compress the SRH, and support 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 May 6, 2020.

Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     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  . . . . . . . . .   7
     5.1.  Procedures of SR-MPLS over IP . . . . . . . . . . . . . .   8
     5.2.  Packet Forwarding . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 [I-D.ietf-spring-segment-routing-mpls].  It also can
   be applied to IPv6 data plane by encoding a list of 128-bits SIDs in
   IPv6 Segment Routing Extension Header (SRH)




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   [I-D.ietf-6man-segment-routing-header].  Applicability of 32-bits SID
   that may represent an IPv4 address has not been defined.

   SR extensions to Interior Gateway Protocols (IGP), IS-IS
   [I-D.ietf-isis-segment-routing-extensions], OSPF
   [I-D.ietf-ospf-segment-routing-extensions], and OSPFv3
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions], 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.

   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 with the following values:

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

      0b01 - 32-bits SID, an IPv4 address;

      0b10 - 32-bits SID, an MPLS label in leftmost 20-bits, rightmost
      12-bits for context information used by the label forwarding
      entry.  The context information could be U-SID function code.

      0b11 - reserved for future use.

   Entries of the segment list in the SRH MUST be of the same length.






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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 2.

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

                Figure 2: 32-bits long U-SIDs Encapsulation

   The SR-MPLS and SRv6 interworking is illustrated in Figure 3.  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
   [I-D.ietf-spring-segment-routing-mpls].








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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)|
              +--------------+    +--------------+    +--------------+
              |   USID(F)    |    |   USID(F)    |    |   USID(F)    |
              +--------------+    +--------------+    +--------------+
              |Label(service)|    |Label(service)|    |Label(service)|
              +--------------+    +--------------+    +--------------+
              |    Payload   | -> |    Payload   | -> |    Payload   |
              +--------------+    +--------------+    +--------------+


                  Figure 3: 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.  Figure 4 demonstrates an example of the packet
   forwarding, where U-SID is an MPLS label.

   The controller may assign the end-to-end SR tunnel U-SIDs (from A to
   F), and another method is outside the scope of this document.

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 beneficila that U-SID type matches to an MPLS label.
   In that case, an ILM (Incoming Label Map) entry can be used to map a



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   U-SID to an IPv6 address.  As result, it is not necessary to
   introduce a new type of index-based mapping table.  For ILM entry of
   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 copied to DA is remote node loopback IPv6 address.

   Operations oon an MPLS label of U-SID type are the same as those
   defined in [I-D.ietf-mpls-sr-over-ip].  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 [I-D.ietf-mpls-sr-over-ip] 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 target node that originated the FEC (Forwarding
   Equivalence Class), and on each airway node along the segment list,
   UDP header is frequently removed and put again.  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.  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 (Penutimate Segment
   Popping) may be used, that is different from SR-MPLS over IP/UDP
   method [I-D.ietf-mpls-sr-over-ip], 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 [I-D.ietf-mpls-sr-over-ip].  But it differs from that in FIB
   action and segment list processing.  For completeness, we repeat the
   description of [I-D.ietf-mpls-sr-over-ip] 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 4: Packet Forwarding Example

   In the example shown in Figure 4, 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, 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 get the next
   segment (label) within SRH according to SL 2, searches ILM entry by
   the next label, get the FEC 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 according to SRH.SL 1, looks up ILM entry
   by the next label, gets the FEC information, a loopback IPv6 address
   of H, and then copies it to IP DA and transmits the packet.  Because



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   the value of SRH.SL is 0, the SRH can be removed if the Prefix-SID of
   H is set to PSP.

6.  IANA Considerations

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

7.  Security Considerations

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

8.  Acknowledgements

   TBD

9.  Normative References

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and d. daniel.voyer@bell.ca, "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-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., and B. Decraene, "IS-IS Extensions for
              Segment Routing", draft-ietf-isis-segment-routing-
              extensions-25 (work in progress), May 2019.

   [I-D.ietf-mpls-sr-over-ip]
              Xu, X., Bryant, S., Farrel, A., Hassan, S., Henderickx,
              W., and Z. Li, "SR-MPLS over IP", draft-ietf-mpls-sr-over-
              ip-07 (work in progress), June 2019.

   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]
              Psenak, P. and S. Previdi, "OSPFv3 Extensions for Segment
              Routing", draft-ietf-ospf-ospfv3-segment-routing-
              extensions-23 (work in progress), January 2019.





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   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-27 (work in progress), December 2018.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-22
              (work in progress), May 2019.

   [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>.

   [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>.

Authors' Addresses

   Cheng Weiqiang
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com


   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com






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   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


   Shay
   Broadcom
   Israel

   Email: shay.zadok@broadcom.com










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