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Versions: 00 01

Network Working Group                                            M. Chen
Internet-Draft                                                   X. Geng
Intended status: Informational                                    Huawei
Expires: November 8, 2019                                          Z. Li
                                                            China Mobile
                                                             May 7, 2019


               Segment Routing (SR) Based Bounded Latency
             draft-chen-detnet-sr-based-bounded-latency-01

Abstract

   One of the goals of DetNet is to provide bounded end-to-end latency
   for critical flows.  This document defines how to leverage Segment
   Routing (SR) to implement bounded latency.  Specifically, the SR
   Identifier (SID) is used to specify transmission time (cycles) of a
   packet.  When forwarding devices along the path follow the
   instructions carried in the packet, the bounded latency is achieved.
   This is called Cycle Specified Queuing and Forwarding (CSQF) in this
   document.

   Since SR is a source routing technology, no per-flow state is
   maintained at intermediate and egress nodes, SR-based CSQF naturally
   supports flow aggregation that is deemed to be a key capability to
   allow DetNet to scale to large networks.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 November 8, 2019.

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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cycle Specified Queuing and Forwarding  . . . . . . . . . . .   4
     3.1.  CSQF Basic Concepts . . . . . . . . . . . . . . . . . . .   4
     3.2.  CSQF Queuing Model  . . . . . . . . . . . . . . . . . . .   5
     3.3.  CSQF Timing Model . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Congestion Protection and Resource Reservation  . . . . .   8
     3.5.  An Example of CSQF  . . . . . . . . . . . . . . . . . . .   9
   4.  Segment Routing Extensions for CSQF . . . . . . . . . . . . .  10
     4.1.  Time Aware Adjacency Segment(TA-Adj-SID)  . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Deterministic Networking (DetNet) [I-D.ietf-detnet-architecture] is
   defined to provide end-to-end bounded latency and extremely low
   packet loss rates for critical flows.  For a specific path, the end-
   to-end latency consists of two parts: 1) the accumulated latency on
   the wire, 2) the accumulated latency of nodes along the path.  The
   former can be considered as constant once the path has been
   determined.  The latter is contributed by the latency within each
   node along the path.  So, to guarantee the end-to-end bounded
   latency, control the bounded latency within a node is the key.  If



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   every node along the path can guarantee bounded latency, then end-to-
   end bounded latency can be achieved.

   [I-D.finn-detnet-bounded-latency] gives a framework that describes
   how bounded latency and zero congestion loss are achieved.  It
   introduces a parameterized timing model that can be used by DetNet
   solutions by selecting a corresponding Quality of Service (QoS)
   algorithm and resource reservation algorithm to achieve the bounded
   latency and zero congestion loss goal.

   This document defines how to leverage Segment Routing (SR) [RFC8402]
   to implement bounded latency, which is called Time Aware Segment
   Routing(TA-SR).  A segment is associated with a topological
   instruction, which instruct a node to forward the packet via a
   specific outgoing interface, as it is defined in [RFC8402].  At the
   same time, the segment is also associated with DetNet bounded latency
   service.  Specifically, the segment ID(SID) is used to carry and
   specify the "sending time" of a packet, and some mechanisms can be
   used to ensure that the packet will be transmitted in that specified
   period of sending time, which is called Time Aware Segment
   Routing(TA-SR).

   The TA-SR architecture supports any type of control plane:
   distributed (IS-IS or OSPF or BGP), centralized (NETCONF or PCEP or
   BGP), or hybrid (PCEP or BGP).

   The TA-SR architecture can be instantiated on various data planes,
   including TA-SR over MPLS (TA-SR MPLS) or TA-SR over IPv6 (TA-SRv6).

2.  Terminology

   All the terminologies used in this document are extensions of
   [RFC8402].

   Time Aware Segment:

   Time Aware SID:

   TA-SR MPLS SID:

   TA-SRv6 SID:

   TA-SR Domain:

   TA-SR Globle Block (SRGB):

   TA-SR Local Block (SRGB):




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   TA-Adjacency Segment:

   Forwarding Time Base: besides: the node uses the SID as an entry to
   get the Egress interface with Forwarding Information Base(FIB);
   Similarl, the node can use the SID as an entry to get the sending
   time of the packet, with Forwarding Time Base.

3.  Cycle Specified Queuing and Forwarding

3.1.  CSQF Basic Concepts

   By specifying the sending cycle of a packet at a node and making sure
   that the packet will be transmitted in that cycle, CSQF can achieve
   bounded latency within the node.  By specifying the sending cycle at
   every node along a path, the end-to-end bounded latency can be
   achieved.

   To support CSQF, similar to Cyclic Queuing and Forwarding (CQF)
   [IEEE802.1Qch], the sending time of an output interface of a node is
   divided into a series of equal time intervals with the duration of T.
   Each time interval is called a "cycle", and each cycle corresponds to
   a queue.  During a cycle, only the corresponding queue is open and
   all the packets in that queue will be transmitted.  CSQF can not only
   control the bounded latency at every node along a path, but regulate
   the traffic at each node as planned.  Therefore, no congestion will
   occur.

   Figure 1 provides an overview of CSQF.

           +---+   +---+   +---+   +---+
           | A |---| B |---| C |---| D |
           +-+-+   +---+   +---+   +---+

   A   |---X---+-------+-------+-------+-------+-------+-------|

   B   |-------+-------+---X---+-------+-------+-------+-------|

   C   |-------+-------+-------+-------+---X---+-------+-------|

   E   |-------+-------+-------+-------+-------+-------+---X---|

        cycle1  cycle2  cycle3   cycle4  cycle5  cycle6  cycle7

        DetNet path: A->B->C->D
        Specified cycle list of packet X: <1, 3, 5, 7>

                    Figure 1: CSQF Overview




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   CSQF has the following characteristics:

   o  The sending time (cycle) of a packet at each node along a path is
      specified so that the packet will be transmitted in the specified
      cycles, hence to guarantee the end-to-end bounded latency.

   o  The specified cycles are calculated by fully considering the link
      delay, processing delay and the available cycle resources,
      resulting in no bandwidth waste and no congestion (cycle-based
      traffic regulation).

   o  Segment routing (SR) is used.  Specifically, a SID is used to
      indicate in which cycle and to which output interface that a
      packet is specified to transmit, and an SR SID list is used to
      carry the specified cycles along a path.  With SR, there is no
      per-flow states maintained at the intermediate and egress node.
      As a result, scalability is greatly improved compared to a
      solution that maintains flow state at each hop.

   o  Flow aggregation is naturally supported by introducing SR and
      cycle-based scheduling.

3.2.  CSQF Queuing Model

   In Cyclic Queuing and Forwarding (CQF) [IEEE802.1Qch], time is
   divided into numbered time intervals, and each time interval is
   called a cycle; the critical traffic is then transmitted and queued
   for transmission along a path in a cyclic manner.  With CQF, the
   delays experienced by a given packet are as follows:

   o  The maximum end-to-end delay = (N+1) * T;

   o  The minimum end-to-end delay = (N-1) * T;

   o  Where the N is the number of hops and T is the duration of the
      cycle.

   CQF assumes that a packet is transmitted from an upstream node in a
   cycle and the packet must be received at the downstream node in the
   same cycle, and it must be transmitted in the next cycle to the
   nexthop node.  This assumption leads to very low bandwidth
   utilization when the link delay, processing delay, etc., factors
   cannot be considered as trivial.  To guarantee this assumption, more
   bandwidth has to be reserved as a guard band for each cycle, and the
   effective bandwidth for DetNet service will be greatly reduced.

   CSQF improves on CQF by explicitly specifying the sending cycles at
   every node along the path.  This relieves the limitation that the



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   sending (at the upstream node) and receiving (at the downstream node)
   have to be in the same cycle.  For CSQF, the cycle to use depends on
   traffic planning and path calculation.  The path calculation will
   consider the available cycle resources, bandwidth, and delay
   constraints.

              +--------+          +--------+        +--------+
   Queue1     |   SQ   -->      -->   TQ   |      -->   RQ   |
              +--------+          +--------+        +--------+
   Queue2   -->   RQ   |          |   SQ   -->    -->   TQ   |
              +--------+          +--------+        +--------+
   Queue3   -->   TQ   |        -->   RQ   |        |   SQ   -->
              +--------+          +--------+        +--------+
               Cycle 1              Cycle2            Cycle3

                       +--->SQ---->RQ---->TQ---->
                       |                        |
                       +------------<-----------+

                     Figure 2: CSQF Queuing Model


   For CSQF, three queues (in theory, two or more queues work as well)
   for each output interface are used.  During a particular cycle, only
   one queue is open and the packets in that queue will be transmitted.
   This queue is called the sending queue (SQ).  The other two queues
   are closed and can enqueue packets.  One of them is called the
   receiving queue (RQ).  The third queue is called the tolerating queue
   (TQ).

   The RQ is used for receiving the packets that are expected to be
   transmitted in the next cycle.  The TQ is used for tolerating the
   packets that come a bit early due to processing delay variation
   (processing jitter) or other reasons (e.g., packets are not
   transmitted as required by the traffic specification).  Both RQ and
   TQ can have the capability to absorb a certain amount of processing
   jitter and traffic bursts.  The upper bound of the absorbing capacity
   is 2T.  In order to increase the jitter/burst absorbing capacity, a
   four or more-queue model can be used.  If the processing delay and
   traffic bursts are small, two-queue model works as well.

   The roles of the three queues are not fixed, and on the contrary,
   they rotate with each cycle change.  As showed in Figure 2, during
   cycle 1, queue 1 is SQ, queue 2 is RG and queue 3 is TQ; during cycle
   2, queue 1 is TQ, queue 2 is SQ and queue 3 is RQ, during cycle 3,
   queue 1 is RQ, queue 2 is TQ and queue 3 is SQ.  That means, for a
   particular queue, its role will rotate as "...->SQ->RQ->TQ->SQ->...",
   the starting role of a queue can be any one of the three roles.



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   In CSQF, a cycle corresponds to a queue.  There are several ways to
   do cycle to queue mapping.  The simplest mapping between cycles and
   queues is 1:1 mapping.  There could be N:1 mapping, but that requires
   more identifiers, which in the case of segment routing, would require
   more SIDs.  This document does not specify which mapping should be
   used.  The mapping choice is left to the operator.

3.3.  CSQF Timing Model

           DetNet relay node A          DetNet relay node B
          +-------------------+        +-------------------+
          |    Reg.  Queue    |        |    Reg.  Queue    |
          |   +-+-+ +-+-+-+   |        |   +-+-+ +-+-+-+   |
       -->+   | | | | | | +   +------->+   | | | | | | +   +-->
          |   +-+-+ +-+-+-+   |        |   +-+-+ +-+-+-+   |
          |                   |        |                   |
          +-------------------+        +-------------------+
       -->|<->|<-->|<---->|<->|<------>|<->|<-->|<---->|<->|<--
       2,3  4   5      6   1     2,3     4   5      6    1   2,3
              1: Output delay       3: Preemption delay
              2: Link delay         4: Processing delay
              5: Regulation delay   6: Queuing delay.

                    Figure 3: Timing model for DetNet

   The DetNet timing model in Figure 3 is defined in
   [I-D.finn-detnet-bounded-latency].  It details the delays that a
   packet can experience from hop to hop.  There are six delays, the
   detailed explanation of which can be found in
   [I-D.finn-detnet-bounded-latency].  This document simplifies the
   above model as follows:

           DetNet relay node A          DetNet relay node B
          +-------------------+        +-------------------+
          |    Reg.  Queue    |        |    Reg.  Queue    |
          |   +-+-+ +-+-+-+   |        |   +-+-+ +-+-+-+   |
       -->+   | | | | | | +   +------->+   | | | | | | +   +-->
          |   +-+-+ +-+-+-+   |        |   +-+-+ +-+-+-+   |
          |                   |        |                   |
          +-------------------+        +-------------------+
       -->|<->|<------------->|<------>|<->|<------------->|<--
        2   3        1             2     3         1         2
                  1: Queuing delay
                  2: Link delay
                  3: Processing delay

             Figure 4: Simplified Timing model for DetNet




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   In this simplified timing model, only three delays are defined.  The
   queuing delay in this new model includes the output delay, regulation
   delay, and queuing delay that are defined in the DetNet timing model
   (Figure 3).  The link delay defined in this document includes the
   link delay and the preemption delay defined in
   [I-D.finn-detnet-bounded-latency].  The processing delay is the same
   as defined in [I-D.finn-detnet-bounded-latency].

   To further simplify the model, it assumes that the link delay only
   depends on the distance of the link.  Once the DetNet path has been
   determined, the link delay can be considered as constant.  The
   processing delay and queuing delay are variable but have their upper
   bounds.

   For the processing delay, there are two bounds: minimum processing
   delay (Min-P-Delay) and maximum processing delay (Max-P-Delay).

   o  Thus, the maximum processing jitter (Max-P-Jitter) = Max-P-Delay -
      Min-P-Delay.

   As described in Section 2.2, both the RQ and TQ can be used for
   absorbing processing jitter, and the upper bound of the absorbing
   capacity is 2T.  So, if the processing jitter is less than 2T, the
   three-queue model can work.  Otherwise, more buffer is needed to
   absorb the jitter, through increasing the duration of the cycle or by
   adding more queues.  Increasing the duration of the cycles is
   equivalent to increasing the depth of the queues (adding more buffer
   for each queue).

   With above, for CSQF, the delays experienced by a given packet are as
   follows:

   o  The maximum end-to-end delay = Link delay + N * (Max-P-Delay +
      2T);

   o  The maximum end-to-end jitter = 2T;

   o  Where N is the number of hops and T is the duration of a cycle.

3.4.  Congestion Protection and Resource Reservation

   Congestion protection is the key for bounded latency and zero
   congestion loss.  An essential component of DetNet is Traffic
   Engineering (TE), so that dedicated resources can be reserved for the
   exclusive use of DetNet flows.  To avoid congestion, two or more
   flows must be prevented from contending for the same resource.  For
   normal TE, the critical resource is bandwidth, but in the case of
   CSQF, the critical resource is interface occupation time.  Bandwidth



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   is an average value, which can generally guarantee the quality of
   service generally, but bursts and congestion may still occur.  By
   comparison, the interface occupation time is an absolute value, which
   can avoid packet packets conflicting for the same resource by
   controller computation and time allocation for different flows.  The
   unit of time allocation is the cycle, and a Traffic Specification,
   the flow transmission description, is necessary for the computation.

   CSQF uses segment routing SIDs to carry the time allocation
   information (the cycle), and it ensures that a node can schedule
   different packets without conflict and forward the packets at the
   proper time.  The resource reservation is not explicitly implemented
   by a control plane protocol, such as Resource Reservation Protocol -
   Traffic Engineering (RSVP-TE) or Stream Reservation Protocol (SRP).
   Rather, it is guaranteed by the SR controller, which maintains the
   status of different flows and time occupation of all the network
   devices in the domain.  This is called the Virtual Resource
   Reservation (VRR) in this document.

3.5.  An Example of CSQF

           +---+    +---+    +---+    +---+
           | A |----| B |----| C |----| E |
           +-+-+    +---+    +-+-+    +---+
             |      +---+      |
             +------| D |------+
                    +---+

   A   |---X---+-------+-------+-------+-------+-------+-------|

   B   |-------+-------+---X---+-------+-------+-------+-------|

   C   |-------+-------+-------+-------+---X---+-------+-------|

   E   |-------+-------+-------+-------+-------+-------+---X---|

        cycle1  cycle2  cycle3   cycle4  cycle5  cycle6  cycle7

        DetNet path: A->B->C->E
        Specified cycle list <1, 3, 5, 7>

                    Figure 5: CSQF Example


   As showed in Figure 5, there is a DetNet path (A->B->C->E), and a
   packet (X) is expected to be transmitted in cycle 1 at node A, in
   cycle 3 at node B, in cycle 5 at node C and in cycle 7 at node E.  A




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   cycle list <1, 3, 5, 7> is attached to the packet, and the packet
   will be transmitted along the path as the specified cycles.

   Given the topology as above, assume the duration of a cycle is 10us;
   the link delays between nodes are the same (e.g., 100us); the minimum
   processing delay at each node = 10us, the maximum processing delay at
   each node is 20us, so the maximum processing jitter is 10us.

   For a given packet that is transmitted along the path(A->B->C->D->E),
   the experienced maximum end-to-end delay is:

      (N-1) * link delay + N * (maximum processing delay + 2T)

      = 3*100 + 4* 40

      = 460 (us)

   The maximum end-to-end jitter is always 2T (20us).

4.  Segment Routing Extensions for CSQF

   This document defines a new segment that is called a Cycle Segment,
   which is used to identify a cycle.  A Cycle Segment is a local
   segment and is allocated from the Segment Routing Local Block
   (SRLB)[RFC8402].

   A Cycle Segment has two meanings: 1) identify an interface/link, just
   like the adjacency segment does; 2) identify a cycle of the
   interface/link.  To specify to which interface and in which cycle a
   packet should be transmitted, it just needs to attach a Cycle Segment
   to the packet.  By attaching a list of Cycle Segments to a packet, it
   can not only implement the explicit route of the packet that is
   required by DetNet [I-D.ietf-detnet-architecture], but also specify
   the sending cycle at each node along the path without maintaining
   per-flow states at the intermediate and egress nodes.  Hence, it
   naturally supports flow aggregation, and that allows DetNet to
   support large number of DetNet flows and scale to large networks.

   Normally, several SR SIDs are required to be allocated for each CSQF
   capable interface.  How many SIDs are allocated depends on how many
   cycles are used.  Given a three-queue model and a 1:1 cycle to queue
   mapping is used, three SIDs will be allocated for each CSQF capable
   interface.  For example, given node A, SR-MPLS SIDs 1001, 1002, and
   1003 are allocated to one of its interfaces.  SID 1001 identifies
   cycle 1, SID 1002 identifies cycle 2, SID 1003 identifies cycle 3.

   The SR [RFC8402] can be instantiated on various data planes.  There
   are two data-plane instantiations of SR: SR over MPLS (SR-MPLS) and



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   SR over IPv6 (SRv6).  Both SR-MPLS and SRv6 SIDs can be used for CSQF
   cycle identification.  The mapping (IGP extensions) between a cycle
   and a SID will be defined in a separate document.

4.1.  Time Aware Adjacency Segment(TA-Adj-SID)

   An Time Aware Adjacency segment is an IGP segment attached to a
   specified sending time of a unidirectional adjacency, which
   inheriting all the definitions of Adjacency segment defined in
   [RFC8402], adding new capability:

   When a node binds a group of AT-Adj-SIDs V1-Vn to a local data-link
   L, the node MUST install the following FIB entry:

      Incoming Active Segment: V1-Vn

      Ingress Operation: NEXT

      Egress Interface: L

   When a node binds an TA-Adj-SID V1 to sending time: Cycle 1, the node
   MUST install the following Forwarding Time Base (FTB) entry:

      Incoming Active Segment: V1

      Sending Time: Cycle 1

      Output Queue: Queue 1

   So a packet with TA-Adj-SID V1 will be transmitted go through output
   queue 1 of egress interface L within cycle 1.

5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

6.  Security Considerations

7.  Acknowledgements

   The authors would like to thank Andrew G.  Malis, Norman Finn for his
   review, suggestion and comments to this document.






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8.  References

8.1.  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>.

8.2.  Informative References

   [I-D.finn-detnet-bounded-latency]
              Finn, N., Boudec, J., Mohammadpour, E., Zhang, J., Varga,
              B., and J. Farkas, "DetNet Bounded Latency", draft-finn-
              detnet-bounded-latency-03 (work in progress), March 2019.

   [I-D.geng-detnet-conf-yang]
              Geng, X., Chen, M., Li, Z., and R. Rahman, "DetNet
              Configuration YANG Model", draft-geng-detnet-conf-yang-06
              (work in progress), October 2018.

   [I-D.geng-detnet-info-distribution]
              Geng, X., Chen, M., and Z. Li, "IGP-TE Extensions for
              DetNet Information Distribution", draft-geng-detnet-info-
              distribution-03 (work in progress), October 2018.

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-12 (work in progress), March 2019.

   [IEEE802.1Qch]
              IEEE, "IEEE, "Cyclic Queuing and Forwarding (IEEE Draft
              P802.1Qch)", 2017,
              <http://www.ieee802.org/1/files/private/ch-drafts/>.",
              2016.

   [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

   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com



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   Xuesong Geng
   Huawei

   Email: gengxuesong@huawei.com


   Zhenqiang Li
   China Mobile

   Email: lizhenqiang@chinamobile.com









































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