Stateful Path Computation Element (PCE) Inter-domain Considerations
draft-dhody-pce-stateful-pce-interdomain-05

Versions: 00 01 02 03 04 05                                             
PCE Working Group                                               D. Dhody
Internet-Draft                                                  X. Zhang
Intended status: Informational                       Huawei Technologies
Expires: March 22, 2018                               September 18, 2017


  Stateful Path Computation Element (PCE) Inter-domain Considerations
              draft-dhody-pce-stateful-pce-interdomain-05

Abstract

   A stateful Path Computation Element (PCE) maintains information about
   Label Switched Path (LSP) characteristics and resource usage within a
   network in order to provide traffic engineering path calculations for
   its associated Path Computation Clients (PCCs).  Furthermore, PCEs
   are used to compute shortest constrained Traffic Engineering Label
   Switched Paths (TE LSPs) in Multiprotocol Label Switching (MPLS) and
   Generalized MPLS (GMPLS) networks across multiple domains.

   This document describes general considerations for the deployment of
   stateful PCE(s) in inter-domain scenarios including inter-area and
   inter-AS.  The inter-layer considerations will be described in a
   separate document.  This document does not specify any extensions to
   PCEP.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 22, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  LSP State Synchronization . . . . . . . . . . . . . . . .   4
   3.  Stateful PCE Deployments  . . . . . . . . . . . . . . . . . .   4
     3.1.  Single Stateful PCE, Multiple Domains . . . . . . . . . .   5
     3.2.  Multiple Stateful PCE, Multiple Domains . . . . . . . . .   5
       3.2.1.  Per Domain Path Computation . . . . . . . . . . . . .   6
       3.2.2.  Backward-Recursive PCE-based Computation  . . . . . .   7
       3.2.3.  Hierarchical PCE  . . . . . . . . . . . . . . . . . .   8
   4.  Other Considerations  . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Delegation  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  PCE-initiated LSP . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .   9
     6.1.  Control of Function and Policy  . . . . . . . . . . . . .   9
     6.2.  Information and Data Models . . . . . . . . . . . . . . .   9
     6.3.  Liveness Detection and Monitoring . . . . . . . . . . . .   9
     6.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  10
     6.5.  Requirements On Other Protocols . . . . . . . . . . . . .  10
     6.6.  Impact On Network Operations  . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Contributor Addresses  . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The Path Computation Element communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform path
   computations in response to Path Computation Clients' (PCCs)
   requests.





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   [RFC8051] describes general considerations for a stateful PCE
   deployment and examines its applicability and benefits, as well as
   its challenges and limitations through a number of use cases.
   [I-D.ietf-pce-stateful-pce] describes a set of extensions to PCEP to
   provide stateful control.  A stateful PCE has access to not only the
   information carried by the network's Interior Gateway Protocol (IGP),
   but also the set of active paths and their reserved resources for its
   computations.  The additional state allows the PCE to compute
   constrained paths while considering individual LSPs and their
   interactions.

   The ability to compute shortest constrained TE LSPs in Multiprotocol
   Label Switching (MPLS) and Generalized MPLS (GMPLS) networks across
   multiple domains has been identified as a key motivation for PCE
   development.  In this context, a domain is a collection of network
   elements within a common sphere of address management or path
   computational responsibility such as an Interior Gateway Protocol
   (IGP) area or an Autonomous Systems (AS).

   This document presents general considerations for stateful PCE(s)
   deployment in multi-domain scenarios.

2.  Overview

   A stateful PCE maintains two sets of information for use in path
   computation.  The first is the Traffic Engineering Database (TED)
   which includes the topology and resource state in the network.  The
   second is the LSP State Database (LSP-DB), in which a PCE stores
   attributes of all active LSPs in the network, such as their paths
   through the network, bandwidth/resource usage, switching types and
   LSP constraints.  This state information allows the PCE to compute
   constrained paths while considering individual LSPs and their inter-
   dependency.  [I-D.ietf-pce-stateful-pce] applies equally to MPLS-TE
   and GMPLS LSPs and distinguishes between an active and a passive
   stateful PCE.  A passive stateful PCE uses LSP state information to
   optimize path computations but does not actively update LSP state.
   In contrast, an active stateful PCE may issue recommendations to the
   network.  For example, an active stateful PCE may update LSP
   parameters for those LSPs that have been delegated control over to
   the PCE by its PCCs.

   The capability to compute the routes of end-to-end inter-domain MPLS-
   TE LSPs is expressed as requirements in [RFC4105] and [RFC4216] and
   may be realized by PCE(s).  PCEs may use one of the following
   mechanisms to compute end-to-end paths:

   o  a per-domain path computation technique [RFC5152];




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   o  a Backward-Recursive PCE-based Computation (BRPC) mechanism
      [RFC5441];

   o  a Hierarchical PCE mechanism [RFC6805];

   This document examines the stateful PCE inter-domain considerations
   for all of these mechanisms.

2.1.  LSP State Synchronization

   The population of the LSP-DB using information received from PCCs
   (ingress LSR) is supported by the stateful PCE extensions defined in
   [I-D.ietf-pce-stateful-pce] , i.e., via LSP state report messages.

   The inter-domain LSP state is synchronized to the ingress-PCE from
   the ingress LSR (PCC), but this PCC cannot synchronize to other PCEs
   (in transit or egress domains), thus other mechanism must be
   investigated for this purpose.

   Either the boundary node of the other domains, would need to
   synchronize the state of LSP passing though it to the PCE, or a
   mechanism for synchronization of inter-domain LSPs between the PCEs
   is required.  The former would require small change in the existing
   state synchronization and reporting where a border node acts as a
   PCC.  The later could use the mechanism described in
   [I-D.litkowski-pce-state-sync] can be used between the PCEs to
   synchronize the inter-domain LSP state between each other.  Further
   section provide various considerations for this choice.

3.  Stateful PCE Deployments

   There are multiple models to perform PCE-based inter-domain path
   computation:

   o  A single PCE

   o  Multiple PCEs

      *  without inter-PCE communication

      *  with inter-PCE communication

   This section describe stateful PCE considerations for each of these
   deployment models.







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3.1.  Single Stateful PCE, Multiple Domains

   In this model, inter-domain path computation is performed by a single
   stateful PCE that has topology visibility into all domains.  The
   inter-domain LSP state is synchronized to the PCE from the ingress
   LSR (PCC) itself.  The PCC may also choose to delegate control over
   this LSP to the PCE.  Thus this model is similar to a single domain
   in all aspects.

   Following figure show an example of inter-area case comprising of
   Area 0,1 and 2.  A single stateful PCE is deployed for all areas.

                                  *******
                                  * PCE *
                                  *******

                         !                      !
                         !                      !
         A----B----C----ABR1----D----E----F----ABR2----G----H----I
         |    |    |     |      |    |    |     |      |    |    |
         |    |    |     |      |    |    |     |      |    |    |
         J----K----L----ABR3----M----N----O----ABR4----P----Q----R
                         !                      !
              Area 1     !         Area 0       !      Area 2

   In this model, PCE has visibility into the topology (TED) of all
   domains as well as the state of all active LSPs (LSP-DB) including
   inter-domain LSPs.  This model is thus well suited to take advantage
   of all stateful PCE capabilities.

   It should be noted that in some deployments, a single stateful PCE
   may not be possible because of administrative and confidentiality
   concerns.

3.2.  Multiple Stateful PCE, Multiple Domains

   In this model, there is at least one PCE per domain, and each PCE has
   topology (TED) visibility restricted to its own domain.  The inter-
   domain LSP state is synchronized to the ingress-PCE from the ingress
   LSR (PCC), but this PCC may not be able to synchronize to other PCEs
   (in transit or egress domains).  This PCC may also choose to delegate
   control over this LSP to the Ingress-PCE, which may issue inter-
   domain path computation or re-optimization request to other PCEs.  An
   inter-domain LSP that originates in a domain, is synchronized to the
   PCE in that domain.  A new procedure is needed to synchronize state
   of inter-domain LSP that do not originate in the domain.  In other
   words, inter-domain LSP state should also be synchronized to transit




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   and egress PCEs as the inter-domain LSP traverse through those
   domains.

   Following figure show an example of inter-AS case comprising of AS
   100 and AS 200.  A stateful PCE is deployed per AS.

                  ********                      ********
                  * PCE1 *                      * PCE2 *
                  ********                      ********

                              !          !
                              !          !
              A----B----C----ASBR1------ASBR2----D----E----F
              |    |    |     |          |       |    |    |
              |    |    |     |          |       |    |    |
              G----H----I----ASBR3------ASBR4----J----K----L
                              !          !
                   AS100      !          !       AS200

   In order to conceal the information, a PCE may use path-key based
   confidentiality mechanisms as per [RFC5520].

   This section further describes considerations with respect to each of
   the inter-domain path computation techniques.

3.2.1.  Per Domain Path Computation

   The per domain path computation technique [RFC5152] is based on
   Multiple PCE Path Computation without Inter-PCE Communication Model
   as described in [RFC4655].  It defines a method where the path is
   computed during the signaling process (on a per-domain basis).  The
   entry Boundary Node (BN) of each domain is responsible for performing
   the path computation for the section of the LSP that crosses the
   domain, or for requesting that a PCE for that domain computes that
   piece of the path.

   The ingress LSR would synchronize the state to the ingress PCE,
   further the entry boundary nodes should also synchronize the state of
   inter-domain LSP to transit and egress PCEs.  Note that the BN on the
   path of an LSP can probably see the path (through the Record Route
   object in RSVP-TE signaling [RFC3209]) and knows the bandwidth
   reserved for the LSP.  Thus each entry BN along the path could be
   made responsible to synchronize the LSP state to the transit/egress
   PCE(s).

   Since the stateful PCE(s) do not communicate during this inter-domain
   path computation technique and each entry BN would perform path




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   computation via Path Computation Request (PCReq) and Reply (PCRep)
   messages, a passive stateful PCE is well suited for this case.

   In case of delegation to the ingress PCE (active stateful PCE), it
   would be capable of loose path computation only and make updates to
   the ingress LSR with this limited visibility.  The entry BN would
   perform path computation via Path Computation Request and Reply
   messages (and thus rely on the passive stateful mode).  Thus the
   inter-domain LSP is delegated only to the ingress PCE.

3.2.2.  Backward-Recursive PCE-based Computation

   The BRPC [RFC5441] technique is based on Multiple PCE Path
   Computation with Inter-PCE Communication Model as described in
   [RFC4655].  It involves cooperation and communication between PCEs in
   order to compute an optimal end-to-end path across multiple domains.
   The sequence of domains to be traversed may be known before the path
   computation, but it can also be used when the domain path is unknown
   and determined during path computation.

   As described in Section 3.2.1, the entry boundary nodes may
   synchronize the state of inter-domain LSPs to transit and egress
   PCEs.  An alternative approach may be for each PCE to synchronize the
   state along the path across domains, i.e., each PCE would report the
   state to the next PCE(s) in the adjacent domain along the domain
   sequence of the inter-domain path.  A mechanism similar to state-sync
   described in [I-D.litkowski-pce-state-sync] may be utilized for this
   purpose.

   Some path segment in the end to end path may also be hidden via path-
   key as per [RFC5520] during state synchronization.

   In case of passive path computation request to the ingress PCE from
   the ingress LSR the BRPC path computation procedure is applied to
   compute end-to-end path by using PCReq and PCRep messages among
   stateful PCE(s) in passive mode.

   In case of delegation to the ingress PCE (active stateful PCE), the
   ingress PCE may trigger the end-to-end path computation via the same
   BRPC procedure using the path computation request and reply messages
   among stateful PCE(s) (acting in passive mode).  For re-optimization
   the ingress PCE still rely on the same BRPC procedure triggered by
   the ingress PCE.  Ultimately the inter-domain LSP is delegated to the
   ingress PCE and only the ingress PCE can trigger end-to-end (E2E)
   path re-optimization with help of transit/egress PCE using the BRPC
   procedure, based on the result the ingress PCE would issue updates to
   the inter-domain LSP.




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3.2.3.  Hierarchical PCE

   In H-PCE [RFC6805] architecture, the parent PCE is used to compute a
   multi-domain path based on the domain connectivity information.  The
   parent PCE may be requested to provide a end-to-end path or only the
   sequence of domains.

   As described in Section 3.2.1 and Section 3.2.2, the entry boundary
   nodes may synchronize the state of inter-domain LSP to transit and
   egress child PCEs.  In this case, it might not be possible to
   synchronize state to the parent PCE.  If the parent PCE provides the
   sequence of domains and BRPC procedure is used to get the E2E path,
   each PCE may be responsible to synchronize the state along the path
   across domains similar to Section 3.2.2.  An alternative approach may
   be for ingress PCE to synchronize LSP state with the Parent PCE and
   it may further synchronize the state to the child PCE(s) along the
   path across domains, i.e. parent PCE would report the state to the
   child PCE(s) along the domain sequence.

   Some path segment in the end to end path may also be hidden via path-
   key as per [RFC5520] during state synchronization.

   In case of passive path computation request to the ingress PCE from
   the ingress LSR, the H-PCE path computation procedure is applied to
   compute sequence of domains or end-to-end path by using PCReq and
   PCRep messages among stateful PCE(s) in passive mode.

   In case of delegation to the ingress PCE (active stateful PCE), the
   ingress child PCE may further delegate to parent PCE.  See
   [I-D.ietf-pce-stateful-hpce] for details.

4.  Other Considerations

4.1.  Delegation

   As noted in this document, the inter-domain LSP is delegated to the
   ingress PCE and only the ingress PCE can issue updates to the inter-
   domain LSP.  The ingress PCE is responsible to trigger E2E path re-
   optimization.

   Thus the ingress PCE can recommend updation for all aspects of the
   inter-domain LSP including the segment of path in another domain
   (which it may have computed with the help of other cooperating PCEs).
   These interaction between PCEs for the inter-domain path computation
   are done using PCReq/PCRep messages (i.e., in a passive mode).

   The transit/egress PCE cannot update any attribute of the inter-
   domain LSP on its own as it may not have any interaction with the



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   ingress LSR.  A mechanism may be developed for transit/egress PCE to
   inform the ingress PCE to trigger E2E re-optimization and choose to
   update the inter-domain LSP based on the result.  Also the ingress
   PCE may use combination of local information and events along with
   some external mechanism (management / monitoring interface) to
   trigger E2E path re-optimization.

   Though ingress PCE can recommend update for path segments in other
   domains, the entry boundary node of that domain can apply policy
   control during signaling as explained in [RFC4105] and [RFC4216].

4.2.  PCE-initiated LSP

   [I-D.ietf-pce-pce-initiated-lsp] describes setup, maintenance and
   teardown of PCE-initiated LSPs under the stateful PCE model, without
   the need for local configuration on the PCC.  Similar to LSP
   updation, the inter-domain LSP can be initiated by the ingress PCE
   using the PCInitiate message to the ingress LSR.  Note that per-
   domain LSP may also be initiated by respective domain's PCE and
   stitched together but this is out of scope for this document.

5.  Security Considerations

   The security considerations are as per [RFC5440] and
   [I-D.ietf-pce-stateful-pce].  Any multi-domain operation necessarily
   involves the exchange of information across domain boundaries.  This
   may represent a significant security and confidentiality risk
   especially when the domains are controlled by different commercial
   entities.  PCEP allows individual PCEs to maintain confidentiality of
   their domain path information by using path-keys [RFC5520].

6.  Manageability Considerations

6.1.  Control of Function and Policy

   Mechanisms defined in this document do not imply any new control of
   function and policy requirements.

6.2.  Information and Data Models

   [RFC7420] describes the PCEP MIB, there are no new MIB Objects for
   this document.

6.3.  Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].



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6.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440].

6.5.  Requirements On Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.

6.6.  Impact On Network Operations

   Mechanisms defined in this document do not have any impact on network
   operations in addition to those already listed in [RFC5440].

7.  IANA Considerations

   This is an informational document and has no IANA considerations.

8.  Acknowledgments

   The authors would like to that Young Lee, Haomian Zheng, Fatai Zhang
   for this comments.

9.  References

9.1.  Normative References

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-21 (work in progress), June 2017.

9.2.  Informative References

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.






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   [RFC4105]  Le Roux, J., Ed., Vasseur, J., Ed., and J. Boyle, Ed.,
              "Requirements for Inter-Area MPLS Traffic Engineering",
              RFC 4105, DOI 10.17487/RFC4105, June 2005,
              <https://www.rfc-editor.org/info/rfc4105>.

   [RFC4216]  Zhang, R., Ed. and J. Vasseur, Ed., "MPLS Inter-Autonomous
              System (AS) Traffic Engineering (TE) Requirements",
              RFC 4216, DOI 10.17487/RFC4216, November 2005,
              <https://www.rfc-editor.org/info/rfc4216>.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
              Per-Domain Path Computation Method for Establishing Inter-
              Domain Traffic Engineering (TE) Label Switched Paths
              (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
              <https://www.rfc-editor.org/info/rfc5152>.

   [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
              "A Backward-Recursive PCE-Based Computation (BRPC)
              Procedure to Compute Shortest Constrained Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 5441,
              DOI 10.17487/RFC5441, April 2009,
              <https://www.rfc-editor.org/info/rfc5441>.

   [RFC5520]  Bradford, R., Ed., Vasseur, JP., and A. Farrel,
              "Preserving Topology Confidentiality in Inter-Domain Path
              Computation Using a Path-Key-Based Mechanism", RFC 5520,
              DOI 10.17487/RFC5520, April 2009,
              <https://www.rfc-editor.org/info/rfc5520>.

   [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
              DOI 10.17487/RFC6805, November 2012,
              <https://www.rfc-editor.org/info/rfc6805>.

   [RFC7420]  Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Communication Protocol
              (PCEP) Management Information Base (MIB) Module",
              RFC 7420, DOI 10.17487/RFC7420, December 2014,
              <https://www.rfc-editor.org/info/rfc7420>.






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   [RFC8051]  Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
              Stateful Path Computation Element (PCE)", RFC 8051,
              DOI 10.17487/RFC8051, January 2017,
              <https://www.rfc-editor.org/info/rfc8051>.

   [I-D.ietf-pce-pce-initiated-lsp]
              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-ietf-pce-pce-initiated-lsp-10 (work in
              progress), June 2017.

   [I-D.ietf-pce-stateful-hpce]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., King, D.,
              and O. Dios, "Hierarchical Stateful Path Computation
              Element (PCE).", draft-ietf-pce-stateful-hpce-01 (work in
              progress), June 2017.

   [I-D.litkowski-pce-state-sync]
              Litkowski, S., Sivabalan, S., and D. Dhody, "Inter
              Stateful Path Computation Element communication
              procedures", draft-litkowski-pce-state-sync-02 (work in
              progress), August 2017.





























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Appendix A.  Contributor Addresses

   Udayasree Palle
   Huawei Technologies
   Divyasree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: udayasreereddy@gmail.com


   Avantika
   Huawei Technologies
   Divyasree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: s.avantika.avantika@gmail.com



Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyasree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: dhruv.ietf@gmail.com


   Xian Zhang
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R.China

   EMail: zhang.xian@huawei.com












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