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Path Computation Element (PCE) Protocol Extensions for Stateful PCE Usage in GMPLS-controlled Networks

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Authors Xian Zhang , Young Lee , Fatai Zhang , Ramon Casellas , Oscar Gonzalez de Dios , Zafar Ali
Last updated 2015-01-07
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PCE Working Group                                            Xian Zhang 
Internet-Draft                                                Young Lee 
Intended status: Standards Track                            Fatai Zhang 
                                                         Ramon Casellas 
                                                 Oscar Gonzalez de Dios 
                                                         Telefonica I+D 
                                                              Zafar Ali 
                                                          Cisco Systems 
Expires: July 8, 2015                                   January 8, 2015 

   Path Computation Element (PCE) Protocol Extensions for Stateful PCE 
                   Usage in GMPLS-controlled Networks 


   The Path Computation Element (PCE) facilitates Traffic Engineering 
   (TE) based path calculation in large, multi-domain, multi-region, or 
   multi-layer networks. [Stateful-PCE] provides the fundamental PCE 
   communication Protocol (PCEP) extensions needed to support stateful 
   PCE functions, without specifying the technology-specific extensions. 
   This memo provides extensions required for PCEP so as to enable the 
   usage of a stateful PCE capability in GMPLS-controlled networks. 

Status of this Memo 

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

   Internet-Drafts are working documents of the Internet Engineering   
   Task Force (IETF), its areas, and its working groups.  Note that   
   other groups may also distribute working documents as Internet-   

   Internet-Drafts are draft documents valid for a maximum of six 
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   documents at any   time.  It is inappropriate to use Internet-Drafts 

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   as reference   material or to cite them other than as "work in 

   The list of current Internet-Drafts can be accessed at 

   The list of Internet-Draft Shadow Directories can be accessed at 

   This Internet-Draft will expire on July 8, 2015. 

Copyright Notice 

   Copyright (c) 2014 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 
   ( 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. 


Conventions used in this document 

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   document are to be interpreted as described in RFC-2119 [RFC2119]. 

Table of Contents 

   Table of Contents .............................................. 2 
   1. Introduction ................................................ 3 
   2. PCEP Extensions ............................................. 4 
      2.1. Overview of Requirements................................ 4 
      2.2. Stateful PCE Capability Advertisement ...................4 
         2.2.1. PCE Capability Advertisement in Multi-layer Networks 5 
      2.3. LSP Delegation in GMPLS-controlled Networks ............. 6 
      2.4. LSP Synchronization in GMPLS-controlled Networks......... 6 
      2.5. Modification of Existing PCEP Messages and Procedures.... 8 
         2.5.1. Use cases ......................................... 9 
         2.5.2. Modification for LSP Re-optimization ............... 9 
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         2.5.3. Modification for Route Exclusion .................. 10 
   3. IANA Considerations ........................................ 11 
      3.1. New PCEP Error Codes................................... 11 
      3.2. New Subobject for the Exclude Route Object ............. 11 
   4. Manageability Considerations................................ 12 
      4.1. Requirements on Other Protocols and Functional Components12 
   5. Security Considerations..................................... 12 
   6. Acknowledgement ............................................ 12 
   7. References ................................................. 12 
      7.1. Normative References................................... 12 
      7.2. Informative References................................. 13 
   8. Contributors' Address....................................... 13 
   Authors' Addresses ............................................ 14 

1. Introduction 

   [RFC 4655] presents the architecture of a Path Computation Element 
   (PCE)-based model for computing Multiprotocol Label Switching (MPLS) 
   and Generalized MPLS (GMPLS) Traffic Engineering Label Switched 
   Paths (TE LSPs).  To perform such a constrained computation, a PCE 
   stores the network topology (i.e., TE links and nodes) and resource 
   information (i.e., TE attributes) in its TE Database (TED).  Such a 
   PCE is usually referred as a stateless PCE. To request path 
   computation services to a PCE, [RFC 5440] defines the PCE 
   communication Protocol (PCEP) for interaction between a Path 
   Computation Client (PCC) and a PCE, or between two PCEs.  PCEP as 
   specified in [RFC 5440] mainly focuses on MPLS networks and the PCEP 
   extensions needed for GMPLS-controlled networks are provided in 

   Stateful PCEs are shown to be helpful in many application scenarios, 
   in both MPLS and GMPLS networks, as illustrated in [Stateful-APP].  
   In order for these applications to able to exploit the capability of 
   stateful PCEs, extensions to PCEP are required.  

   [Stateful-PCE] provides the fundamental extensions needed for 
   stateful PCE to support general functionality, but leaves out the 
   specification for technology-specific objects/TLVs.  Complementarily, 
   this document focuses on the extensions that are necessary in order 
   for the deployment of stateful PCEs in GMPLS-controlled networks.  

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2. PCEP Extensions  

2.1. Overview of Requirements 

   This section notes the main functional requirements for PCEP 
   extensions to support stateful PCE for use in GMPLS-controlled 
   networks, based on the description in [Stateful-APP].  Many 
   requirements are common across a variety of network types (e.g., 
   MPLS-TE networks and GMPLS networks) and the protocol extensions to 
   meet the requirements are already described in [Stateful-PCE].  This 
   document does not repeat the description of those protocol 
   extensions.  Other requirements that are also common across a 
   variety of network types do not currently have protocol extensions 
   defined in [Stateful-PCE].  In these cases, this document presents 
   protocol extensions for discussion by the PCE working group and 
   potential inclusion in [Stateful-PCE].  In addition, this document 
   presents protocol extensions for a set of requirements which are 
   specific to the use of a stateful PCE in a GMPLS-controlled network. 

   The basic requirements are as follows: 

   o  Advertisement of the stateful PCE capability.  This generic 
      requirement is covered in Section 7.1.1. of [Stateful-PCE].  
      Section 2.2. of this document discusses other potential extensions 
      for this functionality. 

   o  LSP delegation is already covered in Section 5.5. of [Stateful-
      PCE].  Section 2.3. of this document provides extension for its 
      application in GMPLS-controlled networks.  Moreover, further   
      discussion of some generic details that may need additional 
      consideration is provided.  

   o  LSP state synchronization and LSP state report. This is a generic 
      requirement already covered in Section 5.4. of [Stateful-PCE].  
      However, there are further extensions required specifically for 
      GMPLS-controlled networks and discussed in Section 2.4.  Reference 
      to LSPs by identifiers is discussed in Section 7.3. of [Stateful-
      PCE].  This feature can be applied to reduce the data carried in 
      PCEP messages.  Use cases and additional Error Codes are necessary, 
      as described in Section 2.5. of this draft. 

2.2. Stateful PCE Capability Advertisement 

   Whether a PCE has stateful capability or not can be advertised 
   during the PCEP session establishment process. It can also be 
   advertised through routing protocols as described in [RFC5088]. In 
   either case, the following additional aspects should also be 
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 2.2.1. PCE Capability Advertisement in Multi-layer Networks 

   In multi-layer network scenarios, such as an IP-over-optical network, 
   if there are dedicated PCEs responsible for each layer, then the 
   PCCs should be informed of which PCEs they should synchronize their 
   LSP states with, as well as send path computation requests to.  The 
   Layer-Cap TLV defined in [INTER-LAYER] can be used to indicate which 
   layer a PCE is in charge of. (Editor's note: this change is 
   currently not included in the current version of the [INTER-LAYER] 
   draft. It is expected that it will be included in its next version.) 
   This TLV is optional and MAY be carried in the OPEN object.  It is 
   RECOMMMENDED that a PCC synchronizes its LSP states with the same 
   PCEs that it can use for path computation in a multi-layer network. 
   In a single layer, this TLV MAY not be used.  However, if the PCE 
   capability discovery depends on IGP and if an IGP instance spans 
   across multiple layers, this TLV is still needed.  

   Alternatively, the extension to current OSPF PCED TLV and IS-IS PCED 
   sub-TLV are needed.  A new domain-type denoting the layer 
   information can be defined: 

   domain-type: T.B.D. (suggested value: 3) 

   which denotes the network layer information, in which a stateful PCE 
   has the stateful capability. 

   When it is carried in PCE-DOMAIN sub-TLV, it denotes the layer for 
   which a PCE is responsible for path computation as well as LSP state 
   synchronization.  When carried in the PCE-NEIG-DOMAIN sub-TLV, it 
   denotes its adjacent layers for which a PCE can compute paths and 
   synchronize the LSP states.  The DOMAIN-ID information can be 
   represented using the following format, to denote the layer 

    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      
   | LSP Enc. Type | Switching Type|             Reserved          |     
   The IS-IS PCE-DOMAIN sub-TLV is extended to have the following 

      TYPE:   3 
      LENGTH: Variable 

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      VALUE:  This is composed of one octet indicating the domain-type 
    (area ID, AS Number or Network Layer) and a variable length IS-IS 
    area ID, 32-bit AS number, or a 32-Bit Network-Layer, with encoding 
    specified above identifying a network layer where the PCE has 
    visibility and can compute paths. 
    A new domain-type value is defined:  
    Value   Meaning  
    3       Network Layer 
2.3. LSP Delegation in GMPLS-controlled Networks  

   To enable the PCE to control an LSP, the PCUpd message is defined in 
   [Stateful-PCE].  However, the specification of technology specific 
   extensions is not covered.  The following defines the <path> 
   descriptor, present in the PCUpd message, that should be used in 
   GMPLS-controlled networks: 



         <attribute-list> ::= [<LSPA>] 



         <metric-list>::= <METRIC>[<metric-list>] 

   BANDWIDTH object used in the attribute-list is defined in [PCEP-
   GMPLS]. Additional TLVs defined for <LSPA> object in [PCEP-GMPLS] 
   MAY also need to be included. 

   LSP parameter update controlled by a stateful PCE in a multi-domain 
   network is complex and requires well-defined operational procedures 
   as well as protocol design and is out of scope of this document and 
   left for further study. 

2.4. LSP Synchronization in GMPLS-controlled Networks 

   For LSP state synchronization of stateful PCEs in GMPLS networks, 
   the LSP attributes, such as its bandwidth, associated route as well 
   as protection information etc, should be updated by PCCs to PCE LSP 
   database (LSP-DB). Note the LSP state synchronization described in 
   this document denotes both the bulk LSP report at the initialization 
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   phase as well as the LSP state report afterwards described in 

   As per [Stateful-PCE], it does not cover technology-specific 
   specification for state synchronization. Therefore, extensions of 
   PCEP for stateful PCE usage in GMPLS networks are required. For LSP 
   state synchronization, the objects/TLVs that should be used for 
   stateful PCE in GMPLS networks are defined in [PCEP-GMPLS] and are 
   briefly summarized as below:  

   o BANDWIDTH (Generalized BANDWIDTH Object Type) 

   o END-POINTS (Generalized END-POINTS Object Type) 


   o Use of IF_ID_ERROR_SPEC. [Stateful-PCE] section 7.3.4. only 
   considers  RSVP ERROR_SPEC TLVs. GMPLS extends this to also support 
   IF_ID_ERROR_SPEC, for example, to report about failed unnumbered 

   o Extended objects to support the inclusion of the label and 
   unnumbered links.  

   Per [Stateful-PCE], the Path Computation Report (PCRpt) message is 
   defined for LSP state synchronization purposes. PCRpt is used by a 
   PCC to report one or more of its LSPs to a stateful PCE. However, 
   the <path> descriptor is technology-specific and left undefined.   

   For LSP state synchronization in GMPLS-controlled networks, the 
   encoding of the <path> descriptor is defined as follows: 

   <path>::=<ERO><attribute-list> [<RRO>] 


         <attribute-list> ::= [<END-POINTS>]  






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         <metric-list>::= <METRIC>[<metric-list>] 

   The objects included in the <path> descriptor can be found in 
   [RFC5440], [PCEP-GMPLS] and [RFC5521]. 

   For all the objects presented in this section, the P and I bit MUST 
   be set to 0 since they are only used by a PCC to report its LSP 

   In GMPLS-controlled networks, the <ERO> object may include a list of 
   the label sub-object for SDH/SONET, OTN and DWDM networks. It may 
   also include a list of unnumbered interface IDs to denote the 
   allocated resource. The <RRO>, <IRO> and <XRO> objects MAY include 
   unnumbered interface IDs and labels for networks such as OTN and WDM 

   If the LSP being reported is a protecting LSP, the <PROTECTION-
   ATTRIBUTE> TLV MUST be included in the <LSPA> object to denote its 
   attributes and restrictions. Moreover, if the status of the 
   protecting LSP changes from non-operational to operational, this 
   should be synchronized to the stateful PCE. For example, in 1:1 
   protection, the combination of S=0, P=1 and O=0 denotes the 
   protecting path is set up already but not used for carrying traffic. 
   Upon the working path failure, the operational status of the 
   aforementioned protecting LSP changes to in-use (i.e., O=1). This 
   information should be synchronized with a stateful PCE through a 
   PCRpt message. 

   The object type used here for <BANDWIDTH> and <END-POINTS> objects 
   MUST be the ones defined in [PCEP-GMPLS]. The <END-POINTS> are used 
   to report the end-points address associated with the LSP being 
   reported since the <ERO> may not carry such information. 


2.5.  Modification of Existing PCEP Messages and Procedures  

   One of the advantages mentioned in [Stateful-APP] is that the 
   stateful nature of a PCE simplifies the information conveyed in PCEP 
   messages, notably between PCC and PCE, since it is possible to refer 
   to PCE managed state for active LSPs. To be more specific, with a 
   stateful PCE, it is possible to refer to a LSP with a unique 
   identifier in the scope of the PCC-PCEP session and thus use such 
   identifier to refer to that LSP.  

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 2.5.1. Use cases 

   Use Case 1: Assuming a stateful PCE's LSP-DB is up-to-date, a PCC 
   (e.g. NMS) requesting for a re-optimization of one or several LSPs 
   can send the request with ''R'' bit set and only provides the relevant 
   LSP unique identifiers. 

   Upon receiving the PCReq message, PCE should be able to correlate 
   with one or multiple LSPs with their detailed state information and 
   carry out optimization accordingly.  

   The handling of RP object specified in [RFC5440] is stated as 

   ''The absence of an RRO in the PCReq message for a non-zero-bandwidth 
   TE LSP (when the R bit of the RP object is set) MUST trigger the 
   sending of a PCErr message with Error-Type="Required Object Missing" 
   and Error-value="RRO Object missing for re-optimization." 

   If a PCE has stateful capabilities, and such capabilities have been 
   negotiated and advertised, specific rules given in [RFC5440] may 
   need to be relaxed. In particular, the re-optimization case: if the 
   re-optimization request refers to a given LSP state, and the RRO 
   information is available, the PCE can proceed. 

   Use Case 2: in order to set up a LSP which has a constraint that its 
   route should not use resources used by one or more existing LSPs, a 
   PCC can send a PCReq with the identifiers of these LSPs. A stateful 
   PCE should be able to find the corresponding route and resource 
   information so as to meet the constraints set by the requesting PCC. 
   Hence, the LSP identifier TLV defined in [Stateful-PCE], encoded as 
   a subobject, can be used in XRO object for this purpose. Note that 
   if the PCC is a node in the network, the constraint LSP ID 
   information will be confined to the LSPs initiated by itself. 

 2.5.2. Modification for LSP Re-optimization 

   For re-optimization, upon receiving a path computation request and 
   the ''R'' bit is set, the stateful PCE SHOULD still perform the re-
   optimization in the following two cases: 

   Case 1: the existing bandwidth and route information of the to-be-
   optimized LSP is provided in the path computation request. This 
   information should be provided via <BANDWIDTH>, <ERO> objects. 

   Case 2: the existing bandwidth and route information can be found 
   locally in its LSP-DB. In this case, the PCRep and PCReq messages 
   need to be modified to carry LSP identifiers. This is specified in 
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   [Stateful-PCE]. The stateful PCE can find this information using the 
   per-node LSP ID (e.g., PLSP-ID defined in [Stateful-PCE]) together 
   with the PCC's address. 

   If no LSP state information is available to carry out re-
   optimization, the stateful PCE should report the error ''LSP state 
   information unavailable for the LSP re-optimization'' (Error Type = 
   T.B.D., Error value= T.B.D.). 

 2.5.3. Modification for Route Exclusion 

   A LSP identifier sub-object is defined and its format 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 
     |X|Type (TBD.) |     Length    |   Attributes  |    Flag        |  
     |               PLSP-ID                   |      Reserved       |  
       X bit and Attribute fields are defined in [RFC5521].  
       X bit:  indicates whether the exclusion is mandatory (X=1) and 
   MUST be accommodated, or desired (X=0) and SHOULD be accommodated.  
      Type: Subobject Type for a per-node LSP identifier.  
      Length: The Length contains the total length of the subobject in 
   bytes, including the Type and Length fields.  
      Attributes: indicates how the exclusion object is to be 
   interpreted. Currently, Interface (Attributes = 0), Node (Attributes 
   =1) and SRLG (Attributes =2) are defined in [RFC5521] and this 
   document does not define new values. 
      Flags: is used to further specify the exclusion constraint with 
   regard to the LSP. Currently, no values are defined. 
      PLSP-ID: This is the identifier given to a LSP and it is unique on 
   a node basis. It is defined in [Stateful-PCE]. 
     Reserved: Reserved fields within subobjects MUST be transmitted as 
   zero and SHOULD be ignored on receipt. 
   One or multiple of these sub-objects can be present in the XRO 
   object. When a stateful PCE receives a path computation request 
   carrying this sub-object, it should find relevant information of 
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   these LSPs and preclude the resource during the path computation 
   process. If a stateful PCE cannot recognize one or more of the 
   received LSP identifiers, it should reply PCErr saying ''the LSP 
   state information for route exclusion purpose cannot be found'' 
   (Error-type = T.B.D., Error-value= T.B.D.). Optionally, it may 
   provide with the unrecognized identifier information to the 
   requesting PCC. 

3. IANA Considerations 

   IANA is requested to allocate new Types for the TLV/Object defined 
   in this document. 

3.1. New PCEP Error Codes 

   IANA is requested to make the following allocation in the "PCEP-
   ERROR Object Error Types and Values" registry.  The values here are 
   suggested for use by IANA. 

   Error Type        Meaning                                Reference 

   21                LSP state information missing           

   Error-value 1:    LSP state information unavailable  [This document] 

                     for the LSP re-optimization 

   Error-value 2:   LSP state information for route  

                    exclusion purpose cannot be found   [This document] 


3.2. New Subobject for the Exclude Route Object 

   The "PCEP Parameters" registry contains a subregistry "PCEP Objects" 
   with an entry for the XRO object (Exclude Route Object). 

   IANA is requested to add a further sub-object that can be carried in   
   the XRO as follows: 

   Value                      Description                Reference 

   TBD (suggested value: 5) LSP identifier sub-object [this document] 

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4. Manageability Considerations 

   The description and functionality specifications presented related 
   to stateful PCEs should also comply with the manageability 
   specifications covered in Section 8 of [RFC4655]. Furthermore, a 
   further list of manageability issues presented in [Stateful-PCE] 
   should also be considered. 

   Additional considerations are presented in the next sections. 

4.1. Requirements on Other Protocols and Functional Components 

   When the detailed route information is included for LSP state 
   synchronization (either at the initial stage or during LSP state 
   report process), this require the ingress node of an LSP carry the 
   RRO object in order to enable the collection of such information.  

5. Security Considerations 

   The security issues presented in [RFC5440] and [Stateful-PCE] apply 
   to this document.  

6. Acknowledgement 

   We would like to thank Adrian Farrel and Cyril Margaria for the 
   useful comments and discussions. 

7. References 

7.1. Normative References 

   [RFC2119] Bradner, S., "Key words for use in RFCs to indicate 
             requirements levels", RFC 2119, March 1997.  

   [RFC4655] Farrel, A., Vasseur, J.-P., and Ash, J., "A Path 
             Computation Element (PCE)-Based Architecture", RFC 4655, 
             August 2006. 

   [RFC5440] Vasseur, J.-P., and Le Roux, JL., "Path Computation 
             Element (PCE) Communication Protocol (PCEP)", RFC 5440, 
             March 2009. 

   [RFC5088] Le Roux, JL., Vasseur, J.-P., Ikejiri, Y., Zhang, R., 
             ''OSPF Protocol Extensions for Path Computation Element 
             (PCE) Discovery'', RFC 5088, January 2008. 

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   [RFC5089]  Le Roux, JL., Vasseur, JP., Ikejiri, Y., and R. Zhang,            
             "IS-IS Protocol Extensions for Path Computation Element            
             (PCE) Discovery", RFC 5089, January 2008. 

   [INTER-LAYER] Oki, E., Takeda, Tomonori, Le Roux, JL., Farrel, A., 
             Zhang, F., ''Extensions to the Path Computation Element 
             communication Protocol (PCEP) for Inter-Layer MPLS and 
             GMPLS Traffic Engineering'', draft-ietf-pce-inter-layer-ext, 
             work in progress. 

   [Stateful-PCE]Crabbe, E., Medved, J., Varga, R., Minei, I., ''PCEP 
             Extensions for Stateful PCE'', draft-ietf-pce-stateful-pce, 
             work in progress. 

   [PCEP-GMPLS] Margaria, C., Gonzalez de Dios, O., Zhang, F., ''PCEP 
             extensions for GMPLS'', draft-ietf-pce-gmpls-pcep-
             extensions, work in progress. 

7.2. Informative References 

   [Stateful-APP] Zhang, X., Minei, I., et al, "Applicability of 
             Stateful Path Computation Element (PCE) ", draft-ietf-pce-
             stateful-pce-app, work in progress. 

8. Contributors' Address 

   Dhruv Dhody 
   Huawei Technology 
   Leela Palace 
   Bangalore, Karnataka 560008 

   Yi Lin 
   Huawei Technologies 
   F3-5-B R&D Center, Huawei Base 
   Bantian, Longgang District 
   Shenzhen 518129 P.R.China 
   Phone: +86-755-28972914 

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Authors' Addresses 

   Xian Zhang 
   Huawei Technologies 
   F3-5-B R&D Center, Huawei Base 
   Bantian, Longgang District 
   Shenzhen 518129 P.R.China 
   Phone: +86-755-28972645 
   Young Lee 
   1700 Alma Drive, Suite 100 
   Plano, TX  75075 
   Phone: +1 972 509 5599 x2240 
   Fax:   +1 469 229 5397 
   Fatai Zhang 
   F3-5-B R&D Center, Huawei Base 
   Bantian, Longgang District 
   P.R. China 
   Phone: +86-755-28972912 
   Ramon Casellas 
   Av. Carl Friedrich Gauss n7 
   Castelldefels, Barcelona 08860 
   Oscar Gonzalez de Dios  
   Telefonica Investigacion y Desarrollo 
   Emilio Vargas 6 
   Madrid,   28045 
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   Phone: +34 913374013 
   Zafar Ali 
   Cisco Systems 

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