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PCE Working Group                                                  Q. Wu
Internet-Draft                                                  D. Dhody
Intended status: Standards Track                                  Huawei
Expires: February 13, 2014                                       D. King
                                                      Old Dog Consulting
                                                                D. Lopez
                                                          Telefonica I+D
                                                         August 12, 2013


 Path Computation Element (PCE) Discovery using Domain Name System(DNS)
                   draft-wu-pce-dns-pce-discovery-02

Abstract

   Discovery of the Path Computation Element (PCE) within an IGP area or
   routing domain is possible using OSPF [RFC5088] and IS-IS [RFC5089].
   However, in some deployment scenarios PCEs may not wish, or be able,
   to participate within the IGP process, therefore it would be
   beneficial for the Path Computation Client (PCC) (or other PCEs) to
   discover PCEs via an alternative mechanism to those proposed in
   [RFC5088] and [RFC5089].

   This document specifies the requirements, use cases, procedures and
   extensions to support discovery via DNS for PCE.

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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 13, 2014.

Copyright Notice

   Copyright (c) 2013 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
   (http://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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions used in this document  . . . . . . . . . . . . . .  5
   3.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Outside the Routing Domain . . . . . . . . . . . . . . . .  6
     3.2.  Query-Response v/s Advertisement . . . . . . . . . . . . .  7
     3.3.  Network Address Translation Gateway  . . . . . . . . . . .  7
   4.  Other Considerations . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Load Sharing of Path Computation Requests  . . . . . . . .  8
   5.  Discovering a Path Computation Element . . . . . . . . . . . .  9
     5.1.  Determining the PCE Service and transport protocol . . . . 10
     5.2.  Determining the IP Address of the PCE  . . . . . . . . . . 10
     5.3.  Determining path computation scope,capability,the PCE
           domains and Neighbor PCE domains . . . . . . . . . . . . . 11
     5.4.  Relationship between PCE-Domain and DNS Domain-Name  . . . 12
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18















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1.  Introduction

   The Path Computation Element Communication Protocol (PCEP) is a
   transaction-based protocol carried over TCP [RFC4655].  In order to
   be able to direct path computation requests to the Path Computation
   Element (PCE), a Path Computation Client (PCC) (or other PCEs) needs
   to know the location and capability of a PCE.

   In a network where an IGP is used and where the PCE participates in
   the IGP, discovery mechanisms exist for PCC (or PCE) to learn the
   identity and capability of each PCE.  [RFC5088] defines a PCE
   Discovery (PCED) TLV carried in an OSPF Router LSA.  Similarly,
   [RFC5089] defines the PCED sub-TLV for use in PCE Discovery using
   IS-IS.  Scope of the advertisement is limited to IGP area/level or
   Autonomous System (AS).

   However in certain scenarios not all PCEs will participate in the IGP
   instance, section 3 (Motivation) outlines a number of use cases.  In
   these cases, current PCE Discovery mechanisms are therefore not
   appropriate and another PCE discovery function would be required.

   This document describes PCE discovery via DNS.  The mechanism with
   which DNS comes to know about the PCE and its capability is out of
   scope of this document.

1.1.  Terminology

   The following terminology is used in this document.

   Domain:  As per [RFC4655], any collection of network elements within
      a common sphere of address management or path computational
      responsibility.  Examples of domains include Interior Gateway
      Protocol (IGP) areas and Autonomous Systems (ASs).

   Domain-Name:  TBD.

1.2.  Requirements

   As described in [RFC4674], the PCE Discovery information should at
   least be composed of:

   o  The PCE location: an IPv4 and/or IPv6 address that is used to
      reach the PCE.  It is RECOMMENDED to use an address that is always
      reachable if there is any connectivity to the PCE;

   o  The PCE path computation scope (i.e., intra-layer, inter-area,
      inter-AS, or inter-layer);




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   o  The set of one or more PCE-Domain(s) into which the PCE has
      visibility and for which the PCE can compute paths;

   o  The set of zero, one, or more neighbor PCE-Domain(s) toward which
      the PCE can compute paths;

   that allows PCCs to select appropriate PCEs:

   This document specifies an extension to DNS for the above PCE
   information discovery, which is complements the existing discovery
   mechanism.








































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2.  Conventions used in this document

   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 RFC2119 [RFC2119].














































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3.  Motivation

   This section discusses in more detail the motivation and use cases
   for an alternative DNS based PCE discovery mechanism.

3.1.  Outside the Routing Domain

   When the PCE is a router participating in the Interior Gateway
   Protocol (IGP), or even a server participating passively in the IGP,
   with all PCEP speakers in the same routing domain, a simple and
   efficient way to announce PCEs consists of using IGP flooding.

   But the existing mechanism does not work in following situations:

   Inter-AS:  Per domain path computation mechanism [RFC5152] or
      Backward recursive path computation (BRPC) [RFC5441] MAY be used
      by cooperating PCEs to compute inter-domain path.  In which case
      these cooperating PCEs should be known to other PCEs.  In case of
      inter-AS where the PCEs do not participate in a common IGP, the
      existing IGP discovery mechanism cannot be used to discover
      inter-AS PCE.

   Hierarchy of PCE:  The H-PCE [RFC6805] architecture does not require
      disclosure of internals of a child domain to the parent PCE.  It
      may be necessary for a third party to manage the parent PCEs
      according to commercial and policy agreements from each of the
      participating service providers [PCE-QUESTION].  [RFC6805]
      specifies that a child PCE must be configured with the address of
      its parent PCE in order for it to interact with its parent PCE.
      However handling changes in parent PCE identities and coping with
      failure events would be an issue for a configured system.  There
      is no scope for parent PCEs to advertise their presence to child
      PCEs when they are not a part of the same routing domain.

   BGP:  [I.D.draft-ietf-idr-ls-distribution] describes a mechanism by
      which links state and traffic engineering information can be
      collected from networks and shared with external components using
      the BGP routing protocol.  An external PCE MAY use this mechanism
      to populate its TED and not take part in the same IGP routing
      domain.

   NMS/OSS:  PCE server MAY gain the knowledge of Topology information
      from some management system (e.g.,NMS/OSS) and not take part in
      the same routing domain.  Also note that in some case PCC may not
      be a router and instead be a management system like NMS and may
      not be able to discover PCE via IGP discovery.





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3.2.  Query-Response v/s Advertisement

   Advertisement based IGP PCE discovery [RFC5088] and [RFC5089] floods
   the PCE information to an area, a subset of areas or to a full
   routing domain.  By the very nature of flooding and advertisements it
   generates unwanted traffic and may lead to unnecessary advertisement,
   especially when PCE information needs frequent changes.

   DNS is a query-response based mechanism, a client (say PCC) can use
   DNS to discover a PCE only when it needs it and does not require any
   other node in the network to be involved.

   Incase of Intermittent PCEP session, where PCEP sessions are
   systematically open and closed for each PCEP request, a DNS based
   query-response mechanism is more suitable.  One may utilize DNS based
   load-balancing and recovery functions.

3.3.  Network Address Translation Gateway

   PCEP uses TCP as the transport [RFC5440].  To secure TCP connection
   that underlay PCEP sessions, TLS can be used besides using TCP-MD5
   [RFC2385] and TCP-AUTH [RFC5295].  When PCC and PCE support TCP-MD5
   or TCP-AUTH while NAT does not, TCP connection establishment fails.
   When NAT gateway is in presence, a TCP or TCP/TLS connection can be
   opened by Interactive Connectivity Establishment (ICE) [RFC5245] for
   the purpose of connectivity checks.  However the TCP connection
   cannot be established in cases where one of the peers is behind a NAT
   with connection-dependent filtering properties [RFC5382].  Therefore
   IGP discovery is limited within an IGP domain and cannot be used in
   this case.





















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4.  Other Considerations

4.1.  Load Sharing of Path Computation Requests

   Multiple PCE servers can be present in a single network domain for
   redundancy.  DNS supports inherent load balancing where multiple PCEs
   (with different IP addresses) are known in DNS for a single PCE
   server name and are hidden from the PCC.

   In an IGP advertisement based PCE discovery, one learns of all the
   PCEs and it is the job of the PCC to do load-balancing.

   A DNS based load-balancing mechanism works well in case of
   Intermittent PCEP sessions and request are load-balanced among PCEs
   similar to HTTP request without any complexity at the client.




































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5.  Discovering a Path Computation Element

   The Dynamic Delegation Discovery System (DDDS) [RFC3401] is used to
   implement lazy binding of strings to data, in order to support
   dynamically configured delegation systems.  The DDDS functions by
   mapping some unique string to data stored within a DDDS database by
   iteratively applying string transformation rules until a terminal
   condition is reached.  When DDDS uses DNS as a distributed database
   of rules, these rules are encoded using the Naming Authority Pointer
   (NAPTR) Resource Record (RR).  One of these rules is the First Well
   Known Rule, which says where the process starts.

   In current specifications, the First Well Known Rule in a DDDS
   application [RFC3403] is assumed to be fixed, i.e., the domain in the
   tree where the lookups are to be routed to, is known.  This document
   proposes the input to the First Well Known Rule to be dynamic, based
   on the search path the resolver discovers or is configured to use.

   The search path of the resolver can either be pre-configured, or
   discovered using DHCP.

   When the PCC or other PCEs needs to discover PCEs in the domain into
   which the PCEP speaker has visibility (e.g.,local domain), the input
   to the First Well Known Rule MUST be the domain the PCC knows, which
   is assumed to be pre-configured in the PCC or discovered using DHCP.

   When the PCC needs to discover PCE in the other domain (e.g., AS,
   Parent PCE in the parent domain)into which the PCC has no visibility,
   it SHOULD know the domain name of that domain and use DHCP to
   discover IP address of the PCE in that domain that provides path
   computation service along with some PCE location information useful
   to a PCC for PCE selection, and contact it directly.  In some
   instances, the discovery may result in a per protocol/application
   list of domain names that are then used as starting points for the
   subsequent NAPTR lookups.  If neither the IP address nor other PCE
   location information can be discovered with the above procedure, the
   PCC MAY request a domain search list, as described in [RFC3397] and
   [RFC3646], and use it as input to the DDDS application.

   When the PCC does not find valid domain names using the procedures
   above, it MUST stop the attempt to discover any PCE.

   The dynamic rule described above SHOULD NOT be used for discovering
   services other than Path computation services described in this
   document, unless stated otherwise by a future specification.

   The procedures defined here result in an IP address, PCE domain,
   neighboring PCE domain and PCE Computation Scope where the PCC can



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   contact the PCE that hosts the service the PCC is looking for.

5.1.  Determining the PCE Service and transport protocol

   The PCC should know the service identifier for the Path Computation
   Discovery service.  The service identifier for the Path Computation
   Discovery service is defined as "PCED", The PCE supporting "PCED"
   service MUST support only TCP as transport, as described in
   [RFC5440].

   The services relevant for the task of transport protocol selection
   are those with NAPTR service fields with values "ID+M2X", where ID is
   the service identifier defined in the previous section, and X is a
   letter that corresponds to a transport protocol supported by the
   domain.  This specification only defines M2T for TCP.  This document
   also establishes an IANA registry for mappings of NAPTR service name
   to transport protocol.

   These NAPTR [RFC3403] records provide a mapping from a domain to the
   SRV [RFC2782] record for contacting a PCE with the specific transport
   protocol in the NAPTR services field.  The resource record MUST
   contain an empty regular expression and a replacement value, which
   indicates the domain name where the SRV record for that particular
   transport protocol can be found.  As per [RFC3403], the client
   discards any records whose services fields are not applicable.

   The PCC MUST discard any service fields that identify a resolution
   service whose value is not "M2T", for values of T that indicate TCP
   transport protocols supported by the client.  The NAPTR processing as
   described in RFC 3403 will result in the discovery of the most
   preferred transport protocol of the PCE that is supported by the
   client, as well as an SRV record for the PCE.

5.2.  Determining the IP Address of the PCE

   As an example, consider a client that wishes to find PCED service in
   the example.com domain.  The client performs a NAPTR query for that
   domain, and the following NAPTR records are returned:

   Order Pref Flags  Service     Regexp       Replacement
    IN NAPTR  50   50   "s"  "PCED"    ""
    _PCED._tcp.example.com
    IN NAPTR  90   50   "s"  "PCED+M2T"    ""
    _PCED._tcp.example.com

   This indicates that the domain does have a PCE providing Path
   Computation services over TCP, in that order of preference.  Since
   the client only supports TCP, TCP will be used, targeted to a host



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   determined by an SRV lookup of _PCED._tcp.example.com.  That lookup
   would return:

      ;;  Priority  Weight    Port        Target
   IN  SRV    0        1      XXXX   server1.example.com
   IN  SRV    0        2      XXXX   server2.example.com

   where XXXX represents the port number at which the service is
   reachable.

   Note that the regexp field in the NAPTR example above is empty.  The
   regexp field MUST NOT be used when discovering path computation
   services, as its usage can be complex and error prone.  Also, the
   discovery of the path computation service does not require the
   flexibility provided by this field over a static target present in
   the TARGET field.

   If the client is already configured with the information about which
   transport protocol is used for a path computation service in a
   particular domain, it can directly perform an SRV query for that
   specific transport using the service identifier of the path
   computation Service.  For example, if the client knows that it should
   be using TCP for path computation service, it can perform a SRV query
   directly for_PCED._tcp.example.com.

   Once the server providing the desired service and the transport
   protocol has been determined, the next step is to determine the IP
   address.

   According to the specification of SRV RRs in [RFC2782], the TARGET
   field is a fully qualified domain name (FQDN) that MUST have one or
   more address records; the FQDN must not be an alias, i.e., there MUST
   NOT be a CNAME or DNAME RR at this name.  Unless the SRV DNS query
   already has reported a sufficient number of these address records in
   the Additional Data section of the DNS response (as recommended by
   [RFC2782]), the PCC needs to perform A and/or AAAA record lookup(s)
   of the domain name, as appropriate.  The result will be a list of IP
   addresses, each of which can be contacted using the transport
   protocol determined previously.

5.3.  Determining path computation scope,capability,the PCE domains and
      Neighbor PCE domains

   DNS servers MAY use DNS TXT record to give additional information
   about PCE service and add TXT record to the additional information
   section that are relevant to the answer and have the same
   authenticity as the data (Generally this will be made up of A and SRV
   records)in the answer section.  The additional information includes



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   path computation scope, capability, the PCE domains and Neighbor PCE
   domains associated with the PCE. the PCC MAY inspect those Additional
   Information section and be capable of handling responses from
   nameservers that never fill in the Additional Information part of a
   response.

5.4.  Relationship between PCE-Domain and DNS Domain-Name

   As per [RFC4655], PCE-Domain is a collection of network elements
   within a common sphere of address management or path computational
   responsibility.  Examples of PCE-domains include Interior Gateway
   Protocol (IGP) areas and Autonomous Systems (ASs).  The DNS domain-
   name should have a mechanism to link the IGP area or AS to the DNS
   namespace.

   Editors Note - To be discussed further



































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6.  IANA Considerations

   The usage of NAPTR records described here requires well-known values
   for the service fields for the transport supported by Path
   Computation Services.  The table of mappings from service field
   values to transport protocols is to be maintained by IANA.

   The registration in the RFC MUST include the following information:

   Service Field: The service field being registered.

   Protocol: The specific transport protocol associated with that
   service field.  This MUST include the name and acronym for the
   protocol, along with reference to a document that describes the
   transport protocol.

   Name and Contact Information: The name, address, email address,
   and telephone number for the person performing the registration.

   The following values have been placed into the registry:

   Service Fields                    Protocol
      PCED+M2T                        TCP

   New Service Fields are to be added via Standards Action as defined in
   [RFC5226].

   IANA is also requested to register PCED as service name in the
   Protocol and Service Names registry.






















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7.  Security Considerations

   It is believed that this proposed DNS extension introduces no new
   security considerations (i.e., A list of known threats to services
   using DNS) beyond those described in [RFC3833].  For most of those
   identified threats, the DNS Security Extensions [RFC4033] does
   provide protection.  It is therefore recommended to consider the
   usage of DNSSEC [RFC4033] and the aspects of DNSSEC Operational
   Practices [RFC6781] when deploying Path Computation Services.

   In deployments where DNSSEC usage is not feasible, measures should be
   taken to protect against forged DNS responses and cache poisoning as
   much as possible.  Efforts in this direction are documented in
   [RFC5452].

   Where inputs to the procedure described in this document are fed via
   DHCP, DHCP vulnerabilities can also cause issues.  For instance, the
   inability to authenticate DHCP discovery results may lead to the Path
   Computation service results also being incorrect, even if the DNS
   process was secured.































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8.  Acknowledges

   The author would like to thank Claire Bi,Ning Kong, Liang Xia,
   Stephane Bortzmeyer,Yi Yang, Ted Lemon and Adrian Farrel for their
   review and comments that help improvement to this document.














































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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997.

   [RFC2782]  Gulbrandsen, A., "A DNS RR for specifying the location of
              services (DNS SRV)", RFC 2782, February 2000.

   [RFC3397]  Aboba, B., "Dynamic Host Configuration Protocol (DHCP)
              Domain Search Option", RFC 3397, November 2002.

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC4033]  Arends, R., "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

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

   [RFC4674]  Droms, R., "Requirements for Path Computation Element
              (PCE) Discovery", RFC 4674, December 2003.

   [RFC4848]  Daigle, D., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, April 2007.

   [RFC5226]  Narten, T., "Guidelines for Writing an IANA Considerations
              Section in RFCs", RFC 5226, May 2008.

   [RFC5440]  Le Roux, JL., "Path Computation Element (PCE)
              Communication Protocol (PCEP)", RFC 5440, April 2007.

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              December 2012.

   [RFC6805]  King, D. and A. Farrel, "The Application of the Path
              Computation Element Architecture to the Determination of a
              Sequence of Domains in MPLS and GMPLS", RFC 6805,
              November 2012.



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9.2.  Informative References

   [ALTO]     Kiesel, S., "ALTO Server Discovery",
              ID draft-ietf-alto-server-discovery-08, March 2013.

   [BGP-LS]   Gredler, H., "North-Bound Distribution of Link-State and
              TE Information using BGP",
              ID draft-ietf-idr-ls-distribution-03, May 2013.

   [PCE-QUESTION]
              Farrel, A., "Unanswered Questions in the Path Computation
              Element Architecture",
              ID http://tools.ietf.org/html/draft-ietf-pce-questions-00,
              July 2013.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

   [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part One: The Comprehensive DDDS", RFC 3401, October 2002.

   [RFC3833]  Atkins, D., "Threat Analysis of the Domain Name System
              (DNS)", RFC 3833, August 2004.

   [RFC5088]  Le Roux, JL., "OSPF Protocol Extensions for Path
              Computation Element (PCE) Discovery", RFC 5088,
              January 2008.

   [RFC5089]  Le Roux, JL., "IS-IS Protocol Extensions for Path
              Computation Element (PCE) Discovery", RFC 5089,
              January 2008.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5295]  Touch, J., "The TCP Authentication Option", RFC 5295,
              June 2010.

   [RFC5382]  Guha, S., "NAT Behavioral Requirements for TCP", RFC 5382,
              October 2008.

   [RFC5452]  Hubert, A., "Measures for Making DNS More Resilient
              against Forged Answers", RFC 5452, January 2009.






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Authors' Addresses

   Qin Wu
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   Email: sunseawq@huawei.com


   Dhruv Dhody
   Huawei
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   Email: dhruv.dhody@huawei.com


   Daniel King
   Old Dog Consulting
   UK

   Email: daniel@olddog.co.uk


   Diego R. Lopez
   Telefonica I+D


   Email: diego@tid.es



















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