DHC WG                                                            Y. Cui
Internet-Draft                                                    Q. Sun
Intended status: Standards Track                     Tsinghua University
Expires: August 20, 2015                                       I. Farrer
                                                     Deutsche Telekom AG
                                                                  Y. Lee
                                                                  Q. Sun
                                                           China Telecom
                                                            M. Boucadair
                                                          France Telecom
                                                       February 16, 2015

              Dynamic Allocation of Shared IPv4 Addresses


   This memo describes the dynamic allocation of shared IPv4 addresses
   to clients using DHCPv4.  Address sharing allows a single IPv4
   address to be allocated to multiple active clients simultaneously,
   each client being differentiated by a unique set of transport layer
   source port numbers.  The necessary changes to existing DHCPv4 client
   and server behavior are described and a new DHCPv4 option for
   provisioning clients with shared IPv4 addresses is included.

   Due to the nature of IP address sharing, some limitations to its
   applicability are necessary.  This memo describes these limitations
   and recommends suitable architectures and technologies where address
   sharing may be utilized.

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 August 20, 2015.

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Copyright Notice

   Copyright (c) 2015 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
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Functional Overview . . . . . . . . . . . . . . . . . . . . .   3
   5.  Client-Server Interaction . . . . . . . . . . . . . . . . . .   4
   6.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Restrictions to Client Usage of a Shared IPv4 Address . .   6
   7.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Leasing Shared and Non-Shared IPv4 Addresses from a
           Single DHCP 4o6 Server  . . . . . . . . . . . . . . . . .   8
   8.  DHCPv4 Port Parameters Option . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     9.1.  Denial-of-Service . . . . . . . . . . . . . . . . . . . .   9
     9.2.  Port Randomization  . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The shortage of available public IPv4 addresses means that it is not
   always possible for operators to allocate a full IPv4 address to
   every connected device.  This problem is particularly acute whilst an
   operator is migrating from their existing, native IPv4 network to a
   native IPv6 network with IPv4 provided as an overlay service.  During
   this phase, public IPv4 addresses are needed to provide for both
   existing and transition networks.

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   Two main types of solutions have emerged to address the problem (see
   Appendix A of [RFC6269]):

   1.  Deploying Carrier Grade Network Address Translation devices
       (CGNAT, [RFC6888]).
   2.  Distributing the same public IPv4 address to multiple clients
       differentiated by non-overlapping layer 4 port sets.

   This memo focuses on the second category of solutions.

   [RFC7341] introduces a "DHCP 4o6 Server", which offers dynamic
   leasing for IPv4 addresses to clients as in DHCPv4 [RFC2131] but
   transported within a DHCPv6 message flow.  This memo specifies a new
   DHCPv4 option: OPTION_V4_PORTPARAMS, and describes how it can be used
   for the dynamic leasing of shared IPv4 addresses.

   Although DHCPv4 over DHCPv6 is used as the underlying DHCPv4
   transport mechanism throughout this document, OPTION_V4_PORTPARAMS as
   a DHCPv4 option may also be used in other solutions, if required.

   This extension is only suitable for specific architectures based on
   the Address plus Port model (A+P) [RFC6346] such as
   [I-D.ietf-softwire-lw4over6] and certain configurations of

2.  Requirements Language

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

3.  Terminology

   This document makes use of the following terms:

   Shared IPv4 address:  An IPv4 address with a restricted layer 4 port
                         set.  Connections sourced from the shared
                         address MUST use source ports within the
                         assigned port set.
   Port Set ID (PSID):   Identifier for a range of ports assigned to a
                         DHCP client.

4.  Functional Overview

   Functionally, the dynamic allocation of shared IPv4 addresses by the
   DHCP 4o6 Server is similar to dynamic allocation process for 'full'
   IPv4 addresses described in [RFC2131].  The essential difference is
   that the DHCP 4o6 Server MAY allocate the same IPv4 address to more

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   than one DHCP 4o6 client simultaneously, providing that each shared
   address allocation also includes a range of layer 4 source ports
   unique to that address (i.e., the combined tuple of IPv4 address and
   Port Set ID MUST be unique for each active lease).

   The DHCP 4o6 client implements OPTION_V4_PORTPARAMS (described
   below), which is a DHCPv4 option containing PSID (Port Set ID)
   information.  The client includes this option within the Parameter
   Request List option [RFC2132] in its DHCPv4 DHCPDISCOVER and
   DHCPREQUEST messages, indicating its support for shared, dynamic
   address leasing to the DHCP 4o6 server.

   OPTION_V4_PORTPARAMS is also implemented by the server to identify
   clients that support shared, dynamic address leasing.  With this
   option, the server can dynamically allocate PSIDs to clients and
   maintain shared IPv4 address leases.  The server then manages unique
   client leases based the IPv4 address and PSID tuple, instead of using
   only the IPv4 address.

   In the event that a dynamic, shared addressing capable client
   receives more than one DHCP 4o6 offer, where a received offer does
   not contain OPTION_V4_PORTPARAMS (i.e.,j is an offer for a full IPv4
   address), then the client SHOULD prefer the full IPv4 offer over the
   shared IPv4 address offer(s), unless specifically configured

5.  Client-Server Interaction

   The following DHCPv4 message flow is transported within the
   DHCPv4-query and DHCPv4-response messages as in DHCPv4 over DHCPv6

   1.  When the client constructs the DHCPv4 DHCPDISCOVER message to be
       transported within the DHCPv4-query message, the DHCPDISCOVER
       message MUST include the client identifier option (constructed as
       per [RFC4361]) and the Parameter Request List (PRL) option with
   2.  DHCP 4o6 Servers that receive the DHCPDISCOVER message and
       support shared IPv4 addresses respond with a DHCPOFFER message as
       usual with the shared IPv4 address in the 'yiaddr' field and MUST
       add an OPTION_V4_PORTPARAMS option containing an available
       restricted port set.  If the DHCPDISCOVER included an
       OPTION_V4_PORTPARAMS option containing a non-zero PSID-Len field,
       the DHCP 4o6 Server MAY allocate a port set of the requested size
       to the client (depending on policy).  The DHCPOFFER message is
       then encapsulated in the DHCPv4-response message and sent to the

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   3.  The client evaluates all received DHCPOFFER messages and selects
       one (e.g., based on the configuration parameters received, such
       as the size of the offered port set).  The client then sends a
       DHCPREQUEST encapsulated in the DHCPv4-query message containing
       the corresponding OPTION_V4_PORTPARAMS received in the DHCPOFFER
   4.  The server identified in the DHCPREQUEST message creates a
       binding for the client.  The binding includes the client
       identifier, the IPv4 address and the PSID.  These parameters are
       used by both the server and the client to identify a lease in any
       DHCP message.  The server MUST respond with a DHCPACK message
       containing OPTION_V4_PORTPARAMS for the requesting client.
   5.  On receipt of the DHCPACK message with the configuration
       parameters, the client MUST NOT perform an in-use probe on the
       address, such as ARPing for a duplicate allocated address.
   6.  If the client chooses to relinquish its lease by sending a
       DHCPRELEASE message, the client MUST include the leased network
       address and OPTION_V4_PORTPARAMS (with the allocated PSID) to
       identify the lease to be released.

   In the case that the client has stored the previously allocated
   address and restricted port set, the logic described in Section 3.2
   of [RFC2131] MUST be followed on the condition that the client's
   source IPv6 address for DHCP 4o6 does not change.  Note, this
   corresponds to INIT-REBOOT state defiend in [RFC2131].  The client
   MUST include the OPTION_V4_PORTPARAMS with the requested port set
   information in the message flow, which starts with a DHCPREQUEST
   message.  If the client's DHCP 4o6 IPv6 source address is changed for
   any reason, the client MUST re-initiate the DHCP 4o6 shared-address
   provisioning process by sending a DHCPDISCOVER message.

6.  Client Behavior

   A DHCP 4o6 client discovering for a shared IPv4 address MUST include
   the OPTION_V4_PORTPARAMS option code in the Parameter Request List
   option.  If a client has been successfully allocated and IPv4 address
   and PSID previously, the client MAY include in the DHCPDISCOVER
   message the 'requested IP address' option along with an
   OPTION_V4_PORTPARAMS to request that a specific IPv4 address and PSID
   be re-assigned.  Alternatively, the client MAY omit the 'requested IP
   address' option, but include an OPTION_V4_PORTPARAMS with a non-zero
   value in only the PSID-Len field, as a hint to the server for the
   preferred size of the port set.

   A client that requests OPTION_V4_PORTPARAMS, but receives DHCPOFFER
   and DHCPACK messages without OPTION_V4_PORTPARAMS SHOULD proceed as
   defined in [RFC7341] and configure a full IPv4 address with no
   address sharing.

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   When receiving a DHCPACK message containing OPTION_V4_PORTPARAMS, the
   client MUST use the received explicit PSID for configuring the
   interface for which the DHCP 4o6 request was made.

   The client MUST NOT probe a newly received IPv4 address (e.g., using
   ARP) to see if it is in use by another host.

   When the client renews or releases its DHCP lease, it MUST put the
   values of offset, PSID length and PSID into OPTION_V4_PORTPARAMS, and
   send it to the server within corresponding DHCPv4 messages that are
   conveyed through DHCPv4-query message.

   In the event that the client's DHCP 4o6 IPv6 source address is
   changed for any reason, the client MUST re-initiate the DHCP 4o6
   shared-address provisioning process by sending a DHCPDISCOVER

6.1.  Restrictions to Client Usage of a Shared IPv4 Address

   As a single IPv4 address is being shared between a number of
   different clients, the allocated shared address is only suitable for
   certain uses.  The client MUST implement a function to ensure that
   only the allocated layer 4 ports of the shared IPv4 address are used
   for sourcing new connections, or accepting inbound connections.

   The client MUST apply the following rules for all traffic destined to
   or originating from the shared IPv4 address:

   o  The client MUST use only port-aware protocols (e.g., TCP, UDP,
      DCCP etc.) or ICMP implementing [RFC5508].
   o  All connections originating from the shared IPv4 address MUST use
      a source port taken from the allocated restricted port set.
   o  The client MUST NOT accept inbound connections on ports outside of
      the allocated restricted port set.

   In order to prevent addressing conflicts which could arise from the
   allocation of the same IPv4 address, the client MUST NOT configure
   the received restricted IPv4 address on-link.

   The mechanism by which a client implements the above rules is out of
   the scope of this document.

   In the event that the DHCPv4 over DHCPv6 configuration mechanism
   fails for any reason, the client MUST NOT configure an IPv4 link-
   local address [RFC3927] (taken from the range).

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7.  Server Behavior

   The DHCP 4o6 Server MUST NOT reply with OPTION_V4_PORTPARAMS unless
   the client has explicitly listed the option code in the Parameter
   Request List (Option 55) [RFC2132].

   The DHCP 4o6 Server SHOULD reply with OPTION_V4_PORTPARAMS if the
   client includes OPTION_V4_PORTPARAMS in its Parameter Request List.
   In order to achieve the dynamic management of shared IPv4 addresses,
   the server MUST implement an address and port-set pool that provides
   the same function as the address pool in a regular DHCP server.  The
   server MUST use the combination of address and PSID as the key for
   maintaining the state of a lease, and for searching for an available
   lease for assignment.  The leasing database MUST include the IPv4
   address, PSID and client identifier of the requesting client.

   When a server receives a DHCPDISCOVER message with
   OPTION_V4_PORTPARAMS in the Parameter Request List option, the server
   determines an IPv4 address with a PSID for the requesting client.  If
   an IPv4 address with a PSID is available, the server SHOULD follow
   the logic below to select which specific address and PSID to
   provision to the client.  The logic is similar to that in
   Section 4.3.1 of [RFC2131].

   o  The client's current address with the PSID as recorded in the
      client's current lease binding, ELSE
   o  The client's previous address with PSID as recorded in the
      client's (expired or released) binding, if that address with PSID
      is in the server's pool of available addresses and PSIDs, and not
      already allocated, ELSE
   o  The address requested in the 'Requested IP Address' option along
      with the PSID parameters requested in the OPTION_V4_PORTPARAMS, if
      that pair of address and PSID is valid and not already allocated,
   o  A new address with a PSID allocated from the server's pool of
      available addresses and PSIDs.

   Upon receipt of a DHCPRELEASE message with OPTION_V4_PORTPARAMS, the
   server searches for the lease using the address in the 'ciaddr' field
   and the PSID information in the OPTION_V4_PORTPARAMS, and marks the
   lease as unallocated.

   The port-set assignment MUST be coupled with the address assignment
   process.  Therefore the server MUST assign the address and port set
   in the same DHCP message.

   When defining the pools of IPv4 addresses and PSIDs which are
   available to lease to clients, the server MUST implement a mechanism

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   to reserve some port ranges (e.g., 'well-known-ports' 0-1023) from
   allocation to clients.  The reservation policy SHOULD be

7.1.  Leasing Shared and Non-Shared IPv4 Addresses from a Single DHCP
      4o6 Server

   A single DHCP 4o6 server may serve clients that do not support
   OPTION_V4_PORTPARAMS as well as those that do.  As the rules for the
   allocation of shared addresses differ from the rules for full IPv4
   address assignment, the DHCP 4o6 server MUST implement a mechanism to
   ensure that clients not supporting OPTION_V4_PORTPARAMS do not
   receive shared addresses.  For example, two separate IPv4 addressing
   pools could be used, one of which allocates IPv4 addresses and PSIDs
   only to clients that have requested them.

   If the server is only configured with address pools for shared
   address allocation, it MUST discard requests that do not contain
   OPTION_V4_PORTPARAMS in the Parameter Request List option.

8.  DHCPv4 Port Parameters Option

   The DHCPv4 Port Parameters Option uses the same fields as the S46
   Port Parameters Option described in Section 4.5 of
   [I-D.ietf-softwire-map-dhcp], implemented as a DHCPv4 option.  This
   is to maintain compatibility with existing port set implementations.

   The format of OPTION_V4_PORTPARAMS is shown in Figure 1.

                 0                             1
                 0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
                |      option-code      |     option-len        |
                |         offset        |       PSID-len        |
                |                     PSID                      |

                  Figure 1: DHCPv4 Port Parameters Option

   o  option-code: OPTION_V4_PORTPARAMS (TBA)
   o  option-len: 4
   o  offset: (PSID offset) 8 bits long field that specifies the numeric
      value for the excluded port range/offset bits (A-bits), as per
      section 5.1 of [I-D.ietf-softwire-map].  Allowed values are
      between 0 and 15, with the default value being 6 for MAP based

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      implementations.  This parameter is unused by a Lightweight 4over6
      client and should be set to 0.
   o  PSID-len: Bit length value of the number of significant bits in
      the PSID field (also known as 'k').  When set to 0, the PSID field
      is to be ignored.  After the first 'a' bits, there are k bits in
      the port number representing the value of PSID.  Subsequently, the
      address sharing ratio would be 2^k.
   o  PSID: Explicit 16-bit (unsigned word) PSID value.  The PSID value
      algorithmically identifies a set of ports assigned to a client.
      The first k-bits on the left of this 2-octets field is the PSID
      value.  The remaining (16-k) bits on the right are padding zeros.

   [I-D.ietf-softwire-map] Section 5.1 provides a full description of
   how the PSID is interpreted by the client.

   In order to exclude the system ports ([RFC6335]) or ports reserved by
   ISPs, the former port-sets that contain well-known ports MUST NOT be
   assigned unless the operator has explicitly configured otherwise
   (e.g., by allocating a full IPv4 address).

9.  Security Considerations

   The security considerations in [RFC2131] and [RFC7341] are to be
   considered.  Additional considerations are elaborated in the
   following sub-sections.

9.1.  Denial-of-Service

   The solution is vulnerable to DoS attacks when used on a shared
   medium or when access network authentication is not a prerequisite to
   IP address assignment.  The solution SHOULD only be used on point-to-
   point links, tunnels, and/or in environments where authentication at
   the link layer is performed before IP address assignment.  It is not
   suitable for network access over shared mediums.

9.2.  Port Randomization

   Preserving port randomization [RFC6056] may be more or less difficult
   depending on the address sharing ratio (i.e., the size of the port
   space assigned to a client).  The host can only randomize the ports
   inside a fixed port range [RFC6269].

   More discussion to improve the robustness of TCP against Blind In-
   Window Attacks can be found at [RFC5961].  Other means than the
   (IPv4) source port randomization to provide protection against
   attacks should be used (e.g., use [RFC5961] to improve the robustness
   of TCP against Blind In-Window Attacks, use IPv6).

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   A proposal to preserve the entropy when selecting port is discussed
   in [I-D.bajko-pripaddrassign].

10.  IANA Considerations

   IANA is requested to assign the following new DHCPv4 Option Code in
   the registry maintained in http://www.iana.org/assignments/bootp-

            Option Name Value Data   Meaning
   -------------------- ----- ------ -----------------------------------
   OPTION_V4_PORTPARAMS TBA   4      This option is used to configure a
                                     set of ports bound to a shared IPv4

11.  Acknowledgements

   This document is merged from [I-D.sun-dhc-port-set-option] and

   The authors would like to thank Peng Wu, Gabor Bajko, Teemu
   Savolainen, Ted Lemon, Tina Tsou, Pierre Levis, Cong Liu and Marcin
   Siodelski, for their contributions to this work.

12.  References

12.1.  Normative References

              Cui, Y., Qiong, Q., Boucadair, M., Tsou, T., Lee, Y., and
              I. Farrer, "Lightweight 4over6: An Extension to the DS-
              Lite Architecture", draft-ietf-softwire-lw4over6-13 (work
              in progress), November 2014.

              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP)", draft-ietf-softwire-map-12
              (work in progress), November 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

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   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961, August

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056, January

   [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport", RFC
              7341, August 2014.

12.2.  Informative References

              Bajko, G., Savolainen, T., Boucadair, M., and P. Levis,
              "Port Restricted IP Address Assignment", draft-bajko-
              pripaddrassign-04 (work in progress), April 2012.

              Farrer, I., "Dynamic Allocation of Shared IPv4 Addresses
              using DHCPv4 over DHCPv6", draft-farrer-dhc-shared-
              address-lease-00 (work in progress), June 2013.

              Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              configuration of Softwire Address and Port Mapped
              Clients", draft-ietf-softwire-map-dhcp-11 (work in
              progress), November 2014.

              Qiong, Q., Lee, Y., Sun, Q., Bajko, G., and M. Boucadair,
              "Dynamic Host Configuration Protocol (DHCP) Option for
              Port Set Assignment", draft-sun-dhc-port-set-option-02
              (work in progress), October 2013.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927, May

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   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.

   [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269, June

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165, RFC
              6335, August 2011.

   [RFC6346]  Bush, R., "The Address plus Port (A+P) Approach to the
              IPv4 Address Shortage", RFC 6346, August 2011.

   [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common Requirements for Carrier-Grade NATs
              (CGNs)", BCP 127, RFC 6888, April 2013.

Authors' Addresses

   Yong Cui
   Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6260-3059
   Email: yong@csnet1.cs.tsinghua.edu.cn

   Qi Sun
   Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6278-5822
   Email: sunqi@csnet1.cs.tsinghua.edu.cn

   Ian Farrer
   Deutsche Telekom AG
   CTO-ATI, Landgrabenweg 151
   Bonn, NRW  53227

   Email: ian.farrer@telekom.de

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   Yiu L. Lee
   One Comcast Center
   Philadelphia  PA 19103

   Email: yiu_lee@cable.comcast.com

   Qiong Sun
   China Telecom
   Room 708, No.118, Xizhimennei Street
   Beijing  100035
   P.R. China

   Phone: +86-10-58552936
   Email: sunqiong@ctbri.com.cn

   Mohamed Boucadair
   France Telecom
   2330 Central Expressway
   Rennes  35000

   Email: mohamed.boucadair@orange.com

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