Network Working Group                                             Y. Cui
Internet-Draft                                                     J. Wu
Intended status: Standards Track                                   P. Wu
Expires: March 11, 2012                              Tsinghua University
                                                                 C. Metz
                                                     Cisco Systems, Inc.
                                                              O. Vautrin
                                                        Juniper Networks
                                                                  Y. Lee
                                                       September 8, 2011

                  Public IPv4 over Access IPv6 Network


   This draft proposes a mechanism for bidirectional IPv4 communication
   between IPv4 Internet and end hosts or IPv4 networks sited in IPv6
   access network.  This mechanism follows the softwire hub and spoke
   model and uses IPv4-over-IPv6 tunnel as basic method to traverse IPv6
   network.  By allocating public IPv4 addresses to end hosts/networks
   in IPv6, it can achieve IPv4 end-to-end bidirectional communication
   between these hosts/networks and IPv4 Internet.  This mechanism is an
   IPv4 access method for hosts and IPv4 networks sited in IPv6.

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

   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 March 11, 2012.

Copyright Notice

   Copyright (c) 2011 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
   ( 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
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Deployment Scenario  . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Scenario and requirements  . . . . . . . . . . . . . . . .  4
     4.2.  Use cases  . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Public 4over6 Mechanism  . . . . . . . . . . . . . . . . . . .  6
     5.1.  Address allocation and mapping maintenance . . . . . . . .  6
     5.2.  4over6 initiator behavior  . . . . . . . . . . . . . . . .  7
       5.2.1.  Host initiator . . . . . . . . . . . . . . . . . . . .  8
       5.2.2.  CPE initiator  . . . . . . . . . . . . . . . . . . . .  8
     5.3.  4over6 concentrator behavior . . . . . . . . . . . . . . .  8
   6.  Technical Advantages . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   Global IPv4 addresses are running out fast.  Meanwhile, the demand
   for IP address is still growing and may even burst in potential
   circumstances like "Internet of Things".  To satisfy the end users,
   operators have to push IPv6 to the front, by building IPv6 networks
   and providing IPv6 services.

   When IPv6-only networks are widely deployed, users of those networks
   will probably still need IPv4 connectivity.  This is because part of
   Internet will stay IPv4-only for a long time, and network users in
   IPv6-only networks will communicate with network users sited in the
   IPv4-only part of Internet.  This demand could eventually decrease
   with the general IPv6 adoption.

   Network operators should provide IPv4 services to IPv6 users to
   satisfy their demand, usually through tunnels.  This type of IPv4
   services differ in provisioned IPv4 addresses.  If the users can't
   get public IPv4 addresses (e.g., new network users join an ISP which
   don't have enough unused IPv4 addresses), they have to use private
   IPv4 addresses on the client side, and IPv4-private-to-public
   translation is required on the carrier side, as is described in Dual-
   stack Lite[RFC6333].  Otherwise the users can get public IPv4
   addresses, and use them for IPv4 communication.  In this case,
   translation on the carrier side won't be necessary.  The network
   users and operators can avoid all the issues raised by translation,
   such as ALG, NAT traversal, state maintenance, etc.  Note that this
   "public IPv4" situation is actually quite common.  There're
   approximatively 2^32 network users who are using or can potentially
   get public IPv4 addresses.  Most of them will switch to IPv6 sooner
   or later, and will require IPv4 services for a significant period
   after the switching.  This draft focuses on this situation, i.e., to
   provide IPv4 access for users in IPv6 networks, where public IPv4
   addresses are still available for allocation.

2.  Requirements Language

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

3.  Terminology

   Public 4over6: Public 4over6 is the mechanism proposed by this draft.
   Generally, Public 4over6 supports bidirectional communication between
   IPv4 Internet and IPv4 hosts or local networks in IPv6 access
   network, by leveraging IPv4-in-IPv6 tunnel and public IPv4 address

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   4over6 initiator: in Public 4over6 mechanism, 4over6 initiator is the
   IPv4-in-IPv6 tunnel initiator located on the user side of IPv6
   network.  The 4over6 initiator can be either a dual-stack capable
   host or a dual-stack CPE device.  In the former case, the host has
   both IPv4 and IPv6 stack but is provisioned with IPv6 access only.
   In the latter case, the CPE has both IPv6 interface for access to ISP
   network and IPv4 interface for local network connection; hosts in the
   local network can be IPv4-only.

   4over6 concentrator: in Public 4over6 mechanism, 4over6 concentrator
   is the IPv4-in-IPv6 tunnel concentrator located in IPv6 ISP network.
   It's a dual-stack router which connects to both the IPv6 network and
   IPv4 Internet.

4.  Deployment Scenario

4.1.  Scenario and requirements

   The general scenario of Public 4over6 is shown in Figure 1.  Users in
   an IPv6 network take IPv6 as their native service.  Some users are
   end hosts which face the ISP network directly, while others are local
   networks behind CPEs, such as a home LAN, an enterprise network, etc.
   The ISP network is IPv6-only rather than dual-stack, which means that
   ISP can't provide native IPv4 access to its users; however, it's
   acceptable that one or more routers on the carrier side become dual-
   stack and get connected to IPv4 Internet.  So if network users want
   to connect to IPv4, these dual-stack routers will be their

                    |    IPv6 ISP Network     |
                 +------+                     |
                 |host: |                     |
                 |initi-|                     |
                 |ator  |=================+-------+   +-----------+
                 +------+                 |4over6 |   |   IPv4    |
                    |      IPv4-in-IPv6   |Concen-|---| Internet  |
   +----------+  +------+                 |trator |   |           |
   |local IPv4|--|CPE:  |=================+-------+   +-----------+
   | network  |  |initi-|                     |
   +----------+  |ator  |                     |
                 +------+                     |
                    |                         |

   Figure 1 Public 4over6 scenario

   Before getting into any technical details, the communication

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   requirements should be stated.  The first one is that, 4over6 users
   require IPv4-to-IPv4 communication with the IPv4 Internet.  An IPv4
   access service is needed rather than an IPv6-to-IPv4 translation
   service.  (IPv6-to-IPv4 communication is out of the scope of this

   Second, 4over6 users require public IPv4 addresses rather than
   private addresses.  Public IPv4 address means there's no IPv4 CGN
   along the path, so the acquired IPv4 service is better.  In
   particular, some hosts may be application servers, public address
   works better for reasons like straightforward access, direct DNS
   registration, no stateful mapping maintenance on CGN, etc.  For the
   direct-connected host case, each host should get one public IPv4
   address.  For the local IPv4 network case, the CPE can get a public
   IPv4 address and runs an IPv4 NAT for the local network.  Here a
   local NAT is still much better than the situation that involves a
   CGN, since this NAT is in local network and can be configured and
   managed by the users.

   Third, translation is not preferred in this scenario.  If this IPv4-
   to-IPv4 communication is achieved by IPv4-IPv6 translation, it'll
   need double translation along the path, one from IPv4 to IPv6 and the
   other from IPv6 back to IPv4.  This would be quite complicated,
   especially in addressing.  Contrarily a tunnel can achieve the IPv4-
   over-IPv6 traversing easily.  Therefore this draft follows the hub
   and spoke softwire model.

   Moreover, the ISP would probably like to keep their IPv4 and IPv6
   addressing and routing separated when provisioning IPv4 over IPv6.
   Then the ISP can manage the native IPv6 network more easily and
   independently, and also provision IPv4 in a flexible, on-demand way.
   The cost is that the concentrator needs to maintain per-user address
   mapping state, which would be described in detail.

4.2.  Use cases

   Public 4over6 can be applicable in several practical cases.  The
   first one is that ISPs which still own enough IPv4 addresses switch
   to IPv6.  The ISPs can deploy public 4over6 to preserve IPv4 service
   for the customers.  This case is actually quite common.  The majority
   of the wired end users today get Internet access with public IPv4
   address.  When their ISPs switch to IPv6, these users can still use
   the same amount of IPv4 addresses for IPv4 access.  Public 4over6 can
   leverage these addresses and offer tunneled IPv4 access.

   The second case is ISPs which don't have enough IPv4 addresses any
   more switch to IPv6.  For these ISPs, dual-stack lite is so far the
   most mature solution to provision IPv4 over IPv6.  In dual-stack

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   lite, end users use private IPv4 addresses, experience a 44CGN and
   hence some service degradation.  As long as the end users use public
   IPv4 addresses, all CGN issues can be avoided and the IPv4 service
   can be full bi-directional.  In other words, Public 4over6 can be
   deployed along with DS-lite, to provide a value-added service.
   Common users adopt DS-lite to communicate with IPv4 while high-end
   users adopt Public 4over6.  The two mechanisms can actually be
   coupled easily.

   There is also a special situation in the second case that the end
   users are IPv4 application servers.  In this situation, public
   address brings significant convenience.  The DNS registration can be
   direct using dedicated address; the access of application clients can
   be straightforward with no translation; there's no need to reserve
   and maintain session state on the CGN, and no well-known port
   collision will come up.  So it's better to have servers adopt Public
   4over6 for IPv4 access when they're located in IPv6 network.

   Following the principle of Public 4over6, it's also possible to
   achieve address multiplexing and save IPv4 addresses.  There're
   already efforts on this subject, see
   [I-D.cui-softwire-b4-translated-ds-lite] and [I-D.sun-v6ops-laft6].
   The basic idea is that instead of allocating a full IPv4 address to
   every end user, the ISP can allocate an IPv4 address with restricted
   port range to every end user.

   Besides, the draft would like to be explicit about the scope of
   direct-connected host case and CPE case.  The host case is clear: the
   host is directly connected to IPv6 network, but the protocol stack on
   the host support IPv4 too.  As to the CPE case, this draft would like
   to only focus on the case that the local network behind the CPE is
   private IPv4.  If the users want to run public IPv4 in the local
   network, then they can either run dual-stack and thus turn into
   direct-connected host case(likely home LAN situation), or they can
   acquire address blocks from the ISP and build configured tunnel or
   softwire mesh[RFC5565] with the ISP network(likely enterprise network
   situation). 4over6 concentrator can be implemented to be compatible
   with the latter case too, though.

5.  Public 4over6 Mechanism

5.1.  Address allocation and mapping maintenance

   Public 4over6 can be generally considered as IPv4-over-IPv6 hub and
   spoke tunnel using public IPv4 address.  Each 4over6 initiator will
   use public IPv4 address for IPv4-over-IPv6 communication.  As is
   described above, in the host initiator case, every host will get one
   IPv4 address; in the CPE case, every CPE will get one IPv4 address,

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   which will be shared by hosts behind the CPE.  The key problem here
   is IPv4 address allocation over IPv6 network, from ISP device(s) to
   separated 4over6 initiators.

   There're two possibilities here.  One is DHCPv4 over IPv6, and the
   other is static configuration.  DHCPv4 over IPv6 is achieved by
   performing DHCPv4 over IPv6 network between 4over6 concentrator and
   4over6 initiators.  [I-D.cui-softwire-dhcp-over-tunnel] describes the
   solution to achieve this by building DHCPv4 procedure on IPv4-in-IPv6
   tunnel.  An evolved draft describing performing DHCPv4 directly on
   IPv6 environment is coming out soon in DHC working group.  As to
   static configuration, 4over6 users and the ISP operators should
   negotiate beforehand to authorize the IPv4 address.  Application
   servers can falls into this case.  Public 4over6 supports both
   address allocation manners.

   Along with IPv4 address allocation, Public 4over6 should maintain the
   IPv4-IPv6 address mappings on the concentrator.  In this type of
   address mapping, the IPv4 address is the public IPv4 address
   allocated to a 4over6 initiator, and the IPv6 addresses is the
   initiator's IPv6 address.  This mapping is used to provide correct
   encapsulation destination address for the concentrator.

   If the address is allocated through static configuration, the
   concentrator should install the mapping manually when assigning the
   address, and delete the mapping manually when recycling the address.
   Else the address is allocated by DHCPv4, the concentrator should
   participate in the DHCP procedure, either run a DHCPv4 server to
   dynamically allocate public addresses to 4over6 initiators, or
   perform the DHCP relay functions and leave the actual address
   allocation job to a dedicated DHCPv4 server located in IPv4.  When
   allocating an IPv4 address(to be more precise, when sending back a
   DHCP ACK message to a 4over6 intiator), the concentrator should
   install a mapping entry of the allocated IPv4 address and the
   initiator's IPv6 address into the address mapping table.  This entry
   should be deleted when receiving a DHCP RE:EASE of that IPv4 address,
   or the lease of that IPv4 address expires.  The latter case requires
   the concentrator to maintain the lifetime of the address leases,
   i.e., the mapping entries.

5.2.  4over6 initiator behavior

   4over6 initiator has an IPv6 interface connected to the IPv6 ISP
   network, and a tunnel interface to support IPv4-in-IPv6
   encapsulation.  In CPE case, it has at least one IPv4 interface
   connected to IPv4 local network.

   4over6 initiator should learn the 4over6 concentrator's IPv6 address

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   beforehand.  For example, if the initiator gets its IPv6 address by
   DHCPv6, it can get the 4over6 concentrator's IPv6 address through a
   DHCPv6 option[RFC6334].

5.2.1.  Host initiator

   When the initiator is a direct-connected host, it assigns the
   allocated public IPv4 address to its tunnel interface.  The host uses
   this address for IPv4 communication.  If the host acquires this
   address through DHCP, it should support DHCPv4 over IPv6.

   For IPv4 data traffic, the host performs the IPv4-in-IPv6
   encapsulation and decapsulation on the tunnel interface.  When
   sending out an IPv4 packet, it performs the encapsulation, using the
   IPv6 address of the 4over6 concentrator as the IPv6 destination
   address, and its own IPv6 address as the IPv6 source address.  The
   encapsulated packet will be forwarded to the IPv6 network.  The
   decapsulation on 4over6 initiator is simple.  When receiving an IPv4-
   in-IPv6 packet, the initiator just drops the IPv6 header, and hands
   it to upper layer.

5.2.2.  CPE initiator

   The CPE case is quite similar to the host initiator case.  The CPE
   assigns the allocated IPv4 address to its tunnel interface.  The CPE
   should support DHCPv4 over IPv6 if it acquires this address through
   DHCP.  The local IPv4 network won't take part in the public IPv4
   allocation; instead, end hosts will use private IPv4 addresses,
   possibly allocated by the CPE.

   On data plan, the CPE can be viewed as a regular IPv4 NAT(using
   tunnel interface as the NAT outside interface) cascaded with a tunnel
   initiator.  For IPv4 data packets received from the local network,
   the CPE translates these packets, using the tunnel interface address
   as the source address, and then encapsulates the translated packet
   into IPv6, using the concentrator's IPv6 address as the destination
   address, the CPE's IPv6 address as source address.  For IPv6 data
   packet received from the IPv6 network, the CPE performs decapsulation
   and IPv4 public-to-private translation.  As to the CPE itself, it
   uses the public, tunnel interface address to communicate with the
   IPv4 Internet, and the private, IPv4 interface address to communicate
   with the local network.

5.3.  4over6 concentrator behavior

   4over6 concentrator represents the IPv4-IPv6 border router working as
   the remote tunnel endpoint for 4over6 initiators, with its IPv6
   interface connected to the IPv6 network, IPv4 interface connected to

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   the IPv4 Internet, and a tunnel interface supporting IPv4-in-IPv6
   encapsulation and decapsulation.  There's no CGN on the 4over6
   concentrator, it won't perform any translation function; instead,
   4over6 concentrator maintains an IPv4-IPv6 address mapping table for
   IPv4 data encapsulation.

   4over6 concentrator maintains the IPv4-IPv6 address mapping of 4over6
   initiators.  Besides manual configration of address mappings, it runs
   either a DHCP relay or a DHCP server which support DHCPv4 over IPv6,
   for 4over6 initiators.  When sending out a DHCP ACK, the concentrator
   reads the allocated IPv4 address and the IPv6 destination address
   from the packet, installs the mapping entry into the mapping table or
   renews it if it already exists.  When the lifetime of a mapping
   entry/a lease of allocated address expires, or when the concentrator
   receives a DHCP RELEASE of allocated address, the concentrator
   deletes the corresponding mapping entry from the table.  The mapping
   entry is used to provide correct encapsulation destination address
   for concentrator encapsulation.  As long as the entry exists in the
   table, the concentrator can encapsulate inbound IPv4 packets destined
   to the initiator, with the initiator's IPv6 address as IPv6

   On the IPv6 side, 4over6 concentrator decapsulates IPv4-in-IPv6
   packets coming from 4over6 initiators.  It removes the IPv6 header of
   every IPv4-in-IPv6 packet and forwards it to the IPv4 Internet.  On
   the IPv4 side, the concentrator encapsulates the IPv4 packets
   destined to 4over6 initiators.  When performing the IPv4-in-IPv6
   encapsulation, the concentrator uses its own IPv6 address as the IPv6
   source address, uses the IPv4 destination address in the packet to
   look up IPv6 destination address in the address mapping table.  After
   the encapsulation, the concentrator sends the IPv6 packet on its IPv6
   interface to reach an initiator.

   The 4over6 concentrator, or its upstream router should advertise the
   IPv4 prefix which contains the IPv4 addresses of 4over6 users to the
   IPv4 side, in order to make these initiators reachable on IPv4

   Since the concentrator has to maintain the IPv4-IPv6 address mapping
   table, the concentrator is stateful in IP level.  Note that this
   table will be much smaller than a CGN table, as there is no port
   information involved.

6.  Technical Advantages

   Public 4over6 provides a method for users in IPv6 network to
   communicate with IPv4.  In many scenarios, this can be viewed as an
   alternative to IPv6-IPv4 translation mechanisms which have well-known

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   limitations described in [RFC4966].

   Since a 4over6 initiator uses a public IPv4 address, Public 4over6
   supports full bidirectional communication between IPv4 Internet and
   hosts/IPv4 networks in IPv6 access network.  In particular, it
   supports the servers in IPv6 network to provide IPv4 application
   service transparently.

   Public 4over6 provides IPv4 access over IPv6 network while keeps
   IPv4-IPv6 addressing and routing separated.  Therefore the ISP can
   manage the native IPv6 network independently without the influence of
   IPv4-over-IPv6 requirements, and also provision IPv4 in a flexible,
   on-demand way.

   Public 4over6 supports dynamic reuse of a single IPv4 address between
   multiple subscribers based on their dynamic requirement of
   communication with IPv4 Internet.  A subscriber will request a public
   IPv4 address for a period of time only when it need to communicate
   with IPv4 Internet.  Besides, in the CPE case, one public IPv4
   address will be shared by the local network.  So Public 4over6 can
   improve the reuse rate of IPv4 addresses.

   Public 4over6 is suited for network users/ISPs which can still get/
   provide public IPv4 addresses.  Dual-stack lite is suited for network
   users/ISPs which can no longer get/provide public IPv4 addresses.  By
   combining Public 4over6 and Dual-stack lite, the IPv4-over-IPv6 Hub
   and spoke problem can be well solved.

7.  Security Considerations

   The 4over6 concentrator should support ways to limit service only to
   registered customers.  The first step is to allocate IPv4 addresses
   only to registered customers.  One simple way is to fliter on the
   IPv6 source addresses of incoming DHCP packets and only respond to
   the ones with the IPv6 source address range of the customers.  The
   concentrator can also perform authentication during DHCP, for
   example, based on the MAC address of the initiators.  As to data
   packets, the concentrator can implement an IPv6 ingress filter on the
   tunnel interface to accept only the IPv6 address range defined in the

8.  References

8.1.  Normative References

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

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                                             BCP 14, RFC 2119,
                                             March 1997.

   [RFC4925]                                 Li, X., Dawkins, S., Ward,
                                             D., and A. Durand,
                                             "Softwire Problem
                                             Statement", RFC 4925,
                                             July 2007.

   [RFC4966]                                 Aoun, C. and E. Davies,
                                             "Reasons to Move the
                                             Network Address Translator
                                             - Protocol Translator
                                             (NAT-PT) to Historic
                                             Status", RFC 4966,
                                             July 2007.

   [RFC5549]                                 Le Faucheur, F. and E.
                                             Rosen, "Advertising IPv4
                                             Network Layer Reachability
                                             Information with an IPv6
                                             Next Hop", RFC 5549,
                                             May 2009.

   [RFC5565]                                 Wu, J., Cui, Y., Metz, C.,
                                             and E. Rosen, "Softwire
                                             Mesh Framework", RFC 5565,
                                             June 2009.

   [RFC6333]                                 Durand, A., Droms, R.,
                                             Woodyatt, J., and Y. Lee,
                                             "Dual-Stack Lite Broadband
                                             Deployments Following IPv4
                                             Exhaustion", RFC 6333,
                                             August 2011.

   [RFC6334]                                 Hankins, D. and T.
                                             Mrugalski, "Dynamic Host
                                             Configuration Protocol for
                                             IPv6 (DHCPv6) Option for
                                             Dual-Stack Lite", RFC 6334,
                                             August 2011.

8.2.  Informative References

   [I-D.cui-softwire-b4-translated-ds-lite]  Cui, Y., Wu, J., Wu, P.,
                                             Sun, Q., Xie, C., and C.
                                             Zhou, "Lightweight 4over6

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                                             in access network", draft-
                                             ds-lite-01 (work in
                                             progress), July 2011.

   [I-D.cui-softwire-dhcp-over-tunnel]       Cui, Y., Wu, P., and J. Wu,
                                             "DHCPv4 Behavior over IP-IP
                                             tunnel", draft-cui-
                                             01 (work in progress),
                                             July 2011.

   [I-D.sun-v6ops-laft6]                     Sun, Q. and C. Xie, "LAFT6:
                                             Lightweight address family
                                             transition for IPv6",
                                             (work in progress),
                                             March 2011.

Authors' Addresses

   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6260-3059

   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6278-5983

   Peng Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6278-5822

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   Chris Metz
   Cisco Systems, Inc.
   3700 Cisco Way
   San Jose, CA  95134


   Olivier Vautrin
   Juniper Networks
   1194 N Mathilda Avenue
   Sunnyvale, CA  94089


   Yiu L. Lee
   One Comcast Center
   Philadelphia, PA  19103


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