Network Working Group X. Xu
Internet-Draft Huawei Technologies Co.,Ltd
Intended status: Informational M. Boucadair
Expires: April 23, 2011 France Telecom
Y. Lee
Comcast
G. Chen
China Mobile
October 20, 2010
Redundancy Requirements and Framework for Stateful Network Address
Translators (NAT)
draft-xu-behave-stateful-nat-standby-06
Abstract
This document defines a set of requirements and a framework for
ensuring redundancy for stateful Network Address Translators (NAT),
including NAT44, NAT64 and NAT46.
Requirements Language
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 RFC 2119 [RFC2119].
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 April 23, 2011.
Copyright Notice
Copyright (c) 2010 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
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Acronyms . . . . . . . . . . . . . . . . . . . 3
2.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
3. Reference Architecture . . . . . . . . . . . . . . . . . . . . 5
4. Dynamic and Static States . . . . . . . . . . . . . . . . . . 6
5. Overview of Redundancy Mechanisms . . . . . . . . . . . . . . 6
6. Cold Standby Mode . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Internal Realm . . . . . . . . . . . . . . . . . . . . . . 8
6.2. External Realm . . . . . . . . . . . . . . . . . . . . . . 8
6.3. NAT Reachability Announcement . . . . . . . . . . . . . . 9
7. Hot Standby Mode . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Internal Realm . . . . . . . . . . . . . . . . . . . . . . 10
7.2. External Realm . . . . . . . . . . . . . . . . . . . . . . 10
7.3. NAT Reachability Announcement . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. State Synchronization Protocol Considerations . . . . 12
Appendix B. Election Protocol Considerations . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Network Address Translation (NAT) has been used as an efficient way
to share the same IPv4 address among several hosts. Recently, due to
IPv4 address shortage, several proposals have been elaborated to rely
on Carrier Grade NAT (CGN, a.k.a.- LSN for Large Scale NAT) (e.g.,
[I-D.shirasaki-nat444-isp-shared-addr],
[I-D.ietf-softwire-dual-stack-lite] and
[I-D.ietf-behave-v6v4-xlate-stateful]). In such models, CGN function
(which may be embedded in a router or be deployed in standalone
devices) is deployed within large-scale networks, such as ISP
networks or enterprise ones, where a large number of customers are
located. These customers within a network which is served by a
single CGN device may experience service degradation due to the
presence of the single point of failure or loss or state information.
Therefore, redundancy capabilities of the CGN devices are strongly
desired in order to deliver highly available services to customers.
Failure detection and repair time must be therefore shortened.
This document describes a framework of redundancy for stateful NAT
including: NAT44 including DS-Lite, NAT64 and NAT46.
The main purpose of this document is to analyze means to ensure high
availability in environments where carrier grade NAT44, NAT64 and
NAT46 are deployed. Some engineering recommendations are provided
for the selection of the IPv6 prefix to build IPv4-Embedded IPv6
addresses [I-D.ietf-behave-address-format] and the routing
configuration.
Except dealing with the exceptional failures (e.g., power outage, OS
crash-down or link failure, etc.), the redundancy mechanism described
in this document can also be used for planned maintenance operations
(i.e., graceful shutdown of the Primary NAT due to maintenance
needs).
Unless otherwise mentioned, NAT and CGN terms throughout this
document, pertain to stateful NAT and stateful CGN. Stateless NAT is
out of the scope of this document.
2. Terminology and Acronyms
2.1. Acronyms
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CGN Carrier Grade NAT
LSN Large Scale NAT
DS-Lite Dual Stack Lite
AFTR Address Family Transition Router
NAT Network Address Translation
ISP Internet Service Provider
2.2. Terminology
This document makes use of the terms defined in [RFC2663]. Below are
provided terms specific to this document:
o CGN (Carrier Grade NAT) or LSN (Large Scale NAT): a NAT device
placed within a large-scale network (e.g., ISP network, enterprise
network, or campus network). CGN may be placed at the boundary
between the large-scale private network and the public Internet,
between a private network and a large-scale public network or
between two heterogeneous IP realms (i.e., IPv4 and IPv6).
o CGN internal address realm (internal realm for short): a realm
internal to the CGN.
For NAT44, the internal realm refers to the private networks.
For NAT64, the internal realm means IPv6 network or IPv6
Internet.
For NAT46, the internal realm refers to IPv4 network or IPv4
Internet.
For DS-Lite, the internal address realm is assumed to be
private IPv4 addresses even if the transport mode used to
convey exchanged traffic is IPv6. A DS-Lite CGN device
(a.k.a., Address Family Transition Router) is a special NAT44
function which uses the IPv6 address as a means to de-multiplex
users sharing the same IPv4 address
[I-D.ietf-softwire-dual-stack-lite].
o The hosts located in the internal realm are called internal hosts,
and the addresses used in the internal realm are called internal
addresses.
o CGN external address realm (external realm for short): a realm
external to the CGN.
For NAT44, the external realm refers to the IPv4 Internet.
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For NAT64, the external realm means the IPv4 Internet or IPv4
network.
For NAT46, the external realm refers to the IPv6 Internet or
IPv6 network.
o The hosts located in the external realm are called external hosts,
and the addresses used in the external realm are called external
addresses.
o Internal address pool: an address pool used for assigning internal
addresses to represent the external hosts in the internal realm.
This address pool is specific to NAT46 and NAT64.
For NAT46, the CGN will allocate one internal address (which is
an IPv4 address) from the pool to an external IPv6 host and map
the external IPv6 host's IPv6 address to this IPv4 address.
For NAT64, the CGN internal address pool is the Prefix64
[I-D.ietf-behave-address-format]. Prefix64 is used for
synthesizing internal IPv6 addresses to represent external IPv4
hosts in the internal realm.
o External address pool: an address pool used by the CGN for
assigning external addresses to the internal hosts.
For NAT44 and NAT64, the external address pool contains a set
of public IPv4 addresses.
For NAT46, the external IPv6 address pool is the Prefix64.
Prefix64 is used by the CGN for synthesizing the external IPv6
addresses to represent internal IPv4 hosts in the external
realm.
o CPE (Customer Premises Equipment): a device which is used to
interconnect the customer premise with the service provider's
network.
3. Reference Architecture
In a typical operational scenario, as illustrated in Figure 1, two
NAT devices are deployed for redundancy purposes. This is the
reference architecture for the mechanisms we describe in this memo.
Note that these mechanisms are also suitable in scenarios where more
than two NAT devices are used.
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+-------------------------+ +-----------------------+
| | | |
| +-+-----+-+ |
| | NAT-A | |
+----+-------------+ +-+-----+-+ +-------------+ |
| Internal Host | | | |External Host| |
+----+-------------+ | | +-------------+ |
| +-+-----+-+ |
| | NAT-B | |
| Internal realm +-+-----+-+ External realm |
| | | |
+-------------------------+ +-----------------------+
Figure 1: General Scenario of Dual NAT Routers
The redundancy mechanisms for NAT44, NAT46 and NAT64 are almost
identical. In all cases, the NAT device or the immediate router of
the NAT device announces the reachability of the NAT device to the
external realm. The slight difference is the NAT reachability
information. For example, NAT64 announces an IPv6 route for the
Prefix64; NAT44 announces an IPv4 default route; DS-Lite AFTR
announces an IPv6 route pointing to itself; and NAT46 announces a
route for its internal address pool. This difference does not affect
the general redundancy mechanisms, so the mechanisms described in
this memo can be applied to NAT44, NAT64 and NAT46 devices.
4. Dynamic and Static States
The NAT states mentioned in this document only mean those NAT states
which are created dynamically by outgoing packets, rather than those
static NAT states which are configured manually or with automatic
means such as UPnP or PCP. For those static NAT states (a.k.a., port
forwarding entries), they are essentially part of the configuration
data.
Port forwarding entries SHOULD be stored in permanent storage
whatever the deployed redundancy mode.
5. Overview of Redundancy Mechanisms
The fundamental principle of NAT redundancy is to make two or more
NAT devices function as a redundancy group, and select one as the
Primary NAT and the other(s) as the Backup NAT through a dedicated
election procedure or manual configuration.
In the nominal regime, traffic exchanged between one host in the
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internal realm and another host in the external realm is handled by
the Primary NAT. Once the Primary NAT is out of service, the Backup
NAT with the highest priority (if several Backup NAT devices are
deployed) takes over and provides the NAT services to the internal
hosts. This Backup NAT is then identified as new Primary NAT. Once
the former Primary NAT became operational, it could either preempt
the role of Primary NAT or stay as a candidate in the redundancy
group. This is part of administrative policies and out of scope of
this memo.
In order to implement the aforementioned procedure, means to detect
and to notify the failure of the Primary NAT to the redundancy group
SHOULD be activated.
To ensure a coherent behavior when NAT device fails, this document
assumes that both Primary and Backup NAT devices are managed by the
same administrative domain. Thus, consistent configuration policies
SHOULD be enforced in all devices. Note that the election process
MUST be deterministic and does not lead to ambiguous situation where
two or more NAT devices become Primary NAT. Moreover, the failover
SHOULD be quick to ensure service continuity and keep end-users from
perceiving service unavailability.
Three redundancy modes are described hereafter: the cold standby, the
hot standby and the partial hot standby modes:
1. The cold standby mode is simple. The NAT states are not
replicated from the Primary NAT to the Backup NAT. When the
Primary NAT fails, all the existing established sessions will be
flushed out. The internal hosts are required to re-establish
sessions to the external hosts;
2. The hot standby mode keeps established sessions while failover
happens. NAT states are replicated from the Primary NAT to the
Backup NAT. When the Primary NAT fails, the Backup NAT will take
over all the existing established sessions. The internal hosts
are not required to re-establish sessions to the external hosts.
3. The partial hot standby mode is a flavor of the hot standby mode
described above. It is used to avoid replicating NAT states of
trivial sessions (e.g., short lifetime sessions) while achieving
hot standby for significant sessions (e.g., critical protocols or
applications, long lifetime sessions etc.). Criteria for
sessions to be replicated on backup NATs SHOULD be explicitly
configured on the NAT devices of a redundancy group.
The following sections provide more information about the cold
standby and the hot standby modes.
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6. Cold Standby Mode
6.1. Internal Realm
The internal addresses used to represent the external hosts in the
internal realm SHOULD be retained after the NAT failover. The
following assesses how this requirement is met in each NAT flavor:
o For NAT44 and DS-Lite, the external hosts' internal addresses
(i.e., the addresses used to represent the external hosts in the
internal realm) are unchanged (i.e., not NAT-ed). Therefore, the
above requirement is met without additional work.
o For NAT64, the NAT devices belonging to a redundancy group SHOULD
be configured with an identical Prefix64. Since the NAT64 uses
stateless address translation for the external hosts, using the
same Prefix64 in the Backup NAT can guarantee the internal hosts
to see the same internal addresses for the external hosts.
o For NAT46, NAT devices in a redundancy group SHOULD be configured
with an identical IPv4 address pool. A subset of translation
state information SHOULD be synchronized among these NAT devices
through a dedicated state synchronization protocol such as
[I-D.xu-behave-nat-state-sync]. This translation state ensures
that the Backup NAT, once taking over as a Primary NAT, will
assign the same IPv4 addresses to the external IPv6 hosts for the
internal IPv4 hosts.
6.2. External Realm
Each NAT device in a NAT redundancy group is configured with a
different external address pool. A route to that external pool is
then announced into the external realm by the NAT device or the NAT
immediate router.
o For NAT44, DS-Lite and NAT64: NAT devices SHOULD be configured
with different external IPv4 address pools. These address pools
are not overlapped. Otherwise, when the Primary NAT fails and the
Backup NAT takes over the Primary NAT, a NAT collision may happen.
For example, assuming a Primary NAT NAT-ed internal host Host-A to
IPv4-A. IPv4-A is an address which belongs to the external
address pool. If the Backup NAT after taking over the primary NAT
was configured with the same pool, the Backup NAT MAY assign the
same IPv4-A to another internal host Host-B. So, Host-B may
receive datagrams originally targeted for Host-A. This might
cause confusion to Host-B. In addition, by using different
external address pools on each NAT device, incoming datagrams of a
given session from the external hosts are ensured to always
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traverse through the Backup NAT device after the Primary NAT
failover happens.
o For NAT46, the issue occurred in NAT44 and NAT64 cases will not
happen. NAT46 relies on stateless address translation for the
internal hosts. The Primary and Backup NAT SHOULD use the same
external Prefix64, hence the external hosts can use the Backup
NAT46 to reach the internal hosts. In Cold Standby mechanism, the
Primary and Backup NAT MAY use different Prefix64s. In contrast,
the Primary and Backup NAT in Hot Standby mechanism MUST use an
identical Prefix64.
6.3. NAT Reachability Announcement
In order to force the IP datagrams from the internal realm to always
traverse through the Primary NAT to the external realm, the Primary
NAT SHOULD announce into the internal realm a route towards the
external realm.
o For NAT44, the Primary NAT announces an IPv4 default route into
the internal realm.
o For DS-Lite, the Primary NAT announces a host route into the
internal realm.
o For NAT64, the Primary NAT announces a route for the Prefix64 into
the internal realm.
o For NAT46, the Primary NAT announces a route for the internal
address pool into the internal realm (If the internal address pool
can be aggregated to one prefix).
The Primary NAT SHOULD attempt to withdraw its previously announced
routes when it ceases the Primary role due to pre-configured
conditions, e.g.- it loses the IP connectivity to the external realm.
When the Primary NAT fails and the Backup NAT takes over, datagrams
from the internal hosts destined for the external realm SHOULD pass
through the Backup NAT. Hence, when the Backup NAT is manually
configured to switch over to become the Primary NAT, the Backup NAT
(or associated router) SHOULD announce the same route into the
internal realm, but the routing cost of this route MUST be set to a
higher value than the route announced by the Primary NAT.
Alternatively, the Primary NAT announces several more specific routes
into the internal realm while the Backup NAT announces an aggregate
route. Taken the NAT46 as an example, assuming the internal address
pool is 10.0.0.0/8, the Primary NAT announces two more specific
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routes to 10.0.0.0/9 and 10.128.0.0/9 respectively while the Backup
NAT announces an aggregate route to 10.0.0.0/8. In case the Primary
NAT and the Backup NAT are automatically elected through a dedicated
election process, the Backup NAT would be elected as a new Primary
NAT once the old Primary one fails, so it is not necessary for the
Backup NAT to make the above route announcements until it is elected
as a new Primary NAT.
In order for the external hosts to traverse through the NAT to reach
the internal hosts, the Primary and Backup NAT SHOULD announce a
route of its own external address pool into the external realm.
7. Hot Standby Mode
7.1. Internal Realm
The procedure is identical to Section 6.1.
7.2. External Realm
To preserve the established sessions during the failover and to keep
the internal addresses unchanged for the external hosts, the external
addresses for the internal hosts MUST also be preserved. To preserve
the external address of the internal host after NAT-ed, the NAT
devices in a redundancy group MUST use an identical external address
pool. In addition, they MUST assign the same external address (or
address/port pair) to a given internal host.
o For NAT46, the Primary NAT and Backup NAT MUST use an identical
Prefix64.
o For NAT44, DS-Lite and NAT64, the NAT devices in a redundancy
group MUST use the same external address pool and the translation
states on the Primary NAT device MUST be synchronized to the
Backup NAT(s) in a timely fashion.
7.3. NAT Reachability Announcement
In order to force IP datagrams between the internal realm and the
external realm always traverse through the Primary NAT, the Primary
NAT (or its associated router) SHOULD announce into the internal
realm a route towards the external realm and announce into the
external realm a route towards the external address pool.
Once the connectivity to either the external realm or the internal
realm is lost, the Primary NAT device itself or a third party SHOULD
attempt to withdraw the above routes. If the Primary NAT and the
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Backup NAT are specified manually, the Backup NAT device (or its
associated router) SHOULD also announce those routes, but with higher
enough cost or larger granularity, so as to prepare for the failover.
When the Primary NAT fails, the datagrams towards the external realm
will pass through the Backup NAT device. In case the Primary NAT and
the Backup are automatically elected through a dedicated election
procedure, the Backup NAT would be elected as a new Primary NAT when
the old Primary NAT device fails. Consequently, it is not necessary
for the Backup NAT to make the above route announcement until it is
elected as a new Primary NAT.
8. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
9. Security Considerations
TBD.
10. Acknowledgements
The author would like to thank Dan Wing and Dave Thaler for their
insightful comments and reviews, and thank Dacheng Zhang and Xuewei
Wang for their valuable editorial reviews.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.ietf-behave-address-format]
Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators",
draft-ietf-behave-address-format-10 (work in progress),
August 2010.
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[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-12 (work in
progress), July 2010.
[I-D.ietf-softwire-dual-stack-lite]
Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", draft-ietf-softwire-dual-stack-lite-06 (work
in progress), August 2010.
[I-D.shirasaki-nat444-isp-shared-addr]
Shirasaki, Y., Miyakawa, S., Nakagawa, A., Yamaguchi, J.,
and H. Ashida, "NAT444 addressing models",
draft-shirasaki-nat444-isp-shared-addr-04 (work in
progress), July 2010.
[I-D.xu-behave-nat-state-sync]
Cheng, D. and X. Xu, "NAT State Synchronization Using
SCSP", draft-xu-behave-nat-state-sync-02 (work in
progress), August 2010.
[RFC2334] Luciani, J., Armitage, G., Halpern, J., and N. Doraswamy,
"Server Cache Synchronization Protocol (SCSP)", RFC 2334,
April 1998.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling",
RFC 4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC5798] Nadas, S., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798, March 2010.
Appendix A. State Synchronization Protocol Considerations
[I-D.xu-behave-nat-state-sync] defines a candidate solution to NAT
state synchronization by using Server Cache Synchronization Protocol
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(SCSP) [RFC2334]. For more information about the proposed solution,
refer to [I-D.xu-behave-nat-state-sync].
[[Note: What to do with this section?]]
Appendix B. Election Protocol Considerations
[[Note: What to do with this section?]]
An election process and associated protocol(s) can be used to
automatically elect one NAT device among a NAT redundancy group as
the Primary NAT and the others as Backup NATs. Once the Primary NAT
fails, the Backup NAT with the highest priority SHOULD take over the
Primary NAT role after a short delay. The election protocol is also
used to track the connectivity to the external realm and the internal
realm. Once connections to the external realm or the internal realm
lost, the NAT device is not qualified to be the Primary NAT and it
will withdraw the route towards the external realm announced
previously. In the case of hot standby, it SHOULD also withdraw the
route towards the external address pool.
As an implementation example, VRRP [RFC5798] can be used as the
automatic election protocol. In addition, an interface tracking
mechanism can also be used to adjust the priority to influence the
election results.
If two NAT devices are directly connected via an Ethernet network,
VRRP can run directly on the Ethernet interfaces. Otherwise, some
extra configuration or protocol changes need to be implemented. One
option is to create conditions for VRRP to run among these devices.
For example, to create a VPLS [RFC4761][RFC4762] instance and enable
IP functions and run VRRP on those VLAN interfaces which are bound to
that VPLS instance. If enabling IP on those interfaces is not
supported, the following trick to realize the same goal, but at a
cost of consuming two physical interfaces on each NAT router: create
a VPLS instance among a set of NAT devices, and on each of them one
Ethernet interface is bound to that VPLS instance, and another IP-
enabled Ethernet interface is locally connected with that interface.
Then VRRP can run on those IP enabled Ethernet interfaces which are
all connected to that VPLS instance. Another option is to extend
VRRP so that VRRP neighbors can be specified manually and VRRP
messages can be exchanged directly among VRRP neighbors in unicast.
VRRP is only an implementation example of the election process.
Other protocols MAY be used to manage the roles of Primary and
Backup.
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Authors' Addresses
Xiaohu Xu
Huawei Technologies Co.,Ltd
KuiKe Building, No.9 Xinxi Rd.,
Hai-Dian District, Beijing 100085
P.R. China
Email: xuxh@huawei.com
Mohamed Boucadair
France Telecom
Rennes
France
Email: mohamed.boucadair@orange-ftgroup.com
Yiu L. Lee
Comcast
Email: yiu_lee@cable.comcast.com
URI: http://www.comcast.com
Gang Chen
China Mobile
53A,Xibianmennei Ave.
Beijing, 100053
P.R.China
Email: phdgang@gmail.com
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