RFC7648: Port Control Protocol (PCP) Proxy Function
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Related keywords:
(address referral)
(AFTR)
(ALG offload)
(CGN)
(control outbound connection)
(discover port number)
(DS-Lite)
(external IP address)
(firewall)
(incoming connection)
(NAT)
(NAT cascading)
(NAT control)
(NAT64)
(PCP client)
(PCP server)
(pinholing)
(port forwarding)
(port mapping)
(referral)
(running a server behind NAT)
Internet Engineering Task Force (IETF) S. Perreault
Request for Comments: 7648 Jive Communications
Category: Standards Track M. Boucadair
ISSN: 2070-1721 France Telecom
R. Penno
D. Wing
Cisco
S. Cheshire
Apple
September 2015
Port Control Protocol (PCP) Proxy Function
Abstract
This document specifies a new Port Control Protocol (PCP) functional
element: the PCP proxy. The PCP proxy relays PCP requests received
from PCP clients to upstream PCP server(s). A typical deployment
usage of this function is to help establish successful PCP
communications for PCP clients that cannot be configured with the
address of a PCP server located more than one hop away.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7648.
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RFC 7648 PCP Proxy September 2015
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
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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. Use Case: The NAT Cascade . . . . . . . . . . . . . . . . 4
1.2. Use Case: The PCP Relay . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Operation of the PCP Proxy . . . . . . . . . . . . . . . . . 6
3.1. Optimized Hairpin Routing . . . . . . . . . . . . . . . . 8
3.2. Termination of Recursion . . . . . . . . . . . . . . . . 9
3.3. Source Address for PCP Requests Sent Upstream . . . . . . 10
3.4. Unknown Opcodes and Options . . . . . . . . . . . . . . . 10
3.4.1. No NAT Is Co-located with the PCP Proxy . . . . . . . 10
3.4.2. PCP Proxy Co-located with a NAT Function . . . . . . 10
3.5. Mapping Repair . . . . . . . . . . . . . . . . . . . . . 11
3.6. Multiple PCP Servers . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Normative References . . . . . . . . . . . . . . . . . . 12
5.2. Informative References . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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RFC 7648 PCP Proxy September 2015
1. Introduction
This document defines a new Port Control Protocol (PCP) [RFC6887]
functional element: the PCP proxy. As shown in Figure 1, the
PCP proxy is logically equivalent to a PCP client back-to-back with a
PCP server. The "glue" between the two is what is specified in this
document. Other than that "glue", the server and the client behave
exactly like their regular counterparts.
The PCP proxy is responsible for relaying PCP messages received from
PCP clients to upstream PCP servers and vice versa.
Whether or not the PCP proxy is co-located with a flow-aware function
(e.g., NAT, firewall) is deployment specific.
.................
+------+ : +------+------+ : +------+
|Client|-------:-|Server|Client|-:----|Server|
+------+ : +------+------+ : +------+
: Proxy :
.................
Figure 1: Reference Architecture
This document assumes a hop-by-hop PCP authentication scheme. That
is, referring to Figure 1, the leftmost PCP client authenticates with
the PCP proxy, while the PCP proxy authenticates with the upstream
server. Note that in some deployments, PCP authentication may only
be enabled between the PCP proxy and an upstream PCP server (e.g., a
customer premises host may not authenticate with the PCP proxy, but
the PCP proxy may authenticate with the PCP server). The hop-by-hop
authentication scheme is more suitable from a deployment standpoint.
Furthermore, it allows implementations to easily support a PCP proxy
that alters PCP messages (e.g., strips a PCP option, modifies a
PCP field).
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1.1. Use Case: The NAT Cascade
In today's world, with public routable IPv4 addresses becoming less
readily available, it is increasingly common for customers to receive
a private address from their Internet Service Provider (ISP), and the
ISP uses a NAT gateway of its own to translate those packets before
sending them out onto the public Internet. This means that there is
likely to be more than one NAT on the path between client machines
and the public Internet:
o If a residential customer receives a translated address from their
ISP and then installs their own residential NAT gateway to share
that address between multiple client devices in their home, then
there are at least two NAT gateways on the path between client
devices and the public Internet.
o If a mobile phone customer receives a translated address from
their mobile phone carrier and uses "Personal Hotspot" or
"Internet Sharing" software on their mobile phone to make Wireless
LAN (WLAN) Internet access available to other client devices, then
there are at least two NAT gateways on the path between those
client devices and the public Internet.
o If a hotel guest connects a portable WLAN gateway to their hotel
room's Ethernet port to share their room's Internet connection
between their phone and their laptop computer, then packets from
the client devices may traverse the hotel guest's portable NAT,
the hotel network's NAT, and the ISP's NAT before reaching the
public Internet.
While it is possible, in theory, that client devices could somehow
discover all the NATs on the path and communicate with each one
separately using PCP [RFC6887], in practice it is not clear how
client devices would reliably learn this information. Since the NAT
gateways are installed and operated by different individuals and
organizations, no single entity has knowledge of all the NATs on the
path. Also, even if a client device could somehow know all the NATs
on the path, requiring a client device to communicate separately with
all of them imposes unreasonable complexity on PCP clients, many of
which are expected to be simple low-cost devices.
In addition, this goes against the spirit of NAT gateways. The main
purpose of a NAT gateway is to make multiple downstream client
devices appear, from the point of view of everything upstream of the
NAT gateway, to be a single client device. In the same spirit, it
makes sense for a PCP-capable NAT gateway to make multiple downstream
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client devices requesting port mappings appear, from the point of
view of everything upstream of the NAT gateway, to be a single client
device requesting port mappings.
1.2. Use Case: The PCP Relay
Another envisioned use case of the PCP proxy is to help establish
successful PCP communications for PCP clients that cannot be
configured with the address of a PCP server located more than one hop
away. A PCP proxy can, for instance, be embedded in a CPE (Customer
Premises Equipment) while the PCP server is located in a network
operated by an ISP. This is illustrated in Figure 2.
|
+------+ |
|Client|--+
+------+ | +-----+ +------+
+--|Proxy|--------<ISP network>----------|Server|
+------+ | +-----+ +------+
|Client|--+ CPE
+------+ |
|
LAN
Figure 2: PCP Relay Use Case
This works because the proxy's server side is listening on the
address used as a default gateway by the clients. The clients use
that address as a fallback when discovering the PCP server's address.
The proxy picks up the requests and forwards them upstream to the
ISP's PCP server, with whose address it has been provisioned through
regular PCP client provisioning means.
This particular use case assumes that provisioning the server's
address on the CPE is feasible while doing it on the clients in the
LAN is not, which is what makes the PCP proxy valuable.
An alternative way to contact an upstream PCP server that may be
several hops away is to use a well-known anycast address
[PCP-ANYCAST], but that technique can be problematic when multiple
PCP servers are to be contacted [PCP-DEPLOY].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
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Where this document uses the terms "upstream" and "downstream", the
term "upstream" refers to the direction outbound packets travel
towards the public Internet, and the term "downstream" refers to the
direction inbound packets travel from the public Internet towards
client systems. Typically, when a home user views a web site, their
computer sends an outbound TCP SYN packet upstream towards the public
Internet, and an inbound downstream TCP SYN ACK reply comes back from
the public Internet.
3. Operation of the PCP Proxy
Upon receipt of a PCP mapping-creation request from a downstream
PCP client, a PCP proxy first examines its local mapping table to see
if it already has a valid active mapping matching the internal
address and internal port (and in the case of PEER requests, the
remote peer) given in the request.
If the PCP proxy does not already have a valid active mapping for
this mapping-creation request, then it allocates an available port on
its external interface. We assume for the sake of this description
that the address of its external interface is itself a private
address, subject to translation by an upstream NAT. The PCP proxy
then constructs an appropriate corresponding PCP request of its own
(as described below) and sends it to its upstream NAT, and the newly
created local mapping is considered temporary until a confirming
reply is received from the upstream PCP server.
If the PCP proxy does already have a valid active mapping for this
mapping-creation request and the lifetime remaining on the local
mapping is at least 3/4 of the lifetime requested by the PCP client,
then the PCP proxy SHOULD send an immediate reply giving the
outermost external address and port (previously learned using PCP
recursively, as described below) and the actual lifetime remaining
for this mapping. If the lifetime remaining on the local mapping is
less than 3/4 of the lifetime requested by the PCP client, then the
PCP proxy MUST generate an upstream request as described below.
For mapping-deletion requests (lifetime = 0), the local mapping, if
any, is deleted, and then (regardless of whether or not a local
mapping existed) a corresponding upstream request is generated.
The PCP proxy knows the destination IP address for its upstream
PCP request using the same means that are available for provisioning
a PCP client. In particular, the PCP proxy MUST follow the procedure
defined in Section 8.1 of the PCP specification [RFC6887] to discover
its PCP server. This does not preclude other means from being used
in addition.
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In the upstream PCP request:
o The PCP client's IP address and internal port are the PCP proxy's
own external address and port just allocated for this mapping.
o The suggested external address and port in the upstream
PCP request SHOULD be copied from the original PCP request. On a
typical renewal request, this will be the outermost external
address and port previously learned by the client.
o The requested lifetime is as requested by the client if it falls
within the acceptable range for this PCP server; otherwise, it
SHOULD be capped to appropriate minimum and maximum values
configured for this PCP server.
o The mapping nonce is copied from the original PCP request.
o For PEER requests, the remote peer IP address and port are copied
from the original PCP request.
Upon receipt of a PCP reply giving the outermost (i.e., publicly
routable) external address, port, and lifetime, the PCP proxy records
this information in its own mapping table and relays the information
to the requesting downstream PCP client in a PCP reply. The
PCP proxy therefore records, among other things, the following
information in its mapping table:
o Client's internal address and port.
o External address and port allocated by this PCP proxy.
o Outermost external address and port allocated by the upstream
PCP server.
o Mapping lifetime (also dictated by the upstream PCP server).
o Mapping nonce.
In the downstream PCP reply:
o The lifetime is as granted by the upstream PCP server, or less if
the granted lifetime exceeds the maximum lifetime this PCP server
is configured to grant. If the proxy chooses to grant a
downstream lifetime greater than the lifetime granted by the
upstream PCP server (which is NOT RECOMMENDED), then this
PCP proxy MUST take responsibility for renewing the upstream
mapping itself.
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o The Epoch Time is this PCP proxy's Epoch Time, not the Epoch Time
of the upstream PCP server. Each PCP server has its own
independent Epoch Time. However, if the Epoch Time received from
the upstream PCP server indicates a loss of state in that
PCP server, the PCP proxy can either (1) recreate the lost
mappings itself or (2) reset its own Epoch Time to cause its
downstream clients to perform such state repairs themselves. A
PCP proxy MUST NOT simply copy the upstream PCP server's
Epoch Time into its downstream PCP replies, because if it suffers
its own state loss it needs the ability to communicate that state
loss to clients. Thus, each PCP server has its own independent
Epoch Time. However, as a convenience, a downstream PCP proxy may
simply choose to reset its own Epoch Time whenever it detects that
its upstream PCP server has lost state. Thus, in this case, the
PCP proxy's Epoch Time always resets whenever its upstream
PCP server loses state; it may reset at other times as well.
o The mapping nonce is copied from the reply received from the
upstream PCP server.
o The assigned external port and assigned external IP address are
copied from the reply received from the upstream PCP server (i.e.,
they are the outermost external IP address and port, not the
locally assigned external address and port). By recursive
application of this procedure, the outermost external IP address
and port are relayed from the outermost NAT, through one or more
intervening PCP proxies, until they ultimately reach the
downstream client.
o For PEER requests, the remote peer IP address and port are copied
from the reply received from the upstream PCP server.
3.1. Optimized Hairpin Routing
A PCP proxy SHOULD implement optimized hairpin routing. What this
means is the following:
o If a PCP proxy observes an outgoing packet arriving on its
internal interface that is addressed to an external address and
port appearing in the NAT gateway's own mapping table, then the
NAT gateway SHOULD (after creating a new outbound mapping if one
does not already exist) rewrite the packet appropriately and
deliver it to the internal client to which that external address
and port are currently allocated.
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o Similarly, if a PCP proxy observes an outgoing packet arriving on
its internal interface that is addressed to an *outermost*
external address and port appearing in the NAT gateway's own
mapping table, then the NAT gateway SHOULD do as described above:
create a new outbound mapping if one does not already exist, and
then rewrite the packet appropriately and deliver it to the
internal client to which that outermost external address and port
are currently allocated. This is not necessary for successful
communication, but it provides efficiency. Without this optimized
hairpin routing, the packet will be delivered all the way to the
outermost NAT gateway, which will then perform standard hairpin
translation and send it back. Using knowledge of the outermost
external address and port, this rewriting can be anticipated and
performed locally. This rewriting technique will typically offer
higher throughput and lower latency than sending packets all the
way to the outermost NAT gateway and back.
Note that traffic counters maintained by an upstream PCP server will
differ from the counters of a PCP proxy implementing optimized
hairpin routing.
3.2. Termination of Recursion
Any recursive algorithm needs a mechanism to terminate the recursion
at the appropriate point. This termination of recursion can be
achieved in a variety of ways. The following (non-exhaustive)
examples are provided for illustration purposes:
o An ISP's PCP-controlled gateway (which may embed a NAT, firewall,
or any function that can be controlled with PCP) could be
configured to know that it is the outermost PCP-controlled
gateway, and consequently it does not need to relay PCP requests
upstream.
o A PCP-controlled gateway could determine automatically that if its
external address is not one of the known private addresses
[RFC1918] [RFC6598], then its external address is a public
routable IP address, and consequently it does not need to relay
PCP requests upstream.
o Recursion may be terminated if there is no explicit list of
PCP servers configured (manually, using DHCP [RFC7291], or
otherwise) or if its default router is not responsive to
PCP requests.
o Recursion may also be terminated if the upstream PCP-controlled
device does not embed a PCP proxy.
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3.3. Source Address for PCP Requests Sent Upstream
As with a regular PCP server, the PCP-controlled device can be a NAT,
a firewall, or even some sort of hybrid. In particular, a PCP proxy
that simply relays all requests upstream can be thought of as the
degenerate case of a PCP server controlling a wide-open firewall
back-to-back with a regular PCP client.
One important property of the PCP-controlled device will affect the
PCP proxy's behavior: when the proxy's server part instructs the
device to create a mapping, that mapping's external address may or
may not be one that belongs to the proxy node.
o When the mapping's external address belongs to the proxy node, as
would presumably be the case for a NAT, then the proxy's client
side sends out an upstream PCP request using the mapping's
external IP address as the source.
o When the mapping's external address does not belong to the proxy
node, as would presumably be the case for a firewall, then the
proxy's client side needs to install upstream mappings on behalf
of its downstream clients. To do this, it MUST insert a
THIRD_PARTY option in its upstream PCP request carrying the
mapping's external address.
Note that hybrid PCP-controlled devices may create NAT-like mappings
in some circumstances and firewall-like mappings in others. A proxy
controlling such a device would adjust its behavior dynamically,
depending on the kind of mapping created.
3.4. Unknown Opcodes and Options
3.4.1. No NAT Is Co-located with the PCP Proxy
When no NAT is co-located with the PCP proxy, the port numbers
included in received PCP messages (from the PCP server or
PCP client(s)) are not altered by the PCP proxy. The PCP proxy
relays to the PCP server unknown options and Opcodes because there is
no reachability failure risk.
3.4.2. PCP Proxy Co-located with a NAT Function
By default, the proxy MUST relay unknown Opcodes and mandatory-to-
process unknown options. Rejecting unknown options and Opcodes has
the drawback of preventing a PCP client from making use of new
capabilities offered by the PCP server but not supported by the
PCP proxy, even if no IP address and/or port is included in the
option/Opcode.
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Because PCP messages with an unknown Opcode or mandatory-to-process
unknown options can carry a hidden internal address or internal port
that will not be translated, a PCP proxy MUST be configurable to
disable relaying unknown Opcodes and mandatory-to-process unknown
options. If the PCP proxy is configured to disable relaying unknown
Opcodes and mandatory-to-process unknown options, the PCP proxy MUST
behave as follows:
o a PCP proxy co-located with a NAT MUST reject, via an
UNSUPP_OPCODE error response, a received request with an unknown
Opcode.
o a PCP proxy co-located with a NAT MUST reject, via an
UNSUPP_OPTION error response, a received request with a mandatory-
to-process unknown option.
3.5. Mapping Repair
ANNOUNCE requests received from PCP clients are handled locally; as
such, these requests MUST NOT be relayed to the provisioned
PCP server.
Upon receipt of an unsolicited ANNOUNCE response from a PCP server,
the PCP proxy proceeds to renew the mappings and checks to see
whether or not there are changes compared to a local cache if it is
maintained by the PCP proxy. If no change is detected, no
unsolicited ANNOUNCE is generated towards PCP clients. If a change
is detected, the PCP proxy MUST generate unsolicited ANNOUNCE
message(s) to appropriate PCP clients. If the PCP proxy does not
maintain a local cache for the mappings, unsolicited multicast
ANNOUNCE messages are sent to PCP clients.
Upon change of its external IP address, the PCP proxy SHOULD renew
the mappings it maintained. If the PCP server assigns a different
external port, the PCP proxy SHOULD follow the PCP mapping repair
procedure [RFC6887]. This can be achieved only if a full state table
is maintained by the PCP proxy.
3.6. Multiple PCP Servers
A PCP proxy MAY handle multiple PCP servers at the same time. Each
PCP server is associated with its own epoch value. PCP clients are
not aware of the presence of multiple PCP servers.
Following the PCP Server Selection process [RFC7488], if several
PCP servers are configured to the PCP proxy, it will contact in
parallel all these PCP servers.
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In some contexts (e.g., PCP-controlled Carrier-Grade NATs (CGNs)),
the PCP proxy MAY load-balance the PCP clients among available
PCP servers. The PCP proxy MUST ensure that requests of a given
PCP client are relayed to the same PCP server.
The PCP proxy MAY rely on some fields (e.g., Zone-ID [PCP-ZONES]) in
the PCP request to redirect the request to a given PCP server.
4. Security Considerations
The PCP proxy MUST follow the security considerations detailed in the
PCP specification [RFC6887] for both the client and server side.
Section 3.3 specifies the cases where a THIRD_PARTY option is
inserted by the PCP proxy. In those cases, ways to prevent a
malicious user from creating mappings on behalf of a third party must
be employed as discussed in Section 13.1 of the PCP specification
[RFC6887]. In particular, THIRD_PARTY options MUST NOT be enabled
unless the network on which the PCP messages are to be sent is fully
trusted (via physical or cryptographic security, or both) -- for
example, if access control lists (ACLs) are installed on the
PCP proxy, the PCP server, and the network between them so that those
ACLs allow only communications from a trusted PCP proxy to the
PCP server.
A received request carrying an unknown Opcode or option SHOULD be
dropped (or, in the case of an unknown option that is not mandatory
to process, the option SHOULD be removed) if it is not compatible
with security controls provisioned to the PCP proxy.
The device embedding the PCP proxy MAY block PCP requests directly
sent to the upstream PCP server(s). This can be enforced using ACLs.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<http://www.rfc-editor.org/info/rfc6887>.
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RFC 7648 PCP Proxy September 2015
5.2. Informative References
[PCP-ANYCAST]
Kiesel, S., Penno, R., and S. Cheshire, "Port Control
Protocol (PCP) Anycast Addresses", Work in Progress,
draft-ietf-pcp-anycast-07, August 2015.
[PCP-DEPLOY]
Boucadair, M., "Port Control Protocol (PCP) Deployment
Models", Work in Progress,
draft-boucadair-pcp-deployment-cases-03, July 2014.
[PCP-ZONES]
Penno, R., "PCP Support for Multi-Zone Environments", Work
in Progress, draft-penno-pcp-zones-01, October 2011.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<http://www.rfc-editor.org/info/rfc1918>.
[RFC6598] Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
Space", BCP 153, RFC 6598, DOI 10.17487/RFC6598, April
2012, <http://www.rfc-editor.org/info/rfc6598>.
[RFC7291] Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
the Port Control Protocol (PCP)", RFC 7291,
DOI 10.17487/RFC7291, July 2014,
<http://www.rfc-editor.org/info/rfc7291>.
[RFC7488] Boucadair, M., Penno, R., Wing, D., Patil, P., and T.
Reddy, "Port Control Protocol (PCP) Server Selection",
RFC 7488, DOI 10.17487/RFC7488, March 2015,
<http://www.rfc-editor.org/info/rfc7488>.
Acknowledgements
Many thanks to C. Zhou, T. Reddy, and D. Thaler for their review and
comments.
Special thanks to F. Dupont, who contributed to this document.
Perreault, et al. Standards Track [Page 13]
RFC 7648 PCP Proxy September 2015
Authors' Addresses
Simon Perreault
Jive Communications
Quebec, QC
Canada
Email: sperreault@jive.com
Mohamed Boucadair
France Telecom
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Reinaldo Penno
Cisco
United States
Email: repenno@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
United States
Email: dwing@cisco.com
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
United States
Phone: +1 408 974 3207
Email: cheshire@apple.com
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