PCP Extension for Third Party Authorization
draft-wing-pcp-third-party-authz-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Dan Wing , Tirumaleswar Reddy.K , Prashanth Patil , Reinaldo Penno | ||
| Last updated | 2013-05-24 | ||
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draft-wing-pcp-third-party-authz-00
PCP D. Wing
Internet-Draft T. Reddy
Intended status: Standards Track P. Patil
Expires: November 26, 2013 R. Penno
Cisco
May 25, 2013
PCP Extension for Third Party Authorization
draft-wing-pcp-third-party-authz-00
Abstract
It is often desirable for an application server to permit a flow
across a firewall, as happens today when a firewall includes an
Application Layer Gateway (ALG) function. However, an ALG has
several weaknesses.
This document describes a cryptographic technique for an application
server to permit a flow across a firewall. This technique uses OAuth
and a new PCP option.
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 November 26, 2013.
Copyright Notice
Copyright (c) 2013 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
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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. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 4
5. Obtaining Token Using OAuth . . . . . . . . . . . . . . . . . 6
5.1. Generating ACCESS_TOKEN option . . . . . . . . . . . . . 9
5.2. PCP server processing ACCESS_TOKEN option . . . . . . . . 9
5.3. Processing the PCP response . . . . . . . . . . . . . . . 11
6. PCP Server and Proxy behavior . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
It is desirable for a third party to permit flows across a firewall.
A typical use-case is a SIP proxy (which is aware of legitimate
calls) which is not co-located with a firewall. Today this
functionality is provided by a firewall implementing a SIP-aware
Application Layer Gateway function, which examines the SIP signaling
to that SIP proxy and opens the appropriate pinholes for the RTP
media. This has disadvantages, as described in detail in section
Section 3.
This draft addresses the requirement "Third Party Authorization"
explained in section 4 of [I-D.reddy-pcp-auth-req].
This document proposes that a PCP [RFC6887] client communicate with
an OAuth Authorization Server to obtain a cryptographic token for its
media flow. That token is included in the PCP request and validated
by the PCP server.
Note: There is no relationship with the THIRD_PARTY option defined in
[RFC6887], which serves a different purpose.
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2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
WebRTC Server: Web server which supports WebRTC.
3. Problem Statement
To protect networks using real-time communications, firewalls or
session border controllers are typically deployed. Firewalls usually
include Application Layer Gateway functionality, which intercepts and
analyzes the session signaling traffic such as the Session Initiation
Protocol (SIP) [RFC3261] and creates dynamic mapping to permit the
corresponding media traffic. In particular, a firewall extracts
media transport addresses, transport protocol and ports from the
session description and creates a dynamic mapping for media to flow
through. This model has the following problems:
1. It does not work if the session signaling is end-to-end encrypted
(say, using TLS).
2. It does not work if the firewall does not understand the session
signaling protocol, or extensions to the protocol, used by the
endpoints.
3. It does not work if the session signaling and media traverse
different firewalls (e.g., signaling exits a network via one
firewall whereas media exits a network via a different firewall)
When an enterprise deploys a WebRTC [I-D.ietf-rtcweb-overview] or SIP
server, the above problems are relevant because:
1. The session signaling between the WebRTC application running in
the browser and the web server will use TLS.
2. WebRTC does not enforce a particular session signaling protocol;
therefore, the firewall is unlikely to understand the signaling
protocol.
3. The session signaling and the peer-to-peer media may traverse
different firewalls.
As a result the firewall will block media traffic.
A mitigation to the problems above is for the enterprise to deploy a
TURN server on their DMZ, and have WebRTC clients use that TURN
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server. The use-case explained in Section 4.2.5.1 of
[I-D.ietf-rtcweb-use-cases-and-requirements] refers to deploying a
TURN [RFC5766] server to audit all media sessions from inside the
company premises to any external peer.
However, using TURN for all such communication causes some problems
for the enterprise network administrator :
o Enterprise firewalls would typically have granular policies to
permit call initiated using selected WebRTC servers (Dr. Good) it
trusts and block others (Dr. Evil).
o TURN server is just providing the 5-tuple details (source IP
address, destination IP address, protocol number, source port
number, and destination port number) but no other details of the
WebRTC or SIP server being used to establish the call.
o TURN server could increase media latency as explained in section
4.1.2.2 of [RFC5245] and high-end TURN server would be needed (For
example when TLS-over-TCP transport is used between the client and
the server) to cater to all such calls.
o TURN server could either be located in the DMZ of the enterprise
network or located in the public Internet. If the TURN server is
located in the public Internet it comes at a high cost to the
provider of the TURN server, since the server typically needs a
high-bandwidth connection to the Internet as explained in the
Introduction of [RFC5766]. As a consequence, it is best to use a
TURN server only when a direct communication path cannot be found.
When the client and a peer use ICE to determine the communication
path, ICE will use hole punching techniques to search for a direct
path first and only use a TURN server when a direct path cannot be
found.
o Some of the other limitations of TURN explained in section 2.6 of
[RFC5766] are, the value of the Diffserv field may not be
preserved, the Explicit Congestion Notification (ECN) field may be
reset etc.
4. Solution Overview
In the below topology, the main functional elements involved are :
=========================
| WebRTC Server |
=========================
| 3rd Party Network
|
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|
==================
| WAN |-----+-+----+---+----+-+---
================== |
| |
| |
| |
+-------+-------+ |
| Firewall - | |
| PCP Server | |
+-------+-------+ |
| |
| |
Branch office | | Mobile Network
-+-+-----+-----------+-+-----+-------- ----+-+-------+------
| |
+-+------+ +--------+
| Alice | | Bob |
+--------+ +--------+
Users : Alice, Bob
Figure 1: WebRTC server in a different administrative domain
In the topology, a WebRTC Server is deployed in a third party network
trusted by the Enterprise. For the two endpoints to successfully
establish media sessions, firewall need to permit the ICE [RFC5245]
connectivity checks and subsequent media traffic.
In such a scenario the specification proposes the following technique
to solve the problem :
1. The PCP client authenticates tests the PCP server by sending its
first request without any authorization. If the PCP server
returns an AUTHORIZATION_REQUIRED error message, the PCP client
concludes that the PCP server is mandating the use of third party
authorization.
2. The PCP client then obtains a cryptographic token from an OAuth
2.0 server.
3. The PCP client sends a PCP request including cryptographic token
in the TOKEN_ACCESS option, defined below. Alternatively, the
PCP client could first obtain a cryptographic token from the
OAuth 2.0 server and send the PCP request with the
TOKEN_ACCESS_option by default.
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4. The PCP server uses the TOKEN_ACCESS option to perform third
party authorization.
This technique can also be used by a PCP capable NAT to permit MAP
request from the PCP client so that the client can learn the External
IP Addresses and Ports using MAP request/response. If server
reflexive candidates learnt using STUN [RFC5389] and External IP
addresses/Ports learnt using PCP are different then the candidates
learnt via PCP are encoded in the ICE offer and answer just like the
server reflexive candidates.
This technique can be used by any other Application Function trusted
by the network to permit time-bound, encrypted, peer-to-peer traffic.
5. Obtaining Token Using OAuth
This section explains OAuth 2.0 authorization framework [RFC6749] to
solve the "Third Party Authorization" requirement explained in
section 4 of [I-D.reddy-pcp-auth-req].
The following mapping of OAuth concepts to PCP is used :
+----------------------+----------------------------+
| OAuth | PCP |
+======================+============================+
| Client | PCP Client |
+----------------------+----------------------------+
| Resource owner | Authorization Server. For |
| | example the WebRTC server |
+----------------------+----------------------------+
| Authorization server | Authorization server. |
+----------------------+----------------------------+
| Resource server | PCP Server |
+----------------------+----------------------------+
Figure 2: OAuth terminology mapped to PCP terminology
Using the Oauth 2.0 authorization framework, a PCP client (third-
party application) obtains limited access to a PCP server (resource
server) on behalf of the WebRTC server (resource owner or
authorization server). The PCP client requests access to resources
controlled by the resource owner (WebRTC server) and hosted by the
resource server (PCP server). The PCP client obtains an access
token, lifetime, and other access attributes like the PCP options and
opcodes that the PCP client is permitted to use from the
authorization server. The PCP client conveys the token in PCP
ACESS_TOKEN option to access the protected resources hosted by the
resource server (PCP server). The PCP server validates the token and
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takes appropriate action e.g., allow the PCP request to create
mappings on the PCP server.
+---------------+
^| +-------+
+--------->| Authorization | |
| | Server | |
| | | | Authorization
| | |<--+ | (e.g Permit MAP/PEER)
(1) | +---------------+ | | (4)
Access | | |
Token | Get Token | |
Request | Resource | |
| (3) | |
| | |
| | V
+-------+---+ +-+----=-----+
| | (2) | |
| | PCP Request + Access | |
| PCP | Token | PCP |
| Client |---------------------->| Server |
| | | |
| | | |
+-----------+ +------------+
Figure 3: Interactions
OAuth in [RFC6749] defines four grant types. This specification uses
the OAuth grant type "Implicit" explained in section 1.3.2 of
[RFC6749] where the PCP client is issued an access token directly.
The scope of the access token explained in section 3.3 of [RFC6749]
MUST be PCP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Option Code=TBD| Reserved | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain Name Length | Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Domain Name |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| key id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| access token length | Reserved3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| access token |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: PCP ACCESS_TOKEN Option
The fields are described below:
Option Length: 16 bits. Indicates the length of the enclosed data,
in octets.
Domain Name Length: Length of the 'Domain Name' field in octets.
Server Domain Name: The domain name of the Authorized Server that
generated the access token.
Timestamp: 64-bit unsigned integer field containing a timestamp.
The value indicates the number of seconds since January 1, 1970,
00:00 UTC, by using a fixed point format. In this format, the
integer number of seconds is contained in the first 48 bits of the
field, and the remaining 16 bits indicate the number of 1/64K
fractions of a second (Native format - Unix).
Lifetime: The lifetime of the access token since the response was
generated, in seconds. For example, the value 3600 indicates one
hour. The Lifetime value SHOULD be equal to the "expires_in"
parameter defined in section 4.2.2 of [RFC6749].
key id key id, which is an identifier generated by the authorization
server. It generates this key id by computing a hash over the
access token using SHA-1 and truncating the hash to 96 bits
(retaining the left most bits).
access token length: Length of the access token field in octets.
OAuth does not impose any limitation on the length of the access
token but since PCP messages cannot exceed 1100 octets (Section 7
of [RFC6887]), length of the access token needs to be restricted
to fit within the maximum PCP message size. The access token is
defined in section 1.4 of [RFC6749]. TBD : what is the
recommended/maximum token length for PCP. We need a discussion of
this maximum length and analysis of what that means
access token: The access token issued by the authorization server.
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Option Name: ACCESS_TOKEN
Number: TBA in the mandatory-to-process range (IANA)
Purpose: This option conveys the token granted by authorization
server for third party authorization.
Valid for Opcodes: MAP, PEER
Length: Variable, but MUST NOT be 0.
May appear in : request. May appear in response only if it appeared
in the associated request.
Maximum occurrences : 1
5.1. Generating ACCESS_TOKEN option
The mechanism used by the OAuth client to obtain a token from the
OAuth authorization server is outside the scope of this document.
The OAuth client could obtain the token via in-band signaling or an
exclusive out-of-band protocol. This specification uses the token
type Handle described in [RFC6819]. A handle token is a reference to
some internal data structure within the OAuth authorization server;
the internal data structure contains the attributes of the token such
as allowed PCP Opcode or PCP Option, etc. The PCP client, after
receiving the access token from the OAuth authorization server,
generates the ACCESS_TOKEN option which is included in the PCP
request to the PCP server.
5.2. PCP server processing ACCESS_TOKEN option
The PCP server performs processing in the order of the paragraphs
below.
When the PCP server receives a PCP request with ACCESS_TOKEN option,
it will verify that the access token is valid. To address replay
attacks, the PCP server should perform the following check :
When a PCP request with ACCESS_TOKEN Option is received, the received
timestamp (TSnew in the Timestamp field) is checked and the
cryptographic token is accepted if the timestamp is recent enough to
the reception time of the PCP request, RDnew :
Lifetime + Delta > abs(RDnew - TSnew)
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The recommended value for the allowed Delta is 5 seconds. If the
timestamp is NOT within the boundaries then discard the PCP request
with AUTHORIZATION_INVALID error response.
After the validation described above, the PCP server communicates
with the authorization server in order to validate the token and
obtain token-bound data. The mechanism for communication is outside
the scope of this document. The PCP server makes a request to the
authorization server to validate the token but produces no other data
with the request. If the token is successfully validated, the
authorization server just returns the token bound authorization data
in response. The PCP server then matches this authorization data
with what is requested in the PCP request sent by the PCP client. If
the authorization sets match, the PCP server honors the PCP request
made by the PCP client.
If the token is invalid or the request exceeds what is authorized by
the token then the PCP server generates AUTHORIZATION_INVALID error
response. An example might be that an OAuth authorization server
permits creating 5 mappings, and the PCP request made by the client
is trying to create a 6th mapping.
Handle token type was selected for the following reasons :
1. Authorization Server can inform the PCP server to revoke the
access token after the call is terminated. This mechanism
ensures that even if the PCP client does not close the dynamic
mapping created, the PCP server based on the revocation
notification from the Authorization Server can close the dynamic
mapping.
2. PCP-controlled Firewall with restrictive policies may also want
to validate with the Authorization Server if the selected
candidate pairs in the final offer/answer match the 5-tuple {dest
addr, source addr, protocol, dest port, source port} sessions
traversing the Firewall. This validation ensures that the PCP
client is using the token only to send and receive the media
streams finalized in the call to the remote peer. Thus PCP
server can make sure that the token cannot be used for anything
else.
3. If PCP authentication [I-D.ietf-pcp-authentication] is used then
the PCP server may also validate with the authorization server if
the access token is issued and used by the same user or not.
Another approach, not discussed in this document, is a self-contained
token, where all the information necessary to authenticate the
validity of the token is contained within the token itself. This
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approach has the benefit of avoiding a protocol between the PCP
server and the OAuth authentication server for token validation, thus
reducing latency. However, this approach has the drawback of needing
a large PCP packet to accommodate the token and requiring the
authorization server generate its message integrity over exactly the
PCP fields, in the same order, that will be sent by the PCP client.
Because PCP messages are limited to 1100 octets, using the handle
approach is more flexible and the trade-off for additional latency is
reasonable. The other disadvantages of self-contained token, such as
difficulties with revocation etc, are discussed in[RFC6819].
Note to implementors : If the additional latency needs to be avoided
and it is permissible to create a PCP mapping briefly for PCP
clients, an implementation could create PCP mappings while the token
is being validated. The PCP server could create a mapping
immediately, send a PCP response and in parallel start verification
of the token. If the verification request times out or returns a
failure response, the PCP mapping can be destroyed and a PCP mapping
update is sent to the PCP client. The PCP server while waiting for
the validation response to arrive from Authorization server can
either drop or buffer the traffic matching the mapping created.
5.3. Processing the PCP response
Upon receiving a PCP response, the PCP client performs the normal
processing described in Section 8.3 of [RFC6887]. If the PCP
response was SUCCESS (0), the PCP server has determined that the
token is valid. If the PCP response was AUTHORIZATION_INVALID, it
indicates that the token could be invalid, expired or the PCP request
exceeded what is authorized by the token.
6. PCP Server and Proxy behavior
The ACCESS_TOKEN option is mandatory-to-process (its most significant
bit is clear). Thus, per existing behavior described in [RFC6887], a
PCP server receiving this option MUST return the error
MALFORMED_OPTION if the option contents are malformed, or
UNSUPP_OPTION if the option is unrecognized, unimplemented, or
disabled, or if the client is not authorized to use the option.
PCP Proxy MUST follow the rules mentioned in section of 7 of
[I-D.ietf-pcp-proxy] when processing ACCESS_TOKEN option.
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7. Security Considerations
Security considerations discussed in [RFC6887] and PCP authentication
[I-D.ietf-pcp-authentication] are to be taken into account.
PCP server will delete explicit dynamic mappings after lifetime of
the cryptographic token expires. The PCP server could maintain the
mapping for an extra Delta seconds. The Delta value (e.g 10 seconds)
would be locally configured on the PCP server. PCP client must
obtain a new cryptographic token from the authorization server when
the current token becomes invalid or expires. The PCP client must
propagate the new cryptographic token to the PCP server to refresh
the lifetime of the mappings before the current token becomes invalid
or expires. PCP server in addition to timestamp checking can also
maintain a cache of used kid as an effective countermeasure against
replay attacks.
8. IANA Considerations
We request IANA register the PCP option ACCESS_TOKEN and the result
codes AUTHORIZATION_REQUIRED and AUTHORIZATION_INVALID in
[pcp-registry].
9. Acknowledgements
Authors would like to thank Charles Eckel and Paul Jones for their
comments and review.
10. References
10.1. Normative References
[I-D.ietf-pcp-authentication]
Wasserman, M., Hartman, S., and D. Zhang, "Port Control
Protocol (PCP) Authentication Mechanism", draft-ietf-pcp-
authentication-01 (work in progress), October 2012.
[I-D.ietf-pcp-proxy]
Boucadair, M., Penno, R., and D. Wing, "Port Control
Protocol (PCP) Proxy Function", draft-ietf-pcp-proxy-02
(work in progress), February 2013.
[I-D.reddy-pcp-auth-req]
Reddy, T., Patil, P., Wing, D., and R. Penno, "PCP
Authentication Requirements", draft-reddy-pcp-auth-req-03
(work in progress), May 2013.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, October 2011.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
2013.
[pcp-registry]
IANA, , "Port Control Protocol (PCP) Parameters", May
2013, <http://www.iana.org/assignments/pcp-parameters/pcp-
parameters.xml>.
10.2. Informative References
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-06 (work
in progress), February 2013.
[I-D.ietf-rtcweb-use-cases-and-requirements]
Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use-cases and Requirements", draft-
ietf-rtcweb-use-cases-and-requirements-10 (work in
progress), December 2012.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
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[RFC6342] Koodli, R., "Mobile Networks Considerations for IPv6
Deployment", RFC 6342, August 2011.
[RFC6819] Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
January 2013.
Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Prashanth Patil
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marthalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: praspati@cisco.com
Reinaldo Penno
Cisco Systems, Inc.
170 West Tasman Drive
San Jose 95134
USA
Email: repenno@cisco.com
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