Network Working Group W. Marshall, Ed.
Request for Comments: 3313 AT&T
Category: Informational January 2003
Private Session Initiation Protocol (SIP) Extensions
for Media Authorization
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the need for Quality of Service (QoS) and
media authorization and defines a Session Initiation Protocol (SIP)
extension that can be used to integrate QoS admission control with
call signaling and help guard against denial of service attacks. The
use of this extension is only applicable in administrative domains,
or among federations of administrative domains with previously
agreed-upon policies, where both the SIP proxy authorizing the QoS,
and the policy control of the underlying network providing the QoS,
belong to that administrative domain or federation of domains.
Marshall, Ed. Informational [Page 1]
RFC 3313 SIP Extensions for Media Authorization January 2003
Table of Contents
1. Scope of Applicability......................................... 22. Conventions Used in this Document.............................. 33. Background and Motivation...................................... 34. Overview....................................................... 45. Changes to SIP to Support Media Authorization.................. 45.1 SIP Header Extension....................................... 55.2 SIP Procedures............................................. 55.2.1 User Agent Client (UAC)................................ 65.2.2 User Agent Server (UAS)................................ 65.2.3 Originating Proxy (OP)................................. 75.2.4 Destination Proxy (DP)................................. 76. Examples....................................................... 86.1 Requesting Bandwidth via RSVP Messaging.................... 86.1.1 User Agent Client Side................................. 86.1.2 User Agent Server Side................................. 107. Advantages of the Proposed Approach............................ 128. Security Considerations........................................ 139. IANA Considerations............................................ 1310. Notice Regarding Intellectual Property Rights................. 1311. Normative References.......................................... 1412. Informative References........................................ 1413. Contributors.................................................. 1514. Acknowledgments............................................... 1515. Editor's Address.............................................. 1516. Full Copyright Statement...................................... 16
This document defines a SIP extension that can be used to integrate
QoS admission control with call signaling and help guard against
denial of service attacks. The use of this extension is only
applicable in administrative domains, or among federations of
administrative domains with previously agreed-upon policies, where
both the SIP proxy authorizing the QoS, and the policy control of the
underlying network providing the QoS, belong to that administrative
domain or federation of domains. Furthermore, the mechanism is
generally incompatible with end-to-end encryption of message bodies
that describe media sessions.
This is in contrast with general Internet principles, which separate
data transport from applications. Thus, the solution described in
this document is not applicable to the Internet at large. Despite
these limitations, there are sufficiently useful specialized
deployments that meet the assumptions described above, and can accept
the limitations that result, to warrant informational publication of
this mechanism. An example deployment would be a closed network,
Marshall, Ed. Informational [Page 2]
RFC 3313 SIP Extensions for Media Authorization January 2003
which emulates a traditional circuit switched telephone network.
This document specifies a private header, facilitating use in these
specialized configurations.
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 [2].
Current IP telephony systems assume a perfect world in which there is
either an unlimited amount of bandwidth, or network layer Quality of
Service (QoS) is provided without any kind of policy control.
However, the reality is that end-to-end bandwidth is not unlimited
and uncontrolled access to QoS, in general, is unlikely. The primary
reason for this is that QoS provides preferential treatment of one
flow, at the expense of another. Consequently, it is important to
have policy control over whether a given flow should have access to
QoS. This will not only enable fairness in general, but can also
prevent denial of service attacks.
In this document, we are concerned with providing QoS for media
streams established via the Session Initiation Protocol (SIP) [3].
We assume an architecture that integrates call signaling with media
authorization, as illustrated in the Figure below. The solid lines
(A and B) show interfaces, whereas the dotted line (C) illustrates
the QoS enabled media flow:
+---------+
| Proxy |
+--------->| |
| +---------+
| ^
A)| B) |
| { }
| |
| v
v +------+
+------+ C) | Edge |
| UA |........|router|......
+------+ +------+
Figure 1 - Basic Architecture
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In this architecture, we assume a SIP UA connected to a QoS enabled
network with an edge router acting as a Policy Enforcement Point
(PEP) [6]. We further assume that a SIP UA that wishes to obtain QoS
initiates sessions through a proxy which can interface with the QoS
policy control for the data network being used. We will refer to
such a proxy as a QoS enabled proxy. We assume that the SIP UA needs
to present an authorization token to the network in order to obtain
Quality of Service (C). The SIP UA obtains this authorization token
via SIP (A) from the QoS enabled proxy by means of an extension SIP
header, defined in this document. The proxy, in turn, communicates
either directly with the edge router or with a Policy Decision Point
(PDP - not shown) [6] in order to obtain a suitable authorization
token for the UA.
Examples of access data networks, where such a QoS enabled proxy
could be used, include DOCSIS based cable networks and 3rd generation
(3G) wireless networks.
A session that needs to obtain QoS for the media streams in
accordance with our basic architecture described above goes through
the following steps.
The SIP UA sends an INVITE to the QoS enabled proxy, which for each
resulting dialog includes one or more media authorization tokens in
all unreliable provisional responses (except 100), the first reliable
1xx or 2xx response, and all retransmissions of that reliable
response for the dialog. When the UA requests QoS, it includes the
media authorization tokens with the resource reservation.
A SIP UA may also receive an INVITE from its QoS enabled proxy which
includes one or more media authorization tokens. In that case, when
the UA requests QoS, it includes the media authorization tokens with
the resource reservation. The resource reservation mechanism is not
part of SIP and is not described within the scope of this document.
This document defines a private SIP header extension to support a
media authorization scheme. In this architecture, a QoS enabled SIP
proxy supplies the UA with one or more authorization tokens which are
to be used in QoS requests. The extension defined allows network QoS
resources to be authorized by the QoS enabled SIP proxy.
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RFC 3313 SIP Extensions for Media Authorization January 2003
A new P-Media-Authorization general header field is defined. The P-
Media-Authorization header field contains one or more media
authorization tokens which are to be included in subsequent resource
reservations for the media flows associated with the session, that
is, passed to an independent resource reservation mechanism, which is
not specified here. The media authorization tokens are used for
authorizing QoS for the media stream(s). The P-Media-Authorization
header field is described by the following ABNF [4]:
P-Media-Authorization = "P-Media-Authorization" HCOLON
P-Media-Authorization-Token
*(COMMA P-Media-Authorization-Token)
P-Media-Authorization-Token = 1*HEXDIG
Table 1 below is an extension of tables 2 and 3 in [3] for the new
header field defined here. For informational purposes, this table
also includes relevant entries for standards track extension methods
published at the time this document was published. The INFO, PRACK,
UPDATE, and SUBSCRIBE and NOTIFY methods are defined respectively in
[11], [9], [12], and [10].
Where proxy ACK BYE CAN INV OPT REG
P-Media-Authorization R ad o - - o - -
P-Media-Authorization 2xx ad - - - o - -
P-Media-Authorization 101-199 ad - - - o - -
Where proxy INF PRA UPD SUB NOT
P-Media-Authorization R ad - o o - -
P-Media-Authorization 2xx ad - o o - -
Table 1: Summary of header fields.
The P-Media-Authorization header field can be used only in SIP
requests or responses that can carry a SIP offer or answer. This
naturally keeps the scope of this header field narrow.
This section defines SIP [3] procedures for usage in media
authorization compatible systems, from the point of view of the
authorizing QoS.
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The initial SIP INVITE message, mid-call messages that result in
network QoS resource changes, and mid-call changes in call
destination should be authorized. These SIP messages are sent
through the QoS enabled proxies to receive this authorization. In
order to authorize QoS, the QoS enabled SIP proxy MAY need to inspect
message bodies that describe the media streams (e.g., SDP). Hence,
it is recommended (as may be appropriate within the applicability
scope in Section 1 of this document) that such message bodies not be
encrypted end-to-end.
The P-Media-Authorization-Token, which is contained in the P-Media-
Authorization header, is included for each dialog in all unreliable
provisional responses (except 100), the first reliable 1xx or 2xx
response, and all retransmissions of that reliable response for the
dialog sent by the QoS enabled SIP proxy to the UAC.
The UAC should use all the P-Media-Authorization-Tokens from the most
recent request/response that contained the P-Media-Authorization
header when requesting QoS for the associated media stream(s). This
applies to both initial and subsequent refresh reservation messages
(for example, in an RSVP-based reservation system). A reservation
function within the UAC should convert each string of hex digits into
binary, and utilize each result as a Policy-Element, as defined in
RFC 2750 [5] (excluding Length, but including P-Type which is
included in each token). These Policy-Elements would typically
contain the authorizing entity and credentials, and be used in an
RSVP request for media data stream QoS resources.
The User Agent Server receives the P-Media-Authorization-Token in an
INVITE (or other) message from the QoS enabled SIP proxy. If the
response contains a message body that describes media streams for
which the UA desires QoS, it is recommended (as may be appropriate
within the applicability scope in Section 1 of this document) that
this message body not be encrypted end-to-end.
The UAS should use all the P-Media-Authorization-Tokens from the most
recent request/response that contained the P-Media-Authorization
header when requesting QoS for the associated media stream(s). This
applies both to initial and subsequent refresh reservation messages
(for example, in an RSVP-based reservation system). A reservation
function within the UAS should convert each string of hex digits into
binary, and utilize each result as a Policy-Element, as defined in
RFC 2750 [5] (excluding Length, but including P-Type which is
included in each token). These Policy-Elements would typically
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contain the authorizing entity and credentials, and be used in an
RSVP request for media data stream QoS resources.
When the originating QoS enabled proxy (OP) receives an INVITE (or
other) message from the UAC, the proxy authenticates the caller, and
verifies that the caller is authorized to receive QoS.
In cooperation with an originating Policy Decision Point (PDP-o), the
OP obtains and/or generates one or more media authorization tokens.
These contain sufficient information for the UAC to get the
authorized QoS for the media streams. Each media authorization token
is formatted as a Policy-Element, as defined in RFC 2750 [5]
(excluding Length, but including P-Type which is included in each
token), and then converted to a string of hex digits to form a P-
Media-Authorization-Token. The proxy's resource management function
may inspect message bodies that describe the media streams (e.g.,
SDP), in both requests and responses in order to decide what QoS to
authorize.
For each dialog that results from the INVITE (or other) message
received from the UAC, the originating proxy must add a P-Media-
Authorization header with the P-Media-Authorization-Token in all
unreliable provisional responses (except 100), the first reliable 1xx
or 2xx response, and all retransmissions of that reliable response
the proxy sends to the UAC, if that response may result in network
QoS changes. A response with an SDP may result in such changes.
The Destination QoS Enabled Proxy (DP) verifies that the called party
is authorized to receive QoS.
In cooperation with a terminating Policy Decision Point (PDP-t), the
DP obtains and/or generates a media authorization token that contains
sufficient information for the UAS to get the authorized QoS for the
media streams. The media authorization token is formatted as a
Policy-Element, as defined in RFC 2750 [5] (excluding Length, but
including P-Type which is included in each token), and then converted
to a string of hex digits to form a P-Media-Authorization-Token. The
proxy's resource management function may inspect message bodies that
describe the media streams (e.g., SDP), in both requests and
responses in order to decide what QoS to authorize.
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The Destination Proxy must add the P-Media-Authorization header with
the P-Media-Authorization-Token in the INVITE (or other) request that
it sends to the UAS if that message may result in network QoS
changes. A message with an SDP body may result in such changes.
Below we provide an example of how the P-Media-Authorization header
field can be used in conjunction with the Resource Reservation
Protocol (RSVP) [7]. The example assumes that an offer arrives in
the initial INVITE and an answer arrives in a reliable provisional
response [9], which contains an SDP description of the media flow.
Figure 2 presents a high-level overview of a basic call flow with
media authorization from the viewpoint of the UAC. Some policy
interactions have been omitted for brevity.
When a user goes off-hook and dials a telephone number, the UAC
collects the dialed digits and sends the initial (1)INVITE message to
the originating SIP proxy.
The originating SIP proxy (OP) authenticates the user/UAC and
forwards the (2)INVITE message to the proper SIP proxy.
Assuming the call is not forwarded, the terminating end-point sends a
(3)18x response to the initial INVITE via OP. Included in this
response is an indication of the negotiated bandwidth requirement for
the connection (in the form of an SDP description [8]).
When OP receives the (3)18x, it has sufficient information regarding
the end-points, bandwidth and characteristics of the media exchange.
It initiates a Policy-Setup message to PDP-o, (4)AuthProfile.
The PDP-o stores the authorized media description in its local store,
generates an authorization token that points to this description, and
returns the authorization token to the OP, (5)AuthToken.
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UAC ER-o PDP-o OP
|(1)INVITE | | | Client Authentication
|------------------------------------------->| and Call Authoriz.
| | | | (2)INVITE
| | | |-------------->
| | | | (3)18x
| | |(4)AuthProfile |<--------------
| | |<--------------|
| | |(5)AuthToken |
| | |-------------->| Auth. Token put into
| | | (6)18x | P-Media-Authorization
|<-------------------------------------------| header extension.
|---(7)PRACK-------------------------------->|
| |--(8)PRACK---->
| |<-(9)200 (PRACK)
|<--(10)200 (PRACK)--------------------------|
| | | |
|Copies the RSVP policy object |
|from the P-Media-Authorization |
|(11)RSVP-PATH | |
|----------->| (12)REQ | |
| |-------------->| Using the Auth-Token and Authorized
| | (13)DEC | Profile that is set by the SIP Proxy
| |<--------------| the PDP makes the decision
| | | |(14)RSVP-PATH
| |------------------------------------------------>
| | | |(15)RSVP-PATH
|<--------------------------------------------------------------
|Copies the RSVP policy object |
|from the P-Media-Authorization |
|(16)RSVP-RESV | |
|----------->| (17)REQ | |
| |-------------->| Using the Auth-Token and Authorized
| | (18)DEC | Profile that is set by the SIP Proxy
| |<--------------| the PDP makes the decision
| | | |(19)RSVP-RESV
| |--------------------------------------------------->
| | | |(20)RSVP-RESV
|<----------------------------------------------------------------
| | | |
Figure 2 - Media Authorization with RSVP (UAC)
The OP includes the authorization token in the P-Media-Authorization
header extension of the (6)18x message.
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Upon receipt of the (6)18x message, the UAC stores the media
authorization token from the P-Media-Authorization header. Also, the
UAC acknowledges the 18x message by sending a (7)PRACK message, which
is responded to with (10) 200.
Before sending any media, the UAC requests QoS by sending an
(11)RSVP-PATH message, which includes the previously stored P-Media-
Authorization-Token as a Policy-Element.
ER-o, upon receipt of the (11)RSVP-PATH message, checks the
authorization through a PDP-o COPS message exchange, (12)REQ. PDP-o
checks the authorization using the stored authorized media
description that was linked to the authorization token it returned to
OP. If authorization is successful, PDP-o returns an "install"
Decision, (13)DEC.
ER-o checks the admissibility for the request, and if admission
succeeds, it forwards the (14)RSVP-PATH message.
Once UAC receives the (15)RSVP-PATH message from UAS, it sends the
(16)RSVP-RESV message to reserve the network resources.
ER-o, upon receiving the (16)RSVP-RESV message checks the
authorization through a PDP-o COPS message exchange, (17)REQ. PDP-o
checks the authorization using the stored authorized media
description that was linked to the authorization token it returned to
OP. If authorization is successful, PDP-o returns an "install"
Decision, (18)DEC.
ER-o checks the admissibility for the request, and if admission
succeeds, it forwards the (19)RSVP-RESV message.
Upon receiving the (20)RSVP-RESV message, network resources have been
reserved in both directions.
Figure 3 presents a high-level overview of a call flow with media
authorization from the viewpoint of the UAS. Some policy
interactions have been omitted for brevity.
Since the destination SIP proxy (DP) has sufficient information
regarding the endpoints, bandwidth, and characteristics of the media
exchange, it initiates a Policy-Setup message to the terminating
Policy Decision Point (PDP-t) on receipt of the (1)INVITE.
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RFC 3313 SIP Extensions for Media Authorization January 2003
UAS ER-t PDP-t DP
| | | | (1)INVITE
| | | |<--------------
| | | | Proxy Authentication
| | | (2)AuthProfile| and Call Authoriz.
| | |<--------------|
| | | (3)AuthToken |
| | |-------------->| Auth. Token put into
| | | (4)INVITE | P-Media-Authorization
|<------------------------------------------| header extension
| (5)18x | | |
|------------------------------------------>| (6)18x
|Copies the RSVP policy object |-------------->
|from the P-Media-Authorization |
|(7)RSVP-PATH | |
|---------->| (8)REQ | |
| |-------------->| Using the Auth-Token and Authorized
| | (9)DEC | Profile that is set by the SIP Proxy
| |<--------------| the PDP makes the decision
| | | |(10)RSVP-PATH
| |-------------------------------------------------->
| | | |(11)RSVP-PATH
|<--------------------------------------------------------------
|Copies the RSVP policy object |
|from the P-Media-Authorization |
| (12)RSVP-RESV | |
|---------->| | |
| | (13)REQ | |
| |-------------->| Using the Auth-Token and Authorized
| | (14)DEC | Profile that is set by the SIP Proxy
| |<--------------| the PDP makes the decision
| | | |(15)RSVP-RESV
| |--------------------------------------------------->
| | | |(16)RSVP-RESV
|<---------------------------------------------------------------
| | | |<-(17)PRACK---------
|<--(18)PRACK ------------------------------|
|---(19)200 (PRACK) ----------------------->|
| | | |--(20)200 (PRACK)-->
| | | |
Figure 3 - Media Authorization with RSVP (UAS)
PDP-t stores the authorized media description in its local store,
generates an authorization token that points to this description, and
returns the authorization token to DP. The token is placed in the
(4)INVITE message and forwarded to the UAS.
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Assuming that the call is not forwarded, the UAS sends a (5)18x
response to the initial INVITE message, which is forwarded back to
UAC. At the same time, the UAS sends a (7)RSVP-PATH message which
includes the previously stored P-Media-Authorization-Token as a
Policy-Element.
ER-t, upon receiving the (7)RSVP-PATH message checks the
authorization through a PDP-t COPS message exchange. PDP-t checks
the authorization using the stored authorized media description that
was linked to the authorization token it returned to DP. If
authorization is successful, PDP-t returns an "install" Decision,
(9)DEC.
ER-t checks the admissibility for the request, and if admission
succeeds, it forwards the (10)RSVP-PATH message.
Once the UAS receives the (11)RSVP-PATH message, it sends the
(12)RSVP-RESV message to reserve the network resources.
ER-t, upon reception of the (12)RSVP-RESV message, checks the
authorization through a PDP-t COPS message exchange. PDP-t checks
the authorization using the stored authorized media description that
was linked to the authorization token that it returned to DP. If
authorization is successful, PDP-t returns an "install" Decision,
(14)DEC.
ER-t checks the admissibility for the request and if admission
succeeds, it forwards the (15)RSVP-RESV message.
Upon receiving the (16)RSVP-RESV message, network resources have been
reserved in both directions.
For completeness, we show the (17)PRACK message for the (5) 18x
response and the resulting (19) 200 response acknowledging the PRACK.
The use of media authorization makes it possible to control the usage
of network resources. In turn, this makes IP Telephony more robust
against denial of service attacks and various kinds of service
frauds. By using the authorization capability, the number of flows,
and the amount of network resources reserved can be controlled,
thereby making the IP Telephony system dependable in the presence of
scarce resources.
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RFC 3313 SIP Extensions for Media Authorization January 2003
In order to control access to QoS, a QoS enabled proxy should
authenticate the UA before providing it with a media authorization
token. Both the method and policy associated with such
authentication are outside the scope of this document, however it
could, for example, be done by using standard SIP authentication
mechanisms, as described in [3].
Media authorization tokens sent in the P-Media-Authorization header
from a QoS enabled proxy to a UA MUST be protected from eavesdropping
and tampering. This can, for example, be done through a mechanism
such as IPSec or TLS. However, this will only provide hop-by-hop
security. If there is one or more intermediaries (e.g., proxies),
between the UA and the QoS enabled proxy, these intermediaries will
have access to the P-Media-Authorization header field value, thereby
compromising confidentiality and integrity. This will enable both
theft-of-service and denial-of-service attacks against the UA.
Consequently, the P-Media-Authorization header field MUST NOT be
available to any untrusted intermediary in the clear or without
integrity protection. There is currently no mechanism defined in SIP
that would satisfy these requirements. Until such a mechanism
exists, proxies MUST NOT send P-Media-Authorization headers through
untrusted intermediaries, which might reveal or modify the contents
of this header. (Note that S/MIME-based encryption in SIP is not
available to proxy servers, as proxies are not allowed to add message
bodies.)
QoS enabled proxies may need to inspect message bodies describing
media streams (e.g., SDP). Consequently, such message bodies should
not be encrypted. In turn, this will prevent end-to-end
confidentiality of the said message bodies, which lowers the overall
security possible.
This document defines a new private SIP header for media
authorization, "P-Media-Authorization". This header has been
registered by the IANA in the SIP header registry, using the RFC
number of this document as its reference.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
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RFC 3313 SIP Extensions for Media Authorization January 2003
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] 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.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[5] Herzog, S., "RSVP Extensions for Policy Control", RFC 2750,
January 2000.
[6] Yavatkar, R., Pendarakis, D. and R. Guerin, "A Framework for
Policy-based Admission Control", RFC 2753, January 2000.
[7] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
"Resource Reservation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[8] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[9] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262, June
2002.
[10] Roach, A. B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[11] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[12] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, September 2002.
Marshall, Ed. Informational [Page 14]
RFC 3313 SIP Extensions for Media Authorization January 2003
The following people contributed significantly and were co-authors on
earlier versions of this document:
Bill Marshall (AT&T), K. K. Ramakrishnan (AT&T), Ed Miller
(Terayon), Glenn Russell (CableLabs), Burcak Beser (Juniper
Networks), Mike Mannette (3Com), Kurt Steinbrenner (3Com), Dave
Oran (Cisco), Flemming Andreasen (Cisco), John Pickens (Com21),
Poornima Lalwaney (Nokia), Jon Fellows (Copper Mountain Networks),
Doc Evans (Arris), and Keith Kelly (NetSpeak).
The Distributed Call Signaling work in the PacketCable project is the
work of a large number of people, representing many different
companies. The contributors would like to recognize and thank the
following for their assistance: John Wheeler, Motorola; David
Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jay Strater,
Jeff Ollis, Clive Holborow, Motorola; Doug Newlin, Guido Schuster,
Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; Farzi Khazai,
Nortel; John Chapman, Bill Guckel, Michael Ramalho, Cisco; Chuck
Kalmanek, Doug Nortz, John Lawser, James Cheng, Tung-Hai Hsiao,
Partho Mishra, AT&T; Telcordia Technologies; and Lucent Cable
Communications. Dean Willis and Rohan Mahy provided valuable
feedback as well.
Bill Marshall
AT&T
Florham Park, NJ 07932
EMail: wtm@research.att.com
Marshall, Ed. Informational [Page 15]
RFC 3313 SIP Extensions for Media Authorization January 2003
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
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Marshall, Ed. Informational [Page 16]