Network Working Group A. Johnston
Internet-Draft Avaya
Intended status: Standards Track J. Rafferty
Expires: March 23, 2012 Dialogic
September 20, 2011
A Mechanism for Transporting User to User Call Control Information in
SIP
draft-ietf-cuss-sip-uui-02
Abstract
There is a need for applications using SIP to exchange User to User
Information (UUI) data during session establishment. This
information, known as call control UUI, is a small piece of data
inserted by an application initiating the session, and utilized by an
application accepting the session. This data is opaque to SIP and
its function is unrelated to any basic SIP function. This document
defines a new SIP header field, User-to-User, to transport UUI, along
with an extension mechanism.
Status of this Memo
This Internet-Draft is submitted to IETF 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
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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 March 23, 2012.
Copyright Notice
Copyright (c) 2011 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
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described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements Discussion . . . . . . . . . . . . . . . . . . . 3
4. Normative Definition . . . . . . . . . . . . . . . . . . . . . 5
4.1. Syntax for UUI Header Field . . . . . . . . . . . . . . . 5
4.2. Source Identity of UUI . . . . . . . . . . . . . . . . . . 6
5. Guidelines for Usages of the UUI Mechanism . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Registration of Header Field . . . . . . . . . . . . . . . 8
6.2. Registration of Header Field Parameters . . . . . . . . . 9
6.3. Registration of SIP Option Tag . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Appendix - Other Possible Mechanisms . . . . . . . . . . . . . 10
8.1. Why INFO is Not Used . . . . . . . . . . . . . . . . . . . 10
8.2. Why Other Protocol Encapsulation UUI Mechanisms are
Not Used . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.3. MIME body Approach . . . . . . . . . . . . . . . . . . . . 11
8.4. URI Parameter . . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Informative References . . . . . . . . . . . . . . . . . . 13
10.2. Normative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Overview
This document describes the transport of User to User Information
(UUI) using SIP [RFC3261]. Specifically, we discuss a mechanism for
the transport of general application UUI and also for the transport
of call control related ITU-T Q.931 User to User Information Element
(UU IE) [Q931] and ITU-T Q.763 User to User Information Parameter
[Q763] data in SIP. UUI is widely used in the PSTN today in contact
centers and call centers which are transitioning away from ISDN to
SIP. This extension will also be used for native SIP endpoints
implementing similar services and interworking with ISDN services.
This mechanism was designed to meet the use cases, requirements, and
call flows for SIP call control UUI detailed in
[I-D.ietf-cuss-sip-uui-reqs]. All references to requirement numbers
(REQ-N) and figure numbers refer to this document.
The mechanism chosen is a new SIP header field, along with a new SIP
option tag. The header field carries the UUI information, along with
parameters indicating the encoding of the UUI, the application user
of the UUI, and optionally the content of the UUI. The header field
can be escaped into URIs supporting referral and redirection
scenarios. In these scenarios, History-Info is used to indicate the
inserter of the UUI. The SIP option tag is used to indicate support
for the header field. Support for the header field indicates that a
UA is able to extract the information in the UUI and pass it up the
protocol stack. Individual applications using the UUI mechanism will
utlize media feature tags to indicate that a UA supports a particular
application user of UUI. Guidelines for defininig usages of this
mechanism are provided.
2. Terminology
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 BCP 14, RFC 2119
[RFC2119].
3. Requirements Discussion
This section describes how the User-to-User header field meets the
requirements in [I-D.ietf-cuss-sip-uui-reqs]. The header field can
be included in INVITE requests and responses and BYE requests and
responses, meeting REQ-1 and REQ-2.
For redirection and referral use cases and REQ-3, the header field
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would be escaped into the Contact or Refer-To URI. Currently, UAs
that support attended transfer support the ability to escape a
Replaces header field into a Refer-To URI, and when acting upon this
URI add the Replaces header field to the triggered INVITE. This
logic and behavior is identical for the UUI header field. The UA
processing the REFER or the 3xx to the INVITE will need to support
the UUI mechanism, as UAs in general do not process unknown escaped
header fields.
Since SIP proxy forwarding and retargeting does not affect header
fields, the header field meets REQ-4.
The UUI header field will carry the UUI data and not a pointer to the
data, so REQ-5 is met.
Since the basic design of the UUI header field is similar to the ISDN
UUI service, interworking with PSTN protocols will be straightforward
and will be documented in a separate specification, meeting REQ-6.
Requirements REQ-7, REQ-8, and REQ-10 relate to discovery of the
mechanism and supported applications. REQ-7 relates to support of
the UUI header field, while REQ-8 relates to routing based on support
of the UUI header field. REQ-7 is met by defining a new SIP option
tag 'uui'. The use of a 'Require: uui' in a request, or 'Supported:
uui' in an OPTIONS response could be used to require or discover
support of the mechanism. The presence of a Supported:uui or
Require:uui header field can be used by proxies to route to an
appropriate UA, meeting REQ-8. REQ-10 is met by creating a new class
of SIP feature tags. For example, the feature tag 'sip.uui-isdn'
could be used to indicate support of the ISDN UUI service, or
'sip.uui-app1' could be used to indicate support for a particular
application, app1.
Proxies commonly apply policy to the presence of certain SIP header
fields in requests by either passing them or removing them from
requests. REQ-9 is met by allowing proxies and other intermediaries
to remove UUI header fields in a request or response based on policy.
Carrying UUI data elements of at least 129 octets is trivial in the
UUI header field, meeting REQ-11. Note that very large UUI elements
should be avoided, as SIP header fields have traditionally not been
large.
To meet REQ-12 in redirection and referral use cases, History-Info
[I-D.ietf-sipcore-rfc4244bis] can be used. In these retargeting
cases, the changed Request-URI will be recorded in the History-Info
header field along with the identity of the element that performed
the retargeting.
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The requirement for integrity protection in REQ-13 could be met by
the use of an S/MIME signature over a subset of header fields, as
defined in Section 23.4 of RFC 3261 "SIP Header Privacy and Integrity
using S/MIME: Tunneling SIP". The requirement of REQ-14 for end-to-
end privacy could be met using S/MIME or using encryption at the
application layer. Note that the use of S/MIME to secure the UUI
will result in an additional body being added to the request.
Hopwise TLS allows the header field to meet REQ-15 for hop-by-hop
security.
4. Normative Definition
This document defines a new SIP header field "User-to-User" to
transport call control UUI to meet the requirements in
[I-D.ietf-cuss-sip-uui-reqs].
To help tag and identify the UUI used with this header field, "app",
"content", and "encoding" parameters are defined. The "app"
parameter identifies the application which generates and consumes the
UUI information. For the case of interworking with the ISDN UUI
Service, the application is unknown, so a value to indicate ISDN UUI
Service interworking will be defined. If the "app" parameter is not
present, interworking with the ISDN UUI Service MUST be assumed. The
"content" parameter identifies the actual content of the UUI data.
If not present, the content MUST be assumed to be unknown as it is in
the ISDN UUI Service. For newly defined applications using the SIP
UUI service, a "content" value MUST be defined and SHOULD be used.
The "encoding" parameter indicates the method of encoding the
information in the UUI. This specification only defines
"encoding=hex". If the "encoding" parameter is not present, "hex"
MUST be assumed.
UUI data is considered an opaque series of octets. This mechanism
SHOULD NOT be used to convey a URL or URI: the Call-Info header field
[RFC3261] is used for this purpose.
4.1. Syntax for UUI Header Field
The User-to-User header field can be present in INVITE requests and
responses only and in BYE requests and responses. Note that only
end-to-end responses can be used, e.g. 1xx, 2xx, and 3xx responses.
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 2234 and extends RFC 3261.
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UUI = "User-to-User" HCOLON uui-data *(SEMI uui-param)
uui-data = token
uui-param = enc-param / cont-param / app-param / generic-param
enc-param = "encoding" EQUAL ("hex" / token)
cont-param = "content" EQUAL token
app-param = "app" EQUAL token
A single User-to-User header field may be present in a request or a
response. Any size limitations on the UUI for a particular purpose
must be defined by that application.
4.2. Source Identity of UUI
It is important for the recipient of UUI to know the identity of the
UA that inserted the UUI. In a request without any History-Info
[I-D.ietf-sipcore-rfc4244bis] header field, the inserter of the UUI
will be the UA identified by the URI in the From header field. In a
request with a History-Info header field, the recipient needs to
parse the Targeted-to-URIs present (hi-targeted-to-uri) to see if any
escaped User-to-User header fields are present. If an escaped User-
to-User header field is present and matches the UUI in the request,
it indicates that redirection has taken place which has resulted in
the UUI inclusion in the request. The inserter of the UUI will be
the UA identified by the Targeted-to-URI of the History-Info element
prior to the element with the escaped UUI. In a response, the
inserter of the UUI will be the UA identified in the To header field
of the response. Note that any updates to this identity by use of
the SIP Connected Identity extension [RFC4916] will update this
information.
For an example of History-Info and redirection, consider Figure 2
from [I-D.ietf-cuss-sip-uui-reqs] where the Originating UA is Carol,
the Redirector Bob, and the Terminating UA Alice. The INVITE F4
containing UUI could be:
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INVITE sips:alice@example.com SIP/2.0
Via: SIP/2.0/TLS lab.example.com:5061
;branch=z9hG4bKnashds9
To: Bob <sips:bob@example.com>
From: Carol <sips:carol@example.com>;tag=323sf33k2
Call-ID: dfaosidfoiwe83ifkdf
Max-Forwards: 70
Contact: <sips:carol@lab.example.com>
Supported: histinfo
User-to-User: 342342ef34;encoding=hex
History-Info: <sips:bob@example.com>;index=1
<allOneLine>
History-Info: <sips:alice@example.com?Reason=SIP%3Bcause%3D302
&User-to-User=342342ef34%3Bencoding%3Dhex>;index=1.1;rc=1
</allOneLine>
Without the redirection captured in the History-Info, Alice would
conclude the UUI was inserted by Carol. However, the History-Info
containing UUI (index=1.1) indicates that the inserter was Bob
(index=1).
To enable the inserter identity of UUI, UAs supporting this mechanism
SHOULD support History-Info [I-D.ietf-sipcore-rfc4244bis] and include
Supported: histinfo in all requests and responses.
5. Guidelines for Usages of the UUI Mechanism
All applications using this SIP UUI mechanism must publish a
standards track RFC which describes the usage. The SIP UUI mechanism
is applicable in the following situations:
1. The information is generated and consumed by an application
during session setup using SIP, but the application is not
necessarily SIP aware.
2. The behavior of SIP entities that support it is not
significantly changed (as discussed in Section 4 of [RFC5727]).
3. User Agent Clients (UAC) and User Agent Servers (UAS) are the
generators and consumers of the UUI data. Proxies may route based
on the application tag but not otherwise be involved.
4. Intermediary elements or proxies can remove or insert UUI
information
This mechanism is not applicable in the following situations:
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1. The behavior of SIP entities that support it is significantly
changed (as discussed in Section 4 of [RFC5727]).
2. The information is generated and consumed at the SIP layer by
SIP elements.
3. SIP elements besides the UAC and UAS might be interested in
consuming (beyond adding or removing) the information.
4. There are specific privacy issues involved with the
information being transported (e.g., geolocation or emergency-
related information).
This specification defines only the value of "hex" for the "encoding"
parameter. New values can be defined and added to the IANA registry
with a standards track RFC, which needs to discuss the issues in this
section.
New "encoding" values must reference a common encoding scheme or
define the exact new encoding scheme.
New "content" values must describe the content of the UUI and give
some example use cases. The default "encoding" and other allowed
encoding methods must be defined for this new content.
New "app" values must describe the new application which is utilizing
the UUI data and give some example use cases. The default "content"
value and other allowed contents must be defined for this new
purpose. Any restrictions on the size of the UUI data must be
described for the new application. In addition, it is recommended
that a Media Feature tag be defined per RFC 3840 [RFC3840] to
indicate support for this application usage of UUI. For example, the
media feature tag sip.uui-app1 could be defined to indicate support
for an app named app1. The definition of a new SIP feature tag for
this application is NOT RECOMMENDED unless there are other SIP
behaviors needed to implement this feature.
For an example UUI usage definition, see
[I-D.drage-cuss-sip-uui-isdn].
6. IANA Considerations
6.1. Registration of Header Field
This document defines a new SIP header field named "User-to-User".
The following row shall be added to the "Header Fields" section of
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the SIP parameter registry:
+------------------+--------------+-----------+
| Header Name | Compact Form | Reference |
+------------------+--------------+-----------+
| User-to-User | | [RFCXXXX] |
+------------------+--------------+-----------+
Editor's Note: [RFCXXXX] should be replaced with the designation of
this document.
6.2. Registration of Header Field Parameters
This document defines the parameters for the header field defined in
the preceding section. The header field "User-to-User" can contain
the parameters "encoding", "content", and "purpose".
The following rows shall be added to the "Header Field Parameters and
Parameter Values" section of the SIP parameter registry:
+------------------+----------------+-------------------+-----------+
| Header Field | Parameter Name | Predefined Values | Reference |
+------------------+----------------+-------------------+-----------+
| User-to-User | encoding | hex | [RFCXXXX] |
+------------------+----------------+-------------------+-----------+
Editor's Note: [RFCXXXX] should be replaced with the designation of
this document.
6.3. Registration of SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of [RFC3261].
This document defines the SIP option tag "uui".
The following row has been added to the "Option Tags" section of the
SIP Parameter Registry:
+------------+------------------------------------------+-----------+
| Name | Description | Reference |
+------------+------------------------------------------+-----------+
| uui | This option tag is used to indicate that | [RFCXXXX] |
| | a UA supports and understands the | |
| | User-to-User header field. | |
+------------+------------------------------------------+-----------+
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Editor's Note: [RFCXXXX] should be replaced with the designation of
this document.
7. Security Considerations
User to user information can potentially carry sensitive information
that might require privacy or integrity protection. Standard
deployed SIP security mechanisms such as TLS transport, offer these
properties on a hop-by-hop basis. To preserve multi-hop or end-to-
end confidentiality and integrity of UUI, approaches using S/MIME can
be used, as discussed in the draft. However, the lack of deployment
of these mechanisms means that applications can not in general rely
on them. As such, applications are encouraged to utilize their own
security mechanisms.
8. Appendix - Other Possible Mechanisms
Two other possible mechanisms for transporting UUI will be described:
MIME body and URI parameter transport.
8.1. Why INFO is Not Used
Since the INFO method [RFC6086], was developed for ISUP interworking
of user-to-user information, it might seem to be the logical choice
here. For non-call control user-to-user information, INFO can be
utilized for end to end transport. However, for transport of call
control user-to-user information, INFO can not be used. As the call
flows in [I-D.ietf-cuss-sip-uui-reqs] show, the information is
related to an attempt to establish a session and must be passed with
the session setup request (INVITE), responses to that INVITE, or
session termination requests. As a result, it is not possible to use
INFO in these cases.
8.2. Why Other Protocol Encapsulation UUI Mechanisms are Not Used
Other protocols have the ability to transport UUI information. For
example, consider the ITU-T Q.931 User to User Information Element
(UU IE) [Q931] and the ITU-T Q.763 User to User Information Parameter
[Q763]. In addition, NSS (Narrowband Signaling System) [Q1980] is
also able to transport UUI information. Should one of these
protocols be in use, and present in both User Agents, then utilizing
these other protocols to transport UUI might be a logical solution.
Essentially, this is just adding an additional layer in the protocol
stack. In these cases, SIP is not transporting the UUI; it is
encapsulating another protocol, and that protocol is transporting the
UUI. Once a mechanism to transport that other protocol using SIP
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exists, the UUI transport function is essentially obtained without
any additional effort or work.
However, the authors believe that SIP needs to have its own native
UUI transport mechanism. It is not reasonable for a SIP UA to have
to implement another entire protocol (either ISDN or NSS, for
example) just to get the very simple UUI transport service. Of
course, this work does not preclude anyone from using other protocols
with SIP to transport UUI information.
8.3. MIME body Approach
One method of transport is to use a MIME body. This is in keeping
with the SIP-T architecture [RFC3372] in which MIME bodies are used
to transport ISUP information. Since the INVITE will normally have
an SDP message body, the resulting INVITE with SDP and UUI will be
multipart MIME. This is not ideal as many SIP UAs do not support
multipart MIME INVITEs.
A bigger problem is the insertion of a UUI message body by a redirect
server or in a REFER. The body would need to be encoded in the
Contact URI of the 3xx response or the Refer-To URI of a REFER.
Currently, the authors are not aware of any UAs that support this
capability today for any body type. As such, the complete set of
semantics for this operation would need to be determined and defined.
Some issues will need to be resolved, such as, do all the Content-*
header fields have to be escaped as well? And, what if the escaped
Content-Length does not agree with the escaped body?
Since proxies cannot remove a body from a request or response, it is
not at all clear how this mechanism could meet REQ-9.
The requirement for integrity protection could be met by the use of
an S/MIME signature over the body, as defined in Section 23.3 of RFC
3261 "Securing MIME bodies". Alternatively, this could be achieved
using RFC 4474 [RFC4474]. The requirement for end-to-end privacy
could be met using S/MIME encryption or using encryption at the
application layer. However, note that neither S/MIME or RFC 4474
enjoys deployment in SIP today.
An example:
<allOneLine>
Contact: <sip:+12125551212@gateway.example.com?Content-Type=
application/uui&body=ZeGl9i2icVqaNVailT6F5iJ90m6mvuTS4OK05M0vDk0Q4Xs>
</allOneLine>
Note that the <allOneLine> tag convention from SIP Torture Test
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Messages [RFC4475] is used to show that there are no line breaks in
the actual message syntax.
As such, the MIME body approach meets REQ-1, REQ-2, REQ-4, REQ-5,
REQ-7, REQ-11, REQ-13, and REQ-14. Meeting REQ-12 seems possible,
although the authors do not have a specific mechanism to propose.
Meeting REQ-3 is problematic, but not impossible for this mechanism.
However, this mechanism does not seem to be able to meet REQ-9.
8.4. URI Parameter
Another proposed approach is to encode the UUI as a URI parameter.
This UUI parameter could be included in a Request-URI or in the
Contact URI or Refer-To URI. It is not clear how it could be
transported in a responses which does not have a Request-URI, or in
BYE requests or responses.
<allOneLine>
Contact: <sip:+12125551212@gateway.example.com;uui=ZeGl9i2icVqaNVailT6
F5iJ90m6mvuTS4OK05M0vDk0Q4Xs>
</allOneLine>
An INVITE sent to this Contact URI would contain UUI in the Request-
URI of the INVITE. The URI parameter has a drawback in that a URI
parameter carried in a Request-URI will not survive retargeting by a
proxy as shown in Figure 2 of [I-D.ietf-cuss-sip-uui-reqs]. That is,
if the URI is included with an Address of Record instead of a Contact
URI, the URI parameter in the Reqeuest-URI will not be copied over to
the Contact URI, resulting in the loss of the information. Note that
if this same URI was present in a Refer-To header field, the same
loss of information would occur.
The URI parameter approach would meet REQ-3, REQ-5, REQ-7, REQ-9, and
REQ-11. It is possible the approach could meet REQ-12 and REQ-13.
The mechanism does not appear to meet REQ-1, REQ-2, REQ-4, and
REQ-14.
9. Acknowledgements
Joanne McMillen was a major contributor and co-author of earlier
versions of this document. Thanks to Spencer Dawkins, Keith Drage,
Vijay Gurbani, and Laura Liess for their review of the document. The
authors wish to thank Francois Audet, Denis Alexeitsev, Paul Kyzivat,
Cullen Jennings, and Mahalingam Mani for their comments.
10. References
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10.1. Informative References
[Q763] "ITU-T Q.763 Signaling System No. 7 - ISDN user part
formats and codes",
http://www.itu.int/rec/T-REC-Q.931-199805-I/en .
[Q931] "ITU-T Q.931 User to User Information Element (UU IE)",
http://www.itu.int/rec/T-REC-Q.931-199805-I/en .
[ETSI] "ETSI ETS 300 207-1 Ed.1 (1994), Integrated Services
Digital Network (ISDN); Diversion supplementary
services".
[RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol
for Telephones (SIP-T): Context and Architectures",
BCP 63, RFC 3372, September 2002.
[RFC6086] Holmberg, C., Burger, E., and H. Kaplan, "Session
Initiation Protocol (SIP) INFO Method and Package
Framework", RFC 6086, January 2011.
[RFC4475] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
and H. Schulzrinne, "Session Initiation Protocol (SIP)
Torture Test Messages", RFC 4475, May 2006.
[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change Process
for the Session Initiation Protocol (SIP) and the Real-
time Applications and Infrastructure Area", BCP 67,
RFC 5727, March 2010.
[I-D.drage-cuss-sip-uui-isdn]
Drage, K. and A. Johnston, "Interworking ISDN Call Control
User Information with SIP",
draft-drage-cuss-sip-uui-isdn-01 (work in progress),
September 2011.
[Q1980] "ITU-T Q.1980.1 The Narrowband Signalling Syntax (NSS) -
Syntax Definition", http://www.itu.int/itudoc/itu-t/aap/
sg11aap/history/q1980.1/q1980.1.html .
10.2. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
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June 2002.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.
[I-D.ietf-cuss-sip-uui-reqs]
Johnston, A. and L. Liess, "Problem Statement and
Requirements for Transporting User to User Call Control
Information in SIP", draft-ietf-cuss-sip-uui-reqs-05 (work
in progress), September 2011.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[I-D.ietf-sipcore-rfc4244bis]
Barnes, M., Audet, F., Schubert, S., Gmbh, D., and C.
Holmberg, "An Extension to the Session Initiation Protocol
(SIP) for Request History Information",
draft-ietf-sipcore-rfc4244bis-05 (work in progress),
April 2011.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
Authors' Addresses
Alan Johnston
Avaya
St. Louis, MO 63124
Email: alan.b.johnston@gmail.com
James Rafferty
Dialogic
Email: james.rafferty@dialogic.com
Johnston & Rafferty Expires March 23, 2012 [Page 14]
