Network Working Group A. Johnston, Ed.
Internet-Draft Avaya
Intended status: Standards Track J. McMillen
Expires: March 20, 2011 Unaffiliated
J. Rafferty
Dialogic
September 16, 2010
A Mechanism for Transporting User to User Call Control Information in
SIP
draft-johnston-cuss-sip-uui-00
Abstract
The need for applications using SIP to exchange User to User
Information (UUI) data during session establishment has been
discussed. Several approaches to transporting call control User to
User Information (UUI) data in SIP have been proposed. As networks
move to SIP it is important that applications requiring this data can
continue to function in SIP networks as well as the ability to
interwork with this ISDN service for end-to- end transparency. This
document discusses three mechanisms to meet the requirements defined
in the Requirements for SIP Call Control UUI document. A new SIP
header field which bests meets these requirements is proposed.
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 20, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Possible Mechanisms . . . . . . . . . . . . . . . . . . . . . 3
3.1. Why INFO is Not Used . . . . . . . . . . . . . . . . . . . 3
3.2. Why Other Protocol Encapsulation UUI Mechanisms are
Not Used . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.3. MIME body Approach . . . . . . . . . . . . . . . . . . . . 4
3.4. URI Parameter . . . . . . . . . . . . . . . . . . . . . . 5
3.5. Header Field Approach . . . . . . . . . . . . . . . . . . 5
4. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Syntax for UUI Header Field . . . . . . . . . . . . . . . . . 7
5.1. Definition of New Parameter Values . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Registration of Header Field . . . . . . . . . . . . . . . 8
6.2. Registration of Header Field Parameters . . . . . . . . . 8
6.3. Registration of SIP Option Tag . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Informative References . . . . . . . . . . . . . . . . . . 10
9.2. Normative References . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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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.
The definition, use cases, requirements, and call flows for SIP call
control UUI is discussed in [johnston-cuss-sip-uui-reqs]. All
references to requirement numbers (REQ-N) and figure numbers refer to
this draft.
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. Possible Mechanisms
Three possible mechanisms for transporting UUI will be described:
MIME body, URI parameter, and header field transport.
3.1. Why INFO is Not Used
Since the INFO method [RFC2976], 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 the previous section 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.
3.2. Why Other Protocol Encapsulation UUI Mechanisms are Not Used
Other protocols have the ability to transport UUI information. For
example, consider ITU-T Q.931 User to User Information Element (UU
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IE) [Q931] and ITU-T Q.763 User to User Information Parameter [Q763],
as discussed in the requirements draft. 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
make a lot of sense. Essentially, this is just adding an additional
layer in the protocol stack. SIP is not transporting the UUI, it is
encapsulating another protocol, and that protocol is transporting the
UUI. Once there is a mechanism to transport that other protocol
using SIP, 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.
3.3. MIME body Approach
One method of transport is to transport the UUI information as 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, no UAs support this capability today, and even defining
this is problematic. For example, do all the Content-* header fields
have to be escaped as well? What if the escaped Content-Length does
not agree with the escaped body?
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
Messages [RFC4475] is used to show that there are no line breaks in
the actual message syntax.
The MIME body approach meets REQs 1-5. However, it does not meet
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REQ-6 as support for Multipart MIME and escaped bodies in URIs is
uncommon in SIP UAs.
3.4. URI Parameter
Another proposed approach is to encode the UUI as a URI parameter
into the Contact or Refer-To URI.
<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 [johnston-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. As a
result, this approach does not meet REQ-4. Note that if this same
URI was present in a Refer-To header field, the same loss of
information would occur.
3.5. Header Field Approach
Another approach that has been proposed is to use a header field to
transport the UUI information. The header field would be included in
INVITE requests and responses and BYE requests and responses, and
would pass transparently through proxies. For redirection, the
header field would be escaped into the Contact or Refer-To URI. This
is commonly supported in UAs due to call transfer use cases. As a
result, the header field approach supports REQs 1-7. In order to
meet REQ- 8, a SIP feature tag is needed which can be included in
Supported and Require header fields.
The Call-Info header field is related to the UUI information.
However, there are a number of important differences:
o Call-Info is typically used for rendering to the user. While some
of the UUI information may ultimately be rendered to the user,
most of the UUI information will be consumed by the end device or
by an application server.
o Call-Info usually contains a URI pointer to the information
instead of the actual information itself which does not meet
REQ-5. It could be possible to use a data URI to carry the UUI
directly in this header field.
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o The use of Call-Info for interworking to and from ISDN networks
seems problematic.
Overall, the overloading of the Call-Info header field for carrying
interworked UUI does not seem like a good idea. A separate header
field allows for clear policy and authorization rules to be used.
For these reasons, a separate header field needs to be defined,
described here as User-to-User. For example, here is an example
User-to-User header field from message F1 in Figure 1 of
[johnston-cuss-sip-uui-reqs]:
User-to-User: 56a390f3d2b7310023a;encoding=hex;purpose=isdn-interwork
;content=isdn-uui
For example, here is an escaped User-to-User header field from the
redirection response F2 of Figure 3:
<allOneLine>
Contact: <sip:+12125551212@gateway.example.com?User-to-User=
56a390f3d2b7310023a%3Bencoding%3Dhex%3Bpurpose%3Disdn-interwork%3B
content%3Disdn-uui>
</allOneLine>
The resulting INVITE F5 would contain:
User-to-User: 56a390f3d2b7310023a;encoding=hex;purpose=isdn-interwork
;content=isdn-uui
An escaped User-to-User header field from the REFER message response
F1 of Figure 4:
<allOneLine>
Refer-To: <sip:+12125551212@gateway.example.com?User-to-User=
56a390f3d2b7310023a%3Bencoding%3Dhex%3Bpurpose%3Disdn-interwork%3B
content%3Disdn-uui>
</allOneLine>
This would result in the INVITE F4 containing:
User-to-User: 56a390f3d2b7310023a;encoding=hex;purpose=isdn-interwork
;content=isdn-uui
4. Recommendation
The recommendation is to define a new SIP header field "User-to-User"
to transport call control UUI since this mechanism best supports the
requirements in [johnston-cuss-sip-uui-reqs]. There are also
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existing implementations and running code for this header field
approach. A SIP feature tag "uui" also needs to be defined so that
it can be used in Supported and Require header fields to meet REQ-8.
To help tag and identify the UUI used with this header field,
"purpose", "content", and "encoding" parameters are defined. This
specification only defines encodings of hex and IA5. Other
specifications can define other purposes and contents for this header
field per the requirements of this document.
5. 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.
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 2234 and extends RFC 3261.
UUI = "User-to-User" HCOLON uui-data *(SEMI uui-param)
uui-data = token
uui-param = enc-param | cont-param | purp-param | generic-param
enc-param = "encoding=" ("hex" | "IA5" | token)
cont-param = "content=" token
purp-param = "purpose=" token
The parameter "encoding=hex" means hexadecimal encoding. The
parameter "encoding=IA5" means Internet Alphabet Number 5 encoding,
also known as ITU-T T.50 [ia5]. If the encoding parameter is not
present, the default value of "hex" MUST be assumed. Other encoding
methods of encoding MAY also be standardized.
User-to-User header fields with different purpose parameters may be
present in a request or response. The number of User-to-User header
fields which may be present in a request or response is defined for a
particular purpose (application). Any size limitations on the UUI
for a particular purpose must be defined by that purpose.
5.1. Definition of New Parameter Values
This specification defines only the values of "hex", "IA5", and 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.
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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 "purpose" values must describe the new purpose 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 purpose.
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
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] |
+------------------+----------------+-------------------+-----------+
| User-to-User | encoding | IA5 | [RFCXXXX] |
+------------------+----------------+-------------------+-----------+
Editor's Note: [RFCXXXX] should be replaced with the designation of
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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. | |
+------------+------------------------------------------+-----------+
Editor's Note: [RFCXXXX] should be replaced with the designation of
this document.
7. Security Considerations
User to user information can be exchanged over SIP on a hop-by-hop or
end-to-end basis. In some cases, UUI may carry privacy information
that would require confidentiality and message integrity. Standard
SIP security mechanisms, viz., based on TLS, offer these properties
per-hop. To preserve multi-hop or end-end confidentiality and
integrity, S/MIME profile MUST be utilized. Since the security
requirements and key management of the UUI information are likely to
be quite different from the SIP signaling transport, another approach
would be for the UUI information to be encrypted before being passed
to SIP for transport.
Received User-to-User information should only be trusted if it is
authenticated or if it is received within a trust domain. For
example, Spec-T, defined in [RFC3324] could be used to define a trust
domain. When utilized by a gateway to map information to or from
ISDN Q.931 and ISUP Q.763, appropriate policy should be applied based
on the PSTN trust domain.
8. Acknowledgements
Thanks to Spencer Dawkins, Keith Drage, Vijay Gurbani, and Laura
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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.
9. References
9.1. 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.
[RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976,
October 2000.
[RFC4475] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
and H. Schulzrinne, "Session Initiation Protocol (SIP)
Torture Test Messages", RFC 4475, May 2006.
[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 .
9.2. 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,
June 2002.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.
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[johnston-cuss-sip-uui-reqs]
Johnston, A., McMillen, J., and L. Liess, "Problem
Statement and Requirements for Transporting User to User
Call Control Information in SIP",
draft-johnston-cuss-sip-uui-reqs-00 .
[ia5] "T.50 : International Reference Alphabet (IRA) (Formerly
International Alphabet No. 5 or IA5) - Information
technology - 7-bit coded character set for information
interchange",
http://www.itu.int/rec/T-REC-T.50-199209-I/en .
Authors' Addresses
Alan Johnston (editor)
Avaya
St. Louis, MO 63124
Email: alan.b.johnston@gmail.com
Joanne McMillen
Unaffiliated
Email: c.joanne.mcmillen@gmail.com
James Rafferty
Dialogic
Email: james.rafferty@dialogic.com
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