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

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This Internet-Draft will expire on March 20, 2011.

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Table of Contents

1.  Overview
2.  Terminology
3.  Possible Mechanisms
    3.1.  Why INFO is Not Used
    3.2.  Why Other Protocol Encapsulation UUI Mechanisms are Not Used
    3.3.  MIME body Approach
    3.4.  URI Parameter
    3.5.  Header Field Approach
4.  Recommendation
5.  Syntax for UUI Header Field
    5.1.  Definition of New Parameter Values
6.  IANA Considerations
    6.1.  Registration of Header Field
    6.2.  Registration of Header Field Parameters
    6.3.  Registration of SIP Option Tag
7.  Security Considerations
8.  Acknowledgements
9.  References
    9.1.  Informative References
    9.2.  Normative References
§  Authors' Addresses

1.  Overview

This document describes the transport of User to User Information (UUI) using SIP [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.). 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] (, “ITU-T Q.931 User to User Information Element (UU IE),” .) and ITU-T Q.763 User to User Information Parameter [Q763] (, “ITU-T Q.763 Signaling System No. 7 - ISDN user part formats and codes,” .) 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] (Johnston, A., McMillen, J., and L. Liess, “Problem Statement and Requirements for Transporting User to User Call Control Information in SIP,” .). 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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] (Donovan, S., “The SIP INFO Method,” October 2000.), 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 IE) [Q931] (, “ITU-T Q.931 User to User Information Element (UU IE),” .) and ITU-T Q.763 User to User Information Parameter [Q763] (, “ITU-T Q.763 Signaling System No. 7 - ISDN user part formats and codes,” .), as discussed in the requirements draft. In addition, NSS (Narrowband Signaling System) [Q1980] (, “ITU-T Q.1980.1 The Narrowband Signalling Syntax (NSS) - Syntax Definition,” .) 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] (Vemuri, A. and J. Peterson, “Session Initiation Protocol for Telephones (SIP-T): Context and Architectures,” September 2002.) 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 (Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J., and H. Schulzrinne, “Session Initiation Protocol (SIP) Torture Test Messages,” May 2006.) [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 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] (Johnston, A., McMillen, J., and L. Liess, “Problem Statement and Requirements for Transporting User to User Call Control Information in SIP,” .). 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:

• 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.
• 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.
• 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] (Johnston, A., McMillen, J., and L. Liess, “Problem Statement and Requirements for Transporting User to User Call Control Information in SIP,” .):

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] (Johnston, A., McMillen, J., and L. Liess, “Problem Statement and Requirements for Transporting User to User Call Control Information in SIP,” .). There are also 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] (, “T.50 : International Reference Alphabet (IRA) (Formerly International Alphabet No. 5 or IA5) - Information technology - 7-bit coded character set for information interchange,” .). 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.

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 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] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.).

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] (Watson, M., “Short Term Requirements for Network Asserted Identity,” November 2002.) 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 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

9.2. Normative References

Authors' Addresses