Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)
draft-hanes-dispatch-fax-capability-03

Abstract

This document defines and registers with IANA the new 'fax' media feature tag for use with SIP. Currently, fax calls are indistinguishable from voice at call setup. Consequently, fax calls can be routed to SIP user agents that are not fax capable. A 'fax' media feature tag implemented in conjunction with RFC 3841 [RFC3841] allows for more accurate fax call routing.

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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Motivation
• 4. Example
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

Fax communications in the Session Initiation Protocol (SIP) are handled in a "voice first" manner. Indications that a user desires to use a fax transport protocol, such as ITU-T T.38[T38], to send a fax are not known during the call setup. The call is set up as a voice call first and then only after it is connected, does a switchover to the T.38[T38] protocol occur. This is problematic in that fax calls can be routed inadvertently to SIP user agents that are not fax capable.

To ensure that fax calls are routed to fax capable SIP user agents, an implementation of caller preferences defined in RFC 3841 [RFC3841] is necessary. Feature preferences are a part of RFC 3841 [RFC3841] that would allow UAs to express their preference for receiving fax communications. Subsequently SIP servers take these preferences into account to increase the likelihood that fax calls land at fax capable SIP user agents.

This document defines the 'fax' media feature tag for use in the SIP tree as per Section 12.1 of RFC 3840 [RFC3840]. This feature tag will be applied per RFC 3841 [RFC3841] as a feature preference for fax capable UAs.

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 [RFC2119].

3. Motivation

In the majority of circumstances, it is preferred that capabilities be handled in the SDP portion of the SIP communication. However, fax is somewhat unique in that the ultimate intention of the call is not accurately signaled in the initial SDP exchange. Fax is one of the few situations where a media feature tag indicating a capability is highly predictive of the ultimate communication request that will be made in the near future but is not indicated by the current SDP.

Specifically, indications of T.38[T38] or any other fax transport protocol in the call are not known when the call is setup. Fax calls are always considered voice calls until after they are connected. This results in increased chances of fax calls being received by SIP user agents not capable of handling fax transmissions.

For example, Alice wants to send a fax to Bob. Bob registers two SIP UAs. The first SIP UA is not fax capable but the second one supports the T.38[T38] fax protocol. Currently, SIP servers are unable to know when the call starts that Alice prefers a fax capable SIP UA to handle her call. Additionally, the SIP servers are also not aware of which of Bob's SIP UAs are fax capable.

An implementation of RFC3841 [RFC3841] changes this scenario and feature preferences are used to resolve this issue. With RFC 3841 [RFC3841], Alice can express up front that she prefers a T.38[T38] fax capable SIP UA for this call. At the same time, Bob's SIP UAs have expressed their fax capabilities as well during registration. Now when Alice places a fax call to Bob, the call is appropriately routed to Bob's fax capable SIP UA.

4. Example

Bob registers with the fax media feature tag. The message flow is shown in FFigure 1:

          
SIP Registrar                    Bob's SIP UA                             	  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                               |
      |          REGISTER F1          |
      |<------------------------------|
      |                               |
      |           200 OK F2           |
      |------------------------------>|
      |                               |
		

Figure 1: Fax Media Feature Tag SIP Registration Example

          
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP bob-TP@example.com;branch=z9hG4bK309475a2
From: <sip:bob-tp@example.com>;tag=a6c85cf
To: <sip:bob-tp@pexample.com>
Call-ID: a84b4c76e66710
Max-Forwards: 70
CSeq: 116 REGISTER
Contact: <sip:bob-tp@example.com;transport=tcp>;+sip.fax="t38"
Expires: 3600
	

F1 REGISTER Bob -> Registrar

The registrar responds with a 200 OK:

          
SIP/2.0 200 OK
From: <sip:bob-tp@example.com>;tag=a6c85cf
To: <sip:bob-tp@example.com>;tag=1263390604
Contact: <sip:bob-tp@example.com;transport=tcp>;+sip.fax="t38"
Expires: 120
Call-ID: a84b4c76e66710
Via: SIP/2.0/TCP bob-TP@example.com;branch=z9hG4bK309475a2
CSeq: 116 REGISTER
Expires: 3600

F2 200 OK Registrar -> Bob

          
INVITE sip:UserY@example.com SIP/2.0
From: sip:UserX@operator.com
To: sip:UserY@example.com
Accept-Contact: *;+sip.fax="t38"
Content-Type: application/sdp

Callers desiring to express a preference for fax will include the sip.fax media feature tag in the Accept-Contact header of their INVITE.

5. Security Considerations

The security considerations related to the use of media feature tags from Section 11.1 of RFC 3840 [RFC3840] apply.

6. IANA Considerations

This specification adds a new media feature tag to the SIP Media Feature Tag Registration Tree per the procedures defined in RFC 2506 [RFC2506] and RFC 3840 [RFC3840].

Media feature tag name:

sip.fax

ASN.1 Identifier:

1.3.6.1.8.4.{PH}

Summary of the media feature indicated by this tag:

This feature tag indicates whether a communications device supports the ITU-T T.38[T38] fax protocol ("t38") or the passthrough method of fax transmission using the ITU-T G.711[G711] audio codec ("passthrough").

Values appropriate for use with this feature tag:

Token with an equality relationship. Values are:

t38:

The device supports the image/t38 media type [RFC3326] and implements ITU-T T.38[T38] for transporting the ITU-T T.30[T30] and ITU-T T.4[T4] fax data over IP.

passthrough:

The device supports the audio/pcmu and audio/pcma media types [RFC4856] for transporting ITU-T T.30[T30] and ITU-T T.4[T4] fax data using the ITU-T G.711[G711] audio codec. Additional implementation recommendations are in ITU-T V.152[V152] Sections 6 and 6.1.

The feature tag is intended primarily for use in the following applications, protocols, services, or negotiation mechanisms:

This feature tag is most useful in a communications application for the early identification of a FoIP call.

Examples of typical use:

Ensuring a fax call is routed to a fax capable SIP UA.

Related standards or documents:

RFCXXXX

Security Considerations:

Security considerations for this media feature tag are discussed in Section 5 of this document.

[[NOTE TO RFC EDITOR: Please change {PH} above to the correct identifier for this entry in the IANA registry for iso.org.dod.internet.features.sip-tree (1.3.6.1.8.4)]]

[[NOTE TO RFC EDITOR: Please change XXXX to the number assigned to this specification, and remove this paragraph on publication.]]

7. Acknowledgements

This document is a result of the unique cooperation between the SIP Forum and the i3 Forum who embarked on a groundbreaking international test program for FoIP to improve the interoperability and reliability of fax communications over IP networks, especially tandem networks. The authors would like to acknowledge the effort and dedication of all the members of the Fax-over-IP (FoIP) Task Group in the SIP Forum and the communications carriers of the I3 Forum that contributed to this global effort.

This memo has benefited from the discussion and review of the DISPATCH working group, especially the detailed and thoughtful comments and corrections of Dan Wing, Paul Kyzivat, Christer Holmberg, Charles Eckel, and Dale Worley.

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses