Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)
draft-hanes-dispatch-fax-capability-05

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 initiation. Consequently, fax calls can be routed to SIP user agents that are not fax capable. A 'fax' media feature tag implemented in conjunction with caller preferences allows for more accurate fax call routing.

Status of This Memo

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This Internet-Draft will expire on May 10, 2013.

Copyright Notice

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) [RFC3261] 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 when the initial INVITE message is sent. 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 (UAs) 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 Session Description Protocol (SDP) portion of the SIP [RFC3261] 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 initiated by an INVITE message. 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 Figure 1:

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

          
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
	

          
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

          
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