Registration of the Real-time-text Media Feature Tag

Media feature tags for use in SIP headers are introduced in RFC 3840 [1]. They are used with the Session Initiation Protocol (SIP). When negotiating a text communication medium in a session, a user may find it very desirable to achieve a text communication path with real-time text capabilities. In order to support routing of negotiating to support this preference, an indication in a SIP media-feature tag can be used. That media-feature tag is defined in this specification. One value is given to the parameter for indication of full simultaneous two way capabilities of the text communication. When interworking with legacy telecommunication devices through a gateway, an IP based text phone using SIP may be required to limit its capabilities to match those devices. For example, V.21 based textphones are full duplex in transport, but have varying handling of the presentation. Some types merge the two sources in one window. Some have a kind of irc-like display with labels in front of the text from different participants. And yet some do a split in two windows of real-time text from each direction. In order for a SIP-based text capable device to display an appropriate user interface when interacting with one of these legacy devices, it is necessary to convey a parameter indicating the limited capability of the legacy device. It is also of interest to route calls to the most capable device. When it is interacting with a legacy device, an IP text phone may receive an offer that contains the 'real-time-text' tag indicating some restriction. That tag then acts as a cue to configure the user interface appropriately, although there is nothing in the generated answer to indicate that this has been done. Similiarly, if an answer is received that contains a 'real-time-text' tag, that indicates that the remote device has limited capabilities, and the user-interface should present some indication of this to the user. More information on handling of real-time text is found in Framework for real-time text over IP using the Session Initiation Protocol. This memo defines such a parameter.

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 RFC 2119.

This specification defines a Media Feature tag 'real-time-text'. Its formatting in SIP headers is described in the following: Media Feature Tag name: sip.real-time-text ASN.1 Identifier: y.y. Summary of the media feature indicated by this tag: The sip.real-time-text media feature tag indicates whether a communications device can simultaneously handle the real-time text medium and audio in its user interface or towards attached devices, or if limitations must be applied by users who communicate with the device. Values appropriate for use with this feature tag: Token with an equality relationship. Defined values are: The device can handle real-time text simultaneously in both directions and if it supports audio, audio and text can also be handled simultaneously. (default) The device requires some ordered turn-taking to take place. It is not specified how strict this turntaking must be, and it is left to the users to figure out what level of turntaking signaling is needed. The device can handle simultaneous transmission of text in both directions, but there is a risk that the display is made in one window so that it requires "soft" turntaking indications in order to get a readable display. Simultaneous transmission can be used for interruptions and alerting. If audio is supported, it is handled alternating with text. The device cannot handle real-time text transmission in both directions in its user interface or towards attached devices. Strict turntaking must be applied by the users and is usually signalled by agreed turntaking indicators in the text medium. If audio is supported, it shall only be used alternating with text. 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 describing the capabilities of a SIP multimedia communication device, SIP application servers implementing services including real-time text, or a PSTN textphone gateway, connecting SIP devices with legacy textphones, with the intention both to support routing of the call and indication to the SIP device user if turntaking must be applied. Examples of typical use: Choosing to communicate with a native, real-time text capable device rather than a legacy textphone behind a gateway, when a selection is possible. Instruct the user of the SIP device about the need to apply turntaking habits as used with legacy textphones when the call is made through a gateway with that kind of device. Related standards or documents: RFC XXXX. Security Considerations: Security considerations for this media feature tag are discussed in Section 11.1 of RFC 3840.

The following example shows the use of the 'text-turntaking' tag in a SIP message.

;audio;text;text-turntaking=soft Content-Length: 0 ]]>

Security considerations for this media feature tag are discussed in Section 11.1 of RFC 3840.

The need for this registration was discussed with Barry Dingle, Gonzalo Camarillo , and Colin Perkins.