Network Working Group C. Perkins
Internet-Draft University of Glasgow
Expires: January 25, 2007 July 24, 2006
The SDP 'txp' Attribute
draft-perkins-mmusic-sdp-txp-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
This memo defines a new Session Description Protocol (SDP) attribute
'txp'. This is used to indicate that a text device has limited
presentation capabilities.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The SDP 'txp' Attribute . . . . . . . . . . . . . . . . . . . . 3
4. The 'txp' Attribute in the Offer/Answer Model . . . . . . . . . 4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Relation to the SDP Content Attribute . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
10. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
*** This draft is a strawman proposal for discussion purposes ***
The Session Description Protocol (SDP) is a protocol that is intended
for describing multimedia sessions for the purposes of session
announcement, session invitation, and other forms of multimedia
session initiation. One of the most typical use cases of SDP is the
one where it is used with the Session Initiation Protocol (SIP).
When interworking with legacy devices through a gateway, an IP based
text phone using SIP/SDP may be required to limit its capabilities to
match those devices. For example, V.21 textphones are full duplex in
transport, but have varying handling of the presentation. Some
merges the two sources in one window. Some have a kind of irc-like
display with labels in front of the parties text. And yet some do a
split in two windows of real-time text from each direction.
In order for an IP-based text phone 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. This memo defines such a parameter.
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 RFC 2119 [1].
3. The SDP 'txp' Attribute
This specification defines a new media-level value attribute, 'txp'.
Its formatting in SDP is described by the following BNF:
txp-attribute = "a=txp"
The 'txp' attribute indicates that the media stream is originated
from a textphone with some presentation limitation. This limited
device capability makes it probable that the user should apply formal
turntaking habits that are common among text telephone users in the
PSTN. The indication should be used for an indication in the user
interface.
A typical use case is a connection where one endpoint is an analog
textphone of a kind that merges text from both ends in the same
window, and the other one is a native IP based real time text capable
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terminal. The human user of the IP terminal need to change behaviour
when this indication is received, and apply formal turn-taking
habits. They may also need to figure out to what extent it is
possible to interrupt the other party if the need arises, because
that possibility varies between textphone types.
4. The 'txp' Attribute in the Offer/Answer Model
When it is interacting with a legacy device, an IP text phone may
receive an offer that contains the 'txp' attribute. That attribute
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 (*** should there be? ***). Similiarly, if an
answer is received that contains a 'txp' attribute, that indicates
that the remote device has limited capabilities, and the user-
interface should present some indication of this to the user.
This specification does not define a means to discover whether or not
the peer endpoint understands the 'txp' attribute. Indeed, the 'txp'
attribute is informative only at the offer/answer model level. The
fact that the peer endpoint does not understand the 'txp' attribute
does not keep the media session from being established. The only
consequence is that user interaction may be initially disrupted,
since the user interface will not be configured to match the
capabilities of legacy devices, and users will have to intuit that
turn taking is needed.
Since the 'txp' attribute does not have to be understood, an SDP
answer MAY contain 'txp' attributes even if none were present in the
offer. Similarly, the answer MAY contain no 'txp' attributes even if
they were present in the offer.
Use of the 'txp' attribute where SDP is used in the declarative
style, for example with the Session Announcement Protocol, is for
further study.
5. Example
The following example shows the use of the 'txp' attribute with SDP.
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v=0
o=Alice 292742730 29277831 IN IP4 131.163.72.4
s=-
c=IN IP4 131.164.74.2
t=0 0
m=text 52886 RTP/AVP 100
a=rtpmap:100 t140/8000
a=txp
6. Relation to the SDP Content Attribute
There is a proposal to use the SDP Content Attribute to signal that a
text device has limited capabilities, using "a=content:txp". This is
not an appropriate use of the content attribute, since the content
attribute is intended to describe only the content of a media stream,
not to define the capabilities of the device that is generating that
stream.
7. Security Considerations
An attacker may attempt to add, modify, or remove 'txp' attributes
from a session description. This could result in an application
behaving in an undesirable way. So, it is strongly RECOMMENDED that
integrity protection be applied to the SDP session descriptions. For
session descriptions carried in SIP, S/MIME is the natural choice to
provide such end-to-end integrity protection, as described in RFC
3261. Other applications MAY use a different form of integrity
protection.
8. IANA Considerations
The new SDP attribute "txp" is registered (see Section 3). This is a
media level attribute and is not dependent on charset.
9. Acknowledgements
Most of the text in this draft is taken from the SDP Content
Attribute draft (draft-ietf-mmusic-sdp-media-content-04.txt). Other
text comes from suggestions on the MMUSIC mailing list by Gunnar
Hellstrom and Arnoud van Wijk.
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10. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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Author's Address
Colin Perkins
University of Glasgow
Department of Computing Science
17 Lilybank Gardens
Glasgow G12 8QQ
UK
Email: csp@csperkins.org
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