WG MMUSIC Keith Drage, Ed.
Internet Draft Juergen Stoetzer-Bradler
Intended status: Standards track Albrecht Schwarz
Expires: February 2016 Alcatel-Lucent
August 10, 2015
T.140 Text Conversation over Data Channels
draft-schwarz-mmusic-t140-usage-data-channel-00.txt
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Abstract
This document specifies how the ITU-T Protocol for multimedia
application text conversation (Recommendation ITU-T T.140) can be
instantiated as a data channel sub-protocol, using the SDP
offer/answer exchange-based external negotiation defined in [I-
D.ietf-mmusic-data-channel-sdpneg]. Two network configurations are
documented: a WebRTC end-to-end configuration (connecting two T.140
over data channel endpoints), and a gateway configuration
(connecting an T.140 over data channel endpoint with an T.140 over
RTP/UDP endpoint).
Table of Contents
1. Introduction...................................................3
1.1. Motivation................................................3
1.2. Framework of WebRTC data applications.....................4
2. Conventions....................................................4
2.1. Prescriptive language.....................................4
2.2. Notation..................................................4
3. Terminology and abbreviations..................................4
3.1. Terminology used..........................................4
3.2. Abbreviations used........................................5
4. Prinicples.....................................................6
4.1. T.140 Data Channel........................................6
4.2. Session Mapping...........................................6
5. End-to-End Configuration.......................................6
5.1. Basic T.140 Support.......................................6
5.1.1. Session Negotiation..................................6
5.1.1.1. Use of dcmap Attribute..........................6
5.1.1.2. Use of dcsa Attribute...........................7
5.1.1.3. Example SDP Negotiation.........................7
5.1.2. Session Opening......................................7
5.1.3. Data Framing.........................................7
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5.1.4. Data Sending and Reporting...........................7
5.1.5. Session Closing......................................7
5.2. Support of T.140-specific Functions.......................7
6. Gateway Configurations.........................................8
6.1. Introduction..............................................8
6.2. Gateway-embedded Interworking Functions for T.140.........8
6.2.1. WebRTC T.140 text to IP text telephony in text
conversation mode...........................................8
6.2.2. WebRTC T.140 text to IP text telephony in text relay
mode........................................................8
6.2.3. WebRTC T.140 text to IP text telephony in text pass-
through mode................................................9
6.2.4. WebRTC T.140 text to PSTN text telephony.............9
6.3. Example gateway configuration in more detail..............9
7. Security Considerations.......................................10
8. IANA Considerations...........................................10
9. References....................................................10
9.1. Normative References.....................................10
9.2. Informative References...................................11
10. Acknowledgments..............................................11
11. CHANGE LOG...................................................13
11.1. Initial draft-schwarz-mmusic-t140-usage-data-channel-01.13
11.1.1. Status.............................................13
11.1.2. Changes against "draft-ietf-mmusic-msrp-usage-data-
channel-01"................................................13
1. Introduction
1.1. Motivation
Recommendation ITU-T T.140 [ITU-T T.140] defines a protocol for text
conversation, also known as realtime text or text telephony. The
native transport for IP networks is based on the Real-time Transport
Protocol (RTP; see [RFC4103]) due to its inherent realtime
characteristics, similar as conversational audio and video services.
WebRTC text conversation is based on T.140, but considered as "data
traffic" component (despite the fact of the native RTP-based
transport in non-WebRTC environments), see [I-D.ietf-rtcweb-????].
This document defines the SDP-based negotiation and transport of the
T.140 protocol over data channels, where a data channel is a bi-
directional communication channel running on top of SCTP/DTLS (as
per [I-D.ietf-rtcweb-data-channel]) and where T.140 is instantiated
as a sub-protocol of this data channel.
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Considering an T.140 endpoint being an T.140 application that uses
data channel from WebRTC specifications [I-D.ietf-rtcweb-data-
channel], this document describes two configurations where the other
endpoint is respectively either another T.140 over data channel
endpoint (e.g., a WebRTC application) or an T.140 endpoint using
native RTP transport.
1.2. Framework of WebRTC data applications
There are multiple IP application protocols which using WebRTC data
channels as transport, such as MSRP or BFCP besides T.140. The SDP-
based indication and negotiation of such WebRTC data applications at
call control signalling level follows common principles. The first
WebRTC application from this suite is/was the MSRP-based instant
messaging service for WebRTC, see [I-D.ietf-mmusic-msrp-usage-data-
channel]. This specification for T.140 was derived from that
document and uses an aligned clause structuring.
It may be noted that the T.140 protocol as such is much simpler in
comparison to the MSRP, which requires an extensive set of SDP
elements (in comparison to T.140) for the description of specific
MSRP services and their protocol parameter settings.
2. Conventions
2.1. Prescriptive language
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 [RFC2119].
2.2. Notation
The brief notation "T.140" is used as a synonym for the text
conversation protocol according to [ITU-T T.140].
3. Terminology and abbreviations
3.1. Terminology used
This document uses the following terms:
Data channel: A WebRTC data channel as specified in [I-D.ietf-
rtcweb-data-channel].
T.140 data channel: A data channel specifically used to transport
the text and presentation control information of one T.140 session.
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External negotiation: Data channel negotiation based on out-of-band
or in-band mechanisms other than the Data Channel Establishment
Protocol specified in [I-D.ietf-rtcweb-data-protocol].
In-band: Transmission through the peer-to-peer SCTP association.
Out-of-band: Transmission through the call control signaling path,
e.g., using JSEP [I-D.ietf-rtcweb-jsep] and the SDP Offer/Answer
model [RFC3264].
Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of the
SDP offerer, the peer is the SDP answerer. From the perspective of
the SDP answerer, the peer is the SDP offerer
3.2. Abbreviations used
DS #####
DTLS Datagram Transport Layer Security
GCP Gateway Control Protocol
ITU-T International Telecommunication Union Telecommunication
Standardization Sector
IWF Interworking Function
JSEP #####
MG (H.248) Media Gateway
MGC (H.248) Media Gateway Controller
RTP Real-time Transport Protocol
SCTP Stream Control Transmission Protocol
SDP Session Description Protocol
SIP Session Initiation Protocol
TCP Transmission Control Protocol
TDM (Synchronous) Time Division Multiplexing
ToIP Text over IP
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TLS Transport Layer Security
UA User Agent
UDP User Datagram Protocol
VBD Voiceband Data
4. Prinicples
4.1. T.140 Data Channel
In this document, an T.140 data channel is a data channel for which
the instantiated sub-protocol is "t140", and where the T.140-related
negotiation is done as part of the SDP-based external negotiation
method defined in [I-D.ietf-mmusic-data-channel-sdpneg].
4.2. Session Mapping
In this design, the T.140 session maps to the SCTP association and
the "SCTP stream pair" assigned to the data channel, and each T.140
session maps to one data channel exactly.
5. End-to-End Configuration
This section describes the network configuration where each T.140
endpoint is running T.140 over a data channel.
5.1. Basic T.140 Support
5.1.1. Session Negotiation
5.1.1.1. Use of dcmap Attribute
The SDP offer shall include a dcmap attribute line (defined in [I-
D.ietf-mmusic-data-channel-sdpneg], within the media description for
the SCTP association for each T.140 data channel session to be
negotiated.
The attribute includes the following data channel parameters:
o "label=" labelstring
o "subprotocol=" "t140"
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The labelstring is set by the T.140 application according to [I-
D.ietf-mmusic-data-channel-sdpneg]. The max-retr, max-time and
ordered parameters shall not be used.
Rest of the SDP offer/answer procedures are per [I-D.ietf-mmusic-
data-channel-sdpneg].
The following is an example of the dcmap attribute for an T.140
session to be negotiated (on default SCTP port 5000) with stream=3
and without any label:
a=dcmap:3 subprotocol="t140"
5.1.1.2. Use of dcsa Attribute
FIXTHIS
5.1.1.3. Example SDP Negotiation
FIXTHIS
5.1.2. Session Opening
FIXTHIS
5.1.3. Data Framing
FIXTHIS
5.1.4. Data Sending and Reporting
FIXTHIS
5.1.5. Session Closing
FIXTHIS
5.2. Support of T.140-specific Functions
FIXTHIS (e.g., T.140 specific reliable transport requirements)
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6. Gateway Configurations
6.1. Introduction
This section describes the network configuration where one T.140
endpoint uses data channels as T.140 transport, the other T.140
endpoint uses a different protocol stack. There is consequently an
interworking function (IWF) in the media plane required, associated
with correspondent interworking in the signalling plane. Such type
of IWFs are provided by gateway devices, given here by WebRTC
gateways as defined by [I-D.ietf-rtcweb-gateways], [ITU-T H.248.94],
[3GPP 29.334], etc.
6.2. Gateway-embedded Interworking Functions for T.140
There are a plethora of interworking functions knowns for T.140
[ITU-T T.140] due to the long history and usage of this service in
legacy packet-switched and circuit-switched networks. Some examples
are indicated below.
6.2.1. WebRTC T.140 text to IP text telephony in text conversation mode
Service "IP text telephony in text conversation mode":
Transport service according to [RFC4103].
Media plane IWF:
"T.140-over-SCTP stream-over-SCTP association-over-DTLS-over-L4"
to "T.140-over-RTP/UDP"
6.2.2. WebRTC T.140 text to IP text telephony in text relay mode
Service "IP text telephony in text relay mode":
Transport of T.140 over IP networks, according to [ITU-T V.151].
Also known as Text-over-IP (ToIP).
Media plane IWF:
"T.140-over-SCTP stream-over-SCTP association-over-DTLS-over-L4"
to "T.140-over-IP TLP-UDP".
NOTE: [ITU-T V.151] uses the concept of an "IP Transport Layer
Protocol" (IP-TLP) above the native IP transport layer, which
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again allows different framing protocol options (such as RTP-based
or SPRT-based framing (Simple Packet Relay Transport, see Annex B
of [ITU-T V.150.1])).
6.2.3. WebRTC T.140 text to IP text telephony in text pass-through mode
Service "IP text telephony in text pass-through mode":
Transport of analogue PSTN text telephones signals over PCM
encoded voice over IP networks, according to [ITU-T V.152]. Also
known as Voiceband-over-IP (VBDoIP).
Media plane IWF:
"T.140-over-SCTP stream-over-SCTP association-over-DTLS-over-L4"
to "text/modem-over-G.711-over-RTP/UDP".
NOTE: This is the VBD mode according to [ITU-T V.152], using ITU-T
G.711 as VBD codec (i.e., a different codec configuration as in
comparison to G.711 as audio codec).
6.2.4. WebRTC T.140 text to PSTN text telephony
Service "PSTN text telephony":
Transport of analogue PSTN text telephones signals over a)
analogue lines or b) circuit-switched 64-kbit/s channels.
Media plane IWF:
"T.140-over-SCTP stream-over-SCTP association-over-DTLS-over-L4"
to "text/modem-over-DS0/TDM" (in case of circuit-switched
networks).
6.3. Example gateway configuration in more detail
The signalling plane and media plane interworking functions are
elaborated in more detail at the example of a WebRTC gateway with
support of interworking to IP text telephony in text conversation
mode.
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This example describes the network configuration where one T.140
endpoint uses data channels as T.140 transport, the other T.140
endpoint uses RTP/UDP connections as T.140 transport, and the two
T.140 endpoints interwork via an appropriate interworking function.
FIXTHIS - details
7. Security Considerations
FIXTHIS
8. IANA Considerations
FIXTHIS
9. References
9.1. Normative References
[RFC2119] RFC 2119 (03/1997), "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14.
[RFC3264] RFC 3264 (06/2002), "An Offer/Answer Model with the
Session Description Protocol (SDP)".
[RFC4103] RFC 4103 (06/2005), "RTP Payload for Text Conversation".
[RFC4566] RFC 4566 (07/2006), "SDP: Session Description Protocol".
[I-D.ietf-rtcweb-data-protocol] draft-ietf-rtcweb-data-channel
(##/2015), "WebRTC Data Channel Establishment Protocol".
[I-D.ietf-rtcweb-data-channel]
http://tools.ietf.org/wg/rtcweb/draft-ietf-rtcweb-data-
channel/draft-ietf-rtcweb-data-channel (##/2015), "WebRTC
Data Channels".
[I-D.ietf-mmusic-data-channel-sdpneg] draft-ietf-mmusic-data-
channel-sdpneg (##/2015), "SDP-based Data Channel
Negotiation".
[I-D.ietf-mmusic-msrp-usage-data-channel] draft-ietf-mmusic-msrp-
usage-data-channel (##/2015), "MSRP over Data Channels".
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[ITU-T T.140] Recommendation ITU-T T.140 (02/1998), "Protocol for
multimedia application text conversation".
Free copy via:
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-
T.140-199802-I!!PDF-E&type=items
9.2. Informative References
[I-D.ietf-rtcweb-gateways] draft-ietf-rtcweb-gateways (##/2015),
"WebRTC Gateways".
[ITU-T H.248.94] Recommendation ITU-T H.248.94 (10/2015), "Web-
based real-time communication services - H.248 protocol
support and profile guidelines".
Status: still work in progress in ITU-T SG16 Question 3
Free copy via: FIXTHIS
[ITU-T V.151] Recommendation ITU-T V.151 (05/2006), "Procedures for
the end-to-end connection of analogue PSTN text telephones
over an IP network utilizing text relay".
Free copy via:
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-
V.151-200605-I!!PDF-E&type=items
[ITU-T V.152] Recommendation ITU-T V.152 (09/2010), "Procedures for
supporting voice-band data over IP networks".
Free copy via:
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-
V.152-201009-I!!PDF-E&type=items
[3GPP 29.334] 3GPP TS 29.334 Release 13 (2015), "Digital cellular
telecommunications system (Phase 2+); Universal Mobile
Telecommunications System (UMTS); LTE; IMS Application
Level Gateway (IMS-ALG) - IMS Access Gateway (IMS-AGW); Iq
Interface; Stage 3".
Free copy via:
ftp://ftp.3gpp.org/specs/archive/29_series/29.334/
10. Acknowledgments
FIXTHIS
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Authors' Addresses
Keith Drage (editor)
Alcatel-Lucent
Quadrant, Stonehill Green, Westlea
Swindon
UK
Email: keith.drage@alcatel-lucent.com
Dr. Juergen Stoetzer-Bradler
Alcatel-Lucent
Lorenzstrasse 10
D-70435 Stuttgart
GERMANY
Email: Juergen.Stoetzer-Bradler@alcatel-lucent.com
Dr. Albrecht Schwarz
Alcatel-Lucent
Lorenzstrasse 10
D-70435 Stuttgart
GERMANY
Email: Albrecht.Schwarz@alcatel-lucent.com
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11. CHANGE LOG
11.1. Initial draft-schwarz-mmusic-t140-usage-data-channel-01
11.1.1. Status
o The initial document represents a skeleton where still a number
of clauses are still empty.
o The intention is to propose a document structure aligned with the
MSRP draft, and to emphasize WebRTC gateway flavours.
11.1.2. Changes against "draft-ietf-mmusic-msrp-usage-data-channel-01"
o Replace protocol "MSRP" by "T.140" plus protocol specific
adaptations
o Initial scope of gateway section
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