Internet DRAFT - draft-ietf-mmusic-t140-usage-data-channel
draft-ietf-mmusic-t140-usage-data-channel
MMUSIC Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 8373 (if approved) G. Hellstrom
Intended status: Standards TrackGunnar Hellstrom Accessible Communicatio
Expires: October 12, 2020 April 10, 2020
T.140 Real-time Text Conversation over WebRTC Data Channels
draft-ietf-mmusic-t140-usage-data-channel-14
Abstract
This document specifies how a WebRTC data channel can be used as a
transport mechanism for Real-time text using the ITU-T Protocol for
multimedia application text conversation (Recommendation ITU-T
T.140), and how the SDP offer/answer mechanism can be used to
negotiate such data channel, referred to as T.140 data channel. This
document updates RFC 8373 to specify its use with WebRTC data
channels.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 12, 2020.
Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. WebRTC Data Channel Considerations . . . . . . . . . . . . . 4
4. SDP Considerations . . . . . . . . . . . . . . . . . . . . . 4
4.1. Use of dcmap Attribute . . . . . . . . . . . . . . . . . 4
4.2. Use of dcsa Attribute . . . . . . . . . . . . . . . . . . 5
4.2.1. Maximum Character Transmission Rate . . . . . . . . . 5
4.2.2. Real-time Text Conversation Languages . . . . . . . . 6
4.2.3. Real-time Text Direction . . . . . . . . . . . . . . 7
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9
5. T.140 Considerations . . . . . . . . . . . . . . . . . . . . 10
5.1. Session Layer Functions . . . . . . . . . . . . . . . . . 10
5.2. Data Encoding and Sending . . . . . . . . . . . . . . . . 11
5.3. Data Buffering . . . . . . . . . . . . . . . . . . . . . 11
5.4. Loss of T140blocks . . . . . . . . . . . . . . . . . . . 11
5.5. Multi-party Considerations . . . . . . . . . . . . . . . 12
6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 12
7. Update to RFC 8373 . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 14
9.1. Subprotocol Identifier t140 . . . . . . . . . . . . . . . 14
9.2. SDP fmtp Attribute . . . . . . . . . . . . . . . . . . . 15
9.3. SDP Language Attributes . . . . . . . . . . . . . . . . . 15
9.4. SDP Media Direction Attributes . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
The ITU-T Protocol for multimedia application text conversation
(Recommendation ITU-T T.140) [T140] defines a protocol for text
conversation, also known as real-time text. The transport used for
IP networks is the "RTP Payload for Text Conversation" [RFC4103]
mechanism, based on the Real-time Transport Protocol (RTP) [RFC3550].
This document specifies how a WebRTC data channel
[I-D.ietf-rtcweb-data-channel] can be used as a transport mechanism
for T.140, and how the SDP offer/answer mechanism for data channels
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[I-D.ietf-mmusic-data-channel-sdpneg] can be used to negotiate such a
data channel.
In this document, a T.140 data channel refers to a WebRTC data
channel for which the instantiated sub-protocol is "t140", and where
the channel is negotiated using the SDP-based external negotiation
method [I-D.ietf-mmusic-data-channel-sdpneg].
NOTE: The decision to transport real-time text using a WebRTC data
channel, instead of using RTP based transport [RFC4103], is motivated
by use-case "U-C 5: Real-time text chat during an audio and/or video
call with an individual or with multiple people in a conference", see
Section 3.2 of [I-D.ietf-rtcweb-data-channel].
The brief notation "T.140" is used as a name for the text
conversation protocol according to [T140].
Real-time text is intended to be entered by human users from a
keyboard, handwriting recognition, voice recognition or any other
input method. The rate of character entry is usually at a level of a
few characters per second or less.
Section 3 defines the generic data channel properties for a T.140
data channel, and Section 4 defines how they are conveyed in an SDP
dcmap attribute. While this document defines how to establish a
T.140 data channel using the SDP-based external negotiation method
[I-D.ietf-mmusic-data-channel-sdpneg], the generic T.140 and gateway
considerations defined in Section 3, Section 5 and Section 6 of this
document can also be applied when a T.140 data channel is established
using another mechanism (e.g., the mechanism defined in
[I-D.ietf-rtcweb-data-protocol]). Section 5 of
[I-D.ietf-mmusic-data-channel-sdpneg] defines the mapping between the
SDP dcmap attribute parameters and the protocol parameters used in
[I-D.ietf-rtcweb-data-protocol].
This document updates [RFC8373], by defining how the SDP hlang-send
and hlang-recv attributes are used for the "application/webrtc-
datachannel" media type.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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3. WebRTC Data Channel Considerations
The following WebRTC data channel property values
[I-D.ietf-rtcweb-data-channel] apply to a T.140 data channel:
+--------------------------+-------------------------------+
| Subprotocol Identifier | t140 |
| Transmission reliability | reliable |
| Transmission order | in-order |
| Label | See Section 4.1 and Section 6 |
+--------------------------+-------------------------------+
NOTE: T.140 requires the transport channel to provide transmission of
real-time text without duplication and in original order. Therefore,
T.140 does not specify reliable and ordered transmission of T.140
data on the application layer. Instead, when RTP based transport is
used, the RTP sequence number is used to detect packet loss and out-
of-order packets, and a redundancy mechanism is used to achieve
reliable delivery of T.140 data. By using the WebRTC data channel
reliable and in-order transmission features
[I-D.ietf-rtcweb-data-channel] for the T.140 data channel, there is
no need for a redundancy mechanism or a mechanism to detect data loss
and out-of-order delivery at the application level. The latency
characteristics of the T.140 data channel is also regarded to be
sufficient to meet the application requirements of T.140.
4. SDP Considerations
The generic SDP considerations, including the SDP Offer/Answer
procedures [RFC3264], for negotiating a WebRTC data channel are
defined in [I-D.ietf-mmusic-data-channel-sdpneg]. This section, and
its subsections, define the SDP considerations that are specific to a
T.140 data channel, identified by an SDP 'dcmap' attribute
[I-D.ietf-mmusic-data-channel-sdpneg] with a "t140" attribute
parameter value.
4.1. Use of dcmap Attribute
An offerer and answerer MUST, in each offer and answer, include an
SDP 'dcmap' attribute [I-D.ietf-mmusic-data-channel-sdpneg] in the
SDP media description (m= section) [I-D.ietf-mmusic-rfc4566bis]
describing the SCTP association [RFC4960] used to realize the T.140
data channel.
The offerer and answerer MUST include the subprotocol attribute
parameter, with a "t140" parameter value, in the 'dcmap' attribute
value.
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The offerer and answerer MAY include the priority attribute parameter
and the label attribute parameter in the 'dcmap' attribute value, as
specified in [I-D.ietf-mmusic-data-channel-sdpneg].
NOTE: As specified in [I-D.ietf-rtcweb-data-channel], when a data
channel is negotiated using the mechanism defined in
[I-D.ietf-rtcweb-data-protocol], the label attribute parameter value
has to be the same in both directions. That rule also applies to
data channels negotiated using the mechanism defined in this
document.
The offerer and answerer MUST NOT include the max-retr or the max-
time attribute parameters in the 'dcmap' attribute. If either of
those attribute parameters is received in an offer, the answerer MUST
reject the offer. If either of those attribute parameters is
received in an answer the offerer MUST NOT accept the answer.
Instead, the answerer MUST take appropriate actions, e.g., by sending
a new offer without a T.140 data channel, or by terminating the
session.
If the ordered attribute parameter is included in the 'dcmap'
attribute, it MUST be assigned the value 'true'.
Below is an example of the 'dcmap' attribute for a T.140 data channel
with stream id=3 and without any label:
a=dcmap:3 subprotocol="t140"
4.2. Use of dcsa Attribute
An offerer and answerer can, in each offer and answer, include one or
more SDP 'dcsa' attributes [I-D.ietf-mmusic-data-channel-sdpneg] in
the m= section describing the SCTP association used to realize the
T.140 data channel.
If an offerer or answerer receives a 'dcsa' attribute that contains
an SDP attribute which usage has not been defined for a T.140 data
channel, the offerer or answerer should ignore the 'dcsa' attribute,
following the rules in Section 6.7 of
[I-D.ietf-mmusic-data-channel-sdpneg].
4.2.1. Maximum Character Transmission Rate
A 'dcsa' attribute can contain the SDP 'fmtp' attribute used to
indicate a maximum character transmission rate [RFC4103]. The 'cps'
attribute parameter is used to indicate the maximum character
transmission rate that the endpoint that includes the attribute is
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able to receive, and the value is used as a mean value in characters
per second over any 10-second interval.
If the 'fmtp' attribute is included, the 'format' attribute parameter
MUST be set to "t140".
If no 'fmtp' attribute with a 'cps' attribute parameter is included,
the default value of 30 applies [RFC4103].
The offerer and answerer MAY modify the 'cps' attribute parameter
value in subsequent offers and answers.
This document does not define any other usage of the 'fmtp' attribute
for a T.140 channel. If an offerer or answerer receives a 'dcsa'
attribute that contains an 'fmtp' attribute that is not according to
the procedure above, the offerer or answerer MUST ignore the 'dcsa'
attribute.
NOTE: The 'cps' attribute parameter is especially useful when a T.140
data channel endpoint is acting as a gateway (Section 6) and is
interworking with a T.140 transport mechanism that have restrictions
on how many characters can be sent per second.
If an endpoint receives text at a higher rate than it can handle,
e.g., because the sending endpoint does not support the 'cps'
attribute parameter, it SHOULD either indicate to the sending
endpoint that it is not willing to receive more text, using the
direction attributes (Section 4.2.3), or use a flow control mechanism
to reduce the rate. However, in certain applications, e.g. emergency
services, it is important to regain human interaction as soon as
possible, and it might therefore be more appropriate to simply
discard the received overflow, insert a mark for loss [T140ad1], and
continue to process the received text as soon as possible.
NOTE: At the time of writing this specification, the standardized API
for WebRTC data channels does not support flow control. Should such
be available at some point, a receiving endpoint might use it in
order to slow down the rate of text received from the sending
endpoint.
4.2.2. Real-time Text Conversation Languages
'dcsa' attributes can contain the SDP 'hlang-send' and 'hlang-recv'
attributes [RFC8373] to negotiate the language to be used for the
real-time text conversation.
For a T.140 data channel, the modality is "written" [RFC8373].
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4.2.3. Real-time Text Direction
'dcsa' attributes can contain the SDP 'sendonly', 'recvonly',
'sendrecv' and 'inactive' attributes [I-D.ietf-mmusic-rfc4566bis] to
negotiate the direction in which text can be transmitted in a real-
time text conversation.
NOTE: A WebRTC data channel is always bi-directional. The usage of
the 'dcsa' attribute only affects the direction in which
implementations are allowed to transmit text on a T.140 data channel.
The offer/answer rules for the direction attributes are based on the
rules for unicast streams defined in [RFC3264], as described below.
Note that the rules only apply to the direction attributes.
Session-level direction attributes [I-D.ietf-mmusic-rfc4566bis] have
no impact on a T.140 data channel.
4.2.3.1. Generating an Offer
If the offerer wishes to both send and receive text on a T.140 data
channel, it SHOULD mark the data channel as sendrecv with a
'sendrecv' attribute inside a 'dcsa' attribute. If the offerer does
not explicitly mark the data channel, an implicit 'sendrecv'
attribute inside a 'dcsa' attribute is applied by default.
If the offerer wishes to only send text on a T.140 data channel, it
MUST mark the data channel as sendonly with a 'sendonly' attribute
inside a 'dcsa' attribute.
If the offerer wishes to only receive text on a T.140 data channel,
it MUST mark the data channel as recvonly with a 'recvonly' attribute
inside a 'dcsa' attribute.
If the offerer wishes to neither send nor receive text on a T.140
data channel, it MUST mark the data channel as inactive with an
'inactive' attribute inside a 'dcsa' attribute.
If the offerer has marked a data channel as sendrecv (or if the
offerer did not explicitly mark the data channel) or recvonly, it
MUST be prepared to receive T.140 data as soon as the state of the
T.140 data channel allows it.
4.2.3.2. Generating an Answer
When the answerer accepts an offer, and marks the direction of the
text in the corresponding answer, the direction is based on the
marking (or the lack of explicit marking) in the offer.
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If the offerer explicitly marked the data channel as sendrecv, or if
the offerer did not mark the data channel, the answerer SHOULD mark
the data channel as sendrecv, sendonly, recvonly or inactive with a
'sendrecv', 'sendonly', 'recvonly' or 'inactive' attribute
respectively inside a 'dcsa' attribute. If the answerer does not
explicitly mark the data channel, an implicit 'sendrecv' attribute
inside a 'dcsa' attribute is applied by default.
If the offerer marked the data channel as sendonly, the answerer MUST
mark the data channel as recvonly or inactive with a 'recvonly' or
'inactive' attribute respectively inside a 'dcsa' attribute.
If the offerer marked the data channel as recvonly, the answerer MUST
mark the data channel as sendonly or inactive with a 'sendonly' or
'inactive' attribute respectively inside a 'dcsa' attribute.
If the offerer marked the data channel as inactive, the answerer MUST
mark the data channel as inactive with an 'inactive' attribute inside
a 'dcsa' attribute.
If the answerer has marked a data channel as sendrecv or recvonly, it
MUST be prepared to receive data as soon as the state of the T.140
data channel allows transmission of data.
4.2.3.3. Offerer Receiving an Answer
When the offerer receives an answer to the offer and the answerer has
marked a data channel as sendrecv (or the answerer did not mark the
data channel) or recvonly in the answer, the offerer can start
sending T.140 data as soon as the state of the T.140 data channel
allows it. If the answerer has marked the data channel as inactive
or sendonly, the offerer MUST NOT send any T.140 data.
If the answerer has not marked the direction of a T.140 data channel
in accordance with the procedures above, it is RECOMMENDED that the
offerer does not process that as an error situation, but rather
assume that the answerer might both send and receive T.140 data on
the data channel.
4.2.3.4. Modify Text Direction
If an endpoint wishes to modify a previously negotiated text
direction in an ongoing session, it MUST initiate an offer that
indicates the new direction, following the rules in Section 4.2.3.1.
If the answerer accepts the offer it follows the procedures in
Section 4.2.3.2.
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4.3. Examples
Below is an example of an m= section of an offer for a T.140 data
channel offering real-time text conversation in Spanish and
Esperanto, and an m= section in the associated answer accepting
Esperanto. The maximum character transmission rate is set to 20. As
the offerer and answerer have not explicitly indicated the real-time
text direction, the default direction "sendrecv" applies.
Offer:
m=application 911 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:db8::3
a=max-message-size:1000
a=sctp-port 5000
a=setup:actpass
a=dcmap:2 label="ACME customer service";subprotocol="t140"
a=dcsa:2 fmtp:t140 cps=20
a=dcsa:2 hlang-send:es eo
a=dcsa:2 hlang-recv:es eo
Answer:
m=application 2004 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:db8::1
a=max-message-size:1000
a=sctp-port 6000
a=setup:passive
a=dcmap:2 label="ACME customer service";subprotocol="t140"
a=dcsa:2 fmtp:t140 cps=20
a=dcsa:2 hlang-send:eo
a=dcsa:2 hlang-recv:eo
Below is an example of an m= section of an offer for a T.140 data
channel where the offerer wishes to only receive real-time text, and
an m= section in the associated answer indicating that the answerer
will only send real-time text. No maximum character transmission
rate is indicated. No preference for the language to be used for the
real-time text conversation is indicated.
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Offer:
m=application 1400 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:db8::3
a=max-message-size:1000
a=sctp-port 5000
a=setup:actpass
a=dcmap:2 label="ACME customer service";subprotocol="t140"
a=dcsa:2 recvonly
Answer:
m=application 2400 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:db8::1
a=max-message-size:1000
a=sctp-port 6000
a=setup:passive
a=dcmap:2 label="ACME customer service";subprotocol="t140"
a=dcsa:2 sendonly
5. T.140 Considerations
5.1. Session Layer Functions
Section 6.1 of [T140] describes the generic T.140 session control
functions at a high-level in a signalling protocol independent
manner. The list below describes how the functions are realized when
using a T.140 data channel.
o Prepare session: An endpoint can indicate its support of T.140
data channels using signalling specific means (e.g., using SIP
OPTIONS [RFC3261]), or by indicating the support in an offer or
answer (Section 4)
o Initiate session: An offer used to request the establishment of a
T.140 data channel (Section 4)
o Accept session: An answer used to accept a request to establish a
T.140 data channel (Section 4)
o Deny session: An answer used to reject a request to establish a
T.140 data channel, using the generic procedures for rejecting a
data channel [I-D.ietf-mmusic-data-channel-sdpneg]
o Disconnect session: An offer or answer used to disable a
previously established T.140 data channel, using the generic
procedures for closing a data channel
[I-D.ietf-mmusic-data-channel-sdpneg]
o Data: Data sent on an established T.140 data channel (Section 5.2)
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5.2. Data Encoding and Sending
T.140 text is encoded and framed as T140blocks [RFC4103].
Each T140block is sent on the SCTP stream [RFC4960] used to realize
the T.140 data channel using standard T.140 transmission procedures
[T140]. One or more T140blocks can be sent in a single SCTP user
message [RFC4960]. Unlike RTP based transport for real-time text
[RFC4103], T.140 data channels do not use redundant transmission of
text. The reason for this is that the T.140 data channel achieves
robust transmission by using the "reliable" mode of the data channel.
Data sending procedures conform to [T140].
See Section 8 of [T140] for coding details.
NOTE: The T.140 coding details contain information on optional
control codes for controlling the presentation which may not be
supported by the presentation level of the receiving application.
The receiving application is expected to handle reception of such
T.140 control codes appropriately (e.g. ignore and skip them) even if
their effect on the presentation is not supported.
5.3. Data Buffering
As described in [T140], buffering can be used to reduce overhead,
with the maximum assigned transmission interval of T140blocks from
the buffer being 500 ms as long as there is text to send.
Buffering MAY also be used for staying within the maximum character
transmission rate (Section 4.2).
An implementation needs to take the user requirements for smooth flow
and low latency in real-time text conversation into consideration
when assigning a transmission interval. It is RECOMMENDED to use the
default transmission interval of 300 milliseconds [RFC4103], for
T.140 data channels. Implementers might also use lower values for
specific applications requiring low latency, taking the increased
overhead in consideration.
5.4. Loss of T140blocks
In case of network failure or congestion, T.140 data channels might
fail and get torn down. If this happens but the session sustains, it
is RECOMMENDED that implementations try to reestablish the T.140 data
channels. As a T.140 data channel does not provide a mechanism for
the receiver to identify retransmitted T140blocks after channel
reestablishment, the sending endpoint MUST NOT retransmit T140blocks.
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Similarly, a receiver SHOULD indicate to the user that there has been
a channel reestablishment, and that text might have been lost. This
MAY be done by inserting the missing text markers [T140ad1] or in any
other way evident to the user.
NOTE: If the SCTP association [RFC4960] used to realize the T.140
data channel fails and gets torn down, it needs to be re-established
before the T.140 data channel can be reestablished. The procedures
after the reestablishment of the T.140 data channel defined in this
section apply no matter if only the T.140 data channel, or the whole
SCTP association, got torn down.
5.5. Multi-party Considerations
If an implementation needs to support multi-party scenarios, the
implementation needs to support multiple simultaneous T.140 data
channels, one for each remote party. At the time of writing this
document, this is true even in scenarios where each participant
communicates via a centralized conference server. The reason is
that, unlike RTP media, WebRTC data channels and the T.140 protocol
do not support the indication of the source of T.140 data. The SDP
'dcmap' attribute label attribute parameter (Section 4.1) can be used
by the offerer to provide additional information about each T.140
data channel, and help implementations to distinguish between them.
NOTE: Future extensions to T.140, or to the T140block, might allow
indicating the source of T.140 data, in which case it might be
possible to use a single T.140 data channel to transport data from
multiple remote sources. The usage of a single T.140 data channel,
without any protocol extensions, would require the conference server
to only forward real-time text from one source at any given time, and
e.g., include human readable text labels in the real-time text stream
that indicate the source whenever the conference server switches the
source. This would allow the receiver to present real-time text from
different sources separately. The procedures of such mechanism are
outside the scope of this document.
6. Gateway Considerations
A number of real-time text transports and protocols have been defined
for both packet-switched and circuit-switched networks. Many are
based on the ITU-T T.140 protocol on application and presentation
level [T140]. At the time of writing this document, some mechanisms
are no longer used, as the technologies they use have been obsoleted,
while others are still in use.
When performing interworking between T.140 data channels and real-
time text in other transports and protocols, a number of factors need
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to be considered. At the time of writing this document, the most
common IP-based real-time text transport is the RTP based mechanism
defined in [RFC4103]. While this document does not define a complete
interworking solution, this list below provides some guidance and
considerations to take into account when designing a gateway for
interworking between T.140 data channels and RTP-based T.140
transport:
o For each T.140 data channel there is an RTP stream for real-time
text [RFC4103]. Redundancy is by default declared and used on the
RTP stream. There is no redundancy on the T.140 data channel, but
the reliable property [I-D.ietf-mmusic-data-channel-sdpneg] is set
on it.
o During a normal text flow, T140blocks received from one network
are forwarded towards the other network. Keep-alive traffic is
handled by lower layers on the T.140 data channel. A gateway
might have to extract keep-alives from incoming RTP streams, and
MAY generate keep-alives on outgoing RTP streams.
o If the gateway detects or suspects loss of data on the RTP stream,
and the lost data has not been retrieved using a redundancy
mechanism, the gateway SHOULD insert the T.140 missing text marker
[T140ad1] in the data sent on the outgoing T.140 data channel.
o If the gateway detects that the T.140 data channel has failed and
got torn down, once the data channel has been reestablished the
gateway SHOULD insert the T.140 missing text marker [T140ad1] in
the data sent on the outgoing RTP stream if it detects or suspects
that data sent by the remote T.140 data channel endpoint was lost.
o If the gateway detects that the T.140 data channel has failed and
got torn down, once the data channel has been reestablished the
gateway SHOULD insert the T.140 missing text marker [T140ad1] in
the data sent on the outgoing T.140 data channel if it detects or
suspects that data sent or to be sent on the T.140 data channel
was lost during the failure.
o The gateway MUST indicate the same text transmission direction
(Section 4.2.3) on the T.140 data channel and the RTP stream.
NOTE: In order for the gateway to insert a missing text marker, or to
perform other actions that require that the gateway has access to the
T.140 data, the T.140 data cannot be encrypted end-to-end between the
T.140 data channel endpoint and the RTP endpoint. At the time of
writing this document, no mechanism to provide such end-to-end
encryption is defined.
7. Update to RFC 8373
This document updates RFC 8373 [RFC8373], by defining how the SDP
hlang-send and hlang-recv attributes are used for the "application/
webrtc-datachannel" media type.
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SDP offerers and answerers MUST NOT include the attributes directly
in the m= section associated with the 'application/webrtc-
datachannel' media type. Instead, the attributes MUST be associated
with individual data channels, using the SDP 'dcsa' attribute. A
specification that defines a subprotocol that uses the attributes
MUST specify the modality for that subprotocol, or how to retrieve
the modality if the subprotocol supports multiple modalities. The
subprotocol is indicated using the SDP 'dcmap' attribute.
8. Security Considerations
The generic WebRTC security considerations are defined in
[I-D.ietf-rtcweb-security-arch] and [I-D.ietf-rtcweb-security].
The generic security considerations for WebRTC data channels are
defined in [I-D.ietf-rtcweb-data-channel]. As data channels are
always encrypted by design, the T.140 data channels will also be
encrypted.
The generic security considerations for the SDP-based external
negotiation method are defined in
[I-D.ietf-mmusic-data-channel-sdpneg]. There are no additional T.140
data channel specific security considerations.
When performing interworking between T.140 data channels and real-
time text and the RTP based mechanism defined in [RFC4103], in order
for a gateway to insert a missing text marker, or to perform other
actions that require that the gateway has access to the T.140 data,
the T.140 data cannot be encrypted end-to-end between the T.140 data
channel endpoint and the RTP endpoint.
9. IANA considerations
[RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
RFC number of this document.]
9.1. Subprotocol Identifier t140
This document adds the subprotocol identifier "t140" to the
"WebSocket Subprotocol Name Registry" as follows:
+--------------------------+----------------------------+
| Subprotocol Identifier: | t140 |
| Subprotocol Common Name: | ITU-T T.140 Real-Time Text |
| Subprotocol Definition: | RFCXXXX |
| Reference: | RFCXXXX |
+--------------------------+----------------------------+
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9.2. SDP fmtp Attribute
This document defines the usage of the SDP 'fmtp' attribute, if this
attribute is included in an SDP 'dcsa' attribute and associated with
an T.140 real-time text session over a WebRTC data channel. The
usage is defined in Section 4.2.1.
The usage level "dcsa(t140)" is added to the registration of the SDP
'fmtp' attribute in the Session Description Protocol (SDP) Parameters
registry as follows:
+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | fmtp |
| Usage level: | dcsa(t140) |
| Purpose: | Indicate format parameters for a T.140 |
| | data channel, such as maximum character |
| | transmission rates. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
9.3. SDP Language Attributes
This document modifies the usage of the SDP 'hlang-send' and 'hlang-
recv' attributes, if these attributes are included in SDP 'dcsa'
attributes associated with an T.140 data channel. The modified usage
is described in Section 4.2.2.
The usage level "dcsa(t140)" is added to the registration of the SDP
'hland-send' attribute in the Session Description Protocol (SDP)
Parameters registry as follows:
+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | hlang-send |
| Usage level: | dcsa(t140) |
| Purpose: | Negotiate the language to be used on a |
| | T.140 data channel. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
The usage level "dcsa(t140)" is added to the registration of the SDP
'hland-recv' attribute in the Session Description Protocol (SDP)
Parameters registry as follows:
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+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | hlang-recv |
| Usage level: | dcsa(t140) |
| Purpose: | Negotiate the language to be used on a |
| | T.140 data channel. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
9.4. SDP Media Direction Attributes
This document modifies the usage of the SDP 'sendonly', 'recvonly',
'sendrecv' and 'inactive' attributes, if these attributes are
included in SDP 'dcsa' attributes associated T.140 data channel. The
modified usage is described in Section 4.2.3.
The usage level "dcsa(t140)" is added to the registration of the SDP
'sendonly' attribute in the Session Description Protocol (SDP)
Parameters registry as follows:
+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | sendonly |
| Usage level: | dcsa(t140) |
| Purpose: | Negotiate the direction in which real- |
| | time text can be sent on a T.140 data |
| | channel. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
The usage level "dcsa(t140)" is added to the registration of the SDP
'recvonly' attribute in the Session Description Protocol (SDP)
Parameters registry as follows:
+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | recvonly |
| Usage level: | dcsa(t140) |
| Purpose: | Negotiate the direction in which real- |
| | time text can be sent on a T.140 data |
| | channel. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
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The usage level "dcsa(t140)" is added to the registration of the SDP
'sendrecv' attribute in the Session Description Protocol (SDP)
Parameters registry as follows:
+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | sendrecv |
| Usage level: | dcsa(t140) |
| Purpose: | Negotiate the direction in which real- |
| | time text can be sent on a T.140 data |
| | channel. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
The usage level "dcsa(t140)" is added to the registration of the SDP
'inactive' attribute in the Session Description Protocol (SDP)
Parameters registry as follows:
+-----------------------+-------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | inactive |
| Usage level: | dcsa(t140) |
| Purpose: | Negotiate the direction in which real- |
| | time text can be sent on a T.140 data |
| | channel. |
| Reference: | RFCXXXX |
+-----------------------+-------------------------------------------+
10. Acknowledgements
This document is based on an earlier Internet draft edited by Keith
Drage, Juergen Stoetzer-Bradler and Albrecht Schwarz.
Thomas Belling provided useful comments on the initial (pre-
submission) version of the draft. Paul Kyzivat and Bernard Aboba
provided comments on the draft.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
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Internet-Draft T.140 Data Channel April 2020
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002, <https://www.rfc-
editor.org/info/rfc3264>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<https://www.rfc-editor.org/info/rfc4103>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8373] Gellens, R., "Negotiating Human Language in Real-Time
Communications", RFC 8373, DOI 10.17487/RFC8373, May 2018,
<https://www.rfc-editor.org/info/rfc8373>.
[I-D.ietf-mmusic-rfc4566bis]
Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", draft-ietf-mmusic-
rfc4566bis-37 (work in progress), August 2019.
[I-D.ietf-rtcweb-data-channel]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-13 (work in
progress), January 2015.
[I-D.ietf-mmusic-data-channel-sdpneg]
Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
Even, "SDP-based Data Channel Negotiation", draft-ietf-
mmusic-data-channel-sdpneg-28 (work in progress), May
2019.
[I-D.ietf-rtcweb-security-arch]
Rescorla, E., "WebRTC Security Architecture", draft-ietf-
rtcweb-security-arch-20 (work in progress), July 2019.
[I-D.ietf-rtcweb-security]
Rescorla, E., "Security Considerations for WebRTC", draft-
ietf-rtcweb-security-12 (work in progress), July 2019.
[T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
multimedia application text conversation", February 1998.
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Internet-Draft T.140 Data Channel April 2020
[T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
Protocol for multimedia application text conversation",
February 2000.
11.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002, <https://www.rfc-
editor.org/info/rfc3261>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[I-D.ietf-rtcweb-data-protocol]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel
Establishment Protocol", draft-ietf-rtcweb-data-
protocol-09 (work in progress), January 2015.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Gunnar Hellstrom
Gunnar Hellstrom Accessible Communication
Esplanaden 30
Vendelso 136 70
Sweden
Email: gunnar.hellstrom@ghaccess.se
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