draft-ietf-avtcore-multi-party-rtt-mix-06.txt   draft-ietf-avtcore-multi-party-rtt-mix-07.txt 
AVTCore G. Hellstrom AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: RFC 4102, RFC 4103 (if approved) 11 June 2020 Updates: RFC 4102, RFC 4103 (if approved) 12 July 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 13 December 2020 Expires: 13 January 2021
RTP-mixer formatting of multi-party Real-time text RTP-mixer formatting of multi-party Real-time text
draft-ietf-avtcore-multi-party-rtt-mix-06 draft-ietf-avtcore-multi-party-rtt-mix-07
Abstract Abstract
Real-time text mixers for multi-party sessions need to identify the Real-time text mixers for multi-party sessions need to identify the
source of each transmitted group of text so that the text can be source of each transmitted group of text so that the text can be
presented by endpoints in suitable grouping with other text from the presented by endpoints in suitable grouping with other text from the
same source. same source.
Regional regulatory requirements specify provision of real-time text Regional regulatory requirements specify provision of real-time text
in multi-party calls. RFC 4103 mixer implementations can use in multi-party calls. RFC 4103 mixer implementations can use
traditional RTP functions for source identification, but the mixer traditional RTP functions for source identification, but the mixer
source switching performance is limited when using the default source switching performance is limited when using the default
transmission with redundancy. transmission with redundancy.
An enhancement for RFC 4103 real-time text mixing is provided in this Enhancements for RFC 4103 real-time text mixing is provided in this
document, suitable for a centralized conference model that enables document, suitable for a centralized conference model that enables
source identification and efficient source switching. The intended source identification and source switching. The intended use is for
use is for real-time text mixers and multi-party-aware participant real-time text mixers and multi-party-aware participant endpoints.
endpoints. The mechanism builds on use of the CSRC list in the RTP Two mechanisms are provided. The mechanisms builds on use of the
packet and an extended packet format "text/rex". CSRC list in the RTP packet for source identification. One method
makes use of the same "text/red" format as for two-party sessions,
while the other makes use of an extended packet format "text/rex" for
more efficient transmission.
A capability exchange is specified so that it can be verified that a A capability exchange is specified so that it can be verified that a
participant can handle the multi-party coded real-time text stream. participant can handle the multi-party coded real-time text stream.
The capability is indicated by the media subtype "text/rex". The capability for one method is by use of a media attribute a=rtt-
mix-rtp-mixer. The other method is indicated by the media subtype
"text/rex".
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103]
A brief description about how a mixer can format text for the case A brief description about how a mixer can format text for the case
when the endpoint is not multi-party aware is also provided. when the endpoint is not multi-party aware is also provided.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
skipping to change at page 2, line 10 skipping to change at page 2, line 15
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 13 December 2020. This Internet-Draft will expire on 13 January 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Selected solution and considered alternative . . . . . . 5 1.1. Selected solution and considered alternative . . . . . . 5
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Intended application . . . . . . . . . . . . . . . . . . 6 1.3. Intended application . . . . . . . . . . . . . . . . . . 7
2. Use of fields in the RTP packets . . . . . . . . . . . . . . 6 2. Specified solutions . . . . . . . . . . . . . . . . . . . . . 7
3. Actions at transmission by a mixer . . . . . . . . . . . . . 9 2.1. Negotiated use of the RFC 4103 format for multi-party in a
3.1. Initial BOM transmission . . . . . . . . . . . . . . . . 9 single RTP stream . . . . . . . . . . . . . . . . . . . . 7
3.2. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 9 2.2. Use of an extended packet format "text/rex" with text from
3.3. Transmission interval . . . . . . . . . . . . . . . . . . 10 multiple sources . . . . . . . . . . . . . . . . . . . . 17
3.4. Do not send received text to the originating source . . . 10 2.3. Mixing for multi-party unaware endpoints . . . . . . . . 35
3.5. Clean incoming text . . . . . . . . . . . . . . . . . . . 10 3. Presentation level considerations . . . . . . . . . . . . . . 36
3.6. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 10 3.1. Presentation by multi-party aware endpoints . . . . . . . 36
3.7. Text placement in packets . . . . . . . . . . . . . . . . 10 3.2. Multi-party mixing for multi-party unaware endpoints . . 38
3.8. Maximum number of sources per packet . . . . . . . . . . 11 4. Gateway Considerations . . . . . . . . . . . . . . . . . . . 44
3.9. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 11 4.1. Gateway considerations with Textphones (e.g. TTYs). . . 44
3.10. Creation of the redundancy . . . . . . . . . . . . . . . 11 4.2. Gateway considerations with WebRTC. . . . . . . . . . . . 45
3.11. Timer offset fields . . . . . . . . . . . . . . . . . . . 12 5. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 45
3.12. Other RTP header fields . . . . . . . . . . . . . . . . . 12 6. Congestion considerations . . . . . . . . . . . . . . . . . . 46
3.13. Pause in transmission . . . . . . . . . . . . . . . . . . 12 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 46
4. Actions at reception . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
4.1. Multi-party vs two-party use . . . . . . . . . . . . . . 12 8.1. Registration of the "rtt-mix-rtp-mixer" sdp media
4.2. Level of redundancy . . . . . . . . . . . . . . . . . . . 13 attribute . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3. Extracting text and handling recovery and loss . . . . . 13 8.2. Registration of "text/rex" media subtype . . . . . . . . 47
4.4. Delete BOM . . . . . . . . . . . . . . . . . . . . . . . 14 9. Security Considerations . . . . . . . . . . . . . . . . . . . 47
4.5. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 14 10. Change history . . . . . . . . . . . . . . . . . . . . . . . 47
10.1. Changes included in
5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 14 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 47
6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 14 10.2. Changes included in
7. Usage without redundancy . . . . . . . . . . . . . . . . . . 14 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 47
8. Use with SIP centralized conferencing framework . . . . . . . 15 10.3. Changes included in
9. Conference control . . . . . . . . . . . . . . . . . . . . . 15 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 48
10. Media Subtype Registration . . . . . . . . . . . . . . . . . 15 10.4. Changes included in
11. SDP considerations . . . . . . . . . . . . . . . . . . . . . 17 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 48
11.1. Mapping of media parameters to sdp . . . . . . . . . . . 18 10.5. Changes included in
11.2. Security for session control and media . . . . . . . . . 18 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 48
11.3. SDP offer/answer examples . . . . . . . . . . . . . . . 18 10.6. Changes included in
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 20 draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 49
13. Performance considerations . . . . . . . . . . . . . . . . . 23 10.7. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 49
14. Presentation level considerations . . . . . . . . . . . . . . 23 10.8. Changes from
14.1. Presentation by multi-party aware endpoints . . . . . . 24
14.2. Multi-party mixing for multi-party unaware endpoints . . 26
15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 32
15.1. Gateway considerations with Textphones (e.g. TTYs). . . 32
15.2. Gateway considerations with WebRTC. . . . . . . . . . . 33
16. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 33
17. Congestion considerations . . . . . . . . . . . . . . . . . . 33
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
20. Security Considerations . . . . . . . . . . . . . . . . . . . 34
21. Change history . . . . . . . . . . . . . . . . . . . . . . . 34
21.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 34
21.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 34
21.3. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 35
21.4. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 35
21.5. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 36
21.6. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 36
21.7. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-03 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 36 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 50
21.8. Changes from 10.9. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 50
21.9. Changes from 10.10. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 -02 . . . . . . . . . . . . . . . . . . . . . . . . . . 50
21.10. Changes from 10.11. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 38 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 51
22. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 51
22.1. Normative References . . . . . . . . . . . . . . . . . . 38 11.1. Normative References . . . . . . . . . . . . . . . . . . 51
22.2. Informative References . . . . . . . . . . . . . . . . . 40 11.2. Informative References . . . . . . . . . . . . . . . . . 53
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 40 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 53
1. Introduction 1. Introduction
RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for
transmission of real-time text (RTT) and the "text/t140" format. It transmission of real-time text (RTT) and the "text/t140" format. It
also specifies a redundancy format "text/red" for increased also specifies a redundancy format "text/red" for increased
robustness. RFC 4102 [RFC4102] registers the "text/red" format. robustness. RFC 4102 [RFC4102] registers the "text/red" format.
Regional regulatory requirements specify provision of real-time text Regional regulatory requirements specify provision of real-time text
in multi-party calls. in multi-party calls.
Real-time text is usually provided together with audio and sometimes Real-time text is usually provided together with audio and sometimes
with video in conversational sessions. with video in conversational sessions.
The redundancy scheme of RFC 4103 [RFC4103] enables efficient The redundancy scheme of RFC 4103 [RFC4103] enables efficient
transmission of redundant text in packets together with new text. transmission of redundant text in packets together with new text.
However the redundant header format has no source indicators for the However the redundancy header format has no source indicators for the
redundant transmissions. An assumption has had to be made that the redundant transmissions. An assumption has had to be made that the
redundant parts in a packet are from the same source as the new text. redundant parts in a packet are from the same source as the new text.
The recommended transmission is one new and two redundant generations The recommended transmission is one new and two redundant generations
of text (T140blocks) in each packet and the recommended transmission of text (T140blocks) in each packet and the recommended transmission
interval is 300 ms. interval is 300 ms.
A mixer, selecting between text input from different sources and A mixer, selecting between text input from different sources and
transmitting it in a common stream needs to make sure that the transmitting it in a common stream needs to make sure that the
receiver can assign the received text to the proper sources for receiver can assign the received text to the proper sources for
presentation. Therefore, using RFC 4103 without any extra rule for presentation. Therefore, using RFC 4103 without any extra rule for
source identification, the mixer needs to stop sending new text from source identification, the mixer needs to stop sending new text from
one source and then make sure that all text so far has been sent with one source and then make sure that all text so far has been sent with
all intended redundancy levels (usually two) before switching to all intended redundancy levels (usually two) before switching to
another source. That causes the very long time of one second to another source. That causes the long time of one second to switch
switch between transmission of text from one source to text from between transmission of text from one source to text from another
another source. Both the total throughput and the switching source when using the default transmission interval 300 ms. Both the
performance in the mixer is too low for most applications. total throughput and the switching performance in the mixer would be
too low for most applications. However by shorting the transmission
interval to 100 ms, good performance is achieved for up to 3
simultaneously sending sources and usable performance for up to 5
simultaneously sending sources. This method is negotiated through an
sdp media attribute "rtt-mix-rtp-mixer".
A more efficient source identification scheme requires that each A more efficient source identification scheme requires that each
redundant T140block has its source individually preserved. This redundant T140block has its source individually preserved. This
document introduces a source indicator by specific rules for document introduces a source indicator by specific rules for
populating the CSRC-list and the data header in the RTP-packet. populating the CSRC-list and the data header in the RTP-packet.
An extended packet format "text/rex" is specified for this purpose, An extended packet format "text/rex" is specified for this purpose,
providing the possibility to include text from up to 16 sources in providing the possibility to include text from up to 15 sources in
each packet in order to enhance mixer source switching performance. each packet in order to enhance mixer source switching performance.
By these extensions, the performance requirements on multi-party By these extensions, the performance requirements on multi-party
mixing for real-time text are exceeded by the solution in this mixing for real-time text are exceeded by the "text/rex" solution in
document. this document.
A negotiation mechanism can therefore be based on selection between A negotiation mechanism can therefore be based on selection of the
the "text/red" and the "text/rex" media formats for verification that "text/red" with media attribute "rtt-mix-rtp-mixer" or the "text/rex"
the receiver is able to handle the multi-party coded stream. media format for verification that the parties are able to handle a
multi-party coded stream and agreeing on which method to use.
A fall-back mixing procedure is specified for cases when the A fall-back mixing procedure is specified for cases when the
negotiation results in "text/red" being the only common submedia negotiation results in "text/red" without the "rtt-mix attribute"
format. being the only common submedia format.
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format for the multi-party mixing case introducing an attribute for indicating multi-party capability, and
and more strict rules for the source indications. an extended packet format for the multi-party mixing case and more
strict rules for the source indications.
1.1. Selected solution and considered alternative 1.1. Selected solution and considered alternative
The mechanism specified in this document makes use of the RTP mixer A number of alternatives were considered when searching an efficient
model specified in RFC3550[RFC3550]. From some points of view, use multi-party method for real-time text. This section explains a few
of the RTP translator model specified in RFC 3550 would be more of them briefly.
efficient, because then the text packets can pass the translator with
only minor modification. However, there may be a lack of support for One RTP stream per source, sent in the same RTP session with
the translator model in existing RTP implementations, and therefore "text/red" format. From some points of view, use of multiple RTP
the more common RTP-mixer model was selected. The translator model streams, one for each source, sent in the same RTP session, called
would also easier cause congestion if many users send text the RTP translator model in RFC 3550 [RFC3550], would be
simultaneously. efficient, and use exactly the same packet format as RFC 4103, the
same payload type and a simple SDP declaration. However, there is
currently lack of support for multi-stream RTP in certain
implementation technologies. The multi-stream solution would also
cause more overhead than a single RTP stream solution "text/rex"
specified in this document and more the more simultaneous sending
participants there are.
The "text/red" format in RFC 4103 with shorter transmission
interval, and indicating source in CSRC. The "text/red" format with
"text/t140" payload in a single RTP stream can be sent with 100 ms
packet intervals instead of the regular 300 ms. The source is
indicated in the CSRC field. Source switching can then be done
every 300 ms while simultaneous transmission occurs. With two
participants sending text simultaneously, the switching and
transmission performance is good. With three or more
simultaneously sending participants, there will be a noticable
jerkiness in text presentation, more the more participants who
send text simultaneously. With three sending participants, the
jerkiness will be about 450 ms, and with five, about 1350 ms.
Text sent from a source at the end of the period its text is sent
by the mixer will have close to zero extra delay. Recent text
will be presented with no or low delay. The 1350 ms jerkiness
will be noticable and slightly unpleasant, but corresponds in time
to what typing humans often cause by hesitation or changing
position while typing. A benefit of this method is that no new
packet format needs to be introduced and implemented. Since
simultaneous typing by more than two parties is rare, and in many
applications also more than three parties in a call is rare, this
method can be used successfully without its limitations becoming
annoying. Negotiation is based on a new sdp media attribute "rtt-
mix-rtp-mixer".
The "text/rex" packet format with up to 15 sources in one packet. Th
e mechanism called "text/rex" specified in this document makes use
of the RTP mixer model specified in RFC3550[RFC3550]. Text from
up to 15 sources can be included in each packet. Packets are
normally sent every 300 ms. The mean delay will be 150 ms. The
sources are indicated in the CSRC list of the RTP packets. A new
redundancy packet format is specified, named "text/rex".
The presentation planned by the mixer for multi-party unaware
endpoints. It is desirable to have a method that does not require
any modifications in existing user devices implementing RFC 4103
for RTT without explicit support of multi-party sessions. This is
possible by having the mixer insert a new line and a text
formatted source label before each switch of text source in the
stream. Switch of source can only be done in places in the text
where it does not disturb the perception of the contents. Text
from only one source can be presented in real time at a time. The
delay will therefore be varying. The method has also other
limitations, but is included in this document as a fallback
method. In calls where parties take turns properly by ending
their entries with a new line, the limitations will have limited
influence on the user experience. while only two parties send
text, these two will see the text in real time with no delay.
1.2. Nomenclature 1.2. Nomenclature
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC, RTCP, RTP- The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC, RTCP, RTP-
mixer, RTP-translator are explained in [RFC3550] mixer, RTP-translator are explained in [RFC3550]
skipping to change at page 6, line 7 skipping to change at page 7, line 7
"TTY" stands for a text telephone type used in North America. "TTY" stands for a text telephone type used in North America.
"WebRTC" stands for web based communication specified by W3C and "WebRTC" stands for web based communication specified by W3C and
IETF. IETF.
"DTLS-SRTP" stnds for security specified in RFC 5764 [RFC5764]. "DTLS-SRTP" stnds for security specified in RFC 5764 [RFC5764].
1.3. Intended application 1.3. Intended application
The format for multi-party real-time text is primarily intended for The methods for multi-party real-time text are primarily intended for
use in transmission between mixers and endpoints in centralised use in transmission between mixers and endpoints in centralised
mixing configurations. It is also applicable between endpoints as mixing configurations. It is also applicable between endpoints as
well as between mixers. well as between mixers. An often mentioned application is for
emergency service calls with real-time text and voice, where a
calltaker want to make an attended handover of a call to another
agent, and stay observing in the session. Multimedia conference
sessions with support for participants to contribute in text is
another application. Conferences with central support for speech-to-
text conversion is yet another mentioned application.
2. Use of fields in the RTP packets In all these applications, normally only one participant at a time
will send long text utterances. In some cases, one other participant
will occasionally contribute with a longer comment simultaneously.
That may also happen in some rare cases when text is interpreted to
text in another language in a conference. Apart from these cases,
other participants are only expected to contribute with very brief
utterings while others are sending text.
Text is supposed to be human generated, by some text input means,
such as typing on a keyboard or using speech-to-text technology.
Occasional small cut-and-paste operations may appear even if that is
not the initial purpose of real-time text.
The real-time characteristics of real-time text is essential for the
participants to be able to contribute to a conversation. If the text
is too much delayed from typing a letter to its presentation, then,
in some conference situations, the opportunity to comment will be
gone and someone else will grab the turn. A delay of more than one
second in such situations is an obstacle for good conversation.
2. Specified solutions
2.1. Negotiated use of the RFC 4103 format for multi-party in a single
RTP stream
This section specifies use of the current format specified in
[RFC4103] for true multi-party real-time text. It is an update of
RFC 4103 by a clarification on one way to use it in the multi-party
situation. It is done by completing a negotiation for this kind of
multi-party capability and by indicating source in the CSRC element
in the RTP packets. Please use [RFC4103] as reference when reading
the following description.
2.1.1. Negotiation for use of this method
RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
redundancy format "text/red" for increased robustness of real-time
text transmission. This document updates RFC 4102[RFC4102] and RFC
4103[RFC4103] by introducing a capability negotiation for handling
multi-party real-time text. The capability negotiation is based on
use of the sdp media attribute "rtt-mix-rtp-mixer".
The syntax is as follows:
a=rtt-mix-rtp-mixer
A transmitting party SHALL send text according to the multi-party
format only when the negotiation for this method was successful and
when the CC field in the RTP packet is 1. In all other cases, the
packets SHALL be populated as for a two-party session.
2.1.2. Use of fields in the RTP packets
The CC field SHALL show the number of members in the CSRC list, which
is one (1) in transmissions from a mixer involved in a multi-party
session, and otherwise 0.
When transmitted from a mixer during a multi-party session, a CSRC
list is included in the packet. The single member in the CSRC-list
SHALL contain the SSRC of the source of the T140blocks in the packet.
When redundancy is used, the recommended level of redundancy is to
use one primary and two redundant generations of T140blocks. In some
cases, a primary or redundant T140block is empty, but is still
represented by a member in the redundancy header.
From other aspects, the contents of the RTP packts are equal to what
is specified in RFC 4103.
2.1.3. Transmission of multi-party contents
As soon as a participant is known to participate in a session and
being available for text reception, a Unicode BOM character SHALL be
sent to it according to the procedures in this section. If the
transmitter is a mixer, then the source of this character SHALL be
indicated to be the mixer itself.
2.1.4. Keep-alive
After that, the transmitter SHALL send keep-alive traffic to the
receivers at regular intervals when no other traffic has occurred
during that interval if that is decided for the actual connection.
Recommendations for keep-alive can be found in RFC 6263[RFC6263].
2.1.5. Transmission interval
A "text/red" transmitter SHOULD send packets distributed in time as
long as there is something (new or redundant T140blocks) to transmit.
The maximum transmission interval SHOULD then be 300 ms. It is
RECOMMENDED to send next packet to a receiver as soon as new text to
that receiver is available, as long as the time after the latest sent
packet to the same receiver is more than or equal to 100 ms, and also
the maximum character rate to the receiver is not exceeded. The
intention is to keep the latency low while keeping a good protection
against text loss in bursty packet loss conditions. New and
redundant text from one source MAY be transmitted in the same packet.
Text from different sources MUST NOT be transmitted in the same
packet.
2.1.6. Do not send received text to the originating source
Text received from a participant SHOULD NOT be included in
transmission to that participant.
2.1.7. Clean incoming text
A mixer SHALL handle reception and recovery of packet loss, marking
of possible text loss and deletion of 'BOM' characters from each
participant before queueing received text for transmission to
receiving participants.
2.1.8. Redundancy
The transmitting party using redundancy SHALL send redundant
repetitions of T140blocks aleady transmitted in earlier packets. The
number of redundant generations of T140blocks to include in
transmitted packets SHALL be deducted from the SDP negotiation. It
SHOULD be set to the minimum of the number declared by the two
parties negotiating a connection.
2.1.9. Text placement in packets
At time of transmission, the mixer SHALL populate the RTP packet with
all T140blocks queued for transmission originating from the source in
turn for transmission as long as this is not in conflict with the
allowed number of characters per second or the maximum packet size.
The SSRC of the source shall be placed as the member in the CSRC-
list.
2.1.10. Maximum number of sources per packet
When text from more than one source is available for transmission,
the mixer SHALL let the sources take turns in having their text
transmitted. When switching from transmission of one source to allow
another source to have its text sent, all intended redundant
generations of the last text from the current source MUST be
transmitted before text from another source can be transmitted.
Actively transmitting sources SHOULD be allowed to take turns as
frequently as possible to have their text transmitted. That implies
that with the recommended redundancy, the mixer SHALL send primary
text and two packets with redundant text from the current source
before text from another source is transmitted. The source with the
oldest received text in the mixer SHOULD be next in turn to get all
its available text transmitted.
Note: The CSRC-list in an RTP packet only includes the participant
who's text is included in text blocks. It is not the same as the
total list of participants in a conference. With audio and video
media, the CSRC-list would often contain all participants who are not
muted whereas text participants that don't type are completely silent
and thus are not represented in RTP packet CSRC-lists once their text
have been transmitted as primary and the intended number of redundant
generations.
2.1.11. Empty T140blocks
If no unsent T140blocks were available for a source at the time of
populating a packet, but T140blocks are available which have not yet
been sent the full intended number of redundant transmissions, then
the primary T140block for that source is composed of an empty
T140block, and populated (without taking up any length) in a packet
for transmission. The corresponding SSRC SHALL be placed as usual in
its place in the CSRC-list.
2.1.12. Creation of the redundancy
The primary T140block from a source in the latest transmitted packet
is used to populate the first redundant T140block for that source.
The first redundant T140block for that source from the latest
transmission is placed as the second redundant T140block.
Usually this is the level of redundancy used. If a higher number of
redundancy is negotiated, then the procedure SHALL be maintained
until all available redundant levels of T140blocks are placed in the
packet. If a receiver has negotiated a lower number of "text/red"
generations, then that level shall be the maximum used by the
transmitter.
2.1.13. Timer offset fields
The timestamp offset values are inserted in the data header, with the
time offset from the RTP timestamp in the packet when the
corresponding T140block was sent from its original source as primary.
The timestamp offsets are expressed in the same clock tick units as
the RTP timestamp.
The timestamp offset values for empty T140blocks have no relevance
but SHOULD be assigned realistic values.
2.1.14. Other RTP header fields
The number of members in the CSRC list ( 0 or 1) shall be placed in
the "CC" header field. Only mixers place value 1 in the "CC" field.
The current time SHALL be inserted in the timestamp.
The SSRC of the mixer for the RTT session SHALL be inserted in the
SSRC field of the RTP header.
The M-bit shall be handled as specified in [RFC4103].
2.1.15. Pause in transmission
When there is no new T140block to transmit, and no redundant
T140block that has not been retransmitted the intended number of
times from any source, the transmission process can stop until either
new T140blocks arrive, or a keep-alive method calls for transmission
of keep-alive packets.
2.1.16. RTCP considerations
A mixer SHALL send RTCP reports with SDES, CNAME and NAME information
about the sources in the multi-party call. This makes it possible
for participants to compose a suitable label for text from each
source.
2.1.17. Reception of multi-party contents
The "text/red" receiver included in an endpoint with presentation
functions will receive RTP packets in the single stream from the
mixer, and SHALL distribute the T140blocks for presentation in
presentation areas for each source. Other receiver roles, such as
gateways or chained mixers are also feasible, and requires
consideration if the stream shall just be forwarded, or distributed
based on the different sources.
2.1.17.1. Multi-party vs two-party use
If the "CC" field value of a received packet is 1, it indicates that
multi-party transmission is active, and the receiver MUST be prepared
to act on the source according to its role. If the CC value is 0,
the connection is point-to-point.
2.1.17.2. Level of redundancy
The used level of redundancy generations SHALL be evaluated from the
received packet contents. The number of generations (including the
primary) is equal to the number of members in the redundancy header.
2.1.17.3. Extracting text and handling recovery and loss
The RTP sequence numbers of the received packets SHALL be monitored
for gaps and packets out of order.
As long as the sequence is correct, each packet SHALL be unpacked in
order. The T140blocks SHALL be extracted from the primary area, and
the corresponding SSRC SHALL be extracted from the CSRC list and used
for assigning the new T140block to the correct presentation areas (or
correspondingly for other receiver roles).
If a sequence number gap appears and is still there after some
defined time for jitter resolution, T140data SHALL be recovered from
redundant data. If the gap is wider than the number of generations
of redundant T140blocks in the packet, then a t140block SHALL be
created with a marker for possible text loss [T140ad1] and assigned
to the SSRC of the transmitter as a general input from the mixer
because in general it is not possible to deduct from which source(s)
text was lost. It is in some cases possible to deduct that no text
was lost even for a gap wider than the redundancy generations, and in
some cases it can be concluded which source that likely had loss.
Therefore, the receiver MAY insert the marker for possible text loss
[T140ad1] in the presentation area corresponding to the source which
may have had loss.
Then, the T140block in the received packet SHALL be retrieved
beginning with the highest redundant generation, and assigning it to
the presentation area of that source. Finally the primary T140block
SHALL be retrieved from the packet and similarly assigned to the
corresponding presentation area for the source.
If the sequence number gap was equal to or less than the number of
redundancy generations in the received packet, a missing text marker
SHALL NOT be inserted, and instead the T140block and the SSRC fully
recovered from the redundancy information and the CSRC-list in the
way indicated above.
2.1.17.4. Delete BOM
Unicode character "BOM" is used as a start indication and sometimes
used as a filler or keep alive by transmission implementations.
These SHALL be deleted on reception.
2.1.17.5. Empty T140blocks
Empty T140blocks are included as fillers for unused redundancy levels
in the packets. They just do not provide any contents and do not
contribute to the received streams.
2.1.18. Performance considerations
This solution has good performance up to three participants
simultaneously sending text. At higher numbers of participants
simultaneously sending text, a jerkiness is visible in the
presentation of text. With five participants simultaneously
transmitting text, the jerkiness is about 1400 ms. Evenso, the
transmission of text catches up, so there is no resulting delay
introduced. The solution is therefore suitable for emergency service
use, relay service use, and small or well-managed larger multimedia
conferences. It is only less suitable for large conferences with a
high number of participants sending text simultaneously. It should
be noted that it is only the number of users sending text within the
same moment that causes jerkiness, not the total number of users with
RTT capability.
2.1.19. Offer/answer considerations
A party which has negotiated the "rtt-mix-rtp-mixer" sdp media
attribute MUST populate the CSRC-list and format the packets
according to this section if it acts as an rtp-mixer and sends multi-
party text.
A party which has negotiated the the "rtt-mix-rtp-mixer" sdp media
attribute MUST interpret the contents of the CSRC-list and the
packets according to this section in received rtp packets in the
corresponding RTP stream.
A party performing as a mixer, which has not negotiated the "rtt-mix-
rtp-mixer" sdp media attribute, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD, if nothing else
is specified for the application, format transmitted text to that
participant to be suitable to present on a multi-party unaware
endpoint as further specified in section Section 3.2.
A party not performing as a mixer MUST not include the CSRC list.
2.1.20. Security for session control and media
Security SHOULD be applied on both session control and media. In
applications where legacy endpoints without security may exist, a
negotiation between security and no security SHOULD be applied. If
no other security solution is mandated by the application, then RFC
8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media
security with DTLS. Most SDP examples below are for simplicity
expressed without the security additions. The principles (but not
all details) for applying DTLS-SRTP security is shown in a couple of
the following examples.
2.1.21. SDP offer/answer examples
This sections shows some examples of SDP for session negotiation of
the real-time text media in SIP sessions. Audio is usually provided
in the same session, and sometimes also video. The examples only
show the part of importance for the real-time text media.
Offer example for "text/red" format and multi-party support:
m=text 11000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
a=rtt-mix-rtp-mixer
Answer example from a multi-party capable device
m=text 11000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
a=rtt-mix-rtp-mixer
Offer example for "text/red" format including multi-party
and security:
a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
a=rtt-mix-rtp-mixer
The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media
line still indicating RTP/AVP.
Answer example from multi-party capable device with security
a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
a=rtt-mix-rtp-mixer
With the "fingerprint" the device acknowledges use of SRTP/DTLS.
Answer example from a multi-party unaware device that also
does not support security:
m=text 12000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
2.1.22. Packet sequence example
This example shows a symbolic flow of packets from a mixer with loss
and recovery. A and B are sources of RTT. P indicates primary data.
R1 is first redundant generation data and R2 is second redundant
generation data. A1, B1, A2 etc are text chunks (T140blocks)
received from the respective sources. X indicates dropped packet
between the mixer and a receiver.
|----------------|
|Seq no 1 |
|CC=1 |
|CSRC list A |
|R2: A1 |
|R1: A2 |
|P: A3 |
|----------------|
Assuming that earlier packets ( with text A1 and A2) were received in
sequence, text A3 is received from packet 1 and assigned to reception
area A. The mixer is now assumed to have received text from source B
and need to prepare for sending that text. First it must send the
redundant generations of text A1.
|----------------|
|Seq no 2 |
|CC=1 |
|CSRC list A |
|R2 A2 |
|R1: A3 |
|P: Empty |
|----------------|
Nothing needs to be retrieved from this packet.
X----------------|
X Seq no 3 |
X CC=1 |
X CSRC list A |
X R2: A3 |
X R1: Empty |
X P: Empty |
X----------------|
Packet 3 is assumed to be dropped in network problems
X----------------|
X Seq no 4 |
X CC=1 |
X CSRC list B |
X R2: Empty |
X R1: Empty |
X P2: B1 |
X----------------|
Packet 4 contains text from B, assumed dropped in network problems.
The mixer is assumed to have received text from A on turn to send.
Sending of text from B must therefore be temporarily ended by
sending redundancy twice.
X----------------|
X Seq no 5 |
X CC=1 |
X CSRC list B |
X R2: Empty |
X R1: B1 |
X P: Empty |
X----------------|
Packet 5 is assumed to be dropped in network problems
|----------------|
|Seq no 6 |
|CC=1 |
|CSRC list B |
| R2: B1 |
| R1: Empty |
| P: Empty |
|----------------|
Packet 6 is received. The latest received sequence number was 2.
Recovery is therefore tried for 3,4,5. There is no coverage for seq
no 3. But knowing that A1 must have been sent as R2 in packet 3, it
can be concluded that nothing was lost.
For seqno 4, text B1 is recovered from the second generation
redundancy and appended to the reception area of B. For seqno 5,
nothing needs to be recovered. No primary text is available in
packet 6.
After this sequence, A3 and B1 have been received. In this case no
text was lost. Even if also packet 2 was lost, it can be concluded
that no text was lost.
If also packets 1 and 2 were lost, there would be a need to create a
marker for possibly lost text (U'FFFD) [T140ad1], inserted generally
and possibly also in text sequences A and B.
2.2. Use of an extended packet format "text/rex" with text from
multiple sources
The method specified in this section called "text/rex" has higher
performance than the previous method. Text from up to 15 sources can
be included in each packet. This may be of value in large non-
managed conferences.
2.2.1. Use of fields in the RTP packets
RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
redundancy format "text/red" for increased robustness of real-time redundancy format "text/red" for increased robustness of real-time
text transmission. This document updates RFC 4102[RFC4102] and RFC text transmission. This document updates RFC 4102[RFC4102] and RFC
4103[RFC4103] by introducing a format "text/rex" with a rule for 4103[RFC4103] by introducing a format "text/rex" with a rule for
populating and using the CSRC-list in the RTP packet and extending populating and using the CSRC-list in the RTP packet and extending
the redundancy header to be called a data header. This is done in the redundancy header to be called a data header. This is done in
order to enhance the performance in multi-party RTT sessions. order to enhance the performance in multi-party RTT sessions.
The "text/rex" format can be seen as an "n-tuple" of the "text/red" The "text/rex" format can be seen as an "n-tuple" variant of the
format intended to carry text information from up to 16 sources per "text/red" format intended to carry text information from up to 15
packet. sources per packet.
The CC field SHALL show the number of members in the CSRC list, which The CC field SHALL show the number of members in the CSRC list, which
is one per source represented in the packet. is one per source represented in the packet.
When transmitted from a mixer, a CSRC list is included in the packet. When transmitted from a mixer, a CSRC list is included in the packet.
The members in the CSRC-list SHALL contain the SSRCs of the sources The members in the CSRC-list SHALL contain the SSRCs of the sources
of the T140blocks in the packet. The order of the CSRC members MUST of the T140blocks in the packet. The order of the CSRC members MUST
be the same as the order of the sources of the data header fields and be the same as the order of the sources of the data header fields and
the T140blocks. When redundancy is used, text from all included the T140blocks. When redundancy is used, text from all included
sources MUST have the same number of redundant generations. The sources MUST have the same number of redundant generations. The
skipping to change at page 8, line 5 skipping to change at page 19, line 30
The high values appear only in exceptional cases, e.g. when some The high values appear only in exceptional cases, e.g. when some
data has been held in order to keep the text flow under the data has been held in order to keep the text flow under the
Characters Per Second (CPS) limit. Characters Per Second (CPS) limit.
block length: 10 bits Length in bytes of the corresponding data block length: 10 bits Length in bytes of the corresponding data
block excluding the header. block excluding the header.
The header for the final block has a zero F bit, and apart from that The header for the final block has a zero F bit, and apart from that
the same fields as other data headers. the same fields as other data headers.
Note: This document has a packet format that is similar to that of Note: The "text/rex" packet format is similar to that of RFC 2198
RFC 2198 [RFC2198] but is different from some aspects. RFC 2198 [RFC2198] but is different from some aspects. RFC 2198 associates
associates the whole of the CSRC-list with the primary data and the whole of the CSRC-list with the primary data and assumes that the
assumes that the same list applies to reconstructed redundant data. same list applies to reconstructed redundant data. In this section a
In this document a T140block is associated with exactly one CSRC list T140block is associated with exactly one CSRC list member as
member as described above. Also RFC 2198 [RFC2198] anticipates described above. Also RFC 2198 [RFC2198] anticipates infrequent
infrequent change to CSRCs; implementers should be aware that the change to CSRCs; implementers should be aware that the order of the
order of the CSRC-list according to this document will vary during CSRC-list according to this section will vary during transitions
transitions between transmission from the mixer of text originated by between transmission from the mixer of text originated by different
different participants. Another difference is that the last member participants. Another difference is that the last member in the data
in the data header area in RFC 2198 [RFC2198] only contains the header area in RFC 2198 [RFC2198] only contains the payload type
payload type number while in this document it has the same format as number while in this section it has the same format as all other
all other entries in the data header. entries in the data header.
The picture below shows a typical "text/rex" RTP packet with multi- The picture below shows a typical "text/rex" RTP packet with multi-
party RTT contents from three sources and coding according to this party RTT contents from three sources and coding according to this
document. section.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC=3 |M| "REX" PT | RTP sequence number | |V=2|P|X| CC=3 |M| "REX" PT | RTP sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of packet creation | | timestamp of packet creation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier | | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
skipping to change at page 9, line 34 skipping to change at page 21, line 16
A-P, B-P, C-P are primary data from A, B and C. A-P, B-P, C-P are primary data from A, B and C.
A-R1, B-R1, C-R1 are first redundant generation data from A, B and C. A-R1, B-R1, C-R1 are first redundant generation data from A, B and C.
A-R2, B-R2, C-R2 are first redundant generation data from A, B and C. A-R2, B-R2, C-R2 are first redundant generation data from A, B and C.
In a real case, some of the data headers will likely indicate a zero In a real case, some of the data headers will likely indicate a zero
block length, and no corresponding T.140 data. block length, and no corresponding T.140 data.
3. Actions at transmission by a mixer 2.2.2. Actions at transmission by a mixer
3.1. Initial BOM transmission 2.2.2.1. Initial BOM transmission
As soon as a participant is known to participate in a session and As soon as a participant is known to participate in a session and
being available for text reception, a Unicode BOM character SHALL be being available for text reception, a Unicode "BOM" character SHALL
sent to it according to the procedures in this document. If the be sent to it according to the procedures in this section. If the
transmitter is a mixer, then the source of this character SHALL be transmitter is a mixer, then the source of this character SHALL be
indicated to be the mixer itself. indicated to be the mixer itself.
3.2. Keep-alive 2.2.2.2. Keep-alive
After that, the transmitter SHALL send keep-alive traffic to the After that, the transmitter SHALL send keep-alive traffic to the
receivers at regular intervals when no other traffic has occurred receivers at regular intervals when no other traffic has occurred
during that interval if that is decided for the actual connection. during that interval if that is decided for the actual connection.
Recommendations for keep-alive can be found in RFC 6263[RFC6263]. Recommendations for keep-alive can be found in RFC 6263[RFC6263].
3.3. Transmission interval 2.2.2.3. Transmission interval
A "text/rex" transmitter SHOULD send packets distributed in time as A "text/rex" transmitter SHOULD send packets distributed in time as
long as there is something (new or redundant T140blocks) to transmit. long as there is something (new or redundant T140blocks) to transmit.
The maximum transmission interval SHOULD then be 300 ms. It is The maximum transmission interval SHOULD then be 300 ms. It is
RECOMMENDED to send a packet to a receiver as soon as new text to RECOMMENDED to send a packet to a receiver as soon as new text to
that receiver is available, as long as the time after the latest sent that receiver is available, as long as the time after the latest sent
packet to the same receiver is more than 150 ms, and also the maximum packet to the same receiver is more than 150 ms, and also the maximum
character rate to the receiver is not exceeded. The intention is to character rate to the receiver is not exceeded. The intention is to
keep the latency low while keeping a good protection against text keep the latency low while keeping a good protection against text
loss in bursty packet loss conditions. loss in bursty packet loss conditions.
3.4. Do not send received text to the originating source 2.2.2.4. Do not send received text to the originating source
Text received from a participant SHOULD NOT be included in Text received from a participant SHOULD NOT be included in
transmission to that participant. transmission to that participant.
3.5. Clean incoming text 2.2.2.5. Clean incoming text
A mixer SHALL handle reception and recovery of packet loss, marking A mixer SHALL handle reception and recovery of packet loss, marking
of possible text loss and deletion of 'BOM' characters from each of possible text loss and deletion of 'BOM' characters from each
participant before queueing received text for transmission to participant before queueing received text for transmission to
receiving participants. receiving participants.
3.6. Redundancy 2.2.2.6. Redundancy
The transmitting party using redundancy SHALL send redundant The transmitting party using redundancy SHALL send redundant
repetitions of T140blocks aleady transmitted in earlier packets. The repetitions of T140blocks aleady transmitted in earlier packets. The
number of redundant generations of T140blocks to include in number of redundant generations of T140blocks to include in
transmitted packets SHALL be deducted from the SDP negotiation. It transmitted packets SHALL be deducted from the SDP negotiation. It
SHOULD be set to the minimum of the number declared by the two SHOULD be set to the minimum of the number declared by the two
parties negotiating a connection. The same number of redundant parties negotiating a connection. The same number of redundant
generations MUST be used for text from all sources when it is generations MUST be used for text from all sources when it is
transmitted to a receiver. The number of generations sent to a transmitted to a receiver. The number of generations sent to a
receiver SHALL be the same during the whole session unless it is receiver SHALL be the same during the whole session unless it is
modified by session renegotiation. modified by session renegotiation.
3.7. Text placement in packets 2.2.2.7. Text placement in packets
At time of transmission, the mixer SHALL populate the RTP packet with At time of transmission, the mixer SHALL populate the RTP packet with
T140blocks combined from all T140blocks queued for transmission T140blocks combined from all T140blocks queued for transmission
originating from each source as long as this is not in conflict with originating from each source as long as this is not in conflict with
the allowed number of characters per second or the maximum packet the allowed number of characters per second or the maximum packet
size. These T140blocks SHALL be placed in the packet interleaved size. These T140blocks SHALL be placed in the packet interleaved
with redundant T140blocks and new T140blocks from other sources. The with redundant T140blocks and new T140blocks from other sources. The
SSRC of each source shall be placed as a member in the CSRC-list at a SSRC of each source shall be placed as a member in the CSRC-list at a
place corresponding to the place of its T140blocks in the packet. place corresponding to the place of its T140blocks in the packet.
3.8. Maximum number of sources per packet 2.2.2.8. Maximum number of sources per packet
Text from a maximum of 16 sources MAY be included in a packet. The Text from a maximum of 15 sources MAY be included in a packet. The
reason for this limitation is the maximum number of CSRC list members reason for this limitation is the maximum number of CSRC list members
allowed in a packet. If text from more sources need to be allowed in a packet. If text from more sources need to be
transmitted, the mixer MAY let the sources take turns in having their transmitted, the mixer MAY let the sources take turns in having their
text transmitted. When stopping transmission of one source to allow text transmitted. When stopping transmission of one source to allow
another source to have its text sent, all intended redundant another source to have its text sent, all intended redundant
generations of the last text from the source to be stopped MUST be generations of the last text from the source to be stopped MUST be
transmitted before text from another source can be transmitted. transmitted before text from another source can be transmitted.
Actively transmitting sources SHOULD be allowed to take turns with Actively transmitting sources SHOULD be allowed to take turns with
short intervals to have their text transmitted. short intervals to have their text transmitted.
Note: The CSRC-list in an RTP packet only includes participants who's Note: The CSRC-list in an RTP packet only includes participants who's
text is included in text blocks. It is not the same as the total text is included in text blocks. It is not the same as the total
list of participants in a conference. With audio and video media, list of participants in a conference. With audio and video media,
the CSRC-list would often contain all participants who are not muted the CSRC-list would often contain all participants who are not muted
whereas text participants that don't type are completely silent and whereas text participants that don't type are completely silent and
thus are not represented in RTP packet CSRC-lists once their text thus are not represented in RTP packet CSRC-lists once their text
have been transmitted as primary and the intended number of redundant have been transmitted as primary and the intended number of redundant
generations. generations.
3.9. Empty T140blocks 2.2.2.9. Empty T140blocks
If no unsent T140blocks were available for a source at the time of If no unsent T140blocks were available for a source at the time of
populating a packet, but T140blocks are available which have not yet populating a packet, but T140blocks are available which have not yet
been sent the full intended number of redundant transmissions, then been sent the full intended number of redundant transmissions, then
the primary T140block for that source is composed of an empty the primary T140block for that source is composed of an empty
T140block, and populated (without taking up any length) in a packet T140block, and populated (without taking up any length) in a packet
for transmission. The corresponding SSRC SHALL be placed in its for transmission. The corresponding SSRC SHALL be placed in its
place in the CSRC-list. place in the CSRC-list.
3.10. Creation of the redundancy 2.2.2.10. Creation of the redundancy
The primary T140block from each source in the latest transmitted The primary T140block from each source in the latest transmitted
packet is used to populate the first redundant T140block for that packet is used to populate the first redundant T140block for that
source. The first redundant T140block for that source from the source. The first redundant T140block for that source from the
latest transmission is placed as the second redundant T140block latest transmission is placed as the second redundant T140block.
source.
Usually this is the level of redundancy used. If a higher number of Usually this is the level of redundancy used. If a higher number of
redundancy is negotiated, then the procedure SHALL be maintained redundancy is negotiated, then the procedure SHALL be maintained
until all available redundant levels of T140blocks and their sources until all available redundant levels of T140blocks and their sources
are placed in the packet. If a receiver has negotiated a lower are placed in the packet. If a receiver has negotiated a lower
number of "text/rex" generations, then that level shall be the number of "text/rex" generations, then that level shall be the
maximum used by the transmitter. maximum used by the transmitter.
3.11. Timer offset fields 2.2.2.11. Timer offset fields
The timer offset values are inserted in the data header, with the The timer offset values are inserted in the data header, with the
time offset from the RTP timestamp in the packet when the time offset from the RTP timestamp in the packet when the
corresponding T140block was sent from its original source as primary. corresponding T140block was sent from its original source as primary.
The timer offsets are expressed in the same clock tick units as the The timer offsets are expressed in the same clock tick units as the
RTP timestamp. RTP timestamp.
The timestamp offset values for empty T140blocks have no relevance The timestamp offset values for empty T140blocks have no relevance
but SHOULD be assigned realistic values. but SHOULD be assigned realistic values.
3.12. Other RTP header fields 2.2.2.12. Other RTP header fields
The number of members in the CSRC list shall be placed in the "CC" The number of members in the CSRC list shall be placed in the "CC"
header field. Only mixers place values >0 in the "CC" field. header field. Only mixers place values >0 in the "CC" field.
The current time SHALL be inserted in the timestamp. The current time SHALL be inserted in the timestamp.
The SSRC of the mixer for the RTT session SHALL be inserted in the The SSRC of the mixer for the RTT session SHALL be inserted in the
SSRC field of the RTP header. SSRC field of the RTP header.
3.13. Pause in transmission The M-bit SHALL be set to 1 first in the session and after a pause.
2.2.2.13. Pause in transmission
When there is no new T140block to transmit, and no redundant When there is no new T140block to transmit, and no redundant
T140block that has not been retransmitted the intended number of T140block that has not been retransmitted the intended number of
times, the transmission process can stop until either new T140blocks times, the transmission process can stop until either new T140blocks
arrive, or a keep-alive method calls for transmission of keep-alive arrive, or a keep-alive method calls for transmission of keep-alive
packets. packets.
4. Actions at reception 2.2.3. Actions at reception
The "text/rex" receiver included in an endpoint with presentation The "text/rex" receiver included in an endpoint with presentation
functions will receive RTP packets in the single stream from the functions will receive RTP packets in the single stream from the
mixer, and SHALL distribute the T140blocks for presentation in mixer, and SHALL distribute the T140blocks for presentation in
presentation areas for each source. Other receiver roles, such as presentation areas for each source. Other receiver roles, such as
gateways or chained mixers are also feasible, and requires gateways or chained mixers are also feasible, and requires
consideration if the stream shall just be forwarded, or distributed consideration if the stream shall just be forwarded, or distributed
based on the different sources. based on the different sources.
4.1. Multi-party vs two-party use 2.2.3.1. Multi-party vs two-party use
If the "CC" field value of a received packet is >0, it indicates that If the "CC" field value of a received packet is >0, it indicates that
multi-party transmission is active, and the receiver MUST be prepared multi-party transmission is active, and the receiver MUST be prepared
to act on the different sources according to its role. If the CC to act on the different sources according to its role. If the CC
value is 0, the connection is point-to-point. value is 0, the transmission is point-to-point.
4.2. Level of redundancy 2.2.3.2. Level of redundancy
The used level of redundancy generations SHALL be evaluated from the The used level of redundancy generations SHALL be evaluated from the
received packet contents. If the CC value is 0, the number of received packet contents. If the CC value is 0, the number of
generations (including the primary) is equal to the number of members generations (including the primary) is equal to the number of members
in the data header. If the CC value is >0, the number of generations in the data header. If the CC value is >0, the number of generations
(including the primary) is equal to the number of members in the data (including the primary) is equal to the number of members in the data
header divided by the CC value. If the remainder from the division header divided by the CC value. If the remainder from the division
is >0, then the packet is malformed and SHALL cause an error is >0, then the packet is malformed and SHALL cause an error
indication in the receiver. indication in the receiver.
4.3. Extracting text and handling recovery and loss 2.2.3.3. Extracting text and handling recovery and loss
The RTP sequence numbers of the received packets SHALL be monitored The RTP sequence numbers of the received packets SHALL be monitored
for gaps and packets out of order. for gaps and packets out of order.
As long as the sequence is correct, each packet SHALL be unpacked in As long as the sequence is correct, each packet SHALL be unpacked in
order. The T140blocks SHALL be extracted from the primary areas, and order. The T140blocks SHALL be extracted from the primary areas, and
the corresponding SSRCs SHALL be extracted from the corresponding the corresponding SSRCs SHALL be extracted from the corresponding
positions in the CSRC list and used for assigning the new T140block positions in the CSRC list and used for assigning the new T140block
to the correct presentation areas (or correspondingly). to the correct presentation areas (or correspondingly).
If a sequence number gap appears and is still there after some If a sequence number gap appears and is still there after some
defined time for jitter resolution, T140data SHALL be recovered from defined time for jitter resolution, T140data SHALL be recovered from
redundant data. If the gap is wider than the number of generations redundant data. If the gap is wider than the number of generations
of redundant T140blocks in the packet, then a t140block SHALL be of redundant T140blocks in the packet, then a t140block SHALL be
created with a marker for text loss [T140ad1] and assigned to the created with a marker for possible text loss [T140ad1] and assigned
SSRC of the transmitter as a general input from the mixer because in to the SSRC of the transmitter as a general input from the mixer
general it is not possible to deduct from which sources text was because in general it is not possible to deduct from which sources
lost. text was lost. It is however likely that the sources which had loss
were active in transmission just before or after the sequence number
gap. Therefore, the receiver MAY insert the marker for possible text
loss [T140ad1] in the presentation areas corresponding to the sources
which had text in the packets just before and after the gap.
Then, the T140blocks in the received packet SHALL be retrieved Then, the T140blocks in the received packet SHALL be retrieved
beginning with the highest redundant generation, grouping them with beginning with the highest redundant generation, grouping them with
the corresponding SSRC from the CSRC-list and assigning them to the the corresponding SSRC from the CSRC-list and assigning them to the
presentation areas per source. Finally the primary T140blocks SHALL presentation areas per source. Finally the primary T140blocks SHALL
be retrieved from the packet and similarly their sources retrieved be retrieved from the packet and similarly their sources retrieved
from the corresponding positions in the CSRC-list, and then assigned from the corresponding positions in the CSRC-list, and then assigned
to the corresponding presentation areas for the sources. to the corresponding presentation areas for the sources.
If the sequence number gap was equal to or less than the number of If the sequence number gap was equal to or less than the number of
redundancy generations in the received packet, a missing text marker redundancy generations in the received packet, a missing text marker
SHALL NOT be inserted, and instead the T140blocks and their SSRCs SHALL NOT be inserted, and instead the T140blocks and their SSRCs
fully recovered from the redundancy information and the CSRC-list in fully recovered from the redundancy information and the CSRC-list in
the way indicated above. the way indicated above.
4.4. Delete BOM 2.2.3.4. Delete BOM
Unicode character BOM is used as a start indication and sometimes Unicode character "BOM" is used as a start indication and sometimes
used as a filler or keep alive by transmission implementations. used as a filler or keep alive by transmission implementations.
These SHALL be deleted on reception. These SHALL be deleted on reception.
4.5. Empty T140blocks 2.2.3.5. Empty T140blocks
Empty T140blocks are included as fillers for unused redundancy levels Empty T140blocks are included as fillers for unused redundancy levels
in the packets. They just do not provide any contents and do not in the packets. They just do not provide any contents and do not
contribute to the received streams. contribute to the received streams.
5. RTCP considerations 2.2.4. RTCP considerations
A mixer SHALL send RTCP reports with SDES, CNAME and NAME information A mixer SHALL send RTCP reports with SDES, CNAME and NAME information
about the sources in the multi-party call. This makes it possible about the sources in the multi-party call. This makes it possible
for participants to compose a suitable label for text from each for participants to compose a suitable label for text from each
source. source.
6. Chained operation 2.2.5. Chained operation
By strictly applying the rules for "text/rex" packet format by all By strictly applying the rules for "text/rex" packet format by all
conforming devices, mixers MAY be arranged in chains. conforming devices, mixers MAY be arranged in chains.
7. Usage without redundancy 2.2.6. Usage without redundancy
The "text/rex" format SHALL be used also for multi-party The "text/rex" format SHALL be used also for multi-party
communication when the redundancy mechanism is not used. That MAY be communication when the redundancy mechanism is not used. That MAY be
the case when robustness in transmission is provided by some other the case when robustness in transmission is provided by some other
means than by redundancy. All aspects of this document SHALL be means than by redundancy. All aspects of this section SHALL be
applied except the redundant generations in transmission. applied except the redundant generations in transmission.
The "text/rex" format SHOULD thus be used for multi-party operation, The "text/rex" format SHOULD thus be used for multi-party operation,
also when some other protection against packet loss is utilized, for also when some other protection against packet loss is utilized, for
example a reliable network or transport. The format is also suitable example a reliable network or transport. The format is also suitable
to be used for point-to-point operation. to be used for point-to-point operation.
8. Use with SIP centralized conferencing framework 2.2.7. Use with SIP centralized conferencing framework
The SIP conferencing framework, mainly specified in RFC The SIP conferencing framework, mainly specified in RFC
4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable 4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
for coordinating sessions including multi-party RTT. The RTT stream for coordinating sessions including multi-party RTT. The RTT stream
between the mixer and a participant is one and the same during the between the mixer and a participant is one and the same during the
conference. Participants get announced by notifications when conference. Participants get announced by notifications when
participants are joining or leaving, and further user information may participants are joining or leaving, and further user information may
be provided. The SSRC of the text to expect from joined users MAY be be provided. The SSRC of the text to expect from joined users MAY be
included in a notification. The notifications MAY be used both for included in a notification. The notifications MAY be used both for
security purposes and for translation to a label for presentation to security purposes and for translation to a label for presentation to
other users. other users.
9. Conference control 2.2.8. Conference control
In managed conferences, control of the real-time text media SHOULD be In managed conferences, control of the real-time text media SHOULD be
provided in the same way as other for media, e.g. for muting and provided in the same way as other for media, e.g. for muting and
unmuting by the direction attributes in SDP [RFC4566]. unmuting by the direction attributes in SDP [RFC4566].
Note that floor control functions may be of value for RTT users as Note that floor control functions may be of value for RTT users as
well as for users of other media in a conference. well as for users of other media in a conference.
10. Media Subtype Registration 2.2.9. Media Subtype Registration
This registration is done using the template defined in [RFC6838] and This registration is done using the template defined in [RFC6838] and
following [RFC4855]. following [RFC4855].
Type name: Type name:
text text
Subtype name: Subtype name:
rex rex
skipping to change at page 16, line 30 skipping to change at page 28, line 9
m=text 49170 RTP/AVP 98 100 m=text 49170 RTP/AVP 98 100
a=rtpmap:98 rex/1000 a=rtpmap:98 rex/1000
a=fmtp:98 100/100/100 a=fmtp:98 100/100/100
a=rtpmap:100 t140/1000 a=rtpmap:100 t140/1000
a=fmtp:100 cps=200 a=fmtp:100 cps=200
Encoding considerations: Encoding considerations:
binary; see Section 4.8 of [RFC6838]. binary; see Section 4.8 of [RFC6838].
Security considerations: Security considerations:
See Section 20 of RFC xxxx. [RFC Editor: Upon publication as an See Section 9 of RFC xxxx. [RFC Editor: Upon publication as an
RFC, please replace "XXXX" with the number assigned to this RFC, please replace "XXXX" with the number assigned to this
document and remove this note.] document and remove this note.]
Interoperability considerations: Interoperability considerations:
None. None.
Published specification: Published specification:
RFC XXXX. [RFC Editor: Upon publication as an RFC, please replace RFC XXXX. [RFC Editor: Upon publication as an RFC, please replace
"XXXX" with the number assigned to this document and remove this "XXXX" with the number assigned to this document and remove this
note.] note.]
skipping to change at page 17, line 21 skipping to change at page 29, line 5
Restrictions on usage: Restrictions on usage:
This media type depends on RTP framing, and hence is only defined This media type depends on RTP framing, and hence is only defined
for transfer via RTP [RFC3550]. for transfer via RTP [RFC3550].
Author: Author:
Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se> Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>
Change controller: Change controller:
IETF AVTCore Working Group delegated from the IESG. IETF AVTCore Working Group delegated from the IESG.
11. SDP considerations 2.2.10. SDP considerations
There are receiving RTT implementations which implement RFC 4103 There are receiving RTT implementations which implement RFC 4103
[RFC4103] but not the source separation by the CSRC. Sending mixed [RFC4103] but not the source separation by the CSRC. Sending mixed
text according to the usual CSRC convention from RFC 2198 [RFC2198] text according to the usual CSRC convention from RFC 2198 [RFC2198]
to a device implementing only RFC 4103 [RFC4103] would risk to lead to a device implementing only RFC 4103 [RFC4103] and no multi-party
to unreadable presented text. Therefore, in order to negotiate RTT mechanism would risk to lead to unreadable presented text.
mixing capability according to this document, all devices supporting Therefore, in order to negotiate RTT mixing capability according to
this document for multi-party aware participants SHALL include an SDP the "text/rtx" method, all devices supporting "text/rex"" for multi-
media format "text/rex" in the SDP [RFC4566], indicating this party aware participants SHALL include an SDP media format "text/rex"
capability in offers and answers. Multi-party streams using the in the SDP [RFC4566], indicating this format in offers and answers.
coding of this document intended for multi-party aware endpoints MUST Multi-party streams using the coding of this section intended for
NOT be sent to devices which have not indicated the "text/rex" multi-party aware endpoints MUST NOT be sent to devices which have
format. not indicated the "text/rex" format.
Implementations not understanding the "text/rex" format MUST ignore Implementations not understanding the "text/rex" format MUST ignore
it according to common SDP rules. it according to common SDP rules.
The SDP media format defined here, is named "rex", for extended The SDP media format defined here, is named "rex", for extended
"red". It is intended to be used in "text" media descriptions with "red". It is intended to be used in "text" media descriptions with
"text/rex" and "text/t140" formats. Both formats MUST be declared "text/rex" and "text/t140" formats. Both formats MUST be declared
for the "text/rex" format to be used. It indicates capability to use for the "text/rex" format to be used. It indicates capability to use
source indications in the CSRC list and the packet format according source indications in the CSRC list and the packet format according
to this document. It also indicates ability to receive 150 real-time to this section. It also indicates ability to receive 150 real-time
text characters per second by default. text characters per second by default.
11.1. Mapping of media parameters to sdp 2.2.10.1. Mapping of media parameters to sdp
The information carried in the media type registration has a specific The information carried in the media type registration has a specific
mapping to fields in the Session Description Protocol (SDP) , which mapping to fields in the Session Description Protocol (SDP) , which
is commonly used to describe RTP sessions. When SDP RFC 4566 is commonly used to describe RTP sessions. When SDP RFC 4566
[RFC4566]is used to specify sessions employing the "text/rex" format, [RFC4566]is used to specify sessions employing the "text/rex" format,
the mapping is as follows: the mapping is as follows:
* The media type ("text") goes in SDP "m=" as the media name. * The media type ("text") goes in SDP "m=" as the media name.
* The media subtype (payload format name) goes in SDP "a=rtpmap" as * The media subtype (payload format name) goes in SDP "a=rtpmap" as
the encoding name. The RTP clock rate in "a=rtpmap" MUST be 1000 the encoding name. The RTP clock rate in "a=rtpmap" MUST be 1000
for "text/rex". for "text/rex".
* When the payload type is used with redundancy, the level of * When the payload type is used with redundancy, the level of
redundancy is shown by the number of elements in the slash- redundancy is shown by the number of elements in the slash-
separated payload type list in the "fmtp" parameter of the "text/ separated payload type list in the "fmtp" parameter of the "text/
rex" media format. rex" media format.
11.2. Security for session control and media 2.2.10.2. Security for session control and media
Security SHOULD be applied on both session control and media. In Security SHOULD be applied on both session control and media. In
applications where legacy endpoints without security may exist, a applications where legacy endpoints without security may exist, a
negotiation between security and no security SHOULD be applied. If negotiation between security and no security SHOULD be applied. If
no other security solution is mandated by the application, then RFC no other security solution is mandated by the application, then RFC
8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media 8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media
security with DTLS. Most SDP examples below are for simplicity security with DTLS. Most SDP examples below are for simplicity
expressed without the security additions. The principles (but not expressed without the security additions. The principles (but not
all details) for applying DTLS-SRTP security is shown in a couple of all details) for applying DTLS-SRTP security is shown in a couple of
the following examples. the following examples.
11.3. SDP offer/answer examples 2.2.10.3. SDP offer/answer examples
This sections shows some examples of SDP for session negotiation of This sections shows some examples of SDP for session negotiation of
the real-time text media in SIP sessions. Audio is usually provided the real-time text media in SIP sessions. Audio is usually provided
in the same session, and sometimes also video. The examples only in the same session, and sometimes also video. The examples only
show the part of importance for the real-time text media. show the part of importance for the real-time text media.
Offer example for just multi-party capability: Offer example for just "text/rex" multi-party capability :
m=text 11000 RTP/AVP 101 98 m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
Answer example from a multi-party capable device Answer example from a multi-party capable device
m=text 12000 RTP/AVP 101 98 m=text 12000 RTP/AVP 101 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
Offer example for both traditional "text/red" and multi-party format: Offer example for "text/red" and "text/rex" multi-party support:
m=text 11000 RTP/AVP 101 100 98 m=text 11000 RTP/AVP 101 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
a=rtt-mix-rtp-mixer
Answer example from a multi-party capable device Answer example from multi-party capable device using "text/rex".
m=text 11000 RTP/AVP 101 98 m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
Offer example for both traditional "text/red" and multi-party format Offer example for both traditional "text/red" and multi-party format
including security: including security:
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 101 100 98 m=text 11000 RTP/AVP 101 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
a=rtt-mix-rtp-mixer
The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media
line still indicating RTP/AVP. line still indicating RTP/AVP.
Answer example from a multi-party capable device including security Answer example from a multi-party capable device including security
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 101 98 m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
skipping to change at page 20, line 14 skipping to change at page 31, line 39
Answer example from a multi-party unaware device that also Answer example from a multi-party unaware device that also
does not support security: does not support security:
m=text 12000 RTP/AVP 100 98 m=text 12000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
A party which has negotiated the "text/rex" format MUST populate the A party which has negotiated the "text/rex" format MUST populate the
CSRC-list and format the packets according to this document if it CSRC-list and format the packets according to this section if it acts
acts as an rtp-mixer and sends multi-party text. as an rtp-mixer and sends multi-party text.
A party which has negotiated the "text/rex" capability MUST interpret A party which has negotiated the "text/rex" capability MUST interpret
the contents of the CSRC-list and the packets according to this the contents of the CSRC-list and the packets according to this
document in received rtp packets using the corresponding payload section in received rtp packets using the corresponding payload type.
type.
A party performing as a mixer, which has not negotiated the "text/ A party performing as a mixer, which has not negotiated the "text/
rex" format, but negotiated a "text/red" or "text/t140" format in a rex" format, but negotiated a "text/red" or "text/t140" format in a
session with a participant SHOULD, if nothing else is specified for session with a participant SHOULD, if nothing else is specified for
the application, format transmitted text to that participant to be the application, format transmitted text to that participant to be
suitable to present on a multi-party unaware endpoint as further suitable to present on a multi-party unaware endpoint as further
specified in section Section 14.2. specified in section Section 3.2.
A party not performing as a mixer MUST not include the CSRC list if A party not performing as a mixer MUST not include the CSRC list if
it has a single source of text. it has a single source of text.
12. Examples 2.2.10.4. Packet examples
This example shows a symbolic flow of packets from a mixer with loss This example shows a symbolic flow of packets from a mixer with loss
and recovery. A, B and C are sources of RTT. M is the mixer. Pn and recovery. A, B and C are sources of RTT. M is the mixer. Pn
indicates primary data in source group "n". Rn1 is first redundant indicates primary data in source group "n". Rn1 is first redundant
generation data and Rn2 is second redundant generation data in source generation data and Rn2 is second redundant generation data in source
group "n". A1, B1, A2 etc are text chunks (T140blocks) received from group "n". A1, B1, A2 etc are text chunks (T140blocks) received from
the respective sources. X indicates dropped packet between the mixer the respective sources. X indicates dropped packet between the mixer
and a receiver. and a receiver.
|----------------| |----------------|
skipping to change at page 22, line 38 skipping to change at page 34, line 22
| R21: B2 | | R21: B2 |
| P2: B3 | | P2: B3 |
| R32: A3 | | R32: A3 |
| R31: A4 | | R31: A4 |
| P3: A5 | | P3: A5 |
|----------------| |----------------|
Packet 6 is received. The latest received sequence number was 2. Packet 6 is received. The latest received sequence number was 2.
Recovery is therefore tried for 3,4,5. But there is no coverage for Recovery is therefore tried for 3,4,5. But there is no coverage for
seq no 3. A missing text mark (U'FFFD) [T140ad1] is created and seq no 3. A missing text mark (U'FFFD) [T140ad1] is created and
appended to the common mixer reception area. For seqno 4, texts C2, appended to the common mixer reception area. A missing text mark
B1 and A3 are recovered from the second generation redundancy and (U'FFFD) MAY also be appended in all streams which had text in the
appended to their respective reception areas. For seqno 5, texts B2 packets before and after the gap. That is in this case after A1, and
and A4 are recovered from the first generation redundancy and C1, and before B1.
appended to their respective reception areas. Primary text B3 and A5
are received and appended to their respective reception areas. For seqno 4, texts C2, B1 and A3 are recovered from the second
generation redundancy and appended to their respective reception
areas. For seqno 5, texts B2 and A4 are recovered from the first
generation redundancy and appended to their respective reception
areas. Primary text B3 and A5 are received and appended to their
respective reception areas.
After this sequence, the following has been received: A1,A3, A4, A5; After this sequence, the following has been received: A1,A3, A4, A5;
B1, B2, B3; C1, C2. A possible loss is indicated by the missing text B1, B2, B3; C1, C2. A possible loss is indicated by the general
mark in time between A1 and A3. missing text mark in time between A1 and A3, and in the streams after
A1 and C1 and before B1.
With only one or two packets lost, there would not be any need to With only one or two packets lost, there would not be any need to
create a missing text marker, and all text would be recovered. create a missing text marker, and all text would be recovered.
It will be a design decision how to present the missing text markers It will be a design decision how to present the missing text markers
assigned to the mixer as a source. assigned to the mixer as a source.
13. Performance considerations 2.2.10.5. Performance considerations
This document allows new text from up to 16 sources per packet. A This method allows new text from up to 15 sources per packet. A
mixer implementing the specification will normally cause a latency of mixer implementing the specification will normally cause a latency of
0 to 150 milliseconds in text from up to 16 simultaneous sources. 0 to 150 milliseconds in text from up to 15 simultaneous sources.
This performance meets well the realistic requirements for conference This performance meets well the realistic requirements for conference
and conversational applications for which up to 5 simultaneous and conversational applications for which up to 5 simultaneous
sources should not be delayed more than 500 milliseconds by a mixer. sources should not be delayed more than 500 milliseconds by a mixer.
In order to achieve good performance, a receiver for multi-party In order to achieve good performance, a receiver for multi-party
calls SHOULD declare a sufficient CPS value for the "text/t140" calls SHOULD declare a sufficient CPS value for the "text/t140"
format in SDP for the number of allowable characters per second. format in SDP for the number of allowable characters per second.
As comparison, if the "text/red" format would be used for multi-party As comparison, if the "text/red" format would be used for multi-party
communication with its default timing and redundancy, 5 communication with its default timing and redundancy, 5
simultaneously sending parties would cause jerky presentation of the simultaneously sending parties would cause jerky presentation of the
skipping to change at page 23, line 43 skipping to change at page 35, line 40
The value MAY be modified in the CPS parameter of the FMTP attribute The value MAY be modified in the CPS parameter of the FMTP attribute
in the media section for the "text/t140" media. A mixer combining in the media section for the "text/t140" media. A mixer combining
real-time text from a number of sources may have a higher combined real-time text from a number of sources may have a higher combined
flow of text coming from the sources. Endpoints SHOULD therefore flow of text coming from the sources. Endpoints SHOULD therefore
specify a suitable higher value for the CPS parameter, corresponding specify a suitable higher value for the CPS parameter, corresponding
to its real reception capability. A value for CPS of 150 is the to its real reception capability. A value for CPS of 150 is the
default for the "text/t140" stream in the "text/rex" format. See RFC default for the "text/t140" stream in the "text/rex" format. See RFC
4103 [RFC4103] for the format and use of the CPS parameter. The same 4103 [RFC4103] for the format and use of the CPS parameter. The same
rules apply for the "text/rex" format except for the default value. rules apply for the "text/rex" format except for the default value.
14. Presentation level considerations 2.3. Mixing for multi-party unaware endpoints
A method is specified in this section for cases when the
participating endpoint does not implement any solution for multi-
party presentation of real-time text. The solution requires the
mixer to insert text dividers and readable labels and only send text
from one source at a time until a suitable point appears for source
change. This solution is a fallback method with functional
limitations that acts on the presentation level and is further
specified in Section 3.2.
3. Presentation level considerations
ITU-T T.140 [T140] provides the presentation level requirements for ITU-T T.140 [T140] provides the presentation level requirements for
the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for
erasure and other formatting functions and has the following general erasure and other formatting functions and has the following general
statement for the presentation: statement for the presentation:
"The display of text from the members of the conversation should be "The display of text from the members of the conversation should be
arranged so that the text from each participant is clearly readable, arranged so that the text from each participant is clearly readable,
and its source and the relative timing of entered text is visualized and its source and the relative timing of entered text is visualized
in the display. Mechanisms for looking back in the contents from the in the display. Mechanisms for looking back in the contents from the
skipping to change at page 24, line 30 skipping to change at page 36, line 41
text to be grouped in presentation. The characters "CRLF" may be text to be grouped in presentation. The characters "CRLF" may be
used by other implementations as replacement for Line Separator. The used by other implementations as replacement for Line Separator. The
"CRLF" combination SHALL be erased by just one erasing action, just "CRLF" combination SHALL be erased by just one erasing action, just
as the Line Separator. Presentation functions are allowed to group as the Line Separator. Presentation functions are allowed to group
text for presentation in smaller groups than the line separators text for presentation in smaller groups than the line separators
imply and present such groups with source indication together with imply and present such groups with source indication together with
text groups from other sources (see the following presentation text groups from other sources (see the following presentation
examples). Erasure has no specific limit by any delimiter in the examples). Erasure has no specific limit by any delimiter in the
text stream. text stream.
14.1. Presentation by multi-party aware endpoints 3.1. Presentation by multi-party aware endpoints
A multi-party aware receiving party, presenting real-time text MUST A multi-party aware receiving party, presenting real-time text MUST
separate text from different sources and present them in separate separate text from different sources and present them in separate
presentation fields. The receiving party MAY separate presentation presentation fields. The receiving party MAY separate presentation
of parts of text from a source in readable groups based on other of parts of text from a source in readable groups based on other
criteria than line separator and merge these groups in the criteria than line separator and merge these groups in the
presentation area when it benefits the user to most easily find and presentation area when it benefits the user to most easily find and
read text from the different participants. The criteria MAY e.g. be read text from the different participants. The criteria MAY e.g. be
a received comma, full stop, or other phrase delimiters, or a long a received comma, full stop, or other phrase delimiters, or a long
pause. pause.
skipping to change at page 26, line 21 skipping to change at page 38, line 23
| |for the seminar? | | | |for the seminar? | |
|Eve, will you do | | | |Eve, will you do | | |
|your presentation on| | | |your presentation on| | |
|Friday? |Yes, Friday at 10. | | |Friday? |Yes, Friday at 10. | |
|Fine, wo | |We need to meet befo | |Fine, wo | |We need to meet befo |
|___________________________________________________________________| |___________________________________________________________________|
Figure 4: An example of a coordinated column-view of a three-party Figure 4: An example of a coordinated column-view of a three-party
session with entries ordered vertically in approximate time-order. session with entries ordered vertically in approximate time-order.
14.2. Multi-party mixing for multi-party unaware endpoints 3.2. Multi-party mixing for multi-party unaware endpoints
When the mixer has indicated multi-party capability by the "text/rex" When the mixer has indicated multi-party capability by the "rtt-mix-
format in an SDP negotiation, but the multi-party capability rtp-mixer" sdp attribute or the "text/rex" format in an SDP
negotiation fails with an endpoint, then the agreed "text/red" or negotiation, but the multi-party capability negotiation fails with an
"text/t140" format SHALL be used and the mixer SHOULD compose a best- endpoint, then the agreed "text/red" or "text/t140" format SHALL be
effort presentation of multi-party real-time text in one stream used and the mixer SHOULD compose a best-effort presentation of
intended to be presented by an endpoint with no multi-party multi-party real-time text in one stream intended to be presented by
awareness. an endpoint with no multi-party awareness.
This presentation format has functional limitations and SHOULD be This presentation format has functional limitations and SHOULD be
used only to enable participation in multi-party calls by legacy used only to enable participation in multi-party calls by legacy
deployed endpoints implementing only RFC 4103 without the multi-party deployed endpoints implementing only RFC 4103 without any multi-party
extension specified in this document. extensions specified in this document.
The principles and procedures below do not specify any new protocol The principles and procedures below do not specify any new protocol
elements. They are instead composed from the information in ITU-T elements. They are instead composed from the information in ITU-T
T.140 [T140] and an ambition to provide a best effort presentation on T.140 [T140] and an ambition to provide a best effort presentation on
an endpoint which has functions only for two-party calls. an endpoint which has functions only for two-party calls.
The mixer mixing for multi-party unaware endpoints SHALL compose a The mixer mixing for multi-party unaware endpoints SHALL compose a
simulated limited multi-party RTT view suitable for presentation in simulated limited multi-party RTT view suitable for presentation in
one presentation area. The mixer SHALL group text in suitable groups one presentation area. The mixer SHALL group text in suitable groups
and prepare for presentation of them by inserting a new line between and prepare for presentation of them by inserting a new line between
them if the transmitted text did not already end with a new line. A them if the transmitted text did not already end with a new line. A
presentable label SHOULD be composed and sent for the source presentable label SHOULD be composed and sent for the source
initially in the session and after each source switch. With this initially in the session and after each source switch. With this
procedure the time for source switching is depending on the actions procedure the time for source switching is depending on the actions
of the users. In order to expedite source switch, a user can for of the users. In order to expedite source switch, a user can for
example end its turn with a new line. example end its turn with a new line.
14.2.1. Actions by the mixer at reception from the call participants 3.2.1. Actions by the mixer at reception from the call participants
When text is received by the mixer from the different participants, When text is received by the mixer from the different participants,
the mixer SHALL recover text from redundancy if any packets are lost. the mixer SHALL recover text from redundancy if any packets are lost.
The mark for lost text [T140ad1] SHOULD be inserted in the stream if The mark for lost text [T140ad1] SHOULD be inserted in the stream if
unrecoverable loss appears. Any Unicode "BOM" characters, possibly unrecoverable loss appears. Any Unicode "BOM" characters, possibly
used for keep-alive shall be deleted. The time of creation of text used for keep-alive shall be deleted. The time of creation of text
(retrieved from the RTP timestamp) SHALL be stored together with the (retrieved from the RTP timestamp) SHALL be stored together with the
received text from each source in queues for transmission to the received text from each source in queues for transmission to the
recipients. recipients.
14.2.2. Actions by the mixer for transmission to the recipients 3.2.2. Actions by the mixer for transmission to the recipients
The following procedure SHOULD be applied for each recipient of The following procedure SHOULD be applied for each recipient of
multi-part text from the mixer. multi-part text from the mixer.
The text for transmission SHOULD be formatted by the mixer for each The text for transmission SHOULD be formatted by the mixer for each
receiving user for presentation in one single presentation area. receiving user for presentation in one single presentation area.
Text received from a participant SHOULD NOT be included in Text received from a participant SHOULD NOT be included in
transmission to that participant. When there is text available for transmission to that participant. When there is text available for
transmission from the mixer to a receiving party from more than one transmission from the mixer to a receiving party from more than one
participant, the mixer SHOULD switch between transmission of text participant, the mixer SHOULD switch between transmission of text
skipping to change at page 28, line 31 skipping to change at page 40, line 31
code stored as the status for the current source before the source code stored as the status for the current source before the source
switch is done, a reset of SGR shall be sent by the sequence SGR 0 switch is done, a reset of SGR shall be sent by the sequence SGR 0
[009B 0000 006D] after the new line and before the new label during a [009B 0000 006D] after the new line and before the new label during a
source switch. See SGR below for an explanation. This transmission source switch. See SGR below for an explanation. This transmission
does not influence the display count. does not influence the display count.
If there is an SGR code stored for the new source after the source If there is an SGR code stored for the new source after the source
switch, that SGR code SHOULD be transmitted to the recipient before switch, that SGR code SHOULD be transmitted to the recipient before
the label. This transmission does not influence the display count. the label. This transmission does not influence the display count.
14.2.3. Actions on transmission of text 3.2.3. Actions on transmission of text
Text from a source sent to the recipient SHOULD increase the display Text from a source sent to the recipient SHOULD increase the display
count by one per transmitted character. count by one per transmitted character.
14.2.4. Actions on transmission of control codes 3.2.4. Actions on transmission of control codes
The following control codes specified by T.140 require specific The following control codes specified by T.140 require specific
actions. They SHOULD cause specific considerations in the mixer. actions. They SHOULD cause specific considerations in the mixer.
Note that the codes presented here are expressed in UCS-16, while Note that the codes presented here are expressed in UCS-16, while
transmission is made in UTF-8 transform of these codes. transmission is made in UTF-8 transform of these codes.
BEL 0007 Bell Alert in session, provides for alerting during an BEL 0007 Bell Alert in session, provides for alerting during an
active session. The display count SHOULD not be altered. active session. The display count SHOULD not be altered.
NEW LINE 2028 Line separator. Check and perform a source switch if NEW LINE 2028 Line separator. Check and perform a source switch if
skipping to change at page 30, line 16 skipping to change at page 42, line 16
up to the end of the leading label after a source switch, then the up to the end of the leading label after a source switch, then the
mixer must not transmit more backspaces. Instead it is mixer must not transmit more backspaces. Instead it is
RECOMMENDED that a letter "X" is inserted in the text stream for RECOMMENDED that a letter "X" is inserted in the text stream for
each backspace as an indication of the intent to erase more. A each backspace as an indication of the intent to erase more. A
new line is usually coded by a Line Separator, but the character new line is usually coded by a Line Separator, but the character
combination "CRLF" MAY be used instead. Erasure of a new line is combination "CRLF" MAY be used instead. Erasure of a new line is
in both cases done by just one erasing action (Backspace). If the in both cases done by just one erasing action (Backspace). If the
display count has a positive value it is decreased by one when the display count has a positive value it is decreased by one when the
BS is sent. If the display count is at zero, it is not altered. BS is sent. If the display count is at zero, it is not altered.
14.2.5. Packet transmission 3.2.5. Packet transmission
A mixer transmitting to a multi-party unaware terminal SHOULD send A mixer transmitting to a multi-party unaware terminal SHOULD send
primary data only from one source per packet. The SSRC SHOULD be the primary data only from one source per packet. The SSRC SHOULD be the
SSRC of the mixer. The CSRC list SHOULD contain one member and be SSRC of the mixer. The CSRC list SHOULD contain one member and be
the SSRC of the source of the primary data. the SSRC of the source of the primary data.
14.2.6. Functional limitations 3.2.6. Functional limitations
When a multi-party unaware endpoint presents a conversation in one When a multi-party unaware endpoint presents a conversation in one
display area in a chat style, it inserts source indications for display area in a chat style, it inserts source indications for
remote text and local user text as they are merged in completed text remote text and local user text as they are merged in completed text
groups. When an endpoint using this layout receives and presents groups. When an endpoint using this layout receives and presents
text mixed for multi-party unaware endpoints, there will be two text mixed for multi-party unaware endpoints, there will be two
levels of source indicators for the received text; one generated by levels of source indicators for the received text; one generated by
the mixer and inserted in a label after each source switch, and the mixer and inserted in a label after each source switch, and
another generated by the receiving endpoint and inserted after each another generated by the receiving endpoint and inserted after each
switch between local and remote source in the presentation area. switch between local and remote source in the presentation area.
skipping to change at page 31, line 10 skipping to change at page 43, line 10
Text loss because of network errors may hit the label between entries Text loss because of network errors may hit the label between entries
from different parties, causing risk for misunderstanding from which from different parties, causing risk for misunderstanding from which
source a piece of text is. source a piece of text is.
These facts makes it strongly RECOMMENDED to implement multi-party These facts makes it strongly RECOMMENDED to implement multi-party
awareness in RTT endpoints. The use of the mixing method for multi- awareness in RTT endpoints. The use of the mixing method for multi-
party-unaware endpoints should be left for use with endpoints which party-unaware endpoints should be left for use with endpoints which
are impossible to upgrade to become multi-party aware. are impossible to upgrade to become multi-party aware.
14.2.7. Example views of presentation on multi-party unaware endpoints 3.2.7. Example views of presentation on multi-party unaware endpoints
The following pictures are examples of the view on a participant's The following pictures are examples of the view on a participant's
display for the multi-party-unaware case. display for the multi-party-unaware case.
_________________________________________________ _________________________________________________
| Conference | Alice | | Conference | Alice |
|________________________|_________________________| |________________________|_________________________|
| |I will arrive by TGV. | | |I will arrive by TGV. |
|[Bob]:My flight is to |Convenient to the main | |[Bob]:My flight is to |Convenient to the main |
|Orly. |station. | |Orly. |station. |
skipping to change at page 32, line 31 skipping to change at page 44, line 31
|[Eve] But I need to be back to | | |[Eve] But I need to be back to | |
| the hotel by 11 because I need | | | the hotel by 11 because I need | |
| |-| | |-|
|______________________________________________|v| |______________________________________________|v|
| of course, I underst | | of course, I underst |
|________________________________________________| |________________________________________________|
Figure 6: An example of a view of the multi-party unaware Figure 6: An example of a view of the multi-party unaware
presentation in chat style. Alice is the local user. presentation in chat style. Alice is the local user.
15. Gateway Considerations 4. Gateway Considerations
15.1. Gateway considerations with Textphones (e.g. TTYs). 4.1. Gateway considerations with Textphones (e.g. TTYs).
Multi-party RTT sessions may involve gateways of different kinds. Multi-party RTT sessions may involve gateways of different kinds.
Gateways involved in setting up sessions SHALL correctly reflect the Gateways involved in setting up sessions SHALL correctly reflect the
multi-party capability or unawareness of the combination of the multi-party capability or unawareness of the combination of the
gateway and the remote endpoint beyond the gateway. gateway and the remote endpoint beyond the gateway.
One case that may occur is a gateway to PSTN for communication with One case that may occur is a gateway to PSTN for communication with
textphones (e.g. TTYs). Textphones are limited devices with no textphones (e.g. TTYs). Textphones are limited devices with no
multi-party awareness, and it SHOULD therefore be suitable for the multi-party awareness, and it SHOULD therefore be suitable for the
gateway to not indicate multi-party awareness for that case. Another gateway to not indicate multi-party awareness for that case. Another
solution is that the gateway indicates multi-party capability towards solution is that the gateway indicates multi-party capability towards
the mixer, and includes the multi-party mixer function for multi- the mixer, and includes the multi-party mixer function for multi-
party unaware endpoints itself. This solution makes it possible to party unaware endpoints itself. This solution makes it possible to
make adaptations for the functional limitations of the textphone make adaptations for the functional limitations of the textphone
(TTY). (TTY).
More information on gateways to textphones (TTYs) is found in RFC More information on gateways to textphones (TTYs) is found in RFC
5194[RFC5194] 5194[RFC5194]
15.2. Gateway considerations with WebRTC. 4.2. Gateway considerations with WebRTC.
Gateway operation to real-time text in WebRTC may also be required. Gateway operation to real-time text in WebRTC may also be required.
In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data- In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data-
channel[I-D.ietf-mmusic-t140-usage-data-channel]. channel[I-D.ietf-mmusic-t140-usage-data-channel].
A multi-party bridge may have functionality for communicating by RTT A multi-party bridge may have functionality for communicating by RTT
both in RTP streams with RTT and WebRTC t140 data channels. Other both in RTP streams with RTT and WebRTC t140 data channels. Other
configurations may consist of a multi-party bridge with either configurations may consist of a multi-party bridge with either
technology for RTT transport and a separate gateway for conversion of technology for RTT transport and a separate gateway for conversion of
the text communication streams between RTP and t140 data channel. the text communication streams between RTP and t140 data channel.
skipping to change at page 33, line 39 skipping to change at page 45, line 39
rtt, a new t140 channel SHOULD be established to WebRTC users with rtt, a new t140 channel SHOULD be established to WebRTC users with
the label parameter composed from the NAME field in RTCP on the RTP the label parameter composed from the NAME field in RTCP on the RTP
side. side.
When a new participant has entered the multi-party session with RTT When a new participant has entered the multi-party session with RTT
transport in a WebRTC t140 data channel, the new participant SHOULD transport in a WebRTC t140 data channel, the new participant SHOULD
be announced by a notification to RTP users. The label parameter be announced by a notification to RTP users. The label parameter
from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP
side, or other available session information. side, or other available session information.
16. Updates to RFC 4102 and RFC 4103 5. Updates to RFC 4102 and RFC 4103
This document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by This document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format "text/rex" for the multi-party introducing an sdp media attribute "rtt-mix-rtp-mixer" for
mixing case and more strict rules for the use of redundancy, and negotiation of multi-party mixing capability with the [RFC4103]
population of the CSRC list in the packets. Implications for the format and an extended packet format "text/rex" for the enhanced
CSRC list use from RFC 2198[RFC2198] are hereby not in effect. performance multi-party mixing case and more strict rules for the use
of redundancy, and population of the CSRC list in the packets.
Implications for the CSRC list use from RFC 2198[RFC2198] is not in
effect for the "text/rex" format.
17. Congestion considerations The update is in line with the statement in RFC 4103 section 4,
saying that "Forward Error Correction mechanisms, ..., or any other
mechanism with the purpose of increasing the reliability of text
transmission, MAY be used as an alternative or complement to
redundancy."
6. Congestion considerations
The congestion considerations and recommended actions from RFC 4103 The congestion considerations and recommended actions from RFC 4103
[RFC4103] are valid also in multi-party situations. [RFC4103] are valid also in multi-party situations.
The first action in case of congestion SHOULD be to temporarily The first action in case of congestion SHOULD be to temporarily
increase the transmission interval up to one second. increase the transmission interval up to two seconds.
18. Acknowledgements 7. Acknowledgements
James Hamlin for format input. James Hamlin for format input.
19. IANA Considerations 8. IANA Considerations
8.1. Registration of the "rtt-mix-rtp-mixer" sdp media attribute
[RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
RFC number of this document.]
IANA is asked to register the new sdp attribute "rtt-mix-rtp-mixer".
Contact name: IESG
Contact email: iesg@ietf.org
Attribute name: rtt-mix-rtp-mixer
Attribute syntax: a=rtt-mix-rtp-mixer
Attribute semantics: See RFCXXXX Section 2.1.1
Attribute value: none
Usage level: media
Purpose: Indicate support by mixer or endpoint of multi-party mixing
for real-time text transmission, using a common RTP-stream for
transmission of text from a number of sources mixed with one
source at a time and the source indicated in a single CSRC-list
member.
Charset Dependent: no
O/A procedure: See RFCXXXX Section 2.1.19
Mux Category: normal
Reference: RFCXXXX
8.2. Registration of "text/rex" media subtype
The IANA is requested to register the media type "text/rex" as The IANA is requested to register the media type "text/rex" as
specified in Section 10. The media type is also requested to be specified in Section 2.2.9. The media type is also requested to be
added to the IANA registry for "RTP Payload Format Media Types" added to the IANA registry for "RTP Payload Format Media Types"
<http://www.iana.org/assignments/rtp-parameters>. <http://www.iana.org/assignments/rtp-parameters>.
20. Security Considerations 9. Security Considerations
The RTP-mixer model requires the mixer to be allowed to decrypt, pack The RTP-mixer model requires the mixer to be allowed to decrypt, pack
and encrypt secured text from the conference participants. Therefore and encrypt secured text from the conference participants. Therefore
the mixer needs to be trusted. This is similar to the situation for the mixer needs to be trusted. This is similar to the situation for
central mixers of audio and video. central mixers of audio and video.
The requirement to transfer information about the user in RTCP The requirement to transfer information about the user in RTCP
reports in SDES, CNAME and NAME fields, and in conference reports in SDES, CNAME and NAME fields, and in conference
notifications, for creation of labels may have privacy concerns as notifications, for creation of labels may have privacy concerns as
already stated in RFC 3550 [RFC3550], and may be restricted of already stated in RFC 3550 [RFC3550], and may be restricted of
privacy reasons. The receiving user will then get a more symbolic privacy reasons. The receiving user will then get a more symbolic
label for the source. label for the source.
21. Change history 10. Change history
21.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 10.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07
Added a method based on the "text/red" format and single source per
packet, negotiated by the "rtt-mix-rtp-mixer" sdp attribute.
Added reasoning and recommendation about indication of loss.
The highest number of sources in one packet is 15, not 16. Changed.
Added in information on update to RFC 4103 that RFC 4103 explicitly
allows addition of FEC method. The redundancy is a kind of forward
error correction..
10.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06
Improved definitions list format. Improved definitions list format.
The format of the media subtype parameters is made to match the The format of the media subtype parameters is made to match the
requirements. requirements.
The mapping of media subtype parameters to sdp is included. The mapping of media subtype parameters to sdp is included.
The CPS parameter belongs to the t140 subtype and does not need to be The CPS parameter belongs to the t140 subtype and does not need to be
registered here. registered here.
21.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 10.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05
nomenclature and editorial improvements nomenclature and editorial improvements
"this document" used consistently to refer to this document. "this document" used consistently to refer to this document.
21.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 10.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04
'Redundancy header' renamed to 'data header'. 'Redundancy header' renamed to 'data header'.
More clarifications added. More clarifications added.
Language and figure number corrections. Language and figure number corrections.
21.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 10.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03
Mention possible need to mute and raise hands as for other media. Mention possible need to mute and raise hands as for other media.
---done ---- ---done ----
Make sure that use in two-party calls is also possible and explained. Make sure that use in two-party calls is also possible and explained.
- may need more wording - - may need more wording -
Clarify the RTT is often used together with other media. --done-- Clarify the RTT is often used together with other media. --done--
Tell that text mixing is N-1. A users own text is not received in Tell that text mixing is N-1. A users own text is not received in
skipping to change at page 36, line 4 skipping to change at page 49, line 15
simultaneous sending users and introduced delay 16, 150 vs simultaneous sending users and introduced delay 16, 150 vs
requirements 5 vs 500. -done -- requirements 5 vs 500. -done --
Clarify redundancy level per connection. -done- Clarify redundancy level per connection. -done-
Timestamp also for the last data header. To make it possible for all Timestamp also for the last data header. To make it possible for all
text to have time offset as for transmission from the source. Make text to have time offset as for transmission from the source. Make
that header equal to the others. -done- that header equal to the others. -done-
Mixer always use the CSRC list, even for its own BOM. -done- Mixer always use the CSRC list, even for its own BOM. -done-
Combine all talk about transmission interval (300 ms vs when text has Combine all talk about transmission interval (300 ms vs when text has
arrived) in section 3 in one paragraph or close to each other. -done- arrived) in section 3 in one paragraph or close to each other. -done-
Documents the goal of good performance with low delay for 5 Documents the goal of good performance with low delay for 5
simultaneous typers in the introduction. -done- simultaneous typers in the introduction. -done-
Describe better that only primary text shall be sent on to receivers. Describe better that only primary text shall be sent on to receivers.
Redundancy and loss must be resolved by the mixer. -done- Redundancy and loss must be resolved by the mixer. -done-
21.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 10.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02
SDP and better description and visibility of security by OSRTP RFC SDP and better description and visibility of security by OSRTP RFC
8634 needed. 8634 needed.
The description of gatewaying to WebRTC extended. The description of gatewaying to WebRTC extended.
The description of the data header in the packet is improved. The description of the data header in the packet is improved.
21.6. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 10.7. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01
2,5,6 More efficient format "text/rex" introduced and attribute 2,5,6 More efficient format "text/rex" introduced and attribute
a=rtt-mix deleted. a=rtt-mix deleted.
3. Brief about use of OSRTP for security included- More needed. 3. Brief about use of OSRTP for security included- More needed.
4. Brief motivation for the solution and why not rtp-translator is 4. Brief motivation for the solution and why not rtp-translator is
used added to intro. used added to intro.
7. More limitations for the multi-party unaware mixing method 7. More limitations for the multi-party unaware mixing method
inserted. inserted.
8. Updates to RFC 4102 and 4103 more clearly expressed. 8. Updates to RFC 4102 and 4103 more clearly expressed.
9. Gateway to WebRTC started. More needed. 9. Gateway to WebRTC started. More needed.
21.7. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 10.8. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00 draft-ietf-avtcore-multi-party-rtt-mix-00
Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00 Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00
Replaced CDATA in IANA registration table with better coding. Replaced CDATA in IANA registration table with better coding.
Converted to xml2rfc version 3. Converted to xml2rfc version 3.
21.8. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to 10.9. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03 -03
Changed company and e-mail of the author. Changed company and e-mail of the author.
Changed title to "RTP-mixer formatting of multi-party Real-time text" Changed title to "RTP-mixer formatting of multi-party Real-time text"
to better match contents. to better match contents.
Check and modification where needed of use of RFC 2119 words SHALL Check and modification where needed of use of RFC 2119 words SHALL
etc. etc.
More about the CC value in sections on transmitters and receivers so More about the CC value in sections on transmitters and receivers so
that 1-to-1 sessions do not use the mixer format. that 1-to-1 sessions do not use the mixer format.
Enhanced section on presentation for multi-party-unaware endpoints Enhanced section on presentation for multi-party-unaware endpoints
A paragraph recommending CPS=150 inserted in the performance section. A paragraph recommending CPS=150 inserted in the performance section.
21.9. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to 10.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01
-02 to -02
In Abstract and 1. Introduction: Introduced wording about regulatory In Abstract and 1. Introduction: Introduced wording about regulatory
requirements. requirements.
In section 5: The transmission interval is decreased to 100 ms when In section 5: The transmission interval is decreased to 100 ms when
there is text from more than one source to transmit. there is text from more than one source to transmit.
In section 11 about SDP negotiation, a SHOULD-requirement is In section 11 about SDP negotiation, a SHOULD-requirement is
introduced that the mixer should make a mix for multi-party unaware introduced that the mixer should make a mix for multi-party unaware
endpoints if the negotiation is not successful. And a reference to a endpoints if the negotiation is not successful. And a reference to a
skipping to change at page 38, line 14 skipping to change at page 51, line 23
In chapter 9. "Use with SIP centralized conferencing framework" the In chapter 9. "Use with SIP centralized conferencing framework" the
following note is inserted: Note: The CSRC-list in an RTP packet only following note is inserted: Note: The CSRC-list in an RTP packet only
includes participants who's text is included in one or more text includes participants who's text is included in one or more text
blocks. It is not the same as the list of participants in a blocks. It is not the same as the list of participants in a
conference. With audio and video media, the CSRC-list would often conference. With audio and video media, the CSRC-list would often
contain all participants who are not muted whereas text participants contain all participants who are not muted whereas text participants
that don't type are completely silent and so don't show up in RTP that don't type are completely silent and so don't show up in RTP
packet CSRC-lists. packet CSRC-lists.
21.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 10.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00
to -01 to -01
Editorial cleanup. Editorial cleanup.
Changed capability indication from fmtp-parameter to SDP attribute Changed capability indication from fmtp-parameter to SDP attribute
"rtt-mix". "rtt-mix".
Swapped order of redundancy elements in the example to match reality. Swapped order of redundancy elements in the example to match reality.
Increased the SDP negotiation section Increased the SDP negotiation section
22. References 11. References
22.1. Normative References 11.1. Normative References
[I-D.ietf-mmusic-t140-usage-data-channel] [I-D.ietf-mmusic-t140-usage-data-channel]
Holmberg, C. and G. Hellstrom, "T.140 Real-time Text Holmberg, C. and G. Hellstrom, "T.140 Real-time Text
Conversation over WebRTC Data Channels", Work in Progress, Conversation over WebRTC Data Channels", Work in Progress,
Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel- Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel-
14, 10 April 2020, <https://tools.ietf.org/html/draft- 14, 10 April 2020, <https://tools.ietf.org/html/draft-
ietf-mmusic-t140-usage-data-channel-14>. ietf-mmusic-t140-usage-data-channel-14>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
skipping to change at page 40, line 5 skipping to change at page 53, line 20
[T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
multimedia application text conversation", February 1998, multimedia application text conversation", February 1998,
<https://www.itu.int/rec/T-REC-T.140-199802-I/en>. <https://www.itu.int/rec/T-REC-T.140-199802-I/en>.
[T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
Protocol for multimedia application text conversation", Protocol for multimedia application text conversation",
February 2000, February 2000,
<https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>. <https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>.
22.2. Informative References 11.2. Informative References
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the [RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353, Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006, DOI 10.17487/RFC4353, February 2006,
<https://www.rfc-editor.org/info/rfc4353>. <https://www.rfc-editor.org/info/rfc4353>.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, DOI 10.17487/RFC4575, August Conference State", RFC 4575, DOI 10.17487/RFC4575, August
2006, <https://www.rfc-editor.org/info/rfc4575>. 2006, <https://www.rfc-editor.org/info/rfc4575>.
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