draft-ietf-avtcore-multi-party-rtt-mix-07.txt   draft-ietf-avtcore-multi-party-rtt-mix-08.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) 12 July 2020 Updates: RFC 4103 (if approved) 12 August 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 13 January 2021 Expires: 13 February 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-07 draft-ietf-avtcore-multi-party-rtt-mix-08
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 characteristics with redundancy.
Enhancements 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 source switching. The intended use is for source identification and source switching. The intended use is for
real-time text mixers and multi-party-aware participant endpoints. real-time text mixers and multi-party-aware participant endpoints.
Two mechanisms are provided. The mechanisms builds on use of the The specified mechanism build on the standard use of the CSRC list in
CSRC list in the RTP packet for source identification. One method the RTP packet for source identification. The method makes use of
makes use of the same "text/red" format as for two-party sessions, 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 for one method is by use of a media attribute a=rtt- The capability is indicated by use of a media attribute "rtt-mix-rtp-
mix-rtp-mixer. The other method is indicated by the media subtype mixer".
"text/rex".
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] The document updates RFC 4103[RFC4103]
A brief description about how a mixer can format text for the case A specifications of how a mixer can format text for the case when the
when the endpoint is not multi-party aware is also provided. 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.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
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
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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 January 2021. This Internet-Draft will expire on 13 February 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Selected solution and considered alternative . . . . . . 5 1.1. Selected solution and considered alternative . . . . . . 4
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Intended application . . . . . . . . . . . . . . . . . . 7 1.3. Intended application . . . . . . . . . . . . . . . . . . 7
2. Specified solutions . . . . . . . . . . . . . . . . . . . . . 7 2. Specified solutions . . . . . . . . . . . . . . . . . . . . . 7
2.1. Negotiated use of the RFC 4103 format for multi-party in a 2.1. Negotiated use of the RFC 4103 format for multi-party in a
single RTP stream . . . . . . . . . . . . . . . . . . . . 7 single RTP stream . . . . . . . . . . . . . . . . . . . . 7
2.2. Use of an extended packet format "text/rex" with text from 2.2. Mixing for multi-party unaware endpoints . . . . . . . . 19
multiple sources . . . . . . . . . . . . . . . . . . . . 17 3. Presentation level considerations . . . . . . . . . . . . . . 19
2.3. Mixing for multi-party unaware endpoints . . . . . . . . 35 3.1. Presentation by multi-party aware endpoints . . . . . . . 20
3. Presentation level considerations . . . . . . . . . . . . . . 36 3.2. Multi-party mixing for multi-party unaware endpoints . . 22
3.1. Presentation by multi-party aware endpoints . . . . . . . 36 4. Gateway Considerations . . . . . . . . . . . . . . . . . . . 27
3.2. Multi-party mixing for multi-party unaware endpoints . . 38 4.1. Gateway considerations with Textphones (e.g. TTYs). . . 28
4. Gateway Considerations . . . . . . . . . . . . . . . . . . . 44 4.2. Gateway considerations with WebRTC. . . . . . . . . . . . 28
4.1. Gateway considerations with Textphones (e.g. TTYs). . . 44 5. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 29
4.2. Gateway considerations with WebRTC. . . . . . . . . . . . 45 6. Congestion considerations . . . . . . . . . . . . . . . . . . 29
5. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 45 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
6. Congestion considerations . . . . . . . . . . . . . . . . . . 46 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 46
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
8.1. Registration of the "rtt-mix-rtp-mixer" sdp media 8.1. Registration of the "rtt-mix-rtp-mixer" sdp media
attribute . . . . . . . . . . . . . . . . . . . . . . . . 46 attribute . . . . . . . . . . . . . . . . . . . . . . . . 29
8.2. Registration of "text/rex" media subtype . . . . . . . . 47
9. Security Considerations . . . . . . . . . . . . . . . . . . . 47 9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10. Change history . . . . . . . . . . . . . . . . . . . . . . . 47 10. Change history . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Changes included in 10.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 47 draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 30
10.2. Changes included in 10.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 47 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 31
10.3. Changes included in 10.3. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 48 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 31
10.4. Changes included in 10.4. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 48 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 31
10.5. Changes included in 10.5. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 48 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 31
10.6. Changes included in 10.6. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 49 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 31
10.7. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 49 10.7. Changes included in
10.8. Changes from draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 32
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 10.8. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 33
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 50
10.9. Changes from 10.9. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 50 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 33
10.10. Changes from 10.10. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 50 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.11. Changes from 10.11. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.12. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 51 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 34
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 51 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
11.1. Normative References . . . . . . . . . . . . . . . . . . 51 11.1. Normative References . . . . . . . . . . . . . . . . . . 35
11.2. Informative References . . . . . . . . . . . . . . . . . 53 11.2. Informative References . . . . . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 53 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36
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 redundancy 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 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 sent so far has been sent
all intended redundancy levels (usually two) before switching to with all intended redundancy levels (usually two) before switching to
another source. That causes the long time of one second to switch another source. That causes the long time of one second to switch
between transmission of text from one source to text from another between transmission of text from one source to text from another
source when using the default transmission interval 300 ms. Both the source when using the default transmission interval 300 ms. Both the
total throughput and the switching performance in the mixer would be total throughput and the switching performance in the mixer would be
too low for most applications. However by shorting the transmission too low for most applications. However by shorting the transmission
interval to 100 ms, good performance is achieved for up to 3 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 and usable performance for up to 5
simultaneously sending sources. This method is negotiated through an simultaneously sending sources. Capability to use this method is
sdp media attribute "rtt-mix-rtp-mixer". indicated by an sdp media attribute "rtt-mix-rtp-mixer".
A more efficient source identification scheme requires that each
redundant T140block has its source individually preserved. This
document introduces a source indicator by specific rules for
populating the CSRC-list and the data header in the RTP-packet.
An extended packet format "text/rex" is specified for this purpose,
providing the possibility to include text from up to 15 sources in
each packet in order to enhance mixer source switching performance.
By these extensions, the performance requirements on multi-party
mixing for real-time text are exceeded by the "text/rex" solution in
this document.
A negotiation mechanism can therefore be based on selection of the A negotiation mechanism can therefore be based on selection of the
"text/red" with media attribute "rtt-mix-rtp-mixer" or the "text/rex" "text/red" with media attribute "rtt-mix-rtp-mixer" for verification
media format for verification that the parties are able to handle a that the parties are able to handle a multi-party coded stream and
multi-party coded stream and agreeing on which method to use. agreeing on using that method.
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" without the "rtt-mix attribute" negotiation results in "text/red" without the "rtt-mix-rtp-mixer"
being the only common submedia format. attribute being the only common format for real-time text.
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by The document updates RFC 4103[RFC4103] by introducing an attribute
introducing an attribute for indicating multi-party capability, and for indicating capability for the multi-party mixing case and rules
an extended packet format for the multi-party mixing case and more for source indications and source switching.
strict rules for the source indications.
1.1. Selected solution and considered alternative 1.1. Selected solution and considered alternative
A number of alternatives were considered when searching an efficient A number of alternatives were considered when searching an efficient
multi-party method for real-time text. This section explains a few and easily implemented multi-party method for real-time text. This
of them briefly. section explains a few of them briefly.
One RTP stream per source, sent in the same RTP session with One RTP stream per source, sent in the same RTP session with
"text/red" format. From some points of view, use of multiple RTP "text/red" format.
streams, one for each source, sent in the same RTP session, called From some points of view, use of multiple RTP streams, one for
the RTP translator model in RFC 3550 [RFC3550], would be each source, sent in the same RTP session, called the RTP
efficient, and use exactly the same packet format as RFC 4103, the translator model in [RFC3550], would be efficient, and use exactly
same payload type and a simple SDP declaration. However, there is the same packet format as [RFC4103], the same payload type and a
currently lack of support for multi-stream RTP in certain simple SDP declaration. However, there is currently lack of
implementation technologies. The multi-stream solution would also support for multi-stream RTP in certain implementation
cause more overhead than a single RTP stream solution "text/rex" technologies. This fact made it not included in this
specified in this document and more the more simultaneous sending specification.
participants there are.
The "text/red" format in RFC 4103 with shorter transmission The "text/red" format in RFC 4103 with shorter transmission
interval, and indicating source in CSRC. The "text/red" format with interval, and indicating source in CSRC.
"text/t140" payload in a single RTP stream can be sent with 100 ms The "text/red" format with "text/t140" payload in a single RTP
packet intervals instead of the regular 300 ms. The source is stream can be sent with 100 ms packet intervals instead of the
indicated in the CSRC field. Source switching can then be done regular 300 ms. The source is indicated in the CSRC field.
every 300 ms while simultaneous transmission occurs. With two Source switching can then be done every 300 ms while simultaneous
participants sending text simultaneously, the switching and transmission occurs. With two participants sending text
transmission performance is good. With three or more simultaneously, the switching and transmission performance is
simultaneously sending participants, there will be a noticable good. With three simultaneously sending participants, there will
jerkiness in text presentation, more the more participants who be a noticable jerkiness in text presentation. The jerkiness will
send text simultaneously. With three sending participants, the be more expressed the more participants who send text
jerkiness will be about 450 ms, and with five, about 1350 ms. simultaneously. With five sending participants, the jerkiness
Text sent from a source at the end of the period its text is sent will be about 1400 ms. Text sent from a source at the end of the
by the mixer will have close to zero extra delay. Recent text period its text is sent by the mixer will have close to zero extra
will be presented with no or low delay. The 1350 ms jerkiness delay. Recent text will be presented with no or low delay. The
will be noticable and slightly unpleasant, but corresponds in time 1400 ms jerkiness will be noticable and slightly unpleasant, but
to what typing humans often cause by hesitation or changing corresponds in time to what typing humans often cause by
position while typing. A benefit of this method is that no new hesitation or changing position while typing. A benefit of this
packet format needs to be introduced and implemented. Since method is that no new packet format needs to be introduced and
simultaneous typing by more than two parties is rare, and in many implemented. Since simultaneous typing by more than two parties
applications also more than three parties in a call is rare, this is rare, and in most applications also more than three parties in
method can be used successfully without its limitations becoming a call is rare, this method can be used successfully without its
annoying. Negotiation is based on a new sdp media attribute "rtt- limitations becoming annoying. Negotiation is based on a new sdp
mix-rtp-mixer". media attribute "rtt-mix-rtp-mixer". This method is selected to
be the main one specified in this document.
The "text/rex" packet format with up to 15 sources in one packet. Th A new "text" media subtype with up to 15 sources in each packet.
e mechanism called "text/rex" specified in this document makes use The mechanism makes use of the RTP mixer model specified in
of the RTP mixer model specified in RFC3550[RFC3550]. Text from RFC3550[RFC3550]. Text from up to 15 sources can be included in
up to 15 sources can be included in each packet. Packets are each packet. Packets are normally sent every 300 ms. The mean
normally sent every 300 ms. The mean delay will be 150 ms. The delay will be 150 ms. The sources are indicated in strict order
sources are indicated in the CSRC list of the RTP packets. A new in the CSRC list of the RTP packets. A new redundancy packet
redundancy packet format is specified, named "text/rex". format is specified. This method would result in good
performance, but would require standardisation and implementation
of new releases in the target technologies that would take more
time than desirable to complete. It was therefore not selected to
be included in this specification.
The presentation planned by the mixer for multi-party unaware The presentation planned by the mixer for multi-party unaware
endpoints. It is desirable to have a method that does not require endpoints.
any modifications in existing user devices implementing RFC 4103 It is desirable to have a method that does not require any
for RTT without explicit support of multi-party sessions. This is 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 possible by having the mixer insert a new line and a text
formatted source label before each switch of text source in the formatted source label before each switch of text source in the
stream. Switch of source can only be done in places in the text stream. Switch of source can only be done in places in the text
where it does not disturb the perception of the contents. 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 from only one source can be presented in real time at a time. The
delay will therefore be varying. The method has also other delay will therefore be varying. The method has also other
limitations, but is included in this document as a fallback limitations, but is included in this document as a fallback
method. In calls where parties take turns properly by ending method. In calls where parties take turns properly by ending
their entries with a new line, the limitations will have limited their entries with a new line, the limitations will have limited
influence on the user experience. while only two parties send influence on the user experience. while only two parties send
text, these two will see the text in real time with no delay. text, these two will see the text in real time with no delay.
This method is specified as a fallback method in this
specification.
RTT transport in WebRTC
Transport of real-time text in the WebRTC technology is specified
to use the WebRTC data channel in
[I-D.ietf-mmusic-t140-usage-data-channel]. That spcification
contains a section briefly describing its use in multi-party
sessions. The focus of this specification is RTP transport.
Therefore, even if the WebRTC transport provides good multi-party
performance, it is just mentioned in this specification in
relation to providing gateways with multi-party capabilities
between RTP and WebRTC technologies.
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 7, 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 methods for multi-party real-time text are primarily intended for The method for multi-party real-time text documented in this
use in transmission between mixers and endpoints in centralised specification is primarily intended for use in transmission between
mixing configurations. It is also applicable between endpoints as mixers and endpoints in centralised mixing configurations. It is
well as between mixers. An often mentioned application is for also applicable between mixers. An often mentioned application is
emergency service calls with real-time text and voice, where a for emergency service calls with real-time text and voice, where a
calltaker want to make an attended handover of a call to another calltaker want to make an attended handover of a call to another
agent, and stay observing in the session. Multimedia conference agent, and stay observing the session. Multimedia conference
sessions with support for participants to contribute in text is sessions with support for participants to contribute in text is
another application. Conferences with central support for speech-to- another application. Conferences with central support for speech-to-
text conversion is yet another mentioned application. text conversion is yet another mentioned application.
In all these applications, normally only one participant at a time In all these applications, normally only one participant at a time
will send long text utterances. In some cases, one other participant will send long text utterances. In some cases, one other participant
will occasionally contribute with a longer comment simultaneously. will occasionally contribute with a longer comment simultaneously.
That may also happen in some rare cases when text is interpreted to That may also happen in some rare cases when text is interpreted to
text in another language in a conference. Apart from these cases, text in another language in a conference. Apart from these cases,
other participants are only expected to contribute with very brief other participants are only expected to contribute with very brief
skipping to change at page 7, line 48 skipping to change at page 7, line 48
2. Specified solutions 2. Specified solutions
2.1. Negotiated use of the RFC 4103 format for multi-party in a single 2.1. Negotiated use of the RFC 4103 format for multi-party in a single
RTP stream RTP stream
This section specifies use of the current format specified in This section specifies use of the current format specified in
[RFC4103] for true multi-party real-time text. It is an update of [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 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 situation. It is done by completing a negotiation for this kind of
multi-party capability and by indicating source in the CSRC element multi-party capability and by indicating source in the CSRC element
in the RTP packets. Please use [RFC4103] as reference when reading in the RTP packets.
the following description.
Please use [RFC4103] as reference when reading the following
description.
2.1.1. Negotiation for use of this method 2.1.1. Negotiation for use of this method
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 4103[RFC4103] by
4103[RFC4103] by introducing a capability negotiation for handling introducing a capability negotiation for handling multi-party real-
multi-party real-time text. The capability negotiation is based on time text. The capability negotiation is based on use of the sdp
use of the sdp media attribute "rtt-mix-rtp-mixer". media attribute "rtt-mix-rtp-mixer".
The syntax is as follows: The syntax is as follows:
a=rtt-mix-rtp-mixer "a=rtt-mix-rtp-mixer"
A transmitting party SHALL send text according to the multi-party A transmitting party SHALL send text according to the multi-party
format only when the negotiation for this method was successful and 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 when the CC field in the RTP packet is set to 1. In all other cases,
packets SHALL be populated as for a two-party session. the packets SHALL be populated and interpreted as for a two-party
session.
2.1.2. Use of fields in the RTP packets 2.1.2. Use of fields in the RTP packets
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 (1) in transmissions from a mixer involved in a multi-party SHALL be one (1) in transmissions from a mixer involved in a multi-
session, and otherwise 0. party session, and otherwise 0.
When transmitted from a mixer during a multi-party session, a CSRC 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 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. SHALL contain the SSRC of the source of the T140blocks in the packet.
When redundancy is used, the recommended level of redundancy is to When redundancy is used, the recommended level of redundancy is to
use one primary and two redundant generations of T140blocks. In some use one primary and two redundant generations of T140blocks. In some
cases, a primary or redundant T140block is empty, but is still cases, a primary or redundant T140block is empty, but is still
represented by a member in the redundancy header. represented by a member in the redundancy header.
From other aspects, the contents of the RTP packts are equal to what From other aspects, the contents of the RTP packts are equal to what
is specified in RFC 4103. is specified in [RFC4103].
2.1.3. Transmission of multi-party contents 2.1.3. Transmission of multi-party contents
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 be
sent to it according to the procedures in this section. If the 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.
2.1.4. Keep-alive 2.1.4. 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 [RFC6263].
2.1.5. Transmission interval 2.1.5. Transmission interval
A "text/red" transmitter SHOULD send packets distributed in time as A "text/red" transmitter in a mixer SHOULD send packets distributed
long as there is something (new or redundant T140blocks) to transmit. in time as long as there is something (new or redundant T140blocks)
The maximum transmission interval SHOULD then be 300 ms. It is to transmit. The maximum transmission interval SHOULD then be 300
RECOMMENDED to send next packet to a receiver as soon as new text to ms. It is RECOMMENDED to send next packet to a receiver as soon as
that receiver is available, as long as the time after the latest sent new text to that receiver is available, as long as the time after the
packet to the same receiver is more than or equal to 100 ms, and also latest sent packet to the same receiver is more than or equal to 100
the maximum character rate to the receiver is not exceeded. The ms, and also the maximum character rate to the receiver is not
intention is to keep the latency low while keeping a good protection exceeded. The intention is to keep the latency low while keeping a
against text loss in bursty packet loss conditions. New and good protection against text loss in bursty packet loss conditions.
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 2.1.6. Only one source per packet
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.
2.1.7. 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.
2.1.7. Clean incoming text 2.1.8. 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.
2.1.8. Redundancy 2.1.9. 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.
number of redundant generations of T140blocks to include in
The 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. parties negotiating a connection.
2.1.9. Text placement in packets 2.1.10. 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
all T140blocks queued for transmission originating from the source in all T140blocks queued for transmission originating from the source in
turn for transmission as long as this is not in conflict with the turn for transmission as long as this is not in conflict with the
allowed number of characters per second or the maximum packet size. allowed number of characters per second ("CPS") or the maximum packet
The SSRC of the source shall be placed as the member in the CSRC- size. The SSRC of the source shall be placed as the only member in
list. the CSRC-list.
2.1.10. Maximum number of sources per packet 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. Source switching
When text from more than one source is available for transmission, When text from more than one source is available for transmission,
the mixer SHALL let the sources take turns in having their text the mixer SHALL let the sources take turns in having their text
transmitted. When switching from transmission of one source to allow transmitted. When switching from 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 current source MUST be generations of the last text from the current source 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 as Actively transmitting sources SHOULD be allowed to take turns as
frequently as possible to have their text transmitted. That implies frequently as possible to have their text transmitted. That implies
that with the recommended redundancy, the mixer SHALL send primary that with the recommended redundancy, the mixer SHALL send primary
text and two packets with redundant text from the current source text and two packets with redundant text from the current source
before text from another source is transmitted. The source with the 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 oldest text received in the mixer SHOULD be next in turn to get all
its available text transmitted. its available text transmitted.
Note: The CSRC-list in an RTP packet only includes the participant 2.1.12. Empty T140blocks
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 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 as usual in for transmission. The corresponding SSRC SHALL be placed as usual in
its place in the CSRC-list. its place in the CSRC-list.
2.1.12. Creation of the redundancy 2.1.13. Creation of the redundancy
The primary T140block from a source in the latest transmitted packet The primary T140block from a source in the latest transmitted packet
is used to populate the first redundant T140block for that source. is used to populate the first redundant T140block for that source.
The first redundant T140block for that source from the latest The first redundant T140block for that source from the latest
transmission is placed as the second redundant T140block. transmission is placed as the second redundant T140block.
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 are placed in the until all available redundant levels of T140blocks are placed in the
packet. If a receiver has negotiated a lower number of "text/red" packet. If a receiver has negotiated a lower number of "text/red"
generations, then that level shall be the maximum used by the generations, then that level shall be the maximum used by the
transmitter. transmitter.
2.1.13. Timer offset fields 2.1.14. Timer offset fields
The timestamp offset values are inserted in the data header, with the The timestamp offset values are inserted in the redundancy header,
time offset from the RTP timestamp in the packet when the with 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 timestamp offsets are expressed in the same clock tick units as The timestamp offsets are expressed in the same clock tick units as
the RTP timestamp. the 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.
2.1.14. Other RTP header fields 2.1.15. Other RTP header fields
The number of members in the CSRC list ( 0 or 1) shall be placed in 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 "CC" header field. Only mixers place value 1 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.
The M-bit shall be handled as specified in [RFC4103]. The M-bit shall be handled as specified in [RFC4103].
2.1.15. Pause in transmission 2.1.16. 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 from any source, the transmission process can stop until either times from any source, the transmission process can stop until either
new T140blocks arrive, or a keep-alive method calls for transmission new T140blocks arrive, or a keep-alive method calls for transmission
of keep-alive packets. of keep-alive packets.
2.1.16. RTCP considerations 2.1.17. 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.
2.1.17. Reception of multi-party contents Integrity considerations SHALL be considered when composing these
fields.
2.1.18. Reception of multi-party contents
The "text/red" receiver included in an endpoint with presentation The "text/red" 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.
2.1.17.1. Multi-party vs two-party use 2.1.18.1. Multi-party vs two-party use
If the "CC" field value of a received packet is 1, it indicates that 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 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, to act on the source according to its role. If the CC value is 0,
the connection is point-to-point. the connection is point-to-point.
2.1.17.2. Level of redundancy 2.1.18.2. 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. The number of generations (including the received packet contents. The number of generations (including the
primary) is equal to the number of members in the redundancy header. primary) is equal to the number of members in the redundancy header.
2.1.17.3. Extracting text and handling recovery and loss 2.1.18.3. 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 area, and order. The T140blocks SHALL be extracted from the primary area, and
the corresponding SSRC SHALL be extracted from the CSRC list and used the corresponding SSRC SHALL be extracted from the CSRC list and used
for assigning the new T140block to the correct presentation areas (or for assigning the new T140block to the correct presentation areas (or
correspondingly for other receiver roles). correspondingly for other receiver roles).
skipping to change at page 13, line 11 skipping to change at page 13, line 26
the presentation area of that source. Finally the primary T140block the presentation area of that source. Finally the primary T140block
SHALL be retrieved from the packet and similarly assigned to the SHALL be retrieved from the packet and similarly assigned to the
corresponding presentation area for the source. corresponding presentation area for the source.
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 T140block and the SSRC fully SHALL NOT be inserted, and instead the T140block and the SSRC fully
recovered from the redundancy information and the CSRC-list in the recovered from the redundancy information and the CSRC-list in the
way indicated above. way indicated above.
2.1.17.4. Delete BOM 2.1.18.4. 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.
2.1.17.5. Empty T140blocks 2.1.18.5. 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.
2.1.18. Performance considerations 2.1.19. Performance considerations
This solution has good performance up to three participants This solution has good performance for up to three participants
simultaneously sending text. At higher numbers of participants simultaneously sending text. At higher numbers of participants
simultaneously sending text, a jerkiness is visible in the simultaneously sending text, a jerkiness is visible in the
presentation of text. With five participants simultaneously presentation of text. With five participants simultaneously
transmitting text, the jerkiness is about 1400 ms. Evenso, the transmitting text, the jerkiness is about 1400 ms. Evenso, the
transmission of text catches up, so there is no resulting delay transmission of text catches up, so there is no resulting total delay
introduced. The solution is therefore suitable for emergency service introduced. The solution is therefore suitable for emergency service
use, relay service use, and small or well-managed larger multimedia use, relay service use, and small or well-managed larger multimedia
conferences. It is only less suitable for large conferences with a conferences. Only in large unmanaged conferences with a high number
high number of participants sending text simultaneously. It should of participants there may on very rare occasions appear situations
be noted that it is only the number of users sending text within the when many participants happen to send text simultaneously, resulting
same moment that causes jerkiness, not the total number of users with in unpleasantly long switching times. It should be noted that it is
RTT capability. 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 2.1.20. Offer/answer considerations
A party which has negotiated the "rtt-mix-rtp-mixer" sdp media A party which has negotiated the "rtt-mix-rtp-mixer" sdp media
attribute MUST populate the CSRC-list and format the packets attribute MUST populate the CSRC-list and format the packets
according to this section if it acts as an rtp-mixer and sends multi- according to this section if it acts as an rtp-mixer and sends multi-
party text. party text.
A party which has negotiated the the "rtt-mix-rtp-mixer" sdp media A party which has negotiated the the "rtt-mix-rtp-mixer" sdp media
attribute MUST interpret the contents of the CSRC-list and the attribute MUST interpret the contents of the "CC" field the CSRC-list
packets according to this section in received rtp packets in the and the packets according to this section in received rtp packets in
corresponding RTP stream. the corresponding RTP stream.
A party performing as a mixer, which has not negotiated the "rtt-mix- 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/ rtp-mixer" sdp media attribute, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD, if nothing else t140" format in a session with a participant SHOULD, if nothing else
is specified for the application, format transmitted text to that is specified for the application, format transmitted text to that
participant to be suitable to present on a multi-party unaware participant to be suitable to present on a multi-party unaware
endpoint as further specified in section Section 3.2. endpoint as further specified in section Section 3.2.
A party not performing as a mixer MUST not include the CSRC list. A party not performing as a mixer MUST not include the CSRC list.
2.1.20. Security for session control and media 2.1.21. 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.
2.1.21. SDP offer/answer examples 2.1.22. 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 "text/red" format and multi-party support: Offer example for "text/red" format and multi-party support:
m=text 11000 RTP/AVP 100 98 m=text 11000 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=rtt-mix-rtp-mixer a=rtt-mix-rtp-mixer
Answer example from a multi-party capable device Answer example from a multi-party capable device
m=text 11000 RTP/AVP 100 98 m=text 14000 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=rtt-mix-rtp-mixer a=rtt-mix-rtp-mixer
Offer example for "text/red" format including multi-party Offer example for "text/red" format including multi-party
and security: and 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 100 98 m=text 11000 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=rtt-mix-rtp-mixer 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 multi-party capable device with security Answer example from a multi-party capable device with 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 100 98 m=text 16000 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=rtt-mix-rtp-mixer a=rtt-mix-rtp-mixer
With the "fingerprint" the device acknowledges use of SRTP/DTLS. With the "fingerprint" the device acknowledges use of SRTP/DTLS.
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
2.1.22. Packet sequence example 2.1.23. Packet sequence example from a source switch
This example shows a symbolic flow of packets from a mixer with loss This example shows a symbolic flow of packets from a mixer including
and recovery. A and B are sources of RTT. P indicates primary data. loss and recovery. The sequence includes a source switch. A and B
R1 is first redundant generation data and R2 is second redundant are sources of RTT. P indicates primary data. R1 is first redundant
generation data. A1, B1, A2 etc are text chunks (T140blocks) generation data and R2 is second redundant generation data. A1, B1,
received from the respective sources. X indicates dropped packet A2 etc are text chunks (T140blocks) received from the respective
between the mixer and a receiver. sources. X indicates dropped packet between the mixer and a
receiver.
|----------------| |----------------|
|Seq no 1 | |Seq no 1 |
|CC=1 | |CC=1 |
|CSRC list A | |CSRC list A |
|R2: A1 | |R2: A1 |
|R1: A2 | |R1: A2 |
|P: A3 | |P: A3 |
|----------------| |----------------|
Assuming that earlier packets ( with text A1 and A2) were received in Assuming that earlier packets ( with text A1 and A2) were received in
sequence, text A3 is received from packet 1 and assigned to reception 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 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 and need to prepare for sending that text. First it must send the
redundant generations of text A1. redundant generations of text A2 and A3.
|----------------| |----------------|
|Seq no 2 | |Seq no 2 |
|CC=1 | |CC=1 |
|CSRC list A | |CSRC list A |
|R2 A2 | |R2 A2 |
|R1: A3 | |R1: A3 |
|P: Empty | |P: Empty |
|----------------| |----------------|
Nothing needs to be retrieved from this packet. Nothing needs to be retrieved from this packet.
X----------------| X----------------|
X Seq no 3 | X Seq no 3 |
X CC=1 | X CC=1 |
X CSRC list A | X CSRC list A |
X R2: A3 | X R2: A3 |
X R1: Empty | X R1: Empty |
X P: Empty | X P: Empty |
X----------------| X----------------|
Packet 3 is assumed to be dropped in network problems Packet 3 is assumed to be dropped in network problems. It was the
last packet with contents from A before the source switch.
X----------------| X----------------|
X Seq no 4 | X Seq no 4 |
X CC=1 | X CC=1 |
X CSRC list B | X CSRC list B |
X R2: Empty | X R2: Empty |
X R1: Empty | X R1: Empty |
X P2: B1 | X P2: B1 |
X----------------| X----------------|
Packet 4 contains text from B, assumed dropped in network problems. Packet 4 contains text from B, assumed dropped in network problems.
skipping to change at page 17, line 42 skipping to change at page 18, line 9
packet 6. packet 6.
After this sequence, A3 and B1 have been received. In this case no 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 text was lost. Even if also packet 2 was lost, it can be concluded
that no text was lost. that no text was lost.
If also packets 1 and 2 were lost, there would be a need to create a 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 marker for possibly lost text (U'FFFD) [T140ad1], inserted generally
and possibly also in text sequences A and B. and possibly also in text sequences A and B.
2.2. Use of an extended packet format "text/rex" with text from 2.1.24. Use with SIP centralized conferencing framework
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
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 format "text/rex" with a rule for
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
order to enhance the performance in multi-party RTT sessions.
The "text/rex" format can be seen as an "n-tuple" variant of the
"text/red" format intended to carry text information from up to 15
sources per packet.
The CC field SHALL show the number of members in the CSRC list, which
is one per source represented 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
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
the T140blocks. When redundancy is used, text from all included
sources MUST have the same number of redundant generations. The
primary, first redundant, second redundant and possible further
redundant generations of T140blocks MUST be grouped per source in the
packet in "source groups". 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 data header.
The RTP header is followed by one or more source groups of data
headers: one header for each text block to be included. Each of
these data headers provides the timestamp offset and length of the
corresponding data block, in addition to the payload type number
corresponding to the payload format "text/t140". The data headers
are followed by the data fields carrying T140blocks from the sources.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| block PT | timestamp offset | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The bits in the data header.
The bits in the data header are specified as follows:
F: 1 bit First bit in header indicates whether another header block
follows. It has value 1 if further header blocks follow, and
value 0 if this is the last header block.
block PT: 7 bits RTP payload type number for this block,
corresponding to the t140 payload type from the RTPMAP SDP
attribute.
timestamp offset: 14 bits Unsigned offset of timestamp of this block
relative to the timestamp given in the RTP header. The offset is
a time to be subtracted from the current timestamp to determine
the timestamp of the data when the latest part of this block was
sent from the original source. If the timestamp offset would be
>15 000, it SHALL be set to 15 000. For redundant data, the
resulting time is the time when the data was sent as primary from
the original source. If the value would be >15 000, then it SHALL
be set to 15 000 plus 300 times the redundancy level of the data.
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
Characters Per Second (CPS) limit.
block length: 10 bits Length in bytes of the corresponding data
block excluding the header.
The header for the final block has a zero F bit, and apart from that
the same fields as other data headers.
Note: The "text/rex" packet format is similar to that of RFC 2198
[RFC2198] but is different from some aspects. RFC 2198 associates
the whole of the CSRC-list with the primary data and assumes that the
same list applies to reconstructed redundant data. In this section a
T140block is associated with exactly one CSRC list member as
described above. Also RFC 2198 [RFC2198] anticipates infrequent
change to CSRCs; implementers should be aware that the order of the
CSRC-list according to this section will vary during transitions
between transmission from the mixer of text originated by different
participants. Another difference is that the last member in the data
header area in RFC 2198 [RFC2198] only contains the payload type
number while in this section it has the same format as all other
entries in the data header.
The picture below shows a typical "text/rex" RTP packet with multi-
party RTT contents from three sources and coding according to this
section.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC=3 |M| "REX" PT | RTP sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of packet creation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CSRC list member 1 = SSRC of source of "A" |
| CSRC list member 2 = SSRC of source of "B" |
| CSRC list member 3 = SSRC of source of "C" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestmp offset of "A-R2" |"A-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "A-R1" |"A-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "A-P" |"A-P" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "B-R2" |"B-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "B-R1" |"B-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "B-P" | "B-P" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "C-R2" |"C-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "C-R1" |"C-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT |timestamp offset of "C-P" |"C-P" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "A-R2" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |"A-R1" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|"A-P" T.140 encoded primary | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "B-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "B-R1" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "B-P" T.140 encoded primary data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-R1" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2:A "text/rex" packet with text from three sources A, B, 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-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
block length, and no corresponding T.140 data.
2.2.2. Actions at transmission by a mixer
2.2.2.1. Initial BOM transmission
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.
2.2.2.2. 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].
2.2.2.3. Transmission interval
A "text/rex" 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 a 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 150 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.
2.2.2.4. Do not send received text to the originating source
Text received from a participant SHOULD NOT be included in
transmission to that participant.
2.2.2.5. 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.
2.2.2.6. 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. The same number of redundant
generations MUST be used for text from all sources when it is
transmitted to a receiver. The number of generations sent to a
receiver SHALL be the same during the whole session unless it is
modified by session renegotiation.
2.2.2.7. Text placement in packets
At time of transmission, the mixer SHALL populate the RTP packet with
T140blocks combined from all T140blocks queued for transmission
originating from each source as long as this is not in conflict with
the allowed number of characters per second or the maximum packet
size. These T140blocks SHALL be placed in the packet interleaved
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
place corresponding to the place of its T140blocks in the packet.
2.2.2.8. Maximum number of sources per packet
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
allowed in a packet. If text from more sources need to be
transmitted, the mixer MAY let the sources take turns in having their
text transmitted. When stopping transmission of one source to allow
another source to have its text sent, all intended redundant
generations of the last text from the source to be stopped MUST be
transmitted before text from another source can be transmitted.
Actively transmitting sources SHOULD be allowed to take turns with
short intervals to have their text transmitted.
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
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.
2.2.2.9. 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 in its
place in the CSRC-list.
2.2.2.10. Creation of the redundancy
The primary T140block from each 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 and their sources
are placed in the packet. If a receiver has negotiated a lower
number of "text/rex" generations, then that level shall be the
maximum used by the transmitter.
2.2.2.11. Timer offset fields
The timer 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 timer 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.
2.2.2.12. Other RTP header fields
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.
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 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
T140block that has not been retransmitted the intended number of
times, the transmission process can stop until either new T140blocks
arrive, or a keep-alive method calls for transmission of keep-alive
packets.
2.2.3. Actions at reception
The "text/rex" 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.
2.2.3.1. Multi-party vs two-party use
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
to act on the different sources according to its role. If the CC
value is 0, the transmission is point-to-point.
2.2.3.2. Level of redundancy
The used level of redundancy generations SHALL be evaluated from the
received packet contents. If the CC value is 0, the number of
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
(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
is >0, then the packet is malformed and SHALL cause an error
indication in the receiver.
2.2.3.3. 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 areas, and
the corresponding SSRCs SHALL be extracted from the corresponding
positions in the CSRC list and used for assigning the new T140block
to the correct presentation areas (or correspondingly).
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 sources
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
beginning with the highest redundant generation, grouping them with
the corresponding SSRC from the CSRC-list and assigning them to the
presentation areas per source. Finally the primary T140blocks SHALL
be retrieved from the packet and similarly their sources retrieved
from the corresponding positions in the CSRC-list, and then assigned
to the corresponding presentation areas for the sources.
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 T140blocks and their SSRCs
fully recovered from the redundancy information and the CSRC-list in
the way indicated above.
2.2.3.4. 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.
2.2.3.5. 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.
2.2.4. 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.
2.2.5. Chained operation
By strictly applying the rules for "text/rex" packet format by all
conforming devices, mixers MAY be arranged in chains.
2.2.6. Usage without redundancy
The "text/rex" format SHALL be used also for multi-party
communication when the redundancy mechanism is not used. That MAY be
the case when robustness in transmission is provided by some other
means than by redundancy. All aspects of this section SHALL be
applied except the redundant generations in transmission.
The "text/rex" format SHOULD thus be used for multi-party operation,
also when some other protection against packet loss is utilized, for
example a reliable network or transport. The format is also suitable
to be used for point-to-point operation.
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.
2.2.8. Conference control 2.1.25. 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.
2.2.9. Media Subtype Registration 2.1.26. Maximum character rate "CPS"
This registration is done using the template defined in [RFC6838] and
following [RFC4855].
Type name:
text
Subtype name:
rex
Required parameters:
rate:
The RTP timestamp (clock) rate. The only valid value is 1000.
pt:
a comma-separated list of RTP payload types. Because comma is
a special character, the list must be a quoted-string (enclosed
in double quotes). Each list element is a mapping of the
dynamic payload type number to an embedded Content-type
specification for the payload format corresponding to the
payload type. The format of the mapping is:
payload-type-number "=" content-type
If the content-type string includes a comma, then the content-
type string MUST be a quoted-string. If the content- type
string does not include a comma, it MAY still be quoted. Since
it is part of the list which must itself be a quoted- string,
that means the quotation marks MUST be quoted with backslash
quoting as specified in RFC 2045. If the content- type string
itself contains a quoted-string, then the requirement for
backslash quoting is recursively applied. To specify the text/
rex payload format in SDP, the pt parameter is mapped to an
a=fmtp attribute by eliminating the parameter name (pt) and
changing the commas to slashes. For example:
pt = " = \"text/t140;cps=200,text/t140,text/t140\" "
Implies the following sdp
m=text 49170 RTP/AVP 98 100
a=rtpmap:98 rex/1000
a=fmtp:98 100/100/100
a=rtpmap:100 t140/1000
a=fmtp:100 cps=200
Encoding considerations:
binary; see Section 4.8 of [RFC6838].
Security considerations:
See Section 9 of RFC xxxx. [RFC Editor: Upon publication as an
RFC, please replace "XXXX" with the number assigned to this
document and remove this note.]
Interoperability considerations:
None.
Published specification:
RFC XXXX. [RFC Editor: Upon publication as an RFC, please replace
"XXXX" with the number assigned to this document and remove this
note.]
Applications which use this media type:
For example: Text conferencing tools, multimedia conferencing
tools.Real-time conversational tools.
Fragment identifier considerations:
N/A.
Additional information:
None.
Person & email address to contact for further information:
Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>
Intended usage:
COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only defined
for transfer via RTP [RFC3550].
Author:
Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>
Change controller:
IETF AVTCore Working Group delegated from the IESG.
2.2.10. SDP considerations
There are receiving RTT implementations which implement RFC 4103
[RFC4103] but not the source separation by the CSRC. Sending mixed
text according to the usual CSRC convention from RFC 2198 [RFC2198]
to a device implementing only RFC 4103 [RFC4103] and no multi-party
mechanism would risk to lead to unreadable presented text.
Therefore, in order to negotiate RTT mixing capability according to
the "text/rtx" method, all devices supporting "text/rex"" for multi-
party aware participants SHALL include an SDP media format "text/rex"
in the SDP [RFC4566], indicating this format in offers and answers.
Multi-party streams using the coding of this section intended for
multi-party aware endpoints MUST NOT be sent to devices which have
not indicated the "text/rex" format.
Implementations not understanding the "text/rex" format MUST ignore
it according to common SDP rules.
The SDP media format defined here, is named "rex", for extended
"red". It is intended to be used in "text" media descriptions with
"text/rex" and "text/t140" formats. Both formats MUST be declared
for the "text/rex" format to be used. It indicates capability to use
source indications in the CSRC list and the packet format according
to this section. It also indicates ability to receive 150 real-time
text characters per second by default.
2.2.10.1. Mapping of media parameters to sdp
The information carried in the media type registration has a specific
mapping to fields in the Session Description Protocol (SDP) , which
is commonly used to describe RTP sessions. When SDP RFC 4566
[RFC4566]is used to specify sessions employing the "text/rex" format,
the mapping is as follows:
* 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 encoding name. The RTP clock rate in "a=rtpmap" MUST be 1000
for "text/rex".
* When the payload type is used with redundancy, the level of
redundancy is shown by the number of elements in the slash-
separated payload type list in the "fmtp" parameter of the "text/
rex" media format.
2.2.10.2. 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.
2.2.10.3. 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 just "text/rex" multi-party capability :
m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Answer example from a multi-party capable device
m=text 12000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Offer example for "text/red" and "text/rex" multi-party support:
m=text 11000 RTP/AVP 101 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=rtpmap:101 rex/1000
a=fmtp:100 98/98/98
a=fmtp:101 98/98/98
a=rtt-mix-rtp-mixer
Answer example from multi-party capable device using "text/rex".
m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Offer example for both traditional "text/red" and multi-party format
including 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 101 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=rtpmap:101 rex/1000
a=fmtp:100 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
line still indicating RTP/AVP.
Answer example from a multi-party capable device including 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 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
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
A party which has negotiated the "text/rex" format 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 "text/rex" capability MUST interpret
the contents of the CSRC-list and the packets according to this
section in received rtp packets using the corresponding payload type.
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
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 if
it has a single source of text.
2.2.10.4. Packet examples
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
indicates primary data in source group "n". Rn1 is first redundant
generation data and Rn2 is second redundant generation data in source
group "n". 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 |
|R12: Empty |
|R11: Empty |
|P1: A1 |
|----------------|
Assuming that earlier packets were received in sequence, text A1 is
received from packet 1 and assigned to reception area A.
|----------------|
|Seq no 2 |
|CC=3 |
|CSRC list C,A |
|R12 Empty |
|R11:Empty |
|P1: C1 |
|R22 Empty |
|R21: A1 |
|P2: Empty |
|----------------|
Text C1 is received from packet 2 and assigned to reception area C.
X----------------|
X Seq no 3 |
X CC=2 |
X CSRC list C,A |
X R12: Empty |
X R11: C1 |
X P1: Empty |
X R22: A1 |
X R21: Empty |
X P2: A2 |
X----------------|
Packet 3 is assumed to be dropped in network problems
X----------------|
X Seq no 4 |
X CC=3 |
X CSRC list C,B,A|
X R12: Empty |
X R11: Empty |
X P1: C2 |
X R22: Empty |
X R21: Empty |
X P2: B1 |
X R32: Empty |
X R31: A2 |
X P3: A3 |
X----------------|
Packet 4 is assumed to be dropped in network problems
X----------------|
X Seq no 5 |
X CC=3 |
X CSRC list C,B,A|
X R12: Empty |
X R11: C2 |
X P1: Empty |
X R22: Empty |
X R21: B1 |
X P2: B2 |
X R32: A2 |
X R31: A3 |
X P3: A4 |
X----------------|
Packet 5 is assumed to be dropped in network problems
|----------------|
|Seq no 6 |
|CC=3 |
|CSRC list C,B,A |
| R12: C2 |
| R11: Empty |
| P1: Empty |
| R22: B1 |
| R21: B2 |
| P2: B3 |
| R32: A3 |
| R31: A4 |
| P3: A5 |
|----------------|
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
seq no 3. A missing text mark (U'FFFD) [T140ad1] is created and
appended to the common mixer reception area. A missing text mark
(U'FFFD) MAY also be appended in all streams which had text in the
packets before and after the gap. That is in this case after A1, and
C1, and before B1.
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;
B1, B2, B3; C1, C2. A possible loss is indicated by the general
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
create a missing text marker, and all text would be recovered.
It will be a design decision how to present the missing text markers
assigned to the mixer as a source.
2.2.10.5. Performance considerations
This method allows new text from up to 15 sources per packet. A
mixer implementing the specification will normally cause a latency of
0 to 150 milliseconds in text from up to 15 simultaneous sources.
This performance meets well the realistic requirements for conference
and conversational applications for which up to 5 simultaneous
sources should not be delayed more than 500 milliseconds by a mixer.
In order to achieve good performance, a receiver for multi-party
calls SHOULD declare a sufficient CPS value for the "text/t140"
format in SDP for the number of allowable characters per second.
As comparison, if the "text/red" format would be used for multi-party
communication with its default timing and redundancy, 5
simultaneously sending parties would cause jerky presentation of the
text from them in text spurts with 5 seconds intervals. With a
reduction of the transmission interval to 150 ms, the time between
text spurts for 5 simultaneous sending parties would be 2.5 seconds.
Five simultaneous sending parties may occasionally occur in a
conference with one or two main sending parties and three parties
giving very brief comments.
The default maximum rate of reception of "text/t140" real-time text The default maximum rate of reception of "text/t140" real-time text
is in RFC 4103 [RFC4103] specified to be 30 characters per second. is in RFC 4103 [RFC4103] specified to be 30 characters per second.
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 occasionally have a
flow of text coming from the sources. Endpoints SHOULD therefore higher combined flow of text coming from the sources. Endpoints
specify a suitable higher value for the CPS parameter, corresponding SHOULD therefore specify a suitable higher value for the CPS
to its real reception capability. A value for CPS of 150 is the parameter, corresponding to its real reception capability. A value
default for the "text/t140" stream in the "text/rex" format. See RFC for "CPS" of 90 is the default for the "text/t140" stream in the
4103 [RFC4103] for the format and use of the CPS parameter. The same "text/red" format when multi-party real-time text is negotiated. See
rules apply for the "text/rex" format except for the default value. RFC 4103 [RFC4103] for the format and use of the CPS parameter. The
same rules apply for the multi-party case except for the default
value.
2.3. Mixing for multi-party unaware endpoints 2.2. Mixing for multi-party unaware endpoints
A method is specified in this section for cases when the A method is specified in this section for cases when the
participating endpoint does not implement any solution for multi- participating endpoint does not implement any solution for multi-
party presentation of real-time text. The solution requires the party presentation of real-time text. The solution requires the
mixer to insert text dividers and readable labels and only send text 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 from one source at a time until a suitable point appears for source
change. This solution is a fallback method with functional change. This solution is a fallback method with functional
limitations that acts on the presentation level and is further limitations that acts on the presentation level and is further
specified in Section 3.2. specified in Section 3.2.
skipping to change at page 38, line 19 skipping to change at page 22, line 4
|____________________|______________________|_______________________| |____________________|______________________|_______________________|
| | |I will arrive by TGV. | | | |I will arrive by TGV. |
|My flight is to Orly| |Convenient to the main | |My flight is to Orly| |Convenient to the main |
| |Hi all, can we plan |station. | | |Hi all, can we plan |station. |
| |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.
3.2. 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 "rtt-mix- When the mixer has indicated multi-party capability by the "rtt-mix-
rtp-mixer" sdp attribute or the "text/rex" format in an SDP rtp-mixer" sdp attribute in an SDP negotiation, but the multi-party
negotiation, but the multi-party capability negotiation fails with an capability negotiation fails with an endpoint, then the agreed "text/
endpoint, then the agreed "text/red" or "text/t140" format SHALL be red" or "text/t140" format SHALL be used and the mixer SHOULD compose
used and the mixer SHOULD compose a best-effort presentation of a best-effort presentation of multi-party real-time text in one
multi-party real-time text in one stream intended to be presented by stream intended to be presented by an endpoint with no multi-party
an endpoint with no multi-party awareness. 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 any multi-party deployed endpoints implementing only RFC 4103 without any multi-party
extensions 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.
skipping to change at page 39, line 40 skipping to change at page 23, line 29
When switching source, the mixer SHOULD insert a line separator if When switching source, the mixer SHOULD insert a line separator if
the already transmitted text did not end with a new line (line the already transmitted text did not end with a new line (line
separator or CRLF). A label SHOULD be composed from information in separator or CRLF). A label SHOULD be composed from information in
the CNAME and NAME fields in RTCP reports from the participant to the CNAME and NAME fields in RTCP reports from the participant to
have its text transmitted, or from other session information for that have its text transmitted, or from other session information for that
user. The label SHOULD be delimited by suitable characters (e.g. '[ user. The label SHOULD be delimited by suitable characters (e.g. '[
]') and transmitted. The CSRC SHOULD indicate the selected source. ]') and transmitted. The CSRC SHOULD indicate the selected source.
Then text from that selected participant SHOULD be transmitted until Then text from that selected participant SHOULD be transmitted until
a new suitable point for switching source is reached. a new suitable point for switching source is reached.
Integrity considerations SHALL be taken when composing the label.
Seeking a suitable point for switching source SHOULD be done when Seeking a suitable point for switching source SHOULD be done when
there is older text waiting for transmission from any party than the there is older text waiting for transmission from any party than the
age of the last transmitted text. Suitable points for switching are: age of the last transmitted text. Suitable points for switching are:
* A completed phrase ended by comma * A completed phrase ended by comma
* A completed sentence * A completed sentence
* A new line (line separator or CRLF) * A new line (line separator or CRLF)
skipping to change at page 45, line 8 skipping to change at page 28, line 27
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]
4.2. 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
channel[I-D.ietf-mmusic-t140-usage-data-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.
In WebRTC, it is assumed that for a multi-party session, one t140 In WebRTC, it is assumed that for a multi-party session, one t140
data channel is established for each source from a gateway or bridge data channel is established for each source from a gateway or bridge
to each participant. Each participant also has a data channel with to each participant. Each participant also has a data channel with
skipping to change at page 45, line 39 skipping to change at page 29, line 11
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.
5. Updates to RFC 4102 and RFC 4103 5. Updates to RFC 4103
This document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an sdp media attribute "rtt-mix-rtp-mixer" for
negotiation of multi-party mixing capability with the [RFC4103]
format and an extended packet format "text/rex" for the enhanced
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.
The update is in line with the statement in RFC 4103 section 4, This document updates RFC 4103[RFC4103] by introducing an sdp media
saying that "Forward Error Correction mechanisms, ..., or any other attribute "rtt-mix-rtp-mixer" for negotiation of multi-party mixing
mechanism with the purpose of increasing the reliability of text capability with the [RFC4103] format, and by specifying the rules for
transmission, MAY be used as an alternative or complement to packets when multi-party capability is negotiated and in use.
redundancy."
6. Congestion considerations 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 two seconds. increase the transmission interval up to two seconds.
If the unlikely situation appears that more than 20 participants in a
conference send text simultaneously, it will take more than 7 seconds
between presentation of text from each of these participants. More
time than that can cause confusion in the session. It is therefore
RECOMMENDED that the mixer discards such text in excess inserts a
general indication of possible text loss [T140ad1] in the session.
If the main text contributor is indicated in any way, the mixer MAY
avoid deleting text from that participant.
7. Acknowledgements 7. Acknowledgements
James Hamlin for format input. James Hamlin for format and performance aspects.
8. IANA Considerations 8. IANA Considerations
8.1. Registration of the "rtt-mix-rtp-mixer" sdp media attribute 8.1. Registration of the "rtt-mix-rtp-mixer" sdp media attribute
[RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
RFC number of this document.] RFC number of this document.]
IANA is asked to register the new sdp attribute "rtt-mix-rtp-mixer". IANA is asked to register the new sdp attribute "rtt-mix-rtp-mixer".
skipping to change at page 46, line 37 skipping to change at page 30, line 4
[RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
RFC number of this document.] RFC number of this document.]
IANA is asked to register the new sdp attribute "rtt-mix-rtp-mixer". IANA is asked to register the new sdp attribute "rtt-mix-rtp-mixer".
Contact name: IESG Contact name: IESG
Contact email: iesg@ietf.org Contact email: iesg@ietf.org
Attribute name: rtt-mix-rtp-mixer Attribute name: rtt-mix-rtp-mixer
Attribute syntax: a=rtt-mix-rtp-mixer Attribute syntax: a=rtt-mix-rtp-mixer
Attribute semantics: See RFCXXXX Section 2.1.1 Attribute semantics: See RFCXXXX Section 2.1.1
Attribute value: none Attribute value: none
Usage level: media Usage level: media
Purpose: Indicate support by mixer or endpoint of multi-party mixing Purpose: Indicate support by mixer and endpoint of multi-party
for real-time text transmission, using a common RTP-stream for mixing for real-time text transmission, using a common RTP-stream
transmission of text from a number of sources mixed with one 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 source at a time and the source indicated in a single CSRC-list
member. member.
Charset Dependent: no Charset Dependent: no
O/A procedure: See RFCXXXX Section 2.1.19
O/A procedure: See RFCXXXX Section 2.1.20
Mux Category: normal Mux Category: normal
Reference: RFCXXXX Reference: RFCXXXX
8.2. Registration of "text/rex" media subtype
The IANA is requested to register the media type "text/rex" as
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"
<http://www.iana.org/assignments/rtp-parameters>.
9. 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.
10. Change history 10. Change history
10.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07 10.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08
Deleted the method requiring a new packet format "text/rex" because
of the longer standardization and implementation period it needs.
Focus on use of RFC 4103 text/red format with shorter transmission
interval, and source indicated in CSRC.
10.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07
Added a method based on the "text/red" format and single source per Added a method based on the "text/red" format and single source per
packet, negotiated by the "rtt-mix-rtp-mixer" sdp attribute. packet, negotiated by the "rtt-mix-rtp-mixer" sdp attribute.
Added reasoning and recommendation about indication of loss. Added reasoning and recommendation about indication of loss.
The highest number of sources in one packet is 15, not 16. Changed. 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 Added in information on update to RFC 4103 that RFC 4103 explicitly
allows addition of FEC method. The redundancy is a kind of forward allows addition of FEC method. The redundancy is a kind of forward
error correction.. error correction..
10.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 10.3. 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.
10.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 10.4. 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.
10.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 10.5. 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.
10.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 10.6. 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
the mix. -done- the mix. -done-
In 3. correct the interval to: A "text/rex" transmitter SHOULD send In 3. correct the interval to: A "text/rex" transmitter SHOULD send
packets distributed in time as long as there is something (new or packets distributed in time as long as there is something (new or
redundant T140blocks) to transmit. The maximum transmission interval redundant T140blocks) to transmit. The maximum transmission interval
SHOULD then be 300 ms. It is RECOMMENDED to send a packet to a SHOULD then be 300 ms. It is RECOMMENDED to send a packet to a
receiver as soon as new text to that receiver is available, as long 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 as the time after the latest sent packet to the same receiver is more
than 150 ms, and also the maximum character rate to the receiver is than 150 ms, and also the maximum character rate to the receiver is
skipping to change at page 49, line 25 skipping to change at page 32, line 44
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-
10.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 10.7. 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.
10.7. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 10.8. 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.
10.8. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 10.9. 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.
10.9. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to 10.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02
-03 to -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.
10.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 10.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01
to -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
skipping to change at page 51, line 23 skipping to change at page 34, line 45
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.
10.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 10.12. 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
skipping to change at page 52, line 5 skipping to change at page 35, line 25
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,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
DOI 10.17487/RFC2198, September 1997,
<https://www.rfc-editor.org/info/rfc2198>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>. July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type", [RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type",
RFC 4102, DOI 10.17487/RFC4102, June 2005, RFC 4102, DOI 10.17487/RFC4102, June 2005,
<https://www.rfc-editor.org/info/rfc4102>. <https://www.rfc-editor.org/info/rfc4102>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<https://www.rfc-editor.org/info/rfc4103>. <https://www.rfc-editor.org/info/rfc4103>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566, Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>. July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<https://www.rfc-editor.org/info/rfc4855>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010, DOI 10.17487/RFC5764, May 2010,
<https://www.rfc-editor.org/info/rfc5764>. <https://www.rfc-editor.org/info/rfc5764>.
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263, Control Protocol (RTCP) Flows", RFC 6263,
DOI 10.17487/RFC6263, June 2011, DOI 10.17487/RFC6263, June 2011,
<https://www.rfc-editor.org/info/rfc6263>. <https://www.rfc-editor.org/info/rfc6263>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T. [RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
Stach, "An Opportunistic Approach for Secure Real-time Stach, "An Opportunistic Approach for Secure Real-time
Transport Protocol (OSRTP)", RFC 8643, Transport Protocol (OSRTP)", RFC 8643,
DOI 10.17487/RFC8643, August 2019, DOI 10.17487/RFC8643, August 2019,
<https://www.rfc-editor.org/info/rfc8643>. <https://www.rfc-editor.org/info/rfc8643>.
[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>.
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