draft-ietf-avtcore-multi-party-rtt-mix-08.txt   draft-ietf-avtcore-multi-party-rtt-mix-09.txt 
AVTCore G. Hellstrom AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: RFC 4103 (if approved) 12 August 2020 Updates: RFC 4103 (if approved) 13 October 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 13 February 2021 Expires: 16 April 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-08 draft-ietf-avtcore-multi-party-rtt-mix-09
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
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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.
The specified mechanism build on the standard use of the CSRC list in The specified mechanism build on the standard use of the CSRC list in
the RTP packet for source identification. The method makes use of the RTP packet for source identification. The method makes use of
the same "text/red" format as for two-party sessions. the same "text/red" format as for two-party sessions.
A capability exchange is specified so that it can be verified that a A capability exchange is specified so that it can be verified that a
participant can handle the multi-party coded real-time text stream. participant can handle the multi-party coded real-time text stream.
The capability is indicated by use of a media attribute "rtt-mix-rtp- The capability is indicated by use of a media attribute "rtt-mixer".
mixer".
The document updates RFC 4103[RFC4103] The document updates RFC 4103[RFC4103]
A specifications of how a mixer can format text for the case when the A specifications of how a mixer can format text for the case 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.
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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 February 2021. This Internet-Draft will expire on 16 April 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License. provided without warranty as described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Selected solution and considered alternative . . . . . . 4 1.1. Selected solution and considered alternative . . . . . . 5
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Intended application . . . . . . . . . . . . . . . . . . 7 1.3. Intended application . . . . . . . . . . . . . . . . . . 7
2. Specified solutions . . . . . . . . . . . . . . . . . . . . . 7 2. Specified solutions . . . . . . . . . . . . . . . . . . . . . 8
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 . . . . . . . . . . . . . . . . . . . . 8
2.2. Mixing for multi-party unaware endpoints . . . . . . . . 19 2.2. Mixing for multi-party unaware endpoints . . . . . . . . 8
3. Presentation level considerations . . . . . . . . . . . . . . 19 3. Details for the multi-party aware mixing case . . . . . . . . 8
3.1. Presentation by multi-party aware endpoints . . . . . . . 20 3.1. Offer/answer considerations . . . . . . . . . . . . . . . 9
3.2. Multi-party mixing for multi-party unaware endpoints . . 22 3.2. Use of fields in the RTP packets . . . . . . . . . . . . 9
4. Gateway Considerations . . . . . . . . . . . . . . . . . . . 27 3.3. Initial transmission of a BOM character . . . . . . . . . 10
4.1. Gateway considerations with Textphones (e.g. TTYs). . . 28 3.4. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Gateway considerations with WebRTC. . . . . . . . . . . . 28 3.5. Transmission interval . . . . . . . . . . . . . . . . . . 10
5. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 29 3.6. Only one source per packet . . . . . . . . . . . . . . . 10
6. Congestion considerations . . . . . . . . . . . . . . . . . . 29 3.7. Do not send received text to the originating source . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 3.8. Clean incoming text . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 3.9. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Registration of the "rtt-mix-rtp-mixer" sdp media 3.10. Text placement in packets . . . . . . . . . . . . . . . . 11
attribute . . . . . . . . . . . . . . . . . . . . . . . . 29 3.11. Source switching . . . . . . . . . . . . . . . . . . . . 11
3.12. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30 3.13. Creation of the redundancy . . . . . . . . . . . . . . . 12
10. Change history . . . . . . . . . . . . . . . . . . . . . . . 30 3.14. Timer offset fields . . . . . . . . . . . . . . . . . . . 12
10.1. Changes included in 3.15. Other RTP header fields . . . . . . . . . . . . . . . . . 12
draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 30 3.16. Pause in transmission . . . . . . . . . . . . . . . . . . 13
10.2. Changes included in 3.17. RTCP considerations . . . . . . . . . . . . . . . . . . . 13
3.18. Reception of multi-party contents . . . . . . . . . . . . 13
3.19. Performance considerations . . . . . . . . . . . . . . . 15
3.20. Security for session control and media . . . . . . . . . 15
3.21. SDP offer/answer examples . . . . . . . . . . . . . . . . 15
3.22. Packet sequence example from a source switch . . . . . . 17
3.23. Maximum character rate "CPS" . . . . . . . . . . . . . . 19
4. Presentation level considerations . . . . . . . . . . . . . . 19
4.1. Presentation by multi-party aware endpoints . . . . . . . 20
4.2. Multi-party mixing for multi-party unaware endpoints . . 22
5. Relation to Conference Control . . . . . . . . . . . . . . . 27
5.1. Use with SIP centralized conferencing framework . . . . . 28
5.2. Conference control . . . . . . . . . . . . . . . . . . . 28
6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 28
6.1. Gateway considerations with Textphones (e.g. TTYs). . . 28
6.2. Gateway considerations with WebRTC. . . . . . . . . . . . 29
7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 29
8. Congestion considerations . . . . . . . . . . . . . . . . . . 29
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
10.1. Registration of the "rtt-mixer" sdp media attribute . . 30
11. Security Considerations . . . . . . . . . . . . . . . . . . . 31
12. Change history . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 31
12.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 31
12.3. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 31
10.3. Changes included in 12.4. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 32
10.4. Changes included in 12.5. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 32
10.5. Changes included in 12.6. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 32
10.6. Changes included in 12.7. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 32
10.7. Changes included in 12.8. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 33
10.8. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 33 12.9. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 33
10.9. Changes from 12.10. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-03 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 33 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 34
10.10. Changes from
12.11. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 33 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.11. Changes from 12.12. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 34 -02 . . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.12. Changes from 12.13. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 34 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 35
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
11.1. Normative References . . . . . . . . . . . . . . . . . . 35 13.1. Normative References . . . . . . . . . . . . . . . . . . 35
11.2. Informative References . . . . . . . . . . . . . . . . . 36 13.2. Informative References . . . . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 37
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.
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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 sent so far has been sent one source and then make sure that all text sent so far has been sent
with 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 shortening the
interval to 100 ms, good performance is achieved for up to 3 transmission interval to 100 ms, good performance is achieved for up
simultaneously sending sources and usable performance for up to 5 to 3 simultaneously sending sources and usable performance for up to
simultaneously sending sources. Capability to use this method is 5 simultaneously sending sources. Capability to use this method is
indicated by an sdp media attribute "rtt-mix-rtp-mixer". indicated by an sdp media attribute "rtt-mixer".
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" for verification "text/red" with media attribute "rtt-mixer" for verification that the
that the parties are able to handle a multi-party coded stream and parties are able to handle a multi-party coded stream and agreeing on
agreeing on using that method. 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-rtp-mixer" negotiation results in "text/red" without the "rtt-mixer" attribute
attribute being the only common format for real-time text. being the only common format for real-time text.
The document updates RFC 4103[RFC4103] by introducing an attribute The document updates RFC 4103[RFC4103] by introducing an attribute
for indicating capability for the multi-party mixing case and rules for indicating capability for the multi-party mixing case and rules
for source indications and source switching. for source indications and source switching.
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
and easily implemented multi-party method for real-time text. This and easily implemented multi-party method for real-time text. This
section explains a few 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. "text/red" format.
From some points of view, use of multiple RTP streams, one for From some points of view, use of multiple RTP streams, one for
each source, sent in the same RTP session, called the RTP each source, sent in the same RTP session, called the RTP
translator model in [RFC3550], would be efficient, and use exactly translator model in [RFC3550], would be efficient, and use exactly
the same packet format as [RFC4103], the same payload type and a the same packet format as [RFC4103], the same payload type and a
simple SDP declaration. However, there is currently lack of simple SDP declaration. However, there is currently lack of
support for multi-stream RTP in certain implementation support for multi-stream RTP in certain implementation
technologies. This fact made it not included in this technologies. This fact made it not included in this document.
specification.
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. interval, and indicating source in CSRC.
The "text/red" format with "text/t140" payload in a single RTP The "text/red" format with "text/t140" payload in a single RTP
stream can be sent with 100 ms packet intervals instead of the stream can be sent with 100 ms packet intervals instead of the
regular 300 ms. The source is indicated in the CSRC field. regular 300 ms. The source is indicated in the CSRC field.
Source switching can then be done every 300 ms while simultaneous Source switching can then be done every 300 ms while simultaneous
transmission occurs. With two participants sending text transmission occurs. With two participants sending text
simultaneously, the switching and transmission performance is simultaneously, the switching and transmission performance is
good. With three simultaneously sending participants, there will good. With three simultaneously sending participants, there will
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period its text is sent by the mixer will have close to zero extra period its text is sent by the mixer will have close to zero extra
delay. Recent text will be presented with no or low delay. The delay. Recent text will be presented with no or low delay. The
1400 ms jerkiness will be noticable and slightly unpleasant, but 1400 ms jerkiness will be noticable and slightly unpleasant, but
corresponds in time to what typing humans often cause by corresponds in time to what typing humans often cause by
hesitation or changing position while typing. A benefit of this hesitation or changing position while typing. A benefit of this
method is that no new packet format needs to be introduced and method is that no new packet format needs to be introduced and
implemented. Since simultaneous typing by more than two parties implemented. Since simultaneous typing by more than two parties
is rare, and in most applications also more than three parties in is rare, and in most applications also more than three parties in
a call is rare, this method can be used successfully without its a call is rare, this method can be used successfully without its
limitations becoming annoying. Negotiation is based on a new sdp limitations becoming annoying. Negotiation is based on a new sdp
media attribute "rtt-mix-rtp-mixer". This method is selected to media attribute "rtt-mixer". This method is selected to be the
be the main one specified in this document. main one specified in this document.
A new "text" media subtype with up to 15 sources in each packet. A new "text" media subtype with up to 15 sources in each packet.
The mechanism makes use of the RTP mixer model specified in The mechanism makes use of the RTP mixer model specified in
RFC3550[RFC3550]. Text from up to 15 sources can be included in RFC3550[RFC3550]. Text from up to 15 sources can be included in
each packet. Packets are normally sent every 300 ms. The mean each packet. Packets are normally sent every 300 ms. The mean
delay will be 150 ms. The sources are indicated in strict order delay will be 150 ms. The sources are indicated in strict order
in the CSRC list of the RTP packets. A new redundancy packet in the CSRC list of the RTP packets. A new redundancy packet
format is specified. This method would result in good format is specified. This method would result in good
performance, but would require standardisation and implementation performance, but would require standardisation and implementation
of new releases in the target technologies that would take more of new releases in the target technologies that would take more
time than desirable to complete. It was therefore not selected to time than desirable to complete. It was therefore not selected to
be included in this specification. be included in this document.
The presentation planned by the mixer for multi-party unaware The presentation planned by the mixer for multi-party unaware
endpoints. endpoints.
It is desirable to have a method that does not require any It is desirable to have a method that does not require any
modifications in existing user devices implementing RFC 4103 for modifications in existing user devices implementing RFC 4103 for
RTT without explicit support of multi-party sessions. This is 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 This method is specified as a fallback method in this document.
specification.
RTT transport in WebRTC RTT transport in WebRTC
Transport of real-time text in the WebRTC technology is specified Transport of real-time text in the WebRTC technology is specified
to use the WebRTC data channel in to use the WebRTC data channel in
[I-D.ietf-mmusic-t140-usage-data-channel]. That spcification [I-D.ietf-mmusic-t140-usage-data-channel]. That spcification
contains a section briefly describing its use in multi-party contains a section briefly describing its use in multi-party
sessions. The focus of this specification is RTP transport. sessions. The focus of this document is RTP transport.
Therefore, even if the WebRTC transport provides good multi-party Therefore, even if the WebRTC transport provides good multi-party
performance, it is just mentioned in this specification in performance, it is just mentioned in this document in relation to
relation to providing gateways with multi-party capabilities providing gateways with multi-party capabilities between RTP and
between RTP and WebRTC technologies. 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]
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"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 method for multi-party real-time text documented in this The method for multi-party real-time text specified in this document
specification is primarily intended for use in transmission between is primarily intended for use in transmission between mixers and
mixers and endpoints in centralised mixing configurations. It is endpoints in centralised mixing configurations. It is also
also applicable between mixers. An often mentioned application is applicable between mixers. An often mentioned application is for
for emergency service calls with real-time text and voice, where a 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 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
skipping to change at page 7, line 43 skipping to change at page 8, line 17
is too much delayed from typing a letter to its presentation, then, is too much delayed from typing a letter to its presentation, then,
in some conference situations, the opportunity to comment will be in some conference situations, the opportunity to comment will be
gone and someone else will grab the turn. A delay of more than one gone and someone else will grab the turn. A delay of more than one
second in such situations is an obstacle for good conversation. second in such situations is an obstacle for good conversation.
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 The main purpose of this document is to specify the use of the
[RFC4103] for true multi-party real-time text. It is an update of current format for real-time text in [RFC4103] for true multi-party
RFC 4103 by a clarification on one way to use it in the multi-party real-time text. It is an update of RFC 4103 by a clarification on
situation. It is done by completing a negotiation for this kind of one way to use it in the multi-party situation. It is done by
multi-party capability and by indicating source in the CSRC element completing a negotiation for this kind of multi-party capability and
in the RTP packets. by indicating source in the CSRC element in the RTP packets.
Please use [RFC4103] as reference when reading the following The detailed procedures for the multi-party capable case are
description. specified in Section 3
2.1.1. Negotiation for use of this method Please use [RFC4103] as reference when reading the specification.
2.2. Mixing for multi-party unaware endpoints
A method is also specified in this document for cases when the
endpoint participating in a multi-party call does not implement any
solution for multi-party presentation of real-time text. The
solution requires the mixer to insert text dividers and readable
labels and only send text from one source at a time until a suitable
point appears for source change. This solution is a fallback method
with functional limitations that acts on the presentation level.
A party performing as a mixer, which has not negotiated the "rtt-
mixer" sdp media attribute, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD, if nothing else
is specified for the application, format transmitted text to that
participant to be suitable to present on a multi-party unaware
endpoint as further specified in Section 4.2.
3. Details for the multi-party aware mixing case
3.1. Offer/answer considerations
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 4103[RFC4103] by text transmission. This document updates RFC 4103[RFC4103] by
introducing a capability negotiation for handling multi-party real- introducing a capability negotiation for handling multi-party real-
time text. The capability negotiation is based on use of the sdp time text and a way to indicate the source of thansmitted text. The
media attribute "rtt-mix-rtp-mixer". capability negotiation is based on use of the sdp media attribute
"rtt-mixer".
The syntax is as follows: The syntax is as follows:
"a=rtt-mix-rtp-mixer" "a=rtt-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 set to 1. In all other cases, when the CC field in the RTP packet is set to 1. In all other cases,
the packets SHALL be populated and interpreted as for a two-party the packets SHALL be populated and interpreted as for a two-party
session. session.
2.1.2. Use of fields in the RTP packets A party which has negotiated the "rtt-mixer" sdp media attribute MUST
populate the CSRC-list and format the packets according to Section 3
if it acts as an rtp-mixer and sends multi-party text.
A party which has negotiated the "rtt-mixer" sdp media attribute MUST
interpret the contents of the "CC" field the CSRC-list and the
packets according to Section 3 in received rtp packets in the
corresponding RTP stream.
A party not performing as a mixer MUST not include the CSRC list.
3.2. 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
SHALL be one (1) in transmissions from a mixer involved in a multi- SHALL be one (1) in transmissions from a mixer involved in a multi-
party 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 SHALL be included in the packet. The single member in the CSRC-
SHALL contain the SSRC of the source of the T140blocks in the packet. list SHALL contain the SSRC of the source of the T140blocks in the
When redundancy is used, the recommended level of redundancy is to packet. When redundancy is used, the recommended level of redundancy
use one primary and two redundant generations of T140blocks. In some is to use one primary and two redundant generations of T140blocks.
cases, a primary or redundant T140block is empty, but is still In some cases, a primary or redundant T140block is empty, but is
represented by a member in the redundancy header. still 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 packets are equal to what
is specified in [RFC4103]. is specified in [RFC4103].
2.1.3. Transmission of multi-party contents 3.3. Initial transmission of a BOM character
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 Note that the BOM character SHALL be transmitted with the same
redundancy procedures as any other text.
3.4. 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 [RFC6263]. Recommendations for keep-alive can be found in [RFC6263].
2.1.5. Transmission interval 3.5. Transmission interval
A "text/red" transmitter in a mixer SHOULD send packets distributed A "text/red" transmitter in a mixer SHOULD send packets distributed
in time as long as there is something (new or redundant T140blocks) in time as long as there is something (new or redundant T140blocks)
to transmit. The maximum transmission interval SHOULD then be 300 to transmit. The maximum transmission interval SHOULD then be 300
ms. It is RECOMMENDED to send next packet to a receiver as soon as ms. It is RECOMMENDED to send next packet to a receiver as soon as
new text to that receiver is available, as long as the time after the new text to that receiver is available, as long as the time after the
latest sent packet to the same receiver is more than or equal to 100 latest sent packet to the same receiver is more than or equal to 100
ms, and also the maximum character rate to the receiver is not ms, and also the maximum character rate to the receiver is not
exceeded. The intention is to keep the latency low while keeping a exceeded. The intention is to keep the latency low while keeping a
good protection against text loss in bursty packet loss conditions. good protection against text loss in bursty packet loss conditions.
2.1.6. Only one source per packet 3.6. Only one source per packet
New and redundant text from one source MAY be transmitted in the same New and redundant text from one source MAY be transmitted in the same
packet. Text from different sources MUST NOT be transmitted in the packet. Text from different sources MUST NOT be transmitted in the
same packet. same packet.
2.1.7. Do not send received text to the originating source 3.7. Do not send received text to the originating source
Text received from a participant SHOULD NOT be included in Text received to a mixer from a participant SHOULD NOT be included in
transmission to that participant. transmission from the mixer to that participant.
2.1.8. Clean incoming text 3.8. Clean incoming text
A mixer SHALL handle reception and recovery of packet loss, marking A mixer SHALL handle reception, recovery of packet loss, marking of
of possible text loss and deletion of 'BOM' characters from each 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.
2.1.9. Redundancy 3.9. Redundancy
The transmitting party using redundancy SHALL send redundant A transmitting party using redundancy SHALL send redundant
repetitions of T140blocks aleady transmitted in earlier packets. repetitions of T140blocks aleady transmitted in earlier packets.
The 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 deduced 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.
2.1.10. Text placement in packets 3.10. 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 ("CPS") or the maximum packet allowed number of characters per second ("CPS") or the maximum packet
size. The SSRC of the source shall be placed as the only member in size. The SSRC of the source shall be placed as the only member in
the CSRC-list. the CSRC-list.
Note: The CSRC-list in an RTP packet only includes the participant 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 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 total list of participants in a conference. With audio and video
media, the CSRC-list would often contain all participants who are not media, the CSRC-list would often contain all participants who are not
muted whereas text participants that don't type are completely silent muted whereas text participants that don't type are completely silent
and thus are not represented in RTP packet CSRC-lists once their text and thus are not represented in RTP packet CSRC-lists.
have been transmitted as primary and the intended number of redundant
generations.
2.1.11. Source switching 3.11. 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 get turn to have its text sent, all intended
generations of the last text from the current source MUST be redundant generations of the last text from the current source MUST
transmitted before text from another source can be transmitted. be transmitted before text from another source is 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 text received 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.
2.1.12. Empty T140blocks 3.12. 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.
2.1.13. Creation of the redundancy The first packet in the session, the first after a source switch and
the first after a pause SHALL be poulated with the available
T140blocks for the source in turn to be sent as primary, and empty
T140blocks for the agreed number of redundancy generations.
3.13. 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.
2.1.14. Timer offset fields 3.14. Timer offset fields
The timestamp offset values are inserted in the redundancy header, The timestamp offset values are inserted in the redundancy header,
with the 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.
2.1.15. Other RTP header fields 3.15. 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].
2.1.16. Pause in transmission 3.16. 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.
2.1.17. RTCP considerations 3.17. 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.
Integrity considerations SHALL be considered when composing these Integrity considerations SHALL be considered when composing these
fields. fields.
2.1.18. Reception of multi-party contents 3.18. 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.
2.1.18.1. Multi-party vs two-party use 3.18.1. 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.
2.1.18.2. Level of redundancy 3.18.2. 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.
2.1.18.3. Extracting text and handling recovery and loss 3.18.3. 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 area (or
correspondingly for other receiver roles). correspondingly for other receiver roles).
If a sequence number gap appears and is still there after some If a sequence number gap appears and is still there after some
defined time for jitter resolution, T140data SHALL be recovered from defined time for jitter resolution, T140data SHALL be recovered from
redundant data. If the gap is wider than the number of generations redundant data. If the gap is wider than the number of generations
of redundant T140blocks in the packet, then a t140block SHALL be of redundant T140blocks in the packet and it is concluded that
created with a marker for possible text loss [T140ad1] and assigned t140blocks MAY have been lost, then a t140block SHALL be created with
to the SSRC of the transmitter as a general input from the mixer a marker for possible text loss [T140ad1] and assigned to the SSRC of
because in general it is not possible to deduct from which source(s) the transmitter as a general input from the mixer because in general
text was lost. It is in some cases possible to deduct that no text it is not possible to deduce from which source(s) text was lost. It
was lost even for a gap wider than the redundancy generations, and in is in some cases possible to deduce that no text was lost even for a
some cases it can be concluded which source that likely had loss. gap wider than the redundancy generations, and in some cases it can
Therefore, the receiver MAY insert the marker for possible text loss be concluded which source that likely had loss. Therefore, the
[T140ad1] in the presentation area corresponding to the source which receiver MAY insert the marker for possible text loss [T140ad1] in
may have had loss. the presentation area corresponding to the source which may have had
loss. See Section 3.22
Then, the T140block in the received packet SHALL be retrieved Then, the T140block in the received packet SHALL be retrieved
beginning with the highest redundant generation, and assigning it to beginning with the highest redundant generation, and assigning it to
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 and primary information and the CSRC-
way indicated above. list in the way indicated above.
2.1.18.4. Delete BOM 3.18.4. 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.
2.1.18.5. Empty T140blocks 3.18.5. 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.
2.1.19. Performance considerations 3.19. Performance considerations
This solution has good performance for 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 total 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. Only in large unmanaged conferences with a high number conferences. Only in large unmanaged conferences with a high number
of participants there may on very rare occasions appear situations of participants there may on very rare occasions appear situations
when many participants happen to send text simultaneously, resulting when many participants happen to send text simultaneously, resulting
in unpleasantly long switching times. It should be noted that it is in unpleasantly long switching times. It should be noted that it is
only the number of users sending text within the same moment that only the number of users sending text within the same moment that
causes jerkiness, not the total number of users with RTT capability. causes jerkiness, not the total number of users with RTT capability.
2.1.20. Offer/answer considerations 3.20. Security for session control and media
A party which has negotiated the "rtt-mix-rtp-mixer" sdp media
attribute MUST populate the CSRC-list and format the packets
according to this section if it acts as an rtp-mixer and sends multi-
party text.
A party which has negotiated the the "rtt-mix-rtp-mixer" sdp media
attribute MUST interpret the contents of the "CC" field the CSRC-list
and the packets according to this section in received rtp packets in
the corresponding RTP stream.
A party performing as a mixer, which has not negotiated the "rtt-mix-
rtp-mixer" sdp media attribute, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD, if nothing else
is specified for the application, format transmitted text to that
participant to be suitable to present on a multi-party unaware
endpoint as further specified in section Section 3.2.
A party not performing as a mixer MUST not include the CSRC list.
2.1.21. 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.
2.1.22. SDP offer/answer examples 3.21. 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-mixer
Answer example from a multi-party capable device Answer example from a multi-party capable device
m=text 14000 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-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-mixer
The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media
line still indicating RTP/AVP. line still indicating RTP/AVP.
Answer example from a multi-party capable device 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 16000 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-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
2.1.23. Packet sequence example from a source switch 3.22. Packet sequence example from a source switch
This example shows a symbolic flow of packets from a mixer including This example shows a symbolic flow of packets from a mixer including
loss and recovery. The sequence includes a source switch. A and B loss and recovery. The sequence includes a source switch. A and B
are sources of RTT. P indicates primary data. R1 is first redundant are sources of RTT. P indicates primary data. R1 is first redundant
generation data and R2 is second redundant generation data. A1, B1, generation data and R2 is second redundant generation data. A1, B1,
A2 etc are text chunks (T140blocks) received from the respective A2 etc are text chunks (T140blocks) received from the respective
sources. X indicates dropped packet between the mixer and a sources. X indicates dropped packet between the mixer and a
receiver. receiver. The session is assumed to use original and two redundant
generations of RTT.
|----------------| |----------------|
|Seq no 1 | |Seq no 101 |
|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 101 and assigned to
area A. The mixer is now assumed to have received text from source B reception area A. The mixer is now assumed to have received text
and need to prepare for sending that text. First it must send the from source B and need to prepare for sending that text. First it
redundant generations of text A2 and A3. must send the redundant generations of text A2 and A3.
|----------------| |----------------|
|Seq no 2 | |Seq no 102 |
|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. But the receiver
needs to remember temporarily that it was received.
X----------------| X----------------|
X Seq no 3 | X Seq no 103 |
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. It was the Packet 103 is assumed to be dropped in network problems. It was the
last packet with contents from A before the source switch. last packet with contents from A before the source switch.
X----------------| X----------------|
X Seq no 4 | X Seq no 104 |
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 104 contains text from B, assumed dropped in network
The mixer is assumed to have received text from A on turn to send. problems. The mixer is assumed to have received text from A
Sending of text from B must therefore be temporarily ended by on turn to send. Sending of text from B must therefore be
sending redundancy twice. temporarily ended by sending redundancy twice.
X----------------| X----------------|
X Seq no 5 | X Seq no 105 |
X CC=1 | X CC=1 |
X CSRC list B | X CSRC list B |
X R2: Empty | X R2: Empty |
X R1: B1 | X R1: B1 |
X P: Empty | X P: Empty |
X----------------| X----------------|
Packet 5 is assumed to be dropped in network problems Packet 105 is assumed to be dropped in network problems
|----------------| |----------------|
|Seq no 6 | |Seq no 106 |
|CC=1 | |CC=1 |
|CSRC list B | |CSRC list B |
| R2: B1 | | R2: B1 |
| R1: Empty | | R1: Empty |
| P: Empty | | P: Empty |
|----------------| |----------------|
Packet 6 is received. The latest received sequence number was 2. Packet 106 is received. The latest received sequence number was 102.
Recovery is therefore tried for 3,4,5. There is no coverage for seq Recovery is therefore tried for 103,104,105. There is no coverage
no 3. But knowing that A1 must have been sent as R2 in packet 3, it for seq no 103. But knowing that A3 was received as R1 in packet
can be concluded that nothing was lost. 102, it can be concluded that packet 103 just contained A3 as R2 and
A3 was already received and nothing was lost in packet 103.
For seqno 4, text B1 is recovered from the second generation For seqno 104, text B1 is recovered from the second generation
redundancy and appended to the reception area of B. For seqno 5, redundancy in packet 106 and appended to the reception area of B.
nothing needs to be recovered. No primary text is available in For seqno 105, nothing needs to be recovered. No primary text is
packet 6. available in packet 106.
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 102 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 101 and 102 were lost, there would be a need to
marker for possibly lost text (U'FFFD) [T140ad1], inserted generally create a marker for possibly lost text (U'FFFD) [T140ad1], inserted
and possibly also in text sequences A and B. generally and possibly also in text sequences A and B.
2.1.24. Use with SIP centralized conferencing framework
The SIP conferencing framework, mainly specified in RFC
4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
for coordinating sessions including multi-party RTT. The RTT stream
between the mixer and a participant is one and the same during the
conference. Participants get announced by notifications when
participants are joining or leaving, and further user information may
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
security purposes and for translation to a label for presentation to
other users.
2.1.25. Conference control
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
unmuting by the direction attributes in SDP [RFC4566].
Note that floor control functions may be of value for RTT users as The general conclusion is that if CSRC is equal before and after a
well as for users of other media in a conference. gap, only a gap of two can be recovered. If CSRC is different before
and after the gap, a gap of 4 can be recovered if the last packet
before the gap contained only R2 data, a gap of 3 can be recovered if
the last packet before the gap contained R1 data and a gap of 2 can
be recovered if the last packet received before the gap contained
primary data. For larger gaps than the above, a mark for possible
loss needs to be inserted in the received stream from the mixer.
2.1.26. Maximum character rate "CPS" 3.23. Maximum character rate "CPS"
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 occasionally have a real-time text from a number of sources may occasionally have a
higher combined flow of text coming from the sources. Endpoints higher combined flow of text coming from the sources. Endpoints
SHOULD therefore specify a suitable higher value for the CPS SHOULD therefore specify a suitable higher value for the CPS
parameter, corresponding to its real reception capability. A value parameter, corresponding to its real reception capability. A value
for "CPS" of 90 is the default for the "text/t140" stream in the for "CPS" of 90 is the default for the "text/t140" stream in the
"text/red" format when multi-party real-time text is negotiated. See "text/red" format when multi-party real-time text is negotiated. See
RFC 4103 [RFC4103] for the format and use of the CPS parameter. The 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 same rules apply for the multi-party case except for the default
value. value.
2.2. Mixing for multi-party unaware endpoints 4. Presentation level considerations
A method is specified in this section for cases when the
participating endpoint does not implement any solution for multi-
party presentation of real-time text. The solution requires the
mixer to insert text dividers and readable labels and only send text
from one source at a time until a suitable point appears for source
change. This solution is a fallback method with functional
limitations that acts on the presentation level and is further
specified in Section 3.2.
3. Presentation level considerations
ITU-T T.140 [T140] provides the presentation level requirements for ITU-T T.140 [T140] provides the presentation level requirements for
the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for
erasure and other formatting functions and has the following general erasure and other formatting functions and has the following general
statement for the presentation: statement for the presentation:
"The display of text from the members of the conversation should be "The display of text from the members of the conversation should be
arranged so that the text from each participant is clearly readable, arranged so that the text from each participant is clearly readable,
and its source and the relative timing of entered text is visualized and its source and the relative timing of entered text is visualized
in the display. Mechanisms for looking back in the contents from the in the display. Mechanisms for looking back in the contents from the
skipping to change at page 20, line 5 skipping to change at page 20, line 21
text to be grouped in presentation. The characters "CRLF" may be text to be grouped in presentation. The characters "CRLF" may be
used by other implementations as replacement for Line Separator. The used by other implementations as replacement for Line Separator. The
"CRLF" combination SHALL be erased by just one erasing action, just "CRLF" combination SHALL be erased by just one erasing action, just
as the Line Separator. Presentation functions are allowed to group as the Line Separator. Presentation functions are allowed to group
text for presentation in smaller groups than the line separators text for presentation in smaller groups than the line separators
imply and present such groups with source indication together with imply and present such groups with source indication together with
text groups from other sources (see the following presentation text groups from other sources (see the following presentation
examples). Erasure has no specific limit by any delimiter in the examples). Erasure has no specific limit by any delimiter in the
text stream. text stream.
3.1. Presentation by multi-party aware endpoints 4.1. Presentation by multi-party aware endpoints
A multi-party aware receiving party, presenting real-time text MUST A multi-party aware receiving party, presenting real-time text MUST
separate text from different sources and present them in separate separate text from different sources and present them in separate
presentation fields. The receiving party MAY separate presentation presentation fields. The receiving party MAY separate presentation
of parts of text from a source in readable groups based on other of parts of text from a source in readable groups based on other
criteria than line separator and merge these groups in the criteria than line separator and merge these groups in the
presentation area when it benefits the user to most easily find and presentation area when it benefits the user to most easily find and
read text from the different participants. The criteria MAY e.g. be read text from the different participants. The criteria MAY e.g. be
a received comma, full stop, or other phrase delimiters, or a long a received comma, full stop, or other phrase delimiters, or a long
pause. pause.
When text is received from multiple original sources simultaneously, When text is received from multiple original sources, the
the presentation SHOULD provide a view where text is added in presentation SHOULD provide a view where text is added in multiple
multiple places simultaneously. presentation fields.
If the presentation presents text from different sources in one If the presentation presents text from different sources in one
common area, the presenting endpoint SHOULD insert text from the common area, the presenting endpoint SHOULD insert text from the
local user ended at suitable points merged with received text to local user ended at suitable points merged with received text to
indicate the relative timing for when the text groups were completed. indicate the relative timing for when the text groups were completed.
In this presentation mode, the receiving endpoint SHALL present the In this presentation mode, the receiving endpoint SHALL present the
source of the different groups of text. source of the different groups of text.
A view of a three-party RTT call in chat style is shown in this A view of a three-party RTT call in chat style is shown in this
example . example .
skipping to change at page 22, line 7 skipping to change at page 22, line 7
| |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 4.2. 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-
rtp-mixer" sdp attribute in an SDP negotiation, but the multi-party mixer" sdp attribute in an SDP negotiation, but the multi-party
capability negotiation fails with an endpoint, then the agreed "text/ capability negotiation fails with an endpoint, then the agreed "text/
red" or "text/t140" format SHALL be used and the mixer SHOULD compose red" or "text/t140" format SHALL be used and the mixer SHOULD compose
a best-effort presentation of multi-party real-time text in one a best-effort presentation of multi-party real-time text in one
stream intended to be presented by an endpoint with no multi-party stream intended to be presented by 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.
skipping to change at page 22, line 38 skipping to change at page 22, line 38
simulated limited multi-party RTT view suitable for presentation in simulated limited multi-party RTT view suitable for presentation in
one presentation area. The mixer SHALL group text in suitable groups one presentation area. The mixer SHALL group text in suitable groups
and prepare for presentation of them by inserting a new line between and prepare for presentation of them by inserting a new line between
them if the transmitted text did not already end with a new line. A them if the transmitted text did not already end with a new line. A
presentable label SHOULD be composed and sent for the source presentable label SHOULD be composed and sent for the source
initially in the session and after each source switch. With this initially in the session and after each source switch. With this
procedure the time for source switching is depending on the actions procedure the time for source switching is depending on the actions
of the users. In order to expedite source switch, a user can for of the users. In order to expedite source switch, a user can for
example end its turn with a new line. example end its turn with a new line.
3.2.1. Actions by the mixer at reception from the call participants 4.2.1. Actions by the mixer at reception from the call participants
When text is received by the mixer from the different participants, When text is received by the mixer from the different participants,
the mixer SHALL recover text from redundancy if any packets are lost. the mixer SHALL recover text from redundancy if any packets are lost.
The mark for lost text [T140ad1] SHOULD be inserted in the stream if The mark for lost text [T140ad1] SHOULD be inserted in the stream if
unrecoverable loss appears. Any Unicode "BOM" characters, possibly unrecoverable loss appears. Any Unicode "BOM" characters, possibly
used for keep-alive shall be deleted. The time of creation of text used for keep-alive shall be deleted. The time of creation of text
(retrieved from the RTP timestamp) SHALL be stored together with the (retrieved from the RTP timestamp) SHALL be stored together with the
received text from each source in queues for transmission to the received text from each source in queues for transmission to the
recipients. recipients.
3.2.2. Actions by the mixer for transmission to the recipients 4.2.2. Actions by the mixer for transmission to the recipients
The following procedure SHOULD be applied for each recipient of The following procedure SHOULD be applied for each recipient of
multi-part text from the mixer. multi-part text from the mixer.
The text for transmission SHOULD be formatted by the mixer for each The text for transmission SHOULD be formatted by the mixer for each
receiving user for presentation in one single presentation area. receiving user for presentation in one single presentation area.
Text received from a participant SHOULD NOT be included in Text received from a participant SHOULD NOT be included in
transmission to that participant. When there is text available for transmission to that participant. When there is text available for
transmission from the mixer to a receiving party from more than one transmission from the mixer to a receiving party from more than one
participant, the mixer SHOULD switch between transmission of text participant, the mixer SHOULD switch between transmission of text
skipping to change at page 24, line 23 skipping to change at page 24, line 23
code stored as the status for the current source before the source code stored as the status for the current source before the source
switch is done, a reset of SGR shall be sent by the sequence SGR 0 switch is done, a reset of SGR shall be sent by the sequence SGR 0
[009B 0000 006D] after the new line and before the new label during a [009B 0000 006D] after the new line and before the new label during a
source switch. See SGR below for an explanation. This transmission source switch. See SGR below for an explanation. This transmission
does not influence the display count. does not influence the display count.
If there is an SGR code stored for the new source after the source If there is an SGR code stored for the new source after the source
switch, that SGR code SHOULD be transmitted to the recipient before switch, that SGR code SHOULD be transmitted to the recipient before
the label. This transmission does not influence the display count. the label. This transmission does not influence the display count.
3.2.3. Actions on transmission of text 4.2.3. Actions on transmission of text
Text from a source sent to the recipient SHOULD increase the display Text from a source sent to the recipient SHOULD increase the display
count by one per transmitted character. count by one per transmitted character.
3.2.4. Actions on transmission of control codes 4.2.4. Actions on transmission of control codes
The following control codes specified by T.140 require specific The following control codes specified by T.140 require specific
actions. They SHOULD cause specific considerations in the mixer. actions. They SHOULD cause specific considerations in the mixer.
Note that the codes presented here are expressed in UCS-16, while Note that the codes presented here are expressed in UCS-16, while
transmission is made in UTF-8 transform of these codes. transmission is made in UTF-8 transform of these codes.
BEL 0007 Bell Alert in session, provides for alerting during an BEL 0007 Bell Alert in session, provides for alerting during an
active session. The display count SHOULD not be altered. active session. The display count SHOULD not be altered.
NEW LINE 2028 Line separator. Check and perform a source switch if NEW LINE 2028 Line separator. Check and perform a source switch if
skipping to change at page 25, line 46 skipping to change at page 25, line 46
up to the end of the leading label after a source switch, then the up to the end of the leading label after a source switch, then the
mixer must not transmit more backspaces. Instead it is mixer must not transmit more backspaces. Instead it is
RECOMMENDED that a letter "X" is inserted in the text stream for RECOMMENDED that a letter "X" is inserted in the text stream for
each backspace as an indication of the intent to erase more. A each backspace as an indication of the intent to erase more. A
new line is usually coded by a Line Separator, but the character new line is usually coded by a Line Separator, but the character
combination "CRLF" MAY be used instead. Erasure of a new line is combination "CRLF" MAY be used instead. Erasure of a new line is
in both cases done by just one erasing action (Backspace). If the in both cases done by just one erasing action (Backspace). If the
display count has a positive value it is decreased by one when the display count has a positive value it is decreased by one when the
BS is sent. If the display count is at zero, it is not altered. BS is sent. If the display count is at zero, it is not altered.
3.2.5. Packet transmission 4.2.5. Packet transmission
A mixer transmitting to a multi-party unaware terminal SHOULD send A mixer transmitting to a multi-party unaware terminal SHOULD send
primary data only from one source per packet. The SSRC SHOULD be the primary data only from one source per packet. The SSRC SHOULD be the
SSRC of the mixer. The CSRC list SHOULD contain one member and be SSRC of the mixer. The CSRC list SHOULD contain one member and be
the SSRC of the source of the primary data. the SSRC of the source of the primary data.
3.2.6. Functional limitations 4.2.6. Functional limitations
When a multi-party unaware endpoint presents a conversation in one When a multi-party unaware endpoint presents a conversation in one
display area in a chat style, it inserts source indications for display area in a chat style, it inserts source indications for
remote text and local user text as they are merged in completed text remote text and local user text as they are merged in completed text
groups. When an endpoint using this layout receives and presents groups. When an endpoint using this layout receives and presents
text mixed for multi-party unaware endpoints, there will be two text mixed for multi-party unaware endpoints, there will be two
levels of source indicators for the received text; one generated by levels of source indicators for the received text; one generated by
the mixer and inserted in a label after each source switch, and the mixer and inserted in a label after each source switch, and
another generated by the receiving endpoint and inserted after each another generated by the receiving endpoint and inserted after each
switch between local and remote source in the presentation area. switch between local and remote source in the presentation area.
skipping to change at page 26, line 38 skipping to change at page 26, line 38
Text loss because of network errors may hit the label between entries Text loss because of network errors may hit the label between entries
from different parties, causing risk for misunderstanding from which from different parties, causing risk for misunderstanding from which
source a piece of text is. source a piece of text is.
These facts makes it strongly RECOMMENDED to implement multi-party These facts makes it strongly RECOMMENDED to implement multi-party
awareness in RTT endpoints. The use of the mixing method for multi- awareness in RTT endpoints. The use of the mixing method for multi-
party-unaware endpoints should be left for use with endpoints which party-unaware endpoints should be left for use with endpoints which
are impossible to upgrade to become multi-party aware. are impossible to upgrade to become multi-party aware.
3.2.7. Example views of presentation on multi-party unaware endpoints 4.2.7. Example views of presentation on multi-party unaware endpoints
The following pictures are examples of the view on a participant's The following pictures are examples of the view on a participant's
display for the multi-party-unaware case. display for the multi-party-unaware case.
_________________________________________________ _________________________________________________
| Conference | Alice | | Conference | Alice |
|________________________|_________________________| |________________________|_________________________|
| |I will arrive by TGV. | | |I will arrive by TGV. |
|[Bob]:My flight is to |Convenient to the main | |[Bob]:My flight is to |Convenient to the main |
|Orly. |station. | |Orly. |station. |
skipping to change at page 27, line 51 skipping to change at page 27, line 51
|[Eve] But I need to be back to | | |[Eve] But I need to be back to | |
| the hotel by 11 because I need | | | the hotel by 11 because I need | |
| |-| | |-|
|______________________________________________|v| |______________________________________________|v|
| of course, I underst | | of course, I underst |
|________________________________________________| |________________________________________________|
Figure 6: An example of a view of the multi-party unaware Figure 6: An example of a view of the multi-party unaware
presentation in chat style. Alice is the local user. presentation in chat style. Alice is the local user.
4. Gateway Considerations 5. Relation to Conference Control
4.1. Gateway considerations with Textphones (e.g. TTYs). 5.1. Use with SIP centralized conferencing framework
The SIP conferencing framework, mainly specified in RFC
4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
for coordinating sessions including multi-party RTT. The RTT stream
between the mixer and a participant is one and the same during the
conference. Participants get announced by notifications when
participants are joining or leaving, and further user information may
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
security purposes and for translation to a label for presentation to
other users.
5.2. Conference control
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
unmuting by the direction attributes in SDP [RFC4566].
Note that floor control functions may be of value for RTT users as
well as for users of other media in a conference.
6. Gateway Considerations
6.1. Gateway considerations with Textphones (e.g. TTYs).
Multi-party RTT sessions may involve gateways of different kinds. Multi-party RTT sessions may involve gateways of different kinds.
Gateways involved in setting up sessions SHALL correctly reflect the Gateways involved in setting up sessions SHALL correctly reflect the
multi-party capability or unawareness of the combination of the multi-party capability or unawareness of the combination of the
gateway and the remote endpoint beyond the gateway. gateway and the remote endpoint beyond the gateway.
One case that may occur is a gateway to PSTN for communication with One case that may occur is a gateway to PSTN for communication with
textphones (e.g. TTYs). Textphones are limited devices with no textphones (e.g. TTYs). Textphones are limited devices with no
multi-party awareness, and it SHOULD therefore be suitable for the multi-party awareness, and it SHOULD therefore be suitable for the
gateway to not indicate multi-party awareness for that case. Another gateway to not indicate multi-party awareness for that case. Another
solution is that the gateway indicates multi-party capability towards solution is that the gateway indicates multi-party capability towards
the mixer, and includes the multi-party mixer function for multi- the mixer, and includes the multi-party mixer function for multi-
party unaware endpoints itself. This solution makes it possible to party unaware endpoints itself. This solution makes it possible to
make adaptations for the functional limitations of the textphone make adaptations for the functional limitations of the textphone
(TTY). (TTY).
More information on gateways to textphones (TTYs) is found in RFC More information on gateways to textphones (TTYs) is found in RFC
5194[RFC5194] 5194[RFC5194]
4.2. Gateway considerations with WebRTC. 6.2. 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 In WebRTC, RTT is specified in
[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.
skipping to change at page 29, line 11 skipping to change at page 29, line 39
rtt, a new t140 channel SHOULD be established to WebRTC users with rtt, a new t140 channel SHOULD be established to WebRTC users with
the label parameter composed from the NAME field in RTCP on the RTP the label parameter composed from the NAME field in RTCP on the RTP
side. side.
When a new participant has entered the multi-party session with RTT When a new participant has entered the multi-party session with RTT
transport in a WebRTC t140 data channel, the new participant SHOULD transport in a WebRTC t140 data channel, the new participant SHOULD
be announced by a notification to RTP users. The label parameter be announced by a notification to RTP users. The label parameter
from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP
side, or other available session information. side, or other available session information.
5. Updates to RFC 4103 7. Updates to RFC 4103
This document updates RFC 4103[RFC4103] by introducing an sdp media This document updates RFC 4103[RFC4103] by introducing an sdp media
attribute "rtt-mix-rtp-mixer" for negotiation of multi-party mixing attribute "rtt-mixer" for negotiation of multi-party mixing
capability with the [RFC4103] format, and by specifying the rules for capability with the [RFC4103] format, and by specifying the rules for
packets when multi-party capability is negotiated and in use. packets when multi-party capability is negotiated and in use.
6. Congestion considerations 8. 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 If the unlikely situation appears that more than 20 participants in a
conference send text simultaneously, it will take more than 7 seconds conference send text simultaneously, it will take more than 7 seconds
between presentation of text from each of these participants. More between presentation of text from each of these participants. More
time than that can cause confusion in the session. It is therefore time than that can cause confusion in the session. It is therefore
RECOMMENDED that the mixer discards such text in excess inserts a RECOMMENDED that the mixer discards such text in excess inserts a
general indication of possible text loss [T140ad1] in the session. general indication of possible text loss [T140ad1] in the session.
If the main text contributor is indicated in any way, the mixer MAY If the main text contributor is indicated in any way, the mixer MAY
avoid deleting text from that participant. avoid deleting text from that participant.
7. Acknowledgements 9. Acknowledgements
James Hamlin for format and performance aspects. James Hamlin for format and performance aspects.
8. IANA Considerations 10. IANA Considerations
8.1. Registration of the "rtt-mix-rtp-mixer" sdp media attribute 10.1. Registration of the "rtt-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-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-mixer
Attribute syntax: a=rtt-mix-rtp-mixer
Attribute semantics: See RFCXXXX Section 2.1.1 Attribute semantics: See RFCXXXX Section 3.1
Attribute value: none Attribute value: none
Usage level: media Usage level: media
Purpose: Indicate support by mixer and endpoint of multi-party Purpose: Indicate support by mixer and endpoint of multi-party
mixing for real-time text transmission, using a common RTP-stream mixing for real-time text transmission, using a common RTP-stream
for 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 2.1.20 O/A procedure: See RFCXXXX Section 3.1
Mux Category: normal Mux Category: normal
Reference: RFCXXXX Reference: RFCXXXX
9. Security Considerations 11. 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 12. Change history
10.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08 12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09
Changed name on the SDP media attribute to "rtt-mixer"
Restructure of section 2 for balance between aware and unaware cases.
Moved conference control to own section.
Improved clarification of recovery and loss in the packet sequence
example.
A number of editorial corrections and improvements.
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08
Deleted the method requiring a new packet format "text/rex" because Deleted the method requiring a new packet format "text/rex" because
of the longer standardization and implementation period it needs. of the longer standardization and implementation period it needs.
Focus on use of RFC 4103 text/red format with shorter transmission Focus on use of RFC 4103 text/red format with shorter transmission
interval, and source indicated in CSRC. interval, and source indicated in CSRC.
10.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07 12.3. 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-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..
10.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 12.4. 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.
10.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 12.5. 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.
10.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 12.6. 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.
10.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 12.7. 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 32, line 44 skipping to change at page 33, line 36
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-
10.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 12.8. 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.
10.8. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 12.9. 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.
10.9. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 12.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03
draft-ietf-avtcore-multi-party-rtt-mix-00 to 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.
10.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 12.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02
to -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.
10.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 12.12. 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 34, line 45 skipping to change at page 35, line 36
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.
10.12. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 12.13. 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
11. References 13. References
11.1. Normative References 13.1. Normative References
[I-D.ietf-mmusic-t140-usage-data-channel] [I-D.ietf-mmusic-t140-usage-data-channel]
Holmberg, C. and G. Hellstrom, "T.140 Real-time Text Holmberg, C. and G. Hellstrom, "T.140 Real-time Text
Conversation over WebRTC Data Channels", Work in Progress, Conversation over WebRTC Data Channels", Work in Progress,
Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel- Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel-
14, 10 April 2020, <https://tools.ietf.org/html/draft- 14, 10 April 2020, <https://tools.ietf.org/html/draft-
ietf-mmusic-t140-usage-data-channel-14>. ietf-mmusic-t140-usage-data-channel-14>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
skipping to change at page 36, line 20 skipping to change at page 37, line 14
[T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
multimedia application text conversation", February 1998, multimedia application text conversation", February 1998,
<https://www.itu.int/rec/T-REC-T.140-199802-I/en>. <https://www.itu.int/rec/T-REC-T.140-199802-I/en>.
[T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
Protocol for multimedia application text conversation", Protocol for multimedia application text conversation",
February 2000, February 2000,
<https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>. <https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>.
11.2. Informative References 13.2. Informative References
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the [RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353, Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006, DOI 10.17487/RFC4353, February 2006,
<https://www.rfc-editor.org/info/rfc4353>. <https://www.rfc-editor.org/info/rfc4353>.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, DOI 10.17487/RFC4575, August Conference State", RFC 4575, DOI 10.17487/RFC4575, August
2006, <https://www.rfc-editor.org/info/rfc4575>. 2006, <https://www.rfc-editor.org/info/rfc4575>.
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