draft-ietf-avtcore-multi-party-rtt-mix-16.txt   draft-ietf-avtcore-multi-party-rtt-mix-17.txt 
AVTCore G. Hellstrom AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: 4103 (if approved) 1 May 2021 Updates: 4103 (if approved) 7 May 2021
Intended status: Standards Track Intended status: Standards Track
Expires: 2 November 2021 Expires: 8 November 2021
RTP-mixer formatting of multi-party Real-time text RTP-mixer formatting of multiparty Real-time text
draft-ietf-avtcore-multi-party-rtt-mix-16 draft-ietf-avtcore-multi-party-rtt-mix-17
Abstract Abstract
Enhancements for RFC 4103 real-time text mixing are provided in this This document provides enhancements for RFC 4103 real-time text
document, suitable for a centralized conference model that enables mixing suitable for a centralized conference model that enables
source identification and rapidly interleaved transmission of text source identification and rapidly interleaved transmission of text
from different sources. The intended use is for real-time text from different sources. The intended use is for real-time text
mixers and participant endpoints capable of providing an efficient mixers and participant endpoints capable of providing an efficient
presentation or other treatment of a multi-party real-time text presentation or other treatment of a multiparty real-time text
session. The specified mechanism builds on the standard use of the session. The specified mechanism builds on the standard use of the
CSRC list in the RTP packet for source identification. The method CSRC list in the RTP packet for source identification. The method
makes use of the same "text/t140" and "text/red" formats as for two- makes use of the same "text/t140" and "text/red" formats as for two-
party sessions. party sessions.
Solutions using multiple RTP streams in the same RTP session are Solutions using multiple RTP streams in the same RTP session are
briefly mentioned, as they could have some benefits over the RTP- briefly mentioned, as they could have some benefits over the RTP-
mixer model. The possibility to implement the solution in a wide mixer model. The possibility to implement the solution in a wide
range of existing RTP implementations made the RTP-mixer model be range of existing RTP implementations made the RTP-mixer model be
selected to be fully specified in this document. selected to be fully specified in this document.
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
mixer and a participant can handle the multi-party coded real-time mixer and a participant can handle the multiparty-coded real-time
text stream using the RTP-mixer method. The capability is indicated text stream using the RTP-mixer method. The capability is indicated
by use of an SDP media attribute "rtt-mixer". by use of an SDP media attribute "rtt-mixer".
The document updates RFC 4103 "RTP Payload for Text Conversation". The document updates RFC 4103 "RTP Payload for Text Conversation".
A specification of how a mixer can format text for the case when the A specification of how a mixer can format text for the case when the
endpoint is not multi-party aware is also provided. endpoint is not multiparty-aware is also provided.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 November 2021. This Internet-Draft will expire on 8 November 2021.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Selected solution and considered alternatives . . . . . . 6 1.2. Selected solution and considered alternatives . . . . . . 7
1.3. Intended application . . . . . . . . . . . . . . . . . . 9 1.3. Intended application . . . . . . . . . . . . . . . . . . 9
2. Overview of the two specified solutions and selection of 2. Overview of the two specified solutions and selection of
method . . . . . . . . . . . . . . . . . . . . . . . . . 10 method . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1. The RTP-mixer based solution for multi-party aware 2.1. The RTP-mixer-based solution for multiparty-aware
endpoints . . . . . . . . . . . . . . . . . . . . . . . . 10 endpoints . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2. Mixing for multi-party unaware endpoints . . . . . . . . 10 2.2. Mixing for multiparty-unaware endpoints . . . . . . . . . 11
2.3. Offer/answer considerations . . . . . . . . . . . . . . . 11 2.3. Offer/answer considerations . . . . . . . . . . . . . . . 11
2.4. Actions depending on capability negotiation result . . . 12 2.4. Actions depending on capability negotiation result . . . 13
3. Details for the RTP-mixer based mixing method for multi-party 3. Details for the RTP-mixer-based mixing method for
aware endpoints . . . . . . . . . . . . . . . . . . . . . 13 multiparty-aware endpoints . . . . . . . . . . . . . . . 13
3.1. Use of fields in the RTP packets . . . . . . . . . . . . 13 3.1. Use of fields in the RTP packets . . . . . . . . . . . . 13
3.2. Initial transmission of a BOM character . . . . . . . . . 13 3.2. Initial transmission of a BOM character . . . . . . . . . 14
3.3. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 14 3.3. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 14
3.4. Transmission interval . . . . . . . . . . . . . . . . . . 14 3.4. Transmission interval . . . . . . . . . . . . . . . . . . 14
3.5. Only one source per packet . . . . . . . . . . . . . . . 14 3.5. Only one source per packet . . . . . . . . . . . . . . . 15
3.6. Do not send received text to the originating source . . . 14 3.6. Do not send received text to the originating source . . . 15
3.7. Clean incoming text . . . . . . . . . . . . . . . . . . . 15 3.7. Clean incoming text . . . . . . . . . . . . . . . . . . . 15
3.8. Redundant transmission principles . . . . . . . . . . . . 15 3.8. Redundant transmission principles . . . . . . . . . . . . 15
3.9. Interleaving text from different sources . . . . . . . . 15 3.9. Interleaving text from different sources . . . . . . . . 15
3.10. Text placement in packets . . . . . . . . . . . . . . . . 15 3.10. Text placement in packets . . . . . . . . . . . . . . . . 16
3.11. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 16 3.11. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 16
3.12. Creation of the redundancy . . . . . . . . . . . . . . . 16 3.12. Creation of the redundancy . . . . . . . . . . . . . . . 17
3.13. Timer offset fields . . . . . . . . . . . . . . . . . . . 17 3.13. Timer offset fields . . . . . . . . . . . . . . . . . . . 17
3.14. Other RTP header fields . . . . . . . . . . . . . . . . . 17 3.14. Other RTP header fields . . . . . . . . . . . . . . . . . 17
3.15. Pause in transmission . . . . . . . . . . . . . . . . . . 17 3.15. Pause in transmission . . . . . . . . . . . . . . . . . . 18
3.16. RTCP considerations . . . . . . . . . . . . . . . . . . . 17 3.16. RTCP considerations . . . . . . . . . . . . . . . . . . . 18
3.17. Reception of multi-party contents . . . . . . . . . . . . 18 3.17. Reception of multiparty contents . . . . . . . . . . . . 18
3.18. Performance considerations . . . . . . . . . . . . . . . 20 3.18. Performance considerations . . . . . . . . . . . . . . . 20
3.19. Security for session control and media . . . . . . . . . 20 3.19. Security for session control and media . . . . . . . . . 20
3.20. SDP offer/answer examples . . . . . . . . . . . . . . . . 21 3.20. SDP offer/answer examples . . . . . . . . . . . . . . . . 21
3.21. Packet sequence example from interleaved transmission . . 22 3.21. Packet sequence example from interleaved transmission . . 22
3.22. Maximum character rate "CPS" . . . . . . . . . . . . . . 25 3.22. Maximum character rate "CPS" . . . . . . . . . . . . . . 25
4. Presentation level considerations . . . . . . . . . . . . . . 25 4. Presentation level considerations . . . . . . . . . . . . . . 25
4.1. Presentation by multi-party aware endpoints . . . . . . . 26 4.1. Presentation by multiparty-aware endpoints . . . . . . . 26
4.2. Multi-party mixing for multi-party unaware endpoints . . 28 4.2. multiparty mixing for multiparty-unaware endpoints . . . 28
5. Relation to Conference Control . . . . . . . . . . . . . . . 34 5. Relation to Conference Control . . . . . . . . . . . . . . . 34
5.1. Use with SIP centralized conferencing framework . . . . . 34 5.1. Use with SIP centralized conferencing framework . . . . . 34
5.2. Conference control . . . . . . . . . . . . . . . . . . . 34 5.2. Conference control . . . . . . . . . . . . . . . . . . . 35
6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 34 6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 35
6.1. Gateway considerations with Textphones (e.g. TTYs). . . 34 6.1. Gateway considerations with Textphones . . . . . . . . . 35
6.2. Gateway considerations with WebRTC. . . . . . . . . . . . 35 6.2. Gateway considerations with WebRTC . . . . . . . . . . . 35
7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 36 7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 36
8. Congestion considerations . . . . . . . . . . . . . . . . . . 36 8. Congestion considerations . . . . . . . . . . . . . . . . . . 36
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
10.1. Registration of the "rtt-mixer" SDP media attribute . . 36 10.1. Registration of the "rtt-mixer" SDP media attribute . . 37
11. Security Considerations . . . . . . . . . . . . . . . . . . . 37 11. Security Considerations . . . . . . . . . . . . . . . . . . . 38
12. Change history . . . . . . . . . . . . . . . . . . . . . . . 38 12. Change history . . . . . . . . . . . . . . . . . . . . . . . 38
12.1. Changes included in 12.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-16 . . . . . . . 38 draft-ietf-avtcore-multi-party-rtt-mix-17 . . . . . . . 38
12.2. Changes included in 12.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-15 . . . . . . . 38 draft-ietf-avtcore-multi-party-rtt-mix-16 . . . . . . . 38
12.3. Changes included in 12.3. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-14 . . . . . . . 38 draft-ietf-avtcore-multi-party-rtt-mix-15 . . . . . . . 38
12.4. Changes included in 12.4. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-13 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-14 . . . . . . . 39
12.5. Changes included in 12.5. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-12 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-13 . . . . . . . 39
12.6. Changes included in 12.6. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-11 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-12 . . . . . . . 39
12.7. Changes included in 12.7. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-10 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-11 . . . . . . . 40
12.8. Changes included in 12.8. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-10 . . . . . . . 40
12.9. Changes included in 12.9. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 40
12.10. Changes included in 12.10. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 40
12.11. Changes included in 12.11. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 41
12.12. Changes included in 12.12. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 41
12.13. Changes included in 12.13. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 41
12.14. Changes included in 12.14. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 41
12.15. Changes included in 12.15. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 42 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 41
12.16. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 42 12.16. Changes included in
12.17. Changes from draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 43
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 12.17. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 43
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 42
12.18. Changes from 12.18. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 43
12.19. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 43 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 43
12.19. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 43
12.20. Changes from 12.20. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 44
12.21. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 44 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 45
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 44 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
13.1. Normative References . . . . . . . . . . . . . . . . . . 44 13.1. Normative References . . . . . . . . . . . . . . . . . . 45
13.2. Informative References . . . . . . . . . . . . . . . . . 46 13.2. Informative References . . . . . . . . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 46 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 47
1. Introduction 1. Introduction
"RTP Payload for Text Conversation" [RFC4103] specifies use of RTP "RTP Payload for Text Conversation" [RFC4103] specifies use of RTP
[RFC3550] for transmission of real-time text (RTT) and the "text/ [RFC3550] for transmission of real-time text (RTT) and the "text/
t140" format. It also specifies a redundancy format "text/red" for t140" format. It also specifies a redundancy format "text/red" for
increased robustness. The "text/red" format is registered in increased robustness. The "text/red" format is registered in
[RFC4102]. [RFC4102].
Real-time text is usually provided together with audio and sometimes Real-time text is usually provided together with audio and sometimes
with video in conversational sessions. with video in conversational sessions.
A requirement related to multi-party sessions from the presentation A requirement related to multiparty sessions from the presentation
level standard T.140 [T140] for real-time text is: "The display of level standard T.140 [T140] for real-time text is: "The display of
text from the members of the conversation should be arranged so that text from the members of the conversation should be arranged so that
the text from each participant is clearly readable, and its source the text from each participant is clearly readable, and its source
and the relative timing of entered text is visualized in the and the relative timing of entered text is visualized in the
display." display."
Another requirement is that the mixing procedure must not introduce Another requirement is that the mixing procedure must not introduce
delays in the text streams that are experienced to be disturbing the delays in the text streams that are experienced to be disturbing the
real-time experience of the receiving users. real-time experience of the receiving users.
Use of RTT is increasing, and specifically, use in emergency calls is Use of RTT is increasing, and specifically, use in emergency calls is
increasing. Emergency call use requires multi-party mixing. RFC increasing. Emergency call use requires multiparty mixing. RFC 4103
4103 "RTP Payload for Text Conversation" mixer implementations can "RTP Payload for Text Conversation" mixer implementations can use
use traditional RTP functions for source identification, but the traditional RTP functions for source identification, but the
performance of the mixer when giving turns for the different sources performance of the mixer when giving turns for the different sources
to transmit is limited when using the default transmission to transmit is limited when using the default transmission
characteristics with redundancy. characteristics with redundancy.
The redundancy scheme of [RFC4103] enables efficient transmission of The redundancy scheme of [RFC4103] enables efficient transmission of
earlier transmitted redundant text in packets together with new text. earlier transmitted redundant text in packets together with new text.
However the redundancy header format has no source indicators for the However the redundancy header format has no source indicators for the
redundant transmissions. The redundant parts in a packet must redundant transmissions. The redundant parts in a packet must
therefore be from the same source as the new text. The recommended therefore be from the same source as the new text. The recommended
transmission is one new and two redundant generations of text transmission is one new and two redundant generations of text
(T140blocks) in each packet and the recommended transmission interval (T140blocks) in each packet and the recommended transmission interval
for two-party use is 300 ms. for two-party use is 300 ms.
Real-time text mixers for multi-party sessions need to include the Real-time text mixers for multiparty sessions need to include the
source with each transmitted group of text from a conference source with each transmitted group of text from a conference
participant so that the text can be transmitted interleaved with text participant so that the text can be transmitted interleaved with text
groups from different sources in the rate they are created. This groups from different sources at the rate they are created. This
enables the text groups to be presented by endpoints in suitable enables the text groups to be presented by endpoints in suitable
grouping with other text from the same source. grouping with other text from the same source.
The presentation can then be arranged so that text from different The presentation can then be arranged so that text from different
sources can be presented in real-time and easily read. At the same sources can be presented in real-time and easily read. At the same
time it is possible for a reading user to perceive approximately when time it is possible for a reading user to perceive approximately when
the text was created in real time by the different parties. The the text was created in real time by the different parties. The
transmission and mixing is intended to be done in a general way so transmission and mixing is intended to be done in a general way so
that presentation can be arranged in a layout decided by the that presentation can be arranged in a layout decided by the
endpoint. endpoint.
There are existing implementations of RFC 4103 in endpoints without There are existing implementations of RFC 4103 in endpoints without
the updates from this document. These will not be able to receive the updates from this document. These will not be able to receive
and present real-time text mixed for multi-party aware endpoints. and present real-time text mixed for multiparty-aware endpoints.
A negotiation mechanism is therefore needed for verification if the A negotiation mechanism is therefore needed for verification if the
parties are able to handle a common method for multi-party parties are able to handle a common method for multiparty
transmission and agreeing on using that method. transmission and agreeing on using that method.
A fall-back mixing procedure is also needed for cases when the A fallback mixing procedure is also needed for cases when the
negotiation result indicates that a receiving endpoint is not capable negotiation result indicates that a receiving endpoint is not capable
of handling the mixed format. Multi-party unaware endpoints would of handling the mixed format. Multiparty-unaware endpoints would
possibly otherwise present all received multi-party mixed text as if possibly otherwise present all received multiparty mixed text as if
it came from the same source regardless of any accompanying source it came from the same source regardless of any accompanying source
indication coded in fields in the packet. Or they may have any other indication coded in fields in the packet. Or they may have other
undesirable way of acting on the multi-party content. The fall-back undesirable ways of acting on the multiparty content. The fallback
method is called the mixing procedure for multi-party unaware method is called the mixing procedure for multiparty-unaware
endpoints. The fall-back method is naturally not expected to meet endpoints. The fallback method is naturally not expected to meet all
all performance requirements placed on the mixing procedure for performance requirements placed on the mixing procedure for
multi-party aware endpoints. multiparty-aware endpoints.
The document updates [RFC4103] by introducing an attribute for The document updates [RFC4103] by introducing an attribute for
indicating capability for the RTP-mixer based multi-party mixing case indicating capability for the RTP-mixer-based multiparty mixing case
and rules for source indications and interleaving of text from and rules for source indications and interleaving of text from
different sources. different sources.
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown above. capitals, as shown above.
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 defined in [RFC3550]. mixer, RTP-translator are defined in [RFC3550].
The term "T140block" is defined in [RFC4103] to contain one or more The term "T140block" is defined in [RFC4103] to contain one or more
T.140 code elements. T.140 code elements.
"TTY" stands for a text telephone type used in North America. "TTY" stands for a textphone type used in North America.
"WebRTC" stands for web based communication specified by W3C and "WebRTC" stands for web-based real-time communication specified by
IETF. See [RFC8825]. W3C and IETF. See [RFC8825].
"DTLS-SRTP" stands for security specified in [RFC5764]. "DTLS-SRTP" is a DTLS extension for use with SRTP/SRTCP specified in
[RFC5764].
"multi-party aware" stands for an endpoint receiving real-time text "multiparty-aware" describes an endpoint receiving real-time text
from multiple sources through a common conference mixer being able to from multiple sources through a common conference mixer being able to
present the text in real-time separated by source and presented so present the text in real-time, separated by source, and presented so
that a user can get an impression of the approximate relative timing that a user can get an impression of the approximate relative timing
of text from different parties. of text from different parties.
"multi-party unaware" stands for an endpoint not itself being able to "multiparty-unaware" describes an endpoint not itself being able to
separate text from different sources when received through a common separate text from different sources when received through a common
conference mixer. conference mixer.
1.2. Selected solution and considered alternatives 1.2. Selected solution and considered alternatives
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 multiparty method for real-time text. This
section explains a few of them briefly. section explains a few of them briefly.
Multiple RTP streams, one per participant. Multiple RTP streams, one per participant
One RTP stream per source would be sent in the same RTP session One RTP stream per source would be sent in the same RTP session
with the "text/red" format. From some points of view, use of with the "text/red" format. From some points of view, use of
multiple RTP streams, one for each source, sent in the same RTP multiple RTP streams, one for each source, sent in the same RTP
session would be efficient, and would use exactly the same packet session would be efficient, and would use exactly the same packet
format as [RFC4103] and the same payload type. A couple of format as [RFC4103] and the same payload type. A couple of
relevant scenarios using multiple RTP-streams are specified in relevant scenarios using multiple RTP-streams are specified in
"RTP Topologies" [RFC7667]. One possibility of special interest "RTP Topologies" [RFC7667]. One possibility of special interest
is the Selective Forwarding Middlebox (SFM) topology specified in is the Selective Forwarding Middlebox (SFM) topology specified in
RFC 7667 section 3.7 that could enable end to end encryption. In RFC 7667 section 3.7 that could enable end-to-end encryption. In
contrast to audio and video, real-time text is only transmitted contrast to audio and video, real-time text is only transmitted
when the users actually transmit information. Thus an SFM when the users actually transmit information. Thus an SFM
solution would not need to exclude any party from transmission solution would not need to exclude any party from transmission
under normal conditions. In order to allow the mixer to convey under normal conditions. In order to allow the mixer to convey
the packets with the payload preserved and encrypted, an SFM the packets with the payload preserved and encrypted, an SFM
solution would need to act on some specific characteristics of the solution would need to act on some specific characteristics of the
"text/red" format. The redundancy headers are part of the "text/red" format. The redundancy headers are part of the
payload, so the receiver would need to just assume that the payload, so the receiver would need to just assume that the
payload type number in the redundancy header is for "text/t140". payload type number in the redundancy header is for "text/t140".
The characters per second parameter (CPS) would need to act per The characters per second parameter (CPS) would need to act per
stream. The relation between the SSRC and the source would need stream. The relation between the SSRC and the source would need
to be conveyed in some specified way, e.g. in the CSRC. Recovery to be conveyed in some specified way, e.g., in the CSRC. Recovery
and loss detection would preferably be based on sequence number and loss detection would preferably be based on sequence number
gap detection. Thus sequence number gaps in the incoming stream gap detection. Thus sequence number gaps in the incoming stream
to the mixer would need to be reflected in the stream to the to the mixer would need to be reflected in the stream to the
participant and no new gaps created by the mixer. However, the participant, with no new gaps created by the mixer. However, the
RTP implementation in both mixers and endpoints need to support RTP implementation in both mixers and endpoints need to support
multiple streams in the same RTP session in order to use this multiple streams in the same RTP session in order to use this
mechanism. For best deployment opportunity, it should be possible mechanism. For best deployment opportunity, it should be possible
to upgrade existing endpoint solutions to be multi-party aware to upgrade existing endpoint solutions to be multiparty-aware with
with a reasonable effort. There is currently a lack of support a reasonable effort. There is currently a lack of support for
for multi-stream RTP in certain implementation technologies. This multi-stream RTP in certain implementations. This fact led to
fact made this solution only briefly mentioned in this document as this solution being only briefly mentioned in this document as an
an option for further study. option for further study.
RTP-mixer based method for multi-party aware endpoints. RTP-mixer-based method for multiparty-aware endpoints
The "text/red" format in RFC 4103 is sent with shorter The "text/red" format in RFC 4103 is sent with a shorter
transmission interval with the RTP-mixer method and indicating transmission interval with the RTP-mixer method and indicating the
source in CSRC. The "text/red" format with "text/t140" payload in source in the CSRC field. The "text/red" format with a "text/
a single RTP stream can be sent when text is available from the t140" payload in a single RTP stream can be sent when text is
call participants instead of at the regular 300 ms. The source is available from the call participants instead of at the regular 300
indicated in the CSRC field. Transmission of packets with text ms. Transmission of packets with text from different sources can
from different sources can then be done smoothly while then be done smoothly while simultaneous transmission occurs as
simultaneous transmission occurs as long as it is not limited by long as it is not limited by the maximum character rate "CPS".
the maximum character rate "CPS". With ten participants sending With ten participants sending text simultaneously, the switching
text simultaneously, the switching and transmission performance is and transmission performance is good. With more simultaneously
good. With more simultaneously sending participants, and sending participants, and with receivers having the default
receivers with default capacity there will be a noticeable capacity there will be a noticeable jerkiness and delay in text
jerkiness and delay in text presentation. The jerkiness will be presentation. The jerkiness will be more expressed the more
more expressed the more participants who send text simultaneously. participants who send text simultaneously. Two seconds jerkiness
Two seconds jerkiness will be noticeable and slightly unpleasant, will be noticeable and slightly unpleasant, but it corresponds in
but corresponds in time to what typing humans often cause by time to what typing humans often cause by hesitation or changing
hesitation or changing position while typing. A benefit of this position while typing. A benefit of this method is that no new
method is that no new packet format needs to be introduced and packet format needs to be introduced and implemented. Since
implemented. Since simultaneous typing by more than two parties simultaneous typing by more than two parties is very rare, this
is very rare, this method can be used successfully with good method can be used successfully with good performance. Recovery
performance. Recovery of text in case of packet loss is based on of text in case of packet loss is based on analysis of timestamps
analysis of timestamps of received redundancy versus earlier of received redundancy versus earlier received text. Negotiation
received text. Negotiation is based on a new SDP media attribute is based on a new SDP media attribute "rtt-mixer". This method is
"rtt-mixer". This method is selected to be the main one specified selected to be the main one specified in this document.
in this document.
Multiple sources per packet. Multiple sources per packet
A new "text" media subtype would be specified with up to 15 A new "text" media subtype would be specified with up to 15
sources in each packet. The mechanism would make use of the RTP sources in each packet. The mechanism would make use of the RTP
mixer model specified in RTP [RFC3550]. Text from up to 15 mixer model specified in RTP [RFC3550]. Text from up to 15
sources can be included in each packet. Packets are normally sent sources can be included in each packet. Packets are normally sent
every 300 ms. The mean delay will be 150 ms. The sources are with 300 ms intervals. The mean delay will be 150 ms. The
indicated in strict order in the CSRC list of the RTP packets. A sources are indicated in strict order in the CSRC list of the RTP
new redundancy packet format is specified. This method would packets. A new redundancy packet format is specified. This
result in good performance, but would require standardisation and method would result in good performance, but would require
implementation of new releases in the target technologies that standardisation and implementation of new releases in the target
would take more time than desirable to complete. It was therefore technologies that would take more time than desirable to complete.
not selected to be included in this document. It was therefore not selected to be included in this document.
Mixing for multi-party unaware endpoints Mixing for multiparty-unaware endpoints
Presentation of text from multiple parties is prepared by the Presentation of text from multiple parties is prepared by the
mixer in one single stream. It is desirable to have a method that mixer in one single stream. It is desirable to have a method that
does not require any modifications in existing user devices does not require any modifications in existing user devices
implementing RFC 4103 for RTT without explicit support of multi- implementing RFC 4103 for RTT without explicit support of
party sessions. This is possible by having the mixer insert a new multiparty sessions. This is possible by having the mixer insert
line and a text formatted source label before each switch of text a new line and a text formatted source label before each switch of
source in the stream. Switch of source can only be done in places text source in the stream. Switch of source can only be done in
in the text where it does not disturb the perception of the places in the text where it does not disturb the perception of the
contents. Text from only one source can be presented in real time contents. Text from only one source can be presented in real time
at a time. The delay will therefore be varying. The method also at a time. The delay will therefore vary. The method also has
has other limitations, but is included in this document as a other limitations, but is included in this document as a fallback
fallback method. In calls where parties take turns properly by method. In calls where parties take turns properly by ending
ending their entries with a new line, the limitations will have their entries with a new line, the limitations will have limited
limited influence on the user experience. while only two parties influence on the user experience. when only two parties send text,
send text, these two will see the text in real time with no delay. these two will see the text in real time with no delay. This
This method is specified as a fallback method in this document. method is specified as a fallback method in this document.
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 [RFC8865]. That specification to use the WebRTC data channel in [RFC8865]. That specification
contains a section briefly describing its use in multi-party contains a section briefly describing its use in multiparty
sessions. The focus of this document 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 multiparty
performance, it is just mentioned in this document in relation to performance, it is just mentioned in this document in relation to
providing gateways with multi-party capabilities between RTP and providing gateways with multiparty capabilities between RTP and
WebRTC technologies. WebRTC technologies.
1.3. Intended application 1.3. Intended application
The method for multi-party real-time text specified in this document The method for multiparty real-time text specified in this document
is primarily intended for use in transmission between mixers and is primarily intended for use in transmission between mixers and
endpoints in centralised mixing configurations. It is also endpoints in centralised mixing configurations. It is also
applicable between mixers. An often mentioned application is for applicable between mixers. An often mentioned application is for
emergency service calls with real-time text and voice, where a emergency service calls with real-time text and voice, where a call
calltaker wants to make an attended handover of a call to another taker wants to make an attended handover of a call to another agent,
agent, and stay observing the session. Multimedia conference and stay to observe the session. Multimedia conference sessions with
sessions with support for participants to contribute in text is support for participants to contribute in text is another
another application. Conferences with central support for speech-to- application. Conferences with central support for speech-to-text
text conversion is yet another mentioned application. conversion is yet another mentioned application.
In all these applications, normally only one participant at a time In all these applications, normally only one participant at a time
will send long text utterances. In some cases, one other participant will send long text utterances. In some cases, one other participant
will occasionally contribute with a longer comment simultaneously. will occasionally contribute with a longer comment simultaneously.
That may also happen in some rare cases when text is interpreted to That may also happen in some rare cases when text is interpreted to
text in another language in a conference. Apart from these cases, text in another language in a conference. Apart from these cases,
other participants are only expected to contribute with very brief other participants are only expected to contribute with very brief
utterings while others are sending text. utterings while others are sending text.
Users expect that the text they send is presented in real-time in a Users expect that the text they send is presented in real-time in a
skipping to change at page 10, line 10 skipping to change at page 10, line 27
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. Overview of the two specified solutions and selection of method 2. Overview of the two specified solutions and selection of method
This section contains a brief introduction of the two methods This section contains a brief introduction of the two methods
specified in this document. specified in this document.
2.1. The RTP-mixer based solution for multi-party aware endpoints 2.1. The RTP-mixer-based solution for multiparty-aware endpoints
This method specifies negotiated use of the RFC 4103 format for This method specifies negotiated use of the RFC 4103 format for
multi-party transmission in a single RTP stream. The main purpose of multiparty transmission in a single RTP stream. The main purpose of
this document is to specify a method for true multi-party real-time this document is to specify a method for true multiparty real-time
text mixing for multi-party aware endpoints that can be widely text mixing for multiparty-aware endpoints that can be widely
deployed. The RTP-mixer based method makes use of the current format deployed. The RTP-mixer-based method makes use of the current format
for real-time text in [RFC4103]. It is an update of RFC 4103 by a for real-time text in [RFC4103]. It is an update of RFC 4103 by a
clarification on one way to use it in the multi-party situation. clarification on one way to use it in the multiparty situation. That
That is done by completing a negotiation for this kind of multi-party is done by completing a negotiation for this kind of multiparty
capability and by interleaving packets from different sources. The capability and by interleaving packets from different sources. The
source is indicated in the CSRC element in the RTP packets. Specific source is indicated in the CSRC element in the RTP packets. Specific
considerations are made to be able to recover text after packet loss. considerations are made to be able to recover text after packet loss.
The detailed procedures for the RTP-mixer based multi-party aware The detailed procedures for the RTP-mixer-based multiparty-aware case
case are specified in Section 3. are specified in Section 3.
Please use [RFC4103] as reference when reading the specification. Please use [RFC4103] as reference when reading the specification.
2.2. Mixing for multi-party unaware endpoints 2.2. Mixing for multiparty-unaware endpoints
A method is also specified in this document for cases when the A method is also specified in this document for cases when the
endpoint participating in a multi-party call does not itself endpoint participating in a multiparty call does not itself implement
implement any solution, or not the same, as the mixer. The method any solution, or not the same, as the mixer. The method requires the
requires the mixer to insert text dividers and readable labels and mixer to insert text dividers and readable labels and only send text
only send text from one source at a time until a suitable point from one source at a time until a suitable point appears for source
appears for source change. This solution is a fallback method with change. This solution is a fallback method with functional
functional limitations. It acts on the presentation level. limitations. It acts on the presentation level.
A party acting as a mixer, which has not negotiated any method for A party acting as a mixer, which has not negotiated any method for
true multi-party RTT handling, but negotiated a "text/red" or "text/ true multiparty RTT handling, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD in order to t140" format in a session with a participant SHOULD in order to
maintain interoperability, if nothing else is specified for the maintain interoperability, if nothing else is specified for the
application, format transmitted text to that participant to be application, format transmitted text to that participant to be
suitable to present on a multi-party unaware endpoint as further suitable to present on a multiparty-unaware endpoint as further
specified in Section 4.2. specified in Section 4.2.
2.3. Offer/answer considerations 2.3. Offer/answer considerations
RTP Payload for Text Conversation [RFC4103] specifies use of RTP RTP Payload for Text Conversation [RFC4103] specifies use of RTP
[RFC3550], and a redundancy format "text/red" for increased [RFC3550], and a redundancy format "text/red" for increased
robustness of real-time text transmission. This document updates robustness of real-time text transmission. This document updates
[RFC4103] by introducing a capability negotiation for handling multi- [RFC4103] by introducing a capability negotiation for handling
party real-time text, a way to indicate the source of transmitted multiparty real-time text, a way to indicate the source of
text, and rules for efficient timing of the transmissions interleaved transmitted text, and rules for efficient timing of the transmissions
from different sources. interleaved from different sources.
The capability negotiation for the "RTP-mixer based multi-party The capability negotiation for the "RTP-mixer-based multiparty
method" is based on use of the SDP media attribute "rtt-mixer". method" is based on use of the SDP media attribute "rtt-mixer".
The syntax is as follows: The syntax is as follows:
"a=rtt-mixer" "a=rtt-mixer"
If any other method for RTP-based multi-party real-time text gets If any other method for RTP-based multiparty real-time text gets
specified by additional work, it is assumed that it will be specified by additional work, it is assumed that it will be
recognized by some specific SDP feature exchange. recognized by some specific SDP feature exchange.
2.3.1. Initial offer 2.3.1. Initial offer
A party intending to set up a session and being willing to use the A party intending to set up a session and being willing to use the
RTP-mixer based method of this specification for sending or receiving RTP-mixer-based method of this specification for sending or receiving
or both sending and receiving real-time text SHALL include the "rtt- or both sending and receiving real-time text SHALL include the "rtt-
mixer" SDP attribute in the corresponding "text" media section in the mixer" SDP attribute in the corresponding "text" media section in the
initial offer. initial offer.
The party MAY indicate capability for both the RTP-mixer based method The party MAY indicate capability for both the RTP-mixer-based method
of this specification and other methods. of this specification and other methods.
When the offeror has sent the offer including the "rtt-mixer" When the offeror has sent the offer including the "rtt-mixer"
attribute, it MUST be prepared to receive and handle real-time text attribute, it MUST be prepared to receive and handle real-time text
formatted according to both the method for multi-party aware parties formatted according to both the method for multiparty-aware parties
specified in Section 3 in this specification and two-party formatted specified in Section 3 in this specification and two-party formatted
real-time text. real-time text.
2.3.2. Answering the offer 2.3.2. Answering the offer
A party receiving an offer containing the "rtt-mixer" SDP attribute A party receiving an offer containing the "rtt-mixer" SDP attribute
and being willing to use the RTP-mixer based method of this and being willing to use the RTP-mixer-based method of this
specification for sending or receiving or both sending and receiving specification for sending or receiving or both sending and receiving
SHALL include the "rtt-mixer" SDP attribute in the corresponding SHALL include the "rtt-mixer" SDP attribute in the corresponding
"text" media section in the answer. "text" media section in the answer.
If the offer did not contain the "rtt-mixer" attribute, the answer If the offer did not contain the "rtt-mixer" attribute, the answer
MUST NOT contain the "rtt-mixer" attribute. MUST NOT contain the "rtt-mixer" attribute.
An answer MUST NOT include acceptance of more than one method for An answer MUST NOT include acceptance of more than one method for
multi-party real-time text in the same RTP session. multiparty real-time text in the same RTP session.
When the answer including acceptance is transmitted, the answerer When the answer including acceptance is transmitted, the answerer
MUST be prepared to act on received text in the negotiated session MUST be prepared to act on received text in the negotiated session
according to the method for multi-party aware parties specified in according to the method for multiparty-aware parties specified in
Section 3 of this specification. Reception of text for a two-party Section 3 of this specification. Reception of text for a two-party
session SHALL also be supported. session SHALL also be supported.
2.3.3. Offeror processing the answer 2.3.3. Offeror processing the answer
When the answer is processed by the offeror, it MUST act as specified When the answer is processed by the offeror, it MUST act as specified
in Section 2.4 in Section 2.4
2.3.4. Modifying a session 2.3.4. Modifying a session
A session MAY be modified at any time by any party offering a A session MAY be modified at any time by any party offering a
modified SDP with or without the "rtt-mixer" SDP attribute expressing modified SDP with or without the "rtt-mixer" SDP attribute expressing
a desired change in the support of multi-party real-time text. a desired change in the support of multiparty real-time text.
If the modified offer adds indication of support for multi-party If the modified offer adds indication of support for multiparty real-
real-time text by including the "rtt-mixer" SDP attribute, the time text by including the "rtt-mixer" SDP attribute, the procedures
procedures specified in the previous subsections SHALL be applied. specified in the previous subsections SHALL be applied.
If the modified offer deletes indication of support for multi-party If the modified offer deletes indication of support for multiparty
real-time text by excluding the "rtt-mixer" SDP attribute, the answer real-time text by excluding the "rtt-mixer" SDP attribute, the answer
MUST NOT contain the "rtt-mixer" attribute, and both parties SHALL MUST NOT contain the "rtt-mixer" attribute, and both parties SHALL
after processing the SDP exchange NOT send real-time text formatted after processing the SDP exchange NOT send real-time text formatted
for multi-party aware parties according to this specification. for multiparty-aware parties according to this specification.
2.4. Actions depending on capability negotiation result 2.4. Actions depending on capability negotiation result
A transmitting party SHALL send text according to the RTP-mixer based A transmitting party SHALL send text according to the RTP-mixer-based
multi-party method only when the negotiation for that method was multiparty method only when the negotiation for that method was
successful and when it conveys text for another source. In all other successful and when it conveys text for another source. In all other
cases, the packets SHALL be populated and interpreted as for a two- cases, the packets SHALL be populated and interpreted as for a two-
party session. party session.
A party which has negotiated the "rtt-mixer" SDP media attribute MUST A party which has negotiated the "rtt-mixer" SDP media attribute MUST
populate the CSRC-list and format the packets according to Section 3 populate the CSRC-list, and format the packets according to Section 3
if it acts as an rtp-mixer and sends multi-party text. if it acts as an rtp-mixer and sends multiparty text.
A party which has negotiated the "rtt-mixer" SDP media attribute MUST A party which has negotiated the "rtt-mixer" SDP media attribute MUST
interpret the contents of the "CC" field, the CSRC-list and the interpret the contents of the "CC" field, the CSRC-list and the
packets according to Section 3 in received RTP packets in the packets according to Section 3 in received RTP packets in the
corresponding RTP stream. corresponding RTP stream.
A party which has not successfully completed the negotiation of the A party which has not successfully completed the negotiation of the
"rtt-mixer" SDP media attribute MUST NOT transmit packets interleaved "rtt-mixer" SDP media attribute MUST NOT transmit packets interleaved
from different sources in the same RTP stream as specified in from different sources in the same RTP stream as specified in
Section 3. If the party is a mixer and did declare the "rtt-mixer" Section 3. If the party is a mixer and did declare the "rtt-mixer"
SDP media attribute, it SHOULD perform the procedure for multi-party SDP media attribute, it SHOULD perform the procedure for multiparty-
unaware endpoints. If the party is not a mixer, it SHOULD transmit unaware endpoints. If the party is not a mixer, it SHOULD transmit
as in a two-party session according to [RFC4103]. as in a two-party session according to [RFC4103].
3. Details for the RTP-mixer based mixing method for multi-party aware 3. Details for the RTP-mixer-based mixing method for multiparty-aware
endpoints endpoints
3.1. Use of fields in the RTP packets 3.1. 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 when conveying text SHALL be one (1) in transmissions from a mixer when conveying text
from other sources in a multi-party session, and otherwise 0. from other sources in a multiparty session, and otherwise 0.
When text is conveyed by a mixer during a multi-party session, a CSRC When text is conveyed by a mixer during a multiparty session, a CSRC
list SHALL be included in the packet. The single member in the CSRC- list SHALL be included in the packet. The single member in the CSRC-
list SHALL contain the SSRC of the source of the T140blocks in the list SHALL contain the SSRC of the source of the T140blocks in the
packet. packet.
When redundancy is used, the RECOMMENDED level of redundancy is to When redundancy is used, the RECOMMENDED level of redundancy is to
use one primary and two redundant generations of T140blocks. In some use one primary and two redundant generations of T140blocks. In some
cases, a primary or redundant T140block is empty, but is still cases, a primary or redundant T140block is empty, but is still
represented by a member in the redundancy header. represented by a member in the redundancy header.
From other aspects, the contents of the RTP packets are equal to what In other regards, the contents of the RTP packets are equal to what
is specified in [RFC4103]. is specified in [RFC4103].
3.2. Initial transmission of a BOM character 3.2. Initial transmission of a BOM character
As soon as a participant is known to participate in a session with As soon as a participant is known to participate in a session with
another entity and is available for text reception, a Unicode BOM another entity and is available for text reception, a Unicode BOM
character SHALL be sent to it by the other entity according to the character SHALL be sent to it by the other entity according to the
procedures in this section. If the transmitter is a mixer, then the procedures in this section. If the transmitter is a mixer, then the
source of this character SHALL be indicated to be the mixer itself. source of this character SHALL be indicated to be the mixer itself.
skipping to change at page 14, line 23 skipping to change at page 14, line 34
3.4. Transmission interval 3.4. Transmission interval
A "text/red" or "text/t140" transmitter in a mixer SHALL send packets A "text/red" or "text/t140" transmitter in a mixer SHALL send packets
distributed in time as long as there is something (new or redundant distributed in time as long as there is something (new or redundant
T140blocks) to transmit. The maximum transmission interval SHALL T140blocks) to transmit. The maximum transmission interval SHALL
then be 330 ms, when no other limitations cause a longer interval to then be 330 ms, when no other limitations cause a longer interval to
be temporarily used. It is RECOMMENDED to send the next packet to a be temporarily used. It is RECOMMENDED to send the next 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 maximum character rate ("CPS") to the receiver is not exceeded as the maximum character rate ("CPS") to the receiver is not exceeded
during any 10 second interval. The intention of these time intervals during any 10-second interval. The intention of these time intervals
is to keep the latency low and network load limited while keeping a is to keep the latency low and network load limited while keeping
good protection against text loss in bursty packet loss conditions. good protection against text loss in bursty packet loss conditions.
The main purpose of the 330 ms interval is for timing of redundant The main purpose of the 330 ms interval is for timing of redundant
transmission, when no new text from the same source is available. transmission, when no new text from the same source is available.
If the "CPS" value is reached, longer transmission intervals SHALL be If the "CPS" value is reached, longer transmission intervals SHALL be
applied and only part of the text queued for transmission sent at end applied and only as much of the text queued for transmission SHALL be
of each transmission interval, until the transmission rate falls sent at the end of each transmission interval that can be allowed
under the "CPS" value again. See also Section 8 without exceeding the "CPS" value, until the transmission rate falls
under the "CPS" value again. Division of text for partial
transmission MUST then be made at T140block borders. See also
Section 8
For a transmitter not acting in a mixer, the transmission interval For a transmitter not acting as a mixer, the transmission interval
principles from [RFC4103] apply, and the transmission interval SHALL principles from [RFC4103] apply, and the transmission interval SHALL
be 300 ms. be 300 ms.
3.5. Only one source per packet 3.5. Only one source per packet
New text and redundant copies of earlier text from one source SHALL New text and redundant copies of earlier text from one source SHALL
be transmitted in the same packet if available for transmission at be transmitted in the same packet if available for transmission at
the same time. Text from different sources MUST NOT be transmitted the same time. Text from different sources MUST NOT be transmitted
in the same packet. in the same packet.
3.6. Do not send received text to the originating source 3.6. Do not send received text to the originating source
Text received by a mixer from a participant SHOULD NOT be included in Text received by a mixer from a participant SHOULD NOT be included in
transmission from the mixer to that participant, because the normal transmission from the mixer to that participant, because the normal
behavior of the endpoint is to present locally produced locally. behavior of the endpoint is to present locally-produced text locally.
3.7. Clean incoming text 3.7. Clean incoming text
A mixer SHALL handle reception, recovery from packet loss, deletion A mixer SHALL handle reception, recovery from packet loss, deletion
of superfluous redundancy, marking of possible text loss and deletion of superfluous redundancy, marking of possible text loss and deletion
of 'BOM' characters from each participant before queueing received of 'BOM' characters from each participant before queueing received
text for transmission to receiving participants. text for transmission to receiving participants.
3.8. Redundant transmission principles 3.8. Redundant transmission principles
A transmitting party using redundancy SHALL send redundant A transmitting party using redundancy SHALL send redundant
repetitions of T140blocks already transmitted in earlier packets. repetitions of T140blocks already 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 deduced from the SDP negotiation. It transmitted packets SHALL be deduced from the SDP negotiation. It
SHALL be set to the minimum of the number declared by the two parties SHALL be set to the minimum of the number declared by the two parties
negotiating a connection. It is RECOMMENDED to declare and transmit negotiating a connection. It is RECOMMENDED to declare and transmit
one original and two redundant generations of the T140blocks, because one original and two redundant generations of the T140blocks because
that provides good protection against text loss in case of packet that provides good protection against text loss in case of packet
loss, and low overhead. loss, and low overhead.
3.9. Interleaving text from different sources 3.9. Interleaving text from different sources
When text from more than one source is available for transmission When text from more than one source is available for transmission
from a mixer, the mixer SHALL let the sources take turns in having from a mixer, the mixer SHALL let the sources take turns in having
their text transmitted. their text transmitted.
The source with the oldest text received in the mixer or oldest The source with the oldest text received in the mixer or oldest
skipping to change at page 15, line 47 skipping to change at page 16, line 15
3.10. Text placement in packets 3.10. Text placement in packets
The mixer SHALL compose and transmit an RTP packet to a receiver when The mixer SHALL compose and transmit an RTP packet to a receiver when
one of the following conditions has occurred: one of the following conditions has occurred:
* There is unsent text available for transmission to that receiver. * There is unsent text available for transmission to that receiver.
* 330 ms has passed since already transmitted text was queued for * 330 ms has passed since already transmitted text was queued for
transmission as redundant text. transmission as redundant text.
At time of transmission, the mixer SHALL populate the RTP packet with At the time of transmission, the mixer SHALL populate the RTP packet
all T140blocks queued for transmission originating from the source in with all T140blocks queued for transmission originating from the
turn for transmission as long as this is not in conflict with the source in turn for transmission as long as this is not in conflict
allowed number of characters per second ("CPS") or the maximum packet with the allowed number of characters per second ("CPS") or the
size. In this way, the latency of the latest received text is kept maximum packet size. In this way, the latency of the latest received
low even in moments of simultaneous transmission from many sources. text is kept low even in moments of simultaneous transmission from
many sources.
Redundant text SHALL also be included. See Section 3.12 Redundant text SHALL also be included, and the assessment of how much
new text can be included within the maximum packet size MUST take
into account that the redundancy has priority to be transmitted in
its entirety. See Section 3.12
The SSRC of the source SHALL be placed as the only member in the The SSRC of the source SHALL be placed as the only member in the
CSRC-list. 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
whose text is included in text blocks. It is not the same as the whose 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. and thus are not represented in RTP packet CSRC-lists.
skipping to change at page 16, line 27 skipping to change at page 16, line 48
3.11. Empty T140blocks 3.11. Empty T140blocks
If no unsent T140blocks were available for a source at the time of If no unsent T140blocks were available for a source at the time of
populating a packet, but T140blocks are available which have not yet populating a packet, but T140blocks are available which have not yet
been sent the full intended number of redundant transmissions, then been sent the full intended number of redundant transmissions, then
the primary T140block for that source is composed of an empty the primary T140block for that source is composed of an empty
T140block, and populated (without taking up any length) in a packet T140block, and populated (without taking up any length) in a packet
for transmission. The corresponding SSRC SHALL be placed as usual in for transmission. The corresponding SSRC SHALL be placed as usual in
its place in the CSRC-list. its place in the CSRC-list.
The first packet in the session, the first after a source switch and The first packet in the session, the first after a source switch, and
the first after a pause SHALL be poulated with the available the first after a pause SHALL be populated with the available
T140blocks for the source in turn to be sent as primary, and empty T140blocks for the source in turn to be sent as primary, and empty
T140blocks for the agreed number of redundancy generations. T140blocks for the agreed number of redundancy generations.
3.12. Creation of the redundancy 3.12. 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 saved for populating the first redundant T140block for that source is saved for populating the first redundant T140block for that source
in next transmission of text from that source. The first redundant in the next transmission of text from that source. The first
T140block for that source from the latest transmission is saved for redundant T140block for that source from the latest transmission is
populating the second redundant T140block in next transmission of saved for populating the second redundant T140block in the next
text from that source. transmission of text from that source.
Usually this is the level of redundancy used. If a higher number of Usually this is the level of redundancy used. If a higher level 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.
The T140blocks saved for transmission as redundant data are assigned The T140blocks saved for transmission as redundant data are assigned
a planned transmission time 330 ms after the current time, but SHOULD a planned transmission time 330 ms after the current time, but SHOULD
be transmitted earlier if new text for the same source gets in turn be transmitted earlier if new text for the same source gets in turn
for transmission before that time. for transmission before that time.
skipping to change at page 17, line 19 skipping to change at page 17, line 40
when the corresponding T140block was sent as primary. when the corresponding T140block was sent 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.
3.14. Other RTP header fields 3.14. 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.
A value of "0" indicates that the source is the transmitting device A value of "0" indicates that the source is the transmitting device
itself and that the source is indicated by the SSRC field. This itself and that the source is indicated by the SSRC field. This
value is used by endpoints, and by mixers sending data that it is value is used by endpoints, and by mixers sending self-sourced data.
source of itself.
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].
3.15. Pause in transmission 3.15. 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 SHALL be stopped times from any source, the transmission process SHALL be stopped
until either new T140blocks arrive, or a keep-alive method calls for until either new T140blocks arrive, or a keep-alive method calls for
transmission of keep-alive packets. transmission of keep-alive packets.
3.16. RTCP considerations 3.16. 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
about the sources in the multi-party call. This makes it possible information about the sources in the multiparty call. This makes it
for participants to compose a suitable label for text from each possible for participants to compose a suitable label for text from
source. each source.
Integrity SHALL be considered when composing these fields. They Integrity SHALL be considered when composing these fields. They
contain name and address information that may be sensitive to contain name and address information that may be sensitive to
transmit in its entirety e.g. to unauthenticated participants. transmit in its entirety e.g., to unauthenticated participants.
Similar considerations SHALL be taken as for other media. Similar considerations SHALL be taken as for other media.
3.17. Reception of multi-party contents 3.17. Reception of multiparty 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.
3.17.1. Acting on the source of the packet contents 3.17.1. Acting on the source of the packet contents
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
the text is conveyed from a source indicated in the single member in the text is conveyed from a source indicated in the single member in
the CSRC-list, and the receiver MUST act on the source according to the CSRC-list, and the receiver MUST act on the source according to
its role. If the CC value is 0, the source is indicated in the SSRC its role. If the CC value is 0, the source is indicated in the SSRC
field. field.
3.17.2. Detection and indication of possible text loss 3.17.2. Detection and indication of possible text loss
The RTP sequence numbers of the received packets SHALL be monitored The receiver SHALL monitor the RTP sequence numbers of the received
for gaps and packets out of order. If a sequence number gap appears packets for gaps and packets out of order. If a sequence number gap
and still exists after some defined short time for jitter resolution, appears and still exists after some defined short time for jitter
the packets in the gap SHALL be regarded as lost. resolution, the packets in the gap SHALL be regarded as lost.
If it is known that only one source is active in the RTP session, If it is known that only one source is active in the RTP session,
then it is likely that a gap equal to or larger than the agreed then it is likely that a gap equal to or larger than the agreed
number of redundancy generations (including the primary) causes text number of redundancy generations (including the primary) causes text
loss. In that case a t140block SHALL be created with a marker for loss. In that case, the receiver SHALL create a t140block with a
possible text loss [T140ad1] and assigned to the source and inserted marker for possible text loss [T140ad1] and associate it with the
in the reception buffer for that source. source and insert it in the reception buffer for that source.
If it is known that more than one source is active in the RTP If it is known that more than one source is active in the RTP
session, then it is not possible in general to evaluate if text was session, then it is not possible in general to evaluate if text was
lost when packets were lost. With two active sources and the lost when packets were lost. With two active sources and the
recommended number of redundancy generations (3), it can take a gap recommended number of redundancy generations (3), it can take a gap
of five consecutive lost packets until any text may be lost, but text of five consecutive lost packets until any text may be lost, but text
loss can also appear if three non-consecutive packets are lost when loss can also appear if three non-consecutive packets are lost when
they contained consecutive data from the same source. A simple they contained consecutive data from the same source. A simple
method to decide when there is risk for resulting text loss is to method to decide when there is risk for resulting text loss is to
evaluate if three or more packets were lost within one second. If evaluate if three or more packets were lost within one second. If
this simple method is used, then a t140block SHOULD be created with a this simple method is used, then a t140block SHOULD be created with a
marker for possible text loss [T140ad1] and assigned to the SSRC of marker for possible text loss [T140ad1] and associated with the SSRC
the transmitter as a general input from the mixer. of the transmitter as a general input from the mixer.
Implementations MAY apply more refined methods for more reliable Implementations MAY apply more refined methods for more reliable
detection of if text was lost or not. Any refined method SHALL detection of whether text was lost or not. Any refined method SHOULD
prefer marking possible loss rather than not marking when it is prefer marking possible loss rather than not marking when it is
uncertain if there was loss. uncertain if there was loss.
3.17.3. Extracting text and handling recovery 3.17.3. Extracting text and handling recovery
When applying the following procedures, the effects MUST be When applying the following procedures, the effects MUST be
considered of possible timestamp wrap around and the RTP session considered of possible timestamp wrap around and the RTP session
possibly changing SSRC. possibly changing SSRC.
When a packet is received in an RTP session using the packetization When a packet is received in an RTP session using the packetization
for multi-party aware endpoints, its T140blocks SHALL be extracted in for multiparty-aware endpoints, its T140blocks SHALL be extracted in
the following way. The description is adapted to the default the following way. The description is adapted to the default
redundancy case using the original and two redundant generations. redundancy case using the original and two redundant generations.
The source SHALL be extracted from the CSRC-list if available, The source SHALL be extracted from the CSRC-list if available,
otherwise from the SSRC. otherwise from the SSRC.
If the received packet is the first packet received from the source, If the received packet is the first packet received from the source,
then all T140blocks in the packet SHALL be retrieved and assigned to then all T140blocks in the packet SHALL be retrieved and assigned to
a receive buffer for the source beginning with the second generation a receive buffer for the source beginning with the second generation
redundancy, continuing with the first generation redundancy and redundancy, continuing with the first generation redundancy and
skipping to change at page 20, line 8 skipping to change at page 20, line 24
3.17.4. Delete 'BOM' 3.17.4. 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 after extraction from received packets. These SHALL be deleted after extraction from received packets.
3.18. Performance considerations 3.18. Performance considerations
This solution has good performance with low text delays as long as This solution has good performance with low text delays as long as
the sum of characters per second during any 10 second interval sent the sum of characters per second during any 10-second interval sent
from a number of simultaneously sending participants to a receiving from a number of simultaneously sending participants to a receiving
participant does not reach the 'CPS' value. At higher numbers of participant does not reach the 'CPS' value. At higher numbers of
characters per second sent, a jerkiness is visible in the characters per second sent, a jerkiness is visible in the
presentation of text. The solution is therefore suitable for presentation of text. The solution is therefore suitable for
emergency service use, relay service use, and small or well-managed emergency service use, relay service use, and small or well-managed
larger multimedia conferences. Only in large unmanaged conferences larger multimedia conferences. Only in large unmanaged conferences
with a high number of participants there may on very rare occasions with a high number of participants there may on very rare occasions
appear situations when many participants happen to send text appear situations when many participants happen to send text
simultaneously, resulting in unpleasantly jerky presentation of text simultaneously, resulting in unpleasantly jerky presentation of text
from each sending participant. It should be noted that it is only from each sending participant. It should be noted that it is only
the number of users sending text within the same moment that causes the number of users sending text within the same moment that causes
jerkiness, not the total number of users with RTT capability. jerkiness, not the total number of users with RTT capability.
3.19. Security for session control and media 3.19. Security for session control and media
Security SHOULD be applied when possible regarding the capabilities Security SHOULD be applied when possible regarding the capabilities
of the participating devices by use of SIP over TLS by default of the participating devices by use of SIP over TLS by default
according to [RFC5630] section 3.1.3 on session control level and by according to [RFC5630] section 3.1.3 on the session control level and
default using DTLS-SRTP [RFC5764] on media level. In applications by default using DTLS-SRTP [RFC5764] on the media level. In
where legacy endpoints without security may exist, a negotiation applications where legacy endpoints without security may exist, a
SHOULD be performed to decide if security by encryption on media negotiation SHOULD be performed to decide if encryption on the media
level will be applied. If no other security solution is mandated for level will be applied. If no other security solution is mandated for
the application, then OSRTP [RFC8643] is a suitable method be applied the application, then OSRTP [RFC8643] is a suitable method to be
to negotiate SRTP media security with DTLS. Most SDP examples below applied to negotiate SRTP media security with DTLS. Most SDP
are for simplicity expressed without the security additions. The examples below are for simplicity expressed without the security
principles (but not all details) for applying DTLS-SRTP [RFC5764] additions. The principles (but not all details) for applying DTLS-
security is shown in a couple of the following examples. SRTP [RFC5764] security are shown in a couple of the following
examples.
This document contains two mixing procedures which imply different This document contains two mixing procedures which imply different
security levels. The mixing for conference-unaware endpoints has security levels. The mixing for conference-unaware endpoints has
lower security level than the mixing method for conference-aware lower security level than the mixing method for conference-aware
endpoints, because there may be an opportunity for a malicious mixer endpoints, because there may be an opportunity for a malicious mixer
or a middleman to masquerade the source labels accompanying the text or a middleman to masquerade the source labels accompanying the text
streams in text format. This is especially true if support of un- streams in text format. This is especially true if support of un-
encrypted SIP and media is supported because of lack of such support encrypted SIP and media is supported because of lack of such support
in the target endpoints. However, the mixing for conference-aware in the target endpoints. However, the mixing for conference-aware
endpoints as specified here also requires that the mixer can be endpoints as specified here also requires that the mixer can be
trusted. End to end encryption would require further work and could trusted. Further general security considerations are covered in
be based on WebRTC as specified in Section 1.2. Section 11.
End-to-end encryption would require further work and could be based
on WebRTC as specified in Section 1.2.
3.20. SDP offer/answer examples 3.20. SDP offer/answer examples
This section shows some examples of SDP for session negotiation of This section 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. The show the part of importance for the real-time text media. The
examples relate to the single RTP stream mixing for multi-party aware examples relate to the single RTP stream mixing for multiparty-aware
endpoints and for multi-party unaware endpoints. endpoints and for multiparty-unaware endpoints.
Note: Multi-party RTT MAY also be provided through other methods, Note: Multiparty RTT MAY also be provided through other methods,
e.g. by a Selective Forwarding Middlebox (SFM). In that case, the e.g., by a Selective Forwarding Middlebox (SFM). In that case, the
SDP of the offer will include something specific for that method, and SDP of the offer will include something specific for that method,
an answer acknowledging the use of that method would accept it by e.g., an SDP attribute or another media format. An answer selecting
something specific included in the SDP. The offer may contain also the use of that method would accept it by a corresponding
the "rtt-mixer" SDP media attribute for the main RTT media when the acknowledgement included in the SDP. The offer may contain also the
offeror has capability for both multi-party methods, while an answer, "rtt-mixer" SDP media attribute for the main RTT media when the
offeror has capability for both multiparty methods, while an answer,
selecting to use SFM will not include the "rtt-mixer" SDP media selecting to use SFM will not include the "rtt-mixer" SDP media
attribute. attribute.
Offer example for "text/red" format and multi-party support: Offer example for "text/red" format and multiparty 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-mixer a=rtt-mixer
Answer example from a multi-party capable device Answer example from a multiparty-aware 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-mixer a=rtt-mixer
Offer example for "text/red" format including multi-party Offer example for "text/red" format including multiparty
and security: and security:
a=fingerprint: (fingerprint1) a=fingerprint: (fingerprint1)
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-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.
Note: For brevity, the entire value of the SDP fingerprint attribute Note: For brevity, the entire value of the SDP fingerprint attribute
is not shown in this and the following example. is not shown in this and the following example.
Answer example from a multi-party capable device with security Answer example from a multiparty-aware device with security
a=fingerprint: (fingerprint2) a=fingerprint: (fingerprint2)
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-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 multiparty-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
3.21. Packet sequence example from interleaved transmission 3.21. Packet sequence example from interleaved transmission
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 interleaved transmission of loss and recovery. The sequence includes interleaved transmission of
text from two RTT sources A and B. P indicates primary data. R1 is text from two RTT sources A and B. P indicates primary data. R1 is
first redundant generation data and R2 is the second redundant first redundant generation data and R2 is the second redundant
generation data. A1, B1, A2 etc are text chunks (T140blocks) generation data. A1, B1, A2 etc. are text chunks (T140blocks)
received from the respective sources and sent on to the receiver by received from the respective sources and sent on to the receiver by
the mixer. X indicates dropped packet between the mixer and a the mixer. X indicates a dropped packet between the mixer and a
receiver. The session is assumed to use original and two redundant receiver. The session is assumed to use original and two redundant
generations of RTT. generations of RTT.
|-----------------------| |-----------------------|
|Seq no 101, Time=20400 | |Seq no 101, Time=20400 |
|CC=1 | |CC=1 |
|CSRC list A | |CSRC list A |
|R2: A1, Offset=600 | |R2: A1, Offset=600 |
|R1: A2, Offset=300 | |R1: A2, Offset=300 |
|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 101 and assigned to sequence, text A3 is received from packet 101 and assigned to
reception area A. The mixer is now assumed to have received text reception buffer A. The mixer is now assumed to have received
from source B 100 ms after packet 101 and will send that text. initial text from source B 100 ms after packet 101 and will send that
Transmission of A2 and A3 as redundancy is planned for 330 ms after text. Transmission of A2 and A3 as redundancy is planned for 330 ms
packet 101 if no new text from A is ready to be sent before that. after packet 101 if no new text from A is ready to be sent before
that.
|-----------------------| |-----------------------|
|Seq no 102, Time=20500 | |Seq no 102, Time=20500 |
|CC=1 | |CC=1 |
|CSRC list B | |CSRC list B |
|R2 Empty, Offset=600 | |R2 Empty, Offset=600 |
|R1: Empty, Offset=300 | |R1: Empty, Offset=300 |
|P: B1 | |P: B1 |
|-----------------------| |-----------------------|
Packet 102 is received. Packet 102 is received.
skipping to change at page 23, line 38 skipping to change at page 24, line 14
X------------------------| X------------------------|
X Seq no 104, Timer=20830| X Seq no 104, Timer=20830|
X CC=1 | X CC=1 |
X CSRC list B | X CSRC list B |
X R2: Empty, Offset=600 | X R2: Empty, Offset=600 |
X R1: B1, Offset=300 | X R1: B1, Offset=300 |
X P: B2 | X P: B2 |
X------------------------| X------------------------|
Packet 104 contains text from B, including new B2 and Packet 104 contains text from B, including new B2 and
redundant B1. It is assumed dropped in network redundant B1. It is assumed dropped due to network
problems. problems.
The mixer has A3 redundancy to send but no new text The mixer has A3 redundancy to send but no new text
appears from A and therefore the redundancy is sent appears from A and therefore the redundancy is sent
330 ms after the previous packet with text from A. 330 ms after the previous packet with text from A.
|------------------------| |------------------------|
| Seq no 105, Timer=21060| | Seq no 105, Timer=21060|
| CC=1 | | CC=1 |
| CSRC list A | | CSRC list A |
| R2: A3, Offset=660 | | R2: A3, Offset=660 |
| R1: Empty, Offset=330 | | R1: Empty, Offset=330 |
| P: Empty | | P: Empty |
|------------------------| |------------------------|
Packet 105 is received. Packet 105 is received.
A gap for lost 103, and 104 is detected. A gap for lost packets 103 and 104 is detected.
Assume that no other loss was detected the last second. Assume that no other loss was detected during the last second.
Then it can be concluded that nothing was totally lost. Then it can be concluded that nothing was totally lost.
R2 is checked. Its original time was 21040-660=20400. R2 is checked. Its original time was 21060-660=20400.
A packet with text from A was received with that A packet with text from A was received with that
timestamp, so nothing needs to be recovered. timestamp, so nothing needs to be recovered.
B1 and B2 still needs to be transmitted as redundancy. B1 and B2 still need to be transmitted as redundancy.
This is planned 330 ms after packet 105. That This is planned 330 ms after packet 105. That
would be at 21150. would be at 21160.
|-----------------------| |-----------------------|
|Seq no 106, Timer=21160| |Seq no 106, Timer=21160|
|CC=1 | |CC=1 |
|CSRC list B | |CSRC list B |
| R2: B1, Offset=660 | | R2: B1, Offset=660 |
| R1: B2, Offset=330 | | R1: B2, Offset=330 |
| P: Empty | | P: Empty |
|-----------------------| |-----------------------|
skipping to change at page 25, line 16 skipping to change at page 25, line 28
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 [RFC4103] specified to be 30 characters per second. The value is in [RFC4103] specified to be 30 characters per second. The value
MAY be modified in the "CPS" parameter of the FMTP attribute in the MAY be modified in the "CPS" parameter of the FMTP attribute in the
media section for the "text/t140" media. A mixer combining real-time media section for the "text/t140" media. A mixer combining real-time
text from a number of sources may occasionally have a higher combined text from a number of sources may occasionally have a higher combined
flow of text coming from the sources. Endpoints SHOULD therefore flow of text coming from the sources. Endpoints SHOULD therefore
specify a suitable higher value for the "CPS" parameter, specify a suitable higher value for the "CPS" parameter,
corresponding to its real reception capability. A value for "CPS" of corresponding to its real reception capability. A value for "CPS" of
90 SHALL be the default for the "text/t140" stream in the "text/red" 90 SHALL be the default for the "text/t140" stream in the "text/red"
format when multi-party real-time text is negotiated. See [RFC4103] format when multiparty real-time text is negotiated. See [RFC4103]
for the format and use of the "CPS" parameter. The same rules apply for the format and use of the "CPS" parameter. The same rules apply
for the multi-party case except for the default value. for the multiparty case except for the default value.
4. Presentation level considerations 4. Presentation level considerations
"Protocol for multimedia application text conversation" [T140] "Protocol for multimedia application text conversation" [T140]
provides the presentation level requirements for the [RFC4103] provides the presentation level requirements for the [RFC4103]
transport. Functions for erasure and other formatting functions and transport. Functions for erasure and other formatting functions are
are specified in [T140] which has the following general statement for specified in [T140] which has the following general statement for the
the presentation: 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
current session should be provided. The text should be displayed as current session should be provided. The text should be displayed as
soon as it is received." soon as it is received."
Strict application of [T140] is of essence for the interoperability Strict application of [T140] is of essence for the interoperability
of real-time text implementations and to fulfill the intention that of real-time text implementations and to fulfill the intention that
the session participants have the same information of the text the session participants have the same information conveyed in the
contents of the conversation without necessarily having the exact text contents of the conversation without necessarily having the
same layout of the conversation. exact same layout of the conversation.
[T140] specifies a set of presentation control codes to include in [T140] specifies a set of presentation control codes to include in
the stream. Some of them are optional. Implementations MUST be able the stream. Some of them are optional. Implementations MUST be able
to ignore optional control codes that they do not support. to ignore optional control codes that they do not support.
There is no strict "message" concept in real-time text. The Unicode There is no strict "message" concept in real-time text. The Unicode
Line Separator character SHALL be used as a separator allowing a part Line Separator character SHALL be used as a separator allowing a part
of received text to be grouped in presentation. The characters of received text to be grouped in presentation. The characters
"CRLF" may be used by other implementations as replacement for Line "CRLF" may be used by other implementations as a replacement for Line
Separator. The "CRLF" combination SHALL be erased by just one Separator. The "CRLF" combination SHALL be erased by just one
erasing action, just as the Line Separator. Presentation functions erasing action, the same as the Line Separator. Presentation
are allowed to group text for presentation in smaller groups than the functions are allowed to group text for presentation in smaller
line separators imply and present such groups with source indication groups than the line separators imply and present such groups with
together with text groups from other sources (see the following source indication together with text groups from other sources (see
presentation examples). Erasure has no specific limit by any the following presentation examples). Erasure has no specific limit
delimiter in the text stream. by any delimiter in the text stream.
4.1. Presentation by multi-party aware endpoints 4.1. Presentation by multiparty-aware endpoints
A multi-party aware receiving party, presenting real-time text MUST A multiparty-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, the When text is received from multiple original sources, the
presentation SHALL provide a view where text is added in multiple presentation SHALL provide a view where text is added in multiple
presentation fields. 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
skipping to change at page 28, line 7 skipping to change at page 28, 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.
4.2. Multi-party mixing for multi-party unaware endpoints 4.2. multiparty mixing for multiparty-unaware endpoints
When the mixer has indicated RTT multi-party capability in an SDP When the mixer has indicated RTT multiparty capability in an SDP
negotiation, but the multi-party capability negotiation fails with an negotiation, but the multiparty capability negotiation fails with an
endpoint, then the agreed "text/red" or "text/t140" format SHALL be endpoint, then the agreed "text/red" or "text/t140" format SHALL be
used and the mixer SHOULD compose a best-effort presentation of used and the mixer SHOULD compose a best-effort presentation of
multi-party real-time text in one stream intended to be presented by multiparty real-time text in one stream intended to be presented by
an endpoint with no multi-party awareness, when that is desired in an endpoint with no multiparty awareness, when that is desired in the
the actual implementation. The following specifies a procedure which actual implementation. The following specifies a procedure which MAY
MAY be applied in that situation. be applied in that situation.
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 multiparty calls by legacy
deployed endpoints implementing only RFC 4103 without any multi-party deployed endpoints implementing only RFC 4103 without any multiparty
extensions specified in this document. extensions specified in this document.
The principles and procedures below do not specify any new protocol The principles and procedures below do not specify any new protocol
elements. They are instead composed from the information in [T140] elements. They are instead composed from the information in [T140]
and an ambition to provide a best effort presentation on an endpoint and an ambition to provide a best-effort presentation on an endpoint
which has functions only for two-party calls. which has functions only for two-party calls.
The mixer mixing for multi-party unaware endpoints SHALL compose a The mixer mixing for multiparty-unaware endpoints SHALL compose a
simulated limited multi-party RTT view suitable for presentation in simulated, limited multiparty 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 SHALL be composed and sent for the source initially presentable label SHALL be composed and sent for the source initially
in the session and after each source switch. With this procedure the in the session and after each source switch. With this procedure the
time for switching from transmission of text from one source to time for switching from transmission of text from one source to
transmission of text from another source is depending on the actions transmission of text from another source depends on the actions of
of the users. In order to expedite source switch, a user can for the users. In order to expedite source switching, a user can, for
example end its turn with a new line. example, end its turn with a new line.
4.2.1. 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] SHALL be inserted in the stream if The mark for lost text [T140ad1] SHALL 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 in order to be able to evaluate text loss. recipients in order to be able to evaluate text loss.
4.2.2. Actions by the mixer for transmission to the recipients 4.2.2. Actions by the mixer for transmission to the recipients
The following procedure SHALL be applied for each multi-party unaware The following procedure SHALL be applied for each multiparty-unaware
recipient of multi-party text from the mixer. recipient of multiparty text from the mixer.
The text for transmission SHALL be formatted by the mixer for each The text for transmission SHALL 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 because it is usually presented transmission to that participant because it is usually presented
locally at transmission time. When there is text available for locally at transmission time. 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 SHALL switch between transmission of text from participant, the mixer SHALL switch between transmission of text from
the different sources at suitable points in the transmitted stream. the different sources at suitable points in the transmitted stream.
When switching source, the mixer SHALL insert a line separator if the When switching source, the mixer SHALL insert a line separator if the
already transmitted text did not end with a new line (line separator already transmitted text did not end with a new line (line separator
or CRLF). A label SHALL be composed from information in the CNAME or CRLF). A label SHALL be composed from information in the CNAME
and NAME fields in RTCP reports from the participant to have its text and NAME fields in RTCP reports from the participant to have its text
transmitted, or from other session information for that user. The transmitted, or from other session information for that user. The
label SHALL be delimited by suitable characters (e.g. '[ ]') and label SHALL be delimited by suitable characters (e.g., '[ ]') and
transmitted. The CSRC SHALL indicate the selected source. Then text transmitted. The CSRC SHALL indicate the selected source. Then text
from that selected participant SHALL be transmitted until a new from that selected participant SHALL be transmitted until a new
suitable point for switching source is reached. suitable point for switching source is reached.
Integrity considerations SHALL be taken when composing the label. Information available to the mixer for composing the label may
contain sensitive personal information that SHOULD not be revealed in
sessions not securely authenticated and protected. Integrity
considerations regarding how much personal information is included in
the label SHOULD therefore be taken when composing the label.
Seeking a suitable point for switching source SHALL be done when Seeking a suitable point for switching source SHALL be done when
there is older text waiting for transmission from any party than the there is older text waiting for transmission from any party than the
age of the last transmitted text. Suitable points for switching are: age of the last transmitted text. Suitable points for switching are:
* A completed phrase ended by comma * A completed phrase ended by comma
* A completed sentence * A completed sentence
* A new line (line separator or CRLF) * A new line (line separator or CRLF)
* A long pause (e.g. > 10 seconds) in received text from the * A long pause (e.g., > 10 seconds) in received text from the
currently transmitted source currently transmitted source
* If text from one participant has been transmitted with text from * If text from one participant has been transmitted with text from
other sources waiting for transmission for a long time (e.g. > 1 other sources waiting for transmission for a long time (e.g., > 1
minute) and none of the other suitable points for switching has minute) and none of the other suitable points for switching has
occurred, a source switch MAY be forced by the mixer at next word occurred, a source switch MAY be forced by the mixer at the next
delimiter, and also if even a word delimiter does not occur within word delimiter, and also even if a word delimiter does not occur
a time (e.g. 15 seconds) after the scan for word delimiter within a time (e.g., 15 seconds) after the scan for a word
started. delimiter started.
When switching source, the source which has the oldest text in queue When switching source, the source which has the oldest text in queue
SHALL be selected to be transmitted. A character display count SHALL SHALL be selected to be transmitted. A character display count SHALL
be maintained for the currently transmitted source, starting at zero be maintained for the currently transmitted source, starting at zero
after the label is transmitted for the currently transmitted source. after the label is transmitted for the currently transmitted source.
The status SHALL be maintained for the latest control code for Select The status SHALL be maintained for the latest control code for Select
Graphic Rendition (SGR) from each source. If there is an SGR code Graphic Rendition (SGR) from each source. If there is an SGR code
stored as the status for the current source before the source switch 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 [009B is done, a reset of SGR SHALL be sent by the sequence SGR 0 [009B
skipping to change at page 30, line 32 skipping to change at page 30, line 40
4.2.3. Actions on transmission of text 4.2.3. Actions on transmission of text
Text from a source sent to the recipient SHALL increase the display Text from a source sent to the recipient SHALL increase the display
count by one per transmitted character. count by one per transmitted character.
4.2.4. 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 SHALL cause specific considerations in the mixer. actions. They SHALL 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 the UTF-8 encoding 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 SHALL NOT be altered. active session. The display count SHALL 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
appropriate. Increase display count by 1. appropriate. Increase the display count by 1.
CR LF 000D 000A A supported, but not preferred way of requesting a CR LF 000D 000A A supported but not preferred way of requesting a
new line. Check and perform a source switch if appropriate. new line. Check and perform a source switch if appropriate.
Increase display count by 1. Increase the display count by 1.
INT ESC 0061 Interrupt (used to initiate mode negotiation INT ESC 0061 Interrupt (used to initiate the mode negotiation
procedure). The display count SHALL NOT be altered. procedure). The display count SHALL NOT be altered.
SGR 009B Ps 006D Select graphic rendition. Ps is rendition SGR 009B Ps 006D Select graphic rendition. Ps is the rendition
parameters specified in ISO 6429. The display count SHALL NOT be parameters specified in ISO 6429. The display count SHALL NOT be
altered. The SGR code SHOULD be stored for the current source. altered. The SGR code SHOULD be stored for the current source.
SOS 0098 Start of string, used as a general protocol element SOS 0098 Start of string, used as a general protocol element
introducer, followed by a maximum 256 bytes string and the ST. introducer, followed by a maximum 256-byte string and the ST. The
The display count SHALL NOT be altered. display count SHALL NOT be altered.
ST 009C String terminator, end of SOS string. The display count ST 009C String terminator, end of SOS string. The display count
SHALL NOT be altered. SHALL NOT be altered.
ESC 001B Escape - used in control strings. The display count SHALL ESC 001B Escape - used in control strings. The display count SHALL
NOT be altered for the complete escape code. NOT be altered for the complete escape code.
Byte order mark "BOM" (U+FEFF) "Zero width, no break space", used Byte order mark "BOM" (U+FEFF) "Zero width, no break space", used
for synchronization and keep-alive, SHALL be deleted from incoming for synchronization and keep-alive. It SHALL be deleted from
streams. It SHALL also be sent first after session establishment incoming streams. It SHALL also be sent first after session
to the recipient. The display count SHALL NOT be altered. establishment to the recipient. The display count SHALL NOT be
altered.
Missing text mark (U+FFFD) "Replacement character", represented as a Missing text mark (U+FFFD) "Replacement character", represented as a
question mark in a rhombus, or if that is not feasible, replaced question mark in a rhombus, or if that is not feasible, replaced
by an apostrophe ', marks place in stream of possible text loss. by an apostrophe '. It marks the place in the stream of possible
This mark SHALL be inserted by the reception procedure in case of text loss. This mark SHALL be inserted by the reception procedure
unrecoverable loss of packets. The display count SHALL be in case of unrecoverable loss of packets. The display count SHALL
increased by one when sent as for any other character. be increased by one when sent as for any other character.
SGR If a control code for selecting graphic rendition (SGR), other SGR If a control code for selecting graphic rendition (SGR) other
than reset of the graphic rendition (SGR 0) is sent to a than reset of the graphic rendition (SGR 0) is sent to a
recipient, that control code SHALL also be stored as status for recipient, that control code SHALL also be stored as the status
the source in the storage for SGR status. If a reset graphic for the source in the storage for SGR status. If a reset graphic
rendition (SGR 0) originated from a source is sent, then the SGR rendition (SGR 0) originating from a source is sent, then the SGR
status storage for that source SHALL be cleared. The display status storage for that source SHALL be cleared. The display
count SHALL NOT be increased. count SHALL NOT be increased.
BS (U+0008) Back Space, intended to erase the last entered character BS (U+0008) Back Space, intended to erase the last entered character
by a source. Erasure by backspace cannot always be performed as by a source. Erasure by backspace cannot always be performed as
the erasing party intended. If an erasing action erases all text the erasing party intended. If an erasing action erases all text
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 SHALL be decreased by one display count has a positive value it SHALL be decreased by one
when the BS is sent. If the display count is at zero, it SHALL when the BS is sent. If the display count is at zero, it SHALL
NOT not altered. NOT be altered.
4.2.5. Packet transmission 4.2.5. Packet transmission
A mixer transmitting to a multi-party unaware terminal SHALL send A mixer transmitting to a multiparty-unaware terminal SHALL send
primary data only from one source per packet. The SSRC SHALL be the primary data only from one source per packet. The SSRC SHALL be the
SSRC of the mixer. The CSRC list SHALL contain one member and be the SSRC of the mixer. The CSRC list SHALL contain one member and be the
SSRC of the source of the primary data. SSRC of the source of the primary data.
4.2.6. Functional limitations 4.2.6. Functional limitations
When a multi-party unaware endpoint presents a conversation in one When a multiparty-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 multiparty-unaware endpoints, there will be two levels
levels of source indicators for the received text; one generated by of source indicators for the received text; one generated by the
the mixer and inserted in a label after each source switch, and mixer and inserted in a label after each source switch, and another
another generated by the receiving endpoint and inserted after each generated by the receiving endpoint and inserted after each switch
switch between local and remote source in the presentation area. between local and remote source in the presentation area. This will
This will waste display space and look inconsistent to the reader. waste display space and look inconsistent to the reader.
New text can be presented only from one source at a time. Switch of New text can be presented only from one source at a time. Switch of
source to be presented takes place at suitable places in the text, source to be presented takes place at suitable places in the text,
such as end of phrase, end of sentence, line separator and such as end of phrase, end of sentence, line separator and
inactivity. Therefore the time to switch to present waiting text inactivity. Therefore the time to switch to present waiting text
from other sources may become long and will vary and depend on the from other sources may become long and will vary and depend on the
actions of the currently presented source. actions of the currently presented source.
Erasure can only be done up to the latest source switch. If a user Erasure can only be done up to the latest source switch. If a user
tries to erase more text, the erasing actions will be presented as tries to erase more text, the erasing actions will be presented as
letter X after the label. letter X after the label.
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 make it strongly RECOMMENDED to implement multi-party These facts make it strongly RECOMMENDED to implement multiparty
awareness in RTT endpoints. The use of the mixing method for multi- awareness in RTT endpoints. The use of the mixing method for
party-unaware endpoints should be left for use with endpoints which multiparty-unaware endpoints should be left for use with endpoints
are impossible to upgrade to become multi-party aware. which are impossible to upgrade to become multiparty-aware.
4.2.7. Example views of presentation on multi-party unaware endpoints 4.2.7. Example views of presentation on multiparty-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 multiparty-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. |
|[Eve]:Hi all, can we | | |[Eve]:Hi all, can we | |
|plan for the seminar. | | |plan for the seminar. | |
| | | | | |
|[Bob]:Eve, will you do | | |[Bob]:Eve, will you do | |
|your presentation on | | |your presentation on | |
|Friday? | | |Friday? | |
|[Eve]:Yes, Friday at 10.| | |[Eve]:Yes, Friday at 10.| |
|[Bob]: Fine, wo |We need to meet befo | |[Bob]: Fine, wo |We need to meet befo |
|________________________|_________________________| |________________________|_________________________|
Figure 5: Alice who has a conference-unaware client is receiving the Figure 5: Alice who has a conference-unaware client is receiving the
multi-party real-time text in a single-stream. multiparty real-time text in a single-stream.
This figure shows how a coordinated column view MAY be presented on This figure shows how a coordinated column view MAY be presented on
Alice's device in a view with two-columns. The mixer inserts labels Alice's device in a view with two-columns. The mixer inserts labels
to show how the sources alternate in the column with received text. to show how the sources alternate in the column with received text.
The mixer alternates between the sources at suitable points in the The mixer alternates between the sources at suitable points in the
text exchange so that text entries from each party can be text exchange so that text entries from each party can be
conveniently read. conveniently read.
_________________________________________________ _________________________________________________
| |^| | |^|
skipping to change at page 34, line 5 skipping to change at page 34, line 28
| | | | | |
|[Eve] we can have dinner and then walk | | |[Eve] we can have dinner and then walk | |
| | | | | |
|[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 multiparty-unaware presentation
presentation in chat style. Alice is the local user. in chat style. Alice is the local user.
In this view, there is a tradition in receiving applications to In this view, there is a tradition in receiving applications to
include a label showing the source of the text, here shown with include a label showing the source of the text, here shown with
parenthesis "()". The mixer also inserts source labels for the parenthesis "()". The mixer also inserts source labels for the
multi-party call participants, here shown with brackets "[]". multiparty call participants, here shown with brackets "[]".
5. Relation to Conference Control 5. Relation to Conference Control
5.1. Use with SIP centralized conferencing framework 5.1. Use with SIP centralized conferencing framework
The SIP conferencing framework, mainly specified in [RFC4353], The SIP conferencing framework, mainly specified in [RFC4353],
[RFC4579] and [RFC4575] is suitable for coordinating sessions [RFC4579] and [RFC4575] is suitable for coordinating sessions
including multi-party RTT. The RTT stream between the mixer and a including multiparty RTT. The RTT stream between the mixer and a
participant is one and the same during the conference. Participants participant is one and the same during the conference. Participants
get announced by notifications when participants are joining or get announced by notifications when participants are joining or
leaving, and further user information may be provided. The SSRC of leaving, and further user information may be provided. The SSRC of
the text to expect from joined users MAY be included in a the text to expect from joined users MAY be included in a
notification. The notifications MAY be used both for security notification. The notifications MAY be used both for security
purposes and for translation to a label for presentation to other purposes and for translation to a label for presentation to other
users. users.
5.2. Conference control 5.2. Conference control
In managed conferences, control of the real-time text media SHOULD be In managed conferences, control of the real-time text media SHOULD be
provided in the same way as other for media, e.g. for muting and provided in the same way as other for media, e.g., for muting and
unmuting by the direction attributes in SDP [RFC8866]. unmuting by the direction attributes in SDP [RFC8866].
Note that floor control functions may be of value for RTT users as Note that floor control functions may be of value for RTT users as
well as for users of other media in a conference. well as for users of other media in a conference.
6. Gateway Considerations 6. Gateway Considerations
6.1. Gateway considerations with Textphones (e.g. TTYs). 6.1. Gateway considerations with Textphones
Multi-party RTT sessions may involve gateways of different kinds. multiparty 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 multiparty 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 multiparty awareness, and it SHOULD therefore be suitable for the
gateway to not indicate multi-party awareness for that case. Another gateway to not indicate multiparty awareness for that case. Another
solution is that the gateway indicates multi-party capability towards solution is that the gateway indicates multiparty capability towards
the mixer, and includes the multi-party mixer function for multi- the mixer, and includes the multiparty mixer function for multiparty-
party unaware endpoints itself. This solution makes it possible to unaware endpoints itself. This solution makes it possible to adapt
make adaptations for the functional limitations of the textphone to the functional limitations of the textphone.
(TTY).
More information on gateways to textphones (TTYs) is found in More information on gateways to textphones is found in [RFC5194]
[RFC5194]
6.2. 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 [RFC8865]. In WebRTC, RTT is specified in [RFC8865].
A multi-party bridge may have functionality for communicating by RTT A multiparty bridge may have functionality for communicating by RTT
both in RTP streams with RTT and WebRTC T.140 data channels. Other both in RTP streams with RTT and WebRTC T.140 data channels. Other
configurations may consist of a multi-party bridge with either configurations may consist of a multiparty 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 T.140 data channel. the text communication streams between RTP and T.140 data channel.
In WebRTC, it is assumed that for a multi-party session, one T.140 In WebRTC, it is assumed that for a multiparty session, one T.140
data channel is established for each source from a gateway or bridge data channel is established for each source from a gateway or bridge
to each participant. Each participant also has a data channel with to each participant. Each participant also has a data channel with a
two-way connection with the gateway or bridge. two-way connection with the gateway or bridge.
The t140 channel used both ways is for text from the WebRTC user and The T.140 data channel used both ways is for text from the WebRTC
from the bridge or gateway itself to the WebRTC user. The label user and from the bridge or gateway itself to the WebRTC user. The
parameter of this t140 channel is used as NAME field in RTCP to label parameter of this T.140 data channel is used as the NAME field
participants on the RTP side. The other t140 channels are only for in RTCP to participants on the RTP side. The other T.140 data
text from other participants to the WebRTC user. channels are only for text from other participants to the WebRTC
user.
When a new participant has entered the session with RTP transport of When a new participant has entered the session with RTP transport of
RTT, a new T.140 channel SHOULD be established to WebRTC users with RTT, a new T.140 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 multiparty session with RTT
transport in a WebRTC T.140 data channel, the new participant SHOULD transport in a WebRTC T.140 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.
When a participant on the RTP side disappears, the corresponding When a participant on the RTP side is disconnected from the
T.140 data channel(s) SHOULD be closed. multiparty session, the corresponding T.140 data channel(s) SHOULD be
closed.
When a WebRTC user of T.140 data channels disconnects from the mixer, When a WebRTC user of T.140 data channels disconnects from the mixer,
the corresponding RTP streams or sources in an RTP-mixed stream the corresponding RTP streams or sources in an RTP-mixed stream
SHOULD be closed. SHOULD be closed.
T.140 data channels MAY be opened and closed by negotiation or T.140 data channels MAY be opened and closed by negotiation or
renegotiation of the session or by any other valid means as specified renegotiation of the session or by any other valid means as specified
in section 1 of [RFC8865]. in section 1 of [RFC8865].
7. Updates to RFC 4103 7. Updates to RFC 4103
This document updates [RFC4103] by introducing an SDP media attribute This document updates [RFC4103] by introducing an SDP media attribute
"rtt-mixer" for negotiation of multi-party mixing capability with the "rtt-mixer" for negotiation of multiparty-mixing capability with the
[RFC4103] format, and by specifying the rules for packets when multi- [RFC4103] format, and by specifying the rules for packets when
party capability is negotiated and in use. multiparty capability is negotiated and in use.
8. Congestion considerations 8. Congestion considerations
The congestion considerations and recommended actions from [RFC4103] The congestion considerations and recommended actions from [RFC4103]
are valid also in multi-party situations. are also valid in multiparty situations.
The first action in case of congestion SHALL be to temporarily The first action in case of congestion SHALL be to temporarily
increase the transmission interval up to two seconds. increase the transmission interval up to two seconds.
If the very unlikely situation appears that many participants in a If the very unlikely situation appears that many participants in a
conference send text simultaneously for a long period, a delay may conference send text simultaneously for a long period, a delay may
build up for presentation of text at the receivers if the limitation build up for presentation of text at the receivers if the limitation
in characters per second("CPS") to be transmitted to the participants in characters per second ("CPS") to be transmitted to the
is exceeded. More delay than 7 seconds can cause confusion in the participants is exceeded. More delay than 7 seconds can cause
session. It is therefore RECOMMENDED that an RTP-mixer based mixer confusion in the session. It is therefore RECOMMENDED that an RTP-
discards such text in excess and inserts a general indication of mixer-based mixer discards such text causing excessive delays and
possible text loss [T140ad1] in the session. If the main text inserts a general indication of possible text loss [T140ad1] in the
contributor is indicated in any way, the mixer MAY avoid deleting session. If the main text contributor is indicated in any way, the
text from that participant. It should however be noted that human mixer MAY avoid deleting text from that participant. It should
creation of text normally contains pauses, when the transmission can however be noted that human creation of text normally contains
catch up, so that the transmission overload situations are expected pauses, when the transmission can catch up, so that the transmission
to be very rare. overload situations are expected to be very rare.
9. Acknowledgements 9. Acknowledgements
James Hamlin for format and performance aspects. James Hamlin for format and performance aspects.
10. IANA Considerations 10. IANA Considerations
10.1. Registration of the "rtt-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
skipping to change at page 37, line 18 skipping to change at page 37, line 37
Contact email: iesg@ietf.org Contact email: iesg@ietf.org
Attribute name: rtt-mixer Attribute name: rtt-mixer
Attribute semantics: See RFCXXXX Section 2.3 Attribute semantics: See RFCXXXX Section 2.3
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 multiparty mixing
mixing for real-time text transmission, using a common RTP-stream for real-time text transmission, using a common RTP-stream for
for transmission of text from a number of sources mixed with one 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 2.3 O/A procedure: See RFCXXXX Section 2.3
Mux Category: normal Mux Category: normal
Reference: RFCXXXX Reference: RFCXXXX
11. 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,
and encrypt secured text from the conference participants. Therefore pack, and encrypt secured text from the conference participants.
the mixer needs to be trusted. This is similar to the situation for Therefore the mixer needs to be trusted to achieve security in
central mixers of audio and video. confidentiality and integrity. This situation is similar to the
situation for handling audio and video media in centralized mixers.
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 for already stated in RFC 3550 [RFC3550], and may be restricted for
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.
Participants with malicious intentions may appear and e.g. disturb Participants with malicious intentions may appear and e.g., disturb
the multi-party session by a continuous flow of text, or masquerade the multiparty session by emitting a continuous flow of text. They
as text from other participants. Counteractions should be to require may also send text that appears to originate from other participants.
secure signaling, media and authentication, and to provide higher Counteractions should be to require secure signaling, media and
level conference functions e.g. for blocking and expelling authentication, and to provide higher-layer conference functions
participants. e.g., for blocking, muting, and expelling participants.
Further security considerations specific for this application are Further security considerations specific for this application are
specified in section Section 3.19. specified in Section 3.19.
12. Change history 12. Change history
[RFC Editor: Please remove this section prior to publication.] [RFC Editor: Please remove this section prior to publication.]
12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-16 12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-17
Actions on Gen-ART review comments.
Actions on SecDir review comments.
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-16
Improvements in the offer/answer considerations section by adding Improvements in the offer/answer considerations section by adding
subsections for each phase in the negotiation as requested by IANA subsections for each phase in the negotiation as requested by IANA
expert review. expert review.
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-15 12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-15
Actions on review comments from Jurgen Schonwalder: Actions on review comments from Jurgen Schonwalder:
A bit more about congestion situations and that they are expected to A bit more about congestion situations and that they are expected to
be very rare. be very rare.
Explanation of differences in security between the conference-aware Explanation of differences in security between the conference-aware
and the conference-unaware case added in security section. and the conference-unaware case added in security section.
Presentation examples with suource labels made less confusing, and Presentation examples with suource labels made less confusing, and
explained. explained.
Reference to T.140 inserted at first mentioning of T.140. Reference to T.140 inserted at first mentioning of T.140.
Reference to RFC 8825 inserted to explain WebRTC Reference to RFC 8825 inserted to explain WebRTC
Nit in wording in terminology section adjusted. Nit in wording in terminology section adjusted.
12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-14 12.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-14
Changes from comments by Murray Cucherawy during AD review. Changes from comments by Murray Kucherawy during AD review.
Many SHOULD in section 4.2 on multi-party unaware mixing changed to Many SHOULD in section 4.2 on multiparty-unaware mixing changed to
SHALL, and the whole section instead specified to be optional SHALL, and the whole section instead specified to be optional
depending on the application. depending on the application.
Some SHOULD in section 3 either explained or changed to SHALL. Some SHOULD in section 3 either explained or changed to SHALL.
In order to have explainable conditions behind SHOULDs, the In order to have explainable conditions behind SHOULDs, the
transmission interval in 3.4 is changed to as soon as text is transmission interval in 3.4 is changed to as soon as text is
available as a main principle. The call participants send with 300 available as a main principle. The call participants send with 300
ms interval so that will create realistic load conditions anyway. ms interval so that will create realistic load conditions anyway.
12.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-13 12.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-13
Changed year to 2021. Changed year to 2021.
Changed reference to draft on RTT in WebRTC to recently published RFC Changed reference to draft on RTT in WebRTC to recently published RFC
8865. 8865.
Changed label brackets in example from "[]" to "()" to avoid nits Changed label brackets in example from "[]" to "()" to avoid nits
comment. comment.
Changed reference "RFC 4566" to recently published "RFC 8866" Changed reference "RFC 4566" to recently published "RFC 8866"
12.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-12 12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-12
Changes according to responses on comments from Brian Rosen in Changes according to responses on comments from Brian Rosen in
Avtcore list on 2020-12-05 and -06. Avtcore list on 2020-12-05 and -06.
Changes according to responses to comments by Bernard Aboba in Changes according to responses to comments by Bernard Aboba in
avtcore list 2020-12-06. avtcore list 2020-12-06.
Introduction of an optiona RTP multi-stream mixing method for further Introduction of an optiona RTP multi-stream mixing method for further
study as proposed by Bernard Aboba. study as proposed by Bernard Aboba.
Changes clarifying how to open and close T.140 data channels included Changes clarifying how to open and close T.140 data channels included
in 6.2 after comments by Lorenzo Miniero. in 6.2 after comments by Lorenzo Miniero.
Changes to satisfy nits check. Some "not" changed to "NOT" in Changes to satisfy nits check. Some "not" changed to "NOT" in
normative wording combinations. Some lower case normative words normative wording combinations. Some lower case normative words
changed to upper case. A normative reference deleted from the changed to upper case. A normative reference deleted from the
abstract. Two informative documents moved from normative references abstract. Two informative documents moved from normative references
to informative references. to informative references.
12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-11 12.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-11
Timestamps and timestamp offsets added to the packet examples in Timestamps and timestamp offsets added to the packet examples in
section 3.23, and the description corrected. section 3.23, and the description corrected.
A number of minor corrections added in sections 3.10 - 3.23. A number of minor corrections added in sections 3.10 - 3.23.
12.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-10 12.8. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-10
The packet composition was modified for interleaving packets from The packet composition was modified for interleaving packets from
different sources. different sources.
The packet reception was modified for the new interleaving method. The packet reception was modified for the new interleaving method.
The packet sequence examples was adjusted for the new interleaving The packet sequence examples was adjusted for the new interleaving
method. method.
Modifications according to responses to Brian Rosen of 2020-11-03 Modifications according to responses to Brian Rosen of 2020-11-03
12.8. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09 12.9. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09
Changed name on the SDP media attribute to "rtt-mixer" Changed name on the SDP media attribute to "rtt-mixer"
Restructure of section 2 for balance between aware and unaware cases. Restructure of section 2 for balance between aware and unaware cases.
Moved conference control to own section. Moved conference control to own section.
Improved clarification of recovery and loss in the packet sequence Improved clarification of recovery and loss in the packet sequence
example. example.
A number of editorial corrections and improvements. A number of editorial corrections and improvements.
12.9. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08 12.10. 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.
12.10. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07 12.11. 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-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..
12.11. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 12.12. 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 The "CPS" parameter belongs to the t140 subtype and does not need to
be registered here. be registered here.
12.12. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 12.13. 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.
12.13. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 12.14. 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.
12.14. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 12.15. 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
skipping to change at page 42, line 19 skipping to change at page 43, line 5
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-
12.15. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 12.16. 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.
12.16. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 12.17. 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 multiparty-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.
12.17. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 12.18. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03
to 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.
12.18. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 12.19. 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 multiparty-unaware endpoints
A paragraph recommending CPS=150 inserted in the performance section. A paragraph recommending CPS=150 inserted in the performance section.
12.19. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 12.20. 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 multiparty-unaware
endpoints if the negotiation is not successful. And a reference to a endpoints if the negotiation is not successful. And a reference to a
later chapter about it. later chapter about it.
The presentation considerations chapter 14 is extended with more The presentation considerations chapter 14 is extended with more
information about presentation on multi-party aware endpoints, and a information about presentation on multiparty-aware endpoints, and a
new section on the multi-party unaware mixing with low functionality new section on the multiparty-unaware mixing with low functionality
but SHOULD a be implemented in mixers. Presentation examples are but SHOULD a be implemented in mixers. Presentation examples are
added. added.
A short chapter 15 on gateway considerations is introduced. A short chapter 15 on gateway considerations is introduced.
Clarification about the text/t140 format included in chapter 10. Clarification about the text/t140 format included in chapter 10.
This sentence added to the chapter 10 about use without redundancy. This sentence added to the chapter 10 about use without redundancy.
"The text/red format SHOULD be used unless some other protection "The text/red format SHOULD be used unless some other protection
against packet loss is utilized, for example a reliable network or against packet loss is utilized, for example a reliable network or
skipping to change at page 44, line 15 skipping to change at page 45, line 5
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.
12.20. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 12.21. 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
 End of changes. 237 change blocks. 
447 lines changed or deleted 471 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/