draft-ietf-avtcore-multi-party-rtt-mix-18.txt   draft-ietf-avtcore-multi-party-rtt-mix-19.txt 
AVTCore G. Hellstrom AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: 4103 (if approved) 17 May 2021 Updates: 4103 (if approved) 25 May 2021
Intended status: Standards Track Intended status: Standards Track
Expires: 18 November 2021 Expires: 26 November 2021
RTP-mixer formatting of multiparty Real-time text RTP-mixer formatting of multiparty Real-time text
draft-ietf-avtcore-multi-party-rtt-mix-18 draft-ietf-avtcore-multi-party-rtt-mix-19
Abstract Abstract
This document provides enhancements for RFC 4103 real-time text This document provides enhancements for RFC 4103 real-time text
mixing 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 multiparty 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 Contributing Source (CSRC) list in the Realtime Protocol (RTP) packet
makes use of the same "text/t140" and "text/red" formats as for two- for source identification. The method makes use of the same "text/
party sessions. t140" and "text/red" formats as for two-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 multiparty-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 RFC 8866 Session Description Protocol (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 multiparty-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.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 18 November 2021. This Internet-Draft will expire on 26 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
skipping to change at page 2, line 47 skipping to change at page 2, line 52
2.3. Offer/answer considerations . . . . . . . . . . . . . . . 11 2.3. Offer/answer considerations . . . . . . . . . . . . . . . 11
2.4. Actions depending on capability negotiation result . . . 13 2.4. Actions depending on capability negotiation result . . . 13
3. Details for the RTP-mixer-based mixing method for 3. Details for the RTP-mixer-based mixing method for
multiparty-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 . . . . . . . . . 14 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 . . . . . . . . . . . . . . . 15 3.5. Only one source per packet . . . . . . . . . . . . . . . 15
3.6. Do not send received text to the originating source . . . 15 3.6. Do not send received text to the originating source . . . 15
3.7. Clean incoming text . . . . . . . . . . . . . . . . . . . 15 3.7. Clean incoming text . . . . . . . . . . . . . . . . . . . 16
3.8. Redundant transmission principles . . . . . . . . . . . . 15 3.8. Redundant transmission principles . . . . . . . . . . . . 16
3.9. Interleaving text from different sources . . . . . . . . 15 3.9. Text placement in packets . . . . . . . . . . . . . . . . 16
3.10. Text placement in packets . . . . . . . . . . . . . . . . 16 3.10. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 17
3.11. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 16 3.11. Creation of the redundancy . . . . . . . . . . . . . . . 17
3.12. Creation of the redundancy . . . . . . . . . . . . . . . 17 3.12. Timer offset fields . . . . . . . . . . . . . . . . . . . 18
3.13. Timer offset fields . . . . . . . . . . . . . . . . . . . 17 3.13. Other RTP header fields . . . . . . . . . . . . . . . . . 18
3.14. Other RTP header fields . . . . . . . . . . . . . . . . . 17 3.14. Pause in transmission . . . . . . . . . . . . . . . . . . 18
3.15. Pause in transmission . . . . . . . . . . . . . . . . . . 18 3.15. RTCP considerations . . . . . . . . . . . . . . . . . . . 19
3.16. RTCP considerations . . . . . . . . . . . . . . . . . . . 18 3.16. Reception of multiparty contents . . . . . . . . . . . . 19
3.17. Reception of multiparty contents . . . . . . . . . . . . 18 3.17. Performance considerations . . . . . . . . . . . . . . . 21
3.18. Performance considerations . . . . . . . . . . . . . . . 20 3.18. Security for session control and media . . . . . . . . . 21
3.19. Security for session control and media . . . . . . . . . 20 3.19. SDP offer/answer examples . . . . . . . . . . . . . . . . 22
3.20. SDP offer/answer examples . . . . . . . . . . . . . . . . 21 3.20. Packet sequence example from interleaved transmission . . 23
3.21. Packet sequence example from interleaved transmission . . 22 3.21. Maximum character rate "cps" . . . . . . . . . . . . . . 26
3.22. Maximum character rate "CPS" . . . . . . . . . . . . . . 25 4. Presentation level considerations . . . . . . . . . . . . . . 26
4. Presentation level considerations . . . . . . . . . . . . . . 25 4.1. Presentation by multiparty-aware endpoints . . . . . . . 27
4.1. Presentation by multiparty-aware endpoints . . . . . . . 26 4.2. Multiparty mixing for multiparty-unaware endpoints . . . 29
4.2. multiparty mixing for multiparty-unaware endpoints . . . 28 5. Relation to Conference Control . . . . . . . . . . . . . . . 35
5. Relation to Conference Control . . . . . . . . . . . . . . . 34 5.1. Use with SIP centralized conferencing framework . . . . . 36
5.1. Use with SIP centralized conferencing framework . . . . . 34 5.2. Conference control . . . . . . . . . . . . . . . . . . . 36
5.2. Conference control . . . . . . . . . . . . . . . . . . . 35 6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 36
6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 35 6.1. Gateway considerations with Textphones . . . . . . . . . 36
6.1. Gateway considerations with Textphones . . . . . . . . . 35 6.2. Gateway considerations with WebRTC . . . . . . . . . . . 36
6.2. Gateway considerations with WebRTC . . . . . . . . . . . 35 7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 37
7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 36 8. Congestion considerations . . . . . . . . . . . . . . . . . . 38
8. Congestion considerations . . . . . . . . . . . . . . . . . . 36 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 10.1. Registration of the "rtt-mixer" SDP media attribute . . 38
10.1. Registration of the "rtt-mixer" SDP media attribute . . 37 11. Security Considerations . . . . . . . . . . . . . . . . . . . 39
11. Security Considerations . . . . . . . . . . . . . . . . . . . 38 12. Change history . . . . . . . . . . . . . . . . . . . . . . . 40
12. Change history . . . . . . . . . . . . . . . . . . . . . . . 38
12.1. Changes included in 12.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-18 . . . . . . . 38 draft-ietf-avtcore-multi-party-rtt-mix-19 . . . . . . . 40
12.2. Changes included in 12.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-17 . . . . . . . 38 draft-ietf-avtcore-multi-party-rtt-mix-18 . . . . . . . 40
12.3. Changes included in 12.3. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-16 . . . . . . . 38 draft-ietf-avtcore-multi-party-rtt-mix-17 . . . . . . . 40
12.4. Changes included in 12.4. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-15 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-16 . . . . . . . 40
12.5. Changes included in 12.5. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-14 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-15 . . . . . . . 40
12.6. Changes included in 12.6. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-13 . . . . . . . 39 draft-ietf-avtcore-multi-party-rtt-mix-14 . . . . . . . 41
12.7. Changes included in 12.7. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-12 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-13 . . . . . . . 41
12.8. Changes included in 12.8. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-11 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-12 . . . . . . . 41
12.9. Changes included in 12.9. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-10 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-11 . . . . . . . 42
12.10. Changes included in 12.10. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 40 draft-ietf-avtcore-multi-party-rtt-mix-10 . . . . . . . 42
12.11. Changes included in 12.11. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 42
12.12. Changes included in 12.12. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 43
12.13. Changes included in 12.13. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 43
12.14. Changes included in 12.14. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 43
12.15. Changes included in 12.15. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 41 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 43
12.16. Changes included in 12.16. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 42 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 43
12.17. Changes included in 12.17. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 43 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 44
12.18. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 43 12.18. Changes included in
12.19. Changes from draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 45
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 12.19. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 45
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 43
12.20. Changes from 12.20. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 43 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 45
12.21. Changes from 12.21. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 44 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 45
12.22. Changes from 12.22. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 46
12.23. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 45 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 47
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 45 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
13.1. Normative References . . . . . . . . . . . . . . . . . . 45 13.1. Normative References . . . . . . . . . . . . . . . . . . 47
13.2. Informative References . . . . . . . . . . . . . . . . . 46 13.2. Informative References . . . . . . . . . . . . . . . . . 48
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 47 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 49
1. Introduction 1. Introduction
"RTP Payload for Text Conversation" [RFC4103] specifies use of RTP "RTP Payload for Text Conversation" [RFC4103] specifies use of the
[RFC3550] for transmission of real-time text (RTT) and the "text/ Real-Time Transport Protocol (RTP) [RFC3550] for transmission of
t140" format. It also specifies a redundancy format "text/red" for real-time text (RTT) and the "text/t140" format. It also specifies a
increased robustness. The "text/red" format is registered in redundancy format "text/red" for increased robustness. The "text/
[RFC4102]. red" format is registered in [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 multiparty 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."
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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 multiparty 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 multiparty mixing. RFC 4103 increasing. Emergency call use requires multiparty mixing because it
"RTP Payload for Text Conversation" mixer implementations can use is common that one agent needs to transfer the call to another
traditional RTP functions for source identification, but the specialized agent but is obliged to stay on the call at least to
verify that the transfer was successful. Mixer implementations for
RFC 4103 "RTP Payload for Text Conversation" can use traditional RFC
3550 RTP functions for mixing and 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 However, the redundancy header format has no source indicators for
the redundant transmissions. The redundant parts in a packet must the 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
skipping to change at page 6, line 13 skipping to change at page 6, line 26
possibly otherwise present all received multiparty 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 other indication coded in fields in the packet. Or they may have other
undesirable ways of acting on the multiparty content. The fallback undesirable ways of acting on the multiparty content. The fallback
method is called the mixing procedure for multiparty-unaware method is called the mixing procedure for multiparty-unaware
endpoints. The fallback method is naturally not expected to meet all endpoints. The fallback method is naturally not expected to meet all
performance requirements placed on the mixing procedure for performance requirements placed on the mixing procedure for
multiparty-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 multiparty mixing case declaring support of the RTP-mixer-based multiparty mixing case and
and rules for source indications and interleaving of text from rules for source indications and interleaving of text from different
different sources. 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 Source Description (SDES), Canonical name (CNAME), Name
(NAME), Synchronization Source (SSRC), Contributing Source (CSRC),
CSRC list, CSRC count [CC], Real-Time control protocol (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 textphone type used in North America. "TTY" stands for a textphone type used in North America.
"WebRTC" stands for web-based real-time communication specified by Web based real-time communication (WebRTC) is specified by the World
W3C and IETF. See [RFC8825]. Wide Web Consortium (W3C) and IETF. See [RFC8825].
"DTLS-SRTP" is a DTLS extension for use with SRTP/SRTCP specified in "DTLS-SRTP" is a Datagram Transport Layer Security (DTLS) extension
[RFC5764]. for use with Secure Real-Time Transport Protocol/Secure Real-Time
Control Protocol (SRTP/SRTCP) specified in [RFC5764].
"multiparty-aware" describes 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.
"multiparty-unaware" describes 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.
skipping to change at page 7, line 30 skipping to change at page 7, line 44
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, with 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 multiparty-aware with to upgrade existing endpoint solutions to be multiparty-aware with
skipping to change at page 8, line 5 skipping to change at page 8, line 19
option for further study. option for further study.
RTP-mixer-based method for multiparty-aware endpoints RTP-mixer-based method for multiparty-aware endpoints
The "text/red" format in RFC 4103 is sent with a shorter The "text/red" format in RFC 4103 is sent with a shorter
transmission interval with the RTP-mixer method and indicating the transmission interval with the RTP-mixer method and indicating the
source in the CSRC field. The "text/red" format with a "text/ source in the CSRC field. The "text/red" format with a "text/
t140" payload in a single RTP stream can be sent when text is t140" payload in a single RTP stream can be sent when text is
available from the call participants instead of at the regular 300 available from the call participants instead of at the regular 300
ms. Transmission of packets with text from different sources can ms. Transmission of packets with text from different sources can
then be done smoothly while simultaneous transmission occurs as then be done smoothly while simultaneous transmission occurs as
long as it is not limited by the maximum character rate "CPS". long as it is not limited by the maximum character rate "cps".
With ten participants sending text simultaneously, the switching With ten participants sending text simultaneously, the switching
and transmission performance is good. With more simultaneously and transmission performance is good. With more simultaneously
sending participants, and with receivers having the default sending participants, and with receivers having the default
capacity there will be a noticeable jerkiness and delay in text capacity there will be a noticeable jerkiness and delay in text
presentation. The jerkiness will be more expressed the more presentation. The jerkiness will be more expressed the more
participants who send text simultaneously. Two seconds jerkiness participants who send text simultaneously. Two seconds jerkiness
will be noticeable and slightly unpleasant, but it corresponds in will be noticeable and slightly unpleasant, but it corresponds in
time to what typing humans often cause by hesitation or changing time to what typing humans often cause by hesitation or changing
position while typing. A benefit of this method is that no new position while typing. A benefit of this method is that no new
packet format needs to be introduced and implemented. Since packet format needs to be introduced and implemented. Since
simultaneous typing by more than two parties is very rare, this simultaneous typing by more than two parties is expected to be
method can be used successfully with good performance. Recovery very rare as described in Section 1.3, this method can be used
of text in case of packet loss is based on analysis of timestamps successfully with good performance. Recovery of text in case of
of received redundancy versus earlier received text. Negotiation packet loss is based on analysis of timestamps of received
is based on a new SDP media attribute "rtt-mixer". This method is redundancy versus earlier received text. Negotiation is based on
selected to be the main one specified in this document. a new SDP media attribute "rtt-mixer". This method is selected to
be the main one specified 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]. The sources are indicated
in strict order in the CSRC list of the RTP packets. The CSRC
list can have up to 15 members. Therefore, 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
with 300 ms intervals. The mean delay will be 150 ms. The with 300 ms intervals. The mean delay will be 150 ms. A new
sources are indicated in strict order in the CSRC list of the RTP redundancy packet format is specified. This method would result
packets. A new redundancy packet format is specified. This in good performance, but would require standardization and
method would result in good performance, but would require implementation of new releases in the target technologies that
standardization 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 multiparty-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 implementing RFC 4103 for RTT without explicit support of
multiparty sessions. This is possible by having the mixer insert multiparty sessions. This is possible by having the mixer insert
a new line and a text formatted source label before each switch of a new line and a text formatted source label before each switch of
text source in the stream. Switch of source can only be done in text source in the stream. Switch of source can only be done in
places in the text where it does not disturb the perception of the places in the text where it does not disturb the perception of the
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2.2. Mixing for multiparty-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 multiparty call does not itself implement endpoint participating in a multiparty call does not itself implement
any solution, or not the same, as the mixer. The method requires the any solution, or not the same, as the mixer. The method requires the
mixer to insert text dividers and readable labels and only send text mixer to insert text dividers and readable labels and only send text
from one source at a time until a suitable point appears for source from one source at a time until a suitable point appears for source
change. This solution is a fallback method with functional change. This solution is a fallback method with functional
limitations. 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 mixer SHOULD by default format and transmit text to a call
true multiparty RTT handling, but negotiated a "text/red" or "text/ participant to be suitable to present on a multiparty-unaware
t140" format in a session with a participant SHOULD in order to endpoint which has not negotiated any method for true multiparty RTT
maintain interoperability, if nothing else is specified for the handling, but negotiated a "text/red" or "text/t140" format in a
application, format transmitted text to that participant to be session. This SHOULD be done if nothing else is specified for the
suitable to present on a multiparty-unaware endpoint as further application in order to maintain interoperability. Section 4.2
specified in Section 4.2. specifies how this mixing is done.
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 [RFC4103] by introducing a capability negotiation for handling
multiparty real-time text, a way to indicate the source of multiparty real-time text, a way to indicate the source of
transmitted text, and rules for efficient timing of the transmissions transmitted text, and rules for efficient timing of the transmissions
interleaved from different sources. interleaved from different sources.
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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.
Even when the "rtt-mixer" attribute is successfully negotiated, the
parties MAY send and receive two-party coded real-time text.
An answer MUST NOT include acceptance of more than one method for An answer MUST NOT include acceptance of more than one method for
multiparty 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 multiparty-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
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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 multiparty real-time text. a desired change in the support of multiparty real-time text.
If the modified offer adds indication of support for multiparty real- If the modified offer adds indication of support for multiparty real-
time text by including the "rtt-mixer" SDP attribute, the procedures time text by including the "rtt-mixer" SDP attribute, the 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 multiparty 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. After processing this
after processing the SDP exchange NOT send real-time text formatted SDP exchange, the parties MUST NOT send real-time text formatted for
for multiparty-aware parties according to this specification. 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
multiparty 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
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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.
In other regards, 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 Byte-
character SHALL be sent to it by the other entity according to the Order Mark (BOM) character SHALL be sent to it by the other entity
procedures in this section. If the transmitter is a mixer, then the according to the procedures in this section. This is useful in many
source of this character SHALL be indicated to be the mixer itself. configurations to open ports and firewalls and setting up the
connection between the application and the network. If the
transmitter is a mixer, then the source of this character SHALL be
indicated to be the mixer itself.
Note that the BOM character SHALL be transmitted with the same Note that the BOM character SHALL be transmitted with the same
redundancy procedures as any other text. redundancy procedures as any other text.
3.3. Keep-alive 3.3. Keep-alive
After that, the transmitter SHALL send keep-alive traffic to the After that, the transmitter SHALL send keep-alive traffic to the
receiver(s) at regular intervals when no other traffic has occurred receiver(s) at regular intervals when no other traffic has occurred
during that interval, if that is decided for the actual connection. during that interval, if that is decided for the actual connection.
It is RECOMMENDED to use the keep-alive solution from [RFC6263]. The It is RECOMMENDED to use the keep-alive solution from [RFC6263]. The
consent check of [RFC7675] is a possible alternative if it is used consent check of [RFC7675] is a possible alternative if it is used
anyway for other reasons. anyway for other reasons.
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 between
then be 330 ms, when no other limitations cause a longer interval to text transmissions from the same source SHALL then be 330 ms, when no
be temporarily used. It is RECOMMENDED to send the next packet to a other limitations cause a longer interval to be temporarily used. It
receiver as soon as new text to that receiver is available, as long is RECOMMENDED to send the next packet to a receiver as soon as new
as the maximum character rate ("CPS") to the receiver is not exceeded text to that receiver is available, as long as the mean character
during any 10-second interval. The intention of these time intervals rate of new text to the receiver calculated over the last 10 one-
is to keep the latency low and network load limited while keeping second intervals does not exceed the "cps" value of the receiver.
good protection against text loss in bursty packet loss conditions. The intention is to keep the latency low and network load limited
The main purpose of the 330 ms interval is for timing of redundant while keeping good protection against text loss in bursty packet loss
transmission, when no new text from the same source is available. conditions. The main purpose of the 330 ms interval is for timing of
redundant transmission, when no new text from the same source is
available.
If the "CPS" value is reached, longer transmission intervals SHALL be The reason for the value 330 ms is that many sources of text will
applied and only as much of the text queued for transmission SHALL be transmit new text with 300 ms intervals during periods of continuous
sent at the end of each transmission interval that can be allowed user typing, and then reception in the mixer of such new text will
without exceeding the "CPS" value, until the transmission rate falls cause a combined transmission of the new text and the unsent
under the "CPS" value again. Division of text for partial redundancy from the previous transmission. Only when the user stops
transmission MUST then be made at T140block borders. See also typing, the 330 ms interval will be applied to send the redundancy.
Section 8
If the Characters Per Second (cps) value is reached, a longer
transmission interval SHALL be applied for text from all sources as
specified in [RFC4103] and only as much of the text queued for
transmission SHALL be sent at the end of each transmission interval
as can be allowed without exceeding the "cps" value. Division of
text for partial transmission MUST then be made at T140block borders.
When the transmission rate falls under the "cps" value again, the
transmission intervals SHALL be returned to 330 ms and transmission
of new text SHALL return to be made as soon as new text is available.
NOTE: that extending the transmission intervals during high load
periods does not change the number of characters to be conveyed. It
just evens out the load in time and reduces the number of packets per
second. With human created conversational text, the sending user
will eventually take a pause letting transmission catch up.
See also Section 8.
For a transmitter not acting as 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 normal transmission interval
be 300 ms. SHALL 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 text 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 as specified in
[RFC4103] for single-party sources and Section 3.16 for multiparty
sources (chained mixers).
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. Text placement in packets
When text from more than one source is available for transmission
from a mixer, the mixer SHALL let the sources take turns in having
their text transmitted.
The source with the oldest text received in the mixer or oldest The mixer SHALL compose and transmit an RTP packet to a receiver when
redundant text SHALL be next in turn to get all its available unsent one or more of the following conditions have occurred:
text transmitted. Any redundant repetitions of earlier transmitted
text not yet sent the intended number of times SHALL be included as
redundant retransmission in the transmission.
3.10. Text placement in packets * The transmission interval is the normal 330 ms and there is newly
received unsent text available for transmission to that receiver.
The mixer SHALL compose and transmit an RTP packet to a receiver when * The current transmission interval has passed and is longer than
one of the following conditions has occurred: the normal 330 ms and there is newly received unsent text
available for transmission to that receiver.
* There is unsent text available for transmission to that receiver. * The current transmission interval ( normally 330 ms) has passed
since already transmitted text was queued for transmission as
redundant text.
* 330 ms has passed since already transmitted text was queued for The principles from [RFC4103] apply for populating the header, the
transmission as redundant text. redundancy header and the data in the packet with specifics specified
here and in the following sections.
At the time of transmission, the mixer SHALL populate the RTP packet At the time of transmission, the mixer SHALL populate the RTP packet
with all T140blocks queued for transmission originating from the with all T140blocks queued for transmission originating from the
source in turn for transmission as long as this is not in conflict source in turn for transmission as long as this is not in conflict
with the allowed number of characters per second ("CPS") or the with the allowed number of characters per second ("cps") or the
maximum packet size. In this way, the latency of the latest received maximum packet size. In this way, the latency of the latest received
text is kept low even in moments of simultaneous transmission from text is kept low even in moments of simultaneous transmission from
many sources. many sources.
Redundant text SHALL also be included, and the assessment of how much Redundant text SHALL also be included, and the assessment of how much
new text can be included within the maximum packet size MUST take new text can be included within the maximum packet size MUST take
into account that the redundancy has priority to be transmitted in into account that the redundancy has priority to be transmitted in
its entirety. See Section 3.12 its entirety. See Section 3.4
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.
3.11. Empty T140blocks 3.10. 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 populated 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 3.11. 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 the next transmission of text from that source. The first in the next transmission of text from that source. The first
redundant T140block for that source from the latest transmission is redundant T140block for that source from the latest transmission is
saved for populating the second redundant T140block in the next saved for populating the second redundant T140block in the next
transmission of text from that source. transmission of text from that source.
Usually this is the level of redundancy used. If a higher level 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.
3.13. Timer offset fields 3.12. Timer offset fields
The timestamp offset values SHALL be inserted in the redundancy The timestamp offset values SHALL be inserted in the redundancy
header, with the time offset from the RTP timestamp in the packet header, with the time offset from the RTP timestamp in the packet
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 3.13. 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 self-sourced data. value is used by endpoints, and by mixers sending self-sourced data.
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 header field SHALL contain the SSRC of the RTP session where
SSRC field of the RTP header. the packet will be transmitted.
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 3.14. 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 3.15. RTCP considerations
A mixer SHALL send RTCP reports with SDES, CNAME, and NAME A mixer SHALL send RTCP reports with SDES, CNAME, and NAME
information about the sources in the multiparty call. This makes it information about the sources in the multiparty call. This makes it
possible for participants to compose a suitable label for text from possible for participants to compose a suitable label for text from
each source. each source.
Confidentiality SHALL be considered when composing these fields. Privacy considerations SHALL be taken when composing these fields.
They contain name and address information that may be sensitive to They 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.
3.17. Reception of multiparty contents 3.16. 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. They require
consideration if the stream shall just be forwarded, or distributed considerations 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 3.16.1. 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 3.16.2. Detection and indication of possible text loss
The receiver SHALL monitor the RTP sequence numbers of the received The receiver SHALL monitor the RTP sequence numbers of the received
packets for gaps and packets out of order. If a sequence number gap packets for gaps and packets out of order. If a sequence number gap
appears and still exists after some defined short time for jitter appears and still exists after some defined short time for jitter and
resolution, the packets in the gap SHALL be regarded as lost. reordering 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, the receiver SHALL create a t140block with a loss. In that case, the receiver SHALL create a t140block with a
marker for possible text loss [T140ad1] and associate it with the marker for possible text loss [T140ad1] and associate it with the
source and insert it 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 associated with the SSRC marker for possible text loss [T140ad1] and associated with the SSRC
of the transmitter as a general input from the mixer. of the RTP session 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 whether text was lost or not. Any refined method SHOULD 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 3.16.3. 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 multiparty-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.
skipping to change at page 20, line 12 skipping to change at page 21, line 12
data has contents. The redundant data has contents in the first data has contents. The redundant data has contents in the first
received packet from a source only after initial packet loss. received packet from a source only after initial packet loss.
If the packet is not the first packet from a source, then if the If the packet is not the first packet from a source, then if the
second generation redundant data is available, its timestamp SHALL be second generation redundant data is available, its timestamp SHALL be
created by subtracting its timestamp offset from the RTP timestamp. created by subtracting its timestamp offset from the RTP timestamp.
If the resulting timestamp is later than the latest retrieved data If the resulting timestamp is later than the latest retrieved data
from the same source, then the redundant data SHALL be retrieved and from the same source, then the redundant data SHALL be retrieved and
appended to the receive buffer. The process SHALL be continued in appended to the receive buffer. The process SHALL be continued in
the same way for the first generation redundant data. After that, the same way for the first generation redundant data. After that,
the primary data SHALL be retrieved from the packet and appended to the timestamp of the packet SHALL be compared with the timestamp of
the receive buffer for the source. the latest retrieved data from the same source and if it is later,
then the primary data SHALL be retrieved from the packet and appended
to the receive buffer for the source.
3.17.4. Delete 'BOM' 3.16.4. 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 3.17. 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 mean number of characters per second sent during any 10-second
from a number of simultaneously sending participants to a receiving interval from a number of simultaneously sending participants to a
participant does not reach the 'CPS' value. At higher numbers of receiving participant, does not reach the "cps" value. At higher
characters per second sent, a jerkiness is visible in the numbers of sent characters per second, 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. In such circumstances, the result may be
from each sending participant. It should be noted that it is only unpleasantly jerky presentation of text from each sending
the number of users sending text within the same moment that causes participant. It should be noted that it is only the number of users
jerkiness, not the total number of users with RTT capability. sending text within the same moment that causes jerkiness, not the
total number of users with RTT capability.
3.19. Security for session control and media 3.18. Security for session control and media
Security SHOULD be applied when possible regarding the capabilities Security mechanisms to provide confidentiality and integrity
of the participating devices by use of SIP over TLS by default protection and peer authentication SHOULD be applied when possible
according to [RFC5630] section 3.1.3 on the session control level and regarding the capabilities of the participating devices by use of SIP
by default using DTLS-SRTP [RFC5764] on the media level. In over TLS by default according to [RFC5630] section 3.1.3 on the
applications where legacy endpoints without security may exist, a session control level and by default using DTLS-SRTP [RFC5764] on the
negotiation SHOULD be performed to decide if encryption on the media media level. In applications where legacy endpoints without security
level will be applied. If no other security solution is mandated for are allowed, a negotiation SHOULD be performed to decide if
the application, then OSRTP [RFC8643] is a suitable method to be encryption on the media level will be applied. If no other security
applied to negotiate SRTP media security with DTLS. Most SDP solution is mandated for the application, then OSRTP [RFC8643] is a
examples below are for simplicity expressed without the security suitable method to be applied to negotiate SRTP media security with
additions. The principles (but not all details) for applying DTLS- DTLS. Most SDP examples below are for simplicity expressed without
SRTP [RFC5764] security are shown in a couple of the following the security additions. The principles (but not all details) for
examples. applying DTLS-SRTP [RFC5764] security are shown in a couple of the
following examples.
This document contains two mixing procedures which imply different Further general security considerations are covered in Section 11.
security levels. The mixing for conference-unaware endpoints has
lower security level than the mixing method for conference-aware
endpoints, because there may be an opportunity for a malicious mixer
or a middleman to masquerade the source labels accompanying the text
streams in text format. This is especially true if support of un-
encrypted SIP and media is supported because of lack of such support
in the target endpoints. However, the mixing for conference-aware
endpoints as specified here also requires that the mixer can be
trusted. Further general security considerations are covered in
Section 11.
End-to-end encryption would require further work and could be based End-to-end encryption would require further work and could be based
on WebRTC as specified in Section 1.2. on WebRTC as specified in Section 1.2 or on double encryption as
specified in [RFC8723].
3.20. SDP offer/answer examples 3.19. 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 multiparty-aware examples relate to the single RTP stream mixing for multiparty-aware
endpoints and for multiparty-unaware endpoints. endpoints and for multiparty-unaware endpoints.
Note: Multiparty 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
skipping to change at page 22, line 45 skipping to change at page 23, line 38
With the "fingerprint" the device acknowledges use of SRTP/DTLS. With the "fingerprint" the device acknowledges use of SRTP/DTLS.
Answer example from a multiparty-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 3.20. 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 a 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.
skipping to change at page 24, line 6 skipping to change at page 25, line 6
X CSRC list A | X CSRC list A |
X R2: A2, Offset=630 | X R2: A2, Offset=630 |
X R1: A3, Offset=330 | X R1: A3, Offset=330 |
X P: Empty | X P: Empty |
X------------------------| X------------------------|
Packet 103 is assumed to be lost due to network problems. Packet 103 is assumed to be lost due to network problems.
It contains redundancy for A. Sending A3 as second level It contains redundancy for A. Sending A3 as second level
redundancy is planned for 330 ms after packet 103. redundancy is planned for 330 ms after packet 103.
X------------------------| X------------------------|
X Seq no 104, Timer=20830| X Seq no 104, Timer=20800|
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 due to 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
skipping to change at page 24, line 38 skipping to change at page 25, line 38
Packet 105 is received. Packet 105 is received.
A gap for lost packets 103 and 104 is detected. A gap for lost packets 103 and 104 is detected.
Assume that no other loss was detected during 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 21060-660=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 need 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 104. That
would be at 21160. would be at 21130.
|-----------------------| |-----------------------|
|Seq no 106, Timer=21160| |Seq no 106, Timer=21130|
|CC=1 | |CC=1 |
|CSRC list B | |CSRC list B |
| R2: B1, Offset=660 | | R2: B1, Offset=630 |
| R1: B2, Offset=330 | | R1: B2, Offset=330 |
| P: Empty | | P: Empty |
|-----------------------| |-----------------------|
Packet 106 is received. Packet 106 is received.
The second level redundancy in packet 106 is B1 and has timestamp The second level redundancy in packet 106 is B1 and has timestamp
offset 660 ms. The timestamp of packet 106 minus 660 is 20500 which offset 630 ms. The timestamp of packet 106 minus 630 is 20500 which
is the timestamp of packet 102 THAT was received. So B1 does not is the timestamp of packet 102 that was received. So B1 does not
need to be retrieved. The first level redundancy in packet 106 has need to be retrieved. The first level redundancy in packet 106 has
offset 330. The timestamp of packet 106 minus 330 is 20830. That is offset 330. The timestamp of packet 106 minus 330 is 20800. That is
later than the latest received packet with source B. Therefore B2 is later than the latest received packet with source B. Therefore B2 is
retrieved and assigned to the input buffer for source B. No primary retrieved and assigned to the input buffer for source B. No primary
is available in packet 106. is available in packet 106.
After this sequence, A3 and B1 and B2 have been received. In this After this sequence, A3 and B1 and B2 have been received. In this
case no text was lost. case no text was lost.
3.22. Maximum character rate "CPS" 3.21. Maximum character rate "cps"
The default maximum rate of reception of "text/t140" real-time text The default maximum rate of reception of "text/t140" real-time text
is in [RFC4103] specified to be 30 characters per second. The actual is in [RFC4103] specified to be 30 characters per second. The actual
rate is calculated without regard to any redundant text transmission rate is calculated without regard to any redundant text transmission
and is in the multiparty case evaluated for all sources contributing and is in the multiparty case evaluated for all sources contributing
to transmission to a receiver. The value MAY be modified in the to transmission to a receiver. The value MAY be modified in the
"CPS" parameter of the FMTP attribute in the media section for the "cps" parameter of the FMTP attribute in the media section for the
"text/t140" media. A mixer combining real-time text from a number of "text/t140" media. A mixer combining real-time text from a number of
sources may occasionally have a higher combined flow of text coming sources may occasionally have a higher combined flow of text coming
from the sources. Endpoints SHOULD therefore specify a suitable from the sources. Endpoints SHOULD therefore specify a suitable
higher value for the "CPS" parameter, corresponding to its real higher value for the "cps" parameter, corresponding to its real
reception capability. A value for "CPS" of 90 SHALL be the default reception capability. A value for "cps" of 90 SHALL be the default
for the "text/t140" stream in the "text/red" format when multiparty for the "text/t140" stream in the "text/red" format when multiparty
real-time text is negotiated. See [RFC4103] for the format and use real-time text is negotiated. See [RFC4103] for the format and use
of the "CPS" parameter. The same rules apply for the multiparty case of the "cps" parameter. The same rules apply for the multiparty case
except for the default value. 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 are transport. Functions for erasure and other formatting functions are
specified in [T140] which has the following general statement for the specified in [T140] which has the following general statement for the
presentation: presentation:
skipping to change at page 26, line 11 skipping to change at page 27, line 11
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 conveyed in the the session participants have the same information conveyed in the
text contents of the conversation without necessarily having the text contents of the conversation without necessarily having the
exact 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 ignore
to ignore optional control codes that they do not support. 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 a 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, the same as the Line Separator. Presentation erasing action, the same as the Line Separator. Presentation
functions are allowed to group text for presentation in smaller functions are allowed to group text for presentation in smaller
groups than the line separators imply and present such groups with groups than the line separators imply and present such groups with
source indication together with text groups from other sources (see source indication together with text groups from other sources (see
skipping to change at page 28, line 21 skipping to change at page 29, line 21
| |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. multiparty mixing for multiparty-unaware endpoints 4.2. Multiparty mixing for multiparty-unaware endpoints
When the mixer has indicated RTT multiparty capability in an SDP When the mixer has indicated RTT multiparty capability in an SDP
negotiation, but the multiparty 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
multiparty 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 multiparty awareness, when that is desired in the an endpoint with no multiparty awareness, when that is desired in the
actual implementation. The following specifies a procedure which MAY actual implementation. The following specifies a procedure which MAY
be applied in that situation. be applied in that situation.
skipping to change at page 28, line 45 skipping to change at page 30, line 8
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 of information from [T140] and elements. They are instead composed of information from [T140] and
an ambition to provide a best-effort presentation on an endpoint an ambition to provide a best-effort presentation on an endpoint
which has functions originally intended only for two-party calls. which has functions originally intended only for two-party calls.
The mixer mixing for multiparty-unaware endpoints SHALL compose a The mixer mixing for multiparty-unaware endpoints SHALL compose a
simulated, limited multiparty 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 line separator
them if the transmitted text did not already end with a new line. A between them if the transmitted text did not already end with a new
presentable label SHALL be composed and sent for the source initially line (line separator or CRLF). A presentable label SHALL be composed
in the session and after each source switch. With this procedure the and sent for the source initially in the session and after each
time for switching from transmission of text from one source to source switch. With this procedure the time for switching from
transmission of text from another source depends on the actions of transmission of text from one source to transmission of text from
the users. In order to expedite source switching, a user can, for another source depends on the actions of the users. In order to
example, end its turn with a new line. expedite source switching, a user can, for 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
skipping to change at page 29, line 41 skipping to change at page 31, line 6
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 of information in the CNAME and or CRLF). A label SHALL be composed of information in the CNAME and
NAME fields in RTCP reports from the participant to have its text 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.
Information available to the mixer for composing the label may Information available to the mixer for composing the label may
contain sensitive personal information that SHOULD not be revealed in contain sensitive personal information that SHOULD NOT be revealed in
sessions not securely authenticated and protected. Integrity sessions not securely authenticated and confidentiality protected.
considerations regarding how much personal information is included in Privacy considerations regarding how much personal information is
the label SHOULD therefore be taken when composing the label. 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 the next occurred, a source switch MAY be forced by the mixer at the next
word delimiter, and also even if a word delimiter does not occur word delimiter, and also even if a word delimiter does not occur
skipping to change at page 33, line 32 skipping to change at page 35, line 4
|[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
multiparty 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.
_________________________________________________ _________________________________________________
| |^| | |^|
|(Alice) Hi, Alice here. |-| |(Alice) Hi, Alice here. |-|
| | | | | |
|(mix)[Bob)] Bob as well. | | |(mix)[Bob)] Bob as well. | |
| | | | | |
|[Eve] Hi, this is Eve, calling from Paris | | |[Eve] Hi, this is Eve, calling from Paris | |
| I thought you should be here. | | | I thought you should be here. | |
| | | | | |
|(Alice) I am coming on Thursday, my | | |(Alice) I am coming on Thursday, my | |
| performance is not until Friday morning.| | | performance is not until Friday morning.| |
| | | | | |
|(mix)[Bob] And I on Wednesday evening. | | |(mix)[Bob] And I on Wednesday evening. | |
| | | | | |
|[Eve] we can have dinner and then walk | | |[Eve] we can have dinner and then walk | |
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Figure 6: An example of a view of the multiparty-unaware presentation Figure 6: An example of a view of the multiparty-unaware 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
multiparty 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 Session Initiation Protocol (SIP) conferencing framework, mainly
[RFC4579] and [RFC4575] is suitable for coordinating sessions specified in [RFC4353], [RFC4579] and [RFC4575] is suitable for
including multiparty RTT. The RTT stream between the mixer and a coordinating sessions including multiparty RTT. The RTT stream
participant is one and the same during the conference. Participants between the mixer and a participant is one and the same during the
get announced by notifications when participants are joining or conference. Participants get announced by notifications when
leaving, and further user information may be provided. The SSRC of participants are joining or leaving, and further user information may
the text to expect from joined users MAY be included in a be provided. The SSRC of the text to expect from joined users MAY be
notification. The notifications MAY be used both for security included in a notification. The notifications MAY be used both for
purposes and for translation to a label for presentation to other security purposes and for translation to a label for presentation to
users. other 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 6.1. Gateway considerations with Textphones
multiparty 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
multiparty 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 Public Switched Telephone
textphones (e.g., TTYs). Textphones are limited devices with no Network (PSTN) for communication with textphones (e.g., TTYs).
multiparty awareness, and it SHOULD therefore be suitable for the Textphones are limited devices with no multiparty awareness, and it
gateway to not indicate multiparty awareness for that case. Another SHOULD therefore be suitable for the gateway to not indicate
solution is that the gateway indicates multiparty capability towards multiparty awareness for that case. Another solution is that the
the mixer, and includes the multiparty mixer function for multiparty- gateway indicates multiparty capability towards the mixer, and
unaware endpoints itself. This solution makes it possible to adapt includes the multiparty mixer function for multiparty-unaware
to the functional limitations of the textphone. endpoints itself. This solution makes it possible to adapt to the
functional limitations of the textphone.
More information on gateways to textphones is found in [RFC5194] More information on gateways to textphones is found in [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 multiparty 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
skipping to change at page 36, line 49 skipping to change at page 38, line 16
The congestion considerations and recommended actions from [RFC4103] The congestion considerations and recommended actions from [RFC4103]
are also valid in multiparty situations. are also valid in multiparty situations.
The time values SHALL then be applied per source of text sent to a The time values SHALL then be applied per source of text sent to a
receiver. receiver.
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 in characters per second ("cps") to be transmitted to the
participants is exceeded. More delay than 7 seconds can cause participants is exceeded. More delay than 7 seconds can cause
confusion in the session. It is therefore RECOMMENDED that an RTP- confusion in the session. It is therefore RECOMMENDED that an RTP-
mixer-based mixer discards such text causing excessive delays and mixer-based mixer discards such text causing excessive delays and
inserts a general indication of possible text loss [T140ad1] in the inserts a general indication of possible text loss [T140ad1] in the
session. If the main text contributor is indicated in any way, the session. If the main text contributor is indicated in any way, the
mixer MAY avoid deleting text from that participant. It should mixer MAY avoid deleting text from that participant. It should
however be noted that human creation of text normally contains however be noted that human creation of text normally contains
pauses, when the transmission can catch up, so that the transmission pauses, when the transmission can catch up, so that the transmission
overload situations are expected to be very rare. overload situations are expected to be very rare.
skipping to change at page 38, line 9 skipping to change at page 39, line 21
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, The RTP-mixer model requires the mixer to be allowed to decrypt,
pack, and encrypt secured text from the conference participants. pack, and encrypt secured text from the conference participants.
Therefore, the mixer needs to be trusted to achieve security in Therefore, the mixer needs to be trusted to maintain confidentiality
confidentiality and integrity. This situation is similar to the and integrity of the RTT data. This situation is similar to the
situation for handling audio and video media in centralized mixers. 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, may have privacy concerns as already stated in RFC
already stated in RFC 3550 [RFC3550], and may be restricted for 3550 [RFC3550], and may be restricted for privacy reasons. When used
privacy reasons. The receiving user will then get a more symbolic for creation of readable labels in the presentation, the receiving
label for the source. user will then get a more symbolic label for the source.
The services available through the RTT mixer may have special
interest for deaf and hard-of-hearing persons. Some users may want
to refrain from revealing such characteristics broadly in
conferences. The design of the conference systems where the mixer is
included MAY need to be made with confidentiality of such
characteristics in mind.
Participants with malicious intentions may appear and e.g., disturb Participants with malicious intentions may appear and e.g., disturb
the multiparty session by emitting a continuous flow of text. They the multiparty session by emitting a continuous flow of text. They
may also send text that appears to originate from other participants. may also send text that appears to originate from other participants.
Counteractions should be to require secure signaling, media and Counteractions should be to require secure signaling, media and
authentication, and to provide higher-layer conference functions authentication, and to provide higher-layer conference functions
e.g., for blocking, muting, and expelling participants. e.g., for blocking, muting, and expelling participants.
Participants with malicious intentions may also try to disturb the
presentation by sending incomplete or malformed control codes.
Handling of text from the different sources by the receivers MUST
therefore be well separated so that the effects of such actions only
affect text from the source causing the action.
Care should be taken that if use of the mixer is allowed for users
both with and without security procedures, opens for possible attacks
by both unauthenticated call participants and even eavesdropping and
manipulating of content non-participants.
As already stated in Section 3.18, security in media SHOULD be
applied by using DTLS-SRTP [RFC5764] on the media level.
Further security considerations specific for this application are Further security considerations specific for this application are
specified in Section 3.19. specified in Section 3.18.
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-18 12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-19
Edits because of comments in a review by Francesca Palombini.
Edits because of comments from Benjamin Kaduk.
Proposed to not change anything because of Robert Wilton's comments.
Two added sentences in the security section to meet comments by Roman
Danyliw.
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-18
Edits of nits as proposed in a review by Lars Eggert. Edits of nits as proposed in a review by Lars Eggert.
Edits as response to review by Martin Duke. Edits as response to review by Martin Duke.
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-17 12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-17
Actions on Gen-ART review comments. Actions on Gen-ART review comments.
Actions on SecDir review comments. Actions on SecDir review comments.
12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-16 12.4. 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.
12.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-15 12.5. 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.
12.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-14 12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-14
Changes from comments by Murray Kucherawy during AD review. Changes from comments by Murray Kucherawy during AD review.
Many SHOULD in section 4.2 on multiparty-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.
12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-13 12.7. 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"
12.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-12 12.8. 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.
12.8. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-11 12.9. 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.
12.9. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-10 12.10. 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
12.10. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09 12.11. 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.
12.11. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08 12.12. 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.
12.12. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07 12.13. 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.
12.13. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 12.14. 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.
12.14. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 12.15. 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.
12.15. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 12.16. 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.
12.16. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 12.17. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03
Mention possible need to mute and raise hands as for other media. Mention possible need to mute and raise hands as for other media.
---done ---- ---done ----
Make sure that use in two-party calls is also possible and explained. Make sure that use in two-party calls is also possible and explained.
- may need more wording - - may need more wording -
Clarify the RTT is often used together with other media. --done-- Clarify the RTT is often used together with other media. --done--
Tell that text mixing is N-1. A users own text is not received in Tell that text mixing is N-1. A users own text is not received in
skipping to change at page 43, line 4 skipping to change at page 44, line 48
Clarify redundancy level per connection. -done- Clarify redundancy level per connection. -done-
Timestamp also for the last data header. To make it possible for all Timestamp also for the last data header. To make it possible for all
text to have time offset as for transmission from the source. Make text to have time offset as for transmission from the source. Make
that header equal to the others. -done- that header equal to the others. -done-
Mixer always use the CSRC list, even for its own BOM. -done- Mixer always use the CSRC list, even for its own BOM. -done-
Combine all talk about transmission interval (300 ms vs when text has Combine all talk about transmission interval (300 ms vs when text has
arrived) in section 3 in one paragraph or close to each other. -done- arrived) in section 3 in one paragraph or close to each other. -done-
Documents the goal of good performance with low delay for 5 Documents the goal of good performance with low delay for 5
simultaneous typers in the introduction. -done- simultaneous typers in the introduction. -done-
Describe better that only primary text shall be sent on to receivers. Describe better that only primary text shall be sent on to receivers.
Redundancy and loss must be resolved by the mixer. -done- Redundancy and loss must be resolved by the mixer. -done-
12.17. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 12.18. 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.
12.18. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 12.19. 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 multiparty-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.
12.19. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 12.20. 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.
12.20. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 12.21. 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 multiparty-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.
12.21. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 12.22. 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 multiparty-unaware introduced that the mixer should make a mix for multiparty-unaware
skipping to change at page 45, line 14 skipping to change at page 47, 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 whose text is included in one or more text includes participants whose 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.
12.22. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 12.23. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00
to -01 to -01
Editorial cleanup. Editorial cleanup.
Changed capability indication from fmtp-parameter to SDP attribute Changed capability indication from fmtp-parameter to SDP attribute
"rtt-mix". "rtt-mix".
Swapped order of redundancy elements in the example to match reality. Swapped order of redundancy elements in the example to match reality.
Increased the SDP negotiation section Increased the SDP negotiation section
skipping to change at page 46, line 26 skipping to change at page 48, line 20
[RFC7675] Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M. [RFC7675] Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M.
Thomson, "Session Traversal Utilities for NAT (STUN) Usage Thomson, "Session Traversal Utilities for NAT (STUN) Usage
for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675, for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675,
October 2015, <https://www.rfc-editor.org/info/rfc7675>. October 2015, <https://www.rfc-editor.org/info/rfc7675>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for
Browser-Based Applications", RFC 8825,
DOI 10.17487/RFC8825, January 2021,
<https://www.rfc-editor.org/info/rfc8825>.
[RFC8865] Holmberg, C. and G. Hellström, "T.140 Real-Time Text [RFC8865] Holmberg, C. and G. Hellström, "T.140 Real-Time Text
Conversation over WebRTC Data Channels", RFC 8865, Conversation over WebRTC Data Channels", RFC 8865,
DOI 10.17487/RFC8865, January 2021, DOI 10.17487/RFC8865, January 2021,
<https://www.rfc-editor.org/info/rfc8865>. <https://www.rfc-editor.org/info/rfc8865>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: [RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866, Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021, DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>. <https://www.rfc-editor.org/info/rfc8866>.
skipping to change at page 47, line 35 skipping to change at page 49, line 25
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667, [RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015, DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/info/rfc7667>. <https://www.rfc-editor.org/info/rfc7667>.
[RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T. [RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
Stach, "An Opportunistic Approach for Secure Real-time Stach, "An Opportunistic Approach for Secure Real-time
Transport Protocol (OSRTP)", RFC 8643, Transport Protocol (OSRTP)", RFC 8643,
DOI 10.17487/RFC8643, August 2019, DOI 10.17487/RFC8643, August 2019,
<https://www.rfc-editor.org/info/rfc8643>. <https://www.rfc-editor.org/info/rfc8643>.
[RFC8723] Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
"Double Encryption Procedures for the Secure Real-Time
Transport Protocol (SRTP)", RFC 8723,
DOI 10.17487/RFC8723, April 2020,
<https://www.rfc-editor.org/info/rfc8723>.
[RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for
Browser-Based Applications", RFC 8825,
DOI 10.17487/RFC8825, January 2021,
<https://www.rfc-editor.org/info/rfc8825>.
Author's Address Author's Address
Gunnar Hellstrom Gunnar Hellstrom
Gunnar Hellstrom Accessible Communication Gunnar Hellstrom Accessible Communication
SE-13670 Vendelso SE-13670 Vendelso
Sweden Sweden
Email: gunnar.hellstrom@ghaccess.se Email: gunnar.hellstrom@ghaccess.se
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