draft-ietf-avtcore-multi-party-rtt-mix-01.txt   draft-ietf-avtcore-multi-party-rtt-mix-02.txt 
AVTCore G. Hellstrom AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: RFC 4102, RFC 4103 (if approved) 14 May 2020 Updates: RFC 4102, RFC 4103 (if approved) 20 May 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 15 November 2020 Expires: 21 November 2020
RTP-mixer formatting of multi-party Real-time text RTP-mixer formatting of multi-party Real-time text
draft-ietf-avtcore-multi-party-rtt-mix-01 draft-ietf-avtcore-multi-party-rtt-mix-02
Abstract Abstract
Real-time text mixers for multi-party sessions need to identify the Real-time text mixers for multi-party sessions need to identify the
source of each transmitted group of text so that the text can be source of each transmitted group of text so that the text can be
presented by endpoints in suitable grouping with other text from the presented by endpoints in suitable grouping with other text from the
same source. same source.
Regional regulatory requirements specify provision of real-time text Regional regulatory requirements specify provision of real-time text
in multi-party calls. RFC 4103 mixer implementations can use in multi-party calls. RFC 4103 mixer implementations can use
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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 15 November 2020. This Internet-Draft will expire on 21 November 2020.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
skipping to change at page 2, line 33 skipping to change at page 2, line 33
as described in Section 4.e of the Trust Legal Provisions and are as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License. provided without warranty as described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Selected solution and considered alternative . . . . . . 4 1.1. Selected solution and considered alternative . . . . . . 4
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Intended application . . . . . . . . . . . . . . . . . . 5 1.3. Intended application . . . . . . . . . . . . . . . . . . 5
2. Use of fields in the RTP packets . . . . . . . . . . . . . . 5 2. Use of fields in the RTP packets . . . . . . . . . . . . . . 5
3. Actions at transmission by a mixer . . . . . . . . . . . . . 7 3. Actions at transmission by a mixer . . . . . . . . . . . . . 8
4. Actions at reception . . . . . . . . . . . . . . . . . . . . 9 4. Actions at reception . . . . . . . . . . . . . . . . . . . . 10
5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 10 5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 10 6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 11
7. Usage without redundancy . . . . . . . . . . . . . . . . . . 10 7. Usage without redundancy . . . . . . . . . . . . . . . . . . 12
8. Use with SIP centralized conferencing framework . . . . . . . 11 8. Use with SIP centralized conferencing framework . . . . . . . 12
9. Media Subtype Registration . . . . . . . . . . . . . . . . . 11 9. Media Subtype Registration . . . . . . . . . . . . . . . . . 12
10. SDP considerations . . . . . . . . . . . . . . . . . . . . . 12 10. SDP considerations . . . . . . . . . . . . . . . . . . . . . 14
11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14 10.1. Security for session control and media . . . . . . . . . 14
12. Performance considerations . . . . . . . . . . . . . . . . . 17 10.2. SDP offer/answer examples . . . . . . . . . . . . . . . 14
13. Presentation level considerations . . . . . . . . . . . . . . 17 11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
13.1. Presentation by multi-party aware endpoints . . . . . . 18 12. Performance considerations . . . . . . . . . . . . . . . . . 19
13.2. Multi-party mixing for multi-party unaware endpoints . . 20 13. Presentation level considerations . . . . . . . . . . . . . . 19
14. Gateway Considerations . . . . . . . . . . . . . . . . . . . 26 13.1. Presentation by multi-party aware endpoints . . . . . . 20
15. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 26 13.2. Multi-party mixing for multi-party unaware endpoints . . 22
16. Congestion considerations . . . . . . . . . . . . . . . . . . 26 14. Gateway Considerations . . . . . . . . . . . . . . . . . . . 27
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26 14.1. Gateway considerations with Textphones (e.g. TTYs). . . 28
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 14.2. Gateway considerations with WebRTC. . . . . . . . . . . 28
19. Security Considerations . . . . . . . . . . . . . . . . . . . 27 15. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 29
20. Change history . . . . . . . . . . . . . . . . . . . . . . . 27 16. Congestion considerations . . . . . . . . . . . . . . . . . . 29
20.1. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 27 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
20.2. Changes from 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 19. Security Considerations . . . . . . . . . . . . . . . . . . . 29
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 27 20. Change history . . . . . . . . . . . . . . . . . . . . . . . 29
20.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 30
20.2. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 30
20.3. Changes from 20.3. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 27 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 30
20.4. Changes from 20.4. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 28 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 30
20.5. Changes from 20.5. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 31
20.6. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 29 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 32
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 21. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
21.1. Normative References . . . . . . . . . . . . . . . . . . 29 21.1. Normative References . . . . . . . . . . . . . . . . . . 32
21.2. Informative References . . . . . . . . . . . . . . . . . 30 21.2. Informative References . . . . . . . . . . . . . . . . . 33
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 31 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction 1. Introduction
RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for
transmission of real-time text (RTT) and the "text/t140" format. It transmission of real-time text (RTT) and the "text/t140" format. It
also specifies a redundancy format "text/red" for increased also specifies a redundancy format "text/red" for increased
robustness. RFC 4102 [RFC4102] registers the "text/red" format. robustness. RFC 4102 [RFC4102] registers the "text/red" format.
Regional regulatory requirements specify provision of real-time text Regional regulatory requirements specify provision of real-time text
in multi-party calls. in multi-party calls.
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1.3. Intended application 1.3. Intended application
The scheme for identification of source of redundant transmissions is The scheme for identification of source of redundant transmissions is
intended for transmission from entities taking the mixer role in intended for transmission from entities taking the mixer role in
centralised mixing configurations for RTT. It is intended for centralised mixing configurations for RTT. It is intended for
reception by both endpoints and mixers. reception by both endpoints and mixers.
2. Use of fields in the RTP packets 2. Use of fields in the RTP packets
RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
redundancy format "text/red" for increased robustness. This redundancy format "text/red" for increased robustness of real-time
specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by text transmission. The current specification updates RFC
introducing a rule for populating and using the CSRC-list in the RTP 4102[RFC4102] and RFC 4103[RFC4103] by introducing a rule for
packet and extending the redundancy header in order to enhance the populating and using the CSRC-list in the RTP packet and extending
performance in multi-party RTT sessions. the redundancy header in order to enhance the performance in multi-
party RTT sessions.
The "text/rex" format is an "n-tuple" of the "text/red" format
intended to carry text information from up to 16 sources per packet.
The CSRC members, the redundancy header and the T140block areas SHALL
be populated with a number of the same fields as they are specified
in RFC 4103[RFC4103] appended in the same order as the sources appear
in the CSRC list. The most recent text should be placed as the last
text element.
When transmitted from a mixer, the members in the CSRC-list SHALL When transmitted from a mixer, the members in the CSRC-list SHALL
contain the SSRCs of the sources of the T140blocks in the packet. contain the SSRCs of the sources of the T140blocks in the packet.
When redundancy is used, text from all included sources MUST have the The order of the CSRC members MUST be the same as the order of
same number of redundancy generations. The primary, second and sources of the T140blocks and redundant header fields. When
further redundant generations of T140blocks are grouped per source in redundancy is used, text from all included sources MUST have the same
the packet in "source groups" and in the same order as the SSRCs of number of redundancy generations. The primary, first redundant,
the sources in the CSRC list. ( the recommended level of redundancy second redundant and possible further redundant generations of
is to use one primary and two redundant generations of T140blocks.) T140blocks MUST be grouped per source in the packet in "source
In some cases, a primary or redundant T140block is empty, but is groups". The recommended level of redundancy is to use one primary
still represented by a member in the redundancy header. and two redundant generations of T140blocks. In some cases, a
primary or redundant T140block is empty, but is still represented by
a member in the redundancy header.
The CC field SHALL show the number of members in the CSRC list. The CC field SHALL show the number of members in the CSRC list.
The RTP header is followed by one or more source groups of data block
headers: one header for each text block to be included. Each of
these headers except the last provides the timestamp offset and
length of the corresponding data block, in addition to the payload
type number corresponding to the payload format "text/t140". The
data block headers are followed by the data fields carrying
T140blocks from the sources. Finally, the most recent T140block (if
any) follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| block PT | timestamp offset | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The bits in the header are specified as follows:
F: 1 bit First bit in header indicates whether another header block
follows. It has value 1 if further header blocks follow, and
value 0 if this is the last header block.
block PT: 7 bits RTP payload type number for this block,
corresponding to the t140 payload type.
timestamp offset: 14 bits Unsigned offset of timestamp of this block
relative to timestamp given in RTP header. The offset is a time
to be subtracted from the current timestamp to determine the
timestamp of the data when this block was received to the mixer.
For primary data from a mixer, the resulting time is the time when
the data or the youngest part of the data was received by the
mixer. For primary data from an endpoint, only one primary data
is included last in the packet. If the timestamp offset would be
>15 000, it is set to 15 000. For redundant data, the resulting
time the resulting time is the time when the data was sent as
primary. If the value would be >15 000, then it SHALL be set to
15 000 plus 300 times the redundancy level of the data. The high
values appear only in exceptional cases, e.g. when some data has
been held in order to keep the text flow under the cps limit.
block length: 10 bits Length in bytes of the corresponding data
block excluding header.
The last data block is placed last in the packet. The timestamp and
block-length fields are derived from the header of this block, since
they may be determined from the RTP header and overall packet length.
The header for the final block comprises only a zero F bit, and the
block payload type number information corresponding to the t140
format, a total of 8 bits. This is illustrated in the figure below:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0| Block PT |
+-+-+-+-+-+-+-+-+
This specification departs from section 4 of RFC 2198 [RFC2198] which This specification departs from section 4 of RFC 2198 [RFC2198] which
associates the whole of the CSRC-list with the primary data and associates the whole of the CSRC-list with the primary data and
assumes that the same list applies to reconstructed redundant data. assumes that the same list applies to reconstructed redundant data.
In the present specification a T140block is associated with exactly In the present specification a T140block is associated with exactly
one CSRC list member as described above. Also RFC 2198 [RFC2198] one CSRC list member as described above. Also RFC 2198 [RFC2198]
anticipates infrequent change to CSRCs; implementers should be aware anticipates infrequent change to CSRCs; implementers should be aware
that the order of the CSRC-list according to this specification will that the order of the CSRC-list according to this specification will
vary during transitions between transmission from the mixer of text vary during transitions between transmission from the mixer of text
originated by different participants. originated by different participants.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-P" T.140 encoded primary data | | "C-P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1:A 'text/rex' packet with text from three sources A, B, C. Figure 1:A 'text/rex' packet with text from three sources A, B, C.
3. Actions at transmission by a mixer 3. Actions at transmission by a mixer
A "text/rex" transmitter is sending packets at least at regular A "text/rex" transmitter is sending packets at least at regular
transmission intervals as long as there is something (new or transmission intervals as long as there is something (new or
redundant T140blocks) to transmit. The default transmission interval redundant T140blocks) to transmit. The default transmission interval
for point-to-point operation is 300 ms. that SHOULD be applied is 300 ms.
As soon as a participant is known to participate in a session and As soon as a participant is known to participate in a session and
being available for text reception, a Unicode BOM character SHALL be being available for text reception, a Unicode BOM character SHALL be
sent to it according to the procedures in the present document. sent to it according to the procedures in the present document.
After that, the transmitter SHALL send keep-alive traffic to the After that, the transmitter SHALL send keep-alive traffic to the
receivers at regular intervals when no other traffic has occurred receivers at regular intervals when no other traffic has occurred
during that interval if that is decided for the actual connection. during that interval if that is decided for the actual connection.
Recommendations for keep-alive can be found in RFC 6263[RFC6263]. Recommendations for keep-alive can be found in RFC 6263[RFC6263].
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according to common SDP rules. according to common SDP rules.
The sdp media format defined here, is named "rex", for extended The sdp media format defined here, is named "rex", for extended
redundancy. It is intended to be used in "text" media descriptions redundancy. It is intended to be used in "text" media descriptions
with "text/rex" and "text/t140" formats. Both formats MUST be with "text/rex" and "text/t140" formats. Both formats MUST be
declared for the "text/rex" format to be used. It indicates declared for the "text/rex" format to be used. It indicates
capability to use source indications in the CSRC list and the packet capability to use source indications in the CSRC list and the packet
format according to the present specification. It also indicates format according to the present specification. It also indicates
ability to receive 150 real-time text characters per second. ability to receive 150 real-time text characters per second.
Security SHOULD be applied. If no other security solution is 10.1. Security for session control and media
mandated by the application then RFC 8643 [RFC8643] MUST be applied.
The SDP examples below are expressed without the security additions
for simplicity. EDITOR NOTE - more about security------
SDP offer/answer examples Security SHOULD be applied on both session control and media. In
applications where legacy endpoints without security may exist, a
negotiation between security and no security SHOULD be applied. If
no other security solution is mandated by the application, then RFC
8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media
security with DTLS. Most SDP examples below are expressed without
the security additions for simplicity. The principles (but not all
details) for applying DTLS-SRTP security is shown in a couple of the
following examples.
10.2. SDP offer/answer examples
Offer example for just multi-party capability: Offer example for just multi-party capability:
m=text 11000 RTP/AVP 101 98 m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
Answer example from a multi-party capable device Answer example from a multi-party capable device
m=text 12000 RTP/AVP 101 98 m=text 12000 RTP/AVP 101 98
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a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
Answer example from a multi-party capable device Answer example from a multi-party capable device
m=text 11000 RTP/AVP 101 98 m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000 a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98 a=fmtp:101 98/98/98
Answer example from a multi-party unaware device: Offer example for both traditional "text/red" and multi-party format
including security:
a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 101 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=rtpmap:101 rex/1000
a=fmtp:100 98/98/98
a=fmtp:101 98/98/98
The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media
line still indicating RTP/AVP.
Answer example from a multi-party capable device including security
a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
With the "fingerprint" the device acknowledges use of SRTP/DTLS.
Answer example from a multi-party unaware device that also
does not support security:
m=text 12000 RTP/AVP 100 98 m=text 12000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
A party who has negotiated the "text/rex" format MUST populate the A party who has negotiated the "text/rex" format MUST populate the
CSRC-list and format the packets according to the present CSRC-list and format the packets according to the present
specification if it acts as an rtp-mixer and sends multi-party text. specification if it acts as an rtp-mixer and sends multi-party text.
skipping to change at page 26, line 6 skipping to change at page 28, line 4
| the hotel by 11 because I need |-| | the hotel by 11 because I need |-|
| |-| | |-|
|______________________________________________|v| |______________________________________________|v|
| of course, I underst | | of course, I underst |
|________________________________________________| |________________________________________________|
Figure 5: An example of a view of the multi-party unaware Figure 5: An example of a view of the multi-party unaware
presentation in chat style. Alice is the local user. presentation in chat style. Alice is the local user.
14. Gateway Considerations 14. Gateway Considerations
14.1. Gateway considerations with Textphones (e.g. TTYs).
Multi-party RTT sessions may involve gateways of different kinds. Multi-party RTT sessions may involve gateways of different kinds.
Gateways involved in setting up sessions SHALL correctly reflect the Gateways involved in setting up sessions SHALL correctly reflect the
multi-party capability or unawareness of the combination of the multi-party capability or unawareness of the combination of the
gateway and the remote endpoint beyond the gateway. gateway and the remote endpoint beyond the gateway.
One case that may occur is a gateway to PSTN for communication with One case that may occur is a gateway to PSTN for communication with
textphones (e.g. TTYs). Textphones are limited devices with no textphones (e.g. TTYs). Textphones are limited devices with no
multi-party awareness, and it SHOULD therefore be suitable for the multi-party awareness, and it SHOULD therefore be suitable for the
gateway to not indicate multi-party awareness for that case. Another gateway to not indicate multi-party awareness for that case. Another
solution is that the gateway indicates multi-party capability towards solution is that the gateway indicates multi-party capability towards
the mixer, and includes the multi-party mixer function for multi- the mixer, and includes the multi-party mixer function for multi-
party unaware endpoints itself. This solution makes it possible to party unaware endpoints itself. This solution makes it possible to
make adaptations for the functional limitations of the textphone make adaptations for the functional limitations of the textphone
(TTY). (TTY).
14.2. Gateway considerations with WebRTC.
Gateway operation to real-time text in WebRTC may also be required. Gateway operation to real-time text in WebRTC may also be required.
In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data- In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data-
channel[I-D.ietf-mmusic-t140-usage-data-channel]. EDITOR NOTE ----- channel[I-D.ietf-mmusic-t140-usage-data-channel].
more about this is needed------------
A multi-party bridge may have functionality for communicating by RTT
in both RTP streams with RTT and WebRTC t140 data channels. Other
configurations may consist of a multi-party bridge with either
technology for RTT transport and a separate gateway for conversion of
the text communication streams between RTP and t140 data channel.
In WebRTC, it is assumed that for a multi-party session, one t140
data channel is established for each source from a gateway or bridge
to each participant. Each participant also has a data channel with
two-way connection with the gateway or bridge.
The t140 channel used both ways is for text from the WebRTC user and
from the bridge or gateway itself to the WebRTC user. The label
parameter of this t140 channel is used as NAME field in RTCP to
participants on the RTP side. The other t140 channels are only for
text from other participants to the WebRTC user.
When a new participant has entered the session with RTP transport of
rtt, a new t140 channel SHOULD be established to WebRTC users with
the label parameter composed from the NAME field in RTCP on the RTP
side.
When a new participant has entered the multi-party session with RTT
transport in a WebRTC t140 data channel, the new participant SHOULD
be announced by a notification to RTP users. The label parameter
from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP
side.
15. Updates to RFC 4102 and RFC 4103 15. Updates to RFC 4102 and RFC 4103
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format 'text/rex' for the multi-party introducing an extended packet format 'text/rex' for the multi-party
mixing case and more strict rules for the use of redundancy, and mixing case and more strict rules for the use of redundancy, and
population of the CSRC list in the packets. Implications for the population of the CSRC list in the packets. Implications for the
CSRC list use from RFC 2198[RFC2198] are hereby not in effect. CSRC list use from RFC 2198[RFC2198] are hereby not in effect.
16. Congestion considerations 16. Congestion considerations
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the mixer needs to be trusted. This is similar to the situation for the mixer needs to be trusted. This is similar to the situation for
central mixers of audio and video. central mixers of audio and video.
The requirement to transfer information about the user in RTCP The requirement to transfer information about the user in RTCP
reports in SDES, CNAME and NAME fields for creation of labels may reports in SDES, CNAME and NAME fields for creation of labels may
have privacy concerns as already stated in RFC 3550 [RFC3550], and have privacy concerns as already stated in RFC 3550 [RFC3550], and
may be restricted of privacy reasons. The receiving user will then may be restricted of privacy reasons. The receiving user will then
get a more symbolic label for the source. get a more symbolic label for the source.
20. Change history 20. Change history
20.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02
20.1. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 SDP and better description and visibility of security by OSRTP RFC
8634 needed.
The description of gatewaying to WebRTC extended.
The description of the redundancy header in the packet is improved.
20.2. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01
2,5,6 More efficient format text/rex introduced and attribute a=rtt- 2,5,6 More efficient format text/rex introduced and attribute a=rtt-
mix deleted. mix deleted.
3. Brief about use of OSRTP for security included- More needed. 3. Brief about use of OSRTP for security included- More needed.
4. Brief motivation for the solution and why not rtp-translator is 4. Brief motivation for the solution and why not rtp-translator is
used added to intro. used added to intro.
7. More limitations for the multi-party unaware mixing method 7. More limitations for the multi-party unaware mixing method
inserted. inserted.
8. Updates to RFC 4102 and 4103 more clearly expressed. 8. Updates to RFC 4102 and 4103 more clearly expressed.
9. Gateway to WebRTC started. More needed. 9. Gateway to WebRTC started. More needed.
20.2. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 20.3. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00 draft-ietf-avtcore-multi-party-rtt-mix-00
Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00 Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00
Replaced CDATA in IANA registration table with better coding. Replaced CDATA in IANA registration table with better coding.
Converted to xml2rfc version 3. Converted to xml2rfc version 3.
20.3. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to 20.4. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03 -03
Changed company and e-mail of the author. Changed company and e-mail of the author.
Changed title to "RTP-mixer formatting of multi-party Real-time text" Changed title to "RTP-mixer formatting of multi-party Real-time text"
to better match contents. to better match contents.
Check and modification where needed of use of RFC 2119 words SHALL Check and modification where needed of use of RFC 2119 words SHALL
etc. etc.
More about the CC value in sections on transmitters and receivers so More about the CC value in sections on transmitters and receivers so
that 1-to-1 sessions do not use the mixer format. that 1-to-1 sessions do not use the mixer format.
Enhanced section on presentation for multi-party-unaware endpoints Enhanced section on presentation for multi-party-unaware endpoints
A paragraph recommending CPS=150 inserted in the performance section. A paragraph recommending CPS=150 inserted in the performance section.
20.4. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to 20.5. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 -02
In Abstract and 1. Introduction: Introduced wording about regulatory In Abstract and 1. Introduction: Introduced wording about regulatory
requirements. requirements.
In section 5: The transmission interval is decreased to 100 ms when In section 5: The transmission interval is decreased to 100 ms when
there is text from more than one source to transmit. there is text from more than one source to transmit.
In section 11 about sdp negotiation, a SHOULD-requirement is In section 11 about sdp negotiation, a SHOULD-requirement is
introduced that the mixer should make a mix for multi-party unaware introduced that the mixer should make a mix for multi-party unaware
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In chapter 9. "Use with SIP centralized conferencing framework" the In chapter 9. "Use with SIP centralized conferencing framework" the
following note is inserted: Note: The CSRC-list in an RTP packet only following note is inserted: Note: The CSRC-list in an RTP packet only
includes participants who's text is included in one or more text includes participants who's text is included in one or more text
blocks. It is not the same as the list of participants in a blocks. It is not the same as the list of participants in a
conference. With audio and video media, the CSRC-list would often conference. With audio and video media, the CSRC-list would often
contain all participants who are not muted whereas text participants contain all participants who are not muted whereas text participants
that don't type are completely silent and so don't show up in RTP that don't type are completely silent and so don't show up in RTP
packet CSRC-lists. packet CSRC-lists.
20.5. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 to 20.6. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 -01
Editorial cleanup. Editorial cleanup.
Changed capability indication from fmtp-parameter to sdp attribute Changed capability indication from fmtp-parameter to sdp attribute
"rtt-mix". "rtt-mix".
Swapped order of redundancy elements in the example to match reality. Swapped order of redundancy elements in the example to match reality.
Increased the SDP negotiation section Increased the SDP negotiation section
 End of changes. 25 change blocks. 
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