draft-ietf-avtcore-multi-party-rtt-mix-00.txt   draft-ietf-avtcore-multi-party-rtt-mix-01.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) 7 May 2020 Updates: RFC 4102, RFC 4103 (if approved) 14 May 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 8 November 2020 Expires: 15 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-00 draft-ietf-avtcore-multi-party-rtt-mix-01
Abstract Abstract
Real-time text mixers for multi-party sessions need to identify the Real-time text mixers for multi-party sessions need to identify the
source of each transmitted group of text so that the text can be source of each transmitted group of text so that the text can be
presented by endpoints in suitable grouping with other text from the presented by endpoints in suitable grouping with other text from the
same source. same source.
Regional regulatory requirements specify provision of real-time text Regional regulatory requirements specify provision of real-time text
in multi-party calls. RFC 4103 mixer implementations can use in multi-party calls. RFC 4103 mixer implementations can use
traditional RTP functions for source identification, but the mixer traditional RTP functions for source identification, but the mixer
source switching performance is limited when using the default source switching performance is limited when using the default
transmission with redundancy. transmission with redundancy.
An enhancement for RFC 4103 real-time text mixing is provided in the An enhancement for RFC 4103 real-time text mixing is provided in the
present specification, suitable for a centralized conference model present specification, suitable for a centralized conference model
that enables source identification and efficient source switching. that enables source identification and efficient source switching.
The intended use is for real-time text mixers and multi-party-aware The intended use is for real-time text mixers and multi-party-aware
participant endpoints. The mechanism builds on use of the CSRC list participant endpoints. The mechanism builds on use of the CSRC list
in the RTP packet. in the RTP packet and an extended packet format 'text/rex'.
A capability exchange is specified so that it can be verified that a A capability exchange is specified so that it can be verified that a
participant can handle the multi-party coded real-time text stream. participant can handle the multi-party coded real-time text stream.
The capability is indicated by an sdp media attribute "rtt-mix". The capability is indicated by the media subtype "text/rex".
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103]
A brief description about how a mixer can format text for the case A brief description about how a mixer can format text for the case
when the endpoint is not multi-party aware is also provided. when the endpoint is not multi-party aware is also provided.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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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 8 November 2020. This Internet-Draft will expire on 15 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.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Selected solution and considered alternative . . . . . . 4
3. Intended application . . . . . . . . . . . . . . . . . . . . 4 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 4
4. Use of fields in the RTP packets . . . . . . . . . . . . . . 4 1.3. Intended application . . . . . . . . . . . . . . . . . . 5
5. Actions at transmission by a mixer . . . . . . . . . . . . . 5 2. Use of fields in the RTP packets . . . . . . . . . . . . . . 5
6. Actions at reception . . . . . . . . . . . . . . . . . . . . 7 3. Actions at transmission by a mixer . . . . . . . . . . . . . 7
7. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 8 4. Actions at reception . . . . . . . . . . . . . . . . . . . . 9
8. Chained operation . . . . . . . . . . . . . . . . . . . . . . 8 5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Usage without redundancy . . . . . . . . . . . . . . . . . . 8 6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 10
10. Use with SIP centralized conferencing framework . . . . . . . 8 7. Usage without redundancy . . . . . . . . . . . . . . . . . . 10
11. SDP Capability negotiation . . . . . . . . . . . . . . . . . 9 8. Use with SIP centralized conferencing framework . . . . . . . 11
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Media Subtype Registration . . . . . . . . . . . . . . . . . 11
13. Performance considerations . . . . . . . . . . . . . . . . . 12 10. SDP considerations . . . . . . . . . . . . . . . . . . . . . 12
14. Presentation level considerations . . . . . . . . . . . . . . 12 11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
14.1. Presentation by multi-party aware endpoints . . . . . . 13 12. Performance considerations . . . . . . . . . . . . . . . . . 17
14.2. Multi-party mixing for multi-party unaware endpoints . . 15 13. Presentation level considerations . . . . . . . . . . . . . . 17
15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 20 13.1. Presentation by multi-party aware endpoints . . . . . . 18
16. Congestion considerations . . . . . . . . . . . . . . . . . . 21 13.2. Multi-party mixing for multi-party unaware endpoints . . 20
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21 14. Gateway Considerations . . . . . . . . . . . . . . . . . . . 26
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 15. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 26
19. Security Considerations . . . . . . . . . . . . . . . . . . . 21 16. Congestion considerations . . . . . . . . . . . . . . . . . . 26
20. Change history . . . . . . . . . . . . . . . . . . . . . . . 22 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
20.1. Changes from 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 19. Security Considerations . . . . . . . . . . . . . . . . . . . 27
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 22 20. Change history . . . . . . . . . . . . . . . . . . . . . . . 27
20.1. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 27
20.2. Changes from 20.2. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 22 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 27
20.3. Changes from 20.3. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 22 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 27
20.4. Changes from 20.4. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 28
20.5. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 23 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 29
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 21. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
21.1. Normative References . . . . . . . . . . . . . . . . . . 23 21.1. Normative References . . . . . . . . . . . . . . . . . . 29
21.2. Informative References . . . . . . . . . . . . . . . . . 24 21.2. Informative References . . . . . . . . . . . . . . . . . 30
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 31
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.
skipping to change at page 3, line 40 skipping to change at page 3, line 43
format has no source indicators for the redundant transmissions. An format has no source indicators for the redundant transmissions. An
assumption has had to be made that the redundant parts in a packet assumption has had to be made that the redundant parts in a packet
are from the same source as the new text. The recommended are from the same source as the new text. The recommended
transmission is one new and two redundant generations of text transmission is one new and two redundant generations of text
(T140blocks) in each packet and the recommended transmission interval (T140blocks) in each packet and the recommended transmission interval
is 300 ms. is 300 ms.
A mixer, selecting between text input from different sources and A mixer, selecting between text input from different sources and
transmitting it in a common stream needs to make sure that the transmitting it in a common stream needs to make sure that the
receiver can assign the received text to the proper sources for receiver can assign the received text to the proper sources for
presentation. Therefore, without any extra rule for source presentation. Therefore, using RFC 4103 without any extra rule for
identification, the mixer needs to stop sending new text from that source identification, the mixer needs to stop sending new text from
source and then make sure that all text so far has been sent with all that source and then make sure that all text so far has been sent
intended redundancy levels (usually two) before switching source. with all intended redundancy levels (usually two) before switching
That causes the very long time of one second to switch between source. That causes the very long time of one second to switch
transmission of text from one source to text from another source. between transmission of text from one source to text from another
Both the total throughput and the switching performance in the mixer source. Both the total throughput and the switching performance in
is too low for most applications. the mixer is too low for most applications.
A more efficient source identification scheme requires that each A more efficient source identification scheme requires that each
redundant T140block has its source individually preserved. The redundant T140block has its source individually preserved. The
present specification introduces a source indicator by specific rules present specification introduces a source indicator by specific rules
for populating the CSRC-list in the RTP-packet. for populating the CSRC-list and the redundancy header in the RTP-
packet.
A negotiation mechanism is also introduced for verification that the An extended packet format 'text/rex' is specified for this purpose,
receiver is able to handle the multi-party coded stream. providing the possibility to include text from up to 16 sources in
each packet in order to enhance mixer source switching performance.
A negotiation mechanism can therefore be based on selection between
the "text/red" and the "text/rex" media formats for verification that
the receiver is able to handle the multi-party coded stream.
A fall-back mixing procedure is specified for cases when the A fall-back mixing procedure is specified for cases when the
negotiation fails. negotiation results in "text/red" being the only common submedia
format.
2. Nomenclature The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format for the multi-party mixing case
and more strict rules for the use of redundancy.
1.1. Selected solution and considered alternative
The mechanism specified in the present document makes use of the RTP
mixer model specified in RFC3550[RFC3550]. From some points of view,
use of the RTP translator model specified in RFC 3550 would be more
efficient, because then the text packets can pass the translator with
only minor modification. However, there may be a lack of support for
the translator model in existing RTP implementations, and therefore
the more common RTP-mixer model was selected.
1.2. Nomenclature
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained
in [RFC3550] in [RFC3550]
The term "T140block" is defined in RFC 4103 [RFC4103] to contain one The term "T140block" is defined in RFC 4103 [RFC4103] to contain one
or more T.140 code elements. or more T.140 code elements.
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.
4. 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. This
specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing a rule for populating and using the CSRC-list in the RTP introducing a rule for populating and using the CSRC-list in the RTP
packet in order to enhance the performance in multi-party RTT packet and extending the redundancy header in order to enhance the
sessions. performance in multi-party RTT sessions.
When transmitted from a mixer, the first member in the CSRC-list When transmitted from a mixer, the members in the CSRC-list SHALL
SHALL contain the SSRC of the source of the primary T140block in the contain the SSRCs of the sources of the T140blocks in the packet.
packet. The second and further members in the CSRC-list SHALL When redundancy is used, text from all included sources MUST have the
contain the SSRC of the source of the first, second, etc redundant same number of redundancy generations. The primary, second and
generations of T140blocks included in the packet. ( the recommended further redundant generations of T140blocks are grouped per source in
level of redundancy is to use one primary and two redundant the packet in "source groups" and in the same order as the SSRCs of
generations of T140blocks.) In some cases, a primary or redundant the sources in the CSRC list. ( the recommended level of redundancy
T140block is empty, but is still represented by a member in the is to use one primary and two redundant generations of T140blocks.)
redundancy header. For such cases, the corresponding CSRC-list In some cases, a primary or redundant T140block is empty, but is
member MUST also be included. 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.
Note: This specification departs from section 4 of RFC 2198 [RFC2198] This specification departs from section 4 of RFC 2198 [RFC2198] which
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.
The picture below shows a typical RTP packet with multi-party RTT The picture below shows a typical 'text/rex' RTP packet with multi-
contents and coding according to the present specification. party RTT contents and coding according to the present specification.
0 1 2 3 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC=3 |M| "RED" PT | sequence number of primary | |V=2|P|X| CC=3 |M| "REX" PT | RTP sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" | | timestamp of packet creation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier | | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CSRC list member 1 = SSRC of source of "P" | | CSRC list member 1 = SSRC of source of "A" |
| CSRC list member 2 = SSRC of source of "R1" | | CSRC list member 2 = SSRC of source of "B" |
| CSRC list member 3 = SSRC of source of "R2" | | CSRC list member 3 = SSRC of source of "C" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R2" | "R2" block length | |1| T140 PT |timestmp offset of "A-R2" |"A-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R1" | "R1" block length | |1| T140 PT |timestamp offset of "A-R1" |"A-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | "R2" T.140 encoded redundant data | |1| T140 PT | timestamp offset of "A-P" |"A-P" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "B-R2" |"B-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "B-R1" |"B-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "B-P" | "B-P" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "C-R2" |"C-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "C-R1" |"C-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | "A-R2" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "R1" T.140 encoded redundant data | | | | "A-R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "P" T.140 encoded primary data | | "A-P" T.140 encoded primary data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
Figure 1: text/red packet with sources indicated in the CSRC-list. | "B-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "B-R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "B-P" T.140 encoded primary data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "C-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "C-R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1:A 'text/rex' packet with text from three sources A, B, C.
5. Actions at transmission by a mixer 3. Actions at transmission by a mixer
A "text/red" transmitter is usually sending packets at a regular A "text/rex" transmitter is sending packets at least at regular
transmission interval as long as there is something (new or redundant transmission intervals as long as there is something (new or
T140blocks) to transmit. The default transmission interval for redundant T140blocks) to transmit. The default transmission interval
point-to-point operation is 300 ms. for point-to-point operation is 300 ms.
For multi-party operation, the transmission interval from mixers As soon as a participant is known to participate in a session and
SHOULD be decreased to 100 ms for periods when there is text being available for text reception, a Unicode BOM character SHALL be
available for transmission from more than two sources. It is also sent to it according to the procedures in the present document.
allowed for the mixer to send a packet as soon as text has been
received from a source as long as the maximum number of characters
per second indicated by the recipient is not exceeded, and also the
number of packets sent per second to a recipient is kept under a
specified number. This number SHALL be 10 if no other limit is
applied for the application. The mixer has its own SSRC, and its own
RTP sequence number series. At time of transmission, the mixer SHALL
populate the RTP packet with a T140block combined from all T140blocks
queued for transmission originating from next source in turn for
getting its text transmitted as long as this is not in conflict with
the allowed number of characters per second. This T140block shall be
placed in the primary area of the packet. The SSRC of its source
shall be placed as the first member in the CSRC-list. The current
time SHALL be inserted in the timestamp.
If no unsent T140blocks were available at this time, but T140blocks After that, the transmitter SHALL send keep-alive traffic to the
are available which have not yet been sent the full intended number receivers at regular intervals when no other traffic has occurred
of redundant transmissions, then the primary T140block is composed of during that interval if that is decided for the actual connection.
an empty T140block, and populated (without taking up any length) in a Recommendations for keep-alive can be found in RFC 6263[RFC6263].
packet for transmission. The SSRC of the mixer is included in the
first place of the CSRC-list.
The primary T140block, in the latest transmission is used to populate For multi-party operation, it is RECOMMENDED that the mixer sends a
the first redundant T140block, and its source is placed as the second packet to each receiver as soon as text has been received from a
member of the CSRC-list. The first redundant T140block from the source as long as the maximum number of characters per second
latest transmission is placed as the second redundant T140block, and indicated by the recipient is not exceeded, and also the number of
the corresponding CSRC placed in its place in the CSRC-list. packets sent per second to a recipient is kept under a specified
number. This number SHALL be 10 if no other limit is applied for the
application. The intention is to keep the latency introduced by the
mixer low.
The mixer has its own SSRC, and its own RTP sequence number series.
The number of redundant generations of T140blocks to include in
transmitted packets SHALL be deducted from the SDP negotiation. It
SHOULD be set to the minimum of the number declared by the receiver
and the transmitter. The same number of redundant generations MUST
be used for all sources in the transmissions. The number of
generations sent to a receiver SHALL be the same during the whole
session unless it is modified by session renegotiation.
At time of transmission, the mixer SHALL populate the RTP packet with
T140blocks combined from all T140blocks queued for transmission
originating from each source as long as this is not in conflict with
the allowed number of characters per second or the maximum packet
size. These T140blocks SHALL be placed in the packet interleaved
with redundant T140blocks and new T140blocks from other sources. The
primary and redundant T140blocks from each source are grouped
together in "source groups" appearing in the packet in the same order
as the corresponding CSRC list member and redundancy header members
for the source group. The format of the redundancy header members
and the T140blocks in each source group are the same as for the
single source possible in the "text/red" format. The SSRC of each
source shall be placed as a member in the CSRC-list at a place
corresponding to the place of its T140blocks in the packet. The
current time SHALL be inserted in the timestamp. The timestamp
offset values for empty T140blocks have no relevance but SHOULD be
assigned realistic values.
Text from a maximum of 16 sources MAY be included in a packet. The
reason for this limitation is the maximum number of CSRC list
members. If text from more sources need to be transmitted, the mixer
MAY let the sources take turns in having their text transmitted.
When stopping transmission of one source to allow another source to
have its text sent, all intended redundant generations of the last
text from the source to be stopped MUST be transmitted before text
from another source can be transmitted. Actively transmitting
sources SHOULD be allowed to take turns with short intervals to have
their text transmitted.
If no unsent T140blocks were available at this time for a source, but
T140blocks are available which have not yet been sent the full
intended number of redundant transmissions, then the primary
T140block for that source is composed of an empty T140block, and
populated (without taking up any length) in a packet for
transmission. The corresponding SSRC SHALL be placed in its place in
the CSRC-list.
The primary T140block from each source in the latest transmission is
used to populate the first redundant T140block for that source. The
first redundant T140block for that source from the latest
transmission is placed as the second redundant T140block source.
Usually this is the level of redundancy used. If a higher number of Usually this is the level of redundancy used. If a higher number of
redundancy is used, then the procedure is maintained until all redundancy is negotiated, then the procedure SHALL be maintained
available redundant levels of T140blocks and their sources are placed until all available redundant levels of T140blocks and their sources
in the packet. If a receiver has negotiated a lower number of text/ are placed in the packet. If a receiver has negotiated a lower
red generations, then that level shall be the maximum used by the number of text/rex generations, then that level shall be the maximum
transmitter. used by the transmitter.
The timer offset values are inserted in the redundancy header, with The timer offset values are inserted in the redundancy header, with
the time offset from when the corresponding T140block was sent as the time offset from the RTP timestamp in the packet when the
original. corresponding T140block was sent from the mixer as primary.
The number of members in the CSRC list shall be placed in the "CC" The number of members in the CSRC list shall be placed in the "CC"
header field. Only mixers place values >0 in the "CC" field. header field. Only mixers place values >0 in the "CC" field.
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, the transmission process can stop until either new T140blocks times, the transmission process can stop until either new T140blocks
arrive, or a keep-alive method calls for transmission of keep-alive arrive, or a keep-alive method calls for transmission of keep-alive
packets. packets.
6. Actions at reception 4. Actions at reception
The enhanced "text/red" receiver included in an endpoint with The "text/rex" receiver included in an endpoint with presentation
presentation functions will receive RTP packets in the single stream functions will receive RTP packets in the single stream from the
from the mixer, and shall distribute the T140blocks for presentation mixer, and SHALL distribute the T140blocks for presentation in
in presentation areas for each source. Other tasks for receivers, presentation areas for each source. Other receiver roles, such as
such as gateways or chained mixers are also feasible, and requires gateways or chained mixers are also feasible, and requires
consideration if the stream shall just be forwarded, or a consideration if the stream shall just be forwarded, or distributed
distribution based on different sources is needed. based on the different sources.
If the "CC" field value of a received packet is >1, it indicates the If the "CC" field value of a received packet is >1, it indicates that
used level of redundancy for the current packet and that the enhanced multi-party transmission is active, and the receiver MUST act on the
packet format is used. If the CC field value is 0 or 1, it indicates different sources according to its role. If the CC field value is 1,
that the enhanced format is not used. If the CC value is 1, the CSRC it indicates that there is a mixer between the source and the
indicates the source of primary data. receiver, but that there is only one source at the moment. If the CC
value is 0, the connection is point-to-point.
The used level of redundancy generations SHALL be evaluated from the
received packet contents. If the CC value is 0, the number of
generations (including the primary) is equal to the number of members
in the redundancy header. If the CC value is >0, the number of
generations (including the primary) is equal to the number of members
in the redundancy header divided by the CC value. If the remainder
from the division is >0, then the packet is malformed and SHALL cause
an error indication in the receiver.
The RTP sequence numbers of the received packets SHALL be monitored The RTP sequence numbers of the received packets SHALL be monitored
for gaps and packets out of order. for gaps and packets out of order.
As long as the sequence is correct, each packet SHALL be unpacked in As long as the sequence is correct, each packet SHALL be unpacked in
order. The T140blocks SHALL be extracted from the primary area, and order. The T140blocks SHALL be extracted from the primary areas, and
the corresponding SSRC SHALL be extracted from the first position in the corresponding SSRCs SHALL be extracted from the corresponding
the CSRC list and used for assigning the new T140block to the correct positions in the CSRC list and used for assigning the new T140block
presentation area (or correspondingly). to the correct presentation areas (or correspondingly).
If a sequence number gap appears and is still there after some If a sequence number gap appears and is still there after some
defined time for jitter resolution, T140data SHALL be recovered from defined time for jitter resolution, T140data SHALL be recovered from
redundant data. If the gap is wider than the number of generations redundant data. If the gap is wider than the number of generations
of redundant T140blocks in the packet, then a t140block SHALL be of redundant T140blocks in the packet, then a t140block SHALL be
created with a marker for text loss [T140ad1] and assigned to the created with a marker for text loss [T140ad1] and assigned to the
SSRC of the transmitter as a general input from the mixer. SSRC of the transmitter as a general input from the mixer because in
general it is not possible to deduct from which sources text was
lost.
Then, the T140blocks in the received packet SHALL be retrieved Then, the T140blocks in the received packet SHALL be retrieved
beginning with the highest redundant generation, grouping them with beginning with the highest redundant generation, grouping them with
the corresponding SSRC from the CSRC-list and assigning them to the the corresponding SSRC from the CSRC-list and assigning them to the
presentation areas per source. Finally the primary T140block SHALL presentation areas per source. Finally the primary T140blocks SHALL
be retrieved from the packet and similarly its source retrieved from be retrieved from the packet and similarly their sources retrieved
the first position in the CSRC-list, and then assigned to the from the corresponding positions in the CSRC-list, and then assigned
corresponding presentation area for that source. to the corresponding presentation areas for the sources.
If the sequence number gap was equal to or less than the number of If the sequence number gap was equal to or less than the number of
redundancy generations in the received packet, a missing text marker redundancy generations in the received packet, a missing text marker
SHALL NOT be inserted, and instead the T140blocks and their SSRCs SHALL NOT be inserted, and instead the T140blocks and their SSRCs
fully recovered from the redundancy information and the CSRC-list in fully recovered from the redundancy information and the CSRC-list in
the way indicated above. the way indicated above.
Unicode character BOM is sometimes used as a filler or keep alive by Unicode character BOM is used as a start indication and sometimes
transmission implementations. These SHALL be deleted on reception. used as a filler or keep alive by transmission implementations.
These SHALL be deleted on reception.
Note that empty T140blocks are sometimes included in the packets. Empty T140blocks are included as fillers for unused redundancy levels
They just do not provide any contents. in the packets. They just do not provide any contents and do not
contribute to the received streams.
7. RTCP considerations 5. RTCP considerations
A mixer SHALL send RTCP reports with SDES, CNAME and NAME information A mixer SHALL send RTCP reports with SDES, CNAME and NAME information
about the sources in the multi-party call. This makes it possible about the sources in the multi-party call. This makes it possible
for participants to compose a suitable label for text from each for participants to compose a suitable label for text from each
source. source.
8. Chained operation 6. Chained operation
By strictly applying the rules for CSRC-list population by all By strictly applying the rules for "text/rex" packet format by all
conforming devices, mixers MAY be arranged in chains. conforming devices, mixers MAY be arranged in chains.
9. Usage without redundancy 7. Usage without redundancy
The CSRC list member SHALL be used as source indicator also for cases
when the "text/t140" format is used from a mixer. That MAY be the
case when robustness in transmission is provided by some other means
than by redundancy and the "text/red" format. All aspects of this
memo SHALL be applied except the redundant generations in
transmission.
For the use case without redundancy using the "text/t140" format, the The "text/rex" format SHALL be used also for multi-party
"CC" field in the RTP packet shall have the value 1, and the CSRC communication when the redundancy mechanism is not used. That MAY be
list SHALL contain one member when sent from a mixer. the case when robustness in transmission is provided by some other
means than by redundancy. All aspects of the present document SHALL
be applied except the redundant generations in transmission.
The "text/red" format SHOULD be used unless some other protection The "text/rex" format SHOULD thus be used for multi-party operation,
against packet loss is utilized, for example a reliable network or also when some other protection against packet loss is utilized, for
transport. example a reliable network or transport. The format is also allowed
to be used for point-to-point operation.
10. Use with SIP centralized conferencing framework 8. Use with SIP centralized conferencing framework
The SIP conferencing framework, mainly specified in RFC The SIP conferencing framework, mainly specified in RFC
4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable 4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
for coordinating sessions including multi-party RTT. The RTT stream for coordinating sessions including multi-party RTT. The RTT stream
between the mixer and a participant is one and the same during the between the mixer and a participant is one and the same during the
conference. Participants get announced by notifications when conference. Participants get announced by notifications when
participants are joining or leaving, and further user information may participants are joining or leaving, and further user information may
be provided. The SSRC of the text to expect from joined users can be be provided. The SSRC of the text to expect from joined users MAY be
included in a notification. This can 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.
Note: The CSRC-list in an RTP packet only includes participants who's Note: The CSRC-list in an RTP packet only includes participants who's
text is included in one or more text blocks. It is not the same as text is included in one or more text blocks. It is not the same as
the list of participants in a conference. With audio and video the 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.
11. SDP Capability negotiation 9. Media Subtype Registration
There are RTT implementations which implement RFC 4103 [RFC4103] but This registration is done using the template defined in [RFC6838] and
not the present specification for real-time awareness. Sending mixed following [RFC4855].
text according to the present specification to a device implementing
only RFC 4103 [RFC4103] would lead to unreadable presented text.
Therefore, in order to negotiate RTT mixing capability according to
the present specification, all devices supporting the present
specification for multi-party aware participants SHALL include an sdp
media attribute "rtt-mix" in the sdp, indicating this capability in
offers and answers. Multi-party streams using the coding of the
present specification intended for multi-party aware endpoints MUST
NOT be sent to devices which have not indicated the "rtt-mix"
capability.
Implementations not understanding this parameter MUST ignore it Type name: text
Subtype name: rex
Required parameters: rate: The RTP timestamp (clock) rate. The
only valid value is 1000.
pt: A slash-separated list with the payload
type number(pt) for the primary text, the first redundant text,
the second redundant text etc, that the receiver is capable to
receive.
Optional parameter: cps: This parameter is used to signal the
capabilities of a receiver implementation. It indicates the
maximum number of characters that may be received per second
measured over a period of 10 seconds. The default value is 150.
Encoding considerations: binary; see Section 4.8 of [RFC6838].
Security considerations: See Section 19 of RFC xxxx. [RFC Editor:
Upon publication as an RFC, please replace "XXXX" with the number
assigned to this document and remove this note.]
Interoperability considerations: None.
Published specification: RFC XXXX. [RFC Editor: Upon publication as
an RFC, please replace "XXXX" with the number assigned to this
document and remove this note.]
Applications which use this media type: For example: Text
conferencing tools, multimedia conferencing tools.
Fragment identifier considerations: N/A.
Additional information: None.
Person & email address to contact for further information: Gunnar
Hellstrom <gunnar.hellstrom@ghaccess.se>
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing, and
hence is only defined for transfer via RTP [RFC3550].
Author: Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>
Change controller: IETF AVTCore Working Group delegated from the
IESG.
10. SDP considerations
There are receiving RTT implementations which implement RFC 4103
[RFC4103] but not the source separation by the CSRC. Sending mixed
text according to the usual CSRC convention from RFC 2198 [RFC2198]
to a device implementing only RFC 4103 [RFC4103] would risk to lead
to unreadable presented text. Therefore, in order to negotiate RTT
mixing capability according to the present specification, all devices
supporting the present specification for multi-party aware
participants SHALL include an sdp media format "text/rex" in the sdp,
indicating this capability in offers and answers. Multi-party
streams using the coding of the present specification intended for
multi-party aware endpoints MUST NOT be sent to devices which have
not indicated the "text/rex" format.
Implementations not understanding this format MUST ignore it
according to common SDP rules. according to common SDP rules.
The sdp media attribute defined here, is named "rtt-mix". It has no The sdp media format defined here, is named "rex", for extended
parameters. It is intended to be used in "text" media descriptions redundancy. It is intended to be used in "text" media descriptions
with "text/red" and "text/t140" formats. It indicates capability to with "text/rex" and "text/t140" formats. Both formats MUST be
use source indications in the CSRC list according to the present declared for the "text/rex" format to be used. It indicates
specification. It also indicates ability to receive 10 real-time capability to use source indications in the CSRC list and the packet
text packets per second. format according to the present specification. It also indicates
ability to receive 150 real-time text characters per second.
Syntax: Security SHOULD be applied. If no other security solution is
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------
a=rtt-mix SDP offer/answer examples
The attribute is used in offer/answer procedures in a declarative Offer example for just multi-party capability:
way. Both parties express their capability to use sources in the
CSRC list as specified in the present specification.
A party who has expressed the "rtt-mix" capability MUST populate the m=text 11000 RTP/AVP 101 98
CSRC-list according to the present specification if it acts as an a=rtpmap:98 t140/1000
rtp-mixer and sends to a party who has expressed the "rtt-mix" a=rtpmap:101 rex/1000
capability. a=fmtp:101 98/98/98
A party who has expressed the "rtt-mix" capability MUST interpret the Answer example from a multi-party capable device
contents of the CSRC-list according to the present specification in m=text 12000 RTP/AVP 101 98
received rtp packets from parties who have expressed "rtt-mix" a=rtpmap:98 t140/1000
capability . a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
A party MUST NOT transmit packets with the format for multi-party Offer example for both traditional "text/red" and multi-party format:
aware participants according to the present specification to a party
who has not expressed "rtt-mix" capability.
A party performing as a mixer, which has expressed the "rtt-mix" m=text 11000 RTP/AVP 101 100 98
capability, but not received "rtt-mix" capability indication in a 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
Answer example from a multi-party capable device
m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Answer example from a multi-party unaware device:
m=text 12000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
A party who has negotiated the "text/rex" format MUST populate the
CSRC-list and format the packets according to the present
specification if it acts as an rtp-mixer and sends multi-party text.
A party who has negotiated the "text/rex" capability MUST interpret
the contents of the CSRC-list and the packets according to the
present specification in received rtp packets using the corresponding
payload type.
A party performing as a mixer, which has not negotiated the "text/
rex" format, but negotiated a "text/red" or "text/t140" format in a
session with a participant SHOULD, if nothing else is specified for session with a participant SHOULD, if nothing else is specified for
the application, format transmitted text to that participant to be the application, format transmitted text to that participant to be
suitable to present on a multi-party unaware endpoint as further suitable to present on a multi-party unaware endpoint as further
specified in section Section 14.2. specified in section Section 13.2.
12. Examples 11. Examples
This example shows a symbolic flow of packets from a mixer with loss This example shows a symbolic flow of packets from a mixer with loss
and recovery. A, B and C are sources of RTT. M is the mixer. P and recovery. A, B and C are sources of RTT. M is the mixer. Pn
indicates primary data. R1 is first redundant generation data and R2 indicates primary data in source group "n". Rn1 is first redundant
is second redundant generation data. A1, B1, A2 etc are text chunks generation data and Rn2 is second redundant generation data in source
(T140blocks) received from the respective sources. X indicates group "n". A1, B1, A2 etc are text chunks (T140blocks) received from
dropped packet between the mixer and a receiver. the respective sources. X indicates dropped packet between the mixer
and a receiver.
|----------------| |----------------|
|Seq no 1 | |Seq no 1 |
|CSRC list A,M,B | |CC=1 |
|R2 B99 | |CSRC list A |
|R1: Empty | |R12: Empty |
|P: A1 | |R11: Empty |
|P1: A1 |
|----------------| |----------------|
Assuming that earlier packets were received in sequence, text A1 is Assuming that earlier packets were received in sequence, text A1 is
received from packet 1 and assigned to reception area A. received from packet 1 and assigned to reception area A.
|----------------| |----------------|
|Seq no 2 | |Seq no 2 |
|CC=3 | |CC=3 |
|CSRC list C,A,M | |CSRC list C,A |
|R2 Empty | |R12 Empty |
|R1: A1 | |R11:Empty |
|P: C1 | |P1: C1 |
|R22 Empty |
|R21: A1 |
|P2: Empty |
|----------------| |----------------|
Text C1 is received from packet 2 and assigned to reception area C. Text C1 is received from packet 2 and assigned to reception area C.
X----------------| X----------------|
X Seq no 3 | X Seq no 3 |
X CC=3 | X CC=2 |
X CSRC list A,C,A| X CSRC list C,A |
X R2: A1 | X R12: Empty |
X R1: C1 | X R11: C1 |
X P: A2 | X P1: Empty |
X R22: A1 |
X R21: Empty |
X P2: A2 |
X----------------| X----------------|
Packet 3 is assumed to be dropped in network problems Packet 3 is assumed to be dropped in network problems
X----------------| X----------------|
X Seq no 4 | X Seq no 4 |
X CC=3 | X CC=3 |
X CSRC list B,A,C| X CSRC list C,B,A|
X R2: C1 | X R12: Empty |
X R1: A2 | X R11: Empty |
X P: B1 | X P1: C2 |
X R22: Empty |
X R21: Empty |
X P2: B1 |
X R32: Empty |
X R31: A2 |
X P3: A3 |
X----------------| X----------------|
Packet 4 is assumed to be dropped in network problems Packet 4 is assumed to be dropped in network problems
X----------------| X----------------|
X Seq no 5 | X Seq no 5 |
X CC=3 | X CC=3 |
X CSRC list A,B,A| X CSRC list C,B,A|
X R2: A2 | X R12: Empty |
X R1: B1 | X R11: C2 |
X P: A3 | X P1: Empty |
X R22: Empty |
X R21: B1 |
X P2: B2 |
X R32: A2 |
X R31: A3 |
X P3: A4 |
X----------------| X----------------|
Packet 5 is assumed to be dropped in network problems Packet 5 is assumed to be dropped in network problems
|----------------| |----------------|
|Seq no 6 | |Seq no 6 |
|CC=3 | |CC=3 |
|CSRC list C,A,B | |CSRC list C,B,A |
|R2: B1 | | R12: C2 |
|R1: A3 | | R11: Empty |
|P: C2 | | P1: Empty |
| R22: B1 |
| R21: B2 |
| P2: B3 |
| R32: A3 |
| R31: A4 |
| P3: A5 |
|----------------| |----------------|
Packet 6 is received. The latest received sequence number was 2. Packet 6 is received. The latest received sequence number was 2.
Recovery is therefore tried for 3,4,5. But there is no coverage Recovery is therefore tried for 3,4,5. But there is no coverage
for seq no 3. A missing text mark (U'FFFD) is created and for seq no 3. A missing text mark (U'FFFD) is created and
appended to the mixer reception area. appended to the common mixer reception area.
For seqno 4, text B1 is recovered and appended to reception area B. For seqno 4, texts C2, B1 and A3 are recovered and appended to
For seqno 5, text A3 is recovered and appended to reception area A. their respective reception areas.
Primary text C2 is received and appended to reception area C. For seqno 5, texts B2 and A4 are recovered and appended to their
respective reception areas.
Primary text B3 and A5 are received and appended to their
respective reception areas.
With only one or two packets lost, there would not be any need to With only one or two packets lost, there would not be any need to
create a missing text marker, and all text would be recovered. create a missing text marker, and all text would be recovered.
It will be a design decision how to present the missing text markers It will be a design decision how to present the missing text markers
assigned to the mixer as a source. assigned to the mixer as a source.
13. Performance considerations 12. Performance considerations
This specification allows new text from one source per packet. This specification allows new text from 16 sources per packet.
Packets SHOULD be transmitted with timed intervals. The default Packets SHOULD be transmitted with a maximum time interval when there
transmission interval is 300 ms for RFC 4103[RFC4103], which is is text to be transmitted (either primary or redundant) . The default
suitable for transmission from single sources. However when more maximum transmission interval is 300 ms. It is also RECOMMENDED for
sources contribute to the flow, a shorter transmission interval MAY the mixer to send a packet as soon as text has been received from a
be applied. The transmission interval is therefore RECOMMENDED to be source as long as the maximum number of characters per second
100 ms for mixers when there is text from more than two sources
available for transmission to the same recipient.It is also allowed
for the mixer to send a packet as soon as text has been received from
a source as long as the maximum number of characters per second
indicated by the recipient is not exceeded, and also the number of indicated by the recipient is not exceeded, and also the number of
packets sent per second to a recipient is kept under a number. In packets sent per second to a recipient is kept under a number. In
order to achieve good performance, a receiver for multi-party calls order to achieve good performance, a receiver for multi-party calls
SHOULD declare a sufficient CPS value in SDP for the number of SHOULD declare a sufficient CPS value in SDP for the number of
allowable characters per second. These characteristics provide for allowable characters per second. These characteristics provide for
smooth flow of text with acceptable latency from at least 5 sources smooth flow of text with acceptable latency from at least 32 sources
simultaneously. simultaneously.
The default maximum rate of reception of real-time text is in RFC The default maximum rate of reception of real-time text is in RFC
4103 [RFC4103] specified to be 30 characters per second. The value 4103 [RFC4103] specified to be 30 characters per second. The value
MAY be modified in the CPS parameter of the FMTP attribute in the MAY be modified in the CPS parameter of the FMTP attribute in the
media section for RFC 4103. A mixer combining real-time text from a media section for RFC 4103. A mixer combining real-time text from a
number of sources may have a higher combined flow of text coming from number of sources may have a higher combined flow of text coming from
the sources. Endpoints SHOULD therefore specify a suitable higher the sources. Endpoints SHOULD therefore specify a suitable higher
value for the CPS parameter, corresponding to its real reception value for the CPS parameter, corresponding to its real reception
capability. A value for CPS of 150 is RECOMMENDED because it would capability. A value for CPS of 150 is the default for the "text/rex"
be suitable for reception in most multi-party sessions, and a format. See RFC 4103 [RFC4103] for the format and use of the CPS
realistic value for the capacity in most implementation environments. parameter. The same rules apply for the "text/rex" format except for
See RFC 4103 [RFC4103] for the format and use of the CPS parameter. the default value.
14. Presentation level considerations 13. Presentation level considerations
ITU-T T.140 [T140] provides the presentation level requirements for ITU-T T.140 [T140] provides the presentation level requirements for
the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for
erasure and other formatting functions and has the following general erasure and other formatting functions and has the following general
statement for the presentation: statement for the presentation:
"The display of text from the members of the conversation should be "The display of text from the members of the conversation should be
arranged so that the text from each participant is clearly readable, arranged so that the text from each participant is clearly readable,
and its source and the relative timing of entered text is visualized and its source and the relative timing of entered text is visualized
in the display. Mechanisms for looking back in the contents from the in the display. Mechanisms for looking back in the contents from the
skipping to change at page 13, line 28 skipping to change at page 18, line 28
text to be grouped in presentation. The characters "CRLF" may be text to be grouped in presentation. The characters "CRLF" may be
used by other implementations as replacement for Line Separator. The used by other implementations as replacement for Line Separator. The
"CRLF" combination SHALL be erased by just one erasing action, just "CRLF" combination SHALL be erased by just one erasing action, just
as the Line Separator. Presentation functions are allowed to group as the Line Separator. Presentation functions are allowed to group
text for presentation in smaller groups than the line separators text for presentation in smaller groups than the line separators
imply and present such groups with source indication together with imply and present such groups with source indication together with
text groups from other sources (see the following presentation text groups from other sources (see the following presentation
examples). Erasure has no specific limit by any delimiter in the examples). Erasure has no specific limit by any delimiter in the
text stream. text stream.
14.1. Presentation by multi-party aware endpoints 13.1. Presentation by multi-party aware endpoints
A multi-party aware receiving party, presenting real-time text MUST A multi-party aware receiving party, presenting real-time text MUST
separate text from different sources and present them in separate separate text from different sources and present them in separate
presentation areas. The receiving party MAY separate presentation of presentation areas. The receiving party MAY separate presentation of
parts of text from a source in readable groups based on other parts of text from a source in readable groups based on other
criteria than line separator and merge these groups in the criteria than line separator and merge these groups in the
presentation area when it benefits the user to most easily find and presentation area when it benefits the user to most easily find and
read text from the different participants. The criteria MAY e.g. be read text from the different participants. The criteria MAY e.g. be
a received comma, full stop, or other phrase delimiters, or a long a received comma, full stop, or other phrase delimiters, or a long
pause. pause.
skipping to change at page 14, line 37 skipping to change at page 19, line 37
|[Eve] we can have dinner and then take a walk | | |[Eve] we can have dinner and then take a walk | |
| | | | | |
| <Eve-typing> But I need to be back to | | | <Eve-typing> But I need to be back to | |
| the hotel by 11 because I need | | | the hotel by 11 because I need | |
| |-| | |-|
| <Bob-typing> I wou |-| | <Bob-typing> I wou |-|
|______________________________________________|v| |______________________________________________|v|
| of course, I underst | | of course, I underst |
|________________________________________________| |________________________________________________|
Figure 1: Example of a three-party call with chat style.
Figure 2: Example of a three-party RTT call presented in chat style. Figure 2: Example of a three-party RTT call presented in chat style.
Other presentation styles than the chat style may be arranged. Other presentation styles than the chat style may be arranged.
This figure shows how a coordinated column view MAY be presented. This figure shows how a coordinated column view MAY be presented.
_____________________________________________________________________ _____________________________________________________________________
| Bob | Eve | Alice | | Bob | Eve | Alice |
|____________________|______________________|_______________________| |____________________|______________________|_______________________|
| | |I will arrive by TGV. | | | |I will arrive by TGV. |
skipping to change at page 15, line 21 skipping to change at page 20, 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 3: An example of a coordinated column-view of a three-party Figure 3: 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.
14.2. Multi-party mixing for multi-party unaware endpoints 13.2. Multi-party mixing for multi-party unaware endpoints
When the mixer has indicated multi-party capability in an sdp When the mixer has indicated multi-party capability in an sdp
negotiation, but the multi-party capability negotiation fails with an negotiation, but the multi-party capability negotiation fails with an
endpoint, then the mixer SHOULD compose a best-effort presentation of endpoint, then the mixer SHOULD compose a best-effort presentation of
multi-party real-time text in one stream intended to be presented by multi-party real-time text in one stream intended to be presented by
an endpoint with no multi-party awareness. an endpoint with no multi-party awareness.
This presentation format has functional limitations and SHOULD be This presentation format has functional limitations and SHOULD be
used only to enable participation in multi-party calls by legacy used only to enable participation in multi-party calls by legacy
deployed endpoints. deployed endpoints.
skipping to change at page 16, line 5 skipping to change at page 21, line 5
simulated limited multi-party RTT view suitable for presentation in simulated limited multi-party RTT view suitable for presentation in
one presentation area. The mixer SHALL group text in suitable groups one presentation area. The mixer SHALL group text in suitable groups
and prepare for presentation of them by inserting a new line betwwen and prepare for presentation of them by inserting a new line betwwen
them if the transmitted text did not already end with a new line. A them if the transmitted text did not already end with a new line. A
presentable label SHOULD be composed and sent for the source presentable label SHOULD be composed and sent for the source
initially in the session and after each source switch. With this initially in the session and after each source switch. With this
procedure the time for source switching is depending on the actions procedure the time for source switching is depending on the actions
of the users. In order to expedite source switch, a user can for of the users. In order to expedite source switch, a user can for
example end its turn with a new line. example end its turn with a new line.
14.2.1. Actions by the mixer at reception from the call participants 13.2.1. Actions by the mixer at reception from the call participants
When text is received by the mixer from the different participants, When text is received by the mixer from the different participants,
the mixer SHALL recover text from redundancy if any packets are lost. the mixer SHALL recover text from redundancy if any packets are lost.
The mark for lost text [T140ad1] SHOULD be inserted in the stream if The mark for lost text [T140ad1] SHOULD be inserted in the stream if
unrecoverable loss appears. Any Unicode BOM characters, possibly unrecoverable loss appears. Any Unicode BOM characters, possibly
used for keep-alive shall be deleted. The time of arrival of text used for keep-alive shall be deleted. The time of arrival of text
SHALL be stored together with the received text from each source in a SHALL be stored together with the received text from each source in a
queue for transmission to the recipients. queue for transmission to the recipients.
14.2.2. Actions by the mixer for transmission to the recipients 13.2.2. Actions by the mixer for transmission to the recipients
The following procedure SHOULD be applied for each recipient of The following procedure SHOULD be applied for each recipient of
multi-part text from the mixer. multi-part text from the mixer.
The text for transmission SHOULD be formatted by the mixer for each The text for transmission SHOULD be formatted by the mixer for each
receiving user for presentation in one single presentation area. receiving user for presentation in one single presentation area.
Text received from a participant SHOULD NOT be included in Text received from a participant SHOULD NOT be included in
transmission to that participant. When there is text available for transmission to that participant. When there is text available for
transmission from the mixer to a receiving party from more than one transmission from the mixer to a receiving party from more than one
participant, the mixer SHOULD switch between transmission of text participant, the mixer SHOULD switch between transmission of text
skipping to change at page 17, line 30 skipping to change at page 22, line 30
Graphic Rendition (SGR) from each source. If there is an SGR code Graphic Rendition (SGR) from each source. If there is an SGR code
stored for the current source before the source switch is done, a stored for the current source before the source switch is done, a
reset of SGR shall be sent by the sequence SGR 0 [009B 0000 006D] reset of SGR shall be sent by the sequence SGR 0 [009B 0000 006D]
after the new line and before the new label during a source switch. after the new line and before the new label during a source switch.
See SGR below for an explanation. This transmission does not See SGR below for an explanation. This transmission does not
influence the display count. If there is an SGR code stored for the influence the display count. If there is an SGR code stored for the
new source after the source switch, that SGR code SHOULD be new source after the source switch, that SGR code SHOULD be
transmitted to the recipient before the label. This transmission transmitted to the recipient before the label. This transmission
does not influence the display count. does not influence the display count.
14.2.3. Actions on transmission of text 13.2.3. Actions on transmission of text
Text from a source sent to the recipient SHOULD increase the display Text from a source sent to the recipient SHOULD increase the display
count by one per transmitted character. count by one per transmitted character.
14.2.4. Actions on transmission of control codes 13.2.4. Actions on transmission of control codes
The following control codes specified by T.140 require specific The following control codes specified by T.140 require specific
actions. They SHOULD cause specific considerations in the mixer. actions. They SHOULD cause specific considerations in the mixer.
Note that the codes presented here are expressed in UCS-16, while Note that the codes presented here are expressed in UCS-16, while
transmission is made in UTF-8 transform of these codes. transmission is made in UTF-8 transform of these codes.
BEL 0007 Bell Alert in session, provides for alerting during an BEL 0007 Bell Alert in session, provides for alerting during an
active session. The display count SHOULD not be altered. active session. The display count SHOULD not be altered.
NEW LINE 2028 Line separator. Check and perform a source switch if NEW LINE 2028 Line separator. Check and perform a source switch if
skipping to change at page 19, line 5 skipping to change at page 24, line 5
to the end of the leading label after a source switch, then the to the end of the leading label after a source switch, then the
mixer must not transmit more backspaces. Instead it is mixer must not transmit more backspaces. Instead it is
RECOMMENDED that a letter "X" is inserted in the text stream for RECOMMENDED that a letter "X" is inserted in the text stream for
each backspace as an indication of the intent to erase more. A each backspace as an indication of the intent to erase more. A
new line is usually coded by a Line Separator, but the character new line is usually coded by a Line Separator, but the character
combination "CRLF" MAY be used instead. Erasure of a new line is combination "CRLF" MAY be used instead. Erasure of a new line is
in both cases done by just one erasing action (Backspace). If the in both cases done by just one erasing action (Backspace). If the
display count has a positive value it is decreased by one when the display count has a positive value it is decreased by one when the
BS is sent. If the display count is at zero, it is not altered. BS is sent. If the display count is at zero, it is not altered.
14.2.5. Packet transmission 13.2.5. Packet transmission
A mixer transmitting to a multi-party unaware terminal SHOULD send A mixer transmitting to a multi-party unaware terminal SHOULD send
primary data only from one source per packet. The SSRC SHOULD be the primary data only from one source per packet. The SSRC SHOULD be the
SSRC of the mixer. The CSRC list SHOULD contain one member and be SSRC of the mixer. The CSRC list SHOULD contain one member and be
the SSRC of the source of the primary data. the SSRC of the source of the primary data.
14.2.6. Functional limitations 13.2.6. Functional limitations
When a multi-party unaware endpoint presents a conversation in one When a multi-party unaware endpoint presents a conversation in one
display area in a chat style, it inserts source indications for display area in a chat style, it inserts source indications for
remote text and local user text as they are merged in completed text remote text and local user text as they are merged in completed text
groups. When an endpoint using this layout receives and presents groups. When an endpoint using this layout receives and presents
text mixed for multi-party unaware endpoints, there will be two text mixed for multi-party unaware endpoints, there will be two
levels of source indicators for the received text; one generated by levels of source indicators for the received text; one generated by
the mixer and inserted in a label after each source switch, and the mixer and inserted in a label after each source switch, and
another generated by the receiving endpoint and inserted after each another generated by the receiving endpoint and inserted after each
switch between local and remote source in the presentation area. switch between local and remote source in the presentation area.
This will waste display space and look inconsistent to the reader. This will waste display space and look inconsistent to the reader.
This fact, combined with the slowness in source switching and the New text can be presented only from one source at a time. Switch of
limited support of erasure makes it strongly RECOMMENDED to implement source to be presented takes place at suitable places in the text,
multi-party awareness in RTT endpoints. The use of the mixing method such as end of phrase, end of sentence, line separator and
for multi-party-unaware endpoints should be left for use with inactivity. Therefore the time to switch to present waiting text
endpoints which are impossible to upgrade to become multi-party from other sources may become long and will vary and depend on the
aware. actions of the currently presented source.
14.2.7. Example views of presentation on multi-party unaware endpoints Erasure can only be done up to the latest source switch. If a user
tries to erase more text, the erasing actions will be presented as
letter X after the label.
These facts makes it strongly RECOMMENDED to implement multi-party
awareness in RTT endpoints. The use of the mixing method for multi-
party-unaware endpoints should be left for use with endpoints which
are impossible to upgrade to become multi-party aware.
13.2.7. Example views of presentation on multi-party unaware endpoints
The following pictures are examples of the view on a participant's The following pictures are examples of the view on a participant's
display for the multi-party-unaware case. display for the multi-party-unaware case.
_________________________________________________ _________________________________________________
| Conference | Alice | | Conference | Alice |
|________________________|_________________________| |________________________|_________________________|
| |I will arrive by TGV. | | |I will arrive by TGV. |
|[Bob]:My flight is to |Convenient to the main | |[Bob]:My flight is to |Convenient to the main |
|Orly. |station. | |Orly. |station. |
skipping to change at page 20, line 35 skipping to change at page 26, line 5
|[Eve] But I need to be back to | | |[Eve] But I need to be back to | |
| the hotel by 11 because I need |-| | the hotel by 11 because I need |-|
| |-| | |-|
|______________________________________________|v| |______________________________________________|v|
| of course, I underst | | of course, I underst |
|________________________________________________| |________________________________________________|
Figure 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.
15. Gateway Considerations 14. Gateway Considerations
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).
Gateway operation to real-time text in WebRTC may also be required.
In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data-
channel[I-D.ietf-mmusic-t140-usage-data-channel]. EDITOR NOTE -----
more about this is needed------------
15. Updates to RFC 4102 and RFC 4103
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format 'text/rex' for the multi-party
mixing case and more strict rules for the use of redundancy, and
population of the CSRC list in the packets. Implications for the
CSRC list use from RFC 2198[RFC2198] are hereby not in effect.
16. Congestion considerations 16. Congestion considerations
The congestion considerations and recommended actions from RFC 4103 The congestion considerations and recommended actions from RFC 4103
[RFC4103] are valid also in multi-party situations. [RFC4103] are valid also in multi-party situations.
17. Acknowledgements 17. Acknowledgements
18. IANA Considerations James Hamlin for format input.
[RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
RFC number of this document.]
IANA is asked to register the new sdp attribute "rtt-mix".
+---------------------+----------------------------------------+ 18. IANA Considerations
| Contact name: | IESG |
+---------------------+----------------------------------------+
| Contact email: | iesg@ietf.org |
+---------------------+----------------------------------------+
| Attribute name: | rtt-mix |
+---------------------+----------------------------------------+
| Attribute syntax | a=rtt-mix |
+---------------------+----------------------------------------+
| Attribute semantics | See RFCXXXX Section 11 |
+---------------------+----------------------------------------+
| Attribute value | - |
+---------------------+----------------------------------------+
| Usage level: | media |
+---------------------+----------------------------------------+
| Purpose: | Indicate support for the rtp-mixer |
| | format for real-time text transmission |
+---------------------+----------------------------------------+
| O/A procedure | Declarative |
+---------------------+----------------------------------------+
| Mux Category | normal |
+---------------------+----------------------------------------+
| Reference: | RFCXXXX |
+---------------------+----------------------------------------+
Table 1 The IANA is requested to register the media type registration "text/
rex" as specified in Section 9. The media type is also requested to
be added to the IANA registry for "RTP Payload Format Media Types"
<http://www.iana.org/assignments/rtp-parameters>.
19. Security Considerations 19. Security Considerations
The RTP-mixer model requires the mixer to be allowed to decrypt, pack The RTP-mixer model requires the mixer to be allowed to decrypt, pack
and encrypt secured text from the conference participants. Therefore and encrypt secured text from the conference participants. Therefore
the mixer needs to be trusted. This is similar to the situation for the mixer needs to be trusted. This is similar to the situation for
central mixers of audio and video. central mixers of audio and video.
The requirement to transfer information about the user in RTCP The requirement to transfer information about the user in RTCP
reports in SDES, CNAME and NAME fields 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 from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 20.1. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01
2,5,6 More efficient format text/rex introduced and attribute a=rtt-
mix deleted.
3. Brief about use of OSRTP for security included- More needed.
4. Brief motivation for the solution and why not rtp-translator is
used added to intro.
7. More limitations for the multi-party unaware mixing method
inserted.
8. Updates to RFC 4102 and 4103 more clearly expressed.
9. Gateway to WebRTC started. More needed.
20.2. 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.
20.2. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to 20.3. 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.
20.3. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to 20.4. 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
skipping to change at page 23, line 31 skipping to change at page 29, line 14
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.
20.4. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 to 20.5. 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
21. References 21. References
21.1. Normative References 21.1. Normative References
[I-D.ietf-mmusic-t140-usage-data-channel]
Holmberg, C. and G. Hellstrom, "T.140 Real-time Text
Conversation over WebRTC Data Channels", Work in Progress,
Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel-
14, 10 April 2020, <https://tools.ietf.org/html/draft-
ietf-mmusic-t140-usage-data-channel-14>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse- Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
DOI 10.17487/RFC2198, September 1997, DOI 10.17487/RFC2198, September 1997,
<https://www.rfc-editor.org/info/rfc2198>. <https://www.rfc-editor.org/info/rfc2198>.
skipping to change at page 24, line 24 skipping to change at page 30, line 13
July 2003, <https://www.rfc-editor.org/info/rfc3550>. July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type", [RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type",
RFC 4102, DOI 10.17487/RFC4102, June 2005, RFC 4102, DOI 10.17487/RFC4102, June 2005,
<https://www.rfc-editor.org/info/rfc4102>. <https://www.rfc-editor.org/info/rfc4102>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<https://www.rfc-editor.org/info/rfc4103>. <https://www.rfc-editor.org/info/rfc4103>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<https://www.rfc-editor.org/info/rfc4855>.
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263,
DOI 10.17487/RFC6263, June 2011,
<https://www.rfc-editor.org/info/rfc6263>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
Stach, "An Opportunistic Approach for Secure Real-time
Transport Protocol (OSRTP)", RFC 8643,
DOI 10.17487/RFC8643, August 2019,
<https://www.rfc-editor.org/info/rfc8643>.
[T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
multimedia application text conversation", February 1998, multimedia application text conversation", February 1998,
<https://www.itu.int/rec/T-REC-T.140-199802-I/en>. <https://www.itu.int/rec/T-REC-T.140-199802-I/en>.
[T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
Protocol for multimedia application text conversation", Protocol for multimedia application text conversation",
February 2000, February 2000,
<https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>. <https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>.
21.2. Informative References 21.2. Informative References
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