draft-ietf-avtcore-multi-party-rtt-mix-03.txt   draft-ietf-avtcore-multi-party-rtt-mix-04.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) 27 May 2020 Updates: RFC 4102, RFC 4103 (if approved) 1 June 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 28 November 2020 Expires: 3 December 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-03 draft-ietf-avtcore-multi-party-rtt-mix-04
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
skipping to change at page 2, line 10 skipping to change at page 2, line 10
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 28 November 2020. This Internet-Draft will expire on 3 December 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
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Selected solution and considered alternative . . . . . . 4 1.1. Selected solution and considered alternative . . . . . . 5
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . 8 3. Actions at transmission by a mixer . . . . . . . . . . . . . 8
4. Actions at reception . . . . . . . . . . . . . . . . . . . . 11 3.1. Initial BOM transmission . . . . . . . . . . . . . . . . 8
5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 12 3.2. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 9
6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 12 3.3. Transmission interval . . . . . . . . . . . . . . . . . . 9
7. Usage without redundancy . . . . . . . . . . . . . . . . . . 12 3.4. Do not send to the same source . . . . . . . . . . . . . 9
8. Use with SIP centralized conferencing framework . . . . . . . 13 3.5. Clean incoming text . . . . . . . . . . . . . . . . . . . 9
9. Conference control . . . . . . . . . . . . . . . . . . . . . 13 3.6. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 9
10. Media Subtype Registration . . . . . . . . . . . . . . . . . 13 3.7. Text placement in packets . . . . . . . . . . . . . . . . 10
11. SDP considerations . . . . . . . . . . . . . . . . . . . . . 14 3.8. Maximum number of sources per packet . . . . . . . . . . 10
11.1. Security for session control and media . . . . . . . . . 15 3.9. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 10
11.2. SDP offer/answer examples . . . . . . . . . . . . . . . 15 3.10. Creation of the redundancy . . . . . . . . . . . . . . . 11
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.11. Timer offset fields . . . . . . . . . . . . . . . . . . . 11
13. Performance considerations . . . . . . . . . . . . . . . . . 20 3.12. Other RTP header fields . . . . . . . . . . . . . . . . . 11
14. Presentation level considerations . . . . . . . . . . . . . . 20 3.13. Pause in transmission . . . . . . . . . . . . . . . . . . 11
14.1. Presentation by multi-party aware endpoints . . . . . . 21 4. Actions at reception . . . . . . . . . . . . . . . . . . . . 12
14.2. Multi-party mixing for multi-party unaware endpoints . . 23 4.1. Multi-party vs two-party use . . . . . . . . . . . . . . 12
15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 28 4.2. Level of redundancy . . . . . . . . . . . . . . . . . . . 12
15.1. Gateway considerations with Textphones (e.g. TTYs). . . 29 4.3. Extracting text and handling recovery and loss . . . . . 12
15.2. Gateway considerations with WebRTC. . . . . . . . . . . 29 4.4. Delete BOM . . . . . . . . . . . . . . . . . . . . . . . 13
16. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 30 4.5. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 13
17. Congestion considerations . . . . . . . . . . . . . . . . . . 30
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 13
19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 13
20. Security Considerations . . . . . . . . . . . . . . . . . . . 30 7. Usage without redundancy . . . . . . . . . . . . . . . . . . 13
21. Change history . . . . . . . . . . . . . . . . . . . . . . . 30 8. Use with SIP centralized conferencing framework . . . . . . . 14
9. Conference control . . . . . . . . . . . . . . . . . . . . . 14
10. Media Subtype Registration . . . . . . . . . . . . . . . . . 14
11. SDP considerations . . . . . . . . . . . . . . . . . . . . . 15
11.1. Security for session control and media . . . . . . . . . 16
11.2. SDP offer/answer examples . . . . . . . . . . . . . . . 16
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
13. Performance considerations . . . . . . . . . . . . . . . . . 21
14. Presentation level considerations . . . . . . . . . . . . . . 21
14.1. Presentation by multi-party aware endpoints . . . . . . 22
14.2. Multi-party mixing for multi-party unaware endpoints . . 24
15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 30
15.1. Gateway considerations with Textphones (e.g. TTYs). . . 30
15.2. Gateway considerations with WebRTC. . . . . . . . . . . 31
16. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 31
17. Congestion considerations . . . . . . . . . . . . . . . . . . 31
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
20. Security Considerations . . . . . . . . . . . . . . . . . . . 32
21. Change history . . . . . . . . . . . . . . . . . . . . . . . 32
21.1. Changes included in 21.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 32
21.2. Changes included in 21.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 32
21.3. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 32 21.3. Changes included in
21.4. Changes from draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 33
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 21.4. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 34
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 32
21.5. Changes from 21.5. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 34
21.6. Changes from 21.6. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 33 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 34
21.7. Changes from 21.7. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 35
21.8. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 34 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 35
22. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 22. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
22.1. Normative References . . . . . . . . . . . . . . . . . . 34 22.1. Normative References . . . . . . . . . . . . . . . . . . 36
22.2. Informative References . . . . . . . . . . . . . . . . . 35 22.2. Informative References . . . . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 38
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.
Real-time text is usually provided together with audio and sometimes Real-time text is usually provided together with audio and sometimes
with video in conversational sessions. with video in conversational sessions.
The redundancy scheme enables efficient transmission of redundant The redundancy scheme of RFC 4103 [RFC4103] enables efficient
text in packets together with new text. However the redundant header transmission of redundant text in packets together with new text.
format has no source indicators for the redundant transmissions. An However the redundant header format has no source indicators for the
assumption has had to be made that the redundant parts in a packet redundant transmissions. An assumption has had to be made that the
are from the same source as the new text. The recommended redundant parts in a packet are from the same source as the new text.
transmission is one new and two redundant generations of text The recommended transmission is one new and two redundant generations
(T140blocks) in each packet and the recommended transmission interval of text (T140blocks) in each packet and the recommended transmission
is 300 ms. interval 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, using RFC 4103 without any extra rule for presentation. Therefore, using RFC 4103 without any extra rule for
source identification, the mixer needs to stop sending new text from source identification, the mixer needs to stop sending new text from
that source and then make sure that all text so far has been sent one source and then make sure that all text so far has been sent with
with all intended redundancy levels (usually two) before switching all intended redundancy levels (usually two) before switching to
source. That causes the very long time of one second to switch another source. That causes the very long time of one second to
between transmission of text from one source to text from another switch between transmission of text from one source to text from
source. Both the total throughput and the switching performance in another source. Both the total throughput and the switching
the mixer is too low for most applications. performance in 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 and the redundancy header in the RTP- for populating the CSRC-list and the data header in the RTP-packet.
packet.
An extended packet format 'text/rex' is specified for this purpose, An extended packet format 'text/rex' is specified for this purpose,
providing the possibility to include text from up to 16 sources in providing the possibility to include text from up to 16 sources in
each packet in order to enhance mixer source switching performance. each packet in order to enhance mixer source switching performance.
By these extensions, the performance requirements on multi-party By these extensions, the performance requirements on multi-party
mixing for real-time text are exceeded by the solution in the present mixing for real-time text are exceeded by the solution in the present
document. document.
A negotiation mechanism can therefore be based on selection between A negotiation mechanism can therefore be based on selection between
the "text/red" and the "text/rex" media formats for verification that the "text/red" and the "text/rex" media formats for verification that
skipping to change at page 5, line 21 skipping to change at page 5, line 41
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.
1.3. Intended application 1.3. Intended application
The format for multi-party real-time text is primarily intended for The format for multi-party real-time text is primarily intended for
use in transmission between mixers and endpoints in centralised use in transmission between mixers and endpoints in centralised
mixing configurations. It is also applicable between endpoint as mixing configurations. It is also applicable between endpoints as
well as between mixers. well as between 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 of real-time redundancy format "text/red" for increased robustness of real-time
text transmission. The current specification updates RFC text transmission. The current specification updates RFC
4102[RFC4102] and RFC 4103[RFC4103] by introducing a rule for 4102[RFC4102] and RFC 4103[RFC4103] by introducing a rule for
populating and using the CSRC-list in the RTP packet and extending populating and using the CSRC-list in the RTP packet and extending
the redundancy header in order to enhance the performance in multi- the redundancy header to be called a data header. This is done in
party RTT sessions. order to enhance the performance in multi-party RTT sessions.
The "text/rex" format is an "n-tuple" of the "text/red" format The "text/rex" format can be seen as an "n-tuple" of the "text/red"
intended to carry text information from up to 16 sources per packet. format intended to carry text information from up to 16 sources per
The CSRC members, the redundancy header and the T140block areas SHALL packet.
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.
The order of the CSRC members MUST be the same as the order of The order of the CSRC members MUST be the same as the order of
sources of the T140blocks and redundant header fields. When sources of the T140blocks and redundant header fields. When
redundancy is used, text from all included sources MUST have the same redundancy is used, text from all included sources MUST have the same
number of redundancy generations. The primary, first redundant, number of redundant generations. The primary, first redundant,
second redundant and possible further redundant generations of second redundant and possible further redundant generations of
T140blocks MUST be grouped per source in the packet in "source T140blocks MUST be grouped per source in the packet in "source
groups". The recommended level of redundancy is to use one primary groups". The recommended level of redundancy is to use one primary
and two redundant generations of T140blocks. In some cases, a and two redundant generations of T140blocks. In some cases, a
primary or redundant T140block is empty, but is still represented by primary or redundant T140block is empty, but is still represented by
a member in the redundancy header. a member in the data 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 The RTP header is followed by one or more source groups of data
headers: one header for each text block to be included. Each of headers: one header for each text block to be included. Each of
these headers provides the timestamp offset and length of the these data headers provides the timestamp offset and length of the
corresponding data block, in addition to the payload type number corresponding data block, in addition to the payload type number
corresponding to the payload format "text/t140". The data block corresponding to the payload format "text/t140". The data headers
headers are followed by the data fields carrying T140blocks from the are followed by the data fields carrying T140blocks from the sources.
sources. Finally, the most recent T140block (if any) follows.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| block PT | timestamp offset | block length | |F| block PT | timestamp offset | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The bits in the header are specified as follows: Figure 1: The bits in the data header.
The bits in the data header are specified as follows:
F: 1 bit First bit in header indicates whether another header block F: 1 bit First bit in header indicates whether another header block
follows. It has value 1 if further header blocks follow, and follows. It has value 1 if further header blocks follow, and
value 0 if this is the last header block. value 0 if this is the last header block.
block PT: 7 bits RTP payload type number for this block, block PT: 7 bits RTP payload type number for this block,
corresponding to the t140 payload type. corresponding to the t140 payload type from the RTPMAP SDP
attribute.
timestamp offset: 14 bits Unsigned offset of timestamp of this block timestamp offset: 14 bits Unsigned offset of timestamp of this block
relative to timestamp given in RTP header. The offset is a time relative to the timestamp given in the RTP header. The offset is
to be subtracted from the current timestamp to determine the a time to be subtracted from the current timestamp to determine
timestamp of the data when the latest part of this block was sent the timestamp of the data when the latest part of this block was
from the original source. If the timestamp offset would be >15 sent from the original source. If the timestamp offset would be
000, it is set to 15 000. For redundant data, the resulting time >15 000, it SHALL be set to 15 000. For redundant data, the
is the time when the data was sent as primary from the original resulting time is the time when the data was sent as primary from
source. If the value would be >15 000, then it SHALL be set to 15 the original source. If the value would be >15 000, then it SHALL
000 plus 300 times the redundancy level of the data. The high be set to 15 000 plus 300 times the redundancy level of the data.
values appear only in exceptional cases, e.g. when some data has The high values appear only in exceptional cases, e.g. when some
been held in order to keep the text flow under the cps limit. data has been held in order to keep the text flow under the
Characters Per Second (CPS) limit.
block length: 10 bits Length in bytes of the corresponding data block length: 10 bits Length in bytes of the corresponding data
block excluding header. block excluding the header.
The last T140block is placed last in the packet. The header for the The last T140block is placed last in the packet. The header for the
final block has a zero F bit, and apart from that the same fields as final block has a zero F bit, and apart from that the same fields as
other data headers. other data headers.
This specification has a packet format that is similar to that of RFC Note: This specification has a packet format that is similar to that
2198 [RFC2198] but is different from some aspects. RFC 2198 of RFC 2198 [RFC2198] but is different from some aspects. RFC 2198
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. Another difference is that the originated by different participants. Another difference is that the
last member in the data header area in RFC 2198 [RFC2198] only last member in the data header area in RFC 2198 [RFC2198] only
contains the payload type number while in the current specification contains the payload type number while in the current specification
it has the same format as all other entries in the data header. it has the same format as all other entries in the data header.
The picture below shows a typical 'text/rex' RTP packet with multi- The picture below shows a typical 'text/rex' RTP packet with multi-
party RTT contents and coding according to the present specification. party RTT contents from three sources 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| "REX" PT | RTP sequence number | |V=2|P|X| CC=3 |M| "REX" PT | RTP sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of packet creation | | timestamp of packet creation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier | | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
skipping to change at page 8, line 40 skipping to change at page 8, line 39
| "B-R1" T.140 encoded redundant data | | "B-R1" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "B-P" T.140 encoded primary data | | | "B-P" T.140 encoded primary data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-R2" T.140 encoded redundant data | | | "C-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-R1" T.140 encoded redundant data | | "C-R1" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "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 2: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
3.1. Initial BOM transmission
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. If sent to it according to the procedures in the present document. If
the transmitter is a mixer, then the source of this character SHALL the transmitter is a mixer, then the source of this character SHALL
be indicated to be the mixer itself. be indicated to be the mixer itself.
3.2. Keep-alive
After that, the transmitter SHALL send keep-alive traffic to the After that, the transmitter SHALL send keep-alive traffic to the
receivers at regular intervals when no other traffic has occurred receivers at regular intervals when no other traffic has occurred
during that interval if that is decided for the actual connection. during that interval if that is decided for the actual connection.
Recommendations for keep-alive can be found in RFC 6263[RFC6263]. Recommendations for keep-alive can be found in RFC 6263[RFC6263].
3.3. Transmission interval
A "text/rex" transmitter SHOULD send packets distributed in time as A "text/rex" transmitter SHOULD send packets distributed in time as
long as there is something (new or redundant T140blocks) to transmit. long as there is something (new or redundant T140blocks) to transmit.
The maximum transmission interval SHOULD then be 300 ms. It is The maximum transmission interval SHOULD then be 300 ms. It is
RECOMMENDED to send a packet to a receiver as soon as new text to RECOMMENDED to send a packet to a receiver as soon as new text to
that receiver is available, as long as the time after the latest sent that receiver is available, as long as the time after the latest sent
packet to the same receiver is more than 150 ms, and also the maximum packet to the same receiver is more than 150 ms, and also the maximum
character rate to the receiver is not exceeded. The intention is to character rate to the receiver is not exceeded. The intention is to
keep the latency low while keeping a good protection against text keep the latency low while keeping a good protection against text
loss in bursty packet loss conditions. loss in bursty packet loss conditions.
3.4. Do not send to the same source
Text received from a participant SHOULD NOT be included in Text received from a participant SHOULD NOT be included in
transmission to that participant. transmission to that participant.
The mixer has its own SSRC, and its own RTP sequence number series. 3.5. Clean incoming text
A mixer SHALL handle reception and recovery of packet loss, marking A mixer SHALL handle reception and recovery of packet loss, marking
of possible text loss and deletion of 'BOM' characters from each of possible text loss and deletion of 'BOM' characters from each
participant before queueing received text for transmission to participant before queueing received text for transmission to
receiving participants. receiving participants.
The transmitting party sends redundant repetitions of T140blocks 3.6. Redundancy
aleady transmitted in earlier packets. The number of redundant
generations of T140blocks to include in transmitted packets SHALL be The transmitting party using redundancy SHALL send redundant
deducted from the SDP negotiation. It SHOULD be set to the minimum repetitions of T140blocks aleady transmitted in earlier packets. The
of the number declared by the two parties negotiating a connection. number of redundant generations of T140blocks to include in
The same number of redundant generations MUST be used for text from transmitted packets SHALL be deducted from the SDP negotiation. It
all sources when it is transmitted to a receiver. The number of SHOULD be set to the minimum of the number declared by the two
generations sent to a receiver SHALL be the same during the whole parties negotiating a connection. The same number of redundant
session unless it is modified by session renegotiation. generations MUST be used for text from all sources when it is
transmitted to a receiver. The number of generations sent to a
receiver SHALL be the same during the whole session unless it is
modified by session renegotiation.
3.7. Text placement in packets
At time of transmission, the mixer SHALL populate the RTP packet with At time of transmission, the mixer SHALL populate the RTP packet with
T140blocks combined from all T140blocks queued for transmission T140blocks combined from all T140blocks queued for transmission
originating from each source as long as this is not in conflict with originating from each source as long as this is not in conflict with
the allowed number of characters per second or the maximum packet the allowed number of characters per second or the maximum packet
size. These T140blocks SHALL be placed in the packet interleaved size. These T140blocks SHALL be placed in the packet interleaved
with redundant T140blocks and new T140blocks from other sources. The with redundant T140blocks and new T140blocks from other sources. The
primary and redundant T140blocks from each source are grouped SSRC of each source shall be placed as a member in the CSRC-list at a
together in "source groups" appearing in the packet in the same order place corresponding to the place of its T140blocks in the packet.
as the corresponding CSRC list member and redundancy header members
for the source group. The format of the redundancy header members 3.8. Maximum number of sources per packet
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 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 reason for this limitation is the maximum number of CSRC list
members. If text from more sources need to be transmitted, the mixer members. If text from more sources need to be transmitted, the mixer
MAY let the sources take turns in having their text transmitted. MAY let the sources take turns in having their text transmitted.
When stopping transmission of one source to allow another source to When stopping transmission of one source to allow another source to
have its text sent, all intended redundant generations of the last have its text sent, all intended redundant generations of the last
text from the source to be stopped MUST be transmitted before text text from the source to be stopped MUST be transmitted before text
from another source can be transmitted. Actively transmitting from another source can be transmitted. Actively transmitting
sources SHOULD be allowed to take turns with short intervals to have sources SHOULD be allowed to take turns with short intervals to have
their text transmitted. their text transmitted.
Note: The CSRC-list in an RTP packet only includes participants who's
text is included in text blocks. It is not the same as the total
list of participants in a conference. With audio and video media,
the CSRC-list would often contain all participants who are not muted
whereas text participants that don't type are completely silent and
thus are not represented in RTP packet CSRC-lists once their text
have been transmitted as primary and the intended number of redundant
generations.
3.9. Empty T140blocks
If no unsent T140blocks were available at this time for a source, but If no unsent T140blocks were available at this time for a source, but
T140blocks are available which have not yet been sent the full T140blocks are available which have not yet been sent the full
intended number of redundant transmissions, then the primary intended number of redundant transmissions, then the primary
T140block for that source is composed of an empty T140block, and T140block for that source is composed of an empty T140block, and
populated (without taking up any length) in a packet for populated (without taking up any length) in a packet for
transmission. The corresponding SSRC SHALL be placed in its place in transmission. The corresponding SSRC SHALL be placed in its place in
the CSRC-list. the CSRC-list.
3.10. Creation of the redundancy
The primary T140block from each source in the latest transmission is The primary T140block from each source in the latest transmission is
used to populate the first redundant T140block for that source. The used to populate the first redundant T140block for that source. The
first redundant T140block for that source from the latest first redundant T140block for that source from the latest
transmission is placed as the second redundant T140block source. 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 negotiated, then the procedure SHALL be maintained redundancy is negotiated, then the procedure SHALL be maintained
until all available redundant levels of T140blocks and their sources until all available redundant levels of T140blocks and their sources
are placed in the packet. If a receiver has negotiated a lower are placed in the packet. If a receiver has negotiated a lower
number of text/rex generations, then that level shall be the maximum number of text/rex generations, then that level shall be the maximum
used by the transmitter. used by the transmitter.
The timer offset values are inserted in the redundancy header, with 3.11. Timer offset fields
the time offset from the RTP timestamp in the packet when the
The timer offset values are inserted in the data header, with the
time offset from the RTP timestamp in the packet when the
corresponding T140block was sent from its original source as primary. corresponding T140block was sent from its original source as primary.
The timestamp offset values for empty T140blocks have no relevance
but SHOULD be assigned realistic values.
3.12. Other RTP header fields
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.
The current time SHALL be inserted in the timestamp.
The SSRC of the mixer for the RTT session SHALL be inserted in the
SSRC field of the RTP header.
3.13. Pause in transmission
When there is no new T140block to transmit, and no redundant When there is no new T140block to transmit, and no redundant
T140block that has not been retransmitted the intended number of T140block that has not been retransmitted the intended number of
times, 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.
4. Actions at reception 4. Actions at reception
The "text/rex" receiver included in an endpoint with presentation The "text/rex" receiver included in an endpoint with presentation
functions will receive RTP packets in the single stream from the functions will receive RTP packets in the single stream from the
mixer, and SHALL distribute the T140blocks for presentation in mixer, and SHALL distribute the T140blocks for presentation in
presentation areas for each source. Other receiver roles, such as presentation areas for each source. Other receiver roles, such as
gateways or chained mixers are also feasible, and requires gateways or chained mixers are also feasible, and requires
consideration if the stream shall just be forwarded, or distributed consideration if the stream shall just be forwarded, or distributed
based on the different sources. based on the different sources.
If the "CC" field value of a received packet is >1, it indicates that 4.1. Multi-party vs two-party use
multi-party transmission is active, and the receiver MUST act on the
different sources according to its role. If the CC field value is 1, If the "CC" field value of a received packet is >0, it indicates that
it indicates that there is a mixer between the source and the multi-party transmission is active, and the receiver MUST be prepared
receiver, but that there is only one source at the moment. If the CC to act on the different sources according to its role. If the CC
value is 0, the connection is point-to-point. value is 0, the connection is point-to-point.
4.2. Level of redundancy
The used level of redundancy generations SHALL be evaluated from the The used level of redundancy generations SHALL be evaluated from the
received packet contents. If the CC value is 0, the number of received packet contents. If the CC value is 0, the number of
generations (including the primary) is equal to the number of members generations (including the primary) is equal to the number of members
in the redundancy header. If the CC value is >0, the number of in the data header. If the CC value is >0, the number of generations
generations (including the primary) is equal to the number of members (including the primary) is equal to the number of members in the data
in the redundancy header divided by the CC value. If the remainder header divided by the CC value. If the remainder from the division
from the division is >0, then the packet is malformed and SHALL cause is >0, then the packet is malformed and SHALL cause an error
an error indication in the receiver. indication in the receiver.
4.3. Extracting text and handling recovery and loss
The RTP sequence numbers of the received packets SHALL be monitored The RTP sequence numbers of the received packets SHALL be monitored
for gaps and packets out of order. for gaps and packets out of order.
As long as the sequence is correct, each packet SHALL be unpacked in As long as the sequence is correct, each packet SHALL be unpacked in
order. The T140blocks SHALL be extracted from the primary areas, and order. The T140blocks SHALL be extracted from the primary areas, and
the corresponding SSRCs SHALL be extracted from the corresponding the corresponding SSRCs SHALL be extracted from the corresponding
positions in the CSRC list and used for assigning the new T140block positions in the CSRC list and used for assigning the new T140block
to the correct presentation areas (or correspondingly). to the correct presentation areas (or correspondingly).
skipping to change at page 12, line 14 skipping to change at page 13, line 19
be retrieved from the packet and similarly their sources retrieved be retrieved from the packet and similarly their sources retrieved
from the corresponding positions in the CSRC-list, and then assigned from the corresponding positions in the CSRC-list, and then assigned
to the corresponding presentation areas for the sources. 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.
4.4. Delete BOM
Unicode character BOM is used as a start indication and sometimes Unicode character BOM is used as a start indication and sometimes
used as a filler or keep alive by transmission implementations. used as a filler or keep alive by transmission implementations.
These SHALL be deleted on reception. These SHALL be deleted on reception.
4.5. Empty T140blocks
Empty T140blocks are included as fillers for unused redundancy levels Empty T140blocks are included as fillers for unused redundancy levels
in the packets. They just do not provide any contents and do not in the packets. They just do not provide any contents and do not
contribute to the received streams. contribute to the received streams.
5. 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.
skipping to change at page 13, line 18 skipping to change at page 14, line 23
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 MAY be be provided. The SSRC of the text to expect from joined users MAY be
included in a notification. The notifications MAY be used both for included in a notification. The notifications MAY be used both for
security purposes and for translation to a label for presentation to security purposes and for translation to a label for presentation to
other users. other users.
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
the total list of participants in a conference. With audio and video
media, the CSRC-list would often contain all participants who are not
muted whereas text participants that don't type are completely silent
and thus are not represented in RTP packet CSRC-lists.
9. Conference control 9. Conference control
In managed conferences, control of the real-time text media SHOULD be In managed conferences, control of the real-time text media SHOULD be
provided in the same way as other for media, e.g. for muting and provided in the same way as other for media, e.g. for muting and
unmuting by the direction attributes in sdp [RFC4566]. unmuting by the direction attributes in SDP [RFC4566].
Note that floor control functions may be of value for RTT users as Note that floor control functions may be of value for RTT users as
well as for users of other media in a conference. well as for users of other media in a conference.
10. Media Subtype Registration 10. Media Subtype Registration
This registration is done using the template defined in [RFC6838] and This registration is done using the template defined in [RFC6838] and
following [RFC4855]. following [RFC4855].
Type name: text Type name: text
skipping to change at page 14, line 21 skipping to change at page 15, line 18
Upon publication as an RFC, please replace "XXXX" with the number Upon publication as an RFC, please replace "XXXX" with the number
assigned to this document and remove this note.] assigned to this document and remove this note.]
Interoperability considerations: None. Interoperability considerations: None.
Published specification: RFC XXXX. [RFC Editor: Upon publication as Published specification: RFC XXXX. [RFC Editor: Upon publication as
an RFC, please replace "XXXX" with the number assigned to this an RFC, please replace "XXXX" with the number assigned to this
document and remove this note.] document and remove this note.]
Applications which use this media type: For example: Text Applications which use this media type: For example: Text
conferencing tools, multimedia conferencing tools. conferencing tools, multimedia conferencing tools.Real-time
conversational tools.
Fragment identifier considerations: N/A. Fragment identifier considerations: N/A.
Additional information: None. Additional information: None.
Person & email address to contact for further information: Gunnar Person & email address to contact for further information: Gunnar
Hellstrom <gunnar.hellstrom@ghaccess.se> Hellstrom <gunnar.hellstrom@ghaccess.se>
Intended usage: COMMON Intended usage: COMMON
skipping to change at page 14, line 49 skipping to change at page 15, line 47
11. SDP considerations 11. SDP considerations
There are receiving RTT implementations which implement RFC 4103 There are receiving RTT implementations which implement RFC 4103
[RFC4103] but not the source separation by the CSRC. Sending mixed [RFC4103] but not the source separation by the CSRC. Sending mixed
text according to the usual CSRC convention from RFC 2198 [RFC2198] text according to the usual CSRC convention from RFC 2198 [RFC2198]
to a device implementing only RFC 4103 [RFC4103] would risk to lead to a device implementing only RFC 4103 [RFC4103] would risk to lead
to unreadable presented text. Therefore, in order to negotiate RTT to unreadable presented text. Therefore, in order to negotiate RTT
mixing capability according to the present specification, all devices mixing capability according to the present specification, all devices
supporting the present specification for multi-party aware supporting the present specification for multi-party aware
participants SHALL include an sdp media format "text/rex" in the sdp, participants SHALL include an SDP media format "text/rex" in the SDP
indicating this capability in offers and answers. Multi-party [RFC4566], indicating this capability in offers and answers. Multi-
streams using the coding of the present specification intended for party streams using the coding of the present specification intended
multi-party aware endpoints MUST NOT be sent to devices which have for multi-party aware endpoints MUST NOT be sent to devices which
not indicated the "text/rex" format. have not indicated the "text/rex" format.
Implementations not understanding this format MUST ignore it Implementations not understanding this format MUST ignore it
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 "red". It is intended to be used in "text" media descriptions with
with "text/rex" and "text/t140" formats. Both formats MUST be "text/rex" and "text/t140" formats. Both formats MUST be declared
declared for the "text/rex" format to be used. It indicates for the "text/rex" format to be used. It indicates capability to use
capability to use source indications in the CSRC list and the packet source indications in the CSRC list and the packet format according
format according to the present specification. It also indicates to the present specification. It also indicates ability to receive
ability to receive 150 real-time text characters per second. 150 real-time text characters per second by default.
11.1. Security for session control and media 11.1. Security for session control and media
Security SHOULD be applied on both session control and media. In Security SHOULD be applied on both session control and media. In
applications where legacy endpoints without security may exist, a applications where legacy endpoints without security may exist, a
negotiation between security and no security SHOULD be applied. If negotiation between security and no security SHOULD be applied. If
no other security solution is mandated by the application, then RFC no other security solution is mandated by the application, then RFC
8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media 8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media
security with DTLS. Most SDP examples below are expressed without security with DTLS. Most SDP examples below are expressed without
the security additions for simplicity. The principles (but not all the security additions for simplicity. The principles (but not all
details) for applying DTLS-SRTP security is shown in a couple of the details) for applying DTLS-SRTP security is shown in a couple of the
following examples. following examples.
11.2. SDP offer/answer examples 11.2. SDP offer/answer examples
This sections shows some examples of SDP for session negotiation of This sections shows some examples of SDP for session negotiation of
the real-time text media in SIP sessions. In the same session the real-time text media in SIP sessions. Audio is usually provided
usually audio is provided and sometimes also video. The examples in the same session, and sometimes also video. The examples only
only show the part of importance for the real-time text media. show the part of importance for the real-time text media.
Offer example for 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
skipping to change at page 17, line 5 skipping to change at page 18, line 5
With the "fingerprint" the device acknowledges use of SRTP/DTLS. With the "fingerprint" the device acknowledges use of SRTP/DTLS.
Answer example from a multi-party unaware device that also Answer example from a multi-party unaware device that also
does not support security: does not support security:
m=text 12000 RTP/AVP 100 98 m=text 12000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
A party who has negotiated the "text/rex" format MUST populate the A party which 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.
A party who has negotiated the "text/rex" capability MUST interpret A party which has negotiated the "text/rex" capability MUST interpret
the contents of the CSRC-list and the packets according to the the contents of the CSRC-list and the packets according to the
present specification in received rtp packets using the corresponding present specification in received rtp packets using the corresponding
payload type. payload type.
A party performing as a mixer, which has not negotiated the "text/ 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 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 14.2.
A party not performing as a mixer MUST not include the CSRC list if
it has a single source of text.
12. Examples 12. 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. Pn and recovery. A, B and C are sources of RTT. M is the mixer. Pn
indicates primary data in source group "n". Rn1 is first redundant indicates primary data in source group "n". Rn1 is first redundant
generation data and Rn2 is second redundant generation data in source generation data and Rn2 is second redundant generation data in source
group "n". A1, B1, A2 etc are text chunks (T140blocks) received from group "n". A1, B1, A2 etc are text chunks (T140blocks) received from
the respective sources. X indicates dropped packet between the mixer the respective sources. X indicates dropped packet between the mixer
and a receiver. and a receiver.
skipping to change at page 19, line 34 skipping to change at page 20, line 34
| R12: C2 | | R12: C2 |
| R11: Empty | | R11: Empty |
| P1: Empty | | P1: Empty |
| R22: B1 | | R22: B1 |
| R21: B2 | | R21: B2 |
| P2: B3 | | P2: B3 |
| R32: A3 | | R32: A3 |
| R31: A4 | | R31: A4 |
| P3: A5 | | P3: A5 |
|----------------| |----------------|
Packet 6 is received. The latest received sequence number was 2.
Recovery is therefore tried for 3,4,5. But there is no coverage Packet 6 is received. The latest received sequence number was 2.
for seq no 3. A missing text mark (U'FFFD) is created and Recovery is therefore tried for 3,4,5. But there is no coverage for
appended to the common mixer reception area. seq no 3. A missing text mark (U'FFFD) [T140ad1] is created and
For seqno 4, texts C2, B1 and A3 are recovered and appended to appended to the common mixer reception area. For seqno 4, texts C2,
their respective reception areas. B1 and A3 are recovered from the second generation redundancy and
For seqno 5, texts B2 and A4 are recovered and appended to their appended to their respective reception areas. For seqno 5, texts B2
respective reception areas. and A4 are recovered from the first generation redundancy and
Primary text B3 and A5 are received and appended to their appended to their respective reception areas. Primary text B3 and A5
respective reception areas. are received and appended to their respective reception areas.
After this sequence, the following has been received: A1,A3, A4, A5;
B1, B2, B3; C1, C2. A possible loss is indicated by the missing text
mark in time between A1 and A3.
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 13. Performance considerations
This specification allows new text from up to 16 sources per packet. This specification allows new text from up to 16 sources per packet.
A mixer implementing the specification will normally cause a latency A mixer implementing the specification will normally cause a latency
of 0 to 150 milliseconds in text from up to 16 simultaneous sources. of 0 to 150 milliseconds in text from up to 16 simultaneous sources.
This performance meets well the realistic requirements for conference This performance meets well the realistic requirements for conference
applications for which up to 5 simultaneous sources should not be and conversational applications for which up to 5 simultaneous
delayed more than 500 milliseconds by a mixer. In order to achieve sources should not be delayed more than 500 milliseconds by a mixer.
good performance, a receiver for multi-party calls SHOULD declare a In order to achieve good performance, a receiver for multi-party
sufficient CPS value in SDP for the number of allowable characters calls SHOULD declare a sufficient CPS value in SDP for the number of
per second. allowable characters per second.
As comparison, if the "text/red" format would be used for multi-party
communication with its default timing and redundancy, 5
simultaneously sending parties would cause jerky presentation of the
text from them in text spurts with 5 seconds intervals. With a
reduction of the transmission interval to 150 ms, the time between
text spurts for 5 simultaneous sending parties would be 2.5 seconds.
Five simultaneous sending parties may occasionally occur in a
conference with one or two main sending parties and three parties
giving very brief comments.
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 the default for the "text/rex" capability. A value for CPS of 150 is the default for the "text/rex"
format. See RFC 4103 [RFC4103] for the format and use of the CPS format. See RFC 4103 [RFC4103] for the format and use of the CPS
skipping to change at page 21, line 33 skipping to change at page 22, line 44
A multi-party aware receiving party, presenting real-time text MUST A multi-party aware receiving party, presenting real-time text MUST
separate text from different sources and present them in separate separate text from different sources and present them in separate
presentation fields. The receiving party MAY separate presentation presentation fields. The receiving party MAY separate presentation
of parts of text from a source in readable groups based on other of parts of text from a source in readable groups based on other
criteria than line separator and merge these groups in the criteria than line separator and merge these groups in the
presentation area when it benefits the user to most easily find and presentation area when it benefits the user to most easily find and
read text from the different participants. The criteria MAY e.g. be read text from the different participants. The criteria MAY e.g. be
a received comma, full stop, or other phrase delimiters, or a long a received comma, full stop, or other phrase delimiters, or a long
pause. pause.
When text is received from multiple original sources, the When text is received from multiple original sources simultaneously,
presentation SHOULD provide a view where text is added in multiple the presentation SHOULD provide a view where text is added in
places simultaneously. multiple places simultaneously.
If the presentation presents text from different sources in one If the presentation presents text from different sources in one
common area, the presenting endpoint SHOULD insert text from the common area, the presenting endpoint SHOULD insert text from the
local user ended at suitable points merged with received text to local user ended at suitable points merged with received text to
indicate the relative timing for when the text groups were completed. indicate the relative timing for when the text groups were completed.
In this presentation mode, the receiving endpoint SHALL present the In this presentation mode, the receiving endpoint SHALL present the
source of the different groups of text. source of the different groups of text.
A view of a three-party RTT call in chat style is shown in this A view of a three-party RTT call in chat style is shown in this
example . example .
_________________________________________________ _________________________________________________
| | | | |^|
|[Alice] Hi, Alice here. | | |[Alice] Hi, Alice here. | |
| | | | | |
|[Bob] Bob as well. | | |[Bob] Bob as well. | |
| | | | | |
|[Eve] Hi, this is Eve, calling from Paris. | | |[Eve] Hi, this is Eve, calling from Paris. | |
| I thought you should be here. | | | I thought you should be here. | |
| | | | | |
|[Alice] I am coming on Thursday, my | | |[Alice] I am coming on Thursday, my | |
| performance is not until Friday morning.| | | performance is not until Friday morning.| |
| | | | | |
|[Bob] And I on Wednesday evening. | | |[Bob] And I on Wednesday evening. | |
| | | | | |
|[Alice] Can we meet on Thursday evening? | | |[Alice] Can we meet on Thursday evening? | |
| | | | | |
|[Eve] Yes, definitely. How about 7pm. | | |[Eve] Yes, definitely. How about 7pm. | |
| at the entrance of the restaurant | | | at the entrance of the restaurant | |
| Le Lion Blanc? | | | Le Lion Blanc? | |
|[Eve] we can have dinner and then take a walk | | |[Eve] we can have dinner and then take a walk |-|
| | | |______________________________________________|V|
| <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 2: Example of a three-party RTT call presented in chat style. Figure 3: Example of a three-party RTT call presented in chat style
seen at participant 'Alice's endpoint.
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. |
|My flight is to Orly| |Convenient to the main | |My flight is to Orly| |Convenient to the main |
| |Hi all, can we plan |station. | | |Hi all, can we plan |station. |
| |for the seminar? | | | |for the seminar? | |
|Eve, will you do | | | |Eve, will you do | | |
|your presentation on| | | |your presentation on| | |
|Friday? |Yes, Friday at 10. | | |Friday? |Yes, Friday at 10. | |
|Fine, wo | |We need to meet befo | |Fine, wo | |We need to meet befo |
|___________________________________________________________________| |___________________________________________________________________|
Figure 3: An example of a coordinated column-view of a three-party
Figure 4: An example of a coordinated column-view of a three-party
session with entries ordered vertically in approximate time-order. session with entries ordered vertically in approximate time-order.
14.2. Multi-party mixing for multi-party unaware endpoints 14.2. 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 implementing only RFC 4103 without the multi-party
extension specified in the current document.
The principles and procedures below do not specify any new protocol The principles and procedures below do not specify any new protocol
elements or behaviors. They are instead composed from the elements or behaviors. They are instead composed from the
information in ITU-T T.140 [T140] and an ambition to provide a best information in ITU-T T.140 [T140] and an ambition to provide a best
effort presentation on an endpoint which has functions only for two- effort presentation on an endpoint which has functions only for two-
party calls. party calls.
The mixer mixing for multi-party unaware endpoints SHALL compose a The mixer mixing for multi-party unaware endpoints SHALL compose a
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 between
them if the transmitted text did not already end with a new line. A them if the transmitted text did not already end with a new line. A
presentable label SHOULD be composed and sent for the source presentable label SHOULD be composed and sent for the source
initially in the session and after each source switch. With this initially in the session and after each source switch. With this
procedure the time for source switching is depending on the actions procedure the time for source switching is depending on the actions
of the users. In order to expedite source switch, a user can for of the users. In order to expedite source switch, a user can for
example end its turn with a new line. example end its turn with a new line.
14.2.1. Actions by the mixer at reception from the call participants 14.2.1. 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 creation of text
SHALL be stored together with the received text from each source in a (retrieved from the RTP timestamp) SHALL be stored together with the
queue for transmission to the recipients. received text from each source in queues for transmission to the
recipients.
14.2.2. Actions by the mixer for transmission to the recipients 14.2.2. 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
skipping to change at page 24, line 39 skipping to change at page 26, line 7
* A completed sentence * A completed sentence
* A new line (line separator or CRLF) * A new line (line separator or CRLF)
* A long pause (e.g. > 10 seconds) in received text from the * A long pause (e.g. > 10 seconds) in received text from the
currently transmitted source currently transmitted source
* If text from one participant has been transmitted with text from * If text from one participant has been transmitted with text from
other sources waiting for transmission for a long time (e.g. > 1 other sources waiting for transmission for a long time (e.g. > 1
minute) and none of the suitable points for switching has minute) and none of the other suitable points for switching has
occurred, a source switch MAY be forced by the mixer at next word occurred, a source switch MAY be forced by the mixer at next word
delimiter, and also if even a word delimiter does not occur within delimiter, and also if even a word delimiter does not occur within
a time (e.g. 15 seconds) after the scan for word delimiter a time (e.g. 15 seconds) after the scan for word delimiter
started. started.
When switching source, the source which has the oldest text in queue When switching source, the source which has the oldest text in queue
SHOULD be selected to be transmitted. A character display count SHOULD be selected to be transmitted. A character display count
SHOULD be maintained for the currently transmitted source, starting SHOULD be maintained for the currently transmitted source, starting
at zero after the label is transmitted for the currently transmitted at zero after the label is transmitted for the currently transmitted
source. source.
There SHOULD be a storage for the latest control code for Select The status SHOULD be maintained for the latest control code for
Graphic Rendition (SGR) from each source. If there is an SGR code Select Graphic Rendition (SGR) from each source. If there is an SGR
stored for the current source before the source switch is done, a code stored as the status for the current source before the source
reset of SGR shall be sent by the sequence SGR 0 [009B 0000 006D] switch is done, a reset of SGR shall be sent by the sequence SGR 0
after the new line and before the new label during a source switch. [009B 0000 006D] after the new line and before the new label during a
See SGR below for an explanation. This transmission does not source switch. See SGR below for an explanation. This transmission
influence the display count. If there is an SGR code stored for the
new source after the source switch, that SGR code SHOULD be
transmitted to the recipient before the label. This transmission
does not influence the display count. does not influence the display count.
If there is an SGR code stored for the new source after the source
switch, that SGR code SHOULD be transmitted to the recipient before
the label. This transmission does not influence the display count.
14.2.3. Actions on transmission of text 14.2.3. 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 14.2.4. 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
skipping to change at page 26, line 6 skipping to change at page 27, line 22
SOS 0098 Start of string, used as a general protocol element SOS 0098 Start of string, used as a general protocol element
introducer, followed by a maximum 256 bytes string and the ST. introducer, followed by a maximum 256 bytes string and the ST.
The display count SHOULD not be altered. The display count SHOULD not be altered.
ST 009C String terminator, end of SOS string. The display count ST 009C String terminator, end of SOS string. The display count
SHOULD not be altered. SHOULD not be altered.
ESC 001B Escape - used in control strings. The display count SHOULD ESC 001B Escape - used in control strings. The display count SHOULD
not be altered for the complete escape code. not be altered for the complete escape code.
Byte order mark FEFF Zero width, no break space, used for Byte order mark 'BOM* FEFF Zero width, no break space, used for
synchronization and keep-alive. SHOULD be deleted from incoming synchronization and keep-alive. SHOULD be deleted from incoming
streams. Shall be sent first after session establishment to the streams. Shall be sent first after session establishment to the
recipient. The display count shall not be altered. recipient. The display count shall not be altered.
Missing text mark FFFD Replacement character, marks place in stream Missing text mark FFFD Replacement character, represented as a
of possible text loss. SHOULD be inserted by the reception question mark in a rhombus, or if that is not feasible, replaced
procedure in case of unrecoverable loss of packets. The display by an apostrophe ', marks place in stream of possible text loss.
count SHOULD be increased by one when sent as for any other SHOULD be inserted by the reception procedure in case of
character. unrecoverable loss of packets. The display count SHOULD be
increased by one when sent as for any other character.
SGR If a control code for selecting graphic rendition (SGR), other SGR If a control code for selecting graphic rendition (SGR), other
than reset of the graphic rendition (SGR 0) is sent to a than reset of the graphic rendition (SGR 0) is sent to a
recipient, that control code shall also be stored for the source recipient, that control code shall also be stored as status for
in the storage for SGR. If a reset graphic rendition (SGR 0) the source in the storage for SGR status. If a reset graphic
originated from a source is sent, then the SGR storage for that rendition (SGR 0) originated from a source is sent, then the SGR
source shall be cleared. The display count shall not be status storage for that source shall be cleared. The display
increased. count shall not be increased.
BS 0008 Back Space, intended to erase the last entered character by BS 0008 Back Space, intended to erase the last entered character by
a source. Erasure by backspace cannot always be performed as the a source. Erasure by backspace cannot always be performed as the
erasing party intended. If an erasing action erases all text up erasing party intended. If an erasing action erases all text up
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
skipping to change at page 27, line 29 skipping to change at page 28, line 47
source to be presented takes place at suitable places in the text, source to be presented takes place at suitable places in the text,
such as end of phrase, end of sentence, line separator and such as end of phrase, end of sentence, line separator and
inactivity. Therefore the time to switch to present waiting text inactivity. Therefore the time to switch to present waiting text
from other sources may become long and will vary and depend on the from other sources may become long and will vary and depend on the
actions of the currently presented source. actions of the currently presented source.
Erasure can only be done up to the latest source switch. If a user Erasure can only be done up to the latest source switch. If a user
tries to erase more text, the erasing actions will be presented as tries to erase more text, the erasing actions will be presented as
letter X after the label. letter X after the label.
Text loss because of network errors may hit the label between entries
from different parties, causing risk for misunderstanding from which
source a piece of text is.
These facts makes it strongly RECOMMENDED to implement multi-party These facts makes it strongly RECOMMENDED to implement multi-party
awareness in RTT endpoints. The use of the mixing method for multi- awareness in RTT endpoints. The use of the mixing method for multi-
party-unaware endpoints should be left for use with endpoints which party-unaware endpoints should be left for use with endpoints which
are impossible to upgrade to become multi-party aware. are impossible to upgrade to become multi-party aware.
14.2.7. Example views of presentation on multi-party unaware endpoints 14.2.7. 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.
skipping to change at page 28, line 21 skipping to change at page 29, line 31
|[Eve]:Hi all, can we | | |[Eve]:Hi all, can we | |
|plan for the seminar. | | |plan for the seminar. | |
| | | | | |
|[Bob]:Eve, will you do | | |[Bob]:Eve, will you do | |
|your presentation on | | |your presentation on | |
|Friday? | | |Friday? | |
|[Eve]:Yes, Friday at 10.| | |[Eve]:Yes, Friday at 10.| |
|[Bob]: Fine, wo |We need to meet befo | |[Bob]: Fine, wo |We need to meet befo |
|________________________|_________________________| |________________________|_________________________|
Figure 4: Alice who has a conference-unaware client is receiving the Figure 5: Alice who has a conference-unaware client is receiving the
multi-party real-time text in a single-stream. This figure shows how multi-party real-time text in a single-stream. This figure shows how
a coordinated column view MAY be presented on Alice's device. a coordinated column view MAY be presented on Alice's device.
_________________________________________________ _________________________________________________
| |^| | |^|
|[Alice] Hi, Alice here. | | |[Alice] Hi, Alice here. | |
| | | | | |
|[mix][Bob] Bob as well. | | |[mix][Bob] Bob as well. | |
| | | | | |
|[Eve] Hi, this is Eve, calling from Paris | | |[Eve] Hi, this is Eve, calling from Paris | |
skipping to change at page 28, line 48 skipping to change at page 30, line 28
| | | | | |
|[Eve] we can have dinner and then walk | | |[Eve] we can have dinner and then walk | |
| | | | | |
|[Eve] But I need to be back to | | |[Eve] But I need to be back to | |
| the hotel by 11 because I need |-| | the hotel by 11 because I need |-|
| |-| | |-|
|______________________________________________|v| |______________________________________________|v|
| of course, I underst | | of course, I underst |
|________________________________________________| |________________________________________________|
Figure 5: An example of a view of the multi-party unaware Figure 6: An example of a view of the multi-party unaware
presentation in chat style. Alice is the local user. presentation in chat style. Alice is the local user.
15. Gateway Considerations 15. Gateway Considerations
15.1. Gateway considerations with Textphones (e.g. TTYs). 15.1. 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
skipping to change at page 29, line 21 skipping to change at page 30, line 50
One case that may occur is a gateway to PSTN for communication with One case that may occur is a gateway to PSTN for communication with
textphones (e.g. TTYs). Textphones are limited devices with no textphones (e.g. TTYs). Textphones are limited devices with no
multi-party awareness, and it SHOULD therefore be suitable for the multi-party awareness, and it SHOULD therefore be suitable for the
gateway to not indicate multi-party awareness for that case. Another gateway to not indicate multi-party awareness for that case. Another
solution is that the gateway indicates multi-party capability towards solution is that the gateway indicates multi-party capability towards
the mixer, and includes the multi-party mixer function for multi- the mixer, and includes the multi-party mixer function for multi-
party unaware endpoints itself. This solution makes it possible to party unaware endpoints itself. This solution makes it possible to
make adaptations for the functional limitations of the textphone make adaptations for the functional limitations of the textphone
(TTY). (TTY).
More information on gateways to textphones (TTYs) is found in RFC
5194[RFC5194]
15.2. Gateway considerations with WebRTC. 15.2. 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]. channel[I-D.ietf-mmusic-t140-usage-data-channel].
A multi-party bridge may have functionality for communicating by RTT A multi-party bridge may have functionality for communicating by RTT
in both RTP streams with RTT and WebRTC t140 data channels. Other both in RTP streams with RTT and WebRTC t140 data channels. Other
configurations may consist of a multi-party bridge with either configurations may consist of a multi-party bridge with either
technology for RTT transport and a separate gateway for conversion of technology for RTT transport and a separate gateway for conversion of
the text communication streams between RTP and t140 data channel. the text communication streams between RTP and t140 data channel.
In WebRTC, it is assumed that for a multi-party session, one t140 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 data channel is established for each source from a gateway or bridge
to each participant. Each participant also has a data channel with to each participant. Each participant also has a data channel with
two-way connection with the gateway or bridge. two-way connection with the gateway or bridge.
The t140 channel used both ways is for text from the WebRTC user and The t140 channel used both ways is for text from the WebRTC user and
skipping to change at page 30, line 9 skipping to change at page 31, line 37
When a new participant has entered the session with RTP transport of When a new participant has entered the session with RTP transport of
rtt, a new t140 channel SHOULD be established to WebRTC users with rtt, a new t140 channel SHOULD be established to WebRTC users with
the label parameter composed from the NAME field in RTCP on the RTP the label parameter composed from the NAME field in RTCP on the RTP
side. side.
When a new participant has entered the multi-party session with RTT When a new participant has entered the multi-party session with RTT
transport in a WebRTC t140 data channel, the new participant SHOULD transport in a WebRTC t140 data channel, the new participant SHOULD
be announced by a notification to RTP users. The label parameter be announced by a notification to RTP users. The label parameter
from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP
side. side, or other available session information.
16. Updates to RFC 4102 and RFC 4103 16. Updates to RFC 4102 and RFC 4103
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by This 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.
17. Congestion considerations 17. 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.
skipping to change at page 31, line 4 skipping to change at page 32, line 30
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.
21. Change history 21. Change history
21.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03
21.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04
'Redundancy header' renamed to 'data header'.
More clarifications added.
Language and figure number corrections.
21.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03
Mention possible need to mute and raise hands as for other media. Mention possible need to mute and raise hands as for other media.
---done ---- ---done ----
Make sure that use in two-party calls is also possible and explained. Make sure that use in two-party calls is also possible and explained.
- may need more wording - - may need more wording -
Clarify the RTT is often used together with other media. --done-- Clarify the RTT is often used together with other media. --done--
Tell that text mixing is N-1. A users own text is not received in Tell that text mixing is N-1. A users own text is not received in
skipping to change at page 32, line 4 skipping to change at page 33, line 37
text to have time offset as for transmission from the source. Make text to have time offset as for transmission from the source. Make
that header equal to the others. -done- that header equal to the others. -done-
Mixer always use the CSRC list, even for its own BOM. -done- Mixer always use the CSRC list, even for its own BOM. -done-
Combine all talk about transmission interval (300 ms vs when text has Combine all talk about transmission interval (300 ms vs when text has
arrived) in section 3 in one paragraph or close to each other. -done- arrived) in section 3 in one paragraph or close to each other. -done-
Documents the goal of good performance with low delay for 5 Documents the goal of good performance with low delay for 5
simultaneous typers in the introduction. -done- simultaneous typers in the introduction. -done-
Describe better that only primary text shall be sent on to receivers. Describe better that only primary text shall be sent on to receivers.
Redundancy and loss must be resolved by the mixer. -done- Redundancy and loss must be resolved by the mixer. -done-
21.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 21.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02
SDP and better description and visibility of security by OSRTP RFC SDP and better description and visibility of security by OSRTP RFC
8634 needed. 8634 needed.
The description of gatewaying to WebRTC extended. The description of gatewaying to WebRTC extended.
The description of the redundancy header in the packet is improved. The description of the data header in the packet is improved.
21.3. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 21.4. 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.
21.4. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 21.5. 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.
21.5. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to 21.6. 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.
21.6. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to 21.7. 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
endpoints if the negotiation is not successful. And a reference to a endpoints if the negotiation is not successful. And a reference to a
later chapter about it. later chapter about it.
The presentation considerations chapter 14 is extended with more The presentation considerations chapter 14 is extended with more
information about presentation on multi-party aware endpoints, and a information about presentation on multi-party aware endpoints, and a
new section on the multi-party unaware mixing with low functionality new section on the multi-party unaware mixing with low functionality
but SHOULD a be implemented in mixers. Presentation examples are but SHOULD a be implemented in mixers. Presentation examples are
added. added.
skipping to change at page 34, line 5 skipping to change at page 35, line 45
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.
21.7. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 to 21.8. 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
22. References 22. References
22.1. Normative References 22.1. Normative References
skipping to change at page 36, line 10 skipping to change at page 37, line 48
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, DOI 10.17487/RFC4575, August Conference State", RFC 4575, DOI 10.17487/RFC4575, August
2006, <https://www.rfc-editor.org/info/rfc4575>. 2006, <https://www.rfc-editor.org/info/rfc4575>.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol [RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents", (SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006, BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,
<https://www.rfc-editor.org/info/rfc4579>. <https://www.rfc-editor.org/info/rfc4579>.
[RFC5194] van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real-
Time Text over IP Using the Session Initiation Protocol
(SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008,
<https://www.rfc-editor.org/info/rfc5194>.
Author's Address Author's Address
Gunnar Hellstrom Gunnar Hellstrom
Gunnar Hellstrom Accessible Communication Gunnar Hellstrom Accessible Communication
Esplanaden 30 Esplanaden 30
SE-13670 Vendelso SE-13670 Vendelso
Sweden Sweden
Email: gunnar.hellstrom@ghaccess.se Email: gunnar.hellstrom@ghaccess.se
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