draft-ietf-avtcore-multi-party-rtt-mix-09.txt   draft-ietf-avtcore-multi-party-rtt-mix-10.txt 
AVTCore G. Hellstrom AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: RFC 4103 (if approved) 13 October 2020 Updates: RFC 4103 (if approved) 18 November 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 16 April 2021 Expires: 22 May 2021
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-09 draft-ietf-avtcore-multi-party-rtt-mix-10
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, while new text from other sources is also presented in
readable grouping as received interleaved in real-time.
Regional regulatory requirements specify provision of real-time text Regional regulatory requirements specify provision of real-time text
in multi-party calls. RFC 4103 mixer implementations can use in multi-party calls. RFC 4103 mixer implementations can use
traditional RTP functions for source identification, but the mixer traditional RTP functions for source identification, but the mixer
source switching performance is limited when using the default source switching performance is limited when using the default
transmission characteristics with redundancy. transmission characteristics with redundancy.
Enhancements for RFC 4103 real-time text mixing is provided in this Enhancements for RFC 4103 real-time text mixing is provided in this
document, suitable for a centralized conference model that enables document, suitable for a centralized conference model that enables
source identification and source switching. The intended use is for source identification and source switching. The intended use is for
real-time text mixers and multi-party-aware participant endpoints. real-time text mixers and multi-party-aware participant endpoints.
The specified mechanism build on the standard use of the CSRC list in The specified mechanism build on the standard use of the CSRC list in
the RTP packet for source identification. The method makes use of the RTP packet for source identification. The method makes use of
the same "text/red" format as for two-party sessions. the same "text/t140" and "text/red" formats as for two-party
sessions.
A capability exchange is specified so that it can be verified that a A capability exchange is specified so that it can be verified that a
participant can handle the multi-party coded real-time text stream. participant can handle the multi-party coded real-time text stream.
The capability is indicated by use of a media attribute "rtt-mixer". The capability is indicated by use of a media attribute "rtt-mixer".
The document updates RFC 4103[RFC4103] The document updates RFC 4103[RFC4103]
A specifications of how a mixer can format text for the case when the A specifications of how a mixer can format text for the case when the
endpoint is not multi-party aware is also provided. endpoint is not multi-party aware is also provided.
skipping to change at page 2, line 15 skipping to change at page 2, line 15
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This Internet-Draft will expire on 16 April 2021. This Internet-Draft will expire on 22 May 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Selected solution and considered alternative . . . . . . 5 1.1. Selected solution and considered alternative . . . . . . 5
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 7 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Intended application . . . . . . . . . . . . . . . . . . 7 1.3. Intended application . . . . . . . . . . . . . . . . . . 8
2. Specified solutions . . . . . . . . . . . . . . . . . . . . . 8 2. Overview over the two specified solutions . . . . . . . . . . 8
2.1. Negotiated use of the RFC 4103 format for multi-party in a 2.1. Negotiated use of the RFC 4103 format for multi-party
single RTP stream . . . . . . . . . . . . . . . . . . . . 8 transmission in a single RTP stream . . . . . . . . . . . 9
2.2. Mixing for multi-party unaware endpoints . . . . . . . . 8 2.2. Mixing for multi-party unaware endpoints . . . . . . . . 9
3. Details for the multi-party aware mixing case . . . . . . . . 8 3. Details for the multi-party aware mixing case . . . . . . . . 9
3.1. Offer/answer considerations . . . . . . . . . . . . . . . 9 3.1. Offer/answer considerations . . . . . . . . . . . . . . . 9
3.2. Use of fields in the RTP packets . . . . . . . . . . . . 9 3.2. Actions depending on capability negotiation result . . . 10
3.3. Initial transmission of a BOM character . . . . . . . . . 10 3.3. Use of fields in the RTP packets . . . . . . . . . . . . 10
3.4. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 10 3.4. Initial transmission of a BOM character . . . . . . . . . 11
3.5. Transmission interval . . . . . . . . . . . . . . . . . . 10 3.5. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 11
3.6. Only one source per packet . . . . . . . . . . . . . . . 10 3.6. Transmission interval . . . . . . . . . . . . . . . . . . 11
3.7. Do not send received text to the originating source . . . 10 3.7. Only one source per packet . . . . . . . . . . . . . . . 11
3.8. Clean incoming text . . . . . . . . . . . . . . . . . . . 10 3.8. Do not send received text to the originating source . . . 11
3.9. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 11 3.9. Clean incoming text . . . . . . . . . . . . . . . . . . . 11
3.10. Text placement in packets . . . . . . . . . . . . . . . . 11 3.10. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 12
3.11. Source switching . . . . . . . . . . . . . . . . . . . . 11 3.11. Source switching . . . . . . . . . . . . . . . . . . . . 12
3.12. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 12 3.12. Text placement in packets . . . . . . . . . . . . . . . . 12
3.13. Creation of the redundancy . . . . . . . . . . . . . . . 12 3.13. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 13
3.14. Timer offset fields . . . . . . . . . . . . . . . . . . . 12 3.14. Creation of the redundancy . . . . . . . . . . . . . . . 13
3.15. Other RTP header fields . . . . . . . . . . . . . . . . . 12 3.15. Timer offset fields . . . . . . . . . . . . . . . . . . . 13
3.16. Pause in transmission . . . . . . . . . . . . . . . . . . 13 3.16. Other RTP header fields . . . . . . . . . . . . . . . . . 14
3.17. RTCP considerations . . . . . . . . . . . . . . . . . . . 13 3.17. Pause in transmission . . . . . . . . . . . . . . . . . . 14
3.18. Reception of multi-party contents . . . . . . . . . . . . 13 3.18. RTCP considerations . . . . . . . . . . . . . . . . . . . 14
3.19. Performance considerations . . . . . . . . . . . . . . . 15 3.19. Reception of multi-party contents . . . . . . . . . . . . 15
3.20. Security for session control and media . . . . . . . . . 15 3.20. Performance considerations . . . . . . . . . . . . . . . 17
3.21. SDP offer/answer examples . . . . . . . . . . . . . . . . 15 3.21. Security for session control and media . . . . . . . . . 17
3.22. Packet sequence example from a source switch . . . . . . 17 3.22. SDP offer/answer examples . . . . . . . . . . . . . . . . 18
3.23. Maximum character rate "CPS" . . . . . . . . . . . . . . 19 3.23. Packet sequence example from a source switch . . . . . . 19
4. Presentation level considerations . . . . . . . . . . . . . . 19 3.24. Maximum character rate "CPS" . . . . . . . . . . . . . . 21
4.1. Presentation by multi-party aware endpoints . . . . . . . 20 4. Presentation level considerations . . . . . . . . . . . . . . 21
4.2. Multi-party mixing for multi-party unaware endpoints . . 22 4.1. Presentation by multi-party aware endpoints . . . . . . . 22
5. Relation to Conference Control . . . . . . . . . . . . . . . 27 4.2. Multi-party mixing for multi-party unaware endpoints . . 24
5.1. Use with SIP centralized conferencing framework . . . . . 28 5. Relation to Conference Control . . . . . . . . . . . . . . . 29
5.2. Conference control . . . . . . . . . . . . . . . . . . . 28 5.1. Use with SIP centralized conferencing framework . . . . . 30
6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 28 5.2. Conference control . . . . . . . . . . . . . . . . . . . 30
6.1. Gateway considerations with Textphones (e.g. TTYs). . . 28 6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 30
6.2. Gateway considerations with WebRTC. . . . . . . . . . . . 29 6.1. Gateway considerations with Textphones (e.g. TTYs). . . 30
7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 29 6.2. Gateway considerations with WebRTC. . . . . . . . . . . . 31
8. Congestion considerations . . . . . . . . . . . . . . . . . . 29 7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 31
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 8. Congestion considerations . . . . . . . . . . . . . . . . . . 31
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Registration of the "rtt-mixer" sdp media attribute . . 30 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
11. Security Considerations . . . . . . . . . . . . . . . . . . . 31 10.1. Registration of the "rtt-mixer" sdp media attribute . . 32
12. Change history . . . . . . . . . . . . . . . . . . . . . . . 31 11. Security Considerations . . . . . . . . . . . . . . . . . . . 33
12. Change history . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Changes included in 12.1. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-10 . . . . . . . 33
12.2. Changes included in 12.2. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 33
12.3. Changes included in 12.3. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 31 draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 34
12.4. Changes included in 12.4. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 34
12.5. Changes included in 12.5. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 34
12.6. Changes included in 12.6. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 34
12.7. Changes included in 12.7. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 32 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 34
12.8. Changes included in 12.8. Changes included in
draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 33 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 35
12.9. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 33 12.9. Changes included in
12.10. Changes from draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 36
draft-hellstrom-avtcore-multi-party-rtt-source-03 to 12.10. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 36
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 34
12.11. Changes from 12.11. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to draft-hellstrom-avtcore-multi-party-rtt-source-03 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 34 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 36
12.12. Changes from 12.12. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 34 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 36
12.13. Changes from 12.13. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 37
12.14. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 35 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 38
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
13.1. Normative References . . . . . . . . . . . . . . . . . . 35 13.1. Normative References . . . . . . . . . . . . . . . . . . 38
13.2. Informative References . . . . . . . . . . . . . . . . . 37 13.2. Informative References . . . . . . . . . . . . . . . . . 39
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 37 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 40
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.
A requirement related to multi-party sessions from the presentation
level standard T.140 for real-time text is: "The display of text from
the members of the conversation should be arranged so that the text
from each participant is clearly readable, and its source and the
relative timing of entered text is visualized in the display."
Another requirement is that the mixing procedure must not introduce
delays in the text streams that are experienced disturbing the real-
time experience of the receiving users.
The redundancy scheme of RFC 4103 [RFC4103] enables efficient The redundancy scheme of RFC 4103 [RFC4103] enables efficient
transmission of redundant text in packets together with new text. transmission of redundant text in packets together with new text.
However the redundancy header format has no source indicators for the However the redundancy header format has no source indicators for the
redundant transmissions. An assumption has to be made that the redundant transmissions. The redundant parts in a packet must
redundant parts in a packet are from the same source as the new text. therefore be from the same source as the new text. The recommended
The recommended transmission is one new and two redundant generations transmission is one new and two redundant generations of text
of text (T140blocks) in each packet and the recommended transmission (T140blocks) in each packet and the recommended transmission interval
interval is 300 ms. for two-party use is 300 ms.
A mixer, selecting between text input from different sources and Real-time text mixers for multi-party sessions therefore need to
transmitting it in a common stream needs to make sure that the insert the source of each transmitted group of text from a conference
receiver can assign the received text to the proper sources for participant so that the text can be transmitted interleaved with text
presentation. Therefore, using RFC 4103 without any extra rule for groups from different sources in the rate they are created. This
source identification, the mixer needs to stop sending new text from enables the text groups to be presented by endpoints in suitable
one source and then make sure that all text sent so far has been sent grouping with other text from the same source. The presentation can
with all intended redundancy levels (usually two) before switching to then be arranged so that text from different sources can be presented
another source. That causes the long time of one second to switch in real-time and easily read while it is possible for a reading user
between transmission of text from one source to text from another to also perceive approximately when the text was created in real time
source when using the default transmission interval 300 ms. Both the by the different parties. The transmission and mixing is intended to
total throughput and the switching performance in the mixer would be be done in a general way so that presentation can be arranged in a
too low for most applications. However by shortening the layout decided by the endpoint.
transmission interval to 100 ms, good performance is achieved for up
to 3 simultaneously sending sources and usable performance for up to
5 simultaneously sending sources. Capability to use this method is
indicated by an sdp media attribute "rtt-mixer".
A negotiation mechanism can therefore be based on selection of the There are existing implementations of RFC 4103 without the updates
"text/red" with media attribute "rtt-mixer" for verification that the from this document. These will not be able to receive and present
real-time text mixed for multi-party aware endpoints.
A negotiation mechanism is therefore needed for verification if the
parties are able to handle a multi-party coded stream and agreeing on parties are able to handle a multi-party coded stream and agreeing on
using that method. using that method.
A fall-back mixing procedure is specified for cases when the A fall-back mixing procedure is also needed for cases when the
negotiation results in "text/red" without the "rtt-mixer" attribute negotiation result indicates that a receiving endpoint is not capable
being the only common format for real-time text. of handling the mixed format. This method is called the mixing
procedure for multi-party unaware endpoints. The fall-back method is
naturally not expected to meet all performance requirements placed on
the mixing procedure for multi-party aware endpoints.
The document updates RFC 4103[RFC4103] by introducing an attribute The document updates RFC 4103[RFC4103] by introducing an attribute
for indicating capability for the multi-party mixing case and rules for indicating capability for the multi-party mixing case and rules
for source indications and source switching. for source indications and source switching.
1.1. Selected solution and considered alternative 1.1. Selected solution and considered alternative
A number of alternatives were considered when searching an efficient A number of alternatives were considered when searching an efficient
and easily implemented multi-party method for real-time text. This and easily implemented multi-party method for real-time text. This
section explains a few of them briefly. section explains a few of them briefly.
One RTP stream per source, sent in the same RTP session with One RTP stream per source, sent in the same RTP session with
"text/red" format. "text/red" format.
From some points of view, use of multiple RTP streams, one for From some points of view, use of multiple RTP streams, one for
each source, sent in the same RTP session, called the RTP each source, sent in the same RTP session, called the RTP
translator model in [RFC3550], would be efficient, and use exactly translator model in [RFC3550], would be efficient, and use exactly
the same packet format as [RFC4103], the same payload type and a the same packet format as [RFC4103], the same payload type and a
simple SDP declaration. However, there is currently lack of simple SDP declaration. However, the RTP implementation in both
support for multi-stream RTP in certain implementation mixers and endpoints need to support multiple streams in the same
technologies. This fact made it not included in this document. RTP session in order to use this mechanism. For best deployment
opportunity, it should be possible to upgrade existing endpoint
solutions to be multi-party aware with a reasonable effort. There
is currently a lack of support for multi-stream RTP in certain
implementation technologies. This fact made this solution not
selected for inclusion in this document.
The "text/red" format in RFC 4103 with shorter transmission The "text/red" format in RFC 4103 with shorter transmission
interval, and indicating source in CSRC. interval, and indicating source in CSRC.
The "text/red" format with "text/t140" payload in a single RTP The "text/red" format with "text/t140" payload in a single RTP
stream can be sent with 100 ms packet intervals instead of the stream can be sent with 100 ms packet intervals instead of the
regular 300 ms. The source is indicated in the CSRC field. regular 300 ms. The source is indicated in the CSRC field.
Source switching can then be done every 300 ms while simultaneous Source switching can then be done every 100 ms while simultaneous
transmission occurs. With two participants sending text transmission occurs. With five participants sending text
simultaneously, the switching and transmission performance is simultaneously, the switching and transmission performance is
good. With three simultaneously sending participants, there will good. With more simultaneously sending participants, there will
be a noticable jerkiness in text presentation. The jerkiness will be a noticable jerkiness in text presentation. The jerkiness will
be more expressed the more participants who send text be more expressed the more participants who send text
simultaneously. With five sending participants, the jerkiness simultaneously. With ten sending participants, the jerkiness will
will be about 1400 ms. Text sent from a source at the end of the be about one second. Text sent from a source at the end of the
period its text is sent by the mixer will have close to zero extra period its text is sent by the mixer will have close to zero extra
delay. Recent text will be presented with no or low delay. The delay. Recent text will be presented with no or low delay. The
1400 ms jerkiness will be noticable and slightly unpleasant, but one second jerkiness will be noticable and slightly unpleasant,
corresponds in time to what typing humans often cause by but corresponds in time to what typing humans often cause by
hesitation or changing position while typing. A benefit of this hesitation or changing position while typing. A benefit of this
method is that no new packet format needs to be introduced and method is that no new packet format needs to be introduced and
implemented. Since simultaneous typing by more than two parties implemented. Since simultaneous typing by more than two parties
is rare, and in most applications also more than three parties in is very rare, and in most applications also more than three
a call is rare, this method can be used successfully without its parties in a call is rare, this method can be used successfully
limitations becoming annoying. Negotiation is based on a new sdp with good performance. Recovery of text in case of packet loss is
media attribute "rtt-mixer". This method is selected to be the based on analysis of timestamps of received redundancy versus
main one specified in this document. earlier received text. Negotiation is based on a new sdp media
attribute "rtt-mixer". This method is selected to be the main one
specified in this document.
A new "text" media subtype with up to 15 sources in each packet. A new "text" media subtype with up to 15 sources in each packet.
The mechanism makes use of the RTP mixer model specified in The mechanism makes use of the RTP mixer model specified in
RFC3550[RFC3550]. Text from up to 15 sources can be included in RFC3550[RFC3550]. Text from up to 15 sources can be included in
each packet. Packets are normally sent every 300 ms. The mean each packet. Packets are normally sent every 300 ms. The mean
delay will be 150 ms. The sources are indicated in strict order delay will be 150 ms. The sources are indicated in strict order
in the CSRC list of the RTP packets. A new redundancy packet in the CSRC list of the RTP packets. A new redundancy packet
format is specified. This method would result in good format is specified. This method would result in good
performance, but would require standardisation and implementation performance, but would require standardisation and implementation
of new releases in the target technologies that would take more of new releases in the target technologies that would take more
skipping to change at page 7, line 25 skipping to change at page 7, line 47
mixer, RTP-translator are explained in [RFC3550] mixer, RTP-translator are explained 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.
"TTY" stands for a text telephone type used in North America. "TTY" stands for a text telephone type used in North America.
"WebRTC" stands for web based communication specified by W3C and "WebRTC" stands for web based communication specified by W3C and
IETF. IETF.
"DTLS-SRTP" stnds for security specified in RFC 5764 [RFC5764]. "DTLS-SRTP" stands for security specified in RFC 5764 [RFC5764].
"multi-party aware" stands for an endpoint receiving real-time text
from multiple sources through a common conference mixer being able to
present the text in real-time separated by source and presented so
that a user can get an impression of the approximate relative timing
of text from different parties.
"multi-party unaware" stands for an endpoint not itself being able to
separate text from different sources when received through a common
conference mixer.
1.3. Intended application 1.3. Intended application
The method for multi-party real-time text specified in this document The method for multi-party real-time text specified in this document
is primarily intended for use in transmission between mixers and is primarily intended for use in transmission between mixers and
endpoints in centralised mixing configurations. It is also endpoints in centralised mixing configurations. It is also
applicable between mixers. An often mentioned application is for applicable between mixers. An often mentioned application is for
emergency service calls with real-time text and voice, where a emergency service calls with real-time text and voice, where a
calltaker want to make an attended handover of a call to another calltaker want to make an attended handover of a call to another
agent, and stay observing the session. Multimedia conference agent, and stay observing the session. Multimedia conference
skipping to change at page 8, line 12 skipping to change at page 8, line 48
Occasional small cut-and-paste operations may appear even if that is Occasional small cut-and-paste operations may appear even if that is
not the initial purpose of real-time text. not the initial purpose of real-time text.
The real-time characteristics of real-time text is essential for the The real-time characteristics of real-time text is essential for the
participants to be able to contribute to a conversation. If the text participants to be able to contribute to a conversation. If the text
is too much delayed from typing a letter to its presentation, then, is too much delayed from typing a letter to its presentation, then,
in some conference situations, the opportunity to comment will be in some conference situations, the opportunity to comment will be
gone and someone else will grab the turn. A delay of more than one gone and someone else will grab the turn. A delay of more than one
second in such situations is an obstacle for good conversation. second in such situations is an obstacle for good conversation.
2. Specified solutions 2. Overview over the two specified solutions
2.1. Negotiated use of the RFC 4103 format for multi-party in a single This section contains a brief introduction of the two methods
RTP stream specified in this document.
The main purpose of this document is to specify the use of the 2.1. Negotiated use of the RFC 4103 format for multi-party transmission
current format for real-time text in [RFC4103] for true multi-party in a single RTP stream
real-time text. It is an update of RFC 4103 by a clarification on
one way to use it in the multi-party situation. It is done by
completing a negotiation for this kind of multi-party capability and
by indicating source in the CSRC element in the RTP packets.
The detailed procedures for the multi-party capable case are The main purpose of this document is to specify a method for true
specified in Section 3 multi-party real-time text mixing for multi-party aware endpoints.
The method use of the current format for real-time text in [RFC4103].
It is an update of RFC 4103 by a clarification on one way to use it
in the multi-party situation. It is done by completing a negotiation
for this kind of multi-party capability and by indicating source in
the CSRC element in the RTP packets. Specific considerations are
made to be able to recover text after packet loss.
The detailed procedures for the multi-party aware case are specified
in Section 3
Please use [RFC4103] as reference when reading the specification. Please use [RFC4103] as reference when reading the specification.
2.2. Mixing for multi-party unaware endpoints 2.2. Mixing for multi-party unaware endpoints
A method is also specified in this document for cases when the A method is also specified in this document for cases when the
endpoint participating in a multi-party call does not implement any endpoint participating in a multi-party call does not itself
solution for multi-party presentation of real-time text. The implement any solution for multi-party handling of real-time text.
solution requires the mixer to insert text dividers and readable The solution requires the mixer to insert text dividers and readable
labels and only send text from one source at a time until a suitable labels and only send text from one source at a time until a suitable
point appears for source change. This solution is a fallback method point appears for source change. This solution is a fallback method
with functional limitations that acts on the presentation level. with functional limitations that acts on the presentation level.
A party performing as a mixer, which has not negotiated the "rtt- A party performing as a mixer, which has not negotiated the "rtt-
mixer" sdp media attribute, but negotiated a "text/red" or "text/ mixer" sdp media attribute, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD, if nothing else t140" format in a session with a participant SHOULD, if nothing else
is specified for the application, format transmitted text to that is specified for the application, format transmitted text to that
participant to be suitable to present on a multi-party unaware participant to be suitable to present on a multi-party unaware
endpoint as further specified in Section 4.2. endpoint as further specified in Section 4.2.
skipping to change at page 9, line 4 skipping to change at page 9, line 40
with functional limitations that acts on the presentation level. with functional limitations that acts on the presentation level.
A party performing as a mixer, which has not negotiated the "rtt- A party performing as a mixer, which has not negotiated the "rtt-
mixer" sdp media attribute, but negotiated a "text/red" or "text/ mixer" sdp media attribute, but negotiated a "text/red" or "text/
t140" format in a session with a participant SHOULD, if nothing else t140" format in a session with a participant SHOULD, if nothing else
is specified for the application, format transmitted text to that is specified for the application, format transmitted text to that
participant to be suitable to present on a multi-party unaware participant to be suitable to present on a multi-party unaware
endpoint as further specified in Section 4.2. endpoint as further specified in Section 4.2.
3. Details for the multi-party aware mixing case 3. Details for the multi-party aware mixing case
3.1. Offer/answer considerations 3.1. Offer/answer considerations
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. This document updates RFC 4103[RFC4103] by text transmission. This document updates RFC 4103[RFC4103] by
introducing a capability negotiation for handling multi-party real- introducing a capability negotiation for handling multi-party real-
time text and a way to indicate the source of thansmitted text. The time text, a way to indicate the source of transmitted text, and
capability negotiation is based on use of the sdp media attribute rules for efficient timing of the transmissions.
The capability negotiation is based on use of the sdp media attribute
"rtt-mixer". "rtt-mixer".
Both parties shall indicate their capability in a session setup or
modification, and evaluate the capability of the counterpart.
The syntax is as follows: The syntax is as follows:
"a=rtt-mixer" "a=rtt-mixer"
3.2. Actions depending on capability negotiation result
A transmitting party SHALL send text according to the multi-party A transmitting party SHALL send text according to the multi-party
format only when the negotiation for this method was successful and format only when the negotiation for this method was successful and
when the CC field in the RTP packet is set to 1. In all other cases, when the CC field in the RTP packet is set to 1. In all other cases,
the packets SHALL be populated and interpreted as for a two-party the packets SHALL be populated and interpreted as for a two-party
session. session.
A party which has negotiated the "rtt-mixer" sdp media attribute MUST A party which has negotiated the "rtt-mixer" sdp media attribute MUST
populate the CSRC-list and format the packets according to Section 3 populate the CSRC-list and format the packets according to Section 3
if it acts as an rtp-mixer and sends multi-party text. if it acts as an rtp-mixer and sends multi-party text.
A party which has negotiated the "rtt-mixer" sdp media attribute MUST A party which has negotiated the "rtt-mixer" sdp media attribute MUST
interpret the contents of the "CC" field the CSRC-list and the interpret the contents of the "CC" field the CSRC-list and the
packets according to Section 3 in received rtp packets in the packets according to Section 3 in received rtp packets in the
corresponding RTP stream. corresponding RTP stream.
A party not performing as a mixer MUST not include the CSRC list. A party not performing as a mixer MUST not include the CSRC list.
3.2. Use of fields in the RTP packets 3.3. Use of fields in the RTP packets
The CC field SHALL show the number of members in the CSRC list, which The CC field SHALL show the number of members in the CSRC list, which
SHALL be one (1) in transmissions from a mixer involved in a multi- SHALL be one (1) in transmissions from a mixer involved in a multi-
party session, and otherwise 0. party session, and otherwise 0.
When transmitted from a mixer during a multi-party session, a CSRC When transmitted from a mixer during a multi-party session, a CSRC
list SHALL be included in the packet. The single member in the CSRC- list SHALL be included in the packet. The single member in the CSRC-
list SHALL contain the SSRC of the source of the T140blocks in the list SHALL contain the SSRC of the source of the T140blocks in the
packet. When redundancy is used, the recommended level of redundancy packet. When redundancy is used, the recommended level of redundancy
is to use one primary and two redundant generations of T140blocks. is to use one primary and two redundant generations of T140blocks.
In some cases, a primary or redundant T140block is empty, but is In some cases, a primary or redundant T140block is empty, but is
still represented by a member in the redundancy header. still represented by a member in the redundancy header.
From other aspects, the contents of the RTP packets are equal to what From other aspects, the contents of the RTP packets are equal to what
is specified in [RFC4103]. is specified in [RFC4103].
3.3. Initial transmission of a BOM character 3.4. Initial transmission of a BOM character
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 with
being available for text reception, a Unicode BOM character SHALL be another entity and being available for text reception, a Unicode BOM
sent to it according to the procedures in this section. If the character SHALL be sent to it by the other entity according to the
transmitter is a mixer, then the source of this character SHALL be procedures in this section. If the transmitter is a mixer, then the
indicated to be the mixer itself. source of this character SHALL be indicated to be the mixer itself.
Note that the BOM character SHALL be transmitted with the same Note that the BOM character SHALL be transmitted with the same
redundancy procedures as any other text. redundancy procedures as any other text.
3.4. Keep-alive 3.5. 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 receiver(s) at regular intervals when no other traffic has occurred
during that interval if that is decided for the actual connection. during that interval, if that is decided for the actual connection.
Recommendations for keep-alive can be found in [RFC6263]. Recommendations for keep-alive can be found in [RFC6263].
3.5. Transmission interval 3.6. Transmission interval
A "text/red" transmitter in a mixer SHOULD send packets distributed A "text/red" transmitter in a mixer SHOULD send packets distributed
in time as long as there is something (new or redundant T140blocks) in time as long as there is something (new or redundant T140blocks)
to transmit. The maximum transmission interval SHOULD then be 300 to transmit. The maximum transmission interval SHOULD then be 330
ms. It is RECOMMENDED to send next packet to a receiver as soon as ms. It is RECOMMENDED to send next packet to a receiver as soon as
new text to that receiver is available, as long as the time after the new text to that receiver is available, as long as the time after the
latest sent packet to the same receiver is more than or equal to 100 latest sent packet to the same receiver is more than or equal to 100
ms, and also the maximum character rate to the receiver is not ms, and also the maximum character rate to the receiver is not
exceeded. The intention is to keep the latency low while keeping a exceeded. The intention is to keep the latency low and network load
good protection against text loss in bursty packet loss conditions. limited while keeping a good protection against text loss in bursty
packet loss conditions.
3.6. Only one source per packet 3.7. Only one source per packet
New and redundant text from one source MAY be transmitted in the same New and redundant text from one source SHALL be transmitted in the
packet. Text from different sources MUST NOT be transmitted in the same packet if available for transmission at the same time. Text
same packet. from different sources MUST NOT be transmitted in the same packet.
3.7. Do not send received text to the originating source 3.8. Do not send received text to the originating source
Text received to a mixer from a participant SHOULD NOT be included in Text received to a mixer from a participant SHOULD NOT be included in
transmission from the mixer to that participant. transmission from the mixer to that participant.
3.8. Clean incoming text 3.9. Clean incoming text
A mixer SHALL handle reception, recovery of packet loss, marking of A mixer SHALL handle reception, recovery of packet loss, deletion of
possible text loss and deletion of 'BOM' characters from each superfluous redundancy, marking of possible text loss and deletion of
participant before queueing received text for transmission to 'BOM' characters from each participant before queueing received text
receiving participants. for transmission to receiving participants.
3.9. Redundancy 3.10. Redundancy
A transmitting party using redundancy SHALL send redundant A transmitting party using redundancy SHALL send redundant
repetitions of T140blocks aleady transmitted in earlier packets. repetitions of T140blocks already transmitted in earlier packets.
The number of redundant generations of T140blocks to include in The number of redundant generations of T140blocks to include in
transmitted packets SHALL be deduced from the SDP negotiation. It transmitted packets SHALL be deduced from the SDP negotiation. It
SHOULD be set to the minimum of the number declared by the two SHOULD be set to the minimum of the number declared by the two
parties negotiating a connection. parties negotiating a connection. It is RECOMMENDED to transmit one
original and two redundant generations of the T140blocks.
3.10. Text placement in packets 3.11. Source switching
When text from more than one source is available for transmission
from a mixer, the mixer SHALL let the sources take turns in having
their text transmitted.
The source with the oldest received text in the mixer SHOULD be next
in turn to get all its available unsent text transmitted.
3.12. Text placement in packets
The mixer SHOULD compose and transmit an RTP packet to a receiver
when one of the following conditions has occurred:
* 100 ms has passed since the latest transmission to that receiver,
and there is unsent text available for transmission.
* New text has arrived and more than 100 ms has passed since latest
transmission to that receiver.
* 330 ms has passed since already transmitted text was queued for
transmission as redundant text, and more than 100 ms has passed
since the latest transmission to that receiver, and the redundant
text is still not sent.
At time of transmission, the mixer SHALL populate the RTP packet with At time of transmission, the mixer SHALL populate the RTP packet with
all T140blocks queued for transmission originating from the source in all T140blocks queued for transmission originating from the source in
turn for transmission as long as this is not in conflict with the turn for transmission as long as this is not in conflict with the
allowed number of characters per second ("CPS") or the maximum packet allowed number of characters per second ("CPS") or the maximum packet
size. The SSRC of the source shall be placed as the only member in size. In this way, the latency of the latest received text is kept
the CSRC-list. low even in moments of simultaneous transmission from many sources.
The SSRC of the source shall be placed as the only member in the
CSRC-list.
Note: The CSRC-list in an RTP packet only includes the participant Note: The CSRC-list in an RTP packet only includes the participant
who's text is included in text blocks. It is not the same as the 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 total list of participants in a conference. With audio and video
media, the CSRC-list would often contain all participants who are not media, the CSRC-list would often contain all participants who are not
muted whereas text participants that don't type are completely silent muted whereas text participants that don't type are completely silent
and thus are not represented in RTP packet CSRC-lists. and thus are not represented in RTP packet CSRC-lists.
3.11. Source switching 3.13. Empty T140blocks
When text from more than one source is available for transmission,
the mixer SHALL let the sources take turns in having their text
transmitted. When switching from transmission of one source to allow
another source to get turn to have its text sent, all intended
redundant generations of the last text from the current source MUST
be transmitted before text from another source is transmitted.
Actively transmitting sources SHOULD be allowed to take turns as
frequently as possible to have their text transmitted. That implies
that with the recommended redundancy, the mixer SHALL send primary
text and two packets with redundant text from the current source
before text from another source is transmitted. The source with the
oldest text received in the mixer SHOULD be next in turn to get all
its available text transmitted.
3.12. Empty T140blocks
If no unsent T140blocks were available for a source at the time of If no unsent T140blocks were available for a source at the time of
populating a packet, but T140blocks are available which have not yet populating a packet, but T140blocks are available which have not yet
been sent the full intended number of redundant transmissions, then been sent the full intended number of redundant transmissions, then
the primary T140block for that source is composed of an empty the primary T140block for that source is composed of an empty
T140block, and populated (without taking up any length) in a packet T140block, and populated (without taking up any length) in a packet
for transmission. The corresponding SSRC SHALL be placed as usual in for transmission. The corresponding SSRC SHALL be placed as usual in
its place in the CSRC-list. its place in the CSRC-list.
The first packet in the session, the first after a source switch and The first packet in the session, the first after a source switch and
the first after a pause SHALL be poulated with the available the first after a pause SHALL be poulated with the available
T140blocks for the source in turn to be sent as primary, and empty T140blocks for the source in turn to be sent as primary, and empty
T140blocks for the agreed number of redundancy generations. T140blocks for the agreed number of redundancy generations.
3.13. Creation of the redundancy 3.14. Creation of the redundancy
The primary T140block from a source in the latest transmitted packet The primary T140block from a source in the latest transmitted packet
is used to populate the first redundant T140block for that source. is saved for populating the first redundant T140block for that source
The first redundant T140block for that source from the latest in next transmission of text from that source. The first redundant
transmission is placed as the second redundant T140block. T140block for that source from the latest transmission is saved for
populating the second redundant T140block in next transmission of
text from that source.
Usually this is the level of redundancy used. If a higher number of Usually this is the level of redundancy used. If a higher 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 are placed in the until all available redundant levels of T140blocks are placed in the
packet. If a receiver has negotiated a lower number of "text/red" packet. If a receiver has negotiated a lower number of "text/red"
generations, then that level shall be the maximum used by the generations, then that level shall be the maximum used by the
transmitter. transmitter.
3.14. Timer offset fields The T140blocks saved for transmission as redundant data are assigned
a planned transmission time 330 ms after the current time.
3.15. Timer offset fields
The timestamp offset values are inserted in the redundancy header, The timestamp offset values are inserted in the redundancy header,
with the time offset from the RTP timestamp in the packet when the 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 as primary.
The timestamp offsets are expressed in the same clock tick units as The timestamp offsets are expressed in the same clock tick units as
the RTP timestamp. the RTP timestamp.
The timestamp offset values for empty T140blocks have no relevance The timestamp offset values for empty T140blocks have no relevance
but SHOULD be assigned realistic values. but SHOULD be assigned realistic values.
3.15. Other RTP header fields 3.16. Other RTP header fields
The number of members in the CSRC list ( 0 or 1) shall be placed in The number of members in the CSRC list ( 0 or 1) shall be placed in
the "CC" header field. Only mixers place value 1 in the "CC" field. the "CC" header field. Only mixers place value 1 in the "CC" field.
A value of "0" indicates that the source is the transmitting device
itself and that the source is indicated by the SSRC field. This
value is used by endpoints, and by mixers sending data that it is
source of itself.
The current time SHALL be inserted in the timestamp. The current time SHALL be inserted in the timestamp.
The SSRC of the mixer for the RTT session SHALL be inserted in the The SSRC of the mixer for the RTT session SHALL be inserted in the
SSRC field of the RTP header. SSRC field of the RTP header.
The M-bit shall be handled as specified in [RFC4103]. The M-bit shall be handled as specified in [RFC4103].
3.16. Pause in transmission 3.17. Pause in transmission
When there is no new T140block to transmit, and no redundant When there is no new T140block to transmit, and no redundant
T140block that has not been retransmitted the intended number of T140block that has not been retransmitted the intended number of
times from any source, the transmission process can stop until either times from any source, the transmission process can stop until either
new T140blocks arrive, or a keep-alive method calls for transmission new T140blocks arrive, or a keep-alive method calls for transmission
of keep-alive packets. of keep-alive packets.
3.17. RTCP considerations 3.18. 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.
Integrity considerations SHALL be considered when composing these Integrity considerations SHALL be considered when composing these
fields. fields.
3.18. Reception of multi-party contents 3.19. Reception of multi-party contents
The "text/red" receiver included in an endpoint with presentation The "text/red" receiver included in an endpoint with presentation
functions will receive RTP packets in the single stream from the functions will receive RTP packets in the single stream from the
mixer, and SHALL distribute the T140blocks for presentation in mixer, and SHALL distribute the T140blocks for presentation in
presentation areas for each source. Other receiver roles, such as presentation areas for each source. Other receiver roles, such as
gateways or chained mixers are also feasible, and requires gateways or chained mixers are also feasible, and requires
consideration if the stream shall just be forwarded, or distributed consideration if the stream shall just be forwarded, or distributed
based on the different sources. based on the different sources.
3.18.1. Multi-party vs two-party use 3.19.1. Multi-party vs two-party use
If the "CC" field value of a received packet is 1, it indicates that If the "CC" field value of a received packet is 1, it indicates that
multi-party transmission is active, and the receiver MUST be prepared multi-party transmission is active, and the receiver MUST be prepared
to act on the source according to its role. If the CC value is 0, to act on the source according to its role. If the CC value is 0,
the connection is point-to-point. the connection is point-to-point.
3.18.2. Level of redundancy 3.19.2. 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. The number of generations (including the received packet contents. The number of generations (including the
primary) is equal to the number of members in the redundancy header. primary) is equal to the number of members in the redundancy header.
3.18.3. Extracting text and handling recovery and loss 3.19.3. Empty T140blocks
Empty T140blocks are included as fillers for unused primary or
redundancy levels in the packets. They just do not provide any
contents and do not contribute to the received streams.
3.19.4. Detection and indication of possible text 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. If a sequence number gap appears
and still exists after some defined short time for jitter resolution,
the packets in the gap SHALL be regarded lost.
As long as the sequence is correct, each packet SHALL be unpacked in If it is known that only one source is active in the RTP session,
order. The T140blocks SHALL be extracted from the primary area, and then it is likely that a gap equal to or larger than the agreed
the corresponding SSRC SHALL be extracted from the CSRC list and used number of redundancy generations causes text loss. In that case a
for assigning the new T140block to the correct presentation area (or t140block SHALL be created with a marker for possible text loss
correspondingly for other receiver roles). [T140ad1] and assigned to the source and inserted in the reception
buffer for that source.
If a sequence number gap appears and is still there after some If it is known that more than one source is active in the RTP
defined time for jitter resolution, T140data SHALL be recovered from session, then it is not possible in general to evaluate if text was
redundant data. If the gap is wider than the number of generations lost when packets were lost. With two active sources and the
of redundant T140blocks in the packet and it is concluded that recommended number of redundancy generations (3), it can take a gap
t140blocks MAY have been lost, then a t140block SHALL be created with of five lost packets until any text may be lost, but text loss can
a marker for possible text loss [T140ad1] and assigned to the SSRC of also appear if three non-consecutive packets are lost when they
the transmitter as a general input from the mixer because in general contained consecutive data from the same source. A simple method to
it is not possible to deduce from which source(s) text was lost. It decide when there is risk for resulting text loss is to evaluate if
is in some cases possible to deduce that no text was lost even for a three or more packets were lost within one second. Then a t140block
gap wider than the redundancy generations, and in some cases it can SHALL be created with a marker for possible text loss [T140ad1] and
be concluded which source that likely had loss. Therefore, the assigned to the SSRC of the transmitter as a general input from the
receiver MAY insert the marker for possible text loss [T140ad1] in mixer.
the presentation area corresponding to the source which may have had
loss. See Section 3.22
Then, the T140block in the received packet SHALL be retrieved Implementations MAY apply more refined methods for more reliable
beginning with the highest redundant generation, and assigning it to detection of if text was lost or not. Any refined method SHOULD
the presentation area of that source. Finally the primary T140block rather falsely mark possible loss when there was no loss instead of
SHALL be retrieved from the packet and similarly assigned to the not marking possible loss when there was loss.
corresponding presentation area for the source.
If the sequence number gap was equal to or less than the number of 3.19.5. Extracting text and handling recovery
redundancy generations in the received packet, a missing text marker
SHALL NOT be inserted, and instead the T140block and the SSRC fully
recovered from the redundancy and primary information and the CSRC-
list in the way indicated above.
3.18.4. Delete 'BOM' When applying the following procedures, the effects MUST be
considered of possible timestamp wrap around and the RTP session
possibly changing SSRC.
Unicode character 'BOM' is used as a start indication and sometimes When a packet is received in an RTP session using the packetization
used as a filler or keep alive by transmission implementations. for multi-party aware endpoints, its T140blocks SHALL be extracted in
These SHALL be deleted on reception. the following way. The description is adapted to the default
redundancy case using the original and two redundant generations.
3.18.5. Empty T140blocks When applying the following procedures, the effects MUST be
considered of possible timestamp wrap around and the RTP session
possibly changing SSRC.
Empty T140blocks are included as fillers for unused redundancy levels The source SHALL be extracted from the CSRC-list if available,
in the packets. They just do not provide any contents and do not otherwise from the SSRC.
contribute to the received streams.
3.19. Performance considerations If the received packet is the first packet received from the source,
then all T140blocks in the packet SHALL be retrieved and assigned to
a receive buffer for the source beginning with the second generation
redundancy, continuing with the first generation redundancy and
finally the primary.
This solution has good performance for up to three participants Note that the normal case is that in the first packet, only the
primary data has contents. The redundant data has contents in the
first received packet from a source only after initial packet loss.
If the packet is not the first packet from a source, then if the
second generation redundant data is available, its timestamp SHALL be
created by subtracting its timestamp offset from the RTP timestamp.
If the resulting timestamp is later than the latest retrieved data
from the same source, then the redundant data SHALL be retrieved and
appended to the receive buffer. The process SHALL be continued in
the same way for the first generation redundant data. After that,
the primary data SHALL be retrieved from the packet and appended to
the receive buffer for the source.
3.19.6. Delete 'BOM'
Unicode character 'BOM' is used as a start indication and sometimes
used as a filler or keep alive by transmission implementations.
These SHALL be deleted after extraction from received packets.
3.20. Performance considerations
This solution has good performance for up to five participants
simultaneously sending text. At higher numbers of participants simultaneously sending text. At higher numbers of participants
simultaneously sending text, a jerkiness is visible in the simultaneously sending text, a jerkiness is visible in the
presentation of text. With five participants simultaneously presentation of text. With ten participants simultaneously
transmitting text, the jerkiness is about 1400 ms. Evenso, the transmitting text, the jerkiness is about one second. Even so, the
transmission of text catches up, so there is no resulting total delay transmission of text catches up, so there is no resulting total delay
introduced. The solution is therefore suitable for emergency service introduced. The solution is therefore suitable for emergency service
use, relay service use, and small or well-managed larger multimedia use, relay service use, and small or well-managed larger multimedia
conferences. Only in large unmanaged conferences with a high number conferences. Only in large unmanaged conferences with a high number
of participants there may on very rare occasions appear situations of participants there may on very rare occasions appear situations
when many participants happen to send text simultaneously, resulting when many participants happen to send text simultaneously, resulting
in unpleasantly long switching times. It should be noted that it is in unpleasantly long switching times. It should be noted that it is
only the number of users sending text within the same moment that only the number of users sending text within the same moment that
causes jerkiness, not the total number of users with RTT capability. causes jerkiness, not the total number of users with RTT capability.
3.20. Security for session control and media 3.21. 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 SHOULD be performed to decide if security by encryption
no other security solution is mandated by the application, then RFC will be applied. If no other security solution is mandated for the
8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media application, then RFC 8643 OSRTP[RFC8643] SHOULD be applied to
security with DTLS. Most SDP examples below are for simplicity negotiate SRTP media security with DTLS. Most SDP examples below are
expressed without the security additions. The principles (but not for simplicity expressed without the security additions. The
all details) for applying DTLS-SRTP security is shown in a couple of principles (but not all details) for applying DTLS-SRTP security is
the following examples. shown in a couple of the following examples.
3.21. SDP offer/answer examples 3.22. 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. Audio is usually provided the real-time text media in SIP sessions. Audio is usually provided
in the same session, and sometimes also video. The examples only in the same session, and sometimes also video. The examples only
show the part of importance for the real-time text media. show the part of importance for the real-time text media.
Offer example for "text/red" format and multi-party support: Offer example for "text/red" format and multi-party support:
m=text 11000 RTP/AVP 100 98 m=text 11000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
skipping to change at page 16, line 22 skipping to change at page 18, line 29
Answer example from a multi-party capable device Answer example from a multi-party capable device
m=text 14000 RTP/AVP 100 98 m=text 14000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
a=rtt-mixer a=rtt-mixer
Offer example for "text/red" format including multi-party Offer example for "text/red" format including multi-party
and security: and security:
a=fingerprint: SHA-1 \ a=fingerprint: (fingerprint1)
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 11000 RTP/AVP 100 98 m=text 11000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
a=rtt-mixer a=rtt-mixer
The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media The "fingerprint" is sufficient to offer DTLS-SRTP, with the media
line still indicating RTP/AVP. line still indicating RTP/AVP.
Note: For brevity, the entire value of the SDP fingerprint attribute
is not shown in this and the following example.
Answer example from a multi-party capable device with security Answer example from a multi-party capable device with security
a=fingerprint: SHA-1 \ a=fingerprint: (fingerprint2)
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=text 16000 RTP/AVP 100 98 m=text 16000 RTP/AVP 100 98
a=rtpmap:98 t140/1000 a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000 a=rtpmap:100 red/1000
a=fmtp:100 98/98/98 a=fmtp:100 98/98/98
a=rtt-mixer a=rtt-mixer
With the "fingerprint" the device acknowledges use of SRTP/DTLS. With the "fingerprint" the device acknowledges use of SRTP/DTLS.
Answer example from a multi-party unaware device that also Answer example from a 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
3.22. Packet sequence example from a source switch 3.23. Packet sequence example from a source switch
This example shows a symbolic flow of packets from a mixer including This example shows a symbolic flow of packets from a mixer including
loss and recovery. The sequence includes a source switch. A and B loss and recovery. The sequence includes a source switch. A and B
are sources of RTT. P indicates primary data. R1 is first redundant are sources of RTT. P indicates primary data. R1 is first redundant
generation data and R2 is second redundant generation data. A1, B1, generation data and R2 is second redundant generation data. A1, B1,
A2 etc are text chunks (T140blocks) received from the respective A2 etc are text chunks (T140blocks) received from the respective
sources. X indicates dropped packet between the mixer and a sources. X indicates dropped packet between the mixer and a
receiver. The session is assumed to use original and two redundant receiver. The session is assumed to use original and two redundant
generations of RTT. generations of RTT.
skipping to change at page 17, line 28 skipping to change at page 19, line 36
|CC=1 | |CC=1 |
|CSRC list A | |CSRC list A |
|R2: A1 | |R2: A1 |
|R1: A2 | |R1: A2 |
|P: A3 | |P: A3 |
|----------------| |----------------|
Assuming that earlier packets ( with text A1 and A2) were received in Assuming that earlier packets ( with text A1 and A2) were received in
sequence, text A3 is received from packet 101 and assigned to sequence, text A3 is received from packet 101 and assigned to
reception area A. The mixer is now assumed to have received text reception area A. The mixer is now assumed to have received text
from source B and need to prepare for sending that text. First it from source B and will send that text 100 ms after packet 101.
must send the redundant generations of text A2 and A3. Transmission of A2 and A3 as redundancy is planned for 200 ms after
packet 101.
|----------------| |----------------|
|Seq no 102 | |Seq no 102 |
|CC=1 | |CC=1 |
|CSRC list A | |CSRC list B |
|R2 A2 | |R2 Empty |
|R1: A3 | |R1: Empty |
|P: Empty | |P: B1 |
|----------------| |----------------|
Nothing needs to be retrieved from this packet. But the receiver B1 is retrieved from this packet. Redundant transmission of
needs to remember temporarily that it was received. B1 is planned 200 ms after packet 102.
X----------------| X----------------|
X Seq no 103 | X Seq no 103 |
X CC=1 | X CC=1 |
X CSRC list A | X CSRC list A |
X R2: A3 | X R2: A2 |
X R1: Empty | X R1: A3 |
X P: Empty | X P: Empty |
X----------------| X----------------|
Packet 103 is assumed to be dropped in network problems. It was the Packet 103 is assumed to be dropped in network problems. It
last packet with contents from A before the source switch. contains redundancy for A. Sending A3 as second level
redundancy is planned for 100 ms after packet 104.
X----------------| X----------------|
X Seq no 104 | X Seq no 104 |
X CC=1 | X CC=1 |
X CSRC list B | X CSRC list B |
X R2: Empty | X R2: Empty |
X R1: Empty | X R1: B1 |
X P2: B1 | X P2: B2 |
X----------------| X----------------|
Packet 104 contains text from B, assumed dropped in network Packet 104 contains text from B, assumed dropped in network
problems. The mixer is assumed to have received text from A problems. The mixer has A3 redundancy to send and plans it
on turn to send. Sending of text from B must therefore be after 100 ms.
temporarily ended by sending redundancy twice.
X----------------| X----------------|
X Seq no 105 | X Seq no 105 |
X CC=1 | X CC=1 |
X CSRC list B | X CSRC list A |
X R2: Empty | X R2: A3 |
X R1: B1 | X R1: Empty |
X P: Empty | X P: Empty |
X----------------| X----------------|
Packet 105 is assumed to be dropped in network problems Packet 105 is assumed to be dropped in network problems
B1 and B2 still needs to be transmitted as redundancy.
This is planned 320 ms after packet 105.
|----------------| |----------------|
|Seq no 106 | |Seq no 106 |
|CC=1 | |CC=1 |
|CSRC list B | |CSRC list B |
| R2: B1 | | R2: B1 |
| R1: Empty | | R1: B2 |
| P: Empty | | P: Empty |
|----------------| |----------------|
Packet 106 is received. The latest received sequence number was 102. Packet 106 is received 320 ms after packet 105. The latest received
Recovery is therefore tried for 103,104,105. There is no coverage sequence number was 102. 103,104,105 were lost. Three packets were
for seq no 103. But knowing that A3 was received as R1 in packet thus lost during less than one second. The simple rule for detection
102, it can be concluded that packet 103 just contained A3 as R2 and of text loss then results in that an indicator for possible loss
A3 was already received and nothing was lost in packet 103. (U'FFFD) [T140ad1], should be inserted generally for the mixer.
For seqno 104, text B1 is recovered from the second generation
redundancy in packet 106 and appended to the reception area of B.
For seqno 105, nothing needs to be recovered. No primary text is
available in packet 106.
After this sequence, A3 and B1 have been received. In this case no
text was lost. Even if also packet 102 was lost, it can be concluded
that no text was lost.
If also packets 101 and 102 were lost, there would be a need to The second level redundancy in packet 106 is B1 and has timestamp
create a marker for possibly lost text (U'FFFD) [T140ad1], inserted offset 620 ms. The timestamp of packet 106 minus 620 is the
generally and possibly also in text sequences A and B. timestamp of packet 101 which was received. So B1 does not need to
be retrieved. The first level redundancy in packet 106 is 420. The
timestamp of packet 106 minus 420 is later than the latest received
packet with source A. Therefore B2 is retrieved and assigned to the
input buffer for source B. No primary is available in packet 106
The general conclusion is that if CSRC is equal before and after a After this sequence, A3 and B1 and B2 have been received. In this
gap, only a gap of two can be recovered. If CSRC is different before case no text was lost.
and after the gap, a gap of 4 can be recovered if the last packet
before the gap contained only R2 data, a gap of 3 can be recovered if
the last packet before the gap contained R1 data and a gap of 2 can
be recovered if the last packet received before the gap contained
primary data. For larger gaps than the above, a mark for possible
loss needs to be inserted in the received stream from the mixer.
3.23. Maximum character rate "CPS" 3.24. Maximum character rate "CPS"
The default maximum rate of reception of "text/t140" real-time text The default maximum rate of reception of "text/t140" real-time text
is in RFC 4103 [RFC4103] specified to be 30 characters per second. is in RFC 4103 [RFC4103] specified to be 30 characters per second.
The value MAY be modified in the CPS parameter of the FMTP attribute The value MAY be modified in the CPS parameter of the FMTP attribute
in the media section for the "text/t140" media. A mixer combining in the media section for the "text/t140" media. A mixer combining
real-time text from a number of sources may occasionally have a real-time text from a number of sources may occasionally have a
higher combined flow of text coming from the sources. Endpoints higher combined flow of text coming from the sources. Endpoints
SHOULD therefore specify a suitable higher value for the CPS SHOULD therefore specify a suitable higher value for the CPS
parameter, corresponding to its real reception capability. A value parameter, corresponding to its real reception capability. A value
for "CPS" of 90 is the default for the "text/t140" stream in the for "CPS" of 90 is the default for the "text/t140" stream in the
skipping to change at page 31, line 19 skipping to change at page 33, line 19
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, and in conference reports in SDES, CNAME and NAME fields, and in conference
notifications, for creation of labels may have privacy concerns as notifications, for creation of labels may have privacy concerns as
already stated in RFC 3550 [RFC3550], and may be restricted of already stated in RFC 3550 [RFC3550], and may be restricted of
privacy reasons. The receiving user will then get a more symbolic privacy reasons. The receiving user will then get a more symbolic
label for the source. label for the source.
Participants with malicious intentions may appear and e.g. disturb
the multi-party session by a continuous flow of text, or masquerading
as text from other participants. Counteractions should be to require
secure signaling, media and authentication, and to provide higher
level conference functions e.g. for blocking and expelling
participants.
12. Change history 12. Change history
12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09 12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-10
The packet composition was modified for interleaving packets from
different sources.
The packet reception was modified for the new interleaving method.
The packet sequence examples was adjusted for the new interleaving
method.
Modifications according to responses to Brian Rosen of 2020-11-03
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09
Changed name on the SDP media attribute to "rtt-mixer" Changed name on the SDP media attribute to "rtt-mixer"
Restructure of section 2 for balance between aware and unaware cases. Restructure of section 2 for balance between aware and unaware cases.
Moved conference control to own section. Moved conference control to own section.
Improved clarification of recovery and loss in the packet sequence Improved clarification of recovery and loss in the packet sequence
example. example.
A number of editorial corrections and improvements. A number of editorial corrections and improvements.
12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08 12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08
Deleted the method requiring a new packet format "text/rex" because Deleted the method requiring a new packet format "text/rex" because
of the longer standardization and implementation period it needs. of the longer standardization and implementation period it needs.
Focus on use of RFC 4103 text/red format with shorter transmission Focus on use of RFC 4103 text/red format with shorter transmission
interval, and source indicated in CSRC. interval, and source indicated in CSRC.
12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07 12.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07
Added a method based on the "text/red" format and single source per Added a method based on the "text/red" format and single source per
packet, negotiated by the "rtt-mixer" sdp attribute. packet, negotiated by the "rtt-mixer" sdp attribute.
Added reasoning and recommendation about indication of loss. Added reasoning and recommendation about indication of loss.
The highest number of sources in one packet is 15, not 16. Changed. The highest number of sources in one packet is 15, not 16. Changed.
Added in information on update to RFC 4103 that RFC 4103 explicitly Added in information on update to RFC 4103 that RFC 4103 explicitly
allows addition of FEC method. The redundancy is a kind of forward allows addition of FEC method. The redundancy is a kind of forward
error correction.. error correction..
12.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 12.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06
Improved definitions list format. Improved definitions list format.
The format of the media subtype parameters is made to match the The format of the media subtype parameters is made to match the
requirements. requirements.
The mapping of media subtype parameters to sdp is included. The mapping of media subtype parameters to sdp is included.
The CPS parameter belongs to the t140 subtype and does not need to be The CPS parameter belongs to the t140 subtype and does not need to be
registered here. registered here.
12.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05
nomenclature and editorial improvements nomenclature and editorial improvements
"this document" used consistently to refer to this document. "this document" used consistently to refer to this document.
12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 12.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04
'Redundancy header' renamed to 'data header'. 'Redundancy header' renamed to 'data header'.
More clarifications added. More clarifications added.
Language and figure number corrections. Language and figure number corrections.
12.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 12.8. 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 33, line 32 skipping to change at page 36, line 4
text to have time offset as for transmission from the source. Make text to have time offset as for transmission from the source. Make
that header equal to the others. -done- that header equal to the others. -done-
Mixer always use the CSRC list, even for its own BOM. -done- Mixer always use the CSRC list, even for its own BOM. -done-
Combine all talk about transmission interval (300 ms vs when text has Combine all talk about transmission interval (300 ms vs when text has
arrived) in section 3 in one paragraph or close to each other. -done- arrived) in section 3 in one paragraph or close to each other. -done-
Documents the goal of good performance with low delay for 5 Documents the goal of good performance with low delay for 5
simultaneous typers in the introduction. -done- simultaneous typers in the introduction. -done-
Describe better that only primary text shall be sent on to receivers. Describe better that only primary text shall be sent on to receivers.
Redundancy and loss must be resolved by the mixer. -done- Redundancy and loss must be resolved by the mixer. -done-
12.8. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 12.9. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02
SDP and better description and visibility of security by OSRTP RFC SDP and better description and visibility of security by OSRTP RFC
8634 needed. 8634 needed.
The description of gatewaying to WebRTC extended. The description of gatewaying to WebRTC extended.
The description of the data header in the packet is improved. The description of the data header in the packet is improved.
12.9. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 12.10. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01
2,5,6 More efficient format "text/rex" introduced and attribute 2,5,6 More efficient format "text/rex" introduced and attribute
a=rtt-mix deleted. a=rtt-mix deleted.
3. Brief about use of OSRTP for security included- More needed. 3. Brief about use of OSRTP for security included- More needed.
4. Brief motivation for the solution and why not rtp-translator is 4. Brief motivation for the solution and why not rtp-translator is
used added to intro. used added to intro.
7. More limitations for the multi-party unaware mixing method 7. More limitations for the 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.
12.10. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 12.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03
to draft-ietf-avtcore-multi-party-rtt-mix-00 to draft-ietf-avtcore-multi-party-rtt-mix-00
Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00 Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00
Replaced CDATA in IANA registration table with better coding. Replaced CDATA in IANA registration table with better coding.
Converted to xml2rfc version 3. Converted to xml2rfc version 3.
12.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 12.12. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02
to -03 to -03
Changed company and e-mail of the author. Changed company and e-mail of the author.
Changed title to "RTP-mixer formatting of multi-party Real-time text" Changed title to "RTP-mixer formatting of multi-party Real-time text"
to better match contents. to better match contents.
Check and modification where needed of use of RFC 2119 words SHALL Check and modification where needed of use of RFC 2119 words SHALL
etc. etc.
More about the CC value in sections on transmitters and receivers so More about the CC value in sections on transmitters and receivers so
that 1-to-1 sessions do not use the mixer format. that 1-to-1 sessions do not use the mixer format.
Enhanced section on presentation for multi-party-unaware endpoints Enhanced section on presentation for multi-party-unaware endpoints
A paragraph recommending CPS=150 inserted in the performance section. A paragraph recommending CPS=150 inserted in the performance section.
12.12. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 12.13. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01
to -02 to -02
In Abstract and 1. Introduction: Introduced wording about regulatory In Abstract and 1. Introduction: Introduced wording about regulatory
requirements. requirements.
In section 5: The transmission interval is decreased to 100 ms when In section 5: The transmission interval is decreased to 100 ms when
there is text from more than one source to transmit. there is text from more than one source to transmit.
In section 11 about SDP negotiation, a SHOULD-requirement is In section 11 about SDP negotiation, a SHOULD-requirement is
introduced that the mixer should make a mix for multi-party unaware introduced that the mixer should make a mix for multi-party unaware
skipping to change at page 35, line 36 skipping to change at page 38, line 5
In chapter 9. "Use with SIP centralized conferencing framework" the In chapter 9. "Use with SIP centralized conferencing framework" the
following note is inserted: Note: The CSRC-list in an RTP packet only following note is inserted: Note: The CSRC-list in an RTP packet only
includes participants who's text is included in one or more text includes participants who's text is included in one or more text
blocks. It is not the same as the list of participants in a blocks. It is not the same as the list of participants in a
conference. With audio and video media, the CSRC-list would often conference. With audio and video media, the CSRC-list would often
contain all participants who are not muted whereas text participants contain all participants who are not muted whereas text participants
that don't type are completely silent and so don't show up in RTP that don't type are completely silent and so don't show up in RTP
packet CSRC-lists. packet CSRC-lists.
12.13. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 12.14. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00
to -01 to -01
Editorial cleanup. Editorial cleanup.
Changed capability indication from fmtp-parameter to SDP attribute Changed capability indication from fmtp-parameter to SDP attribute
"rtt-mix". "rtt-mix".
Swapped order of redundancy elements in the example to match reality. Swapped order of redundancy elements in the example to match reality.
Increased the SDP negotiation section Increased the SDP negotiation section
skipping to change at page 37, line 40 skipping to change at page 40, line 9
[RFC5194] van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real- [RFC5194] van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real-
Time Text over IP Using the Session Initiation Protocol Time Text over IP Using the Session Initiation Protocol
(SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008, (SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008,
<https://www.rfc-editor.org/info/rfc5194>. <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
SE-13670 Vendelso SE-13670 Vendelso
Sweden Sweden
Email: gunnar.hellstrom@ghaccess.se Email: gunnar.hellstrom@ghaccess.se
 End of changes. 122 change blocks. 
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