Internet DRAFT - draft-hellstrom-avtcore-multi-party-rtt-source
draft-hellstrom-avtcore-multi-party-rtt-source
Network Working Group G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: RFC 4102, RFC 4103 (if approved) 9 April 2020
Intended status: Standards Track
Expires: 11 October 2020
RTP-mixer formatting of multi-party Real-time text
draft-hellstrom-avtcore-multi-party-rtt-source-03
Abstract
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
presented by endpoints in suitable grouping with other text from the
same source.
Regional regulatory requirements specify provision of real-time text
in multi-party calls. RFC 4103 mixer implementations can use
traditional RTP functions for source identification, but the mixer
source switching performance is limited when using the default
transmission with redundancy.
An enhancement for RFC 4103 real-time text mixing is provided in the
present specification, suitable for a centralized conference model
that enables source identification and efficient source switching.
The intended use is for real-time text mixers and multi-party-aware
participant endpoints. The mechanism builds on use of the CSRC list
in the RTP packet.
A capability exchange is specified so that it can be verified that a
participant can handle the multi-party coded real-time text stream.
The capability is indicated by an sdp media attribute "rtt-mix".
A brief description about how a mixer can format text for the case
when the endpoint is not multi-party aware is also provided.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 October 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Intended application . . . . . . . . . . . . . . . . . . . . 4
4. Use of fields in the RTP packets . . . . . . . . . . . . . . 4
5. Actions at transmission by a mixer . . . . . . . . . . . . . 5
6. Actions at reception . . . . . . . . . . . . . . . . . . . . 6
7. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Chained operation . . . . . . . . . . . . . . . . . . . . . . 8
9. Usage without redundancy . . . . . . . . . . . . . . . . . . 8
10. Use with SIP centralized conferencing framework . . . . . . . 8
11. SDP Capability negotiation . . . . . . . . . . . . . . . . . 9
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
13. Performance considerations . . . . . . . . . . . . . . . . . 12
14. Presentation level considerations . . . . . . . . . . . . . . 12
14.1. Presentation by multi-party aware endpoints . . . . . . 13
14.2. Multi-party mixing for multi-party unaware endpoints . . 15
15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 20
16. Congestion considerations . . . . . . . . . . . . . . . . . . 21
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
19. Security Considerations . . . . . . . . . . . . . . . . . . . 21
20. Change history . . . . . . . . . . . . . . . . . . . . . . . 21
20.1. Changes from version -02 to -03 . . . . . . . . . . . . 21
20.2. Changes from version -01 to -02 . . . . . . . . . . . . 22
20.3. Changes from version -00 to -01 . . . . . . . . . . . . 23
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21. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
21.1. Normative References . . . . . . . . . . . . . . . . . . 23
21.2. Informative References . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for
transmission of real-time text (RTT) and the "text/t140" format. It
also specifies a redundancy format "text/red" for increased
robustness. RFC 4102 [RFC4102] registers the "text/red" format.
Regional regulatory requirements specify provision of real-time text
in multi-party calls.
The redundancy scheme enables efficient transmission of redundant
text in packets together with new text. However the redundant header
format has no source indicators for the redundant transmissions. An
assumption has had to be made that the redundant parts in a packet
are from the same source as the new text. The recommended
transmission is one new and two redundant generations of text
(T140blocks) in each packet and the recommended transmission interval
is 300 ms.
A mixer, selecting between text input from different sources and
transmitting it in a common stream needs to make sure that the
receiver can assign the received text to the proper sources for
presentation. Therefore, without any extra rule for 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 with all
intended redundancy levels (usually two) before switching source.
That causes the very long time of one second to switch between
transmission of text from one source to text from another source.
Both the total throughput and the switching performance in the mixer
is too low for most applications.
A more efficient source identification scheme requires that each
redundant T140block has its source individually preserved. The
present specification introduces a source indicator by specific rules
for populating the CSRC-list in the RTP-packet.
A negotiation mechanism is also introduced for verification that the
receiver is able to handle the multi-party coded stream.
A fall-back mixing procedure is specified for cases when the
negotiation fails.
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2. Nomenclature
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained
in [RFC3550]
The term "T140block" is defined in RFC 4103 [RFC4103] to contain one
or more T.140 code elements.
3. Intended application
The scheme for identification of source of redundant transmissions is
intended for transmission from entities taking the mixer role in
centralised mixing configurations for RTT. It is intended for
reception by both endpoints and mixers.
4. Use of fields in the RTP packets
RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
redundancy format "text/red" for increased robustness. This
specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing a rule for populating and using the CSRC-list in the RTP
packet in order to enhance the performance in multi-party RTT
sessions.
When transmitted from a mixer, the first member in the CSRC-list
SHALL contain the SSRC of the source of the primary T140block in the
packet. The second and further members in the CSRC-list SHALL
contain the SSRC of the source of the first, second, etc redundant
generations of T140blocks included in the packet. ( the recommended
level of redundancy is to use one primary and two redundant
generations of T140blocks.) In some cases, a primary or redundant
T140block is empty, but is still represented by a member in the
redundancy header. For such cases, the corresponding CSRC-list
member MUST also be included.
The CC field SHALL show the number of members in the CSRC list.
Note: This specification departs from section 4 of RFC 2198 [RFC2198]
which associates the whole of the CSRC-list with the primary data and
assumes that the same list applies to reconstructed redundant data.
In the present specification a T140block is associated with exactly
one CSRC list member as described above. Also RFC 2198 [RFC2198]
anticipates infrequent change to CSRCs; implementers should be aware
that the order of the CSRC-list according to this specification will
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vary during transitions between transmission from the mixer of text
originated by different participants.
The picture below shows a typical RTP packet with multi-party RTT
contents and coding according to the present specification.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC=3 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CSRC list member 1 = SSRC of source of "P" |
| CSRC list member 2 = SSRC of source of "R1" |
| CSRC list member 3 = SSRC of source of "R2" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R2" | "R2" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R1" | "R1" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | "R2" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+
| "P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: text/red packet with sources indicated in the CSRC-list.
5. Actions at transmission by a mixer
A "text/red" transmitter is usually sending packets at a regular
transmission interval as long as there is something (new or redundant
T140blocks) to transmit. The default transmission interval for
point-to-point operation is 300 ms.
For multi-party operation, the transmission interval from mixers
SHOULD be decreased to 100 ms for periods when there is text
available for transmission from more than two sources. It is also
allowed for the mixer to send a packet as soon as text has been
received from a source as long as the maximum number of characters
per second indicated by the recipient is not exceeded, and also the
number of packets sent per second to a recipient is kept under a
specified number. This number SHALL be 10 if no other limit is
applied for the application. The mixer has its own SSRC, and its own
RTP sequence number series. At time of transmission, the mixer SHALL
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populate the RTP packet with a T140block combined from all T140blocks
queued for transmission originating from next source in turn for
getting its text transmitted as long as this is not in conflict with
the allowed number of characters per second. This T140block shall be
placed in the primary area of the packet. The SSRC of its source
shall be placed as the first member in the CSRC-list. The current
time SHALL be inserted in the timestamp.
If no unsent T140blocks were available at this time, but T140blocks
are available which have not yet been sent the full intended number
of redundant transmissions, then the primary T140block is composed of
an empty T140block, and populated (without taking up any length) in a
packet for transmission. The SSRC of the mixer is included in the
first place of the CSRC-list.
The primary T140block, in the latest transmission is used to populate
the first redundant T140block, and its source is placed as the second
member of the CSRC-list. The first redundant T140block from the
latest transmission is placed as the second redundant T140block, and
the corresponding CSRC placed in its place in the CSRC-list.
Usually this is the level of redundancy used. If a higher number of
redundancy is used, then the procedure is maintained until all
available redundant levels of T140blocks and their sources are placed
in the packet. If a receiver has negotiated a lower number of text/
red generations, then that level shall be the maximum used by the
transmitter.
The timer offset values are inserted in the redundancy header, with
the time offset from when the corresponding T140block was sent as
original.
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.
When there is no new T140block to transmit, and no redundant
T140block that has not been retransmitted the intended number of
times, the transmission process can stop until either new T140blocks
arrive, or a keep-alive method calls for transmission of keep-alive
packets.
6. Actions at reception
The enhanced "text/red" receiver included in an endpoint with
presentation functions will receive RTP packets in the single stream
from the mixer, and shall distribute the T140blocks for presentation
in presentation areas for each source. Other tasks for receivers,
such as gateways or chained mixers are also feasible, and requires
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consideration if the stream shall just be forwarded, or a
distribution based on different sources is needed.
If the "CC" field value of a received packet is >1, it indicates the
used level of redundancy for the current packet and that the enhanced
packet format is used. If the CC field value is 0 or 1, it indicates
that the enhanced format is not used. If the CC value is 1, the CSRC
indicates the source of primary data.
The RTP sequence numbers of the received packets SHALL be monitored
for gaps and packets out of order.
As long as the sequence is correct, each packet SHALL be unpacked in
order. The T140blocks SHALL be extracted from the primary area, and
the corresponding SSRC SHALL be extracted from the first position in
the CSRC list and used for assigning the new T140block to the correct
presentation area (or correspondingly).
If a sequence number gap appears and is still there after some
defined time for jitter resolution, T140data SHALL be recovered from
redundant data. If the gap is wider than the number of generations
of redundant T140blocks in the packet, then a t140block SHALL be
created with a marker for text loss [T140ad1] and assigned to the
SSRC of the transmitter as a general input from the mixer.
Then, the T140blocks in the received packet SHALL be retrieved
beginning with the highest redundant generation, grouping them with
the corresponding SSRC from the CSRC-list and assigning them to the
presentation areas per source. Finally the primary T140block SHALL
be retrieved from the packet and similarly its source retrieved from
the first position in the CSRC-list, and then assigned to the
corresponding presentation area for that source.
If the sequence number gap was equal to or less than the number of
redundancy generations in the received packet, a missing text marker
SHALL NOT be inserted, and instead the T140blocks and their SSRCs
fully recovered from the redundancy information and the CSRC-list in
the way indicated above.
Unicode character BOM is sometimes used as a filler or keep alive by
transmission implementations. These SHALL be deleted on reception.
Note that empty T140blocks are sometimes included in the packets.
They just do not provide any contents.
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7. RTCP considerations
A mixer SHALL send RTCP reports with SDES, CNAME and NAME information
about the sources in the multi-party call. This makes it possible
for participants to compose a suitable label for text from each
source.
8. Chained operation
By strictly applying the rules for CSRC-list population by all
conforming devices, mixers MAY be arranged in chains.
9. Usage without redundancy
The CSRC list member SHALL be used as source indicator also for cases
when the "text/t140" format is used from a mixer. That MAY be the
case when robustness in transmission is provided by some other means
than by redundancy and the "text/red" format. All aspects of this
memo SHALL be applied except the redundant generations in
transmission.
For the use case without redundancy using the "text/t140" format, the
"CC" field in the RTP packet shall have the value 1, and the CSRC
list SHALL contain one member when sent from a mixer.
The "text/red" format SHOULD be used unless some other protection
against packet loss is utilized, for example a reliable network or
transport.
10. Use with SIP centralized conferencing framework
The SIP conferencing framework, mainly specified in RFC
4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
for coordinating sessions including multi-party RTT. The RTT stream
between the mixer and a participant is one and the same during the
conference. Participants get announced by notifications when
participants are joining or leaving, and further user information may
be provided. The SSRC of the text to expect from joined users can be
included in a notification. This can be used both for security
purposes and for translation to a label for presentation to 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 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.
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11. SDP Capability negotiation
There are RTT implementations which implement RFC 4103 [RFC4103] but
not the present specification for real-time awareness. Sending mixed
text according to the present specification to a device implementing
only RFC 4103 [RFC4103] would lead to unreadable presented text.
Therefore, in order to negotiate RTT mixing capability according to
the present specification, all devices supporting the present
specification for multi-party aware participants SHALL include an sdp
media attribute "rtt-mix" in the sdp, indicating this capability in
offers and answers. Multi-party streams using the coding of the
present specification intended for multi-party aware endpoints MUST
NOT be sent to devices which have not indicated the "rtt-mix"
capability.
Implementations not understanding this parameter MUST ignore it
according to common SDP rules.
The sdp media attribute defined here, is named "rtt-mix". It has no
parameters. It is intended to be used in "text" media descriptions
with "text/red" and "text/t140" formats. It indicates capability to
use source indications in the CSRC list according to the present
specification. It also indicates ability to receive 10 real-time
text packets per second.
Syntax:
a=rtt-mix
The attribute is used in offer/answer procedures in a declarative
way. Both parties express their capability to use sources in the
CSRC list as specified in the present specification.
A party who has expressed the "rtt-mix" capability MUST populate the
CSRC-list according to the present specification if it acts as an
rtp-mixer and sends to a party who has expressed the "rtt-mix"
capability.
A party who has expressed the "rtt-mix" capability MUST interpret the
contents of the CSRC-list according to the present specification in
received rtp packets from parties who have expressed "rtt-mix"
capability .
A party MUST NOT transmit packets with the format for multi-party
aware participants according to the present specification to a party
who has not expressed "rtt-mix" capability.
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A party performing as a mixer, which has expressed the "rtt-mix"
capability, but not received "rtt-mix" capability indication in a
session with a participant SHOULD, if nothing else is specified for
the application, format transmitted text to that participant to be
suitable to present on a multi-party unaware endpoint as further
specified in section Section 14.2.
12. Examples
This example shows a symbolic flow of packets from a mixer with loss
and recovery. A, B and C are sources of RTT. M is the mixer. P
indicates primary data. R1 is first redundant generation data and R2
is second redundant generation data. A1, B1, A2 etc are text chunks
(T140blocks) received from the respective sources. X indicates
dropped packet between the mixer and a receiver.
|----------------|
|Seq no 1 |
|CSRC list A,M,B |
|R2 B99 |
|R1: Empty |
|P: A1 |
|----------------|
Assuming that earlier packets were received in sequence, text A1 is
received from packet 1 and assigned to reception area A.
|----------------|
|Seq no 2 |
|CC=3 |
|CSRC list C,A,M |
|R2 Empty |
|R1: A1 |
|P: C1 |
|----------------|
Text C1 is received from packet 2 and assigned to reception area C.
X----------------|
X Seq no 3 |
X CC=3 |
X CSRC list A,C,A|
X R2: A1 |
X R1: C1 |
X P: A2 |
X----------------|
Packet 3 is assumed to be dropped in network problems
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X----------------|
X Seq no 4 |
X CC=3 |
X CSRC list B,A,C|
X R2: C1 |
X R1: A2 |
X P: B1 |
X----------------|
Packet 4 is assumed to be dropped in network problems
X----------------|
X Seq no 5 |
X CC=3 |
X CSRC list A,B,A|
X R2: A2 |
X R1: B1 |
X P: A3 |
X----------------|
Packet 5 is assumed to be dropped in network problems
|----------------|
|Seq no 6 |
|CC=3 |
|CSRC list C,A,B |
|R2: B1 |
|R1: A3 |
|P: C2 |
|----------------|
Packet 6 is received. The latest received sequence number was 2.
Recovery is therefore tried for 3,4,5. But there is no coverage
for seq no 3. A missing text mark (U'FFFD) is created and
appended to the mixer reception area.
For seqno 4, text B1 is recovered and appended to reception area B.
For seqno 5, text A3 is recovered and appended to reception area A.
Primary text C2 is received and appended to reception area C.
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.
It will be a design decision how to present the missing text markers
assigned to the mixer as a source.
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13. Performance considerations
This specification allows new text from one source per packet.
Packets SHOULD be transmitted with timed intervals. The default
transmission interval is 300 ms for RFC 4103[RFC4103], which is
suitable for transmission from single sources. However when more
sources contribute to the flow, a shorter transmission interval MAY
be applied. The transmission interval is therefore RECOMMENDED to be
100 ms for mixers when there is text from more than two sources
available for transmission to the same recipient.It is also allowed
for the mixer to send a packet as soon as text has been received from
a source as long as the maximum number of characters per second
indicated by the recipient is not exceeded, and also the number of
packets sent per second to a recipient is kept under a number. In
order to achieve good performance, a receiver for multi-party calls
SHOULD declare a sufficient CPS value in SDP for the number of
allowable characters per second. These characteristics provide for
smooth flow of text with acceptable latency from at least 5 sources
simultaneously.
The default maximum rate of reception of real-time text 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 in the
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
the sources. Endpoints SHOULD therefore specify a suitable higher
value for the CPS parameter, corresponding to its real reception
capability. A value for CPS of 150 is RECOMMENDED because it would
be suitable for reception in most multi-party sessions, and a
realistic value for the capacity in most implementation environments.
See RFC 4103 [RFC4103] for the format and use of the CPS parameter.
14. Presentation level considerations
ITU-T T.140 [T140] provides the presentation level requirements for
the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for
erasure and other formatting functions and has the following general
statement for the presentation:
"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. Mechanisms for looking back in the contents from the
current session should be provided. The text should be displayed as
soon as it is received."
Strict application of T.140 [T140] is of essence for the
interoperability of real-time text implementations and to fulfill the
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intention that the session participants have the same information of
the text contents of the conversation without necessarily having the
exact same layout of the conversation. This also includes the
ability to ignore optional presentation control codes not supported
by a receiving application.
T.140 [T140] specifies a set of presentation control codes to include
in the stream. Some of them are optional. Implementations MUST be
able to ignore optional control codes that they do not support.
There is no strict "message" concept in real-time text. Line
Separator SHALL be used as a separator allowing a part of received
text to be grouped in presentation. The characters "CRLF" may be
used by other implementations as replacement for Line Separator. The
"CRLF" combination SHALL be erased by just one erasing action, just
as the Line Separator. Presentation functions are allowed to group
text for presentation in smaller groups than the line separators
imply and present such groups with source indication together with
text groups from other sources (see the following presentation
examples). Erasure has no specific limit by any delimiter in the
text stream.
14.1. Presentation by multi-party aware endpoints
A multi-party aware receiving party, presenting real-time text MUST
separate text from different sources and present them in separate
presentation areas. The receiving party MAY separate presentation of
parts of text from a source in readable groups based on other
criteria than line separator and merge these groups in the
presentation area when it benefits the user to most easily find and
read text from the different participants. The criteria MAY e.g. be
a received comma, full stop, or other phrase delimiters, or a long
pause.
When text is received from multiple original sources, the
presentation SHOULD provide a view where text is added in multiple
places simultaneously.
If the presentation presents text from different sources in one
common area, the presenting endpoint SHOULD insert text from the
local user ended at suitable points merged with received text to
indicate the relative timing for when the text groups were completed.
In this presentation mode, the receiving endpoint SHALL present the
source of the different groups of text.
A view of a three-party RTT call in chat style is shown in this
example .
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_________________________________________________
| | |
|[Alice] Hi, Alice here. | |
| | |
|[Bob] Bob as well. | |
| | |
|[Eve] Hi, this is Eve, calling from Paris. | |
| I thought you should be here. | |
| | |
|[Alice] I am coming on Thursday, my | |
| performance is not until Friday morning.| |
| | |
|[Bob] And I on Wednesday evening. | |
| | |
|[Alice] Can we meet on Thursday evening? | |
| | |
|[Eve] Yes, definitely. How about 7pm. | |
| at the entrance of the restaurant | |
| Le Lion Blanc? | |
|[Eve] we can have dinner and then take a walk | |
| | |
| <Eve-typing> But I need to be back to | |
| the hotel by 11 because I need | |
| |-|
| <Bob-typing> I wou |-|
|______________________________________________|v|
| of course, I underst |
|________________________________________________|
Figure 1: Example of a three-party call with chat style.
Figure 2: Example of a three-party RTT call presented in chat style.
Other presentation styles than the chat style may be arranged.
This figure shows how a coordinated column view MAY be presented.
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_____________________________________________________________________
| Bob | Eve | Alice |
|____________________|______________________|_______________________|
| | |I will arrive by TGV. |
|My flight is to Orly| |Convenient to the main |
| |Hi all, can we plan |station. |
| |for the seminar? | |
|Eve, will you do | | |
|your presentation on| | |
|Friday? |Yes, Friday at 10. | |
|Fine, wo | |We need to meet befo |
|___________________________________________________________________|
Figure 3: An example of a coordinated column-view of a three-party
session with entries ordered vertically in approximate time-order.
14.2. Multi-party mixing for multi-party unaware endpoints
When the mixer has indicated multi-party capability in an sdp
negotiation, but the multi-party capability negotiation fails with an
endpoint, then the mixer SHOULD compose a best-effort presentation of
multi-party real-time text in one stream intended to be presented by
an endpoint with no multi-party awareness.
This presentation format has functional limitations and SHOULD be
used only to enable participation in multi-party calls by legacy
deployed endpoints.
The principles and procedures below do not specify any new protocol
elements or behaviors. They are instead composed from the
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-
party calls.
The mixer mixing for multi-party unaware endpoints SHALL compose a
simulated limited multi-party RTT view suitable for presentation in
one presentation area. The mixer SHALL group text in suitable groups
and prepare for presentation of them by inserting a new line betwwen
them if the transmitted text did not already end with a new line. A
presentable label SHOULD be composed and sent for the source
initially in the session and after each source switch. With this
procedure the time for source switching is depending on the actions
of the users. In order to expedite source switch, a user can for
example end its turn with a new line.
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14.2.1. Actions by the mixer at reception from the call 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 mark for lost text [T140ad1] SHOULD be inserted in the stream if
unrecoverable loss appears. Any Unicode BOM characters, possibly
used for keep-alive shall be deleted. The time of arrival of text
SHALL be stored together with the received text from each source in a
queue 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
multi-part text from the mixer.
The text for transmission SHOULD be formatted by the mixer for each
receiving user for presentation in one single presentation area.
Text received from a participant SHOULD NOT be included in
transmission to that participant. When there is text available for
transmission from the mixer to a receiving party from more than one
participant, the mixer SHOULD switch between transmission of text
from the different sources at suitable points in the transmitted
stream.
When switching source, the mixer SHOULD insert a line separator if
the already transmitted text did not end with a new line (line
separator or CRLF). A label SHOULD be composed from information in
the CNAME and NAME fields in RTCP reports from the participant to
have its text transmitted, or from other session information for that
user. The label SHOULD be delimited by suitable characters (e.g. '[
]') and transmitted. The CSRC SHOULD indicate the selected source.
Then text from that selected participant SHOULD be transmitted until
a new suitable point for switching source is reached.
Seeking a suitable point for switching source SHOULD be done when
there is older text waiting for transmission from any party than the
age of the last transmitted text. Suitable points for switching are:
* A completed phrase ended by comma
* A completed sentence
* A new line (line separator or CRLF)
* A long pause (e.g. > 10 seconds) in received text from the
currently transmitted source
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* If text from one participant has been transmitted with text from
other sources waiting for transmission for a long time (e.g. > 1
minute) and none of the suitable points for switching has
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
a time (e.g. 15 seconds) after the scan for word delimiter
started.
When switching source, the source which has the oldest text in queue
SHOULD be selected to be transmitted. A character display count
SHOULD be maintained for the currently transmitted source, starting
at zero after the label is transmitted for the currently transmitted
source.
There SHOULD be a storage for the latest control code for Select
Graphic Rendition (SGR) from each source. If there is an SGR code
stored for the current source before the source switch is done, a
reset of SGR shall be sent by the sequence SGR 0 [009B 0000 006D]
after the new line and before the new label during a source switch.
See SGR below for an explanation. This transmission 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
Text from a source sent to the recipient SHOULD increase the display
count by one per transmitted character.
14.2.4. Actions on transmission of control codes
The following control codes specified by T.140 require specific
actions. They SHOULD cause specific considerations in the mixer.
Note that the codes presented here are expressed in UCS-16, while
transmission is made in UTF-8 transform of these codes.
BEL 0007 Bell Alert in session, provides for alerting during an
active session. The display count SHOULD not be altered.
NEW LINE 2028 Line separator. Check and perform a source switch if
appropriate. Increase display count by 1.
CR LF 000D 000A A supported, but not preferred way of requesting a
new line. Check and perform a source switch if appropriate.
Increase display count by 1.
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INT ESC 0061 Interrupt (used to initiate mode negotiation
procedure). The display count SHOULD not be altered.
SGR 009B Ps 006D Select graphic rendition. Ps is rendition
parameters specified in ISO 6429. The display count SHOULD not be
altered. The SGR code SHOULD be stored for the current source.
SOS 0098 Start of string, used as a general protocol element
introducer, followed by a maximum 256 bytes string and the ST.
The display count SHOULD not be altered.
ST 009C String terminator, end of SOS string. The display count
SHOULD not be altered.
ESC 001B Escape - used in control strings. The display count SHOULD
not be altered for the complete escape code.
Byte order mark FEFF Zero width, no break space, used for
synchronization and keep-alive. SHOULD be deleted from incoming
streams. Shall be sent first after session establishment to the
recipient. The display count shall not be altered.
Missing text mark FFFD Replacement character, marks place in stream
of possible text loss. SHOULD be inserted by the reception
procedure in case of 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
than reset of the graphic rendition (SGR 0) is sent to a
recipient, that control code shall also be stored for the source
in the storage for SGR. If a reset graphic rendition (SGR 0)
originated from a source is sent, then the SGR storage for that
source shall be cleared. The display count shall not be
increased.
BS 0008 Back Space, intended to erase the last entered character by
a source. Erasure by backspace cannot always be performed as the
erasing party intended. If an erasing action erases all text up
to the end of the leading label after a source switch, then the
mixer must not transmit more backspaces. Instead it is
RECOMMENDED that a letter "X" is inserted in the text stream for
each backspace as an indication of the intent to erase more. A
new line is usually coded by a Line Separator, but the character
combination "CRLF" MAY be used instead. Erasure of a new line is
in both cases done by just one erasing action (Backspace). If the
display count has a positive value it is decreased by one when the
BS is sent. If the display count is at zero, it is not altered.
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14.2.5. Packet transmission
A mixer transmitting to a multi-party unaware terminal SHOULD send
primary data only from one source per packet. The SSRC SHOULD be the
SSRC of the mixer. The CSRC list SHOULD contain one member and be
the SSRC of the source of the primary data.
14.2.6. Functional limitations
When a multi-party unaware endpoint presents a conversation in one
display area in a chat style, it inserts source indications for
remote text and local user text as they are merged in completed text
groups. When an endpoint using this layout receives and presents
text mixed for multi-party unaware endpoints, there will be two
levels of source indicators for the received text; one generated by
the mixer and inserted in a label after each source switch, and
another generated by the receiving endpoint and inserted after each
switch between local and remote source in the presentation area.
This will waste display space and look inconsistent to the reader.
This fact, combined with the slowness in source switching and the
limited support of erasure makes it strongly RECOMMENDED to implement
multi-party awareness in RTT endpoints. The use of the mixing method
for multi-party-unaware endpoints should be left for use with
endpoints which are impossible to upgrade to become multi-party
aware.
14.2.7. Example views of presentation on multi-party unaware endpoints
The following pictures are examples of the view on a participant's
display for the multi-party-unaware case.
_________________________________________________
| Conference | Alice |
|________________________|_________________________|
| |I will arrive by TGV. |
|[Bob]:My flight is to |Convenient to the main |
|Orly. |station. |
|[Eve]:Hi all, can we | |
|plan for the seminar. | |
| | |
|[Bob]:Eve, will you do | |
|your presentation on | |
|Friday? | |
|[Eve]:Yes, Friday at 10.| |
|[Bob]: Fine, wo |We need to meet befo |
|________________________|_________________________|
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Figure 4: Alice who has a conference-unaware client is receiving the
multi-party real-time text in a single-stream. This figure shows how
a coordinated column view MAY be presented on Alice's device.
_________________________________________________
| |^|
|[Alice] Hi, Alice here. | |
| | |
|[mix][Bob] Bob as well. | |
| | |
|[Eve] Hi, this is Eve, calling from Paris | |
| I thought you should be here. | |
| | |
|[Alice] I am coming on Thursday, my | |
| performance is not until Friday morning.| |
| | |
|[mix][Bob] And I on Wednesday evening. | |
| | |
|[Eve] we can have dinner and then walk | |
| | |
|[Eve] But I need to be back to | |
| the hotel by 11 because I need |-|
| |-|
|______________________________________________|v|
| of course, I underst |
|________________________________________________|
Figure 5: An example of a view of the multi-party unaware
presentation in chat style. Alice is the local user.
15. Gateway Considerations
Multi-party RTT sessions may involve gateways of different kinds.
Gateways involved in setting up sessions SHALL correctly reflect the
multi-party capability or unawareness of the combination of the
gateway and the remote endpoint beyond the gateway.
One case that may occur is a gateway to PSTN for communication with
textphones (e.g. TTYs). Textphones are limited devices with no
multi-party awareness, and it SHOULD therefore be suitable for the
gateway to not indicate multi-party awareness for that case. Another
solution is that the gateway indicates multi-party capability towards
the mixer, and includes the multi-party mixer function for multi-
party unaware endpoints itself. This solution makes it possible to
make adaptations for the functional limitations of the textphone
(TTY).
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16. Congestion considerations
The congestion considerations and recommended actions from RFC 4103
[RFC4103] are valid also in multi-party situations.
17. Acknowledgements
18. IANA Considerations
[RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
RFC number of this document.]
IANA is asked to register the new sdp attribute "rtt-mix".
+---------------------+------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | rtt-mix |
| Attribute syntax | a=rtt-mix |
| Attribute semantics | See RFCXXXX section 11. |
| Attribute value | - |
| Usage level: | media |
| Purpose: | Indicate support for the rtp-mixer format|
| | for real-time text transmission |
| O/A procedure | Declarative |
| Mux Category | normal |
| Reference: | RFCXXXX |
+---------------------+------------------------------------------+
19. Security Considerations
The RTP-mixer model requires the mixer to be allowed to decrypt, pack
and encrypt secured text from the conference participants. Therefore
the mixer needs to be trusted. This is similar to the situation for
central mixers of audio and video.
The requirement to transfer information about the user in RTCP
reports in SDES, CNAME and NAME fields for creation of labels may
have privacy concerns as already stated in RFC 3550 [RFC3550], and
may be restricted of privacy reasons. The receiving user will then
get a more symbolic label for the source.
20. Change history
20.1. Changes from version -02 to -03
Changed company and e-mail of the author.
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Changed title to "RTP-mixer formatting of multi-party Real-time text"
to better match contents.
Check and modification where needed of use of RFC 2119 words SHALL
etc.
More about the CC value in sections on transmitters and receivers so
that 1-to-1 sessions do not use the mixer format.
Enhanced section on presentation for multi-party-unaware endpoints
A paragraph recommending CPS=150 inserted in the performance section.
20.2. Changes from version -01 to -02
In Abstract and 1. Introduction: Introduced wording about regulatory
requirements.
In section 5: The transmission interval is decreased to 100 ms when
there is text from more than one source to transmit.
In section 11 about sdp negotiation, a SHOULD-requirement is
introduced that the mixer should make a mix for multi-party unaware
endpoints if the negotiation is not successful. And a reference to a
later chapter about it.
The presentation considerations chapter 14 is extended with more
information about presentation on multi-party aware endpoints, and a
new section on the multi-party unaware mixing with low functionality
but SHOULD a be implemented in mixers. Presentation examples are
added.
A short chapter 15 on gateway considerations is introduced.
Clarification about the text/t140 format included in chapter 10.
This sentence added to the chapter 10 about use without redundancy.
"The text/red format SHOULD be used unless some other protection
against packet loss is utilized, for example a reliable network or
transport."
Note about deviation from RFC 2198 added in chapter 4.
In chapter 9. "Use with SIP centralized conferencing framework" the
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
blocks. It is not the same as the list of participants in a
conference. With audio and video media, the CSRC-list would often
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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
packet CSRC-lists.
20.3. Changes from version -00 to -01
Editorial cleanup.
Changed capability indication from fmtp-parameter to sdp attribute
"rtt-mix".
Swapped order of redundancy elements in the example to match reality.
Increased the SDP negotiation section
21. References
21.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
DOI 10.17487/RFC2198, September 1997,
<https://www.rfc-editor.org/info/rfc2198>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type",
RFC 4102, DOI 10.17487/RFC4102, June 2005,
<https://www.rfc-editor.org/info/rfc4102>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<https://www.rfc-editor.org/info/rfc4103>.
[T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
multimedia application text conversation", February 1998,
<https://www.itu.int/rec/T-REC-T.140-199802-I/en>.
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[T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
Protocol for multimedia application text conversation",
February 2000,
<https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>.
21.2. Informative References
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006,
<https://www.rfc-editor.org/info/rfc4353>.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, DOI 10.17487/RFC4575, August
2006, <https://www.rfc-editor.org/info/rfc4575>.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,
<https://www.rfc-editor.org/info/rfc4579>.
Author's Address
Gunnar Hellstrom
Gunnar Hellstrom Accessible Communication
Esplanaden 30
SE-13670 Vendelso
Sweden
Email: gunnar.hellstrom@ghaccess.se
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