RTP-mixer formatting of multi-party Real-time text
draft-ietf-avtcore-multi-party-rtt-mix-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9071.
|
|
|---|---|---|---|
| Author | Gunnar Hellstrom | ||
| Last updated | 2020-05-14 (Latest revision 2020-05-08) | ||
| Replaces | draft-hellstrom-avtcore-multi-party-rtt-source | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Reviews |
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draft-ietf-avtcore-multi-party-rtt-mix-01
AVTCore G. Hellstrom
Internet-Draft Gunnar Hellstrom Accessible Communication
Updates: RFC 4102, RFC 4103 (if approved) 14 May 2020
Intended status: Standards Track
Expires: 15 November 2020
RTP-mixer formatting of multi-party Real-time text
draft-ietf-avtcore-multi-party-rtt-mix-01
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 and an extended packet format 'text/rex'.
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 the media subtype "text/rex".
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103]
A brief description about how a mixer can format text for the case
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 15 November 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/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Selected solution and considered alternative . . . . . . 4
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Intended application . . . . . . . . . . . . . . . . . . 5
2. Use of fields in the RTP packets . . . . . . . . . . . . . . 5
3. Actions at transmission by a mixer . . . . . . . . . . . . . 7
4. Actions at reception . . . . . . . . . . . . . . . . . . . . 9
5. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Chained operation . . . . . . . . . . . . . . . . . . . . . . 10
7. Usage without redundancy . . . . . . . . . . . . . . . . . . 10
8. Use with SIP centralized conferencing framework . . . . . . . 11
9. Media Subtype Registration . . . . . . . . . . . . . . . . . 11
10. SDP considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12. Performance considerations . . . . . . . . . . . . . . . . . 17
13. Presentation level considerations . . . . . . . . . . . . . . 17
13.1. Presentation by multi-party aware endpoints . . . . . . 18
13.2. Multi-party mixing for multi-party unaware endpoints . . 20
14. Gateway Considerations . . . . . . . . . . . . . . . . . . . 26
15. Updates to RFC 4102 and RFC 4103 . . . . . . . . . . . . . . 26
16. Congestion considerations . . . . . . . . . . . . . . . . . . 26
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
19. Security Considerations . . . . . . . . . . . . . . . . . . . 27
20. Change history . . . . . . . . . . . . . . . . . . . . . . . 27
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20.1. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 27
20.2. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 27
20.3. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03 . . . . . . . . . . . . . . . . . . . . . . . . . . 27
20.4. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-01 to
-02 . . . . . . . . . . . . . . . . . . . . . . . . . . 28
20.5. Changes from
draft-hellstrom-avtcore-multi-party-rtt-source-00 to
-01 . . . . . . . . . . . . . . . . . . . . . . . . . . 29
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
21.1. Normative References . . . . . . . . . . . . . . . . . . 29
21.2. Informative References . . . . . . . . . . . . . . . . . 30
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 31
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, using RFC 4103 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.
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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 and the redundancy header in the RTP-
packet.
An extended packet format 'text/rex' is specified for this purpose,
providing the possibility to include text from up to 16 sources in
each packet in order to enhance mixer source switching performance.
A negotiation mechanism can therefore be based on selection between
the "text/red" and the "text/rex" media formats for verification that
the receiver is able to handle the multi-party coded stream.
A fall-back mixing procedure is specified for cases when the
negotiation results in "text/red" being the only common submedia
format.
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format for the multi-party mixing case
and more strict rules for the use of redundancy.
1.1. Selected solution and considered alternative
The mechanism specified in the present document makes use of the RTP
mixer model specified in RFC3550[RFC3550]. From some points of view,
use of the RTP translator model specified in RFC 3550 would be more
efficient, because then the text packets can pass the translator with
only minor modification. However, there may be a lack of support for
the translator model in existing RTP implementations, and therefore
the more common RTP-mixer model was selected.
1.2. Nomenclature
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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.
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1.3. 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.
2. 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 and extending the redundancy header in order to enhance the
performance in multi-party RTT sessions.
When transmitted from a mixer, the members in the CSRC-list SHALL
contain the SSRCs of the sources of the T140blocks in the packet.
When redundancy is used, text from all included sources MUST have the
same number of redundancy generations. The primary, second and
further redundant generations of T140blocks are grouped per source in
the packet in "source groups" and in the same order as the SSRCs of
the sources in the CSRC list. ( 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.
The CC field SHALL show the number of members in the CSRC list.
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
vary during transitions between transmission from the mixer of text
originated by different participants.
The picture below shows a typical 'text/rex' 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| "REX" PT | RTP sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of packet creation |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CSRC list member 1 = SSRC of source of "A" |
| CSRC list member 2 = SSRC of source of "B" |
| CSRC list member 3 = SSRC of source of "C" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestmp offset of "A-R2" |"A-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "A-R1" |"A-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "A-P" |"A-P" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "B-R2" |"B-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "B-R1" |"B-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "B-P" | "B-P" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "C-R2" |"C-R2" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT |timestamp offset of "C-R1" |"C-R1" block length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | "A-R2" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "A-R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "A-P" T.140 encoded primary data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "B-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "B-R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "B-P" T.140 encoded primary data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| "C-R2" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| | "C-R1" T.140 encoded redundant data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "C-P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1:A 'text/rex' packet with text from three sources A, B, C.
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3. Actions at transmission by a mixer
A "text/rex" transmitter is sending packets at least at regular
transmission intervals as long as there is something (new or
redundant T140blocks) to transmit. The default transmission interval
for point-to-point operation is 300 ms.
As soon as a participant is known to participate in a session and
being available for text reception, a Unicode BOM character SHALL be
sent to it according to the procedures in the present document.
After that, the transmitter SHALL send keep-alive traffic to the
receivers at regular intervals when no other traffic has occurred
during that interval if that is decided for the actual connection.
Recommendations for keep-alive can be found in RFC 6263[RFC6263].
For multi-party operation, it is RECOMMENDED that the mixer sends a
packet to each receiver 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 intention is to keep the latency introduced by the
mixer low.
The mixer has its own SSRC, and its own RTP sequence number series.
The number of redundant generations of T140blocks to include in
transmitted packets SHALL be deducted from the SDP negotiation. It
SHOULD be set to the minimum of the number declared by the receiver
and the transmitter. The same number of redundant generations MUST
be used for all sources in the transmissions. The number of
generations sent to a receiver SHALL be the same during the whole
session unless it is modified by session renegotiation.
At time of transmission, the mixer SHALL populate the RTP packet with
T140blocks combined from all T140blocks queued for transmission
originating from each source as long as this is not in conflict with
the allowed number of characters per second or the maximum packet
size. These T140blocks SHALL be placed in the packet interleaved
with redundant T140blocks and new T140blocks from other sources. The
primary and redundant T140blocks from each source are grouped
together in "source groups" appearing in the packet in the same order
as the corresponding CSRC list member and redundancy header members
for the source group. The format of the redundancy header members
and the T140blocks in each source group are the same as for the
single source possible in the "text/red" format. The SSRC of each
source shall be placed as a member in the CSRC-list at a place
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corresponding to the place of its T140blocks in the packet. The
current time SHALL be inserted in the timestamp. The timestamp
offset values for empty T140blocks have no relevance but SHOULD be
assigned realistic values.
Text from a maximum of 16 sources MAY be included in a packet. The
reason for this limitation is the maximum number of CSRC list
members. If text from more sources need to be transmitted, the mixer
MAY let the sources take turns in having their text transmitted.
When stopping transmission of one source to allow another source to
have its text sent, all intended redundant generations of the last
text from the source to be stopped MUST be transmitted before text
from another source can be transmitted. Actively transmitting
sources SHOULD be allowed to take turns with short intervals to have
their text transmitted.
If no unsent T140blocks were available at this time for a source, but
T140blocks are available which have not yet been sent the full
intended number of redundant transmissions, then the primary
T140block for that source is composed of an empty T140block, and
populated (without taking up any length) in a packet for
transmission. The corresponding SSRC SHALL be placed in its place in
the CSRC-list.
The primary T140block from each source in the latest transmission is
used to populate the first redundant T140block for that source. The
first redundant T140block for that source from the latest
transmission is placed as the second redundant T140block source.
Usually this is the level of redundancy used. If a higher number of
redundancy is negotiated, then the procedure SHALL be 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/rex 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 the RTP timestamp in the packet when the
corresponding T140block was sent from the mixer as primary.
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.
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4. Actions at reception
The "text/rex" 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 receiver roles, such as
gateways or chained mixers are also feasible, and requires
consideration if the stream shall just be forwarded, or distributed
based on the different sources.
If the "CC" field value of a received packet is >1, it indicates that
multi-party transmission is active, and the receiver MUST act on the
different sources according to its role. If the CC field value is 1,
it indicates that there is a mixer between the source and the
receiver, but that there is only one source at the moment. If the CC
value is 0, the connection is point-to-point.
The used level of redundancy generations SHALL be evaluated from the
received packet contents. If the CC value is 0, the number of
generations (including the primary) is equal to the number of members
in the redundancy header. If the CC value is >0, the number of
generations (including the primary) is equal to the number of members
in the redundancy header divided by the CC value. If the remainder
from the division is >0, then the packet is malformed and SHALL cause
an error indication in the receiver.
The RTP sequence numbers of the received packets SHALL be monitored
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 areas, and
the corresponding SSRCs SHALL be extracted from the corresponding
positions in the CSRC list and used for assigning the new T140block
to the correct presentation areas (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 because in
general it is not possible to deduct from which sources text was
lost.
Then, the T140blocks in the received packet SHALL be retrieved
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 T140blocks SHALL
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be retrieved from the packet and similarly their sources retrieved
from the corresponding positions in the CSRC-list, and then assigned
to the corresponding presentation areas for the sources.
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 used as a start indication and sometimes
used as a filler or keep alive by transmission implementations.
These SHALL be deleted on reception.
Empty T140blocks are included as fillers for unused redundancy levels
in the packets. They just do not provide any contents and do not
contribute to the received streams.
5. 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.
6. Chained operation
By strictly applying the rules for "text/rex" packet format by all
conforming devices, mixers MAY be arranged in chains.
7. Usage without redundancy
The "text/rex" format SHALL be used also for multi-party
communication when the redundancy mechanism is not used. That MAY be
the case when robustness in transmission is provided by some other
means than by redundancy. All aspects of the present document SHALL
be applied except the redundant generations in transmission.
The "text/rex" format SHOULD thus be used for multi-party operation,
also when some other protection against packet loss is utilized, for
example a reliable network or transport. The format is also allowed
to be used for point-to-point operation.
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8. 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 MAY be
included in a notification. The notifications MAY 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 total list of participants in a conference. With audio and video
media, the CSRC-list would often contain all participants who are not
muted whereas text participants that don't type are completely silent
and thus are not represented in RTP packet CSRC-lists.
9. Media Subtype Registration
This registration is done using the template defined in [RFC6838] and
following [RFC4855].
Type name: text
Subtype name: rex
Required parameters: rate: The RTP timestamp (clock) rate. The
only valid value is 1000.
pt: A slash-separated list with the payload
type number(pt) for the primary text, the first redundant text,
the second redundant text etc, that the receiver is capable to
receive.
Optional parameter: cps: This parameter is used to signal the
capabilities of a receiver implementation. It indicates the
maximum number of characters that may be received per second
measured over a period of 10 seconds. The default value is 150.
Encoding considerations: binary; see Section 4.8 of [RFC6838].
Security considerations: See Section 19 of RFC xxxx. [RFC Editor:
Upon publication as an RFC, please replace "XXXX" with the number
assigned to this document and remove this note.]
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Interoperability considerations: None.
Published specification: RFC XXXX. [RFC Editor: Upon publication as
an RFC, please replace "XXXX" with the number assigned to this
document and remove this note.]
Applications which use this media type: For example: Text
conferencing tools, multimedia conferencing tools.
Fragment identifier considerations: N/A.
Additional information: None.
Person & email address to contact for further information: Gunnar
Hellstrom <gunnar.hellstrom@ghaccess.se>
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing, and
hence is only defined for transfer via RTP [RFC3550].
Author: Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>
Change controller: IETF AVTCore Working Group delegated from the
IESG.
10. SDP considerations
There are receiving RTT implementations which implement RFC 4103
[RFC4103] but not the source separation by the CSRC. Sending mixed
text according to the usual CSRC convention from RFC 2198 [RFC2198]
to a device implementing only RFC 4103 [RFC4103] would risk to lead
to unreadable presented text. Therefore, in order to negotiate RTT
mixing capability according to the present specification, all devices
supporting the present specification for multi-party aware
participants SHALL include an sdp media format "text/rex" in the sdp,
indicating this capability in offers and answers. Multi-party
streams using the coding of the present specification intended for
multi-party aware endpoints MUST NOT be sent to devices which have
not indicated the "text/rex" format.
Implementations not understanding this format MUST ignore it
according to common SDP rules.
The sdp media format defined here, is named "rex", for extended
redundancy. It is intended to be used in "text" media descriptions
with "text/rex" and "text/t140" formats. Both formats MUST be
declared for the "text/rex" format to be used. It indicates
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capability to use source indications in the CSRC list and the packet
format according to the present specification. It also indicates
ability to receive 150 real-time text characters per second.
Security SHOULD be applied. If no other security solution is
mandated by the application then RFC 8643 [RFC8643] MUST be applied.
The SDP examples below are expressed without the security additions
for simplicity. EDITOR NOTE - more about security------
SDP offer/answer examples
Offer example for just multi-party capability:
m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Answer example from a multi-party capable device
m=text 12000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Offer example for both traditional "text/red" and multi-party format:
m=text 11000 RTP/AVP 101 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=rtpmap:101 rex/1000
a=fmtp:100 98/98/98
a=fmtp:101 98/98/98
Answer example from a multi-party capable device
m=text 11000 RTP/AVP 101 98
a=rtpmap:98 t140/1000
a=rtpmap:101 rex/1000
a=fmtp:101 98/98/98
Answer example from a multi-party unaware device:
m=text 12000 RTP/AVP 100 98
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
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A party who has negotiated the "text/rex" format MUST populate the
CSRC-list and format the packets according to the present
specification if it acts as an rtp-mixer and sends multi-party text.
A party who has negotiated the "text/rex" capability MUST interpret
the contents of the CSRC-list and the packets according to the
present specification in received rtp packets using the corresponding
payload type.
A party performing as a mixer, which has not negotiated the "text/
rex" format, but negotiated a "text/red" or "text/t140" format in a
session with a participant SHOULD, if nothing else is specified for
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 13.2.
11. 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. Pn
indicates primary data in source group "n". Rn1 is first redundant
generation data and Rn2 is second redundant generation data in source
group "n". 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 |
|CC=1 |
|CSRC list A |
|R12: Empty |
|R11: Empty |
|P1: A1 |
|----------------|
Assuming that earlier packets were received in sequence, text A1 is
received from packet 1 and assigned to reception area A.
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|----------------|
|Seq no 2 |
|CC=3 |
|CSRC list C,A |
|R12 Empty |
|R11:Empty |
|P1: C1 |
|R22 Empty |
|R21: A1 |
|P2: Empty |
|----------------|
Text C1 is received from packet 2 and assigned to reception area C.
X----------------|
X Seq no 3 |
X CC=2 |
X CSRC list C,A |
X R12: Empty |
X R11: C1 |
X P1: Empty |
X R22: A1 |
X R21: Empty |
X P2: A2 |
X----------------|
Packet 3 is assumed to be dropped in network problems
X----------------|
X Seq no 4 |
X CC=3 |
X CSRC list C,B,A|
X R12: Empty |
X R11: Empty |
X P1: C2 |
X R22: Empty |
X R21: Empty |
X P2: B1 |
X R32: Empty |
X R31: A2 |
X P3: A3 |
X----------------|
Packet 4 is assumed to be dropped in network problems
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X----------------|
X Seq no 5 |
X CC=3 |
X CSRC list C,B,A|
X R12: Empty |
X R11: C2 |
X P1: Empty |
X R22: Empty |
X R21: B1 |
X P2: B2 |
X R32: A2 |
X R31: A3 |
X P3: A4 |
X----------------|
Packet 5 is assumed to be dropped in network problems
|----------------|
|Seq no 6 |
|CC=3 |
|CSRC list C,B,A |
| R12: C2 |
| R11: Empty |
| P1: Empty |
| R22: B1 |
| R21: B2 |
| P2: B3 |
| R32: A3 |
| R31: A4 |
| P3: A5 |
|----------------|
Packet 6 is received. The latest received sequence number was 2.
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 common mixer reception area.
For seqno 4, texts C2, B1 and A3 are recovered and appended to
their respective reception areas.
For seqno 5, texts B2 and A4 are recovered and appended to their
respective reception areas.
Primary text B3 and A5 are received and appended to their
respective reception areas.
With only one or two packets lost, there would not be any need to
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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12. Performance considerations
This specification allows new text from 16 sources per packet.
Packets SHOULD be transmitted with a maximum time interval when there
is text to be transmitted (either primary or redundant) . The default
maximum transmission interval is 300 ms. It is also RECOMMENDED 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 32 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 the default for the "text/rex"
format. See RFC 4103 [RFC4103] for the format and use of the CPS
parameter. The same rules apply for the "text/rex" format except for
the default value.
13. 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."
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Strict application of T.140 [T140] is of essence for the
interoperability of real-time text implementations and to fulfill the
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.
13.1. 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.
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A view of a three-party RTT call in chat style is shown in this
example .
_________________________________________________
| | |
|[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 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.
13.2. 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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13.2.1. 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.
13.2.2. 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.
13.2.3. Actions on transmission of text
Text from a source sent to the recipient SHOULD increase the display
count by one per transmitted character.
13.2.4. 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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13.2.5. 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.
13.2.6. 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.
New text can be presented only from one source at a time. Switch of
source to be presented takes place at suitable places in the text,
such as end of phrase, end of sentence, line separator and
inactivity. Therefore the time to switch to present waiting text
from other sources may become long and will vary and depend on the
actions of the currently presented source.
Erasure can only be done up to the latest source switch. If a user
tries to erase more text, the erasing actions will be presented as
letter X after the label.
These facts makes it strongly RECOMMENDED to implement multi-party
awareness in RTT endpoints. The use of the mixing method for multi-
party-unaware endpoints should be left for use with endpoints which
are impossible to upgrade to become multi-party aware.
13.2.7. Example views of presentation on multi-party unaware endpoints
The following pictures are examples of the view on a participant's
display for the multi-party-unaware case.
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_________________________________________________
| 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 |
|________________________|_________________________|
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.
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14. 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).
Gateway operation to real-time text in WebRTC may also be required.
In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data-
channel[I-D.ietf-mmusic-t140-usage-data-channel]. EDITOR NOTE -----
more about this is needed------------
15. Updates to RFC 4102 and RFC 4103
The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
introducing an extended packet format 'text/rex' for the multi-party
mixing case and more strict rules for the use of redundancy, and
population of the CSRC list in the packets. Implications for the
CSRC list use from RFC 2198[RFC2198] are hereby not in effect.
16. Congestion considerations
The congestion considerations and recommended actions from RFC 4103
[RFC4103] are valid also in multi-party situations.
17. Acknowledgements
James Hamlin for format input.
18. IANA Considerations
The IANA is requested to register the media type registration "text/
rex" as specified in Section 9. The media type is also requested to
be added to the IANA registry for "RTP Payload Format Media Types"
<http://www.iana.org/assignments/rtp-parameters>.
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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 to draft-ietf-avtcore-multi-party-rtt-mix-01
2,5,6 More efficient format text/rex introduced and attribute a=rtt-
mix deleted.
3. Brief about use of OSRTP for security included- More needed.
4. Brief motivation for the solution and why not rtp-translator is
used added to intro.
7. More limitations for the multi-party unaware mixing method
inserted.
8. Updates to RFC 4102 and 4103 more clearly expressed.
9. Gateway to WebRTC started. More needed.
20.2. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to
draft-ietf-avtcore-multi-party-rtt-mix-00
Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00
Replaced CDATA in IANA registration table with better coding.
Converted to xml2rfc version 3.
20.3. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to
-03
Changed company and e-mail of the author.
Changed title to "RTP-mixer formatting of multi-party Real-time text"
to better match contents.
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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.
20.4. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-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.
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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
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.
20.5. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-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
[I-D.ietf-mmusic-t140-usage-data-channel]
Holmberg, C. and G. Hellstrom, "T.140 Real-time Text
Conversation over WebRTC Data Channels", Work in Progress,
Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel-
14, 10 April 2020, <https://tools.ietf.org/html/draft-
ietf-mmusic-t140-usage-data-channel-14>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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>.
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Internet-Draft RTP-mixer format for multi-party RTT May 2020
[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>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<https://www.rfc-editor.org/info/rfc4855>.
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263,
DOI 10.17487/RFC6263, June 2011,
<https://www.rfc-editor.org/info/rfc6263>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
Stach, "An Opportunistic Approach for Secure Real-time
Transport Protocol (OSRTP)", RFC 8643,
DOI 10.17487/RFC8643, August 2019,
<https://www.rfc-editor.org/info/rfc8643>.
[T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
multimedia application text conversation", February 1998,
<https://www.itu.int/rec/T-REC-T.140-199802-I/en>.
[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>.
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Internet-Draft RTP-mixer format for multi-party RTT May 2020
[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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