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Indicating source of multi-party Real-time text
draft-hellstrom-avtcore-multi-party-rtt-source-00

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Document	Type	This is an older version of an Internet-Draft whose latest revision state is "Replaced".
	Author	Gunnar Hellstrom
	Last updated	2020-02-26
	Replaced by	draft-ietf-avtcore-multi-party-rtt-mix, draft-ietf-avtcore-multi-party-rtt-mix, RFC 9071
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draft-hellstrom-avtcore-multi-party-rtt-source-00

Network Working Group GH. Hellstrom
Internet-Draft Omnitor
Updates: RFC 4102, RFC 4103 (if February 26, 2020
approved)
Intended status: Standards Track
Expires: August 29, 2020

Indicating source of multi-party Real-time text
draft-hellstrom-avtcore-multi-party-rtt-source-00

Abstract

Real-time text mixers need to identify the source of each transmitted
text chunk so that it can be presented in suitable grouping with
other text from the same source. An enhancement for RFC 4103 real-
time text is provided, suitable for a centralized conference model
that enables source identification, and provision of text mixers and
conference-enabled participants. 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 a format
parameter "rtp-mix".

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/.

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 August 29, 2020.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents

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(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
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Intended application . . . . . . . . . . . . . . . . . . . . 3
4. Use of fields in the RTP packets . . . . . . . . . . . . . . 3
5. Actions at transmission by a mixer . . . . . . . . . . . . . 4
6. Actions at reception . . . . . . . . . . . . . . . . . . . . 5
7. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Chained operation . . . . . . . . . . . . . . . . . . . . . . 6
9. Use with SIP centralized conferencing framework . . . . . . . 6
10. Usage without redundancy . . . . . . . . . . . . . . . . . . 7
11. SDP considerations . . . . . . . . . . . . . . . . . . . . . 7
12. Capability negotiation . . . . . . . . . . . . . . . . . . . 7
13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
14. Performance considerations . . . . . . . . . . . . . . . . . 9
15. Presentation level considerations . . . . . . . . . . . . . . 9
16. Congestion considerations . . . . . . . . . . . . . . . . . . 10
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
18.1. Addition of the rtp-mix fmtp-parameter to the text/red
format . . . . . . . . . . . . . . . . . . . . . . . . . 10
18.2. Addition of the rtp-mix fmtp-parameter to the text/t140
format . . . . . . . . . . . . . . . . . . . . . . . . . 11
19. Security Considerations . . . . . . . . . . . . . . . . . . . 11
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
20.1. Normative References . . . . . . . . . . . . . . . . . . 12
20.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550] for
transmission of RTT and the text/t140 format. It also specifies a
redundancy scheme text/red for increased robustness. RFC 4102
[RFC4102] registers the text/red format. The redundancy scheme
enables very 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

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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 need to make sure that the receiver can assign the
received text to the proper sources for presentation. Therefore, in
order to switch source, 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). 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.

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 participants and mixers.

4. Use of fields in the RTP packets

RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
redundancy scheme 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 to enhance the use in multi-party RTT sessions.

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 generation T140blocks included in the

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packet. ( the recommended level of redundancy is to use one primary
and two redundant generations of T140blocks.) In some cases, the
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.

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 | Note CC=3 for three sources in this packet.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CSRC list member 1 = SSRC of source of "P" | This is the CSRC list.
| 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. 300 ms is the default transmission interval,
but shorter intervals may be considered for specific cases. The
transmitter has its own SSRC, and its own RTP sequence number series.
At time of transmission, the RTP packet SHALL be populated with next
T140block queued for transmission from any of the active sources.
This T140block shall be placed in the primary area of the packet.
When performing as a mixer, the SSRC of its source shall be placed as
the first member in the CSRC-list. The current time is inserted and
the timestamp.

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If no unsent T140blocks were available, at this time, but T140blocks
available which have not been yet been sent the full intended number
of redundant transmissions, then The primary T140block is created by
an empty T140block, and populated in a packet for transmission. The
SSRC of the transmitter 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 now 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. (usually one or two times the transmission interval).

The number of members in the CSRC list shall be place in the CC
header 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 if a keep-alive method calls for transmission of keep-
alive packets.

6. Actions at reception

The enhanced text/red receiver shall receive RTP packets in the
single stream from the transmitter, and distribute the T140blocks for
presentation in presentation areas for each source. Other tasks for
receivers, such as gateways or chained mixers are also feasible.

The CC field of the received packets indicate the used level of
redundancy for the current packet.

The RTP sequence numbers of the received packets are monitored for
gaps or packets out of order.

As long as the sequence is correct, each packet is unpacked in order.
The T140blocks are extracted from the primary area, and the
corresponding SSRC is extracted from the first position in the CSRC

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list and used for assigning the new T140block to the correct input
area (or correspondingly).

If a sequence number gap appears and is still there after some short
defined time for jitter resolution, T140data needs to be recovered
from redundant data. If the gap is wider than the number of
generations of redundant T140blocks in the packet, then a t140block
is 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 are retrieved beginning with the
highest redundant generation, grouping them with the corresponding
SSRC from the CSRC-list and assigning them to further processing per
source. Finally the primary T140block is retrieved from the packet
and similarly its source retrieved from the first position in the
CSRC-list, and then assigned to the corresponding input handling for
that source.

If the sequence number gap was equal or less than the number of
redundancy generations in the received packet, no missing text marker
shall be inserted, and instead the T140blocks and their SSRCs
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 should be deleted on reception.

Note that empty T140blocks are sometimes included in the packets.
They just do not provide any contents.

7. RTCP considerations

A mixer should send RTCP reports with SDES, CNAME and NAME
information about the sources in the conference. This makes it
possible for participants to compose a suitable label for text from
each source. This is as an alternative to what may provided in the
conference notification.

8. Chained operation

By strictly applying the rules for CSRC-list population by all
conforming devices, mixers can be arranged in chains.

9. 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. Even if the RTT
stream is one and the same during the conference, the participants

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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
preparation of an SSRC to label translation for presentation to other
users.

10. Usage without redundancy

The CSRC list member should be used as source indicator also for
cases when the text/t140 format is used. 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 applies
except the redundant generations in transmission.

For the use case without redundancy, the CC field in the RTP packet
shall have the value 1, and the CSRC list contain one member.

11. SDP considerations

A new FMTP parameter for the text/red and text/t140 formats is
defined here, named "rtp-mix", without any parameter.

Syntax, shown together with the other fmtp parameters of text/red.

a=fmtp:<format> cps=<integer>;rtp-mix

The "cps" and "rtp-mix" parameters may appear in any order.

12. Capability negotiation

There are RTT implementations which implement RFC 4103 [RFC4103] but
not the present specification. Sending mixed text according to the
present specification to a device implementing only RFC 4103
[RFC4103] would lead to unreadable output. Therefore, in order to
negotiate RTT Mixing capability according to the present
specification, All devices supporting the present specification shall
include this fmtp parameter in offers and answers. Multi-party
streams using the coding in this specification must not be sent to
devices who have not negotiated the rtp-mix capability.

Format, as added to the format for the text/red fmtp parameters.

a=fmtp:<format> cps=<integer>;rtp-mix

Implementations not understanding this parameter should ignore it
according to common SDP rules.

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13. Examples

A symbolic flow of packets from a mixer is shown with loss and
recovery.

----------------
Seq no 1
CSRC list A,M,B
P: A1
R1: Empty
R2 B99
----------------

Assuming that earlier packets were received in sequence, text A1 is
received and assigned to reception area A.

----------------
Seq no 2
CC=3
CSRC list C,A,M
P: C1
R1: A1
R2 Empty
----------------
Text C1 is received and assigned to reception area C.

----------------
X Seq no 3
X CC=3
X CSRC list A,C,A
X P: A2
X R1: C1
X R2: A1
----------------
Assumed to be dropped in network problems

----------------
X Seq no 4
X CC=3
X CSRC list B,A,C
X P: B1
X R1: A2
X R2: C1
----------------
Assumed to be dropped in network problems

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----------------
X Seq no 5
X CC=3
X CSRC list A,B,A
X P: A3
X R1: B1
X R2: A2
----------------
Assumed to be dropped in network problems

----------------
Seq no 6
CC=3
CSRC list C,A,B
P: C2
R1: A3
R2: B1
----------------
Latest received sequence number was 2. Recovery is tried for 3,4,5.
But there is no coverage for seq no 3. A missing text mark is created and assigned to the mixer reception area.
For seqno 4, text B1 is recovered and assigned to reception area B.
For seqno 5, text A3 is recovered and assigned to reception area A.
Primary text C2 is received and assigned 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.

14. Performance considerations

This specification allows new text from one source per packet.
Packets are transmitted with timed intervals. The default
transmission interval is 300 ms for RFC 4103[RFC4103], and is
suitable for transmission from single sources. However when more
sources contribute to the flow, a shorter transmission interval may
be applicable. The transmission interval is therefore recommended to
be 100 ms for mixers. This interval provides for smooth flow of text
from 5 sources simultaneously.

15. 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 and this general statement for the
presentation:

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"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."

There is no strict "message" concept in real-time text. Line
separator is used as a separator allowing a part of received text to
be grouped in presentation. The receiving party may separate
presentation of parts of text from a source in readable groups based
on other criteria when it benefits the user to most easily find new
text or correlated text from different parties.

Further presentation level considerations are out of scope for this
document.

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.]

18.1. Addition of the rtp-mix fmtp-parameter to the text/red format

This document adds the format parameter "rtp-mix" to the parameters
for the a=fmtp attribute of the text/red format. The parameter has
no values. It is an indication of the capability to use the rtp-
mixer format in a real-time text session. The parameter can occur
with other fmtp parameters in any order. The usage is specified in
section "negotiation" above. The usage adds to the usage of the fmtp
attribute defined in RFC 4102 [RFC4102] and RFC 4103 [RFC4103].

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18.2. Addition of the rtp-mix fmtp-parameter to the text/t140 format

This document adds the format parameter "rtp-mix" to the parameters
for the a=fmtp attribute of the text/t140 format. The parameter has
no values. It is an indication of the capability to use the rtp-
mixer format in a real-time text session. The parameter can occur
with other fmtp parameters in any order. The usage is specified in
section "negotiation" above. The usage adds to the usage of the fmtp
attribute defined in RFC 4102 and RFC 4103.

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.

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20. References

20.1. 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>.

[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.

[T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
Protocol for multimedia application text conversation",
February 2000.

20.2. 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>.

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Author's Address

Gunnar Hellstrom
Omnitor
Esplanaden 30
Vendelso 13670
Sweden

Phone: +46708204288
Email: gunnar.hellstrom@omnitor.se

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Indicating source of multi-party Real-time text
draft-hellstrom-avtcore-multi-party-rtt-source-00