Network Working Group G. Hellstrom
Internet-Draft Omnitor
Updates: RFC 4102, RFC 4103 (if February 29, 2020
approved)
Intended status: Standards Track
Expires: September 1, 2020
Indicating source of multi-party Real-time text
draft-hellstrom-avtcore-multi-party-rtt-source-01
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, for use by 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 an sdp
media attribute "rtt-mix".
Status of This Memo
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This Internet-Draft will expire on September 1, 2020.
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document authors. All rights reserved.
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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 Capability negotiation . . . . . . . . . . . . . . . . . 7
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
13. Performance considerations . . . . . . . . . . . . . . . . . 9
14. Presentation level considerations . . . . . . . . . . . . . . 10
15. Congestion considerations . . . . . . . . . . . . . . . . . . 10
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
18. Security Considerations . . . . . . . . . . . . . . . . . . . 11
19. Change history . . . . . . . . . . . . . . . . . . . . . . . 11
19.1. Changes from version -00 to -01 . . . . . . . . . . . . 11
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
20.1. Normative References . . . . . . . . . . . . . . . . . . 11
20.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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. 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
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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, 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) in order to switch 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.
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 performance 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 redundant generation T140blocks included in
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the 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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. 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 placed 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.
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. The RTT stream
is one and the same during the conference. Participants get
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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 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 an sdp media attribute "rtt-mix" indicating this capability
in offers and answers. Multi-party streams using the coding of this
specification must not be sent to devices who have not indicated the
"rtt-mix" capability.
Implementations not understanding this parameter MUST ignore it
according to common SDP rules.
An sdp media attribute is defined here, named "rtt-mix", without any
parameter. 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 this
specification. 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 this specification.
A party who has expressed the "rtt-mix" capability MUST populate the
CSRC-list according to this specification if it acts as an rtp-mixer
and sends to a party who has expressed the "rtt-mix" capability.
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A party who has expressed the "rtt-mix" capability MUST interpret the
contents of the CSRC-list according to this specification in received
rtp packets from parties who have expressed "rtt-mix" capability .
A party MUST NOT transmit packets with redundancy format according to
this specification to a party who has not expressed "rtt-mix"
capability.
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 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 and assigned to reception area C.
----------------
X Seq no 3
X CC=3
X CSRC list A,C,A
X R2: A1
X R1: C1
X P: A2
----------------
Assumed to be dropped in network problems
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----------------
X Seq no 4
X CC=3
X CSRC list B,A,C
X R2: C1
X R1: A2
X P: B1
----------------
Assumed to be dropped in network problems
----------------
X Seq no 5
X CC=3
X CSRC list A,B,A
X R2: A2
X R1: B1
X P: A3
----------------
Assumed to be dropped in network problems
----------------
Seq no 6
CC=3
CSRC list C,A,B
R2: B1
R1: A3
P: C2
----------------
The latest received sequence number before 6 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 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.
13. 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
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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.
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 and has this 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."
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 (as comma, full stop, or other phrase delimiters,
or a long pause) 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.
15. Congestion considerations
The congestion considerations and recommended actions from RFC 4103
[RFC4103] are valid also in multi-party situations.
16. Acknowledgements
17. 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".
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+---------------------+------------------------------------------+
| Contact name: | IESG |
| Contact email: | iesg@ietf.org |
| Attribute name: | rtt-mix |
| Attribute syntax | a=rtt-mix |
| Attribute semantics | See RFCXXXX section |
| 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 |
+---------------------+------------------------------------------+
18. 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.
19. Change history
19.1. 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
20. References
20.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[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, .
[RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type",
RFC 4102, DOI 10.17487/RFC4102, June 2005,
.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
.
[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,
.
[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, .
[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,
.
Author's Address
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Gunnar Hellstrom
Omnitor
Esplanaden 30
Vendelso 13670
Sweden
Phone: +46708204288
Email: gunnar.hellstrom@omnitor.se
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