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AVTCore                                                     G. Hellstrom
Internet-Draft                 Gunnar Hellstrom Accessible Communication
Updates: RFC 4102, RFC 4103 (if approved)                     7 May 2020
Intended status: Standards Track
Expires: 8 November 2020


           RTP-mixer formatting of multi-party Real-time text
               draft-ietf-avtcore-multi-party-rtt-mix-00

Abstract

   Real-time text mixers for multi-party sessions need to identify the
   source of each transmitted group of text so that the text can be
   presented by endpoints in suitable grouping with other text from the
   same source.

   Regional regulatory requirements specify provision of real-time text
   in multi-party calls.  RFC 4103 mixer implementations can use
   traditional RTP functions for source identification, but the mixer
   source switching performance is limited when using the default
   transmission with redundancy.

   An enhancement for RFC 4103 real-time text mixing is provided in the
   present specification, suitable for a centralized conference model
   that enables source identification and efficient source switching.
   The intended use is for real-time text mixers and multi-party-aware
   participant endpoints.  The mechanism builds on use of the CSRC list
   in the RTP packet.

   A capability exchange is specified so that it can be verified that a
   participant can handle the multi-party coded real-time text stream.
   The capability is indicated by an sdp media attribute "rtt-mix".

   A brief description about how a mixer can format text for the case
   when the endpoint is not multi-party aware is also provided.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.





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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 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
   2.  Nomenclature  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Intended application  . . . . . . . . . . . . . . . . . . . .   4
   4.  Use of fields in the RTP packets  . . . . . . . . . . . . . .   4
   5.  Actions at transmission by a mixer  . . . . . . . . . . . . .   5
   6.  Actions at reception  . . . . . . . . . . . . . . . . . . . .   7
   7.  RTCP considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Chained operation . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Usage without redundancy  . . . . . . . . . . . . . . . . . .   8
   10. Use with SIP centralized conferencing framework . . . . . . .   8
   11. SDP Capability negotiation  . . . . . . . . . . . . . . . . .   9
   12. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   13. Performance considerations  . . . . . . . . . . . . . . . . .  12
   14. Presentation level considerations . . . . . . . . . . . . . .  12
     14.1.  Presentation by multi-party aware endpoints  . . . . . .  13
     14.2.  Multi-party mixing for multi-party unaware endpoints . .  15
   15. Gateway Considerations  . . . . . . . . . . . . . . . . . . .  20
   16. Congestion considerations . . . . . . . . . . . . . . . . . .  21
   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   20. Change history  . . . . . . . . . . . . . . . . . . . . . . .  22
     20.1.  Changes from
            draft-hellstrom-avtcore-multi-party-rtt-source-03 to
            draft-ietf-avtcore-multi-party-rtt-mix-00  . . . . . . .  22



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     20.2.  Changes from
            draft-hellstrom-avtcore-multi-party-rtt-source-02 to
            -03  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     20.3.  Changes from
            draft-hellstrom-avtcore-multi-party-rtt-source-01 to
            -02  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     20.4.  Changes from
            draft-hellstrom-avtcore-multi-party-rtt-source-00 to
            -01  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
   21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     21.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     21.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for
   transmission of real-time text (RTT) and the "text/t140" format.  It
   also specifies a redundancy format "text/red" for increased
   robustness.  RFC 4102 [RFC4102] registers the "text/red" format.
   Regional regulatory requirements specify provision of real-time text
   in multi-party calls.

   The redundancy scheme enables efficient transmission of redundant
   text in packets together with new text.  However the redundant header
   format has no source indicators for the redundant transmissions.  An
   assumption has had to be made that the redundant parts in a packet
   are from the same source as the new text.  The recommended
   transmission is one new and two redundant generations of text
   (T140blocks) in each packet and the recommended transmission interval
   is 300 ms.

   A mixer, selecting between text input from different sources and
   transmitting it in a common stream needs to make sure that the
   receiver can assign the received text to the proper sources for
   presentation.  Therefore, without any extra rule for source
   identification, the mixer needs to stop sending new text from that
   source and then make sure that all text so far has been sent with all
   intended redundancy levels (usually two) before switching source.
   That causes the very long time of one second to switch between
   transmission of text from one source to text from another source.
   Both the total throughput and the switching performance in the mixer
   is too low for most applications.

   A more efficient source identification scheme requires that each
   redundant T140block has its source individually preserved.  The
   present specification introduces a source indicator by specific rules
   for populating the CSRC-list in the RTP-packet.



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   A negotiation mechanism is also introduced for verification that the
   receiver is able to handle the multi-party coded stream.

   A fall-back mixing procedure is specified for cases when the
   negotiation fails.

2.  Nomenclature

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained
   in [RFC3550]

   The term "T140block" is defined in RFC 4103 [RFC4103] to contain one
   or more T.140 code elements.

3.  Intended application

   The scheme for identification of source of redundant transmissions is
   intended for transmission from entities taking the mixer role in
   centralised mixing configurations for RTT.  It is intended for
   reception by both endpoints and mixers.

4.  Use of fields in the RTP packets

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
   redundancy format "text/red" for increased robustness.  This
   specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
   introducing a rule for populating and using the CSRC-list in the RTP
   packet in order to enhance the performance in multi-party RTT
   sessions.

   When transmitted from a mixer, the first member in the CSRC-list
   SHALL contain the SSRC of the source of the primary T140block in the
   packet.  The second and further members in the CSRC-list SHALL
   contain the SSRC of the source of the first, second, etc redundant
   generations of T140blocks included in the packet. ( the recommended
   level of redundancy is to use one primary and two redundant
   generations of T140blocks.)  In some cases, a primary or redundant
   T140block is empty, but is still represented by a member in the
   redundancy header.  For such cases, the corresponding CSRC-list
   member MUST also be included.

   The CC field SHALL show the number of members in the CSRC list.





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   Note: This specification departs from section 4 of RFC 2198 [RFC2198]
   which associates the whole of the CSRC-list with the primary data and
   assumes that the same list applies to reconstructed redundant data.
   In the present specification a T140block is associated with exactly
   one CSRC list member as described above.  Also RFC 2198 [RFC2198]
   anticipates infrequent change to CSRCs; implementers should be aware
   that the order of the CSRC-list according to this specification will
   vary during transitions between transmission from the mixer of text
   originated by different participants.

   The picture below shows a typical RTP packet with multi-party RTT
   contents and coding according to the present specification.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X| CC=3  |M|  "RED" PT   |   sequence number of primary  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               timestamp of primary encoding "P"               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |  CSRC list member 1 = SSRC of source of "P"                   |
      |  CSRC list member 2 = SSRC of source of "R1"                  |
      |  CSRC list member 3 = SSRC of source of "R2"                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |  timestamp offset of "R2" | "R2" block length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |  timestamp offset of "R1" | "R1" block length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   T140 PT   | "R2" T.140 encoded redundant data             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
      |   |  "R1" T.140 encoded redundant data        |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         +-+-+-+
      |              "P" T.140 encoded primary data             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Figure 1: text/red packet with sources indicated in the CSRC-list.

5.  Actions at transmission by a mixer

   A "text/red" transmitter is usually sending packets at a regular
   transmission interval as long as there is something (new or redundant
   T140blocks) to transmit.  The default transmission interval for
   point-to-point operation is 300 ms.

   For multi-party operation, the transmission interval from mixers
   SHOULD be decreased to 100 ms for periods when there is text
   available for transmission from more than two sources.  It is also



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   allowed for the mixer to send a packet as soon as text has been
   received from a source as long as the maximum number of characters
   per second indicated by the recipient is not exceeded, and also the
   number of packets sent per second to a recipient is kept under a
   specified number.  This number SHALL be 10 if no other limit is
   applied for the application.  The mixer has its own SSRC, and its own
   RTP sequence number series.  At time of transmission, the mixer SHALL
   populate the RTP packet with a T140block combined from all T140blocks
   queued for transmission originating from next source in turn for
   getting its text transmitted as long as this is not in conflict with
   the allowed number of characters per second.  This T140block shall be
   placed in the primary area of the packet.  The SSRC of its source
   shall be placed as the first member in the CSRC-list.  The current
   time SHALL be inserted in the timestamp.

   If no unsent T140blocks were available at this time, but T140blocks
   are available which have not yet been sent the full intended number
   of redundant transmissions, then the primary T140block is composed of
   an empty T140block, and populated (without taking up any length) in a
   packet for transmission.  The SSRC of the mixer is included in the
   first place of the CSRC-list.

   The primary T140block, in the latest transmission is used to populate
   the first redundant T140block, and its source is placed as the second
   member of the CSRC-list.  The first redundant T140block from the
   latest transmission is placed as the second redundant T140block, and
   the corresponding CSRC placed in its place in the CSRC-list.

   Usually this is the level of redundancy used.  If a higher number of
   redundancy is used, then the procedure is maintained until all
   available redundant levels of T140blocks and their sources are placed
   in the packet.  If a receiver has negotiated a lower number of text/
   red generations, then that level shall be the maximum used by the
   transmitter.

   The timer offset values are inserted in the redundancy header, with
   the time offset from when the corresponding T140block was sent as
   original.

   The number of members in the CSRC list shall be placed in the "CC"
   header field.  Only mixers place values >0 in the "CC" field.

   When there is no new T140block to transmit, and no redundant
   T140block that has not been retransmitted the intended number of
   times, the transmission process can stop until either new T140blocks
   arrive, or a keep-alive method calls for transmission of keep-alive
   packets.




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6.  Actions at reception

   The enhanced "text/red" receiver included in an endpoint with
   presentation functions will receive RTP packets in the single stream
   from the mixer, and shall distribute the T140blocks for presentation
   in presentation areas for each source.  Other tasks for receivers,
   such as gateways or chained mixers are also feasible, and requires
   consideration if the stream shall just be forwarded, or a
   distribution based on different sources is needed.

   If the "CC" field value of a received packet is >1, it indicates the
   used level of redundancy for the current packet and that the enhanced
   packet format is used.  If the CC field value is 0 or 1, it indicates
   that the enhanced format is not used.  If the CC value is 1, the CSRC
   indicates the source of primary data.

   The RTP sequence numbers of the received packets SHALL be monitored
   for gaps and packets out of order.

   As long as the sequence is correct, each packet SHALL be unpacked in
   order.  The T140blocks SHALL be extracted from the primary area, and
   the corresponding SSRC SHALL be extracted from the first position in
   the CSRC list and used for assigning the new T140block to the correct
   presentation area (or correspondingly).

   If a sequence number gap appears and is still there after some
   defined time for jitter resolution, T140data SHALL be recovered from
   redundant data.  If the gap is wider than the number of generations
   of redundant T140blocks in the packet, then a t140block SHALL be
   created with a marker for text loss [T140ad1] and assigned to the
   SSRC of the transmitter as a general input from the mixer.

   Then, the T140blocks in the received packet SHALL be retrieved
   beginning with the highest redundant generation, grouping them with
   the corresponding SSRC from the CSRC-list and assigning them to the
   presentation areas per source.  Finally the primary T140block SHALL
   be retrieved from the packet and similarly its source retrieved from
   the first position in the CSRC-list, and then assigned to the
   corresponding presentation area for that source.

   If the sequence number gap was equal to or less than the number of
   redundancy generations in the received packet, a missing text marker
   SHALL NOT be inserted, and instead the T140blocks and their SSRCs
   fully recovered from the redundancy information and the CSRC-list in
   the way indicated above.

   Unicode character BOM is sometimes used as a filler or keep alive by
   transmission implementations.  These SHALL be deleted on reception.



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

7.  RTCP considerations

   A mixer SHALL send RTCP reports with SDES, CNAME and NAME information
   about the sources in the multi-party call.  This makes it possible
   for participants to compose a suitable label for text from each
   source.

8.  Chained operation

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

9.  Usage without redundancy

   The CSRC list member SHALL be used as source indicator also for cases
   when the "text/t140" format is used from a mixer.  That MAY be the
   case when robustness in transmission is provided by some other means
   than by redundancy and the "text/red" format.  All aspects of this
   memo SHALL be applied except the redundant generations in
   transmission.

   For the use case without redundancy using the "text/t140" format, the
   "CC" field in the RTP packet shall have the value 1, and the CSRC
   list SHALL contain one member when sent from a mixer.

   The "text/red" format SHOULD be used unless some other protection
   against packet loss is utilized, for example a reliable network or
   transport.

10.  Use with SIP centralized conferencing framework

   The SIP conferencing framework, mainly specified in RFC
   4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
   for coordinating sessions including multi-party RTT.  The RTT stream
   between the mixer and a participant is one and the same during the
   conference.  Participants get announced by notifications when
   participants are joining or leaving, and further user information may
   be provided.  The SSRC of the text to expect from joined users can be
   included in a notification.  This can be used both for security
   purposes and for translation to a label for presentation to other
   users.

   Note: The CSRC-list in an RTP packet only includes participants who's
   text is included in one or more text blocks.  It is not the same as
   the list of participants in a conference.  With audio and video



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   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.

11.  SDP Capability negotiation

   There are RTT implementations which implement RFC 4103 [RFC4103] but
   not the present specification for real-time awareness.  Sending mixed
   text according to the present specification to a device implementing
   only RFC 4103 [RFC4103] would lead to unreadable presented text.
   Therefore, in order to negotiate RTT mixing capability according to
   the present specification, all devices supporting the present
   specification for multi-party aware participants SHALL include an sdp
   media attribute "rtt-mix" in the sdp, indicating this capability in
   offers and answers.  Multi-party streams using the coding of the
   present specification intended for multi-party aware endpoints MUST
   NOT be sent to devices which have not indicated the "rtt-mix"
   capability.

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

   The sdp media attribute defined here, is named "rtt-mix".  It has no
   parameters.  It is intended to be used in "text" media descriptions
   with "text/red" and "text/t140" formats.  It indicates capability to
   use source indications in the CSRC list according to the present
   specification.  It also indicates ability to receive 10 real-time
   text packets per second.

   Syntax:

   a=rtt-mix

   The attribute is used in offer/answer procedures in a declarative
   way.  Both parties express their capability to use sources in the
   CSRC list as specified in the present specification.

   A party who has expressed the "rtt-mix" capability MUST populate the
   CSRC-list according to the present specification if it acts as an
   rtp-mixer and sends to a party who has expressed the "rtt-mix"
   capability.

   A party who has expressed the "rtt-mix" capability MUST interpret the
   contents of the CSRC-list according to the present specification in
   received rtp packets from parties who have expressed "rtt-mix"
   capability .





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   A party MUST NOT transmit packets with the format for multi-party
   aware participants according to the present specification to a party
   who has not expressed "rtt-mix" capability.

   A party performing as a mixer, which has expressed the "rtt-mix"
   capability, but not received "rtt-mix" capability indication in a
   session with a participant SHOULD, if nothing else is specified for
   the application, format transmitted text to that participant to be
   suitable to present on a multi-party unaware endpoint as further
   specified in section Section 14.2.

12.  Examples

   This example shows a symbolic flow of packets from a mixer with loss
   and recovery.  A, B and C are sources of RTT.  M is the mixer.  P
   indicates primary data.  R1 is first redundant generation data and R2
   is second redundant generation data.  A1, B1, A2 etc are text chunks
   (T140blocks) received from the respective sources.  X indicates
   dropped packet between the mixer and a receiver.

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

   Assuming that earlier packets were received in sequence, text A1 is
   received from packet 1 and assigned to reception area A.

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











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

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

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

   |----------------|
   |Seq no 6        |
   |CC=3            |
   |CSRC list C,A,B |
   |R2: B1          |
   |R1: A3          |
   |P: C2           |
   |----------------|
   Packet 6 is received. The latest received sequence number was 2.
   Recovery is therefore tried for 3,4,5. But there is no coverage
   for seq no 3. A missing text mark (U'FFFD) is created and
   appended to the mixer reception area.
   For seqno 4, text B1 is recovered and appended to reception area B.
   For seqno 5, text A3 is recovered and appended to reception area A.
   Primary text C2 is received and appended to reception area C.

   With only one or two packets lost, there would not be any need to
   create a missing text marker, and all text would be recovered.



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   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 SHOULD be transmitted with timed intervals.  The default
   transmission interval is 300 ms for RFC 4103[RFC4103], which is
   suitable for transmission from single sources.  However when more
   sources contribute to the flow, a shorter transmission interval MAY
   be applied.  The transmission interval is therefore RECOMMENDED to be
   100 ms for mixers when there is text from more than two sources
   available for transmission to the same recipient.It is also allowed
   for the mixer to send a packet as soon as text has been received from
   a source as long as the maximum number of characters per second
   indicated by the recipient is not exceeded, and also the number of
   packets sent per second to a recipient is kept under a number.  In
   order to achieve good performance, a receiver for multi-party calls
   SHOULD declare a sufficient CPS value in SDP for the number of
   allowable characters per second.  These characteristics provide for
   smooth flow of text with acceptable latency from at least 5 sources
   simultaneously.

   The default maximum rate of reception of real-time text is in RFC
   4103 [RFC4103] specified to be 30 characters per second.  The value
   MAY be modified in the CPS parameter of the FMTP attribute in the
   media section for RFC 4103.  A mixer combining real-time text from a
   number of sources may have a higher combined flow of text coming from
   the sources.  Endpoints SHOULD therefore specify a suitable higher
   value for the CPS parameter, corresponding to its real reception
   capability.  A value for CPS of 150 is RECOMMENDED because it would
   be suitable for reception in most multi-party sessions, and a
   realistic value for the capacity in most implementation environments.
   See RFC 4103 [RFC4103] for the format and use of the CPS parameter.

14.  Presentation level considerations

   ITU-T T.140 [T140] provides the presentation level requirements for
   the RFC 4103 [RFC4103] transport.  T.140 [T140] has functions for
   erasure and other formatting functions and has the following general
   statement for the presentation:

   "The display of text from the members of the conversation should be
   arranged so that the text from each participant is clearly readable,
   and its source and the relative timing of entered text is visualized
   in the display.  Mechanisms for looking back in the contents from the
   current session should be provided.  The text should be displayed as
   soon as it is received."



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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.

14.1.  Presentation by multi-party aware endpoints

   A multi-party aware receiving party, presenting real-time text MUST
   separate text from different sources and present them in separate
   presentation areas.  The receiving party MAY separate presentation of
   parts of text from a source in readable groups based on other
   criteria than line separator and merge these groups in the
   presentation area when it benefits the user to most easily find and
   read text from the different participants.  The criteria MAY e.g. be
   a received comma, full stop, or other phrase delimiters, or a long
   pause.

   When text is received from multiple original sources, the
   presentation SHOULD provide a view where text is added in multiple
   places simultaneously.

   If the presentation presents text from different sources in one
   common area, the presenting endpoint SHOULD insert text from the
   local user ended at suitable points merged with received text to
   indicate the relative timing for when the text groups were completed.
   In this presentation mode, the receiving endpoint SHALL present the
   source of the different groups of text.





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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 1: Example of a three-party call with chat style.

   Figure 2: Example of a three-party RTT call presented in chat style.

   Other presentation styles than the chat style may be arranged.

   This figure shows how a coordinated column view MAY be presented.












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   _____________________________________________________________________
   |       Bob          |       Eve            |       Alice           |
   |____________________|______________________|_______________________|
   |                    |                      |I will arrive by TGV.  |
   |My flight is to Orly|                      |Convenient to the main |
   |                    |Hi all, can we plan   |station.               |
   |                    |for the seminar?      |                       |
   |Eve, will you do    |                      |                       |
   |your presentation on|                      |                       |
   |Friday?             |Yes, Friday at 10.    |                       |
   |Fine, wo            |                      |We need to meet befo   |
   |___________________________________________________________________|

   Figure 3: An example of a coordinated column-view of a three-party
   session with entries ordered vertically in approximate time-order.

14.2.  Multi-party mixing for multi-party unaware endpoints

   When the mixer has indicated multi-party capability in an sdp
   negotiation, but the multi-party capability negotiation fails with an
   endpoint, then the mixer SHOULD compose a best-effort presentation of
   multi-party real-time text in one stream intended to be presented by
   an endpoint with no multi-party awareness.

   This presentation format has functional limitations and SHOULD be
   used only to enable participation in multi-party calls by legacy
   deployed endpoints.

   The principles and procedures below do not specify any new protocol
   elements or behaviors.  They are instead composed from the
   information in ITU-T T.140 [T140] and an ambition to provide a best
   effort presentation on an endpoint which has functions only for two-
   party calls.

   The mixer mixing for multi-party unaware endpoints SHALL compose a
   simulated limited multi-party RTT view suitable for presentation in
   one presentation area.  The mixer SHALL group text in suitable groups
   and prepare for presentation of them by inserting a new line betwwen
   them if the transmitted text did not already end with a new line.  A
   presentable label SHOULD be composed and sent for the source
   initially in the session and after each source switch.  With this
   procedure the time for source switching is depending on the actions
   of the users.  In order to expedite source switch, a user can for
   example end its turn with a new line.







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14.2.1.  Actions by the mixer at reception from the call participants

   When text is received by the mixer from the different participants,
   the mixer SHALL recover text from redundancy if any packets are lost.
   The mark for lost text [T140ad1] SHOULD be inserted in the stream if
   unrecoverable loss appears.  Any Unicode BOM characters, possibly
   used for keep-alive shall be deleted.  The time of arrival of text
   SHALL be stored together with the received text from each source in a
   queue for transmission to the recipients.

14.2.2.  Actions by the mixer for transmission to the recipients

   The following procedure SHOULD be applied for each recipient of
   multi-part text from the mixer.

   The text for transmission SHOULD be formatted by the mixer for each
   receiving user for presentation in one single presentation area.
   Text received from a participant SHOULD NOT be included in
   transmission to that participant.  When there is text available for
   transmission from the mixer to a receiving party from more than one
   participant, the mixer SHOULD switch between transmission of text
   from the different sources at suitable points in the transmitted
   stream.

   When switching source, the mixer SHOULD insert a line separator if
   the already transmitted text did not end with a new line (line
   separator or CRLF).  A label SHOULD be composed from information in
   the CNAME and NAME fields in RTCP reports from the participant to
   have its text transmitted, or from other session information for that
   user.  The label SHOULD be delimited by suitable characters (e.g. '[
   ]') and transmitted.  The CSRC SHOULD indicate the selected source.
   Then text from that selected participant SHOULD be transmitted until
   a new suitable point for switching source is reached.

   Seeking a suitable point for switching source SHOULD be done when
   there is older text waiting for transmission from any party than the
   age of the last transmitted text.  Suitable points for switching are:

   *  A completed phrase ended by comma

   *  A completed sentence

   *  A new line (line separator or CRLF)

   *  A long pause (e.g. > 10 seconds) in received text from the
      currently transmitted source





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   *  If text from one participant has been transmitted with text from
      other sources waiting for transmission for a long time (e.g. > 1
      minute) and none of the suitable points for switching has
      occurred, a source switch MAY be forced by the mixer at next word
      delimiter, and also if even a word delimiter does not occur within
      a time (e.g. 15 seconds) after the scan for word delimiter
      started.

   When switching source, the source which has the oldest text in queue
   SHOULD be selected to be transmitted.  A character display count
   SHOULD be maintained for the currently transmitted source, starting
   at zero after the label is transmitted for the currently transmitted
   source.

   There SHOULD be a storage for the latest control code for Select
   Graphic Rendition (SGR) from each source.  If there is an SGR code
   stored for the current source before the source switch is done, a
   reset of SGR shall be sent by the sequence SGR 0 [009B 0000 006D]
   after the new line and before the new label during a source switch.
   See SGR below for an explanation.  This transmission does not
   influence the display count.  If there is an SGR code stored for the
   new source after the source switch, that SGR code SHOULD be
   transmitted to the recipient before the label.  This transmission
   does not influence the display count.

14.2.3.  Actions on transmission of text

   Text from a source sent to the recipient SHOULD increase the display
   count by one per transmitted character.

14.2.4.  Actions on transmission of control codes

   The following control codes specified by T.140 require specific
   actions.  They SHOULD cause specific considerations in the mixer.
   Note that the codes presented here are expressed in UCS-16, while
   transmission is made in UTF-8 transform of these codes.

   BEL 0007 Bell  Alert in session, provides for alerting during an
      active session.  The display count SHOULD not be altered.

   NEW LINE 2028  Line separator.  Check and perform a source switch if
      appropriate.  Increase display count by 1.

   CR LF 000D 000A  A supported, but not preferred way of requesting a
      new line.  Check and perform a source switch if appropriate.
      Increase display count by 1.





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   INT ESC 0061  Interrupt (used to initiate mode negotiation
      procedure).  The display count SHOULD not be altered.

   SGR 009B Ps 006D  Select graphic rendition.  Ps is rendition
      parameters specified in ISO 6429.  The display count SHOULD not be
      altered.  The SGR code SHOULD be stored for the current source.

   SOS 0098  Start of string, used as a general protocol element
      introducer, followed by a maximum 256 bytes string and the ST.
      The display count SHOULD not be altered.

   ST 009C  String terminator, end of SOS string.  The display count
      SHOULD not be altered.

   ESC 001B  Escape - used in control strings.  The display count SHOULD
      not be altered for the complete escape code.

   Byte order mark FEFF  Zero width, no break space, used for
      synchronization and keep-alive.  SHOULD be deleted from incoming
      streams.  Shall be sent first after session establishment to the
      recipient.  The display count shall not be altered.

   Missing text mark FFFD  Replacement character, marks place in stream
      of possible text loss.  SHOULD be inserted by the reception
      procedure in case of unrecoverable loss of packets.  The display
      count SHOULD be increased by one when sent as for any other
      character.

   SGR  If a control code for selecting graphic rendition (SGR), other
      than reset of the graphic rendition (SGR 0) is sent to a
      recipient, that control code shall also be stored for the source
      in the storage for SGR.  If a reset graphic rendition (SGR 0)
      originated from a source is sent, then the SGR storage for that
      source shall be cleared.  The display count shall not be
      increased.

   BS 0008  Back Space, intended to erase the last entered character by
      a source.  Erasure by backspace cannot always be performed as the
      erasing party intended.  If an erasing action erases all text up
      to the end of the leading label after a source switch, then the
      mixer must not transmit more backspaces.  Instead it is
      RECOMMENDED that a letter "X" is inserted in the text stream for
      each backspace as an indication of the intent to erase more.  A
      new line is usually coded by a Line Separator, but the character
      combination "CRLF" MAY be used instead.  Erasure of a new line is
      in both cases done by just one erasing action (Backspace).  If the
      display count has a positive value it is decreased by one when the
      BS is sent.  If the display count is at zero, it is not altered.



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14.2.5.  Packet transmission

   A mixer transmitting to a multi-party unaware terminal SHOULD send
   primary data only from one source per packet.  The SSRC SHOULD be the
   SSRC of the mixer.  The CSRC list SHOULD contain one member and be
   the SSRC of the source of the primary data.

14.2.6.  Functional limitations

   When a multi-party unaware endpoint presents a conversation in one
   display area in a chat style, it inserts source indications for
   remote text and local user text as they are merged in completed text
   groups.  When an endpoint using this layout receives and presents
   text mixed for multi-party unaware endpoints, there will be two
   levels of source indicators for the received text; one generated by
   the mixer and inserted in a label after each source switch, and
   another generated by the receiving endpoint and inserted after each
   switch between local and remote source in the presentation area.
   This will waste display space and look inconsistent to the reader.

   This fact, combined with the slowness in source switching and the
   limited support of erasure makes it strongly RECOMMENDED to implement
   multi-party awareness in RTT endpoints.  The use of the mixing method
   for multi-party-unaware endpoints should be left for use with
   endpoints which are impossible to upgrade to become multi-party
   aware.

14.2.7.  Example views of presentation on multi-party unaware endpoints

   The following pictures are examples of the view on a participant's
   display for the multi-party-unaware case.


     _________________________________________________
    |       Conference       |          Alice          |
    |________________________|_________________________|
    |                        |I will arrive by TGV.    |
    |[Bob]:My flight is to   |Convenient to the main   |
    |Orly.                   |station.                 |
    |[Eve]:Hi all, can we    |                         |
    |plan for the seminar.   |                         |
    |                        |                         |
    |[Bob]:Eve, will you do  |                         |
    |your presentation on    |                         |
    |Friday?                 |                         |
    |[Eve]:Yes, Friday at 10.|                         |
    |[Bob]: Fine, wo         |We need to meet befo     |
    |________________________|_________________________|



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   Figure 4: Alice who has a conference-unaware client is receiving the
   multi-party real-time text in a single-stream.  This figure shows how
   a coordinated column view MAY be presented on Alice's device.

     _________________________________________________
    |                                              |^|
    |[Alice] Hi, Alice here.                       | |
    |                                              | |
    |[mix][Bob] Bob as well.                       | |
    |                                              | |
    |[Eve] Hi, this is Eve, calling from Paris     | |
    |      I thought you should be here.           | |
    |                                              | |
    |[Alice] I am coming on Thursday, my           | |
    |      performance is not until Friday morning.| |
    |                                              | |
    |[mix][Bob] And I on Wednesday evening.        | |
    |                                              | |
    |[Eve] we can have dinner and then walk        | |
    |                                              | |
    |[Eve] But I need to be back to                | |
    |    the hotel by 11 because I need            |-|
    |                                              |-|
    |______________________________________________|v|
    | of course, I underst                           |
    |________________________________________________|

   Figure 5: An example of a view of the multi-party unaware
   presentation in chat style.  Alice is the local user.

15.  Gateway Considerations

   Multi-party RTT sessions may involve gateways of different kinds.
   Gateways involved in setting up sessions SHALL correctly reflect the
   multi-party capability or unawareness of the combination of the
   gateway and the remote endpoint beyond the gateway.

   One case that may occur is a gateway to PSTN for communication with
   textphones (e.g.  TTYs).  Textphones are limited devices with no
   multi-party awareness, and it SHOULD therefore be suitable for the
   gateway to not indicate multi-party awareness for that case.  Another
   solution is that the gateway indicates multi-party capability towards
   the mixer, and includes the multi-party mixer function for multi-
   party unaware endpoints itself.  This solution makes it possible to
   make adaptations for the functional limitations of the textphone
   (TTY).





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16.  Congestion considerations

   The congestion considerations and recommended actions from RFC 4103
   [RFC4103] are valid also in multi-party situations.

17.  Acknowledgements

18.  IANA Considerations

   [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
   RFC number of this document.]

   IANA is asked to register the new sdp attribute "rtt-mix".

     +---------------------+----------------------------------------+
     | Contact name:       | IESG                                   |
     +---------------------+----------------------------------------+
     | Contact email:      | iesg@ietf.org                          |
     +---------------------+----------------------------------------+
     | Attribute name:     | rtt-mix                                |
     +---------------------+----------------------------------------+
     | Attribute syntax    | a=rtt-mix                              |
     +---------------------+----------------------------------------+
     | Attribute semantics | See RFCXXXX Section 11                 |
     +---------------------+----------------------------------------+
     | Attribute value     | -                                      |
     +---------------------+----------------------------------------+
     | Usage level:        | media                                  |
     +---------------------+----------------------------------------+
     | Purpose:            | Indicate support for the rtp-mixer     |
     |                     | format for real-time text transmission |
     +---------------------+----------------------------------------+
     | O/A procedure       | Declarative                            |
     +---------------------+----------------------------------------+
     | Mux Category        | normal                                 |
     +---------------------+----------------------------------------+
     | Reference:          | RFCXXXX                                |
     +---------------------+----------------------------------------+

                                 Table 1

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.




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   The requirement to transfer information about the user in RTCP
   reports in SDES, CNAME and NAME fields for creation of labels may
   have privacy concerns as already stated in RFC 3550 [RFC3550], and
   may be restricted of privacy reasons.  The receiving user will then
   get a more symbolic label for the source.

20.  Change history

20.1.  Changes from 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.

20.2.  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.

   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.

20.3.  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.



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   The presentation considerations chapter 14 is extended with more
   information about presentation on multi-party aware endpoints, and a
   new section on the multi-party unaware mixing with low functionality
   but SHOULD a be implemented in mixers.  Presentation examples are
   added.

   A short chapter 15 on gateway considerations is introduced.

   Clarification about the text/t140 format included in chapter 10.

   This sentence added to the chapter 10 about use without redundancy.
   "The text/red format SHOULD be used unless some other protection
   against packet loss is utilized, for example a reliable network or
   transport."

   Note about deviation from RFC 2198 added in chapter 4.

   In chapter 9.  "Use with SIP centralized conferencing framework" the
   following note is inserted: Note: The CSRC-list in an RTP packet only
   includes participants who's text is included in one or more text
   blocks.  It is not the same as the list of participants in a
   conference.  With audio and video media, the CSRC-list would often
   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.


20.4.  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

   [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>.




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   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse-
              Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
              DOI 10.17487/RFC2198, September 1997,
              <https://www.rfc-editor.org/info/rfc2198>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC4102]  Jones, P., "Registration of the text/red MIME Sub-Type",
              RFC 4102, DOI 10.17487/RFC4102, June 2005,
              <https://www.rfc-editor.org/info/rfc4102>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [T140]     ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
              multimedia application text conversation", February 1998,
              <https://www.itu.int/rec/T-REC-T.140-199802-I/en>.

   [T140ad1]  ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
              Protocol for multimedia application text conversation",
              February 2000,
              <https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>.

21.2.  Informative References

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,
              <https://www.rfc-editor.org/info/rfc4353>.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
              Session Initiation Protocol (SIP) Event Package for
              Conference State", RFC 4575, DOI 10.17487/RFC4575, August
              2006, <https://www.rfc-editor.org/info/rfc4575>.

   [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,
              <https://www.rfc-editor.org/info/rfc4579>.

Author's Address





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   Gunnar Hellstrom
   Gunnar Hellstrom Accessible Communication
   Esplanaden 30
   SE-13670 Vendelso
   Sweden

   Email: gunnar.hellstrom@ghaccess.se












































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