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AVTCore                                                     G. Hellstrom
Internet-Draft                 Gunnar Hellstrom Accessible Communication
Updates: RFC 4102, RFC 4103 (if approved)                   11 June 2020
Intended status: Standards Track
Expires: 13 December 2020


           RTP-mixer formatting of multi-party Real-time text
               draft-ietf-avtcore-multi-party-rtt-mix-06

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 this
   document, suitable for a centralized conference model that enables
   source identification and efficient source switching.  The intended
   use is for real-time text mixers and multi-party-aware participant
   endpoints.  The mechanism builds on use of the CSRC list in the RTP
   packet and an extended packet format "text/rex".

   A capability exchange is specified so that it can be verified that a
   participant can handle the multi-party coded real-time text stream.
   The capability is indicated by the media subtype "text/rex".

   The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103]

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

Status of This Memo

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

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



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

   This Internet-Draft will expire on 13 December 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Selected solution and considered alternative  . . . . . .   5
     1.2.  Nomenclature  . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Intended application  . . . . . . . . . . . . . . . . . .   6
   2.  Use of fields in the RTP packets  . . . . . . . . . . . . . .   6
   3.  Actions at transmission by a mixer  . . . . . . . . . . . . .   9
     3.1.  Initial BOM transmission  . . . . . . . . . . . . . . . .   9
     3.2.  Keep-alive  . . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Transmission interval . . . . . . . . . . . . . . . . . .  10
     3.4.  Do not send received text to the originating source . . .  10
     3.5.  Clean incoming text . . . . . . . . . . . . . . . . . . .  10
     3.6.  Redundancy  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.7.  Text placement in packets . . . . . . . . . . . . . . . .  10
     3.8.  Maximum number of sources per packet  . . . . . . . . . .  11
     3.9.  Empty T140blocks  . . . . . . . . . . . . . . . . . . . .  11
     3.10. Creation of the redundancy  . . . . . . . . . . . . . . .  11
     3.11. Timer offset fields . . . . . . . . . . . . . . . . . . .  12
     3.12. Other RTP header fields . . . . . . . . . . . . . . . . .  12
     3.13. Pause in transmission . . . . . . . . . . . . . . . . . .  12
   4.  Actions at reception  . . . . . . . . . . . . . . . . . . . .  12
     4.1.  Multi-party vs two-party use  . . . . . . . . . . . . . .  12
     4.2.  Level of redundancy . . . . . . . . . . . . . . . . . . .  13
     4.3.  Extracting text and handling recovery and loss  . . . . .  13
     4.4.  Delete BOM  . . . . . . . . . . . . . . . . . . . . . . .  14
     4.5.  Empty T140blocks  . . . . . . . . . . . . . . . . . . . .  14



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   5.  RTCP considerations . . . . . . . . . . . . . . . . . . . . .  14
   6.  Chained operation . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Usage without redundancy  . . . . . . . . . . . . . . . . . .  14
   8.  Use with SIP centralized conferencing framework . . . . . . .  15
   9.  Conference control  . . . . . . . . . . . . . . . . . . . . .  15
   10. Media Subtype Registration  . . . . . . . . . . . . . . . . .  15
   11. SDP considerations  . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Mapping of media parameters to sdp . . . . . . . . . . .  18
     11.2.  Security for session control and media . . . . . . . . .  18
     11.3.  SDP offer/answer examples  . . . . . . . . . . . . . . .  18
   12. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   13. Performance considerations  . . . . . . . . . . . . . . . . .  23
   14. Presentation level considerations . . . . . . . . . . . . . .  23
     14.1.  Presentation by multi-party aware endpoints  . . . . . .  24
     14.2.  Multi-party mixing for multi-party unaware endpoints . .  26
   15. Gateway Considerations  . . . . . . . . . . . . . . . . . . .  32
     15.1.  Gateway considerations with Textphones (e.g.  TTYs). . .  32
     15.2.  Gateway considerations with WebRTC.  . . . . . . . . . .  33
   16. Updates to RFC 4102 and RFC 4103  . . . . . . . . . . . . . .  33
   17. Congestion considerations . . . . . . . . . . . . . . . . . .  33
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  34
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   20. Security Considerations . . . . . . . . . . . . . . . . . . .  34
   21. Change history  . . . . . . . . . . . . . . . . . . . . . . .  34
     21.1.  Changes included in
             draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . .  34
     21.2.  Changes included in
             draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . .  34
     21.3.  Changes included in
             draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . .  35
     21.4.  Changes included in
             draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . .  35
     21.5.  Changes included in
             draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . .  36
     21.6.  Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . .  36
     21.7.  Changes from
             draft-hellstrom-avtcore-multi-party-rtt-source-03 to
             draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . .  36
     21.8.  Changes from
             draft-hellstrom-avtcore-multi-party-rtt-source-02 to
             -03 . . . . . . . . . . . . . . . . . . . . . . . . . .  37
     21.9.  Changes from
             draft-hellstrom-avtcore-multi-party-rtt-source-01 to
             -02 . . . . . . . . . . . . . . . . . . . . . . . . . .  37
     21.10. Changes from
             draft-hellstrom-avtcore-multi-party-rtt-source-00 to
             -01 . . . . . . . . . . . . . . . . . . . . . . . . . .  38
   22. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38



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     22.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     22.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  40

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.

   Real-time text is usually provided together with audio and sometimes
   with video in conversational sessions.

   The redundancy scheme of RFC 4103 [RFC4103] enables efficient
   transmission of redundant text in packets together with new text.
   However the redundant header format has no source indicators for the
   redundant transmissions.  An assumption has had to be made that the
   redundant parts in a packet are from the same source as the new text.
   The recommended transmission is one new and two redundant generations
   of text (T140blocks) in each packet and the recommended transmission
   interval is 300 ms.

   A mixer, selecting between text input from different sources and
   transmitting it in a common stream needs to make sure that the
   receiver can assign the received text to the proper sources for
   presentation.  Therefore, using RFC 4103 without any extra rule for
   source identification, the mixer needs to stop sending new text from
   one source and then make sure that all text so far has been sent with
   all intended redundancy levels (usually two) before switching to
   another 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.  This
   document introduces a source indicator by specific rules for
   populating the CSRC-list and the data header in the RTP-packet.

   An extended packet format "text/rex" is specified for this purpose,
   providing the possibility to include text from up to 16 sources in
   each packet in order to enhance mixer source switching performance.
   By these extensions, the performance requirements on multi-party
   mixing for real-time text are exceeded by the solution in this
   document.



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   A negotiation mechanism can therefore be based on selection between
   the "text/red" and the "text/rex" media formats for verification that
   the receiver is able to handle the multi-party coded stream.

   A fall-back mixing procedure is specified for cases when the
   negotiation results in "text/red" being the only common submedia
   format.

   The document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
   introducing an extended packet format for the multi-party mixing case
   and more strict rules for the source indications.

1.1.  Selected solution and considered alternative

   The mechanism specified in this document makes use of the RTP mixer
   model specified in RFC3550[RFC3550].  From some points of view, use
   of the RTP translator model specified in RFC 3550 would be more
   efficient, because then the text packets can pass the translator with
   only minor modification.  However, there may be a lack of support for
   the translator model in existing RTP implementations, and therefore
   the more common RTP-mixer model was selected.  The translator model
   would also easier cause congestion if many users send text
   simultaneously.

1.2.  Nomenclature

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

   The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC, RTCP, RTP-
   mixer, RTP-translator are explained in [RFC3550]

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

   "TTY" stands for a text telephone type used in North America.

   "WebRTC" stands for web based communication specified by W3C and
   IETF.

   "DTLS-SRTP" stnds for security specified in RFC 5764 [RFC5764].









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1.3.  Intended application

   The format for multi-party real-time text is primarily intended for
   use in transmission between mixers and endpoints in centralised
   mixing configurations.  It is also applicable between endpoints as
   well as between mixers.

2.  Use of fields in the RTP packets

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
   redundancy format "text/red" for increased robustness of real-time
   text transmission.  This document updates RFC 4102[RFC4102] and RFC
   4103[RFC4103] by introducing a format "text/rex" with a rule for
   populating and using the CSRC-list in the RTP packet and extending
   the redundancy header to be called a data header.  This is done in
   order to enhance the performance in multi-party RTT sessions.

   The "text/rex" format can be seen as an "n-tuple" of the "text/red"
   format intended to carry text information from up to 16 sources per
   packet.

   The CC field SHALL show the number of members in the CSRC list, which
   is one per source represented in the packet.

   When transmitted from a mixer, a CSRC list is included in the packet.
   The members in the CSRC-list SHALL contain the SSRCs of the sources
   of the T140blocks in the packet.  The order of the CSRC members MUST
   be the same as the order of the sources of the data header fields and
   the T140blocks.  When redundancy is used, text from all included
   sources MUST have the same number of redundant generations.  The
   primary, first redundant, second redundant and possible further
   redundant generations of T140blocks MUST be grouped per source in the
   packet in "source groups".  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 data header.

   The RTP header is followed by one or more source groups of data
   headers: one header for each text block to be included.  Each of
   these data headers provides the timestamp offset and length of the
   corresponding data block, in addition to the payload type number
   corresponding to the payload format "text/t140".  The data headers
   are followed by the data fields carrying T140blocks from the sources.








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     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|   block PT  |  timestamp offset         |   block length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: The bits in the data header.

   The bits in the data header are specified as follows:

   F:  1 bit First bit in header indicates whether another header block
      follows.  It has value 1 if further header blocks follow, and
      value 0 if this is the last header block.

   block PT:  7 bits RTP payload type number for this block,
      corresponding to the t140 payload type from the RTPMAP SDP
      attribute.

   timestamp offset:  14 bits Unsigned offset of timestamp of this block
      relative to the timestamp given in the RTP header.  The offset is
      a time to be subtracted from the current timestamp to determine
      the timestamp of the data when the latest part of this block was
      sent from the original source.  If the timestamp offset would be
      >15 000, it SHALL be set to 15 000.  For redundant data, the
      resulting time is the time when the data was sent as primary from
      the original source.  If the value would be >15 000, then it SHALL
      be set to 15 000 plus 300 times the redundancy level of the data.
      The high values appear only in exceptional cases, e.g. when some
      data has been held in order to keep the text flow under the
      Characters Per Second (CPS) limit.

   block length:  10 bits Length in bytes of the corresponding data
      block excluding the header.

   The header for the final block has a zero F bit, and apart from that
   the same fields as other data headers.















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   Note: This document has a packet format that is similar to that of
   RFC 2198 [RFC2198] but is different from some aspects.  RFC 2198
   associates the whole of the CSRC-list with the primary data and
   assumes that the same list applies to reconstructed redundant data.
   In this document 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 document will vary during
   transitions between transmission from the mixer of text originated by
   different participants.  Another difference is that the last member
   in the data header area in RFC 2198 [RFC2198] only contains the
   payload type number while in this document it has the same format as
   all other entries in the data header.

   The picture below shows a typical "text/rex" RTP packet with multi-
   party RTT contents from three sources and coding according to this
   document.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X| CC=3  |M|  "REX" PT   |   RTP sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               timestamp of packet creation                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |  CSRC list member 1 = SSRC of source of "A"                   |
      |  CSRC list member 2 = SSRC of source of "B"                   |
      |  CSRC list member 3 = SSRC of source of "C"                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |timestmp offset of "A-R2"  |"A-R2" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |timestamp offset of "A-R1" |"A-R1" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   | timestamp offset of "A-P" |"A-P" block length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |timestamp offset of "B-R2" |"B-R2" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |timestamp offset of "B-R1" |"B-R1" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   | timestamp offset of "B-P" | "B-P" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |timestamp offset of "C-R2" |"C-R2" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|   T140 PT   |timestamp offset of "C-R1" |"C-R1" block length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   T140 PT   |timestamp offset of "C-P"  |"C-P" block length |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | "A-R2" T.140 encoded redundant data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |"A-R1" T.140 encoded redundant data            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |"A-P" T.140 encoded primary    |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     "B-R2" T.140 encoded redundant data       |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      "B-R1" T.140 encoded redundant data                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | "B-P" T.140 encoded primary data              |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     "C-R2" T.140 encoded redundant data       |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      "C-R1" T.140 encoded redundant data                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      "C-P" T.140 encoded primary data         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Figure 2:A "text/rex" packet with text from three sources A, B, C.

   A-P, B-P, C-P are primary data from A, B and C.

   A-R1, B-R1, C-R1 are first redundant generation data from A, B and C.

   A-R2, B-R2, C-R2 are first redundant generation data from A, B and C.

   In a real case, some of the data headers will likely indicate a zero
   block length, and no corresponding T.140 data.

3.  Actions at transmission by a mixer

3.1.  Initial BOM transmission

   As soon as a participant is known to participate in a session and
   being available for text reception, a Unicode BOM character SHALL be
   sent to it according to the procedures in this document.  If the
   transmitter is a mixer, then the source of this character SHALL be
   indicated to be the mixer itself.

3.2.  Keep-alive

   After that, the transmitter SHALL send keep-alive traffic to the
   receivers at regular intervals when no other traffic has occurred
   during that interval if that is decided for the actual connection.
   Recommendations for keep-alive can be found in RFC 6263[RFC6263].





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3.3.  Transmission interval

   A "text/rex" transmitter SHOULD send packets distributed in time as
   long as there is something (new or redundant T140blocks) to transmit.
   The maximum transmission interval SHOULD then be 300 ms.  It is
   RECOMMENDED to send a packet to a receiver as soon as new text to
   that receiver is available, as long as the time after the latest sent
   packet to the same receiver is more than 150 ms, and also the maximum
   character rate to the receiver is not exceeded.  The intention is to
   keep the latency low while keeping a good protection against text
   loss in bursty packet loss conditions.

3.4.  Do not send received text to the originating source

   Text received from a participant SHOULD NOT be included in
   transmission to that participant.

3.5.  Clean incoming text

   A mixer SHALL handle reception and recovery of packet loss, marking
   of possible text loss and deletion of 'BOM' characters from each
   participant before queueing received text for transmission to
   receiving participants.

3.6.  Redundancy

   The transmitting party using redundancy SHALL send redundant
   repetitions of T140blocks aleady transmitted in earlier packets.  The
   number of redundant generations of T140blocks to include in
   transmitted packets SHALL be deducted from the SDP negotiation.  It
   SHOULD be set to the minimum of the number declared by the two
   parties negotiating a connection.  The same number of redundant
   generations MUST be used for text from all sources when it is
   transmitted to a receiver.  The number of generations sent to a
   receiver SHALL be the same during the whole session unless it is
   modified by session renegotiation.

3.7.  Text placement in packets

   At time of transmission, the mixer SHALL populate the RTP packet with
   T140blocks combined from all T140blocks queued for transmission
   originating from each source as long as this is not in conflict with
   the allowed number of characters per second or the maximum packet
   size.  These T140blocks SHALL be placed in the packet interleaved
   with redundant T140blocks and new T140blocks from other sources.  The
   SSRC of each source shall be placed as a member in the CSRC-list at a
   place corresponding to the place of its T140blocks in the packet.




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3.8.  Maximum number of sources per packet

   Text from a maximum of 16 sources MAY be included in a packet.  The
   reason for this limitation is the maximum number of CSRC list members
   allowed in a packet.  If text from more sources need to be
   transmitted, the mixer MAY let the sources take turns in having their
   text transmitted.  When stopping transmission of one source to allow
   another source to have its text sent, all intended redundant
   generations of the last text from the source to be stopped MUST be
   transmitted before text from another source can be transmitted.
   Actively transmitting sources SHOULD be allowed to take turns with
   short intervals to have their text transmitted.

   Note: The CSRC-list in an RTP packet only includes participants who's
   text is included in text blocks.  It is not the same as the total
   list of participants in a conference.  With audio and video media,
   the CSRC-list would often contain all participants who are not muted
   whereas text participants that don't type are completely silent and
   thus are not represented in RTP packet CSRC-lists once their text
   have been transmitted as primary and the intended number of redundant
   generations.

3.9.  Empty T140blocks

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

3.10.  Creation of the redundancy

   The primary T140block from each source in the latest transmitted
   packet is used to populate the first redundant T140block for that
   source.  The first redundant T140block for that source from the
   latest transmission is placed as the second redundant T140block
   source.

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





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3.11.  Timer offset fields

   The timer offset values are inserted in the data header, with the
   time offset from the RTP timestamp in the packet when the
   corresponding T140block was sent from its original source as primary.

   The timer offsets are expressed in the same clock tick units as the
   RTP timestamp.

   The timestamp offset values for empty T140blocks have no relevance
   but SHOULD be assigned realistic values.

3.12.  Other RTP header fields

   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.

   The current time SHALL be inserted in the timestamp.

   The SSRC of the mixer for the RTT session SHALL be inserted in the
   SSRC field of the RTP header.

3.13.  Pause in transmission

   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.

4.  Actions at reception

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

4.1.  Multi-party vs two-party use

   If the "CC" field value of a received packet is >0, it indicates that
   multi-party transmission is active, and the receiver MUST be prepared
   to act on the different sources according to its role.  If the CC
   value is 0, the connection is point-to-point.





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4.2.  Level of redundancy

   The used level of redundancy generations SHALL be evaluated from the
   received packet contents.  If the CC value is 0, the number of
   generations (including the primary) is equal to the number of members
   in the data header.  If the CC value is >0, the number of generations
   (including the primary) is equal to the number of members in the data
   header divided by the CC value.  If the remainder from the division
   is >0, then the packet is malformed and SHALL cause an error
   indication in the receiver.

4.3.  Extracting text and handling recovery and loss

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

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

   If a sequence number gap appears and is still there after some
   defined time for jitter resolution, T140data SHALL be recovered from
   redundant data.  If the gap is wider than the number of generations
   of redundant T140blocks in the packet, then a t140block SHALL be
   created with a marker for text loss [T140ad1] and assigned to the
   SSRC of the transmitter as a general input from the mixer because in
   general it is not possible to deduct from which sources text was
   lost.

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

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







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4.4.  Delete BOM

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

4.5.  Empty T140blocks

   Empty T140blocks are included as fillers for unused redundancy levels
   in the packets.  They just do not provide any contents and do not
   contribute to the received streams.

5.  RTCP considerations

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

6.  Chained operation

   By strictly applying the rules for "text/rex" packet format by all
   conforming devices, mixers MAY be arranged in chains.

7.  Usage without redundancy

   The "text/rex" format SHALL be used also for multi-party
   communication when the redundancy mechanism is not used.  That MAY be
   the case when robustness in transmission is provided by some other
   means than by redundancy.  All aspects of this document SHALL be
   applied except the redundant generations in transmission.

   The "text/rex" format SHOULD thus be used for multi-party operation,
   also when some other protection against packet loss is utilized, for
   example a reliable network or transport.  The format is also suitable
   to be used for point-to-point operation.















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8.  Use with SIP centralized conferencing framework

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

9.  Conference control

   In managed conferences, control of the real-time text media SHOULD be
   provided in the same way as other for media, e.g. for muting and
   unmuting by the direction attributes in SDP [RFC4566].

   Note that floor control functions may be of value for RTT users as
   well as for users of other media in a conference.

10.  Media Subtype Registration

   This registration is done using the template defined in [RFC6838] and
   following [RFC4855].

   Type name:
      text

   Subtype name:
      rex

   Required parameters:
      rate:
         The RTP timestamp (clock) rate.  The only valid value is 1000.

      pt:
         a comma-separated list of RTP payload types.  Because comma is
         a special character, the list must be a quoted-string (enclosed
         in double quotes).  Each list element is a mapping of the
         dynamic payload type number to an embedded Content-type
         specification for the payload format corresponding to the
         payload type.  The format of the mapping is:

         payload-type-number "=" content-type





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         If the content-type string includes a comma, then the content-
         type string MUST be a quoted-string.  If the content- type
         string does not include a comma, it MAY still be quoted.  Since
         it is part of the list which must itself be a quoted- string,
         that means the quotation marks MUST be quoted with backslash
         quoting as specified in RFC 2045.  If the content- type string
         itself contains a quoted-string, then the requirement for
         backslash quoting is recursively applied.  To specify the text/
         rex payload format in SDP, the pt parameter is mapped to an
         a=fmtp attribute by eliminating the parameter name (pt) and
         changing the commas to slashes.  For example:

         pt = " = \"text/t140;cps=200,text/t140,text/t140\" "

         Implies the following sdp

                 m=text 49170 RTP/AVP 98 100
                 a=rtpmap:98 rex/1000
                 a=fmtp:98 100/100/100
                 a=rtpmap:100 t140/1000
                 a=fmtp:100 cps=200

   Encoding considerations:
      binary; see Section 4.8 of [RFC6838].

   Security considerations:
      See Section 20 of RFC xxxx.  [RFC Editor: Upon publication as an
      RFC, please replace "XXXX" with the number assigned to this
      document and remove this note.]

   Interoperability considerations:
      None.

   Published specification:
      RFC XXXX.  [RFC Editor: Upon publication as an RFC, please replace
      "XXXX" with the number assigned to this document and remove this
      note.]

   Applications which use this media type:
      For example: Text conferencing tools, multimedia conferencing
      tools.Real-time conversational tools.

   Fragment identifier considerations:
      N/A.

   Additional information:
      None.




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   Person & email address to contact for further information:
      Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>

   Intended usage:
      COMMON

   Restrictions on usage:
      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP [RFC3550].

   Author:
      Gunnar Hellstrom <gunnar.hellstrom@ghaccess.se>

   Change controller:
      IETF AVTCore Working Group delegated from the IESG.

11.  SDP considerations

   There are receiving RTT implementations which implement RFC 4103
   [RFC4103] but not the source separation by the CSRC.  Sending mixed
   text according to the usual CSRC convention from RFC 2198 [RFC2198]
   to a device implementing only RFC 4103 [RFC4103] would risk to lead
   to unreadable presented text.  Therefore, in order to negotiate RTT
   mixing capability according to this document, all devices supporting
   this document for multi-party aware participants SHALL include an SDP
   media format "text/rex" in the SDP [RFC4566], indicating this
   capability in offers and answers.  Multi-party streams using the
   coding of this document intended for multi-party aware endpoints MUST
   NOT be sent to devices which have not indicated the "text/rex"
   format.

   Implementations not understanding the "text/rex" format MUST ignore
   it according to common SDP rules.

   The SDP media format defined here, is named "rex", for extended
   "red".  It is intended to be used in "text" media descriptions with
   "text/rex" and "text/t140" formats.  Both formats MUST be declared
   for the "text/rex" format to be used.  It indicates capability to use
   source indications in the CSRC list and the packet format according
   to this document.  It also indicates ability to receive 150 real-time
   text characters per second by default.










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11.1.  Mapping of media parameters to sdp

   The information carried in the media type registration has a specific
   mapping to fields in the Session Description Protocol (SDP) , which
   is commonly used to describe RTP sessions.  When SDP RFC 4566
   [RFC4566]is used to specify sessions employing the "text/rex" format,
   the mapping is as follows:

   *  The media type ("text") goes in SDP "m=" as the media name.

   *  The media subtype (payload format name) goes in SDP "a=rtpmap" as
      the encoding name.  The RTP clock rate in "a=rtpmap" MUST be 1000
      for "text/rex".

   *  When the payload type is used with redundancy, the level of
      redundancy is shown by the number of elements in the slash-
      separated payload type list in the "fmtp" parameter of the "text/
      rex" media format.

11.2.  Security for session control and media

   Security SHOULD be applied on both session control and media.  In
   applications where legacy endpoints without security may exist, a
   negotiation between security and no security SHOULD be applied.  If
   no other security solution is mandated by the application, then RFC
   8643 OSRTP[RFC8643] SHOULD be applied to negotiate SRTP media
   security with DTLS.  Most SDP examples below are for simplicity
   expressed without the security additions.  The principles (but not
   all details) for applying DTLS-SRTP security is shown in a couple of
   the following examples.

11.3.  SDP offer/answer examples

   This sections shows some examples of SDP for session negotiation of
   the real-time text media in SIP sessions.  Audio is usually provided
   in the same session, and sometimes also video.  The examples only
   show the part of importance for the real-time text media.

    Offer example for just multi-party capability:

         m=text 11000 RTP/AVP 101 98
         a=rtpmap:98 t140/1000
         a=rtpmap:101 rex/1000
         a=fmtp:101 98/98/98







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    Answer example  from a multi-party capable device
         m=text 12000 RTP/AVP 101 98
         a=rtpmap:98 t140/1000
         a=rtpmap:101 rex/1000
         a=fmtp:101 98/98/98

   Offer example for both traditional "text/red" and multi-party format:

         m=text 11000 RTP/AVP 101 100 98
         a=rtpmap:98 t140/1000
         a=rtpmap:100 red/1000
         a=rtpmap:101 rex/1000
         a=fmtp:100 98/98/98
         a=fmtp:101 98/98/98

    Answer example  from a multi-party capable device
         m=text 11000 RTP/AVP 101 98
         a=rtpmap:98 t140/1000
         a=rtpmap:101 rex/1000
         a=fmtp:101 98/98/98

    Offer example for both traditional "text/red" and multi-party format
    including security:
          a=fingerprint: SHA-1 \
          4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
          m=text 11000 RTP/AVP 101 100 98
          a=rtpmap:98 t140/1000
          a=rtpmap:100 red/1000
          a=rtpmap:101 rex/1000
          a=fmtp:100 98/98/98
          a=fmtp:101 98/98/98

   The "Fingerprint" is sufficient to offer DTLS-SRTP, with the media
   line still indicating RTP/AVP.

     Answer example from a multi-party capable device including security
          a=fingerprint: SHA-1 \
          FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
          m=text 11000 RTP/AVP 101 98
          a=rtpmap:98 t140/1000
          a=rtpmap:101 rex/1000
          a=fmtp:101 98/98/98

   With the "fingerprint" the device acknowledges use of SRTP/DTLS.







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   Answer example from a multi-party unaware device that also
   does not support security:

         m=text 12000 RTP/AVP 100 98
         a=rtpmap:98 t140/1000
         a=rtpmap:100 red/1000
         a=fmtp:100 98/98/98

   A party which has negotiated the "text/rex" format MUST populate the
   CSRC-list and format the packets according to this document if it
   acts as an rtp-mixer and sends multi-party text.

   A party which has negotiated the "text/rex" capability MUST interpret
   the contents of the CSRC-list and the packets according to this
   document in received rtp packets using the corresponding payload
   type.

   A party performing as a mixer, which has not negotiated the "text/
   rex" format, but negotiated a "text/red" or "text/t140" format in a
   session with a participant SHOULD, if nothing else is specified for
   the application, format transmitted text to that participant to be
   suitable to present on a multi-party unaware endpoint as further
   specified in section Section 14.2.

   A party not performing as a mixer MUST not include the CSRC list if
   it has a single source of text.

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.  Pn
   indicates primary data in source group "n".  Rn1 is first redundant
   generation data and Rn2 is second redundant generation data in source
   group "n".  A1, B1, A2 etc are text chunks (T140blocks) received from
   the respective sources.  X indicates dropped packet between the mixer
   and a receiver.

   |----------------|
   |Seq no 1        |
   |CC=1            |
   |CSRC list A     |
   |R12: Empty      |
   |R11: Empty      |
   |P1: A1          |
   |----------------|

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



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   |----------------|
   |Seq no 2        |
   |CC=3            |
   |CSRC list C,A   |
   |R12 Empty       |
   |R11:Empty       |
   |P1: C1          |
   |R22 Empty       |
   |R21: A1         |
   |P2: Empty       |
   |----------------|
   Text C1 is received from packet 2 and assigned to reception area C.

   X----------------|
   X Seq no 3       |
   X CC=2           |
   X CSRC list C,A  |
   X R12: Empty     |
   X R11: C1        |
   X P1:  Empty     |
   X R22: A1        |
   X R21: Empty     |
   X P2:  A2        |
   X----------------|
   Packet 3 is assumed to be dropped in network problems

   X----------------|
   X Seq no 4       |
   X CC=3           |
   X CSRC list C,B,A|
   X R12: Empty     |
   X R11: Empty     |
   X P1: C2         |
   X R22: Empty     |
   X R21: Empty     |
   X P2: B1         |
   X R32: Empty     |
   X R31: A2        |
   X P3:  A3        |
   X----------------|
   Packet 4 is assumed to be dropped in network problems










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   X----------------|
   X Seq no 5       |
   X CC=3           |
   X CSRC list C,B,A|
   X R12: Empty     |
   X R11: C2        |
   X P1: Empty      |
   X R22: Empty     |
   X R21: B1        |
   X P2: B2         |
   X R32: A2        |
   X R31: A3        |
   X P3:  A4        |
   X----------------|
   Packet 5 is assumed to be dropped in network problems

   |----------------|
   |Seq no 6        |
   |CC=3            |
   |CSRC list C,B,A |
   | R12: C2        |
   | R11: Empty     |
   | P1: Empty      |
   | R22: B1        |
   | R21: B2        |
   | P2:  B3        |
   | R32: A3        |
   | R31: A4        |
   | P3:  A5        |
   |----------------|

   Packet 6 is received.  The latest received sequence number was 2.
   Recovery is therefore tried for 3,4,5.  But there is no coverage for
   seq no 3.  A missing text mark (U'FFFD) [T140ad1] is created and
   appended to the common mixer reception area.  For seqno 4, texts C2,
   B1 and A3 are recovered from the second generation redundancy and
   appended to their respective reception areas.  For seqno 5, texts B2
   and A4 are recovered from the first generation redundancy and
   appended to their respective reception areas.  Primary text B3 and A5
   are received and appended to their respective reception areas.

   After this sequence, the following has been received: A1,A3, A4, A5;
   B1, B2, B3; C1, C2.  A possible loss is indicated by the missing text
   mark in time between A1 and A3.

   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 document allows new text from up to 16 sources per packet.  A
   mixer implementing the specification will normally cause a latency of
   0 to 150 milliseconds in text from up to 16 simultaneous sources.
   This performance meets well the realistic requirements for conference
   and conversational applications for which up to 5 simultaneous
   sources should not be delayed more than 500 milliseconds by a mixer.
   In order to achieve good performance, a receiver for multi-party
   calls SHOULD declare a sufficient CPS value for the "text/t140"
   format in SDP for the number of allowable characters per second.

   As comparison, if the "text/red" format would be used for multi-party
   communication with its default timing and redundancy, 5
   simultaneously sending parties would cause jerky presentation of the
   text from them in text spurts with 5 seconds intervals.  With a
   reduction of the transmission interval to 150 ms, the time between
   text spurts for 5 simultaneous sending parties would be 2.5 seconds.

   Five simultaneous sending parties may occasionally occur in a
   conference with one or two main sending parties and three parties
   giving very brief comments.

   The default maximum rate of reception of "text/t140" 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 the "text/t140" media.  A mixer combining
   real-time text from a number of sources may have a higher combined
   flow of text coming from the sources.  Endpoints SHOULD therefore
   specify a suitable higher value for the CPS parameter, corresponding
   to its real reception capability.  A value for CPS of 150 is the
   default for the "text/t140" stream in the "text/rex" format.  See RFC
   4103 [RFC4103] for the format and use of the CPS parameter.  The same
   rules apply for the "text/rex" format except for the default value.

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



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   in the display.  Mechanisms for looking back in the contents from the
   current session should be provided.  The text should be displayed as
   soon as it is received."

   Strict application of T.140 [T140] is of essence for the
   interoperability of real-time text implementations and to fulfill the
   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.

   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 fields.  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 simultaneously,
   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 |-|
                |______________________________________________|v|
                | <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 3: Example of a three-party RTT call presented in chat style
   seen at participant 'Alice's endpoint.

   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 4: 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 by the "text/rex"
   format in an SDP negotiation, but the multi-party capability
   negotiation fails with an endpoint, then the agreed "text/red" or
   "text/t140" format SHALL be used and 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 implementing only RFC 4103 without the multi-party
   extension specified in this document.

   The principles and procedures below do not specify any new protocol
   elements.  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 between
   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 creation of text
   (retrieved from the RTP timestamp) SHALL be stored together with the
   received text from each source in queues 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 other 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.

   The status SHOULD be maintained for the latest control code for
   Select Graphic Rendition (SGR) from each source.  If there is an SGR
   code stored as the status 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 "BOM" (U+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 (U+FFFD)  "Replacement character", represented as a
      question mark in a rhombus, or if that is not feasible, replaced
      by an apostrophe ', 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 as status for
      the source in the storage for SGR status.  If a reset graphic
      rendition (SGR 0) originated from a source is sent, then the SGR
      status storage for that source shall be cleared.  The display
      count shall not be increased.

   BS (U+0008)  Back Space, intended to erase the last entered character













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      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.

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.

   New text can be presented only from one source at a time.  Switch of
   source to be presented takes place at suitable places in the text,
   such as end of phrase, end of sentence, line separator and
   inactivity.  Therefore the time to switch to present waiting text
   from other sources may become long and will vary and depend on the
   actions of the currently presented source.

   Erasure can only be done up to the latest source switch.  If a user
   tries to erase more text, the erasing actions will be presented as
   letter X after the label.

   Text loss because of network errors may hit the label between entries
   from different parties, causing risk for misunderstanding from which
   source a piece of text is.





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   These facts makes it strongly RECOMMENDED to implement multi-party
   awareness in RTT endpoints.  The use of the mixing method for multi-
   party-unaware endpoints should be left for use with endpoints which
   are impossible to upgrade to become multi-party aware.

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     |
    |________________________|_________________________|

   Figure 5: 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.





















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     _________________________________________________
    |                                              |^|
    |[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 6: An example of a view of the multi-party unaware
   presentation in chat style.  Alice is the local user.

15.  Gateway Considerations

15.1.  Gateway considerations with Textphones (e.g.  TTYs).

   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).

   More information on gateways to textphones (TTYs) is found in RFC
   5194[RFC5194]




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15.2.  Gateway considerations with WebRTC.

   Gateway operation to real-time text in WebRTC may also be required.
   In WebRTC, RTT is specified in draft-ietf-mmusic-t140-usage-data-
   channel[I-D.ietf-mmusic-t140-usage-data-channel].

   A multi-party bridge may have functionality for communicating by RTT
   both in RTP streams with RTT and WebRTC t140 data channels.  Other
   configurations may consist of a multi-party bridge with either
   technology for RTT transport and a separate gateway for conversion of
   the text communication streams between RTP and t140 data channel.

   In WebRTC, it is assumed that for a multi-party session, one t140
   data channel is established for each source from a gateway or bridge
   to each participant.  Each participant also has a data channel with
   two-way connection with the gateway or bridge.

   The t140 channel used both ways is for text from the WebRTC user and
   from the bridge or gateway itself to the WebRTC user.  The label
   parameter of this t140 channel is used as NAME field in RTCP to
   participants on the RTP side.  The other t140 channels are only for
   text from other participants to the WebRTC user.

   When a new participant has entered the session with RTP transport of
   rtt, a new t140 channel SHOULD be established to WebRTC users with
   the label parameter composed from the NAME field in RTCP on the RTP
   side.

   When a new participant has entered the multi-party session with RTT
   transport in a WebRTC t140 data channel, the new participant SHOULD
   be announced by a notification to RTP users.  The label parameter
   from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP
   side, or other available session information.

16.  Updates to RFC 4102 and RFC 4103

   This document updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
   introducing an extended packet format "text/rex" for the multi-party
   mixing case and more strict rules for the use of redundancy, and
   population of the CSRC list in the packets.  Implications for the
   CSRC list use from RFC 2198[RFC2198] are hereby not in effect.

17.  Congestion considerations

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





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   The first action in case of congestion SHOULD be to temporarily
   increase the transmission interval up to one second.

18.  Acknowledgements

   James Hamlin for format input.

19.  IANA Considerations

   The IANA is requested to register the media type "text/rex" as
   specified in Section 10.  The media type is also requested to be
   added to the IANA registry for "RTP Payload Format Media Types"
   <http://www.iana.org/assignments/rtp-parameters>.

20.  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, and in conference
   notifications, 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.

21.  Change history

21.1.  Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06

   Improved definitions list format.

   The format of the media subtype parameters is made to match the
   requirements.

   The mapping of media subtype parameters to sdp is included.

   The CPS parameter belongs to the t140 subtype and does not need to be
   registered here.

21.2.  Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05

   nomenclature and editorial improvements

   "this document" used consistently to refer to this document.




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21.3.  Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04

   'Redundancy header' renamed to 'data header'.

   More clarifications added.

   Language and figure number corrections.

21.4.  Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03

   Mention possible need to mute and raise hands as for other media.
   ---done ----

   Make sure that use in two-party calls is also possible and explained.
   - may need more wording -

   Clarify the RTT is often used together with other media. --done--

   Tell that text mixing is N-1.  A users own text is not received in
   the mix. -done-

   In 3. correct the interval to: A "text/rex" transmitter SHOULD send
   packets distributed in time as long as there is something (new or
   redundant T140blocks) to transmit.  The maximum transmission interval
   SHOULD then be 300 ms.  It is RECOMMENDED to send a packet to a
   receiver as soon as new text to that receiver is available, as long
   as the time after the latest sent packet to the same receiver is more
   than 150 ms, and also the maximum character rate to the receiver is
   not exceeded.  The intention is to keep the latency low while keeping
   a good protection against text loss in bursty packet loss conditions.
   -done-

   In 1.3 say that the format is used both ways. -done-

   In 13.1 change presentation area to presentation field so that reader
   does not think it shall be totally separated. -done-

   In Performance and intro, tell the performance in number of
   simultaneous sending users and introduced delay 16, 150 vs
   requirements 5 vs 500. -done --

   Clarify redundancy level per connection.  -done-

   Timestamp also for the last data header.  To make it possible for all
   text to have time offset as for transmission from the source.  Make
   that header equal to the others. -done-

   Mixer always use the CSRC list, even for its own BOM. -done-



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   Combine all talk about transmission interval (300 ms vs when text has
   arrived) in section 3 in one paragraph or close to each other. -done-

   Documents the goal of good performance with low delay for 5
   simultaneous typers in the introduction. -done-

   Describe better that only primary text shall be sent on to receivers.
   Redundancy and loss must be resolved by the mixer. -done-

21.5.  Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02

   SDP and better description and visibility of security by OSRTP RFC
   8634 needed.

   The description of gatewaying to WebRTC extended.

   The description of the data header in the packet is improved.

21.6.  Changes to draft-ietf-avtcore-multi-party-rtt-mix-01

   2,5,6 More efficient format "text/rex" introduced and attribute
   a=rtt-mix deleted.

   3.  Brief about use of OSRTP for security included- More needed.

   4.  Brief motivation for the solution and why not rtp-translator is
   used added to intro.

   7.  More limitations for the multi-party unaware mixing method
   inserted.

   8.  Updates to RFC 4102 and 4103 more clearly expressed.

   9.  Gateway to WebRTC started.  More needed.

21.7.  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.








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21.8.  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.

21.9.  Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to
       -02

   In Abstract and 1.  Introduction: Introduced wording about regulatory
   requirements.

   In section 5: The transmission interval is decreased to 100 ms when
   there is text from more than one source to transmit.

   In section 11 about SDP negotiation, a SHOULD-requirement is
   introduced that the mixer should make a mix for multi-party unaware
   endpoints if the negotiation is not successful.  And a reference to a
   later chapter about it.

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

   A short chapter 15 on gateway considerations is introduced.

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

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

   Note about deviation from RFC 2198 added in chapter 4.



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   In chapter 9.  "Use with SIP centralized conferencing framework" the
   following note is inserted: Note: The CSRC-list in an RTP packet only
   includes participants who's text is included in one or more text
   blocks.  It is not the same as the list of participants in a
   conference.  With audio and video media, the CSRC-list would often
   contain all participants who are not muted whereas text participants
   that don't type are completely silent and so don't show up in RTP
   packet CSRC-lists.


21.10.  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

22.  References

22.1.  Normative References

   [I-D.ietf-mmusic-t140-usage-data-channel]
              Holmberg, C. and G. Hellstrom, "T.140 Real-time Text
              Conversation over WebRTC Data Channels", Work in Progress,
              Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel-
              14, 10 April 2020, <https://tools.ietf.org/html/draft-
              ietf-mmusic-t140-usage-data-channel-14>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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



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   [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>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://www.rfc-editor.org/info/rfc4855>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC6263]  Marjou, X. and A. Sollaud, "Application Mechanism for
              Keeping Alive the NAT Mappings Associated with RTP / RTP
              Control Protocol (RTCP) Flows", RFC 6263,
              DOI 10.17487/RFC6263, June 2011,
              <https://www.rfc-editor.org/info/rfc6263>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC8643]  Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
              Stach, "An Opportunistic Approach for Secure Real-time
              Transport Protocol (OSRTP)", RFC 8643,
              DOI 10.17487/RFC8643, August 2019,
              <https://www.rfc-editor.org/info/rfc8643>.

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

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




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Internet-Draft    RTP-mixer format for multi-party RTT         June 2020


22.2.  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>.

   [RFC5194]  van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real-
              Time Text over IP Using the Session Initiation Protocol
              (SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008,
              <https://www.rfc-editor.org/info/rfc5194>.

Author's Address

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

   Email: gunnar.hellstrom@ghaccess.se




















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