[Docs] [txt|pdf|xml] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02

Network Working Group                                       G. Hellstrom
Internet-Draft                                                   Omnitor
Updates: RFC 4102, RFC 4103 (if                           March 26, 2020
         approved)
Intended status: Standards Track
Expires: September 27, 2020


            Indicating source of multi-party Real-time text
           draft-hellstrom-avtcore-multi-party-rtt-source-02

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/.




Hellstrom              Expires September 27, 2020               [Page 1]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   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 September 27, 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  . . . . . . . . . . . . . . . . . . . .   6
   7.  RTCP considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Chained operation . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Usage without redundancy  . . . . . . . . . . . . . . . . . .   8
   10. Use with SIP centralized conferencing framework . . . . . . .   8
   11. SDP Capability negotiation  . . . . . . . . . . . . . . . . .   8
   12. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   13. Performance considerations  . . . . . . . . . . . . . . . . .  11
   14. Presentation level considerations . . . . . . . . . . . . . .  12
     14.1.  Presentation by multi-party aware endpoints  . . . . . .  12
     14.2.  Multi-party mixing for multi-party unaware endpoints . .  14
   15. Gateway Considerations  . . . . . . . . . . . . . . . . . . .  17
   16. Congestion considerations . . . . . . . . . . . . . . . . . .  17
   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   20. Change history  . . . . . . . . . . . . . . . . . . . . . . .  18
     20.1.  Changes from version -00 to -01  . . . . . . . . . . . .  18
     20.2.  Changes from version -01 to -02  . . . . . . . . . . . .  18



Hellstrom              Expires September 27, 2020               [Page 2]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     21.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     21.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

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.

   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 briefly specified for cases when the
   negotiation fails.






Hellstrom              Expires September 27, 2020               [Page 3]


Internet-Draft     Indicate source of multi-party RTT         March 2020


2.  Nomenclature

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

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

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

3.  Intended application

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

4.  Use of fields in the RTP packets

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
   redundancy 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.

   The first member in the CSRC-list shall contain the SSRC of the
   source of the primary T140block in the packet.  The second and
   further members in the CSRC-list shall contain the SSRC of the source
   of the first, second, etc redundant generation T140blocks included in
   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.

   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 this specification a text block 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




Hellstrom              Expires September 27, 2020               [Page 4]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   rotate 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 should be
   decreased to 100 ms for periods when there is text available for
   transmission from more than one source.  The mixer has its own SSRC,
   and its own RTP sequence number series.  At time of transmission, the
   RTP packet SHALL be populated 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




Hellstrom              Expires September 27, 2020               [Page 5]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   SSRC of its source shall be placed as the first member in the CSRC-
   list.  The current time is 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 now placed as the second redundant T140block,
   and the corresponding CSRC placed in its place in the CSRC-list.

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

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

   The number of members in the CSRC list shall be placed in the "CC"
   header field.

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

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.

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



Hellstrom              Expires September 27, 2020               [Page 6]


Internet-Draft     Indicate source of multi-party RTT         March 2020


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

   As long as the sequence is correct, each packet is unpacked in order.
   The T140blocks are extracted from the primary area, and the
   corresponding SSRC is extracted from the first position in the CSRC
   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 needs to be recovered
   from redundant data.  If the gap is wider than the number of
   generations of redundant T140blocks in the packet, then a t140block
   is created with a marker for text loss [T140ad1] and assigned to the
   SSRC of the transmitter as a general input from the mixer.

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

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

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

7.  RTCP considerations

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

8.  Chained operation

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




Hellstrom              Expires September 27, 2020               [Page 7]


Internet-Draft     Indicate source of multi-party RTT         March 2020


9.  Usage without redundancy

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

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

   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
   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.  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 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




Hellstrom              Expires September 27, 2020               [Page 8]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   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 this
   specification.  It also indicates ability to receive RTT packets with
   an interval of 100 ms.

   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 .

   A party MUST NOT transmit packets with the multi-party format
   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



Hellstrom              Expires September 27, 2020               [Page 9]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   (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.

   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








Hellstrom              Expires September 27, 2020              [Page 10]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   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 before 6 was 2.
Recovery is therefore tried for 3,4,5.
But there is no coverage for seq no 3. A missing text mark (U'FFFD)
is created and 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.

   It will be a design decision how to present the missing text markers
   assigned to the mixer as a source.

13.  Performance considerations

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







Hellstrom              Expires September 27, 2020              [Page 11]


Internet-Draft     Indicate source of multi-party RTT         March 2020


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."

   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.  It is essential that
   implementations are able to ignore

   There is no strict "message" concept in real-time text.  Line
   separator is used as a separator allowing a part of received text to
   be grouped in presentation.  The characters CR LF are allowed as
   replacement for Line Separator.  The CR LF combination is erased by
   just one erasing action.  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.



Hellstrom              Expires September 27, 2020              [Page 12]


Internet-Draft     Indicate source of multi-party RTT         March 2020


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

   If the presentation presents text from different sources in one
   common area, the 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.

   A view of a three-party RTT call in chat style is shown in this
   example .

                 _________________________________________________
                |                                              | |
                |[Alice] Hi, Alice here.                       | |
                |                                              | |
                |[Bob] Bob as well.                            | |
                |                                              | |
                |[Eve] Hi, this is Eve, calling from Paris.    | |
                |      I thought you should be here.           | |
                |                                              | |
                |[Alice] I am coming on Thursday, my           | |
                |      performance is not until Friday morning.| |
                |                                              | |
                |[Bob] And I on Wednesday evening.             | |
                |                                              | |
                |[Alice] Can we meet on Thursday evening?      | |
                |                                              | |
                |[Eve] Yes, definitely. How about 7pm.         | |
                |     at the entrance of the restaurant        | |
                |     Le Lion Blanc?                           | |
                |[Eve] we can have dinner and then take a walk | |
                |                                              | |
                | <Eve-typing> But I need to be back to        | |
                |    the hotel by 11 because I need            | |
                |                                              |-|
                | <Bob-typing> I wou                           |-|
                |______________________________________________|v|
                | of course, I underst                           |
                |________________________________________________|

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

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

   Other presentation styles than the chat style may be arranged.



Hellstrom              Expires September 27, 2020              [Page 13]


Internet-Draft     Indicate source of multi-party RTT         March 2020


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

   _____________________________________________________________________
   |       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 severe functional limitations and should
   be used only to enable participation in multi-party calls by legacy
   deployed endpoints.

   The text should be formatted by the mixer for presentation in one
   single presentation area.  When there is text available for
   transmission from the mixer to the participant for more than one
   participant, the mixer shall switch between transmission of text from
   different sources at suitable points in the transmitted stream.  When
   switching source, the mixer shall insert a line separator and a label
   composed from information in the CNAME and NAME fields in RTCP
   reports from the participant to have its text transmitted.  The label
   SHALL be delimited by suitable characters.  Then text from that
   selected participant shall be transmitted until a new suitable point
   for switching source is reached.

   Suitable points for switching are: a completed phrase ended by comma,
   a completed sentence, a line separator or other valid code for new
   line.  A long pause (e.g. > 10 seconds) in received text from the
   currently transmitted source is also a suitable switching point.  If
   text from one participant has been transmitted with text from other
   sources waiting for transmission for a long time (e.g. > 1 minute)



Hellstrom              Expires September 27, 2020              [Page 14]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   and none of the suitable points for switching has occurred, a source
   switch MAY be forced by the mixer at next word separator.  When
   switching source, the text that has been waiting longest should be
   selected to be transmitted.

   Erasure by backspace can not always be performed as the erasing party
   intended.  If an erasing action erases all text up to 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 CR LF may be used instead.  Erasure of a
   new line is in both cases done by just one erasing action
   (Backspace).

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

   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 multi-party-
   unaware mixing method should be left for use with endpoints which are
   impossible to upgrade to become multi-party aware.

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












Hellstrom              Expires September 27, 2020              [Page 15]


Internet-Draft     Indicate source of multi-party RTT         March 2020


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

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

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

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






Hellstrom              Expires September 27, 2020              [Page 16]


Internet-Draft     Indicate source of multi-party RTT         March 2020


15.  Gateway Considerations

   Multi-party RTT sessions may involve gateways of different kinds.  It
   is essential that gateways involved in setting up sessions 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 would therefore be suitable for the
   gateway to not indicate multi-party awareness for that case.

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                                  |
      +---------------------+------------------------------------------+

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.



Hellstrom              Expires September 27, 2020              [Page 17]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   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 version -00 to -01

   Editorial cleanup.

   Changed capability indication from fmtp-parameter to sdp attribute
   "rtt-mix".

   Swapped order of redundancy elements in the example to match reality.

   Increased the SDP negotiation section

20.2.  Changes from version -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.



Hellstrom              Expires September 27, 2020              [Page 18]


Internet-Draft     Indicate source of multi-party RTT         March 2020


   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.  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>.

   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J., 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.

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








Hellstrom              Expires September 27, 2020              [Page 19]


Internet-Draft     Indicate source of multi-party RTT         March 2020


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

   Gunnar Hellstrom
   Omnitor
   Esplanaden 30
   Vendelso  13670
   Sweden

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
























Hellstrom              Expires September 27, 2020              [Page 20]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/