Network Working Group                                       G. Hellstrom
Internet-Draft                                                   Omnitor
Updates: RFC 4102, RFC 4103 (if                        February 29, 2020
Intended status: Standards Track
Expires: September 1, 2020

            Indicating source of multi-party Real-time text


   Real-time text mixers need to identify the source of each transmitted
   text chunk so that it can be presented in suitable grouping with
   other text from the same source.  An enhancement for RFC 4103 real-
   time text is provided, suitable for a centralized conference model
   that enables source identification, for use by text mixers and
   conference-enabled participants.  The mechanism builds on use of the
   CSRC list in the RTP packet.  A capability exchange is specified so
   that it can be verified that a participant can handle the multi-party
   coded real-time text stream.  The capability is indicated by an sdp
   media attribute "rtt-mix".

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 1, 2020.

Copyright Notice

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   document authors.  All rights reserved.

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

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Nomenclature  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Intended application  . . . . . . . . . . . . . . . . . . . .   3
   4.  Use of fields in the RTP packets  . . . . . . . . . . . . . .   3
   5.  Actions at transmission by a mixer  . . . . . . . . . . . . .   4
   6.  Actions at reception  . . . . . . . . . . . . . . . . . . . .   5
   7.  RTCP considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Chained operation . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Use with SIP centralized conferencing framework . . . . . . .   6
   10. Usage without redundancy  . . . . . . . . . . . . . . . . . .   7
   11. SDP Capability negotiation  . . . . . . . . . . . . . . . . .   7
   12. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   13. Performance considerations  . . . . . . . . . . . . . . . . .   9
   14. Presentation level considerations . . . . . . . . . . . . . .  10
   15. Congestion considerations . . . . . . . . . . . . . . . . . .  10
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   19. Change history  . . . . . . . . . . . . . . . . . . . . . . .  11
     19.1.  Changes from version -00 to -01  . . . . . . . . . . . .  11
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     20.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     20.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550] for
   transmission of real-time text (RTT) and the "text/t140" format.  It
   also specifies a redundancy format "text/red" for increased
   robustness.  RFC 4102 [RFC4102] registers the "text/red" format.  The
   redundancy scheme enables efficient transmission of redundant text in
   packets together with new text.  However the redundant header format
   has no source indicators for the redundant transmissions.  An
   assumption has had to be made that the redundant parts in a packet
   are from the same source as the new text.  The recommended
   transmission is one new and two redundant generations of text
   (T140blocks) in each packet and the recommended transmission interval

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   is 300 ms.  A mixer, selecting between text input from different
   sources and transmitting it in a common stream need to make sure that
   the receiver can assign the received text to the proper sources for
   presentation.  Therefore, without any extra rule for source
   identification, the mixer needs to stop sending new text from that
   source and then make sure that all text so far has been sent with all
   intended redundancy levels (usually two) in order to switch source.
   That causes the very long time of one second to switch between
   transmission of text from one source to text from another source.
   Both the total throughput and the switching performance in the mixer
   is too low for most applications.

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

2.  Nomenclature

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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

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

3.  Intended application

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

4.  Use of fields in the RTP packets

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

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

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   the packet. ( the recommended level of redundancy is to use one
   primary and two redundant generations of T140blocks.)  In some cases,
   the primary or redundant T140block is empty, but is still represented
   by a member in the redundancy header.  For such cases, the
   corresponding CSRC-list member MUST also be included.

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

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

5.  Actions at transmission by a mixer

   A text/red transmitter is usually sending packets at a regular
   transmission interval as long as there is something (new or redundant
   T140blocks) to transmit. 300 ms is the default transmission interval,
   but shorter intervals may be considered for specific cases.  The
   transmitter has its own SSRC, and its own RTP sequence number series.
   At time of transmission, the RTP packet SHALL be populated with next
   T140block queued for transmission from any of the active sources.
   This T140block shall be placed in the primary area of the packet.
   When performing as a mixer, the SSRC of its source shall be placed as
   the first member in the CSRC-list.  The current time is inserted and
   the timestamp.

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   If no unsent T140blocks were available, at this time, but T140blocks
   available which have not been yet been sent the full intended number
   of redundant transmissions, then The primary T140block is created by
   an empty T140block, and populated in a packet for transmission.  The
   SSRC of the transmitter is included in the first place of the CSRC-

   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

   The timer offset values are inserted in the redundancy header, with
   the time offset from when the corresponding T140block was sent as
   original. (usually one or two times the transmission interval).

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

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

6.  Actions at reception

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

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

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

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

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

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

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

   If the sequence number gap was equal or less than the number of
   redundancy generations in the received packet, no missing text marker
   shall be inserted, and instead the T140blocks and their SSRCs
   recovered from the redundancy information and the CSRC-list in the
   way indicated above.

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

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

7.  RTCP considerations

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

8.  Chained operation

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

9.  Use with SIP centralized conferencing framework

   The SIP conferencing framework, mainly specified in RFC
   4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
   for coordinating sessions including multi-party RTT.  The RTT stream
   is one and the same during the conference.  Participants get

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   announced by notifications when participants are joining or leaving,
   and further user information may be provided.  The SSRC of the text
   to expect from joined users can be included in a notification.  This
   can be used both for security purposes and for preparation of an SSRC
   to label translation for presentation to other users.

10.  Usage without redundancy

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

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

11.  SDP Capability negotiation

   There are RTT implementations which implement RFC 4103 [RFC4103] but
   not the present specification.  Sending mixed text according to the
   present specification to a device implementing only RFC 4103
   [RFC4103] would lead to unreadable output.  Therefore, in order to
   negotiate RTT Mixing capability according to the present
   specification, all devices supporting the present specification shall
   include an sdp media attribute "rtt-mix" indicating this capability
   in offers and answers.  Multi-party streams using the coding of this
   specification must not be sent to devices who have not indicated the
   "rtt-mix" capability.

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

   An sdp media attribute is defined here, named "rtt-mix", without any
   parameter.  It is intended to be used in "text" media descriptions
   with "text/red" and "text/t140" formats.  It indicates capability to
   use source indications in the CSRC list according to this
   specification.  Syntax:


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

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

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

   A party MUST NOT transmit packets with redundancy format according to
   this specification to a party who has not expressed "rtt-mix"

12.  Examples

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

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

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

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

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

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

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

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

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

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

13.  Performance considerations

   This specification allows new text from one source per packet.
   Packets are transmitted with timed intervals.  The default
   transmission interval is 300 ms for RFC 4103[RFC4103], and is
   suitable for transmission from single sources.  However when more

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   sources contribute to the flow, a shorter transmission interval may
   be applicable.  The transmission interval is therefore recommended to
   be 100 ms for mixers.  This interval provides for smooth flow of text
   from 5 sources simultaneously.

14.  Presentation level considerations

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

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

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

   Further presentation level considerations are out of scope for this

15.  Congestion considerations

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

16.  Acknowledgements

17.  IANA Considerations

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

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

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      | Contact name:       | IESG                                     |
      | Contact email:      |                            |
      | Attribute name:     | rtt-mix                                  |
      | Attribute syntax    | a=rtt-mix                                |
      | Attribute semantics | See RFCXXXX section <xref target="nego"/>|
      | Attribute value     | -                                        |
      | Usage level:        | media                                    |
      | Purpose:            | Indicate support for the rtp-mixer format|
      |                     | for real-time text transmission          |
      | O/A procedure       | Declarative                              |
      | Mux Category        | normal                                   |
      | Reference:          | RFCXXXX                                  |

18.  Security Considerations

   The RTP-mixer model requires the mixer to be allowed to decrypt, pack
   and encrypt secured text from the conference participants.  Therefore
   the mixer needs to be trusted.  This is similar to the situation for
   central mixers of audio and video.

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

19.  Change history

19.1.  Changes from version -00 to -01

   Editorial cleanup.

   Changed capability indication from fmtp-parameter to sdp attribute

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

   Increased the SDP negotiation section

20.  References

20.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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

   [RFC4102]  Jones, P., "Registration of the text/red MIME Sub-Type",
              RFC 4102, DOI 10.17487/RFC4102, June 2005,

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,

   [T140]     ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
              multimedia application text conversation", February 1998.

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

20.2.  Informative References

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
              Session Initiation Protocol (SIP) Event Package for
              Conference State", RFC 4575, DOI 10.17487/RFC4575, August
              2006, <>.

   [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,

Author's Address

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   Gunnar Hellstrom
   Esplanaden 30
   Vendelso  13670

   Phone: +46708204288

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