RTP-mixer formatting of multi-party Real-time text

1.1. Selected solution and considered alternative

A number of alternatives were considered when searching an efficient multi-party method for real-time text. This section explains a few of them briefly.¶

One RTP stream per source, sent in the same RTP session with "text/red" format.

From some points of view, use of multiple RTP streams, one for each source, sent in the same RTP session, called the RTP translator model in RFC 3550 [RFC3550], would be efficient, and use exactly the same packet format as RFC 4103, the same payload type and a simple SDP declaration. However, there is currently lack of support for multi-stream RTP in certain implementation technologies. The multi-stream solution would also cause more overhead than a single RTP stream solution "text/rex" specified in this document and more the more simultaneous sending participants there are.¶

The "text/red" format in RFC 4103 with shorter transmission interval, and indicating source in CSRC.

The "text/red" format with "text/t140" payload in a single RTP stream can be sent with 100 ms packet intervals instead of the regular 300 ms. The source is indicated in the CSRC field. Source switching can then be done every 300 ms while simultaneous transmission occurs. With two participants sending text simultaneously, the switching and transmission performance is good. With three or more simultaneously sending participants, there will be a noticable jerkiness in text presentation, more the more participants who send text simultaneously. With three sending participants, the jerkiness will be about 450 ms, and with five, about 1350 ms. Text sent from a source at the end of the period its text is sent by the mixer will have close to zero extra delay. Recent text will be presented with no or low delay. The 1350 ms jerkiness will be noticable and slightly unpleasant, but corresponds in time to what typing humans often cause by hesitation or changing position while typing. A benefit of this method is that no new packet format needs to be introduced and implemented. Since simultaneous typing by more than two parties is rare, and in many applications also more than three parties in a call is rare, this method can be used successfully without its limitations becoming annoying. Negotiation is based on a new sdp media attribute "rtt-mix-rtp-mixer".¶

The "text/rex" packet format with up to 15 sources in one packet.

The mechanism called "text/rex" specified in this document makes use of the RTP mixer model specified in RFC3550[RFC3550]. Text from up to 15 sources can be included in each packet. Packets are normally sent every 300 ms. The mean delay will be 150 ms. The sources are indicated in the CSRC list of the RTP packets. A new redundancy packet format is specified, named "text/rex".¶

The presentation planned by the mixer for multi-party unaware endpoints.

It is desirable to have a method that does not require any modifications in existing user devices implementing RFC 4103 for RTT without explicit support of multi-party sessions. This is possible by having the mixer insert a new line and a text formatted source label before each switch of text source in the stream. Switch of source can only be done in places in the text where it does not disturb the perception of the contents. Text from only one source can be presented in real time at a time. The delay will therefore be varying. The method has also other limitations, but is included in this document as a fallback method. In calls where parties take turns properly by ending their entries with a new line, the limitations will have limited influence on the user experience. while only two parties send text, these two will see the text in real time with no delay.¶

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].¶

1.3. Intended application

The methods for multi-party real-time text are 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. An often mentioned application is for emergency service calls with real-time text and voice, where a calltaker want to make an attended handover of a call to another agent, and stay observing in the session. Multimedia conference sessions with support for participants to contribute in text is another application. Conferences with central support for speech-to-text conversion is yet another mentioned application.¶

In all these applications, normally only one participant at a time will send long text utterances. In some cases, one other participant will occasionally contribute with a longer comment simultaneously. That may also happen in some rare cases when text is interpreted to text in another language in a conference. Apart from these cases, other participants are only expected to contribute with very brief utterings while others are sending text.¶

Text is supposed to be human generated, by some text input means, such as typing on a keyboard or using speech-to-text technology. Occasional small cut-and-paste operations may appear even if that is not the initial purpose of real-time text.¶

The real-time characteristics of real-time text is essential for the participants to be able to contribute to a conversation. If the text is too much delayed from typing a letter to its presentation, then, in some conference situations, the opportunity to comment will be gone and someone else will grab the turn. A delay of more than one second in such situations is an obstacle for good conversation.¶

2.1. Negotiated use of the RFC 4103 format for multi-party in a single RTP stream

This section specifies use of the current format specified in [RFC4103] for true multi-party real-time text. It is an update of RFC 4103 by a clarification on one way to use it in the multi-party situation. It is done by completing a negotiation for this kind of multi-party capability and by indicating source in the CSRC element in the RTP packets. Please use [RFC4103] as reference when reading the following description.¶

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

        m=text 11000 RTP/AVP 100 98
        a=rtpmap:98 t140/1000
        a=rtpmap:100 red/1000
        a=fmtp:100 98/98/98
        a=rtt-mix-rtp-mixer

   Answer example  from a multi-party capable device
        m=text 11000 RTP/AVP 100 98
        a=rtpmap:98 t140/1000
        a=rtpmap:100 red/1000
        a=fmtp:100 98/98/98
        a=rtt-mix-rtp-mixer

   Offer example for "text/red" format including multi-party
   and 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 100 98
         a=rtpmap:98 t140/1000
         a=rtpmap:100 red/1000
         a=fmtp:100 98/98/98
         a=rtt-mix-rtp-mixer

    Answer example from multi-party capable device with 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 100 98
         a=rtpmap:98 t140/1000
         a=rtpmap:100 red/1000
         a=fmtp:100 98/98/98
         a=rtt-mix-rtp-mixer

  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

  |----------------|
  |Seq no 1        |
  |CC=1            |
  |CSRC list A     |
  |R2: A1          |
  |R1: A2          |
  |P:  A3          |
  |----------------|

  |----------------|
  |Seq no 2        |
  |CC=1            |
  |CSRC list A     |
  |R2  A2          |
  |R1: A3          |
  |P: Empty        |
  |----------------|
  Nothing needs to be retrieved from this packet.

  X----------------|
  X Seq no 3       |
  X CC=1           |
  X CSRC list A    |
  X R2: A3         |
  X R1: Empty      |
  X P:  Empty      |
  X----------------|
  Packet 3 is assumed to be dropped in network problems

  X----------------|
  X Seq no 4       |
  X CC=1           |
  X CSRC list B    |
  X R2: Empty      |
  X R1: Empty      |
  X P2: B1         |
  X----------------|
  Packet 4 contains text from B, assumed dropped in network problems.
  The mixer is assumed to have received text from A on turn to send.
  Sending of text from B must therefore be temporarily ended by
  sending redundancy twice.

  X----------------|
  X Seq no 5       |
  X CC=1           |
  X CSRC list B    |
  X R2: Empty      |
  X R1: B1         |
  X P:  Empty      |
  X----------------|
  Packet 5 is assumed to be dropped in network problems

  |----------------|
  |Seq no 6        |
  |CC=1            |
  |CSRC list B     |
  | R2: B1         |
  | R1: Empty      |
  | P:  Empty      |
  |----------------|

2.2. Use of an extended packet format "text/rex" with text from multiple sources

The method specified in this section called "text/rex" has higher performance than the previous method. Text from up to 15 sources can be included in each packet. This may be of value in large non-managed conferences.¶

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

    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 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | "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.

 Offer example for just "text/rex" 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

 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 "text/red" and "text/rex" multi-party support:

      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
      a=rtt-mix-rtp-mixer

 Answer example from multi-party capable device using "text/rex".
      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
       a=rtt-mix-rtp-mixer

  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

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

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

|----------------|
|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

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        |
|----------------|

2.3. Mixing for multi-party unaware endpoints

A method is specified in this section for cases when the participating endpoint does not implement any solution for multi-party presentation of real-time text. The solution requires the mixer to insert text dividers and readable labels and only send text from one source at a time until a suitable point appears for source change. This solution is a fallback method with functional limitations that acts on the presentation level and is further specified in Section 3.2.¶

3.1. 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.¶

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.¶

_____________________________________________________________________
|       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.¶

4.1. 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]¶

4.2. 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.¶

10.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07

Added a method based on the "text/red" format and single source per packet, negotiated by the "rtt-mix-rtp-mixer" sdp attribute.¶

Added reasoning and recommendation about indication of loss.¶

The highest number of sources in one packet is 15, not 16. Changed.¶

Added in information on update to RFC 4103 that RFC 4103 explicitly allows addition of FEC method. The redundancy is a kind of forward error correction..¶

10.2. 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.¶

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

nomenclature and editorial improvements¶

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

10.4. 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.¶

10.5. 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-¶

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-¶

10.6. 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.¶

10.7. 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.¶

10.8. 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.¶

10.9. 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.¶

10.10. 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.¶

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.¶

10.11. 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¶

11.1. 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>.

[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>.

11.2. 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>.