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

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 and an extended packet format 'text/rex'.¶

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

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

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

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, using RFC 4103 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 and the redundancy header in the RTP-packet.¶

An extended packet format 'text/rex' is specified for this purpose, providing the possibility to include text from up to 16 sources in each packet in order to enhance mixer source switching performance.¶

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

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

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

2. Use of fields in the RTP packets

RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a redundancy format "text/red" for increased robustness. 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 and extending the redundancy header in order to enhance the performance in multi-party RTT sessions.¶

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

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

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

The picture below shows a typical 'text/rex' 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|  "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   | "A-R2" T.140 encoded redundant data           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
   |   |  "A-R1" T.140 encoded redundant data  |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
   | "A-P" T.140 encoded primary data  |                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
   |     "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 1:A 'text/rex' packet with text from three sources A, B, C.

3. Actions at transmission by a mixer

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

As soon as a participant is known to participate in a session and being available for text reception, a Unicode BOM character SHALL be sent to it according to the procedures in the present document.¶

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

For multi-party operation, it is RECOMMENDED that the mixer sends a packet to each receiver as soon as text has been received from a source as long as the maximum number of characters per second indicated by the recipient is not exceeded, and also the number of packets sent per second to a recipient is kept under a specified number. This number SHALL be 10 if no other limit is applied for the application. The intention is to keep the latency introduced by the mixer low.¶

The mixer has its own SSRC, and its own RTP sequence number series.¶

The number of redundant generations of T140blocks to include in transmitted packets SHALL be deducted from the SDP negotiation. It SHOULD be set to the minimum of the number declared by the receiver and the transmitter. The same number of redundant generations MUST be used for all sources in the transmissions. The number of generations sent to a receiver SHALL be the same during the whole session unless it is modified by session renegotiation.¶

At time of transmission, the mixer SHALL populate the RTP packet with T140blocks combined from all T140blocks queued for transmission originating from each source as long as this is not in conflict with the allowed number of characters per second or the maximum packet size. These T140blocks SHALL be placed in the packet interleaved with redundant T140blocks and new T140blocks from other sources. The primary and redundant T140blocks from each source are grouped together in "source groups" appearing in the packet in the same order as the corresponding CSRC list member and redundancy header members for the source group. The format of the redundancy header members and the T140blocks in each source group are the same as for the single source possible in the "text/red" format. The SSRC of each source shall be placed as a member in the CSRC-list at a place corresponding to the place of its T140blocks in the packet. The current time SHALL be inserted in the timestamp. The timestamp offset values for empty T140blocks have no relevance but SHOULD be assigned realistic values.¶

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

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

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

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

The timer offset values are inserted in the redundancy header, with the time offset from the RTP timestamp in the packet when the corresponding T140block was sent from the mixer as primary.¶

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

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

4. Actions at reception

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

If the "CC" field value of a received packet is >1, it indicates that multi-party transmission is active, and the receiver MUST act on the different sources according to its role. If the CC field value is 1, it indicates that there is a mixer between the source and the receiver, but that there is only one source at the moment. If the CC value is 0, the connection is point-to-point.¶

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

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

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

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

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

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

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

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

5. RTCP considerations

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

6. Chained operation

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

7. Usage without redundancy

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

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

8. Use with SIP centralized conferencing framework

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

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 total list of participants in a conference. With audio and video media, the CSRC-list would often contain all participants who are not muted whereas text participants that don't type are completely silent and thus are not represented in RTP packet CSRC-lists.¶

11. Examples

This example shows a symbolic flow of packets from a mixer with loss and recovery. A, B and C are sources of RTT. M is the mixer. Pn indicates primary data in source group "n". Rn1 is first redundant generation data and Rn2 is second redundant generation data in source group "n". A1, B1, A2 etc are text chunks (T140blocks) received from the respective sources. X indicates dropped packet between the mixer and a receiver.¶

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

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

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

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.¶

12. Performance considerations

This specification allows new text from 16 sources per packet. Packets SHOULD be transmitted with a maximum time interval when there is text to be transmitted (either primary or redundant) . The default maximum transmission interval is 300 ms. It is also RECOMMENDED for the mixer to send a packet as soon as text has been received from a source as long as the maximum number of characters per second indicated by the recipient is not exceeded, and also the number of packets sent per second to a recipient is kept under a number. In order to achieve good performance, a receiver for multi-party calls SHOULD declare a sufficient CPS value in SDP for the number of allowable characters per second. These characteristics provide for smooth flow of text with acceptable latency from at least 32 sources simultaneously.¶

The default maximum rate of reception of real-time text is in RFC 4103 [RFC4103] specified to be 30 characters per second. The value MAY be modified in the CPS parameter of the FMTP attribute in the media section for RFC 4103. A mixer combining real-time text from a number of sources may have a higher combined flow of text coming from the sources. Endpoints SHOULD therefore specify a suitable higher value for the CPS parameter, corresponding to its real reception capability. A value for CPS of 150 is the default for the "text/rex" format. See RFC 4103 [RFC4103] for the format and use of the CPS parameter. The same rules apply for the "text/rex" format except for the default value.¶

13. 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. Implementations MUST be able to ignore optional control codes that they do not support.¶

There is no strict "message" concept in real-time text. Line Separator SHALL be used as a separator allowing a part of received text to be grouped in presentation. The characters "CRLF" may be used by other implementations as replacement for Line Separator. The "CRLF" combination SHALL be erased by just one erasing action, just as the Line Separator. Presentation functions are allowed to group text for presentation in smaller groups than the line separators imply and present such groups with source indication together with text groups from other sources (see the following presentation examples). Erasure has no specific limit by any delimiter in the text stream.¶

              _________________________________________________
             |                                              | |
             |[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                           |
             |________________________________________________|

_____________________________________________________________________
|       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   |
|___________________________________________________________________|

  _________________________________________________
 |       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     |
 |________________________|_________________________|

  _________________________________________________
 |                                              |^|
 |[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                           |
 |________________________________________________|

14. Gateway Considerations

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

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

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]. EDITOR NOTE ----- more about this is needed------------¶

15. Updates to RFC 4102 and RFC 4103

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

16. Congestion considerations

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

20. Change history

21. References