Re: [MMUSIC] T.140 in Data Channel scenarios

[Gunnar Hellström <[e-mail]>]

Hi,

Hi Bernard,



Gunnar will probably be able to give a better answer, but below are some comments from me.



>At the W3C TPAC 2019 meeting last week, draft-ietf-mmusic-t140-usage-data-channel came up as part of a discussion of Accessible RTC Use Cases:

>https://www.w3.org/WAI/APA/wiki/Accessible_RTC_Use_Cases

>

>Here are a few of the questions that came up.

>

>Section 3

>

>       +--------------------------+-------------------------------+

>       | Subprotocol Identifier   | t140                          |

>       | Transmission reliability | reliable                      |

>       | Transmission order       | in-order                      |

>       | Label                    | See Section 4.1<https://tools.ietf.org/html/draft-ietf-mmusic-t140-usage-data-channel-05#section-4.1> and Section 6<https://tools.ietf.org/html/draft-ietf-mmusic-t140-usage-data-channel-05#section-6> |

>       +--------------------------+-------------------------------+

>

> Are there any situations in which unreliable or partially reliable transport might be appropriate?

> For example, some participants envisaged scenarios in which low latency communications might be

>  desirable, such as in emergencies, and questioned whether it might make sense to use the maxPacketLifetime

> or maxRetransmissions parameters.

>

> So there was a question about whether it might ever make sense to support an unreliable data channel (possibly with redundancy).



T.140 requires the transport mechanism to keep packets in order, and without duplication. When using RTP, the sequence number is used to detect out-of-order packets and loss of packets. You don’t have that mechanism in a data channel, and T.140 itself does not contain a sequence number.



When using RTP, reliability can provided by sending of redundant data, or by using FEC. FEC is RTP-specific, so you can’t use that on a data channel.



I don’t think the redundancy mechanism can be used on the data channel either, because it uses the RTP header timestamp.



And, you can not re-transmit T.140 text on a data channel (T.140 RTP packets are not re-transmitted either), because the receiver will not be able to detect it (again, because T.140 does not contain a sequence number).



Of course we could have defined an “envelope” for the data channel T.140 text, with sequence numbers. But, the idea was to simply send the T.140 text as the data channel itself provides delivery and reliability.



[GH] I agree with Christer. There is a risk that during very severe conditions, the reliable SCTP association makes a number of retries for up to 30 seconds and then breaks,

But the chance to get understandable text through in such conditions by sending in an unreliable channel with redundancy is probably not much higher. And it would require to for example use RTP and RFC 4103 on top of an unreliable data channel, a complication I would not like to introduce.

I made a bit more comments on this on August 21.





---



>Lost information (compared with RTP)

>

>We had some questions relating to information "lost in translation" between realtime text

>and data channel.  This includes aspects of the RTP header, including SSRCs, timestamps and sequence numbers.



Yes, that information is lost.



There were some discussions about including SSRC somehow, in order to support conferences where a mixer uses a single data channel for text received from multiple remote users, but we decided that it is outside the scope of this draft.


[GH] The Stream-ID can be seen as "an SSRC", and the Label can be seen as a CNAME in one direction. Sequence number and timestamp are usually only used on transport level, and something similar is likely used in the reliable data channel transport, but not reported to the application.

A concern though: If the reliable channel breaks because of bad conditions, can we then really know on both sides what has been transmitted so that if we get a successful reconnection we can avoid resending already received text?


Regards

Gunnar





Regards,



Christer