Re: [AVTCORE] Adam Roach's Discuss on draft-ietf-payload-rtp-ttml-03: (with DISCUSS and COMMENT)

["Roni Even (A)" <[e-mail]>]

James,
The OSN is the original sequence number
Roni

-----Original Message-----
From: Roni Even (A) 
Sent: Wednesday, October 16, 2019 4:19 PM
To: 'James Sandford'; Adam Roach; The IESG
Cc: [e-mail]; [e-mail]; [e-mail]
Subject: RE: Adam Roach's Discuss on draft-ietf-payload-rtp-ttml-03: (with DISCUSS and COMMENT)

Hi James,
About packet loss, If the intention is to use retransmission look at RFC4588, it encapsulate the original RTP so no need for changes in ttml. You just need to signal support in the SDP.

Roni Even

-----Original Message-----
From: James Sandford [mailto:[e-mail]]
Sent: Wednesday, October 16, 2019 3:45 PM
To: Adam Roach; The IESG
Cc: [e-mail]; Roni Even (A); [e-mail]; [e-mail]
Subject: RE: Adam Roach's Discuss on draft-ietf-payload-rtp-ttml-03: (with DISCUSS and COMMENT)

Responses in-line.

Regards,
James

>----------------------------------------------------------------------
>DISCUSS:
>----------------------------------------------------------------------
>
>Thanks for the work everyone put into this document. I think it's not 
>quite ready to publish, due to one ambiguity, one critical missing 
>feature, and the lack of guidance around fragmentation. I also have two 
>comments that I consider very important, although they don't quite rise 
>to the level of blocking publication.
>
>As always, it's possible that my DISCUSS points are off-base, and I'd 
>be happy to be corrected if I've misunderstood anything here.
>
>-----------------------------------------------------------------------
>----
>
>§4.1:
>
>
>>     When the document spans more
>>     than one RTP packet, the entire document is obtained by
>>     concatenating User Data Words from each contributing packet in
>>     ascending order of Sequence Number.
>
>This is underspecified, in that it doesn't make it clear whether it 
>would be valid to split a single UTF-8 or UTF-16 character between RTP 
>packets, and it is nearly certain that different implementations will 
>make different assumptions on this point, leading to interop failures.
>For example, the UTF-8 encoding of '¢' is 0xC2 0xA2. Would it be valid 
>to place the "0xC2" in one packet and the "0xA2" in a subsequent packet?
>
>Without specifying this, it is quite likely that some implementations 
>will use, e.g., UTF-8 strings to accumulate the contents of RTP 
>packets; and most such libraries will emit errors or exhibit unexpected 
>behavior if units of less than a character are added at any time.  (The 
>same point holds for splitting a UTF-16 byte across packets).
>
>I don't think it much matters which choice you make (explicitly 
>allowing or explicitly forbidding splitting characters between 
>packets), but it does need to be explicit. I have a slight personal 
>preference for requiring that characters cannot be split (both for ease 
>of implementation on the receiving end and to more smoothly handle 
>missing data due to extended packet loss), but leave it to the authors and working group to decide.

Thanks for highlighting this. I also have a slight preference for not splitting characters.

>-----------------------------------------------------------------------
>----
>
>Unlike other definitions to convey non-loss-resilient data on RTP 
>streams, this document had no defined mechanism to deal with packet 
>loss. This makes it unusable on the public Internet, where packet loss 
>is an inevitable feature of the network. The existing text-in-RTP 
>specifications define procedures to deal with such loss (see, e.g., RFC 4103 section 4 and RFC 4396 section 5).

I think RFC 4396 Section 5 could be adapted for our purposes. The main issue is the TTML payload document currently makes use of the Sequence Number to identify the order of document fragments. A re-transmitted fragment would have a different Sequence Number so there would be no way to match equivalent packets. A potential solution is to use the 16 Reserved bits for a new counter that performs the task of identifying fragment order.

How would this affect the progress of this document? While it represents a major change to the layout of the payload, the mechanisms used will be identical.

>-----------------------------------------------------------------------
>----
>
>This format is rather unique in that it, alone among all other RTP text 
>formats, is designed to send monolithic documents that may stretch into 
>the multiple kilobyte range.  While fragmentation is mentioned as a 
>possibility, the document provides no implementation guidance about 
>when to fragment documents, and what sizes each fragment should assume.
>RFC 4396 section 4.4 is an example of the kind of information I would 
>expect to see in a document like this, with emphasis on the fact that 
>TTML documents are going to frequently exceed the PTMU for a typical network connection.

I would not be opposed to an equivalent section to this. I'll work on adapting the referenced section.

>----------------------------------------------------------------------
>COMMENT:
>----------------------------------------------------------------------
>
>§1:
>
>>  TTML (Timed Text Markup Language)[TTML2] is a media type for 
>> describing timed text such as closed captions (also known as
>>  subtitles) in television workflows or broadcasts as XML.
>
>Although superficially similar, there are important distinctions 
>between subtitles (intended to help a hearing audience exclusively with 
>spoken dialog, typically because the audio is in a different language 
>or otherwise difficult to
>understand) and closed captions (intended to aid deaf or 
>hard-of-hearing viewers by providing a direct, word-for-word 
>transcription of dialog as well as descriptions of all other audio 
>present). Calling one "also known as" the other is incorrect.
>
>I suggest rephrasing as:
>
>   TTML (Timed Text Markup Language)[TTML2] is a media type for
>   describing timed text such as closed captions and subtitles
>   in television workflows or broadcasts as XML.

This isn't strictly true. At least in the UK, the term subtitles is used as a catch-all term to describe both translation subtitles/those to support unintelligible speech, and subtitles for the deaf and hard of hearing. That said, I have no objection to the alternative wording.

>-----------------------------------------------------------------------
>----
>
>§4.2.1.1:
>
>>  The TTML document instance MUST use the "media" value of the 
>> "ttp:timeBase" parameter attribute on the root element.
>
>This statement makes an assumption that the 
>"http://www.w3.org/ns/ttml#parameter" namespace MUST be mapped to the "ttp"
>prefix, which is both bad form and probably not what is intended. I 
>suggest rephrasing as:
>
>   The TTML document instance MUST include a "timeBase" element from
>   the "http://www.w3.org/ns/ttml#parameter" namespace containing
>   the value "media".

I have spoken to Nigel Megitt (chair of the W3C Timed Text Working Group) about this. He noted that "timeBase" is an attribute not an element. The following alternative is verbose but unambiguous:

    The TTML document instance's root "tt" element in the "http://www.w3.org/ns/ttml" namespace MUST include a "timeBase" attribute in the "http://www.w3.org/ns/ttml#parameter" namespace containing the value "media".