RTP Payload for Timed Text Markup Language (TTML)
draft-ietf-payload-rtp-ttml-06

Thanks for addressing my DISCUSS and COMMENT points. I have preserved
them below for posterity.

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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.

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§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.

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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).

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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.

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§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.

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§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".