Subject: [AVT] retransmission notes ...

From: John Lazzaro <[email-masked]>
Date: Thu, 2 Jan 2003 12:07:02 -0800

Hi everyone,

	Recently read over draft-ietf-avt-rtp-retransmission-03.txt,
a few observations:

  [1] The apt SDP usage seems to indicate that a session cannot
  assign the same PTYPE value for two streams in a session, even
  if these streams both use the same codec + parameters.

  [2] The semantics for multiple payload numbers on an m= line
  shown in Section 8.4 seem to imply that the streams corresponding
  to both payload numbers on the m= line are sent simultaneously.

I'm wondering about compatibility issues regarding these two points.

For example, RFC 3388 Section 7.4 (on FID semanics) seems to use the
ability of two streams in a session to have the same PTYPE number to
execute some of its semantics. So, [1] may make it impossible to use
these semantics and retransmission in the same session. Although, the
lack of dynamic payload examples in 3388 makes it hard to say for sure
if this is an issue -- maybe parsers of FID SDP are supposed to track
down the dynamic payload definitions and compare mime-types?

I can't come up with a complementary example for [2], but I listed it
in case someone else reading can ... (maybe offer/answer semantics?).
Usually, only one of the PTYPEs on the m= line is being sent at one
time, but senders may switch between them, but [2] above is different.

--

Overall, the document reads well. Editorially, the last sentence in
the first full paragraph on page 7 could use a rewrite, as could the
second paragraph Section 6.3 -- both are unclear unless you already
know what the text is trying to say.

-------------------------------------------------------------------------
John Lazzaro -- Research Specialist -- CS Division -- EECS -- UC Berkeley
lazzaro [at] cs [dot] berkeley [dot] edu     www.cs.berkeley.edu/~lazzaro
-------------------------------------------------------------------------
_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

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From: [email-masked]
Date: Mon, 6 Jan 2003 16:05:02 -0600

John,

>   if these streams both use the same codec + parameters.

No. If two streams use the same codec and parameters, they can use the same payload type value. The confusion may stem from the fact that declaring a single payload type in the m line is enough to send multiple RTP streams of this payload type in the same session. In other words, you don't need to declare the payload type twice if you want to send two streams. Note that in this case, one must be sure to associate a retransmission stream with the correct original stream as explained in Section 5.3.

>   to both payload numbers on the m= line are sent simultaneously.

There's no such  requirement. Both payload types could be sent concurrently or there could be switching between different payload types in the RTP stream.

> execute some of its semantics. So, [1] may make it impossible to use

I don't get this point.

> these semantics and retransmission in the same session. Although, the

This is probably true. You can use the FID semantics for only one purpose within a session e.g. radio bearer selection or DTMF tones as explained in the FID document or retransmission. I think it's a limitation we can live with.

> time, but senders may switch between them, but [2] above is different.

Do the answers to point [1] and [2] above answer these objections to the use of the FID semantics for the grouping of the original and retransmission session?

Let me know if anything is still unclear.

> know what the text is trying to say.

Thanks for pointing this out. We'll try to improve the text.

David.

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From: John Lazzaro <[email-masked]>
Date: Mon, 6 Jan 2003 15:56:20 -0800

>  [email-masked] writes:
>
> No. If two streams use the same codec and parameters, they can use the
> same payload type value.

Oh, OK -- then the five.example.com example in Section 7.4.1 of RFC 3388
should work OK in conjunction with retransmission ...