Subject: RTP interoperability statements for draft standard

From: Colin Perkins <[email-masked]>
Date: Fri, 03 Dec 1999 16:19:49 +0000

To the AVT working group,

As noted in the AVT meeting at the Washington DC IETF, it is necessary to
collect interoperability reports for RTP implementations before we can
progress the RTP specification and audio/video profile to draft standard.
Two internet-drafts describe the type of interoperability statement required:
	draft-ietf-avt-rtp-interop-02.txt
	draft-ietf-avt-profile-interop-00.txt
and a third draft (draft-ietf-avt-rtptest-02.txt) describes possible
testing strategies.

This message is a plea for vendors with RTP implementations to make
available results of their interoperability tests, in order that we
can advance the specifications. Results of the tests to date are 
available from http://www-mice.cs.ucl.ac.uk/multimedia/misc/avt/RTPinterop/
Please send additions to this table to me (if you have a problem with
making this information publically available, please contact me privately
and we can discuss alternatives).

Thanks,
Colin

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From: "Ram Kordale" <[email-masked]>
Date: Fri, 3 Dec 1999 15:26:06 -0800

Could someone please clarify the following "conflict" I see in the spec?

In the event of data loss, RFC 2250 has mechanisms to recover at the slice
level(basically by discarding RTP packets until you find one with the
BeginningOfSlice bit set to 1).

However, if you lose a few RTP packets, one is **really forced** to skip
until the beginning of the next GOP!

This is an incredible amount of loss or I have misunderstood the spec. Could
someone please clarify?

Details:
-------

Consider the following segment of MPEG-1 system stream.
...GOP_H P11 P12 P13 ... P1n GOP_H P21 P22 P23 ...

GOP_H refers to GOP Headers. Pij refers to picture j of the ith GOP.

For simplicity, assume that the stream and display order are the same. Thus,
Pij will have a "Temporal Reference" of j. For simplicity, assume each
picture is split into 5 RTP packets.

Suppose I receive RTP packets containing all of the first GOP header and
P11 and I receive 1 RTP packet containing the first "piece" of P12.

Suppose the next RTP packet I received contains the "Temporal Reference"
of 3, how do I know which GOP this picture belongs to. In other words, how
do I know it belongs to P13 or P23 or in general Pi3.

If I cannot know, I will have to insert a broken GOP thus essentially
losing data until the next GOP header is received.

Thanks for your help.
Ram

-- 
Ram Kordale
Advanced Media Products