Subject: [AVT] Clarifications required on 3984

From: Archna Mittal <[email-masked]>
Date: Mon, 06 Jun 2005 14:46:47 +0530

Hi All,
   Currently, I am working on RFC 3984. I need some clarifications 
regarding the handling of Fragmented Units in case of loss of one FU. 
RFC says (Section 5.8, 2nd last paragraph):

"If a fragmentation unit is lost, the receiver SHOULD discard all   
following fragmentation units in transmission order corresponding to the 
same fragmented NAL unit"

What I am confused by the word , "FOLLOWING".

It can be interpreted 2 ways:
1) I should discard the all the following FUs. What about already 
received FUs. Should I pass these FUs to the Application?
2) Should I discard all the FUs? (i.e. nothing to pass to the application)

Any pointers are welcome.

Thanks & Regards,
-Archna
_______________________________________________
Audio/Video Transport Working Group
[email-masked]
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From: "Stephan Wenger" <[email-masked]>
Date: Mon, 6 Jun 2005 19:34:46 +0300

Hi Archna,

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of Archna
Mittal
[.]

It can be interpreted 2 ways:
1) I should discard the all the following FUs. What about already received
FUs. Should I pass these FUs to the Application?
2) Should I discard all the FUs? (i.e. nothing to pass to the application)

I believe, the language of the RFC is quite clear.  Option 1.  Whether your
decoder can meaningfully handle the resulting partly received NAL units is
an entirely different question.

The most straightforward implementation of an RTP receiver/decoder pair
would perhaps want to discard all NAL unit fragments of a given NAL unit,
when a single fragment is lost (regardless of its position).  A more
sophisticated decoder may include means to identify the final completely
coded macroblock in a fragment.  For such a decoder, it may be of advantage
to decode all correctly received macroblocks in the first consecutively and
correctly received fragments, and that is what the sentence in the RFC
refers to.  In practice, I would hope that your question (and your choice
between those alternatives) turns out to be rather irrelevant. because I
would hope that you never receive fragments when you are in a seriously
error prone environment.

Side note: We included fragmentation for the support of a few very specific
scenarios, as mentioned in the RFC.  Unless you are implementing one of
these scenarios, I would strongly suggest NOT using fragmentation in your
sender (if you are implementing a sender as well.  Except in the mentioned,
exotic scenarios, the use of appropriately sized slices will almost always
yield better results.  

Regards,

Stephan

Thanks & Regards,
-Archna