Subject: Header Extensions

From: Peter Parnes <[email-masked]>
Date: Sun, 02 Apr 2000 18:21:06 +0200

Hi

I would like to get a clarification on how header extensions should be
handled. 

How should an application behave if it receives an RTP video packet (say
H.261) and that packet has an unknown header extension? Should the
receiving application ignore the whole packet or should it just jump
over the header extension part and continue handling the packet as if
there wasn't any extension at all? 

/P

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From: Stephen Casner <[email-masked]>
Date: Mon, 3 Apr 2000 01:40:16 -0700 (Pacific Daylight Time)

Peter,

> there wasn't any extension at all? 

That decision is up to the application; either choice is acceptable.
The length field is explicit in the header extension definition to
allow the header extension be ignorable.  That is the choice I would
normally make.

Was your question triggered by the comment during the recent AVT
meeting about the vat and vic applications having demonstrated
interoperability on the header extension feature by simply dropping
all packets with the X bit set?  As I said, that is an acceptable
response.  In this particular case, I believe it was an expression of
policy by the implementors to say that use of header extensions should
be minimized.
							-- Steve

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From: Peter Parnes <[email-masked]>
Date: Mon, 03 Apr 2000 20:01:33 +0200

My question was triggered by VIC's behavior although I had completely
missed the comments on the last AVT meeting.  

Anyhow, the application for using the extension header is in a
application I call mRobot, a simple echo application for audio, video
(and the other tools that Marratech Pro provides like chat, Web, file
dist and WB). I use the ext header to identify that the data has been
echoed via a mRobot to prevent loops if more than one mRobot is started
on the same group-port. Just looking at the RTCP is not a solution as it
can be lost (especially if a loop was created and the mRobots start
eating up all the bandwidth :-). The usage of the mRobot is to easier
debug mcast connectivity for end users by having a number of mRobots in
the same session at different geographical locations. 

As UCL seem to be the happy developers of vic these days, do you have
any plans to change vic to just ignore the extension header and not the
whole packet? 

-P