Subject: mrouted tunnel, video Ok, audio not

From: Markus Buchhorn <[email-masked]>
Date: Mon, 17 Aug 1998 12:50:57 +1000

Hi All

We've found a problem very similar to what others have reported - two
subnets joined by a tunnel (here from a Sun 2.6 to an SGI IRIX 6.2, mrouted
3.8 or 3.9b3), where the video is shared no problem at all, but audio
appears to not cross the tunnel.

We did a bit more searching after we saw similar (unsolved) problems
mentioned on the mbone list. What we found in our case indicates not an
mrouted problem but something funny with vat...

We have two subnets (C and M) each with two machines as clients, and an
mrouter on each subnet connected by a single tunnel, th=32, metric 1.
mrinfo shows the right things as does mtrace apparently (I'm not one of the
dozen who understand mtrace in detail :-) )

After reading some of the comments we moved the tunnel to various
machines/architectures but always got the same behaviour.

Video (vic) between the two subnets is seen by all four clients. An audio
signal from vat is however only seen by the clients on the local subnet.
When we snoop'ed the network traffic we in fact see the audio from the
remote subnet being sent onto the local subnet - but vat is not picking it
up. 

i.e. client on subnet C sends audio to 224.2.165.185/22366, which the other
client on subnet C can see. client on subnet M runs vat on that address and
doesn't see the client from subnet C. snoop on the M client shows packets
arriving to that group and port though. Running another client on subnet M
shows both of the clients on the LAN, but not the one from subnet C - even
though the packets are there on their interfaces....

So then I looked at how we ran vat. I always ran it from sdr on subnet C,
whereas on subnet M the user ran it by hand (vat -t ttl group/port). When
the user ran it the same way as I did (as sdr starts it) everything
suddenly worked...

vat -f pcm2 -r -I 1 -t 63 -C test session 224.2.165.185/22366

The '-I' refers to the conference bus, the '-r' isn't described in my vat
man page :-) Are either of those likely to stop vat's talking to each other
if not copied at both ends ? I also presumed that the audio format was
irrelevant, since I understood that vat can pick up what encoding is being
used and decode appropriately. 

Can anybody explain what is happening ? 

This may be a trap that others have fallen into... 

Cheers,
	Markus

Markus Buchhorn,  Advanced Computational Systems CRC     | Ph: +61 2 62798810
email: [email-masked], snail: ACSys, RSISE Bldg,|Fax: +61 2 62798602
Australian National University, Canberra 0200, Australia |Mobile: 0417 281429

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From: Bill Fenner <[email-masked]>
Date: Sun, 16 Aug 1998 21:39:53 PDT

The "-r" flag causes vat to use RTPv2; absence of it means to use
the old vat encapsulation.  A vat invoked with "-r" will indeed
fail to interoperate with a vat that was not invoked with "-r".

  Bill