Mac OS X - Multicast UDP not working?

[Johnny Turpin]

Cool! I did some snooping around here trying to find out why we have some
encoders spitting out quite large UDP packets, but I haven't really been
able to come to any conclusion other than it was because our QA was testing
the encoders with lots of different values, and apparently some legacy
(older) decoders expected larger UDP packet sizes.

I agree that having UDP packets split across ethernet frames is not the most
efficient way of transmitting data, but its looks like there are encoders
out there (like ours and v-brick) which allow this option. Anyway, thanks
for adding the option to VLC.

-johnny

> -----Original Message-----
> From: Christophe Massiot [mailto:massiot at via.ecp.fr]
> Sent: Friday, July 19, 2002 2:30 PM
> To: vlc at videolan.org
> Subject: RE: Mac OS X - Multicast UDP not working?
> 
> 
> At 14:27 -0400 19/07/2002, Talbert, Scott wrote :
> 
> >I don't know why exactly VLC expects UDP packets to be < 1500 
> >bytes.  I suggested (to Christophe, on the VLC team) that they be 
> >able to handle packets up to 64K in size (the UDP limit) but he has 
> >not responded yet.
> 
> VLC expects UDP packets to be < 1500, because I don't see the point 
> in sending bigger packets on Ethernet networks. It will just increase 
> fragmentation for no gain. But anyway since people are actually doing 
> it, we'll have to find a solution.
> 
> I needed some thinking and I finally decided not to follow your 
> advice. The reason I'm skeptical is because as per the VLC structure, 
> packets are always allocated with DEFAULT_MTU size. But if the server 
> sends 1500-bytes packets, we lose 65536-1500 = 64036 bytes. There can 
> be thousands of packets in the decoder buffers, so we would explode 
> our current (quite low) memory requirements. A workaround would be to 
> memcpy the big network buffer to a buffer allocated with the right 
> size, but it has always been against our policy to add unnecessary 
> memcpy()s, for obvious performance reasons.
> 
> So what I have done is add a --mtu option. By default it is still 
> 1500 bytes, but you can change this value without recompiling. This 
> option should be added to the GUI Preferences windows, so that it can 
> be changed permanently.
> 
> -- 
> Christophe Massiot.
> 
> -- 
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