netdev - Re: UDP splice

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Date:	Mon, 24 Jun 2013 19:01:19 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Ricardo Landim <ricardolan@...il.com>, netdev@...r.kernel.org
Subject: Re: UDP splice

On Mon, Jun 24, 2013 at 05:02:56PM +0100, Ben Hutchings wrote:
> On Mon, 2013-06-24 at 17:51 +0200, Hannes Frederic Sowa wrote:
> > On Mon, Jun 24, 2013 at 04:42:34PM +0100, Ben Hutchings wrote:
> > > splice() works with streams, but UDP is a message-oriented protocol.
> > > How would a UDP implementation of splice() decide where to put the
> > > message boundaries, or to distinguish the messages?
> > 
> > Splicing a pipe to udp socket should work nontheless. Splicing from udp socket
> > to pipe does not work.
> 
> Should it?  I suppose it could work for UDP-based protocols that include
> their own framing, but I can't see that it's generally useful.

It is the same with sendfile: One splice-call would result in one udp
message and could span several ip fragments. So, message boundaries
would be maintained. splice is good for large data but the generation of
fragments defeat its use here.

I don't know what would be most suitable to use for copying udp fragments
from one socket to another socket. I would have tried recvmmsg/sendmmsg.

Greetings,

  Hannes

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