netdev - Re: UDP path MTU discovery

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Date:	Tue, 30 Mar 2010 08:16:27 +0200
From:	"Edgar E. Iglesias" <edgar.iglesias@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Templin, Fred L" <Fred.L.Templin@...ing.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Rick Jones <rick.jones2@...com>, Glen Turner <gdt@....id.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: UDP path MTU discovery

On Tue, Mar 30, 2010 at 07:20:44AM +0200, Andi Kleen wrote:
> On Mon, Mar 29, 2010 at 04:38:49PM -0700, Templin, Fred L wrote:
> > > 1) 4096 bytes UDP messages... well...
> > > 2) Using regular TCP for DNS servers... well...
> > > 
> > > I believe some guys were pushing TCPCT (Cookie Transactions) for this
> > > case ( http://tools.ietf.org/html/draft-simpson-tcpct-00.html )
> > > 
> > > (That is, using an enhanced TCP for long DNS queries... but not only for
> > > DNS...)
> > 
> > IPv4 gets by this by setting DF=0 in the IP header, and
> > lets the network fragment the packet if necessary. IPv6 can
> > similarly get by this by having the sending host fragment
> > the large UDP packet into IPv6 fragments no longer than
> > 1280 bytes each.
> 
> That's true -- in theory the UDP app unwilling/unable to do proper ptmudisc 
> could set the path mtu to 1280 + header and still keep path mtu discovery off 
> and then just fragment. 
> 
> Drawback would be of course suboptimal network use with too small MTUs
> in the common case.
>
> Right now there is no right socket option to set the path mtu. We
> have a IP_MTU option, but it only works for getting the MTU.
> That's because the PMTU is in the routing cache entry and shared
> by multiple sockets. Presumably one could add a special case
> with an MTU in the socket overriding the one in the destination entry.

Sorry I'm not following you here.. Why do you need to set the MTU?

IIUC:
UDP is supposed to preserve datagram boundaries, so the sender should
when seeing an EMSGSIZE, read the PMTU and avoid sending further UDP
packets larger than that. Userspace has control over the UDP datagram
size. If it can, the app will also at this point retransmit any recent
packets that went out larger than the fresh PMTU.

If you don't want to hassle with all of that, the app can stick to
1280 (or I guess for the extreme/lazy cases turn on fragmentation)..

Cheers
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