netdev - Re: UDP path MTU discovery

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Date:	Sun, 4 Apr 2010 12:25:20 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Glen Turner <gdt@....id.au>
Cc:	Andi Kleen <andi@...stfloor.org>, Rick Jones <rick.jones2@...com>,
	netdev@...r.kernel.org
Subject: Re: UDP path MTU discovery

On Fri, Apr 02, 2010 at 04:11:58PM +1030, Glen Turner wrote:
> On Thu, 2010-04-01 at 02:55 +0200, Andi Kleen wrote:
> > > What we need is an API for an instant notification that a ICMP Packet
> > > Too Big message has arrived concerning the socket.
> > 
> > That already exists of course: IP_RECVERR
> 
> Hi Andi,
> 
> So what should I code?  The suggested EMSGSIZE or your suggestion
> of grabbing all returning ICMP and parsing it?  Noting that the

You don't need to parse any ICMPs, the kernel does that for you. 
See the documentation of IP_RECVERR in ip(7). The MTU is in ee_info

First you need to enable path mtu discovery for the socket
using IP_MTU_DISCOVER.

So you can either keep track of the MTU yourself based on extended
errors coming out of IP_RECVERR, or ask the kernel using IP_MTU when
the socket is connected or simply lower when you see a EMSGSIZE. It's also 
possible to do this with a dummy socket that gets connected/unconnected too.

> second choice is pretty ugly.  That both seem specific to Linux is
> frustrating, but that is life -- adding support for an operating
> system seems to inevitably add #ifdefs for this sort of code.

Well when the other OS see the need they will hopefully add similar
interfaces, with some luck even compatible to the ones in Linux.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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