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Re: IPsec and Path MTU

To: "David S. Miller" <davem@xxxxxxxxxx>
Subject: Re: IPsec and Path MTU
From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Fri, 18 Jun 2004 09:12:41 +1000
Cc: kuznet@xxxxxxxxxxxxx, jmorris@xxxxxxxxxx, netdev@xxxxxxxxxxx
In-reply-to: <20040617152921.730892c7.davem@xxxxxxxxxx>
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On Thu, Jun 17, 2004 at 03:29:21PM -0700, David S. Miller wrote:
> On Fri, 18 Jun 2004 07:38:32 +1000
> Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> > Suppose that the MTU of is 1500, and that the calculated MTU
> > for the bundle is 1430.
> > 
> > If there is a host on the Internet or behind some sort
> > a VPN where the path from to it has an MTU of 1200,
> > then by sending a 1430-byte packet to from,
> > we will get back an ICMP packet saying that the largest MTU for
> > is 1200.
> > 
> > This will be successfully stored in the route entry.  But the route
> > entry's MTU is not used at all since the MTU of the bundle is deduced
> > from the MTU of the path,  So we'll continue to send large
> > packets to
> This is what Alexey is talking about.  When we send a packet out for
> an IPSEC rule, we have to remember the inner (per-transform pre-tunnel)
> IP addresses (keyed by outer IP address and ESP/AH spi) in order to get
> the ICMP PMTU messages handled correctly.  We don't do this right now,
> it's difficult and complicated work.

Right, that's *what* Alexey is talking about.  But it's *not* what I'm
talking about :)

In my case, the ICMP message is not coming from the remote IPsec gateway
or a router in front of it.  It's coming from a host behind it.  So
the original IP header is in the ICMP message, in the clear.
> It's an issue not specific to making the gateway be the sender of
> the packet, it's an issue with tunnels in all cases currently.

Correct.  But before we get to that, let's fix the simple case first.

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