netdev - Re: [PATCH net] gre: Fix MTU sizing check for gretap tunnels

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Message-ID: <CALnjE+oXbT+J01A-9GRxAAVuANaVA6ifF5_oc6ZSWRY2TTVUBw@mail.gmail.com>
Date:	Thu, 11 Jul 2013 16:19:17 -0700
From:	Pravin Shelar <pshelar@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	netdev@...r.kernel.org, jesse@...ira.com, davem@...emloft.net
Subject: Re: [PATCH net] gre: Fix MTU sizing check for gretap tunnels

On Thu, Jul 11, 2013 at 3:45 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2013-07-11 at 15:24 -0700, Pravin Shelar wrote:
>> On Thu, Jul 11, 2013 at 1:12 PM, Alexander Duyck
>> <alexander.h.duyck@...el.com> wrote:
>> > This change fixes an MTU sizing issue seen with gretap tunnels when non-gso
>> > packets are sent from the interface.
>> >
>> > In my case I was able to reproduce the issue by simply sending a ping of
>> > 1421 bytes with the gretap interface created on a device with a standard
>> > 1500 mtu.
>> >
>> > This fix is based on the fact that the tunnel mtu is already adjusted by
>> > dev->hard_header_len so it would make sense that any packets being compared
>> > against that mtu should also be adjusted by hard_header_len and the tunnel
>> > header instead of just the tunnel header.
>> >
>> we can simplify code by not doing dev->hard_header_len adjustment to tunnel-mtu.
>>
>> And right thing would be adjusting tunnel-mtu according to rt->dst.dev
>> header-len so that we get mtu for out going path.
>
> What's the mtu value we want to put in the ICMP message ?
>
>
I think it should be max payload that tunnel-device can take for that
route. Something like (route_mtu - (tunnel_header_len + iph_len +
route_dev->header_len))

gre is been using dev->hard_header_len rather than
rt_dev->hard_header_len for long time which does not look right.
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