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Date:   Tue, 30 Jun 2020 10:27:12 -0700
From:   Jakub Kicinski <>
To:     Oliver Herms <>
Subject: Re: [PATCH v3] IPv4: Tunnel: Fix effective path mtu calculation

On Tue, 30 Jun 2020 12:21:14 +0200 Oliver Herms wrote:
> On 30.06.20 08:22, Jakub Kicinski wrote:
> > On Fri, 26 Jun 2020 00:44:35 +0200 Oliver Herms wrote:  
> >> The calculation of the effective tunnel mtu, that is used to create
> >> mtu exceptions if necessary, is currently not done correctly. This
> >> leads to unnecessary entries in the IPv6 route cache for any
> >> packet send through the tunnel.
> >>
> >> The root cause is, that "dev->hard_header_len" is subtracted from the
> >> tunnel destionations path mtu. Thus subtracting too much, if
> >> dev->hard_header_len is filled in. This is that case for SIT tunnels
> >> where hard_header_len is the underlyings dev hard_header_len (e.g. 14
> >> for ethernet) + 20 bytes IP header (see net/ipv6/sit.c:1091).  
> > 
> > It seems like SIT possibly got missed in evolution of the ip_tunnel
> > code? It seems to duplicate a lot of code, including pmtu checking.
> > Doesn't call ip_tunnel_init()...  
> Are you open for patches cleaning this up?

Certainly! Maybe some of the oddities are justified, but cleanup /
re-aligning with the rest of ip_tunnels would be nice.

Not sure how much of it is qualifying as a bug, so perhaps two series
would be needed - one for net / stable with bug fixes and another of
pure cleanups for net-next?

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