netdev - Re: [PATCH] xfrm: do not set IPv4 DF flag when encapsulating IPv6 frames <= 1280 bytes.

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Date:   Mon, 30 May 2022 09:14:24 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
CC:     Linux Network Development Mailing List <netdev@...r.kernel.org>,
        "Lorenzo Colitti" <lorenzo@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Lina Wang <lina.wang@...iatek.com>
Subject: Re: [PATCH] xfrm: do not set IPv4 DF flag when encapsulating IPv6
 frames <= 1280 bytes.

On Sat, May 28, 2022 at 02:25:59AM -0700, Maciej Żenczykowski wrote:
> On Wed, May 25, 2022 at 11:51 PM Steffen Klassert
> <steffen.klassert@...unet.com> wrote:
> >
> > On Wed, May 18, 2022 at 02:05:48PM -0700, Maciej Żenczykowski wrote:
> > > From: Maciej Żenczykowski <maze@...gle.com>
> > >
> > > One may want to have DF set on large packets to support discovering
> > > path mtu and limiting the size of generated packets (hence not
> > > setting the XFRM_STATE_NOPMTUDISC tunnel flag), while still
> > > supporting networks that are incapable of carrying even minimal
> > > sized IPv6 frames (post encapsulation).
> > >
> > > Having IPv4 Don't Frag bit set on encapsulated IPv6 frames that
> > > are not larger than the minimum IPv6 mtu of 1280 isn't useful,
> > > because the resulting ICMP Fragmentation Required error isn't
> > > actionable (even assuming you receive it) because IPv6 will not
> > > drop it's path mtu below 1280 anyway.  While the IPv4 stack
> > > could prefrag the packets post encap, this requires the ICMP
> > > error to be successfully delivered and causes a loss of the
> > > original IPv6 frame (thus requiring a retransmit and latency
> > > hit).  Luckily with IPv4 if we simply don't set the DF flag,
> > > we'll just make further fragmenting the packets some other
> > > router's problems.
> > >
> > > We'll still learn the correct IPv4 path mtu through encapsulation
> > > of larger IPv6 frames.
> > >
> > > I'm still not convinced this patch is entirely sufficient to make
> > > everything happy... but I don't see how it could possibly
> > > make things worse.
> > >
> > > See also recent:
> > >   4ff2980b6bd2 'xfrm: fix tunnel model fragmentation behavior'
> > > and friends
> > >
> > > Bug: 203183943
> > > Cc: Lorenzo Colitti <lorenzo@...gle.com>
> > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Lina Wang <lina.wang@...iatek.com>
> > > Cc: Steffen Klassert <steffen.klassert@...unet.com>
> > > Signed-off-by: Maciej Zenczykowski <maze@...gle.com>
> >
> > Applied, thanks a lot!
> 
> Thanks.
> 
> Is this published somewhere, since I'd lack to backport it to Android
> Common Kernel 5.10+, but can't find a sha1 (yet?)

Actually I applied it, but forgot to push it out.
It is now in the ipsec tree:

git://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec.git