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

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Message-ID: <20220523144702.GY680067@gauss3.secunet.de>
Date:   Mon, 23 May 2022 16:47:02 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
CC:     Maciej Żenczykowski <maze@...gle.com>,
        "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 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

To what does this bug number refer to? Bugzilla? Please make that clear
if you want to have this number in the commit message.

Thanks!