netdev - Re: [PATCH] Revert "xfrm: xfrm_state_mtu should return at least 1280 for ipv6"

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Message-ID: <20220201064639.GS1223722@gauss3.secunet.de>
Date:   Tue, 1 Feb 2022 07:46:39 +0100
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Jiri Bohac <jbohac@...e.cz>
CC:     Sabrina Dubroca <sd@...asysnail.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "Mike Maloney" <maloneykernel@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH] Revert "xfrm: xfrm_state_mtu should return at least 1280
 for ipv6"

On Wed, Jan 26, 2022 at 04:00:18PM +0100, Jiri Bohac wrote:
> This reverts commit b515d2637276a3810d6595e10ab02c13bfd0b63a.
> 
> Commit b515d2637276a3810d6595e10ab02c13bfd0b63a ("xfrm: xfrm_state_mtu
> should return at least 1280 for ipv6") in v5.14 breaks the TCP MSS
> calculation in ipsec transport mode, resulting complete stalls of TCP
> connections. This happens when the (P)MTU is 1280 or slighly larger.
> 
> The desired formula for the MSS is:
> MSS = (MTU - ESP_overhead) - IP header - TCP header
> 
> However, the above commit clamps the (MTU - ESP_overhead) to a
> minimum of 1280, turning the formula into
> MSS = max(MTU - ESP overhead, 1280) -  IP header - TCP header
> 
> With the (P)MTU near 1280, the calculated MSS is too large and the
> resulting TCP packets never make it to the destination because they
> are over the actual PMTU.
> 
> The above commit also causes suboptimal double fragmentation in
> xfrm tunnel mode, as described in
> https://lore.kernel.org/netdev/20210429202529.codhwpc7w6kbudug@dwarf.suse.cz/
> 
> The original problem the above commit was trying to fix is now fixed
> by commit 6596a0229541270fb8d38d989f91b78838e5e9da ("xfrm: fix MTU
> regression").
> 
> Signed-off-by: Jiri Bohac <jbohac@...e.cz>

Applied, thanks Jiri!