From: Sabrina Dubroca <sd@queasysnail.net>
To: Jiri Bohac <jbohac@suse.cz>
Cc: Mike Maloney <maloneykernel@gmail.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
davem@davemloft.net, netdev@vger.kernel.org,
Steffen Klassert <steffen.klassert@secunet.com>,
Herbert Xu <herbert@gondor.apana.org.au>
Subject: Re: [RFC PATCH] fix xfrm MTU regression
Date: Sat, 1 May 2021 12:23:40 +0200 [thread overview]
Message-ID: <YI0sLJ4rOAULgojz@hog> (raw)
In-Reply-To: <20210429202529.codhwpc7w6kbudug@dwarf.suse.cz>
2021-04-29, 22:25:29 +0200, Jiri Bohac wrote:
> On Thu, Apr 29, 2021 at 09:48:09PM +0200, Sabrina Dubroca wrote:
> > That should be fixed with commit b515d2637276 ("xfrm: xfrm_state_mtu
> > should return at least 1280 for ipv6"), currently in Steffen's ipsec
> > tree:
> > https://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec.git/commit/?id=b515d2637276
>
> Thanks, that is interesting! The patch makes my large (-s 1400) pings inside
> ESP pass through a 1280-MTU link on an intermediary router but in a suboptimal
> double-fragmented way. tcpdump on the router shows:
>
> 22:09:44.556452 IP6 2001:db8:ffff::1 > 2001:db8:ffff:1::1: frag (0|1232) ESP(spi=0x00000001,seq=0xdd), length 1232
> 22:09:44.566269 IP6 2001:db8:ffff::1 > 2001:db8:ffff:1::1: frag (1232|100)
> 22:09:44.566553 IP6 2001:db8:ffff::1 > 2001:db8:ffff:1::1: ESP(spi=0x00000001,seq=0xde), length 276
>
> I.e. the ping is fragmented into two ESP packets and the first ESP packet is then fragmented again.
It's a bit ugly, but I don't think we can do any better. We're going
through the stack twice in tunnel mode. The first pass (before xfrm)
we fragment according to the PMTU (adjusted to IPV6_MIN_MTU, because
MTUs lower than that are illegal in IPv6). The second time (after
xfrm), the first ESP packet is too big so we fragment it. This
behavior is consistent with a vti device running over a network with
MTU=1280 (which doesn't seem to work without my patch).
In transport mode, we're only going through the stack once, so we
don't see this double fragmentation.
I think my patch is correct, because without it we have IPv6 dsts
going around the kernel with an associated MTU smaller than
IPV6_MIN_MTU.
--
Sabrina
next prev parent reply other threads:[~2021-05-01 10:24 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-29 17:02 [RFC PATCH] fix xfrm MTU regression Jiri Bohac
2021-04-29 19:48 ` Sabrina Dubroca
2021-04-29 20:25 ` Jiri Bohac
2021-05-01 10:23 ` Sabrina Dubroca [this message]
2021-04-30 5:36 ` [RFC PATCH v2] " Jiri Bohac
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