netdev - [ipsec/xfrm] IPv6 fragmentation/path-mtu

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Date:   Mon, 18 Mar 2019 01:03:09 +0100
From:   Bram Yvahk <bram-yvahk@...l.wizbit.be>
To:     netdev@...r.kernel.org
CC:     steffen.klassert@...unet.com, herbert@...dor.apana.org.au,
        davem@...emloft.net
Subject: [ipsec/xfrm] IPv6 fragmentation/path-mtu

When playing a bit with IPv6 and XFRM I ran into a possible
issue/edge case.

In my testing I used linux 4.14.95, I was planning on testing this
with latest kernel and investigating this a bit more deeply but so
far I've not yet been able to do so... Only reason why I'm already
submitting this message is because there is a 'Linux IPsec workshop'
next week.

When path-mtu between the two ipsec gateways is 1280 (i.e. minimum
IPv6 mtu) and when a client in the network attempts to send a larger
message then it receives a ICMPv6 PKT_TOOBIG message.
The problem: mtu field in the message is set to 1198... This is lower
then the minimum IPv6 mtu and the client seems to ignore it.

(What I think should happen in this particular case: do not send a
 PKT_TOOBIG to the client but instead transmit fragmented IPv6 ESP
 packets to accommodate the path-mtu)