netdev - Re: [PATCH net] vxlan: calculate correct header length for GPE

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Date: Wed, 19 Jul 2023 21:08:28 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jiri Benc <jbenc@...hat.com>
Cc: netdev@...r.kernel.org, Jesse Brandeburg <jesse.brandeburg@...el.com>,
 Tony Nguyen <anthony.l.nguyen@...el.com>
Subject: Re: [PATCH net] vxlan: calculate correct header length for GPE

On Tue, 18 Jul 2023 12:50:13 +0200 Jiri Benc wrote:
> This causes problems in skb_tunnel_check_pmtu, where incorrect PMTU is
> cached. If the VXLAN-GPE interface has MTU 1464 set (with the underlying
> interface having the usual MTU of 1500), a TCP stream sent over the
> tunnel is first segmented to 1514 byte frames only to be immediatelly
> followed by a resend with 1500 bytes frames, before the other side even
> has a chance to ack them.

Sounds like we are overly conservative, assuming the header will be
larger than it ends up being. But you're saying it leads to oversized,
not undersized packets?