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

[Paolo Abeni <pabeni@redhat.com>]

On Thu, 2023-07-20 at 10:17 +0200, Jiri Benc wrote:
> On Wed, 19 Jul 2023 21:08:28 -0700, Jakub Kicinski wrote:
> > 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?
> 
> Sorry for not providing enough details. The packets are actually
> correctly sized, initially. Then a lower, incorrect PMTU is cached.
> 
> In the collect_md mode (which is the only mode that VXLAN-GPE
> supports), there's no magic auto-setting of the tunnel interface MTU.
> It can't be, since the destination and thus the underlying interface
> may be different for each packet.
> 
> So, the administrator is responsible for setting the correct tunnel
> interface MTU. Apparently, the administrators are capable enough to
> calculate that the maximum MTU for VXLAN-GPE is (their_lower_MTU - 36).
> They set the tunnel interface MTU to 1464. If you run a TCP stream over
> such interface, it's then segmented according to the MTU 1464, i.e.
> producing 1514 bytes frames. Which is okay, this still fits the lower
> MTU.
> 
> However, skb_tunnel_check_pmtu (called from vxlan_xmit_one) uses 50 as
> the header size and thus incorrectly calculates the frame size to be
> 1528. This leads to ICMP too big message being generated (locally),
> PMTU of 1450 to be cached and the TCP stream to be resegmented.
> 
> The fix is to use the correct actual header size, especially for
> skb_tunnel_check_pmtu calculation.
> 
> Should I resend with more detailed patch description?

I guess there is not such a thing as a "too verbose commit message", so
I would say: yes please!

Thanks!

Paolo