netdev - Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets

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Date:   Sat, 18 Jul 2020 19:58:50 +0200
From:   Stefano Brivio <sbrivio@...hat.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
        aconole@...hat.com
Subject: Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu
 discovery on encap sockets

On Sat, 18 Jul 2020 11:02:46 -0600
David Ahern <dsahern@...il.com> wrote:

> On 7/18/20 12:56 AM, Stefano Brivio wrote:
> > On Fri, 17 Jul 2020 09:04:51 -0600
> > David Ahern <dsahern@...il.com> wrote:
> >   
> >> On 7/17/20 6:27 AM, Stefano Brivio wrote:  
> >>>>    
> >>>>> Note that this doesn't work as it is because of a number of reasons
> >>>>> (skb doesn't have a dst, pkt_type is not PACKET_HOST), and perhaps we
> >>>>> shouldn't be using icmp_send(), but at a glance that looks simpler.      
> >>>>
> >>>> Yes, it also requires that the bridge has IP connectivity
> >>>> to reach the inner ip, which might not be the case.    
> >>>
> >>> If the VXLAN endpoint is a port of the bridge, that needs to be the
> >>> case, right? Otherwise the VXLAN endpoint can't be reached.
> >>>     
> >>>>> Another slight preference I have towards this idea is that the only
> >>>>> known way we can break PMTU discovery right now is by using a bridge,
> >>>>> so fixing the problem there looks more future-proof than addressing any
> >>>>> kind of tunnel with this problem. I think FoU and GUE would hit the
> >>>>> same problem, I don't know about IP tunnels, sticking that selftest
> >>>>> snippet to whatever other test in pmtu.sh should tell.      
> >>>>
> >>>> Every type of bridge port that needs to add additional header on egress
> >>>> has this problem in the bridge scenario once the peer of the IP tunnel
> >>>> signals a PMTU event.    
> >>>
> >>> Yes :(  
> >>
> >> The vxlan/tunnel device knows it is a bridge port, and it knows it is
> >> going to push a udp and ip{v6} header. So why not use that information
> >> in setting / updating the MTU? That's what I was getting at on Monday
> >> with my comment about lwtunnel_headroom equivalent.  
> > 
> > If I understand correctly, you're proposing something similar to my
> > earlier draft from:
> > 
> > 	<20200713003813.01f2d5d3@...sabeth>
> > 	https://lore.kernel.org/netdev/20200713003813.01f2d5d3@elisabeth/
> > 
> > the problem with it is that it wouldn't help: the MTU is already set to
> > the right value for both port and bridge in the case Florian originally
> > reported.  
> 
> I am definitely hand waving; I have not had time to create a setup
> showing the problem. Is there a reproducer using only namespaces?

And I'm laser pointing: check the bottom of that email ;)

-- 
Stefano