On Mon, 7 Jun 2021 16:46:17 +0500 Roman Mamedov <r...@romanrm.net> wrote:
> On Mon, 7 Jun 2021 13:27:10 +0200 > "Jason A. Donenfeld" <ja...@zx2c4.com> wrote: > > > Can you walk me through your use case a bit more, so I can wrap my mind > > around the requirements? > > > > ingress --plain--> wireguard --wireguard[plain]--> vxlan > > --vxlan[wireguard[plain]]--> egress > > Not sure I understand your scheme correctly. In any case, the path of a > packet would be... > > On peer 1: > > * plain Ethernet -> wrapped into VXLAN -> encrypted into WireGuard > > On peer 2: > > * decrypted from WireGuard -> unwrapped from VXLAN -> plain Ethernet > > > So my question is, why can't you set wireguard's MTU to 80 bytes less > > than vxlan's MTU? What's preventing that or making it infeasible? > > To transparently bridge two Ethernet LANs, a VXLAN interface needs to join an > L2 bridge. All interfaces that are members of a bridge must have the same MTU. > > As such, br0 members on both sides: > eth0 (MTU 1500) > vx0 (MTU 1500) > > VXLAN transports full L2 frames encapsulating them into UDP. To fit the > full 1500-byte packet and accounting for VXLAN and related IP overheads, > the resulting packet size is 1574 bytes. > > So this same host that just generated the 1574-byte encapsulated VXLAN packet > with something it received via its eth0 port, now needs to send it further to > its WG peer(s). For this to succeed, the in-tunnel WG MTU needs to be 1574 or > more, not 1412 or 1420, as VXLAN itself can't be fragmented[1]; or even if it > could, that would mean a much worse overhead ratio than currently. > > [1] https://datatracker.ietf.org/doc/html/rfc7348#section-4.3 In case you are not convinced by this case, would you consider at least allowing fragmentation when WG's in-tunnel MTU is set to >=1500? Because this is the user effectively saying "yes I know this is not gonna fit in one packet, I want to rely on WG packets being fragmented", but without the need for extra knobs. -- With respect, Roman