UDP checksums and inband control messages

Rouven Czerwinski rouven at czerwinskis.de
Tue Feb 20 07:36:25 CET 2018


Derek Fawcus <dfawcus+lists-wireguard at employees.org> writes:

> I see from the code that currently the kernel UDP sockets
> have checksums enabled.  I also note the message from
> November speculating upon if in band control messages
> should be added.
> One thing I was pondering recently in the context of UDP tunnelling,
> is that one doesn't really need to have UDP checksums on the
> encapsulating packet, since they payload itself is eventually
> protected by its IPv4 header checksum, and likewise its transport
> payload being protected by its own TCP/UDP/etc checksum.
> [OK - some exceptions, but valid to a first approximation]

This is only correct for IPv4, IPv6 does not have a checksum field
(coincidentally because all high level protocols implement their own

> In the case of a crypto tunnel tunnel when using a
> verified / authenticated crypto algorithm, any lower level
> UDP checksum is even more redundent.

Yes, but computing the UDP Checksum is really inexpensive in contrast to
the authentication done on a packet with the correct checksum.

> The one place where UDP checksums would stil seem to be useful
> is for any in band control messages, if they were themselves
> not covered by the crypto layer.  i.e. c.f. OpenVPN and
> its payload vs control messages.
> Which then got me to thinking that one could sort of cheat,
> and use the checksum field in the UDP header as an indicator
> of if the payload is control or data.  All zero bits for
> data, none zero (including 0xffff) for control.
> This would also have the advantage that if one is using a system
> without support for h/w checksum offload, one gets to save a
> bit of CPU;  however that may or may not be significant depending
> upon just if/when packet memory if touched, and by which cores
> in a system.  i.e. I'm pondering a non Linux kernel implementation.
> So - thoughts?  Is it worth doing something like this for wireguard?

Rouven Czerwinski

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