Subject: Re: Skipping TCP / UDP / IP checksums on loopback traffic
To: Jonathan Stone <>
From: Nathan J. Williams <>
List: tech-net
Date: 10/25/2004 13:05:41
Jonathan Stone <> writes:

> See Jeffrey Hsu's comment about ``faithfully mirror[ing] the same
> processing done for physical interfaces'.

Can you give a more exact pointer for those of us who don't generally
keep up with the networking literature?

> Some of us who tweak the network stack use lo0 as a
> measurement-proxy for the stack itself, independent of the vagaries
> of any particular NIC or LAN.  In my own little corner of the world,
> that's the primary purpose for lo0.


> If you *want* a high-speed, low-overhead local IPC transport, then use
> PF_LOCAL. That's what it's for.

This seems like a philosophical point as much as a practical one, and
I can imagine [but will not make] an argument that lo0 is the
"natural" local transport and that PF_LOCAL is redundant, and that
your benchmarking usage is not the case we should be accomodating.

> >If not, I would like to advocate
> >its removal, because from where I stand there's no reason for this
> >behavior to be optional.
> I see no compelling reason [aside from relative performance in bogus
> benchmarks, which I think we agree a bogus reason] to elide the
> checksum at all.

Well, either way, we seem to be in complete agreement that the sysctl
is a bad idea.

        - Nathan