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SubjectRe: [PATCH 0/10] af_unix: add multicast and filtering features to AF_UNIX
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Hi David

On Mon, 2012-02-27 at 14:05 -0500, David Miller wrote:
> From: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
> Date: Mon, 27 Feb 2012 15:00:06 +0100
>
> > Primary for performance reasons. D-bus is an IPC system for processes in
> > the same machine so traversing the whole TCP/IP stack seems a little
> > overkill to me.
>
> You haven't actually tested what the cost of this actually is, so what
> you're saying is mere speculation. In many cases TCP/UDP over
> loopback is actually faster than AF_UNIX.
>
you're right we haven't tested this, but because of the other points in
Javier's mail, which are the special semantics we need for this to fit
the D-Bus usage:

> - total order is guaranteed: If sender A sends a message before B,
then
> receiver C and D should both get message A first and then B.
>
> - slow readers: dropping packets vs blocking the sender. Although
> datagrams are not reliable on IP, datagrams on Unix sockets are
never
> lost. So if one receiver has its buffer full the sender is blocked
> instead of dropping packets. That way we guarantee a reliable
> communication channel.
>
> - multicast group acess control: controlling who can join the
multicast
> group.
>
> - multicast on loopback is not supported: which means we have to use a
> NIC (i.e: eth0).

Because of all of this, UDP/IP multicast wasn't even considered as an
option. We might be wrong in some/all of those, so could you please
comment on them to check if that's so?

thanks



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