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SubjectRe: [PATCH 0/10] af_unix: add multicast and filtering features to AF_UNIX
On Fri, 2 Mar 2012, David Miller wrote:

> From: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
> Date: Fri, 2 Mar 2012 10:39:24 +0200
>
>> Like I said before there is many projects using AF_UNIX as IPC
>> transport, the documentation actually induces people to use for this
>> purpose, and many would benefit from being able to do multicast.
>
> You can't have it both ways.
>
> If it's useful for many applications, then many applications would
> benefit from a userland library that solved the problem using
> existing facilities such as IP multicast.

I missed the start of this discussion (but did see the lwn.net article on
it)

as I understand it, they are looking for some features that are not in IP
multicast (or at least not as I understand it)

1. reliable delivery

2. in-order delivery

3. sender blocking on recipients rather than dropping messages when the
channel is full.

IP multicast definantly does not do #3, and as far as I understand it, is
essentially UDP to multiple recipients, and UDP does not provide either #1
or #2

Yes, this could be done entirely in userspace (with something like 0MQ as
I see others mentioning), and I don't understand the Android aversion to
any userspace daemons, but with all of that being said, I do think that a
kernel-based mechanism that supports having iptables type filters on it
would be a very nice thing to have (and should be able to re-use a lot of
existing code that would end up being duplicated if this is done in a
userspace daemon)

now it may be that some of the requirements may result in error O_PONY or
O_SANITY (the sender blocking seems like a potential problem, but that
may possibly make sense as a configurable option)

David Lang


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