netdev - Re: [PATCH 0/10] af_unix: add multicast and filtering features to AF

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Message-ID: <alpine.DEB.2.02.1203021405040.7143@asgard.lang.hm>
Date:	Fri, 2 Mar 2012 14:19:45 -0800 (PST)
From:	david@...g.hm
To:	David Miller <davem@...emloft.net>
cc:	luiz.dentz@...il.com, eric.dumazet@...il.com,
	javier.martinez@...labora.co.uk, rodrigo.moya@...labora.co.uk,
	javier@...labora.co.uk, lennart@...ttering.net,
	kay.sievers@...y.org, alban.crequy@...labora.co.uk,
	bart.cerneels@...labora.co.uk, sjoerd.simons@...labora.co.uk,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [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@...il.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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