Le jeudi 01 mars 2012 à 14:56 +0100, Javier Martinez Canillas a écrit :

We could use AF_INET multicast on a local machine but we need some
ordering and control flow requirements that are not guaranteed on UDP
multicast over IP. That's why we thought to add a new address family
AF_MCAST.

It seems application logic and complexity pushed into kernel, for a very
single user (even if used in a lot of products) : D-Bus

To make it a general local multicast solution and not being too specific
we added some flags to control its behavior like
MCAST_MREQ_DROP_WHEN_FULL to decide to either block the sender or drop
the packet when one receiver has its queue full.

I am only wondering how many lines this is going to add in kernel for a
complete implementation, given your performance expectations, flow
control, reliability, not counting all security issues (ancillary
messages and so on)

In case of IP_MULTICAST_LOOP, we could allow the sender to sleep if
receiver queue is full, with a bit of tweaking in stack (current
implementation uses loopback re-inject, so requires softirq handling).

In fact, we could use a new IP_MULTICAST_LOCAL option, so that sender
processing doesnt trigger a softirq handler at all and is allowed to
sleep if needed. For example skb allocations could use GFP_KERNEL
instead of current GFP_ATOMIC ones in udp mcast .

I dont know, maybe it would be a smaller patch.