IP_MAX_MEMBERSHIPS

[Robert Watson]

On Thu, 13 Apr 2006, Bruce M Simpson wrote:

> On Thu, Apr 13, 2006 at 07:32:14AM +0100, Bruce M Simpson wrote:
>> On Wed, Apr 12, 2006 at 10:34:13PM -0400, Stephen Clark wrote:
>>> Can anyone tell me why the maximum number of members in a multicast
>>> group is set at
>>> 20?
>>> Are there issues with increasing this number?
>
> Ah. Your original question appears not to have been answered. My bad.
>
> I do not see a limit on the members of a multicast group on FreeBSD either 
> for individual sockets or for multicast forwarding based on my reading of 
> the source code.

We have a bounded size array of possible memberships for each socket in the 
per-pcb multicast option structure.  I don't know of any reason why changing 
this limit would break anything, although in the end testing is good.  If the 
limit is raised too much, we face significant wasted kernel memory if it's not 
actually used, and could think about adding code to scale the array size up as 
needed, but it's not clear that's useful unless the maximum is increased 
significantly.

A typical reason one might need more memberships is if one has a multicast 
service that needs to work on all available interfaces, and one has a lot of 
interfaces (i.e., lots of vlans).

Robert N M Watson