Re: IGMP join/leave time variability

[Alan Cox]

> >From this, I infer that there should be _no_ initial delay on sending 
> the IGMP join.  In fact, a quick peek at the source confirms this: 
> (net/ipv4/igmp.c):
> 
> #define IGMP_Initial_Report_Delay               (1*HZ)
> 
> /* IGMP_Initial_Report_Delay is not from IGMP specs!
>  * IGMP specs require to report membership immediately after
>  * joining a group, but we delay the first report by a
>  * small interval. It seems more natural and still does not
>  * contradict to specs provided this delay is small enough.
>  */
> 
> But this "small interval" is actually very noticeable in our application.

I suspect the small interval for the first one should be 1 not 1*HZ. That
would keep a little bit of jitter which is good to avoid the multicast
receive/join group problem

[Lots of clients all running an app listening for multicast packets, one
 packet says 'do xyz on this group' and they all then send joins at the same
 instant]

Alan