Subject: Re: question about function igmp_stop_timer() in net/ipv4/igmp.c

From: Andrew Lunn <[email-masked]>
Date: Mon, 24 Oct 2016 17:32:00 +0200

On Mon, Oct 24, 2016 at 07:50:12PM +0800, Dongpo Li wrote:
> Hello
> 
> We encountered a multicast problem when two set-top box(STB) join the same multicast group and leave.
> The two boxes can join the same multicast group
> but only one box can send the IGMP leave group message when leave,
> the other box does not send the IGMP leave message.
> Our boxes use the IGMP version 2.
> 
> I added some debug info and found the whole procedure is like this:
> (1) Box A joins the multicast group 225.1.101.145 and send the IGMP v2 membership report(join group).
> (2) Box B joins the same multicast group 225.1.101.145 and also send the IGMP v2 membership report(join group).
> (3) Box A receives the IGMP membership report from Box B and kernel calls igmp_heard_report().
>     This function will call igmp_stop_timer(im).
>     In function igmp_stop_timer(im), it tries to delete IGMP timer and does the following:
>         im->tm_running = 0;
>         im->reporter = 0;
> (4) Box A leaves the multicast group 225.1.101.145 and kernel calls
>     ip_mc_leave_group -> ip_mc_dec_group -> igmp_group_dropped.
>     But in function igmp_group_dropped(), the im->reporter is 0, so the kernel does not send the IGMP leave message.

RFC 2236 says:

2.  Introduction

   The Internet Group Management Protocol (IGMP) is used by IP hosts to
   report their multicast group memberships to any immediately-
   neighboring multicast routers.

Are Box A or B multicast routers?

    Andrew