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

From: Dongpo Li <[email-masked]>
Date: Tue, 25 Oct 2016 19:50:53 +0800

On 2016/10/25 15:39, Andrew Lunn wrote:
> On Tue, Oct 25, 2016 at 09:13:54AM +0800, Dongpo Li wrote:
>> Hi Andrew,
>>
>> On 2016/10/24 23:32, Andrew Lunn wrote:
>>> 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?
>> Thank you for your comments.
>> Both Box A and B are IP hosts, not multicast routers.
>> And the RFC says: IGMP is used by "IP hosts" to report their multicast group membership.
> 
> They report their membership to gateways, not to each other. The
> gateway will then arrange for multicast traffic for the group from
> other subnets to be forwarded to this subnet. You don't need IGMP to
> receive local traffic.
> 
> Also, this timer is to do with responding to IGMP querier, typically
> the multicast gateway. The querier keeps track of if a group is in use
> within a subnet. It listens to group joins and optionally leaves. It
> also periodically sends out IGMP querier requests, asking who is
> interested in what groups. The hosts use a random delay before
> answering, and if some other hosts replies about a group they are a
> member of, they don't send a response themselves. One is enough.
> 
Of course, this timer is to do with responding to IGMP querier.
I mean, if the timer has expired, the host must have sent the IGMP responding message,
maybe IGMP join group message or responding router's IGMP querier.
So the function igmp_stop_timer() should not set im->reporter to 0 if the timer has expired.
Otherwise the host can't send IGMP leave message when it leaves the group because the im->reporter is 0.

    Regards,
    Dongpo

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