Bridge IGMP snooping question

Bridge IGMP snooping question

[Thomas Martitz]

Hello Linux folks,

I have one question regarding IGMP snooping on bridges, specifically 
about leaving multicast groups.

br_multicast_leave_group() has this check early on:

if (!netif_running(br->dev) ||
         (port && port->state == BR_STATE_DISABLED) ||
         timer_pending(&querier->timer)) /* <- THIS */
                goto out;

I'm wondering why the bridge code prevents group leaves if if the 
querier timer is pending. From my understanding the timer acts as an 
indication whether the local system is the network's querier or not (if 
pending then another router is querier). Therefore this check prevents 
to leave groups if there is another querier.

I'm wondering what's the rationale for this, if any. It seems to be that 
the decision whether an attached client gets mc forwarded is independent 
on whether the local system is querier or not. Git log does not reveal 
the answer as this check is there from the beginning.

PS: This behavior conflicts with something I'm working.

Thank you very much in advance.

Best regards.

Bridge IGMP snooping question

[Thomas Martitz]

Hello Linux folks,

I have one question regarding IGMP snooping on bridges, specifically 
about leaving multicast groups.

br_multicast_leave_group() has this check early on:

if (!netif_running(br->dev) ||
         (port && port->state == BR_STATE_DISABLED) ||
         timer_pending(&querier->timer)) /* <- THIS */
                goto out;

I'm wondering why the bridge code prevents group leaves if if the 
querier timer is pending. From my understanding the timer acts as an 
indication whether the local system is the network's querier or not (if 
pending then another router is querier). Therefore this check prevents 
to leave groups if there is another querier.

I'm wondering what's the rationale for this, if any. It seems to be that 
the decision whether an attached client gets mc forwarded is independent 
on whether the local system is querier or not. Git log does not reveal 
the answer as this check is there from the beginning.

PS: This behavior conflicts with something I'm working.

Thank you very much in advance.

Best regards.

Re: Bridge IGMP snooping question

[Herbert Xu]

On Thu, Oct 02, 2014 at 10:13:05AM +0200, Thomas Martitz wrote:
> Hello Linux folks,
> 
> I have one question regarding IGMP snooping on bridges, specifically
> about leaving multicast groups.
> 
> br_multicast_leave_group() has this check early on:
> 
> if (!netif_running(br->dev) ||
>         (port && port->state == BR_STATE_DISABLED) ||
>         timer_pending(&querier->timer)) /* <- THIS */
>                goto out;
> 
> I'm wondering why the bridge code prevents group leaves if if the
> querier timer is pending. From my understanding the timer acts as an
> indication whether the local system is the network's querier or not
> (if pending then another router is querier). Therefore this check
> prevents to leave groups if there is another querier.
> 
> I'm wondering what's the rationale for this, if any. It seems to be
> that the decision whether an attached client gets mc forwarded is
> independent on whether the local system is querier or not. Git log
> does not reveal the answer as this check is there from the
> beginning.
> 
> PS: This behavior conflicts with something I'm working.

You can't just leave a group because you received a single
leave message.  There may be another group member still listening
on that port.

To make sure no listeners are left a query message must be sent.
If there is another querier on the network then they are meant to
send such a query message which means that all we have to do is
passively wait for the timeout to expire.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt