> -----Original Message-----
> From: Jakub Raczyński <j.raczyn...@elpromaelectronics.com>
> Sent: Friday, February 18, 2022 1:08 AM
> To: linuxptp-users@lists.sourceforge.net
> Subject: Re: [Linuxptp-users] Multicast group (re)join issue vs IGMP snooping
> 
> > 18.02.2022 00:51 Richard Cochran <richardcoch...@gmail.com> wrote:
> > On Thu, Feb 17, 2022 at 05:26:15PM +0100, Janusz Użycki wrote:
> >
> > > From our observations for IPv4 (L4) E2E it still applies to ptp4l. On 
> > > Cisco
> > > and Netgear switches (even L3 set as L2 switch) IGMP snooping must be
> > > disabled for proper PTP multicast work.
> >
> > So their "IGMP snooping" feature is broken.  Ask them to fix it.
> 
> I disagree. As far as I can tell, IGMP resending is done by kernel from 
> version 4.9
> upon notification. So basically, ptp4l will not work with any switch using 
> IGMP in
> any way on older kernels. We probably should assume that devices with older
> kernel are all EOL but it is not uncommon for them to be still used. So maybe
> some annotation on what kernel version will ptp4l work or what are required
> kernel modules.
> 
> > Cisco's PTP switch has issues.  Have you looked at the residence
> > times?  I don't feel like implementing workarounds for buggy >hardware.
> 
> And is depending on kernel features for ptp4l to work properly a good way? I 
> do
> not think it is hardware issue here, but rather dependence on kernel version
> and/or modules.
> 

ptp4l already has a lot of "it works better on newer kernel" bits, due to 
interfacing with Linux kernel interfaces so directly.  I think the earliest 
technically working version is something from the 3.x series (3.5?) but I don't 
see a problem with documenting a newer version as recommended with the 
justifications of which features we rely on.

I think I agree with Richard that this is the kernel's problem, and that we 
shouldn't try to forcibly re-send IGMP messages ourselves (in fact, there is no 
real interface to do this in the socket options.. There's simply a join and 
leave group option, but no mention of needing to re-join periodically or what 
attempting to join an already joined group would do).

Thanks,
Jake

> Best regards
> Jakub
> 
> 
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