On 2/17/2022 8:26 AM, Janusz Użycki wrote:
> Hi,
> 
> Does the issue still concern PTP for Linux:
> "Dear IEEE 1588 implementers: don't forget about IGMP if you do 
> multicast! « The PTP guy (logdown.com) " 
> http://theptpguy.logdown.com/posts/2015/08/31/dear-ieee-1588-implementers-remember-about-igmp
>  
> ?
> 
>  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.
> ptp4l joins to multicast group once on start (only once, ie. it triggers 
> Linux kernel once to send IGMP packet):
> udp.c: err = setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &req, 
> sizeof(req));
> What is impact such implementation when interface goes down and up 
> again, or on network recrossing? There is no periodic rejoin package 
> observed.

I think we at least need to re-send these when the link changes, though
we perhaps were thinking that the kernel does this for us? Maybe thats a
bug or maybe we have to set an option?

There is no mention about whether we would need to re-issue this socket
option in the "man 7 ip" manual page.


> For IPv6 likely there is no problem but we have not tested.
> Maybe it is Linux kernel issue. However for comparison ptpd2 implements 
> periodic multicast group (re)join (optionmaster_igmp_refresh_interval):
> src/dep/net.c:netInitMulticastIPv4() 
> https://github.com/ptpd/ptpd/blob/master/src/dep/net.c#L605 
> https://github.com/ptpd/ptpd/blob/master/src/dep/net.c#L2278 
> https://github.com/ptpd/ptpd/blob/master/src/protocol.c#L607 
> https://github.com/ptpd/ptpd/blob/master/src/protocol.c#L1120 Other 
> proprietary PTP devices send IGMP packets periodically every 1-2s and 
> work with the switches despite IGMP snooping enabled...
> 

This sounds like what we should be doing as well.

> best regards
> Janusz
> 
> 
> 
> 
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