On 24/07/13 14:38, William Manley wrote:

If an IGMP join packet is lost you will not receive data sent to the
multicast group so if no data arrives from that multicast group in a
period of time after the IGMP join a second IGMP join will be sent.  The
delay between joins is the "IGMP Unsolicited Report Interval".

Previously this value was hard coded to be chosen randomly between 0-10s.
This can be too long for some use-cases, such as IPTV as it can cause
channel change to be slow in the presence of packet loss.

The value 10s has come from IGMPv2 RFC2236, which was reduced to 1s in
IGMPv3 RFC3376.  This patch makes the kernel use the 1s value from the
later RFC if we are operating in IGMPv3 mode.  IGMPv2 behaviour is
unaffected.

Tested with Wireshark and a simple program to join a (non-existant)
multicast group.  The distribution of timings for the second join differ
based upon setting /proc/sys/net/ipv4/conf/eth0/force_igmp_version.
---
  net/ipv4/igmp.c |   17 ++++++++++++++---
  1 file changed, 14 insertions(+), 3 deletions(-)

I'm also working on a patch which builds on this adding a procfs knob 
similar to force_igmp_version but called 
"force_igmp_unsolicited_report_interval".  This will allow the interval 
to be overridden from user-space independent of the version of IGMP in 
use as for our use-case we want the interval to be short even for IGMPv2.

Thanks

Will