RE: IGMP router behavior during LMQT period

Hi,

 

In IGMP version 3, explicit Host Tracking is done by the router.

Please refer to Appendix B of RFC 3376 (Summary of Changes from IGMPv2),
here it mentions many differences. However, the below mentioned excerpt
from the RFC are relevant for your question. 

 

*	Hosts no longer perform suppression, to simplify implementations
and permit explicit membership tracking.

 

Another excerpt supporting the explicit host tracking at router in case
of IGMP v3 RFC 3376 Section A.2 (Host Suppression)

 

In IGMPv1 and IGMPv2, a host would cancel sending a pending membership
reports if a similar report was observed from another member on the
network.  In IGMPv3, this suppression of host membership reports has
been removed.  The following points explain the reasons behind this
decision.

*	Routers may want to track per-host membership status on an
interface.  This allows routers to implement fast leaves (e.g., for
layered multicast congestion control schemes) as well as track
membership status for possible accounting purposes.

 

Regards,

Ashish

 

________________________________

From: Princy Elizabeth [mailto:[email protected]] 
Sent: Tuesday, September 18, 2007 10:38 AM
To: [email protected]
Subject: [magma] IGMP router behavior during LMQT period

 

Hi,

 

I had a question regarding the expected router behavior during the last
member query timeout(LMQT) period. Consider 2 IGMPv3 hosts, H1 and H2
sitting on a LAN. Both have subscribed to a group G1. Host H1 now sends
a leave for the group G1. On receiving this the querier would reduce its
GMI to LMQT and schedule RV number of group specific queries to be sent,
one every last member query interval (LMQI) seconds. Since IGMPv3 state
change records are retransmitted by the host to guard against packet
loss, H1 would retransmit its Leave (TO_INCLUDE{}) after the unsolicited
report interval. What should the router do on receiving this leave? 

 

a) Should it reset the group timer LMQT again and again send out RV
number of group specific queries 

OR

b) Should it simply ignore this leave?

 

If it is (b), what should it do if it gets a leave from the other host
H2 also during this LMQT period(i.e. H2 leaves the group immediately
after sending an overriding join in response to the first group specific
query)? Shouldn't that be treated as described in (a)? But how can the
router distinguish between leaves from 2 different hosts unless it is
explicitly tracking hosts?

 

Thanks,

Princy

  

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