From mboxrd@z Thu Jan  1 00:00:00 1970
From: Denys Fedoryschenko <denys@visp.net.lb>
Subject: Re: Multicast packet loss
Date: Fri, 30 Jan 2009 21:17:35 +0200
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To: Kenny Chang <kchang@athenacr.com>
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On Friday 30 January 2009 19:49:48 Kenny Chang wrote:
> Hi all,
>
> We've been having some issues with multicast packet loss, we were wondering
> if anyone knows anything about the behavior we're seeing.

I didn't work much on multicast, but i have heavy unicast udp streaming (PEP 
for satellite).

First thing to check
net.core.wmem_max = 131071
net.core.wmem_default = 124928
net.ipv4.udp_mem = 379008       505344  758016

Usually they are too small by default.

next
netstat -s

Important part
Udp:
    1263992126 packets received
    260196 packets to unknown port received.
    627001 packet receive errors
    74235906 packets sent
    RcvbufErrors: 56683
    SndbufErrors: 4295851


in your case     SndbufErrors matter.