From: "James W. Mills" <jmills@...>
Date: 2007-04-12T03:12:45+09:00
Subject: Missing packets in multicast receiver.

Hello list.

I need to use multicast in a project I am working on.  I would like to 
have a multicast sender send a large amount of multicast data, and a 
multicast receiver receive it.

I used the example I found here:
http://onestepback.org/index.cgi/Tech/Ruby/MulticastingInRuby.red
to get started.

What I want initially is to be able to blast packets to a multicast 
group.  I wrote a simple multicast sender that prints a message every 
(almost) MB of data it sends.  I also wrote a simple receiver that does 
the same thing for every (almost) MB of data it receives.  Here they are.

Sender:
##############################
require 'socket'

MULTICAST_ADDR = "225.0.0.1"
PORT= 1200

begin
  socket = UDPSocket.open
  socket.setsockopt(Socket::IPPROTO_IP, Socket::IP_TTL, [1].pack('i'))
  data_source = File.open '/dev/zero'
  data_length = 0
  loop do
    data = data_source.read(1024)
    data_length += data.length
    if (data_length % 102400) == 0
      puts data_length
    end
    socket.send(data, 0, MULTICAST_ADDR, PORT)
  end
ensure
  socket.close
end
##############################

Receiver:

##############################
require 'socket'
require 'ipaddr'

MULTICAST_ADDR = "225.0.0.1"
PORT = 1200

ip =  IPAddr.new(MULTICAST_ADDR).hton + IPAddr.new("0.0.0.0").hton
sock = UDPSocket.new
sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_ADD_MEMBERSHIP, ip)
sock.bind(Socket::INADDR_ANY, PORT)
start = Time.now
data_length = 0
begin
while true
  msg, info = sock.recvfrom(1024)
  data_length += msg.length
  if data_length % 102400 == 0
    puts data_length
  end
end
ensure
  sock.close
  puts "Ran for #{Time.now - start}"
end
##############################

If you start the receiver, then start the sender, you will see that the 
numbers do not line up at all.  The receiver gets one MB for every 10 or 
so MB that is sent.

Now, please don't fillet me over the coals for this, but I have the 
receiver written in perl as well.  The perl code does the same thing as 
the Ruby code was meant to.  Here it is.

##############################
use IO::Socket::Multicast;

# create a new UDP socket ready to read datagrams on port 1100
my $s = IO::Socket::Multicast->new(LocalPort=>1200);

# Add a multicast group
$s->mcast_add('225.0.0.1');

# now receive some multicast data

$data = "";
$t=0;

while(true) {
    $s->recv($data,1024);
    $t += length $data;
        if($t % 102400 == 0) {
        warn "$t";
    }
}
##############################

If you run the perl receiver against the Ruby sender, the programs print 
out the same messages, as expected.  I am very curious to know what I am 
doing wrong with the Ruby receiver that is causing it to miss so much 
data.  I suspect the line:

sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_ADD_MEMBERSHIP, ip)

in the receiver might hold the answer, but so far nothing I have tried 
has made any difference.  I find it interesting that it is using TCP 
instead of UDP/DGRAM, but I was unable to get it to work with IPPROTO_UDP...


Help is very much appreciated!

Thanks,
~james