title: How can you reduce the transmission size of a packet being sent via multicast?
body: <p>I have a few questions (all related), so I'll first explain the context and then ask. My primary question is the title, but beyond that I think I just need clarification and confirmation on whether my understanding is correct.</p>
  
  <p>We have some developers who are exchanging data between servers via multicast. They are sending an ~8k text file that is being transmitted as a single, large packet and other servers are listening to the multicast address to receive the file as an update. Naturally, this is getting broken up into fragments and, given it's multicast, if a fragment is lost then the whole packet is lost (which is what is happening, frequently).</p>
  
  <p>When a 25MB file (powerpoint for example) is sent via email -- how is it broken down? My understanding was always that the application layer (FTP for example) would calculate the file size, break it into individual packets, and send them (while keeping track of the packets getting sent)... now if it sent it and somewhere along the path there was a smaller MTU, the router (at layer 3/IP) would fragment this. Once the fragments arrived, it would be reassembled at layer 3, then further up at layer 7, all the packets would be reassembled (for example by an FTP server). Do I understand that right?</p>
  
  <p>If my understanding is right, I know that with TCP there is PMTU (Path MTU) which can check the entire path (RFC 1191) for the lowest MTU size and use that before transmission. Given that multicast uses UDP for the transport layer, I wasn't sure if there was a way for the sending host to check for this. Not only is there a problem with it being UDP, but the fact the destination would be a multicast address... so even if it could check the path, it would only know the half of the path between the sender and the multicast address, not the half of the path between the multicast address and the listener. Or is there a way to check this and still reduce the size of the transmitted packet?</p>
  
  <p>Ultimately, I believe they need to reduce the size of the packets being sent and I assume this would be done at a higher layer (something in the software), so as opposed to an 8k packet, it would be 6 packets that the application tracks... though even with this 'solution' I'm not sure if it's feasible with multicast. Is it?</p>