David, The main reason P2P is moving toward reliable-flow-controlled-UDP is that UDP allows for widely available straight forward techniques to route around NATs in NAT-to-NAT file delivery scenarios. I believe this was covered in the thread, but it may be such common knowledge by now that we only refer to it implicitly. Mangling TCP to implement similar traversal techniques is a substantially more difficult task. Though not impossible at all, it's a tricky bit of hacking you'll need to do to make it work. Travis -----Original Message----- From: p2p-hackers-bounces@zgp.org [mailto:p2p-hackers-bounces@zgp.org] On Behalf Of David Barrett Sent: Friday, November 26, 2004 5:45 PM To: P2P Hackers Subject: [p2p-hackers] Why UDP and not TCP? We've had a long-ranging discussion on how to overcome UDP's inherently unreliable nature, but I'm confused: what overwhelming benefits do you see to UDP that can't be found in TCP? Elsewhere, I've heard the general arguments: 1) UDP is faster (ie, lower latency) 2) UDP is more efficient (ie, lower bandwidth) 3) UDP is easier (ie, no TCP shutdown issues) 4) UDP is more scalable (ie, no inbound connection limits) However, it seems these arguments are only really true if in the application: (from http://www.atlasindia.com/multicast.htm) - Messages require no acknowledgement - Messages between hosts are sporadic or irregular - Reliability is implemented at the process level. Reliable file transfer (the impetus for our discussion, I think) doesn't seem to be a good match for the above criteria. Indeed, it would seem to me that in this situation: 1) Latency is less important than throughput 2) TCP/UDP are similarly efficient because the payload will likely dwarf any packet overhead 3) A custom reliability layer in software is harder than a standardized, worldwide, off-the-shelf reliability layer implemented in hardware 4) The user will run out of bandwidth faster than simultaneous TCP inbound connections. At least, that's what my view tells me. What am I missing? Is there another angle to the UDP/TCP protocol selection that I'm not seeing? I've seen mention of congestion -- does UDP somehow help resolve this? Alternatively, do you find yourself forced to use UDP against your will? I really don't want to start a religious war, but I would like to know what holes exist in my reasoning above. Thanks! -david _______________________________________________ p2p-hackers mailing list p2p-hackers@zgp.org http://zgp.org/mailman/listinfo/p2p-hackers _______________________________________________ Here is a web page listing P2P Conferences: http://www.neurogrid.net/twiki/bin/view/Main/PeerToPeerConferences _______________________________________________ p2p-hackers mailing list p2p-hackers@zgp.org http://zgp.org/mailman/listinfo/p2p-hackers _______________________________________________ Here is a web page listing P2P Conferences: http://www.neurogrid.net/twiki/bin/view/Main/PeerToPeerConferences ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> ______________________________________________________________ ICBM: 48.07078, 11.61144 http://www.leitl.org 8B29F6BE: 099D 78BA 2FD3 B014 B08A 7779 75B0 2443 8B29 F6BE http://moleculardevices.org http://nanomachines.net [demime 1.01d removed an attachment of type application/pgp-signature]