port blocking, throttling, and scrambling

[alphabeater]

http://yro.slashdot.org/yro/02/07/14....shtml?tid=153

roadrunner in texas is, apparently, blocking the fasttrack port.

http://homepage.ntlworld.com/j.bucha...x/blocked.html

this site is devoted to solving a similar problem with isps limiting traffic on winmx's ports. on top of this, people have been reporting similar things since the early days of opennap servers. some isps seem very eager to limit what their customers can and can't use their internet access for - and some are almost monopolies in their areas, too.

the way the tcp (as well as udp) protocol works is a key vulnerability of decentralised p2p. for a program to act as a server (ie. receive any incoming connections), it must listen on a port. fasttrack (kazaa/grokster) uses port 1214, winmx uses 6699 and 6257, opennap uses 8888, gnutella uses 6346, and even freenet has to use a standard port, 8481. although the port is usually configurable, about half the peers in the network need to use the standard port for the network to remain stable - otherwise, they can't find each other.

one solution would be to have users enter a port number (this is the solution widely adopted on opennap, server : port lists instead of just server lists). this can, however, become cumbersome.

thinking about this, i realised - what is the one thing that both the server and client will know before establishing a connection? the ip addresses of both. so, instead of ever using a default tcp port, ports for individual peers could be worked out using a algorithm known by all peers.

for example, if my ip address was 64.124.41.39...

each number in an ip is 1 to 255
4 x 250 = 1000
so i could take 5 from each number, making 59.119.36.34
add these numbers together, 59 + 119 + 36 + 34 = 248
multiply by 10, 248 x 10 = 2480

so my p2p program would listen for incoming connections on port 2480 - this could be worked out when the program was started, and then quickly worked out by any peer encountering mine just before it connected to me. i think that this technique would stop isps from quickly and easily throttling or blocking certain ports used by known p2p programs.

simple, but effective.