What is better for instant messenger TCP or UDP?

Question

I need to implement client/server instant messenger using pure sockets in Java lang.
The server should serve large number of clients and I need to decide which sockets should I use - TCP or UDP.
Thanks, Costa.

Answer 1

TCP

Reason:

TCP: "There is absolute guarantee that the data transferred remains intact and arrives in the same order in which it was sent."

UDP: "There is no guarantee that the messages or packets sent would reach at all."

Learn more at: http://www.diffen.com/difference/TCP_vs_UDP

Would you want your chat message possibly lost?

Edit: I missed the part about "large chat program". I think because of the nature of the chat program it needs to be a TCP server, I cannot imagine the actual text content sent by users over a UDP protocol.

The max limit for TCP servers is 65536 connections at the same time. If you really need to go past that number you could create a dispatcher server that would send incoming connects to the appropriate server depending on current server loads.

Answer 2

You could use both. Use TCP for exchanging the actual messages, (so no data lost and streaming large messages, (eg. containing jpegs etc), is possible. Use UDP only for sending short 'connectNow' messages to clients for which there are messages queued. The clients could have states like (NotLoggedIn, TCPconnected, TCPdisconnected, LoggedOut) with various timeouts to control the state transitions as well as the normal message-exchange events. The UDP 'connectNow' message would instruct clients in 'TCPdisconnected' to connect and so move to 'TCPconnected', where they would stay, exchanging messages, until some inactivity timer instructs the client to disconnect for now. This would, of course, be unreliable and so you may wish to repeat the 'connectNow' message every X seconds for N times until the client connects. The client should, in any case, attempt a poll every X minutes, just in case...