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...

Answer 3

It depends whether the user needs to know if the messages have been delivered to the server. UDP packets have no inherent acknowledgement. If the client sends an IM message to the server and it gets lost in transit, neither the client or the server will know about it.

(The short answer is "use TCP" ... but it is worth thinking through the design implications for yourself.)

Answer 4

TCP would give you reliability, which is certainly desirable when during instant messaging -- you would not want messages to be dropped during converstation.

However, if you intend on using group messaging, then you might end up using mulitcast. For such cases, UDP would be the right chioce since UDP can handle point to multipoint. Using TCP for multicast applications would be hard since now the sender would have to keep track of retransmissions/sending rate for multiple receivers. One alternative could be to use TCP for point-to-point chat and use UDP for group messaging.