sockets networking tcp/ip and ports some clarifications

Question

I am in the process of developing a peer to peer app, I am a bit confused by the following scenario:

Lets say my application will use an outgoing port 1863 - which is also used for msn messenger(if this is not the port lets assume it is)

Now, client executes my app and connects to my server at port 1863.

I am a bit confused if this is going to produce any problems. I know that 2 apps can use same port for outgoing communication. But what happens to the data coming back?

Also, does my client need to open port for my app to run correctly??

Answer 1

I know that 2 apps can use same port for outgoing communication. But what happens to the data coming back?

That's exactly the problem the source port solves. The peer can always differentiate between 2 connections based on it. When it sends replies, what was the source port now becomes the destination port allowing the original receiver to correctly pass data to the rightful processes.