I’m writing this for my friend support
David and the rest, guys, you dont understand nothing about networking and all related about messaging, beleive me. I respect you all but it seems you’re trying to talk about things that you dont understand.
Now, lecture about how does your computer work with other servers, i’ll take real life example to finally explain how it works.
Lets imagine that girl is aol server and boy is client(your aim messenger or regdate program),
so boy(client) tells by his mouth to girl(server) “HELLO MY DEAR”,
girl(server) can listen him by ears(port 5190),
ok she heard him because girl(server) has open ears(port 5190 on server(girl)),
and she(girl, server) replies him by her mouth “HI SWEETY DAVID”,
boy has to have his own ears(his local port) to listen her right? if he won’t has it the client(boy) will not hear what she talking about.
so if this one clear let me tell you about some technical details.
All new and old technical discoveries have logic which was taken from the real life else if there is no logic then there is no any point to make it.
and that example about girl and boy shows it by simple but to understand.
Now first of all to connect to aol server your aim clinet sends hello packet to one of aol authentication servers to his remote port 5190, then if your password is good server replies to your aim local port that its ok to enter our aol network and gives to you unique cookie and another server address with same port which will work with your aim client all the time until you disconnect. So, aim understands it and sends second hello packet to server and port what was received by first time. If your cookie is good then you are ONLINE and at this time server begins to send packets to your local port with different info of you including your server side contact list and etc. If your local ports will be closed then you could only send hello packets but you will never hear what server says to you because you dont have ears or you closed it and yu will never be ONLINE.
And doesnt matter what is the number of local port, the point is it should be to get replies. your aim messenger uses another one or couple. My program when she running watching for 3000 port and if its free to use then it waits there server replies if it will be busy by another program then it will try to open 3001 and etc else my program will now work. May be you dont understand how could I write protocol like aim oscar and dont beleive it that one man could write program like aim messenger, then you can use my other programs but almost all of them concern only icq. So don’t be afraid I realy busy man and dont care about backdoors, trojans viruses and your screennames. If you afraid then like said my friend dont use it, call to aol time warner and ask birthday of you screenname.
Sorry for mistakes, but my english is not such perfect like yours.
If you have some ideas about how to write aim usefull tool using oscar protocol then welcome to my icq.

ps: http://iserverd.khstu.ru/oscar/
usefull site for u

little addition, I runned icq and aim, so what we see at netstat -b
Active Connections (inf_gth computer)
Proto Local Address Foreign Address State PID
TCP admins:1460 64.12.25.20:5190 ESTABLISHED 1908 [Icq.exe]
TCP admins:1503 172.30.36.23:netbios-ssn ESTABLISHED 4 [System]
TCP admins:2287 64.12.30.200:5190 ESTABLISHED 6328 [aim.exe]
TCP admins:2293 64.12.29.181:5190 ESTABLISHED 6328 [aim.exe]

Your firewalls are set by default to allow to aim to open any ports therefor you so worried about port 3000 because your firewalls dont know what the my program is 😉