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tcp connection

i have two computer A & B. both are use dial-up connection connected to internet, but A use ISP a, B use ISP b.
from A i can make a tcp connection to B, but I can't make a tcp connection from B to A.
the tcp connection can only be initiated by Computer A. why?
how can i initiate a tcp connection from B?
as i know, icq use tcp connection between users, i can do message chat between the above A & B, it doesn't matter which one start the chat.
why icq can make a tcp connection from B to A and I can't establish tcp connection if i run my own program. why?
xiamAsked:
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xiamAuthor Commented:
Edited text of question.
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jhanceCommented:
First, ICQ is not peer-to-peer.  It's client-server so when running ICQ, there is never a direct connection between your computers A and B.  A has a connection to the ICQ server and B has a connection to the ICQ sever and the server routes the data between the two.

When dialed up, some ISPs block incoming network connections as a security measure and/or they run their client traffic through a firewall or proxy.  This would prevent B from contacting A directly.  You should check with ISP b to see if this is the case.

A tool that can be useful for determining where traffic is going is TRACERT.  Open a DOS window and A and type:

TRACERT a.b.c.d

where a.b.c.d is the IP address of the computer (in this case B) that you want to try and contact.  It should show you the hosts that the traffic go through.  Then try the same thing on B while giving it A's IP address.  See where the packets are getting lost.
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xiamAuthor Commented:
Dear Jhance,
thank you for the answer.
but i read somewhere, the icq chat is done by a tcp connection between users directly. it is not go through icq server.
i hope you or someone can make it clear how does this icq connection bwteen users done.

thanks,
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jhanceCommented:
>but i read somewhere, the icq chat is
>done by a tcp connection between users
>directly. it is not go through icq
>server.

>i hope you or someone can make it
>clear how does this icq connection
>bwteen users done.

I'll stand by what I said already and say that whatever you read was incorrect.  ICQ requires a server and all traffic goes through the server.  There is no direct client to client connection.
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