Why will a TCP Server send a FIN immediately after accepting a connection?

Question

From the ethreal packet capture, I see the following behaviour which appears quite strange to me:

Client --> Server  [SYN]
Server --> Client  [SYN, ACK]
Client --> Server  [ACK]
Server --> Client  [FIN, ACK]
Client --> Server  [ACK]
Client --> Server  [TCP Segment of a reassembled PDU] (I don't know what this means)
Server --> Client  [RST]

Any ideas as to why this could be happening?

Also, the Server Port is 6000. Could that cause any problem?

My other doubts:

1. Why is there a FIN, ACK? Shouldn't it be only FIN? What is the meaning of the ACK in that message?
2. Shouldn't there be a FIN from Client also?

EDIT: After some more analysis, I found if the number of file descriptors have exceeded the limit then a FIN is sent by the Server. But, in this case it doesn't appear that the file descriptors have exceeded the limit. For what other scenarios can this happen?

Answer 1

Upon deep analysis, the following was found to be the reason of the problem:

When a Client tries TCP connect, even if the server is not currently calling accept, the connection will pass. This will happen if server has called 'listen' function and it will keep accepting the connections till backlog limit is reached.

But, if the application process exceeds the limit of max file descriptors it can use, then when server calls accept, then it realizes that there are no file descriptors available to be allocated for the socket and fails the accept call and the TCP connection sending a FIN to other side.

I just though of posting this finding here. I am still leaving the accepted answer as that of Habbie's.

Thanks to all those who answered this question.