Question

I’m trying to implement a communication between a legacy system and a Linux system but I constantly get one of the following scenarios:

(The legacy system is server, the Linux is client)

Function recv(2) returns 0 (the peer has performed an orderly shutdown.)
> SYN
< SYN, ACK
> ACK
< PSH, ACK  (the data)
> FIN, ACK
< ACK
> RST
< FIN, ACK
> RST
> RST

Function connect(2) returns -1 (error)
> SYN
< RST, ACK

When the server have send its data, the client should answer with data, but instead I get a “FIN, ACK”
Why is it like this? How should I interpret this? I’m not that familiar with TCP at this level

Answer 1

When the server have send its data, the client should answer with data, but I instead get a “FIN, ACK” Why is it like this? How should I interpret this?

It could be that once the server has sent the data (line 4) the client closes the socket or terminates prematurely and the operating system closes its socket and sends FIN (line 5). The server replies to FIN with ACK but the client has ceased to exist already and its operating system responds with RST. (I would expect the client OS to silently ignore and discard any TCP segments arriving for a closed connection during the notorious TIME-WAIT state, but that doesn’t happen for some reason.)

http://en.wikipedia.org/wiki/Transmission_Control_Protocol#Connection_termination:

Some host TCP stacks may implement a half-duplex close sequence, as Linux or HP-UX do. If such a host actively closes a connection but still has not read all the incoming data the stack already received from the link, this host sends a RST instead of a FIN (Section 4.2.2.13 in RFC 1122). This allows a TCP application to be sure the remote application has read all the data the former sent—waiting the FIN from the remote side, when it actively closes the connection. However, the remote TCP stack cannot distinguish between a Connection Aborting RST and this Data Loss RST. Both cause the remote stack to throw away all the data it received, but that the application still didn’t read