Re: Socket.Send() succeeds, but remote end point closed the connection

yvan wrote:
Hi,

Here is my client/server scenario:
Step1: Client connects to server and sends data sucessfully (using Socket.Send()).
Step2: Server gracefully exists (calls Socket.Shutdown() and Socket.Close()). I see the server connection status go from ESTABLISHED to FIN_WAIT2, the client connection go from ESTABLISHED to WAIT_CLOSE. Step3: client.Send() succeeds and return the number of bytes sent (over the closed connection!)
Step4: same as step3, Send() throws, which is expected, but was expected in step3

Question 1: Shouldn't step3 behave like step4? If step3 can happen, then the transport is not reliable.
Question 2: Is there a socket status event the client can subscribe to?
....

This is by design of TCP protocol.

What you describe is not graceful close by server. In CLOSE_WAIT state, send is allowed. CLOSE_WAIT means that other side has shutdown it's outgoing side of connection (you will not receive any data on that connection), but you may send until you shutdown your side (remember that tcp is full-duplex). Server should not exit or call close on socket until it has receive shutdown from other side (it has to make transition from FIN_WAIT2 to TIME_WAIT state).

Gracefull close is:
Server calls Shutdown with SD_SEND
client receives close notification (read will return 0)
client may send data and call shutdown with SD_SEND when finished
server must loop in read until it receives close notification (receive 0 from read). Only then it may close socket.

Only if you close gracefully, you are sure data is delivered. Please note that there are linger options that can be set which affect this Check in MSDN:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winsock/winsock/graceful_shutdown_linger_options_and_socket_closure_2.asp

Send does not ensure data is sent, it ensures only it is buffered by the OS (TCP protocol). IOW, number of bytes returned from send call > 0 does not mean data is received by other side.


Regards,
Goran


.

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