Question

I made an experiment:
A server listens on port 8804 accepts a connection of a client and then send data to the client endless. I shutdown the network.

1. When I run netstat -anotp | grep 8804 ,it shows that the connection is “ESTABLISHED” on both server and client , but there is no data transmission.
2. After a while , the server throw an error : “Connection time out”
3. netstat -anotp | grep 8804 and found that the client is still “ESTABLISHED”

So:
1. Why does the server which is blocked on the system call “write” throw the “Connection timeout” error. Why not the client ?
2. How to let the client find the connection is shutdown actually.
3. Why are the server and client’s statuses both “ESTABLISHED” when the network does not work ?

Thanks for your answer !

Answer 1

1. Your server is expecting TCP ACKs for individual data segments that it sends to the client; however, the client has no idea how long the server’s data is. Since you shutdown the network the server no longer gets ACKs from the client. Result: Connection timeout on the server (See Note 1)
2. Use TCP Keepalives on your socket (See Note 2)
3. You have not enabled TCP Keepalives. If you are using python, you can do so like this (assuming your socket is named s):

# Do this before you accept() anything on the socket
s.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

EDIT:
Since you’re using C, a link to the Linux TCP Keepalives Howto

NOTES

1. RFC 1122: Section 4.2.3.5 "TCP Connection Failures"
2. RFC 1122: Section 4.2.3.6 "TCP Keepalives"