I have a client server situation where the client opens a TCP socket to the server, and sometimes long periods of time will pass with no data being sent between them. I have encountered an issue where the server tries to send data to the client, and it seems to be successful, but the client never receives it, and after a few minutes, it looks like the client then gets disconnected.
Do I need to send some kind of keep alive packet every once in a while?
Edit: To note, this is with peers on the same computer. The computer is behind a NAT, that forwards a range of ports used to this computer. The client that connects with the server opens the connection via DNS. i.e. it uses the mydomain.net & port to connect.
On Windows, sockets with no data sent are a big source for trouble in many applications and must be handled correctly.
The problem is, that SO_KEEPALIVE’s period can be set system-wide (otherwise, a default is useless two hours) or with the later winsock API.
Therefore, many applications do send some occasional byte of data every now and then (to be disregarded by the peer) only to make the network layer declare disconnection after ACK is not received (after all due retransmissions done by the layer and ack timeout).
Answering your question: no, the sockets do not disconnect automatically.
Yet, you must be careful with the above issue. What complicates it further is that testing this behavior is very hard. For example, if you set everything correctly and you expect to detect disconnection properly, you cannot test it by disconnecting the physical layer. This is because the NIC will sense the carrier loss and the socket layer will signal to close all application sockets that relied on it. A good way to test it is connect two computers with 3 legs and two switches in between, disconnecting the middle leg, thus preventing carrier loss but still physically disconnecting the machines.