5.15 Crashing and Rebooting of Server Host

In this scenario, we will establish a connection between the client and server and then assume the server host crashes and reboots. In the previous section, the server host was still down when we sent it data. Here, we will let the server host reboot before sending it data. The easiest way to simulate this is to establish the connection, disconnect the server from the network, shut down the server host and then reboot it, and then reconnect the server host to the network. We do not want the client to see the server host shut down (which we will cover in Section 5.16).

As stated in the previous section, if the client is not actively sending data to the server when the server host crashes, the client is not aware that the server host has crashed. (This assumes we are not using the SO_KEEPALIVE socket option.) The following steps take place:

1. We start the server and then the client. We type a line to verify that the connection is established.

2. The server host crashes and reboots.

3. We type a line of input to the client, which is sent as a TCP data segment to the server host.

4. When the server host reboots after crashing, its TCP loses all information about connections that existed before the crash. Therefore, the server TCP responds to the received data segment from the client with an RST.

5. Our client is blocked in the call to readline when the RST is received, causing readline to return the error ECONNRESET .

If it is important for our client to detect the crashing of the server host, even if the client is not actively sending data, then some other technique (such as the SO_KEEPALIVE socket option or some client/server heartbeat function) is required.