connected over a network via TCP/IP, TCP Keepalive Packets can be used to determine if the connection is still valid, and terminate it if needed. Most hosts that support TCP also support TCP Keepalive. Each host (or peer) periodically sends a TCP packet to its peer which solicits a response. If a certain number of keepalives are sent and no response (ACK) is received then the sending host will terminate the connection from its end. If a connection has been terminated due to a TCP Keepalive time-out and the other host eventually sends a packet for the old connection, the host that terminated the connection will send a packet with the RST flag set to signal the other host that the old connection is no longer active. This will force the other host to terminate its end of the connection so a new connection can be established. Typically TCP Keepalives are sent every 45 or 60 seconds on an idle TCP connection, and the connection is dropped after 3 sequential ACKs are missed. This varies by host, e.g. by default Windows PCs send the first TCP Keepalive packet after 7200000ms (2 hours), then sends 5 Keepalives at 1000ms intervals, dropping the connection if there is no response to any of the Keepalive packets. Keepalive on higher layers Since TCP keepalive is optional, various protocols (e.g. SMB and TLS) implement their own keep-alive feature on top of TCP. It is also common for protocols which maintain a session over a connectionless protocol, e.g. OpenVPN over UDP, to implement their own keep-alive. Other uses HTTP keepalive The Hypertext Transfer Protocol uses the keyword "Keep-Alive" in the "Connection" header to signal that the connection should be kept open for further messages (this is the default in HTTP 1.1, but in HTTP 1.0 the default was to use a new connection for each request/reply pair). Despite the similar name, this function is entirely unrelated. Other devices "Keep-alive" devices are used in automotive repair to maintain battery voltage for devices in the vehicle when the battery is disconnected or changed, usually by plugging in a small battery to the vehicle's 12 volt power outlet. A typical application is preventing the vehicle's radio or other device from going to "code" mode (security lockout) during vehicle repair. Typically a lower voltage source, such as a 9–volt battery, is sufficient for the purpose. Electric clocks often have battery-powered