keptalive

The term "keptalive" refers to a mechanism used in network communication protocols to maintain an active connection between two endpoints, typically a client and a server. This technique is essential in scenarios where prolonged periods of inactivity might otherwise lead to the premature termination of a connection, such as in TCP (Transmission Control Protocol) sessions.

In TCP, a keptalive feature is implemented to detect and recover from dead connections. When enabled, the operating system periodically sends small packets of data to the remote host to check if the connection is still active. If no response is received within a specified timeout period, the system assumes the connection is no longer viable and takes appropriate action, such as closing the connection or notifying the application.

Keptalive is particularly useful in long-running applications, such as web servers, databases, or remote administration tools, where connections may remain idle for extended periods. Without this feature, stale connections could persist, consuming unnecessary resources or causing issues when the connection is eventually reused. The keptalive mechanism helps prevent such problems by ensuring that only valid connections are maintained.

The configuration of keptalive parameters, including the interval between probes and the number of retries before declaring a connection dead, varies by operating system and network stack implementation. For example, in Linux, these settings can be adjusted via kernel parameters like `tcp_keepalive_time`, `tcp_keepalive_intvl`, and `tcp_keepalive_probes`. Similarly, Windows and other systems provide their own methods for tuning these values.

While keptalive is primarily associated with TCP, similar concepts exist in other protocols, such as UDP (User Datagram Protocol) with keepalive extensions or application-layer protocols like HTTP/1.1, which may use persistent connections. The exact behavior depends on the protocol implementation and the requirements of the communication environment.