Keptalives

Keptalives, often written as keepalives, refer to mechanisms that periodically send signals to verify that a network or application connection remains active. They are used across transport protocols, application protocols, and network devices to detect broken links and prevent idle resources from being dropped by intermediaries such as NAT gateways, load balancers, or firewalls.

In practice there are two broad forms: transport-layer keepalives built into protocols such as TCP or UDP, and application-level keepalives implemented by the software using the connection. Transport keepalives typically involve the sender sending a small probe after a period of inactivity and expecting a response; if responses fail after a configured number of probes, the connection may be considered dead. Application keepalives include mechanisms such as HTTP connection keep-alive headers, WebSocket ping/pong messages, and periodic heartbeat messages in custom protocols.

Configuration and behavior vary by system and protocol. TCP keepalive settings are often configurable, controlling idle time before probes, the interval between probes, and the maximum number of probes. Application keepalives may be tuned by timeouts in the server or client, heartbeat intervals, and response expectations. These mechanisms help maintain state in NATs and firewalls and reduce the time to detect failed peers, enabling faster recovery or reestablishment of connections. However, they add network traffic and can cause spurious disconnects if paths are asymmetric or rate-limited, and may broaden an attack surface when abused for probe-based denial of service.

In practice, keepalives are part of connection management and are chosen based on reliability needs, network conditions, and resource constraints. They are documented in protocol specifications for HTTP, WebSocket, TCP, and other transport protocols, and implemented in operating systems and middleware.