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Keepalive Signal Definition and Meaning

In networking, a keepalive signal is a packet that is sent to maintain a connection between two points even though data is not being transmitted.

Transmission control protocol (TCP) is one protocol that can use keepalive packets to maintain a connection between a client (typically a computer or another end-user device) and a server so it can process multiple HTTP requests without needing to repeat the three-step handshake. The keepalive signal reduces latency issues for the client and also helps the efficiency of the broader network by conserving bandwidth that can be used elsewhere and requiring less overall maintenance for a single connection.

Keepalive signals can also be used to validate that a connection is still secure. When a client sends a keepalive signal that is not acknowledged within a designated length of time by the server (or vice-versa), it will reattempt a limited number of transmissions before terminating the connection and requiring a new three-step handshake to be initiated. This prevents packets from being received out of order, incomplete, or damaged—one of the main benefits of using TCP instead of UDP.

UDP, SMB, TLS, NAT and SIP are among the protocols that can use keepalive signals, and some persistent connections, like VPNs, can use them too. HTTP code also has a Keep-Alive function that works at the browser level, but it is unrelated to the TCP-IP function.

Keepalive signals have several different configurations:

  • The duration of time between transmissions
  • The total number of transmissions before timeout
  • The number of retransmission attempts after a packet has not been received
  • The interval between retransmissions

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