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A keepalive is a signal sent from one device to another to maintain a connection between the two devices. This may be between a client and a server, but it could apply to any number of devices or technologies. Keepalives are used in network environments to maintain an open communication pathway, or to regularly check the status of a connection to a remote device.
Keepalives are all about keeping a line of communication open. The normal situation is that a connection is made and then closes right away. A keepalive maintains that connection in an active status for a set amount of time.
One way to understand keepalives is to think of a telephone conversation. When speaking to others on the phone, it is customary to avoid long pauses. If there is silence for a while, one may say, “Are you still there?” and wait for an answer. When the other person says, “Yes, I’m still here,” they have kept the conversation alive.
Networks use protocols to set up and maintain conversations between devices. To keep these protocols active – perhaps in order to send more data or to check if a link is still working – a keepalive may be sent at predefined intervals. Keepalives may use configurable parameters, such as frame size, interval between signals, number of retries, and timeouts. Keepalives keep network connections alive.
There are many technologies that use keepalives. TCP keepalive packets check to see if a connection is still valid. HTTP keepalives maintain browser connections while data is being transferred. Keepalives may be used with session interface protocol (SIP), spanning tree protocol (STP), server message block (SMB), transport layer security (TLS), or any number of protocols where network designers may find them useful.