“Keepalive” is a signal sent between two connected systems to keep the connection active. It is used to test a connection or prevent it from being terminated, and can be used to prevent automatic termination by servers. The signal contains no data and little bandwidth is used.
The term “keepalive” refers to a process by which a signal is sent between two connected systems, often over a network such as the Internet, to keep the connection between these two systems active. When a computer user, for example, accesses a web page, his computer sends a request for information from the computer, or client, to the server for that web page. After accessing the information and sending the information to the client by the server, there is usually a certain period of time after which the connection between client and server will be dropped. When a keepalive signal is sent, however, it keeps that connection alive.
In general, the term “keepalive” is typically used to refer to both the signal being sent and the sending process. The signal itself usually contains no data other than the response request from the server, which means that little bandwidth is used to send this type of message. Once the server receives the signal, it responds with an acknowledgment signal, or ACK, which is received by the client. Programming a keepalive protocol on a client typically establishes a timer that determines when the next message is sent.
There are several reasons for using a keepalive signal, although it is typically used to test a connection or to prevent it from being terminated. When used to test a connection, the signal is sent by the client based on a timer set in the script for the keepalive protocol. As long as the signal is recognized, the system continues to operate normally. If the signal is not received, however, a notification can be given to the client user indicating that the connection is no longer active.
Many networks, especially large networks like the Internet, are designed with servers that automatically drop a client’s connection after a set amount of time. This is done to allow for the creation of new connections, since client-server connections are finite in number. A keepalive signal can also be used to prevent this automatic termination from occurring. When the client sends a keepalive signal to the server, it indicates that the connection is still in use, which prevents the server from automatically terminating the connection. This is often done on websites that allow persistent connections, although this is not a guaranteed method of ensuring continuous connectivity.
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