Keepalive packet#

Keepalive packet is simply an ACK with the sequence number set to one less than the current sequence number for the connection. A host receiving one of these ACKs responds with an ACK for the current sequence number. Keep-alives can be used to verify that the computer at the remote end of a connection is still available. The keepalive packet contains null data. In an Ethernet network, a keepalive frame length is 60 bytes, while the server response to this, also a null data frame, is 54 bytes. There are three parameters related to keepalive:

· Keepalive time is the duration between two keepalive transmissions in idle condition. TCP keepalive period is required to be configurable and by default is set to no less than 2 hours.

· Keepalive interval is the duration between two successive keepalive retransmissions, if acknowledgement to the previous keepalive transmission is not received.

· Keepalive retry is the number of retransmissions to be carried out before declaring that remote end is not available

TCP keep-alives can be sent once every KeepAliveTime (defaults to 7,200,000 milliseconds or two hours) if no other data or higher-level keep-alives have been carried over the TCP connection. If there is no response to a keep-alive, it is repeated once every KeepAliveInterval seconds.