Axel AX4000 User's Manual - Manual (Page 95)

Appendix
AXEL Office Server
89
A.8 - KEEPALIVE FUNCTION
The keepalive is a mechanism that allows a TCP/IP device to regularly check its
TCP/IP connection status.
A.8.1 - Why a Keepalive Function is Needed?
A TCP/IP device is not able to detect in real time a network problem (remote
host down, cable problem, etc).
A network problem can only be detected when data is sent and the destination
host acknowledgement is not received. In this case, when the
acknowledgement is not received, the remote TCP/IP device is considered
'unreachable' and its associated resources are released. This release is
performed few minutes after the data was sent.
A.8.2 - Example of Potential Issue
Take for example an Office Server channel, set as a telnet server with the
DialOut/IP driver under Windows. The Windows host opens a connection to the
Office Server in order to associate a Windows COM port the Office Server.
In the event of a (network) problem Windows will react and the DialOut/IP
connection will be closed. But the disconnection request will not be received by
the Office Server (the TCP/IP link is broken). A 'Zombie' connection is now
associated with the Office Server channel. Even After the network problem has
been repaired, further Window’s connection requests will be refused by the
Office Server (the channel is still associated with the zombie connection).
Note: the Office Server channel will be released, either on Office Server reboot
or when data is sent to Windows (the network problem will be detected).
A.8.3 - Keepalive Mechanism
The principle of keepalive is the Office Server checks regularly the TCP/IP
connection status. These automatic checks are performed when the TPC/IP
connection is idle.