Axel AX4000 User's Manual - Manual (Page 96)

Appendix
90
AXEL Office Server
To check a connection, the Office Server performs the following operations:
- if the connection is idle during a certain delay (i.e. no incoming network
frame), a 'keepalive probe' is sent by the Office Server,
- if no response is received after 4 seconds, another 'keepalive probe' is
sent,
- if no response is received after 8 'keepalive probes', the device is
considered to be 'unreachable' (the associated connection is closed and
the channel is released).
When the TCP/IP device is unreachable, 32 seconds (8x4) are needed to
detect the trouble and to release the channel.
A.8.4 - Enabling the Keepalive
When a network service is set ([Serial Lines] →[Line X]→[Network Service]),
the TCP Keepalive Timeout parameter is requested. The options are:
- 0 (default value): keepalive is disabled.
- from 1 to 99: keepalive is enabled. This number is the delay (in minutes)
needed to release a channel associated with an unreachable host.
Example: the TCP Keepalive Timeout is set to 5. If the TCP/IP connection is
idle during 4 minutes 28 seconds, the first 'keepalive probe' is sent:
- If the TCP/IP connection is still 'alive', the checking process is stopped.
- If the TCP/IP host is really unreachable, 32 seconds are needed to
complete checking. So, after 5 minutes (4 minutes 28 seconds + 32
seconds) the channel is released.
A.8.5 - When Routers are Used
Under default settings (keepalive disabled), the Office Server is "silent” with
TCP/IP inactivity. But with keepalive enabled a regular data flow is generated
during TCP/IP inactivity. (The 'keepalive probes').
With ISDN routers (which automatically drop the phone line) this regularly data
flow will prevent the router from hanging-up. In this scenario the keepalive can
cause very expensive phone bills.
Note: some routers are able to intercept 'keepalive probes' and to simulate the
TCP/IP device response. This function (keepalive spoofing) avoids extra data