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Supervision
Introduction
This note describes which provisions are taken by NetModule Routers to guarantee always-on and always-connected
functionality.
The NetModule Routers are designed for high reliability and 24×7 operations. To guarantee best connection availability several levels and methods of supervision are used.
Depending on your hardware model, WAN links can be made up of either Wireless Wide Area Network (WWAN), Wireless LAN (WLAN), Ethernet or PPP over Ethernet (PPPoE) connections.
The highest level of monitoring is the Ping Supervision which is individually configurable for each active (up) link as well generic for any WWAN link. The user can define up to two host which are pinged every “ping interval” seconds. If there is no response of at least one of the two hosts within “ping timeout” milliseconds a retry is done using the “retry interval” time up to “max. number of trial fails”. If still there is no answer the defined emergency action is executed. It's recommended to set “restart link services” on each link pinging the next hop e.g. the access point for a WiFi client link. Restarting the link service means, that it will be tried to re-establish the link e.g. redial on a modem connection. Emergency action “none” means that the link is defined as (inactive) down. If you are using a VPN you should also enable the dead pear detection or keep alive mechanisms. “ANY” is a shortcut for all WAN interfaces.
The WWAN Manager who is responsible for the control of the mobile network modems checks every 15 seconds if the connection is still established. Different informations like configurable signal strength, network registration status of service type are used to determine the status of the modem links.
The task of the Link Manager is to verify if a link is up or down and to select highest priority link as the so-called hotlink which holds the default route for outgoing packets. Every 5 seconds he checks for each active link if the following conditions are met:
| Condition | WWAN | WLAN | ETH | PPPoE |
|---|---|---|---|---|
| Modem is registered | X | |||
| Registered with valid service type | X | |||
| Valid SIM state | X | |||
| Sufficient signal strength | X | X | ||
| Client is associated | X | |||
| Client is authenticated | X | |||
| Valid DHCP address retrieved | X | X | X | X |
| Link is up and holds address | X | X | X | X |
| Ping check succeeded | X | X | X | X |
If at least one condition of a link is not met, the link manager counts the number of subsequent failures. WWAN links are marked as down after 5 subsequent failures, PPPoE after three, WiFi and Ethernet after one. The highest priority link which is up will become the so-called hotlink which holds the default route for outgoing packets. Of course the link manager will change the hotlink as soon a higher priority interface comes up.
Finaly a Hardware Watchdog is available which will restart the device if he is not retriggered after x seconds. The Router Suppervision deamon checks permanently if all components of the router application are still alive. So in the unlikely case that the operating systems or the router software has an internal error the system restarts completely by rebooting. This is the only condition where the system will reboot except “reboot system” is configured in the link supervision.
Summary
- Level 4: VPN dead peer detection/keep alive
- Is VPN link over WAN link healthy? See also Supervision of a VPN Tunnel.
- Level 3: Link level Ping Supervision
- Is link to next network node healthy?
- Level 2: Modem/WiFi Supervision
- Is modem ok, device registered and a sufficient signal available ?
- Level 1: Link Supervison
- Is physical link up or physical connection ok?
- Level 0: Hardware Watchdog
- Is router firmware and operating system running ok?
Configuration
