Digi SureLink™ is an optional feature that monitors the integrity of an established network connection. Several settings and tests can be configured to perform a selected test to examine the functional integrity of the network connection, and take action to recover the connection in the event that it is lost.
Currently SureLink is supported for cellular communications only. The SureLink feature and its settings are off by default, and must be enabled and configured.
The Link Integrity Monitoring settings can be configured to perform a selected test to examine the functional integrity of the network connection, and take action to recover the connection in the event that it is lost.
Link integrity monitoring settings:
Enables or disables the link integrity monitoring tests. If this setting is enabled, the other Link Integrity Monitoring settings may be configured and are used to verify the functional integrity of the mobile connection. The default is off (disabled).
There are several tests available:
– Ping Test
– TCP Connection Test
– DNS Lookup Test
– Device Cloud Connection Test
You can use these tests to demonstrate that two-way communication is working over the mobile connection. For the link integrity tests to provide meaningful results, the remote or target hosts must be accessible over the mobile connection and not through the LAN interface of the device (if it has one). That is, the settings should be configured to guarantee that the mobile connection is actually being tested.
Enables or disables the use of “ping” (ICMP) as a test to verify the integrity of the mobile connection. The test is successful if a valid ping reply is received in response to the ping request sent. The ping test actually sends up to 30 ping requests, one per second, to test the link. When a valid reply is received, the test completes successfully and immediately. If a reply is received for the first request sent, there is no
need to send the other two requests.
Two destination hosts may be configured for this test. If the first host fails to reply to all three ping requests, the same test is attempted to the second host. If neither host replies to any of the ping requests sent, the test fails. The primary and secondary addresses may
be either IP addresses or fully qualified domain names.
– Primary Address: First host to test.
– Secondary Address: Second host to test (if the first host fails)
*TCP Connection Test:
Enables or disables the creation of a new TCP connection as a test to verify the integrity of the mobile connection. The test is successful if a TCP connection is established to a specified remote host and port number. If the remote host actively refuses the connection request, the test is considered a failure. The TCP connection test waits up to 30 seconds for the connection to be established or refused.
When the TCP connection is established, the test completes successfully, and the TCP connection is closed immediately.
Two destination hosts may be configured for this test. If the first host fails to establish (or refuse) the TCP connection, the same test is attempted to the second host. If neither host successfully establishes (or refuses) the TCP connection, the test fails. The primary
and secondary addresses may be either IP addresses or fully qualified domain names.
– TCP Port: The TCP port number to connect to on the remote host. The default is 80.
*DNS Lookup Test:
Enables or disables the use of a Domain Name Server (DNS) lookup as a test to verify the integrity of the mobile connection. The test is successful if a valid reply is received from a DNS server. Typically, this means the hostname is successfully
“resolved” to an IP address by a DNS server. But even a reply such as “not found” or “name does not exist” is acceptable as a successful test result, since that demonstrates successful two-way communication over the mobile connection. When a valid reply is
received, the test completes successfully and immediately.
The DNS servers used in this test for the hostname lookup, are the primary and secondary DNS servers obtained from the mobile network when the mobile PPP connection is first established. These addresses can be viewed on the
Administration > Mobile Status page.
*Device Cloud Connection Test:
Enables or disables verification of the Device Cloud connection. The test is successful if a connection can be established to the configured Digi Device Cloud server, and if keep-alive messages can be exchanged with the server. The test fails if a connection
cannot be established, the connection is closed, or if keep-alive messages stop. The Device Cloud server is configured on the Configuration - Device Cloud Connectivity page.
*Repeat the selected link integrity test every N seconds:
Specifies the interval, in seconds, at which the selected test is initiated (repeated). This value must be between 10
and 65535. The default is 240. This setting is used by Ping, TCP Connection, and DNS Lookup tests only, not Device Cloud Connection.
If the configured interval is less time than it takes a test to complete, the next test will not be initiated until the previous (current) test has completed.
*Test only when idle if no data is received for the above period of time:
Specifies that the test repeat interval (above) is to be used as an idle period interval. That is, initiate the selected link integrity test only after no data has been received for the specified interval of time. This changes the behavior of the test in that the test interval varies according to
the presence of other data received from the mobile connection. This setting is used by Ping, TCP Connection, and DNS Lookup tests only, not Device Cloud Connection.
Although using this idle option may result in less data being exchanged over the mobile connection, it also prevents the link integrity tests from running as often to verify the true bi-directional state of that connection.
*Reset or Reconnect the link after the following number of consecutive link integrity
Specifies that after the configured number of consecutive link integrity test failures, the mobile connection should be either reset or reconnected:
– Reset: The cellular modem is reset.
– Reconnect: The mobile interface is reconnected.
This value must be between 1 and 255. The default is 3. When the mobile connection is reestablished, the “consecutive failures” counter is reset to zero.
If the mobile connection is disconnected for any reason (including not as a result of a link integrity test failure), the consecutive failures count is reset to zero when the mobile connection is reestablished.
*Primary Test Hostname:
Secondary Test Hostname:
These settings apply to Ping, TCP Connection, and DNS Lookup tests only. Two hostnames may be configured for the link integrity monitoring test. Hostnames can be specified in the form of domain names or IP addresses. If the first hostname fails to get a
reply, the same test is attempted for the second hostname. If no reply is received for either hostname, the test fails. The primary and secondary DNS names should be fully qualified domain names. Note that the reverse lookup of an IP address is possible, but
that is usually unlikely to succeed in returning a name. Still, such a reverse lookup can be used to demonstrate the integrity of the mobile connection.
– Primary Test Hostname: The first hostname to look up.
– Secondary Test Hostname: The second hostname to look up (if the first hostname
Last updated: Aug 23, 2017