The `keepalive` feature in libosmo-mgcp-client allows scheduling periodical
queries on the MGCP layer in order to make sure it is reachable and hence
obtain information on the state of the MGCP link.
This patch only uses it to print the status on the VTY, but it will be used
too in a follow-up commit by the MGW Pool when picking an MGW from the pool:
MGWs whose link is considered to be DOWN are skipped.
The feature consists of:
- A `keepalive request-interval` which will trigger a transmission of an MGCP
AuditEndpoint command targeting endpoint with name `keepalive request-endpoint`.
This interval is updated every time any message is transmitted in the MGCP
link, meaning the MGCP AuditEndpoint message is only triggered if no message
has been transmitted since `keepalive request-interval` seconds ago.
- A `keepalive timeout` considering the MGW to be non-reachable (link DOWN) if
no message is received over that amount of time.
The `keepalive` parameters are to be preferrably configured so that
"keepalive request-interval" * 2 < "keepalive timeout".
Example VTY configuration of `keepalive` feature in libosmo-mgcp-client:
keepalive request-interval 20 <1>
keepalive request-endpoint null <2>
keepalive timeout 50 <3>
<1> Transmit an MGCP AuditEndpoint message to the MGW if no message has been
sent to it over last 10 seconds
<2> The MGCP AuditEndpoint targets the `null` endpoint. This is a special
endpoint available at OsmoMGW for those purposes, but any available
endpoint can be configured and used instead.
<3> Consider the MGCP link to be DOWN if no message is received from the
MGW over the last 50 seconds
NOTE: The `keepalive` feature is disabled by default, and must be explicitly
configured in order to enable it.