tcpwatch: A network packet arrival deadline monitor.

Tcpwatch lets you specify a tcpdump/bpf filter and a time interval in
milliseconds. If a packet matching the filter arrives within the time
interval, nothing happens. If it does not arrive within the interval,
an outage is signalled to stdout or syslog. Once a packet matches
again, the outage is over, a message is signalled to stdout or syslog,
and the deadline timer is reset.

Usage: tcpwatch -i <iface> [options]
Options:
  -w n   Set deadline to n milliseconds.
  -D     Run in foreground. Log to stderr.