Hold time is the interval(in milliseconds) between the
time of receiving the last HELLO ACK message and the time to
assume that the cable interface is out of service.
In the case of Working, it is the interval in milliseconds
between the time when the Working receives a HELLO
message and the time to assume that Protect
has failed.
In the case of Protect, it is the interval in
milliseconds between the time when the Protect sends
last HELLO message without seeing HELLO_ACK and the
time to assume that Working has failed.
It is carried by HELLO message and is used by Protect
and all Working.