FAQ: What are "advisory signals" and "cascade signals" and how do they work?
This article applies to Domain Time II.
Last Updated: 12 June 2015
What are "advisory signals" and "cascade signals" and how do they work?
Cascade and advisory signals are UDP packets sent to port 9909, used by Domain Time II to implement the
Cascading Time Hierarchy.
Clients normally view cascade signals as a mandatory requests for them to synchronize. Level 1 cascades
are unicast (master directly to server). All other cascades and advisories are broadcast.
Advisory signals are identical to cascade signals in terms of packet size, data, and hierarchy levels,
but have an additional bit set in the packet telling the client (or downline server) that synchronization is
recommended instead of mandatory.
As of version 5.x, you may disable cascades and advisories from the Domain Time Server applet, on the Domain Role property page.
On older versions you may change the Send Advisories registry setting
to False. Version 5.x and later Clients may be set to ignore cascades and advisories from the Domain Time Client applet, on the Advanced property page.
Advisories are sent under the following circumstances:
- After a master has successfully synchronized its slaves, it will send a "master changed" advisory (a Level 1
cascade trigger with the advisory bit set). This advisory is treated as a cascade trigger by any clients
that cannot discover a slave. Clients who have received a cascade trigger from a slave will ignore this
- Slaves send advisories after they have checked with the master and changed their time by 5 milliseconds
or more during a non-cascade synchronization. If the slave synced because it received a Level 1 cascade
trigger, it will send a Level 2 cascade instead of a Level 2 advisory. Clients who are using the slave
sending a Level 2 advisory will resynchronize if they believe their own clocks may be off by an amount
significant enough to warrant checking. Each client determines whether or not to sync upon receipt of
an advisory by examining how many other advisories it has seen recently from that or other servers, and
how recently it has corrected its clock.