This article applies to Domain Time II.

Last updated: 5/23/2002

Question

What are "advisory signals" and "cascade signals" and how do they work?

Answer

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. You may disable advisories on any server by changing the Send Advisories registry setting to False. Advisories are enabled by default.

Advisories are sent under the following circumstances:

1. 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 advisory.

2. 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.