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10.5.3.7 Message Rate Settings
The decision between the different general mode options is mainly dictated on the network environment in which
the PTP infrastructure is installed. In addition to the mode selection, a number of intervals for certain types of
PTP network messages needs to be defined. In most cases, the default values as defined in the standard are a
safe bet, but there are applications and scenarios where a custom message rate is required.
A possible example is a situation where the PTP infrastructure is integrated within an environment with high
network load. In this case, the PTP packets can be affected by the effect of packet delay variation (PDV). An
increase of the PTP message rate(s) can avoid synchronization problems due to packet queuing within non-PTP
compliant switches which might cause false measurements. At higher rates, these false measurements can be
detected and corrected faster as compared to lower rates at the cost of increased traffic.
The message rates for the following message types can be changed:
1) ANNOUNCE messages
2) SYNC/FOLLOWUP messages
3) (P)DELAY_REQUEST messages
10.5.3.8 ANNOUNCE Messages
These PTP messages are used to inform the PTP network participants about existing and available master
clock devices. They include a number of values that indicate the potential synchronization accuracy.
The procedure used to decide which of the available devices (that could become masters) is selected is called the
“best master clock algorithm” (BMCA). The values that are used in this BMCA are read from the ANNOUNCE
messages that potential masters send out periodically.
The rate at which these messages are sent out are directly affecting the time that is required by a slave
device to select a master and to switch to a different master in case the selected one fails.
Multiple devices can simultaneously transmit ANNOUNCE messages during periods in which no master has
been selected (yet). This happens for example when a PTP network is powered up, i.e. all devices are starting
to work at the same time. In this case all devices that consider themselves (based on their configuration and
status) being capable of providing synchronization to all the other PTP devices will start to send out AN-
NOUNCE messages. They will receive the other candidates’ ANNOUNCE messages as well and perform the
BMCA. If they determine that another candidate is more suitable to become the master clock, they stop sending
ANNOUNCE messages and either become slave devices or go into "PASSIVE" mode, waiting for the selected
master to stop sending ANNOUNCE messages. This is determined to be the case when no ANNOUNCE
message is received within 3 ANNOUNCE message intervals.
As an example, if the ANNOUNCE interval has been configured to be 2 seconds (one message every 2 sec-
onds, the default value), the master is considered to have failed when no message has been received for 6 seconds.
In order to choose a master (a backup master clock or the primary one during initialization) the devices require
to receive at least two consecutive ANNOUNCE messages. Continuing our example, it would take the 6 seconds
to determine that the current master has failed and another 4 seconds to select the new one. That means an
ANNOUNCE interval of 2 seconds translates into at least 10 seconds of “switching time” and 4 seconds of
“initial master clock selection time”. So, choosing a shorter ANNOUNCE message interval will allow a faster
switching to a backup master clock, but it can lead to false positives when the chosen interval is too short for
the network environment.
72 Date: 22nd June 2020 microSync