Communications - October 2012

running oscillator); the “priority” as determined by the local PTP administrators (which is used as an override mechanism and not required to be set for proper operation); the device’s clock ID (which typically includes the device’s MAC address); and other attributes used by the BMC.

Having every PTP device run the BMC and process Announce messages means administrators can simply power on a system and have a network of time-synchronized devices automatically configured for optimal performance, regardless of spontaneous network topology changes.

Boundary clocks. Because switches and routers effectively segment a PTP network, PTP introduced boundary clocks as a means of distributing a master clock to different parts of the network. The PTP standard describes a boundary clock as containing a single timekeeper disciplined by PTP but having multiple PTP ports in a domain. A port may serve as either the source of time (a PTP master) to devices attached to it or one that synchronizes the timekeeper (a PTP slave) to some other clock connected to that port. A boundary clock can be implemented to replace a traditional network switch or router in larger networks that are normally segmented by such devices. Because boundary clocks differ in operation from the PTP clocks previously described in this article, PTP differentiates the two by referring to them as either ordinary clocks or boundary clocks.

Each port of a boundary clock can be thought of as a separate ordinary clock instance that shares a single timekeeper with the boundary clock’s other ordinary clock instances. Only one port on the device can be in the slave state, which eliminates contentious use of the device’s timekeeper (two ports trying to adjust the time, for example). All other ports are considered masters to the devices on their respective segments.

The existence of boundary clocks re-
quires PTP to use the term grandmaster
to describe the master to the entire PTP
network, since the slaves on a bound-
ary-clock port consider the boundary
clock to be their master. Each master
port is responsible for handling the
same duties as an ordinary clock mas-
ter, which effectively hides all of the
slaves from the boundary clock’s mas-
ter. Likewise, a slave ordinary clock (or
another boundary clock with the con-
nected port in the slave state) is hidden
from the PTP hierarchy “above” the
boundary clock. A boundary clock does
not pass the PTP synchronization mes-
sages from its slaves “up” to its master.
Without this behavior, a grandmaster
would be responsible for processing
Delay Request messages and issuing
Delay Response messages from and to,
respectively, every slave device on the
entire PTP network. In most cases it
would not be able to run the protocol
stack effectively.

For example, if the grandmaster goes offline, the next most eligible slave device can announce itself as master (once its BMC algorithm has determined it’s appropriate to do so), and the boundary clock will transition the port connected to that slave to the slave state. That boundary clock will then have a port that was once in the slave state now in the master state to other ordinary and boundary clocks. Those clocks will then evaluate that new master with the BMC algorithm and transition appropriately, repeating this process for the rest of the hierarchy. Depending on the network topology, this situation may not be ideal as the number of hops in between this new master and a slave will have increased by one (the boundary clock connected