IEEE 1588 aims at creating a master-slave timing distribution model across PTP clocks. In IEEE 1588, a clock is a node that implements PTP. In that node, a “PTP port” is a physical, logical, or virtual interface supporting PTP and handling PTP communications. To define the state or function of the port, IEEE 1588 defines state machines as part of its Best Master Clock Algorithm [BMCA] process.

Best Master Clock Algorithm

The BMCA actually has two key functions in a clock. Based on the information received on its PTP Port(s), the BMCA of a clock will:

• define the master-slave hierarchy of the PTP distribution
• select the best master (the clock source that it will synchronise to)

NOTE: Be aware of the distinction between the port state and the master clock selection, though both are handled as part of BMCA.

The BMCA is designed in such a way that a local clock determines which of all the clocks (including itself) is the “best”, then it decides on the state of all the ports (master/slave/passive). Clocks do not negotiate who should be master or slave – instead each computes the state of it’s own ports. The algorithm, by design, avoids configurations with two masters, no masters, or ones that thrash.

PTP ports can be in 1 of 3 states (the above diagram only considers GM, Slave, and intermediate port states. The role and type of the intermediate clocks isn’t considered yet, simply assume that they run the BMCA);

1. Master – providing timing information to downstream clocks
2. Slave – learning timing from an upstream (master) clock
3. Passive – preventing a loop in the PTP topology

The master port of a clock transmits its packet-based clock signal in the form of PTP messages to the slave port or another clock. Slave ports allow a clock to synchronise with the root of the hierarchy, the ultimate master clock e.g. the Grandmaster [GM] clock. A passive port prevents timing loops or conflicting master ports in the same hierarchy, when there are multiple valid transport network paths (consider an L3 network – no ports blocked due to STP). The state of the port, and the selected clock source (best master or master port) can change over time.

In a steady state network, the BMCA processed by the clocks establishes a hierarchy up to the root, i.e. a GM, and ultimately to slave clocks, the leaves of the hierarchy.

The BMCA will avoid creating timing loops through valid transport network paths e.g. ports that are not blocked by spanning tree. In the case of any changes e.g. device or link failure, the BMCA might have to re-establish the hierarchy and/or reselect the master.

How do we select the “best” clock?

The best master clock algorithm compares one clock to another by comparing datasets (my dataset, vs current best dataset, vs. the dataset just received) that represent those clocks. The table below shows the parameters used to compare any two PTP clocks.

GM selection can be manipulated using Priority1 and Priority2 values. Typically a true Grand Master device (Meinberg, etc.) will have a low Priority1, with the Priority2 value used to select a preferred active Grand Master device.

We would expect devices with a stable clock (for example the -CL variant of Arista’s 7150s), close to the GMs in the hierarchy to have an intermediate Priority1 and 2 values e.g. 127 – this way they would only become the GM if both “true” GM devices fail.

Slave devices (servers, etc.) would have the highest Priority value configured e.g. 255 to ensure that they never become the GM for the timing domain.

PTP Domains

IEEE 1588 defines the idea of a PTP Domain. These domains can be used for different purposes, e.g. technical or administrative, at the convenience of the user.  The standard defines the protocol and rules but does not define usage. A PTP domain corresponds to one GM delivering its clock signal over the hierarchy established for that domain. Simply put, a PTP domain corresponds to a unique hierarchy of two or more clocks synchronised to the same GM.

The spec allows a clock device to support multiple domains, but a PTP port within that device can only be part of a single domain. Logically, because a PTP communication path is established between two PTP ports, a PTP path belongs to one PTP domain and can never span multiple PTP domains.

A transport network supports multiple PTP domains and a physical network interface can convey multiple PTP paths depending on vendor implementation. Remember, a PTP port is a logical description that can be translated into a physical or virtual interface in a clock implementation. The specification allows for a PTP port to be assigned to a physical port, a VLAN or a loopback interface.

A network or a clock device can support multiple domains on the condition that each domain remains separate. This separation can be physical or logical, as distinct communication networks, VLANs or clocks, and could lead to more complex and expensive design implementation, particularly for maintaining different timescales in single clocks.

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