PTP (Precision Time Protocol) is a protocol that enables clocks to be synchronised with a theoretical accuracy of the order of a nanosecond. PTP is the protocol to be used for all sensitive applications (energy, industry, audiovisual, finance, transport, and so on). This protocol works on local networks, but it is also able to operate over very long distances. This makes it particularly useful for extensive networks with several remote sites.
When a network becomes very extensive, it is possible that not all clocks are used for the same purposes and that they do not require the same quality of synchronisation. With no way of dividing a network, it is mandatory to maintain the highest accuracy for all clocks, which involves unnecessary costs. Indeed, the PTP protocol only provides one master clock (GMC, for Grandmaster Clock) by network.
The concept of PTP domain
To address this issue, the PTP protocol can define domains within a network. By default, all network machines are in domain 0. However, it is possible to partition the network by adding other domains. PTP is able to define up to 256 domains, from domain 0 to domain 255.
Each domain will be identified by the hardware as an independent network and will have its own GMC, to which slave clocks of the domain will be able to synchronise. The different PTP domains can use the same physical network. Messages from a different domain than theirs are simply being ignored by the machines of a PTP domain.
Domains in practice
Each PTP domain can be seen as a temporal subnetwork of the physical network. As PTP domains are logically isolated from each other, the master clock of a domain will not synchronise the slave clocks of another domain.
Time servers can manage PTP domains as long as they use at least the first version of the protocol. It is worth mentioning that the first version of the protocol has been standardised in 2002 (IEEE 1588-2002 standard of the IETF), and the second version in 2008 (IEEE 1588-2008).
In order to define the domain to which a clock belongs, the subdomain name field is used in the first version of the protocol, then the domain name field in subsequent versions. This field is transmitted in all PTP messages exchanged, which enable equipment to filter the messages intended for a domain other than their own.
Advantages and drawbacks of separating domains
Separating a PTP network into several domains features several advantages.
As each domain is configured independently, it will have its own Grandmaster Clock.
This shortens synchronisation distances and avoids the need to introduce additional delays. Nevertheless, as each domain must have its own GMC, this implies that the system must have high-accuracy clocks (at least one per domain).
Besides, since domains are isolated from each other, the failure of a GMC only affects one domain. However, if the different PTP domains share the same physical network, a physical failure of a time server (router) could potentially affect all domains.
The implications of logical segmentation
As segmentation is logical and not physical, an equipment can belong to several PTP domains at the same time. This enables overcoming the isolation of the different domains. Indeed, it is not mandatory for a clock to have the same role in each domain. A clock can be slave for domain 1 and master for domain 2. Therefore, this clock will be updated by the GMC of domain 1 and will broadcast its timestamps on domain 2. In this way, although domain 1 ignores the messages of domain 2 and vice versa, the time of domain 1 can be used for synchronising domain 2. This solution can be used temporarily when the GMC of domain 2 breaks down. The clock that replaces the GMC is no longer connected to a reference clock but simply to the master clock of another domain.
The overlap between the different domains also makes it possible to supervise the grandmaster clocks of each domain, thus enabling detection of potential failures. If a slave machine belongs to all domains, then it will receive all timestamps. As a result, if not all clocks are synchronised, it is possible via a majority vote to see which master clock drifts over time.
To conclude
By configuring rigorously the PTP domains on a network, it is possible to match a time network to the synchronisation requirements of the system applications. This additional configuration is inexpensive and enables guaranteeing the best efficiency within the network.
With more than 150 years of expertise in time management and present in more than 140 countries, Bodet Time is a major French leader in time synchronisation and time frequency. PTP Netsilon time servers allow all equipment present on a network to be synchronised with the right time.