Arista Systems: PTP Best Practices for Media and Entertainment and Broadcast Centers (June 2020)

There are many different ways to distribute PTP across a facility (or multiple facilities) for the purpose of providing timing to various transmitters/receivers (slave devices). PTP is transported over multicast from a master clock (Grandmaster) and this multicast group propagates throughout the network to all endpoints that are requesting it.

There are many possible variations to a However, when designing a functional PTP distribution network especially in the Media and Entertainment (M&E) space where there are possibly hundreds of slave devices, scattered throughout a facility, scalability and CPU utilization of the endpoints must be taken into account. It is recommended that the platforms selected for an M&E deployment, and more specifically SMPTE-2110, support Boundary Clock and that this feature be enabled. In addition to those reasons, there is a simplistic approach to relying on Boundary Clock, both in configuration and troubleshooting, as shown in the first Case Study. Without Boundary Clock, in a routed environment, an RP will be required, which inevitably means that Anycast or a Bootstrap Router will be required - adding to the complexity of the design. Also, ensuring that the network design supports boundary clock is vital and should be the first topic of discussion when designing a PTP distribution network

https://www.arista.com/assets/data/pdf/Whitepapers/ME-PTP-White-Paper.pdf

The table below represents the different range of values per message type or domain based on AES67 and 2059-2 profiles, and their overlap. However, the most common values for these message types are:

Announce Interval = 0

Sync Interval = -3

Delay Request Interval = -3

 

What is a Boundary Clock? Within PTP, a Boundary Clock (BC) is a device that allows for the synchronization of multiple devices, spanned across multiple subnets, while at the same time preventing the back and forth messaging between each and every slave. Switches supporting PTP boundary mode synchronize with grand masters and provide timing and drift information to upstream and downstream devices.A Grandmaster is connected to a switch, and that switch interface will become a slave to its grandmaster. All other interfaces on the switch that are PTP enabled, will be a master for its connected endpoint device. This endpoint device can be a transmitter, receiver, or even another switch. A downstream switch will follow the same pattern mentioned above, whereby the interface joining the two switches together (on the GM switch) will be the master to the downstream switch’s slave.

 

What is a Transparent Clock? Transparent Clocking devices remove PTP message jitter (the result of their own queues) differently. The multicast or unicast PTP messages are passed through using entirely normal unicast or multicast forwarding, but the precise time that each packet takes to transit the switch is recorded, and this time (correction factor) is included in the PTP messaging as it passes to the intended recipient. The end-point can now use the correction factor to compensate for the delay through the switch (or multiple Transparent Clock switch hops as well).

 

Recommendation: Boundary Clock Wherever possible, configuring a PTP distribution network utilizing Boundary Clock mode is recommended for the following reasons:

Security: For each interface on an Arista switch, there is a configuration option called “ptp role master”. This ensures that the devices connected to that interface can never become the GM f

Message Suppression: Since each interface is a master only to the device connected to it, the delay request messages (sent from slave to master) do not leak to other switch interfaces on the local or connected switches. All PTP messages terminate at the boundary clock.

Scalability: PTP aware devices have a limit to the number of messages that can be received per second, including the grandmasters. Thousands of messages a second could cause devices to lock up. Since the Boundary Clock absorbs many of these messages, the overall PTP distribution network can grow and expand without the concern of increasing the number of messages traversing the network.

Clock Recovery: Boundary switches continue providing timing data even if a master clock becomes incapacitated. Once the master clock has recovered or a backup has taken over, the network recalibrates without losing a step, thus ensuring consistent, reliable broadcast operations.

 

Troubleshooting tips

show ptp (displays which interfaces are in the following modes: Master, Slave, Passive, Disabled ; Grandmaster ID, # steps away from GM, etc

 

show ptp parent (displays ID and parameters of PTP parent)

show ptp clock (similar to ‘show ptp’)

show ptp interface ethernet # counters (displays counters of required PTP messages, such as sync messages sent/received, delay-req sent/received)

show ptp monitor (displays up to 100 entries of offset from master, mean path delay, and skew)

Analysis of PTP Packets Arista switches, being linux based, allow for on-board tcpdump captures that can be exported and read in packet analysis tools such as Wireshark. This is a fantastic tool for verifying that the PTP packets have the appropriate data to ensure a steady system timing loc