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    Precision Time Protocol for Synchronization in Broadcast-over-IP

    July 19, 2021

    In every broadcasting application, all devices must exchange media data continuously while remaining tightly synchronized with each other at all times. That’s why, as broadcasting facilities worldwide make the transition from serial digital interface (SDI) to partial or complete reliance on IP technology, accurate and reliable time transfer over Ethernet has become key. In the July issue of the SMPTE Motion Imaging Journal, authors Nikolaus Kerö and Thomas Kernen present an in-depth discussion of the challenges in their paper, “A Multipronged Approach to Effectively Secure SMPTE ST 2059 Time Transfer.” https://ieeexplore.ieee.org/document/9470359

     

    SMPTE has specified in its ST 2059 standards the use of Precision Time Protocol (PTP), as defined in the IEEE 1588 standards for media signal generation and synchronization. While proven to deliver an accuracy within 1 μs for all types of networks, PTP’s vulnerability to error is a major concern in maintaining an uninterrupted, bidirectional message flow. Therefore a series of enhancements has been developed to ensure robust, resilient and secure synchronization.

     

    The paper’s authors consider four possible PTP Error Conditions that can occur in the network, and offer ways the broadcasting industry can efficiently use and benefit from the new enhancements of the ST 2059 standard:

     

    1. The PTP event message flow is interrupted in either direction or altogether. Countermeasures include an improved clock adjustment process to counter transient disruptions, and extended monitoring of all PTP devices.
    2. The device receives the wrong time information. Countermeasures include source diversity, frequency diversity, basic reception filters, and signal strength and diversity filters
    3. The quality of the time information is inaccurate or deteriorated, typically due to one or more network devices being transiently or permanently overloaded. Here, the quality of the control loop implementation of the PTP boundary clock has a significant influence on the overall quality of the time transfer within the network
    4. The device is overwhelmed with a high number of PTP messages leading to a denial of service and disabling its capability to the adjust its clock. To partially circumvent this situation, the ST 2059 standard strongly recommends reverting to a mixed mode, where only the common messages for all nodes (announce, sync) are sent as multicast, while all node-specific messages (delay request and responses) use unicast. 

    To make things even more challenging, the four error conditions are not mutually exclusive, and each can occur either permanently and transiently. These failures can occur either inadvertently due to misconfigured or malfunctioning PTP devices somewhere in the network, or be caused deliberately as a result of a malicious attack against the integrity of the timing network. Here, the authors propose basic security countermeasures against malicious attacks:

     

    • Basic Security Aspects. A crucial first step in every IP network is the use of an Authentication, Authorization, and Accounting (AAA) model enforced throughout the complete system. As a first step, all PTP messages must originate from a reliable and traceable source. It is also important that all devices are configured to accept PTP announce messages only from validated sources.
    • Use of IPv6 to Help Secure the PTP Infrastructure. Internet Protocol version 6(IPv6) is the most recent version of the Internet Protocol (IP). While IPv6 as a transport for PTP messages has yet to gain traction in SMPTE ST 2059-driven environments, other industries where it is being rolled out for the latest generation of telecom systems have proven its resilience.
    • Securing the Payload or the Transport. Many protocols have security built into their specification, as was the case with the 2008 revision of the IEEE 1588 standard.
    • Continuous Monitoring: Regardless of which countermeasures are put in place to prevent malicious attacks, continuous monitoring is still the most effective security method to deploy.

    In broadcasting applications, even transient faults which jeopardize the integrity of the time transfer mechanism must be avoided at all costs. As the authors of this article show, PTP is a highly effective and accurate means of synchronizing devices connected to each other via an Ethernet network, and their resilience can be further improved using state-of-the-art cryptographic methods, supported by the latest version of the IEEE 1588 standard.

     

    Interested in digging deeper into synchronization using Precision Time Protocol? Read the complete article in July’s issue of the SMPTE Motion Imaging Journal.https://www.smpte.org/motion-imaging-journal

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