Implementing a time server for video surveillance purposes

The need for time synchronisation and its constraints

A video surveillance system is a set of equipment used independently to monitor and record all activities in a specific area. This set includes IP-based cameras, video recorders and management systems for controlling and accessing recordings. As they are mainly used for security purposes, it is paramount that these systems follow three principles:

• Accuracy. Accurate timestamps enable the causal chain of events to be accurately reconstructed, which is crucial for post-incident investigation and analysis.
• Legal admissibility. Video recordings with accurate and synchronised timestamps are more likely to be legally admissible. Inconsistencies in time can compromise the validity of evidence.
• Efficiency. Correct synchronisation improves coordination between different security devices, thus making system management and maintenance easier.

It is then necessary to implement a timestamping mechanism that is as reliable and accurate as possible. However, time synchronisation features some constraints: in fact, the following constraints should be observed:

• Using NTP (Network Time Protocol). In France for example, the ANSSI (French National Agency for the Security of Information Systems) recommends using the NTP protocol to ensure accurate clock synchronisation. It should be noted that a more simplified version of NTP (SNTP) observes the same levels of accuracy.
• Secure management. Management of timestamping devices must be secured by using authentication and encryption mechanisms to prevent attacks. It is possible to use the NTS protocol which allows to add such mechanisms to the NTP protocol.
• Supervision. Implementing ongoing control mechanisms enables time drifts to be detected (and therefore quickly corrected).

To ensure efficient time synchronisation while observing all these constraints, choosing the most suitable time server must be carefully considered.

Choosing and configuring a time server

It is recommended to use a time server compatible with NTP, that is secure and uses a reliable reference time source (for example, the UTC time provided by atomic clocks).

Using public NTP servers should be avoided as they pose security risks. Indeed, since public time servers require constant connection to the Internet, they can be easily compromised or become ineffective. As a result, the best practice is to choose a local NTP server on the network, which will enable total control over the time source.

The following key steps are to be observed when implementing a local time server:

• Firstly, the NTP server must be installed in the network, with a reliable connection to a time reference source.
• The next step is to configure video surveillance systems (cameras, recorders, and so on), so that they can synchronise with the NTP server. This is an easy step for modern equipment and Netsilon time servers. Once the configuration is carried out, it is required to make sure that the equipment and the time server are secure. It is especially recommended to implement robust authentication mechanisms.
• The last step consists in implementing supervision tools to monitor synchronisation accuracy and quickly detect any time drift.

To guarantee ongoing availability of services, it is recommended to use redundant time sources. For example, in addition to main NTP servers, backup servers can be used to ensure synchronisation even in the event of failure of the main time source.

By observing all these steps, the resulting synchronisation process will guarantee the reliability of video recordings and will meet both legal and security requirements.