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Timestamping with Director xStream Pro

 

Timestamping packets has long been the key to accurate timing analysis when tuning network performance. Lately it has become especially critical in the financial sector due to the severe impact of even microseconds of latency on automated high-speed trading transactions. Since 2007, Net Optics has offered timestamping in its iTap access product line. Recently we brought the feature into our network controller line with the Director xStream Pro. The timestamp applied by Director xStream Pro uses a new, flexible, easy-to-use format that is explained in this post.

When timestamping is enabled in any of Director xStream Pro’s eight ProPorts (the top row of ports in the chassis), a 12-byte timestamp and a new CRC are appended to each packet that passes through the port. The timestamp records the precise time that the first bit of the packet arrived at the input port—this point is critical, as products that timestamp at the outgoing tool port lose accuracy due to variable delays through the device.

The timestamp format is diagrammed below.

As you can see, the first four bytes of the timestamp are a 32-bit binary value in seconds. The second four bytes are a 32-bit binary value representing tenths of microseconds; this field rolls over (returns to zero) when it reaches  0×98967F or 1 second. The final four bytes are reserved for use when higher-precision timestamping becomes available, making the timestamp format capable of supporting a resolution of 0.1 picoseconds.

Some examples of the timestamp are:

00 00 00 01   00 00 00 00   00 00 00 00 = 1 second

00 00 00 00   00 00 00 0a   00 00 00 00 = 1 microsecond

00 00 00 1b   00 96 ff   ff     00 00 00 00 = 27.9895935 seconds

The timestamp can easily be decoded by a protocol analyzer or other monitoring tool. A Wireshark capture of a timestamped packet is shown below.

 

Note that the packet’s original CRC is preserved. If a packet arrives with a bad CRC, the timestamp adds a good CRC on the end and forwards the packet to the monitoring tool. The tool can reference the original CRC to validate the packet. (When timestamping is NOT enabled, Director xStream Pro drops packets that arrive with bad CRCs.)

The timestamp is generated by a free-running 1 Mhz counter, providing microsecond  precision of the relative timing between packets arriving on any timestamping ProPorts in the chassis. Left by itself, the counter can drift slightly with time. To prevent drift, a pulse-per-second signal from an precision time source such as GPS can be applied to the BNC connector labeled GPS on the rear panel. Moreover, if multiple chassis are sync’d to the same time source, the timestamps will provide accurate relative timing for packets arriving at different chassis.

With the high-precision, input-port-based timestamping of Director xStream Pro, you no longer need to worry about adding a network controller switch between the traffic and your timing analysis tools. Director xStream Pro’s timestamps always provide your analyzer with precise timing of the packets on the wire, regardless of any delays introduced along the monitoring path.

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