• Achieving Optimal Timestamp Accuracy on 7150 Platforms

 
 
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Background

The launch of the Arista 7150S in 2012 delivered the industry’s first product to offer high speed networking with high granularity packet time stamping in an Ethernet switch/router platform. This article will review how time stamping has evolved since.

Arista first conceived of the capability to leverage the 7150S to intersect with an emerging network monitoring market requirement that would improve the ability to capture the proper order of traffic captured by network analysis tools over the out of band (OOB) tap aggregation (aka network packet broker) infrastructure.

Applying timestamps to packets at the network ingress ports of the tap aggregator switches enabled capture tools to overcome the inaccuracy of applying timestamps when packets finally reached capture tools. In an OOB tap aggregation network packets are forwarded over multiple hops and physical links of different lengths before being aggregated through networking tiers towards the capture, logging and tool ports. At this point the original relative arrival time is long lost as variable delays have been applied and the end to end visibility to a flow cannot be reconstructed accurately.

Evolution of Time stamping

Over time, the set of requirements for time stamping has evolved beyond a simple relative timestamp to requiring support for packets to be timestamped with absolute UTC time – synchronized to a master clock source.

Arista responded with a unique solution to enable the translation of the silicon’s native hardware time stamping capability to UTC timestamps. This involved enhancements in both EOS software and the system FPGA to generate a ‘keyframe’ that allowed post-processing of captured data to convert hardware timestamps to PCAP friendly nanosecond scale timestamps.

Working closely with customers and 3rd party tool vendors, the keyframe concept has itself evolved to provide incremental information to allow increasingly high quality time resolution.

For reasons of backwards compatibility the default original keyframe configurations are retained, however the advanced features, documented here can be configured on demand, (assuming the receiver is capable of taking advantage of the new metadata fields).

Recommendation

While these fields have been available since 2013/14, due to the growth of interest in time stamping some customers with existing timestamp/keyframe decoding mechanisms may not yet take advantage of the newer fields and capability.

Arista recommends that, for optimal performance and accuracy in decoding timestamps generated by the 7150, customers review the linked documentation and work with their local monitoring solution vendors or Arista engineering teams to ensure the optimal settings are enabled.

 

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