Core competency

We initially set up the cNodes to record 95th percentile jitter, meaning the value of jitter measurements within 95% of one another. Because of a configuration error, we turned off recording this measurement about a third of the way into the test window. For the period we did measure, 95th percentile jitter looked very good: Typically, it was around 40 to 100 microsec for all providers - close to the average jitter and nowhere near the level that would harm any application.

While it's unfortunate that we couldn't present 95th percentile jitter for the whole test duration, we can say that nearly all jitter measurements appear to be close to the average values. Jitter numbers were spot-checked for all seven ISPs for the entire test period, and most maximum jitter values are close to the averages - certainly well below the threshold where application performance might suffer.

Looking at loss

Packet loss is another key measure of network health. While certain applications can tolerate loss, it's generally the case that dropped packets degrade performance in some way.

A few dropped packets can cause TCP retransmissions, and this will reduce throughput and increase latency. Drop enough packets, and TCP sessions will time out. This is especially critical considering that more than 90% of Internet traffic runs over TCP.

Most ISPs kept loss minimal - for some ISPs, packet loss amounted to less than 0.01% of all packets transmitted.

C&W led all ISPs with a perfect score on packet loss. C&W's network delivered every one of the 462 million packets we fed its routers during the normal test window. And even though we officially measured loss only during normal operations, C&W's network also was perfect during maintenance periods.

The Level 3 and Savvis networks also delivered virtually all the 676 million packets they carried. Some loss existed for both these ISPs during normal operations. However, the amounts were so small that after rounding, the results equate to 0.00% loss.

The other ISPs experienced higher loss, although "higher" is a relative term. In the worst case - on three Verio circuits sending traffic to New York - packet loss ranged from 0.12% to 0.14%.

Curiously, packet loss only affected traffic headed to New York from Verio's Chicago, Dallas and Palo Alto POPs. Traffic from New York to these locations wasn't affected.

As noted, Verio attributes the problems with three OC-48 circuits at an unspecified location. Verio fixed the problems, and packet loss fell sharply after that. If we factor out the misbehaving OC-48s, Verio's average loss on all other circuits amounts to 0.01% of all traffic.