I previously posted about measuring TCP round-trip times from my web server to its clients. Last night I quickly added code to my existing site traffic monitor to perform this task, as the experimental throwaway to validate my design idea. I have not yet designed the data store, only the collection of round-trip times. To see what it’s doing, I syslog the rtt measurements. It appears to be working fine. Here’s some data from Google’s crawlers prowling my site:
May 2 21:37:23 www sitetrafficd[2318]: [I] 66.249.66.43:45825 rtt 123.2 ms
May 2 21:38:49 www sitetrafficd[2318]: [I] 66.249.66.57:38926 rtt 123.6 ms
May 2 21:38:49 www sitetrafficd[2318]: [I] 66.249.66.43:38085 rtt 123.5 ms
May 2 21:40:16 www sitetrafficd[2318]: [I] 66.249.66.143:39725 rtt 137.8 ms
May 2 21:40:16 www sitetrafficd[2318]: [I] 66.249.66.143:37657 rtt 126.2 ms
May 2 21:41:25 www sitetrafficd[2318]: [I] 66.249.66.204:47961 rtt 160.9 ms
May 2 21:41:25 www sitetrafficd[2318]: [I] 66.249.66.143:45623 rtt 121.1 ms
May 2 21:41:47 www sitetrafficd[2318]: [I] 66.249.66.60:36603 rtt 142 ms
May 2 21:42:15 www sitetrafficd[2318]: [I] 66.249.66.204:48875 rtt 123.6 ms
May 2 21:43:15 www sitetrafficd[2318]: [I] 66.249.66.43:56275 rtt 125.8 ms
May 2 21:44:42 www sitetrafficd[2318]: [I] 66.249.66.57:49966 rtt 124.1 ms
May 2 21:44:42 www sitetrafficd[2318]: [I] 66.249.66.204:53209 rtt 122.9 ms
May 2 21:45:59 www sitetrafficd[2318]: [I] 66.249.66.238:46595 rtt 123.8 ms
May 2 21:47:27 www sitetrafficd[2318]: [I] 66.249.66.60:60241 rtt 142.2 ms
I believe I can call the raw measurement design valid. It’s a bonus that it was not difficult to add to my existing data collection application. It’s another bonus that the data collection remains fairly lightweight in user space. My collection process has a resident set size just over 4M, and that’s on a 64-bit machine. My round-trip time measurement resolution is microseconds, and I’m using the timestamps from pcap to reduce scheduler-induced variance. Since I’m not using any special kernel facilities directly, this code should port fairly easily to OS X and Linux.
May 3, 2012
I added the ability to track SYN to SYN ACK round-trip time, so I can run data collection on my desktop or gateway and characterize connections where I’m acting as the TCP client.