Measuring TCP round-trip times, part 1: random thought

I would like to start measuring TCP round-trip times from my web server. This could potentially be done on either my web server or my firewall. But given that I’m already sniffing related packets on my web server for other purposes, it makes sense to do the work there, possibly in the same process.

The idea is simple, and surely unoriginal: measure the time between my server’s SYN ACK and the client’s ACK of my SYN ACK (the last 2/3 of a TCP handshake). Record the wall time, the client IP address, and the time between the transmission of my SYN ACK and the reception of the client’s ACK of my SYN ACK.

In the not too distant future, I will upgrade my desktop machine to FreeBSD 9.0-STABLE. At that point I’ll start writing code that utilizes use the new h_ertt(4) kernel module.

Much of what I want is actually client-anonymous: an idea of the distribution of network distance of the visitors of my web site. I will want a facility to deal with crawlers, since they’re of less interest to me than human eyeballs and are likely to skew some statistics.

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