# The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

## Sunday, November 01, 2020

### What ping times and the speed of light have in common

Back when I was wearing a network admin hat, about ten to fifteen years ago, one question I tried answering was how to determine the time it took a network packet to travel one way. The `ping` command measures the round trip time:

```[spc]lucy:~>ping -c 5 brevard.conman.org
PING brevard.conman.org (66.252.224.242) 56(84) bytes of data.
64 bytes from brevard.conman.org (66.252.224.242): icmp_seq=0 ttl=49 time=26.7 ms
64 bytes from brevard.conman.org (66.252.224.242): icmp_seq=1 ttl=49 time=26.1 ms
64 bytes from brevard.conman.org (66.252.224.242): icmp_seq=2 ttl=49 time=26.9 ms
64 bytes from brevard.conman.org (66.252.224.242): icmp_seq=3 ttl=49 time=27.0 ms
64 bytes from brevard.conman.org (66.252.224.242): icmp_seq=4 ttl=49 time=27.1 ms

--- brevard.conman.org ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4004ms
rtt min/avg/max/mdev = 26.163/26.832/27.161/0.362 ms, pipe 2
[spc]lucy:~>
```

So, can one assume that the one way time is a bit over 13ms? Not really, because asymetric routing—where the path to a remote destination does not match the path from said remote destination—does exist. I've seen it happen several times while wearing my network admin hat. Yes, you can attempt to synchronize the clocks on the two systems—it's not easy, but it is possible if you work hard to achieve it. Over time, I found it easier to just use the round trip time as reported by `ping` and assume the path is symetrical.

I was reminded of that when I watched “Why no one has measured the speed of light.” It never occured to me that scientists haven't actually measured the speed of light at 299,792,458 meters per second, but used the round trip time to calculate it and just assume the timing is symetrical because it's impossible to synchronize clocks between two sites (for the purpose of measuring the speed of light). Kind of mind blowing if you ask me.

## Obligatory Miscellaneous

You have my permission to link freely to any entry here. Go ahead, I won't bite. I promise.

The dates are the permanent links to that day's entries (or entry, if there is only one entry). The titles are the permanent links to that entry only. The format for the links are simple: Start with the base link for this site: http://boston.conman.org/, then add the date you are interested in, say 2000/08/01, so that would make the final URL:

http://boston.conman.org/2000/08/01

You can also specify the entire month by leaving off the day portion. You can even select an arbitrary portion of time.

You may also note subtle shading of the links and that's intentional: the “closer” the link is (relative to the page) the “brighter” it appears. It's an experiment in using color shading to denote the distance a link is from here. If you don't notice it, don't worry; it's not all that important.

It is assumed that every brand name, slogan, corporate name, symbol, design element, et cetera mentioned in these pages is a protected and/or trademarked entity, the sole property of its owner(s), and acknowledgement of this status is implied.