Wednesday November 29, 2006

NTP over HTTP

en.wikipedia.org -> clevervest.com -> rkeene.org :

The problem - you know how inaccurate your PC clock is. You would like the clock to keep correct time, perhaps for your appointments, meetings and so on. But the (corporate) firewall you're behind may block almost everything except HTTP for web surfing. So you cannot use normal NTP (Network Time Protocol) clients.

The solution - use a HTP (HTTP Time Protocol) client. It makes use of the fact that the HTTP headers include a date/time field. For example, if I do a telnet www.microsoft.com 80, I'll get something like this :

HTTP/1.1 200 OK
Date: Wed, 29 Nov 2006 06:28:26 GMT
Server: Microsoft-IIS/6.0
P3P: CP="ALL IND DSP COR ADM CONo CUR CUSo IVAo IVDo PSA PSD TAI TELo OUR SAMo CNT COM INT NAV ONL PHY PRE PUR UNI"
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 32304

So, while *you* cannot access NTP directly, you *hope* that some of these web servers by major corporations have their clocks set by NTP correctly. And that they transmit this information in the HTTP headers. By taking an average reading from a few web servers, the client *hopes* to get some kind of reasonably accurate reading of the current time. So it is NTP over HTTP - well, sort of. HTP client download here.

(2006-11-29 14:38:29 SGT) [Tech] Permalink

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