[email protected] said: > On the other hand PTP is evolving to be a future protocol for time transfer. > Nowadays it is superior than NTP in the LAN environment.
"Superior" is an interesting word. I'm not familiar with the details of current PTP implementations. I am reasonably familiar with the NTP reference package which is shipped with most Linux/*BSD distributions and also runs on Windows. (The Windows default is to use SNTP, S for Simple or maybe Stupid. You have to go out of your way to install ntpd on Windows.) NTP is intended for use in the big bad Internet. Much of the logic/code in ntpd is trying to dance around broken clocks or broken servers or crazy networks. It has a crypto option, but it's not used very often. (There are plenty of problems before you get to one that needs crypto.) After the classic ntp client-server/request-response exchange, the client has 4 time stamps. When the request left the client. When the request arrived at the server. When the response left the server. When the response arrived back at the client. The middle two are using the server's clock. The first and last use the clients clock. The reason that you need 2 server times is that the server may be busy (for example doing crypto crunching) or whatever. Normally both server time stamps are very close together. If you assume that both clocks are accurate, that lets you measure network timings in both directions. NTP assumes the network transit times are symmetrical. That lets you calculate the clock offset. I have a slow DSL line. If I start a big download, the queuing delay on my DSL line jumps up to a second or so on average. (lots of noise) If I go web browsing and hit a page with lots of graphics, the queuing delays can almost get to 4 seconds. That basically screws up the symmetrical assumption. If you are interested in this area, you should study and understand this web page: http://www.eecis.udel.edu/~mills/ntp/html/huffpuff.html Clients using NIST servers sometimes see this problem with the other sign. They have queuing delays at the input to the server because zillions of clients are pounding on the server. The input (to-server) link sometimes gets overloaded, even with 100 megabit Ethernet connections. (maybe faster...) That's the top edge of the wedge diagram rather than my bottom edge. --------- PTP is basically making the network transit times more accurate than "symmetrical" by measuring them. Each box that processes a packet updates the packet with the processing/queuing delays. I think the PTP geeks are working on crypto. I'm not tracking the details. -- These are my opinions. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
