What is the difference between NTP and SNTP - PDF by sja20118

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                                                                                                                        07.21.04
                         What is the difference between NTP and SNTP?
(The following is a response to a question made by Mr. Keith Wing, Supervisor for Customer Service, at
Spectracom Corporation.)

Question: As far as we can tell, the Time Server we have only supports SNTP. The Web site information we
found says NTP, our manual isn’t too specific and the documentation to set up a windows client says to use
SNTP. We tried unsuccessfully to get our routers to connect via NTP. Can you help us?

Mr. Wing’s response: NTP (Network Time Protocol) and SNTP (Simple Network Time Protocol) are very
similar TCP/IP protocols in that they use the same time packet from the Ethernet Time Server message to
compute accurate time. The time stamp that the Time Server sends out and the procedures we use are the same
whether NTP (i.e. full implementation NTP) is being used, or if SNTP is being used.

The difference between NTP and SNTP is in the time synchronization program running on each individual PC
(Server or Workstations). The time program, whether it is a Windows built-in program like W32Time (Which
uses the SNTP protocol) or a third party add-on, determines which protocol is being used - not the Spectracom
Ethernet Time Server. The Ethernet Time Server does not care which protocol is being used. The difference
between NTP and SNTP is in the error checking and the actual correction to the time itself.

The NTP algorithm is much more complicated than the SNTP algorithm. NTP normally uses multiple time
servers to verify the time and then controls the slew rate of the PC. The algorithm determines if the values are
accurate using several methods including fudge factors and identifying time servers that don't agree with the
other time servers. It then speeds up or slows down the PC's drift rate so that (1) the PC's time is always correct
and (2) there won't be any subsequent time jumps after the initial correction.

Unlike NTP, SNTP usually uses just one Ethernet Time Server to calculate the time and then it "jumps" the
system time to the calculated time. It can, however, have back-up Ethernet Time Servers in case one is not
available. During each interval, it determines whether the time is off enough to make a correction and if it is, it
applies the correction.

Specific to routers, there are a couple of things to keep in mind and a couple of things you can do.

First of all, to synchronize to an Ethernet Time Server, the router must be able to “see” the IP address of the
time server. If there are multiple gateways on the network, the immediate gateway needs to be programmed
into the Spectracom Time Server. All gateways, after the immediate one has been programmed, have to be
added to the routing tables. To make sure that the routers can see the IP address of the time server, try pinging
the IP address. It will most likely respond to the ping. Pings usually get through even if the tables are not set
correctly.

The next thing to try is to make sure Telnet is enabled, at least for this test, in the Time Server. From the router,
using the command prompt, try to telnet to the time server using the command: telnet (IP address of time
Server) 9999 <enter>, where 9999 is the port. The response from the unit should be the software version and a
prompt for the user to hit the enter key to go into the setup mode. If it does not respond to telnet, the port was
not entered (9999 at the end of the command), Telnet is disabled in the time server or the IP address is not
getting through the gateways. Try this command on a PC in the same subnet as the Ethernet Time Server and
see if it responds. If it does, the issue is with the address getting through the Gateways. If a firewall is
installed, port 123 has to be open to let NTP/SNTP packets through.

The last thing that I can think of is if these are Cisco routers, you may need to enable MD5 encryption in the
time server. Cisco uses MD5 but MD5 encryption may not be a requirement. The Spectracom Time Server
manual includes instructions on how to set up and use the MD5 feature.

								
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