CERIAS NTP Service

Need a Time Server?

If you have contacted us because you need a time server, you can use our servers subject to the contraints discussed below. We offer two stratum 2 servers for general off-campus use, and three stratum 1 servers for time-setting for larger enterprises and those needing the most accurate time. Several of our stratum 2 servers are part of the NTP Pool, and that service may provide a good option if you need one or more time servers, in addition to (or in place of) our servers. Other servers are listed at the time servers site.

About

We provide a set of NTP servers for use inside and outside of Purdue University. These consist of three stratum 1 time servers (GPS and CDMA-based) for any Internet users, 2 additional stratum 2 servers for external users, and an additional stratum 2 server for internal Purdue users.

• The stratum 1 servers are currently two Symmetricom^® TymServe^™ model 2100LD servers, which use GPS signals for their time standard, and one EndRun Technologies Tempus LX^™ CDMA-based server, which uses cell-phone timing signals as a time standard. Positioning of the GPS antennas is such that 3 satellites are almost always in view, and we sometimes sync with 5. The location of the CDMA-based server is such that we have an extremely strong signal. Expected deviation of the time signal is less than 1 microsecond relative to UTC at the servers.
• The stratum 2 servers for external users are both V3/V4 servers running on AMD-based Linux servers. The stratum 2 server for internal Purdue usage is running on our Cisco 7500 series router.

Four of these servers are on the same LAN, and the fifth server (Tier I) is in a different building on a different LAN, for fault tolerance. Another three internal servers are sync'd to those five servers, to a total of 10 external stratum 1 servers in the eastern/central U.S., and are in a peered clock-loop with each other and a set of external stratum 2 servers. Our set of six stratum 2 servers are therefore sync'd against a total of 13 stratum 1 servers (GPS, ACTS, CDMA, PSC, WWV, and USNO sources), plus peered with at least a dozen non-CERIAS stratum 2 servers. Our WWW and FTP servers are two of these stratum 2 servers, and help keep the file times and logs in sync on those services; they are available for public use.

Eight of our nine server hosts are located within the same physical racks, within 3 meters of each other (physical), with UPS power and restricted access. (For reference, the machines are located at 40:25:33.3N, 86:54:54.6W (GPS).) The ninth is located in a nearby building, also in a restricted location with UPS. All nine have mutual MD5 keying enabled to reject spoofing.

The result is a highly stable time source with (we hope!) exceedingly small deviation.
(This may be over-engineered, but we like to be thorough!)

With that said, we make no guarantees about this service. The servers may go down at any time, or become unreachable to users outside Purdue. We will try to maintain reasonable availability, but any service is provided on an "as is" basis as a courtesy. We make no warranties about the accuracy of the time signal beyond our good-faith belief that it is as accurate as can be achieved using our stock hardware and software.

Access as a client

If you wish to use our time servers, we ask that you observe the following:

For users ON THE PURDUE WEST LAFAYETTE CAMPUS only:

• We request that you point your NTP requests at horologe.cerias.purdue.edu. This will be the best connection (lowest delay and jitter) for almost all Purdue campus users. It is a stratum 2 server with a direct connection to our stratum 1 time servers and it is present in the clock-net described above. For on-campus use there should be no discernable difference in time accuracy between this server and the stratum 1 servers. In fact, to get to the stratum 1 servers your packets would need to transit an additional router and switch, so horologe should have less delay and jitter.
• Hosts in Computer Science are a special case, and they should reference andromeda.cs.purdue.edu as a time source.
• Campus hosts that must reference a Stratum I host should reference andromeda.cs.purdue.edu, darkcity.cerias.purdue.edu, or caspak.cerias.purdue.edu.
• There is no need to inform us or register to use these servers from on campus.

For users NOT AT PURDUE:

• Please send email to ntp-request@cerias.purdue.edu if you intend to use our time servers. Let us know which machine/domain will be involved, and an email address if we need to contact you. Please indicate which server you will be using.
• Use the DNS name of our servers rather than the IP addresses. The IP addresses may change without warning but the names will remain constant. If we need to change our servers for some reason, use of the DNS names instead of the IP addresses should make any such change transparent.
• If you have a slow connection or long round-trip delay to our server, then pick another NTP server or establish your own level 1 server (see below). This will give you better time sync. Currently, network access to server "tack" seems to have less delay than to "tick" or "tock" but they should work equally well for all clients.
• Instead of having multiple machines connect to our servers, establish your own stratum 2 or 3 server. Have a single machine connect to our servers to set its time, then have all your local machines use it as their time source. This should work better by reducing the aggregate overall network delay. It also reduces load on our servers!
• All of our servers should provide nearly identical time service. We therefore ask outside users to use ONE (and only one) of our servers for a time base.
• Clients synchronizing a server for a group of hosts or a domain should use one of the stratum 1 servers: These servers are named tick.cerias.purdue.edu, tock.cerias.purdue.edu and tack.cerias.purdue.edu.
• Clients synchronizing a personal machine or a home network should use one of the stratum 2 servers: These servers are named ftp.cerias.purdue.edu and www.cerias.purdue.edu.
• Please do not overpoll our servers. That is, leave the frequency at which you check to something reasonable -- if your internal clock is so bad as to require a resync every few minutes, then you should get a better machine or your own time server! Standard NTP suggests that a good onboard clock with appropriate NTP settings will settle into a pattern of polling about once every 1024 seconds (maxpoll = 10).

Peering

If you have one or more stratum 2 servers and wish to peer them with our stratum 2 servers (as contrasted with acting as clients), then contact ntp-request@cerias.purdue.edu.

Authenticated Time

If you have need of authenticated time (using MD5, as defined in the NTP protocol) then we can exchange key information to enable this. Contact ntp-request@cerias.purdue.edu. We currently do not have autokeying set up on any of the servers.

Establishing your own server

All the Stratum 1 servers we use come as rack-mountable units with an external antenna. They are generally easy to set up, although antenna placement is important: GPS antennas usually require outside placement with a clear view of most of the sky, although we have ours near the peak of a building attic. Antennas mounted outside also need lightning arrestors installed for safety purposes. CDMA units work from cell phone signals, so anywhere inside where there is a good cellphone signal is appropriate.

We have had a generally very good experience with our TymServe products (the products work out of the box, but we have had slow -- and not always complete -- responses to some of our technical questions). All the software was in place and required minimal configuration. These products sync to GPS satellite signals.

We recently added an EndRun Tempus LX server. This also was extremely easy to set up and run -- less than 10 minutes from out of the box to locked and serving a signal! The documentation is good, and the interface is simple. We are very pleased with this server, too. EndRun makes servers that provide a time signal from either GPS or CDMA signals; ours is a CDMA unit.

Establishing your own time server might be a good move, especially if you need a secure time source, because you can run it inside your firewall. Cost for a system is generally a few thousand dollars for US customers. Options are available to get more precise local clock crystals for times when the external time source is unavailable, but that adds expense. For more details, you can visit Symmetricom or EndRun Technologies.

For More Information

Lots more information on NTP, including the latest software, is available at http://www.ntp.org/.

Last modified December 25 2006.