Perhaps one of the most overlooked elements of network management is that of time synchronization. If its importance in the overall health of the network were better understood, then perhaps it would be paid more attention. In this article, I'll discuss time synchronization and the Network Time Protocol.
Why Synchronization Matters
In a local area network (LAN), time synchronization is important because it affects components such as file systems and applications. If the time being issued to the server by the system hardware clock is incorrect, it is quite possible for corruption to occur within applications, particularly in complex systems such as databases. In wide area networks (WANs), time synchronization is even more essential. The distributed nature of WANs greatly increases the probability of an incorrect timestamp, not to mention the fact that WANs often span time zones, further complicating the issue.
One of the best (and most used) examples of the importance of time synchronization is that of e-mail. Imagine receiving an e-mail, the timestamp of which indicates that it was received before it was sent. Very confusing. Another more frightening example includes the computers used by air traffic controllers, but we won't even go there. So, how does an operating system get the wrong time?
For the most part, operating systems take their time from the local hardware clock of the system on which they are loaded. Although hardware clocks have improved in terms of accuracy and reliability, they are still prone to inaccuracies. In addition, the one-to-one relationship of the operating system and the machine on which it is running means that it is very possible for two different systems on a network to have different times. What is needed is a mechanism that allows systems to synchronize themselves with a reliable time source and subsequently with each other. The mechanism is the Network Time Protocol (NTP).
Guidelines pertaining to the use of Network Time Protocol time sources are available on the Internet. A document describing these Rules of Engagement, along with a list of public time servers, can be found here.
NTP operates over UDP on port 123. If you're using a firewall, you may need to change the firewall configuration so that NTP traffic can flow through.
Network Time Protocol
NTP is not a new protocol; in fact, it's been around since the 1980s. The current version of NTP, version 4, is relatively new, and previous versions are still well supported. Great care is taken to ensure that new versions of NTP are backward compatible. The generic nature of NTP means that it is platform independent, and NTP support is available for almost all popular platforms including Linux, Unix, Windows NT/2000, Novell NetWare, Windows 95/98, Mac, as well as other networking devices such as routers. There is even a version for Palm! In many cases, shareware and freeware versions of NTP server and client software are available. Some of these use the lighter Simple NTP (SNTP) protocol, which is based on standard NTP but has less overhead.
Before time can be synchronized by NTP, the correct time must first be ascertained. One of the most popular methods of obtaining this information is from Internet-based public time servers. The servers are structured in a tiered model, with those at the top tier designed to be the most accurate. These top-level Internet time servers are known as Primary, or Stratum-1, time servers. Stratum-1 servers provide accurate time by synchronizing with reliable sources such as the Global Positioning System or purpose-specific radio broadcasts. To ensure that these Primary time servers are not overwhelmed with requests, a number of other servers are also configured as Secondary, or Stratum-2, time servers. Although there may be small differences in time between the Stratum 1 and 2 servers, the possible change is limited and makes no difference to most networks.