|
|
|
|
|
|
|
2.7.3.4.3
Net Time in Detail |
|
|
|
|
|
|
|
|
I'll look at some Internet time services in further detail. There are two primary advantages to using the Net for time services at the moment: |
|
|
|
|
|
|
|
|
Cost. Synchronizing a PC can be done without a long-distance call to Boulder, Colorado. |
|
|
|
|
|
|
|
|
Standardization. Using NIST time allows machines in a network to standardize timekeeping. |
|
|
|
|
|
|
|
|
The single disadvantage is that accuracy is restricted to approximately one second. For many applications, this is more than adequate. |
|
|
|
|
|
|
|
|
Internet standards are described by a document known as a "Request for Comment," universally truncated by Net cognoscenti to RFC. These documents are numbered in chronological order. |
|
|
|
|
|
|
|
|
The first Net time services were described in RFC 868, issued in May 1983. RFC 868 defines a simple standard, in which port 37 is defined as the time server port. A time server monitors port 37 for a connection via Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). Connections return a 32-bit binary number representing the number of seconds since midnight of A.D. 1900 January 1. |
|
|
|
|
|
|
|
|
This is a shareware program written by H.C. Mingham-Smith of Berkshire, England, and performs this connection and necessary conversion. TARDIS v2.0a works under Windows 3.1 with a Winsock connection to the Internet. TARDIS can be found in file TARDIS2A.ZIP, available via ftp at ftp.cica.indiana.edu in directory /pub/pc/win3/winsock. |
|
|
|
|
|
|
|
|
TARDIS functions with any RFC 868 time server, such as the NIST server in Boulder (time_A.timefreq.bldrdoc.gov), or its military cousin at the U.S. Naval Observatory (tycho.usno.navy.mil). A useful feature of TARDIS is the simple user interface for accepting time zone and daylight savings data. |
|
|
|
|
|
|
|
|
A more ambitious version, NTTARDIS, was released in September 1994 for Windows NT and can also act as a time server. The file NTTARDIS.ZIP is available from ftp.cica.indiana.edu in directory /pub/pc/win3/nt. |
|
|
|
|
|
|
|
|
RFC 868 has two problems: |
|
|
|
|
|
|
|
|
Time is expressed in terms of integer seconds. |
|
|
|
|
|
|
|
|
Transmission delays through the network are ignored. |
|
|
|
|
|