Installing and Administering Internet Services

312 Chapter7
Configuring the Network Time Protocol (NTP)
Advanced NTP Topics
type), m = multicast, b= broadcast, - = netaddr (usually 0).
• The when column shows the number of seconds since the remote host
response was received.
• The poll (poll period) column shows the polling interval to the
remote host, as determined by xntpd. You can define the minimum
polling interval with the minpoll option in the peer, server, or
broadcast definitions in the /etc/ntp.conf file. Some of the popular
values for network connections include 512 and 1024 seconds
(approximately 8 mins. and 17 mins.). Systems with external clocks,
like GPS, should poll every 64 seconds or less.
• The reach (reachability) column shows how successful attempts to
reach the server are. This is a 8-bit shift register with the most recent
probe in the 2^0 position. The value 001 indicates the most recent
probe was answered, while 357 indicates one probe wasnot answered.
The value 377 indicates all of the recent probes have been answered.
• The delay (round trip time) column shows how long (in milliseconds)
it took for the reply packet to come back in response to the query sent
to the server.
• The offset (time difference) column shows how different (in
milliseconds) the server’s clock and the client’s clock are from one
another. Note that when this number exceeds 128, NTP makes an
adjustment and the message “synchronization lost” appears in the log
file.
• The disp (dispersion) column shows how much the “offset”
measurement varies between samples. This is an error-bound
estimate. The dispersion is a primary measure of network service
quality.