| This MIB module defines a MIB which provides
mechanisms to monitor an NTP server.
The MIB is derived from the Technical Report
#Management of the NTP with SNMP# TR No. 98-09
authored by A.S. Sethi and Dave Mills in the
University of Delaware.
Below is a brief overview of NTP system architecture
and implementation model. This will help understand
the objects defined below and their relationships.
NTP Intro:
The Network Time Protocol (NTP) Version 3, is used to
synchronize timekeeping among a set of distributed
time servers and clients. The service model is based
on a returnable-time design which depends only on
measured clock offsets, but does not require reliable
message delivery. The synchronization subnet uses a
self-organizing, hierarchical master-slave
configuration, with synchronization paths determined
by a minimum-weight spanning tree. While multiple
masters (primary servers) may exist, there is no
requirement for an election protocol.
System Archiecture:
In the NTP model a number of primary reference
sources, synchronized by wire or radio to national
standards, are connected to widely accessible
resources, such as backbone gateways, and operated as
primary time servers. The purpose of NTP is to convey
timekeeping information from these servers to other
time servers via the Internet and also to cross-check
clocks and mitigate errors due to equipment or
propagation failures. Some number of local-net hosts
or gateways, acting as secondary time servers, run NTP
with one or more of the primary servers. In order to
reduce the protocol overhead, the secondary servers
distribute time via NTP to the remaining local-net
hosts. In the interest of reliability, selected hosts
can be equipped with less accurate but less expensive
radio clocks and used for backup in case of failure of
the primary and/or secondary servers or communication
paths between them.
NTP is designed to produce three products: clock
offset, round-trip delay and dispersion, all of which
are relative to a selected reference clock. Clock
offset represents the amount to adjust the local clock
to bring it into correspondence with the reference
clock. Roundtrip delay provides the capability to
launch a message to arrive at the reference clock at a
specified time. Dispersion represents the maximum
error of the local clock relative to the reference
clock. Since most host time servers will synchronize
via another peer time server, there are two components
in each of these three products, those determined by
the peer relative to the primary reference source of
standard time and those measured by the host relative
to the peer. Each of these components are maintained
separately in the protocol in order to facilitate
error control and management of the subnet itself.
They provide not only precision measurements of offset
and delay, but also definitive maximum error bounds,
so that the user interface can determine not only the
time, but the quality of the time as well.
Implementation Model:
In what may be the most common client/server model a
client sends an NTP message to one or more servers and
processes the replies as received. The server
interchanges addresses and ports, overwrites certain
fields in the message, recalculates the checksum and
returns the message immediately. Information included
in the NTP message allows the client to determine the
server time with respect to local time and adjust the
local clock accordingly. In addition, the message
includes information to calculate the expected
timekeeping accuracy and reliability, as well as
select the best from possibly several servers.
While the client/server model may suffice for use on
local nets involving a public server and perhaps many
workstation clients, the full generality of NTP
requires distributed participation of a number of
client/servers or peers arranged in a dynamically
reconfigurable, hierarchically distributed
configuration. It also requires sophisticated
algorithms for association management, data
manipulation and local-clock control.
Glossary:
1. Host: Refers to an instantiation of the NTP
protocol on a local processor.
2. Peer: Refers to an instantiation of the NTP
protocol on a remote processor connected by
a network path from the local host. |
