Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 1667)

NTP Introduction and Configuration
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 61.3
The same message format is used for both requests and replies. When a request is
received, the server interchanges addresses and ports, fills in or overwrites certain fields in
the message, recalculates the checksum, and returns it immediately. The information
included in the NTP message allows each client/ server peer to determine the
timekeeping characteristics of its peers, including the expected accuracies of their clocks.
Each peer uses this information and selects the best time from possibly several other
clocks, updates the local clock, and estimates its accuracy.
There is no provision in NTP for peer discovery, acquisition, or authentication. Data
integrity is provided by the IP and UDP checksums. No reachability, circuit-management,
duplicate-detection, or retransmission facilities are provided or necessary.
By its very nature clock synchronization requires long periods of time (hours or days) and
multiple comparisons in order to maintain accurate timekeeping. The more comparisons
performed, the greater the accuracy of the timekeeping.
NTP on the Switch
The implementation of NTP on the switch is based on the following RFCs:
■ RFC 958, Network Time Protocol (NTP)
■ RFC 1305, Network Time Protocol (Version 3) Specification, Implementation and
Analysis
■ RFC 1510, The Kerberos Network Authentication Service (V5)
Two modes of operation are supported: client and server. The switch is in client mode
most of the time where it polls the configured peer at least once every preconfigured
minimum time period.
The peer that the switch refers to must be a more accurate clock source than the switch
itself or another switch directly connected to a more accurate clock source. The switch
operates as a secondary time server. It cannot operate as a primary time server unless the
primary clock source is operating in server mode. A primary clock source usually operates
in broadcast mode, which is not supported by the switch’s implementation of NTP. When
the switch receives a valid reply from the peer, it synchronizes its own internal clock
according to the information from the reply.
If the switch receives a synchronization request from an NTP client, it temporarily changes
to server mode. It replies to the request with the current time from the switch’s internal
clock along with other information useful for synchronization. The switch’s internal clock is
accurate to 0.005 seconds.