The optional NTP protocol elements for NTP autoconfiguration and NTP autodiscovery can significantly simplify installation. The NTP specification for these is defined such that they are truly optional for both client and server. In the event that a client cannot find an NTP server automatically using these services, it can use the DHCP optional information or manually configured information to find a server. Support for these services is recommended but not mandatory.
This transaction exists primarily as a means of documenting whether particular models of equipment support the automatic discovery. This lets installation and operation plan their DHCP and equipment installation procedures in advance.
This applies to any client that needs the correct time, or that needs to have its time stamps synchronized with those of another system. The accuracy of synchronization is determined by details of the configuration and implementation of the network and NTP servers at any specific site.
Both the NTP and SNTP clients shall utilize the NTP server information if it is provided by DHCP and NTP services have not been found using autodiscovery. Manual configuration shall be provided as a backup. Autodiscovery or DHCP are preferred.
Figure G.1-1 Find NTP Servers
DHCP Server Provides UTC offset, provides list of NTP servers
DHCP Client Receives UTC offset and list of NTP servers
NTP Client Maintains client clock
SNTP Client Maintains client clock
NTP Servers External time servers. These may have connections to other time servers, and may be synchronized with national time sources.
The DHCP server may have provided a list of NTP servers or one may be obtained through optional NTP discovery mechanisms. If this list is empty and no manually configured NTP server address is present, the client shall select its internal clock as the time source (see below). If the list is not empty, the client shall attempt to maintain time synchronization with all those NTP servers. The client may attempt to use the multi-cast, manycast, and broadcast options as defined in RFC-1305. It shall utilize the point to point synchronization option if these are not available. The synchronization shall be in compliance with either RFC-1305 (NTP) or RFC-2030 (SNTP).
If the application requires time synchronization of better than 1s mean error, the client should use NTP. SNTP cannot ensure a more accurate time synchronization.
The DHCP server may have provided a UTC offset between the local time at the machine and UTC. If this is missing, the UTC offset will be obtained in a device specific manner (e.g. service, CMOS). If the UTC offset is provided, the client shall use this offset for converting between UTC and local time.
If there is no UTC offset information from the DHCP server, then the NTP client will use its preset or service set UTC offset.
If there is no NTP time server, then the NTP client will select its internal battery clock as the source of UTC. These may have substantial errors. This also means that when there are multiple systems but no NTP source, the multiple systems will not attempt to synchronize with one another.
The local battery clock time is set to UTC, or the local operating system has proper support to manage both battery clock time, NTP clock time, and system clock time. The NTP time is always in UTC.
The client will remain synchronized with its selected time source. In an environment with one or more NTP servers, this will be good time synchronization. In the absence of NTP servers, the selected source will be the internal client clock, with all its attendant errors.