DICOM PS3.15 2020c - Security and System Management Profiles

G Time Synchronization Profiles

G.1 Basic Time Synchronization Profile

The Basic Time Synchronization Profile defines services to synchronize the clocks on multiple computers. It employs the Network Time Protocol (NTP) services that have been used for this purpose by many other disciplines. NTP permits synchronization to a local server that provides a local time source, and synchronization to a variety of external time services. The accuracy and precision controls are not explicitly part of the protocol. They are determined in large part by the selection of clock hardware and network topology.

An extensive discussion of implementation strategies for NTP can be found at http://www.ntp.org.

The Basic Time Synchronization Profile applies to the actors DHCP Client, DHCP Server, SNTP Client, NTP Client and NTP Server. The mandatory and optional transactions are described in the table and sections below.

Table G.1-1. Basic Time Synchronization Profile

Actor

Transaction

Optionality

Section

NTP Server

Maintain Time

M

G.1.2

Find NTP Servers

O

G.1.1

NTP Client

Maintain Time

M

G.1.2

Find NTP Servers

O

G.1.1

SNTP Client

Maintain Time

M

G.1.2

DHCP Server

Find NTP Servers

O

G.1.1

DCHP Client

Find NTP Servers

M

G.1.1


G.1.1 Find NTP Servers

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.

G.1.1.1 Scope

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.

G.1.1.2 Use Case Roles

Find NTP Servers

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.

G.1.1.3 Referenced Standards

[RFC 2030] Simple Network Time Protocol (SNTP) Version 4

[RFC 5905] Network Time Protocol Version 4: Protocol and Algorithms Specification

[RFC 5906] Network Time Protocol Version 4: Autokey Specification

[RFC 8633] Network Time Protocol Best Current Practices

G.1.1.4 Basic Course of Events.

The NTP Client uses a list of NTP Servers, which may be:

  • obtained through optional NTP discovery mechanisms (see [RFC 5905]Section 3.1),

  • provided by a DHCP Server, see Annex F on DHCP, and/or

  • manually configured.

If the list is not empty, the client shall attempt to maintain time synchronization with at least one of the NTP servers. The client synchronization shall be in compliance with either [RFC 5905] (NTP) or [RFC 2030] (SNTP). If the list is empty, the client may choose an alternative method of time synchronization.

SNTP provides much lower accuracy than NTP. If time synchronization of better than 1s mean error is required, the client should use NTP. [RFC 5905] and [RFC 8633] discuss implementation and accuracy considerations.

A DHCP server may provide to the DHCP Client a UTC offset between the local time at the machine and UTC, which the client shall use for converting between UTC and local time.

G.1.1.5 Alternative Paths

If there is no UTC offset information from the DHCP or NTP server, then the UTC offset will be obtained in a device specific manner (e.g., service, CMOS, internal battery clock).

G.1.1.6 Assumptions

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.

G.1.1.7 Postconditions

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.

G.1.2 Maintain Time

G.1.2.1 Scope

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.

G.1.2.2 Use Case Roles

Maintain Time

Figure G.2-1. Maintain Time


NTP/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.

G.1.2.3 Referenced Standards

[RFC 2030] Simple Network Time Protocol (SNTP) Version 4

[RFC 2827] Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing

[RFC 5905] Network Time Protocol Version 4: Protocol and Algorithms Specification

[RFC 5906] Network Time Protocol Version 4: Autokey Specification

[RFC 8633] Network Time Protocol Best Current Practices

G.1.2.4 Basic Course of Events.

The detail on Maintain Time transactions is described in [RFC 5905] and [RFC 2030]. The most common and the mandatory minimum mode for NTP operation uses a series of messages between client and servers. The client sends requests to the servers, which fill in time related fields in a response, and the client performs optimal estimation of the present time based on that information. The RFCs deal with issues of lost messages, estimation formulae, etc. Once the clocks are in synchronization these message exchanges typically stabilize at roughly 1000 second intervals.

The client machine uses the time estimate to maintain the internal operating system clock. This clock is then used by applications that need time information. This approach eliminates the application visible difference between synchronized and unsynchronized time. The RFCs provide guidance on proper implementations.

G.1.3 NTP Security Considerations (Informative)

NTP security considerations ([RFC 5905] Section 8, [RFC 5906], and [RFC 8633]) may be applicable based on site-specific environment and threat considerations. Locations with NTP Servers should also consider [RFC 2827] and implementing access controls on the use of the server.

Security Policies and Procedures for NTP are maintained at http://www.nwtime.org/security-policy/ as part of the Network Time Foundation.

G.1.4 NTP Implementation Considerations (Informative)

NTP compliant servers always support both NTP and SNTP clients. The difference is one of synchronization accuracy, not communications compatibility. Although in theory both NTP and SNTP clients could run at the same time on the same system, this is not recommended. The SNTP updates will simply degrade the time accuracy. When other time protocol clients, such as IRIG, are also being used, these clients must be coordinated with the NTP client to avoid synchronization problems.

These and other considerations, such as multiple clock types, accuracy implications, and configuration alternatives, are documented at http://www.ntp.org.

G.1.5 Conformance

The Conformance Statement for the NTP Server and NTP Client shall state whether secure transactions ([RFC 5906]) are supported.

The Conformance Statement for the NTP Server shall state whether it is also an NTP Client.

The Conformance Statement for the NTP Client shall state how it manages time when no NTP Server is available.

DICOM PS3.15 2020c - Security and System Management Profiles