NTP100 – Rev 2.2.0 Oct-12
9
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®
, Inc.
NTP Addressing Modes
Unicast
The NTP100 supports the unicast method of NTP packets transfer. Unicast method involves direct transfer of requested
information from the NTP server to the NTP client based on a query or NTP time request. The unicast method is supported
simultaneously when either the broadcast or multicast modes are selected.
Broadcast
The NTP100 supports the broadcasting of NTP packets. This feature is useful in situation where network administration may
wish to avoid the network traffic created by a large number of clients making periodic NTP requests, or in situations where
such periodic requests end up synchronized is such a manner as to exceed the NTP100’s ability to reply. The broadcast mode
is a widespread or open-ended broadcast, not intended for any specific IP address.
The NTP100 provides NTP [UDP] broadcasts using the broadcast address [255.255.255.255].
Note that some firewalls and routers will not forward UDP broadcasts by default. Security configurations may need to be
adjusted to allow the UDP broadcast packets to pass on the configured port.
Multicast
The NTP100 also supports multicast addressing of NTP packets as a recently added feature. As opposed to broadcast mode,
which is a widespread or open-ended broadcast of NTP packets whereby, data is sent to every possible receiver (client),.
Multicasting is useful because it conserves bandwidth. It does this by replicating packets only as needed within the network
to send them only to receivers (clients) that want them, thereby not transmitting unnecessary packets.
The concept of a group is crucial to multicasting. Every multicast requires a multicast group; the sender (or source) transmits
to the group address, and only members of the group can receive the multicast data. A group is defined by a Class D address.
The NTP100 does not restrict the use of the multicast address assignment and supports the full range of class D multicast
addresses or groups from 224.0.0.0 to 239.255.255.255 . These groups or class D address ranges for multicasting are
defined and governed by RFC3171
, IANA IPv4 Multicast Guidelines.
Typically, the multicast address range 224.0.1.0 - 224.0.1.255 (224.0.1/24) [ Internetwork Control Block] is utilized for
NTP traffic, however, please refer to the RFC3171
for your specific application and implementation.
The Internet Group Management Protocol (IGMP) is a protocol that controls group membership for individual hosts. This
protocol only operates in a LAN setting, but is required if you wish to be able to join a multicast group on a host. IGMP is
defined in RFC 2236
.
Note: Use of the multicast addressing method requires the use of routers & switches and other network devices that support
the Internet Group Management Protocol (IGMP). In addition, the IGMP mode must be enabled and configured for
multicasting addressing to be implemented properly. The implementation of multicasting addressing is beyond the scope of
support available from Masterclock. Please ensure that your network system components are capable of , and configured
properly for IGMP before utilizing the multicast addressing feature.
Note: You will need to check with your firewall vendor to determine how to enable multicast traffic through a firewall. In
addition, you may want to read RFC 2588: IP Multicast and Firewalls
.
Anycast
The NTP100 does not currently provide anycast capability.
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