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NTP Addressing Modes
Unicast (Query)
The network clock 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 network clock 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 ND clock listens for NTP [UDP] broadcasts using the broadcast address [255.255.255.255].
[Note: 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 network clock 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, and group membership, 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 network clock 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 which support the Internet Group Management Protocol (IGMP). In addition, the IGMP mode
must be enabled and configured for multicasting addressing to be implemented properly. Please ensure
®
ND Clocks User Manual - Rev 2.0.1 Sep-08
7
Symmetricom
, Inc.
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