Ntp Addressing Modes - Symmetricom ND-CLK User Manual

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 time servers
ability to reply. The broadcast mode is a widespread or open-ended broadcast, not
intended for any specific IP address.
The network 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.
ND-CLK User Manual- Rev 1.0.1 - Sep-08
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