10.5.3.3 Multicast or Unicast
The initial version of PTP (IEEE 1588-2002 also known as PTPv1) was a multicast-only protocol. Multicast
mode has the great advantage that the master clock needs to send only one SYNC packet to a Multicast address
and it is received by all slave devices that listen to that multicast address.
In version 2 of the protocol (IEEE 1588-2008) the unicast mode was introduced in addition to the multicast
mode. In unicast mode, the master has to send one packet each to every slave device, requiring much more CPU
performance on the master and producing orders of magnitudes more traffic.
On the other hand, some switches might block multicast traffic, so that in certain environments, Unicast mode
has to be used.
10.5.3.4 Two-Step or One-Step
The PTP protocol requires the master to periodically send SYNC messages to the slave devices. The hardware
time stamping approach of PTP requires that the master records the exact time when such a SYNC packet
is going on the network wire and needs to communicate this time stamp to the slaves. This can be achieved
by either sending this time stamp in a separate packet (a so-called FOLLOW-UP message) or by directly
manipulating the outgoing SYNC message, writing the hardware time stamp directly into the packet just before
it leaves the network port.
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Date: 22nd June 2020
microSync