6 Precision Time Protocol (PTP) / IEEE1588
6.2.3 End-To-End (E2E) or Peer-To-Peer (P2P) Delay Measurements
In addition to receiving the SYNC/FOLLOWUP messages a PTP slave device needs to be able to measure the
network delay, i.e. the time it took the SYNC message to traverse the network path between the master and
the slave. This delay is required to correct the received time information accordingly and it is measured by
the slave in a configured interval (more about the message intervals later). A delay measurement is performed
by sending a so-called DELAY_REQUEST to the master which timestamps it and returns the timestamp in a
DELAY_RESPONSE message.
IEEE 1588-2008 offers two different mechanisms for performing the delay measurements. A slave can either
measure the delay all the way to the master, this is called End-To-End (or E2E in short) or to its direct network
neighbors (which would in almost all cases be a switch – or t wo in a redundant setup), using the Peer-To-Peer
delay measurement mechanism (P2P). The delay measurements of all links between the master and the slave
are then added and accumulated while a SYNC packet is traversing the network.
The advantage of this method is that it can dramatically reduce the degradation of accuracy after topology
changes. For example: in a redundant network ring topology the network delay will be affected when the ring
breaks open and network traffic needs to be redirected and flows into the other direction. A PTP slave in a
sync infrastructure using E2E would in this case apply the wrong delay correction calculations until it performs
the next delay measurement (and finds out that the network path delay has changed). The same scenario in a
P2P setup would see much less time error, because the delay of all changed network links were already available.
The drawback: the P2P approach requires that all involved PTP devices and all switches support this mecha-
nism. A switch/hub without P2P support would in the best case simply pass the so-called PDELAY messages
through and as a result degrade the accuracy of the delay measurements. In the worst case it would block/drop
the PDELAY messages completely, which effectively would result in no delay measurements at all.
So, E2E is the only available choice if you are running PTP traffic through non-PTP-aware switches. It is a
reasonable choice if you are not using redundant network topologies or can accept that the delay measurements
are wrong for a certain amount of time.
SyncBox N2X Date: 17th September 2019 21
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