End to End means the latency through the network is calculated directly between master and slave. This allows for PTP to operate over networks with Transparent clocks, non-PTP aware network switches, or a combination of both. The slaves sends delay requests upstream to the master, the master responds, and the network latency is calculated. The calculation is then applied by the slave to the received Sync message.
Why use End to End?
The advantage of this method is that the network nodes do not need to be PTP compliant, which reduces the limitations on hardware that can be used in the network. As end to end is measuring the delay between the master and slave any delay added to the system by the network nodes will be accounted for. However; the accuracy achieved through a network implementing End to End delay mechanism, will not be as good as the same topology implementing Peer to Peer delay mechanism with transparent clocks. Also note that End to End delay mechanism generates a larger volume of network traffic when multiple slaves being present on the network.