at will degrade the ac- curacy of the recovered phase. This is illustrated in Figure 12-5, where the asymmetric timestamp- ing points are marked as tingress_skewed and tegress_skewed- Reference plane Timestamping unit tegress Transmit tegress-skewer ATSU asymmetry tingress Receive {ingress-skewed Figure 12-5 Reference Plane for Timestamping Unit Finally, at the minimum TSU resolution, it should be possible to identify each individual packet on a link by a timestamp unique to that packet. This requires that the timestamp resolution must be less than the time it takes to transmit a minimum-size Ethernet frame of 64 bytes. Of course, the time taken to transmit that 64-byte (or 512-bit) packet depends on the link speed. Including 8 bytes of preamble and 12 bytes of minimum inter-frame gap, the smallest frame be- comes 84 bytes (or 672 bits) on the wire. It takes 67.2 ns to transmit 672 bits on a 10-Gbps link, 16.8 ns on a 40-Gbps link, and 6.72 ns on a 100-Gbps link. So, for a TSU supporting 100-Gbps in- terfaces, a minimum timestamp resolution of 6 ns is required. Software: Control Plane, Servo, and Packet Processing Designing and implementing a PTP-aware NE requires extensive software support as well as good hardware. These software modules need to be based on a flexible architecture SO they can be adap- ted to constantly evolving recommendations from standards development organizations (such as ITU-T and IEEE). From a software point of view, three key elements are required by a PTP-aware NE. The first ele- ment is the PTP protocol stack that usually runs on the host processor as a software module. The list that follows covers some features of the PTP protocol stack functionality (the list is not com- plete) to provide an idea of the functionality implemented in a PTP protocol stack. It must be able to Support configuration of different PTP clock node types (such as ordinary clock (OC), bound- ary clock (BC), and SO on). Set up the PTP packet generator and receiver based on the node's PTP configuration and indic- a