an the former due to its access to physical layer time stamping (e.g. it defines packet arrival precisely as halfway up the lead- ing edge of the first bit) and to its defining on-path support, that is, PTP-specific mechanisms implemented along the path taken by the PTP packets. NTP is a client-server protocol (the client requests service from an NTP server, and the server maintains no information on the clients) while PTP is master-slave (the master sends information to the slaves). In all such protocols the delay from the master or server to the slave or client must be measured in order to offset the time-of-day announcement. In NTP this measurement is merged with the announcements, while PTP separates the two functions in order to enable announcements at a higher rate (and even multicast). The standard technique measures round-trip delay and assuming symmetry between the two directions divides by two. Furthermore, the non-negligible residence time in the responding element must be taken into account. The calculation involves four time stamps. The client/slave initiates the exchange; t1 is the time (according to the client/slave's clock) that the protocol packet was sent originally; t2 and t3 are the time (according to in the server/master's clock) that the server/master receives that packet and transmits its response packet respectively; and t4 is the time (according to the client/slave's clock) that the client/slave receives the response packet. Assuming symmetry, the estimate of the one-way delay is: Delay =12(t4-t1-(t3 -t2)) The problem is that this calculated one-way delay varies from packet to packet due to queuing delays in intermediate switches between master and slave, and infrequently due to routing changes. PTP enables cancelation of these effects through either of two mechanisms: A transparent clock (TC) is an intermediate switch that can measure its total residence time and add it to the accumulated time through the network. A boundary clock (BC) is an intermediate switch that like a slave discipli