g Task Force (IETF) was RFC 868, "Time Pro- tocol," which allows a user to query time on a node (via the rtime(3) system call on Linux systems). The main packet-based protocols in use today are NTP and PTP. See Chapter 7 for a comparison of these methods. NTP was first proposed as RFC 958 in the id-1980s, and it is now up to its fourth generation. For telecommunications, NTP is mainly used to carry time of day, although there are applications using it to carry frequency for mobile. The Simple Network Time Protocol (SNTP) is a lighter version subset of the standard NTP, used where the performance of the full NTP implementation is not needed. NTPv4 and SNTP are defined in RFC 5905, and together they are probably the most popu- lar and widely deployed time transport used worldwide (mainly for setting system time on devices connected to data networks such as the Internet). In order to carry frequency using packets, one method that has been widely deployed is based on one of the PTP telecom profiles specifically designed for this role. See the section "G.8265.1- Telecom Profile for Frequency Distribution" in Chapter 7 for details of this approach. Other meth- ods, such as ACR, exist to carry frequency over packets, and they are covered in detail in Chapter 9. However, when it comes to carrying time and phase, the best-in-class tool available is a packet- based distribution method based on the PTP protocol. PTP and all its profiles are covered extens- lively in Chapter 7, SO refer there for all the details. Basically, when there is not a radio solution for distributing time, the key alternative is a two-way time transfer protocol on a packet network, using 3.4.1 Transport and Signaling of Quality Levels No matter whether the clock is distributed over the physical media or with packet-based methods, an additional mechanism is required to exchange clock quality information and traceability of syn- chronization signals. Depending on the method that is used in the timing distribution network, there are different methods to