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==== Clock ==== Timestamps are measurements of a device clock. Most device clock components conform to standard requirements for stratum 3 clocks, such as G-1244-CORE[ref_id:gr_1244_core] or ITU-T G.812[ref_id:g812] type IV. The device clock SHOULD conform to one of these and report this. Before using the common terms in this domain, we first define them: * Resolution is the quantity below which two samples are seen as equal. It is defined as a time interval (e.g., nsec). The range of values that can be expressed is defined in terms of wrap-around time. From this, a minimum bit-width can be derived. Resolution itself is not an integer storage size, however. * Precision is the distribution of measurements. It indicates repeatability of measurements, and is affected by read uncertainty. Precision is also expressed as a time interval. * Accuracy is the offset from the true value. A perfectly precise measurement may have a constant offset. In this context, for instance the offset from the measurement plane from the reference plane. Clock resolution and precision MUST be 10 ns or better. The clock MUST NOT drift more than 10 ppm. This may require a temperature controlled device (TXCO, OXCO or otherwise), but implementation is not prescribed. The clock must have a wraparound no worse than the 64-bit PTPv1 format, which is 2^32 seconds or roughly 136 years. The counter MUST be monotonically non-decreasing. That is, causality must be maintained: any packet B measured after another packet A at the same measurement plane cannot have a timestamp lower than the timestamp of A. A packet passing through two measurement planes X and Y (such as PHY Tx and Rx when looping through a switch) must have a timestamp at Y greater than or equal to the timestamp at X. Timestamps may be equal in particular if transmission rate is higher than clock accuracy. <span id="clock-synchronization"></span>
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