This Recommendation relays synchronization aspects in packet networks. In particular@ it specifies the hypothetical reference model (HRM) and the PDV network limits applicable when frequency synchronization is carried via packets and is recovered according to the adaptive clock recovery method as defined in Recommendations ITU-T G.8261 and ITU-T G.8260. It specifies the minimum equipment tolerance to packet delay variation in terms of the metrics defined in Recommendation ITU-T G.8260 at the boundary of these packet networks. Two main applications are addressed in this Recommendation: the distribution of a synchronization network clock signal via a packet based method (e.g.@ using PTP or NTP packets and using an adaptive approach)@ and the distribution of a service clock signal over a packet network according to an adaptive clock recovery method (e.g.@ clock recovery of a circuit emulation service (CES) using an adaptive method)@ however the details on the CES network limits are for further study. Both one-way and two-way methods are considered. The packet networks that are in the scope of this Recommendation are currently limited to the following scenarios: ? Ethernet ([IEEE 802.3]@ [IEEE 802.1D]@ [IEEE 802.1 ad]@ [IEEE 802.1Q]@ [IEEE 802.1Qay]) ? MPLS ([IETF RFC 3031]@ [ITU-T G.8110]) ? IP ([IETF RFC 791] and [IETF RFC 2460]) The physical layer that is relevant to this specification is the Ethernet media types as defined in IEEE Standard 802.3 ??C2008. Other physical layers can be relevant and may be addressed in future versions of this Recommendation.