at reserves resources in order to guar- antee SLOs. It does not specify a full OAM suite or APS. MPLS-TP is a transport network-specific flavor. Its defining characteristic is the defini- tion of QoS protocols and true APS (instead of FRR); it may operate without IP forward- ing or routing protocols. 5. MPLS-SR is the newest addition. MPLS segment routing is implemented by a stack of MPLS labels, which are popped in order to reveal the next hop, similar to (now all but deprecated) source routing but without the security issues of enabling an end user dictate forwarding behavior. 6 Enabling Technologies It must be noted that Ethernet defines both the physical (L1) and data-link (L2) layers, while MPLS (as part of the IP suite) does not define a physical layer. Therefore, it frequently occurs that MPLS or IP avail themselves of Ethernet for the lower layers. However, we differenti- ate between this use of Ethernet as a "dumb pipe," and the use of Carrier Ethernet as a carrier grade networking technology. Yet, it may happen that the differentiation is some- times blurred, with some functionality being carried out in the Ethernet underlay network (e.g. timing, physical layer fault detection, etc.), and other functionalities carried out by the higher layer. 6.6.5.3 Protocol Stacks for User Traffic In this subsection we detail the construction of the forwarding plane packets as seen in var- ious points in the transport network. For concreteness we will assume in the following that the PDU session type is IP, and that all the data-link and physical interfaces are Ethernet. We describe packet structure according to the layering convention rather than the packet order convention, SO that higher layers appear first, and the rightmost header is the first to be transmitted. NG-RAN user traffic (e.g. from either DU or CU) is delivered as GTP-U packets, which are themselves encapsulated in UDP/IP (note that 2G and 3G used TDM or ATM, and that these may still be required to be supported). Similarly, signaling traffic is encapsu