yer-four port numbers. MPLS natively carries only IP or MPLS itself (the latter using the label stack) but may transport a wide variety of payloads via pseudowire mechanisms. It should be noted that MPLS packets are not self-describing, and thus there is no way of discovering the traffic type by packet inspection. Although 3GPP defines three PDU session types for 5G, namely, IP, Ethernet, and unstructured, and the transport network may handle traffic from various gNB functional split options, in practice all 5G packet transport networks will be required to transport IP over Ethernet. The split option 8 fronthaul traffic (not being explicitly specified for 5G) will generally be transported as CPRI (CPRI) over dark fiber or OTN, and the low-level split defined by O-RAN (see Section 4.5) is expected to be much more popular due to its reduced data rate. This latter split may be encapsulated in IP using eCPRI (eCPRI). For CU/DU split (see Section 4.2) traffic will be GTP in UDP in IP; the split option 1 (also not being explicitly specified for 5G) may use be IP in GTP over UDP/IP; handoff to third-party packet networks will avail itself of pure IP. All of these may be carried over the aforementioned alternative lower layers, such as double-tagged Ethernet, MPLS, Ethernet pseudowires over MPLS, and often seemingly ridiculous combinations of these are regularly encountered. For the foreseeable future CSGs will be required to support 4G (both fronthaul and back- haul), and perhaps 3G (IP or ATM) and even 2G (TDM). In addition, non-3GPP IP (e.g. Wi-Fi) and other sources of IP traffic (e.g. residential) may all be in the mix. Some wholesalers, especially those with Carrier Ethernet networks, may prefer not to terminate the Ethernet underlay over which the IP traffic is delivered and employ Ether- net pseudowires (RFC 4448) instead. In such cases, user Ethernet PDUs may be handled natively. 6.6.5.4.3 Controlling Forwarding Behavior It is often the case that network traversal needs to be more nuanced than simply ensuring pac