ket delivery. This is most often the case when there are alternate paths with quite dif- ferent E2E QoS parameters, although other factors may also be influential (e.g. paths with quite different costs). Additionally, network slicing requires separation and appropriate for- warding of flows belonging to different slices, and issues specific to it will be discussed in the next subsection. Two types of path computation are used in the control plane: 1. Distributed routing where forwarding devices exchange information between them- selves and each independently builds a forwarding information base (FIB). 2. Centralized path computation (network management, SDN) where an omniscient "God box" uses graph optimization algorithms to compute paths, and disseminates these to the forwarding elements. Independently of these, two types of QoS handling need to be considered in the forwarding plane: 6.6 Transport Network 1. Hard QoS (AKA IntServ, traffic engineering [TE]) where a combination of connection admission control (CAC) and resource reservation provides hard QoS guarantees. 2. Soft QoS (AKA DiffServ, traffic conditioning) where packets are afforded differ- ential treatment according to their priority and discard eligibility, and schedul- ing/queuing/policing/shaping algorithms provide statistical QoS performance. Ethernet, IP, and MPLS were all originally best effort with no QoS handling, but each devel- oped such handling over time. Hard QoS was proposed for VoIP in the form of Resource Reservation Protocol (RSVP), but was never widely used. DiffServ IP is common based on the 6-bit DiffServ code point (DSCP) field in the IP header. Carrier Ethernet implements soft QoS based on the 3-bit priority code point (PCP, colloquially called priority bits or P-bits) and discard eligibility indicator (DEI). MPLS-TE adopted the traffic engineering approach by extending RSVP to RSVP-TE (RFC 3209). Ethernet, IP, and MPLS have traditionally utilized various distributed control protocols to learn how to forward, while in Carrier Ethe