rvable forwarding behavior applied at a DiffServ-compliant node to a DiffServ behavior aggregate. The IETF's development of DiffServ in the 1990s was intended to address the following difficulties with IntServ and RSVP: Scalability. Maintaining states by routers in high-speed networks is difficult due to the very large number of flows. Flexible service models. IntServ includes only two classes, and there was a need to provide more qualitative service classes and "relative" service distinction. Simpler signaling (compared with RSVP). Many applications and users may only want to specify a more qualitative notion of service. Which are DiffServ functions? Well, it provides only simple functions in the core network (forwarding according to the PHB) and relatively complex functions at edge routers (or hosts). A DiffServ-capable host or first DiffServ-capable router requires the following actions (Figure 2.13): Classification. Network edge node marks packets according to classification rules to be specified (manually set by network administrators, or by some protocol); Traffic conditioning. Network edge node may delay and then forward, or they may discard packets. In DiffServ, IP packets are marked in the IP headers, SO this is truly a network-layer QoS mechanism (there is no need to read or change upper layer headers such as proto- cols on the transport or application layer). The marking in IP headers is completed by 2.7 Integrated Services (IntServ) and Differentiated Services (DiffServ) Edge routers router mark packets DiffServ domain router router Per hob behavior of DiffServ DiffServ - Differentiated services Figure 2.13 Differentiated services (DiffServ) functions. Meter Packets Forward Classifier Marker Shaper/ dropper Figure 2.14 Classification and conditioning of IP traffic. the edge routers in the DiffServ network domain (that is the network that implements DiffServ architecture) by marking specified fields in the IP headers: ToS field in the IPv4 header, and DSCP in the IPv6 header. It may be desirable to lim