is simplest, but it does not provide any QoS differentiation between calls/sessions or aggregates of IP traffic. However, if there is enough bandwidth available, best-effort can also provide services with satisfactory quality to end-users. When comparing IntServ and DiffServ, the main difference is in the scalability of the two approaches. DiffServ is much more scalable because all routers keep information per traffic class (the number of traffic classes is limited and it is not related to traffic volume), while in the IntServ architecture the network nodes (routers) make first reservation of resources regarding required QoS and then maintain this information in network nodes on a per flow basis, 2 Internet QoS Table 2.2 Comparison of best-effort, DiffServ, and IntServ. Best-effort DiffServ IntServ Service Connectivity Per aggregate isolation Per flow isolation No isolation Per aggregate guarantee Per flow guarantee No guarantees Service scope End-to-end Domain End-to-end Complexity No setup Long-term setup Per flow setup Scalability Highly scalable (nodes Scalable (edge routers Not scalable (each maintain only routing maintain per aggregate state; router maintains per state) core routers per class state) flow state) which may lead to scalability problems (e.g. when there are many flows in parallel, which is typical in core and transport or transit networks). 2.8 QoS with Multi-Protocol Label Switching (MPLS) The main IETF approach standardized for QoS traffic management in transport net- works is MPLS, with its standard defined in 2001 [27]. In IP transport networks with implemented MPLS (a typical scenario in transport IP-based networks in the first two decades of the twenty-first century), label switching is used instead of routing based on IP addresses. So, QoS is not implemented with DiffServ fields in the IPv4 or IPv6 headers, but by adding information via label headers (between the MAC header on layer 2 and the IP header on layer 3). MPLS works between OSI layers 2 and 3 (below the IP and above the MAC) an