ser devices (i.e. hosts, including clients and servers). Then the 2 Internet QoS question is how to provide QoS guarantees to this NRT traffic, which is TCP-based. That can be accomplished either by adding certain QoS mechanisms in an initially best-effort Internet network or by ensuring that sufficient capacity is available. So, something that was the main Internet networking paradigm (simple network which uses the best-effort principle for transfer of packets, and complex hosts which deal with congestion that may occur anywhere along the path between the endpoints of communication by using TCP at the end-hosts) is becoming one of the main obstacles for provision of QoS in the network end-to-end for NRT traffic. 2.4 QoS on Different Protocols Layers The networking protocols in all-IP telecommunication networks are based on IP on the network layer and on TCP or UDP on the transport protocol layer. The lowest protocol layer in the protocol stack which functions end-to-end is the network layer, where the IP is positioned in all hosts and network nodes (e.g. routers). End-to-end means that the sending hosts creates an IP packet which may be received as a whole (IP packet) through the Internet by the receiving host. Of course, certain types of network nodes may change the contents of the IP packet, such as NAT (network address translation) gateways or proxies along the path. So, network layer QoS is related to the IP in both IPv4 and IPv6. However, protocols over and below the IP layer influence the QoS being provided. The performance of IP-based services depends upon the performance of other proto- col layers below or above the IP. Regarding the lower protocol layers (below IP), these belong to certain links (e.g. between two network elements such as routers and switches, or between network elements and end-hosts) which provide connection-oriented or connectionless transport for the IP layer (i.e. IP packets). In general, links between hosts and/or nodes in telecommunication networks may be based on different types 