routers to packets belonging to different flows. For The Internet Multimedia Conferencing Architecture instance, datagrams from a real-time flow can be tapped for forwarding ahead of datagrams from a low-priority flow. Routers handling a particular flow will need additional information in order to forward its datagrams properly: namely information on how to distinguish the datagrams in one flow from datagrams belonging to other flows and what class of treatment each datagram is due. A router might prioritize all datagrams with a certain destination IP address and a certain UDP destination port number, while continuing best efforts for datagrams with a different destination. In the latter case, a router handles several flows and therefore has to implement different packet filters in order to classify the datagrams it receives. The default is best effort, which is what packets that do not belong to any flow known by the router receive. Services Available The integrated services architecture currently pro- vides two services: controlled-load and guaranteed services. They represent two levels of better-than-best-effort services (Figure 3-17). Packets benefiting from controlled-load services are given priority over best-effort traffic. Therefore, even if the network is congested with best- effort traffic, the controlled-load service delivers the datagrams as if the network were under moderate load. This service does not guarantee any particular bandwidth or delay for a certain flow; it simply ensures that packets get superior treatment. Guaranteed service, as the name suggest, provides a certain bandwidth and a delay bound for a particular flow. Therefore, the jitter observed for guaranteed traffic is small to negligible. Router Figure 3-17 Guaranteed service Packet filter in a router. IP packet IP packet Incoming interface Cotnrolled-Load service Outgoing interface IP packet IP packet Packet IP packet IP packet IP packet IP packet filter Best-effort service IP packet IP packet Chapter 3 Because a router cannot grant 