high-priority treatment to an unlimited amount of traffic, routers need the capability for admission control and resource reservation. Upon receiving a request for handling a new flow, a router checks whether it has enough available resources to accept it without impacting other flows in progress. If the QoS requested for the flow can be granted, it reserves resources for the newcomer. State Information Stored in the Network We've just seen that routers must store information about flows in order to differentiate data- grams properly. This implies that the network stores state information. Pre- viously, we argued that doing SO flies in the face of the IP paradigm, pushing the intelligence to the end systems and storing as little state as possible in the network. The trade-off for less information is more robust systems that tolerate network failures better. Acknowledging the value of the paradigm and the need to make exceptions to it on well-defined occasions, reserva- tion merging and soft states help minimize the problems that exceptions can cause. Reservation Merging Figures 3-18, 3-19, and 3-20 show how reservation merging is performed in a multicast group. We saw in Figure 3-13 how the shared distribution tree was calculated for that topology. In Figure 3-18, we now assume that 131.160.1.112 is the sender and the other hosts, 153.88.251.19 and 138.85.27.10, are the receivers. Receiver 138.85.27.10 requests a certain QoS for its incoming flow. The routers in the path store the necessary state in their packet filters and the QoS requested is honored (Figure 3-19). Now receiver 153.88.251.19 also requests QoS for the flow it is receiving. However, the second host won't need to request QoS for the entire path from the sender because a QoS reservation is already present in part of the path for the same flow. Therefore, when 153.88.251.19 requests QoS, state infor- mation stored in the routers in the path that was already provisioning QoS for the first host does not increase. New state information is limited to th