ose routers in the path from 153.88.251.19 to the main distribution tree (Figure 3-20). The Internet Multimedia Conferencing Architecture Figure 3-18 153.88.251.19 Distribution tree with 131.160.1.112 131.160.1.112 acting as a sender. ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER 138.85.27.10 Figure 3-19 153.88.251.19 138.85.27.10 requests QoS for 131.160.1.112 the flow. ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER QoS requested by 138.85.27.10 138.85.27.10 Best-effort service Chapter 3 Figure 3-20 153.88.251.19 153.88.251.1 19 also requests QoS. 131.160.1.112 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER QoS requested by 138.85.27.10 138.85.27.10 QoS requested by 153.88.251.19 Soft States Implementing soft states increases the system robustness. Soft states store state information temporarily, after which the router removes all the state it was storing. Under this system, if the state information is not refreshed periodically, it will expire and enable the release of all the resources reserved in the router. Soft states are typically refreshed by send- ing a message to the router with the proper information, whereas hard states are stored permanently and require a release-of-state command to relinquish resources. ReSerVation Protocol (RSVP) ReSerVation Protocol (RSVP) [RFC 2205] is the protocol used for resource reservation in the network. RSVP installs the necessary state in the routers and refreshes it periodically. Reservations for a particular flow are initiated by its receiver; the messages that store state from the receiver to the sender are called RESV messages. However, IP datagrams from the receiver to the sender do not typically follow the same path as datagrams in the opposite direction. Thus, an RSVP message (PATH message) from the sender to the receiver of the flow has to The Internet Multimedia Conferencing Architecture be sent prior to resource reservation in order to scout out which path the datagrams belonging to the flow will follow. PATH messages contain the path that RESV messages m