flows. Each flow will then be further logically assigned a priority level (in conjunction with queuing struc- tures) and then mapped onto classes. Each class is mapped onto one of many ports. Some architec- tures even go SO far as to make a point of splitting ports into a two-layer hierarchy of virtual ports and physical ports. Each class maps into one of multiple virtual ports and each virtual port can then map to one of multiple physical ports. The idea of hierarchical traffic-management granularity should now be a bit clearer. This example can be summarized as follows: Flows Classes Virtual ports Physical ports The RSVP (RFC 2205) was one of the first protocols that attempted to provide some sense of rudi- mentary QoS or guaranteed bandwidth. It is currently used on IP networks. It works by requesting from downstream network equipment that network resources and capacity be reserved in advance- in other words, before a specific flow of traffic is sent out into the network. Its mechanisms are sim- ple compared to the requirements of the complex realm of QoS. It may or may not receive a positive signal. The signaling works in the following way. The originator of traffic (transmitter) A sends a PATH message to receiver B that describes the characteristics and needs of the upcoming traffic. Every intermediate router will forward this message to its downstream peer on the next hop all the way to the receiver. If receiver B accepts PATH message, it will issue a RESV message back to the transmitter that is requesting network resources for the flow. The RESV message now travels backwards on the same link hopping from one router to the other and hopefully making it eventually back to transmitter A. If a router cannot honor the request, then an error message is sent to the receiver and the signaling handshake is ended with- out setting up a connection between A and B. If every stage has honored the request, the appropriate buffering and bandwidth resources are set up at each router to accommodate the oncoming flow. IntSe