QoS: Creating Inequality In An Equal World Your Bandwidth Is Waiting To reserve a given level of bandwidth end to end, all devices in the chain from source to destination must support RSVP. In practice, an RSVP-enabled host will request a certain service level from the network--essentially all the routers in the path from source to destination. The network will agree to this request and reserve the requested bandwidth or terminate the requested session. Within the RSVP host software, admission control and policy control modules are executed. Admission control determines if the node can obtain sufficient resources to supply the requested QoS. Policy control checks whether the application has administrative permission to make this reservation. If both these checks succeed, the host RSVP software sets parameters in a packet classifier and packet scheduler to obtain the desired QoS from the network. The packet classifier defines the QoS level for every packe t, and the packet scheduler orders packets for transmission to obtain the QoS. The RSVP request for a given QoS level is made by an RSVP-aware appl ication (in the Microsoft Windows world, this is made via calls to the WinSock 2 library), requesting that a certain service model define the bandwidth required or the delay tolerated. In practice, only two service models are widely deployed: guaranteed service and controlled-load service. The guaranteed-service model ensures that the delay restrictions requested by a host originating an RSVP call are met. The controlled-load service model makes no guarantees, but admits new RSVP connections only up to the point where service starts to deteriorate. Beyond that, new connection requests are denied. Controlled load is the model implemented by Cisco when it demonstrated its RSVP offering at last fall's NetWorld+Interop show. Using an RSVP-enabled version of Intel Corp.'s ProShare presenter application displaying video, an RSVP session was established through an internetwork of Cisco routers, and the video stream remained intact. With RSVP disabled, the video stream through the internetwork was disrupted and did not display properly. RSVP is not a routing protocol. It uses routing table entries generated by other protocols, such as the Routing Information Protocol (RIP) or Open Shortest Path First (OSPF), to determine the next router in sequence to deliver packets to. Also, RSVP will adapt to new routes as they appear. It must be conceded that RSVP delivers better QoS performance over point-to-point links than over LAN connections. This is because the network represents a queue of data that is not directly under the control of the device making RSVP guarantees. For example, a router can offer guarantees for one RSVP stream on one connected network, but the router does not know the loads or timing of loads that neighboring systems will present. Step in Time RTP is an applications-layer protocol that uses time stamps and sequence information in its header to recover from delay variations and packet loss in a stream of traffic transmitted on an internetwork. An RTP session works best when established wi thin an RSVP connection (see diagram above). In this arrangement, the RSVP connection is established by a network device requesting a QoS from the network. Once this RSVP connection is established, the application in the device requesting the connection can use RTP to deliver video and other delay-sensitive data. As a network administrator, you would be involved only in setting up RSVP parameters: RTP is in the realm of application programmers. Sign Right Up Multicast is fundamentally different than unicast (point-to-point) and broadcast communications. Central to the theme of multicast communication is that a recipient has to actively join the group of hosts receiving the multicast. The multicast is sent once by the originator, is routable (pure broadcasts are not), and is not sent to segments where hosts have not registered to receive t he multicast. To successfully implement multicast features, both hosts and routers must have multicast capability. On hosts, the Internet Group Multicast Protocol (IGMP) is now part of Windows. In routers, there are several routing protocols that facilitate multicast routing: the Distance Vector Multicast Routing Protocol (DBMP), Protocol Independent Multicast (PIM) and Multicast Open Shortest Path Routing (MOSPR). An alternative approach to IP Multicast is to use switched VLANs to direct broadcasts to the desired workstations. This is less flexible than the IP Multicast option, since it requires you to add workstations to the VLAN carrying the broadcast signal--a host cannot dynamically subscribe to a multicast group and have the data stream routed to it.

Finding Unified Architectural Diversity by David A. Zimmer Updated May 12, 1997