Implementing QoS enables real-time programs to make the most efficient use of network bandwidth. The goal of a QoS implementation is a guaranteed delivery system for network traffic, such as IP packets.
The set of mechanisms QoS uses to set up a delivery system for network traffic includes services and protocols. The following SlideShow describes these mechanisms.
RSVP enables end nodes to communicate with each QoS-aware network device (included in the hop path between RSVP session members), and negotiates QoS parameters and network usage admission. The RSVP protocol is used to exchange PATH and RESV messages with the network. A PATH message, which the sender initiates, describes the QoS parameters of the traffic, the sender's address, and the destination address of the traffic. The reserve (RESV) message, returned by the receiver, describes the QoS parameters of the traffic to be received. When the sender receives the RESV message, the QoS data flow begins.
The QoS service provider constructs and periodically updates the PATH and RESV messages on behalf of an application. You can also configure sending applications, such as those controlling multicast transmissions, to begin sending immediately on a best-effort basis, which is then upgraded to QoS on receipt of the RESV message. The following image illustrates a basic QoS connection.
For example, a sending host that wants to reserve bandwidth sends path messages toward the intended recipient, through an RSVP-enabled Windows Sockets (WinSock) service provider. These path messages, which describe the bandwidth requirements and relevant parameters of the data to be sent, are propagated to all intermediate routers along the path.
A receiving host confirms the flow and network path by sending RESV messages back through the network, describing the bandwidth characteristics of data from the sender. As these reserve messages propagate back toward the sender, intermediate routers decide whether or not to accept the proposed reservation and commit bandwidth resources. If all routers commit the required resources, the sender can begin sending data.
The next lesson wraps up this module.