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Quality of Service (QoS)

QoS in packet-switched networks in general, and IP networks in particular, has been an important area of networking research over the past several decades. However, even though there have been rich research results in this area, relatively little has been put into practice in deployed networks. The IP header has 8 bits dedicated to the Type of Service (TOS) field, but the TOS field has not been used until recently; even now, the use of TOS to indicate class of service is not common.

Queuing disciplines can also discriminate between service classes. Research has shown that packet delay can be bounded with a generalized processor sharing service discipline, yet no network delay guarantee is based on the queuing arrangements along the packet transit path.

In general, QoS is needed when the demand for network resources outpaces the availability of resources. Therefore, policies are implemented to control whether a service request can proceed. If a request is allowed, efforts are made to ensure that the requested resources are available for it. The control mechanisms determining whether to admit the request, and the control mechanisms for subsequent policy instrumentation, are not free. If the cost of the control mechanisms outweighs the benefits derived from such control, there is no incentive to provide the control. The economics of the Internet service industry have not yet provided the incentives to move away from the current best-effort, flat-pricing service model.

Unlike security, which can be applied at the tunnel endpoints only, QoS must be applied at every hop that a packet visits. If the QoS guarantee is not applied at any one of the intermediate hops, the effort to provide QoS by all other devices along the transit path becomes moot. The fact that an IP network is connectionless also poses a major problem for QoS. In an IP network, the intermediate routers along the packet transit path are not required to keep state information, including the parameters used to enforce the QoS guarantees.

MPLS—together with its signaling protocols, either RSVP-TE (Resource Reservation Protocol-Traffic Engineering) or CR-LDP (Constraint-based Routing using the LDP)—is the important mechanism for enabling QoS in the Internet. The signaling mechanism establishes state information within the network. MPLS provides the traffic classification mechanism so that IP packets can be classified to correspond to the state established within the network. Various traffic shaping and queuing mechanisms (such as token bucket and weighted fair queuing) are then applied to the classified traffic according to the state information, thus guaranteeing that the service provided is the service requested.

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