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In a bandwidth-managed network in which QoS (quality of service) is being provided, a mechanism is required that can estimate the level of QoS that a new user session will need and whether there is enough bandwidth available to service that session. If bandwidth is available, the session is admitted. Think about getting a seat on an airplane. The reservation agent provides admission by issuing a ticket if there are seats available.

There will be competition among sessions and flows for network bandwidth. An easy method for admission control is to service flows based on arrival time. This is often called CAC (capacity-based admission control). With CAC, new flows are indiscriminately admitted until capacity is exhausted (First Come First Admitted). New flows are not admitted until capacity is available.

Another method is to allow some flows to preempt others based on priority settings. RFC 2751 (Signaled Preemption Priority Policy Element, January 2000) and RFC 2815 (Integrated Service Mappings on IEEE 802 Networks, May 2000) describe admission control in a priority-queued system. However, adding traffic to a higher priority queue can affect the performance of lower-priority classes, so priority queued systems must use sophisticated admission control algorithms.

One aspect of admission control is the ability to monitor, control, and enforce the use of network resources and services with policy-based management. The criteria for policy-based management includes identifying users and applications or identifying traffic based on how, when, and where it enters the network. RFC 2753 (A Framework for Policy-based Admission Control, January 2000) describes a policy-based control framework for admission control.

RFC 2814 (Subnet Bandwidth Manager: A Protocol for RSVP-based Admission Control over IEEE 802-style networks, May 2000) describes a signaling method and protocol for LAN-based admission control over RSVP flows.

RFC 2816 (A Framework for Integrated Services Over Shared and Switched IEEE 802 LAN Technologies, May 2000) discusses methods for supporting RSVP in LAN environments. It describes a BA (bandwidth allocator) that is responsible for performing admission control and maintaining state about the allocation of resources in the subnet. End stations request services such as bandwidth reservation that are processed by the BA.