The services offered by established Internet protocols are unsuitable for real-time applications because
variable queueing delays and packet loss because of congestion can occur at any time.
Integrating real-time QoS mechanisms, which can control end-to-end packet delay, requires admission control
and network management facilities that give network operators the ability to control the sharing of
bandwidth and other QoS resources among different traffic classes. This form of network
operation is known as controlled link sharing.
Research activities, such as the ``Integrated Services Packet Network'' described in [CSZ92], were undertaken to improve the situation. The result is laid down in RFC 1633 [RFC1633]: the Integrated Services (IS) in the Internet Architecture. Braden, Clark, and Shenker, the authors of RFC 1633, construct an Internet architecture that offers controlled link sharing, real-time services, and best-effort services for traditional applications with no real-time requirements.
Although this new architecture substantially changes the service model of the Internet in theory, the migration to the new structure should not prove very difficult because the changes are incorporated in the prevailing IP architecture as an extension of the original architecture.
The authors of [RFC1633] emphasise the separation of the new service model, which
defines the externally visible behaviour of the network (see Section 
), from the
proposed reference implementation framework.
The reference implementation framework consists of a traffic control module (see Section
), which enforces different qualities of service, and a
reservation setup protocol (see Section ), which creates and maintains flow-specific
state on each host and router along the paths of a flow.