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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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a specific b<strong>and</strong>width, a maximum packet delay, or a maximum packet loss rate.<br />

In the IS implementation, the information from Tspec <strong>and</strong> Rspec is used in the<br />

packet scheduler.<br />

8.2.2 Controlled Load Service<br />

The Controlled Load Service is intended to support the class of applications that<br />

are highly sensitive to overloaded conditions in the Internet, such as real-time<br />

applications. These applications work well on underloaded networks, but<br />

degrade quickly under overloaded conditions. If an application uses the<br />

Controlled Load Service, the performance of a specific data flow does not<br />

degrade if the network load increases.<br />

The Controlled Load Service offers only one service level, which is intentionally<br />

minimal. There are no optional features or capabilities in the specification. The<br />

service offers only a single function. It approximates best-effort service over<br />

lightly loaded networks. This means that applications that make QoS<br />

reservations using Controlled Load Services are provided with service closely<br />

equivalent to the service provided to uncontrolled (best-effort) traffic under lightly<br />

loaded conditions. In this context, lightly loaded conditions means that a very<br />

high percentage of transmitted packets will be successfully delivered to the<br />

destination, <strong>and</strong> the transit delay for a very high percentage of the delivered<br />

packets will not greatly exceed the minimum transit delay.<br />

Each router in a network that accepts requests for Controlled Load Services<br />

must ensure that adequate b<strong>and</strong>width <strong>and</strong> packet processing resources are<br />

available to h<strong>and</strong>le QoS reservation requests. This can be realized with active<br />

admission control. Before a router accepts a new QoS reservation, represented<br />

by the Tspec, it must consider all important resources, such as link b<strong>and</strong>width,<br />

router or switch port buffer space, <strong>and</strong> computational capacity of the packet<br />

forwarding.<br />

The Controlled Load Service class does not accept or make use of specific target<br />

values for control parameters, such as b<strong>and</strong>width, delay, or loss. Applications<br />

that use Controlled Load Services must guard against small amounts of packet<br />

loss <strong>and</strong> packet delays.<br />

QoS reservations using Controlled Load Services need to provide a Tspec that<br />

consists of the token bucket parameters r <strong>and</strong> b, as well as the minimum policed<br />

unit m <strong>and</strong> the maximum packet size M. An Rspec is not necessary, because<br />

Controlled Load Services does not provide functions to reserve a fixed<br />

b<strong>and</strong>width or guarantee minimum packet delays. Controlled Load Service<br />

provides QoS control only for traffic that conforms to the Tspec that was provided<br />

at setup time. This means that the service guarantees only apply for packets that<br />

294 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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