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.The flow descriptor defines the traffic and QoS characteristics for aspecific flow of data packets.In the IS specifications, the flow descriptor consist ofa filter specification (filterspec) and a flow specification (flowspec).Flow DescriptorFilterspec FlowspecTspec Rspec3376a\3376F7S2Figure 285.Flow DescriptorThe filterspec is used to identify the packets that belong to a specific flow with thesender IP address and source port.The information from the filterspec is used inthe packet classifier.The flowspec contains a set of parameters that are called theinvocation information.It is possible to assort the invocation information in twogroups:Traffic Specification (Tspec)Servive Request Specification (Rspec)508 TCP/IP Tutorial and Technical Overview The Tspec describes the traffic characteristics of the requested service.In the ISmodel, this Tspec is represented with a token bucket filter.This principle defines adata-flow control mechanism that adds characters (token) in periodical timeintervals into a buffer (bucket) and allows a data packet to leave the sender only ifthere are at least as many tokens in the bucket as the packet length of the datapacket.This strategy allows a precise control of the time interval between two datapackets on the network.The token bucket system is specified by two parameters:the token rate r, which represents the rate at which tokens are placed into thebucket and the bucket capacity b.Both r and b must be positive.rbData Packet Data Flow3376a\3376F7S3Figure 286.Token Bucket FilterThe parameter r specifies the long term data rate and is measured in bytes of IPdatagrams per second.The value of this parameter can range from 1 byte persecond to 40 terabytes per second.The parameter b specifies the burst data rateallowed by the system and is measured in bytes.The value of this parameter canrange from 1 byte to 250 gigabytes.The range of values allowed for theseparameters is intentionally large to be prepared for future network technologies.The network elements are not expected to support the full range of the values.Traffic that passes the token bucket filter must obey the rule that over all timeperiods T, the amount of data sent does not exceed rT+b, where r and b are thetoken bucket parameters.Two other token bucket parameters are also part of the Tspec.The minimumpoliced unit m and the maximum packet size M.The parameter m specifies theminimum IP datagram size in bytes.Smaller packets are counted against the tokenbucket filter as being of size m.The parameter M specifies the maximum packetsize in bytes that conforms to the Tspec.Network elements must reject a servicerequest if the requested maximum packet size is larger than the MTU size of thelink.Summarizing, the token bucket filter is a policing function that isolates thepackets that conform to the traffic specifications from the ones that do not conform.The Service Request Specification (Rspec) specifies the Quality of Service theapplication wants to request for a specific flow.This information depends on thetype of service and the needs of the QoS requesting application.It may consist ofa specific bandwidth, a maximum packet delay or a maximum packet loss rate.Inthe IS implementation, the information from Tspec and Rspec is used in the packetscheduler.Chapter 10.Quality of Service 509 10.2.1.1 Controlled Load ServiceThe Controlled Load Service is intended to support the class of applications thatare highly sensitive to overloaded conditions in the Internet, such as real-timeapplications.These applications are working well on unloaded networks butdegrade quickly under overloaded conditions.If an application uses the ControlledLoad Service, the performance of a specific data flow does not degrade if thenetwork load increases.The Controlled Load Service offers only one service level which is intentionallyminimal.There are no optional features or capabilities in the specification.Theservice offers only a single function.It approximates best-effort service over lightlyloaded networks.This means that applications that make QoS reservations usingControlled Load Services are provided with service closely equivalent to the serviceprovided to uncontrolled (best-effort) traffic under lightly loaded conditions
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