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Cytat
Do celu tam się wysiada. Lec Stanisław Jerzy (pierw. de Tusch-Letz, 1909-1966)
A bogowie grają w kości i nie pytają wcale czy chcesz przyłączyć się do gry (. . . ) Bogowie kpią sobie z twojego poukładanego życia (. . . ) nie przejmują się zbytnio ani naszymi planami na przyszłość ani oczekiwaniami. Gdzieś we wszechświecie rzucają kości i przypadkiem wypada twoja kolej. I odtąd zwyciężyć lub przegrać - to tylko kwestia szczęścia. Borys Pasternak
Idąc po kurzych jajach nie podskakuj. Przysłowie szkockie
I Herkules nie poradzi przeciwko wielu.
Dialog półinteligentów równa się monologowi ćwierćinteligenta. Stanisław Jerzy Lec (pierw. de Tusch - Letz, 1909-1966)
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.MarkerDS markers set the DS byte of the incoming IP packets to a particularbit pattern.The PHB is set in the first 6 bits of the DS byte so that themarked packets are forwarded inside of the DS domain accordingly tothe SLA between service provider and customer.Shaper/DropperPacket shapers and droppers cause conformance to some configuredtraffic properties, for example a token bucket filter as described in10.2.1, Service Classes on page 508.They use different methods tobring the stream into compliance with a traffic profile.Shapers delaysome or all of the packets.A shaper usually has a finite-size buffer, andpackets may be discarded if there is not sufficient buffer space to holdthe delayed packets.Droppers discard some or all of the packets.Thisprocess is know as policing the stream.A dropper can be implementedas a special case of a shaper by setting the shaper buffer size to zeropackets.The traffic conditioner is mainly used in DS boundary components but it can alsobe implemented in an interior component.Figure 299 shows the cooperation of thetraffic conditioner components.MeterShaper/MarkerPackets ClassifierDropper3376A\3376F7SHFigure 299.DS Traffic ConditionerThe traffic conditioner in a boundary component makes sure that packets thattransit the domain are correctly marked to select a PHB from one of the PHBChapter 10.Quality of Service 527 groups supported within the domain.This is necessary because different DSdomains can have different groups of PHBs, which means that the same entry inthe DS byte can be interpreted variably in different domains.For example, in the first domain a packet traverses, all routers have four queueswith different queue priorities (0-3).Packets with a PHB value of three are routedwith the highest priority.But in the next domain the packet travels through, allrouters have eight different queues and all packets with the PHB value of seven arerouted with the highest priority.The packet that was forwarded in the first domainwith high priority has only medium priority in the second domain.This may violatethe SLA contract between customer and service provider.Therefore, the trafficconditioner in the boundary router that connects the two domains, must assure thatthe PHB value is remarked from three to seven if the packet travels from the first tothe second domain.Figure 300 shows an example for the remarking of datapackets that travel trough two different domains.Boundary Router Interior RouterInterior Router7 7 7 733 3 6 625 514 43 32 21 1Classifying Remarking0 03 3 73376X\3376F7SJFigure 300.Remarking of Data PacketsIf a data packet travels through multiple domains, the DS byte can be remarked atevery boundary component to guarantee the QoS that was contracted in the SLA.The SLA contains the details of the Traffic Conditioning Agreement (TCA) thatspecifies classifier rules and temporal properties of a traffic stream.The TCAcontains information how metering, marking, discarding and shaping of packetsmust be done in the traffic conditioner to fulfill the SLA.The TCA information mustbe available in all boundary components of a DS network to guarantee that packetspassing different DS domains receives the same service in each domain.10.3.1.4 DS Interior ComponentsThe interior components inside of a DS domain select the forwarding behavior forpackets based on their DS byte.The interior component is usually a router thatcontains a traffic prioritization algorithm.Because the value of the DS byte normallydoesn't change inside of a DS domain, all interior routers must use the same trafficforwarding policies to comply with the QoS agreement.Data packets with differentPHB values in the DS byte receive different QoSs according to the QoS definitionsfor this PHB.Because all interior routers in a domain use the same policyfunctions for incoming traffic, the traffic conditioning inside of an interior node isdone only by a packet classifier.It selects packets based on their PHB value orother IP header fields and forwards the packets to the queue management andscheduling instance of the node.Figure 301 on page 529 shows the trafficconditioning in an interior node.528 TCP/IP Tutorial and Technical Overview Interior RouterDS Byte Queue Management/PacketsClassifier Scheduler3376A\3376F7SFFigure 301.DS Interior ComponentTraffic classifying and prioritized routing is done in every interior component of aDS domain.After a data packet has crossed a domain, it reaches the boundaryrouter of the next domain and gets possibly remarked to cross this domain with therequested QoS.10.3.1.5 Source DomainsThe IETF DS working group defines a source domain as the domain that containsone or more nodes which originate the traffic that receives a particular service.Traffic sources and intermediate nodes within a source domain can perform trafficclassification and conditioning functions.The traffic that is sent from a sourcedomain may be marked by the traffic sources directly or by intermediate nodesbefore leaving the source domain.In this context it is important to understand that the first PHB marking of the datapackets is not done by the sending application itself.Applications do not notice theavailability of Differentiated Services in a network.Therefore, applications using DSnetworks must not be rewritten to support DS.This is an important difference toIntegrated Services, where most applications support the RSVP protocol directlywhereby some code changes are necessary.The first PHB marking of packets that are sent from an application can be done inthe source host or the first router the packet passes.The packets are identifiedwith their IP address and source port.For example, a customer has an SLA with aservice provider that guarantees a higher priority for the packets sent by an audioapplication.The audio application sends the data packets through a specific portand can be recognized in multi-field classifiers.This classifier type recognizes theIP address and port number of a packet and can distinguish the packets fromdifferent applications.If the host contains a traffic conditioner with an MF classifier,the IP packet can be marked with the appropriate PHB value and consequentlyreceives the QoSs that are requested by the customer.If the host doesn't containa traffic conditioner, the initial marking of the packets is done by the first router inthe source domain that supports traffic conditioning.Figure 302 on page 530shows the initial marking of a packet inside of a host and a router.Chapter 10.Quality of Service 529 Host HostApplication ApplicationData DataTCP TCPTCP Data TCP DataIPClassifier IPIPIP TCP DataTCP DataDSMarkerByteClassifierRouterIPTCP DataDSMarkerByteRouterInitial marking in the hostInitial marking in the router3376X\3376F7SLFigure 302.Initial Marking of Data PacketsIn our example, the DS network has the policy that the packets from the audioapplication should have higher priority than other packets.The sender host canmark the DS field of all outgoing packets with a DS codepoint that indicates higherpriority.Alternatively, the first-hop router directly connected to the sender's hostmay classify the traffic and mark the packets with the correct DS codepoint.Thesource DS domain is responsible for ensuring that the aggregated traffic towards itsprovider DS domain conforms to the SLA between customer and service provider
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