QUALITYOF SERVICE · P ART 1 - PacketLife.net

1 Jeremy OFSERVICE PART1 quality of SERVICE ModelsLayer 2 QoS MarkingsMediumEthernetClass of SERVICE (CoS)NameType3-bit field in headerFrame RelayDiscard Eligibility (DE)1-bit drop eligibility flagBest Effort No QoS policies are implementedIntegrated services (IntServ)Resource Reservation Protocol (RSVP) is used to reserve bandwidth per-flow across all nodes in a pathDifferentiated services (DiffServ)Packets are individually classified and marked; policy decisions are made independently by each node in a pathIP Type of SERVICE (TOS) Loss Priority (CLP)Traffic Class (TC)1-bit drop eligibility flag3-bit field compatible with QoS MarkingsIP PrecedenceThe first three bits of the IP TOS field; limited to 8 traffic classesDifferentiated services Code Point (DSCP)The first six bits of the IP TOS are evaluated to provide more granular classification; backward-compatible with IP PrecedenceQoS FlowchartHardware QueueQueuingDecisionSchedulerSoftware QueueNoYesSoftware QueueSoftware QueueHWQueueFull?

2 TerminologyPer-Hop Behavior (PHB)The individual QoS action performed at each independent DiffServ nodeTrust Boundary Beyond this, inbound QoS markings are not trustedTail Drop Occurs when a packet is dropped because a queue is fullPolicingImposes an artificial ceiling on the amount of bandwidth that may be consumed; traffic exceeding the policer rate is reclassified or droppedShapingSimilar to policing but buffers excess traffic for delayed transmission; makes more efficient use of bandwidth but introduces a delayDSCP Per-Hop BehaviorsClass Selector (CS) Backward-compatible with IP Precedence valuesAssured Forwarding (AF) Four classes with variable drop preferencesExpedited Forwarding (EF) Priority queuing for delay-sensitive trafficCongestion AvoidanceRandom Early Detection (RED)Packets are randomly dropped before a queue is full to prevent tail drop; mitigates TCP synchronizationWeighted RED (WRED)RED with the added capability of recognizing prioritized traffic based on its markingTCP SynchronizationFlows adjust TCP window sizes in synch, making inefficient use of a linkClass-Based WRED (CBWRED)WRED employed inside a class-based WFQ (CBWFQ) Jeremy OFSERVICE PART2 Queuing ComparisonDefault on Interfaces>2 MbpsFIFON umber of Queues1 Configurable ClassesBandwidth AllocationProvides for Minimal DelayModern ImplementationNoAutomaticNoYesNoPQ4 YesAutomaticYesNoNoCQConfiguredYesConfig uredNoNo<=2 MbpsWFQD ynamicNoAutomaticNoNoNoCBWFQC onfiguredYesConfiguredNoYesNoLLQC onfiguredYesConfiguredYesYesFirst In First Out (FIFO)Priority Queuing (PQ)LLQ Config Example!

3 Match packets by DSCP valueclass-map match-all Voicematch dscp ef!class-map match-all Call-Signalingmatch dscp cs3!class-map match-any Critical-Appsmatch dscp af21 af22!! Match packets by access listclass-map match-all Scavengermatch access-group name OtherClass Definitionspolicy-map Fooclass Voice! Priority queue policed to 33%priority percent 33class Call-Signaling! Allocate 5% of bandwidthbandwidth percent 5class Critical-Appsbandwidth percent 20! Extend queue size to 96 packetsqueue-limit 96class Scavenger! Police to 64 kbpspolice cir 64000conform-action transmit exceed-action drop class class-default! Enable WFQfair-queue! Enable WRED random-detectPolicy Creationinterface Serial0! Apply the policy in or outservice-policy output FooPolicy ApplicationLLQ Config Exampleshow policy-map [interface]Show interfaceshow queue <interface>HighMediumNormalLowHardwareQueueHardware QueueTxRingCustom Queuing (CQ)Weighted Fair Queuing (WFQ) Packets are transmitted in the order they are processed No prioritization is provided Default queuing method on high-speed (>2 Mbps) interfaces Configurable with the tx-ring-limitinterface config command Provides four static queues which cannot be reconfigured Higher-priority queues are always emptied before lower-priority queues Lower-priority queues are at risk of bandwidth starvation Rotates through queues using Weighted Round Robin (WRR)

4 Processes a configurable number of bytes from each queue per turn Prevents queue starvation but does not provide for delay-sensitive traffic Queues are dynamically created per flow to ensure fair processing Statistically drops packets from aggressive flows more often No support for delay-sensitive trafficClass-Based WFQ (CBWFQ)Low Latency Queuing (LLQ) WFQ with administratively configured queues Each queue is allocated an amount/percentage of bandwidth No support for delay-sensitive traffic CBWFQ with the addition of a policed strict-priority queue Highly configurable while still supporting delay-sensitive trafficFlow 1 Flow 2 Flow B/cycle4500 B/cycle1500 B/cycleQueue AQueue BQueue CHardwareQueue512 Kbps Min1024 Kbps MinRemainderQueue AQueue BDefaultHardwareQueue512 Kbps Min1024 Kbps MinRemainderQueue AQueue BDefault512 Kbps MaxPriorityHardwareQueueShow mls qos