Overview Of QoS In IP And MPLS Networks - NANOG

Overview of QoS inPacket-based IP and MPLS NetworksParesh ShahUtpal MukhopadhyayaArun SathiamurthiNANOG 20061

Agenda Introduction QoS Service Models DiffServ QoS Techniques MPLS QoS SummaryNANOG 20062

Introduction QoS Service Models DiffServ QoS Techniques MPLS QoS SummaryNANOG 20063

What is Quality of Service?“Qo S re pre s e nts the s e t o f te c hnique s ne c e s s ary tomanag e ne two rk bandwidth, de lay, jitter, and pac ke tlo s s .Fro m a bus ine s s pe rs pe c tive , it is e s s e ntial to as s urethat the c ritic al applic atio ns are g uarante e d thene two rk re s o urc e s the y ne e d, de s pite varyingne two rk traffic lo ad.”NANOG 20064

Traffic Characterization Identify traffic sources and types Need for appropriate handling Realtime and Non-realtime Voice (Delay sensitive) Video (Bandwidth intensive) Data (Loss sensitive) HTTP, FTP, SMTP Bursty and Constant type Multi-service traffic: IP, MPLS Single or Multiple flows of the same typeNANOG 20065

QoS Requirements Traffic influencing parameters Latency, Jitter, Loss Management of finite resources Rate Control Queuing and Scheduling Congestion Management Admission Control Routing Control Traffic protection Service Level Agreement (SLA) per-flow aggregatedNANOG 20066

QoS TriangleIdentify Traffic typeDetermine QoSparametersApply QoS techniqueNANOG 20067

QoS Approaches Fine-grained approach flow-based (individual flows) Coarse-grained approach aggregated (large number of flows) Leads to two different QoS ModelsNANOG 20068

Introduction QoS Service Models DiffServ QoS Techniques MPLS and QoS SummaryNANOG 20069

QoS Service Models Best effort (No QoS) Integrated services (Hard QoS) Differentiated services (Soft QoS)NANOG 200610

Best Effort Model – Traditional Internet “We’ll do the best we can”But messages may be lost en route Traditional datagram model Not a traditional telephone company modelPay for what you want, and get exactly thatNANOG 200611

Integrated Services Model IntServ Architecture (RFC 1633) Hard QoS Guarantees per-flow QoS Strict Bandwidth Reservations Needs Signaling to accomplish Path Reservation– Resource Reservation Protocol RSVP (RFC 2205)– PATH/RESV messages Admission Control Must be configured on every router along the path Works well on small-scale– Has issues with scaling with large number of flows– Requires devices to retain state informationNANOG 200612

Differentiated Services Model DiffServ Architecture – RFC 2475 Scales well with large flows through aggregation Creates a means for traffic conditioning (TC) Defines per-hop behavior (PHB) Edge nodes perform TC– Allows core routers to do more important processing tasks Tough to predict end-to-end behavior– Especially with multiple DiffServ Domains– DiffServ implementation versus Capacity planningNANOG 200613

Differentiated Services ArchitectureDiffServ APHBTraffic Conditioning Agreement (TCA)Per-Hop Behavior g/DroppingNANOG 200614

Introduction QoS Service Models DiffServ QoS Techniques MPLS and QoS SummaryNANOG 200615

IETF DiffServ Model Re-define TOS byte in IP header to Differentiated ServicesCode Point (DSCP) Uses 6 bits to categorize traffic into “Behavior Aggregates” Defines a number of “Per Hop Behaviors” applied to links Two-Ingredient Recipe: Condition the Traffic at the Edges Invoke the PHBs in the CoreNANOG 200616

IP TOS vs IP DSCPNANOG 200617

Diffserv Class SelectorNANOG 200618

DiffServ Traffic r/Dropper Classifier: Selects a packet in a traffic stream based on the content of some portion ofthe packet header Meter: Checks compliance to traffic parameters (eg Token Bucket) and passes result tothe marker and shaper/dropper to trigger a particular action for in/out of profile packets Marker: Writes/rewrites DSCP Shaper: Delays some packets to be compliant with a profile Dropper: Discards some or all of the packets in a traffic stream in order to bring thestream into compliance with a traffic profileNANOG 200619

Classification and Marking Classification– Identification based on field(s) in a packet– Flow identification parameters Src/Dest. Address, Source/Dest. Port, Protocol– IP Precedence / DSCP based Marking– Marking/Coloring packets to indicate class– Application marked or node configured IP Precedence or DSCP MPLS EXP Other instances (FR-DE and ATM-CLP)NANOG 200620

Traffic Metering Traffic Rate Management in network boundary nodes Traffic Metering measures trafficDoes not alter traffic characteristicsReports compliance results to Shaper or Dropper Uses Token Bucket Scheme to measure traffic– Mean or Committed Information Rate– Conformed Burst size– Extended Burst sizeNANOG 200621

Policing and Shaping PoliceSends conforming traffic and allows burstsDrops non-conforming traffic (due to lack of tokens)Provision for Packet re-marking Shaping– Smoothes traffic but increases overall latency– Buffers packets when tokens are exhaustedNANOG 200622

Policing and ShapingNANOG 200623

Policing Uses the token bucket scheme Tokens added to the bucket at the committed rate Depth of the bucket determines the burst size Packets arriving with sufficient tokens in the bucket aresaid to conform Packets arriving with insufficient tokens in the bucket aresaid to exceedNANOG 200624

Token Bucket in PolicingTokensB—Burst SizepOverflowTokensp—Token Arrival RateBPacketsArrivingConformExceedNANOG 200625

Shaping Uses the token bucket scheme Smoothes bursty traffic to meet CIR through buffering Queued Packets transmitted as tokens are availableNANOG 200626

Token Bucket in Traffic ShapingNANOG 200627

Per-Hop Behavior (PHB) PHB relates to resource allocation for a flow Resource allocation is typically Bandwidth Queuing / Scheduling mechanisms:– FIFO / WFQ / MWRR / MDRR PHB also involves determining a packet drop policy Congestion avoidance schemes – primary techniqueRED / WREDNANOG 200628

Queuing/Scheduling Scheduling mechanisms guarantee BW for flows More bandwidth guarantee means dequeue more from onequeue or set of queues. De-queue depends on weights allocated to queuesNANOG 200629

Congestion Avoidance/Management When there is congestion what should we do?Tail drop i.e. Packets dropped due to Max Queue LengthDrop selectively but based on IP Prec / DSCP bit Congestion control mechanisms for TCP traffic– Adaptive– Dominant transport protocolNANOG 200630

The Problem of CongestionControlled CongestionThroughputUncontrolled CongestionCongestion Uncontrolled, congestion will seriously degrade performanceThe system buffers fill upPackets are dropped, resulting in retransmissionsThis causes more packet loss and increased latencyThe problem builds on itselfNANOG 200631

TCP traffic and Congestion Congestion window based on slow-startSender / Receive negotiation Packet loss indicator of congestion– Congestion window re-sizing– Source throttles trafficNANOG 200632

Global SynchronizationQueueUtilization100%TimeTail Drop3 Traffic Flows Startat Different TimesAnother Traffic FlowStarts at This Point Global synchronization is many connections going through TCP Slow-Startmode at the same timeNANOG 200633

Random Early Detect (RED) A congestion avoidance algorithm Designed to work with a transport protocol like TCP Minimize packet delay jitter by controlling average queue size Uses Packet drop probability and Avg. Queue size Avoids global synchronization of many connectionsNANOG 200634

RED—Packet-Drop Probability Packets are dropped sufficiently frequently to controlthe average queue size The probability that a packet is dropped from aconnection is proportional to the amountof packets sent by the connectionNANOG 200635

holdNANOG 2006MaximumThresholdAverageQueueSize36

Weighted RED (WRED) WRED combines RED with IP Precedence or DSCPto implement multiple service classes Each service class has a defined min and maxthresholds, and drop ratesNANOG 200637

WRED Service Profile ExampleTwo ServiceLevels are Shown;Up to SixCan Be fileAdjustableStandardMinNANOG ageQueueSizePremiumMax38

When Should WRED be Used? Where the bulk of your traffic is TCP as oppose toUDPOnly TCP will react to a packet drop; UDP will notNANOG 200639

Introduction QoS Service Models DiffServ QoS Techniques MPLS and QoS SummaryNANOG 200640

MPLS DiffservNANOG 200641

MPLS DiffServ Architecture MPLS does NOT define new QoS architectures MPLS QoS uses Differentiated Services (DiffServ) architecturedefined for IP QoS (RFC 2475) MPLS DiffServ is defined in RFC3270NANOG 200642

DiffServ Scalability via AggregationNANOG 200643

What’s Unchanged in MPLS DiffServ When Compared to IP DiffServ– Functional components (TCA/PHB) and where they are usedClassification, marking, policing, and shaping at networkboundariesBuffer management and packet scheduling mechanisms usedto implement PHB– PHB definitions EF: low delay/jitter/loss AF: low loss BE: No guarantees (best effort)NANOG 200644

What’s new in MPLS DiffServ ?IP DiffServ Domain Prec/DSCP field is not directly visible to MPLS Label Switch Routers (theyforward based on MPLS Header and EXP field) Information on DiffServ must be made visible to LSR in MPLS Header usingEXP field / Label. How do we map DSCP into EXP ? Interaction between them.NANOG 200645

DSCP to EXP MappingRFC3270 does not recommend specific EXP values for DS PHBs(EF/AF/CS)NANOG 200646

MPLS DiffServ – RFC 3270 Problem: IP DSCP 6 bits while MPLS EXP 3bits Solution: where8 or less PHBs are used, those can bemapped into EXP field use “E-LSPs with preconfiguredmapping” Solution: where more than 8 PHBs are used in core,those need to be mapped in both “label and EXP” ”LLSPs” are neededNANOG 200647

Types of Label Switched Paths Both E-LSP and L-LSP can use LDP or RSVP for label distributionNANOG 200648

MPLS DiffServ TopologyNANOG 200649

MPLS DiffServ Tunneling Modes Based on RFC 3270 ModesUniformShort-PipePipeNANOG 200650

Uniform Mode Assume the entire admin domain of a ServiceProvider is under a single DiffServ domain Then, it is likely a requirement to keep thecolouring information uniform (keep it when goingfrom IP to IP, IP to MPLS, MPLS to MPLS, MPLS toIP).NANOG 200651

Uniform ModeNANOG 200652

Short-Pipe Mode Assume an ISP network implementing a DiffServPolicy Assume its customer network implementinganother policy Requirement:Transparency: the customer wants to preserve its DSCPintactUniformity: within the IP/MPLS backbone, the SP wants tohave a uniform diffserv domainNANOG 200653

Short-Pipe Mode The PHB of the topmost popped label is copied into the new top label Note that policy applied on the egress interface of the egress PE is based on the DSCP of thecustomer, hence the ‘short-pipe’ naming.NANOG 200654

Pipe Mode Exactly the same case as Short-Pipe However, the SP wants to drive the outboundclassification for WFQ/WRED on the egressinterface from a PE to a CE based on its DiffServpolicy (EXP)NANOG 200655

Pipe Mode The PHBs of the topmost popped label is copied into the new top label Classification is based on mpls-exp field (EXP 0) of the topmost received MPLS frameNANOG 200656

MPLS TE and DiffservNANOG 200657

BW Optimization and Congestion Mgmt. inParallelTE DiffServ Spread Traffic around with more flexibility than theIGP Offers Reserve per-class bandwidth, sort of Manage Unfairness During Temporary CongestionNANOG 200658

Why TE: Shortest Path and Congestion20MbpsTraffic to igE(1Gbps)R740MbpsTraffic to R5NANOG 200659

The TE Solution20MbpsTraffic to R5R3R820Mbps trafficto R5 from R8R4R5R240Mbps trafficto R1 from R8R1R6R740MbpsTraffic to R5 MPLS Labels can be used to engineer explicit paths Tunnels are UNI-DIRECTIONALNormal path: R8 R2 R3 R4 R5Tunnel path: R1 R2 R6 R7 R4NANOG 200660

How MPLS TE Works Explicit routing Constrained-based routing Admission control Protection capabilities RSVP-TE to establish LSPs ISIS and OSPF extensions toadvertise link attributesMid-pointTail endHead endNANOG 200661