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1NANOG 2006
Overview of QoS inPacket-based IP and MPLS Networks
Paresh ShahUtpal Mukhopadhyaya
Arun Sathiamurthi
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2NANOG 2006
Agenda
• Introduction• QoS Service Models• DiffServ QoS Techniques• MPLS QoS• Summary
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3NANOG 2006
• Introduction• QoS Service ModelsQoS Service Models• DiffServ QoS TechniquesDiffServ QoS Techniques• MPLS QoSMPLS QoS• SummarySummary
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4NANOG 2006
What is Quality of Service?
“
”
QoS repres ents the s e t o f techniques neces s ary to manage network bandwidth, de lay, jitter, and packet los s .
From a bus ines s pers pective , it is es s ential to as s ure that the critical applications are guaranteed the network res ources they need, des pite varying network traffic load.
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5NANOG 2006
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 type
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6NANOG 2006
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• aggregated
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7NANOG 2006
QoS Triangle
Identify Traffic type
Apply QoS technique
Determine QoS parameters
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8NANOG 2006
QoS Approaches
• Fine-grained approach• flow-based (individual flows)
• Coarse-grained approach• aggregated (large number of flows)
• Leads to two different QoS Models
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9NANOG 2006
• Introduction• QoS Service ModelsQoS Service Models• DiffServ QoS TechniquesDiffServ QoS Techniques• MPLS and QoSMPLS and QoS• SummarySummary
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10NANOG 2006
QoS Service Models
• Best effort (No QoS)• Integrated services (Hard QoS)• Differentiated services (Soft QoS)
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11NANOG 2006
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 model
Pay for what you want, and get exactly that
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12NANOG 2006
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 information
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13NANOG 2006
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 planning
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14NANOG 2006
Differentiated Services Architecture
Traffic Conditioning Agreement (TCA)Classification/Marking/Policing/Shaping
Per-Hop Behavior (PHB)Queuing/Dropping
Ingress Node
Interior Node
Egress Node
TCAPHB
PHB TCAPHB
DiffServ Domain
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15NANOG 2006
• Introduction• QoS Service ModelsQoS Service Models• DiffServ QoS TechniquesDiffServ QoS Techniques• MPLS and QoSMPLS and QoS• SummarySummary
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16NANOG 2006
IETF DiffServ Model
• Re-define TOS byte in IP header to Differentiated Services Code 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 Core
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17NANOG 2006
IP TOS vs IP DSCP
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18NANOG 2006
Diffserv Class Selector
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19NANOG 2006
DiffServ Traffic Conditioner
Classifier Marker Shaper/Dropper
Meter
Packets
Shaped
• Classifier: Selects a packet in a traffic stream based on the content of some portion of the packet header
• Meter: Checks compliance to traffic parameters (eg Token Bucket) and passes result to the 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 the
stream into compliance with a traffic profile
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20NANOG 2006
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)
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21NANOG 2006
Traffic Metering
• Traffic Rate Management in network boundary nodes• Traffic Metering measures traffic
Does not alter traffic characteristics
Reports compliance results to Shaper or Dropper
• Uses Token Bucket Scheme to measure traffic– Mean or Committed Information Rate– Conformed Burst size– Extended Burst size
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22NANOG 2006
Policing and Shaping
• PoliceSends conforming traffic and allows bursts
Drops 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 exhausted
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23NANOG 2006
Policing and Shaping
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24NANOG 2006
Policing
• Uses the token bucket schemetoken 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 are
said to conform• Packets arriving with insufficient tokens in the bucket are
said to exceed
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25NANOG 2006
Token Bucket in Policing
p
Tokens
BOverflowTokens
PacketsArriving Conform
Exceed
B—Burst Sizep—Token Arrival Rate
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26NANOG 2006
Shaping
• Uses the token bucket schemetoken bucket scheme • Smoothes bursty traffic to meet CIR through buffering• Queued Packets transmitted as tokens are available
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27NANOG 2006
Token Bucket in Traffic Shaping
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28NANOG 2006
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 technique
RED / WRED
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29NANOG 2006
Queuing/Scheduling
• Scheduling mechanisms guarantee BW for flows• More bandwidth guarantee means dequeue more from one
queue or set of queues.• De-queue depends on weights allocated to queues
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30NANOG 2006
Congestion Avoidance/Management
• When there is congestion what should we do?Tail drop i.e. Packets dropped due to Max Queue Length
Drop selectively but based on IP Prec / DSCP bit
• Congestion control mechanisms for TCP traffic– Adaptive– Dominant transport protocol
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31NANOG 2006
The Problem of Congestion
• 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 itself
Throughput
Congestion
Controlled CongestionControlled Congestion
Uncontrolled CongestionUncontrolled Congestion
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32NANOG 2006
TCP traffic and Congestion
• Congestion window based on slow-startSender / Receive negotiation
• Packet loss indicator of congestion– Congestion window re-sizing– Source throttles traffic
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33NANOG 2006
Global Synchronization
Time
Queue Utilization100%
Tail Drop
3 Traffic Flows Start at Different Times
Another Traffic FlowStarts at This Point
• Global synchronization is many connections going through TCP Slow-Start mode at the same time
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34NANOG 2006
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 connections
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35NANOG 2006
RED—Packet-Drop Probability
• Packets are dropped sufficiently frequently to control the average queue size
• The probability that a packet is dropped from a connection is proportional to the amount of packets sent by the connection
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36NANOG 2006
RED
Packet DiscardProbability
AverageQueue Size
MaximumThreshold
MinimumThreshold
Adjustable
1
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37NANOG 2006
Weighted RED (WRED)
• WRED combines REDRED with IP PrecedenceIP Precedence or DSCP to implement multiple service classes
• Each service class has a defined min and max thresholds, and drop rates
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38NANOG 2006
WRED Service Profile Example
Packet DiscardProbability
1
AverageQueue Size
Two Service Levels are Shown;
Up to SixCan Be Defined
StandardMin
StandardMax
PremiumMin
PremiumMax
StandardServiceProfile Premium
ServiceProfile
Adjustable
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39NANOG 2006
When Should WRED be Used?
• Where the bulk of your traffic is TCP as oppose to UDP
Only TCP will react to a packet drop; UDP will notOnly TCP will react to a packet drop; UDP will not
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40NANOG 2006
• Introduction• QoS Service ModelsQoS Service Models• DiffServ QoS TechniquesDiffServ QoS Techniques• MPLS and QoSMPLS and QoS• SummarySummary
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41NANOG 2006
MPLS Diffserv
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42NANOG 2006
MPLS DiffServ Architecture
• MPLS does NOT define new QoS architectures• MPLS QoS uses Differentiated Services (DiffServ) architecture
defined for IP QoS (RFC 2475)• MPLS DiffServ is defined in RFC3270
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43NANOG 2006
DiffServ Scalability via Aggregation
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44NANOG 2006
What’s Unchanged in MPLS DiffServ• When Compared to IP DiffServ
– Functional components (TCA/PHB) and where they are used
Classification, marking, policing, and shaping at network boundaries
Buffer management and packet scheduling mechanisms used to implement PHB
– PHB definitions• EF: low delay/jitter/loss• AF: low loss• BE: No guarantees (best effort)
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45NANOG 2006
What’s new in MPLS DiffServ ?IP DiffServ Domain
• Prec/DSCP field is not directly visible to MPLS Label Switch Routers (they forward based on MPLS Header and EXP field)
• Information on DiffServ must be made visible to LSR in MPLS Header using EXP field / Label.
• How do we map DSCP into EXP ? Interaction between them.
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46NANOG 2006
DSCP to EXP Mapping
RFC3270 does not recommend specific EXP values for DS PHBs (EF/AF/CS)
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47NANOG 2006
MPLS DiffServ – RFC 3270
• Problem: IP DSCP = 6 bits while MPLS EXP = 3bits• Solution: where 8 or less PHBs are used, those can be
mapped into EXP fielduse “E-LSPs with preconfigured mapping”
• Solution: where more than 8 PHBs are used in core, those need to be mapped in both “label and EXP” ”L-LSPs” are needed
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48NANOG 2006
Types of Label Switched Paths
• Both E-LSP and L-LSP can use LDP or RSVP for label distribution
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49NANOG 2006
MPLS DiffServ Topology
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50NANOG 2006
MPLS DiffServ Tunneling Modes
• Based on RFC 3270• Modes
Uniform
Short-Pipe
Pipe
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51NANOG 2006
Uniform Mode
• Assume the entire admin domain of a Service Provider is under a single DiffServ domain
• Then, it is likely a requirement to keep the colouring information uniform (keep it when going from IP to IP, IP to MPLS, MPLS to MPLS, MPLS to IP).
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52NANOG 2006
Uniform Mode
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53NANOG 2006
Short-Pipe Mode
• Assume an ISP network implementing a DiffServ Policy
• Assume its customer network implementing another policy
• Requirement: Transparency: the customer wants to preserve its DSCP intact Uniformity: within the IP/MPLS backbone, the SP wants to have a uniform diffserv domain
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54NANOG 2006
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 the customer, hence the ‘short-pipe’ naming.
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55NANOG 2006
Pipe Mode
• Exactly the same case as Short-Pipe• However, the SP wants to drive the outbound
classification for WFQ/WRED on the egress interface from a PE to a CE based on its DiffServ policy (EXP)
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56NANOG 2006
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 frame
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57NANOG 2006
MPLS TE and Diffserv
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58NANOG 2006
BW Optimization and Congestion Mgmt. in Parallel
TE + DiffServ
•Spread Traffic around with more flexibility than the IGP Offers• Reserve per-class bandwidth, sort of• Manage Unfairness During Temporary Congestion
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59NANOG 2006
Why TE: Shortest Path and Congestion
R8
R2
R6
R3
R4
R7
R5
R1
OC3(155Mbps)
OC3(155Mbps)E3
(34Mbps)
GigE(1Gbps)
GigE(1Gbps)
GigE(1Gbps)
20MbpsTraffic to R5
40MbpsTraffic to R5
60MbpsAggregate
26MbpsDrops!
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60NANOG 2006
The TE Solution
• MPLS Labels can be used to engineer explicit paths• Tunnels are UNI-DIRECTIONAL
Normal path: R8 R2 R3 R4 R5Tunnel path: R1 R2 R6 R7 R4
R8
R2
R6
R3
R4
R7
R5
R1
40MbpsTraffic to R5
20Mbps trafficto R5 from R8
40Mbps trafficto R1 from R8
20MbpsTraffic to R5
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61NANOG 2006
How MPLS TE Works
Head end
Mid-point Tail end
• Explicit routing• Constrained-based routing• Admission control• Protection capabilities• RSVP-TE to establish LSPs• ISIS and OSPF extensions to
advertise link attributes
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62NANOG 2006
DiffServ-Aware TE (DS-TE)
• Regular TE allows for one reservable bandwidth amount per link
• DS-TE allows for more than one reservable bandwidth amount per link
• Brings per-class dimension to TE• Basic idea: connect PHB class bandwidth to DS-TE
bandwidth sub-pool
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63NANOG 2006
DiffServ-Aware TE
• Per-class constrained-based routing
• Per-class admission control
Best-Effort TE LSPLow-Latency TE LSP with Reserved BW
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64NANOG 2006
DiffServ-Aware TE
• Link BW distributed in pools or BW Constraints (BC)
• Up to 8 BW pools• Different BW pool models
Maximum Reservable Bandwidth
(MRB)
DS-TE BW Allocation
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65NANOG 2006
DS-TE BW Pools – Maximum Allocation Model (MAM)
BC0: 20% Best Effort BC1: 50% PremiumBC2: 30% Voice
All classes
Maximum Reservable Bandwidth
(MRB)
BC2
BC1
BC0
Class2
Class1
Class3
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66NANOG 2006
DS-TE BW Pools – Russian Dolls Model (RDM)
BC2
BC1
BC0
• BW pool applies to one or more classes
• Global BW pool (BC0) equals MRB
• BC0..BCn used for computing unreserved BW for class n
All classes
(class1+
Class2 +
Class3)Class2 +
Class3Class3
Maximum Reservable Bandwidth
(MRB)
BC0: MRB Best Effort + Premium + VoiceBC1: 50% Premium + VoiceBC2: 30% Voice
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67NANOG 2006
Aggregate TE in DiffServ Network
Traffic on TE tunnel follows DiffServ per hop behaviours
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68NANOG 2006
DiffServ TE
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69NANOG 2006
DS TE and QoS
“DiffServ TE does not preclude the necessity of configuring PHB QoS in the TE path. DiffServ TE operates in conjunction with QoS mechanisms”
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70NANOG 2006
• Introduction• QoS Service ModelsQoS Service Models• DiffServ QoS TechniquesDiffServ QoS Techniques• MPLS QoSMPLS QoS• SummarySummary
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71NANOG 2006
Summary
• QoS techniquesEffective allocation of network resources
• IP Diff ServService Differentiation
• MPLS & Diff ServBuilds scalable networks for SP
• DiffServ Tunneling ModesScalable and flexible QoS optionsSupports Draft Tunneling Mode RFC
• Diff Serv TEProvides strict point-to-point guaranteesPipe Model
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72NANOG 2006
Q & A