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QoS / CoS in the LANQoS / CoS in the LAN

Byron D. Early Chad D. Burnham

University of DenverUTS - Network Services

WestNet – January 15, 2004ASU – Tempe, AZ

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QoS / CoS Definition

• Techniques to enhance network performance for traffic types deemed essential to your institution’s business model:– Bandwidth– Delay– Jitter– Packet Loss

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“Managed Unfairness”

• Goal: predictable end-to-end service levels for selected (“preferred”) traffic– Prioritizing: “preferential packet forwarding”

given to selected network traffic types at the expense of lower priority traffic

– Preferential Treatment Based On:• Traffic type• Institution’s business model (“mission-critical”)

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QoS / CoS Parameters

• Bandwidth:– Bandwidth Management:

•Does not create additional bandwidth•“Reallocate” existing bandwidth to

satisfy requirements of applications•Weakest link determines maximum

available bandwidth

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QoS / CoS Parameters

• Delay (3 Major Types):– Processing: encode/decode; queuing– Serialization: transmission onto circuit– End-to-End: total packet/frame delay

from source-to-destination

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QoS/CoS Parameters (cont.)

• Jitter: “delay variations” from one frame/packet to another for a given flow

• Packet Loss: packets/frames lost in “forwarding path”– Buffer overflows– Transmissions errors– QoS: Traffic policing

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QoS / CoS Parameters (cont.)

• Acceptable Delays (typical):– Telephony: < 150 ms– Video Conferencing (VC): < 500 ms

• Encoding / Decoding: 125-250 ms (each)• WAN Transit: 50-100+ ms• LAN Transit: < 1-5 ms (per node)• Jitter: < 20% on one-way delay

– H.323 Pt-to-Pt: ~300 ms

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Application Requirements

experpt from Cisco “IP QoS”, 2002 by Zdravko Nikolov

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Congestion & Performance

• Network Traffic: unpredictable & “bursty” nature fundamentally drives need for QoS/CoS

• Transmission Queues:– Limited size transmit buffers need overfill

protection• “Tail Drop”: full transmit queue drops all

incoming packets (inefficient TCP windowing)• Interface Queues use QoS to intelligently

manage which packets are dropped

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Interface Queues

• “Intelligently” protect transmit queues from being overwhelmed

• QoS/CoS Techniques: should impact traffic only under CONGESTED conditions– IP Precedence (ToS)– Class-based Weighted Fair Queuing (CBWFQ)– Low Latency Queuing (LLQ)– Etc.

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Why QoS in a Switched Environment?

• Increasing Bandwidth is not a panacea:– High Cost: prohibitive for higher-speed

links– Does not solve “TCP windowing” issue of

taking as much bandwidth as possible– Interactive traffic: requires low delay &

jitter (VoIP, VC)

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Initial QoS Planning

• Identify “congestion points” in campus LAN hierarchy– Switch “uplink speeds”– LAN-to-LAN speed mismatches

• Classify critical applications requiring preferential forwarding in your environment

• Implement QoS techniques at congestion points to match traffic requirements

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Types of QoS / CoS

• Best Effort (BE): no QoS applied to packet/frames along forwarding path– default behavior

• Integrated Services Model (IntServ): end-station or network node signals network neighbors with QoS request

• Differentiated Services Model (Diffserv): network recognizes traffic classes requiring QoS

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Types of QoS / CoS (cont.)

• IntServ & DiffServ models can also be used in combination to achieve end-to-end QoS

• True end-to-end QoS requires by all devices along forwarding path

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IntServ: RSVPIntServ: RSVP

• RFC 1633 / 2205-2215 (RSVP)• Resource Reservation Protocol (RSVP):

– Identifies application (flow)– Signaling determines if required network

resources are available– Admission Control determines if application

(flow) will be granted resources• Common Open Policy Service (COPS; RFC 2748-

2753) offloads admission control to “central policy server”

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IntServ: RSVP (cont.)IntServ: RSVP (cont.)

• RSVP Process:– Sender sends path message to receiver

about QoS capabilities of intermediate nodes– Receiver processes and generates

“upstream” request to reserve resources– UNI-Directional Process (requires each end

point to reserve resources)– Uses existing mechanisms (WFQ, etc)

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Differentiated ServicesDifferentiated Services

• RFC 2475 (DiffServ)• Most Generally Accepted QoS Model• Different Services to Different Traffic

types - that can scale!• Uses Packet Classification and

Marking [DSFIELD]

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• Packet Classification– Layer 2 & Layer 3 – ACL,URL,MIME Type, NBAR – to identify traffic– Perform as close as possible to source

• Packet Marking– Based on Classification (used to distinguish)– Marking is carried throughout network– Scalable: Deployed on 1st Layer-3-capable

device (Limiting burden on core devices)

Differentiated Services - (cont.)Differentiated Services - (cont.)

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Differentiated Services - (cont.)Differentiated Services - (cont.)

• Congestion Management– Isolates and prioritizes various classes

of traffic– Re-ordering of packet transmissions– Impacts delay and jitter– Egress function (CBWFQ & LLQ)

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Differentiated Services - (cont.)Differentiated Services - (cont.)

• Congestion Avoidance– TCP Based – cause a smaller TCP Window– Weighted Random Early Detection (WRED)– Random dropping to prevent exhaustion of

queue • “Tail-drop” Condition

– Uses DiffServ Code point (DSCP) or IP Precedence

• Traffic Conditioning

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Differentiated Services - (cont.)Differentiated Services - (cont.)

• Traffic ConditioningPolicers

• Drop packets exceeding specified rate• UDP does not re-transmit dropped packets• Better for VoIP• Cisco: CAR

Shapers• Limits rate of packets using buffers• Adds delay which is not good for VoIP & VC• Cisco: GTS, FRTS, Class-based etc

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DiffServ - Per Hop Behavior DiffServ - Per Hop Behavior **(PHB)****(PHB)**

• RFC 2475 – Foundation of DiffServ• Forwarding Behavior applied @ each DS-

complaint node to a DS “behavior aggregate” (BA)– BA: Collection of packets with the same DiffServ

Code Point traversing a node in a given direction

• Based on single or multiple criteria• MF Classifier (MF): Source/Destination address, DS

field, Protocol ID, Ports

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DiffServ – DSCPDiffServ – DSCP“Code Points”“Code Points”

• RFC 2474 – Field Format• Obsoletes RFC 791

– ToS – IP Precedence

• Code Points are backward compatible• Default configs = recommended

mappings

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Diffserv Diffserv Assured Forwarding (AF) – Assured Forwarding (AF) –

PHB TypePHB Type

• RFC 2597 • 12 recommended Code Points

– 4 independent classes each having 3 Levels of “drop precedence”

Class Low Drop Medium Drop High Drop

AF1 001010 (AF11) 001100 (AF12) 001110 (AF13)

AF2 010010 (AF21) 010100 (AF22) 010110 (AF23)

AF3 011010 (AF31) 011100 (AF32) 011110 (AF33)

AF4 100010 (AF41) 100100 (AF42) 100110 (AF43)

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Diffserv Diffserv DS Field FormatDS Field Format

• IP Header Comparison: IP Precedence/ToS & DS Code points

BIT 0 1 2 3 4 5 6 7IPv4 (RFC 791) UnusedDS Field

IP Precedence "ToS Bits"DSCP ECN

• In IPv6 = “Traffic Class” Octet

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DiffServ: Expedited Forwarding (EF)DiffServ: Expedited Forwarding (EF)

• RFC 2598• Node forwards packet ASAP

– DSCP 46 (101110)• Real-time traffic requiring low delay & jitter• Marking Mechanisms:

– CAR, policy-based Routing, Dial Peers, Class-based marking, Class-based Policer

• Cisco: LLQ– single strict priority queue extends CBWFQ

• Risk: Too much EF traffic can lead to “starvation” of non EF traffic! – Police EF traffic rate

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Classification, Marking & MappingClassification, Marking & Mapping

• Layer 2 CoS frames are classified and marked in the “ISL” or “802.1Q” header

• Frames passing from L2 to L3 lose header information

• Mapping Problem between L2 & L3:– 64 DSCP Values (0-63)– 8 CoS Value (0-7)– Groups of DSCP values must be mapped to

single CoS values

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QoS / CoS “Trust Concepts”QoS / CoS “Trust Concepts”

• How ingress packets are handled on interfaces• End-User-Ports:

– Generally treated as “untrusted” by network administrators because OS allow users to set CoS values

– Switch changes CoS to Best Effort (0) when frame is forwarded

• Switch-to-Switch, Switch-to-Router & Switch-to-IP Phone:– Usually treated as “trusted” by network

administrators & CoS value is unchanged

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Layer 2 CoS MarkingLayer 2 CoS Marking

•Layer 2 ISL Frame

•ISL CoS: uses 3 least significant bits of “user field” in ISL header

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Layer 2 CoS Marking (cont.)Layer 2 CoS Marking (cont.)

•Layer 2 802.1q/p Frame

•802.1q/p CoS: uses 3 bits of “user priority” portion of “tag field”

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QoS / CoS Summary TableQoS / CoS Summary Table

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References

• Cisco Catalyst QoS: Quality of Service in Campus Networks– Michael Flannagan, Richard Froom & Kevin Turek– ISBN#1-58705-120-6

• IP QoS (Cisco, 2002)– Zdravko Nikolov (znikolov@cisco.com)

• Polycomm User Group Presentation:– http://www.pug.com/conference/2003_Conference/Presentations/A1-QoS-an

d_CoS.pdf– Kris Acharya, Optimal Systems, Inc.

(on assignment at Pfizer, Inc.)– September 15th, 2003

• Eva Heinold - CCCSC München - eva.heinold@hp.com– http://www.decus.de/slides/sy2003/08_04/1g02.pdf

• Jeff Caruso: Network World– http://www.nwfusion.com/newsletters/lans/2003/1215lan1.html