QoS / CoS in the LAN

1. QoS / CoS in the LAN Byron D. Early Chad D. Burnham University of Denver UTS - Network Services WestNet – January 15, 2004 ASU – Tempe, AZ

2. QoS / CoS Definition • Techniques to enhance network performance for traffic types deemed essential to your institution’s business model: • Bandwidth • Delay • Jitter • Packet Loss

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

4. 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

5. 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

6. 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

7. 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

8. Application Requirements experpt from Cisco “IP QoS”, 2002 by Zdravko Nikolov

9. 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

10. 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.

11. 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)

12. 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

13. 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

14. 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

15. IntServ: 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”

16. 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)

17. Differentiated Services • RFC 2475 (DiffServ) • Most Generally Accepted QoS Model • Different Services to Different Traffic types - that can scale! • Uses Packet Classification and Marking [DSFIELD]

18. Differentiated Services - (cont.) • 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)

19. 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)

20. 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

21. Differentiated Services - (cont.) • Traffic Conditioning Policers • 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

22. DiffServ - Per Hop Behavior **(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

23. DiffServ – DSCP“Code Points” • RFC 2474 – Field Format • Obsoletes RFC 791 • ToS – IP Precedence • Code Points are backward compatible • Default configs = recommended mappings

24. Diffserv Assured Forwarding (AF) – PHB Type • RFC 2597 • 12 recommended Code Points • 4 independent classes each having 3 Levels of “drop precedence”

25. Diffserv DS Field Format • IP Header Comparison: IP Precedence/ToS & DS Code points • In IPv6 = “Traffic Class” Octet

26. 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

27. Classification, 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

28. 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

29. Layer 2 CoS Marking • Layer 2 ISL Frame • ISL CoS: uses 3 least significant bits of “user field” in ISL header

30. 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”

31. QoS / CoS Summary Table

32. 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-and_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