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 priority traffic –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: ms (each) WAN Transit: 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 / (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 ) 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 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 1 st Layer-3-capable device (Limiting burden on core devices) Differentiated Services - (cont.)

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 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 recommended Code Points –4 independent classes each having 3 Levels of “drop precedence” ClassLow DropMedium DropHigh Drop AF (AF11) (AF12) (AF13) AF (AF21) (AF22) (AF23) AF (AF31) (AF32) (AF33) AF (AF41) (AF42) (AF43)

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 q/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# IP QoS (Cisco, 2002) –Zdravko Nikolov Polycomm User Group Presentation: – /A1-QoS-and_CoS.pdfhttp:// /A1-QoS-and_CoS.pdf –Kris Acharya, Optimal Systems, Inc. (on assignment at Pfizer, Inc.) –September 15th, 2003 Eva Heinold - CCCSC München - – Jeff Caruso: Network World –