1. ECE544: Communication Networks-II, Spring 2009 H. Liu Lecture 9 (QoS) Includes teaching materials from D. Raychaudhuri, L. Peterson

2. Today’s Lecture Congestion control in best effort networks –Basic principles & mechanisms –FQ, WFQ, congestion feedback, TCP, RED Quality-of-service (QoS) –Mechanisms (traffic shaping, admission control, reservation, priority queuing) –RSVP Intserv and Diffserv, RIO –Comparison to ATM (CBR, VBR; ABR)

3. Congestion Control & QoS in Packet Networks Congestion control – reactive methods used in best effort networks –Packet scheduling at network nodes –Feedback congestion control End-to-end Hop-by-hop QoS control – proactive methods used for premium or guaranteed services: –Source traffic shaping & policing at entry points –Priority queuing and packet drop at routers –End-to-end reservation and admission control

4. Network Congestion All networks have saturating throughput –Reduction in performance beyond max capacity –Need to keep input load below G 0 –Also must avoid unstable equilibrium point in overload region Overload region Normal operating Point (G 0 ) Capacity Limit S max Offered Traffic (G) Thru Traffic margin Congestion control policies Unstable network load Stable network load lines with congestion control

5. Queue Scheduling A queue scheduler employs 2 strategies: –Scheduling discipline: Which packet to serve (transmit) next –Drop policy: Which packet to drop next (when required)

6. FIFO Queuing FIFO:first-in-first-out (or FCFS: first- come-first-serve) Arriving packets get dropped when queue is full regardless of flow or importance - implies drop-tail Important distinction: –FIFO: scheduling discipline –Drop-tail: drop policy

7. Fair Queuing Main idea: –maintain a separate queue for each flow currently flowing through router –router services queues in Round-Robin fashion

8. FQ illustration Flow 1 Flow 2 Flow n I/P O/P Variation: Weighted Fair Queuing (WFQ)

9. Some Complications Packets are of different length We really need bit-by-bit round-robin (RR) FQ simulates bit-by-bit RR –Not feasible to interleave bits!

10. Bit-by-bit RR Single flow: suppose clock ticks when a bit is transmitted. For packet i: –Pi: length, A i = arrival time, Si: begin transmit time, F i : finish transmit time. F i = S i +P i –F i = max (F i-1, A i ) + P i Multiple flows: clock ticks when a bit from all active flows is transmitted, that is, the clock advances by one tick when n bits are transmitted (assuming n flows) –calculate Fi for each packet –transmit packet with earliest Fi RR is only simulated, packet in transmission is not interrupted. If n flows have data to transmit, each gets 1/nth bandwidth.

11. Bit-by-bit RR Flow 1 Flow 2 Pkt 2-1=3 units Pkt 1-1=2 units Pkt 2-2=2 units Pkt 1-2= 1 unit Pkt 1-3= 1 unit Channel clock -  1-1 P(1,1) = 2 P(1,2) = 1 P(1,3) = 1 P(2,1) = 3 P(2,2) = 2 Start with A(*,*)=0 (all pkts arrive at T=0) F(1,1) = 1 F(1,2) = 1.5 F(1,3) = 2 F(2,1) = 1.5 F(2,2) = 2.5 Fi = max (Fi- 1, Ai) + Pi 2-11-21-32-2

12. Weighted Fair Queuing (WFQ) Weighted Fair Queuing (WFQ): assign a weight to each flow –Assume transmitting w q bits each time the router serves queue q (simulate in packet scheduling) –Control the percentage of the link’s bandwidth that a flow will get The bandwidth that flow q gets (n active queues sending data): –FQ is a special case of WFQ with a weight of 1 for each queue

13. Congestion Control and Congestion Avoidance TCP’s “blind” approach: –Detect congestion (loss) after it happens and back off on offered rate –Increase load trying to maximize utilization until loss occurs Source Rate (bps) Congestion detected (via packet loss) Time-out Alternatively: –We can try to predict congestion and reduce rate before packets start being discarded –This is called congestion avoidance

14. Congestion Control via Router Feedback Router has unified view of queuing behavior Routers can distinguish between propagation and persistent queuing delays Routers can decide on transient congestion, based on workload

15. Solving the Full Queues Problem Router monitor the load Drop (or mark) packets before queue becomes full (early drop) Intuition: notify senders of incipient congestion –Simple example: If qlen > drop level, drop (or mark) each new packet with a fixed probability p Does not control misbehaving users

16. Random Early Detection (RED) Motivation: –High bw-delay flows have large queues to accommodate transient congestion –TCP detects congestion from loss - after queues have built up and increase delay Aim: –Keep throughput high and delay low –Accommodate bursts

17. Random Early Detection (RED) Detect incipient congestion, allow bursts Keep power (throughput/delay) high –keep average queue size low –assume hosts respond to lost packets Avoid window synchronization –randomly mark packets, instead of dropping Avoid bias against bursty traffic Some protection against ill-behaved users

18. RED Algorithm Maintain running average of queue length If avg < min th do nothing –Low queuing, send packets through If avg > max th, drop all packets –Protection from misbehaving sources Else randomly drop (or mark) some packets in a manner proportional to queue length –Notify sources of incipient congestion

19. RED Operation Min threshold Max threshold Average queue length minthreshmaxthresh MaxP 1.0 Avg length P(drop) If AvgLen <= MinThreshold –Queue the packet If MinThreshold < Avglen < MaxThreshold –Calculate probability P –Drop the arriving packets with probability P If Avglen >= MaxThreshold –Drop the arriving packet AveLen = (1-W) x AveLen + W x SampleLen Count:# of newly arriving packets that have been queued (not dropped) while AvgLen has been between the two thresholds –Count increases, P increases –Make drop more evenly distributed (Avoid bias against bursty traffic)

20. Explicit Congestion Control Router can signal the congestion by marking packets instead of dropping using RED –Set the ECN bit (bit 7 of the IP TOS field) in IP header The destination copies the ECN bit into the ACK sent back to the source The source TCP responds to the ECN bit set in the same way as a packet drop VersionHLen TOSLength IdentFlagsOffset TTLProtocolChecksum SourceAddr DestinationAddr Options (variable) Pad (variable) 048161931 Data ToS field => now used for DiffServ and ECN –Bits 0-5: Differentiated Services Code Point (DSCP) –Bit 6: ECN-capable –Bit 7: ECN

21. Quality of Service Outline Realtime Applications Integrated Services Differentiated Services

22. Realtime Applications Require “deliver on time” assurances –must come from inside the network –Example application (audio) –sample voice once every 125us –each sample has a playback time –packets experience variable delay in network –add constant factor to playback time: playback point –Use initial buffering delay to compensate jitter, but result in longer end-to-end delay Microphone Speaker Sampler, A D converter Buffer, D A

23. Playback Buffer Sequence number Packet generation Network delay Buffer Playback Time Packet arrival

24. Example Distribution of Delays 1 2 3 Packets (%) 90%97%98%99% 15020010050 Delay (milliseconds)

25. Application requirements & Services Classes Different application requirements –Elastic: no restrict delay requirements, traditional data –Real-time: delay bound, jitter, loss Loss: intolerant or tolerant to some loss Delay: adaptive (e.g. lengthening/shortening the silence between words, playing back video a little slower, etc) or not adaptive Data rate: adaptive (e.g. reduce video quality by compressing video more) or not adaptive Different application requirements=>different service classes (not only best effort anymore) A network that can provide these different levels of service is said to support QoS –Integrated service: fine-grained approach, provide QoS to individual applications or flows Allow individual application flows to specify their needs to the routers using an explicit signaling mechanism (RSVP) Scalability is an issue –Differentiated Service: coarse-grained approach, provide QoS to several classes of data or aggregated traffic Assign packets into a small number of classes that receive differentiated treatment in the routers

26. Taxonomy of applications Applications Elastic Real-Time Loss, delay tolerant Intolerant Interactive Non-adaptiveadaptiveNon-adaptive Delay adaptive Rate adaptive Rate adaptive Asynchronous Interactive-bulk

27. Components of Integrated Services architecture Flowspec: information of the flow traffic characteristics and its service Reservations (includes reservation signaling protocol) Admission control based on flow description and current load Scheduling to meet the reservation Traffic shaping at edges to fit reservation Traffic policing to mark or drop non-conforming traffic Some application adaptation

28. Types of guarantees Absolute bound on delay and jitter Absolute bound on delay only Statistical bound on delay No quantitative delay bound but admission control and preferential treatment None

29. Internet service classes proposed by IETF Guaranteed service –firm bounds on e2e delays and bandwidth Controlled load –“a QoS closely approximating the QoS that same flow would receive from an unloaded network element, but uses capacity (admission) control to assure that this service is received even when the network element is overloaded” Use a queuing mechanism such as WFQ to isolate the controlled load traffic from other traffic Admission control to limit the total amount of controlled load traffic Best effort

30. Overview of mechanisms Flow specification (flowspec) –type of service we require Admission control –can the network provide the requested service? Resource reservation protocol –RSVP Packet scheduling

31. Flowspecs Tspec: describes the flow’s traffic characteristics Rspec: describes the service requested from the network

32. Token bucket filter Described by 2 parameters: –token rate r: rate of tokens placed in the bucket –bucket depth B: capacity of the bucket Operation: –tokens are placed in bucket at rate r –if bucket fills, tokens are discarded –sending a packet of size P uses P tokens –if bucket has P tokens, packet sent at max rate, else must wait for tokens to accumulate

33. Token bucket operation tokens Packet overflow tokens Packet Enough tokens packet goes through, tokens removed Not enough tokens - wait for tokens to accumulate

34. TB characteristics On the long run, rate is limited to r On the short run, a burst of size B can be sent at peak data rate Amount of traffic entering at interval T is bounded by: –traffic = B + r*T Information useful to admission algorithm

35. Token bucket specs BW Time 1 2 123 Flow A Flow B Flow A: r = 1 Mbps, B=1 byte Flow B: r = 1 Mbps, B=1MB

36. Admission control When new flow request arrives, look at Rspec and Tspec and decide whether to admit or reject –Can it provide the desired service requested by the flow, given the currently available resources without causing any previously admitted flow to receive worse service that agreed? Not policing

37. Reservation protocol: RSVP Upper layer protocols and applications IP Link layer modules ICMPIGMPRSVP IP service interface Link layer service interface

38. RSVP Used on connectionless networks Relies on soft state: reservations must be refreshed and do not have to be explicitly deleted Aims to support multicast as effectively as unicast flows - mcast apps good candidates for real-time, and are heterogeneous Receiver-oriented approach

39. Basic message types PATH message RESV message CONFIRMATIONmessage –generated only upon request –unicast to receiver when RESV reaches node with established state TEARDOWN message ERROR message (if path or RESV fails)

40. Making a reservation Receivers make reservation Before making a reservation, receiver must know: –type of traffic sender will send (Tspec) –path the sender’s packets will follow Both can be accomplished by sending PATH messages by the sender

41. PATH messages PATH messages carry sender’s Tspec and sent from the sender to the receiver Record the path from the sender to the receiver in the PATH message Receivers send RESV messages that follow reverse path and setup reservations If reservation cannot be made, user gets an error

42. PATH and RESV messages R Sender 1 Sender 2 receiver 1 receiver 2 RR R PATH RESV RESV (merged)

43. Soft State Allow increasing or decreasing the level of resource reservation Adapt to link or router failure and topology changes –Routing protocol makes routing changes, RSVP adjusts reservation state –In absence of route or membership changes, periodic PATH and RESV msgs refresh established reservation state –When change, new PATH msgs follow new path, new RESV msgs set reservation –Non-refreshed state times out automatically

44. Router handling of RESV messages If new request rejected, send error message If admitted: –install packet filter into forwarding dbase –pass flow parameters to scheduler –activate packet policing if needed –forward RESV msg upstream

45. Packet classifying and scheduling Each arriving packet must be: –classified: associated with the application reservation Examining up to five fields in the packet: source + destination address, protocol number, source + destination port –scheduled: managed in the queue so that it receives the requested service implementation not specified in the service model

46. RSVP and multicast Reservations from multiple receivers for a single sender are merged together at branching points Reservations for multiple senders may not be added up: –audio conference, not many talk at same time –Different reservation styles: Reserve resources for all speakers Reserve resources for any n speakers Reserve resources for speakers A and B only

47. RSVP versus ATM (Q.2931) RSVP –receiver generates reservation –soft state (refresh/timeout) –separate from route establishment –QoS can change dynamically –receiver heterogeneity ATM –sender generates connection request –hard state (explicit delete) –concurrent with route establishment –QoS is static for life of connection –uniform QoS to all receivers

48. ATM Service Categories CBR –Constant Bit Rate –Continuous flow of data with tight bounds on delay and delay variation rt-VBR –Real-Time Variable Bit Rate –Variable bandwidth with tight bounds on delay and delay variation nrt-VBR –Non-Real-Time Variable Bit Rate –Variable bandwidth with tight bound on cell loss UBR –Unspecified Bit Rate –No guarantees (i.e., best effort delivery) ABR –Available Bit Rate –Flow control on source with tight bound on cell loss

49. Traffic Shaping Traffic shaping: control traffic in order to conform to the traffic contract by delaying packets to meet certain criteria. –To optimize or guarantee performance (lower latency, higher usable bandwidth) –commonly applied at the network edges to control traffic entering the network, but can also be applied by the traffic source or in the network Token bucket –tokens are placed in bucket at rate r –if bucket fills, tokens are discarded –sending a packet of size P uses P tokens –if bucket has P tokens, packet sent at max rate, –else must wait for tokens to accumulate

50. Traffic Policing Traffic policing: router monitors the flow traffic for conformity with a traffic contract –Drop or tag the packets not conforming to the TSpec that used to make the reservation –to enforce compliance with that contract Token bucket –tokens are placed in bucket at rate r –if bucket fills, tokens are discarded –When a packet of p bytes arrives, p tokens are removed from the bucket, and the packet is sent to the network. –If fewer than p tokens are available, no tokens are removed from the bucket, and the packet is considered to be non- conformant. –Drop or mark the non-conformant packets

51. Differentiated Services (DiffServ) Analogy: –airline service, first class, coach, various restrictions on coach as a function of payment Best-effort expected to make up bulk of traffic, but revenue from “premium” service important to economic base (will pay for more plentiful bandwidth overall) –Not motivated by real-time, motivated by economics and assurances

52. Differentiated Services (cont) Divide traffic into a small number of classes and allocates resource on a per-class basis, instead of individual flows –Scalable –No need for reservations: just mark packets E.g. packet marking can be done at administrative boundaries before injecting packets into network –Significant savings in signaling, much simpler overall

53. IP DiffServ IP packets carry 6-bit service code points (DSCP) –Potentially support 64- different classes of services –In implementation, the number of DSCPs used in a network is much smaller, e.g. simple two- class network VersionHLen TOSLength IdentFlagsOffset TTLProtocolChecksum SourceAddr DestinationAddr Options (variable) Pad (variable) 048161931 Data ToS field => now used for DiffServ and ECN –Bits 0-5: Differentiated Services Code Point (DSCP) –Bit 6: ECN-capable –Bit 7: ECN

54. DiffServ Where and how to set the DSCP value –Host set DSCP based on knowledge of application requirements –Router at an administrative boundary set DSCP according to a certain policy What does a router do when it receives a packet marked with a certain DSCP –Routers map DSCP to per-hop-behavior (PHB) –PHBs can be standard or local –Standard PHBs include Default: No special treatment or best effort Expedited forwarding (EF): should be forwarded with low delay and loss Assured forwarding (AF): Multiple classes, each class with multiple drop ppreference

55. Expedited Forwarding (EF) A router should forward the EF packets with minimal delay and low loss To implement, put all EF packets in a dedicated EF queue and ensure that the arrival rate of packets to the queue is less than the service rate –Configure the router at the edge of an administrative domain to allow a certain max rate of EF packets arrivals into the domain Limit the EF packet rate < the bandwidth of the slowest link in the domain –Give EF packet queue high priority, e.g. WFQ with a high weight to EF queue

56. Assured Forwarding (AF) A set of AF PHBs is defined AFxy: –x: class, typically determine a queue for the packet –y:drop preference –For example: AF11, AF12 and AF13 packets are put into the same queue, but AF13 would have a greater chance to be dropped due to congestion. AF11 and AF2y would go to different queues 12 AF PHBs are recommended by IETF, 4 AF classes with three drop preference levels each

57. An Example DiffServ Network Implementation Support Two-type DiffServ –Premium (expedited) service: (type P) User sends within profile, network commits to delivery with the agreed profile –Classify packet in one of two queues, use priority –Shaping at trust boundaries only, using token bucket Allocate resource based on the agreed peak rate –conservative, virtual wire services –unused premium goes to best effort (subsidy!) –Assured service: (type A) based on expected capacity usage profiles traffic unlikely to be dropped if user maintains profile. Out- of-profile traffic marked

58. Premium traffic flow first hop router internal router border router host border router ISP Company A Unmarked packet flow Packets in premium flows have bit set Premium packet flow restricted to R bytes/sec

59. 2-bit differentiated service Use precedence field to encode P & A type packets P packets are queued at higher priority than ordinary best effort A packets treated preferentially wrt dropping probability in the normal queue Leaf and border routers have input and output tasks - other routers just output

60. Leaf router input functionality Clear A & P bits Packet classifier Marker 1 Marker N Forwarding engine Arriving packet Best effort Flow 1 Flow N Marker: Leaf routers have traffic profiles - they classify packets based on packet header (destination addr & port, source addr & port, protocol ID, etc) –If no profile present, pass as best effort –If profile is for A: mark in-profile packets with A, forward others unmarked –If profile is for P: delay out-of -profile packets to shape into profile

61. Markers to implement two different services Wait for token Set P bit Packet input Packet output Drop on overflow P service Test if token Set A bit token No token Packet input Packet output A service

62. Router output interface for two-bit architecture 2 queues: P packets on higher priority queue Lower priority queue implements RED “In or Out” scheme (RIO) P-bit set? If A-bit set incr A_cnt High-priority Q Low-priority Q If A-bit set decr A_cnt RIO queue management Packets out yes no

63. Border router input interface Profile Meters Arriving packet Is packet marked? Token available? Token available? Clear A-bit Drop packet Forwarding engine A set P set token Not marked no At border routers, profile meters test marked flows: –drop P packets out of profile –unmark A packets

64. Red with In or Out (RIO) Similar to RED, but with two separate probability curves Has two classes, “In” and “Out” (of profile) “Out” class has lower Minthresh, so packets are dropped from this class first As avg queue length increases, “in” packets are dropped MaxP 1.0 Min out Min in Max in Max out P(drop) AvgLen

65. 65 Today’s Homework Chapter 6 -6.13 -6.32 -6.43 -6.44 -6.45 Chapter 4 –4.66 –4.67