Corso di Reti di Calcolatori II a.a

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1 a.a Tecniche ed Architetture per la QoS (slide in lingua inglese) Simon Pietro Romano

2 Credits Il materiale didattico utilizzato per questa lezione è stato preparato in collaborazione con i Professori:» Stefano Avallone» Roberto Canonico» Antonio Pescapè» Giorgio Ventre Alcune slide sono tratte dal materiale didattico messo a disposizione dei docenti sul sito ufficiale del libro di testo (cfr prossima slide)

3 Nota di copyright per le slide di Kurose/Ross Nota per l utilizzo: Abbiamo preparato queste slide con l intenzione di renderle disponibili a tutti (professori, studenti, lettori). Sono in formato PowerPoint in modo che voi possiate aggiungere e cancellare slide (compresa questa) o modificarne il contenuto in base alle vostre esigenze. Come potete facilmente immaginare, da parte nostra abbiamo fatto un sacco di lavoro. In cambio, vi chiediamo solo di rispettare le seguenti condizioni: se utilizzate queste slide (ad esempio, in aula) in una forma sostanzialmente inalterata, fate riferimento alla fonte (dopo tutto, ci piacerebbe che la gente usasse il nostro libro!) se rendete disponibili queste slide in una forma sostanzialmente inalterata su un sito web, indicate che si tratta di un adattamento (o che sono identiche) delle nostre slide, e inserite la nota relativa al copyright. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Reti di calcolatori e Internet: Un approccio top-down 4 a edizione Jim Kurose, Keith Ross Pearson Paravia Bruno Mondadori Spa 2008

4 Why Better-than-Best-Effort (QoS)? To support a wider range of applications» Real-time, Multimedia, etc To develop sustainable economic models and new private networking services» Current flat priced models, and best-effort services do not cut it for businesses

5 Quality of Service: What is it? Multimedia applications: network audio and video QoS network provides application with level of performance needed for application to function

6 What is QoS? Better performance as described by a set of parameters or measured by a set of metrics Generic parameters:» Bandwidth» Delay, Delay-jitter» Packet loss rate (or loss probability) Transport/Application-specific parameters:» Timeouts» Percentage of important packets lost

7 What is QoS (contd)? These parameters can be measured at several granularities:» micro flow, aggregate flow, population QoS considered better if» a) more parameters can be specified» b) QoS can be specified at a fine-granularity QoS spectrum: Best Effort Leased Line

8 Fundamental Problems FIFO Scheduling Discipline B B In a FIFO service discipline, the performance assigned to one flow is convoluted with the arrivals of packets from all other flows!» Can t get QoS with a free-for-all» Need to use new scheduling disciplines which provide isolation of performance from arrival rates of background traffic

9 QoS Big Picture: Control/Data Planes Control Plane: Signaling + Admission Control or SLA (Contracting) + Provisioning/Traffic Engineering Router Workstation Router Internetwork or WAN Router Workstation Data Plane: Traffic conditioning (shaping, policing, marking etc) + Traffic Classification + Scheduling, Buffer management

10 QoS Components QoS set aside resources for premium services QoS components:» a) Specification of premium services: (service/service level agreement design)» b) How much resources to set aside? (admission control/provisioning)» c) How to ensure network resource utilization, do load balancing, flexibly manage traffic aggregates and paths? (QoS routing, traffic engineering)

11 QoS Components (..continued)» d) How to actually set aside these resources in a distributed manner? (signaling, provisioning, policy)» e) How to deliver the service when the traffic actually comes in (claim/police resources)? (traffic shaping, classification, scheduling)» f) How to monitor quality, account and price these services? (network mgmt, accounting, billing, pricing)

12 How to upgrade the Internet for QoS? Approach: de-couple end-system evolution from network evolution End-to-end protocols: RTP, H.323 etc to spur the growth of adaptive multimedia applications» Assume best-effort or better-than-best-effort clouds Network protocols: IntServ, Diffserv, RSVP, MPLS, COPS» To support better-than-best-effort capabilities at the network (IP) level

13 QOS SPECIFICATION, TRAFFIC, SERVICE CHARACTERIZATION, BASIC MECHANISMS

14 Service Specification Loss: probability that a flow s packet is lost Delay: time it takes a packet s flow to get from source to destination Delay jitter: maximum difference between the delays experienced by two packets of the flow Bandwidth: maximum rate at which the soource can send traffic

15 Hard Real Time: Guaranteed Services Service contract» Network to client: guarantee a deterministic upper bound on delay for each packet in a session» Client to network: the session does not send more than it specifies Algorithm support» Admission control based on worst-case analysis» Per flow classification/scheduling at routers

16 Soft Real Time: Controlled Load Service Service contract:» Network to client: similar performance as an unloaded best-effort network» Client to network: the session does not send more than it specifies Algorithm Support» Admission control based on measurement of aggregates» Scheduling for aggregate possible

17 Traffic and Service Characterization To quantify a service one has two know» Flow s traffic arrival» Service provided by the router, i.e., resources reserved at each router Examples:» Traffic characterization: token bucket» Service provided by router: fix rate and fix buffer space Characterized by a service model (service curve framework)

18 Token Bucket Characterized by three parameters (b, r, R)» b token depth» r average arrival rate» R maximum arrival rate (e.g., R might be link capacity) A bit is transmitted only when there is an available token» When a bit is transmitted exactly one token is consumed r tokens per second bits b tokens b*r/(r-r) slope R slope r <= R bps regulator time

19 Characterizing a Source by Token Bucket Arrival curve maximum amount of bits transmitted by time t Use token bucket to bound the arrival curve bps bits Arrival curve time time

20 Example Arrival curve maximum amount of bits transmitted by time t Use token bucket to bound the arrival curve bits (b=1,r=1,r=2) Arrival curve bps time size of time interval

21 Per-hop Reservation Given b,r,r and per-hop delay d Allocate bandwidth r a and buffer space B a so as to guarantee d slope r a bits slope r Arrival curve b d B a

22 What is a Service Model? external process offered traffic Network element (connection oriented) delivered traffic The QoS measures (delay, throughput, loss, cost) depend on offered traffic, and possibly other external processes. A service model attempts to characterize the relationship between offered traffic, delivered traffic, and possibly other external processes.

23 Arrival and Departure Process R in Network Element R out bits R in (t) = arrival process = amount of data arriving up to time t delay buffer R out (t) = departure process = amount of data departing up to time t t

24 Traffic Envelope (Arrival Curve) Maximum amount of service that a flow can send during an interval of time t b(t) = Envelope slope = max average rate Burstiness Constraint slope = peak rate t

25 Mechanisms: Traffic Shaping/Policing Token bucket: limits input to specified Burst Size (b) and Average Rate (r).» Traffic sent over any time T <= r*t + b» a.k.a Linear bounded arrival process (LBAP) Excess traffic may be queued, marked BLUE, or simply dropped

26 Leaky bucket

27 Leaky bucket vs token bucket (1/2) Input to a leaky bucket Output from a leaky bucket Output from a token bucket with capacity of 250 KB

28 Leaky bucket vs token bucket (2/2) Output from a token bucket with capacity of 500 KB Output from a token bucket with capacity of 750 KB Output from a 500KB token bucket feeding a 10-MB/sec leaky bucket

29 SCHEDULING

30 Mechanisms: Queuing/Scheduling Traffic Sources $$$$$$ $$$ $ Traffic Classes Class A Class B Class C Use a few bits in header to indicate which queue (class) a packet goes into (also branded as Class of Service CoS) High $$ users classified into high priority queues, which also may be less populated lower delay and low likelihood of packet drop Ideas: priority, round-robin, classification, aggregation,...

31 Mechanisms: Buffer Mgmt/Priority Drop Drop RED and BLUE packets Drop only BLUE packets Ideas: packet marking, queue thresholds, differential dropping, buffer assignments

32 Packet Scheduling Decide when and what packet to send on output link» Usually implemented at output interface flow 1 1 Classifier flow 2 Scheduler 2 flow n Buffer management

33 Focus: Scheduling Policies Priority Queuing: classes have different priorities; class may depend on explicit marking or other header info, e.g. IP source or destination, TCP Port numbers, etc. Transmit a packet from the highest priority class with a non-empty queue Preemptive and non-preemptive versions

34 Scheduling Policies (more) Round Robin: scan class queues serving one from each class that has a non-empty queue

35 Round-Robin Discussion Advantages: protection among flows» Misbehaving flows will not affect the performance of well-behaving flows Misbehaving flow a flow that does not implement any congestion control» FIFO does not have such a property Disadvantages:» More complex than FIFO: per flow queue/state» Biased toward large packets a flow receives service proportional to the number of packets

36 Generalized Processor Sharing (GPS) Assume a fluid model of traffic» Visit each non-empty queue in turn (RR)» Serve infinitesimal from each» Leads to max-min fairness GPS is un-implementable!» We cannot serve infinitesimals, only packets

37 Generalized Processor Sharing A work conserving GPS is defined as Wi ( t, t + w i dt) = W ( t, t + j B( t ) dt) w j i B( t) where» w i weight of flow i» W i (t 1, t 2 ) total service received by flow i during [t 1, t 2 )» W(t 1, t 2 ) total service allocated to all flows during [t 1, t 2 )» B(t) number of flows backlogged

38 Bit-by-bit Round Robin Single flow: clock ticks when a bit is transmitted. For packet i:» P i = length, A i = arrival time, S i = begin transmit time, F i = finish transmit time» F i = S i +P i = max (F i-1, A i ) + P i Multiple flows: clock ticks when a bit from all active flows is transmitted round number» Can calculate F i for each packet if number of flows is known at all times This can be complicated

39 Packet Approximation of Fluid System Standard techniques of approximating fluid GPS» Select packet that finishes first in GPS assuming that there are no future arrivals Important properties of GPS» Finishing order of packets currently in system independent of future arrivals Implementation based on virtual time» Assign virtual finish time to each packet upon arrival» Packets served in increasing order of virtual times

40 Fair Queuing (FQ) Idea: serve packets in the order in which they would have finished transmission in the fluid flow system Mapping bit-by-bit schedule onto packet transmission schedule Transmit packet with the lowest F i at any given time Variation: Weighted Fair Queuing (WFQ)» Assign different weights to different flows..

41 QoS ARCHITECTURES

42 Parekh-Gallager theorem Let a connection be allocated weights at each WFQ scheduler along its path, so that the least bandwidth it is allocated is g Let it be leaky-bucket regulated such that # bits sent in time [t 1, t 2 ] <= g(t 2 - t 1 ) + σ Let the connection pass through K schedulers, where the k th scheduler has a rate r(k) Let the largest packet size in the network be P end _ to _ end _ delay σ / g + K 1 P / g + K k = 1 k = 1 P / r ( k )

43 Significance P-G Theorem shows that WFQ scheduling can provide end-to-end delay bounds in a network of multiplexed bottlenecks» WFQ provides both bandwidth and delay guarantees» Bound holds regardless of cross traffic behavior (isolation)» Needs shapers at the entrance of the network Can be generalized for networks where schedulers are variants of WFQ, and the link service rate changes over time

44 Stateless vs. Stateful QoS Solutions Stateless solutions routers maintain no fine grained state about traffic scalable, robust weak services Stateful solutions routers maintain per-flow state powerful services guaranteed services + high resource utilization fine grained differentiation protection much less scalable and robust

45 Existing Solutions QoS Stateful Tenet [Ferrari & Verma 89] IntServ [Clark et al 91] ATM [late 80s] Stateless Diffserv - [Clark & Wroclawski 97] - [Nichols et al 97] Network support for congestion control Round Robin [Nagle 85] Fair Queueing [Demers et al 89] Flow Random Early Drop (FRED) [Lin & Morris 97] DecBit [Ramkrishnan & Jain 88] Random Early Detection (RED) [Floyd & Jacobson 93] BLUE [Feng et al 99] REM [Low at al 00]

46 Integrated Services (IntServ) An architecture for providing QOS guarantees in IP networks for individual application sessions Relies on resource reservation, and routers need to maintain state information of allocated resources and respond to new Call setup requests

47 IntServ: reference model HOST ROUTER Application Reservation Setup Protocol Reservation Setup Protocol Routing Daemon Packet Classifier Packet Scheduler Packet Classifier Packet Scheduler Admission Control

48 Resource Reservation Protocol: RSVP Receiver RESV PATH Sender

49 RSVP: Internet Signaling Creates and maintains distributed reservation state De-coupled from routing Receiver-initiated: scales for multicast Soft-state: reservation times out unless refreshed Latest paths:» discovered through PATH messages (forward direction)» used by RESV messages (reverse direction)

50 RSVP operation SENDING END SYSTEM RECEIVING END SYSTEM Application INTERMEDIATE NETWORK ELEMENT Application PATH PATH RSVP API SENDER_TSPEC Initial ADSPEC Merged FLOWSPEC Traffic Control SENDER_TSPEC Updated ADSPEC FLOWSPEC FILTERSPEC API RESV RSVP RESV RSVP RESERVATION QoS (GS, CL) Receiver TSPEC [Receiver RSPEC]

51 RSVP: object formats Sender Sender Tspec : characterization of the traffic being generated FlowSpec Receiver TSpec Description of the kind of traffic which the reservation has to be applied to Receiver Receiver RSpec Desired QoS class and associated parameters FilterSpec Selects a subset of the flow s packets, based either on source or on a higherlayer protocol

52 Call Admission Session must first declare its QOS requirement and characterize the traffic it will send through the network R-spec: defines the QOS being requested T-spec: defines the traffic characteristics A signaling protocol is needed to carry the R-spec and T-spec to the routers where reservation is required» RSVP is the leading candidate for such signaling protocol

53 Stateful Solution: Guaranteed Services Achieve per-flow bandwidth and delay guarantees» Example: guarantee 1MBps and < 100 ms delay to a flow Sender Receiver

54 Guaranteed Service: a graphical interpretation.. Slope = r E G delay F b L Slope = p i Σ j=1 buffer H C j / R + D j Slope = R

55 Stateful Solution: Guaranteed Services Allocate resources - perform per-flow admission control Sender Receiver

56 Stateful Solution: Guaranteed Services Install per-flow state Sender Receiver

57 Stateful Solution: Guaranteed Services Challenge: maintain per-flow state consistent Sender Receiver

58 Stateful Solution: Guaranteed Services Per-flow classification Sender Receiver

59 Stateful Solution: Guaranteed Services Per-flow buffer management Sender Receiver

60 Stateful Solution: Guaranteed Services Per-flow scheduling Sender Receiver

61 Stateful Solution Complexity Data path» Per-flow classification» Per-flow buffer management» Per-flow scheduling Control path» install and maintain per-flow state for data and control paths Classifier Per-flow State flow 1 flow 2 flow n Buffer management Scheduler output interface

62 Stateless vs. Stateful Stateless solutions are more» scalable» robust Stateful solutions provide more powerful and flexible services» guaranteed services + high resource utilization» fine grained differentiation» protection

63 Differentiated Services (Diffserv) Intended to address the following difficulties with Intserv and RSVP» Scalability: maintaining states by routers in high speed networks is difficult due to the very large number of flows» Flexible Service Models: Intserv has only two classes want to provide more qualitative service classes; want to provide relative service distinction (Platinum, Gold, Silver, )» Simpler signaling (than RSVP): many applications and users may only want to specify a more qualitative notion of service

64 Differentiated Services Model Ingress Edge Router Interior Router Egress Edge Router Edge routers: traffic conditioning (policing, marking, dropping), SLA negotiation» Set values in DS-byte in IP header based upon negotiated service and observed traffic Interior routers: traffic classification and forwarding (near stateless core!)» Use DS-byte as index into forwarding table

65 Diffserv Architecture Edge router: - per-flow traffic management - marks packets as inprofile and out-profile b r marking scheduling... Core router: - per class traffic management - buffering and scheduling based on marking at edge - preference given to in-profile packets - Assured Forwarding

66 Packet format support Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6: renamed as DS 6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive 2 bits are currently unused

67 Traffic Conditioning It may be desirable to limit traffic injection rate of some class user declares traffic profile (e.g., rate and burst size) traffic is metered and shaped if nonconforming

68 Per-hop Behavior (PHB) PHB: name for interior router data-plane functions» Includes scheduling, buff. mgmt, shaping etc Logical spec: PHB does not specify mechanisms to use to ensure performance behavior Examples:» Class A gets x% of outgoing link bandwidth over time intervals of a specified length» Class A packets leave first before packets from class B

69 PHB (contd) PHBs specified:» Expedited Forwarding: departure rate of packets from a class equals or exceeds a specified rate (logical link with a minimum guaranteed rate) Emulates leased-line behavior» Assured Forwarding: 4 classes, each guaranteed a minimum amount of bandwidth and buffering; each with three drop preference partitions Emulates frame-relay behavior

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