Traditional queuing behaviour in routers. Scheduling and queue management. Questions. Scheduling mechanisms. Scheduling [1] Scheduling [2]

Transcription

1 Traditioal queuig behaviour i routers Schedulig ad queue maagemet Data trasfer: datagrams: idividual packets o recogitio of flows coectioless: o sigallig Forwardig: based o per-datagram, forwardig table look-ups o examiatio of type of traffic o priority traffic Traffic patters Questios How do we modify router schedulig behaviour to support QoS? What are the alteratives to FCFS? How do we deal with cogestio? Schedulig mechaisms Schedulig [] Schedulig [2] Service request at server: e.g. packet at router iputs Service order: which service request (packe to service first? Scheduler: decides service order (based o policy/algorithm) maages service (outpu queues Router (etwork packet hadlig server): service: packet forwardig scheduled resource: output queues service requests: packets arrivig o iput lies Simple router schematic Iput lies: o iput bufferig Packet classifier: policy-based classificatio Correct output queue: forwardig/routig tables switchig fabric output buffer (queue) Scheduler: which output queue serviced ext forwardig / routig policy packet classifier(s) forwardig / routig tables switchig fabric scheduler output buffer(s)

2 FCFS schedulig Coservatio law [] Null packet classifier Packets queued to outputs i order they arrive No packet differetiatio No otio of flows of packets Aytime a packet arrives, it is serviced as soo as possible: FCFS is a work-coservig scheduler FCFS is work-coservig: ot idle if packets waitig Reduce delay of oe flow, icrease the delay of oe or more others We ca ot give all flows a lower delay tha they would get uder FCFS N " = $ q = C $ = # µ $ : mea lik utlisatio q : mea delay due to scheduler C : costat [s] # : mea packet rate[p/s] µ : mea per! packet servicerate[s/p] Coservatio law [2] No-work-coservig schedulers Example µ : 0.ms/p (fixed) Flow f: λ : 0p/s q : 0.ms ρ q = 0-7 s Flow f2: λ 2 : 0p/s q 2 : 0.ms ρ 2 q 2 = 0-7 s C = s Chage f: λ : 5p/s q : 0.s ρ q = s For f2 this meas: decrease λ 2? decrease q 2? Note the trade-off for f2: delay vs. throughput Chage service rate (µ ): chage service priority No-work coservig disciplies: ca be idle eve if packets waitig allows smoothig of packet flows Do ot serve packet as soo as it arrives: wait util packet is eligible for trasmissio Eligibility: fixed time per router, or fixed time across etwork Less jitter Makes dowstream traffic more predictable: output flow is cotrolled less bursty traffic Less buffer space: router: output queues ed-system: de-jitter buffers Higher ed-to-ed delay Complex i practise may require time sychroisatio at routers Schedulig: requiremets The max-mi fair share criteria Ease of implemetatio: simple fast high-speed etworks low complexity/state implemetatio i hardware Fairess ad protectio: local fairess: max-mi local fairess global fairess protect ay flow from the (mis)behaviour of ay other Performace bouds: per-flow bouds determiistic (guarateed) statistical/probabilistic data rate, delay, jitter, loss Admissio cotrol: (if required) should be easy to implemet should be efficiet i use Flows are allocated resource i order of icreasig demad Flows get o more tha they eed Flows which have ot bee allocated as they demad get a equal share of the available resource Weighted max-mi fair share possible If max-mi fair provides protectio m = mi( x, M )!! N C " mi i M = N " + C : capacity of resource (maximum resource) m : actual resourceallocatio to flow x : resource demad by flow, x! x L! x M : resource available to flow # " = Example: C = 0, four flow with demads of 2, 2.6, 4, 5 actual resource allocatios are 2, 2.6, 2.7, N 2

3 Schedulig: dimesios Simple priority queuig Priority levels: how may levels? higher priority queues services first ca cause starvatio lower priority queues Work-coservig or ot: must decide if delay/jitter cotrol required is cost of implemetatio of delay/jitter cotrol i etwork acceptable? Degree of aggregatio: flow graularity per applicatio flow? per user? per ed-system? cost vs. cotrol Servicig withi a queue: FCFS withi queue? check for other parameters? added processig overhead queue maagemet K queues: k K queue k + has greater priority tha queue k higher priority queues serviced first Very simple to implemet Low processig overhead Relative priority: o determiistic performace bouds Fairess ad protectio: ot max-mi fair: starvatio of low priority queues Geeralised processor sharig (GPS) GPS relative ad absolute fairess Work-coservig Provides max-mi fair share Ca provide weighted max-mi fair share Not implemetable: used as a referece for comparig other schedulers serves a ifiitesimally small amout of data from flow i Visits flows roud-robi ) # # N $, # i # N $, " j, $, j) ) : weight give to flow $, :service to flow i i iterval [ôô,t flow i has a o! empty queue Use fairess boud to evaluate GPS emulatios (GPS-like schedulers) Relative fairess boud: fairess of scheduler with respect to other flows it is servicig Absolute fairess boud: fairess of scheduler compared to GPS for the same flow j, RFB = " j) G( AFB = " : actual service for flow i i [%, t] G( : GPS service for flow i i [%, t] = mi{ ), L, K)} $ ( r( = N $ ( j, # j= $ ( : weight give to flow i at router k r( :service rate of router k! i! N flow umber! k! K router umber Weighted roud-robi (WRR) Deficit roud-robi (DRR) Simplest attempt at GPS Queues visited roudrobi i proportio to weights assiged Differet mea packet sizes: weight divided by mea packet size for each queue Mea packets size upredictable: may cause ufairess Service is fair over log timescales: must have more tha oe visit to each flow/queue short-lived flows? small weights? large umber of flows? DRR does ot eed to kow mea packet size Each queue has deficit couter (dc): iitially zero Scheduler attempts to serve oe quatum of data from a o-empty queue: packet at head served if size quatum + dc dc quatum + dc size else dc += quatum Queues ot served durig roud build up credits : oly o-empty queues Quatum ormally set to max expected packet size: esures oe packet per roud, per o-empty queue RFB: 3T/r (T = max pkt service time, r = lik rate) Works best for: small packet size small umber of flows 3

4 Weighted Fair Queuig (WFQ) [] Weighted Fair Queuig (WFQ) [2] Based o GPS: GPS emulatio to produce fiish-umbers for packets i queue Simplificatio: GPS emulatio serves packets bit-by-bit roud-robi Fiish-umber: the time packet would have completed service uder (bit-by-bi GPS packets tagged with fiishumber smallest fiish-umber across queues served first Roud-umber: executio of roud by bitby-bit roud-robi server fiish-umber calculated from roud umber If queue is empty: fiish-umber is: umber of bits i packet + roud-umber If queue o-empty: fiish-umber is: highest curret fiish umber for queue + umber of bits i packet F( = max{ F( k ",, R( } + P( F( : fiish - umber for packet k P( : o flow i arrivig at time t size of packet arrivig at time t k o flow i R( : roud - umber at time t F! ( = max{ F! ( k ",, R( } +!( : weight give to flow i P(!( Rate of chage of R( depeds o umber of active flows (ad their weights) As R( chages, so packets will be served at differet rates Flow completes (empty queue): oe less flow i roud, so R icreases more quickly so, more flows complete R icreases more quickly etc. iterated deletio problem WFQ eeds to evaluate R each time packet arrives or leaves: processig overhead Weighted Fair Queuig (WFQ) [3] Class-Based Queuig Buffer drop policy: packet arrives at full queue drop packets already i queued, i order of decreasig fiishumber Ca be used for: best-effort queuig providig guarateed data rate ad determiistic ed-to-ed delay WFQ used i real world Alteratives also available: self-clocked fair-queuig (SCFQ) worst-case fair weighted fair queuig (WF 2 Q) Hierarchical lik sharig: lik capacity is shared class-based allocatio policy-based class selectio Class hierarchy: assig capacity/priority to each ode ode ca borrow ay spare capacity from paret fie-graied flows possible Note: this is a queuig mechaism: requires use of a scheduler app % RT X appn NRT 00% root (li 40% 30% 30% 30% 0% Y 25% RT RT real-time NRT o-real-time Z 5% NRT Queue maagemet [] Queue maagemet ad cogestio cotrol Schedulig: which output queue to visit which packet to trasmit from output queue Queue maagemet: esurig buffers are available: memory maagemet orgaisig packets withi queue packet droppig whe queue is full cogestio cotrol 4

5 Queue maagemet [2] Packet droppig policies Cogestio: misbehavig sources source sychroisatio routig istability etwork failure causig re-routig cogestio could hurt may flows: aggregatio Drop packets: drop ew packets util queue clears? admit ew packets, drop existig packets i queue? Drop-from-tail: easy to implemet delayed packets at withi queue may expire Drop-from-head: old packets purged first good for real time better for TCP Radom drop: fair if all sources behavig misbehavig sources more heavily pealised Flush queue: drop all packets i queue simple flows should back-off iefficiet Itelliget drop: based o level 4 iformatio may eed a lot of state iformatio should be fairer Ed system reactio to packet drops No-real-time TCP: packet drop cogestio slow dow trasmissio slow start cogestio avoidace etwork is happy! Real-time UDP: packet drop fill-i at receiver?? applicatio-level cogestio cotrol required flow data rate adaptatio ot be suited to audio/video? real-time flows may ot adapt hurts adaptive flows Queue maagemet could protect adaptive flows: smart queue maagemet required RED [] Radom Early Detectio: spot cogestio before it happes drop packet pre-emptive cogestio sigal source slows dow prevets real cogestio Which packets to drop? moitor flows cost i state ad processig overhead vs. overall performace of the etwork RED [2] Probability of packet drop queue legth Queue legth value expoetial average: smooths reactio to small bursts puishes sustaied heavy traffic Packets ca be dropped or marked as offedig : RED-aware routers more likely to drop offedig packets Source must be adaptive: OK for TCP real-time traffic UDP? TCP-like adaptatio for real-time flows Mechaisms like RED require adaptive sources How to idicate cogestio? packet drop OK for TCP packet drop hurts real-time flows use ECN? Adaptatio mechaisms: layered audio/video codecs TCP is uicast: real-time ca be multicast 5

6 Schedulig ad queue maagemet: Discussio Summary Fairess ad protectio: queue overflow cogestio feedback from router: packet drop? Scalability: graularity of flow speed of operatio Flow adaptatio: o-real time: TCP real-time? Aggregatio: graularity of cotrol graularity of service amout of router state lack of protectio Sigallig: set-up of router state iform router about a flow explicit cogestio otificatio? Schedulig mechaisms work-coservig vs. o-work-coservig Schedulig requiremets Schedulig dimesios Queue maagemet Cogestio cotrol 6