Congestion Control & Resource Allocation

Transcription

1 Congestion Control & Resource Allocation Proper Use of Networks Congestion A system is loaded beyond its capacity In a lightly loaded network, network throughput and delay increase linearly as offered load increases Delay Throughput Beyond the breakup point, throughput decreases even though offered load increases Delay increases exponentially Offered Load

2 Congestion Control & Resource Allocation The only solution to the congestion problem is to throttle the packets entered into the network Little s theorem Two ways to solve the congestion problem Congestion control Resource allocation Congestion control Send packets, if congestion occurs reduce the sending rate Mostly performed at hosts Resource allocation Request resources (e.g. bandwidth) before sending packets Limits the sending rate to the agreed amount SNU SCONE lab. 3 Evaluation of CC Algorithms Efficiency & Fairness Easy to achieve one while scarifying the other Efficiency How efficiently(properly) uses network resources? In most cases, as throughput increases, delay increases also Power = Throughput / Delay Consider M/M/1/ system Throughput = λ Delay = 1/(μ- λ) Power = λ (μ- λ) SNU SCONE lab. Too pessimistic

3 Fairness No unanimously agreed definition Are resources fairly allocated to users? f1 f2 How to allocate bandwidth to f1 and f2? One popular fairness index 2 2 f( x, x,..., x ) ( x) n x 1 2 n i i Xi: throughput of flow i SNU INC lab. 5 Congestion Control TCP provides a reliable transport service Layer 4 error control mechanism based on Go-Back-N ARQ ACK, window size, TimeOut, Differences of Layer 2/4 Layer 2 Layer 4 Network structure Single link Many physical networks Sender One (P2P) Many Resource sharing None, MAC Congestion control Characteristics of layer 4 Some parts of networks are congested while other parts are not Indirect sensing 6

4 Taxonomy Closed-loop Control mechanism Monitor the network condition Feedback Adapt the sending rate to the S observed network condition Congestion detection (Monitor) Explicit vs Implicit Inflow control Window vs. Rate Response (ACK) R SNU SCONE lab. 7 Congestion Detection How to obtain network information? Explicit Network (routers) informs the network status explicitly Good performance Overhead of information acquisition Implicit Guess the network condition by looking at various symptoms Less accurate, but simpler & smaller overhead SNU SCONE lab. 8

5 Input Control - 1 How to regulate the packet inflow? Window based Like the ARQ window Limit the number of packets that can be sent without ACKs Vary window size according to the network conditions Window size & Rate? N=λ T S D Plusses for window No need for fine-grained timer Self-limiting System 9 Input Control - 2 Rate-based Adjust the packet sending rate Needs a timer Plusses for rate Better control (finer grain) De-coupling of congestion control and error control Safeness of control mechanism Robust against error, malfunction, SNU SCONE lab. 10

6 TCP Congestion Control TCP Network Model Empire state building elevators Two elevators, one from ground to the 80 th floor and another from the 80 th to observation deck Only the first has a long line If a path is congested, only one link has a (long) queue Single bottleneck link S R 12

7 TCP Network Model - 2 Constant RTT RTTs are about the same whether the path is congested or not Routers have a very small buffer (e.g. buffer size is < 1000 packets) Link speed is ~1 Tbps Queueing delay can be ignored for long distance paths Constant RTT assumption is not valid anymore Buffer bloat problem SNU SCONE lab. 13 TCP CC Background The Internet was fully operational since 1980 At the beginning, no congestion control, just flow control A sender transmit as much bytes as possible up to AW (Advertised Window) Congestion Van Jacobson introduced the congestion control mechanism in late 1980 cwnd: Congestion window AW: Advertised Window (Or rwnd) S D rcvbuf: Socket interface receiver buffer AW = rcvbuf occupied bytes SNU SCONE lab. 14

8 TCP Error Control TCP provides end-to-end reliable delivery service Mechanism Go-back-N ARQ Selective repeat Learn SACK (Selective ACK) option For each segment, a receiver returns ACK to a sender Learn delayed ACK option Sender measures RTT DATA segment S R ACK segment SNU SCONE lab. 15 Today s News Topics Layer 4 TCP CC (Nov. 15): PD 6.3 Advanced. CC (Nov. 17): PD 6.4 TCP, UDP (Nov. 22): PD: 5.1, 5.2 SNU SCONE lab. 16

9 TCP Congestion Control - Feedback Implicit feedback What are the symptoms of network congestion? Large RTT Packet loss Out-of-order delivery (missing segment) A congested node (router) receives packets more than its capacity Buffer will increase and packets will be dropped eventually A sender waits for an ACK after sending a segment If an ACK arrives before TO, judge the network is not congested In the case of TO, guess the segment was dropped due to network congestion Is the feedback reliable? 17 TCP Congestion Control - AIMD Additive Increase (AI) Increase window size (cwnd) by one at each RTT when all ACKs arrive before TO Not congested Increment rule inc = MSS * (MSS / cwnd) cwnd += inc Note: cwnd unit is byte MSS: Max. Segment Size Multiplicative Decrease (MD) TO Congested Decrease cwnd in the case of TO cwnd = cwnd / 2 TCP may drop cwnd to 1 MSS SNU SCONE lab.

10 Behavior of AIMD Oscillating on the fairness line f2 Fairness Line S1 f1 R1 Efficiency Line S2 f2 R2 RTT1 = RTT2 f1 Redo for RTT1 = 2*RTT2 SNU SCONE lab. 19 Slow Start AI is too conservative Takes one RTT to increase cwnd one MSS Let RTT =0.1 sec. & BW = 10 Gbps, How long will it take until fully use the BW? (MSS=10,000 bits) Slow start Increase cwnd multiplicatively Two situations to apply the slow start Initialization No network information Recovery from TO Use previous network information Multiplicative increase up to threshold = (half of the cwnd prior to the TO) 20

11 No transmission waiting for Ack Single packet loss during AI Example Trace Multiple packet drops Suppose the available BW is 20 KB If cwnd is 16, then all packets will be delivered Slow start increases cwnd to KB would be lost TO when cwnd = 22 Slow start up to 11 and then AI Problem: Coarse timer granularity => Waste bandwidth waiting for timeouts SNU INC lab. 21 Fast Retransmission Solve the problem of long TO Differences? btw Receiving duplicate ACKs Out-of-order delivery Packet loss SNU SCONE lab. 22

12 Fast Retransmission - 2 Fast retransmit If duplicate ACKs arrive, retransmit unacked segments immediately not waiting for TO How many duplicate ACKs trigger FR? Window size? SNU SCONE lab. 23 TCP Tahoe Basic AIMD Slow start & Fast retransmission 24

13 Fast Recovery & TCP Reno Mechanism Remove the slow start phase in the case of Fast Retransmit Go directly to half the last CW Increase cwnd additively from the threshold TCP Reno In addition to TCP Tahoe, add fast recovery and (header prediction + delayed ACK) mechanisms What is Header Prediction? SNU SCONE lab. 25 Congestion Avoidance Protocols

14 Congestion Avoidance Resource Allocation Congestion Avoidance Congestion Control Prevent congestion Detect the symptoms when congestion may occur soon and adjust the sending rate Many methods rely on (explicit) feedbacks from routers SNU SCONE lab. 27 DECbit Flow Control Mechanism Every packet has a bit in header Intermediate routers set congestion bit if average queue length >=? The destination node copies the congestion bit to ACK The sender monitors ACKs and adjust cwnd according to AIMD How to judge Congestion? DECbit Condition to trigger congestion avoidance? SNU SCONE lab. 28

15 Average Queue Length Computed over queue regeneration cycles Balance between sensitivity and stability If AvgLen > 1, mark the bit with 50% probability SNU SCONE lab. 29 Source Actions Observe bits over past + present window size Should not take control actions too fast! Wait for past change to take effect If more than 50% set, then decrease window, else increase Additive increase, multiplicative decrease cwnd = cwnd + 1 cwnd = * cwnd SNU SCONE lab. 30

16 RED (Random Early Detection) TCP Feedback is generated when a packet is dropped Drop packets in advance Early random drop Drop packets before the buffer is full Early feedback to avoid congestion Implicit (Indirect) feedback No need to change the TCP protocol (hosts) SNU SCONE lab. 31 Packet Drop - 1 Adjust drop probability according to the severity of congestion (Queue length) Average queue length AvgLen=(1-W)*AvgLen + W*SampleLen C B A Queueing mechanism Case A enqueue the packet Case B drop packet with probability P (AvgLen,..) Case C drop packet 32

17 TCP Vegas Congestion Queued packets increases How to estimate the number of packets queued inside the network? Idea: symptoms that congestion will happen soon RTT is growing Sending rate flattens SNU SCONE lab. 33 N=λ T N λ NQ SNU SCONE lab. 34

18 TCP Vegas Estimate the # of packets in the queue S R Number of packets (bytes) in the network (= cwnd) = In-transit packets (Ns) + Queued packets (NQ) Time in the system (= RTT) We know them = In-transit time + Queueing time What other values can we know(estimate)? In-transit time = the RTT when there is no queueing BaseRTT Approximate it w/ the Minimum RTT that s been observed SNU SCONE lab. 35 TCP Vegas - 2 N= λ T cwnd = λ RTT S λ Ns Ts N= λ T Ns = λ BaseRTT Current input rate (sending rate) = cwnd / RTT ( = λ) Number of in-transit packets (Ns) = λ * BaseRTT NQ = N-Ns = cwnd - λ * BaseRTT = cwnd (cwnd/rtt) * BaseRTT = cwnd * BaseRTT * (1/BaseRTT 1/RTT) = BaseRTT * (cwnd/basertt cwnd/rtt) = BaseRTT * (Expected Rate ActualRate) λ NQ TQ R SNU INC lab. 36

19 TCP Vegas Adjustment Diff = ExpectedRate - ActualRate if Diff < -->increase CW linearly else if Diff > -->decrease CW linearly else -->leave CW unchanged SNU SCONE lab. 37 Expected Rate-α Expected Rate-β Expected Rate Actual Rate SNU SCONE lab. 38

20 Packet Pair Assumption Routers use round-robin scheduling 1 n 2 1 How to measure (my) capacity at a bottleneck link? Send two packets back to back Then, spacing between packets at receiver (= ack spacing) = 1/(rate of slowest server) SNU SCONE lab. 39 Packet Pair SNU SCONE lab. 40

21 Algorithm 1/r Sending rate Bottleneck link rate 1/b S R N(cwnd) = Nq + Ns 1/b Nq / Ns = # packets in bottleneck buffer/in service(transit) b = bottleneck rate Ns = RTT*b Nq = cwnd - RTT*b (assuming no losses) Let Tq is the target queue length cwnd = r*rtt = Nq + RTT*b r: sending rate To have the target queue length, adjust input rate to r r *RTT = Tq + RTT*b r(k+1) = r(k) + (Tq -Nq)/RTT SNU SCONE lab. 41 Resource Allocation QoS(Quality of Service)

22 QoS (Quality of Service) Real-time service Service that requires strict delay and loss performance Ex: A voice call requires the delay of less than ms Performance aspects Delay Loss Jitter Bandwidth SNU SCONE lab. 43 Application Taxonomy Applications Elastic Real Time Tolerate occasional losses? Intolerant Tolerant Adapt to network condition? Nonadaptive Adaptive Rate Adaptive Adjust sending rate Delay Adaptive Adjust playback delay SNU SCONE lab. 44

23 Queueing Queue in data structure Queue in network When a packet arrives at a router The router determines an outgoing link & enqueues the packet to the interface s buffer Waiting for its service according to the queueing principle Queueing Queueing is a complex mechanism Structure of buffers(queues) Methods of inserting arriving packets to queues Discard of packets Scheduling Service order: Who will be served first? SNU SCONE lab. 45 FIFO & Drop Tail Simplest queueing mechanism A single queue that is shared by all flows (sources) Serve the oldest packet first Drop tail When the buffer is full, discard new packets FIFO problems No isolation (separation) A user that generates many packets get more services No priority All packets are equally treated SNU SCONE lab. 46

24 Multiple Queues Isolation Multiple queues each dedicated to Priority Flow What is a flow?? A packet is inserted to the queue of its priority/flow Insertion & dropping to/from each queue may be same as a single queue Scheduling Priority scheduling Serve all high priority packets before serving low priority packets Fair scheduling Serve packets fairly (equally) 47 RR (Round Robin) Fair service Allocate resources equally to all customers Round Robin (RR) Serve each queue once per round What is the service (scheduling) unit? Packet by packet RR Send one packet from each flow per round Unfair if packet sizes are different Bit by bit RR Send one bit from each flow per round Fair but Impossible How to emulate bit by bit RR? 48

25 FQ (Fair Queueing) Emulation of bit by bit RR Virtual clock An imaginary clock that moves one tick when one bit from all active queues are transmitted Compute the finish time of each packets and transmit the packet with the earliest finish time No preemption Finish a transmission once it started Finish time: Fi Fi = Si + Pi Start time: Si Si = max (Fi-1, Ai) SNU SCONE lab. 49 FQ 2 WFQ(Weighted Fair Queueing) - Allocate different weights to flows Implementation - Computation of virtual clock - Selection of the earliest finish time packet : Heap sorting, O(log N) There are thousands of simultaneous flows!! 50

26 QoS Support Architectures Mechanisms to support QoS Separation (Classification) Different treatment (Scheduling) Resource reservation Blocking & Regulation (Policing) Two QoS support approaches IntServ (Integrated Service) architecture Fine-grained, Reserve resources for each flow Strict QoS support DiffServ (Differentiated Service) architecture Coarse-grained, Group flows into a priority class Scalability SNU SCONE lab. 51 IntServ Classify traffic into three categories GS (Guaranteed Service) Strict support of QoS requirements e.g. VoIP CS (Controlled-load Service) Performance of lightly loaded network e.g. Streaming BS (Best-effort Service) Mechanisms Admission control & Reservation Classification Scheduling Policing (Regulation) SNU SCONE lab. 52

27 Call Admission Control (CAC) Procedure Before sending packets, examine if the network can support the flow Admit the flow if there are enough resources necessary for QoS support at each link on the end-to-end path Flowspec A flow should specify how much traffic it will generate & the level of QoS Performance requirements (Rspec) For simplicity, use deterministic requirements No packet loss w/ a maximum allowable delay Traffic characteristics (Tspec) Characteristics of traffic that the flow generate SNU SCONE lab. 53 Traffic Specification CBR(Constant Bit Rate) and VBR(Variable) Traffic generated by realtime applications are highly variable Leaky bucket representation Burst size = bucket size Average flow rate = token rate Tokens stored more than σ are spilled over Tokens enter at rate ρ σ bytes Stop if there are not enough tokens Size s packet consumes s tokens A(t) σ + ρ t SNU SCONE lab. 54

28 Parekh-Gallager Theorem How to satisfy the strictest QoS requirement? Worst-case end-to-end delay (no packet loss) Assume that bw are allocated to a flow at each WFQ scheduler along its path, so that the least bw it is allocated is g Let it be leaky-bucket regulated such that # bits sent in time [t 1, t 2 ] <= ρ(t 2 -t 1 ) + Let the connection pass through K routers(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 / g K 1 P / g K k 1 k 1 P / r ( k ) SNU SCONE lab. 55 RSVP (Resource reservation Protocol) A signaling protocol that reserves bandwidth (buffer) for QoS guarantee on the best-effort Internet Examine if the links on the end-to-end path have enough resources Parekh-Gallager Theorem Features Soft state Multicast Receiver-oriented Filter SNU SCONE lab. 56

29 RSVP Procedure Sender initiates a session by sending PATH message to receiver Includes Tspec Route pinning Receiver determines the supportable traffic parameter and desirable QoS level Receiver determines the resource requirements (Rspec) and returns RESV message along the pinned route Intermediate nodes determine if they can support the flowspec (Tspec + Rspec) Maintain states for packet classification and resource reservation SNU SCONE lab. 57 DiffServ Motivations Scalable QoS support network IntServ is too complicated Emphasize good network planning Well-engineered networks usually provide good performance Minimize traffic control Architecture Distribute functions to core devices and edge devices Edge devices Flow by flow processing Classification Packet marking/shaping Core devices Simple priority queueing based on packet classes SNU SCONE lab. 58

30 DiffServ Service classes EF (Expedited Forwarding) Transmit the packet before any other packets AF (Assured Forwarding) Usually receive good performance BF Mechanisms Admission control For EF and AF services Regulator EF Marker AF SNU SCONE lab. 59 DiffServ Edge Router Edge router Identify flows Packet classification Regulation Discard violating packets Marking Mark violating packets as OUT (Out of profile) packets Flow 1 Classifier Flow N Marker Shaper SNU SCONE lab. 60

31 DiffServ Core Router Process packets based on classes PHB(Per Hop Behavior) EF Priority Queueing AF RIO (RED with In and Out) EF AF, BF SNU SCONE lab. 61 RIO Apply different parameters for IN and OUT packets P(drop) 1.0 MaxP AvgLen Min out Min in Max out Max in SNU SCONE lab. 62