EECS122 Lecture 5 Department of Electrical Engineering and Computer Sciences University of California Berkeley Performance Motivation Timing Diagrams Metrics Evaluation Techniques EECS 122 UCB 2 Performance Motivation Understanding Network Behavior Improving Protocols Verifying Correctness of Implementation Detecting Faults Choosing Provider Feasibility of Applications Monitoring Service Level Agreements Billing EECS 122 UCB 3 Performance Timing Sending one packet Illustration Examples Queuing Queuing example Store and forward S F Multiple packets Cut Through Fluid View EECS 122 UCB 4 Timing One Packet R bps P bits T seconds T P R Time P R Transmission time T Propagation time L speed L km x time km Time km 3 3 s in free space 4 s in copper EECS 122 UCB 5 s in fiber 5 Timing Illustration Transmission Line Transmitter Receiver Signal One bit Each bit takes 1 R seconds to be transmitted The bits take T seconds to propagate EECS 122 UCB 6 Timing Examples T P R P 1KByte R 1Gbps 100km fiber T T 500 s P R 8 s P R T T P R EECS 122 UCB P R P 1KByte R 100Mbps 1km fiber T 5 s P R 80 s 7 Timing Queuing Link P bits R bps Q T seconds Q R Q R queuing delay load dependent T P R EECS 122 UCB Time 8 Timing Queuing ExampleR bps P bits Link 1 kbit packets R 1Mbps Q 1ms Time T t Q R P R for a packet that arrives at t 1ms Time t Let Tn Q R P R for packet n T1 1ms T2 1 5ms EECS 122 UCB T3 2ms T4 1ms T5 1 5ms T6 1ms 9 Timing Store and Forward System 10Mbps EECS 122 UCB 5Mbps 100Mbps 10Mbps 10 Timing S F Multiple System 10Mbps EECS 122 UCB 5Mbps 100Mbps 10Mbps 11 Timing Cut Through System R1 10Mbps R2 10Mbps Header Start forwarding as soon as the header is received Note What if R2 R1 EECS 122 UCB 12 Timing A Fluid View System A t rate a t bits in 0 t X t Rate R D t rate d t bits in 0 t a t d t X t EECS 122 UCB 13 Performance Metrics Throughput Delay EECS 122 UCB 14 Metrics Throughput Definitions Example 1 Connection Example 2 Link Fluctuations Measurements EECS 122 UCB 15 Throughput Definitions Roughly throughput bit rate e g 120Kbits second More precisely Throughput of a connection or of a link total number of bits during some period t t T divided by T Bandwidth of a link link rate bit rate of the transmitter Note misnomer but common usage Utilization of a link throughput of the link link rate Bit rate units 1Kbps 103bps 1Mbps 106bps 1Gbps 109bps For memory 1Kbytes 210 bytes 1 024 bytes 1MBytes 220 bytes Some rates are expressed in packet per second pps relevant when the bottleneck is the header processing EECS 122 UCB 16 Throughput Connection Connection Send W bits window size Wait for ACKs Repeat Assume that the round trip time is RTT seconds Throughput W RTT bps Numerical Example W 64KBytes 512 kbits 512x1 024 524 288 bits RTT 200ms Throughput W T 2 6Mbps EECS 122 UCB Source Destination K RTT K RTT Time 17 Throughput Link 1Mbps link sends 1 000 bit packets Mbps Rate every s 1 0 5 Rate every 40 ms Utilization 50 EECS 122 UCB Time t 18 Throughput Fluctuations Rate varies over time Throughput over last T seconds max mean min EECS 122 UCB Time t 19 Measurements TCP Keep track of number of bytes received Let R t number of bits in 0 t Throughput over last T seconds R t R t T T Link Device has counter with number of bytes received calculate as above EECS 122 UCB 20 Metrics Delay Definitions Illustration 1 Illustration 2 Little s Result Measurements Example 1 Measurements Example 2 EECS 122 UCB 21 Delay Definitions Delay Latency of bit packet file from A to B The time required for bit packet file to go from A to B Jitter Variability in delay Round Trip Time RTT Two way delay from sender to receiver and back Bandwidth Delay Product Product of bw and delay indicates storage capacity of network EECS 122 UCB 22 Delay Illustration 1 2 1 S D Latest bit seen by time t at point 1 at point 2 n Delay of bit n EECS 122 UCB 23 Delay Illustration 2 1 2 S D Packet arrival times at 1 1 20 ms 2 Packet arrival times at 2 EECS 122 UCB Max delay 100 ms Min delay 40 ms Jitter 60 ms 24 Delay Little s Result N 1 S T N T N 1 2 D X t S area S T 1 T N integral of X t 1 X t dt S T 1 T N N T T N T T Average occupancy average delay x average arrival rate EECS 122 UCB 25 Delay Measurements 1 A Good Epoch EECS 122 UCB 26 Delay Measurements 2 A Worse Epoch EECS 122 UCB 27 Performance Evaluation Techniques Models Analysis Models Simulations Measurements EECS 122 UCB 28 Evaluation Analysis Example M M 1 Queue Arrivals are Poisson with rate Service times are exponentially distributed with mean 1 Average delay per packet T 1 1 1 where utilization For instance 1 1ms and 80 Q 5ms EECS 122 UCB 29 Evaluation Simulation Model of traffic Model of routers links Simulation Time Driven X n state at time n X n 1 f X n event at time n Event Driven Y n state after event n E n n th event T n time when event n occurs Y n 1 T n 1 g Y n T n E n Key Step Output analysis estimates confidence intervals EECS 122 UCB 30 Evaluation Measurements Design Experiment Representative Output Analysis EECS 122 UCB 31
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