EE 122 Network Performance Evaluation September 7 2004 Overview Motivations Timing diagrams Metrics Evaluation techniques 2 Motivations Understanding network behavior Improving protocols Verifying correctness of implementation Detecting faults Monitor service level agreements Choosing providers Billing 3 Outline Motivations Timing diagrams Metrics Evaluation techniques 4 Timing Diagrams Sending one packet Queueing Switching Store and forward Cut through Fluid view 5 Definitions Link bandwidth capacity maximum rate in bps at which the sender can send data along the link Propagation delay time it takes the signal to travel from source to destination Packet transmission time time it takes the sender to transmit all bits of the packet Queuing delay time the packet need to wait before being transmitted because the queue was not empty when it arrived Processing Time time it takes a router switch to process the packet header manage memory etc 6 Sending One Packet R bits per second bps Bandwidth R bps Propagation delay T sec T seconds P bits Transmission time P R T time Propagation delay T Length speed of light 1 speed 3 3 usec in free space 4 usec in copper 5 usec in fiber 7 Sending one Packet Examples P 1 Kbyte R 1 Gbps 100 Km fiber T 500 usec P R 8 usec T P R T P R time T P R T P 1 Kbyte R 100 Mbps 1 Km fiber T 5 usec P R 80 usec P R time 8 Queueing The queue has Q bits when packet arrives packet has to wait for the queue to drain before being transmitted P bits Q bits Capacity R bps Propagation delay T sec Queueing delay Q R P R T time 9 Queueing Example P bits Q bits P 1 Kbit R 1 Mbps P R 1 ms Packet arrival Time ms 0 0 5 1 7 7 5 Delay for for packet packet that that arrives arrives at at time time t t d t d t Q t R Q t R P R P R bits in queue Q t Delay 2 Kb 1 5 Kb 1 Kb 0 5 Kb Time packet 1 d 0 1ms packet 2 d 0 5 1 5ms packet 3 d 1 2ms 10 Switching Store and Forward A packet is stored enqueued before being forwarded sent 10 Mbps Sender time 5 Mbps 100 Mbps 10 Mbps Receiver 11 Store and Forward Multiple Packet Example 10 Mbps Sender time 5 Mbps 100 Mbps 10 Mbps Receiver 12 Switching Cut Through A packet starts being forwarded sent as soon as its header is received R1 10 Mbps R2 10 Mbps Receiver Sender Header time What happens if R2 R1 13 Outline Motivations Timing diagrams Metrics Throughput Delay Evaluation techniques 14 Throughput Throughput of a connection or link total number of bits successfully transmitted during some period t t T divided by T Link utilization throughput of the link link rate Bit rate units 1Kbps 103bps 1Mbps 106bps 1 Gbps 109bps For memory 1 Kbyte 210 bytes 1024 bytes Some rates are expressed in packets per second pps relevant for routers switches where the bottleneck is the header processing 15 Example Windows Based Flow Control Source Connection Send W bits window size Wait for ACKs Repeat RTT RTT Assume the round triptime is RTT seconds Throughput W RTT bps Numerical example RTT W 64 Kbytes RTT 200 ms Throughput W T 2 6 Mbps Destination time 16 Throughput Fluctuations Throughput may vary over time Throughput max mean min Time 17 Delay 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 bandwidth and delay storage capacity of network 18 Delay Illustration 1 1 Sender Latest bit seen by time t 2 Receiver at point 2 at point 1 Delay time 19 Delay Illustration 2 1 Sender 2 Receiver Packet arrival times at 1 1 2 Delay Packet arrival times at 2 time 20 Little s Theorem Assume a system e g a queue at which packets arrive at rate a t Let d i be the delay of packet i i e time packet i spends in the system What is the average number of packets in the system d i delay of packet i a t arrival rate system Intuition Assume arrival rate is a 1 packet per second and the delay of each packet is s 5 seconds What is the average number of packets in the system 21 Little s Theorem 1 Latest bit seen by time t d i delay of packet i x t number of packets in transit in the system at time t Sender 2 Receiver x t time T What is the system occupancy i e average number of packets in transit between 1 and 2 22 Little s Theorem 1 Latest bit seen by time t d i delay of packet i x t number of packets in transit in the system at time t Sender 2 Receiver x t S area time T Average occupancy S T 23 Little s Theorem 1 Latest bit seen by time t d i delay of packet i x t number of packets in transit in the system at time t 2 Sender Receiver S N S N 1 P d N 1 x t S area time T S S 1 S 2 S N P d 1 d 2 d N 24 Little s Theorem 1 Latest bit seen by time t d i delay of packet i x t number of packets in transit in the system at time t 2 Sender Receiver S N P S N 1 d N 1 x t S area time T Average S T P d 1 d 2 d N T occupancy P N T d 1 d 2 d N N Average arrival time Average delay 25 Little s Theorem 1 Latest bit seen by time t d i delay of packet i x t number of packets in transit in the system at time t 2 Sender Receiver S N a i S N 1 d N 1 x t S area time T Average occupancy average arrival rate x average delay 26 Outline Motivations Timing diagrams Metrics Evaluation techniques 27 Evaluation Techniques Measurements gather data from a real network e g ping www berkeley edu realistic specific Simulations run a program that pretends to be a real network e g NS network simulator Nachos OS simulator Models analysis write some equations from which we can derive conclusions general may not be realistic Usually use combination of methods 28 Analysis Example M M 1 Queue a s Arrivals are Poisson with rate a Service times are exponentially distributed with mean 1 s s rate at which packets depart from a full queue Average delay per packet T 1 s a 1 a 1 u where u a s utilization Numerical example 1 a 1ms u 80 Q 5ms 29 Simulation Model of traffic Model of routers links Simulation …
View Full Document
Unlocking...