Computer Networks Performance Metrics

Performance Metrics Outline Generic Performance Metrics Network performance Measures Components of Hop and End-to-End Packet Delay Other Performance Measures Computer Networks Performance Metrics2

Computer Networks Computer Networks Performance Metrics Host B Host C Host L Host D Host E Host G Host J Host A Host H Host F Host M W TX Y Z nodes AP W1 W2 W3 W4

How do Loss and Delay occur? packets queue in router buffers when the sum of the arriving packets at the router exceeds the output link capacity. Computer Networks Performance Metrics packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers 1517

Simple Queueing Model Computer Networks Performance Metrics5 Customer Arrivals Queue Server

Router Node Computer Networks Performance Metrics 6 packet node Outgoing Link Router Buffer Server packet Incoming Link

Utilization :: ◦ The percentage of time a device is busy servicing a “customer”. Throughput :: ◦ The number of jobs processed by the “system” per unit time. Response time :: ◦ The time required to receive the response of a request (round-trip time (RTT) ). Delay :: ◦ The time required to perform an operation. Computer Networks Performance Metrics7 Generic Performance Metrics

Network Performance Measures Channel utilization :: The average fraction of time a channel is busy [e.g. Util. = 80%] ◦ Note: Channel utilization is often referred to as channel efficiency, if overhead is not taken into account (i.e., it is excluded from the useful bits sent), Throughput:: The amount of data that can be sent by the channel per unit time (bits/sec) [e.g. Tput. = 10 Mbps] Computer Networks Performance Metrics8

Throughput Throughput:: rate at which bits transferred between sender and receiver ◦ instantaneous: rate at a given point in time ◦ average: rate over longer period of time Computer Networks Performance Metrics9 server, with file of F bits to send to client link capacity R s bits/sec link capacity R c bits/sec pipe that can carry fluid at rate R s bits/sec) pipe that can carry fluid at rate R c bits/sec) server sends bits (fluid) into pipe

Throughput (more) R s < R c What is average end-end throughput? Computer Networks Performance Metrics10 R s bits/sec R c bits/sec R s > R c What is average end-end throughput? R s bits/sec R c bits/sec link on end-end path that constrains end-end throughput bottleneck link

Throughput: Internet Scenario per-connection end-end throughput: x connections (fairly) share backbone link R bits/sec. Bandwidth is divided among the connections. What’s the main bottleneck Min{R c,R s,R/x} Computer Networks Performance Metrics11 RsRs RsRs RsRs RcRc RcRc RcRc R

End-to-End Packet Delay Computer Networks Performance Metrics12 End-to-end delay includes multiple hop link delays. The sum of delays of the connecting links. Tanenbaum

Hop Delay Components Nodal Processing Queuing Data Transmission Propagation Computer Networks Performance Metrics13

Four Sources of Packet Delay Processing Delay  Checking for bit errors  Determination of output link  Moving packet from input queue to output queue  Table lookup time (see routing algorithms) Computer Networks Performance Metrics 14 A B propagation transmission nodal processing queueing 1.Processing delay [PROC] Between the time the packet is correctly received at the head node of the incoming link and the time the packet is assigned to an outgoing link queue for transmission.

Four Sources of Packet Delay Computer Networks Performance Metrics 15 A B propagation transmission nodal processing queueing Queueing delay  Time waiting at output link for transmission  Depends on congestion level of router 2.Queuing delay [QUEUE] Between the time the packet is assigned to a queue for transmission and the time it starts being transmitted. Between the time the packet is assigned to a queue for transmission and the time it starts being transmitted. During this time, the packet waits while other packets in the transmission queue are transmitted. During this time, the packet waits while other packets in the transmission queue are transmitted.

Transmission delay: R=link bandwidth (bps) L=packet length (bits) Time required to send bits into link = L/R (sec.) Computer Networks Performance Metrics 16 A B propagation transmission nodal processing queueing Four Sources of Packet Delay 3.Transmission delay [TRANS] Between the times that the first and last bits of the packet are transmitted. Depends on available bandwidth.

Propagation delay: d = Length of physical link (met.) s = Propagation speed in medium (met./sec.) Propagation delay = d/s (sec.) Depends on the physical distance and medium Computer Networks Performance Metrics 17 A B propagation transmission nodal processing queueing Four Sources of Packet Delay 4. Propagation delay [PROP] Between the time the last bit is transmitted at the head node of the link queue and the time the last bit is received at the next router.

End-to-end Delay End-to-end packet delay :: The time required for delivering a packet from the source node to the destination Host (or node). Most often, we are interested in the packet delay within the communication subnets. This delay is the sum of the delays on each subnet link traversed by the packet. Computer Networks Performance Metrics18

Link (Nodal) Delay d proc = processing delay ◦ typically a few microsecs or less d queue = queuing delay ◦ depends on congestion d trans = transmission delay ◦ = L/R, significant for narrow bandwidth channels d prop = propagation delay ◦ a few microsecs to tens of msecs Computer Networks Performance Metrics19

End-to-End Packet Delay End-to-End delay = Sum of ALL link packet delays. End-to-End can be defined either: ◦ From Host-to-Host ◦ From end-to-end nodes within the subnet. Computer Networks Performance Metrics20 E-T-E delay=  (Link Delay) Link Delay= PROC+QUEUE+TRANS+PROP

End-to-End Packet Delay Computer Networks Performance Metrics21 End-to-end delay includes multiple hop link delays.

Queueing Delay R=Link (Channel) bandwidth (bits/s) L=Packet length (bits) a=Average packet arrival rate (p/s) Computer Networks Performance Metrics 22 Traffic Intensity = La/R  La/R~0: Average queuing delay small  La/R  1: Delays become large  La/R> or =1: More work arriving than can be handled. The rate at which bits arrive exceeds the rate bits can be transmitted.

Network Performance Measures Latency :: ◦ usually implies the minimum possible delay. Latency assumes no queueing and no contention encountered along the path. Goodput :: ◦ measured at the receiver ◦ the number of useful traffic delivered by the network to a certain destination, per unit of time. ◦ File transmission (file size/transfer time) ◦ No Retransmitted packets ◦ No protocol overhead ◦ No dropped packets ◦ Goodput  Throughput Computer Networks Performance Metrics23

Network Performance Measures Fairness :: ◦ Fairness determines whether users or applications are receiving a fair share of system resources. ◦ There are several mathematical and conceptual definitions of fairness. Jain’s fairness (Raj Jain) ◦ n : number of users ◦ x i : the resource share allotted to ith user ◦ ranges from 1/n (worst case) to 1 (best case) ◦ it is maximum when all users receive the same allocation. ◦ is k/n when k users equally share the resource, and the other n − k users receive zero allocation. Computer Networks Performance Metrics24

Network Performance Measures ◦ Other fairness Definitions (e.g., Max-min) ? Computer Networks Performance Metrics25

Network Performance Measures Quality of Service (QoS) :: ◦ QoS refers to a broad collection of networking technologies and techniques. ◦ The goal of QoS is to provide guarantees on the ability of a network to deliver predictable results. ◦ Several QoS-based network performance factors are  availability (uptime)  error rate (reliability)  bandwidth (throughput)  latency (delay) Computer Networks Performance Metrics26

Network Performance Metrics Packet Delivery Ratio (PDR):: ◦ The ratio of the number of received packets to the total number of sent packets ◦ PDR= Packets Received / Packets Sent ◦ Packets sent/received values should be from the same layer. i.e. all from application layer or network layer No Duplicate, No Retransmission Packet loss rate:: ◦ The percentage of the lost or dropped packets. Link layer retransmission rates:: ◦ The percentage of DL layer frames that are retransmitted. Computer Networks Performance Metrics27

Packet Loss Packet loss is the failure of one or more transmitted packets to arrive at their destinations. Packet loss reasons: ◦ Low signal strength ◦ Interference ◦ Congestion ◦ Overburdened network Queue (buffer) capacity is finite, and the packets arriving to the full queue are dropped. Computer Networks Performance Metrics 28 A B packet being transmitted packet arriving to full buffer is dropped. buffer (waiting area)

Performance Metrics Summary The three most general performance measures are : utilization, throughput and response time. In computer networks, end-to-end delay is an important performance metric. Queuing models are used to analyze and estimate computer network performance. Computer Networks Performance Metrics29

Performance Metrics Summary Other useful metrics include: latency, goodput, fairness and QoS metrics such as Availability, reliability. In wireless networks, delivery ratio, packet loss rate and link layer retransmission rates are valuable network measures. Computer Networks Performance Metrics30