计算机网络英文课件：lecture-04-Delay, loss, and throughput

1、Computer Networking Lecture 3Quantifying Delay, Loss and Throughput延时，丢包，吞吐量延时，丢包，吞吐量March 3, 2016Dr. Qingyao WSome slides are from lectures of Prof. J.F Kurose and K.W. RossHow do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link capacity packets q

2、ueue, wait for turnABpacket being transmitted (delay)packets queueing (delay)free (available) buffers: arriving packets dropped (loss) if no free buffersFour sources of packet delay 1. nodal processing: check bit errors determine output linkABpropagationtransmissionnodalprocessingqueueing 2. queuein

3、g time waiting at output link for transmission depends on congestion level of routerDelay in packet-switched networks3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R4. Propagation delay: d = length of physical link s = propagation speed in medium

4、 (2x108 m/sec) propagation delay = d/sABpropagationtransmissionnodalprocessingqueueingNote: s and R are very different quantities!Caravan analogy cars “propagate” at 100 km/hr toll booth takes 12 sec to service car (transmission time) carbit; caravan packet Q: How long until caravan is lined up befo

5、re 2nd toll booth?toll boothtoll booth10-car caravan100 km100 kmCaravan analogy cars “propagate” at 100 km/hr toll booth takes 12 sec to service car (transmission time) carbit; caravan packet Q: How long until caravan is lined up before 2nd toll booth? Time to “push” entire caravan through toll boot

6、h onto highway = 12*10 = 120 sec Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutestoll boothtoll booth10-car caravan100 km100 kmCaravan analogy (more) Cars now “propagate” at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2n

7、d booth before all cars serviced at 1st booth?toll boothtoll booth10-car caravan100 km100 kmCaravan analogy (more) Cars now “propagate” at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth? Yes! After 7 min, 1st car at 2nd b

8、ooth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! See Ethernet applet at AWL Web sitetoll boothtoll booth10-car caravan100 km100 kmEnd-to-End delay What about End-to-End delay?Queueing delay (revisited) R=link bandwidth (

9、bps) L=packet length (bits) a=average packet arrival ratetraffic intensity = La/R(流量强度）流量强度） La/R 0: average queueing delay small La/R - 1: delays become large La/R 1: more “work” arriving than can be serviced, average delay infinite!Review: Nodal delayproptransqueueprocnodalddddd“Real” Internet del

10、ays and routes What do “real” Internet delay & loss look like? Traceroute tracert program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path towards destination router i will return pa

11、ckets to sender sender times interval between transmission and reply.3 probes3 probes3 probes“Real” Internet delays and routes1cs-gw(54)1(45)1(30)6ms5ms5ms4jn1-at1-0-0-(29)16ms11ms13

12、ms5jn1-so7-0-0-(36)21()22(6)22ms22ms22ms853(53)104ms109ms106ms9de2-(29)(0)113ms121ms114ms11renater-(4)112ms114ms112ms12nio-n2.cssi.ren

13、ater.fr(3)111ms114ms116ms13nice.cssi.renater.fr(02)123ms125ms124ms14r3t2-nice.cssi.renater.fr(10)126ms126ms124ms15eurecom-(4)135ms128ms133ms165(5)126ms128ms126ms17*18*19fantasia.eurecom.fr(42)132ms128ms136 mstracero

14、ute: to www.eurecom.frThree delay measurements from to * means no response (probe lost, router not replying)trans-oceaniclinkPacket loss queue (aka buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (aka l

15、ost) lost packet may be retransmitted by previous node, by source end system, or not at allABpacket being transmittedpacket arriving tofull buffer is lostbuffer (waiting area)Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given p

16、oint in time average: rate over long(er) period of timeserver, withfile of F bits to send to clientlink capacity Rs bits/seclink capacity Rc bits/sec pipe that can carryfluid at rate Rs bits/sec) pipe that can carryfluid at rate Rc bits/sec)server sends bits (fluid) into pipeThroughput (more) Rs Rc What is average end-end throughput?Rs bits/sec Rc bits/seclink on end-en