1 Delay,'loss,'layered'architectures'Computer'Networks'Delay,'loss'and'throughput'Layered'architectures'Delay,'loss,'layered'architectures'How'do'loss'and'delay'occur?'packets'queue'in'router'buffers''• packet'arrival'rate'to'link'exceeds'output'link'capacity'• packets'queue,'wait'for'tu rn'A B packet'being'transmiBed'(delay)'packets'queueing'(delay)'free'(available)'buffers:'arriving'packets''dropped'(loss)'if'no'free'buffers'2 Delay,'loss,'layered'architectures'Fo ur'sou rces'of'packet'delay'1. Nodal'processing:''– check'bit'errors'– determine'output'link'2. Queueing'– Kme'waiKng'at'output'link'for'transmission''– depends'on'congesKon'level'of'router'3. Transmission'delay:'– aka'store'&'forward'delay'– R=link'bandwidth'(bps)'– L=packet'length'(bits)'– Kme'to'send'bits'into'link'='L/R'2. PropagaKon'delay:'– d'='length'of'physical'link'– s'='propagaKon'speed'in'medium'(~2x108'm/sec)'– propagaKon'delay'='d/s'A B propagation transmission nodal processing queueing Note:'s'and'R'are'very'different'quanKKes!'Delay,'loss,'layered'architectures'Caravan'analogy'• cars'“propagate”'at''100'km/hr'• toll'booth'takes'12'sec'to'service'car'(trans missi on'Kme)'• car~bit;'caravan'~'packet'• Q:'How'long'unKl'caravan'is'lined'up'before'2nd'toll'booth?'• Time'to'“push”'enKre'caravan'through'toll'booth'onto'highway'='12*10'='120'sec'• Time'for'last'car'to'propagate'from'1st'to'2nd'tol l'b oth:'100km/(100km/hr)='1'hr'• A:'62'minutes'• 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?'• 1st'bit'of'packet'can'arrive'at'2nd'router'before'packet'is'fully'transmiBed'at'1st'router!'toll booth toll booth ten-car caravan 100 km 100 km3 Delay,'loss,'layered'architectures'Nodal'delay'• dproc'='processing'delay'– typically'a'few'microsecs'or'less'• dqueue'='queuing'delay'– depends'on'congesKon'• dtrans'='transmission'delay'– ='L/R,'significant'for'low_speed'links'• dprop'='propagaKon'delay'– a'few'microsecs'to'hundreds'of'msecs'Delay,'loss,'layered'architectures'Queueing'delay'(revisited)'• R=link'bandwidth'(bps)'• L=packet'length'(bits)'• a=average'packet'arrival'rate'traffic'intensity'='La/R'• 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!'4 Delay,'loss,'layered'architectures'“Real”'Internet'delays'and'routes'• What'do'“real”'Internet'd elay'&'loss'look'like?''• Traceroute'program:'provides'delay'measurement'from'source'to'router'along'end_end'Internet'path'towards'desKnaKon.''For'all 'i:'– sends'three'packets'that'will'reach'router'i'on'path'towards'desKnaKon'– router'i'will'return'packets'to'sender'– sender'Kmes'interval'between'transmission'and'reply.'3 probes 3 probes 3 probes Delay,'loss,'layered'architectures'“Real”'Internet'delays'and'routes'1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms traceroute: gaia.cs.umass.edu to www.eurecom.fr Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu * means no response (probe lost, router not replying) trans-oceanic link5 Delay,'loss,'layered'architectures'Packet'loss'• queue'(aka'buffer)'preceding'link'in'buffer'has'finite'capacity'• packet'arriving'to'full'queue'dropped'(aka'lost)'• lost'packet'may'be'retransmiBed'by'previous'node,'by'source'end'system,'or'not'at'all'A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area) Delay,'loss,'layered'architectures'Throughput'• throughput:'rate'(bits/Kme'unit)'at'which'bits'transferred'between'sender/receiver'– instantaneous:'rate'at'given'point'in'Kme'– average:'rate'over'longer'period'of'Kme'server, with file of F bits to send to client link capacity Rs bits/sec link capacity Rc bits/sec pipe that can carry fluid at rate Rs bits/sec) pipe that can carry fluid at rate Rc bits/sec) server sends bits (fluid) into pipe6 Delay,'loss,'layered'architectures'Throughput'(more)'• Rs'<'Rc''What'is'average'end_end'throughput?' Rs bits/sec Rc bits/sec • Rs'>'Rc''What'is'average'end_end'throughput?'Rs bits/sec Rc bits/sec link'on'end_end'path'that'constrains''end_end'throughput'bo7leneck'link'Delay,'loss,'layered'architectures'Throughput:'Internet'scenario'• per_connecKon'end_end'throughput:'min(Rc,Rs,R/10)'• in'pracKce:'Rc'or'Rs'is'ocen'boBleneck'10 connections (fairly) share backbone bottleneck link R bits/sec Rs Rs Rs Rc Rc Rc R7 Delay,'loss,'layered'architectures'Protocol'“Layers”'Networks'are'complex!''• many'“pieces”:'– hosts'– routers'– links'of'various'media'– applicaKons'– protocols'– hardware,'socware'QuesKon:''Is'there'any'hope'of'organizing'structure'of'network?'Or'at'least'our'discussion'of'networks?'Delay,'loss,'layered'architectures'OrganizaKon'of'air'travel'• a'series'of'steps'ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing airplane routing8