How do loss and delay occur?

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Network Performance Modelling NWEN302 Computer Network Design

How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) A free (available) buffers: arriving packets dropped (loss) if no free buffers packets queueing (delay) B © Winston Seah NWEN302 Computer Network Design

Four sources of packet delay 1. nodal processing: check bit errors determine output link 2. queueing: time waiting at output link for transmission depends on congestion level of router A B propagation transmission nodal processing queueing © Winston Seah NWEN302 Computer Network Design

Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s Note: s and R are very different quantities! A B propagation transmission nodal processing queueing © Winston Seah NWEN302 Computer Network Design

NWEN302 Computer Network Design Caravan analogy toll booth ten-car caravan 100 km cars “propagate” at 100 km/hr toll booth takes 12 sec to service car (transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? Time to “push” entire caravan through toll booth 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 minutes © Winston Seah NWEN302 Computer Network Design

Caravan analogy (more) toll booth ten-car caravan 100 km 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 booth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! © Winston Seah NWEN302 Computer Network Design

NWEN302 Computer Network Design Nodal delay dproc = processing delay typically a few microsecs or less dqueue = queuing delay depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs © Winston Seah NWEN302 Computer Network Design

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! © Winston Seah NWEN302 Computer Network Design

“Real” Internet delays and routes What do “real” Internet delay & loss look like? Traceroute 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 packets to sender sender times interval between transmission and reply. 3 probes 3 probes 3 probes © Winston Seah NWEN302 Computer Network Design

“Real” Internet delays and routes traceroute: gaia.cs.umass.edu to www.eurecom.fr Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu 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 trans-oceanic link * means no response (probe lost, router not replying) © Winston Seah NWEN302 Computer Network Design

NWEN302 Computer Network Design Packet loss queue (aka buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (lost) lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) packet being transmitted A B packet arriving to full buffer is lost © Winston Seah NWEN302 Computer Network Design

NWEN302 Computer Network Design Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time pipe that can carry fluid at rate Rc bits/sec) pipe that can carry fluid at rate Rs bits/sec) server sends bits (fluid) into pipe link capacity Rs bits/sec link capacity Rc bits/sec server, with file of F bits to send to client © Winston Seah NWEN302 Computer Network Design

NWEN302 Computer Network Design Throughput (more) Rs < Rc What is average end-end throughput? Rc bits/sec Rs 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 bottleneck link © Winston Seah NWEN302 Computer Network Design

Throughput: Internet scenario per-connection end-end throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck Rs Rs Rs R Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec Rc © Winston Seah NWEN302 Computer Network Design