11 Introduction Introduction M/M/1 Queueing delay (revisited) - PDF document

Introduction Introduction Four sources of packet delay Delay in packet-switched networks 3. Transmission delay: 4. Propagation delay:  1. nodal processing:  2. queueing  R=link bandwidth (bps)  d = length of physical link  check bit errors  time waiting at output  L=packet length (bits)  s = propagation speed in link for transmission  determine output link medium (~2x10 8 m/sec)  time to send bits into  depends on congestion level of router link = L/R  propagation delay = d/s Note: s and R are very transmission A different quantities! propagation transmission A propagation B nodal B queueing processing nodal processing queueing CSE Department CSE Department 61 62 Introduction Introduction Caravan analogy Caravan analogy (more) 100 km 100 km 100 km 100 km ten-car ten-car toll toll toll toll caravan caravan booth booth booth booth  Time to “push” entire  Yes! After 7 min, 1st car  cars “propagate” at  Cars now “propagate” at caravan through toll at 2nd booth and 3 cars 100 km/hr 1000 km/hr booth onto highway = still at 1st booth.  toll booth takes 12 sec to 12*10 = 120 sec  Toll booth now takes 1  1st bit of packet can service car (transmission min to service a car  Time for last car to arrive at 2nd router time) propagate from 1st to  Q: Will cars arrive to before packet is fully  car~bit; caravan ~ packet 2nd toll both: 2nd booth before all transmitted at 1st router!  Q: How long until caravan 100km/(100km/hr)= 1 hr cars serviced at 1st  See Ethernet applet at AWL is lined up before 2nd toll booth? Web site  A: 62 minutes booth? CSE Department CSE Department 63 64 Introduction Introduction Exercise problem! Nodal delay     d d d d d nodal proc queue trans prop  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 low-speed links  d prop = propagation delay  a few microsecs to hundreds of msecs CSE Department CSE Department 65 66 11

Introduction Introduction M/M/1 Queueing delay (revisited)  R=link bandwidth (bps)  L=packet length (bits)  a=average packet arrival rate traffic intensity = La/R Expected number  La/R ~ 0: average queueing delay small  La/R -> 1: delays become large  La/R > 1: more “work” arriving than can be Expected time serviced, average delay infinite! CSE Department CSE Department 67 68 Introduction Introduction “Real” Internet delays and routes “Real” Internet delays and routes traceroute: gaia.cs.umass.edu to www.eurecom.fr  What do “real” Internet delay & loss look like? Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu  Traceroute program: provides delay 1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms measurement from source to router along end-end 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms Internet path towards destination. For all i: 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  sends three packets that will reach router i on path 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms trans-oceanic towards destination 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 link  router i will return packets to sender 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  sender times interval between transmission and reply. 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 3 probes 3 probes 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 * * * 3 probes * means no response (probe lost, router not replying) 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms CSE Department CSE Department 69 70 Introduction Introduction tracert Packet loss  queue (aka buffer) preceding link in buffer has finite capacity  packet arriving to full queue dropped (aka lost)  lost packet may be retransmitted by previous node, by source end system, or not at all buffer packet being transmitted (waiting area) A B packet arriving to full buffer is lost http://www.linkwan.com/gb/broadmeter/tracemap/traceroute.asp CSE Department CSE Department 71 72 12

Introduction Introduction Throughput Throughput (more)  throughput: rate (bits/time unit) at which  R s < R c What is average end-end throughput? bits transferred between sender/receiver  instantaneous: rate at given point in time R s bits/sec R c bits/sec  average: rate over longer period of time  R s > R c What is average end-end throughput? R s bits/sec R c bits/sec link capacity link capacity server, with pipe that can carry server sends bits pipe that can carry bottleneck link R s bits/sec R c bits/sec file of F bits fluid at rate (fluid) into pipe fluid at rate to send to client R s bits/sec) R c bits/sec) link on end-end path that constrains end-end throughput CSE Department CSE Department 73 74 Introduction Introduction Roadmap Throughput: Internet scenario  What is the Internet?  per-connection  Network edge R s end-end  end systems, access networks, links R s R s throughput:  Network core min(R c ,R s ,R/10)  circuit switching, packet switching, network R structure  Delay, loss and throughput in packet- R c R c  in practice: R c or switched networks R s is often R c  Protocol layers, service models bottleneck  Networks under attack: security  History 10 connections (fairly) share backbone bottleneck link R bits/sec CSE Department CSE Department 75 76 Introduction Introduction Protocol “Layers” Internet protocol stack Networks are complex!  application: supporting network applications  many “pieces”: application  FTP, SMTP, HTTP  hosts Question:  transport: process-process data  routers transport Is there any hope of transfer  links of various organizing structure of  TCP, UDP network media network?  network: routing of datagrams from  applications link source to destination  protocols Or at least our discussion  IP, routing protocols of networks?  hardware, physical  link: data transfer between software neighboring network elements  PPP, Ethernet  physical: bits “on the wire” CSE Department CSE Department 77 78 13