Introduction1-1Chapter 1: roadmap1.1 What isthe Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service modelsIntroduction1-2Internet structure: network of networks roughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity/level3, Sprint, AT&T, QWest), national/international coverage treat each other as equalsTier 1 ISPTier 1 ISPTier 1 ISPTier-1 providers interconnect (peer) privatelyNAPTier-1 providers also interconnect at public network access points (NAPs)Introduction1-3Tier-1 ISP: e.g., SprintSprint US backbone networkIntroduction1-4Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPsTier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customeroftier-1 providerTier-2 ISPs also peer privately with each other, interconnect at NAPIntroduction1-5Internet structure: network of networks “Tier-3” ISPs and local ISPs  last hop (“access”) network (closest to end systems)Tier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPLocal and tier-3 ISPs are customersofhigher tier ISPsconnecting them to rest of InternetIntroduction1-6Internet structure: network of networks a packet passes through many networks!Tier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPIntroduction1-7Chapter 1: roadmap1.1 What isthe Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service modelsIntroduction1-8How do loss and delay occur?packets queuein router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turnABpacket being transmitted (delay)packets queueing (delay)free (available) buffers: arriving packets dropped (loss) if no free buffersIntroduction1-9Four sources of packet delay 1. nodal processing: check bit errors determine output linkABpropagationtransmissionnodalprocessingqueueing 2. queueing time waiting at output link for transmission  depends on congestion level of routerIntroduction1-10Delay 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 (~2x108m/sec) propagation delay = d/sABpropagationtransmissionnodalprocessingqueueingNote: s and R are very different quantities!Introduction1-11Nodal 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 msecsproptransqueueprocnodalddddd+++=Introduction1-12Queueing delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet arrival ratetraffic 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!Introduction1-13“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 ion path towards destination router iwill return packets to sender sender times interval between transmission and reply.3 probes3 probes3 probesIntroduction1-14“Real” Internet delays and routes1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms4 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 ms7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms17 * * *18 * * *19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 mstraceroute: gaia.cs.umass.edu to www.eurecom.frThree delay measements from gaia.cs.umass.edu to cs-gw.cs.umass.edu* means no reponse (probe lost, router not replying)trans-oceaniclinkIntroduction1-15Packet loss queue (aka buffer) preceding link in buffer has finite capacity when packet arrives to full queue, packet is dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not retransmitted at allIntroduction1-16Chapter 1: roadmap1.1 What isthe Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service modelsIntroduction1-17Protocol “Layers”Networks are complex!  many “pieces”: hosts routers links of various media applications protocols hardware, softwareQuestion:Is there any hope of organizingstructure of network?Or at least our discussion of networks?Introduction1-18Why layering?Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in one procedure doesn’t affect rest of system