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1、Chapter 1: roadmap1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History1.4.1 Access networksAccess networks can be loosely classifi
2、ed into three categories: residential access company access (university, company) wireless accessKeep in mind: bandwidth (bits per second) of access network? shared or dedicated?Access networks are the physical links that connect an end system to its edge router, which is the first router on a path
3、from the end system to any other distant end system.1. Residential access: 1) point to point access Dialup via modem up to 56Kbps direct access to router (often less) Cant surf and phone at same time: cant be “always on” ADSL: asymmetric digital subscriber line (不对称数字用户线) up to 1 Mbps upstream (toda
4、y typically 1: more “work” arriving than can be serviced, average delay infinite!6. “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
5、 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 probes3 probes3 probes6. “Real” Internet delays and routes1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.
6、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- (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0- (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
7、 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2- (62.40.96.129) 109 ms 102 ms 104 ms 10 (62.40.96.50) 113 ms 121 ms 114 ms 11 renater- (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
8、 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom- (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 mstraceroute: gaia.cs.umass.edu to
9、www.eurecom.frThree delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu * means no response (probe lost, router not replying)trans-oceanic link6. “Real” Internet delays and routes实践练习1. http:/www.traceroute.org/ 英文站点2. Packet 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 all
