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(WAN) ATM MPLS MANET, WSN, WMN (Multimedia) (Peer-to-Peer)

millions of connected computing devices: hosts = end systems running network apps4

Home networkInstitutional networkMobile networkGlobal ISPRegional ISProuter

PCserverwirelesslaptopcellular handheld

wiredlinksaccess pointscommunication linksfiber, copper, radio, satellitetransmission rate = bandwidthrouters: forward packets (chunks of data)4A closer look at network structure:network edge: applications and hosts5

access networks, physical media: wired, wireless communication links network core: interconnected routersnetwork of networks

5The network edge:end systems (hosts):run application programse.g. Web, emailat edge of network6

client/serverpeer-peerclient/server modelclient host requests, receives service from always-on servere.g. Web browser/server; email client/serverpeer-peer model: minimal (or no) use of dedicated serverse.g. Skype, BitTorrent6Access networks and physical mediaQ: How to connect end systems to edge router?residential access netsinstitutional access networks (school, company): LANmobile access networksKeep in mind: bandwidth (bits per second) of access network?shared or dedicated?7

7Local area networkscompany/univ local area network (LAN) connects end system to edge routerEthernet: 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernetmodern configuration: end systems connect into Ethernet switch

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8Wireless access networksshared wireless access network connects end system to routervia base station aka access pointwireless LANs:802.11b/g (WiFi): 11 or 54 Mbpswider-area wireless accessprovided by operators~1Mbps over cellular systemWiMAX (10s Mbps) over wide area9

basestationmobilehostsrouter9The Network Coremesh of interconnected routersthe fundamental question: how is data transferred through net?circuit switching: dedicated circuit per call: telephone netpacket-switching: data sent thru net in discrete chunks10

10Internet protocol stackapplication: supporting network applications (FTP, SMTP, HTTP)transport: process-process data transfer (TCP, UDP)network: routing of datagrams from source to destinationIP, routing protocolslink: data transfer between neighboring network elementsPPP, Ethernetphysical: bits on the wire

1111Encapsulation12source

applicationtransportnetworklinkphysicalHtHnMsegmentHtdatagramdestination

applicationtransportnetworklinkphysicalHtHnHlMHtHnMHtMMnetworklinkphysicallinkphysicalHtHnHlMHtHnMHtHnMHtHnHlMrouterswitchmessageMHtMHnframe12error control (detection & recovery)13

EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction

ARQ: automatic request repeatStop and WaitSliding Window

Two Key Network-Layer Functionsforwarding: move packets from routers input to appropriate router outputrouting: determine route taken by packets from source to dest. routing algorithms

14151230111value in arrivingpackets headerrouting algorithmlocal forwarding tableheader valueoutput link01000101011110013221Interplay between routing and forwardingRoutingGraph abstraction for routing algorithms:graph nodes are routersgraph edges are physical linkslink cost: delay, $ cost, or congestion level16Goal: determine good path(sequence of routers) thru network from source to dest.Routing protocolAEDCBF2213112535good path:typically means minimum cost pathother defs possibleRouting: only two approaches used in practiceGlobal:all routers have complete topology, link cost infolink state algorithms: use Dijkstras algorithm to find shortest path from given router to all destinationsDecentralized: router knows physically-connected neighbors, link costs to neighborsiterative process of computation, exchange of info with neighborsdistance vector algorithmsa self-stabilizing algorithm (well see these later)17Addressing: network layerIP address: 32-bit identifier for host, router interface interface: connection between host, router and physical linkrouters typically have multiple interfaceshost may have multiple interfacesIP addresses associated with interface, not host, router18

223.1.1.1223.1.1.2223.1.1.3223.1.1.4223.1.2.9

223.1.2.2223.1.2.1

223.1.3.2223.1.3.1223.1.3.27223.1.1.1 = 11011111 00000001 00000001 00000001223111IP AddressingIP address: network part (high order bits)host part (low order bits) whats a network ? (from IP address perspective)device interfaces with same network part of IP addresscan physically reach each other without intervening router19

223.1.1.1223.1.1.2223.1.1.3223.1.1.4223.1.2.9

223.1.2.2223.1.2.1

223.1.3.2223.1.3.1223.1.3.27network consisting of 3 IP networks(for IP addresses starting with 223, first 24 bits are network address)LANLANsbus topology popular through mid 90stoday: star topology prevailsactive switch in center, each spoke runs a (separate) Ethernet protocolwireless LANS: 802.1120

bus: coaxial cable

switch

starshared RF (e.g., 802.11 WiFi)

20LAN Addresses

21Each adapter on LAN has unique LAN address (also has an IP address)LAN (or MAC or physical) address: used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM

Question: why separateMAC and IP addresses?ARP: Address Resolution ProtocolEach IP node (host, router) on LAN has ARP tableARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)22Question: how to determineMAC address of Bknowing Bs IP address?

1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53 LAN137.196.7.23137.196.7.78137.196.7.14137.196.7.8822ARP protocol: Same LAN (network)A wants to send datagram to B, and Bs MAC address not in As ARP table.A broadcasts ARP query packet, containing B's IP address dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC addressframe sent to As MAC address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshedARP is plug-and-play:nodes create their ARP tables without intervention from net administrator2323Addressing: routing to another LANwalkthrough: send datagram from A to B via R assume A knows Bs IP address

two ARP tables in router R, one for each IP network (LAN)

24R1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112

111.111.111.111

A74-29-9C-E8-FF-55

222.222.222.22188-B2-2F-54-1A-0F

B222.222.222.22249-BD-D2-C7-56-2A24A creates IP datagram with source A, destination B A uses ARP to get Rs MAC address for 111.111.111.110A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagramAs NIC sends frame Rs NIC receives frame R removes IP datagram from Ethernet frame, sees its destined to BR uses ARP to get Bs MAC address R creates frame containing A-to-B IP datagram sends to B25R1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112

111.111.111.111

A74-29-9C-E8-FF-55

222.222.222.22188-B2-2F-54-1A-0F

B222.222.222.22249-BD-D2-C7-56-2AThis is a really importantexample make sure youunderstand!25