3 مطالب درس مروری بر شبکه های کامپیوتری و اینترنت شبکه های گسترده (WAN) ATMMPLSشبکه های بی سیمانتقال بی سیمشبکه های MANET, WSN, WMNشبکه های چندرسانه ای (Multimedia)شبکه های نظیر به نظیر (Peer-to-Peer)
4 مروری بر شبکه های کامپیوتری و اینترنت PCserverwirelesslaptopcellularhandheldmillions of connected computing devices: hosts = end systemsrunning network appsHome networkInstitutional networkMobile networkGlobal ISPRegional ISPcommunication linksfiber, copper, radio, satellitetransmission rate = bandwidthwiredlinksaccesspointsrouters: forward packets (chunks of data)router
5 A closer look at network structure: network edge: applications and hostsaccess networks, physical media: wired, wireless communication linksnetwork core:interconnected routersnetwork of networks
6 The network edge: client/server model peer-peer model: end systems (hosts):run application programse.g. Web,at “edge of network”peer-peerclient/serverclient/server modelclient host requests, receives service from always-on servere.g. Web browser/server; client/serverpeer-peer model:minimal (or no) use of dedicated serverse.g. Skype, BitTorrent
7 Access networks and physical media Q: 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?
8 Local 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
9 Wireless access networks shared wireless access network connects end system to routervia base station aka “access point”wireless LANs:802.11b/g (WiFi): 11 or 54 Mbpswider-area wireless accessprovided by operators~1Mbps over cellular systemWiMAX (10’s Mbps) over wide arearouterbasestationmobilehosts
10 The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net?circuit switching: dedicated circuit per call: telephone netpacket-switching: data sent thru net in discrete “chunks”
11 Internet protocol stack application: 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”
12 Encapsulation destination source application transport network link messageMapplicationtransportnetworklinkphysicalsegmentHtMHtdatagramHtHnMHnframeHtHnHlMlinkphysicalswitchdestinationnetworklinkphysicalHtHnMHtHnHlMMapplicationtransportnetworklinkphysicalHtHnMHtMHtHnMrouterHtHnHlM
13 error control (detection & recovery) EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fieldsError detection not 100% reliable!protocol may miss some errors, but rarelylarger EDC field yields better detection and correctionARQ: automatic request repeatStop and WaitSliding Window
14 Two Key Network-Layer Functions forwarding: move packets from router’s input to appropriate router outputrouting: determine route taken by packets from source to dest.routing algorithms
15 Interplay between routing and forwarding 1230111value in arrivingpacket’s headerrouting algorithmlocal forwarding tableheader valueoutput link010001011001
16 Routing Graph abstraction for routing algorithms: Routing protocolGoal: determine “good” path(sequence of routers) thrunetwork from source to dest.53BC25A21F3Graph abstraction for routing algorithms:graph nodes are routersgraph edges are physical linkslink cost: delay, $ cost, or congestion level12DE1“good” path:typically means minimum cost pathother def’s possible
17 Routing: only two approaches used in practice Global:all routers have complete topology, link cost info“link state” algorithms: use Dijkstra’s 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 neighbors“distance vector” algorithmsa ‘self-stabilizing algorithm’ (we’ll see these later)
18 Addressing: network layer IP address: 32-bit identifier for host, router interfaceinterface: connection between host, router and physical linkrouter’s typically have multiple interfaceshost may have multiple interfacesIP addresses associated with interface, not host, router=223111
19 IP Addressing IP address: network part (high order bits)host part (low order bits)what’s a network ? (from IP address perspective)device interfaces with same network part of IP addresscan physically reach each other without intervening routerLANnetwork consisting of 3 IP networks(for IP addresses starting with 223,first 24 bits are network address)
20 LANs bus topology popular through mid 90s today: star topology prevailsactive switch in center, each “spoke” runs a (separate) Ethernet protocolwireless LANS:shared RF(e.g., WiFi)switchstarbus: coaxial cable
21 LAN AddressesEach 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 ROMQuestion: why separateMAC and IP addresses?
22 ARP: Address Resolution Protocol Question: how to determineMAC address of Bknowing B’s IP address?Each 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)1A-2F-BB ADLAN71-65-F7-2B-08-5358-23-D7-FA-20-B00C-C4-11-6F-E3-98
23 ARP protocol: Same LAN (network) A wants to send datagram to B, and B’s MAC address not in A’s ARP table.A broadcasts ARP query packet, containing B's IP addressdest MAC address = FF-FF- FF-FF-FF-FFall machines on LAN receive ARP queryB receives ARP packet, replies to A with its (B's) MAC addressframe sent to A’s 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 administrator
24 Addressing: routing to another LAN walkthrough: send datagram from A to B via Rassume A knows B’s IP addresstwo ARP tables in router R, one for each IP network (LAN)R1A-23-F9-CD-06-9BE6-E BB-4BCC-49-DE-D0-AB-7DAC-E8-FF-5588-B2-2F-54-1A-0FB49-BD-D2-C7-56-2A
25 A B A creates IP datagram with source A, destination B A uses ARP to get R’s MAC address forA creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagramA’s NIC sends frameR’s NIC receives frameR removes IP datagram from Ethernet frame, sees its destined to BR uses ARP to get B’s MAC addressR creates frame containing A-to-B IP datagram sends to BThis is a really importantexample – make sure youunderstand!R1A-23-F9-CD-06-9BE6-E BB-4BCC-49-DE-D0-AB-7DAC-E8-FF-5588-B2-2F-54-1A-0FB49-BD-D2-C7-56-2A
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