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شبکه های کامپیوتری پیشرفته

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2 شبکه های کامپیوتری پیشرفته

3 مطالب درس مروری بر شبکه های کامپیوتری و اینترنت شبکه های گسترده (WAN)
ATM MPLS شبکه های بی سیم انتقال بی سیم شبکه های MANET, WSN, WMN شبکه های چندرسانه ای (Multimedia) شبکه های نظیر به نظیر (Peer-to-Peer)

4 مروری بر شبکه های کامپیوتری و اینترنت
PC server wireless laptop cellular handheld millions of connected computing devices: hosts = end systems running network apps Home network Institutional network Mobile network Global ISP Regional ISP communication links fiber, copper, radio, satellite transmission rate = bandwidth wired links access points routers: forward packets (chunks of data) router

5 A closer look at network structure:
network edge: applications and hosts access networks, physical media: wired, wireless communication links network core: interconnected routers network of networks

6 The network edge: client/server model peer-peer model:
end systems (hosts): run application programs e.g. Web, at “edge of network” peer-peer client/server client/server model client host requests, receives service from always-on server e.g. Web browser/server; client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent

7 Access networks and physical media
Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company): LAN mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated?

8 Local area networks company/univ local area network (LAN) connects end system to edge router Ethernet: 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet modern configuration: end systems connect into Ethernet switch

9 Wireless access networks
shared wireless access network connects end system to router via base station aka “access point” wireless LANs: 802.11b/g (WiFi): 11 or 54 Mbps wider-area wireless access provided by operators ~1Mbps over cellular system WiMAX (10’s Mbps) over wide area router base station mobile hosts

10 The Network Core mesh of interconnected routers
the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-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 destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire”

12 Encapsulation destination source application transport network link
message M application transport network link physical segment Ht M Ht datagram Ht Hn M Hn frame Ht Hn Hl M link physical switch destination network link physical Ht Hn M Ht Hn Hl M M application transport network link physical Ht Hn M Ht M Ht Hn M router Ht Hn Hl M

13 error control (detection & recovery)
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 repeat Stop and Wait Sliding Window

14 Two Key Network-Layer Functions
forwarding: move packets from router’s input to appropriate router output routing: determine route taken by packets from source to dest. routing algorithms

15 Interplay between routing and forwarding
1 2 3 0111 value in arriving packet’s header routing algorithm local forwarding table header value output link 0100 0101 1001

16 Routing Graph abstraction for routing algorithms:
Routing protocol Goal: determine “good” path (sequence of routers) thru network from source to dest. 5 3 B C 2 5 A 2 1 F 3 Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level 1 2 D E 1 “good” path: typically means minimum cost path other 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 destinations Decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors “distance vector” algorithms a ‘self-stabilizing algorithm’ (we’ll see these later)

18 Addressing: network layer
IP address: 32-bit identifier for host, router interface interface: connection between host, router and physical link router’s typically have multiple interfaces host may have multiple interfaces IP addresses associated with interface, not host, router = 223 1 1 1

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 address can physically reach each other without intervening router LAN network 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 prevails active switch in center, each “spoke” runs a (separate) Ethernet protocol wireless LANS: shared RF (e.g., WiFi) switch star bus: coaxial cable

21 LAN Addresses Each 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 separate MAC and IP addresses?

22 ARP: Address Resolution Protocol
Question: how to determine MAC address of B knowing B’s IP address? Each IP node (host, router) on LAN has ARP table ARP 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 AD LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-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 address dest MAC address = FF-FF- FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame 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 refreshed ARP 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 R assume A knows B’s IP address two ARP tables in router R, one for each IP network (LAN) R 1A-23-F9-CD-06-9B E6-E BB-4B CC-49-DE-D0-AB-7D A C-E8-FF-55 88-B2-2F-54-1A-0F B 49-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 for A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s NIC sends frame R’s NIC receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B This is a really important example – make sure you understand! R 1A-23-F9-CD-06-9B E6-E BB-4B CC-49-DE-D0-AB-7D A C-E8-FF-55 88-B2-2F-54-1A-0F B 49-BD-D2-C7-56-2A


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