شبکه های کامپیوتری پیشرفته مطالب درس مروری بر شبکه های...
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کامپیوتری های شبکهپیشرفته
درس مطالب
اینترنت و کامپیوتری های شبکه بر مروری
( گسترده های (WANشبکه ATM MPLS
سیم بی های شبکه
سیم بی انتقال
سیم بی های شبکه
های MANET, WSN, WMNشبکه
( ای چندرسانه های (Multimediaشبکه( نظیر به نظیر های (Peer-to-Peerشبکه
اینترنت و کامپیوتری های شبکه بر مروری
millions of connected
computing devices: hosts = end systems
running network apps
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Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wirelesslaptop
cellular handheld
wiredlinks
access points
communication linksfiber, copper, radio,
satellitetransmission rate =
bandwidth
routers: forward packets (chunks of data)
A closer look at network structure: network edge: applications
and hosts
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access networks, physical media: wired, wireless
communication links network core:
interconnected routersnetwork of networks
The network edge:end systems (hosts):
run application programs
e.g. Web, email
at “edge of network”
6
client/server
peer-peer
client/server modelclient host requests,
receives service from always-on server
e.g. Web browser/server; email client/server peer-peer model:
minimal (or no) use of dedicated servers
e.g. Skype, BitTorrent
Access networks and physical media
Q: How to connect end systems to edge router?
residential access netsinstitutional access
networks (school, company): LAN
mobile access networks
Keep in mind :bandwidth (bits per
second) of access network?
shared or dedicated? 7
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
8
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
Mbps
wider-area wireless accessprovided by operators
~1Mbps over cellular system
WiMAX (10’s Mbps) over wide area
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basestation
mobilehosts
router
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”
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Internet protocol stackapplication: supporting network
applications (FTP, SMTP, HTTP)transport: process-process data
transfer (TCP, UDP)network: routing of datagrams
from source to destinationIP, routing protocols
link: data transfer between neighboring network elements
PPP, Ethernet
physical: bits “on the wire”11
Encapsulation
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source
application
transportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
message M
Ht M
Hn
frame
error control (detection & recovery)
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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
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
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15
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Routing
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links
link cost: delay, $ cost, or congestion
level16
Goal: determine “good” path
(sequence of routers )thru network from source to
dest.
Routing protocol
A
ED
CB
F
22
13
1
1
2
53
5
“good” path:typically means
minimum cost pathother def’s possible
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)17
Addressing: network layerIP address: 32-bit
identifier for host, router interface
interface: connection between host, router
and physical linkrouter’s typically have
multiple interfaces
host may have multiple interfaces
IP addresses associated with interface, not
host, router
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223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
IP AddressingIP 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
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223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 IP networks(for IP addresses starting with 223 ,
first 24 bits are network address)
LAN
LANs
bus topology popular through mid 90s
today: star topology prevails
active switch in center, each “spoke” runs a (separate) Ethernet protocol
wireless LANS: 802.11
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bus: coaxial cableswitch
star
shared RF( e.g., 802.11 WiFi)
LAN Addresses
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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 separateMAC and IP addresses?
ARP: Address Resolution Protocol
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)
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Question: how to determineMAC address of B
knowing B’s IP address?
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
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 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 refreshed
ARP is “plug-and-play:”nodes create their ARP
tables without intervention from net
administrator23
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)
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R
1A-23-F9-CD-06-9B
222.222.222.220111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
A74-29-9C-E8-FF-55
222.222.222.221
88-B2-2F-54-1A-0F
B222.222.222.222
49-BD-D2-C7-56-2A
A creates IP datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110
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
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R
1A-23-F9-CD-06-9B
222.222.222.220111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
A74-29-9C-E8-FF-55
222.222.222.221
88-B2-2F-54-1A-0F
B222.222.222.222
49-BD-D2-C7-56-2A
This is a really importantexample – make sure you
understand!
