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IP: The Internet Protocol
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Spring 2002Computer Networks
Applications
LAN---Bus Topology Each computer
is connected to a cable. Coordinates with others to send a message
BUS
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Spring 2002Computer Networks
Applications
Issues in Sharing Communication Media Generally, most networks types involve
sharing: Bus, ring in LANs High speed backbone in WANs
Granting one party exclusive access may block all others for intolerable periods of time; Ex: to send a file of 5megabytes over a
network that may transfer 56,000 bits/s will require 12 minutes.
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Spring 2002Computer Networks
Applications
Solution: Divide a message in small blocks, called
packets; Computers take turns in sending packets; insures fair, prompt access to the shared
resources; Ex. a packet of 8000 bits is transmitted in 0.143
s over a network that transfers 56,000 bits/s. easier to detect and recover from errors
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Spring 2002Computer Networks
Applications
Packets Transmission All computers connected to a network have a unique
number, called address; Each type of network uses its own format for addresses:
from few digits to as much as 16 digits; At the beginning of a packet, there is a header which
contains the addresses of the sender and the destination; The hardware (NIC card) looks at every packet; if its for
the local machines it copies it;
A
B
C
DDc c D
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Spring 2002Computer Networks
Applications
Packets Transmission (cont.)
Packets are not all the same size any size can be sent up to a maximum Ex: a keystroke can be sent as one
individual packet. Sender and receiver have to agree
on how to specify the beginning and ending of each frame;
Ex: have dedicated characters for beginning and end;
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Spring 2002Computer Networks
Applications
Packets Transmission (cont.)
Packet transmission is very fast: Typical LAN transfer ~ large 1000
packets/s Packet switching systems adapt
automatically as computers start/end sending data fair access to shared resources
Most networks, including Internet are a packet switching system
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Spring 2002Computer Networks
Applications
Internet: a Collection of Disparate Networks Different goals:
Speed, cost, distance; Different standards for:
Expected carrier; Coding bits; Detecting and recovering from errors; Protocols for transmitting messages: bus,
token ring,… Packets sizes, and encoding for the start/end
of packets,… Types of computer addresses
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Spring 2002Computer Networks
Applications
Solution: Routers
RouterRouter
Router
High speed connection
Routers: computers design to interconnect different networks
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Spring 2002Computer Networks
Applications
Solution: Internet Protocol (IP)
IP hides the details of physical networks
IP specifies: Packet format; How routers should forward packets Define address format
Every computer connected to the Internet must run IP software
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Spring 2002Computer Networks
Applications
The illusion of a single network The internet concept:
(a) the illusion of a single network that IP provides;
(b) the underlying physical structure.
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Spring 2002Computer Networks
Applications
Datagrams A packet following IP specification is called
a datagram; The header of a datagram contains the
addresses of the sender and the destination;
But, each network type: defines its own packet format; Accepts and delivers only packets that adhere
to its own format. How can IP datagrams be sent across
networks that do not recognize IP format?
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Spring 2002Computer Networks
Applications
Datagrams (cont.) A datagram travels across a given network
inside that network packet; When the packet arrives at the next router,
the router opens the packet and extracts the datagram;
If the datagram has to be sent to another network, the router:
Creates a new network packet; Encloses the datagram inside that packet; Sends it to the next router along the path;
If the datagram arrived at the destination: the receiver processes it.
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Spring 2002Computer Networks
Applications
Datagram transmission
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Spring 2002Computer Networks
Applications
Internet Addressing To make datagram routing and delivery
possible, each computer is assigned a unique address, called Internet address or IP address;
Each address is a 32-bit binary number; To make routing efficient, each address is
divided into two parts: a prefix and a suffix; Prefix: identifies the physical network to
which the computer is attached; Suffix: identifies each computer attached to
that network.
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Spring 2002Computer Networks
Applications
Internet Addressing (cont.) To insure uniqueness:
two networks cannot be assigned the same address
network numbers are distributed by a centralized authority, called Internet Assigned Number Authority
two computers on the same network cannot be assigned the same number
suffixes can be assigned locally without global coordination
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Spring 2002Computer Networks
Applications
Dotted decimal notation Convenient way to express IP
addresses Each 8-bit section represented as a
decimal number; Uses periods to separate the sections;
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Spring 2002Computer Networks
Applications
Forwarding a datagram Each router along the path, uses the
destination address to determine the next hop to which it has to be sent.
Each IP router keeps relevant information into a routing table;
Each entry specifies a destination and the next hop used to reach it;
Each destination is a network (an internet contains over 1000 times more hosts than networks)
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Spring 2002Computer Networks
Applications
Routing table---an example
(a)An internet formed by 4 networks and 3 routers;(b)The conceptual routing table of router 2
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Spring 2002Computer Networks
Applications
Address resolution IP addresses are abstractions provided by
software---differ usually from hardware addresses;
Mapping between a hardware address and an IP address is called address resolution;
Is used by routers/computers when need to send a packet on the same physical network.
Address resolution techniques: Table lookup: mappings are stored in memory, which the
software searches Message exchange: a computer sends a message that
requests an address binding, and another computer sends a reply that contains the requested info.
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Spring 2002Computer Networks
Applications
Address resolution with table lookup
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Spring 2002Computer Networks
Applications
An example trip through an internet
x
Y
R1
R2
R3