Transcript The Internet Protocol

IP: The Internet Protocol
LAN---Bus Topology
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Each computer
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is connected to a cable.
Coordinates with others to send a message
BUS
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Issues in Sharing
Communication Media
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Generally, most networks types involve
sharing:
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Bus, ring in LANs
High speed backbone in WANs
Granting one party exclusive access may
block all others for intolerable periods of
time;
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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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Solution:
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Divide a message in small blocks, called
packets;
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Computers take turns in sending packets;
insures fair, prompt access to the shared
resources;
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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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Packets Transmission
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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;
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Packets Transmission (cont.)
Packets are not all the same size
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Sender and receiver have to agree on how
to specify the beginning and ending of
each frame;
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any size can be sent up to a maximum
Ex: a keystroke can be sent as one individual
packet.
Ex: have dedicated characters for beginning
and end;
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Packets Transmission (cont.)
Packet transmission is very fast:
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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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Internet: a Collection of
Disparate Networks
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Different goals:
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Speed, cost, distance;
Different standards for:
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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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Solution: Routers
Router
High speed
connection
Router
Router
Routers: computers design to interconnect different networks
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Solution: Internet Protocol (IP)
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IP hides the details of physical networks
IP specifies:
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Packet format;
How routers should forward packets
Define address format
Every computer connected to the
Internet must run IP software
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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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Datagrams
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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:
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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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Datagrams (cont.)
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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:
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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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Datagram transmission
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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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Internet Addressing (cont.)
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To insure uniqueness:
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two networks cannot be assigned the
same address
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network numbers are distributed by a
centralized authority, called Internet Assigned
Number Authority
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two computers on the same network
cannot be assigned the same number
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suffixes can be assigned locally without global
coordination
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Dotted decimal notation
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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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Forwarding a datagram
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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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Routing table---an example
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(a) An internet formed by 4 networks and 3 routers;
(b) The conceptual routing table of router 2
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Address resolution
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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:
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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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Address resolution with table
lookup
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An example trip through an
internet
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R1
x
R2
R3
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Y