Transcript Packet Switching

Packet Switching
Packet is a unit of data that can be
transferred at one time.
Packet includes a header and the data.
Packets are of variable sizes with a
maximum size limit.
All data is transferred across the Internet
in packets.
Packet Switching (contd.)
Packet switching is the process of routing
communications traffic in the form of data
packets from a sender to the (correct)
receiver.
LANs and WANs use packet switching.
Packet switching avoids delays; short
messages need not wait for long transfers
to complete.
Packet Switching (contd.)
Packet switching permits multiple pairs of
computers to communicate
simultaneously in a shared network.
Computers take turns sending packets.
Interface hardware handles sharing
automatically.
Packet Switching (contd.)
Each device connected to a network
contains a small computer that handles
the communication.
Network software on the sender side
divides data into packets.
Network software on the receiver side
reassembles the packets.
Internet - A Network of
Networks
Interconnection of multiple packet
switched networks.
Multiple packet switching technologies different speed, distances, & cost
characteristics
Internet provides
a mechanism to interconnect arbitrary
networks
software to transfer data across the
connections
Routers
Computers with special purpose
software that interconnects networks.
Forward packets from one computer to
another.
Routing is the process of selecting a
network over which to send a packet.
Router can interconnect diverse
technologies, for example, a LAN to a
WAN.
Backbone Network & Sites
A major WAN to which other networks
attach is called a backbone network.
Example: vBNS - very high-speed Backbone
Network System (by MCI in 1995).
Sites reached by the backbone are called
backbone sites.
At each backbone site, a router connects
a LAN to the backbone.
Wide Area Backbone
LAN at site 1
net at 2
net at 3
LAN at site 2
LAN at site 3
Router
Wide Area Backbone
LAN at site 4
LAN at site 5
LAN at site 6
net at 6
LAN at site 7
net at 7
Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.
IP - Internet Protocol
A Communication Protocol is an
agreement that specifies a common
language two computers use to
exchange messages.
A protocol specifies
exact format & meaning of each message
conditions under which a message can be
sent
how a computer should respond to a
message
IP - Internet Protocol (contd.)
IP specifies how a packet must be
formed, and how a router must forward it
to its destination.
IP software is needed on a computer
connected to the Internet.
IP software is memory resident.
Internet packets are called IP datagrams.
IP - Internet Protocol (contd.)
IP transforms a collection of networks and
routers into a seamless communication
system.
IP makes the complex internal physical
structure of the Internet transparent to
the end user.
Internal Structure
Router connecting networks a and b
Network a
a
b
c
f
e
d
Computer attached
to network d
Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.
IP Datagrams & Network Packets
IP datagram defines a standard format for
all Internet packets.
IP datagram travels inside a network
packet.
IP datagram is data within a network.
Router creates a new network packet for
transmission across another network.
Datagram is processed by software on the
destination computer.
Internet or IP Address
IP address is a unique number assigned
to a computer.
Computer stores an IP address in 4 bytes.
IP address is displayed as 4 decimal
numbers separated by periods,
139.78.100.1
IP addresses are not random; prefix is the
same for all computers on the same
network.
Routing Example
Computer C2
Router 1
C2
Computer C3
a
b
C1
c
Router 5
Router 2
C3
d
e
f
Computer C1
Router 4
Router 3
Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.
TCP - Transmission Control
Protocol
In addition to IP software, most of the
computers that connect to the Internet
also run TCP software.
TCP/IP - an entire set of Internet
communication protocols.
TCP enables two computer programs to
communicate across the Internet connect, exchange data, and disconnect.
TCP (contd.)
TCP makes the Internet reliable.
Checks for lost datagrams that were
discarded by routers.
Puts incoming datagrams in the order they
were sent.
Checks for duplicate datagrams.
TCP handles the problem of lost
datagrams by using timers and
acknowledgements.
TCP (contd.)
TCP software on the receiver side sends
an acknowledgement back to the sender.
TCP on the sender side starts a timer
when it sends data. If an ack. arrives
before the timer expires then the TCP
cancels the timer, else it retransmits
another copy.
TCP’s timer mechanism adapts to
“distance” of destination and Internet
traffic delays.
Names for Computers
Users prefer alphabetic names to IP
addresses.
Each computer on the Internet must have
a unique name.
Names with many parts
Full Name = local name + organization
suffix
E.g. www.okstate.edu = www + okstate.edu
Full Name = computer name + dept. name
+ organization suffix
Names for Computers (contd.)
Domain Name System (DNS) is a software
that translates a domain name to an IP
address.
DNS uses the client/server approach;
application program contacts the domain
name server to get the IP address.
Domain name servers store names of
computers at only one company or
enterprise.
Names for Computers (contd.)
Computer name lookup is automatic.
Application asks a local domain name server
and if needed this server then contacts a
remote domain name server.
There is no correspondence between the
parts of a domain name and the parts of
its IP address.
Reasons for Internet’s success
IP provides flexibility
Accommodates many types of hardware WAN or LAN, high-speed or slow, wired or
wireless, etc.
TCP/IP standards specify how to send IP
datagrams on each type of network.
TCP provides reliability
Automatically adapts to Internet conditions
Makes reliable communication possible even
during periods of congestion.