Transcript Packets, How do they Work

Packets
How do they
Work
Packets, How do they Work
Lets pretend for a minute we are going to send this
email. The size of this email is 3.5 kilobits.
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Source Computer
OSI Model
96 bits
Application Layer
Header
896 bits Max
Data
32 bits
Trailer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Datalink Layer
The Application Layer setup our first
packet. It is important to note, the packets
on our network are of fix-length, 1024bits.
896 bits of DATA ONLY are entered into
the packet at this stage.
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Source Computer
OSI Model
Application Layer
96 bits
Presentation Layer
Header
896 bits Max
Data
32 bits
Trailer
Session Layer
Transport Layer
Network Layer
Datalink Layer
This layer uses a set form of translation
that allows the data to be interpreted
properly. This layer can add encryption for
security reasons.
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Source Computer
OSI Model
Application Layer
Presentation Layer
96 bits
Session Layer
Header
896 bits Max
Data
32 bits
Trailer
Transport Layer
Network Layer
Datalink Layer
Physical Layer
Author: Rodger Burgess
This layer allows users on different
machines to establish sessions between
them by directing the information to the
correct destination, and identifying the
source to the destination
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Source Computer
OSI Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Datalink Layer
Physical Layer
Author: Rodger Burgess
The basic function of the transport layer,
is to accept data from the session layer,
split it up into smaller units of data
32 bits
96 bits
Header
896 bits Max data
Trailer
segments pass these to the network layer
and ensure that the pieces all arrive
correctly at the other end and combines
incoming segments into a continuous file.
29th September 2008 © Copyrights Reserved
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Packets, How do they Work
OSI Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Source Computer
We now proceed to the Network Layer
where DATA is entered into the Header. IP
of Source, IP of Destination, Protocols and
Packet Numbering. From this layer
ROUTING can take place
32 bits
96 bits
Header
896 bits Max data
Trailer
Datalink Layer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
OSI Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Source Computer
Now to the Datalink and Physical Layers.
The Datalink can check for traffic
currently on the media. It is here where
most errors are detected and corrected.
Physical Layer lets our DATA enter the
media of our network.
Network Layer
Datalink Layer
32 bits
96 bits
Header
896 bits Max data
Trailer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
OSI Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Destination Computer
The Packet has arrived at its Destination
Computer. The Datalink excepts the
packet and checks with the Network Layer
if this packet is intended for us. It then
checks for errors and notifies the Source
Computer that it has received the packet.
Network Layer
Datalink Layer
32 bits
96 bits
Header
896 bits Max data
Trailer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
OSI Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Destination Computer
The Network Layer extracts the
information from the Header and
acknowledges that this packet is for this
computer or sends it back down to the
Datalink Layer for retransmission.
32 bits
96 bits
Header
896 bits Max data
Trailer
Datalink Layer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
OSI Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Destination Computer
The Transport Layer extracts the
information from the Header and places
each receiving Packet into correct order
32 bits
96 bits
Header
896 bits Max data
Trailer
Network Layer
Datalink Layer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Destination Computer
OSI Model
Application Layer
Presentation Layer
96 bits
Session Layer
Header
32 bits
896 bits Max data
Trailer
Transport Layer
Network Layer
Datalink Layer
Physical Layer
Author: Rodger Burgess
Here we establish sessions between
machines and manage the
communications between the source and
destination computer.
29th September 2008 © Copyrights Reserved
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Packets, How do they Work
Destination Computer
OSI Model
Application Layer
Presentation Layer
96 bits
Header
896 bits Max data
Trailer
Session Layer
Transport Layer
Network Layer
Datalink Layer
Physical Layer
Author: Rodger Burgess
If we had encrypted the data a the source
computer, then we will have to decrypt the
data here. We are really concerned with the
syntax and semantics of the information
transmitted here.
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Destination Computer
OSI Model
96 bits
Application Layer
Header
32 bits
896 bits Max data
Trailer
Presentation Layer
Session Layer
Transport Layer
The Application Layer extracts the DATA
and present it to the user.
Network Layer
Datalink Layer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
1
Packets, How do they Work
Destination Computer
OSI Model
96 bits
Application Layer
Header
32 bits
896 bits Max data
Trailer
Presentation Layer
Session Layer
Transport Layer
Network Layer
How many PACKETS were sent to the
Destination Computer so that the USER
could read the entire message.
Datalink Layer
Physical Layer
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
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Packets, How do they Work
1
2
3
96 bits
896 bits Max
32 bits
Header
Data
Trailer
96 bits
896 bits Max
32 bits
Header
Data
Trailer
96 bits
896 bits Max
32 bits
Header
Data
Trailer
96 bits
4
Author: Rodger Burgess
Header
812 bits Max
Data
29th September 2008 © Copyrights Reserved
32 bits
Trailer
1
Packets, How do they Work
A Point to Remember
If the packet size of our network is set to fix-length of 1024
bits, what problems could this cause to the following?
Problems
1. File size is 4.5 kilobits
2. File size is 6275 bits
3. File size is 500 bits
What would happen in these cases.
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
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Packets, How do they Work
A Point to Remember
Problems
1. Here we would have sent 6 packets with the last packet
being 20 bits in size.
2. Here we would have sent 8 packets with the last packet
being 2 bits in size.
3. Here we would have sent only 1 packet with the total
amount of data.
Did you come up with this?
Author: Rodger Burgess
If you did can you see a
problem looming?
29th September 2008 © Copyrights Reserved
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Packets, How do they Work
Remember This
Your network PACKETS were set to FIXED LENGTH of
1024bits.
Problems
1. Yes 6 packets were sent, but the last packet ONLY had 148
bits in a packet of 1024 bits.
2. Yes 8 packets were sent, but the last packet ONLY had 130
bits.
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
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Packets, How do they Work
Here are more ISO Flow Diagrams for the
class to discuss.
I hope by now you have grasped the concept
of PACKETS on a network.
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
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Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
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Author: Rodger Burgess
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The End
Author: Rodger Burgess
29th September 2008 © Copyrights Reserved
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