03-20-2002
You are partially correct! The data does traverse down the OSI model on the source node.......across the network medium.......to the destination node....and back up the OSI model.
BUT, transmissions do not have to actually start at the application level. Have you ever heard of two or three tier routers? They only use part of the OSI model.....hence two or three layers are transmitted....not all 7.
It goes like this:
Data to be sent in this datagram = XXXXXXXXXX
Layer 7 puts on application specific headers:
AAXXXXXXXXXXA
Then Layer 6 for presentation:
PPAAXXXXXXXXXXAP
Then Layer 5 does the same on a session basis:
SSPPAAXXXXXXXXXXAPS
Layer 4 for transport portion:
TTSSPPAAXXXXXXXXXXAPST
Layer 3 goes next:
NNTTSSPPAAXXXXXXXXXXAPSTN
Layer 2 puts on the Data Link layer stuff:
DDNNTTSSPPAAXXXXXXXXXXAPSTND
The finally the Physical layer:
PPDDNNTTSSPPAAXXXXXXXXXXAPSTNDP
This is then transmitted. At the destination...each layer strips the corresponding layer and sends it up to the next layer....etc.
This is very basic...it's more complex than this tutorial...each layer does not have the same size of headers...and such...but this gets the idea across.
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LEARN ABOUT DEBIAN
net::write::layer4
Net::Write::Layer4(3pm) User Contributed Perl Documentation Net::Write::Layer4(3pm)
NAME
Net::Write::Layer4 - object for a transport layer (layer 4) descriptor
SYNOPSIS
use Net::Write::Layer qw(:constants);
use Net::Write::Layer4;
my $desc = Net::Write::Layer4->new(
dst => '192.168.0.1',
protocol => NW_IPPROTO_TCP,
family => NW_AF_INET,
);
$desc->open;
$desc->send('G'x666);
$desc->close;
DESCRIPTION
This is the class for creating a layer 4 descriptor.
ATTRIBUTES
dst The target IPv4 or IPv6 address we will send frames to.
family
Address family, see Net::Write::Layer CONSTANTS section.
protocol
Transport layer protocol to use, see Net::Write::Layer CONSTANTS section.
METHODS
new Object constructor. You MUST pass a valid dst attribute. Default values:
protocol: NW_IPPROTO_TCP
family: NW_AF_INET
open
Open the interface.
send (scalar)
Send raw data to the network.
close
Close the descriptor.
CAVEATS
Does not work at all under Win32 systems. They can't send frames at layer 4.
SEE ALSO
Net::Write::Layer
AUTHOR
Patrice <GomoR> Auffret
COPYRIGHT AND LICENSE
Copyright (c) 2006-2009, Patrice <GomoR> Auffret
You may distribute this module under the terms of the Artistic license. See LICENSE.Artistic file in the source distribution archive.
perl v5.10.1 2009-06-10 Net::Write::Layer4(3pm)