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Full Discussion: The OSI-model
Special Forums IP Networking The OSI-model Post 17863 by thomas.jones on Wednesday 20th of March 2002 07:38:06 PM
Old 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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ticlts(7D)							      Devices								ticlts(7D)

NAME
ticlts, ticots, ticotsord - loopback transport providers SYNOPSIS
#include <sys/ticlts.h> #include <sys/ticots.h> #include <sys/ticotsord.h> DESCRIPTION
The devices known as ticlts, ticots, and ticotsord are ``loopback transport providers,'' that is, stand-alone networks at the transport level. Loopback transport providers are transport providers in every sense except one: only one host (the local machine) is ``connected to'' a loopback network. Loopback transports present a TPI (STREAMS-level) interface to application processes and are intended to be accessed via the TLI (application-level) interface. They are implemented as clone devices and support address spaces consisting of ``flex- addresses,'' that is, arbitrary sequences of octets of length > 0, represented by a netbuf structure. ticlts is a datagram-mode transport provider. It offers (connectionless) service of type T_CLTS. Its default address size is TCL_DEFAULTAD- DRSZ. ticlts prints the following error messages (see t_rcvuderr(3NSL)): TCL_BADADDR bad address specification TCL_BADOPT bad option specification TCL_NOPEER bound TCL_PEERBADSTATE peer in wrong state ticots is a virtual circuit-mode transport provider. It offers (connection-oriented) service of type T_COTS. Its default address size is TCO_DEFAULTADDRSZ. ticots prints the following disconnect messages (see t_rcvdis(3NSL)): TCO_NOPEER no listener on destination address TCO_PEERNOROOMONQ peer has no room on connect queue TCO_PEERBADSTATE peer in wrong state TCO_PEERINITIATED peer-initiated disconnect TCO_PROVIDERINITIATED provider-initiated disconnect ticotsord is a virtual circuit-mode transport provider, offering service of type T_COTS_ORD (connection-oriented service with orderly release). Its default address size is TCOO_DEFAULTADDRSZ. ticotsord prints the following disconnect messages (see t_rcvdis(3NSL)): TCOO_NOPEER no listener on destination address TCOO_PEERNOROOMONQ peer has no room on connect queue TCOO_PEERBADSTATE peer in wrong state TCOO_PEERINITIATED provider-initiated disconnect TCOO_PROVIDERINITIATED peer-initiated disconnect USAGE
Loopback transports support a local IPC mechanism through the TLI interface. Applications implemented in a transport provider-independent manner on a client-server model using this IPC are transparently transportable to networked environments. Transport provider-independent applications must not include the headers listed in the synopsis section above. In particular, the options are (like all transport provider options) provider dependent. ticlts and ticots support the same service types (T_CLTS and T_COTS) supported by the OSI transport-level model. ticotsord supports the same service type (T_COTSORD) supported by the TCP/IP model. FILES
/dev/ticlts /dev/ticots /dev/ticotsord SEE ALSO
t_rcvdis(3NSL), t_rcvuderr(3NSL) SunOS 5.11 3 Jul 1990 ticlts(7D)
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