| Document: FSC-0080
  | Version:  001
  | Date:     01 Mar 1995
  |
  | Mikael Ståldal, 2:201/337


                  Describing FidoNet with a layered model
                     Mikael Ståldal, 2:201/337@FidoNet

Introduction
============
FTS-1 tries to describe FidoNet with the OSI model, but I don't think that
description is any good.

I have tried to make a better description of FidoNet with the OSI model, but
without success. I don't think that's possible.

Instead I made my own layered model, inspired from OSI. I use seven layers
with the same name as in OSI, but with Session and Transport swapped.


Why this model?
===============
The main goal with this model is to make FidoNet more flexible make it
easier to change into newer and better protocols and data structures.

It should be possible to change one layer without affecting the others.


Description of the layers
=========================

Physical layer
--------------
The telephone network (PSTN).


Link layer
----------
Modem with protocols like V.22 and V.32. This layer also include real-time
error correction (V.42, MNP4) and data compression (V.42bis, MNP5).

Serial communication hardware (UART).


Network layer
-------------
Serial communications drivers (FOSSIL) and file transfer protocols
(Zmodem etc). Note that this layer can be divided into two sub-layers, the
file transfer protocol on top on the serial communication driver; but upper
layers can interact with the serial communication driver directly.


Session layer
-------------
Session handshake protocols (YooHoo, EMSI) implemented in mailers. This
layer can reliably send files directly between two systems. It doesn't
perform any routing. It doesn't know about NetMail, EchoMail etc.

The upper layers uses logical addresses (node numbers), this layer performs
address resolution (often by using a nodelist) to obtain the physical address
(telephone number) necessary to establish a connection.

The upper layers can tell if a file should be sent immediately, when
appropriate or be placed on hold.

When files are received, this layer tells the upper layers.


Transport layer
---------------
This layer handles routing and transport of NetMail, EchoMail and file
attaches. The format of mailpackets is defined in this layer, except the
internal structure of a packed message.

A mail processor takes received NetMail and EchoMail and places it in the
local message base. It also looks there for messages to send.


Presentation layer
------------------
Here are the internal structure of a packed message defined.

In TYPE-2, this layer is totally mixed up with the Transport layer. In new
packet formats, they will hopefully be separated.


Application layer
-----------------
Here are the local message base (stored messages) defined.


Interaction between layers
==========================

Interaction between Session layer and Transport layer
-----------------------------------------------------
The Session and Transport layers does only interact between sessions, not
during them. The reason is that is should be possible to keep them in
different programs and implement it in single tasking operating systems like
MS-DOS. This doesn't prevent the Session and Transport layers from running
simultaneously in a multitasking system.


Comments
========
There is one problem with this model, how to describe the File Request
server function. Sending File Requests are no problems, that's just a File
Attach with a *.REQ file. The File Request server function have to be
implemented directly in the Session layer.