From: hcb@world.std.com (Howard C Berkowitz)
Subject: FAQ you were afraid to ask
Organization: The World Public Access UNIX, Brookline, MA
Date: Tue, 22 Dec 1992 17:09:46 GMT

The Seven Deadly Layers

Among the most frequent questions I'm asked in OSI teaching 
is "do I need to know what all the layers do?"  This is 
especially true of management audiences, who _need_ to know 
the power centers.   They may not know what a layer is, but 
they know there are seven of them and they don't want a 
single one to go unsupervised. :-)

Over the years, I have found a useful analogy.  Educational 
theory suggests we should start with something that the 
student knows, and build from there.

Therefore, I ask management audiences to reflect not on 
theoretical network architecture, but on sin.  Specifically, 
I ask them to consider the Seven Deadly Sins.

These sins  have definite relevance to the OSI Reference 
Model.  The "most popular" deadly sins are analogies for the 
layers most important for non-developers to know about.

Audiences think of sins in a fairly consistent way.  
Approximately 75% immediately think of Lust.

Lust, clearly, relates directly to the Physical Layer.  It 
is essential to be aware of the function of the Lust Layer, 
for that defines how to Plug In.  [1]

Most of the remaining audience splits among Avarice and 
Gluttony.  These also are important in OSI.  

Avarice, or Greed, is often realized as the Bottom Line in 
business.  One is closer to understanding the Tao of OSI 
when one realizes that it places the Bottom Line (i.e., what 
OSI does for real user applications programs) on Top.  The 
top of the Avarice Layer is the Service Access Point to the 
Application, or Avarice, Layer. [Note 2]

Those members of the audience who thought first of Gluttony 
also have some understanding of OSI.  Gluttony deals with 
establishing a relationship between a mouth entity and a 
food entity; Network deals with the next course while 
Transport deals with the end goal of dessert.

Users really need to know the functions of Application, 
Transport/Network (as the distinction blurs here), and 
Physical.  International Standardized Profiles follow this 
model:  Application is the visible part of A-, B-, or 
Avarice Profiles; Transport and Network define T- and U-
profiles, and Physical deals with the bottom of T- and U-
profiles.  These four components also, for instructional 
purposes, nicely describe the major protocol levels of the 
Internet Protocol Suite:  application protocols, TCP and 
UDP,IP, and interface protocols.

There is always one in the audience, however, who thinks of 
Sloth.  

Sloth is a difficult sin. How does one confess it?  "Bless 
me, I have slothed?"  "Forgive me for committing sloth?  How 
can I commit not doing something?"

Since Sloth is a sin we really have trouble talking 
about,and involves not doing useful things, it is a relevant 
analogy to the Session Layer.  Both Sloth and Session are 
needed for theological completeness, but their relevance to 
the ordinary sinner or the OSI user is fairly limited.[Note 
3].

 
----- 

[Note 1]    When presenting these analogies at an IEEE 
conference in New York, a woman's clear voice rang out from 
the back of the room, "Well, I'm glad SOME standards body is 
defining how to plug in things correctly.  God knows most 
male engineers don't understand that at all."

[Note 2]  This part of the analogy can continue into 
Application Service Elements:  ACSE, the Avarice Control 
Service Element; ROSE, the Remote Organization Submission 
Element; etc.

[Note 3]  After their first reading of Presentation Context 
Negotionation and ASN.1 Basic Encoding Rules, some nominate 
the sin of Pride as the proper analogy for the Presentation 
Layer.