[chapter-delegates] Teaching analogies

[Timo Kiravuo]

> From: Veni Markovski <veni at veni.com>
> DHCP
> You wake up after wild drinking. Your first words are "Who am I?", "Where 

Nice, except with DHCP you first ask "Can anybody tell me who am I?"
and then you reply to the first person to answer back "OK, you tell me
who am I". Otherwise you would have several servers giving a lease to
you.

I have used the postal system as an analogy to the Internet and it
seems to work pretty well: 

The postal system provides IP and all the layers below it, which you
really don't know about. This means that the postal system has the
right to delay, change order, lose, destroy etc. of any of your mail.
Absolutely no guarantee of delivery.

When you order a magazine with a serial story (insert the name of a
local magazine, I use Donald Duck and Don Rosa's stories, which are
very popular in Finland), the postal system tries to deliver the
magazine, but then you have your mother/secretary/somebody put the
magazines in order, so that you can read the whole story at one go and
in the right order. So your mother is the TCP.

Now here is the first important analogy. If one magazine is missing,
your mom does not complain to the post office, but contacts the
publisher of the magazine and the publisher will send a new one.
Actually the postal system is so stupid (stateless, connectionless)
that they bring you a magazine every week, but if you ask them, they
know nothing about it. The connection is between you and the publisher
and it only exists because both you and the publisher know about it
(hold the state of the connection).

Then I usually go on to explain that there is no way to know how my
mail is routed and how it is transported, but I know how to address a
letter and put it into the postbox and the post office knows how to
deliver the mail to me through my mail slot. These are the APIs, the
second important analogy. And when my mail goes into the mail bag the
SDU is encapsulated into a PDU.

This analogy extends pretty nicely. A colleague of mine noted that
Christmas cards are Pings and courier services provide QoS. And it is
easy to explain the purpose of layered protocols, when you explain how
difficult it would be for each publisher to transport their magazines
by themselves. 

I usually start by explaining the application, TCP and IP layers
technically and then I go over the same stuff by using the analogy.
The views of comprehension on people's faces are nice. 

kiravuo
ISOC.fi