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It's amazing how stories
and analogies can

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help us understand concepts.

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So as a precursor to the Open
System Interconnect model--

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the OSI model--

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I want to talk with you for
a few minutes about a story

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regarding how two
devices, or two entities,

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could communicate
with each other,

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using The Tale of Two Kings.

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I'd like you to imagine
that King A wants

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to invite King B, who lives
in a different kingdom,

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over to lunch.

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Now, do you suppose King A
has to go out by himself,

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and ride across the
countryside to do

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the invitation personally?

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The answer probably is, no.

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He's got a whole bunch of
people that work for him.

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So in this story, King A is
going to call for his scribe.

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(IMITATES TRUMPET)
"Bring me my scribe!"

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And the scribe is an
entity that is providing

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some service to the King.

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In our story, the scribe's
going to write down

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the words the King
speaks, which is

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going to include the invitation
to King B to come to lunch.

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And once the scribe has
provided this service

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and written this
information down,

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the scribe is way too
high on the pecking order

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to have to go out himself
to deliver the message.

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What the scribe's going
to do, the scribe's

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going to call on the
secret agent in kingdom A--

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Mr. 007 himself-- to go
ahead and also provide some

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functionality.

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For example, 007 may provide
encryption, or compression,

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or translation of that
language, or parts of it,

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to make it more
compatible with King B.

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And once the secret agent
007 has done its work,

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it will hand it over
to the attorney--

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the lawyer on King A's side,
and the lawyer's responsible

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for the management and
the affairs of King A--

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including communications
between King A and other kings.

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So assuming the attorney
says, OK, yes, I'm

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willing to let this
message go, the lawyer

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gives a stamp of approval,
and hands it down

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to the middle manager.

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Now the middle manager's job
is to-- based on the type

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of message being sent--

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perhaps send it reliably.

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Or if it doesn't have to have
a receipt that comes back--

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unreliably.

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Or if the message
is just too big,

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the middle manager may decide
to chop up the message,

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and then label those messages.

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For example, this is part one
of three, and two of three,

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and three of three.

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And when that middle
manager has done his job,

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it's going to hand that
message or messages--

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based on how it
chopped them up--

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down to the mailroom.

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Now, we might
think to ourselves,

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hmm, what does the mailroom do?

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Well, they handle mail.

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It's going to have a
couple of addresses on it.

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It's going to have an
address on the upper left.

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For example, that's
the From address.

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It'd say, from Kingdom A.
And it's also very likely

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going to have a To address.

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For example, to Kingdom B-- to
King B specifically at Kingdom

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B.

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So after the mailroom has
the appropriate labels

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to the message or messages, the
mailroom then hands these over

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to the envelope stuffers.

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Now, one of the
questions might be,

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which is the best type
of envelope to use?

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And that would also depend
on what type of carrier

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are we going to use.

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Are we using FedEx, or UPS, or
USPS, or some other delivery

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service?

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So if we're going
to be using FedEx,

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it's very likely we're going
to use a FedEx envelope.

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So the envelope stuffers
are placing the messages

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into the appropriate envelope.

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And then our last step
from Kingdom A's side

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is the actual FedEx
truck that's going

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to take our message or
messages, and route them

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or deliver them to Kingdom B.

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Now, when these
messages are delivered--

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in our example it's by FedEx--

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the FedEx driver himself
or herself doesn't just

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run up to King B, hey,
here's some stuff for you.

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No.

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King B has the same complement
of individuals and staff

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that King A had, including
attorneys, and secret agents,

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and middle managers, and mail
rooms, and envelope stuffers.

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So when the delivery happens
by FedEx over at Kingdom B

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on the right, that information
is processed from the bottom

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up, starting with the physical
delivery of the messages,

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and then the envelope stuffers,
which are now opening contents

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and handing the results over to
the mailroom-- which says, yup,

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this is our kingdom,
it's the right address--

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who then hand the message
up to the middle manager

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at King B's side.

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And the middle manager
at King B's side

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might be a little bit nervous
if it sees, oh my goodness, this

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is one of three.

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Where's two of three,
and three or three?

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It might hold on
to that information

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until it receives part two of
three and part three of three,

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reassemble all those together
in one package, if you will,

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and then hand them
up to the attorney.

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Then the attorney, who manages
the sessions and affairs

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for King B, can
decide whether or not

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we want to go ahead and continue
processing these and accepting

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these messages.

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And if so, it hands it over
to 007 on King B's side.

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So on King A's side, if 007
did features like encryption

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and compression on
the receiving side,

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the 007 function would
do things like decryption

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and decompression.

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And when 007 on the receiving
side is done with his work,

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he can hand that
up to the scribe.

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And then the scribe providing
this service to King B,

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would simply read
off the message,

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which would say, "King
A would like to know

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if you'd like to go to lunch."

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And if King B
replies, we're going

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to follow that same
logical path from King B,

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going all the way
down his staff,

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having the message
delivered by FedEx,

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and then be received all
the way up on King A's side

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until King A gets the reply.

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Now, your mission, should
you choose to accept it,

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is to take this story that
we've gone through together,

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and I'd like you to share
it with somebody-- a friend,

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or a colleague, or a
coworker, or a loved one.

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Because by telling this
story, and remembering

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each of the staff
members in the kingdoms,

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that's going to help a
boatload as we take a deeper

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look into the concept of
the OSI reference model,

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and how two devices can
communicate across the network.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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