It's amazing how stories and analogies can
help us understand concepts.
So as a precursor to the Open System Interconnect model--
the OSI model--
I want to talk with you for a few minutes about a story
regarding how two devices, or two entities,
could communicate with each other,
using The Tale of Two Kings.
I'd like you to imagine that King A wants
to invite King B, who lives in a different kingdom,
over to lunch.
Now, do you suppose King A has to go out by himself,
and ride across the countryside to do
the invitation personally?
The answer probably is, no.
He's got a whole bunch of people that work for him.
So in this story, King A is going to call for his scribe.
(IMITATES TRUMPET) "Bring me my scribe!"
And the scribe is an entity that is providing
some service to the King.
In our story, the scribe's going to write down
the words the King speaks, which is
going to include the invitation to King B to come to lunch.
And once the scribe has provided this service
and written this information down,
the scribe is way too high on the pecking order
to have to go out himself to deliver the message.
What the scribe's going to do, the scribe's
going to call on the secret agent in kingdom A--
Mr. 007 himself-- to go ahead and also provide some
functionality.
For example, 007 may provide encryption, or compression,
or translation of that language, or parts of it,
to make it more compatible with King B.
And once the secret agent 007 has done its work,
it will hand it over to the attorney--
the lawyer on King A's side, and the lawyer's responsible
for the management and the affairs of King A--
including communications between King A and other kings.
So assuming the attorney says, OK, yes, I'm
willing to let this message go, the lawyer
gives a stamp of approval, and hands it down
to the middle manager.
Now the middle manager's job is to-- based on the type
of message being sent--
perhaps send it reliably.
Or if it doesn't have to have a receipt that comes back--
unreliably.
Or if the message is just too big,
the middle manager may decide to chop up the message,
and then label those messages.
For example, this is part one of three, and two of three,
and three of three.
And when that middle manager has done his job,
it's going to hand that message or messages--
based on how it chopped them up--
down to the mailroom.
Now, we might think to ourselves,
hmm, what does the mailroom do?
Well, they handle mail.
It's going to have a couple of addresses on it.
It's going to have an address on the upper left.
For example, that's the From address.
It'd say, from Kingdom A. And it's also very likely
going to have a To address.
For example, to Kingdom B-- to King B specifically at Kingdom
B.
So after the mailroom has the appropriate labels
to the message or messages, the mailroom then hands these over
to the envelope stuffers.
Now, one of the questions might be,
which is the best type of envelope to use?
And that would also depend on what type of carrier
are we going to use.
Are we using FedEx, or UPS, or USPS, or some other delivery
service?
So if we're going to be using FedEx,
it's very likely we're going to use a FedEx envelope.
So the envelope stuffers are placing the messages
into the appropriate envelope.
And then our last step from Kingdom A's side
is the actual FedEx truck that's going
to take our message or messages, and route them
or deliver them to Kingdom B.
Now, when these messages are delivered--
in our example it's by FedEx--
the FedEx driver himself or herself doesn't just
run up to King B, hey, here's some stuff for you.
No.
King B has the same complement of individuals and staff
that King A had, including attorneys, and secret agents,
and middle managers, and mail rooms, and envelope stuffers.
So when the delivery happens by FedEx over at Kingdom B
on the right, that information is processed from the bottom
up, starting with the physical delivery of the messages,
and then the envelope stuffers, which are now opening contents
and handing the results over to the mailroom-- which says, yup,
this is our kingdom, it's the right address--
who then hand the message up to the middle manager
at King B's side.
And the middle manager at King B's side
might be a little bit nervous if it sees, oh my goodness, this
is one of three.
Where's two of three, and three or three?
It might hold on to that information
until it receives part two of three and part three of three,
reassemble all those together in one package, if you will,
and then hand them up to the attorney.
Then the attorney, who manages the sessions and affairs
for King B, can decide whether or not
we want to go ahead and continue processing these and accepting
these messages.
And if so, it hands it over to 007 on King B's side.
So on King A's side, if 007 did features like encryption
and compression on the receiving side,
the 007 function would do things like decryption
and decompression.
And when 007 on the receiving side is done with his work,
he can hand that up to the scribe.
And then the scribe providing this service to King B,
would simply read off the message,
which would say, "King A would like to know
if you'd like to go to lunch."
And if King B replies, we're going
to follow that same logical path from King B,
going all the way down his staff,
having the message delivered by FedEx,
and then be received all the way up on King A's side
until King A gets the reply.
Now, your mission, should you choose to accept it,
is to take this story that we've gone through together,
and I'd like you to share it with somebody-- a friend,
or a colleague, or a coworker, or a loved one.
Because by telling this story, and remembering
each of the staff members in the kingdoms,
that's going to help a boatload as we take a deeper
look into the concept of the OSI reference model,
and how two devices can communicate across the network.
I hope this has been informative for you,
and I'd like to thank you for viewing.