Exam
Adminitrivia
Final Exam on Thursday No Makeups given without prior permission Test is open notes. No electronic devices permitted.
Cell phone Laptops Anything else that can communicate
Layers Revisited
Scenario One
Alice works at the Albany office of MegaCorp Alice wants to send a memo to Alex, her counterpart at the Glens Falls office of Megacorp Lets consider what Alice might do
Max
Alice gives the memo to Max, her assistant and tells him to deliver it to Alex
Max
Max gets in his car He drives to Glens Falls He finds the MegaCorp building in Glens Falls He finds the floor and office Alice works on He takes the elevator to the floor, finds the office and delivers the memo
Max
If Alice sends five memos a day to Alex, he's going to be very busy If we send a separate car with every memo, MegaCorp is going to go broke.
What Max Needs to Know
How to drive How to get to Glens Falls How to find the MegaCorp Building in Glens Falls How to find the floor and office Alex works on How to run an elevator
Scenario Two
Scenario Two
Alice has a memo for Alex in the Glens Falls office Alice puts the memo in an envelope, and addresses it to Alex Alice gives the envelope to Ted in the mailroom
Ted in the Mailroom
Ted takes the envelope, and all the other interoffice mail for the Glens Falls office, puts it in an Express Mail package Ted addresses the package to Theo, his counterpart in the Glen's falls Mailroom Ted give the package to Paula at the Post office
Paula at the Post Office
Paula puts the package in a bag with all the other express mail going to the Glens Falls post office The bag is picked up, and delivered to Peter, Paula's counterpart at the Glens Falls Post Office
Peter at the Post Office
Peter removes the package from the bag, and puts it with the other mail for the Glen's Falls MegaCorp office The mail is delivered to Theo, in the Mailroom at Glens Falls office
Theo in the Mailroom
Theo opens the package She finds the memo for Alex She delivers the memo to Alex
Finally
Alex opens the envelope and reads the Memo
Alice in the Office
Alice knows
That Alex works for MegaCorp How to send a message to Alex using Ted in the Mailroom
Alice does not know
How to use Express Mail Whether Ted uses Express Mail or UPS
Ted in the Mailroom
Ted knows
How to use the Post Office to send a package to Theo in the mailroom at Glen's Falls
Ted does not know
How the Post Office gets packages to Glens Falls Where Glens Falls is How Theo gets the memo to Alex
Paula at the Post Office
Paula Knows
How to get mail to Peter in the Glen's Falls post office
Paula does not know
How Peter gets the mail to Theo How Theo gets the memo to Alex
Peter at the Post Office
Peter knows
How to get mail to Theo in the Mailroom
Peter does not know
How Theo gets the memo to Alex
Theo in the Mailroom
Theo knows
The floor and office Alex works on How to get the memo to Alex
Overview
Each person only does one piece of the job, and knows little or nothing of how the other jobs are done. Each person knows how to pass mail to the person above them, and how to send mail to their counterpart, or peer, using the services provided by the person below them.
Networking
Overview
Network communication is application to application
Messages are sent from one application running on a machine Messages are received by another application running on another machine
This is a complicated job
Overview
In programming, when we have a complicated job, we divide it into many smaller easy to write jobs This is called, "divide and conquer"
Overview
In networking we divide the job into "layers" As in the example, each layer does one part of the job. Each layer accepts and sends messages to the layer above it. Each layer uses the services of the layer below to send messages to its peer running on the other machine.
Overview
The collection of layers is called a Network Stack There are many stacks. The one in most common usage is TCP/IP
Network Stack
The layers of the network stack include
Application Transport Network Link
Some models have more layers, we need not be concerned with them
Application Layer
Application Layer
The application layer is the top layer of the stack The Application Layer
Accepts messages from the application addressed to another application on another machine Prepares the for messages sending Uses the services of the Transport layer to send the messages to the Application Layer on the other machine.
Application Layer
When recieveing messages
Application layer gets a message from the transport layer It unpacks the message to get it ready to deliver to the application Its transmits the message to the application
Application Layer
Knows
How to communicate with applications running on its machine How to prepare messages
Break into packets
How to send messages using the Transport Layer How to receive messages from the Transport Layer
Application Layer
Does not know
How the Transport layer delivers the messageto the other machine How the Network works
The TCP/IP stack has no Application Layer Applicaton layer is shared between the Operating System and the application
Transport Layer
Transport Layer
On transmit, the Transport Layer receives messages from the Application Layer The Transport layer moves messages to the transport layer on the other machine Messages are divided into packets The transport layer insures all packets are delivered in the order received from the Application layer
Transport Layer
The Transport layer sends messages to the Transport layer using services provided by the Network Layer On receive
The Transport Layer recieves messages from the Network layer.
It insures that all parts of the data are received It delivers the data to the Application Layer
Transport Layer
Knows
How to send and receive messages from the Application Layer How to coordinate with the transport layer on the other machine to make sure all parts of the message are received and correctly ordered How to use use the Network layer to send and receive messages to the other machine
Transport Layer
Does not know
How the network works, (what protocol is used) What the application is doing How the machines are linked together
Network Layer
Network Layer
On transmit, the Network Layer recieves messages from the Application Layer The network layer knows how the network is put together and how to get messages to the other machine
This is called routing
Network Layer
The Network layer uses services provided by the Link layer to communicate with the Network Layer on the other machine. On receive the Network layer receives messages from the link layer
If the messages are for this machine it passes them to the Transport Layer If the message is for another machine it forwards it down the next step on the route
Network Layer
Knows
How the network is organized, so it knows how to route messages How to communicate with the Transport Layer How to use the services of...