Introduction

Introduction to Computer Networking

Network Applications

Read and write data over network

Dominant model now is : Bidirectional reliable byte stream connection

• One side read what the other writes
• Operate in both direction
• Reliable(Work on a connection)

For example the HTTP protocol works like this

• Client open a connection to the server, the server accept the connection
• The client write the request to connection, the server read it from the connection
• The server write back the response to the connection, the client then read from the connection

The 4 layer Internet Model

Applications want to “use” the internet, they just care about read and write data, they don’t care how the data is transmitted. So they use layers as build blocks to reuse other people’s work rather than do all the thing from scratch

The model describe Internet operations in hierarchy

The link layer

The link layer is responsible for transmit the package

What is a package(aka datagram)?

*----------------------------------------*
|    Data              |  from   |   to  |
*----------------------------------------*

• Link layer accept the package from network layer

• Transmit one step forward, and then hand the package up to network layer

• Network layer check whether itself is the destination of the package

  if (to_address == my_address){
      accept();
  }
  else{
      give_back_to_link_layer_and_transmit_to_next();
  }

  The Network layer

The network layer is “special”, If you want to use the Internet, you must use IP(Internet Protocal)

• IP makes a best-effort attempt to deliver the package to its destination, but NO guarantee.
• IP datagrams CAN be lost, CAN be delivered out-of-order and CAN be corrupted

If you want your data to transmit in a more controlled way, you should use another protocal running on top of IP —- TCP

The Transport layer

“Control” how the datagrams are transmitted

TCP(Transmit Control Protocol) do the “missing job” for IP

• If the datagrams are out-of-order, TCP will reorder them into the original way
• If there are missing and corrupted packages, TCP will ask the sender to resend they

However, not every application want a reliable transmit, they can use other protocals, for example UDP(User Datagram Protocol)

The Application layer

bidirectional byte stream

They can define their own protocols about how to interpret these bytes

Summary

• 4 layer model follows a horizontal way

  Each layer pretend to talk “directly” to its peer layer, not knowing about what happens in the lower layers

• 4 layer model is an implementation of the OSI 7 layer model

  

  The only legacy to know about OSI model is the numbering system

  Sometimes Application layer is refered as “layer 7”, link layer is refered as “layer 2” etc.