Data Encapsulation, a Critical concept to be understood
When starting with protocols that work at the upper layers of the network models, each set of data is wrapped inside the next lower layer protocol, similar to wrapping letters inside an envelope. The application creates the data, then the transport layer wraps that data inside its format, then the network layer wraps the data, and finally the link (ethernet) layer encapsulates the data and transmits it.
To continue, you should understand the definition of a client and server with regards to networking. If you are a server, you will provide services to a client, in much the same way as a private investigator would provide
services to their clients. A client will contact the server, and ask for service, which the server will then provide.
The service may be as simple as sending a single block of data back to the client. Since there are many clients, a server must be constantly ready to receive client requests, even though it may already be working with other clients. Usually the client program will operate on one computer, while the server program will operate on another computer, although programs can be written to be both a client and a server.
Lets say you write a client chat program and a server chat program to be used by two people to send messages between their machines. You run the server program on machine B, and the client program on machine A. Tom is on machine A and George is on machine B. George's machine is always ready to be contacted, but cannot initiate a contact. Therefore if George wants to talk to Tom, he cannot, until Tom contacts him. Tom, of course can initiate contact at any time. Now you decide to solve the problem and merge the functionality of the two programs into one, so both parties may contact the other. This program is now a client/server program which operates both as a client and a server. You write your code so when one side initiates contact, he will get a dialog box, and a dialog box will pop up on the other side. At the time contact is initiated, a socket is opened between the two machines and a virtual connection is established. The program will let the user (Tom) type text into the dialog window, and hit send. When the user hits send, roughly the following will happen.
1. Your program will pass Tom's typed text in a buffer, to the socket. This happens on machine A.
2. The underlying software (Code in a library called by a function your program used to send the data)
supporting the socket puts the data inside a TCP data packet. This means that a TCP header will be added
to the data. This header contains a source and destination port number along with some other information
and a checksum. Deamon programs (Daemon definition at the bottom of this page) may also work at this
level to sort packages based on port number (hence the TCP wrapper program in UNIX and Linux).
3. The TCP packet will be placed inside an IP data packet with a source and destination IP address along
with some other data for network management. This may be done by a combination of your library
function, the operating system and supporting programs.
4. The IP data packet is placed inside an ethernet data packet. This data packet includes the destination and
source address of the network interface cards (NIC) on the two computers. The address here is the
hardware address of the respective cards and is called the MAC address.
5. The ethernet packet is transmitted over the network line.
6. Assuming there is a direct connection between the two computers, the network interface card on machine
B, will recognize its MAC address and grab the data.
7. The IP data packet will be extracted from the ethernet data packet. A combination of deamons and the
operating system will perform this operation.
8. The TCP data packet will be extracted from the IP data packet. A combination of deamons, the operating
system, and libraries called by your program will perform this function.
9. The data will be extracted from the TCP packet. Your program will then display the retrieved data (text) in
the text display window for George to read.
Be aware that for the sake of simplicity, we are excluding details such as error management, routing, and
identifying the hardware address of the NIC on the computer intended to receive the data. Also we are not
mentioning the possible rejection of service based on a packet's port number or sender's IP address.
A deamon program is a program that runs in the background on a computer operating system. It is used to
perform various tasks including server functions. It is usually started when the operating system is booted, but a user or administrator may be able to start or stop a daemon at any time.
