You should be aware of the fact, that when
talking about networking you will hear the word "protocol" all the time.
A D V E R T I S E M E N T
This is because protocols are sets of standards that define all
operations within a network. They define how various operations are to
be performed. They may even define how devices outside the network can
interact with the network. Protocols define everything from basic
networking data structures, to higher level application programs. They
define various services and utility programs. Protocols operate at many
layers of the network models described below. There are protocols
considered to be transport protocols such as TCP and UDP. Other
protocols work at the network layer of the OSI network model shown
below, and some protocols work at several of the network layers.
Protocols are outlined in Request for Comments (RFCs). At the end of
this document is a list of protocols and associated RFC
numbers.Protocols. Although RFCs define protocols not all RFCs define
protocols but may define other requirements for the internet such as RFC
1543 which provides information about the preparation of RFCs. The
following RFCs are very central to the TCP/IP protocol.
RFC 1122 - Defines host requirements of the TCP/IP suite of
protocols covering the link, network (IP), and transport (TCP, UDP)
RFC 1123 - The companion RFC to 1122 covering requirements for
internet hosts at the application layer
RFC 1812 - Defines requirements for internet gateways which are
There are several network models which you may hear about but the one
you will hear about most is the ISO network model described below. You
should realize, however that there are others such as:
The internet layered protocol
The TCP/IP 4 layered protocol
The Microsoft networking protocol
If you don't like any of these models, feel free to invent your own along
with your own networking scheme of course, and add it to the list above. You can
call it "The MyName Protocol". Ever wonder why networking can be so complex and
confusing? Welcome to the world of free enterprise!
The OSI Network Model Standard
The International Standards Organization (ISO) has defined a standard called
the Open Systems Interconnection (OSI) reference model. This is a seven layer
architecture listed below. Each layer is considered to be responsible for a
different part of the communications. This concept was developed to accommodate
changes in technology. The layers are arranged here from the lower levels
starting with the physical (hardware) to the higher levels.
Physical Layer - The actual hardware.
Data Link Layer - Data transfer method (802x ethernet). Puts data in
frames and ensures error free transmission. Also controls the timing of the
network transmission. Adds frame type, address, and error control
information. IEEE divided this layer into the two following sublayers.
Logical Link control (LLC) - Maintains the Link between two
computers by establishing Service Access Points (SAPs) which are a
series of interface points. IEEE 802.2.
Media Access Control (MAC) - Used to coordinate the sending of data
between computers. The 802.3, 4, 5, and 12 standards apply to this
layer. If you hear someone talking about the MAC address of a network
card, they are referring to the hardware address of the card.
Network Layer - IP network protocol. Routes messages using the best path
Transport Layer - TCP, UDP. Ensures properly sequenced and error free
Session Layer - The user's interface to the network. Determines when the
session is begun or opened, how long it is used, and when it is closed.
Controls the transmission of data during the session. Supports security and
name lookup enabling computers to locate each other.
Presentation Layer - ASCII or EBCDEC data syntax. Makes the type of data
transparent to the layers around it. Used to translate date to computer
specific format such as byte ordering. It may include compression. It
prepares the data, either for the network or the application depending on
the direction it is going.
Application Layer - Provides services software applications need.
Provides the ability for user applications to interact with the network.
Many protocol stacks overlap the borders of the seven layer model by
operating at multiple layers of the model. File Transport Protocol (FTP) and
telnet both work at the application, presentation, and the session layers.
The Internet, TCP/IP, DOD Model
This model is sometimes called the DOD model since it was designed for the
department of defense It is also called the TCP/IP four layer protocol, or the
internet protocol. It has the following layers:
Link - Device driver and interface card which maps to the data link and
physical layer of the OSI model.
Network - Corresponds to the network layer of the OSI model and includes
the IP, ICMP, and IGMP protocols.
Transport - Corresponds to the transport layer and includes the TCP and
Application - Corresponds to the OSI Session, Presentation and
Application layers and includes FTP, Telnet, ping, Rlogin, rsh, TFTP, SMTP,
SNMP, DNS, your program, etc.
Please note the four layer TCP/IP protocol. Each layer has a set of data that it
The Link layer corresponds to the hardware, including the device driver
and interface card. The link layer has data packets associated with it
depending on the type of network being used such as ARCnet, Token ring or
ethernet. In our case, we will be talking about ethernet.
The network layer manages the movement of packets around the network and
includes IP, ICMP, and IGMP. It is responsible for making sure that packages
reach their destinations, and if they don't, reporting errors.
The transport layer is the mechanism used for two computers to exchange
data with regards to software. The two types of protocols that are the
transport mechanisms are TCP and UDP. There are also other types of
protocols for systems other than TCP/IP but we will talk about TCP and UDP
in this document.
The application layer refers to networking protocols that are used to
support various services such as FTP, Telnet, BOOTP, etc. Note here to avoid
confusion, that the application layer is generally referring to protocols
such as FTP, telnet, ping, and other programs designed for specific purposes
which are governed by a specific set of protocols defined with RFC's
(request for comments). However a program that you may write can define its
own data structure to send between your client and server program so long as
the program you run on both the client and server machine understand your
protocol. For example when your program opens a socket to another machine,
it is using TCP protocol, but the data you send depends on how you structure
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
Your program will pass Tom's typed text in a buffer, to the socket. This
happens on machine A.
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).
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.
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.
The ethernet packet is transmitted over the network line.
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 IP data packet will be extracted from the ethernet data packet. A
combination of deamons and the operating system will perform this operation.
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.
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
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
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.