What does TCP/IP mean?
TCP/IP is a suite of protocols. The acronym TCP/IP means "Transmission Control Protocol/Internet
Protocol" and is pronounced "T-C-P-I-P". It comes from the names of the two major protocols in the suite of
protocols, i.e. the TCP and IP protocols).
In some ways, TCP/IP represents all communication rules for the internet and is based on the IP addressing
notion, i.e. the idea of providing an IP address for each machine on the network so as to be able to route
data packets. Given that the TCP/IP protocol suite was originally created with a military purpose, it is
designed to respond to a certain number of criteria, including:
Splitting messages into packets;
Use of an address system;
Routing data over the network;
Error detection in data transmissions.
Knowledge of the collection of TCP/IP protocols is not essential for a simple user, just as a viewer does not
need to know how his television or audiovisual networks work. However, for people who want to
administrate or support a TCP/IP network, knowledge of it is vital.
The difference between standard and implementation
Overall TCP/IP brings together two notions:
The notion of standard: TCP/IP represents the way in which communications are carried out over a
network.
The notion of implementation: the designation TCP/IP is often extended to software based on the
TCP/IP protocol. TCP/IP is in fact a model which network application developers use. The applications
are therefore implementations of the TCP/IP protocol.
TCP/IP is a layer model
So as to be able to apply the TCP/IP model to any machine, i.e. independently of the operating system, the
TCP/IP system of protocols has been broken down into several modules each carrying out a specific task.
Furthermore, these modules carry out these tasks one after the other in a specific order, so there is stratified
system, this is the reason we talk about a layer model.
The term layer is used to reflect the fact that the data which travels over the network crosses several
protocol levels. So, data (information packets) which circulates over the network is processed successively
by each layer, which adds an item of information (called a header) then sends it to the next layer.
The TCP/IP model is very close to the OSI model (model comprising of 7 layers) which was developed by
the international standards organisation (ISO) in order to standardise communications between computers.
Presentation of the OSI model
OSI means Open Systems Interconnection. This model was established by ISO in order to implement a
communications standard between computers on a network, that is the rules which manage communications
between computers. In fact, when networks began, each manufacturer had its own system (we talk of a
proprietary system) so many incompatible networks coexisted. This is the why it was necessary to establish
a standard.
The role of the OSI model is to standardise communication between machines so that different
manufacturers can develop compatible (software or hardware) products (as long as they strictly observe the
OSI model).
The importance of a layer system
The aim of a system in layers is to separate the problem into different parts (the layers) depending on their
level of abstraction.
Each layer of the model communicates with an adjacent level (above or below it). Each layer therefore uses
the services of the lower layers and supplies them to that of the upper layer.
The OSI model
The OSI model is a model which comprises of 7 layers, while the TCP/IP model only has 4. In reality the
TCP/IP model was developed around about the same time as the OSI model which is why it is inspired by it
but does not completely conform to the specifications of the OSI model. The layers of the OSI model are as
follows:
Level Old model New model
Level 7<><>
Level 6<><>
Level 5<><>
Level 4<><>
Level 3<><>
Level 2< Liaison><>
Level 1<><>
The physical layer defines the way in which data is physically converted into digital signals on the
communication media (electric pulses, light modulation, etc.).
The data link layer defines the interface with the network interface card and the sharing of the
transmission media.
The network layer makes it possible to manage addressing and routing of data, i.e. their path via the
network
The transport layer is in charge of data transport, its division into packets and management of
potential transmission errors.
The session layer defines the opening and destruction of communication sessions between networked
machines.
The presentation layer defines the format of data handled by the application layer (its representation
and potentially its compression and encryption) independently of the system.
The application layer provides the interface with applications. It is therefore the closest level to the
users, managed directly by the software.
The TCP/IP model
The TCP/IP model, inspired by the OSI model, also uses the modular approach (use of modules or layers)
but only contains four:
Application layer<>
< Transport><>
< Internet><>
Network Access layer<>
As can be seen, the layers of the TCP/IP model have much more diverse tasks that those of the OSI model,
considering that certain layers from the TCP/IP model correspond to several levels of the OSI model.
The roles of the different layers are as follows:
Network Access layer: specifies the form in which data must be routed whichever type of network is
used.
Internet layer: responsible for supplying the data packet (datagram)
Transport layer: provides the routing data, along with the mechanisms making it possible to know the
status of the transmission
Application layer: incorporates standard network applications (Telnet, SMTP, FTP, ...)
Here are the main protocols which make up the TCP/IP suite:
Network applications
TCP or UDP
IP, ARP, RARP
FTS, FDDI, PPP, Ethernet, Token ring
Data encapsulation
During a transmission, data crosses each one of the layers at the level of the originator machine. At each
layer, a piece of information is added to the data packet, this is a header, a collection of information which
guarantees transmission. At the level of the recipient machine, when passing through each layer, the header
is read, then deleted. So, upon its receipt, the message is in its original state...
At each level, the data packet changes aspect, because a header is added to it, so the designations change
according to the layers:
The data packet is called a message at Application layer level
The message is then encapsulated in the form of a segment in the Transport layer
Once the segment is encapsulated in the Internet layer it takes the name of datagram
Finally, we talk about a frame at the Network Access layer level
Network Access layer
The network access layer is the first layer of the TCP/IP stack, it offers the ability to access whichever
physical network, i.e. the resources to be implemented so as to transmit data via a network.
So, the network access layer contains all specifications relating to the transmission of data over a physical
network, when it is a local area network (Token ring, Ethernet, FDDI), connected by telephone line or any
other type of link to a network. It deals with the following concepts:
Routing data over the connection
Coordination of the data transmission (synchronisation)
Data format
Signal conversion (analogue/digital)
Error detection on arrival
Fortunately all these specifications are transparent in the eyes of the user, because all these tasks are in
fact carried out by the operating system, while the hardware drivers allow connection to the network (e.g.
network card driver).
The Internet layer
The Internet layer is the "most important" layer (they are all important in their way) because it is this which
defines the datagrams and manages the IP addressing notions.
It enables the routing of datagrams (data packets) to remote machines along with the management of their
division and assembly upon receipt.
The Internet layer contains 5 protocols:
The IP protocol
The ARP protocol
The ICMP protocol
The RARP protocol
The IGMP protocol
The first three protocols are the most important protocols for this layer...
The Transport layer
The protocols for the preceding layers make it possible to send information from one machine to another.
The transport layer enables applications running on remote machines to communicate. The problem is
identifying these applications.
In fact, depending on the machine and its operating system, the application may be a program, task,
process...
Furthermore, the name of the application may vary from system to system, that is why a numbering system
has been put in place so as to be able to associate an application type with a data type, these identifiers are
called ports.
The transport layer contains two protocols enabling two applications to exchange data independently of the
type of network taken (i.e. independently of the lower layers), these are the following two protocols:
TCP, a connection orientated protocol which provides error detection
UDP, a connectionless orientated protocol where error detection is outdated
The Application layer
The application layer is located at the top of the TCP/IP protocol layers. This one contains the network
applications which make it possible to communicate using the lower layers.
The software in this layer therefore communicates using one of the two protocols of the layer below (the
transport layer), i.e. TCP or UDP.
There are different types of applications for this layer, but the majority are network services, or applications
supplied to the user to provide the interface with the operating system. They can be classed according to the
services that they provide:
File and print management services (transfer)
Network connection services
Remote connection services
Various Internet utilities
