The Internet Protocol (IP) formally specifies the format
to Internet packets, called Datagrams, and informally embodies
the ideas of connectionless delivery.
IP provides three important definitions. First, the IP
protocol defines the basic unit of data transfer used throughout
a TCP/IP Internet. Thus it specifies the exact format of
all data as it passes across a TCP/IP Internet. Second,
IP software performs the routing function, choosing a path
over which data will be sent. Third, in addition to the
precise, formal specification of data formats and routing,
IP includes a set of rules that embody the idea of unreliable
As shown above, the IP datagram is divided into header and
data areas. Among other information, the datagram header contains
the source and destination IP addresses, fragmentation control,
precedence, and a checksum used to catch transmission errors.
Besides fixed-length fields, each datagram header can contain
an options field. The options field is a variable length, depending
on the number and type of options used as well as the size of
the data area allocated for each option. Intended to help monitor
and control the Internet, options allow one to specify or record
routing information, or to gather timestamps as the datagram
traverses an Internet.
(ii) Routing IP Datagrams:
IP routing consists of deciding where to send a datagram
based on its destination IP address. Direct delivery is possible
if the destination machine lies on a network to which the sending
machine attaches. If the sender cannot reach the destination
directly, the sender must forward the datagram to a router.
The general paradigm is that hosts send indirectly routed datagrams
to the nearest router; the datagrams travel through the Internet
from router to router until they can be delivered directly across
one physical network.
The above figure depicts an example of a singly homed host
that must route datagrams. The host must choose to send a datagram
either to route R1 or to route R2, because each router provides
the best path to some destinations.
Direct and Indirect Delivery:
The routing can be divided into two forms: direct delivery
and indirect delivery. Direct delivery, the transmission of
a datagram from one machine across a single physical network
directly to another, is the basis on which all Internet communication
rests. Two machines can engage in direct delivery only if they
both attach directly to the same underlying physical transmission
system. Indirect delivery occurs when the destination is not
on a directly attached network, forcing the sender to pass the
datagram to a router for delivery.
Routing with IP Addresses:
When IP software looks up a route, the algorithm produces
the IP address of the next machine to which the datagram should
be sent; IP passes the datagram and next hop address to network
interface software. Transmission of a datagram from one machine
to next always involves encapsulating the datagram in a physical
frame, mapping the next hop internet address to a physical address,
and sending the frame using the underlying hardware.
The Internet routing algorithm is a table driven and uses
only IP addresses. Although it is possible for a routing table
to contain a host-specific destination address, most routing
tables contain only network addresses, keeping routing tables
small. Using a default route can also help keep a routing table
small, especially for hosts that can access only one router.
(iii) Error and Control Messages (ICMP):
The Internet Control Message Protocol allows router to send
error or control messages to other routers or hosts; ICMP provides
communication between the Internet Protocol software on one
machine and the Internet Protocol software on another. ICMP
includes source quench messages that retard the rate of transmission,
redirect messages that request a host to change its routing
table, and echo request/reply messages that hosts can use to
determine whether a destination can be reached. An ICMP message
travels in the data area of an IP datagram and has three fixed-length
fields at the beginning of the message: an ICMP message type
field, a code field, and an ICMP checksum field. The message
type determines the format of the rest of the message as well
as its meaning.
When a datagram causes an error, ICMP can only report the
error condition back to the original source of the datagram;
the source must relate the error to an individual application
program or take other action to correct the problem.
6. Protocol Layering
Protocols are the standards that specify how data is represented
when being transferred from one machine to another. Protocols
specify how the transfer occurs, how errors are detected, and
how acknowledgements are passed. To simplify protocol design
and implementation, communication stacks are segregated into
layers that can be solved independently. Each layer is assigned
a separate protocol.
The idea of layering is fundamental because it provides a
conceptual framework for protocol design.
ISO 7-Layer Reference Model:
7 Application layer
Application Layer is responsible for managing the communication
between the applications.
6 Presentation Layer
Presentation Layer provides common operations on the structure
of data being exchanged.
5 Session Layer
Session Layer provides the control structure for managing
Terminating sessions. (Connections)
4 Transport Layer
Transport Layer provides reliable transparent data transfer
between end points. (there may be more than two end points).
Provide error recovery and flow control.
3 Network Layer
Network Layer makes the upper layers independent of the data
transmission, switching technologies, and topology of the network.
Determine which path (or paths) in the network that
a given unit of data will take (routing).
2 Link Layer
Link Layer provides reliable transfer across the physical
Establishes the beginning and end of blocks of data
(with synchronization when necessary)
Error detection and possibly correction;
1 Physical Layer
Physical Layer provides transmission of unstructured bits
across the physical medium.
The TCP/IP Model:
The TCP/IP model does not exactly match the OSI model. There
is no universal agreement regarding how to describe TCP/IP with
a layered model but it is generally agreed that there are fewer
levels than the seven layers of the OSI model. The layers of
the TCP/IP model are defined as follows:
In TCP/IP the Application Layer also includes the OSI Presentation
Layer and Session Layer. In this document an application is
any process that occurs above the Transport Layer. This includes
all of the processes that involve user interaction. The application
determines the presentation of the data and controls the session.
In TCP/IP the terms socket and port are used to
describe the path over which applications communicate. There
are numerous application level protocols in TCP/IP, including
Simple Mail Transfer Protocol (SMTP) and Post Office Protocol
(POP) used for e-mail, Hyper Text Transfer Protocol (https)
used for the World-Wide-Web, and File Transfer Protocol (FTP).
Most application level protocols are associated with one or
more port number.
In TCP/IP there are two Transport Layer protocols. The Transmission
Control Protocol (TCP) guarantees that information is received
as it was sent. The User Datagram Protocol (UDP) performs no
end-to-end reliability checks.
In the OSI Reference Model the Network Layer isolates the
upper layer protocols from the details of the underlying network
and manages the connections across the network. The Internet
Protocol (IP) is normally described as the TCP/IP Network Layer.
Because of the Inter-Networking emphasis of TCP/IP this is commonly
referred to as the Internet Layer. All upper and lower layer
communications travel through IP as they are passed through
the TCP/IP protocol stack.
Network Access Layer
In TCP/IP the Data Link Layer and Physical Layer are normally
grouped together. TCP/IP makes use of existing Data Link and
Physical Layer standards rather than defining its own. Most
RFCs that refer to the Data Link Layer describe how IP utilizes
existing data link protocols such as Ethernet, Token Ring, FDDI,
HSSI, and ATM. The Physical Layer typically defines the characteristics
of the hardware that carries the communication signal. This
describes attributes such as pin configurations, voltage levels,
and cable requirements.