Introduction to transport layer

Transport-Layer services
The transport layer is responsible for providing services to the application layer; it receives
services from the network layer.
Process-to-Process Communication
? The first duty of a transport-layer protocol is to provide process-to-process communication.
A process is an application-layer entity (running program) that uses the services of the
transport layer.
? The network layer is responsible for communication at the computer level (host-to-host
communication). A network layer protocol can deliver the message only to the destination
computer. However, this is an incomplete delivery. The message still needs to be handed to
the correct process. The following Figure shows the domains of a network layer and a
transport layer.
Addressing: Port Numbers
? A remote computer can run several server programs at the same time, just as several local
computers can run one or more client programs at the same time. For communication, we
must define the:
? Local host, Local process, Remote host and Remote process
? The local host and the remote host are defined using IP addresses. To define the processes,
we need second identifiers called port numbers. In the TCP/IP protocol suite, the port
numbers are integers between 0 and 65,535.
? The client program defines itself with a port number, called the ephemeral port number
(chosen randomly). The word ephemeral means short lived.
? The server process must also define itself with a port number (called well-known port
numbers). This port number, however, cannot be chosen randomly.
? The destination IP address defines the host among the different hosts in the world. After the
host has been selected, the port number defines one of the processes on this particular host.
2
ICANN Ranges Internet Corporation for Assigned Names and Numbers
ICANN has divided the port numbers into three ranges: well-known, registered, and dynamic
(or private)
? Well-known ports. The ports ranging from 0 to 1,023 are assigned and controlled by
ICANN..
? Registered ports. The ports ranging from 1,024 to 49,151 are not assigned or controlled
by ICANN. They can only be registered with ICANN to prevent duplication.
? Dynamic ports. The ports ranging from 49,152 to 65,535 are neither controlled nor
registered. They can be used as temporary or private port numbers. The original
recommendation was that the ephemeral port numbers for clients be chosen from this range.
However, most systems do not follow this recommendation.
Socket Addresses
- A transport-layer protocol in the TCP suite needs both the IP address and the port number, at
each end, to make a connection. The combination of an IP address and a port number is called a
socket address.
- To use the services of transport layer in the Internet, we need a pair of socket addresses: the
client socket address and the server socket address. These four pieces of information are part of
the network-layer packet header and the transport-layer packet header. The first header contains
the IP addresses; the second header contains the port numbers.