Transport Primitives:-

The transport service primitives allow the transport layer users such as application programs to access the transport device. Each transport service has it’s own access primitives. The transport service is similar to network services but there are some important differences.

  • The main difference is that the connection oriented transport service is reliable.
  • The second difference between  the network services and transport services is that for whom the services are intended for.
  • The transport services are seen by many programs and programmers. Hence, the transport service is convenient and easy to use.
  • The transport interface allows the application programs to establish,use and release connections.

Primitives for a simple transport service:-

  1. Name:- LISTEN

TPDU sent:- None

Meaning:- Block until some process                          tries to connect.

2.      Name:- CONNECT

TPDU sent:- Connection request

Meaning:- Actively attempt to                                establish a connection.

3.      Name:- SEND

TPDU:- Data

Meaning:- Send data

4.       Name:- RECEIVE

TPDU:- none

Meaning:- Block until a data TPDU                            arrives.

5.         Name:- DISCONNECT

TPDU:- Disconnection request

Meaning:- Replace the connection

What Is a TPDU?

The message sent from a transport entity to a transport entity is called as transport protocol or TPDU.

Nesting of TPDU’S , Packets and Frames:

  • The TPDU’S which are exchanged by the transport layer are contained in the packets that are exchanged by the network layer. These packets are in turn contained in the frames which exchanged by the data link layer.
  • When a frame arrives, the data link layer processes the frame header and passes the contents of the frame payload field to the network entity.
  • The network entity processes the packet header and passes the contents of the packet payload to the transport entity. This is called Nesting.


The socket interface was originally based on UNIX . It defines a set of system calls or procedures. The communication structure that we need in socket programming is called a socket. A socket acts as an end point. Two processes can communicate if and only if they have a socket at each end.

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