Osi(5)

Transport Layer

The transport layer accepts messages of arbitrary length from the session layer, segments them into packets, submits them to the network layer for transmission, and finally reassembles the packets at the destination. The transport layer records a sequence number in each packet, and uses the sequence numbers for detecting lost packets and for ensuring that messages are reconstructed in the correct sequence.

Transport layer guarantees transmission of data from one end to the other.

The overall function of transport layer is to provide transport service of the quality required by the session layer entities, when quality is specified in terms of throughput, delay and other quality measuring parameters.

The relationship of the transport layer to the network layer and session layer is shown in fig

Functions of the Transport Layer are

1. Segmenting and Reassembly

The transport layer creates packets out of the messages received from the application layer. Packetizing is a process of dividing a long message into smaller ones.

A header is also added to each packet.

After packetizing, it (transport layer) submits these packets to network layer for transmission and finally reassembles the packets at the destination.

2.             Sequence Numbering

The transport layer records a sequence number in each packet and uses the sequence number for detecting lost packets and for ensuring that messages are reconstructed in the correct sequence.

3.             Connection Control

Transport layer protocols are divided into two categories:

(a) Connection oriented delivery: A connection oriented transport layer protocol establishes a connection i.e. virtual path between sender and receiver.

(b) Connection less delivery: A connectionless transport protocol will treat each packet independently. There is no connection between them. Each packet can take its own different route.

4.             Flow Control

We know that data linK layer can provide the flow control. Similarly transport layer also can provide flow control. But this flow control is performed end to end rather than across a single link.

5.             Error Control

The transport layer can provide error control as well. But error control at transport layer is performed end to end rather than across a single link. Error correction is generally achieved through retransmission.

6.             Multiplexing if Needed

For optimum use of network connection, the transport entity may assign several end-to-end transport connection to the network connection. This is known as multiplexing.

Session Layer

• This layer provides establishing, maintaining and terminating a session or dialogue between two end systems. It creates the session, manages the packets sent back and forth during the session and terminates the session.

• The dialogue type/session type is also specified by it i.e. half duplex or full duplex.

• The session layer also provides for data synchronization and check pointing so that in the event of the network failure, only the data sent after the point of failure need be resent.

•This layer also controls the dialog between two processes determining who can transmit and who can receive at what point during the communication.

The relationship of the session layer to the transport layer and the presentation layer is shown in figure.

Functions of the Session Layer

1. Session Establishment

The main function of session layer is to provide and establish connections between session users, known as session. A session between two computers can be established for Communication, file transfer, for remote login or for some other purpose.

2. Mode of session

It is the most important function of session layer. The exchange of data between user entities may either be two way alternate (half duplex) or two way simultaneous (full duplex).

3.             Dialog Management

In most of the cases the session connections are full duplex but the upper layers some times communicate in half duplex modes too. In such cases the session layer has to keep track of whose turn it is to talk and is known as dialog management.

4. Synchronization

The session layer allows a process to add check point i.e. synchronization points into a stream of data. In case of crash during the transmission of the data can be retransmitted from the check point instead of retransmitting it from the start.