Transport Layer

The transport layer is responsible for source to destination delivery of the entire message. The transport layer ensures that the whole message arrive intact and in order and check all the packets relation to each other. Also check the error control and flow control at source to destination level.
For additional security, network layer may create the connection between two end ports. It involves the etablishment, data transfer and connection release.
Transport layer have following responsibilities;

• Service point addressing; As compters run many programs at the same time. So here source to destination delivery is not from one computer to other but it is froma specific program on one computer to a specific program on the other. Therefore transport layer header must include a service point address or port address.The transport layer gets the entire message to the correct program running on computer.
• A mesasge is divided into transmittable segmentrs, each segment contains a sequence number. These number used in reassembling.
• The transport layer can be connectionless or connection oriented. In connectionless it simply deliver packet to network layer at destination but in connection oriented it makes connection with the transport layer and then deliver packets.
• Like data link layer it is responsible for flow control. Here flow control is end to end not on a single link.
• Like data link layer it is responsible for error control. It makes sure that the entire message is delivered correctly at the destination without any error.