Transport Layer
Transport Layer
The basic 'function' of the transport layer is to accept data from above, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done, efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware technology.
The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. However, other possible kinds of transport service are the transporting of isolated messages, with no guarantee about the order of delivery, and the broadcasting of messages to multiple destinations. The type' of service is determined when the connection is established. (As an aside, an error-free channel is impossible to achieve; what people really mean by this term is that the error rate is low enough to ignore in practice.
The transport layer is a true end-to-end layer, all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control messages. In the lower layers, the protocols are between each machine and its immediate neighbors, and not between the ultimate source and destination machines, which may be separated by many routers. The difference between layers 1 through 3, which are chained, and layers 4 through 7, which are end-to-end.
The responsibilities of the transport layer are as follows
» Host-to-host message delivery : Ensuring that all the packets of a message sent by a source node arrive at the intended destination.
» Application to application communication : The transport layer enables communication between two applications running on different computers.
» Segmentation and reassembly : The transport layer breaks a message into packets, numbers them by adding sequence numbers at the destination to reassemble the original message.
» Connection : The transport layer might create a logical connection between the source and the destination for the duration of the complete message transfer for better control over the message transfer.