Routing refers to the process of identifying the possible paths that you may use to transmit your
message, and selecting the best path based on criteria that you specify. Routing is a service provided by the
network layer.
The network layer usually does not make any guarantee of the end-to-end correctness of the message
transmission. Because computers on the network can fail at any time, and new computers can be added to the
network at any time, it is possible for frame routes to change in midmessage. As a result, frames of a message
can arrive out of order, or frames can be duplicated, or frames can be lost.
In most cases, we care that the entire message be transmitted correctly, so another layer of protocol provides
that guarantee. Ensuring not only that the message will be successfully passed between pairs of computers,
but also that the message will be received correctly at the destination, is the job of the transport layer protocol.
The transport protocol keeps track of the sequencing of frames in a message, and insures that regardless of
whatever bad things may happen during transmission, the receiver will get all the frames, in the correct order,
without duplications.
The OSI model provides three higher layers: the session, presentation, and application layers.
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