$1.$ What are the functions of the transport layer?

$2.$ Why are two protocols, TCP and UDP, defined at the transport layer instead of just TCP$?$

$3.$ Why is segmentation useful?

$4.$ How is reliability at the transport layer different from the reliability provided by the data$-$link layer?

$5.$ What are the important potential problems with reliability that are handled by the transport layer?

$6.$ How does TCP provide reliability?

$7.$ What is flow control?

$8.$ Why is it more useful to allow the receiver to control flow speed, rather than the sender$?$

$9.$ How is flow control implemented?

$10.$ What is sliding window in the context of flow control? Describe its operation.

$11.$ What is multiplexing in the context of the transport layer?

$12.$ How is the multiplexing at the transport layer different from the multiplexing at the physical layer?