Kurose and Ross Edition 6 Exercise 2 Question P3 (Page No 171)

Question

Consider an HTTP client that wants to retrieve a Web document at a given URL. The IP address of the HTTP server is initially unknown. What transport and application-layer protocols besides HTTP are needed in this scenario?

Answer 1

HTTP is an application layer protocol. HTTP needs TCP as its transport layer protocol.
To establish a TCP connection, socket address of the server is needed. To perform this , another application layer protocol is needed.

This application layer protocol is DNS. DNS stands for Domain Name System. DNS uses UDP as its transport layer protocol.

DNS is used to map url to IP address.

Using DNS, client will get the IP address of the server. As HTTP is a standard application layer protocol, it has well known port number which is 80. This IP address + port number forms the socket address of the server.
Now TCP connection will be established and HTTP request message will be send to server. Then server will respond with the web document.

Thus except HTTP, application layer protocol used is DNS. Transport layer protocols used are TCP and UDP.