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?

Short answer

DNS for domain resolution and TCP for HTTP data retrieval are needed.

Step-by-step solution

Identify the Requirements

To retrieve a web document at a given URL, the client must first translate the domain name into an IP address.

Determine Protocol for Domain Resolution

The Domain Name System (DNS) protocol is required to resolve the URL's domain name into an IP address, as the IP address of the HTTP server is initially unknown.

Application-Layer Protocol

HTTP itself operates at the application layer to facilitate web document retrieval once the IP address is known.

Transport Layer Protocol

The Transmission Control Protocol (TCP) is used at the transport layer to establish a reliable connection between the client and the server for both DNS and HTTP data transmission.

Key concepts

DNS

When you want to visit a website, you often type in a domain name, like example.com, rather than an IP address. But computers need IP addresses to locate servers on the internet. This is where the Domain Name System (DNS) comes in handy. It acts like the internet's phonebook, translating human-readable domain names into numeric IP addresses that machines understand.
When the IP address of a website is unknown, the DNS resolves or maps the desired domain name to its corresponding IP address. This process ensures that your request gets directed to the correct server.
There are different types of DNS servers involved in this process:

• Root Nameservers: These are the first step in translating human-readable domain names into IP addresses.
• Top-Level Domain (TLD) Servers: They manage the top-level domains like .com, .org, etc., giving directions to the appropriate authoritative DNS server.
• Authoritative Nameservers: They are the ones that store the actual DNS records, including the IP addresses of particular domains.

DNS functions ensure that you can navigate seamlessly while browsing the web, linking domain names to their respective digital home addresses.

TCP

When communicating over the internet, a reliable stream of data is essential, and that’s where the Transmission Control Protocol (TCP) shines. It operates at the transport layer of the Internet Protocol (IP) suite and ensures reliable, ordered, and error-checked delivery of data.
One of TCP's core tasks is establishing a connection between a client (like your computer) and a server. This connection starts with a process famously known as the "three-way handshake":

• SYN: The client sends a synchronization packet to the server.
• SYN-ACK: The server acknowledges by sending a SYN-ACK packet back to the client.
• ACK: The client sends an acknowledgment back to the server, and the connection is established.

After this handshake, data, such as web pages or other requested documents, is transmitted. TCP ensures that all packets arrive at their destination correctly, and if anything is amiss, it can retransmit data. This makes it perfect for HTTP, guaranteeing that web content is delivered accurately and completely.
Overall, TCP is crucial for maintaining the integrity and reliability of data as it moves across the web.

Web Document Retrieval

After successfully resolving the website's domain name to an IP address and establishing a reliable connection with TCP, the final step in exploring a web resource involves the retrieval process of the web document. This primarily happens through the HTTP protocol.
HTTP, or HyperText Transfer Protocol, facilitates the fetching and displaying of web content such as web pages. When a browser requests a page, it sends an HTTP request to the server hosting the website. This request includes various details such as the resource being requested, the method to use (like GET for requesting data or POST for sending data), and information about the client making the request.
The server then processes this request and responds with an HTTP response, which could contain the requested page, an error message, or other relevant data.
HTTP is stateless, meaning each request from a client to a server is treated as an independent transaction that is unrelated to any previous request. Even though this design simplifies and accelerates transactions, it also means additional measures (like cookies and sessions) are needed to maintain state information across multiple requests, making browsing a seamless experience.
Thus, the combination of resolving addresses with DNS, establishing connections with TCP, and retrieving data with HTTP forms the backbone of web document retrieval that we experience every day.