45th BCS — Written (CSE-971), Exam: 31.01.2024প্রশ্ন ও সমাধান

HTTP (HyperText Transfer Protocol): an application-layer protocol used to request and transfer web resources (HTML, JSON, images) between a client (browser) and a server, typically running over TCP. It is stateless and request/response based, using methods such as GET, POST, PUT and DELETE.

DNS (Domain Name System): a hierarchical, distributed naming system that translates human-readable domain names (e.g. mygov.bd) into machine-usable IP addresses, using a client–server architecture of resolvers and name servers.

How DNS resolves mygov.bd:

sequenceDiagram
    participant Br as Browser
    participant Res as Recursive Resolver (ISP)
    participant Root as Root Server
    participant TLD as .bd TLD Server
    participant Auth as Authoritative NS (mygov.bd)

    Br->>Res: Resolve mygov.bd?
    Res->>Res: Check cache (miss)
    Res->>Root: Who handles .bd?
    Root-->>Res: Referral to .bd TLD server
    Res->>TLD: Who handles mygov.bd?
    TLD-->>Res: Referral to mygov.bd's authoritative NS
    Res->>Auth: What is the A record for mygov.bd?
    Auth-->>Res: IP address (e.g. 203.0.113.10)
    Res-->>Br: IP address (cached with TTL)
    Br->>Br: Open TCP + send HTTP GET to that IP

Steps in words:

1. The browser first checks its own and the OS's local DNS cache.
2. On a cache miss, it queries the ISP's recursive resolver.
3. The resolver asks a root server, which refers it to the .bd TLD server.
4. The TLD server refers it to the authoritative name server for mygov.bd.
5. The authoritative server returns the actual A record (IP address).
6. The resolver caches the result (for the record's TTL) and returns it to the browser.
7. The browser opens a TCP connection to that IP and sends an HTTP GET request to fetch the page.

Exam takeaway: DNS resolution is a hierarchical, recursive/iterative lookup (root → TLD → authoritative), and results are cached using TTL to avoid repeating the whole chain on every request.