Bangladesh Water Development Board Assistant Programmer Written Test: 2018প্রশ্ন ও সমাধান

Definition: the OSI (Open Systems Interconnection) model is a 7-layer conceptual framework standardizing how data moves from one application, across a network, to another — each layer provides services to the one above it and uses services from the one below.

graph TD
  L7[7. Application] --> L6[6. Presentation]
  L6 --> L5[5. Session]
  L5 --> L4[4. Transport]
  L4 --> L3[3. Network]
  L3 --> L2[2. Data Link]
  L2 --> L1[1. Physical]

Layer	Responsibility
Application	User-facing services (HTTP, FTP, email) — user interaction
Presentation	Data translation, encryption, compression
Session	Establishes/manages/terminates dialogue sessions
Transport	End-to-end reliable delivery, segmentation (TCP/UDP) — key for data transfer
Network	Logical addressing and routing (IP)
Data Link	Node-to-node framing, MAC addressing, error detection — key for data transfer on a link
Physical	Raw bit transmission over the medium (cables, signals)

Most important for the two named tasks: the Application layer is where the user directly interacts with the network (browsers, email clients); the Data Link (and Transport) layers are most responsible for the actual mechanics of moving data reliably between connected nodes/hosts.

Exam takeaway: remember the layers bottom-up as "Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application).

Aspect	DNS Server	DNS Cache
What it is	A dedicated server holding an authoritative/recursive database mapping hostnames to IP addresses	A temporary local store (in the OS, browser or resolver) of recently resolved hostname→IP results
Location	Runs on ISP/organization infrastructure, globally distributed	Lives on the end-user's own machine/router/browser
Lifetime	Persistent, continuously maintained	Temporary — entries expire after a Time-To-Live (TTL)
Role	Resolves a name for the first time by querying the DNS hierarchy	Answers repeat lookups instantly without contacting any server

sequenceDiagram
  participant U as User Browser
  participant C as DNS Cache
  participant S as DNS Server
  U->>C: Look up www.example.com
  alt Cache hit
    C-->>U: Return cached IP instantly
  else Cache miss
    C->>S: Query DNS Server
    S-->>C: Return IP, cache it with TTL
    C-->>U: Return IP
  end

Importance of DNS cache in the WWW: it avoids repeating a slow, multi-hop DNS-server query for every single request, dramatically speeding up page loads and reducing load on the global DNS infrastructure — without it, every website visit would need a fresh round trip through the DNS hierarchy.

Exam takeaway: the DNS server is the authoritative source of truth; the DNS cache is a speed optimization that stores its recent answers locally until the TTL expires.