Application Layer Practice Questions

In P2P architecture (e.g., BitTorrent), each peer acts as both client and server - it downloads from and uploads to other peers. As more peers join: demand increases (more downloaders) but supply also increases (more uploaders), causing the system to self-scale. In client-server architecture, adding more clients increases load on the server, requiring expensive server upgrades. P2P distribution time for a file of size F with N peers and server upload rate u_s: min(F/u_s, F/d_min, NF/(u_s + Σu_i)) - grows very slowly with N.

DNS (Domain Name System) is an application layer protocol that translates human-readable domain names (e.g., www.example.com) into IP addresses (e.g., 192.0.2.1) that routers can use to forward packets. DNS operates primarily over UDP on port 53 (TCP for large responses or zone transfers). It uses a distributed hierarchical database organized into zones, with root servers, TLD servers, and authoritative name servers.

Since the local resolver has a cached entry for example.com's authoritative name server, it can skip the root and TLD lookups. It directly queries example.com's authoritative name server for 'mail.example.com' - this is 1 additional query. The authoritative server returns the IP address of mail.example.com. Without any cache, 3 queries would be needed (root → TLD → authoritative). With the authoritative server cached, only 1 query to the authoritative server is needed.

In Active mode FTP, the server initiates the data connection back to the client. In Passive mode, the client initiates both the control and data connections to the server.

FTP uses two separate TCP connections: the control connection on port 21 persists throughout the session and carries commands (USER, PASS, LIST, RETR, STOR, etc.) and responses. The data connection on port 20 (in active mode) is opened separately for each file transfer or directory listing and closed afterward. In passive mode (PASV), the server opens a random high port for data and the client connects to it - useful when the client is behind a NAT or firewall. Port 21 for control and port 20 for data (active mode) are the standard defaults.

POP3 (Post Office Protocol 3, port 110): downloads email to the local client and by default deletes it from the server. It is simple and works well when email is accessed from a single device. IMAP (Internet Message Access Protocol, port 143): email remains on the server and the client synchronizes with it. IMAP supports folders, search, and partial message retrieval - ideal for accessing email from multiple devices (phone, laptop, web) since messages are always on the server. Both use TCP for reliable delivery.

Since HTTP is stateless (each request is independent), cookies are used to maintain session state. They are stored on the client and sent with subsequent requests to the same server.