Computer Networks Interview Questions and Answers

TCP and UDP are transport-layer protocols with opposite trade-offs.

TCP is connection-oriented. It sets up a connection with a three-way handshake, numbers every segment, acknowledges delivery, retransmits lost data, orders bytes and applies flow and congestion control. It is used when correctness matters: HTTP, SMTP, SSH and database connections.

UDP is connectionless. It sends independent datagrams with no handshake, acknowledgements or ordering, so delivery is best-effort. It is used when low latency matters more than reliability: DNS, DHCP, VoIP, video streaming and online games. QUIC adds reliability and encryption on top of UDP in user space.

TCP: SYN, SYN-ACK, ACK -> ordered, reliable byte stream
UDP: datagram -> fire and forget

TCP's guarantees cost latency and header overhead; UDP is cheaper but application code must handle loss, duplication and reordering when required.

The browser parses the URL, then finds the server's IP: it checks its cache, the OS cache, /etc/hosts, then asks a resolver, which recurses from the root servers through the top-level domain to the authoritative name server. Meanwhile a TCP connection is opened to port 443 with the three-way handshake.

For HTTPS, a TLS handshake follows: the client sends supported ciphers and a random value, the server returns its certificate, they verify it against trusted CAs, agree on keys via ECDHE and switch to encrypted records. Modern TLS 1.3 does this in one round trip.

The browser then sends an HTTP request with method, path, headers and cookies. The server may hit a load balancer, a cache and application code, and returns a status and body. The browser parses HTML, builds the DOM, fetches subresources, builds the CSSOM, runs layout and paint, and executes JavaScript. All of these stages are visible in browser developer tools.

The OSI model is a seven-layer reference model: Physical, Data Link, Network, Transport, Session, Presentation and Application. The TCP/IP model as taught is four layers: Link, Internet, Transport and Application.

OSI                     TCP/IP
7 Application  }        Application (HTTP, DNS, TLS)
6 Presentation }   ->
5 Session      }        Transport (TCP, UDP)
4 Transport             Internet (IP, ICMP)
3 Network               Link (Ethernet, Wi-Fi, ARP)
2 Data Link
1 Physical

OSI is a conceptual framework that names what each layer should do and helps when reasoning about problems and protocols. TCP/IP is the model actually implemented by the Internet, and it merges the upper layers and leaves some OSI concerns, such as encryption, to the application or to TLS. In troubleshooting people still speak in OSI layers: a layer 2 problem means a switching or MAC issue, while layer 7 means the application payload.