Computer Networking: Key Interview Questions and Learning Points

Understanding the TCP/IP Model and Common Protocols

The TCP/IP model, a foundational concept for internet communication, consists of four layers, each serving a distinct purpose in the transmission of data over the network. Let’s break down the protocols associated with each layer and explore their functionalities.

Application Layer Protocols

1. HTTP (Hypertext Transfer Protocol): The standard protocol used for transferring web pages and resources over the internet.
2. SMTP (Simple Mail Transfer Protocol): Used for sending emails between servers.
3. POP3/IMAP (Post Office Protocol/Internet Message Access Protocol): Protocols for retrieving emails from a server.
4. FTP (File Transfer Protocol): Used for transferring files between client and server.
5. Telnet: A protocol for remote command-line access.
6. SSH (Secure Shell Protocol): A secure version of Telnet, providing encrypted communication.
7. RTP (Real-Time Transport Protocol): Used for real-time communication, such as audio and video streaming.
8. DNS (Domain Name System): A system that translates domain names into IP addresses.

Transport Layer Protocols

1. TCP (Transmission Control Protocol):

   • Provides reliable data transmission by ensuring data packets are delivered in the correct order.
   • Features like flow control, congestion control, and error detection make TCP reliable.
   • TCP Segments: Data is segmented, numbered, and acknowledged, ensuring data integrity.

2. UDP (User Datagram Protocol):

   • Unlike TCP, UDP is connectionless, offering faster transmission at the cost of reliability.
   • RDT (Reliable Data Transfer) protocols can be built on top of UDP to handle reliability.

Network Layer Protocols

1. IP (Internet Protocol): The core protocol responsible for addressing and routing data packets across networks.
2. ARP (Address Resolution Protocol): Resolves IP addresses to MAC addresses in local networks.
3. ICMP (Internet Control Message Protocol): Used for sending control messages like error reporting (e.g., "ping" command).
4. NAT (Network Address Translation): Translates private IP addresses into public IP addresses and vice versa.
5. OSPF (Open Shortest Path First): A link-state routing protocol used in large enterprise networks.
6. RIP (Routing Information Protocol): A distance-vector routing protocol, often used in smaller networks.
7. BGP (Border Gateway Protocol): Used for routing data between different networks, particularly in large-scale networks like the internet.

Link Layer Protocols

1. Error Detection: Protocols ensure that data received is error-free, typically using checksums or CRCs.
2. Multiplexing: Technologies that enable multiple communications to share a single transmission medium.
3. CSMA/CD (Carrier Sense Multiple Access with Collision Detection): A protocol used in Ethernet networks to handle data collisions.
4. MAC (Media Access Control): Ensures that data is properly addressed and delivered on a physical network.
5. Ethernet: The most common local area network (LAN) technology.

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HTTP Protocols Breakdown

Request and Response Messages

• Request Message: Contains a request line (method, URL, HTTP version), headers (additional information like Host, User-Agent), and an optional body (data for methods like POST).

• Response Message: Contains a status line (HTTP version, status code, status message), headers, and an optional body (data like HTML or JSON content).

Common HTTP Status Codes:

• 2xx: Successful requests (e.g., 200 OK).
• 3xx: Redirection (e.g., 301 Moved Permanently).
• 4xx: Client error (e.g., 404 Not Found).
• 5xx: Server error (e.g., 500 Internal Server Error).

HTTP Methods:

• GET: Retrieves data from the server.
• POST: Sends data to the server.
• PUT: Updates existing data.
• DELETE: Removes data.
• HEAD: Fetches headers without the body.

GET vs POST:

• GET is used to fetch resources, typically data retrieval.
• POST is used to send data to the server, typically for creating or updating resources.

HTTP Connections:

• Short Connection: Each request/response cycle requires a new TCP connection.
• Long Connection: A persistent TCP connection is used to send multiple requests and responses, reducing overhead (enabled by Keep-Alive).

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HTTP/1.1 vs HTTP/2 vs HTTP/3

• HTTP/1.1:

  • Uses long connections and pipelining for performance improvement but faces head-of-line blocking.

• HTTP/2:

  • Header Compression (HPACK) reduces redundancy.
  • Binary Protocol improves parsing efficiency.
  • Multiplexing enables parallel data transfer to avoid blocking.

• HTTP/3:

  • Built on QUIC, uses UDP for faster connection establishment.
  • Eliminates head-of-line blocking and allows connection migration for seamless transitions between networks.

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HTTPS (Secure HTTP)

• Port: HTTP uses port 80, while HTTPS uses port 443.
• Encryption: HTTPS uses SSL/TLS to encrypt data, ensuring secure communication between client and server.
• Authentication: Requires a digital certificate from a trusted Certificate Authority (CA).

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TCP Overview

Why TCP Requires Three-Way Handshake:

• Prevent Duplicate Connections: Ensures old or duplicate connections don't interfere.
• Synchronizes Sequence Numbers: Both ends agree on the initial sequence numbers for reliable communication.
• Resource Efficiency: Prevents wastage of resources by establishing a connection only when necessary.

TCP’s Reliability:

• Sequence Numbers: Each byte of data has a unique sequence number to ensure data is in the correct order.
• Acknowledgments: The receiver sends back an acknowledgment to the sender confirming data receipt.
• Timeouts and Retransmissions: Ensures reliability by resending data if acknowledgment isn’t received.
• Flow Control: Prevents congestion by regulating the data rate based on receiver’s capacity.
• Congestion Control: Reduces transmission rate if network congestion is detected.

TCP vs UDP:

• TCP: Reliable, connection-oriented, guarantees delivery, in-order delivery, and error-checking.
• UDP: Faster, connectionless, no guarantees, better for real-time applications.

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DNS (Domain Name System)

DNS translates human-readable domain names into IP addresses. It’s a distributed system that enables clients to access resources using domain names, such as "www.example.com", instead of having to remember numeric IP addresses.

DNS Resolution Process:

1. Client queries local DNS for the IP address of a domain.
2. If not cached, the local DNS queries the root DNS server.
3. The root DNS directs to the TLD server (e.g., for .com domains).
4. The TLD server directs to the authoritative DNS server for the domain.
5. The authoritative DNS responds with the IP address.
6. The local DNS returns the IP to the client.

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Conclusion

TCP/IP protocols are integral to how the internet operates, with each layer providing distinct services and ensuring that data is transmitted reliably, securely, and efficiently. Understanding these protocols is key for building, maintaining, and troubleshooting network applications.

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