Computer Networks (AD-602) Unit 5 Notes - RGPV

How unit 5 is examined

Application-layer protocols (HTTP, FTP, SSH, email, DNS, SNMP) and the basics of cryptography; email, modern ciphers, DNS and SNMP carry the marks.

WWW and HTTP

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. The WWW is a collection of linked hypertext documents served by web servers and viewed in browsers; HTTP (HyperText Transfer Protocol) is the application-layer protocol that carries them.

Key points.

HTTP follows the client-server request-response model: the browser sends a request and the server returns a response with a status code (200 OK, 404 Not Found).
It runs over TCP on port 80 (HTTPS uses port 443 with TLS).
Common methods are GET (fetch), POST (send data), PUT (store), DELETE (remove) and HEAD (headers only).
HTTP is stateless, so the server keeps no memory between requests; cookies are used to add state.
Example: GET /index.html HTTP/1.1 with Host: www.rgpv.ac.in returns HTTP/1.1 200 OK followed by the page.

Asked: [7 marks] (Jun 2025) Explain the following application layer protocols: i) HTTP ii) DNS

FTP

Definition. FTP (File Transfer Protocol) copies files between a client and a server over TCP.

Key points.

It uses two connections: a control connection on port 21 for commands and replies, and a data connection on port 20 for the file.
The user must log in with a username and password, and these travel unencrypted.
Because control and data are kept separate, commands can be sent while a transfer is running.
TFTP is the simpler UDP-based version without login.

SSH

Definition. SSH (Secure Shell) gives a secure remote login and command session over an insecure network, on TCP port 22.

Key points.

It replaces Telnet, which sent passwords in plain text, by encrypting the whole session.
The server is authenticated by its host key, and the user by password or public-key pair.
It provides confidentiality and integrity, and supports secure file copy (SCP, SFTP) and port forwarding.

Email (SMTP, MIME, IMAP)

Definition. Email is an application-layer service in which a user agent composes a message, mail servers relay it using SMTP, and the receiver fetches it using a mailbox-access protocol such as IMAP.

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Key points.

SMTP (Simple Mail Transfer Protocol) is a push protocol that moves mail from the sender's user agent to its server and between servers, over TCP port 25.
SMTP commands are text lines such as HELO, MAIL FROM, RCPT TO, DATA and QUIT, and each is answered by a numeric reply code (250 means OK).
SMTP transfers only 7-bit ASCII text, so it cannot carry images, audio or non-English text directly.
MIME (Multipurpose Internet Mail Extensions) extends the message format by adding headers such as MIME-Version, Content-Type and Content-Transfer-Encoding, so binary data is converted to ASCII (for example Base64) and sent by SMTP.
MIME allows attachments, multiple parts, and content types like text/html, image/jpeg and audio/mpeg.
IMAP (Internet Message Access Protocol, port 143) is a pull protocol that lets the receiver read mail stored on the server.
IMAP keeps mail on the server, supports folders, partial download and searching, and syncs the same mailbox across many devices; POP3 (port 110) instead downloads and usually deletes it.
Advantages are fast, cheap and asynchronous delivery; limitations are spam and lack of built-in security, so TLS is added.

Answer frame. Open with the definition of email and its three protocols; draw the flow diagram; then develop SMTP, MIME and IMAP in that order (points 1-7), naming port and function for each; close with one line: SMTP sends, MIME formats, IMAP retrieves. The 14-mark question needs any two of four topics, so answer this one plus one more.

Asked: [14 marks] (Jun 2025) Write a short note on (any two): a) SMTP, MIME and IMAP email protocols b) Bellman-ford algorithm c) IEEE Standards 802 series d) Types of transmission media (guided and unguided)

DNS

Definition. <mark>The Domain Name System is a hierarchical, distributed naming database that translates domain names into IP addresses and back.</mark>

Key points.

It lets users remember names such as www.rgpv.ac.in instead of numeric IP addresses, and uses UDP port 53 (TCP for large transfers).
The hierarchy is root, top-level domain (.in, .com), then authoritative servers for each domain.
Forward lookup: the resolver asks root, then TLD, then the authoritative server, which returns the IP; answers are cached.
Reverse lookup maps an IP to a name using PTR records under in-addr.arpa.
Record types: A (IPv4), AAAA (IPv6), MX (mail server), NS (name server), CNAME (alias), PTR.

Asked: [7 marks] (May 2024) What is the Domain Name System (DNS) and what role does it play in computer networking? Explain how DNS translates domain names to IP addresses and vice versa.

Network Management (SNMP)

Definition. SNMP (Simple Network Management Protocol) is an application-layer protocol that lets a manager monitor and control network devices such as routers and switches.

Key points.

Architecture has a manager (the management station), agents running on each device, and the MIB (Management Information Base), the database of variables the agent exposes.
It runs over UDP on ports 161 (agent) and 162 (traps).
PDUs: GetRequest reads a value, GetNextRequest reads the next, SetRequest changes a value, Response returns the result, and Trap is an unsolicited alarm from the agent.
Functions are monitoring (traffic, CPU), configuration (setting values), and fault management (traps on failures).

Asked: [7 marks] (May 2023) Explain Simple Network Management Protocol (SNMP) and its functions.

Introduction to security

Definition. Network security protects data and systems from unauthorised access, alteration and loss.

Key points.

The CIA triad: confidentiality (only authorised users read data), integrity (data is not altered) and availability (service is usable when needed).
Attacks are passive (eavesdropping, traffic analysis) or active (masquerade, replay, modification, denial of service).
Cryptography, firewalls and authentication are the main defences.

Traditional Ciphers

Definition. <mark>Traditional ciphers are character-oriented ciphers that hide plaintext by substituting or rearranging letters using a secret key.</mark>

Key points.

Substitution replaces letters: Caesar shifts each letter by a fixed key k, so with k = 3 HELLO becomes KHOOR.
Monoalphabetic cipher uses one arbitrary letter-to-letter mapping (26! keys) but is broken by letter-frequency analysis.
Polyalphabetic ciphers (Vigenere) use several mappings, hiding frequencies.
Transposition keeps letters but changes positions: rail fence with depth 2 writes MEETMENOW in two rows and reads M E M N W then E T E O, giving MEMNWETEO.
Columnar transposition writes the text in rows and reads columns in key order.

Asked: [7 marks] (May 2023) What are Traditional ciphers? What are the different techniques used in traditional cipher? Explain.

Modern Ciphers

Definition. Modern ciphers are bit-oriented ciphers that process binary data as block ciphers (DES, AES) or stream ciphers, and include public-key systems such as RSA.

Key points.

Block ciphers encrypt fixed blocks (DES 64-bit block with 56-bit key; AES 128-bit block with 128/192/256-bit key); stream ciphers encrypt bit by bit.
Symmetric encryption uses one shared secret key; asymmetric uses a public key to encrypt and a private key to decrypt.
Modern ciphers have huge key spaces (2^128 for AES) versus 26! for monoalphabetic, so brute force is impractical.
They give diffusion and confusion, so one plaintext change alters many ciphertext bits and frequency analysis fails.
They work on bits by computer, so they encrypt any data, and are fast.
Public-key ciphers solve key distribution and enable digital signatures.

Basis	Symmetric	Asymmetric
Keys	One shared secret	Public and private pair
Speed	Fast	Slower
Problem	Key distribution	Computation cost
Examples	DES, AES	RSA
Basis	Traditional	Modern
---	---	---
Unit	Characters	Bits
Key space	Small	Very large
Security	Broken by frequency analysis	Resists it

Answer frame. Define modern ciphers; give both tables; for Q3 stress advantages (points 3-5), for Q4 the symmetric-asymmetric table first; close: modern ciphers give strong, fast, key-managed security.

Asked: [7 marks] (May 2024) Define modern ciphers and explain their advantages over traditional ciphers. Asked: [7 marks] (Jun 2025) Explain the differences between symmetric and asymmetric encryption techniques. How do modern ciphers improve network security compared to traditional ciphers?

Message Integrity and Authentication

Definition. Message integrity ensures a message is unchanged in transit, and authentication proves who sent it.

Key points.

A hash function (MD5, SHA) turns a message into a fixed-size digest; any change in the message changes the digest.
A MAC (message authentication code) is a hash computed with a shared secret key, so it proves both integrity and sender.
A digital signature is a hash encrypted with the sender's private key, so anyone can verify it with the public key and the sender cannot deny it.

Last-minute revision

HTTP: port 80, stateless, GET/POST/PUT/DELETE; HTTPS port 443.
FTP: control port 21, data port 20.
SSH: port 22, secure replacement for Telnet.
SMTP port 25 (push), IMAP port 143 (pull), POP3 port 110; MIME allows non-ASCII content.
DNS: port 53, records A, AAAA, MX, NS, CNAME, PTR.
SNMP: manager, agent, MIB; ports 161/162; PDUs Get, Set, Response, Trap.
CIA triad: confidentiality, integrity, availability.
Caesar: HELLO with k = 3 gives KHOOR; monoalphabetic has 26! keys.
DES 64-bit block, 56-bit key; AES 128-bit block, key 128/192/256.
Symmetric one key and fast; asymmetric two keys and slower.
MAC uses a shared key; digital signature uses a private key.

Memory hooks

SMTP Sends, MIME Makes-ready, IMAP Inspects on server.
Ports: FTP 20/21, SSH 22, SMTP 25, DNS 53, HTTP 80, IMAP 143, SNMP 161.
SNMP = Manager asks, Agent answers, MIB stores.
Substitution swaps letters, transposition moves them.
CIA = Confidential, Intact, Available.

Coverage checklist

WWW and HTTP: Jun 2025 HTTP and DNS question.
FTP: definition and ports.
SSH: definition and port.
Email (SMTP, MIME, IMAP): Jun 2025 short note.
DNS: May 2024 DNS question and Jun 2025 part ii.
Network Management (SNMP): May 2023 SNMP question.
Introduction to security: CIA triad and attacks.
Traditional Ciphers: May 2023 question.
Modern Ciphers: May 2024 and Jun 2025 questions.
Message Integrity and Authentication: hash, MAC, signature.

Account

Computer Networks (AD-602) - Unit 5 Short Notes

How unit 5 is examined

WWW and HTTP

FTP

SSH

Email (SMTP, MIME, IMAP)

DNS

Network Management (SNMP)

Introduction to security

Traditional Ciphers

Modern Ciphers

Message Integrity and Authentication

Last-minute revision

Memory hooks

Coverage checklist