UNIT 5: COMPUTER NETWORKS - EXAM-FOCUSED SHORT NOTES

Based on RGPV past papers (2022-2025), prioritizing 🔴 High (7m) and 🟡 Medium (4-5m) mark questions.

I. FOUNDATIONS & REFERENCE MODELS

ISO-OSI Reference Model

• 7 Layers (Bottom-Up):

  1. Physical: Bits over medium (Repeater, Hub).

  2. Data Link: Frames, MAC addressing, error control (Bridge, Switch).

  3. Network: Packets, routing, logical addressing (Router).

  4. Transport: End-to-end, segmentation, flow/error control (TCP/UDP).

  5. Session: Dialog control, synchronization.

  6. Presentation: Translation, encryption, compression (SSL/TLS).

  7. Application: User interface, network services (HTTP, SMTP, DNS).

• Critique: Theoretical, complex, protocol mapping not strict, implementation late.

TCP/IP Protocol Suite (4 Layers)

1. Network Interface (Link): Combines OSI Physical & Data Link.

2. Internet: OSI Network layer (IP, ICMP, ARP).

3. Transport: OSI Transport layer (TCP, UDP).

4. Application: OSI Session, Presentation, Application combined.

• Key Difference: TCP/IP is protocol-centric and practical; OSI is model-centric and theoretical.

📌 Comparison Table: OSI vs TCP/IP

| Feature | OSI Model | TCP/IP Model |

| :--- | :--- | :--- |

| Layers | 7 | 4 |

| Approach | Theoretical, generic | Practical, protocol-based |

| Layer 1-2 | Separate Physical & Data Link | Combined Network Interface |

| Layer 3 | Network | Internet |

| Layer 4 | Transport | Transport |

| Layers 5-7 | Session, Presentation, Application | Combined Application |

| Protocols | Not defined (e.g., OSI protocols failed) | Well-defined (IP, TCP, UDP, HTTP) |

| Usage | Reference, teaching | Internet, real-world |

Connection-Oriented vs. Connectionless Services

II. PHYSICAL LAYER & TRANSMISSION MEDIA

Channel Capacity Theorems

1. Nyquist Formula (Noiseless Channel):

$$C = 2B \log_2 M \text{ (bps)}$$

*   $C$: Max bit rate, $B$: Bandwidth (Hz), $M$: Signal levels.

*   **Example:** $$\displaystyle B=3kHz $$, $$\displaystyle M=4 \Rightarrow C = 2 \times 3000 \times \log_2 4 = 12000 \text{ bps} $$.

2. Shannon's Theorem (Noisy Channel):

$$C = B \log_2 (1 + \text{SNR}) \text{ (bps)}$$

*   $C$: Channel capacity, $B$: Bandwidth, $\text{SNR}$: Signal-to-Noise Ratio (linear, not dB).

*   **Example:** $$\displaystyle B=4kHz $$, $$\displaystyle \text{SNR}=1000 \Rightarrow C \approx 4000 \times 9.97 \approx 39880 \text{ bps} $$.

> **📌 Key:** Nyquist gives max rate for *given M* in noiseless case; Shannon gives *absolute max* for noisy channel.

Transmission Media

Multiplexing

• FDM: Different frequencies for different signals (Radio, TV).

• TDM:

  • Synchronous TDM: Fixed time slots, even if no data (wasteful).

  • Statistical TDM: Dynamic slot allocation based on demand (efficient).

  • Frame Size (TDM): Sum of all source rates × slot duration.

III. DATA LINK LAYER

Error Detection & Correction

📌 CRC Computation Steps:

1. Append $r$ zeros to data (where $r$ = degree of $G(x)$).

2. Perform binary polynomial division (XOR).

3. Remainder (length $r$) is CRC checksum.

4. Transmit: Original data + CRC.

5. Receiver divides by same $G(x)$; remainder 0 = no error.

Sliding Window Protocols

📌 Piggybacking: Attaching ACK to data frame going in reverse direction (improves efficiency vs separate ACK frames).

Multiple Access Protocols (MAC)

Random Access (Contention-Based)

• CSMA Persistence:

  • 1-persistent: Sense idle → transmit immediately (high collision).

  • Non-persistent: Sense busy → wait random time (lower collision, delay).

  • p-persistent: Sense idle → transmit with prob $p$; defer with $1-p$.

• Binary Exponential Backoff (BEB): After $$\displaystyle k^{th} $$ collision, wait random $$\displaystyle [0, 2^k-1] $$ slot times. $k$ capped (e.g., 10 for Ethernet).

• CSMA/CD Min Frame Size: $$\displaystyle T_{trans} \geq 2 \times T_{prop} $$ to detect collision.

$$\text{Min Frame Size} = 2 \times \text{Propagation Distance} \times \text{Signal Speed} \times \text{Bandwidth}$$

Controlled Access

• Reservation: Reserve slots in future frame.

• Polling: Master polls slaves (primary-secondary).

• Token Passing: Token circulates; holder transmits (Token Ring, FDDI).

LAN Standards (IEEE 802)

📌 CSMA/CA (WLAN): Uses collision avoidance: DIFS/SIFS intervals, NAV (virtual carrier sense), RTS/CTS for hidden nodes.

Bit & Byte Stuffing

• Bit Stuffing (HDLC-like): Flag = 01111110. After 5 consecutive '1's, insert '0'. Receiver deletes after 5 '1's.

  • Example: Data 01111110 → Transmit 011111010 (stuff '0' after 5 '1's).

• Byte Stuffing (Character-oriented): Escape character (e.g., DLE). If data byte = flag or escape, insert escape before it.

IV. NETWORK LAYER

IP Addressing (IPv4)

• Classful (Legacy):

  | Class | First Bits | Network ID | Host ID | Default Mask | Range | | :--- | :--- | :--- | :--- | :--- | :--- | | A | 0 | 8 bits | 24 bits | 255.0.0.0 | 1.0.0.0 - 126.255.255.255 | | B | 10 | 16 bits | 16 bits | 255.255.0.0 | 128.0.0.0 - 191.255.255.255 | | C | 110 | 24 bits | 8 bits | 255.255.255.0 | 192.0.0.0 - 223.255.255.255 | | D | 1110 | - | - | - | Multicast (224-239) | | E | 1111 | - | - | - | Experimental (240-255) |

  • Limitations: Wasted addresses, no flexibility, routing table explosion.

• CIDR (Classless): a.b.c.d/n where $n$ = network prefix bits. Allows variable-length subnetting.

• Subnetting: Divide network into smaller subnets.

  • Steps: 1. Determine required subnets/hosts. 2. Borrow bits from host part. 3. New mask = original mask + borrowed bits. 4. Calculate block size = $$\displaystyle 2^{\text{host-bits}} $$. 5. Subnet addresses = multiples of block size. 6. Range: Subnet+1 to Subnet+block-2. Broadcast = Subnet+block-1.

  • Example: 192.168.10.0/24 → 4 subnets. Borrow 2 bits → /26. Block size = $$\displaystyle 2^{6}=64 $$. Subnets: 192.168.10.0/26, 64/26, 128/26, 192/26.

• Supernetting (Aggregation): Combine contiguous networks into larger one (reduce routing table size). e.g., 192.168.0.0/24 + 192.168.1.0/24 → 192.168.0.0/23.

IPv6

• Motivation: Address exhaustion, no built-in security, complex header.

• Header: Fixed 40 bytes. Fields: Version, Traffic Class, Flow Label, Payload Length, Next Header, Hop Limit, Source/Dest Address (128 bits).

• Features: 128-bit address (hex, colon-separated), no checksum, extension headers, stateless autoconfiguration, IPSec mandatory.

• IPv4 vs IPv6:

  | IPv4 | IPv6 | | :--- | :--- | | 32-bit address | 128-bit address | | Header: 20-60 bytes (variable) | Header: 40 bytes (fixed) | | Checksum in header | No header checksum | | Fragmentation by routers & hosts | Only by source host | | ARP required | Neighbor Discovery (ICMPv6) | | Manual/DHCP config | Stateless autoconfig |

Routing Algorithms

Static Routing

• Shortest Path (Dijkstra's):

  1. Label source node as permanent ($P$), others as tentative ($T$) with cost.

  2. Select $T$ node with smallest cost, make permanent.

  3. Update costs of neighbors of new permanent node.

  4. Repeat until destination permanent.

  • Limitations: Manual, slow to adapt to changes, single metric.

Dynamic Routing

• Distance Vector (DVR - Bellman-Ford):

  • Each node maintains distance vector (cost to each dest via each neighbor).

  • Periodic/triggered update: send own vector to neighbors.

  • Bellman-Ford equation: $$\displaystyle D_x(y) = \min_v \{ c(x,v) + D_v(y) \} $$

  • Problems: Count-to-infinity, routing loops, slow convergence.

  • Example: RIP uses hop count (max 15 hops).

• Link State (LSR):

  1. Discover neighbors: Hello packets.

  2. Measure cost: Typically delay.

  3. Flood LSPs: Each node sends its link state to all others (reliable flooding).

  4. Compute paths: Each node runs Dijkstra on complete graph.

  • Advantages: Fast convergence, no loops, global view.

  • Disadvantages: High overhead (flooding), memory intensive.

📌 DVR vs LSR:

| Aspect | DVR | LSR |

| :--- | :--- | :--- |

| Info Shared | Distance vectors (to all dests) | Link state (to neighbors only) |

| Algorithm | Bellman-Ford (iterative) | Dijkstra (on complete topology) |

| Convergence | Slow, count-to-infinity | Fast |

| Overhead | Periodic full updates | Flooding on change (event-driven) |

| Scalability | Poor (RIP) | Better (OSPF) |

| Loop Prevention | Hard (split horizon, poison reverse) | Inherent (no loops in shortest path) |

Network Layer Protocols & Devices

• ARP (Address Resolution Protocol): Maps IP → MAC. Broadcast: "Who has IP X? Tell Y (MAC)." Unicast reply.

• RARP (Reverse ARP): Maps MAC → IP (diskless boot). Obsolete (replaced by BOOTP/DHCP).

• ICMP (Internet Control Message Protocol): Error & diagnostic messages.

  • Types: 0 (Echo Reply), 3 (Destination Unreachable), 8 (Echo Request), 11 (Time Exceeded).

  • Tools: ping (Echo Request/Reply), traceroute (Time Exceeded with TTL expiry).

• Virtual Circuits vs Datagram Subnets:

  | Virtual Circuit (VC) | Datagram | | :--- | :--- | | Connection setup before data. | No setup; each packet independent. | | Path fixed; all packets follow same route. | Each packet routed independently. | | Packets ordered; sequencing optional. | Packets may arrive out-of-order. | | Router state per VC (table). | Stateless routers (only dest address). | | Example: Frame Relay, ATM. | Example: IP (Internet). |

Congestion Control (Network Layer)

• Causes: Slow processors, low bandwidth, bursty traffic, poor routing.

• Open-loop: Prevent congestion (policing, shaping, admission control).

• Closed-loop: Detect & alleviate (backpressure, choke packets, load shedding).

• Traffic Shaping:

  • Leaky Bucket: Constant output rate, burst input → smooths burst.

  • Token Bucket: Tokens accumulate (rate $r$, capacity $b$). Packet needs token to send → allows burst up to $b$.

  • Comparison: Leaky Bucket = rigid output; Token Bucket = bursty output allowed.

V. DATA LINK LAYER DEVICES & INTERCONNECTION

📌 Bridge vs Switch: Switch = hardware-based, multi-port bridge with higher performance and more ports.

VI. TRANSPORT LAYER

Transport Layer Protocols

• UDP (User Datagram Protocol):

  • Features: Connectionless, unreliable, no flow/error control, minimal overhead.

  • Header (8 bytes): Source Port (16), Dest Port (16), Length (16), Checksum (16).

  • Applications: DNS, VoIP, streaming, DHCP.

• TCP (Transmission Control Protocol):

  • Features: Connection-oriented, reliable, flow/error control, congestion control, full-duplex.

  • Header (20-60 bytes):

    • Source/Dest Port (16 each)

    • Seq # (32), ACK # (32)

    • Flags (6 bits): URG, ACK, PSH, RST, SYN, FIN

    • Window Size (16): Receiver's buffer space.

    • Checksum (16), Urgent Pointer (16)

    • Options (variable)

TCP Connection Management

• Establishment (Three-Way Handshake):

  1. SYN: Client → Server (SYN=1, Seq=x).

  2. SYN-ACK: Server → Client (SYN=1, ACK=1, Seq=y, Ack=x+1).

  3. ACK: Client → Server (ACK=1, Seq=x+1, Ack=y+1).

  • States: CLOSED → SYN_SENT → SYN_RCVD → ESTABLISHED.

• Termination (Four-Way Handshake - Graceful):

  1. FIN: A → B (FIN=1, Seq=u).

  2. ACK: B → A (ACK=1, Ack=u+1).

  3. FIN: B → A (FIN=1, Seq=v, Ack=u+1).

  4. ACK: A → B (ACK=1, Ack=v+1).

  • TIME_WAIT: A waits 2×MSL (Max Segment Lifetime) after last ACK to ensure B received ACK and to handle delayed duplicates.

  📌 Why Graceful Termination? Ensures all data delivered, resources released, prevents old duplicate segments from interfering.

TCP Congestion Control

• Goal: Regulate sender's rate based on network congestion.

• Variables: cwnd (congestion window, sender limit), ssthresh (slow start threshold).

• Phases:

  1. Slow Start: cwnd starts at 1 MSS. Doubles each RTT (exponential) until cwnd >= ssthresh or loss.

  2. Congestion Avoidance: cwnd increases by 1 MSS per RTT (linear - AIMD).

  3. Fast Retransmit: On 3 duplicate ACKs → retransmit missing segment without waiting for timeout.

  4. Fast Recovery: ssthresh = cwnd / 2, cwnd = ssthresh + 3, then linear increase on dup ACKs.

• On Timeout: ssthresh = cwnd / 2, cwnd = 1, restart slow start.

• AIMD (Additive Increase Multiplicative Decrease): Increase linearly, cut window in half on loss → fair, stable.

VII. APPLICATION LAYER

Domain Name System (DNS)

• Need: Hierarchical, distributed database translating domain names ↔ IP addresses.

• Hierarchy: Root → TLD (.com, .org, country-code) → Authoritative (e.g., rgpv.ac.in).

• Resolution:

  • Recursive Query: Resolver asks server; server returns final answer (or error).

  • Iterative Query: Server returns next server to ask (or answer if known).

• Caching: Resolvers/servers cache responses with TTL.

• Resource Records (RR):

  • A: IPv4 address.

  • AAAA: IPv6 address.

  • MX: Mail exchange server.

  • CNAME: Canonical name (alias).

  • PTR: Pointer (reverse lookup).

Electronic Mail (Email)

• Architecture:

  • MUA (Mail User Agent): User interface (Outlook, Thunderbird).

  • MTA (Mail Transfer Agent): Server-to-server transfer (SMTP).

  • MDA (Mail Delivery Agent): Local delivery to mailbox.

• SMTP (Simple Mail Transfer Protocol):

  • Push protocol (MUA → MTA, MTA → MTA).

  • Commands: HELO, MAIL FROM, RCPT TO, DATA, QUIT.

  • Port: 25 (plain), 587 (submission).

  • Limitation: 7-bit ASCII only → MIME (Multipurpose Internet Mail Extensions) for binary/text encoding.

• Mail Retrieval (Pull):

  • POP3 (Post Office Protocol v3): Downloads & deletes from server (simple).

  • IMAP (Internet Message Access Protocol): Keeps mail on server; syncs folders (modern).

  • Ports: POP3: 110, IMAP: 143.

World Wide Web (WWW) & HTTP

• Architecture: Client (Browser) ↔ Server (HTTP server).

• HTTP (Hypertext Transfer Protocol):

  • Stateless: Server does not remember clients (solves with cookies/sessions).

  • Request/Response: ASCII text.

  • Methods: GET (fetch), POST (submit data), HEAD, PUT, DELETE.

  • Headers: Host, User-Agent, Content-Type, Cookie.

  • Persistent vs Non-Persistent:

    • Non-persistent (HTTP/1.0): One TCP connection per object → high overhead.

    • Persistent (HTTP/1.1): One TCP connection for multiple objects (pipelining).

• HTTPS: HTTP over TLS/SSL (port 443). Provides encryption, server authentication.

Simple Network Management Protocol (SNMP)

• Architecture:

  • Manager: Central console (NMS).

  • Agent: Software on managed device.

  • MIB (Management Information Base): Database of managed objects (variables).

• Operations: GET, GETNEXT, SET, TRAP (asynchronous alert from agent).

• Versions: SNMPv1 (basic), SNMPv2c (bulk get, 64-bit counters), SNMPv3 (security).

Other Application Layer Protocols (Short Notes)

• PPP (Point-to-Point Protocol):

  • Features: Byte-oriented, error detection (FCS), authentication (PAP/CHAP), multilink, compression.

  • Phases: Link establishment (LCP), Authentication (optional), Network layer protocol (NCP).

  • Frame Format: Flag (7E) | Address (FF) | Control (03) | Protocol | Data | FCS | Flag.

• HDLC (High-Level Data Link Control):

  • Bit-oriented, synchronous.

  • Frame: Flag (7E) | Address | Control | Data | FCS | Flag.

  • Control Field: Defines frame type (I-frame, S-frame, U-frame).

  • Modes: NRM (primary-secondary), ARM, ABM (balanced).

• VLANs (Virtual LANs):

  • Concept: Logical segmentation of LAN into broadcast domains.

  • Benefits: Security, broadcast control, flexibility.

  • Tagging (IEEE 802.1Q): 4-byte VLAN tag inserted in Ethernet frame (TPID, TCI with VLAN ID, priority).

VIII. CROSS-LAYER & MISCELLANEOUS TOPICS

Switching Techniques

Network Topologies

📌 Star vs Mesh: Star is cost-effective for LANs; Mesh is for backbone/wan due to high reliability.

Cryptography (Basics)

• Symmetric: Same key for encryption/decryption (AES, DES). Fast, key distribution problem.

• Asymmetric: Public/private key pair (RSA). Slower, solves key distribution.

• Encryption: Plaintext + Key → Ciphertext.

• Decryption: Ciphertext + Key → Plaintext.

• Digital Signature: Hash of message encrypted with sender's private key (provides authentication, non-repudiation).

MLMA (Multiple Access with Collision Avoidance)

• Mentioned in May 2023 paper. Likely refers to protocols like CSMA/CA (used in 802.11 WLAN) where stations avoid collisions via techniques like RTS/CTS, NAV, DIFS/SIFS timing, rather than detecting them (CD not possible in wireless).

📝 Final Exam Tips:

1. Diagrams: Always draw OSI/TCP-IP models, TCP header, IPv4 header, Ethernet frame, Token Ring frame for 7m questions.

2. Calculations: Practice CRC polynomial division, subnetting, Nyquist/Shannon, CSMA/CD min frame size, ALOHA throughput.

3. Algorithms: Be ready to step through Dijkstra and Bellman-Ford with a small graph.

4. Comparisons: Use tables for OSI/TCP-IP, DVR/LSR, IPv4/IPv6, TCP/UDP, Guided/Unguided, Circuit/Packet.

5. Protocol Details: Know TCP states, DNS resolution steps, SMTP commands, PPP phases, HDLC frame types.

6. Common Pitfalls:

   • CRC: Remember to multiply data by $$\displaystyle x^r $$ before division.

   • Subnetting: First address = network ID, last = broadcast, usable = in between.

   • CSMA/CD: Min frame size ensures collision detection during transmission.

   • TCP TIME_WAIT: Prevents old duplicates, ensures ACK delivery.

   • DVR: Count-to-infinity problem; use split horizon/poison reverse to mitigate.

\boxed{\text{Revise past paper questions from 2022-2025 for pattern and repeated topics.}}