Computer Networks (CY-403) - Unit 3 Short Notes

UNIT 3: COMPUTER NETWORKS

I. INTRODUCTION TO COMPUTER NETWORKS

Definition: A computer network is an interconnected collection of autonomous computers that communicate and share resources.

Key Components:

• Hosts/End Systems: Sources/destinations of data (computers, servers).

• Routers: Forward packets between networks (Network layer).

• Links/Communication Channels: Guided (cables) or unguided (wireless) media.

• Protocols: Rules governing data exchange (e.g., TCP, IP, HTTP).

Network Types (by scale):

Network Criteria:

• Performance: Throughput, delay, jitter, bandwidth.

• Reliability: Delivery accuracy, fault tolerance (redundant paths).

• Security: Confidentiality, integrity, access control.

[!TIP] Exam Focus: Be ready to differentiate network types by geographical scope and typical technologies. Performance metrics often appear in numerical problems (delay, throughput).

II. REFERENCE MODELS

A. OSI REFERENCE MODEL (7 LAYERS)

Peer-to-Peer Communication: Each layer at source communicates with its peer at destination using protocol data units (PDUs).

Layer Functions:

1. Physical: Bits over medium (voltage, timing). Devices: repeaters, hubs.

2. Data Link: Frames, error control (CRC), flow control. MAC addresses. Devices: switches, bridges.

3. Network: Packets, routing, logical addressing (IP). Device: router.

4. Transport: Segments, end-to-end reliability, flow control. Ports. Protocols: TCP, UDP.

5. Session: Dialog control (synchronization), checkpointing.

6. Presentation: Syntax/semantics (encryption, compression, MIME).

7. Application: User services (HTTP, FTP, SMTP).

Service Primitives (between adjacent layers):

• REQUEST (upper → lower)

• INDICATION (lower → upper, event notification)

• RESPONSE (upper → lower, after indication)

• CONFIRM (lower → upper, after request)

Critique/Limitations:

• Complexity: Too many layers, some redundant.

• Implementation: Protocol stack overhead; some layers (Session, Presentation) merged in practice.

• Timing: Designed before Internet boom; not optimized for modern needs.

OSI vs TCP/IP Comparison:

[!TIP] Exam Focus: Know all 7 OSI layers in order (mnemonic: "All People Seem To Need Data Processing"). Contrast OSI's theoretical separation with TCP/IP's pragmatic merging. TCP/IP's Internet layer = OSI's Network layer.

B. TCP/IP REFERENCE MODEL

Layers & Protocols:

1. Link Layer (Network Interface): Ethernet (802.3), Wi-Fi (802.11), PPP, ARP.

2. Internet Layer: IP (IPv4/IPv6), ICMP, IGMP. Core: packet routing.

3. Transport Layer: TCP (reliable), UDP (unreliable).

4. Application Layer: HTTP, FTP, SMTP, DNS, SNMP (merges OSI's Session/Presentation/Application).

Advantages:

• Practical: Protocols are defined alongside model.

• Scalable: Connectionless IP simplifies routing.

• Interoperable: Open standards (RFCs) ensure multi-vendor compatibility.

• Robust: Best-effort delivery; reliability added at endpoints (TCP).

Disadvantages:

• Model-Layer Mismatch: Doesn't clearly separate concerns (e.g., no clear Session layer).

• Security: Not designed with security in mind (add-ons like TLS/SSL).

• Complexity: Many protocols at Application layer; no strict layering enforcement.

Standardization Process:

• RFCs (Request for Comments): Primary documents (e.g., RFC 791 for IPv4).

• IETF (Internet Engineering Task Force): Develops and promotes Internet standards.

• IESG (Internet Engineering Steering Group): Approves RFCs as standards.

• STD series: Track of approved standards (e.g., STD 5 for TCP).

[!TIP] Exam Focus: Memorize the 4 TCP/IP layers and key protocols per layer. Understand why TCP/IP succeeded over OSI (pragmatism, timing, open standards). RFCs/IETF role is often asked.

III. PHYSICAL LAYER

A. TRANSMISSION MEDIA

Guided Media (signals confined to a physical path):

Unguided Media (wireless propagation):

• Radio Waves: Omnidirectional, penetrate walls (Wi-Fi, Bluetooth, cellular).

• Microwaves: Directional, line-of-sight, high BW (satellite, point-to-point links).

• Infrared: Short range, line-of-sight, secure (remote controls, IrDA).

• Satellite: Geostationary (36,000 km, ~0.5s delay) vs LEO (low delay).

Broadband Technologies:

• DSL (Digital Subscriber Line): Uses existing phone lines; ADSL (asymmetric), VDSL (higher speed, shorter distance).

• Cable Modem: Shared coaxial cable; DOCSIS standards; contention-based.

• FTTH (Fiber to the Home): Direct fiber; highest speed (GPON, EPON).

B. DATA RATE AND CAPACITY

Nyquist Formula (noiseless channel):

$$ \text{Max bit rate} = 2B \log_2 M \text{ bps} $$

• $B$ = bandwidth (Hz)

• $M$ = number of signal levels (discrete)

Example: $$\displaystyle B=3 $$ kHz, $$\displaystyle M=4 $$ (2 bits/signal) → $$\displaystyle 2 \times 3000 \times \log_2 4 = 12,000 $$ bps.

Shannon's Theorem (noisy channel):

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

• $C$ = channel capacity (max error-free data rate)

• $B$ = bandwidth (Hz)

• $\text{SNR}$ = signal-to-noise ratio (linear, not dB)

Note: SNR in dB → linear: $$\displaystyle \text{SNR}_{\text{linear}} = 10^{\text{SNR}_{\text{dB}}/10} $$

[!TIP] Exam Focus: Nyquist assumes no noise; Shannon gives theoretical max for noisy channels. Know how to convert SNR from dB. Questions often ask: "If signal levels double, how does Nyquist rate change?" (Answer: increases by $$\displaystyle \log_2 2 = 1 $$ bit/signal).

C. MULTIPLEXING

Frequency Division Multiplexing (FDM):

• Each signal modulated to different frequency bands.

• Guard bands prevent interference.

• Used in radio/TV broadcasting, DSL (separate voice/data bands).

Time Division Multiplexing (TDM):

• Synchronous TDM: Fixed time slots; if a source has no data, slot is empty (wasted).

• Statistical TDM: Slots allocated dynamically; more efficient.

• Frame Size Calculation:

  • For $n$ sources, each with $k$ bits per slot, plus overhead $h$ bits/frame:

$$ \text{Frame size} = n \times k + h \text{ bits} $$

• Data rate = frame size / frame time.

Example: 3 sources (100, 200, 150 bits/slot), 1 framing bit → frame size = $$\displaystyle 3 \times 200 + 1 = 601 $$ bits (max slot size = 200). If frame time = 125 µs → data rate = $$\displaystyle 601 / 125 \times 10^{-6} = 4.808 $$ Mbps.

D. LINE CODING

Manchester Encoding:

• Transition in middle of bit period: low→high for '1', high→low for '0' (or vice versa, convention varies).

• Self-clocking (synchronization built-in).

• Example: 010 → ↓↑↑↓ (if 1=low→high, 0=high→low).

Differential Manchester:

• Transition at start of bit period always present.

• Meaning: Presence/absence of mid-bit transition encodes bit.

  • '0': transition at start and middle.

  • '1': transition only at start.

• More immune to noise (inversion doesn't change meaning).

[!TIP] Exam Focus: Draw waveforms for given binary strings. Manchester has a transition every half-bit; Differential Manchester has a transition at every bit boundary plus mid-bit for '0'.

IV. DATA LINK LAYER

A. FUNCTIONS AND SERVICES

• Framing: Delineate frame boundaries (bit/byte stuffing).

• Physical Addressing: MAC addresses.

• Error Control: Detection (CRC), correction (Hamming, FEC).

• Flow Control: Prevent fast sender overwhelming slow receiver.

• Link Management: Establish, maintain, release links.

• Access Control: Multiple access on shared media (MAC protocols).

B. ERROR DETECTION AND CORRECTION

Parity Check:

• Vertical: Parity bit per data unit (byte/word).

• Horizontal: Parity across multiple units (detects burst errors).

• Limitation: Only detects odd number of bit errors; cannot correct.

CRC (Cyclic Redundancy Check):

• Generator Polynomial $C(x)$: Divisor (e.g., $10011$ for CRC-4).

• Codeword Generation:

  1. Append $r$ zeros to data $D(x)$ ($$\displaystyle r = \deg C(x) $$).

  2. Divide $$\displaystyle D(x) \cdot x^r $$ by $C(x)$ (modulo-2 division).

  3. Remainder $R(x)$ is CRC checksum.

  4. Transmit $$\displaystyle D(x) \cdot x^r + R(x) $$.

• Error Detection: Receiver divides by same $C(x)$; non-zero remainder → error.

Example: Data 1101011011, $$\displaystyle C(x)=10011 $$ (CRC-4).

• Append 4 zeros: 11010110110000.

• Divide by 10011 (XOR division).

• Remainder = 1100 → transmitted codeword = 11010110111100.

Hamming Code (single-bit error correction):

• $k$ data bits, $r$ parity bits where $$\displaystyle 2^r \ge k + r + 1 $$.

• Parity bits at positions $$\displaystyle 2^i $$ (1,2,4,8,...).

• Each parity bit covers specific data bits (based on binary index).

• Receiver: syndrome calculation → error position.

Hybrid ARQ:

• Combines ARQ (retransmission on error) + FEC (forward error correction).

• Types:

  • Type I: Send FEC code; correct errors without retransmission if possible.

  • Type II: Incremental redundancy; send parity bits only if NACK received.

C. DATA LINK PROTOCOLS

HDLC (High-Level Data Link Control):

• Frame Format:

  
  Flag (01111110) | Address | Control | Info | FCS | Flag
  
  

• Operational Modes:

  • NRM (Normal Response Mode): Primary (master) initiates, secondary responds.

  • ABM (Asynchronous Balanced Mode): Both stations equal (peer-to-peer); used in PPP.

  • ARM (Asynchronous Response Mode): Secondary can transmit without permission.

PPP (Point-to-Point Protocol):

• Phases: Link establishment (LCP), authentication (PAP/CHAP), network layer (NCP), termination.

• Frame Format:

  
  Flag (7E) | Address (FF) | Control (03) | Protocol | Payload | FCS | Flag
  
  

  • Protocol field: 0x0021 (IP), 0xC021 (LCP), etc.

• Comparison with SLIP:

  | Feature | SLIP | PPP | |---------|------|-----| | Multi-protocol | No (only IP) | Yes (IP, IPX, AppleTalk) | | Error detection | No | Yes (FCS) | | Authentication | No | Yes (PAP/CHAP) | | Dynamic IP assignment | No | Yes (via IPCP) | | Link configuration | No | Yes (LCP) |

SLIP Limitations: No error detection, no multi-protocol, no dynamic IP, no compression.

Frame Relay (brief):

• Architecture: Virtual circuits (PVCs/SVCs), no error correction (only error detection), high-speed packet switching.

• Functions: Frame delimiting, congestion notification (FECN/BECN bits), minimal processing.

D. FRAMING AND STUFFING

Bit Stuffing (HDLC-like):

• Flag = 01111110.

• Sender: Insert '0' after five consecutive '1's in data to avoid flag mimicry.

• Receiver: Remove '0' after five '1's.

Example: Data 01111110 → stuffed 011111010 (insert 0 after five 1s).

Byte Stuffing (character-oriented, e.g., PPP in byte mode):

• Escape character (e.g., 0x7D) used.

• If data byte = flag (0x7E) or escape (0x7D), send 0x7D followed by XOR'd byte.

Example: Data 0x7E → transmit 0x7D 0x5E (since 0x7E XOR 0x20 = 0x5E).

E. FLOW AND ERROR CONTROL

Stop-and-Wait Protocol:

• Sender sends 1 frame, waits for ACK before next.

• Efficiency $$\displaystyle \eta = \frac{1}{1 + 2a} $$ where $$\displaystyle a = \frac{\text{propagation delay}}{\text{transmission time}} $$.

• Buffer Management: Sender needs 1 buffer; receiver needs 1 buffer (or discards duplicate ACKs).

• Limitation: Low utilization for long propagation delays (high $a$).

Sliding Window Protocol:

• Go-Back-N (GBN):

  • Sender window size $$\displaystyle W_s $$, receiver window $$\displaystyle W_r = 1 $$.

  • Cumulative ACKs: ACK for $N$ means all frames $\le N$ received correctly.

  • On timeout or duplicate ACK, retransmit all unacknowledged frames from $N+1$ onward.

  • Utilization $$\displaystyle U = \frac{W_s}{1+2a} $$ (if $$\displaystyle W_s \le 2a+1 $$); else $U \approx 1$.

• Selective Repeat (SR):

  • Both windows size $$\displaystyle W_s = W_r \le 2^{k-1} $$ ($k$ = seq bits).

  • Individual ACKs; retransmit only missing frames.

  • Receiver buffers out-of-order frames.

  • More efficient than GBN but complex (requires buffering).

Piggybacking:

• Attaching ACK to data frames going in reverse direction (instead of separate ACK frames).

• Improves efficiency in bidirectional communication (e.g., in full-duplex links).

F. MULTIPLE ACCESS PROTOCOLS (MAC)

Static vs Dynamic Channel Allocation:

• Static: Fixed assignment (FDMA, TDMA). Wastes bandwidth if inactive.

• Dynamic: On-demand (ALOHA, CSMA). Better for bursty traffic.

Random Access Protocols:

• Pure ALOHA:

  • Transmit anytime; collisions possible.

  • Vulnerable period = $$\displaystyle 2 \times T_{\text{frame}} $$ (any overlap).

  • Throughput $$\displaystyle S = G e^{-2G} $$ ($G$ = offered load).

  • Max throughput = $1/(2e) \approx 0.184$ at $$\displaystyle G=0.5 $$.

• Slotted ALOHA:

  • Time slots = frame time; transmit only at slot start.

  • Vulnerable period = $$\displaystyle T_{\text{frame}} $$.

  • Throughput $$\displaystyle S = G e^{-G} $$.

  • Max throughput = $1/e \approx 0.368$ at $$\displaystyle G=1 $$.

Carrier Sense Multiple Access (CSMA):

• Sense channel before transmitting.

• 1-persistent: If idle, transmit immediately; if busy, wait until idle then transmit (high collision probability).

• Non-persistent: If busy, wait random time, then sense again (reduces collisions, increases delay).

• p-persistent (for slotted): If idle, transmit with probability $p$, else defer to next slot.

• Binary Exponential Backoff (BEB) (Ethernet):

  • After $i$-th collision, wait random $$\displaystyle k \times T_{\text{slot}} $$, $$\displaystyle k \in [0, 2^i-1] $$.

  • Example: After 3 collisions, $k \in [0,7]$.

CSMA/CD (Collision Detection):

• Used in wired Ethernet (bus/star).

• Jam signal on collision; abort transmission.

• Minimum Frame Size to ensure collision detection:

$$ \text{Min frame size} \ge 2 \times \text{propagation delay} \times \text{bandwidth} $$

• Must transmit for $2 \times \text{round-trip propagation time}$ to detect collision.

Example: 2 km cable, signal speed $$\displaystyle 2 \times 10^8 $$ m/s → propagation delay = $$\displaystyle 2 \times 10^3 / 2 \times 10^8 = 10 \mu s $$. Round-trip = $20 \mu s$. Bandwidth 10 Mbps → min frame = $$\displaystyle 20 \times 10^{-6} \times 10 \times 10^6 = 200 $$ bits = 25 bytes.

CSMA/CA (Collision Avoidance):

• Used in wireless (Wi-Fi 802.11).

• RTS/CTS (optional): Request-to-send / Clear-to-send handshake to avoid hidden terminal problem.

• DIFS/SIFS: Inter-frame spaces; higher priority for short frames (ACK).

G. LAN STANDARDS (IEEE 802)

MAC Addressing (48-bit, IEEE 802):

• Format: AA:BB:CC:DD:EE:FF (hex).

  • OUI (first 3 bytes): Assigned to manufacturer.

  • NIC specific (last 3 bytes): Manufacturer-assigned.

• Types:

  • Unicast: Specific station (LSB of first byte = 0).

  • Multicast: Group of stations (LSB = 1, e.g., 01:00:5E for IPv4 multicast).

  • Broadcast: FF:FF:FF:FF:FF:FF (all stations on LAN).

[!TIP] Exam Focus: Compare Ethernet vs Token Ring vs Wi-Fi access methods. Know CSMA/CD minimum frame calculation. MAC address types: unicast (individual), multicast (group), broadcast (all). OUI concept.

V. NETWORK LAYER

A. FUNCTIONS AND DESIGN ISSUES

• Routing: Path selection (source, intermediate routers).

• Forwarding: Move packet from input to output link (per-hop).

• Logical Addressing: IP addresses (hierarchical).

• Fragmentation/Reassembly: MTU differences.

• Congestion Control: Avoid gridlock (cooperation with Transport layer).

• Inter-networking: Connect heterogeneous networks (via routers).

B. ROUTING ALGORITHMS

Distance Vector Routing (DVR):

• Bellman-Ford Equation:

$$ D_x(y) = \min_{v \in \text{neighbors}} \left\{ c(x,v) + D_v(y) \right\} $$

• $$\displaystyle D_x(y) $$ = cost from $x$ to $y$ as known by $x$.

• $c(x,v)$ = cost of link $(x,v)$.

• RIP (Routing Information Protocol):

  • Metric = hop count (max 15 hops).

  • Updates every 30 sec (or triggered).

  • Count-to-infinity problem: Slow convergence; solutions: split horizon, poison reverse.

• Example: Given neighbor vectors, compute new distance vector.

Link State Routing (LSR):

• Dijkstra's Algorithm (Shortest Path First):

  1. Initialize: $$\displaystyle L(1)=0 $$, $$\displaystyle L(v)=\infty $$ for others; $$\displaystyle N' = \{1\} $$.

  2. Find $w \notin N'$ with smallest $L(w)$; add to $N'$.

  3. Update $L(v)$ for $v \in N - N'$: $$\displaystyle L(v) = \min(L(v), L(w) + c(w,v)) $$.

  4. Repeat until all nodes in $N'$.

• LSP Flooding: Each router broadcasts its link state to all others (reliable flooding).

• Database Synchronization: Each router has identical link-state database (LSDB).

• OSPF (Open Shortest Path First) uses LSR.

Comparison:

C. CONGESTION CONTROL

Causes: Buffer overflow, slow processors, low bandwidth. Principles:

• Open-loop: Prevent congestion before it happens (traffic shaping, admission control).

• Closed-loop: Detect and recover (feedback-based).

Techniques:

• Traffic Shaping:

  • Leaky Bucket: Fixed output rate; bursty input smoothed; may discard excess.

  • Token Bucket: Tokens arrive at rate $r$; burst up to $b$ tokens allowed; more flexible.

    • Throughput: Up to $b + r \times T$ in time $T$.

• Load Shedding: Discard packets when overloaded.

  • RED (Random Early Detection): Probabilistically drop packets before queue full to avoid global sync.

• QoS (Quality of Service):

  • IntServ (Integrated Services): Resource reservation (RSVP) per flow; fine-grained but not scalable.

  • DiffServ (Differentiated Services): Per-hop behavior (PHB) based on DS field (6 bits); scalable (EF, AF, BE classes).

D. IP ADDRESSING (IPv4)

Classful Addressing (legacy):

Limitations:

• Address depletion: Class A/B waste addresses; Class C too small.

• Routing table explosion: Every classful network in global table.

• No flexibility: Cannot subnet within class without extra mask.

CIDR (Classless Inter-Domain Routing):

• Notation: a.b.c.d/n (e.g., 192.168.1.0/24).

• Address Aggregation (Supernetting): Combine contiguous prefixes into larger block (reduce routing table).

• Subnetting:

  • Given network N with mask M, create k subnets → need $n$ bits where $$\displaystyle 2^n \ge k $$.

  • New subnet mask = old mask + $n$ bits.

  • Subnet address: Network address incremented by block size = $$\displaystyle 2^{32-n} $$.

  • Range: First usable = subnet + 1; last usable = subnet + block size - 2; broadcast = subnet + block size - 1.

Example: 192.168.10.0/24 → 4 subnets → need 2 bits → new mask /26 (255.255.255.192). Block size = $$\displaystyle 2^{32-26}=64 $$.

• Subnet 0: 192.168.10.0/26 → range 192.168.10.1 – 192.168.10.62, broadcast 192.168.10.63.

• Subnet 1: 192.168.10.64/26 → range 65–126, broadcast 127.

• Subnet 2: 192.168.10.128/26 → range 129–190, broadcast 191.

• Subnet 3: 192.168.10.192/26 → range 193–254, broadcast 255.

Special Addresses:

• Network address: All host bits 0 (not assignable).

• Broadcast address: All host bits 1 (not assignable).

• Private Addresses (RFC 1918):

  • 10.0.0.0/8

  • 172.16.0.0/12 (172.16.0.0 – 172.31.255.255)

  • 192.168.0.0/16

• Loopback: 127.0.0.0/8 (e.g., 127.0.0.1).

E. IPv6

Motivations:

• Address space: 128-bit ($$\displaystyle 3.4 \times 10^{38} $$ addresses).

• Simplified header: Fixed 40-byte header; no checksum, options minimal.

• Extension headers: Optional, after base header (routing, fragmentation, security).

• Autoconfiguration: SLAAC (Stateless Address Autoconfiguration).

• Built-in security: IPsec mandatory (but not always used).

Header Format (40 bytes):

Version (4) | Traffic Class (8) | Flow Label (20)

Payload Length (16) | Next Header (8) | Hop Limit (8)

Source Address (128)

Destination Address (128)

• Next Header: Indicates extension header or upper layer (TCP=6, UDP=17).

• Hop Limit: Like TTL.

• Flow Label: For QoS (identify packet flow).

Address Representation:

• Hexadecimal, colon-separated: 2001:0db8:85a3:0000:0000:8a2e:0370:7334.

• Zero compression: Consecutive zeros → :: (once only).

  • Example: 2001:db8::1 (compresses middle zeros).

• IPv4-mapped IPv6: ::ffff:192.0.2.128.

Transition Mechanisms:

• Dual Stack: Host runs both IPv4 & IPv6.

• Tunneling: Encapsulate IPv6 in IPv4 (6to4, Teredo).

• Translation: NAT-PT (deprecated), NAT64/DNS64.

IPv4 vs IPv6 Comparison:

F. NETWORK LAYER PROTOCOLS

ARP (Address Resolution Protocol):

• Purpose: Map IP address → MAC address (on same LAN).

• Operation:

  1. Host checks ARP cache; if miss, broadcasts ARP request: "Who has IP X? Tell Y (MAC_y)."

  2. Owner responds with ARP reply (unicast): "IP X is at MAC_x."

  3. Both update ARP caches.

• Proxy ARP: Router answers ARP for another host (makes remote host appear local).

• Cache: Entries timeout (typically 60 sec).

RARP (Reverse ARP):

• Purpose: Diskless workstation gets its IP from server (MAC → IP).

• Operation: Station broadcasts RARP request with its MAC; RARP server replies with IP.

• Replaced by: BOOTP, then DHCP (more features).

ICMP (Internet Control Message Protocol):

• Role: Network diagnostics, error reporting (not for user data).

• Message Types:

  • Destination Unreachable (Type 3): No route, port unreachable, etc.

  • Time Exceeded (Type 11): TTL expired (used by traceroute).

  • Echo Request/Reply (Type 8/0): Used by ping.

  • Redirect (Type 5): Suggest better next-hop.

  • Source Quench (deprecated): Congestion control.

• Encapsulation: Inside IP datagram (Next Header = 1).

G. FRAGMENTATION AND REASSEMBLY

Need: Different networks have different MTU (Maximum Transmission Unit). IP must handle packets larger than next link's MTU.

Fields in IPv4 Header:

• Identification: Same for all fragments of a packet.

• Flags:

  • DF (Don't Fragment): If set and packet too large → drop + ICMP error.

  • MF (More Fragments): Set on all but last fragment.

• Fragment Offset: 13-bit; offset in 8-byte units from start of original packet.

  • All fragments except last have MF=1; offset = (byte position)/8.

Reassembly: Done only at destination host (routers do not reassemble). If fragments missing within timeout (typically 60 sec), entire packet discarded.

Example: Original 4000-byte packet (20-byte header), MTU 1500 bytes.

• Max data per fragment = 1480 bytes (multiple of 8).

• Fragment 1: offset 0, MF=1, data 0–1479.

• Fragment 2: offset 185 (1480/8), MF=1, data 1480–2959.

• Fragment 3: offset 370 (2960/8), MF=0, data 2960–3959 (last fragment, 1040 bytes).

H. ROUTING TYPES

• Unicast Routing: One-to-one; standard routing tables.

• Broadcast Routing:

  • Flooding: Send to all neighbors (duplicates, infinite loops); use sequence numbers or spanning tree to control.

  • Multi-destination routing: Each router copies packet to multiple outputs.

  • Reverse Path Forwarding (RPF): Forward only if incoming interface is on shortest path back to source.

• Multicast Routing:

  • Source-specific tree vs Group-shared tree.

  • Protocols: DVMRP (Distance Vector Multicast Routing Protocol), PIM (Protocol Independent Multicast) in sparse/dense modes.

  • IGMP (Internet Group Management Protocol): Hosts join/leave multicast groups on LAN.

VI. TRANSPORT LAYER

A. SERVICES

Multiplexing/Demultiplexing:

• Sender: Host assigns port number (16-bit) to identify application process.

• Receiver: Transport layer uses (dest port, source port) + IP addresses to demultiplex to correct socket.

B. USER DATAGRAM PROTOCOL (UDP)

Header Format (8 bytes):

Source Port (16) | Dest Port (16)

Length (16)      | Checksum (16)

• Length: Header + data (min 8 bytes).

• Checksum: Optional in IPv4; mandatory in IPv6. Covers pseudo-header (src IP, dst IP, protocol, UDP length).

• Applications: DNS (port 53), VoIP, streaming, DHCP (68/67), SNMP (161).

C. TRANSMISSION CONTROL PROTOCOL (TCP)

Header Format (20–60 bytes):

Source Port (16) | Dest Port (16)

Sequence Number (32)

Acknowledgment Number (32)

Data Offset (4) | Reserved (6) | Flags (6) | Window (16)

Checksum (16) | Urgent Pointer (16)

[Options (variable)] | Padding

• Flags:

  • URG: Urgent pointer valid.

  • ACK: Acknowledgment number valid (set after SYN).

  • PSH: Push function (deliver immediately).

  • RST: Reset connection.

  • SYN: Synchronize sequence numbers (connection setup).

  • FIN: Terminate connection.

• Window: Receiver's advertised window size (rwnd) for flow control.

• Urgent Pointer: Offset to urgent data (if URG set).

TCP Header Analysis from Hex Dump (example from past paper):

05320017 00000001 00000000 500207FF 00000000

• Source port: 0532 (hex) = 1330 (dec).

• Dest port: 0017 = 23 (dec) → Telnet.

• Seq number: 00000001 = 1.

• Ack number: 00000000 = 0 (no ACK yet, likely SYN).

• Header length: 5 (first 4 bits) → 5 × 4 = 20 bytes (no options).

• Flags: 02 (SYN) + 07FF? Actually flags in 13th–14th bytes: 02 (binary 000010) → SYN set.

• Window: 07FF = 2047.

Connection Establishment (Three-Way Handshake):

1. Client → Server: SYN=1, seq=x.

2. Server → Client: SYN=1, ACK=1, seq=y, ack=x+1.

3. Client → Server: ACK=1, seq=x+1, ack=y+1.

• State diagram: CLOSED → SYN_SENT → ESTABLISHED (client); LISTEN → SYN_RCVD → ESTABLISHED (server).

Connection Termination (Four-Way Handshake):

• Either side can close:

  1. Fin sender: FIN=1, seq=u → FIN_WAIT_1.

  2. Other: ACK=1, ack=u+1 → CLOSE_WAIT; sends its own FIN=1, seq=v → FIN_WAIT_2.

  3. First side: ACK=1, ack=v+1 → TIME_WAIT (2×MSL wait).

  4. Other: ACK=1, ack=v+1 → CLOSED.

• TIME_WAIT (2×MSL = 2×Maximum Segment Lifetime):

  • Ensures last ACK received (retransmit if lost).

  • Allows old duplicate segments to expire (prevents confusion with new connection).

• Graceful release: Important to avoid data loss; both sides close independently.

Flow Control:

• Sliding window: Receiver advertises rwnd in header (window field).

• Sender must not send more than $\min(\text{cwnd}, \text{rwnd})$ bytes.

• rwnd updated by receiver based on buffer availability.

Congestion Control (avoid congestion collapse):

• Variables:

  • cwnd (congestion window): Sender's limit based on network congestion.

  • ssthresh (slow start threshold): Threshold to switch from slow start to congestion avoidance.

• Algorithms:

  1. Slow Start: Initially cwnd = 1 MSS; double per RTT until ssthresh or loss.

  2. Congestion Avoidance: After ssthresh, increase cwnd by 1 MSS per RTT (additive increase).

  3. Fast Retransmit: On 3 duplicate ACKs, retransmit missing segment immediately (no wait for timeout).

  4. Fast Recovery: After fast retransmit, set ssthresh = cwnd/2, cwnd = ssthresh + 3, then increase by 1 per duplicate ACK; on new ACK, set cwnd = ssthresh (exit recovery).

• Timeout: cwnd = 1; ssthresh = cwnd/2.

[!TIP] Exam Focus: Draw TCP state diagram (CLOSED, LISTEN, SYN_SENT, SYN_RCVD, ESTABLISHED, FIN_WAIT_1/2, TIME_WAIT, CLOSE_WAIT, LAST_ACK, CLOSING). Know cwnd/ssthresh evolution in congestion control (graph). Explain why TIME_WAIT is necessary (2MSL rule).

D. TCP vs UDP COMPARISON

VII. APPLICATION LAYER

A. DOMAIN NAME SYSTEM (DNS)

Purpose: Translate human-readable domain names (e.g., www.rgpv.ac.in) to IP addresses.

Hierarchical Structure:

Root servers (.) → TLD servers (.com, .org, .in) → Authoritative servers (rgpv.ac.in) → Local/Recursive resolvers

• Root: 13 logical servers (anycast), know TLD servers.

• TLD: Manage domains under them (.com managed by Verisign).

• Authoritative: Host-specific records for a domain.

• Local DNS Server: ISP-provided; caches answers; recursive/iterative queries.

Resolution Process:

1. Recursive query (client → local DNS): Local DNS handles entire lookup.

2. Iterative queries (local DNS → others):

   • Local DNS asks root → root returns TLD server for .com.

   • Local DNS asks TLD → returns authoritative for example.com.

   • Local DNS asks authoritative → returns IP.

3. Caching: Each server caches records with TTL (Time to Live); reduces latency and traffic.

Resource Records (RR):

• A: IPv4 address (example.com → 93.184.216.34).

• AAAA: IPv6 address.

• MX: Mail exchange (priority, hostname).

• CNAME: Canonical name (alias).

• NS: Name server for domain.

• PTR: Pointer (reverse DNS, IP → name).

Example: nslookup www.google.com → queries local DNS, which may cache or perform iterative lookup.

B. ELECTRONIC MAIL

Architecture:

• MUA (Mail User Agent): Outlook, Thunderbird (compose/read).

• MTA (Mail Transfer Agent): Sendmail, Postfix (transfer between servers).

• MDA (Mail Delivery Agent): Procmail, deliver to mailbox (mbox, Maildir).

SMTP (Simple Mail Transfer Protocol) (port 25):

• Operation (client → server):

  
  HELO client.example.com
  
  MAIL FROM:<[email protected]>
  
  RCPT TO:<[email protected]>
  
  DATA
  
  Subject: Test
  
  ...
  
  .
  
  QUIT
  
  

• ESMTP (Extended SMTP): EHLO command; extensions (SIZE, STARTTLS, 8BITMIME).

• STARTTLS: Upgrade to TLS for encryption.

Message Format (RFC 5322):

Header:

From: [email protected]

To: [email protected]

Subject: Hello

Date: ...

MIME-Version: 1.0

Content-Type: multipart/mixed; boundary="----=_Part_0_123"

Body:
------=_Part_0_123

Content-Type: text/plain

Hello Bob.
------=_Part_0_123

Content-Type: image/jpeg

Content-Transfer-Encoding: base64

... (base64 data) ...
------=_Part_0_123--

Retrieval Protocols:

• POP3 (Post Office Protocol v3) (port 110/995 TLS):

  • Download-and-delete (or keep) model; simple; no server-side folders.

• IMAP (Internet Message Access Protocol) (port 143/993 TLS):

  • Manipulate messages on server; folders; partial fetch; better for multiple clients.

C. WORLD WIDE WEB (WWW)

Components:

• URL (Uniform Resource Locator): scheme://host:port/path?query#fragment (e.g., https://www.example.com:443/index.html?q=test#section1).

• HTML (HyperText Markup Language): Markup language for documents.

• HTTP (HyperText Transfer Protocol): Application layer protocol.

• Browser: Client (Chrome, Firefox).

• Web Server: Apache, Nginx.

HTTP:

• Request/Response Model: Stateless (each request independent).

• Methods:

  • GET: Retrieve data.

  • POST: Submit data (form, upload).

  • PUT, DELETE, HEAD, OPTIONS (RESTful).

• Versions:

  • HTTP/1.0: New TCP connection per request (inefficient).

  • HTTP/1.1: Persistent connections (keep-alive), pipelining, chunked encoding.

  • HTTP/2: Multiplexing over single TCP, header compression (HPACK), server push.

  • HTTP/3: Over QUIC (UDP-based), reduced latency.

• Cookies: Set-Cookie header; stateful sessions (shopping cart, login).

D. FILE TRANSFER PROTOCOL (FTP)

Connections:

• Control connection: Port 21, persistent, commands/responses (ASCII).

• Data connection: Port 20 (active mode) or dynamic (passive mode), binary, per file transfer.

Active Mode:

• Client opens port 20 → server connects from port 20 to client's specified port.

• Issue: Firewalls often block incoming connections.

Passive Mode:

• Client sends PASV; server opens random port and tells client; client connects to that port.

• Firewall-friendly.

Common Commands:

• USER, PASS, LIST, RETR (retrieve), STOR (store), QUIT.

E. SIMPLE NETWORK MANAGEMENT PROTOCOL (SNMP)

Components:

• Manager: Central monitoring system (e.g., SolarWinds, Nagios).

• Agent: Software on managed device (router, switch) collecting data.

• MIB (Management Information Base): Database of managed objects (OIDs, hierarchical tree).

Operations:

• GET: Retrieve value.

• SET: Modify value.

• GETNEXT/GETBULK: Traverse MIB.

• TRAP/INFORM: Asynchronous notifications (agent → manager). INFORM requires ACK; TRAP does not.

Versions:

• SNMPv1: Community-based security (plaintext "community string"); limited.

• SNMPv2c: Community-based; GETBULK, INFORM; still insecure.

• SNMPv3: User-based security (USM): authentication (MD5/SHA), encryption (DES/AES); access control (VACM).

VIII. NETWORK DEVICES AND INTERCONNECTION

Bridges and Switches:

• Transparent Bridging:

  1. Learning: Record (MAC, port) from source address of incoming frames.

  2. Forwarding: If dest MAC in table → forward to that port; else flood (except incoming).

  3. Filtering: If source and dest on same port → discard.

• Switching Methods:

  • Store-and-Forward: Entire frame received, check FCS, then forward (reliable, latency).

  • Cut-Through: Forward as soon as dest address read (low latency, no error check).

  • Fragment-Free: Wait for first 64 bytes (collision window in Ethernet) → hybrid.

• VLANs (Virtual LANs):

  • Purpose: Segmentation (security, broadcast control) without physical separation.

  • 802.1Q Tagging: Insert 4-byte VLAN tag in Ethernet frame (TPID=0x8100, VLAN ID 12 bits, priority 3 bits).

  • Trunk ports: Carry multiple VLANs (tagged); access ports: single VLAN (untagged).

Routers:

• Functions:

  • Routing: Compute best path (routing protocols).

  • Forwarding: Lookup destination IP in routing table → next-hop/interface.

  • NAT (Network Address Translation): Map private IPs to public IP (conserves IPv4 addresses).

• Routing Table: Destination network, next-hop, interface, metric.

Remote Bridging:

• Concept: Connect two LANs over a WAN link (e.g., two Ethernet segments connected via serial link).

• Challenges in Large LANs:

  • Latency: Long propagation delays over WAN break CSMA/CD (min frame size may exceed MTU).

  • Scalability: Broadcast storms across remote links; spanning tree may block links.

  • Solutions: Use routers instead (break broadcast domains), tunneling (e.g., GRE), or VLANs over trunk.

[!TIP] Exam Focus: Switch vs hub vs router differences (layer, forwarding basis). VLAN tagging (802.1Q) format. Remote bridging issues: CSMA/CD fails over long WAN due to propagation delay exceeding min frame time.

IX. SWITCHING TECHNIQUES

Comparison:

• Circuit vs Packet:

  • Circuit: Guaranteed bandwidth, low delay, inefficient for bursty traffic.

  • Packet: Efficient sharing, variable delay, congestion possible.

• Datagram vs VC:

  • Datagram: No setup, each packet routed independently (robust to failures, out-of-order).

  • VC: Setup overhead, in-order delivery, easier QoS.

X. NETWORK TOPOLOGIES

Full vs Partial Mesh:

• Full Mesh: $n(n-1)/2$ links (every node connected to every other).

• Partial Mesh: Only critical nodes fully connected; others have redundant paths.

[!TIP] Exam Focus: Compare topologies by cost, reliability, scalability, fault tolerance. Star most common in Ethernet; ring in Token Ring/FDDI; mesh in WANs/backbones.

XI. SECURITY AND MANAGEMENT (BRIEF)

Cryptography Basics:

• Symmetric (AES, DES): Same key for encrypt/decrypt; fast; key distribution problem.

• Asymmetric (RSA, ECC): Public/private key; slow; used for key exchange, digital signatures.

• Hashing (SHA-256, MD5): One-way; integrity check.

• Digital Signatures: Hash + encrypt with private key → authenticity, non-repudiation.

SNMP (covered in Application Layer): Use SNMPv3 for security (authentication, encryption).

END OF UNIT 3 NOTES