Computer Networks (CY-403) - Unit 5 Short Notes

UNIT 5: COMPUTER NETWORKS - COMPREHENSIVE NOTES

I. NETWORK ARCHITECTURE & FUNDAMENTALS

A. Reference Models

ISO-OSI Reference Model (7 Layers)

A theoretical framework for network communication, where each layer provides services to the layer above it.

TCP/IP Protocol Suite (4/5-Layer Model)

Practical model that underpins the modern Internet.

[!TIP] OSI vs. TCP/IP Key Difference: OSI is a prescriptive reference model (what should be done). TCP/IP is a descriptive protocol suite (what is done). TCP/IP's layers are not strictly aligned with OSI's.

Comparison: OSI vs. TCP/IP Model

Connection-Oriented vs. Connectionless Services

Service Primitives

Operations a layer provides to the layer above it:

1. REQUEST: Entity asks for service.

2. INDICATION: Event notification to the entity.

3. RESPONSE: Entity provides answer to indication.

4. CONFIRM: Result of a request returned to requester.

B. Network Types & Topologies

Network Types by Scale

Physical Topologies

C. Switching Techniques

[!TIP] Key Exam Point: Packet Switching (Datagram) is the foundation of the Internet (IP). Virtual Circuits are used in Frame Relay & ATM.

II. PHYSICAL LAYER & TRANSMISSION MEDIA

A. Guided Media (Wired)

B. Unguided Media (Wireless)

C. Signal & Transmission Concepts

• Bandwidth (Hz): Frequency range a medium can support.

• Data Rate (bps): Bits transmitted per second.

• Throughput (bps): Actual achieved data rate (affected by protocol overhead, congestion).

• Propagation Delay (s): Time for a bit to travel from source to destination. Propagation Delay = Distance / Propagation Speed.

Key Formulas

1. Nyquist Theorem (Noiseless Channel):

$$ \text{Max Bit Rate} = 2 \times B \times \log_2(M) $$

Where `B` = bandwidth (Hz), `M` = number of signal levels.

> \boxed{\text{Increasing signal levels } M \text{ increases data rate but reduces noise immunity.}}

2. Shannon's Theorem (Noisy Channel):

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

Where `C` = channel capacity (bps), `SNR` = signal-to-noise ratio (linear, not dB).

> \boxed{\text{Defines the absolute maximum theoretical data rate for a noisy channel.}}

D. Multiplexing

E. Line Coding (Digital-to-Digital)

III. DATA LINK LAYER

A. Functions & Services

Framing

• Byte/Flag Oriented: Use special flag byte (e.g., 01111110 in HDLC) to delimit frame. Requires Byte Stuffing (insert ESC after flag).

• Bit Oriented: Use bit pattern flag (e.g., 01111110). Requires Bit Stuffing (insert 0 after five consecutive 1s).

Error Detection & Correction

[!TIP] CRC Calculation Steps:

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

2. Perform modulo-2 division (XOR) by G(x).

3. Remainder R(x) is the CRC.

4. Transmit original data + R(x).

Flow & Error Control (ARQ)

Window Size Constraints:

• GBN: N ≤ 2^k - 1 (where k = sequence number bits).

• SR: N ≤ 2^{k-1}.

B. Data Link Protocols

HDLC (High-Level Data Link Control)

• Frame Format: Flag 01111110, Address, Control, Info, FCS, Flag.

• Modes: Normal (NRM), Asynchronous (ARM). Uses Supervisory (S) frames for ACK/NAK.

• Operation: Bit stuffing, CRC-32, supports full-duplex.

PPP (Point-to-Point Protocol)

• Phases: Link Establishment (LCP), Authentication (PAP/CHAP), Network Layer Protocol (NCP), Termination.

• Frame Format: Flag, Address (11111111), Control (00000011), Protocol (e.g., 0x0021 for IP), Data, FCS, Flag.

• Features: Simple, supports multiple protocols, authentication.

SLIP (Serial Line IP)

• Limitations: No error detection, no authentication, only supports IP, requires static IPs. Obsolete, replaced by PPP.

C. Sliding Window Protocols

Stop-and-Wait

• Sender sends 1 frame, waits for ACK.

• Efficiency: η = T_Tx / (T_Tx + 2 * T_prop + T_ack).

• Problem: Inefficient for long propagation delays (high T_prop).

Go-Back-N (GBN)

• Sender can have up to N unacknowledged frames.

• Receiver only accepts in-order frames; out-of-order frames are discarded.

• Timeout: Retransmit all frames from lost frame onward.

• Utilization: U = N / (1 + 2a) where a = T_prop / T_Tx.

Selective Repeat (SR)

• Sender & receiver windows of size N.

• Receiver buffers out-of-order frames.

• Individual ACKs; sender retransmits only missing frames.

• Window Size Rule: N ≤ 2^{k-1} to avoid sequence number ambiguity.

Piggybacking

• Attaching ACK to a data frame going in the reverse direction (instead of separate ACK frame). Improves efficiency in full-duplex.

D. Bit/Byte Stuffing

• Bit Stuffing (HDLC-like): After flag 01111110, insert 0 after five consecutive 1s in data. At receiver, remove 0 after five 1s.

• Byte Stuffing (PPP-like): Use ESC character (0x7D). If data byte equals FLAG or ESC, insert ESC before it. Receiver removes ESC.

IV. MEDIUM ACCESS CONTROL (MAC) SUB-LAYER

A. Channel Allocation

B. Random Access (Contention) Protocols

[!TIP] CSMA/CD Minimum Frame Size: Must be at least 2 * T_prop * Bandwidth to ensure collision detection before transmission ends.

$$ \text{Min Frame Size} \geq 2 \times \text{Propagation Delay} \times \text{Bandwidth} $$

C. Controlled Access Protocols

• Reservation: Stations reserve future slots.

• Polling: Central controller (master) polls each station.

• Token Passing: Token (control frame) passed sequentially; only token holder can transmit.

D. IEEE 802 Standards & LAN Technologies

E. Bridging & Switching

[!TIP] Switch vs. Bridge: Switch is essentially a multi-port bridge with higher port density, typically supporting full-duplex and higher speeds.

Remote Bridging

Connecting geographically separated LANs via WAN links (e.g., leased lines). Challenges: High latency breaks token passing/TCP efficiency, spanning tree may block redundant links, expensive.

V. NETWORK LAYER

A. Functions & Design Issues

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

• Routing: Determine end-to-end path (routing algorithms).

• Congestion Control: Prevent overload (network layer: packet discarding, load shedding).

• Fragmentation & Reassembly: Break packets to fit MTU (Maximum Transmission Unit) of outgoing link. Fields in IP header: Identification, Flags (DF, MF), Fragment Offset.

B. Routing Algorithms

Optimality Principle

Any optimal path from A to B consists of optimal subpaths from any intermediate node X to B.

Shortest Path Routing (Dijkstra's Algorithm - Link State)

1. Start with source node S. Set cost(S)=0, cost(others)=∞.

2. Find node X with smallest tentative cost not yet permanent.

3. Make cost(X) permanent.

4. Update costs of neighbors Y of X: if cost(X) + cost(X,Y) < cost(Y), update cost(Y) and set predecessor.

5. Repeat until all nodes permanent or destination found.

Advantages: Fast convergence, global view. Limitations: High overhead (link-state advertisements), memory intensive.

Distance Vector Routing (Bellman-Ford)

1. Each node maintains a distance vector (cost to all destinations via each neighbor).

2. Periodically (or on change), send entire vector to direct neighbors.

3. On receipt, apply Bellman-Ford equation:

$$ D_x(y) = \min_v \{ c(x,v) + D_v(y) \} $$

(Cost from `x` to `y` = min over neighbors `v` of [cost(x,v) + cost(v to y)]).

4. Update own table if a better path found.

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

Solutions: Split horizon (don't advertise route back to source), poison reverse (advertise infinite metric back), triggered updates.

Comparison: Link State vs. Distance Vector

C. IP Addressing (IPv4)

Classful Addressing (Legacy)

Limitations: Wasted addresses (large blocks for small orgs), no flexibility, routing table explosion.

Subnetting

• Goal: Divide a large network (/n) into smaller logical sub-networks.

• Process:

  1. Determine number of subnets needed (S). Find s such that 2^s ≥ S. New mask = original n + s.

  2. List all subnets by incrementing the subnet part.

  3. For each subnet: Subnet ID (all host bits 0), Valid Host Range (Subnet ID+1 to Broadcast-1), Broadcast Address (all host bits 1).

• Example: 192.168.10.0/24 into 4 subnets.

  • s=2 (since 2^2=4). New mask: /26 or 255.255.255.192.

  • Increment = 2^{32-26} = 64.

  • Subnets: 192.168.10.0/26, 192.168.10.64/26, 192.168.10.128/26, 192.168.10.192/26.

  • First subnet hosts: 192.168.10.1 - 192.168.10.62, broadcast 192.168.10.63.

CIDR (Classless Inter-Domain Routing)

• Notation: a.b.c.d/n (prefix length n).

• Aggregation/Supernetting: Combine multiple contiguous prefixes into one larger prefix to reduce routing table size.

  • Example: 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24 → 192.168.0.0/22.

ARP (Address Resolution Protocol)

• Need: Map IP address (Layer 3) to MAC address (Layer 2) on a local network.

• Operation:

  1. Host A with IP IP_A wants to send to IP_B on same LAN.

  2. A checks ARP cache. If miss, broadcasts ARP Request: "Who has IP_B? Tell IP_A (MAC_A)."

  3. Host B (with IP_B) unicasts ARP Reply: "IP_B is at MAC_B" to A.

  4. A caches IP_B → MAC_B mapping.

RARP (Reverse ARP)

• Diskless machine (no storage) knows its MAC, needs IP. Broadcasts RARP request. RARP server replies with IP.

ICMP (Internet Control Message Protocol)

• Role: Network layer protocol for error reporting & diagnostics (not for user data).

• Common Messages:

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

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

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

  • Redirect (Type 5): Suggest better route.

D. IPv6

IPv6 Address Types:

• Unicast: Single interface.

• Multicast: Group of interfaces.

• Anycast: Nearest of multiple interfaces (same address).

Transition Strategies:

• Dual Stack: Host/run both IPv4 & IPv6.

• Tunneling: Encapsulate IPv6 packet inside IPv4 packet.

• Header Translation: Convert IPv6 header to IPv4 (and vice versa).

VI. TRANSPORT LAYER

A. Functions & Services

• Process-to-Process Communication: Uses port numbers (16-bit) to identify application layer processes.

• Multiplexing/Demultiplexing: Multiple application processes share one transport connection.

• Connection Control: Connection-oriented (TCP) vs. Connectionless (UDP).

• Flow Control: Receiver-driven (prevents sender from overwhelming receiver). TCP uses sliding window.

• Congestion Control: Network-wide issue (prevent overload). Open-loop (prevent) vs. Closed-loop (detect & recover).

• Load Shedding: Discarding packets during congestion.

Congestion Control Techniques

[!TIP] Leaky Bucket vs. Token Bucket: Leaky bucket enforces strict average rate. Token bucket allows bursts up to a limit, then enforces average rate.

B. User Datagram Protocol (UDP)

• Connectionless, unreliable, no-frills.

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

• Applications: DNS, streaming media, VoIP, TFTP, SNMP (where speed > reliability).

C. Transmission Control Protocol (TCP)

• Connection-oriented, reliable, byte-stream.

• Header Format (20-60 bytes):

  • Source/Dest Port: 16 bits each.

  • Sequence Number: Byte number of first byte in segment.

  • Acknowledgment Number: Next expected byte number (cumulative ACK).

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

  • Window Size: Receiver's available buffer (flow control).

  • Checksum: Covers header+data+pseudo-header.

  • Urgent Pointer: Valid if URG set.

  • Options: MSS, window scaling, timestamps.

Connection Management

1. Three-Way Handshake (Establishment):

   • A → B: SYN=1, seq=x

   • B → A: SYN=1, ACK=1, seq=y, ack=x+1

   • A → B: ACK=1, seq=x+1, ack=y+1

   Why 3-way? Prevents old duplicate connection initiations (security). SYN consumes a sequence number.

2. Four-Way Handshake (Termination - Graceful Release):

   • A → B: FIN=1, seq=u

   • B → A: ACK=1, ack=u+1 (may still send data)

   • B → A: FIN=1, seq=v

   • A → B: ACK=1, ack=v+1

   Why 4-way? TCP is full-duplex. Each direction closed independently. TIME_WAIT state (2MSL) ensures last ACK received & old duplicates expire.

TCP Congestion Control

1. Slow Start: cwnd starts at 1 MSS. On each ACK, cwnd += 1 MSS. Exponential growth until ssthresh.

2. Congestion Avoidance: After cwnd ≥ ssthresh, cwnd += 1 MSS per RTT (linear growth).

3. Fast Retransmit: On 3 duplicate ACKs, retransmit missing segment without waiting for timeout. Set ssthresh = cwnd / 2, cwnd = ssthresh + 3 MSS.

4. Fast Recovery: After fast retransmit, for each duplicate ACK, cwnd += 1 MSS. On new ACK (for new data), set cwnd = ssthresh (exit fast recovery).

D. Comparison: TCP vs. UDP

VII. APPLICATION LAYER

A. Domain Name System (DNS)

• Need: Map human-readable domain names (www.rgpvonline.com) to IP addresses.

• Hierarchical Structure:

  • Root Servers (13 sets): Top-level.

  • TLD Servers: .com, .org, .in, country-code TLDs.

  • Authoritative Servers: Hold records for specific domains.

  • Local DNS Resolver: Usually at ISP or organization.

• Resolution Process:

  1. Resolver queries root server for com TLD server.

  2. Queries com TLD server for rgpvonline.com authoritative server.

  3. Queries authoritative server for www.rgpvonline.com A record.

  4. Returns IP to resolver → client.

  • Iterative Query: Server returns best known answer (referral to another server).

  • Recursive Query: Server takes full responsibility to find answer (usually from resolver to local server).

• Caching: Resolver caches answers with TTL (Time-To-Live). Servers also cache.

• Resource Records (RR): A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail server), NS (name server).

B. Electronic Mail (Email)

Architecture

• User Agent (UA): Email client (Outlook, Thunderbird).

• Message Transfer Agent (MTA): Mail server (Sendmail, Postfix). SMTP between MTAs.

• Mail Delivery Agent (MDA): Delivers to user's mailbox (e.g., procmail).

Protocols

• SMTP (Simple Mail Transfer Protocol):

  • Push protocol (port 25). ASCII command/response.

  • ESMTP (Extended SMTP): Adds STARTTLS, authentication, MIME.

  • Limitation: Body must be 7-bit ASCII.

• MIME (Multipurpose Internet Mail Extensions):

  • Encodes non-ASCII (binary, images) into 7-bit ASCII using Base64 or quoted-printable.

  • Defines multipart messages (attachments).

• POP3 (Post Office Protocol v3):

  • Pull protocol (port 110). Download-and-delete model. Simple, no remote mailbox management.

• IMAP (Internet Message Access Protocol):

  • Pull protocol (port 143/993). Remote mailbox management. Messages stay on server; can create folders, search.

C. World Wide Web (WWW) & HTTP

• HTTP (Hypertext Transfer Protocol):

  • Request/Response model (client-server).

  • Stateless: Server does not retain user state between requests (cookies/sessions used for state).

  • Methods: GET (retrieve), POST (submit data), PUT (upload), DELETE.

  • Status Codes: 1xx (informational), 2xx (success), 3xx (redirect), 4xx (client error), 5xx (server error).

  • Connections: Non-persistent (one request/response per TCP connection) vs. Persistent (HTTP/1.1 default, multiple requests per connection).

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

D. Other Application Layer Protocols

• SNMP (Simple Network Management Protocol):

  • Manager (NMS) ↔ Agent (managed device).

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

  • Operations: GET, SET, GETNEXT, TRAP (asynchronous notification).

• FTP (File Transfer Protocol):

  • Separate control (port 21) & data (port 20) connections.

  • Active Mode: Server connects to client's data port.

  • Passive Mode: Client connects to server's data port (firewall-friendly).

• Cryptography (Basic Concepts):

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

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

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

  • Certificate: Digital document binding public key to entity, signed by CA (Certificate Authority).

VIII. PRACTICAL PROBLEMS & CALCULATIONS

A. Nyquist Theorem

Problem: Noiseless channel, bandwidth B = 3 kHz, M = 4 signal levels. Find max bit rate.

Solution:

$$ R_{max} = 2 \times 3000 \times \log_2(4) = 6000 \times 2 = 12000 \text{ bps} = 12 \text{ kbps} $$

If M increases, R_max increases logarithmically.

B. CRC Computation

Problem: Data D(x) = x^5 + x^4 + x^2 + 1 (binary 1101011011), Generator G(x) = x^3 + x + 1 (binary 1011). Find transmitted codeword.

Solution:

1. Data: 1101011011 (10 bits). Append 3 zeros → 1101011011000.

2. Divide by 1011 (modulo-2):

   1100010101 (quotient)

  ---------------

1011 ) 1101011011000

   1011

   -----

    1111

    1011

    -----

     1000

     1011

     -----

      0110

      0000

      ----

       1101

       1011

       -----

        0110

        0000

        -----

         1100

         1011

         -----

          0110 → Remainder R(x) = 110

3. CRC = 110.

4. Transmitted Codeword: Original data + CRC = 1101011011 + 110 = 1101011011110.

C. Subnetting

Problem: Divide 192.168.10.0/24 into 4 subnets.

Solution:

1. Need 2^s ≥ 4 → s=2. New mask: /26 = 255.255.255.192.

2. Block size = 2^{32-26} = 64.

3. Subnets:

*   **Subnet 0:** `192.168.10.0/26` <br> Hosts: `192.168.10.1` - `192.168.10.62` <br> Broadcast: `192.168.10.63`

*   **Subnet 1:** `192.168.10.64/26` <br> Hosts: `192.168.10.65` - `192.168.10.126` <br> Broadcast: `192.168.10.127`

*   **Subnet 2:** `192.168.10.128/26` <br> Hosts: `192.168.10.129` - `192.168.10.190` <br> Broadcast: `192.168.10.191`

*   **Subnet 3:** `192.168.10.192/26` <br> Hosts: `192.168.10.193` - `192.168.10.254` <br> Broadcast: `192.168.10.255`

D. Throughput & Efficiency

1. Pure ALOHA Throughput: S = G * e^{-2G}. Max S ≈ 0.184 at G=0.5.

2. Slotted ALOHA Throughput: S = G * e^{-G}. Max S ≈ 0.368 at G=1.

3. Stop-and-Wait Efficiency:

$$ \eta = \frac{T_{Tx}}{T_{Tx} + 2 T_{prop} + T_{ack}} $$

For high bandwidth-delay product, `η` is very low.

4. Go-Back-N Utilization:

$$ U = \frac{N}{1 + 2a} \quad \text{where } a = T_{prop} / T_{Tx} $$

Max `N` limited by sequence number space.

E. CSMA/CD Minimum Frame Size

Problem: LAN length 2 km, bandwidth 10^7 bps, signal speed 2×10^8 m/s. Find min frame size.

Solution:

1. Round-trip propagation delay: T_{prop} = Distance / Speed = 2000 / (2×10^8) = 10^{-5} s.

2. Round-trip time: 2 * T_{prop} = 2 × 10^{-5} s.

3. Min frame transmission time must ≥ round-trip time:

`T_Tx ≥ 2 * T_prop`

`Frame Size / Bandwidth ≥ 2 × 10^{-5}`

`Frame Size ≥ 10^7 × 2 × 10^{-5} = 200 bits`.

Answer: Minimum frame size = 200 bits (or 25 bytes).

F. TDM Frame Size & Data Rate

Problem: Multiplex 3 channels (2 kbps, 3 kbps, 4 kbps) using synchronous TDM. Find frame size (bits) and data rate.

Solution:

1. Each channel gets 1 time slot per frame.

2. To avoid waste, each slot must carry enough bits for the slowest channel? No, in synchronous TDM, slots are fixed size. Typically, slot size = bits per sample. If channels are byte-oriented, slot size might be 8 bits.

• Assumption: Each channel provides 1 byte (8 bits) per frame.

3. Frame Size: 3 slots × 8 bits = 24 bits.

4. Frame Rate: Must match fastest channel's sampling rate. Fastest channel = 4 kbps. If 1 byte/sample → 4000 samples/sec → 4000 frames/sec.

5. Data Rate: Frame Rate × Frame Size = 4000 × 24 = 96,000 bps = 96 kbps.

G. TCP Header Analysis (Hex Dump)

Dump: 05320017 00000001 00000000 500207FF 00000000

Break into 32-bit words:

1. 05320017 → Source Port: 0x0532 = 1330 decimal. Dest Port: 0x0017 = 23 decimal (Telnet).

2. 00000001 → Seq = 1.

3. 00000000 → Ack = 0 (no ACK, connection setup).

4. 500207FF → Header length: 0x50 = 80 bytes? No, 0x50 in hex = 80 decimal → 80/4 = 20 words → 20 bytes (standard). Flags: 0x07 = 00000111 → URG, ACK, PSH set? Wait, 0x07 in lower byte? Actually, 500207FF: 50=HLEN, 02=flags? Let's decode properly:

*   First 4 bits: `5` → `5 * 4 = 20` bytes.

*   Next 6 bits: Reserved (0).

*   Next 6 bits: Flags. `02` in hex = `00000010` binary → only **SYN** flag set? Actually, byte `02` = `00000010`, so bit positions: URG(0), ACK(0), PSH(0), RST(0), SYN(1), FIN(0). So **SYN** set.

*   Window size: `0x07FF` = **2047**.

5. 00000000 → Checksum, Urgent Pointer (0).

Summary:

• Source Port: 1330

• Dest Port: 23 (Telnet)

• Seq: 1

• Ack: 0

• Header Len: 20 bytes

• Flags: SYN (connection request)

• Window Size: 2047

IX. SHORT NOTE TOPICS (FREQUENT 3-4m QUESTIONS)

ARP, RARP, and ICMP Protocols

• ARP: Maps IP → MAC on local LAN. Uses broadcast request, unicast reply. Cache entries timeout.

• RARP: Diskless machine (knows MAC) gets IP from RARP server. Obsolete (replaced by BOOTP/DHCP).

• ICMP: Network layer error/diagnostic protocol. ping (Echo Request/Reply), traceroute (Time Exceeded), Destination Unreachable.

PPP, SLIP, and HDLC Protocols

Guided and Unguided Transmission Media

• Guided: Copper wire (Twisted Pair, Coaxial), Fiber Optic. Signals confined to physical path.

• Unguided: Radio, Microwave, Infrared. Signals propagate through air/space, no physical conductor.

Virtual LAN (VLAN)

• Need: Segment a physical LAN into multiple broadcast domains for security, performance, management.

• Operation: Switch ports assigned to VLANs. Frames tagged with VLAN ID (IEEE 802.1Q header). Traffic confined to same VLAN unless via router.

• Benefits: Reduced broadcast traffic, improved security, flexible user grouping.

HTTP

• Application layer protocol for World Wide Web.

• Request/Response model. Stateless (cookies/sessions for state).

• Methods: GET, POST, PUT, DELETE.

• Status Codes: 200 OK, 404 Not Found, 500 Server Error.

• HTTPS: HTTP over TLS/SSL (port 443).

UDP

• Connectionless, unreliable, no-frills transport protocol.

• Header: Source Port, Dest Port, Length, Checksum (optional in IPv4, mandatory in IPv6).

• Use: DNS, streaming, VoIP, where speed > reliability or application handles reliability.

Cryptography

• Symmetric: Single shared key (AES, DES). Fast, key distribution issue.

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

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

• Certificate: Digital document (signed by CA) binding public key to entity.

FDM and TDM

• FDM: Different signals on different frequencies (radio channels, cable TV).

• TDM: Different signals on different time slots. Synchronous (fixed slots), Statistical (dynamic allocation).

Token Ring & FDDI

• Token Ring (802.5): Ring topology, token passing access. Single token circulates; only holder transmits. Beaconing for ring recovery. 4/16 Mbps.

• FDDI: High-speed (100 Mbps) dual-ring (primary/secondary) using fiber. Token passing like Token Ring but faster, more reliable (dual-ring fault tolerance).

SMTP

• Push protocol for email transfer between MTAs (port 25).

• ASCII command/response (e.g., HELO, MAIL FROM, RCPT TO, DATA).

• ESMTP: Extensions (STARTTLS, AUTH, SIZE).

• Limitation: 7-bit ASCII only → MIME used for binary.

Congestion Control

• Need: Prevent network overload (packet loss, high delay).

• Network Layer: Packet discarding, load shedding.

• Transport Layer (TCP): Closed-loop feedback control.

  • Slow Start: Exponential cwnd growth.

  • Congestion Avoidance: Linear cwnd growth.

  • Fast Retransmit/Recovery: React to duplicate ACKs.

• Techniques: Leaky Bucket, Token Bucket (traffic shaping).

DNS

• Hierarchical, distributed database mapping domain names ↔ IP addresses.

• Components: Root servers, TLD servers, Authoritative servers, Resolvers.

• Resolution: Iterative/recursive queries, caching with TTL.

• RR Types: A, AAAA, CNAME, MX, NS.

SNMP

• Network management protocol (application layer).

• Architecture: Manager (NMS) ↔ Agent (managed device).

• MIB: Database of managed objects (variables).

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

MLMA (Multiple Access with Collision Avoidance)

• Refers to CSMA/CA (used in IEEE 802.11 Wi-Fi).

• Principle: Avoid collisions rather than detect them (hard in wireless).

• Mechanisms:

  • Physical Carrier Sense: CCA (Clear Channel Assessment).

  • Virtual Carrier Sense: NAV (Network Allocation Vector) from RTS/CTS.

  • RTS/CTS: Optional handshake to reserve channel for long frames.

  • DIFS/SIFS: Inter-frame spacing priorities.

Broadcast and Multicast Routing

• Broadcast Routing: Send to all nodes in network.

  • Flooding: Simple but causes broadcast storm.

  • Reverse Path Forwarding (RPF): Router forwards broadcast only if received on shortest path from source.

• Multicast Routing: Send to group of interested receivers.

  • DVMRP: Distance Vector Multicast Routing Protocol (uses reverse path).

  • PIM: Protocol Independent Multicast (Sparse/Dense modes).

  • MOSPF: Multicast extension to OSPF.

Frame Relay (Protocol Architecture)

• Layer 2 (Data Link) WAN protocol, packet-switched.

• Connection-oriented (virtual circuits: PVCs/SVCs).

• Functions:

  • Frame Format: Flag, DLCI (virtual circuit ID), Control, Data, FCS, Flag.

  • No error correction (only error detection, relies on higher layers).

  • Congestion Control: Discard (DE bit), congestion notification (FECN/BECN bits).

• Devices: DTE (customer router), DCE (switch).

Router and Gateways

• Router (Layer 3): Connects different networks (IP subnets). Forwards based on IP address. Uses routing tables. Contains broadcast domains.

• Gateway (Layer 5-7): Protocol converter between dissimilar networks (e.g., Email gateway, VoIP gateway, application-level gateway).

Connection Establishment and Release (TCP)

• Establishment (3-Way Handshake):

  1. A → B: SYN=1, seq=x

  2. B → A: SYN=1, ACK=1, seq=y, ack=x+1

  3. A → B: ACK=1, seq=x+1, ack=y+1

  Why 3-way? Prevents old duplicate SYN attacks. SYN consumes a sequence number.

• Release (4-Way Handshake - Graceful):

  1. A → B: FIN=1, seq=u

  2. B → A: ACK=1, ack=u+1 (may still send data)

  3. B → A: FIN=1, seq=v

  4. A → B: ACK=1, ack=v+1

  Why 4-way? TCP full-duplex; each direction closed independently. TIME_WAIT (2MSL) ensures last ACK delivered & old duplicates expire.

Electronic Mail Working

1. Composition: User writes email in UA (User Agent).

2. Transfer: UA hands message to MTA (Mail Transfer Agent) via SMTP.

3. Routing: MTAs use DNS to find destination domain's MX record (mail server). SMTP hops between MTAs.

4. Delivery: Destination MTA stores message in user's mailbox (via MDA).

5. Retrieval: User retrieves via POP3 (download-and-delete) or IMAP (remote management).

QoS Techniques in Internetworking

• Need: Provide preferential treatment (low delay, jitter, loss) for real-time traffic (VoIP, video).

• Techniques:

  • Scheduling: Priority queuing, Weighted Fair Queuing (WFQ).

  • Traffic Shaping: Leaky Bucket, Token Bucket (smooth bursty traffic).

  • Resource Reservation: RSVP (Resource Reservation Protocol) - hosts request QoS from routers.

  • DiffServ (Differentiated Services): Per-hop behavior (PHB) based on DSCP bits in IP header. Simple, scalable.

  • IntServ (Integrated Services): Per-flow state & reservation (not scalable for Internet).

Leaky Bucket vs. Token Bucket

Pure vs. Slotted ALOHA