Computer Networking

OSI vs. TCP/IP Model Mapping:

Single-Mode vs. Multi-Mode Fiber Optic Cables:

• Single-Mode Fiber (SMF): Features an extremely narrow core diameter ($8 - 10\ \mu\text{m}$). Emits a single wavelength beam of laser light without modal dispersion, enabling long-haul telecommunication distances ($> 40 - 100\text{ km}$).
• Multi-Mode Fiber (MMF): Features a much wider core diameter ($50 - 62.5\ \mu\text{m}$). Allows multiple light rays (modes) from LED sources to propagate simultaneously with modal dispersion, suitable for short-range campus LAN backbones ($< 550\text{ m}$).

ARP & ARP Cache Poisoning:

• Role: Maps a known 32-bit Logical IP address to a physical 48-bit Media Access Control (MAC) hardware address on a local broadcast network link.
• ARP Poisoning (Spoofing): A malicious attack where an adversary broadcasts forged gratuitous ARP reply packets associating the attacker’s MAC address with the default gateway’s IP address, intercepting all outbound subnet traffic (Man-In-The-Middle attack).

Bandwidth-Delay Product (BDP):

• Significance: Measures the maximum volume of unacknowledged data “in flight” that can fill the network pipe at any instant. For maximum throughput, TCP’s receive window size ($RWIN$) must be configured at least equal to the BDP.

RFC 1918 Private IPv4 Address Ranges:

1. Class A: 10.0.0.0 to 10.255.255.255 (10.0.0.0/8, $16,777,216$ addresses).
2. Class B: 172.16.0.0 to 172.31.255.255 (172.16.0.0/12, $1,048,576$ addresses).
3. Class C: 192.168.0.0 to 192.168.255.255 (192.168.0.0/16, $65,536$ addresses).

Cyclic Redundancy Check (CRC) Polynomial Division

1. Formulate Generator and Appended Message:

• Generator Polynomial: $G(x) = x^4 + x^1 + x^0 \implies \mathbf{10011_2}$ (Degree $r = 4$).
• Data message $M = \mathbf{1101011011_2}$.
• Append $r = 4$ zeros: $M' = \mathbf{11010110110000_2}$.

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2. Modulo-2 Binary Polynomial Division (XOR):

          1100001010  (Quotient)
        -------------------------
10011 ) 11010110110000
        10011
        -----
         10011
         10011
         -----
          000010110
              10011
              -----
               010100
                10011
                -----
                 01110 (Remainder)

• Remainder after final division step: $\mathbf{1110_2}$ (4 bits).
• CRC Checksum: $\mathbf{1110_2}$.

3. Transmitted Codeword:

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Protocol Workflow:

1. Carrier Sense: Station listens to shared medium before transmitting. If idle, it transmits immediately (1-persistent). If busy, it waits until line becomes idle.
2. Collision Detection: While transmitting, station monitors line voltage. If signal voltage exceeds collision threshold, transmission collides.
3. Jamming Signal: Station immediately halts packet transmission and broadcasts a 32-to-48 bit Jamming Signal alerting all stations on the cable.
4. Binary Exponential Backoff:
   • Station increments collision counter $k = \min(attempt, 10)$.
   • Chooses random delay slot $r \in \{0, 1, 2, \dots, 2^k - 1\}$.
   • Waits backoff time $T_{\text{backoff}} = r \times 51.2\ \mu\text{s}$ (slot time) before re-attempting.

TCP Connection Establishment and Teardown

1. Three-Way Handshake (Connection Establishment):

Client (SYN-SENT)                           Server (LISTEN -> SYN-RCVD)
      |                                              |
      | --- 1. SYN=1, Seq=1000 --------------------> |
      | <--- 2. SYN=1, ACK=1, Seq=5000, Ack=1001 --- |
      |                                              |
(ESTABLISHED)                                  (ESTABLISHED)
      | --- 3. ACK=1, Seq=1001, Ack=5001 ----------> |

• Step 1: Client initiates by sending SYN with initial sequence number ($ISN_c = 1000$).
• Step 2: Server acknowledges ($Ack = 1001$) and responds with its own $ISN_s = 5000$ (SYN-ACK).
• Step 3: Client acknowledges ($Ack = 5001$). Connection established!

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2. Four-Way Teardown (Connection Termination):

1. Client sends FIN $\to$ Server replies ACK (Client enters FIN-WAIT-2).
2. Server finishes sending remaining data and sends FIN.
3. Client replies ACK and enters TIME-WAIT state ($2 \times MSL$) to guarantee packet drain before port reuse.

Distance Vector (RIP) vs. Link State (OSPF)

DNS Resolution Process: Recursive vs. Iterative

DNS maps human-friendly hostnames (e.g., tu.edu.np) to machine routable IP addresses.

Client Browser ---> (Recursive) ---> Local DNS Resolver (ISP / 8.8.8.8)
                                        |  ^
                1. Iterative: Where is .np? |  | 2. Root Nameserver (.root)
                                        |  v
                3. Iterative: Where is edu.np?|  | 4. TLD Nameserver (.np)
                                        |  v
                5. Iterative: Where is tu.edu.np? 6. Authoritative Nameserver (tu.edu.np)

• Recursive Query: Client offloads full burden of resolution onto the Local DNS Resolver. The resolver must return either the resolved IP address or a hard failure error.
• Iterative Query: Resolver queries nameservers sequentially (Root $\to$ TLD $\to$ Authoritative). If a nameserver cannot answer, it refers the resolver to the next authoritative server down the tree.

Network Address Translation (NAT) & Port Address Translation (PAT)

PAT (NAT Overload): Maps thousands of internal private IP addresses to a single public IP address by translating and tracking unique Layer 4 TCP/UDP port numbers.

Private LAN (192.168.1.0/24)             PAT Router (Public IP: 202.70.64.1)       Internet
Host A: 192.168.1.10:49152 --------> NAT Translation Table:
Host B: 192.168.1.25:52114 --------> [ 192.168.1.10:49152 <-> 202.70.64.1:10001 ] ---> Web Server
                                     [ 192.168.1.25:52114 <-> 202.70.64.1:10002 ]

How 500 Hosts Share One IP:

1. TCP/UDP ports provide a 16-bit address space ($2^{16} = 65,535$ ports).
2. The router assigns each outbound internal connection a unique ephemeral public source port ($10,000 - 65,000$).
3. When response packets return, the router inspects destination port, consults its active NAT translation state table, and rewrites the header back to the corresponding private IP and port.

Comprehensive VLSM Subnet Design: 192.168.10.0/24

Order Departments by Decreasing Host Requirements:

1. Dept A (60 hosts): Needs $2^h - 2 \ge 60 \implies h = 6$ host bits ($2^6 - 2 = 62$). Mask $= 32 - 6 = \mathbf{/26}$.
2. Dept B (28 hosts): Needs $2^h - 2 \ge 28 \implies h = 5$ host bits ($2^5 - 2 = 30$). Mask $= 32 - 5 = \mathbf{/27}$.
3. Dept C (14 hosts): Needs $2^h - 2 \ge 14 \implies h = 4$ host bits ($2^4 - 2 = 14$). Mask $= 32 - 4 = \mathbf{/28}$.
4. WAN Link (2 hosts): Needs $2^h - 2 \ge 2 \implies h = 2$ host bits ($2^2 - 2 = 2$). Mask $= 32 - 2 = \mathbf{/30}$.

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VLSM Subnet Allocation Table:

Address Space Efficiency:

• Total Addresses Used $= 64 + 32 + 16 + 4 = 116$ addresses.
• Remaining Unallocated Space $= 192.168.10.116$ to $192.168.10.255$ ($140$ contiguous addresses available for future enterprise branch expansions).

TCP Congestion Control Architecture & Window Trajectory

TCP prevents network collapse through four tightly coupled algorithms:

cwnd (MSS)
  ^
16|                   (ssthresh = 16)
  |                       /-----------\  (Triple Duplicate ACK at Round 12)
12|                      /             \  --> Fast Recovery: ssthresh = 7, cwnd = 7
  |                     / (Congestion   \___________
 8|                    /   Avoidance:                \
  |                   /    +1 MSS/RTT)                \  (Timeout Loss at Round 16)
 4|       /----------/                                 \
  |      / (Slow Start:                                 \
 1|_____/   Doubles each RTT)                            \_____ (Restart Slow Start: cwnd=1)
  +------------------------------------------------------------------> Transmission Rounds
  0     2     4     6     8     10    12    14    16    18    20

1. Detailed Phase Explanations:

1. Slow Start ($cwnd < ssthresh$):
   • Starts with $cwnd = 1\text{ MSS}$ (or $2 - 4\text{ MSS}$).
   • For every received ACK, $cwnd$ increases by $1\text{ MSS}$ $\implies$ Doubles exponentially every RTT:

     Swipe to see the full formula↔

     $$cwnd_{t+1} = 2 \times cwnd_t$$
2. Congestion Avoidance ($cwnd \ge ssthresh$):
   • Growth throttles to an Additive Increase: $cwnd$ increases by $1\text{ MSS}$ per full RTT:

     Swipe to see the full formula↔

     $$cwnd_{t+1} = cwnd_t + \frac{1}{cwnd_t}$$
3. Fast Retransmit & Fast Recovery (Triple Duplicate ACKs):
   • Arrival of 3 identical duplicate ACKs indicates packet loss while network still has packet flow capacity.
   • Fast Retransmit: Re-transmits missing segment immediately without waiting for retransmission timer to expire.
   • Fast Recovery (TCP Reno):

     Swipe to see the full formula↔

     $$ssthresh = \frac{cwnd}{2} = \frac{14}{2} = 7\text{ MSS}$$

     Swipe to see the full formula↔

     $$cwnd = ssthresh + 3\text{ MSS} = 10\text{ MSS}$$
     Resumes Additive Increase from 7 MSS without resetting to 1 MSS!
4. Timeout Event (Catastrophic Congestion):
   • If a retransmission timer expires (timeout loss):

     Swipe to see the full formula↔

     $$ssthresh = \frac{cwnd}{2}$$

     Swipe to see the full formula↔

     $$cwnd = 1\text{ MSS} \quad \text{(Full reset to Slow Start)}$$