⚡ Cheatsheet: Computer Networks

Delays

• Transmission (\(T_t\)): \(L / B\) (Length / Bandwidth).
• Propagation (\(T_p\)): \(Distance / Speed\).
• Round Trip Time (RTT): \(2 \times T_p\).
• Total Time: \(T_t + T_p + T_{queue} + T_{proc}\).

Flow Control

Protocol	Efficiency (\(\eta\))	Window Sizes (\(W_s, W_r\))
Stop & Wait	\(\frac{1}{1+2a}\)	\(1, 1\)
GBN	\(\frac{N}{1+2a}\)	\(N, 1\)
SR	\(\frac{N}{1+2a}\)	\(N, N\)

• Here \(a = T_p / T_t\). Max throughput = Efficiency \(\times\) Bandwidth.

IP Addressing (IPv4)

• Class A: 0... (NetID: 8 bits)
• Class B: 10... (NetID: 16 bits)
• Class C: 110... (NetID: 24 bits)
• Subnetting: Borrow \(x\) bits from Host ID. Subnets = \(2^x\). Hosts = \(2^h - 2\).
• CIDR: /24 means 24 bits Network, 8 bits Host (254 hosts).

TCP/UDP

• TCP Header: Min 20 Bytes, Max 60 Bytes.
• UDP Header: 8 Bytes (Source Port, Dest Port, Len, Checksum).
• Sequence Numbers: Byte-stream based. \(Ack \# = Next Expected Byte\).
• Congestion:
• Slow start: Doubles CWND every RTT.
• Congestion Avoidance: Linear increase (+1 MSS).
• Threshold: On timeout, \(Threshold = CWND/2\), \(CWND = 1\).

⚠️ Common Traps

• Bandwidth-Delay Product: \(B \times RTT\). Amount of data "in flight". Vital for window sizing.
• Manchester Encoding: Used in Ethernet. Baud rate = \(2 \times\) Bit rate.
• Fragment Offset: Measured in 8-byte blocks. \(Offset = 100 \implies 800\) bytes skipped.
• Checksum: Detects errors, does NOT correct them. CRC is usually Link Layer.