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-Computer Networks Final Exam Cheatsheet
-
-Use this as a fast recall guide. Focus on bolded terms, key formulas, and contrasts.
-
-1) Internet Basics
-- Protocol = message formats + order + actions
-- Edge vs Core: apps at edge; switches/routers in core
-- Packet switching: statistical multiplexing, store-and-forward
-- Circuit switching: reserved resources (FDM/TDM)
-- Delay types: d_proc + d_queue + d_trans(L/R) + d_prop(d/s)
-- Traffic intensity ρ = (L·a)/R → keep ρ < 1
-- Bottleneck link determines end-to-end throughput
-
-2) Layers and Encapsulation
-- Layers: App, Transport, Network, Link, Physical
-- Units: msg, segment, datagram, frame, bits
-- Encapsulation: each layer adds header; strip on receive
-
-3) Application Layer
-- Arch: Client–Server (always-on server) vs P2P (self-scaling)
-- HTTP: stateless; persistent vs non-persistent; cookies add state
-- DNS: hierarchical (root→TLD→authoritative); records A, NS, CNAME, MX
-- Email: SMTP push, IMAP access; typically over TCP
-- Web cache: proxy reduces RTT/bw; conditional GET uses If-Modified-Since/ETag
-- Streaming/DASH: client adapts bitrate; manifests; CDNs
-
-4) Transport
-- UDP: simple, no conn, best-effort; checksum
-- Reliable transfer building blocks: checksum, seq#, ACK/NAK, timer, retransmit
-- Pipelining: GBN (cum ACK, retransmit from loss), SR (selective ACK/buffer)
-- TCP: byte-stream, seq/ACK numbers (cumulative), flow control (rwnd)
-- RTT/Timeout: EstRTT=0.875·EstRTT+0.125·Sample; DevRTT EWMA; TO=EstRTT+4·Dev
-- Fast retransmit: on 3 dup ACKs
-- Congestion control: AIMD; slow start (exp), avoidance (lin), fast recovery
-- Throughput approx: (0.75·W)/RTT; sending rate ≈ cwnd/RTT
-- Variants: CUBIC (default Linux), BBR (model-based), Vegas (delay-based)
-- QUIC: UDP-based, integrated TLS, streams, faster startup, no HOL
-
-5) Network Layer (Data Plane)
-- Forwarding vs routing: local vs global
-- Longest prefix match; FIB lookup fast (TCAM)
-- IPv4 header key fields: TTL, Protocol, Src/Dst, checksum
-- Subnet/CIDR: a.b.c.d/x; route aggregation
-- DHCP: discover→offer→request→ack
-- NAT: private net uses one public IP; pros/cons (breaks E2E)
-- IPv6: 128-bit, fixed 40B header; no fragmentation; extension headers
-- Tunneling: IPv6-in-IPv4 during transition
-- Queues: HOL blocking at inputs; buffer sizing ≈ BDP/√N; bufferbloat
-- Schedulers: FIFO, priority, RR, WFQ
-
-6) Network Layer (Control Plane)
-- Link-state (Dijkstra): global view; flood LSAs; OSPF
-- Distance-vector (Bellman-Ford): neighbor exchange; count-to-infinity; poisoned reverse
-- BGP: interdomain, policy-first; attributes: LOCAL_PREF, AS_PATH, MED; hot-potato
-- SDN: centralized controller, match-action (OpenFlow), stats, events
-- NETCONF/YANG: model-driven config; SNMP for monitoring
-
-7) Link Layer and LANs
-- Services: framing, link access, error detection/correction
-- Error detection: parity (weak), checksum (software), CRC (strong)
-- Multiple access:
-  - TDM/FDM/CDMA (partitioning)
-  - Slotted/Pure ALOHA (random)
-  - CSMA/CD (wired); CSMA/CA (Wi‑Fi)
-- Ethernet: frame = preamble | dst | src | type | data (≥46) | CRC
-- Switches: self-learning MAC table; no loops → STP or use routing
-- VLANs: port-based segmentation; 802.1Q tag on trunks
-- MPLS: labels for fast fwd/TE/VPNs
-- Data centers: multi-tier/Clos, ECMP, SDN control, load balancers
-
-8) Wireless and Mobile
-- Wireless impairments: path loss, interference, multipath
-- SNR↑ → BER↓; higher rate → needs higher SNR
-- Hidden terminals; RTS/CTS mitigates
-- 802.11 MAC: CSMA/CA with DIFS/backoff/SIFS/ACK; rate adaptation; power save
-- Cellular LTE/5G: eNodeB/gNodeB; core (MME/HSS, SGW/PGW); tunnels; OFDM; handover
-- Mobility: home vs visited nets; indirect (triangle) vs direct routing; Mobile IP
-
-9) Security
-- Goals: Confidentiality, Integrity, Authentication, Availability
-- Symmetric crypto: AES; block vs stream; CBC with IV
-- Public key: RSA basics; use for key exchange + signatures
-- Hash/MAC: cryptographic hash; HMAC for integrity/auth
-- Digital signatures: sign hash with priv key; verify with pub key
-- TLS: handshake (certs, key exchange), key derivation, record MAC+encrypt
-- IPsec: SAs, ESP/AH, tunnel vs transport
-- Operational: firewalls (packet/stateful/app), IDS (sig/anomaly), defense-in-depth
-
-Key Formulas and Quick Facts
-- d_trans = L/R; d_prop = d/s; d_nodal = d_proc + d_queue + d_trans + d_prop
-- Traffic intensity ρ = (L·a)/R → if ρ ≥ 1 queues blow up
-- Throughput path = min link rate; BDP = R·RTT (bits in flight) → needed window/buffer
-- TCP timers: EstRTT, DevRTT, TO = EstRTT + 4·DevRTT
-- TCP cwnd sawtooth: avg ≈ 0.75·Wmax
-
-Typical Exam Comparisons
-- Packet vs circuit switching
-- TCP vs UDP; GBN vs SR; CSMA/CD vs CSMA/CA
-- OSPF (LS) vs RIP (DV); BGP purpose vs OSPF
-- NAT pros/cons; IPv4 vs IPv6 headers
-- Web cache pros; conditional GET
-- Firewall types; IDS methods; TLS vs IPsec
-
-Pitfalls and Gotchas
-- Confuse propagation vs transmission delay
-- RTT vs one-way; BDP units (bits!)
-- HTTP persistent reduces RTTs; cookies add state to stateless HTTP
-- Longest prefix match chooses most specific route
-- CRC detect/correct: detect bursts ≤ r bits; not correction by itself
-- Wi‑Fi cannot do collision detection (half-duplex radios, hidden nodes)
-
-Numbers to Remember
-- Ethernet min frame 64B; MTU 1500B
-- Wi‑Fi interframe: SIFS < DIFS; slot times vary by PHY
-- Common ports: HTTP 80/443, SMTP 25/587, DNS 53, IMAP 143/993
-- IPv4 TTL decrement each hop; typical speed in fiber ≈ 2e8 m/s
-
-Last‑Minute Checklist
-- Can you compute d_trans, d_prop, BDP, TCP window/throughput?
-- Can you explain cookies, DNS hierarchy, and conditional GET?
-- Can you run Dijkstra step-by-step and do longest prefix match?
-- Do you know TCP handshake/flags, congestion control phases?
-- Can you compare OSPF vs BGP and explain policy in BGP?
-- Do you remember CSMA/CA timing and RTS/CTS sequence?
-- Can you outline TLS handshake and IPsec tunnel structure?