Master Networking Fundamentals: From OSI Layers to TCP/IP, Security, and Wireless

Chapter 1 – Overview

Fundamental terms:

Link – the physical medium connecting nodes (copper, fiber, satellite, etc.).

Node – a computer, hub, switch, router, or similar device.

Protocol – rules governing message format, meaning, order, and actions.

Service – a layer provides services to the layer above while using services from the layer below.

Entity – any hardware or software process that can send or receive information.

Peer entity – two entities at the same layer that exchange PDU units.

PDU – protocol data unit, the data unit exchanged between peer layers.

Network models:

C/S (client‑server) – client requests service, server provides it.

B/S (browser‑server) – web browser acts as client.

P2P – each host can be both client and server.

Network scope classifications:

LAN – local area network (≈1 km).

MAN – metropolitan area network (5–50 km).

WAN – wide area network (tens to thousands of km).

PAN – personal area network (≈10 m).

Performance parameters (rate, bandwidth, throughput, delay, RTT, utilization, etc.) with sub‑delays (transmission, propagation, processing, queuing).

Chapter 2 – Internet Composition

The Internet consists of an edge part (hosts, mobile devices, super‑computers) and a core part (interconnected routers). The core performs routing and forwarding, while the edge runs applications.

Chapter 3 – Switching Techniques

Circuit Switching

Classic telephone network; a dedicated path is established, used, then released. Uses three phases: call setup, communication, call release. Multiplexing shares a link among multiple circuits.

Packet Switching

Messages are split into packets; each packet receives a header with control information and may travel different paths. At the destination, packets are reassembled.

Datagram Switching

Each whole datagram is stored at a node, a forwarding table is consulted, and the datagram is sent to the next node.

Virtual‑Circuit Switching

A logical path is pre‑established; packets carry a virtual‑circuit identifier instead of a full destination address.

Chapter 4 – TCP/IP Architecture

The TCP/IP model has four layers (link, internet, transport, application). Data is encapsulated at each layer with a header (and sometimes a trailer at the link layer).

During de‑encapsulation, each layer removes its header; the link layer also removes the trailer.

Chapter 5 – Physical Layer

Signal Coding

NRZ (non‑return‑to‑zero) uses high level for 1 and low level for 0. Manchester coding embeds a transition in the middle of each bit (1 → high→low, 0 → low→high).

Factors Influencing Signal Distortion

Transmission rate.

Transmission distance.

Medium type.

Noise and interference.

Transmission Media

Twisted pair – 4‑pair, shielded or unshielded, straight‑through vs crossover.

Coaxial cable – 50 Ω for LAN/digital, 75 Ω for TV/analog.

Fiber optic – single‑mode (long distance, laser source) and multi‑mode (short distance, LED source).

Wireless – radio waves.

Multiplexing Techniques

FDM – divide bandwidth into separate frequency bands.

TDM – divide time into equal slots; each user gets a slot.

Statistical TDM – slots allocated dynamically based on demand.

WDM – wavelength division multiplexing for optical fibers.

CDMA – code division multiple access; orthogonal codes allow simultaneous use of the same frequency.

Broadband Access Technologies

ADSL – uses existing telephone lines, frequency‑division multiplexing.

HFC – hybrid fiber‑coaxial, uses cable TV infrastructure.

FTTx – fiber to the home (FTTH), building (FTTB), or curb (FTTC).

Chapter 6 – Transport Layer

The transport layer offers end‑to‑end, process‑to‑process communication, hiding lower‑layer details.

UDP

Supports unicast, multicast, broadcast.

Connectionless, unreliable.

Header fields: source port, destination port, length, checksum.

Checksum calculation: set field to zero, sum 16‑bit words with carry, take one's complement, place result in checksum field. Receiver performs the same sum; a result of 0 indicates no error.

TCP

Connection‑oriented, reliable, ordered delivery.

Header fields: source port, destination port, sequence number, acknowledgment number, data offset, flags (URG, ACK, PSH, RST, SYN, FIN), window size, checksum, urgent pointer, options.

Reliability mechanisms include timeout retransmission (RTO = SRTT + 4·RTTVAR), cumulative acknowledgments, and fast retransmit after three duplicate ACKs.

Flow control uses a sliding window (sender window, receiver advertised window). Deadlock is avoided with a persist timer that probes a zero‑window receiver.

Congestion control:

Slow start – cwnd doubles each RTT until ssthresh.

Congestion avoidance – cwnd increases linearly (by one MSS per RTT).

On timeout, ssthresh = cwnd/2, cwnd = 1 (restart slow start).

Connection establishment – three‑way handshake (SYN, SYN‑ACK, ACK). Connection termination – four‑way handshake (FIN, ACK, FIN, ACK).

Chapter 7 – Application Layer

DNS

Maps domain names to IP addresses, supports aliases and load balancing.

Hierarchical structure: root → top‑level → authoritative → local name servers.

Four server types: root, TLD, authoritative, and local resolver.

Iterative resolution – resolver queries each level step by step.

Recursive resolution – resolver performs the full lookup on behalf of the client.

Caching reduces latency; commands: ipconfig /displaydns, ipconfig /flushdns.

FTP

Control connection on TCP port 21 for commands (login, directory navigation).

Data connection on TCP port 20 for file transfer.

Anonymous FTP can be accessed via command line, web browser (ftp://), or dedicated clients.

HTTP

Provides the protocol for web browsers and servers.

URL syntax: scheme://host[:port]/path.

Typical exchange: client sends GET request, server replies with status line, headers, and optional body.

HTTP/1.1 defaults to persistent connections and pipelining, reducing round‑trip times.

Cookies enable stateful interactions (session tracking, personalization).

Web document types: static (fixed files), dynamic (generated by CGI/servlets), and active (client‑side scripts).

Email

Components: mail user agent, mail server, SMTP for sending, POP3/IMAP for retrieval.

Message format includes headers (From, To, Subject, etc.) and body; MIME extends it for attachments and non‑ASCII content.

Web‑based mail uses HTTP/HTTPS interfaces.

DHCP

Provides dynamic IP configuration (address, subnet mask, default gateway, DNS servers).

Four‑message exchange: DISCOVER, OFFER, REQUEST, ACK.

Lease time defines how long a client may use an address; renewal occurs at 50 % of lease.

Relay agents allow DHCP servers to serve clients on different subnets.

P2P File Distribution

Peers exchange file chunks using a tit‑for‑tat strategy: each peer prefers the fastest four neighbors, re‑evaluates every 10 s, and randomly contacts another peer every 30 s to discover better sources.

Chapter 8 – Network Security

Common attacks: eavesdropping, data tampering, malware, DoS/DDoS.

Symmetric encryption – same secret key for encryption and decryption.

Public‑key (asymmetric) encryption – each user has a public key (PK) and a private key (SK). Sender encrypts with receiver’s PK; receiver decrypts with its SK.

Digital signatures provide authentication, integrity, and non‑repudiation (hash the message, encrypt hash with sender’s private key).

Key distribution mechanisms: KDC (Kerberos) for symmetric keys, CA (certificate authority) for public‑key certificates.

Chapter 9 – Wireless LAN

Infrastructure WLAN – stations communicate via an Access Point (AP).

Ad‑hoc WLAN – stations communicate directly without an AP; routing protocols are required.

Wireless sensor networks – low‑power, low‑bandwidth, often used in IoT.

CSMA/CA operation: carrier sense, DIFS wait, random backoff, transmission, SIFS ACK.

Hidden‑node problem can cause collisions despite CSMA/CA.

802.11 frame format includes four address fields (RA, TA, DA, SA) and control fields.

Chapter 10 – Miscellaneous Topics

Address Lengths

MAC – 48 bits (6 bytes).

IPv4 – 32 bits (4 bytes).

IPv6 – 128 bits (16 bytes).

Port – 16 bits.

Header Sizes

Ethernet frame header+trailer = 18 bytes.

IPv4 header = 20 bytes + optional 0‑40 bytes.

IPv6 base header = 40 bytes.

Error Checking

Frame CRC – polynomial division, remainder appended; receiver checks for zero remainder.

IPv4, TCP, UDP checksums – one's complement sum of 16‑bit words (including pseudo‑header for TCP/UDP).

Routing Protocols

RIP – distance‑vector, hop count metric, max 15 hops.

OSPF – link‑state, Dijkstra shortest‑path tree, area hierarchy.

BGP – path‑vector, inter‑AS routing, TCP port 179.

MPLS – label‑switched paths; ingress router pushes a label, core routers forward based on label, egress router pops the label.

Switching Types

Circuit switching – dedicated path, used in telephone networks.

Datagram switching – each packet routed independently.

Packet switching – packets are segmented from a message and routed separately.

TCP vs UDP

TCP – connection‑oriented, reliable, flow and congestion control.

UDP – connectionless, best‑effort delivery, suitable for streaming, DNS, etc.

IPv4 vs IPv6

IPv6 expands address space, simplifies header, removes checksum, adds flow label and priority.

Address representation: colon‑hex, zero‑compression, CIDR.

Transition mechanisms: dual‑stack hosts, tunneling.

P2P vs C/S

P2P – peers act as both client and server, no central authority.

C/S – distinct client requests and server provides services.

Search Engines

Full‑text search (Google, Bing, Baidu).

Directory‑based search (Yahoo, Sina, Sohu, NetEase).

CSMA/CD vs CSMA/CA

CSMA/CD – used in wired Ethernet; detects collisions after they occur.

CSMA/CA – used in Wi‑Fi; attempts to avoid collisions via backoff and RTS/CTS.

Common Attack Vectors

Worms, trojans, logic bombs, backdoors, rogue software.

Eavesdropping – intercepting traffic.

Denial‑of‑service – overwhelming a target with traffic.

Chapter 11 – Frequently Used Commands

ipconfig /all

, /displaydns, /flushdns, /release, /renew. ping -n count, -l size, -t for continuous ping. tracert – shows the path to a destination. arp -a, -d, -s – view, clear, or set static ARP entries.

Chapter 12 – Terminology Glossary

ISP – Internet Service Provider.

IXP – Internet Exchange Point.

Hub – multi‑port repeater.

LAN, MAN, WAN – local, metropolitan, and wide area networks.

WLAN – wireless LAN; VLAN – virtual LAN.

P2P – peer‑to‑peer networking; C/S – client‑server model.

CSMA/CD – carrier sense multiple access with collision detection (wired).

CSMA/CA – carrier sense multiple access with collision avoidance (wireless).

LiFi – visible‑light communication.

WiFi – IEEE 802.11 wireless LAN.

ADSL – asymmetric digital subscriber line.

HFC – hybrid fiber‑coaxial.

FTTx – fiber‑to‑the‑X (FTTH, FTTB, FTTC).

URL – uniform resource locator.

VPN – virtual private network.

IPSec – suite for securing IP traffic.

NAT – network address translation.

ICMP – Internet Control Message Protocol.

IGMP – Internet Group Management Protocol.

MSS – maximum segment size (TCP).

BGP – border gateway protocol.

AS – autonomous system.

HTTPS – HTTP over TLS/SSL.

MPLS – multi‑protocol label switching.

AP – access point; SSID – service set identifier.

AdHoc – peer‑to‑peer wireless mode.

Blockchain – distributed ledger technology.