Key Concepts of Computer Networks – Fundamentals, Protocols and Layered Architecture

The author revisits a previously published list of 89 core networking concepts, re‑grouping them by chapter to make the material easier to follow.

Basic Network Concepts

主机 (host) refers to any device that can connect to a network, also called an 端系统 (end system), such as phones, tablets, TVs, game consoles or cars.

通信链路 (communication link) is a physical path formed by cables (coaxial, twisted‑pair, fiber) that interconnect devices.

传输速率 (transfer rate) measured in bit/s quantifies how fast data moves from one end system to another.

分组 (packet) is a data fragment created by a sending end system, each with a header, that traverses the network.

转发表 (forwarding table) stores the mapping of packet destinations inside a router.

路由器 (router) connects LANs and WANs, maintains a routing table and selects the optimal path for packets at the network layer.

交换机 (switch) operates at the data‑link layer, providing dedicated full‑duplex links between two nodes.

集线器 (hub) is a physical‑layer device that broadcasts incoming frames to all ports; it works in half‑duplex mode and is largely obsolete.

半双工模式 (half‑duplex) allows only one device to transmit at a time, while 全双工模式 (full‑duplex) permits simultaneous two‑way communication.

路径 (path) is the sequence of links and switches a packet traverses.

因特网服务商 (ISP) are the three major Chinese operators: China Mobile, China Telecom, China Unicom.

网络协议 (network protocol) defines the rules for data exchange.

IP (Internet Protocol) specifies the packet format used between routers and end systems.

TCP/IP 协议簇 (TCP/IP suite) includes TCP, IP, ICMP, ARP, UDP, DNS, SMTP, etc.

丢包 (packet loss) describes the phenomenon where a packet disappears in transit.

吞吐量 (throughput) is the amount of data successfully transferred per unit time.

报文 (message) usually refers to an application‑layer packet; 报文段 (segment) is a transport‑layer packet; 数据报 (datagram) is a network‑layer packet; 帧 (frame) is a link‑layer packet.

电路交换 (circuit switching) reserves a dedicated channel for the whole session, offering reliability but low utilization.

报文交换 (message switching) stores the whole message at an intermediate node before forwarding.

分组交换 (packet switching) forwards packets independently without reserving a channel, achieving higher resource efficiency.

带宽 (bandwidth) denotes the maximum data rate of a link, usually expressed in bps.

频分复用 (frequency division multiplexing) and 时分复用 (time division multiplexing) are multiplexing techniques for analog and digital signals respectively.

时延 (delay) = transmission + propagation + processing + queuing delays.

单播 (unicast), 广播 (broadcast) and 多播 (multicast) describe one‑to‑one, one‑to‑all and one‑to‑many delivery modes.

Application Layer

应用程序体系结构 (application architecture) includes Client‑Server (CS) and Peer‑to‑Peer (P2P).

客户‑服务体系 (client‑server model) separates clients that request services from servers that provide them; typical server is a Web 服务器 serving browsers.

P2P 体系 (P2P) eliminates a central server; each peer can act as both client and server.

Key concepts: 进程 (process), 分布式应用程序 (distributed application), 套接字接口 (socket API), 客户端 (client), 服务器 (server), IP 地址 (IP address), 端口号 (port number), URI , URL .

Web‑related terms: HTML , Web 页面 (Web page), Web 服务器 , CDN (Content Delivery Network) and its variants (专用 CDN, 第三方 CDN), WAF (Web Application Firewall), WebService , HTTP , Session , Cookie , SMTP , POP3 , DNS .

Transport Layer

可靠数据传输 (reliable data transfer) guarantees error‑free delivery.

非持续连接 (non‑persistent) opens a new TCP connection for each request; 持续连接 (persistent) reuses the same connection.

传输控制协议 (TCP) provides a reliable, connection‑oriented service with mechanisms such as three‑way handshake, flow control (sliding window), congestion control, fast retransmit, selective acknowledgment, and four‑way termination.

用户数据报协议 (UDP) offers connectionless, best‑effort delivery.

Important TCP fields and flags: SYN , ACK , FIN , MSS (Maximum Segment Size), MTU (Maximum Transmission Unit), 冗余 ACK , 快速重传 , 选择确认 , 滑动窗口 , 发送缓存 , 接收缓存 .

State diagrams for the three‑way handshake (LISTEN, SYN‑SEND, SYN‑RECEIVED, ESTABLISHED) and four‑way termination (FIN‑WAIT‑1, FIN‑WAIT‑2, CLOSE‑WAIT, CLOSING, LAST‑ACK, TIME‑WAIT, CLOSED).

Network Layer

路由选择算法 (routing algorithm) decides the path a packet takes.

转发 (forwarding) moves a packet from an input link to the appropriate output link.

Routing techniques: IPv4 (32‑bit addresses, connectionless), IPv6 (128‑bit addresses, larger address space).

Address resolution: ARP (maps IP to MAC), RARP (maps MAC to IP), 代理 ARP (proxy ARP).

Control protocols: ICMP (error messages), DHCP (dynamic host configuration), NAT (Network Address Translation) with its conversion table and NAT traversal.

Routing protocols: OSPF (link‑state), BGP (inter‑domain), IGP , EGP , RIP (distance‑vector).

Data Link & Physical Layers

Key devices: 节点 (node), 链路 (link), MAC 协议 (Media Access Control), 以太网 (Ethernet), VLAN (Virtual LAN), 基站 (base station).

Error detection/correction: 奇偶校验位 (parity), 校验和 (checksum), 循环冗余检测 (CRC), 向前纠错 (FEC).

Medium access methods: CSMA/CD (collision detection), 共享介质型网络 (shared‑medium), 非共享介质型网络 (non‑shared), 令牌环 (token ring).

Switching functions: 过滤 (filtering), 转发 (forwarding), 交换机表 (switching table), MPLS (Multiprotocol Label Switching).

Network Security

Four security goals: confidentiality, integrity, endpoint authentication, and operational security.

Cryptographic basics: plaintext, ciphertext, encryption/decryption algorithms, keys, symmetric encryption, block ciphers, stream ciphers, public key, private key, CA (Certificate Authority).

Security devices: 防火墙 (firewall) – packet filter, stateful filter, application‑level gateway; 入侵检测系统 (IDS) – alerts on malicious traffic; 入侵防止系统 (IPS) – blocks malicious traffic.

Conclusion

The article compiles a curated index of the most important networking concepts, providing a quick‑reference guide for students and practitioners. Readers are encouraged to use it as a lookup when encountering unfamiliar terms, and the author invites feedback and sharing.