Comprehensive Overview of Computer Networks and Core Protocols

1. Computer Network Overview

Networks are ubiquitous, ranging from telephone and television networks to computer networks, which combine computing and communication technologies.

1.1 Network Classification

By scope: WAN, MAN, LAN

By users: public networks, private networks

1.2 Layered Architecture

The TCP/IP four‑layer model and the OSI seven‑layer model are compared, highlighting the principle that each layer should be independent, flexible, and fully decoupled.

1.3 Design Principles of Layered Architecture

Layers are independent.

Each layer provides sufficient flexibility.

Layers are completely decoupled.

1.4 Performance Metrics

Key metrics include data rate (bps), various delays (transmission, propagation, queuing, processing), and round‑trip time (RTT).

2. Physical Layer

The physical layer connects devices and transmits bit streams, providing a reliable physical medium for upper layers.

Repeater (amplifier): regenerates signals within the same LAN.

Hub: multi‑port repeater, operates half‑duplex.

Channel types: simplex, half‑duplex, and full‑duplex.

3. Data Link Layer

3.1 Overview

Provides reliable transmission over the physical layer, handling framing, flow control, error detection, and retransmission. Main services include physical address mapping, framing, and error checking.

3.2 Error Detection

Techniques: parity check (limited to single‑bit errors) and cyclic redundancy check (CRC).

3.3 Maximum Transmission Unit (MTU)

Defines the maximum frame size; the path MTU is the smallest MTU along a route.

3.4 Ethernet Protocol

Ethernet uses 48‑bit MAC addresses and provides a widely used LAN technology. Frame structure includes type, destination/source MAC, payload (46‑1500 bytes), and CRC.

4. Network Layer

Responsible for end‑to‑end packet delivery, routing, and addressing. Core protocols: IP, ICMP, ARP, RARP, and routing devices such as routers.

4.1 IP Protocol Details

IP creates a virtual inter‑connected network, handling addressing, routing, and basic error control. Header fields include version, header length, total length, TTL, and protocol identifier.

4.2 IP Forwarding Process

Illustrates how routers forward IP packets based on routing tables.

4.3 Subnetting

Describes class‑A, B, C address structures and the purpose of classes D and E.

4.4 NAT

Network Address Translation allows multiple private hosts to share a single public IP, translating addresses and maintaining a NAT table.

4.5 ARP & RARP

ARP maps IP addresses to MAC addresses; RARP performs the reverse mapping.

4.6 ICMP

ICMP reports errors and diagnostics, used by tools like ping and traceroute.

4.7 Routing Overview

Discusses routing algorithms (link‑state, distance‑vector), autonomous systems, static vs. dynamic routing, and protocols such as RIP, OSPF, and BGP.

5. Transport Layer

Provides end‑to‑end data segmentation, reliability, flow control, and congestion control.

5.1 UDP

Connectionless, low‑overhead protocol with a simple 8‑byte header (source port, destination port, length, checksum).

5.2 TCP

Connection‑oriented, reliable protocol offering segmentation, reassembly, flow control, congestion control, and error detection.

Three‑way handshake (SYN, SYN‑ACK, ACK) establishes a connection.

Four‑step termination (FIN, ACK) gracefully closes a connection.

Congestion control uses slow start, congestion avoidance, fast retransmit, and fast recovery.

第一次：客户端发送 SYN（seq=x）进入 SYN_SEND 状态

第二次：服务器回复 SYN+ACK（seq=y, ack=x+1）进入 SYN_RCVD 状态

第三次：客户端发送 ACK（seq=x+1, ack=y+1）进入 ESTABLISHED 状态

6. Application Layer

Provides network services to applications. Key protocols include DNS, DHCP, HTTP/HTTPS, FTP, Telnet, SMTP, POP3.

6.1 DNS

Translates domain names to IP addresses using a hierarchical lookup (cache → hosts file → DNS servers).

6.2 DHCP

Automatically assigns IP addresses to hosts on a LAN using UDP.

6.3 HTTP & HTTPS

HTTP is a reliable, TCP‑based protocol for web communication; HTTPS adds SSL/TLS encryption for security.

Original source: https://blog.csdn.net/Royalic/article/details/119985591