Comprehensive Overview of Computer Network Architecture and Protocols

1. Computer Network Overview

Networks exist everywhere around us, from telephone and television networks to computer networks, and even internal biological networks such as the nervous and digestive systems. The most typical example is the computer network, which combines computer technology and communication technology.

2. Network Classification

According to scope: WAN (Wide Area Network), MAN (Metropolitan Area Network), LAN (Local Area Network). According to users: public networks and private networks.

3. Layered Structure of Computer Networks

The TCP/IP four‑layer model and the OSI seven‑layer model are compared.

3.1 Design Principles of Layered Architecture

Each layer is independent of the others.

Each layer must have sufficient flexibility.

Layers are completely decoupled.

3.2 Performance Indicators

Rate (bps), delay (transmission, propagation, queuing, processing), round‑trip time (RTT).

4. Physical Layer

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

Repeater (also called amplifier): regenerates signals within the same LAN; both ports must use the same protocol; follows the 5‑4‑3 rule in 10BASE‑5 Ethernet.

Hub: a multi‑port repeater, operates half‑duplex, cannot isolate collision or broadcast domains.

Basic concepts of channels: a channel carries information in one direction; a communication circuit contains a sending channel and a receiving channel.

Simplex channel – one‑way communication.

Half‑duplex channel – two‑way but not simultaneous.

Full‑duplex channel – two‑way simultaneous communication.

5. Data Link Layer

5.1 Overview

The data link layer provides reliable transmission of network‑layer data to adjacent nodes, handling physical addressing, framing, flow control, error detection, and retransmission.

5.2 Error Detection

Parity check (limited to single‑bit errors) and CRC (Cyclic Redundancy Check).

5.3 Maximum Transmission Unit (MTU)

Frames have a size limit defined by MTU; the path MTU is determined by the smallest MTU along the route.

5.4 Ethernet Protocol Details

Each device has a unique 48‑bit MAC address expressed in hexadecimal. Ethernet (IEEE 802.3) is a widely used LAN technology offering high data rates and low hardware cost.

Type field – identifies the upper‑layer protocol (2 bytes).

Destination and source MAC addresses – each 6 bytes.

Data – encapsulated upper‑layer packet (46‑1500 bytes).

CRC – 4‑byte cyclic redundancy check.

Minimum Ethernet frame – 64 bytes (including 18‑byte header, 46‑byte minimum payload).

6. Network Layer

The network layer provides end‑to‑end data transport, handling addressing and routing. The core protocol is IP, which offers an unreliable, connectionless service.

IP (Internet Protocol)

ICMP (Internet Control Message Protocol)

ARP (Address Resolution Protocol)

RARP (Reverse ARP)

Routers are the primary devices at this layer.

6.1 IP Protocol Details

IP creates a virtual interconnection that hides physical network differences. Key fields include version (IPv4/IPv6), header length, total length, TTL, and protocol identifier (e.g., TCP, UDP).

6.2 IP Forwarding Process

6.3 Subnetting

Class A (8‑bit network, 24‑bit host), Class B (16‑bit network, 16‑bit host), Class C (24‑bit network, 8‑bit host); Class D for multicast, Class E reserved.

6.4 NAT (Network Address Translation)

NAT allows multiple private hosts to share a single public IP, reducing address consumption but adding complexity. It rewrites source addresses for outbound packets and restores destination addresses for inbound packets using a translation table.

6.5 ARP and RARP

ARP maps a 32‑bit IP address to a 48‑bit MAC address dynamically; RARP performs the reverse mapping.

6.6 ICMP Details

ICMP reports errors and abnormal conditions; its messages are encapsulated within IP datagrams.

Common applications: Ping for fault diagnosis and Traceroute for path discovery.

7. Transport Layer

The transport layer segments upper‑layer data and provides reliable (TCP) or unreliable (UDP) end‑to‑end delivery, handling flow control, error control, and congestion control.

7.1 UDP Details

UDP is a simple, connectionless, datagram‑oriented protocol with minimal header overhead and no congestion control.

Header (8 bytes): Source Port | Destination Port | Length | Checksum.

7.2 TCP Details

TCP provides reliable, connection‑oriented byte‑stream transmission with flow control, congestion control, and error detection.

Key functions include segmentation/reassembly, flow control via sliding window, congestion control (slow start, congestion avoidance, fast retransmit, fast recovery), and three‑way handshake for connection establishment.

First handshake: client sends SYN (seq=x).

Second handshake: server replies with SYN+ACK (seq=y, ack=x+1).

Third handshake: client sends ACK (seq=x+1, ack=y+1).

7.3 Connection Termination (Four‑Way Handshake)

First: client sends FIN (seq=x).

Second: server ACKs (ACK, ack=x+1).

Third: server sends FIN (seq=y).

Fourth: client ACKs (ACK, ack=y+1).

8. Application Layer

The application layer provides interfaces for operating systems and network applications. Main protocols include FTP, Telnet, DNS, SMTP, POP3, and HTTP/HTTPS.

8.1 DNS Details

DNS maps human‑readable domain names to IP addresses. Resolution order: browser cache → hosts file → router cache → DNS servers (iterative/recursive).

8.2 DHCP Details

DHCP automatically assigns IP addresses to hosts on a LAN using UDP.

8.3 HTTP and HTTPS Details

HTTP is a reliable, TCP‑based protocol for transferring hypertext. HTTPS adds SSL/TLS encryption on port 443 for confidentiality and integrity.

End of article.