01. HCLA review

OSI Reference Model (Open Systems Interconnection Reference Model) – OSI/RM

• The core idea of ​​the OSI reference model - layering

Application layer—Provides various application services and converts abstract language into code

Presentation layer - encoding converted to binary

Session layer — maintains a session connection between the web application and the web server

Transport layer - use port numbers to realize end-to-end transmission to distinguish different services

​ The port number (0-65535) is divided into:

1. Static port number (1-1023):

​ A port number represents a service and has a permanent binding relationship

​ http： 80 8080

​ https：443

2. Dynamic port number (1024-65535):

​ A port number represents a service, and has a temporary binding relationship

​0 - special port number - reserved port number (used in network programming to represent all port numbers)

Transfer Protocol

• reliable transmission

  ​ Reliable Mechanism: Determine Retransmission Sorting Flow Control

• TCP - Transmission Control Protocol

  ​ Connection-oriented reliable transport protocol

• UDP - User Datagram Protocol

  ​ Non-connection-oriented unreliable transport protocol

Connection-oriented - three-way handshake, four-way wave

three handshake

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waved four times

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FIN – request to disconnect

ACK – acknowledgment

Network layer - logical addressing by IP address

IP address is a logical address

​IPV4 address

​IPV6 address

The method of obtaining the target IP address:

1. Directly know the IP address of the other party

2. Obtain IP address through domain name

3. Direct access through the app

4. Broadcasting

Data Link Layer—Physical addressing through MAC addresses

The MAC address consists of 48 binary bits. The serial numbers programmed into all chips when they leave the factory.

MAC address features:

1. Unique in the world

2. Unified format

The method of obtaining the target MAC address - ARP protocol - Address Resolution Protocol - Obtaining another address through one address

ARP

working principle:

​ First, the host sends an ARP request packet in the form of broadcast. Request MAC address by IP address. Because it is a broadcast frame, all devices in the broadcast domain can receive this request. After receiving it, the device first records the correspondence between the source IP and the source MAC address in the data packet in the local ARP cache table, and then looks at the requested IP address. If the request is not its own local IP address, it will be directly discarded; if the request is its own local MAC address, it will respond with ARP. Afterwards, when sending data, it will first check the local ARP cache table, if there is a record in the table, it will directly send according to the record. If there is no record, then send an ARP request to obtain the MAC address.

Classification:

Forward ARP: Obtain MAC address through IP address

Reverse ARP: Obtain IP address through MAC address.

Free ARP: 1, detect address conflict; 2, self-introduction

Physical layer - processing and transmission of electrical signals

TCP/IP Reference Model - Protocol Suite

TCP/IP Reference Model

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TCP/IP Standard Model

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PUD — Protocol Data Unit

• Application Layer – Datagrams
• Transport Layer – Data Segment
• Network Layer – Packets
• Data Link Layer – Data Frame
• Physical layer – bitstream

Encapsulation and Decapsulation

Encapsulation – the process of adding data at each layer in order to achieve the purpose of that layer

• Application Layer – Encapsulation at the application layer depends on the protocols working at the application layer
• Transport layer – encapsulation port number (TCP, UDP)
• Network layer – encapsulates IP address (IP protocol)
• Data Link Layer – Encapsulating MAC Address (Ethernet Protocol)
• physical layer

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cross-layer encapsulation

TCP/IP can support cross-layer encapsulation, OSI does not support it (cross-layer encapsulation occurs only when directly connected devices are transmitted in close distance)

• Encapsulation Across Four Layers – OSPF – 89
• Encapsulation across three and four layers – STP (IEEE 802.3)

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cross-layer encapsulation

TCP/IP can support cross-layer encapsulation, OSI does not support it (cross-layer encapsulation occurs only when directly connected devices are transmitted in close distance)

• Encapsulation Across Four Layers – OSPF – 89
• Encapsulation across three and four layers – STP (IEEE 802.3)