This is the first day of my participation in the Gwen Challenge in November. Check out the details: the last Gwen Challenge in 2021

This article is the author’s personal summary and paraphrase after learning “Illustrated TCPIP(5th edition). Unizhiki Grid”

What is the OSI

The OSI is the Open System Interconnection Reference Model, a conceptual Model developed by the International Organization for Standardization (ISO) as a standard framework for trying to network computers around the world.

Let’s start with an image from the book



I may still be confused, but let me give you my own understanding of each layer, which is the main content of this blog post

  • Application layer: software and application layer operation. (For example, mail sending and receiving in mail software, and mail storage)

  • Presentation layer: Focus on the specific representation of data, such as converting a computer’s “specific data format” to “standard data format common to the network”, or reverse conversion to maintain data consistency between heterogeneous models. An encoding format is appended to the header message. (Text transcoding)

  • Session layer: Information about the order of data transfer is appended to the header message to determine how to transmit. (Send 5 emails, establish one connection per email, or send 5 emails with one connection)

  • Transport Layer: This layer is really responsible for moving data across the network. Creating logical communication between the two hosts is its primary function for establishing or disconnecting connections. Confirm the data received, responsible for resend. The data to be transmitted is appended to the header message for identification.

  • Network layer: Sends data from the sending host to the receiving host in an interconnected network environment. The network layer sends the data and address information it receives from the upper layer to the data link layer for subsequent processing.

  • The data link layer and physical layer: the physical layer, data of 0, 1, converted into voltage and pulse light to the physical transmission medium, and directly connected to each other between devices using address to realize transmission (MAC address, namely physical or hardware address), will put in this layer contains the MAC address information forwarded to the first attached to the network layer data, send it to the network.

  • Note: Both the network layer and the data link layer send data to the receiver based on the target address, but the network layer is responsible for sending the whole data to the final target address, while the data link layer is only responsible for sending data within a segment.

The “header” mentioned above is similar to the request header and corresponding header in the HTTP request, which contains some data such as text code, client type and so on. The information in the “header” is added in these layers. (The flow chart of a network communication is as follows)

The sender transmits data sequentially from layer 7 and 6 to layer 1 from top to bottom, while the receiver transmits data hierarchically from layer 1 and 2 to layer 7 from bottom to top to each upper level. At each layer, data transmitted from the previous layer can be processed with “header” information that is necessary for the current layer’s protocol. Then the receiving end separates the data “head” and “content” of the received data, forwards the data to the upper layer, and finally restores the data of the sending end to its original state.

OSI versus TCP/IP

  1. OSI has seven layers, while TCP/IP simplifies the high-level protocol and reduces the level of communication to four layers, improving communication efficiency.
  2. OSI is an idealized model that defines the reference model first, then the protocol, which can be difficult to implement. TCP/IP was the first protocol to define, so it quickly became the de facto mainstream network protocol standard. (OSI is like Esperanto, TCP/IP is like English.)
  3. Communication with other computers through the Internet must comply with the TCP/IP protocol.