Five layer network architecture

The five layer network architecture is divided into ： application layer 、 Transport layer 、 The network layer 、 Data link layer 、 The physical layer . The functions of each layer are as follows ：

1. The fifth floor —— application layer (application layer)

   (1) application layer (application layer)： Is the highest in the architecture . Directly provide services for the user's application process .

   (2) There are many application layer protocols in the Internet , For example, it supports World Wide Web applications HTTP agreement , Email enabled SMTP agreement , Supporting file transfer FTP Protocol, etc .

2. The fourth level —— Transport layer (transport layer)

   (1) Transport layer (transport layer)： Responsible for providing services for communication between processes in two hosts . Because a host can run multiple processes at the same time , So the transport layer has Reuse and Divide up The function of .

   a. Reuse , It means that multiple application layer processes can use the services of the following transport layer at the same time .

   b. Divide up , It is to deliver the received information to the corresponding processes in the above application layer .

   (2) The transport layer mainly uses the following two protocols ： **(1) Transmission control protocol TCP(Transmission Control Protocol)：** Connection oriented , The unit of data transmission is message segment , Able to provide reliable delivery . **(2) User packet protocol UDP(User Datagram Protocol)：** No connection , The unit of data transmission is user datagram , There is no guarantee of reliable delivery , Only available “ Do your best to deliver ”.

3. The third level —— The network layer (network layer)

   The network layer (network layer) It mainly includes the following two tasks ：

   (1) Responsible for providing communication services for different hosts in packet switching network . When sending data , The network layer transmits the remaining message segments or user datagrams in the transport layer . stay TCP/IP In the system , Because the network layer uses IP agreement , So grouping is also called IP The datagram , Or simply datagram .

   (2) Choose the right route , Group that causes the source host transport layer to be passed down , Can find the destination host through the router in the network .

4. The second floor —— Data link layer (data link layer)

   ** Data link layer (data link layer)：** Often referred to as the link layer , We know , Data transfer between two hosts , It's always transmitted over a link , in other words , Data transfer between two adjacent nodes is direct ( Point to point ), At this time, we need to use a special link layer protocol .

   When transferring data between two adjacent nodes , The data link layer hands over the network layer IP Datagram assembly into frame (framing), On the link between two adjacent nodes “ transparent ” To transmit data in a frame .

   Each frame contains the data and the necessary control information ( Such as synchronous information 、 Address information 、 Error control, etc ). Typical frame length is hundreds of bytes to more than one thousand bytes .

   Be careful ：” transparent ” It's a very important term . It said , Something that actually exists appears as if it doesn't exist .” Transfer data transparently at the data link layer ” It means that any bit combination data that is unable to pass through this data link layer . therefore , For the data being transmitted , These data are “ Can't see ” Data link layer . Or say , The data link layer is transparent to this data . (1) When receiving data , The control information enables the receiver to know which bit a frame starts from and ends at . such , After receiving a frame, the data link layer , You can extract the data from it , Hand over to the network layer . (2) The control information also enables the receiver to detect whether there is any error in the received frame . If there is a mistake , The data link layer simply discards the frame in error , In order to avoid further transmission waste of network resources . If you need to correct your mistakes , It's on the transport floor TCP Agreement to complete .

5. first floor —— The physical layer (physical layer)

   ** The physical layer (physical layer)：** The unit of data transmitted on the physical layer is bit . The task of the physical layer is to transparently transmit bitstreams .