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<>1. Three elements of network protocol
* grammar ： Structure or format of data and control information .
* semantics ： What kind of control information needs to be sent , What kind of action to complete and how to respond .
* synchronization ： A detailed description of the order in which events are implemented .
<>2. Advantages and disadvantages of stratification
<>3. Architecture with five layer protocol
* application layer
The task is to complete the specified network application through the interaction between application processes . Application layer protocol defines the rules of communication and interaction between application processes . The data unit that the application layer interacts with is called message .
* Transport layer
The task is to provide common data transmission service for the communication between processes in two hosts .
The following two protocols are mainly used ：
* Transmission control protocol TCP： Provides connection oriented , Reliable data transmission service , The unit of data transmission is the segment .
* User datagram protocol UDP： Provides connectionless , Best effort data transmission service （ The reliability of data transmission is not guaranteed ）, The unit of data transmission is user datagram .
* network layer
Responsible for providing communication services for different hosts in packet switching network .
* data link layer
When data is transmitted between two adjacent nodes, the data link layer will IP It was handed over from the floor IP Datagrams are assembled into frames , Transmitting frames between two adjacent nodes . Each frame contains data and necessary control information .
The control information also enables the receiver to detect whether there is an error in the received frame , If there is a mistake, discard it .
* physical layer
The unit of data transmitted on the physical layer is bits .
be careful ： It is often mentioned now TCP/IP It does not necessarily mean the two separate agreements , It's about the whole thing TCP/IP protocol family .
<>4. The process of data transmission
* The application process data is transferred to the application layer first , Add the application layer header , Become application layer PDU（ Protocol data unit ）
* Add the head of the transport layer to the transport layer , Become transport layer message
* Add network layer header to network layer , become IP Datagram （ Or group ）
* IP Datagrams are transmitted to the data link layer , Add the head and tail of the data link layer , It becomes the data link layer frame
* The data link layer frame is then transmitted to the physical layer , The bottom physical layer carries the bit stream to the physical media .
* And then layer by layer of peeling up .
<>5. Services and agreements
* The agreement is horizontal , The protocol is the rule that controls the communication between peer entities .
* Service is Vertical , That is, services are provided from the lower layer to the upper layer through the interface between layers .
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