Deng Geqi case of mind - first met five network model

　　We all often hear Http protocol, TCP / IP protocol, UDP protocol and so many agreements, these are some of both familiar and unfamiliar words, many students do not understand these protocols is doing what? Tasty? No agreement okay? These agreements What is the difference? Today we'll talk about this topic more interesting - for the content, to talk about the people around me - Deng brother.
　　Brother Deng as saying his life, stable long period waves, the waves will be stable for a long time. One day, you as brother to brother Deng introduced a girlfriend named: Zhao hammer - Deng brother although usually smoking, drinking hot head, but the character is quite introverted - so decided exchanged letters -
　　this time Deng brother and hammer sister is equivalent to two or two software programs, Deng brother wanted to hammer sister messenger, as if two or software programs to communicate, if we took the communication between the browser and the server, for example, then Deng Costa Rica is the equivalent of the browser, is the equivalent of a hammer sister program on the server, we call these programs or software application layer
　　so thought Tong Deng brother got to deliver the letter, but as a brother to worry about thinking the letter sent to the wrong Tong people, so advance for a signal to hammer Deng brother and sister.
　　This signal is like an application layer protocol, such as: Http agreement, of course, there are many application layer protocols, including DNS protocol (DNS), FTP protocol (file transfer), SMTP protocol (Message Transfer), etc.
　　sent to the wrong if the messenger the place, thinking Tong sent the letter to her sister's next door neighbor hammer Pharaoh, saying the last part of the signal Si Tong: "the king cover ground tiger." The other side will feel this person sick mind, certainly will not close. Only able to recognize the cypher of the people, will close this letter.
　　Http protocol like to recognize the program to interpret the data content Http protocol in the transmission
　　then the letter to Deng brother Tong think the process needs to be considered - transmission of letters Deng brother this time.
　　At this time of the establishment and management of data transmission connections in the world of the network transport layer we call
　　this time Deng brother may have many options, you can move a small Mazar sat at the door, etc., can also eat and so on, but also side can sleep while waiting, or simply unequal.
　　Each of these approaches are a way to call it a protocol, which is a transport layer protocol
　　we just give two examples. Whether one is believed to have wood there, now go to sleep. After all, great age to maintenance. Even thought the messenger halfway Tong in the letter was lost, no matter ~
　　in this way in the world network, which we call: UDP protocol, UDP protocol is to do its utmost to ensure that the data, but does not guarantee that the data will be able to to the. We often use Ping command to test whether the communication between the host, the principle is the UDP protocol. In this way the letter messenger lost the case, what we call in the network: packet loss
　　Another way is to move a little brother Deng Mazar, sitting in the doorway waiting for a reply hammer, etc. ~ sister's, if Si Tong in halfway the letter was lost, Deng brother to write another one sent in the past.
　　In this way, the network, which we call: TCP protocol, TCP protocol will ensure the correctness, will ensure that the data sequence data.
　　Deng chose to move a little brother Mazar waiting in two ways, with the TV drama GEEK-like smile.
　　After thinking Tong handed the letter to find a good line, the letter sent in the past. So I opened the XX map, Q: Who is the most handsome man in the world -? Map: handsome so many years, do not you tired? After (are interested can go look at the map tease ~) Si Tong got inner satisfaction, began looking to hammer home the most suitable route sister. Deng 601 Columbia Street pests living in the chamber 138, the hammer sister lives flowery Street, Room 502 100.
　　Here are the equivalent of each building housing connected to a computer network, each house are the equivalent of a port on the computer, and network traffic is like a network computer world. In the world of the network, which we call the network layer. In the real world we have to find a person's address we need is the address and house number, address and house number has a certain format, this format analogy to the online world is a kind of agreement, which we call the network layer protocol. We usually use IP plus port to locate the position of a program in the network, this approach is a kind of network layer protocol, IP protocol we call
　　Sitong find the best route on the map, to first to A, then to B, then C, the final hammer to his sister's house.
　　We call this layer the data link layer, a data link layer which it is divided into two portions or sub-called two layers, a map is displayed on the route we call: Logical Link Control sublayer, another route is the real world, we call: MAC sublayer, which I am sure you are familiar with the abbreviation layer --MAC, MAC address is often said that
　　in the actual process of messengers, the bus needs to be done , subway, bicycle riding, etc. share, needs the bus, bus cards, the rider needs to scan the sub-code.
　　This level in the network we call the physical layer, physical layer protocol there are many, we will not list them -
　　and now this letter has to think from Tong Deng brother from the hands of the hands, thinking Tong also take vehicles on the way messenger. After thinking Tong car station to get off, and then locate the address hammers his sister's house and the house number, then knocked on the door, and a hammer to signal his sister.
　　This process is the process from the application layer to the physical layer. However, when the data to the other computer is a process from the physical to the application layer, after arriving by car, the first under the bus, that is, from the physical layer to the data link layer, which is thought Tong has been in the data link layer the last link in the. And then find the corresponding floor door according to house number, which is the data link layer to the network layer process. After finding the door, knock on the door of his sister's house a hammer, which is the network layer to the transport layer process, and then hammer sister to sign, which is the process of transport layer to the application layer of
　　good will - So far we have five network the basic structure of the model finished, we need to remember, five network models are: application layer, transport layer, network layer, data link layer and physical layer.
　　Five-layer network model during data transmission, first at the application layer A transport layer to the network layer to the data link layer to the physical layer, the data to be transported at B, and then from the physical layer to the data at the B link layer to the network layer to the transport layer to the application layer. Each protocol layer essentially sets out a special data format. So when the data is sent, you need to disassemble the layers of the agreement before you can get the data inside.
　　This process is like when we go out first and then wear socks shoes, take off your shoes after a home first and then the socks off the same. We could not remove their shoes socks directly ~ So when we message to the destination, the protocol layers is removed, to remove the outermost first physical layer protocol, and then gradually removed layer by layer, and finally removing the application layer protocol.
　　In addition to the five-layer network model there are seven layer network model, the seven-layer network model, what is it? The application layer is divided into three, namely: the application layer, presentation layer, and session layer. Out of the new session layer and the presentation layer is no corresponding protocol. The presentation layer is generally do data formatting, encryption or other operations. Deng brother would say the letter, afraid of being peek, then use Oracle to write, so afraid of other people to read. It is said that the situation layer to do.