TCP / IP. True World Model
The last lecture we said that the OSI 7 layer model is just a theoretical model, and it only needs to guarantee 7 layers.
The function can be realized, and the actual hierarchy does not need to be divided into 7 layers. And if you really have 7 floors. So the speed of data processing
It will be much slower. In practical applications. Use the most use of the DOD model. Also become TCP / IP protocol cluster DOD model (Department of DEFANSE Model US Department of Defense) As the name suggestion, is the US defense
A network model for design. The earliest used in Arpanet. These words may speak in the first chapter of many textbooks.
However, the general textbook has not been said to the DOD model and the OSI model correspondence. Or a lot is blurred or wrong
. Here I will describe the 2 associated relationships. There are 7 floors in theosi model we know, and the DOD model is
There is only 4 layers. Below is the corresponding relationship osi dod7.application ┐ 6.presentation | -> 4. Application / process5.session ┘ 4.Transport ---> 3. Host to host3.network ---> 2. Internet2.data Link ┬-> 1. network access1.physical ┘
Since I won't take the character. So some is hard to see. In fact, the Layer 4 of the Layer 1 of the Layer 1 of the Layer 1 OSI 5.6.7 corresponds to the 4th floor of DOD. In fact, this is more easily memorable due to physical layer and data link. The layer is very close. So it is divided into one. Then the top is in turn, the top
A large piece is an application layer (processing layer) Now we have an actual model available. However, we generally describe a device or protocol
Time to use OSI's model, such as when we discuss Switch, he will say that he is a 2-story setting.
Prepare. The router is a 3-storey device, there will be some special devices, such as 3 floors, 4-layer switches.
Some are classified using the OSI model. This is not confused.
We have been heard of TCP or UDP. What is SMTP.POP3. Which of these agreements are defined?
That? The next picture will give you a very clear concept (can't be dramatically: d).
4. Application HTTP, FTP, Telnet, SNMP, SMTP, POP3, DNS, etc.
3.Host to Hosttcp, UDP
2.InetNeticmp, ARP, RARP, IP
1.Network AccessetEthernet, Fastethernet, Token Ring, etc.
Well ... this is clear. Let's take a look at the bottom to look at the first thing. Including Ethernet, fast Ethernet, and now there are a protocol, etc.
These protocols specify the number of cables. The connection method, etc. The physical layer of the physical layer. There is also the bottom layer using MAC communication
Wait.
Next is IP.ARP. These. I also said in the OSI model. Through the IP address. We are in the forward package
There is no need to know the location of the specific target. The router will naturally be forwarded according to routing tables. The last stop is slow
Slow delivery. The ultimate goal is achieved. The ARP protocol is converted between IP and MAC.
I was mentioned in the previous chapter. Because of the router, IP, the entire network really can cover the world. So this layer
It is also easy to remember if you are called Internet.
Wow.TCP, UDP is our most, he is a control network connection. In OSI
Transport layer. In the DOD is the Host to Host end. Meaning is actually the same. That is in 2
A virtual communication channel is built between the computer.
The top floor is endless. All the final application layers are here, you can even define you.
I have a type of protocol. These are all possible. Because the layer of it is provided to the developer
The above is only standardized.
TCP Baotou Source Port 16-bit Target Port 16-bit serial number 32-bit response number 32-bit TCP head length 4 bit RESERVED 6-bit control code 6-bit window size 16-bit offset 16 bit checksum 16-bit option 32 bits (can Select) So we got the smallest size of the TCP header. It is 20 bytes.
UDP Baotou Source Port 16-bit destination port 16-bit length 16-bit checksum 16 grace ... UDP package is a lot. It is true. Because UDP is non-reliable. The original intention is as fast as possible
So the packet is sent out. So the UDP protocol is very streamlined. There is a problem, it seems that there is no IP address in these heads. No IP address, where to send it
• Yes. You observe very careful. TCP and UDP's head do not have any IP information. Let's want TCP
And which layer belonging to DOD? Yes! It is a third layer. And IP is located in the second floor of the model. That is they
Although the two are associated. But it is not the same layer. An important rule of the model is. When the sender sends a data, the last layer will pass the data to the next layer.
Waiting. The package of the previous layer has become the data portion of the next layer. The receiving end accepts data. The next layer will be derived from the head information of the layer.
Then let's take a look at the actual example: If we send data AAA to another. Then the data will be added to the SMTP head.
Become [SMTP] AAA. Send it to the TCP layer. Become [TCP] [SMTP] AAA. Send it to the Internet layer
[IP] [TCP] [SMTP] AAA. Then become [Mac] [IP] [TCP] [SMTP] AAA to send to the router via ENTERNET or FASTENTERNET. The router replaces its own MAC address.
Go up. Transfer to the next level. This is a long journey. Eventually this data stream arrives at the target machine.
The target machine begins with the next layer. Remove Mac, become [IP] [TCP] [SMTP] AAA to the IP layer, grace, ratio
I am sending it to me this IP. After you go, become [TCP] [SMTP] AAA.TCP to receive the checksum, yes.
On [SMTP] AAA. Finally the SMTP protocol explained. Get aaa.
The Long March finally ended. We also sent AAA to the target machine. Everyone should understand why TCP Baotou
And there is no IP address inside the UDP header? Because IP is located below the next layer. TACP and UDP Baotou information are used as
The data segment of the IP package is transmitted.
The IP layer can no matter that much. He only took the agreement of his layer, that is, the data from the above layer added himself.
The head is passed to the next layer. Turn the data from the following layer. Transmit to the top layer.
Every layer is doing so. Perfect fit has completed the end journey of the packet.
Mac. End Address [World in Canned: TCP / IP Base Lecture] https://www.xfocus.net/bbs/index.php?act=st&f=2&t=46119
