Network configuration and usage of FreeBSD

In the late 60, the United States Department of Defense (DoD) was interested in establishing a packet-switched WAN, whose basic idea was that the local network failure would not affect the normal operation of other parts of the network by connecting dispersed networks across multiple geographies. Mainly because of the cold war, they called for the establishment of a military network that would operate normally in the event of attacks in some areas.

The Research Institute, ARPA, began the study of a network project called ARPANET, which requires communication between different computers, and different computers run different operating systems, so there is a problem of how different systems communicate with the same rules. The most important issue, therefore, is to develop standard protocols that all systems comply with, allowing applications to be communicated to applications and independent of specific computer platforms. As a result, they developed standard protocols that are independent of specific computer systems to enable communication between various computers.

Because there are multiple applications, the protocol is not one, but a set of protocols. The TCP protocol and the IP protocol are the most important of the two of these protocols that they develop, so it is common to call the entire protocol family by TCP/IP protocol, which is the current protocol standard for Internet use.

The success of TCP/IP is inseparable from BSD Unix, because TCP/IP is platform-independent, while the university's Research institute used BSD Unix primarily, so the University of California, Berkeley, first implemented TCP/IP in BSD Unix. So this technology quickly spread to the use of BSD Unix University, TCP/IP has been rapidly promoted, more and more networks use this protocol, more and more networks through this Protocol to interconnect, and gradually form the internet today.

DOD Reference Model

The reference model is a visual representation of how computers communicate, and it organizes each step required to communicate in a hierarchical manner. If one step is responsible for the specific transmission of the physical line, it is considered to belong to the lower level in the model, and conversely, if a step is to deal with a specific application, it is considered to belong to the high-level of the model, with a logical relationship between the upper and lower layers, and some intermediate levels.

The reason for dividing the communication process into different levels is because the communication process is a complex process, for users, can use WWW browsing information, can send e-mail to meet the needs, they only care about these applications, but not the specific communication process, and some other software resides in the background to deal with the necessary tasks in the communication process , such as establishing a connection, sending data, and so on, without caring about how the data is sent out; there are some hardware dedicated to the transmission of the load signal; So different hardware and software have hierarchical call relationship, which naturally forms the concept of hierarchy, and the reference model is to differentiate and define these levels.

In the reference model, the physical transfer of the data is transmitted by the application--the highest layer of the model--to the lower level, up to the transport protocol layer where the data is transmitted, through which the data is transferred to the destination computer through physical media such as cables, optical cables, microwaves, etc. When the data arrives at the destination computer, it passes up the model and eventually arrives at the application in the destination computer. This process is the physical process of transferring data.

A simple example can be used to explain this communication process. For example, the transfer of letters between friends is another easier to understand the process of communication, the process of the sender, the recipient, the Postman, Mail and so on. By sending the letter to the postman, the postman sends the letter through the mail to the other city, and the postman delivers the letter to the addressee. This allows the process to be divided hierarchically, the sender and the recipient can be regarded as a level, they use the postal service, can be regarded as the application layer; All levels of postal staff is a level, the specific processing of communication work, can be called the communication layer, and the mail can be viewed as another level, processing specific mail work, called the transport layer; This is a simple example, the specific network communication process is much more complicated.

As you can see from the diagram above, although the transfer of physical data from top to bottom, and then down to the next computer application, if you do not consider how the low-level is specifically handled, you can see the communication between each layer and the corresponding layer in the other computer. This makes it possible to have logical connection communication and data transfer between each layer and the corresponding layer of another computer. If you use the previous example of a letter of return, it means that the sender only considers the addressee, and the recipient only considers the sender, regardless of how the postman handles the letter, logically the communication between the friends.