Specific introduction
Indicates Layer Located on the sixth floor of the OSI hierarchy, one of its main functions is to provide a common language for herself. In order to perform interoperability. This type of service is required because the data representation of different computer architecture is different. Differentiated with transparent data from the fifth layer, the representation layer is to process all problems associated with data representation and transportation, including conversion, encryption, and compression. Each computer may have its own indicated internal method, such as the ASCII code and the EBCDIC code, so it is necessary to indicate layer agreement to ensure that different computers can be understood from each other.
, for example, , the IBM host uses EBCDIC encoding, and most of the PCs use ASCII code. In this case, it is necessary to represent a layer to complete this conversion. If you want to remember this 6th floor with as little words, it is "a universal data format".
Representation The service provided by the application layer has three contents
syntax conversion: syntax conversion involves the conversion of code conversion and character set, modification of data format, adaptation of data structure operations , Data compression, data encryption, etc.
Syntax Selection: Syntax Selection is a syntax that provides an initial selection and subsequently modifying this option.
Database: Use the service established by the service provided by the session layer to manage data transport and synchronization control over this coupled, and normal or abnormal termination of the connection.
actual function
representing the function of the layer
network security and confidentiality management; text compression and packaging; Virtual Terminal Protocol (VTP).
Syntax Conversion: Convert abstract syntax into transfer syntax, and implement the opposite conversion in the other party (the transfer syntax is converted into an abstract syntax). The content involved has code conversion, character conversion, data format modification, and adaptation, data compression, encryption, and so on.
Syntax Negotiation: The appropriate context is determined by applying the appropriate context according to the requirements of the application layer, ie the transfer syntax is determined.
Connection Management: Including the establishment of a session layer service to establish a connection to manage data transport and synchronization control over this connection (using the session layer corresponding to the service), and terminate this connection normally or abnormally.
Through the previous introduction, we can see that the 5th floor below the session has completed the end-to-end data transfer, and is reliable, there is no error transfer. But the data transfer is only means of means, and it is ultimately. To achieve the use of data. Since the various systems are not exactly the same as the definition of the data, the most readily understood examples are keyboards, and the meaning of certain keys on it is different in many systems. This naturally utilizes other systems The data has caused obstacles. The representation layer and the application layer are responsible for eliminating this obstacle.
For user data, it can be analyzed from both sides, one is the data meaning called semantics. The other is the representation of the data, called the grammatical. Like text, graphics, sound, variety, compression, encryption, etc. It belongs to the scope of grammar. The layer has designed three 15 functional units, where the context management function unit is communication. The data coding rules between the users so that the two sides have a consistent data form, which can know each other.
OSI representation layer
It has set DP8822, DP8823 for services, protocols, and text communications , DIS6937 / 2 and other series of criteria. Represents a layer of translation between the layer, and the application and the network, mainly solving the syntax representation of user information, that is, provides formatted representation and conversion data services. The compression, decompression, encryption, and decryption are completed in this layer.
Working principle
In the representation layer, the data will be formatted in accordance with the scheme that the network can understand; this format is different from the type of network used. Represents the decryption and encryption of layer management data, such as system password processing If you query your bank account on the Internet, use is a secure connection. Your account data is encrypted before the transmission, the other end of the network, the representation layer will decrypt the received data. In addition, the representation layer protocol is also decoded and encoded to the image and file format information.
encryption is divided into link encryption and end-to-end encryption. For the representation of the layer, the participated encryption is an end-to-end encryption, and the information is automatically encrypted by the sender and enters the TCP / IP packet package, and then enter the Internet as unread and unrecognized data. After arriving at the destination, it is automatically fully decrypted to become readable data. End-to-end encryption faces the high-level body of the network, not for information encryption, the protocol information is transmitted in a clear text, and the user data does not need to decrypt in the central node.