OSI Reference Model and various layer functions, the difference between TCP and UDP

The OSI Reference Model: The ISO/IEC 7498 standard defines a 7-tier architecture model for network interconnection, the Open Systems Interconnect Reference Model.

The OSI Reference Model defines the hierarchical structure of open systems, the interrelationships between hierarchies, and the possible services that are included in each layer. The OSI service definition provides a detailed description of the services provided by each layer, but does not address the implementation of the interface. The OSI Reference Model is not a standard , but a conceptual framework used in the development of standards.

The 1 Physical layer is the lowest level of the OSI reference Model. The physical layer uses the transmission medium to establish, manage and release the physical connection between the communication hosts, realizes the transparent transmission of the bitstream, and provides the data transmission service for the link layer. The data transmission unit of the physical layer is bit.

2 Data Link Layer
1) The bottom of the data link layer is the physical layer, and the adjacent upper layers are the network layer.
2) The Data link layer provides a bit-stream transmission based on the physical layer, by establishing the data
Link connection, the use of error control and flow control methods, so that the wrong physical lines into error-free data link. 3) Data transmission Unit is the frame of the link layer.

3 Network Layer
1) The network layer adjacent to the bottom is the data link layer, high-level is the transport layer.

2) The network layer chooses the appropriate transmission through the communication subnet through the routing algorithm for the packet
Path for traffic control, congestion control, and network interconnection. 3) The data transmission unit of the network layer is grouped.

4 Transport Layer
1) The transport layer is adjacent to the network layer, the upper level is the session layer.
2) The Transport layer provides a reliable end for process communication distributed over different geo-location computers
-End connection and data transfer services.
3) The transport Layer shields the details of the underlying data communication to the upper layers. 4) The transmission layer of the data transmission unit is a message.

5 Session Layer
1) The next level of the conversation layer is the transport layer, and the upper level is the presentation layer.
2) The session layer is responsible for maintaining the establishment, management, and termination of the connection between the two session hosts.
And the exchange of data.

6 Presentation Layer
1) The underlying layer adjacent to the presentation is the session layer, and the upper level is the application layer.
2) The presentation layer is responsible for data format transformation between communication systems, data encryption and decryption,
Data compression and recovery.

7 Application Layer
1) The application layer is the highest level of the reference Model
2) Application layer implementation of communication process control between applications that work together

The difference between TCP and UDP

TCP is like a telephone, the two must have an uninterrupted path, the data can not reach each other, the other side has been waiting, unless the other side directly hung up, first said first, after said after to, there is Order.

UDP is like sending a letter, the sender just sends, regardless of. But you must specify the address of the other person on your envelope. There is no access between the sender and the addressee, by the post office. It may have been a long time since the letter was sent, it may not have arrived at all, and the post is not necessarily after.

TCP is a Transmission Control protocol that provides a connection-oriented, reliable byte-stream service. Before customers can exchange data with the server, a TCP connection must be established between the two parties before the data is transferred. TCP provides timeouts, discards duplicate data, verifies data, and traffic control to ensure that data can be from one end to the other.

UDP is a User Datagram protocol and a simple transport layer protocol for datagram. UDP does not provide reliability, it simply sends the application to the IP layer of data sent out, but does not guarantee that they can reach the destination. Since UDP does not have to establish a connection between the client and the server before transmission, and there is no time-out retransmission mechanism, the transmission speed is fast.

OSI Reference Model and various layer functions, the difference between TCP and UDP