TCP/IP的五层层次模型

Application layer Handles information transfer between two network applications, including functions such as security checks, identification of the participating machines, and initiation of the data exchange.

Presentation layer Handles data formatting, including issues such as whether lines end in carriage return/line feed (CR/LF) or just carriage return (CR), whether data is to be compressed or encrypted, and so forth.

Session layer Manages the connection between cooperating applications, including high-level synchronization and monitoring of which application is "talking" and which is "listening."

Transport layer On the client, this layer divides messages into packets and assigns them sequence numbers to ensure that they are all received in the proper order. On the destination, it assembles packets that have been received. It also shields the session layer from the effects of changes in hardware.

Network layer Creates packet headers and handles routing, congestion control, and internetworking. It is the highest layer that understands the network's topology—that is, the physical configuration of the machines in the network, any limitations in bandwidth, and so on.