网络课件第一章讲解.pptVIP

* 1.3 Network Software 1.3.1 Protocol Hierarchies Figure 1-13. Layers, protocols, and interfaces. * 1.3 Network Software ● a protocol is an agreement between the communicating parties on how communication is to proceed. ● The entities comprising the corresponding layers on different machines are called peers. ● To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. * ● A set of layers and protocols is called a network architecture. ● Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services the lower layer makes available to the upper one. 1.3 Network Software ● A list of protocols used by a certain system, one protocol per layer, is called a protocol stack. * Figure 1-15. Example information flow supporting virtual communication in layer 5. 1.3 Network Software * 1.3.2 Design Issues for the Layers ●addressing ● error control ● flow control ● multiplexing and demultiplexing ● routing * 1.3.3 Connection-Oriented and Connectionless Services ● Connection-oriented: telephone system ① first establish a connection ② do a lot communication ③ release the connection ● Connectionless: The postal model: your data is put into some kind of envelope on which the destination address has been written. The envelope + contents is sent to the destination, and that’s it. ● Each service can be characterized by a quality of service. * Figure 1-16. Six different types of service. 1.3.3 Connection-Oriented and Connectionless Services * 1.3.4 Service Primitives A service is formally specified by a set of primitives (operations) available to a user process to access the service. Figure 1-17. Five service primitives for implementing a simple connection-oriented service. * 1.3.5 The Relationship of Services to Protocols A service is a set of primitives (operations) that a layer provides to the layer above it. the lower layer