What is the difference between agreement and service?

Explain a concept first ： Entity ！

Entity (entity)： Any hardware or software process that can send or receive information , An entity is a specific software module .

Understanding agreements and services

1、 agreement Is to control two peer entities （ Or multiple entities ） Of the communication rules aggregate .
The agreement has three elements ：
（1） grammar ： That is, the format of data and control information .
（2） semantics ： The operation to be completed by the sender or receiver , That is, what kind of control information needs to be sent , What actions to complete and what responses to make
（3） Sync ： That is, a detailed description of the sequence of event implementation
Under the control of the protocol , The communication between two peer entities enables this layer to provide services to the upper layer . To implement this layer protocol , You also need to use the services provided by the following layer .
2、 service It's vertical , Provide services from the bottom to the top .
3、 The difference between agreement and service ：
（1） The implementation of the protocol ensures that it can provide services like the upper layer . The service users of this layer can only see the service, but not the following protocol . The following protocol is transparent to the above service users .
（2） Agreement is “ Horizontal ”, That is, the protocol controls two peer entities （ Between peers ） The rules of communication .
But the service is “ Vertical ”, That is, service is The lower Through interlayer interface Up to the top Provided . The upper layer uses the services provided by the lower layer by exchanging some commands with the lower layer , These orders are in OSI Called service primitives in .