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  1. agreement

    The rules, standards, or conventions established for the exchange of == data in a network are called network protocols, which are horizontal collections of rules governing the communication between two (or more) peer entities. There is no agreement between unequal entities

    The protocol consists of syntax, semantics, and synchronization:

    1. The syntax specifies the format of the data to be transmitted
    2. The semantics specify the functionality to be accomplished
    3. Synchronization specifies the order in which operations are performed
  2. interface

    Interface is the connection point of information exchange between two adjacent layers in the same node, and it is an internal regulation of a system. Each layer can define interfaces only between adjacent layers, not across layers. On a typical interface, entities at two adjacent layers of the same node interact through a service access point (SAP). Services are provided to the upper layer through SAP, where layer N +1 can access layer N services

  3. service

    A service is a function call provided by the lower layer to the immediate upper layer and is vertical

    Note: “Protocol” is “horizontal”, i.e. protocol is the rule that controls communication between peer entities

    The service is “vertical”, that is, the service is provided to the upper layer through the next layer interface

    Only functions that are “visible” to a higher level of entity are called services

    The services provided by computer networks can be divided into three categories:

    1. Connection-oriented and connectionless services

      Connection-oriented: Connection-oriented is divided into three phases. The first is the establishment of a connection, in which a request is made to establish a connection. Only after the connection is successfully established can the data transfer begin, which is the second stage. When the data transfer is complete, the connection must be released. Such as TCP

      No connection: Data is transferred directly, which does not ensure communication reliability. Such as IP, UDP

    2. Reliable and unreliable services

      Reliable service: the network has error correction, error detection, and response mechanisms to ensure the correct and reliable transmission of data to the destination

      Unreliable service: it means that the network only tries its best to transmit data correctly and reliably, but cannot guarantee that the data can be delivered correctly and reliably to the destination

      Note: For unreliable services, users can take measures to change unreliable services into reliable services

    3. Answer service and no answer service

      Responder service: the receiver gives corresponding response to the sender after receiving the data. The response is automatically realized by the transmission system, not by users

      No reply service: the receiver does not automatically reply the data received.