1.5 Protocol level and service model

In the previous sections we covered many concepts, such as hosts, routers, rich link media, applications, protocols….. It’s as if these cluttered things make up the current network, but can they be organized, like a clear desktop, so that people can see at a glance? We are going to bring up today’s topic, the hierarchy of networks.

1.5.1 Protocol Layer

layeredBenefit is to reduce the complexity of the system, for the only need to direct application of the lower layer of the content, and don’t need to care about its implementation details, when the upper problems will not affect the lower level, we look back at some two hosts communicate with each other, the process of the source host sends the needed information data, through the link with all kinds of packet switches to convey to the destination host, Let’s draw a picture of itWe see five levels along the way, and the message sent at the application layer is calledmessageParsed once through the transport layerMessage segmentIt is then parsed into datagrams at the network layer and into datagrams at the link layerframeAnd finally at the physical levelBits in frameIt is sent out through a link to a route or line switch to reach the destination hostThe inverse resolutionBecomes the original packet. We see that each layer is parsed once and sent to the next layer, and each layer is parsed in what we would normally call a parsed manneragreementThis parsing is also called:encapsulationA group has different names at different levels, because when it passes through each layer, it is encapsulated with relevant information belonging to the layer, that is, the necessary information mentioned above. Thus, each hierarchical grouping has two types of fields: header fields and payloads; The payload is the packet data from the previous layer, and the header field is the necessary information added to the layer. Packets are continuously encapsulated to implement the related functions specified by the protocols of each layer, which differ from each otherNetwork protocol stack, as shown below.

1.5.2OSI 7 layer model

The OSI 7 layer model has only two more layers than the network layer, mainly to facilitate other implementations of applications.