Design and implementation of tcp/ip series overview

1、 layered

1.1. Concept of layering

Network layering is the sending or forwarding of data to be completed by network nodes 、 Pack or unpack , Control the loading or removal of information , It is completed by different hardware and software modules . In this way, the complex problem of communication and network interconnection can be simplified .
Layered design allows developers to implement different parts of the system separately , It is usually done by professionals in different fields , The most commonly mentioned concept of protocol layering is based on an international organization for standardization （ISO） Open systems interconnection standards defined （ Referred to as OSI） Model .
OSI The model mainly includes seven logical layers , Namely ： The physical layer 、 The link layer 、 The network layer 、 Transport layer 、 The session layer 、 The presentation layer 、 application layer .
（1） The physical layer ： It mainly involves physical entities , Specify connector 、 Data rate and how to encode bits on some media . It also describes low-level error detection and correction and frequency allocation .
（2） The link layer ： Specify the method of communication over a single link ,
（3） The network layer : Specify the method of multi hop communication through potentially different types of link layer networks . For packet networks , It describes Abstract packet format and standard addressing structure .
（4） Transport layer : Specify the method of connection or association between multiple programs running in the same computer system . If not achieved elsewhere , This layer can realize reliable transmission .
（5） The session layer : Specify the method of a communication session consisting of multiple connections , It may involve closing the connection 、 Restart the connection and checkpoint process . for example ISO X.255 Is a session layer protocol .
（6） The presentation layer : Methods for data representation formats and conversion rules for applications . Typical examples are characters from EBCDIC Convert to ASCII code . Encryption is sometimes related to this layer , It may also be on other layers .
（7） application layer : Methods to complete some user initialization tasks . The application layer is usually designed and implemented by application developers .

2. Reuse in layered implementation 、 Decompose and encapsulate

        A major advantage of layering is the ability to reuse protocols , This form of reuse allows multiple protocols to coexist in the same infrastructure , It also allows multiple instances of the same protocol object to exist at the same time , And will not be confused .
        Multiplexing can happen at different layers , And there are different types of identifiers in each layer , Used to determine which protocol or flow information belongs to , When a protocol data unit of a layer （PDU） When the object of is carried by the lower level , This process is called encapsulation at adjacent lower layers . Encapsulation is often used with layering .