TCP/IP based application programming interface

In "TCP/IP Basics: Data Encapsulation", we mentioned that each layer in the network layer interacts with each other through interfaces and provides services. The interface between the application layer and the user is called the Application Programming Interface (API). API is actually a collection of functions, or a collection of definitions and protocols. Regardless of the type of collection, its essence is to shield users from implementation details and complexity through abstraction.

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The main purpose of an API is to provide application developers with the ability to access a set of routines based on certain software or hardware without having to access the source code or understand the details of the internal working mechanism.

From the user's perspective, the application program interface is presented as a series of API functions that users can use to develop network applications.

From a network perspective, the application programming interface provides users with a set of methods that they can use to send service requests, information, and data to the application layer. Each layer in the network responds in turn, ultimately completing network data transmission.

The role of API is mainly reflected in the following aspects.

• Remote Procedure Call (RPC): Communication between programs is achieved through procedures (or tasks) acting on shared data buffers.
• Standard Query Language (SQL): is a standard query language for accessing data, which enables data sharing between applications through a common database.
• File transfer: Share data between applications by sending formatted files.
• Information delivery: refers to small formatted messages between loosely coupled or tightly coupled applications, which enables data sharing through direct communication between programs.

There are many application programming interfaces at the application layer, and they are developing rapidly. The more common ones include Socket, FTP, HTTP, and Telnet.

These interfaces can be divided into four categories:

• Remote Procedure Call (RPC) interface
• Data query interface
• File Class Interface
• Data communication interface

For example, the FTP protocol belongs to the file interface, based on which users can share and transfer files across the network. Socket and HTTP can be attributed to data communication interfaces, based on which users can develop network communication applications and web page interactive programs.

From the perspective of programming development, whether it is FTP, HTTP or Telnet, they are all application layer protocols developed based on the Socket interface. They are further encapsulation and abstraction of the Socket interface, thereby providing users with a higher level of services and interfaces.

There are two commonly used socket types: stream socket (SOCK_STREAM) and datagram socket (SOCK_DGRAM).

• A stream socket is a connection-oriented socket that is designed for connection-oriented TCP service applications.
• A datagram socket is a connectionless socket, corresponding to the connectionless UDP service application.

In terms of user interface, there is also the transport layer TLI interface, which was developed by AT&T and is sometimes called XTI. It is an application program interface provided by the transport layer for users and can be used to develop applications at the transport layer.