[network planning] 2.1.2 transport layer services that can be selected by the application

This article is based on 《 computer network —— top-down approach 》

1. Process of communication

At application level , The subjects who communicate with each other are process , A process can be considered as a program running in the end system .

The interface between a process and a computer network

The two processes communicate with each other through the network to form a communication process pair , The two processes send messages to each other . Process through a process called ** Socket （socket）** Software interface to send or receive messages to or from the network . Application layer developers can control everything about sockets in the application layer , But there is little control over the socket at the transport layer end .

2. Transportation services available for the application

Service is a function provided by the lower layer to the upper layer . The application layer can select the services provided by the transportation layer . The application layer can provide the following four types of services for the transportation layer ： Reliable data transportation 、 throughput 、 Timing and security

1. Reliable data transportation

Packets may be lost in the computer network , Or some bits in the packet are discarded after being damaged . But some applications in the application layer （ Like email 、 File transfer, etc ） If packet loss occurs , It will lead to disastrous consequences . therefore , The transport layer needs to provide... To the application layer Reliable data transmission service , Ensure that the data can reach the destination . Of course , For example, applications such as video conferencing allow a small number of messages to be lost Tolerable lost applications , You can also choose not to use reliable data transmission services .

2. throughput

The transport layer protocol can provide the application layer with guaranteed available throughput at a certain rate . This service can meet Bandwidth sensitive applications , For example, the Internet telephony application is based on 32kbps Is encoded with care , You can apply for more than through this service 32kbps Stable throughput . Elastic applications （ Like email , File transfer ） There is no minimum requirement for throughput , You can not use this service .

3. timing

Some applications, for example, require delay for online games and live broadcasting , The game application has a delay higher than 100ms You will have a bad experience , therefore , The transport layer provides timing services for the application layer , It can guarantee the delivery of messages within a certain time .

4. Security

The transport layer can also provide security services .

3. Application layer protocol

Application layer protocol defines how applications running on different end systems should transmit messages to each other , Specifically

• Type of message exchanged , Such as request message and corresponding message
• Syntax of various message types , For example, how to describe each field in the message
• Determines when and how a process sends messages , Specifies how to respond to messages