TCP/IP— Transport layer

The application layer is the interface for applications to enter the network , After the application data is encapsulated, it enters the transport layer . The transport layer defines end-to-end connectivity between host applications . The two most common protocols in the transport layer are transmission control protocol TCP（Transmission Control Protocol） And user datagram protocol UDP（User Datagram Protocol）.

What is the port number ？

When the computer receives a packet , How to distinguish which application this packet belongs to ？

Add a new port number in the transport layer , Used to distinguish different programs . Some web applications are extremely common , The port number is also used to distinguish different network services .（ The port number corresponds to the upper layer protocol , The agreement corresponds to different services ）

Common port numbers （ be based on C/S Model services ）

Transport layer Overview

• The transport layer provides End to end The connection of
• The network layer provides Point to point The connection of
  

Transport layer protocol

TCP and UDP Packet structure comparison

TCP Session creation — Three handshakes ：

TCP Termination of session — Four waves

TCP and UDP Comparison of

Transmission control protocol （TCP）	User datagram protocol （UDP）
Connection oriented	There is no connection
Reliable transmission	Best effort transmission
Support flow control and window mechanism	No flow control and window mechanism
HTTP、FTP etc.	TFTP、DNS、DHCP etc.