Overview: the transport layer provides communication services to the application layer above it, which belongs to the highest level of communication-oriented part and the lowest level of user function;

Provides logical communication between processes running on different hosts.

Functions of the transport layer:

The transport layer provides logical communication between application processes.

Multiplexing and demultiplexing:

Multiplexing: Different application processes of the sender can use the same transport layer protocol to transmit data.

Separation: The transport layer of the receiver can deliver these data to the destination application process correctly after stripping the message header.

The transport layer also performs error detection (header and data parts) on the received message.

Two different transport protocols are provided, namely TCP with plane connection and UDP without connection.

Screen the details of the underlying network core (such as network topology, routing protocol, etc.). ) from high-level users, so that the application process appears to have an end-to-end logical communication channel between two transport layer entities.

2. Addressing and port of transport layer

The function of the port: the port can identify the application process in the host, let various application processes in the application layer pass their data down to the transport layer through the port, and let the transport layer know that the data in its message segment is passed up to the corresponding process in the application layer through the port.

Ports only have local meaning, and there is no connection between the same ports of different computers in the Internet.

Software ports and hardware ports:

Software port: the abstract protocol port between the layers of the protocol stack, which is the address where various protocol processes of the application layer and the transport entity layer interact.

Hardware port: the interface between different hardware devices.

Port number:

Port number used by the server:

Familiar with port numbers (0 ~ 1023): FTP 2 1, TELNET23, SMTP25, DNS53, TFTP69, HTTP80, SNMP 16 1.

Registration port number (1024 ~ 49151) The use of these ports must be registered in IANA to prevent duplication.

The port number used by the client (49 152 ~ 65535) is also called transient port (also called temporary port number): this port number is dynamically selected only when the client process is running.

After the communication, the client port number just used does not exist, so this port number can be used by other client processes.

Socket:

Background: IP address is used to identify and distinguish different hosts, and port number is used to identify and distinguish different application processes in a host.

In the network, the socket combination of the sender and the receiver is used to identify the endpoint:

Socket = (host IP address, port number)

Unique symbol of the host and its application (process) in the network.

Is actually a communication endpoint.