TCP and UDP

TCP and UDP It's all transport layer protocols , They all belong to TCP/IP Protocol family

One 、UDP

UDP The full name is User datagram protocol , In the Internet, it's connected with TCP The protocol is also used to process packets , It's a connectionless protocol . stay OSI In the model On the transport layer , be in IP The next level of agreement .UDP Packet Grouping is not provided 、 Disadvantages of assembly and sequencing ; namely , When the message is sent , It is impossible to know whether it arrived safely and completely .

1） Facing connectionless
UDP You don't need to be with TCP Before sending data Three handshakes Connected , Data can be sent at any time . And it's just a data message carrier , There will be no splitting and splicing operation for data message .

• At the sending end , The application layer transfers data to the transport layer ,UDP Only one... Will be added to the data UDP Head sign , And then it goes to the network layer
• At the receiving end , The network layer passes the data to the transport layer ,UDP Will only remove IP The message header will not have any splicing operation , Then it is passed to the application layer

2）UDP Provides unicast , multicast , The function of broadcasting .

3） Message oriented
The sender's UDP Message delivered to the application , After adding the header, it will be delivered down to IP layer .UDP For the messages handed in by the application layer , Neither merge , It doesn't break up , But keep the boundaries of these messages . therefore , The application must choose the appropriate size of the message .

4） Unreliability

• First of all, unreliability is reflected in On no connection , There is no need to establish a connection when communicating .
• also The data received will be transmitted , also No backup data , Sending data is also It doesn't care whether the other party has received the data correctly .
• Moreover, the network environment is good or bad , however UDP Because there's no congestion control , Always send data at a constant speed . Even if the network conditions are not good , It will not adjust the transmission rate . In this way The disadvantage is that packet loss may occur when the network conditions are not good , however The advantage is that in some real-time scenarios （ For example, conference call ） You need to use UDP instead of TCP.

5） Small overhead , Very efficient when transmitting data messages
UDP The cost of the head is small , Only 8 byte , comparison TCP At least 20 There are far fewer bytes , It's very efficient to transmit data messages .

Two 、TCP

TCP The full name is Transmission control protocol It's a connection oriented 、 reliable 、 Transport layer communication protocol based on byte stream .TCP It's connection-oriented 、 Reliable Streaming Protocol （ Flow is a continuous data structure ）.

1） Connection oriented
Connection oriented , It means that a connection must be established at both ends before sending data . The method of establishing the connection is Three handshakes To establish a connection , Lay a foundation for reliable data transmission .

2） Only unicast transmission is supported
Every one of them TCP A transport connection can only have two endpoints , Only point-to-point data transmission ; Multicast is not supported 、 Broadcast transmission mode .

3） Byte stream oriented
TCP Unlike UDP Just like that, messages are transmitted independently , It is transmitted in byte stream mode without retaining the message boundary .

4） Reliable transmission
Judge packet loss 、 The error code depends on TCP Of Segment number as well as Confirmation no. .TCP In order to ensure the reliability of message transmission , Just give each package a serial number , At the same time, the serial number also ensures the sequential reception of packets sent to the receiving entity . Then the receiving entity sends a corresponding acknowledgement to the bytes that have been successfully received (ACK); If the sender entity has a reasonable round-trip delay (RTT) No confirmation has been received within , So the corresponding data （ Suppose we lost ） Will be retransmitted .

5） Provide congestion control
When the network appears congestion When ,TCP It can reduce the cost of injecting data into the network rate and Number , Ease congestion .

6） Provide full duplex communication
TCP Allow applications on both sides of the communication to send data at any time , because TCP There are buffers at both ends of the connection , Used to temporarily store data for two-way communication . Of course ,TCP You can send a segment of data immediately , You can also cache a period of time to send more data segments at a time （ The maximum segment size depends on MSS）

3、 ... and 、TCP and UDP The difference between

	UDP	TCP
Is it connected	There is no connection	Connection oriented
reliable	Unreliable transmission , No flow control and congestion control	Reliable transmission （ Data order and correctness ）, Using flow control and congestion control
Number of connected objects	Support one-to-one , One to many , Many to one and many to many interactive communication	It can only be one-to-one communication
transport	Message oriented	Byte stream oriented
First cost	Small overhead of the first part , only 8 byte	The first is the smallest 20 byte , Maximum 60 byte
Applicable scenario	For real-time applications , For example, video conference 、 live broadcast	For applications requiring reliable transmission , For example, file transfer

Four 、TCP and UDP Usage scenarios of

TCP Application scenarios ：
Efficiency requirements are relatively low , But the scenes with relatively high requirements for accuracy . Because the data needs to be confirmed in the transmission 、 retransmission 、 Sorting and other operations , By contrast, efficiency is not UDP high . for example ： File transfer （ High accuracy and high requirements 、 But the speed can be relatively slow ）、 Take mail 、 Remote login .

UDP Application scenarios ：
Efficiency requirements are relatively high , Scenes with relatively low accuracy requirements . for example ：QQ Chat 、 Online video 、 Voice over the Internet （ im , High speed requirement , But it's not a big problem to have occasional interruptions , And the resend mechanism can't be used here at all ）、 Broadcast communications （ radio broadcast 、 multicast ）.

5、 ... and 、 Data flow diagram