User datagram protocol UDP

Author: intellectuals
Computer science
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Copyright notice ： This paper is about CSDN Blogger 「 Pu Shangqing sky 」 The original article of

Knowledge point

UDP brief introduction

Content

• UDP yes OSI A connectionless transport layer protocol in the reference model , It is mainly used in the transmission which does not require the packet to arrive in sequence , The checking and sorting of packet transmission order is completed by the application layer , Provide simple and unreliable transaction oriented information transfer services .UDP The agreement is basically IP The interface between protocol and upper protocol . A process can have multiple port numbers （ The port number and Socket Object association , There can be more than one... In a process Socket object ）, But a port number can only correspond to one process （ There are special cases ）
• UDP Provide connectionless communication , And there is no reliability guarantee for the transmission of data packets , Suitable for transmitting a small amount of data at a time ,UDP The application layer is responsible for the reliability of the transmission . frequently-used UDP Port number ：53（DNS）,69（TFTP）,161（SNMP）, Use UDP The agreement includes ：TFTP,SNMP,NFS,DNS,BOOTP.
• UDP The message has no reliability guarantee , Sequence assurance , Flow control fields, etc , Poor reliability , But because of UDP Protocol has fewer control options , Small delay in data transmission , High data transmission efficiency , Suitable for applications with low reliability requirements , Or applications that can guarantee reliability , Such as DNS,TFTP,SNMP.
• Many applications only support DUP, Such as ： Multimedia data stream , No additional data will be generated , Don't resend even if you know that there are broken packets . When transmission performance is emphasized instead of transmission integrity , Such as ： Audio and multimedia applications ,UDP Is the best choice . When the data transmission time is very short , So that the previous connection process becomes the main body of the whole flow ,UDP It's also a good choice

Actual application site

• In the choice UDP As a transport protocol, we must be careful , In a very unsatisfactory network quality environment ,UDP Protocol packets will be lost seriously , But because of UDP Characteristics of ： It belongs to connectionless protocol , Therefore, the consumption of resources is small , The advantage of fast processing speed , So usually audio , Video and ordinary data are used in transmission UDP More , Because even if they occasionally lose oneortwo packets , It will not have too much impact on the acceptance results , For example, we chat with QQ It is used UDP agreement
• In the field of field measurement and control , For distributed controllers , Detector, etc , Its application environment is relatively harsh , In this way, different requirements are put forward for earth ah transmission data , Such as real-time , Anti interference , Safety and so on . Based on this , In field communication , If an application wants to transmit a set of data to another node in the network , May by UDP The process adds a header to the data and sends it to IP process ,UDP The protocol eliminates the process of establishing and dismantling connections and cancels the retransmission inspection mechanism , It can reach a relatively high communication rate

Message format

• UDP The newspaper is headed by 4 Domains make up , Each of these domains accounts for 2 Bytes , Specific include ： Source port number , Destination port number , Datagram length , Check value

Port number

• The party sending the data （ It can be client or server ） take UDP Packets are sent through the source port , The data receiving party receives data through the target port . Some network applications can only use static ports reserved or registered in advance ; Other network applications can use unregistered dynamic ports . because UDP The header uses two bytes to store the port number , So the valid range of port number 0-65535
• Division of port number

0-1023 Well known port number ,HTTP,FTP,SSH These widely used application layer protocols , Their port numbers are fixed
1024-65535： Port number dynamically assigned by the operating system , The port number of the client is assigned by the operating system from this range

• Well known port number

SSH The server ：22 port
FTP The server ：21 port
TELNET The server ：23 port
HTTP The server ：80 port
HTTPS The server ：443 port

length

• Datagram length refers to the total number of words and bytes including header and data part . Because the header length is fixed , Therefore, this field is mainly used to calculate the variable length data part （ Also known as data load ）. The maximum length of datagram varies according to the operating environment . In theory , The maximum length of datagram including header is 65535 byte . however , Some practical applications often limit the size of datagrams , Sometimes it will be reduced to 8192 byte
• So there's a problem ,UDP There is one in the first part of the agreement 2 Maximum length of bits , That is to say a UDP The maximum length of data that can be transmitted is （65535 byte ） about 64K（ contain UDP The first one ）, However 64K In today's Internet Environment , It's a very small number , If we need to transmit more data than 64K, You need to manually subcontract at the application layer , Send... Multiple times , And manually assemble at the receiving end , And manual disassembly and assembly will have great BUG risk , This has to be used TCP transmission

Check value

• UDP The protocol uses the check value in the header to ensure the security of data . The check value is first calculated by a special algorithm at the data sender , After delivery to the receiver , You need to recalculate . If a datagram is tampered with by a third party or damaged due to line noise during retransmission , The verification value calculation of the sender and the receiver does not match , thus UDP The protocol can check for errors
• Introduce a common check value MD5

Its characteristic is
1. No matter how long the data is , Got MD5 It's a fixed length
2. Calculated from data MD5 Value is very convenient and simple , But by MD5 It is almost impossible to push back data
3. Even if the data is changed a little ,MD5 The value of will change greatly

UDP The main features

There is no connection

• UDP It's a connectionless protocol , Before transmitting data , No connection is established between the source end and the terminal , When it wants to transmit , Simply grab the data from the application , And throw it on the Internet as soon as possible . At the sending end ,UDP The speed of data transmission is only the speed generated by the application , The capacity of the computer and the limitation of the transmission bandwidth ; At the receiving end ,UDP Put each message in the queue , The application reads one message segment at a time from the queue
• Because the transmission data does not establish a connection , So there is no need to maintain the connection state , Including sending and receiving status, etc , Therefore, a server can transmit the same message to multiple airliners at the same time

unreliable

• There is no confirmation mechanism , There is no retransmission mechanism , If it's due to a network failure , It will cause that the paragraph cannot be sent to the other party , Of course UDP The protocol layer will not return any error information to the application layer
• although UDP It's an unreliable agreement , But it's an ideal protocol for distributing information . for example , Report the stock market on the screen , Display aviation information, etc .UDP Also used for routing information RIP Modify the routing table in . In these applications , If a message is lost , In a few seconds, another new message will replace it .UDP Widely used in multimedia applications

For datagram

• UDP It's message oriented .
• Application layer to UDP Message of ,UDP After adding the first part, it will be delivered to the lower layer . Neither split , It doesn't merge , But keep the boundaries of these messages , therefore , The application needs to select the appropriate message size
• for example ： The sender calls once sendto, send out 100 Bytes , Then the receiver must also call the corresponding one recvfrom, Accept 100 Bytes , Instead of calling in cycles 10 Time recvfrom, Each acceptance 10 Bytes
• UDP The buffer .
• UDP In the true sense of the United States and Europe Send buffer , call sendto Will be handed over directly to the kernel , The kernel transmits the data to the network layer protocol , Carry out subsequent transmission actions
• UDP have Accept buffer But this accept buffer does not guarantee the received UDP The order and delivery of the newspaper UDP The order of reporting is the same , If the buffer is full , Arrive again UDP The data will be discarded . therefore ,UDP Of socket Can read , Can also write , This is full duplex .

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