What is the transport layer protocol tcp/udp???

Introduction to transport layer protocol

1、TCP agreement

tcp Transmission control protocol （TCP,Transmission Control Protocol） It is a transport protocol specially designed to provide reliable end-to-end byte stream on unreliable Internet .

2、TCP Message segment

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Source port number ： The port number of the sender process .  Target port number ： The port number of the receiving process .  Serial number ： The sender will number each byte , It is convenient for the receiving party to correctly reorganize .  When TCP When receiving data bytes from the process , Divide them into data segments and store them in the send cache , And number each byte . When the data reaches the destination , The receiver will rearrange the data according to this sequence number , Make sure the data is correct .  Confirmation no. ： Confirmation message to the sender .  When the receiver responds to the message, it will use it to tell the sender that the data segments of this serial number have been received , If the confirmation number is X, It means before X-1 All data segments have been received .  The length of the first ： Used to represent TCP Packet header length , commonly 20 byte , But the length of the head can be extended up to 60 byte .

Control bits ：

• URG:  Emergency position . Emergency pointer valid bit , Use with emergency pointer .

• ACK:  Confirmation bit . When ACK=1 when , Confirm that the serial number field is valid , by 0 when , Invalid .

• PSH:  Urgent position . Sign bit is 1 when , The receiver is required to deliver the data segment to the application layer as soon as possible .

• RST： Reset bit . When RST The value is 1 when , Notify to re-establish TCP Connect .

• SYN： Sync （ Connect ） position . Synchronization sequence number bit ,TCP When you need to establish a connection, set this value to 1.

• FIN:  Off position . When TCP When you need to disconnect to complete data transmission , The party proposing to disconnect sets this value to 1.

  Window size ： Indicate the number of locally acceptable data segments . The size of this value is variable , When the network is unobstructed, the receiving end response message will increase the window value to speed up the transmission speed , When the network is unstable, reducing this value can ensure the reliable transmission of network data ,TCP The flow control in depends on changing the window size . For example, the download speed is a few from the beginning KB Gradually rise to a few MB The process of . For controlling network speed .  The checksum ： For error control . The scope of field verification includes header and data . The data segment will be checked and calculated when it is sent and when it reaches the destination , If the values are inconsistent , The data will be considered corrupted , The receiver will discard the data .  Pointer to an emergency ： and URG In combination with , When URG=1 Effective when .  Options ： stay TCP The first can have up to 40 Optional information for bytes . for example , Maximum segment length MSS.MSS Tell them TCP： The maximum data field of the message segment that my cache can receive is MSS Bytes .

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3.TCP Connect

 TCP The process of establishing a connection is called three handshakes

​  For example, host A Want to communicate with the host B Connect , host A Send... First SYN message （SYN=1） Request to establish a connection , host A Serial number Seq=100. host B Send after receiving SYN and ACK message , Namely (ACK=1) Confirm and also (SYN=1) Request to establish a connection host B Seq=300,Ack=101（x+1）, This Ack It's a confirmation number , Represents the front host A Of x Has been received . host A Received the host B Request to establish a connection , It's also going on （ACK=1） confirm , And reply Seq=101（x+1）,Ack=301（y+1） On behalf of the host B Of y Received . TCP Four waves disconnected

​ host A Want to communicate with the host B disconnect , Send... First （FIN=1） Request disconnection and （ACK=1） confirm . host B reply （ACK=1） Agree to disconnect , Then the host B Also want to disconnect from the host A The connection of , send out （FIN=1） and （ACK=1） confirm . host A（ACK=1） Make sure to disconnect .  host B Agree to disconnect the reply ACK=1 It is called semi disconnected state , At this time, the host computer A Unable to continue connecting to the host B, But the host B Can still give the host A send data .( Download offline )  Half off state , At this time, the host computer A Unable to continue connecting to the host B, But the host B Can still give the host A send data .( Download offline ) 2、UCP agreement   shortcoming ： There is no connection , unreliable   advantage ： The cost is small , Efficient  UDP length ： Used to indicate UDP The total length of , Add data to the header   The checksum ： To complete the right UDP Error checking of data , It is UDP The only reliable mechanism provided by the protocol

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