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TCP and UDP Differences between
1,TCP Connection oriented protocol ,UDP Is a connectionless protocol ;
2,TCP Protocol transmission is reliable ,UDP Protocol transmission “ do everything in one's power ”;
3,TCP Flow control is possible ,UDP no way ;
4,TCP Can be segmented ,UDP no way ;
5,TCP Slow transmission speed , Large occupation of resources ;UDP Fast transmission speed , Small resource occupation .
What is connection oriented ： Before formal data transmission , Pass the prepared agreement first , Establish a point-to-point connection , Then transfer the data .
TCP Message format ：（ Back ）
TCP --- Variable length head
seq Serial number ,ack Confirm serial number （ The lengths are 4 Bytes ） Used to ensure TCP Three handshakes and four waves
header length ： Data offset ,4 position , The value of this field is TCP Head （ Include options ） Length divided by 4.
Pointer flag meaning ：
URG： Emergency flag bit , Set 1 activation , Bring emergency data to the forefront , Corresponding emergency pointer , The emergency pointer indicates which bit the emergency data goes to , That is, the data before the pointer is the data that needs urgent processing （ General setting 0, Data transmission for emergency priority processing ）
ACK： Confirmation mark bit , Used to determine the reply message , Linkage with confirmation serial number , Set 1 Confirm serial number activation when , The confirmation sequence number is different from the confirmation mark bit , Confirm that the serial number is ack.
PSH： Express Lane , Set 1 You don't have to queue up when , Indicates that data is being transmitted
RST： Data reset information identifier （ Used to close the connection abnormally , Forced shutdown , unwanted ack confirm ）
SYN： Request flag bit , Request connection setup 1 .
FIN：TCP End identifier of the connection , Indicates the end of the connection .
Pseudo Header Check ---
Encapsulating the network layer 12 Verify the contents of bytes together ：32 Bit source IP address ,32 target IP address ,8 Bit agreement number ,8 Bit reservation ,16 Bit message length , One TCP The smallest data transmission unit is 20 byte .
UDP:
UDP Only source and destination port numbers and UDP Length and check core , So one UDP The minimum unit of data is 8 Bytes .
TCP and UDP The biggest difference between data transmission protocols is TCP Need to establish reliable connection oriented （ Three handshakes and four waves ）.
TCP Three handshakes （ That is, it interacts by sending three data packets ）：
First handshake ：Client Flag bit SYN Set as 1, Randomly generate a value seq=X, And send the packet to Server,Client get into SYN_SENT state , wait for Server confirm . The Second Handshake ：Server Flag bit after receiving data packet SYN=1 know Client Request to establish connection ,Server Flag bit SYN and ACK All set to 1, Confirm serial number activation ack=X+1, Because the serial number sent for the first time is X, So confirm the serial number ack=X+1, And randomly generate a value seq=Y, And send the packet to Client To confirm the connection request . Third handshake ：Client After receiving the confirmation , inspect ack Is it X+1,ACK Is it 1, If correct, the flag bit will be ACK Set as 1,ack=Y+1, And send the packet to Server,Server inspect ack Is it Y+1,ACK Is it 1, If correct, the connection is established successfully , Complete three handshakes , subsequently Client And Server Data transfer can begin between .
TCP Four waves ：
First wave ：Client Send a FIN, Used to close Client reach Server Data transfer . Second wave ：Server received FIN after , Send a ACK to Client, Confirm that the serial number is the received serial number +1（ And SYN identical , One FIN Occupy a sequence number ）. Third wave ：Server Send a FIN, Used to close Server reach Client Data transfer . Fourth wave ：Client received FIN after ,Client get into TIME_WAIT state , Then send a ACK to Server, Confirm that the serial number is the received serial number +1,Server get into CLOSED state , Complete four waves .
Why does it take three handshakes and four waves :
When the client sends a wave request , The data has been sent , The server still has data to send , Until the server sends the data , The server sends the last packet FIN, At this time, the client sends ACK information , The server will be closed . Data transmission is not required to establish a connection .
TCP Guarantee mechanism of transmission reliability ： confirm , Retransmission , sort , Flow control （ Sliding window mechanism ）
Network layer encapsulation ：-IP address -IP Message format ：（TCP and IP All variable length heads ）
TTL： The packet is forwarded every time it passes through the router ,TTL The value will be reduced by one , When in a packet TTL Value is 0 Time , The router will no longer forward the packet , It will be discarded directly .
8 Bit protocol ：TCP--6 UDP--17 ICMP--1
ip Slice :
MTU --- Maximum transmission unit --- 1500 byte （ Maximum length of a data segment , The largest transmission unit in the physical layer ）
MSS --- Maximum segment length --- 1460（MSS Not included TCP and IP Protocol header length ）
encapsulation ： Right to left .
Unpacking ： From left to right .
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