Wireshark packet analysis TCP, FTP

TCP brief introduction

TCP protocol

TCP Responsible for application software （ Like your browser ） Communication with network software .

IP Responsible for communication between computers .

TCP Be responsible for data segmentation and loading IP package , And then recombine them when they arrive .

IP Responsible for sending packets to recipients .
tcp Sign a

SYN： Sync flag bit , be used for 2 When a host wants to establish a connection , The first packet sent , stay TCP Three handshakes , The first two handshakes will have SYN Sign a , As above, TCP Three handshakes .

ACK： Confirmation bit , It is used to indicate that the previous packet sent by the other party has been received successfully , As you can see in the picture above , The second handshake ,Receiver The package sent contains ACK and SYN,ACK Is to tell Sender, You just sent me SYN package , I have received .

FIN： The sender sends FIN Sign a , The purpose is to tell the recipient , This is the last bag .

URG：urgent flag Used to inform receiver Process this package should be given priority , Put the other bags aside .

PSH：push flag Function also urgent equally , priority , It's just that the package program with this flag directly handles , Do not enter the receive cache .

RST：reset flag It's from receiver Send to sender It's in my bag , Indicates that reconnection is needed . Need to reset .

ECE：ECN The response flag is used in TCP3 A handshake indicates a TCP The end is to have ECN Functional , And indicate the received TCP Bag IP The head of the ECN Set to 11.

CWR： Congestion window reduction flag is set by the sending host , Used to indicate that it has received settings ECE logo TCP package . The congestion window is being TCP An internal variable maintained , Used to manage send window size .