TCP three handshakes and four waves

The third handshake is to prove that both parties have the ability to send response messages

such as

A Want to B Establishing a connection

If A B Both sides are normal

Establishing a connection

1. A towards B send out syn, B Received , here B know A Can send messages normally
2. B towards A send out syn and ack, A Received , here A know B Can receive... Normally (ack), It can also be sent normally (syn), but B I don't know yet A Can you receive it normally
3. Last A towards B send out ack, Prove that you can receive . thus A B Both parties have confirmed that the other party can send messages normally

The fourth wave is to ensure that the data has been completely sent

Close the connection

1. A towards B send out , I'm going to close it , At that time A There must be no data to B Sent
2. B towards A send out ack Indicates that a shutdown request has been received , But this time B There may be data that hasn't been sent yet
3. B Active direction A send out fin It means that my data has been completely sent , Cut it off . At this time, both parties have no data to send to each other
4. A send out ack confirm , About to disconnect