TCP three handshakes and four waves

Three handshakes

1. In one sentence, summarize the necessity of three handshakes ？
   Both the client and the server should confirm that the other party can send , Acceptable .
2. Why not shake hands twice ？
   The second handshake ： When the server receives the connection establishment request , send out ack+syn package , That is, the second handshake is completed （ here , The server just sent ack+syn package , I don't know if it will be accepted correctly ）
   therefore , Two handshakes can cause the following problems .

• After the second handshake, the server establishes a connection （ Need to allocate resources such as memory ）, But sent ack+syn The loss of , The client cannot establish a connection , The server is always waiting . Waste of resources .
• After the second handshake, the server establishes a connection , Then the client sends a connection request , But the request is blocked on the network , The client sent another connection request , After the second request is received by the server , The first connection request also reached the server , Cause the old connection to be responded .

3. The third handshake can carry data , You can also not carry data .

Four waves

1. In one sentence, summarize the necessity of four waves ？
   Both the client and the server confirm that they have no data to send , No data to accept , And after the disconnection signal is received , Only then can it be disconnected .
2. Why not wave three times ？
   When the server sends for the second time FIN when , Then the server is in a state where no data can be received , The state of having no data to send . The server needs to ensure the disconnection signal FIN Received by the client , To disconnect , So you need a fourth handshake . otherwise , The client has been waiting . The client received FIN when , It is necessary to ensure that the disconnection signal ACK Received by the server , So delay 2MSL.
3. wait for 2MSL The function of rear disconnection ？
   wait for 2MSL Then disconnect , Ensure that the server receives the last ACK. If not delayed 2MSL Immediately disconnect and then send a new request , Just disconnect the signal ACK And not received by the server . The server will repeatedly send FIN, Wait for the last time ACK, But the client will only send new requests at this time syn package , The server will think that the request code is wrong . Reset connection . Make sure ACK Received , The subsequent connection will not be affected .
4. Make sure for the last time ACK To be received , Why not wait 1MSL?
   1MSL Is the maximum life cycle of the message . Waiting time ensures When the last time ACK When not received , Retransmitted by the server FIN It will be recognized by the client again .2MSL Enough , Longer waiting times are not necessary .