Why is TCP a triple handshake

Reference resources

TCP Why three handshakes , Not twice or four times ？

1. tcp Three handshake process

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It can be seen from the picture that ,tcp The key to three handshakes is , Serial number seq Exchange confirmation of .

2. Why not two handshakes

Because for clients and servers , Both parties to each other Serial number The confirmation of is the key to reliable transmission . The process of two handshakes is as follows ：

1. A Send a sync signal SYN + A's Initial sequence number
2. B Send a sync signal SYN + B's Initial sequence number + B's ACK sequence number

When the second step is completed , We can Guarantee B Known A The serial number of , Because the second step is only executed after the first step is successful . but There is no guarantee A know B The serial number of , Because the transmission in the second step may fail .

The two handshakes are omitted in the figure , Sent SYN=1 The details of the

After two handshakes , Because we assumed ,tcp Only two handshakes , Then when the second handshake is completed ,B You have to think that the connection has been established , No matter whether the transmission of the second handshake is successful . that , If the transmission of the second handshake fails ,A You won't receive it B The serial number of , You can't be sure B The data transmission of starts from . At this time ,B towards A Sent some data (TCP It's full duplex communication , So the server B You can take the initiative to the client A Send a message ), The serial number is attached 20000, By A received ：

here A There are two embarrassing choices ：

1. A Can keep packets . however A even B I don't know the starting serial number of , When will this data package be saved , To reply ACK Well ？
2. A Can reply to ACK.ACK Means that the byte data before its serial number has been received , But A even B I don't know the starting serial number of ： The starting serial number may be 100, It can also be 1000,A It is impossible to determine which bytes of sequence number are missing , Also dare not reply ACK 了 .

therefore , No matter which choice , It's all inappropriate . This is precisely because A No confirmation B The serial number of . And in the tcp in , By three handshakes , And packet loss processing mechanism ,A and B Will make sure that their serial number is received by the other party .

3. tcp Handling of packet loss in three handshakes

Copy from TCP Why three handshakes , Not twice or four times ？ tcp In addition to using three handshakes , We should also deal with packet loss accidents properly , In order to make sure A、B Transmission and confirmation of serial numbers of both parties .

1. If the first bag , namely A issue B Of SYN Lost on the way , Didn't arrive B A Will periodically timeout retransmission , Until receipt B The confirmation of
2. If the second package , namely B issue A Of SYN +ACK Lost on the way , Didn't arrive A B Will periodically timeout retransmission , Until receipt A The confirmation of
3. If the third bag , namely A issue B Of ACK Lost on the way , Didn't arrive B A End of hair ACK, Think unilaterally of TCP by Established state , and B It is obvious that TCP by Active state ： a. Suppose that there is no data to send from either side ,B Will periodically timeout retransmission , Until receipt A The confirmation of , When received B Of TCP Connect Also for the Established state , Two way contract can be contracted . b. Suppose this time A There's data to send ,B received A Of Data + ACK, Naturally, it will switch to established state , And accept A Of Data( Hair ACK At the same time, by the way Data close ). c. Assume B There's data to send , The data can't be sent , It will be retransmitted periodically SYN + ACK, Until receipt A To send the data ( received A The confirmation of , The connection has not been established , You can't transmit data ).

such ,tcp Three handshakes of , Plus the processing mechanism of packet loss , To ensure that A、B Confirmation of serial numbers of both parties . This establishes the basis for reliable transmission .

4. Why not four handshakes

Combine the second and third steps of the four handshakes , Three handshakes . In order to improve efficiency , Yes, you can merge 2、3 Step by step .