TCP Three and four waves of

Three handshakes

① The client sends out the plan to establish TCP Connection request message , In the header of the message SYN position =1, It indicates that the message cannot carry data , But it takes a sequence number (seq).TCP Is oriented to a byte stream , In a TCP Each byte in the byte stream transferred in a connection is numbered in order , This number is the serial number (seq). Select an initial sequence number in the request to establish connection message seq=X, But do not carry data . here ,TCP The client process enters SYN-SENT( Sync sent ） state .
② After the server listening port receives the request , If you agree to establish a connection , Send a confirmation to the client . In the confirmation message segment SYN Bit and ACK All bits are set to 1, It also consumes a serial number and selects an initial serial number seq=Y.ACK position =1 It indicates that the confirmation number in the message is valid , The confirmation number is ack=X+1, Indicates that the next sequence number you want the client to transmit is X+1（ Because the client has sent a serial number in the first step x Message of ）. here TCP The server process enters SYN-RCVD（ Sync received ） state .
③ After the client receives the confirmation from the server , Also send a confirmation to the server , Therefore, the... Of the confirmation message segment is first set ACK Set as 1, Confirmation no. ack=Y+1, Indicates that you want the next sequence number transmitted on the server side to be Y+1（ Because the server has sent a serial number in the previous step y Message of ）, And my serial number is seq=X+1. here ,TCP Connection established , Both sides enter ESTABLISHED（ Connection established ） state .
SYN: Request link FIN: break link ACK: reply RST: Reset connection PSH: Push data URG: Pointer to an emergency

Use popular words to describe the above three handshake processes ！

① client ： The server , Can we establish a connection ？
② The server ： B: yes, you can , Let's make a connection ！
③ client ： received , Make a connection ！ Then build TCP The connection is successful if it is four handshakes , It's like this
① client ： The server , Can we establish a connection ？② The server ： B: yes, you can , Let's make a connection ！③ client ： received , Let's make a connection ！④ The server ： received , Let's make a connection ！

1 Bitstream protocol

Suitable for extremely bad network conditions

back off n agreement

Back to n individual All of them will be retransmitted later

Choose retransmission protocol

Which one is lost Retransmit which

Why does the connection handshake need three times

Why should there be a third handshake , because b I wonder if this bag is a The sultry bag , May be a One of the multiple Teaser packets sent , This is a late bag , There is no need to accept .“ Three handshakes ” Is the purpose of “ In order to prevent the invalid connection request message segment from suddenly transferring to the server , So there's a mistake ”

• Two handshakes , The client knows that the server has the ability to receive and send information , But the server only knows that the client has the ability to send information , But I don't know that the client has the ability to receive information , If you add the second 3 The second handshake , The server knows that the client has the ability to receive information （ Because the client receives the message and replies to the server ）.

• Now consider such a situation , Due to network congestion , The first connection request message segment sent by the client （ The first handshake ） In some network nodes for a long time , It will take some time to reach the server .

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• Then, because the client does not receive the confirmation message from the server , The client considers this connection request message segment （ The first handshake ） It doesn't work , Then resend the connection request message , This time, I didn't stay in the network because of network congestion , Successfully sent to the server , Because it's two handshakes , So after shaking hands twice , The connection is established .
  

• The client thinks that the initial request connection message stuck in the network due to network congestion has failed , It doesn't actually fail , Just stay in the network due to the congestion of the network , here , The network is not blocked , It's unblocked again , The message successfully arrived at the server , The server thinks that the client should communicate with the server TCP The connection of , So the server responds to the connection request message , Agree to establish a connection . Assume no third handshake , So as long as the server sends out a confirmation , A new connection is established .

• Because now the client does not send out the request to establish the connection , Therefore, the server-side confirmation will not be ignored , It will not send data to the server . But the server side thinks that the new transportation connection has been established , And has been waiting for the client to send data . Many resources on the server side are wasted .

therefore , A third handshake is needed to solve the problem , The third handshake is the confirmation message sent by the client to the server , If the server does not receive the reply confirmation message from the client after the specified time , Therefore, resources will not be allocated to clients , therefore , This connection has been abandoned , It won't consume the resources on the server .

Four waves

Simply put, don't write , Don't read and confirm , Stop writing and let others confirm .


TIME_WAIT Two functions

1. Ensure that the connection is terminated normally
2. Allow old repeating segments to disappear in the network

Why? TIME-WAIT Status needs to pass 2msl（ Maximum message lifetime ） Before returning CLOSE state

Although all four messages have been sent , Can enter the CLOSE state , But the network may have the illusion of unreliability , Maybe the last one ACK The loss of ,TIME-WAIT Status is used to resend messages that may be lost .
wait for 2msl The meaning of ： Reliable termination TCP Connect 、 Promise to be late TCP The message has enough time to be recognized and discarded ;
1、 Guarantee A Sent ACK Be able to get to B, This message may be lost , be in LAST-ACK Of B I can't receive the message sent to myself FIN and ACK Message confirmation ,B Over time retransmission FIN and ACK message , So in 2msl Received this retransmission in time FIN+ACK message , next A Retransmission once ;
2、 In the meantime , Define the connector for this connection （ Customer's IP Address and port number , Server's IP Address and port number ） It can no longer be used , This connection can only be in 2msl It can be used only after it is finished ;
3、 In order to make old packets disappear in the network due to expiration , Each specific TCP You must select a message maximum lifetime MSL, It is the longest time in the network before any message field is discarded .

Why wave four times

Refer to the three grip mobile phone system , It takes at least three waves , If only three times , The client sends the data request to disconnect , And the server may not also send data , If you return at the same time ACK and FIN To the client , disconnect , May cause data damage ; If sent first ACK, Wait again B Send the data after sending FIN and ACK, The integrity of the transmitted data can be guaranteed .
tcp It's full duplex mode , Received FIN It means that no data will be sent again , But you can still send data .

TCP How to realize flow control

TCP Use sliding window protocol for flow control . Imagine sending data at a high rate on the sending side and receiving it at a low rate on the receiving side , In order to ensure that the data is not lost , Flow control is clearly needed , Coordinate the working rhythm of both sides of communication . So called sliding window , Can be understood as the size of the buffer that the receiver can provide .TCP Use a sliding window to tell the sender how much buffer it can provide for the data it sends , This protocol can speed up data transmission , Improve network throughput .

TCP How to implement congestion control

Congestion control is also called window based congestion control , So , The sender maintains a window called a congestion window cwnd(congestion window) State variable of , The size of the congestion window depends on how congested the network is , And changing dynamically , The sender makes its sending window equal to congestion window ( notes ： Here we assume that the receiver always has enough cache space , Therefore, the size of the transmission window depends on the congestion degree of the network , In fact, the real window value = min( Receiver window , Congestion window ) The sender controls the congestion window cwnd The principle of ： As long as the network is not congested , The congestion window can be increased , In order to send out more groups . But as long as there is congestion or possible congestion in the network , We have to reduce the congestion window , To reduce the number of packets injected into the network , So that the congestion of link network .

How to know that there is congestion in the network ？

1. When congestion occurs in the network , The router will drop the packet , Therefore, as long as the sender does not receive the confirmation message that should arrive in time , In other words, as long as there is a timeout , We can guess that the network may be congested ,( Now the transmission quality of communication lines is generally very good , The probability of discarding due to transmission and output errors is very small ( Far less than 1%)) therefore , The basis for judging network congestion is whether there is a timeout .
2. The receiver establishes a fast retransmission mechanism , If a bag is lost , Then continue to send retransmission request for the subsequent packet ; Once the sender receives three identical acknowledgments , I knew that there was an error after the package , Retransmit the bag immediately ;