Three handshakes and four waves of TCP protocol and why------ One two pandas

TCP Triple connection process

I haven't reviewed for a long time TCP Three handshakes, four waves , Review the .

SYN、RST It is used for connection establishment and clearing , contain SYN The message is called SYN Message segment .
When establishing a connection ：
First step , Client's TCP First to the server side TCP Send a special TCP message . The message segment does not contain application layer data . The header of the message contains a flag bit SYN Be set to 1, At the same time, the customer service end selects a random serial number seq（client_isn） And put it at the beginning TCP SYN In the message segment , This message segment will be sent to the server . In a message segment SYN=1 Indicates that this is a connection establishment message ,seq Is the random serial number of the client .

The second step , once TCPSYN Of the message segment ip Datagram arrives at server host , The server will take it out of the datagram TCPSYN Message segment , For the sake of TCP Distribute TCP Caching and variables , And send the message segment allowing connection to the client . First, the flag bit of the message segment to be replied SYN Set as 1, Secondly, the first confirmation number field of the message segment is set to client_isn+1, namely ack=x+1; Finally, the server will select its own initial serial number （server_isn） namely seq=y, And place it in TCP In the sequence number field of the header of the message segment .（ This message segment that allows connection actually indicates ：“ I have received your SYN grouping , The group has an initial sequence number clien_isn. I agree to establish a connection . My own serial number is server_isn”）, This segment is called SYNACK Message segment . Of the message segment SYN=1 Yes indicates that this is a connection establishment message ,ACK It is the confirmation of the client message segment ,seq=y The server sends its own initial serial number ,ack=x+1 It refers to the sequence number of the next byte expected to be received from the other party .

The third step , Upon receipt of SYNACK After the message , The client will also allocate cache and variables to the connection . The client sends another message segment to the server . This message segment confirms the connection allowed message segment of the server , The client will value server_isn+1 Placed in TCP To complete this work in the header confirmation field of the message segment .SYN Be set to 0, Because the connection has been established .
among ACK It is the confirmation of the server-side message segment ,seq=x+1 It is the sequence number of the next message transmitted to the other party , That is to synchronize the serial number given after receiving the server message last time ,ack=y+1 Is the sequence number of the next byte expected from the server .

Why do I need three connections , Can't secondary connection ？

The necessity of triple connection ：
The reason a ： To prevent duplicate connections

TCP The third handshake is mainly used to prevent the repeated connection caused by the old connection .

Without a third handshake , The server can only choose to establish a connection or reject a connection , But the server doesn't know whether the connection is illegal , So every time I receive a SYN It's going to create a new connection , The server will establish multiple redundant invalid links , Cause unnecessary waste of resources （SYN Flooding attack ）. if TCP It's three handshakes , Then the customer receives ack=x+1 You can determine whether the current connection is a historical connection , If it's a historical connection , The customer service terminal will send a termination message RST, Tell the server to terminate this connection , If it is not a historical connection , Then send instructions to the server to establish the connection .

Popular said ：
If the client makes a connection request , However, due to network fluctuation, the server did not receive the request for connection from the client , So the client sends a connection request again , The client and server establish a connection after two handshakes . Both sides begin to transmit data , After the data transmission is completed , Both parties disconnect . After a while , The connection request that was originally suspended in the network transmission arrived at the server . The server thinks that the client has sent a new connection request , So it sends a confirmation message segment to the client , Agree to establish a connection ( Two handshakes only require the server to send a confirmation message segment , Just set up the connection ). At this time, the server is waiting for the data sent by the client , Has been wasting system resources .

Reason two ： To synchronize the initialization sequence

First, the client sends a seq=200 To the server , The server will return a message after receiving it ack=200+1 The confirmation of , When the customer service end sends many datagrams to the server , Because of the Internet , Packets do not arrive in the order they were sent . When the server confirms the received packets, it will only confirm one by one , Like sending ack=900+1 Confirmation datagram for , Just before 900 The datagram of serial number has been received . If only received seq=900 And the next one to arrive is seq=1000 Data packets of , Generally, it will be discarded or kept until all out of order data is reported .

TCP It is a stable network transmission protocol , It needs a serial number to ensure its stability . This serial number is used to prevent duplication or omission of datagrams , And solve the problem of reverse datagram order . When the client sends a message segment with initial serial number to the server , The server needs to answer a question that has received the initial serial number ACK Reply message , The datagram sent by the server also contains the initial serial number of the server , You also need to get a reply from the client , In this way, the serial numbers of both parties can be synchronized .

TCP Four waves

When the client application process sends a command to close the connection , client TCP It will send a message to the server TCP Send a message segment with a special flag bit , This special flag bit is FIN=1; When the server receives this message segment, it sends a confirmation message segment to the sender , The server then sends its own termination segment , Its FIN The flag bit is also set to 1. Last , The client confirms the termination message of the server . This time TCP All connected resources are released on both hosts .
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Self study notes .