Network packet capturing to understand the establishment process of TCP triple handshake

  TCP The three handshakes in sometimes are troublesome to understand , Let's learn about it by grabbing bags tcp The process of establishing a connection

  The basic network topology is as follows , We capture packets on either of the two ports of the router

  After the data packet is crawled, it looks like this , This is the packet of three handshakes we want to see .

  At the same time, the schematic diagram of three handshakes in the network is also put up for our comparison .

  This is the first packet TCP Details of , Through the picture, we can see three handshakes. First, the client initiates the connection , Send a seq= 6509（ The starting serial number ）, And in the identification bit SYN Set a packet to Server The server .

Above is the second , When Server After receiving the data packet, the server replies with seq=6549（ New serial number ）,ack=6510（ Acknowledge receipt of the last packet ）, In the identification bit SYN、ACK The set packet is sent to Client client .

        
Above is the last packet , When Client The client will reply again after receiving the packet seq=6510（ New serial number ）,ack=6550（ Acknowledge receipt of the last packet ）, In the identification bit ACK The set packet is sent to Server The server .

  By the above operation , One TCP The connection is established , The client and Server The servers can communicate and send data normally .