Shulou(Shulou.com)06/01 Report--

What this article shares with you is about the three-way handshake and four waves of TCP and their status. The editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article. Without saying much, let's take a look at it.

Three-way handshake:

TCP's three-way handshake process:

The C side sends the SYN (SYN=1) flag and sends a SEQ number (SEQ is a random number) to indicate that you want to connect with the S side. (C: SYN_SENT S: LISTEN)

After receiving the packet containing (SYN=1), the S side indicates that it wants to accept the packet, then replies with an ACK number (ACK=SEQ+1), and then replies with a SYN=1,ACK=1 packet as well. (C:SYN_SENT, S:SYN_REVD)

The C side replies with an ACK number packet (ACK=SEQ+1), ACK= 1, and then the connection is established, and the rest of the work is to transmit. (C: ESTABLISHED S:ESTABLISHED)

Question:

There was a misunderstanding in this place that SYN and SYN number were the same thing. As a result, they were confused and did not really understand this three-way handshake. As a result, it was too late to repent. Add a few more questions

You won't take it for granted.

In the figure, you can see the first. First, 10.0.0.61 sends a SYN packet to 10.1.3.32, and the flag bit of SYN is 1, and a Seq number=2856901853 is sent.

The second article: 10.1.3.32 received the request, returned a SYN+ACK packet, and confirmation and connection package, Ack number=2856901853+1, and then randomly sent a random number of Seq=3836060639.

Third: 10.0.0.61 received the request, returned an ACK packet, ACK number=3836060639+1, and then sent a Seq= random number.

Through the above three, a stateful connection is established, followed by a process of data transmission.

The essence of the three-way handshake is that the two sides reliably exchange a Seq number, through which the order of subsequent transmissions is confirmed.

Wave four times:

The process of waving four times, the first important reminder is a sentence in Baidu encyclopedia, and TCP is full-duplex, so both sides need to close the connection.

Suppose A has already passed all the data.

A sends a packet with the FIN flag bit 1, indicating that it has completed the transmission.

B sends an ACK to indicate that he already knows and closes the connection from A-> B.

After a while, B also sent all the data, and then B sent a packet marked FIN 1, and then passed it to A.

A looks like a FIN sign, indicating that a disconnection signal from B has been received, and then A sends an ACK=1 packet to formally disconnect all connections.

Take a closer look at this description and find that there are several problems. first of all, A sent a FIN=1 packet, which only means that A has passed all the messages that should be transmitted. As for whether B receives it or not, A does not care whether it is received or not, or whether it knows.

So, obviously, things should not be like the above, like the three-way handshake, there must be the intervention of Seq number.

The correct process is as follows:

A sends a packet of FIN=1 and has a Seq number. (A:FIN_WAIT_1 B:ESTABLISHED)

After receiving it, B returns a packet of ACK=1 and a packet of Ack number= Seq number+1. (a: FIN_WAIT_1 B:CLOSE_WAIT)

After the transmission is completed, like another connection, send a FIN=1 packet and carry a Seq number at the same time. (A:FIN_WAIT_2 B:LAST_ACK)

When A receives it, it returns a packet of ACK=1 and sends the packet of Ack number = seq number + 1 to A. (A:TIME_WAIT B:CLOSED)

These are the three-way handshake and four waves of TCP and their status. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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