Why is ack=seq+1 when TCP shakes hands three times

from Telnet The agreement says

This example comes from 《 Top down method of computer network 》 The first 6 edition 3.5.2 Section .

Above picture , Because the mainframe A Sent a character C Host computer B, So as the receiver B Feedback of ACK by 42+1.

Above picture , Because the mainframe B Sent a character C Host computer A, So as the receiver A Feedback of ACK by 79+1.

And in the last transmission , Because I didn't carry any data , actually Seq = 43 It doesn't make sense . But because of this fixed field, you have to put a value in it , So put a Next Sequence Number Here is the most suitable . That is to say, the serial number is not consumed in this transmission .
The only meaningful thing is ACK = 80, This tells the host B： I have successfully accepted At least the serial number is 79 And the length is 1 The data of .

Possible next transmission

If the next time is the host A Send data again , Packet Seq It will be 43, Because the last time the host A The sending of did not consume the serial number （ namely Len = 0）, So the mainframe A The sequence number of the sent packet should still be from 43 Start .

If the next time is the host B Send data again , Packet ACK It will be 43, Because the last time the host A The sending of did not consume the serial number （ namely Len = 0）, So the mainframe B Or expect the other party to 43 The starting sequence number .

Three handshakes ACK

This is the three handshakes between this machine and Baidu .

First look at the first two packets , The first packet is Len = 0, But the second packet ACK Still Seq = 0 On the basis of 1 了 . That's strange , Because according to the logic of the previous chapter , Not carrying data , that ACK It should be equal to Seq Of .

Look at the last two packets , alike , The first packet is Len = 0, But the second packet ACK Still Seq = 0 On the basis of 1 了 .

explain

The server responds to the client with a sequence number of zero, as this is its first packet in this TCP session, and a relative acknowledgement number of 1. The acknowledgement number is set to 1 to indicate the receipt of the client’s SYN flag in packet #1.
Notice that the acknowledgement number has been increased by 1 although no payload data has yet been sent by the client. This is because the presence of the SYN or FIN flag in a received packet triggers an increase of 1 in the sequence. (This does not interfere with the accounting of payload data, because packets with the SYN or FIN flag set do not carry a payload.)

In short , Feedback from the receiver ACK The reason to add 1, Because the sender contains SYN Flag bit or FIN Sign a .

in other words , As long as the sender contains SYN Flag bit or FIN Sign a , Even if it does not contain data , The receiving party must also consider The sender consumed a serial number .

in addition , with SYN Flag bit or FIN Message segment of flag bit （ Three handshakes and four waves ）, It is also not allowed to carry data .