"Learning Notes" connection and release of TCP

First, we introduce some key signals of TCP packets.

The ACK:TCP agreement stipulates that only ack=1 is valid and that all messages sent after the connection is established must have an ACK of 1
SYN (synchronization): Used to synchronize the sequence number when the connection is established. When Syn=1 and ack=0, it indicates that this is a connection request message. If the other party agrees to establish the connection, the Syn=1 and ack=1 should be made in the response message. Therefore, the SYN 1 indicates that this is a connection request or a connection acceptance message.
FIN (Finis) is the end, which means to release a connection. When FIN = 1 o'clock, it indicates that the sender's data for this segment has been sent, and that the connection is required to be released.

Three-time handshake for TCP connections

First, the client makes a request to connect the Syn=1 ack=0, TCP rules syn=1 can not carry data, but to consume a sequence number, so declare their serial number is seq=x.
The Server then responds with confirmation that syn=1 ack=1 seq=y, ack=x+1.
Then the Client again to confirm, but no SYN, this is ack=1, seq=x+1, ack=y+1.
Then the connection is established.

TCP Connection Release process

When customer A does not have something to send to release a This side of the connection, a will send a message (no data), where FIN is set to 1, Server B receives a letter to the application, then a side of the connection has been closed, that a no longer send information (but still can receive information). A received B confirmation after entering the waiting state, waiting for the B request to release the connection, b after the data sent to a request connection release, is also expressed with fin=1, and with ACK = U+1 (), a received after reply to a confirmation message, and entered the time_wait state, waiting for 2MSL time.
Why wait?
For this situation: B to a send FIN = 1 release connection request, but this message is missing, a does not receive a confirmation message, B timeout will be re-transmitted, then a in Wait_time can also receive this request, then reply to a confirmation on the line. (A when a request for FIN = 1 is received, Wait_time is re-credited).

"Learning Notes" connection and release of TCP