TCP FIN 字节值？

Question

My question is really two parts:

From the sender's perspective,

1. in addition to TCP header FIN flag, is it true that TCP layer injects (or more like appending since it should be at the end of the stream) an artificial byte in the stream, meaning this byte is part of the TCP payload?
2. if so, what is the value of this byte?

From the receiver's perspective,

3. both TCP layer and application needs to know this FIN flag/byte. So is it true that TCP layer only looks at the FIN flag, without special handling the byte in the stream?
4. How is the application notified? By the FIN flag, OR, by this special byte in stream?
5. When is the application notified? Right upon TCP layer receiving the segment with the FIN flag, OR, when that segment eventually bubbles up the receiver's TCP buffer?
6. If the application does not get special notification until the segment that has the FIN flag eventually bubbles up TCP buffer, it means TCP layer has to somehow mark the buffer, since the TCP header should be stripped already. So how does it mark the FIN?

Answer 1

in addition to TCP header FIN flag, is it true that TCP layer injects (or more like appending since it should be at the end of the stream) an artificial byte in the stream, meaning this byte is part of the TCP payload?

No. There is no actual byte injected but only the TCP sequence number is increased so that it is clear that the ACK is for the FIN and not some previous data.

This means also that if the FIN is received no special byte is put into the socket buffer and delivered to the application but that the socket buffer is marked as done. A read by the application on an empty and "done" socket buffer will return that there are no more data in the buffer and never will be and thus the application knows that the peer has stopped sending (ie socket shutdown for writing or socket closed).