问题发现：
发现FTP-DATA数据传输完，TCP的挥手似乎只有两次
实际发现FTP-DATA报文中，TCP层flags中携带了FIN标志
piggyback FIN
问题转化为
TCP packet中如果有FIN flag，该报文还能携带data数据么？
答案是肯定的
RFC793中描述：
The SYN and FIN
are the only controls requiring this protection, and these controls
are used only at connection opening and closing. For sequence number
purposes, the SYN is considered to occur before the first actual data
octet of the segment in which it occurs, while the FIN is considered
to occur after the last actual data octet in a segment in which it
occurs.
按道理应该是可以再TCP FIN报文中插入data数据的。
有个资料中显示ACK也可携带data，这叫piggybacking
pdf
那么FIN报文中携带数据
是不是叫piggyback FIN？