*** TCP ***     
offset  length  description
0       2       src port
2       2       dst port 
4       4       sequence
8       4       acknowledgement
12      2       data offset(4) / reserved(6) / flags(6) : UAPRSF (so F=1)
14      2       window
16      2       checksum
18      2       urgent pointer
20      3       options
23      1       padding
24      1-      data