rfc 8008135

<tcp layer> +------------------------+
 |preamble |                        |
 +---------+--------  --------------+
 |            syn header            |
 |                                  |
 |      +----------------+----+-----+
 |      |   syn packet   |----> XXXXXXXXXX
 |      +----------------+----+-----+    XXX
 |      | dataz 0201010100101010101 |      XX
 |      +---------------------------+  loop-X d-loop
 |      | 010101010101010101NOPNOPNO|       X
 |      +------------+--------------+      XX
 |      |            | fork child synh's XXX
 +------+--+ <-------+---------+ <---+XXXX