[Question]about the commit tcp: randomize timestamps on syncookies

Hi,
when i read the code about the commit
84b114b98452c431299d99c135f751659e517acb tcp: randomize timestamps on
syncookie, i cannot understand the code
in cookie_v4_check：

tsoff = secure_tcp_ts_off(...)
tcp_opt.rcv_tsecr -= tsoff


in my view, this tsoff is different with that which we used when we send SYNACK,

why rcv_tsecr can directly be subtracted by tsoff? any thought behind it?

any feedback is welcome, thanks.

Regards

Xin.