One question for tcp_input function

[ren maosheng]

Hi folks,

I have a question about below source code when I study the stack:

 if (acked > so->so_snd.sb_cc) {
                        tp->snd_wnd -= so->so_snd.sb_cc;
====================>
                        sbdrop_locked(&so->so_snd, (int)so->so_snd.sb_cc);
                        ourfinisacked = 1;
                } else {
                        sbdrop_locked(&so->so_snd, acked);
                        tp->snd_wnd -= acked;
=====================>
                        ourfinisacked = 0;
                }

We reduce the snd_wnd by acked number, I investigate the RFC793 and didn't
find tcp will need to do this.

Could you please shed a light on this? Is there any reason freebsd stack
considering doing this?  Thanks a lot!

Best Regards,
Ren Maosheng