cuaa0&ttyd0's bug?

hi,folks:
It seems that the ttyd0 isn't the dialin line to login , and
the cuaa0 like is both the dialin/out device!under 4.9 above
and 5.X .but the ttyd0 work well under 4.8.
here is my test:
I wanna direct connected two bsd box via a null-modem cable,on
a box ,I congfiured the ttys as follow:
# The 'dialup' keyword identifies dialin lines to login, fingerd etc.
ttyd0 "/usr/libexec/getty std.9600" dialup on secure
ttyd1 "/usr/libexec/getty std.9600" dialup off secure
and kill -HUP 1
on the other box , I use cu connenct the box:
cu -l /dev/cuaa0
connected
and then , nothing more i could recv!
but if I confirgure the ttys as below:
# The 'dialup' keyword identifies dialin lines to login, fingerd etc.
cuaa0 "/usr/libexec/getty std.9600" dialup on secure
ttyd1 "/usr/libexec/getty std.9600" dialup off secure
and do it like before i do , it worked!
why???? It's a bug???
thanks in advance
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--wsk