IPsec inbound processing

[827 <827@nu.edu.pk>]

The RFC 2401's section 5.2.1 says about selecting an SA / SA bundle for
Inbound IP traffic. I am at the moment quite confused about the sequence of
searching and using an SA. 

Why is it so that we first directly look for an SA from SAD using the
selector valus of the packet? Why not we directly refer to SPD than get the
SA pointer from there (SPD) to look it in SAD. 

And if it is not the way i have written it, how an inbound ip packet is
processed?

Regards.

Siddique
FAST - NU Pakistan