tripple des

[Timothy Newsham <newsham@wiliki.eng.hawaii.edu>]

Crypto question:
why was the following chosen for tripple DES :
   EN(DE(EN(data,k1),k2),k3);   

The encryption would involve passing data through IP,
then doing 16 rounds forward with k1,
(factoring out the IP-1 and IP)
then doing 16 rounds backwards with k2
(factoring out the next IP-1 and IP)
then doing 16 rounds forward with k3
then going through IP-1

How would this compare with
   EN(EN(EN(data,k1),k2),k3);

which goes through IP,  does 16 rounds each with k1, k2 then
k3, then IP-1 ?

The only difference is that the key scheduler rotates backwards
(or another interpretation keys used in reverse order) for the
second stage.

Does anyone know the rationale behind this?