tripple des

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?