Hi,
I transfered my L1B to H1B with new employer. I want to start my Green Card process with my new employer. My Old L1B employer(Indian MNC) is not giving me experience letter . I was with him around 4.5 years. is it mandatory to have experience letter from previous employer for GC Process ? In which stage of GC process experience letters are required ? what happens if one can fail to present experience letter ? what all options i have to prove my employment with previous employer other than experience letter ?

Thanks,