.s2p file format
For truly passive interconnects and devices, you will likely not see too much difference between S12 and S21. However, for automated extraction you should probably correct the Touchstone (R) files to have the same ordering.
The IBIS Open Forum worked with Agilent some time ago to produce a draft specification document for Touchstone(R), as Touchstone(R) is supported by the IBIS Interconnect Modeling (ICM) Specification. You can find the draft specification document below:
**broken link removed**
I hope this helps!
- Michael Mirmak
Intel Corp.
Chair, EIA IBIS Open Forum