What is the value of the fpad used in the NMAC?
If you look at the original publication of NMAC &amp; HMAC you will notice that there is no such padding in NMAC and in HMAC the constants are XORed with the key.

I don't know where this fpad comes from. It's not mentioned in the HMAC RFC either.

Found it! It is mentioned in the paper with the update security proofs. But there it is stated that it doesn't need to fulfill any requirements (p. 7). I assume that it is not defined in practice because the practical implementation of NMAC is HMAC and, as also stated in the above paper, leaving it out entirely is fine.