NMAC uses an fpad value that is appended to the output of the Cascade function in the penultimate step of the algorithm (as depicted here, p.63) before it is sent to the last step of the NMAC algorithm to produce final tag.
I found that opad and ipad values used in HMAC are one-block constants (0x5c5c5c…5c5c and 0x363636…3636 respectively). I could not find the same for the NMAC's fpad.
What is the value of the fpad? Is it fixed and well-defined? Is it expected to be defined by the standard/cryptosystem that may decide to employ NMAC?