Network Direct SDK/DDK may be obtained as a nuget package (preferred) from:

https://www.nuget.org/packages/NetworkDirect

or downloaded from:

https://www.microsoft.com/en-us/download/details.aspx?id=36043
on page press Download button and select NetworkDirect_DDK.zip.

Extract header files from downloaded
NetworkDirect_DDK.zip:\NetDirect\include\ file into this directory,
or add path to NetDirect headers into VS include paths