The standard definition for the FTP protocol is RFC 959.

You can also try _TCP/IP Illustrated_ (several volumes) by
W. Richard Stevens.  Although I wish I had this complete
set, I don't, so can't vouch for whether there's adequate
information to implement an FTP server.

No doubt you can also get the C source code from several
sources as a reference as well.

Terry J. Westley, Software Development Manager
Veridian Engineering, Information Warfare
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Author of TASH, an Ada binding to Tcl/Tk.
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