The notion of allowing continuation lines to start with arbitrary text solves a minor problem for a few server FTP implementers at the expense of creating a major problem for all user FTP implementers. Given that multi-line replies are allowed to be nested, the fact that the maximum allowed level of nesting is left unspecified creates a hardship for implementers of user FTPs. This hardship is somewhat easily solved on a machine that has hardware stacks, but not so for other machines. This prohibits sending data between the final end of record and the end of file, but does not specify what the receiver of data is to do if this rule is broken. A major complaint about the protocol concerns the fact that the writer of an FTP user process must handle a considerable number of special cases merely to determine whether or not the last command sent was successful.
Different commands have different success reply codes.
There are several types of replies that are extraneous from the point of view of a user FTP process, and their reply codes have no characteristic that makes them easily distinguishable. If it is desired to communicate more detailed information, the text of the reply may used for this purpose.