that document talk about PRET which is implemented almost exactly like mog suggestion.
for specific implementation ftp client could use OPTS command.
Quote:
Introduce a new command called the Pre Transfer (PRET) command (see further description below in the section on new commands under consideration). This command would allow arbitrary state information to be set for a single transfer before any other command were issued. This option has the advantage that state information other than the filename could be provided. File size is one option that comes to mind. However, this system allows information not anticipated today to be made available.
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PRET PRETransfer Command
FTP has no way of specifying critical information (such as filename) before the start of a transfer. The introduction of PRET (pre-transfer) is designed to fix this inadequacy.
The PRET commands sets state for the *next* transfer only. It does not set global state, and is tightly coupled to a single STOR/RETR command.
PRET <RETR/STOR> [attribute list]
arguments:
----------
<RETR/STOR>:
The first argument MUST be the string "RETR" or the string "STOR". In order to make key decisions the server must know what type of transfer this command is in regard to.
[attribute list]:
The attribute list contians comma seperated attribute value pairs. The attribute is defined by the specific server implementation. This allows it to be modified as new requirements come up.
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