In IIS FTP(v5.0), REST command has no effect!!

hi,

I am developing an FTP client using MFC/socket. My program can connect to the server running under Linux, Serv-U, and IIS FTP, and download/upload files from the server. But i found that the IIS server does not work well with REST command. Whatever i send REST to set the restart position, the server sends me the WHOLE file. But in response to the REST command, it says ok by replying
350 Restarting at xxxxx.

My program works all well when it connects to another kind of server.
Could anyone tell me what may be wrong in my program? Thanks.
And could anyone tell me what will cause the IIS FTP server to ignore the REST command?

here's some content of the FTP session,
TYPE I
200 Type set to I.
PASV
227 Entering Passive Mode (61,141,***,***,16,73)
REST 526108
350 Restarting at 526108.
RETR ASG1.dcm

125 Data connection already open; Transfer starting.
226 Transfer complete.