pkgsrc: distfile fetching issues on a slow connection

[Dmitry Matveev <me%dmitrymatveev.co.uk@localhost>]

Hello,

I have noticed that when I am using a slow 3g Internet connection, some 
heavy distfiles are re-downloaded for many times during the standard 
"make install clean" procedure. The server could just terminate a 
connection, but looks like it is not my case: according to the logs, 
file is fetched completely.

A sample log:

    150 Opening BINARY mode data connection for 
'qt-everywhere-opensource-src-4.7.4.tar.gz' (220388303 bytes).
    100% 
|***************************************************************|   210 
MiB   52.95 KiB/s    00:00 ETA

    421 Service not available, remote server timed out. Connection 
closed.
    220388303 bytes received in  1:08:43 (52.18 KiB/s)
    ftp: No control connection for command
    fetch: Unable to fetch expected file 
qt-everywhere-opensource-src-4.7.4.tar.gz

And then it repeats.

What does the script await from the server after fetching and why it 
restarts the downloading even if the file has been fetched completely?

Dmitry