On Sun, 05 Aug 2001 19:39:21 +1200, Franck Martin <franck(_at_)sopac(_dot_)org>
said:
So ESMTP could have an extension called RESUME
The protocol would be standard, except that before DATA and after RCPT
TO, the sending server who has identified the RESUME keyword could ask a
RESUME session...
For FTP and HTTP, most transfers are *DOWNLOADS* - which means that the server
has *already* committed space on stable storage to hold the file on at least
a semi-permanent basis. If it *is* an upload you are trying to resume, the
user is most likely uploading to *his* storage space, which is likely
quota-protecteed in some way.
On the other hand, SMTP is a store-and-forward system where the files are
kept in essentially *temporary* storage. As such, the general design
philosophy is to throw away the temporary files if a transfer fails, so
as to free up queue space.
This in and of itself is not a problem, until you realize that quite possibly
more than one file may need to be saved in case the person will *someday*
try to RESUME the transfer. Now.. a real-life case in point.
Our mail server is getting hit *thousands* of times a day with variants
of the SirCam virus. Many of these transfers fail. Currently, our mail
server would just toss the files related to the failed transfer - if it
supported RESUME it would have to keep them around.
It's call a 'denial of service attack'. Feed the victim lots of large files
they have to keep around, and then leave and watch the fun as their mail
server runs out of queue space.
And that's why there isn't a RESUME.
--
Valdis Kletnieks
Operating Systems Analyst
Virginia Tech
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