Re: Abort data transfer?

On 19 Oct 2009, at 16:03, David MacQuigg wrote:

> Thanks to all who answered this question.
>
> Hector Santos wrote:
> > SM wrote:
> > > Hi David,
> > > At 14:29 17-10-2009, David MacQuigg wrote:
> > > > Is it permissible to abort data transfer before the end of data,  
> > > > say after you have received all headers, and you know the DKIM  
> > > > sig is bad?
> > > > RFC-5321 section 3.3 says:
> > > >
> > > >  If the verb is initially accepted and the 354 reply issued, the  
> > > > DATA
> > > >  command should fail only if the mail transaction was incomplete  
> > > > (for
> > > >  example, no recipients), if resources were unavailable  
> > > > (including, of
> > > >  course, the server unexpectedly becoming unavailable), or if the
> > > >  server determines that the message should be rejected for policy  
> > > > or
> > > >  other reasons.
> > > That paragraph is about the DATA command.  You haven't received  
> > > any headers yet.  You don't know whether the DKIM signature is  
> > > bad.  This is more of a SMTP question though.  You can do anything  
> > > you wish for policy reasons.  As long as you adhere to the RFC  
> > > 5321, you won't cause interoperability or hard to debug problems.   
> > > I suggest not aborting the data transfer to avoid such problems.
> > +1. Aborted Data transfers can cause retransmissions.  
> > Unfortunately, it is better to receive the entire payload and then  
> > issue a negative response.
> Forget about DKIM.  What if there is some other reason to abort,  
> like the payload is too large?  We cannot just continue receiving  
> data forever.  Is there a "permissible" way within SMTP to abort  
> during data, or should I just ignore these ambiguous requirements,  
> and either: 1) Send a TCP reset, or 2) Let the transmitter hang.
My implementations note the reason for rejection, then switch the  
event handler for the connection to one that just reads each buffer  
and then throws it away.  There is nothing more that can be done.   
Even trying to inform the sender by writing the error to it while it  
is sending won't necessarily work, because it might not choose to look  
at its inbuffers until it's quite finished spewing.  And in any case  
that's standards violation territory you're wandering into.
So you'll just have to do what you can to keep the connection alive  
for the data to come in, then reject it and hopefully at that point  
you can resume the dialogue to finish up and ensure the sender will  
leave you alone.  Anything else is basically suboptimal.
Cheers,
Sabahattin