Martin,
We are on the same page, I think. I am not sure a single SMTP
message can have multiple true recipients because SMTP clients I have
seen would usually send multiple copies, one to each To:/Cc: address.
(SMTP gurus please correct me). Fan out is possible when recipient is
an alias (management(_at_)company(_dot_)com) that SMTP server expands into
unfiltered bob(_at_)company(_dot_)com and filtered
rob(_at_)company(_dot_)com(_dot_) As far as I
can see, that expansion can happen at the OPES client or at the OPES
server (in theory), but the difference is minor.
Obviously, this optimization feature complicates the protocol. Special
care needs to be taken to insure that all expanded recipients get the
message even if, for example, the OPES server goes down in the middle
of the processing, after sending the first (marked, unfiltered)
response to the first group of recipients. This complicates
client-server negotiations: they need to agree on how many responses
and to what recipients there will be so that OPES client can take over
if OPES server fails (and save the messages to yet unprocessed
recipients, for example).
Do you think the optimization is worth the complexity? Can you think
of other, non-SMTP related uses where multiple responses to a single
OPES request would be useful?
N.B. A third way to implement what you want is to allow multiple
"recipient" addresses in an OPES request to mean that there are
actually multiple requests, _without_ duplicating the request on the
wire. Same for responses. This approach could be called a "lazy copy"
optimization. The difference is subtle, but it causes fewer protocol
complications than allowing (at the protocol level) multiple responses
to a single request.
Thank you,
Alex.
On Mon, 17 Feb 2003, Martin Stecher wrote:
Hi Hilarie and Alex,
I think that we are not talking about the same SMTP filtering
scenario. Let me build on the virus filtering example and describe a
concrete use case:
There is a SMTP MTA dealing as the incoming mail server of a
company. It is OPES enabled and uses a callout service to do virus
filtering on all the incoming messages. Every individual in the
company can select his/her preference: Infected messages can either
be replaced by an alert message or can be repaired. This property is
kept in a directory service for the company. The OPES SMTP device is
a general implementation which is not aware of virus scanning
techniques and especially not on the user preferences, it does not
access the directory service at all, but the callout service does.
Now the OPES device receives an email messsage containing a virus
which is sent to 20 recipients within the company. It is a single
message!
The OPES client forwards this one message to the callout service.
That one looks up the AV filter preference in the company's
directory service. The callout service detects the virus and sees
that 18 recipients want to have the message repaired and 2
recipients prefer to get the virus replaced by an alert.
From this point on two copies of the SMTP message are required with
different content. One to be sent to 18 recipients the other to the
other 2.
How could this be solved?
- By enabling the OPES protocol to return two copies of an application
message in response to one filtering request
- By requiring that the OPES edge device ensures to pre-generate copies
of the application message where each copy guarantees to result in
only a single response.
Martin