Re[2]: Intent to revive "expires" header from draft-ietf-mailext-new-fields-15

[Chris Haynes <chris@xxxxxxxxxxxxxxxxx>]

On Monday, July 28, 2008 at 5:58:31 PM, Bill McQuillan wrote:

> An Observation:

> Conceptually there is a difference between how long the information in the
> message is thought to be relevant versus how long the message itself is
> thought to be relevant.

> Perhaps some of the controversy here is due to conflating these two things.

Quite. 

To put it another way, there is continuing confusion over whether the originator is communicating an expiry date to the mail transport / storage system, or to the human recipient of the message.

IMHO there can only be two possible 'meanings' attached to the header.

If it is intended to be received and understood by the mail transportation system I contend that the only possible semantic can be:

1) "After the given date the mail system MAY discard this message." 

(This is regardless of whether or not it has been delivered and/or if the recipient has seen it.)



The only other possible use of the 'Expires' header (so far as an RFC822 group is concerned) is as a transparent communication between originator and recipient. In this case:

2A) The mail transportation system MUST transport the header without modification and 
2B) MUST NOT pay any attention to its presence or absence.



It's my assertion that, with the architecture of today's mail systems, these are the only two possible semantics we can discuss here.

Anything else is _incapable_ of being defined, implemented and used in an unambiguous, universal way; it would have to cross the architectural boundary between originating (non-technical) human and receiving technical machine.

Debate...

Chris Haynes