Re: 2822upd-06 Sender

On 2/4/08 at 3:08 AM +0100, Frank Ellermann wrote:

>Hi, on the DKIM list a poster claimed that Sender is OPTIONAL.

Of course, that's obviously false. Sender is not OPTIONAL (defined in 
section 5 of RFC 2119). There are cases where it is REQUIRED (section 
1 of 2119) and cases where "there may exist valid reasons in 
particular circumstances to" not have a Sender, "but the full 
implications must be understood and carefully weighed before choosing 
a different course" (section 3 of 2119). As you say:

>1:  from multiple authors    => MUST (table 3.6 and in 3.6.2)
>2:  one author = transmitter => SHOULD NOT (redundant, 3.6.2)
>3:  otherwise SHOULD (3.6.2)
>
>I don't get case (3), why is it only a SHOULD ?  For anything that's 
>not covered by (2: one author = transmitter) I'd expect a MUST:
>
>- Otherwise, both fields SHOULD appear.
>+ Otherwise, both fields MUST appear.
>
>What could be a plausible excuse to ignore the SHOULD in (3) ?

Two that I can come up with off the top of my head:

- The agent responsible for the actual transmission of the message is 
the company lawyer who vets every e-mail message before sending it 
out. But the lawyer's identity should not be revealed to the rest of 
the world for security reasons.

- The agent responsible for transmission is *really* hard to 
determine. A particular system may have an audit log which can figure 
out who sent it later, but that information is unavailable when the 
message itself is being sent.

It certainly lowers interoperability to not have a Sender when there 
is a single author and we know the transmitter is different from the 
author, but only insofar as we have lost information. But it does not 
eliminate interoperability. Not having a Sender when there are two 
authors *definitely* can damage interoperability since you simply 
*can't* determine any value for the Sender.

pr
-- 
Pete Resnick <http://www.qualcomm.com/~presnick/>
Qualcomm Incorporated - Direct phone: (858)651-4478, Fax: (858)651-1102