op=sub

Hi, for an interesting article by John Levine in CLEAR see:

<http://mipassoc.org/pipermail/ietf-clear/2004-November/000165.html>

Here's how I understand this article for our purposes (copied
from a minor update of the "op" proposal skipping old stuff):

* version 0.3
  - removed rfc822
  - removed pra
  - added   sub
* to do
  - use a style suited for a separate Internet draft based on
    SPF, this text won't be a future chapter 6.3 in SPF drafts.

6.3  op: options

   options = "op=" name *( "." name )

   The op modifier introduces a dot-separated list of optional
   properties.  New properties can be defined in additional
   documents in the same way as new modifiers.

   An initial set of optional properties is defined below:
   "helo", "trusted", "auth", and "sub".
[...]
6.3.4  The optional "sub" property

   SPF implementatios are expected to determine the "zone cut"
   for a given domain if it does not have its own sender policy.

   The strategy outlined in [RfC 2181] chapter 6 for this task
   is not necessarily the best strategy.  A simple way to find
   a sender policy above any given domain is to walk up the tree
   by removing labels left to right.  This procedure could find
   a sender policy for a domain below the wanted "zone cut".

   The "sub" property instructs implementations that the found
   sender policy MUST NOT be used for subdomains.  Because SPF
   implementations are free to ignore all options SPF records
   at the "zone cut" SHOULD NOT specify the "sub" property.
-- 
See <http://purl.net/xyzzy/home/test/> for complete text, bye.