policy expiry and update issues

In order to cope with policy update problems and the non-ambiguity problems
during policy transition phases (cf. section 2.4.1), the element EXPIRY
could be slightly modified and a new element LAUNCH could be introduced.

Actually, specifying relative expiry times longer than one day makes it
impossible to switch to a new policy and to be sure that there are no
cached policies no more identical to the newly deployed policy.

This issue could be addressed by allowing a simultaneous use of the ?date?
and the ?max-age? attributes of the EXPIRY element with the semantics:
?this policy / policy reference file is valid for the duration specified by
?max-age? but not after the date specified by ?date??.

In addition, the introduction of a new element LAUNCH as a companion to
EXPIRY could streamline the policy transition process. An attribute ?date?
indicates since when the policy will be valid. In written text privacy
policies, it is usual to indicate when they will come into effect. Multiple
policies with non-overlapping validity periods could be specified for a
same URI to handle policy transition.

Best wishes, Sören Preibusch.