2.0 and 3.0 compatibility

[Erik Rissanen <erik@axiomatics.com>]

All,

Since the meeting earlier today, I've been thinking about the backwards 
compatibility issue. I think I have a solution which will always work.

What we are aiming for is to be able to mix 2.0 and 3.0 policies and 
requests in a single PDP implementation. There are four cases to consider.

The cases of a 2.0 policy against a 2.0 request and a 3.0 policy against 
a 3.0 request are obviously not issues.

Now, consider a 3.0 policy against a 2.0 request. In this case we can 
upgrade the 2.0 request into an 3.0 request and the 3.0 policy should 
work fine. The upgrade procedure is to simply create attribute 
categories for the subject, resource, action, environment and any 
particular subject categories. The resource content document goes in the 
content element of the resource category and any xpath operation in the 
3.0 policy should work as long it refers to the standard categories. If 
it does not refer to the standard categories, it won't apply to the 2.0 
request anyway.

The 2.0 policy and 3.0 request is a bit trickier. As we have discussed 
at length earlier, it is very difficult to upgrade a 2.0 policy into 3.0 
in general due to the problem of rewriting xpath expressions.

However, it seems to me that it is easy to downgrade the 3.0 request 
into a 2.0 request. Initially I thought it would not be possible since 
the 3.0 request is more expressive than the 2.0 request, but it turns 
out it doesn't matter.

What we do is that we create a 2.0 request from the 3.0 request as 
follows: From the standard resource, action and environment categories 
in the 3.0 request we create the resource, action and environment 
elements of the 2.0 request. The remaining 3.0 attribute categories all 
go into the subject element as subject categories.

The treatment of the resource, action and environment should be clear. 
But what about all the categories which could mistakenly be made into 
subject categories? There are two cases to consider:

1. The 2.0 policy refers to the 3.0 category.
2. The 2.0 policy does not refer to the 3.0 category.

In case 1, this can only happen if the 3.0 category is actually a 
subject category, so this means we have made a correct translation.

In case 2, the mistranslation will not have any effect on the decision.

Anyway, please review my thinking. It's midnight here now and I am up 
only because I cannot sleep due to excess caffeine intake earlier today. 
I might have missed something here. :-)

Regards,
Erik