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Re: a question on SPP draft ...


> I realize this, but this would seem to be less efficient than checking
> against an ordered list.  Or am I mistaken?

Well, yes and no.

If you linearly search a decorrelated list of policy rules, on average
it will be less efficient since there will be a larger list of more
complex policies to search.  No argument there.

However, there are at least two things you need to also consider when
looking at efficiency here.  

1) Searching no longer has to be linear.  You can attempt to optimize
your searches.  For example, you can keep a cache of the most recently
used policies, or move the most often used policies to the top of the
list.  Since they are decorrelated, these (and I'm sure other) techniques
can be utilized to speed up searching.

2) The process of setting up an end-to-end communication is much more
than just looking up SPD entries.  Decorrelation was introduced into
SPP to enable caching of policies from other policy servers, since the
policy exchange and resolution process is fairly expensive.

I don't have numbers to show that decorrelation is worthwhile
system-wide from an efficiency standpoint, but the benefit of being
able to cache policies for resolution seem to intuitively make a 
big difference.

> Eg. the correlated, ordered list:
>     1. ->
>     2.   (catch all)
> turns into the decorrelated
>     1. ->
>     2. OR
> If I am using the decorrelated version and I have already checked step
> 1, the range checking is redundant.  I would like to optimize this
> redundancy from the policy checking that is actually used.
> In addition, when doing an IKE negotiation to establish SAs that
> these policies require, the correlated (2) is negotiable whereas
> the decorrelated (2) is not (IKE does not, currently, support
> unions of ranges for instance).

The decorrelated rule (2) can be broken into two rules that can be

Besides, negotiating ranges with correlated policy rules in IKE can
encounter the same dangers as caching correlated policies in SPP,
so it's much safer to negotiate decorrelated policies in IKE.