Re: predictable IP ID

Savochkin Andrey Vladimirovich (saw@msu.ru)
Tue, 5 Oct 1999 20:41:06 +0400


Hi,

Well, unlike Alexey I accept opinions even when a work has been done :-)

On Mon, Oct 04, 1999 at 10:25:19PM +0200, Andi Kleen wrote:
> The secure RND can run with a few cache lines, the AVL inetpeer
> code needs a potentially unlimited number of cache lines. Given
> the current CPU/memory speed ratio, I think the OpenBSD approach
> looks much better (my first vote for the per-dst counter was based
> on the assumptions that it is free because the routing cache can be
> used). Also I think the secure RND is needed anyways to generate

I thought that the IDs can be stored in the routing cache directly.
Alexey explained me why they should not and I agreed.

BTW, OpenBSD solution sucks. After some analisys now I'm able to predict the
whole sequence taking 7 consecutive IDs. You may send me an example and
I'll send you a sequence of the next IDs :-)

Now we have a strong RND based generator for the first ID for a peer. But
its fairly slow (MD4 hash) and can't be used for every packet. Strength of
the generator and requirements for a guaranteed long period contradicts each
other.

> the initial per destination ip counter (so unless you always talk
> to the same host the overhead is the same + some ugly code)

Andrey

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