Re: [tcpm] question about TCP-AO and rekeying

[Joe Touch <[e-mail]>]

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Eric Rescorla wrote:
> At Tue, 16 Jun 2009 23:24:55 -0700,
> Joe Touch wrote:
>> Eric Rescorla wrote:
>> ...
>>>> However, the invariant is twofold:
>>>>
>>>> 	a) for a given packet, only one MKT applies
>>>>
>>>> 	b) for two endpoints with multiple MKTs,
>>>> 	the *same* MKT applies.
>>> I don't see that this is true. As I understand the current design
>>> there's no reason that both sides can't use different MKTs
>>> indefinitely.
>> Each side can use a different MKT to transmit. However, if side A uses
>> MKT X to transmit, then side B needs to know to use MKT X to receive. If
>> side A matches MKT X on transmit and side B matches MKT Y on receive,
>> then there's a problem for that connection.
>>
>> So let's rephrase, recognizing that there are two MKTs at any given time
>> (one for transmit on each side, and the same pair for receive on the
>> opposite side).
>>
>> b) for two endpoints, if a given packet matches MKT on one side during
>> transmit, it must match the corresponding MKT on the other side during
>> receive.
> 
> Right, but this doesn't require ordering or non-overlapping, as far as
> I can tell.  It merely requires that at any time there only be one MKT
> corresponding to any given socketpair/key-id.

That is only possible with either non-overlapping or ordering to resolve
overlaps. Note that the MKT has to be unique for incoming packets given
a socketpair/keyid. For outgoing SYNs, the MKT has to be unique within
just a socketpair, because the keyid hasn't been determined yet.

If we prevent overlaps altogether, we may impact utility. We won't be
able to have things like:

	A -> B use MKT X

	A -> * use MKT Y

	* -> * use MKT Z

In this case, A->B matches all three, but having a default 'catchall'
can be very useful, if not expected.

If we allow overlap resolution, then we need to ensure that both sides
resolve to the same result.

Joe
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