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

[Eric Rescorla <[e-mail]>]

At Fri, 19 Jun 2009 10:06:36 -0700,
Joe Touch wrote:
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> Eric Rescorla wrote:
> ...
> > OK, I fear we're talking past each other.
> > 
> > My claim is that the system must enforce the following invariant:
> > 
> > * There must be only one MKT defined that matches any given set
> >   of parameters: (s-ip, d-ip, s-port, s-port, key-id).
> > 
> > This includes wildcards and ranges.
> 
> That one is a lot easier, since you're including the key-id. That's
> available only for either established connections (e.g., bound to the
> socket after connection establishment for outgoing, or indicated in the
> segment for incoming).
> 
> For the initial outgoing SYNs from legacy applications, we need the MKT
> match to be unique per:
> 
> 	(s-ip, d-ip, s-port, d-port)
> 
> This is the case that's causing the problem for two cases:

You've said that before, but I don't understand why you think it's a
problem. If there are two valid MKTs for a given packet, each side can
pick whichever one they want independently and everything will work.

The match only needs to be unique in the sense that you can't protect
a single packet with two keys. It does not need to be unique and
deterministic in the sense that each side needs to be unambiguously
determine that the *same* key applies to each packet.


> The TSAD/TAPD had a notion of grouping MKTs together when patterns
> overlapped, so that (b) would allow selection of a 'currently designated
> primary' to be used for outgoing SYNs. Getting rid of that structure -
> which you claimed was an implementation issue - is omitting structure in
> the key groupings that allows (b), and also enables (a).
> 
> If we omit the TSAD/TAPD, how do we express the notion that a given key
> is preferred for outgoing SYNs?

We don't. The implementations do it however they want (and I can think of
a number of plausible implementations) . As long as a valid key is selected,
things will work.

-Ekr