Hi

This may have been discussed before, but I haven’t found such discussion. 
Apologies in advance if this is a stupid question.

Suppose we have to VPN peers configured to set up a tunnel between them. 
Suppose further that the IKE SAs are significantly longer-lived than the IPsec 
SAs.

PFS is configured on both sides, but there are no matching groups (perhaps GW-1 
is configured with only group 19, while GW-2 is configured only with group 20).

When the tunnel is first set up, it is negotiated in the IKE_AUTH exchange. 
Diffie-Hellman is not performed, so the mismatched configuration is not 
detected - traffic flows through the tunnel.

After a while, one of the gateways attempts to rekey the tunnel, or else create 
a new tunnel with the same peer. This time the tunnel is set up using the 
CREATE_CHILD_SA exchange. The SA payload will contain the wrong DH group and 
the exchange will fail, resulting in traffic flow stopping.

As far as I can tell, this behavior is consistent with the RFC, but the user 
experience is very strange. Traffic should either flow or not flow - it should 
not stop at rekeying.

Am I missing something?

Thanks

Yoav

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