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One obvious approach would be not to sync after every exchange (that could be a lot of messages), but sync, say, every N seconds (say, N=5) in one big batch (for all IKE_SAs that changed in the last N seconds). Most of the time, almost all IKE_SAs are just sitting there idle (so IKEv2 message ID counters don't change). In case of failure, the stand-by device would have out-of-date information for some small percentage of IKE_SAs (those whose counters changed since last sync) , but that's always going to be the case (for exchanges where something more happened just before/during the failure).
I haven't done the math, though, so I don't know what value of N would result in both acceptable bandwidth and acceptable failure rate for the stand-by (depends on how many messages your typical IKE_SAs have per hour on average)...
From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of ext Kalyani Garigipati (kagarigi)
Sent: 03 September, 2009 16:07
Subject: [IPsec] Ikev2 HA message Id Issue
In Ikev2 HA, there is an issue with the message Id and window size. Standby device-----------------------active device----------------------------------Peer device
The active device participating in the exchange with the peer will update its message id counters as per the exchanges done. This info cannot be synced to the stand-by device for every exchange done since that would take up all the bandwidth and is not an efficient way.
The stand-by device when it becomes active will start with the message Id as 1 and this will not be accepted by the peer, since its message Id counters are different. Hence a solution is required to sync the message Id counters to the standby device.
I think solution 1 should be implemented with Ikev2. Please give your comments