if enable ssl communication between the SA devices, any impact on the performance?
Yes there is a performance overhead although this should be negligible on Series 4/4S appliances, I suppose the question is, do you need to secure transmissions between the appliances? if so then you require the SSL option to be enabled (isn't by default).
Yes, correct, between appliances. Default it's not enabled for most of the services like decoder, concentrator, broker and etc.
If enable, what's the performance impact? I assume between decoder and concentrator will be impact the most.
The impact would entirely depend on configuration,hardware,software version and throughput (log/packet??), without understanding this it is not possible to specify the exact performance hit. As the chattiest communication is between the Decoder and Concentrator then, yes this is where the highest impact would be.
what are the dependency regarding configuration,hardware,software version and throughput (log/packet)?
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