SSL client certificate authentication frequency and SSL Session ID

Hi,

I'm trying to write a very simple iRule to insert SSL Session ID value in HTTP request header. I require the client authentication in the Virtual Server sslclient profile and I've noticed that the F5 generate and send the Session ID value in the ServerHello SSL message only if a configure the frequency for the client authentication to a value of "once", if I set it to "always" then the Session ID is always empty.

I guess that when I set it to "always" the Session ID value is empty because the F5 doesn't want to reuse the session in any case and prefers a full handshake with the client, but I'm not sure of this.

Can anyone explain why the SSL Session ID is empty or has a value depending on the client authentication frequency parameter?

Thanks!