[Users] key exchange failed with MIKEY

Hi everybody! I set up my Openser with TLS for signaling and it works well. I use Minisip as softphone and it registers successufully over TLS. But I have the following problem when I try to make a call using SRTP: Openser denies the INVITE cause its size is more than 2048 and then it makes an error as "Message too big". Even if I try to bypass this error in the configuration, the INVITE is still denied. In fact, I saw that Minisip uses MIKEY for the key exchange, and put it in the SDP body, and then it makes quite big but I don't understand why the size must be under 2048. Does someone have any idea to help me? Thanks Greg