[libsrtp] SRTP Payload

Hello, I want to ask about the payload from SRTP. Here I use the 128-bit AES_CM encryption feature. What I want to ask is for every srtp_protect / srtp_protect_mki function call, does that represent an SRTP packet?

Then in the srtp_protect_mki module, the srtp_encrypt function will be called which is related to the srtp_aes_icm_encrypt function. Then what I want to ask is what is the payload size which is the parameter of the function? Will selection of the AES_CM_128 bit feature affect the payload length of each SRTP packet?

Thank you