The security of the data (authentication, confidentiality, integrity and forward secrecy) is handled by the transport security protocol, BTP. The same protocol is used for all transports. BTP is an obfuscated protocol: to anyone except the intended sender and recipient, all data is indistinguishable from random. So BTP's first job is to let the recipient know who sent the data, so the recipient can use the right key to authenticate and decrypt it.