We now move from the transport protocol layers to a detailed discussion of the middleware protocols. In this chapter we discuss RFCOMM, the Bluetooth serial port emulation protocol, and the Bluetooth Service Discovery Protocol, or SDP. Version 1.0 of the core specification (volume 1) devotes nearly 90 pages to these two protocols. As with the other detailed discussions of portions of the specification, this chapter attempts to reveal the motivation and thought process behind the development of these protocols. While the important elements of RFCOMM and SDP are examined here, this material focuses on the design basis for the protocols and thus is not a substitute for the specification itself.