The following references can be handy to anyone wanting a nuts and bolts understanding of TS communication. If you know of any not covered here, please feel free to drop me a line - aka(at)mvps.org.
Matthew Chapman of RDesktop fame has the most complete public protocol documentation in the form of the RDesktop source code.
He has collected the links for the ITU-T drafts and RFCs that are somewhat relevant; Although the links are here, I strongly suggest checking out the RDesktop site for more up-to-date information.
Application Sharing Protocol
Multipoint Communication Service
ISO DP 8073 Transport Protocol
ISO Transport on TCP
Although the RDP protocol is not open in the traditional sense, Microsoft does allow software vendors access to the specification at no initial cost.
If the vendor later produces a product which uses the RDP protocol, under some circumstances it may need to be licensed from Microsoft; that can be done by one-time fee payment.
Remote Desktop Protocol Specification Now Available Under License