There have been some design changes in RDS (remote desktop services) and in RDC (remote desktop client).  Let’s start by discussing the legacy RDP.  RDP is a layered binary-encoded protocol that runs on a lossless connection-oriented transport.  Some of RDP’s layers originated in code derived from the NetMeeting project, which in turn had roots in ITU standards (such as T.120).  As RDP has evolved, the layers have diverged significantly from those standards for performance reasons as headers have been collapsed down and bit-packed for efficiency.  Many of the core design choices in RDP have been made to optimize for performance – bandwidth on the wire as well as encode/decode speed. Many key-protocol structures are hand-designed to explicitly use the least amount of space possible.

The RDP protocol streams graphics in a server-push model with lazy updates to increase the opportunity for the server to coalesce multiple updates into one output packet.  The RDP protocol is driven at the server by a scheduler that takes into account factors like user-input activity (mouse/keyboard) and can accelerate graphics output to provide a smoother interactive experience.  RDP bandwidth use is variable depending on the scenario and design choices are made to ensure that it is ’quiet’ on the wire (no keep-alives or other packets are sent when the screen is not updating).  Below is the legacy RDP Architecture:

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