Some history - LCS2005 let you do appsharing from communicator via the netmeeting client.  Netmeeting used T.120 underneath as a data conferencing and appsharing protocol.  Worked pretty well for a handful of people - didn't scale well past a dozen or so but people really liked having that appshare button to click on in the Communicator dialog window.  NetMeeting was tough to use without putting up directory servers or Exchange Conferencing Service so this worked well for the ad-hoc small meeting use case.

We moved away from NetMeeting in Vista and so in OCS2007 appsharing was gone from Communicator.  In its place we added the full featured web conferencing Live Meeting client with a very similar feature set to the Live Meeting service.  Here you can do appsharing and much more - whiteboard, polls, markup, ppt, etc.  You could escalate to a LM from Communicator and the LM client would launch inviting everyone into a meet now session.  This scaled up nicely becuase we had a new OCS server side MCU handling it (DATAMCU).  PSOM - Persistent Shared Object Model is the protocol behind LM originally developed by PlaceWare.  PSOM is much more efficient on the network than T.120 with estimates of 10kbps for a typical participant.  PSOM uses TCP port 8057 and is encrypted with TLS.

So now we're at OCS 2007 R2 and what do we do - add back in click-to-appshare right within the Communicator client via RDP.  The first version of RDP was based on T.128 - same family of ITU standards that Netmeeting was based on!  Little bit of dejavu here but RDP is much better than netmeeting - very efficient and very mature protocol that's been around and tweaked since NT4.0.  The standard listening port for RDP is 3389 but OCS treats RDP as real-time media which means we wrap it up in RTP and the payload is RDP.  We turn off RDP encryption since we encrypt the RTP payload for SRTP.  We treat it like any other media type.  By the way that's why we dont support you virtualizing CWA since it translates RTP to JPGs for app sharing to browser clients (we dont support virtualizing any roles that handle media).

I had a customer approach me about RDP capabilities for sharing 3d images and depending on the application it is much better suited for such things than PSOM but keep in mind it is still a protocol for remote viewing/control and wont work too well for video at 30fps.  There is a new appsharing MCU to scale this up (RDPMCU) so multiparty works fine although only one person can be in control at a time.  It also doesn't have the full capabilites of PSOM like annotations for example so the LiveMeeting client is better there.  I gotta say though I just love having the capability back right in OC and I use it all the time for quick ad-hoc sharing/collab.  So there's the formula - no more netmeeting and R2 adding RDP in addition to PSOM - RDP+PSOM-NetMeeting=R2!

 Here are some links on how RDP works and optimizations we have made in the protocol:

MSDN Protocol Doc

Top 10 RDP Protocol Misconceptions - Part 1

Top 10 RDP Protocol Misconceptions - Part 2

RDP Performance Whitepaper