There are 3 Thinwire bitmap encoding modes used in current XenApp and XenDesktop Server OS and Desktop OS VDA graphics remoting:

• Full screen H.264
• Thinwire Plus
• Thinwire Plus with selective H.264

(Legacy GDI remoting uses the XPDM Remoting driver and so does not use a Thinwire bitmap encoder)
 
Current Encoding Method (XenApp & XenDesktop 7.16 and lower):
In a typical desktop session, most of the imagery consists of simple graphics or text regions. When any of the above bitmap encoding modes are used, Thinwire selects these areas for lossless encoding using the 2DRLE codec. At the Receiver (client) end, these same elements are decoded (using the Receiver-side 2DRLE decoder) for session display.
 
New MDRLE Encoding Method:
For XenApp & XenDesktop VDA version 7.17, a new higher compression ratio MDRLE encoder has been added. The Citrix HDX product developers have reworked the MDRLE codec so that it consumes less bandwidth in typical desktop sessions; when compared with 2DRLE.
 
MDRLE Codec Benefits:
Lower bandwidth usually means an improvement in session interactivity (especially on shared or constrained links) and ultimately reduced costs.
It is expected bandwidth consumption when using the MDRLE codec will be about 15% less than when compared with XenApp and XenDesktop 7.15 LTSR.
 
MDRLE Codec Configuration:
No configuration is required to avail of the MDRLE codec benefits. Instead, the VDA will negotiate the Receiver-side decoding capabilities and use VDA MDRLE encoding and Receiver MDRLE decoding, if supported. If Receiver does not support MDRLE decoding, the VDA will automatically fallback to 2DRLE encoding.
 
Minimum Requirements:

• XenApp and XenDesktop 7.17 version VDAs.
• Receiver for Windows version 4.11
• (in time other Receiver types will include MDRLE codec support)