1.3 Protocol Overview (Synopsis)

The Remote Desktop Protocol: NSCodec Codec Extension reduces the bandwidth associated with desktop remoting by efficiently compressing 24 bits per pixel (bpp) and 32 bpp images. This is achieved by using the NSCodec bitmap codec. This bitmap codec is based on the bitmap compression techniques introduced in [MS-RDPEGDI] section 3.1.9.

The [MS-RDPBCGR] PDUs that encapsulate [MS-RDPNSC] structures are summarized in the following figure.

Figure 1: Encapsulation and sequencing of NSCodec settings and bitmaps

[MS-RDPNSC] settings are encapsulated in an NSCodec Capability Set (section 2.2.1), which is ultimately transported in a server-to-client Demand Active PDU ([MS-RDPBCGR] section 2.2.1.13.1) or client-to-server Confirm Active PDU ([MS-RDPBCGR] section 2.2.1.13.2). The Demand Active PDU and Confirm Active PDU are transmitted during the Capabilities Exchange Phase of the RDP Connection Sequence ([MS-RDPBCGR] section 1.3.1.1).

When the RDP Connection Sequence has run to completion, bitmap images of the user's session are transmitted from the server to the client ([MS-RDPBCGR] section 1.3.6). NSCodec compression techniques (section 3.1.8) and structures (section 2.2.2) are used to efficiently transport these bitmaps so that they can be rendered on the client. NSCodec-compressed bitmaps that cannot be cached are sent encapsulated in Set Surface Bits Surface Commands ([MS-RDPBCGR] section 2.2.9.2.1), which are ultimately transported in a server-to-client Fast-Path Surface Commands Update ([MS-RDPBCGR] section 2.2.9.1.2.1.10). NSCodec-compressed bitmaps that can be cached are sent encapsulated in Cache Bitmap – Revision 3 ([MS-RDPEGDI] section 2.2.2.2.1.2.8) Secondary Drawing Orders, which are ultimately transported in a server-to-client Fast-Path Orders Update ([MS-RDPEGDI] section 2.2.2.2). Bitmap caching is discussed in [MS-RDPEGDI] section 3.1.1.1.1.