1.7 Versioning and Capability Negotiation
Capability negotiation for RDP is essentially the same as for T.128. The server advertises its capabilities in a Demand Active PDU sent to the client, and the client advertises its capabilities in the follow-up Confirm Active PDU (see the Capability Exchange phase in section 184.108.40.206). Capability sets are packaged in a combined capability set structure. This structure contains a count of the number of capability sets, followed by the contents of the individual capability sets.
Figure 5: Combined Capability Set structure
Information exchanged in the capability sets includes data such as supported PDUs and drawing orders, desktop dimensions, and allowed color depths, input device support, cache structures and feature support. The client and server do not violate any peer capabilities when sending data on the wire. This ensures that all RDP traffic on the wire is consistent with expectations and can be processed by each party.
Early capability information (in the form of a bitmask) is advertised by the client as part of the data which it sends to the server during the Basic Settings Exchange phase. This information is intended for capabilities that need to be advertised prior to the actual Capability Exchange phase. For example, support for the Set Error Info PDU is established before the Licensing phase of the connection sequence, which occurs before the Capability Exchange phase (section 220.127.116.11). This is necessary because the server has to be aware of how errors can be communicated back to the client.
The client and server data exchanged during the Basic Settings Exchange phase in the RDP Connection Sequence (section 18.104.22.168) includes an RDP version number (consisting of a major and minor field). However, this version information does not accurately reflect the version of RDP being used by RDP 5.0, 5.1, 5.2, 6.0, 6.1, 7.0, 7.1, 8.0, and 8.1 clients because they all advertise a minor version of "4").
The build number of the client is also available as part of the data the client sends to the server during the Basic Settings Exchange phase. However, this value is implementation-dependent and is not necessarily consistent across the spectrum of RDP clients manufactured by different vendors.