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The Device Session Property Access (DSPA) Protocol is used to exchange name-value pairs between the host computer and the device for Audio-Visual (A/V) and device-capability properties.
This protocol uses the Device Services Lightweight Remoting (DSLR) Protocol, specified in [MS-DSLR] to enable the remoting of services between the two devices over a reliable point-to-point channel.
The Property Bag service (section 2.2.1) messages, are implemented and offered by the device (acting in this case as the stub) while the host computer acts as the proxy, in DSLR nomenclatures. For a more detailed definition of these roles, please refer to [MS-DSLR]). The Property Bag service contains the following messages or functions:
GetStringProperty (section 2.2.1.1): This function is used to get a string value for the given property described by the property name.
GetDWORDProperty (section 2.2.1.3): This function is used to get a DWORD value for the given property described by the property name.
SetDWORDProperty (section 2.2.1.2): This function is used to set a DWORD value for the given property described by the property name.
As described previously, at any given time, the host computer will act as a client (proxy in DSLR terminology, which invokes the service remotely) and client device will act as a server (stub, which performs the request). In this document, we will refer to the client device as the stub and the host computer as the proxy, but for the sake of simplicity and consistency, in general cases we will always refer the host computer as the "host" and the client device as the "client".
The following block diagram shows the relationship between the host device (that is, the host computer) and the extender device (client).
Figure 1: Device Session Property Access Protocol block diagram