Extensibility
Provisions are made for extending constants and structures both in a
device-independent way and in a device-specific (vendor-specific) way. In constants that
are scalar enumerations, a range of values is reserved for future common
extensions. The remainder of values are identified as device specific. A vendor can
define meanings for these values in any way desired. Their interpretation is
keyed to the
extension ID provided in the
LINEDEVCAPS data structure. For constants that are defined as bit flags, a range of
low-order bits are reserved, where the high-order bits can be extension specific. It
is recommended that extended values and bit arrays use bits from the highest
value or high-order bit down. This leaves the option to move the border between
the common portion and extension portion if there is a need to do so in the
future. Extensions to data structures are assigned a variably sized field with
size/offset being part of the fixed part. TAPI describes for each data structure
what device-specific extensions are allowed.
In addition to recognizing a specific extension ID, the application must
negotiate the extension version number that the application and the service provider
will operate under. This is done in the second version negotiation phase of
the
lineGetDevCaps function.
An extension ID is a globally unique identifier. There is no central registry
for extension IDs. Instead, they are generated locally by the manufacturer by a
utility that is available with the toolkit. The number is made up of parts
such as a unique LAN address, time of day, and random number, to guarantee global
uniqueness. Globally Unique Identifiers are designed to be distinguishable from
HP/DEC universally unique identifiers and are thus fully compatible with them.
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