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This section describes how STREAMS multiplexing configurations are created and also discusses multiplexing drivers.
Earlier, Streams were described as linear connections of modules, where each invocation of a module is connected to at most one upstream module and one downstream module. While this configuration is suitable for many applications, others require the ability to multiplex Streams in a variety of configurations. Typical examples are terminal window facilities, and internetworking protocols (which might route data over several subnetworks).
``Many-to-one multiplexor'' shows an example of a multiplexor that multiplexes data from several upper Streams over a single lower Stream. An upper Stream is one that is upstream from a multiplexor, and a lower Stream is one that is downstream from a multiplexor. A terminal windowing facility might be implemented in this fashion, where each upper Stream is associated with a separate window.
Many-to-one multiplexor
``One-to-many multiplexor'' shows a second type of multiplexor that might route data from a single upper Stream to one of several lower Streams. An internetworking protocol could take this form, where each lower Stream links the protocol to a different physical network.
One-to-many multiplexor
``Many-to-many multiplexor'' shows a third type of multiplexor that might route data from one of many upper Streams to one of many lower Streams.
Many-to-many multiplexor
The STREAMS mechanism supports the multiplexing of Streams through special pseudo-device drivers. Using a linking facility, users can dynamically build, maintain, and dismantle multiplexed Stream configurations. Simple configurations like the ones shown in ``Many-to-one multiplexor'' through ``Many-to-many multiplexor'' can be further combined to form complex, multilevel, multiplexed Stream configurations.
STREAMS multiplexing configurations are created in the kernel by interconnecting multiple Streams. Conceptually, there are two kinds of multiplexors: upper and lower multiplexors. Lower multiplexors have multiple lower Streams between device drivers and the multiplexor, and upper multiplexors have multiple upper Streams between user processes and the multiplexor.
``Internet multiplexing stream'' is an example of the multiplexor configuration that typically occurs where internetworking functions are included in the system. This configuration contains three hardware device drivers. The IP (Internet Protocol) is a multiplexor.
The IP multiplexor switches messages among the lower Streams or sends them upstream to user processes in the system. In this example, the multiplexor expects to see the same interface downstream to Module 1, Module 2, and Driver 3.
Internet multiplexing stream
``Internet multiplexing stream'' depicts the IP multiplexor as part of a larger configuration. The multiplexor configuration, shown in the dashed rectangle, generally has an upper multiplexor and additional modules. Multiplexors can also be cascaded below the IP multiplexor driver if the device drivers are replaced by multiplexor drivers.
``X.25 multiplexing stream'' shows a multiplexor configuration where the multiplexor (or multiplexing driver) routes messages between the lower Stream and one upper Stream. This Stream performs X.25 multiplexing to multiple independent Switched Virtual Circuit (SVC) and Permanent Virtual Circuit (PVC) user processes. Upper multiplexors are a specific application of standard STREAMS facilities that support multiple minor devices in a device driver. This figure also shows that more complex configurations can be built by having one or more multiplexed drivers below and multiple modules above an upper multiplexor.
Developers can choose either upper or lower multiplexing, or both, when designing their applications. For example, a window multiplexor would have a similar configuration to the X.25 configuration of ``X.25 multiplexing stream'', with a window driver replacing the Packet Layer, a tty driver replacing the driver XYZ, and the child processes of the terminal process replacing the user processes. Although the X.25 and window multiplexing Streams have similar configurations, their multiplexor drivers would differ significantly. The IP multiplexor in ``Internet multiplexing stream'' has a different configuration than the X.25 multiplexor, and the driver would implement its own set of processing and routing requirements in each configuration.
X.25 multiplexing stream
In addition to upper and lower multiplexors, you can create more complex configurations by connecting Streams containing multiplexors to other multiplexor drivers. With such a diversity of needs for multiplexors, it is not possible to provide general purpose multiplexor drivers. Rather, STREAMS provides a general purpose multiplexing facility, which allows users to set up the intermodule/driver plumbing to create multiplexor configurations of generally unlimited interconnection.