Manipulating modules

STREAMS provides the capabilities to manipulate modules from the user level, to interchange modules with common service interfaces, and to change the service interface to a STREAMS user process. These capabilities yield further benefits when implementing networking services and protocols, including:

• User level programs can be independent of underlying protocols and physical communication media.

• Network architectures and higher level protocols can be independent of underlying protocols, drivers, and physical communication media.

• Higher level services can be created by selecting and connecting lower level services and protocols.

The following examples show the benefits of STREAMS capabilities for creating service interfaces and manipulating modules. These examples are only illustrations and do not necessarily reflect real situations.

Protocol portability

``X.25 multiplexing stream'' shows how the same X.25 protocol module can be used with different drivers on different machines by implementing compatible service interfaces. The X.25 protocol module interfaces are Connection Oriented Network Service (CONS) and Link Access Protocol - Balanced (LAPB).

X.25 multiplexing stream

Protocol substitution

Alternate protocol modules (and device drivers) can be interchanged on the same machine if they are implemented to an equivalent service interface.

Protocol migration

``Protocol migration'' illustrates how STREAMS can move functions between kernel software and front-end firmware. A common downstream service interface allows the transport protocol module to be independent of the number or type of modules below. The same transport module connects without change to either an X.25 module or X.25 driver that has the same service interface.

By shifting functions between software and firmware, developers can produce cost effective, functionally equivalent systems over a wide range of configurations. They can rapidly incorporate technological advances. The same transport protocol module can be used on a lower capacity machine, where economics may preclude the use of front-end hardware, and also on a larger scale system where a front-end is economically justified.

Protocol migration

Module reusability

``Module reusability'' shows the same canonical module (for example, one that provides delete and kill processing on character strings) reused in two different Streams. This module is typically implemented as a filter, with no downstream service interface. In both cases, a tty interface is presented to the Stream's user process because the module is nearest to the Stream head.

Module reusability