HP 800 HP DLPI Programmer's Guide - Page 25

Introduction to DLPI The Data Link Layer Physical Attachment Identification The physical point of attachment (PPA in Figure 1-2) is the point at which a system attaches itself to a physical communications medium. All communication on that physical medium funnels through the PPA. On systems where a DLS provider supports more than one physical medium, the DLS user must identify which medium it will communicate through. A PPA is identified by a unique PPA identifier. For media that supports physical layer multiplexing of multiple channels over a single physical medium (such as the B and D channels of ISDN), the PPA identifier must identify the specific channel over which communication will occur. Two styles of DLS provider are defined by DLPI, distinguished by the way they enable a DLS user to choose a particular PPA. The style 1 provider assigns a PPA based on the major/minor device the DLS user opened. This style of provider is appropriate when few PPAs will be supported. If the number of PPAs a DLS provider will support is large, a style 2 provider implementation is more suitable. The style 2 provider requires a DLS user to explicitly identify the desired PPA using a special attach service primitive. For a style 2 driver, open(2) creates a stream between the DLS user and DLS provider, and the attach primitive then associates a particular PPA with that stream. The format of the PPA identifier is specific to the DLS provider. DLPI provides a mechanism to get and/or modify the physical address. The primitives to handle these functions are described in Chapter 2. The physical address value can be modified in a postattached state. This modifies the value for all streams for that provider for a particular PPA. The DLS user uses the supported primitives, DL_ATTACH_REQ, DL_BIND_REQ, DL_ENABMULTI_REQ, and DL_PROMISCON_REQ, to define a set of enabled physical and SAP address components on a per-stream basis. The DL_ATTACH_REQ primitive addresses PPA identification (physical medium identification). It is invalid for a DLS provider to ever send upstream a data message for which the DLS user on that stream has not requested. The provider is responsible for enforcing the isolation of SAP and physical address space effects on a per-stream basis through necessary means. Chapter 1 25