Introduction to DLPI
DLPI Services
transfer is neither acknowledged nor confirmed, and there is no end-to-end flow control provided. As such, the connectionless data transfer service cannot guarantee reliable delivery of data. However a specific DLS provider can provide assurance that messages will not be lost, duplicated, or reordered.
DL_UNITDATA_REQ conveys one DLSDU to the DLS provider.
DL_UNITDATA_IND conveys one DLSDU to the DLS user. The normal flow of messages is illustrated in Figure 1-27.
Figure 1-27 Message Flow: Connectionless Data Transfer
DL_UNITDATA request 
DL_UNITDATA indication
Error Reporting Service
The connectionless-mode error reporting service may be used to notify a DLS user that a previously sent data unit either produced an error or could not be delivered. This service does not, however, guarantee that an error indication will be issued for every undeliverable data unit.
Figure 1-28 Connectionless-Mode Error Reporting
DL_UDERROR indication 
Raw-mode ServicesThe raw-mode services enable a DLS user to transfer packets containing complete MAC and LLC headers to a peer DLS user. The raw-mode service does not guarantee reliable delivery of data units between peer DLS users (e.g. lack of flow control may cause buffer resource shortages that result in data being discarded).
The DLS user requests the raw-mode services by setting the service mode in the DL_BIND_REQ to DL_HP_RAWDLS.