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Data Link Provider Interface (DLPI)

Data Link Provider Interface (DLPI)
Copyright © 1997 The Open Group

Introduction

Overview

This document specifies a STREAMS kernel-level instantiation of the ISO Data Link Service Definition ISO/IEC 8886 (see referenced documents) and Logical Link Control ISO/IEC 8802-2 (LLC) (see referenced documents). Where the two standards do not conform, ISO/IEC 8886 prevails.

The Data Link Provider Interface (DLPI) enables a data link service user to access and use any of a variety of conforming data link service providers without special knowledge of the provider's protocol.

Specifically, the interface is intended to support X.25 LAPB, BX.25 level 2, SDLC, ISDN LAPD, Ethernet, CSMA/CD, FDDI, token ring, token bus, Bisync, Frame Relay, ATM, Fiber Channel and HIPPI. Among the expected data link service users are implementations of the OSI network layer and SNA path control. The interface specifies access to data link service providers, and does not define a specific protocol implementation. Thus, issues of network management, protocol performance, and performance analysis tools are beyond the scope of this document and should be addressed by specific implementations of a data link provider. However, accompanying each provider implementation should be information that describes the protocol-specific behavior of that provider. Currently, there are plans to come up with a set of implementors' agreements/guidelines for common data link providers. These agreements will address issues such as DLSAP address space, subsequent addresses, PPA access and control, QOS, supported services etc.

Readers should note that the list of data links supported by the interface may be augmented and is published on the World-Wide Web in the DLPI Corrigendum U019 at http://www.xopen.org (under Sales and Ordering). This Corrigendum will contain relevant additional information such as recognised implementors' agreements/guidelines for common data link providers. A printed copy of this Web page may also be obtained from The Open Group on request (address on inside front page of this specification).

This specification assumes the reader is familiar with OSI Reference Model (see referenced documents) terminology, OSI Data Link Services, and STREAMS. Although there are no formal standards for a STREAMS environment, extensive descriptions of STREAMS and STREAMS programming can be found in the referenced document STREAMS.

Model of the Data Link Layer

The data link layer (layer 2 in the OSI Reference Model) is responsible for the transmission and error-free delivery of bits of information over a physical communications medium.

The model of the data link layer is presented here to describe concepts that are used throughout the specification of DLPI. It is described in terms of an inter face architecture, as well as addressing concepts needed to identify different components of that architecture. The description of the model assumes familiarity with the OSI Reference Model.

Model of the Service Interface

Each layer of the OSI Reference Model has two standards:

DLPI is an implementation of the first type of standard. It specifies an interfa ce to the services of the data link layer. Abstract View of DLPI depicts the abstract view of DLPI.

Figure: Abstract View of DLPI

The data link interface is the boundary between the network and data link layers of the OSI Reference Model. The network layer entity is the user of the services of the data link interface (DLS user), and the data link layer entity is the provider of those services (DLS provider). This interface consists of a set of primitives which provide access to the data link layer services, plus the rules for using those primitives (state transition rules). A data link interface service primitive might request a particular service or indicate a pending event.

To provide uniformity among the various UNIX system networking products, service interfaces that map to the Data Link and Transport Layers of the OSI Reference Model have been developed. The service primitives that make up these interfaces are defined as STREAMS messages that are transferred between the user and provider of the service. DLPI is one such kernel-level interface, and is targeted for STREAMS protocol modules that either use or provide data link services. In addition, user programs that wish to access a STREAMS-based data link provider directly may do so using the putmsg() and getmsg() system calls.

Referring to the abstract view of DLPI (see Abstract View of DLPI ), the DLS provider is configured as a STREAMS driver, and the DLS user accesses the provider using open() to establish a stream to the DLS provider. The stream acts as a communication endpoint between a DLS user and the DLS provider. After the stream is created, the DLS user and DLS provider communicate via the messages presented later in this specification.

DLPI is intended to free data link users from specific knowledge of the characteristics of the data link provider. Specifically, the definition of DLPI hopes to achieve the goal of allowing a DLS user to be implemented independently of a specific communications medium. Any data link provider (supporting any communications medium) that conforms to the DLPI specification may be substituted beneath the DLS user to provide the data link services. Support of a new DLS provider should not require any changes to the implementation of the DLS user.

Modes of Communication

The data link provider interface supports three modes of communication:

The connection mode is circuit-oriented and enables data to be transferred over a pre-established connection in a sequenced manner. Data may be lost or corrupted in this service mode, however, due to provider-initiated resynchronization or connection aborts.

The connectionless mode is message-oriented and supports data transfer in self-contained units with no logical relationship required between units. Because there is no acknowledgement of each data unit transmission, this service mode can be unreliable in the most general case. However, a specific DLS provider can provide assurance that messages will not be lost, duplicated, or reordered.

The acknowledged connectionless mode provides the means by which a data link user can send data and request the return of data at the same time. Although the exchange service is connectionless, in-sequence delivery is guaranteed for data sent by the initiating station. The data unit transfer is point-to-point.

Connection-mode Service

The connection-mode service is characterized by four phases of communication:
Local Management
This phase enables a DLS user to initialize a stream for use in communication and establish an identity with the DLS provider.
Connection Establishment
This phase enables two DLS users to establish a data link connection between them to exchange data. One user (the calling DLS user) initiates the connection establishment procedures, while another user (the called DLS user) waits for incoming connect requests. The called DLS user is identified by an address associated with its stream (see DLPI Addressing ).

A called DLS user may either accept or deny a request for a data link connection. If the request is accepted, a connection is established between the DLS users and they enter the data transfer phase. For both the calling and called DLS users, only one connection may be established per stream. Thus, the stream is the communication endpoint for a data link connection.

The called DLS user may choose to accept a connection on the stream where it received the connect request, or it may open a new stream to the DLS provider and accept the connection on this new, responding stream. By accepting the connection on a separate stream, the initial stream can be designated as a listening stream through which all connect requests will be processed.

As each request arrives, a new stream (communication endpoint) can be opened to handle the connection, enabling subsequent requests to be queued on a single stream until they can be processed.

Data Transfer
In this phase, the DLS users are considered peers and may exchange data simultaneously in both directions over an established data link connection. Either DLS user may send data to its peer DLS user at any time. Data sent by a DLS user is guaranteed to be delivered to the remote user in the order in which it was sent.
Connection Release
This phase enables either the DLS user, or the DLS provider, to break an established connection. The release procedure is considered abortive, so any data that has not reached the destination user when the connection is released may be discarded by the DLS provider.

Connectionless-mode Service

The connectionless mode service does not use the connection establishment and release phases of the connection-mode service. The local management phase is still required to initialize a stream. Once initialized, however, the connectionless data transfer phase is immediately entered. Because there is no established connection, however, the connectionless data transfer phase requires the DLS user to identify the destination of each data unit to be transferred. The destination DLS user is identified by the address associated with that user (see DLPI Addressing ).

Connectionless data transfer does not guarantee that data units will be delivered to the destination user in the order in which they were sent. Furthermore, it does not guarantee that a given data unit will reach the destination DLS user, although a given DLS provider may provide assurance that data will not be lost.

Acknowledged Connectionless-mode Service

The acknowledged connectionless mode service also does not use the connection establishment and release phases of the connection-mode service. The local management phase is still required to initialize a stream. Once initialized, the acknowledged connectionless data transfer phase is immediately entered.

Acknowledged connectionless data transfer guarantees that data units will be delivered to the destination user in the order in which they were sent. A data link user entity can send a data unit to the destination DLS User, request a previously prepared data unit from the destination DLS User, or exchange data units.

DLPI Addressing

Each user of DLPI must establish an identity to communicate with other data link users. This identity consists of two pieces. Firstly, the DLS user must somehow identify the physical medium over which it will communicate. This is particularly evident on systems that are attached to multiple physical media. Secondly, the DLS user must register itself with the DLS provider so that the provider can deliver protocol data units destined for that user. Data Link Addressing Components illustrates the components of this identification approach, which are explained below.

Figure: Data Link Addressing Components

Physical Attachment Identification

The physical point of attachment (PPA in Data Link Addressing Components ) 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 support 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. One possible implementation of a style 1 driver would reserve a major device for each PPA the data link driver would support. This would allow the STREAMS clone open feature to be used for each PPA configured. 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, the open() 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, and should be described in the provider-specific addendum documentation.

DLPI provides a mechanism to get or modify the physical address. The primitives to handle these functions are described in Primitives for Management Services . The physical address value can be modified in a post-attached state. This would modify the value for all streams for that provider for a particular PPA. The physical address cannot be modified if even a single stream for that PPA is in the bound state.

The DLS User uses the supported primitives (DL_ATTACH_REQ, DL_BIND_REQ, DL_ENABMULTI_REQ, DL_PROMISCON_REQ) to define a set of enabled physical and SAP address components on a per Stream basis. 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 burden is on the provider to enforce by any means that it chooses the isolation of SAP and physical address space effects on a per-stream basis.

Data Link User Identification

A data link user's identity is established by associating it with a data link service access point (DLSAP), which is the point through which the user will communicate with the data link provider. A DLSAP is identified by a DLSAP address.

The DLSAP address identifies a particular data link service access point that is associated with a stream (communication endpoint). A bind service primitive enables a DLS user to either choose a specific DLSAP by specifying its DLSAP address, or to determine the DLSAP associated with a stream by retrieving the bound DLSAP address. This DLSAP address can then be used by other DLS users to access a specific DLS user. The format of the DLSAP address is specific to the DLS provider, and should be described in the provider-specific addendum documentation. However, DLPI provides a mechanism for decomposing the DLSAP address into component pieces. The DL_INFO_ACK primitive returns the length of the SAP component of the DLSAP address, along with the total length of the DLSAP address.

Certain DLS Providers require the capability of binding on multiple DLSAP addresses. This can be achieved through subsequent binding of DLSAP addresses. DLPI supports peer and hierarchical binding of DLSAPs. When the User requests peer addressing, the DLSAP specified in a subsequent bind may be used in lieu of the DLSAP bound in the DL_BIND_REQ. This allows for a choice to be made between a number of DLSAPs on a stream when determining traffic based on DLSAP values. An emample of this would be to specify various ether_type values as DLSAPs. The DL_BIND_REQ, for example, could be issued with ether_type value of IP, and a subsequent bind could be issued with ether_type value of ARP. The Provider may now multiplex off of the ether_type field and allow for either IP or ARP traffic to be sent up this stream.

When the DLS User requests hierarchical binding, the subsequent bind specifies a DLSAP that will be used in addition to the DLSAP bound using a DL_BIND_REQ. This allows additional information to be specified, which will be used in a header or for demultiplexing. An example of this would be to use hierarchical bind to specify the OUI (Organizationally Unique Identifier) to be used by SNAP.

If a DLS Provider supports peer subsequent bind operations, the first SAP that is bound is used as the source SAP when there is ambiguity. DLPI supports the ability to associate several streams with a single DLSAP, where each stream may be a unique data link connection endpoint. However, not all DLS providers can support such configurations because some DLS providers may have no mechanism beyond the DLSAP address for distinguishing multiple connections. In such cases, the provider will restrict the DLS user to one stream per DLSAP.

Connection Management Stream

The earlier description of the connection-mode service assumed that a DLS user bound a DLSAP to the stream it would use to receive connect requests. In some instances, however, it is expected that a given service may be accessed through any one of several DLSAPs. To handle this scenario, a separate stream would be required for each possible destination DLSAP, regardless of whether any DLS user actually requested a connection to that DLSAP. Obvious resource problems can result in this scenario.

To obviate the need for tying up system resources for all possible destination DLSAPs, a connection management stream utility is defined in DLPI. A connection management stream is one that receives any connect requests that are not destined for currently bound DLSAPs capable of receiving connect indications. With this mechanism, a special listener can handle incoming connect requests intended for a set of DLSAPs, by opening a connection management stream to the DLS provider which will retrieve all connect requests arriving through a particular PPA. In the model, therefore, there may be a connection management stream per PPA.

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