PWG Mail Archive: Proposed Printer MIB discussion of Referenced Groups

Proposed Printer MIB discussion of Referenced Groups

Bill Wagner (bwagner@digprod.com)
Sun, 3 Nov 1996 23:29:54 -0500


Recent suggestions for additions to the Printer MIB have related to
information obtainable from objects in the Host Resource MIB. This
suggests that the degree to which the Host Resource MIB is to be
implemented in the printer MIB and perhaps the dual function of the Host
Resource MIB are unclear. The inconsistent degree of Host Resource MIB
implementation may be a result of the somewhat non definitive statement
of requirement in paragraph 3.2 of the RFC compounded by the ambiguity
about what may be covered in the Host Resource MIB versus what shall be
covered, combined with the complexity and lack of utility in covering
all devices.

The confusion is additional compounded by the fact that the Host
Resource MIB both deals with a superset of `printer' (specifically
multiple printers sharing a common network node) where the printers are
listed devices, and a constituent component of printers, specifically
the System controller. This dual roll is made possible by the
prtStorageRef and prtDeviceRef tables in the printer MIB,
which-"associate particular storage and device entries to printers". The
printer itself is a device in the hrDevice table, perhaps one of several
printer devices.

There also seems inconsistent use of the interfaces group of MIB to
identify non-network interfaces in a printer. This may be because the
MIB II interface group states that each "interface is thought of as
being attached to a `subnetwork'", because the correlation of interfaces
listed in MIB-II to the printers listed in the hrDevice table is
confusing, or because it did not seem necessary. However, the
interfaces correlation in necessary for channels, and for the
identification of job path in the Jobs monitoring MIB.

I propose adding the following paragraphs (or some refined, corrected,
sanitized version) under section 3. I do not necessarily subscribe to
the inclusion of all this `stuff', but I believe that this proposed
addition explicitly states what is currently implicitly required. I
believe the RFC must explicitly define what is to be included and give a
little help in understanding the rather Byzantine structure. If my
interpretation is incorrect, then I welcome corrections.


Bill Wagner, DPI



3. Groups from other MIB Specifications

This section identifies the groups from other MIB's that shall be
supported to supplement and complete a printer MIB implementation. The
section also describes some of the less obvious characteristics of the
Printer MIB structure that are related to the inclusion of these other
MIB groups.

3.1. System Group

All objects in the system group of MIB-II (RFC 1213) shall be
implemented; however, as described in paragraph 3.4, implementers must
carefully consider what constitutes the system.

3.2. System Controller

The Storage and Device Groups of the Host Resources MIB (RFC 1514)
shall be implemented to support the printer(s) system controller, and
any supporting devices. If deemed appropriate by the implementer, other
Groups of the Host Resources MIB (System, Running Software, Running
Software Performance, and Installed Software) may be implemented.
Because of the structure of the Host Resources MIB, the devices
constituting the system controller are at the same level as the printer.
See paragraph 3.4 for an outline of the implications of this.

3.3. Interface Group

All objects in the Interfaces Group of MIB-II (RFC 1213) shall be
implemented for all print information interfaces to the printer,
including non-network interfaces.

3.4 Implications of the use of Groups from other MIB's

In structuring the Printer MIB, it is inconvenient to follow directly
the hierarchical structure implicit in the printer block diagram (Figure
2). There are two reasons for this:

1. Figure 2 suggests that the printer interface to the network
is through the interfaces group. It is generally required that this
network node is supported by an implementation MIB-II (RFC1213).
However, the network node may support one printer or several. And
further, the SNMP agent may be within the `system controller' part of
the functional printer, or it may be completely external to the printer
as represented on the diagram.. Therefore, the relationship between the
MIB-II defined network node, the agent implementing the MIB's and the
functional printer may not be consistent with the structure suggested
in figure 2.

2. The printer controller is, in many cases, a full fledged
computer containing many of the resources of a standard host computer
in a workstation. This includes devices such as the memory, interfaces,
network, and indeed, printer. The Host Resources MIB has well developed
structures for such devices. However, the Host Resource MIB only deals
with devices associated with a single `host', and it considers the
printer to be a part of this host on the same level as memory,
processor, and other devices considered part of the System Controller of
the printer.

It was convenient therefore to conceive of a `host' associated with the
SNMP agent and with the network node by which the agent and ultimately
the printer(s) communicate with the network. All host-resource devices
communicating though this network node (including the one or more
printers that are to be characertized) are considered part of the host
and are supported by implementation of the Host Resources Device Group
and the Host Resource Storage group. The system group of MIB -II
relates to this `host'. The printer (s) are identified as part of the
device table in the Host Resources MIB.

Further, not only are the printer and the host resource devices
constituting the System Controller of the printer at the same level, if
there are multiple printers, these printers and the Host Resource
devices constituting these printer are all at the same level, whether
dedicated to one printer or shared. The functional hierarchy implicit
in the printer block diagram is therefore flattened with respect to host
resource devices.

3.4.1 Host Resource MIB Device Group
For each instance of a host resource device operating from a given
network node, including the printer(s), the device is listed in the Host
Resource MIB device table with the following attributes.
hrDeviceIndex
hrDeviceType
hrDeviceDescr
hrDeviceID
hrDeviceStatus
hrDeviceErrors
The Device Description, Device ID and Device Status listed in this
table for each printer identify and characterize that printer. The
hrDevice index for each printer is included as an indexing value for
almost all variables in the printer MIB. In the case of multiple
printers, the printer MIB appears as a composite MIB for all printers
considered part of this host. Each table of the printer MIB which
includes hrDeviceIndex as an index will contain the variables for each
printer.

Non printer devices listed in the table are associated with one or more
listed printer devices by the prtDeviceRefTable in the printer MIB.
This table, as most in the Printer MIB, is indexed by hrDeviceIndex; but
unlike most of the other tables where the devices of interest are
printers, the devices of interest for this table are non-printer
devices. The only accessible variable for each row in this table is the
device number of the printer device which is associated with the indexed
non printer device. The table includes a second index,
prtDeviceRefSeqNumber, which allows a listed device to be associated
with multiple printer devices.

For example, a fully integrated printer may contain, as part of its
system controller,
hrDeviceProcessor ,
hrDeviceNetwork
hrDeviceDiskStorage
hrDeviceParallelPort
hrDeviceSerialPort
hrDeviceVolatileMemory
hrDeviceNonVolatileMemory

Ideally, these must all be listed as devices in the virtual host, along
with the printer (hrDevicePrinter) itself. Therefore, in this example
eight devices would be included with hrDevice Index values of 1-8. Since
there is but one printer, the prtDeviceRefTable in the printer MIB
would contain 7 entries, each with a value identifying the printer
hrDeviceIndex. Because there is only one printer, devices are not shared
and the prtDeviceRefSeqNumber index is (1) in all cases.

Further, the Host Resource MIB defines device specific tables to be
supported for certain devices. These devices, and the primary
significance of the additional table(s) are:

hrProcessorTable: identification and significant characteristics of
processor
hrNetworkTable: correlates a network device to a key to ifIndex
(MIB-II).
hrPrinterTable and hrPrinterErrorTable: the mechanism communicating the
status of each printer
hrDiskStorageTable: identifies disk access and media type and capacity
hrPartitionTable: partitions on long term storage devices
hrFSTable: identifies local file system type, characteristics and
pertinent parameters

3.4.2 Host Resource Storage Group

Program and data storage exist both as physical devices in the Host
Resource Device Table, and as logical storage areas supported in the
Host Resource Storage Group. Logical storage is listed and assigned an
index in the hrStorageTable. Storage is correlated to specific printers
by the prtStorageRefTable in the printer MIB. This table is indexed by
hrStorageIndex. The only accessible variable for each row in this table
is the device number of the printer device which is associated with the
indexed storage. The table includes a second index,
prtStorageRefSeqNumber, which allows logical storage to be associated
with multiple printer devices.

3.4.3 MIB-II Interface Group

The interfaces by which the printer receives print data are identified
within the Interfaces table of MIB-II. In the case of multiple printers,
the network interface for the `host' as well as all of the interfaces
for all printers are listed in this table.
The interfaces may also be listed as devices in the Host Resource
Device table. Network Port devices are identified by ifIndex to
correlate them back to the MIB-II Interface table; no such provision
exists for "serial" and "parallel" ports. Interfaces listed in the Host
Resource device table may be correlated to specific printers in the
`host' by the prtDeviceRefTable in the printer MIB; this may be useful
if there are multiple printers.
The ifIndex is also used to identify the interface associated with each
channel in the Printer MIB Channels group. Therefore, specific
interfaces are also correlated back to specific printers via the
channels mechanism.