Another revision. I would really value insightful comments, corrections to factual errors and supplemental information. Have I missed some major pro or con? Thanks in advance.
-- snip -- 4.2 Available Solutions There are several ways to do accounting. A brief overview will here be given of the solutions available, noting some pros and cons. The solutions discussed are pre-printing, proprietary software, Hewlett-Packard's Printer Job Language (PJL), Adobe's PostScript language and the Simple Network Management Protocol (SNMP). 4.2.1 Pre-printing These solutions are those where the counting of impressions takes place in pure software implementations before printing and the actual printer hardware is not involved at all. Two examples will be given here. In modern Microsoft Windows operating systems, applications call the Graphics Device Interface (GDI) to print [Win2k-ProResKit]. The GDI calls the printer driver for information, which is then used by the GDI to create a job. GDI then knows the number of impressions of a job and then delivers it to the spooler. If print output is produced in RAW format, the GDI is not used. The spooler logs the number of impressions in the event log where other software then can check for accounting information. By bypassing the GDI and sending a RAW job directly to the spooler, the spooler will log the job with zero impressions. Ghostscript is a cross-platform interpreter for PostScript. It can be used to count the number of impressions of a job before it is sent to the printer. According to the grapevine, there exist ways to fool Ghostscript into miscounting. Any pre-printing solution suffers from the fact that the printer may not actually print all pages. For example, the printer could jam, or run out of paper or toner. The user would still get charged. This means that a pre-printing solution does not fulfill Requirement 5. 4.2.2 Proprietary Software Several printer manufacturers offer proprietary software solutions that sometimes only work with their printers. These run on certain platforms only and often do not offer open access to data. As a packaged solution, one can say that these does not fulfill Requirement 2 nor Requirement 3, and sometimes not even Requirement 1. 4.2.3 PJL Hewlett-Packard's Printer Job Language (PJL) is used by many existing accounting applications. A connection is held open to the printer after the job has been sent to it, periodically querying for whether the job is done or not. Before the job and when the job finishes, the total number of impressions is read. The number of impressions used by the job can then be calculated. The PJL commands JOB, EOJ, ECHO and USTATUS can be used to retrieve the total number of impressions and check on printer status [PJL]. Pros of this solution is that almost all printers on the market today support PJL according to their specifications. According to the grapevine, the support is in reality sketchy. What the commands return and when, while clearly specified in the technical reference manual, differs. One has to make sure that the commands really work and return what they are supposed to. It is hard to know whether the number of impressions is actually correct. Writing a general implementation that works with several printer models from different manufacturers could turn into a game of rules and exceptions. Consequently, Requirement 1 and Requirement 5 are not clearly fulfilled. 4.2.4 PostScript Adobe's PostScript language is an interpretive programming language with powerful graphics capabilities [PS]. Its primary application is to describe the appearance of text, graphical shapes, and sampled images on printed or displayed pages. PostScript language level 2 implementations version 2011 and greater, and PostScript language level 3 implemenations feature a new system parameter PageCount [PS]. But not all products do necessarily support it. PageCount tracks the number of pages that have been successfully processed since manufacture, counting the number of copies for each showpage (multiplicated with the number of pages requested for every showpage). Pages not physically printed are counted, making the count inaccurate during the job, but accurate after adjustment at the end of the job. This is what the specification say at least. Just as with PJL, a connection is held open to the printer after the job has been sent to it. A control sequence is sent in order to retrieve the status of the printer. Whether you get the status this way depends on the printer. Some printers, according to reports on the LPRng mailing list, stop responding when they run out of paper. If one manages to verify that the printer has finished with the job, commands can be issued to retrieve the system parameter PageCount. By using the PageCount value from before the job and the one retrieved after the job, the number of impressions used during the job can be calculated. PostScript is often supported by accounting applications as an alternative, or supplement, to PJL. On the pro side for this solution is that many printers support PostScript today. They though have different implementations and support different language levels. Whether they support the needed features may be uncertain. One may have to use different PostScript code with each printer for it to work correctly. From this one can see that Requirement 1 and Requirement 5 are not clearly fulfilled. 4.2.5 SNMP The Simple Network Management Protocol (SNMP) is a definition of a protocol by which management information for a network element may be inspected or altered by logically remote users [RFC1157]. Together with companion memos which describe the structure of management information along with the management information base (MIB), these documents provide a simple, workable architecture and system for managing TCP/IP-based networks. Two of these companion memos are of particular interest to us. The Host Resources MIB provides two status objects, hrDeviceStatus and hrPrinterStatus, which describe many of the states of a printer [RFC2790]. The Printer MIB provides another object, prtMarkerLifeCount, which gives us the count of the number of units of measure counted during the life of the printer using units specified in the object prtMarkerCounterUnit [RFC1759]. SNMP is independent of the process of sending the content of the job to the printer. By using the two status objects to monitor whether the printer is done printing, and the counter object to retrieve the number of impressions before and after a job, an accurate number of actual printed pages can be calculated. The counter is located in the printer and counts only the number of impressions actually printed. Support for SNMP in printers has become more important and has seen an upsurge since generic SNMP-based monitoring tools have become increasingly popular. Most printers on the market today support the two MIBs needed for the SNMP solution. There are exceptions though, but the consensus seems to be that more printers work with the SNMP solution compared to the PJL solution. Requirement 1 and Requirement 5 are fulfilled as long as one makes sure the printers in use (or being acquired) have proper SNMP and MIB support. 4.2.6 Conclusion With its increasing support in printers, the SNMP solution seems to be the best option. The same general implementation would work with several printer models from different manufacturers, as long as the printer supports SNMP and mentioned MIBs properly. As the SNMP solution is independent of the process of sending the content of the job to the printer, it reduces the number of places where things could go wrong. The SNMP solution will be discussed in more detail in Section [sec:Accounting--SNMP]. -- snip -- ----------------------------------------------------------------------------- YOU MUST BE A LIST MEMBER IN ORDER TO POST TO THE LPRNG MAILING LIST The address you post from MUST be your subscription address If you need help, send email to [EMAIL PROTECTED] (or lprng-requests or lprng-digest-requests) with the word 'help' in the body. 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