 |
An Overview of the
Common UNIX Printing
System,
Version 1.1
July 10, 2000
Michael Sweet, Easy Software
Products
Copyright 1998-2003, All Rights
Reserved. |
This whitepaper describes the Common UNIX Printing SystemTM
("CUPSTM"), a portable and extensible printing system for
UNIX®. CUPS is being developed by Easy Software Products, a software firm
located in Hollywood, Maryland that has been selling commercial software for
UNIX since 1993 through more than 40 distributors serving over 80 countries
worldwide.
Additional information on CUPS is available on the World Wide Web at "http://www.cups.org/".
Background
Printing within UNIX has historically been done using one of two printing
systems - the Berkeley Line Printer Daemon ("LPD") [RFC1179] and the AT&T
Line Printer system. These printing systems were designed in the 70's for
printing text to line printers; vendors have since added varying levels of
support for other types of printers.
Replacements for these printing systems have emerged [LPRng, Palladin, PLP],
however none of the replacements change the fundamental capabilities of these
systems.
Over the last few years several attempts at developing a standard printing
interface have been made, including the draft POSIX Printing standard developed
by the Institute of Electrical and Electronics Engineers, Inc. ("IEEE")
[IEEE-1387.4] and Internet Printing Protocol ("IPP") developed by the Internet
Engineering Task Force ("IETF") through the Printer Working Group ("PWG")
[IETF-IPP]. The POSIX printing standard defines a common set of command-line
tools as well as a C interface for printer administration and print jobs, but
has been shelved by the IEEE.
The Internet Printing Protocol defines extensions to the HyperText Transport
Protocol 1.1 [RFC2616] to provide support for remote printing services. IPP/1.0
was accepted by the IETF as an experimental Request For Comments [RFC] document
in October of 1999. Since then the Printer Working Group has developed an
updated set of specifications for IPP/1.1 which have been accepted by the IETF
and are awaiting publication as proposed standards. Unlike POSIX Printing, IPP
enjoys widespread industry support and is poised to become the standard network
printing solution for all operating systems.
CUPS uses IPP/1.1 to provide a complete, modern printing system for UNIX that
can be extended to support new printers, devices, and protocols while providing
compatibility with existing UNIX applications. CUPS is free software provided
under the terms of the GNU General Public License and GNU Library General Public
License.
History
The first production release of CUPS (based on IPP/1.0) was released in
October of 1999. Since then, we have released several patch updates to the
original CUPS 1.0 release that addressed security, portability, and bugs found,
but no new functionality was added to improve the stability of the CUPS code.
CUPS 1.1 is based on IPP/1.1 and adds many of the functional enhancements
that have been requested by our users. As with 1.0, CUPS 1.1 will be followed by
patch releases that address any problems found with the software but add no new
features.
Design Overview
Like most printing systems, CUPS is designed around a central print
scheduling process that dispatches print jobs, processes administrative
commands, provides printer status information to local and remote programs, and
informs users as needed. Figure 1 shows the basic organization of CUPS.
Figure 1 - CUPS Block Diagram
Scheduler
The scheduler is a HTTP/1.1 server application that handles HTTP requests.
Besides handling printer requests via IPP POST requests, the scheduler also acts
as a full-featured web server for documentation, status monitoring, and
administration.
The scheduler also manages a list of available printers on the LAN and
dispatches print jobs as needed using the appropriate filters and backends.
Configuration Files
The configuration files consist of:
- The HTTP server configuration file.
- Printer and class definition files.
- MIME type and conversion rule files.
- PostScript Printer Description ("PPD") files.
The HTTP server configuration file is purposely similar to the Apache server
configuration file and defines all of the access control properties for the
server.
The printer and class definition files list the available printer queues and
classes. Printer classes are collections of printers. Jobs sent to a class are
forwarded to the first available printer in the class, round-robin fashion.
The MIME type files list the supported MIME types (text/plain,
application/postscript, etc.) and "magic" rules for automatically detecting the
format of a file. These are used by the HTTP server to determine the
Content-Type field for GET and HEAD requests and by the IPP
request handler to determine the file type when a Print-Job or
Send-File request is received with a document-format of
application/octet-stream.
The MIME conversion rule files list the available filters. The filters are
used when a job is dispatched so that an application can send a convenient file
format to the printing system which then converts the document into a printable
format as needed. Each filter has a relative cost associated with it, and the
filtering algorithm chooses the set of filters that will convert the file to the
needed format with the lowest total "cost".
The PPD files describe the capabilities of all printers, not just PostScript
printers. There is one PPD file for each printer. PPD files for non-PostScript
printers define additional filters through cupsFilter attributes to
support printer drivers.
CUPS API
The CUPS API contains CUPS-specific convenience functions for queuing print
jobs, getting printer information, accessing resources via HTTP and IPP, and
manipulating PPD files. Unlike the rest of CUPS, the CUPS API is provided under
the terms of the GNU LGPL so it may be used by non-GPL applications.
Berkeley and System V Commands
CUPS provides the System V and Berkeley command-line interfaces for
submitting jobs and checking the printer status. The lpstat and
lpc status commands also show network printers ("printer@server")
when printer browsing is enabled.
The System V administation commands are supplied for managing printers and
classes. The Berkeley printer administration tool (lpc) is only
supported in a "read-only" mode to check the current status of the printer
queues and scheduler.
Filters
A filter program reads from the standard input or from a file if a filename
is supplied. All filters must support a common set of options including printer
name, job ID, username, job title, number of copies, and job options. All output
is sent to the standard output.
Filters are provided for many file formats and include image file and
PostScript raster filters that support non-PostScript printers. Multiple filters
are run in parallel to produce the required output format.
The PostScript raster filter is based on the GNU Ghostscript 5.50 core.
Instead of using the Ghostscript printer drivers and front-end, the CUPS filter
uses a generic raster printer driver and CUPS-compliant front-end to support any
kind of raster printer. This allows the same printer driver filter to be used
for printing raster data from any filter.
CUPS Imaging
The CUPS Imaging library provides functions for managing large images, doing
colorspace conversion and color management, scaling images for printing, and
managing raster page streams. It is used by the CUPS image file filters, the
PostScript RIP, and all raster printers drivers.
Backends
A backend program is a special filter that sends print data to a device or
network connection. Backends for parallel, serial, USB, LPD, IPP, and AppSocket
(JetDirect) connections are provided in CUPS 1.1.
SAMBA version 2.0.6 and higher includes a SMB backend
(smbspool(1)) that can be used with CUPS 1.0 or 1.1 for printing to
Windows.
Network Printing
Traditionally, network printing has been one of the hardest things to get
working under UNIX. One reason is because each vendor added their own extensions
to the LPD protocol (the previous standard for network printing), making
cross-platform printing difficult if not impossible.
Another reason is that you have to administer every network printer on every
client machine. In some cases you can "clone" the printer configuration from a
"master" client to each of the others, but even that can be time-consuming and
error-prone. Something better is needed.
CUPS provides "printer browsing", which allows clients to automatically see
and use printers from any server on a LAN. This means that you only need to
configure the server and the clients will automatically see the printers and
classes on it.
In addition, CUPS can automatically merge multiple identical network printers
into "implicit classes". This allows clients to send jobs to the implicit class
and have them print on the first available printer or server. In addition,
failsafe and load-balancing functions are enabled simply by defining the same
printer on multiple servers!
New Features in CUPS 1.1
CUPS 1.1 includes many new features and capabilities:
- Backends
- Banner Page
Support
- Digest
Authentication
- Directory Services
- Directory Structure
Changes
- Documentation
- Drivers
- Filters
- IPP Support
- Job
Persistence
- LPD Client Support
- User-Defined Printers
and Options
- Web Administration
Interface
CUPS 1.1 implements a new backend interface for retrieving a list of
available devices for CUPS clients. This allows administration interfaces to
query the CUPS scheduler for a list of available devices, automatically
configure printers if the device identification information is available, and
present the user with a list of available devices rather than relying on the
user to know what devices are configured on the system.
The new release also includes a backend for USB printers under *BSD and
Linux. Support for USB under Solaris 8 will be provided in a subsequent patch
release.
CUPS 1.1 includes support for banner pages at the beginning and end of a job.
Banner pages may be of any file format and support variable substitution for job
titles, usernames, etc. Default banner pages are associated with each printer
and can be overridden with command-line options by the user.
Digest authentication provides a more secure method of authenticating access
to the printing system. Unlike Basic authentication, Digest authentication does
not send passwords "in the clear" so it is more difficult to gain unauthorized
access to your system.
CUPS 1.1 implements Digest authentication using a special MD5 password file
instead of the UNIX password file. This file is managed using the new
lppasswd command.
CUPS 1.1 adds new directory service ("printer browsing") features to make
using CUPS on large LANs and WANs easier. You can now poll a remote server for
printer information and relay it to the LAN as well as restrict what printer
information is processed (e.g. to "hide" servers, domains, or networks that you
don't want to see.)
CUPS 1.1 now uses a directory structure that complies with the Filesystem
Hierarchy Standard ("FHS"), version 2.0. This should make integration into
existing Linux and *BSD distributions a lot easier.
The CUPS 1.1 documentation has gone through many revisions, including a
completely rewritten administrators manual, a new programmers manual, and an IPP
implementation reference manual.
CUPS 1.1 includes drivers for EPSON dot-matrix and inkjet printers. As with
the HP PCL drivers, the EPSON drivers don't necessarily provide the best
possible output for each printer but should provide adequate printing quality
for general day-to-day printing.
CUPS 1.1 includes new image, PostScript, PDF, and text filters. The image
filters have been upgraded to support Windows BMP and Alias PIX files.
The PostScript filter is now based off GNU Ghostscript 5.50. The new filter
provides much better performance with higher-resolution printers and supports
most Level 3 PostScript language features.
The new PDF filter is based off the excellent Xpdf software from Derek
Noonburg and supports automatic page scaling. The new filter is a faster,
smaller, more reliable replacement for the GNU Ghostscript PDF filtering that
was used in CUPS 1.0.
The new text filter now supports bidirectional text and can embed fonts as
needed.
Probably the least visible portion of CUPS is the IPP support. CUPS 1.1
implements all of the required IPP/1.1 operations and attributes and most of the
optional ones. The optional Create-Job and Send-File operations are now
implemented, allowing for better System V printing system compatibility (one job
ID per lp command) and support for banner pages.
CUPS 1.1 supports job persistence. This means that jobs are preserved even
after a reboot, a feature that was sorely missing from CUPS 1.0.
In addition, CUPS 1.1 allows you to keep job information after the job has
printed. The basic post-job persistence mode provides a job history (number of
pages printed, time job was printed, etc.) but does not preserve the actual job
files. This can be changed to discard all information after a job is printed or
keep the job files after printing so you can reprint a job at some later time.
By popular request, CUPS 1.1 supports LPD-based clients using a new
mini-daemon that handles LPD requests and passes them on to the main server.
CUPS 1.1 includes support for user-defined printers and options via a new
lpoptions command. User-defined printers are special instances of
the available printers (e.g. "printer/instance" or "printer@server/instance")
that can have their own default options such as media size, resolution, and so
forth. The lpoptions command can also be used to set a different
default printer queue.
CUPS 1.0 provided a simple class, job, and printer monitoring interface for
web browsers. CUPS 1.1 replaces this interface with an enhanced administration
interface that allows you to add, modify, delete, configure, and control
classes, jobs, and printers.
Software Using CUPS
A lot has happened since CUPS 1.0 came out, and many software packages are
supporting CUPS. We have contributed code to the SAMBA team to support CUPS, and
parts of that are already available in SAMBA 2.0.6 and 2.0.7. With any luck the
final pieces that provide a complete integration with SAMBA will be available in
the next release of SAMBA.
Two graphical interfaces have appeared on the scene that use CUPS as well.
The KUPS project provides a KDE-based interface for CUPS and can be found at:
http://kups.sourceforge.net/
The X Printing Panel ("XPP") project provides a graphical printing panel for
CUPS and can be found at:
http://www.phy.uni-bayreuth.de/till/xpp
Numerous other filters, drivers, tutorials, etc. have been made available on
the CUPS Links web page, available at:
http://www.cups.org/links.php
Finally, our own ESP Print Pro software uses CUPS to provide drivers for
thousands of printers and can be found at:
http://www.easysw.com/printpro
Operating Systems Using CUPS
One of our goals has always been to get as many UNIX/Linux distributions
using CUPS as possible. Debian is currently providing CUPS as part of its stable
distribution, and many other distributions are considering it in their next
releases.
Summary
The Common UNIX Printing System provides a modern printing interface for UNIX
applications that is both flexible and user-friendly. The software provides
System V and Berkeley compatible command-line interfaces to ensure compatibility
with existing applications. CUPS 1.1 adds many new features that make it an even
better choice for printing under UNIX.
Who to Contact
For more information on CUPS please contact us at:
Attn: CUPS Information
Easy Software Products
44141 Airport View Drive Suite 204
Hollywood, Maryland 20636-3111 USA
+1.301.373.9600
[email protected]
References
- IEEE-1387.4
- System Administration - Part 4: Printing Interfaces (draft)
- IETF-IPP
- Internet Printing Protocol/1.1
- LPRng
- An enhanced, extended, and portable implementation of the Berkeley LPR
print spooler functionality
- Palladin
- A printing system developed at the Massachussetts Institute of Technology
- PLP
- The Portable Line Printer spooler system
- RFC1179
- Line Printer Daemon Protocol
- RFC2046
- Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types
- RFC2616
- Hypertext Transfer Protocol -- HTTP/1.1
Trademarks
The Common UNIX Printing System, CUPS, and the CUPS logo are the trademark
property of Easy Software Products. All other trademarks are the property of
their respective owners.
