The Cable Modem HOWTO

Howard Shane

<hshane[AT]austin.rr.com>
Revision History                                                             
Revision 0.9           2004-07-30            Revised by: jhs                 
Completed draft.                                                             


This document was written to assist the Linux user in configuring a cable
modem for internet access using a cable network.

-----------------------------------------------------------------------------
Table of Contents
1. Introduction
    1.1. Copyright Information
    1.2. Disclaimer
    1.3. New Versions
    1.4. Credits
    1.5. Feedback
    1.6. Conventions Used in this Document
   
   
2. Prerequisites
    2.1. Networking and Ethernet Support
    2.2. Ethernet Support
    2.3. USB Interface Support
    2.4. The Modem Device
    2.5. The DHCP Client
   
   
3. Configuration
    3.1. Ready to Start With an Ethernet-Modem connection
    3.2. Grabbing A Specific Interface
    3.3. Using the USB interface instead of an Ethernet card
   
   
4. Notes on Specific Hardware
    4.1. Ambit Modems
    4.2. Broadcom Cable Modems
    4.3. Ericson PipeRider Modems
    4.4. Motorola SurfBoard Modems
    4.5. RCA (Tompson) Modems
    4.6. Terayon Modems
    4.7. Toshiba PCX-XXXX Cable Modems
    4.8. Webstar Modems
   
   
5. Troubleshooting
A. Gnu Free Documentation License

1. Introduction

This document was written to assist the Linux user in setting up their cable
modem, and includes information on configuring a DHCP client, enabling the
device with or without USB support and troubleshooting. I have successfully
configured Motorola and Toshiba brand cable modems with the methods herein,
and others have informed me that the configuration methods of other
manufacturers' hardware is almost identical. A mostly-complete laundry list
of hardware specific tidbits is addressed in Section 4; if you have a model
working that is not listing or have some specific informtion that you think
will be helpful, please send it along via the address in Section 1.5
-----------------------------------------------------------------------------

1.1. Copyright Information

This document is Copyright 2004 by Howard Shane.

Permission is granted to copy, distribute and/or modify this document under
the terms of the GNU Free Documentation License, Version 1.2 or any later
version published by the Free Software Foundation with no Invariant Sections,
no Front-Cover Texts, and no Back-Cover Texts. A copy of the license can be
found in Appendix A.
-----------------------------------------------------------------------------

1.2. Disclaimer

No liability for the contents of this document can be accepted. Use the
concepts, examples and other content entirely at your own risk. As this is a
new edition, there may be technical or other inaccuracies that may result in
the loss of irreplaceable data. In any case, proceed with caution, and
realize that although errors are highly unlikely, the author can accept no
responsibility for them.

All copyrights are held by their by their respective owners, unless
specifically noted otherwise. Use of a term in this document should not be
regarded as affecting the validity of any trademark or service mark.

Naming of particular products or brands should not be seen as endorsements.
-----------------------------------------------------------------------------

1.3. New Versions

This is the initial release.

The latest version number of this document can be found [http://
www.hshanemd.net/docs/HOWTOS/Cable-Modem/] here.
-----------------------------------------------------------------------------

1.4. Credits

I would like to thank Brad Hards, the primary author of the Linux CDCEther
kernel driver for graciously volunteering several useful bits of information,
as well as Rob McGee and Jess Portnoy for encouraging me to morph this
document into the new Cable-Modem HOWTO and contributing useful suggestions.

Also, I would like to thank Marla, who has stood by me confidently even when
the outlook was dimmest and others unforgiving. I'm yours forever.
-----------------------------------------------------------------------------

1.5. Feedback

Please send any additions or comments pertaining to this document to the
following email address: <hshane[AT]austin.rr.com>.
-----------------------------------------------------------------------------

1.6. Conventions Used in this Document

The following conventions are used in this document and are outlined here for
those who may not yet have a complete understanding of how to access and
control the underlying operating system in Linux, which is almost always the
bash shell.

First, filenames are referenced in a paragraph like so: /path/file

Commands in Linux are executed (or 'called') at the command prompt, otherwise
known as the 'command line.' If you are in the non-graphical (text-based)
environment you will usually be presented the bash shell prompt which is a
dollar sign:
+---------------------------------------------------------------------------+
|$                                                                          |
+---------------------------------------------------------------------------+
...or the hash mark:
+---------------------------------------------------------------------------+
|#                                                                          |
+---------------------------------------------------------------------------+
...if you have logged in as root or have acquired root, or 'superuser'
privileges. You can also access the bash shell in the X window system,
otherwise known as X or X11, with an [http://invisible-island.net/xterm/]
xterm or similar X-terminal-emulator. Commands to be performed at the bash
prompt, but referenced in a paragraph of this document, usually look like
this: do this now

Commands and/or the resulting output of commands may also be outlined with
screen output in their own paragraph or heading:

+---------------------------------------------------------------------------+
|$                                                                          |
|date Sun Jul 27 22:37:11 CDT 2003                                          |
+---------------------------------------------------------------------------+

When a command is written in front of the bash prompt (e.g. $ date above), it
is assumed the [Return] or [Enter] key has been depressed after the command,
possibly followed by the output (e.g., the date).
-----------------------------------------------------------------------------

2. Prerequisites

2.1. Networking and Ethernet Support

Most, if not all, cable modems are easily configured for use with Linux. Most
cable modems have two possible interfaces for connection to your computer:
Ethernet or USB. If your version of Linux is like most, there is very little
you will need to do to to get your cable modem working with an Ethernet
interface as long as you have support for networking and the driver for your
specific networking hardware installed.
-----------------------------------------------------------------------------

2.2. Ethernet Support

An open Ethernet interface is the most likely means that a cable provider
will use or recommend when setting up your cable internet service. Your ISP
may even deliver and/or install a new NIC card to access the modem if you
have no open Ethernet cards available.The cable connects to the modem, which
then connects to your NIC using ordinary 10BaseT/100BaseT Ethernet cable. If
you are uncertain about any of the above, and your modem and/or NIC are not
already physically installed for you, read the [http://www.tldp.org/HOWTO/
Ethernet-HOWTO.html] Ethernet HOWTO and your cable modem's instruction manual
before proceeding any further.

Warning Do not try to install an NIC by yourself unless you are already      
        familiar with the process or are assisted by someone who is.         

For software support, most base installs of Linux distributions come enabled
for TCP/IP networking and common Ethernet hardware drivers 'out of the box,'
so if you are using a preconfigured kernel from a boxed or downloaded version
of a Linux distribution you can probably skip to Section 2.4. If you don't
know what a kernel is and/or are curious about it, see the [http://
www.tldp.org/HOWTO/Kernel-HOWTO.html] Kernel HOWTO for more information.

The (Linux kernel) networking support requirements for use of a cable modem
include the following, and which you will need to select using whatever
kernel configuration front-end application (whether make config, make
menuconfig or make xconfig) prior to compiling a custom kernel. For 2.4.x and
earlier kernels under the heading 'Networking options' you will require 'TCP/
IP Networking' along with 'Network Device Support,' go to 'Ethernet (10 or
100Mbit) Support' and enable your specific NIC driver. If you use the 2.6.x
series, the heading 'Drivers' has a sub-heading 'Networking Support,' where
you should enable 'Network Device Support' and 'Ethernet Connection Support/
Ethernet (10 or 100Mbit), as well as your specific NIC hardware driver.

If you are interested in how to configure the USB interface go to Section 2.3
; otherwise you can go to Section 2.4.
-----------------------------------------------------------------------------

2.3. USB Interface Support

There seems to be no reliable data regarding testing bandwidth or performance
of an Ethernet connection vs. the USB port of cable modems. In theory they
should be equivalent, though some believe there is poorer performance (at
least in Linux) using USB. Unless you have no way to use an Ethernet
interface, have no ability to add a cheap NIC, or are just plain determined,
there is probably no advantage to using USB. If so, you will need the
following, in addition to Networking support outlined above: 1) USB Subsystem
support 2) your host controller driver (UHCI, OHCI or EHCI) and one of the
following:

Kernel Version 2.4.x

  * CDC Ethernet Support
   

For kernel 2.6.x you will need

  * Multi-purpose USB Networking Framework (CONFIG_USB_USBNET) with
   
  * CDC Ethernet support (CONFIG_USB_CDCETHER)
   

See Section 3.3 for instructions on how to load modules when time to test the
modem.
-----------------------------------------------------------------------------

2.4. The Modem Device

First, plug in and turn on the cable modem. Connect your Ethernet card to the
modem with 10BaseT/100BaseT cable into the non-USB interface if this was not
already done for you. Be sure the modem isn't on standby mode or turned off;
this should be obvious looking at the LEDs. The standby button is on the top
of the device on most models. Note that your cable internet provider should
be able to tell remotely whether your modem is connected and functioning
properly, which is helpful to differentiate hardware from configuration
problems on your end. They will also need the MAC (Media Access Control)
hardware address of your modem to allow the device access to their network.
If at any time you substitute one modem for another you will need to inform
them so the MAC address can be updated and your access to the cable network
restored.

Your modem will be assigned an IP address once connected for the first time
that may remain the same or may change periodically depending on the IP
address turnover of your ISP's DHCP server and how long you remain offline if
you disconnect.
-----------------------------------------------------------------------------

2.5. The DHCP Client

2.5.1. Installation on a Debian System

Debian requires you have a DHCP client package installed and running. As of
this writing there are two user-space programs for this. In Woody (stable),
there is the dhcp-client package, automatically installed as a part of the
base packages as /sbin/dhclient. For Sarge (testing) and up, this has been
replaced by the dhcpcd package. The latter has its configuration files under
/etc/dhcpc, but nothing really needs to be modified if you have only one
Ethernet card you will be using for the cable internet service. The dhcpcd
daemon is easily installed for those using testing branch as root with 
apt-get install dhcpcd .
-----------------------------------------------------------------------------

2.5.2. Installing on .rpm- or .tgz-Based Systems

For .rpm- or .tgz-based distributions, I offer the following link that walks
you through the setup of a DHCP client, in the [http://www.tldp.org/HOWTO/
mini/DHCP/x74.html] DHCP mini-HOWTO.

Just run /sbin/dhclient or whichever client you use to get a dynamic IP
address.
-----------------------------------------------------------------------------

3. Configuration

-----------------------------------------------------------------------------
3.1. Ready to Start With an Ethernet-Modem connection

Once you are plugged into the system, your modem is on and your dhcp server
is running as outlined in Section 2.5, you should be provided your own IP
address that doesn't change unless you drop the lease (i.e. are offline) for
a while. To confirm that the DHCP client is working and you have a new IP
address, execute (as root) ifconfig without any other arguments, and you
should see the following:

+---------------------------------------------------------------------------------------+
|eth0 Link encap:Ethernet HWaddr 00:D0:09:DE:D4:6F                                      |
|        inet addr:66.190.XXX.XXX Bcast:255.255.255.255 Mask:255.255.255.0 UP BROADCAST |
|        RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2591777 errors:0 dropped:0      |
|        overruns:0 frame:0 TX packets:5589 errors:0 dropped:0 overruns:0 carrier:0     |
|        collisions:0 txqueuelen:100 RX bytes:168673636 (160.8 MiB) TX bytes:1752872    |
|        (1.6 MiB) Interrupt:12 Base address:0xc400                                     |
+---------------------------------------------------------------------------------------+

+-------------------------------------------------------------------------------------+
|lo Link encap:Local Loopback                                                         |
|        inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX |
|        packets:5168 errors:0 dropped:0 overruns:0 frame:0 TX packets:5168 errors:0  |
|        dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1695104    |
|        (1.6 MiB) TX bytes:1695104 (1.6 MiB)                                         |
+-------------------------------------------------------------------------------------+

...which shows the system loopback device, lo, and also eth0, the cable modem
having successfully acquired an IP address (the 'inet addr') provided by our
cable internet service provider.

A simple Ethernet-based configuration should be complete at this step. If you
have more than one Ethernet interface available and want to select a specific
one for the cable service, go on to Section 3.2.
-----------------------------------------------------------------------------

3.2. Grabbing A Specific Interface

Let's say we want to select a specific Ethernet interface (ethX) to be the
recipient of the DHCP cable internet service. If you run ifconfig as root you
get a list of open devices:

+-----------------------------------------------------------------------------------------+
|eth0 Link encap:Ethernet                                                                 |
|HWaddr 00:D0:09:DE:D4:6F                                                                 |
|        inet addr:192.168.1.1 Bcast:192.168.1.255 Mask:255.255.255.0 BROADCAST RUNNING   |
|        MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0   |
|        TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 |
|        RX bytes:0 (0.0 b) TX bytes:0 (0.0 b) Interrupt:12 Base address:0xc400           |
+-----------------------------------------------------------------------------------------+

+-------------------------------------------------------------------------------------+
|lo Link encap:Local Loopback                                                         |
|        inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX |
|        packets:5168 errors:0 dropped:0 overruns:0 frame:0 TX packets:5168 errors:0  |
|        dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1695104    |
|        (1.6 MiB) TX bytes:1695104 (1.6 MiB)                                         |
+-------------------------------------------------------------------------------------+

...where eth0 is a standard NIC that we have preconfigured to the IP address
192.168.1.1.

Note the HWaddr field, or hardware address, on the first line. This is the
same as the MAC, or Media Access Control address, and is how we will specify
the interface for each action.
-----------------------------------------------------------------------------

3.2.1. Manually Assigning MAC to Interface Name

Whatever distribution you use, you can always use the command line to
designate an Ethernet device (via the MAC) to be interface 'ethX' with or
without a particular IP address. This may be necessary when you change your
settings, swap out hardware or for just plain testing. Use ifconfig:

+---------------------------------------------------------------------------+
|# ifconfig ethX hw ether 00:D0:09:DE:D4:6F up                              |
+---------------------------------------------------------------------------+

You can check if it worked by calling ifconfig without options after your
next reboot. See man ifconfig for more options.
-----------------------------------------------------------------------------

3.2.2. Automatic Configuration of Devices

The method for automatically assigning a particular interface name ethX, to a
specific hardware interface (Media Access Control address) differs by
distribution.
-----------------------------------------------------------------------------

3.2.2.1. Debian

If you are running a Debian system, you can alter the /etc/network/interfaces
file to look like this:

+---------------------------------------------------------------------------------------+
|        # /etc/network/interfaces -- configuration file for ifup(8), ifdown(8) # The   |
|        loopback interface auto lo iface lo inet loopback auto eth0 iface eth0 inet    |
|        static address 192.168.1.1 netmask 255.255.255.0 network 192.168.1.0 hwaddress |
|        ether 00:D0:09:DE:D4:6F                                                        |
|                                                                                       |
|        auto eth1 iface eth1 inet dhcp hwaddress ether 00:04:BD:DE:42:0B               |
+---------------------------------------------------------------------------------------+

The auto eth0 and auto eth1 are required to have the interfaces configured at
boot-up. Note that some versions of dhcp clients by default always grab eth0
for the dhcpc interface. So even after doing all the above, unless you
specifically run /sbin/dhcpcd-bin eth1 it won't work. The easy way to do this
at boot up is to make an init script to load the dhcp address to the correct
interface, which should be placed in /etc/init.d. Then run update-rc.d as
root with the correct arguments (see man update-rc.d).
-----------------------------------------------------------------------------

3.2.2.2. Red Hat

The file to edit will of the form ifcfg-ethX, located in /etc/sysconfig/
network-scripts/. The contents are typically of the form:

+---------------------------------------------------------------------------+
| DEVICE=eth1                                                               |
|# Device name. This is the interface your cable modem will use.            |
|ONBOOT=yes                                                                 |
|# load at boot BOOTPROTO=dhcp                                              |
|# sets the device to receive an IP address from a DHCP server              |
|HWADDR=00:04:BD:DE:42:0B                                                   |
|# the hardware MAC                                                         |
+---------------------------------------------------------------------------+

Edit the relevant sections with your favorite text editor program as root to
reflect your interface name and MAC and save it. If the file doesn't exist
for the interface you need you can create it, for example ifcfg-eth1. You can
then use ifconfig eth1 up from the command line to use it immediately. Upon
reboot the correct hardware settings should be loaded automatically.
-----------------------------------------------------------------------------

3.2.2.3. Slackware

Slackware has a configuration utility named 'netconfig' that you can use at
the command line as root to modify your network settings and configure
interfaces by static IP addresses or dhcp. You can rerun it at any time to
reconfigure things once you have installed your modem.
-----------------------------------------------------------------------------

3.2.2.4. Other Distributions

Feel free to contribute if you have distribution-specific information to add;
see Section 1.5. Consult your distribution documentation where appropriate.
-----------------------------------------------------------------------------

3.3. Using the USB interface instead of an Ethernet card

3.3.1. USB CDCEther

If you wish to use the USB interface to accept data you will need USB
subsystem support in your kernel as discussed in Section 2.1, whether
USB-ohci, USB-ehci, or whatever USB host controller driver required by your
system. For a more in-depth discussion of this, I direct you to the Linux-USB
project site.

In addition the CDCEther (Communications Device Class Ethernet) protocol
common to cable modems.

There are two ways your kernel may support a device driver: either by being
statically (always on) compiled within that kernel or loaded (at boot time or
later by a script or manually) as a module. You can find out if a driver is
statically compiled in the kernel by checking the log of messages at boot up
using dmesg | less on the command line. Experienced users may want to take an
easier route and check the configuration file for the currently running
kernel in /boot, usually of the form config-2.X.x, which is a text file that
can be grepped. On the command line, try grep -e followed by 
CONFIG_USB_CDCETHER in 2.4.x kernels or CONFIG_USB_ETH in 2.6.x kernels
followed by the filename; you should see a '=y or =m at the end of this
string for static or modular status, respectively.

The output of dmesg | less should reveal something similar to the following

+---------------------------------------------------------------------------+
| Mar 2 11:00:52 K7 kernel: CDCEther.c:                                     |
|0.98.6 7 Jan 2002 Brad Hards and another                                   |
|Mar 2 11:00:52 K7 kernel: usb.c: registered new driver CDCEther            |
+---------------------------------------------------------------------------+

in the case of a statically compiled driver or a module loaded at boot. 

If you don't see it, to find out if your kernel has already loaded the
CDCEther (Communications Device Class Ethernet) driver as a module, on the
command line issue lsmod as root. You should see the module CDCEther listed
for 2.4.x kernels, or alternatively usbnet for 2.6.x kernels.

If you don't see it listed with lsmod try loading the module directly as
root; for kernel 2.4.x this would be of the form
+---------------------------------------------------------------------------+
| # modprobe CDCEther                                                       |
+---------------------------------------------------------------------------+
whereas for 2.6.x the command would be
+---------------------------------------------------------------------------+
| # modprobe usbnet                                                         |
+---------------------------------------------------------------------------+

If all goes well you should see something like the following at the end of
your system log files (/var/log/messages or /var/log/syslog, you may need to
be root to view these) or at the end of dmesg | less from the command line:

+---------------------------------------------------------------------------+
| Mar 2 11:00:52 K7 kernel: CDCEther.c:                                     |
|0.98.6 7 Jan 2002 Brad Hards and another                                   |
|Mar 2 11:00:52 K7 kernel: usb.c: registered new driver CDCEther            |
+---------------------------------------------------------------------------+

If all goes well you should see something like the following in your system
log files or at the end of dmesg:  

+---------------------------------------------------------------------------+
| Mar 2 11:00:52 K7 kernel: CDCEther.c:                                     |
|0.98.6 7 Jan 2002 Brad Hards and another                                   |
|Mar 2 11:00:52 K7 kernel: usb.c: registered new driver CDCEther            |
+---------------------------------------------------------------------------+

If you are unsuccessful with any of the above, and are determined to use the
USB conduit of this device, you will need to recompile your kernel to support
it. You will need the 2.4.3 kernel or later. For detailed instructions on
recompiling your kernel, I direct you to the Kernel-HOWTO. The options
required to get USB interface support working on this device have been
addressed in Section 2.1.
-----------------------------------------------------------------------------

4. Notes on Specific Hardware

Thanks to Jess Portnoy and others for much of the information below. Also,
there are some notes specifically pertaining to configuring a cable modem and
cable internet service providers in Israel on [http://www.freewebs.com/
linuxnet/CableUSBLinuxEn.html] Jess' webpage for those that live there. I
have personally subscribed to two large cable ISPs in the U.S. without any
trouble using the instructions outlined above. The previous incarnation of
this document had a large catalogue of ISPs listed by geography, but as a
rule the identity of the upstream provider should not affect configuration.
If your cable ISP claims not to support linux, it is not because of any
technical difficulties related to the operating system. Just don't expect
them to configure if for you on your end like they do for most other users
(that's why you're reading this document, right?).
-----------------------------------------------------------------------------

4.1. Ambit Modems

Supported by Ethernet and CDCEther.
-----------------------------------------------------------------------------

4.2. Broadcom Cable Modems

Supported by Ethernet and CDCEther.
-----------------------------------------------------------------------------

4.3. Ericson PipeRider Modems

Supported by Ethernet and CDCEther.
-----------------------------------------------------------------------------

4.4. Motorola SurfBoard Modems

This model has enjoyed widespread use by cable providers. There is a separate
[http://tldp.org/HOWTO/Motorola-Surfboard-Modem/index.html] HOWTO, written by
your humble author. Supported by both Ethernet and USB CDCEther. Refer to
that document for any hardware specific questions. The generic instructions
above generally work.
-----------------------------------------------------------------------------

4.5. RCA (Tompson) Modems

Supported with Ethernet, CDCEther untested.
-----------------------------------------------------------------------------

4.6. Terayon Modems

At minimum the tj715 is known to be supported using Ethernet and CDCEther.
-----------------------------------------------------------------------------

4.7. Toshiba PCX-XXXX Cable Modems

The four X's in the name indicate model number. Works with Ethernet, CDCEther
untested. 'out-of-the-box' with the above instructions.
-----------------------------------------------------------------------------

4.8. Webstar Modems

Supported with Ethernet, CDCEther untested.
-----------------------------------------------------------------------------

5. Troubleshooting

Q: I get kicked offline about once every 4 days, for no apparent reason, and
get the following error, or something similar, in the kernel log:
+-----------------------------------------------------------------------------------------+
| Feb 20 10:05:12 K7 kernel: CDCEther.c: rx status -110 Feb 20 10:05:12 K7 kernel:        |
|CDCEther.c: no repsonse in BULK IN Feb 20 10:05:12 K7 kernel: CDCEther.c: rx status -110 |
|Feb 20 10:05:12 K7 kernel: CDCEther.c: no repsonse in BULK IN Feb 20 10:05:12 K7 kernel: |
|CDCEther.c: rx status -110 Feb 20 10:05:12 K7 kernel: CDCEther.c: no repsonse in BULK    |
|IN Feb 20 10:05:12 K7 kernel: CDCEther.c: rx status -110                                 |
+-----------------------------------------------------------------------------------------+

A: There are a number of reasons this may be happening, and future updates to
the CDCEther driver may solve some of them. At least one user on the
Linux-USB-user mailing list noticed that on at least one occasion data sent
to the modem from upstream by the cable provider has triggered it. Also, the
modem itself is very sensitive to power interruptions and can lose the
connection if this occurs. The fix is to run ifdown ethX, where ethX is the
Ethernet interface (eth0, eth1 etc.) to clear out any remaining settings that
are hung, then remove the module with rmmod CDCEther, reinsert the CDCEther
module and then ifup ethX . A reboot may be necessary if this doesn't fix the
problem. If none of these work you probably have a real service interruption.

Q: I get the following messages on boot-up; are they errors?

+-------------------------------------------------------------------------------------+
| Can't use                                                                           |
|SetEthernetMulticastFilters request Mar 2 11:00:52 K7 kernel: CDCEther.c: Ethernet   |
|information found at device configuration.  Trying to use it anyway.  Mar 2 11:00:52 |
|K7 kernel: CDCEther.c: Imperfect filtering support - need sw hashing                 |
+-------------------------------------------------------------------------------------+

A: No. The multicast message is pertaining to Multicast support in the
kernel, which is optional and not necessary for the proper functioning of
this modem. The message about 'Ethernet Information' is a design bug in the
modem and can be ignored. As for the 'Imperfect filtering support, to quote
Brad Hards:

"This is a bit difficult to explain - I assume that you know what
multicasting is - when you join a multicast group, this can be handled by the
networking device so that other multicast traffic doesn't cause interrupts.
That is called 'perfect filtering.' However sometimes the number of multicast
addresses exceeds the number of filters that you have. This leads to
'imperfect filtering,' which can cut down the number of interrupts, but you
still need to do some work in the networking stack. Then you get to the
typical cable modem implementation, and there is not filtering at all. Every
multicast packet goes to the host to be filtered. This doesn't normally
matter though, because the cable modem is a point to point link."
-----------------------------------------------------------------------------

A. Gnu Free Documentation License

Version 1.2, November 2002

Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 59 Temple Place,
Suite 330, Boston, MA 02111-1307 USA Everyone is permitted to copy and
distribute verbatim copies of this license document, but changing it is not
allowed.

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that the Document is released under this License. If a section does not fit
the above definition of Secondary then it is not allowed to be designated as
Invariant. The Document may contain zero Invariant Sections. If the Document
does not identify any Invariant Sections then there are none.

The "Cover Texts" are certain short passages of text that are listed, as
Front-Cover Texts or Back-Cover Texts, in the notice that says that the
Document is released under this License. A Front-Cover Text may be at most 5
words, and a Back-Cover Text may be at most 25 words.

A "Transparent" copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the general
public, that is suitable for revising the document straightforwardly with
generic text editors or (for images composed of pixels) generic paint
programs or (for drawings) some widely available drawing editor, and that is
suitable for input to text formatters or for automatic translation to a
variety of formats suitable for input to text formatters. A copy made in an
otherwise Transparent file format whose markup, or absence of markup, has
been arranged to thwart or discourage subsequent modification by readers is
not Transparent. An image format is not Transparent if used for any
substantial amount of text. A copy that is not "Transparent" is called
"Opaque".

Examples of suitable formats for Transparent copies include plain ASCII
without markup, Texinfo input format, LaTeX input format, SGML or XML using a
publicly available DTD, and standard-conforming simple HTML, PostScript or
PDF designed for human modification. Examples of transparent image formats
include PNG, XCF and JPG. Opaque formats include proprietary formats that can
be read and edited only by proprietary word processors, SGML or XML for which
the DTD and/or processing tools are not generally available, and the
machine-generated HTML, PostScript or PDF produced by some word processors
for output purposes only.

The "Title Page" means, for a printed book, the title page itself, plus such
following pages as are needed to hold, legibly, the material this License
requires to appear in the title page. For works in formats which do not have
any title page as such, "Title Page" means the text near the most prominent
appearance of the work's title, preceding the beginning of the body of the
text.

A section "Entitled XYZ" means a named subunit of the Document whose title
either is precisely XYZ or contains XYZ in parentheses following text that
translates XYZ in another language. (Here XYZ stands for a specific section
name mentioned below, such as "Acknowledgements", "Dedications",
"Endorsements", or "History".) To "Preserve the Title" of such a section when
you modify the Document means that it remains a section "Entitled XYZ"
according to this definition.

The Document may include Warranty Disclaimers next to the notice which states
that this License applies to the Document. These Warranty Disclaimers are
considered to be included by reference in this License, but only as regards
disclaiming warranties: any other implication that these Warranty Disclaimers
may have is void and has no effect on the meaning of this License.

 2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either commercially
or noncommercially, provided that this License, the copyright notices, and
the license notice saying this License applies to the Document are reproduced
in all copies, and that you add no other conditions whatsoever to those of
this License. You may not use technical measures to obstruct or control the
reading or further copying of the copies you make or distribute. However, you
may accept compensation in exchange for copies. If you distribute a large
enough number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and you may
publicly display copies.

 3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have printed
covers) of the Document, numbering more than 100, and the Document's license
notice requires Cover Texts, you must enclose the copies in covers that
carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the
front cover, and Back-Cover Texts on the back cover. Both covers must also
clearly and legibly identify you as the publisher of these copies. The front
cover must present the full title with all words of the title equally
prominent and visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve the
title of the Document and satisfy these conditions, can be treated as
verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit legibly, you
should put the first ones listed (as many as fit reasonably) on the actual
cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more
than 100, you must either include a machine-readable Transparent copy along
with each Opaque copy, or state in or with each Opaque copy a
computer-network location from which the general network-using public has
access to download using public-standard network protocols a complete
Transparent copy of the Document, free of added material. If you use the
latter option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this Transparent
copy will remain thus accessible at the stated location until at least one
year after the last time you distribute an Opaque copy (directly or through
your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give them
a chance to provide you with an updated version of the Document.

 4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under the
conditions of sections 2 and 3 above, provided that you release the Modified
Version under precisely this License, with the Modified Version filling the
role of the Document, thus licensing distribution and modification of the
Modified Version to whoever possesses a copy of it. In addition, you must do
these things in the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title distinct from
that of the Document, and from those of previous versions (which should, if
there were any, be listed in the History section of the Document). You may
use the same title as a previous version if the original publisher of that
version gives permission.

B. List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified Version,
together with at least five of the principal authors of the Document (all of
its principal authors, if it has fewer than five), unless they release you
from this requirement.

C. State on the Title page the name of the publisher of the Modified Version,
as the publisher.

D. Preserve all the copyright notices of the Document.

E. Add an appropriate copyright notice for your modifications adjacent to the
other copyright notices.

F. Include, immediately after the copyright notices, a license notice giving
the public permission to use the Modified Version under the terms of this
License, in the form shown in the Addendum below.

G. Preserve in that license notice the full lists of Invariant Sections and
required Cover Texts given in the Document's license notice.

H. Include an unaltered copy of this License.

I. Preserve the section Entitled "History", Preserve its Title, and add to it
an item stating at least the title, year, new authors, and publisher of the
Modified Version as given on the Title Page. If there is no section Entitled
"History" in the Document, create one stating the title, year, authors, and
publisher of the Document as given on its Title Page, then add an item
describing the Modified Version as stated in the previous sentence.

J. Preserve the network location, if any, given in the Document for public
access to a Transparent copy of the Document, and likewise the network
locations given in the Document for previous versions it was based on. These
may be placed in the "History" section. You may omit a network location for a
work that was published at least four years before the Document itself, or if
the original publisher of the version it refers to gives permission.

K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the
Title of the section, and preserve in the section all the substance and tone
of each of the contributor acknowledgements and/or dedications given therein.

L. Preserve all the Invariant Sections of the Document, unaltered in their
text and in their titles. Section numbers or the equivalent are not
considered part of the section titles.

M. Delete any section Entitled "Endorsements". Such a section may not be
included in the Modified Version.

N. Do not retitle any existing section to be Entitled "Endorsements" or to
conflict in title with any Invariant Section.

O. Preserve any Warranty Disclaimers. If the Modified Version includes new
front-matter sections or appendices that qualify as Secondary Sections and
contain no material copied from the Document, you may at your option
designate some or all of these sections as invariant. To do this, add their
titles to the list of Invariant Sections in the Modified Version's license
notice. These titles must be distinct from any other section titles. You may
add a section Entitled "Endorsements", provided it contains nothing but
endorsements of your Modified Version by various parties--for example,
statements of peer review or that the text has been approved by an
organization as the authoritative definition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list of
Cover Texts in the Modified Version. Only one passage of Front-Cover Text and
one of Back-Cover Text may be added by (or through arrangements made by) any
one entity. If the Document already includes a cover text for the same cover,
previously added by you or by arrangement made by the same entity you are
acting on behalf of, you may not add another; but you may replace the old
one, on explicit permission from the previous publisher that added the old
one.

The author(s) and publisher(s) of the Document do not by this License give
permission to use their names for publicity for or to assert or imply
endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified versions,
provided that you include in the combination all of the Invariant Sections of
all of the original documents, unmodified, and list them all as Invariant
Sections of your combined work in its license notice, and that you preserve
all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and multiple
identical Invariant Sections may be replaced with a single copy. If there are
multiple Invariant Sections with the same name but different contents, make
the title of each such section unique by adding at the end of it, in
parentheses, the name of the original author or publisher of that section if
known, or else a unique number. Make the same adjustment to the section
titles in the list of Invariant Sections in the license notice of the
combined work.

In the combination, you must combine any sections Entitled "History" in the
various original documents, forming one section Entitled "History"; likewise
combine any sections Entitled "Acknowledgements", and any sections Entitled
"Dedications". You must delete all sections Entitled "Endorsements".

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in the
collection, provided that you follow the rules of this License for verbatim
copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it
individually under this License, provided you insert a copy of this License
into the extracted document, and follow this License in all other respects
regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and
independent documents or works, in or on a volume of a storage or
distribution medium, is called an "aggregate" if the copyright resulting from
the compilation is not used to limit the legal rights of the compilation's
users beyond what the individual works permit. When the Document is included
in an aggregate, this License does not apply to the other works in the
aggregate which are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of
the Document, then if the Document is less than one half of the entire
aggregate, the Document's Cover Texts may be placed on covers that bracket
the Document within the aggregate, or the electronic equivalent of covers if
the Document is in electronic form. Otherwise they must appear on printed
covers that bracket the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may distribute
translations of the Document under the terms of section 4. Replacing
Invariant Sections with translations requires special permission from their
copyright holders, but you may include translations of some or all Invariant
Sections in addition to the original versions of these Invariant Sections.
You may include a translation of this License, and all the license notices in
the Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions of
those notices and disclaimers. In case of a disagreement between the
translation and the original version of this License or a notice or
disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements", "Dedications",
or "History", the requirement (section 4) to Preserve its Title (section 1)
will typically require changing the actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as
expressly provided for under this License. Any other attempt to copy, modify,
sublicense or distribute the Document is void, and will automatically
terminate your rights under this License. However, parties who have received
copies, or rights, from you under this License will not have their licenses
terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU
Free Documentation License from time to time. Such new versions will be
similar in spirit to the present version, but may differ in detail to address
new problems or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the
Document specifies that a particular numbered version of this License "or any
later version" applies to it, you have the option of following the terms and
conditions either of that specified version or of any later version that has
been published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may choose
any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the
License in the document and put the following copyright and license notices
just after the title page:

Copyright (c) YEAR YOUR NAME. Permission is granted to copy, distribute and/
or modify this document under the terms of the GNU Free Documentation
License, Version 1.2 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover Texts, and no
Back-Cover Texts. A copy of the license is included in the section entitled
"GNU Free Documentation License".

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the "with...Texts." line with this: with the Invariant Sections being
LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the
Back-Cover Texts being LIST. If you have Invariant Sections without Cover
Texts, or some other combination of the three, merge those two alternatives
to suit the situation.

If your document contains nontrivial examples of program code, we recommend
releasing these examples in parallel under your choice of free software
license, such as the GNU General Public License, to permit their use in free
software.
