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Multiple CPE Provisioning Options
Thinking ahead of supporting multiple CPE's within the home
By: Bruce Bahlmann - Contributing Author (your
feedback
is important to us!)
Created: January 24, 1999
Note: For help designing/implementing your multi-CPE cable modem service or for developing tools to help you improve or implement such a program contact Birds-Eye.Net.
Overview:
There exists a need to enable cable companies (MSOs) to support local area networks
(LANs) via the use of a cable modem that supports multiple customer premise equipment
(CPE). The purpose of this paper is to provide information that will allow the reader to
identify the issues of providing this capability, become familiar with various methods
that have been proposed, and compare these methods so as to provide the necessary
information needed to make future decisions on which method makes the most dollars &
sense.
Issues Identified:
When multiple CPEs are connected the resulting entity represent several issues that
provisioning systems must address to support this functionality. These issues are
summarized in Table 1.0.
| Changing NICs |
Changing out customer network interface cards (NIC) can make
an existing customer appear as though they are a "new" customer – unknown
to the system. |
| Static IPs |
Vulnerability to customers who configure their CPE with a
static IP addresses. |
| Isolation |
Preventing customer traffic that is not destine for the
Internet or another MSO customer LAN from entering the broadband. |
| Selective Access |
Enabling a subset of customer NICs access to network while
allowing the remaining to network (i.e. don’t allow printers to get routable IP
addresses if they don’t need one). |
| DNS Support |
Providing persistent (bahlmann.mn.mediaone.net) or vanity
(bahlmann.com) domain name service (DNS) to selected CPEs. |
| Legacy Support |
Data over cable service interface specification (DOCSIS)
cable modem solution must also support large population of Legacy cable modems (LANCity
revision 3). |
| Troubleshooting |
MSO’s must have the ability to match a NIC with a
particular customer for the purpose of troubleshooting (i.e. with IP address in hand the
MSO must be able to "quickly" determine the customer associated with that IP
address). |
| Supported Devices |
Will option support different devices, if so, what
constitutes a supported device? If a device can obtain a dynamic IP address does that mean
the customer can now plug it in and use it on his/her LAN? |
| Supported Services |
Are support services included with option, if so, what is the
scope and depth of an MSO offering of multiple CPE support (i.e. does it provide Tier 2
support only personal computers running select versions of software or does it open up and
say any device that is capable of obtaining a dynamic IP address). |
| Self-Install |
Providing the customer with the ability to install this
capability without MSO intervention as well as integrate well with non-multiple CPE self
installs. |
| Timing |
Does support for multiple CPE require a particular sequence
or can events needed to set up this option happen in any order? |
| Connectivity |
Can connectivity interrupt or break multiple CPE support.
What happens in the event communications are lost? |
| Bridging Table |
>Cable modem learns devices when it plugged into a
network. If the cable modem is "programmed" for a single CPE, this may (or may
not) be the desired one that the customer wants to access the Internet. |
| Renumbering |
Renumbering the network should be handled seamlessly by the
option or else it could cause problems in general network operations. |
Table 1.0 Issues relating to supporting multiple
CPEs.
With these issues in mind the goal should be to provide a "solution" that
most effectively addresses the provisioning strategy of the MSO. The overall provisioning
strategy is needed as a model to guide decisions about building provisioning systems. The
provisioning strategy does not suggest particular solutions or particular vendors but
limits the many possibilities to only the ones that more closely match the mission and
objective of the MSO in regards to providing provisioning services. Some basic
provisioning strategies are listed below (Table 1.1) to provide a basis on which to
introduce the topic of this paper (DHCP Cable Modem) however, it is highly suggested that
the reader refer to their respective company’s provisioning strategy.
| Modular |
Provisioning services must be modular thus
enabling many vendors to compete for the desired functionality. |
| Standards-Based |
Provisioning servers should employ well honed
standards for operation as well as interoperation. |
| Scalability |
Provisioning servers must be able to scale up as
needed. For example, the entry level architecture should last some number of years (or
some number of customers) with follow on augmentation that is stated from the beginning
(before its purchased – preferably) being available as the need arises. |
Table 1.1 Common MSO provisioning strategies
Several individuals and organizations have attempted to condense the above stated
issues and come up with a "virtual" option that would enable MSO’s to
support multiple CPEs. These options, and their variants, are discussed individually in
the next section.
Provisioning Options:
The systems capable of supporting multiple CPEs center around the following three
options:
- Manipulation of MENS
- Relay Agent Information Option (Option 84)
- Cable Modem DHCP Server
Each of these options will be discussed in brief detail
Manipulation of MENS:
Modem centric model that utilizes a configurable parameter (common in both DOCSIS and
Legacy cable modems) to dictate the maximum number ethernet nodes supported (MENS). This
option enables a modem to limit the number of network devices that can access the MSO
network (and its provisioning servers). For example if this is property is set to one,
only a single NIC can access the MSO network through that modem (the first NIC learned by
the cable modem). There are two ways of fulfilling this option as far as limiting the
number of NICs that are allowed to pass data through the modem once it has been set to
some number. Either the modem is allowed to control this on its own (default) or one could
pre-load NICs and or IP addresses. If the default option is used the modem internally
learns NICs of network devices connected to the modem and permit these devices to pass
data through the modem. In networks where the number of network devices exceeds the number
of devices the modem will support, there will inevitably be an access problem (one or
more, of the authorized computers will not be able to use our network because they were
not learned by the modem).
Relay Agent Information Option (Option 84):
An end-to-end system that takes advantage of a DHCP option implemented by select cable
modem termination system (CMTS) vendors where by the DOCSIS cable modem ethernet address
is joined with the broadcast messages from any CPEs connected to it. The rest of the
system involves various modifications to the provisioning systems to correctly map the
relationship created at the CMTS to the desired number of CPEs to which the customer has
subscribed.
Cable Modem DHCP Server
The basic idea behind this option is to enable DHCP server functionality on the
ethernet port of a cable modem. This option would provide multiple CPE functionality in
two different flavors:
- Network Address Translation (NAT)
- Proxy DHCP services
Using NAT along with DHCP services, the cable modem would allow multiple CPEs Internet
access via a single routable IP address. This would require the cable modem to obtain two
IP addresses: one for the cable modem (to support SNMP etc.) which is usually a private IP
address, and the another which it would use to grant CPE devices access the Internet.
Furthermore, this method would establish a 1-to-1 relationship with each cable modem and
some number of CPEs. This desired relationship would be such that a single cable modem
ethernet address could represent both the CM and the CPE. Since the cable modem already
has two ethernet addresses, the second ethernet address could be used to obtain the
routable IP address needed for NAT. Since these ethernet address pairs are sequential,
this allows a single cable modem ethernet address to represent both CPE and cable modem (a
highly attractive feature of this model is that no CPE NICs would need to be managed). Any
DNS services would be mapped to the initial port identified by the client ID
"primary". In this way, the customer could easily change which CPE is assigned
their hostname by which client has the ID of "primary".
Using proxy DHCP services without NAT enables the cable modem to carry out DHCP
requests for a certain number of CPEs and hand out private IP addresses to the other CPEs.
This model, however, breaks the 1-to-1 relationship and would require that the cable modem
some how be told how many IP addresses to allow through (similar to MENS). Unfortunately a
ton of logic would be needed by the cable modem to support this option such that things
like renumbering could happen seamlessly (clearing learned DHCP requests allowing the
cable modem’s DHCP server to request some number of new IP addresses from the
"MSO" provisioning server in preparation for use by clients. One negative note,
is that multiple leases would need to be assigned to the cable modem for itself as well as
the PCs behind it.
Some features common to both DHCP server options are:
- DHCP services in the cable modem handle normal everyday traffic required by CPEs –
this traffic is handled quickly and most efficiently at the customer premises
significantly reducing the load on the primary DHCP server.
- Store and forward approach allows a cable modem that has been momentarily disconnected
from the main provisioning server to maintain its CPEs until their lease runs out or its
told by the CMTS otherwise (changing subnets during a renumbering).
- Causes all cable modems to request DHCP addresses for their clients regardless of
whether they are up or down (a particularly useful feature when renumbering and for
walking DNS changes).
Option Comparison:
To better understand the options previously discussed, each option is examined for
whether or not it addresses the issues introduced in this document. Also examined is what
requirements and additional equipment and configurations are needed to operationally
support the option. To compare the latter, each option is examined in terms of what is
needed to from each service and what category would the modification/enhancement of the
hardware/software be classified. For purposes of "initially" examining these
options, this latter category is subjective and the reader should be advised as such.
Table 2.0 below, provides a summary of the options described above.
| Issue: |
MENS |
DHCP Info Option |
Cable Modem DHCP |
| Changing NIC |
Y (optional) |
Y |
Y |
| Static IPs |
Y (if pre-stuffing used) |
N |
Y |
| Isolation |
Y (if pre-stuffing used) |
N |
Y |
| Selective Access |
Y |
Y |
Y |
| DNS Support |
Y |
Y |
Y |
| Legacy Support |
Y |
N |
N (could be added) |
| Troubleshooting |
Y (via 2 lookups) |
Y (via 2 lookups) |
Y (single lookup) |
| Supported Devices |
N (restricted) |
Y (over extend MSO) |
Y (don’t care) |
| Supported Services |
N |
Y |
Y (don’t care) |
| Self Install |
Y |
Y (if modified) |
Y |
| Timing |
N |
Y |
N |
| Connectivity |
Y |
Y |
N |
| Bridging Table |
Y |
N |
N |
| Renumbering |
N |
Y |
N* |
Note private network assigned to customer would never have to change
(only when adding new clients).
| Modification Needed: |
MENS |
DHCP Info Option |
Cable Modem DHCP |
| Provisioning Server |
New Service Groups |
New Logic |
None |
| TFTP Server (non-OTF) |
None |
None |
None |
| CMTS |
None |
Hybrid Relay Agent |
None |
Concerns about Charging by the Connection:
Due to the complexities involved in creating a model that is configurable for specific
numbers of CPEs in the home, it may make more sense (as discussed in previous document Multi-user
Residential Cable Modem) to NOT implement this functionality as another financial
opportunity but rather as a value add. In leu of making the customer increasingly paying
for additional connections "through the same pipe" provided by the cable modem
more customers will be enticed to seek out increasingly simple proxy applications that
abound on the Internet. By do this the customer is moving into a more complex network
setup within their home where by creating a much more difficult problem to solve by an MSO
tier 2 organization if problems arise. Providing this service at no additional cost will
get more power users on board with the technology and steal profits from perspective proxy
and NAT vendors by basically giving this functionality away as part of the basic service.
Running multiple CPEs without a gateway/proxy is a very simple network to troubleshoot.
Instead, MSO should seek revenues from add on services and tier the different types of
connections and bandwidths. We currently have the ability to run customers with private or
routable IP addresses. This option alone could further extend our availability of routable
IP addresses which could be associated with more expensive services. We could also offer
varying throughput services. Combining IP with throughput you would yield a full array of
connection service level options that are available today. Layering different types of
services on top of connectivity for any number of devices in the home or office provides a
solid business model can simplify support costs.
References:
Multi-user
Residential Cable Modem, Bruce Bahlmann, December 4, 1997
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