Network Working Group D. Jones
Request for Comments: 3256 YAS Corporation
Category: Standards Track R. Woundy
The DOCSIS (Data-Over-Cable Service Interface Specifications) Device
Class DHCP (Dynamic Host Configuration Protocol) Relay Agent
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document proposes a new sub-option to the DHCP (Dynamic Host
Configuration Protocol) Relay Agent Information Option. This new
sub-option is for use with DOCSIS (Data-Over-Cable Service Interface
Specifications) cable modems and describes a "device class" to which
the cable modem belongs. The cable modem signals its device class
information to the Relay Agent using DOCSIS signaling, and the Relay
Agent forwards the device class information to the DHCP Server which
can then make a policy decision based on it.
The "Relay Agent Information" Option is described in  and includes
several Relay Agent Information sub-options. This RFC proposes an
additional sub-option for use with DOCSIS cable modems. This sub-
option is added by DHCP relay agents which terminate cable modems.
The sub-option encodes an identifier of the device class to which the
cable modem belongs. It is intended for use by DHCP servers to make
policy decisions based on the device class of the host.
The motivation for using a Relay Agent Information sub-option, rather
than a new or existing DHCP option, is the introduction of CPE
Controlled Cable Modems (CCCMs) . In an implementation of a CCCM,
the modem firmware controls DOCSIS signaling, but the attached
computer (CPE) manages other protocol activities -- particularly DHCP
client message handling. The assumption of this document is that it
Jones & Woundy Standards Track [Page 1]RFC 3256 The DOCSIS Device Class DHCP April 2002
is better to trust the operation of the CCCM firmware, than to trust
the operation of CCCM software running on the attached computer
(e.g., a standard PC).
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as described in RFC 2119 .
2. DOCSIS Device Class Sub-option
The DOCSIS RFI specification  specifies the Device Class encoding
within the payload of the Device Class Identification Request (DCI-
REQ) message. The relay agent MUST pass the Device Class value
unchanged to the DHCP server. Possible uses of this field include:
o host endpoint information
o host hardware capabilities
o host software capabilities
o host options information
DOCSIS defines the Device Class to be a 32-bit field where individual
bits represent individual attributes of the CM. Bit #0 is the least
significant bit of the field. Bits are set to 1 to select the
attributes defined below.
bit #0 - CPE Controlled Cable Modem (CCCM)
bits #1-31 - Reserved and set to zero
The DOCSIS Device Class sub-option is coded as follows:
SubOpt Len Device Class
| 4 | 4 | d1 | d2 | d3 | d4 |
The DHCP server needs to understand the meaning of this sub-option in
order to offer different policy options in its reply to the host.
DHCP servers MAY use the device class for IP and other parameter
assignment policies for cable modems.
Jones & Woundy Standards Track [Page 2]RFC 3256 The DOCSIS Device Class DHCP April 20023. Security Considerations
Operation of the DHCP Relay Agent Information Option relies on an
implied trusted relationship between the DHCP relay agent and the
DHCP server. The discussion of security considerations for the DHCP
relay agent information option  apply to this sub-option as well.
Operation of the DOCSIS Device Class sub-option relies on an implied