Network Working Group D. Jones
Request for Comments: 3256 YAS Corporation
Category: Standards Track R. Woundy
AT&T Broadband
April 2002
The DOCSIS (Data-Over-Cable Service Interface Specifications) Device
Class DHCP (Dynamic Host Configuration Protocol) Relay Agent
Information Sub-option
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.
Abstract
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.
1. Introduction
The "Relay Agent Information" Option is described in [1] 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) [2]. 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 [4].
2. DOCSIS Device Class Sub-option
The DOCSIS RFI specification [3] 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 2002
3. 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 [1] apply to this sub-option as well.
Operation of the DOCSIS Device Class sub-option relies on an implied
trusted relationship between the DHCP client (i.e., the cable modem)
and the DHCP relay agent, through DOCSIS signaling. According to
DOCSIS specifications [2], the cable modem firmware always controls
DOCSIS signaling, but cannot control DHCP client message handling
(e.g., CCCMs). This document assumes that the cable modem firmware
is trustworthy for DOCSIS signaling information.
This document introduces a new identifier, the DOCSIS Device Class
sub-option, that is provided by the relay agent device and is assumed
to be trusted. Cryptographic or other techniques to authenticate the
device class are beyond the scope of this document.
4. IANA Considerations
IANA has assigned a value of 4 from the DHCP Relay Agent Sub-options
space [RFC 3046] for the DOCSIS Device Class sub-option defined in
section 2.
5. References
[1] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[2] "Data-Over-Cable Service Interface Specifications: Cable Modem
to Customer Premise Equipment Interface Specification SP-CMCI-
I07-020301", DOCSIS, March 2002, http://www.cablemodem.com.
[3] "Data-Over-Cable Service Interface Specifications: Cable Modem
Radio Frequency Interface Specification SP-RFIv1.1-I08-020301",
DOCSIS, March 2002, http://www.cablemodem.com.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Jones & Woundy Standards Track [Page 3]RFC 3256 The DOCSIS Device Class DHCP April 2002
6. Authors' Addresses
Doug Jones
YAS Corporation
300 Brickstone Square
Andover, MA 01810
Phone: (303) 661-3823
EMail: doug@yas.com
Rich Woundy
AT&T Broadband
27 Industrial Avenue
Chelmsford, MA 01824
Phone: (978) 244-4010
EMail: rwoundy@broadband.att.com
Jones & Woundy Standards Track [Page 4]
RFC 3256 The DOCSIS Device Class DHCP April 2002
7. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Jones & Woundy Standards Track [Page 5]
