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rfc:rfc3256

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
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 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
 copyrights defined in the Internet Standards process must be
 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.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Jones & Woundy Standards Track [Page 5]

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