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Network Working Group                                           D. JonesRequest for Comments: 3256                               YAS CorporationCategory: Standards Track                                      R. Woundy                                                          AT&T Broadband                                                              April 2002The DOCSIS (Data-Over-Cable Service Interface Specifications) DeviceClass DHCP (Dynamic Host Configuration Protocol) Relay AgentInformation Sub-optionStatus 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 itJones & 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 inRFC 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 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 [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 insection 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 20026.  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.comJones & Woundy              Standards Track                     [Page 4]

RFC 3256              The DOCSIS Device Class DHCP            April 20027.  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]