    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

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       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.

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    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.

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    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

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    7.  Full Copyright Statement

       Copyright (C) The Internet Society (2002).  All Rights Reserved.

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    Acknowledgement

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

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