The DOCSIS (Data-Over-Cable Service Interface Specifications) Device Class DHCP (Dynamic Host Configuration Protocol) Relay Agent Information Sub-option
Extracted elements (7)
A Relay Agent Information sub-option was chosen over a new or existing DHCP option because CPE Controlled Cable Modems (CCCMs) have modem firmware controlling DOCSIS signaling while the attached CPE manages DHCP. Trusting the modem firmware for device class signaling is considered more secure than trusting software on the attached computer.
DHCP servers MAY use the device class for IP and other parameter assignment policies for cable modems.
The relay agent MUST pass the Device Class value unchanged to the DHCP server. The value is obtained from the Device Class Identification Request (DCI-REQ) DOCSIS signaling message.
The DOCSIS Device Class sub-option is assigned sub-option code 4 within the DHCP Relay Agent Information Option space. It carries a 32-bit Device Class field (d1-d4) identifying attributes of the cable modem, with bit #0 indicating a CPE Controlled Cable Modem (CCCM) and bits #1-31 reserved and set to zero.
IANA assigned sub-option code 4 from the DHCP Relay Agent Sub-options space (RFC 3046) for the DOCSIS Device Class sub-option defined in this document.
Operation relies on an implied trusted relationship between the DHCP relay agent and the DHCP server, and between the cable modem and the relay agent via DOCSIS signaling. The device class identifier provided by the relay agent is assumed to be trusted, but cryptographic or other authentication techniques for the device class are outside the scope of this document.
The sub-option is encoded as: SubOpt=4 (1 octet), Len=4 (1 octet), followed by 4 octets (d1-d4) carrying the 32-bit Device Class value. Total length is 6 octets.