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As part of the uBR relay agent implementation, the uBR captures and gleans information from DHCP requests and responses, in order to associate subscriber PC MAC addresses, DHCP server-assigned IP addresses, and subscriber cable modem MAC addresses. This information gleaning allows the uBR to avoid downstream ARP broadcasts, to perform IP source address verification of upstream datagrams, and to encrypt unicast downstream traffic by means of the DOCSIS Baseline Privacy protocol.
Unfortunately, the uBR does not capture all DHCP state information through DHCP gleaning. The uBR cannot glean from any DHCP unicast messages (particularly renewals and releases), since these messages are not processed by the uBR relay agent, and since capturing these messages would require special processing that would degrade the uBR forwarding performance of all IP traffic. Furthermore, this DHCP state information does not persist across uBR reboots or replacements. Consequently, the only reliable source of DHCP state information for the uBR is the DHCP server.
The DHCP server supports a new DHCP message, similar to a DHCPINFORM message. The DHCPLEASEQUERY message's number in DHCP Message Type (option 53) is 13.
The DHCPLEASEQUERY message is sent from a DHCP client (that is, the uBR) to the DHCP server. It always yields either a DHCPACK or DHCPNAK response from the server. Note that a different DHCP server that does not support this message is likely to drop the packet.
The following sections describe the pertinent message contents of the DHCPLEASEQUERY message.
The values in the htype, hlen, and chaddr fields are set to 0. The DHCPLEASEQUERY message searchrd for IP addresses, not for MAC addresses. The DHCPACK message encodes the physical address of the lease owner in the htype, hlen, and chaddr fields, and includes the values of the options requested (if available) in the Parameter Request List of the DHCPLEASEQUERY message. See the "Parameter Request List Option" for more information.
The ciaddr field is set to the IP address of the lease to be searched.
The DHCP server replies to the DHCPLEASEQUERY message with a DHCPACK message if the server has definitive information about the lease indicated by the ciaddr field.
The server replies with a DHCPNAK message if the server does not have definitive information about the lease indicated by the ciaddr field.
Like all other DHCPACK or DHCPNAK packets, the server unicasts the result to the ciaddr unless there is a giaddr field value present. In this case the server unicasts the result to the giaddr.
The giaddr field is set to the IP address of the DHCP client. The giaddr field is independent of the ciaddr to be searched.
The Parameter Request List option is set to the options of interest to the requestor, which can include the IP Address Lease Time (option 51) and the Relay Agent Information option (82).
If the IP Address Lease Time is specified in the Parameter Request List, the DHCP server returns this option in the DHCPACK with the remaining time until lease expiration. The server does not return this option if the lease is not leased. This allows the DHCP client to determine whether the lease is currently leased and for how long.
The DHCP server uses information from the lease state for the DHCPACK option values. The value for the IP Address Lease Time in the DHCPACK is the number of seconds that remain until the current lease expires. The value for the Relay Agent Information option in the DHCPACK is the value of the Relay Agent Information option from the most recent DHCPDISCOVER or DHCPREQUEST message from the client to this DHCP server.
To accommodate DHCPLEASEQUERY messages sent to a DHCP failover backup server when the master server is down, the master server must communicate the Relay Agent Information option values to the backup server. To accomplish this task, it uses DHCP failover update messages.
Posted: Thu Feb 3 10:38:03 PST 2000
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