About the SCE-Sniffer DHCP LEG
Revised: July 28, 2009, OL-15752-05
This module describes the Service Control Management Suite (SCMS) Subscriber Manager (SM) SCE-Sniffer DHCP Login Event Generator (LEG) software module.
•Information About the SCE-Sniffer DHCP LEG
•Information About SCE-Sniffer DHCP LEG Functionality
Information About the SCE-Sniffer DHCP LEG
The SCMS SM SCE-Sniffer DHCP LEG is a software module that receives Raw Data Record (RDR) messages containing DHCP information from Service Control Engine (SCE) devices configured with a DHCP sniffer service. The SCE-Sniffer DHCP LEG is an extension of the SM software and runs as part of the SM.
SCE-Sniffer DHCP LEG Operation
The SCE device analyzes DHCP traffic and reports the DHCP transactions to the SM device using the RDR protocol. The SM extracts the modem MAC address, the CPE IP address, and optionally, the subscriber package information from the RDR, and triggers a logon or logout operation to the SM.
Figure 11-1 represents the operation of the SCE-Sniffer DHCP LEG:
Figure 11-1 SCE-Sniffer DHCP LEG Operation
Information About SCE-Sniffer DHCP LEG Functionality
The SCE devices analyze the DHCP ACK packets of DHCP transactions and send the information to the SCE-Sniffer DHCP LEG that resides on the SM. The LEG performs login and logout operations to the SM using the information sent from the SCE devices. The DHCP transactions that are relevant for the operation of the LEG are initial logon, lease extension, and release.
•DHCP Initial Logon Transaction
•DHCP Lease Extension Transaction
•DHCP Release Transaction
DHCP Initial Logon Transaction
The following is a detailed description of the attributes extracted from the DHCP initial logon transaction:
For cable environments—The subscriber ID is the modem MAC address, which you extract from option 82 (Remote-ID sub-option of the DHCP Relay Agent Information Option). Therefore, for a successful logon operation, it is required that option 82 contains the modem MAC address in the DHCP initial logon transaction. If option 82 is missing, it is not possible to perform a logon operation. Furthermore, the value of option 82 is compared with the haddr field to identify modem transactions and not login the modem IP address to the SM.
For non-cable DHCP environments—The LEG supports using other DHCP options for the subscriber ID. If the DHCP option does not exist in the packet, it is possible to use the IP address as a fallback. In this case, the subscriber ID is in the format IP_a.b.c.d.
The chain of decisions regarding the subscriber-ID is as follows:
a. Use the configured DHCP option as the subscriber-ID if it exists.
b. Otherwise, if the fallback to IP is enabled, use the IP address.
c. Otherwise, attempt to extend the lease based solely on the IP address. (This will only work if the IP address is in the database).
Each subscriber might have multiple IP addresses, depending on the number of CPE devices connected to the modem. A logon operation is triggered for each assigned IP address in the DHCP message.
If the transaction correlates to a CPE device, the assigned IP address for that CPE device is added to the SM database. The IP address of the modem is not added to the SM database. If the transaction correlates to a modem device, no IP mappings are added to the SM database, but a logon operation is performed anyway to update package information.
If the transaction correlates to a CPE device, the assigned IP is added to the SM database with a lease time taken from option 51 (lease time option). Note that option 51 must contain the lease time; otherwise no logon operation is performed.
The policy information is assigned according to configurable options in the DHCP message. The LEG includes a component that converts the package information data from the DHCP packet to a subscriber package ID. If the packet does not contain package information, it is possible to log in the subscriber with a default package, or log in the subscriber with no package information at all.
After extracting the above information, the LEG performs a logon operation to the SM.
DHCP Lease Extension Transaction
The same attributes are extracted from the DHCP lease extension transaction as for the DHCP initial logon transaction, but the existence of option 82 is not required. If the modem MAC address cannot be retrieved from option 82, the SM database is queried for this information.
DHCP Release Transaction
The DHCP release transaction is handled differently to the other DHCP transactions. If the transaction correlates to a CPE device, the LEG performs an SM logout operation with the IP address of the CPE, which appears as a released IP address in the packet itself.
Note A logout operation is also performed when the lease time of the subscriber is expired, and the SM is configured to perform auto logouts. Release transactions also trigger logout operations, but do not replace the auto logout mechanism of the SM.