About the DHCP Lease Query LEG

This module describes the Subscriber Manager DHCP Lease Query LEG software module and the terms and concepts relevant to the DHCP Lease Query LEG.

Information About the DHCP Lease Query LEG

The SCMS SM DHCP Lease Query LEG is a software module that handles pull-requests from the different SCE platforms in the network. The LEG queries the DHCP server using a DHCP Lease-Query transaction. The DHCP Lease Query LEG can be run on the SM server or on the SCE device. If you use an SM, the LEG must be used on the SM, not on the SCE.

DHCP Lease Query LEG Operation

The subscriber's traffic (1) triggers a pull-request from the SCE (2). The SM receives the request for processing. If the SM does not find a subscriber with a matching IP address in the subscriber database, it passes the pull-request to the DHCP Lease Query LEG (3). The LEG queries the DHCP server. If the server finds a match for the IP address in its database, the server replies with the subscriber information (4). The LEG performs a login operation (5). This operation updates the subscriber database based on the received information and logs the subscriber into the SCE (6), which triggered the pull-request.

If desired, install the DHCP Lease Query LEG directly on the SCE device to integrate the SCE with DHCP servers without the use of an SM server. The following diagram represents the operation of the LEG when installed on the SCE device:

The subscriber's traffic (1) triggers a pull-request from the SCE (2). The Lease Query LEG receives the request and queries the DHCP server. If the server finds a match for the IP address in its database, the server replies with the subscriber information (3). Based on the received information, the LEG responds to the SCE with a pull-response, which includes the subscriber ID and the IP address lease-time returned from the DHCP server (4).

Note An Internet Engineering Task Force (IETF) standard defines the DHCP Lease-Query transaction. The LEG supports the RFC 4388 standard. For more information, see the IETF website.

Terms and Concepts

The following terms and concepts are necessary to understand the DHCP Lease Query LEG and SM configuration and operation. Additional information can be found in the Cisco SCMS Subscriber Manager User Guide .

LEG (Login Event Generator)

A software component that performs subscriber login and logout operations on the SM/SCE. The LEG handles dynamic subscriber integration.

Cable/Satellite Modem

A data modem that provides Internet access over cable and satellite networks. The modem usually corresponds to a single subscriber of the Internet Service Provider (ISP).

CPE (Customer Premise Equipment)

Any equipment that an end-user can connect to the network through a modem. The end-user usually owns multiple CPE devices that are used to connect to the Internet through a single modem.

DHCP Lease Query Transaction

The DHCP Lease Query transaction is a DHCP transaction with special message types that enable, among other things, clients to query DHCP servers regarding the owner and the lease-expiration-time of an IP address.

An IETF standard defines the DHCP Lease-Query transaction. For more information, see the IETF website.

Subscriber Mappings

The SCE platform requires mappings between the network IDs (IP addresses) of the flows it encounters and the subscriber IDs. The SM database contains the network IDs that map to the subscriber IDs. The SCE network-ID-to-subscriber mappings are constantly updated from the SM database.

The main function of the DHCP Lease Query LEG is to provide the SM/SCE with network-ID-to-subscriber mappings in real time.

Pull-request

A message sent from an SCE device to the SM or the LEG when it identifies the use of a new subscriber IP address in the network. The SM uses the IP address provided in this message to query the database to retrieve the subscriber data of the subscriber associated with this address and to send its data to the SCE.

Subscriber Domain

The SM provides the option of partitioning SCE platforms and subscribers into subscriber domains. A subscriber domain is a group of SCE platforms that share a group of subscribers. Subscriber domains can be configured using the SM configuration file and can be viewed using the SM Command-Line Utility (CLU).

For additional information about domains and domain aliases, see the "Configuration File Options" chapter of the Cisco SCMS Subscriber Manager User Guide .

Subscriber Package

A subscriber policy package usually defines the policy enforced by Cisco SCMS solutions on each subscriber. The DHCP Lease Query LEG can handle the package ID in any of the following ways:

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Set the package ID according to configurable options of the DHCP initial login or lease extension transactions

Information About DHCP Lease Query LEG Functionality

DHCP Lease Query LEG Process

The LEG processes DHCP Lease-Query transactions to the DHCP server using the IP address indicated in the pull-request from the SCE. The DHCP server replies whether there is an active lease (DHCPLEASEACTIVE message) for this IP address and provides information about the subscriber associated with this IP address according to a list of options requested by the LEG. By default, the LEG requests the lease time and the modem MAC and adds package association related options if needed.

The DHCP Lease Query LEG supports up to two redundant DHCP servers. The LEG identifies a server failure by counting the consecutive requests that time out. After a configurable threshold of timed-out requests, the LEG starts to send the requests to the recently activated server, which was previously in standby. The LEG does not return to the failed server until the activated server fails.

When installing the LEG on the SM server, it runs with the privileges assigned to the user pcube on this machine. On UNIX platforms, because only the super-user (root) can open ports under 1024, the LEG cannot open the DHCP ports. To solve this problem, a simple application is supplied with the LEG, which forwards the DHCP packets between the LEG and the DHCP servers. This application is the DHCP Forwarder, which is described in the DHCP Forwarder Application module.

When installing the LEG on the SCE device, there is no need to use the DHCP Forwarder application.

DHCP Lease Query Transaction

The DHCP Lease Query transaction is a DHCP transaction where the client (LEG) sends a DHCPLEASEQUERY message to the server, indicating the information it wants to query. The LEG only queries the IP address. The server can reply with several types of messages; for example a DHCPLEASEACTIVE message means that an active lease was found and the request information is supplied, or a DHCPLEASEUNASSIGNED message means this IP is currently not assigned to any subscriber.

The following is a detailed description of the attributes extracted from the DHCP Lease Query transaction :

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Subscriber ID—By default, 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, the DHCP server must support and store option 82 for each Customer Premise Equipment (CPE). This default can be overwritten by configuration. Furthermore, the LEG can assign the IP address as a fallback if the option does not exist in the server's response. This fallback is disabled by default.

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Lease time—The assigned IP is added to the SM or SCE database with a lease time taken from option 51. Note that if option 51 does not appear in the DHCPLEASEACTIVE reply, an infinite lease time is assigned for this IP address.

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Package—Configurable options in the DHCP message determine how to assign the package information. 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. The package options are assumed to be encoded as strings.

After extracting the preceding information, the LEG logs the subscriber into the SM/SCE.

Installation and Usage

The DHCP Lease Query LEG is an external component (PQI file) that can be installed on the SM or SCE device.

Package Contents

The DHCP Lease Query LEG distribution is part of the SM LEG distribution. The DHCP Forwarder application distribution and installation script are also part of the SM LEG distribution.

The LEG installation package includes a set of configuration files and command-line utilities for the LEG.

The SCMS SM LEG distribution file contains the DHCP Lease Query LEG distribution, which is located in the Lease_Query_LEG directory. Cisco supplies a DHCP Lease Query LEG distribution package. The following table describes the package contents:

Table 1-1 File Layout of the DHCP Lease Query LEG Distribution Package
Root	Folder (under root)	File name	Notes
pkg-ext-dir
		Install	LEG installation procedure description
		install-forwarder.sh	DHCP Forwarder installation script
		linux-def.sh	Linux-specific definitions
		solaris-def.sh	Solaris-specific definitions
		dhcp_forwarder.tar.gz	DHCP Forwarder distribution
		sm-common.sh	General utility script
	sce
		dhcp_pkg.cfg	Default configuration file for package association
		leaseq.pqi	DHCP Lease Query LEG distribution
	sm
		leaseq.pqi	DHCP Lease Query LEG distribution

Note The directory to which the distribution is extracted is referred to as pkg-ext-dir .

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