1Presentation_ID © 1999, Cisco Systems, Inc.

DHC Working Group

DHCP Lease Query

Richard Woundy

Cisco Systems

March 29, 2000

2Presentation_ID © 1999, Cisco Systems, Inc.

What is DHCP Lease Query?

• A lightweight method for relay agents to get “location information” from the DHCP server(s)

• A message designed for the needs of broadband access concentrators (e.g. DOCSIS CMTS, DVB INA, DSL AC)

• A DHCP message that does not modify server lease state (like DHCPINFORM)

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Location Information

• Often includes the following information:

Device hardware (MAC) address

Port/virtual circuit that leads to the device

Hardware address of the intervening subscriber modem

• Can be used for both downstream transmission, and upstream verification

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Location Information in a Cable Access Network

DHCP Server

Access ConcentratorSubscriber ModemsComputers

24.128.1.1

24.128.1.2

24.128.1.3

24.128.1.4

Access Concentrators use location information for:

•Choosing specific broadband access network

•Encrypting traffic for specific downstream subscriber modem

•Forwarding traffic to specific subscriber modem

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Why Propose a New DHCP Message - 1

• Use broadcast ARP

Chatty on public network

Vulnerable to subscriber spoofing

• Capture information from relayed DHCP messages (gleaning)

Process starts from scratch with reboot or replacement of relay agent

Relay agent misses unicast DHCP messages (e.g. Renewals)

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Why Propose a New DHCP Message - 2

• Leverage DHCP Server MIB

Access concentrators act as SNMP agents, but not as SNMP managers

• Leverage DHCP LDAP Schema

Access concentrators don’t act as LDAP clients

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DHCP Lease Query Exchange

• DHCP Lease Query message

Ciaddr refers to IP address lease to query

Giaddr refers to requestor (i.e. access concentrator)

Parameter request list includes IP Address Lease Time option (51) and Relay Agent Information option (82)

• DHCP Lease Query response - DHCPACK or DHCPNAK

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DHCP Lease Query Example - DOCSIS CMTS

• CMTS receives packet to forward downstream across cable

CMTS has no local location information

• CMTS sends DHCP Lease Query, gets DHCPACK

Chaddr contains the PC MAC address

Option 82 contains subscriber modem info

• CMTS transmits packet using BPI

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Interactions with Lease Query

• Relay Agent Gleaning

Gleaning state replaces Lease Query state

• Lease Query with Failover

Access concentrator sends Lease Query messages to multiple DHCP servers

Failover BNDUPD messages need to include option 82

• Lease Query uses DHCP Authentication

10Presentation_ID © 1999, Cisco Systems, Inc.

DHCP Lease Query Action Items

• DHCP Lease Query implemented in Cisco uBR, Cisco Network Registrar

• DHCP Lease Query internet draft posted March 10th

• Determine working group sentiment at Adelaide IETF, from mailing lists

• Update DHCP Lease Query draft depending on technical comments and standards-track decision