1. DHCP Lease Query DHC Working Group Richard Woundy Cisco Systems
March 29, 2000
Phone example
Laptop at friend’s house
Copyright 1998 American Internet Corporation, Company Confidential

2. 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)

3. 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

4. Location Information in a Cable Access Network
Computers
Subscriber Modems
Access Concentrator
DHCP Server
Access Concentrators use location information for:
Choosing specific broadband access network
Encrypting traffic for specific downstream subscriber modem
Forwarding traffic to specific subscriber modem

5. 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)

6. 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

7. 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

8. 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

9. 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

10. 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