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

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)

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

Location Information in a Cable Access Network Computers Subscriber Modems Access Concentrator 24.128.1.1 DHCP Server 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

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)

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

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

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

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

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