Transcript What is DHCP Lease Query?

DHC Working Group
DHCP Lease Query
Richard Woundy
Cisco Systems
March 29, 2000
Presentation_ID
© 1999, Cisco Systems, Inc.
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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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© 1999, Cisco Systems, Inc.
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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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© 1999, Cisco Systems, Inc.
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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
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© 1999, Cisco Systems, Inc.
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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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© 1999, Cisco Systems, Inc.
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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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© 1999, Cisco Systems, Inc.
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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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© 1999, Cisco Systems, Inc.
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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
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© 1999, Cisco Systems, Inc.
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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
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