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DHC Working Group

DHC Working Group. DHCP Lease Query Richard Woundy Cisco Systems March 29, 2000. What is DHCP Lease Query?. A lightweight method for relay agents to get “location information” from the DHCP server(s)

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DHC Working Group

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  1. DHC Working Group DHCP Lease Query Richard Woundy Cisco Systems March 29, 2000

  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 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

  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

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