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DHCP: Dual-Stack Issues draft-ietf-dhc-dual-stack-01

DHCP: Dual-Stack Issues draft-ietf-dhc-dual-stack-01. Tim Chown tjc@ecs.soton.ac.uk dhc WG, IETF 60, San Diego, August 2, 2004. The crux. Nodes in a dual-stack environment may require IPv4 and IPv6 configuration (addresses and/or options).

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DHCP: Dual-Stack Issues draft-ietf-dhc-dual-stack-01

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  1. DHCP: Dual-Stack Issuesdraft-ietf-dhc-dual-stack-01 Tim Chown tjc@ecs.soton.ac.uk dhc WG, IETF 60, San Diego, August 2, 2004

  2. The crux • Nodes in a dual-stack environment may require IPv4 and IPv6 configuration (addresses and/or options). • Should we advocate separate DHCPv4 and DHCPv6, or add options to DHCPv6 to handle serving DHCPv4 information? • If we choose to have separate servers, how do we ensure the multiple sources of configuration information are handled by the clients?

  3. DHCP servers • DHCPv4 (RFC2131) • DHCP for IPv4 • DHCPv6 (RFC3315) • For IPv6 nodes using full stateful autoconfiguration for address and other configuration options • Stateless DHCPv6 (RFC3736) • For IPv6 nodes using IPv6 stateless address autoconfiguration (RFC2462), only using DHCP for configuration options (DNS, etc)

  4. Configuration information • For example: • IPv4 address • IPv6 address • DNS server • NTP server • NIS server • DNS search path • May receive IPv4 and/or IPv6 addresses for configuration options

  5. Issue 1: multiple responses • How to handle multiple responses? • Use most recent data? • Prefer DHCPv6 served data or option addresses? • Round robin? • In some cases this may not be an issue, e.g. two different DNS servers should give consistent responses to DNS queries. • There may be other sources of configuration data, e.g. NIS, manually configured files, etc.

  6. Issue 2: administration • It may be the case that DHCPv4 and DHCPv6 servers are maintained by different administrative entities • This can be argued to be a multihoming issue?

  7. Issue 3: multiple interfaces • IPv4 and/or IPv6 may be run on different node interfaces • So are the received configuration data and settings per interface or per node? • DHCPv6 has the DHCP Unique Identifier (DUID) concept to supercede MAC address, DHCP for IPv4 is introducing this concept • draft-ietf-dhc-3315id-for-v4-02

  8. Issue 4: DNS load balancing • The DHCP server returns different DNS data to different nodes to load balance between multiple local resolvers • May be problematic if trying to balance with DHCPv4 and DHPCv6 servers both used • Could apply to other services, e.g. NTP

  9. Issue 5: DNS search path • The search path may vary for administrative reasons • For example, new IPv6 services may be offered for foo.com under *.ipv6.foo.com

  10. Issue 6: Protocol startup • (This has been added in -01 draft) • What happens if the IPv6 interface (transport) is started after DHCPv4 was used to configure the client? • Should that data be discarded, or merged with any subsequent DHCPv6 response • Is the timing issue important?

  11. Issue 7: DHCP option variations • Some DHCPv4 options aren’t present in DHCPv6 • IP version limitations exist, e.g. only IPv6 servers may be in an IPv6 NTP option • DHCP and DHCPv6 option numbers may be different • Some sites may use IPv4-mapped addresses in DHCPv6-based options - is this a bad thing? • Should DHCPv6 options exist that can carry IPv4 and IPv6 addresses?

  12. Two solutions • Run separate DHCP and DHCPv6 servers • Run a single DHCPv6 server, and add capability to serve IPv4 addresses and options • Can we agree on a preferred approach?

  13. Separate servers (1) • Servers may or may not be on same node • Server configuration data could be generated from a single database • Implies client has heuristics to handle (merge) multiple server responses • In some cases inconsistencies won’t matter • Need a per-host preference method, e.g. “Prefer DHCPv6” • Need a method to merge “list” responses, e.g. “alternate, DHCPv6 first” • How to handle merging names and addresses?

  14. Separate servers (2) • If we take this approach, we need to identify situations where differences in DHCPv4 and DHCPv6 responses may impact node behaviour • We must place some faith in the site administrator to configure the DHCPv4 and DHCPv6 servers consistently • But we need to be confident that functionality is retained (e.g. DNS load balancing) • If we take this path, we need to complete this task

  15. Single DHCPv6 server (1) • Have just one (DHCPv6) server • Modify DHCPv6 to return IPv4 information (over IPv6 transport/lookup) • Possibility is hinted at in RFC3315 • Single server and transport leads to simplicity and predictability, and less failure modes • Do we want dual-stack nodes to use separate DHCPv4 and DHCPv6 servers for the next 10 or 20 years?

  16. Single DHCPv6 server (2) • A number of potential problems/issues arise: • IPv4-only nodes can’t use DHCPv4 any more; if you do run DHCPv4 for these nodes, you then still have duplication of information • An IPv6-only node can’t use DHCPv4 responses • What if a responding DHCPv6 server isn’t able or configured to serve IPv4 information? • If IPv4 addresses are allocated from DHCPv4 and DHPCv6 servers, more stress is placed on valuable IPv4 address space • The DHCPv6 specification will become more complex and bloated to enable this capability

  17. Way forward? • Can we agree one path? • The list seems to lean towards separate servers, but it’s not clear the consensus is strong? • If we adopt the two server approach, we need to do more analysis of inconsistency impact, methods to specify preferences, etc. • There is one early implementation of preferences • Should we abstract the multihoming/dna issues? • Need to answer these two and edit to -02 version before any WGLC could be considered

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