Subject: DHCPv6 Appendix A

From: Jim Bound <[email-masked]>
Date: Wed, 30 Aug 2000 08:41:43 -0400

Folks,

This will be pretty relevant for the design meeting too.  
Also parts like Multicast, Reconfigure, DNS Updates, etc were all part of the
inherent design of DHCPv6 not add-ons.

I also want to stress the "integration" of Stateful in this case DHCPv6
with the stateless code base deployed now in the market for IPv6 and
that it is also useful for IPv6 Transition.  The code Francis Dupont
sent out also questions the statement about complexity objectively, but
then Francis is a key IPv6 implementor to and understands IPv6 very
well.  This code base should carry some weight in our thinking process.

Also Mike Carney is one of the key implementors of DHCPv4, who clearly
has a good handle on both protocols and why DHCPv6 vs DHCPv4
engineering trade-offs were made.

A. Comparison between DHCPv4 and DHCPv6

   This appendix is provided for readers who will find it useful to see
   a model and architecture comparison between DHCPv4 [7, 1] and DHCPv6.
   There are three key reasons for the differences:

     o IPv6 inherently supports a new model and architecture for
       communications and autoconfiguration of addresses.

     o DHCPv6 benefits from the new IPv6 features.

     o New features were added to support the expected evolution and
       the existence of more complicated Internet network service
       requirements.

   IPv6 Architecture/Model Changes:

     o The link-local address permits a node to have an address
       immediately when the node boots, which means all clients have a
       source IP address at all times to locate an on-link server or
       relay.

     o The need for BOOTP compatibility and the broadcast flag have been
       removed.

     o Multicast and address scoping in IPv6 permit the design of
       discovery packets that would inherently define their range by the
       multicast address for the function required.

     o Stateful autoconfiguration has to coexist and integrate with
       stateless autoconfiguration supporting Duplicate Address
       Detection and the two IPv6 lifetimes, to facilitate the dynamic
       renumbering of addresses and the management of those addresses.

     o Multiple addresses per interface are inherently supported in
       IPv6.

     o Some DHCPv4 options are unnecessary now because the configuration
       parameters are either obtained through IPv6 Neighbor Discovery or
       the Service Location protocol [16].

   DHCPv6 Architecture/Model Changes:

     o The message type is the first byte in the packet.

     o IPv6 Address allocations are now handled in a message extension
       as opposed to the message header.

     o Client/Server bindings are now mandatory and take advantage of
       the client's link-local address to always permit communications
       either directly from an on-link server, or from a off-link server
       through an on-link relay.

     o Servers are discovered by a client Solicit, followed by a server
       Advertise message

     o The client will know if the server is on-link or off-link.

     o The on-link relay may locate off-link server addresses from
       system configuration or by the use of a site-wide multicast
       packet.

     o ACKs and NAKs are not used.

     o The server assumes the client receives its responses unless it
       receives a retransmission of the same client request.  This
       permits recovery in the case where the network has faulted.

     o Clients can issue multiple, unrelated Request messages to the
       same or different servers.

     o The function of DHCPINFORM is inherent in the new packet design;
       a client can request configuration parameters other than IPv6
       addresses in the optional extension headers.

     o Clients MUST listen to their UDP port for the new Reconfigure
       message from servers.

     o New extensions have been defined.

   With the changes just enumerated, we can support new user features,
   including

     o Configuration of Dynamic Updates to DNS

     o Address deprecation, for dynamic renumbering.

     o Relays can be preconfigured with server addresses, or use of
       multicast.

     o Authentication

     o Clients can ask for multiple IP addresses.

     o Addresses can be reclaimed using the Reconfigure-init message.

     o Integration between stateless and stateful address
       autoconfiguration.

     o Enabling relays to locate off-link servers.

/jim