[dhcwg] Two message transaction in DHCPv6

Based on a conversation with Mark Stapp, here's a proposal for allowing a 
two message exchange for configuration. prefix and address assignment.  The 
goal of this proposal is to allow, under certain circumstances, a client to 
complete its initial DHCPv6 transaction with two messages instead of the 
current four messages, Solicit-Advertise-Request-Reply.  The circumstances 
under which this mechanism is designed to work are: only one server will 
respond to the client Solicit; the server will commit the prefixes and 
addresses to the client immediately.  The proposed mechanism also allows a 
client to indicate whether it is willing to participate in a two message 
transaction, and allows a server to force a four message transaction 
(according to local policy).

Here's the proposal...

A new TMT option (two message transaction) is defined.  The TMT option can 
only be included in a Solicit message.  There is no data associated with 
the TMT option.

The client includes a TMT option in its Solicit message, indicating its 
willingness to perform a two message transaction.

If the server that receives a Solicit with a TMT option has a policy to 
perform the two message transaction, it determines the client 
configuration, commits the assignment of any prefixes or addresses, and 
sends the configuration to the client in a Reply message.

If the server that receives a Solicit with a TMT option does not have a 
policy to perform the two message transaction, the server ignores the TMT 
option and responds with an Advertise.

If the client receives a Reply, it records the configuration and is done 
(and may ignore any received Advertise messages).  If the client receives 
one or more Advertise messages but not Reply message, the client sends a 
Request to continue the four message transaction.

I intend to include this mechanism in the -24 rev of the DHCPv6 spec, which 
I'm working on now.  Comments welcome...

- Ralph

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