[DNA] Simplification of DNAv4 specification

[Bernard Aboba <bernard_aboba@hotmail.com>]

It has been suggested that the DNAv4 specification can be simplified by 
recommending that implementations simultaneously test reachability to one or 
more networks in parallel with attempting to obtain a configuration via 
DHCPv4. While -15 allows this, it is optional rather than recommended 
behavior, and so the specification still has to accomodate the case where an 
implementation attempts to obtain configuration serially.

By assuming that the implementation attempts to obtain configuration by 
multiple mechanisms in parallel, it can be guaranteed that DNAv4 will not 
take longer than DHCPv4 to obtain a configuration. It also simplifies the 
DNAv4 client implementation, since the client can determine the set of 
networks for which it has an operable configuration, and select a subset of 
that for reachability testing without having to consider link layer hints 
other than "link up".

For example, if a host has a still-valid DHCP leases on three networks, it 
can do 3 reachability tests in parallel with attempting to obtain a 
configuration via DHCPv4.

The downside of this approach is the extra traffic. However, it appears that 
this risk can be mitigated via rate limiting, jittering and exponential 
backoff. Overall,  the benefits of this approach, in terms of a simplified 
implementation and improved performance and robustness seem to be worth the 
costs.

Comments welcome.  A strawman draft is available here:
http://www.drizzle.com/~aboba/DNA/draft-ietf-dhc-dna-ipv4-16.txt