(IPng 4997) Re: Last Call: IP Version 6 Addressing Architecture to Proposed Standard

brian's reason is exactly the goal which was in mind:

to bound the maximum complexity of the default-free region
at values believed to be viable with some margin.

the margin is important because even routers normally
though to be "default-free" will probably carry a significant
number of more-specific prefixes for optimizing paths,
both internal to a TLA and between TLAs.

and note, once again, the issue is not the size of the default-free
region, but the complexity of the topology, which determines how
many copies of the full default-free region one must examine before
arriving at the forwarding table with one entry per TLA.

it is now routine to see an announced prefix 15 times via
different paths, only one of which must be selected for use.
the complexity of the topology is only expected to increase,
both internally and externally, so it is not unreasonable to
attempt to bound the size of the set as that is the only 
parameter which is in any sense "tunable".

as for 13 - anything smaller was felt to be clearly too small,
and it becomes harder and harder to argue for bigger numbers
in light of the complexity management which is mandatory.

if anyone expects a magic formula which says "13" and not something
else, you won't get it.  what is very clear is that it is pretty
easy for it to to "too big", and then it eats into the other
topology bits which have their own set of long arguments.
would 14 work - certainly.  

Like everything else, 13 is an engineering compromise - chosen
to balance one set of considerations against a bunch of others,
and after ruminating over it a long time, the consensus was
13 was the best choice.

and as someone else pointed out, the TLA space can be expanded
laterally into other reserved areas, so there are more available.

now, to look at things from a different vantage point....

I think one deep issue here is that the IPv6 address design, in
some sense, appears to threaten the existing registries.  the design
assumes that each TLA act as a registry for its region of the address
space, leaving the registries to only allocate TLAs, which will be
infrequent.

just so.

but given the work required to run a registry, existing registries with
good track records would be in an excellent position to compete for the
right to provide registration services for *any* TLA or delegation within
a TLA, not just ones they assigned.  

this is a natural fall-out of attempting to provide an addressing structure
which can pave the way for making the network self-organizing.  if that
were really the case, unlike today, there would be little need for
registry organizations (little - not "no", but little) which use humans
clerical workers to execute a simple resource allocation algorithm for the
world's largest distributed computer.

	-mo