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

Daniel, 

There were two basic arguments for limiting the length of the TLA
prefix:

1) Routing table size.  All TLA have to be in all tables in all 
   defaultless routers.  Thus, the number of TLAs becomes a minorant
   of the size of the tables.

2) Address space size.  The current document only specifies 1/8th
   of the address space.  If it turns out that there is a need for
   many more TLA's in three years, it will actually be possible
   to make some.

The particular value chosen, 13 bits, as only one advantage.  The
combination of format and TLA fits in 16 bits, which provides for easy
writing of routing prefixes.

The point that you note re: allocation practices is, however, perfectly
true.  ISPs should not receive a TLA 'a priori", they should graduate
to it.  If I remeber correctly, an ISP that starts operating in Europe
today receives a /19, and will gradually receive more space as it
connects more customers.  An ISP that starts operating with IPv6 should
receive one or several NLAs from established transit providers; it should
only receive a global entry in the routing tables once it has passed
some "graduation" criteria.

-- 
Christian Huitema