The "anti-SL" arguments are primarily arguments aainst using ambiguous addresses.
Ambiguous addresses are a royal pain in hosts that connect to multiple sites, either
simultaneously or over time -- the applications need extra logic, and that creates
bugs. But we clearly have an issue in the case of disconnected sites, intermittently
connected sites, and ad hoc networks.
The "let's pick a prefix" argument is probably OK for large "managed" sites. In fact,
most of the large sites have at least one IPv4 address and can pick a prefix; they
could even obtain a provisional allocation from a friendly ISP. But this leaves out
the small sites, the ad hoc networks, the unmanaged sites. However, if we just look at
these small sites, we can easily get unambiguous *link* prefixes of the form:
<some-16-bit-prefix><unique 48 bit number>::/64
In a small site, these prefixes can be autoconfigured by routers, and then published
in the IGP. If there are several routers on the same link, they can either elect a
master prefix or just advertise one prefix each. Having unique per-link prefixes has
quite a few advantages:
- We get actual zero-configuration, a site can be just switched on.
- Local connectivity can be used for adding a global addressing plan when the
site joins the Internet.
- Hosts can be multihomed at will; there is enough information in the address
to find the right exit.
- The addresses remain valid if a site is split, or if two sites are merged.
- Unreachability is enforced by firewalls, not by bits in the address.
- Since the link prefix is a /64, there is zero chance of having a nasty ISP
leak it to the Internet.
- If the /16 is well known, it can be plugged as "least preferred" in the
address selection rules.
Is anyone interested in pursuing this design?
-- Christian Huitema
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