Last week I recapped the results of some recent work I've done with my colleagues modeling Internet performance. In addition to assessing capacity and demand, we looked at what you could call "the logical Internet": the scalability of such protocol-layer features as addressing, routing, multihoming, and mobility.

The news there is pretty bad: Internet scalability is reaching its limits rapidly because of architectural issues inherent in the design of the 'Net. And IPv6 -- since the mid-1990s touted as the fix -- patently fails to fix the problem.

To see why, look closely at addressing. There are three types of names and addresses necessary for a complete architecture: application names, which are location independent and indicate what is to be accessed; network-node addresses, which are location dependent and route independent, and indicate where the accessed application is; and point-of-attachment addresses, which may or may not be location dependent but are route dependent and describe how to get there. 

A major problem with Internet architecture is that it names the same thing twice: Media-access-control (MAC) addresses and IP addresses both name the point of attachment, but there are no defined mechanisms for creating either network-node or application addresses.

Essentially, that means Internet architecture includes just the "how," not the "where" and "what." So what? This makes it incredibly cumbersome to implement such functions as multihoming (connecting to multiple networks simultaneously for load-balancing, greater performance or redundancy) or mobility (roaming across multiple networks). And IPv6 doesn't fix these weaknesses, it just throws a spotlight on them.

Take multihoming. In today's Internet, a URL first must resolve to an IP address, then to a well-known port. If a system has multiple interfaces (such as when it's multihomed), it has multiple aggregate-able IP addresses. The routers can't tell, however, that these different addresses go to the same place (because, again, there's no defined mechanism for doing so). So, the system has to be assigned a non-aggregate-able address, which increases everyone's router-table size. In practice, that means that either most users can't multihome, or routing tables must increase dramatically.