5.3 Merge window, part 1

While problem 1 is common to IPv6 during the transition period, the problem with variable length addresses with 1:1 IPv4 compatibility for the source + destination are 4 bytes case is that the transition period is effectively infinite - there is never a penalty for refusing to migrate, whereas in IPv6 land, there is a penalty for failure to migrate once a tipping point is reached. For example, today it is the case that if you care about the performance of your servers when accessed via a mobile phone, you need to support IPv6, because for significant subsets of mobile users, IPv4 goes via a remote NAT, while IPv6 takes the shortest route.

And variable length addresses always have a performance or cost penalty in hardware, which never goes away. For a fixed size address, the router simply reads the address and acts on it. For a variable length address, the router has to read the length, read the first chunk of the address, mask off any parts of the first chunk of address that aren't valid, attempt to act on it, and then if the needed part of the address is longer than the chunk, repeat for the next chunk. Worse, if you're not cautious, router manufacturers will attempt to "get away" with not handling the full complexity - e.g. only route on the first N bits, and ignore the rest of the address - and if those routers become common, you've effectively shrunk the routable component of the address. We've seen this in IPv4 in the 80s, where routers fell back to a slow path if the routing prefix was too long (more than 16 bits), and we've seen this in IPv6 routers that only route on the first 64 bits of the address. Variable length addressing just makes this harder, because you also have to handle the pain that 32 bit "1.1.1.1" is not guaranteed to route to the same place as 64 bit "1.1.1.1/32", which is not guaranteed to route to the same place as 128 bit "1.1.1.1/32" (well, unless you remove the requirement that 32 bit "1.1.1.1" routes to the same place as IPv4 "1.1.1.1").

This extra complexity is inherent to variable length addressing, and makes the hardware more complex; in turn, this means that you either need more complex hardware to handle lookups in the same number of clock cycles, or you need more clock cycles to do the same lookup. Fixed length addresses avoid this - you always read a fixed size chunk and then act on it.