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Comment by exelius | Hacker News Reader

Well, kind of... It's less about not being able to grok IPv6 and more about these routers having limited memory available. IPv6 addresses take up 4x as much memory as IPv4 addresses, and that adds up pretty quickly in a routing table. As routers are typically integrated devices, you can't just go "add more RAM", so sticking with IPv4 effectively quadruples your capacity.

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Is that mixing up two different classes of devices?

Home embedded routers run into problems due to the overhead of state tracking and buffers for all the simultaneous connections. If they support 10k simultaneous connections, and they track 2x ip addresses for each, the additional overhead of 2x 12 bytes per ipv6 connection is 240kB memory. Buffers are going to be much, much more than that, and buffer requirements are about the same for ipv4 and ipv6.

Routers that have full BGP routing tables tend to be serious routers with TCAM. That's expensive and that does put pressure on BGP routing table sizes, to accommodate older and cheaper [real] routers. However, ignoring ipv6 is simply not an option, so there's no point in dreaming about how staying with ipv4 forever would allow quadruple the routing table capacity for a set amount of TCAM.

I was actually referring more to the core routers with TCAM than home routers. And I agree, ignoring IPv6 isn't an option, but it is a viable option to encourage workarounds. Which is exactly what's happened; so most ISPs that support IPv6 do 6to4 tunneling at the edge.

That's a workaround? Interior routers within the ISP don't need to hold the global routing table, so there's no real TCAM pressure on them, right?

As you venture down into the realm of cheap consumer routers, you run out of ROM to hold the code implementing IPv6 long before you run out of RAM.

It can be even worse than that. A significant number of routers forward IPv4 packets in hardware but fall back to a software implementation for IPv6. So it might happen that the real bottleneck is not memory but throughput.

As for sticking with IPv4 for increased capacity, it depends. Private networks might get away with it for their internal traffic, but no major ISP nowadays will deliberately choose not to route IPv6 for their customers.

The IPv4 address table needs 3x as many routes as a non-fragmented address space like IPv6 would, so it's a much smaller difference than it sounds like.

During the initial few years, it was also about hardware limitations. The Cisco kit we had in the early 2000s featured hardware accelerated IPv4 handling but fell back to the host CPU for IPv6. Even a simple ping test would show the difference since the software path was seen as a fallback for anomalies and they'd provisioned the CPU accordingly.

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