Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 10:29 UTC (Fri) by paulj (subscriber, #341)
In reply to: Should distributors disable IPv4-mapped IPv6? by farnz
Parent article: Should distributors disable IPv4-mapped IPv6?

I think my reply to raven667 goes into that: http://lwn.net/Articles/690723/

Once the legacy space is out, further assignments must of course be from a prefix that is constant in the legacy space. It would be the assigning authority that determines that.

As to what stops you advertising other people's space - or greater prefixes spanning many assigned spaces, well nothing really stops you technically in BGP as used today. However, there are socio-political-commercial checks. E.g., what stops you advertising 2001::/16 to todays public Inter6net?

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 10:33 UTC (Fri) by farnz (subscriber, #17727) [Link] (2 responses)

What stops me not advertising 2002:192.0.2.0::/48 at all in the IPvN space, and just advertising 192.0.2.0 in the IPv4 space, thus allowing me to hijack any suballocations? In IPv6, it's simple - if I control 192.0.2.0, I control the entirety of 2002:192.0.2.0::/48 anyway, and thus hijacking it isn't an issue.

And, from the description you're giving of post-runout allocations, we'd effectively sacrifice 32 bits of address as "dead" - especially since people are likely to optimize their IPvN routing to go down fast paths if those bits are the static "no matching v4" prefix, and to just route over IPv4 otherwise, forcing people who want to switch off IPv4 routing to continue to take part in the v4 network indefinitely, or lose reachability.

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 10:49 UTC (Fri) by paulj (subscriber, #341) [Link] (1 responses)

You're talking about the case where legacy has run out, and further allocations all come from X::/32, right? And you're asking, what stops you advertising X/32 in the IPv4 network?

Well, what stops you advertising /any/ prefix X in IPv4 today that you don't have a right to advertise? What you're asking is exactly equivalent to "What stops me advertising 64/8?" or "what stops me advertising 184/8?" It's an interesting discussion, but not specific to transition mechanisms for extending IP address bits.

As for sacrificing dead bits, why do you think they have to be /32? There's no reason we couldn't have used foresight in the 90s to reserve a /8 in the v4 space for the extended space. Where I wrote "further assignments must of course be from a prefix that is constant in the legacy space" I didn't intend that to mean that prefix would have to be the full width of the legacy space.

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 10:59 UTC (Fri) by farnz (subscriber, #17727) [Link]

Because I fully expect router vendors to do the same sort of shit as they do today, and do anything to win benchmarks. If you can be 0.01% faster by special-casing IPvN to the "extended" prefix, and using IPv4 routing for the remainder of the IPv4 network, that's what you'll do, and you'll blame other people when it breaks, right up until you're proven to be at fault.

Thus, I pay the pain of IPv4 routing for much, much longer than I need to - I may have access to far better IPvN connectivity (e.g. he.net were doing some incredible - Cogent-beating - deals on IPv6-only transit at one point), but I'm stuck with IPv4 indefinitely.