In the first Terastream blog post I mentioned Deutsche Telekom decided to use an IPv6-only access network. Does that mean they decided to go down the T-Mobile route and deployed NAT64 + 464XLAT? That combo wouldn’t work well for them, and they couldn’t use MAP-E due to lack of IP address space, so they deployed yet another translation mechanism – Lightweight 4over6.
Most IPv6 residential deployments get stuck due to CPE challenges – old CPEs don’t support IPv6, it was hard to get IPv6-capable CPEs a few years ago, and it was impossible to get CPEs that supported the encapsulation/translation mechanism you wanted to use.
Deutsche Telekom found a great solution to the whole CPE conundrum: they have their own CPEs, but (if I understood the PLNOG presentation correctly) they also built OpenWRT distribution that includes lw4over6, and plan to open-source it.
Every single ISP-scale translation or encapsulation solution requires large head-end boxes (Address Family Translation Router or AFTR in DS-Lite lingo). Deutsche Telekom decided to replace those boxes with a scale-out architecture with limited shared state (that’s why they use lw4over6 and not DS-Lite). They run numerous lwAFTR instances in the data center adjacent to a core router.
Scale-out mechanisms are extremely simple: the lw4over6 traffic arriving from the CPE routers and the return IPv4 traffic are spread across the available lwAFTR instances using the traditional 5-tuple load balancing mechanism.
Even though lwAFTR instances keep per-CPE state (mapping between IPv4 port range(s) and IPv6 CPE address), they don’t keep per-session state (NAT44 is done by the CPE). The return traffic (IPv4 to CPE) thus doesn’t have to go through the same lwAFTR instance as the outbound traffic.
Instead of investing heavily in unproven emerging technologies, Deutsche Telekom engineers designed an extremely simple and scalable access network offering IPv6 and IPv4 access. The scale-out virtual appliance architecture adapts automatically to changing user demands, minimizes the initial investment, and grows in sync with the growing user base … and they managed to do all of that with traditional technologies and standard products.