I’m very pleased to report that my SIIT-DC RFCs were published by the IETF last week. If you’re interested in learning how to operate an IPv6-only data centre while ensuring that IPv4-only Internet users will remain able to access the services hosted in it, you should really check them out.
Start out with Stateless IP/ICMP Translation for IPv6 Data Center Environments (RFC 7755). This document describes the core functionality of SIIT-DC and the reasons why it was conceived.
If you think that you can’t possibly make your data centre IPv6-only yet because you still need to support few legacy IPv4-only applications or devices, continue with RFC 7756. This document describes how the basic SIIT-DC architecture can be extended to support IPv4-only applications and devices, allowing them to live happily in an otherwise IPv6-only network.
The third and final document is Explicit Address Mappings for Stateless IP/ICMP Translation (RFC 7757). This extends the previously existing SIIT protocol, making it flexible enough to support SIIT-DC. This extension is not specific to SIIT-DC; other IPv6 transition technologies such as 464XLAT and IVI also make use of it. Unless you’re implementing an IPv4/IPv6 translation device, you can safely skip RFC 7757. That said, if you want a deeper understanding on how SIIT-DC works, I recommend you take the time to read RFC 7757 too.
So what is SIIT-DC, exactly?
SIIT-DC is a novel approach to the IPv6 transition that we’ve developed here at Redpill Linpro. It facilitates the use of IPv6-only data centre environments in the transition period where a significant portion of the Internet remains IPv4-only. One could quite accurately say that SIIT-DC delivers «IPv4-as-a-Service» for data centre operators.
In a nutshell, SIIT-DC works like this: when an IPv4 packet is sent to a service hosted in a data centre (such as a web site), that packet is intercepted by a device called an SIIT-DC Border Relay (BR) as soon as it reaches the data centre. The BR translates the IPv4 packet to IPv6, after which it is forwarded to the IPv6 web server just like any other IPv6 packet. The server’s reply gets routed back to a BR, where it is translated from IPv6 to IPv4, and forwarded through the IPv4 Internet back to the client. Neither the client nor the server need to know that translation between IPv4 and IPv6 is taking place; the IPv4 client thinks it’s talking to a regular IPv4 server, while the IPv6 server thinks it’s talking to a regular IPv6 client.
There are several reasons why an operator might find SIIT-DC an appealing approach. In no particular order:
- It facilitates IPv6 deployment without accumulation of IPv4 technical debt. The operator can simply switch from IPv4 to IPv6, rather than committing to operate IPv6 in parallel with IPv4 for the unforseeable future (i.e., dual stack). This greatly reduces complexity and operational overhead.
- It doesn’t require the native IPv6 infrastructure to be built in a certain way. Any IPv6 network is compatible with SIIT-DC. It does not touch native IPv6 traffic from IPv6-enabled users. This means that when the IPv4 protocol eventually falls into disuse, no migration project will be necessary - SIIT-DC can be safely removed without any impact to the IPv6 infrastructure.
- It maximises the utilisation of the operator’s public IPv4 addresses. If all the operator has available is a /24, every single of those 256 addresses can be used to provide Internet-facing services and applications. No addresses go to waste due to them being assigned to routers or backend servers (which do not need to communicate with the public Internet). It is no longer necessary to waste addresses by rounding up IPv4 LAN prefix sizes to the nearest power of two. Never again will it be necessary to expand a server LAN prefix, as it will be IPv6-only and thus practically infinitely large.
- Unlike IPv4 NAT, it is completely stateless. Therefore, it scales in the same way as a standard IP router: the only metrics that matter are packets-per-second and bits-per-second. Its stateless nature makes it trivial to deploy; the BRs can be located anywhere in the IPv6 network. It is possible to spread the load between multiple BRs using standard techniques such as anycast or ECMP. High availability and redundancy are easily accomplished with the use of standard IP routing protocols.
- Unlike some kinds of IPv4 NAT, it doesn’t hide the source address of IPv4 users. Thus, the IPv6-only application servers remain able to perform tasks which depend on the client’s source address, such as geo-location or abuse logging.
- It allows for IPv4-only applications or devices to be hosted in an otherwise IPv6-only data centre. This is accomplished through an optional component called a SIIT-DC Edge Relay. This is what is being described in RFC 7756.
The history of SIIT-DC
I think it was around the year 2008 that it dawned on me that Redpill Linpro’s IPv4 resources would not last forever. At some point in the future we would inevitably be prevented from expanding our infrastructure based on IPv4. It was clear that we needed to come up with a plan on how to deal with that situation well ahead of time. IPv6 obviously needed to be part of that plan, but exactly how wasn’t clear at all.
Conventional wisdom at the time told us that dual stack, i.e., running IPv4 in parallel with IPv6, was the solution. We did some pilot projects, but the results were discouraging. In particular, these problems quickly became apparent:
- It would not prevent us from running out of IPv4. After all, dual stack requires just as many IPv4 addresses as single-stack IPv4.
- IPv4 would continue to become an ever more entrenched part of our infrastructure. Every new IPv4-using service or application would inevitably make a future IPv4 sunsetting project even more difficult to pull off.
- Server and application operators simply didn’t like running two networking protocols in parallel. Dual stack greatly increased complexity: it became necessary to duplicate service configuration, firewall rules, monitoring targets, and so on, just in order to support both protocols equally well. This duplication in turn created lots of new possibilities of things going wrong, reducing reliability and uptime. And when something did go wrong, troubleshooting the issue required more time. Single stack was therefore seen as superior to dual stack.
It was clear that we needed a better approach based on single-stack IPv6, but we were unable to find an already existing one which solved all of our problems.
One of the things that we evaluated, though, was Stateless IP/ICMP Translation (RFC 6145). SIIT looked promising, but it had some significant shortcomings (which RFC 7757’s Problem Statement section elaborates on). In its then-current state, SIIT simply wasn’t flexible enough to be up to the task we had in mind for it. However, we did identify a way SIIT could be improved in order to facilitate our IPv6-only data centre use case. This improvement is what RFC 7757 ended up describing.
I believe the first time I presented the idea of SIIT-DC (under the working name «RFC 6145 ++») in public was at IIS.se’s World IPv6 Day seminar back in June 2011. In case you’re interested in a little bit of «history in the making», the slides (starting at page 34) and video (starting at 34:15) from that event are still available.
A few months later we had a working proof of concept (based on TAYGA) running. By January 2012 I had enough confidence in it to move our corporate home page www.redpill-linpro.com to it, where it has remained since. I didn’t ask for permission…but fortunately I didn’t have to ask for forgiveness either - to this day there have been zero complaints!
The solution turned out to work remarkably well, so in keeping with our open source philosophy we decided to document exactly how it worked so that the entire Internet community could benefit from it. To that end, my very first Internet-Draft, draft-anderson-siit-dc-00, was submitted to the IETF in November 2012. I must admit I greatly underestimated the amount of work that would be necessary from that point on…
The document was eventually adopted by the IPv6 Operations working group (v6ops) and split into three different documents, each covering relatively independent areas of functionality. Then began multiple cycles of peer review and feedback by the working group followed by updates and refinements. I’d especially like to thank Fred Baker, chair of the v6ops working group, for helping out a lot during the process. For a newcomer like me, the IETF procedures can certainly appear rather daunting, but thanks to Fred’s guidance it went very smoothly.
One particularly significant event happened in early 2015, when Alberto Leiva Popper from NIC México joined in the effort as a co-author of RFC 7757-to-be (which describes the specifics of the updated SIIT algorithm). Alberto is the lead developer of Jool, an open-source IPv4/IPv6 translator for the Linux kernel. Thanks to his efforts, RFC 7757-to-be (and, by extension, SIIT-DC) was quickly implemented in Jool, which really helped move things along. The IETF considers the availability of running code to be of utmost importance when considering a proposed new Internet standard, and Jool fit the bill perfectly.
For the record, we decommissioned our old TAYGA-based SIIT-DC BRs in favour of new ones based on Jool as soon as we could. This was a great success - our Jool BRs are currently handling IPv4 connectivity for hundreds of IPv6-only services and applications, and the number is rapidly growing. We’re very grateful to Alberto and NIC México for all the great work they’ve done with Jool - it’s an absolutely fantastic piece of software. I encourage anyone interested in IPv6 transition to download it and try it out.
In late 2015 the documents reached IETF consensus, after which they were sent to the RFC Editor. They did a great job with helping improve the language, fixing inconsistencies, pointing out unclear or ambiguous sentences, and so on. When that was done, the only remaining thing was to publish the documents - which, as I mentioned before, happened last week.
It feels great to have crossed the finish line with these documents, and writing them has certainly been an very interesting exercise. It is also nice to prove that it is possible for regular operators to provide meaningful contributions to the IETF - you don’t have to be an academic or work for one of the big network equipment vendors. That said, it has taken considerable effort, so I certainly look forward to being able to focus fully on my work as a network engineer again. I promise that’s going to result in more good IPv6 news in 2016…watch this space!