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Snabb lwAFTR

What's a lwAFTR?

Lightweight 4-over-6 (lw4o6) is an IPv6 transition mechanism, specified as RFC 7596. An lwAFTR is the internet-facing component of an lw4o6 implementation.

Snabb lwAFTR allows a network operator to run a pure IPv6 network internally, while providing interoperability with the IPv4 internet. Each customer IPv6 address may be associated with a limited range of ports on an IPv4 address. Restricting port ranges allows an ISP to serve more customers with a smaller IPv4 address space, which keeps legacy IPv4 costs low.

The mapping between IPv4 addresses and customers is done in such a way that the lwAFTR instance only needs to know about the mapping between each assigned IPv6 address and an IPv4 address and port range. In particular, an lwAFTR doesn't need to keep per-flow state, lowering complexity and cost. This also means that lwAFTR scales horizontally; multiple lwAFTR functions can service the same set of customers, and any flow can be processed by any lwAFTR function in the node.

See a talk!

Katerina Barone-Adesi and Andy Wingo gave a talk on Snabb and the lwAFTR at FOSDEM 2016! Eventually there will be a video here: https://fosdem.org/2016/schedule/event/snabbswitch/

In the meantime, you might like to check out the slides.


The Snabb lwAFTR has a fully functional data plane that can encapsulate and decapsulate traffic at line rate over two 10 Gb NICs. It supports arbitrarily large binding tables, IPv4 address sharing using the port-set ID scheme, VLAN tagging, fragmentation, reassembly, NDP, and implements all of RFC 7596 including hairpinning and configurable ICMP error handling.

An lwAFTR is just one part of a lw4o6 deployment. The routers that directly serve the users (the customer premise equipment, or CPE boxes; e.g. running OpenWRT) need to do the job of terminating a softwire to the lwAFTR. The piece of software on the CPE that does this is called the B4, or in the case of lw4o6 the lwB4. Each B4 needs to be deployed with the IPv6 address of the lwAFTR, the IPv6 address of the B4, and the corresponding IPv4 address and PSID. In a real deployment, probably you will use DHCPv6 or some big NETCONF management system to configure both the lwAFTR and the CPE.

The lwAFTR only has a data plane for now; you need some external control plane to update its configuration. Or, you do what we do now, and you configure it all at the command like with little text files :)

Getting started

Building the lwAFTR

Building the lwAFTR is pretty simple. At a shell, just check out the right branch of Snabb, type make, and you're done!

git clone https://github.com/Igalia/snabb.git
cd snabb
git checkout lwaftr_starfruit

That's all! You'll find a self-contained snabb binary in your current directory that you can copy whereever you like.

We're working on merging to upstream snabb; follow the progress in this GitHub issue.

Run the end-to-end tests

The Snabb lwAFTR has a set of tests which run the lwAFTR, feeding it in packets on its IPv4 and IPv6 interfaces and recording the packets that it gets in reply, checking that the output is exactly what we expect.

To run these tests:

( cd src/program/lwaftr/tests/end-to-end; sudo ./end-to-end.sh && sudo ./end-to-end-vlan.sh )

This test suite includes tests for traffic class mapping, hairpinning (including for ICMP), fragmentation, and so on. They do not require access to a NIC.


There are a lot of configuration knobs! See the Configuration page, for general configuration, and Binding table page, for more on binding tables.

Running the lwAFTR

You have a binding table and a configuration: great, you're finally ready to run the lwAFTR! The only tricky part is making sure you're using the right network interfaces. See Running, and be sure to check Performance to make sure you're getting all the lwAFTR can give.

The lwAFTR processes traffic between any NIC supported by Snabb, which mainly means Intel 82599 10 Gb adapters. It's also possible to run on virtualized adapters using the virtio-net support that just landed in Snabb. See Virtualization, for more on how to get the lwAFTR working on virtualized network interfaces.







Compatibility and interoperability

RFC Compliance

Discovery of next-hop L2 addresses via NDP

Change Log