GSoC 2017 – Add MPTCP support in LEDE/OpenWRT trunk – Final

Brief summary

In the first post (beginning of the GSoC 2017 project) I made a few checkpoints to complete at the end of the summer. Now I don’t copy them here, but the good news is all of them completed successfully. The main goal was a very simple transparent multipath Wi-Fi link bandwidth aggregation. The proof in the video above. And the details below.

What has been done in August

Because everything tested successfully on a virtual environment, the next step was port everything into a real, LEDE based physical test environment. The first step was to build the LEDE with MPTCP support to the routers. It went without any problem and I have installed it to Netgear R7000 and Netgear R7800 routers. These are quite powerful SOHO routers, R7000 with 1.4GHz- and R7800 with 1.7GHz dual core CPU. But R7800 using more recent architecture so it seems like more than twice as fast as R7000. So I installed ss-redir to R7800 and ss-server on R7000 and configured as before.
On the client, every TCP traffic redirected to ss-redir in the iptables PREROUTING chain (except where the destination is the same LAN as the source). When this happens, the client’s TCP flow from LAN just gets split into two MPTCP sub-flow on the two WAN, which is two Wi-Fi bridge connection in our case. I use some old Ubiquiti devices (2 NanoStation M5 and 2 NanoStation Loco M5) as You see on the video just to try out if it works. When I experimented with ss-server and ss-redir with simple UTP cable connection, it turned out the encryption is very slow even in these powerful CPU-s. I get 700Mbps between the two router (measured with iperf3) but when encryption turned on it slows down 50Mbps or less (depends on the type of the cipher). I decided to fork shadowsocks-libev and make a version which makes the encryption optional. I also created a custom package feed for my LEDE fork which is contains that version. So if You clone MPTCP LEDE and update the feeds, shadowsocks-libev-nocrypto packages are available in the menuconfig. This helps the connection over the ss-redir/ss-server become much faster.
On the server, there is no special config, just an ss-server and static WAN IP addresses with a DHCP server. Every other device (the router with the client, and the 4 Wi-Fi bridges) got the addresses and gateways from DHCP. This makes the configuration very comfortable.

Simple topology of the multipath Wi-Fi bridging topology

I configured the Wi-Fi station pairs to different bands: 5180MHz and 5700MHz to make sure they are not interfering with each other. Then I started the test! As You can see in the figure (and on the video, but it’s not as clear because of my small desk and extra cables for the PoE injectors) I connected one LAN client to each router. One of them is my PC and the another is my notebook, each of them run iperf3. Very important, as I mentioned in my previous post, none of them have any special configuration! Just plugged into the LAN port of the router and that’s it. During the iperf transmission, I unplug one Wi-Fi bridge (from path #1 VLAN) from the router: the iperf session continues, only the throughput slows down to half, from 40Mbps to 20Mbps. This is the expected result: one MPTCP sub-flow torn down between the routers, but the another still alive and functional. When I plug the bridge back in and get the IP address over DHCP, another MPTCP sub-flow builds back over the recovered Wi-Fi bridge and the throughput goes back to 40Mbps.

Potential use-case and deployment

This is a small proof-of-concept testbed but I think this project maybe works on real-life Wi-Fi mesh networks. It is not hard to imagine a mesh network with multiple available paths between the intermediate router devices. Another use-case is to speed up point-to-point rooftop Wi-Fi links – with this You might be beat Ubiquity airFiber24‘s speed with multiple cheaper bridges :-). As I presented there is a realizable gain for the user with minimal configuration on the routers and no configuration on the end devices. In my opinion, the throughput depends on the CPU performance and not on the number of TCP flows. I also verified this with my virtual environment. So if there is many clients and many TCP flow completely fine but for high throughput, the setup require powerful (x86 if possible) hardware.

Future plans

The work in the GSoC 2017 completed, but there are some thing what has to be done in the future. The most important is the UDP or another kind of traffic. Currently, this is singlepath, routed through the default gateway. There is a MPT application (MultiPath Tunnel, like a multipath VPN without encryption) which is suitable for UDP traffic and handles many paths with different weight value (use paths in different ratios). Another interesting approach is the MPUDP kernel module + OpenVPN but this is a small “hack” for research purposes at this moment.
Sadly the current implementation of shadowsocks-libev is single threaded, using only one CPU core. I would like to make it multithreaded if possible in the near future. I will maintain the MPTCP LEDE repo as long as possible in the future and my shadowsocks fork. It depends on resources, but I would like to make a repo for my feed which contains the compiled packages. And yes, the feed now only contains one application, I would like to improve it with other MPTCP related stuff. The ride never ends, the work continues!

I would like to say thanks for Freifunk to adopt this project and my mentors – Benjamin Henrion and Claudio Pisa for their ideas and help! And of course to Google, for make this project possible.

MPTCP LEDE on github: https://github.com/spyff/lede-mptcp
Feed for packages on github: https://github.com/spyff/packages
Shadowsocks-libev-nocrypto on github: https://github.com/spyff/shadowsocks-libev-nocrypto
Blogpost with the tutorial and detailed configuration: https://spyff.github.io/mptcp/2017/08/27/transparent-mptcp-proxy/

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