V2X networking and Linux

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Despite the prominence that Linux plays in automotive consortia like GENIVI and the long list of car-industry heavyweights involved in projects like Automotive Grade Linux, there are still plenty of areas in the automotive market where Linux and open-source software are forced to play catch-up. One of these areas is so-called V2X networking—a blanket term that encapsulates a handful of related initiatives, namely vehicle-to-vehicle (V2V) communication and vehicle-to-infrastructure (V2I) communication. At the 2015 Automotive Linux Summit in Tokyo, Hong-Jong Jeong from ZENOME gave a talk on the basics of current V2X networking projects and some insights into how Linux developers can get started.

The main barrier to participating in V2X development, he explained, is that the companies spearheading the work have only released hardware in "black box" form: proprietary, undocumented radio modules, often in sealed enclosures. The literal black-box approach makes developing device drivers virtually impossible, of course, and it does not help that the devices themselves tend to be prohibitively expensive as well.

Nevertheless, there is published standards work to draw on, and progress is being made. ZENOME has been concentrating its efforts on V2V. As of today, there are three main standards in use for V2V networking that are designed to function together, and they are close to final approval in the US and in Europe. The lowest layer is 802.11p, an extension to the existing WiFi standard. On top of this is the IEEE 1609 family, which defines a multi-layer stack for the Wireless Access in Vehicular Environments (WAVE) architecture. Finally, the top level is defined in SAE J2735, the Dedicated Short Range Communications (DSRC) Message Set Dictionary.

Broadly speaking, the standards fit together as follows. V2V exchanges are, due to the nature of moving vehicles, short and transient. So 802.11p sets out some exceptions to the normal 802.11 WiFi physical and access-control layers: links between vehicles take place Outside the Context of a BSS (basic service set), so they are referred to as "OCB." A BSS would be the network defined by an access point or the ad-hoc network created in WiFi ad-hoc mode; these conventions are not established between vehicles. There are no authentication or association steps at the access-control level: all participants transmit a wildcard BSSID. At the physical layer, 802.11p standard defines seven 10MHz-wide channels in the 5.9GHz range for V2V networking, with two reserved for emergency and safety messages.

At the level above 802.11p, the WAVE specification defines a connectionless, UDP-like message protocol. The header includes a service provider field, but no IP address, since the participants may have no Internet gateway at all. The top of the stack is the DSRC Message Set, which defines a set of well-known messages that vehicles may transmit and are expected to understand. Jeong discussed several of the messages, which include emergency vehicle alerts, road-side incident alerts, and map data. The most common message type, however, is the "basic safety message," which is a general status report vehicles are expected to emit periodically.

As far as Linux support is concerned, Jeong noted that, as of 3.19, there is an OCB mode patch for 802.11p in the mainline kernel. A team at the Czech Technical University in Prague and developers at Volkswagen have written patches adding the necessary support to mac80211, cfg80211, and iw. With those patches, he said, it is possible to implement raw socket support.

Acquiring a radio device capable of transmitting or receiving 802.11p has been more difficult. Through some trial and error, ZENOME has found two 802.11a chipsets with open-source drivers that can be modified to work with 802.11p's narrower channels. Both are mini-PCIe cards from Atheros, and the company has been testing them with the Ventana GW5100 Single Board Computer from Gateworks.

Using that hardware and software combination, it is possible to set up a machine that captures WAVE packets and decodes DRSC messages. ZENOME maintains a Yocto layer called meta-v2x with the configuration to receive WAVE packets and hand them up to an (included) user-space daemon that decodes the DSRC message content. At the moment, he emphasized, it is still a work in progress. But the team has tested it against four models of V2V black-box radio from the two leading manufacturers, capturing traffic with tcpdump and Wireshark.

The next steps that the team is exploring include a kernel-space module for handling WAVE messages, a priority queue (which would sort incoming messages based on their emergency status), and some sort of API for applications.

So there is clearly quite a way to go. Even with ZENOME's work, it is only possible to receive V2V messages, and only on two specific hardware devices. Transmitting V2V is yet to come, as is V2I networking and further V2X ideas—Jeong mentioned one recent addition to the V2X fold: V2P networking, for vehicle-to-pedestrian messaging. There is a lot of interest in these networking systems, both from the "intelligent road infrastructure" side, which envisions signs and highway markers that constantly update passersby about nearby conditions, and from the autonomous-vehicle side, which is focused on safety and collision avoidance.

It is, perhaps, unfortunate that the Linux community is forced to hack together compatible radios from other networking equipment, but if experience teaches anything, it is that Linux will always catch up if the need is real enough. Given the time and resources being poured into automotive Linux efforts at present, it is probably only a matter of time before our favorite operating system is on par with the proprietary V2X vendors' products.

[The author would like to thank the Linux Foundation for travel assistance to attend ALS 2015.]