Double Tall Iced Mocha, Lite on the Chocolate

So I am probably the last person to post something about Field Day 2006 but eh, better late then never. The main idea behind field day is to rapidly deploy a wireless network at various points in the city and do so without relying the grid for communication or power. We do it mostly for a reason to geek out, but being able to this in a state of emergency would be very usefull.

There were three physical sites that participated:

• Magnolia Park
• Alki Beach
• Elliot Bay

All sites were powered via battery. At Alki we have a deep cycle battery that powered our wireless node and 5 or so laptops for the entire day. We arrived at the site around 11am and had the tent, power and network up and running by 12:30.

Probably the most interesting part of the day was playing with OLSR in this situation. Most nodes were Soekris based Metrix boxes running Pyramid Linux. We used OLSR to handle routing for the network. OLSR is the “Optimized Link State Routing” protocol it is a link state routing protocol that is intended for use in MANETs (mobile ad-hoc networks) also commonly referred to as “mesh” networks. It advertised all the host routes for individual nodes running OLSR and provided HNA for subnets that contained hosts not running OLSR. In addition it also provided HNA for networks that Internet gateways. HNA stands for Host and Network Association messages, which are basically routing advertisements for non-mesh nodes.

One major technical issue we spent a lot of time trouble shooting was the Elliot bay node. It was primarly connected via a 802.11 client bridge device. This device had at least one machine behind it. The problem was quite confusing we could ping a node behind the client bridge no problem, we recieved routing information from it, however when we attempted to route packets through the node behind the client bridge they were silently dropped. We verified via packet traces that they left one radio interface however they never reached the machine behind the client bridge. This was because packets being routed through the client bridge had a destination address of a host the client bridge did not know about. Since 802.11 has no specification for bridging in client (STA) or ad-hoc mode, client bridges get around this by using proxy arp and since it knows nothing about the destination address in the packet to be routed it does pass it. A work around was to set the mac address of the bridge to match the mac address of the host behind it running OLSR. I am still not sure why this worked, but have not had the time to think it through.

Other field day posts:

• Brian
• Casey
• Eric
• Matt
• Dan