i2c support

[beriberikix]

Just watched the presentation from ROSCon 2015 Hamburg, which mention TTL UART is a future goal. What about i2c? That seems like a good target as most MCUs have onboard i2c and it is a great solution for multi-node systems (RS485, also mentioned, is great but less abundant in non-automation situations.)

[codebot]

Hi @beriberikix , yeah i2c and uarts and other lower-level, non-natively-internetworkable physical layers (including, perhaps most commonly, USB) are definitely in the wish-list and the informal longer-term roadmap. It will be a bit tricky because these protocols tend to have smaller physical frame sizes and/or smaller logical datagrams than Ethernet/UDP4, which was what RTPS was originally designed for. So there will have to be some adaptation of the protocol and generic forwarding / routing work on whatever nearby host can actually reach other networks. But I think the principles of RTPS have a lot of running room and it will be fabulous to adapt them to lower-cost, simpler physical layers. But for now I'm working on finishing up a near-total rewrite of fastRTPS which follows the UML of the specification much more closely, and is generally much cleaner code. This is currently shown as the "history_cache" branch in this repo. Once that gets put back together (it's getting close...) and our near-term RTPS-over-UDP4 needs are handled, it will be fun to do some blue-sky dreaming about how to adapt this stuff to other physical layers.