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Friday, November 02, 2012

Windows Virtual PC IPv6: Part 3

The original title of the first post in the this series was "Windows Virtual PC Wireless IPv6", but dropped the "Wireless" before publishing. At that point, I had Part 1 and Part 2 written and knew the problem was with the wireless card versus the wired card, but didn't yet know the root cause. And since I had exhausted my troubleshooting tools (tcpdump and network based analysis) and wasn't about to dive into wireless driver software reverse engineering, I figured I may never have the exact answer. Instead, I focused the posts on discussing the problem and how it relates to the question, "is 'X' IPv6 ready?"

I did want to close the loop and exhaust all possible angles. So I called my friend and colleague Randy who had experience with wireless bridging / routing and virtual machines from a previous client. I remember he would lament about the issues he faced and custom drivers and wireless cards needed to get things only partially working. Since I was only an observer, I didn't have the intimate details, so now I sought them out over a phone call.

I explained my set up and issue and Randy immediately knew the problem. It wasn't IPv6 specific nor was it related to Windows Virtual PC. In fact, it wasn't even the wireless card or its respective drivers (per se), it was the 802.11 protocol itself. Randy explained the technical issues and gave me the proper Google terms to search to flush out the entire explanation.

And sure enough, after our call I started finding links such as:

that detail the IPv6 neighbor discovery problem I encountered when using wireless bridging. In my case, it wasn't a standalone bridge device, but the virtual network bridge when using a virtual machine. Note that in my testing the issue happened with Windows Virtual PC, but Randy's previous work involved VMware, which also had the issue. Again, this is protocol / driver related, not specific to IPv6 (network) or application layer software.

Further complicating the matter was the "incorrect" 'tcpdump' decodes. While the Ethernet header in the packets I posted showed source and destination MAC addresses, it wasn't the full picture. Wireless 802.11 has a different frame format than wired 802.3 Ethernet. However, the *pcap (winpcap in my case) libraries can't decode the 802.11 header if the wireless driver software doesn't pass it along. With my stock Broadcom wireless on a Dell laptop running Windows 7, I was seeing the "fake" header.

This final link from the OpenWRT project does a good job visualizing what is happening. Again, the link details physical bridges versus the virtual bridge introduced with machine virtualization software, but the same holds true.

So what does all this mean? Depending on how you look at this problem, it could be hardware, protocol or software related. For me, it manifested when trying IPv6 on Windows Virtual PC - so that was my first thought. It turns out the answer was much more involved and complicated than I imagined. However, broken IPv6 connectivity was the symptom and the casual user experiencing that will probably just say, "the network is down." No amount of planning is going to identify these unique use cases until they occur - hopefully during a pilot, more likely during production. You're going to need some smart people when you start rolling out IPv6.

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