Broadcast Storm

Learning about networking is not very fun unless you actually get to see it in action, which is the opportunity that I gave my apprentice today. A few months ago, the corporate network at the place where I am doing public service and paying my dues, went down when a low-level tech accidentally connected the network in a loop.

When that happens, it becomes possible to send packets round and round in a loop. This is not so much a problem with proper network connections because the packets will be terminated at the destination. However, it is a problem for broadcast packets as they could possibly be broadcast in circles – ad infinitum.

First, I got two computers with two network ports each. Then, I bridged the ports together so that br0 on each computer bridged eth0 and eth1 together. This is the example configuration for Debian networking:

auto br0
iface br0 inet static
bridge_ports eth0 eth1
bridge_stp off

Next, I connected one network port from each PC together with a LAN cable. I asked my apprentice to observe the lights on the network ports. They blinked a few times when the cables were first attached due to hand-shaking of the network capabilities. Then, all activity stopped.

So, I initiated a broadcast packet by performing a ping to a random IP address. Because the PC does not know the destination machine, it will send an Address Resolution Protocol (ARP) broadcast packet asking for the destination machine for a reply. We could have just as easily done this with an arping command instead.

The lights on the network ports began to blink with activity and we managed to observe things. After my apprentice got a good look, I stopped the pinging and we confirmed that all activity stopped.

Then, we connected the other two network ports from each PC together with a second LAN cable. Now, we have got ourselves a network loop. Again the lights blinked a few times during hand-shaking and then all activity stopped. I initiated another broadcast and the lights began to blink again. So far so good.

Now, it was time for the broadcast storm. I stopped the pinging like before but this time around, the lights did not stop blinking. The broadcast packet was moving round the network in a loop. My apprentice got all excited that the network ports were blinking rapidly even when there was no application sending out any activity.

Then, I disconnected one port and the blinking stopped – magic!

Finally, I showed my apprentice how the network would work with STP enabled. I changed the settings above and set bridge_stp on instead, restarted the network stack and reconnected the network cable. With STP in place, there would be periodic network activity when the STP packets were sent out but these were easily distinguished from the crazy blinking broadcast storm.

I repeated the whole experiment again, but this time, the broadcast packets stopped after the pinging application was stopped – success!

I think that this would make networking subjects more interesting. This experiment was fairly simple to conduct and took nary five minutes. It would demonstrate networking concepts in a very visual and visceral manner. It does away with the whole idea of imagining how network packets moved through a network. Beautiful simplicity.

IPv6 Stress

I was reading this article on seven IPv6 myths today and a random thought entered my head. The article seemed to stress that the only real difference that IPv6 makes in this world is extending the address space from 32-bits to 128-bits. This is a 4x increase in the address space currently used.

This instantly triggered a yellow-flag in my head. This also means that there will be a marked increase in the memory space on networking devices, coupled with an increase in memory bandwidth consumption on the said devices.

What many people may not know is that on a networking device like a router, routes need to be maintained for the duration of a connection. This is usually maintained as some sort of table in the memory. When a network packet comes into a router, its header is examined for the destination address. This is then used to look-up the route to take in the routing table. Then, the network packet is forwarded onto the route on its way to the destination.

So, an IPv6 routing table would need to quadruple its memory space requirements. This would not be so bad if we didn’t take the real-world performance of memory into account. It would also mean that the amount of memory transfers would be quadrupled as well, assuming similar table and lookup patterns.

While I have not researched into this, I do think that there is room for innovation here. We could possibly change the routing table structure by dropping off some of the bits or to partition it up into directories indexed by the significant bits of the address or to hash the IP addresses in such a way as to reduce memory requirements.

This may not mean much to a lot of people but if the route look-up process slows down by a factor of four, it will significantly affect the latency of the network even if its effects on throughput may be minimal. Latency is very important for certain applications, anything that requires near real-time feedback like first-person shooters.