The Treehouse Blog

So, at work today we received two boxes from Vendor C shipped via Vendor U that contained identical parts. It was immediately observed that the one box was far heavier than it should have been. Perhaps they shipped both of the items in the one box, and manuals and such in the other? Stranger things have happened. And, well, had. Both boxes contained what they were meant to. The heavier one had a plastic-wrapped white object in the bottom that looked like shrink-wrapped documentation – at least, until you tried to move it. It had a Vendor U shipping label indicating a weight of 37lbs and was about a foot square and an inch thick. Apparently this piece of solid steel encountered the Vendor C box and penetrated it during shipment (judging from the correctly sized hole in the side of the box, noticed afterwards). Vendor U is sending someone to pick up the stowaway package, and the Vendor C gear looks to be undamaged. Still. Weird.

There are good networking problems. These are the kind that something happens that you don’t understand. You mull over it, eventually diagram out the different layers of the various protocol stacks, and discover that things are working exactly as designed, just not as you expected. Ian and I uncovered an example of this kind of problem a few years back at CTI. There was a periodic surge in broadcast traffic on our LAN destined for a specific box. After ruling out all of the various NAT rules and such going on, we eventually found the simple cause. The box was not sending any packets of its own. So, the switch had no forwarding entry for it. So it did what switches are supposed to do in that case: broadcast the packet. Coming to those kind of conclusions is fun.

There are at least three kinds of bad networking problems. The first is when something is evil by design. An infamous example of this would be the error measurements in the DS1-MIB. Absolutely useless when it comes to making time-series graphs, or setting event thresholds. The design seems to have been one of convenience, since the quantities represented date back to the first of the DS1 channel banks. Anachronistic design sucks.

Another type of evil networking problem is one that is seemingly random. Everyone knows that one of the first steps in troubleshooting is knowing what steps are needed to reproduce the problem. With random events, you’re essentially screwed. Turn up the debugging until it gets painful and wait for the event to recur, and pray you captured something useful when and if it does. DDOS attacks and the like can fall into this category, especially for those of us without the ability to do meaningful flow logging. Tracking down random problems is evil.

The third category of networking problems I feel like discussing this evening is the unreasonable problem. This is the non sequitur, the problem that makes you yell futilely at your terminal or coworker, from the complete and utter ridiculousness of it. If you manage to solve one of these, you might end up with a good networking problem, as described above. Or you might want to take a sledge hammer to a piece of hardware. Let’s explore some examples:

Near the end of my CTI experience, a certain Astrocom CSU/DSU was observed having a most unlikely problem. If I remember correctly, it somehow would drop packets over a certain size. A most improbable feat considering that your average CSU/DSU should not really have any concept of what a packet is, let alone be able to drop one. Patrick offered a bounty on the problem, but as far as I know, it was never solved.

The last example is the reason I am writing tonight. Last summer at Doug’s LAN party, I had difficulties copying large files to my desktop machine. I eventually blamed it on my patch cord, but by that time we were packing things up, so this was never really tested. Even before this, I was having trouble sending large print jobs to my printer. I quickly blamed this on the network card in the printer, or some postscript oddities, but never came to a solution.

Later, when trying to use my desktop as a file server for a CentOS install, I realized that my network issue with my desktop was ongoing. Traffic analysis indicated that during a high speed transfer coming from my desktop to another machine, the connection would stop passing packets. Packets on other TCP connections between the same machines were unaffected, but subsequent retries of packets associated with the dead connection were getting dropped somewhere. Since this seemed to be connection related, firewall settings were verified and found to be fine. I let the problem fester, as it was not causing any day-to-day difficulties, since my desktop isn’t ordinarily sending large amounts of data, just receiving.

So, this evening I was talking with Doug about printer stuff, and he made a connection that I had been missing. Was my printer problem related to my network problem? And what about all of those problems with NFS in the recent past? Yes, they all sound like candidates. With that late realization, I delved into the annoying network problem. I replaced the network card. The problem persisted. The cabling was ruled out. And the problem was narrowed down to a switch. A specific switch port, to be precise. Now, I’m not exactly sure how one of my NetGear GS506 gigabit switches is managing to drop packets belonging to a specific connection when that connection begins spouting lots of traffic in a certain direction, but that’s exactly what it appears to be doing. And it’s reproducible. So yes, a problem as insidious as this problem should be solved with the sledge, but some tape over port 1 of the switch should do. Thanks for the insight, Doug! And if you see a problem like this, check the switch, even though it doesn’t make any sense.

Internet connectivity in the US, particularly rural areas, is awful. And there is really no excuse for it. The Paradox of the Best Network provides some insight into this (thanks to Patrick for the link, via his blog). I also envision general office buildings in the future that provide telecommuters a way to get out of the house and have access to shared resources. And everyone working in the building might be working for a distinct company across the globe. In the mean time, my 1Mbit with extra evil shaping connection to Kuhn and my 11Mbit wireless link to Chris will have to suffice.

No structure here… just some random goings-on:

For the last several kernel updates for FC4, my DVD sharing using GNBD hasn’t worked. I guess those special ioctl()s aren’t getting translated over or something. And NFS or SMB sharing an encrypted DVD just doesn’t do anything good at all (ignoring the fact that NFS seems really sucky with the latest FC4 updates). So, after months of not being able to watch DVDs, I gave up, and bought a USB drive cage to attach a DVD drive to Oak. Works perfectly. Despite performance issues and cabling evilness, I still can’t completely rule out a stack of USB drives RAIDed as a bulk storage solution, especially with all of the device mapper coolness in Linux. Too early to be thinking of that, though, as the computer storage fund hasn’t matured yet, despite the fact that Oak is at 99% capacity.

As Doug has mentioned, Asterisk and VoIP is still pretty neat. I’ve setup a teliax account, since they have pricing like nufone with a whole lot of rate centers worth of DIDs (they’re essentially a Level3 reseller). So, we just need to get some VPN’ing set up. I’m in desperate need of some UT, and I think VPN might be a useful substitute for a LAN party this winter.

My grandfather (on my Dad’s side) has been in the hospital on and off for more than a month now. Currently he has pneumonia, is very weak, and not entirely coherent. Your prayers would be appreciated for what could be a difficult Christmas season for the family.

Things at Ship are going fairly well. I did shoot myself in the foot with the “ip arp inspection” feature of the Sup720 this week though. Does 15 pps of ARP traffic seem like a good default threshold for shutting down trunk ports to you? Me neither. Of course, I asked that question after two ports had been err-disabled. Hopefully Tim and I will get to do a real test of some VMPS soon, too.

I’ve added the first WAN link to the thtech network: 802.11g to Chris’s house. We used garden hose as conduit for about 250′ of CAT5e from my shed to the WAP. The WAP was mounted on a pole inside of a tupperware container. Ethernet handled the distance fine, but a trivial POE implementation arrived at by splicing the power adapter that came with the WAP into the CAT5 didn’t work. There was too much resistance on the line for the 5v power to overcome. So, we tried stepping up to 12v with a DC-DC converter on the other end. This would result in the WAP partially powering up, but continually rebooting. I figured that this was due to the WAP drawing too much power when it tried to power up the radio. I purchased a higher capacity DC-DC converter at WalMart, and all has been well. We’re currently getting 600 KB/s, but earlier tests had us at about 1.1MB/s. We’ll have to do some antenna tweaking to try to remedy that.