Agylen

It all started when my iMac started turning itself off randomly and in the end it decided that it didn’t want to tun itself on anymore. I suspect a problem with the power supply, so right before Easter I brought it in for check and possibly repair. Haven’t heard anything about it, since then.

Then, right after Easter, my DSL connection suddenly became much slower. More like crawling, actually. Slow like a 56k modem that was losing a good deal of packets, which translates to being basically unusable.

Since I have three computers at home and they all have the same problem, I immediately excluded the possibility that all of them had started going mad at once. I also excluded the WiFi base station from the loop, to no avail. I replaced all cables, still no luck. I even got a new modem/router, since the one I had was left over from the previous provider and I couldn’t rule out the possibility that it had started malfunctioning during the Easter break. Same problem.

Calling technical support, of course, entails being kept on hold for 15 minutes on average, listening to an endless loop of Lily Allen’s LDN (I hate that song), then, if you’re lucky enough to be able to talk to someone, they will tell you that, for slowness problems, you have to execute an online speed test from their website (which I did but could not complete, since it times out at around 5%, when the average speed was measured in the vicinity of 0.1kbps) and report the results.

Incidentally, this is the same company that left me out in the cold for a week when switching providers. I’m tempted to switch again, but I’m afraid no company is actually able to provide a decent service.

Having ruled out computers, cabling, the router and the WiFi network as possible culprits, I’m starting to suspect some sort of TCP-related problem. A couple clues:

• Speed seems to be extremely low only when downloading. Uploading is generally blazingly fast.
• Downloads seem to start normally but then crawl to a halt after a few seconds or fractions of a second.
• Some applications are apparently unaffected by the problem. Most notably, I can conduct a Skype convestation without interruptions. I even tried Joost and, even though it doesn’t play anything and times out and disconnects after a while, it is almost able to sustain an inbound stream of a few hundred KB/s for half a minute or so.
• Ping times are absolutely normal.

This behavior makes me think that it’s not a line noise problem either, otherwise all types of traffic would be similarly affected, I think. It looks like what’s mostly affected is long running TCP connections, and only in the downstream direction. P2P applications, which probably use something different, I don’t know, are much less affected. ICMP stuff (ping) is not affected at all.

So I suspected some TCP fragmentation problem, and indeed, after mucking around with the MTU and MRU parameters of the router’s WAN (DSL) interface, I got somewhere: with MTU and MRU set at a value of 1000 (they were 1400 and 1492, respectively, before) my DLS line still feels like a 56k analog modem, but without packet loss. This means that I can at least navigate most “lightweight” websites and read email. Big deal!

Now, if anyone is reading this and has some clues about how to diagnose TCP problems and how to determine the optimal values of MTU, MRU and maybe some of the other, more obscure, configurable parameters in my router (D-Link DSL-G624T), I’ll be eternally grateful.

Last but not the least, last Friday the domain of this website disappeared completely from its DNS provider and I couldn’t get someone to resolve the situation until yesterday in the afternoon.

All in all, a very frustrating period.