* Trying to use FTP on a PPPoE ADSL service
Over the past two newsletters, we’ve been discussing some of the ins and outs of transitioning to Point-to-Point Protocol over Ethernet (PPPoE) as the authentication scheme for DSL service. From an operational perspective, the biggest challenge so far was that when switching from cable modem to DSL, the router had to be reconfigured. Of course, the simplest solution would probably be to purchase yet another router for almost-free, and switch the cable from the LAN switch to the router rather than from the router to the cable or DSL modem.
But then Steve hit the wall when an important application – FTP – started hanging on a somewhat regular basis. The FTP app had been working for years. (In fact, the application has a copyright date of 1992-1998. But it works. Or at least it used to.)
The first time that a large upload began on the newly configured PPPoE ADSL service, the application choked on a large file transfer. Being the empiricist that he is, Steve suspected the DSL service as the culprit, but he had to get the file uploaded so he “simply” switched back over to the cable modem, reconfiguring the router en route. A day or so later, there was a break in the action so the experiment could continue. Guess what? It still failed on the DSL link.
So Steve decided to take both a top-down and bottom-up approach. After e-mailing the DSL provider’s director of analyst and customer relations, whom he happened to know, Steve also started down the help desk path.
At the help desk, after repeatedly explaining what FTP was – that it really wasn’t e-mail or Web browsing – Steve got to a level of tech support that at least understood the problem. Thirty minutes into the process, Steve was informed that indeed this was a known problem and that a trouble ticket had been open on it for about a week. If the problem persisted for a few more days, then he should call back. (Thank goodness Steve thought to ask what the trouble ticket number was in order to avoid the 30 minutes of relatively efficiently getting to this point.)
But the top-down approach actually solved the problem. Relatively soon, Steve got a call from the national tech support folks, and within about 15 minutes of working with a knowledgeable tech, it was determined that PPPoE actually adds about 6 to 8 bytes to the packet size, and with a large transmission, we were hitting some packets that exceeded the proper packet size. A quick adjustment was made to the MTU (maximum transmission unit) size on Windows XP to go from the default of most NICs of about 1,480-bytes down to 1,300-bytes. And, automatically, the connection started working.
There are a couple of lessons here. Had it not been for the “inside track,” this would have remained an issue for days if not weeks. Also, “transparent transitions” are not always what they seem. Glass is “transparent,” but don’t try running through it.
