lundi, février 05, 2007

Know your Encapsulation!

This is, effectively, a continuation of what may well become a running thread in this blog: Luis discovers crap about IT arcana that took him all day to figure out. The most recent discovery, entitled Know your MTU!, was that there were physical problems with France Telecom's lines that were creating a great deal of packet loss; the solution was to reduce Maximum Transmission Units (MTUs) to reduce packet fragmentation.

This time around, it was encapsulation. One of our network routers--and the one that is usually the most stable--suddenly dropped off the grid at some point during the weekend and steadfastly refused to reconnect. When I plugged the output from the ADSL modem into my laptop and tried to make the PPPoE (PPP over Ethernet) connection, it still wouldn't connect. When I used (our ISP)'s low-speed phone modem connection, it connected perfectly (although painfully slowly). Clearly there wasn't a system-wide problem, because our other two routers were working fine. I got into the range of another router and looked online to check the status of our accounts with our ISP. Everything was in order, so there was no reason to suspect that the account had been cancelled. Finally, I spent a bit of time reading through reams of postings to the user's support forums. After reading what seemed like a million threads entitled "I can't !@#$ing connect!!!!1!" I finally noticed that one (seemingly knowledgeable) person was advising another member to set up his ADSL connection using PPPoA (PPP over ATM). Essentially, PPPoA uses a different form of encapsulation (i.e., how it formats packets and how it receives them) that offers slightly less overhead, and thus slightly better transmission rates--but at the risk of being a bit less flexible and reliable viz. PPPoE.

Aaaaaanyway. Aside from the PPPoA, the setup this guy was describing was identical to mine, so I hacked into the ADSL modem and set it to PPPoA. Bing! Everything was right back online. Unfortunately, the wireless router isn't able to do PPPoA, so I couldn't leave the ADSL modem on bridge-mode and concentrate the ADSL and WiFi management into one router. Instead, I re-set the ADSL modem in PPPoA mode, running a DHCP server in the subnet, and then set up the wireless router to take an IP address from the ADSL modem and split that into another DHCP subnet as Alas, it's a bit inefficient, in that our packets now have to pass through two internal subnets (and thus there's a lot of port translation going on).

That was pretty much my day. I slept in a bit as I wasn't feeling very well, dedicated most of my day to fixing this mess, and then went grocery shopping. I used some of the Keema Masala that I had kicking around for a fantastic ground beef curry with spicy jasmine rice, and then I made a version of my friend Erika's vinegar-and-oil beets, which are in fact a version of the amazing beets served at Lula Café in Chicago. Delicious! I'll be peeing purple for days, I'm sure.

Aucun commentaire: