This is interesting timing for this article to come out about Comcast blocking traffic.

Comcast is messing with TCP handshakes

Just yesterday, one of my clients contacting me saying that everyone in their office couldn’t get email. I manage the server which hosts their email. I quickly checked my email to see if it worked for me (it did.)
Eventually in the troubleshooting process, I had them hit the website which resides on the same server. No dice. I have quick access to 2 other systems which use Comcast, so I logged in and tried connecting. One worked, one didn’t. I also got reports of some sites not working from other people on Comcast connections yesterday.

In analyzing the problem, I found that connections weren’t being established from these “problem” locations. The filtering was happening with the handshake. A 3 step handshake goes like this:

  1. Computer1: “Hello, Computer2 are you there?”
  2. Computer2: “I’m here. Computer1 are you there?”
  3. Computer1: “I’m here.”

Then things start to run over the connection. Here is a more technical description of TCP connections. Yesterday I watched the network traffic coming and going from my server and found that Comcast was filtering step 2 so that the first reply never got back. What a bunch of jerks.

I’m guessing that the cause of this is that their peer-to-peer filtering monster that went out of control and started blocking legitimate traffic (I don’t run any P2P software on any of my networks.)

I signed up for an Internet connection, not a Comcast-filtered Internet connection. I wish I had other options where I live. I’m rooting for Utopia and will research my ISP options thoroughly if we ever move to a different house.