Good and important story in the Machinist, Salon.com’s tech blog, about Associated Press’s discovery that Comcast is selectively interfering with its user’s data transfer.

Comcast, the AP determined, actively manages data on its network by using software to essentially masquerade as its subscribers’ machines. When non-Comcast Internet subscribers request files from your Comcast-connected machine — as happens in peer-to-peer file-sharing applications — Comcast’s technology steps in and tells the non-Comcast subscriber you’re not available.

No one disputes that Comcast has the right to manage data transfer to increase performance for all its subscribers. The objection is that the method is dishonest. According to the AP story, Comcast’s software inspects data coming into the network, and if the data appears to be peer-to-peer file transfer between a subscriber and a non-subscriber, the software sends out forged data packets to the two computers.

Each PC gets a message invisible to the user that looks like it comes from the other computer, telling it to stop communicating. But neither message originated from the other computer — it comes from Comcast. If it were a telephone conversation, it would be like the operator breaking into the conversation, telling each talker in the voice of the other: “Sorry, I have to hang up. Good bye.”

Of course, Comcast doesn’t mention in its advertising or in its contract that it is blocking this kind of use of its service. In both, Comcast emphasizes the high speeds and the unlimitedness of its services, and the company is perfectly happy to take the money from subscribers who want the service precisely in order to use peer-to-peer file sharing.

I’m already down on Comcast from my experiences trying to email their subscribers. I am a technical editor for a small publisher, and as a result I send and receive a lot of attachments, mostly Word docs and PDF files, to and from authors. Well, now and then when I email an attachment to an author on Comcast, I get a message back saying that the message couldn’t be delivered because my company has been identified as a spammer.

A spammer. Right. You may have noticed what a problem you have been having lately with all that unsolicited email in your inbox urging you to buy study materials that are claimed to help you pass your licensing exam in structural engineering. You know the ones: Get stronger, deeper understanding — fast! Last longer under time pressure! Insert joke about stress analysis of steel members here!

Turns out, we get on this list because from time to time we send out email promotions to people who have signed up to receive them, and these promotions keep triggering whatever spam detection software Comcast has set up. It never seems to trigger anybody else’s, just Comcast’s, and they respond by blocking all messages over a certain very small size. Whenever this happens, I usually just forward the attachment to one of my several personal email accounts and then send it along from there, but it’s a pain in the butt.

Well, our marketing director calls Comcast up and says, look, we’re a small company and everyone who receives our promotions has signed up for them, please take us off your list of blocked addresses. And they do, and after a half a day or a day passes, I can send attachments again. Until the next time it happens again. And again. And again.