ISPs and filtering: the sorting office analogy

With all of the recent parliament and newspaper interest in ISPs filtering content - specifically with reference to blocking particular sites, it is worth spelling out why compelling them to filter certain web sites is useless.

Imagine that Royal Mail was prohibited from delivering mail to domestic addresses from certain senders.

A very simple analogy is to think of ISPs like the sorting department in the post office. Let us consider only postcards and letters.

Postcards are "open" and the sorting office can see both the sender and recipient as well as their content. As letters are in envelopes, the sorting office can see the sender's address (on the back) and the recipient's address on the front. However, they cannot read the contents of the envelope.

Web traffic is similar to this. Ordinary web traffic (e.g. BBC News) appears to the ISP like a postcard. They can see the sender and receiver as well as the article being read.

Encrypted web traffic is a little more like a letter. The ISP can see the sender and receiver but cannot read the data that was sent. This means that content can only be blocked on the basis of sender or receiver. Hence, an ISP wishing to block a porn site hosted on a commercial web server that hosts both a news site and a porn site would have to block both - meaning the news site would be inaccessible.

This is not very fair on the news site which becomes a victim of censorship in this case.

It is worth noting that this is a considerable simplification and does not refer to all of the technical issues surrounding filtering but is a useful model for demonstrating a principle.