"China has several gigabytes per second of traffic to deal with and has a lot more international businesses," he says. "They can't be as heavy-handed with their filtration. The Iranians aren't as concerned about that… so they get to use all these fancy toys that, if the Chinese used them, could cripple their economy."

With that in mind, this article will look at five of the most commonly-used technologies the Iranian government has been using to stifle dissent, as outlined in Sennhauser's paper.

IP Blocking

IP Blocking is one of the most basic methods that governments such as Iran use for censorship, as it simply prevents all packets going to or from targeted IP addresses. Sennhauser says that this was how the government banned access to the BBC's Persian news services and how it took down websites critical of the election.

But while these sorts of operations are relatively simple to execute, they don't tackle the problem of individual communications between users, especially if the users have set up multi-hop circuits that use multiple servers to create a proxy ring.

Traffic Classification (QoS)

This is a much more sophisticated method of blocking traffic than IP blocking, as governments can halt any file sent through a certain type of protocol, such as FTP. Because the government knows that FTP transfers are most often sent through TCP port 21, they can simply limit the bandwidth available on that port and throttle transfers. Sennhauser says that this type of traffic shaping practice is the most common one used by governments today, as "it is not too resource intensive and is fairly easy to set up."

Shallow Packet Inspection

Shallow packet inspection is basically a blunter, broader version of the deep packet inspection (DPI) technique that is used to block packets based on their content. But unlike DPI, which intercepts packets and inspects their fingerprints, headers and payloads, shallow packet inspection makes broad generalities about traffic based solely on checking out the packet header. Although shallow packet inspection can't provide the Iranian government with the same detailed traffic assessments as DPI, Sennhauser says that it is much better at handling volume than DPI.

"It's a less refined tool, but it can also deal with a lot more traffic than true DPI can," he explains. "Shallow packet inspection is more judging a book by its cover. If a packet says that it's SSL (Secure Sockets Layer) in the header, then a shallow packet inspector takes it at face value."

Sennhauser notes, however, that this is a double-edged sword. If a user disguises their SSL packets as FTP packets in the header, the shallow packet inspector won't be able to tell the difference.

Packet Fingerprinting

This is a slightly more refined method of throttling packets than shallow packet inspection, as it looks not only at the packet header but at its length, frequency of transmission and other characteristics to make a rough determination of its content. Sennhauser says the government can use this technique to better classify packets and not throttle traffic sent out by key businesses.

"A lot of things don't explicitly say what they are. For example, a lot of VPN traffic is indistinguishable from SSH traffic, which means that it would be throttled if SSH was," he says. "But what if businesses relied on VPN connections? You'd move the system to fingerprinting, where the two are easily distinguishable."

Deep Packet Inspection / Packet Content Filtering

DPI is the most refined method that the government has for blocking Internet traffic. As mentioned above, deep packet inspectors examine not only a packet's header but also its payload. This gives governments the ability to filter packets at a more surgical level than any of the other techniques discussed so far.

"Viewing a packet's contents doesn't tell you much on its own, especially if it's encrypted," he says. "But combining it with the knowledge gained from fingerprinting and shallow packet inspection, it is usually more than enough to figure out what sort of traffic you're looking at."

There are downsides to using DPI, of course: it's much more complicated to run and is far more labor-intensive than other traffic-shaping technologies. But on the other hand, Sennhauser says there's no magic bullet for getting around DPI as users can usually only temporarily elude it by "finding flaws in their system." And even this won't help for long, as the government can simply correct their system's flaws once they're discovered.

"Once they fix the flaw, you've lost unless you can figure out some real way to circumvent it," Sennhauser notes.

Endgame still unclear

Sennhauser says that the government has employed these technologies smartly despite being caught flat-footed by the initial furor after the election. Indeed, he thinks the only reason that Iran hasn't yet completely shut down dissidents' communications is that they've had to fight with an army of hackers who tirelessly search for flaws in their system.

"It really is an arms race," he says. "They create a problem, we circumvent it, they create another, we get around that one. This continues on until the need to do so is removed. The circumstances which will end the competition aren't clear yet."