II. Network Neutrality and the Evolving Internet
2.4 Packet Inspection and the Growing Threat of Discrimination
2.4.1 The Mechanics of Network Discrimination
To understand how network discrimination works in practice, we must first understand how data traverses the network. Picture a scenario where end host A and B are respectively located on separate networks X and Y, which are connected through Z. When host A sends a packet to B, the data is transferred from network X, through Z, to network Y, via a series of routers and switches along the network. Whenever a router receives a packet, it must first determine which outgoing link to send it on. If the link is available, the packet is sent on its way. If the link is busy, the packet is queued in a buffer, and waits its turn to use the link. If the buffer is full, which happens when the network is overloaded, the packet may be dropped34.
In the original application-agnostic Internet, all packets were transferred on a first come first serve basis. In an application aware network, the system has far more choices when it comes to deciding what to do with the packet. In the paper Nuts and Bolts of Network
Neutrality
35, Edward Felten describes some of the different approaches network owners may take, which we adapt here.A. Best Efforts or Absolute Non-Discrimination
Absolute non-discrimination is where the network does not discriminate at all between the single bits that pass through it. Every individual packet transmitted through the system is treated in exactly the same way, on a first-come-first-serve basis, regardless of its properties.
This was referred to as a “best-efforts” service, whereby the network would attempt to deliver any packet based on its best guess and best effort as to how to get it to its destination. When a
34 According to the TCP/IP protocol, a dropped packet signals to the sending end host that the link is congested, and a well behaved host will then back off and reduce the rate of transmission until the link returns to an uncongested state.
35 Edward W. Felten, Nuts and Bolts of Network Neutrality (Aug. 2006), http://itpolicy. princeton.
edu/pub/neutrality. pdf.
link buffer is full and a new packet comes in, the router has several choices: (1) it can drop the new incoming packet, or (2) it can allow it into the queue by dropping another packet in the queue, likely the oldest packet in the queue, if not some other packet at random. In such a scenario, any packet has an equal chance of being dropped.
B. Minimal Discrimination
There are, however, no rules requiring the router to drop packets in a certain way. In fact, a router can discard packets in any way it pleases. Minimal discrimination is a scenario whereby the network assigns priorities to packets in the queue. When necessary, rather than dropping packets at random, or based on their order of arrival, the router will drop packets with the lowest priority first. For example, whenever the buffer is full, the router may decide to drop P2P packets first. Felten calls this “minimal” discrimination36, because it only discriminates against certain types of packets when the network is congested and therefore cannot serve all packets at once. Most of the time, when the network is not congested, there is no difference between treatment of higher and lower priority packets.
C. Non-Minimal Discrimination
There is another type of implementation, however, in which the routers may selectively discard low priority packets even if there is enough capacity on the network to deliver them.
For example, the router may be set to reserve 50% of the network’s capacity for high priority packets. When the percentage of lower priority packets reaches the threshold, they may face being dropped, even if the remaining 50% stays idle. Felten calls this kind of discrimination
“non-minimal,” because it artificially restricts certain packets to an arbitrary percentage of
36 See id. at 2.
capacity. 37
D. Delay Discrimination
Another type of discrimination possible is delay discrimination. This type of discrimination can happen in conjunction with minimal and non-minimal discrimination.
Unlike the previous two types of discrimination, which are executed through the dropping of packets, this type of discrimination works through the reordering of packets. Just as the Internet Protocol does not specify what in what order packets should be dropped, it likewise does not specify the order in which they should be sent. While routers generally route packets on a first-come-first-serve basis, it is equally acceptable to send packets in a different order.
For example, a router could allow high priority packets to always cut in front of the line, or advance through the queue at a faster pace. Low priority packets therefore experience an extra delay when passing through the router, much like humans do when people cut in line. This delay is known as “latency.” Another consequence of delay discrimination is that packets may be sent out of order, or experience different delays. This variation in delay is known as
“jitter.” 38
E. Absolute Discrimination
This is the most extreme type of discrimination, and in practice this is synonymous with blocking. What happens is that certain types of packets are categorically blocked when they pass through the router, regardless of whether or not there is a link available, or if there is a buffer queue. For example, a network provider with incentives to block Internet voice services could decide to drop all voice packets passing through their network, rendering the network unusable for VoIP.
37 See id.
38 See id. at 3.