I’ve been writing lately about the nuts and bolts of network discrimination. Today I want to continue that discussion by talking about how the Internet responds to congestion, and how network discrimination might affect that response. As usual, I’ll simplify the story a bit to spare you a lengthy dissertation on network management, but I won’t mislead you about the fundamental issues.
I described previously how network congestion causes Internet routers to discard some data packets. When data packets arrive at a router faster than the router can forward those packets along the appropriate outgoing links, the packets will pile up in the router’s memory. Eventually the memory will fill up, and the router will have to drop (i.e., discard) some packets.
Every dropped packet has some computer at the edge of the network waiting for it. Eventually the waiting computer and its communication partner will figure out that the packet must have been dropped. From this, they will deduce that the network is congested. So they will re-send the dropped packet, but in response to the probable congestion they will slow down the rate at which they transmit data. Once enough packets are dropped, and enough computers slow down their packet transmission, the congestion will clear up.
This is a very indirect way of coping with congestion – drop packets, wait for endpoint computers to notice the missing packets and respond by slowing down – but it works pretty well.
One interesting aspect of this system is that it is voluntary – the system relies on endpoint computers to slow down when they see congestion, but nothing forces them to do so. We can think of this as a kind of deal between endpoint computers, in which each one promises to slow down if its packets are dropped.
But there is an incentive to defect from this deal. Suppose that you defect – when your packets are dropped you keep on sending packets as fast as you can – but everybody else keeps the deal. When your packets are dropped, the congestion will continue. Then other people’s packets will be dropped, until enough of them slow down and the congestion eases. By ignoring the congestion signals you are getting more than your fair share of the network.
Despite the incentive to defect, most people keep the deal by using networking software that slows down as expected in response to congestion. Why is this? We could debate the reasons, but it seems safe to say that there is a sort of social contract by which users cooperate with their peers, and software vendors cooperate by writing software that causes users to keep the deal.
One of the reasons users comply, I think, is a sense of fairness. If I believe that the burdens of congestion control fall pretty equally on everybody, at least in the long run, then it seems fair to me to slow down my own transmissions when my turn comes. One time I might be the one whose packets get dropped, so I will slow down. Another time, by chance, somebody else’s packets may be dropped, so it will be their turn to slow down. Everybody gets their turn.
(Note: I’m not claiming that the average user has thought through these issues carefully. But many software providers have made decisions about what to do, and those decisions factor in users’ wants and needs. Software developers act as proxies for users in making these decisions. Perhaps this point will get more discussion in the comments.)
But now suppose that the network starts singling out some people and dropping their packets first. Now the burden of congestion control falls heavily on them – they have to slow down and others can just keep going. Suddenly the I’ll-slow-down-if-you-do deal doesn’t seem so fair, and the designated victims are more likely to defect from the deal and just keep sending data even when the network tells them to slow down.
The implications for network discrimination should now be pretty clear. If the network discriminates by sending misleading signals about congestion, and sending them preferentially to certain machines or certain applications, the incentive for those machines and applications to stick to the social contract and do their share to control congestion, will weaken. Will this lead to a wave of defections that destroys the Net? Probably not, but I can’t be sure. I do think this is something we should think about.
We should also listen to the broader lesson of this analysis. If the network discriminates, users and applications will react by changing their behavior. Discrimination will have secondary effects, and we had better think carefully about what they will be.
[Note for networking geeks: Yes, I know about RED, and congestion signaling, non-TCP protocols that don’t do backoff, and so on. I hope you’ll agree that despite all of those real-world complications, the basic argument in this post is valid.]