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diff --git a/doc/contrib/incentives.txt b/doc/contrib/incentives.txt deleted file mode 100644 index 850a0d01e..000000000 --- a/doc/contrib/incentives.txt +++ /dev/null @@ -1,479 +0,0 @@ - - Tor Incentives Design Brainstorms - -1. Goals: what do we want to achieve with an incentive scheme? - -1.1. Encourage users to provide good relay service (throughput, latency). -1.2. Encourage users to allow traffic to exit the Tor network from - their node. - -2. Approaches to learning who should get priority. - -2.1. "Hard" or quantitative reputation tracking. - - In this design, we track the number of bytes and throughput in and - out of nodes we interact with. When a node asks to send or receive - bytes, we provide service proportional to our current record of the - node's value. One approach is to let each circuit be either a normal - circuit or a premium circuit, and nodes can "spend" their value by - sending and receiving bytes on premium circuits: see section 4.1 for - details of this design. Another approach (section 4.2) would treat - all traffic from the node with the same priority class, and so nodes - that provide resources will get and provide better service on average. - - This approach could be complemented with an anonymous e-cash - implementation to let people spend reputations gained from one context - in another context. - -2.2. "Soft" or qualitative reputation tracking. - - Rather than accounting for every byte (if I owe you a byte, I don't - owe it anymore once you've spent it), instead I keep a general opinion - about each server: my opinion increases when they do good work for me, - and it decays with time, but it does not decrease as they send traffic. - Therefore we reward servers who provide value to the system without - nickle and diming them at each step. We also let them benefit from - relaying traffic for others without having to "reserve" some of the - payment for their own use. See section 4.3 for a possible design. - -2.3. Centralized opinions from the reputation servers. - - The above approaches are complex and we don't have all the answers - for them yet. A simpler approach is just to let some central set - of trusted servers (say, the Tor directory servers) measure whether - people are contributing to the network, and provide a signal about - which servers should be rewarded. They can even do the measurements - via Tor so servers can't easily perform only when they're being - tested. See section 4.4. - -2.4. Reputation servers that aggregate opinions. - - The option above has the directory servers doing all of the - measurements. This doesn't scale. We can set it up so we have "deputy - testers" -- trusted other nodes that do performance testing and report - their results. - - If we want to be really adventurous, we could even - accept claims from every Tor user and build a complex weighting / - reputation system to decide which claims are "probably" right. - One possible way to implement the latter is something similar to - EigenTrust [http://www.stanford.edu/~sdkamvar/papers/eigentrust.pdf], - where the opinion of nodes with high reputation more is weighted - higher. - -3. Related issues we need to keep in mind. - -3.1. Relay and exit configuration needs to be easy and usable. - - Implicit in all of the above designs is the need to make it easy to - run a Tor server out of the box. We need to make it stable on all - common platforms (including XP), it needs to detect its available - bandwidth and not overreach that, and it needs to help the operator - through opening up ports on his firewall. Then we need a slick GUI - that lets people click a button or two rather than editing text files. - - Once we've done all this, we'll hit our first big question: is - most of the barrier to growth caused by the unusability of the current - software? If so, are the rest of these incentive schemes superfluous? - -3.2. The network effect: how many nodes will you interact with? - - One of the concerns with pairwise reputation systems is that as the - network gets thousands of servers, the chance that you're going to - interact with a given server decreases. So if 90% of interactions - don't have any prior information, the "local" incentive schemes above - are going to degrade. This doesn't mean they're pointless -- it just - means we need to be aware that this is a limitation, and plan in the - background for what step to take next. (It seems that e-cash solutions - would scale better, though they have issues of their own.) - -3.3. Guard nodes - - As of Tor 0.1.1.11, Tor users pick from a small set of semi-permanent - "guard nodes" for their first hop of each circuit. This seems like it - would have a big impact on pairwise reputation systems since you - will only be cashing in on your reputation to a few people, and it is - unlikely that a given pair of nodes will use each other as guard nodes. - - What does this imply? For one, it means that we don't care at all - about the opinions of most of the servers out there -- we should - focus on keeping our guard nodes happy with us. - - One conclusion from that is that our design needs to judge performance - not just through direct interaction (beginning of the circuit) but - also through indirect interaction (middle of the circuit). That way - you can never be sure when your guards are measuring you. - - Both 3.2 and 3.3 may be solved by having a global notion of reputation, - as in 2.3 and 2.4. However, computing the global reputation from local - views could be expensive (O(n^2)) when the network is really large. - -3.4. Restricted topology: benefits and roadmap. - - As the Tor network continues to grow, we will need to make design - changes to the network topology so that each node does not need - to maintain connections to an unbounded number of other nodes. For - anonymity's sake, we may partition the network such that all - the nodes have the same belief about the divisions and each node is - in only one partition. (The alternative is that every user fetches - his own random subset of the overall node list -- this is bad because - of intersection attacks.) - - Therefore the "network horizon" for each user will stay bounded, - which helps against the above issues in 3.2 and 3.3. - - It could be that the core of long-lived servers will all get to know - each other, and so the critical point that decides whether you get - good service is whether the core likes you. Or perhaps it will turn - out to work some other way. - - A special case here is the social network, where the network isn't - partitioned randomly but instead based on some external properties. - Social network topologies can provide incentives in other ways, because - people may be more inclined to help out their friends, and more willing - to relay traffic if most of the traffic they are relaying comes - from their friends. It also opens the door for out-of-band incentive - schemes because of the out-of-band links in the graph. - -3.5. Profit-maximizing vs. Altruism. - - There are some interesting game theory questions here. - - First, in a volunteer culture, success is measured in public utility - or in public esteem. If we add a reward mechanism, there's a risk that - reward-maximizing behavior will surpass utility- or esteem-maximizing - behavior. - - Specifically, if most of our servers right now are relaying traffic - for the good of the community, we may actually *lose* those volunteers - if we turn the act of relaying traffic into a selfish act. - - I am not too worried about this issue for now, since we're aiming - for an incentive scheme so effective that it produces tens of - thousands of new servers. - -3.6. What part of the node's performance do you measure? - - We keep referring to having a node measure how well the other nodes - receive bytes. But don't leeching clients receive bytes just as well - as servers? - - Further, many transactions in Tor involve fetching lots of - bytes and not sending very many. So it seems that we want to turn - things around: we need to measure how quickly a node is _sending_ - us bytes, and then only send it bytes in proportion to that. - - However, a sneaky user could simply connect to a node and send some - traffic through it, and voila, he has performed for the network. This - is no good. The first fix is that we only count if you're receiving - bytes "backwards" in the circuit. Now the sneaky user needs to - construct a circuit such that his node appears later in the circuit, - and then send some bytes back quickly. - - Maybe that complexity is sufficient to deter most lazy users. Or - maybe it's an argument in favor of a more penny-counting reputation - approach. - - Addendum: I was more thinking of measuring based on who is the service - provider and service receiver for the circuit. Say Alice builds a - circuit to Bob. Then Bob is providing service to Alice, since he - otherwise wouldn't need to spend his bandwidth. So traffic in either - direction should be charged to Alice. Of course, the same attack would - work, namely, Bob could cheat by sending bytes back quickly. So someone - close to the origin needs to detect this and close the circuit, if - necessary. -JN - -3.7. What is the appropriate resource balance for servers vs. clients? - - If we build a good incentive system, we'll still need to tune it - to provide the right bandwidth allocation -- if we reserve too much - bandwidth for fast servers, then we're wasting some potential, but - if we reserve too little, then fewer people will opt to become servers. - In fact, finding an optimum balance is especially hard because it's - a moving target: the better our incentive mechanism (and the lower - the barrier to setup), the more servers there will be. How do we find - the right balance? - - One answer is that it doesn't have to be perfect: we can err on the - side of providing extra resources to servers. Then we will achieve our - desired goal -- when people complain about speed, we can tell them to - run a server, and they will in fact get better performance. - -3.8. Anonymity attack: fast connections probably come from good servers. - - If only fast servers can consistently get good performance in the - network, they will stand out. "Oh, that connection probably came from - one of the top ten servers in the network." Intersection attacks over - time can improve the certainty of the attack. - - I'm not too worried about this. First, in periods of low activity, - many different people might be getting good performance. This dirties - the intersection attack. Second, with many of these schemes, we will - still be uncertain whether the fast node originated the traffic, or - was the entry node for some other lucky user -- and we already accept - this level of attack in other cases such as the Murdoch-Danezis attack - [http://freehaven.net/anonbib/#torta05]. - -3.9. How do we allocate bandwidth over the course of a second? - - This may be a simple matter of engineering, but it still needs to be - addressed. Our current token bucket design refills each bucket once a - second. If we have N tokens in our bucket, and we don't know ahead of - time how many connections are going to want to send out how many bytes, - how do we balance providing quick service to the traffic that is - already here compared to providing service to potential high-importance - future traffic? - - If we have only two classes of service, here is a simple design: - At each point, when we are 1/t through the second, the total number - of non-priority bytes we are willing to send out is N/t. Thus if N - priority bytes are waiting at the beginning of the second, we drain - our whole bucket then, and otherwise we provide some delayed service - to the non-priority bytes. - - Does this design expand to cover the case of three priority classes? - Ideally we'd give each remote server its own priority number. Or - hopefully there's an easy design in the literature to point to -- - this is clearly not my field. - - Is our current flow control mechanism (each circuit and each stream - start out with a certain window, and once they've exhausted it they - need to receive an ack before they can send more) going to have - problems with this new design now that we'll be queueing more bytes - for less preferred nodes? If it turns out we do, the first fix is - to have the windows start out at zero rather than start out full -- - it will slow down the startup phase but protect us better. - - While we have outgoing cells queued for a given server, we have the - option of reordering them based on the priority of the previous hop. - Is this going to turn out to be useful? If we're the exit node (that - is, there is no previous hop) what priority do those cells get? - - Should we do this prioritizing just for sending out bytes (as I've - described here) or would it help to do it also for receiving bytes? - See next section. - -3.10. Different-priority cells arriving on the same TCP connection. - - In some of the proposed designs, servers want to give specific circuits - priority rather than having all circuits from them get the same class - of service. - - Since Tor uses TCP's flow control for rate limiting, this constraints - our design choices -- it is easy to give different TCP connections - different priorities, but it is hard to give different cells on the - same connection priority, because you have to read them to know what - priority they're supposed to get. - - There are several possible solutions though. First is that we rely on - the sender to reorder them so the highest priority cells (circuits) are - more often first. Second is that if we open two TCP connections -- one - for the high-priority cells, and one for the low-priority cells. (But - this prevents us from changing the priority of a circuit because - we would need to migrate it from one connection to the other.) A - third approach is to remember which connections have recently sent - us high-priority cells, and preferentially read from those connections. - - Hopefully we can get away with not solving this section at all. But if - necessary, we can consult Ed Knightly, a Professor at Rice - [http://www.ece.rice.edu/~knightly/], for his extensive experience on - networking QoS. - -3.11. Global reputation system: Congestion on high reputation servers? - - If the notion of reputation is global (as in 2.3 or 2.4), circuits that - go through successive high reputation servers would be the fastest and - most reliable. This would incentivize everyone, regardless of their own - reputation, to choose only the highest reputation servers in its - circuits, causing an over-congestion on those servers. - - One could argue, though, that once those servers are over-congested, - their bandwidth per circuit drops, which would in turn lower their - reputation in the future. A question is whether this would overall - stabilize. - - Another possible way is to keep a cap on reputation. In this way, a - fraction of servers would have the same high reputation, thus balancing - such load. - -3.12. Another anonymity attack: learning from service levels. - - If reputation is local, it may be possible for an evil node to learn - the identity of the origin through provision of differential service. - For instance, the evil node provides crappy bandwidth to everyone, - until it finds a circuit that it wants to trace the origin, then it - provides good bandwidth. Now, as only those directly or indirectly - observing this circuit would like the evil node, it can test each node - by building a circuit via each node to another evil node. If the - bandwidth is high, it is (somewhat) likely that the node was a part of - the circuit. - - This problem does not exist if the reputation is global and nodes only - follow the global reputation, i.e., completely ignore their own view. - -3.13. DoS through high priority traffic. - - Assume there is an evil node with high reputation (or high value on - Alice) and this evil node wants to deny the service to Alice. What it - needs to do is to send a lot of traffic to Alice. To Alice, all traffic - from this evil node is of high priority. If the choice of circuits are - too based toward high priority circuits, Alice would spend most of her - available bandwidth on this circuit, thus providing poor bandwidth to - everyone else. Everyone else would start to dislike Alice, making it - even harder for her to forward other nodes' traffic. This could cause - Alice to have a low reputation, and the only high bandwidth circuit - Alice could use would be via the evil node. - -3.14. If you run a fast server, can you run your client elsewhere? - - A lot of people want to run a fast server at a colocation facility, - and then reap the rewards using their cablemodem or DSL Tor client. - - If we use anonymous micropayments, where reputation can literally - be transferred, this is trivial. - - If we pick a design where servers accrue reputation and can only - use it themselves, though, the clients can configure the servers as - their entry nodes and "inherit" their reputation. In this approach - we would let servers configure a set of IP addresses or keys that get - "like local" service. - -4. Sample designs. - -4.1. Two classes of service for circuits. - - Whenever a circuit is built, it is specified by the origin which class, - either "premium" or "normal", this circuit belongs. A premium circuit - gets preferred treatment at each node. A node "spends" its value, which - it earned a priori by providing service, to the next node by sending - and receiving bytes. Once a node has overspent its values, the circuit - cannot stay as premium. It either breaks or converts into a normal - circuit. Each node also reserves a small portion of bandwidth for - normal circuits to prevent starvation. - - Pro: Even if a node has no value to spend, it can still use normal - circuits. This allow casual user to use Tor without forcing them to run - a server. - - Pro: Nodes have incentive to forward traffic as quick and as much as - possible to accumulate value. - - Con: There is no proactive method for a node to rebalance its debt. It - has to wait until there happens to be a circuit in the opposite - direction. - - Con: A node needs to build circuits in such a way that each node in the - circuit has to have good values to the next node. This requires - non-local knowledge and makes circuits less reliable as the values are - used up in the circuit. - - Con: May discourage nodes to forward traffic in some circuits, as they - worry about spending more useful values to get less useful values in - return. - -4.2. Treat all the traffic from the node with the same service; - hard reputation system. - - This design is similar to 4.1, except that instead of having two - classes of circuits, there is only one. All the circuits are - prioritized based on the value of the interacting node. - - Pro: It is simpler to design and give priority based on connections, - not circuits. - - Con: A node only needs to keep a few guard nodes happy to forward their - traffic. - - Con: Same as in 4.1, may discourage nodes to forward traffic in some - circuits, as they worry about spending more useful values to get less - useful values in return. - -4.3. Treat all the traffic from the node with the same service; - soft reputation system. - - Rather than a guaranteed system with accounting (as 4.1 and 4.2), - we instead try for a best-effort system. All bytes are in the same - class of service. You keep track of other Tors by key, and give them - service proportional to the service they have given you. That is, in - the past when you have tried to push bytes through them, you track the - number of bytes and the average bandwidth, and use that to weight the - priority of their connections if they try to push bytes through you. - - Now you're going to get minimum service if you don't ever push bytes - for other people, and you get increasingly improved service the more - active you are. We should have memories fade over time (we'll have - to tune that, which could be quite hard). - - Pro: Sybil attacks are pointless because new identities get lowest - priority. - - Pro: Smoothly handles periods of both low and high network load. Rather - than keeping track of the ratio/difference between what he's done for - you and what you've done for him, simply keep track of what he's done - for you, and give him priority based on that. - - Based on 3.3 above, it seems we should reward all the nodes in our - path, not just the first one -- otherwise the node can provide good - service only to its guards. On the other hand, there might be a - second-order effect where you want nodes to like you so that *when* - your guards choose you for a circuit, they'll be able to get good - performance. This tradeoff needs more simulation/analysis. - - This approach focuses on incenting people to relay traffic, but it - doesn't do much for incenting them to allow exits. It may help in - one way through: if there are few exits, then they will attract a - lot of use, so lots of people will like them, so when they try to - use the network they will find their first hop to be particularly - pleasant. After that they're like the rest of the world though. (An - alternative would be to reward exit nodes with higher values. At the - extreme, we could even ask the directory servers to suggest the extra - values, based on the current availability of exit nodes.) - - Pro: this is a pretty easy design to add; and it can be phased in - incrementally simply by having new nodes behave differently. - -4.4. Centralized opinions from the reputation servers. - - Have a set of official measurers who spot-check servers from the - directory to see if they really do offer roughly the bandwidth - they advertise. Include these observations in the directory. (For - simplicity, the directory servers could be the measurers.) Then Tor - servers give priority to other servers. We'd like to weight the - priority by advertised bandwidth to encourage people to donate more, - but it seems hard to distinguish between a slow server and a busy - server. - - The spot-checking can be done anonymously to prevent selectively - performing only for the measurers, because hey, we have an anonymity - network. - - We could also reward exit nodes by giving them better priority, but - like above this only will affect their first hop. Another problem - is that it's darn hard to spot-check whether a server allows exits - to all the pieces of the Internet that it claims to. If necessary, - perhaps this can be solved by a distributed reporting mechanism, - where clients that can reach a site from one exit but not another - anonymously submit that site to the measurers, who verify. - - A last problem is that since directory servers will be doing their - tests directly (easy to detect) or indirectly (through other Tor - servers), then we know that we can get away with poor performance for - people that aren't listed in the directory. Maybe we can turn this - around and call it a feature though -- another reason to get listed - in the directory. - -5. Recommendations and next steps. - -5.1. Simulation. - - For simulation trace, we can use two: one is what we obtained from Tor - and one from existing web traces. - - We want to simulate all the four cases in 4.1-4. For 4.4, we may want - to look at two variations: (1) the directory servers check the - bandwidth themselves through Tor; (2) each node reports their perceived - values on other nodes, while the directory servers use EigenTrust to - compute global reputation and broadcast those. - -5.2. Deploying into existing Tor network. - |