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[tor-commits] [tor/release-0.2.4] Remove incentives.txt from tor git; putting it into torspec.
commit 18da1e0cf268bb56adc1a45fa4877e6a1bd2b470
Author: Nick Mathewson <nickm@xxxxxxxxxxxxxx>
Date: Fri Mar 15 11:25:45 2013 -0400
Remove incentives.txt from tor git; putting it into torspec.
---
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diff --git a/doc/contrib/incentives.txt b/doc/contrib/incentives.txt
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-
- 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.
-
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