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clean up part of the incentives discussion.

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much work still remains.


svn:r5983
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Roger Dingledine 2006-02-12 10:34:31 +00:00
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doc/incentives.txt
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@ -21,6 +21,10 @@
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 in 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
@ -53,7 +57,7 @@
3. Related issues we need to keep in mind.
3.1. Relay and exit needs to be easy and usable.
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
@ -62,7 +66,7 @@
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 need to face the big question: is
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?
@ -70,11 +74,12 @@
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 in 90% of interactions
you're acting for the first time, the "local" incentive schemes above
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.
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
@ -82,7 +87,7 @@
"guard nodes" for their first hop of each circuit. This seems to 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 both use the other as guard nodes.
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
@ -98,7 +103,7 @@
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're going to partition the network such that all
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
@ -114,7 +119,11 @@
A special case here is the social network, where the network isn't
partitioned randomly but instead based on some external properties.
More on this later.
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 only their friends are relaying through them. 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.
@ -136,17 +145,20 @@
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 many transactions in Tor involve fetching lots of
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 can _send_
us bytes, and then only send it bytes in proportion to that.
There's an obvious attack though: a sneaky user could simply connect
to a node and send some traffic through it. Voila, he has performed
for the network. This is no good. The first fix is that we only count
if you're sending 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.
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
@ -158,22 +170,52 @@
to provide the right bandwidth allocation -- if we reserve too much
bandwidth for fast servers, then we're wasting some potential, but we
if we reserve too little, then fewer people will opt to become servers.
How do we find the right balance?
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. 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.
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 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 accept is N/t. Thus if N
priority bytes arrive 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.
4. Sample designs.
@ -232,7 +274,8 @@
servers weight priority for other servers depending on advertised
bandwidth, giving particularly low priority to connections not
listed or that failed their spot-checks. The spot-checking can be
done anonymously, because hey, we have an anonymity network.
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
@ -241,7 +284,9 @@
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.
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.