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another rev of dir-spec.txt

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svn:r3398
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Roger Dingledine
2005-01-21 08:16:02 +00:00
parent 4826718eac
commit d4d131cc83
1 changed files with 139 additions and 58 deletions
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doc/dir-spec.txt
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@@ -20,89 +20,160 @@ In particular:
1a. We want to let clients learn about new servers from anywhere
and build circuits through them if they wish. This means that
Tor nodes need to be able to Extend to nodes they don't already
know about. This is already implemented, but see the 'Extend policy'
issue below.
know about.
1b. We want to provide a robust (available) and not-too-centralized
1b. We want to let servers limit the addresses and ports they're
willing to extend to. This is necessary e.g. for middleman nodes
who have jerks trying to extend from them to badmafia.com:80 all
day long and it's drawing attention.
1b'. While we're at it, we also want to handle servers that *can't*
extend to some addresses/ports, e.g. because they're behind NAT or
otherwise firewalled. (See section 5 below.)
1c. We want to provide a robust (available) and not-too-centralized
mechanism for tracking network status (which nodes are up and working)
and admission (which nodes are "recommended" for certain uses).
1c. [optional] We want to permit servers that can't route to all other
servers, e.g. because they're behind NAT or otherwise firewalled.*
2. Assumptions.
People get the code from us, and they trust us (or our gpg keys, or
something down the trust chain that's equivalent).
2a. People get the code from us, and they trust us (or our gpg keys, or
something down the trust chain that's equivalent).
Even if the software allows humans to change the client configuration,
most of them will use the default that's provided, so we should provide
one that is the right balance of robust and safe.
2b. Even if the software allows humans to change the client configuration,
most of them will use the default that's provided. so we should
provide one that is the right balance of robust and safe. That is,
we need to hard-code enough "first introduction" locations that new
clients will always have an available way to get connected.
Assume that Sybil attackers can produce only a limited number of
independent-looking nodes.
2c. Assume that the current "ask them to email us and see if it seems
suspiciously related to previous emails" approach will not catch
the strong Sybil attackers. Therefore, assume the Sybil attackers
we do want to defend against can produce only a limited number of
not-obviously-on-the-same-subnet nodes.
Roger has only a limited amount of time for approving nodes, and doesn't
want to be the time bottleneck anyway.
2d. Roger has only a limited amount of time for approving nodes; shouldn't
be the time bottleneck anyway; and is doing a poor job at keeping
out some adversaries.
We can trust servers to accurately report their characteristics (uptime,
capacity, exit policies, etc), as long as we have some mechanism for
notifying clients when we notice that they're lying.
2e. Some people would be willing to offer servers but will be put off
by the need to send us mail and identify themselves.
2e'. Some evil people will avoid doing evil things based on the perception
(however true or false) that there are humans monitoring the network
and discouraging evil behavior.
2e''. Some people will trust the network, and the code, more if they
have the perception that there are trustworthy humans guiding the
deployed network.
There exists a "main" core Internet in which most locations can access
most locations. We'll focus on it first.
2f. We can trust servers to accurately report their characteristics
(uptime, capacity, exit policies, etc), as long as we have some
mechanism for notifying clients when we notice that they're lying.
2g. There exists a "main" core Internet in which most locations can access
most locations. We'll focus on it (first).
3. Some notes on how to achieve.
We ship with S (e.g. 3) seed keys.
We ship with N (e.g. 20) introducer locations and fingerprints.
We ship with some set of signed timestamped certs for those introducers.
Piece one: (required)
Introducers serve signed network-status pages, listing their opinions
of network status and which routers are good.
We ship with N (e.g. 20) directory server locations and fingerprints.
They also serve descriptors in some way. These don't need to be signed by
the introducers, since they're self-signed and timestamped by each server.
Directory servers serve signed network-status pages, listing their
opinions of network status and which routers are good (see 4a below).
A DHT is not so appropriate for distributing server descriptors as long
as we expect each client to plan to collect all of them periodically. It
would seem that each introducer might as well just keep its own
big pile of descriptors, and they synchronize (pull) from each other
periodically. Clients then get network-status pages from a threshold of
introducers, fetch enough of the server descriptors to make them happy,
and proceed as now. Anything wrong with this?
Dirservers collect and provide server descriptors as well. These don't
need to be signed by the dirservers, since they're self-certifying
and timestamped.
Notice that this doesn't preclude other approaches to discovering
different concurrent Tor networks. For example, a Tor network inside
China could ship Tor with a different torrc and poof, they're using
a different set of seed keys and a different set of introducers. Some
smarter clients could be made to learn about both networks, and be told
which nodes bridge the networks.
(In theory the dirservers don't need to be the ones serving the
descriptors, but in practice the dirservers would need to point people
at the place that does, so for simplicity let's assume that they do.)
Clients then get network-status pages from a threshold of dirservers,
fetch enough of the corresponding server descriptors to make them happy,
and proceed as now.
Piece two: (optional)
We ship with S (e.g. 3) seed keys (trust anchors), and ship with
signed timestamped certs for each dirserver. Dirservers also serve a
list of certs, maybe including a "publish all certs since time foo"
functionality. If at least two seeds agree about something, then it
is so.
Now dirservers can be added, and revoked, without requiring users to
upgrade to a new version. If we only ship with dirserver locations
and not fingerprints, it also means that dirservers can rotate their
signing keys transparently.
But, keeping track of the seed keys becomes a critical security issue;
and rotating them in a backward-compatible way adds complexity.
Piece three: (optional)
Notice that this doesn't preclude other approaches to discovering
different concurrent Tor networks. For example, a Tor network inside
China could ship Tor with a different torrc and poof, they're using
a different set of dirservers. Some smarter clients could be made to
learn about both networks, and be told which nodes bridge the networks.
...
4. Unresolved issues.
4a. How do the dirservers decide whether to recommend a server? We
could have them do it based on contact from the human, but by
assumptions 2c and 2d above, that's going to be less effective, and
more of a hassle, as we scale up. Thus I propose that they simply
do some basic automatic measuring themselves, starting with the
current "are they connected to me" measurement, and that's all
that is done.
We could blacklist as we notice evil servers, but then we're in
the same boat all the irc networks are in. We could whitelist as we
notice new servers, and stop whitelisting (maybe rolling back a bit)
once an attack is in progress. If we assume humans aren't particularly
good at this anyway, we could just do automated delayed whitelisting,
and have a "you're under attack" switch the human can enable for a
while to start acting more conservatively.
Once upon a time we collected contact info for servers, which was
mainly used to remind people that their servers are down and could
they please restart. Now that we have a critical mass of servers,
I've stopped doing that reminding. So contact info is less important.
4b. What do we do about recommended-versions? Do we need a threshold of
dirservers to claim that your version is obsolete before you believe
them? Or do we make it have less effect -- e.g. print a warning but
never actually quit? Coordinating all the humans to upgrade their
recommended-version strings at once seems bad. Maybe if we have
seeds, the seeds can sign a recommended-version and upload it to
the dirservers.
4c. What does it mean to bind a nickname to a key? What if each dirserver
does it differently, so one nickname corresponds to several keys?
Maybe the solution is that nickname<=>key bindings should be
individually configured by clients in their torrc (if they want to
refer to nicknames in their torrc), and we stop thinking of nicknames
as globally unique.
4d. What new features need to be added to server descriptors so they
remain compact yet support new functionality? Section 5 is a start
of discussion of one answer to this.
4. Unresolved:
- What new features need to be added to server descriptors so they
remain compact yet support new functionality?
- How do we compactly describe seeds, introducers, and certs? Does
Tor have built-in defaults still, that can be overridden?
- How much cert functionality do we want in our PKI? Can we revoke
introducers, or is that done by releasing a new version of the code?
- By what mechanism will new servers contact the humans who run
introducers, so they can be approved?
- Is our network growing because of peoples' trust in Roger? Will it
grow the same way, or as robustly, or more robustly, with no
figurehead?
- 'Extend policies' -- middleman doesn't really mean middleman, alas.
----------
(*) Regarding "Blossom: an unstructured overlay network for end-to-end
5. Regarding "Blossom: an unstructured overlay network for end-to-end
connectivity."
In this section we address possible solutions to the problem of how to allow
Tor routers in different transport domains to communicate.
[Can we have a one-sentence definition of transport domain here? If there
are 5 servers on the Internet as we know it and suddenly one link between
a pair of them catches fire, how many transport domains are involved now?
What if one link is down permanently but the rest work? Is "in the same
transport domain as" a symmetric property?]
First, we presume that for every interface between transport domains A and B,
one Tor router T_A exists in transport domain A, one Tor router T_B exists in
transport domain B, and (without loss of generality) T_A can open a persistent
@@ -122,7 +193,12 @@ As in regular Tor, each Blossom router pushes its descriptor to directory
servers. These directory servers can be within the same transport domain, but
they need not be. The trick is that if a directory server is in another
transport domain, then that directory server must know through which Tor
routers to send messages destined for the Tor router in question. Descriptors
routers to send messages destined for the Tor router in question.
[We are assuming that routers in the non-primary transport domain (the
primary one being the one with dirservers) know how to get to the primary
transport domain, either through Tor or other voodoo, to publish to the
hard-coded dirservers.]
Descriptors
for Blossom routers held by the directory server must contain a special field
for specifying a path through the overlay (i.e. an ordered list of router
names/IDs) to a router in a foreign transport domain. (This field may be a set
@@ -144,6 +220,7 @@ appended to each path. If the new router used approach (b), then the directory
server will define the same path(s) in the descriptors for the routers
specified in the list. The directory server will then insert the newly defined
path into the descriptor from the router.
[Dirservers can't modify server descriptors; they're self-certifying. -RD]
If all directory servers are within the same transport domain, then the problem
is solved: routers can exist within multiple transport domains, and as long as
@@ -173,3 +250,7 @@ to that used by BGP (may be a simple distance-vector route selection procedure
that only takes into account path length, or may be more complex to avoid
loops, cache results, etc.) in order to choose the best one.
[How does this work with exit policies (how do we enumerate all resources
in our transport domain?), and translating resources that we want to
get to to servers that can reach them?]