tor/doc/dir-spec.txt

$Id$

                      Tor network discovery protocol

0. Scope

This document proposes a way of doing more distributed network discovery
while maintaining some amount of admission control. We don't recommend
you implement this as-is; it needs more discussion.

Terminology:
  - Client: The Tor component that chooses paths.
  - Server: A relay node that passes traffic along.

1. Goals.

We want more decentralized discovery for network topology and status.
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.

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).

2. Assumptions.

2a. People get the code from us, and they trust us (or our gpg keys, or
    something down the trust chain that's equivalent).

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.

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.

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.

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.

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.

Piece one: (required)

  We ship with N (e.g. 20) directory server locations and fingerprints.

  Directory servers serve signed network-status pages, listing their
  opinions of network status and which routers are good (see 4a below).

  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.

  (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. Also,
  dirserver locations must be at least somewhere static, since each lost
  dirserver degrades reachability for old clients. So as the dirserver
  list rolls over we have no choice but to put out new versions.


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.



5. Regarding "Blossom: an unstructured overlay network for end-to-end
connectivity."

Define "transport domain" as a set of nodes who can all mutually name each
other directly, using transport-layer (e.g. HOST:PORT) naming.

Define "clique" as a set of nodes who can all mutually contact each other directly,
using transport-layer (e.g. HOST:PORT) naming.

Neither transport domains and cliques form a partition of the set of all nodes.
Just as cliques may overlap in theoretical graphs, transport domains and
cliques may overlap in the context of Blossom.

In this section we address possible solutions to the problem of how to allow
Tor routers in different transport domains to communicate.

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
connection to T_B.  Any Tor traffic between the two routers will occur over
this connection, which effectively renders the routers equal partners in
bridging between the two transport domains.  We refer to the established link
between two transport domains as a "bridge" (we use this term because there is
no serious possibility of confusion with the notion of a layer 2 bridge).

Next, suppose that the universe consists of transport domains connected by
persistent connections in this manner.  An individual router can open multiple
connections to routers within the same foreign transport domain, and it can
establish separate connections to routers within multiple foreign transport
domains.

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.

Blossom routers can advertise themselves to other transport domains in two
ways:

(1) Directly push the descriptor to a directory server in the other transport
domain.  This probably works particularly well if the other transport domain is
"the Internet", or if there are hard-coded directory servers in "the Internet".
The router has the responsibility to inform the directory server about which
routers can be used to reach it.

(2) Push the descriptor to a directory server in the same transport domain.
This is the easiest solution for the router, but it relies upon the existence
of a directory server in the same transport domain that is capable of
communicating with directory servers in the remote transport domain.  In order
for this to work, some individual Tor routers must have published their
descriptors in remote transport domains (i.e. followed the first option) in
order to provide a link by which directory servers can communiate
bidirectionally.

If all directory servers are within the same transport domain, then approach
(1) is sufficient: routers can exist within multiple transport domains, and as
long as the network of transport domains is fully connected by bridges, any
router will be able to access any other router in a foreign transport domain
simply by extending along the path specified by the directory server.  However,
we want the system to be truly decentralized, which means not electing any
particular transport domain to be the master domain in which entries are
published.

This is the explanation for (2): in order for a directory server to share
information with a directory server in a foreign transport domain to which it
cannot speak directly, it must use Tor, which means referring to the other
directory server by using a router in the foreign transport domain.  However,
in order to use Tor, it must be able to reach that router, which means that a
descriptor for that router must exist in its table, along with a means of
reaching it.  Therefore, in order for a mutual exchange of information between
routers in transport domain A and those in transport domain B to be possible,
when routers in transport domain A cannot establish direct connections with
routers in transport domain B, then some router in transport domain B must have
pushed its descriptor to a directory server in transport domain A, so that the
directory server in transport domain A can use that router to reach the
directory server in transport domain B.

Descriptors for Blossom routers are read-only, as for regular Tor routers, so
directory servers cannot modify them.  However, Tor directory servers also
publish a "network-status" page that provide information about which nodes are
up and which are not.  Directory servers could provide an additional field for
Blossom nodes.  For each Blossom node, the directory server specifies a set of
paths (may be only one) 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
of paths rather than a single path.)

A new router publishing to a directory server in a foreign transport should
include a list of routers.  This list should be either:

a. ...a list of routers to which the router has persistent connections, or, if
the new router does not have any persistent connections,

b. ...a (not necessarily exhaustive) list of fellow routers that are in the
same transport domain.

The directory server will be able to use this information to derive a path to
the new router, as follows.  If the new router used approach (a), then the
directory server will define the set of paths to the new router as union of the
set of paths to the routers on the list with the name of the last hop appended
to each path.  If the new router used approach (b), then the directory server
will define the paths to the new router as the union of the set of paths to the
routers specified in the list.  The directory server will then insert the newly
defined path into the field in the network-status page from the router.

When confronted with the choice of multiple different paths to reach the same
router, the Blossom nodes may use a route selection protocol similar in design
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.

If a .exit name is not provided, then a path will be chosen whose nodes are all
among the set of nodes provided by the directory server that are believed to be
in the same transport domain (i.e. no explicit path).  Thus, there should be no
surprises to the client.  All routers should be careful to define their exit
policies carefully, with the knowledge that clients from potentially any
transport domain could access that which is not explicitly restricted.