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tor/doc/dir-spec.txt
Nick Mathewson 2f53d86707 remove historical stuff from dir-spec.txt 
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2005-09-27 16:37:11 +00:00

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$Id$
Tor directory protocol for 0.1.1.x series
0. Scope and preliminaries
This document should eventually be merged into tor-spec.txt and replace
the existing notes on directories.
This is not a finalized version; what we actually wind up implementing
may be very different from the system described here.
0.1. Goals
There are several problems with the way Tor handles directories right
now:
1. Directories are very large and use a lot of bandwidth.
2. Every directory server is a single point of failure.
3. Requiring every client to know every server won't scale.
4. Requiring every directory cache to know every server won't scale.
5. Our current "verified server" system is kind of nonsensical.
6. Getting more directory servers adds more points of failure and
worsens possible partitioning attacks.
This design tries to solve every problem except problems 3 and 4, and to
be compatible with likely eventual solutions to problems 3 and 4.
1. Outline
There is no longer any such thing as a "signed directory". Instead,
directory servers sign a very compressed 'network status' object that
lists the current descriptors and their status, and router descriptors
continue to be self-signed by servers. Clients download network status
listings periodically, and download router descriptors as needed. ORs
upload descriptors relatively infrequently.
There are multiple directory servers. Rather than doing anything
complicated to coordinate themselves, clients simply rotate through them
in order, and only use servers that most of the last several directory
servers like.
2. Router descriptors
The router descriptor format is unchanged from tor-spec.txt.
ORs SHOULD generate a new router descriptor whenever any of the
following events have occurred:
- A period of time (18 hrs by default) has passed since the last
time a descriptor was generated.
- A descriptor field other than bandwidth or uptime has changed.
- Bandwidth has changed by more than +/- 50% from the last time a
descriptor was generated, and at least a given interval of time
(20 mins by default) has passed since then.
- Uptime has been reset.
After generating a descriptor, ORs upload it to every directory
server they know.
3. Network status
Directory servers generate, sign, and compress a network-status document
as needed. As an optimization, they may rate-limit the number of such
documents generated to once every few seconds. Directory servers should
rate-limit at least to the point where these documents are generated no
faster than once per second.
The network status document contains a preamble, a set of router status
entries, and a signature, in that order.
We use the same meta-format as used for directories and router descriptors
in "tor-spec.txt".
The preamble contains:
"network-status-version" -- A document format version. For this
specification, the version is "2".
"dir-source" -- The hostname, current IP address, and directory
port of the directory server, separated by spaces.
"fingerprint" -- A base16-encoded hash of the signing key's
fingerprint, with no additional spaces added.
"contact" -- An arbitrary string describing how to contact the
directory server's administrator. Administrators should include at
least an email address and a PGP fingerprint.
"dir-signing-key" -- The directory server's public signing key.
"client-versions" -- A comma-separated list of recommended client versions.
"server-versions" -- A comma-separated list of recommended server versions.
"published" -- The publication time for this network-status object.
"dir-options" -- A set of flags separated by spaces:
"Names" if this directory server performs name bindings.
"Versions" if this directory server recommends software versions.
The dir-options entry is optional. The "-versions" entries are required if
the "Versions" flag is present. The other entries are required and must
appear exactly once. The "network-status-version" entry must appear first;
the others may appear in any order.
For each router, the router entry contains: (This format is designed for
conciseness.)
"r" -- followed by the following elements, separated by spaces:
- The OR's nickname,
- A hash of its identity key, encoded in base64, with trailing =
signs removed.
- A hash of its most recent descriptor, encoded in base64, with
trailing = signs removed. (The hash is calculated as for
computing the signature of a descriptor.)
- The publication time of its most recent descriptor.
- An IP
- An OR port
- A directory port (or "0" for none")
"s" -- A series of space-separated status flags:
"Exit" if the router is useful for building general-purpose exit
circuits.
"Stable" if the router tends to stay up for a long time.
"Fast" if the router has high bandwidth.
"Running" if the router is currently usable.
"Named" if the router's identity-nickname mapping is canonical.
"Valid" if the router has been 'validated'.
"Authority" if the router is a directory authority.
The "r" entry for each router must appear first and is required. The
's" entry is optional. Unrecognized flags, or extra elements on the
"r" line must be ignored.
The signature section contains:
"directory-signature". A signature of the rest of the document using
the directory server's signing key.
We compress the network status list with zlib before transmitting it.
4. Directory server operation
By default, directory servers remember all non-expired, non-superseded OR
descriptors that they have seen.
For each OR, a directory server remembers whether the OR was running and
functional the last time they tried to connect to it, and possibly other
liveness information.
Directory server administrators may label some servers or IPs as
blacklisted, and elect not to include them in their network-status lists.
Thus, the network-status list includes all non-blacklisted,
non-expired, non-superseded descriptors for ORs that the directory has
observed at least once to be running.
Directory server administrators may decide to support name binding. If
they do, then they must maintain a file of nickname-to-identity-key
mappings, and try to keep this file consistent with other directory
servers. If they don't, they act as clients, and report bindings made by
other directory servers (name X is bound to identity Y if at least one
binding directory lists it, and no directory binds X to some other Y'.)
The authoritative network-status published by a host should be available at:
http://<hostname>/tor/status/authority.z
An authoritative network-status published by another host with fingerprint
<F> should be available at:
http://<hostname>/tor/status/fp/<F>.z
An authoritative network-status published by other hosts with fingerprints
<F1>,<F2>,<F3> should be available at:
http://<hostname>/tor/status/fp/<F1>+<F2>+<F3>.z
The most recent network-status documents from all known authoritative
directories, concatenated, should be available at:
http://<hostname>/tor/status/all.z
The most recent descriptor for a server whose identity key has a
fingerprint of <F> should be available at:
http://<hostname>/tor/server/fp/<F>.z
The most recent descriptors for servers have fingerprints <F1>,<F2>,<F3>
should be available at:
http://<hostname>/tor/server/fp/<F1>+<F2>+<F3>.z
The most recent descriptor for this server should be at:
http://<hostname>/tor/server/authority.z
A concatenated set of the most recent descriptors for all known servers
should be available at:
http://<hostname>/tor/server/all.z
For debugging, directories MAY expose non-compressed objects at URLs like
the above, but without the final ".z".
Clients MUST handle compressed concatenated information in two forms:
- A concatenated list of zlib-compressed objects.
- A zlib-compressed concatenated list of objects.
Directory servers MAY generate either format: the former requires less
CPU, but the latter requires less bandwidth.
4.1. Caching
Directory caches (most ORs) regularly download network status documents,
and republish them at a URL based on the directory server's identity key:
http://<hostname>/tor/status/<identity fingerprint>.z
A concatenated list of all network-status documents should be available at:
http://<hostname>/tor/status/all.z
4.2. Compression
5. Client operation
Every OP or OR, including directory servers, acts as a client to the
directory protocol.
Each client maintains a list of trusted directory servers. Periodically
(currently every 20 minutes), the client downloads a new network status. It
chooses the directory server from which its current information is most
out-of-date, and retries on failure until it finds a running server.
When choosing ORs to build circuits, clients proceed as follows:
- A server is "listed" if it is listed by more than half of the "live"
network status documents the clients have downloaded. (A network
status is "live" if it is the most recently downloaded network status
document for a given directory server, and the server is a directory
server trusted by the client, and the network-status document is no
more than D (say, 10) days old.)
- A server is "valid" is it is listed as valid by more than half of the
"live" downloaded" network-status document.
- A server is "running" if it is listed as running by more than
half of the "recent" downloaded network-status documents.
(A network status is "recent" if it was published in the last
60 minutes. If there are fewer than 3 such documents, the most
recently published 3 are "recent." If there are fewer than 3 in all,
all are "recent.")
Clients store network status documents so long as they are live.
5.1. Scheduling network status downloads
This download scheduling algorithm implements the approach described above
in a relatively low-state fashion. It reflects the current Tor
implementation.
Clients maintain a list of authorities; each client tries to keep the same
list, in the same order.
Periodically, on startup, and on HUP, clients check whether they need to
download fresh network status documents. The approach is as follows:
- If we have under X network status documents newer than OLD, we choose a
member of the list at random and try download XX documents starting
with that member's.
- Otherwise, if we have no network status documents newer than NEW, we
check to see which authority's document we retrieved most recently,
and try to retrieve the next authority's document. If we can't, we
try the next authority in sequence, and so on.
5.2. Managing naming
In order to provide human-memorable names for individual server
identities, some directory servers bind names to IDs. Clients handle
names in two ways:
If a client is encountering a name it has not mapped before:
If all the "binding" networks-status documents the client has so far
received same claim that the name binds to some identity X, and the
client has received at least three network-status documents, the client
maps the name to X.
If a client is encountering a name it has mapped before:
It uses the last-mapped identity value, unless all of the "binding"
network status documents bind the name to some other identity.
5.3. Notes on what we do now.
THIS SECTION SHOULD BE FOLDED INTO THE EARLIER SECTIONS; THEY ARE WRONG;
THIS IS RIGHT.
All downloaded networkstatuses are discarded once they are 10 days old (by
published date).
Authdirs download each others' networkstatus every
AUTHORITY_NS_CACHE_INTERVAL minutes (currently 10).
Directory caches download authorities' networkstatus every
NONAUTHORITY_NS_CACHE_INTERVAL minutes (currently 10).
Clients always try to replace any networkstatus received over
NETWORKSTATUS_MAX_VALIDITY ago (currently 2 days). Also, when the most
recently received networkstatus is more than
NETWORKSTATUS_CLIENT_DL_INTERVAL (30 minutes) old, and we do not have any
open directory connections fetching a networkstatus, clients try to
download the networkstatus on their list after the most recently received
networkstatus, skipping failed networkstatuses. A networkstatus is
"failed" if NETWORKSTATUS_N_ALLOWABLE_FAILURES (3) attempts in a row have
all failed.
We do not update router statuses if we have less than half of the
networkstatuses.
A networkstatus is "live" if it is the most recent we have received signed
by a given trusted authority.
A networkstatus is "recent" if it is "live" and:
- it was received in the last DEFAULT_RUNNING_INTERVAL (currently 60
minutes)
OR - it was one of the MIN_TO_INFLUENCE_RUNNING (3) most recently received
networkstatuses.
Authorities always believe their own opinion as to a router's status. For
other tors:
- a router is valid if more than half of the live networkstatuses think
it's valid.
- a router is named if more than half of the live networkstatuses from
naming authorities think it's named, and they all think it has the
same name.
- a router is running if more than half of the recent networkstatuses
think it's running.
Everyone downloads router descriptors as follows:
- If any networkstatus lists a more recently published routerdesc with a
different descriptor digest, and no more than
MAX_ROUTERDESC_DOWNLOAD_FAILURES attempts to retrieve that routerdesc
have failed, then that routerdesc is "downloadable".
- Every DirFetchInterval, or whenever a request for routerdescs returns
no routerdescs, we launch a set of requests for all downloadable
routerdescs. We divide the downloadable routerdescs into groups of no
more than DL_PER_REQUEST, and send a request for each group to
directory servers chosen independently.
- We also launch a request as above when a request for routerdescs
fails and we have no directory connections fetching routerdescs.
TODO Specify here:
- When to 0-out failure count for networkstatus?
- Drop fallback to download-all. Also, always split download.
- For versions: if you're listed by more than half of live versioning
networkstatuses, good. if less than half of networkstatuses are live,
don't do anything. If half are live, and half of less of the
versioning ones list you, warn. Only warn once every 24 hours.
- For names: warn if an unnamed router is specified by nickname.
Rate-limit these warnings.
- Also, don't believe N->K if another naming authdir says N->K'.
- Revise naming rule: N->K is true if any naming directory says N->K,
and no other naming directory says N->K' or N'->K.
- Minimum info to build circuits.
- Revise: always split requests when we have too little info to build
circuits.
- Describe when router is "out of date". (Any dirserver says so.)
- Change rule from "do not launch new connections when one exists" to
"do not request any fingerprint that we're currently requesting."
- Launch new connections every minute, plus whenever a download fails.
- Reset routerdesc failure count after 60 minutes, or when
when network comes back on after absence.
- Make "I didn't get the one I thought was most recent" a failure.
- Retry these every 5 minutes if you're a client.
- Mirrors should retry these harder and more often.
- If we have a routerdesc for Bob, and he says, "I'm 0.1.0.x", don't
fetch a new one if it was published in the last 2 hours. (??)
6. Remaining issues
Client-knowledge partitioning is worrisome. Most versions of this don't
seem to be worse than the Danezis-Murdoch tracing attack, since an
attacker can't do more than deduce probable exits from entries (or vice
versa). But what about when the client connects to A and B but in a
different order? How bad can it be partitioned based on its knowledge?
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