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tor/doc/spec/dir-spec-v2.txt
Nick Mathewson 9031bbd4d8 r13982@catbus: nickm | 2007-07-29 01:31:53 -0400 
Actually, we missed a rule about what routers to prefer: first prefer authority to non-authority, *then* running, *then* bandwidth.


svn:r10969
2007-07-29 05:32:03 +00:00

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$Id$
Tor directory protocol, version 2
0. Scope and preliminaries
This directory protocol is used by Tor version 0.1.1.x and 0.1.2.x. See
dir-spec-v1.txt for information on earlier versions, and dir-spec.txt
for information on later versions.
0.1. Goals and motivation
There were several problems with the way Tor handles directory information
in version 0.1.0.x and earlier. Here are the problems we try to fix with
this new design, already implemented in 0.1.1.x:
1. Directories were very large and use up a lot of bandwidth: clients
downloaded descriptors for all router several times an hour.
2. Every directory authority was a trust bottleneck: if a single
directory authority lied, it could make clients believe for a time an
arbitrarily distorted view of the Tor network.
3. Our current "verified server" system is kind of nonsensical.
4. Getting more directory authorities would add more points of failure
and worsen possible partitioning attacks.
There are two problems that remain unaddressed by this design.
5. Requiring every client to know about every router won't scale.
6. Requiring every directory cache to know every router won't scale.
We attempt to fix 1-4 here, and to build a solution that will work when we
figure out an answer for 5. We haven't thought at all about what to do
about 6.
1. Outline
There is a small set (say, around 10) of semi-trusted directory
authorities. A default list of authorities is shipped with the Tor
software. Users can change this list, but are encouraged not to do so, in
order to avoid partitioning attacks.
Routers periodically upload signed "descriptors" to the directory
authorities describing their keys, capabilities, and other information.
Routers may act as directory mirrors (also called "caches"), to reduce
load on the directory authorities. They announce this in their
descriptors.
Each directory authority periodically generates and signs a compact
"network status" document that lists that authority's view of the current
descriptors and status for known routers, but which does not include the
descriptors themselves.
Directory mirrors download, cache, and re-serve network-status documents
to clients.
Clients, directory mirrors, and directory authorities all use
network-status documents to find out when their list of routers is
out-of-date. If it is, they download any missing router descriptors.
Clients download missing descriptors from mirrors; mirrors and authorities
download from authorities. Descriptors are downloaded by the hash of the
descriptor, not by the server's identity key: this prevents servers from
attacking clients by giving them descriptors nobody else uses.
All directory information is uploaded and downloaded with HTTP.
Coordination among directory authorities is done client-side: clients
compute a vote-like algorithm among the network-status documents they
have, and base their decisions on the result.
1.1. What's different from 0.1.0.x?
Clients used to download a signed concatenated set of router descriptors
(called a "directory") from directory mirrors, regardless of which
descriptors had changed.
Between downloading directories, clients would download "network-status"
documents that would list which servers were supposed to running.
Clients would always believe the most recently published network-status
document they were served.
Routers used to upload fresh descriptors all the time, whether their keys
and other information had changed or not.
1.2. Document meta-format
Router descriptors, directories, and running-routers documents all obey the
following lightweight extensible information format.
The highest level object is a Document, which consists of one or more
Items. Every Item begins with a KeywordLine, followed by one or more
Objects. A KeywordLine begins with a Keyword, optionally followed by
whitespace and more non-newline characters, and ends with a newline. A
Keyword is a sequence of one or more characters in the set [A-Za-z0-9-].
An Object is a block of encoded data in pseudo-Open-PGP-style
armor. (cf. RFC 2440)
More formally:
Document ::= (Item | NL)+
Item ::= KeywordLine Object*
KeywordLine ::= Keyword NL | Keyword WS ArgumentsChar+ NL
Keyword = KeywordChar+
KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
ArgumentChar ::= any printing ASCII character except NL.
WS = (SP | TAB)+
Object ::= BeginLine Base-64-encoded-data EndLine
BeginLine ::= "-----BEGIN " Keyword "-----" NL
EndLine ::= "-----END " Keyword "-----" NL
The BeginLine and EndLine of an Object must use the same keyword.
When interpreting a Document, software MUST ignore any KeywordLine that
starts with a keyword it doesn't recognize; future implementations MUST NOT
require current clients to understand any KeywordLine not currently
described.
The "opt" keyword was used until Tor 0.1.2.5-alpha for non-critical future
extensions. All implementations MUST ignore any item of the form "opt
keyword ....." when they would not recognize "keyword ....."; and MUST
treat "opt keyword ....." as synonymous with "keyword ......" when keyword
is recognized.
Implementations before 0.1.2.5-alpha rejected any document with a
KeywordLine that started with a keyword that they didn't recognize.
Implementations MUST prefix items not recognized by older versions of Tor
with an "opt" until those versions of Tor are obsolete.
Other implementations that want to extend Tor's directory format MAY
introduce their own items. The keywords for extension items SHOULD start
with the characters "x-" or "X-", to guarantee that they will not conflict
with keywords used by future versions of Tor.
2. Router operation
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.
- Its uptime has been reset (by restarting).
After generating a descriptor, ORs upload it to every directory
authority they know, by posting it to the URL
http://<hostname:port>/tor/
2.1. Router descriptor format
Every router descriptor MUST start with a "router" Item; MUST end with a
"router-signature" Item and an extra NL; and MUST contain exactly one
instance of each of the following Items: "published" "onion-key"
"signing-key" "bandwidth".
A router descriptor MAY have zero or one of each of the following Items,
but MUST NOT have more than one: "contact", "uptime", "fingerprint",
"hibernating", "read-history", "write-history", "eventdns", "platform",
"family".
Additionally, a router descriptor MAY contain any number of "accept",
"reject", and "opt" Items. Other than "router" and "router-signature",
the items may appear in any order.
The items' formats are as follows:
"router" nickname address ORPort SocksPort DirPort
Indicates the beginning of a router descriptor. "address" must be an
IPv4 address in dotted-quad format. The last three numbers indicate
the TCP ports at which this OR exposes functionality. ORPort is a port
at which this OR accepts TLS connections for the main OR protocol;
SocksPort is deprecated and should always be 0; and DirPort is the
port at which this OR accepts directory-related HTTP connections. If
any port is not supported, the value 0 is given instead of a port
number.
"bandwidth" bandwidth-avg bandwidth-burst bandwidth-observed
Estimated bandwidth for this router, in bytes per second. The
"average" bandwidth is the volume per second that the OR is willing to
sustain over long periods; the "burst" bandwidth is the volume that
the OR is willing to sustain in very short intervals. The "observed"
value is an estimate of the capacity this server can handle. The
server remembers the max bandwidth sustained output over any ten
second period in the past day, and another sustained input. The
"observed" value is the lesser of these two numbers.
"platform" string
A human-readable string describing the system on which this OR is
running. This MAY include the operating system, and SHOULD include
the name and version of the software implementing the Tor protocol.
"published" YYYY-MM-DD HH:MM:SS
The time, in GMT, when this descriptor was generated.
"fingerprint"
A fingerprint (a HASH_LEN-byte of asn1 encoded public key, encoded in
hex, with a single space after every 4 characters) for this router's
identity key. A descriptor is considered invalid (and MUST be
rejected) if the fingerprint line does not match the public key.
[We didn't start parsing this line until Tor 0.1.0.6-rc; it should
be marked with "opt" until earlier versions of Tor are obsolete.]
"hibernating" 0|1
If the value is 1, then the Tor server was hibernating when the
descriptor was published, and shouldn't be used to build circuits.
[We didn't start parsing this line until Tor 0.1.0.6-rc; it should be
marked with "opt" until earlier versions of Tor are obsolete.]
"uptime"
The number of seconds that this OR process has been running.
"onion-key" NL a public key in PEM format
This key is used to encrypt EXTEND cells for this OR. The key MUST be
accepted for at least 1 week after any new key is published in a
subsequent descriptor.
"signing-key" NL a public key in PEM format
The OR's long-term identity key.
"accept" exitpattern
"reject" exitpattern
These lines describe the rules that an OR follows when
deciding whether to allow a new stream to a given address. The
'exitpattern' syntax is described below. The rules are considered in
order; if no rule matches, the address will be accepted. For clarity,
the last such entry SHOULD be accept *:* or reject *:*.
"router-signature" NL Signature NL
The "SIGNATURE" object contains a signature of the PKCS1-padded
hash of the entire router descriptor, taken from the beginning of the
"router" line, through the newline after the "router-signature" line.
The router descriptor is invalid unless the signature is performed
with the router's identity key.
"contact" info NL
Describes a way to contact the server's administrator, preferably
including an email address and a PGP key fingerprint.
"family" names NL
'Names' is a space-separated list of server nicknames or
hexdigests. If two ORs list one another in their "family" entries,
then OPs should treat them as a single OR for the purpose of path
selection.
For example, if node A's descriptor contains "family B", and node B's
descriptor contains "family A", then node A and node B should never
be used on the same circuit.
"read-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
"write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
Declare how much bandwidth the OR has used recently. Usage is divided
into intervals of NSEC seconds. The YYYY-MM-DD HH:MM:SS field
defines the end of the most recent interval. The numbers are the
number of bytes used in the most recent intervals, ordered from
oldest to newest.
[We didn't start parsing these lines until Tor 0.1.0.6-rc; they should
be marked with "opt" until earlier versions of Tor are obsolete.]
"eventdns" bool NL
Declare whether this version of Tor is using the newer enhanced
dns logic. Versions of Tor without eventdns SHOULD NOT be used for
reverse hostname lookups.
[All versions of Tor before 0.1.2.2-alpha should be assumed to have
this option set to 0 if it is not present. All Tor versions at
0.1.2.2-alpha or later should be assumed to have this option set to
1 if it is not present. Until 0.1.2.1-alpha-dev, this option was
not generated, even when eventdns was in use. Versions of Tor
before 0.1.2.1-alpha-dev did not parse this option, so it should be
marked "opt". With 0.2.0.1-alpha, the old 'dnsworker' logic has
been removed, rendering this option of historical interest only.]
2.2. Nonterminals in router descriptors
nickname ::= between 1 and 19 alphanumeric characters, case-insensitive.
hexdigest ::= a '$', followed by 20 hexadecimal characters.
[Represents a server by the digest of its identity key.]
exitpattern ::= addrspec ":" portspec
portspec ::= "*" | port | port "-" port
port ::= an integer between 1 and 65535, inclusive.
[Some implementations incorrectly generate ports with value 0.
Implementations SHOULD accept this, and SHOULD NOT generate it.]
addrspec ::= "*" | ip4spec | ip6spec
ipv4spec ::= ip4 | ip4 "/" num_ip4_bits | ip4 "/" ip4mask
ip4 ::= an IPv4 address in dotted-quad format
ip4mask ::= an IPv4 mask in dotted-quad format
num_ip4_bits ::= an integer between 0 and 32
ip6spec ::= ip6 | ip6 "/" num_ip6_bits
ip6 ::= an IPv6 address, surrounded by square brackets.
num_ip6_bits ::= an integer between 0 and 128
bool ::= "0" | "1"
Ports are required; if they are not included in the router
line, they must appear in the "ports" lines.
3. Network status format
Directory authorities generate, sign, and compress network-status
documents. Directory servers SHOULD generate a fresh network-status
document when the contents of such a document would be different from the
last one generated, and some time (at least one second, possibly longer)
has passed since the last one was generated.
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". Implementations MAY insert blank lines
for clarity between sections; these blank lines are ignored.
Implementations MUST NOT depend on blank lines in any particular location.
As used here, "whitespace" is a sequence of 1 or more tab or space
characters.
The preamble contains:
"network-status-version" -- A document format version. For this
specification, the version is "2".
"dir-source" -- The authority's hostname, current IP address, and
directory port, all separated by whitespace.
"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, in any order, separated by whitespace:
"Names" if this directory authority performs name bindings.
"Versions" if this directory authority recommends software versions.
"BadExits" if the directory authority flags nodes that it believes
are performing incorrectly as exit nodes.
"BadDirectories" if the directory authority flags nodes that it
believes are performing incorrectly as directory caches.
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. Implementations MUST ignore
additional arguments to the items above, and MUST ignore unrecognized
flags.
For each router, the router entry contains: (This format is designed for
conciseness.)
"r" -- followed by the following elements, in order, separated by
whitespace:
- 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, in the form
YYYY-MM-DD HH:MM:SS, in GMT.
- An IP address
- An OR port
- A directory port (or "0" for none")
"s" -- A series of whitespace-separated status flags, in any order:
"Authority" if the router is a directory authority.
"BadExit" if the router is believed to be useless as an exit node
(because its ISP censors it, because it is behind a restrictive
proxy, or for some similar reason).
"BadDirectory" if the router is believed to be useless as a
directory cache (because its directory port isn't working,
its bandwidth is always throttled, or for some similar
reason).
"Exit" if the router is useful for building general-purpose exit
circuits.
"Fast" if the router is suitable for high-bandwidth circuits.
"Guard" if the router is suitable for use as an entry guard.
"Named" if the router's identity-nickname mapping is canonical,
and this authority binds names.
"Stable" if the router is suitable for long-lived circuits.
"Running" if the router is currently usable.
"Valid" if the router has been 'validated'.
"V2Dir" if the router implements this protocol.
"v" -- The version of the Tor protocol that this server is running. If
the value begins with "Tor" SP, the rest of the string is a Tor
version number, and the protocol is "The Tor protocol as supported
by the given version of Tor." Otherwise, if the value begins with
some other string, Tor has upgraded to a more sophisticated
protocol versioning system, and the protocol is "a version of the
Tor protocol more recent than any we recognize."
The "r" entry for each router must appear first and is required. The
"s" entry is optional (see Section 3.1 below for how the flags are
decided). Unrecognized flags on the "s" line and extra elements
on the "r" line must be ignored. The "v" line is optional; it was not
supported until 0.1.2.5-alpha, and it must be preceded with an "opt"
until all earlier versions of Tor are obsolete.
The signature section contains:
"directory-signature" nickname-of-dirserver NL Signature
Signature is a signature of this network-status document
(the document up until the signature, including the line
"directory-signature <nick>\n"), using the directory authority's
signing key.
We compress the network status list with zlib before transmitting it.
3.1. Establishing server status
(This section describes how directory authorities choose which status
flags to apply to routers, as of Tor 0.1.1.18-rc. Later directory
authorities MAY do things differently, so long as clients keep working
well. Clients MUST NOT depend on the exact behaviors in this section.)
In the below definitions, a router is considered "active" if it is
running, valid, and not hibernating.
"Valid" -- a router is 'Valid' if it is running a version of Tor not
known to be broken, and the directory authority has not blacklisted
it as suspicious.
"Named" -- Directory authority 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 authorities. If they don't, they act as clients, and
report bindings made by other directory authorities (name X is bound to
identity Y if at least one binding directory lists it, and no directory
binds X to some other Y'.) A router is called 'Named' if the router
believes the given name should be bound to the given key.
"Running" -- A router is 'Running' if the authority managed to connect to
it successfully within the last 30 minutes.
"Stable" -- A router is 'Stable' if it is active, and either its
uptime is at least the median uptime for known active routers, or
its uptime is at least 30 days. Routers are never called stable if
they are running a version of Tor known to drop circuits stupidly.
(0.1.1.10-alpha through 0.1.1.16-rc are stupid this way.)
"Fast" -- A router is 'Fast' if it is active, and its bandwidth is
in the top 7/8ths for known active routers.
"Guard" -- A router is a possible 'Guard' if it is 'Stable' and its
bandwidth is above median for known active routers. If the total
bandwidth of active non-BadExit Exit servers is less than one third
of the total bandwidth of all active servers, no Exit is listed as
a Guard.
"Authority" -- A router is called an 'Authority' if the authority
generating the network-status document believes it is an authority.
"V2Dir" -- A router supports the v2 directory protocol if it has an open
directory port, and it is running a version of the directory protocol that
supports the functionality clients need. (Currently, this is
0.1.1.9-alpha or later.)
Directory server administrators may label some servers or IPs as
blacklisted, and elect not to include them in their network-status lists.
Authorities SHOULD 'disable' any servers in excess of 3 on any single IP.
When there are more than 3 to choose from, authorities should first prefer
authorities to non-authorities, then prefer Running to non-Running, and
then prefer high-bandwidth to low-bandwidth. To 'disable' a server, the
authority *should* advertise it without the Running or Valid flag.
Thus, the network-status list includes all non-blacklisted,
non-expired, non-superseded descriptors.
4. Directory server operation
All directory authorities and directory mirrors ("directory servers")
implement this section, except as noted.
4.1. Accepting uploads (authorities only)
When a router posts a signed descriptor to a directory authority, the
authority first checks whether it is well-formed and correctly
self-signed. If it is, the authority next verifies that the nickname
question is already assigned to a router with a different public key.
Finally, the authority MAY check that the router is not blacklisted
because of its key, IP, or another reason.
If the descriptor passes these tests, and the authority does not already
have a descriptor for a router with this public key, it accepts the
descriptor and remembers it.
If the authority _does_ have a descriptor with the same public key, the
newly uploaded descriptor is remembered if its publication time is more
recent than the most recent old descriptor for that router, and either:
- There are non-cosmetic differences between the old descriptor and the
new one.
- Enough time has passed between the descriptors' publication times.
(Currently, 12 hours.)
Differences between router descriptors are "non-cosmetic" if they would be
sufficient to force an upload as described in section 2 above.
Note that the "cosmetic difference" test only applies to uploaded
descriptors, not to descriptors that the authority downloads from other
authorities.
4.2. Downloading network-status documents (authorities and caches)
All directory servers (authorities and mirrors) try to keep a fresh
set of network-status documents from every authority. To do so,
every 5 minutes, each authority asks every other authority for its
most recent network-status document. Every 15 minutes, each mirror
picks a random authority and asks it for the most recent network-status
documents for all the authorities the authority knows about (including
the chosen authority itself).
Directory servers and mirrors remember and serve the most recent
network-status document they have from each authority. Other
network-status documents don't need to be stored. If the most recent
network-status document is over 10 days old, it is discarded anyway.
Mirrors SHOULD store and serve network-status documents from authorities
they don't recognize, but SHOULD NOT use such documents for any other
purpose. Mirrors SHOULD discard network-status documents older than 48
hours.
4.3. Downloading and storing router descriptors (authorities and caches)
Periodically (currently, every 10 seconds), directory servers check
whether there are any specific descriptors (as identified by descriptor
hash in a network-status document) that they do not have and that they
are not currently trying to download.
If so, the directory server launches requests to the authorities for these
descriptors, such that each authority is only asked for descriptors listed
in its most recent network-status. When more than one authority lists the
descriptor, we choose which to ask at random.
If one of these downloads fails, we do not try to download that descriptor
from the authority that failed to serve it again unless we receive a newer
network-status from that authority that lists the same descriptor.
Directory servers must potentially cache multiple descriptors for each
router. Servers must not discard any descriptor listed by any current
network-status document from any authority. If there is enough space to
store additional descriptors, servers SHOULD try to hold those which
clients are likely to download the most. (Currently, this is judged
based on the interval for which each descriptor seemed newest.)
Authorities SHOULD NOT download descriptors for routers that they would
immediately reject for reasons listed in 3.1.
4.4. HTTP URLs
"Fingerprints" in these URLs are base-16-encoded SHA1 hashes.
The authoritative network-status published by a host should be available at:
http://<hostname>/tor/status/authority.z
The network-status published by a host with fingerprint
<F> should be available at:
http://<hostname>/tor/status/fp/<F>.z
The network-status documents published by 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 authorities,
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 with identity fingerprints
<F1>,<F2>,<F3> should be available at:
http://<hostname>/tor/server/fp/<F1>+<F2>+<F3>.z
(NOTE: Implementations SHOULD NOT download descriptors by identity key
fingerprint. This allows a corrupted server (in collusion with a cache) to
provide a unique descriptor to a client, and thereby partition that client
from the rest of the network.)
The server descriptor with (descriptor) digest <D> (in hex) should be
available at:
http://<hostname>/tor/server/d/<D>.z
The most recent descriptors with digests <D1>,<D2>,<D3> should be
available at:
http://<hostname>/tor/server/d/<D1>+<D2>+<D3>.z
The most recent descriptor for this server should be at:
http://<hostname>/tor/server/authority.z
[Nothing in the Tor protocol uses this resource yet, but it is useful
for debugging purposes. Also, the official Tor implementations
(starting at 0.1.1.x) use this resource to test whether a server's
own DirPort is reachable.]
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 SHOULD 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.
Clients SHOULD use upper case letters (A-F) when base16-encoding
fingerprints. Servers MUST accept both upper and lower case fingerprints
in requests.
5. Client operation: downloading information
Every Tor that is not a directory server (that is, those that do
not have a DirPort set) implements this section.
5.1. Downloading network-status documents
Each client maintains an ordered list of directory authorities.
Insofar as possible, clients SHOULD all use the same ordered list.
For each network-status document a client has, it keeps track of its
publication time *and* the time when the client retrieved it. Clients
consider a network-status document "live" if it was published within the
last 24 hours.
Clients try to have a live network-status document hours from *every*
authority, and try to periodically get new network-status documents from
each authority in rotation as follows:
If a client is missing a live network-status document for any
authority, it tries to fetch it from a directory cache. On failure,
the client waits briefly, then tries that network-status document
again from another cache. The client does not build circuits until it
has live network-status documents from more than half the authorities
it trusts, and it has descriptors for more than 1/4 of the routers
that it believes are running.
If the most recently _retrieved_ network-status document is over 30
minutes old, the client attempts to download a network-status document.
When choosing which documents to download, clients treat their list of
directory authorities as a circular ring, and begin with the authority
appearing immediately after the authority for their most recently
retrieved network-status document. If this attempt fails (either it
fails to download at all, or the one it gets is not as good as the
one it has), the client retries at other caches several times, before
moving on to the next network-status document in sequence.
Clients discard all network-status documents over 24 hours old.
If enough mirrors (currently 4) claim not to have a given network status,
we stop trying to download that authority's network-status, until we
download a new network-status that makes us believe that the authority in
question is running. Clients should wait a little longer after each
failure.
Clients SHOULD try to batch as many network-status requests as possible
into each HTTP GET.
(Note: clients can and should pick caches based on the network-status
information they have: once they have first fetched network-status info
from an authority, they should not need to go to the authority directly
again.)
5.2. Downloading and storing router descriptors
Clients try to have the best descriptor for each router. A descriptor is
"best" if:
* It is the most recently published descriptor listed for that router
by at least two network-status documents.
OR,
* No descriptor for that router is listed by two or more
network-status documents, and it is the most recently published
descriptor listed by any network-status document.
Periodically (currently every 10 seconds) clients check whether there are
any "downloadable" descriptors. A descriptor is downloadable if:
- It is the "best" descriptor for some router.
- The descriptor was published at least 10 minutes in the past.
(This prevents clients from trying to fetch descriptors that the
mirrors have probably not yet retrieved and cached.)
- The client does not currently have it.
- The client is not currently trying to download it.
- The client would not discard it immediately upon receiving it.
- The client thinks it is running and valid (see 6.1 below).
If at least 16 known routers have downloadable descriptors, or if
enough time (currently 10 minutes) has passed since the last time the
client tried to download descriptors, it launches requests for all
downloadable descriptors, as described in 5.3 below.
When a descriptor download fails, the client notes it, and does not
consider the descriptor downloadable again until a certain amount of time
has passed. (Currently 0 seconds for the first failure, 60 seconds for the
second, 5 minutes for the third, 10 minutes for the fourth, and 1 day
thereafter.) Periodically (currently once an hour) clients reset the
failure count.
No descriptors are downloaded until the client has downloaded more than
half of the network-status documents.
Clients retain the most recent descriptor they have downloaded for each
router so long as it is not too old (currently, 48 hours), OR so long as
it is recommended by at least one networkstatus AND no "better"
descriptor has been downloaded. [Versions of Tor before 0.1.2.3-alpha
would discard descriptors simply for being published too far in the past.]
[The code seems to discard descriptors in all cases after they're 5
days old. True? -RD]
5.3. Managing downloads
When a client has no live network-status documents, it downloads
network-status documents from a randomly chosen authority. In all other
cases, the client downloads from mirrors randomly chosen from among those
believed to be V2 directory servers. (This information comes from the
network-status documents; see 6 below.)
When downloading multiple router descriptors, the client chooses multiple
mirrors so that:
- At least 3 different mirrors are used, except when this would result
in more than one request for under 4 descriptors.
- No more than 128 descriptors are requested from a single mirror.
- Otherwise, as few mirrors as possible are used.
After choosing mirrors, the client divides the descriptors among them
randomly.
After receiving any response client MUST discard any network-status
documents and descriptors that it did not request.
6. Using directory information
Everyone besides directory authorities uses the approaches in this section
to decide which servers to use and what their keys are likely to be.
(Directory authorities just believe their own opinions, as in 3.1 above.)
6.1. Choosing routers for circuits.
Tor implementations only pay attention to "live" network-status documents.
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 1 day old.
For time-sensitive information, Tor implementations focus on "recent"
network-status documents. A network status is "recent" if it is live, and
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.")
Circuits SHOULD NOT be built until the client has enough directory
information: network-statuses (or failed attempts to download
network-statuses) for all authorities, network-statuses for at more than
half of the authorites, and descriptors for at least 1/4 of the servers
believed to be running.
A server is "listed" if it is included by more than half of the live
network status documents. Clients SHOULD NOT use unlisted servers.
Clients believe the flags "Valid", "Exit", "Fast", "Guard", "Stable", and
"V2Dir" about a given router when they are asserted by more than half of
the live network-status documents. Clients believe the flag "Running" if
it is listed by more than half of the recent network-status documents.
These flags are used as follows:
- Clients SHOULD NOT use non-'Valid' or non-'Running' routers unless
requested to do so.
- Clients SHOULD NOT use non-'Fast' routers for any purpose other than
very-low-bandwidth circuits (such as introduction circuits).
- Clients SHOULD NOT use non-'Stable' routers for circuits that are
likely to need to be open for a very long time (such as those used for
IRC or SSH connections).
- Clients SHOULD NOT choose non-'Guard' nodes when picking entry guard
nodes.
- Clients SHOULD NOT download directory information from non-'V2Dir'
caches.
6.2. 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:
When a client encounters a name it has not mapped before:
If all the live "Naming" network-status documents the client has
claim that the name binds to some identity ID, and the client has at
least three live network-status documents, the client maps the name to
ID.
When a user tries to refer to a router with a name that does not have a
mapping under the above rules, the implementation SHOULD warn the user.
After giving the warning, the implementation MAY use a router that at
least one Naming authority maps the name to, so long as no other naming
authority maps that name to a different router. If no Naming authority
maps the name to a router, the implementation MAY use any router that
advertises the name.
Not every router needs a nickname. When a router doesn't configure a
nickname, it publishes with the default nickname "Unnamed". Authorities
SHOULD NOT ever mark a router with this nickname as Named; client software
SHOULD NOT ever use a router in response to a user request for a router
called "Unnamed".
6.3. Software versions
An implementation of Tor SHOULD warn when it has fetched (or has
attempted to fetch and failed four consecutive times) a network-status
for each authority, and it is running a software version
not listed on more than half of the live "Versioning" network-status
documents.
6.4. Warning about a router's status.
If a router tries to publish its descriptor to a Naming authority
that has its nickname mapped to another key, the router SHOULD
warn the operator that it is either using the wrong key or is using
an already claimed nickname.
If a router has fetched (or attempted to fetch and failed four
consecutive times) a network-status for every authority, and at
least one of the authorities is "Naming", and no live "Naming"
authorities publish a binding for the router's nickname, the
router MAY remind the operator that the chosen nickname is not
bound to this key at the authorities, and suggest contacting the
authority operators.
...
6.5. Router protocol versions
A client should believe that a router supports a given feature if that
feature is supported by the router or protocol versions in more than half
of the live networkstatus's "v" entries for that router. In other words,
if the "v" entries for some router are:
v Tor 0.0.8pre1 (from authority 1)
v Tor 0.1.2.11 (from authority 2)
v FutureProtocolDescription 99 (from authority 3)
then the client should believe that the router supports any feature
supported by 0.1.2.11.
This is currently equivalent to believing the median declared version for
a router in all live networkstatuses.
7. Standards compliance
All clients and servers MUST support HTTP 1.0.
7.1. HTTP headers
Servers MAY set the Content-Length: header. Servers SHOULD set
Content-Encoding to "deflate" or "identity".
Servers MAY include an X-Your-Address-Is: header, whose value is the
apparent IP address of the client connecting to them (as a dotted quad).
For directory connections tunneled over a BEGIN_DIR stream, servers SHOULD
report the IP from which the circuit carrying the BEGIN_DIR stream reached
them. [Servers before version 0.1.2.5-alpha reported 127.0.0.1 for all
BEGIN_DIR-tunneled connections.]
Servers SHOULD disable caching of multiple network statuses or multiple
router descriptors. Servers MAY enable caching of single descriptors,
single network statuses, the list of all router descriptors, a v1
directory, or a v1 running routers document. XXX mention times.
7.2. HTTP status codes
XXX We should write down what return codes dirservers send in what situations.
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