$Id$ Tor directory protocol, version 3 0. Scope and preliminaries This directory protocol is used by Tor version 0.2.0.x-alpha and later. See dir-spec-v1.txt for information on the protocol used up to the 0.1.0.x series, and dir-spec-v2.txt for information on the protocol used by the 0.1.1.x and 0.1.2.x series. Caches and authorities must still support older versions of the directory protocols, until the versions of Tor that require them are finally out of commission. See Section XXXX on backward compatibility. This document merges and supersedes the following proposals: 101 Voting on the Tor Directory System 103 Splitting identity key from regularly used signing key 104 Long and Short Router Descriptors AS OF 3 MAY 2007, THIS SPECIFICATION HAS NOT YET BEEN COMPLETELY IMPLEMENTED. 0.1. History The earliest versions of Onion Routing shipped with a list of known routers and their keys. When the set of routers changed, users needed to fetch a new list. The Version 1 Directory protocol -------------------------------- [XXX say which versions added what.] Early versions of Tor introduced "Directory authorities": servers that served signed "directory" documents containing a list of signed "router descriptors", along with short summary of the status of each router. Thus, clients could get up-to-date information on the state of the network automatically, and be certain that they list they were getting was attested by a trusted directory authority. Later versions added directory caches, which download directories from the authorities and serve them to clients. Non-caches fetch from the caches in preference to fetching from the authorities, thus distributing bandwidth requirements. Also added during the version 1 directory protocol were "router status" documents: short documents that listed only the up/down status of the routers on the network, rather than a complete list of all the descriptors. Clients and caches would fetch these documents far more frequently than they would fetch full directories. The Version 2 Directory Protocol -------------------------------- During the Tor 0.1.1.x series, Tor revised its handling of directory documents in order to address two major problems: * Directories had grown quite large (over 1MB), and most directory downloads consisted mainly of router descriptors that clients already had. * Every directory authorities 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. (Clients trusted the most recent signed document they downloaded.) Thus, adding more authorities would make the system less secure, not more. To address these, we extended the directory protocol so that authorities now published signed "network status" documents. Each network status listed, for every router in the network: a hash of its identity key, a hash of its most recent descriptor, and a summary of what the authority believed about its status. Clients would download the authorities' network status documents in turn, and believe statements about routers iff they were attested to by more than half of the authorities. Instead of downloading all router descriptors at once, clients downloaded only the descriptors that they did not have. Descriptors were indexed by their digests, in order to prevent malicious caches from giving different versions of a router descriptor to different clients. Routers began working harder to upload new descriptors only when their contents were substantially changed. 0.2. Goals of the version 3 protocol Version 3 of the Tor directory protocol tries to solve the following issues: * A great deal of bandwidth used to transmit router descriptors was used by two fields that are not actually used by Tor routers. We save about 60% by moving them into a separate document that most clients do not fetch or use. * It was possible under certain perverse circumstances for clients to download an unusual set of network status documents, thus partitioning themselves from clients who have a more recent and/or typical set of documents. Even under the best of circumstances, clients were sensitive to the ages of the network status documents they downloaded. Therefore, instead of having the clients correlate multiple network status documents, we have the authorities collectively vote on a single consensus network status document. * The most sensitive data in the entire network (the identity keys of the directory authorities) needed to be stored unencrypted so that the authorities can sign network-status documents on the fly. Now, the authorities' identity keys are stored offline, and used to certify medium-term signing keys that can be rotated. 0.3. Some Remaining questions Things we could solve on a v3 timeframe: The SHA-1 hash is showing its age. We should do something about our dependency on it. We could probably future-proof ourselves here in this revision, at least so far as documents from the authorities are concerned. Too many things about the authorities are hardcoded by IP. Perhaps we should start accepting longer identity keys for routers too. Things to solve eventually: Requiring every client to know about every router won't scale forever. Requiring every directory cache to know every router won't scale forever. 1. Outline There is a small set (say, around 5-10) 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. Every authority has a very-secret, long-term "Authority Identity Key". This is stored encrypted and/or offline, and is used to sign "key certificate" documents. Every key certificate contains a medium-term (3-12 months) "authority signing key", that is used by the authority to sign other directory information. (Note that the authority identity key is distinct from the router identity key that the authority uses in its role as an ordinary router.) Routers periodically upload signed "routers descriptors" to the directory authorities describing their keys, capabilities, and other information. Routers may also upload signed "extra info documents" containing information that is not required for the Tor protocol. Directory authorities serve router descriptors indexed by router identity, or by hash of the descriptor. Routers may act as directory caches to reduce load on the directory authorities. They announce this in their descriptors. Periodically, each directory authority periodically generates a view of the current descriptors and status for known routers. They send a signed summary of this view (a "status vote") to the other authorities. The authorities compute the result of this vote, and sign a "consensus status" document containing the result of the vote. Directory caches download, cache, and re-serve consensus documents. Clients, directory caches, and directory authorities all use consensus documents to find out when their list of routers is out-of-date. (Directory authorities also use vote statuses.) 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. [Authorities also generate and caches also cache documents produced and used by earlier versions of this protocol; see section XXX for notes.] 1.1. What's different from version 2? Clients used to download a multiple network status documents, corresponding roughly to "status votes" above. They would compute the result of the vote on the client side. Authorities used sign documents using the same private keys they used for their roles as routers. This forced them to keep these extremely sensitive keys in memory unencrypted. All of the information in extra-info documents used to be kept in the main descriptors. 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. When generating documents that need to be read by older versions of Tor, implementations MUST prefix items not recognized by older versions of Tor with an "opt" until those versions of Tor are obsolete. [Note that key certificates, status vote documents, extra info documents, and status consensus documents will never by read by older versions of Tor.] 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. In our document descriptions below, we tag Items with a multiplicity in brackets. Possible tags are: "At start, exactly once": These items MUST occur in every instance of the document type, and MUST appear exactly once, and MUST be the first item in their documents. "Exactly once": These items MUST occur exactly one time in every instance of the document type. "At end, exactly once": These items MUST occur in every instance of the document type, and MUST appear exactly once, and MUST be the last item in their documents. "At most once": These items MAY occur zero or one times in any instance of the document type, but MUST NOT occur more than once. "Any number": These items MAY occur zero, one, or more times in any instance of the document type. "Once or more": These items MUST occur at least once in any instance of the document type, and MAY occur more. 1.3. Signing documents Every signable document below is signed in a similar manner, using a given "Initial Item", a final "Signature Item", a digest algorithm, and a signing key. The Initial Item must be the first item in the document. The Signature Item has the following format: [arguments] NL SIGNATURE NL The "SIGNATURE" Object contains a signature (using the signing key) of the PKCS1-padded digest of the entire document, taken from the beginning of the Initial item, through the newline after the Signature Item's keyword and its arguments. Unless otherwise, the digest algorithm is SHA-1. All documents are invalid unless signed with the correct signing key. The "Digest" of a document, unless stated otherwise, is its digest *as signed by this signature scheme*. 2. Router operation and formats ORs SHOULD generate a new router descriptor and a new extra-info document 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 them to every directory authority they know, by posting them (in order) to the URL http:///tor/ 2.1. Router descriptor format Router descriptors consist of the following items. For backward compatibility, there should be an extra NL at the end of each router descriptor. In lines that take multiple arguments, extra arguments SHOULD be accepted and ignored. "router" nickname address ORPort SOCKSPort DirPort NL [At start, exactly once.] 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 NL [Exactly once] 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 NL [At most once] 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 NL [Exactly once] The time, in GMT, when this descriptor (and its corresponding extra-info document if any) was generated. "fingerprint" fingerprint NL [At most once] 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" bool NL [At most once] 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" number NL [At most once] The number of seconds that this OR process has been running. "onion-key" NL a public key in PEM format [Exactly once] 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. It MUST be 1024 bits. "signing-key" NL a public key in PEM format [Exactly once] The OR's long-term identity key. It MUST be 1024 bits. "accept" exitpattern NL "reject" exitpattern NL [Any number] 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 [At end, exactly once] 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 [At most once] Describes a way to contact the server's administrator, preferably including an email address and a PGP key fingerprint. "family" names NL [At most once] '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 [At most once] "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL [At most once] 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.] [See also migration notes in section 2.2.1.] "eventdns" bool NL [At most once] 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 some future version, the old 'dnsworker' logic will be removed, rendering this option of historical interest only.] "caches-extra-info" 0|1 NL [At most once.] True if this router is a directory cache that provides extra-info documents. If absent, the value should be treated as false. [Versions before 0.2.0.1-alpha don't recognize this, and versions before 0.1.2.5-alpha will reject descriptors containing it unless it is prefixed with "opt"; it should be so prefixed until these versions are obsolete.] "extra-info-digest" digest NL [At most once] "Digest" is a hex-encoded digest (using upper-case characters) of the router's extra-info document, as signed in the router's extra-info. (If this field is absent, the router is not uploading a corresponding extra-info document.) [Versions before 0.2.0.1-alpha don't recognize this, and versions before 0.1.2.5-alpha will reject descriptors containing it unless it is prefixed with "opt"; it should be so prefixed until these versions are obsolete.] 2.2. Extra-info documents Extra-info documents consist of the following items: "extra-info" Nickname Fingerprint NL [At start, exactly once.] Identifies what router this is an extra info descriptor for. Fingerprint is encoded in hex (using upper-case letters), with no spaces. "published" [Exactly once.] The time, in GMT, when this document (and its corresponding router descriptor if any) was generated. It MUST match the published time in the corresponding router descriptor. "read-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL [At most once.] "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL [At most once.] As documented in 2.1 above. See migration notes in section 2.2.1. "router-signature" NL Signature NL [At end, exactly once.] A document signature as documented in section 1.3, using the initial item "extra-info" and the final item "router-signature", signed with the router's identity key. 2.2.1. Moving history fields to extra-info documents. Tools that want to use the read-history and write-history values SHOULD download extra-info documents as well as router descriptors. Such tools SHOULD accept history values from both sources; if they appear in both documents, the values in the extra-info documents are authoritative. At some future time, to save space, new versions of Tor will no longer generate router descriptors containing read-history or write-history. Tools should continue to accept read-history and write-history values in router descriptors produced by older versions of Tor. 2.3. 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" 3. Formats produced by directory authorities. Every authority has two keys used in this protocol: a signing key, and an authority identity key. (Authorities also have a router identity key used in their role as a router and by earlier versions of the directory protocol.) The identity key is used from time to time to sign new key certificates using new signing keys; it is very sensitive. The signing key is used to sign key certificates and status documents. There are three kinds of documents generated by directory authorities: Key certificates Status votes Status consensuses Each is discussed below. 3.1. Key certificates Key certificates consist of the following items: "dir-key-certificate-version" version NL [At start, exactly once.] Determines the version of the key certificate. MUST be "3" for the protocol described in this document. Implementations MUST reject formats they don't understand. "fingerprint" fingerprint NL [Exactly once.] Hexadecimal encoding without spaces based on the authority's identity key. "dir-identity-key" NL a public key in PEM format [Exactly once.] The long-term authority identity key for this authority. This key SHOULD be at least 2048 bits long; it MUST NOT be shorter than 1024 bits. "dir-key-published" YYYY-MM-DD HH:MM:SS NL [Exactly once.] The time (in GMT) when this document and corresponding key were last generated. "dir-key-expires" YYYY-MM-DD HH:MM:SS NL [Exactly once.] A time (in GMT) after which this key is no longer valid. "dir-signing-key" NL a key in PEM format [Exactly once.] The directory server's public signing key. This key MUST be at least 1024 bits, and MAY be longer. "dir-key-certification" NL Signature NL [At end, exactly once.] A document signature as documented in section 1.3, using the initial item "dir-key-certificate-version" and the final item "dir-key-certification", signed with the authority identity key. Authorities MUST generate a new signing key and corresponding certificate before the key expires. 3.2. Vote and consensus status documents Votes and consensuses are more strictly formatted then other documents in this specification, since different authorities must be able to generate exactly the same consensus given the same set of votes. The procedure for deciding when to generate vote and consensus status documents are described in section XXX below. Status documents contain a preamble, an authority section, a list of router status entries, and one more footers signature, in that order. Unlike other formats described above, a SP in these documents must be a single space character (hex 20). Some items appear only in votes, and some items appear only in consensuses. Unless specified, items occur in both. The preamble contains the following items. They MUST occur in the order given here: "network-status-version" SP version NL. [At start, exactly once.] A document format version. For this specification, the version is "3". "vote-status" SP type NL [Exactly once.] The status MUST be "vote" or "consensus", depending on the type of the document. "published" SP YYYY-MM-DD SP HH:MM:SS NL [Exactly once.] The publication time for this status document (if a vote), or the start of the period for this vote (if a consensus). "valid-until" [Exactly once.] A time after which this vote or consensus will no longer be valid. "client-versions" SP VersionList NL [At most once.] A comma-separated list of recommended client versions, in ascending order. If absent, no opinion is held about client versions. "server-versions" SP VersionList NL [At most once.] A comma-separated list of recommended server versions, in ascending order. If absent, no opinion is held about server versions. "known-flags" SP FlagList NL [Exactly once.] A space-separated list of all of the flags that this document might contain. A flag is "known" either because the authority knows about them and might set them (if in a vote), or because enough votes were counted for the consensus for an authoritative opinion to have been formed about their status. The authority section of a vote contains the following items, followed in turn by the authority's current key certificate: "dir-source" SP nickname SP identity SP address SP IP SP dirport NL [Exactly once, at start] Describes this authority. The nickname is a convenient identifier for the authority. The identity is a hex fingerprint of the authority's current identity key. The address is the server's hostname. The IP is the server's current IP address, and dirport is its current directory port. "contact" SP string NL [At most once.] An arbitrary string describing how to contact the directory server's administrator. Administrators should include at least an email address and a PGP fingerprint. The authority section of a consensus contains groups the following items, in the order given, with one group for each authority that contributed to the consensus: "dir-source" SP nickname SP address SP IP SP dirport NL [Exactly once, at start] As in the authority section of a vote. "contact" SP string NL [At most once.] As in the authority section of a vote. "fingerprint" SP fingerprint NL [Exactly once.] A hex fingerprint, without spaces, of the authority's current identity key. "vote-digest" SP digest NL [Exactly once.] A digest of the vote from the authority that contributed to this consensus. Each router status entry contains the following items. Router status entries are sorted in ascending order by identity digest. "r" SP nickname SP identity SP digest SP publication SP IP SP ORPort SP DirPort NL [At start, exactly once.] "Nickname" is the OR's nickname. "Identity" is a hash of its identity key, encoded in base64, with trailing equals sign(s) removed. "Digest" is a hash of its most recent descriptor (as signed), encoded in base64 as "identity". "Publication" is the publication time of its most recent descriptor, in the form YYYY-MM-DD HH:MM:SS, in GMT. "IP" is its current IP address; ORPort is its current OR port, "DirPort" is it's current directory port, or "0" for "none". "s" SP Flags NL [At most once.] A series of space-separated status flags, in alphabetical order. Currently documented flags are: "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" SP version NL [At most once.] 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 signature section contains the following item, which appears Exactly Once for a vote, and At Least Once for a consensus. "directory-signature" SP identity SP digest NL Signature This is a signature of the status document, with the initial item "network-status-version", and the signature item "directory-signature", using the signing key. (In this case, we take the hash through the _space_ after directory-signature, not the newline: this ensures that all authorities sign the same thing.) "identity" is the hex-encoded digest of the authority identity key of the signing authority, and "digest" is the hex-encoded digest of the current authority signing key of the signing authority. 3.3. Deciding how to vote. (This section describes how directory authorities choose which status flags to apply to routers, as of Tor 0.2.0.0-alpha-dev. 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. Thus, the network-status list includes all non-blacklisted, non-expired, non-superseded descriptors. 3.4. Computing a consensus from a set of votes Given a set of votes, authorities compute the contents of the consensus document as follows: The "published" is the latest of all published times on the votes. The "valid-until" is the earliest of all valid-until times on the votes. "client-versions" and "server-versions" are sorted in ascending order; A version is recommended in the consensus if it is recommended by more than half of the voting authorities that included a client-versions or server-versions lines in their votes. The authority item groups (dir-source, contact, fignerprint, vote-digest) are taken from the votes of the voting authorities. These groups are sorted by the digests of the authorities identity keys, in ascending order. A router status entry is included in the result if it is included by more than half of the authorities (total authorities, not just those whose votes we have). A router entry has a flag set if it is included by more than half of the authorities who care about that flag. Two router entries are "the same" if they have the same identity digest. We use whatever descriptor digest is attested to by the most authorities among the voters, breaking ties in favor of the one with the most recent publication time. The signatures at the end of the document appear are sorted in ascending order by identity digest. [CUTOFF HERE. STUFF BELOW THIS POINT HAS NOT YET BEEN UPDATED FROM V2.] 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:///tor/status/authority.z The network-status published by a host with fingerprint should be available at: http:///tor/status/fp/.z The network-status documents published by hosts with fingerprints ,, should be available at: http:///tor/status/fp/++.z The most recent network-status documents from all known authorities, concatenated, should be available at: http:///tor/status/all.z The most recent descriptor for a server whose identity key has a fingerprint of should be available at: http:///tor/server/fp/.z The most recent descriptors for servers with identity fingerprints ,, should be available at: http:///tor/server/fp/++.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 (in hex) should be available at: http:///tor/server/d/.z The most recent descriptors with digests ,, should be available at: http:///tor/server/d/++.z The most recent descriptor for this server should be at: http:///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:///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, 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. 8. Backward compatibility and migration plans