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-
-                          BridgeDB specification
-
-                             Karsten Loesing
-                              Nick Mathewson
-
-Table of Contents
-
-    0. Preliminaries
-    1. Importing bridge network statuses and bridge descriptors
-        1.1. Parsing bridge network statuses
-        1.2. Parsing bridge descriptors
-        1.3. Parsing extra-info documents
-    2. Assigning bridges to distributors
-    3. Giving out bridges upon requests
-    4. Selecting bridges to be given out based on IP addresses
-    5. Selecting bridges to be given out based on email addresses
-    6. Selecting unallocated bridges to be stored in file buckets
-    7. Displaying Bridge Information
-    8. Writing bridge assignments for statistics
-
-0. Preliminaries
-
-   This document specifies how BridgeDB processes bridge descriptor files
-   to learn about new bridges, maintains persistent assignments of bridges
-   to distributors, and decides which bridges to give out upon user
-   requests.
-
-   Some of the decisions here may be suboptimal: this document is meant to
-   specify current behavior as of August 2013, not to specify ideal
-   behavior.
-
-1. Importing bridge network statuses and bridge descriptors
-
-   BridgeDB learns about bridges by parsing bridge network statuses,
-   bridge descriptors, and extra info documents as specified in Tor's
-   directory protocol. BridgeDB parses one bridge network status file
-   first and at least one bridge descriptor file and potentially one extra
-   info file afterwards.
-
-   BridgeDB scans its files on sighup.
-
-   BridgeDB does not validate signatures on descriptors or networkstatus
-   files: the operator needs to make sure that these documents have come
-   from a Tor instance that did the validation for us.
-
-1.1. Parsing bridge network statuses
-
-   Bridge network status documents contain the information of which bridges
-   are known to the bridge authority and which flags the bridge authority
-   assigns to them.
-   We expect bridge network statuses to contain at least the following two
-   lines for every bridge in the given order (format fully specified in Tor's
-   directory protocol):
-
-      "r" SP nickname SP identity SP digest SP publication SP IP SP ORPort
-          SP DirPort NL
-      "a" SP address ":" port NL (no more than 8 instances)
-      "s" SP Flags NL
-
-   BridgeDB parses the identity and the publication timestamp from the "r"
-   line, the OR address(es) and ORPort(s) from the "a" line(s), and the
-   assigned flags from the "s" line, specifically checking the assignment
-   of the "Running" and "Stable" flags.
-   BridgeDB memorizes all bridges that have the Running flag as the set of
-   running bridges that can be given out to bridge users.
-   BridgeDB memorizes assigned flags if it wants to ensure that sets of
-   bridges given out should contain at least a given number of bridges
-   with these flags.
-
-1.2. Parsing bridge descriptors
-
-   BridgeDB learns about a bridge's most recent IP address and OR port
-   from parsing bridge descriptors.
-   In theory, both IP address and OR port of a bridge are also contained
-   in the "r" line of the bridge network status, so there is no mandatory
-   reason for parsing bridge descriptors.  But the functionality described
-   in this section is still implemented in case we need data from the
-   bridge descriptor in the future.
-
-   Bridge descriptor files may contain one or more bridge descriptors.
-   We expect a bridge descriptor to contain at least the following lines in
-   the stated order:
-
-      "@purpose" SP purpose NL
-      "router" SP nickname SP IP SP ORPort SP SOCKSPort SP DirPort NL
-      "published" SP timestamp
-      ["opt" SP] "fingerprint" SP fingerprint NL
-      "router-signature" NL Signature NL
-
-   BridgeDB parses the purpose, IP, ORPort, nickname, and fingerprint
-   from these lines.
-   BridgeDB skips bridge descriptors if the fingerprint is not contained
-   in the bridge network status parsed earlier or if the bridge does not
-   have the Running flag.
-   BridgeDB discards bridge descriptors which have a different purpose
-   than "bridge". BridgeDB can be configured to only accept descriptors
-   with another purpose or not discard descriptors based on purpose at
-   all.
-   BridgeDB memorizes the IP addresses and OR ports of the remaining
-   bridges.
-   If there is more than one bridge descriptor with the same fingerprint,
-   BridgeDB memorizes the IP address and OR port of the most recently
-   parsed bridge descriptor.
-   If BridgeDB does not find a bridge descriptor for a bridge contained in
-   the bridge network status parsed before, it does not add that bridge
-   to the set of bridges to be given out to bridge users.
-
-1.3. Parsing extra-info documents
-
-   BridgeDB learns if a bridge supports a pluggable transport by parsing
-   extra-info documents.
-   Extra-info documents contain the name of the bridge (but only if it is
-   named), the bridge's fingerprint, the type of pluggable transport(s) it
-   supports, and the IP address and port number on which each transport
-   listens, respectively.
-
-   Extra-info documents may contain zero or more entries per bridge. We expect
-   an extra-info entry to contain the following lines in the stated order:
-
-      "extra-info" SP name SP fingerprint NL
-      "transport" SP transport SP IP ":" PORT ARGS NL
-
-   BridgeDB parses the fingerprint, transport type, IP address, port and any
-   arguments that are specified on these lines. BridgeDB skips the name. If
-   the fingerprint is invalid, BridgeDB skips the entry.  BridgeDB memorizes
-   the transport type, IP address, port number, and any arguments that are be
-   provided and then it assigns them to the corresponding bridge based on the
-   fingerprint. Arguments are comma-separated and are of the form k=v,k=v.
-   Bridges that do not have an associated extra-info entry are not invalid.
-
-2. Assigning bridges to distributors
-
-   A "distributor" is a mechanism by which bridges are given (or not
-   given) to clients.  The current distributors are "email", "https",
-   and "unallocated".
-
-   BridgeDB assigns bridges to distributors based on an HMAC hash of the
-   bridge's ID and a secret and makes these assignments persistent.
-   Persistence is achieved by using a database to map node ID to
-   distributor.
-   Each bridge is assigned to exactly one distributor (including
-   the "unallocated" distributor).
-   BridgeDB may be configured to support only a non-empty subset of the
-   distributors specified in this document.
-   BridgeDB may be configured to use different probabilities for assigning
-   new bridges to distributors.
-   BridgeDB does not change existing assignments of bridges to
-   distributors, even if probabilities for assigning bridges to
-   distributors change or distributors are disabled entirely.
-
-3. Giving out bridges upon requests
-
-   Upon receiving a client request, a BridgeDB distributor provides a
-   subset of the bridges assigned to it.
-   BridgeDB only gives out bridges that are contained in the most recently
-   parsed bridge network status and that have the Running flag set (see
-   Section 1).
-   BridgeDB may be configured to give out a different number of bridges
-   (typically 4) depending on the distributor.
-   BridgeDB may define an arbitrary number of rules. These rules may
-   specify the criteria by which a bridge is selected. Specifically,
-   the available rules restrict the IP address version, OR port number,
-   transport type, bridge relay flag, or country in which the bridge
-   should not be blocked.
-
-4. Selecting bridges to be given out based on IP addresses
-
-   BridgeDB may be configured to support one or more distributors which
-   gives out bridges based on the requestor's IP address.  Currently, this
-   is how the HTTPS distributor works.
-   The goal is to avoid handing out all the bridges to users in a similar
-   IP space and time.
-# Someone else should look at proposals/ideas/old/xxx-bridge-disbursement
-# to see if this section is missing relevant pieces from it.  -KL
-
-   BridgeDB fixes the set of bridges to be returned for a defined time
-   period.
-   BridgeDB considers all IP addresses coming from the same /24 network
-   as the same IP address and returns the same set of bridges. From here on,
-   this non-unique address will be referred to as the IP address's 'area'.
-   BridgeDB divides the IP address space equally into a small number of
-# Note, changed term from "areas" to "disjoint clusters" -MF
-   disjoint clusters (typically 4) and returns different results for requests
-   coming from addresses that are placed into different clusters.
-# I found that BridgeDB is not strict in returning only bridges for a
-# given area.  If a ring is empty, it considers the next one.  Is this
-# expected behavior?  -KL
-#
-# This does not appear to be the case, anymore. If a ring is empty, then
-# BridgeDB simply returns an empty set of bridges. -MF
-#
-# I also found that BridgeDB does not make the assignment to areas
-# persistent in the database.  So, if we change the number of rings, it
-# will assign bridges to other rings.  I assume this is okay?  -KL
-   BridgeDB maintains a list of proxy IP addresses and returns the same
-   set of bridges to requests coming from these IP addresses.
-   The bridges returned to proxy IP addresses do not come from the same
-   set as those for the general IP address space.
-
-   BridgeDB can be configured to include bridge fingerprints in replies
-   along with bridge IP addresses and OR ports.
-   BridgeDB can be configured to display a CAPTCHA which the user must solve
-   prior to returning the requested bridges.
-
-   The current algorithm is as follows.  An IP-based distributor splits
-   the bridges uniformly into a set of "rings" based on an HMAC of their
-   ID.  Some of these rings are "area" rings for parts of IP space; some
-   are "category" rings for categories of IPs (like proxies).  When a
-   client makes a request from an IP, the distributor first sees whether
-   the IP is in one of the categories it knows.  If so, the distributor
-   returns an IP from the category rings.  If not, the distributor
-   maps the IP into an "area" (that is, a /24), and then uses an HMAC to
-   map the area to one of the area rings.
-
-   When the IP-based distributor determines from which area ring it is handing
-   out bridges, it identifies which rules it will use to choose appropriate
-   bridges. Using this information, it searches its cache of rings for one
-   that already adheres to the criteria specified in this request. If one
-   exists, then BridgeDB maps the current "epoch" (N-hour period) and the
-   IP's area (/24) to a point on the ring based on HMAC, and hands out
-   bridges at that point. If a ring does not already exist which satisfies this
-   request, then a new ring is created and filled with bridges that fulfill
-   the requirements. This ring is then used to select bridges as described.
-
-   "Mapping X to Y based on an HMAC" above means one of the following:
-
-      - We keep all of the elements of Y in some order, with a mapping
-        from all 160-bit strings to positions in Y.
-      - We take an HMAC of X using some fixed string as a key to get a
-        160-bit value.  We then map that value to the next position of Y.
-
-   When giving out bridges based on a position in a ring, BridgeDB first
-   looks at flag requirements and port requirements.  For example,
-   BridgeDB may be configured to "Give out at least L bridges with port
-   443, and at least M bridges with Stable, and at most N bridges
-   total."  To do this, BridgeDB combines to the results:
-
-      - The first L bridges in the ring after the position that have the
-        port 443, and
-      - The first M bridges in the ring after the position that have the
-        flag stable and that it has not already decided to give out, and
-      - The first N-L-M bridges in the ring after the position that it
-        has not already decided to give out.
-
-    After BridgeDB selects appropriate bridges to return to the requestor, it
-    then prioritises the ordering of them in a list so that as many criteria
-    are fulfilled as possible within the first few bridges. This list is then
-    truncated to N bridges, if possible. N is currently defined as a
-    piecewise function of the number of bridges in the ring such that:
-
-             /
-            |  1,   if len(ring) < 20
-            |
-        N = |  2,   if 20 <= len(ring) <= 100
-            |
-            |  3,   if 100 <= len(ring)
-             \
-
-    The bridges in this sublist, containing no more than N bridges, are the
-    bridges returned to the requestor.
-
-5. Selecting bridges to be given out based on email addresses
-
-   BridgeDB can be configured to support one or more distributors that are
-   giving out bridges based on the requestor's email address.  Currently,
-   this is how the email distributor works.
-   The goal is to bootstrap based on one or more popular email service's
-   sybil prevention algorithms.
-# Someone else should look at proposals/ideas/old/xxx-bridge-disbursement
-# to see if this section is missing relevant pieces from it.  -KL
-
-   BridgeDB rejects email addresses containing other characters than the
-   ones that RFC2822 allows.
-   BridgeDB may be configured to reject email addresses containing other
-   characters it might not process correctly.
-# I don't think we do this, is it worthwhile? -MF
-   BridgeDB rejects email addresses coming from other domains than a
-   configured set of permitted domains.
-   BridgeDB normalizes email addresses by removing "." characters and by
-   removing parts after the first "+" character.
-   BridgeDB can be configured to discard requests that do not have the
-   value "pass" in their X-DKIM-Authentication-Result header or does not
-   have this header.  The X-DKIM-Authentication-Result header is set by
-   the incoming mail stack that needs to check DKIM authentication.
-
-   BridgeDB does not return a new set of bridges to the same email address
-   until a given time period (typically a few hours) has passed.
-# Why don't we fix the bridges we give out for a global 3-hour time period
-# like we do for IP addresses?  This way we could avoid storing email
-# addresses.  -KL
-# The 3-hour value is probably much too short anyway.  If we take longer
-# time values, then people get new bridges when bridges show up, as
-# opposed to then we decide to reset the bridges we give them.  (Yes, this
-# problem exists for the IP distributor). -NM
-# I'm afraid I don't fully understand what you mean here.  Can you
-# elaborate?  -KL
-#
-# Assuming an average churn rate, if we use short time periods, then a
-# requestor will receive new bridges based on rate-limiting and will (likely)
-# eventually work their way around the ring; eventually exhausting all bridges
-# available to them from this distributor. If we use a longer time period,
-# then each time the period expires there will be more bridges in the ring
-# thus reducing the likelihood of all bridges being blocked and increasing
-# the time and effort required to enumerate all bridges. (This is my
-# understanding, not from Nick) -MF
-# Also, we presently need the cache to prevent replays and because if a user
-# sent multiple requests with different criteria in each then we would leak
-# additional bridges otherwise. -MF
-   BridgeDB can be configured to include bridge fingerprints in replies
-   along with bridge IP addresses and OR ports.
-   BridgeDB can be configured to sign all replies using a PGP signing key.
-   BridgeDB periodically discards old email-address-to-bridge mappings.
-   BridgeDB rejects too frequent email requests coming from the same
-   normalized address.
-
-   To map previously unseen email addresses to a set of bridges, BridgeDB
-   proceeds as follows:
-
-     - It normalizes the email address as above, by stripping out dots,
-       removing all of the localpart after the +, and putting it all
-       in lowercase.  (Example: "John.Doe+bridges@example.COM" becomes
-       "johndoe@example.com".)
-     - It maps an HMAC of the normalized address to a position on its ring
-       of bridges.
-     - It hands out bridges starting at that position, based on the
-       port/flag requirements, as specified at the end of section 4.
-
-    See section 4 for the details of how bridges are selected from the ring
-    and returned to the requestor.
-
-6. Selecting unallocated bridges to be stored in file buckets
-
-# Kaner should have a look at this section. -NM
-
-   BridgeDB can be configured to reserve a subset of bridges and not give
-   them out via one of the distributors.
-   BridgeDB assigns reserved bridges to one or more file buckets of fixed
-   sizes and write these file buckets to disk for manual distribution.
-   BridgeDB ensures that a file bucket always contains the requested
-   number of running bridges.
-   If the requested number of bridges in a file bucket is reduced or the
-   file bucket is not required anymore, the unassigned bridges are
-   returned to the reserved set of bridges.
-   If a bridge stops running, BridgeDB replaces it with another bridge
-   from the reserved set of bridges.
-# I'm not sure if there's a design bug in file buckets.  What happens if
-# we add a bridge X to file bucket A, and X goes offline?  We would add
-# another bridge Y to file bucket A.  OK, but what if A comes back?  We
-# cannot put it back in file bucket A, because it's full.  Are we going to
-# add it to a different file bucket?  Doesn't that mean that most bridges
-# will be contained in most file buckets over time?  -KL
-#
-# This should be handled the same as if the file bucket is reduced in size.
-# If X returns, then it should be added to the appropriate distributor. -MF
-
-7. Displaying Bridge Information
-
-  After bridges are selected using one of the methods described in
-  Sections 4 - 6, they are output in one of two formats. Bridges are
-  formatted as:
-
-      <address:port> NL
-
-  Pluggable transports are formatted as:
-
-      <transportname> SP <address:port> [SP arglist] NL
-
-  where arglist is an optional space-separated list of key-value pairs in
-  the form of k=v.
-
-  Previously, each line was prepended with the "bridge" keyword, such as
-
-      "bridge" SP <address:port> NL
-
-      "bridge" SP <transportname> SP <address:port> [SP arglist] NL
-
-# We don't do this anymore because Vidalia and TorLauncher don't expect it.
-# See the commit message for b70347a9c5fd769c6d5d0c0eb5171ace2999a736.
-
-8. Writing bridge assignments for statistics
-
-   BridgeDB can be configured to write bridge assignments to disk for
-   statistical analysis.
-   The start of a bridge assignment is marked by the following line:
-
-      "bridge-pool-assignment" SP YYYY-MM-DD HH:MM:SS NL
-
-   YYYY-MM-DD HH:MM:SS is the time, in UTC, when BridgeDB has completed
-   loading new bridges and assigning them to distributors.
-
-   For every running bridge there is a line with the following format:
-
-      fingerprint SP distributor (SP key "=" value)* NL
-
-   The distributor is one out of "email", "https", or "unallocated".
-
-   Both "email" and "https" distributors support adding keys for "port",
-   "flag" and "transport". Respectively, the port number, flag name, and
-   transport types are the values. These are used to indicate that
-   a bridge matches certain port, flag, transport criteria of requests.
-
-   The "https" distributor also allows the key "ring" with a number as
-   value to indicate to which IP address area the bridge is returned.
-
-   The "unallocated" distributor allows the key "bucket" with the file
-   bucket name as value to indicate which file bucket a bridge is assigned
-   to.
-