commit d72ac9afe19c04d0c09479d9005eb31bc5348d14
Author: Isis Lovecruft <isis(a)torproject.org>
Date: Fri Dec 13 07:42:09 2013 +0000
Move specs and proposals into doc/ directory.
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+
+ BridgeDB specification
+
+ Karsten Loesing
+ Nick Mathewson
+
+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(a)example.COM" becomes
+ "johndoe(a)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.
+
