commit d72ac9afe19c04d0c09479d9005eb31bc5348d14 Author: Isis Lovecruft isis@torproject.org Date: Fri Dec 13 07:42:09 2013 +0000
Move specs and proposals into doc/ directory. --- doc/bridge-db-spec.txt | 391 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 391 insertions(+)
diff --git a/doc/bridge-db-spec.txt b/doc/bridge-db-spec.txt new file mode 100644 index 0000000..c897226 --- /dev/null +++ b/doc/bridge-db-spec.txt @@ -0,0 +1,391 @@ + + 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@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. +