[tor-commits] [torspec/master] Add draft proposal of new guard selection algorithm.
nickm at torproject.org
nickm at torproject.org
Thu Oct 29 21:19:56 UTC 2015
commit c9a9acb28f8b3ec536e2ff38f87fd2892d813f64
Author: Isis Lovecruft <isis at torproject.org>
Date: Thu Oct 29 20:32:10 2015 +0000
Add draft proposal of new guard selection algorithm.
---
proposals/xxx-guard-selection.txt | 303 +++++++++++++++++++++++++++++++++++++
1 file changed, 303 insertions(+)
diff --git a/proposals/xxx-guard-selection.txt b/proposals/xxx-guard-selection.txt
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+Filename: xxx-guard-selection.txt
+Title: New Guard Selection Behaviour
+Author: Isis Lovecruft, George Kadianakis
+Created: 2015-10-28
+Status: Draft
+Extends: 241-suspicious-guard-turnover.txt
+
+
+§1. Overview
+
+ In addition to the concerns regarding path bias attacks, namely that the
+ space from which guards are selected by some specific client should not
+ consist of the entirely of nodes with the Guard flag (cf. §1 of proposal
+ #247), several additional concerns with respect to guard selection behaviour
+ remain. This proposal outlines a new entry guard selection algorithm, which
+ additionally addresses the following concerns:
+
+ - Heuristics and algorithms for determining how and which guard(s)
+ is(/are) chosen should be kept as simple and easy to understand as
+ possible.
+
+ - Clients in censored regions or who are behind a fascist firewall who
+ connect to the Tor network should not experience any significant
+ disadvantage in terms of reachability or usability.
+
+ - Tor should make a best attempt at discovering the most appropriate
+ behaviour, with as little user input and configuration as possible.
+
+
+§2. Design
+
+ Alice, an OP attempting to connect to the Tor network, should undertake the
+ following steps to determine information about the local network and to
+ select (some) appropriate entry guards. In the following scenario, it is
+ assumed that Alice has already obtained a recent, valid, and verifiable
+ consensus document.
+
+ Before attempting the guard selection procedure, Alice initialises the guard
+ data structures and prepopulates the guardlist structures, including the
+ UTOPIC_GUARDLIST and DYSTOPIC_GUARDLIST (cf. §XXX). Additionally, the
+ structures have been designed to make updates efficient both in terms of
+ memory and time, in order that these and other portions of the code which
+ require an up-to-date guard structure are capable of obtaining such.
+
+ 0. Determine if the local network is potentially accessible.
+
+ Alice should attempt to discover if the local network is up or down,
+ based upon information such as the availability of network interfaces
+ and configured routing tables. See #16120. [0]
+
+ [XXX: This section needs to be fleshed out more. I'm ignoring it for
+ now, but since others have expressed interest in doing this, I've added
+ this preliminary step. —isis]
+
+ 1. Check that we have not already attempted to add too many guards
+ (cf. proposal #241).
+
+ 2. Then, if the PRIMARY_GUARDS on our list are marked offline, the
+ algorithm attempts to retry them, to ensure that they were not flagged
+ offline erroneously when the network was down. This retry attempt
+ happens only once every 20 mins to avoid infinite loops.
+
+ [Should we do an exponential decay on the retry as s7r suggested? —isis]
+
+ 3. Take the list of all available and fitting entry guards and return the
+ top one in the list.
+
+ 4. If there were no available entry guards, the algorithm adds a new entry
+ guard and returns it. [XXX detail what "adding" means]
+
+ 5. Go through the steps 1-4 above algorithm, using the UTOPIC_GUARDLIST.
+
+ 5.a. When the GUARDLIST_FAILOVER_THRESHOLD of the UTOPIC_GUARDLIST has
+ been tried (without success), Alice should begin trying steps 1-4
+ with entry guards from the DYSTOPIC_GUARDLIST as well. Further,
+ if no nodes from UTOPIC_GUARDLIST work, and it appears that the
+ DYSTOPIC_GUARDLIST nodes are accessible, Alice should make a note
+ to herself that she is possibly behind a fascist firewall.
+
+ 5.b. If no nodes from either the UTOPIC_GUARDLIST or the
+ DYSTOPIC_GUARDLIST are working, Alice should make a note to
+ herself that the network has potentially gone down. Alice should
+ then schedule, at exponentially decaying times, to rerun steps 0-5.
+
+ [XXX Should we do step 0? Or just 1-4? Should we retain any
+ previous assumptions about FascistFirewall? —isis]
+
+ 6. [XXX Insert potential other fallback mechanisms, e.g. switching to
+ using bridges? —isis]
+
+
+§3. New Data Structures, Consensus Parameters, & Configurable Variables
+
+§3.1. Consensus Parameters & Configurable Variables
+
+ Variables marked with an asterisk (*) SHOULD be consensus parameters.
+
+ DYSTOPIC_GUARDS ¹
+ All nodes listed in the most recent consensus which are marked with
+ the Guard flag and which advertise their ORPort(s) on 80, 443, or any
+ other addresses and/or ports controllable via the FirewallPorts and
+ ReachableAddresses configuration options.
+
+ UTOPIC_GUARDS
+ All nodes listed in the most recent consensus which are marked with
+ the Guard flag and which do NOT advertise their ORPort(s) on 80, 443,
+ or any other addresses and/or ports controllable via the FirewallPorts
+ and ReachableAddresses configuration options.
+
+ PRIMARY_GUARDS *
+ The number of first, active, PRIMARY_GUARDS on either the
+ UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST as "primary". We will go to
+ extra lengths to ensure that we connect to one of our primary guards,
+ before we fall back to a lower priority guard. By "active" we mean that
+ we only consider guards that are present in the latest consensus as
+ primary.
+
+ UTOPIC_GUARDS_ATTEMPTED_THRESHOLD *
+ DYSTOPIC_GUARDS_ATTEMPTED_THRESHOLD *
+ These thresholds limit the amount of guards from the UTOPIC_GUARDS and
+ DYSTOPIC_GUARDS which should be partitioned into a single
+ UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST respectively. Thus, this
+ represents the maximum percentage of each of UTOPIC_GUARDS and
+ DYSTOPIC_GUARDS respectively which we will attempt to connect to. If
+ this threshold is hit we assume that we are offline, filtered, or under
+ a path bias attack by a LAN adversary.
+
+ There are currently 1600 guards in the network. We allow the user to
+ attempt 80 of them before failing (5% of the guards). With regards to
+ filternet reachability, there are 450 guards on ports 80 or 443, so the
+ probability of picking such a guard guard here should be high.
+
+ This logic is not based on bandwidth, but rather on the number of
+ relays which possess the Guard flag. This is for three reasons: First,
+ because each possible *_GUARDLIST is roughly equivalent to others of
+ the same category in terms of bandwidth, it should be unlikely [XXX How
+ unlikely? —isis] for an OP to select a guardset which contains less
+ nodes of high bandwidth (or vice versa). Second, the path-bias attacks
+ detailed in proposal #241 are best mitigated through limiting the
+ number of possible entry guards which an OP might attempt to use, and
+ varying the level of security an OP can expect based solely upon the
+ fact that the OP picked a higher number of low-bandwidth entry guards
+ rather than a lower number of high-bandwidth entry guards seems like a
+ rather cruel and unusual punishment in addition to the misfortune of
+ already having slower entry guards. Third, we favour simplicity in the
+ redesign of the guard selection algorithm, and introducing bandwidth
+ weight fraction computations seems like an excellent way to
+ overcomplicate the design and implementation.
+
+
+§3.2. Data Structures
+
+ UTOPIC_GUARDLIST
+ DYSTOPIC_GUARDLIST
+ These lists consist of a subset of UTOPIC_GUARDS and DYSTOPIC_GUARDS
+ respectively. The guards in these guardlists are the only guards to
+ which we will attempt connecting.
+
+ When an OP is attempting to connect to the network, she will construct
+ hashring structure containing all potential guard nodes from both
+ UTOPIC_GUARDS and DYSTOPIC_GUARDS. The nodes SHOULD BE inserted into
+ the structure some number of times proportional to their consensus
+ bandwidth weight. From this, the client will hash some information
+ about themselves [XXX what info should we use? —isis] and, from that,
+ choose #P number of points on the ring, where #P is
+ {UTOPIC,DYSTOPIC}_GUARDLIST_ATTEMPTED_THRESHOLD proportion of the
+ total number of unique relays inserted (if a duplicate is selected, it
+ is discarded). These selected nodes comprise the
+ {UTOPIC,DYSTOPIC}_GUARDLIST for (first) entry guards. (We say "first"
+ in order to distinguish between entry guards and the vanguards
+ proposed for hidden services in proposal #247.)
+
+ [Perhaps we want some better terminology for this. Suggestions
+ welcome. —isis]
+
+ Each GUARDLIST SHOULD have the property that the total sum of
+ bandwidth weights for the nodes contained within it is roughly equal
+ to each other guardlist of the same type (i.e. one UTOPIC_GUARDLIST is
+ roughly equivalent in terms of bandwidth to another UTOPIC_GUARDLIST,
+ but necessarily equivalent to a DYSTOPIC_GUARDLIST).
+
+ For space and time efficiency reasons, implementations of the
+ GUARDLISTs SHOULD support prepopulation(), update(), insert(), and
+ remove() functions. A second data structure design consideration is
+ that the amount of "shifting" — that is, the differential between
+ constructed hashrings as nodes are inserted or removed (read: ORs
+ falling in and out of the network consensus) — SHOULD be minimised in
+ order to reduce the resources required for hashring update upon
+ receiving a newer consensus.
+
+ The implementation we propose is to use a Consistent Hashring,
+ modified to dynamically allocate replications in proportion to
+ fraction of total bandwidth weight. As with a normal Consistent
+ Hashring, replications determine the number times the relay is
+ inserted into the hashring. The algorithm goes like this:
+
+ router ↠⊥
+ key ↠0
+ replications ↠0
+ bw_weight_total ↠0
+ while router ∈ GUARDLIST:
+ | bw_weight_total ↠bw_weight_total + BW(router)
+ while router ∈ GUARDLIST:
+ | replications ↠FLOOR(CONSENSUS_WEIGHT_FRACTION(BW(router), bw_total) * T)
+ | factor ↠(S / replications)
+ | while replications != 0:
+ | | key ↠(TOINT(HMAC(ID)[:X] * replications * factor) mod S
+ | | INSERT(key, router)
+ | | replications <- replications - 1
+
+ where:
+
+ - BW is a function for extracting the value of an OR's `w bandwith=`
+ weight line from the consensus,
+ - GUARDLIST is either UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST,
+ - CONSENSUS_WEIGHT_FRACTION is a function for computing a router's
+ consensus weight in relation to the summation of consensus weights
+ (bw_total),
+ - T is some arbitrary number for translating a router's consensus
+ weight fraction into the number of replications,
+ - H is some collision-resistant hash digest,
+ - S is the total possible hash space of H (e.g. for SHA-1, with
+ digest sizes of 160 bits, this would be 2^160),
+ - HMAC is a keyed message authentication code which utilises H,
+ - ID is an hexadecimal string containing the hash of the router's
+ public identity key,
+ - X is some (arbitrary) number of bytes to (optionally) truncate the
+ output of the HMAC to,
+ - S[:X] signifies truncation of S, some array of bytes, to a
+ sub-array containing X bytes, starting from the first byte and
+ continuing up to and including the Xth byte, such that the
+ returned sub-array is X bytes in length.
+ - INSERT is an algorithm for inserting items into the hashring,
+ - TOINT convert hexadecimal to decimal integers,
+
+ For routers A and B, where B has a little bit more bandwidth than A,
+ this gets you a hashring which looks like this:
+
+ B-´¯¯`-BA
+ A,` `.
+ / \
+ B| |B
+ \ /
+ `. ,´A
+ AB--__--´B
+
+ When B disappears, A remains in the same positions:
+
+ _-´¯¯`-_A
+ A,` `.
+ / \
+ | |
+ \ /
+ `. ,´A
+ A`--__--´
+
+ And similarly if B disappears:
+
+ B-´¯¯`-B
+ ,` `.
+ / \
+ B| |B
+ \ /
+ `. ,´
+ B--__--´B
+
+ Thus, no "shifting" problems, and recalculation of the hashring when a
+ new consensus arrives via the update() function is much more time
+ efficient.
+
+ Alternatively, for a faster and simpler algorithm, but non-uniform
+ distribution of the keys, one could remove the "factor" and replace
+ the derivation of "key" in the algorithm above with:
+
+ key ↠HMAC(ID || replications)[:X]
+
+ A reference implementation in Python is available². [1]
+
+
+§4. Footnotes
+
+¹ "Dystopic" was chosen because those are the guards you should choose from if
+ you're behind a FascistFirewall.
+
+² One tiny caveat being that the ConsistentHashring class doesn't dynamically
+ assign replication count by bandwidth weight; it gets initialised with the
+ number of replications. However, nothing in the current implementation
+ prevents you from doing:
+ >>> h = ConsistentHashring('SuperSecureKey', replications=6)
+ >>> h.insert(A)
+ >>> h.replications = 23
+ >>> h.insert(B)
+ >>> h.replications = 42
+ >>> h.insert(C)
+
+
+§5. References
+
+ [0]: https://trac.torproject.org/projects/tor/ticket/16120
+ [1]: https://gitweb.torproject.org/user/isis/bridgedb.git/tree/bridgedb/hashring.py?id=949d33e8#n481
+
+
+-*- coding: utf-8 -*-
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