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Commit 7af0aa25 authored by Mike Perry's avatar Mike Perry
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Update dir-spec.txt with new weight constraints.

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doc/spec/dir-spec.txt
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......@@ -1632,6 +1632,7 @@
"7" -- Provides keyword=integer pairs of consensus parameters
"8" -- Provides microdescriptor summaries
"9" -- Provides weights for selecting flagged routers in paths
"10" -- Fixes edge case bugs in router flag selection weights
Before generating a consensus, an authority must decide which consensus
method to use. To do this, it looks for the highest version number
......@@ -1694,22 +1695,25 @@
Wme*E + Wee*E == E (aka: Wee = 1-Wme)
We are short 2 constraints with the above set. The remaining constraints
come from examining different cases of network load.
come from examining different cases of network load. The following
constraints are used in consensus method 10 and above. There are another
incorrect and obsolete set of constraints used for these same cases in
consensus method 9. For those, see dir-spec.txt in Tor 0.2.2.10-alpha
to 0.2.2.16-alpha.
Case 1: E >= T/3 && G >= T/3 (Neither Exit nor Guard Scarce)
In this case, the additional two constraints are: Wme*E == Wmd*D and
Wgd == 0, which maximizes Exit-flagged bandwidth in the middle position.
In this case, the additional two constraints are: Wmg == Wmd,
Wed == 1/3.
This leads to the solution:
Wgg = (weight_scale*(D+E+G+M))/(3*G)
Wmd = (weight_scale*(2*D + 2*E - G - M))/(6*D)
Wme = (weight_scale*(2*D + 2*E - G - M))/(6*E)
Wee = (weight_scale*(-2*D + 4*E + G + M))/(6*E)
Wmg = weight_scale - Wgg
Wed = weight_scale - Wmd
Wgd = 0
Wgd = weight_scale/3
Wed = weight_scale/3
Wmd = weight_scale/3
Wee = (weight_scale*(E+G+M))/(3*E)
Wme = weight_scale - Wee
Wmg = (weight_scale*(2*G-E-M))/(3*G)
Wgg = weight_scale - Wmg
Case 2: E < T/3 && G < T/3 (Both are scarce)
......@@ -1733,25 +1737,35 @@
Subcase b: R+D >= S
In this case, if M <= T/3, we have enough bandwidth to try to achieve
a balancing condition, and add the constraints Wgg == 1 and
Wme*E == Wmd*D:
a balancing condition.
Wgg = weight_scale
Wgd = (weight_scale*(D + E - 2*G + M))/(3*D) (T/3 >= G (Ok))
Wmd = (weight_scale*(D + E + G - 2*M))/(6*D) (T/3 >= M)
Wme = (weight_scale*(D + E + G - 2*M))/(6*E)
Wee = (weight_scale*(-D + 5*E - G + 2*M))/(6*E) (2E+M >= T/3)
Wmg = 0;
Wed = weight_scale - Wgd - Wmd
Add constraints Wgg = 1, Wmd == Wgd to maximize bandwidth in the guard
position while still allowing exits to be used as middle nodes:
If M >= T/3, the above solution will not be valid (one of the weights
will be < 0 or > 1). In this case, we use:
Wee = (weight_scale*(E - G + M))/E
Wed = (weight_scale*(D - 2*E + 4*G - 2*M))/(3*D)
Wme = (weight_scale*(G-M))/E
Wmg = 0
Wgg = weight_scale
Wmd = (weight_scale - Wed)/2
Wgd = (weight_scale - Wed)/2
If this system ends up with any values out of range (ie negative, or
above weight_scale), use the constraints Wgg == 1 and Wee == 1, since
both those positions are scarce:
Wgg = weight_scale
Wee = weight_scale
Wmg = Wme = Wmd = 0
Wgd = (weight_scale*(D+E-G))/(2*D)
Wed = weight_scale - Wgd
Wed = (weight_scale*(D - 2*E + G + M))/(3*D)
Wmd = (weight_Scale*(D - 2*M + G + E))/(3*D)
Wme = 0
Wmg = 0
Wgd = weight_scale - Wed - Wmd
If M > T/3, then the Wmd weight above will become negative. Set it to 0
in this case:
Wmd = 0
Wgd = weight_scale - Wed
Case 3: One of E < T/3 or G < T/3
......@@ -1759,36 +1773,44 @@
Subcase a: (S+D) < T/3:
if G=S:
Wgg = Wgd = weight_scale;
Wmd = Wed = Wmg = 0;
Wme = (weight_scale*(E-M))/(2*E);
Wee = weight_scale-Wme;
Wgg = Wgd = weight_scale;
Wmd = Wed = Wmg = 0;
// Minor subcase, if E is more scarce than M,
// keep its bandwidth in place.
if (E < M) Wme = 0;
else Wme = (weight_scale*(E-M))/(2*E);
Wee = weight_scale-Wme;
if E=S:
Wee = Wed = weight_scale;
Wmd = Wgd = Wmg = 0;
Wmg = (weight_scale*(G-M))/(2*G);
Wgg = weight_scale-Wmg;
Wee = Wed = weight_scale;
Wmd = Wgd = Wme = 0;
// Minor subcase, if G is more scarce than M,
// keep its bandwidth in place.
if (G < M) Wmg = 0;
else Wmg = (weight_scale*(G-M))/(2*G);
Wgg = weight_scale-Wmg;
Subcase b: (S+D) >= T/3
if G=S:
Add constraints Wmg = 0, Wme*E == Wmd*D to maximize exit bandwidth
in the middle position:
Wgd = (weight_scale*(D + E - 2*G + M))/(3*D);
Wmd = (weight_scale*(D + E + G - 2*M))/(6*D);
Wme = (weight_scale*(D + E + G - 2*M))/(6*E);
Wee = (weight_scale*(-D + 5*E - G + 2*M))/(6*E);
Wgg = weight_scale;
Wmg = 0;
Wed = weight_scale - Wgd - Wmd;
Add constraints Wgg = 1, Wmd == Wed to maximize bandwidth
in the guard position, while still allowing exits to be
used as middle nodes:
Wgg = weight_scale
Wgd = (weight_scale*(D - 2*G + E + M))/(3*D)
Wmg = 0
Wee = (weight_scale*(E+M))/(2*E)
Wme = weight_scale - Wee
Wmd = (weight_scale - Wgd)/2
Wed = (weight_scale - Wgd)/2
if E=S:
Add constraints Wgd = 0, Wme*E == Wmd*D:
Wgg = (weight_scale*(D + E + G + M))/(3*G);
Wmd = (weight_scale*(2*D + 2*E - G - M))/(6*D);
Wme = (weight_scale*(2*D + 2*E - G - M))/(6*E);
Wee = (weight_scale*(-2*D + 4*E + G + M))/(6*E);
Wgd = 0;
Add constraints Wee == 1, Wmd == Wgd to maximize bandwidth
in the exit position:
Wee = weight_scale;
Wed = (weight_scale*(D - 2*E + G + M))/(3*D);
Wme = 0;
Wgg = (weight_scale*(G+M))/(2*G);
Wmg = weight_scale - Wgg;
Wed = weight_scale - Wmd;
Wmd = (weight_scale - Wed)/2;
Wgd = (weight_scale - Wed)/2;
To ensure consensus, all calculations are performed using integer math
with a fixed precision determined by the bwweightscale consensus
......
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