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Update specs for weight computation and use.

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Mike Perry 2010-02-14 18:16:06 -08:00
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commit 5d4f5ef07d
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178
doc/spec/dir-spec.txt
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@ -1304,8 +1304,42 @@
or does not support (if 'reject') for exit to "most
addresses".
The signature section contains the following item, which appears
Exactly Once for a vote, and At Least Once for a consensus.
The footer section is delineated in all votes and consensuses supporting
consensus method 9 and above with the following:
"directory-footer" NL
It contains two subsections, a bandwidths-weights line and a
directory-signature.
The bandwidths-weight line appears At Most Once for a consensus. It does
not appear in votes.
"bandwidth-weights" SP "Wed=" INT SP "Wee=" INT SP "Weg=" INT SP
"Wem=" INT SP "Wgd=" INT SP "Wgg=" INT SP "Wgm=" INT SP
"Wmd=" INT SP "Wme=" INT SP "Wmg=" INT SP "Wmm=" INT SP NL
These values represent the weights to apply to router bandwidths during
path selection. They are sorted in alphabetical order in the list. The
integer values are divided by BW_WEIGHT_SCALE=10000 or the consensus
param "bwweightscale". They are:
Wgg - Weight for Guard-flagged nodes in the guard position
Wgm - Weight for non-flagged nodes in the guard Position
Wgd - Weight for Guard+Exit-flagged nodes in the guard Position
Wmg - Weight for Guard-flagged nodes in the middle Position
Wmm - Weight for non-flagged nodes in the middle Position
Wme - Weight for Exit-flagged nodes in the middle Position
Wmd - Weight for Guard+Exit flagged nodes in the middle Position
Weg - Weight for Guard flagged nodes in the exit Position
Wem - Weight for non-flagged nodes in the exit Position
Wee - Weight for Exit-flagged nodes in the exit Position
Wed - Weight for Guard+Exit-flagged nodes in the exit Position
These values are calculated as specified in Section 3.4.3.
The signature contains the following item, which appears Exactly Once
for a vote, and At Least Once for a consensus.
"directory-signature" SP identity SP signing-key-digest NL Signature
@ -1586,6 +1620,146 @@
use an accept-style summary and list as much of the port list as is
possible within these 1000 bytes. [XXXX be more specific.]
3.4.3. Computing Bandwidth Weights
Let weight_scale = 10000
Let G be the total bandwidth for Guard-flagged nodes.
Let M be the total bandwidth for non-flagged nodes.
Let E be the total bandwidth for Exit-flagged nodes.
Let D be the total bandwidth for Guard+Exit-flagged nodes.
Let T = G+M+E+D
Let Wgd be the weight for choosing a Guard+Exit for the guard position.
Let Wmd be the weight for choosing a Guard+Exit for the middle position.
Let Wed be the weight for choosing a Guard+Exit for the exit position.
Let Wme be the weight for choosing an Exit for the middle position.
Let Wmg be the weight for choosing a Guard for the middle position.
Let Wgg be the weight for choosing a Guard for the guard position.
Let Wee be the weight for choosing an Exit for the exit position.
Balanced network conditions then arise from solutions to the following
system of equations:
Wgg*G + Wgd*D == M + Wmd*D + Wme*E + Wmg*G (guard bw = middle bw)
Wgg*G + Wgd*D == Wee*E + Wed*D (guard bw = exit bw)
Wed*D + Wmd*D + Wgd*D == D (aka: Wed+Wmd+Wdg = 1)
Wmg*G + Wgg*G == G (aka: Wgg = 1-Wmg)
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.
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.
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
Case 2: E < T/3 && G < T/3 (Both are scarce)
Let R denote the more scarce class (Rare) between Guard vs Exit.
Let S denote the less scarce class.
Subcase a: R+D < S
In this subcase, we simply devote all of D bandwidth to the
scarce class.
Wgg = Wee = weight_scale
Wmg = Wme = Wmd = 0;
if E < G:
Wed = weight_scale
Wgd = 0
else:
Wed = 0
Wgd = weight_scale
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:
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
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:
Wgg = weight_scale
Wee = weight_scale
Wmg = Wme = Wmd = 0
Wgd = (weight_scale*(D+E-G))/(2*D)
Wed = weight_scale - Wgd
Case 3: One of E < T/3 or G < T/3
Let S be the scarce class (of E or G).
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;
if E=S:
Wee = Wed = weight_scale;
Wmd = Wgd = Wmg = 0;
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;
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;
Wmg = weight_scale - Wgg;
Wed = weight_scale - Wmd;
To ensure consensus, all calculations are performed using integer math
with a fixed precision determined by the bwweightscale consensus
parameter (defaults at 10000).
For future balancing improvements, Tor clients support 11 additional weights
for directory requests and middle weighting. These weights are currently
set at 1.0, with the exception of the following groups of assignments:
Directory requests use middle weights:
Wbd=Wmd, Wbg=Wmg, Wbe=Wme, Wbm=Wmm
Handle bridges and strange exit policies:
Wgm=Wgg, Wem=Wee, Weg=Wed
3.5. Detached signatures
Assuming full connectivity, every authority should compute and sign the

46
doc/spec/path-spec.txt
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@ -192,23 +192,41 @@ of their choices.
below)
- XXXX Choosing the length
For circuits that do not need to be "fast", when choosing among
multiple candidates for a path element, we choose randomly.
For "fast" circuits, we only choose nodes with the Fast flag. For
non-"fast" circuits, all nodes are eligible.
For "fast" circuits, we pick a given router as an exit with probability
proportional to its bandwidth.
For all circuits, we weight node selection according to router bandwidth.
For non-exit positions on "fast" circuits, we pick routers as above, but
we weight the bandwidth of Exit-flagged nodes depending
on the fraction of bandwidth available from non-Exit nodes. Call the
total bandwidth for Exit nodes under consideration E,
and the total bandwidth for all nodes under
consideration T. If E<T/3, we do not consider Exit-flagged nodes.
Otherwise, we weight their bandwidth with the factor (E-T/3)/E. This
ensures that bandwidth is evenly distributed over nodes in 3-hop paths.
We also weight the bandwidth of Exit and Guard flagged nodes depending on
the fraction of total bandwidth that they make up and depending upon the
position they are being selected for.
Similarly, guard nodes are weighted by the factor (G-T/3)/G, and not
considered for non-guard positions if this value is less than 0.
These weights are published in the consensus, and are computed as described
in Section 3.4.3 of dir-spec.txt. They are:
Wgg - Weight for Guard-flagged nodes in the guard position
Wgm - Weight for non-flagged nodes in the guard Position
Wgd - Weight for Guard+Exit-flagged nodes in the guard Position
Wmg - Weight for Guard-flagged nodes in the middle Position
Wmm - Weight for non-flagged nodes in the middle Position
Wme - Weight for Exit-flagged nodes in the middle Position
Wmd - Weight for Guard+Exit flagged nodes in the middle Position
Weg - Weight for Guard flagged nodes in the exit Position
Wem - Weight for non-flagged nodes in the exit Position
Wee - Weight for Exit-flagged nodes in the exit Position
Wed - Weight for Guard+Exit-flagged nodes in the exit Position
Wgb - Weight for BEGIN_DIR-supporting Guard-flagged nodes
Wmb - Weight for BEGIN_DIR-supporting non-flagged nodes
Web - Weight for BEGIN_DIR-supporting Exit-flagged nodes
Wdb - Weight for BEGIN_DIR-supporting Guard+Exit-flagged nodes
Wbg - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Wbm - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Wbe - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Wbd - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Additionally, we may be building circuits with one or more requests in
mind. Each kind of request puts certain constraints on paths: