Re: [or-talk] where are the exit nodes gone?

[Scott Bennett <bennett@xxxxxxxxxx>]

     On Sun, 11 Apr 2010 06:12:43 -0500 (CDT) I wrote:
>Hi Olaf,
>     On Sun, 11 Apr 2010 12:11:36 +0200 Olaf Selke <olaf.selke@xxxxxxxxxxxx>
>wrote:
>>Scott Bennett schrieb:
>>
>>>      Observed by what?  If it has anything to do with the numbers
>>>  given in the consensus documents, then the only value such graphs
>>> would have would be for the purpose of comparing those graphs with the
>>> values reported by the relays themselves.  The values in the consensus
>>> documents alone are, a priori, worthless.
>>
>>yes, the max and the burst bandwidth are not so much worth for statistic
>
>     You did say "observed", not "advertised".
>
>>purposes. As I mentioned some says ago, "MaxAdvertisedBandwidth 2500 KB"
>>config option leads to an real average bandwidth (measured by mrtg) of
>>about 16000 KB on blutmagie exit. A higher MaxAdvertisedBandwidth value
>
>     Remember, there are exactly two vantage points from which valid
>observations can be made, no more and no less.  One is from inside your
>system's networking stack (including packet filter software).  The other
>is inside your tor relay's process.  Unless the value of "about 16000 KB"
>(/s) comes from one of those two sources, then I simply don't believe the
>so-called measurement, and neither should you.  Such a measurement means,
>at best, only that "it's probably a relatively big number when compared
>to the rest of such numbers in the consensus, and the real number is
>almost certainly larger than this number".
>
>>is killing the cpu with the number of new conns/s.

     I see I missed the implication in Olaf's main complaint above, which
is that the authorities are advertising more capacity for his node than
his node is advertising.  Checking the current (i.e., valid-after
2010-04-11 10:00:00) consensus, I see that the authorities have decided
to volunteer 2560 KB/s on behalf of node blutmagie, which is indeed greater
than 2500 KB/s, though only 2.4% greater.  (For some reason, my current
directory files don't seem to contain a descriptor for blutmagie at all.
I don't know why, but I assume that it will prove to be a temporary
situation.)  In any case, if a consensus document volunteers any capacity
exceeding the smallest of a node's BandwidthRate, RelayBandwidthRate, or
MaxAdvertisedBandwidth, then I believe it should be documented and reported
as a bug in the authority code.
>>
>>Is it possible to use the average observed bandwidth reported by the
>>relays? Knowing the number of exit relays doesn't help very much without
>
>     No, not at the present time because that is not reported by the relays.
>What a relay reports is the highest minimum number of bytes handled in any
>one second in a ten-second sliding window within the the past 24 hours.
>That value is then devalued considerably by the fact that the 24-hour
>periods are not normally consecutive, but rather are overlapped by roughly
>six hours at each end, so that only the middle twelve of the 24 hours are
>represented exclusively in a measurement reporting period.
>     The whole reporting setup is wrong and needs to be revamped from
>scratch in order to get a system that works properly.  As I've noted before,
>the very first and most critical thing to be done is the design separation
>of throughput capacity (which the clients need to know) from actual service
>rendered (which only some humans want to know).  The rest cannot even be
>begun until that much is done.
>
>>knowing about the total provided bandwidth.
>>
>     Probably the best data (i.e., not as bad as any of the other values
>reported) for that purpose would be found in the extra-info documents.
>Divide each field by 900 s to get the average rates in B/s.  One good
>thing about the numbers in the extra-info documents is that both "bytes
>read" and "bytes written" are reported.

     The other things I missed in Olaf's remarks above are *exit* usage
and *exit* capacity.  If tor ever get proper reporting of throughput
capacity, then adding up the capacities of all exit nodes in the consensus
or, arguably, the current directory, would yield the total exit capacity
because it matters not whether data leave a node for a true destination or
just to another node.
     But the total exit usage question cannot be answered at present because
nothing reports that information at present.  Whether tor is keeping such
information locally but not reporting it either locally or to some authority,
I don't know.  If it is, then adding a few lines to write the information to
a log every so often should be fairly trivial to do.


                                  Scott Bennett, Comm. ASMELG, CFIAG
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