[tor-dev] Gap between advertised and utilized bandwidth

[Karsten Loesing]

Hi George,

I found this in my IRC backlog from November 7, 2014:

20:34 #tor-dev: < asn> karsten: how should one read these graphs
https://metrics.torproject.org/bandwidth.html#bandwidth-flags ?
20:34 #tor-dev: < asn> karsten: can the gap between two lines be
                       interpreted as available (unused) bw?
20:35 #tor-dev: < asn> karsten: so the gap between the yellow lines
                       is the available guard bw?
20:35 #tor-dev: < asn> karsten: but there is not that much available
                       exit bw, right? so it's not entirely accurate.

These are all fine questions.

First of all, the gap between two lines is *supposed* to be interpreted
as available (unused) bandwidth.  Advertised bandwidth is the number of
bytes that all relay together were willing and capable to handle.  (It's
the minimum of the three bandwidth numbers contained in server
descriptors.)  Bandwidth history is the number of bytes that all relays
together actually handled.  (It's the sum of byte histories contained in
extra-info descriptors.)  So, the difference of the two should be the
number of bytes that was left unused, in theory.

However, I can't say how accurate this comparison is.  If I had the time
to investigate, I'd start by downloading the latest descriptor archives
from CollecTor and comparing advertised bandwidths and bandwidth
histories per relay.  If you or somebody else wants to do that I might
be able to give them a hand.

All the best,
Karsten