Missing measurements on RIPE Atlas

0.99 MB·

by  Wenqin Shao

Missing measurements on RIPE Atlas
A view from probe connection activity
Wenqin Shao*, Jean-Louis Rougier*, François Devienne† and Mateusz Viste†
*TelecomParisTech,† Border6
ABSTRACT 1.00
7 Weshowthatitiscommontolosesomedatapointsformea- 0.75
1 surementsscheduledatregularintervalonRIPEAtlas. The DF0.50
0 temporalcorrelationbetweenmissingmeasurementsandcon- C
0.25
2
nectioneventsareanalyzed,inthepursuitofunderstanding
0.00
n reasonsbehindsuchmissings. Tooursurprise,abigpartof 0 2 10 100 1000 10800
a measurementsarelostwhileprobesareconnected. # of missing measurement
J
Figure 1: CDF of total missing length per probe.

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1. INTRODUCTION
WeobservedtheRIPEAtlasplatformforonemonth,

] RIPE Atlas, a public measurement platform, enables from 2016-06-01 to 2016-07-01 UTC. All the v3 probes
I
N various application such as performance monitoring [8, firstconnectedbeforethebeginningdate(11613ofthem)
18], anomalies detection [9, 17, 14], peering and IXP are considered. Connection events (id 7000) and built-
.
s measurements [4, 16] etc. As it continues to gain pop- inPingmeasurementstoDNSb-root(id1010),ahighly
c
[ ularity among network operators and researchers, its available destination, are collected [1]. Controllers and

measurement quality becomes a natural concern. It is the ping destination are not within the same network.
1
nowknownthatloadhaveobviousimpactsonmeasure- Controller logs the moments at which probes connects
v
8 ment precision and scheduling [10, 6]. to and disconnects from it. The built-in ping mea-
3 We focus on data completeness, another aspect of surement is scheduled on every probe at 4min interval.
9 measurementqualitythatreceivedlessattentionsofar. 10800 ping results are thus expected from each probe
0
Missingmeasurementscancausevariousundesiredcon- within the month. 7353 probes, out of the available
0
sequences. Apart from widening confidence interval of 11613, had Ping measurements during this period.
.
1 inference[9],itrequiresingeneralmethodologicaladap-
0 3. MISSINGATFIRSTGLANCE
tations, e.g. in spectrum analysis [5, 15, 19], otherwise
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1 biased estimation would be expected [7]. 4440probes(60.4%)missnomorethan2datapoints,
: One obvious reason of missing is that the probe is which is totally legitimate, as random jitter is added
v
not running (properly), e.g. power off [2]. As long as to each single measurement to avoid synchronization
i
X a probe is powered, it tries to maintain a connection to among probes. For the rest, the missing length spans
r acontrollertoreportmeasurementsandreceiveassign- a wide range according to Fig. 1. 1358 (18.5%) probes
a
ments. Therefore the probe connection activity pro- miss more than 10% of the total measurements (i.e. 72
vides a good indication of the probe availability, and hours over a month).
is used in current investigation conducted by RIPE on
probe OS stability [13, 11, 20]. 4. CROSSWITHCONNECTIONEVENTS
In order to infer other possible causes, we crossed
Several reasons may contribute to the disconnection
the measurement timestamps with the moments probe
ofaprobetoitscontroller: 1)probenotworking(prop-
connects to and disconnects from a Atlas controller.
erly); 2) network issues preventing the connection; 3)
If measurement missing coincides with the probe dis-
controller not available, e.g. during maintenance [3].
connection, chances are that the probe is dysfunctional
Meanwhile, the last two reasons shall not prevent a
during the missing. However, if measurements are lost
probe from performing built-in measurements, as the
whiletheprobeiswellconnected,something‘abnormal’
results can be unreachable or timeout, and be stored
should be expected, beyond the known probe OS issue.
locally on the probe [12]. That is to say, missing does
not necessarily occur when probe is disconnected, but is
2. DATACOLLECTION
unexpected while probe is connected.
1
4.1 Overlapwithconnectedperiod
l overlapped l shifted
10800
s
nt (D/C, C) 5793 the rest 8314
me 1000
e
ur 100 1d
s
# mea 102 T ... 4m1ihn
H
shifted overlapped G 0sec
Figure 2: Missing length distribution. RI
... −4min
We count, for each missing segment, the number of −1h
−1d
missingmeasurementsthatoccurredduringaconnected
cpoeurinotd.byWtheeolbetnaginththofemovisesrilnagp sreagtimoenbtys.diTvihdeindgistthriis- −1d−1h−4min 0sec 4min1h 1d −1d−1h−4min 0sec 4min1h 1d
bution of overlap ratio is concentrated at the two ends, Time from LEFT edge to the nearest connection event
0 and 1. For the convenience of illustration, we cut Figure 3: (D/C, C) stands for missing segments more
missing segments into two groups, one with overlap ra- closelycorrelatedwithdisconnectedperiod. Numberof
tio ≤ 0.5, denoted as shifted, the other with the rest, concerned missing segments is given in the title. Neg-
denotedasoverlapped. Measurementmissingthatover- ative time distance means the edge happens before the
laps connected period is ‘unexpected’. connection event and vice verse.
The two groups demonstrate different length distri- Ping interval (4min), and recovery tends to take place
bution profiles, Fig. 2. 15391 missing segments are 4 minute after the connection. Such strong correlation
observed. 10292 (66.87%) missing segments are over- with probe disconnected period indicates that probe
lapped with connected period. They are mostly short dysfunction is probably the cause.
in length. 5560 of them last no more than 2 measure- However,thebeginningofsuchmissingsegments(D/C,
ments. Onepossibleexplanationisthatthesemeasure- C) can as well be dislocated from disconnection event.
ments are skipped due to scheduling or load issues [2, At the left end of the graph, measurements are lost
10]. Meanwhile, 2490 of them are equal to or longer longbeforethedisconnection,whichwefind‘abnormal’,
than 1 hour, involving only 620 probes, for which we even though the recovery is near connection.
believe that the previous explanation hardly applies. Totherightend, measurementsonlybegintobelost
Missing segments shifted from connected period are a long time after the probe is disconnected. One pos-
more likely to be long. This is possibly due to the v3 sible explanation is that measurements are first stored
probe OS stability issue still under investigation. It is locally after disconnection from controller [12]. Then
known to be responsible for long term probe discon- new measurements are lost after local storage is full.
nection and requires manual operation to recover the Contrary to the compact distribution in (D/C, C)
probe [12, 13, 11, 20]. sub-graph, the majority of the rest missing segments
spreadsalongthetimeaxis. Thedistancesfromleftand
4.2 Temporalcorrelation right edge to connection events are highly correlated,
suggesting that both left and right edges are on the
Toobtainasubtlerview,weseektofindout: whendo
same side of a same connection event. We note that
measurementsbegintobelostandwhenaretheyrecov-
thesemeasurementsweremostlylostwhileprobeswere
ered? Are these moments close to connection events?
well connected, i.e. overlapped.
First, we define the left edge as the last measure-
ment before a period of measurement absence, and the
5. CONCLUSION
right edge, accordingly, the first measurement after re-
covery. We then calculate the time interval separating Inouranalysiscoveringalargenumberofprobesover
these edges and the closest connection events. We also one month, only 60% of v3 Atlas probes have complete
identify the nature of these events, and mark ‘D/C’ for measurements. Around1/3missingsegmentsappearto
disconnection, ‘C’ for connection. closelycorrelatedtodisconnectedperiod. TheprobeOS
1284 missing segments locate at the beginning or the stability issue might have contributed to such missings,
end ofthe observation period. Theyare unavailablefor as suggested by the heavy tail of the missing lengths.
this analysis as only one edge can be observed. For the However,theremaining2/3ofmissingsoccurredwhile
rest with both edges, 5793 missing segments’ left edge probes are connected. Half of them are no more than
is closer to a disconnection event and the right edge is 2 measurements in length, and are thus likely to be
closer to a connection event, according to Fig. 3 sub- caused by scheduling issues. However, around 25% of
graph(D/C,C).Judgingfromthedensitycontour,last this category lasts long(≥ 1h). The reason of such an
measurement most likely precedes disconnection by a unexpected behavior is still to be determined.
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