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Path Transparency Observatory

Path Transparency Observatory

An open-source, public repository for measurement of path transparency and
impairments on new and existing network protocols in the Internet.


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Custom Query

Please specify your query criteria and then click on 'Run Query' to submit the query. Your query will be submitted to a queue and you'll be provided a link to retrieve the results.

Grouping (optional)

Select additional criteria to group the results data (in separate rows in results table and as input for graphical representation) or leave empty.

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Path criteria (optional)

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Name/identifier string
Name/identifier string

Overview of current conditions for observations in the PTO

ecn.connectivity.worksTCP connections with and without ECN were established successfully.
brokenA TCP connection without ECN negotiation attempt was established, but not with ECN.
transientA TCP connection with ECN negotiation was established, but not without. Since there is no reasonable model for a host that only responds to ECN-enable TCP handshakes at this time, we clearly consider this case as a transient measurement error.
offlineNo connection attempt to the target succeeded.
ecn.connectivity.superworks, broken, transient, offlineEach corresponding condition has been observed three times in three parallel trials.
weirdDifferent conditions have been observed in the three parallel trials.
ecn.negotiation_attempt.succeededECN was successfully negotiated.
failedA TCP connection with ECN negotiation attempt was established, but ECN was not successfully negotiated.
ecn.ipmark.ce.seenAt least one CE mark observed on this connection.
ect_zero.seenAt least one ECT(0) mark observed on this connection.
ect_zero.seenAt least one ECT(1) mark observed on this connection.
ecn.path_dependent.weakA host is flagged as weakly path dependent if their is at least one works and at least one broken super condition for it from different vantage points.
strictA host is flagged as strictly path dependent if it is weakly path dependent and there are no transient, offline, or weird super conditions for it.
strongA host is flagged as strongly path dependent if it is strictly path dependent but there are at least two broken super conditions associated with it.
ecn.side_dependent.weakA host is flagged as weakly site dependent, if there are broken but no works and no transient super conditions associated with it.
strongA host is flagged as strongly site dependent, if there are only broken super conditions for it.


path conditions

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and graphical rendering of the results


raw data

to contibute measurments

on path transparency



from different data sets

to generate path transparency conditions

What is Path Transparency?

Given the increasing deployment of middleboxes in the Internet, we see an ossification of the protocol stack that makes it difficult to deploy new protocols or protocol extensions. The data provided by this observatory assists to quantify impairments on path transparency when traffic is altered or treated differently on a network path based on the protocol stack in use.

You can use the query web-frontend to query our observation database and get graphical rendering of the results!

We also provide a Path Transparency Observatory API (PAPI) to run custom queries on the observation data base of the PTO based on a custom-tailored query language, called Interesting Query Language (IQL). PAPI offers additional functionality enabling more complex queries than provided by our web-frontend. Some API methods are not public and require an API key. To receive an API key please contact us.

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January 2017: Webservers that negotiated ECN successfully

ecn.negotiation_attempt.failed [22.3%]ecn.negotiat....succeeded [77.7%]024367130

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January 2017: Paths with connectivity problems if TCP attempts to negotiate ECN

ecn.connectivity.transient [0.1%]ecn.connectivity.broken [0.2%]ecn.connectivity.offline [2%]ecn.connectivity.works [97.7%]031398734

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Explicit Congestion Notification (ECN)

Currently the PTO only host observation on impairments for Explicit Congestion Notification (ECN), an IP/TCP extension for congestion signaling instead of drop. Despite initial deployment problems blocking ECN usage, we have measurement increasing support of ECN by webservers and Apple announced in April 2016 that iOS and macOS devices will probabilistic attempts to negotiate ECN by default on the client side.

The PTO provides access to our latest measurement data that complements a series of measurement started in 2014. Older data as well as measurement data exposing impairments on other protocols and protocol extension are currently in processing and will be successively added to the PTO over the next couple of month. Stay tuned!

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