Monday August 15th we will be updating the NDT unified views to provide researchers with access to the row filter logic used to exclude rows that are not believed to be representative measurements of network performance. This change enables interested researchers to better understand MLab’s data filtering and to easily craft alternate filters to verify our assumptions, or to filter data to address questions other than network throughput.
The “nofilter” versions of the unified views compute and expose all of the filtering criteria columns, but do not perform filtering of the data.
The new unified views are also faster, but there are still opportunities for further improvement.
Changes to the schema
The schema for the new unified views are forward compatible with the current production views. Existing queries using fully supported features (i.e. no fields starting with underscore) should run without modification.
The new unified views also expose additional metadata, including implementation details for the client, server, parser and wire protocol; locations of raw data, and in some cases, pointers to the source code. Documentation of this new meta data will be released in the coming months.
Changes to the data
The new unified download view has new default row filter logic that does not exclude flows with non-network bottlenecks, such as CPU or buffering limits at the client. The old unified_download view excluded flows that did not exhibit any network congestion feedback, such as packet loss or queuing delay, because without feedback their performance was determined by something other than the network. Dave Clark pointed out that this introduces a sample bias against faster networks.
To improve transparency, we also introduce “nofilter” versions of the unified views which expose the multiple row filtering criteria. We describe these in more detail below.
This release does not change any of the values for a.MeanThroughputMbps or other data values, and only increases the number of rows, i.e. tests considered to be valid.
Prerelease versions of the new unified views are available for testing. Researchers using non-standard columns, custom unified views, or who are concerned about changes to the data are encouraged to test with the prototype unified views. See: Release notes for new prototype unified views
The simpleset test is to edit
FROM lines and replace:
then, your queries should just run.
Comparing the Data across changes
The nofilter versions of the unified views provide the columns IsValidBest and IsValid2021, which are the new and old row selection predicates, respectively. These can be used to replicate legacy queries or to implement differential queries. For example:
SELECT COUNT (*) AS TotalRows, COUNTIF (IsValidBest) AS ValidBestRows, COUNTIF (IsValid2021) AS Valid2021Rows, COUNTIF (IsValidBest != IsValid2021) AS DeltaRows FROM `mlab-collaboration.mm_preproduction.unified_downloads_nofilter` -- before Aug 15th -- `measuremement-lab.ndt.unified_downloads_nofilter` -- after Aug 15th WHERE date = '2022-02-15' AND (IsValidBest OR IsValid2021)
Note that IsValid2021 is a reconstruction of the production unified views as of early 2022. The reconstruction matches very closely: averaging within 1 row per day for the first 12+ years of the project. (Mismatches are most likely corner cases, such as tests that straddle midnight). In 2022 we started testing NDT in Google Cloud, and have since discovered that the 2021 production views inappropriately exclude some NDT tests from GCP because the tests used RFC1918 addresses (net 10/8, etc). This bug has not been reconstructed in IsValid2021, but only affects a small sample of data from virtual servers running in canary mode. The new isValidBest is correct for this data.
Since the new views are backwards compatible with the old views, the old documentation is still technically correct, but does not describe how to make use of the new filter logic or improved metadata. Expect this documentation to be added in the coming months.
 Clark, David D. and Wedeman, Sara, Measurement, Meaning and Purpose: Exploring the M-Lab NDT Dataset (August 2, 2021). Available at SSRN: https://ssrn.com/abstract=3898339 or http://dx.doi.org/10.2139/ssrn.3898339