[atlas] DNS RTT over TCP: twice as long than UDP?

Folks,

This page at Atlas provides a great visualization on the RTT of DNS
queries over TCP/UDP to the Root DNS servers:

https://atlas.ripe.net/results/maps/root-server-performance/

We can see that the RTT of TCP queries  is at least twice as long than
UDP ones (which, according to the page " TCP is expected to be 2-3 times
" longer.

I cannot replicate these results in my setup, even in large scale from
various vantage points: I always get very similar results for either TCP
or UDP. Example:

UDP:
   * $ dig  example.nl @ns1.dns.nl
      Query time: 8 msec

TCP:
   * $dig +tcp example.nl @ns1.dns.nl
      Query time: 9 msec

I think there might be something with the definition of RTT.

My hypothesis is that the field *rtt* on DNS queries on Atlas[1], for
TCP, is, in fact, measuring 2 RTTs: the RTT of the TCP handshake, and
the RTT of query/response itself.

By definition,  however, RTT is "the length of time it takes for a
signal to be sent plus the length of time it takes for an
acknowledgement of that signal to be received."[2]. So if DNS TCP
measurements starts measuring from the SYN packet, that', in fact, be
two RTTs. Is this the case with Atlas?

I know it may sound a bit like nitpicking, but I just want to be sure of
what's exactly being measured.

thanks a lot,

/giovane



[1] https://atlas.ripe.net/docs/data_struct/#v4750_dns
[2] https://en.wikipedia.org/wiki/Round-trip_delay_time