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
On Fri, Jul 05, 2019 at 12:33:25PM +0000, Giovane Moura <giovane.moura@sidn.nl> wrote a message of 26 lines which said:
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.
I assume (I didn't check with tcpdump) that Atlas starts the clock when the SYN packet leaves, and dig when the DNS request leaves. That would explain your observation. Both make sense, and I disagree when you say that the second method is the only right one. In practice, which method is the most relevant depends on whether you use persistent TCP connections or not.
On 2019/07/05 14:33 , Giovane Moura wrote:
https://atlas.ripe.net/results/maps/root-server-performance/
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?
The use of RTT on that page is wrong. The DNS measurement reports response time. Philip
participants (3)
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Giovane Moura -
Philip Homburg -
Stephane Bortzmeyer