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I'm pretty sure that c-ares is already doing this next server as a
parallel query, just the default timeout isn't where you expect. If
you set it lower, it will start a second request at that point the
timeout is hit, but if the first request responds, it will still use
that response if the next server on the list hasn't yet responded
.... its been a while since I looked at the code, but that seems to
be what I recall. What c-ares does NOT have is an overall query
timeout ... that has been requested previously, but it doesn't
currently exist (though I agree it should). The logic for retries
once it hits the end of the list of nameservers is a bit weird so
predicting when a query will return a failed result is basically
impossible from what I recall. So this seems to be converging on
what I originally suggested then, except now it sounds like also
adding the ability to set an overall query timeout.<br>
<br>
<br>
<div class="moz-cite-prefix">On 1/19/22 7:04 PM, Dmitry Karpov via
c-ares wrote:<br>
</div>
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<div class="WordSection1">
<p class="MsoNormal">> Again, there's a reason happy
eyeballs doesn't just hammer all endpoints returned from
getaddrinfo() simultaneously, I'd think the same reasoning
would go for DNS servers ... be kind ... start a second query
after a short delay if we haven't received a response yet
(e.g. 200ms). <o:p></o:p></p>
<p class="MsoNormal">> It doesn't make sense to hammer more
than 1 DNS server if they're all responsive, you just doubled
the network load for DNS for no reason.<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal">Very true! But in my parallel approach, I
didn’t mean to start all parallel queries simultaneously.<br>
I didn’t nail the details, but obviously such approach should
be similar to the Happy Eyeballs even for single stacks.<br>
<br>
So, parallel queries in the parallel approach should be
started with some small delays like 200ms in Happy Eyeballs,
but the whole name resolution should be controlled by one
constant and deterministic timeout – i.e. 5s, which shouldn’t
depend on the number of the name servers in the list, as it is
currently the case with c-ares.<br>
In my use cases, using c-ares with libcurl, I see different
name resolution timeouts: 5s, 15s,… depending on a number of
bad name servers in the list, which cause some my time
critical services to fail.<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal">And we can’t just use 200ms as a DNS
timeout per name server and iterate name servers sequentially,
because there are high-latency satellite links with big RTTs,
which require 2s and sometimes more for name resolutions.<o:p></o:p></p>
<p class="MsoNormal">That’s why the parallel approach (with
delays between parallel queries) seems to me as a better
solution for bad name servers than the sequential one.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But as I said, any improvements in this
area will be very welcomed c-ares extensions, especially if
they help libcurl with c-ares, used by a lot of people, to
better handle issues with bad name servers.<br>
<br>
Thanks,<br>
Dmitry Karpov<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b>From:</b> Brad House
<a class="moz-txt-link-rfc2396E" href="mailto:brad@brad-house.com"><brad@brad-house.com></a> <br>
<b>Sent:</b> Wednesday, January 19, 2022 2:37 PM<br>
<b>To:</b> c-ares discussions <a class="moz-txt-link-rfc2396E" href="mailto:c-ares@lists.haxx.se"><c-ares@lists.haxx.se></a><br>
<b>Cc:</b> Dmitry Karpov <a class="moz-txt-link-rfc2396E" href="mailto:dkarpov@roku.com"><dkarpov@roku.com></a><br>
<b>Subject:</b> Re: Feature request for parallel queries
for name servers from different protocol families (IPv4 vs
IPv6)<o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">I guess it
always depends on the design of whatever is using c-ares. In
my own use cases, I have a single ares_channel running on an
event loop and enqueue my lookups to there ... so it keeps
state. Nothing with thread local storage or anything, just
dispatching to that event loop for any DNS queries that need
to be performed. The single ares_channel can handle multiple
simultaneous DNS queries.
<br>
<br>
Also, since there is a proposed feedback loop, if a DNS server
is no longer reachable, it will re-sort the list for any
future requests, so it would only impact a single request (ok,
well, whatever number of requests came in before the timeout
or error occurred).<br>
<br>
Again, there's a reason happy eyeballs doesn't just hammer all
endpoints returned from getaddrinfo() simultaneously, I'd
think the same reasoning would go for DNS servers ... be kind
... start a second query after a short delay if we haven't
received a response yet (e.g. 200ms). It doesn't make sense
to hammer more than 1 DNS server if they're all responsive,
you just doubled the network load for DNS for no reason.<br>
<br>
<br>
<o:p></o:p></p>
<div>
<p class="MsoNormal">On 1/19/22 5:25 PM, Dmitry Karpov via
c-ares wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">> I wasn't suggesting this be outside
of c-ares, I was talking about implementing this inside of
c-ares as a simpler alternative to your proposal.<br>
<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal">OK, I got it know. :) <br>
Pre-sorting name servers based on reachability from previous
queries or/and protocol family may help in some cases, but
the sequential approach, even with sorting, still will have
some issues that the parallel approach allows to solve more
efficiently.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">For example, the first query when nothing
is sorted, may cause critical connection timeouts aborting
some applications, and storing name server “reachability
metrics” which name servers will be sorted on will require
either thread local storage (thus requiring each thread to
go through the same “name server discovery” procedure as the
other app threads using c-ares) or some global access to the
metrics data with proper read/write accesses, needed by
multi-threaded apps.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">Also, if run-time conditions change from
the previous query then the sorted list may be not sorted
correctly for the current conditions, and thus not the best
server or even bad server may be tried first, thus
increasing name resolution time.<br>
<br>
The parallel approach, on the other hand, will provide the
fastest name resolution regardless the previous queries, so
it doesn’t need to store any name server metrics and do
pre-processing of the name server list from OS.<o:p></o:p></p>
<p class="MsoNormal"><br>
But I agree that implementing parallel approach may be not
very easy and any improvements in this area will be a very
welcomed extension, anyway.<br>
So, if you think that updated sequential approach with smart
sorting is much easier to implement than the parallel one,
then hopefully we can get it in next c-ares updates.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">Thanks,<br>
Dmitry Karpov<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b>From:</b> Brad House <a
href="mailto:brad@brad-house.com"
moz-do-not-send="true">
<brad@brad-house.com></a> <br>
<b>Sent:</b> Wednesday, January 19, 2022 12:10 PM<br>
<b>To:</b> c-ares discussions <a
href="mailto:c-ares@lists.haxx.se"
moz-do-not-send="true"><c-ares@lists.haxx.se></a><br>
<b>Cc:</b> Dmitry Karpov <a
href="mailto:dkarpov@roku.com" moz-do-not-send="true"><dkarpov@roku.com></a><br>
<b>Subject:</b> Re: Feature request for parallel queries
for name servers from different protocol families (IPv4
vs IPv6)<o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">Commenting
below ...<o:p></o:p></p>
<div>
<p class="MsoNormal">On 1/19/22 2:51 PM, Dmitry Karpov via
c-ares wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">>
Infact, happyeyeballs itself doesn't always do parallel
connection attempts, its an implementation-defined delay
before also attempting the next address in the list.<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">In
case of Happy Eyeballs, a delay between IPv4 and IPv6
connections is constant and typically relatively short –
200-300ms.<br>
But non-functional IPv6 name servers in the server list
may create dynamic delays in connection establishment
which can be very large.<br>
<br>
<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">By
default, c-ares uses 5s timeout per name server, so it
may take 5s and more (if several IPv6 name servers are
in the list) to get to the connection Happy Eyeballs
thus taking much more than expected 200-300ms.<o:p></o:p></p>
</div>
</blockquote>
<p class="MsoNormal"><br>
It would be assumed as part of this patch set, this timer
would be reduced.<br>
<br>
<br>
<br>
<br>
<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><br>
> It would be much easier to stay closer to happy
eyeballs and just sort the dns server list using prior
result success/fail (even upfront sorting using some
algorithm to interleave ipv6/ipv4 in a pattern would
help,
<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">>
maybe with using logic such as from RFC6724 sec 2.1
like we do in ares_getaddrinfo for returned addresses,
but instead of the nameservers themselves).
<br>
<br>
Yes, of course, it is possible that c-ares client can
implement some kind of name server sorting/filtering
logic outside of c-ares and just pass a list of “good”
name servers to c-ares, but in this case it has to be
more involved into the name resolution business than it
would be desired.<o:p></o:p></p>
</div>
</blockquote>
<p class="MsoNormal"><br>
I wasn't suggesting this be outside of c-ares, I was talking
about implementing this inside of c-ares as a simpler
alternative to your proposal.<br>
<br>
-Brad<o:p></o:p></p>
<p class="MsoNormal"><br>
<br>
<o:p></o:p></p>
</blockquote>
<p class="MsoNormal"><o:p> </o:p></p>
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