[dns-operations] More Aggressive prefetch for popular names

Hellqvist, Björn bjorn.hellqvist at teliacompany.com
Mon Apr 15 15:12:00 UTC 2019


Isn't this how normal prefetch works today? If a query is done with TTL - X seconds, then fetch the record. 

When a query is received for cached data which is to expire shortly, named can refresh the data from the authoritative server immediately, ensuring that the cache always has an answer available. 

The prefetch specifies the "trigger" TTL value at which prefetch of the current query will take place: when a cache record with a lower TTL value is encountered during query processing, it will be refreshed.

Bjorn Hellqvist
Senior System Expert (Internet, DNS & Automation)
Telia Company
Solna, Sweden

-----Original Message-----
From: dns-operations <dns-operations-bounces at dns-oarc.net> On Behalf Of Paul Hoffman
Sent: den 11 april 2019 15:54
To: Giovane Moura <giovane.moura at sidn.nl>
Cc: dns-operations at lists.dns-oarc.net
Subject: Re: [dns-operations] More Aggressive prefetch for popular names

On 10 Apr 2019, at 23:56, Giovane Moura wrote:

>> However, if
>> the name was served from the cache during the last 25% of the TTL, 
>> that's a good indication that it will be requested again after the 
>> TTL has expired.
> I think we could really use some large-scale measurement studies on 
> caches to understand their actual behavior. Things in the wild tend to 
> behave sometimes quite differently from what we expect.

Yes, that is becoming obvious to even the most hesitant among us.

> While I agree with the general idea, it would be nice to see if that's 
> what happens in the wild.
> Any resolver ops in here that have some data on this?

Yes, please. This might be tricky to do and require many cache dumps in quick succession, but it would be really valuable to the community.

>> Using this non-aggressive pre-fetching "requested from the cache 
>> during the end of lifetime" rule seems useful to resolver users while 
>> only increasing the authoritative load in the less common cases.
> "only increasing the authoritative load in the less common cases."
> That's where it can get tricky. In theory, yes, but in practice it's 
> hard to estimate the aggregate effects from such policy -- it may as 
> well lead to some unintended collateral damage -- and IMO we need more 
> studies on this.

Fully agree. This could even be aided by some resolver developers adding a debug feature.

--Paul Hoffman
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