On Thu, Sep 7, 2017 at 10:07 PM, Mark Andrews <ma...@isc.org> wrote: > > Part of the problem is that we have one TTL value for both freshness > and don't use beyond. > > This is fixable. It is possible to specify two timer values. It > does require adding signaling between recursive servers and > authoritative servers, on zone transfers and update requests. > > You basically add a additional timer field to every record immediately > after the TTL field. This is only returned if the client has > signalled support for the extended field, I suggest using the last > DNS header bit for this as you can determine how you will parse the > response base on whether the bit is set in the response or not. > This field is used to expire records from the cache and its value > is set to the TTL field if the server has learnt the record from > server that doesn't support the extension. > > The existing TTL field is used for freshness checking. When a query > comes in after that value has expired a freshness check is performed > similar to the existing prefetches that happen today. A TTL of 1 > is returned unless the original TTL was 0 in which case 0 is returned. > > New client - new recursive server - new authservers > > example.com. 300 86400 IN A 1.2.3.4 > > +300 seconds > > example.com. 1 86100 IN A 1.2.3.4 > (background query is in process) > > Old client - new recursive server - new authservers > > example.com. 300 IN A 1.2.3.4 > > +300 seconds > > example.com. 1 IN A 1.2.3.4 > (background query is in process) > > New client - new recusive server - old auth servers > > example.com. 300 300 IN A 1.2.3.4 > > +300 seconds > (record has expired from cache, > new query is performed) > > example.com. 300 300 IN A 1.2.3.4 > > For UPDATE a replacement opcode would be cleanest way to signal the > new format is being used. NOTIMP should be returned by servers > that don't support the new opcode. > > There will be a few broken servers that just echo back the new > header bit. > > This way the authoritative servers still control how long records > are stored for. Dead servers will get a little bit of traffic until > the the refresh completes. If the authorative servers are under > attack the clients still see a answer. > > The alternative is to perform the refresh query and if it fails to > complete within X milliseconds return the cached data rather than > returning the cached data and doing the refresh in the background. > > Mark > > -- > Mark Andrews, ISC > 1 Seymour St., Dundas Valley, NSW 2117, Australia > PHONE: +61 2 9871 4742 INTERNET: ma...@isc.org >
While I like the idea of a "don't use beyond" timer, I think it will be a very long time before it is widely deployed (and actually configured by zone owners), and therefore won't solve our immediate need. It would be great if clients could opt-in, but again I don't see that happening anytime soon. So I would start with resolver-operators deciding what seems best for their clients (which is hat is happening whether we like it or not). Adding client opt-out/opt-in would be good. Signalling to say that a response is stale would be good. Adding the second timer (both per-RR and as a zone default value, like TTL is handled) would be good. On a related note - the SOA "expire" timer tells a slave how long to keep serving "stale" zone data when the master cannot be reached. Would that be a reasonable default value for how long a resolver should serve "stale" data when the authoritative servers cannot be reached? (Currently I think most people set a very high value compared to the TTL.) -- Bob Harold
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