In this episode of PING, APNIC Chief Scientist Geoff Huston and I discuss the behaviour of the DNS system when you have to come up from nothing: the “cold start” where no data exists from prior queries in your “cache”. This stems from a talk given by Ondřej Surý, that Geoff saw at the recent RIPE-92 Meeting held in Edinburgh, in the DNS working group. Cache, is the things you hold on to, from prior work done. It’s a fundamental technique in computer science to avoid the massive disparity in speed between parts of the systems, keeping things you use a lot in “fast” memory close to the CPU, and avoiding having to go to “slow” memory or even worse disk or tape files, to find the data. The size of your cache and it’s speed can have a huge influence on the speed of your program. Comparing CPU performance with and without prior cached data can be very instructive to it’s benefits! -In the DNS, Cache is how you avoid having to wait for a remote system to answer over the network (with all the round-trip delay of question and answer) because you know parts of your answer from the prior queries and answers you hung onto. “Cold Start” is a well known problem in large distributed systems. The problem is not just that industrial systems like coal plants and gas turbines need time to warm up and get sufficient energy to drive the generator, in the so-called “black start” there is the problem of energising the magnetic coils associated with generation of electricity. It’s distinct DC energy, used to make the generator enter the state where the rotational force from the turbine actually makes power. Without a source of power, you cannot make the coils “excite” and so the spinning generator won’t actually make any electrical power. Typically, this DC voltage comes from an independent source like a small diesel generator, or a source of electrical power which hasn’t been affected by the outage. If you contract to supply this contingency power, failure to be available is a major issue. The DNS isn’t an electrical generation network, but it does have this kind of complex dependency, mapping from DNS fully qualified name, to the specific IP addresses bound to the name. The “D” part of DNS stands for “Domain” and the elements of the name in a sequence form “domains” of control (or at least potentially do) separated by the dots in the name which may have a distinct “name server” detailing how things are named under-neath that domain boundary. So a single question “what is www.potaroo.net” has the potential to invoke at least 3 if not more questions, for each “domain” in the name. When you have a cache. The cache of known name-to-address pairs includes the data you need to know the addresses of the nameservers for each domain you have seen, which can tell you the name-to-address pairs for the other things you are asking: either directly or indirectly by passing you on to another system. But if you have no cache, when you ask (for example) the nameservers for .NET and are told they are A.NET and B.NET you are no wiser: how do you find these hosts, when you don’t know how to find .NET? Geoff has been chasing down what he sees exploring this “cold start” behaviour, and what it tells us about how people chose to name their hosts and services, how they provide the “name servers” behind these hosts and services, and how intruding intermediaries, or even trying to work around the risks of a cold start can increase the query burden on the clients worldwide.
