jmdns constants

Question

I have been using JmDNSfor a while now. I could use it for the purposes of my application. Every thing works fine for me (I have "announcer" machines and a "listening" one, and this latter machine can see the other devices and discover their information).

It is true that I've managed to work with the JmDNS jar file, but I did it without totally understanding what is going on in this file. Now I want to know about the effect of using JmDNS for the network traffic. I have consulted the documentation but couldn't manage to discover the signification of the constants, like QUERY_WAIT_INTERVAL, PROBE_THROTTLE_COUNT, etc.

I want to know the default frequency with which the announcer machine sends service announcements.

I also noticed DNS_TTL that was described as follows: "The default TTL is set to 1 hour by the standard, so a record is going to stay in the cache of any listening machine for an hour without need to ping the server again".

I understand that it is the Time To Live of the service to stay in the DNS cache, but I couldn't understand what is intended by "purge the server". Does it mean that the listener has to ask the announcer about a service when the DNS_TTL expires? if so, why do need to have the announcer announce its service every 1s (ANNOUNCE_WAIT_INTERVAL = 1000 milliseconds)?

I am so confused.

Answer 1

The way that the Domain Name System works is basically very simple. Fundamentally it's a tree-like system which starts with the root nameservers. These then delegate name space out to the next level. That level in turn delegates out the next level and so on. For example . is the root, which delegates to .com., which can then delegate out example.com.. (Yes, that trailing . is actually part of the domain name, though you almost never have to use it or see it.

When you load a web page there are usually hundreds of elements that load. This is every image, every JS file, every CSS file, etc. To have your computer request that same domain to IP resolution that many times for one page would make load time unbearable and also create massive unnecessary traffic on the nameserver. Therefore DNS caches. The TTL is how long it caches for. If it's set to 24 hours then when you get an answer for that resolution, that's how long you can hold on to it for before you make another request.

The announcing that you're talking about is the nameserver basically announcing that it's responsible for those domains. You want it constantly stating that so other nameservers know where to go to get the correct (authoritative) data.

Throttling is a term used in many fields and applications and means you're limiting your traffic flow so it doesn't get overloaded.

DNS is actually quite simple to understand once you get the basics down.

Here are a few links that could help you get a better grip of it all:

• Few paragraphs of basic DNS info
• About.com guide
• A few definitions
• Relatively simple and informative PDF from IETF