Hydra

RossO fork at ordersomewherechaos.com
Tue Apr 8 11:46:50 PDT 2003


> Now as to Zeroconf itself (rendevouz as apple is trying to brand it), 
> I am
> not sold on it. Sure DHCP and the like are big ugly beasts but zeroconf
> seems to be able to work best only on small local, very local, scales 
> as
> too big too much will cause the AppleTalk Chatter Effect...

ZC/R is not meant for anything outside of your own LAN. It's great 
behind a NAT.

From: http://www.oreillynet.com/pub/a/wireless/2002/12/20/zeroconf.html

Zeroconf relies on what is called Multicast DNS. Say you are at a party 
and you need to talk to a woman named Suzy. Unicast DNS is like asking 
the host of the party who she is; multicast DNS is like shouting "Is 
Suzy here?" to the whole room.

Naturally, things would get awfully loud if it was a big party, but 
Zeroconf was designed for small local networks, so it isn't a problem. 
And best of all, no one person has to know everyone in the room -- 
being a host is tough, and it's the user who usually ends up with the 
job.

Service discovery uses the same broadcast mechanism, but instead of 
looking for a particular person, you look for capabilities. It's like 
yelling out, "Anyone here know how to mix a Tequila Sunrise?" and 
waiting to see who puts their hand up.

This particular bit of magic is accomplished via a seldom-used DNS 
packet, lovingly named SRV.

The SRV packet was originally designed to locate a service over the 
open Internet. To find Apple's Web server, theoretically, your Web 
browser would query for "_http._tcp.apple.com". If anyone actually used 
the SRV functionality, a DNS server would reply with a list of HTTP 
(Web) servers and some other useful information, such as in which order 
you should try them.

Zeroconf takes this capability and makes it work on the home network. 
Say a word processor wanted to know what printers were on a network. It 
would broadcast a query for "_lpr._tcp.local.arpa" to every device on 
the network (LPR is a common printing protocol). Any printer that heard 
it would respond with a SRV packet with its name and whatever 
information that it felt like giving about its capabilities (color, 
pages per minute, etc.).



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