[Sugar-devel] Notes on service discovery XS/XO

Jonas Smedegaard dr at jones.dk
Mon Apr 20 15:46:55 EDT 2009

Hash: SHA1

On Mon, Apr 20, 2009 at 09:45:28AM -0400, pgf at laptop.org wrote:
>benjamin m. schwartz wrote:
> > Martin Langhoff wrote:
> > > The short of it is that mdns/dns-sd make sense for a small, 
> > > underutilised network of peers. They assume that the network is a 
> > > cheap resource, that broadcast messages are cheap, and that there 
> > > is no coordinating server.
> > 
> > mDNS assumes all of the above things.  DNS-SD does not.  DNS-SD is 
> > perfectly happy to work on a standard DNS server.  From the spec
> > 
> > """
> >    This document proposes no change to the structure of DNS 
> >    messages, and no new operation codes, response codes, resource 
> >    record types, or any other new DNS protocol values. This document 
> >    simply specifies a convention for how existing resource record 
> >    types can be named and structured to facilitate service 
> >    discovery.
> > """
> > (http://files.dns-sd.org/draft-cheshire-dnsext-dns-sd.txt)
>the last i looked at (and actually used) dns-sd to solve the
>discovery problem, it seemed that dns-sd development had stalled. 
>(and i haven't had a reason to look since.)  i believe we used
>code from Sun, which was all i could find at the time, and it
>wasn't what you'd call production ready.  on the other hand, we
>were using it in a somewhat non-standard way -- in fact, we
>switched to mdns soon after because it fit our deployment model
>better, since we didn't really have a central server.  the XS
>model may be a better fit.
>(this was all 3 or 4 years ago, btw.)

Here's my understanding:

  * DNS-SD is a formalized use of DNS records to store services
    (rather than hosts, the most popular use of DNS records).

  * mDNS is DNS over multicast (using DNS-SD to resolve services).

So it seems to me that if you've switched from DNS-SD to mDNS, then in 
fact you are still relying on DNS-SD, just using an additional layer on 
top of it.

A good introduction (assumably more reliable than Wikipedia) is 

  - Jonas

- -- 
* Jonas Smedegaard - idealist og Internet-arkitekt
* Tlf.: +45 40843136  Website: http://dr.jones.dk/

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