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

[pgf at laptop.org]

jonas wrote:
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 > 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).

sigh.

please disregard everything i wrote in the paragraph above.
i was mistakenly referring to DNS-SD when i should have been
referring to SLP ("service location protocol").  we migrated from
SLP to mDNS.  this has nothing to do with anything martin has
proposed for the XS.  sorry!  :-)

paul

 > 
 > 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 
 > http://www.dns-sd.org/
 > 
 > 
 >   - Jonas
 > 
=---------------------
 paul fox, pgf at laptop.org