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

[Jonas Smedegaard]

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On Mon, Apr 20, 2009 at 07:26:30AM -0400, 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)
>
>I'm not particularly knowledgeable about the XS service discovery 
>requirements, nor about DNS, so I can't reasonably tell you to use 
>DNS-SD.
> What I can say is that it seems like it should be workable.

DNS-SD using unicast DNS seems reasonable to me too.

Looking closer at the RFC, the initial service queries do have an added 
overhead in that a layer of indirection is used (not SRV -> A, but 
instead PTR -> SRV + TXT -> A).  But standard DNS optimizations apply, 
so SOA record should allow clients to preserve bandwidth through 
caching.

In other words: Install dnsmasq on the XOs, use plain standard DNS 
internally and on the wire, setup DNS-SD entries in a standard 
nameserver on the XS, and extend Sugar to support DNS-SD.

I'd be happy to help compose standard BIND9 files, if that is what will 
be used on the XS.


     - Jonas

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

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