Subject: Re: dns server discovery using mdns

From: Erik Guttman <[email-masked]>
Date: Wed, 2 May 2001 02:26:22 -0700 (PDT)

> I think Bill is having a wish for a client which want to use a service want
> to find the service on an IP based network without any pre-configuration at
> all.
<snip>
> When talking with Bill, it sounded like he want to do an IP-based version
> of the NBPLookup call in AppleTalk, which could be done over multicast of
> course, and it doesn't have to be a DNS query, and it doesn't have to be a
> DNS response.
 
If we have mDNS (as per draft-ietf-dnsext-mdns-00.txt) and it supports
arbitrary requests, we can conceivably find local networked printers
supporting IPP with the following request -
 
  QNAME="_IPP._TCP.local.arpa.", QTYPE=SRV
 
This assumes (1) mdns servers on printers will respond to QTYPE=SRV
in addition to QTYPE=A and PTR requests (2) the printer is link-local 
(3) we do not care about *which* printer we find on the link (we will 
find them all).
 
SLPv2 (RFC 2608) allows us to find printers that advertise using SLP,
using admin-scope multicast.  SLP queries are attribute based LDAPv3
search filters - so a client can find *specific* printers which match
their requirements and determine what attributes the printer has
without using application-specific attribute querying mechanisms.
 
The *real problem* with service discovery on the internet is that
one has to add software to do it.  There are many ways to do service
configuration (DHCP, DNS SRV RRs, SLP, and non-standard stuff from
microsoft, salutation, etc.)  Which to choose?
 
> What have the zeroconf guys been doing lately?
 
The ZEROCONF WG has been working on completing WG drafts, 2 have gone
through WG last call, 1 new one.
 
We have discussed publishing a 'zeroconf host requirements' RFC (BCP?)
which would list IETF standards track protocols which satisfy the
protocol requirements.  In this doc, for service discovery, we could
list either SLPv2, mDNS with DNS SRV, or both - we have yet to have
that debate.  This *could* help to narrow the debilitating choices by
saying "A conforming zeroconf host implements the following
required/recommended/elective standards..."
 
> Is querying over multicast for available services a bad thing?
 
Definitely not!  There are issues, though (scalability, administrability,
compatible operation in networks without support for multicast, the
service namespace, etc.)
 
We have thought through these issues for SLPv2.  I do not think we
have done so for mDNS, but for link-local use it is probably OK
anyway.  mDNS based service discovery is appealing since it maps
so well onto the use of administered SRV RR deployment in the
wide-area (non-zeroconf) case.  mDNS is simple (appletalk-like),
while SLP has additional complexity (though not much).
 
> I don't think so, but I ask myself whether this is DNS. I don't
> think it is.
 
It is a particular use of DNS formatted messages requiring hosts
that support services respond in a new way.
 
Erik

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