Subject: Re: Requirements for Service Discovery

From: Stuart Cheshire <[email-masked]>
Date: Tue, 6 Aug 2002 21:03:29 -0700

>This brings up an interesting issue.
>Today we have one DNS search list for host names.
>
>But as a user, if I use DNS for discovering services, it isn't
>clear that I want the same search list for each service.

That is true. We could implement multiple different domains of services.

For any protocol I think we can think of ways to make it arbitrarily 
complicated and configurable, but we have to exercise judgement in 
deciding when all the complication serves enough purpose to justify the 
cost. If it is so complicated that it is never implemented, then it is 
hard to argue that the benefit was worth it.

>Forcing a large discountinuity when going from a "small" to a
>"large" network seems counter to this. Compare this with the SLP
>approach where a "large" network implies introducing DAs but not
>doing service discovery in a completely different fashion - that
>discontinuity is much much smaller.

I disagree.

SLP has one discontinuity when going from a small LAN with no scopes 
(i.e. scope "DEFAULT") to a medium-sized network with multiple scopes. 
How does the user find out in which scope they should be looking for 
printers? What I proposed with mDNS-SD is no different.

SLP has another discontinuity when going from a medium-sized network 
(e.g. one corporation) to a larger network (e.g. the whole Internet). It 
just stops working. How would you, Erik Nordmark at Sun, use SLP (not 
DNS) to find the list of public time servers provided by some other 
company, say Apple?

In contrast, the example I gave before, to enumerate my printers using 
DNS-SD, will work from anywhere on this planet that has working Internet 
connectivity:

nslookup -q=ptr _printer._tcp.stuartcheshire.org.

Stuart Cheshire <[email-masked]>
 * Wizard Without Portfolio, Apple Computer
 * Chairman, IETF ZEROCONF
 * www.stuartcheshire.org

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From: Bernard Aboba <[email-masked]>
Date: Tue, 6 Aug 2002 21:35:22 -0700 (PDT)

> DNS) to find the list of public time servers provided by some other

You just root the SLP DAs within the DNS, so that you can  reach the SLP
DAs for a given domain. This is described in the following draft:

http://www.ietf.org/internet-drafts/draft-zhao-slp-remote-da-discovery-03.txt