[dhcwg] Review of Service-Discovery-Type options in DHCP

At the Yokohama meeting I pointed out that many DHCP options carry 
information that might be better served via an explicit service discovery 
mechanism.

Thomas Narten pointed out that the IETF community has not arrived at an 
undisputed consensus about how to do service discovery.

I agree with this in the broad sense, but for simple service discovery 
the world does seem to have arrived at an ad-hoc) solution -- functional 
hostnames in DNS.

Common practice is that most companies have a hostname of the form 
"www.company.com." which resolves to the address of the company's web 
server.

Similar examples are:

ntp.company.com.     (NTP Time Server)
mail.company.com.    (Incoming mail)
smtp.company.com.    (SMTP relay for outgoing mail)
pop.company.com.     (POP mailbox host)
imap.company.com.    (IMAP mail access)

This is an ugly solution for two reasons:
1. It is a misuse of DNS host names to name things that are logical 
services, not hosts.
2. The names are not standardized --
is it "ntp.company.com.", or "time.company.com." ?

Fortunately, DNS already has a solution that addresses these two 
deficiencies: SRV records

1. SRV records explicitly names services, not hosts, and
2. SRV records have a formalized naming convention -- the name of the SRV
   record for NTP service at "company.com." is "_ntp._udp.company.com."

The only missing piece of information that the client need to learn is 
the "company.com." component of the service names it should be looking up.

Perhaps the existing DHCP "Domain Name" option (option code 15) meets 
this need.

If you bring your laptop to Apple, and boot it using DHCP, then the DHCP 
"Domain Name" option will say, "apple.com.", and you can find:

1. A time server by looking up SRV record "_ntp._udp.apple.com."
2. An SMTP relay by looking up SRV record "_smtp._tcp.apple.com."

The people at the Apple offices in Europe get a DHCP "Domain Name" option 
which says, "euro.apple.com.", and they find services by looking up names 
like: "_ntp._udp.euro.apple.com." and "_smtp._tcp.euro.apple.com."

If not the existing "Domain Name" option, perhaps the "Domain Search 
List", or something like it, is the right answer. That way, a single DHCP 
option points you to the place in the DNS name space where all your SRV 
records can be found, instead of every service having to have 
yet-another-separate-option-of-its-own in DHCP.

Stuart Cheshire <cheshire@apple.com>
 * Wizard Without Portfolio, Apple Computer
 * Chairman, IETF ZEROCONF
 * www.stuartcheshire.org

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