One document matched: draft-ymbk-opcode-discover-02.txt-9951.txt

Differences from 02.txt-01.txt

Individual Submission                                      Bill Manning 
draft-ymbk-opcode-discover-02.txt                                   ISI 
							     Paul Vixie
								    ISC
							   Erik Guttman
								    SUN
                                                            06 Jun 2001


                         The DISCOVER opcode 

This document is an Internet-Draft and is subject to all provisions of 
Section 10 of RFC2026 except that the right to produce derivative works 
is not granted.

Comments may be submitted to the group mailing list at "mdns@zocalo.net" 
or the authors.  

Per instructions from a chair of the IETF DNSEXT WG to the moderator of 
the namedropppers@ops.ietf.org mailing list, it is forbidden to discuss 
this draft on that list or in the context of that IETF working group. It
is hoped that members of that WG will review this draft and appeal to 
the WG chairs to open a dialog with the authors to adopt this work as
a working group item.  If events so transpire, it is expected that the
prohibition on discussion will be dropped and the draft will be re-issued
as a WG approved activity, with the draft being subject of all provisions
of Section 10 of RFC 2026.

Distribution of this memo is unlimited.

Internet-Drafts are working documents of the Internet Engineering Task 
Force (IETF), its areas, and its working groups.  Note that other groups 
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and 
may be updated, replaced, or obsoleted by other documents at any time.  It 
is inappropriate to use Internet-Drafts as reference material or to cite 
them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

This work was originally funded under DARPA grant: F30602-99-1-0523

The capitalized keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119

0. Abstract

   The QUERY opcode in the DNS is designed for unicast. With the 
   development of multicast capabilities in the DNS, it is desireable
   to have a more robust opcode for server interactions since a single
   request may result in replies from multiple responders. So DISCOVER
   is defined to deal with replies from multiple responders.
   As such, this document extend the core DNS specifications to allow
   clients to have a method for coping with replies from multiple
   responders. Use of this new opcode may facilitate DNS operations in
   modern networking topologies.

1. DISCOVER works like QUERY except:

        1. it can be sent to a broadcast or multicast destination (QUERY
           isn't defined for non-unicast, and arguably shouldn't be.) 
           While DISCOVER could be used for unicast, what is the point?

        2. the Question section, if present, has <QNAME=zonename,QTYPE=SOA>
           tuples. Future work could augment this structure as follows:
           <QNAME=service,QTYPE=SRV>

        3. if QDCOUNT==0 then only servers willing to do recursion should
           answer. Other servers must silently discard the DISCOVER request.

        4. if QDCOUNT!=0 then only servers who are authoritative for the
           zones named by some QNAME should answer.

        5. responses may echo the request's Question section or leave it blank.

        6. responses have "normal" Answer, Authority, and Additional sections.
           e.g. the response is the same as that to a QUERY. It is desireable
           that zero content answers not be sent to avoid badly formed or
           unfulfilled requests. Responses should be sent to the unicast
           address of the requester and the source address should reflect
           the unicast address of the responder.

Example usage for gethostby{name,addr}-style requestors:

        Compute the zone name of the enclosing in-addr.arpa or ip6.int domain.
        
        DISCOVER whether anyone in-scope is authoritative for this zone.

                If so, query these authoritative servers for local
                in-addr/ip6 names.

        If not, DISCOVER whether there are recursive servers available.

                If so, query these recursive servers for local
                in-addr/ip6 names.

	So, a node will issue a multicast request with the DISCOVER opcode at 
	some particular multicast scope.  Then determine, from the replies, 
	whether there are any DNS servers which are authoritative (or support 
	recursion) for the zone. Replies to DISCOVER requests MUST set the
        Recursion Available (RA) flag in the DNS message header.

	It is important to recognize that a requester must be prepared to 
	receive multiple replies from multiple responders.

        Once one learns a host's FQDN by the above means, repeat the process
        for discovering the closest enclosing authoritative server of such
        local name.

        Cache all NS and A data learned in this process, respecting TTL's.

Usage for SRV requestors:

        Do the gethostbyaddr() and gethostbyname() on one's own link-local
        address, using the above process.

        Assume that the closest enclosing zone for which an authority server
        answers an in-scope DISCOVER packet is "this host's parent domain".

        Compute the SRV name as _service._transport.*.parentdomain.
	
	This is a change to the definition as defined in RFC 1034. 
	A wildcard label ("*") in the QNAME used in a DNS message with 
  	opcode DISCOVER SHOULD be evaluated with special rules.  The 
  	wildcard matches any label for which the DNS server data is
 	authoritative.  For example 'x.*.example.com.' would match 
  	'x.y.example.com.' and 'x.yy.example.com.' provided that the
  	server was authoritative for 'example.com.'  In this particular
	case, we suggest the follwing considerations be made:

   getservbyname() can be satisfied by issuing a request with 
   this computed SRV name.  The servent structure can be 
   populated by values returned from a request as follows:

        s_name    The name of the service, "_service" without the 
                  preceding underscore.
        s_aliases The names returned in the SRV RRs in replies
                  to the query.
        s_port    The port number in the SRV RRs replies to the
                  query.  If these port numbers disagree - one
                  of the port numbers is chosen, and only those
                  names which correspond are returned.
        s_proto   The transport protocol from named by the 
                  "_transport" label, without the preceding 
                  underscore.


        Send SRV query for this name to discovered local authority servers.

     Usage for disconnected networks with no authority servers:

        Hosts should run a "stub server" which acts as though its FQDN is a
        zone name.  Computed SOA gives the host's FQDN as MNAME, "." as the
        ANAME, seconds-since-1Jan2000 as the SERIAL, low constants for EXPIRE
        and the other timers.  Computed NS gives the host's FQDN.  Computed
        glue gives the host's link-local address. Or Hosts may run a
	"DNS stub server" which acts as though its FQDN is a zone name.  The 
	rules governing the behavior of this stub server are given elsewhere 
	[1] [2].

        Such stub servers should answer DISCOVER packets for its zone, and
        will be found by the iterative "discover closest enclosing authority
        server" by DISCOVER clients, either in the gethostbyname() or SRV
        cases described above.  Note that stub servers only answer with
        zone names which match QNAME's, not with zone names which are owned
        by QNAME's.

The only deviation from the DNS[3][4] model is that a host (like, say, a 
printer offering LPD services) has a DNS server which answers authoritatively 
for something which hasn't been delegated to it.  However, the only way that
such DNS servers can be discovered is with a new opcode, DISCOVER, which
is explicitly defined to discover undelegated zones for tightly scoped
purposes.  Therefore this isn't officially a violation of DNS's coherency
principles.


3. IANA Considerations

        As a new opcode, the IANA will need to assign a numeric value
        for the memnonic. The last OPCODE assigned was "5", for UPDATE.
        Test implementations have used OPCODE "6". 
	
4. Security Considerations

        No new security considerations are known to be introduced. Using
        multicast for service discovery has the potential for denial of 
        service. It may also be possible to enable deliberate 
        misconfiguration of clients simply by running a malicious DNS 
        back end resolver.

5. Attribution:
	
	This material was generated in discussions on the mdns mailing list
hosted by Zocalo in March 2000. Paul Vixie crystalized the concepts... 
Stuart Cheshire, Bill Woodcock, Erik Guttman and  were active contributors.

6. Author's Address

   Bill Manning
   PO 12317
   Marina del Rey, CA. 90295
   +1.310.322.8102
   bmanning@karoshi.com

   Paul Vixie
   Internet Software Consortium
   950 Charter Street
   Redwood City, CA 94063
   +1 650 779 7001
   <vixie@isc.org>

   Erik Guttman
   Sun Microsystems
   Eichhölzelstr. 7
   74915 Waibstadt Germany
   +49 6227 356 202
   erik.guttman@sun.com

7. References

[1] draft-ietf-dnsext-mdns-00.txt
[2] draft-manning-dnsext-mdns-00.txt
[3] RFC 1034
[4] RFC 1035

-- 
--bill