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INTERNET DRAFT                                                  Ron Daniel
draft-ietf-urn-naptr-00.txt                 Los Alamos National Laboratory
                                                          Michael Mealling
                                                   Network Solutions, Inc.
                                                             30 Oct., 1996


                Resolution of Uniform Resource Identifiers
                       using the Domain Name System


Status of this Memo
===================

    This document is an Internet-Draft.  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.''

    To learn the current status of any Internet-Draft, please check
    the ``1id-abstracts.txt'' listing contained in the Internet-
    Drafts Shadow Directories on ftp.is.co.za (Africa),
    nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
    ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).

    This draft expires 05 May, 1997.



Abstract:
=========

Uniform Resource Locators (URLs) are the foundation of the World Wide
Web, and are a vital Internet technology. However, they have proven to
be brittle in practice. The basic problem is that URLs typically
identify a particular path to a file on a particular host. There is no
graceful way of changing the path or host once the URL has been
assigned. Neither is there a graceful way of replicating the resource
located by the URL to achieve better network utilization and/or fault
tolerance. Uniform Resource Names (URNs) have been hypothesized as a
adjunct to URLs that would overcome such problems. URNs and URLs
are both instances of a broader class of identifiers known as Uniform
Resource Identifiers (URIs).

This document describes a new DNS Resource Record, NAPTR (Naming
Authority PoinTeR), that provides rules for mapping parts of URIs to
domain names.  By changing the mapping rules, we can change the host
that is contacted to resolve a URI. This will allow a more graceful
handling of URLs over long time periods, and forms the foundation for a
new proposal for Uniform Resource Names.

In addition to locating resolvers, the NAPTR provides for other naming
systems to be grandfathered into the URN world, provides independence
between the name assignment system and the resolution protocol system,
and allows multiple services (Name to Location, Name to Description,
Name to Resource, ...) to be offered.  In conjunction with the SRV RR
proposal [3], the NAPTR record allows those services to be replicated
for the purposes of fault tolerance and load balancing.


Introduction:
=============

Uniform Resource Locators have been a significant advance in locating
resources on the Internet. However, their  brittle nature over time
has been recognized for several years. The Uniform Resource Identifier
working group proposed the development of Uniform Resource Names to serve
as persistent, location-independent identifiers for Internet resources
in order to overcome most of the problems with URLs. RFC-1737 [1] sets
forth requirements on URNs.

During the lifetime of the URI-WG, a number of URN proposals were
generated. The developers of several of those proposals met in a series
of meetings, resulting in a compromise known as the Knoxville framework.
The major principle behind the Knoxville framework is that the resolution
system must be seperate from the way names are assigned. This is in
marked contrast to most URLs, which identify the host to contact and
the protocol to use. Readers are referred to [2] for background on the
Knoxville framework and for additional information on the context and
purpose of this proposal.

Seperating the way names are resolved from the way they are constructed
provides several benefits. It allows multiple naming approaches and
resolution approaches to compete, as it allows different protocols and
resolvers to be used. There is just one problem with such a separation -
how do we resolve a name when it can't give us directions to its
resolver?

For the short term, DNS is the obvious candidate for the resolution
framework, since it is widely deployed and understood. However, it is
not appropriate to use DNS to maintain information on a per-resource
basis. First of all, DNS was never intended to handle that many
records. Second, the limited record size is inappropriate for catalog
information. Third, domain names are not appropriate as URNs.

Therefore our approach is to use DNS to locate "resolvers" that can
provide information on individual resources, potentially including the
resource itself. To accomplish this, we "rewrite" the URI into a domain
name following the rules provided in NAPTR records. Rewrite rules
provide considerable power, which is important when trying to meet the
goals listed above. However, collections of rules can become difficult
to understand. To lessen this problem, the NAPTR rules are *always*
applied to the original URI, *never* to the output of previous rules.

Locating a resolver through the rewrite procedure may take multiple
steps, but the beginning is always the same. Every URI has a
colon-delimited prefix.  NAPTR resolution begins by taking this prefix,
appending the well-known suffix ".urn.net", and querying the DNS for NAPTR
records at that domain name.  Based on the results of this query, zero
or more additional DNS queries may be needed to locate resolvers for the
URI. The details of the conversation between the client and the resolver
thus located are outside the bounds of this draft.
Three brief examples of this procedure are given in the next section.

The NAPTR RR provides the level of indirection needed to keep the naming
system independent of the resolution system, its protocols, and services.
Coupled with the new SRV resource record proposal[3] there is also the
potential for replicating the resolver on multiple hosts, overcoming some
of the most significant problems of URLs. This is an important and subtle
point. Not only do the NAPTR and SRV records allow us to replicate
the resource, we can replicate the resolvers that know about the replicated
resource. Preventing a single point of failure at the resolver level
is a significant benefit. Seperating the resolution procedure from the
way names are constructed has additional benefits. Different resolution
procedures can be used over time, and resolution procedures that are
determined to be useful can be extended to deal with additional namespaces.


Terminology
===========

"Must" or "Shall" - Software that does not behave in the manner that this
           document says it must is not conformant to this document.
"Should" - Software that does not follow the behavior that this document
           says it should may still be conformant, but is probably broken
           in some fundamental way.
"May" -    Implementations may or may not provide the described behavior,
           while still remaining conformant to this document.


Brief overview and examples of the NAPTR RR:
============================================

A detailed description of the NAPTR RR will be given later, but to give
a flavor for the proposal we first give a simple description of the
record and three examples of its use.

The key fields in the NAPTR RR are order, preference, service, flags,
regexp, and replacement:
* The order field specifies the order in which records MUST be processed
  when multiple NAPTR records are returned in response to a single query.
  A naming authority may have delegated a portion of its namespace to
  another agency. Evaluating the NAPTR records in the correct order is
  necessary for delegation to work properly.
* The preference field specifies the order in which records SHOULD
  be processed when multiple NAPTR records have the same value of "order".
  This field lets a service provider specify the order in which resolvers
  are contacted, so that more capable machines are contacted in preference
  to less capable ones.
* The service field specifies the resolution protocol and resolution
  service(s) that will be available if the rewrite specified by the
  regexp or replacement fields is applied. Resolution protocols are
  the protocols used to talk with a resolver. They will be specified in
  other documents. Resolution services are operations such as
  N2R (URN to Resource), N2L (URN to URL), N2C (URN to URC), etc.
  These are discussed in the URN Framework document[2], and their behavior
  in a particular resolution protocol will be given in the specification for
  that protocol.
* The flags field contains modifiers that affect what happens in the
  next DNS lookup, typically for optimizing the process.
* The regexp field is one of two fields used for the rewrite rules, and is
  the core concept of the NAPTR record. The regexp field is a String
  containing a sed-like substitution expression. (The actual grammar for
  the substitution expressions is given later in this draft). The
  substitution expression is applied to the original URN to determine the
  next domain name to be queried. The regexp field should be used when the
  domain name to be generated is conditional on information in the URI. If
  the next domain name is always known, which is anticipated to be a common
  occurrence, the replacement field should be used instead.
* The replacement field is the other field that may be used for the rewrite
  rule. It is an optimization of the rewrite process for the case where the
  next domain name is fixed instead of being conditional on the content of
  the URI. The replacement field is a domain name (subject to compression if
  a DNS sender knows that a given recipient is able to decompress names in
  this RR type's RDATA field). If the rewrite is more complex than a simple
  substitution of a domain name, the replacement field should be set to . and
  the regexp field used.

Note that the client applies all the substitutions and performs all
lookups - this is not handled in DNS servers. Note also that there is no
reason to provide values in both the replacement and regexp field. Only
one should be specified. If a value is specified in both, the replacement
name MUST be used and the regexp string MUST be ignored. It is the belief
of the developers of this document that regexps should rarely be used. The
replacement field seems adequate for the vast majority of situations. Regexps
are only necessary when portions of a namespace are to be delegated to
different resolvers.



Example 1
---------

Consider a URN that uses the hypothetical DUNS namespace. DUNS numbers are
identifiers for approximately 30 million registered businesses around
the world, assigned and maintained by Dunn and Bradstreet. The URN
might look like:

                 urn:duns:002372413:annual-report-1997

The first step in the resolution process is to find out about the DUNS
namespace. The namespace identifier, duns, is extracted from the URN,
prepended to urn.net, and the NAPTRs for duns.urn.net looked up. It might
return records of the form:

duns.urn.net
;;      order pref flags service          regexp        replacement
 IN NAPTR 100  10  "s"  "dunslink+N2L+N2C"  ""   _dunslink._udp.isi.dandb.com
 IN NAPTR 100  20  "s"  "rcds+N2C"          ""   _rcds._udp.isi.dandb.com
 IN NAPTR 100  30  "s"  "http+N2L+N2C+N2R"  ""   _http._tcp.isi.dandb.com

The order field contains equal values, indicating that no name delegation
order has to be followed. The preference field indicates that the provider
would like clients to use the special dunslink protocol, followed by
the RCDS protocol, and that HTTP is offered as a last resort. All the
records specify the "s" flag, which will be explained momentarily.
The service fields say that if we speak dunslink, we will be able to
issue either the N2L or N2C requests to obtain a URL or a URC (description)
of the resource. The Resource Cataloging and Distribution Service (RCDS)[7]
could be used to get a URC for the resource, while HTTP could be used to get
a URL, URC, or the resource itself.  All the records supply the next
domain name to query, none of them need to be rewritten with the aid of
regular expressions.

The general case might require multiple NAPTR rewrites to locate a
resolver, but eventually we will come to the "terminal NAPTR". Once we
have the terminal NAPTR, our next probe into the DNS will be for a SRV
or A record instead of another NAPTR. Rather than probing for a non-existent
NAPTR record to terminate the loop, the flags field is used to indicate
a terminal lookup. If it has a value of "s", the next lookup should
be for SRV RRs, "a" denotes that A records should sought. A "p" flag is
also provided to indicate that the next action is Protocol-specific, but
that looking up another NAPTR will not be part of it.

Since our example RR specified the "s" flag, it was terminal. Our
next action is to lookup SRV RRs for _rcds._udp.isi.dandb.com, which
will tell us hosts that can provide the necessary resolution service.
That lookup might return:

;;                              Pref Weight Port Target
 _rcds._udp.isi.dandb.com IN SRV 0    0    1000 defduns.isi.dandb.com
                          IN SRV 0    0    1000 dbmirror.com.au
                          IN SRV 0    0    1000 ukmirror.com.uk

telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their RCDS server.
(The reader is referred to the SRV proposal [3] for the interpretation
of the fields above).

There is opportunity for significant optimization here. We can return
the SRV records as additional information for terminal NAPTRs
(and the A records as additional information for those SRVs). While this
recursive provision of additional information is not explicitly blessed
in the DNS specifications, it is not forbidden, and BIND does take
advantage of it [4].  This is a significant optimization. In
conjunction with a long TTL for *.urn.net records, the average number
of probes to DNS for resolving DUNS URNs would approach one.
Therefore, DNS server implementors SHOULD provide additional information
with NAPTR responses. The additional information will be either SRV
or A records. If SRV records are available, their A records may be
provided as recursive additional information.

Note that the example NAPTR records above are intended to represent the
reply the client will see. They are not quite identical to what the
domain administrator would put into the zone files. For one thing, the
administrator should supply the trailing '.' character on replacement
domain names. An additional difference will be illustrated later.


Example 2
---------

Consider a URI namespace based on MIME Content-Ids. The URN might look
like this:

        urn:cid:199606121851.1@mordred.gatech.edu

The first step in the resolution process is to find out about the CID
namespace. The namespace identifier, cid, is extracted from the URN,
prepended to urn.net, and the NAPTR for cid.urn.net looked up. It might
return records of the form:

 cid.urn.net
  ;;       order pref flags service       regexp         replacement
   IN NAPTR 100   10   ""     ""    "/.+@([^@]+)/\1/i"         .

We have only one NAPTR response, so ordering the responses is not
a problem.  The replacement field is empty, so we check the regexp
field and use the pattern provided there. We apply that regexp to the
entire URN to see if it matches, which it does.  The \1 part of the
substitution expression returns the string "mordred.gatech.edu". Since
the flags field does not contain "s" or "a", the lookup is not terminal
and our next probe to DNS is for more NAPTR records: lookup(query=NAPTR,
"mordred.gatech.edu").

While mordred could have its very own NAPTR, maintaining those records
on all the machines at a site as large as Georgia Tech would be an
intolerable burden. Instead, a wildcard may be used so that the domain
administrator at Georgia Tech has only a single NAPTR record to
maintain. That record might look like:

*.gatech.edu IN NAPTR
;;       order pref flags service           regexp  replacement
  IN NAPTR 100  50  "s"  "z3950+N2L+N2C"    ""   _z3950._tcp.gatech.edu.
  IN NAPTR 100  50  "s"  "rcds+N2C"          ""   _rcds._udp.gatech.edu.
  IN NAPTR 100  50  "s"  "http+N2L+N2C+N2R"  ""   _http._tcp.gatech.edu.

(Unlike all the other example records in this draft, the one above
is the contents of a zone file, not the response received by the
client. That is so that the wildcard can be seen.)

Continuing with our example, we note that the values of the order and
preference fields are equal in all records, so the client is free to
pick any record. The flags field tells us that these are the last NAPTR
patterns we should see, and after the rewrite (a simple replacement in
this case) we should look up SRV records to get information on the
hosts that can provide the necessary service.

Assuming we prefer the Z39.50 protocol, our lookup might return:

;;                            Pref Weight Port Target
_z3950._tcp.gatech.edu IN SRV 0    0      1000 z3950.gatech.edu
                       IN SRV 0    0      1000 z3950.cc.gatech.edu
                       IN SRV 0    0      1000 z3950.uga.edu

telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their Z39.50 server.

Recall that the regular expression used \1 to extract a domain name
from the CID. There is a significant caveat about the use of
backslashes in DNS zone files. DNS treats backslashes as the escape
character so that '.' can be escaped when necessary. This means that
when a regular expression is entered into the zone file, the
backslashes must be escaped by another backslash.  For the case of the
cid.urn.net record above, the regular expression entered into the zone
file should be "/.+@([^@]+)/\\1/i".  When the client code actually
receives the record, the pattern will have been converted to
"/.+@([^@]+)/\1/i".


Example 3
---------

Even if URN systems were in place now, there would still be a
tremendous number of URLs.  It should be possible to develop a URN
resolution system that can also provide location independence for those
URLs.  This is related to the requirement in [1] to be able to
grandfather in names from other naming systems, such as ISO Formal
Public Identifiers, Library of Congress Call Numbers, ISBNs, ISSNs,
etc.

The NAPTR RR could also be used for URLs that have already been assigned.
Assume we have the URL for a very popular piece of software that the
publisher wishes to mirror at multiple sites around the world:

     http://www.foo.com/software/latest-beta.exe

We extract the prefix, "http", and lookup NAPTR records for
http.urn.net. This might return a record of the form

http.urn.net IN NAPTR
;;  order   pref flags service      regexp             replacement
     100     90   ""      ""   "/.*\/\/([^\/:]+)/\1/i"       .

This expression returns everything after the first double slash and
before the next slash or colon. Backslashes are needed to escape the
forward slash since the forward slash character is what separates the
components of the substitution expression. (Recall from the previous
example that in the zone file, this pattern actually needs to be
entered as "/.*\\/\\/([^\\/:]+)/\\1/i").  Applying this pattern to the
URL extracts "www.foo.com". Looking up NAPTR records for that might
return:

www.foo.com
;;       order pref flags   service  regexp     replacement
 IN NAPTR 100  100  "s"   "http+L2R"   ""    _http._tcp.foo.com
 IN NAPTR 100  100  "s"   "ftp+L2R"    ""    _ftp._tcp.foo.com

Looking up SRV records for _http._tcp.foo.com would return information
on the hosts that foo.com has designated to be its mirror sites. The
client can then pick one for the user.


NAPTR RR Format
===============

The format of the NAPTR RR is given below. The DNS type code for
NAPTR is 104 [this is being changed, we believe it will be assigned 35].

    Domain TTL Class Order Preference Flags Service Regexp Replacement

where:

Domain
       The domain name this resource record refers to.
TTL
       Standard DNS Time To Live field
Class
       Standard DNS meaning
Order
       A 16-bit integer specifying the order in which the NAPTR records
       MUST be processed to ensure correct delegation of portions
       of the namespace over time. Low numbers are processed before
       high numbers, and once a NAPTR is found that "matches" a URN,
       the client MUST NOT consider any NAPTRs with a higher value
       for order.

Preference
       A 16-bit integer which specifies the order in which NAPTR records
       with equal "order" values SHOULD be processed, low numbers
       being processed before high numbers.  This is similar to the
       preference field in an MX record, and is used so domain
       administrators can direct clients towards more capable hosts
       or lighter weight protocols.

Flags
       A String giving flags to control aspects of the rewriting. Flags
       are single characters from the set [A-Z0-9]. The case of the
       alphabetic characters is not significant.

       At this time only three flags, "S", "A", and "P", are defined. "S"
       means that the next lookup should be for SRV records instead of NAPTR
       records. "A" means that the next lookup should be for A records. The
       "P" flag says that the remainder of the resolution shall be carried
       out in a Protocol-specific fashion, and we should not do any more
       DNS queries.

       The remaining alphabetic flags are reserved. The numeric flags may be
       used for local experimentation. The S, A, and P flags are all mutually
       exclusive, and resolution libraries MAY signal an error if more
       than one is given. (Experimental code and code for assiting in the
       creation of NAPTRs would be more likely to signal such an error than
       a client such as a browser). We anticipate that multiple flags will
       be allowed in the future, so implementors MUST NOT assume that the
       flags field can only contain 0 or 1 characters.


Service
       Specifies the resolution service(s) available down this rewrite
       path. It may also specify the particular protocol that is used to
       talk with a resolver. A protocol MUST be specified if the flags field
       states that the NAPTR is terminal. If a protocol is specified, but
       the flags field does not state that the NAPTR is terminal, the next
       lookup MUST be for a NAPTR. The client MAY choose not to perform
       the next lookup if the protocol is unknown, but that behavior MUST NOT
       be relied upon.

       The service field may take any of the values below (using the
       Augmented BNF of RFC 822[5]):

           service_field = [ [protocol] *("+" rs)]
           protocol      = "rcds" / "http" / "hdl" / "rwhois"
           rs            = "N2L" / "N2Ls" / "N2R" / "N2Rs" / "N2C"
                         / "N2Ns" / "L2R" / "L2Ns" / "L2Ls" / "L2C"

       i.e. an optional protocol specification followed by 0 or more
       resolution services. Each resolution service is indicated by
       an initial '+' character.

       Note that the empty string is also a valid service field. This
       will typically be seen at the top levels of a namespace, when it
       is impossible to know what services and protocols will be offered
       by a particular publisher within that name space.

       At this time the known protocols are rcds[7], hdl[8] (binary,
       UDP-based protocols),  http[11] (a textual, TCP-based protocol), and
       rwhois[10] (textual, UDP or TCP based). More will be allowed later.
       The names of the protocols must be formed from the characters [a-Z0-9].
       Case of the characters is not significant.

       The service requests currently allowed are:
             N2L  - Given a URN, return a URL
             N2Ls - Given a URN, return a set of URLs
             N2R  - Given a URN, return an instance of the resource.
             N2Rs - Given a URN, return multiple instances of the resouce,
                    typically encoded using multipart/alternative.
             N2C  - Given a URN, return a collection of meta-information on
                    the named resource. The format of this response is the
                    subject of another document.
             N2Ns - Given a URN, return all URNs that are also identifers
                    for the resource.
             L2R  - Given a URL, return the resource.
             L2Ns - Given a URL, return all the URNs that are identifiers for
                    the resource.
             L2Ls - Given a URL, return all the URLs for instances of
                    of the same resource.
             L2C  - Given a URL, return a description of the resource.

       The actual format of the service request and response will be
       determined by the resolution protocol, and is the subject for other
       documents. Protocols need not offer all services. The labels
       for service requests shall be formed from the set of
       characters [A-Z0-9]. The case of the alphabetic characters is
       not significant.

Regexp
       A STRING containing a substitution expression that is applied to the
       original URI in order to construct the next name to lookup. The grammar
       of the substitution expression is given in the next section.

Replacement
       The next NAME to query for NAPTR, SRV, or A records depending on
       the value of the flags field. As mentioned above, this may be
       compressed.



Substitution Expression Grammar:
================================

The content of the regexp field is a substitution expression. True sed(1)
substitution expressions are not appropriate for use in this application for a
variety of reasons, therefore the contents of the regexp field MUST follow the
grammar below:

  subst_expr   = "/"   ere  "/"  repl  "/"  *flags
  ere          = POSIX Extended Regular Expression (see [9], section 2.8.4)
  repl         = dns_str /  backref / repl dns_str  / repl backref
  dns_str      = ...... which RFC can I cite for this? 1035 seems obsolete .....
  backref      = "\" 1POS_DIGIT
  flags        = "i"
  DNS_CHAR     = "_" / "0" / "1" / ... / "9" / "a" / ... / "z"
  POS_DIGIT    = "1" / "2" / ... / "9"  ; 0 is not an allowed backref value

The result of applying the substitution expression to the original URI shall
be a legal domain name. Since it is possible for the regexp field to be
improperly specified, such that a non-conforming domain name can be
constructed, client software SHOULD verify that the result is a legal
domain name before making queries on it.

Backref expressions "\N" in the repl portion of the substitution expression
are replaced by the (possibly empty) string of characters enclosed by '('
and ')' in the ERE portion of the substitution expression. N is a single
digit from 1 through 9, inclusive. It specifies the N'th backref expression,
the one that begins with the N'th '(' and continues to the matching ')'.
For example, the ERE
                   (A(B(C)DE)(F)G)
has backref expressions:
                    \1  = ABCDEFG
                    \2  = BCDE
                    \3  = C
                    \4  = F
                \5..\9  = error - no matching subexpression

The "i" flag indicates that the ERE matching SHALL be performed in a
case-insensitive fashion. Furthermore, any backref replacements MAY be
normalized to lower case when the "i" flag is given.

Advice to domain administrators:
================================

Beware of regular expressions. Not only are they a pain to get
correct on their own, but there is the previously mentioned interaction
with DNS. Any backslashes in a regexp must be entered twice in a zone
file in order to appear once in a query response. More seriously, the
need for double backslashes has probably not been tested by all
implementors of DNS servers. We anticipate that urn.net will be the
heaviest user of regexps. Only when delegating portions of namespaces
should the typical domain administrator need to use regexps.

On a related note, beware of interactions with the shell when manipulating
regexps from the command line. Since '\' is a common escape character in
shells, there is a good chance that when you think you are saying "\\" you
are actually saying "\".  Similar caveats apply to characters such as
'*', '(', etc.

The URN-WG is discussing the use of international characters in URNs.
The regexp package used may need to be changed from POSIX to one with
the ability to handle UNICODE characters.

The "a" flag allows the next lookup to be for A records rather than
SRV records. Since there is no place for a port specification in the
NAPTR record, when the "A" flag is used the specified protocol must
be running on its default port.


Usage
=====

Pseudocode for a client routine using NAPTRs is given below:

    //
    // findResolver(URN)
    // Given a URN, find a host that can resolve it.
    //
    findResolver(string URN) {
      sprintf(key, "%s.urn.net", extractNS(URN));  // prepend prefix to urn.net
      do {
        rewrite_flag = false;
        terminal = false;
        if (key has been seen) {
          quit with a loop detected error
        }
        add key to list of "seens"
        records = lookup(type=NAPTR, key); // get all NAPTR RRs for 'key'

        sort NAPTR records by "order" field and "preference" field
            (with "order" being more significant than "preference").
        n_naptrs = number of NAPTR records in response.
        curr_order = records[0].order;
        max_order = records[n_naptrs-1].order;

        // Process current batch of NAPTRs according to "order" field.
        for (j=0; j < n_naptrs && records[j].order <= max_order; j++) {
          newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);
          if (!newkey) // Skip to next record if the rewrite didn't match
             continue;
          // We did do a rewrite, shrink max_order to current value
          // so that delegation works properly
          max_order = naptr[j].order;
          // Will we know what to do with the protocol and services
          // specified in the NAPTR? If not, try next record.
          if(!isKnownProto(naptr[j].services)) {
            continue;
          }
          if(!isKnownService(naptr[j].services)) {
            continue;
          }

          // At this point we have a sucessful rewrite and we will know
          // how to speak the protocol and request a known resolution
          // service. Before we do the next lookup, check some
          // optimization possibilities.

          if (strcasecmp(flags, "S")
           || strcasecmp(flags, "P"))
           || strcasecmp(flags, "A")) {
             terminal = true;
             services = naptr[j].services;
             addnl = any SRV and/or A records returned as additional info
                     for naptr[j].
          }
          key = newkey;
          rewriteflag = true;
          break;
        }
      } while (rewriteflag && !terminal);

      // Did we not find our way to a resolver?
      if (!rewrite_flag) {
         report an error
         return NULL;
      }


      // Leave rest to another protocol?
      if (strcasecmp(flags, "P")) {
         return key as host to talk to;
      }

      // If not, keep plugging
      if (!addnl) { // No SRVs came in as additional info, look them up
        srvs = lookup(type=SRV, key);
      }

      sort SRV records by preference, weight, ...
      foreach (SRV record) { // in order of preference
        try contacting srv[j].target using the protocol and one of the
            resolution service requests from the "services" field of the
            last NAPTR record.
        if (successful)
          return (target, protocol, service);
          // Actually we would probably return a result, but this
          // code was supposed to just tell us a good host to talk to.
      }
      die with an "unable to find a host" error;
    }


Notes:
======
  -  The "urn:" prefix is a matter of religious controversy. Client
     code should handle the cases when it is and is not present.
     Similarly, if regular expressions are used in NAPTR records, they
     should be immune to the presence or absence of an initial "urn:".
  -  A client MUST process multiple NAPTR records in the order specified by
     the "order" field, it MUST NOT simply use the first record that provides
     a known protocol and service combination.
  -  If a record at a particular order matches the URI, but the client
     doesn't know the specified protocol and service, the client SHOULD
     continue to examine records that have the same order. The client
     MUST NOT consider records with a higher value of order. This is
     necessary to make delegation of portions of the namespace work.
     The order field is what lets site administrators say "all requests for
     URIs matching pattern x go to server 1, all others go to server 2".
     (A match is defined as:
        1) The NAPTR provides a replacement domain name
        2) The regular expression matches the URN
     )
  -  When multiple RRs have the same "order", the client should use
     the value of the preference field to select the next NAPTR to
     consider. However, because of preferred protocols or services,
     estimates of network distance and bandwidth, etc. clients
     may use different criteria to sort the records.
  -  If the lookup after a rewrite fails, clients are strongly encouraged
     to report a failure, rather than backing up to pursue other rewrite
     paths.
  -  When a namespace is to be delegated among a set of resolvers, regexps
     must be used. Each regexp appears in a separate NAPTR RR. Administrators
     should do as little delegation as possible, because of limitations on
     the size of DNS responses.
  -  Note that SRV RRs impose additional requirements on clients.


Acknowledgments:
=================

The authors would like to thank Keith Moore for all his consultations
during the development of this draft. We would also like to thank Paul
Vixie for his assistance in debugging our implementation, and his answers
on our questions.


References:
===========

[1] RFC-1737 "Functional Requirements for Uniform Resource Names", Karen
    Sollins and Larry Masinter, Dec. 1994.

[2] draft-daigle-urn-framework-00.txt "A Uniform Resource Naming
    Framework", Leslie Daigle and Patrik Faltstrom, June, 1996.

[3] draft-gulbrandsen-dns-rr-srvcs-03.txt  " A DNS RR for specifying the
    location of services (DNS SRV)",  Arnt Gulbrandsen and Paul Vixie,
    March 1996.

[4] Paul Vixie, personal communication.

[5] RFC-822 "Standard for the Format of ARPA Internet Text Messages",
    Dave H. Crocker, August 1982.

[6] Keith Moore, personal communication.

[7] Keith Moore, "Resource Cataloging and Distribution Service", ???
    (insert as [4], renumber the others?)

[8] Charles Orth, Bill Arms; Handle Resolution Protocol Specification,
    http://www.handle.net/docs/client_spec.html

[9] IEEE Standard for Information Technology - Portable Operating System
    Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1); IEEE Std
    1003.2-1992; The Institute of Electrical and Electronics Engineers;
    New York; 1993. ISBN:1-55937-255-9

[10] ... rwhois spec ...
[11] ... encoding resolution service requests in HTTP...


Security Considerations
=======================
  The use of "urn.net" as the registry for URN namespaces is subject to
  denial of service attacks, as well as other DNS spoofing attacks. The
  interactions with DNSSEC are currently being studied. It is expected
  that NAPTR records will be signed with SIG records once the DNSSEC
  work is deployed.

  The rewrite rules make identifiers from other namespaces subject to
  the same attacks as normal domain names. Since they have not been
  easily resolvable before, this may or may not be considered a problem.

  Regular expressions should be checked for sanity, not blindly passed
  to something like PERL.

Author Contact Information:
===========================

Ron Daniel
Los Alamos National Laboratory
MS B287
Los Alamos, NM, USA, 87545
voice:  +1 505 665 0597
fax:    +1 505 665 4939
email:  rdaniel@lanl.gov


Michael Mealling
Network Solutions
505 Huntmar Park Drive
Herndon, VA  22070
voice: (703) 742-0400
fax: (703) 742-9552
email: michaelm@internic.net
URL: http://www.netsol.com/




    This draft expires 05 May, 1997.

Ron Daniel Jr.                       email: rdaniel@lanl.gov
Advanced Computing Lab               voice: +1 505 665 0597
MS B287                                fax: +1 505 665 4939
Los Alamos National Laboratory        http://www.acl.lanl.gov/~rdaniel/
Los Alamos, NM, USA  87545    obscure_term: "hyponym"


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