Network Working Group M. Mealling
Internet-Draft Verisign
Expires: August 9, 2001 February 8, 2001
Dynamic Delegation Discovery System (DDDS)
draft-ietf-urn-ddds-04
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes the Dynamic Delegation Discovery System
(DDDS). The DDDS defines an abstract algorithm for applying
dynamically retrieved string transformation rules to an
application-unique string. Well-formed transformation rules will
reflect the delegation of management of information associated with
the string. Other documents specify applications and rule databases
with which this algorithm may be used.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Components of a Rule . . . . . . . . . . . . . . . . . . . . . 7
3.2 Substitution Expression Syntax . . . . . . . . . . . . . . . . 8
3.3 The Complete Algorithm . . . . . . . . . . . . . . . . . . . . 9
4. Specifying An Application . . . . . . . . . . . . . . . . . . 11
5. Specifying A Database . . . . . . . . . . . . . . . . . . . . 13
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1 An Automobile Parts Identification System . . . . . . . . . . 15
6.2 A Document Identification Service . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
The Dynamic Delegation Discovery System is used to map some unique
string to data stored within the DDDS by iteratively applying string
transformation rules until a terminal condition is reached. This
document describes the general algorithm, not any particular
application or usage scenario. It is up to other documents to
describe how they use the DDDS algorithms. Two such documents are
RFXXXX[5] which describes the URI Resolution Application and
RFC2916[7] which describes the E.164 Telephone Number to URI Mapping
Application.
The DDDS's history is an evolution from work done by the Uniform
Resource Name Working Group. When Uniform Resource Names[1] were
originally formulated there was the desire to locate an
authoritative server for a URN that (by design) contained no
information about network locations. A system was formulated that
could use a database of rules that could be applied to a URN to find
out information about specific chunks of syntax. This system was
originally called the Resolver Discovery System[2] and only applied
to URNs.
Over time other systems began to apply this same algorithm and
infrastructure to other, non-URN related, systems (see Section 6 for
examples of other ways of using the DDDS). This caused some of the
underlying assumptions to change and need clarification. This
document, which is one of a series, is an update of those original
URN specifications in order to allow new applications and rule
databases to be developed in a standardized manner.
This document, RFC YYYY[4] and RFC XXXX[5] comprise a suite of
specifications based on the generic DDDS algorithm:
o This document describes the generic algorithm, assuming access to
a seperately defined database of transformation rules and a
specification of the actual application that makes use of the
algorithm.
o RFC YYYY describes a rule database that uses the DNS as its data
store.
o RFC XXXX describes how to use the above two specifications for
pulling apart Uniform Resource Identifiers and finding
authoritative metadata servers for those URIs.
These three documents obsolete RFC 2168[8] and RFC 2915[6] as well
as updates RFC 2276[2].
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2. Terminology
The key words "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.
Application Unique String
A string that is the initial input to a DDDS application. The
lexical structure of this string must imply a unique delegation
path, which is analyzed and traced by the repeated selection and
application of Rewrite Rules.
Rewrite Rule
A rule that is applied to an Application Unique String to produce
either a new key to select a new rewrite rule from the rule
database, or a final result string that is returned to the
calling application. Also simply known as a Rule.
First Well Known Rule
This is a rewrite rule that is defined by the application and not
actually in the Rule Database. It is used to produce the first
valid key.
Terminal Rule
A Rewrite Rule that, when used, yields a string that is the final
result of the DDDS process, rather than another database key.
Application
A set of protocols and specifications that specify actual values
for the various generalized parts of the DDDS algorithm. An
Application must define the syntax and semantics of the
Application Unique String, the First Well Known Rule, and one or
more Databases that are valid for the Application.
Rule Database
Any store of Rules such that a unique key can identify a set of
Rules that specify the delegation step used when that particular
Key is used.
Services
A common rule database may be used to associate different
services with a given Application Unique String; e.g. different
protocol functions, different operational characteristics,
geographic segregation, backwards compatibility, etc. Possible
service differences might be message receiving services for
email/fax/voicemail, load balancing over web servers, selection
of a nearby mirror server, cost vs performance trade-offs, etc.
These Services are included as part of a Rule to allow the
Application to make branching decisions based on the
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applicability of one branch or the other from a Service
standpoint.
Flags
Most Applications will require a way for a Rule to signal to the
Application that some Rules provide particular outcomes that
others do not; e.g. different output formats, extensibility
mechanisms, terminal rule signaling, etc. Most Datatabases will
define a Flags field that an Application can use to encode
various values that express these signals.
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3. The Algorithm
The DDDS algorithm is based on the concept of Rewrite Rules. These
rules are collected into a DDDS Rule Database, and accessed by given
unique keys. A given Rule, when applied to an Application Unique
String, transforms that String into new Key that can be used to
retrieve a new Rule from the Rule Database. This new rule is then
re-applied to the original Application Unique String and the cycle
repeats itself until a terminating condition is reached.
It is a fundamental assumption that the Application Unique String
has some kind of regular, lexical structure that the rules can be
applied to. It is an assumption of the DDDS that the lexical element
used to make a delegation decision is simple enough to be contained
within the Application Unique String itself. The DDDS does not solve
the case where a delegation decision is made using knowledge
contained outside the AUS and the Rule (time of day, financial
transactions, rights management, etc).
Diagramatically the algorithm looks like this:
+--------- Application Unique String
| +-----+
| |input|
| +-------+ +---------+
| | First Well Known Rule |
| +-------+ +--------+
| |output|
| +------+
| First Key
| |
| |
| +----<--------------<--------------+
| | |
| key (a DDDS database always |
| +-----+ takes a key and returns |
| |input| a rule) ^
| +---------+ +------------+ |
| | Lookup key in DDDS Database| |
| +---------+ +-----------+ |
| |output| |
| +------+ |
| rule set |
| | |
| | (the input to a rule |
| rule set is the rule and the AUS. ^
| +-----+ The output is always |
+---------------->|input| either a key or the result) |
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+---------------+ +------------------+ |
| Apply Rules to Application Unique String| |
| until non-empty result are obtained | |
| that meet the applications requirements | |
+---------------+ +-----------------+ |
|output| |
+------+ ^
key |
| |
| |
| |
| |
v |
+--------------------------------------+ |
| Was the last matching rule terminal? | No >------+
+--------------------------------------+
Yes (if the rule isn't terminal
| then its output is the new
| key which is used to find a
| new rule set)
|
+------------------------------------+
| The output of the last rule is the |
| result desired by the application |
+------------------------------------+
3.1 Components of a Rule
A Rule is made up of 4 pieces of information:
A Priority
Simply a number used to show which of two otherwise equal rules
may have precedence. This allows the database to express rules
that are equivalent but weighted for load balancing reasons.
A set of Flags
Flags are used to specify attributes of the rule that determine
if this rule is the last one to be applied. The last rule is
called the terminal rule and its output should be the intended
result for the application.
A description of Services
Services are used to specify semantic attributes of a particular
delegation branch. There are many cases where two delegation
branches are identical except that one delegates down to a result
that provides one set of features while another provides some
other set. Features may include operational issues such as load
balancing, geographically based traffic segregation, degraded but
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backwardly compatibile functions for older clients, etc. For
example, two rules may equally apply to a specific delegation
decision for a string. One rule can lead to a terminal rule that
produces information for use in high availability environments
while another may lead to an archival service that may be slower
but is more stable over long periods of time.
A Substitution Expression
This is the actual string modification part of the rule. It is a
combination of a POSIX Extended Regular Expression[3] and a
replacement string similar to Unix sed-style substitution
expression.
3.2 Substitution Expression Syntax
The character set(s) that the substitution expression is in and can
act on are dependent both on the Application and on the Database
being used. An Application must define what the allowed character
sets are for the Application Unique String. A DDDS Database
specification must define what character sets are required for
producing its keys and for how the substitution expression itself is
encoded. The grammar-required characters below only have meaning
once a specific character set is defined for the Database and/or
Application.
The syntax of the Substitution Expression part of the rule is a
sed-style substitution expression. True sed-style substitution
expressions are not appropriate for use in this application for a
variety of reasons, therefore the contents of the regexp field MUST
follow this grammar:
subst-expr = delim-char ere delim-char repl delim-char *flags
delim-char = "/" / "!" / <Any octet not in 'POS-DIGIT' or 'flags'>
; All occurances of a delim_char in a subst_expr must
; be the same character.>
ere = <POSIX Extended Regular Expression>
repl = *(string / backref)
string = *(anychar / escapeddelim)
anychar = <any character other than delim-char>
escapeddelim = "\" delim-char
backref = "\" POS-DIGIT
flags = "i"
POS-DIGIT = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"
The result of applying the substitution expression to the String
MUST result in a key which obeys the rules of the Database (unless
of course it is a Terminal Rule in which case the output follows the
rules of the application). Since it is possible for the regular
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expression to be improperly specified, such that a non-conforming
key can be constructed, client software SHOULD verify that the
result is a legal database key before using it.
Backref expressions 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. This flag
has meaning only when both the Application and Database define a
character set where case insensitivity is valid.
The first character in the substitution expression shall be used as
the character that delimits the components of the substitution
expression. There must be exactly three non-escaped occurrences of
the delimiter character in a substitution expression. Since escaped
occurrences of the delimiter character will be interpreted as
occurrences of that character, digits MUST NOT be used as
delimiters. Backrefs would be confused with literal digits were this
allowed. Similarly, if flags are specified in the substitution
expression, the delimiter character must not also be a flag
character.
3.3 The Complete Algorithm
The following is the exact DDDS algorithm:
1. The First Well Known Rule is applied to the Application Unique
String which produces a Key
2. The Application asks the Database for the ordered set of Rules
that are bound to that Key (see NOTE below on order details)
3. The Substitution Expression for each Rule in the list is applied
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to the Application Unique String until a non-empty string is
yielded. The rule that produced the non-empty string is used for
the next step. If the list is exhausted without a valid match
then the application is notified that no valid output was
available.
4. If the Service description of the rule does not match the
Application requirements, go back to step 3 and continue through
the already retrieved list of rules.
5. If the Flags part of the Rule designate that this Rule is NOT
Terminal, go back to step 2 with the substitution result as the
new Key.
6. Notify the Application that the process has finished and provide
the Application with the Flags and Services part of the Rule
along with the output of the last Substitution Expression.
NOTE: In some applications and/or databases the result set can
express the case where two or more Rules are considered equal. These
Rules are treated as the same Rule, each one possibly having a
Priority which is used to weight a random selection among the
equivalent Rules (this allows for Rules to 'load balance'
themselves).
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4. Specifying An Application
In order for this algorithm to have any usefulness, a specification
must be written describing an application and one or more databases.
In order to specify an application the following pieces of
information are required:
Application Unique String:
This is the only string that the rewrite rules will apply to. The
string must have some regular structure and be unique within the
application such that anyone applying Rules taken from the same
Database will end up with the same Keys. For example, the URI
Resolution application defines the Application Unique String to
be a URI.
No application is allowed to define an Application Unique String
such that the Key obtained by a rewrite rule is treated as the
Application Unique String for input to a new rule. This leads to
sendmail style rewrite rules which are fragile and error prone.
The one single exception to this is when an Application defines
some flag or state where the rules for that application are
suspended and a new DDDS Application or some other arbitrary set
of rules take over. If this is the case then, by definition, none
of these rules apply. One such case can be found in the URI
Resolution application which defines the 'p' flag which states
that the next step is 'protocol specific' and thus outside of the
scope of DDDS.
First Well Known Rule:
This is the first rule that, when applied to the Application
Unique String, produces the first valid Key. It can be expressed
in the same form as a Rule or it can be something more complex.
For example, the URI Resolution application might specify that
the rule is that the sequence of characters in the URI up to but
not including the first colon (the URI scheme) is the first Key.
Valid Databases:
The application can define which Databases are valid. For each
Database the Application must define how the First Well Known
Rule's output (the first Key) is turned into something that is
valid for that Database. For example, the URI Resolution
application could use the Domain Name System (DNS) as a Database.
The operation for turning this first Key into something that was
valid for the database would be to to turn it into some DNS-valid
domain-name. Additionally, for each Database an Application
defines, it must also specify what the valid character sets are
that will produce the correct Keys. In the URI Resolution example
shown here, the character set of a URI is 7 bit ASCII which
matches fairly well with DNS's 8 bit limitation on characters in
its zone files.
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Expected Output:
The Application must define what the expected output of the
Terminal Rule should be. For example, the URI Resolution
application is concerned with finding servers that contain
authoritative data about a given URI. Thus the output of the
terminal rule would be information (hosts, ports, protocols, etc)
that would be used to contact that authoritative server.
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5. Specifying A Database
Additionally, any Application must have at least one corresponding
Database from which to retrieve the Rules. It is important to note
that a given Database may be used by more than one Application. If
this is the case, each rule must be use some combination of its
Services and/or substitution expression to match only those
Application Unique Strings for which it is valid.
A Database specification must include the following pieces of
information:
General Specification:
The Database must have a general specification. This can
reference other standards (SQL, DNS, etc) or it can fully specify
a novel database system. This specification MUST be clear as to
what allowed character sets exist in order to know in which
character set the Keys and Rules are encoded.
Lookup Procedure:
This specifies how a query is formulated and submitted to the
database. In the case of databases that are used for other
purposes (such as DNS), the specification must be clear as to how
a query is formulated specifically for the database to be a DDDS
database. For example, a DNS based Database must specify which
Resource Records or Query Types are used.
Key Format:
If any operations are needed in order to turn a Key into
something that is valid for the database then these must be
clearly defined. For example, in the case of a DNS database, the
Keys must be constructed as valid domain-names.
Rule Format:
The specification for the output format of a rule.
Rule Insertion Procedure:
A specification for how a Rule is inserted into the database.
This can include policy statements about whether or not a Rule is
allowed to be added.
Rule Collision Avoidance:
Since a Database may be used by multiple Applications (ENUM and
URI Resolution for example), the specification must be clear
about how rule collisions will be avoided. There are usually two
methods for handling this: 1) disallow one key from being valid
in two different Applications; 2) if 1 isn't possible then write
the substitution expression such that the regular expression part
contains enough of the Application Unique String as part of its
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match to differentiate between the two Applications.
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6. Examples
The examples given here are for pedagogical purposes only. They are
specifically taken from fictious applications that have not been
specified in any published document.
6.1 An Automobile Parts Identification System
In this example imagine a system setup where by all automobile
manufacturers come together and create a standardized part numbering
system for the various parts (nuts, bolts, frames, instruments, etc)
that make up the automobile manufacturing and repair process. The
problem with such a system is that the auto industry is a very
distributed system where parts are built by various third parties
distributed around the world. In order to find information about a
given part a system must be able to find out who makes that part and
contact them about it.
To facilitate this distributed system the identification number
assigned to a part is assigned hierarchically such that the first 5
digits make up a parts manufacturer ID number. The next 3 digits are
an auto line identifier (Ford, Toyota, etc). The rest of the digits
are assigned by the parts manufacturer according to rules that the
manufacturer decides.
The auto industry decides to use the DDDS to create a distributed
information retrieval system that routes queries to the actual owner
of the data. The industry specifies a database and a query syntax
for retrieving rewrite rules (the APIDA Network) and then specifies
the Auto Parts Identification DDDS Application (APIDA).
The APIDA specification would define the following:
o Application Unique String: the part number
o First Well Known Rule: take the first 5 digits (the manufacturers
ID number) and use that as the Key
o Valid Databases: The APIDA Network
o Expected Output: EDIFAC information about the part
The APIDA Network Database specification would define the following:
o General Specification: a network of EDI enabled databases and
services that, when given a subcomponent of a part number will
return an XML encoded rewrite rule
o Lookup Procedure: following normal APIDA Network protocols, ask
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the network for a rewrite rule for the Key.
o Key Format: no conversion is required
o Rule Format: see APIDA Network documentation for the XML DTD
o Rule Insertion Procedure: determined by the authority that has
control over each section of the part number. I.e. in order to
get a manufacturer ID you must be a member of the Auto Parts
Manufacturers Association
In order to illustrate how the system would work, imagine the part
number "4747301AB7D". The system would take the first 5 digits,
'47473' and ask the network for that Rewrite Rule. This Rule would
be provided by the parts manufacturers database and would allow the
manufacturer to either further sub-delegate the space or point the
querier directly at the EDIFAC information in the system.
In this example let's suppose that the manufacturer returns a Rule
that states that the next 3 digits should be used as part of a query
to their service in order to find a new Rule. This new Rule would
allow the parts manufacturer to further delegate the query to their
parts factories for each auto line. In our example part number the
number '01A' denotes the Toyota line of cars. The Rule that the
manufacturer returns further delegates the query to a supply house
in Japan. This rule also denotes that this Rule is terminal and thus
the result of this last query will be the actual information about
the part.
6.2 A Document Identification Service
This example is very similar to the last since the documents in this
system can simply be thought of as the auto part in the last
example. The difference here is that the information about the
document is kept very close to the author (usually on their
desktop). Thus there is the probability that the number of
delegations can be very deep. Also, in order to keep from having a
large flat space of authors, the authors are organized by
organizations and departments.
Let's suppose that the Application Unique String in this example
looks like the following:
<organization>-<department>-<author>:<project>-<bookcase>-<book>
The Application specification would look like this:
o Application Unique String: the Document ID string given above
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o First Well Known Rule: the characters up to but not including the
first '-' is treated as the first Key.
o Valid Databases: the DIS LDAP Directory
o Expected Output: a record from an LDAP server containing
bibliographic information about the document in XML
The Database specification for the DIS LDAP Directory would look
like this:
o General Specification: the Database uses the LDAP directory
service. Each LDAP server has a record that contains the Rewrite
Rule. Rules refer to other LDAP servers using the LDAP URL scheme.
o Lookup Procedure: using standard LDAP queries, the client asks
the LDAP server for information about the Key.
o Key Format: no conversion is necessary.
o Rule Format: See the LDAP Rewrite Rule specification
o Rule Insertion Procedure: See the procedures published by the
entity that has authority over that section of the DIS tree. The
first section, the organization, is owned by the DIS Agency.
In this example, the first lookup is for the organization's Rule. At
that point the organization may point the client directly at some
large, organization wide database that contains the expected output.
Other organizations may decentralize this process so that Rules end
up delegating the query all the way down to the authors document
management environment of choice.
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7. Security Considerations
This document simply defines the DDDS algorithm and thus, by itself,
does not imply any security issues. It is when this algorithm is
coupled with a Database and an Application that security
considerations can be known well enough to enumerate them beyond
simply saying that dynamic delegation points are a possible point of
attack.
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8. IANA Considerations
This document does not create any requirements on the IANA. Database
and Application specifications may have considerable requirements
but they cannot be enumerated here.
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References
[1] Moats, R., "URN Syntax", RFC 2141, May 1997.
[2] Sollins, K., "Architectural Principles of Uniform Resource Name
Resolution", RFC 2276, January 1998.
[3] The Institute of Electrical and Electronics Engineers, "IEEE
Standard for Information Technology - Portable Operating System
Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1)", IEEE
Std 1003.2-1992, ISBN 1-55937-255-9, January 1993.
[4] Mealling, M., "A DDDS Database Using The Domain Name System",
RFC YYYY, Internet-Draft
draft-ietf-urn-dns-ddds-database-00.txt, May 2000.
[5] Mealling, M., "URI Resolution using the Dynamic Delegation
Discovery System", RFC XXXX, Internet-Draft
draft-ietf-urn-uri-res-ddds-00.txt, July 2000.
[6] Mealling, M. and R.D. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, August 2000.
[7] Faltstrom, P., "E.164 number and DNS", RFC 2916, September 2000.
[8] Daniel, R. and M. Mealling, "Resolution of Uniform Resource
Identifiers using the Domain Name System", RFC 2168, June 1997.
Author's Address
Michael Mealling
Verisign
505 Huntmar Park Drive
Herndon, VA 22070
US
Phone: +1 770 921-2251
EMail: michaelm@netsol.com
URI: http://www.verisign.com
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