Network Working Group L. Daigle
Request for Comments: 2016 P. Deutsch
Category: Experimental B. Heelan
C. Alpaugh
M. Maclachlan
Bunyip Information Systems, Inc.
October 1996
Uniform Resource Agents (URAs)
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
This paper presents an experimental architecture for an agent system
that provides sophisticated Internet information access and
management. Not a generalized architecture for active objects that
roam the Internet, these agents are modeled as extensions of existing
pieces of the Internet information infrastructure. This experimental
agent technology focuses on the necessary information structures to
encapsulate Internet activities into objects that can be activated,
transformed, and combined into larger structured activities.
Acknowledgements
Several people have shared thoughts and viewpoints that have helped
shape the thinking behind this work over the past few years. We'd
like to thank, in particular, Chris Weider, Patrik Faltstrom, Michael
Mealling, Alan Emtage, and the participants in the IETF URI Working
Group for many thought-provoking discussions.
Sima Newell provided insightful comments on the document -- thanks to
her it is much more readable!
Introduction
This document outlines an experimental agent system architecture that
was designed for the purpose of addressing high-level Internet
activities through encapsulation of protocol-specific actions.
Originally presented to the Uniform Resource Identifier (URI) working
group at the IETF, this technology was seen as taking a step beyond
resource location and resource naming. By providing a structured
mechanism for abstracting characteristics of desired information and
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distancing the necessary access incantations from the client, the
notion of a Uniform Resource Agent (URA) was created.
The evolution of Internet information systems has been characterized
by building upon successive layers of encapsulated technologies.
Machine address numbers were devised, and then encapsulated in
advertised machine names, which has allowed the evolution of the
Domain Name System (DNS) RFC1034, RFC1035">[RFC1034, RFC1035]. Protocols were
developed for accessing Internet resources of various descriptions,
and then uniform mechanisms for specifying resource locations,
standardized across protocol types, were developed (URLs) RFC1738">[RFC1738].
Each layer of Internet information primitives has served as the
building blocks for the next level of abstraction and sophistication
of information access, location, discovery and management.
The work described in this paper is an experimental system designed
to take another step in encapsulation. While TCP/IP protocols for
routing, addressing, etc, have permitted the connection and
accessibility of a plethora of information services on the Internet,
these must yet be considered a diverse collection of heterogeneous
resources. The World Wide Web effort is the most successful to date
in attempting to knit these resources into a cohesive whole.
However, the activity best-supported by this structure is (human)
browsing of these resources as documents. The URA initiative
explores the possibility of specifying an activity with the same kind
of precision accorded to resource naming and identification. By
focusing on activities, and not actions, URAs encapsulate resource
access mechanisms based on commonality of information content, not
protocol similarity.
An invoker -- human or otherwise -- may delegate an entire set of
tasks to a fully-instantiated URA. The nature of the tasks is
completely specified by the agent, because it encapsulates knowledge
about relevant Internet resources and the information required in
order to access them. In this way, URAs insulate invokers from the
details of Internet protocols while allowing them to carry out high-
level Internet activities (such as searching a set of web pages and
news groups relevant to a given topic). Also, by formally specifying
a high-level Internet activity in an agent, the same activity can be
repeated at a later date by the same invoker, someone else or even
another agent. Moreover, the agent object may easily be modified to
carry out another related task.
More detail describing the underlying philosophy of this particular
approach can be found in [IIAW95].
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Examples
As a very simple example, consider the client task of subscribing to
a mailing list. There are many mechanisms for providing users with
information necessary to complete a subscription. Currently, all
applications which provide the ability to subscribe to mailing lists
must contain protocol-aware code to carry out the task once the
requisite personal data has been solicited from the user.
Furthermore, any application program that embeds the ability to
subscribe in its code necessarily limits the set of mailing lists to
which a client can subscribe (i.e, to those types foreseen by the
software's creators). If, instead, there is an agent to which this
task can be delegated, all applications can make use of the agent,
and that agent becomes responsible for carrying out the necessary
interactions to complete the subscription. Furthermore, that agent
may be a client to other agents which can supply particular
information about how to subscribe to new types of mail servers, etc.
URAs have been explored as an agent technology to address just these
types of issues.
Relationship to Other Internet Agents
A number of Internet-aware agent and transportable code systems have
become popular -- Java [JAVA], TCL [TCL] and Safe-TCL, Telescript
[TELE], and the TACOMA system [TACOMA], to name a few of them. To
understand the scope of the problem that URAs tackle, it is helpful
to understand how these systems differ from the URA approach. Some
of these agent systems, like Java, focus on providing mechanisms for
creating and distributing (inter)active documents in the World Wide
Web. Others, like TACOMA, have more general intentions of providing
environments for mobile, interacting processes.
While each of these systems makes its individual contribution to
solving the transportation, communication, and security issues
normally associated with agent systems, they yield more objects that
exist within the Internet information space. That is, while they may
permit individual users to have a more sophisticated interaction with
a particular information resource, they do not address the more
general Internet problems of naming, identifying, locating resources,
and locating the same or similar resources again at a later date. It
is this set of problems that URAs specifically set out to address.
In order to create these URA objects that encapsulate a set of
Internet activities, it is necessary to specify their operating
environment and design structure. Together, these form an
experimental architecture for URAs, which can be evaluated in a
preliminary way through a prototype implementation. The remainder of
this paper describes such an experimental architecture, and outlines
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a prototype application built to test the concepts involved in the
creation and execution of URAs.
The Experimental Architecture
The main goal in designing the URA architecture was to provide a
mechanism for separating client need descriptions from the
specifications of mechanisms for satisfying those needs. For
example, from the client's perspective, the need to find MIDI music
files is quite distinct from the particular Internet resource actions
that might be necessary to find them at a given point in time. This
one need might be best met by integrating information from several
very different sources. Also, the client may have the same need on a
different day, but there may be new or different resources to call on
to satisfy it.
A further goal was to provide very structured specifications of the
Internet actions carried out by a particular URA. By making the
structure of an action explicit, it becomes possible to operate on
portions of an agent structure without requiring an understanding of
the complete semantics of its activity.
At the centre of the URA architecture is the concept of a
(persistent) specification of an activity. For purposes that should
become clear as the expected usage of URAs is described in more
detail, we choose to support this concept with the following
requirements of the architecture:
- there is a formalized environment in which these specifications
are examined and executed and otherwise manipulated. This is
referred to as a URAgency.
- the activity specifications are modular, and independent of a
given URAgency environment. Thus, they exist as object constructs
that can be shared amongst URAgencies. There is a standardized
_virtual_ structure of these URA objects, although different
types may exist, with different underlying implementations.
Basic URAgency Requirements
In the most abstract sense, a URAgency is a software system that
manipulates URA objects. In the terminology of objects, a URAgency
identifies the types of URAs it handles, and is responsible for
applying methods to objects of those types. For the purposes of this
experimental work, the only methods it is required to support are
those to get information about a given URA, and to execute a URA.
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The expected result of applying the "get information" method to a URA
is a description of some or all of the URA following the standardized
virtual structure of a URA object, outlined below.
The appropriate way to "execute" a URA is to supply information for
the individual URA data segments (in effect, to permit the creation
of an instance of a virtual object), or to identify a URA instance.
Again, the information is to be supplied in accordance with the
virtual structure below.
A URAgency claiming to handle a particular type of URA must have the
ability to map the implementation structure of that type of URA into
and out of the standard virtual URA structure. The URAgency must also
know how to activate the URA, and it must satisfy any runtime
dependencies for that type of URA.
For example, a URA type may consist of a Pascal program binary which,
when run with particular command line arguments, yields information
in the standard URA object structure. Activating this type of URA
might consist of executing the Pascal binary with an input file
containing all the necessary data segments. A URAgency claiming to
handle this sort of URA type must first be able to provide an
environment to execute the Pascal binary (for whatever platform it
was compiled), and also be able to interact with the Pascal binary
according to these conventions to get information about the URA, or
execute it.
As an alternative example, a URA type may consist of a script in some
interpreted language, with the URA object structure embedded as data
structures within the script. A URAgency handling this type of URA
might have to be able to parse the script to pull out the standard
URA object structure, and provide the script language interpreter for
the purposes of executing the URA.
URA Object Structure
In order to capture the necessary information for carrying out the
type of Internet activity described in the introductory paragraphs of
this document, six basic (virtual) components of a URA object have
been identified. Any implementation of a URA type is expected to be
able to conform to this structure within the context of a URAgency.
The six basic components of a URA object are:
URA HEADER:
Identification of the URA object, including a URA name, type
and abstract, creator name, and the resources required by the
URA.
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ACTIVATION DATA:
Specification of the data elements required to carry out the
URA activity. For example, in the case of an Internet search
for "people", this could include specification of fields for
person name, organization, e-mail address.
TARGETS:
Specification of the URL/URN's to be accessed to carry out the
activity. Note that, until URN's are in common use, the
ability to adjust URLs will be necessary. A key issue for
URAs is the ability to transport them and activate them far
from the creator's originating site. This may have
implications in terms of accessibility of resource sites. For
example, a software search created in Canada will likely
access a Canadian Archie server, and North American ftp sites.
However, an invoker in Australia should not be obliged to edit
the URA object in order to render it relevant in Australia.
The creator, then, can use this section to specify the
expected type of service, with variables for the parts
that can be modified in context (e.g., the host name for an
Archie server, or a mirror ftp site).
EXPERIENCE INFORMATION:
Specification of data elements that are not strictly involved
in conversing with the targets in order to carry out the
agent's activity. This space can be used to store information
from one invocation of a URA instance to the next.
This kind of information could include date of last
execution, or URLs of resources located on a previous
invocation of the agent.
ACTIVITY:
If URAs were strictly data objects, specifying required data
and URL/URN's would suffice to capture the essence of the
composite net interaction. However, the variability of
Internet resource accesses and the scope of what URAs could
accomplish in the net environment seem to suggest the need to
give the creator some means of organizing the instantiation of
the component URL/URN's. Thus, the body of the URA should
contain a scripting mechanism that minimally allows
conditional instantiation of individual URL/URN's. These
conditions could be based on which (content) data elements the
user provided, or accessibility of one URL/URN, etc. It also
provides a mechanism for suggesting scheduling of URL/URN
instantiation.
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The activity is specified by a script or program in a language
specified by the URA type, or by the URA header information.
All the required activation data, targets, and experience
information are referenced by their specification names.
RESPONSE FILTER:
The main purpose of the ACTIVITY module is to specify the
steps necessary to take the ACTIVATION DATA, contact the
TARGETS, and collect responses from those services. The
purpose of the RESPONSE FILTER module is to transform those
responses into the result of the URA invocation. This
transformation may be along the lines of reformatting
some text, or it may be a more elaborate interpretation
such as a relevance rating for a retrieved HTML page.
The response filter is specified by a script or program in a
language specified by the URA type, or by the URA header
information. All the required activation data, targets, and
experience information are referenced by their specification
names.
See Appendix 1 for a more detailed description of the components of a
URA. Appendix 2 contains a sample virtual URA structure.
The Architecture in Action
Having introduced the required capabilities of the URAgency and
virtual structure of URA objects, it is now time to elaborate on the
tasks and interactions that are best supported by URAs.
URAs are constructed by identifying net-based resources of interest
(targets) to carry out a particular task. The activation data
component of a URA is the author's mechanism for specifying (to the
invoker) the elements of information that are required for successful
execution . An invoker creates an instance of a URA object by
providing data that is consistent with, or fills in, this template.
Such an instance encapsulates everything that the agent "needs to
know" in order to contact the specified target(s), make a request of
the resource ("get", "search", etc.) and return a result to the
invoker. This encapsulation is a sophisticated identification of the
task results.
For example, in the case of a mailing list subscription URA, the
creator will identify the target URL for a resource that handles list
subscription (e.g., an HTML form), and specify the data required by
that resource (such as user name, user mail address, and mailing list
identifier). When an invoker provides that information and
instantiates the URA, the resulting object completely encapsulates
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all that is needed in order to subscribe the user -- the subscription
result is identified.
URAs are manipulated through the application of methods. This, in
turn , is governed by the URAgency with which the invoker is
interacting. However, because the virtual structure of URAs is
represented consistently across URA types and URAgencies, a URAgency
can act as one of the targets of a URA. Since methods can be applied
to URAs remotely, URAs can act as invokers of URAs. This can yield a
complex structure of task modules.
For example, a URA designed to carry out a generalized search of
book-selling resources might make use of individual URAs tailored to
each resource. Thus, the top-level URA becomes the orchestrating URA
for access to a number of disparate resources, while being insulated
from the minute details of accessing those resources.
A Prototype Implementation
The experimental work with URAs includes a prototype implementation
of URA objects. These are written in the Tcl scripting language. A
sample prototype Tcl URA can be found in Appendix 3.
The URAgency that was created to handle these URAs is part of the
Silk Desktop Internet Resource Discovery tool. Silk provides a
graphical user interface environment that allows the user to access
and search for Internet information without having to know where to
look or how to look. Silk presents a list of the available URAs to
carry out these activities (e.g., "search for tech reports" or
"hotlist"). For each activity, the user is prompted for the
activation data, and Silk's URAgency executes the URA. The Silk
software also supports the creation and maintenance of URA object
instances. Users can add new URAs by creating new Tcl scripts (per
the guidelines in the "URA Writer's Guide", available with the Silk
software. See [SILK]). The Silk graphical interface hides some of
the mechanics of the underlying URAgency. A more directly-accessible
version of this URAgency will become available.
Conclusions
This work was originally conceived as an extension to the family of
Uniform Resource Identifiers (URIs): Uniform Resource Locators
(URLs), Uniform Resource Characteristics (URCs), and the proposed
Uniform Resource Names (URNs). The approach of formalizing the
characteristics of an information task in a standardized object
structure is seen as a means of identifying a class of resources, and
contributes to the level of abstraction with which users can refer to
Internet resources.
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Although still in its experimental stages, this work has already
evoked interest and shown promise in the area of providing mechanisms
for building more advanced tools to interact with the Internet at a
more sophisticated level than just browsing web pages.
One of the major difficulties that has been faced in developing a
collection of URAs is the brittleness induced by interacting with
services that are primarily geared towards human-users. Small
changes in output formats that are easily discernible by the human
eye can be entirely disruptive to a software client that must apply a
parsing and interpretation mechanism based on placement of cues in
the text. This problem is certainly not unique to URAs -- any
software acting upon results from such a service is affected.
Perhaps there is the need for an evolution of "service entrances" to
information servers on the Internet -- mechanisms for getting "just
the facts" from an information server. Of course, one way to provide
such access is for the service provider to develop and distribute a
URA that interacts with the service. When the service's interface
changes, the service provider will be moved to update the URA that
was built to access it reliably.
Work will continue to develop new types of URAs, as well as other
URAgencies. This will necessitate the creation of URAgency
interaction standards -- the "common virtual URA object structure" is
the first step towards defining a lingua franca among URAs of
disparate types and intention.
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References
[IIAW95] Leslie L. Daigle, Peter Deutsch, "Agents for Internet
Information Clients", CIKM'95 Intelligent Information Agents
Workshop, December 1995.
Available from
[JAVA] "The Java Language: A White Paper" Available from
RFC1034">[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.
RFC1035">[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
RFC1738">[RFC1738] T. Berners-Lee, L. Masinter, M. McCahill, "Uniform Resource
Locators (URL)", RFC 1738, December 1994.
[SILK] Bunyip's Silk project homepage:
[SILKURA] Silk URA information:
[TACOMA] Johansen, D. van Renesse, R. Schneider, F. B., "An
Introduction to the TACOMA Distributed System", Technical Report
95-23, Department of Computer Science, University of Tromso,
Norway, June 1995.
[TCL] Ousterhout, J. K. "Tcl and the Tk Toolkit", Addison Wesley,
1994.
[TELE] White, J. E., "Telescript Technology: The Foundation for the
Electronic Marketplace", General Magic White Paper, General Magic
Inc., 1994.
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Authors' Addresses
Leslie Daigle
Peter Deutsch
Bill Heelan
Chris Alpaugh
Mary Maclachlan
Bunyip Information Systems, Inc.
310 St. Catherine St. West
Suite 300
Montreal, Quebec, CANADA
H2X 2A1
Phone: (514) 875-8611
EMail: ura-bunyip@bunyip.com
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Appendix 1 -- Virtual URA Structure
This appendix contains a BNF-style description of the expected
virtual structure of a URA object. This "virtual structure" acts as
the canonical representation of the information encapsulated in a
given URA. It is expected that more information may optionally be
contained in the elements of the components -- the elements listed
here are offered as the "minimum" or "standard" set.
N.B.:
[]-delimited items are optional
%% denotes a comment
\0 represents the empty string
| is "or"
{} are literal characters
This form is used for convenience and clarity of expression --
whitespace and ordering of individual elements are not considered
significant.
:= {}
:= { URAHDR }
{ ACTDATA }
{ TARG }
{ EXPINFO }
{ ACTSPEC }
{ RESPFILT }
:= { name }
{ author }
{ version }
[ { lang } ]
[ { parent } ]
:= | \0
:= {
{ name }
{ response }
{ prompt }
[ { required } ]
[ { default } ]
}
:= | \0
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:= {
{ name }
{ protocol }
{ url }
[ { } ]
}
:= |
:= %% a complete, valid URL string
(e.g., http://www.bunyip.com/)
:= {
{ scheme }
{ host }
[ { port } ]
{ selector }
}
:= {
{ name }
{ response }
{ prompt }
}
:= {
{ name }
{ response }
{ prompt }
}
:= {
{ name }
{ response }
{ prompt }
}
:= {
{ name }
{ response }
{ prompt }
}
:= {
{ name }
{ response }
}
:=
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:=
%% Without requiring more detail...
:= \n | \0
:= 0 | 1
:=
:=
:=
:=
:=
:=
:=
:=
:=
:=
:=
:= http-get | http-post | ...
:=
:=
:=
:=
:=
:=
:=
:=
:=
:=
:=
:=
:=
Appendix 2 -- Sample Virtual URA
Representation
A valid virtual representation of a Silk Tcl URA is presented below.
The actual URA from which it was drawn is given in Appendix 3.
{
{URAHDR
{name {DejaNews Search}}
{author {Leslie Daigle}}
{version {1.0}}
}
{ACTDATA
{name {Topic Keywords}}
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{prompt {Topic Keywords}}
{response {}}
}
{EXPINFO
{name {Comments}}
{prompt {Comments}}
{response {}}
}
{ACTSPEC
{proc mapResponsesToDejanews {} {
set resp ""
if {[uraAreResponsesSet {Topic Keywords}]} {
lappend resp [list query [uraGetSpecResponse {
Topic Keywords}]]
}
return $resp
}
proc uraRun {} {
global errorInfo
foreach serv [uraListOfServices] {
set u [uraGetServiceURL $serv]
switch -- $serv {
dejanews {
if [catch {
set query [mapResponsesToDejanews]
if {$query != {}} {
set result [uraHTTPPostSearch $u $query]
if {$result != ""} {
set list [dejanews_uraHTTPPostCanonicalize
$result]
puts $list
}
}
}] {
puts stderr $errorInfo
}
}
default {
# can't handle other searches, yet.
} } } }
}
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}
{RESPFILT
{
proc dejanews_uraHTTPPostCanonicalize {htmlRes} {
set result {}
set lines {}
set clause {}
set garb1 ""
set garb2 ""
# Get the body of the result page -- throw away leading and
# trailing URLs
regexp {([^]
*)(.*)
.*}
$htmlRes garb1 garb2 mainres
set lines [split $mainres "\n"]
foreach clause $lines {
if [regexp
{.*(..\/..).*[^"]*)">([^<]*).*([^<]*).*}
$clause garb1 dt relurl desc grp] {
lappend r [list HEADLINE [format "%s (%s, %s)"
[string trim $desc] \
[string trim $grp] $dt]]
lappend r [list URL [format
"http://www.dejanews.com/cgi-bin/%s" $relurl]]
lappend r [list TYPE "text/plain"]
lappend result $r
}
}
return $result
}
}
}
}
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Appendix 3 -- Sample Silk Tcl URA
The following is a valid Silk Tcl URA. For more information on the
implementation and structure of Silk-specific URAs, see the "URA
Writers Guide" that accompanies the distribution of the Silk software
(available from ).
# ----------------------------------------------------------------------
#
# URA initialization
#
# ----------------------------------------------------------------------
#
# Initialize the URA, its search specs and searchable services.
#
# URA init.
set uraDebug 1
uraInit {
{name {DejaNews Search}}
{author {Leslie Daigle}}
{version {1.0}}
{description "This URA will search for UseNet News articles."}
{help "This is help on UseNet News search script."}
}
#
# bug: handling of choices/labels is kind of gross.
#
# Search spec. init.
foreach item {
{
{name {Topic Keywords}}
{field Topic}
{tag STRING}
{description {Keywords to search for in news articles}}
{prompt {Topic Keywords}}
{help {Symbols to look up, separated by spaces.}}
{type STRING}
{subtype {}}
{allowed .*}
{numvals 1}
{required 0}
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{response {}}
{respset 0}
}
} {
uraSearchSpecInit $item
}
uraAnnotationInit {
{help {Enter comments to store with an instance}}
{numvals 1}
{subtype {}}
{response {}}
{name Comments}
{required 0}
{class ANNOTATION}
{type TEXT}
{description {General comments about this URA.}}
{respset 1}
{prompt Comments}
{field {}}
{allowed .*}
}
uraResultInit {
{name {Related Pages}}
{contents { {
{HEADLINE {The DejaNews UseNet search service}}
{TYPE text/plain}
{URL http://www.dejanews.com}
} }}
}
foreach item {
{
{name dejanews}
{protocol http-post}
{url http://marge.dejanews.com/cgi-bin/nph-dnquery}
}
} {
uraServicesInit $item
}
proc dejanews_uraHTTPPostCanonicalize {htmlRes} {
set result {}
set lines {}
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set clause {}
set garb1 ""
set garb2 ""
# Get the body of the result page
# -- throw away leading and trailing URLs
regexp {([^]
*)(.*)
.*} $htmlRes garb1 garb2 mainres
set lines [split $mainres "\n"]
foreach clause $lines {
uraDebugPuts stderr [format "Line: %s" $clause]
if [regexp
{.*(..\/..).*[^"]*)">([^<]*).*([^<]*).*} \
$clause garb1 dt relurl desc grp] {
uraDebugPuts stderr [format
"Date: %s Rel URL: %s Desc: %s Group: %s"
$dt $relurl $desc $grp]
lappend r [list HEADLINE [format "%s (%s, %s)"
[string trim $desc] \
[string trim $grp] $dt]]
lappend r [list URL [format
"http://www.dejanews.com/cgi-bin/%s" $relurl]]
lappend r [list TYPE "text/plain"]
lappend result $r
}
}
return $result
}
# ----------------------------------------------------------------------
#
# Mapping procedures
#
# ----------------------------------------------------------------------
#
# There is one procedure, for each searchable service, to map the search
# spec responses to a form suitable for inclusion into a search URL (or
# whatever form the particular query procedure accepts).
#
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#
#
proc mapResponsesToDejanews {} {
set resp ""
if {[uraAreResponsesSet {Topic Keywords}]} {
lappend resp [list query [uraGetSpecResponse {Topic Keywords}]]
}
return $resp
}
#
# bug: need better error reporting
# (i.e. which searches didn't work and why, etc.)
#
proc uraRun {} {
global errorInfo
foreach serv [uraListOfServices] {
set u [uraGetServiceURL $serv]
switch -- $serv {
dejanews {
if [catch {
set query [mapResponsesToDejanews]
uraDebugPuts stderr [format "%s: query is `%s'."
$serv $query]
if {$query != {}} {
set result [uraHTTPPostSearch $u $query]
if {$result != ""} {
uraDebugPuts stderr [format "%s: result is `%s'."
$serv $result]
set list [dejanews_uraHTTPPostCanonicalize $result]
uraDebugPuts stderr [format "%s: list is `%s'."
$serv $list]
puts $list
}
}
}] {
puts stderr $errorInfo
}
}
default {
# can't handle other searches, yet.
}
Daigle, et. al. Experimental [Page 20]
RFC 2016 Uniform Resource Agents October 1996
}
}
}
Daigle, et. al. Experimental [Page 21]