rfc2016.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.


RFC 2016                Uniform Resource Agents             October 1996


   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
     <http://www.bunyip.com/products/silk/silktree/uratree/iiaw95.ps>

[JAVA] "The Java Language: A White Paper" Available from
     <http://java.sun.com/1.0alpha2/doc/overview/java/index.html>

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
   STD 13, RFC 1034, November 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and
   Specification", STD 13, RFC 1035, November 1987.

[RFC1738] T. Berners-Lee, L. Masinter, M. McCahill, "Uniform Resource
   Locators (URL)", RFC 1738, December 1994.

[SILK] Bunyip's Silk project homepage:
     <http://www.bunyip.com/products/silk/>

[SILKURA] Silk URA information:
     <http://www.bunyip.com/products/silk/silktree/uraintro.html>

[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:  [phone]
   EMail: [e-mail]



































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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.

<VIRTUAL_URA> := {<virtual-ura-structure>}

<virtual-ura-structure> := { URAHDR <ura-header> }
                           { ACTDATA <activation-data> }
                           { TARG <targets> }
                           { EXPINFO <experience information> }
                           { ACTSPEC <activity> }
                           { RESPFILT <response filter> }

<ura-header> := { name <ura-name> }
                { author <ura-author> }
                { version <ura-version> }
                [ { lang <lang-dependencies> } ]
                [ { parent <parent-of-instance> } ]



<activation-data> := <act-data-element><activation-data> | \0

<act-data-element> := {
                       { name <data-elt-name> }
                       { response <data-elt-value> }
                       { prompt <data-elt-prompt> }
                       [ { required <boolean> } ]
                       [ { default <data-default-val> } ]
                      }

<targets> := <target-service><targets> | \0



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<target-service> := {
                     { name <targ-url> }
                     { protocol <url-protocol> }
                     { url <url-spec> }
                     [ { <url-type-specific-data> } ]
                    }

<url-spec> :=  <complete-url> | <url-constructor>

<complete-url> := %% a complete, valid URL string
                     (e.g., http://www.bunyip.com/)

<url-constructor> := {
                      { scheme <url-scheme-spec> }
                      { host <url-host-spec> }
                      [ { port <url-port-spec> } ]
                      { selector <url-selector-spec> }
                     }

<url-scheme-spec> := {
                      { name <scheme-name> }
                      { response <scheme-value> }
                      { prompt <scheme-prompt> }
                     }
<url-host-spec> := {
                      { name <host-name> }
                      { response <host-value> }
                      { prompt <host-prompt> }
                   }
<url-port-spec> := {
                      { name <port-name> }
                      { response <port-value> }
                      { prompt <port-prompt> }
                   }
<url-selector-spec> := {
                      { name <selector-name> }
                      { response <selector-value> }
                      { prompt <selector-prompt> }
                       }


<experience information> := {
                             { name <data-elt-name> }
                             { response <data-elt-value> }
                            }

<activity> :=  <compound-string>




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<response filter> := <compound-string>


%% Without requiring more detail...

<compound-string> := <string>\n<compound-string> | \0
<boolean> := 0 | 1
<ura-name> := <string>
<ura-author> := <string>
<ura-version> := <string>
<lang-dependencies> := <string>
<parent-of-instance> := <string>
<data-elt-name> := <string>
<data-elt-value> := <string>
<data-elt-prompt> := <string>
<data-elt-default> := <string>
<data-default-val> := <string>
<targ-url> := <string>
<url-protocol> := http-get | http-post | ...
<url-type-specific-data> := <string>
<scheme-name> := <string>
<scheme-value> := <string>
<scheme-prompt> := <string>
<host-name> := <string>
<host-value> := <string>
<host-prompt> := <string>
<port-name> := <string>
<port-value> := <string>
<port-prompt> := <string>
<url-selector-name> := <string>
<url-selector-value> := <string>
<url-selector-prompt> := <string>

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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RFC 2016                Uniform Resource Agents             October 1996