[Search] [txt|pdfized|bibtex] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02                                                      
Open Pluggable Edge Services                                     A. Beck
Internet-Draft                                       Lucent Technologies
Expires: March 16, 2004                                      A. Rousskov
                                                 The Measurement Factory
                                                      September 16, 2003


                     P: Message Processing Language
                       draft-ietf-opes-rules-p-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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

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

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

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

   This Internet-Draft will expire on March 16, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   P is a simple configuration language designed for specification of
   message processing instructions at application proxies. P can be used
   to instruct an intermediary how to manipulate the application message
   being proxied. Such instructions are needed in an Open Pluggable Edge
   Services (OPES) context.









Beck & Rousskov          Expires March 16, 2004                 [Page 1]


Internet-Draft       P: Message Processing Language       September 2003


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Language elements  . . . . . . . . . . . . . . . . . . . . . .  6
   3.1 Objects  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.2 Operators  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.3 Expressions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.4 Statements . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.5 Assignments  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Modules  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  OPES Services  . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   A.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   B.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
       Normative References . . . . . . . . . . . . . . . . . . . . . 19
       Informative References . . . . . . . . . . . . . . . . . . . . 20
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20
       Intellectual Property and Copyright Statements . . . . . . . . 21






























Beck & Rousskov          Expires March 16, 2004                 [Page 2]


Internet-Draft       P: Message Processing Language       September 2003


1. Introduction

   The Open Pluggable Edge Services (OPES) architecture
   [I-D.ietf-opes-architecture], enables cooperative application
   services (OPES services) between a data provider, a data consumer,
   and zero or more OPES processors.  The application services under
   consideration analyze and possibly transform application-level
   messages exchanged between the data provider and the data consumer.
   OPES processors need to be told what services are to be applied to
   what application messages. P language can be used for this
   configuration task.

   In other words, P language primary objective is to express statements
   similar to:

                if message meets criteria C,
                then apply service S;

                                Figure 1

   Thus, P programs mostly deal with formulating message-dependent
   conditions and executing services.

   P design attempts to satisfy several conflicting goals:

   flexibility: Application intermediaries deal with a wide range of
      applications and protocols (SMTP, HTTP, RTSP, IM, etc.). The
      language must be able to accommodate virtually all known tasks in
      selecting a desired adaptation service for a message of a known
      application protocol (and conceivable future applications).

   efficiency: Language interpretation must be efficient enough to be
      comparable with other message processing overheads at a typical
      application proxy (e.g., interpreting HTTP headers to determine
      response cachability).

   simplicity: Typical configurations must be easy to write and
      understand for a typical OPES system administrator.

   correctness: Many message handling configurations are written without
      direct access to intermediaries that will use those
      configurations.  The extent of off-line (compile-time) correctness
      checks should catch all syntax errors and many common semantic
      errors such as undefined values and type conflicts.

   compactness: It is possible that some processing instructions will be
      piggybacked as headers/metadata to messages they refer to, placing
      stringent size requirements on language code.



Beck & Rousskov          Expires March 16, 2004                 [Page 3]


Internet-Draft       P: Message Processing Language       September 2003


   security: It should be difficult if not impossible to write malicious
      code that would result in security vulnerability of compliant
      language interpreter.

   While P addresses OPES needs, its design is meant to be applicable
   for a variety of similar intermediary configuration tasks such as
   access control list (ACL) specification and message routing in proxy
   meshes or load-balancing environments.

   P design is based on a minimal useful subset of features from several
   programming languages such as R (S), Smalltalk, and C++. Technically
   speaking, P is a single-assignment, lazy evaluation, strongly typed
   functional programming language.






































Beck & Rousskov          Expires March 16, 2004                 [Page 4]


Internet-Draft       P: Message Processing Language       September 2003


2. Syntax

   P syntax is defined by the following Augmented Backus-Naur Form
   (ABNF) [RFC2234]:

   code = *(statement ";")

   statement = assignment / function-call / if-statement

   assignment = identifier ":=" expression

   if-statement = "if" "(" expression ")" "{" code "}"

   expression =
        name / function-call / "{" code "}"
        ... ; more to be defined (logical and arithmetic expressions)

   name = identifier *( "." identifier)

   function-call = name "(" [params] ")"

   params = expression *( "," expression)

   identifier = ALPHA *(ALPHA / DIGIT / "_")

   ... ; more primitives to be defined as needed


                                Figure 2

   XXX: add /* comments */.




















Beck & Rousskov          Expires March 16, 2004                 [Page 5]


Internet-Draft       P: Message Processing Language       September 2003


3. Language elements

3.1 Objects

   P is centered around the concept of an "object" that is similar to
   objects from other object-oriented languages. An object is,
   essentially, a piece of data or information. The value of an object
   is indistinguishable from the object itself. Object type is defined
   by the semantics of applicable operations and manipulations.  Almost
   everything in P is an object, even a piece of code. Here are a few P
   objects, listed one per line:

        0
        "http://www.ietf.org/"
        Core
        { a := 1/0; }

   Many objects contain other objects, often called members.  Members
   are accessible by their name, using the member access operator (".").
   Member access operator has a single parameter: the name of the member
   to access. All P objects support "." operator, but not all objects
   have members. Here are a few examples:

        Http.message.headers
        Core.interpreter.stop
        "string".nosuchmember

   Many objects support operators other than member access. For example,
   member objects that support function call "()" operator are often
   call methods.

        Http.message.headers.have(header)
        Core.interpreter.stop()
        1 / 0
        "string" + "string"

   P operators are described in Section 3.2. below.

   P does not have built-in facilities for describing object types. When
   writing a P program, only objects known to interpreter (e.g., Core)
   and objects generated by known objects (e.g., Core.import("Http"))
   can be used. P supports loadable modules that can be used to add
   objects to support new application protocols.  In fact, P core
   supports no application protocols directly. Instead, modules like
   "Http" can be used to process messages depending on application
   protocol being proxied.

   No default (silent) object type conversion is supported. However,



Beck & Rousskov          Expires March 16, 2004                 [Page 6]


Internet-Draft       P: Message Processing Language       September 2003


   explicit conversion (casting) is rarely needed because many methods
   are polymorphic (can work with several object types).

3.2 Operators

   Several operators are used in P to denote common operations.  These
   symbols are deemed to improve readability of P code as compared to
   their spelled-out-in-English counterparts.

   P Operators

   +--------------+----------------------------------------------------+
   |   operator   | default semantics                                  |
   +--------------+----------------------------------------------------+
   |    A == B    | A is semantically equal to B; does not modify A or |
   |              | B.                                                 |
   |              |                                                    |
   |    A != B    | semantical inequality, same as !(A == B).          |
   |              |                                                    |
   |      !A      | logical negation, same as (A == false)             |
   |              |                                                    |
   |    A and B   | logical concatenation, same as !(!A or !B)         |
   |              |                                                    |
   |    A or B    | logical disjunction (inclusive), same as !(!A or   |
   |              | !B)                                                |
   |              |                                                    |
   |     A + B    | sum of A and B; does not modify A or B.            |
   |              |                                                    |
   |     A * B    | product of A and B; does not modify A or B.        |
   |              |                                                    |
   |     A - B    | difference between A and B; does not modify A or   |
   |              | B.                                                 |
   |              |                                                    |
   |     A / B    | ratio of A to B; does not modify A or B.           |
   |              |                                                    |
   |      A.n     | access to A's member named n; does not modify A;   |
   |              | fails if A has no member named n.                  |
   |              |                                                    |
   |    A(...)    | object A is to perform a function call with zero   |
   |              | or more parameters; may modify A and/or parameters |
   +--------------+----------------------------------------------------+

   Operator precedence defines natural evaluation order used in
   mathematics and many programming languages. In the following list,
   operators are ordered based on their precedence. Operators with
   smaller precedence index are evaluated first. Operators with the same
   precedence index are evaluated in the left-to-right order of
   occurrence in an expression.



Beck & Rousskov          Expires March 16, 2004                 [Page 7]


Internet-Draft       P: Message Processing Language       September 2003


   1.  .

   2.  ()

   3.  !

   4.  * /

   5.  + -

   6.  == !=

   7.  and

   8.  or

   Except for the member access operator ("."). operators do not have to
   be supported by an object. Moreover, operator semantics may differ
   from one object to another (or even from one invocation to another
   for the same object though the latter is unlikely to be common in
   practice). Object writers SHOULD follow common operator semantics and
   MUST document actual operator semantics when adding support for these
   operators to their objects. The interpreter MUST NOT allow object
   writers to change operator precedence.

   Operators are not global special symbols but are passed to the object
   for interpretation, along with their parameters. Applying an operator
   is semantically equivalent to calling an object method. For example,
   the following three expressions are equivalent:

        a + b + c
        (a.+(b)) + c
        (a.+(b)).+(c)

                                Figure 6

   The "a + b + c" form is preferred for purely visual reasons.  Core P
   module provides basic objects and operators for them (e.g., boolean
   and integer). Application-specific modules usually provide
   applications-specific objects; those objects usually have
   application-specific methods and may not have methods to support
   operations common for basic types. For example, an Http module
   supplies an HTTP header object that does not have a "*" method.

3.3 Expressions

   P expressions are used in if-statements to specify the condition for
   the if-statement body to be interpreted.



Beck & Rousskov          Expires March 16, 2004                 [Page 8]


Internet-Draft       P: Message Processing Language       September 2003


        if (Http.request.method == "GET" and time.current() > time.noon) {
                ...
        }

                                Figure 7

   Evaluation of an expression stops when the value of an expression is
   known and cannot be changed by further evaluation. This
   short-circuiting optimization technique is common to many programming
   languages. In the following example, the value of A will never be
   interpreted when C is interpreted, regardless of the context where C
   is used:

        C := false and A;
        if (C) { ... };
        if (!C) { ... };
        ...

                                Figure 8


3.4 Statements

   Objects are manipulated using if-statements and function-calls.

        if (Http.request.method == "GET") {
                Services.applyOne(serviceFoo);
        }

                                Figure 9


3.5 Assignments

   Most procedural programming languages use variables to store
   intermediate processing results. In such languages, a variable is
   essentially a named piece of memory that can be assigned a value and
   can be updated with new values as needed. P does not have such
   variables. Instead, P uses a "single assignment" approach: an
   expression can be tagged with a name and that name can be reused many
   times in the program. On the surface, this is equivalent to having
   all "traditional" variables declared as "constant". The following two
   if-statements are semantically equivalent in P:

        if (Http.request.headers.have(Http.makeHeader("Client-IP"))) {...}

        h := Http.makeHeader("Client-IP");
        hs := Http.request.headers();



Beck & Rousskov          Expires March 16, 2004                 [Page 9]


Internet-Draft       P: Message Processing Language       September 2003


        if (hs.have(h)) {...}

                               Figure 10

   If the expression changes, a new name must be used to tag the new
   expression. After an assignment statement, the value of the name is
   not the value of the expression, but the expression itself.  Thus,
   the following two code fragments are equivalent and make no sense in
   P (the first fragment would make sense in languages such as C++):

        h := Http.makeHeader("Client-IP");
        h := Http.makeHeader("Server-IP");

        h := Http.makeHeader("Client-IP");
        Http.makeHeader("Client-IP") := Http.makeHeader("Server-IP");

                               Figure 11

   The interpreter can but does not have to evaluate the expression
   named in the assignment statement until the name is actually used in
   an expression that requires evaluation (e.g., as a parameter of a
   function call statement). This allows for optional performance
   optimizations where only used expressions are evaluated.

   P does not have user-defined functions. However, some code reuse is
   possible because P code is a valid expression and, hence, can be
   named and reused:

        code := { ... complicated service action ... };
        if (condition1) { code; };
        ...
        if (condition2) { code; };

                               Figure 12

   XXX: document whether expression has to be evaluated in the
   assignment context or use context. Document name scope.














Beck & Rousskov          Expires March 16, 2004                [Page 10]


Internet-Draft       P: Message Processing Language       September 2003


4. Modules

   Application-specific support is available in P via modules.  Basic P
   primitives such as integer types and boolean operations comprise the
   Core module.  Module is an object. The Core modules supplies the
   following methods to manipulate other modules:

   Core.import("M"): load a module called "M" and return it as the
      result.

   Core.lookup(M): start looking up unresolved attributes and method
      identifiers in a previously loaded module M.

   The Core module is assumed to be loaded (and being looked up) before
   the interpretation starts.

   XXX: document lookup conflict resolution.


































Beck & Rousskov          Expires March 16, 2004                [Page 11]


Internet-Draft       P: Message Processing Language       September 2003


5. OPES Services

   Services module contains basic attributes and methods for searching
   and executing OPES services:

   Services.findOne(URI): returns a service object that corresponds to
      the specified URI. Fails if no corresponding object exists.

   Services.applyOne(service, ...): applies the specified service to the
      current application message and optionally supplies
      service-specific application parameters. XXX: should parameters
      include the part of the message to be modified or just services
      metadata?

   Here is a service application example for a German to French
   translation service:

        Http := import("Http");
        if (Http.response.language_is("german")) {
                service := Services.find("opes://services/tran/german/french");
                service.toDialect("southern");
                Services.applyOne(service, Http.request.headers);
        }

                               Figure 13

   XXX: explain how failures are propagated and can be handled

   XXX: add Core.interpreter.stop and Core.interpreter.restart methods.






















Beck & Rousskov          Expires March 16, 2004                [Page 12]


Internet-Draft       P: Message Processing Language       September 2003


6. Failures

   Virtually any P statement may fail: expression denominator may be
   zero, named members may not exist, objects may not support applied
   operators, service execution may fail, interpreter may ran out of
   resources during an assignment, etc. A failure immediately stops
   interpretation of the first surrounding code block and assigns that
   block a boolean value of false.

   If the failed block is a part of a larger expression, the interpreter
   MUST continue evaluating the expression containing the failed block
   using usual expression evaluating rules, including short-circuiting
   boolean expressions. If the failed block is a stand-alone statement,
   that statement fails and the failure is propagated using the above
   rules.  If the implicit code block surrounding the program fails
   (XXX: document or require an implicit surrounding block like XML
   does), the entire P program interpretation terminates with a failure.

   Failure propagation rules allow to catch failures, similar to an
   exception mechanisms in languages like C++ or Java, except that P
   exceptions are not objects (they carry no information). For example,
   here is a simple way to introduce a backup/failover service:

        {
                ...
                Services.applyOne(unsafeService);
        } or {
                ...
                Services.applyOne(failoverService);
        };

                               Figure 14

   The following example illustrates how a failure-prone service can be
   retried twice if needed:

                code := {
                        /* code executing the service */
                };
                code or code or code;

                               Figure 15

   It is possible to force the interpreter to fail using the
   "Core.interpreter.fail(reason)" call. This is handy when there is a
   logical failure that the interpreter cannot detect on its own:

                {



Beck & Rousskov          Expires March 16, 2004                [Page 13]


Internet-Draft       P: Message Processing Language       September 2003


                        /* large piece of code executing several services,
                           each manipulating the current HTTP message ... */

                        /* checkpoint */
                        if (!Http.message.headers.have("Content-Length")) {
                                Core.interpreter.fail("services did not set CL");
                        }

                        /* OK, continue message manipulation ... */
                } or {
                        /* recover from failure ... */
                }

                               Figure 16





































Beck & Rousskov          Expires March 16, 2004                [Page 14]


Internet-Draft       P: Message Processing Language       September 2003


7. Security Considerations

   XXX: document non-obvious vulnerabilities: too many names, too deep
   nesting, invalid math, too much error logging; execution of
   unauthorized services, unauthorized exposure of sensitive information
   to authorized services.













































Beck & Rousskov          Expires March 16, 2004                [Page 15]


Internet-Draft       P: Message Processing Language       September 2003


8. Compliance

   XXX: define what a compliant interpreter is.
















































Beck & Rousskov          Expires March 16, 2004                [Page 16]


Internet-Draft       P: Message Processing Language       September 2003


Appendix A. Examples

   This appendix contains half-baked examples to illustrate P usage in
   common OPES environments. Example themes are taken from
   [I-D.beck-opes-irml] to ease the comparison with IRML.

   Here is a data provider example:

        interpreter.languageVersion("1.0"); // fails if incompatible

        Http := import("Http");
        lookup(Http);

        // Is the requested web document our home page?
        isHome := request.uri.looksLikeHome();

        // Does the user send us a specific cookie?
        cookie := makeHeader("Cookie", "sew=23");
        haveCookie := request.headers.have(cookie);

        if (isHome and haveCookie) {
                Services := import("Services");
                service := Services.findOne("opes://local.net/add-lcl-content");
                service.clientIp(request.clientIp);
                Services.applyOne(service);
        }

                               Figure 17

   Here is a data consumer example:

        Services := import("Services");
        service := Services.findOne("opes://privacy.net/priv-serv");
        service.action("remove-referer");
        Services.applyOne(service);

                               Figure 18














Beck & Rousskov          Expires March 16, 2004                [Page 17]


Internet-Draft       P: Message Processing Language       September 2003


Appendix B. Change Log

   Internal WG revision control IDs: $RCSfile: rules-lang.xml,v $
   $Revision: 1.5 $.















































Beck & Rousskov          Expires March 16, 2004                [Page 18]


Internet-Draft       P: Message Processing Language       September 2003


Normative References

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [I-D.ietf-opes-architecture]
              Barbir, A., "An Architecture for Open Pluggable Edge
              Services (OPES)", draft-ietf-opes-architecture-04 (work in
              progress), December 2002.










































Beck & Rousskov          Expires March 16, 2004                [Page 19]


Internet-Draft       P: Message Processing Language       September 2003


Informative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [I-D.beck-opes-irml]
              Beck, A. and M. Hofmann, "IRML: A Rule Specification
              Language for Intermediary Services",
              draft-beck-opes-irml-03 (work in progress), June 2003.


Authors' Addresses

   Andre Beck
   Lucent Technologies
   101 Crawfords Corner Rd.
   Holmdel, NJ
   US

   Phone: +1 732 332-5983
   EMail: abeck@bell-labs.com


   Alex Rousskov
   The Measurement Factory

   EMail: rousskov@measurement-factory.com
   URI:   http://www.measurement-factory.com/






















Beck & Rousskov          Expires March 16, 2004                [Page 20]


Internet-Draft       P: Message Processing Language       September 2003


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights. Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11. Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard. Please address the information to the IETF Executive
   Director.


Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



Beck & Rousskov          Expires March 16, 2004                [Page 21]


Internet-Draft       P: Message Processing Language       September 2003


   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































Beck & Rousskov          Expires March 16, 2004                [Page 22]