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
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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.
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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
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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.
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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.
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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 */.
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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,
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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.
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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.
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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();
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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.
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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.
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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.
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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:
{
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/* 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
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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.
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8. Compliance
XXX: define what a compliant interpreter is.
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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
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Appendix B. Change Log
Internal WG revision control IDs: $RCSfile: rules-lang.xml,v $
$Revision: 1.5 $.
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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.
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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/
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