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Network Working Group S. Waldbusser
Request for Comments: 4011 Nextbeacon
Category: Standards Track J. Saperia
JDS Consulting, Inc.
T. Hongal
Riverstone Networks, Inc.
March 2005
Policy Based Management MIB
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo defines a portion of the Management Information Base (MIB)
for use with network management protocols in TCP/IP-based internets.
In particular, this MIB defines objects that enable policy-based
monitoring and management of Simple Network Management Protocol
(SNMP) infrastructures, a scripting language, and a script execution
environment.
Table of Contents
1. The Internet-Standard Management Framework .................. 3
2. Overview .................................................... 4
3. Policy-Based Management Architecture ........................ 4
4. Policy-Based Management Execution Environment ............... 10
4.1. Terminology ........................................... 10
4.2. Execution Environment - Elements of Procedure ......... 10
4.3. Element Discovery ..................................... 11
4.3.1. Implementation Notes .......................... 12
4.4. Element Filtering ..................................... 13
4.4.1. Implementation Notes .......................... 13
4.5. Policy Enforcement .................................... 13
4.5.1. Implementation Notes .......................... 14
5. The PolicyScript Language ................................... 14
5.1. Formal Definition ..................................... 15
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RFC 4011 Policy Based Management MIB March 2005
5.2. Variables ............................................. 18
5.2.1. The Var Class ................................. 19
5.3. PolicyScript QuickStart Guide ......................... 23
5.3.1. Quickstart for C Programmers .................. 25
5.3.2. Quickstart for Perl Programmers ............... 25
5.3.3. Quickstart for TCL Programmers ................ 25
5.3.4. Quickstart for Python Programmers ............. 26
5.3.5. Quickstart for JavaScript/ECMAScript/JScript
Programmers ................................... 26
5.4. PolicyScript Script Return Values ..................... 26
6. Index Information for `this element' ........................ 27
7. Library Functions ........................................... 28
8. Base Function Library ....................................... 29
8.1. SNMP Library Functions ................................ 29
8.1.1. SNMP Operations on Non-Local Systems .......... 30
8.1.2. Form of SNMP Values ........................... 32
8.1.3. Convenience SNMP Functions .................... 34
8.1.3.1. getVar() ............................ 34
8.1.3.2. exists() ............................ 34
8.1.3.3. setVar() ............................ 35
8.1.3.4. searchColumn() ...................... 36
8.1.3.5. setRowStatus() ...................... 38
8.1.3.6. createRow() ......................... 39
8.1.3.7. counterRate() ....................... 42
8.1.4. General SNMP Functions ........................ 44
8.1.4.1. newPDU() ............................ 45
8.1.4.2. writeVar() .......................... 45
8.1.4.3. readVar() ........................... 46
8.1.4.4. snmpSend() .......................... 47
8.1.4.5. readError() ......................... 48
8.1.4.6. writeBulkParameters() ............... 48
8.1.5. Constants for SNMP Library Functions .......... 49
8.2. Policy Library Functions .............................. 51
8.2.1. elementName() ................................. 51
8.2.2. elementAddress() .............................. 51
8.2.3. elementContext() .............................. 52
8.2.4. ec() .......................................... 52
8.2.5. ev() .......................................... 52
8.2.6. roleMatch() ................................... 52
8.2.7. Scratchpad Functions .......................... 53
8.2.8. setScratchpad() ............................... 55
8.2.9. getScratchpad() ............................... 56
8.2.10. signalError() ................................. 57
8.2.11. defer() ....................................... 57
8.2.12. fail() ........................................ 58
8.2.13. getParameters() ............................... 58
8.3. Utility Library Functions ............................. 59
8.3.1. regexp() ...................................... 59
Waldbusser, et al. Standards Track [Page 2]
RFC 4011 Policy Based Management MIB March 2005
8.3.2. regexpReplace() ............................... 60
8.3.3. oidlen() ...................................... 60
8.3.4. oidncmp() ..................................... 60
8.3.5. inSubtree() ................................... 60
8.3.6. subid() ....................................... 61
8.3.7. subidWrite() .................................. 61
8.3.8. oidSplice() ................................... 61
8.3.9. parseIndex() .................................. 62
8.3.10. stringToDotted() .............................. 63
8.3.11. integer() ..................................... 64
8.3.12. string() ...................................... 64
8.3.13. type() ........................................ 64
8.3.14. chr() ......................................... 64
8.3.15. ord() ......................................... 64
8.3.16. substr() ...................................... 65
8.4. General Functions ..................................... 65
9. International String Library ................................ 65
9.1. stringprep() .......................................... 66
9.1.1. Stringprep Profile ............................ 66
9.2. utf8Strlen() .......................................... 67
9.3. utf8Chr() ............................................. 68
9.4. utf8Ord() ............................................. 68
9.5. utf8Substr() .......................................... 68
10. Schedule Table .............................................. 69
11. Definitions ................................................. 70
12. Relationship to Other MIB Modules ........................... 113
13. Security Considerations ..................................... 114
14. IANA Considerations ......................................... 117
15. Acknowledgements ............................................ 118
16. References .................................................. 118
16.1. Normative References .................................. 118
16.2. Informative References ................................ 119
Authors' Addresses .............................................. 120
Full Copyright Statement ........................................ 121
1. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to section 7 of
RFC 3410 [16].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. MIB objects are generally
accessed through the Simple Network Management Protocol (SNMP).
Objects in the MIB are defined using the mechanisms defined in the
Structure of Management Information (SMI). This memo specifies a MIB
module that is compliant to the SMIv2, which is described in STD 58,
RFC 2578 [2], STD 58, RFC 2579 [3], and STD 58, RFC 2580 [4].
Waldbusser, et al. Standards Track [Page 3]
RFC 4011 Policy Based Management MIB March 2005
2. Overview
Large IT organizations have developed management strategies to cope
with the extraordinarily large scale and complexity of today's
networks. In particular, they have tried to configure the network as
a whole by describing and implementing high-level business policies,
rather than manage device by device, where orders of magnitude more
decisions (and mistakes) may be made.
The following are examples of "business policies":
- All routers will run code version 6.2.
- On-site contractors will only be connected to ports that are
configured with special security restrictions.
- All voice over cable ports in California must provide free local
calling.
- Apply special forwarding to all ports whose customers have paid for
premium service.
Each of these policies could represent an action applied to hundreds
of thousands of variables.
To automate this practice, customers need software tools that will
implement business policies across their networks, as well as
standard protocols that will ensure that policies can be applied to
all of their devices, regardless of the vendor.
This practice is called Policy-Based Management. This document
defines managed objects for the Simple Network Management Protocol
that are used to distribute policies in a common form throughout the
network.
3. Policy-Based Management Architecture
Policy-based management is the practice of applying management
operations globally on all managed elements that share certain
attributes.
Policies are intended to express a notion of:
if (an element has certain characteristics) then (apply an
operation to that element)
Waldbusser, et al. Standards Track [Page 4]
RFC 4011 Policy Based Management MIB March 2005
Policies take the following normal form:
if (policyCondition) then (policyAction)
A policyCondition is a script that results in a boolean to determine
whether an element is a member of a set of elements upon which an
action is to be performed.
A policyAction is an operation performed on an element or a set of
elements.
These policies are most often executed on or near managed devices
where the elements live (and thus their characteristics may be easily
inspected) and where operations on those elements will be performed.
A management station is responsible for distributing an
organization's policies to all the managed devices in the
infrastructure. The pmPolicyTable provides managed objects for
representing a policy on a managed device.
An element is an instance of a physical or logical entity and is
embodied by a group of related MIB variables, such as all the
variables for interface 7. This enables policies to be expressed
more efficiently and concisely. Elements can also model circuits,
CPUs, queues, processes, systems, etc.
Conceptually, policies are executed in the following manner:
for each element for which policyCondition returns true, execute
policyAction on that element
For example:
If (interface is fast ethernet) then (apply full-duplex mode)
If (interface is access) then (apply security filters)
If (circuit w/gold service paid for) then (apply special queuing)
Each unique combination of policy and element is called an execution
context. Within a particular execution context, the phrase 'this
element' is often used to refer to the associated element, as most
policy operations will be applied to 'this element'. The address of
'this element' contains the object identifier of any attribute of the
element, the SNMP context the element was discovered in, and the
address of the system on which the element was discovered.
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RFC 4011 Policy Based Management MIB March 2005
Policies can manage elements on the same system:
-----------------------------------------------------
| |
| Managed System |
| |
| |
| ------------------ Managed Elements |
| | | interfaces |
| | Policy Manager | manages... circuits |
| | | queues |
| ------------------ processes |
| ... |
| |
-----------------------------------------------------
or they can manage elements on other systems:
--------------------------
| Managed System |
-------------------------- | Managed Elements |
| | | interfaces |
| Management Station or | | circuits |
| Mid-Level Manager | | ... |
| | --------------------------
| ------------------ | manages...
| | Policy Manager | | --------------------------
| ------------------ | | Managed System |
| | | Managed Elements |
-------------------------- | interfaces |
| circuits |
| ... |
--------------------------
...
PolicyConditions have the capability of performing comparison
operations on SNMP variables, logical expressions, and other
functions. Many device characteristics are already defined in MIB
Modules and are easy to include in policyCondition expressions
(ifType == ethernet, frCircuitCommittedBurst < 128K, etc). However,
there are important characteristics that aren't currently in MIB
objects, and, worse, it is not current practice to store this
information on managed devices. Therefore, this document defines MIB
objects for this information. To meet today's needs there are three
missing areas: roles, capabilities, and time.
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Roles
A role is an administratively specified characteristic of a managed
element. As a selector for policies, it determines the applicability
of the policy to a particular managed element.
Some examples of roles are political, financial, legal, geographical,
or architectural characteristics, typically not directly derivable
from information stored on the managed system. For example, "paid
for premium service" or "is plugged into a UPS" are examples of
roles, whereas the "percent utilization of a link" would not be.
Some types of information one would put into a role include the
following:
political - describes the role of a person or group of people, or of
a service that a group of people uses. Examples:
executive, sales, outside-contractor, customer.
If (attached user is executive) then (apply higher bandwidth)
If (attached user is outside-contractor) then (restrict access)
financial/legal - describes what financial consideration was
received. Could also include contractual or legal
considerations. Examples: paid, gold, free, trial,
demo, lifeline.
If (gold service paid for) then (apply special queuing)
geographical - describes the location of an element. Examples:
California, Headquarters, insecure conduit.
If (interface leaves the building) then (apply special security)
architectural - describes the network architects "intent" for an
element. Examples: backup, trunk.
If (interface is backup) then (set ifAdminStatus = down)
Roles in this model are human-defined strings that can be
referenced by policy code. The role table in this MIB may be used
to assign role strings to elements and to view all role string
assignments. Implementation-specific mechanisms may also be used
to assign role strings; however, these assignments must be visible
in the role table. Multiple roles may be assigned to each
element. Because policy code has access to data in MIB objects
that represent the current state of the system and (in contrast)
role strings are more static, it is recommended that role strings
not duplicate information available in MIB objects. Role strings
generally should be used to describe information not accessible in
MIB objects.
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RFC 4011 Policy Based Management MIB March 2005
Policy scripts may inspect role assignments to make decisions
based on whether an element has a particular role assigned to it.
The pmRoleTable allows a management station to learn what roles
exist on a managed system. The management station may choose not
to install policies that depend on a role that does not exist on
any elements in the system. The management station can then
register for notifications of new roles. Upon receipt of a
pmNewRoleNotification, it may choose to install new policies that
make use of that new role.
Capabilities
The capabilities table allows a management station to learn what
capabilities exist on a managed system. The management station
may choose not to install policies that depend on a capability
that does not exist on any elements in the system. The management
station can then register for notifications of new capabilities.
Upon receipt of a pmNewCapabilityNotification, it may choose to
install new policies that make use of that new capability.
Time
Managers may wish to define policies that are intended to apply
for certain periods of time. This might mean that a policy is
installed and is dormant for a period of time, becomes ready, and
then later goes dormant again. Sometimes these time periods will
be regular (Monday-Friday 9-5), and sometimes ad hoc. This MIB
provides a schedule table that can schedule when a policy is ready
and when it is dormant.
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A policy manager contains the following:
-------------------------------------------------------
| Policy Manager |
| |
| ---------------------------------------- |
| | Agent | |
| | | |
| | --------------------------------- | |
| | | Policy Download and Control | | |
| | | pmPolicyTable | | |
| | | pmElementTypeRegTable | | |
| | | pmSchedTable | | |
| | --------------------------------- | |
| | | |
| | --------------------------------- | |
| | | Policy Environment Control | | |
| | | pmRoleTable | | |
| | | pmCapabilitiesTables | | |
| | --------------------------------- | |
| | | |
| | --------------------------------- | |
| | | Policy Monitoring | | |
| | | pmTrackingTables | | |
| | | pmDebuggingTable | | |
| | --------------------------------- | |
| ---------------------------------------- |
| |
| -------------------------------- |
| | Execution Environment | |
| | | |
| | ----------------------- | |
| | | Policy Scheduler | | |
| | ----------------------- | |
| | ----------------------- | |
| | | Language | | |
| | ----------------------- | |
| | ----------------------- | |
| | | Function Library | | |
| | ----------------------- | |
| -------------------------------- |
-------------------------------------------------------
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4. Policy-Based Management Execution Environment
4.1. Terminology
Active Schedule - A schedule specifies certain times that it will be
considered active. A schedule is active during those times.
Valid Policy - A valid policy is a policy that is fully configured
and enabled to run. A valid policy may run unless it is linked to
a schedule entry that says the policy is not currently active.
Ready Policy - A ready policy is a valid policy that either has no
schedule or is linked to a schedule that is currently active.
Precedence Group - Multiple policies can be assigned to a precedence
group with the resulting behavior that for each element, of the
ready policies that match the condition, only the one with the
highest precedence value will be active. For example, if there is
a default bronze policy that applies to any interface and a
special policy for gold interfaces, the higher precedence of the
gold policy will ensure that it is run on gold ports and that the
bronze policy isn't.
Active Execution Context - An active execution context is a pairing
of a ready policy with an element that matches the element type
filter and the policy condition. If there are multiple policies
in the precedence group, it is also necessary that no higher
precedence policy in the group match the policy condition.
Run-Time Exception (RTE) - A run-time exception is a fatal error
caused in language or function processing. If, during the
invocation of a script, a run-time exception occurs, execution of
that script is immediately terminated. If a policyCondition
experiences a run-time exception while processing an element, the
element is not matched by the condition and the associated action
will not be run on that element. A run-time exception can cause
an entry to be added to the pmDebuggingTable and will be reflected
in the pmTrackingPEInfo object.
4.2. Execution Environment - Elements of Procedure
There are several steps performed in order to execute policies in
this environment:
- Element Discovery
- Element Filtering
- Policy Enforcement
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RFC 4011 Policy Based Management MIB March 2005
4.3. Element Discovery
An element is an instance of a physical or logical entity. Examples
of elements include interfaces, circuits, queues, CPUs, and
processes. Sometimes various attributes of an entity will be
described through tables in several standard and proprietary MIB
Modules. As long as the indexing is consistent between these tables,
the entity can be modeled as one element. For example, the ifTable
and the dot3Stats table both contain attributes of interfaces and
share the same index (ifIndex), therefore they can be modeled as one
element type.
The Element Type Registration table allows the manager to learn what
element types are being managed by the system and to register new
types, if necessary. An element type is registered by providing the
OID of an SNMP object (i.e., without the instance). Each SNMP
instance that exists under that object is a distinct element. The
index part of the discovered OID will be supplied to policy
conditions and actions so that this code can inspect and configure
the element. The agent can determine the index portion of discovered
OIDs based on the length of the pmElementTypeRegOIDPrefix for the
portion of the MIB that is being retrieved. For example, if the
OIDPrefix is 'ifEntry', which has 9 subids, the index starts on the
11th subid (skipping the subidentifier for the column; e.g.,
ifSpeed).
For each element that is discovered, the policy condition is called
with the element's name as an argument to see whether the element is
a member of the set the policy acts upon.
Note that agents may automatically configure entries in this table
for frequently used element types (interfaces, circuits, etc.). In
particular, it may configure elements for which discovery is
optimized in one or both of the following ways:
1. The agent may discover elements by scanning internal data
structures as opposed to issuing local SNMP requests. It is
possible to recreate the exact semantics described in this table
even if local SNMP requests are not issued.
2. The agent may receive asynchronous notification of new elements
(for example, "card inserted") and use that information to create
elements instantly rather than through polling. A similar feature
might be available for the deletion of elements.
Note that upon restart, the disposition of agent-installed entries is
described by the pmPolicyStorageType object.
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A special element type "0.0" represents the "system element". "0.0"
represents the single instance of the system itself and provides an
execution context for policies to operate on "the system" and on MIB
objects modeled as scalars. For example, "0.0" gives an execution
context for policy-based selection of the operating system code
version (likely modeled as a scalar MIB object). The element type
"0.0" always exists. As a consequence, no actual discovery will take
place and the pmElementTypeRegMaxLatency object will have no effect
for the "0.0" element type. However, if the "0.0" element type is
not registered in the table, policies will not be executed on the
"0.0" element.
If the agent is discovering elements by polling, it should check for
new elements no less frequently than pmElementTypeRegMaxLatency would
dictate. When an element is first discovered, all policyConditions
are run immediately, and policyConditions that match will have the
associated policyAction run immediately. Subsequently, the
policyCondition will be run regularly for the element, with no more
than pmPolicyConditionMaxLatency milliseconds elapsing between each
invocation. Note that if an implementation has the ability to be
alerted immediately when a particular type of element is created, it
is urged to discover that type of element in this fashion rather than
through polling, resulting in immediate configuration of the
discovered element.
4.3.1. Implementation Notes
Note that although the external behavior of this registration process
is defined in terms of the walking of MIB tables, implementation
strategies may differ. For example, commonly used element types
(such as interface) may have purpose-built element discovery
capability built-in and advertised to managers through an entry in
the pmElementTypeRegTable.
Before registering an element type, a manager is responsible for
inspecting the table to see whether it is already registered (either
by the agent or by another manager). Note that entries that differ
only in the last subid (which specifies which object is an entry) are
effectively duplicates and should be treated as such by the manager.
The system that implements the Policy-Based Management MIB may not
have knowledge of the format of object identifiers in other MIB
Modules. Therefore it is inappropriate for it to check these OIDs
for errors. It is the responsibility of the management station to
register well-formed object identifiers. For example, if an extra
sub-identifier is supplied when the ifTable is registered, no
Waldbusser, et al. Standards Track [Page 12]
RFC 4011 Policy Based Management MIB March 2005
elements will be discovered. Similarly, if a sub-identifier is
missing, every element will be discovered numerous times (once per
column) and none of the element addresses will be well formed.
4.4. Element Filtering
The first step in executing a policy is to see whether the policy is
ready to run based on its schedule. If the pmPolicySchedule object
is equal to zero, there is no schedule defined, and the policy is
always ready. If the pmPolicySchedule object is non-zero, then the
policy is ready only if the referenced schedule group contains at
least one valid schedule entry that is active at the current time.
If the policy is ready, the next step in executing a policy is to see
which elements match the policy condition. The policy condition is
called once for each element and runs to completion. The element's
name is the only argument that is passed to the condition code for
each invocation. No state is remembered within the policy script
from the previous invocation of 'this element' or from the previous
invocation of the policy condition, except for state accessible
through library functions. Two notable examples of these are the
scratchpad functions, which explicitly provide for storing state, and
the SNMP functions, which can store state in local or remote MIB
objects. If any run-time exception occurs, the condition will
terminate immediately for 'this element'. If the condition returns
non-zero, the corresponding policy action will be executed for 'this
element'.
If an element matches a condition and it had not matched that
condition the last time it was checked (or if it is a newly
discovered element), the associated policyAction will be executed
immediately. If the element had matched the condition at the last
check, it will remain in the set of elements whose policyAction will
be run within the policyActionMaxLatency.
4.4.1. Implementation Notes
Whether policy conditions are multi-tasked is an implementation-
dependent matter. Each condition/element combination is conceptually
its own process and can be scheduled sequentially, or two or more
could be run simultaneously.
4.5. Policy Enforcement
For each element that has returned non-zero from the policy
condition, the corresponding policy action is called. The element's
name is the only argument that is passed to the policy action for
each invocation. Except for state accessible from library functions,
Waldbusser, et al. Standards Track [Page 13]
RFC 4011 Policy Based Management MIB March 2005
no state is remembered from the policy condition evaluation, or from
the previous condition/action invocation of 'this element' or from
the previous invocation of the policy condition or action on any
other element. If any run-time exception occurs, the action will
terminate immediately for 'this element'.
4.5.1. Implementation Notes
How policy actions are multi-tasked is an implementation-dependent
matter. Each condition/element combination is conceptually its own
process and can be scheduled sequentially, or two or more could be
run simultaneously.
5. The PolicyScript Language
Policy conditions and policy actions are expressed with the
PolicyScript language. The PolicyScript language is designed to be a
small interpreted language that is simple to understand and
implement; it is designed to be appropriate for writing small scripts
that make up policy conditions and actions.
PolicyScript is intended to be familiar to programmers that know one
of several common languages, including Perl and C. Nominally,
policyScript is a subset of the C language; however, it was desirable
to have access to C++'s operator overloading (solely to aid in
documenting the language). Therefore, PolicyScript is defined
formally as a subset of the C++ language in which many of the
operators are overloaded as part of the "var" class. Note, however,
that a PolicyScript program cannot further overload operators, as the
syntax to specify overloading is not part of the PolicyScript syntax.
A subset was used to provide for easy development of low-cost
interpreters of PolicyScript and to take away language constructs
that are peculiar to the C/C++ languages. For example, it is
expected that both C and Perl programmers will understand the
constructs allowed in PolicyScript.
Some examples of the C/C++ features that are not available are
function definitions, pointer variables, structures, enums, typedefs,
floating point and pre-processor functions (except for comments).
This language is formally defined as a subset of ISO C++ [10] but
only allows constructs that may be expressed in the Extended Backus-
Naur Form (EBNF) documented here. This is because although EBNF
doesn't fully specify syntactical rules (it allows constructs that
are invalid) and doesn't specify semantic rules, it can successfully
be used to define the subset of the language that is required for
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RFC 4011 Policy Based Management MIB March 2005
conformance to this specification. Unless explicitly described
herein, the meaning of any construct expressed in the EBNF can be
found by reference to the ISO C++ standard.
The use of comments and newlines are allowed and encouraged in order
to promote readability of PolicyScript code. Comments begin with
'/*' and end with '*/' or begin with '//' and go until the end of the
line.
One subset is not expressible in the EBNF syntax: all variables
within an instance of a PolicyScript script are within the same
scope. In other words, variables defined in a block delimited with
'{' and '}' are not in a separate scope from variables in the
enclosing block.
PolicyScript code must be expressed in the ASCII character set.
In the EBNF used here, terminals are character set members (singly or
in a sequence) that are enclosed between two single-quote characters
or described as a phrase between '<' and '>' characters.
Nonterminals are a sequence of letters and underscore characters. A
colon (:) following a nonterminal introduces its definition, a
production. In a production, a '|' character separates alternatives.
The '(' and ')' symbols group the enclosed items. The '[' and ']'
symbols indicate that the enclosed items are optional. A '?' symbol
following an item indicates that the item is optional. A '*' symbol
following an item indicates that the item is repeated zero, one, or
more times. A '+' symbol following an item indicates that the item
is repeated one or more times. The symbol '--' begins a comment that
ends at the end of the line.
5.1. Formal Definition
The PolicyScript language follows the syntax and semantics of ISO C++
[10], but is limited to that which can be expressed in the EBNF
below.
The following keywords are reserved words and cannot be used in any
policy script. This prevents someone from using a common keyword in
another language as an identifier in a script, thereby confusing the
meaning of the script. The reserved words are:
auto, case, char, const, default, do, double, enum, extern, float,
goto, inline, int, long, register, short, signed, sizeof, static,
struct, switch, typedef, union, unsigned, void, and volatile.
Waldbusser, et al. Standards Track [Page 15]
RFC 4011 Policy Based Management MIB March 2005
Any syntax error, use of a reserved keyword, reference to an unknown
identifier, improper number of function arguments, error in coercing
an argument to the proper type, exceeding local limitations on string
length, or exceeding local limitations on the total amount of storage
used by local variables will cause an RTE.
PolicyScript permits comments using the comment delimiters, '/*' to
'*/', or the start of comment symbol '//'.
-- Lexical Grammar
letter: '_' | 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
| 'g' | 'h' | 'i' | 'j' | 'k' | 'l' | 'm'
| 'n' | 'o' | 'p' | 'q' | 'r' | 's' | 't'
| 'u' | 'v' | 'w' | 'x' | 'y' | 'z'
| 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
| 'G' | 'H' | 'I' | 'J' | 'K' | 'L' | 'M'
| 'N' | 'O' | 'P' | 'Q' | 'R' | 'S' | 'T'
| 'U' | 'V' | 'W' | 'X' | 'Y' | 'Z'
digit: '0' | '1' | '2' | '3' | '4'
| '5' | '6' | '7' | '8' | '9'
non_zero: '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
oct_digit: '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7'
hex_digit: digit | 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
| 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
escape_seq: '\'' | '\"' | '\?' | '\\'
| '\a' | '\b' | '\f' | '\n'
| '\r' | '\t' | '\v'
| '\' oct_digit+ | '\x' hex_digit+
non_quote: Any character in the ASCII character set
except single quote ('), double quote ("),
backslash ('\'), or newline.
c_char: non_quote | '"' | escape_seq
string_literal: '"' s_char* '"'
s_char: non_quote | ''' | escape_seq
char_constant: ''' c_char '''
decimal_constant: non_zero digit*
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octal_constant: '0' oct_digit*
hex_constant: ( '0x' | '0X' ) hex_digit+
integer_constant: decimal_constant | octal_constant | hex_constant
identifier: letter ( letter | digit )*
-- Phrase Structure Grammar
-- Expressions
primary_expr: identifier | integer_constant | char_constant
| string_literal | '(' expression ')'
postfix_expr: primary_expr
| identifier '(' argument_expression_list? ')'
| postfix_expr '++'
| postfix_expr '--'
| postfix_expr '[' expression ']'
argument_expression_list:
assignment_expr
| argument_expression_list ',' assignment_expr
unary_expr: postfix_expr | unary_op unary_expr
unary_op: '+' | '-' | '~' | '!' | '++' | '--'
binary_expr: unary_expr | binary_expr binary_op unary_expr
binary_op: '||' | '&&' | '|' | '^' | '&' | '!='
| '==' | '>=' | '<=' | '>' | '<' | '>>'
| '<<' | '-' | '+' | '%' | '/' | '*'
assignment_expr: binary_expr
| unary_expr assignment_op assignment_expr
assignment_op: '=' | '*=' | '/=' | '%=' | '+=' | '-='
| '<<=' | '>>=' | '&=' | '^=' | '|='
expression: assignment_expr | expression ',' assignment_expr
-- Declarations
declaration: 'var' declarator_list ';'
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declarator_list: init_declarator
| declarator_list ',' init_declarator
init_declarator: identifier [ '=' assignment_expr ]
-- Statements
statement: declaration
| compound_statement
| expression_statement
| selection_statement
| iteration_statement
| jump_statement
compound_statement: '{' statement* '}'
expression_statement: expression? ';'
selection_statement:
'if' '(' expression ')' statement
| 'if' '(' expression ')' statement 'else' statement
iteration_statement:
'while' '(' expression ')' statement
| 'for' '(' expression? ';' expression? ';' expression? ')'
statement
jump_statement: 'continue' ';'
| 'break' ';'
| 'return' expression? ';'
-- Root production
PolicyScript: statement*
5.2. Variables
To promote shorter scripts and ease in writing them, PolicyScript
provides a loosely typed data class, "var", that can store both
integer and string values. The native C++ types (char, int, etc.)
are thus unnecessary and have not been carried into the subset that
comprises this language. The semantics of the "var" type are modeled
after those of ECMAScript[17].
For example:
var number = 0, name = "IETF";
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This language will be executed in an environment where the following
typedef is declared. (Note that this typedef will not be visible in
the policyCondition or policyAction code.)
typedef ... var;
Although this declaration is expressed here as a typedef, the
'typedef' keyword itself is not available to be used in PolicyScript
code.
5.2.1. The Var Class
A value is an entity that takes on one of two types: string or
integer.
The String type is the set of all finite ordered sequences of zero or
more 8-bit unsigned integer values ("elements"). The string type can
store textual data as well as binary data sequences. Each element is
considered to occupy a position within the sequence. These positions
are indexed with nonnegative integers. The first element (if any) is
at position 0, the next element (if any) at position 1, and so on.
The length of a string is the number of elements (i.e., 8-bit values)
within it. The empty string has length zero and therefore contains
no elements.
The integer type is the set of all integer values in the range
-9223372036854775808 (-2^63) to 18446744073709551615 (2^64-1). If an
integer operation would cause a (positive) overflow, then the result
is returned modulo 2^64. If an integer operation would cause a
(negative) underflow, then the result is undefined. Integer division
rounds toward zero.
Prior to initialization, a var object has type String and a length of
zero.
The policy script runtime system performs automatic type conversion
as needed. To clarify the semantics of certain constructs it is
useful to define a set of conversion operators: ToInteger(),
ToString(), ToBoolean(), and Type(). These operators are not a part
of the language; they are defined here to aid the specification of
the semantics of the language. The conversion operators are
polymorphic; that is, they can accept a value of any standard type.
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ToInteger
The operator ToInteger converts its argument to a value of type
Integer according to the following table:
Integer The result equals the input argument
(no conversion).
String See grammar and note below.
integer_constant The result equals the input argument
(no conversion).
string_literal See grammar and note below.
char_constant See grammar and note below.
ToInteger Applied to Strings
ToInteger applied to the String Type string_literal and to
char_constants applies the following grammar to the input. If the
grammar cannot interpret the string as an expansion of
numeric_string, then an RTE is generated. Note that a numeric_string
that is empty or contains only white space is converted to 0.
-- EBNF for numeric_string
numeric_string : white_space* numeric? white_space*
white_space : <TAB> | <SP> | <NBSP> | <FF> | <VT>
| <CR> | <LF> | <LS> | <PS> | <USP>
numeric : signed_decimal | hex_constant | octal_constant |
enum_decimal
signed_decimal: [ '-' | '+' ] decimal_constant
enum_decimal: [ letter | digit | '-' ]* '(' decimal_constant ')'
-- decimal_constant, hex_constant, and octal_constant are defined
-- in the PolicyScript EBNF described earlier.
Note that when the enum_decimal form is converted, the sequence of
characters before the parenthesis and the pair of parenthesis
themselves are completely ignored, and the decimal_constant inside
the parenthesis is converted. Thus, "frame-relay(32)" translates to
the integer 32.
Although this will make the script more readable than using the
constant "32", the burden is on the code writer to be accurate, as
"ethernet-csmacd(32)" and "frame-relay(999)" will also be accepted.
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ToString
The operator ToString converts its argument to a value of type String
according to the following table:
Integer Return the string containing the decimal
representation of the input argument in
the form of signed_decimal, except that
no leading '+' will be used.
String Return the input argument (no conversion)
integer_constant Return the string containing the decimal
representation of the input argument in the
form of signed_decimal except that no
leading '+' will be used.
string_literal Return the input argument (no conversion)
char_constant Return the string of length one containing
the value of the input argument.
ToBoolean
The operator ToBoolean converts its argument to a value of type
Integer according to the following table:
Integer The result is 0 if the argument is 0.
Otherwise the result is 1.
String The results is 0 if the argument is the
empty string. Otherwise the result is 1.
integer_constant The result is 0 if the argument is 0.
Otherwise the result is 1.
string_literal The result is 0 if the argument is the
empty string. Otherwise the result is 1.
char_constant The result is 1.
Operators
The rules below specify the type conversion rules for the various
operators.
A++: A = ToInteger(A); A++;
A--: A = ToInteger(A); A--;
++A: A = ToInteger(A); ++A;
--A: A = ToInteger(A); --A;
+A: ToInteger(A);
-A: -1 * ToInteger(A);
~A: ToInteger(A);
!A: !ToBoolean(A);
A * B, A - B, A & B, A ^ B , A | B, A << B, A >> B:
ToInteger(A) <operator> ToInteger(B)
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A / B, A % B:
if (ToInteger(B) == 0)
RTE, terminate;
else
ToInteger(A) <operator> ToInteger(B)
A + B:
if (Type(A) == String || Type(B) == String)
ToString(A) concatenated with ToString(B)
else
A + B
Compound Assignment (<operator>=):
Simply follow rules above. Note that type of LHS (Left
Hand Side) may be changed as a result.
A < B, A > B, A <= B, A >= B, A == B, A != B:
if (Type(A) == String && Type(B) == String)
lexically compare strings with strcmp() logic
else
ToInteger(A) <operator> ToInteger(B)
A && B:
if (ToBoolean(A))
ToBoolean(B);
else
false;
A || B:
if (ToBoolean(A))
true;
else
ToBoolean(B);
if(A):
if (ToBoolean(A))
while(A):
while(ToBoolean(A))
for(...; A; ...):
for(...; ToBoolean(A); ...)
A[B] as a RHS (Right Hand Side) value:
if (Type(A) != String
|| ToInteger(B) >= strlen(A))
RTE, terminate;
A[ ToInteger(B) ]
The contents are returned as a string of length one
A[B] = C as a LHS value:
if (Type(A) != String
|| ToInteger(B) >= strlen(A))
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RTE, terminate;
if (strlen(ToString(C)) == 0)
RTE, terminate
A[ ToInteger(B) ] = First octet of ToString(C)
Note that this is only applicable in a simple assignment.
For example, in the expression
"getVar("ifSpeed.1") < 128000"
getVar always returns a string and '128000' is implicitly an integer.
The rules for '<' dictate that if either argument is an integer then
a 'numeric less than' is performed on ToInteger(A) and ToInteger(B).
If "getVar("ifSpeed.1")" returns "64000", the expression can be
translated to:
ToInteger("64000") < ToInteger(128000); or,
64000 < 128000; or,
True
5.3. PolicyScript QuickStart Guide
PolicyScript is designed so that programmers fluent in other
languages can quickly begin to write scripts.
One way to become familiar with a language is to see it in action.
The following nonsensical script exercises most of the PolicyScript
constructs (though it skips some usage options and many arithmetic
operators).
var x, index = 7, str = "Hello World", oid = "ifSpeed.";
x = 0;
while(x < 10){
if (str < "Goodbye") /* string comparison */
continue;
else
break;
x++;
}
if (oidlen(oid) == 10)
oid += "." + index; // append index to oid
for(x = 0; x < 7; x++){
str += "a";
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var y = 12;
index = ((x * 7) + y) % 3;
if (str[6] == 'W')
return index;
}
return;
The following examples are more practical:
For a condition:
// Return 1 if this is an interface and it is tagged
// with the role "gold"
return (inSubtree(elementName(), "ifEntry")
&& roleMatch("gold"))
A condition/action pair:
First, register the Host Resources MIB hrSWRunEntry as a new element
in the pmElementTypeRegTable. This will cause the policy to run for
every process on the system. The token '$*' will be replaced by the
script interpreter with a process index (see Section 7 for a
definition of the '$*' token).
The condition:
// if it's a process and it's an application and it's
// consumed more than 5 minutes of CPU time
return (inSubtree(elementName(), "hrSWRunEntry")
&& getVar("hrSWRunType.$*") == 4 // app, not OS or driver
&& getVar("hrSWRunPerfCPU.$*") > 30000) // 300 seconds
The action:
// Kill it
setVar("hrSWRunStatus.$*", 4, Integer); // invalid(4) kills it
A more substantial action to start an RMON2 host table on interfaces
that match the condition:
var pdu, index;
pdu = newPDU();
writeVar(pdu, 0, "hlHostControlDataSource.*",
"ifIndex." + ev(0), Oid);
writeVar(pdu, 1, "hlHostControlNlMaxDesiredEntries.*", 1000,
Integer);
writeVar(pdu, 2, "hlHostControlAlMaxDesiredEntries.*", 1000,
Integer);
writeVar(pdu, 3, "hlHostControlOwner.*", "policy", String);
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writeVar(pdu, 4, "hlHostControlStatus.*", "active(1)", Integer);
if (createRow(pdu, 5, 4, 20, 65535, index) == 0
|| index == -1)
return;
Because PolicyScript is a least common denominator, it contains
nothing that would astonish programmers familiar with C, C++, Perl,
Tcl, JavaScript, or Python. Although a new programmer may attempt to
use language constructs that aren't available in PolicyScript, s/he
should be able to understand any existing PolicyScript and will
likely know how to use anything that is valid in PolicyScript. The
lists below quickly enumerate the changes of note for programmers
coming from some particular languages. These lists won't describe
the unavailable constructs, but it is easy to see from the definition
above what is available.
5.3.1. Quickstart for C Programmers
- Character constants (i.e., 'c') are treated as one-character
strings, not as integers. So operations such as ('M' - 'A') or (x
+ 'A') will not perform as expected.
- Functions can change the value of arguments even though they are
not pointers (or called like '&arg').
- All variables are in the same scope.
5.3.2. Quickstart for Perl Programmers
- Comments are '/* comment */' and '// till end of line', not '#'.
- No need to put a '