I2NSF                                                            L. Xia
Internet Draft                                             J. Strassner
Intended status: Standard Track                                  Huawei
Expires: April 23, 2019                                       C. Basile
                                                               D. Lopez
                                                       October 23, 2018

                  Information Model of NSFs Capabilities

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   carefully, as they describe your rights and restrictions with
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   This draft defines the concept of an NSF (Network Security Function)
   capability, as well as its information model. Capabilities are a set
   of features that are available from a managed entity, and are
   represented as data that unambiguously characterizes an NSF.
   Capabilities enable management entities to determine the set of
   features from available NSFs that will be used, and simplify the
   management of NSFs.

Table of Contents

   1. Introduction ................................................ 3
   2. Conventions used in this document ........................... 3
      2.1. Acronyms ............................................... 4
   3. Capability Information Model Design ......................... 4
      3.1. Design Principles and ECA Policy Model Overview ........ 5
      3.2. Relation with the External Information Model ........... 8
      3.3. I2NSF Capability Information Model Theory of Operation . 9
         3.3.1. I2NSF Capability Information Model ............... 11
         3.3.2. The SecurityCapability class ..................... 14
      3.4. Modelling NSF Features as Security Capabilities ....... 15
         3.4.1. Matched Policy Rule .............................. 15
         3.4.2. Conflict, Resolution Strategy and Default Action . 16
         3.4.3. I2NSF Condition Clause Operator Types ............ 17
         3.4.4. Uses of the capability information model ......... 19
         3.4.5. A Syntax to Describe the Capability of an NSF .... 19
         3.4.6. Capability Algebra ............................... 20
   4. Considerations on the Practical Use of the CapIM ........... 21
   5. Security Considerations .................................... 22
   6. Contributors ............................................... 22
   7. Acknowledgements ........................................... 23
   8. References ................................................. 23
      8.1. Normative References .................................. 23
      8.2. Informative References ................................ 24

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  1. Introduction

   The rapid development of virtualized systems requires advanced
   security protection in various scenarios. Examples include network
   devices in an enterprise network, User Equipment in a mobile
   network, devices in the Internet of Things, or residential access
   users [RFC8192].

   NSFs produced by multiple security vendors provide various security
   capabilities to customers. Multiple NSFs can be combined together to
   provide security services over the given network traffic, regardless
   of whether the NSFs are implemented as physical or virtual

   Security Capabilities describe the functions that Network Security
   Functions (NSFs) are available to provide for security policy
   enforcement purposes. Security Capabilities are independent of the
   actual security control mechanisms that will implement them.

   Every NSF SHOULD be described with the set of capabilities it
   offers. Security Capabilities enable security functionality to be
   described in a vendor-neutral manner. That is, it is not needed to
   refer to a specific product or technology when designing the
   network; rather, the functions characterized by their capabilities
   are considered. Security Capabilities are a market enabler,
   providing a way to define customized security protection by
   unambiguously describing the security features offered by a given

   This document is organized as follows. Section 2 defines conventions
   and acronyms used. Section 3 discusses the design principles for
   I2NSF capability information model, the related ECA model, and
   provides detailed information model design of I2NSF network security

  2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses terminology defined in [I-D.draft-ietf-i2nsf-
   terminology] for security related and I2NSF scoped terminology.

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  2.1. Acronyms

  I2NSF - Interface to Network Security Functions

  NSF - Network Security Function

  DNF - Disjunctive Normal Form

  3. Capability Information Model Design

   A Capability Information Model (CapIM) is a formalization of the
   functionality that an NSF advertises. This enables the precise
   specification of what an NSF can do in terms of security policy
   enforcement, so that computer-based tasks can unambiguously refer
   to, use, configure, and manage NSFs. Capabilities MUST be defined in
   a vendor- and technology-independent manner (e.g., regardless of the
   differences among vendors and individual products).

   Humans are able to refer to categories of security controls and
   understand each other. For instance, security experts agree on what
   is meant by the terms "NAT", "filtering", and "VPN concentrator".
   As a further example, network security experts unequivocally refer
   to "packet filters" as stateless devices able to allow or deny
   packet forwarding based on various conditions (e.g., source and
   destination IP addresses, source and destination ports, and IP
   protocol type fields) [Alshaer].

   However, more information is required in case of other devices, like
   stateful firewalls or application layer filters. These devices
   filter packets or communications, but there are differences in the
   packets and communications that they can categorize and the states
   they maintain. Humans deal with these differences by asking more
   questions to determine the specific category and functionality of
   the device. Machines can follow a similar approach, which is
   commonly referred to as question-answering [Hirschman] [Galitsky].
   In this context, the CapIM and the derived Data Models provide
   important and rich information sources.

   Analogous considerations can be applied for channel protection
   protocols, where we all understand that they will protect packets by
   means of symmetric algorithms whose keys could have been negotiated
   with asymmetric cryptography, but they may work at different layers
   and support different algorithms and protocols. To ensure
   protection, these protocols apply integrity, optionally
   confidentiality, anti-reply protections, and authenticate peers.

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   The CapIM is intended to clarify these ambiguities by providing a
   formal description of NSF functionality. The set of functions that
   are advertised MAY be restricted according to the privileges of the
   user or application that is viewing those functions. I2NSF
   Capabilities enable unambiguous specification of the security
   capabilities available in a (virtualized) networking environment,
   and their automatic processing by means of computer-based

   This includes enabling the security controller to properly identify
   and manage NSFs, and allow NSFs to properly declare their
   functionality, so that they can be used in the correct way.

  3.1. Design Principles and ECA Policy Model Overview

   This document defines an information model for representing NSF
   capabilities. Some basic design principles for security capabilities
   and the systems that manage them are:

   o Independence: each security capability SHOULD be an independent
      function, with minimum overlap or dependency on other
      capabilities. This enables each security capability to be
      utilized and assembled together freely. More importantly, changes
      to one capability SHOULD NOT affect other capabilities. This
      follows the Single Responsibility Principle [Martin] [OODSRP].

   o Abstraction: each capability MUST be defined in a vendor-
      independent manner.

   o Advertisement: A dedicated, well-known interface MUST be used to
      advertise and register the capabilities of each NSF. This same
      interface MUST be used by other I2NSF Components to determine
      what Capabilities are currently available to them.

   o Execution:  a dedicated, well-known interface MUST be used to
      configure and monitor the use of a capability. This provides a
      standardized ability to describe its functionality, and report
      its processing results. This facilitates multi-vendor

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   o Automation: the system MUST have the ability to auto-discover,
      auto-negotiate, and auto-update its security capabilities (i.e.,
      without human intervention). These features are especially useful
      for the management of a large number of NSFs. They are essential
      for adding smart services (e.g., refinement, analysis, capability
      reasoning, and optimization) to the security scheme employed.
      These features are supported by many design patterns, including
      the Observer Pattern [OODOP], the Mediator Pattern [OODMP], and a
      set of Message Exchange Patterns [Hohpe].

   o Scalability: the management system SHOULD have the capability to
      scale up/down or scale in/out. Thus, it can meet various
      performance requirements derived from changeable network traffic
      or service requests. In addition, security capabilities that are
      affected by scalability changes SHOULD support reporting
      statistics to the security controller to assist its decision on
      whether it needs to invoke scaling or not.

   Based on the above principles, this document defines a capability
   model that enables an NSF to register (and hence advertise) its set
   of capabilities that other I2NSF Components can use. These
   capabilities MAY have their access control restricted by policy;
   this is out of scope for this document. The set of capabilities
   provided by a given set of NSFs unambiguously define the security
   offered by the set of NSFs used. The security controller can compare
   the requirements of users and applications to the set of
   capabilities that are currently available in order to choose which
   capabilities of which NSFs are needed to meet those requirements.
   Note that this choice is independent of vendor, and instead relies
   specifically on the capabilities (i.e., the description) of the
   functions provided.

   Furthermore, when an unknown threat (e.g., zero-day exploits and
   unknown malware) is reported by an NSF, new capabilities may be
   created, and/or existing capabilities may be updated (e.g., by
   updating its signature and algorithm). This results in enhancing the
   existing NSFs (and/or creating new NSFs) to address the new threats.
   New capabilities may be sent to and stored in a centralized
   repository, or stored separately in a vendor's local repository. In
   either case, a standard interface facilitates the update process.
   This document specifies a metadata model that MAY be used to further
   describe and/or prescribe the characteristics and behavior of the
   I2NSF capability model. For example, in this case, metadata could be
   used to describe the updating of the capability, and prescribe the
   particular version that an implementation should use. This initial
   version of the model covers and has been validated to describe NSFs
   that are designed with a set of capabilities (which covers most of

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   the existing NSFs). Checking the behavior of the model with systems
   that change capabilities dynamically at runtime has been extensively
   explored (e.g., impact on automatic registration).

   The "Event-Condition-Action" (ECA) policy model in [RFC8329] is used
   as the basis for the design of the capability model; definitions of
   all I2NSF policy-related terms are also defined in [I-D.draft-ietf-
   i2nsf-terminology]. The following three terms define the structure
   and behavior of an I2NSF imperative policy rule:

   o Event: An Event is defined as any important occurrence in time of
      a change in the system being managed, and/or in the environment
      of the system being managed. When used in the context of I2NSF
      Policy Rules, it is used to determine whether the Condition
      clause of the I2NSF Policy Rule can be evaluated or not. Examples
      of an I2NSF Event include time and user actions (e.g., logon,
      logoff, and actions that violate an ACL).

   o Condition: A condition is defined as a set of attributes,
      features, and/or values that are to be compared with a set of
      known attributes, features, and/or values in order to determine
      whether or not the set of Actions in that (imperative) I2NSF
      Policy Rule can be executed or not. Examples of I2NSF Conditions
      include matching attributes of a packet or flow, and comparing
      the internal state of an NSF to a desired state.

   o Action: An action is used to control and monitor aspects of flow-
      based NSFs when the event and condition clauses are satisfied.
      NSFs provide security functions by executing various Actions.
      Examples of I2NSF Actions include providing intrusion detection
      and/or protection, web and flow filtering, and deep packet
      inspection for packets and flows.

   An I2NSF Policy Rule is made up of three Boolean clauses: an Event
   clause, a Condition clause, and an Action clause. This structure is
   also called an ECA (Event-Condition-Action) Policy Rule. A Boolean
   clause is a logical statement that evaluates to either TRUE or
   FALSE. It may be made up of one or more terms; if more than one term
   is present, then each term in the Boolean clause is combined using
   logical connectives (i.e., AND, OR, and NOT).

   An I2NSF ECA Policy Rule has the following semantics:

          IF <event-clause> is TRUE

             IF <condition-clause> is TRUE

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                THEN execute <action-clause> [constrained by metadata]



   Technically, the "Policy Rule" is really a container that aggregates
   the above three clauses, as well as metadata. Aggregating metadata
   enables business logic to be used to prescribe behavior. For
   example, suppose a particular ECA Policy Rule contains three actions
   (A1, A2, and A3, in that order). Action A2 has a priority of 10;
   actions A1 and A3 have no priority specified. Then, metadata may be
   used to restrict the set of actions that can be executed when the
   event and condition clauses of this ECA Policy Rule are evaluated to
   be TRUE; two examples are: (1) only the first action (A1) is
   executed, and then the policy rule returns to its caller, or (2) all
   actions are executed, starting with the highest priority.

   The above ECA policy model is very general and easily extensible.

  3.2. Relation with the External Information Model

   Note: the symbology used from this point forward is taken from
   section 3.3 of [I-D.draft-ietf-supa-generic-policy-info-model].

   The I2NSF NSF-Facing Interface is used to select and manage the NSFs
   using their capabilities. This is done using the following approach:

   1) Each NSF registers its capabilities with the management system
      through a dedicated interface, and hence, makes its capabilities
      available to the management system;

   2) The security controller compares the needs of the security service
      with the set of capabilities from all available NSFs that it
      manages using the CapIM;

   3) The security controller uses the CapIM to select the final set of
      NSFs to be used;

   4) The security controller takes the above information and creates or
      uses one or more data models from the CapIM to manage the NSFs;

   5) Control and monitoring can then begin.

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   This assumes that an external information model is used to define
   the concept of an ECA Policy Rule and its components (e.g., Event,
   Condition, and Action objects). This enables I2NSF Policy Rules [I-
   D.draft-ietf-i2nsf-terminology] to be sub-classed from an external
   information model.

   The external ECA Information Model supplies at least a set of
   objects that represent a generic ECA Policy Rule, and a set of
   objects that represent Events, Conditions, and Actions that can be
   aggregated by the generic ECA Policy Rule. This enables appropriate
   I2NSF Components to reuse this generic model for different purposes,
   as well as specialize it (i.e., create new model objects) to
   represent concepts that are specific to I2NSF and/or an application
   that is using I2NSF.

   It is assumed that the external ECA Information Model also has the
   ability to aggregate metadata. This enables metadata to be used to
   prescribe and/or describe characteristics and behavior of the ECA
   Policy Rule. Specifically, Capabilities are sub-classed from this
   external metadata model. If the desired Capabilities are already
   defined in the CapIM, then no further action is necessary.
   Otherwise, new Capabilities SHOULD be defined either by defining new
   classes that can wrap existing classes using the decorator pattern
   [Gamma] or by another mechanism (e.g., through sub-classing); the
   parent class of the new Capability SHOULD be either an existing
   CapIM metadata class or a class defined in the external metadata
   information model. In either case, the ECA objects can use the
   existing aggregation between them and the Metadata class to add
   metadata to appropriate ECA objects.

   Detailed descriptions of each portion of the information model are
   given in the following sections.

  3.3. I2NSF Capability Information Model Theory of Operation

   Capabilities are typically used to represent NSF functions that can
   be invoked. Capabilities are objects, and hence, can be used in the
   event, condition, and/or action clauses of an I2NSF ECA Policy Rule.

   The I2NSF CapIM refines a predefined (and external) metadata model;
   the application of I2NSF Capabilities is done by refining a
   predefined (and external) ECA Policy Rule information model that
   defines how to use, manage, or otherwise manipulate a set of
   capabilities. In this approach, an I2NSF Policy Rule is a container
   that is made up of three clauses: an event clause, a condition
   clause, and an action clause. When the I2NSF policy engine receives
   a set of events, it matches those events to events in active ECA

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   Policy Rules. If the event matches, then this triggers the
   evaluation of the condition clause of the matched I2NSF Policy Rule.
   The condition clause is then evaluated; if it matches, then the set
   of actions in the matched I2NSF Policy Rule MAY be executed. The
   operation of each of these clauses MAY be affected by metadata that
   is aggregated by either the ECA Policy Rule and/or by each clause,
   as well as the selected resolution strategy.

   Condition clauses are logical formulas that combine one or more
   conditions that evaluate to a Boolean (i.e., true or false) result.
   The values in a condition clause are built on values received or
   owned by the NSF. For instance, the condition clause 'ip source ==' is true when the IP address is equal to Two or
   more conditions require a formal mechanism to represent how to
   operate on each condition to produce a result. For the purposes of
   this document, every condition clause MUST be expressed in either
   conjunctive or disjunctive normal form. Informally, conjunctive
   normal form expresses a clause as a set of sub-clauses that are
   logically ANDed together, where each sub-clause contains only terms
   that use OR and/or NOT operators). Similarly, disjunctive normal
   form is a set of sub-clauses that are logically ORed together, where
   each sub-clause contains only terms that use AND and/or NOT

   This document defines additional important extensions to both the
   external ECA Policy Rule model and the external Metadata model that
   are used by the I2NSF CapIM; examples include resolution strategy,
   external data, and default actions. All these extensions come from
   the geometric model defined in [Bas12]. A summary of the important
   points of this geometric model follows.

   Formally, given a set of actions in an I2NSF Policy Rule, the
   resolution strategy maps all the possible subsets of actions to an
   outcome. In other words, the resolution strategy is included in an
   I2NSF Policy Rule to decide how to evaluate all the actions from the
   matching I2NSF Policy Rule.

   Some concrete examples of resolution strategy are:

   o First Matching Rule (FMR)

   o Last Matching Rule (LMR)

   o Prioritized Matching Rule (PMR) with Errors (PMRE)

   o Prioritized Matching Rule with No Errors (PMRN)

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   In the above, a PMR strategy is defined as follows:

     1.             Order all actions by their Priority (highest is first, no
        priority is last); actions that have the same priority may be
        appear in any order in their relative location.

     2.             For PMRE:  if any action fails to execute properly, temporarily
        stop execution of all actions. Invoke the error handler of the
        failed action. If the error handler is able to recover from the
        error, then continue execution of any remaining actions; else,
        terminate execution of the ECA Policy Rule.

     3.             For PMRN:  if any action fails to execute properly, stop
        execution of all actions. Invoke the error handler of the failed
        action, but regardless of the result, execution of the ECA
        Policy Rule MUST be terminated.

   Regardless of the resolution strategy, when no rule matches a
   packet, a default action MAY be executed.

   Resolution strategies may use, besides intrinsic rule data (i.e.,
   event, condition, and action clauses), "external data" associated to
   each rule, such as priority, identity of the creator, and creation
   time. Two examples of this are attaching metadata to the policy rule
   and/or its action, and associating the policy rule with another
   class to convey such information.

   3.3.1. I2NSF Capability Information Model

   Figure 1 below shows one example of an external model. This is a
   simplified version of the MEF Policy model [PDO]. For our purposes:

     o  MCMPolicyObject is an abstract class, and is derived from
        MCMManagedEntity [MCM]

     o  MCMPolicyStructure is an abstract superclass for building
        different types of Policy Rules (currently, for I2NSF, only
        imperative (i.e., ECA) Policy Rules are considered)

     o  An I2NSFECAPolicyRule could be subclassed from MCMECAPolicyRule

     o  I2NSF Events, Conditions, and Actions could be subclasses from
        MCMPolicyEvent, MCMPolicyCondition, and MCMPolicyAction

     o  MCMMetaData is aggregated by MCMEntity, which is the superclass
        of MCMManagedEntity. So all Policy objects may aggregate

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                +---------------+           |HasPolicyStructure      |
                |MCMPolicyObject|           |ComponentDecoratorDetail|
                +-------A-------+           +---------------------*--+
                        |                                         *
                        |                                         *
        +---------------+----------------+                        *
        |                                |                        *
+-------+----------+          +----------+----------------+1..*   *
|MCMPolicyStructure|          |MCMPolicyStructureComponent|<------*+
+--------A---------+          +-----------A---------------+        |
         |                                |                        |
         |                   +------------+--------+               |
         |                   |                     |         0..1  ^
 +-------+--------+  +-------+-------+ +-----------+---------------V+
 |MCMECAPolicyRule|  |MCMPolicyClause| |MCMPolicyClauseComponent    |
 +----------------+  +---------------+ |Decorator                   |
          +-------+------+   +---------+--------+  +--------+------+
          |MCMPolicyEvent|   |MCMPolicyCondition|  |MCMPolicyAction|
          +--------------+   +------------------+  +---------------+

        Figure 1 Exemplary External Information Model (from the MEF)

   The CapIM model uses the Decorator Pattern [Gamma]. The decorator
   pattern enables a base object to be "wrapped" by zero or more
   decorator objects. The Decorator MAY attach additional
   characteristics and behavior, in the form of attributes at runtime
   in a transparent manner without requiring recompilation and/or
   redeployment. This is done by using composition instead of
   inheritance. Objects can "wrap" (more formally, extend the interface
   of) an object. In essence, a new object can be built out of pre-
   existing objects.

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   The Decorator Pattern is applied to allow NSF instances to aggregate
   I2NSFSecurityCapability instances. By means of this aggregation, an
   NSF can be associated to the functions it provides in terms of
   security policy enforcement, both at specification time (i.e., when
   a vendor provides a new NSF), statically, when a NSF is added to a
   (virtualized) networking environment, and dynamically, during
   network operations. Figure 2 shows an NSF aggregating zero or more
   SecurityCapabilities. This may be thought of as an NSF possessing
   (or defining) zero or more Security Capabilities. This "possession"
   (or "definition") is represented in UML as an aggregated, called
   HasSecurityCapability. The hasSecurityCapabilityDetail is an
   association class that allows NSF instances to aggregate
   I2NSFSecurityCapability instances. An NSF MAY be described by 0 or
   more SecurityCapabilities.

   Since there can be many types of NSF that have many different types
   of I2NSFSecurityCapabilities, the definition of a SecurityCapability
   must be done using the context of an NSF. This is realized by an
   association class in UML. HasSecurityCapabilityDetail is an
   association class. This yields the following design:

       +-----+0..n                      0..n+--------------------+
       |     |/ \  HasSecurityCapability    |                    |
       | NSF | A ----------+----------------+ SecurityCapability |
       |     |\ /          ^                |                    |
       +-----+             |                +--------------------+
             | HasSecurityCapabilityDetail |
             + ----------------------------+

             Figure 2  Defining SecurityCapabilities of an NSF

   This enables the HasSecurityCapabilityDetail association class to be
   the target of a Policy Rule. That is, the
   HasSecurityCapabilityDetail class has attributes and methods that
   define which I2NSFSecurityCapabilities of this NSF are visible and
   can be used [MCM].

   3.3.2. The SecurityCapability class

   The SecurityCapability class defines the concept of metadata that
   define security-related capabilities. It is subclassed from an
   appropriate class of an external metadata information model.
   Subclasses of the SecurityCapability class can be used to answer the
   following questions:

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     o  What are the events that are caught by the NSF to trigger the
        condition clause evaluation (Event subclass)?

     o  What kind of condition clauses can be specified on the NSF to
        define valid rules? This question splits into two questions:
        (1) what are the conditions that can be specified (Condition
        subclass), and (2) how to build a valid condition clause from a
        set of individual conditions (ClauseEvaluation class).

     o  What are the actions that the NSF can enforce (Action class)?

     o  How to define a correct policy on the NSF?

  3.4. Modelling NSF Features as Security Capabilities

   The capability model proposed in this document is intended to
   provide a formal framework to describe and process the features of
   NSFs, and to evaluate their fitness to a particular purpose,
   expressed in terms of a (possibly complex) policy statement. Using
   the capability model, NSF providers and users can declare and
   manipulate the features of a function or a combination of them.

   Specifically, if the NSF has the classification features needed to
   identify the packets/flows required by a policy, and can enforce the
   needed actions, then that particular NSF is capable of enforcing the
   policy. Therefore, capability selection and usage are based on the
   set of security traffic classification and action features that an
   NSF provides.

   NSFs may also have specific characteristics that automatic processes
   or administrators need to know when they have to generate
   configurations, like the available resolution strategies and the
   possibility to set default actions, which are described in Sections

   3.4.1. Matched Policy Rule

   The concept of a "matched" policy rule is defined as one in which
   its event and condition clauses both evaluate to true. To precisely
   describe what an NSF can do in terms of security, the things need to
   describe are the events it can catch, the conditions it can
   evaluate, and the actions it can enforce.

   Therefore, the properties that to characterize the capabilities of a
   NSF are as below:

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   o Ac is the set of Actions currently available from the NSF;

   o Ec is the set of Events that an NSF can catch. Note that for NSF
      (e.g., a packet filter) that are not able to react to events,
      this set will be empty;

   o Cc is the set of Conditions currently available from the NSF;

   o EVc defines the set of Condition Clause Evaluation Rules that can
      be used at the NSF to decide when the Condition Clause is true
      given the result of the evaluation of the individual Conditions.

   3.4.2. Conflict, Resolution Strategy and Default Action

   Formally, two I2NSF Policy Rules conflict with each other if:

   o the Event Clauses of each evaluate to TRUE;

   o the Condition Clauses of each evaluate to TRUE;

   o the Action Clauses affect the same object in different ways.

   For example, if we have two Policy Rules in the same Policy:

      R1: During 8am-6pm, if traffic is external, then run through FW
      R2: During 7am-8pm, conduct anti-malware investigation

   There is no conflict between R1 and R2, since the actions are
   different. However, consider these two rules:

      R3: During 8am-6pm, John gets GoldService
      R4: During 10am-4pm, FTP from all users gets BronzeService

   R3 and R4 are now in conflict, between the hours of 10am and 4pm,
   because the actions of R3 and R4 are different and apply to the same
   user (i.e., John).

   Conflicts theoretically compromise the correct functioning of
   devices (as happened for routers several year ago). However, NSFs
   have been designed to cope with these issues. Since conflicts are
   originated by simultaneously matching rules, an additional process
   decides the action to be applied, e.g., among the ones the matching
   rule would have enforced. This process is described by means of a
   resolution strategy

   On the other hand, it may happen that, if an event is caught, none
   of the policy rules matches. As a simple case, no rules may match a

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   packet arriving at border firewall. In this case, the packet is
   usually dropped, that is, the firewall has a default behavior to
   manage cases that are not covered by specific rules.

   Therefore, we introduce another security capability that serves to
   characterize valid policies for an NSF that solve conflicts with
   resolution strategies and enforce default actions if no rules match:

   o RSc is the set of Resolution Strategy that can be used to specify
      how to resolve conflicts that occur between the actions of the
      same or different policy rules that are matched and contained in
      this particular NSF;

   o Dc defines the notion of a Default action. This action can be
      either an explicit action that has been chosen {a}, or a set of
      actions {F}, where F is a dummy symbol (i.e., a placeholder
      value) that can be used to indicate that the default action can
      be freely selected by the policy editor. This is denoted as {F} U

   3.4.3. I2NSF Condition Clause Operator Types

   After having analyzed the literature and some existing NSFs, the
   types of Condition clause operators/selectors are categorized as
   exact-match, range-based, regex-based, and custom-match

   Exact-match selectors are (unstructured) sets: elements can only be
   checked for equality, as no order is defined on them.  As an
   example, the protocol type field of the IP header is an unordered
   set of integer values associated to protocols. The assigned protocol
   numbers are maintained by the IANA

   In this selector, it is only meaningful to specify condition clauses
   that use either the "equals" or "not equals operators":

      proto = tcp, udp       (protocol type field equals to TCP or UDP)

      proto != tcp           (protocol type field different from TCP)

   No other operators are allowed on exact-match selectors. For
   example, the following is an invalid condition clause, even if
   protocol types map to integers:

      proto < 62             (invalid condition)

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   Range-based selectors are ordered sets where it is possible to
   naturally specify ranges as they can be easily mapped to integers.
   As an example, the ports in the TCP protocol may be represented
   using a range-based selector (e.g., 1024-65535). For example, the
   following are examples of valid condition clauses:

      source_port = 80

      source_port < 1024

      source_port < 30000 && source_port >= 1024

   We include, in range-based selectors, the category of selectors that
   have been defined by Al-Shaer et al. as "prefix-match" [Alshaer].
   These selectors allow the specification of ranges of values by means
   of simple regular expressions. The typical case is the IP address
   selector (e.g., 10.10.1.*). There is no need to distinguish between
   prefix match and range-based selectors as 10.10.1.* easily maps to

   Another category of selector types includes the regex-based
   selectors, where the matching is performed by using regular
   expressions. This selector type is used frequently at the
   application layer, where data are often represented as strings of
   text. The regex-based selector type also includes string-based
   selectors, where matching is evaluated using string matching
   algorithms (SMA) [Cormen]. Indeed, for our purposes, string matching
   can be mapped to regular expressions, even if in practice SMA are
   much faster. For instance, Squid (http://www.squid-cache.org/), a
   popular Web caching proxy that offers various access control
   capabilities, allows the definition of conditions on URLs that can
   be evaluated with SMA (e.g., dstdomain) or regex matching (e.g.,

   As an example, the condition clause:

      URL = *.website.*

   matches all the URLs that contain a subdomain named website and the
   ones whose path contain the string ".website.". As another example,
   the condition clause:

      MIME_type = video/*

   matches all MIME objects whose type is video.

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   Finally, the idea of a custom check selector is introduced. For
   instance, malware analysis can look for specific patterns, and
   returns a Boolean value if the pattern is found or not.

   In order to be properly used by high-level policy-based processing
   systems (such as reasoning systems and policy translation systems),
   these custom check selectors can be modeled as black-boxes (i.e., a
   function that has a defined set of inputs and outputs for a
   particular state), which provide an associated Boolean output.

   3.4.4. Uses of the capability information model

   The capability information model can be used for two purposes:
   describing the features provided by generic security functions, and
   describing the features provided by specific products. The term
   Generic Network Security Function (GNSF) has been used to define
   generalized categories of NSFs. The idea is to have generic
   components whose behavior is well understood, so that the generic
   component can be used even if it has some vendor-specific functions.

   These generic functions represent a point of interoperability, and
   can be provided by any product that offers the required
   capabilities. GNSF examples include packet filter, URL filter, HTTP
   filter, VPN gateway, anti-virus, anti-malware, content filter,
   monitoring, and anonymity proxy; these will be described later in a
   revision of this draft.

   3.4.5. A Syntax to Describe the Capability of an NSF

   To characterize the behavior of the Policy Rule as specified by the
   CapIM, a NSF can be associated to its security capabilities by means
   of a 6-tuple {Ac, Cc, Ec, RSc, Dc, EVc}, where Ac, Cc, Ec, RSc, Dc,
   and EVc have been described in the previous sections.

   As an example, assume that a packet filter capability, Cap_pf, is
   defined, its, capabilities can be defined as

      Cap_pf = (Apf, Cpf, Epf, RSpf, Dpf, EVpf)


      Apf   =  {Allow, Deny}
      Cpf   =  {IPsrc,IPdst,Psrc,Pdst,protType}
      Epf   =  {}
      RSpf  =  {FMR}

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      Dpf   =  {A1}
      EVpf  =  {DNF}

   3.4.6. Capability Algebra

   Assigning capabilities to a large number of NSFs can be a complex
   (and boring) operation, therefore, this document presents a formal
   way (Capability Algebra) to define templates and manipulate them for

   Before introducing the capability algebra, we will introduce the
   symbols that we will use to represent set operations:

   o "U" is the union operation, A U B returns a new set that includes
      all the elements in A and all the elements in B

   o "\" is the set minus operation, A \ B returns all the elements
      that are in A but not in B.

   Given two sets of capabilities, denoted as cap1=(Ac1,Cc1,
   Ec1,RSc1,Dc1,EVc1) and cap2=(Ac2,Cc2,Ec2,RSc2,Dc2,EVc2)
   two set operations are defined for manipulating capabilities:

   o capability addition: cap1+cap2 = {Ac1 U Ac2, Cc1 U Cc2, Ec1 U
      Ec2, RSc1 U RSc2, Dc1 U DC2, EVc1 U EVc2}

   o capability subtraction: cap_1-cap_2 = {Ac1 \ Ac2, Cc1 \ Cc2, Ec1
      \ Ec2, RSc1 U RSc2, Dc1 U DC2, EVc1 U EVc2}

   In the above formulae, "U" is the set union operator and "\" is the
   set difference operator.

   The addition and subtraction of capabilities are defined as the
   addition (set union) and subtraction (set difference) of both the
   capabilities and their associated actions. Note that the Resolution
   Strategies and Default Actions are added in both cases.

   As an example, assume that a packet filter capability, Cap_pf, is
   defined as in a previous section. Further, assume that a second
   capability, called Cap_time, exists, and that it defines time-based
   conditions. Suppose we need to construct a new generic packet
   filter, Cap_pfgen, that adds time-based conditions to Cap_pf.

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   Conceptually, this is simply the addition of the Cap_pf and Cap_time
   capabilities, as follows:

      Apf   =  {Allow, Deny}
      Cpf   =  {IPsrc,IPdst,Psrc,Pdst,protType}
      Epf   =  {}
      RSpf  =  {FMR}
      Dpf   =  {A1}
      EVpf  =  {DNF}

      Atime =  {Allow, Deny, Log}
      Ctime =  {timestart, timeend, datestart, datestop}
      Etime =  {}
      RStime = {LMR}
      Dtime  = {A2}
      EVtime = {}

   Then, Cap_pfgen is defined as:

        Cpfgen = {Apf U Atime, Cpf U Ctime, Epf U Etime, RSpf U RStime,
                  Dpf U Time, EVpf U EVtime}
               = { {Allow, Deny, Log},
                   {IPsrc,IPdst,Psrc,Pdst,protType} U {timestart,
                  timeend, datestart, datestop},
                   {FMR, LMR},
                   {A1, A2},
                   {DNF} }

   In other words, Cap_pfgen provides three actions (Allow, Deny, Log),
   filters traffic based on a 5-tuple that is logically ANDed with a
   time period, can use either FMR or LMR (but obviously not both), and
   can provide either A1 or A2 (but again, not both) as a default
   action. In any case, multiple conditions will be processed with DNF
   when evaluating the condition clause.

  4. Considerations on the Practical Use of the CapIM

   CapIM is a solid basis for the data models that may serve in I2NSF.
   Nevertheless, a partial re-organization of the data models already
   produced by the WG is expected.

   Currently (A survey on the existing data models in the I2NSF world
   with a couple of lines about it and a list of things that can be

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   The CapIM can benefit from the definition of lists of elements to
   help to easily build the description of the capabilities which
   mainly consists in listing the things it can do:

   o the actions that NSFs can enforce;

   o the conditions that can be specified by at least one NSF.

   Knowing all actions and conditions would be desirable, but covering
   all of them is actually unreasonable. For instance, one-of-the-YANG-
   models lists a high number of protocols that can be a solid basis to
   build this list.

   On the other hand, resolution strategies, and condition clause
   evaluation methods are limited and listing all of them seems

   Finally, Events are too bounded to the systems/domain/architectures,
   therefore, compiling a list is simply impossible.

  5. Security Considerations

   This document defines an object-oriented information model for
   describing policy information that is independent of any specific
   repository, language, or protocol. This document does not define any
   particular system implementation, including a protocol. Hence, it
   does not have any specific security requirements.

  6. Contributors

   The following people contributed to creating this document, and are
   listed below in alphabetical order:

         Antonio Lioy (Politecnico di Torino)

         Dacheng Zhang (Huawei)

         Edward Lopez (Fortinet)

         Fulvio Valenza (Politecnico di Torino)

         Kepeng Li (Alibaba)

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         Luyuan Fang (Microsoft)

         Nicolas Bouthors (QoSmos)

  7. Acknowledgements

   Thanks to Linda Dunbar, Susan Hares and Jaehoon Paul Jeong for the
   discussion and comments.

   This work has been partially supported by the European Commission
   under Horizon 2020 grant agreement no. 700199 "Securing against
   intruders and other threats through a NFV-enabled environment
   (SHIELD)".  This support does not imply endorsement.

  8. References

  8.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
             Syntax Specifications: ABNF", RFC 2234, Internet Mail
             Consortium and Demon Internet Ltd., November 1997.

   [RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
             Network Configuration Protocol (NETCONF)", RFC 6020,
             October 2010.

   [RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
             Used to Form Encoding Rules in Various Routing Protocol
             Specifications", RFC 5511, April 2009.

   [RFC3198]  Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
             M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
             J., and S. Waldbusser, "Terminology for Policy-Based
             Management", RFC 3198, DOI 10.17487/RFC3198,
             November 2001, <http://www.rfc-editor.org/info/rfc3198>.

   [RFC8192]  Hares, S., Lopez, D., Zarny, M., Jacquenet, C., Kumar,
             R., and J. Jeong, "Interface to Network Security Functions
             (I2NSF): Problem Statement and Use Cases", RFC 8192,
             DOI 10.17487/RFC8192, July 2017,

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   [RFC8329]  Lopez, D., Lopez, E., Dunbar, L., Strassner, J. and R.
             Kumar, "Framework for Interface to Network Security
             Functions", RFC 8329, February 2018.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", RFC8174, May 2017.

  8.2. Informative References

   [INCITS359 RBAC]   NIST/INCITS, "American National Standard for
              Information Technology - Role Based Access Control",
              INCITS 359, April, 2003

   [I-D.draft-ietf-i2nsf-terminology] Hares, S., et.al., "Interface to
              Network Security Functions (I2NSF) Terminology", Work in
              Progress, January, 2018

   [I-D.draft-ietf-supa-generic-policy-info-model] Strassner, J.,
              Halpern, J., Coleman, J., "Generic Policy Information
              Model for Simplified Use of Policy Abstractions (SUPA)",
              Work in Progress, May, 2017.

   [Alshaer]  Al Shaer, E. and H. Hamed, "Modeling and management of
              firewall policies", 2004.

   [Bas12]    Basile, C., Cappadonia, A., and A. Lioy, "Network-Level
              Access Control Policy Analysis and Transformation", 2012.

   [Bas15]    Basile, C. and A. Lioy, "Analysis of application-layer
              filtering policies with application to HTTP", 2015.

   [Cormen]   Cormen, T., "Introduction to Algorithms", 2009.

   [Galitsky] Galitsky, B. and Pampapathi, R., "Can many agents answer
              questions better than one", First Monday, 2005;

   [Gamma]    Gamma, E., Helm, R. Johnson, R., Vlissides, J., "Design
              Patterns: Elements of Reusable Object-Oriented
              Software", Addison-Wesley, Nov, 1994.
              ISBN 978-0201633610

   [Hirschman]                   Hirschman, L., and Gaizauskas, R., "Natural Language
              Question Answering: The View from Here", Natural Language
              Engineering 7:4, pgs 275-300, Cambridge University Press,

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   [Hohpe]    Hohpe, G. and Woolf, B., "Enterprise Integration
              Patterns", Addison-Wesley, 2003, ISBN 0-32-120068-3

   [Lunt]     van Lunteren, J. and T. Engbersen, "Fast and scalable
              packet classification", 2003.

   [Martin]   Martin, R.C., "Agile Software Development, Principles,
              Patterns, and Practices", Prentice-Hall, 2002,
              ISBN: 0-13-597444-5

   [MCM]      MEF, "MEF Core Model", Technical Specification MEF X,
              April 2018

   [OODMP]    http://www.oodesign.com/mediator-pattern.html

   [OODSOP]   http://www.oodesign.com/observer-pattern.html

   [OODSRP]   http://www.oodesign.com/single-responsibility-

   [PDO]      MEF, "Policy Driven Orchestration", Technical
              Specification MEF Y, January 2018

   [Taylor]   Taylor, D. and J. Turner, "Scalable packet classification
              using distributed crossproducting of field labels", 2004.

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Authors' Addresses

   Cataldo Basile
   Politecnico di Torino
   Corso Duca degli Abruzzi, 34
   Torino, 10129
   Email: cataldo.basile@polito.it

   Liang Xia (Frank)
   101 Software Avenue, Yuhuatai District
   Nanjing, Jiangsu  210012
   Email: Frank.xialiang@huawei.com

   John Strassner
   2330 Central Expressway
   Santa Clara, CA  95050  USA
   Email: John.sc.Strassner@huawei.com

   Diego R. Lopez
   Telefonica I+D
   Zurbaran, 12
   Madrid,   28010
   Phone: +34 913 129 041
   Email: diego.r.lopez@telefonica.com

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