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Versions: (draft-anderson-forces-model) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 5812

   Internet Draft                               L. Yang
   Expiration: August 2005                           Intel Corp.
   File: draft-ietf-forces-model-04.txt         J. Halpern
   Working Group: ForCES                             Megisto Systems
                                                R. Gopal
                                                     Nokia
                                                A. DeKok
                                                     Infoblox, Inc.
                                                Z. Haraszti
                                                     Clovis Solutions
                                                S. Blake
                                                     Modular Networks
                                                E. Deleganes
                                                     Intel Corp.
                                                August 2005

                      ForCES Forwarding Element Model


                      draft-ietf-forces-model-04.txt


   "By submitting this Internet-Draft, I certify that any applicable
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   or will be disclosed, and any of which I become aware will be
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Internet Draft         ForCES FE Model              August 2005


Abstract

   This document defines the forwarding element (FE) model used in the
   Forwarding and Control Element Separation (ForCES) protocol.  The
   model represents the capabilities, state and configuration of
   forwarding elements within the context of the ForCES protocol, so
   that control elements (CEs) can control the FEs accordingly.  More
   specifically, the model describes the logical functions that are
   present in an FE, what capabilities these functions support, and how
   these functions are or can be interconnected.  This FE model is
   intended to satisfy the model requirements specified in the ForCES
   requirements draft, RFC 3564 [1].  A list of the basic logical
   functional blocks (LFBs) is also defined in the LFB class library to
   aid the effort in defining individual LFBs.

Table of Contents

   Abstract...........................................................2
   1. Definitions.....................................................4
   2. Introduction....................................................5
      2.1. Requirements on the FE model...............................6
      2.2. The FE Model in Relation to FE Implementations.............6
      2.3. The FE Model in Relation to the ForCES Protocol............7
      2.4. Modeling Language for the FE Model.........................7
      2.5. Document Structure.........................................8
   3. FE Model Concepts...............................................8
      3.1. FE Capability Model and State Model........................9
      3.2. LFB (Logical Functional Block) Modeling...................11
         3.2.1. LFB Outputs..........................................13
         3.2.2. LFB Inputs...........................................16
         3.2.3. Packet Type..........................................19
         3.2.4. Metadata.............................................19
         3.2.5. LFB Versioning.......................................26
         3.2.6. LFB Inheritance......................................27
      3.3. FE Datapath Modeling......................................28
         3.3.1. Alternative Approaches for Modeling FE Datapaths.....28
         3.3.2. Configuring the LFB Topology.........................32
   4. Model and Schema for LFB Classes...............................36
      4.1. Namespace.................................................36
      4.2. <LFBLibrary> Element......................................36
      4.3. <load> Element............................................38
      4.4. <frameDefs> Element for Frame Type Declarations...........38
      4.5. <dataTypeDefs> Element for Data Type Definitions..........39
         4.5.1. <typeRef> Element for Aliasing Existing Data Types...41
         4.5.2. <atomic> Element for Deriving New Atomic Types.......42
         4.5.3. <array> Element to Define Arrays.....................42
         4.5.4. <struct> Element to Define Structures................46

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         4.5.5. <union> Element to Define Union Types................47
         4.5.6. Augmentations........................................49
      4.6. <metadataDefs> Element for Metadata Definitions...........50
      4.7. <LFBClassDefs> Element for LFB Class Definitions..........51
         4.7.1. <derivedFrom> Element to Express LFB Inheritance.....52
         4.7.2. <inputPorts> Element to Define LFB Inputs............53
         4.7.3. <outputPorts> Element to Define LFB Outputs..........55
         4.7.4. <attributes> Element to Define LFB Operational
         Attributes..................................................57
         4.7.5. <capabilities> Element to Define LFB Capability
         Attributes..................................................60
         4.7.6. <description> Element for LFB Operational Specification
         ............................................................61
      4.8. XML Schema for LFB Class Library Documents................61
   5. FE Attributes and Capabilities.................................71
      5.1. XML for FEObject Class definition.........................72
      5.2. FE Capabilities...........................................80
         5.2.1. ModifiableLFBTopology................................80
         5.2.2. SupportedLFBs and SupportedLFBType...................80
         5.2.3. SupportedAttributeType...............................82
      5.3. FEAttributes..............................................83
         5.3.1. FEStatus.............................................83
         5.3.2. LFBSelectors and LFBSelectorType.....................83
         5.3.3. LFBTopology and LFBLinkType..........................83
         5.3.4. FENeighbors an FEConfiguredNeighborType..............84
   6. Satisfying the Requirements on FE Model........................85
      6.1. Port Functions............................................86
      6.2. Forwarding Functions......................................86
      6.3. QoS Functions.............................................86
      6.4. Generic Filtering Functions...............................86
      6.5. Vendor Specific Functions.................................86
      6.6. High-Touch Functions......................................87
      6.7. Security Functions........................................87
      6.8. Off-loaded Functions......................................87
      6.9. IPFLOW/PSAMP Functions....................................87
   7. Using the FE model in the ForCES Protocol......................88
      7.1. FE Topology Query.........................................90
      7.2. FE Capability Declarations................................91
      7.3. LFB Topology and Topology Configurability Query...........91
      7.4. LFB Capability Declarations...............................91
      7.5. State Query of LFB Attributes.............................92
      7.6. LFB Attribute Manipulation................................93
      7.7. LFB Topology Re-configuration.............................93
   8. Acknowledgments................................................93
   9. Security Considerations........................................94
   10. Normative References..........................................94
   11. Informative References........................................94

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   12. Authors' Addresses............................................95
   13. Intellectual Property Right...................................96
   14. IANA consideration............................................96
   15. Copyright Statement...........................................96

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC-2119].

1.
  Definitions

   Terminology associated with the ForCES requirements is defined in
   RFC 3564 [1] and is not copied here.  The following list of
   terminology relevant to the FE model is defined in this section.

   FE Model -- The FE model is designed to model the logical processing
   functions of an FE.  The FE model proposed in this document includes
   three components: the modeling of individual logical functional
   blocks (LFB model), the logical interconnection between LFBs (LFB
   topology) and the FE level attributes, including FE capabilities.
   The FE model provides the basis to define the information elements
   exchanged between the CE and the FE in the ForCES protocol.

   Datapath -- A conceptual path taken by packets within the forwarding
   plane inside an FE.  Note that more than one datapath can exist
   within an FE.

   LFB (Logical Functional Block) Class (or type) -- A template
   representing a fine-grained, logically separable and well-defined
   packet processing operation in the datapath.  LFB classes are the
   basic building blocks of the FE model.

   LFB Instance -- As a packet flows through an FE along a datapath, it
   flows through one or multiple LFB instances, where each LFB is an
   instance of a specific LFB class.  Multiple instances of the same
   LFB class can be present in an FE's datapath.  Note that we often
   refer to LFBs without distinguishing between an LFB class and LFB
   instance when we believe the implied reference is obvious for the
   given context.

   LFB Model -- The LFB model describes the content and structures in
   an LFB, plus the associated data definition.  Four types of
   information are defined in the LFB model.  The core part of the LFB
   model is the LFB class definitions; the other three types define the



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   associated data including common data types, supported frame formats
   and metadata.

   LFB Metadata -- Metadata is used to communicate per-packet state
   from one LFB to another, but is not sent across the network.  The FE
   model defines how such metadata is identified, produced and consumed
   by the LFBs, but not how the per-packet state is implemented within
   actual hardware.

   LFB Attribute -- Operational parameters of the LFBs that must be
   visible to the CEs are conceptualized in the FE model as the LFB
   attributes.  The LFB attributes include: flags, single parameter
   arguments, complex arguments, and tables that the CE can read or/and
   write via the ForCES protocol.

   LFB Topology -- A representation of the logical interconnection and
   the placement of LFB instances along the datapath within one FE.
   Sometimes this representation is called intra-FE topology, to be
   distinguished from inter-FE topology.  LFB topology is outside of
   the LFB model, but is part of the FE model.

   FE Topology -- A representation of how multiple FEs within a single
   NE are interconnected.  Sometimes this is called inter-FE topology,
   to be distinguished from intra-FE topology (i.e., LFB topology).  An
   individual FE might not have the global knowledge of the full FE
   topology, but the local view of its connectivity with other FEs is
   considered to be part of the FE model.  The FE topology is
   discovered by the ForCES base protocol or by some other means.

   Inter-FE Topology -- See FE Topology.

   Intra-FE Topology -- See LFB Topology.

   LFB class library -- A set of LFB classes that has been identified
   as the most common functions found in most FEs and hence should be
   defined first by the ForCES Working Group.

2.
  Introduction

   RFC 3746 [2] specifies a framework by which control elements (CEs)
   can configure and manage one or more separate forwarding elements
   (FEs) within a networking element (NE) using the ForCES protocol.
   The ForCES architecture allows Forwarding Elements of varying
   functionality to participate in a ForCES network element.  The
   implication of this varying functionality is that CEs can make only
   minimal assumptions about the functionality provided by FEs in an
   NE.  Before CEs can configure and control the forwarding behavior of

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   FEs, CEs need to query and discover the capabilities and states of
   their FEs.  RFC 3654 [1] mandates that the capabilities, states and
   configuration information be expressed in the form of an FE model.

   RFC 3444 [11] observed that information models (IMs) and data models
   (DMs) are different because they serve different purposes.  "The
   main purpose of an IM is to model managed objects at a conceptual
   level, independent of any specific implementations or protocols
   used".  "DMs, conversely, are defined at a lower level of
   abstraction and include many details.  They are intended for
   implementors and include protocol-specific constructs."  Sometimes
   it is difficult to draw a clear line between the two.  The FE model
   described in this document is primarily an information model, but
   also includes some aspects of a data model, such as explicit
   definitions of the LFB class schema and FE schema.  It is expected
   that this FE model will be used as the basis to define the payload
   for information exchange between the CE and FE in the ForCES
   protocol.

2.1. Requirements on the FE model

   RFC 3654 [1] defines requirements that must be satisfied by a ForCES
   FE model.  To summarize, an FE model must define:
     . Logically separable and distinct packet forwarding operations
        in an FE datapath (logical functional blocks or LFBs);
     . The possible topological relationships (and hence the sequence
        of packet forwarding operations) between the various LFBs;
     . The possible operational capabilities (e.g., capacity limits,
        constraints, optional features, granularity of configuration)
        of each type of LFB;
     . The possible configurable parameters (i.e., attributes) of each
        type of LFB;
     . Metadata that may be exchanged between LFBs.

2.2. The FE Model in Relation to FE Implementations

   The FE model proposed here is based on an abstraction of distinct
   logical functional blocks (LFBs), which are interconnected in a
   directed graph, and receive, process, modify, and transmit packets
   along with metadata.  The FE model should be designed such that
   different implementations of the forwarding datapath can be
   logically mapped onto the model with the functionality and sequence
   of operations correctly captured.  However, the model is not
   intended to directly address how a particular implementation maps to
   an LFB topology.  It is left to the forwarding plane vendors to
   define how the FE functionality is represented using the FE model.



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   Our goal is to design the FE model such that it is flexible enough
   to accommodate most common implementations.

   The LFB topology model for a particular datapath implementation MUST
   correctly capture the sequence of operations on the packet.
   Metadata generation by certain LFBs must always precede any use of
   that metadata by subsequent LFBs in the topology graph; this is
   required for logically consistent operation.  Further, modification
   of packet fields that are subsequently used as inputs for further
   processing must occur in the order specified in the model for that
   particular implementation to ensure correctness.

2.3. The FE Model in Relation to the ForCES Protocol

   The ForCES base protocol is used by the CEs and FEs to maintain the
   communication channel between the CEs and FEs.  The ForCES protocol
   may be used to query and discover the inter-FE topology.  The
   details of a particular datapath implementation inside an FE,
   including the LFB topology, along with the operational capabilities
   and attributes of each individual LFB, are conveyed to the CE within
   information elements in the ForCES protocol.  The model of an LFB
   class should define all of the information that needs to be
   exchanged between an FE and a CE for the proper configuration and
   management of that LFB.

   Specifying the various payloads of the ForCES messages in a
   systematic fashion is difficult without a formal definition of the
   objects being configured and managed (the FE and the LFBs within).
   The FE Model document defines a set of classes and attributes for
   describing and manipulating the state of the LFBs within an FE.
   These class definitions themselves will generally not appear in the
   ForCES protocol.  Rather, ForCES protocol operations will reference
   classes defined in this model, including relevant attributes and the
   defined operations.

   Section 7 provides more detailed discussion on how the FE model
   should be used by the ForCES protocol.

2.4. Modeling Language for the FE Model

   Even though not absolutely required, it is beneficial to use a
   formal data modeling language to represent the conceptual FE model
   described in this document.  Use of a formal language can help to
   enforce consistency and logical compatibility among LFBs.  A full
   specification will be written using such a data modeling language.
   The formal definition of the LFB classes may facilitate the eventual



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   automation of some of the code generation process and the functional
   validation of arbitrary LFB topologies.

   Human readability was the most important factor considered when
   selecting the specification language, whereas encoding, decoding and
   transmission performance was not a selection factor. The encoding
   method for over the wire transport is not dependent on the
   specification language chosen and is outside the scope of this
   document and up to the ForCES protocol to define.

   XML was chosen as the specification language in this document,
   because XML has the advantage of being both human and machine
   readable with widely available tools support.

2.5. Document Structure

   Section 3 provides a conceptual overview of the FE model, laying the
   foundation for the more detailed discussion and specifications in
   the sections that follow.  Section 4 and 5 constitute the core of
   the FE model, detailing the two major components in the FE model:
   LFB model and FE level attributes including capability and LFB
   topology.  Section 6 directly addresses the model requirements
   imposed by the ForCES requirement draft [1] while Section 7 explains
   how the FE model should be used in the ForCES protocol.

3.
  FE Model Concepts

   Some of the important concepts used throughout this document are
   introduced in this section.  Section 3.1 explains the difference
   between a state model and a capability model, and describes how the
   two can be combined in the FE model.  Section 3.2 introduces the
   concept of LFBs (Logical Functional Blocks) as the basic functional
   building blocks in the FE model.  Section 3.3 discusses the logical
   inter-connection and ordering between LFB instances within an FE,
   that is, the LFB topology.

   The FE model proposed in this document is comprised of two major
   components: the LFB model and FE level attributes, including FE
   capabilities and LFB topology.  The LFB model provides the content
   and data structures to define each individual LFB class.  FE
   attributes provide information at the FE level, particularly the
   capabilities of the FE at a coarse level.  Part of the FE level
   information is the LFB topology, which expresses the logical inter-
   connection between the LFB instances along the datapath(s) within
   the FE.  Details of these components are described in Section 4 and
   5.  The intent of this section is to discuss these concepts at the



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   high level and lay the foundation for the detailed description in
   the following sections.

3.1. FE Capability Model and State Model

   The ForCES FE model must describe both a capability and a state
   model.  The FE capability model describes the capabilities and
   capacities of an FE by specifying the variation in functions
   supported and any limitations.  The FE state model describes the
   current state of the FE, that is, the instantaneous values or
   operational behavior of the FE.

   Conceptually, the FE capability model tells the CE which states are
   allowed on an FE, with capacity information indicating certain
   quantitative limits or constraints.  Thus, the CE has general
   knowledge about configurations that are applicable to a particular
   FE.  For example, an FE capability model may describe the FE at a
   coarse level such as:

     . this FE can handle IPv4 and IPv6 forwarding;
     . this FE can perform classification on the following fields:
        source IP address, destination IP address, source port number,
        destination port number, etc;
     . this FE can perform metering;
     . this FE can handle up to N queues (capacity);
     . this FE can add and remove encapsulating headers of types
        including IPSec, GRE, L2TP.

   While one could try and build an object model to fully represent the
   FE capabilities, other efforts found this to be a significant
   undertaking.  The main difficulty arises in describing detailed
   limits, such as the maximum number of classifiers, queues, buffer
   pools, and meters the FE can provide.  We believe that a good
   balance between simplicity and flexibility can be achieved for the
   FE model by combining coarse level capability reporting with an
   error reporting mechanism.  That is, if the CE attempts to instruct
   the FE to set up some specific behavior it cannot support, the FE
   will return an error indicating the problem.  Examples of similar
   approaches include DiffServ PIB [4] and Framework PIB [5].

   The FE state model presents the snapshot view of the FE to the CE.
   For example, using an FE state model, an FE may be described to its
   corresponding CE as the following:
     . on a given port, the packets are classified using a given
        classification filter;
     . the given classifier results in packets being metered in a
        certain way, and then marked in a certain way;

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     . the packets coming from specific markers are delivered into a
        shared queue for handling, while other packets are delivered to
        a different queue;
     . a specific scheduler with specific behavior and parameters will
        service these collected queues.

   Figure 1 shows the concepts of FE state, capabilities and
   configuration in the context of CE-FE communication via the ForCES
   protocol.

        +-------+                                          +-------+
        |       | FE capabilities: what it can/cannot do.  |       |
        |       |<-----------------------------------------|       |
        |       |                                          |       |
        |   CE  | FE state: what it is now.                |  FE   |
        |       |<-----------------------------------------|       |
        |       |                                          |       |
        |       | FE configuration: what it should be.     |       |
        |       |----------------------------------------->|       |
        +-------+                                          +-------+

    Figure 1. Illustration of FE state, capabilities and configuration
        exchange in the context of CE-FE communication via ForCES.

   The concepts relating to LFBs, particularly capability at the LFB
   level and LFB topology will be discussed in the rest of this
   section.

   Capability information at the LFB level is an integral part of the
   LFB model, and is modeled the same way as the other operational
   parameters inside an LFB.  For example, when certain features of an
   LFB class are optional, it must be possible for the CE to determine
   whether those optional features are supported by a given LFB
   instance.  Such capability information can be modeled as a read-only
   attribute in the LFB instance, see Section 4.7.5 for details.

   Capability information at the FE level may describe the LFB classes
   that the FE can instantiate; the number of instances of each that
   can be created; the topological (linkage) limitations between these
   LFB instances, etc.  Section 5 defines the FE level attributes
   including capability information.

   Once the FE capability is described to the CE, the FE state
   information can be represented by two levels.  The first level is
   the logically separable and distinct packet processing functions,
   called Logical Functional Blocks (LFBs).  The second level of
   information describes how these individual LFBs are ordered and

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   placed along the datapath to deliver a complete forwarding plane
   service.  The interconnection and ordering of the LFBs is called LFB
   Topology.  Section 3.2 discusses high level concepts around LFBs,
   whereas Section 3.3 discusses LFB topology issues.

3.2. LFB (Logical Functional Block) Modeling

   Each LFB performs a well-defined action or computation on the
   packets passing through it.  Upon completion of its prescribed
   function, either the packets are modified in certain ways (e.g.,
   decapsulator, marker), or some results are generated and stored,
   often in the form of metadata (e.g., classifier).  Each LFB
   typically performs a single action.  Classifiers, shapers and meters
   are all examples of such LFBs.  Modeling LFBs at such a fine
   granularity allows us to use a small number of LFBs to express the
   higher-order FE functions (such as an IPv4 forwarder) precisely,
   which in turn can describe more complex networking functions and
   vendor implementations of software and hardware.  These LFBs will be
   defined in detail in one or more documents.

   An LFB has one or more inputs, each of which takes a packet P, and
   optionally metadata M; and produces one or more outputs, each of
   which carries a packet P', and optionally metadata M'.  Metadata is
   data associated with the packet in the network processing device
   (router, switch, etc.) and is passed from one LFB to the next, but
   is not sent across the network.  In general, multiple LFBs are
   contained in one FE, as shown in Figure 2, and all the LFBs share
   the same ForCES protocol termination point that implements the
   ForCES protocol logic and maintains the communication channel to and
   from the CE.



















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                           +-----------+
                           |    CE     |
                           +-----------+
                                ^
                                | Fp reference point
                                |
     +--------------------------|-----------------------------------+
     | FE                       |                                   |
     |                          v                                   |
     | +----------------------------------------------------------+ |
     | |                ForCES protocol                           | |
     | |                   termination point                      | |
     | +----------------------------------------------------------+ |
     |           ^                            ^                     |
     |           :                            : Internal control    |
     |           :                            :                     |
     |       +---:----------+             +---:----------|          |
     |       |   :LFB1      |             |   :     LFB2 |          |
     | =====>|   v          |============>|   v          |======>...|
     | Inputs| +----------+ |Outputs      | +----------+ |          |
     | (P,M) | |Attributes| |(P',M')      | |Attributes| |(P",M")   |
     |       | +----------+ |             | +----------+ |          |
     |       +--------------+             +--------------+          |
     |                                                              |
     +--------------------------------------------------------------+

                       Figure 2. Generic LFB Diagram

   An LFB, as shown in Figure 2, has inputs, outputs and attributes
   that can be queried and manipulated by the CE indirectly via an Fp
   reference point (defined in RFC 3746 [2]) and the ForCES protocol
   termination point.  The horizontal axis is in the forwarding plane
   for connecting the inputs and outputs of LFBs within the same FE.
   The vertical axis between the CE and the FE denotes the Fp reference
   point where bidirectional communication between the CE and FE
   occurs: the CE to FE communication is for configuration, control and
   packet injection while FE to CE communication is used for packet re-
   direction to the control plane, monitoring and accounting
   information, errors, etc.  Note that the interaction between the CE
   and the LFB is only abstract and indirect.  The result of such an
   interaction is for the CE to indirectly manipulate the attributes of
   the LFB instances.

   A namespace is used to associate a unique name or ID with each LFB
   class.  The namespace must be extensible so that a new LFB class can
   be added later to accommodate future innovation in the forwarding
   plane.

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   LFB operation must be specified in the model to allow the CE to
   understand the behavior of the forwarding datapath.  For instance,
   the CE must understand at what point in the datapath the IPv4 header
   TTL is decremented.  That is, the CE needs to know if a control
   packet could be delivered to it either before or after this point in
   the datapath.  In addition, the CE must understand where and what
   type of header modifications (e.g., tunnel header append or strip)
   are performed +by the FEs.  Further, the CE must verify that the
   various LFBs along a datapath within an FE are compatible to link
   together.

   There is value to vendors if the operation of LFB classes can be
   expressed in sufficient detail so that physical devices implementing
   different LFB functions can be integrated easily into an FE design.
   Therefore, a semi-formal specification is needed; that is, a text
   description of the LFB operation (human readable), but sufficiently
   specific and unambiguous to allow conformance testing and efficient
   design, so that interoperability between different CEs and FEs can
   be achieved.

   The LFB class model specifies information such as:
     . number of inputs and outputs (and whether they are
        configurable)
     . metadata read/consumed from inputs;
     . metadata produced at the outputs;
     . packet type(s) accepted at the inputs and emitted at the
        outputs;
     . packet content modifications (including encapsulation or
        decapsulation);
     . packet routing criteria (when multiple outputs on an LFB are
        present);
     . packet timing modifications;
     . packet flow ordering modifications;
     . LFB capability information;
     . LFB operational attributes, etc.

   Section 4 of this document provides a detailed discussion of the LFB
   model with a formal specification of LFB class schema.  The rest of
   Section 3.2 only intends to provide a conceptual overview of some
   important issues in LFB modeling, without covering all the specific
   details.

3.2.1. LFB Outputs

   An LFB output is a conceptual port on an LFB that can send
   information to another LFB.  The information is typically a packet

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   and its associated metadata, although in some cases it might consist
   of only metadata, i.e., with no packet data.

   A single LFB output can be connected to only one LFB input.  This is
   required to make the packet flow through the LFB topology
   unambiguously.

   Some LFBs will have a single output, as depicted in Figure 3.a.

      +---------------+               +-----------------+
      |               |               |                 |
      |               |               |             OUT +-->
     ...          OUT +-->           ...                |
      |               |               |    EXCEPTIONOUT +-->
      |               |               |                 |
      +---------------+               +-----------------+

        a. One output               b. Two distinct outputs

      +---------------+               +-----------------+
      |               |               |    EXCEPTIONOUT +-->
      |         OUT:1 +-->            |                 |
     ...        OUT:2 +-->           ...          OUT:1 +-->
      |         ...   +...            |           OUT:2 +-->
      |         OUT:n +-->            |           ...   +...
      +---------------+               |           OUT:n +-->
                                      +-----------------+

     c. One output group       d. One output and one output group

   Figure 3. Examples of LFBs with various output combinations.

   To accommodate a non-trivial LFB topology, multiple LFB outputs are
   needed so that an LFB class can fork the datapath.  Two mechanisms
   are provided for forking: multiple singleton outputs and output
   groups, which can be combined in the same LFB class.

   Multiple separate singleton outputs are defined in an LFB class to
   model a pre-determined number of semantically different outputs.
   That is, the number of outputs must be known when the LFB class is
   defined.  Additional singleton outputs cannot be created at LFB
   instantiation time, nor can they be created on the fly after the LFB
   is instantiated.

   For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one
   output(OUT) to send those packets for which the LPM look-up was
   successful, passing a META_ROUTEID as metadata; and have another

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   output (EXCEPTIONOUT) for sending exception packets when the LPM
   look-up failed.  This example is depicted in Figure 3.b.  Packets
   emitted by these two outputs not only require different downstream
   treatment, but they are a result of two different conditions in the
   LFB and each output carries different metadata.  This concept
   assumes the number of distinct outputs is known when the LFB class
   is defined. For each singleton output, the LFB class definition
   defines the types of frames and metadata the output emits.

   An output group, on the other hand, is used to model the case where
   a flow of similar packets with an identical set of metadata needs to
   be split into multiple paths. In this case, the number of such paths
   is not known when the LFB class is defined because it is not an
   inherent property of the LFB class.  An output group consists of a
   number of outputs, called the output instances of the group, where
   all output instances share the same frame and metadata emission
   definitions (see Figure 3.c).  Each output instance can connect to a
   different downstream LFB, just as if they were separate singleton
   outputs, but the number of output instances can differ between LFB
   instances of the same LFB class.  The class definition may include a
   lower and/or an upper limit on the number of outputs.  In addition,
   for configurable FEs, the FE capability information may define
   further limits on the number of instances in specific output groups
   for certain LFBs.  The actual number of output instances in a group
   is an attribute of the LFB instance, which is read-only for static
   topologies, and read-write for dynamic topologies.  The output
   instances in a group are numbered sequentially, from 0 to N-1, and
   are addressable from within the LFB.  The LFB has a built-in
   mechanism to select one specific output instance for each packet.
   This mechanism is described in the textual definition of the class
   and is typically configurable via some attributes of the LFB.

   For example, consider a re-director LFB, whose sole purpose is to
   direct packets to one of N downstream paths based on one of the
   metadata associated with each arriving packet.  Such an LFB is
   fairly versatile and can be used in many different places in a
   topology.  For example, a redirector can be used to divide the data
   path into an IPv4 and an IPv6 path based on a FRAMETYPE metadata
   (N=2), or to fork into color specific paths after metering using the
   COLOR metadata (red, yellow, green; N=3), etc.

   Using an output group in the above LFB class provides the desired
   flexibility to adapt each instance of this class to the required
   operation.  The metadata to be used as a selector for the output
   instance is a property of the LFB.  For each packet, the value of
   the specified metadata may be used as a direct index to the output



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   instance.  Alternatively, the LFB may have a configurable selector
   table that maps a metadata value to output instance.

   Note that other LFBs may also use the output group concept to build
   in similar adaptive forking capability.  For example, a classifier
   LFB with one input and N outputs can be defined easily by using the
   output group concept.  Alternatively, a classifier LFB with one
   singleton output in combination with an explicit N-output re-
   director LFB models the same processing behavior.  The decision of
   whether to use the output group model for a certain LFB class is
   left to the LFB class designers.

   The model allows the output group be combined with other singleton
   output(s) in the same class, as demonstrated in Figure 3.d.  The LFB
   here has two types of outputs, OUT, for normal packet output, and
   EXCEPTIONOUT for packets that triggered some exception.  The normal
   OUT has multiple instances, thus, it is an output group.

   In summary, the LFB class may define one output, multiple singleton
   outputs, one or more output groups, or a combination thereof.
   Multiple singleton outputs should be used when the LFB must provide
   for forking the datapath, and at least one of the following
   conditions hold:
     . the number of downstream directions are inherent from the
        definition of the class and hence fixed;
     . the frame type and set of metadata emitted on any of the
        outputs are substantially different from what is emitted on
        the other  outputs (i.e., they cannot share frame-type and
        metadata definitions);

   An output group is appropriate when the LFB must provide for forking
   the datapath, and at least one of the following conditions hold:
     . the number of downstream directions is not known when the LFB
        class is defined;
     . the frame type and set of metadata emitted on these outputs are
        sufficiently similar or ideally identical, such they can share
        the same output definition.

3.2.2. LFB Inputs

   An LFB input is a conceptual port on an LFB where the LFB can
   receive information from other LFBs.  The information is typically a
   packet and associated metadata, although in some cases it might
   consist of only metadata, without any packet data.





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   For LFB instances that receive packets from more than one other LFB
   instance (fan-in). There are three ways to model fan-in, all
   supported by the LFB model and can be combined in the same LFB:
     . Implicit multiplexing via a single input
     . Explicit multiplexing via multiple singleton inputs
     . Explicit multiplexing via a group of inputs (input group)

   The simplest form of multiplexing uses a singleton input (Figure
   4.a).  Most LFBs will have only one singleton input.  Multiplexing
   into a single input is possible because the model allows more than
   one LFB output to connect to the same LFB input.  This property
   applies to any LFB input without any special provisions in the LFB
   class.  Multiplexing into a single input is applicable when the
   packets from the upstream LFBs are similar in frame-type and
   accompanying metadata, and require similar processing.  Note that
   this model does not address how potential contention is handled when
   multiple packets arrive simultaneously.  If contention handling
   needs to be explicitly modeled, one of the other two modeling
   solutions must be used.

   The second method to model fan-in uses individually defined
   singleton inputs (Figure 4.b).  This model is meant for situations
   where the LFB needs to handle distinct types of packet streams,
   requiring input-specific handling inside the LFB, and where the
   number of such distinct cases is known when the LFB class is
   defined.  For example, a Layer 2 Decapsulation/Encapsulation LFB may
   have two inputs, one for receiving Layer 2 frames for decapsulation,
   and one for receiving Layer 3 frames for encapsulation.  This LFB
   type expects different frames (L2 vs. L3) at its inputs, each with
   different sets of metadata, and would thus apply different
   processing on frames arriving at these inputs.  This model is
   capable of explicitly addressing packet contention by defining how
   the LFB class handles the contending packets.

                +--------------+       +------------------------+
                | LFB X        +---+   |                        |
                +--------------+   |   |                        |
                                   |   |                        |
                +--------------+   v   |                        |
                | LFB Y        +---+-->|input     Meter LFB     |
                +--------------+   ^   |                        |
                                   |   |                        |
                +--------------+   |   |                        |
                | LFB Z        |---+   |                        |
                +--------------+       +------------------------+

   (a) An LFB connects with multiple upstream LFBs via a single input.

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                +--------------+       +------------------------+
                | LFB X        +---+   |                        |
                +--------------+   +-->|layer2                  |
                +--------------+       |                        |
                | LFB Y        +------>|layer3     LFB          |
                +--------------+       +------------------------+

   (b) An LFB connects with multiple upstream LFBs via two separate
       singleton inputs.

                +--------------+       +------------------------+
                | Queue LFB #1 +---+   |                        |
                +--------------+   |   |                        |
                                   |   |                        |
                +--------------+   +-->|in:0   \                |
                | Queue LFB #2 +------>|in:1   | input group    |
                +--------------+       |...    |                |
                                   +-->|in:N-1 /                |
                ...                |   |                        |
                +--------------+   |   |                        |
                | Queue LFB #N |---+   |     Scheduler LFB      |
                +--------------+       +------------------------+

   (c) A Scheduler LFB uses an input group to differentiate which queue
   LFB packets are coming from.

               Figure 3. Input modeling concepts (examples).

   The third method to model fan-in uses the concept of an input group.
   The concept is similar to the output group introduced in the
   previous section, and is depicted in Figure 4.c.  An input group
   consists of a number of input instances, all sharing the properties
   (same frame and metadata expectations).  The input instances are
   numbered from 0 to N-1.  From the outside, these inputs appear as
   normal inputs, i.e., any compatible upstream LFB can connect its
   output to one of these inputs.  When a packet is presented to the
   LFB at a particular input instance, the index of the input where the
   packet arrived is known to the LFB and this information may be used
   in the internal processing.  For example, the input index can be
   used as a table selector, or as an explicit precedence selector to
   resolve contention.  As with output groups, the number of input
   instances in an input group is not defined in the LFB class.
   However, the class definition may include restrictions on the range
   of possible values.  In addition, if an FE supports configurable
   topologies, it may impose further limitations on the number of
   instances for a particular port group(s) of a particular LFB class.

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   Within these limitations, different instances of the same class may
   have a different number of input instances.  The number of actual
   input instances in the group is an attribute of the LFB class, which
   is read-only for static topologies, and is read-write for
   configurable topologies.

   As an example for the input group, consider the Scheduler LFB
   depicted in Figure 3.c.  Such an LFB receives packets from a number
   of Queue LFBs via a number of input instances, and uses the input
   index information to control contention resolution and scheduling.

   In summary, the LFB class may define one input, multiple singleton
   inputs, one or more input groups, or a combination thereof.  Any
   input allows for implicit multiplexing of similar packet streams via
   connecting multiple outputs to the same input.  Explicit multiple
   singleton inputs are useful when either the contention handling must
   be handled explicitly, or when the LFB class must receive and
   process a known number of distinct types of packet streams.  An
   input group is suitable when contention handling must be modeled
   explicitly, but the number of inputs are not inherent from the class
   (and hence is not known when the class is defined), or when it is
   critical for LFB operation to know exactly on which input the packet
   was received.

3.2.3. Packet Type

   When LFB classes are defined, the input and output packet formats
   (e.g., IPv4, IPv6, Ethernet, etc.) must be specified.  These are the
   types of packets a given LFB input is capable of receiving and
   processing, or a given LFB output is capable of producing.  This
   requires distinct packet types be uniquely labeled with a symbolic
   name and/or ID.

   Note that each LFB has a set of packet types that it operates on,
   but does not care whether the underlying implementation is passing a
   greater portion of the packets.  For example, an IPv4 LFB might only
   operate on IPv4 packets, but the underlying implementation may or
   may not be stripping the L2 header before handing it over -- whether
   that is happening or not is opaque to the CE.

3.2.4. Metadata

   Metadata is the per-packet state that is passed from one LFB to
   another. The metadata is passed with the packet to assist subsequent
   LFBs to process that packet.  The ForCES model captures how the per-
   packet state information is propagated from one LFB to other LFBs.
   Practically, such metadata propagation can happen within one FE, or

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   cross the FE boundary between two interconnected FEs.  We believe
   that the same metadata model can be used for either situation;
   however, our focus here is for intra-FE metadata.

3.2.4.1. Metadata Vocabulary

   Metadata has historically been understood to mean "data about data".
   While this definition is a start, it is inadequate to describe the
   multiple forms of metadata, which may appear within a complex
   network element.  The discussion here categorizes forms of metadata
   by two orthogonal axes.

   The first axis is "internal" versus "external", which describes
   where the metadata exists in the network model or implementation.
   For example, a particular vendor implementation of an IPv4 forwarder
   may make decisions inside of a chip that are not visible externally.
   Those decisions are metadata for the packet that is "internal" to
   the chip.  When a packet is forwarded out of the chip, it may be
   marked with a traffic management header.  That header, which is
   metadata for the packet, is visible outside of the chip, and is
   therefore called "external" metadata.

   The second axis is "implicit" versus "expressed", which specifies
   whether or not the metadata has a visible physical representation.
   For example, the traffic management header described in the previous
   paragraph may be represented as a series of bits in some format, and
   that header is associated with the packet.  Those bits have physical
   representation, and are therefore "expressed" metadata.  If the
   metadata does not have a physical representation, it is called
   "implicit" metadata.  This situation occurs, for example, when a
   particular path through a network device is intended to be traversed
   only by particular kinds of packets, such as an IPv4 router.  An
   implementation may not mark every packet along this path as being of
   type "IPv4", but the intention of the designers is that every packet
   is of that type.  This understanding can be thought of as metadata
   about the packet, which is implicitly attached to the packet through
   the intent of the designers.

   In the ForCES model, we do NOT discuss or represent metadata
   "internal" to vendor implementations of LFBs.  Our focus is solely
   on metadata "external" to the LFBs, and therefore visible in the
   ForCES model.  The metadata discussed within this model may, or may
   not, be visible outside of the particular FE implementing the LFB
   model.  In this regard, the scope of the metadata within ForCES is
   very narrowly defined.




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   Note also that while we define metadata within this model, it is
   only a model.  There is no requirement that vendor implementations
   of ForCES use the exact metadata representations described in this
   document.  The only implementation requirement is that vendors
   implement the ForCES protocol, not the model.

3.2.4.2. Metadata lifecycle within the ForCES model

   Each metadata can be conveniently modeled as a <label, value> pair,
   where the label identifies the type of information, (e.g., "color"),
   and its value holds the actual information (e.g., "red").  The tag
   here is shown as a textual label, but it can be replaced or
   associated with a unique numeric value (identifier).

   The metadata life-cycle is defined in this model using three types
   of events: "write", "read" and "consume".  The first "write"
   implicitly creates and initializes the value of the metadata, and
   hence starts the life-cycle.  The explicit "consume" event
   terminates the life-cycle.  Within the life-cycle, that is, after a
   "write" event, but before the next "consume" event, there can be an
   arbitrary number of "write" and "read" events.  These "read" and
   "write" events can be mixed in an arbitrary order within the life-
   cycle.  Outside of the life-cycle of the metadata, that is, before
   the first "write" event, or between a "consume" event and the next
   "write" event, the metadata should be regarded non-existent or non-
   initialized.  Thus, reading a metadata outside of its life-cycle is
   considered an error.

   To ensure inter-operability between LFBs, the LFB class
   specification must define what metadata the LFB class "reads" or
   "consumes" on its input(s) and what metadata it "produces" on its
   output(s).  For maximum extensibility, this definition should
   neither specify which LFBs the metadata is expected to come from for
   a consumer LFB, nor which LFBs are expected to consume metadata for
   a given producer LFB.

   While it is important to define the metadata types passing between
   LFBs, it is not appropriate to define the exact encoding mechanism
   used by LFBs for that metadata.  Different implementations are
   allowed to use different encoding mechanisms for metadata.  For
   example, one implementation may store metadata in registers or
   shared memory, while another implementation may encode metadata in-
   band as a preamble in the packets.

   At any link between two LFBs, the packet is marked with a finite set
   of active metadata, where active means the metadata is within its
   life-cycle.  There are two corollaries of this model:

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   1. No un-initialized metadata exists in the model.

   2. No more than one occurrence of each metadata tag can be
      associated with a packet at any given time.

3.2.4.3. LFB Operations on Metadata

   When the packet is processed by an LFB (i.e., between the time it is
   received and forwarded by the LFB), the LFB may perform read, write
   and/or consume operations on any active metadata associated with the
   packet.  If the LFB is considered to be a black box, one of the
   following operations is performed on each active metadata.

     . IGNORE:           ignores and forwards the metadata
     . READ:             reads and forwards the metadata
     . READ/RE-WRITE:    reads, over-writes and forwards the metadata
     . WRITE:            writes and forwards the metadata
                          (can also be used to create new metadata)
     . READ-AND-CONSUME: reads and consumes the metadata
     . CONSUME           consumes metadata without reading

   The last two operations terminate the life-cycle of the metadata,
   meaning that the metadata is not forwarded with the packet when the
   packet is sent to the next LFB.

   In our model, a new metadata is generated by an LFB when the LFB
   applies a WRITE operation to a metadata type that was not present
   when the packet was received by the LFB.  Such implicit creation may
   be unintentional by the LFB, that is, the LFB may apply the WRITE
   operation without knowing or caring if the given metadata existed or
   not.  If it existed, the metadata gets over-written; if it did not
   exist, the metadata is created.

   For LFBs that insert packets into the model, WRITE is the only
   meaningful metadata operation.

   For LFBs that remove the packet from the model, they may either
   READ-AND-CONSUME (read) or CONSUME (ignore) each active metadata
   associated with the packet.

3.2.4.4. Metadata Production and Consumption

   For a given metadata on a given packet path, there must be at least
   one producer LFB that creates that metadata and should be at least
   one consumer LFB that needs that metadata.  In this model, the



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   producer and consumer LFBs of a metadata are not required to be
   adjacent.  In addition, there may be multiple producers and
   consumers for the same metadata.  When a packet path involves
   multiple producers of the same metadata, then subsequent producers
   overwrite that metadata value.

   The metadata that is produced by an LFB is specified by the LFB
   class definition on a per output port group basis.  A producer may
   always generate the metadata on the port group, or may generate it
   only under certain conditions.  We call the former an
   "unconditional" metadata, whereas the latter is a "conditional"
   metadata.  In the case of conditional metadata, it should be
   possible to determine from the definition of the LFB when a
   "conditional" metadata is produced.

   The consumer behavior of an LFB, that is, the metadata that the LFB
   needs for its operation, is defined in the LFB class definition on a
   per input port group basis.  An input port group may "require" a
   given metadata, or may treat it as "optional" information.  In the
   latter case, the LFB class definition must explicitly define what
   happens if an optional metadata is not provided.  One approach is to
   specify a default value for each optional metadata, and assume that
   the default value is used if the metadata is not provided with the
   packet.

   When a consumer LFB requires a given metadata, it has dependencies
   on its up-stream LFBs.  That is, the consumer LFB can only function
   if there is at least one producer of that metadata and no
   intermediate LFB consumes the metadata.

   The model should expose these inter-dependencies.  Furthermore, it
   should be possible to take inter-dependencies into consideration
   when constructing LFB topologies, and also that the dependencies can
   be verified when validating topologies.

   For extensibility reasons, the LFB specification should define what
   metadata the LFB requires without specifying which LFB(s) it expects
   a certain metadata to come from.  Similarly, LFBs should specify
   what metadata they produce without specifying which LFBs the
   metadata is meant for.

   When specifying the metadata tags, some harmonization effort must be
   made so that the producer LFB class uses the same tag as its
   intended consumer(s), or vice versa.

3.2.4.5. Fixed, Variable and Configurable Tag


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   When the produced metadata is defined for a given LFB class, most
   metadata will be specified with a fixed tag.  For example, a Rate
   Meter LFB will always produce the "Color" metadata.

   A small subset of LFBs need the capability to produce one or more of
   their metadata with tags that are not fixed in the LFB class
   definition, but instead can be selected per LFB instance.  An
   example of such an LFB class is a Generic Classifier LFB.  We call
   this capability "variable tag metadata production".  If an LFB
   produces metadata with a variable tag, the corresponding LFB
   attribute, called the tag selector, specifies the tag for each such
   metadata.  This mechanism improves the versatility of certain multi-
   purpose LFB classes, since it allows the same LFB class to be used
   in different topologies, producing the right metadata tags according
   to the needs of the topology.

   Depending on the capability of the FE, the tag selector can be
   either a read-only or a read-write attribute.  If the selector is
   read-only, the tag cannot be modified by the CE.  If the selector is
   read-write, the tag can be configured by the CE, hence we call this
   "configurable tag metadata production."  Note that using this
   definition, configurable tag metadata production is a subset of
   variable tag metadata production.

   Similar concepts can be introduced for the consumer LFBs to satisfy
   different metadata needs.  Most LFB classes will specify their
   metadata needs using fixed metadata tags.  For example, a Next Hop
   LFB may always require a "NextHopId" metadata; but the Redirector
   LFB may need a "ClassID" metadata in one instance, and a
   "ProtocolType" metadata in another instance as a basis for selecting
   the right output port.  In this case, an LFB attribute is used to
   provide the required metadata tag at run-time.  This metadata tag
   selector attribute may be read-only or read-write, depending on the
   capabilities of the LFB instance and the FE.

3.2.4.6. Metadata Usage Categories

   Depending on the role and usage of a metadata, various amounts of
   encoding information must be provided when the metadata is defined,
   where some cases offer less flexibility in the value selection than
   others.

   There are three types of metadata related to metadata usage:
     . Relational (or binding) metadata
     . Enumerated metadata
     . Explicit/external value metadata


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   The purpose of the relational metadata is to refer in one LFB
   instance (producer LFB) to a "thing" in another downstream LFB
   instance (consumer LFB), where the "thing" is typically an entry in
   a table attribute of the consumer LFB.

   For example, the Prefix Lookup LFB executes an LPM search using its
   prefix table and resolves to a next-hop reference.  This reference
   needs to be passed as metadata by the Prefix Lookup LFB (producer)
   to the Next Hop LFB (consumer), and must refer to a specific entry
   in the next-hop table within the consumer.

   Expressing and propagating such a binding relationship is probably
   the most common usage of metadata.  One or more objects in the
   producer LFB are bound to a specific object in the consumer LFB.
   Such a relationship is established by the CE explicitly by properly
   configuring the attributes in both LFBs.  Available methods include
   the following:

   The binding may be expressed by tagging the involved objects in both
   LFBs with the same unique, but otherwise arbitrary, identifier.  The
   value of the tag is explicitly configured by the CE by writing the
   value into both LFBs, and this value is also carried by the metadata
   between the LFBs.

   Another way of setting up binding relations is to use a naturally
   occurring unique identifier of the consumer's object as a reference
   and as a value of the metadata (e.g., the array index of a table
   entry).  In this case, the index is either read or inferred by the
   CE by communicating with the consumer LFB.  Once the CE obtains the
   index, it needs to write it into the producer LFB to establish the
   binding.

   Important characteristics of the binding usage of metadata are:

     . The value of the metadata shows up in the CE-FE communication
        for BOTH the consumer and the producer.  That is, the metadata
        value must be carried over the ForCES protocol.  Using the
        tagging technique, the value is WRITTEN to both LFBs.  Using
        the other technique, the value is WRITTEN to only the producer
        LFB and may be READ from the consumer LFB.

     . The metadata value is irrelevant to the CE, the binding is
        simply expressed by using the SAME value at the consumer and
        producer LFBs.

     . Hence the metadata definition is not required to include value
        assignments.  The only exception is when some special value(s)

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        of the metadata must be reserved to convey special events.
        Even though these special cases must be defined with the
        metadata specification, their encoded values can be selected
        arbitrarily.  For example, for the Prefix Lookup LFB example, a
        special value may be reserved to signal the NO-MATCH case, and
        the value of zero may be assigned for this purpose.

   The second class of metadata is the enumerated type.  An example is
   the "Color" metadata that is produced by a Meter LFB. As the name
   suggests, enumerated metadata has a relatively small number of
   possible values, each with a specific meaning.  All possible cases
   must be enumerated when defining this class of metadata.  Although a
   value encoding must be included in the specification, the actual
   values can be selected arbitrarily (e.g., <Red=0, Yellow=1, Green=2>
   and <Red=3, Yellow=2, Green 1> would be both valid encodings, what
   is important is that an encoding is specified).

   The value of the enumerated metadata may or may not be conveyed via
   the ForCES protocol between the CE and FE.

   The third class of metadata is the explicit type.  This refers to
   cases where the metadata value is explicitly used by the consumer
   LFB to change some packet header fields.  In other words, the value
   has a direct and explicit impact on some field and will be visible
   externally when the packet leaves the NE.  Examples are: TTL
   increment given to a Header Modifier LFB, and DSCP value for a
   Remarker LFB.  For explicit metadata, the value encoding must be
   explicitly provided in the metadata definition.  The values cannot
   be selected arbitrarily and should conform to what is commonly
   expected.  For example, a TTL increment metadata should be encoded
   as zero for the no increment case, one for the single increment
   case, etc.  A DSCP metadata should use 0 to encode DSCP=0, 1 to
   encode DSCP=1, etc.

3.2.5. LFB Versioning

   LFB class versioning is a method to enable incremental evolution of
   LFB classes. In general, an FE is not allowed to contain an LFB
   instance for more than one version of a particular class.
   Inheritance (discussed next in Section 3.2.6) has special rules. If
   an FE datapath model containing an LFB instance of a particular
   class C also simultaneously contains an LFB instance of a class C'
   inherited from class C; C could have a different version than C'.

   LFB class versioning is supported by requiring a version string in
   the class definition.  CEs may support multiple versions of a



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   particular LFB class to provide backward compatibility, but FEs are
   not allowed to support more than one version of a particular class.

3.2.6. LFB Inheritance

   LFB class inheritance is supported in the FE model as a method to
   define new LFB classes.  This also allows FE vendors to add vendor-
   specific extensions to standardized LFBs.  An LFB class
   specification MUST specify the base class and version number it
   inherits from (the default is the base LFB class).  Multiple-
   inheritance is not allowed, however, to avoid unnecessary
   complexity.

   Inheritance should be used only when there is significant reuse of
   the base LFB class definition.  A separate LFB class should be
   defined if little or no reuse is possible between the derived and
   the base LFB class.

   An interesting issue related to class inheritance is backward
   compatibility between a descendant and an ancestor class.   Consider
   the following hypothetical scenario where a standardized LFB class
   "L1" exists.  Vendor A builds an FE that implements LFB "L1" and
   vendor B builds a CE that can recognize and operate on LFB "L1".
   Suppose that a new LFB class, "L2", is defined based on the existing
   "L1" class by extending its capabilities incrementally. Let us
   examine the FE backward compatibility issue by considering what
   would happen if vendor B upgrades its FE from "L1" to "L2" and
   vendor C's CE is not changed.  The old L1-based CE can interoperate
   with the new L2-based FE if the derived LFB class "L2" is indeed
   backward compatible with the base class "L1".

   The reverse scenario is a much less problematic case, i.e., when CE
   vendor B upgrades to the new LFB class "L2", but the FE is not
   upgraded.  Note that as long as the CE is capable of working with
   older LFB classes, this problem does not affect the model; hence we
   will use the term "backward compatibility" to refer to the first
   scenario concerning FE backward compatibility.

   Backward compatibility can be designed into the inheritance model by
   constraining LFB inheritance to require the derived class be a
   functional superset of the base class (i.e. the derived class can
   only add functions to the base class, but not remove functions).
   Additionally, the following mechanisms are required to support FE
   backward compatibility:
     1. When detecting an LFB instance of an LFB type that is unknown
        to the CE, the CE MUST be able to query the base class of such
        an LFB from the FE.

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     2. The LFB instance on the FE SHOULD support a backward
        compatibility mode (meaning the LFB instance reverts itself
        back to the base class instance), and the CE SHOULD be able to
        configure the LFB to run in such a mode.

3.3. FE Datapath Modeling

   Packets coming into the FE from ingress ports generally flow through
   multiple LFBs before leaving out of the egress ports.  How an FE
   treats a packet depends on many factors, such as type of the packet
   (e.g., IPv4, IPv6 or MPLS), actual header values, time of arrival,
   etc.  The result of LFB processing may have an impact on how the
   packet is to be treated in downstream LFBs.  This differentiation of
   packet treatment downstream can be conceptualized as having
   alternative datapaths in the FE.  For example, the result of a 6-
   tuple classification performed by a classifier LFB could control
   which rate meter is applied to the packet by a rate meter LFB in a
   later stage in the datapath.

   LFB topology is a directed graph representation of the logical
   datapaths within an FE, with the nodes representing the LFB
   instances and the directed link depicting the packet flow direction
   from one LFB to the next.  Section 3.3.1 discusses how the FE
   datapaths can be modeled as LFB topology; while Section 3.3.2
   focuses on issues related to LFB topology reconfiguration.

3.3.1. Alternative Approaches for Modeling FE Datapaths

   There are two basic ways to express the differentiation in packet
   treatment within an FE, one represents the datapath directly and
   graphically (topological approach) and the other utilizes metadata
   (the encoded state approach).

     . Topological Approach

     Using this approach, differential packet treatment is expressed by
     splitting the LFB topology into alternative paths.  In other
     words, if the result of an LFB must control how the packet is
     further processed, then such an LFB will have separate output
     ports, one for each alternative treatment, connected to separate
     sub-graphs, each expressing the respective treatment downstream.

     . Encoded State Approach

     An alternate way of expressing differential treatment is by using
     metadata.  The result of the operation of an LFB can be encoded in
     a metadata, which is passed along with the packet to downstream

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     LFBs.  A downstream LFB, in turn, can use the metadata and its
     value (e.g., as an index into some table) to determine how to
     treat the packet.

   Theoretically, either approach could substitute for the other, so
   one could consider using a single pure approach to describe all
   datapaths in an FE.  However, neither model by itself results in the
   best representation for all practically relevant cases.  For a given
   FE with certain logical datapaths, applying the two different
   modeling approaches will result in very different looking LFB
   topology graphs.  A model using only the topological approach may
   require a very large graph with many links or paths, and nodes
   (i.e., LFB instances) to express all alternative datapaths.  On the
   other hand, a model using only the encoded state model would be
   restricted to a string of LFBs, which is not an intuitive way to
   describe different datapaths (such as MPLS and IPv4).  Therefore, a
   mix of these two approaches will likely be used for a practical
   model.  In fact, as we illustrate below, the two approaches can be
   mixed even within the same LFB.

   Using a simple example of a classifier with N classification outputs
   followed by other LFBs, Figure 5(a) shows what the LFB topology
   looks like when using the pure topological approach.  Each output
   from the classifier goes to one of the N LFBs where no metadata is
   needed.  The topological approach is simple, straightforward and
   graphically intuitive.  However, if N is large and the N nodes
   following the classifier (LFB#1, LFB#2, ..., LFB#N) all belong to
   the same LFB type (e.g., meter), but each has its own independent
   attributes, the encoded state approach gives a much simpler topology
   representation, as shown in Figure 5(b).  The encoded state approach
   requires that a table of N rows of meter attributes is provided in
   the Meter node itself, with each row representing the attributes for
   one meter instance.  A metadata M is also needed to pass along with
   the packet P from the classifier to the meter, so that the meter can
   use M as a look-up key (index) to find the corresponding row of the
   attributes that should be used for any particular packet P.

   What if those N nodes (LFB#1, LFB#2, ..., LFB#N) are not of the same
   type? For example, if LFB#1 is a queue while the rest are all
   meters, what is the best way to represent such datapaths?  While it
   is still possible to use either the pure topological approach or the
   pure encoded state approach, the natural combination of the two
   appears to be the best option. Figure 5(c) depicts two different
   functional datapaths using the topological approach while leaving
   the N-1 meter instances distinguished by metadata only, as shown in
   Figure 5(c).



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                                        +----------+
                                 P      |   LFB#1  |
                             +--------->|(Attrib-1)|
        +-------------+      |          +----------+
        |            1|------+   P      +----------+
        |            2|---------------->|   LFB#2  |
        | classifier 3|                 |(Attrib-2)|
        |          ...|...              +----------+
        |            N|------+          ...
        +-------------+      |   P      +----------+
                             +--------->|   LFB#N  |
                                        |(Attrib-N)|
                                        +----------+

                5(a) Using pure topological approach

        +-------------+                 +-------------+
        |            1|                 |   Meter     |
        |            2|   (P, M)        | (Attrib-1)  |
        |            3|---------------->| (Attrib-2)  |
        |          ...|                 |   ...       |
        |            N|                 | (Attrib-N)  |
        +-------------+                 +-------------+

          5(b) Using pure encoded state approach to represent the LFB
          topology in 5(a), if LFB#1, LFB#2, ..., and LFB#N are of the
                        same type (e.g., meter).

                                     +-------------+
        +-------------+ (P, M)       | queue       |
        |            1|------------->| (Attrib-1)  |
        |            2|              +-------------+
        |            3| (P, M)       +-------------+
        |          ...|------------->|   Meter     |
        |            N|              | (Attrib-2)  |
        +-------------+              |   ...       |
                                     | (Attrib-N)  |
                                     +-------------+

         5(c) Using a combination of the two, if LFB#1, LFB#2, ..., and
             LFB#N are of different types (e.g., queue and meter).

               Figure 5. An example of how to model FE datapaths

   From this example, we demonstrate that each approach has a distinct
   advantage depending on the situation.  Using the encoded state
   approach, fewer connections are typically needed between a fan-out

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   node and its next LFB instances of the same type because each packet
   carries metadata the following nodes can interpret and hence invoke
   a different packet treatment.  For those cases, a pure topological
   approach forces one to build elaborate graphs with many more
   connections and often results in an unwieldy graph.  On the other
   hand, a topological approach is the most intuitive for representing
   functionally different datapaths.

   For complex topologies, a combination of the two is the most
   flexible.  A general design guideline is provided to indicate which
   approach is best used for a particular situation.  The topological
   approach should primarily be used when the packet datapath forks to
   distinct LFB classes (not just distinct parameterizations of the
   same LFB class), and when the fan-outs do not require changes, such
   as adding/removing LFB outputs, or require only very infrequent
   changes.  Configuration information that needs to change frequently
   should be expressed by using the internal attributes of one or more
   LFBs (and hence using the encoded state approach).

                      +---------------------------------------------+
                      |                                             |
        +----------+  V      +----------+           +------+        |
        |          |  |      |          |if IP-in-IP|      |        |
   ---->| ingress  |->+----->|classifier|---------->|Decap.|---->---+
        | ports    |         |          |----+      |      |
        +----------+         +----------+    |others+------+
                                             |
                                             V
        (a)  The LFB topology with a logical loop

       +-------+   +-----------+            +------+   +-----------+
       |       |   |           |if IP-in-IP |      |   |           |
   --->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|->
       | ports |   |           |----+       |      |   |           |
       +-------+   +-----------+    |others +------+   +-----------+
                                    |
                                    V
        (b)  The LFB topology without the loop utilizing two
             independent classifier instances.

                Figure 6. An LFB topology example.

   It is important to point out that the LFB topology described here is
   the logical topology, not the physical topology of how the FE
   hardware is actually laid out.  Nevertheless, the actual
   implementation may still influence how the functionality is mapped
   to the LFB topology.  Figure 6 shows one simple FE example.  In this

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   example, an IP-in-IP packet from an IPSec application like VPN may
   go to the classifier first and have the classification done based on
   the outer IP header; upon being classified as an IP-in-IP packet,
   the packet is then sent to a decapsulator to strip off the outer IP
   header, followed by a classifier again to perform classification on
   the inner IP header. If the same classifier hardware or software is
   used for both outer and inner IP header classification with the same
   set of filtering rules, a logical loop is naturally present in the
   LFB topology, as shown in Figure 6(a).  However, if the
   classification is implemented by two different pieces of hardware or
   software with different filters (i.e., one set of filters for the
   outer IP header and another set for the inner IP header), then it is
   more natural to model them as two different instances of classifier
   LFB, as shown in Figure 6(b).

   To distinguish between multiple instances of the same LFB class,
   each LFB instance has its own LFB instance ID.  One way to encode
   the LFB instance ID is to encode it as x.y where x is the LFB class
   ID and y is the instance ID within each LFB class.

3.3.2. Configuring the LFB Topology

   While there is little doubt that an individual LFB must be
   configurable, the configurability question is more complicated for
   LFB topology.  Since the LFB topology is really the graphic
   representation of the datapaths within an FE, configuring the LFB
   topology means dynamically changing the datapaths, including
   changing the LFBs along the datapaths on an FE (e.g., creating,
   instantiating or deleting LFBs) and setting up or deleting
   interconnections between outputs of upstream LFBs to inputs of
   downstream LFBs.

   Why would the datapaths on an FE ever change dynamically?  The
   datapaths on an FE are set up by the CE to provide certain data
   plane services (e.g., DiffServ, VPN, etc.) to the Network Element's
   (NE) customers.  The purpose of reconfiguring the datapaths is to
   enable the CE to customize the services the NE is delivering at run
   time.  The CE needs to change the datapaths when the service
   requirements change, such as adding a new customer or when an
   existing customer changes their service.  However, note that not all
   datapath changes result in changes in the LFB topology graph.
   Changes in the graph are dependent on the approach used to map the
   datapaths into LFB topology.  As discussed in 3.3.1, the topological
   approach and encoded state approach can result in very different
   looking LFB topologies for the same datapaths.  In general, an LFB
   topology based on a pure topological approach is likely to
   experience more frequent topology reconfiguration than one based on

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   an encoded state approach.  However, even an LFB topology based
   entirely on an encoded state approach may have to change the
   topology at times, for example, to bypass some LFBs or insert new
   LFBs.  Since a mix of these two approaches is used to model the
   datapaths, LFB topology reconfiguration is considered an important
   aspect of the FE model.

   We want to point out that allowing a configurable LFB topology in
   the FE model does not mandate that all FEs must have this
   capability.  Even if an FE supports configurable LFB topology, the
   FE may impose limitations on what can actually be configured.
   Performance-optimized hardware implementations may have zero or very
   limited configurability, while FE implementations running on network
   processors may provide more flexibility and configurability.  It is
   entirely up to the FE designers to decide whether or not the FE
   actually implements reconfiguration and if so, how much.  Whether a
   simple runtime switch is used to enable or disable (i.e., bypass)
   certain LFBs, or more flexible software reconfiguration is used, is
   implementation detail internal to the FE and outside of the scope of
   FE model.  In either case, the CE(s) must be able to learn the FE's
   configuration capabilities.  Therefore, the FE model must provide a
   mechanism for describing the LFB topology configuration capabilities
   of an FE.  These capabilities may include (see Section 5 for full
   details):
     . Which LFB classes the FE can instantiate
     . Maximum number of instances of the same LFB class that can be
        created
     . Any topological limitations, For example:
          o The maximum number of instances of the same class or any
             class that can be created on any given branch of the graph
          o Ordering restrictions on LFBs (e.g., any instance of LFB
             class A must be always downstream of any instance of LFB
             class B).

   Note that even when the CE is allowed to configure LFB topology for
   the FE, the CE is not expected to be able to interpret an arbitrary
   LFB topology and determine which specific service or application
   (e.g. VPN, DiffServ, etc.) is supported by the FE.  However, once
   the CE understands the coarse capability of an FE, it is the
   responsibility of the CE to configure the LFB topology to implement
   the network service the NE is supposed to provide.  Thus, the
   mapping the CE has to understand is from the high level NE service
   to a specific LFB topology, not the other way around. The CE is not
   expected to have the ultimate intelligence to translate any high
   level service policy into the configuration data for the FEs.
   However, it is conceivable that within a given network service
   domain, a certain amount of intelligence can be programmed into the

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   CE to give the CE a general understanding of the LFBs involved to
   allow the translation from a high level service policy to the low
   level FE configuration to be done automatically.  Note that this is
   considered an implementation issue internal to the control plane and
   outside the scope of the FE model. Therefore, it is not discussed
   any further in this draft.

        +----------+     +-----------+
   ---->| Ingress  |---->|classifier |--------------+
        |          |     |chip       |              |
        +----------+     +-----------+              |
                                                    v
                        +-------------------------------------------+
          +--------+    |   Network Processor                       |
     <----| Egress |    |   +------+    +------+   +-------+        |
          +--------+    |   |Meter |    |Marker|   |Dropper|        |
                ^       |   +------+    +------+   +-------+        |
                |       |                                           |
     +----------+-------+                                           |
     |          |                                                   |
     |    +---------+       +---------+   +------+    +---------+   |
     |    |Forwarder|<------|Scheduler|<--|Queue |    |Counter  |   |
     |    +---------+       +---------+   +------+    +---------+   |
     |--------------------------------------------------------------+

                (a)  The Capability of the FE, reported to the CE

          +-----+    +-------+                      +---+
          |    A|--->|Queue1 |--------------------->|   |
   ------>|     |    +-------+                      |   |  +---+
          |     |                                   |   |  |   |
          |     |    +-------+      +-------+       |   |  |   |
          |    B|--->|Meter1 |----->|Queue2 |------>|   |->|   |
          |     |    |       |      +-------+       |   |  |   |
          |     |    |       |--+                   |   |  |   |
          +-----+    +-------+  |   +-------+       |   |  +---+
        classifier              +-->|Dropper|       |   |  IPv4
                                    +-------+       +---+  Fwd.
                                                 Scheduler

                (b)  One LFB topology as configured by the CE and
                     accepted by the FE







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                                                 Queue1
                      +---+                    +--+
                      |  A|------------------->|  |--+
                   +->|   |                    |  |  |
                   |  |  B|--+  +--+   +--+    +--+  |
                   |  +---+  |  |  |   |  |          |
                   | Meter1  +->|  |-->|  |          |
                   |            |  |   |  |          |
                   |            +--+   +--+          |          Ipv4
                   |         Counter1 Dropper1 Queue2|    +--+  Fwd.
           +---+   |                           +--+  +--->|A |  +-+
           |  A|---+                           |  |------>|B |  | |
    ------>|  B|------------------------------>|  |  +--->|C |->| |->
           |  C|---+                           +--+  | +->|D |  | |
           |  D|-+ |                                 | |  +--+  +-+
           +---+ | |    +---+                  Queue3| | Scheduler
       Classifier1 | |  |  A|------------>       +--+  | |
                   | +->|   |                    |  |--+ |
                   |    |  B|--+  +--+ +-------->|  |    |
                   |    +---+  |  |  | |         +--+    |
                   |  Meter2   +->|  |-+                 |
                   |              |  |                   |
                   |              +--+           Queue4  |
                   |            Marker1          +--+    |
                   +---------------------------->|  |----+
                                                 |  |
                                                 +--+
                (c)  Another LFB topology as configured by the CE and
                     accepted by the FE

        Figure 7. An example of configuring LFB topology.

   Figure 7 shows an example where a QoS-enabled router has several
   line cards that have a few ingress ports and egress ports, a
   specialized classification chip, a network processor containing
   codes for FE blocks like meter, marker, dropper, counter, queue,
   scheduler and Ipv4 forwarder.  Some of the LFB topology is already
   fixed and has to remain static due to the physical layout of the
   line cards.  For example, all of the ingress ports might be hard-
   wired into the classification chip so all packets must flow from the
   ingress port into the classification engine.  On the other hand, the
   LFBs on the network processor and their execution order are
   programmable. However, certain capacity limits and linkage
   constraints could exist between these LFBs. Examples of the capacity
   limits might be: 8 meters; 16 queues in one FE; the scheduler can
   handle at most up to 16 queues; etc.  The linkage constraints might
   dictate that the classification engine may be followed by a meter,

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   marker, dropper, counter, queue or IPv4 forwarder, but not a
   scheduler; queues can only be followed by a scheduler; a scheduler
   must be followed by the IPv4 forwarder; the last LFB in the datapath
   before going into the egress ports must be the IPv4 forwarder, etc.

   Once the FE reports these capabilities and capacity limits to the
   CE, it is now up to the CE to translate the QoS policy into a
   desirable configuration for the FE.  Figure 7(a) depicts the FE
   capability while 7(b) and 7(c) depict two different topologies that
   the CE may request the FE to configure.  Note that both the ingress
   and egress are omitted in (b) and (c) to simplify the
   representation.  The topology in 7(c) is considerably more complex
   than 7(b) but both are feasible within the FE capabilities, and so
   the FE should accept either configuration request from the CE.

4.
  Model and Schema for LFB Classes

   The main goal of the FE model is to provide an abstract, generic,
   modular, implementation-independent representation of the FEs.  This
   is facilitated using the concept of LFBs, which are instantiated
   from LFB classes.  LFB classes and associated definitions will be
   provided in a collection of XML documents. The collection of these
   XML documents is called a LFB class library, and each document is
   called an LFB class library document (or library document, for
   short).  Each of the library documents will conform to the schema
   presented in this section.  The root element of the library document
   is the <LFBLibrary> element.

   It is not expected that library documents will be exchanged between
   FEs and CEs "over-the-wire".  But the model will serve as an
   important reference for the design and development of the CEs
   (software) and FEs (mostly the software part).  It will also serve
   as a design input when specifying the ForCES protocol elements for
   CE-FE communication.

4.1. Namespace

   The LFBLibrary element and all of its sub-elements are defined in
   the following namespace:

      http://ietf.org/forces/1.0/lfbmodel

4.2. <LFBLibrary> Element

   The <LFBLibrary> element serves as a root element of all library
   documents. It contains one or more of the following main blocks:


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     . <frameTypeDefs> for the frame declarations;
     . <dataTypeDefs> for defining common data types;
     . <metadataDefs> for defining metadata, and
     . <LFBClassDefs> for defining LFB classes.

   Each block is optional, that is, one library document may contain
   only metadata definitions, another may contain only LFB class
   definitions, yet another may contain all of the above.

   In addition to the above main blocks, a library document can import
   other library documents if it needs to refer to definitions
   contained in the included document.  This concept is similar to the
   "#include" directive in C.  Importing is expressed by the <load>
   elements, which must precede all the above elements in the document.
   For unique referencing, each LFBLibrary instance document has a
   unique label defined in the "provide" attribute of the LFBLibrary
   element.

   The <LFBLibrary> element also includes an optional <description>
   element, which can be used to provide textual description about the
   library document.

   The following is a skeleton of a library document:

   <?xml version="1.0" encoding="UTF-8"?>
   <LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
     provides="this_library">

     <description>
       ...
     </description>

     <!-- Loading external libraries (optional) -->
     <load library="another_library"/>
     ...

     <!-- FRAME TYPE DEFINITIONS (optional) -->
     <frameTypeDefs>
       ...
     </frameTypeDefs>

     <!-- DATA TYPE DEFINITIONS (optional) -->
     <dataTypeDefs>
       ...

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     </dataTypeDefs>

     <!-- METADATA DEFINITIONS (optional) -->
     <metadataDefs>
       ...
     </metadataDefs>

     <!ùLFB CLASS DEFINITIONS (optional) -->
     <LFBCLassDefs>
       ...
     </LFBCLassDefs>
   </LFBLibrary>


4.3. <load> Element

   This element is used to refer to another LFB library document.
   Similar to the "#include" directive in C, this makes the objects
   (metadata types, data types, etc.) defined in the referred library
   document available for referencing in the current document.

   The load element must contain the label of the library document to
   be included and may contain a URL to specify where the library can
   be retrieved.  The load element can be repeated unlimited times.
   Three examples for the <load> elements:

   <load library="a_library"/>
   <load library="another_library" location="another_lib.xml"/>
   <load library="yetanother_library"
   location="http://www.petrimeat.com/forces/1.0/lfbmodel/lpm.xml"/>

4.4. <frameDefs> Element for Frame Type Declarations

   Frame names are used in the LFB definition to define the types of
   frames the LFB expects at its input port(s) and emits at its output
   port(s).  The <frameDefs> optional element in the library document
   contains one or more <frameDef> elements, each declaring one frame
   type.

   Each frame definition contains a unique name (NMTOKEN) and a brief
   synopsis.  In addition, an optional detailed description may be
   provided.





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   Uniqueness of frame types must be ensured among frame types defined
   in the same library document and in all directly or indirectly
   included library documents.

   The following example defines two frame types:

   <frameDefs>
     <frameDef>
       <name>ipv4</name>
       <synopsis>IPv4 packet</synopsis>
       <description>
         This frame type refers to an IPv4 packet.
       </description>
     </frameDef>
       <frameDef>
       <name>ipv6</name>
       <synopsis>IPv6 packet</synopsis>
       <description>
         This frame type refers to an IPv6 packet.
       </description>
     </frameDef>
     ...
   </frameDefs>

4.5. <dataTypeDefs> Element for Data Type Definitions

   The (optional) <dataTypeDefs> element can be used to define commonly
   used data types. It contains one or more <dataTypeDef> elements,
   each defining a data type with a unique name. Such data types can be
   used in several places in the library documents, including:

      .  Defining other data types
      .  Defining metadata
      .  Defining attributes of LFB classes

   This is similar to the concept of having a common header file for
   shared data types.

   Each <dataTypeDef> element contains a unique name (NMTOKEN), a brief
   synopsis, an optional longer description, and a type definition
   element.  The name must be unique among all data types defined in
   the same library document and in any directly or indirectly included
   library documents. For example:


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   <dataTypeDefs>
     <dataTypeDef>
       <name>ieeemacaddr</name>
       <synopsis>48-bit IEEE MAC address</synopsis>
       ... type definition ...
     </dataTypeDef>
     <dataTypeDef>
       <name>ipv4addr</name>
       <synopsis>IPv4 address</synopsis>
       ... type definition ...
     </dataTypeDef>
     ...
   </dataTypeDefs>

   There are two kinds of data types: atomic and compound.  Atomic data
   types are appropriate for single-value variables (e.g. integer,
   ASCII string, byte array).

   The following built-in atomic data types are provided, but
   additional atomic data types can be defined with the <typeRef> and
   <atomic> elements:

      <name>                   Meaning
      ----                     -------
      char                     8-bit signed integer
      uchar                    8-bit unsigned integer
      int16                    16-bit signed integer
      uint16                   16-bit unsigned integer
      int32                    32-bit signed integer
      uint32                   32-bit unsigned integer
      int64                    64-bit signed integer
      uint64                   64-bit unisgned integer
      boolean                  A true / false value where
                               0 = false, 1 = true
      string[N]                ASCII null-terminated string with
                               buffer of N characters (string max
                               length is N-1)
      byte[N]                  A byte array of N bytes
      octetstring[N]           A buffer of N octets, which may
                               contain fewer than N octets.  Hence
                               the encoded value will always have
                               a length.
      float16                  16-bit floating point number

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      float32                  32-bit IEEE floating point number
      float64                  64-bit IEEE floating point number

   These built-in data types can be readily used to define metadata or
   LFB attributes, but can also be used as building blocks when
   defining new data types.  The boolean data type is defined here
   because it is so common, even though it can be built by sub-ranging
   the uchar data type.

   Compound data types can build on atomic data types and other
   compound data types.  Compound data types can be defined in one of
   four ways.  They may be defined as an array of elements of some
   compound or atomic data type.  They may be a structure of named
   elements of compound or atomic data types (ala C structures).  They
   may be a union of named elements of compound or atomic data types
   (ala C unions).  They may also be defined as augmentations
   (explained below in 4.5.6) of existing compound data types.

   Given that the FORCES protocol will be getting and setting attribute
   values, all atomic data types used here must be able to be conveyed
   in the FORCES protocol.  Further, the FORCES protocol will need a
   mechanism to convey compound data types.  However, the details of
   such representations are for the protocol document to define, not
   the model document.

   For the definition of the actual type in the <dataTypeDef> element,
   the following elements are available: <typeRef>, <atomic>, <array>,
   <struct>, and <union>.

   The predefined type alias is somewhere between the atomic and
   compound data types.  It behaves like a structure, one element of
   which has special behavior.  Given that the special behavior is tied
   to the other parts of the structure, the compound result is treated
   as a predefined construct.

   [EDITOR: How to support augmentation is for further study.]

4.5.1. <typeRef> Element for Aliasing Existing Data Types

   The <typeRef> element refers to an existing data type by its name.
   The referred data type must be defined either in the same library
   document, or in one of the included library documents.  If the
   referred data type is an atomic data type, the newly defined type
   will also be regarded as atomic.  If the referred data type is a
   compound type, the new type will also be compound.  Some usage
   examples follow:

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   <dataTypeDef>
     <name>short</name>
     <synopsis>Alias to int16</synopsis>
     <typeRef>int16</typeRef>
   </dataTypeDef>
   <dataTypeDef>
     <name>ieeemacaddr</name>
     <synopsis>48-bit IEEE MAC address</synopsis>
     <typeRef>byte[6]</typeRef>
   </dataTypeDef>

4.5.2. <atomic> Element for Deriving New Atomic Types

   The <atomic> element allows the definition of a new atomic type from
   an existing atomic type, applying range restrictions and/or
   providing special enumerated values.  Note that the <atomic> element
   can only use atomic types as base types, and its result is always
   another atomic type.

   For example, the following snippet defines a new "dscp" data type:

   <dataTypeDef>
     <name>dscp</name>
     <synopsis>Diffserv code point.</synopsis>
     <atomic>
       <baseType>uchar</baseType>
       <rangeRestriction>
         <allowedRange min="0" max="63"/>
       </rangeRestriction>
       <specialValues>
         <specialValue value="0">
           <name>DSCP-BE</name>
           <synopsis>Best Effort</synopsis>
         </specialValue>
         ...
       </specialValues>
     </atomic>
   </dataTypeDef>

4.5.3. <array> Element to Define Arrays

   The <array> element can be used to create a new compound data type
   as an array of a compound or an atomic data type. The type of the
   array entry can be specified either by referring to an existing type
   (using the <typeRef> element) or defining an unnamed type inside the



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   <array> element using any of the <atomic>, <array>, <struct>, or
   <union> elements.

   The array can be "fixed-size" or "variable-size", which is specified
   by the "type" attribute of the <array> element. The default is
   "variable-size".  For variable size arrays, an optional "max-length"
   attribute specifies the maximum allowed length. This attribute
   should be used to encode semantic limitations, not implementation
   limitations. The latter should be handled by capability attributes
   of LFB classes, and should never be included in data type
   definitions. If the "max-length" attribute is not provided, the
   array is regarded as of unlimited-size.

   For fixed-size arrays, a "length" attribute must be provided that
   specifies the constant size of the array.

   The result of this construct is always a compound type, even if the
   array has a fixed size of 1.

   Arrays can only be subscripted by integers, and will be presumed to
   start with index 0.

   In addition to their subscripts, arrays may be declared to have
   content keys.  Such a declaration has several effects:

     . Any declared key can be used in the ForCES protocol to select
        an element for operations (for details, see the protocol).

     . In any instance of the array, each declared key must be unique
        within that instance.  No two elements of an array may have the
        same values on all the fields which make up a key.

   Each key is declared with a keyID for use in the protocol, where the
   unique key is formed by combining one or more specified key fields.
   To support the case where an array of an atomic type with unique
   values can be referenced by those values, the key field identifier
   may be "*" (i.e., the array entry is the key).  If the value type of
   the array is a structure or an array, then the key is one or more
   fields, each identified by name.  Since the field may be an element
   of the structure, the element of an element of a structure, or
   further nested, the field name is actually a concatenated sequence
   of part identifiers, separated by decimal points (ô.ö).  The syntax
   for key field identification is given following the array examples.

   The following example shows the definition of a fixed size array
   with a pre-defined data type as the array elements:


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   <dataTypeDef>
     <name>dscp-mapping-table</name>
     <synopsis>
       A table of 64 DSCP values, used to re-map code space.
     </synopsis>
     <array type="fixed-size" length="64">
         <typeRef>dscp</typeRef>
     </array>
   </dataTypeDef>

   The following example defines a variable size array with an upper
   limit on its size:

   <dataTypeDef>
     <name>mac-alias-table</name>
     <synopsis>A table with up to 8 IEEE MAC addresses</synopsis>
     <array type="variable-size" max-length="8">
         <typeRef>ieeemacaddr</typeRef>
     </array>
   </dataTypeDef>

   The following example shows the definition of an array with a local
   (unnamed) type definition:

   <dataTypeDef>
     <name>classification-table</name>
     <synopsis>
       A table of classification rules and result opcodes.
     </synopsis>
     <array type="variable-size">
       <struct>
         <element elementID=ö1ö>
           <name>rule</name>
           <synopsis>The rule to match</synopsis>
           <typeRef>classrule</typeRef>
         </element>
         <element ôelementID=ö2ö>
           <name>opcode</name>
           <synopsis>The result code</synopsis>
           <typeRef>opcode</typeRef>
         </element>
       </struct>
     </array>
   </dataTypeDef>

   In the above example, each entry of the array is a <struct> of two
   fields ("rule" and "opcode").

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   The following example shows a table of IP Prefix information that
   can be accessed by a multi-field content key on the IP Address and
   prefix length.  This means that in any instance of this table, no
   two entries can have the same IP address and prefix length.

   <dataTypeDef>
     <name>ipPrefixInfo_table</name>
     <synopsis>
       A table of information about known prefixes
     </synopsis>
     <array type=övariable-sizeö>
       <struct>
         <element elementID=ö1ö>
           <name>address-prefix</name>
           <synopsis>the prefix being described</synopsis>
           <typeRef>ipv4Prefix</typeRef>
         </element>
         <element elementID=ö2ö>
           <name>source</name>
           <synopsis>where is this from</synopsis>
           <typeRef>uint16</typeRef>
         </element>
         <element elementID=ö3ö>
           <name>prefInfo</name>
           <synopsis>the information we care about</synopsis>
           <typeRef>hypothetical-info-type</typeRef>
         </element>
       </struct>
       <key keyID=ö1ö>
         <keyField> address-prefix.ipv4addr </keyField>
         <keyField> address-prefix.prefixlen </keyField>
         <keyField> source </keyField>
       </key>
     </array>
   </dataTypeDef>

   Note that the keyField elements could also have been simply address-
   prefix and source, since all of the fields of address-prefix are
   being used.

4.5.3.1 Key Field References
   In order to use key declarations, one must refer to fields that are
   potentially nested inside other fields in the array.  If there are
   nested arrays, one might even use an array element as a key (but
   great care would be needed to ensure uniqueness.)


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   The key is the combination of the values of each field declared in a
   keyField element.

   Therefore, the value of a keyField element is defined as a
   concatenated Sequence of field identifiers, separated by a "."
   (period) character.  Whitespace is permitted and ignored.

   A valid string for a single field identifier within a keyField
   depends upon the current context.  Initially, in an array key
   declaration, the context is the type of the array.  Progressively,
   the context is whatever type is selected by the field identifiers
   processed so far in the current key field declaration.

   When the current context is an array, (e.g., when declaring a key
   for an array whose content is an array) then the only valid value
   for the field identifier is an explicit number.

   When the current context is a structure, the valid values for the
   field identifiers are the names of the elements of the structure.
   In the special case of declaring a key for an array containing an
   atomic type, where that content is unique and is to be used as a
   key, the value "*" can be used as the single key field identifier.

4.5.4. <struct> Element to Define Structures

   A structure is comprised of a collection of data elements.  Each
   data element has a data type (either an atomic type or an existing
   compound type) and is assigned a name unique within the scope of the
   compound data type being defined.  These serve the same function as
   "struct" in C, etc.

   The actual type of the field can be defined by referring to an
   existing type (using the <typeDef> element), or can be a locally
   defined (unnamed) type created by any of the <atomic>, <array>,
   <struct>, or <union> elements.

   A structure definition is a series of element declarations.  Each
   element carries an elementID for use by the ForCES protocol. In
   addition, the element contains the name, a synopsis, an optional
   description, an optional declaration that the element itself is
   optional, and the typeRef declaration that specifies the element
   type.

   For a dataTypeDefinition of a struct, the structure definition may
   be inherited from, and augment, a previously defined structured type.
   This is indicated by including the derivedFrom attribute on the
   struct declaration.

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   The result of this construct is always regarded a compound type,
   even when the <struct> contains only one field.

   An example:

   <dataTypeDef>
     <name>ipv4prefix</name>
     <synopsis>
       IPv4 prefix defined by an address and a prefix length
     </synopsis>
     <struct>
       <element elementID=ö1ö>
         <name>address</name>
         <synopsis>Address part</synopsis>
         <typeRef>ipv4addr</typeRef>
       </element>
       <element elementID=ö2ö>
         <name>prefixlen</name>
         <synopsis>Prefix length part</synopsis>
         <atomic>
           <baseType>uchar</baseType>
           <rangeRestriction>
             <allowedRange min="0" max="32"/>
           </rangeRestriction>
         </atomic>
       </element>
     </struct>
   </dataTypeDef>

4.5.5. <union> Element to Define Union Types

   Similar to the union declaration in C, this construct allows the
   definition of overlay types.  Its format is identical to the
   <struct> element.

   The result of this construct is always regarded a compound type,
   even when the union contains only one element.

4.5.6 <alias> Element

   It is sometimes necessary to have an element in an LFB or structure
   refer to information in other LFBs.  The <alias> declaration creates
   the constructs for this.  An <alias> element creates a structure
   with fields to control the alias and a field to access the aliased
   information.


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   An <alias> declaration is used wherever a type reference can occur,
   just like a <struct> or <array>.  A typical alias usage would look
   like:

   <dataTypeDef>
     <name>anAliasedVariable</name>
     <synopsis>a reference to something, somewhere</synopsis>
     <alias>the-underlying-type</alias>
   </dataTypeDef>

   This definition declares a type to be used for an alias to things of
   type ôthe-underlying-typeö.  This would more likely occur inside a
   structure rather than directly in a dataTypeDef.

   An alias is a complex structure in order to hold the pieces
   necessary to make it useful.  If "alias" was an actual declared
   structure, and if "tiedReference" was a usable type, the structure
   declaration would look like:

   <dataTypeDef>
     <name>alias</name>
     <synopsis>a reference to information in another LFB</synopsis>
     <struct>
       <element elementID=ö1ö>
         <name>referenceClass</name>
         <synopsis>
           The class of LFB this alias is currently pointing to
         </synopsis>
         <typeRef>uint32</typeRef>
       </element>
       <element elementID=ö2ö>
         <name>referenceInstance</name>
         <synopsis>
           The LFB Instance this alias is currently pointing to
         </synopsis>
         <typeRef>uint32</typeRef>
       </element>
       <element elementID=ö3ö>
         <name>referencePath</name>
         <synopsis>
           The path to the LFB field this alias is currently
           pointing to
         </synopsis>
         <typeRef)octetString[128]</typeRef>
       </element>
       <element elementID=ö4ö>
         <name>referenceClass</name>

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         <synopsis>
           The element to use to follow the alias
         </synopsis>
         <typeRef>tiedReference</typeRef>
       </element>
     </struct>
   </dataTypeDef>

   The referencePath is the identifier in the form used by the ForCES
   protocol for the actual field being referenced.  Its type must be
   the same as the type declared for the alias. (And the FE MUST ensure
   the types are the same on any effort to set the referenceClass,
   referenceInstance, or referencePath elements).  Note that "128" is
   the length limit for paths in the ForCES protocol.)  Thus, by
   setting the three control fields, the CE controls which LFB and the
   field in that LFB, the alias data points to.  Typically, this will
   be information needed by the LFB containing the element with an
   <alias> declaration.

4.5.6. Augmentations

   Compound types can also be defined as augmentations of existing
   compound types.  If the existing compound type is a structure,
   augmentation may add new elements to the type.  The type of an
   existing element can only be replaced with an augmentation derived
   from the current type, an existing element cannot be deleted.  If
   the existing compound type is an array, augmentation means
   augmentation of the array element type.

   One consequence of this is that augmentations are compatible with
   the compound type from which they are derived.  As such,
   augmentations are useful in defining attributes for LFB subclasses
   with backward compatibility.  In addition to adding new attributes
   to a class, the data type of an existing attribute may be replaced
   by an augmentation of that attribute, and still meet the
   compatibility rules for subclasses.

   For example, consider a simple base LFB class A that has only one
   attribute (attr1) of type X.  One way to derive class A1 from A can
   be by simply adding a second attribute (of any type).  Another way
   to derive a class A2 from A can be by replacing the original
   attribute (attr1) in A of type X with one of type Y, where Y is an
   augmentation of X.  Both classes A1 and A2 are backward compatible
   with class A.

   The syntax for augmentations is to include a derivedFrom attribute
   in a structure definition, indicating what structure type is being

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   augmented.  Element names and element IDs within the augmentation
   must not be the same as those in the structure type being augmented.

   [EDITOR: This is a preliminary proposal for handling augmentation of
   structures.]

4.6. <metadataDefs> Element for Metadata Definitions

   The (optional) <metadataDefs> element in the library document
   contains one or more <metadataDef> elements.  Each <metadataDef>
   element defines a metadata.

   Each <metadataDef> element contains a unique name (NMTOKEN).
   Uniqueness is defined to be over all metadata defined in this
   library document and in all directly or indirectly included library
   documents. The <metadataDef> element also contains a brief
   synopsis, an optional detailed description, and a compulsory type
   definition information. Only atomic data types can be used as value
   types for metadata.

   Two forms of type definitions are allowed. The first form uses the
   <typeRef> element to refer to an existing atomic data type defined
   in the <dataTypeDefs> element of the same library document or in one
   of the included library documents. The usage of the <typeRef>
   element is identical to how it is used in the <dataTypeDef>
   elements, except here it can only refer to atomic types.

   [EDITOR: The latter restriction is not yet enforced by the XML
   schema.]

   The second form is an explicit type definition using the <atomic>
   element. This element is used here in the same way as in the
   <dataTypeDef> elements.

   The following example shows both usages:

   <metadataDefs>
     <metadataDef>
       <name>NEXTHOPID</name>
       <synopsis>Refers to a Next Hop entry in NH LFB</synopsis>
       <typeRef>int32</typeRef>
     </metadataDef>
     <metadataDef>
       <name>CLASSID</name>
       <synopsis>
         Result of classification (0 means no match).
       </synopsis>

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       <atomic>
         <baseType>int32</baseType>
         <specialValues>
           <specialValue value="0">
             <name>NOMATCH</name>
             <synopsis>
               Classification didnÆt result in match.
             </synopsis>
           </specialValue>
         </specialValues>
       </atomic>
     </metadataDef>
   </metadataDefs>

4.7. <LFBClassDefs> Element for LFB Class Definitions

   The (optional) <LFBClassDefs> element can be used to define one or
   more LFB classes using <LFBClassDef> elements.  Each <LFBClassDef>
   element defines an LFB class and includes the following elements:
     . <name> provides the symbolic name of the LFB class.  Example:
       "ipv4lpm"
     . <synopsis> provides a short synopsis of the LFB class. Example:
       "IPv4 Longest Prefix Match Lookup LFB"
     . <version> is the version indicator
     . <derivedFrom> is the inheritance indicator
     . <inputPorts> lists the input ports and their specifications
     . <outputPorts> lists the output ports and their specifications
     . <attributes> defines the operational attributes of the LFB
     . <capabilities> defines the capability attributes of the LFB
     . <description> contains the operational specification of the LFB
     . The LFBClassID attribute of the LFBClassDef element defines the
       ID for this class.  These must be globally unique.

   [EDITOR: LFB class names should be unique not only among classes
   defined in this document and in all included documents, but also
   unique across a large collection of libraries.  Obviously some global
   control is needed to ensure such uniqueness.  This subject requires
   further study.  The uniqueness of the class IDs also requires further
   study.]

   Here is a skeleton of an example LFB class definition:

   <LFBClassDefs>
     <LFBClassDef LFBClassID=ö12345ö>
       <name>ipv4lpm</name>
       <synopsis>IPv4 Longest Prefix Match Lookup LFB</synopsis>
       <version>1.0</version>

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       <derivedFrom>baseclass</derivedFrom>

       <inputPorts>
         ...
       </inputPorts>

       <outputPorts>
         ...
       </outputPorts>

       <attributes>
         ...
       </attributes>

       <capabilities>
         ...
       </capabilities>

       <description>
         This LFB represents the IPv4 longest prefix match lookup
         operation.
         The modeled behavior is as follows:
            Blah-blah-blah.
       </description>

     </LFBClassDef>
     ...
   </LFBClassDefs>

   The individual attributes and capabilities will have elementIDs for
   use by the ForCES protocol.  These parallel the elementIDs used in
   structs, and are used the same way.  Attribute and capability
   elementIDs must be unique within the LFB class definition.

   Note that the <name>, <synopsis>, and <version> elements are
   required, all other elements are optional in <LFBClassDef>. However,
   when they are present, they must occur in the above order.

4.7.1. <derivedFrom> Element to Express LFB Inheritance

   The optional <derivedFrom> element can be used to indicate that this
   class is a derivative of some other class.  The content of this
   element must be the unique name (<name>) of another LFB class.  The
   referred LFB class must be defined in the same library document or
   in one of the included library documents.




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   [EDITOR: The <derivedFrom> element will likely need to specify the
   version of the ancestor, which is not included in the schema yet.
   The process and rules of class derivation are still being studied.]

   It is assumed that the derived class is backwards compatible with
   the base class.

4.7.2. <inputPorts> Element to Define LFB Inputs

   The optional <inputPorts> element is used to define input ports.  An
   LFB class may have zero, one, or more inputs.  If the LFB class has
   no input ports, the <inputPorts> element must be omitted.  The
   <inputPorts> element can contain one or more <inputPort> elements,
   one for each port or port-group.  We assume that most LFBs will have
   exactly one input.  Multiple inputs with the same input type are
   modeled as one input group.  Input groups are defined the same way
   as input ports by the <inputPort> element, differentiated only by an
   optional "group" attribute.

   Multiple inputs with different input types should be avoided if
   possible (see discussion in Section 3.2.1).  Some special LFBs will
   have no inputs at all.  For example, a packet generator LFB does not
   need an input.

   Single input ports and input port groups are both defined by the
   <inputPort> element, they are differentiated by only an optional
   "group" attribute.

   The <inputPort> element contains the following elements:
   . <name> provides the symbolic name of the input.  Example: "in".
     Note that this symbolic name must be unique only within the scope
     of the LFB class.
   . <synopsis> contains a brief description of the input.  Example:
     "Normal packet input".
   . <expectation> lists all allowed frame formats.  Example: {"ipv4"
     and "ipv6"}.  Note that this list should refer to names specified
     in the <frameDefs> element of the same library document or in any
     included library documents.  The <expectation> element can also
     provide a list of required metadata.  Example: {"classid",
     "vifid"}.  This list should refer to names of metadata defined in
     the <metadataDefs> element in the same library document or in any
     included library documents.  For each metadata, it must be
     specified whether the metadata is required or optional.  For each
     optional metadata, a default value must be specified, which is
     used by the LFB if the metadata is not provided with a packet.




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   In addition, the optional "group" attribute of the <inputPort>
   element can specify if the port can behave as a port group, i.e., it
   is allowed to be instantiated.  This is indicated by a "yes" value
   (the default value is "no").

   An example <inputPorts> element, defining two input ports, the
   second one being an input port group:

   <inputPorts>
     <inputPort>
       <name>in</name>
       <synopsis>Normal input</synopsis>
       <expectation>
         <frameExpected>
           <ref>ipv4</ref>
           <ref>ipv6</ref>
         </frameExpected>
         <metadataExpected>
           <ref>classid</ref>
           <ref>vifid</ref>
           <ref dependency="optional" defaultValue="0">vrfid</ref>
         </metadataExpected>
       </expectation>
     </inputPort>
     <inputPort group="yes">
       ... another input port ...
     </inputPort>
   </inputPorts>

   For each <inputPort>, the frame type expectations are defined by the
   <frameExpected> element using one or more <ref> elements (see
   example above).  When multiple frame types are listed, it means that
   "one of these" frame types are expected.  A packet of any other
   frame type is regarded as incompatible with this input port of the
   LFB class.  The above example list two frames as expected frame
   types: "ipv4" and "ipv6".

   Metadata expectations are specified by the <metadataExpected>
   element.  In its simplest form, this element can contain a list of
   <ref> elements, each referring to a metadata.  When multiple
   instances of metadata are listed by <ref> elements, it means that
   "all of these" metadata must be received with each packet (except
   metadata that are marked as "optional" by the "dependency" attribute
   of the corresponding <ref> element).  For a metadata that is
   specified "optional", a default value must be provided using the
   "defaultValue" attribute.  The above example lists three metadata as



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   expected metadata, two of which are mandatory ("classid" and
   "vifid"), and one being optional ("vrfid").

   [EDITOR: How to express default values for byte[N] atomic types is
   yet to be defined.]

   The schema also allows for more complex definitions of metadata
   expectations.  For example, using the <one-of> element, a list of
   metadata can be specified to express that at least one of the
   specified metadata must be present with any packet. For example:

   <metadataExpected>
     <one-of>
       <ref>prefixmask</ref>
       <ref>prefixlen</ref>
     </one-of>
   </metadataExpected>

   The above example specifies that either the "prefixmask" or the
   "prefixlen" metadata must be provided with any packet.

   The two forms can also be combined, as it is shown in the following
   example:

   <metadataExpected>
     <ref>classid</ref>
     <ref>vifid</ref>
     <ref dependency="optional" defaultValue="0">vrfid</ref>
     <one-of>
       <ref>prefixmask</ref>
       <ref>prefixlen</ref>
     </one-of>
   </metadataExpected>

   Although the schema is constructed to allow even more complex
   definitions of metadata expectations, we do not discuss those here.

4.7.3. <outputPorts> Element to Define LFB Outputs

   The optional <outputPorts> element is used to define output ports.
   An LFB class may have zero, one, or more outputs.  If the LFB class
   has no output ports, the <outputPorts> element must be omitted.  The
   <outputPorts> element can contain one or more <outputPort> elements,
   one for each port or port-group.  If there are multiple outputs with
   the same output type, we model them as an output port group.  Some
   special LFBs may have no outputs at all (e.g., Dropper).


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   Single output ports and output port groups are both defined by the
   <outputPort> element; they are differentiated by only an optional
   "group" attribute.

   The <outputPort> element contains the following elements:
   . <name> provides the symbolic name of the output.  Example: "out".
     Note that the symbolic name must be unique only within the scope
     of the LFB class.
   . <synopsis> contains a brief description of the output port.
     Example: "Normal packet output".
   . <product> lists the allowed frame formats.  Example: {"ipv4",
     "ipv6"}.  Note that this list should refer to symbols specified in
     the <frameDefs> element in the same library document or in any
     included library documents.  The <product> element may also
     contain the list of emitted (generated) metadata.  Example:
     {"classid", "color"}.  This list should refer to names of metadata
     specified in the <metadataDefs> element in the same library
     document or in any included library documents.  For each generated
     metadata, it should be specified whether the metadata is always
     generated or generated only in certain conditions. This
     information is important when assessing compatibility between
     LFBs.

   In addition, the optional "group" attribute of the <outputPort>
   element can specify if the port can behave as a port group, i.e., it
   is allowed to be instantiated. This is indicated by a "yes" value
   (the default value is "no").

   The following example specifies two output ports, the second being
   an output port group:

   <outputPorts>
     <outputPort>
       <name>out</name>
       <synopsis>Normal output</synopsis>
       <product>
         <frameProduced>
           <ref>ipv4</ref>
           <ref>ipv4bis</ref>
         </frameProduced>
         <metadataProduced>
           <ref>nhid</ref>
           <ref>nhtabid</ref>
         </metadataProduced>
       </product>
     </outputPort>
     <outputPort group="yes">

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       <name>exc</name>
       <synopsis>Exception output port group</synopsis>
       <product>
         <frameProduced>
           <ref>ipv4</ref>
           <ref>ipv4bis</ref>
         </frameProduced>
         <metadataProduced>
           <ref availability="conditional">errorid</ref>
         </metadataProduced>
       </product>
     </outputPort>
   </outputPorts>

   The types of frames and metadata the port produces are defined
   inside the <product> element in each <outputPort>.  Within the
   <product> element, the list of frame types the port produces is
   listed in the <frameProduced> element.  When more than one frame is
   listed, it means that "one of" these frames will be produced.

   The list of metadata that is produced with each packet is listed in
   the optional <metadataProduced> element of the <product>.  In its
   simplest form, this element can contain a list of <ref> elements,
   each referring to a metadata type.  The meaning of such a list is
   that "all of" these metadata are provided with each packet, except
   those that are listed with the optional "availability" attribute set
   to "conditional".  Similar to the <metadataExpected> element of the
   <inputPort>, the <metadataProduced> element supports more complex
   forms, which we do not discuss here further.

4.7.4. <attributes> Element to Define LFB Operational Attributes

   Operational parameters of the LFBs that must be visible to the CEs
   are conceptualized in the model as the LFB attributes.  These
   include, for example, flags, single parameter arguments, complex
   arguments, and tables.  Note that the attributes here refer to only
   those operational parameters of the LFBs that must be visible to the
   CEs.  Other variables that are internal to LFB implementation are
   not regarded as LFB attributes and hence are not covered.

   Some examples for LFB attributes are:
     . Configurable flags and switches selecting between operational
        modes of the LFB
     . Number of inputs or outputs in a port group
     . Metadata CONSUME vs. PROPAGATE mode selectors




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     . Various configurable lookup tables, including interface tables,
        prefix tables, classification tables, DSCP mapping tables, MAC
        address tables, etc.
     . Packet and byte counters
     . Various event counters
     . Number of current inputs or outputs for each input or output
        group
     . Metadata CONSUME/PROPAGATE mode selector

   There may be various access permission restrictions on what the CE
   can do with an LFB attribute.  The following categories may be
   supported:
     . No-access attributes.  This is useful when multiple access
        modes may be defined for a given attribute to allow some
        flexibility for different implementations.
     . Read-only attributes.
     . Read-write attributes.
     . Write-only attributes.  This could be any configurable data for
        which read capability is not provided to the CEs.  (e.g., the
        security key information)
     . Read-reset attributes.  The CE can read and reset this
        resource, but cannot set it to an arbitrary value.  Example:
        Counters.
     . Firing-only attributes.  A write attempt to this resource will
        trigger some specific actions in the LFB, but the actual value
        written is ignored.

   The LFB class may define more than one possible access mode for a
   given attribute (for example, "write-only" and "read-write"), in
   which case it is left to the actual implementation to pick one of
   the modes.  In such cases, a corresponding capability attribute must
   inform the CE about the access mode the actual LFB instance supports
   (see next subsection on capability attributes).

   The attributes of the LFB class are listed in the <attributes>
   element.  Each attribute is defined by an <attribute> element.  An
   <attribute> element contains the following elements:
     . <name> defines the name of the attribute.  This name must be
        unique among the attributes of the LFB class.  Example:
        "version".
     . <synopsis> should provide a brief description of the purpose of
        the attribute.
     . <optional/> indicates that this attribute is optional.
     . The data type of the attribute can be defined either via a
        reference to a predefined data type or providing a local
        definition of the type.  The former is provided by using the
        <typeRef> element, which must refer to the unique name of an

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        existing data type defined in the <dataTypeDefs> element in the
        same library document or in any of the included library
        documents.  When the data type is defined locally (unnamed
        type), one of the following elements can be used: <atomic>,
        <array>, <struct>, and <union>. Their usage is identical to how
        they are used inside <dataTypeDef> elements (see Section 4.5).
     . The optional <defaultValue> element can specify a default value
        for the attribute, which is applied when the LFB is initialized
        or reset.  [EDITOR: A convention to define default values for
        compound data types and byte[N] atomic types is yet to be
        defined.]

   The attribute element also has a mandatory elementID attribute,
   which is a numeric value used by the ForCES protocol.

   In addition to the above elements, the <attribute> element includes
   an optional "access" attribute, which can take any of the following
   values or even a list of these values: "read-only", "read-write",
   "write-only", "read-reset", and "trigger-only". The default access
   mode is "read-write".

   The following example defines two attributes for an LFB:

   <attributes>
     <attribute access="read-only" elementID=ö1ö>
       <name>foo</name>
       <synopsis>number of things</synopsis>
       <typeRef>uint32</typeRef>
     </attribute>
     <attribute access="read-write write-only" elementID=ö2ö>
       <name>bar</name>
       <synopsis>number of this other thing</synopsis>
       <atomic>
         <baseType>uint32</baseType>
         <rangeRestriction>
           <allowedRange min="10" max="2000"/>
         </rangeRestriction>
       </atomic>
       <defaultValue>10</defaultValue>
     </attribute>
   </attributes>

   The first attribute ("foo") is a read-only 32-bit unsigned integer,
   defined by referring to the built-in "uint32" atomic type.  The
   second attribute ("bar") is also an integer, but uses the <atomic>
   element to provide additional range restrictions. This attribute has



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   two possible access modes, "read-write" or "write-only".  A default
   value of 10 is provided.

   Note that not all attributes are likely to exist at all times in a
   particular implementation.  While the capabilities will frequently
   indicate this non-existence, CEs may attempt to reference non-
   existent or non-permitted attributes anyway.  The FORCES protocol
   mechanisms should include appropriate error indicators for this
   case.

   The mechanism defined above for non-supported attributes can also
   apply to attempts to reference non-existent array elements or to set
   read-only elements.

4.7.5. <capabilities> Element to Define LFB Capability Attributes

   The LFB class specification will provide some flexibility for the FE
   implementation regarding how the LFB class is implemented.  For
   example, the class may define some optional features, in which case
   the actual implementation may or may not provide the given feature.
   In these cases the CE must be able to query the LFB instance about
   the availability of the feature.  In addition, the instance may have
   some limitations that are not inherent from the class definition,
   but rather the result of some implementation limitations.  For
   example, an array attribute may be defined in the class definition
   as "unlimited" size, but the physical implementation may impose a
   hard limit on the size of the array.

   Such capability related information is expressed by the capability
   attributes of the LFB class.  The capability attributes are always
   read-only attributes, and they are listed in a separate
   <capabilities> element in the <LFBClassDef>.  The <capabilities>
   element contains one or more <capability> elements, each defining
   one capability attribute.  The format of the <capability> element is
   almost the same as the <attribute> element, it differs in two
   aspects: it lacks the access mode attribute (because it is always
   read-only), and it lacks the <defaultValue> element (because default
   value is not applicable to read-only attributes).

   Some examples of capability attributes:
     . The version of the LFB class that this LFB instance complies
        with;
     . Supported optional features of the LFB class;
     . Maximum number of configurable outputs for an output group;
     . Metadata pass-through limitations of the LFB;
     . Maximum size of configurable attribute tables;
     . Additional range restriction on operational attributes;

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     . Supported access modes of certain attributes (if the access
        mode of an operational attribute is specified as a list of two
        or mode modes).

   The following example lists two capability attributes:

   <capabilities>
     <capability elementID="3">
       <name>version</name>
       <synopsis>
         LFB class version this instance is compliant with.
       </synopsis>
       <typeRef>version</typeRef>
     </capability>
     <capability elementID="4">
       <name>limitBar</name>
       <synopsis>
         Maximum value of the "bar" attribute.
       </synopsis>
       <typeRef>uint16</typeRef>
     </capability>
   </capabilities>

4.7.6. <description> Element for LFB Operational Specification

   The <description> element of the <LFBClass> provides unstructured
   text (in XML sense) to verbally describe what the LFB does.

4.8. XML Schema for LFB Class Library Documents

   <?xml version="1.0" encoding="UTF-8"?>
   <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns="http://ietf.org/forces/1.0/lfbmodel"
    xmlns:lfb="http://ietf.org/forces/1.0/lfbmodel"
    targetNamespace="http://ietf.org/forces/1.0/lfbmodel"
    attributeFormDefault="unqualified"
    elementFormDefault="qualified">
   <xsd:annotation>
     <xsd:documentation xml:lang="en">
     Schema for Defining LFB Classes and associated types (frames,
     data types for LFB attributes, and metadata).
     </xsd:documentation>
   </xsd:annotation>
   <xsd:element name="description" type="xsd:string"/>
   <xsd:element name="synopsis" type="xsd:string"/>
   <!-- Document root element: LFBLibrary -->
   <xsd:element name="LFBLibrary">

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     <xsd:complexType>
       <xsd:sequence>
         <xsd:element ref="description" minOccurs="0"/>
         <xsd:element name="load" type="loadType" minOccurs="0"
                      maxOccurs="unbounded"/>
         <xsd:element name="frameDefs" type="frameDefsType"
                      minOccurs="0"/>
         <xsd:element name="dataTypeDefs" type="dataTypeDefsType"
                      minOccurs="0"/>
         <xsd:element name="metadataDefs" type="metadataDefsType"
                      minOccurs="0"/>
         <xsd:element name="LFBClassDefs" type="LFBClassDefsType"
                      minOccurs="0"/>
       </xsd:sequence>
       <xsd:attribute name="provides" type="xsd:Name" use="required"/>
     </xsd:complexType>
     <!-- Uniqueness constraints -->
     <xsd:key name="frame">
       <xsd:selector xpath="lfb:frameDefs/lfb:frameDef"/>
       <xsd:field xpath="lfb:name"/>
     </xsd:key>
     <xsd:key name="dataType">
       <xsd:selector xpath="lfb:dataTypeDefs/lfb:dataTypeDef"/>
       <xsd:field xpath="lfb:name"/>
     </xsd:key>
     <xsd:key name="metadataDef">
       <xsd:selector xpath="lfb:metadataDefs/lfb:metadataDef"/>
       <xsd:field xpath="lfb:name"/>
     </xsd:key>
     <xsd:key name="LFBClassDef">
       <xsd:selector xpath="lfb:LFBClassDefs/lfb:LFBClassDef"/>
       <xsd:field xpath="lfb:name"/>
     </xsd:key>
   </xsd:element>
   <xsd:complexType name="loadType">
     <xsd:attribute name="library" type="xsd:Name" use="required"/>
     <xsd:attribute name="location" type="xsd:anyURI" use="optional"/>
   </xsd:complexType>
   <xsd:complexType name="frameDefsType">
     <xsd:sequence>
       <xsd:element name="frameDef" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element ref="description" minOccurs="0"/>
           </xsd:sequence>

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         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="dataTypeDefsType">
     <xsd:sequence>
       <xsd:element name="dataTypeDef" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element ref="description" minOccurs="0"/>
             <xsd:group ref="typeDeclarationGroup"/>
           </xsd:sequence>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <!--
      Predefined (built-in) atomic data-types are:
          char, uchar, int16, uint16, int32, uint32, int64, uint64,
          string[N], byte[N], boolean,
          float16, float32, float64
   -->
   <xsd:group name="typeDeclarationGroup">
     <xsd:choice>
       <xsd:element name="typeRef" type="typeRefNMTOKEN"/>
       <xsd:element name="atomic" type="atomicType"/>
       <xsd:element name="array" type="arrayType"/>
       <xsd:element name="struct" type="structType"/>
       <xsd:element name="union" type="structType"/>
       <xsd:element name=öaliasö type="typeRefNMTOKEN"/>
     </xsd:choice>
   </xsd:group>
   <xsd:simpleType name="typeRefNMTOKEN">
     <xsd:restriction base="xsd:token">
       <xsd:pattern value="\c+"/>
       <xsd:pattern value="string\[\d+\]"/>
       <xsd:pattern value="byte\[\d+\]"/>
     </xsd:restriction>
   </xsd:simpleType>
   <xsd:complexType name="atomicType">
     <xsd:sequence>
       <xsd:element name="baseType" type="typeRefNMTOKEN"/>
       <xsd:element name="rangeRestriction"
                    type="rangeRestrictionType minOccurs="0"/>
       <xsd:element name="specialValues" type="specialValuesType"

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                    minOccurs="0"/>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="rangeRestrictionType">
     <xsd:sequence>
       <xsd:element name="allowedRange" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:attribute name="min" type="xsd:integer"
   use="required"/>
           <xsd:attribute name="max" type="xsd:integer"
   use="required"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="specialValuesType">
     <xsd:sequence>
       <xsd:element name="specialValue" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
           </xsd:sequence>
           <xsd:attribute name="value" type="xsd:token"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="arrayType">
     <xsd:sequence>
       <xsd:group ref="typeDeclarationGroup"/>
       <xsd:element name="key" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="keyField" maxOccurs="unbound"
                          type="xsd:string"/>
           </xsd:sequence>
           <xsd:attribute name="keyID" use="required"
                          type="xsd:integer"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
     <xsd:attribute name="type" use="optional"
                    default="variable-size">
       <xsd:simpleType>
         <xsd:restriction base="xsd:string">
           <xsd:enumeration value="fixed-size"/>

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           <xsd:enumeration value="variable-size"/>
         </xsd:restriction>
       </xsd:simpleType>
     </xsd:attribute>
     <xsd:attribute name="length" type="xsd:integer" use="optional"/>
     <xsd:attribute name="maxLength" type="xsd:integer"
                    use="optional"/>
     <!ùdeclare keys to have unique IDs -->
     <xsd:key name="keyID">
       <xsd:selector xpath="lfb:key"/>
       <xsd:field xpath="@keyID"/>
     </xsd:key>
   </xsd:complexType>
   <xsd:complexType name="structType">
     <xsd:sequence>
       <xsd:element name="element" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element name="optional" minOccurs="0"/>
             <xsd:group ref="typeDeclarationGroup"/>
           </xsd:sequence>
           <xsd:attribute name="elementID" use="required"
                          type="xsd:integer"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>  <xsd:attribute name="derivedFrom" use="optional"
                    type="typeRefNMTOKEN"/>
     <!-- key declaration to make elementIDs unique in a struct -->
     <xsd:key name="structElementID">
       <xsd:selector xpath="lfb:element"/>
       <xsd:field xpath="@elementID"/>
     </xsd:key>
   </xsd:complexType>
   <xsd:complexType name="metadataDefsType">
     <xsd:sequence>
       <xsd:element name="metadataDef" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element ref="description" minOccurs="0"/>
             <xsd:choice>
               <xsd:element name="typeRef" type="typeRefNMTOKEN"/>
               <xsd:element name="atomic" type="atomicType"/>
             </xsd:choice>

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           </xsd:sequence>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="LFBClassDefsType">
     <xsd:sequence>
       <xsd:element name="LFBClassDef" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element name="version" type="versionType"/>
             <xsd:element name="derivedFrom" type="xsd:NMTOKEN"
                          minOccurs="0"/>
             <xsd:element name="inputPorts" type="inputPortsType"
                          minOccurs="0"/>
             <xsd:element name="outputPorts" type="outputPortsType"
                          minOccurs="0"/>
             <xsd:element name="attributes" type="LFBAttributesType"
                          minOccurs="0"/>
             <xsd:element name="capabilities"
                          type="LFBCapabilitiesType" minOccurs="0"/>
             <xsd:element ref="description" minOccurs="0"/>
           </xsd:sequence>
           <xsd:attribute name="LFBClassID" use="required"
                          type="xsd:integer"/>
         </xsd:complexType>
         <!-- Key constraint to ensure unique attribute names within
              a class:
         -->
         <xsd:key name="attributes">
           <xsd:selector xpath="lfb:attributes/lfb:attribute"/>
           <xsd:field xpath="lfb:name"/>
         </xsd:key>
         <xsd:key name="capabilities">
           <xsd:selector xpath="lfb:capabilities/lfb:capability"/>
           <xsd:field xpath="lfb:name"/>
         </xsd:key>
         <! ¡ does the above ensure that attributes and capabilities
              have different names?
              If so, the following is the elementID constraint .
         <xsd:key name="attributeIDs">
           <xsd:selector xpath="lfb:attributes/lfb:attribute"/>
           <xsd:field xpath="@elementID"/>
         </xsd:key>
         <xsd:key name="capabilityIDs">

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           <xsd:selector xpath="lfb:attributes/lfb:capability"/>
           <xsd:field xpath="@elementID"/>
         </xsd:key>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:simpleType name="versionType">
     <xsd:restriction base="xsd:NMTOKEN">
       <xsd:pattern value="[1-9][0-9]*\.([1-9][0-9]*|0)"/>
     </xsd:restriction>
   </xsd:simpleType>
   <xsd:complexType name="inputPortsType">
     <xsd:sequence>
       <xsd:element name="inputPort" type="inputPortType"
                    maxOccurs="unbounded"/>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="inputPortType">
     <xsd:sequence>
       <xsd:element name="name" type="xsd:NMTOKEN"/>
       <xsd:element ref="synopsis"/>
       <xsd:element name="expectation" type="portExpectationType"/>
       <xsd:element ref="description" minOccurs="0"/>
     </xsd:sequence>
     <xsd:attribute name="group" type="booleanType" use="optional"
                    default="no"/>
   </xsd:complexType>
   <xsd:complexType name="portExpectationType">
     <xsd:sequence>
       <xsd:element name="frameExpected" minOccurs="0">
         <xsd:complexType>
           <xsd:sequence>
             <!-- ref must refer to a name of a defined frame type -->
             <xsd:element name="ref" type="xsd:string"
                          maxOccurs="unbounded"/>
           </xsd:sequence>
         </xsd:complexType>
       </xsd:element>
       <xsd:element name="metadataExpected" minOccurs="0">
         <xsd:complexType>
           <xsd:choice maxOccurs="unbounded">
             <!-- ref must refer to a name of a defined metadata -->
             <xsd:element name="ref" type="metadataInputRefType"/>
             <xsd:element name="one-of"
                          type="metadataInputChoiceType"/>
           </xsd:choice>
         </xsd:complexType>

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       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="metadataInputChoiceType">
     <xsd:choice minOccurs="2" maxOccurs="unbounded">
       <!-- ref must refer to a name of a defined metadata -->
       <xsd:element name="ref" type="xsd:NMTOKEN"/>
       <xsd:element name="one-of" type="metadataInputChoiceType"/>
       <xsd:element name="metadataSet" type="metadataInputSetType"/>
     </xsd:choice>
   </xsd:complexType>
   <xsd:complexType name="metadataInputSetType">
     <xsd:choice minOccurs="2" maxOccurs="unbounded">
       <!-- ref must refer to a name of a defined metadata -->
       <xsd:element name="ref" type="metadataInputRefType"/>
       <xsd:element name="one-of" type="metadataInputChoiceType"/>
     </xsd:choice>
   </xsd:complexType>
   <xsd:complexType name="metadataInputRefType">
     <xsd:simpleContent>
       <xsd:extension base="xsd:NMTOKEN">
         <xsd:attribute name="dependency" use="optional"
                        default="required">
           <xsd:simpleType>
             <xsd:restriction base="xsd:string">
               <xsd:enumeration value="required"/>
               <xsd:enumeration value="optional"/>
             </xsd:restriction>
           </xsd:simpleType>
         </xsd:attribute>
         <xsd:attribute name="defaultValue" type="xsd:token"
                        use="optional"/>
       </xsd:extension>
     </xsd:simpleContent>
   </xsd:complexType>
   <xsd:complexType name="outputPortsType">
     <xsd:sequence>
       <xsd:element name="outputPort" type="outputPortType"
                    maxOccurs="unbounded"/>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="outputPortType">
     <xsd:sequence>
       <xsd:element name="name" type="xsd:NMTOKEN"/>
       <xsd:element ref="synopsis"/>
       <xsd:element name="product" type="portProductType"/>
       <xsd:element ref="description" minOccurs="0"/>

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     </xsd:sequence>
     <xsd:attribute name="group" type="booleanType" use="optional"
                    default="no"/>
   </xsd:complexType>
   <xsd:complexType name="portProductType">
     <xsd:sequence>
       <xsd:element name="frameProduced">
         <xsd:complexType>
           <xsd:sequence>
             <!-- ref must refer to a name of a defined frame type -->
             <xsd:element name="ref" type="xsd:NMTOKEN"
                          maxOccurs="unbounded"/>
           </xsd:sequence>
         </xsd:complexType>
       </xsd:element>
       <xsd:element name="metadataProduced" minOccurs="0">
         <xsd:complexType>
           <xsd:choice maxOccurs="unbounded">
             <!-- ref must refer to a name of a defined metadata -->
             <xsd:element name="ref" type="metadataOutputRefType"/>
             <xsd:element name="one-of"
                          type="metadataOutputChoiceType"/>
           </xsd:choice>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="metadataOutputChoiceType">
     <xsd:choice minOccurs="2" maxOccurs="unbounded">
       <!-- ref must refer to a name of a defined metadata -->
       <xsd:element name="ref" type="xsd:NMTOKEN"/>
       <xsd:element name="one-of" type="metadataOutputChoiceType"/>
       <xsd:element name="metadataSet" type="metadataOutputSetType"/>
     </xsd:choice>
   </xsd:complexType>
   <xsd:complexType name="metadataOutputSetType">
     <xsd:choice minOccurs="2" maxOccurs="unbounded">
       <!-- ref must refer to a name of a defined metadata -->
       <xsd:element name="ref" type="metadataOutputRefType"/>
       <xsd:element name="one-of" type="metadataOutputChoiceType"/>
     </xsd:choice>
   </xsd:complexType>
   <xsd:complexType name="metadataOutputRefType">
     <xsd:simpleContent>
       <xsd:extension base="xsd:NMTOKEN">
         <xsd:attribute name="availability" use="optional"
                        default="unconditional">

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           <xsd:simpleType>
             <xsd:restriction base="xsd:string">
               <xsd:enumeration value="unconditional"/>
               <xsd:enumeration value="conditional"/>
             </xsd:restriction>
           </xsd:simpleType>
         </xsd:attribute>
       </xsd:extension>
     </xsd:simpleContent>
   </xsd:complexType>
   <xsd:complexType name="LFBAttributesType">
     <xsd:sequence>
       <xsd:element name="attribute" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element ref="description" minOccurs="0"/>
             <xsd:element name="optional" minOccurs="0"/>
             <xsd:group ref="typeDeclarationGroup"/>
             <xsd:element name="defaultValue" type="xsd:token"
                          minOccurs="0"/>
           </xsd:sequence>
           <xsd:attribute name="access" use="optional"
                          default="read-write">
             <xsd:simpleType>
               <xsd:list itemType="accessModeType"/>
             </xsd:simpleType>
           </xsd:attribute>
           <xsd:attribute name="elementID" use="required"
                          type="xsd:integer"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:simpleType name="accessModeType">
     <xsd:restriction base="xsd:NMTOKEN">
       <xsd:enumeration value="read-only"/>
       <xsd:enumeration value="read-write"/>
       <xsd:enumeration value="write-only"/>
       <xsd:enumeration value="read-reset"/>
       <xsd:enumeration value="trigger-only"/>
     </xsd:restriction>
   </xsd:simpleType>
   <xsd:complexType name="LFBCapabilitiesType">
     <xsd:sequence>
       <xsd:element name="capability" maxOccurs="unbounded">

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         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
             <xsd:element ref="description" minOccurs="0"/>
             <xsd:element name="optional" minOccurs="0"/>
             <xsd:group ref="typeDeclarationGroup"/>
           </xsd:sequence>
           <xsd:attribute name="elementID" use="required"
                          type="xsd:integer"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:simpleType name="booleanType">
     <xsd:restriction base="xsd:string">
       <xsd:enumeration value="yes"/>
       <xsd:enumeration value="no"/>
     </xsd:restriction>
   </xsd:simpleType>
   </xsd:schema>

5.
  FE Attributes and Capabilities

   A ForCES forwarding element handles traffic on behalf of a ForCES
   control element.  While the standards will describe the protocol and
   mechanisms for this control, different implementations and different
   instances will have different capabilities.  The CE needs to be able
   to determine what each instance it is responsible for is actually
   capable of doing.  As stated previously, this is an approximation.
   The CE is expected to be prepared to cope with errors in requests
   and variations in detail not captured by the capabilities
   information about an FE.

   In addition to its capabilities, an FE will have attribute
   information that can be used in understanding and controlling the
   forwarding operations.  Some of the attributes will be read only,
   while others will also be writeable.

   In order to make the FE attribute information easily accessible, the
   information will be stored in an LFB.  This LFB will have a class,
   FEObject.  The LFBClassID for this class is 1.  Only one instance of
   this class will ever be present, and the instance ID of that
   instance in the protocol is 1.  Thus, by referencing the elements of
   class:1, instance:1 a CE can get all the information about the FE.
   For model completeness, this LFB Class is described in this section.


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   There will also be an FEProtocol LFB Class.  LFBClassID 2 is
   reserved for that class.  There will be only one instance of that
   class as well.  Details of that class are defined in the ForCES
   protocol document.

5.1. XML for FEObject Class definition

      <?xml version="1.0" encoding="UTF-8"?>
      <LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://ietf.org/forces/1.0/lfbmodel
        provides="FEObject">
        <dataTypeDefs>
          <dataTypeDef>
            <name>LFBAdjacencyLimitType</name>
            <synopsis>Describing the Adjacent LFB</synopsis>
            <struct>
              <element elementID="1">
                <name>NeighborLFB</name>
                <synopsis>ID for that LFB Class</synopsis>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="2">
                <name>ViaPorts</name>
                <synopsis>
                  the ports on which we can connect
                </synopsis>
                <array type="variable-size">
   <!-- It is necessary to define the length limit
        This should be whatever we define elsewhere as the
        limit of a port name
    -->
                  <typeRef>String[40]</typeRef>
                </array>
              </element>
            </struct>
          </dataTypeDef>
          <dataTypeDef>
            <name>PortGroupLimitType</name>
            <synopsis>
              Limits on the number of ports in a given group
            </synopsis>
            <struct>
              <element elementID="1">
                <name>PortGroupName</name>
                <synopsis>Group Name</synopsis>
   <!-- Again, a length limit is needed -->

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                <typeRef>String[4]</typeRef>
              </element>
              <element elementID="2">
                <name>MinPortCount</name>
                <synopsis>Minimum Port Count</synopsis>
                <optional/>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="3">
                <name>MaxPortCount</name>
                <synopsis>Max Port Count</synopsis>
                <optional/>
                <typeRef>uint32</typeRef>
              </element>
            </struct>
          </dataTypeDef>
          <dataTypeDef>
            <name>SupportedLFBType</name>
            <synopsis>table entry for supported LFB</synopsis>
            <struct>
              <element elementID="1">
                <name>LFBName</name>
                <synopsis>
                  The name of a supported LFB Class
                </synopsis>
   <!-- again with the length limit -->
                <typeRef>string[40]</typeRef>
              </element>
              <element elementID="2">
                <name>LFBClassID</name>
                <synopsis>the id of a supported LFB Class</synopsis>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="3">
                <name>LFBOccurrenceLimit</name>
                <synopsis>
                  the upper limit of instances of LFBs of this class
                </synopsis>
                <optional/>
                <typeRef>uint32</typeRef>
              </element>
              <!-- For each port group, how many ports can exist -->
              <element elementID="4">
                <name>PortGroupLimits</name>
                <synopsis>Table of Port Group Limits</synopsis>
                <optional/>
                <array type="variable-size">

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                  <typeRef>PortGroupLimitType</typeRef>
                </array>
              </element>
   <!-- for the named LFB Class, the LFB Classes it may follow -->
              <element elementID="5">
                <name>CanOccurAfters</name>
                <synopsis>
                  List of LFB Classes that this LFB class can follow
                </synopsis>
                <optional/>
                <array type="variable-size">
                  <typeRef>LFBAdjacencyLimitType</typeRef>
                </array>
              </element>
   <!-- for the named LFB Class, the LFB Classes that may follow it
     -->
              <element elementID="6">
                <name>CanOccurBefores</name>
                <synopsis>
                  List of LFB Classes that can follow this LFB class
                </synopsis>
                <optional/>
                <array type="variable-size">
                  <typeRef>LFBAdjacencyLimitType</typeRef>
                </array>
              </element>
            </struct>
          </dataTypeDef>
          <dataTypeDef>
            <name>FEStatusValues</name>
            <synopsis>The possible values of status</synopsis>
            <atomic>
              <baseType>uchar</baseType>
              <specialValues>
                <specialValue value="0">
                  <name> AdminDisable </name>
                  <synopsis>
                    FE is administratively disabled
                </synopsis>
                </specialValue>
                <specialValue value="1">
                  <name>OperDisable</name>
                  <synopsis>FE is operatively disabled</synopsis>
                </specialValue>
                <specialValue value="2">
                  <name> Operenable </name>
                  <synopsis>FE is operating</synopsis>

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                </specialValue>
              </specialValues>
            </atomic>
          </dataTypeDef>
          <dataTypeDef>
            <name>FEConfiguredNeighborType</name>
            <synopsis>Details of the FE's Neighbor</synopsis>
            <struct>
              <element elementID="1">
                <name>NeighborID</name>
                <synopsis>Neighbors FEID</synopsis>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="2">
                <name>interfaceToNeighbor</name>
                <synopsis>
                  FE's interface that connects to this neighbor
                </synopsis>
                <optional/>
   <!-- the length here is the length of interface name.
        It is unfortunate to have to limit it, since it has
        nothing to do with the model
    -->
                <typeRef>String[20]</typeRef>
              </element>
              <element elementID="3">
                <name>neighborNetworkAddress</name>
                <synopsis>The network layer address of the neighbor
                          Presumably, the network type can be
                          determined from the interface information
                </synopsis>
                <typeRef>OctetSting[16]</typeRef>
              </element>
              <element elementID="4">
                <name>neighborMACAdddress</name>
                <synopsis>the media access control address of
                          the neighbor.  Again, it is presumed
                          the type can be determined
                          from the interface information
                </synopsis>
                <typeRef>OctetString[8]</typeRef>
              </element>
            </struct>
          </dataTypeDef>
          <dataTypeDef>
            <name>AccessPermissionValues</name>
            <synopsis>

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              The possible values of attribute access permission
            </synopsis>
   <!-- can this use the access from the schema somehow? -->
            <atomic>
              <baseType>uchar</baseType>
              <specialValues>
                <specialValue value="0">
                  <name>None</name>
                  <synopsis>Access is prohibited</synopsis>
                </specialValue>
                 <specialValue value="1">
                  <name> Read-Only </name>
                  <synopsis>Access is read only</synopsis>
                </specialValue>
                <specialValue value="2">
                  <name>Write-Only</name>
                  <synopsis>
                    The attribute may be written, but not read
                  </synopsis>
                </specialValue>
                <specialValue value="3">
                  <name>Read-Write</name>
                  <synopsis>
                    The attribute may be read or written
                  </synopsis>
                </specialValue>
              </specialValues>
            </atomic>
          </dataTypeDef>
          <dataTypeDef>
            <name>SupportedAttributeType</name>
            <synopsis>
              Mapping between attributes and access modes
            </synopsis>
            <struct>
              <element elementID="1">
                <name>AttributeName</name>
                <synopsis>
                  Name of referenced Attribute
                </synopsis>
   <!-- the length limit issue again -->
                <typeRef>String[40]</typeRef>
              </element>
              <element elementID=ö2ö>
                <name>AttributeID</name>
                <synopsis>
                  The ID in the FE Object of the attribute

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                </synopsis>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="3">
                <name>AccessModes</name>
                <synopsis>Access Modes</synopsis>
                <typeRef>AccessPermissionValues</typeRef>
              </element>
            </struct>
          </dataTypeDef>
          <dataTypeDef>
            <name>LFBSelectorType</name>
            <synopsis>
              Unique identification of a LFB class-instance
            </synopsis>
            <struct>
              <element elementID="1">
                <name>LFBClassID</name>
                <synopsis>LFB Class Identifier</synopsis>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="2">
                <name>LFBInstanceID</name>
                <synopsis>LFB Instance ID</synopsis>
                <typeRef>uint32</typeRef>
              </element>
            </struct>
          </dataTypeDef>
          <dataTypeDef>
            <name>LFBLinkType</name>
            <synopsis>
              Link between two LFB instances of topology
            </synopsis>
            <struct>
              <element elementID="1">
                <name>FromLFBID</name>
                <synopsis>LFB src</synopsis>
                <typeRef>LFBSelector</typeRef>
              </element>
              <element elementID="2">
                <name>FromPortGroup</name>
                <synopsis>src port group</synopsis>
   <!-- again the length limit on strings pops up -->
                <typeRef>String[4]</typeRef>
              </element>
              <element elementID="3">
                <name>FromPortIndex</name>

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                <synopsis>src port index</synopsis>
                <typeRef>uint32</typeRef>
              </element>
              <element elementID="4">
                <name>ToLFBID</name>
                <synopsis>dst LFBID</synopsis>
                <typeRef>LFBSelector</typeRef>
              </element>
              <element elementID="5">
                <name>ToPortGroup</name>
                <synopsis>dst port group</synopsis>
   <!-- again the string length limit -->
                <typeRef>String[40]</typeRef>
              </element>
              <element elementID="6">
                <name>ToPortIndex</name>
                <synopsis>dst port index</synopsis>
                <typeRef>uint32</typeRef>
              </element>
            </struct>
          </dataTypeDef>
        </dataTypeDefs>
        <LFBClassDefs>
          <LFBClassDef LFBClassID="1">
            <name>FEObject</name>
            <synopsis>Core LFB: FE Object</synopsis>
            <capabilities>
              <capability elementID="30">
                <name>ModifiableLFBTopology</name>
                <synopsis>
                  Whether Modifiable LFB is supported
                </synopsis>
                <optional/>
                <typeRef>boolean</typeRef>
              </capability>
              <capability elementID="31">
                <name>SupportedLFBs</name>
                <synopsis>List of all supported LFBs</synopsis>
                <optional/>
                <array type="variable-size">
                  <typeRef>SupportedLFBType</typeRef>
                </array>
              </capability>
              <capability elementID="32">
                <synopsis>List of attribute ACLs</synopsis>
                <optional/>
                <array type="variable-size">

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                  <typeRef>SupportedAttributeType</typeRef>
                </array>
              </capability>
            </capabilities>
            <attributes>
              <attribute access="read-write" elementID="1">
                <name>LFBTopology</name>
                <synopsis>the table of known Topologies</synopsis>
                <array type="variable-size">
                  <typeRef>LFBLinkType</typeRef>
                </array>
              </attribute>
              <attribute access="read-write" elementID="2">
                <name>LFBSelectors</name>
                <synopsis>
                   table of known active LFB classes and
                   instances
                </synopsis>
                <array type="variable-size">
                  <typeRef>LFBSelectorType</typeRef>
                </array>
              </attribute>
              <attribute access="read-write" elementID="3">
                <name>FEName</name>
                <synopsis>name of this FE</synopsis>
                <typeRef>string[40]</typeRef>
              </attribute>
              <attribute access="read-write" elementID="4">
                <name>FEID</name>
                <synopsis>ID of this FE</synopsis>
                <typeRef>uint32</typeRef>
              </attribute>
              <attribute access="read-only" elementID="5">
                <name>FEVendor</name>
                <synopsis>vendor of this FE</synopsis>
                <typeRef>string[40]</typeRef>
              </attribute>
              <attribute access="read-only" elementID="6">
                <name>FEModel</name>
                <synopsis>model of this FE</synopsis>
                <typeRef>string[40]</typeRef>
              </attribute>
              <attribute access="read-only" elementID="7">
                <name>FEState</name>
                <synopsis>model of this FE</synopsis>
                <typeRef>FEStatusValues</typeRef>
              </attribute>

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              <attribute access="read-write" elementID="8">
                <name>FENeighbors</name>
                <synopsis>table of known neighbors</synopsis>
                <array type="variable-size">
                  <typeRef>FEConfiguredNeighborType</typeRef>
                </array>
              </attribute>
            </attributes>
          </LFBClassDef>
        </LFBClassDefs>
      </LFBLibrary>

5.2. FE Capabilities

   The FE Capability information is contained in the capabilities
   element of the class definition.  As described elsewhere, capability
   information is always considered to be read-only.

   The currently defined capabilities are ModifiableLFBTopology,
   SupportedLFBs and SupportedAttributeType.

5.2.1.  ModifiableLFBTopology

   This element has a boolean value that indicates whether the LFB
   topology of the FE may be changed by the CE.  If the element is
   absent, the default value is assumed to be true, and the CE presumes
   the LFB topology may be changed.  If the value is present and set to
   false, the LFB topology of the FE is fixed.  If the topology is
   fixed, the LFBs supported clause may be omitted, and the list of
   supported LFBs is inferred by the CE from the LFB topology
   information.  If the list of supported LFBs is provided when
   ModifiableLFBTopology is false, the CanOccurBefore and CanOccurAfter
   information should be omitted.

5.2.2.  SupportedLFBs and SupportedLFBType

   One capability that the FE should include is the list of supported
   LFB classes.  The SupportedLFBs element, is an array that contains
   the information about each supported LFB Class.  The array structure
   type is defined as the SupportedLFBType dataTypeDef.

   Each occurrence of the SupportedLFBs array element describes an LFB
   class that the FE supports.  In addition to indicating that the FE
   supports the class, FEs with modifiable LFB topology should include
   information about how LFBs of the specified class may be connected
   to other LFBs.  This information should describe which LFB classes
   the specified LFB class may succeed or precede in the LFB topology.

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   The FE should include information as to which port groups may be
   connected to the given adjacent LFB class.  If port group
   information is omitted, it is assumed that all port groups may be
   used.

5.2.2.1. LFBName

   This element has as its value the name of the LFB being described.

5.2.2.2. LFBOccurrenceLimit

   This element, if present, indicates the largest number of instances
   of this LFB class the FE can support.  For FEs that do not have the
   capability to create or destroy LFB instances, this can either be
   omitted or be the same as the number of LFB instances of this class
   contained in the LFB list attribute.

5.2.2.3. PortGroupLimits and PortGroupLimitType

   The PortGroupLimits element is an array of information about the
   port groups supported by the LFB class.  The structure of the port
   group limit information is defined by the PortGroupLimitType
   dataTypeDef.

   Each PortGroupLimits array element contains information describing a
   single port group of the LFB class.  Each array element contains the
   name of the port group in the PortGroupName element, the fewest
   number of ports that can exist in the group in the MinPortCount
   element, and the largest number of ports that can exist in the group
   in the MaxPortCount element.

5.2.2.4.CanOccurAfters and LFBAdjacencyLimitType

   The CanOccurAfters element is an array that contains the list of
   LFBs the described class can occur after.  The array elements are
   defined in the LFBAdjacencyLimitType dataTypeDef.

   The array elements describe a permissible positioning of the
   described LFB class, referred to here as the SupportedLFB.
   Specifically, each array element names an LFB that can topologically
   precede that LFB class.  That is, the SupportedLFB can have an input
   port connected to an output port of an LFB that appears in the
   CanOccurAfters array.  The LFB class that the SupportedLFB can
   follow is identified by the NeighborLFB element of the
   LFBAdjacencyLimitType array element.  If this neighbor can only be
   connected to a specific set of input port groups, then the viaPort
   element is included.  This element occurs once for each input port

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   group of the SupportedLFB that can be connected to an output port of
   the NeighborLFB.

   [e.g., Within a SupportedLFBs element, each array element of the
   CanOccurAfters array must have a unique NeighborLFB, and within each
   array element each viaPort must represent a distinct and valid input
   port group of the SupportedLFB.  The LFB Class definition schema
   does not yet support uniqueness declarations]

5.2.2.5. CanOccurBefores and LFBAdjacencyLimitType

   The CanOccurBefores array holds the information about which LFB
   classes can follow the described class.  Structurally this element
   parallels CanOccurAfters, and uses the same type definition for the
   array element.

   The array elements list those LFB classes that the SupportedLFB may
   precede in the topology.  In this element, the
   viaPort element of the array value represents the output port group
   of the SupportedLFB that may be connected to the NeighborLFB.  As
   with CanOccurAfters, viaPort may occur multiple times if multiple
   output ports may legitimately connect to the given NeighborLFB
   class.

   [And a similar set of uniqueness constraints apply to the
   CanOccurBefore clauses, even though an LFB may occur both in
   CanOccurAfter and CanOccurBefore.]

5.2.2.6. LFBClassCapabilities

   This element contains capability information about the subject LFB
   class whose structure and semantics are defined by the LFB class
   definition.

   [Note:  Important Omissions]

   However, this element does not appear in the definition, because the
   author can not figure out how to write it.

5.2.3.  SupportedAttributeType

   This element serves as a wrapper to hold the information about
   attributed related capabilities.  Specifically, attributes should be
   described in this element if:
     a) they are optional elements in the standard and are supported by
        the FE, or



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     b) the standard allows for a range of access permissions (for
        example, read-only or read-write).

   Each attribute so described is contained in the
   SupportedAttributeType element.  That element contains an
   AttributeName element whose value is the name of the element being
   described, and AttributeID element whose value is the ID in the FE
   Object of the Attribute, and an AccessModes element whose value is
   the list of permissions.

5.3.FEAttributes

   The attributes element is included if the class definition contains
   the attributes of the FE that are not considered "capabilities".
   Some of these attributes are writeable, and some are read-only,
   which should be indicated by the capability information.

   [Editors note - At the moment, the set of attributes is woefully
   incomplete.]

5.3.1.  FEStatus

   This attribute carries the overall state of the FE.  For now, it is
   restricted to the strings AdminDisable, OperDisable and OperEnable.

5.3.2. LFBSelectors and LFBSelectorType

   The LFBSelectors element is an array of information about the LFBs
   currently accessible via ForCES in the FE.  The structure of the LFB
   information is defined by the LFBSelectorType.

   Each entry in the array describes a single LFB instance in the FE.
   The array element contains the numeric class ID of the class of the
   LFB instance and the numeric instance ID for this instance.

5.3.3.  LFBTopology and LFBLinkType

   The optional LFBTopology element contains information about each
   inter-LFB link inside the FE, where each link is described in an
   LFBLinkType element.  The LFBLinkType element contains sufficient
   information to identify precisely the end points of a link.  The
   FromLFBID and ToLFBID fields specify the LFB instances at each end
   of the link, and must reference LFBs in the LFB instance table.  The
   FromPortGroup and ToPortGroup must identify output and input port
   groups defined in the LFB classes of the LFB instances identified by
   FromLFBID and ToLFBID.  The FromPortIndex and ToPortIndex fields
   select the elements from the port groups that this link connects.

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   All links are uniquely identified by the FromLFBID, FromPortGroup,
   and FromPortIndex fields.  Multiple links may have the same ToLFBID,
   ToPortGroup, and ToPortIndex as this model supports fan in of inter-
   LFB links but not fan out.

5.3.4.  FENeighbors an FEConfiguredNeighborType

   The FENeighbors element is an array of information about manually
   configured adjacencies between this FE and other FEs.  The content
   of the array is defined by the FEConfiguredNeighborType element.

   This array is intended to capture information that may be configured
   on the FE and is needed by the CE, where one array entry corresponds
   to each configured neighbor.  Note that this array is not intended
   to represent the results of any discovery protocols, as those will
   have their own LFBs.

   Similarly, the MAC address information in the table is intended to
   be used in situations where neighbors are configured by MAC address.
   Resolution of network layer to MAC address information should be
   captured in ARP LFBs and not duplicated in this table.  Note that
   the same neighbor may be reached through multiple interfaces or at
   multiple addresses.  There is no uniqueness requirement of any sort
   on occurrences of the FENeighbors element.

   Information about the intended forms of exchange with a given
   neighbor is not captured here, only the adjacency information is
   included.

5.3.4.1.NeighborID

   This is the ID in some space meaningful to the CE for the neighbor.
   If this table remains, we probably should add an FEID from the same
   space as an attribute of the FE.

5.3.4.2.NeighborInterface

   This identifies the interface through which the neighbor is reached.

   [Editors note: As the port structures become better defined, the
   type for this should be filled in with the types necessary to
   reference the various possible neighbor interfaces, include physical
   interfaces, logical tunnels, virtual circuits, etc.]

5.3.4.3. NeighborNetworkAddress




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   Neighbor configuration is frequently done on the basis of a network
   layer address.  For neighbors configured in that fashion, this is
   where that address is stored.

5.3.4.4.NeighborMacAddress

   Neighbors are sometimes configured using MAC level addresses
   (Ethernet MAC address, circuit identifiers, etc.)  If such addresses
   are used to configure the adjacency, then that information is stored
   here.  Note that over some ports such as physical point to point
   links or virtual circuits considered as individual interfaces, there
   is no need for either form of address.

6.
  Satisfying the Requirements on FE Model

   This section describes how the proposed FE model meets the
   requirements outlined in Section 5 of RFC 3654 [1].  The
   requirements can be separated into general requirements (Sections 5,
   5.1 - 5.4) and the specification of the minimal set of logical
   functions that the FE model must support (Section 5.5).

   The general requirement on the FE model is that it be able to
   express the logical packet processing capability of the FE, through
   both a capability and a state model.  In addition, the FE model is
   expected to allow flexible implementations and be extensible to
   allow defining new logical functions.

   A major component of the proposed FE model is the Logical Function
   Block (LFB) model.  Each distinct logical function in an FE is
   modeled as an LFB.  Operational parameters of the LFB that must be
   visible to the CE are conceptualized as LFB attributes.  These
   attributes express the capability of the FE and support flexible
   implementations by allowing an FE to specify which optional features
   are supported. The attributes also indicate whether they are
   configurable by the CE for an LFB class.  Configurable attributes
   provide the CE some flexibility in specifying the behavior of an
   LFB.  When multiple LFBs belonging to the same LFB class are
   instantiated on an FE, each of those LFBs could be configured with
   different attribute settings.  By querying the settings of the
   attributes for an instantiated LFB, the CE can determine the state
   of that LFB.

   Instantiated LFBs are interconnected in a directed graph that
   describes the ordering of the functions within an FE.  This directed
   graph is described by the topology model.  The combination of the
   attributes of the instantiated LFBs and the topology describe the
   packet processing functions available on the FE (current state).

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   Another key component of the FE model is the FE attributes. The FE
   attributes are used mainly to describe the capabilities of the FE,
   but they also convey information about the FE state.

   The FE model also includes a definition of the minimal set of LFBs
   that is required by Section 5.5 of RFC 3564[1].  The sections that
   follow provide more detail on the specifics of each of those LFBs.
   Note that the details of the LFBs are contained in a separate LFB
   Class Library document. [EDITOR - need to add a reference to that
   document].

6.1. Port Functions

   The FE model can be used to define a Port LFB class and its
   technology-specific subclasses to map the physical port of the
   device to the LFB model with both static and configurable
   attributes.  The static attributes model the type of port, link
   speed, etc.  The configurable attributes model the addressing,
   administrative status, etc.

6.2. Forwarding Functions

   Because forwarding function is one of the most common and important
   functions in the forwarding plane, it requires special attention in
   modeling to allow design flexibility, implementation efficiency,
   modeling accuracy and configuration simplicity.  Toward that end, it
   is recommended that the core forwarding function being modeled by
   the combination of two LFBs -- Longest Prefix Match (LPM) classifier
   LFB and Next Hop LFB. Special header writer LFB  is also needed to
   take care of TTL decrement and checksum etc.

6.3. QoS Functions

   The LFB class library includes descriptions of the Meter, Queue ,
   Scheduler, Counter and Dropper LFBs to support the QoS functions in
   the forwarding path.  The FE model can also be used to define other
   useful QoS functions as needed.  These LFBs allow the CE to
   manipulate the attributes to model IntServ or DiffServ functions.

6.4. Generic Filtering Functions

   Various combinations of Classifier, Redirector, Meter and Dropper
   LFBs can be used to model a complex set of filtering functions.

6.5. Vendor Specific Functions


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   New LFB classes can be defined according to the LFB model as
   described in Section 4 to support vendor specific functions.  A new
   LFB class can also be derived from an existing LFB class through
   inheritance.

6.6.High-Touch Functions

   High-touch functions are those that take action on the contents or
   headers of a packet based on content other than what is found in the
   IP header.  Examples of such functions include NAT, ALG, firewall,
   tunneling and L7 content recognition.  It is not practical to
   include all possible high-touch functions in the initial LFB library
   due to the number and complexity. However, the flexibility of the
   LFB model and the power of interconnection in LFB topology should
   make it possible to model any high-touch functions.

6.7. Security Functions

   Security functions are not included in the initial LFB class
   library.  However, the FE model is flexible and powerful enough to
   model the types of encryption and/or decryption functions that an FE
   supports and the associated attributes for such functions.

   The IP Security Policy (IPSP) Working Group in the IETF has started
   work in defining the IPSec Policy Information Base [8].  We will try
   to reuse as much of the work as possible.

6.8. Off-loaded Functions

   In addition to the packet processing functions typically found on
   the FEs, some logical functions may also be executed asynchronously
   by some FEs, as directed by a finite-state machine and triggered not
   only by packet events, but by timer events as well.  Examples of
   such functions include; finite-state machine execution required by
   TCP termination or OSPF Hello processing off-loaded from the CE.  By
   defining LFBs for such functions, the FE model is capable of
   expressing these asynchronous functions to allow the CE to take
   advantage of such off-loaded functions on the FEs.

6.9. IPFLOW/PSAMP Functions

   RFC 3917 [9] defines an architecture for IP traffic flow monitoring,
   measuring and exporting.  The LFB model supports statistics
   collection on the LFB by including statistical attributes (Section
   4.7.4) in the LFB class definitions; in addition, special statistics
   collection LFBs such as meter LFBs and counter LFBs can also be used
   to support accounting functions in the FE.

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   [10] describes a framework to define a standard set of capabilities
   for network elements to sample subsets of packets by statistical and
   other methods.  Time event generation, filter LFB, and counter/meter
   LFB are the elements needed to support packet filtering and sampling
   functions -- these elements can all be supported in the FE model.

7.
  Using the FE model in the ForCES Protocol

   The actual model of the forwarding plane in a given NE is something
   the CE must learn and control by communicating with the FEs (or by
   other means).  Most of this communication will happen in the post-
   association phase using the ForCES protocol.  The following types of
   information must be exchanged between CEs and FEs via the ForCES
   protocol:
      1)  FE topology query;
      2)  FE capability declaration;
      3)  LFB topology (per FE) and configuration capabilities query;
      4)  LFB capability declaration;
      5)  State query of LFB attributes;
      6)  Manipulation of LFB attributes;
      7)  LFB topology reconfiguration.

   Items 1) through 5) are query exchanges, where the main flow of
   information is from the FEs to the CEs.  Items 1) through 4) are
   typically queried by the CE(s) in the beginning of the post-
   association (PA) phase, though they may be repeatedly queried at any
   time in the PA phase.  Item 5) (state query) will be used at the
   beginning of the PA phase, and often frequently during the PA phase
   (especially for the query of statistical counters).

   Items 6) and 7) are "command" types of exchanges, where the main
   flow of information is from the CEs to the FEs.  Messages in Item 6)
   (the LFB re-configuration commands) are expected to be used
   frequently.  Item 7) (LFB topology re-configuration) is needed only
   if dynamic LFB topologies are supported by the FEs and it is
   expected to be used infrequently.

   Among the seven types of payload information the ForCES protocol
   carries between CEs and FEs, the FE model covers all of them except
   item 1), which concerns the inter-FE topology.  The FE model focuses
   on the LFB and LFB topology within a single FE.  Since the
   information related to item 1) requires global knowledge about all
   of the FEs and their inter-connection with each other, this exchange
   is part of the ForCES base protocol instead of the FE model.




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   The relationship between the FE model and the seven post-association
   messages are visualized in Figure 9:

                                                    +--------+
                                       ..........-->|   CE   |
                  /----\               .            +--------+
                  \____/ FE Model      .              ^    |
                  |    |................        (1),2 |    | 6, 7
                  |    |  (off-line)   .      3, 4, 5 |    |
                  \____/               .              |    v
                                       .            +--------+
                e.g. RFCs              ..........-->|   FE   |
                                                    +--------+

    Figure 9. Relationship between the FE model and the ForCES protocol
     messages, where (1) is part of the ForCES base protocol, and the
                     rest are defined by the FE model.

   The actual encoding of these messages is defined by the ForCES
   protocol and beyond the scope of the FE model.  Their discussion is
   nevertheless important here for the following reasons:
     . These PA model components have considerable impact on the FE
        model.  For example, some of the above information can be
        represented as attributes of the LFBs, in which case such
        attributes must be defined in the LFB classes.
     . The understanding of the type of information that must be
        exchanged between the FEs and CEs can help to select the
        appropriate protocol format and the actual encoding method
        (such as XML, TLVs).
     . Understanding the frequency of these types of messages should
        influence the selection of the protocol format (efficiency
        considerations).

   An important part of the FE model is the port the FE uses for its
   message exchanges to and from the CE.  In the case that a dedicated
   port is used for CE-FE communication, we propose to use a special
   port LFB, called the CE-FE Port LFB (a subclass of the general Port
   LFB in Section 6.1), to model this dedicated CE-FE port.  The CE-FE
   Port LFB acts as both a source and sink for the traffic from and to
   the CE.  Sometimes the CE-FE traffic does not have its own dedicated
   port, instead the data fabric is shared for the data plane traffic
   and the CE-FE traffic.  A special processing LFB can be used to
   model the ForCES packet encapsulation and decapsulation in such
   cases.

   The remaining sub-sections of this section address each of the seven
   message types.

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7.1. FE Topology Query

   An FE may contain zero, one or more external ingress ports.
   Similarly, an FE may contain zero, one or more external egress
   ports.  In other words, not every FE has to contain any external
   ingress or egress interfaces.  For example, Figure 10 shows two
   cascading FEs.  FE #1 contains one external ingress interface but no
   external egress interface, while FE #2 contains one external egress
   interface but no ingress interface.  It is possible to connect these
   two FEs together via their internal interfaces to achieve the
   complete ingress-to-egress packet processing function. This provides
   the flexibility to spread the functions across multiple FEs and
   interconnect them together later for certain applications.

   While the inter-FE communication protocol is out of scope for
   ForCES, it is up to the CE to query and understand how multiple FEs
   are inter-connected to perform a complete ingress-egress packet
   processing function, such as the one described in Figure 10.  The
   inter-FE topology information may be provided by FEs, may be hard-
   coded into CE, or may be provided by some other entity (e.g., a bus
   manager) independent of the FEs.  So while the ForCES protocol
   supports FE topology query from FEs, it is optional for the CE to
   use it, assuming the CE has other means to gather such topology
   information.

      +-----------------------------------------------------+
      |  +---------+   +------------+   +---------+         |
    input|         |   |            |   |         | output  |
   ---+->| Ingress |-->|Header      |-->|IPv4     |---------+--->+
      |  | port    |   |Decompressor|   |Forwarder| FE      |    |
      |  +---------+   +------------+   +---------+ #1      |    |
      +-----------------------------------------------------+    V
                                                                 |
           +-----------------------<-----------------------------+
           |
           |    +----------------------------------------+
           V    |  +------------+   +----------+         |
           | input |            |   |          | output  |
           +->--+->|Header      |-->| Egress   |---------+-->
                |  |Compressor  |   | port     | FE      |
                |  +------------+   +----------+ #2      |
                +----------------------------------------+

           Figure 10. An example of two FEs connected together.




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   Once the inter-FE topology is discovered by the CE after this query,
   it is assumed that the inter-FE topology remains static.  However,
   it is possible that an FE may go down during the NE operation, or a
   board may be inserted and a new FE activated, so the inter-FE
   topology will be affected.  It is up to the ForCES protocol to
   provide a mechanism for the CE to detect such events and deal with
   the change in FE topology.  FE topology is outside the scope of the
   FE model.

7.2. FE Capability Declarations

   FEs will have many types of limitations.  Some of the limitations
   must be expressed to the CEs as part of the capability model.  The
   CEs must be able to query these capabilities on a per-FE basis.
   Examples:
     . Metadata passing capabilities of the FE.  Understanding these
        capabilities will help the CE to evaluate the feasibility of
        LFB topologies, and hence to determine the availability of
        certain services.
     . Global resource query limitations (applicable to all LFBs of
        the FE).
     . LFB supported by the FE.
     . LFB class instantiation limit.
     . LFB topological limitations (linkage constraint, ordering etc.)

7.3. LFB Topology and Topology Configurability Query

   The ForCES protocol must provide the means for the CEs to discover
   the current set of LFB instances in an FE and the interconnections
   between the LFBs within the FE.  In addition, sufficient information
   should be available to determine whether the FE supports any CE-
   initiated (dynamic) changes to the LFB topology, and if so,
   determine the allowed topologies.  Topology configurability can also
   be considered as part of the FE capability query as described in
   Section 9.3.

7.4. LFB Capability Declarations

   LFB class specifications define a generic set of capabilities.
   When an LFB instance is implemented (instantiated) on a vendor's FE,
   some additional limitations may be introduced.  Note that we discuss
   only those limitations that are within the flexibility of the LFB
   class specification.  That is, the LFB instance will remain
   compliant with the LFB class specification despite these
   limitations.  For example, certain features of an LFB class may be
   optional, in which case it must be possible for the CE to determine
   if an optional feature is supported by a given LFB instance or not.

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   Also, the LFB class definitions will probably contain very few
   quantitative limits (e.g., size of tables), since these limits are
   typically imposed by the implementation.  Therefore, quantitative
   limitations should always be expressed by capability arguments.

   LFB instances in the model of a particular FE implementation will
   possess limitations on the capabilities defined in the corresponding
   LFB class.  The LFB class specifications must define a set of
   capability arguments, and the CE must be able to query the actual
   capabilities of the LFB instance via querying the value of such
   arguments.  The capability query will typically happen when the LFB
   is first detected by the CE.  Capabilities need not be re-queried in
   case of static limitations.  In some cases, however, some
   capabilities may change in time (e.g., as a result of
   adding/removing other LFBs, or configuring certain attributes of
   some other LFB when the LFBs share physical resources), in which
   case additional mechanisms must be implemented to inform the CE
   about the changes.

   The following two broad types of limitations will exist:
     . Qualitative restrictions.  For example, a standardized multi-
        field classifier LFB class may define a large number of
        classification fields, but a given FE may support only a subset
        of those fields.
     . Quantitative restrictions, such as the maximum size of tables,
        etc.

   The capability parameters that can be queried on a given LFB class
   will be part of the LFB class specification.  The capability
   parameters should be regarded as special attributes of the LFB.  The
   actual values of these arguments may be, therefore, obtained using
   the same attribute query mechanisms as used for other LFB
   attributes.

   Capability attributes will typically be read-only arguments, but in
   certain cases they may be configurable.  For example, the size of a
   lookup table may be limited by the hardware (read-only), in other
   cases it may be configurable (read-write, within some hard limits).

   Assuming that capabilities will not change frequently, the
   efficiency of the protocol/schema/encoding is of secondary concern.

7.5. State Query of LFB Attributes

   This feature must be provided by all FEs.  The ForCES protocol and
   the data schema/encoding conveyed by the protocol must together



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   satisfy the following requirements to facilitate state query of the
   LFB attributes:
     . Must permit FE selection.  This is primarily to refer to a
        single FE, but referring to a group of (or all) FEs may
        optional be supported.
     . Must permit LFB instance selection.  This is primarily to refer
        to a single LFB instance of an FE, but optionally addressing of
        a group of LFBs (or all) may be supported.
     . Must support addressing of individual attribute of an LFB.
     . Must provide efficient encoding and decoding of the addressing
        info and the configured data.
     . Must provide efficient data transmission of the attribute state
        over the wire (to minimize communication load on the CE-FE
        link).

7.6. LFB Attribute Manipulation

   This is a place-holder for all operations that the CE will use to
   populate, manipulate, and delete attributes of the LFB instances on
   the FEs.  These operations allow the CE to configure an individual
   LFB instance.

   The same set of requirements as described in Section 9.5 for
   attribute query applies here for attribute manipulation as well.

   Support for various levels of feedback from the FE to the CE (e.g.,
   request received, configuration completed), as well as multi-
   attribute configuration transactions with atomic commit and
   rollback, may be necessary in some circumstances.

   (Editor's note: It remains an open issue as to whether or not other
   methods are needed in addition to "get attribute" and "set
   attribute" (such as multi-attribute transactions).  If the answer to
   that question is yes, it is not clear whether such methods should be
   supported by the FE model itself or the ForCES protocol.)

7.7. LFB Topology Re-configuration

   Operations that will be needed to reconfigure LFB topology:
     . Create a new instance of a given LFB class on a given FE.
     . Connect a given output of LFB x to the given input of LFB y.
     . Disconnect: remove a link between a given output of an LFB and
        a given input of another LFB.
     . Delete a given LFB (automatically removing all interconnects
        to/from the LFB).

8.
  Acknowledgments

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   Many of the colleagues in our companies and participants in the
   ForCES mailing list have provided invaluable input into this work.

9.
  Security Considerations

   The FE model describes the representation and organization of data
   sets and attributes in the FEs.  The ForCES framework document [2]
   provides a comprehensive security analysis for the overall ForCES
   architecture.  For example, the ForCES protocol entities must be
   authenticated per the ForCES requirements before they can access the
   information elements described in this document via ForCES.  Access
   to the information contained in the FE model is accomplished via the
   ForCES protocol, which will be defined in separate documents, and
   thus the security issues will be addressed there.

10.
   Normative References

  [1] Khosravi, H. et al., "Requirements for Separation of IP Control
  and Forwarding", RFC 3654, November 2003.

  [2] Yang, L. et al., "Forwarding and Control Element Separation
  (ForCES) Framework", RFC 3746, April 2004.

11.
   Informative References

   [3] Bernet, Y. et al., "An Informal Management Model for Diffserv
   Routers", RFC 3290, May 2002.

   [4] Chan, K. et al., "Differentiated Services Quality of Service
   Policy Information Base", RFC 3317, March 2003.

   [5] Sahita, R. et al., "Framework Policy Information Base", RFC
   3318, March 2003.

   [6] Moore, B. et al., "Information Model for Describing Network
   Device QoS Datapath Mechanisms", RFC 3670, January 2004.

   [7] Snir, Y. et al., "Policy Framework QoS Information Model", RFC
   3644, Nov 2003.

   [8] Li, M. et al., "IPsec Policy Information Base", work in
   progress, April 2004, <draft-ietf-ipsp-ipsecpib-10.txt>.


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   [9] Quittek, J. et Al., "Requirements for IP Flow Information
   Export", RFC 3917, October 2004.

   [10] Duffield, N., "A Framework for Packet Selection and Reporting",
   work in progress, January 2005, <draft-ietf-psamp-framework-10.txt>.

   [11] Pras, A. and Schoenwaelder, J., RFC 3444 "On the Difference
   between Information Models and Data Models", January 2003.

12.
   Authors' Addresses

   L. Lily Yang
   Intel Corp.
   Mail Stop: JF3-206
   2111 NE 25th Avenue
   Hillsboro, OR 97124, USA
   Phone: +1 503 264 8813
   Email: lily.l.yang@intel.com

   Joel M. Halpern
   Megisto Systems, Inc.
   20251 Century Blvd.
   Germantown, MD 20874-1162, USA
   Phone: +1 301 444-1783
   Email: jhalpern@megisto.com

   Ram Gopal
   Nokia Research Center
   5, Wayside Road,
   Burlington, MA 01803, USA
   Phone: +1 781 993 3685
   Email: ram.gopal@nokia.com

   Alan DeKok
   Infoblox, Inc.
   475 Potrero Ave,
   Sunnyvale CA 94085
   Phone:
   Email: alan.dekok@infoblox.com

   Zsolt Haraszti
   Clovis Solutions
   1310 Redwood Way, Suite B
   Petaluma, CA 94954
   Phone: 707-796-7110
   Email: zsolt@clovissolutions.com



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Internet Draft         ForCES FE Model              August 2005



   Steven Blake
   Modular Networks
   Phone: +1 919 434-1485
   Email: slblake@modularnet.com

   Ellen Deleganes
   Intel Corp.
   Mail Stop: JF3-206
   2111 NE 25th Avenue
   Hillsboro, OR 97124, USA
   Phone: +1 503 712 4173
   Email: ellen.m.deleganes@intel.com


13.
   Intellectual Property Right

   The authors are not aware of any intellectual property right issues
   pertaining to this document.

14.
   IANA consideration

   A namespace is needed to uniquely identify the LFB type in the LFB
   class library.

   Frame type supported on input and output of LFB must also be
   uniquely identified.

   A set of metadata supported by the LFB model must also be uniquely
   identified with names or IDs.

15.
   Copyright Statement

   "Copyright (C) The Internet Society 2005.  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights."

   "This document and the information contained herein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."




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