Internet Draft                               L. Yang
      Expiration: September 2006                        Intel Corp.
      File: draft-ietf-forces-model-06.txt                               J. Halpern
      Expiration: April 2007                            Self
      File: draft-ietf-forces-model-07.txt         E. Deleganes
      Working Group: ForCES                             Megisto Systems
                                                   R. Gopal
                                                   A. DeKok
                                                        Infoblox, Inc.
                                                   Z. Haraszti
                                                        Clovis Solutions
                                                   E. Deleganes                             Intel Corp.
                                                   October 2006

                         ForCES Forwarding Element Model



   Status of this Memo

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      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

      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 Protocol............6
         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........................8
         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 Events...........................................26
            3.2.6. LFB Element Properties...............................27
            3.2.7. LFB Versioning.......................................27
            3.2.8. LFB Inheritance......................................27 Inheritance......................................28
         3.3. FE Datapath Modeling......................................28 Modeling......................................29
            3.3.1. Alternative Approaches for Modeling FE Datapaths.....29
            3.3.2. Configuring the LFB Topology.........................33
      4. Model and Schema for LFB Classes...............................37
         4.1. Namespace.................................................37
         4.2. <LFBLibrary> Element......................................37
         4.3. <load> Element............................................39
         4.4. <frameDefs> Element for Frame Type Declarations...........39
         4.5. <dataTypeDefs> Element for Data Type Definitions..........40
            4.5.1. <typeRef> Element for Aliasing Existing Data Types...42 Types...43
            4.5.2. <atomic> Element for Deriving New Atomic Types.......42 Types.......43
            4.5.3. <array> Element to Define Arrays.....................43 Arrays.....................44
            4.5.4. <struct> Element to Define Structures................47
            4.5.5. <union> Element to Define Union Types................48
            4.5.6. Augmentations........................................48 Augmentations........................................49
         4.6. <metadataDefs> Element for Metadata Definitions...........49 Definitions...........50
         4.7. <LFBClassDefs> Element for LFB Class Definitions..........50 Definitions..........51
            4.7.1. <derivedFrom> Element to Express LFB Inheritance.....52
            4.7.2. <inputPorts> Element to Define LFB Inputs............52 Inputs............53
            4.7.3. <outputPorts> Element to Define LFB Outputs..........55
            4.7.4. <attributes> Element to Define LFB Operational
            4.7.5. <capabilities> Element to Define LFB Capability
            4.7.6. <events> Element for LFB Notification Generation.....60 Generation.....61
            4.7.7. <description> Element for LFB Operational Specification
         4.8. Properties................................................64
            4.8.1. Basic Properties.....................................64
            4.8.2. Array Properties.....................................66
            4.8.3. String Properties....................................66
            4.8.4. Octetstring Properties...............................67
            4.8.5. Event Properties.....................................67
            4.8.6. Alias Properties.....................................70
         4.9. XML Schema for LFB Class Library Documents................70 Documents................71
      5. FE Attributes and Capabilities.................................81 Capabilities.................................82
         5.1. XML for FEObject Class definition.........................81 definition.........................82
         5.2. FE Capabilities...........................................87 Capabilities...........................................89
            5.2.1. ModifiableLFBTopology................................88 ModifiableLFBTopology................................89
            5.2.2. SupportedLFBs and SupportedLFBType...................88 SupportedLFBType...................89
         5.3. FEAttributes..............................................90 FEAttributes..............................................92
            5.3.1. FEStatus.............................................90 FEStatus.............................................92
            5.3.2. LFBSelectors and LFBSelectorType.....................90 LFBSelectorType.....................92
            5.3.3. LFBTopology and LFBLinkType..........................91 LFBLinkType..........................92
            5.3.4. FENeighbors an FEConfiguredNeighborType..............91 and FEConfiguredNeighborType.............93
      6. Satisfying the Requirements on FE Model........................92
         6.1. Port Functions............................................93
         6.2. Forwarding Functions......................................93
         6.3. QoS Functions.............................................93
         6.4. Generic Filtering Functions...............................94
         6.5. Vendor Specific Functions.................................94
         6.6. High-Touch Functions......................................94
         6.7. Security Functions........................................94
         6.8. Off-loaded Functions......................................94
         6.9. IPFLOW/PSAMP Functions....................................95 Model........................93
      7. Using the FE model in the ForCES Protocol......................95 Protocol......................94
         7.1. FE Topology Query.........................................97 Query.........................................96
         7.2. FE Capability Declarations................................98 Declarations................................97
         7.3. LFB Topology and Topology Configurability Query...........98
         7.4. LFB Capability Declarations...............................98
         7.5. State Query of LFB Attributes.............................99
         7.6. LFB Attribute Manipulation...............................100 Manipulation................................99
         7.7. LFB Topology Re-configuration............................100
      8. Example.......................................................100
         8.1. Data Handling............................................108 Handling............................................107
            8.1.1. Setting up a DLCI...................................108
            8.1.2. Error Handling......................................109 Handling......................................108
         8.2. LFB Attributes...........................................109
         8.3. Capabilities.............................................110 Capabilities.............................................109
         8.4. Events...................................................110 Events...................................................109
      9. Acknowledgments...............................................111 IANA Considerations...........................................111
      10. Authors Emeritus.............................................111
      11. Acknowledgments..............................................111
      12. Security Considerations......................................112
      13. Normative References.........................................112
      14. Informative References.......................................112
      15. Authors' Addresses...........................................113
      16. Intellectual Property Right..................................114
      15. IANA consideration...........................................114
      16. Right..................................113
      17. Copyright Statement..........................................114 Statement..........................................113

   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 that
      representing a fine-grained, logically separable aspect of FE
      processing.  Most LFBs relate to packet processing in the data path.
      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
      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.  Metadata is sent between the FE and the CE on
      redirect packets.

      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
      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

   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.
      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
      automation of some of the code generation process and the functional
      validation of arbitrary LFB topologies.  These class definitions
      form the LFB Library.  Documents which describe LFB Classes are
      therefore referred to as LFB Library documents.

      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. This document uses XML
      Schema to define the structure of the LFB Library documents, as
      defined in [12] and [13].  While these LFB Class definitions are not
      sent in the Forces protocol, these definitions comply with the
      recommendations in RFC 3470 [11] on the use of XML in IETF

   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
      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 includes 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

      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].

      There is one common and shared aspect of capability that will be
      handled in a separate fashion.  For all elements of information,
      certain property information is needed.  All elements need
      information as to whether they are supported and if so whether the
      element is readable or writeable.  Based on their type, many
      elements have additional common properties (for example, arrays have
      their current size.)  There is a specific model and protocol
      mechanism for referencing this form of property information about
      elements of the model.

      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;
        . 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

           +-------+                                          +-------+
           |       | 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

      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, the CE MUST be able 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
      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.

                              |    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

      LFB operation is 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

      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
        . metadata read/consumed from inputs;
        . metadata produced at the outputs;
        . packet type(s) accepted at the inputs and emitted at the
        . packet content modifications (including encapsulation or
        . packet routing criteria (when multiple outputs on an LFB are
        . packet timing modifications;
        . packet flow ordering modifications;
        . LFB capability information;
        . Events that can be detected by the LFB, with notification to
           the CE;
        . 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

   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
      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

      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 LFB class definition MUST include the number of
      outputs, implying the number of outputs is 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
      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
      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 to 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.

      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.

                   +--------------+       +------------------------+
                   | 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.
      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
      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. 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.

      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. 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.  In order to allow the CE to
      understand and control the meta-data related operations, the model
      represents each metadata tag as a 32-bit integer.  Each LFB
      definition indicates in its metadata declarations the 32-bit value
      associated with a given metadata tag.  Ensuring consistency of usage
      of tags is important, and outside the scope of the model.

      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:

      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. 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. 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
      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

      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. Fixed, Variable and Configurable Tag

      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.  This selection of tags is variable in
      that the produced output may have any number of different tags.  The
      meaning of the various tags is still defined by the metadata
      declaration associated with the LFB class definition.  This also
      allows the CE to correctly set the tag values in the table to match
      the declared meanings of the metadata tag values.

      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. 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

      There are three types of metadata related to metadata usage:

        . Relational (or binding) metadata
        . Enumerated metadata
        . Explicit/external value metadata

      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

      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)
           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 Events

      During operation, various conditions may occur that can be detected
      by LFBs.  Examples range from link failure or restart to timer
      expiration in special purpose LFBs.  The CE may wish to be notified
      of the occurrence of such events.  The PL protocol provides for such
      notifications.  The LFB definition includes the necessary
      declarations of events.  The declarations include identifiers
      necessary for subscribing to events (so that the CE can indicate to
      the FE which events it wishes to receive) and to indicate in event
      notification messages which event is being reported.

      The declaration of an event defines a condition that an FE can
      detect, and may report.  From a conceptual point of view, event
      processing is split into triggering (the detection of the condition)
      and reporting (the generation of the notification of the event.)  In
      between these two conceptual points there is event filtering.
      Properties associated with the event in the LFB instance can define
      filtering conditions to suppress the reporting of that event.  The
      model thus describes event processing as if events always occur, and
      filtering may suppress reporting.  Implementations may function in
      this manner, or may have more complex logic that eliminates some
      event processing if the reporting would be suppressed.  Any
      implementation producing an effect equivalent to the model
      description is valid.

   3.2.6. LFB Element Properties

      LFBs are made up of elements, containing the information that the CE
      needs to see and / or change about the functioning of the LFB.
      These elements, as described in detail elsewhere, may be basic
      values, complex structures, or tables (containing values,
      structures, or tables.)  Some of these elements are optional.  Some
      elements may be readable or writeable at the discretion of the FE
      implementation.  The CE needs to know these properties.
      Additionally, certain kinds of elements (arrays, aliases, and events
      as of this writing) have additional property information that the CE
      may need to read or write.  This model defines the structure of the
      property information for all defined data types.

      The reports with events are designed to allow for the common,
      closely related information that the CE can be strongly expected to
      need to react to the event.  It is not intended to carry information
      the CE already has, nor large volumes of information, nor
      information related in complex fashions.

   3.2.7. 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
      particular LFB class to provide backward compatibility, but FEs MUST
      NOT support more than one version of a particular class.

      Versioning is not restricted to making backwards compatible changes.
      It is specifically expected to be used to make changes that cannot
      be represented by inheritance.  Often this will be to correct
      errors, and hence may not be backwards compatible.  It may also be
      used to remove elements which are not considered useful
      (particularly if they were previously mandatory, and hence were an
      implementation impediment.)
   3.2.8. 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

      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.
        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 operation controls 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
        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).

                                    P      |   LFB#1  |
           +-------------+      |          +----------+
           |            1|------+   P      +----------+
           |            2|---------------->|   LFB#2  |
           | classifier 3|                 |(Attrib-2)|
           |          ...|...              +----------+
           |            N|------+          ...
           +-------------+      |   P      +----------+
                                +--------->|   LFB#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
      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+------+
           (a)  The LFB topology with a logical loop

          +-------+   +-----------+            +------+   +-----------+
          |       |   |           |if IP-in-IP |      |   |           |
          | ports |   |           |----+       |      |   |           |
          +-------+   +-----------+    |others +------+   +-----------+
           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
      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
      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 are required to 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

        . Which LFB classes the FE can instantiate
        . Maximum number of instances of the same LFB class that can be
        . 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, the CE MUST
      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 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       |              |
           +----------+     +-----------+              |
             +--------+    |   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.

                   (b)  One LFB topology as configured by the CE and
                        accepted by the FE
                         +---+                    +--+
                         |  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 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,
      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

      A namespace is needed to uniquely identify the LFB type in the following namespace:
      class library. The reference to the namespace definition is
      contained in Section 9, IANA Considerations.

   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:

        . <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

      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=""


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

        <!-- FRAME TYPE DEFINITIONS (optional) -->

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

        <!-- METADATA DEFINITIONS (optional) -->

        <!--          -          -           LFB CLASS DEFINITIONS (optional) -->

   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"

   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

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

      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:

          <synopsis>IPv4 packet</synopsis>
            This frame type refers to an IPv4 packet.
          <synopsis>IPv6 packet</synopsis>
            This frame type refers to an IPv6 packet.

   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 attributes of LFB classes

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

      Each <dataTypeDef> element MUST contain 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:

          <synopsis>48-bit IEEE MAC address</synopsis>
          ... type definition ...
          <synopsis>IPv4 address</synopsis>
          ... type definition ...
      There are two kinds of data types: atomic and compound.  Atomic data
      types are appropriate for single-value variables (e.g. integer,
      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 unsigned integer
         boolean                  A true / false value where
                                  0 = false, 1 = true
         string[N]                ASCII null-terminated                A UTF-8 string with
                                  buffer of represented in at most
                                  N characters (string max
                                  length is N-1) Octets.
         string                   ASCII null-terminated                   A UTF-8 string without a configured
                                  storage length limitation limit.
         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
         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  Strings and octetstrings must be conveyed with
      their length, as they are not delimited, and are variable length.

      With regard to strings, this model defines a small set of the actual type
      restrictions and definitions on how they are structured.  String and
      octetstring length limits can be specified in the <dataTypeDef> element,
      the following LFB Class
      definitions.  The element properties for string and octetstring
      elements are available: <typeRef>, <atomic>, <array>,
      <struct>, also contain actual lengths and <union>.

      The predefined type alias length limits.  This
      duplication of limits is somewhere between to allow for implementations with smaller
      limits than the atomic and
      compound data types.  It behaves like a structure, one element maximum limits specified in the LFB Class
      definition.  In all cases, these lengths are specified in octets,
      not in characters.  In terms of
      which has special behavior.  Given that protocol operation, as long as the special behavior
      specified length is tied
      to within the other parts of FE’s supported capabilities, the structure, FE
      stores the compound result is treated
      as contents of a predefined construct.

   4.5.1. <typeRef> Element for Aliasing Existing Data Types

      The <typeRef> element refers to an existing data type string exactly as provided by its name.
      The referred data the CE, and
      returns those contents when requested.  No canonicalization,
      transformations, or equivalences are performed by the FE.  Elements
      of type MUST string (or string[n]) may be defined either used to hold identifiers for
      correlation with elements in other LFBs.  In such cases, an exact
      octet for octet match is used.  No equivalences are used by the same library
      document, FE
      or CE in one of performing that matching.  The ForCES protocol does not
      perform or require validation of the content of UTF-8 strings.
      However, UTF-8 strings SHOULD be encoded in the shortest form to
      avoid potential security issues described in [15].  Any entity
      displaying such strings is expected to perform its own validation
      (for example for correct multi-byte characters, and for ensuring
      that the string does not end in the middle of a multi-byte
      sequence.)  Specific LFB class definitions may restrict the valid
      contents of a string as suited to the particular usage (for example,
      an element that holds a DNS name would be restricted to hold only
      octets valid in such a name.)  FEs should validate the contents of
      SET requests for such restricted elements at the time the set is
      performed, just as range checks for range limited elements are
      performed.  The ForCES protocol behavior defines the normative
      processing for requests using that protocol.

      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.

   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:

        <synopsis>Alias to int16</synopsis>
        <synopsis>48-bit IEEE MAC address</synopsis>

   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 MUST be
      another atomic type.

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

        <synopsis>Diffserv code point.</synopsis>
            <allowedRange min="0" max="63"/>
            <specialValue value="0">
              <synopsis>Best Effort</synopsis>


   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
      <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 MUST always be a compound type, even if
      the array has a fixed size of 1.

      Arrays MUST 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 (".").  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:

          A table of 64 DSCP values, used to re-map code space.
        <array type="fixed-size" length="64">

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

        <synopsis>A table with up to 8 IEEE MAC addresses</synopsis>
        <array type="variable-size" max-length="8">

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

          A table of classification rules and result opcodes.
        <array type="variable-size">
            <element elementID="1">
              <synopsis>The rule to match</synopsis>
            <element elementID="2">
              <synopsis>The result code</synopsis>

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

      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.

          A table of information about known prefixes
        <array type="variable-size">
            <element elementID="1">
              <synopsis>the prefix being described</synopsis>
            <element elementID="2">
                  the protocol or process providing this information
            <element elementID="3">
              <synopsis>the information we care about</synopsis>
          <key keyID="1">
            <keyField> address-prefix.ipv4addr </keyField>
            <keyField> address-prefix.prefixlen </keyField>
            <keyField> source </keyField>

      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. 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.)

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

      Therefore, the value of a keyField element MUST be 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

      For a dataTypeDef 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.

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

      An example:

          IPv4 prefix defined by an address and a prefix length
          <element elementID="1">
            <synopsis>Address part</synopsis>
          <element elementID="2">
            <synopsis>Prefix length part</synopsis>
                <allowedRange min="0" max="32"/>

   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 MUST be 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. The content of an <alias> element MUST be a
      named type.  It can be a base type of a derived type.  The actual
      value referenced by an alias is known as its target.  When a GET or
      SET operation references the alias element, the value of the target
      is returned or replaced.  Write access to an alias element is
      permitted if write access to both the alias and the target are

      The target of an <alias> element is determined by its properties.
      Like all elements, the properties MUST include the support / read /
      write permission for the alias.  In addition, there are several
      fields in the properties which define the target of the alias.
      These fields are the ID of the LFB class of the target, the ID of
      the LFB instance of the target, and a sequence of integers
      representing the path within the target LFB instance to the target
      element.  The type of the target element must match the declared
      type of the alias.  Details of the alias property structure in in the
      section of this document on properties.

      Note that the read / write property of the alias refers to the
      value.  The CE can only determine if it can write the target
      selection properties of the alias by attempting such a write
      operation.  (Property elements do not themselves have properties.)

   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 element in
      a structure definition, indicating what structure type is being
      augmented.  Element names and element IDs within the augmentation
      must not be the same as those in the structure type being augmented.

   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 MUST contain 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 MUST also contain a brief
      synopsis, the mandatory tag value to be used for this metadata, an
      optional detailed description, and a mandatory type definition
      information. Only atomic data types can be used as value types for

      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.
      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:

          <synopsis>Refers to a Next Hop entry in NH LFB</synopsis>
            Result of classification (0 means no match).
              <specialValue value="0">
                  Classification didn’t result in match.

   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 MUST define an LFB class and include the following elements:

        . <name> provides the symbolic name of the LFB class.  Example:
        . <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.
        . <events> defines the events that can be generated by instances
          of this LFB.


      LFB class names should Class Names must be unique not only among classes
      defined in this document and unique, in all included documents, but also
      unique across order to enable other documents
      to reference the classes by name, and to enable human readers to
      understand references to class names.  While a large collection of libraries.  Obviously some global
      control complex naming
      structure could be created, simplicity is needed to ensure such uniqueness.  This subject requires
      further study.  The uniqueness preferred.  As given in the
      IANA considerations section of this document, the class IDs also requires further
      study.] IANA will maintain
      a registry of LFB Class names and Class identifiers, along with a
      reference to the document defining the class.

      Here is a skeleton of an example LFB class definition:

        <LFBClassDef LFBClassID="12345">
          <synopsis>IPv4 Longest Prefix Match Lookup LFB</synopsis>





            This LFB represents the IPv4 longest prefix match lookup
            The modeled behavior is as follows:


      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.

      [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 MUST contain 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.

      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:

          <synopsis>Normal input</synopsis>
              <ref dependency="optional" defaultValue="0">vrfid</ref>
        <inputPort group="yes">
          ... another input port ...

      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 is 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
      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:


      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

        <ref dependency="optional" defaultValue="0">vrfid</ref>

      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).

      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 MUST contain 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

      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:

          <synopsis>Normal output</synopsis>
        <outputPort group="yes">
          <synopsis>Exception output port group</synopsis>
              <ref availability="conditional">errorid</ref>
      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 selector
        . 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

      There may be various access permission restrictions on what the CE
      can do with an LFB attribute.  The following categories may be

        . 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:
        . 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 MUST contain the following elements:

        . <name> defines the name of the attribute.  This name must be
           unique among the attributes of the LFB class.  Example:
        . <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
           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

      The attribute element also MUST have an 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".

      Whether optional elements are supported, and whether elements
      defined as read-write can actually be written can be determined for
      a given LFB instance by the CE by reading the property information
      of that element.

      The following example defines two attributes for an LFB:

        <attribute access="read-only" elementID=”1”> elementID=’’1’’>
          <synopsis>number of things</synopsis>
        <attribute access="read-write write-only" elementID=”2”> elementID=’’2’’>
          <synopsis>number of this other thing</synopsis>
              <allowedRange min="10" max="2000"/>

      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
      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

      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 provides some flexibility for the FE
      implementation regarding how the LFB class is implemented.  For
      example, 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

      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 follow:

        . The version of the LFB class that this LFB instance complies
        . 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;
        . 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:

        <capability elementID="3">
            LFB class version this instance is compliant with.
        <capability elementID="4">
            Maximum value of the "bar" attribute.
   4.7.6. <events> Element for LFB Notification Generation

      The <events> element contains the information about the occurrences
      for which instances of this LFB class can generate notifications to
      the CE.

      The <events> definition needs a baseID attributevalue, which is
      normally <events baseID=”number”>. baseID=’’number’’>.  The value of the baseID is the
      starting element for the path which identifies events.  It must not
      be the same as the elementID of any top level attribute or
      capability of the LFB class.  In derived LFBs (i.e. ones with a
      <derivedFrom> element) where the parent LFB class has an events
      declaration, the baseID must not be present.  Instead, the value
      from the parent class is used.

      [editors note: There is an open issue with regard to how baseID is
      used for an LFBclass and another class derived from it.  Currently,
      the derived class does not declare a baseID.  It may make sense to
      instead to require the baseID attribute and require that it have the
      same value as the parent class events baseID.  Both choices
      (omission or inclusion of baseID in derived classes) leave room for
      errors not covered by the XML Schema.]

      The <events> element contains 0 or more <event> elements, each of
      which declares a single event.  The <event> element has an eventID
      attribute giving the unique ID of the event.  The element will

        . <eventTarget> element indicating which LFB field is tested to
           generate the event;
        . condition element indicating what condition on the field will
           generate the event from a list of defined conditions;
        . <eventReports> element indicating what values are to be
           reported in the notification of the event. <eventTarget> Element

      The <eventTarget> element contains information identifying a field
      in the LFB.  Specifically, the <target> element contains one or more
      <eventField> or <eventSubscript> elements.  These elements represent
      the textual equivalent of a path select component of the LFB. The
      <eventField> element contains the name of an element in the LFB or
      struct.  The first element in a <target> MUST be an <eventField>
      element and MUST name a field in the LFB.  The following element
      MUST identify a valid field within the containing context.  If an
      <eventField> identifies an array, and is not the last element in the
      target, then the next element MUST be an <eventSubscript>.
      <eventSubscript> elements MUST occur only after <eventField> names
      that identifies an array.  An <eventSubscript> may contain a numeric
      value to indicate that this event applies to a specific element of
      the array.  More commonly, the event is being defined across all
      elements of the array.  In that case, <eventSubscript> will contain
      a name.  The name in an <eventSubscript> element is not a field
      name.  It is a variable name for use in the <report> elements of
      this LFB definition.  This name MUST be distinct from any field name
      that can validly occur in the <eventReport> clause.  Hence it SHOULD
      be distinct from any field name used in the LFB or in structures
      used within the LFB.

      The <eventTarget> provides additional components for the path used
      to reference the event.  The path will be the baseID for events,
      followed by the ID for the specific event, followed by a value for
      each <eventSubscript> element in the <eventTarget>.  This will
      identify a specific occurrence of the event.  So, for example, it
      will appear in the event notification LFB.  It is also used for the
      SET-PROPERTY operation to subscribe to a specific event.  A SET-
      PROPERTY of the subscription property (but not of any other
      writeable properties) may be sent by the CE with any prefix of the
      path of the event.  So, for an event defined on a table, a SET-
      PROPERTY with a path of the baseID and the eventID will subscribe
      the CE to all occurrences of that event on any element of the table.
      This is particularly useful for the <eventCreated/> and
      <eventDestroyed/> conditions.  Events using those conditions will
      generally be defined with a field / subscript sequence that
      identifies an array and ends with an <eventSubscript> element.
      Thus, the event notification will indicate which array entry has
      been created or destroyed.  A typical subscriber will subscribe for
      the array, as opposed to a specific element in an array, so it will
      use a shorter path.

      Thus, if there is an LFB with an event baseID of 7, and a specific
      event with an event ID of 8, then one can subscribe to the event by
      referencing the properties of the LFB element with path 7.8.  If the
      event target has no subscripts (for example, it is a simple
      attribute of the LFB) then one can also reference the event
      threshold and filtering properties via the properties on element
      7.8.  If the event target is defined as an element of an array, then
      the target definition will include an <eventSubscript> element.  In
      that case, one can subscribe to the event for the entire array by
      referencing the properties of 7.8.  One can also subscribe to a
      specific element, x, of the array by referencing the subscription
      property of 7.8.x and also access the threshold and filtering
      properties of 7.8.x.  If the event is targeting an element of an
      array within an array, then there will be two (or conceivably more)
      <eventSubscript> elements in the target.  If so, for the case of two
      elements, one would reference the properties of 7.8.x.y to get to
      the threshold and filtering properties of an individual event.

      [Editors note: As currently defined, threshold

      Threshold and filtering conditions can only be applied to individual elements, not entire arrays.  Should
      events.  For events defined on elements of an array, this be changed to
      specification does not allow application to for defining a threshold or filtering
      condition on an array?  If so, we would
      add the complication event for all elements of having it potentially set differently on the
      element and the array as a whole.] an array. <events> Element Conditions

      The condition element represents a condition that triggers a
      notification.  The list of conditions is:

        . <eventCreated/> the target must be an array, ending with a
           subscript indication.  The event is generated when an entry in
           the array is created.  This occurs even if the entry is created
           by CE direction.
        . <eventDeleted/> the target must be an array, ending with a
           subscript indication.  The event is generated when an entry in
           the array is destroyed.  This occurs even if the entry is
           destroyed by CE direction.
        . <eventChanged/> the event is generated whenever the target
           element changes in any way.  For binary attributes such as
           up/down, this reflects a change in state.  It can also be used
           with numeric attributes, in which case any change in value
           results in a detected trigger.
        . <eventGreaterThan/> the event is generated whenever the target
           element becomes greater than the threshold.  The threshold is
           an event property.
        . <eventLessThan/> the event is generated whenever the target
           element becomes less than the threshold.  The threshold is an
           event property.

      As described in the Event Properties section, event items have
      properties associated with them.  These properties include the
      subscription information (indicating whether the CE wishes the FE to
      generate event reports for the event at all, thresholds for events
      related to level crossing, and filtering conditions that may reduce
      the set of event notifications generated by the FE.  Details of the
      filtering conditions that can be applied are given in that section.
      The filtering conditions allow the FE to suppress floods of events
      that could result from oscillation around a condition value.  For FEs
      that do not wish to support filtering, the filter properties can
      either be read only or not supported. <eventReports> Element

      The <eventReports> element of an <event> describes the information
      to be delivered by the FE along with the notification of the
      occurrence of the event.  The <reports> element contains one or more
      <eventReport> elements.  Each <report> element identifies a piece of
      data from the LFB to be reported.  The notification carries that
      data as if the collection of <eventReport> elements had been defined
      in a structure.  Each <eventReport> element thus MUST identify a
      field in the LFB.  The syntax is exactly the same as used in the
      <eventTarget> element, using <eventField> and <eventSubscript>
      elements.  <eventSubcripts> may contain integers.  If they contain
      names, they MUST be names from <eventSubscript> elements of the
      <eventTarget>.  The selection for the report will use the value for
      the subscript that identifies that specific element triggering the
      event.  This can be used to reference the structure / field causing
      the event, or to reference related information in parallel tables.
      This event reporting structure is designed to allow the LFB designer
      to specify information that is likely not known a priori by the CE
      and is likely needed by the CE to process the event.  While the
      structure allows for pointing at large blocks of information (full
      arrays or complex structures) this is not recommended.  Also, the
      variable reference / subscripting in reporting only captures a small
      portion of the kinds of related information.  Chaining through index
      fields stored in a table, for example, is not supported.  In
      general, the <eventReports> mechanism is an optimization for cases
      that have been found to be common, saving the CE from having to
      query for information it needs to understand the event.  It does not
      represent all possible information needs.

      If any elements referenced by the eventReport are optional, then the
      report MUST support optional elements.  Any components which do not
      exist are not reported.

   4.7.7. <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.


      Elements of LFBs have properties which are important to the CE.  The
      most important property is the existence / readability /
      writeability of the element.  Depending up the type of the element,
      other information may be of importance.

      The model provides the definition of the structure of property
      information.  There is a base class of property information.  For
      the array, alias, and event elements there are subclasses of
      property information providing additional fields.  This information
      is accessed by the CE (and updated where applicable) via the PL
      protocol.  While some property information is writeable, there is no
      mechanism currently provided for checking the properties of a
      property element.  Writeability can only be checked by attempting to
      modify the value.


   4.8.1. Basic Properties

      The basic property definition, along with the scalar for
      accessibility is below.  Note that this access permission
      information is generally read-only.

                 The possible values of attribute access permission
                   <specialValue value="0">
                     <synopsis>Access is prohibited</synopsis>
                    <specialValue value="1">
                     <name> Read-Only </name>
                     <synopsis>Access is read only</synopsis>
                   <specialValue value="2">
                       The attribute may be written, but not read
                   <specialValue value="3">
                       The attribute may be read or written

               <synopsis>basic properties, accessibility</synopsis>
                 <element elementID="1">
                       does the element exist, and
                       can it be read or written

   4.8.2. Array Properties

      The properties for an array add a number of important pieces of
      information.  These properties are also read-only.

               <element elementID=”2”> elementID=’’2’’>
                 <synopsis>the number of entries in the array</synopsis>
               <element elementID=”3”> elementID=’’3’’>
                 <synopsis>the last used subscript in the array</synopsis>
               <element elementID=”4”> elementID=’’4’’>
                   The subscript of the first unused array element

  4.8.3 Event

   4.8.3. String Properties

      The properties of a string specify the actual octet length and the
      maximum octet length for the element.  The maximum length is
      included because an FE implementation may limit a string to be
      shorter than the limit in the LFB Class definition.

               <element elementID=’’2’’>
                 <synopsis>the number of octets in the string</synopsis>
               <element elementID=’’3’’>
                   the maximum number of octets in the string

   4.8.4. Octetstring Properties

      The properties of an octetstring specify the actual length and the
      maximum length, since the FE implementation may limit an octetstring
      to be shorter than the LFB Class definition.

               <element elementID=’’2’’>
                   the number of octets in the octetstring
               <element elementID=’’3’’>
                   the maximum number of octets in the octetstring

   4.8.5. Event Properties

      The properties for an event add three (usually) writeable fields.
      One is the subscription field.  0 means no notification is
      generated.  Any non-zero value (typically 1 is used) means that a
      notification is generated.  The hysteresis field is used to suppress
      generation of notifications for oscillations around a condition
      value, and is described in the text for events.  The threshold field
      is used for the <eventGreaterThan/> and <eventLessThan/> conditions.
      It indicates the value to compare the event target against.  Using
      the properties allows the CE to set the level of interest.  FEs
      which do not supporting setting the threshold for events will make
      this field read-only.

               <element elementID=”2”> elementID=’’2’’>
                   has the CE registered to be notified of this event
               <element elementID=”3”> elementID=’’3’’>
                 <synopsis> comparison value for level crossing events
               <element elementID=”4”> elementID=’’4’’>
                 <synopsis> region to suppress event recurrence notices
               <element elementID=”5”> elementID=’’5’’>
                 <synopsis> number of occurrences to suppress
               <element elementID=”6”> elementID=’’6’’>
                 <synopsis> time interval in ms between notifications
           <dataTypeDef> Common Event Filtering

      The event properties have values for controlling several filter
      conditions.  Support of these conditions is optional, but all
      conditions SHOULD be supported.  Events which are reliably known not
      to be subject to rapid occurrence or other concerns may not support
      all filter conditions.

      Currently, three different filter condition variables are defined.
      These are eventCount, eventInterval, and eventHysteris. eventHysteresis.  Setting
      the condition variables to 0 (their default value) means that the
      condition is not checked.

      Conceptually, when an event is triggered, all configured conditions
      are checked.  If no filter conditions are triggered, or if any
      trigger conditions are met, the event notification is generated.  If
      there are filter conditions, and no condition is met, then no event
      notification is generated.  Event filter conditions have reset
      behavior when an event notification is generated.  If any condition
      is passed, and the notification is generated, the the notification reset
      behavior is performed on all conditions, even those which had not
      passed.  This provides a clean definition of the interaction of the
      various event conditions.

      An example of the interaction of conditions is an event with an
      eventCount property set to 5 and an eventInterval property set to
      500 milliseconds.  Suppose that a burst of occurrences of this event
      is detected by the FE.  The first occurrence will cause a
      notification to be sent to the CE.  Then, if four more occurrences
      are detected rapidly (less than 0.5 seconds) they will not result in
      notifications.  If two more occurrences are detected, then the
      second of those will result in a notification.  Alternatively, if
      more than 500 miliseconds milliseconds has passed since the notification and an
      occurrence is detected, that will result in a notification.  In
      either case, the count and time interval suppression is reset no
      matter which condition actually caused the notification. Event Hysteresis Filtering

      Events with numeric conditions can have hysteresis filters applied
      to them.  The hystersis hysteresis level is defined by a property of the
      event.  This allows the FE to notify the CE of the hysteresis
      applied, and if it chooses, the FE can allow the CE to modify the
      hysteresis.  This applies to <eventChanged/> for a numeric field,
      and to <eventGreaterThan/> and <eventLessThan/>.  The content of a
      <variance> element is a numeric value.  When supporting hysteresis,
      the FE MUST track the value of the element and make sure that the
      condition has become untrue by at least the hysteresis from the
      event property.  To be specific, if the hysteresis is V, then

        . For a <eventChanged/> condition, if the last notification was
           for value X, then the <changed/> notification MUST NOT be
           generated until the value reaches X +/- V.
        . For a <eventGreaterThan/> condition with threshold T, once the
           event has been generated at least once it MUST NOT be generated
           again until the field first becomes less than or equal to T – -                                                                       -
           V, and then exceeds T.
        . For a <eventLessThan/> condition with threshold T, once the
           event has been generate at least once it MUST NOT be generated
           again until the field first becomes greater than or equal to T
           + V, and then becomes less than T. Event Count Filtering

      Events may have a count filtering condition.  This property, if set
      to a non-zero value, indicates the number of occurrences of the event
      that should be considered redundant and not result in a notification.
      Thus, if this property is set to 1, and no other conditions apply,
      then every other detected occurrence of the event will result in a
      notification.  This particular meaning is chosen so that the value 1
      has a distinct meaning from the value 0.

      A conceptual implementation (not required) for this might be an
      internal suppression counter.  Whenever an event is triggered, the
      counter is checked.  If the counter is 0, a notification is
      generated.  Whether a notification is generated or not, the counter
      is incremented.  If the counter exceeds the configured value, it is
      reset to 0.  In this conceptual implementation the reset behavior
      when a notification is generated can be thought of as setting the
      counter to 1.

      [Editor’s note: a better description of the conceptual algorithm is
      sought.] Event Time Filtering

      Events may have a time filtering condition.  This property
      represents the minimum time interval (in the absence of some other
      filtering condition being passed) between generating notifications of
      detected events.  This condition MUST only be passed if the time
      since the last notification of the event is longer than the
      configured interval in milliseconds.

      Conceptually, this can be thought of as a stored timestamp which is
      compared with the detection time, or as a timer that is running that
      resets a suppression flag.  In either case, if a notification is
      generated due to passing any condition then the time interval
      detection MUST be restarted.


   4.8.6. Alias Properties

      The properties for an alias add three (usually) writeable fields.
      These combine to identify the target element the subject alias
      refers to.

               <element elementID=”2”> elementID=’’2’’>
                 <synopsis>the class ID of the alias target</synopsis>
               <element elementID=”3”> elementID=’’3’’>
                 <synopsis>the instand instance ID of the alias target</synopsis>
               <element elementID=”4”> elementID=’’4’’>
                   the path to the element target
                   each 4 octets is read as one path element,
                   using the path construction in the PL protocol.

   4.9. XML Schema for LFB Class Library Documents

      <?xml version="1.0" encoding="UTF-8"?>
      <xsd:schema xmlns:xsd=""
        <xsd:documentation xml:lang="en">
        Schema for Defining LFB Classes and associated types (frames,
        data types for LFB attributes, and metadata).
      <xsd:element name="description" type="xsd:string"/>
      <xsd:element name="synopsis" type="xsd:string"/>
      <!-- Document root element: LFBLibrary -->
      <xsd:element name="LFBLibrary">
            <xsd:element ref="description" minOccurs="0"/>
            <xsd:element name="load" type="loadType" minOccurs="0"
            <xsd:element name="frameDefs" type="frameDefsType"
            <xsd:element name="dataTypeDefs" type="dataTypeDefsType"
            <xsd:element name="metadataDefs" type="metadataDefsType"
            <xsd:element name="LFBClassDefs" type="LFBClassDefsType"
          <xsd:attribute name="provides" type="xsd:Name" use="required"/>
        <!-- Uniqueness constraints -->
        <xsd:key name="frame">
          <xsd:selector xpath="lfb:frameDefs/lfb:frameDef"/>
          <xsd:field xpath="lfb:name"/>
        <xsd:key name="dataType">
          <xsd:selector xpath="lfb:dataTypeDefs/lfb:dataTypeDef"/>
          <xsd:field xpath="lfb:name"/>
        <xsd:key name="metadataDef">
          <xsd:selector xpath="lfb:metadataDefs/lfb:metadataDef"/>
          <xsd:field xpath="lfb:name"/>
        <xsd:key name="LFBClassDef">
          <xsd:selector xpath="lfb:LFBClassDefs/lfb:LFBClassDef"/>
          <xsd:field xpath="lfb:name"/>
      <xsd:complexType name="loadType">
        <xsd:attribute name="library" type="xsd:Name" use="required"/>
        <xsd:attribute name="location" type="xsd:anyURI" use="optional"/>
      <xsd:complexType name="frameDefsType">
          <xsd:element name="frameDef" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element ref="description" minOccurs="0"/>
      <xsd:complexType name="dataTypeDefsType">
          <xsd:element name="dataTypeDef" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element ref="description" minOccurs="0"/>
                <xsd:group ref="typeDeclarationGroup"/>
         Predefined (built-in) atomic data-types are:
             char, uchar, int16, uint16, int32, uint32, int64, uint64,
             string[N], string, byte[N], boolean, octetstring[N] octetstring[N],
             float16, float32, float64
      <xsd:group name="typeDeclarationGroup">
          <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="alias" type="typeRefNMTOKEN"/>
      <xsd:simpleType name="typeRefNMTOKEN">
        <xsd:restriction base="xsd:token">
          <xsd:pattern value="\c+"/>
          <xsd:pattern value="string\[\d+\]"/>
          <xsd:pattern value="byte\[\d+\]"/>
          <xsd:pattern value="octetstring\[\d+\]"/>
      <xsd:complexType name="atomicType">
          <xsd:element name="baseType" type="typeRefNMTOKEN"/>
          <xsd:element name="rangeRestriction"
                       type="rangeRestrictionType" minOccurs="0"/>
          <xsd:element name="specialValues" type="specialValuesType"
      <xsd:complexType name="rangeRestrictionType">
          <xsd:element name="allowedRange" maxOccurs="unbounded">
              <xsd:attribute name="min" type="xsd:integer"
              <xsd:attribute name="max" type="xsd:integer"
      <xsd:complexType name="specialValuesType">
          <xsd:element name="specialValue" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
              <xsd:attribute name="value" type="xsd:token"/>
      <xsd:complexType name="arrayType">
          <xsd:group ref="typeDeclarationGroup"/>
          <xsd:element name="contentKey" minOccurs="0"
                <xsd:element name="contentKeyField" maxOccurs="unbounded"
              <xsd:attribute name="contentKeyID" use="required"
            <!--declare keys to have unique IDs -->
            <xsd:key name="contentKeyID">
              <xsd:selector xpath="lfb:contentKey"/>
              <xsd:field xpath="@contentKeyID"/>
        <xsd:attribute name="type" use="optional"
            <xsd:restriction base="xsd:string">
              <xsd:enumeration value="fixed-size"/>
              <xsd:enumeration value="variable-size"/>
        <xsd:attribute name="length" type="xsd:integer" use="optional"/>
        <xsd:attribute name="maxLength" type="xsd:integer"
      <xsd:complexType name="structType">
          <xsd:element name="derivedFrom" type="typeRefNMTOKEN"
          <xsd:element name="element" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element name="optional" minOccurs="0"/>
                <xsd:group ref="typeDeclarationGroup"/>
              <xsd:attribute name="elementID" use="required"
            <!-- key declaration to make elementIDs unique in a struct
            <xsd:key name="structElementID">
              <xsd:selector xpath="lfb:element"/>
              <xsd:field xpath="@elementID"/>
      <xsd:complexType name="metadataDefsType">
          <xsd:element name="metadataDef" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element name="metadataID" type="xsd:integer"/>
                <xsd:element ref="description" minOccurs="0"/>
                  <xsd:element name="typeRef" type="typeRefNMTOKEN"/>
                  <xsd:element name="atomic" type="atomicType"/>
      <xsd:complexType name="LFBClassDefsType">
          <xsd:element name="LFBClassDef" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element name="version" type="versionType"/>
                <xsd:element name="derivedFrom" type="xsd:NMTOKEN"
                <xsd:element name="inputPorts" type="inputPortsType"
                <xsd:element name="outputPorts" type="outputPortsType"
                <xsd:element name="attributes" type="LFBAttributesType"
                <xsd:element name="capabilities"
                             type="LFBCapabilitiesType" minOccurs="0"/>
                <xsd:element name="events"
                             type="eventsType" minOccurs="0"/>
                <xsd:element ref="description" minOccurs="0"/>
              <xsd:attribute name="LFBClassID" use="required"
            <!-- 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 name="capabilities">
              <xsd:selector xpath="lfb:capabilities/lfb:capability"/>
              <xsd:field xpath="lfb:name"/>
            <!-- 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 name="capabilityIDs">
              <xsd:selector xpath="lfb:attributes/lfb:capability"/>
              <xsd:field xpath="@elementID"/>
      <xsd:simpleType name="versionType">
        <xsd:restriction base="xsd:NMTOKEN">
          <xsd:pattern value="[1-9][0-9]*\.([1-9][0-9]*|0)"/>
      <xsd:complexType name="inputPortsType">
          <xsd:element name="inputPort" type="inputPortType"
      <xsd:complexType name="inputPortType">
          <xsd:element name="name" type="xsd:NMTOKEN"/>
          <xsd:element ref="synopsis"/>
          <xsd:element name="expectation" type="portExpectationType"/>
          <xsd:element ref="description" minOccurs="0"/>
        <xsd:attribute name="group" type="booleanType" use="optional"
      <xsd:complexType name="portExpectationType">
          <xsd:element name="frameExpected" minOccurs="0">
                <!-- ref must refer to a name of a defined frame type -->
                <xsd:element name="ref" type="xsd:string"
          <xsd:element name="metadataExpected" minOccurs="0">
              <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"
      <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: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:complexType name="metadataInputRefType">
          <xsd:extension base="xsd:NMTOKEN">
            <xsd:attribute name="dependency" use="optional"
                <xsd:restriction base="xsd:string">
                  <xsd:enumeration value="required"/>
                  <xsd:enumeration value="optional"/>
            <xsd:attribute name="defaultValue" type="xsd:token"
      <xsd:complexType name="outputPortsType">
          <xsd:element name="outputPort" type="outputPortType"
      <xsd:complexType name="outputPortType">
          <xsd:element name="name" type="xsd:NMTOKEN"/>
          <xsd:element ref="synopsis"/>
          <xsd:element name="product" type="portProductType"/>
          <xsd:element ref="description" minOccurs="0"/>
        <xsd:attribute name="group" type="booleanType" use="optional"
      <xsd:complexType name="portProductType">
          <xsd:element name="frameProduced">
                <!-- ref must refer to a name of a defined frame type
                <xsd:element name="ref" type="xsd:NMTOKEN"
          <xsd:element name="metadataProduced" minOccurs="0">
              <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"
      <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: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:complexType name="metadataOutputRefType">
          <xsd:extension base="xsd:NMTOKEN">
            <xsd:attribute name="availability" use="optional"
                <xsd:restriction base="xsd:string">
                  <xsd:enumeration value="unconditional"/>
                  <xsd:enumeration value="conditional"/>
      <xsd:complexType name="LFBAttributesType">
          <xsd:element name="attribute" maxOccurs="unbounded">
                <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"
              <xsd:attribute name="access" use="optional"
                  <xsd:list itemType="accessModeType"/>
              <xsd:attribute name="elementID" use="required"
      <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:complexType name="LFBCapabilitiesType">
          <xsd:element name="capability" maxOccurs="unbounded">
                <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:attribute name="elementID" use="required"
      <xsd:complexType name="eventsType">
          <xsd:element name="event" maxOccurs="unbounded">
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element name="eventTarget" type="eventPathType"/>
                <xsd:element ref="eventCondition"/>
                <xsd:element name="eventReports" type="eventReportsType"
                <xsd:element ref="description" minOccurs="0"/>
              <xsd:attribute name="eventID" use="required"
        <xsd:attribute name="baseID" type="xsd:integer"

      <!-- the substitution group for the event conditions -->
      <xsd:element name="eventCondition" abstract="true"/>
      <xsd:element name="eventCreated"
      <xsd:element name="eventDeleted"
      <xsd:element name="eventChanged"
      <xsd:element name="eventGreaterThan"
      <xsd:element name="eventLessThan"
      <xsd:complexType name="eventPathType">
          <xsd:element ref="eventPathPart" maxOccurs="unbounded"/>
      <!-- the substitution group for the event path parts -->
      <xsd:element name="eventPathPart" type="xsd:string"
      <xsd:element name="eventField" type="xsd:string"
      <xsd:element name="eventSubscript" type="xsd:string"
      <xsd:complexType name="eventReportsType">
          <xsd:element name="eventReport" type="eventPathType"
      <xsd:simpleType name="booleanType">
        <xsd:restriction base="xsd:string">
          <xsd:enumeration value="0"/>
          <xsd:enumeration value="1"/>

   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 MUST 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.

      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=""
      <!--        -        -         xmlns and schemaLocation need to be fixed -->
               <synopsis>Describing the Adjacent LFB</synopsis>
                 <element elementID="1">
                   <synopsis>ID for that LFB Class</synopsis>
                 <element elementID="2">
                     the ports on which we can connect
                   <array type="variable-size">
                 Limits on the number of ports in a given group
                 <element elementID="1">
                   <synopsis>Group Name</synopsis>
                 <element elementID="2">
                   <synopsis>Minimum Port Count</synopsis>
                 <element elementID="3">
                   <synopsis>Max Port Count</synopsis>
               <synopsis>table entry for supported LFB</synopsis>
                 <element elementID="1">
                     The name of a supported LFB Class
                 <element elementID="2">
                   <synopsis>the id of a supported LFB Class</synopsis>
                 <element elementID="3"> elementID=’’3’’>
                     The version of the LFB Class used
                     by this FE.
                 <element elementID="4">
                     the upper limit of instances of LFBs of this class
                 <!-- For each port group, how many ports can exist
                 <element elementID="4"> elementID="5">
                   <synopsis>Table of Port Group Limits</synopsis>
                   <array type="variable-size">
      <!-- for the named LFB Class, the LFB Classes it may follow -->
                 <element elementID="5"> elementID="6">
                     List of LFB Classes that this LFB class can follow
                   <array type="variable-size">
      <!-- for the named LFB Class, the LFB Classes that may follow it
                 <element elementID="6"> elementID="7">
                     List of LFB Classes that can follow this LFB class
                   <array type="variable-size">
               <synopsis>The possible values of status</synopsis>
                   <specialValue value="0">
                       FE is administratively disabled
                   <specialValue value="1">
                     <synopsis>FE is operatively disabled</synopsis>
                   <specialValue value="2">
                     <synopsis>FE is operating</synopsis>
               <synopsis>Details of the FE's Neighbor</synopsis>
                 <element elementID="1">
                   <synopsis>Neighbors FEID</synopsis>
                 <element elementID="2">
                     FE's interface that connects to this neighbor
                 <element elementID="3">
                   <name>NeighborNetworkAddress</name> elementID=’’3’’>
                     The network layer address name of the neighbor.
                      Presumably, the network type can be
                      determined from the interface information.
                 <element elementID="4">
                     The media access control address of on the neighbor.

                     Again, it neighbor to
                     which this FE is presumed the type can be determined
                     from the interface information. adjacent.  This is required
                     In case two FE’s are adjacent on more than
                     one interface.
                 Unique identification of an LFB class-instance
                 <element elementID="1">
                   <synopsis>LFB Class Identifier</synopsis>
                 <element elementID="2">
                   <synopsis>LFB Instance ID</synopsis>
                 Link between two LFB instances of topology
                 <element elementID="1">
                   <synopsis>LFB src</synopsis>
                 <element elementID="2">
                   <synopsis>src port group</synopsis>
                 <element elementID="3">
                   <synopsis>src port index</synopsis>
                 <element elementID="4">
                   <synopsis>dst LFBID</synopsis>
                 <element elementID="5">
                   <synopsis>dst port group</synopsis>
                 <element elementID="6">
                   <synopsis>dst port index</synopsis>
             <LFBClassDef LFBClassID="1">
               <synopsis>Core LFB: FE Object</synopsis>
                 <attribute access="read-write" elementID="1">
                   <synopsis>the table of known Topologies</synopsis>
                   <array type="variable-size">
                 <attribute access="read-write" elementID="2">
                      table of known active LFB classes and
                   <array type="variable-size">
                 <attribute access="read-write" elementID="3">
                   <synopsis>name of this FE</synopsis>
                 <attribute access="read-write" elementID="4">
                   <synopsis>ID of this FE</synopsis>
                 <attribute access="read-only" elementID="5">
                   <synopsis>vendor of this FE</synopsis>
                 <attribute access="read-only" elementID="6">
                   <synopsis>model of this FE</synopsis>
                 <attribute access="read-only" elementID="7">
                   <synopsis>model of this FE</synopsis>
                 <attribute access="read-write" elementID="8">
                   <synopsis>table of known neighbors</synopsis>
                   <array type="variable-size">
                 <capability elementID="30">
                     Whether Modifiable LFB is supported
                 <capability elementID="31">
                   <synopsis>List of all supported LFBs</synopsis>
                   <array type="variable-size">

   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 and
      SupportedLFBs.  Information as to which attributes of the FE LFB are
      supported is accessed by the properties information for those

   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.
      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. LFBName

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

      This element, if present, indicates the largest number of instances LFBClassID

      The numeric ID of this LFB class the FE can support.  For FEs that do not have the
      capability to create or destroy LFB instances, Class being described.  While conceptually
      redundant with the LFB Name, both are included for clarity and to
      allow consistency checking. LFBVersion

      The version string specifying the LFB Class version supported by
      this FE.  As described above in versioning, an FE can support only a
      single version of a given LFB Class. 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. 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

      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. 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
      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] 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

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

      This element contains

      While it would be desirable to include class capability level
      information, this is not included in the model.  While such
      information about belongs in the subject LFB FE Object in the supported class whose structure table,
      the contents of that information would be class specific.  The
      currently expected encoding structures for transferring information
      between the CE and semantics FE are defined by such that allowing completely unspecified
      information would be likely to induce parse errors.  We could
      specify that the information is encoded in an octetstring, but then
      we would have to define the internal format of that octet string.

      As there also are not currently any defined LFB class

      [Note:  Important Omissions]

      However, Class level
      Capabilities that the FE needs to report, this element does information is not appear
      present now, but may be added in a future version of the definition, because FE Protocol
      Object.  (This is an example of a case where versioning, rather than
      inheritance, would be needed, since the
      author FE Object must have class ID
      1 and instance ID 1 so that the protocol behavior can not figure out how to write it. start by
      finding this object.)

   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

   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.
      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 and 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

      While there may be many ways to configure neighbors, the table FE-ID is intended
      the best way for the CE to correlate entities.  And the interface
      identifier (name string) is the best correlator.  The CE will be used in situations where neighbors are configured by MAC address.
      Resolution of network layer
      able to MAC determine the IP address information should be
      captured in ARP LFBs and not duplicated in this table.  Note that media level information about
      the same neighbor may be reached through multiple interfaces or at
      multiple addresses.  There is no uniqueness requirement of any sort
      on occurrences of from the FENeighbors element. neighbor directly.  Omitting that information
      from this table avoids the risk of incorrect double configuration.

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

      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.

      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

      This identifies the various possible neighbor interfaces, include physical
      interfaces, logical tunnels, virtual circuits, etc.] NeighborNetworkAddress

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

      Neighbors are sometimes configured using MAC level addresses
      (Ethernet MAC address, circuit identifiers, etc.)  If such addresses
      are used to configure neighbor through which the adjacency, then that information
      neighbor is stored
      here.  Note that over some ports such as physical point to point
      links or virtual circuits considered as individual interfaces, there reached.  The interface identification is no need for needed when
      either form only one side of address. the adjacency has configuration information,
      or the two FEs are adjacent on more than one interface.

   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).

      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 only the definition of the minimal FE Object LFB
      itself.  Meeting the full set of LFBs
      that is required by Section 5.5 of RFC 3564[1]. working group requirements requires
      other LFBs.  The sections that
      follow provide more detail on the specifics of each of class definitions for 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

   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

      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

   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 will be used
      to support accounting functions provided
      in the FE.

      [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. documents.

   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

         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 inter-FE topology (item 1 above) can be determined by the FE model covers all of them except
      item 1), which concerns CE in
      many ways.  Neither this document nor the inter-FE topology. Forces protocol mandates a
      specific mechanism.  The FE model focuses
      on the LFB Class definition does include the
      capability for an FE to be configured with, and LFB topology within a single FE.  Since provides to the
      information related CE
      in response to item 1) requires global knowledge about all
      of a query, the FEs and their inter-connection with each other, this exchange
      is part identity of its neighbors. There may
      also be defined specific LFB classes and protocols for neighbor
      discovery.  Routing protocols may be used by the ForCES base protocol instead of CE for adjacency
      determination.  The CE may be configured with the FE model. relevant

      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

      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

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

   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

         |  +---------+   +------------+   +---------+         |
       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.

      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.

        . 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.
      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,

      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

      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
      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

   7.6. LFB Attribute Manipulation

      This is a place-holder

      The FE Model provides for all operations that the CE will use to
      populate, manipulate, and delete attributes definition of the LFB instances on
      the FEs.  These operations allow Classes.  Each class
      has a globally unique identifier.  Elements within the CE to configure an individual class are
      assigned identifiers within that scope.  This model also specifies
      that instances of LFB instance. Classes have identifiers.  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 combination of feedback from the FE to the CE (e.g.,
      request received, configuration completed), as well as multi-
      attribute configuration transactions with atomic commit
      class identifiers, instance identifiers, and
      rollback, may be necessary in some circumstances.

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

   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. Example

      This section contains an example LFB definition.  While some
      properties of LFBs are shown by the FE Object LFB, this endeavors to
      show how a data plain LFB might be build.  This example is a
      fictional case of an interface supporting a coarse WDM optical
      interface carry Frame Relay traffic.  The statistical information
      (including error statistics) is omitted.)

      <?xml version="1.0" encoding="UTF-8"?>
      <LFBLibrary xmlns=""
                A frame relay frame, with DLCI without
             <synopsis>An IP Packet</synopsis>
                Information about a single CWDM frequency
              <element elementID="1">
                <synopsis>encoded frequency(channel)</synopsis>
              <element elementID="2">
                <synopsis>state of this frequency</synopsis>
              <element elementID="3">
                <synopsis>current observed power</synopsis>
              <element elementID="4">
                    Information about circuits on this Frequency
                Information about a single Frame Relay circuit
              <element elementID="1">
                <synopsis>DLCI of the circuit</synopsis>
              <element elementID="2">
                <synopsis>state of the circuit</synopsis>
              <element elementID="3">
                <synopsis>is this the LMI circuit</synopsis>
              <element elementID="4">
                    which input / output port is associated
                    with this circuit
                The possible values of status.  Used for both
                administrative and operation status
                <specialValue value="0">
                  <name>Disabled </name>
                  <synopsis>the component is disabled</synopsis>
                <specialValue value="1">
                  <synopsis>FE is operatively disabled</synopsis>
            <synopsis>The DLCI the frame arrived on</synopsis>
            <synopsis>The index of the laser channel</synopsis>
          <LFBClassDef LFBClassID="-255">
            <synopsis>Fictional LFB for Demonstartions</synopsis> Demonstrations</synopsis>
              <inputPort group="yes">
                    Ports for LMI traffic, for transmission
                    Ports for data to be sent on circuits
              <outputPort group="yes">
                    Ports for LMI traffic for processing
              <outputPort group="yes">
                    Ports for Data traffic for processing
              <attribute access="read-write" elementID="1">
                <synopsis>is this port allowed to function</synopsis>
              <attribute access="read-write" elementID="2">
                    table of information per CWDM frequency
                <array type="variable-size">
              <capability elementID="31">
                    whether the port over all is operational
              <capability elementID="32">
                    how many laser frequencies are there
              <capability elementID="33">
                    Total supportable Frame Relay Circuits, across
                    all laser frequencies
            <events baseID="61">
              <event eventID="1">
                    The state of a frequency has changed
                  <!-- report the new state -->
              <event eventID="2">
                <synopsis>A new frequency has appeared</synopsis>
              <event eventID="3">
                    A frequency Table entry has been deleted
              <event eventID="4">
                    there are problems with the laser power level
              <event eventID="5">
                    the state of an Fr circuit on a frequency
                    has changed

   8.1.1. Data Handling

      This LFB is designed to handle data packets coming in from or going
      out to the external world.  It is not a full port, and it lacks many
      useful statistics.  But it serves to show many of the relevant

      Packets arriving without error from the physical interface come in
      on a Frame Relay DLCI on a laser channel.  These two values are used
      by the LFB too look up the handling for the packet.  If the handling
      indicates that the packet is LMI, then the output index is used to
      select an LFB port from the LMItoFE port group.  The packet is sent
      as a full Frame Relay frame (without any bit or byte stuffing) on
      the selected port.  The laser channel and DLCI are sent as meta-
      data, even though the DLCI is also still in the packet.

      Good packets that arrive and are not LMI and have a frame relay type
      indicator of IP are sent as IP packets on the port in the DatatoFE
      port group, using the same index field from the table based on the
      laser channel and DLCI.  The channel and DLCI are attached as meta-
      data for other use (classifiers, for example.)

      The current definition does not specify what to do if the Frame
      Relay type information is not IP.

      Packets arriving on input ports arrive with the Lasesr Laser Channel and
      Frame Relay DLCI as meta-data.  As such, a single input port could
      have been used.  With the structure that is defined (which parallels
      the output structure), the selection of channel and DLCI could be
      restricted by the arriving input port group (LMI vs vs. data) and port
      index.  As an alternative LFB design, the structures could require a
      1-1 relationship between DLCI and LFB port, in which case no meta-
      data would be needed.  This would however be quite complex and
      noisy.  The intermediate level of structure here allows parallelism
      between input and output, without requiring excessive ports.


   8.1.2. Setting up a DLCI

      When a CE chooses to establish a DLCI on a specific laser channel,
      it sends a SET request directed to this LFB.  The request might look

        T = PATH-DATA
          Path: flags = first-avail, length = 4, path = 2, channel, 4
          DataRaw: DLCI, Enable(1), false, out-idx

      Which would esbalish establish the DLCI as enabled, with traffic going to a
      specific element of the output port group DatatoFE.  (The CE would
      ensure that output port is connected to the right place before
      issuing this request.

      The response to the operation would include the actual index
      assigned to this Frame Relay circuit.  This table is structured to
      use separate internal indices and DLCIs.  An alternative design
      could have used the DLCI as index, trading off complexities.

      One could also imagine that the FE has an LMI LFB.  Such an LFB
      would be connected to the LMItoFE and LMIfromFE port groups.  It
      would process LMI information.  It might be the LFBs job to set up
      the frame relay circuits.  The LMI LFB would have an alias entry
      that points to the Frame Relay circuits table it manages, so that it
      can manipulate those entities.


   8.1.3. Error Handling

      The LFB will receive invalid packets over the wire.  Many of these
      will simply result in incrementing counters.  The LFB designer might
      also specify some error rate measures.  This puts more work on the
      FE, but allows for more meaningful alarms.

      There may be some error conditions that should cause parts of the
      packet to be sent to the CE.  The error itself is not something that
      can cause an event in the LFB.  There are two ways this can be

      One way is to define a specific field to count the error, and a
      field in the LFB to hold the required portion of the packet.  The
      field could be defined to hold the portion of the packet from the
      most recent error.  One could then define an event that occurs
      whenever the error count changes, and declare that reporting the
      event includes the LFB field with the packet portion.  For rare but
      extremely critical errors, this is an effective solution.  It
      ensures reliable delivery of the notification.  And it allows the CE
      to control if it wants the notification.  (Use of the event variance
      property would suppress multiple notifications.  It would suppress
      them even if they were many hours apart, so the CE is unlikely to
      use that.)

      Another approach is for the LFB to have a port that connects to a
      redirect sink.  The LFB would attach the laser channel, the DLCI,
      and the error indication as meta-data, and ship the packet to the

      Other aspects of error handling are discussed under events below.

   8.2. LFB Attributes

      This LFB is defined to have two top level attributes.  One reflects
      the administrative state of the LFB.  This allows the CE to disable
      the LFB completely.

      The other attribute is the table of information about the laser
      channels.  It is a variable sized array.  Each array entry contains
      an identifier for what laser frequency this entry is associated
      with, whether that frequency is operational, the power of the laser
      at that frequency, and a table of information about frame relay
      circuits on this frequency.  There is no administrative status since
      a CE can disable an entry simply by removing it.  (Frequency and
      laser power of a non-operational channel are not particularly
      useful.  Knowledge about what frequencies can be supported would be
      a table in the capabilities section.)

      The Frame Relay circuit information contains the DLCI, the
      operational circuit status, whether this circuit is to be treated as
      carrying LMI information, and which port in the output port group of
      the LFB traffic is to be sent to.  As mentioned above, the circuit
      index could, in some designs, be combined with the DLCI.

   8.3. Capabilities

      The capability information for this LFB includes whether the
      underlying interface is operational, how many frequencies are
      supported, and how many total circuits, across all channels, are
      permitted.  The maximum number for a given laser channel can be
      determined from the properties of the FrameRelayCircuits table.  A
      GET-Properties on path will give the CE the properties
      of the array which include the number of entries used, the first
      available entry, and the maximum number of entries permitted.

   8.4. Events

      This LFB is defined to be able to generate several events that the
      CE may be interested in.  There are events to report changes in
      operational state of frequencies, and the creation and deletion of
      frequency entries.  There is an event for changes in status of
      individual frame relay circuits.  So an event notification of would indicate that there had been a circuit status change
      on subscript 11 of the circuit table in subscript 3 of the frequency
      table.  The event report would include the new status of the circuit
      and the DLCI of the circuit.  Arguably, the DLCI is redundant, since
      the CE presumably knows the DLCI based on the circuit index.  It is
      included here to show including two pieces of information in an
      event report.

      As described above, the event declaration defines the event target,
      the event condition, and the event report content.  The event
      properties indicate whether the CE is subscribed to the event, the
      specific threshold for the event, and any filter conditions for the

      Another event shown is a laser power problem.  This event is
      generated whenever the laser falls below the specified threshold.
      Thus, a CE can register for the event of laser power loss on all
      circuits.  It would do this by:

      T = SET-Properties
        Path-TLV: flags=0, length = 2, path = 61.4
          Path-TLV: flags = property-field, length = 1, path = 2
            Content = 1 (register)
          Path-TLV: flags = property-field, length = 1, path = 3
            Content = 15 (threshold)

      This would set the registration for the event on all entries in the
      table.  It would also set the threshold for the event, causing
      reporting if the power falls below 15.  (Presumably, the CE knows
      what the scale is for power, and has chosen 15 as a meaningful
      problem level.)

      If a laser oscillates in power near the 15 mark, one could get a lot
      of notifications.  (If it flips back and forth between 9 and 10,
      each flip down will generate an event.)  Suppose that the CE decides
      to suppress this oscillation somewhat on laser channel 5.  It can do
      this by setting the variance property on that event.  The request
      would look like:

      T = SET-Properties
        Path-TLV: flags=0, length = 3, path = 61.4.5
          Path-TLV: flags = property-field, length = 1, path = 4
            Content = 2 (hysteresis)

      Setting the hysteresis to 2 suppress a lot of spurious
      notifications.  When the level first falls below 10, a notification
      is generated.  If the power level increases to 10 or 11, and then
      falls back below 10, an event will not be generated.  The power has
      to recover to at least 12 and fall back below 10 to generate another
      event.  Once common cause of this form of osciallation oscillation is when the
      actual value is right near the border.  If it is really 9.5, tiny
      changes might flip it back and forth between 9 and 10.  A variance
      level of 1 will suppress this sort of condition.  Many other events
      have osciallations oscillations that are somewhat wider, so larger variance
      settings can be used with those.

   9.  IANA Considerations

      This model creates the need for unique class names and numeric class
      identifiers.  To meet that goal, IANA will maintain a registry of
      LFB Class names, corresponding class identifiers, and the document
      which defines the LFB Class.  The registry policy is simply first
      come first served with regard to LFB Class names.  With regard to
      LFB Class identifiers, identifiers less than 65536 are reserved for
      assignment by RFCs.  Identifiers above 65536 are available for
      assignment on a first come, first served basis.  Registry entries
      must be documented in a stable, publicly available form.

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

      [Editor’s Note: A registry template registry name, and other parts
      required for a new IANA registry are still needed here.]

   10. Authors Emeritus

      The following are the authors who were instrumental in the creation
      of earlier releases of this document.

      Lily Yang, Intel Corp.
      Ram Gopal, Nokia Research Center
      Alan DeKok, Infoblox, Inc.
      Zsolt Haraszti, Clovis Solutions

   11. Acknowledgments

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


   12. 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.


   13. 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.


   14. 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>.

      [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] Hollenbeck, S. et al., "Guidelines for the Use of Extensible
      Markup Language (XML) within IETF Protocols", RFC 3470, January

      [13]  Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
      Schema Part 1: Structures", W3C REC-xmlschema-1, May 2001,

      [14]  Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", W3C
      REC-xmlschema-2, May 2001, <>.

      [15]  Davis, M. and M. Suignard, "UNICODE Security Considerations",
      July 2005,<>.

   15. Authors' Addresses

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

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

      Ram Gopal
      Nokia Research Center
      5, Wayside Road,
      Burlington, MA 01803, USA
      P.O. Box 6049
      Leesburg, VA 20178
      Phone: +1 781 993 3685

      Alan DeKok
      Infoblox, Inc.
      475 Potrero Ave,
      Sunnyvale CA 94085

      Zsolt Haraszti
      Clovis Solutions
      1310 Redwood Way, Suite B
      Petaluma, CA 94954
      Phone: 707-796-7110 703 371 3043

      Ellen Deleganes
      Intel Corp.
      Mail Stop: CO5-156
      15400 NW Greenbrier Parkway
      Beaverton, OR 97006
      Phone: +1 503 677-4996


   16. Intellectual Property Right

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

   15. 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.


   17. Copyright Statement

      "Copyright (C) The Internet Society (2006).  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

      "This document and the information contained herein are provided on