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    Internet Draft                                 L. Yang
    Expiration: July 2004                              Intel Corp.
    File: draft-ietf-forces-model-02.txt           J. Halpern
    Working Group: ForCES                               Megisto Systems
                                                   R. Gopal
                                                        Nokia
                                                   A. DeKok
                                                        IDT Inc.
                                                   Z. Haraszti
                                                   S. Blake
                                                        Ericsson
                                                   E. Deleganes
                                                        Intel Corp.
                                                   February 2004
 
 
                      ForCES Forwarding Element Model
 
 
 
                       draft-ietf-forces-model-02.txt
 
 
 
 
    Status of this Memo
 
    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.  Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF),
    its areas, and its working groups.  Note that other groups may also
    distribute working documents as Internet-Drafts.
 
    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other
    documents at any time.  It is inappropriate to use Internet-Drafts
    as reference material or to cite them other than as ``work in
    progress.''
 
    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt.
 
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.
 
 Abstract
 
    This document defines the forwarding element (FE) model used in the
    Forwarding and Control Plane Separation (ForCES) protocol.  The
    model represents the capabilities, state and configuration of
 
 
 Internet Draft         ForCES FE Model              February 2004
 
 
    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 [1].  A list of the basic logical functional
    blocks (LFBs) is also defined in the LFB class library to aid the
    effort in defining individual LFBs.
 
 Table of Contents
 
    Abstract.........................................................1
    1. Definitions...................................................4
    2. Introduction..................................................6
       2.1. Requirements on the FE model.............................6
       2.2. The FE Model in Relation to FE Implementations...........6
       2.3. The FE Model in Relation to the ForCES Protocol..........7
       2.4. Modeling Language for FE Model...........................8
       2.5. Document Structure.......................................8
    3. FE Model Concepts.............................................8
       3.1. State Model and Capability Model.........................9
       3.2. LFB (Logical Functional Block) Modeling.................11
          3.2.1. LFB Input and Input Group..........................14
          3.2.2. LFB Output and Output Group........................15
          3.2.3. Packet Type........................................16
          3.2.4. Metadata...........................................16
          3.2.5. LFB Versioning.....................................22
          3.2.6. LFB Inheritance....................................23
       3.3. FE Datapath Modeling....................................24
          3.3.1. Alternative Approaches for Modeling FE Datapaths...24
          3.3.2. Configuring the LFB Topology.......................29
    4. Model and Schema for LFB Classes.............................33
       4.1. Namespace...............................................33
       4.2. <LFBLibrary> Element....................................33
       4.3. <load> Element..........................................35
       4.4. <frameDefs> Element for Frame Type Declarations.........35
       4.5. <dataTypeDefs> Element for Data Type Definitions........36
          4.5.1. <typeRef> Element for Aliasing Existing Data Types.38
          4.5.2. <atomic> Element for Deriving New Atomic Types.....39
          4.5.3. <array> Element to Define Arrays...................39
          4.5.4. <struct> Element to Define Structures..............41
          4.5.5. <union> Element to Define Union Types..............42
          4.5.6. Augmentations......................................42
       4.6. <metadataDefs> Element for Metadata Definitions.........43
       4.7. <LFBClassDefs> Element for LFB Class Definitions........44
          4.7.1. <derivedFrom> Element to Express LFB Inheritance...45
 
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          4.7.2. <inputPorts> Element to Define LFB Inputs..........46
          4.7.3. <outputPorts> Element to Define LFB Outputs........48
          4.7.4. <attributes> Element to Define LFB Operational
          Attributes................................................50
          4.7.5. <capabilities> Element to Define LFB Capability
          Attributes................................................53
          4.7.6. <description> Element for LFB Operational
          Specification.............................................54
       4.8. XML Schema for LFB Class Library Documents..............54
    5. FE Attributes and Capabilities...............................63
       5.1. XML Schema for FE Attribute Documents...................64
       5.2. FEDocument..............................................68
          5.2.1. FECapabilities.....................................68
          5.2.2. FEAttributes.......................................71
       5.3. Sample FE Attribute Document............................73
    6. LFB Class Library............................................76
       6.1. Port LFB................................................76
       6.2. L2 Interface LFB........................................77
       6.3. IP interface LFB........................................79
       6.4. Classifier LFB..........................................80
       6.5. Next Hop LFB............................................81
       6.6. Rate Meter LFB..........................................83
       6.7. Redirector (de-MUX) LFB.................................84
       6.8. Packet Header Rewriter LFB..............................84
       6.9. Counter LFB.............................................85
       6.10. Dropper LFB............................................85
       6.11. IPv4 Fragmenter LFB....................................86
       6.12. L2 Address Resolution LFB..............................86
       6.13. Queue LFB..............................................86
       6.14. Scheduler LFB..........................................87
       6.15. MPLS ILM/Decapsulation LFB.............................88
       6.16. MPLS Encapsulation LFB.................................88
       6.17. Tunnel Encapsulation/Decapsulation LFB.................88
       6.18. Replicator LFB.........................................89
    7. Satisfying the Requirements on FE Model......................89
       7.1. Port Functions..........................................90
       7.2. Forwarding Functions....................................90
       7.3. QoS Functions...........................................91
       7.4. Generic Filtering Functions.............................91
       7.5. Vendor Specific Functions...............................91
       7.6. High-Touch Functions....................................91
       7.7. Security Functions......................................91
       7.8. Off-loaded Functions....................................92
       7.9. IPFLOW/PSAMP Functions..................................92
    8. Using the FE model in the ForCES Protocol....................92
       8.1. FE Topology Query.......................................94
       8.2. FE Capability Declarations..............................96
 
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       8.3. LFB Topology and Topology Configurability Query.........96
       8.4. LFB Capability Declarations.............................96
       8.5. State Query of LFB Attributes...........................97
       8.6. LFB Attribute Manipulation..............................98
       8.7. LFB Topology Re-configuration...........................98
    9. Acknowledgments..............................................98
    10. Security Considerations.....................................99
    11. Normative References........................................99
    12. Informative References......................................99
    13. Authors' Addresses.........................................100
    14. Intellectual Property Right................................101
    15. IANA consideration.........................................101
 
 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
 
    A set of terminology associated with the ForCES requirements is
    defined in [1] and is not copied here.  The following list of
    terminology is relevant to the FE model defined in this document.
 
    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 Function Block) class (or type) -- A template
    representing a fine-grained, logically separable and well-defined
    packet processing operation in the datapath.  LFB classes are the
    basic building blocks of the FE model.
 
    LFB (Logical Function Block) Instance -- As a packet flows through
    an FE along a datapath, it flows through one or multiple LFB
    instances, with each implementing an instance of a certain LFB
 
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    class.  There may be multiple instances of the same LFB in an FE's
    datapath.  Note that we often refer to LFBs without distinguishing
    between 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.  There are four types
    of information 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 metadata is encoded within an
    implementation.
 
    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, for example, flags, single
    parameter arguments, complex arguments, and tables that the CE can
    read or/and write via the ForCES protocol.
 
    LFB Topology -- Representation of how the LFB instances are
    logically interconnected and placed along the datapath within one
    FE.  Sometimes it is also 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 the 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 some other
    means.
 
    Inter-FE Topology -- See FE Topology.
 
    Intra-FE Topology -- See LFB Topology.
 
    LFB class library -- A set of LFB classes that is identified as the
    most common functions found in most FEs and hence should be defined
    first by the ForCES Working Group.
 
 
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 2. Introduction
 
    [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.  [1] mandates that the capabilities, states and
    configuration information be expressed in the form of an FE model.
 
    RFC 3444 [11] made the observation 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 first and foremost an information
    model, but it also includes some aspects of a data model, such as
    explicit definitions of the LFB class schema and FE schema.  It is
    expected that this FE model will be used as the basis to define the
    payload for information exchange between the CE and FE in the
    ForCES protocol.
 
 2.1. Requirements on the FE model
 
    [1] defines requirements, which 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
 
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    directed graph, and receive, process, modify, and transmit packets
    along with metadata.  Note that a real forwarding datapath
    implementation should not be constrained by the model.  On the
    contrary, the FE model should be designed such that different
    implementations of the forwarding datapath can all be logically
    mapped onto the model with the functionality and sequence of
    operations correctly captured.  However, the model itself does not
    directly address the issue of 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.
    Nevertheless, we do strive 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,
    modifications 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 would need
    to be exchanged between an FE and a CE for the proper configuration
    and management of that LFB.
 
    Definition of the various payloads of ForCES messages (irrespective
    of the transport protocol ultimately selected) cannot proceed in a
    systematic fashion until a formal definition of the objects being
    configured and managed (the FE and the LFBs within) is undertaken.
    The FE Model document defines a set of classes and attributes for
    describing and manipulating the state of the LFBs of 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
    operations if such are defined).
 
 
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    Section 8 provides more detailed discussion on how the FE model
    should be used by the ForCES protocol.
 
 2.4. Modeling Language for 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 and a full specification will be written
    using such a data modeling language.  Using a formal language can
    help to enforce consistency and logical compatibility among LFBs.
    In addition, the formal definition of the LFB classes has the
    potential to facilitate the eventual automation of some part of the
    code generation process and the functional validation of arbitrary
    LFB topologies.
 
    Human readability was the most important factor considered when
    selecting the specification language.  Encoding, decoding and
    transmission performance was not a selection factor for the
    language because the encoding method for over the wire transport is
    an issue independent of the specification language chosen.  It is
    outside the scope of this document and up to the ForCES protocol to
    define.
 
    XML was chosen as the specification language in this document,
    because XML has the advantage of being both human and machine
    readable with widely available tools support.
 
 2.5. Document Structure
 
    Section 3 provides a conceptual overview of the FE model, laying
    the foundation for the more detailed discussion and specifications
    in the sections that follow. Section 4 and 5 constitute the core of
    the FE model, detailing the two major components in the FE model:
    LFB model and FE level attributes including capability and LFB
    topology.  Section 6 presents a list of LFB classes in the LFB
    class library that will be further specified in separate documents
    according to the FE model presented in Sections 4 and 5.  Section 7
    directly addresses the model requirements imposed by the ForCES
    requirement draft [1] while Section 8 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 how the two can
    be combined in the FE model.  Section 3.2 introduces the concept of
 
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    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: 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 and the capabilities
    about what the FE can or cannot do 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 on these components are
    described in Section 4 and 5.  The intention 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. State Model and Capability Model
 
    The FE capability model describes the capabilities and capacities
    of an FE in terms of variations of functions supported or
    limitations contained.  Conceptually, the FE capability model
    presents the many possible states allowed on an FE with capacity
    information indicating certain quantitative limits or constraints.
    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.
 
    On the other hand, an FE state model describes the current state of
    the FE, that is, the instantaneous values or operational behavior
    of the FE.  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 CE as the following:
       . on a given port the packets are classified using a given
         classification filter;
       . the given classifier results in packets being metered in a
         certain way, and then marked in a certain way;
 
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       . the packets coming from specific markers are delivered into a
         shared queue for handling, while other packets are delivered
         to a different queue;
       . a specific scheduler with specific behavior and parameters
         will service these collected queues.
 
    The information on the capabilities and capacities of the FE helps
    the CE understand the flexibility and limitations of the FE
    functions, so that the CE knows at a coarse level which
    configurations are applicable to the FEs and which ones are not.
    It gets more complicated for the capability model to cope with the
    detailed limits, such as the maximum number of the following items:
    classifiers, queues, buffer pools, and meters the FE can provide.
 
    While one could try to build an object model to fully represent the
    FE capabilities, other efforts have found this to be a significant
    undertaking.  A middle of the road approach is to define coarse-
    grained capabilities and simple capacity measures.  Then, if the CE
    attempts to instruct the FE to set up some specific behavior it is
    not capable of, the FE will return an error indicating the problem.
    Examples of this approach include Framework Policy Information Base
    (PIB) [RFC3318) and Differentiated Services QoS Policy Information
    Base [4].  The capability reporting classes in the DiffServ and
    Framework PIBs are all meant to allow the device to indicate some
    general guidelines about what it can or cannot do, but do not
    necessarily allow it to indicate every possible configuration that
    it can or cannot support.  If a device receives a configuration
    that it cannot implement, it can reject that configuration by
    responding with a failure report.
 
    Figure 1 shows the concepts of FE state, capabilities and
    configuration in the context of CE-FE communication via the ForCES
    protocol.
 
         +-------+                                          +-------+
         |       | FE capabilities: what it can/cannot do.  |       |
         |       |<-----------------------------------------|       |
         |       |                                          |       |
         |   CE  | FE state: what it is now.                |  FE   |
         |       |<-----------------------------------------|       |
         |       |                                          |       |
         |       | FE configuration: what it should be.     |       |
         |       |----------------------------------------->|       |
         +-------+                                          +-------+
 
     Figure 1. Illustration of FE state, capabilities and configuration
         exchange in the context of CE-FE communication via ForCES.
 
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    The ForCES FE model must include both a state model and a
    capability model.  We believe that a good balance between
    simplicity and flexibility can be achieved for the FE model by
    combining the coarse level capability reporting with the error
    reporting mechanism.  Examples of similar approaches include
    DiffServ PIB [4] and Framework PIB [5].
 
    The concepts of LFB and LFB topology will be discussed in the rest
    of this section.  It will become clear that a capability model is
    needed at both the FE level and LFB level.
 
    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, certain features of an LFB
    class may be optional, in which case it must be possible for the CE
    to determine whether or not an optional feature is 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
    the FE can instantiate; the number of instances of each can be
    created; the topological (i.e., 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 distinctive packet processing
    functions, and we call these individual functions Logical
    Functional Blocks (LFBs).  The second level of information is about
    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 discuss high level concepts around LFBs while Section
    3.3 discuss issues around LFB topology.
 
 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 such a function,
    either the packets are modified in certain ways (e.g.,
    decapsulator, marker), or some results are generated and stored,
    probably in the form of metadata (like a classifier). Each LFB
    typically does one thing and one thing only.  Classifiers, shapers,
    meters are all examples of LFBs.  Modeling LFBs at such a fine
 
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    granularity allows us to use a small number of LFBs to create 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. Section 6 provides
    a list of useful LFBs with such granularity.
 
    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 passed from one LFB to the next, but not
    sent across the network.  It is most likely that there are multiple
    LFBs within 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
 
 
 
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    An LFB, as shown in Figure 2, has inputs, outputs and attributes
    that can be queried and manipulated by the CE indirectly via Fp
    reference point (defined in [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 happens: 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 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 new LFB class can
    also be added later to accommodate future innovation in the
    forwarding plane.
 
    LFB operation must be specified in the model to allow the CE to
    understand the behavior of the forwarding datapath.  For instance,
    the CE must understand at what point in the datapath the IPv4
    header TTL is decremented (i.e., it needs to know if a control
    packet could be delivered to the CE 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 various LFBs along a datapath within an FE are compatible to
    link together.
 
    There is value to vendors if the operation of LFB classes can be
    expressed in sufficient detail so that physical devices
    implementing different LFB functions can be integrated easily into
    an FE design.  Therefore, a semi-formal specification is needed;
    that is, a text description of the LFB operation (human readable),
    but sufficiently specific and unambiguous to allow conformance
    testing and efficient design (i.e., eliminate guess-work), so that
    interoperability between different CEs and FEs can be achieved.
 
    The LFB class model specifies information like:
       . number of inputs and outputs (and whether they are
         configurable)
       . metadata read/consumed from inputs;
       . metadata produced at the outputs;
       . packet type(s) accepted at the inputs and emitted at the
         outputs;
 
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       . packet content modifications (including encapsulation or
         decapsulation);
       . packet routing criteria (when multiple outputs on an LFB are
         present);
       . packet timing modifications;
       . packet flow ordering modifications;
       . LFB capability information;
       . LFB operational attributes, etc.
 
    Section 4 of this document provides a detailed discussion of the
    LFB model with a formal specification of LFB class schema.  The
    rest of Section 3.2 only intends to provide a conceptual overview
    of some important issues in LFB modeling, without covering all the
    specific details.
 
 3.2.1. LFB Input and Input Group
 
    An LFB input is a conceptual port of the LFB where the LFB can
    receive information from other LFBs. The information is typically a
    packet (or frame in general) and associated metadata, although in
    some cases it might consist of only metadata, i.e., with a Null-
    packet.
 
    It is inevitable that there will be LFB instances that will receive
    packets from more than one other LFB instances (fan-in).  If these
    fan-in links all carry the same type of information (packet type
    and set of metadata) and require the same processing within the
    LFB, then one input should be sufficient.  If, however, the LFB
    class can receive two or more very different types of input, and
    the processing of these inputs are also very distinct, then that
    may justify the definition of multiple inputs.  But in these cases
    splitting the LFB class into two LFB classes should always be
    considered as an alternative.  In intermediate cases, e.g., where
    the inputs are somewhat different but they require very similar
    processing, the shared input solution should be preferred.  For
    example, if an Ethernet framer LFB is capable of receiving IPv4 and
    IPv6 packets, these can be served by the same LFB input.
 
    Note that we assume the model allows for connecting more than one
    LFB output to a single LFB input directly.  There is no restriction
    on the number of up-stream LFBs connecting their outputs to the
    same input of a single LFB instance.  Note that the behavior of the
    system when multiple packets arrive at such an input simultaneously
    is not defined by the model.  If such behavior needs to be
    described, it can be done either by separating the single input to
    become multiple inputs (one per output), or by inserting other
 
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    appropriate LFBs (such as Queues and possibly Schedulers) between
    the multiple outputs and the single input.
 
    If there are multiple inputs with the same input type, we model
    them as an input group, that is, multiple instances of the same
    input type.  In general, an input group is useful to allow an LFB
    to differentiate packet treatment based on where the packet came
    from.
 
      +----+                                +----+
      |LFB1+---+                            |LFB1+---+
      +----+   |    +---------+             +----+   |    +-----------+
               +--->|in  LFB3 |            input   / +--->|in:1  LFB3 |
      +----+   |    +---------+            group   \ +--->|in:2       |
      |LFB2+---+                            +----+   |    +-----------+
      +----+                                |LFB2+---+
                                            +----+
 
         (a) without input group            (b) with input group
 
                   Figure 3. An example of using input group.
 
    Consider the following two cases in Figure 3(a) and (b).  In Figure
    3(a), the output from two LFBs are directly connected into one
    input of LFB3, assuming that it can be guaranteed that no two
    packets arrive at the same time instance.  If LFB3 must do
    something different based on the source of the packet (LFB1 or
    LFB2), the only way to model that is to make LFB1 and LFB2 pass
    some metadata with different values so that LFB3 can make the
    differentiation based on that metadata.  In Figure 3(b), that
    differentiation can be elegantly expressed within LFB3 using the
    input group concept where the instance id can server as the
    differentiating key.  For example, a scheduler LFB can potentially
    use an input group consisting of a variable number of inputs to
    differentiate the queues from which the packets are coming.
 
 3.2.2. LFB Output and Output Group
 
    An LFB output is a conceptual port of the LFB that can send
    information to some other LFBs.  The information is typically a
    packet (or frame in general) and associated metadata, although in
    some cases it might emit only metadata, i.e., with a Null-packet.
 
    We assume that a single LFB output can be connected to only one LFB
    input (this is required to make the packet flow through the LFB
    topology unambiguous).  Therefore, to allow any non-trivial
    topology, multiple outputs must be allowed for an LFB class.  If
    there are multiple outputs with the same output type, we model them
 
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    as output group, that is, multiple instances of the same output
    type.  For illustration of output group, consider the hypothetical
    LFB in Figure 4.  The LFB has two types of outputs, one of which
    can be instantiated to form an output group.
 
                             +------------------+
                             |           UNPROC +-->
                             |                  |
                             |         PKTOUT:1 +-->   \
                           --> PKTIN   PKTOUT:2 +-->   |
                             |            .     + .    | Output group
                             |            .     + .    |
                             |         PKTOUT:N +-->   /
                             +------------------+
 
                Figure 4. An example of an LFB with output group.
 
    Multiple outputs should mainly be used for functional separation
    where the outputs are connected to very different types of LFBs.
    For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one
    default output to send those packets for which look-up was
    successful (passing a META_ROUTEID as metadata); and have another
    output for sending packets for which the look-up failed.  The
    former output may be connected to a route handler LFB, while the
    latter can be connected to an ICMP response generator LFB or to a
    packet handler LFB that passes the packet up to the CE.
 
 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 that distinct frame 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 it does not care about 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
 
 
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    Metadata is the per-packet state that is passed from one LFB to
    another. The metadata is passed with the packet to assist with
    further processing of that packet.  The ForCES model must capture
    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 both
    situations, however, our focus here is for intra-FE metadata.
 
    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"
    initializes the value of the metadata (implicitly creating and/or
    initializing 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 not
    specify which LFBs the metadata is expected to come from for a
    consumer LFB, or which LFBs are expected to consume metadata for a
    producer LFB.
 
    While it is important to define the metadata types passing between
    LFBs, it is not necessary to define the exact encoding mechanism
    used by LFBs for that metadata. Different implementations are
    allowed to use different encoding mechanisms for metadata.  For
    example, one implementation may store metadata in registers or
    shared memory, while another implementation may encode metadata in-
    band as a preamble in the packets.
 
    At any link between two LFBs, the packet is marked with a finite
    set of active metadata, where active means the metadata is within
    its life-cycle. (i.e., the metadata has been properly initialized
 
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    and has not been consumed yet.) There are two corollaries of this
    model:
 
    1. No uninitialized metadata exists in the model.
 
    2. No more than one occurrence of each metadata tag can be
       associated with a packet at any given time.
 
 
 3.2.4.1. 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 into 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 gets created.
 
    For source-type LFBs (i.e., an LFB that inserts packets into the
    model), WRITE is the only meaningful metadata operation.
 
    Sink-type LFBs (i.e., an LFB that removes the packet from the
    model), may either READ-AND-CONSUME (read) or CONSUME (ignore) each
    active metadata associated with the packet.
 
 3.2.4.2. Metadata Production and Consumption
 
 
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    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 the metadata. In this model, the
    producer and consumer LFBs of a metadata are not required to be
    adjacent. There may be multiple consumers for the same metadata and
    there may be multiple producers of the same metadata. When a packet
    path involves multiple producers of the same metadata, then the
    second, third, etc. producers overwrite that metadata value.
 
 
    The metadata that is produced by an LFB is specified by the LFB
    class definition on a per output port group basis. A producer may
    always generate the metadata on the port group, or may generate it
    only under certain conditions. We call the former an
    "unconditional" metadata, whereas the latter is a "conditional"
    metadata.  In the case of conditional metadata, it should be
    possible to determine from the definition of the LFB when a
    "conditional" metadata is produced.
 
    The consumer behavior of an LFB, that is, the metadata that the LFB
    needs for its operation, is defined in the LFB class definition on
    a per input port group basis. An input port group may "require" a
    given metadata, or may treat it as "optional" information. In the
    latter case, the LFB class definition must explicitly define what
    happens if an optional metadata is not provided. One approach is to
    specify a default value for each optional metadata, and assume that
    the default value is used if the metadata is not provided with the
    packet.
 
    When a consumer 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 this inter-dependency. Furthermore, it
    should be possible to take this inter-dependency into consideration
    when constructing LFB topologies, and also that the dependency 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 expect
    a certain metadata to come from.  Similarly, LFBs should specify
    what metadata they produce without specifying which LFBs the
    metadata is meant for.
 
 
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    When specifying the metadata tags, some harmonization effort must
    be made so that the producer LFB class uses the same tag as its
    intended consumer(s), or vice versa.
 
 
 3.2.4.3. 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 to have 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 variable tag metadata production. If an LFB produces metadata
    with variable tag, a corresponding LFB attribute--called the tag
    selector--specifies the tag for each such metadata. This mechanism
    is to improve the versatility of certain multi-purpose LFB classes,
    since it allows the same LFB class be used in different topologies,
    producing the right metadata tags according to the needs of the
    topology.
 
    Depending on the capability of the FE, the tag selector can be a
    read-only or a read-write attribute. In the former case, the tag
    cannot be modified by the CE. In the latter case the tag can be
    configured by the CE, hence we call this "configurable tag metadata
    production." (Note that in 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
    the 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 to use a "ClassID" metadata in one instance, and a
    "ProtocolType" metadata in another instance as a basis for
    selecting the right output port. In this case, an LFB attribute is
    used to provide the required metadata tag at run-time.  This
    metadata tag selector attribute may be read-only or read-write,
    depending on the capabilities of the LFB instance and the FE.
 
 3.2.4.4. Metadata Usage Categories
 
    Depending on the role and usage of a metadata, various amount of
    encoding information must be provided when the metadata is defined,
 
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    and some cases offer less flexibility in the value selection than
    others.
 
    As far as usage of a metadata is concerned, three types of metadata
    exist:
    - 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 binding relationship is probably
    the most common usage of metadata. One or more objects in the
    producer LFB are related (bound) to a specific object in the
    consumer LFB. Such a relation is established by the CE very
    explicitly, i.e., 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 (written
    by the CE) into both LFBs, and this value is also the value that
    the metadata carries between the LFBs.
 
    Another way of setting up binding relations is to use a naturally
    occurring unique identifier of the consumer's object (for example,
    the array index of a table entry) as a reference (and as a value of
    the metadata. In this case, the index is obtained (read) or
    inferred by the CE by communicating with the consumer LFB. Once the
    CE obtains the index, it needs to plug (write) it into the producer
    LFB to establish the binding.
 
    Important characteristics of the binding usage of metadata are:
 
    - The value of the metadata shows up in the CE-FE communication for
    BOTH the consumer and the producer. That is, the metadata value
    must be carried over the ForCES protocol. Using the tagging
    technique, the value is WRITTEN to both LFBs. Using the other
 
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    technique, the value is WRITTEN to only the producer LFB and may be
    READ from the consumer LFB.
 
    - The actual value is irrelevant for the CE, the binding is simply
    expressed by using the SAME value at the consumer and producer
    LFBs.
 
    - Hence the definition of the metadata does not have 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 and consumed
    by some other LFBs. As the name suggests, enumerated metadata has a
    relatively small number of possible values, each with a very
    specific meaning. All of the 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 value of the metadata is explicitly used by the
    consumer LFB to change some packet header fields. In other words,
    its 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, where the values
    cannot be selected arbitrarily, but rather they should conform to
    what is commonly expected. For example, a TTL increment metadata
    should encode with zero for the no increment case, by one for the
    single increment case, etc. A DSCP metadata should use 0 to encode
    DSCP=0, 1 to encode DSCP=1, etc.
 
 3.2.5. LFB Versioning
 
 
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    LFB class versioning is a method to enable incremental evolution of
    LFB classes.  Unlike inheritance (discussed next in Section 3.2.6),
    where it assumed that an FE datapath model containing an LFB
    instance of a particular class C could also simultaneously contain
    an LFB instance of a class C' inherited from class C; with
    versioning, an FE would not be allowed to contain an LFB instance
    for more than one version of a particular class.
 
    LFB class versioning is supported by requiring a version string in
    the class definition.  CEs may support backwards compatibility
    between multiple versions of a particular LFB class, but FEs are
    not allowed to support more than one single version of a particular
    class.
 
 3.2.6. LFB Inheritance
 
    LFB class inheritance is supported in the FE model as a means of
    defining 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 (with version number) it
    inherits from (with the default being the base LFB class).
    Multiple-inheritance is not allowed, though, to avoid the
    unnecessary complexity.
 
    Inheritance should be used only when there is significant reuse of
    the base LFB class definition.  A separate LFB class should be
    defined if there is not enough reuse 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 there exists a
    standardized LFB class "L1".  Vendor A builds an FE that implements
    LFB "L1" and vendors 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 (for example, by extending its
    capabilities in some incremental way).  Lets first examine the FE
    backward compatibility issue by considering what would happen if
    vendor B upgrades its FE from "L1" to "L2" while 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
 
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    will use the term "backward compatibility" to refer to the first
    scenario concerning FE backward compatibility.
 
    Inheritance can be designed into the model with backward
    compatibility support by constraining the LFB inheritance such that
    the derived class is always a functional superset of the base
    class, i.e., the derived class can only grow on top of the base
    class, but not shrink from it.   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 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 the operation of an LFB may have an
    impact on how the packet is to be treated in further (downstream)
    LFBs and 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) controls what 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 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
    around 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 representing the datapath directly and
    graphically (topological approach) and the other utilizing metadata
    (the encoded state approach).
 
       . Topological Approach
 
 
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       Using this approach, differential packet treatment is expressed
       via actually splitting the LFB topology into alternative paths.
       In other words, if the result of an LFB must control how the
       packet is further processed, then such an LFB will have separate
       output ports (one for each alternative treatment) connected to
       separate sub-graphs (each expressing the respective treatment
       downstream).
 
       . Encoded State Approach
 
       An alternative way of expressing differential treatment is 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
       decide how to treat the packet.
 
    Theoretically, the two approaches can substitute for each other, so
    one may consider using purely one (or the other) approach to
    describe all datapaths in an FE.  However, neither model by itself
    is very useful for practically relevant cases.  For a given FE with
    certain logical datapaths, applying the two different modeling
    approaches would result in very different looking LFB topology
    graphs.  A model using purely the topological approach may require
    a very large graph with many links (i.e., paths) and nodes (i.e.,
    LFB instances) to express all alternative datapaths.  On the other
    hand, a model using purely the encoded state model would be
    restricted to a string of LFBs, which would make it very
    unintuitive to describe very 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 it 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 some other LFBs, Figure 5(a) shows what the LFB
    topology looks like by using the purely topological approach.  Each
    output from the classifier goes to one of the N LFBs followed and
    no metadata is needed here.  The topological approach is simple,
    straightforward and graphically intuitive.  However, if N is large
    and the N nodes followed the classifier (LFB#1, LFB#2, ..., LFB#N)
    all belong to the same LFB type (for example, meter) but each with
    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
 
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    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.
 
    Now what if all the 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 seems the best by representing the two
    different functional datapaths using topological approach while
    leaving the N-1 meter instances distinguished by metadata only, as
    shown in Figure 5(c).
                                         +----------+
                                  P      |   LFB#1  |
                              +--------->|(Attrib-1)|
         +-------------+      |          +----------+
         |            1|------+   P      +----------+
         |            2|---------------->|   LFB#2  |
         | classifier 3|                 |(Attrib-2)|
         |          ...|...              +----------+
         |            N|------+          ...
         +-------------+      |   P      +----------+
                              +--------->|   LFB#N  |
                                         |(Attrib-N)|
                                         +----------+
 
                 5(a) Using pure topological approach
 
         +-------------+                 +-------------+
         |            1|                 |   Meter     |
         |            2|   (P, M)        | (Attrib-1)  |
         |            3|---------------->| (Attrib-2)  |
         |          ...|                 |   ...       |
         |            N|                 | (Attrib-N)  |
         +-------------+                 +-------------+
 
           5(b) Using pure encoded state approach to represent the LFB
          topology in 5(a), if LFB#1, LFB#2, ..., and LFB#N are of the
                        same type (e.g., meter).
 
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                                      +-------------+
         +-------------+ (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 distinct
    advantage for different situations.  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 with it so that 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 a lot more connections and often results in an unwieldy
    graph.  On the other hand, a topological approach is intuitive and
    most useful for representing functionally very different datapaths.
 
    For complex topologies, a combination of the two is the most useful
    and flexible.  Here we provide a general design guideline as to
    what approach is best used for what situation.  The topological
    approach should primarily be used when the packet datapath forks
    into areas with distinct LFB classes (not just distinct
    parameterizations of the same LFB classes), and when the fan-outs
    do not require changes (adding/removing LFB outputs) at all or
    require only very infrequent changes.  Configuration information
    that needs to change frequently should preferably be expressed by
    the internal attributes of one or more LFBs (and hence using the
    encoded state approach).
 
 
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                       +---------------------------------------------+
                       |                                             |
         +----------+  V      +----------+           +------+        |
         |          |  |      |          |if IP-in-IP|      |        |
    ---->| ingress  |->+----->|classifier|---------->|Decap.|---->---+
         | ports    |         |          |----+      |      |
         +----------+         +----------+    |others+------+
                                              |
                                              V
         (a)  The LFB topology with a logical loop
 
 
        +-------+   +-----------+            +------+   +-----------+
        |       |   |           |if IP-in-IP |      |   |           |
    --->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|->
        | ports |   |           |----+       |      |   |           |
        +-------+   +-----------+    |others +------+   +-----------+
                                     |
                                     V
         (b)  The LFB topology without the loop utilizing two
              independent classifier instances.
 
                 Figure 6. An LFB topology example.
 
    It is important to point out that the LFB topology here is the
    logical topology that the packets flow through, not the physical
    topology as determined by how the FE hardware is laid out.
    Nevertheless, the actual implementation may still influence how the
    functionality should be mapped into 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 outer IP header
    while another set for 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 multiple instances of the same LFB class, each LFB
    instance has its own LFB instance ID.  One way to encode the LFB
 
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    instance ID is to encode it as x.y where x is the LFB class ID
    while y is the instance ID within each LFB class.
 
 3.3.2. Configuring the LFB Topology
 
    While there is little doubt that the individual LFB must be
    configurable, the configurability question is more complicated for
    LFB topology.  Since LFB topology is really the graphic
    representation of the datapaths within FE, configuring the LFB
    topology means dynamically changing the datapaths including changes
    to the LFBs along the datapaths on an FE, e.g., creating (i.e.,
    instantiating) or deleting LFBs, 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 is set up by the CE to provide certain data
    plane services (e.g., DiffServ, VPN, etc.) to the NE's 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,
    e.g., when 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, and that is determined
    by the approach we use 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
    totally bypass some LFBs or insert new LFBs.  Since a mix of these
    two approaches is used to model the datapaths, LFB topology
    reconfiguration is considered an important aspect of the FE model.
 
    We want to point out that allowing a configurable LFB topology in
    the FE model does not mandate that all FEs must have such
    capability.  Even if an FE supports configurable LFB topology, it
    is expected that there will be FE-specific limitations on what can
    actually be configured.  Performance-optimized hardware
    implementation 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 such
    reconfiguration and how much.  Whether it is a simple runtime
    switch to enable or disable (i.e., bypass) certain LFBs, or more
 
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    flexible software reconfiguration is all implementation detail
    internal to the FE and outside of the scope of FE model.  In either
    case, the CE(s) must be able to learn the FE's configuration
    capabilities.  Therefore, the FE model must provide a mechanism for
    describing the LFB topology configuration capabilities of an FE.
    These capabilities may include (see Section 5 for full details):
       . What LFB classes can the FE instantiate?
       . How many instances of the same LFB class can be created?
       . What are the topological limitations?  For example:
            o How many instances of the same class or any class 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).
 
    Even if the CE is allowed to configure LFB topology for an FE, how
    can the CE interpret an arbitrary LFB topology (presented to the CE
    by the FE) and know what to do with it?  In other words, how does
    the CE know the mapping between an LFB topology and a particular NE
    service or application (e.g., VPN, DiffServ, etc.)?  We argue that
    first of all, it is unlikely that an FE can support any arbitrary
    LFB topology; secondly, once the CE understands the coarse
    capability of an FE, it is up to the CE to configure the LFB
    topology according to the network service the NE is supposed to
    provide.  So the more important mapping that the CE has to
    understand is from the high level NE service to a specific LFB
    topology, not the other way around.  Do we expect the CE has the
    ultimate intelligence to translate any high level service policy
    into the configuration data for the FEs?  No, but it is conceivable
    that within a given network service domain (like DiffServ), a
    certain amount of intelligence can be programmed into the CE such
    that the CE has a general understanding of the LFBs involved and so
    the translation from a high level service policy to the low level
    FE configuration can be done automatically.  In any event, 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.
 
 
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         +----------+     +-----------+
    ---->| Ingress  |---->|classifier |--------------+
         |          |     |chip       |              |
         +----------+     +-----------+              |
                                                     v
                         +-------------------------------------------+
           +--------+    |   Network Processor                       |
      <----| Egress |    |   +------+    +------+   +-------+        |
           +--------+    |   |Meter |    |Marker|   |Dropper|        |
                 ^       |   +------+    +------+   +-------+        |
                 |       |                                           |
      +----------+-------+                                           |
      |          |                                                   |
      |    +---------+       +---------+   +------+    +---------+   |
      |    |Forwarder|<------|Scheduler|<--|Queue |    |Counter  |   |
      |    +---------+       +---------+   +------+    +---------+   |
      |--------------------------------------------------------------+
 
                 (a)  The Capability of the FE, reported to the CE
 
 
           +-----+    +-------+                      +---+
           |    A|--->|Queue1 |--------------------->|   |
    ------>|     |    +-------+                      |   |  +---+
           |     |                                   |   |  |   |
           |     |    +-------+      +-------+       |   |  |   |
           |    B|--->|Meter1 |----->|Queue2 |------>|   |->|   |
           |     |    |       |      +-------+       |   |  |   |
           |     |    |       |--+                   |   |  |   |
           +-----+    +-------+  |   +-------+       |   |  +---+
         classifier              +-->|Dropper|       |   |  IPv4
                                     +-------+       +---+  Fwd.
                                                  Scheduler
                 (b)  One LFB topology as configured by the CE and
                      accepted by the FE
 
 
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                                                  Queue1
                       +---+                    +--+
                       |  A|------------------->|  |--+
                    +->|   |                    |  |  |
                    |  |  B|--+  +--+   +--+    +--+  |
                    |  +---+  |  |  |   |  |          |
                    | Meter1  +->|  |-->|  |          |
                    |            |  |   |  |          |
                    |            +--+   +--+          |          Ipv4
                    |         Counter1 Dropper1 Queue2|    +--+  Fwd.
            +---+   |                           +--+  +--->|A |  +-+
            |  A|---+                           |  |------>|B |  | |
     ------>|  B|------------------------------>|  |  +--->|C |->| |->
            |  C|---+                           +--+  | +->|D |  | |
            |  D|-+ |                                 | |  +--+  +-+
            +---+ | |    +---+                  Queue3| | Scheduler
        Classifier1 | |  |  A|------------>       +--+  | |
                    | +->|   |                    |  |--+ |
                    |    |  B|--+  +--+ +-------->|  |    |
                    |    +---+  |  |  | |         +--+    |
                    |  Meter2   +->|  |-+                 |
                    |              |  |                   |
                    |              +--+           Queue4  |
                    |            Marker1          +--+    |
                    +---------------------------->|  |----+
                                                  |  |
                                                  +--+
                 (c)  Another LFB topology as configured by the CE and
                      accepted by the FE
 
         Figure 7. An example of configuring LFB topology.
 
    Figure 7 shows an example where a QoS-enabled router has several
    line cards that have a few ingress ports and egress ports, a
    specialized classification chip, a network processor containing
    codes for FE blocks like meter, marker, dropper, counter, queue,
    scheduler and Ipv4 forwarder.  Some of the LFB topology is already
    fixed and has to remain static due to the physical layout of the
    line cards.  For example, all the ingress ports might be already
    hard wired into the classification chip and so all packets must
    follow from the ingress port into the classification engine.  On
    the other hand, the LFBs on the network processor and their
    execution order are programmable, even though there might exist
    certain capacity limits and linkage constraints between these LFBs.
    Examples of the capacity limits might be: there can be no more than
    8 meters; there can be no more than 16 queues in one FE; the
    scheduler can handle at most up to 16 queues; etc.  The linkage
 
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    constraints might dictate that classification engine may be
    followed by a meter, marker, dropper, counter, queue or IPv4
    forwarder, but not 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 such capability and capacity to the CE, it is
    now up to the CE to translate the QoS policy into the desirable
    configuration for the FE.  Figure 7(a) depicts the FE capability
    while 7(b) and 7(c) depict two different topologies that the FE
    might be asked to configure to.  Note that both the ingress and
    egress are omitted in (b) and (c) for simple representation.  The
    topology in 7(c) is considerably more complex than 7(b) but both
    are feasible within the FE capabilities, and so the FE should
    accept either configuration request from the CE.
 
 4. Model and Schema for LFB Classes
 
    The main goal of the FE model is to provide an abstract, generic,
    modular, implementation-independent representation of the FEs. This
    is facilitated using the concept of LFBs which are instantiated
    from LFB classes. LFB classes and associated definitions will be
    provided in a collection of XML documents. The collection of these
    XML documents is called a LFB class library, and each document is
    called an LFB class library document (or library document, for
    short). Each of the library documents will conform to the schema
    presented in this section. The root element of the library document
    is the <LFBLibrary> element.
 
    It is not expected that library documents will be exchanged between
    FEs and CEs "over-the-wire". But the model will serve as an
    important reference for the design and development of the CEs
    (software) and FEs (mostly the software part). It will also serve
    as a design input when specifying the ForCES protocol elements for
    CE-FE communication.
 
 4.1. Namespace
 
    The LFBLibrary element and all of its sub-elements are defined in
    the following namespace:
 
       http://ietf.org/forces/1.0/lfbmodel
 
 4.2. <LFBLibrary> Element
 
    The <LFBLibrary> element serves as a root element of all library
    documents. It contains one or more of the following main blocks:
 
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       . <frameTypeDefs> for the frame declarations;
       . <dataTypeDefs> for defining common data types;
       . <metadataDefs> for defining metadata, and
       . <LFBClassDefs> for defining LFB classes.
 
    Each block is optional, that is, one library may contain only
    metadata defintions, another may contain only LFB class
    definitions, yet another may contain all of the above.
 
    In addition to the above main blocks, a library document can import
    other library documents if it needs to refer to definitions
    contained in the included document. This concept is similar to the
    "#include" directive in C. Importing is expressed by the <load>
    elements, which must precede all the above elements in the
    document. For unique referencing, each LFBLibrary instance document
    has a unique label defined in the "provide" attribute of the
    LFBLibrary element.
 
    The <LFBLibrary> element also includes an optional <description>
    element, which can be used to provide textual description about the
    library.
 
    Following is a skeleton of a library document:
 
    <?xml version="1.0" encoding="UTF-8"?>
    <LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
      provides="this_library">
 
      <description>
        ...
      </description>
 
      <!-- Loading external libraries (optional) -->
      <load library="another_library"/>
      ...
 
      <!-- FRAME TYPE DEFINITIONS (optional) -->
      <frameTypeDefs>
        ...
      </frameTypeDefs>
 
      <!-- DATA TYPE DEFINITIONS (optional) -->
      <dataTypeDefs>
        ...
 
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      </dataTypeDefs>
 
      <!-- METADATA DEFINITIONS (optional) -->
      <metadataDefs>
        ...
      </metadataDefs>
 
      <!—LFB CLASS DEFINITIONS (optional) -->
      <LFBCLassDefs>
        ...
      </LFBCLassDefs>
    </LFBLibrary>
 
 
 4.3. <load> Element
 
    This element is used to refer to another LFB library document.
    Similar to the "include" directive in C, this makes the objects
    (metadata types, data types, etc.) defined in the referred library
    available for referencing in the current document.
 
    The load element must contain the label of the library to be
    included and may contain a URL to specify where the library can be
    retrieved. The load element can be repeated unlimited times. Three
    examples for the <load> elements:
 
    <load library="a_library"/>
    <load library="another_library" location="another_lib.xml"/>
    <load library="yetanother_library"
      location="http://www.petrimeat.com/forces/1.0/lfbmodel/lpm.xml"/>
 
 
 4.4. <frameDefs> Element for Frame Type Declarations
 
    Frame names are used in the LFB definition to define what types of
    frames the LFB expects at its input port(s) and emits at its output
    port(s).  The <frameDefs> optional element in the library document
    contains one or more <frameDef> elements, each declaring one frame
    type.
 
    Each frame definition contains a unique name (NMTOKEN) and a brief
    synopsis.  In addition, an optional detailed description may be
    provided.
 
 
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    Uniqueness of frame types must be ensured among frame types defined
    in the same library document and in all directly or indirectly
    included library documents.
 
    The following example defines two frame types:
 
    <frameDefs>
      <frameDef>
        <name>ipv4</name>
        <synopsis>IPv4 packet</synopsis>
        <description>
          This frame type refers to an IPv4 packet.
        </description>
      </frameDef>
        <frameDef>
        <name>ipv6</name>
        <synopsis>IPv6 packet</synopsis>
        <description>
          This frame type refers to an IPv6 packet.
        </description>
      </frameDef>
      ...
    </frameDefs>
 
 
 4.5. <dataTypeDefs> Element for Data Type Definitions
 
    The (optional) <dataTypeDefs> element can be used to define
    commonly used data types. It contains one or more <dataTypeDef>
    elements, each defining a data type with a unique name. Such data
    types can be used in several places in the library documents,
    including:
 
    .  Defining other data types
    .  Defining metadata
    .  Defining attributes of LFB classes
 
    This is similar to the concept of having a common header file for
    shared data types.
 
    Each <dataTypeDef> element contains a unique name (NMTOKEN), a
    brief
    synopsis, an optional longer description, and a type definition
 
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    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:
 
    <dataTypeDefs>
      <dataTypeDef>
        <name>ieeemacaddr</name>
        <synopsis>48-bit IEEE MAC address</synopsis>
        ... type definition ...
      </dataTypeDef>
      <dataTypeDef>
        <name>ipv4addr</name>
        <synopsis>IPv4 address</synopsis>
        ... type definition ...
      </dataTypeDef>
      ...
    </dataTypeDefs>
 
    There are two kinds of data types: atomic and compound.  Atomic
    data types are appropriate for single-value variables (e.g.
    integer, ASCII string, byte array).
 
    The following built-in atomic data types are provided, but
    additional atomic data types can be defined with the <typeRef> and
    <atomic> elements:
 
       <name>                   Meaning
       ----                     -------
       char                     8-bit signed integer
       uchar                    8-bit unsigned integer
       int16                    16-bit signed integer
       uint16                   16-bit unsigned integer
       int32                    32-bit signed integer
       uint32                   32-bit unsigned integer
       int64                    64-bit signed integer
       uint64                   64-bit unisgned integer
       string[N]                ASCII null-terminated string with
                                buffer of N characters (string max
                                length is N-1)
       byte[N]                  A byte array of N bytes
       float16                  16-bit floating point number
       float32                  32-bit IEEE floating point number
       float64                  64-bit IEEE floating point number
 
 
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    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.
 
    Compound data types can build on atomic data types and other
    compound data types. There are four ways that compound data types
    can be defined.  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, not
    the model documents.
 
    For the definition of the actual type in the <dataTypeDef> element,
    the following elements are available: <typeRef>, <atomic>, <array>,
    <struct>, and <union>.
 
    [EDITOR: How to support augmentation is for further study.]
 
 
 4.5.1. <typeRef> Element for Aliasing Existing Data Types
 
    The <typeRef> element refers to an existing data type by its name.
    The referred data type must be defined either in the same library
    document, or in one of the included library documents. If the
    referred data type is an atomic data type, the newly defined type
    will also be regarded as atomic. If the referred data type is a
    compound type, the new type will also be a compound. Some usage
    examples:
 
    <dataTypeDef>
      <name>short</name>
      <synopsis>Alias to int16</synopsis>
      <typeRef>int16</typeRef>
    </dataTypeDef>
    <dataTypeDef>
      <name><name>ieeemacaddr</name>
      <synopsis>48-bit IEEE MAC address</synopsis>
      <typeRef>byte[6]</typeRef>
    </dataTypeDef>
 
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 4.5.2. <atomic> Element for Deriving New Atomic Types
 
    The <atomic> element allows the definition of a new atomic type
    from an existing atomic type, applying range restrictions and/or
    providing special enumerated values.  Note that the <atomic>
    element can only use atomic types as base types, and its result is
    always another atomic type.
 
    For example, the following snippet defines a new "dscp" data type:
 
    <dataTypeDef>
      <name>dscp</name>
      <synopsis>Diffserv code point.</synopsis>
      <atomic>
        <baseType>uchar</baseType>
        <rangeRestriction>
          <allowedRange min="0" max="63"/>
        </rangeRestriction>
        <specialValues>
          <specialValue value="0">
            <name>DSCP-BE</name>
            <synopsis>Best Effort</synopsis>
          </specialValue>
          ...
        </specialValues>
      </atomic>
    </dataTypeDef>
 
 
 
 4.5.3. <array> Element to Define Arrays
 
    The <array> element can be used to create a new compound data type
    as an array of a compound or an atomic data type. The type of the
    array entry can be specified either by referring to an existing
    type (using the <typeRef> element) or defining an unnamed type
    inside the <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 can specify the maximum allowed length. This
    attribute should be used to encode semantic limitations, and not
    implementation limitations. The latter should be handled by
 
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    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 which
    specifies the constant size of the array.
 
    The result of this construct is always a compound type, even if the
    array has a fixed size of 1.
 
    Arrays can only be subscripted by integers, and will be presumed to
    start with index 0.
 
    The following example shows the definition of a fixed size array
    with pre-defined data type as array elements:
 
    <dataTypeDef>
      <name>dscp-mapping-table</name>
      <synopsys>
        A table of 64 DSCP values, used to re-map code space.
      </synopsis>
      <array type="fixed-size" length="64">
          <typeRef>dscp</typeRef>
      </array>
    </dataTypeDef>
 
    The following example defines a variable size array with an upper
    limit on its size:
 
    <dataTypeDef>
      <name>mac-alias-table </name>
      <synopsys>A table with up to 8 IEEE MAC addresses</synopsis>
      <array type="variable-size" max-length="8">
          <typeRef>ieeemacaddr</typeRef>
      </array>
    </dataTypeDef>
 
    The following example shows the definition of an array with local
    (unnamed) type definition:
 
    <dataTypeDef>
      <name>classification-table</name>
      <synopsys>
        A table of classification rules and result opcodes.
      </synopsis>
      <array type="variable-size">
        <struct>
 
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          <element>
            <name>rule</name>
            <synopsis>The rule to match</synopsis>
            <typeRef>classrule</typeRef>
          </element>
          <element>
            <name>opcode</name>
            <synopsis>The result code</synopsis>
            <typeRef>opcode</typeRef>
          </element>
        </struct>
      </array>
    </dataTypeDef>
 
    In the above example each entry of the array is a <struct> of two
    fileds ("rule" and "opcode").
 
 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.
 
    The result of this construct is always regarded a compound type,
    even if the <struct> contains only one field.
 
    An example:
 
    <dataTypeDef>
      <name>ipv4prefix</name>
      <synopsis>
        IPv4 prefix defined by an address and a prefix length
      </synopsis>
      <struct>
        <element>
          <name>address</name>
          <synopsis>Address part</synopsis>
          <typeRef>ipv4addr</typeRef>
        </element>
        <element>
 
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          <name>prefixlen</name>
          <synopsis>Prefix length part</synopsis>
          <atomic>
            <baseType>uchar</baseType>
            <rangeRestriction>
              <allowedRange min="0" max="32"/>
            </rangeRestriction>
          </atomic>
        </element>
      </struct>
    </dataTypeDef>
 
 
 4.5.5. <union> Element to Define Union Types
 
    Similar to the union declaration in C, this construct allows the
    definition of overlay types.  Its format is identical to the
    <struct> element.
 
    The result of this construct is always regarded a compound type,
    even if the union contains only one element.
 
 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.  They may replace
    the type of an existing element with an augmentation derived from
    the current type.  They may not delete an existing element, nor may
    they replace the type of an existing element with one that is not
    an augmentation of the type that the element has in the basis for
    the augmentation.  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
 
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    augmentation of X.  Both classes A1 and A2 are backward compatible
    with class A.
 
    [EDITOR: How to support the concept of augmentation in the XML
    schema is for further study.]
 
 
 4.6. <metadataDefs> Element for Metadata Definitions
 
    The (optional) <metadataDefs> element in the library document
    contains one or more <metadataDef> elements. Each <metadataDef>
    element defines a metadata.
 
    Each <metadataDef> element contains a unique name (NMTOKEN).
    Uniqueness is defined over all metadata defined in this library
    document and in all directly or indirectly included library
    documents. The <metadataDef> element also contains a brief
    synopsis, an optional detailed description, and a compulsory type
    definition information. Only atomic data types can be used as value
    types for metadata.
 
    Two forms of type definitions are allowed. The first form uses the
    <typeRef> element to refer to an existing atomic data type defined
    in the <dataTypeDefs> element of the same library document or in
    one of the included library documents. The usage of the <typeRef>
    element is identical to how it is used in the <dataTypeDef>
    elements, except here it can only refer to atomic types.
 
    [EDITOR: The latter restriction is not yet enforced by the XML
    schema.]
 
    The second form is an explicit type definition using the <atomic>
    element. This element is used here in the same way as in the
    <dataTypeDef> elements.
 
    The following example shows both usages:
 
    <metadataDefs>
      <metadataDef>
        <name>NEXTHOPID</name>
        <synopsis>Refers to a Next Hop entry in NH LFB</synopsis>
        <typeRef>int32</typeRef>
      </metadataDef>
      <metadataDef>
        <name>CLASSID</name>
        <synopsis>
          Result of classification (0 means no match).
 
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        </synopsis>
        <atomic>
          <baseType>int32</baseType>
          <specialValues>
            <specialValue value="0">
              <name>NOMATCH</name>
              <synopsis>
                Classification didn’t result in match.
              </synopsis>
            </specialValue>
          </specialValues>
        </atomic>
      </metadataDef>
    </metadataDefs>
 
 
 4.7. <LFBClassDefs> Element for LFB Class Definitions
 
    The (optional) <LFBClassDefs> element can be used to define one or
    more LFB classes using <LFBClassDef> elements. Each <LFBClassDef>
    element defines an LFB class and includes the following elements:
      . <name> provides the symbolic name of the LFB class. Example:
         "ipv4lpm"
      . <synopsis> provides a short synopsis of the LFB class.
         Example: "IPv4 Longest Prefix Match Lookup LFB"
      . <version> is the version indicator
      . <derivedFrom> is the inheritance indicator
      . <inputPorts> lists the input ports and their specifications
      . <outputPorts> lists the output ports and their specifications
      . <attributes> defines the operational attributes of the LFB
      . <capabilities> defines the capability attributes of the LFB
      . <description> contains the operational specification of the
         LFB
 
    [EDITOR: LFB class names should be unique not only among classes
    defined in this document and in all included documents, but also
    unique across a large collection of libraries.  Obviously some
    global control is needed to ensure such uniqueness. This subject
    requires further study.]
 
    Here is a skeleton of an example LFB class definition:
 
    <LFBClassDefs>
      <LFBClassDef>
        <name>ipv4lpm</name>
        <synopsis>IPv4 Longest Prefix Match Lookup LFB</synopsis>
        <version>1.0</version>
 
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        <derivedFrom>baseclass</derivedFrom>
 
        <inputPorts>
          ...
        </inputPorts>
 
        <outputPorts>
          ...
        </outputPorts>
 
        <attributes>
          ...
        </attributes>
 
        <capabilities>
          ...
        </capabilities>
 
        <description>
          This LFB represents the IPv4 longest prefix match lookup
          operation.
          The modeled behavior is as follows:
             Blah-blah-blah.
        </description>
 
      </LFBClassDef>
      ...
    </LFBClassDefs>
 
    Except the <name>, <synopsis>, and <version> elements, all other
    elements are optional in <LFBClassDef>, though 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.
 
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 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> elements must be omitted. The
    <inputPorts> element can contain one or more <inputPort> elements,
    one for each port or port-group. We assume that most LFBs will have
    exactly one input. Multiple inputs with the same input type are
    modeled as one input group. Input groups are defined the same way
    as input ports by the <inputPort> element, differentiated only by
    an optional "group" attribute.
 
    Multiple inputs with different input types should be avoided if
    possible (see discussion in Section 3.2.1). Some special LFBs will
    have no inputs at all. For example, a packet generator LFB does not
    need an input.
 
    Single input ports and input port groups are both defined by the
    <inputPort> element, they are differentiated by only an optional
    "group" attribute.
 
    The <inputPort> element contains the following elements:
    . <name> provides the symbolic name of the input.  Example: "in".
       Note that this symbolic name must be unique only within the
       scope of the LFB class.
    . <synopsis> contains a brief description of the input.  Example:
       "Normal packet input".
    . <expectation> lists all allowed frame formats. Example: {"ipv4"
       and "ipv6"}.  Note that this list should refer to names
       specified in the <frameDefs> element of the same library
       document or in any included library documents. The <expectation>
       element can also provide a list of required metadata. Example:
       {"classid", "vifid"}. This list should refer to names of
       metadata defined in the <metadataDefs> element in the same
       library document or in any included library documents. For each
       metadata it must be specified whether the metadata is required
       or optional. For each optional metadata a default value must be
       specified, which is used by the LFB if the metadata is not
       provided with a packet.
 
    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").
 
 
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    An example <inputPorts> element, defining two input ports, the
    second one being an input port group:
 
    <inputPorts>
      <inputPort>
        <name>in</name>
        <synopsis>Normal input</synopsis>
        <expectation>
          <frameExpected>
            <ref>ipv4</ref>
            <ref>ipv6</ref>
          </frameExpected>
          <metadataExpected>
            <ref>classid</ref>
            <ref>vifid</ref>
            <ref dependency="optional" defaultValue="0">vrfid</ref>
          </metadataExpected>
        </expectation>
      </inputPort>
      <inputPort group="yes">
        ... another input port ...
      </inputPort>
    </inputPorts>
 
    For each <inputPort>, the frame type expectations are defined by
    the <frameExpected> element using one or more <ref> elements (see
    example above). When multiple frame types are listed, it means that
    "one of these" frame types are expected. A packet of any other
    frame type is regarded as incompatible with this input port of the
    LFB class. The above example list two frames as expected frame
    types: "ipv4" and "ipv6".
 
    Metadata expectations are specified by the <metadataExpected>
    element. In its simplest form this element can contain a list of
    <ref> elements, each referring to a metadata. When multiple
    instances of metadata are listed by <ref> elements, it means that
    "all of these" metadata must be received with each packet (except
    metadata that are marked as "optional" by the "dependency"
    attribute of the corresponding <ref> element). For a metadata that
    is specified "optional", a default value must be provided using the
    "defaultValue" attribute. The above example lists three metadata as
    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.]
 
 
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    The schema also allows for more complex definitions of metadata
    expectations. For example, using the <one-of> element, a list of
    metadata can be specified to express that at least one of the
    specified metadata must be present with any packet. For example:
 
    <metadataExpected>
      <one-of>
        <ref>prefixmask</ref>
        <ref>prefixlen</ref>
      </one-of>
    </metadataExpected>
 
    The above example specifies that either the "prefixmask" or the
    "prefixlen" metadata must be provided with any packet.
 
    The two forms can also be combined, as it is shown in the following
    example:
 
    <metadataExpected>
      <ref>classid</ref>
      <ref>vifid</ref>
      <ref dependency="optional" defaultValue="0">vrfid</ref>
      <one-of>
        <ref>prefixmask</ref>
        <ref>prefixlen</ref>
      </one-of>
    </metadataExpected>
 
    Although the schema is constructed to allow even more complex
    definition of metadata expectations, we do not discuss these 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.
 
 
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    The <outputPort> element contains the following elements:
    . <name> provides the symbolic name of the output. Example: "out".
       Note that the symbolic name must be unique only within the scope
       of the LFB class.
    . <synopsis> contains a brief description of the output port.
       Example: "Normal packet output".
    . <product> lists the allowed frame formats. Example: {"ipv4",
       "ipv6"}. Note that this list should refer to symbols specified
       in the <frameDefs> element in the same library document or in
       any included library documents. The <product> element may also
       contain the list of emitted (generated) metadata. Example:
       {"classid", "color"}. This list should refer to names of
       metadata specified in the <metadataDefs> element in the same
       library document or in any included library documents. For each
       generated metadata, it should be specified whether the metadata
       is always generated or generated only in certain conditions.
       This information is important when assessing compatibility
       between LFBs.
 
    In addition, the optional "group" attribute of the <outputPort>
    element can specify if the port can behave as a port group, i.e.,
    it is allowed to be instantiated. This is indicated by a "yes"
    value (the default value is "no").
 
    The following example specifies two output ports, the second being
    an output port group:
 
    <outputPorts>
      <outputPort>
        <name>out</name>
        <synopsis>Normal output</synopsis>
        <product>
          <frameProduced>
            <ref>ipv4</ref>
            <ref>ipv4bis</ref>
          </frameProduced>
          <metadataProduced>
            <ref>nhid</ref>
            <ref>nhtabid</ref>
          </metadataProduced>
        </product>
      </outputPort>
      <outputPort group="yes">
        <name>exc</name>
        <synopsis>Exception output port group</synopsis>
        <product>
          <frameProduced>
 
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            <ref>ipv4</ref>
            <ref>ipv4bis</ref>
          </frameProduced>
          <metadataProduced>
            <ref availability="conditional">errorid</ref>
          </metadataProduced>
        </product>
      </outputPort>
    </outputPorts>
 
    What 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 ouputs in a port group
       . Metadata CONSUME vs. PROPAGATE mode selectors
       . 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
 
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       . Number of current inputs or outputs for each input or output
         group
       . Metadata CONSUME/PROPAGATE mode selector
 
    There may be various access permission restrictions on what the CE
    can do with an LFB attribute.  The following categories may be
    supported:
       . No-access attributes.  This is useful when multiple access
         modes maybe defined for a given attribute to allow some
         flexibility for different implementations.
       . Read-only attributes.
       . Read-write attributes.
       . Write-only attributes.  This could be any configurable data
         for which read capability is not provided to the CEs.  (e.g.,
         the security key information)
       . Read-reset attributes.  The CE can read and reset this
         resource, but cannot set it to an arbitrary value. Example:
         Counters.
       . Firing-only attributes.  A write attempt to this resource will
         trigger some specific actions in the LFB, but the actual value
         written is ignored.
 
    The LFB class may define more than one possible access mode for a
    given attribute (for example, "write-only" and "read-write"), in
    which case it is left to the actual implementation to pick one of
    the modes.  In such cases a corresponding capability attribute must
    inform the CE about the access mode the actual LFB instance
    supports (see next subsection on capability attributes).
 
    The attributes of the LFB class are listed in the <attributes>
    element. Each attribute is defined by an <attribute> element. An
    <attribute> element contains the following elements:
       . <name> defines the name of the attribute. This name must be
         unique among the attributes of the LFB class. Example:
         "version".
       . <synopsis> should provide a brief description of the purpose
         of the attribute.
       . 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
 
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         how they are used inside <dataTypeDef> elements (see Section
         4.5).
       . The optional <defaultValue> element can specify a default
         value for the attribute, which is applied when the LFB is
         initialized or reset. [EDITOR: A convention to define default
         values for compound data types and byte[N] atomic types is yet
         to be defined.]
 
    In addition to the above elements, the <attribute> element includes
    an optional "access" attribute, which can take any of the following
    values or even a list of these values: "read-only", "read-write",
    "write-only", "read-reset", and "trigger-only". The default access
    mode is "read-write".
 
    The following example defines two attributes for an LFB:
 
    <attributes>
      <attribute access="read-only">
        <name>foo</name>
        <synopsis>number of things</synopsis>
        <typeRef>uint32</typeRef>
      </attribute>
      <attribute access="read-write write-only">
        <name>bar</name>
        <synopsis>number of this other thing</synopsis>
        <atomic>
          <baseType>uint32</baseType>
          <rangeRestriction>
            <allowedRange min="10" max="2000"/>
          </rangeRestriction>
        </atomic>
        <defaultValue>10</defaultValue>
      </attribute>
    </attributes>
 
    The first attribute ("foo") is a read-only 32-bit unsigned integer,
    defined by referring to the built-in "uint32" atomic type. The
    second attribute ("bar") is also an integer, but uses the <atomic>
    element to provide additional range restrictions. This attribute
    has 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
 
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    mechanisms should include appropriate error indicators for this
    case.
 
    The mechanism defined above for non-supported attributes can also
    apply to attempts to reference non-existent array elements or to
    set read-only elements.
 
 
 4.7.5. <capabilities> Element to Define LFB Capability Attributes
 
    The LFB class specification will provide some flexibility for the
    FE implementation regarding how the LFB class is implemented. For
    example the class may define some features optional, in which case
    the actual implementation may or may not provide the given feature.
    In these cases the CE must be able to query the LFB instance about
    the availability of the feature. In addition, the instance may have
    some limitations that are not inherent from the class definition,
    but rather the result of some implementation limitations. For
    example, an array attribute may be defined in the class definition
    as "unlimited" size, but the physical implementation may impose a
    hard limit on the size of the array.
 
    Such capability related information is expressed by the capability
    attributes of the LFB class. The capability attributes are always
    read-only attributes, and they are listed in a separate
    <capabilities> element in the <LFBClassDef>. The <capabilities>
    element contains one or more <capability> elements, each defining
    one capability attribute. The format of the <capability> element is
    almost the same as the <attribute> element, it differs in two
    aspects: it lacks the access mode attribute (because it is always
    read-only), and it lacks the <defaultValue> element (because
    default value is not applicable to read-only attributes).
 
    Some examples of capability attributes:
       . The version of the LFB class that this LFB instance complies
         with;
       . Supported optional features of the LFB class;
       . Maximum number of configurable outputs for an output group;
       . Metadata pass-through limitations of the LFB;
       . Maximum size of configurable attribute tables;
       . Additional range restriction on operational attributes;
       . 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:
 
 
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    <capabilities>
      <capability>
        <name>version</name>
        <synopsis>
          LFB class version this instance is compliant with.
        </synopsis>
        <typeRef>version</typeRef>
      </capability>
      <capability>
        <name>limitBar</name>
        <synopsis>
          Maximum value of the "bar" attribute.
        </synopsis>
        <typeRef>uint16</typeRef>
      </capability>
    </capabilities>
 
 
 4.7.6. <description> Element for LFB Operational Specification
 
    The <description> element of the <LFBClass> provides unstructured
    text (in XML sense) to verbally describe what the LFB does.
 
 4.8. XML Schema for LFB Class Library Documents
 
    <?xml version="1.0" encoding="UTF-8"?>
    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
     xmlns="http://ietf.org/forces/1.0/lfbmodel"
     xmlns:lfb="http://ietf.org/forces/1.0/lfbmodel"
     targetNamespace="http://ietf.org/forces/1.0/lfbmodel"
     attributeFormDefault="unqualified"
     elementFormDefault="qualified">
    <xsd:annotation>
      <xsd:documentation xml:lang="en">
      Schema for Defining LFB Classes and associated types (frames,
      data types for LFB attributes, and metadata).
      </xsd:documentation>
    </xsd:annotation>
    <xsd:element name="description" type="xsd:string"/>
    <xsd:element name="synopsis" type="xsd:string"/>
    <!-- Document root element: LFBLibrary -->
    <xsd:element name="LFBLibrary">
      <xsd:complexType>
        <xsd:sequence>
          <xsd:element ref="description" minOccurs="0"/>
          <xsd:element name="load" type="loadType" minOccurs="0"
                       maxOccurs="unbounded"/>
          <xsd:element name="frameDefs" type="frameDefsType"
 
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                       minOccurs="0"/>
          <xsd:element name="dataTypeDefs" type="dataTypeDefsType"
                       minOccurs="0"/>
          <xsd:element name="metadataDefs" type="metadataDefsType"
                       minOccurs="0"/>
          <xsd:element name="LFBClassDefs" type="LFBClassDefsType"
                       minOccurs="0"/>
        </xsd:sequence>
        <xsd:attribute name="provides" type="xsd:Name" use="required"/>
      </xsd:complexType>
      <!-- Uniqueness constraints -->
      <xsd:key name="frame">
        <xsd:selector xpath="lfb:frameDefs/lfb:frameDef"/>
        <xsd:field xpath="lfb:name"/>
      </xsd:key>
      <xsd:key name="dataType">
        <xsd:selector xpath="lfb:dataTypeDefs/lfb:dataTypeDef"/>
        <xsd:field xpath="lfb:name"/>
      </xsd:key>
      <xsd:key name="metadataDef">
        <xsd:selector xpath="lfb:metadataDefs/lfb:metadataDef"/>
        <xsd:field xpath="lfb:name"/>
      </xsd:key>
      <xsd:key name="LFBClassDef">
        <xsd:selector xpath="lfb:LFBClassDefs/lfb:LFBClassDef"/>
        <xsd:field xpath="lfb:name"/>
      </xsd:key>
    </xsd:element>
    <xsd:complexType name="loadType">
      <xsd:attribute name="library" type="xsd:Name" use="required"/>
      <xsd:attribute name="location" type="xsd:anyURI" use="optional"/>
    </xsd:complexType>
    <xsd:complexType name="frameDefsType">
      <xsd:sequence>
        <xsd:element name="frameDef" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:element ref="description" minOccurs="0"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="dataTypeDefsType">
      <xsd:sequence>
 
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        <xsd:element name="dataTypeDef" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:element ref="description" minOccurs="0"/>
              <xsd:group ref="typeDeclarationGroup"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <!--
       Predefined (built-in) atomic data-types are:
           char, uchar, int16, uint16, int32, uint32, int64, uint64,
           string[N], byte[N],
           float16, float32, float64
    -->
    <xsd:group name="typeDeclarationGroup">
      <xsd:choice>
        <xsd:element name="typeRef" type="typeRefNMTOKEN"/>
        <xsd:element name="atomic" type="atomicType"/>
        <xsd:element name="array" type="arrayType"/>
        <xsd:element name="struct" type="structType"/>
        <xsd:element name="union" type="structType"/>
      </xsd:choice>
    </xsd:group>
    <xsd:simpleType name="typeRefNMTOKEN">
      <xsd:restriction base="xsd:token">
        <xsd:pattern value="\c+"/>
        <xsd:pattern value="string\[\d+\]"/>
        <xsd:pattern value="byte\[\d+\]"/>
      </xsd:restriction>
    </xsd:simpleType>
    <xsd:complexType name="atomicType">
      <xsd:sequence>
        <xsd:element name="baseType" type="typeRefNMTOKEN"/>
        <xsd:element name="rangeRestriction"
                        type="rangeRestrictionType minOccurs="0"/>
        <xsd:element name="specialValues" type="specialValuesType"
                     minOccurs="0"/>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="rangeRestrictionType">
      <xsd:sequence>
        <xsd:element name="allowedRange" maxOccurs="unbounded">
          <xsd:complexType>
 
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            <xsd:attribute name="min" type="xsd:integer"
    use="required"/>
            <xsd:attribute name="max" type="xsd:integer"
    use="required"/>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="specialValuesType">
      <xsd:sequence>
        <xsd:element name="specialValue" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
            </xsd:sequence>
            <xsd:attribute name="value" type="xsd:token"/>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="arrayType">
      <xsd:sequence>
        <xsd:group ref="typeDeclarationGroup"/>
      </xsd:sequence>
      <xsd:attribute name="type" use="optional"
                     default="variable-size">
        <xsd:simpleType>
          <xsd:restriction base="xsd:string">
            <xsd:enumeration value="fixed-size"/>
            <xsd:enumeration value="variable-size"/>
          </xsd:restriction>
        </xsd:simpleType>
      </xsd:attribute>
      <xsd:attribute name="length" type="xsd:integer" use="optional"/>
      <xsd:attribute name="maxLength" type="xsd:integer"
                     use="optional"/>
    </xsd:complexType>
    <xsd:complexType name="structType">
      <xsd:sequence>
        <xsd:element name="element" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:group ref="typeDeclarationGroup"/>
            </xsd:sequence>
 
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          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="metadataDefsType">
      <xsd:sequence>
        <xsd:element name="metadataDef" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:element ref="description" minOccurs="0"/>
              <xsd:choice>
                <xsd:element name="typeRef" type="typeRefNMTOKEN"/>
                <xsd:element name="atomic" type="atomicType"/>
              </xsd:choice>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="LFBClassDefsType">
      <xsd:sequence>
        <xsd:element name="LFBClassDef" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:element name="version" type="versionType"/>
              <xsd:element name="derivedFrom" type="xsd:NMTOKEN"
                           minOccurs="0"/>
              <xsd:element name="inputPorts" type="inputPortsType"
                           minOccurs="0"/>
              <xsd:element name="outputPorts" type="outputPortsType"
                           minOccurs="0"/>
              <xsd:element name="attributes" type="LFBAttributesType"
                           minOccurs="0"/>
              <xsd:element name="capabilities"
                              type="LFBCapabilitiesType"
 minOccurs="0"/>
              <xsd:element ref="description" minOccurs="0"/>
            </xsd:sequence>
          </xsd:complexType>
          <!-- Key constraint to ensure unique attribute names within
               a class:
          -->
          <xsd:key name="attributes">
 
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            <xsd:selector xpath="lfb:attributes/lfb:attribute"/>
            <xsd:field xpath="lfb:name"/>
          </xsd:key>
          <xsd:key name="capabilities">
            <xsd:selector xpath="lfb:capabilities/lfb:capability"/>
            <xsd:field xpath="lfb:name"/>
          </xsd:key>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:simpleType name="versionType">
      <xsd:restriction base="xsd:NMTOKEN">
        <xsd:pattern value="[1-9][0-9]*\.([1-9][0-9]*|0)"/>
      </xsd:restriction>
    </xsd:simpleType>
    <xsd:complexType name="inputPortsType">
      <xsd:sequence>
        <xsd:element name="inputPort" type="inputPortType"
                     maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="inputPortType">
      <xsd:sequence>
        <xsd:element name="name" type="xsd:NMTOKEN"/>
        <xsd:element ref="synopsis"/>
        <xsd:element name="expectation" type="portExpectationType"/>
        <xsd:element ref="description" minOccurs="0"/>
      </xsd:sequence>
      <xsd:attribute name="group" type="booleanType" use="optional"
                     default="no"/>
    </xsd:complexType>
    <xsd:complexType name="portExpectationType">
      <xsd:sequence>
        <xsd:element name="frameExpected" minOccurs="0">
          <xsd:complexType>
            <xsd:sequence>
              <!-- ref must refer to a name of a defined frame type -->
              <xsd:element name="ref" type="xsd:string"
                           maxOccurs="unbounded"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
        <xsd:element name="metadataExpected" minOccurs="0">
          <xsd:complexType>
            <xsd:choice maxOccurs="unbounded">
              <!-- ref must refer to a name of a defined metadata -->
              <xsd:element name="ref" type="metadataInputRefType"/>
 
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              <xsd:element name="one-of"
                           type="metadataInputChoiceType"/>
            </xsd:choice>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="metadataInputChoiceType">
      <xsd:choice minOccurs="2" maxOccurs="unbounded">
        <!-- ref must refer to a name of a defined metadata -->
        <xsd:element name="ref" type="xsd:NMTOKEN"/>
        <xsd:element name="one-of" type="metadataInputChoiceType"/>
        <xsd:element name="metadataSet" type="metadataInputSetType"/>
      </xsd:choice>
    </xsd:complexType>
    <xsd:complexType name="metadataInputSetType">
      <xsd:choice minOccurs="2" maxOccurs="unbounded">
        <!-- ref must refer to a name of a defined metadata -->
        <xsd:element name="ref" type="metadataInputRefType"/>
        <xsd:element name="one-of" type="metadataInputChoiceType"/>
      </xsd:choice>
    </xsd:complexType>
    <xsd:complexType name="metadataInputRefType">
      <xsd:simpleContent>
        <xsd:extension base="xsd:NMTOKEN">
          <xsd:attribute name="dependency" use="optional"
                         default="required">
            <xsd:simpleType>
              <xsd:restriction base="xsd:string">
                <xsd:enumeration value="required"/>
                <xsd:enumeration value="optional"/>
              </xsd:restriction>
            </xsd:simpleType>
          </xsd:attribute>
          <xsd:attribute name="defaultValue" type="xsd:token"
                         use="optional"/>
        </xsd:extension>
      </xsd:simpleContent>
    </xsd:complexType>
    <xsd:complexType name="outputPortsType">
      <xsd:sequence>
        <xsd:element name="outputPort" type="outputPortType"
                     maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="outputPortType">
      <xsd:sequence>
 
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        <xsd:element name="name" type="xsd:NMTOKEN"/>
        <xsd:element ref="synopsis"/>
        <xsd:element name="product" type="portProductType"/>
        <xsd:element ref="description" minOccurs="0"/>
      </xsd:sequence>
      <xsd:attribute name="group" type="booleanType" use="optional"
                     default="no"/>
    </xsd:complexType>
    <xsd:complexType name="portProductType">
      <xsd:sequence>
        <xsd:element name="frameProduced">
          <xsd:complexType>
            <xsd:sequence>
              <!-- ref must refer to a name of a defined frame type -->
              <xsd:element name="ref" type="xsd:NMTOKEN"
                           maxOccurs="unbounded"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
        <xsd:element name="metadataProduced" minOccurs="0">
          <xsd:complexType>
            <xsd:choice maxOccurs="unbounded">
              <!-- ref must refer to a name of a defined metadata -->
              <xsd:element name="ref" type="metadataOutputRefType"/>
              <xsd:element name="one-of"
                           type="metadataOutputChoiceType"/>
            </xsd:choice>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="metadataOutputChoiceType">
      <xsd:choice minOccurs="2" maxOccurs="unbounded">
        <!-- ref must refer to a name of a defined metadata -->
        <xsd:element name="ref" type="xsd:NMTOKEN"/>
        <xsd:element name="one-of" type="metadataOutputChoiceType"/>
        <xsd:element name="metadataSet" type="metadataOutputSetType"/>
      </xsd:choice>
    </xsd:complexType>
    <xsd:complexType name="metadataOutputSetType">
      <xsd:choice minOccurs="2" maxOccurs="unbounded">
        <!-- ref must refer to a name of a defined metadata -->
        <xsd:element name="ref" type="metadataOutputRefType"/>
        <xsd:element name="one-of" type="metadataOutputChoiceType"/>
      </xsd:choice>
    </xsd:complexType>
    <xsd:complexType name="metadataOutputRefType">
 
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      <xsd:simpleContent>
        <xsd:extension base="xsd:NMTOKEN">
          <xsd:attribute name="availability" use="optional"
                         default="unconditional">
            <xsd:simpleType>
              <xsd:restriction base="xsd:string">
                <xsd:enumeration value="unconditional"/>
                <xsd:enumeration value="conditional"/>
              </xsd:restriction>
            </xsd:simpleType>
          </xsd:attribute>
        </xsd:extension>
      </xsd:simpleContent>
    </xsd:complexType>
    <xsd:complexType name="LFBAttributesType">
      <xsd:sequence>
        <xsd:element name="attribute" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:element ref="description" minOccurs="0"/>
              <xsd:group ref="typeDeclarationGroup"/>
              <xsd:element name="defaultValue" type="xsd:token"
                           minOccurs="0"/>
            </xsd:sequence>
            <xsd:attribute name="access" use="optional"
                           default="read-write">
              <xsd:simpleType>
                <xsd:list itemType="accessModeType"/>
              </xsd:simpleType>
            </xsd:attribute>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:simpleType name="accessModeType">
      <xsd:restriction base="xsd:NMTOKEN">
        <xsd:enumeration value="read-only"/>
        <xsd:enumeration value="read-write"/>
        <xsd:enumeration value="write-only"/>
        <xsd:enumeration value="read-reset"/>
        <xsd:enumeration value="trigger-only"/>
      </xsd:restriction>
    </xsd:simpleType>
    <xsd:complexType name="LFBCapabilitiesType">
      <xsd:sequence>
 
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        <xsd:element name="capability" maxOccurs="unbounded">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="name" type="xsd:NMTOKEN"/>
              <xsd:element ref="synopsis"/>
              <xsd:element ref="description" minOccurs="0"/>
              <xsd:group ref="typeDeclarationGroup"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:simpleType name="booleanType">
      <xsd:restriction base="xsd:string">
        <xsd:enumeration value="yes"/>
        <xsd:enumeration value="no"/>
      </xsd:restriction>
    </xsd:simpleType>
    </xsd:schema>
 
 
 5. FE Attributes and Capabilities
 
    A ForCES forwarding element handles traffic on behalf of a ForCES
    control element.  While the standards will describe the protocol
    and mechanisms for this control, different implementations and
    different instances will have different capabilities.  The CE needs
    to be able to determine what each instance it is responsible for is
    actually capable of doing.  As stated previously, this is an
    approximation.  The CE is expected to be prepared to cope with
    errors in requests and variations in detail not captured by the
    capabilities information about an FE.
 
    In addition to its capabilities, an FE will have some information
    (attributes) 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.
 
    The ForCES protocol will define the actual mechanism for getting
    and setting attribute information.  This model defines the starting
    set of information that will be available.  This definition
    includes the semantics and the structuring of the information.  It
    also provides for extensions to this information.
 
    In order to crisply define the attribute information and structure,
    this document describes the attributes as information in an
    abstract XML document.  Conceptually, each FE contains such a
 
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    document.  The document structure is defined by the XML Schema
    contained in this model.  Operationally, the ForCES protocol refers
    to information contained in that document in order to read or write
    FE attributes and capabilities.  This document is an abstract
    representation of the information.  There is no requirement that
    such a document actually exist in memory.  Unless the ForCES
    protocol calls for transfer of the information in XML, the
    information is not required to ever be represented in the FE in
    XML.  The XML schema serves only to identify the elements and
    structure of the information.
 
    The subsections in this part of the document provide the details on
    this aspect of the FE model.  5.1 gives the XML schema for the
    abstract FE attribute document.  5.2 elaborates on each of the
    defined attributes of the FE, following the hierarchy of the
    schema.  5.3 provides an example XML FE attribute document to
    clarify the meaning of 5.1 and 5.2.
 
 5.1. XML Schema for FE Attribute Documents
 
    <?xml version="1.0" encoding="UTF-8"?>
    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
     <xsd:annotation>
      <xsd:documentation xml:lang="en">
       Schema for the Abstract FE Attributes and Capabilities Document
      </xsd:documentation>
     </xsd:annotation>
 
     <xsd:element name="FEDocument">
      <xsd:complexType>
       <xsd:sequence>
        <xsd:element name="FECapabilities" type="FECapabilitiesType"
             minOccurs="0" maxOccurs="1"/>
        <xsd:element name="FEAttributes" type="FEAttributesType"
             minOccurs="0" maxOccurs="1"/>
       </xsd:sequence>
      </xsd:complexType>
     </xsd:element>
 
     <xsd:complexType name="FECapabilitiesType">
      <xsd:sequence>
       <xsd:element name="ModifiableLFBTopology" type="xsd:boolean"
            minOccurs="0" maxOccurs="1"/>
       <xsd:element name="SupportedLFBs" minOccurs="0" maxOccurs="1">
        <xsd:complexType>
         <xsd:sequence>
 
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          <xsd:element name="SupportedLFB" type="SupportedLFBType"
               minOccurs="1" maxOccurs="unbounded"/>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
       <xsd:element name="SupportedAttributes"
            type="SupportedAttributesType"
            minOccurs="0" maxOccurs="1"/>
      </xsd:sequence>
     </xsd:complexType>
 
     <xsd:complexType name="SupportedLFBType">
      <xsd:sequence>
       <!-- the name of a supported LFB -->
       <xsd:element name="LFBName" type="xsd:NMTOKEN"/>
       <!-- how many of this LFB class can exist -->
       <xsd:element name="LFBOccurrenceLimit"
            type="xsd:nonNegativeInteger" minOccurs="0" maxOccurs="1"/>
       <!-- For each port group, how many ports can exist -->
       <xsd:element name="PortGroupLimits" minOccurs="0" maxOccurs="1">
        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="PortGroupLimit" minOccurs="0"
               maxOccurs="unbounded">
           <xsd:complexType>
            <xsd:sequence>
             <xsd:element name="PortGroupName" type="xsd:NMTOKEN"/>
             <xsd:element name="MinPortCount"
                  type="xsd:nonNegativeInteger"
                  minOccurs="0" maxOccurs="1"/>
             <xsd:element name="MaxPortCount"
                  type="xsd:nonNegativeInteger"
                  minOccurs="0" maxOccurs="1"/>
            </xsd:sequence>
           </xsd:complexType>
          </xsd:element>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
       <!-- for the named LFB Class, the LFB Classes it may follow -->
       <xsd:element name="CanOccurAfters" minOccurs="0" maxOccurs="1">
        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="CanOccurAfter"
               type="LFBAdjacencyLimitType"
               minOccurs="0" maxOccurs="unbounded"/>
         </xsd:sequence>
 
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        </xsd:complexType>
       </xsd:element>
 
       <!-- for the named LFB Class, which LFB Classes may follow -->
       <xsd:element name="CanOccurBefores" minOccurs="0" maxOccurs="1">
        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="CanOccurBefore"
               type="LFBAdjacencyLimitType"
               minOccurs="0" maxOccurs="unbounded"/>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
       <!-- information defined by the Class Definition -->
       <xsd:element name="LFBClassCapabilities" type="xsd:anyType"
            minOccurs="0" maxOccurs="1"/>
      </xsd:sequence>
     </xsd:complexType>
 
     <xsd:complexType name="LFBAdjacencyLimitType">
      <xsd:sequence>
       <xsd:element name="NeighborLFB" type="xsd:NMTOKEN"/>
       <xsd:element name="viaPort" type="xsd:NMTOKEN"
            minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
     </xsd:complexType>
 
     <xsd:complexType name="SupportedAttributesType">
      <xsd:sequence>
       <xsd:element name="SupportedAttribute"
            minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="AttributeName" type="xsd:NMTOKEN"/>
          <xsd:element name="AccessModes" type="xsd:NMTOKEN"/>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
      </xsd:sequence>
     </xsd:complexType>
 
     <xsd:complexType name="FEAttributesType">
      <xsd:sequence>
       <xsd:element name="Vendor" type="xsd:string" minOccurs="0"/>
       <xsd:element name="Model" type="xsd:string" minOccurs="0"/>
       <xsd:element name="FEStatus" type="FEStateType" minOccurs="0"/>
       <xsd:element name="LFBInstances" minOccurs="0" maxOccurs="1">
 
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        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="LFBInstance" minOccurs="0"
               maxOccurs="unbounded">
           <xsd:complexType>
            <xsd:sequence>
             <xsd:element name="LFBClassName" type="xsd:NMTOKEN"/>">
             <xsd:element name="LFBInstanceID" type="xsd:NMTOKEN"/>">
            </xsd:sequence>
           </xsd:complexType>
          </xsd:element>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
       <xsd:element name="LFBTopology" type="LFBTopologyType"
            minOccurs="0" maxOccurs="1"/>
       <xsd:element name="FEConfiguredNeighbors" minOccurs="0"
            maxOccurs="1">
        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="FEConfiguredNeighbor"
               type="FEConfiguredNeighborType"
               minOccurs="0" maxOccurs="unbounded"/>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
      </xsd:sequence>
     </xsd:complexType>
 
     <xsd:complexType name="LFBTopologyType">
      <xsd:sequence>
       <xsd:element name="LFBLink" minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
         <xsd:sequence>
          <xsd:element name="FromLFBID" type="xsd:NMTOKEN"/>
          <xsd:element name="FromPortGroup" type="xsd:NMTOKEN"/>
          <xsd:element name="FromPortIndex"
               type="xsd:nonNegativeInteger"/>
          <xsd:element name="ToLFBID" type="xsd:NMTOKEN"/>
          <xsd:element name="ToPortGroup" type="xsd:NMTOKEN"/>
          <xsd:element name="ToPortIndex"
               type="xsd:nonNegativeInteger"/>
         </xsd:sequence>
        </xsd:complexType>
       </xsd:element>
      </xsd:sequence>
     </xsd:complexType>
 
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     <xsd:complexType name="FEConfiguredNeighborType">
      <xsd:sequence>
       <xsd:element name="NeighborID" type="xsd:anyType"/>
       <xsd:element name="NeighborInterface" type="xsd:anyType"/>
       <xsd:element name="NeighborNetworkAddress" type="xsd:anyType"
            minOccurs="0" maxOccurs="1"/>
       <xsd:element name="NeighborMACAddress" type="xsd:anyType"
            minOccurs="0" maxOccurs="1"/>
      </xsd:sequence>
     </xsd:complexType>
 
     <!-- The values for the simple state attribute -->
     <!-- These should probably be directly encodable in the -->
     <!-- protocol so they may end up numeric instead of strings -->
     <xsd:simpleType name="FEStateType">
      <xsd:restriction base="xsd:NMTOKEN">
       <xsd:enumeration value="AdminDisable"/>
       <xsd:enumeration value="OperDisable"/>
       <xsd:enumeration value="OperEnable"/>
      </xsd:restriction>
     </xsd:simpleType>
 
    </xsd:schema>
 
 5.2. FEDocument
 
    An instance of this document captures the capabilities and FE level
    attribute / state information about a given FE.  Currently, two
    elements are allowed in the FEDocument, FECapabilities and
    FEAttributes.
 
    At the moment, all capability and attribute information in this
    abstract document is defined as optional.  We may wish to mandate
    support for some capability and/or attribute information.
 
    If a protocol using binary encoding of this information is adopted
    by the ForCES working group, then each relevant element defined in
    the schema will have a "ProtocolEncoding" attribute added, with a
    "Fixed" value providing the value that is used in the protocol for
    that element, so that the XML and the on the wire protocol can be
    correlated.
 
 5.2.1.  FECapabilities
 
 
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    This element, which if it occurs must occur only once, contains all
    the capability related information about the FE.  Capability
    information is always considered to be read-only.
 
    The currently defined elements allowed within the FECapabilities
    element are ModifiableLFBTopology, LFBsSupported,
    WriteableAttributes and ReadableAttributes.
 
 5.2.1.1. ModifiableLFBTopology
 
    This element has a boolean value.  This element 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.  In
    that case, 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.
 
 5.2.1.2. SupportedLFBs and SupportedLFB
 
    One capability that the FE should include is the list of supported
    LFB classes. The SupportedLFBs element, which occurs at most once,
    serves as a wrapper for the list of LFB classes supported. Each
    class is described in a SupportedLFB element.
 
    Each occurrence of the SupportedLFB 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.
 
 5.2.1.2.1. LFBName
 
    This element has as its value the name of the LFB being described.
 
 5.2.1.2.2. LFBOccurrenceLimit
 
    This element, if present, indicates the largest number of instances
    of this LFB class the FE can support.  For FEs that do not have the
 
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    capability to create or destroy LFB instances, this can either be
    omitted or be the same as the number of LFB instances of this class
    contained in the LFB list attribute.
 
 5.2.1.2.3. PortGroupLimits and PortGroupLimit
 
    The PortGroupLimits element is the wrapper to hold information
    about the port groups supported by the LFB class. It holds multiple
    occurrences of the PortGroupLimit element.
 
    Each occurrence of the PortGroupLimit element contains the port
    occurrence information for a single port group of the LFB class.
    Each occurrence has the name of the port group in the PortGroupName
    element, the fewest number of ports that can exist in the group in
    the MinPortCount element, and the largest number of ports that can
    exist in the group in the MaxPortCount element.
 
 5.2.1.2.4.CanOccurAfters and CanOccurAfter
 
    The CanOccurAfters element is a wrapper to hold the multiple
    occurrences of the CanOccurAfter permissible placement information.
 
    The CanOccurAfter element describes a permissible positioning of
    the SupportedLFB.  Specifically, it names an LFB that can
    topologically precede the SupportedLFB.  That is, the SupportedLFB
    can have an input port connected to an output port of the LFB that
    it CanOccurAfter. The LFB class that the SupportedLFB can follow is
    identified by the NeighborLFB element of the CanOccurAfter 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 SupportedLFB element, each CanOccurAfter element
    must have a unique NeighborLFB, and within each CanOccurAfter
    element each viaPort must represent a unique and valid input port
    group of the SupportedLFB.  The "unique" clauses for this have not
    yet been added to the schema.]
 
 5.2.1.2.5. CanOccurBefores and CanOccurBefore
 
    The CanOccurBefores element is a wrapper to hold the multiple
    occurrences of the CanOccurBefore permissible placement
    information.
 
    The CanOccurBefore element similarly lists those LFB classes that
    the SupportedLFB may precede in the topology.  In this element, the
 
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    viaPort element represents the output port group of the
    SupportedLFB that may be connected to the NeighborLFB.  As with
    CanOccurAfter, viaPort may occur multiple times if multiple output
    ports may legitimately connect to the given NeighborLFB class.
 
    [And a similar set of uniqueness constraints apply to the
    CanOccurBefore clauses, even though an LFB may occur both in
    CanOccurAfter and CanOccurBefore.]
 
 5.2.1.2.6. LFBClassCapabilities
 
    This element contains capability information about the subject LFB
    class whose structure and semantics are defined by the LFB class
    definition.
 
 5.2.1.3. SupportedAttributes
 
    This element serves as a wrapper to hold the information about
    attributed related capabilities. Specifically, attributes should be
    described in this element if:
       a) they are optional elements in the standard and are supported
         by the FE, or
       b) the standard allows for a range of access permissions (for
         example, read-only or read-write).
 
    Each attribute so described is contained in the SupportedAttributes
    element. That element contains an AttributeName element whose value
    is the name of the element being described and an AccessModes
    element, whose value is the list of permissions.
 
 5.2.2. FEAttributes
 
    The FEAttributes element 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.  At the moment, the set of attributes is
    woefully incomplete.  Each attribute is identified by a unique
    element tag, and the value of the element is the value of the
    attribute.
 
 5.2.2.1. FEStatus
 
    This attribute carries the overall state of the FE.  For now, it is
    restricted to the strings AdminDisable, OperDisable and OperEnable.
 
 5.2.2.2.LFBInstances and LFBInstance
 
 
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    The LFBInstances element serves as a wrapper to hold the multiple
    occurrences of the LFBInstance information about individual LFB
    instances on the FE.
 
    Each occurrence of the LFBInstance element describes a single LFB
    instance.  Each element contains an LFBClassName indicating what
    class this instance has, and an LFBInstanceID indicating the ID
    used for referring to this instance.  For now, the ID uses the
    NMTOKEN construction.  Further protocol work is likely to replace
    this with a range restricted integer.
 
 5.2.2.3. LFBTopology and LFBLink
 
    This optional element contains the information about each inter-LFB
    link inside the FE.  Each link is described in an LFBLink element.
    This element contains sufficient information to identify precisely
    the end points of a link. The FromLFBID and ToLFBID fields indicate
    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 the 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.
 
 5.2.2.4. FEConfiguredNeighbors an FEConfiguredNeighbor
 
    The FEConfiguredNeighbors element is a wrapper to hold the
    configuration information that one or more FEConfiguredNeighbor
    elements convey about the configured FE topology.
 
    The FEConfiguredNeighbor element occurs once for each configured FE
    neighbor the FE knows about.  It should not be filled in based on
    FE level protocol operations.  In general, neighbor discovery
    operation on the FE should be represented and manipulated as an
    LFB.  However, for FEs that include neighbor discovery and do not
    have such an LFB, it is permitted to fill in the information in
    this table based on such protocols.
 
    Similarly, the MAC address information in the table is intended to
    be used in situations where neighbors are configured by MAC
    address.  Resolution of network layer to MAC address information
    should be captured in ARP LFBs, not duplicated in this table.  Note
    that the same neighbor may be reached through multiple interfaces
 
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    or at multiple addresses.  There is no uniqueness requirement of
    any sort on occurrences of the FEConfiguredNeighbor element.
 
    Information about the intended forms of exchange with a given
    neighbor is not captured here, only the adjacency information is
    included.
 
 5.2.2.4.1.NeighborID
 
    This is the ID in some space meaningful to the CE for the neighbor.
    If this table remains, we probably should add an FEID from the same
    space as an attribute of the FE.
 
 5.2.2.4.2.NeighborInterface
 
    This identifies the interface through which the neighbor is
    reached.
 
    [Editors note: As the port structures become better defined, the
    type for this should be filled in with the types necessary to
    reference the various possible neighbor interfaces, include
    physical interfaces, logical tunnels, virtual circuits, etc.]
 
 5.2.2.4.3. NeighborNetworkAddress
 
    Neighbor configuration is frequently done on the basis of a network
    layer address.  For neighbors configured in that fashion, this is
    where that address is stored.
 
 5.2.2.4.4.NeighborMacAddress
 
    Neighbors are sometimes configured using MAC level addresses
    (Ethernet MAC address, circuit identifiers, etc.)  If such
    addresses are used to configure the adjacency, then that
    information is stored here.  Note that over some ports such as
    physical point to point links or virtual circuits considered as
    individual interfaces, there is no need for either form of address.
 
 5.3. Sample FE Attribute Document
 
    <?xml version="1.0">
    <fm:FEDocument xmlns:fm="http://www.ietf.org/...theschema...">
 
     <fm:FECapabilities>
 
      <fm:ModifiableLFBTopology> true </fm:ModifiableLFBTopology>
 
 
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      <fm:SupportedLFBs>
       <fm:SupportedLFB>
        <!-- A simple single-input multi-output classifier -->
        <fm:LFBName> Classifier </fm:LFBName>
        <fm:LFBOccurrenceLimit> 3 </fm:LFBOccurrenceLimit>
 
        <fm:PortGroupLimits>
         <fm:PortGroupLimit>
          <!-- The input port -->
          <fm:PortGroupName> InputPortGroup </fm:PortGroupName>
          <fm:MinPortCount> 1 </fm:MinPortCount>
          <fm:MaxPortCount> 1 </fm:MaxPortCount>
         </fm:PortGroupLimit>
         <fm:PortGroupLimit>
         <!--The normal output ports -->
         <fm:PortGroupName> OutputPortGroup </fm:PortGroupName>
          <fm:MinPortCount> 0 </fm:MinPortCount>
          <fm:MaxPortCount> 32 </fm:MaxPortCount>
         </fm:PortGroupLimit>
         <fm:PortGroupLimit>
          <!-- The optional error port -->
          <fm:PortGroupName> ErrorPortGroup </fm:PortGroupName>
          <fm:MinPortCount> 0 </fm:MinPortCount>
          <fm:MaxPortCount> 1 </fm:MaxPortCount>
         </fm:PortGroupLimit>
        </fm:PortGroupLimits>
        <fm:CanOccurAfters>
         <fm:CanOccurAfter>
          <fm:NeighborLFB> Port </fm:NeighborLFB>
          <!-- omitted viaPort -->
         </fm:CanOccurAfter>
         <fm:CanOccurAfter
          <fm:NeighborLFB> InternalSource </fm:NeighborLFB>
          <!-- omitted viaPort -->
         </fm:CanOccurAfter>
        </fm:CanOccurAfters>
        <fm:CanOccurBefores>
         <fm:CanOccurBefore>
          <fm:NeighborLFB> Marker </fm:NeighborLFB>
          <!-- omitted viaPort -->
         </fm:CanOccurBefore>
        </fm:CanOccurBefores>
       </fm:SupportedLFB>
       <!-- then Supported LFB elements for Port, InternalSource -->
       <!--      Marker, ... -->
      </fm:SupportedLFBs>
 
 
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      <fm:SupportedAttributes>
       <fm:SupportedAttribute>
        <fm:AttributeName> FEStatus  </fm:AttributeName>
        <fm:AccessModes>  read write </fm:AccessModes>
       </fm:SupportedAttribute>
       <fm:SupportedAttribute>
        <fm:AttributeName>  Vendor   </fm:AttributeName>
        <fm:AccessModes>    read     </fm:AccessModes>
       </fm:SupportedAttribute
       <fm:SupportedAttribute>
        <fm:AttributeName>  Model    </fm:AttributeName>
        <fm:AccessModes>    read     </fm:AccessModes>
       </fm:SupportedAttribute>
      </fm:SupportedAttributes>
     </fm:FECapabilities>
 
     <fm:FEAttributes>
      <fm:Vendor> World Wide Widgets </fm:Vendor>
      <fm:Model> Foo Forward Model 6 </fm:Model>
      <fm:FEStatus>       OperEnable </fm:FEStatus>
      <fm:LFBInstances>
       <fm:LFBInstance>
        <fm:LFBClassName> Classifier </fm:LFBClassName>
        <fm:LFBInstanceID> Inst5     </fm:LFBInstanceID>
       </fm:LFBInstance>
       <fm:LFBInstance>
        <fm:LFBClassName> Interface  </fm:LFBClassName>
        <fm:LFBInstanceID> Inst11    </fm:LFBInstanceID>
       </fm:LFBInstance>
       <fm:LFBInstance>
        <fm:LFBClassName> Meter      </fm:LFBClassName>
        <fm:LFBInstanceID> Inst17    </fm:LFBInstanceID>
       </fm:LFBInstance>
      </fm:LFBIntances>
      <fm:LFBTopology>
       <fm:LFBLink>
        <fm:FromLFBID>   Inst11      </fm:fromLFBID>
        <fm:FromPortGroup> IFOnwardGroup </fm:FromPortGroup>
        <fm:FromPortIndex> 1         </fm:FromPortIndex>
        <fm:ToLFBID>     Inst5       </fm:ToLFBID>
        <fm:ToPortGroup> InputPortGroup </fm:ToPortGroup>
        <fm:ToPortIndex> 1           </fm:ToPortIndex>
       </fm:LFBLink>
       <fm:LFBLink>
        <fm:FromLFBID>   Inst5       </fm:fromLFBID>
        <fm:FromPortGroup> OutputGroup </fm:FromPortGroup>
        <fm:FromPortIndex> 1         </fm:FromPortIndex>
 
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        <fm:ToLFBID>     Inst17       </fm:ToLFBID>
        <fm:ToPortGroup> InMeterGroup </fm:ToPortGroup>
        <fm:ToPortIndex> 1           </fm:ToPortIndex>
       </fm:LFBLink>
      </fm:LFBTopology>
     </fm:FEAttributes>
    </fm:FEDocument>
 
 6. LFB Class Library
 
    A set of initial LFB classes are identified here in the LFB class
    library as necessary to build common FE functions.  Some of the LFB
    classes described here are abstract base classes from which
    specific LFB sub-classes will be derived.  Hence, the base classes
    may not be used directly in a particular FE's model, but the sub-
    classes (yet to be defined) could be.  This initial list attempts
    to describe LFB classes at the expected level of granularity.  This
    list is neither exhaustive nor sufficiently detailed.
 
    Several working groups in the IETF have already done some relevant
    work in modeling the provisioning policy data for some of the
    functions we are interested in, for example, the DiffServ
    (Differentiated Services) PIB [4] and IPSec PIB [8].  Whenever
    possible, we have tried to reuse the work done elsewhere instead of
    reinventing the wheel.
 
 6.1. Port LFB
 
    A Port LFB is used to model physical I/O ports on the FE.  It is
    both a source of data "received" by the FE and a sink of data
    "transmitted" by the FE.  The Port LFB contains a number of static
    attributes, which may include, but are not limited to, the
    following items:
       . the number of physical ports on this LFB
       . physical port type
       . physical port link speed (may be variable; e.g., 10/100/1000
         Ethernet).
 
    In addition, the Port LFB contains a number of configurable
    attributes, including:
       . physical port current status (up or down)
       . physical port loopback
       . physical port mapping to L2 interface.
 
    The Port LFB can be sub-classed into technology specific LFB
    classes, with additional static and configurable attributes.
    Examples of possible sub-classes include:
       . Ethernet
 
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       . Packet-over-SONET OC-N
       . ATM-over-SONET/SDN OC-N
       . T3
       . E3
       . T1
       . E1
       . CSIX-L1 switching fabric port (Fi interface)
       . CE-FE port (for Fp interface).
 
    LFB class inheritance can be used to sub-class derived LFB classes
    with additional properties, such as TDM channelization.
 
    The Port LFB "receives" (sources) and "transmits" (sinks) frames in
    technology specific formats (described in the respective LFB class
    definition but not otherwise modeled) into/out of the FE.  Packets
    "received" from a physical port are sourced on (one of) the LFB's
    output port(s), while packets to be "transmitted" on a physical
    port are sinked on (one of) the LFB's input port(s).  The Port LFB
    is unique among LFB classes in that packets accepted on a LFB input
    port are not emitted back out on an LFB output port (except in the
    case of physical port loopback operation).
 
    The Port LFB transmits technology specific L2 frames to
    topologically adjacent LFB instances (i.e., no frame
    decapsulation/encapsulation is modeled in this LFB class).  When
    transmitting a frame to an adjacent downstream LFB, the Port LFB
    provides two items of metadata: the frame length and the L2
    interface identifier.  When receiving frames from an adjacent
    upstream LFB, the frame is accompanied by two items of metadata:
    frame length and outgoing port identifier.
 
    Statistics are not maintained by the Port LFB; statistics
    associated with a particular port may be maintained by an L2
    interface LFB (see Section 6.2).
 
 6.2. L2 Interface LFB
 
    The L2 Interface LFB models an L2 protocol termination.  The L2
    Interface LFB performs two sets of functions: decapsulation and
    demultiplexing as needed on the receive side of an FE, and
    encapsulation and multiplexing as needed on the transmit side.
    Hence the LFB has two distinct sets of inputs and outputs tailored
    for these separate functions.  The L2 Interface LFB is not modeled
    as two separate (receive/transmit) LFBs because there are shared
    attributes between the decapsulation and encapsulation functions.
 
 
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    On the decapsulation input(s), the LFB accepts an L2 protocol
    specific frame, along with frame length and L2 interface metadata.
    The LFB decapsulates the L2 frame by removing any L2
    header/trailers (while simultaneously applying any checksum/CRC
    functions), determines the L2 or L3 protocol type of the next-layer
    packet (based on a PID or Ethertype within the L2 frame header),
    adjusts the frame length metadata, and uses the L2 interface
    metadata to select an L2 interface attribute.  The L2 interface
    attribute supports a number of additional attributes, including:
       . L2 MTU
       . supported next-layer L2 or L3 protocols
       . L2-specific receive counters (byte, packet)
       . counting mode
       . L2 or L3 interface metadata for next-layer packet
       . LFB output port.
    The LFB may support multiple decapsulation output ports within two
    output groups; one for normal forwarding, and one for exception
    packets.  The LFB emits the decapsulated packet along with the
    modified frame length metadata, an L2 or L3 protocol type metadata,
    and an L2 or L3 interface metadata.
 
    On the encapsulation input(s), the LFB accepts a packet along with
    frame length, protocol type, and L2 interface metadata.  The L2
    interface metadata is used to select an L2 interface attribute
    which supports a number of additional attributes, including:
       . L2-specific transmit counters (byte, packet)
       . counting mode (may be taken from receive counters mode)
       . L2 or L3 interface metadata for next-layer frame (we assume
         that L2
       . protocols could be layered on top of an L3 protocol; e.g.,
         L2TP or
       . PWE3), or port metadata.
       . LFB output port.
    The LFB encapsulates the packet using the appropriate L2
    header/trailer and protocol type information (calculating
    checksums/CRCs as necessary), and provides the frame to the next
    LFB along with incremented frame length metadata, updated protocol
    type metadata, and updated interface (or port) metadata, on a
    configurable LFB encapsulation output.
 
    As in the case of the Port LFB, technology specific variants of the
    L2 interface LFB will be sub-classes of the L2 Interface LFB.
    Example sub-classes include:
       . Ethernet/802.1Q
       . PPP
       . ATM AAL5.
 
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    Each sub-class will likely support static and configurable
    attributes specific to the L2 technology; for example the
    Ethernet/802.1Q Interface LFB will support a per-interface MAC
    address attribute.  Note that each technology specific sub-class
    may require additional metadata.  For example, the Ethernet/802.1Q
    Interface LFB requires an outgoing MAC destination address to
    generate an outgoing Ethernet header.
 
    The L2 interface management function is separated into a distinct
    LFB from the Port LFB because L2 encapsulations can be nested
    within frames; e.g., PPP-over-Ethernet-over-ATM AAL5 (PPPoEoA).
 
 6.3. IP interface LFB
 
    The IP Interface LFB models a container for IP interface-specific
    attributes.  These may include:
       . IP protocols supported (IPv4 and/or IPv6)
       . IP MTU
       . interface MIB counters
       . table metadata for associated forwarding tables (LPM,
         multicast)
       . table metadata for associated classification tables.
    The IP Interface LFB also performs basic protocol-specific packet
    heade validation functions (e.g., IP version, IPv4 header length,
    IPv4 header checksum, MTU, TTL=0, etc.).  The IP Interface LFB
    class supports three different L3 protocols: IPv4, IPv6, and MPLS,
    although individual LFB instances might support a subset of these
    protocols, configurable on each interface attribute.
 
    As with the L2 Interface LFB, the IP Interface LFB supports two
    modes of operation: one needed on the receive side of an FE, and
    one on the transmit side, using separate sets of LFB inputs and
    outputs. In the first mode of operation (for FE receive
    processing), the IP Interface LFB accepts IP packets along with
    frame length, L3 protocol type, and interface metadata (possibly
    including additional metadata items such as L2-derived class
    metadata).  The interface metadata is used to select an interface
    attribute, and the protocol type is checked against the protocols
    supported for this interface.  Error checks are applied, including
    whether the particular protocol type is supported on this
    interface, and if no errors occur, the appropriate counters are
    incremented and the protocol type is used to select the outgoing
    LFB output from a set dedicated to the first mode of operation. The
    IP header protocol type/next header field may also be used to
    select an LFB output; for example, IPv4 packets with AH header may
    be directed to a particular next LFB, or IPv6 packets with Hop-by-
    Hop Options.  If errors do occur, the appropriate error counters
 
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    are incremented, and the error type is used to select a specific
    exception LFB output.
 
    In the second mode of operation (for FE transmit processing), the
    IP Interface LFB accepts an IP packet along with frame length,
    protocol type, and interface metadata.  Again, the interface
    metadata is used to select an interface attribute.  The interface
    attribute stores the outgoing L2 or IP interface (e.g., tunnel)
    interface metadata.  The IP MTU of the outgoing interface is
    checked, along with the protocol type of the packet.  If no errors
    occur, the appropriate counters are incremented, and the next level
    interface metadata may be used to select an IP Interface LFB output
    dedicated to the second mode of operation.  Otherwise, the
    appropriate error counters are incremented, and the error type is
    used to select an exception output.
 
    Because the IP Interface LFB is the repository for the interface
    MIB counters, two special pairs of inputs are provided for packets
    which have been selected to be discarded further downstream (one
    each for the receive and transmit counters).  Packets arriving on
    these LFB inputs must be accompanied by frame length and L3
    interface metadata.  An exception output on the LFB should be
    connected to a dropper LFB.
 
 6.4. Classifier LFB
 
    The function of classification is to logically partition packets
    into one of N different classes, based on some sequence of one or
    more mathematical operations applied to the packet and its
    associated metadata. Various LFBs perform an intrinsic
    classification function.  Where this function is a well-defined
    protocol operation, a separate LFB may be defined (e.g., IP
    Interface LFB, which performs header verification).
 
    Several common applications need to classify packets using a
    particular mathematical operation (e.g., longest prefix match (LPM)
    or ternary match) against a fixed set of fields in a packet's
    header plus metadata, or an easily recognized part of the packet
    payload.  Two example applications are classification for
    Differentiated Services or for security processing. Typically the
    packet is evaluated against a potentially large set of rules
    (called "filters") which are processed in a particular order to
    ensure a deterministic result.  This sort of classification
    functionalit is modeled by the Classifier LFB.
 
 
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    The Classifier LFB accepts an input packet and metadata, and
    produces the unmodified packet along with a class metadata, which
    may be used to map the packet to a particular LFB output.
 
    The Classifier LFB supports multiple classifier attributes.  Each
    classifier is parameterized by one or more filters.  Classification
    is performed by selecting the classifier to use on a particular
    packet (e.g., by metadata lookup on a configurable metadata item),
    and by evaluating the selected contents of the accepted packet
    against that classifier's filters.  A filter decides if the input
    packet satisfies particular criteria.  According to [DiffServ], "a
    filter consists of a set of conditions on the component values of a
    packet's classification key (the header values, contents, and
    attributes relevant for classification)".
 
    Note that other LFBs may perform simple classification on the
    packet or its metadata.  The purpose of the Classifier LFB is to
    model an LFB that "digests" large amounts of input data (packet,
    metadata), to produce a "summary" of the classification results, in
    the form of additional (or modified) metadata. Other LFBs can then
    use this summary information to quickly and simply perform trivial
    classification operations.
 
    The Classifier LFB can be sub-classed into several function-
    specific LFB classes which perform common classification functions.
    These may include:
       . Longest Prefix Match (LPM)
       . IP Multicast lookup (S,G)
       . Multifield Exact Match
       . Multifield Ternary Match.
 
 6.5. Next Hop LFB
 
    The Next Hop LFB is used to resolve next hop information following
    a forwarding lookup.  Next Hop information normally includes the
    outgoing interface (or interfaces, in the case of multicast), as
    well as the outgoing IP address(es).  This next hop information
    associated with a forwarding prefix or classification rule is often
    separated into a separate data structure in implementations to
    allow the two pieces of information to be decoupled, because there
    is frequently a fan-in relationship between forwarding prefix/rule
    entries and next hop information, and decoupling them can permit
    more efficient data structure management.
 
    The Next Hop LFB maintains next hop attributes organized into
    multiple next hop tables.  The relevant table for a packet is
 
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    selected based on next hop table metadata.  A set of one or more
    next hop attributes is selected based on next hop index metadata.
    Each next hop attribute stores the following information:
       . a list of one or more outgoing interfaces
       . next hop IP addresses, or, an index to a table of this
         information
       . that is maintained at a downstream LFB
       . a list of outgoing MTUs
       . TTL decrement value
 
    The Next Hop LFB has two primary operations.  The first is to map
    the incoming next hop table and next hop index metadata into a
    configurable next hop attribute.  This mapping may be direct (one
    metadata pair to one next hop attribute). If the next hop index
    metadata selects a set of next hop attributes, final attribute
    resolution depends on a selection algorithm that uses some
    additional metadata, or an internal classification operation, to
    select among a set of possible next hop attributes.  One example is
    weighted next hop selection, where individual packets are mapped to
    particular next hop attributes in the set according to weights and
    to some flow order-preserving function (e.g., such as an address
    pair hash).  Another alternative is class-based next hop selection,
    based on some class metadata.
 
    The second operation is a derivative of the first.  The next hop
    table and next hop index metadata are used to select a set of one
    or more next hop attributes.  Then the outgoing interface values
    stored in those attributes are compared against the incoming
    interface metadata provided to the Next Hop LFB, to determine
    whether the incoming interface is in the set.  This operation, in
    combination with a IP source address forwarding lookup (which
    provides the next hop table/index metadata), can be used to perform
    a reverse path forwarding (RPF) check.
 
    The Next Hop LFB has two inputs: one for normal next hop
    resolution, and one for the incoming interface metadata test (e.g.,
    RPF).  The LFB requires incoming interface, frame length, next hop
    table, and next hop index metadata.  There are two normal output
    groups, one for the normal next hop resolution, and another for the
    RPF check.  No additional metadata is produced for the latter, but
    for the former, the following metadata may be produced:
       . outgoing interface(s)
       . next hop IP address(es)
       . TTL decrement value (if TTL decrement is not performed by the
         Next Hop LFB)
    An alternative mode of operation produces index metadata instead of
    outgoing interface and next hop IP address metadata.  This index
 
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    metadata is used to access a cache of the outgoing interface and
    next hop IP address that may be stored on the egress FE (this
    permits more efficient communication across the Fi interface).
    This index metadata can also be used as input metadata to a MPLS
    Encapsulation LFB.
 
    The Next Hop LFB supports an exception output port group.
    Exception conditions include:
       . RPF test failed
       . No route to host
       . No route to network
       . Packet too big
       . TTL expired
    The mapping between exception conditions and exception outputs is
    configurable, and an exception code metadata is produced on these
    outputs.
 
 6.6. Rate Meter LFB
 
    The Rate Meter LFB is used to meter the packet flow through the LFB
    according to a rate- and time-dependent function.  Packets are
    provided to the Rate Meter LFB along with packet length metadata
    (and optional color metadata) and are associated with a meter
    attribute either statically (based on LFB input) or via some other
    configurable metadata item.  The metering algorithm of the
    associated meter attribute is applied to the packet, using the
    packet length and the current time as inputs, along with previous
    state maintained by the attribute.  A color metadata is associated
    with the packet in accordance with the metering algorithm used.
    The color metadata is optionally emitted with the packet, or used
    to map the packet to a particular LFB output.  Color-aware metering
    algorithms use color metadata if provided with the packet (e.g., by
    a Classifier LFB), or assume a default color value.
 
    The Rate Meter LFB supports a number of static attributes,
    including:
       . supported metering algorithms
       . maximum number of meter attributes.
 
    The Rate Meter LFB supports a number of configurable attributes,
    including:
       . number of LFB inputs
       . number of LFB outputs
       . mapping of LFB input to meter attribute (when mapped
         statically)
       . metadata item to select for mapping to meter attribute
       . mapping of metadata value to meter attribute
 
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       . default meter attribute (when not mapped statically or via
         correct
       . metadata)
       . per-attribute metering algorithm
       . per-attribute metering paramters, including:
       . minimum rate
       . maximum rate
       . burst size
       . color metadata enable
       . mapping of packet color to LFB output.
 
    A Rate Meter LFB can be used to implement a policing function, by
    connecting a LFB output directly to a Dropper LFB, and mapping non-
    conforming (e.g., "red") traffic to that output.
 
 6.7. Redirector (de-MUX) LFB
 
    The Redirector LFB is used to select between alternative datapaths
    based on the value of some metadata item.  The Redirector LFB
    accepts an input packet P, and uses associated metadata item M to
    demultiplex that packet onto one of N outputs; e.g., unicast
    forwarding, multicast, or broadcast.  Configurable attributes
    include:
       . number of LFB output ports (N)
       . metadata item to demultiplex on (M)
       . mapping of metadata value to output port
       . default output port (for un-matched input metadata values).
 
    Note that other LFBs may include demultiplexing functionality
    (i.e., if they have multiple outputs in an output group).  The
    Redirector LFB is especially useful for demultiplexing based on
    metadata items that are not generated or modified by an immediate
    upstream LFB.
 
 6.8. Packet Header Rewriter LFB
 
    The Packet Header Rewriter LFB is used to re-write fields in a
    packet's header.  Function-specific sub-classes of the Packet
    Header Rewriter LFB may be specified as sub-classes of the Modifier
    LFB.  These may include:
       . IPv4 TTL/IPv6 Hop Count
       . IPv4 header checksum
       . DSCP
       . IPv4 NAT
 
    The precise means by which the packet header rewriting functions
    will be specified is TBD.
 
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 6.9. Counter LFB
 
    The Counter LFB is used to maintain packet and/or byte statistics
    on the packet flow through the LFB.  Packets are provided to the
    Counter LFB on an LFB input along with packet length metadata and
    are associated with a count attribute either statically (based on
    the LFB input) or via some other configurable metadata item.  The
    Counter LFB modifies neither the packet nor any associated
    metadata.
 
    The Counter LFB supports a number of static attributes, including:
       . supported counting modes (e.g., byte, packet, both)
       . supported logging modes (e.g., last recorded packet time)
       . maximum number of count attributes
 
    The Counter LFB supports a number of configurable attributes,
    including:
       . number of LFB inputs
       . mapping of LFB input to count attribute (when mapped
         statically)
       . metadata item to select for mapping to count attribute
       . mapping of metadata value to count attribute
       . default count attribute (when not mapped statically or via
         correct
       . metadata)
       . counting mode per-attribute
       . logging mode per-attribute.
 
    The Counter LFB does not perform any time-dependent counting.  The
    time at which a count is made may, however, be logged as part of
    the count attribute.
 
    Other LFBs may maintain internal statistics (e.g., interface LFBs).
    The Counter LFB is especially useful for maintain counts associated
    with QoS policy.
 
 6.10. Dropper LFB
 
    A Dropper LFB has one input, and no outputs.  It discards all
    packets that it accepts without any modification or examination of
    those packets.
 
    The purpose of a Dropper LFB is to allow the description of "sinks"
    within the model, where those sinks do not result in the packet
    being sent into any object external to the model.
 
 
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    The Dropper LFB has no configurable attributes.
 
 6.11. IPv4 Fragmenter LFB
 
    The IPv4 Fragmenter LFB fragments IPv4 packets according to the MTU
    of the outgoing interface.  The IPv4 Fragmenter LFB accepts packets
    with frame length and MTU metadata, and produces a sequence of one
    or more valid IPv4 packets properly fragmented, each along with
    corrected frame length metadata.
 
    The source of the outgoing interface MTU is TBD.  The IPv4
    fragmentation function is not incorporated into the IP Interface
    LFB because forwarding implementations may include additional
    forwarding functions between fragmentation and final output
    interface processing.
 
 6.12. L2 Address Resolution LFB
 
    The L2 Address Resolution LFB is used to map an next hop IP address
    into an L2 address.  The LFB accepts packets with output L2
    interface and next hop IP address metadata, and produces the packet
    along with the correct L2 destination address.  The L2 Address
    Resolution LFB maintains multiple address resolution table
    attributes accessed by the output L2 interface metadata.  Each
    table attribute maintains a set of configurable L2 address
    attributes, accessed by the next hop IP address.
 
    The L2 Address Resolution LFB has a normal output group which
    produces the L2 destination address metadata, as well as an
    exception output. This exception output can be used to divert the
    packet to another LFB (e.g., an ARP/ND Protocol LFB, or a Port LFB
    used to reach the CE) for address resolution.
 
 6.13. Queue LFB
 
    The Queue LFB is used to represent queueing points in the packet
    datapath.  It is always used in combination with one or more
    Scheduler LFBs.  The Queue LFB manages one or more FIFO packet
    queues as configurable attributes.  The Queue LFB provides one or
    more LFB inputs, and packets are mapped from LFB inputs to queues,
    either statically, or via queue metadata.  Each queue attribute is
    mapped one-to-one with a scheduling input on a downstream Scheduler
    LFB.  The Queue LFB provides one or more LFB outputs, along with
    optional scheduling input metadata.
 
    Additional per-queue configurable attributes include the following:
 
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       . maximum depth discard behavior (tail drop/head drop/Active
         Queue Management (AQM))
       . AQM parameters (specific to the AQM algorithm; e.g., RED)
       . Explicit Congestion Notification (ECN) enable.
 
    Packets are provided to the Queue LFB along with a packet length
    metadata and an optional queue metadata.  Because the Queue LFB can
    model sophisticated AQM mechanisms such as per-color marking
    thresholds (e.g., Weighted RED), packets may also be accompanied
    with color metadata.
 
    If ECN is enabled on a queue serving IP packets, then the IP packet
    header is modified if congestion is marked.  A protocol type
    metadata must accompany the packet to indicate the packet protocol
    (e.g., IPv4, IPv6, Ethernet), so that the implementation can
    determine the location of the ECN bits in the header [RFC3168].  In
    the case of IPv4, if congestion is signaled, the header checksum
    must be modified.  The Queue LFB supports a capability to indicate
    whether it corrects the IPv4 header checksum after marking
    congestion experienced.  Support for the checksum fixup is not
    mandatory since the checksum may be recalculated in another LFB
    further downstream.
 
 6.14. Scheduler LFB
 
    The Scheduler LFB is used to perform packet scheduling at queueing
    points in the packet datapath, and hence is always used in
    combination with one or more upstream Queue or Scheduler LFBs.  The
    Scheduler LFB supports one or more logical scheduling inputs.  A
    scheduling input can be mapped one-to-one to a Scheduler LFB input,
    or the scheduling input can be selected via metadata (and both
    mechanisms may be used in combination).
 
    The Scheduler LFB multiplexes its scheduling inputs onto a single
    LFB output, based on its scheduling algorithm along with the per-
    input scheduling configuration.  The packet is not modified during
    the scheduling process.
 
    Packets are provided to the Scheduler LFB along with a packet
    length metadata and an optional scheduling input metadata.
 
    Configurable attributes include:
       . number of logical scheduler inputs
       . number of LFB inputs
       . mapping of LFB input to scheduler input
       . scheduling algorithm
       . per-input scheduling parameters, including:
 
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       . priority
       . minimum service rate
       . maximum service rate
       . burst duration (at maximum service rate).
 
    Hierarchical scheduling configurations can be created by cascading
    two or more Scheduler LFBs.
 
 6.15. MPLS ILM/Decapsulation LFB
 
    The MPLS Incoming Label Map (ILM)/Decapsulation LFB accepts MPLS-
    encapsulated packets, examines (and possibly removes) the top-most
    label, and emits the packet on one output within an output group,
    along with configurable index and class metadata.  The configurable
    metadata can be used as input for an IP Interface LFB, a Next Hop
    LFB, or the same (or another) MPLS ILM/Decapsulation LFB.  This
    allows the FE to terminate, forward, or "pop and lookup" on the
    value of the top-most label.  The LFB maintains a set of ILM table
    attributes indexed by incoming IP interface metadata.  Each ILM
    table entry is an attribute specifying whether to remove the label,
    and which output port to emit the packet on.  An exception output
    is provided for packets with expired TTL.
 
 6.16. MPLS Encapsulation LFB
 
    The MPLS Encapsulation LFB accepts IP or MPLS-encapsulated packets
    and appends an MPLS label stack, which is selected by output
    interface and configurable index metadata.  The TTL of the accepted
    packet is copied from the outermost header into the labels in the
    label stack, and the S bit is set on the bottom label if the
    accepted packet is IP.  The MPLS EXP bits are copied (or mapped)
    according to per-stack attributes.
 
    The MPLS Encapsulation LFB maintains multiple stack table
    attributes indexed by output interface metadata.  Entry attributes
    within a table are indexed by configurable index metadata.  Each
    entry attribute maintains a label stack, along with a configurable
    attribute for EXP bit handling, and possibly class and/or queue
    metadata to emit with the packet.
 
    MPLS ILM/decapsulation and encapsulation functions are modeled in
    separate LFBs because some implementations split these operations
    across FEs.
 
 6.17. Tunnel Encapsulation/Decapsulation LFB
 
 
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    The Tunnel Encapsulation/Decapsulation LFB models tunnel header
    encapsulation and decapsulation/demultiplexing.  The LFB maintains
    separate encapsulation and decapsulation input and output groups.
    The encapsulation input group accepts packets with tunnel metadata,
    appends a tunnel header that is stored in a configurable attribute
    indexed by the tunnel metadata, and emits the packet on an
    encapsulation output.  The decapsulation input group accepts
    packets encapsulated with a tunnel header along with tunnel
    metadata, removes the tunnel header (performing any tunnel-
    protocol-specific classification) according to attributes
    configured on a per-tunnel basis and accessed via the tunnel
    metadata, and emits the packet along with configurable metadata.
    For example, the configurable metadata that is output may be used
    as input interface metadata by a downstream IP or L2 Interface LFB.
    A decapsulation exception output is available and is used in the
    event that decapsulation fails.
 
    The Tunnel Encapsulation/Decapsulation LFB may be sub-classed into
    tunnel-protocol-specific LFBs, including:
       . IP-IP
       . GRE
       . L2TP
       . Generic IPv6 Tunnels
 
 6.18. Replicator LFB
 
    The Replicator LFB is used to replicate accepted packets and emit
    them on one or more outputs in an output group.  Packets are
    accepted along with replicator index metadata.  The LFB maintains
    an attribute table indexed by this metadata.  Each table entry
    attribute specifies the number of times the packet must be
    replicated, the outputs (within the output group) that each
    replicated packet should be emitted on, and configurable metadata
    to be associated with each replicated packet.
 
    The Replicator LFB can be used for multicast replication, or for
    transparent packet interception.
 
 
 7. 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).
 
 
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     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 support flexible implementations by allowing an FE to
     specify supported optional features and to indicate which
     attributes are configurable by the CE for an LFB class (e.g.,
     express the capability of the FE). Configurable attributes also
     provide the CE some flexibility in specifying the behavior of a
     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, one 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 definition of the minimal set of LFBs
    that is required by Section 5.5 of [1]. The sections that follow
    provide more detail on the specifics of each of those LFBs.
 
 7.1. Port Functions
 
    The FE model can be used to define a Port LFB class and its
    technology-specific subclasses (see Section 6.1) 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.
 
 7.2. Forwarding Functions
 
 
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    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 (see Section 6.4) and Next Hop LFB (see Section
    6.5).  Special header writer LFB (see Section 6.8) is also needed
    to take care of TTL decrement and checksum etc.
 
 7.3. QoS Functions
 
    The LFB class library already includes descriptions of the Meter
    (Section 6.6.), Queue (Section 6.13), Scheduler (Section 6.14),
    Counter (Section 6.9) and Dropper (Section 6.10) LFBs to support
    the QoS functions in the forwarding path.  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.
 
 7.4. Generic Filtering Functions
 
    Various combinations of Classifier (Section 6.4), Redirector
    (Section 6.7), Meter (Section 6.6.) and Dropper (Section 6.10) LFBs
    can model a complex set of filtering functions.
 
 7.5. 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
    inheritance.
 
 7.6.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 in Section 6 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.
 
 7.7. Security Functions
 
 
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    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 should
    try to reuse the work as much as we can.
 
 7.8. Off-loaded Functions
 
    In addition to the packet processing functions that are typical to
    find on the FEs, some logical functions may also be executed
    asynchronously by some FEs, according to a certain finite-state
    machine, 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, so
    that the CE may take advantage of such off-loaded functions on the
    FEs.
 
 7.9. IPFLOW/PSAMP Functions
 
    [9] defines 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 LFB (Section 7.2.2) and counter LFB (Section 7.2.1)
    can also be used to support accounting functions 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.
 
 8. Using the FE model in the ForCES Protocol
 
    The actual model of the forwarding plane in a given NE is something
    the CE must learn and control by communicating with the FEs (or by
    other means). Most of this communication will happen in the post-
    association phase using the ForCES protocol. The following types of
    information must be exchanged between CEs and FEs via the ForCES
    protocol:
       1)  FE topology query;
 
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       2)  FE capability declaration;
       3)  LFB topology (per FE) and configuration capabilities query;
       4)  LFB capability declaration;
       5)  State query of LFB attributes;
       6)  Manipulation of LFB attributes;
       7)  LFB topology reconfiguration.
 
    Items 1) through 5) are query exchanges, where the main flow of
    information is from the FEs to the CEs. Items 1) through 4) are
    typically queried by the CE(s) in the beginning of the post-
    association (PA) phase, though they may be repeatedly queried at
    any time in the PA phase. Item 5) (state query) will be used at the
    beginning of the PA phase, and often frequently during the PA phase
    (especially for the query of statistical counters).
 
    Items 6) and 7) are "command" types of exchanges, where the main
    flow of information is from the CEs to the FEs. Messages in Item 6)
    (the LFB re-configuration commands) are expected to be used
    frequently.  Item 7) (LFB topology re-configuration) is needed only
    if dynamic LFB topologies are supported by the FEs and it is
    expected to be used infrequently.
 
    Among the seven types of payload information the ForCES protocol
    carries between CEs and FEs, the FE model covers all of them except
    item 1), which concerns the inter-FE topology.  The FE model
    focuses on the LFB and LFB topology within a single FE.  Since the
    information related to item 1) requires global knowledge about all
    of the FEs and their inter-connection with each other, this
    exchange is part of the ForCES base protocol instead of the FE
    model.
 
    The relationship between the FE model and the seven post-
    association messages are visualized in Figure 9:
 
 
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                                                     +--------+
                                        ..........-->|   CE   |
                   /----\               .            +--------+
                   \____/ FE Model      .              ^    |
                   |    |................        (1),2 |    | 6, 7
                   |    |  (off-line)   .      3, 4, 5 |    |
                   \____/               .              |    v
                                        .            +--------+
                 e.g. RFCs              ..........-->|   FE   |
                                                     +--------+
 
    Figure 9. Relationship between the FE model and the ForCES protocol
      messages, where (1) is part of the ForCES base protocol, and the
                     rest are defined by the FE model.
 
    The actual encoding of these messages is defined by the ForCES
    protocol and beyond the scope of the FE model. Their discussion is
    nevertheless important here for the following reasons:
    .    These PA model components have considerable impact on the FE
    model. For example, some of the above information can be
    represented as attributes of the LFBs, in which case such
    attributes must be defined in the LFB classes.
    .    The understanding of the type of information that must be
    exchanged between the FEs and CEs can help to select the
    appropriate protocol format and the actual encoding method (such as
    XML, TLVs).
    .    Understanding the frequency of these types of messages should
    influence the selection of the protocol format (efficiency
    considerations).
 
    An important part of the FE model is the port the FE uses for its
    message exchanges to and from the CE.  In the case that a dedicated
    port is used for CE-FE communication, we propose to use a special
    port LFB, called the CE-FE Port LFB (a subclass of the general Port
    LFB in Section 6.1), to model this dedicated CE-FE port.  The CE-FE
    Port LFB acts as both a source and sink for the traffic from and to
    the CE.  Sometimes the CE-FE traffic does not have its own
    dedicated port, instead the data fabric is shared for the data
    plane traffic and the CE-FE traffic.  A special processing LFB can
    be used to model the ForCES packet encapsulation and decapsulation
    in such cases.
 
    The remaining sub-sections of this section address each of the
    seven message types.
 
 8.1. FE Topology Query
 
 
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    An FE may contain zero, one or more external ingress ports.
    Similarly, an FE may contain zero, one or more external egress
    ports.  In other words, not every FE has to contain any external
    ingress or egress interfaces.  For example, Figure 10 shows two
    cascading FEs.  FE #1 contains one external ingress interface but
    no external egress interface, while FE #2 contains one external
    egress interface but no ingress interface.  It is possible to
    connect these two FEs together via their internal interfaces to
    achieve the complete ingress-to-egress packet processing function.
    This provides the flexibility to spread the functions across
    multiple FEs and interconnect them together later for certain
    applications.
 
    While the inter-FE communication protocol is out of scope for
    ForCES, it is up to the CE to query and understand how multiple FEs
    are inter-connected to perform a complete ingress-egress packet
    processing function, such as the one described in Figure 10.  The
    inter-FE topology information may be provided by FEs, may be hard-
    coded into CE, or may be provided by some other entity (e.g., a bus
    manager) independent of the FEs.  So while the ForCES protocol
    supports FE topology query from FEs, it is optional for the CE to
    use it, assuming the CE has other means to gather such topology
    information.
 
       +-----------------------------------------------------+
       |  +---------+   +------------+   +---------+         |
     input|         |   |            |   |         | output  |
    ---+->| Ingress |-->|Header      |-->|IPv4     |---------+--->+
       |  | port    |   |Decompressor|   |Forwarder| FE      |    |
       |  +---------+   +------------+   +---------+ #1      |    |
       +-----------------------------------------------------+    V
                                                                  |
            +-----------------------<-----------------------------+
            |
            |    +----------------------------------------+
            V    |  +------------+   +----------+         |
            | input |            |   |          | output  |
            +->--+->|Header      |-->| Egress   |---------+-->
                 |  |Compressor  |   | port     | FE      |
                 |  +------------+   +----------+ #2      |
                 +----------------------------------------+
 
 
            Figure 10. An example of two FEs connected together.
 
 
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    Once the inter-FE topology is discovered by the CE after this
    query, it is assumed that the inter-FE topology remains static.
    However, it is possible that an FE may go down during the NE
    operation, or a board may be inserted and a new FE activated, so
    the inter-FE topology will be affected.  It is up to the ForCES
    protocol to provide a mechanism for the CE to detect such events
    and deal with the change in FE topology.  FE topology is outside
    the scope of the FE model.
 
 8.2. FE Capability Declarations
 
    FEs will have many types of limitations. Some of the limitations
    must be expressed to the CEs as part of the capability model. The
    CEs must be able to query these capabilities on a per-FE basis.
    Examples:
       . Metadata passing capabilities of the FE. Understanding these
         capabilities will help the CE to evaluate the feasibility of
         LFB topologies, and hence to determine the availability of
         certain services.
       . Global resource query limitations (applicable to all LFBs of
         the FE).
       . LFB supported by the FE.
       . LFB class instantiation limit.
       . LFB topological limitations (linkage constraint, ordering
         etc.)
 
 8.3. 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.
 
 8.4. 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
 
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    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,
         etc.
 
    The capability parameters that can be queried on a given LFB class
    will be part of the LFB class specification.  The capability
    parameters should be regarded as special attributes of the LFB. The
    actual values of these arguments may be, therefore, obtained using
    the same attribute query mechanisms as used for other LFB
    attributes.
 
    Capability attributes will typically be read-only arguments, but in
    certain cases they may be configurable. For example, the size of a
    lookup table may be limited by the hardware (read-only), in other
    cases it may be configurable (read-write, within some hard limits).
 
    Assuming that capabilities will not change frequently, the
    efficiency of the protocol/schema/encoding is of secondary concern.
 
 8.5. State Query of LFB Attributes
 
    This feature must be provided by all FEs.  The ForCES protocol and
    the data schema/encoding conveyed by the protocol must together
 
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    satisfy the following requirements to facilitate state query of the
    LFB attributes:
       . Must permit FE selection. This is primarily to refer to a
         single FE, but referring to a group of (or all) FEs may
         optional be supported.
       . Must permit LFB instance selection. This is primarily to refer
         to a single LFB instance of an FE, but optionally addressing
         of a group of LFBs (or all) may be supported.
       . Must support addressing of individual attribute of an LFB.
       . Must provide efficient encoding and decoding of the addressing
         info and the configured data.
       . Must provide efficient data transmission of the attribute
         state over the wire (to minimize communication load on the CE-
         FE link).
 
 8.6. LFB Attribute Manipulation
 
    This is a place-holder for all operations that the CE will use to
    populate, manipulate, and delete attributes of the LFB instances on
    the FEs.  This is how the CE configures an individual LFB instance.
 
    The same set of requirements as described in Section 9.5 for
    attribute query applies here for attribute manipulation as well.
 
    Support for various levels of feedback from the FE to the CE (e.g.,
    request received, configuration completed), as well as multi-
    attribute configuration transactions with atomic commit and
    rollback, may be necessary in some circumstances.
 
    (Editor's note: It remains an open issue as to whether or not other
    methods are needed in addition to "get attribute" and "set
    attribute" (such as multi-attribute transactions).  If the answer
    to that question is yes, it is not clear whether such methods
    should be supported by the FE model itself or the ForCES protocol.)
 
 8.7. 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).
 
 9. Acknowledgments
 
 
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    Many of the colleagues in our companies and participants in the
    ForCES mailing list have provided invaluable input into this work.
 
 10. Security Considerations
 
    The FE model describes the representation and organization of data
    sets and attributes in the FEs.  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.
    The access to the information contained in the FE model is
    accomplished via the ForCES protocol which will be defined in
    separate documents and so the security issues will be addressed
    there.
 
 11. 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", work in progress, November 2003, <draft-ietf-
    forces-framework-13.txt>.
 
 12. 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, January 2003, <draft-ietf-ipsp-ipsecpib-07.txt>.
 
 
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    [9] Quittek, J. et Al., "Requirements for IP Flow Information
    Export", work in progress, January 2004, <draft-ietf-ipfix-reqs-
    15.txt>.
 
    [10] Duffield, N., "A Framework for Passive Packet Measurement ",
    work in progress, December 2003, <draft-ietf-psamp-framework-
    05.txt>.
 
    [11] Pras, A. and Schoenwaelder, J., FRC 3444 "On the Difference
    between Information Models and Data Models", January 2003.
 
 13. Authors' Addresses
 
    L. Lily Yang
    Intel Corp.
    Mail Stop: JF3-206
    2111 NE 25th Avenue
    Hillsboro, OR 97124, USA
    Phone: +1 503 264 8813
    Email: lily.l.yang@intel.com
 
    Joel M. Halpern
    Megisto Systems, Inc.
    20251 Century Blvd.
    Germantown, MD 20874-1162, USA
    Phone: +1 301 444-1783
    Email: jhalpern@megisto.com
 
    Ram Gopal
    Nokia Research Center
    5, Wayside Road,
    Burlington, MA 01803, USA
    Phone: +1 781 993 3685
    Email: ram.gopal@nokia.com
 
    Alan DeKok
    IDT Inc.
    1575 Carling Ave.
    Ottawa, ON K1G 0T3, Canada
    Phone: +1 613 724 6004 ext. 231
    Email: alan.dekok@idt.com
 
    Zsolt Haraszti
    Ericsson
    920 Main Campus Dr, St. 500
    Raleigh, NC  27606, USA
    Phone: +1 919 472 9949
 
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    Email: zsolt.haraszti@ericsson.com
 
    Steven Blake
    Ericsson
    920 Main Campus Dr, St. 500
    Raleigh, NC  27606, USA
    Phone: +1 919 472 9913
    Email: steven.blake@ericsson.com
 
    Ellen Deleganes
    Intel Corp.
    Mail Stop: JF3-206
    2111 NE 25th Avenue
    Hillsboro, OR 97124, USA
    Phone: +1 503 712 4173
    Email: ellen.m.deleganes@intel.com
 
 
 14. 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.
 
 
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