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

Working Group: ForCES                                         J. Halpern
Internet-Draft                                                      Self
Expires: April 9, 2008                                      E. Deleganes
                                                             Intel Corp.
                                                         October 7, 2007


                    ForCES Forwarding Element Model
                     draft-ietf-forces-model-08.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

   Comments are solicited and should be addressed to the working group's
   mailing list at forces@peach.ease.lsoft.com and/or the author(s).

Abstract

   This document defines the forwarding element (FE) model used in the
   Forwarding and Control Element Separation (ForCES) protocol.  The
   model represents the capabilities, state and configuration of
   forwarding elements within the context of the ForCES protocol, so



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   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,RFC3654 [2].


Table of Contents

   1.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Requirements on the FE model  . . . . . . . . . . . . . .   7
     2.2.  The FE Model in Relation to FE Implementations  . . . . .   8
     2.3.  The FE Model in Relation to the ForCES Protocol . . . . .   8
     2.4.  Modeling Language for the FE Model  . . . . . . . . . . .   9
     2.5.  Document Structure  . . . . . . . . . . . . . . . . . . .   9
   3.  FE Model Concepts . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  FE Capability Model and State Model . . . . . . . . . . .  10
     3.2.  LFB (Logical Functional Block) Modeling . . . . . . . . .  13
       3.2.1.  LFB Outputs . . . . . . . . . . . . . . . . . . . . .  16
       3.2.2.  LFB Inputs  . . . . . . . . . . . . . . . . . . . . .  19
       3.2.3.  Packet Type . . . . . . . . . . . . . . . . . . . . .  21
       3.2.4.  Metadata  . . . . . . . . . . . . . . . . . . . . . .  22
       3.2.5.  LFB Events  . . . . . . . . . . . . . . . . . . . . .  29
       3.2.6.  LFB Component Properties  . . . . . . . . . . . . . .  29
       3.2.7.  LFB Versioning  . . . . . . . . . . . . . . . . . . .  30
       3.2.8.  LFB Inheritance . . . . . . . . . . . . . . . . . . .  30
     3.3.  FE Datapath Modeling  . . . . . . . . . . . . . . . . . .  31
       3.3.1.  Alternative Approaches for Modeling FE Datapaths  . .  32
       3.3.2.  Configuring the LFB Topology  . . . . . . . . . . . .  36
   4.  Model and Schema for LFB Classes  . . . . . . . . . . . . . .  40
     4.1.  Namespace . . . . . . . . . . . . . . . . . . . . . . . .  40
     4.2.  <LFBLibrary> Element  . . . . . . . . . . . . . . . . . .  40
     4.3.  <load> Element  . . . . . . . . . . . . . . . . . . . . .  42
     4.4.  <frameDefs> Element for Frame Type Declarations . . . . .  43
     4.5.  <dataTypeDefs> Element for Data Type Definitions  . . . .  43
       4.5.1.  <typeRef> Element for Aliasing Existing Data Types  .  46
       4.5.2.  <atomic> Element for Deriving New Atomic Types  . . .  47
       4.5.3.  <array> Element to Define Arrays  . . . . . . . . . .  47
       4.5.4.  <struct> Element to Define Structures . . . . . . . .  51
       4.5.5.  <union> Element to Define Union Types . . . . . . . .  52
       4.5.6.  <alias> Element . . . . . . . . . . . . . . . . . . .  52
       4.5.7.  Augmentationst  . . . . . . . . . . . . . . . . . . .  53
     4.6.  <metadataDefs> Element for Metadata Definitions . . . . .  54
     4.7.  <LFBClassDefs> Element for LFB Class Definitions  . . . .  55
       4.7.1.  <derivedFrom> Element to Express LFB Inheritance  . .  57
       4.7.2.  <inputPorts> Element to Define LFB Inputs . . . . . .  58



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       4.7.3.  <outputPorts> Element to Define LFB Outputs . . . . .  60
       4.7.4.  <components> Element to Define LFB Operational
               Components  . . . . . . . . . . . . . . . . . . . . .  63
       4.7.5.  <capabilities> Element to Define LFB Capability
               Components  . . . . . . . . . . . . . . . . . . . . .  65
       4.7.6.  <events> Element for LFB Notification Generation  . .  67
       4.7.7.  <description> Element for LFB Operational
               Specification . . . . . . . . . . . . . . . . . . . .  70
     4.8.  Properties  . . . . . . . . . . . . . . . . . . . . . . .  70
       4.8.1.  Basic Properties  . . . . . . . . . . . . . . . . . .  71
       4.8.2.  Array Properties  . . . . . . . . . . . . . . . . . .  73
       4.8.3.  String Properties . . . . . . . . . . . . . . . . . .  73
       4.8.4.  Octetstring Properties  . . . . . . . . . . . . . . .  74
       4.8.5.  Event Properties  . . . . . . . . . . . . . . . . . .  75
       4.8.6.  Alias Properties  . . . . . . . . . . . . . . . . . .  78
     4.9.  XML Schema for LFB Class Library Documents  . . . . . . .  79
   5.  FE Components and Capabilities  . . . . . . . . . . . . . . .  90
     5.1.  XML for FEObject Class definition . . . . . . . . . . . .  91
     5.2.  FE Capabilities . . . . . . . . . . . . . . . . . . . . .  97
       5.2.1.  ModifiableLFBTopology . . . . . . . . . . . . . . . .  97
       5.2.2.  SupportedLFBs and SupportedLFBType  . . . . . . . . .  98
     5.3.  FE Components . . . . . . . . . . . . . . . . . . . . . . 100
       5.3.1.  FEStatus  . . . . . . . . . . . . . . . . . . . . . . 100
       5.3.2.  LFBSelectors and LFBSelectorType  . . . . . . . . . . 100
       5.3.3.  LFBTopology and LFBLinkType . . . . . . . . . . . . . 101
       5.3.4.  FENeighbors and FEConfiguredNeighborType  . . . . . . 101
   6.  Satisfying the Requirements on FE Model . . . . . . . . . . . 102
   7.  Using the FE model in the ForCES Protocol . . . . . . . . . . 103
     7.1.  FE Topology Query . . . . . . . . . . . . . . . . . . . . 105
     7.2.  FE Capability Declarations  . . . . . . . . . . . . . . . 106
     7.3.  LFB Topology and Topology Configurability Query . . . . . 107
     7.4.  LFB Capability Declarations . . . . . . . . . . . . . . . 107
     7.5.  State Query of LFB Attributes . . . . . . . . . . . . . . 108
     7.6.  LFB Component Manipulation  . . . . . . . . . . . . . . . 109
     7.7.  LFB Topology Re-configuration . . . . . . . . . . . . . . 109
   8.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
     8.1.  Data Handling . . . . . . . . . . . . . . . . . . . . . . 116
       8.1.1.  Setting up a DLCI . . . . . . . . . . . . . . . . . . 117
       8.1.2.  Error Handling  . . . . . . . . . . . . . . . . . . . 118
     8.2.  LFB Components  . . . . . . . . . . . . . . . . . . . . . 118
     8.3.  Capabilities  . . . . . . . . . . . . . . . . . . . . . . 119
     8.4.  Events  . . . . . . . . . . . . . . . . . . . . . . . . . 119
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 120
   10. Authors Emeritus  . . . . . . . . . . . . . . . . . . . . . . 121
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 121
   12. Security Considerations . . . . . . . . . . . . . . . . . . . 121
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . . 121
     13.1. Normative References  . . . . . . . . . . . . . . . . . . 121



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     13.2. Informative References  . . . . . . . . . . . . . . . . . 122
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 122
   Intellectual Property and Copyright Statements  . . . . . . . . . 124
















































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1.  Definitions

   The use of compliance terminology (MUST, SHOULD, MAY) is used in
   accordance with RFC2119 [1].  Such terminology is used in describing
   the required behavior of ForCES forwarding elements or control
   elements in supporting or manipulating information described in this
   model.

   Terminology associated with the ForCES requirements is defined in
   RFC3654 [2] and is not copied here.  The following list of
   terminology relevant to the FE model is defined in this section.

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

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

   LFB (Logical Functional Block) Class (or type) -- A template that
   representing a fine-grained, logically separable aspect of FE
   processing.  Most LFBs relate to packet processing in the data path.
   LFB classes are the basic building blocks of the FE model.

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

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

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



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   hardware.  Metadata is sent between the FE and the CE on redirect
   packets.

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

   Structure Component -- Forces allows for complex data structures to
   be used in its data definitions.  Generally, these include tables and
   Structures.  The individual parts which make up a structured set of
   data are referred to as Structure Components.  These can themselves
   be of any valid data type, including tables and structures.

   Component -- Often in describing the forces model and its
   operational, it is useful to refer to the parts of an LFB or
   structure, without regard to what they are part of.  The term
   component by itself will be used to refer to these parts.  If the
   context is unclear, but it is necessary to refer explicitly to either
   LFB Components or Structure Components, then the modifying word will
   be present.  When the correct prefix is clear from context, or when
   no differentiation is needed, no modifier will be used.

   Element -- Element is generally used in this document in accordance
   with the XML usage of the term.  It refers to an XML tagged part of
   an XML document.  For a precise definition, please see the full set
   of XML specifications from the W3C. This term is included in this
   list for completeness, and because earlier versions of this document
   used the term element inconsistently.  The other use of the term
   element is in terms of the FE and CE (Forwarding Element and Control
   Element.)  As those are not textual or data structure items, context
   provides sufficient clarity for this usage.

   Attribute -- Attribute is used in the XML sense of attribute
   information include in an XML tag.

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

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



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   considered to be part of the FE model.  The FE topology is discovered
   by the ForCES base protocol or by some other means.

   Inter-FE Topology -- See FE Topology.

   Intra-FE Topology -- See LFB Topology.

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


2.  Introduction

   RFC3746 [3] 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.RFC3654 [2] mandates that the capabilities, states and
   configuration information be expressed in the form of an FE model.

   RFC3444 [8] observed that information models (IMs) and data models
   (DMs) are different because they serve different purposes.  "The main
   purpose of an IM is to model managed objects at a conceptual level,
   independent of any specific implementations or protocols used".
   "DMs, conversely, are defined at a lower level of abstraction and
   include many details.  They are intended for implementors and include
   protocol-specific constructs."  Sometimes it is difficult to draw a
   clear line between the two.  The FE model described in this document
   is primarily an information model, but also includes some aspects of
   a data model, such as explicit definitions of the LFB class schema
   and FE schema.  It is expected that this FE model will be used as the
   basis to define the payload for information exchange between the CE
   and FE in the ForCES protocol.

2.1.  Requirements on the FE model

   RFC3654 [2]defines requirements that must be satisfied by a ForCES FE
   model.  To summarize, an FE model must define:

   o  Logically separable and distinct packet forwarding operations in
      an FE datapath (logical functional blocks or LFBs);




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   o  The possible topological relationships (and hence the sequence of
      packet forwarding operations) between the various LFBs;

   o  The possible operational capabilities (e.g., capacity limits,
      constraints, optional features, granularity of configuration) of
      each type of LFB;

   o  The possible configurable parameters (i.e., attributes) of each
      type of LFB;

   o  Metadata that may be exchanged between LFBs.

2.2.  The FE Model in Relation to FE Implementations

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

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

2.3.  The FE Model in Relation to the ForCES Protocol

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




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

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

2.4.  Modeling Language for the FE Model

   Even though not absolutely required, it is beneficial to use a formal
   data modeling language to represent the conceptual FE model described
   in this document.  Use of a formal language can help to enforce
   consistency and logical compatibility among LFBs.  A full
   specification will be written using such a data modeling language.
   The formal definition of the LFB classes may facilitate the eventual
   automation of some of the code generation process and the functional
   validation of arbitrary LFB topologies.  These class definitions form
   the LFB Library.  Documents which describe LFB Classes are therefore
   referred to as LFB Library documents.

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

   XML was chosen as the specification language in this document,
   because XML has the advantage of being both human and machine
   readable with widely available tools support.  This document uses XML
   Schema to define the structure of the LFB Library documents, as
   defined in [9] and [10].  While these LFB Class definitions are not
   sent in the Forces protocol, these definitions comply with the
   recommendations in RFC3470 [9] on the use of XML in IETF protocols.

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 Section 5 constitute the core of
   the FE model, detailing the two major aspects of the FE model: a
   general LFB model and a definition of the FE Object LFB, with its



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   components, including FE capabilities and LFB topology information.
   Section 6 directly addresses the model requirements imposed by the
   ForCES requirement draft[1] while Section 7 explains how the FE model
   should be used in the ForCES protocol.


3.  FE Model Concepts

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

   The FE model proposed in this document has two major aspects: the LFB
   model and FE Object defintion whose components include FE capability
   information and LFB topology information.  The LFB model provides the
   content and data structures to define each individual LFB class.  The
   FE Object class defines the components to provide information at the
   FE level, particularly the capabilities of the FE at a coarse level.
   Part of the FE level information is the LFB topology, which expresses
   the logical inter- connection between the LFB instances along the
   datapath(s) within the FE.  Details of these aspects are described in
   Section 4 and Section 5.  The intent of this section is to discuss
   these concepts at the high level and lay the foundation for the
   detailed description in the following sections.

3.1.  FE Capability Model and State Model

   The ForCES FE model includes both a capability and a state model.
   The FE capability model describes the capabilities and capacities of
   an FE by specifying the variation in functions supported and any
   limitations.  The FE state model describes the current state of the
   FE, that is, the instantaneous values or operational behavior of the
   FE.  Equally, this concept applies to LFB classes, where the
   capability information indicates what this FE is capable of providing
   using the specific LFB Class, or even the specific component (such as
   the table size limits.)  Capability information is always read-only,
   as it describes what the FE / LFB can provide, not what the CE has
   requested.

   Conceptually, the FE capability model tells the CE which states are
   allowed on an FE, with capacity information indicating certain
   quantitative limits or constraints.  Thus, the CE has general
   knowledge about configurations that are applicable to a particular



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   FE.  For example, an FE capability model may describe the FE at a
   coarse level such as:

   o  this FE can handle IPv4 and IPv6 forwarding;

   o  this FE can perform classification on the following fields: source
      IP address, destination IP address, source port number,
      destination port number, etc;

   o  this FE can perform metering;

   o  this FE can handle up to N queues (capacity);

   o  this FE can add and remove encapsulating headers of types
      including IPSec, GRE, L2TP.

   While one could try and build an object model to fully represent the
   FE capabilities, other efforts found this to be a significant
   undertaking.  The main difficulty arises in describing detailed
   limits, such as the maximum number of classifiers, queues, buffer
   pools, and meters the FE can provide.  We believe that a good balance
   between simplicity and flexibility can be achieved for the FE model
   by combining coarse level capability reporting with an error
   reporting mechanism.  That is, if the CE attempts to instruct the FE
   to set up some specific behavior it cannot support, the FE will
   return an error indicating the problem.  Examples of similar
   approaches include DiffServ PIB RFC3317 [4] and Framework PIB RFC3318
   [5].

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

   The FE state model presents the snapshot view of the FE to the CE.
   For example, using an FE state model, an FE may be described to its
   corresponding CE as the following:

   o  on a given port, the packets are classified using a given
      classification filter;

   o  the given classifier results in packets being metered in a certain
      way, and then marked in a certain way;



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   o  the packets coming from specific markers are delivered into a
      shared queue for handling, while other packets are delivered to a
      different queue;

   o  a specific scheduler with specific behavior and parameters will
      service these collected queues.

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

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


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

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

   Capability information at the LFB level is an integral part of the
   LFB model, and is modeled the same way as the other operational
   parameters inside an LFB.  For example, when certain features of an
   LFB class are optional, the CE MUST be able to determine whether
   those optional features are supported by a given LFB instance.  Such
   capability information is modeled as either property information, or
   for LFB information not provided by the defined properties, as
   capability components which are inherently read-only.  The schema for
   the definition of LFB classes provides for identifying such
   components.

   Capability information at the FE level may describe the LFB classes
   that the FE can instantiate; the number of instances of each that can
   be created; the topological (linkage) limitations between these LFB
   instances, etc.  Section 5 defines the FE level components including
   capability information.  Since all information is represented as
   LFBs, this is provided by a single instance of the FE Object LFB
   Class.  By using a single instance with a known LFB Class and a known
   instance identification, the Forces Protocol can allow a CE to access



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   this information whenever it needs to, including as part of
   establishing the control of the FE by the CE.

   Once the FE capability is described to the CE, the FE state
   information can be represented by two levels.  The first level is the
   logically separable and distinct packet processing functions, called
   Logical Functional Blocks (LFBs).  The second level of information
   describes how these individual LFBs are ordered and placed along the
   datapath to deliver a complete forwarding plane service.  The
   interconnection and ordering of the LFBs is called LFB Topology.
   Section 3.2 discusses high level concepts around LFBs, whereas
   Section 3.3 discusses LFB topology issues.  This topology information
   is represented as components of the FE Object LFB instance, to allow
   the CE to fetch and manipulate this.

3.2.   LFB (Logical Functional Block) Modeling

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

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












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

                       Figure 2: Generic LFB Diagram

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

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

   LFB operation is specified in the model to allow the CE to understand



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

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

   The LFB class model specifies information such as:

   o  number of inputs and outputs (and whether they are configurable)

   o  metadata read/consumed from inputs;

   o  metadata produced at the outputs;

   o  packet type(s) accepted at the inputs and emitted at the outputs;

   o  packet content modifications (including encapsulation or
      decapsulation);

   o  packet routing criteria (when multiple outputs on an LFB are
      present);

   o  packet timing modifications;

   o  packet flow ordering modifications;

   o  LFB capability information components;

   o  Events that can be detected by the LFB, with notification to the
      CE;

   o  LFB operational components, 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



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   important issues in LFB modeling, without covering all the specific
   details.

3.2.1.  LFB Outputs

   An LFB output is a conceptual port on an LFB that can send
   information to another LFB.  The information is typically a packet
   and its associated metadata, although in some cases it might consist
   of only metadata, i.e., with no packet data.

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

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

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

    a. One output               b. Two distinct outputs

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

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



       Figure 3: Examples of LFBs with various output combinations.

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

   Multiple separate singleton outputs are defined in an LFB class to
   model a pre-determined number of semantically different outputs.



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   That is, the LFB class definition MUST include the number of outputs,
   implying the number of outputs is known when the LFB class is
   defined.  Additional singleton outputs cannot be created at LFB
   instantiation time, nor can they be created on the fly after the LFB
   is instantiated.

   For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one
   output(OUT) to send those packets for which the LPM look-up was
   successful, passing a META_ROUTEID as metadata; and have another
   output (EXCEPTIONOUT) for sending exception packets when the LPM
   look-up failed.  This example is depicted in Figure 3.b.  Packets
   emitted by these two outputs not only require different downstream
   treatment, but they are a result of two different conditions in the
   LFB and each output carries different metadata.  This concept assumes
   the number of distinct outputs is known when the LFB class is
   defined.  For each singleton output, the LFB class definition defines
   the types of frames and metadata the output emits.

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

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



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   (red, yellow, green; N=3), etc.

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

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

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

   In summary, the LFB class may define one output, multiple singleton
   outputs, one or more output groups, or a combination thereof.
   Multiple singleton outputs should be used when the LFB must provide
   for forking the datapath, and at least one of the following
   conditions hold:

   o  the number of downstream directions are inherent from the
      definition of the class and hence fixed;

   o  the frame type and set of metadata emitted on any of the outputs
      are substantially different from what is emitted on the other
      outputs (i.e., they cannot share frame-type and metadata
      definitions);

   An output group is appropriate when the LFB must provide for forking
   the datapath, and at least one of the following conditions hold:

   o  the number of downstream directions is not known when the LFB
      class is defined;

   o  the frame type and set of metadata emitted on these outputs are
      sufficiently similar or ideally identical, such they can share the
      same output definition.



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3.2.2.  LFB Inputs

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

   For LFB instances that receive packets from more than one other LFB
   instance (fan-in).  There are three ways to model fan-in, all
   supported by the LFB model and can be combined in the same LFB:

   o  Implicit multiplexing via a single input

   o  Explicit multiplexing via multiple singleton inputs

   o  Explicit multiplexing via a group of inputs (input group)

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

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








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

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




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

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




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

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






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               Figure 4: Input modeling concepts (examples).

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

   is read-only for static topologies, and is read-write for
   configurable topologies.

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

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

3.2.3.  Packet Type

   When LFB classes are defined, the input and output packet formats
   (e.g., IPv4, IPv6, Ethernet, etc.)  MUST be specified.  These are the



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   types of packets a given LFB input is capable of receiving and
   processing, or a given LFB output is capable of producing.  This
   requires distinct packet types be uniquely labeled with a symbolic
   name and/or ID.

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

3.2.4.  Metadata

   Metadata is the per-packet state that is passed from one LFB to
   another.  The metadata is passed with the packet to assist subsequent
   LFBs to process that packet.  The ForCES model captures how the per-
   packet state information is propagated from one LFB to other LFBs.
   Practically, such metadata propagation can happen within one FE, or
   cross the FE boundary between two interconnected FEs.  We believe
   that the same metadata model can be used for either situation;
   however, our focus here is for intra-FE metadata.

3.2.4.1.  Metadata Vocabulary

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

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

   The second axis is "implicit" versus "expressed", which specifies
   whether or not the metadata has a visible physical representation.
   For example, the traffic management header described in the previous
   paragraph may be represented as a series of bits in some format, and
   that header is associated with the packet.  Those bits have physical
   representation, and are therefore "expressed" metadata.  If the
   metadata does not have a physical representation, it is called



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   "implicit" metadata.  This situation occurs, for example, when a
   particular path through a network device is intended to be traversed
   only by particular kinds of packets, such as an IPv4 router.  An
   implementation may not mark every packet along this path as being of
   type "IPv4", but the intention of the designers is that every packet
   is of that type.  This understanding can be thought of as metadata
   about the packet, which is implicitly attached to the packet through
   the intent of the designers.

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

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

3.2.4.2.  Metadata lifecycle within the ForCES model

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

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

   To ensure inter-operability between LFBs, the LFB class specification
   must define what metadata the LFB class "reads" or "consumes" on its
   input(s) and what metadata it "produces" on its output(s).  For
   maximum extensibility, this definition should neither specify which



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   LFBs the metadata is expected to come from for a consumer LFB, nor
   which LFBs are expected to consume metadata for a given producer LFB.

   While it is important to define the metadata types passing between
   LFBs, it is not appropriate to define the exact encoding mechanism
   used by LFBs for that metadata.  Different implementations are
   allowed to use different encoding mechanisms for metadata.  For
   example, one implementation may store metadata in registers or shared
   memory, while another implementation may encode metadata in- band as
   a preamble in the packets.  In order to allow the CE to understand
   and control the meta-data related operations, the model represents
   each metadata tag as a 32-bit integer.  Each LFB definition indicates
   in its metadata declarations the 32-bit value associated with a given
   metadata tag.  Ensuring consistency of usage of tags is important,
   and outside the scope of the model.

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

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

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

3.2.4.3.  LFB Operations on Metadata

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

      *  IGNORE: ignores and forwards the metadata

      *  READ: reads and forwards the metadata

      *  READ/RE-WRITE: reads, over-writes and forwards the metadata

      *  WRITE: writes and forwards the metadata (can also be used to
         create new metadata)

      *  READ-AND-CONSUME: reads and consumes the metadata

      *  CONSUME consumes metadata without reading

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



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   packet is sent to the next LFB.

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

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

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

3.2.4.4.  Metadata Production and Consumption

   For a given metadata on a given packet path, there MUST be at least
   one producer LFB that creates that metadata and SHOULD be at least
   one consumer LFB that needs that metadata.  In this model, the
   producer and consumer LFBs of a metadata are not required to be
   adjacent.  In addition, there may be multiple producers and consumers
   for the same metadata.  When a packet path involves multiple
   producers of the same metadata, then subsequent producers overwrite
   that metadata value.

   The metadata that is produced by an LFB is specified by the LFB class
   definition on a per output port group basis.  A producer may always
   generate the metadata on the port group, or may generate it only
   under certain conditions.  We call the former an "unconditional"
   metadata, whereas the latter is a "conditional" metadata.  In the
   case of conditional metadata, it should be possible to determine from
   the definition of the LFB when a "conditional" metadata is produced.
   The consumer behavior of an LFB, that is, the metadata that the LFB
   needs for its operation, is defined in the LFB class definition on a
   per input port group basis.  An input port group may "require" a
   given metadata, or may treat it as "optional" information.  In the
   latter case, the LFB class definition MUST explicitly define what
   happens if an optional metadata is not provided.  One approach is to
   specify a default value for each optional metadata, and assume that
   the default value is used if the metadata is not provided with the
   packet.

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



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   LFB consumes the metadata.

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

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

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

3.2.4.5.  Fixed, Variable and Configurable Tag

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

   A small subset of LFBs need the capability to produce one or more of
   their metadata with tags that are not fixed in the LFB class
   definition, but instead can be selected per LFB instance.  An example
   of such an LFB class is a Generic Classifier LFB.  We call this
   capability "variable tag metadata production".  If an LFB

   produces metadata with a variable tag, the corresponding LFB
   attribute, called the tag selector, specifies the tag for each such
   metadata.  This mechanism improves the versatility of certain multi-
   purpose LFB classes, since it allows the same LFB class to be used in
   different topologies, producing the right metadata tags according to
   the needs of the topology.  This selection of tags is variable in
   that the produced output may have any number of different tags.  The
   meaning of the various tags is still defined by the metadata
   declaration associated with the LFB class definition.  This also
   allows the CE to correctly set the tag values in the table to match
   the declared meanings of the metadata tag values.

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



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

3.2.4.6.  Metadata Usage Categories

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

   There are three types of metadata related to metadata usage:

   o  Relational (or binding) metadata

   o  Enumerated metadata

   o  Explicit/external value metadata

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

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

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

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



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   between the LFBs.

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

   Important characteristics of the binding usage of metadata are:

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

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

   o  Hence the metadata definition is not required to include value
      assignments.  The only exception is when some special value(s) of
      the metadata must be reserved to convey special events.  Even
      though these special cases must be defined with the metadata
      specification, their encoded values can be selected arbitrarily.
      For example, for the Prefix Lookup LFB example, a special value
      may be reserved to signal the NO-MATCH case, and the value of zero
      may be assigned for this purpose.

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

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

   The third class of metadata is the explicit type.  This refers to
   cases where the metadata value is explicitly used by the consumer LFB
   to change some packet header fields.  In other words, the value has a



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   direct and explicit impact on some field and will be visible
   externally when the packet leaves the NE.  Examples are: TTL
   increment given to a Header Modifier LFB, and DSCP value for a
   Remarker LFB.  For explicit metadata, the value encoding MUST be
   explicitly provided in the metadata definition.  The values cannot be
   selected arbitrarily and should conform to what is commonly expected.
   For example, a TTL increment metadata should be encoded as zero for
   the no increment case, one for the single increment case, etc.  A
   DSCP metadata should use 0 to encode DSCP=0, 1 to encode DSCP=1, etc.

3.2.5.  LFB Events

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

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

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

3.2.6.   LFB Component Properties

   LFBs are made up of components, containing the information that the
   CE needs to see and / or change about the functioning of the LFB.
   These components, as described in detail elsewhere, may be basic
   values, complex structures (containing multiple components



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   themselves, each of which can be values, structures, or tables), or
   tables (which contain values, structures or tables.)  Some of these
   components are optional.  Some components may be readable or
   writeable at the discretion of the FE implementation.  The CE needs
   to know these properties.  Additionally, certain kinds of components
   (arrays / tables, aliases, and events as of this writing) have
   additional property information that the CE may need to read or
   write.  This model defines the structure of the property information
   for all defined data types.

3.2.7.  LFB Versioning

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

   LFB class versioning is supported by requiring a version string in
   the class definition.  CEs may support multiple versions of a
   particular LFB class to provide backward compatibility, but FEs MUST
   NOT support more than one version of a particular class.

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

3.2.8.  LFB Inheritance

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

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

   An interesting issue related to class inheritance is backward



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

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

   Backward compatibility can be designed into the inheritance model by
   constraining LFB inheritance to require the derived class be a
   functional superset of the base class (i.e. the derived class can
   only add functions to the base class, but not remove functions).
   Additionally, the following mechanisms are required to support FE
   backward compatibility:

   1.  When detecting an LFB instance of an LFB type that is unknown to
       the CE, the CE MUST be able to query the base class of such an
       LFB from the FE.

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

3.3.  FE Datapath Modeling

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



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

3.3.1.  Alternative Approaches for Modeling FE Datapaths

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

   o  Topological Approach

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

   o  Encoded State Approach

   An alternate way of expressing differential treatment is by using
   metadata.  The result of the operation of an LFB can be encoded in a
   metadata, which is passed along with the packet to downstream LFBs.
   A downstream LFB, in turn, can use the metadata and its value (e.g.,
   as an index into some table) to determine how to treat the packet.

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

   Using a simple example of a classifier with N classification outputs
   followed by other LFBs, Figure 5(a) shows what the LFB topology looks



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   like when using the pure topological approach.  Each output from the
   classifier goes to one of the N LFBs where no metadata is needed.
   The topological approach is simple, straightforward and graphically
   intuitive.  However, if N is large and the N nodes following the
   classifier (LFB#1, LFB#2, ..., LFB#N) all belong to the same LFB type
   (e.g., meter), but each has its own independent components, 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 components is provided in the Meter node
   itself, with each row representing the attributes for one meter
   instance.  A metadata M is also needed to pass along with the packet
   P from the classifier to the meter, so that the meter can use M as a
   look-up key (index) to find the corresponding row of the attributes
   that should be used for any particular packet P.

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



























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

   5(a) Using pure topological approach

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

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

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

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

             Figure 5: An example of how to model FE datapaths

   From this example, we demonstrate that each approach has a distinct
   advantage depending on the situation.  Using the encoded state
   approach, fewer connections are typically needed between a fan-out
   node and its next LFB instances of the same type because each packet



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   carries metadata the following nodes can interpret and hence invoke

   a different packet treatment.  For those cases, a pure topological
   approach forces one to build elaborate graphs with many more
   connections and often results in an unwieldy graph.  On the other
   hand, a topological approach is the most intuitive for representing
   functionally different datapaths.

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




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


       +-------+   +-----------+            +------+   +-----------+
       |       |   |           |if IP-in-IP |      |   |           |
   --->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|->
       | ports |   |           |----+       |      |   |           |
       +-------+   +-----------+    |others +------+   +-----------+
                                    |
                                    V

   The LFB topology without the loop utilizing two independent
   classifier instances.

                    Figure 6: An LFB topology example.

   It is important to point out that the LFB topology described here is



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   the logical topology, not the physical topology of how the FE
   hardware is actually laid out.  Nevertheless, the actual
   implementation may still influence how the functionality is mapped to
   the LFB topology.  Figure 6 shows one simple FE example.  In this
   example, an IP-in-IP packet from an IPSec application like VPN may go
   to the classifier first and have the classification done based on the
   outer IP header; upon being classified as an IP-in-IP packet, the
   packet is then sent to a decapsulator to strip off the outer IP

   header, followed by a classifier again to perform classification on
   the inner IP header.  If the same classifier hardware or software is
   used for both outer and inner IP header classification with the same
   set of filtering rules, a logical loop is naturally present in the
   LFB topology, as shown in Figure 6(a).  However, if the
   classification is implemented by two different pieces of hardware or
   software with different filters (i.e., one set of filters for the
   outer IP header and another set for the inner IP header), then it is
   more natural to model them as two different instances of classifier
   LFB, as shown in Figure 6(b).

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

3.3.2.   Configuring the LFB Topology

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

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



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   topologies for the same datapaths.  In general, an LFB topology based
   on a pure topological approach is likely to experience more frequent
   topology reconfiguration than one based on an encoded state approach.
   However, even an LFB topology based entirely on an encoded state
   approach may have to change the topology at times, for example, to
   bypass some LFBs or insert new LFBs.  Since a mix of these two
   approaches is used to model the datapaths, LFB topology
   reconfiguration is considered an important aspect of the FE model.

   We want to point out that allowing a configurable LFB topology in the
   FE model does not mandate that all FEs are required to have this
   capability.  Even if an FE supports configurable LFB topology, the FE
   may impose limitations on what can actually be configured.
   Performance-optimized hardware implementations may have zero or very
   limited configurability, while FE implementations running on network
   processors may provide more flexibility and configurability.  It is
   entirely up to the FE designers to decide whether or not the FE
   actually implements reconfiguration and if so, how much.  Whether a
   simple runtime switch is used to enable or disable (i.e., bypass)
   certain LFBs, or more flexible software reconfiguration is used, is
   implementation detail internal to the FE and outside of the scope of
   FE model.  In either case, the CE(s) MUST be able to learn the FE's
   configuration capabilities.  Therefore, the FE model MUST provide a
   mechanism for describing the LFB topology configuration capabilities
   of an FE.  These capabilities may include (see Section 5 for full
   details):

   o  Which LFB classes the FE can instantiate

   o  Maximum number of instances of the same LFB class that can be
      created

   o  Any topological limitations, For example:

      *  The maximum number of instances of the same class or any class
         that can be created on any given branch of the graph

      *  Ordering restrictions on LFBs (e.g., any instance of LFB class
         A must be always downstream of any instance of LFB class B).

   Note that even when the CE is allowed to configure LFB topology for
   the FE, the CE is not expected to be able to interpret an arbitrary
   LFB topology and determine which specific service or application
   (e.g.  VPN, DiffServ, etc.) is supported by the FE.  However, once
   the CE understands the coarse capability of an FE, the CE MUST
   configure the LFB topology to implement the network service the NE is
   supposed to provide.  Thus, the mapping the CE has to understand is
   from the high level NE service to a specific LFB topology, not the



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   other way around.  The CE is not expected to have the ultimate
   intelligence to translate any high level service policy into the
   configuration data for the FEs.  However, it is conceivable that
   within a given network service domain, a certain amount of
   intelligence can be programmed into the CE to give the CE a general
   understanding of the LFBs involved to allow the translation from a
   high level service policy to the low level FE configuration to be
   done automatically.  Note that this is considered an implementation
   issue internal to the control plane and outside the scope of the FE
   model.  Therefore, it is not discussed any further in this draft.


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

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

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

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




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


             Figure 7: An example of configuring LFB topology

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



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

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


4.   Model and Schema for LFB Classes

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

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

4.1.  Namespace

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

4.2.  <LFBLibrary> Element

   The <LFBLibrary&#x3e element serves as a root element of all library
   documents.  It contains one or more of the following main XML
   elements:




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   o  <frameTypeDefs> for the frame declarations;

   o  <dataTypeDefs> for defining common data types;

   o  <metadataDefs> for defining metadata, and

   o  <LFBClassDefs> for defining LFB classes.

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

   In addition to the above main elements, 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 use of <load> elements,
   which must precede all the above elements in the document.  For
   unique referencing, each LFBLibrary instance document has a unique
   label defined in the "provide" attribute of the LFBLibrary element.

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

   The following is a skeleton of a library document:


























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       <?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>
          ...
         </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
   document available for referencing in the current document.

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




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

4.4.  <frameDefs> Element for Frame Type Declarations

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

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

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

   The following example defines two frame types:

   <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:




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   o  Defining other data types

   o  Defining attributes of LFB classes

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

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

   <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,
   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:

















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          <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 unsigned integer
          boolean                  A true / false value where
                                   0 = false, 1 = true
          string[N]                A UTF-8 string represented in at most
                                   N Octets.
          string                   A UTF-8 string without a configured
                                   storage length limit.
          byte[N]                  A byte array of N bytes
          octetstring[N]           A buffer of N octets, which may
                                   contain fewer than N octets.  Hence
                                   the encoded value will always have
                                   a length.
          float16                  16-bit floating point number
          float32                  32-bit IEEE floating point number
          float64                  64-bit IEEE floating point number

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

   Compound data types can build on atomic data types and other compound
   data types.  Compound data types can be defined in one of four ways.
   They may be defined as an array of components of some compound or
   atomic data type.  They may be a structure of named components of
   compound or atomic data types (ala C structures).  They may be a
   union of named components 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 component
   values, all atomic data types used here must be able to be conveyed
   in the FORCES protocol.  Further, the FORCES protocol will need a
   mechanism to convey compound data types.  However, the details of
   such representations are for the protocol document to define, not the
   model document.  Strings and octetstrings must be conveyed with their
   length, as they are not delimited, and are variable length.




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   With regard to strings, this model defines a small set of
   restrictions and definitions on how they are structured.  String and
   octetstring length limits can be specified in the LFB Class
   definitions.  The component properties for string and octetstring
   components also contain actual lengths and length limits.  This
   duplication of limits is to allow for implementations with smaller
   limits than the maximum limits specified in the LFB Class definition.
   In all cases, these lengths are specified in octets, not in
   characters.  In terms of protocol operation, as long as the specified
   length is within the FE's supported capabilities, the FE stores the
   contents of a string exactly as provided by the CE, and returns those
   contents when requested.  No canonicalization, transformations, or
   equivalences are performed by the FE. components of type string (or
   string[n]) may be used to hold identifiers for correlation with
   components in other LFBs.  In such cases, an exact octet for octet
   match is used.  No equivalences are used by the FE or CE in
   performing that matching.  The ForCES protocol does not perform or
   require validation of the content of UTF-8 strings.  However, UTF-8
   strings SHOULD be encoded in the shortest form to avoid potential
   security issues described in [12].  Any entity displaying such
   strings is expected to perform its own validation (for example for
   correct multi-byte characters, and for ensuring that the string does
   not end in the middle of a multi-byte sequence.)  Specific LFB class
   definitions may restrict the valid contents of a string as suited to
   the particular usage (for example, a component that holds a DNS name
   would be restricted to hold only octets valid in such a name.)  FEs
   should validate the contents of SET requests for such restricted
   components at the time the set is performed, just as range checks for
   range limited components are performed.  The ForCES protocol behavior
   defines the normative processing for requests using that protocol.

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

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

4.5.1.  <typeRef> Element for Aliasing Existing Data Types

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



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   compound type, the new type will also be compound.  Some usage
   examples follow:

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

4.5.2.  <atomic> Element for Deriving New Atomic Types

   The <atomic> element allows the definition of a new atomic type from
   an existing atomic type, applying range restrictions and/or providing
   special enumerated values.  Note that the <atomic> element can only
   use atomic types as base types, and its result MUST be 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.  Depending upon
   context, this document, and others, refer to such arrays as tables or
   arrays interchangeably, without semantic or syntactic implication.



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   The type of the array entry can be specified either by referring to
   an existing type (using the <typeRef> element) or defining an unnamed
   type inside the <array> element using any of the <atomic>, <array>,
   <struct>, or <union> elements.

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

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

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

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

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

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

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

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

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



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

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

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

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

         <dataTypeDef>
           <name>classification-table</name>
           <synopsis>
             A table of classification rules and result opcodes.
           </synopsis>
           <array type="variable-size">
             <struct>
               <component componentID="1">
                 <name>rule</name>
                 <synopsis>The rule to match</synopsis>
                 <typeRef>classrule</typeRef>
               </component>
               <component componentID="2">
                 <name>opcode</name>
                 <synopsis>The result code</synopsis>
                 <typeRef>opcode</typeRef>
               </component>
            </struct>
           </array>
         </dataTypeDef>

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




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

         <dataTypeDef>
           <name>ipPrefixInfo_table</name>
           <synopsis>
             A table of information about known prefixes
           </synopsis>
           <array type="variable-size">
             <struct>
               <component componentID="1">
                 <name>address-prefix</name>
                 <synopsis>the prefix being described</synopsis>
                 <typeRef>ipv4Prefix</typeRef>
               </component>
               <component componentID="2">
                 <name>source</name>
                 <synopsis>
                     the protocol or process providing this information
                 </synopsis>
                 <typeRef>uint16</typeRef>
               </component>
               <component componentID="3">
                 <name>prefInfo</name>
                 <synopsis>the information we care about</synopsis>
                 <typeRef>hypothetical-info-type</typeRef>
               </component>
             </struct>
             <key keyID="1">
               <keyField> address-prefix.ipv4addr </keyField>
               <keyField> address-prefix.prefixlen </keyField>
               <keyField> source </keyField>
             </key>
           </array>
         </dataTypeDef>

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

4.5.3.1.  Key Field References

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



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

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

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

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

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

4.5.4.  <struct> Element to Define Structures

   A structure is comprised of a collection of data components.  Each
   data components 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 component can be defined by referring to an
   existing type (using the <typeDef> element), or can be a locally
   defined (unnamed) type created by any of the <atomic>, <array>,
   <struct>, or <union> elements.

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

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




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

   An example:

   <dataTypeDef>
    <name>ipv4prefix</name>
    <synopsis>
     IPv4 prefix defined by an address and a prefix length
    </synopsis>
    <struct>
     <component componentID="1">
      <name>address</name>
      <synopsis>Address part</synopsis>
      <typeRef>ipv4addr</typeRef>
     </component>
     <component componentID="2">
      <name>prefixlen</name>
      <synopsis>Prefix length part</synopsis>
      <atomic>
       <baseType>uchar</baseType>
       <rangeRestriction>
        <allowedRange min="0" max="32"/>
       </rangeRestriction>
      </atomic>
     </component>
    </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 MUST be a compound type, even when the
   union contains only one element.

4.5.6.  <alias> Element

   It is sometimes necessary to have a component in an LFB or structure
   refer to information (a component) in other LFBs.  The <alias>
   declaration creates the constructs for this.  The content of an
   <alias> element MUST be a named type.  Whatever component the alias
   references (whcih is determined by the alias component properties, as
   described below) that component must be of the same type as that
   declared for the alias.  Thus, when the CE or FE dereferences the
   alias component, the type of the information returned is known.  The



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

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

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

4.5.7.  Augmentationst

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

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

   For example, consider a simple base LFB class A that has only one
   component (comp1) of type X. One way to derive class A1 from A can be
   by simply adding a second component (of any type).  Another way to
   derive a class A2 from A can be by replacing the original component
   (comp1) in A of type X with one of type Y, where Y is an augmentation
   of X. Both classes A1 and A2 are backward compatible with class A.



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   The syntax for augmentations is to include a <derivedFrom> element in
   a structure definition, indicating what structure type is being
   augmented.  Component names and component IDs within the augmentation
   must not be the same as those in the structure type being augmented.

4.6.   <metadataDefs> Element for Metadata Definitions

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

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


















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   <metadataDefs>
    <metadataDef>
     <name>NEXTHOPID</name>
     <synopsis>Refers to a Next Hop entry in NH LFB</synopsis>
     <metadataID>17</metaDataID>
     <typeRef>int32</typeRef>
    </metadataDef>
    <metadataDef>
     <name>CLASSID</name>
     <synopsis>
      Result of classification (0 means no match).
     </synopsis>
     <metadataID>21</metadataID>
     <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 MUST define an LFB class and include the following elements:

   o  <name> provides the symbolic name of the LFB class.  Example:
      "ipv4lpm"

   o  <synopsis> provides a short synopsis of the LFB class.  Example:
      "IPv4 Longest Prefix Match Lookup LFB"

   o  <version> is the version indicator

   o  <derivedFrom> is the inheritance indicator

   o  <inputPorts> lists the input ports and their specifications

   o  <outputPorts> lists the output ports and their specifications





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   o  <components> defines the operational components of the LFB

   o  <capabilities> defines the capability components of the LFB

   o  <description> contains the operational specification of the LFB

   o  The LFBClassID attribute of the LFBClassDef element defines the ID
      for this class.  These must be globally unique.

   o  <events> defines the events that can be generated by instances of
      this LFB.

   LFB Class Names must be unique, in order to enable other documents to
   reference the classes by name, and to enable human readers to
   understand references to class names.  While a complex naming
   structure could be created, simplicity is preferred.  As given in the
   IANA considerations section of this document, the IANA will maintain
   a registry of LFB Class names and Class identifiers, along with a
   reference to the document defining the class.

   Here is a skeleton of an example LFB class definition:






























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   <LFBClassDefs>
    <LFBClassDef LFBClassID="12345">
     <name>ipv4lpm</name>
     <synopsis>IPv4 Longest Prefix Match Lookup LFB</synopsis>
     <version>1.0</version>
     <derivedFrom>baseclass</derivedFrom>

     <inputPorts>
      ...
     </inputPorts>

     <outputPorts>
      ...
     </outputPorts>

     <components>
      ...
     </components>

     <capabilities>
      ...
     </capabilities>

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

    </LFBClassDef>
    ...
   </LFBClassDefs>

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

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

4.7.1.  <derivedFrom> Element to Express LFB Inheritance

   The optional <derivedFrom> element can be used to indicate that this
   class is a derivative of some other class.  The content of this
   element MUST be the unique name (<name>) of another LFB class.  The



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   referred LFB class MUST be defined in the same library document or in
   one of the included library documents.

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

4.7.2.  <inputPorts> Element to Define LFB Inputs

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

   Multiple inputs with different input types should be avoided if
   possible (see discussion in Section 4.7.3).  Some special LFBs will
   have no inputs at all.  For example, a packet generator LFB does not
   need an input.

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

   The <inputPort> element MUST contain the following elements:

   o  <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.

   o  <synopsis> contains a brief description of the input.  Example:
      "Normal packet input".

   o  <expectation> lists all allowed frame formats.  Example: {"ipv4"
      and "ipv6"}.  Note that this list should refer to names specified
      in the <frameDefs> element of the same library document or in any
      included library documents.  The < expectation> element can also
      provide a list of required metadata.  Example: {"classid",
      "vifid"}.  This list should refer to names of metadata defined in
      the <metadataDefs> element in the same library document or in any
      included library documents.  For each metadata, it must be
      specified whether the metadata is required or optional.  For each
      optional metadata, a default value must be specified, which is
      used by the LFB if the metadata is not provided with a packet.




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

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

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

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

   Metadata expectations are specified by the <metadataExpected>
   element.  In its simplest form, this element can contain a list of
   <ref> elements, each referring to a metadata.  When multiple
   instances of metadata are listed by <ref> elements, it means that
   "all of these" metadata must be received with each packet (except
   metadata that are marked as "optional" by the "dependency" attribute
   ot 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").



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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
   definitions of metadata expectations, we do not discuss those here.

4.7.3.  <outputPorts> Element to Define LFB Outputs

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

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

   The <outputPort> element MUST contain the following elements:





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   o  <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.

   o  <synopsis> contains a brief description of the output port.
      Example: "Normal packet output".

   o  <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:
























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   <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>
       <ref>ipv4</ref>
       <ref>ipv4bis</ref>
      </frameProduced>
      <metadataProduced>
       <ref availability="conditional">errorid</ref>
      </metadataProduced>
     </product>
    </outputPort>
   </outputPorts>

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

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






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4.7.4.   <components> Element to Define LFB Operational Components

   Operational parameters of the LFBs that must be visible to the CEs
   are conceptualized in the model as the LFB components.  These
   include, for example, flags, single parameter arguments, complex
   arguments, and tables.  Note that the components 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 components and hence are not covered.

   Some examples for LFB components are:

   o  Configurable flags and switches selecting between operational
      modes of the LFB

   o  Number of inputs or outputs in a port group

   o  Metadata CONSUME vs.PROPAGATE mode selector

   o  Various configurable lookup tables, including interface tables,
      prefix tables, classification tables, DSCP mapping tables, MAC
      address tables, etc.

   o  Packet and byte counters

   o  Various event counters

   o  Number of current inputs or outputs for each input or output group

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

   o  No-access components.  This is useful when multiple access modes
      may be defined for a given component to allow some flexibility for
      different implementations.

   o  Read-only components.

   o  Read-write components.

   o  Write-only components.  This could be any configurable data for
      which read capability is not provided to the CEs. (e.g., the
      security key information)

   o  Read-reset components.  The CE can read and reset this resource,
      but cannot set it to an arbitrary value.  Example: Counters.




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   o  Firing-only components.  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 component (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 property component must inform
   the CE about the access mode the actual LFB instance supports.

   The components of the LFB class are listed in the <components>
   element.  Each component is defined by an <component> element.  An
   <component> element MUST contain the following elements:

   o  <name> defines the name of the component.This name must be unique
      among the components of the LFB class.  Example: "version".

   o  <synopsis> should provide a brief description of the purpose of
      the component.

   o  <optional/> indicates that this component is optional.

   o  The data type of the component can be defined either via a
      reference to a predefined data type or providing a local
      definition of the type.  The former is provided by using the
      <typeRef> element, which must refer to the unique name of an
      existing data type defined in the <dataTypeDefs> element in the
      same library document or in any of the included library documents.
      When the data type is defined locally (unnamed type), one of the
      following elements can be used: <atomic>, <array>, <struct>, and
      <union>.  Their usage is identical to how they are used inside
      <dataTypeDef> elements (see Section 4.5).

   o  The optional <defaultValue> element can specify a default value
      for the component, which is applied when the LFB is initialized or
      reset.

   The <component> element also MUST have an componentID attribute,
   which is a numeric value used by the ForCES protocol.

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

   Whether optional components are supported, and whether components
   defined as read-write can actually be written can be determined for a



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   given LFB instance by the CE by reading the property information of
   that component.

   The following example defines two attributes for an LFB:

   <components>
    <component access="read-only" componentID=1>
     <name>foo</name>
     <synopsis>number of things</synopsis>
     <typeRef>uint32</typeRef>
    </component>
    <component access="read-write write-only" componentID=2>
     <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>
    </component>
   </component>

   The first component ("foo") is a read-only 32-bit unsigned integer,
   defined by referring to the built-in "uint32" atomic type.  The
   second component ("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 components are likely to exist at all times in a
   particular implementation.  While the capabilities will frequently
   indicate this non-existence, CEs may attempt to reference non-
   existent or non-permitted attributes anyway.  The FORCES protocol
   mechanisms should include appropriate error indicators for this case.

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

4.7.5.  <capabilities> Element to Define LFB Capability Components

   The LFB class specification provides some flexibility for the FE
   implementation regarding how the LFB class is implemented.  For
   example, the instance may have some limitations that are not inherent
   from the class definition, but rather the result of some
   implementation limitations.  Some of these limitations are captured



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   by the property information of the LFB components.  The model allows
   for the notion of additional capability information.

   Such capability related information is expressed by the capability
   components of the LFB class.  The capability components 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 component.  The format of the <capability> element is
   almost the same as the <component> 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 components follow:

   o  The version of the LFB class that this LFB instance complies with;

   o  Supported optional features of the LFB class;

   o  Maximum number of configurable outputs for an output group;

   o  Metadata pass-through limitations of the LFB;

   o  Additional range restriction on operational components;

   The following example lists two capability attributes:

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







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4.7.6.  <events> Element for LFB Notification Generation

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

   The <events> element has a baseID attribute value, which is normally
   <events baseID="number">.  The value of the baseID is the starting
   componentID for the path which identifies events.  It must not be the
   same as the componentID of any top level components (including
   capabilities) of the LFB class.  In derived LFBs (i.e. ones with a
   <derivedFrom> element) where the parent LFB class has an events
   declaration, the baseID must not be present in the derived LFB
   <events> element.  Instead, the baseID value from the parent LFB
   class is used.

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

   o  <eventTarget> element indicating which LFB field (component) is
      tested to generate the event;

   o  <condition> element indicating what condition on the field will
      generate the event from a list of defined conditions;

   o  <eventReports> element indicating what values are to be reported
      in the notification of the event.

4.7.6.1.  <eventTarget> Element

   The <eventTarget> element contains information identifying a field in
   the LFB.  Specifically, the <target> element contains one or more
   <eventField> or <eventSubscript> elements.  These elements represent
   the textual equivalent of a path select component of the LFB.  The
   <eventField> element contains the name of a component in the LFB or a
   component nested in an array or struct within the LFB.  The first
   element in a <target> MUST be an <eventField> element and MUST name a
   componet in the LFB.  The following element MUST identify a valid
   component within the containing context.  If an <eventField>
   identifies an array, and is not the last element in the target, then
   the next element MUST be an < eventSubscript> element.
   <eventSubscript> elements MUST occur only after <eventField> names
   that identifies an array component.  An <eventSubscript> may contain
   a numeric value to indicate that this event applies to a specific
   entry (by index) of the array.  More commonly, the event is being
   defined across all elements of the array.  In that case,



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   <eventSubscript> will contain a name.  The name in an
   <eventSubscript> element is not a component name.  It is a variable
   name for use in the <report> elements of this LFB definition.  This
   name MUST be distinct from any component name that can validly occur
   in the <eventReport> clause.  Hence it SHOULD be distinct from any
   component name used in the LFB or in structures used within the LFB.

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

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

   Threshold and filtering conditions can only be applied to individual
   events.  For events defined on elements of an array, this
   specification does not allow for defining a threshold or filtering



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   condition on an event for all elements of an array.

4.7.6.2.  <events> Element Conditions

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

   o  <eventCreated/> the target must be an array, ending with a
      subscript indication.  The event is generated when an entry in the
      array is created.  This occurs even if the entry is created by CE
      direction.

   o  <eventDeleted/> the target must be an array, ending with a
      subscript indication.  The event is generated when an entry in the
      array is destroyed.  This occurs even if the entry is destroyed by
      CE direction.

   o  <eventChanged/> the event is generated whenever the target
      component changes in any way.  For binary components such as up/
      down, this reflects a change in state.  It can also be used with
      numeric attributes, in which case any change in value results in a
      detected trigger.

   o  <eventGreaterThan/> the event is generated whenever the target
      component becomes greater than the threshold.  The threshold is an
      event property.

   o  <eventLessThan/> the event is generated whenever the target
      component becomes less than the threshold.  The threshold is an
      event property.

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

4.7.6.3.  <eventReports> Element

   The <eventReports> element of an <event> describes the information to
   be delivered by the FE along with the notification of the occurrence
   of the event.  The <reports> element contains one or more



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

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

4.7.7.  <description> Element for LFB Operational Specification

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

4.8.  Properties

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

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



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   property element.  Writeability can only be checked by attempting to
   modify the value.

4.8.1.  Basic Properties

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











































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                <dataTypeDef>
                  <name>accessPermissionValues</name>
                  <synopsis>
                    The possible values of attribute access permission
                  </synopsis>
                  <atomic>
                    <baseType>uchar</baseType>
                    <specialValues>
                      <specialValue value="0">
                        <name>None</name>
                        <synopsis>Access is prohibited</synopsis>
                      </specialValue>
                       <specialValue value="1">
                        <name> Read-Only </name>
                        <synopsis>Access is read only</synopsis>
                      </specialValue>
                      <specialValue value="2">
                        <name>Write-Only</name>
                        <synopsis>
                          The attribute may be written, but not read
                        </synopsis>
                      </specialValue>
                      <specialValue value="3">
                        <name>Read-Write</name>
                        <synopsis>
                          The attribute may be read or written
                        </synopsis>
                      </specialValue>
                    </specialValues>
                  </atomic>
                </dataTypeDef>
                <dataTypeDef>
                  <name>baseElementProperties</name>
                  <synopsis>basic properties, accessibility</synopsis>
                  <struct>
                    <component componentID="1">
                      <name>accessibility</name>
                      <synopsis>
                          does the element exist, and
                          can it be read or written
                      </synopsis>
                      <typeRef>accessPermissionValues</typeRef>
                    </component>
                  </struct>
                </dataTypeDef>






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4.8.2.  Array Properties

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

         <dataTypeDef>
           <name>arrayElementProperties</name>
           <struct>
             <derivedFrom>baseElementProperties</derivedFrom>
             <component componentID="2">
               <name>entryCount</name>
               <synopsis>the number of entries in the array</synopsis>
               <typeRef>uint32</typeRef>
             </component>
             <component componentID="3">
               <name>highestUsedSubscript</name>
               <synopsis>the last used subscript in the array</synopsis>
               <typeRef>uint32</typeRef>
             </component>
             <component componentID="4">
               <name>firstUnusedSubscript</name>
               <synopsis>
                 The subscript of the first unused array element
               </synopsis>
               <typeRef>uint32</typeRef>
             </component>
           </struct>
         </dataTypeDef>

4.8.3.  String Properties

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
















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           <dataTypeDef>
             <name>stringElementProperties</name>
             <struct>
               <derivedFrom>baseElementProperties</derivedFrom>
               <component componentID="2">
                 <name>stringLength</name>
                 <synopsis>the number of octets in the string</synopsis>
                 <typeRef>uint32</typeRef>
               </component>
               <component componentID="3">
                 <name>maxStringLength</name>
                 <synopsis>
                   the maximum number of octets in the string
                   </synopsis>
                 <typeRef>uint32</typeRef>
               </component>
             </struct>
           </dataTypeDef>

4.8.4.  Octetstring Properties

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

              <dataTypeDef>
                <name>octetstringElementProperties</name>
                <struct>
                  <derivedFrom>baseElementProperties</derivedFrom>
                  <component componentID="2">
                    <name>octetstringLength</name>
                    <synopsis>
                      the number of octets in the octetstring
                    </synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component componentID="3">
                    <name>maxOctetstringLength</name>
                    <synopsis>
                      the maximum number of octets in the octetstring
                    </synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                </struct>
              </dataTypeDef>






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4.8.5.  Event Properties

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






































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            <dataTypeDef>
              <name>eventElementProperties</name>
              <struct>
                <derivedFrom>baseElementProperties</derivedFrom>
                <component componentID="2">
                  <name>registration</name>
                  <synopsis>
                    has the CE registered to be notified of this event
                  </synopsis>
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="3">
                  <name>threshold</name>
                  <synopsis> comparison value for level crossing events
                  </synopsis>
                  </optional
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="4">
                  <name>eventHysteresis</name>
                  <synopsis> region to suppress event recurrence notices
                  </synopsis>
                  </optional>
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="5">
                  <name>eventCount</name>
                  <synopsis> number of occurrences to suppress
                  </synopsis>
                  </optional>
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="6">
                  <name>eventHysteresis</name>
                  <synopsis> time interval in ms between notifications
                  </synopsis>
                  </optional>
                  <typeRef>uint32</typeRef>
                </component>
              </struct>
            <dataTypeDef>

4.8.5.1.  Common Event Filtering

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



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   all filter conditions.

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

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

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

4.8.5.2.   Event Hysteresis Filtering

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

   o  For a <eventChanged/> condition, if the last notification was for
      value X, then the <changed/> notification MUST NOT be generated
      until the value reaches X +/- V.




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   o  For a <eventGreaterThan/> condition with threshold T, once the
      event has been generated at least once it MUST NOT be generated
      again until the field first becomes less than or equal to T - -V,
      and then exceeds T.

   o  For a <eventLessThan/> condition with threshold T, once the event
      has been generate at least once it MUST NOT be generated again
      until the field first becomes greater than or equal to T + V, and
      then becomes less than T.

4.8.5.3.  Event Count Filtering

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

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

4.8.5.4.  Event Time Filtering

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

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

4.8.6.  Alias Properties

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



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   refers to.

          <dataTypeDef>
            <name>aliasElementProperties</name>
            <struct>
              <derivedFrom>baseElementProperties</derivedFrom>
              <component componentID="2">
                <name>targetLFBClass</name>
                <synopsis>the class ID of the alias target</synopsis>
                <typeRef>uint32</typeRef>
              </component>
              <component componentID="3">
                <name>targetLFBInstance</name>
                <synopsis>the instance ID of the alias target</synopsis>
                <typeRef>uint32</typeRef>
              </component>
              <component componentID="4">
                <name>targetComponentPath</name>
                <synopsis>
                  the path to the component target
                  each 4 octets is read as one path element,
                  using the path construction in the PL protocol.
                </synopsis>
                <typeRef>octetstring[128]</typeRef>
              </component>
            </struct>
          </dataTypeDef>

4.9.  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>



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



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     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="dataTypeDefsType">
     <xsd:sequence>
          <xsd:element name="dataTypeDef" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element ref="description" minOccurs="0"/>
                <xsd:group ref="typeDeclarationGroup"/>
              </xsd:sequence>
            </xsd:complexType>
          </xsd:element>
        </xsd:sequence>
      </xsd:complexType>
      <!--
         Predefined (built-in) atomic data-types are:
             char, uchar, int16, uint16, int32, uint32, int64, uint64,
             string[N], string, byte[N], boolean, octetstring[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:element name="alias" type="typeRefNMTOKEN"/>
        </xsd:choice>
      </xsd:group>
      <xsd:simpleType name="typeRefNMTOKEN">
        <xsd:restriction base="xsd:token">
          <xsd:pattern value="\c+"/>
          <xsd:pattern value="string\[\d+\]"/>
          <xsd:pattern value="byte\[\d+\]"/>
          <xsd:pattern value="octetstring\[\d+\]"/>
        </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>



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      </xsd:complexType>
      <xsd:complexType name="rangeRestrictionType">
        <xsd:sequence>
          <xsd:element name="allowedRange" maxOccurs="unbounded">
            <xsd:complexType>
           <xsd:attribute name="min" type="xsd:integer"
   use="required"/>
           <xsd:attribute name="max" type="xsd:integer"
   use="required"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="specialValuesType">
     <xsd:sequence>
       <xsd:element name="specialValue" maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="name" type="xsd:NMTOKEN"/>
             <xsd:element ref="synopsis"/>
           </xsd:sequence>
           <xsd:attribute name="value" type="xsd:token"/>
         </xsd:complexType>
       </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
   <xsd:complexType name="arrayType">
     <xsd:sequence>
       <xsd:group ref="typeDeclarationGroup"/>
       <xsd:element name="contentKey" minOccurs="0"
                    maxOccurs="unbounded">
         <xsd:complexType>
           <xsd:sequence>
             <xsd:element name="contentKeyField" maxOccurs="unbounded"
                          type="xsd:string"/>
           </xsd:sequence>
           <xsd:attribute name="contentKeyID" use="required"
                          type="xsd:integer"/>
         </xsd:complexType>
         <!--declare keys to have unique IDs -->
         <xsd:key name="contentKeyID">
           <xsd:selector xpath="lfb:contentKey"/>
           <xsd:field xpath="@contentKeyID"/>
         </xsd:key>
       </xsd:element>
     </xsd:sequence>
     <xsd:attribute name="type" use="optional"
                    default="variable-size">



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       <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="derivedFrom" type="typeRefNMTOKEN"
                       minOccurs="0"/>
          <xsd:element name="component" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element name="optional" minOccurs="0"/>
                <xsd:group ref="typeDeclarationGroup"/>
              </xsd:sequence>
              <xsd:attribute name="componentID" use="required"
                             type="xsd:integer"/>
            </xsd:complexType>
            <!-- key declaration to make componentIDs unique in a struct
            -->
            <xsd:key name="structComponentID">
              <xsd:selector xpath="lfb:component"/>
              <xsd:field xpath="@componentID"/>
            </xsd:key>
          </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 name="metadataID" type="xsd:integer"/>
                <xsd:element ref="description" minOccurs="0"/>
                <xsd:choice>
                  <xsd:element name="typeRef" type="typeRefNMTOKEN"/>
                  <xsd:element name="atomic" type="atomicType"/>
                </xsd:choice>



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



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



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          </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>
        <xsd:element name="name" type="xsd:NMTOKEN"/>
        <xsd:element ref="synopsis"/>
        <xsd:element name="product" type="portProductType"/>
        <xsd:element ref="description" minOccurs="0"/>



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



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



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            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element ref="description" minOccurs="0"/>
                <xsd:element name="optional" minOccurs="0"/>
                <xsd:group ref="typeDeclarationGroup"/>
              </xsd:sequence>
              <xsd:attribute name="componentID" use="required"
                             type="xsd:integer"/>
            </xsd:complexType>
          </xsd:element>
        </xsd:sequence>
      </xsd:complexType>
      <xsd:complexType name="eventsType">
        <xsd:sequence>
          <xsd:element name="event" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="name" type="xsd:NMTOKEN"/>
                <xsd:element ref="synopsis"/>
                <xsd:element name="eventTarget" type="eventPathType"/>
                <xsd:element ref="eventCondition"/>
                <xsd:element name="eventReports" type="eventReportsType"
                             minOccurs="0"/>
                <xsd:element ref="description" minOccurs="0"/>
              </xsd:sequence>
              <xsd:attribute name="eventID" use="required"
                             type="xsd:integer"/>
            </xsd:complexType>
          </xsd:element>
        </xsd:sequence>
        <xsd:attribute name="baseID" type="xsd:integer"
                       use="optional"/>
      </xsd:complexType>
      <!-- the substitution group for the event conditions -->
      <xsd:element name="eventCondition" abstract="true"/>
      <xsd:element name="eventCreated"
                  substitutionGroup="eventCondition"/>
      <xsd:element name="eventDeleted"
                  substitutionGroup="eventCondition"/>
      <xsd:element name="eventChanged"
                  substitutionGroup="eventCondition"/>
      <xsd:element name="eventGreaterThan"
                  substitutionGroup="eventCondition"/>
      <xsd:element name="eventLessThan"
                  substitutionGroup="eventCondition"/>
      <xsd:complexType name="eventPathType">



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



5.  FE Components and Capabilities

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

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

   In order to make the FE information easily accessible, the
   information is represented in an LFB.  This LFB has a class,
   FEObject.  The LFBClassID for this class is 1.  Only one instance of
   this class will ever be present, and the instance ID of that instance



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   in the protocol is 1.  Thus, by referencing the components of
   class:1, instance:1 a CE can get the general information about the
   FE.  The FEObject LFB Class is described in this section.

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

5.1.   XML for FEObject Class definition

          <?xml version="1.0" encoding="UTF-8"?>
          <LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://ietf.org/forces/1.0/lfbmodel.xsd"
            provides="FEObject">
       <!-- xmlns and schemaLocation need to be fixed -->
            <dataTypeDefs>
              <dataTypeDef>
                <name>LFBAdjacencyLimitType</name>
                <synopsis>Describing the Adjacent LFB</synopsis>
                <struct>
                  <component componentID="1">
                    <name>NeighborLFB</name>
                    <synopsis>ID for that LFB Class</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component componentID="2">
                    <name>ViaPorts</name>
                    <synopsis>
                      the ports on which we can connect
                    </synopsis>
                    <array type="variable-size">
                      <typeRef>string</typeRef>
                    </array>
                  </component>
                </struct>
              </dataTypeDef>
              <dataTypeDef>
                <name>PortGroupLimitType</name>
                <synopsis>
                  Limits on the number of ports in a given group
                </synopsis>
                <struct>
                  <component componentID="1">
                    <name>PortGroupName</name>
                    <synopsis>Group Name</synopsis>
                    <typeRef>string</typeRef>



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                  </component>
                  <component componentID="2">
                    <name>MinPortCount</name>
                    <synopsis>Minimum Port Count</synopsis>
                    <optional/>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component componentID="3">
                    <name>MaxPortCount</name>
                    <synopsis>Max Port Count</synopsis>
                    <optional/>
                    <typeRef>uint32</typeRef>
                  </component>
                </struct>
              </dataTypeDef>
              <dataTypeDef>
                <name>SupportedLFBType</name>
                <synopsis>table entry for supported LFB</synopsis>
                <struct>
                  <component componentID="1">
                    <name>LFBName</name>
                    <synopsis>
                      The name of a supported LFB Class
                    </synopsis>
                    <typeRef>string</typeRef>
                  </component>
                  <component componentID="2">
                    <name>LFBClassID</name>
                    <synopsis>the id of a supported LFB Class</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component componentID=??3??>
                    <name>LFBVersion</name>
                    <synopsis>
                      The version of the LFB Class used
                      by this FE.
                    </synopsis>
                    <typeRef>string</typeRef>
                  <component componentID="4">
                    <name>LFBOccurrenceLimit</name>
                    <synopsis>
                      the upper limit of instances of LFBs of this class
                    </synopsis>
                    <optional/>
                    <typeRef>uint32</typeRef>
                  </component>
                  <!-- For each port group, how many ports can exist
                  -->



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                  <component componentID="5">
                    <name>PortGroupLimits</name>
                    <synopsis>Table of Port Group Limits</synopsis>
                    <optional/>
                    <array type="variable-size">
                      <typeRef>PortGroupLimitType</typeRef>
                    </array>
                  </component>
       <!-- for the named LFB Class, the LFB Classes it may follow -->
                  <component componentID="6">
                    <name>CanOccurAfters</name>
                    <synopsis>
                      List of LFB Classes that this LFB class can follow
                    </synopsis>
                    <optional/>
                    <array type="variable-size">
                      <typeRef>LFBAdjacencyLimitType</typeRef>
                    </array>
                  </component>
       <!-- for the named LFB Class, the LFB Classes that may follow it
         -->
                  <component componentID="7">
                    <name>CanOccurBefores</name>
                    <synopsis>
                      List of LFB Classes that can follow this LFB class
                    </synopsis>
                    <optional/>
                    <array type="variable-size">
                      <typeRef>LFBAdjacencyLimitType</typeRef>
                    </array>
                  </component>
                </struct>
              </dataTypeDef>
              <dataTypeDef>
                <name>FEStatusValues</name>
                <synopsis>The possible values of status</synopsis>
                <atomic>
                  <baseType>uchar</baseType>
                  <specialValues>
                    <specialValue value="0">
                      <name>AdminDisable</name>
                      <synopsis>
                        FE is administratively disabled
                    </synopsis>
                    </specialValue>
                    <specialValue value="1">
                      <name>OperDisable</name>
                      <synopsis>FE is operatively disabled</synopsis>



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                    </specialValue>
                    <specialValue value="2">
                      <name>OperEnable</name>
                      <synopsis>FE is operating</synopsis>
                    </specialValue>
                  </specialValues>
                </atomic>
              </dataTypeDef>
              <dataTypeDef>
                <name>FEConfiguredNeighborType</name>
                <synopsis>Details of the FE's Neighbor</synopsis>
                <struct>
                  <component componentID="1">
                    <name>NeighborID</name>
                    <synopsis>Neighbors FEID</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component componentID="2">
                    <name>InterfaceToNeighbor</name>
                    <synopsis>
                      FE's interface that connects to this neighbor
                    </synopsis>
                    <optional/>
                    <typeRef>string</typeRef>
                  </component>
                  <component componentID=??3??>
                    <name>NeighborInterface</name>
                    <synopsis>
                      The name of the interface on the neighbor to
                      which this FE is adjacent.  This is required
                      In case two FE?s are adjacent on more than
                      one interface.
                    </synopsis>
                    <optional/>
                    <typeRef>string</typeRef>
                  </component>
                </struct>
              </dataTypeDef>
              <dataTypeDef>
                <name>LFBSelectorType</name>
                <synopsis>
                  Unique identification of an LFB class-instance
                </synopsis>
                <struct>
                  <component componentID="1">
                    <name>LFBClassID</name>
                    <synopsis>LFB Class Identifier</synopsis>
                    <typeRef>uint32</typeRef>



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                  </component>
                  <component componentID="2">
                    <name>LFBInstanceID</name>
                    <synopsis>LFB Instance ID</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                </struct>
              </dataTypeDef>
              <dataTypeDef>
                <name>LFBLinkType</name>
                <synopsis>
                  Link between two LFB instances of topology
                </synopsis>
                <struct>
                  <component componentID="1">
                    <name>FromLFBID</name>
                    <synopsis>LFB src</synopsis>
                    <typeRef>LFBSelectorType</typeRef>
                  </component>
                  <component componentID="2">
                    <name>FromPortGroup</name>
                    <synopsis>src port group</synopsis>
                    <typeRef>string</typeRef>
                  </component>
                  <component componentID="3">
                    <name>FromPortIndex</name>
                    <synopsis>src port index</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component componentID="4">
                    <name>ToLFBID</name>
                    <synopsis>dst LFBID</synopsis>
                    <typeRef>LFBSelectorType</typeRef>
                  </component>
                  <component componentID="5">
                    <name>ToPortGroup</name>
                    <synopsis>dst port group</synopsis>
                    <typeRef>string</typeRef>
                  </component>
                  <component componentID="6">
                    <name>ToPortIndex</name>
                    <synopsis>dst port index</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                </struct>
              </dataTypeDef>
            </dataTypeDefs>
            <LFBClassDefs>



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              <LFBClassDef LFBClassID="1">
                <name>FEObject</name>
                <synopsis>Core LFB: FE Object</synopsis>
                <version>1.0</version>
                <components>
                  <component access="read-write" componentID="1">
                    <name>LFBTopology</name>
                    <synopsis>the table of known Topologies</synopsis>
                    <array type="variable-size">
                      <typeRef>LFBLinkType</typeRef>
                    </array>
                  </component>
                  <component access="read-write" componentID="2">
                    <name>LFBSelectors</name>
                    <synopsis>
                       table of known active LFB classes and
                       instances
                    </synopsis>
                    <array type="variable-size">
                      <typeRef>LFBSelectorType</typeRef>
                    </array>
                  </component>
                  <component access="read-write" componentID="3">
                    <name>FEName</name>
                    <synopsis>name of this FE</synopsis>
                    <typeRef>string[40]</typeRef>
                  </component>
                  <component access="read-write" componentID="4">
                    <name>FEID</name>
                    <synopsis>ID of this FE</synopsis>
                    <typeRef>uint32</typeRef>
                  </component>
                  <component access="read-only" componentID="5">
                    <name>FEVendor</name>
                    <synopsis>vendor of this FE</synopsis>
                    <typeRef>string[40]</typeRef>
                  </component>
                  <component access="read-only" componentID="6">
                    <name>FEModel</name>
                    <synopsis>model of this FE</synopsis>
                    <typeRef>string[40]</typeRef>
                  </component>
                  <component access="read-only" componentID="7">
                    <name>FEState</name>
                    <synopsis>model of this FE</synopsis>
                    <typeRef>FEStatusValues</typeRef>
                  </component>
                  <component access="read-write" componentID="8">



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                    <name>FENeighbors</name>
                    <synopsis>table of known neighbors</synopsis>
                    <array type="variable-size">
                      <typeRef>FEConfiguredNeighborType</typeRef>
                    </array>
                  </component>
                </components>
                <capabilities>
                  <capability componentID="30">
                    <name>ModifiableLFBTopology</name>
                    <synopsis>
                      Whether Modifiable LFB is supported
                    </synopsis>
                    <optional/>
                    <typeRef>boolean</typeRef>
                  </capability>
                  <capability componentID="31">
                    <name>SupportedLFBs</name>
                    <synopsis>List of all supported LFBs</synopsis>
                    <optional/>
                    <array type="variable-size">
                      <typeRef>SupportedLFBType</typeRef>
                    </array>
                  </capability>
                </capabilities>
              </LFBClassDef>
            </LFBClassDefs>
          </LFBLibrary>

5.2.  FE Capabilities

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

   The currently defined capabilities are ModifiableLFBTopology and
   SupportedLFBs.  Information as to which components of the FEObject
   LFB are supported is accessed by the properties information for those
   components.

5.2.1.   ModifiableLFBTopology

   This component has a boolean value that indicates whether the LFB
   topology of the FE may be changed by the CE.  If the component is
   absent, the default value is assumed to be true, and the CE presumes
   the LFB topology may be changed.  If the value is present and set to
   false, the LFB topology of the FE is fixed.  If the topology is
   fixed, the LFBs supported clause may be omitted, and the list of



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   supported LFBs is inferred by the CE from the LFB topology
   information.  If the list of supported LFBs is provided when
   ModifiableLFBTopology is false, the CanOccurBefore and CanOccurAfter
   information should be omitted.

5.2.2.   SupportedLFBs and SupportedLFBType

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

   Each entry in the SupportedLFBs array 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.  This capability
   information on the acceptable ordering and connection of LFBs MAY be
   omitted if the implementor concludes that the actual constraints are
   such that the information would be misleading for the CE.

5.2.2.1.  LFBName

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

5.2.2.2.  LFBClassID

   The numeric ID of the LFB Class being described.  While conceptually
   redundant with the LFB Name, both are included for clarity and to
   allow consistency checking.

5.2.2.3.  LFBVersion

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

5.2.2.4.  LFBOccurrenceLimit

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



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   contained in the LFB list attribute.

5.2.2.5.  PortGroupLimits and PortGroupLimitType

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

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

5.2.2.6.  CanOccurAfters and LFBAdjacencyLimitType

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

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

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

5.2.2.7.  CanOccurBefores and LFBAdjacencyLimitType

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




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   The array entries list those LFB classes that the SupportedLFB may
   precede in the topology.  In this component, the entries in the
   viaPort component of the array value represent the output port groups
   of the SupportedLFB that may be connected to the NeighborLFB.  As
   with CanOccurAfters, viaPort may have multiple entries if multiple
   output ports may legitimately connect to the given NeighborLFB class.

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

5.2.2.8.   LFBClassCapabilitiese

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

   As there also are not currently any defined LFB Class level
   Capabilities that the FE needs to report, this information is not
   present now, but may be added in a future version of the FE Object.
   (This is an example of a case where versioning, rather than
   inheritance, would be needed, since the FE Object must have class ID
   1 and instance ID 1 so that the protocol behavior can start by
   finding this object.)

5.3.  FE Components

   The <components> element is included if the class definition contains
   the definition of the components of the FE Object that are not
   considered "capabilities".  Some of these components are writeable,
   and some are read-only, which is determinable by examining the
   property information of the components.

5.3.1.  FEStatus

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

5.3.2.  LFBSelectors and LFBSelectorType

   The LFBSelectors component is an array of information about the LFBs
   currently accessible via ForCES in the FE.  The structure of the LFB



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   information is defined by the LFBSelectorType dataTypeDef.

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

5.3.3.  LFBTopology and LFBLinkType

   The optional LFBTopology component contains information about each
   inter-LFB link inside the FE, where each link is described in an
   LFBLinkType dataTypeDef.  The LFBLinkType component contains
   sufficient information to identify precisely the end points of a
   link.  The FromLFBID and ToLFBID components specify the LFB instances
   at each end of the link, and MUST reference LFBs in the LFB instance
   table.  The FromPortGroup and ToPortGroup MUST identify output and
   input port groups defined in the LFB classes of the LFB instances
   identified by FromLFBID and ToLFBID.  The FromPortIndex and
   ToPortIndex components select the entries from the port groups that
   this link connects.  All links are uniquely identified by the
   FromLFBID, FromPortGroup, and FromPortIndex fields.  Multiple links
   may have the same ToLFBID, ToPortGroup, and ToPortIndex as this model
   supports fan in of inter- LFB links but not fan out.

5.3.4.   FENeighbors and FEConfiguredNeighborType

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

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

   While there may be many ways to configure neighbors, the FE-ID is the
   best way for the CE to correlate entities.  And the interface
   identifier (name string) is the best correlator.  The CE will be able
   to determine the IP address and media level information about the
   neighbor from the neighbor directly.  Omitting that information from
   this table avoids the risk of incorrect double configuration.

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






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5.3.4.1.  NeighborID

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

5.3.4.2.  InterfaceToNeighbor

   This identifies the interface through which the neighbor is reached.

5.3.4.3.  NeighborInterface

   This identifies the interface on the neighbor through which the
   neighbor is reached.  The interface identification is needed when
   either only one side of the adjacency has configuration information,
   or the two FEs are adjacent on more than one interface.


6.  Satisfying the Requirements on FE Model

   This section describes how the proposed FE model meets the
   requirements outlined in Section 5 of [2].  The requirements can be
   separated into general requirements (Section 5, 5.1 - 5.4) and the
   specification of the minimal set of logical functions that the FE
   model must support (Section 5.5).

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

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

   Instantiated LFBs are interconnected in a directed graph that
   describes the ordering of the functions within an FE.  This directed



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   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 includes only the definition of the FE Object LFB
   itself.  Meeting the full set of working group requirements requires
   other LFBs.  The class definitions for those LFBs will be provided in
   other documents.


7.  Using the FE model in the ForCES Protocol

   The actual model of the forwarding plane in a given NE is something
   the CE must learn and control by communicating with the FEs (or by
   other means).  Most of this communication will happen in the post-
   association phase using the ForCES protocol.  The following types of
   information must be exchanged between CEs and FEs via the ForCES
   protocol:

   1.  FE topology query;

   2.  FE capability declaration;

   3.  LFB topology (per FE) and configuration capabilities query;

   4.  LFB capability declaration;

   5.  State query of LFB attributes;

   6.  Manipulation of LFB attributes;

   7.  LFB topology reconfiguration.

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

   Items 6) and 7) are "command" types of exchanges, where the main flow
   of information is from the CEs to the FEs.  Messages in Item 6) (the
   LFB re-configuration commands) are expected to be used frequently.



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   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.

   The inter-FE topology (item 1 above) can be determined by the CE in
   many ways.  Neither this document nor the Forces protocol mandates a
   specific mechanism.  The LFB Class definition does include the
   capability for an FE to be configured with, and provides to the CE in
   response to a query, the identity of its neighbors.  There may also
   be defined specific LFB classes and protocols for neighbor discovery.
   Routing protocols may be used by the CE for adjacency determination.
   The CE may be configured with the relevant information.

   The relationship between the FE model and the seven post-association
   messages are visualized in Figure 8:



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




    Figure 8: 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:

   o  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.

   o  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



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      as XML, TLVs).

   o  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.

7.1.  FE Topology Query

   An FE may contain zero, one or more external ingress ports.
   Similarly, an FE may contain zero, one or more external egress ports.
   In other words, not every FE has to contain any external ingress or
   egress interfaces.  For example, Figure 9 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 9.  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.







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


            Figure 9: An example of two FEs connected together

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

7.2.  FE Capability Declarations

   FEs will have many types of limitations.  Some of the limitations
   must be expressed to the CEs as part of the capability model.  The
   CEs must be able to query these capabilities on a per-FE basis.
   Examples:

   o  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.

   o  Global resource query limitations (applicable to all LFBs of the
      FE).

   o  LFB supported by the FE.





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   o  LFB class instantiation limit.

   o  LFB topological limitations (linkage constraint, ordering etc.)

7.3.  LFB Topology and Topology Configurability Query

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

7.4.  LFB Capability Declarations

   LFB class specifications define a generic set of capabilities.  When
   an LFB instance is implemented (instantiated) on a vendor's FE, some
   additional limitations may be introduced.  Note that we discuss only
   those limitations that are within the flexibility of the LFB class
   specification.  That is, the LFB instance will remain compliant with
   the LFB class specification despite these limitations.  For example,
   certain features of an LFB class may be optional, in which case it
   must be possible for the CE to determine if an optional feature is
   supported by a given LFB instance or not.  Also, the LFB class
   definitions will probably contain very few quantitative limits (e.g.,
   size of tables), since these limits are typically imposed by the
   implementation.  Therefore, quantitative limitations should always be
   expressed by capability arguments.

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

   The following two broad types of limitations will exist:

   o  Qualitative restrictions.  For example, a standardized multi-
      field classifier LFB class may define a large number of



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      classification fields, but a given FE may support only a subset of
      those fields.

   o  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.

   Much of this restrictive information is captured by the component
   property information, and so can be access uniformly for all
   information within the model.

7.5.  State Query of LFB Attributes

   This feature must be provided by all FEs.  The ForCES protocol and
   the data schema/encoding conveyed by the protocol must together
   satisfy the following requirements to facilitate state query of the
   LFB attributes:

   o  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.

   o  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.

   o  Must support addressing of individual attribute of an LFB.

   o  Must provide efficient encoding and decoding of the addressing
      info and the configured data.

   o  Must provide efficient data transmission of the attribute state
      over the wire (to minimize communication load on the CE-FE link).





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7.6.   LFB Component Manipulation

   The FE Model provides for the definition of LFB Classes.  Each class
   has a globally unique identifier.  Information within the class is
   represented as components and assigned identifiers with the scope of
   that class.  This model also specifies that instances of LFB Classes
   have identifiers.  The combination of class identifiers, instance
   identifiers, and component identifiers are used by the protocol to
   reference the LFB information in the protocol operations.

7.7.  LFB Topology Re-configuration

   Operations that will be needed to reconfigure LFB topology:

   o  Create a new instance of a given LFB class on a given FE.

   o  Connect a given output of LFB x to the given input of LFB y.

   o  Disconnect: remove a link between a given output of an LFB and a
      given input of another LFB.

   o  Delete a given LFB (automatically removing all interconnects to/
      from the LFB).


8.  Example

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

   Later portions of this example include references to protocol
   operations.  The operations described are operations the protocol
   needs to support.  The exact format and fields are purely
   informational here, as the protocol document defines the precise
   syntax and symantics of its operations.

  <?xml version="1.0" encoding="UTF-8"?>
        <LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://ietf.org/forces/1.0/lfbmodel"
         provides="LaserFrameLFB">
          <frameDefs>
            <frameDef>
              <name>FRFrame</name>



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              <synopsis>
                  A frame relay frame, with DLCI without
                  stuffing)
              </synopsis>
            </frameDef>
            <frameDef>
              <name>IPFrame</name>
               <synopsis>An IP Packet</synopsis>
            </frameDef>
          </frameDefs>
          <dataTypeDefs>
            <dataTypeDef>
              <name>frequencyInformationType</name>
              <synopsis>
                  Information about a single CWDM frequency
              </synopsis>
              <struct>
                <component componentID="1">
                  <name>LaserFrequency</name>
                  <synopsis>encoded frequency(channel)</synopsis>
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="2">
                  <name>FrequencyState</name>
                  <synopsis>state of this frequency</synopsis>
                  <typeRef>PortStatusValues</typeRef>
                </component>
                <component componentID="3">
                  <name>LaserPower</name>
                  <synopsis>current observed power</synopsis>
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="4">
                  <name>FrameRelayCircuits</name>
                  <synopsis>
                      Information about circuits on this Frequency
                  </synopsis>
                  <array>
                    <typeRef>frameCircuitsType</typeRef>
                  </array>
                </component>
              </struct>
            </dataTypeDef>
            <dataTypeDef>
              <name>frameCircuitsType</name>
              <synopsis>
                  Information about a single Frame Relay circuit
              </synopsis>



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              <struct>
                <component componentID="1">
                  <name>DLCI</name>
                  <synopsis>DLCI of the circuit</synopsis>
                  <typeRef>uint32</typeRef>
                </component>
                <component componentID="2">
                  <name>CircuitStatus</name>
                  <synopsis>state of the circuit</synopsis>
                  <typeRef>PortStatusValues</typeRef>
                </component>
                <component componentID="3">
                  <name>isLMI</name>
                  <synopsis>is this the LMI circuit</synopsis>
                  <typeRef>boolean</typeRef>
                </component>
                <component componentID="4">
                  <name>associatedPort</name>
                  <synopsis>
                      which input / output port is associated
                      with this circuit
                  </synopsis>
                  <typeRef>uint32</typeRef>
                </component>
              </struct>
            </dataTypeDef>
            <dataTypeDef>
              <name>PortStatusValues</name>
              <synopsis>
                  The possible values of status.  Used for both
                  administrative and operation status
              </synopsis>
              <atomic>
                <baseType>uchar</baseType>
                <specialValues>
                  <specialValue value="0">
                    <name>Disabled </name>
                    <synopsis>the component is disabled</synopsis>
                  </specialValue>
                  <specialValue value="1">
                    <name>Enable</name>
                    <synopsis>FE is operatively disabled</synopsis>
                  </specialValue>
                </specialValues>
              </atomic>
            </dataTypeDef>
          </dataTypeDefs>
          <metadataDefs>



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            <metadataDef>
              <name>DLCI</name>
              <synopsis>The DLCI the frame arrived on</synopsis>
              <metadataID>12</metadataID>
              <typeRef>uint32</typeRef>
            </metadataDef>
            <metadataDef>
              <name>LaserChannel</name>
              <synopsis>The index of the laser channel</synopsis>
              <metadataID>34</metadataID>
              <typeRef>uint32</typeRef>
            </metadataDef>
          </metadataDefs>
          <LFBClassDefs>
            <LFBClassDef LFBClassID="-255">
              <name>FrameLaserLFB</name>
              <synopsis>Fictional LFB for Demonstrations</synopsis>
              <version>1.0</version>
              <inputPorts>
                <inputPort group="yes">
                  <name>LMIfromFE</name>
                  <synopsis>
                      Ports for LMI traffic, for transmission
                  </synopsis>
                  <expectation>
                    <frameExpected>
                      <ref>FRFrame</ref>
                    </frameExpected>
                    <metadataExpected>
                      <ref>DLCI</ref>
                      <ref>LaserChannel</ref>
                    </metadataExpected>
                  </expectation>
                </inputPort>
                <inputPort>
                  <name>DatafromFE</name>
                  <synopsis>
                      Ports for data to be sent on circuits
                  </synopsis>
                  <expectation>
                    <frameExpected>
                      <ref>IPFrame</ref>
                    </frameExpected>
                    <metadataExpected>
                      <ref>DLCI</ref>
                      <ref>LaserChannel</ref>
                    </metadataExpected>
                  </expectation>



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                </inputPort>
              </inputPorts>
              <outputPorts>
                <outputPort group="yes">
                  <name>LMItoFE</name>
                  <synopsis>
                      Ports for LMI traffic for processing
                  </synopsis>
                  <product>
                    <frameProduced>
                      <ref>FRFrame</ref>
                    </frameProduced>
                    <metadataProduced>
                      <ref>DLCI</ref>
                      <ref>LaserChannel</ref>
                    </metadataProduced>
                  </product>
                </outputPort>
                <outputPort group="yes">
                  <name>DatatoFE</name>
                  <synopsis>
                      Ports for Data traffic for processing
                  </synopsis>
                  <product>
                    <frameProduced>
                      <ref>IPFrame</ref>
                    </frameProduced>
                    <metadataProduced>
                      <ref>DLCI</ref>
                      <ref>LaserChannel</ref>
                    </metadataProduced>
                  </product>
                </outputPort>
              </outputPorts>
              <components>
                <component access="read-write" componentID="1">
                  <name>AdminPortState</name>
                  <synopsis>is this port allowed to function</synopsis>
                  <typeRef>PortStatusValues</typeRef>
                </component>
                <component access="read-write" componentID="2">
                  <name>FrequencyInformation</name>
                  <synopsis>
                      table of information per CWDM frequency
                  </synopsis>
                  <array type="variable-size">
                    <typeRef>frequencyInformationType</typeRef>
                  </array>



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                </component>
              </components>
              <capabilities>
                <capability componentID="31">
                  <name>OperationalState</name>
                  <synopsis>
                      whether the port over all is operational
                  </synopsis>
                  <typeRef>PortStatusValues</typeRef>
                </capability>
                <capability componentID="32">
                  <name>MaximumFrequencies</name>
                  <synopsis>
                      how many laser frequencies are there
                  </synopsis>
                  <typeRef>uint16</typeRef>
                </capability>
                <capability componentID="33">
                  <name>MaxTotalCircuits</name>
                  <synopsis>
                      Total supportable Frame Relay Circuits, across
                      all laser frequencies
                  </synopsis>
                  <optional/>
                  <typeRef>uint32</typeRef>
                </capability>
              </capabilities>
              <events baseID="61">
                <event eventID="1">
                  <name>FrequencyState</name>
                  <synopsis>
                      The state of a frequency has changed
                  </synopsis>
                  <eventTarget>
                    <eventField>FrequencyInformation</eventField>
                    <eventSubscript>_FrequencyIndex_</eventSubscript>
                    <eventField>FrequencyState</eventField>
                  </eventTarget>
                  <eventChanged/>
                  <eventReports>
                    <!-- report the new state -->
                    <eventReport>
                      <eventField>FrequencyInformation</eventField>
                      <eventSubscript>_FrequencyIndex_</eventSubscript>
                      <eventField>FrequencyState</eventField>
                    </eventReport>
                  </eventReports>
                </event>



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                <event eventID="2">
                  <name>CreatedFrequency</name>
                  <synopsis>A new frequency has appeared</synopsis>
                  <eventTarget>
                    <eventField>FrequencyInformation></eventField>
                    <eventSubscript>_FrequencyIndex_</eventSubscript>
                  </eventTarget>
                  <eventCreated/>
                  <eventReports>
                    <eventReport>
                      <eventField>FrequencyInformation</eventField>
                      <eventSubscript>_FrequencyIndex_</eventSubscript>
                      <eventField>LaserFrequency</eventField>
                    </eventReport>
                  </eventReports>
                </event>
                <event eventID="3">
                  <name>DeletedFrequency</name>
                  <synopsis>
                      A frequency Table entry has been deleted
                  </synopsis>
                  <eventTarget>
                    <eventField>FrequencyInformation</eventField>
                    <eventSubscript>_FrequencyIndex_</eventSubscript>
                  </eventTarget>
                  <eventDeleted/>
                 </event>
                <event eventID="4">
                  <name>PowerProblem</name>
                  <synopsis>
                      there are problems with the laser power level
                  </synopsis>
                  <eventTarget>
                    <eventField>FrequencyInformation</eventField>
                    <eventSubscript>_FrequencyIndex_</eventSubscript>
                    <eventField>LaserPower</eventField>
                  </eventTarget>
                  <eventLessThan/>
                  <eventReports>
                    <eventReport>
                      <eventField>FrequencyInformation</eventField>
                      <eventSubscript>_FrequencyIndex_</eventSubscript>
                      <eventField>LaserPower</eventField>
                    </eventReport>
                    <eventReport>
                      <eventField>FrequencyInformation</eventField>
                      <eventSubscript>_FrequencyIndex_</eventSubscript>
                      <eventField>LaserFrequency</eventField>



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                    </eventReport>
                  </eventReports>
                </event>
                <event eventID="5">
                  <name>FrameCircuitChanged</name>
                  <synopsis>
                      the state of an Fr circuit on a frequency
                      has changed
                  </synopsis>
                  <eventTarget>
                    <eventField>FrequencyInformation</eventField>
                    <eventSubscript>_FrequencyIndex_</eventSubscript>
                    <eventField>FrameRelayCircuits</eventField>
                    <eventSubscript>FrameCircuitIndex</eventSubscript>
                    <eventField>CircuitStatus</eventField>
                  </eventTarget>
                  <eventChanged/>
                  <eventReports>
                    <eventReport>
                      <eventField>FrequencyInformation</eventField>
                      <eventSubscript>_FrequencyIndex_</eventSubscript>
                      <eventField>FrameRelayCircuits</eventField>
                      <eventSubscript>FrameCircuitIndex</eventSubscript>
                      <eventField>CircuitStatus</eventField>
                    </eventReport>
                    <eventReport>
                      <eventField>FrequencyInformation</eventField>
                      <eventSubscript>_FrequencyIndex_</eventSubscript>
                      <eventField>FrameRelayCircuits</eventField>
                      <eventSubscript>FrameCircuitIndex</eventSubscript>
                      <eventField>DLCI</eventField>
                    </eventReport>
                  </eventReports>
                </event>
              </events>
            </LFBClassDef>
          </LFBClassDefs>
        </LFBLibrary>

8.1.  Data Handling

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

   Packets arriving without error from the physical interface come in on
   a Frame Relay DLCI on a laser channel.  These two values are used by



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

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

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

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

8.1.1.  Setting up a DLCI

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

      T = SET-OPERATION
        T = PATH-DATA
          Path: flags = none, length = 4, path = 2, channel, 4, entryIdx
          DataRaw: DLCI, Enable(1), false, out-idx

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

   The response would confirm the creation of the specified entry.  This
   table is structured to use separate internal indices and DLCIs.  An
   alternative design could have used the DLCI as index, trading off
   complexities.




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

8.1.2.  Error Handling

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

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

   One way is to define a specific component to count the error, and a
   component in the LFB to hold the required portion of the packet.  The
   component could be defined to hold the portion of the packet from the
   most recent error.  One could then define an event that occurs
   whenever the error count changes, and declare that reporting the
   event includes the LFB field with the packet portion.  For rare but
   extremely critical errors, this is an effective solution.  It ensures
   reliable delivery of the notification.  And it allows the CE to
   control if it wants the notification.

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

   Other aspects of error handling are discussed under events below.

8.2.  LFB Components

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

   The other attribute is the table of information about the laser
   channels.  It is a variable sized array.  Each array entry contains
   an identifier for what laser frequency this entry is associated with,
   whether that frequency is operational, the power of the laser at that
   frequency, and a table of information about frame relay circuits on
   this frequency.  There is no administrative status since a CE can
   disable an entry simply by removing it.  (Frequency and laser power



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   of a non-operational channel are not particularly useful.  Knowledge
   about what frequencies can be supported would be a table in the
   capabilities section.)

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

8.3.  Capabilities

   The capability information for this LFB includes whether the
   underlying interface is operational, how many frequencies are
   supported, and how many total circuits, across all channels, are
   permitted.  The maximum number for a given laser channel can be
   determined from the properties of the FrameRelayCircuits table.  A
   GET-Properties on path 2.channel.4 will give the CE the properties of
   the array which include the number of entries used, the first
   available entry, and the maximum number of entries permitted.

8.4.  Events

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

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

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





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

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

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

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

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


9.   IANA Considerations

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



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

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

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


10.  Authors Emeritus

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

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


11.  Acknowledgments

   Many of the colleagues in our companies and participants in the
   ForCES mailing list have provided invaluable input into this work.
   Particular thanks to Jamal Hadi Salim for both his direct efforts on
   this documents, and his energy in ensuring that work on this
   continued.


12.  Security Considerations

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


13.  References

13.1.  Normative References

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



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13.2.  Informative References

   [2]   Khosravi, H. and T. Anderson, "Requirements for Separation of
         IP Control and Forwarding", RFC 3654, November 2003.

   [3]   Yang, L., Dantu, R., Anderson, T., and R. Gopal, "Forwarding
         and Control Element Separation (ForCES) Framework", RFC 3746,
         April 2004.

   [4]   Chan, K., Sahita, R., Hahn, S., and K. McCloghrie,
         "Differentiated Services Quality of Service Policy Information
         Base", RFC 3317, March 2003.

   [5]   Sahita, R., Hahn, S., Chan, K., and K. McCloghrie, "Framework
         Policy Information Base", RFC 3318, March 2003.

   [6]   Li, M., "IPsec Policy Information Base", work in progress,
         draft-ietf -ipsp-spsecpib-10.txt, April 2004.

   [7]   Duffield, N., "A Framework for packet Selection and Reporting",
         draft-ietf -psamp-framework-10.txt, January 2005.

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

   [9]   Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for the
         Use of Extensible Markup Language (XML) within IETF Protocols",
         BCP 70, RFC 3470, January 2003.

   [10]  Thompson, H., Beech, D., Maloney, M., and Mendelsohn, N., "XML
         Schema Part 1: Structures", W3C REC-xmlschema-1,
         http://www.w3.org/TR/ xmlschema-1/, May 2001.

   [11]  Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes",
         W3C REC-xmlschema-2, http://www.w3.org/TR /xmlschema-2/,
         May 2001.

   [12]  Davis, M. and M. Suignard, "UNICODE Security Considerations",
         http://www.unicode.org/ reports/tr36/tr36-3.html, July 2005.












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Authors' Addresses

   Joel Halpern
   Self
   P.O. Box 6049
   Leesburg,, VA  20178


   Phone: +1 703 371 3043
   Email: jmh@joelhalpern.com


   Ellen Deleganes
   Intel Corp.
   Mail Stop: CO5-156  15400 NW Greenbrier Parkway
   Beaverton,, OR  97006


   Phone: +1 503 677-4996
   Email: ellen.m.deleganes@intel.com































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Full Copyright Statement

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