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    Internet Draft                                 L. Yang
    Expiration: April 2004                              Intel Labs
    File: draft-ietf-forces-model-01.txt           J. Halpern
    Working Group: ForCES                               Megisto Systems
                                                   R. Gopal
                                                        Nokia
                                                   A. DeKok
                                                        IDT Inc.
                                                   Z. Haraszti
                                                   S. Blake
                                                        Ericsson
                                                   October 2003
 
 
                      ForCES Forwarding Element Model
 
 
 
                       draft-ietf-forces-model-01.txt
 
 
 
 
    Status of this Memo
 
    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.  Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF),
    its areas, and its working groups.  Note that other groups may also
    distribute working documents as Internet-Drafts.
 
    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other
    documents at any time.  It is inappropriate to use Internet-Drafts
    as reference material or to cite them other than as ``work in
    progress.''
 
    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt.
 
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.
 
 Abstract
 
    This document defines the forwarding element (FE) model used in the
    Forwarding and Control Plane Separation (ForCES) protocol.  The
    model represents the capabilities, state and configuration of
    forwarding elements within the context of the ForCES protocol, so
    that control elements (CEs) can control the FEs accordingly.  More
    specifically, the model describes the logical functions that are
 
 
 
 Internet Draft         ForCES FE Model              October 2003
 
 
    present in an FE, what capabilities these functions support, and
    how these functions are or can be interconnected. This FE model is
    intended to satisfy the model requirements specified in the ForCES
    requirements draft [1].  A list of the basic logical functional
    blocks (LFBs) is also defined in the LFB class library to aid the
    effort in defining individual LFBs.
 
 Table of Contents
 
    Abstract.........................................................1
    1. Definitions...................................................3
    2. Introduction..................................................5
       2.1. Requirements on the FE model.............................6
       2.2. The FE Model in Relation to FE Implementations...........6
       2.3. The FE Model in Relation to the ForCES Protocol..........6
       2.4. Modeling Language for FE Model...........................7
       2.5. Document Structure.......................................8
    3. FE Model Concepts.............................................8
       3.1. State Model and Capability Model.........................8
       3.2. LFB Modeling............................................11
          3.2.1. LFB Input and Input Group..........................13
          3.2.2. LFB Output and Output Group........................15
          3.2.3. Packet Type........................................16
          3.2.4. Metadata...........................................16
          3.2.5. LFB Versioning.....................................18
          3.2.6. LFB Inheritance....................................18
       3.3. FE Datapath Modeling....................................19
          3.3.1. Alternative Approaches for Modeling FE Datapaths...19
          3.3.2. Configuring the LFB Topology.......................23
    4. LFB Model -- LFB and Associated Data Definitions.............27
       4.1. General Data Type Definitions...........................28
          4.1.1. Arrays.............................................29
          4.1.2. Structures.........................................29
          4.1.3. Augmentations......................................30
       4.2. Metadata Definitions....................................30
       4.3. Frame Format Definitions................................30
       4.4. LFB Class Definitions...................................31
          4.4.1. LFB Inheritance....................................31
          4.4.2. LFB Inputs.........................................31
          4.4.3. LFB Outputs........................................32
          4.4.4. LFB Attributes.....................................33
          4.4.5. LFB Operational Specification......................34
    5. LFB Topology Model (To be written)...........................34
    6. FE Level Attributes (To be written)..........................35
    7. LFB Class Library............................................35
       7.1. Port LFB................................................35
       7.2. Dropper LFB.............................................36
 
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       7.3. Redirector (de-MUX) LFB.................................36
       7.4. Scheduler LFB...........................................36
       7.5. Queue LFB...............................................36
       7.6. Counter LFB.............................................37
       7.7. Meter LFB and Policer LFB...............................37
       7.8. Classifier LFB..........................................37
       7.9. Modifier LFB............................................38
       7.10. Packet Header Rewriter LFB.............................38
    8. Satisfying the Requirements on FE Model......................39
       8.1. Port Functions..........................................39
       8.2. Forwarding Functions....................................40
       8.3. QoS Functions...........................................41
       8.4. Generic Filtering Functions.............................41
       8.5. Vendor Specific Functions...............................42
       8.6. High-Touch Functions....................................42
       8.7. Security Functions......................................42
       8.8. Off-loaded Functions....................................43
       8.9. IPFLOW/PSAMP Functions..................................43
    9. Using the FE model in the ForCES Protocol....................43
       9.1. FE Topology Query.......................................45
       9.2. FE Capability Declarations..............................46
       9.3. LFB Topology and Topology Configurability Query.........47
       9.4. LFB Capability Declarations.............................47
       9.5. State Query of LFB Attributes...........................48
       9.6. LFB Attribute Manipulation..............................48
       9.7. LFB Topology Re-configuration...........................49
    10. Acknowledgments.............................................49
    11. Security Considerations.....................................49
    12. Normative References........................................49
    13. Informative References......................................50
    14. Authors' Addresses..........................................50
    15. Intellectual Property Right.................................51
    16. IANA consideration..........................................51
 
 Conventions used in this document
 
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
    this document are to be interpreted as described in [RFC-2119].
 
 
 1. Definitions
 
    A set of terminology associated with the ForCES requirements is
    defined in [1] and is not copied here. The following list of
    terminology is relevant to the FE model defined in this document.
 
 
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    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. There might exist more than one
    datapath within an FE.
 
    LFB (Logical Function Block) class (or type) -- A template
    representing a fine-grained, logically separable and well-defined
    packet processing operation in the datapath.  LFB classes are the
    basic building blocks of the FE model.
 
    LFB (Logical Function Block) Instance -- As a packet flows through
    an FE along a datapath, it flows through one or multiple LFB
    instances, with each implementing an instance of a certain LFB
    class.  There may be multiple instances of the same LFB in an FE's
    datapath.  Note that we often refer to LFBs without distinguishing
    between LFB class and LFB instance when we believe the implied
    reference is obvious for the given context.
 
    LFB Model -- The LFB model describes the content and structures in
    LFB and associated data definition.  There are four types of
    information defined in the LFB model. The core part of the LFB
    model is LFB class definitions while the other three are to define
    the associated data including common data types, supported frame
    formats and metadata.
 
    LFB Metadata -- Metadata is used to communicate per-packet state
    from one LFB to another, but is not sent across the network.  The
    FE model defines how such metadata is identified, produced and
    consumed by the LFBs, but not how metadata is encoded within an
    implementation.
 
    LFB Attribute -- Operational parameters of the LFBs that must be
    visible to the CEs are conceptualized in the FE model as the LFB
    attributes. The LFB attributes include, for example, flags, single
    parameter arguments, complex arguments, and tables that the CE can
    read or/and write via the ForCES protocol.
 
    LFB Topology -- Representation of how the LFB instances are
    logically interconnected and placed along the datapath within one
 
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    FE.  Sometimes it is also called intra-FE topology, to be
    distinguished from inter-FE topology.  LFB topology is outside of
    the LFB model, but part of the FE model.
 
    FE Topology -- Representation of how the multiple FEs in a single
    NE are interconnected.  Sometimes it is called inter-FE topology,
    to be distinguished from intra-FE topology (i.e., LFB topology).
    Individual FE may not have the global knowledge of full FE
    topology, but the local view of its connectivity with other FEs are
    considered part of the FE model.  FE topology is discovered by the
    ForCES base protocol or some other means.
 
    Inter-FE Topology -- See FE Topology.
 
    Intra-FE Topology -- See LFB Topology.
 
    LFB class library -- A set of LFB classes that are identified as
    the most common functions found in most FEs and hence should be
    defined first by the ForCES Working Group.
 
 2. Introduction
 
    [2] specifies a framework by which control elements (CEs) can
    configure and manage one or more separate forwarding elements (FEs)
    within a networking element (NE) using the ForCES protocol.  The
    ForCES architecture allows Forwarding Elements of varying
    functionality to participate in a ForCES network element.  The
    implication of this varying functionality is that CEs can make only
    minimal assumptions about the functionality provided by FEs in a
    NE.  Before CEs can configure and control the forwarding behavior
    of FEs, CEs need to query and discover the capabilities and states
    of their FEs.  [1] mandates that the capabilities, states and
    configuration information be expressed in the form of an FE model.
 
    RFC 3444 [11] made the observation that information models (IMs)
    and data models (DMs) are different because they serve different
    purposes.  "The main purpose of an IM is to model managed objects
    at a conceptual level, independent of any specific implementations
    or protocols used".  "DMs, conversely, are defined at a lower level
    of abstraction and include many details.  They are intended for
    implementors and include protocol-specific constructs."  Sometimes
    it is difficult to draw a clear line between the two.  The FE model
    described in this document is first and foremost an information
    model, but it also has a flavor of a data model as it contains
    explicit definition of the LFB class schema and other data
    structures.  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.
 
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 2.1. Requirements on the FE model
 
    [1] defines requirements which must be satisfied by a ForCES FE
    model.  To summarize, an FE model must define:
       . Logically separable and distinct packet forwarding operations
         in an FE datapath (logical functional blocks or LFBs);
       . The possible topological relationships (and hence the sequence
         of packet forwarding operations) between the various LFBs;
       . The possible operational capabilities (e.g., capacity limits,
         constraints, optional features, granularity of configuration)
         of each type of LFB;
       . The possible configurable parameters (i.e., attributes) of
         each type of LFB;
       . Metadata that may be exchanged between LFBs.
 
 2.2. The FE Model in Relation to FE Implementations
 
    The FE model proposed here is based on an abstraction of distinct
    logical functional blocks (LFBs), interconnected in a directed
    graph, and receiving, processing, modifying, and transmitting
    packets along with metadata.  Note that a real forwarding datapath
    implementation should not be constrained by the model.  On the
    contrary, the FE model should be designed such that different
    implementations of the forwarding datapath can all be logically
    mapped onto the model with the functionality and sequence of
    operations correctly captured.  However, the model itself does not
    directly address the issue of how a particular implementation maps
    to an LFB topology.  This is left to the forwarding plane vendors
    as to how the FE functionality is represented using the FE model.
    Nevertheless, we do strive to design the FE model such that it is
    flexible enough to accommodate most common implementations.
 
    The LFB topology model for a particular datapath implementation
    MUST correctly capture the sequence of operations on the packet.
    Metadata generation (by certain LFBs) must always precede any use
    of that metadata (by subsequent LFBs in the topology graph); this
    is required for logically consistent operation.  Further,
    modifications of packet fields that are subsequently used as inputs
    for further processing must occur in the order specified in the
    model for that particular implementation to ensure correctness.
 
 2.3. The FE Model in Relation to the ForCES Protocol
 
    The ForCES base protocol is used by the CEs and FEs to maintain the
    communication channel between the CEs and FEs.  The ForCES protocol
    may be used to query and discover the inter-FE topology. The
 
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    details of a particular datapath implementation inside an FE
    including the LFB topology, along with the operational capabilities
    and attributes of each individual LFB, are conveyed to the CE
    within information elements in the ForCES protocol.  The model of
    an LFB class should define all of the information that would need
    to be exchanged between an FE and a CE for the proper configuration
    and management of that LFB.
 
    Definition of the various payloads of ForCES messages (irrespective
    of the transport protocol ultimately selected) cannot proceed in a
    systematic fashion until a formal definition of the objects being
    configured and managed (the FE and the LFBs within) is undertaken.
    The FE Model document defines a set of classes and attributes for
    describing and manipulating the state of the LFBs of an FE.  These
    class definitions themselves will generally not appear in the
    Forces protocol.  Rather, Forces protocol operations will
    references classes defined in this model, including relevant
    attributes (and operations if such are defined).
 
    Section 9 provides more detailed discussion on how the FE model
    should be used by the ForCES protocol.
 
 2.4. Modeling Language for FE Model
 
    Even though not absolutely required, it is beneficial to use a
    formal data modeling language to represent the conceptual FE model
    described in this document and a full specification will be written
    using such a data modeling language.  Using a formal language can
    help in enforcing consistency and logical compatibility among LFBs.
    In addition, formal definition of the LFB classes has the potential
    to facilitate the eventual automation of some part of the code
    generation process and the functional validation of arbitrary LFB
    topologies.
 
    The modeling language is used for writing the specification but not
    necessarily for encoding the data over-the-wire between FEs and
    CEs. When selecting the specification language, human readability
    is very important, while there are no performance requirements on
    the language for encoding, decoding, and transmission on the
    language.  XML is used as the specification language in this
    document, because XML has the advantage of being human and machine
    readable with widely available tools support.
 
    The encoding method for over the wire transport is an issue
    independent of the specification language chosen here.  It is
    outside the scope of this document and up to the ForCES protocol to
    define.
 
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 2.5. Document Structure
 
    Section 3 provides conceptual overview of the FE model, laying the
    foundation for the more detailed discussion and specifications in
    the sections that follow. Section 4, 5, and 6 together constitute
    the core of the FE model, detailing the three major components in
    the FE model: LFB model, LFB topology, and FE level attributes
    including capability.  Section 7 presents a list of LFB classes in
    the LFB class library that will be further specified according to
    the FE model presented in earlier Sections (4, 5 and 6).  Section 8
    directly addresses the model requirements imposed by the ForCES
    requirement draft [1] while Section 9 explains how the FE model
    should be used in the ForCES protocol.
 
 3. FE Model Concepts
 
    Some of the most important concepts used throughout this document
    are introduced in this section.  Section 3.1 explains the
    difference between a state model and a capability model, and how
    the two can be combined in the FE model.  Section 3.2 introduces
    the concept of LFBs (Logical Functional Blocks) as the basic
    functional building blocks in the FE model.  Section 3.3 discusses
    the logical inter-connection and ordering between LFB instances
    within an FE, that is, the LFB topology.
 
    The FE model proposed in this document is comprised of these three
    components: LFB model, LFB topology and FE attributes including FE
    capabilities.  The LFB model provides the content and data
    structures to define each individual LFB class; LFB topology
    provides a mean to express the logical inter-connection between the
    LFB instances along the datapath(s) within the FE; and FE
    attributes provide information at the FE level and the capabilities
    about what the FE can or cannot do at a coarse level.   Details on
    each of the three components are described in Section 4, 5 and 6,
    respectively.  The intention of this section is to discuss these
    concepts at the high level and lay the foundation for the detailed
    description in the following sections.
 
 3.1. State Model and Capability Model
 
    The FE capability model describes the capabilities and capacities
    of an FE in terms of variations of functions supported or
    limitations contained.  Conceptually, the FE capability model
    presents the many possible states allowed on an FE with capacity
    information indicating certain quantitative limits or constraints.
 
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    For example, an FE capability model may describe the FE at a coarse
    level such as:
       . this FE can handle IPv4 and IPv6 forwarding;
       . this FE can perform classification on the following fields:
         source IP address, destination IP address, source port number,
         destination port number, etc;
       . this FE can perform metering;
       . this FE can handle up to N queues (capacity);
       . this FE can add and remove encapsulating headers of types
         including IPSec, GRE, L2TP.
 
    On the other hand, an FE state model describes the current state of
    the FE, that is, the instantaneous values or operational behavior
    of the FE. The FE state model presents the snapshot view of the FE
    to the CE. For example, using an FE state model, an FE may be
    described to its CE as the following:
       . on a given port the packets are classified using a given
         classification filter;
       . the given classifier results in packets being metered in a
         certain way, and then marked in a certain way;
       . the packets coming from specific markers are delivered into a
         shared queue for handling, while other packets are delivered
         to a different queue;
       . a specific scheduler with specific behavior and parameters
         will service these collected queues.
 
    The information on the capabilities and capacities of the FE helps
    the CE understand the flexibility and limitations of the FE
    functions, so that the CE knows at a coarse level what
    configurations are applicable to the FEs and what are not.  Where
    it gets more complicated is for the capability model to cope with
    the detailed limits, issues such as how many classifiers the FE can
    handle, how many queues, and how many buffer pools the FE can
    support, how many meters the FE can provide.
 
    While one could try to build an object model for representing
    capabilities in full, other efforts have found this to be a
    significant undertaking. A middle of the road approach is to define
    coarse-grained capabilities and simple capacity measures.  Then, if
    the CE attempts to instruct the FE to set up some specific behavior
    it is not capable of, the FE will return an error indicating the
    problem.  Examples of such approach include Framework Policy
    Information Base (PIB) [RFC3318) and Differentiated Services QoS
    Policy Information Base [4]. The capability reporting classes in
    the DiffServ and Framework PIBs are all meant to allow the device
    to indicate some general guidelines about what it can or cannot do,
    but do not necessarily allow it to indicate every possible
 
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    configuration that it can or cannot support.  If a device receives
    a configuration that it cannot implement, it can reject such
    configuration by replying with a failure report.
 
    Figure 1 shows the concepts of FE state, capabilities and
    configuration in the context of CE-FE communication via 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 ForCES FE model must include both a state model and some flavor
    of a capability model.  We believe that a good balance between
    simplicity and flexibility can be achieved for the FE model by
    combining the coarse level capability reporting with the error
    reporting mechanism.  Examples of similar approach include DiffServ
    PIB [4] and Framework PIB [5].
 
    The concepts of LFB and LFB topology will be discussed in the rest
    of this section.  It will become clear that some flavor of
    capability model is needed at both the FE level and LFB level.
 
    Capability information at the LFB level is an integral part of the
    LFB model, and is modeled the same way as the other operational
    parameters inside an LFB.  For example, certain features of an LFB
    class may be optional, in which case it must be possible for the CE
    to determine if an optional feature is supported by a given LFB
    instance or not.  Such capability information can be modeled as a
    read-only attribute in the LFB instance.  See Section 4.4.4 for
    more details on LFB attributes.
 
    Capability information at the FE level may describe what LFB
    classes the FE can instantiate; how many instances of each can be
    created; the topological (i.e., linkage) limitations between these
    LFB instances, etc.  Section 6 defines the FE level attributes
    including capability information.
 
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    Once the FE capability is described to the CE, the FE state
    information can be represented by two levels.  The first level is
    the logically separable and distinctive packet processing
    functions, and we call these individual functions Logical
    Functional Blocks (LFBs).  The second level of information is about
    how these individual LFBs are ordered and placed along the datapath
    to deliver a complete forwarding plane service.  The
    interconnection and ordering of the LFBs is called LFB Topology.
    Section 3.2 discuss high level concepts around LFBs while Section
    3.3 discuss issues around LFB topology.
 
 3.2. LFB Modeling
 
    Each LFB (Logical Functional Block) performs a well-defined action
    or computation on the packets passing through it.  Upon completion
    of such function, either the packets are modified in certain ways
    (like decapsulator, marker), or some results are generated and
    stored, probably in the form of metadata (like classifier). Each
    LFB typically does one thing and one thing only.  Classifiers,
    shapers, meters are all examples of LFB.  Modeling LFB at such fine
    granularity allows us to use a small number of LFBs to create the
    higher-order FE functions (like IPv4 forwarder) precisely, which in
    turn can describe more complex networking functions and vendor
    implementations of software and hardware.
 
    (Editor's note: We need to revisit the granularity issue around LFB
    later and provide a practical design guideline as how to partition
    the FE functions into LFB classes.  We will gain more insight on
    the subject once we debate and settle on the LFB list in the LFB
    class library, described in Section 7.  So the text around
    granularity here might be revised to reflect the lessons we learn.)
 
    An LFB has one or more inputs, each of which takes a packet P, and
    optionally metadata M; and produces one or more outputs, each of
    which carries a packet P', and optionally metadata M'.  Metadata is
    data associated with the packet in the network processing device
    (router, switch, etc.) and passed between one LFB to the next, but
    not sent across the network.  It is most likely that there are
    multiple LFBs within one FE, as shown in Figure 2, and all the LFBs
    share the same ForCES protocol termination point that implements
    the ForCES protocol logic and maintains the communication channel
    to and from the CE.
 
    An LFB, as shown in Figure 2, has inputs, outputs and attributes
    that can be queried and manipulated by the CE indirectly via Fp
    reference point (defined in [2]) and the ForCES protocol
 
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    termination point.  The horizontal axis is in the forwarding plane
    for connecting the inputs and outputs of LFBs within the same FE.
    The vertical axis between the CE and the FE denotes the Fp
    reference point where bidirectional communication between the CE
    and FE happens: the CE to FE communication is for configuration,
    control and packet injection while the FE to CE is for packet re-
    direction to the control plane, monitoring and accounting
    information, errors, etc.  Note that the interaction between the CE
    and the LFB is only abstract and indirect.  The result of such
    interaction is for the CE to indirectly manipulate the attributes
    of the LFB instances.
 
 
                           +-----------+
                           |     CE    |
                           +-----------+
                                 ^
                                 | Fp reference point
                                 |
      +--------------------------|-----------------------------------+
      | FE                       |                                   |
      |                          v                                   |
      | +----------------------------------------------------------+ |
      | |                ForCES protocol                           | |
      | |                   termination point                      | |
      | +----------------------------------------------------------+ |
      |           ^                           ^                      |
      |           :                           : Internal control     |
      |           :                           :                      |
      |       +---:----------+             +---:----------+          |
      |       |   :LFB1      |             |   :     LFB2 |          |
      | =====>|   v          |============>|   v          |======>...|
      | Inputs| +----------+ |Outputs      | +----------+ |          |
      | (P,M) | |Attributes| |(P',M')      | |Attributes| |(P",M")   |
      |       | +----------+ |             | +----------+ |          |
      |       +--------------+             +--------------+          |
      |                                                              |
      +--------------------------------------------------------------+
 
                       Figure 2. Generic LFB Diagram
 
 
    A namespace is used to associate a unique name or ID with each LFB
    class. The namespace must be extensible so that new LFB class can
    also be added later to accommodate future innovation in the
    forwarding plane.
 
 
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    LFB operation must be specified in the model to allow the CE to
    understand the behavior of the forwarding datapath.  For instance,
    the CE must understand at what point in the datapath the IPv4
    header TTL is decremented (i.e., it needs to know if a control
    packet could be delivered to the CE either before or after this
    point in the datapath).  In addition, the CE must understand where
    and what type of header modifications (e.g., tunnel header append
    or strip) are performed by the FEs.  Further, the CE must verify
    that various LFB 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
    a FE design.  Therefore, semi-formal specification is needed; that
    is, a text description of the LFB operation (human readable), but
    sufficiently specific and unambiguous to allow conformance testing
    and efficient design (i.e., eliminate guess-work), so that
    interoperability between different CEs and FEs can be achieved.
 
    The LFB class model specifies information like:
       . number of inputs and outputs (and whether they are
         configurable)
       . metadata read/consumed from inputs;
       . metadata produced at the outputs;
       . packet type(s) accepted at the inputs and emitted at the
         outputs;
       . packet content modifications (including encapsulation or
         decapsulation);
       . packet routing criteria (when multiple outputs on an LFB are
         present);
       . packet timing modifications;
       . packet flow ordering modifications;
       . LFB capability information;
       . LFB operational attributes, etc.
 
    Section 5 of this document provides detailed discussion on the LFB
    model with a formal specification of LFB class schema.  The rest of
    Section 3.2 here only intends to provide conceptual overview of
    some important issues in LFB modeling, without covering all the
    specific details.
 
 3.2.1. LFB Input and Input Group
 
    An LFB input is a conceptual port of the LFB where the LFB can
    receive information from other LFBs. The information is typically a
    packet (or frame in general) and associated metadata, although in
 
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    some cases it might consist of only metadata, i.e., with a Null-
    packet.
 
    It is inevitable that there will be LFB instances that will receive
    packets from more than one other LFB instances (fan-in).  If these
    fan-in links all carry the same type of information (packet type
    and set of metadata) and require the same processing within the
    LFB, then one input should be sufficient.  If, however, the LFB
    class can receive two or more very different types of input, and
    the processing of these inputs are also very distinct, then that
    may justify the definition of multiple inputs.  But in these cases
    splitting the LFB class into two LFB classes should always be
    considered as an alternative.  In intermediate cases, e.g., where
    the inputs are somewhat different but they require very similar
    processing, the shared input solution should be preferred.  For
    example, if an Ethernet framer LFB is capable of receiving IPv4 and
    IPv6 packets, these can be served by the same LFB input.
 
    Note that we assume the model allows for connecting more than one
    LFB output to a single LFB input directly.  There is no restriction
    on the number of up-stream LFBs connecting their outputs to the
    same input of a single LFB instance.  Note that the behavior of the
    system when multiple packets arrive at such an input simultaneously
    is not defined by the model.  If such behavior needs to be
    described, it can be done either by separating the single input to
    become multiple inputs (one per output), or by inserting other
    appropriate LFBs (such as Queues and possibly Schedulers) between
    the multiple outputs and the single input.
 
    If there are multiple inputs with the same input type, we model
    them as an input group, that is, multiple instances of the same
    input type.  In general, an input group is useful to allow an LFB
    to differentiate packet treatment based on where the packet came
    from.
 
      +----+                                +----+
      |LFB1+---+                            |LFB1+---+
      +----+   |    +---------+             +----+   |    +-----------+
               +--->|in  LFB3 |            input   / +--->|in:1  LFB3 |
      +----+   |    +---------+            group   \ +--->|in:2       |
      |LFB2+---+                            +----+   |    +-----------+
      +----+                                |LFB2+---+
                                            +----+
 
         (a) without input group            (b) with input group
 
                   Figure 3. An example of using input group.
 
 
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    Consider the following two cases in Figure 3(a) and (b).  In Figure
    3(a), the output from two LFBs are directly connected into one
    input of LFB3, assuming that it can be guaranteed no two packets
    arrive at the same time instance.  If LFB3 must do something
    different based on the source of the packet (LFB1 or LFB2), the
    only way to model that is to make LFB1 and LFB2 to pass some
    metadata with different values so that LFB3 can make the
    differentiation based on the metadata.  In Figure 3(b), that
    differentiation can be elegantly expressed within LFB3 using the
    input group concept where the instance id can server as the
    differentiating key.  For example, a scheduler LFB can potentially
    use an input group consisting of a variable number of inputs to
    differentiate the queues from which the packets are coming.
 
 3.2.2. LFB Output and Output Group
 
    An LFB output is a conceptual port of the LFB where it can send
    information to some other LFBs.  The information is typically a
    packet (or frame in general) and associated metadata, although in
    some cases it might emit only metadata,, i.e., with a Null-packet.
 
    We assume that a single LFB output can be connected to only one LFB
    input (this is required to make the packet flow through the LFB
    topology unambiguous).  Therefore, to allow any non-trivial
    topology, multiple outputs must be allowed for an LFB class.  If
    there are multiple outputs with the same output type, we model them
    as output group, that is, multiple instances of the same output
    type.  For illustration of output group, consider the hypothetical
    LFB in Figure 4.  The LFB has two types of outputs, one of which
    can be instantiated to form an output group.
 
                             +------------------+
                             |           UNPROC +-->
                             |                  |
                             |         PKTOUT:1 +-->   \
                           --> PKTIN   PKTOUT:2 +-->   |
                             |            .     + .    | Output group
                             |            .     + .    |
                             |         PKTOUT:N +-->   /
                             +------------------+
 
                Figure 4. An example of an LFB with output group.
 
    Multiple outputs should mainly be used for functional separation
    where the outputs are connected to very different types of LFBs.
    For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one
    default output to send those packets for which look-up was
 
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    successful (passing a META_ROUTEID as metadata); and have another
    output for sending packets for which the look-up failed.  The
    former output may be connected to a route handler LFB, while the
    latter can be connected to an ICMP response generator LFB or to a
    packet handler LFB that passes the packet up to the CE.
 
 3.2.3. Packet Type
 
    When LFB classes are defined, the input and output packet formats
    (e.g., IPv4, IPv6, Ethernet, etc.) must be specified: these are the
    types of packets a given LFB input is capable of receiving and
    processing, or a given LFB output is capable of producing.  This
    requires that distinct frame types be uniquely labeled with a
    symbolic name and/or ID.
 
    Note that each LFB has a set of packet types that it operates on,
    but it does not care about whether the underlying implementation is
    passing a greater portion of the packets.  For example, an IPv4 LFB
    might only operate on IPv4 packets, but the underlying
    implementation may or may not be stripping the L2 header before
    handing it over -- whether that is happening or not is opaque to
    the CE.
 
 
 3.2.4. Metadata
 
    Metadata is used to communicate per-packet state from one LFB to
    another.  To ensure inter-operability among 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 that purpose, metadata types must be identified.
    For example, an META_IFID, passed from a port LFB to an IPv4
    processing LFB (with the IP packet) can be one of the defined
    metadata types.
 
    Symbolic names can be assigned for common metadata types.  In
    addition, additional information such as numeric data type, maximum
    and minimum accepted values, and special values should be defined
    for each metadata value.  Some of these constraints will be defined
    in the LFB class model, and some of them may be specific
    capabilities of a particular LFB instance.
 
    While it is important to define the metadata passing between LFB in
    terms of its name, value and interpretation, it is not necessary to
    define the exact encoding mechanism used by LFBs for metadata.
    Different implementations are allowed to use different encoding
    mechanisms for metadata.  For example, one implementation may store
 
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    metadata in registers or shared memory, while another
    implementation may encode metadata in-band as preamble in the
    packets.
 
    A given LFB may require a certain metadata at its inputs for its
    internal processing.  What should happen with the metadata after it
    is read by the LFB?  In particular, should the metadata be
    propagated along with the packet when the packet is forwarded from
    the LFB to the next LFB, or should it be removed (consumed) by the
    LFB?
 
    In certain cases, passing the metadata along is desirable.  For
    example, a META_CLASSID metadata may denote the result of a
    classification LFB and used in more than one downstream LFBs to
    trigger the proper operation on the packet.  In this case the first
    LFB that uses the META_CLASSID should also allow the META_CLASSID
    to be passed with the packet to the next LFB, and so on.  On the
    other hand, it is easy to see that if metadata is never consumed by
    LFBs, then as the packet trickles through the datapath, a large
    number of metadata will potentially be accumulated by the packet.
 
    We believe that one way to accommodate both scenarios is to specify
    the propagation mode for each element of metadata utilized by an
    LFB class.  Metadata elements which are not propagated are
    specified with the CONSUME mode, while elements which are
    propagated are specified with the PROPAGATE mode.
 
    However, whether a metadata is useful beyond an LFB may depend on
    the actual LFB topology, i.e., what other LFBs are placed
    downstream.  So the propagation mode of metadata should be
    configurable.
 
    A packet may arrive to an LFB with metadata that is not meaningful
    to that LFB, but may be important to some other downstream LFBs.
    To cater to such cases it should be the assumed (default) behavior
    of all LFB classes that they transparently propagate any metadata
    elements that they do not utilize internally.
 
    Actual implementations of LFBs in hardware may have limitations on
    how much metadata they can pass through.  The limitation may be
    expressed in terms of total framesize (packet + metadata), metadata
    total size, number of metadata elements, or a combination of these.
    The limitation may be on the FE level or may be specific to LFBs
    within an FE.  The pass-through capabilities of LFB instances and
    FEs can be queried as part of the capability discovery process.
 
 
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    (Editor's note: The definition of metadata here is only preliminary
    and the authors intend to work on the subject in more detail.
    Input is most welcome.)
 
 3.2.5. LFB Versioning
 
    LFB class versioning is a method to enable incremental evolution of
    LFB classes.  Unlike inheritance (discussed next in Section 3.2.6),
    where it assumed that an FE datapath model containing an LFB
    instance of a particular class C could also simultaneously contain
    an LFB instance of a class C' inherited from class C; with
    versioning, an FE would not be allowed to contain an LFB instance
    for more than one version of a particular class.
 
    LFB class versioning is supported by requiring a version string in
    the class definition.  CEs may support backwards compatibility
    between multiple versions of a particular LFB class, but FEs are
    not allowed to support more than one single version of a particular
    class.
 
 3.2.6. LFB Inheritance
 
    LFB class inheritance is supported in the FE model as a means of
    defining new LFB classes.  This also allows FE vendors to add
    vendor-specific extensions to standardized LFBs.  An LFB class
    specification MUST specify the base class (with version number) it
    inherits from (with the default being the base LFB class).
    Multiple-inheritance is not allowed, though, to avoid the
    unnecessary complexity.
 
    Inheritance should be used only when there is significant reuse of
    the base LFB class definition.  A separate LFB class should be
    defined if there is not enough reuse between the derived and the
    base LFB class.
 
    An interesting issue related to class inheritance is backward
    compatibility (between a descendant and an ancestor class).
    Consider the following hypothetical scenario where there exists a
    standardized LFB class "L1".  Vendor A builds an FE that implements
    LFB "L1" and vendors B builds a CE that can recognize and operate
    on LFB "L1".  Suppose that a new LFB class, "L2", is defined based
    on the existing "L1" class (for example, by extending its
    capabilities in some incremental way).  Lets first examine the FE
    backward compatibility issue by considering what would happen if
    vendor B upgrades its FE from "L1" to "L2" while vendor C's CE is
    not changed.  The old L1-based CE can interoperate with the new L2-
 
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    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.
 
    Inheritance can be designed into the model with backward
    compatibility support by constraining the LFB inheritance such that
    the derived class is always a functional superset of the base
    class, i.e., the derived class can only grow on top of the base
    class, but not shrink from it.   Additionally, the following
    mechanisms are required to support FE backward compatibility:
       1)  When detecting an LFB instance of an LFB type that is
           unknown to the CE, the CE MUST be able to query the base
           class of such an LFB from the FE.
       2)  The LFB instance on the FE SHOULD support a backward
           compatibility mode (meaning the LFB instance reverts itself
           back to the base class instance), and the CE SHOULD be able
           to configure the LFB to run in such mode.
 
 3.3. FE Datapath Modeling
 
    Packets coming into the FE from ingress ports generally flow
    through multiple LFBs before leaving out of the egress ports.  How
    an FE treats a packet depends on many factors, such as type of the
    packet (e.g., IPv4, IPv6 or MPLS), actual header values, time of
    arrival, etc.  The result of the operation of an LFB may have an
    impact on how the packet is to be treated in further (downstream)
    LFBs and this differentiation of packet treatment downstream can be
    conceptualized as having alternative datapaths in the FE.  For
    example, the result of a 6-tuple classification (performed by a
    classifier LFB) controls what rate meter is applied to the packet
    (by a rate meter LFB) in a later stage in the datapath.
 
    LFB topology is a directed graph representation of the logical
    datapaths within an FE, with the nodes representing the LFB
    instances and the directed link the packet flow direction from one
    LFB to the next.  Section 3.3.1 discusses how the FE datapaths can
    be modeled as LFB topology; while Section 3.3.2 focuses on issues
    around LFB topology reconfiguration.
 
 3.3.1. Alternative Approaches for Modeling FE Datapaths
 
 
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    There are two basic ways to express the differentiation in packet
    treatment within an FE, one representing the datapath directly and
    graphically (topological approach) and the other utilizing metadata
    (the encoded state approach).
 
       . Topological Approach
 
       Using this approach, differential packet treatment is expressed
       via actually splitting the LFB topology into alternative paths.
       In other words, if the result of an LFB must control how the
       packet is further processed, then such an LFB will have separate
       output ports (one for each alternative treatment) connected to
       separate sub-graphs (each expressing the respective treatment
       downstream).
 
       . Encoded State Approach
 
       An alternative way of expressing differential treatment is using
       metadata.  The result of the operation of an LFB can be encoded
       in a metadata which is passed along with the packet to
       downstream LFBs.  A downstream LFB, in turn, can use the
       metadata (and its value, e.g., as an index into some table) to
       decide how to treat the packet.
 
    Theoretically, the two approaches can substitute for each other, so
    one may consider using purely one (or the other) approach to
    describe all datapaths in an FE.  However, neither model by itself
    is very useful for practically relevant cases.  For a given FE with
    certain logical datapaths, applying the two different modeling
    approaches would result in very different looking LFB topology
    graphs.  A model using purely the topological approach may require
    a very large graph with many links (i.e., paths) and nodes (i.e.,
    LFB instances) to express all alternative datapaths.  On the other
    hand, a model using purely the encoded state model would be
    restricted to a string of LFBs, which would make it very
    unintuitive to describe very different datapaths (such as MPLS and
    IPv4).  Therefore, a mix of these two approaches will likely be
    used for a practical model.  In fact, as we illustrate it below,
    the two approaches can be mixed even within the same LFB.
 
    Using a simple example of a classifier with N classification
    outputs followed by some other LFBs, Figure 5(a) shows what the LFB
    topology looks like by using the purely topological approach.  Each
    output from the classifier goes to one of the N LFBs followed and
    no metadata is needed here.  The topological approach is simple,
    straightforward and graphically intuitive.  However, if N is large
    and the N nodes followed the classifier (LFB#1, LFB#2, ..., LFB#N)
 
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    all belong to the same LFB type (for example, meter) but each with
    its own independent attributes, the encoded state approach gives a
    much simpler topology representation, as shown in Figure 5(b).  The
    encoded state approach requires that a table of N rows of meter
    attributes is provided in the Meter node itself, with each row
    representing the attributes for one meter instance.  A metadata M
    is also needed to pass along with the packet P from the classifier
    to the meter, so that the meter can use M as a look-up key (index)
    to find the corresponding row of the attributes that should be used
    for any particular packet P.
 
    Now what if all the N nodes (LFB#1, LFB#2, ..., LFB#N) are not of
    the same type? For example, if LFB#1 is a queue while the rest are
    all meters, what is the best way to represent such datapaths?
    While it is still possible to use either the pure topological
    approach or the pure encoded state approach, the natural
    combination of the two seems the best by representing the two
    different functional datapaths using topological approach while
    leaving the N-1 meter instances distinguished by metadata only, as
    shown in Figure 5(c).
                                         +----------+
                                  P      |   LFB#1  |
                              +--------->|(Attrib-1)|
         +-------------+      |          +----------+
         |            1|------+   P      +----------+
         |            2|---------------->|   LFB#2  |
         | classifier 3|                 |(Attrib-2)|
         |          ...|...              +----------+
         |            N|------+          ...
         +-------------+      |   P      +----------+
                              +--------->|   LFB#N  |
                                         |(Attrib-N)|
                                         +----------+
 
                 5(a) Using pure topological approach
 
         +-------------+                 +-------------+
         |            1|                 |   Meter     |
         |            2|   (P, M)        | (Attrib-1)  |
         |            3|---------------->| (Attrib-2)  |
         |          ...|                 |   ...       |
         |            N|                 | (Attrib-N)  |
         +-------------+                 +-------------+
 
           5(b) Using pure encoded state approach to represent the LFB
          topology in 5(a), if LFB#1, LFB#2, ..., and LFB#N are of the
                        same type (e.g., meter).
 
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                                      +-------------+
         +-------------+ (P, M)       | queue       |
         |            1|------------->| (Attrib-1)  |
         |            2|              +-------------+
         |            3| (P, M)       +-------------+
         |          ...|------------->|   Meter     |
         |            N|              | (Attrib-2)  |
         +-------------+              |   ...       |
                                      | (Attrib-N)  |
                                      +-------------+
 
         5(c) Using a combination of the two, if LFB#1, LFB#2, ..., and
              LFB#N are of different types (e.g., queue and meter).
 
                Figure 5. An example of how to model FE datapaths
 
 
    From this example, we demonstrate that each approach has distinct
    advantage for different situations.  Using the encoded state
    approach, fewer connections are typically needed between a fan-out
    node and its next LFB instances of the same type, because each
    packet carries metadata with it so that the following nodes can
    interpret and hence invoke a different packet treatment.  For those
    cases, a pure topological approach forces one to build elaborate
    graphs with a lot more connections and often results in an unwieldy
    graph.  On the other hand, a topological approach is intuitive and
    most useful for representing functionally very different datapaths.
 
    For complex topologies, a combination of the two is the most useful
    and flexible.  Here we provide a general design guideline as to
    what approach is best used for what situation.  The topological
    approach should primarily be used when the packet datapath forks
    into areas with distinct LFB classes (not just distinct
    parameterizations of the same LFB classes), and when the fan-outs
    do not require changes (adding/removing LFB outputs) at all or
    require only very infrequent changes.  Configuration information
    that needs to change frequently should preferably be expressed by
    the internal attributes of one or more LFBs (and hence using the
    encoded state approach).
                       +---------------------------------------------+
                       |                                             |
         +----------+  V      +----------+           +------+        |
         |          |  |      |          |if IP-in-IP|      |        |
    ---->| ingress  |->+----->|classifier|---------->|Decap.|---->---+
         | ports    |         |          |----+      |      |
 
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         +----------+         +----------+    |others+------+
                                              |
                                              V
         (a)  The LFB topology with a logical loop
 
 
        +-------+   +-----------+            +------+   +-----------+
        |       |   |           |if IP-in-IP |      |   |           |
    --->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|->
        | ports |   |           |----+       |      |   |           |
        +-------+   +-----------+    |others +------+   +-----------+
                                     |
                                     V
         (b)  The LFB topology without the loop utilizing two
              independent classifier instances.
 
                 Figure 6. An LFB topology example.
 
    It is important to point out that the LFB topology here is the
    logical topology that the packets flow through, not the physical
    topology as determined by how the FE hardware is laid out.
    Nevertheless, the actual implementation may still influence how the
    functionality should be mapped into the LFB topology.  Figure 6
    shows one simple FE example.  In this example, an IP-in-IP packet
    from an IPSec application like VPN may go to the classifier first
    and have the classification done based on the outer IP header; upon
    being classified as an IP-in-IP packet, the packet is then sent to
    a decapsulator to strip off the outer IP header, followed by a
    classifier again to perform classification on the inner IP header.
    If the same classifier hardware or software is used for both outer
    and inner IP header classification with the same set of filtering
    rules, a logical loop is naturally present in the LFB topology, as
    shown in Figure 6(a).  However, if the classification is
    implemented by two different pieces of hardware or software with
    different filters (i.e., one set of filters for outer IP header
    while another set for inner IP header), then it is more natural to
    model them as two different instances of classifier LFB, as shown
    in Figure 6(b).
 
    To distinguish multiple instances of the same LFB class, each LFB
    instance has its own LFB instance ID.  One way to encode the LFB
    instance ID is to encode it as x.y where x is the LFB class ID
    while y is the instance ID within each LFB class.
 
 3.3.2. Configuring the LFB Topology
 
 
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    While there is little doubt that the individual LFB must be
    configurable, the configurability question is more complicated for
    LFB topology.  Since LFB topology is really the graphic
    representation of the datapaths within FE, configuring the LFB
    topology means dynamically changing the datapaths including changes
    to the LFBs along the datapaths on an FE, e.g., creating (i.e.,
    instantiating) or deleting LFBs, setting up or deleting
    interconnections between outputs of upstream LFBs to inputs of
    downstream LFBs.
 
    Why would the datapaths on an FE ever change dynamically?  The
    datapaths on an FE is set up by the CE to provide certain data
    plane services (e.g., DiffServ, VPN, etc.) to the NE's customers.
    The purpose of reconfiguring the datapaths is to enable the CE to
    customize the services the NE is delivering at run time.  The CE
    needs to change the datapaths when the service requirements change,
    e.g., when adding a new customer, or when an existing customer
    changes their service.  However, note that not all datapath changes
    result in changes in the LFB topology graph, and that is determined
    by the approach we use to map the datapaths into LFB topology.  As
    discussed in 3.3.1, the topological approach and encoded state
    approach can result in very different looking LFB topologies for
    the same datapaths.  In general, an LFB topology based on a pure
    topological approach is likely to experience more frequent topology
    reconfiguration than one based on an encoded state approach.
    However, even an LFB topology based entirely on an encoded state
    approach may have to change the topology at times, for example, to
    totally bypass some LFBs or insert new LFBs.  Since a mix of these
    two approaches is used to model the datapaths, LFB topology
    reconfiguration is considered an important aspect of the FE model.
 
    We want to point out that allowing a configurable LFB topology in
    the FE model does not mandate that all FEs must have such
    capability.  Even if an FE supports configurable LFB topology, it
    is expected that there will be FE-specific limitations on what can
    actually be configured.  Performance-optimized hardware
    implementation may have zero or very limited configurability, while
    FE implementations running on network processors may provide more
    flexibility and configurability.  It is entirely up to the FE
    designers to decide whether or not the FE actually implements such
    reconfiguration and how much.  Whether it is a simple runtime
    switch to enable or disable (i.e., bypass) certain LFBs, or more
    flexible software reconfiguration is all implementation detail
    internal to the FE but 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
 
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    describing the LFB topology configuration capabilities of an FE.
    These capabilities may include (see Section 6 for details):
       . What LFB classes can the FE instantiate?
       . How many instances of the same LFB class can be created?
       . What are the topological limitations?  For example:
            o How many instances of the same class or any class can be
              created on any given branch of the graph?
            o Ordering restrictions on LFBs (e.g., any instance of LFB
              class A must be always downstream of any instance of LFB
              class B).
 
    Even if the CE is allowed to configure LFB topology for an FE, how
    can the CE interpret an arbitrary LFB topology (presented to the CE
    by the FE) and know what to do with it?  In another word, how does
    the CE know the mapping between an LFB topology and a particular NE
    service or application (e.g., VPN, DiffServ, etc.)?  We argue that
    first of all, it is unlikely that an FE can support any arbitrary
    LFB topology; secondly, once the CE understands the coarse
    capability of an FE, it is up to the CE to configure the LFB
    topology according to the network service the NE is supposed to
    provide.  So the more important mapping that the CE has to
    understand is from the high level NE service to a specific LFB
    topology, not the other way around.  Do we expect the CE has the
    ultimate intelligence to translate any high level service policy
    into the configuration data for the FEs?  No, but it is conceivable
    that within a given network service domain (like DiffServ), a
    certain amount of intelligence can be programmed into the CE such
    that the CE has a general understanding of the LFBs involved and so
    the translation from a high level service policy to the low level
    FE configuration can be done automatically.  In any event, this is
    considered an implementation issue internal to the control plane
    and outside the scope of the FE model. Therefore, it is not
    discussed any further in this draft.
 
         +----------+     +-----------+
    ---->| Ingress  |---->|classifier |--------------+
         |          |     |chip       |              |
         +----------+     +-----------+              |
                                                     v
                         +-------------------------------------------+
           +--------+    |   Network Processor                       |
      <----| Egress |    |   +------+    +------+   +-------+        |
           +--------+    |   |Meter |    |Marker|   |Dropper|        |
                 ^       |   +------+    +------+   +-------+        |
                 |       |                                           |
      +----------+-------+                                           |
      |          |                                                   |
 
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      |    +---------+       +---------+   +------+    +---------+   |
      |    |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
 
                                                  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          +--+    |
 
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                    +---------------------------->|  |----+
                                                  |  |
                                                  +--+
                 (c)  Another LFB topology as configured by the CE and
                      accepted by the FE
 
         Figure 7. An example of configuring LFB topology.
 
    Figure 7 shows an example where a QoS-enabled router has several
    line cards that have a few ingress ports and egress ports, a
    specialized classification chip, a network processor containing
    codes for FE blocks like meter, marker, dropper, counter, queue,
    scheduler and Ipv4 forwarder.  Some of the LFB topology is already
    fixed and has to remain static due to the physical layout of the
    line cards.  For example, all the ingress ports might be already
    hard wired into the classification chip and so all packets must
    follow from the ingress port into the classification engine.  On
    the other hand, the LFBs on the network processor and their
    execution order are programmable, even though there might exist
    certain capacity limits and linkage constraints between these LFBs.
    Examples of the capacity limits might be: there can be no more than
    8 meters; there can be no more than 16 queues in one FE; the
    scheduler can handle at most up to 16 queues; etc.  The linkage
    constraints might dictate that classification engine may be
    followed by a meter, marker, dropper, counter, queue or IPv4
    forwarder, but not scheduler; queues can only be followed by a
    scheduler; a scheduler must be followed by the IPv4 forwarder; the
    last LFB in the datapath before going into the egress ports must be
    the IPv4 forwarder, etc.
 
    Once the FE reports such capability and capacity to the CE, it is
    now up to the CE to translate the QoS policy into the desirable
    configuration for the FE.  Figure 7(a) depicts the FE capability
    while 7(b) and 7(c) depict two different topologies that the FE
    might be asked to configure to.  Note that both the ingress and
    egress are omitted in (b) and (c) for simple representation.  The
    topology in 7(c) is considerably more complex than 7(b) but both
    are feasible within the FE capabilities, and so the FE should
    accept either configuration request from the CE.
 
 
 4. LFB Model -- LFB and Associated Data Definitions
 
    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. The LFB model is defined in this section to
 
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    describe the content and structures in LFB and associated data type
    definition.
 
    The core part of the model is the definition of LFB classes.
    Section 4.4 provides more discussion on what will be part of an LFB
    class definition.
 
    Operational parameters of the LFBs that must be visible to the CEs
    are conceptualized in the model as the LFB attributes. These
    include, for example, flags, single parameter arguments, complex
    arguments, and tables.  The definition of the attributes of an LFB
    MUST be part of the LFB class definition. To promote consistent and
    terse definitions of the attributes of LFB classes, commonly used
    attribute types SHOULD be defined in the model outside of the LFB
    class definitions, so that LFB class definitions can "share" these
    type definitions by simply referring to the types. What will
    comprise a data type definition is further discussed in Section
    4.1.
 
    LFBs form a directed graph with each other by sending and receiving
    packets and associated metadata. To provide consistency and logical
    inter-operability among LFB classes, packet types (generic frame
    types) and metadata types MUST BE specified outside of the LFB
    class definitions (but part of the LFB model), so that the LFB
    class definitions can simply refer to these types. These blocks are
    further discussed in Section 4.3 and Section 4.2, respectively.
    In summary, the LFB model will consist of the following four
    categories of definitions:
       1)  Common data type definitions (Section 4.1)
       2)  Metadata definitions (Section 4.2);
       3)  Frame format definitions (Section 4.3);
       4)  LFB class definitions (Section 4.4).
 
    It is not expected that the above information is 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).
 
 4.1. General Data Type Definitions
 
    Data types will be used to describe the LFB attributes (see Section
    4.4.4).  This is similar to the concept of having a common header
    file for shared data types. Data types will include atomic data
    types (e.g. integer, ASCII string), as well as compound or derived
    data types (such as arrays and structures).  Given that the FORCES
    protocol will be getting and setting attribute values, all atomic
    data types used here must be able to be conveyed in the FORCES
 
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    protocol.  Further, the FORCES protocol will need a mechanism to
    convey compound data types.  Details of such representation are for
    the protocol document, not the model documents.
 
    Compound data types can build on atomic data types and other
    compound data types. There are three ways that compound data types
    can be defined.  They may be defined as an array of elements of
    some compound or atomic data type.  They may be a structure of
    named elements of compound or atomic data types (ala C structures).
    They may also be defined as augmentations (explained below in
    4.1.3) of existing compound data types.
 
    In addition, any data type may be used to define a new type by
    restricting the range of values that an instance of the data type
    can take on, and specifying specific semantics that go with that.
    This is similar to the SNMP notion of a textual convention.
 
    For each data type the following information MUST be provided:
    . Symbolic name of data type. Example: "T_IPV4ADDRESS".
    . Actual type declaration.
 
    In addition, a data type definition MAY include the following:
       . Range restrictions.
       . A set of symbolic names for special values. Example:
         "IPV4ADDR_LOOPBACK".
 
    Note that not all attributes will exist at all times in all
    implementations.  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.
 
 4.1.1. Arrays
 
    Compound data types can be defined as arrays of compound or atomic
    data types.  Arrays can only be subscripted by integers, and will
    be presumed to start with subscript 0.  The mechanism defined above
    for non-supported attributes can also apply to attempts to
    reference non-existent array elements or to set non-permitted
    elements.  The valid range of the subscripts of the array must be
    defined either in the definition of the array or in the LFB class
    which uses the compound type definition.
 
 4.1.2. Structures
 
    A structure is comprised of a collection of data elements.  Each
    data element has a data type (either an atomic type or an existing
 
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    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.
 
 4.1.3. Augmentations
 
    Compound types can also be defined as augmentations of existing
    compound types.  If the existing compound type is a structure,
    augmentation may add new elements to the type.  They may replace
    the type of an existing element with an augmentation derived from
    the current type.  They may not delete an existing element, nor may
    they replace the type of an existing element with one that is not
    an augmentation of the type that the element has in the basis for
    the augmentation.  If the existing compound type is an array,
    augmentation means augmentation of the array element type.
 
    One consequence of this is that augmentations are compatible with
    the compound type from which they are derived.  As such,
    augmentations are useful in defining attributes for LFB subclasses
    with backward compatibility.  In addition to adding new attributes
    to a class, the data type of an existing attribute may be replaced
    by an augmentation of that attribute, and still meet the
    compatibility rules for subclasses.
 
    For example, consider a simple base LFB class A that has only one
    attribute (attr1) of type X.  One way to derive class A1 from A can
    be simply adding a second attribute (of any type).  Another way to
    derive a class A2 from A can be replacing the original attribute
    (attr1) in A of type X with one of type Y, where Y is an
    augmentation of X.  Both classes A1 and A2 are backward compatible
    with class A.
 
 4.2. Metadata Definitions
 
    For each metadata type, the following MUST be specified:
       . Metadata symbolic name. Used to refer to the metadata type in
         LFB type specifications.  Example: META_CLASSID.
       . Brief synopsis of the metadata. Example: "Result of
         classification (0 means no match)".
       . Data type and valid range.
 
    In addition, the following information MAY BE part of the metadata
    definition:
       . Symbolic definitions for frequently used or special values of
         the metadata.
 
 4.3. Frame Format Definitions
 
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    This part of the LFB model will list packet types (frame types in
    general) that LFB classes can receive at their inputs and/or emit
    at their outputs.
 
    For each distinct frame type, the following MUST be provided:
       . Symbolic name of frame type. Example: FRAME_IPV4.
       . Brief synopsis of the frame type. Example: "IPv4 packet".
 
 
 4.4. LFB Class Definitions
 
    Each LFB Class definition must provide the following information:
    .    Symbolic name of LFB class. Example: "LFB_IPV4_LPM"
    .    Short synopsis of LFB class. Example: "IPv4 LPM Lookup LFB"
    .    Version indicator
    .    Inheritance indicator (see discussion in Section 4.4.1)
    .    Inputs (see discussion in Section 4.4.2)
    .    Outputs (see discussion in Section 4.4.3)
    .    Attributes (see discussion in Section 4.4.4)
    .    Operational specification (see discussion in Section 4.4.5)
 
 4.4.1. LFB Inheritance
 
    To support LFB class inheritance, the LFB specification must have a
    place holder for indicating the base class and its version.  It is
    assumed that the derived class is backward compatible with the base
    class.
 
 4.4.2. LFB Inputs
 
    An LFB class may have zero, one, or more inputs. We assume that
    most LFBs will have exactly one input.  Multiple inputs with the
    same input type are modeled as one input group.  The input group
    should count as one entry in the input specification.  The number
    of inputs (including input groups) is fixed.
 
    Multiple inputs with different input type should be avoided if
    possible (see discussion in Section 3.2.1).  Some special LFBs will
    have no inputs at all.  For example, a packet generator LFB does
    not need an input.
 
    The LFB class definition MUST specify whether or not the number of
    inputs of the LFB is fixed, and the exact number if fixed. For each
    LFB input (group), the following MUST be specified:
 
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       . Symbolic name of input.  Example: "PKT_IN".  Note that this
         symbolic name must be unique only within the scope of the LFB
         class.
       . Brief synopsis of the input.  Example: "Normal packet input".
       . Indication of whether this input is an input group (i.e., if
         it is allowed to be instantiated).
       . List of allowed frame formats. Example: "{FRAME_IPV4,
         FRAME_IPV6}".  Note that this list should refer to symbols
         specified in the frame definition of the LFB model (see
         Section 4.3).
       . List of required metadata. Example: {META_CLASSID, META_IFID}.
         This list should refer to symbols specified in the metadata
         definition of the LFB model (see Section 4.2). For each
         metadata it should be specified whether the metadata is
         required or optional. For each optional metadata a default
         value MAY BE specified, which is used by the LFB if the
         metadata is not provided at the input.
 
 4.4.3. LFB Outputs
 
    An LFB class may have zero, one, or more outputs.  If there are
    multiple outputs with the same output type, we model them as output
    group.  Some special LFBs may have no outputs at all (e.g.,
    Dropper).
 
    The number of outputs may be fixed for some LFB types and may be
    configurable for others.  The LFB Class definition MUST specify the
    number of outputs (or output types) of the LFB. The output group
    should count as one entry in the output specification, but the
    entry should indicate that instantiation of the output is
    allowed.
 
    For each LFB output (group) the following MUST be specified:
       . Symbolic name of the output. Example: "UNPROC".  In case of an
         output group, the symbolic name is the prefix used to
         construct unique symbols for each output instance. Example:
         "PKTOUT".  Note that the symbolic name must be unique only
         within the scope of the LFB class.
       . Brief synopsis of the output. Example: "Normal packet output".
       . Indication of whether this output is an output group (i.e., if
         it is allowed to be instantiated).
       . List of allowed frame formats. Example: "{FRAME_IPV4,
         FRAME_IPV6}". Note that this list should refer to symbols
         specified in the frame definition of the LFB model (see
         Section 4.3).
       . List of emitted (generated) metadata. Example: {META_CLASSID,
         META_IFID}. This list should refer to symbols specified in the
 
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         metadata definition of the LFB model (see Section 4.2). 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.
 
 4.4.4. LFB Attributes
 
    The operational state of the LFB is modeled by the variables of the
    LFB, collectively called attributes.  Note that the attributes here
    refer to the operational parameters of the LFBs that must be
    visible to the CEs.  The other variables that are internal to LFB
    implementation are not included here in the LFB attributes and are
    not modeled here.
 
    Attribute types will include the following three categories:
       . Capability attributes (see Section 9.4 for more on LFB
         capabilities).  Examples:
        *  Supported optional features of the LFB class;
        *  Maximum number of configurable outputs for an output group;
        *  Metadata pass-through limitations of the LFB;
        *  Maximum size of configurable attribute tables;
        *  Supported access modes of certain attributes (see below).
       . Operational attributes, some of them are configurable by the
         CE, while others might be internally maintained state which
         are read-only for the CE and necessary for the CE to operate
         properly.  Examples:
        *  Configurable flags and switches selecting between
    operational modes of the LFB;
        *  ARP tables;
        *  Number of outputs in an output group;
        *  Metadata CONSUME vs. PROPAGATE mode selector.
       . Statistical attributes (collected by the FE and provided for
         reading to the CE).  Examples:
        *  Packet and byte counters;
        *  Other event counters.
 
    Some of the attributes will be generically available in all LFBs
    while others will be specific to the LFB class.  Examples of
    generic LFB attributes are:
       . LFB class inheritance information (see Section 4.4.1)
       . Number and type of inputs (in case the LFB is self-
         descriptive)
       . Number and type of outputs  (in case the LFB is self-
         descriptive)
       . Number of current outputs for each output group
       . Metadata CONSUME/PROPAGATE mode selector
 
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    There may be various access permission restrictions on what the CE
    can do with an LFB attribute.  The following categories may be
    supported:
       . No-access attributes.  This is useful when multiple access
         modes maybe defined for a given attribute to allow some
         flexibility for different implementations.
       . Read-only attributes.
       . Read-write attributes.
       . Write-only attributes.  This could be any configurable data
         for which read capability is not provided to the CEs.  (??? Do
         we have good example???)
       . Read-reset attributes.  The CE can read and reset this
         resource, but cannot set it to an arbitrary value. Example:
         Counters.
       . Firing-only attributes.  A write attempt to this resource will
         trigger some specific actions in the LFB, but the actual value
         written is ignored.  (??? Example???)
 
    The LFB class may define more than one possible access mode for a
    given attribute (for example, write-only and read-write), in which
    case it is left to the actual implementation to pick one of the
    modes.  In such cases a corresponding capability parameter must
    inform the CE of which mode the actual LFB instance supports.
    The attributes of the LFB class must be defined as a list. For each
    attribute the following information MUST be provided:
       . Reference to the data type (e.g., specified in the generic
         data type block of the LFB model or in an LFB specific data
         type block).
       . Access permission(s).
       . Additional range restrictions (i.e., beyond what is specified
         by the data type definition).
       . Default value. Applied when the LFB is initialized or reset.
 
    The actual structuring of LFB attributes requires further study.
 
 4.4.5. LFB Operational Specification
 
    This section of the model should verbally describe what the LFB
    does.  This will most likely be embedded in an unstructured text
    field in the model.
 
 5. LFB Topology Model (To be written)
 
    (Editor's note: This is a place holder to describe the details on
    how to model LFB topology.)
 
 
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 6. FE Level Attributes (To be written)
 
    (Editor's note: This is a place holder to describe the FE level
    attributes including FE capabilities, for examples:
       . How this FE is connected with other FEs (if known by the FE)?
       . What LFB classes can the FE instantiate?
       . How many instances of the same LFB class can be created?
       . What are the topological limitations?  For example:
            o How many instances of the same class or any class can be
              created on any given branch of the graph?
            o Ordering restrictions on LFBs (e.g., any instance of LFB
              class A must be always downstream of any instance of LFB
              class B).
    )
 
 
 7. LFB Class Library
 
    A set of LFB classes are identified here in the LFB class library
    as necessary to build common FE functions.
 
    Several working groups in the IETF have already done some relevant
    work in modeling the provisioning policy data for some of the
    functions we are interested in, for example, DiffServ
    (Differentiated Services) PIB [4], IPSec PIB [8]. Whenever
    possible, we should try to reuse the work done elsewhere instead of
    reinventing the wheel.
 
 7.1. Port LFB
 
    A Port LFB is used to map a physical port into the LFB model.
 
    The Port LFB maps sources and sinks of packets from outside the LFB
    model onto one logical block which defines and models a physical
    port implementing those functions.
 
    The Port LFB contains a number of configurable parameters, which
    may include, but are not limited to, the following items:
       . the number of ports on this LFB;
       . the sub-interfaces if any;
       . the static attributes of each port (e.g., port type,
         direction, link speed);
       . the configurable attributes of each port (e.g., IP address,
         administrative status);
       . the statistics collected on each port (e.g., number of packets
         received);
 
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       . the current status (up or down).
 
    The Port LFB can have three modes of operation:
       . ingress only
       . egress only
       . hybrid (contains ingress and egress functions)
 
 7.2. Dropper LFB
 
    A dropper LFB has one input, and no outputs.  It discards all
    packets that it receives without any modification or examination of
    those packets.
 
    The purpose of a dropper LFB is to allow the description of "sinks"
    within the model, where those sinks do not result in the packet
    being sent into any object external to the model.
 
 7.3. Redirector (de-MUX) LFB
 
    A redirector LFB has one input, and N outputs.
 
    The purpose of the redirector LFB is to explicitly represent a
    place in the LFB Topology where the redirection process occurs, and
    where it may be configured.
 
    The redirector LFB takes an input packet P, and uses the metadata M
    to redirect that packet to one or more of N outputs, e.g. unicast
    forwarding, multicast, or broadcast.
 
    Note that other LFBs may also have redirecting functionality, if
    they have multiple outputs.
 
 7.4. Scheduler LFB
 
    A Scheduler LFB has multiple inputs and one output.  The purpose of
    the Scheduler LFB is to perform time-dependent packet forwarding.
    The Scheduler LFB multiplexes from its inputs onto its output(s),
    based on internal configuration such as packet priority, etc.  The
    packet is not modified during this process.
 
 7.5. Queue LFB
 
    The Queue LFB has one input and one output.  It takes input packets
    and places them onto queues.  These packets are later forwarded to
    the output(s) of the LFB, based on back-pressure from the next LFB
    which typically is a scheduler LFB.
 
 
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 7.6. Counter LFB
 
    A counter LFB updates its statistical attributes, by counting
    packets, or metadata.  The packet is not modified, and the metadata
    may, or may not, be modified.
 
    The purpose of a Counter LFB is to record simple accounting of
    events on the FE.
 
    A counter LFB is independent of time 't', in that it does not
    perform any time-dependent counting.  The time at which a count is
    made may, however, be associated with that count.
 
 7.7. Meter LFB and Policer LFB
 
    A Meter LFB is a counter LFB that is time dependent.  That is, it
    meters the rate over time at which packets or metadata flow through
    the LFB.  The purpose of the Meter LFB is to record time-dependent
    accounting of events on the FE.
 
    When a Meter LFB has multiple outputs, with one output being a
    marker, or dropping the packet, then the Meter LFB becomes a
    Policer LFB, performing a policing function.
 
 7.8. Classifier LFB
 
    A Classifier LFB uses its attributes to classify the packet into
    one of N different logical classes.
 
    The purpose of a Classifier LFB is to logically partition packets
    into one or more classes.  The result of this partitioning is that
    the Classifier LFB produces metadata that describes the classes
    into which the packet has been partitioned. The packet is not
    modified during this process.
 
    A Classifier LFB takes an input packet and metadata, and produces
    the same packet with new or more metadata.  A classifier is
    parameterized by filters.  Classification is done by matching the
    contents of the incoming packets according to the filters, and the
    result of classification is produced in the form of metadata.  Note
    that this classifier is modeled solely based on its internal
    processing, and not on its inputs and outputs.  The block is a
    single-exit classifier that does NOT physically redirect the
    packet.  In contrast, a DiffServ-like classifier is a 1:N (fan-out)
    device: It takes a single traffic stream as input and generate N
    logically separate traffic streams as output.  That kind of multi-
 
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    exit classifier can be modeled by combining this classifier with a
    redirector (see Section 6.1.6).
 
    A filter decides if input packets match particular criteria. That
    is, it "marks" a packet as either matching, or non-matching to the
    filter criteria. According to [DiffServ], "a filter consists of a
    set of conditions on the component values of a packet's
    classification key (the header values, contents, and attributes
    relevant for classification)".
 
    Note that other FE LFBs MAY perform simple classification on the
    packet or metadata.  The purpose of the FE Classifier LFB is to
    model an LFB that "digests" large amounts of input data (packet,
    metadata), to produce a "summary" of the classification results, in
    the form of additional metadata. Other FE LFBs can then use this
    summary information to quickly and simply perform trivial
    "classifications".
 
    The requirement for a unique and separate FE Classifier LFB comes
    about because it would not make sense to model a classifier LFB
    inside each of every other LFB.  Such a model would be highly
    redundant.  We therefore specifically model a complex
    classification LFB, and explicitly state that other blocks may make
    decisions based on the parameters S, t, and M, but not on P.
 
    Note that a classifier LFB may have multiple outputs.  In that
    case, it may redirect input packets to one (or more) of the
    outputs, and may not associate any metadata with those output
    packets.
 
 7.9. Modifier LFB
 
    A modifier LFB modifies incoming packets and sends them out.
    Usually the metadata is used to determine how to modify the packet.
 
    This LFB is defined in a generic manner, and we expect that
    specific examples of packet and/or metadata modification will be
    described as a subclass of the modifier LFB.
 
    For example, we may have an explicit LFB for packet compression and
    decompression, or for encryption and decryption, or for packet
    encapsulation.  The decision as to how best to model these
    functions will be made based on further investigation of the LFB
    model, and with practical experience using it.
 
 7.10. Packet Header Rewriter LFB
 
 
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    This LFB is used to re-write fields on the packet header, such as
    IPv4 TTL decrementing, checksum calculation, or TCP/IP NAT.
 
    We may want to have multiple LFBs for different kinds of header re-
    writing.
 
 
 8. Satisfying the Requirements on FE Model
 
     (Editor's Note: The text in this section is very preliminary but
     we decide to leave it as is because it is too early to understand
     how to model all the functions as dictated in [1] when Section 7
     is still very much work in progress. This section should be
     revised once Section 7 is more settled.)
 
    A minimum set of FE functions is defined in [1] that must be
    supported by any proposed FE model.  In this section, we
    demonstrate how the three components in FE model as described in
    Section 4, 5, 6 along with the LFB class library defined in Section
    7 can be used to express all the logical functions required in [1].
 
 8.1. Port Functions
 
    Every FE contains a certain number of interfaces (ports), including
    both the inter-NE interfaces and intra-NE interfaces.  The inter-NE
    interfaces are the external interfaces for the NE to
    receive/forward packets from/to the external world.  The intra-NE
    interfaces are used for FE-FE or FE-CE communications.  Same model
    should be used for both the inter-FE and intra-FE interfaces, but
    it is necessary to make the distinction between the two known to
    the CE so that the CE can do different configuration.
 
    The port LFB class is designed to model the specific physical ports
    while the source/sink LFB can be used to model the logical
    interface.
 
    The intra-NE interfaces that are used for FE-FE communications
    should be modeled just like the inter-NE interfaces. The ForCES
    base protocol will include FE topology query so that the CE can
    learn of how the multiple FEs are interconnected via such
    interfaces.  But the intra-NE interfaces that are used for FE-CE
    communications are part of the ForCES protocol entity on the FE and
    so it is not necessary to model them explicitly.  It is assumed
    that every FE will have at least one internal interface to
    communicate to the CE and such interface do not have to be visible
    in the FE model.
 
 
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 8.2. Forwarding Functions
 
    Support for IPv4 and IPv6 unicast and multicast forwarding
    functions must be provided by the model.
 
    Typically, the control plane maintains the Routing Information Base
    (RIB), which contains all the routes discovered by all the routing
    protocols with all kinds of attributes relevant to the routes. The
    forwarding plane uses a different database, the Forwarding
    Information Base (FIB), which contains only the active subset of
    those routes (only the best routes chosen for forwarding) with
    attributes that are only relevant for forwarding. A component in
    the control plane, termed Route Table Manager (RTM), is responsible
    to manage the RIB in the CE and maintain the FIB used by the FEs.
    Therefore, the most important aspect in modeling the forwarding
    functions is the data model for the FIB. The model also needs to
    support the possibility of multiple paths.
 
    At the very minimum, each route in the FIB needs to contain the
    following layer-3 information:
       . the prefix of the destination IP address;
       . the length of the prefix;
       . the number of equal-cost multi-path;
       . the next hop IP address and the egress interface for each
         path.
 
    Another aspect of the forwarding functions is the method to resolve
    a next hop destination IP address into the associated media
    address. There are many ways to resolve Layer 3 to Layer 2 address
    mapping depending upon link layer. For example, in case of Ethernet
    links, the Address Resolution Protocol (ARP, defined in RFC 826) is
    used for IPv4 address resolution.
 
    Assuming a separate table is maintained in the FEs for address
    resolution, the following information is necessary for each address
    resolution entry:
       . the next hop IP address;
       . the media address.
 
    Different implementation may have different ways to maintain the
    FIB and the resolution table. For example, a FIB may consist of two
    separate tables, one to match the prefix to the next hop and the
    other to match the next hop to the egress interface. Another
    implementation may use one table instead.  Our approach of using
    the fine-grained FE blocks to model the forwarding functions allow
    such flexibility.
 
 
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    For example, a combination of a classifier, followed by a modifier
    and a redirector can model the forwarding function.
 
 8.3. QoS Functions
 
    The IETF community has already done lots work in modeling the QoS
    functions in the datapath. The IETF DiffServ working group has
    defined an informal data model [3]for QoS-related functions like
    classification, metering, marking, actions of marking, dropping,
    counting and multiplexing, queueing, etc. The latest work on
    DiffServ PIB (Policy Information Base) [4] defines a set of
    provisioning classes to provide policy control of resources
    implementing the Diferentiated Services Architecture. DiffServ PIB
    also has an element of capability flavor to it. The IETF Policy
    Framework working group is also defining an informational model [6]
    to describe the QoS mechanisms inherent in different network
    devices, including hosts.  This model is intended to be used with
    the QoS Policy Information Model [7] to model how policies can be
    defined to manage and configure the QoS mechanisms present in the
    datapath of devices.
 
    Here is a list of QoS functions that should be supported by the FE
    model:
       . Classifier
       . Meter
       . Marker
       . Dropper
       . Counter
       . Queue and Scheduler
       . Shaper
 
    LFB class library as described in Section 7 already supports most
    of these functions directly.
 
    Note that A shaper should be modeled as a queue feeding a scheduler
    input that is serviced using a non-work-conserving policy.  The
    queue LFB would include multiple FIFO queue resources (selected by
    META_QUEUE_ID) and AQManagers assigned to queues.  The scheduler
    LFB would include multiple input resources with associated service
    policies.  Queue outputs would be bound to scheduler inputs via
    passing META_SCHED_ID with the packet at the output of the queue.
    The metadata is only there to allow correlation in configuration
    parameters between the queueing LFB and the scheduler LFB (assign
    queue X to scheduler input Y by configuring queue X to emit
    META_SCHED_ID Y).
 
 8.4. Generic Filtering Functions
 
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    A combination of classifier, redirector, modifier etc. can model
    complex set of filtering functions. For example, Figure 8
    represents a filtering function that classifies packets into one of
    two logical classes: forward, and drop.  These logical classes are
    represented as meta data M1, and M2.  The re-director uses this
    meta data to re-direct the packet to one of two outputs.  The first
    sinks the packet back into the network.  The second silently drops
    the packets.
 
                 classifier -> redirector  ---M1---  sink
                                           \
                                            \-M2---  dropper
 
                  Figure 8. A filtering function example.
 
 8.5. Vendor Specific Functions
 
    New LFB class can always be defined according to the LFB model as
    described in Section 7 to support vendor specific functions.  New
    LFB class can also be derived from an existing LFB class by
    inheritance.
 
 8.6.High-Touch Functions
 
    High-touch functions are those that take action on the contents or
    headers of a packet based on content other than what is found in
    the IP header.  Examples of such functions include NAT, ALG,
    firewall, tunneling and L7 content recognition.
 
    The ForCES working group first needs to agree upon a small set of
    common high-touch functions with well-defined behavior to be
    included in the LFB class library. Here is a list of candidate
    blocks:
       . NAT
       . Firewall
       . Encapsulator
       . Decapsulator
 
 8.7. Security Functions
 
    The FE model must be able to describe the types of encryption
    and/or decryption functions that an FE supports and the associated
    attributes for such functions.
 
    The IP Security Policy (IPSP) Working Group in the IETF has started
    work in defining the IPSec Policy Information Base [8]. Further
 
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    study on this is needed to determine whether it can be reused here
    and any other additional work is needed.
 
 8.8. Off-loaded Functions
 
    In addition to the packet processing functions that are typical to
    find on the FEs, some logical functions may also be executed
    asynchronously by some FEs, according to a certain finite-state
    machine, triggered not only by packet events, but by timer events
    as well. Examples of such functions include finite-state machine
    execution required by TCP termination or OSPF Hello processing off-
    loaded from the CE. The FE model must be capable of expressing
    these asynchronous functions, so that the CE may take advantage of
    such off-loaded functions on the FEs.
 
    The ForCES working group first needs to agree upon a small set of
    such off-loaded functions with well-understood behavior and
    interactions with the control plane.
 
 8.9. IPFLOW/PSAMP Functions
 
    [9] defines architecture for IP traffic flow monitoring, measuring
    and exporting. The LFB model supports statistics collection on the
    LFB by including statistical attributes (Section 4.4.4) for all the
    LFB class definitions, and meter LFB (Section 7.2.2) and counter
    LFB (Section 7.2.1) can also be used to support accounting
    functions in the FE.
 
    [10] describes a framework to define a standard set of capabilities
    for network elements to sample subsets of packets by statistical
    and other methods.  Time event generation, filter LFB, and
    counter/meter LFB are the elements needed to support packet
    filtering and sampling functions -- these elements are all included
    in the FE model.
 
 9. 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 via 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;
 
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        5)  State query of LFB attributes;
        6)  Manipulation of LFB attributes;
        7)  LFB topology reconfiguration.
 
     Items 1) through 5) are query exchanges, the main flow of
     information being 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" type of exchanges, the main flow of
     information being from the CEs to the FEs. Messages in Item 6)
     (the LFB re-configuration commands) are expected to be used
     frequently.  Item 7) (LFB topology re-configuration) is needed
     only if dynamic LFB topologies are supported by the FEs and it is
     expected to be used infrequently.
 
     Among the seven types of payload information the ForCES protocol
     carries between CEs and FEs, the FE model covers all of them
     except item 1), which concerns the inter-FE topology.  The FE
     model focuses on the LFB and LFB topology within a single FE.
     Since the information of item 1) requires global knowledge about
     all the FEs and their inter-connection with each other, this
     exchange is made part of the ForCES base protocol instead of the
     FE model.
 
     The relationship between the FE model and the seven post-
     association messages are visualized in Figure 9:
 
                                                      +--------+
                                         ..........-->|   CE   |
                    /----\               .            +--------+
                    \____/ FE Model      .              ^    |
                    |    |................        (1),2 |    | 6, 7
                    |    |  (off-line)   .      3, 4, 5 |    |
                    \____/               .              |    v
                                         .            +--------+
                  e.g. RFCs              ..........-->|   FE   |
                                                      +--------+
 
 
      Figure 9. Relationship between 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.
 
 
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     The actual encoding of these messages is defined by the ForCES
     protocol and beyond the scope of the FE model. Their discussion is
     nevertheless important here for the following reasons:
       . These PA model components have considerable impact on the FE
          model. For example, some of the above information can be
          represented as attributes of the LFBs, in which case such
          attributes must be defined in the LFB classes.
       . The understanding of the type of information that must be
          exchanged between the FEs and CEs can help to select the
          appropriate protocol format and the actual encoding method
          (such as XML, TLVs).
       . Understanding the frequency of these types of messages should
          influence the selection of the protocol format (efficiency
          considerations).
 
     The remaining sub-sections of this section address each of the
     seven message types.
 
 9.1. FE Topology Query
 
    (Editor's Note: It is still an open issue where the FE topology
    information query belongs -- it can be either supported as part of
    FE attributes in the FE model, or it can be supported by the ForCES
    protocol explicitly.  Hence the text here is tentative and subject
    to change per WG discussion.)
 
    An FE may contain zero, one or more external ingress ports.
    Similarly, an FE may contain zero, one or more external egress
    ports.  In another word, not every FE has to contain any external
    ingress or egress interfaces.  For example, Figure 10 shows two
    cascading FEs.  FE #1 contains one external ingress interface but
    no external egress interface, while FE #2 contains one external
    egress interface but no ingress interfce.  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, like that described in Figure 10.  The inter-
    FE topology information may be provided by FEs, may be hard-coded
    into CE, or may be provided by some other entity (e.g., a bus
    manager) independent of the FEs.  So while the ForCES protocol
    supports FE topology query from FEs, it is optional for the CE to
 
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    use it, assuming the CE has other means to gather such topology
    information.
 
       +-----------------------------------------------------+
       |  +---------+   +------------+   +---------+         |
     input|         |   |            |   |         | output  |
    ---+->| Ingress |-->|Header      |-->|IPv4     |---------+--->+
       |  | port    |   |Decompressor|   |Forwarder| FE      |    |
       |  +---------+   +------------+   +---------+ #1      |    |
       +-----------------------------------------------------+    V
                                                                  |
            +-----------------------<-----------------------------+
            |
            |    +----------------------------------------+
            V    |  +------------+   +----------+         |
            | input |            |   |          |  output |
            +->--+->|Header      |-->| Egress   |---------+-->
                 |  |Compressor  |   | port     | FE      |
                 |  +------------+   +----------+ #2      |
                 +----------------------------------------+
 
 
            Figure 10. An example of two FEs connected together.
 
    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 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.
 
 9.2. FE Capability Declarations
 
    FEs will have many types of limitations. Some of the limitations
    must be expressed to the CEs as part of the capability model. The
    CEs must be able to query these capabilities on a per-FE basis.
    Examples:
       . Metadata passing capabilities of the FE. Understanding these
         capabilities will help the CE to evaluate the feasibility of
         LFB topologies, and hence to determine the availability of
         certain services.
       . Global resource query limitations (applicable to all LFBs of
         the FE).
       . LFB supported by the FE.
       . LFB class instantiation limit.
 
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       . LFB topological limitations (linkage constraint, ordering
         etc.)
 
 9.3. 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, there should be
    sufficient information provided on whether the FE supports any CE-
    initiated (dynamic) changes to the LFB topology, and if so, what
    are the allowed topologies. Topology configurability can also be
    considered as part of the FE capability query as described in
    Section 9.3.
 
 9.4. LFB Capability Declarations
 
    LFB class specifications will 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 here only limitations that are within the flexibility of
    the LFB class specification, that is, the LFB instance will remain
    compliant with the LFB class specification despite these
    limitations.  For example, certain features of an LFB class may be
    optional, in which case it must be possible for the CE to determine
    if an optional feature is supported by a given LFB instance or not.
    Also, the LFB class definitions will probably contain very few
    quantitative limits (e.g., size of tables), since these limits are
    typically imposed by the implementation. Therefore, quantitative
    limitations should always be expressed by capability arguments.
 
    LFB instances in the model of a particular FE implementation will
    possess limitations on the capabilities defined in the
    corresponding LFB class.  The LFB class specifications must define
    a set of capability arguments, and the CE must be able to query the
    actual capabilities of the LFB instance via querying the value of
    such arguments.  The capability query will typically happen when
    the LFB is first detected by the CE. Capabilities need not be re-
    queried in case of static limitations. In some cases, however, some
    capabilities may change in time (e.g., as a result of
    adding/removing other LFBs, or configuring certain attributes of
    some other LFB when the LFBs share physical resources), in which
    case additional mechanisms must be implemented to inform the CE
    about the changes.
 
    The following two broad types of limitations will exist:
       . Qualitative restrictions.  For example, a standardized multi-
         field classifier LFB class may define a large number of
 
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         classification fields, but a given FE may support only a
         subset of those fields.
       . Quantitative restrictions, such as the maximum size of tables,
         etc.
 
    The capability parameters that can be queried on a given LFB class
    will be part of the LFB class specification.  The capability
    parameters should be regarded as special attributes of the LFB. The
    actual values of these arguments may be, therefore, obtained using
    the same attribute query mechanisms as used for other LFB
    attributes.
 
    Capability attributes will typically be read-only arguments, but in
    certain cases they may be configurable. For example, the size of a
    lookup table may be limited by the hardware (read-only), in other
    cases it may be configurable (read-write, within some hard limits).
 
    Assuming that capabilities will not change frequently, the
    efficiency of the protocol/schema/encoding is of secondary concern.
 
 9.5. State Query of LFB Attributes
 
    This feature must be provided by all FEs.  The ForCES protocol and
    the data schema/encoding conveyed by the protocol must together
    satisfy the following requirements to facilitate state query of the
    LFB attributes:
       . Must permit FE selection. This is primarily to refer to a
         single FE, but referring to a group of (or all) FEs may
         optional be supported.
       . Must permit LFB instance selection. This is primarily to refer
         to a single LFB instance of an FE, but optionally addressing
         of a group of LFBs (or all) may be supported.
       . Must support addressing of individual attribute of an LFB.
       . Must provide efficient encoding and decoding of the addressing
         info and the configured data.
       . Must provide efficient data transmission of the attribute
         state over the wire (to minimize communication load on the CE-
         FE link).
 
 9.6. LFB Attribute Manipulation
 
    This is a place-holder for all operations that the CE will use to
    populate, manipulate, and delete attributes of the LFB instances on
    the FEs.  This is how the CE configures an individual LFB instance.
 
    The same set of requirements as described in Section 9.5 for
    attribute query applies here for attribute manipulation as well.
 
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    Support for various levels of feedback from the FE to the CE (e.g.,
    request received, configuration completed), as well as multi-
    attribute configuration transactions with atomic commit and
    rollback, may be necessary in some circumstances.
 
    (Editor's note: It remains an open issue as to whether or not other
    methods are needed in addition to "get attribute" and "set
    attribute" (such as multi-attribute transactions).  If the answer
    to that question is yes, it is not clear whether such methods
    should be supported by the FE model itself or the ForCES protocol.)
 
 9.7. LFB Topology Re-configuration
 
    Operations that will be needed to reconfigure LFB topology:
       . Create a new instance of a given LFB class on a given FE.
       . Connect a given output of LFB x to the given input of LFB y.
       . Disconnect: remove a link between a given output of an LFB and
         a given input of another LFB.
       . Delete a given LFB (automatically removing all interconnects
         to/from the LFB).
 
 10. Acknowledgments
 
    The authors would also like to thank the following individuals for
    their invaluable technical input: David Putzolu, Hormuzd Khosravi,
    Eric Johnson, David Durham, Andrzej Matejko, T. Sridhar, Jamal Hadi
    Salim, Alex Audu, Gamil Cain.
 
 11. Security Considerations
 
    The FE model describes the representation and organization of data
    sets and attributes in the FEs.  ForCES framework document [2]
    provides a comprehensive security analysis for the overall ForCES
    architecture.  For example, the ForCES protocol entities must be
    authenticated per the ForCES requirements before they can access
    the information elements described in this document via ForCES.
    The access to the information contained in the FE model is
    accomplished via the ForCES protocol which will be defined in
    separate documents and so the security issues will be addressed
    there.
 
 12. Normative References
 
    [1] Khosravi, H. et al., "Requirements for Separation of IP Control
    and Forwarding", work in progress, July 2003, <draft-ietf-forces-
    requirements-10.txt>.
 
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 13. Informative References
 
    [2] Yang, L. et al., "Forwarding and Control Element Separation
    (ForCES) Framework", work in progress, July 2003, <draft-ietf-
    forces-framework-07.txt>.
 
    [3] Bernet, Y. et al., "An Informal Management Model for Diffserv
    Routers", May 2002.
 
    [4] Chan, K. et al., "Differentiated Services Quality of Service
    Policy Information Base", March 2003.
 
    [5] Sahita, R. et al., "Framework Policy Information Base", RFC
    3318, March 2003.
 
    [6] Moore, B. et al., "Information Model for Describing Network
    Device QoS Datapath Mechanisms", work in progress, May 2002,
    <draft-ietf-policy-qos-device-info-model-08.txt>.
 
    [7] Snir, Y. et al., "Policy Framework QoS Information Model", work
    in progress, Nov 2001, <draft-ietf-policy-qos-info-model-04.txt".
 
    [8] Li, M. et al., "IPsec Policy Information Base", work in
    progress, January 2003, <draft-ietf-ipsp-ipsecpib-07.txt>.
 
    [9] Quittek, J. et Al., "Requirements for IP Flow Information
    Export", work in progress, June 2003, <draft-ietf-ipfix-reqs-
    10.txt>.
 
    [10] Duffield, N., "A Framework for Passive Packet Measurement ",
    work in progress, June 2003, <draft-ietf-psamp-framework-03.txt>.
 
    [11] Pras, A. and Schoenwaelder, J., FRC 3444 "On the Difference
    between Information Models and Data Models", January 2003.
 
 14. Authors' Addresses
 
    L. Lily Yang
    Intel Labs
    2111 NE 25th Avenue
    Hillsboro, OR 97124, USA
    Phone: +1 503 264 8813
    Email: lily.l.yang@intel.com
 
 
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    Joel M. Halpern
    Megisto Systems, Inc.
    20251 Century Blvd.
    Germantown, MD 20874-1162, USA
    Phone: +1 301 444-1783
    Email: jhalpern@megisto.com
 
    Ram Gopal
    Nokia Research Center
    5, Wayside Road,
    Burlington, MA 01803, USA
    Phone: +1 781 993 3685
    Email: ram.gopal@nokia.com
 
    Alan DeKok
    IDT Inc.
    1575 Carling Ave.
    Ottawa, ON K1G 0T3, Canada
    Phone: +1 613 724 6004 ext. 231
    Email: alan.dekok@idt.com
 
    Zsolt Haraszti
    Ericsson
    920 Main Campus Dr, St. 500
    Raleigh, NC  27606, USA
    Phone: +1 919 472 9949
    Email: zsolt.haraszti@ericsson.com
 
    Steven Blake
    Ericsson
    920 Main Campus Dr, St. 500
    Raleigh, NC  27606, USA
    Phone: +1 919 472 9913
    Email: steven.blake@ericsson.com
 
 15. Intellectual Property Right
 
    The authors are not aware of any intellectual property right issues
    pertaining to this document.
 
 16. IANA consideration
 
    A namespace is needed to uniquely identify the LFB type in the LFB
    class library.
 
    Frame type supported on input and output of LFB must also be
    uniquely identified.
 
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    A set of metadata supported by the LFB model must also be uniquely
    identified with names.
 
 
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