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RFC 5812

 
 
 

Forwarding and Control Element Separation (ForCES) Forwarding Element Model

Part 2 of 5, p. 10 to 41
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3.  ForCES Model Concepts

   Some of the important ForCES concepts used throughout this document
   are introduced in this section.  These include the capability and
   state abstraction, the FE and LFB model construction, and the unique
   addressing of the different model structures.  Details of these
   aspects are described in Section 4 and Section 5.  The intent of this
   section is to discuss these concepts at the high level and lay the
   foundation for the detailed description in the following sections.

   The ForCES FE model includes both a capability and a state
   abstraction.

   o  The FE/LFB capability model describes the capabilities and
      capacities of an FE/LFB by specifying the variation in functions
      supported and any limitations.  Capacity describes the limits of
      specific components (an example would be a table size limit).

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   o  The state model describes the current state of the FE/LFB, that
      is, the instantaneous values or operational behavior of the FE/
      LFB.

   Section 3.1 explains the difference between a capability model and a
   state model, and describes how the two can be combined in the FE
   model.

   The ForCES model construction laid out in this document allows an FE
   to provide information about its structure for operation.  This can
   be thought of as FE-level information and information about the
   individual instances of LFBs provided by the FE.

   o  The ForCES model includes the constructions for defining the class
      of Logical Functional Blocks (LFBs) that an FE may support.  These
      classes are defined in this and other documents.  The definition
      of such a class provides the information content for monitoring
      and controlling instances of the LFB class for ForCES purposes.
      Each LFB model class formally defines the operational LFB
      components, LFB capabilities, and LFB events.  Essentially,
      Section 3.2 introduces the concept of LFBs as the basic functional
      building blocks in the ForCES model.

   o  The FE model also provides the construction necessary to monitor
      and control the FE as a whole for ForCES purposes.  For
      consistency of operation and simplicity, this information is
      represented as an LFB, the FE Object LFB class and a singular LFB
      instance of that class, defined using the LFB model.  The FE
      Object class defines the components to provide information at the
      FE level, particularly the capabilities of the FE at a coarse
      level, i.e., not all possible capabilities or all details about
      the capabilities of the FE.  Part of the FE-level information is
      the LFB topology, which expresses the logical inter-connection
      between the LFB instances along the data path(s) within the FE.
      Section 3.3 discusses the LFB topology.  The FE Object also
      includes information about what LFB classes the FE can support.

   The ForCES model allows for unique identification of the different
   constructs it defines.  This includes identification of the LFB
   classes, and of LFB instances within those classes, as well as
   identification of components within those instances.

   The ForCES protocol [RFC5810] encapsulates target address(es) to
   eventually get to a fine-grained entity being referenced by the CE.
   The addressing hierarchy is broken into the following:

   o  An FE is uniquely identified by a 32-bit FEID.

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   o  Each class of LFB is uniquely identified by a 32-bit LFB ClassID.
      The LFB ClassIDs are global within the network element and may be
      issued by IANA.

   o  Within an FE, there can be multiple instances of each LFB class.
      Each LFB class instance is identified by a 32-bit identifier that
      is unique within a particular LFB class on that FE.

   o  All the components within an LFB instance are further defined
      using 32-bit identifiers.

   Refer to Section 3.3 for more details on addressing.

3.1.  ForCES Capability Model and State Model

   Capability and state modeling applies to both the FE and LFB
   abstraction.

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

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

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

3.1.1.  FE Capability Model and State Model

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

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3.1.1.1.  FE Capability Model

   The FE capability model may be used to describe an FE at a coarse
   level.  For example, an FE might be defined as follows:

   o  the FE can handle IPv4 and IPv6 forwarding;

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

   o  the FE can perform metering;

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

   o  the FE can add and remove encapsulating headers of types including
      IPsec, GRE, L2TP.

   While one could try to build an object model to fully represent the
   FE capabilities, other efforts found this approach to be a
   significant undertaking.  The main difficulty arises in describing
   detailed limits, such as the maximum number of classifiers, queues,
   buffer pools, and meters that the FE can provide.  We believe that a
   good balance between simplicity and flexibility can be achieved for
   the FE model by combining coarse-level-capability reporting with an
   error reporting mechanism.  That is, if the CE attempts to instruct
   the FE to set up some specific behavior it cannot support, the FE
   will return an error indicating the problem.  Examples of similar
   approaches include Diffserv PIB RFC 3317 [RFC3317] and framework PIB
   RFC 3318 [RFC3318].

3.1.1.2.  FE State Model

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

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

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

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

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

3.1.1.3.  LFB Capability and State Model

   Both LFB capability and state information are defined formally using
   the LFB modeling XML schema.

   Capability information at the LFB level is an integral part of the
   LFB model and provides for powerful semantics.  For example, when
   certain features of an LFB class are optional, the CE needs to be
   able to determine whether those optional features are supported by a
   given LFB instance.  The schema for the definition of LFB classes
   provides a means for identifying such components.

   State information is defined formally using LFB component constructs.

3.1.2.  Relating LFB and FE Capability and State Model

   Capability information at the FE level describes the LFB classes that
   the FE can instantiate, the number of instances of each that can be
   created, the topological (linkage) limitations between these LFB
   instances, etc.  Section 5 defines the FE-level components including
   capability information.  Since all information is represented as
   LFBs, this is provided by a single instance of the FE Object LFB
   class.  By using a single instance with a known LFB class and a known
   instance identification, the ForCES protocol can allow a CE to access
   this information whenever it needs to, including while the CE is
   establishing the control of the FE.

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

3.2.   Logical Functional Block (LFB) Modeling

   Each LFB performs a well-defined action or computation on the packets
   passing through it.  Upon completion of its prescribed function,
   either the packets are modified in certain ways (e.g., decapsulator,
   marker), or some results are generated and stored, often in the form

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   of metadata (e.g., classifier).  Each LFB typically performs a single
   action.  Classifiers, shapers, and meters are all examples of such
   LFBs.  Modeling LFBs at such a fine granularity allows us to use a
   small number of LFBs to express the higher-order FE functions (such
   as an IPv4 forwarder) precisely, which in turn can describe more
   complex networking functions and vendor implementations of software
   and hardware.  These fine-grained LFBs will be defined in detail in
   one or more documents to be published separately, using the material
   in this model.

   It is also the case that LFBs may exist in order to provide a set of
   components for control of FE operation by the CE (i.e., a locus of
   control), without tying that control to specific packets or specific
   parts of the data path.  An example of such an LFB is the FE Object,
   which provides the CE with information about the FE as a whole, and
   allows the FE to control some aspects of the FE, such as the data
   path itself.  Such LFBs will not have the packet-oriented properties
   described in this section.

   In general, multiple LFBs are contained in one FE, as shown in
   Figure 2, and all the LFBs share the same ForCES protocol (Fp)
   termination point that implements the ForCES protocol logic and
   maintains the communication channel to and from the CE.

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

                      Figure 2: Generic LFB diagram.

   An LFB, as shown in Figure 2, may have inputs, outputs, and
   components that can be queried and manipulated by the CE via an Fp
   reference point (defined in RFC 3746 [RFC3746]) and the ForCES
   protocol termination point.  The horizontal axis is in the forwarding
   plane for connecting the inputs and outputs of LFBs within the same
   FE.  P (with marks to indicate modification) indicates a data packet,
   while M (with marks to indicate modification) indicates the metadata
   associated with a packet.  The vertical axis between the CE and the
   FE denotes the Fp reference point where bidirectional communication
   between the CE and FE occurs: the CE-to-FE communication is for
   configuration, control, and packet injection, while the FE-to-CE
   communication is used for packet redirection to the control plane,
   reporting of monitoring and accounting information, reporting of
   errors, etc.  Note that the interaction between the CE and the LFB is
   only abstract and indirect.  The result of such an interaction is for
   the CE to manipulate the components of the LFB instances.

   An LFB can have one or more inputs.  Each input takes a pair of a
   packet and its associated metadata.  Depending upon the LFB input
   port definition, the packet or the metadata may be allowed to be

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   empty (or equivalently to not be provided).  When input arrives at an
   LFB, either the packet or its associated metadata must be non-empty
   or there is effectively no input.  (LFB operation generally may be
   triggered by input arrival, by timers, or by other system state.  It
   is only in the case where the goal is to have input drive operation
   that the input must be non-empty.)

   The LFB processes the input, and produces one or more outputs, each
   of which is a pair of a packet and its associated metadata.  Again,
   depending upon the LFB output port definition, either the packet or
   the metadata may be allowed to be empty (or equivalently to be
   absent).  Metadata attached to packets on output may be metadata that
   was received, or may be information about the packet processing that
   may be used by later LFBs in the FEs packet processing.

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

   LFB operation is specified in the model to allow the CE to understand
   the behavior of the forwarding data path.  For instance, the CE needs
   to understand at what point in the data path the IPv4 header TTL is
   decremented by the FE.  That is, the CE needs to know if a control
   packet could be delivered to it either before or after this point in
   the data path.  In addition, the CE needs to understand where and
   what type of header modifications (e.g., tunnel header append or
   strip) are performed by the FEs.  Further, the CE works to verify
   that the various LFBs along a data path within an FE are compatible
   to link together.  Connecting incompatible LFB instances will produce
   a non-working data path.  So the model is designed to provide
   sufficient information for the CE to make this determination.

   Selecting the right granularity for describing the functions of the
   LFBs is an important aspect of this model.  There is value to vendors
   if the operation of LFB classes can be expressed in sufficient detail
   so that physical devices implementing different LFB functions can be
   integrated easily into an FE design.  However, the model, and the
   associated library of LFBs, must not be so detailed and so specific
   as to significantly constrain implementations.  Therefore, a semi-
   formal specification is needed; that is, a text description of the
   LFB operation (human readable), but sufficiently specific and
   unambiguous to allow conformance testing and efficient design, so
   that interoperability between different CEs and FEs can be achieved.

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   The LFB class model specifies the following, among other information:

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

   o  metadata read/consumed from inputs

   o  metadata produced at the outputs

   o  packet types accepted at the inputs and emitted at the outputs

   o  packet content modifications (including encapsulation or
      decapsulation)

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

   o  packet timing modifications

   o  packet flow ordering modifications

   o  LFB capability information components

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

   o  LFB operational components

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

3.2.1.  LFB Outputs

   An LFB output is a conceptual port on an LFB that can send
   information to another LFB.  The information sent on that port is a
   pair of a packet and associated metadata, one of which may be empty.
   (If both were empty, there would be no output.)

   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.

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

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

    a. One output               b. Two distinct outputs

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

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

       Figure 3: Examples of LFBs with various output combinations.

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

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

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

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

   For example, consider a redirector LFB, whose sole purpose is to
   direct packets to one of N downstream paths based on one of the
   metadata associated with each arriving packet.  Such an LFB is fairly
   versatile and can be used in many different places in a topology.
   For example, given LFBs that record the type of packet in a FRAMETYPE
   metadatum, or a packet rate class in a COLOR metadatum, one may uses
   these metadata for branching.  A redirector can be used to divide the
   data path into an IPv4 and an IPv6 path based on a FRAMETYPE
   metadatum (N=2), or to fork into rate-specific paths after metering
   using the COLOR metadatum (red, yellow, green; N=3), etc.

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

   Note that other LFBs may also use the output group concept to build
   in similar adaptive forking capability.  For example, a classifier
   LFB with one input and N outputs can be defined easily by using the
   output group concept.  Alternatively, a classifier LFB with one
   singleton output in combination with an explicit N-output re-director

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   LFB models the same processing behavior.  The decision of whether to
   use the output group model for a certain LFB class is left to the LFB
   class designers.

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

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

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

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

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

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

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

3.2.2.  LFB Inputs

   An LFB input is a conceptual port on an LFB on which the LFB can
   receive information from other LFBs.  The information is typically a
   pair of a packet and its associated metadata.  Either the packet or
   the metadata may for some LFBs and some situations be empty.  They
   cannot both be empty, as then there is no input.

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

   o  Implicit multiplexing via a single input

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   o  Explicit multiplexing via multiple singleton inputs

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

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

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

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

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

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

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

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

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

        Figure 4: Examples of LFBs with various input combinations.

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

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   The number of actual input instances in the group is a component
   defined in the LFB class, which is read-only for static topologies,
   and is read-write for configurable topologies.

   As an example for the input group, consider the Scheduler LFB
   depicted in Figure 4.c.  Such an LFB receives packets from a number
   of Queue LFBs via a number of input instances, and uses the input
   index information to control contention resolution and scheduling.

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

3.2.3.  Packet Type

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

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

3.2.4.  Metadata

   Metadata is state that is passed from one LFB to another alongside a
   packet.  The metadata passed with the packet assists subsequent LFBs
   to process that packet.

   The ForCES model defines metadata as precise atomic definitions in
   the form of label, value pairs.

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   The ForCES model provides to the authors of LFB classes a way to
   formally define how to achieve metadata creation, modification,
   reading, as well as consumption (deletion).

   Inter-FE metadata, i.e., metadata crossing FEs, while it is likely to
   be semantically similar to this metadata, is out of scope for this
   document.

   Section 4 has informal details on metadata.

3.2.4.1.  Metadata Lifecycle within the ForCES Model

   Each metadatum is modeled as a <label, value> pair, where the label
   identifies the type of information (e.g., "color"), and its value
   holds the actual information (e.g., "red").  The label here is shown
   as a textual label, but for protocol processing it is associated with
   a unique numeric value (identifier).

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

3.2.4.2.  Metadata Production and Consumption

   For a given metadatum on a given packet path, there MUST be at least
   one producer LFB that creates that metadatum and SHOULD be at least
   one consumer LFB that needs that metadatum.

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

   The metadata that is produced by an LFB is specified by the LFB class
   definition on a per-output-port-group basis.  A producer may always
   generate the metadata on the port group, or may generate it only
   under certain conditions.  We call the former "unconditional"
   metadata, whereas the latter is "conditional" metadata.  For example,
   deep packet inspection LFB might produce several pieces of metadata
   about the packet.  The first metadatum might be the IP protocol (TCP,
   UDP, SCTP, ...) being carried, and two additional metadata items
   might be the source and destination port number.  These additional
   metadata items are conditional on the value of the first metadatum
   (IP carried protocol) as they are only produced for protocols that

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

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

3.2.4.3.  LFB Operations on Metadata

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

      *  IGNORE: ignores and forwards the metadatum

      *  READ: reads and forwards the metadatum

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

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

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

      *  CONSUME: consumes metadatum without reading

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

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

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

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

3.2.5.  LFB Events

   During operation, various conditions may occur that can be detected
   by LFBs.  Examples range from link failure or restart to timer
   expiration in special purpose LFBs.  The CE may wish to be notified
   of the occurrence of such events.  The description of how such
   messages are sent, and their format, is part of the Forwarding and
   Control Element Separation (ForCES) protocol [RFC5810] document.
   Indicating how such conditions are understood is part of the job of
   this model.

   Events are declared in the LFB class definition.  The LFB event
   declaration constitutes:

   o  a unique 32-bit identifier.

   o  An LFB component that is used to trigger the event.  This entity
      is known as the event target.

   o  A condition that will happen to the event target that will result
      in a generation of an event to the CE.  Examples of a condition
      include something getting created or deleted, a config change,
      etc.

   o  What should be reported to the CE by the FE if the declared
      condition is met.

   The declaration of an event within an LFB class essentially defines
   what part of the LFB component(s) need to be monitored for events,
   what condition on the LFB monitored LFB component an FE should detect
   to trigger such an event, and what to report to the CE when the event
   is triggered.

   While events may be declared by the LFB class definition, runtime
   activity is controlled using built-in event properties using LFB
   component properties (discussed in Section 3.2.6).  A CE subscribes
   to the events on an LFB class instance by setting an event property
   for subscription.  Each event has a subscription property that is by
   default off.  A CE wishing to receive a specific event needs to turn
   on the subscription property at runtime.

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   Event properties also provide semantics for runtime event filtering.
   A CE may set an event property to further suppress events to which it
   has already subscribed.  The LFB model defines such filters to
   include threshold values, hysteresis, time intervals, number of
   events, etc.

   The contents of reports with events are designed to allow for the
   common, closely related information that the CE can be strongly
   expected to need to react to the event.  It is not intended to carry
   information that the CE already has, large volumes of information, or
   information related in complex fashions.

   From a conceptual point of view, at runtime, event processing is
   split into:

   1.  Detection of something happening to the (declared during LFB
       class definition) event target.  Processing the next step happens
       if the CE subscribed (at runtime) to the event.

   2.  Checking of the (declared during LFB class definition) condition
       on the LFB event target.  If the condition is met, proceed with
       the next step.

   3.  Checking (runtime set) event filters if they exist to see if the
       event should be reported or suppressed.  If the event is to be
       reported, proceed to the next step.

   4.  Submitting of the declared report to the CE.

   Section 4.7.6 discusses events in more details.

3.2.6.  Component Properties

   LFBs and structures are made up of components, containing the
   information that the CE needs to see and/or change about the
   functioning of the LFB.  These components, as described in detail in
   Section 4.7, may be basic values, complex structures (containing
   multiple components themselves, each of which can be values,
   structures, or tables), or tables (which contain values, structures,
   or tables).  Components may be defined such that their appearance in
   LFB instances is optional.  Components may be readable or writable at
   the discretion of the FE implementation.  The CE needs to know these
   properties.  Additionally, certain kinds of components (arrays /
   tables, aliases, and events) have additional property information
   that the CE may need to read or write.  This model defines the
   structure of the property information for all defined data types.

   Section 4.8 describes properties in more details.

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3.2.7.  LFB Versioning

   LFB class versioning is a method to enable incremental evolution of
   LFB classes.  In general, an FE is not allowed to contain an LFB
   instance for more than one version of a particular class.
   Inheritance (discussed next in Section 3.2.8) has special rules.  If
   an FE data path model containing an LFB instance of a particular
   class C also simultaneously contains an LFB instance of a class C'
   inherited from class C; C could have a different version than C'.

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

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

3.2.8.  LFB Inheritance

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

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

   An interesting issue related to class inheritance is backward
   compatibility between a descendant and an ancestor class.  Consider
   the following hypothetical scenario where a standardized LFB class
   "L1" exists.  Vendor A builds an FE that implements LFB "L1", and
   vendor B builds a CE that can recognize and operate on LFB "L1".
   Suppose that a new LFB class, "L2", is defined based on the existing
   "L1" class by extending its capabilities incrementally.  Let us
   examine the FE backward-compatibility issue by considering what would
   happen if vendor B upgrades its FE from "L1" to "L2" and vendor C's

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   CE is not changed.  The old L1-based CE can interoperate with the new
   L2-based FE if the derived LFB class "L2" is indeed backward
   compatible with the base class "L1".

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

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

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

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

3.3.  ForCES Model Addressing

   Figure 5 demonstrates the abstraction of the different ForCES model
   entities.  The ForCES protocol provides the mechanism to uniquely
   identify any of the LFB class instance components.

        FE Address = FE01
        +--------------------------------------------------------------+
        |                                                              |
        | +--------------+             +--------------+                |
        | | LFB ClassID 1|             |LFB ClassID 91|                |
        | | InstanceID 3 |============>|InstanceID 3  |======>...      |
        | | +----------+ |             | +----------+ |                |
        | | |Components| |             | |Components| |                |
        | | +----------+ |             | +----------+ |                |
        | +--------------+             +--------------+                |
        |                                                              |
        +--------------------------------------------------------------+

                      Figure 5: FE entity hierarchy.

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   At the top of the addressing hierarchy is the FE identifier.  In the
   example above, the 32-bit FE identifier is illustrated with the
   mnemonic FE01.  The next 32-bit entity selector is the LFB ClassID.
   In the illustration above, two LFB classes with identifiers 1 and 91
   are demonstrated.  The example above further illustrates one instance
   of each of the two classes.  The scope of the 32-bit LFB class
   instance identifier is valid only within the LFB class.  To emphasize
   that point, each of class 1 and 91 has an instance of 3.

   Using the described addressing scheme, a message could be sent to
   address FE01, LFB ClassID 1, LFB InstanceID 3, utilizing the ForCES
   protocol.  However, to be effective, such a message would have to
   target entities within an LFB.  These entities could be carrying
   state, capability, etc.  These are further illustrated in Figure 6
   below.

          LFB Class ID 1,InstanceID 3 Components
          +-------------------------------------+
          |                                     |
          | LFB ComponentID 1                   |
          | +----------------------+            |
          | |                      |            |
          | +----------------------+            |
          |                                     |
          | LFB ComponentID 31                  |
          | +----------------------+            |
          | |                      |            |
          | +----------------------+            |
          |                                     |
          | LFB ComponentID 51                  |
          | +----------------------+            |
          | | LFB ComponentID 89   |            |
          | | +-----------------+  |            |
          | | |                 |  |            |
          | | +-----------------+  |            |
          | +----------------------+            |
          |                                     |
          |                                     |
          +-------------------------------------+

                         Figure 6: LFB hierarchy.

   Figure 6 zooms into the components carried by LFB Class ID 1, LFB
   InstanceID 3 from Figure 5.

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   The example shows three components with 32-bit component identifiers
   1, 31, and 51.  LFB ComponentID 51 is a complex structure
   encapsulating within it an entity with LFB ComponentID 89.  LFB
   ComponentID 89 could be a complex structure itself, but is restricted
   in the example for the sake of clarity.

3.3.1.  Addressing LFB Components: Paths and Keys

   As mentioned above, LFB components could be complex structures, such
   as a table, or even more complex structures such as a table whose
   cells are further tables, etc.  The ForCES model XML schema
   (Section 4) allows for uniquely identifying anything with such
   complexity, utilizing the concept of dot-annotated static paths and
   content addressing of paths as derived from keys.  As an example, if
   LFB ComponentID 51 were a structure, then the path to LFB ComponentID
   89 above will be 51.89.

   LFB ComponentID 51 might represent a table (an array).  In that case,
   to select the LFB component with ID 89 from within the 7th entry of
   the table, one would use the path 51.7.89.  In addition to supporting
   explicit table element selection by including an index in the dotted
   path, the model supports identifying table elements by their
   contents.  This is referred to as using keys, or key indexing.  So,
   as a further example, if ComponentID 51 was a table that was key
   index-able, then a key describing content could also be passed by the
   CE, along with path 51 to select the table, and followed by the path
   89 to select the table structure element, which upon computation by
   the FE would resolve to the LFB ComponentID 89 within the specified
   table entry.

3.4.  FE Data Path Modeling

   Packets coming into the FE from ingress ports generally flow through
   one or more 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), header values, time of arrival, etc.
   The result of LFB processing may have an impact on how the packet is
   to be treated in downstream LFBs.  This differentiation of packet
   treatment downstream can be conceptualized as having alternative data
   paths in the FE.  For example, the result of a 6-tuple classification
   performed by a classifier LFB could control which rate meter is
   applied to the packet by a rate meter LFB in a later stage in the
   data path.

   LFB topology is a directed graph representation of the logical data
   paths within an FE, with the nodes representing the LFB instances and
   the directed link depicting the packet flow direction from one LFB to

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   the next.  Section 3.4.1 discusses how the FE data paths can be
   modeled as LFB topology, while Section 3.4.2 focuses on issues
   related to LFB topology reconfiguration.

3.4.1.  Alternative Approaches for Modeling FE Data Paths

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

   o  Topological Approach

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

   o  Encoded State Approach

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

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

   Using a simple example of a classifier with N classification outputs
   followed by other LFBs, Figure 7.a shows what the LFB topology looks
   like when using the pure topological approach.  Each output from the
   classifier goes to one of the N LFBs where no metadata is needed.
   The topological approach is simple, straightforward, and graphically

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   intuitive.  However, if N is large and the N nodes following the
   classifier (LFB#1, LFB#2, ..., LFB#N) all belong to the same LFB type
   (e.g., meter), but each has its own independent components, the
   encoded state approach gives a much simpler topology representation,
   as shown in Figure 7.b.  The encoded state approach requires that a
   table of N rows of meter components be provided in the Meter node
   itself, with each row representing the attributes for one meter
   instance.  A metadatum M is also needed to pass along with the packet
   P from the classifier to the meter, so that the meter can use M as a
   look-up key (index) to find the corresponding row of the attributes
   that should be used for any particular packet P.

   What if those N nodes (LFB#1, LFB#2, ..., LFB#N) are not of the same
   type?  For example, if LFB#1 is a queue while the rest are all
   meters, what is the best way to represent such data paths?  While it
   is still possible to use either the pure topological approach or the
   pure encoded state approach, the natural combination of the two
   appears to be the best option.  Figure 7.c depicts two different
   functional data paths using the topological approach while leaving
   the N-1 meter instances distinguished by metadata only, as shown in
   Figure 7.c.

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

   (a) Using pure topological approach

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

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

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

   (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 7: An example of how to model FE data paths.

   From this example, we demonstrate that each approach has a distinct
   advantage depending on the situation.  Using the encoded state
   approach, fewer connections are typically needed between a fan-out
   node and its next LFB instances of the same type because each packet
   carries metadata the following nodes can interpret and hence invoke a
   different packet treatment.  For those cases, a pure topological
   approach forces one to build elaborate graphs with many more
   connections and often results in an unwieldy graph.  On the other
   hand, a topological approach is the most intuitive for representing
   functionally different data paths.

   For complex topologies, a combination of the two is the most
   flexible.  A general design guideline is provided to indicate which
   approach is best used for a particular situation.  The topological
   approach should primarily be used when the packet data path forks to
   distinct LFB classes (not just distinct parameterizations of the same
   LFB class), and when the fan-outs do not require changes, such as
   adding/removing LFB outputs, or require only very infrequent changes.

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   Configuration information that needs to change frequently should be
   expressed by using the internal attributes of one or more LFBs (and
   hence using the encoded state approach).

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


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

                    Figure 8: An LFB topology example.

   It is important to point out that the LFB topology described here is
   the logical topology, not the physical topology of how the FE
   hardware is actually laid out.  Nevertheless, the actual
   implementation may still influence how the functionality is mapped to
   the LFB topology.  Figure 8 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 8.a.  However, if the classification
   is implemented by two different pieces of hardware or software with
   different filters (i.e., one set of filters for the outer IP header
   and another set for the inner IP header), then it is more natural to
   model them as two different instances of classifier LFB, as shown in
   Figure 8.b.

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3.4.2.   Configuring the LFB Topology

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

   Why would the data paths on an FE ever change dynamically?  The data
   paths on an FE are set up by the CE to provide certain data plane
   services (e.g., Diffserv, VPN) to the network element's (NE)
   customers.  The purpose of reconfiguring the data paths is to enable
   the CE to customize the services the NE is delivering at run time.
   The CE needs to change the data paths when the service requirements
   change, such as adding a new customer or when an existing customer
   changes their service.  However, note that not all data path changes
   result in changes in the LFB topology graph.  Changes in the graph
   are dependent on the approach used to map the data paths into LFB
   topology.  As discussed in Section 3.4.1, the topological approach
   and encoded state approach can result in very different looking LFB
   topologies for the same data paths.  In general, an LFB topology
   based on a pure topological approach is likely to experience more
   frequent topology reconfiguration than one based on an encoded state
   approach.  However, even an LFB topology based entirely on an encoded
   state approach may have to change the topology at times, for example,
   to bypass some LFBs or insert new LFBs.  Since a mix of these two
   approaches is used to model the data paths, LFB topology
   reconfiguration is considered an important aspect of the FE model.

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

Top      Up      ToC       Page 38 
   provide a mechanism for describing the LFB topology configuration
   capabilities of an FE.  These capabilities may include (see Section 5
   for full details):

   o  Which LFB classes the FE can instantiate

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

   o  Any topological limitations, for example:

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

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

   The CE needs some programming help in order to cope with the range of
   complexity.  In other words, even when the CE is allowed to configure
   LFB topology for the FE, the CE is not expected to be able to
   interpret an arbitrary LFB topology and determine which specific
   service or application (e.g., VPN, Diffserv) is supported by the FE.
   However, once the CE understands the coarse capability of an FE, the
   CE MUST configure the LFB topology to implement the network service
   the NE is supposed to provide.  Thus, the mapping the CE has to
   understand is from the high-level NE service to a specific LFB
   topology, not the other way around.  The CE is not expected to have
   the ultimate intelligence to translate any high-level service policy
   into the configuration data for the FEs.  However, it is conceivable
   that within a given network service domain, a certain amount of
   intelligence can be programmed into the CE to give the CE a general
   understanding of the LFBs involved to allow the translation from a
   high-level service policy to the low-level FE configuration to be
   done automatically.  Note that this is considered an implementation
   issue internal to the control plane and outside the scope of the FE
   model.  Therefore, it is not discussed any further in this document.

Top      Up      ToC       Page 39 
         +----------+     +-----------+
    ---->| Ingress  |---->|classifier |--------------+
         |          |     |chip       |              |
         +----------+     +-----------+              |
                                                     v
                         +-------------------------------------------+
           +--------+    |   Network Processor                       |
      <----| Egress |    |   +------+    +------+   +-------+        |
           +--------+    |   |Meter |    |Marker|   |Dropper|        |
                 ^       |   +------+    +------+   +-------+        |
                 |       |                                           |
      +----------+-------+                                           |
      |          |                                                   |
      |    +---------+       +---------+   +------+    +---------+   |
      |    |Forwarder|<------|Scheduler|<--|Queue |    |Counter  |   |
      |    +---------+       +---------+   +------+    +---------+   |
      +--------------------------------------------------------------+

         Figure 9: The capability of an FE as reported to the CE.

   Figure 9 shows an example where a QoS-enabled (quality-of-service)
   router has several line cards that have a few ingress ports and
   egress ports, a specialized classification chip, and a network
   processor containing codes for FE blocks like meter, marker, dropper,
   counter, queue, scheduler, and IPv4 forwarder.  Some of the LFB
   topology is already fixed and has to remain static due to the
   physical layout of the line cards.  For example, all of the ingress
   ports might be hardwired into the classification chip so all packets
   flow from the ingress port into the classification engine.  On the
   other hand, the LFBs on the network processor and their execution
   order are programmable.  However, certain capacity limits and linkage
   constraints could exist between these LFBs.  Examples of the capacity
   limits might be:

   o  8 meters

   o  16 queues in one FE

   o  the scheduler can handle at most up to 16 queues

   o  The linkage constraints might dictate that:

      *  the classification engine may be followed by:

         +  a meter

         +  marker

Top      Up      ToC       Page 40 
         +  dropper

         +  counter

         +  queue or IPv4 forwarder, but not a scheduler

      *  queues can only be followed by a scheduler

      *  a scheduler must be followed by the IPv4 forwarder

      *  the last LFB in the data path before going into the egress
         ports must be the IPv4 forwarder

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

                 Figure 10: An LFB topology as configured
                     by the CE and accepted by the FE.

   Once the FE reports these capabilities and capacity limits to the CE,
   it is now up to the CE to translate the QoS policy into a desirable
   configuration for the FE.  Figure 9 depicts the FE capability, while
   Figure 10 and Figure 11 depict two different topologies that the CE
   may request the FE to configure.  Note that Figure 11 is not fully
   drawn, as inter-LFB links are included to suggest potential
   complexity, without drawing in the endpoints of all such links.

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                                             Queue1
                     +---+                    +--+
                     |  A|------------------->|  |--+
                  +->|   |                    |  |  |
                  |  |  B|--+  +--+   +--+    +--+  |
                  |  +---+  |  |  |   |  |          |
                  | Meter1  +->|  |-->|  |          |
                  |            |  |   |  |          |
                  |            +--+   +--+          |          IPv4
                  |         Counter1 Dropper1 Queue2|    +--+  Fwd.
          +---+   |                           +--+  +--->|A |  +-+
          |  A|---+                           |  |------>|B |  | |
   ------>|  B|------------------------------>|  |   +-->|C |->| |->
          |  C|---+                           +--+   | +>|D |  | |
          |  D|-+ |                                  | | +--+  +-+
          +---+ | |    +---+                  Queue3 | |Scheduler
      Classifier1 | |  |  A|------------>       +--+ | |
                  | +->|   |                    |  |-+ |
                  |    |  B|--+  +--+ +-------->|  |   |
                  |    +---+  |  |  | |         +--+   |
                  |  Meter2   +->|  |-+                |
                  |              |  |                  |
                  |              +--+           Queue4 |
                  |            Marker1          +--+   |
                  +---------------------------->|  |---+
                                                |  |
                                                +--+

               Figure 11: Another LFB topology as configured
                     by the CE and accepted by the FE.

   Note that both the ingress and egress are omitted in Figure 10 and
   Figure 11 to simplify the representation.  The topology in Figure 11
   is considerably more complex than Figure 10, but both are feasible
   within the FE capabilities, and so the FE should accept either
   configuration request from the CE.



(page 41 continued on part 3)

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