Forwarding and Control Element Separation (ForCES) Forwarding Element Model :: RFC5812
Internet Engineering Task Force (IETF) J. Halpern
Request for Comments: 5812 Self
Category: Standards Track J. Hadi Salim
ISSN: 2070-1721 Znyx Networks
March 2010
Forwarding and Control Element Separation (ForCES)
Forwarding Element Model
Abstract
This document defines the forwarding element (FE) model used in the
Forwarding and Control Element Separation (ForCES) protocol. The
model represents the capabilities, state, and configuration of
forwarding elements within the context of the ForCES protocol, so
that control elements (CEs) can control the FEs accordingly. More
specifically, the model describes the logical functions that are
present in an FE, what capabilities these functions support, and how
these functions are or can be interconnected. This FE model is
intended to satisfy the model requirements specified in RFC 3654.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5812.
Halpern & Hadi Salim Standards Track [Page 1]
RFC 5812 ForCES FE Model March 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Halpern & Hadi Salim Standards Track [Page 2]
RFC 5812 ForCES FE Model March 2010
Table of Contents
1. Introduction ....................................................5
1.1. Requirements on the FE Model ...............................5
1.2. The FE Model in Relation to FE Implementations .............6
1.3. The FE Model in Relation to the ForCES Protocol ............6
1.4. Modeling Language for the FE Model .........................7
1.5. Document Structure .........................................8
2. Definitions .....................................................8
3. ForCES Model Concepts ..........................................10
3.1. ForCES Capability Model and State Model ...................12
3.1.1. FE Capability Model and State Model ................12
3.1.2. Relating LFB and FE Capability and State Model .....14
3.2. Logical Functional Block (LFB) Modeling ...................14
3.2.1. LFB Outputs ........................................18
3.2.2. LFB Inputs .........................................21
3.2.3. Packet Type ........................................24
3.2.4. Metadata ...........................................24
3.2.5. LFB Events .........................................27
3.2.6. Component Properties ...............................28
3.2.7. LFB Versioning .....................................29
3.2.8. LFB Inheritance ....................................29
3.3. ForCES Model Addressing ...................................30
3.3.1. Addressing LFB Components: Paths and Keys ..........32
3.4. FE Data Path Modeling .....................................32
3.4.1. Alternative Approaches for Modeling FE Data Paths ..33
3.4.2. Configuring the LFB Topology .......................37
4. Model and Schema for LFB Classes ...............................41
4.1. Namespace .................................................42
4.2. Element ......................................42
4.3. Element ............................................44
4.4. Element for Frame Type Declarations ...........45
4.5. Element for Data Type Definitions ..........45
4.5.1. Element for Renaming Existing
Data Types .........................................49
4.5.2. Element for Deriving New Atomic Types .....49
4.5.3. Element to Define Arrays ...................50
4.5.4. Element to Define Structures ..............54
4.5.5. Element to Define Union Types ..............56
4.5.6. Element ....................................56
4.5.7. Augmentations ......................................57
4.6. Element for Metadata Definitions ...........58
4.7. Element for LFB Class Definitions ..........59
4.7.1. Element to Express LFB Inheritance ...62
4.7.2. Element to Define LFB Inputs ..........62
4.7.3. Element to Define LFB Outputs ........65
4.7.4. Element to Define LFB
Operational Components .............................67
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4.7.5. Element to Define LFB
Capability Components ..............................70
4.7.6. Element for LFB Notification Generation ...71
4.7.7. Element for LFB Operational
Specification ......................................79
4.8. Properties ................................................79
4.8.1. Basic Properties ...................................79
4.8.2. Array Properties ...................................81
4.8.3. String Properties ..................................81
4.8.4. Octetstring Properties .............................82
4.8.5. Event Properties ...................................83
4.8.6. Alias Properties ...................................87
4.9. XML Schema for LFB Class Library Documents ................88
5. FE Components and Capabilities .................................99
5.1. XML for FEObject Class Definition .........................99
5.2. FE Capabilities ..........................................106
5.2.1. ModifiableLFBTopology .............................106
5.2.2. SupportedLFBs and SupportedLFBType ................106
5.3. FE Components ............................................110
5.3.1. FEState ...........................................110
5.3.2. LFBSelectors and LFBSelectorType ..................110
5.3.3. LFBTopology and LFBLinkType .......................110
5.3.4. FENeighbors and FEConfiguredNeighborType ..........111
6. Satisfying the Requirements on the FE Model ...................111
7. Using the FE Model in the ForCES Protocol .....................112
7.1. FE Topology Query ........................................115
7.2. FE Capability Declarations ...............................116
7.3. LFB Topology and Topology Configurability Query ..........116
7.4. LFB Capability Declarations ..............................116
7.5. State Query of LFB Components ............................118
7.6. LFB Component Manipulation ...............................118
7.7. LFB Topology Reconfiguration .............................118
8. Example LFB Definition ........................................119
8.1. Data Handling ............................................126
8.1.1. Setting Up a DLCI .................................127
8.1.2. Error Handling ....................................127
8.2. LFB Components ...........................................128
8.3. Capabilities .............................................128
8.4. Events ...................................................129
9. IANA Considerations ...........................................130
9.1. URN Namespace Registration ...............................130
9.2. LFB Class Names and LFB Class Identifiers ................130
10. Authors Emeritus .............................................132
11. Acknowledgments ..............................................132
12. Security Considerations ......................................132
13. References ...................................................132
13.1. Normative References ....................................132
13.2. Informative References ..................................133
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RFC 5812 ForCES FE Model March 2010
1. Introduction
RFC 3746 [RFC3746] specifies a framework by which control elements
(CEs) can configure and manage one or more separate forwarding
elements (FEs) within a network element (NE) using the ForCES
protocol. The ForCES architecture allows forwarding elements of
varying functionality to participate in a ForCES network element.
The implication of this varying functionality is that CEs can make
only minimal assumptions about the functionality provided by FEs in
an NE. Before CEs can configure and control the forwarding behavior
of FEs, CEs need to query and discover the capabilities and states of
their FEs. RFC 3654 [RFC3654] mandates that the capabilities, states
and configuration information be expressed in the form of an FE
model.
RFC 3444 [RFC3444] observed that information models (IMs) and data
models (DMs) are different because they serve different purposes.
"The main purpose of an IM is to model managed objects at a
conceptual level, independent of any specific implementations or
protocols used". "DMs, conversely, are defined at a lower level of
abstraction and include many details. They are intended for
implementors and include protocol-specific constructs". Sometimes it
is difficult to draw a clear line between the two. The FE model
described in this document is primarily an information model, but
also includes some aspects of a data model, such as explicit
definitions of the LFB (Logical Functional Block) class schema and FE
schema. It is expected that this FE model will be used as the basis
to define the payload for information exchange between the CE and FE
in the ForCES protocol.
1.1. Requirements on the FE Model
RFC 3654 [RFC3654] defines requirements that must be satisfied by a
ForCES FE model. To summarize, an FE model must define:
o Logically separable and distinct packet forwarding operations in
an FE data path (Logical Functional Blocks or LFBs);
o The possible topological relationships (and hence the sequence of
packet forwarding operations) between the various LFBs;
o The possible operational capabilities (e.g., capacity limits,
constraints, optional features, granularity of configuration) of
each type of LFB;
o The possible configurable parameters (e.g., components) of each
type of LFB; and
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o Metadata that may be exchanged between LFBs.
1.2. The FE Model in Relation to FE Implementations
The FE model proposed here is based on an abstraction using distinct
Logical Functional Blocks (LFBs), which are interconnected in a
directed graph, and receive, process, modify, and transmit packets
along with metadata. The FE model is designed, and any defined LFB
classes should be designed, such that different implementations of
the forwarding data path can be logically mapped onto the model with
the functionality and sequence of operations correctly captured.
However, the model is not intended to directly address how a
particular implementation maps to an LFB topology. It is left to the
forwarding plane vendors to define how the FE functionality is
represented using the FE model. Our goal is to design the FE model
such that it is flexible enough to accommodate most common
implementations.
The LFB topology model for a particular data path implementation must
correctly capture the sequence of operations on the packet. Metadata
generation by certain LFBs MUST always precede any use of that
metadata by subsequent LFBs in the topology graph; this is required
for logically consistent operation. Further, modification of packet
fields that are subsequently used as inputs for further processing
MUST occur in the order specified in the model for that particular
implementation to ensure correctness.
1.3. The FE Model in Relation to the ForCES Protocol
The ForCES base protocol [RFC5810] 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 intra-FE
topology. The details of a particular data path implementation
inside an FE, including the LFB topology, along with the operational
capabilities and attributes of each individual LFB, are conveyed to
the CE within information elements in the ForCES protocol. The model
of an LFB class should define all of the information that needs to be
exchanged between an FE and a CE for the proper configuration and
management of that LFB.
Specifying the various payloads of the ForCES messages in a
systematic fashion is difficult without a formal definition of the
objects being configured and managed (the FE and the LFBs within).
The FE model document defines a set of classes and components for
describing and manipulating the state of the LFBs within an FE.
These class definitions themselves will generally not appear in the
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ForCES protocol. Rather, ForCES protocol operations will reference
classes defined in this model, including relevant components and the
defined operations.
Section 7 provides more detailed discussion on how the FE model
should be used by the ForCES protocol.
1.4. Modeling Language for the FE Model
Even though not absolutely required, it is beneficial to use a formal
data modeling language to represent the conceptual FE model described
in this document. Use of a formal language can help to enforce
consistency and logical compatibility among LFBs. A full
specification will be written using such a data modeling language.
The formal definition of the LFB classes may facilitate the eventual
automation of some of the code generation process and the functional
validation of arbitrary LFB topologies. These class definitions form
the LFB library. Documents that describe LFB classes are therefore
referred to as LFB library documents.
Human readability was the most important factor considered when
selecting the specification language, whereas encoding, decoding, and
transmission performance were not a selection factor. The encoding
method for over-the-wire transport is not dependent on the
specification language chosen and is outside the scope of this
document and up to the ForCES protocol to define.
XML is chosen as the specification language in this document, because
XML has the advantage of being both human and machine readable with
widely available tools support. This document uses an XML schema to
define the structure of the LFB library documents, as defined in
[RFC3470] and [Schema1] and [Schema2]. While these LFB class
definitions are not sent in the ForCES protocol, these definitions
comply with the recommendations in RFC 3470 [RFC3470] on the use of
XML in IETF protocols.
By using an XML schema to define the structure for the LFB library
documents, we have a very clear set of syntactic restrictions to go
with the desired semantic descriptions and restrictions covered in
this document. As a corollary to that, if it is determined that a
change in the syntax is needed, then a new schema will be required.
This would be identified by a different URN to identify the namespace
for such a new schema.
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1.5. Document Structure
Section 3 provides a conceptual overview of the FE model, laying the
foundation for the more detailed discussion and specifications in the
sections that follow. Section 4 and Section 5 constitute the core of
the FE model, detailing the two major aspects of the FE model: a
general LFB model and a definition of the FE Object LFB, with its
components, including FE capabilities and LFB topology information.
Section 6 directly addresses the model requirements imposed by the
ForCES requirements defined in RFC 3654 [RFC3654], while Section 7
explains how the FE model should be used in the ForCES protocol.
2. Definitions
The use of compliance terminology (MUST, SHOULD, MAY, MUST NOT) is
used in accordance with RFC 2119 [RFC2119]. Such terminology is used
in describing the required behavior of ForCES forwarding elements or
control elements in supporting or manipulating information described
in this model.
Terminology associated with the ForCES requirements is defined in RFC
3654 [RFC3654] and is not copied here. The following list of
terminology relevant to the FE model is defined in this section.
FE Model: The FE model is designed to model the logical processing
functions of an FE. The FE model proposed in this document
includes three components; the LFB modeling of individual Logical
Functional Block (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 [RFC5810].
Data Path: A conceptual path taken by packets within the forwarding
plane inside an FE. Note that more than one data path can exist
within an FE.
LFB (Logical Functional Block) Class (or type): A template that
represents a fine-grained, logically separable aspect of FE
processing. Most LFBs relate to packet processing in the data
path. LFB classes are the basic building blocks of the FE model.
LFB Instance: As a packet flows through an FE along a data path, it
flows through one or multiple LFB instances, where each LFB is an
instance of a specific LFB class. Multiple instances of the same
LFB class can be present in an FE's data path. Note that we often
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refer to LFBs without distinguishing between an LFB class and LFB
instance when we believe the implied reference is obvious for the
given context.
LFB Model: The LFB model describes the content and structures in an
LFB, plus the associated data definition. XML is used to provide
a formal definition of the necessary structures for the modeling.
Four types of information are defined in the LFB model. The core
part of the LFB model is the LFB class definitions; the other
three types of information define constructs associated with and
used by the class definition. These are reusable data types,
supported frame (packet) formats, and metadata.
Element: Element is generally used in this document in accordance
with the XML usage of the term. It refers to an XML tagged part
of an XML document. For a precise definition, please see the full
set of XML specifications from the W3C. This term is included in
this list for completeness because the ForCES formal model uses
XML.
Attribute: Attribute is used in the ForCES formal modeling in
accordance with standard XML usage of the term, i.e., to provide
attribute information included in an XML tag.
LFB Metadata: Metadata is used to communicate per-packet state from
one LFB to another, but is not sent across the network. The FE
model defines how such metadata is identified, produced, and
consumed by the LFBs, but not how the per-packet state is
implemented within actual hardware. Metadata is sent between the
FE and the CE on redirect packets.
ForCES Component: A ForCES Component is a well-defined, uniquely
identifiable and addressable ForCES model building block. A
component has a 32-bit ID, name, type, and an optional synopsis
description. These are often referred to simply as components.
LFB Component: An LFB component is a ForCES component that defines
the Operational parameters of the LFBs that must be visible to the
CEs.
Structure Component: A ForCES component that is part of a complex
data structure to be used in LFB data definitions. The individual
parts that make up a structured set of data are referred to as
structure components. These can themselves be of any valid data
type, including tables and structures.
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Property: ForCES components have properties associated with them,
such as readability. Other examples include lengths for variable-
sized components. These properties are accessed by the CE for
reading (or, where appropriate, writing.) Details on the ForCES
properties are found in Section 4.8.
LFB Topology: LFB topology is a representation of the logical
interconnection and the placement of LFB instances along the data
path within one FE. Sometimes this representation is called
intra-FE topology, to be distinguished from inter-FE topology.
LFB topology is outside of the LFB model, but is part of the FE
model.
FE Topology: FE topology is a representation of how multiple FEs
within a single network element (NE) are interconnected.
Sometimes this is called inter-FE topology, to be distinguished
from intra-FE topology (i.e., LFB topology). An individual FE
might not have the global knowledge of the full FE topology, but
the local view of its connectivity with other FEs is considered to
be part of the FE model. The FE topology is discovered by the
ForCES base protocol or by some other means.
Inter-FE Topology: See FE Topology.
Intra-FE Topology: See LFB Topology.
LFB Class Library: The LFB class library is a set of LFB classes
that has been identified as the most common functions found in
most FEs and hence should be defined first by the ForCES Working
Group.
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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RFC 5812 ForCES FE Model March 2010
+-----------+
| 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 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
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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.
Halpern & Hadi Salim Standards Track [Page 38]
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+----------+ +-----------+
---->| 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
Halpern & Hadi Salim Standards Track [Page 39]
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+ 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.
4. Model and Schema for LFB Classes
The main goal of the FE model is to provide an abstract, generic,
modular, implementation-independent representation of the FEs. This
is facilitated using the concept of LFBs, which are instantiated from
LFB classes. LFB classes and associated definitions will be provided
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in a collection of XML documents. The collection of these XML
documents is called an LFB class library, and each document is called
an LFB class library document (or library document, for short). Each
of the library documents MUST conform to the schema presented in this
section. The schema here and the rules for conforming to the schema
are those defined by the W3C in the definitions of XML schema in XML
schema [Schema1] and XML schema DataTypes [Schema2]. The root
element of the library document is the element.
It is not expected that library documents will be exchanged between
FEs and CEs "over-the-wire". But the model will serve as an
important reference for the design and development of the CEs
(software) and FEs (mostly the software part). It will also serve as
a design input when specifying the ForCES protocol elements for CE-FE
communication.
The following sections describe the portions of an LFBLibrary XML
document. The descriptions primarily provide the necessary semantic
information to understand the meaning and uses of the XML elements.
The XML schema below provides the final definition on what elements
are permitted, and their base syntax. Unfortunately, due to the
limitations of English and XML, there are constraints described in
the semantic sections that are not fully captured in the XML schema,
so both sets of information need to be used to build a compliant
library document.
4.1. Namespace
A namespace is needed to uniquely identify the LFB type in the LFB
class library. The reference to the namespace definition is
contained in Section 9, IANA Considerations.
4.2. Element
The element serves as a root element of all library
documents. A library document contains a sequence of top-level
elements. The following is a list of all the elements that can occur
directly in the element. If they occur, they must occur
in the order listed.
o providing a text description of the purpose of the
library document,
o for loading information from other library documents,
o for the frame declarations,
Halpern & Hadi Salim Standards Track [Page 42]
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o for defining common data types,
o for defining metadata, and
o for defining LFB classes.
Each element is optional. One library document may contain only
metadata definitions, another may contain only LFB class definitions,
and yet another may contain all of the above.
A library document can import other library documents if it needs to
refer to definitions contained in the included document. This
concept is similar to the "#include" directive in the C programming
language. Importing is expressed by the use of elements,
which must precede all the above elements in the document. For
unique referencing, each LFBLibrary instance document has a unique
label defined in the "provide" attribute of the LFBLibrary element.
Note that what this performs is a ForCES inclusion, not an XML
inclusion. The semantic content of the library referenced by the
element is included, not the xml content. Also, in terms of
the conceptual processing of elements, the total set of
documents loaded is considered to form a single document for
processing. A given document is included in this set only once, even
if it is referenced by elements several times, even from
several different files. As the processing of LFBLibrary information
is not order dependent, the order for processing loaded elements is
up to the implementor, as long as the total effect is as if all of
the information from all the files were available for referencing
when needed. Note that such computer processing of ForCES model
library documents may be helpful for various implementations, but is
not required to define the libraries, or for the actual operation of
the protocol itself.
The following is a skeleton of a library document:
Halpern & Hadi Salim Standards Track [Page 43]
RFC 5812 ForCES FE Model March 2010
...
...
...
...
4.3. Element
This element is used to refer to another LFB library document.
Similar to the "#include" directive in C, this makes the objects
(metadata types, data types, etc.) defined in the referred library
document available for referencing in the current document.
The load element MUST contain the label of the library document to be
included and MAY contain a URL to specify where the library can be
retrieved. The load element can be repeated unlimited times. Below
are three examples for the elements:
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4.4. Element for Frame Type Declarations
Frame names are used in the LFB definition to define the types of
frames the LFB expects at its input port(s) and emits at its output
port(s). The optional element in the library document
contains one or more elements, each declaring one frame
type.
Each frame definition MUST contain a unique name (NMTOKEN) and a
brief synopsis. In addition, an optional detailed description MAY be
provided.
Uniqueness of frame types MUST be ensured among frame types defined
in the same library document and in all directly or indirectly
included library documents.
The following example defines two frame types:
ipv4
IPv4 packet
This frame type refers to an IPv4 packet.
ipv6
IPv6 packet
This frame type refers to an IPv6 packet.
...
4.5. Element for Data Type Definitions
The (optional) element can be used to define commonly
used data types. It contains one or more elements,
each defining a data type with a unique name. Such data types can be
used in several places in the library documents, including:
Halpern & Hadi Salim Standards Track [Page 45]
RFC 5812 ForCES FE Model March 2010
o Defining other data types
o Defining components of LFB classes
This is similar to the concept of having a common header file for
shared data types.
Each element MUST contain a unique name (NMTOKEN), a
brief synopsis, and a type definition element. The name MUST be
unique among all data types defined in the same library document and
in any directly or indirectly included library documents. The
element MAY also include an optional longer
description, for example:
ieeemacaddr
48-bit IEEE MAC address
... type definition ...
ipv4addr
IPv4 address
... type definition ...
...
There are two kinds of data types: atomic and compound. Atomic data
types are appropriate for single-value variables (e.g., integer,
string, byte array).
The following built-in atomic data types are provided, but additional
atomic data types can be defined with the and
elements:
Halpern & Hadi Salim Standards Track [Page 46]
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Meaning
---- -------
char 8-bit signed integer
uchar 8-bit unsigned integer
int16 16-bit signed integer
uint16 16-bit unsigned integer
int32 32-bit signed integer
uint32 32-bit unsigned integer
int64 64-bit signed integer
uint64 64-bit unsigned integer
boolean A true / false value where
0 = false, 1 = true
string[N] A UTF-8 string represented in at most
N octets
string A UTF-8 string without a configured
storage length limit
byte[N] A byte array of N bytes
octetstring[N] A buffer of N octets, which MAY
contain fewer than N octets. Hence
the encoded value will always have
a length.
float32 32-bit IEEE floating point number
float64 64-bit IEEE floating point number
These built-in data types can be readily used to define metadata or
LFB attributes, but can also be used as building blocks when defining
new data types. The boolean data type is defined here because it is
so common, even though it can be built by sub-ranging the uchar data
type, as defined under atomic types (Section 4.5.2).
Compound data types can build on atomic data types and other compound
data types. Compound data types can be defined in one of four ways.
They may be defined as an array of components of some compound or
atomic data type. They may be a structure of named components of
compound or atomic data types (cf. C structures). They may be a
union of named components of compound or atomic data types (cf. C
unions). They may also be defined as augmentations (explained in
Section 4.5.7) of existing compound data types.
Given that the ForCES protocol will be getting and setting component
values, all atomic data types used here must be able to be conveyed
in the ForCES protocol. Further, the ForCES protocol will need a
mechanism to convey compound data types. However, the details of
such representations are for the ForCES protocol [RFC5810] document
to define, not the model document. Strings and octetstrings must be
conveyed by the protocol with their length, as they are not
delimited, the value does not itself include the length, and these
items are variable length.
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With regard to strings, this model defines a small set of
restrictions and definitions on how they are structured. String and
octetstring length limits can be specified in the LFB class
definitions. The component properties for string and octetstring
components also contain actual lengths and length limits. This
duplication of limits is to allow for implementations with smaller
limits than the maximum limits specified in the LFB class definition.
In all cases, these lengths are specified in octets, not in
characters. In terms of protocol operation, as long as the specified
length is within the FE's supported capabilities, the FE stores the
contents of a string exactly as provided by the CE, and returns those
contents when requested. No canonicalization, transformations, or
equivalences are performed by the FE. Components of type string (or
string[n]) MAY be used to hold identifiers for correlation with
components in other LFBs. In such cases, an exact octet for octet
match is used. No equivalences are used by the FE or CE in
performing that matching. The ForCES protocol [RFC5810] does not
perform or require validation of the content of UTF-8 strings.
However, UTF-8 strings SHOULD be encoded in the shortest form to
avoid potential security issues described in [UNICODE]. Any entity
displaying such strings is expected to perform its own validation
(for example, for correct multi-byte characters, and for ensuring
that the string does not end in the middle of a multi-byte sequence).
Specific LFB class definitions MAY restrict the valid contents of a
string as suited to the particular usage (for example, a component
that holds a DNS name would be restricted to hold only octets valid
in such a name). FEs should validate the contents of SET requests
for such restricted components at the time the set is performed, just
as range checks for range-limited components are performed. The
ForCES protocol behavior defines the normative processing for
requests using that protocol.
For the definition of the actual type in the element,
the following elements are available: , , ,
, and .
The predefined type alias is somewhere between the atomic and
compound data types. Alias is used to allow a component inside an
LFB to be an indirect reference to another component inside the same
or a different LFB class or instance. The alias component behaves
like a structure, one component of which has special behavior. Given
that the special behavior is tied to the other parts of the
structure, the compound result is treated as a predefined construct.
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4.5.1. Element for Renaming Existing Data Types
The element refers to an existing data type by its name.
The referred data type MUST be defined either in the same library
document or in one of the included library documents. If the
referred data type is an atomic data type, the newly defined type
will also be regarded as atomic. If the referred data type is a
compound type, the new type will also be compound. Some usage
examples follow:
short
Alias to int16
int16
ieeemacaddr
48-bit IEEE MAC address
byte[6]
4.5.2. Element for Deriving New Atomic Types
The element allows the definition of a new atomic type from
an existing atomic type, applying range restrictions and/or providing
special enumerated values. Note that the element can only
use atomic types as base types, and its result MUST be another atomic
type.
For example, the following snippet defines a new "dscp" data type:
dscp
Diffserv code point.
uchar
DSCP-BE
Best Effort
...
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4.5.3. Element to Define Arrays
The element can be used to create a new compound data type as
an array of a compound or an atomic data type. Depending upon
context, this document and others refer to such arrays as tables or
arrays interchangeably, without semantic or syntactic implication.
The type of the array entry can be specified either by referring to
an existing type (using the element) or defining an unnamed
type inside the element using any of the , ,
, or elements.
The array can be "fixed-size" or "variable-size", which is specified
by the "type" attribute of the element. The default is
"variable-size". For variable-size arrays, an optional "maxlength"
attribute specifies the maximum allowed length. This attribute
should be used to encode semantic limitations, not implementation
limitations. The latter (support for implementation constraints)
should be handled by capability components of LFB classes, and should
never be included as the maxlength in a data type array that is
regarded as being of unlimited size.
For fixed-size arrays, a "length" attribute MUST be provided that
specifies the constant size of the array.
The result of this construct is always a compound type, even if the
array has a fixed size of 1.
Arrays MUST only be subscripted by integers, and will be presumed to
start with index 0.
In addition to their subscripts, arrays MAY be declared to have
content keys. Such a declaration has several effects:
o Any declared key can be used in the ForCES protocol to select a
component for operations (for details, see the ForCES protocol
[RFC5810]).
o In any instance of the array, each declared key MUST be unique
within that instance. That is, no two components of an array may
have the same values on all the fields that make up a key.
Each key is declared with a keyID for use in the ForCES protocol
[RFC5810], where the unique key is formed by combining one or more
specified key fields. To support the case where an array of an
atomic type with unique values can be referenced by those values, the
key field identifier MAY be "*" (i.e., the array entry is the key).
If the value type of the array is a structure or an array, then the
key is one or more components of the value type, each identified by
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name. Since the field MAY be a component of the contained structure,
a component of a component of a structure, or further nested, the
field name is actually a concatenated sequence of component
identifiers, separated by decimal points ("."). The syntax for key
field identification is given following the array examples.
The following example shows the definition of a fixed-size array with
a predefined data type as the array content type:
dscp-mapping-table
A table of 64 DSCP values, used to re-map code space.
dscp
The following example defines a variable-size array with an upper
limit on its size:
mac-alias-table
A table with up to 8 IEEE MAC addresses
ieeemacaddr
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The following example shows the definition of an array with a local
(unnamed) content type definition:
classification-table
A table of classification rules and result opcodes.
rule
The rule to match
classrule
opcode
The result code
opcode
In the above example, each entry of the array is a of two
components ("rule" and "opcode").
The following example shows a table of IP prefix information that can
be accessed by a multi-field content key on the IP address, prefix
length, and information source. This means that in any instance of
this table, no two entries can have the same IP address, prefix
length, and information source.
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ipPrefixInfo_table
A table of information about known prefixes
address-prefix
the prefix being described
ipv4Prefix
source
the protocol or process providing this information
uint16
prefInfo
the information we care about
hypothetical-info-type
address-prefix.ipv4addr
address-prefix.prefixlen
source
Note that the keyField elements could also have been simply address-
prefix and source, since all of the fields of address-prefix are
being used.
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4.5.3.1. Key Field References
In order to use key declarations, one must refer to components that
are potentially nested inside other components in the array. If
there are nested arrays, one might even use an array element as a key
(but great care would be needed to ensure uniqueness).
The key is the combination of the values of each field declared in a
keyField element.
Therefore, the value of a keyField element MUST be a concatenated
sequence of field identifiers, separated by a "." (period) character.
Whitespace is permitted and ignored.
A valid string for a single field identifier within a keyField
depends upon the current context. Initially, in an array key
declaration, the context is the type of the array. Progressively,
the context is whatever type is selected by the field identifiers
processed so far in the current key field declaration.
When the current context is an array (e.g., when declaring a key for
an array whose content is an array), then the only valid value for
the field identifier is an explicit number.
When the current context is a structure, the valid values for the
field identifiers are the names of the components of the structure.
In the special case of declaring a key for an array containing an
atomic type, where that content is unique and is to be used as a key,
the value "*" MUST be used as the single key field identifier.
In reference array or structure elements, it is possible to construct
keyFields that do not exist. keyField references SHOULD never
reference optional structure components. For references to array
elements, care must be taken to ensure that the necessary array
elements exist when creating or modifying the overall array element.
Failure to do so will result in FEs returning errors on the creation
attempt.
4.5.4. Element to Define Structures
A structure is composed of a collection of data components. Each
data component has a data type (either an atomic type or an existing
compound type) and is assigned a name unique within the scope of the
compound data type being defined. These serve the same function as
"struct" in C, etc. These components are defined using
elements. A element MAY contain an optional derivation
indication, a element. The structure definition MUST
contain a sequence of one or more elements.
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The actual type of the component can be defined by referring to an
existing type (using the element), or can be a locally
defined (unnamed) type created by any of the , ,
, or elements.
The element MUST include a componentID attribute. This
provides the numeric ID for this component, for use by the protocol.
The MUST contain a component name and a synopsis. It MAY
contain a element giving a textual description of the
component. The definition MAY also include an element,
which indicates that the component being defined is optional. The
definition MUST contain elements to define the data type of the
component, as described above.
For a dataTypeDef of a struct, the structure definition MAY be
inherited from, and augment, a previously defined structured type.
This is indicated by including the optional derivedFrom attribute in
the struct declaration before the definition of the augmenting or
replacing components. Section 4.5.7 describes how this is done in
more detail.
The componentID attribute for different components in a structure (or
in an LFB) MUST be distinct. They do not need to be in order, nor do
they need to be sequential. For clarity of human readability, and
ease of maintenance, it is usual to define at least sequential sets
of values. But this is for human ease, not a model or protocol
requirement.
The result of this construct is always a compound type, even when the
contains only one field.
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An example is the following:
ipv4prefix
IPv4 prefix defined by an address and a prefix length
address
Address part
ipv4addr
prefixlen
Prefix length part
uchar
4.5.5. Element to Define Union Types
Similar to the union declaration in C, this construct allows the
definition of overlay types. Its format is identical to the
element.
The result of this construct is always a compound type, even when the
union contains only one element.
4.5.6. Element
It is sometimes necessary to have a component in an LFB or structure
refer to information (a component) in other LFBs. This can, for
example, allow an ARP LFB to share the IP->MAC Address table with the
local transmission LFB, without duplicating information. Similarly,
it could allow a traffic measurement LFB to share information with a
traffic enforcement LFB. The declaration creates the
constructs for this. This construct tells the CE and FE that any
manipulation of the defined data is actually manipulation of data
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defined to exist in some specified part of some other LFB instance.
The content of an element MUST be a named type. Whatever
component the alias references (which is determined by the alias
component properties, as described below), that component must be of
the same type as that declared for the alias. Thus, when the CE or
FE dereferences the alias component, the type of the information
returned is known. The type can be a base type or a derived type.
The actual value referenced by an alias is known as its target. When
a GET or SET operation references the alias element, the value of the
target is returned or replaced. Write access to an alias element is
permitted if write access to both the alias and the target is
permitted.
The target of a component declared by an element is
determined by the information in the component's properties. Like
all components, the properties include the support / read / write
permission for the alias. In addition, there are several fields
(components) in the alias properties that define the target of the
alias. These components are the ID of the LFB class of the target,
the ID of the LFB instance of the target, and a sequence of integers
representing the path within the target LFB instance to the target
component. The type of the target element must match the declared
type of the alias. Details of the alias property structure are
described in Section 4.8 of this document, on properties.
Note that the read / write property of the alias refers to the value.
The CE can only determine if it can write the target selection
properties of the alias by attempting such a write operation.
(Property components do not themselves have properties.)
4.5.7. Augmentations
Compound types can also be defined as augmentations of existing
compound types. If the existing compound type is a structure,
augmentation MAY add new elements to the type. The type of an
existing component MAY be replaced in the definition of an augmenting
structure, but MAY only be replaced with an augmentation derived from
the current type of the existing component. An existing component
cannot be deleted. If the existing compound type is an array,
augmentation means augmentation of the array element type.
Augmentation MUST NOT be applied to unions.
One consequence of this is that augmentations are backward compatible
with the compound type from which they are derived. As such,
augmentations are useful in defining components for LFB subclasses
with backward compatibility. In addition to adding new components to
a class, the data type of an existing component MAY be replaced by an
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augmentation of that component, and still meet the compatibility
rules for subclasses. This compatibility constraint is why
augmentations cannot be applied to unions.
For example, consider a simple base LFB class A that has only one
component (comp1) of type X. One way to derive class A1 from A can
be by simply adding a second component (of any type). Another way to
derive a class A2 from A can be by replacing the original component
(comp1) in A of type X with one of type Y, where Y is an augmentation
of X. Both classes A1 and A2 are backward compatible with class A.
The syntax for augmentations is to include a element in
a structure definition, indicating what structure type is being
augmented. Component names and component IDs for new components
within the augmentation MUST NOT be the same as those in the
structure type being augmented. For those components where the data
type of an existing component is being replaced with a suitable
augmenting data type, the existing component name and component ID
MUST be used in the augmentation. Other than the constraint on
existing elements, there is no requirement that the new component IDs
be sequential with, greater than, or in any other specific
relationship to the existing component IDs except different. It is
expected that using values sequential within an augmentation, and
distinct from the previously used values, will be a common method to
enhance human readability.
4.6. Element for Metadata Definitions
The (optional) element in the library document
contains one or more elements. Each
element defines a metadatum.
Each element MUST contain a unique name (NMTOKEN).
Uniqueness is defined to be over all metadata defined in this library
document and in all directly or indirectly included library
documents. The element MUST also contain a brief
synopsis, the tag value to be used for this metadata, and value type
definition information. Only atomic data types can be used as value
types for metadata. The element MAY contain a detailed
description element.
Two forms of type definitions are allowed. The first form uses the
element to refer to an existing atomic data type defined in
the element of the same library document or in one of
the included library documents. The usage of the element
is identical to how it is used in the elements, except
here it can only refer to atomic types. The latter restriction is
not enforced by the XML schema.
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The second form is an explicit type definition using the
element. This element is used here in the same way as in the
elements.
The following example shows both usages:
NEXTHOPID
Refers to a Next Hop entry in NH LFB
17
int32
CLASSID
Result of classification (0 means no match).
21
int32
NOMATCH
Classification didn't result in match.
4.7. Element for LFB Class Definitions
The (optional) element can be used to define one or
more LFB classes using elements. Each
element MUST define an LFB class and include the following elements:
o provides the symbolic name of the LFB class. Example:
"ipv4lpm".
o provides a short synopsis of the LFB class. Example:
"IPv4 Longest Prefix Match Lookup LFB".
o is the version indicator.
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o is the inheritance indicator.
o lists the input ports and their specifications.
o lists the output ports and their specifications.
o defines the operational components of the LFB.
o defines the capability components of the LFB.
o contains the operational specification of the LFB.
o The LFBClassID attribute of the LFBClassDef element defines the ID
for this class. These must be globally unique.
o defines the events that can be generated by instances of
this LFB.
LFB class names must be unique, in order to enable other documents to
reference the classes by name, and to enable human readers to
understand references to class names. While a complex naming
structure could be created, simplicity is preferred. As given in the
IANA Considerations section of this document, the IANA maintains a
registry of LFB class names and class identifiers, along with a
reference to the document defining the class.
Below is a skeleton of an example LFB class definition. Note that in
order to keep from complicating the XML schema, the order of elements
in the class definition is fixed. Elements, if they appear, must
appear in the order shown.
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ipv4lpm
IPv4 Longest Prefix Match Lookup LFB
1.0
baseclass
...
...
...
...
...
This LFB represents the IPv4 longest prefix match lookup
operation.
The modeled behavior is as follows:
Blah-blah-blah.
...
The individual components and capabilities will have componentIDs for
use by the ForCES protocol. These parallel the componentIDs used in
structs, and are used the same way. Component and capability
componentIDs must be unique within the LFB class definition.
Note that the , , and elements are
required; all other elements are optional in . However,
when they are present, they must occur in the above order.
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The componentID attribute for different items in an LFB class
definition (or components in a struct) MUST be distinct. They do not
need to be in order, nor do they need to be sequential. For clarity
of human readability, and ease of maintenance, it is usual to define
at least sequential sets of values. But this is for human ease, not
a model or protocol requirement.
4.7.1. Element to Express LFB Inheritance
The optional element can be used to indicate that this
class is a derivative of some other class. The content of this
element MUST be the unique name () of another LFB class. The
referred LFB class MUST be defined in the same library document or in
one of the included library documents. In the absence of a
, the class is conceptually derived from the common,
empty, base class.
It is assumed that a derived class is backward compatible with its
base class. A derived class MAY add components to a parent class,
but cannot delete components. This also applies to input and output
ports, events, and capabilities.
4.7.2. Element to Define LFB Inputs
The optional element is used to define input ports. An
LFB class MAY have zero, one, or more inputs. If the LFB class has
no input ports, the element MUST be omitted. The
element can contain one or more elements,
one for each port or port group. We assume that most LFBs will have
exactly one input. Multiple inputs with the same input type are
modeled as one input group. Input groups are defined the same way as
input ports by the element, differentiated only by an
optional "group" attribute.
Multiple inputs with different input types should be avoided if
possible (see discussion in Section 4.7.3). Some special LFBs will
have no inputs at all. For example, a packet generator LFB does not
need an input.
Single input ports and input port groups are both defined by the
element; they are differentiated only by an optional
"group" attribute.
The element MUST contain the following elements:
o provides the symbolic name of the input. Example: "in".
Note that this symbolic name must be unique only within the scope
of the LFB class.
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o contains a brief description of the input. Example:
"Normal packet input".
o lists all allowed frame formats. Example: {"ipv4"
and "ipv6"}. Note that this list should refer to names specified
in the element of the same library document or in any
included library documents. The element can also
provide a list of required metadata. Example: {"classid",
"vpnid"}. This list should refer to names of metadata defined in
the element in the same library document or in any
included library documents. For each metadatum, it must be
specified whether the metadatum is required or optional. For each
optional metadatum, a default value must be specified, which is
used by the LFB if the metadatum is not provided with a packet.
In addition, the optional "group" attribute of the
element can specify if the port can behave as a port group, i.e., it
is allowed to be instantiated. This is indicated by a "true" value
(the default value is "false").
An example element, defining two input ports, the second
one being an input port group is the following:
in
Normal input
[ipv4]
[ipv6]
[classid]
[vifid]
[vrfid]
... another input port ...
For each , the frame type expectations are defined by the
element using one or more [ elements (see example
above). When multiple frame types are listed, it means that "one of
these" frame types is expected. A packet of any other frame type is
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regarded as incompatible with this input port of the LFB class. The
above example lists two frames as expected frame types: "ipv4" and
"ipv6".
Metadata expectations are specified by the
element. In its simplest form, this element can contain a list of
][ elements, each referring to a metadatum. When multiple
instances of metadata are listed by ][ elements, it means that
"all of these" metadata must be received with each packet (except
metadata that are marked as "optional" by the "dependency" attribute
of the corresponding ][ element). For a metadatum that is
specified "optional", a default value MUST be provided using the
"defaultValue" attribute. The above example lists three metadata as
expected metadata, two of which are mandatory ("classid" and
"vifid"), and one being optional ("vrfid").
The schema also allows for more complex definitions of metadata
expectations. For example, using the element, a list of
metadata can be specified to express that at least one of the
specified metadata must be present with any packet. An example is
the following:
][prefixmask]
[prefixlen]
The above example specifies that either the "prefixmask" or the
"prefixlen" metadata must be provided with any packet.
The two forms can also be combined, as shown in the following
example:
[classid]
[vifid]
[vrfid]
[prefixmask]
[prefixlen]
Although the schema is constructed to allow even more complex
definitions of metadata expectations, we do not discuss those here.
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4.7.3. Element to Define LFB Outputs
The optional element is used to define output ports.
An LFB class MAY have zero, one, or more outputs. If the LFB class
has no output ports, the element MUST be omitted. The
element MUST contain one or more elements,
one for each port or port group. If there are multiple outputs with
the same output type, we model them as an output port group. Some
special LFBs have no outputs at all (e.g., Dropper).
Single output ports and output port groups are both defined by the
element; they are differentiated only by an optional
"group" attribute.
The element MUST contain the following elements:
o provides the symbolic name of the output. Example: "out".
Note that the symbolic name must be unique only within the scope
of the LFB class.
o contains a brief description of the output port.
Example: "Normal packet output".
o lists the allowed frame formats. Example: {"ipv4",
"ipv6"}. Note that this list should refer to symbols specified in
the element in the same library document or in any
included library documents. The element MAY also
contain the list of emitted (generated) metadata. Example:
{"classid", "color"}. This list should refer to names of metadata
specified in the element in the same library
document or in any included library documents. For each generated
metadatum, it should be specified whether the metadatum is always
generated or generated only in certain conditions. This
information is important when assessing compatibility between
LFBs.
In addition, the optional "group" attribute of the
element can specify if the port can behave as a port group, i.e., it
is allowed to be instantiated. This is indicated by a "true" value
(the default value is "false").
The following example specifies two output ports, the second being an
output port group:
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out
Normal output
[ipv4]
[ipv4bis]
[nhid]
[nhtabid]
exc
Exception output port group
[ipv4]
[ipv4bis]
[errorid]
The types of frames and metadata the port produces are defined inside
the element in each . Within the
element, the list of frame types the port produces is listed in the
element. When more than one frame is listed, it
means that "one of" these frames will be produced.
The list of metadata that is produced with each packet is listed in
the optional element of the . In its
simplest form, this element can contain a list of [ elements,
each referring to a metadatum type. The meaning of such a list is
that "all of" these metadata are provided with each packet, except
those that are listed with the optional "availability" attribute set
to "conditional". Similar to the element of the
, the element supports more complex
forms, which we do not discuss here further.
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4.7.4. Element to Define LFB Operational Components
Operational parameters of the LFBs that must be visible to the CEs
are conceptualized in the model as the LFB components. These
include, for example, flags, single parameter arguments, complex
arguments, and tables. Note that the components here refer to only
those operational parameters of the LFBs that must be visible to the
CEs. Other variables that are internal to LFB implementation are not
regarded as LFB components and hence are not covered.
Some examples for LFB components are:
o Configurable flags and switches selecting between operational
modes of the LFB
o Number of inputs or outputs in a port group
o Various configurable lookup tables, including interface tables,
prefix tables, classification tables, DSCP mapping tables, MAC
address tables, etc.
o Packet and byte counters
o Various event counters
o Number of current inputs or outputs for each input or output group
The ForCES model supports the definition of access permission
restrictions on what the CE can do with an LFB component. The
following categories are supported by the model:
o No-access components. This is useful for completeness, and to
allow for defining objects that are used by other things, but not
directly referencable by the CE. It is also useful for an FE that
is reporting that certain defined, and typically accessible,
components are not supported for CE access by a reporting FE.
o Read-only components.
o Read-write components.
o Write-only components. This could be any configurable data for
which read capability is not provided to the CEs (e.g., the
security key information).
o Read-reset components. The CE can read and reset this resource,
but cannot set it to an arbitrary value. Example: Counters.
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o Firing-only components. A write attempt to this resource will
trigger some specific actions in the LFB, but the actual value
written is ignored.
The LFB class MUST define only one possible access mode for a given
component.
The components of the LFB class are listed in the
element. Each component is defined by an element. A
element contains some or all of the following elements,
some of which are mandatory:
o MUST occur, and defines the name of the component. This
name must be unique among the components of the LFB class.
Example: "version".
o is also mandatory, and provides a brief description of
the purpose of the component.
o is an optional element, and if present indicates that
this component is optional.
o The data type of the component can be defined either via a
reference to a predefined data type or by providing a local
definition of the type. The former is provided by using the
element, which must refer to the unique name of an
existing data type defined in the element in the
same library document or in any of the included library documents.
When the data type is defined locally (unnamed type), one of the
following elements can be used: , , , or
. Their usage is identical to how they are used inside
elements (see Section 4.5). Some form of data type
definition MUST be included in the component definition.
o The element is optional, and if present is used to
specify a default value for a component. If a default value is
specified, the FE must ensure that the component has that value
when the LFB is initialized or reset. If a default value is not
specified for a component, the CE MUST make no assumptions as to
what the value of the component will be upon initialization. The
CE must either read the value or set the value, if it needs to
know what it is.
o The element MAY also appear. If included, it
provides a longer description of the meaning or usage of the
particular component being defined.
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The element also MUST have a componentID attribute, which
is a numeric value used by the ForCES protocol.
In addition to the above elements, the element includes
an optional "access" attribute, which can take any of the following
values: "read-only", "read-write", "write-only", "read-reset", and
"trigger-only". The default access mode is "read-write".
Whether optional components are supported, and whether components
defined as read-write can actually be written, can be determined for
a given LFB instance by the CE by reading the property information of
that component. An access control setting of "trigger-only" means
that this component is included only for use in event detection.
The following example defines two components for an LFB:
foo
number of things
uint32
bar
number of this other thing
uint32
10
The first component ("foo") is a read-only 32-bit unsigned integer,
defined by referring to the built-in "uint32" atomic type. The
second component ("bar") is also an integer, but uses the
element to provide additional range restrictions. This component has
access mode of read-write allowing it to be both read and written. A
default value of 10 is provided for bar. Although the access for bar
is read-write, some implementations MAY offer only more restrictive
access, and this would be reported in the component properties.
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Note that not all components are likely to exist at all times in a
particular implementation. While the capabilities will frequently
indicate this non-existence, CEs may attempt to reference non-
existent or non-permitted components anyway. The ForCES protocol
mechanisms should include appropriate error indicators for this case.
The mechanism defined above for non-supported components can also
apply to attempts to reference non-existent array elements or to set
read-only components.
4.7.5. Element to Define LFB Capability Components
The LFB class specification provides some flexibility for the FE
implementation regarding how the LFB class is implemented. For
example, the instance may have some limitations that are not inherent
from the class definition, but rather the result of some
implementation limitations. Some of these limitations are captured
by the property information of the LFB components. The model allows
for the notion of additional capability information.
Such capability-related information is expressed by the capability
components of the LFB class. The capability components are always
read-only attributes, and they are listed in a separate
element in the . The
element contains one or more elements, each defining one
capability component. The format of the element is
almost the same as the element. It differs in two
aspects: it lacks the access mode attribute (because it is always
read-only), and it lacks the element (because default
value is not applicable to read-only attributes).
Some examples of capability components follow:
o The version of the LFB class with which this LFB instance complies
o Supported optional features of the LFB class
o Maximum number of configurable outputs for an output group
o Metadata pass-through limitations of the LFB
o Additional range restriction on operational components
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The following example lists two capability attributes:
version
LFB class version this instance is compliant with.
version
limitBar
Maximum value of the "bar" attribute.
uint16
4.7.6. Element for LFB Notification Generation
The element contains the information about the occurrences
for which instances of this LFB class can generate notifications to
the CE. High-level view on the declaration and operation of LFB
events is described in Section 3.2.5.
The element contains 0 or more elements, each of
which declares a single event. The element has an eventID
attribute giving the unique (per LFB class) ID of the event. The
element will include:
o element indicating which LFB field (component) is
tested to generate the event.
o element indicating what condition on the field will
generate the event from a list of defined conditions.
o element indicating what values are to be reported
in the notification of the event.
The example below demonstrates the different constructs.
The element has a baseID attribute value, which is normally
. The value of the baseID is the starting
componentID for the path that identifies events. It must not be the
same as the componentID of any top-level components (including
capabilities) of the LFB class. In derived LFBs (i.e., ones with a
element) where the parent LFB class has an events
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declaration, the baseID must not be present in the derived LFB
element. Instead, the baseID value from the parent LFB
class is used. In the example shown, the baseID is 7.
Foochanged
An example event for a scalar
foo
foo
Goof1changed
An example event for a complex structure
goo
f1
goo
f1
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NewbarEntry
Event for a new entry created on table bar
bar
_barIndex_
bar
_barIndex_
foo
Gah11changed
Event for table gah, entry index 11 changing
gah
11
gah
11
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Gah10field1
Event for table gah, entry index 10, column field1 changing
gah
10
field1
gah
10
4.7.6.1. Element
The element contains information identifying a field in
the LFB that is to be monitored for events.
The element contains one or more s each of
which MAY be followed by one or more elements. Each
of these two elements represents the textual equivalent of a path
select component of the LFB.
The element contains the name of a component in the LFB
or a component nested in an array or structure within the LFB. The
name used in MUST identify a valid component within the
containing LFB context. The first element in an MUST
be an element. In the example shown, four LFB
components foo, goo, bar, and gah are used as s.
In the simple case, an identifies an atomic component.
This is the case illustrated in the event named Foochanged.
is also used to address complex components such as
arrays or structures.
The first defined event, Foochanged, demonstrates how a scalar LFB
component, foo, could be monitored to trigger an event.
The second event, Goof1changed, demonstrates how a member of the
complex structure goo could be monitored to trigger an event.
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The events named NewbarEntry, Gah11changed, and Gah10field1
represent monitoring of arrays bar and gah in differing details.
If an identifies a complex component, then a further
MAY be used to refine the path to the target element.
Defined event Goof1changed demonstrates how a second is
used to point to member f1 of the structure goo.
If an identifies an array, then the following rules
apply:
o elements MUST be present as the next XML element
after an that identifies an array component.
MUST NOT occur other than after an array
reference, as it is only meaningful in that context.
o An contains either:
* A numeric value to indicate that the event applies to a
specific entry (by index) of the array. As an example, event
Gah11changed shows how table gah's index 11 is being targeted
for monitoring.
Or
* It is expected that the more common usage is to have the event
being defined across all elements of the array (i.e., a
wildcard for all indices). In that case, the value of the
MUST be a name rather than a numeric value.
That same name can then be used as the value of
in elements as described below.
An example of a wild card table index is shown in event
NewBarentry where the value is named
_barIndex_
o An MAY follow an to further refine
the path to the target element. (Note: this is in the same spirit
as the case where is used to further refine
in the earlier example of a complex structure example
of Goof1changed.) The example event Gah10field1 illustrates how
the column field1 of table gah is monitored for changes.
It should be emphasized that the name in an element
in defined event NewbarEntry is not a component name. It is a
variable name for use in the elements (described in
Section 4.7.6.3) of the given LFB definition. This name MUST be
distinct from any component name that can validly occur in the
clause.
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4.7.6.2. Element
The event condition element represents a condition that triggers a
notification. The list of conditions is:
: The target must be an array, ending with a
subscript indication. The event is generated when
an entry in the array is created. This occurs even
if the entry is created by CE direction. The event
example NewbarEntry demonstrates the
condition.
: The target must be an array, ending with a
subscript indication. The event is generated when
an entry in the array is destroyed. This occurs
even if the entry is destroyed by CE direction.
: The event is generated whenever the target
component changes in any way. For binary
components such as up/down, this reflects a change
in state. It can also be used with numeric
attributes, in which case any change in value
results in a detected trigger. Event examples
Foochanged, Gah11changed, and Gah10field1
illustrate the condition.
: The event is generated whenever the target
component becomes greater than the threshold.
The threshold is an event property.
: The event is generated whenever the target
component becomes less than the threshold. The
threshold is an event property.
4.7.6.3. Element
The element of an declares the information to
be delivered by the FE along with the notification of the occurrence
of the event.
The element contains one or more
elements. Each element identifies a piece of data from
the LFB class to be reported. The notification carries that data as
if the collection of elements had been defined in a
structure. The syntax is exactly the same as used in the
element, using and ]