Internet Draft L. Yang
Expiration: August 2005 Intel Corp.
File: draft-ietf-forces-model-05.txt J. Halpern
Working Group: ForCES Megisto Systems
R. Gopal
Nokia
A. DeKok
Infoblox, Inc.
Z. Haraszti
Clovis Solutions
S. Blake
Modular Networks
E. Deleganes
Intel Corp.
August 2005
ForCES Forwarding Element Model
draft-ietf-forces-model-05.txt
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Internet Draft ForCES FE Model August 2005
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 the
ForCES requirements draft, RFC 3564 [1]. A list of the basic
logical functional blocks (LFBs) is also defined in the LFB class
library to aid the effort in defining individual LFBs.
Table of Contents
Abstract...........................................................2
1. Definitions.....................................................4
2. Introduction....................................................6
2.1. Requirements on the FE model...............................6
2.2. The FE Model in Relation to FE Implementations.............7
2.3. The FE Model in Relation to the ForCES Protocol............7
2.4. Modeling Language for the FE Model.........................8
2.5. Document Structure.........................................8
3. FE Model Concepts...............................................8
3.1. FE Capability Model and State Model........................9
3.2. LFB (Logical Functional Block) Modeling...................11
3.2.1. LFB Outputs..........................................14
3.2.2. LFB Inputs...........................................17
3.2.3. Packet Type..........................................20
3.2.4. Metadata.............................................21
3.2.5. LFB Events...........................................28
3.2.6. LFB Element Properties...............................28
3.2.7. LFB Versioning.......................................28
3.2.8. LFB Inheritance......................................29
3.3. FE Datapath Modeling......................................30
3.3.1. Alternative Approaches for Modeling FE Datapaths.....30
3.3.2. Configuring the LFB Topology.........................35
4. Model and Schema for LFB Classes...............................39
4.1. Namespace.................................................39
4.2. <LFBLibrary> Element......................................39
4.3. <load> Element............................................41
4.4. <frameDefs> Element for Frame Type Declarations...........41
4.5. <dataTypeDefs> Element for Data Type Definitions..........42
4.5.1. <typeRef> Element for Aliasing Existing Data Types...44
4.5.2. <atomic> Element for Deriving New Atomic Types.......45
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4.5.3. <array> Element to Define Arrays.....................45
4.5.4. <struct> Element to Define Structures................49
4.5.5. <union> Element to Define Union Types................50
4.5.6. Augmentations........................................51
4.6. <metadataDefs> Element for Metadata Definitions...........52
4.7. <LFBClassDefs> Element for LFB Class Definitions..........53
4.7.1. <derivedFrom> Element to Express LFB Inheritance.....54
4.7.2. <inputPorts> Element to Define LFB Inputs............55
4.7.3. <outputPorts> Element to Define LFB Outputs..........57
4.7.4. <attributes> Element to Define LFB Operational
Attributes..................................................59
4.7.5. <capabilities> Element to Define LFB Capability
Attributes..................................................62
4.7.6. <events> Element for LFB Notification Generation.....63
4.7.7. <description> Element for LFB Operational
Specification...............................................67
4.8. Properties................................................67
4.9. XML Schema for LFB Class Library Documents................70
5. FE Attributes and Capabilities.................................81
5.1. XML for FEObject Class definition.........................82
5.2. FE Capabilities...........................................88
5.2.1. ModifiableLFBTopology................................89
5.2.2. SupportedLFBs and SupportedLFBType...................89
5.3. FEAttributes..............................................91
5.3.1. FEStatus.............................................91
5.3.2. LFBSelectors and LFBSelectorType.....................91
5.3.3. LFBTopology and LFBLinkType..........................92
5.3.4. FENeighbors an FEConfiguredNeighborType..............92
6. Satisfying the Requirements on FE Model........................93
6.1. Port Functions............................................94
6.2. Forwarding Functions......................................94
6.3. QoS Functions.............................................95
6.4. Generic Filtering Functions...............................95
6.5. Vendor Specific Functions.................................95
6.6. High-Touch Functions......................................95
6.7. Security Functions........................................95
6.8. Off-loaded Functions......................................96
6.9. IPFLOW/PSAMP Functions....................................96
7. Using the FE model in the ForCES Protocol......................96
7.1. FE Topology Query.........................................98
7.2. FE Capability Declarations...............................100
7.3. LFB Topology and Topology Configurability Query..........100
7.4. LFB Capability Declarations..............................100
7.5. State Query of LFB Attributes............................101
7.6. LFB Attribute Manipulation...............................102
7.7. LFB Topology Re-configuration............................102
8. Example.......................................................103
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8.1. Data Handling............................................110
8.1.1. Setting up a DLCI...................................110
8.1.2. Error Handling......................................111
8.2. LFB Attributes...........................................112
8.3. Capabilities.............................................112
8.4. Events...................................................113
9. Acknowledgments...............................................114
10. Security Considerations......................................114
11. Normative References.........................................114
12. Informative References.......................................114
13. Authors' Addresses...........................................115
14. Intellectual Property Right..................................116
15. IANA consideration...........................................116
16. Copyright Statement..........................................116
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC-2119].
1.
Definitions
Terminology associated with the ForCES requirements is defined in
RFC 3564 [1] and is not copied here. The following list of
terminology relevant to the FE model is defined in this section.
FE Model -- The FE model is designed to model the logical
processing functions of an FE. The FE model proposed in this
document includes three components: the modeling of individual
logical functional blocks (LFB model), the logical interconnection
between LFBs (LFB topology) and the FE level attributes, including
FE capabilities. The FE model provides the basis to define the
information elements exchanged between the CE and the FE in the
ForCES protocol.
Datapath -- A conceptual path taken by packets within the
forwarding plane inside an FE. Note that more than one datapath
can exist within an FE.
LFB (Logical Functional Block) Class (or type) -- A template
representing a fine-grained, logically separable and well-defined
packet processing operation in the datapath. LFB classes are the
basic building blocks of the FE model.
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LFB Instance -- As a packet flows through an FE along a datapath,
it flows through one or multiple LFB instances, where each LFB is
an instance of a specific LFB class. Multiple instances of the
same LFB class can be present in an FE's datapath. Note that we
often refer to LFBs without distinguishing between an LFB class
and LFB instance when we believe the implied reference is obvious
for the given context.
LFB Model -- The LFB model describes the content and structures in
an LFB, plus the associated data definition. Four types of
information are defined in the LFB model. The core part of the
LFB model is the LFB class definitions; the other three types
define the associated data including common data types, supported
frame formats and metadata.
LFB Metadata -- Metadata is used to communicate per-packet state
from one LFB to another, but is not sent across the network. The
FE model defines how such metadata is identified, produced and
consumed by the LFBs, but not how the per-packet state is
implemented within actual hardware.
LFB Attribute -- Operational parameters of the LFBs that must be
visible to the CEs are conceptualized in the FE model as the LFB
attributes. The LFB attributes include: flags, single parameter
arguments, complex arguments, and tables that the CE can read
or/and write via the ForCES protocol.
LFB Topology -- A representation of the logical interconnection
and the placement of LFB instances along the datapath within one
FE. Sometimes this representation is called intra-FE topology, to
be distinguished from inter-FE topology. LFB topology is outside
of the LFB model, but is part of the FE model.
FE Topology -- A representation of how multiple FEs within a
single NE are interconnected. Sometimes this is called inter-FE
topology, to be distinguished from intra-FE topology (i.e., LFB
topology). An individual FE might not have the global knowledge
of the full FE topology, but the local view of its connectivity
with other FEs is 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.
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LFB class library -- A set of LFB classes that has been identified
as the most common functions found in most FEs and hence should be
defined first by the ForCES Working Group.
2.
Introduction
RFC 3746 [2] specifies a framework by which control elements (CEs)
can configure and manage one or more separate forwarding elements
(FEs) within a networking element (NE) using the ForCES protocol.
The ForCES architecture allows Forwarding Elements of varying
functionality to participate in a ForCES network element. The
implication of this varying functionality is that CEs can make
only minimal assumptions about the functionality provided by FEs
in an NE. Before CEs can configure and control the forwarding
behavior of FEs, CEs need to query and discover the capabilities
and states of their FEs. RFC 3654 [1] mandates that the
capabilities, states and configuration information be expressed in
the form of an FE model.
RFC 3444 [11] observed that information models (IMs) and data
models (DMs) are different because they serve different purposes.
"The main purpose of an IM is to model managed objects at a
conceptual level, independent of any specific implementations or
protocols used". "DMs, conversely, are defined at a lower level
of abstraction and include many details. They are intended for
implementors and include protocol-specific constructs." Sometimes
it is difficult to draw a clear line between the two. The FE
model described in this document is primarily an information
model, but also includes some aspects of a data model, such as
explicit definitions of the LFB class schema and FE schema. It is
expected that this FE model will be used as the basis to define
the payload for information exchange between the CE and FE in the
ForCES protocol.
2.1. Requirements on the FE model
RFC 3654 [1] defines requirements that must be satisfied by a
ForCES FE model. To summarize, an FE model must define:
. Logically separable and distinct packet forwarding operations
in an FE datapath (logical functional blocks or LFBs);
. The possible topological relationships (and hence the
sequence of packet forwarding operations) between the various
LFBs;
. The possible operational capabilities (e.g., capacity limits,
constraints, optional features, granularity of configuration)
of each type of LFB;
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. The possible configurable parameters (i.e., attributes) of
each type of LFB;
. Metadata that may be exchanged between LFBs.
2.2. The FE Model in Relation to FE Implementations
The FE model proposed here is based on an abstraction of distinct
logical functional blocks (LFBs), which are interconnected in a
directed graph, and receive, process, modify, and transmit packets
along with metadata. The FE model should be designed such that
different implementations of the forwarding datapath can be
logically mapped onto the model with the functionality and
sequence of operations correctly captured. However, the model is
not intended to directly address how a particular implementation
maps to an LFB topology. It is left to the forwarding plane
vendors to define how the FE functionality is represented using
the FE model. Our goal is to design the FE model such that it is
flexible enough to accommodate most common implementations.
The LFB topology model for a particular datapath implementation
MUST correctly capture the sequence of operations on the packet.
Metadata generation by certain LFBs must always precede any use of
that metadata by subsequent LFBs in the topology graph; this is
required for logically consistent operation. Further,
modification of packet fields that are subsequently used as inputs
for further processing must occur in the order specified in the
model for that particular implementation to ensure correctness.
2.3. The FE Model in Relation to the ForCES Protocol
The ForCES base protocol is used by the CEs and FEs to maintain
the communication channel between the CEs and FEs. The ForCES
protocol may be used to query and discover the inter-FE topology.
The details of a particular datapath implementation inside an FE,
including the LFB topology, along with the operational
capabilities and attributes of each individual LFB, are conveyed
to the CE within information elements in the ForCES protocol. The
model of an LFB class should define all of the information that
needs to be exchanged between an FE and a CE for the proper
configuration and management of that LFB.
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 attributes for
describing and manipulating the state of the LFBs within an FE.
These class definitions themselves will generally not appear in
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the ForCES protocol. Rather, ForCES protocol operations will
reference classes defined in this model, including relevant
attributes and the defined operations.
Section 7 provides more detailed discussion on how the FE model
should be used by the ForCES protocol.
2.4. Modeling Language for the FE Model
Even though not absolutely required, it is beneficial to use a
formal data modeling language to represent the conceptual FE model
described in this document. Use of a formal language can help to
enforce consistency and logical compatibility among LFBs. A full
specification will be written using such a data modeling language.
The formal definition of the LFB classes may facilitate the
eventual automation of some of the code generation process and the
functional validation of arbitrary LFB topologies.
Human readability was the most important factor considered when
selecting the specification language, whereas encoding, decoding
and transmission performance was not a selection factor. The
encoding method for over the wire transport is not dependent on
the specification language chosen and is outside the scope of this
document and up to the ForCES protocol to define.
XML was chosen as the specification language in this document,
because XML has the advantage of being both human and machine
readable with widely available tools support.
2.5. Document Structure
Section 3 provides a conceptual overview of the FE model, laying
the foundation for the more detailed discussion and specifications
in the sections that follow. Section 4 and 5 constitute the core
of the FE model, detailing the two major components in the FE
model: LFB model and FE level attributes including capability and
LFB topology. Section 6 directly addresses the model requirements
imposed by the ForCES requirement draft [1] while Section 7
explains how the FE model should be used in the ForCES protocol.
3.
FE Model Concepts
Some of the important concepts used throughout this document are
introduced in this section. Section 3.1 explains the difference
between a state model and a capability model, and describes how
the two can be combined in the FE model. Section 3.2 introduces
the concept of LFBs (Logical Functional Blocks) as the basic
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functional building blocks in the FE model. Section 3.3 discusses
the logical inter-connection and ordering between LFB instances
within an FE, that is, the LFB topology.
The FE model proposed in this document is comprised of two major
components: the LFB model and FE level attributes, including FE
capabilities and LFB topology. The LFB model provides the content
and data structures to define each individual LFB class. FE
attributes provide information at the FE level, particularly the
capabilities of the FE at a coarse level. Part of the FE level
information is the LFB topology, which expresses the logical
inter-connection between the LFB instances along the datapath(s)
within the FE. Details of these components are described in
Section 4 and 5. The intent of this section is to discuss these
concepts at the high level and lay the foundation for the detailed
description in the following sections.
3.1. FE Capability Model and State Model
The ForCES FE model must describe both a capability and a state
model. The FE capability model describes the capabilities and
capacities of an FE by specifying the variation in functions
supported and any limitations. The FE state model describes the
current state of the FE, that is, the instantaneous values or
operational behavior of the FE.
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. For example, an FE capability model may describe the FE at a
coarse level such as:
. this FE can handle IPv4 and IPv6 forwarding;
. this FE can perform classification on the following fields:
source IP address, destination IP address, source port
number, destination port number, etc;
. this FE can perform metering;
. this FE can handle up to N queues (capacity);
. this FE can add and remove encapsulating headers of types
including IPSec, GRE, L2TP.
While one could try and build an object model to fully represent
the FE capabilities, other efforts found this to be a significant
undertaking. The main difficulty arises in describing detailed
limits, such as the maximum number of classifiers, queues, buffer
pools, and meters the FE can provide. We believe that a good
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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 [4] and
Framework PIB [5].
One common and shared aspect of capability will be handled in a
separate fashion. For all elements of information, certain
property information is needed. All elements need information as
to whether they are supported and if so whether the element is
readable or writeable. Based on their type, many elements have
additional common properties (for example, arrays have their
current size.) There is a specific model and protocol mechanism
for referencing this form of property information about elements
of the model.
The FE state model presents the snapshot view of the FE to the CE.
For example, using an FE state model, an FE may be described to
its corresponding CE as the following:
. on a given port, the packets are classified using a given
classification filter;
. the given classifier results in packets being metered in a
certain way, and then marked in a certain way;
. the packets coming from specific markers are delivered into a
shared queue for handling, while other packets are delivered
to a different queue;
. a specific scheduler with specific behavior and parameters
will service these collected queues.
Figure 1 shows the concepts of FE state, capabilities and
configuration in the context of CE-FE communication via the ForCES
protocol.
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+-------+ +-------+
| | FE capabilities: what it can/cannot do. | |
| |<-----------------------------------------| |
| | | |
| CE | FE state: what it is now. | FE |
| |<-----------------------------------------| |
| | | |
| | FE configuration: what it should be. | |
| |----------------------------------------->| |
+-------+ +-------+
Figure 1. Illustration of FE state, capabilities and configuration
exchange in the context of CE-FE communication via ForCES.
The concepts relating to LFBs, particularly capability at the LFB
level and LFB topology will be discussed in the rest of this
section.
Capability information at the LFB level is an integral part of the
LFB model, and is modeled the same way as the other operational
parameters inside an LFB. For example, when certain features of
an LFB class are optional, it must be possible for the CE to
determine whether those optional features are supported by a given
LFB instance. Such capability information can be modeled as a
read-only attribute in the LFB instance, see Section 4.7.5 for
details.
Capability information at the FE level may describe the LFB
classes 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
attributes including capability information.
Once the FE capability is described to the CE, the FE state
information can be represented by two levels. The first level is
the logically separable and distinct packet processing functions,
called Logical Functional Blocks (LFBs). The second level of
information describes how these individual LFBs are ordered and
placed along the datapath to deliver a complete forwarding plane
service. The interconnection and ordering of the LFBs is called
LFB Topology. Section 3.2 discusses high level concepts around
LFBs, whereas Section 3.3 discusses LFB topology issues.
3.2. LFB (Logical Functional Block) Modeling
Each LFB performs a well-defined action or computation on the
packets passing through it. Upon completion of its prescribed
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function, either the packets are modified in certain ways (e.g.,
decapsulator, marker), or some results are generated and stored,
often in the form of metadata (e.g., classifier). Each LFB
typically performs a single action. Classifiers, shapers and
meters are all examples of such LFBs. Modeling LFBs at such a
fine granularity allows us to use a small number of LFBs to
express the higher-order FE functions (such as an IPv4 forwarder)
precisely, which in turn can describe more complex networking
functions and vendor implementations of software and hardware.
These LFBs will be defined in detail in one or more documents.
An LFB has one or more inputs, each of which takes a packet P, and
optionally metadata M; and produces one or more outputs, each of
which carries a packet P', and optionally metadata M'. Metadata
is data associated with the packet in the network processing
device (router, switch, etc.) and is passed from one LFB to the
next, but is not sent across the network. In general, multiple
LFBs are contained in one FE, as shown in Figure 2, and all the
LFBs share the same ForCES protocol termination point that
implements the ForCES protocol logic and maintains the
communication channel to and from the CE.
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+-----------+
| CE |
+-----------+
^
| Fp reference point
|
+--------------------------|-----------------------------------+
| FE | |
| v |
| +----------------------------------------------------------+ |
| | ForCES protocol | |
| | termination point | |
| +----------------------------------------------------------+ |
| ^ ^ |
| : : Internal control |
| : : |
| +---:----------+ +---:----------| |
| | :LFB1 | | : LFB2 | |
| =====>| v |============>| v |======>...|
| Inputs| +----------+ |Outputs | +----------+ | |
| (P,M) | |Attributes| |(P',M') | |Attributes| |(P",M") |
| | +----------+ | | +----------+ | |
| +--------------+ +--------------+ |
| |
+--------------------------------------------------------------+
Figure 2. Generic LFB Diagram
An LFB, as shown in Figure 2, has inputs, outputs and attributes
that can be queried and manipulated by the CE indirectly via an Fp
reference point (defined in RFC 3746 [2]) and the ForCES protocol
termination point. The horizontal axis is in the forwarding plane
for connecting the inputs and outputs of LFBs within the same FE.
The vertical axis between the CE and the FE denotes the Fp
reference point where bidirectional communication between the CE
and FE occurs: the CE to FE communication is for configuration,
control and packet injection while FE to CE communication is used
for packet re-direction to the control plane, monitoring and
accounting information, errors, etc. Note that the interaction
between the CE and the LFB is only abstract and indirect. The
result of such an interaction is for the CE to indirectly
manipulate the attributes of the LFB instances.
A namespace is used to associate a unique name or ID with each LFB
class. The namespace must be extensible so that a new LFB class
can be added later to accommodate future innovation in the
forwarding plane.
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LFB operation must be specified in the model to allow the CE to
understand the behavior of the forwarding datapath. For instance,
the CE must understand at what point in the datapath the IPv4
header TTL is decremented. That is, the CE needs to know if a
control packet could be delivered to it either before or after
this point in the datapath. In addition, the CE must understand
where and what type of header modifications (e.g., tunnel header
append or strip) are performed +by the FEs. Further, the CE must
verify that the various LFBs along a datapath within an FE are
compatible to link together.
There is value to vendors if the operation of LFB classes can be
expressed in sufficient detail so that physical devices
implementing different LFB functions can be integrated easily into
an FE design. Therefore, a semi-formal specification is needed;
that is, a text description of the LFB operation (human readable),
but sufficiently specific and unambiguous to allow conformance
testing and efficient design, so that interoperability between
different CEs and FEs can be achieved.
The LFB class model specifies information such as:
. number of inputs and outputs (and whether they are
configurable)
. metadata read/consumed from inputs;
. metadata produced at the outputs;
. packet type(s) accepted at the inputs and emitted at the
outputs;
. packet content modifications (including encapsulation or
decapsulation);
. packet routing criteria (when multiple outputs on an LFB are
present);
. packet timing modifications;
. packet flow ordering modifications;
. LFB capability information;
. Events that can be detected by the LFB, with notification to
the CE;
. LFB operational attributes, etc.
Section 4 of this document provides a detailed discussion of the
LFB model with a formal specification of LFB class schema. The
rest of Section 3.2 only intends to provide a conceptual overview
of some important issues in LFB modeling, without covering all the
specific details.
3.2.1. LFB Outputs
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An LFB output is a conceptual port on an LFB that can send
information to another LFB. The information is typically a packet
and its associated metadata, although in some cases it might
consist of only metadata, i.e., with no packet data.
A single LFB output can be connected to only one LFB input. This
is required to make the packet flow through the LFB topology
unambiguously.
Some LFBs will have a single output, as depicted in Figure 3.a.
+---------------+ +-----------------+
| | | |
| | | OUT +-->
... OUT +--> ... |
| | | EXCEPTIONOUT +-->
| | | |
+---------------+ +-----------------+
a. One output b. Two distinct outputs
+---------------+ +-----------------+
| | | EXCEPTIONOUT +-->
| OUT:1 +--> | |
... OUT:2 +--> ... OUT:1 +-->
| ... +... | OUT:2 +-->
| OUT:n +--> | ... +...
+---------------+ | OUT:n +-->
+-----------------+
c. One output group d. One output and one output group
Figure 3. Examples of LFBs with various output combinations.
To accommodate a non-trivial LFB topology, multiple LFB outputs
are needed so that an LFB class can fork the datapath. Two
mechanisms are provided for forking: multiple singleton outputs
and output groups, which can be combined in the same LFB class.
Multiple separate singleton outputs are defined in an LFB class to
model a pre-determined number of semantically different outputs.
That is, the number of outputs must be 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.
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For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have
one output(OUT) to send those packets for which the LPM look-up
was successful, passing a META_ROUTEID as metadata; and have
another output (EXCEPTIONOUT) for sending exception packets when
the LPM look-up failed. This example is depicted in Figure 3.b.
Packets emitted by these two outputs not only require different
downstream treatment, but they are a result of two different
conditions in the LFB and each output carries different metadata.
This concept assumes the number of distinct outputs is known when
the LFB class is defined. For each singleton output, the LFB class
definition defines the types of frames and metadata the output
emits.
An output group, on the other hand, is used to model the case
where a flow of similar packets with an identical set of metadata
needs to be split into multiple paths. In this case, the number of
such paths is not known when the LFB class is defined because it
is not an inherent property of the LFB class. An output group
consists of a number of outputs, called the output instances of
the group, where all output instances share the same frame and
metadata emission definitions (see Figure 3.c). Each output
instance can connect to a different downstream LFB, just as if
they were separate singleton outputs, but the number of output
instances can differ between LFB instances of the same LFB class.
The class definition may include a lower and/or an upper limit on
the number of outputs. In addition, for configurable FEs, the FE
capability information may define further limits on the number of
instances in specific output groups for certain LFBs. The actual
number of output instances in a group is an attribute of the LFB
instance, which is read-only for static topologies, and read-write
for dynamic topologies. The output instances in a group are
numbered sequentially, from 0 to N-1, and are addressable from
within the LFB. The LFB has a built-in mechanism to select one
specific output instance for each packet. This mechanism is
described in the textual definition of the class and is typically
configurable via some attributes of the LFB.
For example, consider a re-director LFB, whose sole purpose is to
direct packets to one of N downstream paths based on one of the
metadata associated with each arriving packet. Such an LFB is
fairly versatile and can be used in many different places in a
topology. For example, a redirector can be used to divide the
data path into an IPv4 and an IPv6 path based on a FRAMETYPE
metadata (N=2), or to fork into color specific paths after
metering using the COLOR metadata (red, yellow, green; N=3), etc.
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Using an output group in the above LFB class provides the desired
flexibility to adapt each instance of this class to the required
operation. The metadata to be used as a selector for the output
instance is a property of the LFB. For each packet, the value of
the specified metadata may be used as a direct index to the output
instance. Alternatively, the LFB may have a configurable selector
table that maps a metadata value to output instance.
Note that other LFBs may also use the output group concept to
build in similar adaptive forking capability. For example, a
classifier LFB with one input and N outputs can be defined easily
by using the output group concept. Alternatively, a classifier
LFB with one singleton output in combination with an explicit N-
output re-director LFB models the same processing behavior. The
decision of whether to use the output group model for a certain
LFB class is left to the LFB class designers.
The model allows the output group be combined with other singleton
output(s) in the same class, as demonstrated in Figure 3.d. The
LFB here has two types of outputs, OUT, for normal packet output,
and EXCEPTIONOUT for packets that triggered some exception. The
normal OUT has multiple instances, thus, it is an output group.
In summary, the LFB class may define one output, multiple
singleton outputs, one or more output groups, or a combination
thereof. Multiple singleton outputs should be used when the LFB
must provide for forking the datapath, and at least one of the
following conditions hold:
. the number of downstream directions are inherent from the
definition of the class and hence fixed;
. the frame type and set of metadata emitted on any of the
outputs are substantially different from what is emitted on
the other outputs (i.e., they cannot share frame-type and
metadata definitions);
An output group is appropriate when the LFB must provide for
forking the datapath, and at least one of the following conditions
hold:
. the number of downstream directions is not known when the LFB
class is defined;
. 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
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An LFB input is a conceptual port on an LFB where the LFB can
receive information from other LFBs. The information is typically
a packet and associated metadata, although in some cases it might
consist of only metadata, without any packet data.
For LFB instances that receive packets from more than one other
LFB instance (fan-in). There are three ways to model fan-in, all
supported by the LFB model and can be combined in the same LFB:
. Implicit multiplexing via a single input
. Explicit multiplexing via multiple singleton inputs
. Explicit multiplexing via a group of inputs (input group)
The simplest form of multiplexing uses a singleton input (Figure
4.a). Most LFBs will have only one singleton input. Multiplexing
into a single input is possible because the model allows more than
one LFB output to connect to the same LFB input. This property
applies to any LFB input without any special provisions in the LFB
class. Multiplexing into a single input is applicable when the
packets from the upstream LFBs are similar in frame-type and
accompanying metadata, and require similar processing. Note that
this model does not address how potential contention is handled
when multiple packets arrive simultaneously. If contention
handling needs to be explicitly modeled, one of the other two
modeling solutions must be used.
The second method to model fan-in uses individually defined
singleton inputs (Figure 4.b). This model is meant for situations
where the LFB needs to handle distinct types of packet streams,
requiring input-specific handling inside the LFB, and where the
number of such distinct cases is known when the LFB class is
defined. For example, a Layer 2 Decapsulation/Encapsulation LFB
may have two inputs, one for receiving Layer 2 frames for
decapsulation, and one for receiving Layer 3 frames for
encapsulation. This LFB type expects different frames (L2 vs. L3)
at its inputs, each with different sets of metadata, and would
thus apply different processing on frames arriving at these
inputs. This model is capable of explicitly addressing packet
contention by defining how the LFB class handles the contending
packets.
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+--------------+ +------------------------+
| LFB X +---+ | |
+--------------+ | | |
| | |
+--------------+ v | |
| LFB Y +---+-->|input Meter LFB |
+--------------+ ^ | |
| | |
+--------------+ | | |
| LFB Z |---+ | |
+--------------+ +------------------------+
(a) An LFB connects with multiple upstream LFBs via a single
input.
+--------------+ +------------------------+
| LFB X +---+ | |
+--------------+ +-->|layer2 |
+--------------+ | |
| LFB Y +------>|layer3 LFB |
+--------------+ +------------------------+
(b) An LFB connects with multiple upstream LFBs via two separate
singleton inputs.
+--------------+ +------------------------+
| Queue LFB #1 +---+ | |
+--------------+ | | |
| | |
+--------------+ +-->|in:0 \ |
| Queue LFB #2 +------>|in:1 | input group |
+--------------+ |... | |
+-->|in:N-1 / |
... | | |
+--------------+ | | |
| Queue LFB #N |---+ | Scheduler LFB |
+--------------+ +------------------------+
(c) A Scheduler LFB uses an input group to differentiate which
queue LFB packets are coming from.
Figure 3. Input modeling concepts (examples).
The third method to model fan-in uses the concept of an input
group. The concept is similar to the output group introduced in
the previous section, and is depicted in Figure 4.c. An input
group consists of a number of input instances, all sharing the
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properties (same frame and metadata expectations). The input
instances are numbered from 0 to N-1. From the outside, these
inputs appear as normal inputs, i.e., any compatible upstream LFB
can connect its output to one of these inputs. When a packet is
presented to the LFB at a particular input instance, the index of
the input where the packet arrived is known to the LFB and this
information may be used in the internal processing. For example,
the input index can be used as a table selector, or as an explicit
precedence selector to resolve contention. As with output groups,
the number of input instances in an input group is not defined in
the LFB class. However, the class definition may include
restrictions on the range of possible values. In addition, if an
FE supports configurable topologies, it may impose further
limitations on the number of instances for a particular port
group(s) of a particular LFB class. Within these limitations,
different instances of the same class may have a different number
of input instances. The number of actual input instances in the
group is an attribute of the LFB class, which is read-only for
static topologies, and is read-write for configurable topologies.
As an example for the input group, consider the Scheduler LFB
depicted in Figure 3.c. Such an LFB receives packets from a
number of Queue LFBs via a number of input instances, and uses the
input index information to control contention resolution and
scheduling.
In summary, the LFB class may define one input, multiple singleton
inputs, one or more input groups, or a combination thereof. Any
input allows for implicit multiplexing of similar packet streams
via connecting multiple outputs to the same input. Explicit
multiple singleton inputs are useful when either the contention
handling must be handled explicitly, or when the LFB class must
receive and process a known number of distinct types of packet
streams. An input group is suitable when contention handling must
be modeled explicitly, but the number of inputs are not inherent
from the class (and hence is not known when the class is defined),
or when it is critical for LFB operation to know exactly on which
input the packet was received.
3.2.3. Packet Type
When LFB classes are defined, the input and output packet formats
(e.g., IPv4, IPv6, Ethernet, etc.) must be specified. These are
the types of packets a given LFB input is capable of receiving and
processing, or a given LFB output is capable of producing. This
requires distinct packet types be uniquely labeled with a symbolic
name and/or ID.
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Note that each LFB has a set of packet types that it operates on,
but does not care whether the underlying implementation is passing
a greater portion of the packets. For example, an IPv4 LFB might
only operate on IPv4 packets, but the underlying implementation
may or may not be stripping the L2 header before handing it over -
- whether that is happening or not is opaque to the CE.
3.2.4. Metadata
Metadata is the per-packet state that is passed from one LFB to
another. The metadata is passed with the packet to assist
subsequent LFBs to process that packet. The ForCES model captures
how the per-packet state information is propagated from one LFB to
other LFBs. Practically, such metadata propagation can happen
within one FE, or cross the FE boundary between two interconnected
FEs. We believe that the same metadata model can be used for
either situation; however, our focus here is for intra-FE
metadata.
3.2.4.1. Metadata Vocabulary
Metadata has historically been understood to mean "data about
data". While this definition is a start, it is inadequate to
describe the multiple forms of metadata, which may appear within a
complex network element. The discussion here categorizes forms of
metadata by two orthogonal axes.
The first axis is "internal" versus "external", which describes
where the metadata exists in the network model or implementation.
For example, a particular vendor implementation of an IPv4
forwarder may make decisions inside of a chip that are not visible
externally. Those decisions are metadata for the packet that is
"internal" to the chip. When a packet is forwarded out of the
chip, it may be marked with a traffic management header. That
header, which is metadata for the packet, is visible outside of
the chip, and is therefore called "external" metadata.
The second axis is "implicit" versus "expressed", which specifies
whether or not the metadata has a visible physical representation.
For example, the traffic management header described in the
previous paragraph may be represented as a series of bits in some
format, and that header is associated with the packet. Those bits
have physical representation, and are therefore "expressed"
metadata. If the metadata does not have a physical
representation, it is called "implicit" metadata. This situation
occurs, for example, when a particular path through a network
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device is intended to be traversed only by particular kinds of
packets, such as an IPv4 router. An implementation may not mark
every packet along this path as being of type "IPv4", but the
intention of the designers is that every packet is of that type.
This understanding can be thought of as metadata about the packet,
which is implicitly attached to the packet through the intent of
the designers.
In the ForCES model, we do NOT discuss or represent metadata
"internal" to vendor implementations of LFBs. Our focus is solely
on metadata "external" to the LFBs, and therefore visible in the
ForCES model. The metadata discussed within this model may, or
may not, be visible outside of the particular FE implementing the
LFB model. In this regard, the scope of the metadata within
ForCES is very narrowly defined.
Note also that while we define metadata within this model, it is
only a model. There is no requirement that vendor implementations
of ForCES use the exact metadata representations described in this
document. The only implementation requirement is that vendors
implement the ForCES protocol, not the model.
3.2.4.2. Metadata lifecycle within the ForCES model
Each metadata can be conveniently modeled as a <label, value>
pair, where the label identifies the type of information, (e.g.,
"color"), and its value holds the actual information (e.g.,
"red"). The tag here is shown as a textual label, but it can be
replaced or associated with a unique numeric value (identifier).
The metadata life-cycle is defined in this model using three types
of events: "write", "read" and "consume". The first "write"
implicitly creates and initializes the value of the metadata, and
hence starts the life-cycle. The explicit "consume" event
terminates the life-cycle. Within the life-cycle, that is, after
a "write" event, but before the next "consume" event, there can be
an arbitrary number of "write" and "read" events. These "read"
and "write" events can be mixed in an arbitrary order within the
life-cycle. Outside of the life-cycle of the metadata, that is,
before the first "write" event, or between a "consume" event and
the next "write" event, the metadata should be regarded non-
existent or non-initialized. Thus, reading a metadata outside of
its life-cycle is considered an error.
To ensure inter-operability between LFBs, the LFB class
specification must define what metadata the LFB class "reads" or
"consumes" on its input(s) and what metadata it "produces" on its
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output(s). For maximum extensibility, this definition should
neither specify 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.
While it is important to define the metadata types passing between
LFBs, it is not appropriate to define the exact encoding mechanism
used by LFBs for that metadata. Different implementations are
allowed to use different encoding mechanisms for metadata. For
example, one implementation may store metadata in registers or
shared memory, while another implementation may encode metadata
in-band as a preamble in the packets.
At any link between two LFBs, the packet is marked with a finite
set of active metadata, where active means the metadata is within
its life-cycle. There are two corollaries of this model:
1. No un-initialized metadata exists in the model.
2. No more than one occurrence of each metadata tag can be
associated with a packet at any given time.
3.2.4.3. LFB Operations on Metadata
When the packet is processed by an LFB (i.e., between the time it
is received and forwarded by the LFB), the LFB may perform read,
write and/or consume operations on any active metadata associated
with the packet. If the LFB is considered to be a black box, one
of the following operations is performed on each active metadata.
. IGNORE: ignores and forwards the metadata
. READ: reads and forwards the metadata
. READ/RE-WRITE: reads, over-writes and forwards the
metadata
. WRITE: writes and forwards the metadata
(can also be used to create new metadata)
. READ-AND-CONSUME: reads and consumes the metadata
. CONSUME consumes metadata without reading
The last two operations terminate the life-cycle of the metadata,
meaning that the metadata is not forwarded with the packet when
the packet is sent to the next LFB.
In our model, a new metadata is generated by an LFB when the LFB
applies a WRITE operation to a metadata type that was not present
when the packet was received by the LFB. Such implicit creation
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may be unintentional by the LFB, that is, the LFB may apply the
WRITE operation without knowing or caring if the given metadata
existed or not. If it existed, the metadata gets over-written; if
it did not exist, the metadata is created.
For LFBs that insert packets into the model, WRITE is the only
meaningful metadata operation.
For LFBs that remove the packet from the model, they may either
READ-AND-CONSUME (read) or CONSUME (ignore) each active metadata
associated with the packet.
3.2.4.4. Metadata Production and Consumption
For a given metadata on a given packet path, there must be at
least one producer LFB that creates that metadata and should be at
least one consumer LFB that needs that metadata. In this model,
the producer and consumer LFBs of a metadata are not required to
be adjacent. In addition, there may be multiple producers and
consumers for the same metadata. When a packet path involves
multiple producers of the same metadata, then subsequent producers
overwrite that metadata value.
The metadata that is produced by an LFB is specified by the LFB
class definition on a per output port group basis. A producer may
always generate the metadata on the port group, or may generate it
only under certain conditions. We call the former an
"unconditional" metadata, whereas the latter is a "conditional"
metadata. In the case of conditional metadata, it should be
possible to determine from the definition of the LFB when a
"conditional" metadata is produced.
The consumer behavior of an LFB, that is, the metadata that the
LFB needs for its operation, is defined in the LFB class
definition on a per input port group basis. An input port group
may "require" a given metadata, or may treat it as "optional"
information. In the latter case, the LFB class definition must
explicitly define what happens if an optional metadata is not
provided. One approach is to specify a default value for each
optional metadata, and assume that the default value is used if
the metadata is not provided with the packet.
When a consumer LFB requires a given metadata, it has dependencies
on its up-stream LFBs. That is, the consumer LFB can only
function if there is at least one producer of that metadata and no
intermediate LFB consumes the metadata.
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The model should expose these inter-dependencies. Furthermore, it
should be possible to take inter-dependencies into consideration
when constructing LFB topologies, and also that the dependencies
can be verified when validating topologies.
For extensibility reasons, the LFB specification should define
what metadata the LFB requires without specifying which LFB(s) it
expects a certain metadata to come from. Similarly, LFBs should
specify what metadata they produce without specifying which LFBs
the metadata is meant for.
When specifying the metadata tags, some harmonization effort must
be made so that the producer LFB class uses the same tag as its
intended consumer(s), or vice versa.
3.2.4.5. Fixed, Variable and Configurable Tag
When the produced metadata is defined for a given LFB class, most
metadata will be specified with a fixed tag. For example, a Rate
Meter LFB will always produce the "Color" metadata.
A small subset of LFBs need the capability to produce one or more
of their metadata with tags that are not fixed in the LFB class
definition, but instead can be selected per LFB instance. An
example of such an LFB class is a Generic Classifier LFB. We call
this capability "variable tag metadata production". If an LFB
produces metadata with a variable tag, the corresponding LFB
attribute, called the tag selector, specifies the tag for each
such metadata. This mechanism improves the versatility of certain
multi-purpose LFB classes, since it allows the same LFB class to
be used in different topologies, producing the right metadata tags
according to the needs of the topology.
Depending on the capability of the FE, the tag selector can be
either a read-only or a read-write attribute. If the selector is
read-only, the tag cannot be modified by the CE. If the selector
is read-write, the tag can be configured by the CE, hence we call
this "configurable tag metadata production." Note that using this
definition, configurable tag metadata production is a subset of
variable tag metadata production.
Similar concepts can be introduced for the consumer LFBs to
satisfy different metadata needs. Most LFB classes will specify
their metadata needs using fixed metadata tags. For example, a
Next Hop LFB may always require a "NextHopId" metadata; but the
Redirector LFB may need a "ClassID" metadata in one instance, and
a "ProtocolType" metadata in another instance as a basis for
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selecting the right output port. In this case, an LFB attribute
is used to provide the required metadata tag at run-time. This
metadata tag selector attribute may be read-only or read-write,
depending on the capabilities of the LFB instance and the FE.
3.2.4.6. Metadata Usage Categories
Depending on the role and usage of a metadata, various amounts of
encoding information must be provided when the metadata is
defined, where some cases offer less flexibility in the value
selection than others.
There are three types of metadata related to metadata usage:
. Relational (or binding) metadata
. Enumerated metadata
. Explicit/external value metadata
The purpose of the relational metadata is to refer in one LFB
instance (producer LFB) to a "thing" in another downstream LFB
instance (consumer LFB), where the "thing" is typically an entry
in a table attribute of the consumer LFB.
For example, the Prefix Lookup LFB executes an LPM search using
its prefix table and resolves to a next-hop reference. This
reference needs to be passed as metadata by the Prefix Lookup LFB
(producer) to the Next Hop LFB (consumer), and must refer to a
specific entry in the next-hop table within the consumer.
Expressing and propagating such a binding relationship is probably
the most common usage of metadata. One or more objects in the
producer LFB are bound to a specific object in the consumer LFB.
Such a relationship is established by the CE explicitly by
properly configuring the attributes in both LFBs. Available
methods include the following:
The binding may be expressed by tagging the involved objects in
both LFBs with the same unique, but otherwise arbitrary,
identifier. The value of the tag is explicitly configured by the
CE by writing the value into both LFBs, and this value is also
carried by the metadata between the LFBs.
Another way of setting up binding relations is to use a naturally
occurring unique identifier of the consumer's object as a
reference and as a value of the metadata (e.g., the array index of
a table entry). In this case, the index is either read or
inferred by the CE by communicating with the consumer LFB. Once
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the CE obtains the index, it needs to write it into the producer
LFB to establish the binding.
Important characteristics of the binding usage of metadata are:
. The value of the metadata shows up in the CE-FE communication
for BOTH the consumer and the producer. That is, the
metadata value must be carried over the ForCES protocol.
Using the tagging technique, the value is WRITTEN to both
LFBs. Using the other technique, the value is WRITTEN to
only the producer LFB and may be READ from the consumer LFB.
. The metadata value is irrelevant to the CE, the binding is
simply expressed by using the SAME value at the consumer and
producer LFBs.
. Hence the metadata definition is not required to include
value assignments. The only exception is when some special
value(s) of the metadata must be reserved to convey special
events. Even though these special cases must be defined with
the metadata specification, their encoded values can be
selected arbitrarily. For example, for the Prefix Lookup LFB
example, a special value may be reserved to signal the NO-
MATCH case, and the value of zero may be assigned for this
purpose.
The second class of metadata is the enumerated type. An example
is the "Color" metadata that is produced by a Meter LFB. As the
name suggests, enumerated metadata has a relatively small number
of possible values, each with a specific meaning. All possible
cases must be enumerated when defining this class of metadata.
Although a value encoding must be included in the specification,
the actual values can be selected arbitrarily (e.g., <Red=0,
Yellow=1, Green=2> and <Red=3, Yellow=2, Green 1> would be both
valid encodings, what is important is that an encoding is
specified).
The value of the enumerated metadata may or may not be conveyed
via the ForCES protocol between the CE and FE.
The third class of metadata is the explicit type. This refers to
cases where the metadata value is explicitly used by the consumer
LFB to change some packet header fields. In other words, the
value has a direct and explicit impact on some field and will be
visible externally when the packet leaves the NE. Examples are:
TTL increment given to a Header Modifier LFB, and DSCP value for a
Remarker LFB. For explicit metadata, the value encoding must be
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explicitly provided in the metadata definition. The values cannot
be selected arbitrarily and should conform to what is commonly
expected. For example, a TTL increment metadata should be encoded
as zero for the no increment case, one for the single increment
case, etc. A DSCP metadata should use 0 to encode DSCP=0, 1 to
encode DSCP=1, etc.
3.2.5. LFB Events
During operation, various conditions may occur that can be
detected by LFBs. Examples range from link failure or restart, to
timer expiration in special purpose LFBs. The CE may wish to be
notified of the occurrence of such events. The PL protocol
provides for such notifications. The LFB definition includes the
necessary declarations of events. The declarations include
identifiers necessary for subscribing to events (so that the CE
can indicate to the FE which events it wishes to receive) and to
indicate in event notification messages which event is being
reported.
3.2.6. LFB Element Properties
LFBs are made up of elements, containing the information that the
CE needs to see and / or change about the functioning of the LFB.
These elements, as described in detail elsewhere, may be basic
values, complex structures, or tables (containing values,
structures, or tables.) Some of these elements are optional.
Some elements may be readable or writeable at the discretion of
the FE implementation. The CE needs to know these properties.
Additionally, certain kinds of elements (arrays, aliases, and
events as of this writing) have additional property information
that the CE may need to read or write. This model defines the
structure of the property information for all defined data types.
The reports with events are designed to allow for the common,
closely related information that the CE can be strongly expected
to need to react to the event. These reports are not intended to
carry information the CE already has, large volumes of
information, nor information related in complex fashions.
3.2.7. LFB Versioning
LFB class versioning is a method to enable incremental evolution
of LFB classes. In general, an FE is not allowed to contain an LFB
instance for more than one version of a particular class.
Inheritance (discussed next in Section 3.2.6) has special rules.
If an FE datapath model containing an LFB instance of a particular
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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
are not allowed to support more than one version of a particular
class.
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 CE is not changed. The old L1-based
CE can interoperate with the new L2-based FE if the derived LFB
class "L2" is indeed backward compatible with the base class "L1".
The reverse scenario is a much less problematic case, i.e., when
CE vendor B upgrades to the new LFB class "L2", but the FE is not
upgraded. Note that as long as the CE is capable of working with
older LFB classes, this problem does not affect the model; hence
we will use the term "backward compatibility" to refer to the
first scenario concerning FE backward compatibility.
Backward compatibility can be designed into the inheritance model
by constraining LFB inheritance to require the derived class be a
functional superset of the base class (i.e. the derived class can
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only add functions to the base class, but not remove functions).
Additionally, the following mechanisms are required to support FE
backward compatibility:
1. When detecting an LFB instance of an LFB type that is
unknown to the CE, the CE MUST be able to query the base
class of such an LFB from the FE.
2. The LFB instance on the FE SHOULD support a backward
compatibility mode (meaning the LFB instance reverts itself
back to the base class instance), and the CE SHOULD be able
to configure the LFB to run in such a mode.
3.3. FE Datapath Modeling
Packets coming into the FE from ingress ports generally flow
through multiple LFBs before leaving out of the egress ports. How
an FE treats a packet depends on many factors, such as type of the
packet (e.g., IPv4, IPv6 or MPLS), actual header values, time of
arrival, etc. The result of LFB processing may have an impact on
how the packet is to be treated in downstream LFBs. This
differentiation of packet treatment downstream can be
conceptualized as having alternative datapaths in the FE. For
example, the result of a 6-tuple classification performed by a
classifier LFB could control which rate meter is applied to the
packet by a rate meter LFB in a later stage in the datapath.
LFB topology is a directed graph representation of the logical
datapaths within an FE, with the nodes representing the LFB
instances and the directed link depicting the packet flow
direction from one LFB to the next. Section 3.3.1 discusses how
the FE datapaths can be modeled as LFB topology; while Section
3.3.2 focuses on issues related to LFB topology reconfiguration.
3.3.1. Alternative Approaches for Modeling FE Datapaths
There are two basic ways to express the differentiation in packet
treatment within an FE, one represents the datapath directly and
graphically (topological approach) and the other utilizes metadata
(the encoded state approach).
. 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 must control how the packet is
further processed, then such an LFB will have separate output
ports, one for each alternative treatment, connected to
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separate sub-graphs, each expressing the respective treatment
downstream.
. Encoded State Approach
An alternate way of expressing differential treatment is by
using metadata. The result of the operation of an LFB can be
encoded in a metadata, which is passed along with the packet to
downstream LFBs. A downstream LFB, in turn, can use the
metadata and its value (e.g., as an index into some table) to
determine how to treat the packet.
Theoretically, either approach could substitute for the other, so
one could consider using a single pure approach to describe all
datapaths in an FE. However, neither model by itself results in
the best representation for all practically relevant cases. For a
given FE with certain logical datapaths, applying the two
different modeling approaches will result in very different
looking LFB topology graphs. A model using only the topological
approach may require a very large graph with many links or paths,
and nodes (i.e., LFB instances) to express all alternative
datapaths. On the other hand, a model using only the encoded
state model would be restricted to a string of LFBs, which is not
an intuitive way to describe different datapaths (such as MPLS and
IPv4). Therefore, a mix of these two approaches will likely be
used for a practical model. In fact, as we illustrate below, the
two approaches can be mixed even within the same LFB.
Using a simple example of a classifier with N classification
outputs followed by other LFBs, Figure 5(a) shows what the LFB
topology looks like when using the pure topological approach.
Each output from the classifier goes to one of the N LFBs where no
metadata is needed. The topological approach is simple,
straightforward and graphically intuitive. However, if N is large
and the N nodes following the classifier (LFB#1, LFB#2, ...,
LFB#N) all belong to the same LFB type (e.g., meter), but each has
its own independent attributes, the encoded state approach gives a
much simpler topology representation, as shown in Figure 5(b).
The encoded state approach requires that a table of N rows of
meter attributes is provided in the Meter node itself, with each
row representing the attributes for one meter instance. A
metadata M is also needed to pass along with the packet P from the
classifier to the meter, so that the meter can use M as a look-up
key (index) to find the corresponding row of the attributes that
should be used for any particular packet P.
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What if those N nodes (LFB#1, LFB#2, ..., LFB#N) are not of the
same type? For example, if LFB#1 is a queue while the rest are all
meters, what is the best way to represent such datapaths? While
it is still possible to use either the pure topological approach
or the pure encoded state approach, the natural combination of the
two appears to be the best option. Figure 5(c) depicts two
different functional datapaths using the topological approach
while leaving the N-1 meter instances distinguished by metadata
only, as shown in Figure 5(c).
+----------+
P | LFB#1 |
+--------->|(Attrib-1)|
+-------------+ | +----------+
| 1|------+ P +----------+
| 2|---------------->| LFB#2 |
| classifier 3| |(Attrib-2)|
| ...|... +----------+
| N|------+ ...
+-------------+ | P +----------+
+--------->| LFB#N |
|(Attrib-N)|
+----------+
5(a) Using pure topological approach
+-------------+ +-------------+
| 1| | Meter |
| 2| (P, M) | (Attrib-1) |
| 3|---------------->| (Attrib-2) |
| ...| | ... |
| N| | (Attrib-N) |
+-------------+ +-------------+
5(b) Using pure encoded state approach to represent the LFB
topology in 5(a), if LFB#1, LFB#2, ..., and LFB#N are of the
same type (e.g., meter).
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+-------------+
+-------------+ (P, M) | queue |
| 1|------------->| (Attrib-1) |
| 2| +-------------+
| 3| (P, M) +-------------+
| ...|------------->| Meter |
| N| | (Attrib-2) |
+-------------+ | ... |
| (Attrib-N) |
+-------------+
5(c) Using a combination of the two, if LFB#1, LFB#2, ...,
and LFB#N are of different types (e.g., queue and meter).
Figure 5. An example of how to model FE datapaths
From this example, we demonstrate that each approach has 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 datapaths.
For complex topologies, a combination of the two is the most
flexible. A general design guideline is provided to indicate
which approach is best used for a particular situation. The
topological approach should primarily be used when the packet
datapath forks to distinct LFB classes (not just distinct
parameterizations of the same LFB class), and when the fan-outs do
not require changes, such as adding/removing LFB outputs, or
require only very infrequent changes. Configuration information
that needs to change frequently should be expressed by using the
internal attributes of one or more LFBs (and hence using the
encoded state approach).
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+---------------------------------------------+
| |
+----------+ V +----------+ +------+ |
| | | | |if IP-in-IP| | |
---->| ingress |->+----->|classifier|---------->|Decap.|---->---+
| ports | | |----+ | |
+----------+ +----------+ |others+------+
|
V
(a) The LFB topology with a logical loop
+-------+ +-----------+ +------+ +-----------+
| | | |if IP-in-IP | | | |
--->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|-
>
| ports | | |----+ | | | |
+-------+ +-----------+ |others +------+ +-----------+
|
V
(b) The LFB topology without the loop utilizing two
independent classifier instances.
Figure 6. An LFB topology example.
It is important to point out that the LFB topology 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 6 shows one simple FE example. In
this example, an IP-in-IP packet from an IPSec application like
VPN may go to the classifier first and have the classification
done based on the outer IP header; upon being classified as an IP-
in-IP packet, the packet is then sent to a decapsulator to strip
off the outer IP header, followed by a classifier again to perform
classification on the inner IP header. If the same classifier
hardware or software is used for both outer and inner IP header
classification with the same set of filtering rules, a logical
loop is naturally present in the LFB topology, as shown in Figure
6(a). However, if the classification is implemented by two
different pieces of hardware or software with different filters
(i.e., one set of filters for the outer IP header and another set
for the inner IP header), then it is more natural to model them as
two different instances of classifier LFB, as shown in Figure
6(b).
To distinguish between multiple instances of the same LFB class,
each LFB instance has its own LFB instance ID. One way to encode
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the LFB instance ID is to encode it as x.y where x is the LFB
class ID and y is the instance ID within each LFB class.
3.3.2. Configuring the LFB Topology
While there is little doubt that an individual LFB must be
configurable, the configurability question is more complicated for
LFB topology. Since the LFB topology is really the graphic
representation of the datapaths within an FE, configuring the LFB
topology means dynamically changing the datapaths, including
changing the LFBs along the datapaths on an FE (e.g., creating,
instantiating or deleting LFBs) and setting up or deleting
interconnections between outputs of upstream LFBs to inputs of
downstream LFBs.
Why would the datapaths on an FE ever change dynamically? The
datapaths on an FE are set up by the CE to provide certain data
plane services (e.g., DiffServ, VPN, etc.) to the Network
Element's (NE) customers. The purpose of reconfiguring the
datapaths is to enable the CE to customize the services the NE is
delivering at run time. The CE needs to change the datapaths when
the service requirements change, such as adding a new customer or
when an existing customer changes their service. However, note
that not all datapath changes result in changes in the LFB
topology graph. Changes in the graph are dependent on the approach
used to map the datapaths into LFB topology. As discussed in
3.3.1, the topological approach and encoded state approach can
result in very different looking LFB topologies for the same
datapaths. In general, an LFB topology based on a pure
topological approach is likely to experience more frequent
topology reconfiguration than one based on an encoded state
approach. However, even an LFB topology based entirely on an
encoded state approach may have to change the topology at times,
for example, to bypass some LFBs or insert new LFBs. Since a mix
of these two approaches is used to model the datapaths, LFB
topology reconfiguration is considered an important aspect of the
FE model.
We want to point out that allowing a configurable LFB topology in
the FE model does not mandate that all FEs must 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
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so, how much. Whether a simple runtime switch is used to enable
or disable (i.e., bypass) certain LFBs, or more flexible software
reconfiguration is used, is implementation detail internal to the
FE and outside of the scope of FE model. In either case, the
CE(s) must be able to learn the FE's configuration capabilities.
Therefore, the FE model must provide a mechanism for describing
the LFB topology configuration capabilities of an FE. These
capabilities may include (see Section 5 for full details):
. Which LFB classes the FE can instantiate
. Maximum number of instances of the same LFB class that can be
created
. Any topological limitations, For example:
o The maximum number of instances of the same class or any
class that can be created on any given branch of the
graph
o Ordering restrictions on LFBs (e.g., any instance of LFB
class A must be always downstream of any instance of LFB
class B).
Note that even when the CE is allowed to configure LFB topology
for the FE, the CE is not expected to be able to interpret an
arbitrary LFB topology and determine which specific service or
application (e.g. VPN, DiffServ, etc.) is supported by the FE.
However, once the CE understands the coarse capability of an FE,
it is the responsibility of the CE to 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
draft.
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+----------+ +-----------+
---->| Ingress |---->|classifier |--------------+
| | |chip | |
+----------+ +-----------+ |
v
+-------------------------------------------+
+--------+ | Network Processor |
<----| Egress | | +------+ +------+ +-------+ |
+--------+ | |Meter | |Marker| |Dropper| |
^ | +------+ +------+ +-------+ |
| | |
+----------+-------+ |
| | |
| +---------+ +---------+ +------+ +---------+ |
| |Forwarder|<------|Scheduler|<--|Queue | |Counter | |
| +---------+ +---------+ +------+ +---------+ |
|--------------------------------------------------------------+
(a) The Capability of the FE, reported to the CE
+-----+ +-------+ +---+
| A|--->|Queue1 |--------------------->| |
------>| | +-------+ | | +---+
| | | | | |
| | +-------+ +-------+ | | | |
| B|--->|Meter1 |----->|Queue2 |------>| |->| |
| | | | +-------+ | | | |
| | | |--+ | | | |
+-----+ +-------+ | +-------+ | | +---+
classifier +-->|Dropper| | | IPv4
+-------+ +---+ Fwd.
Scheduler
(b) One LFB topology as configured by the CE and
accepted by the FE
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Queue1
+---+ +--+
| A|------------------->| |--+
+->| | | | |
| | B|--+ +--+ +--+ +--+ |
| +---+ | | | | | |
| Meter1 +->| |-->| | |
| | | | | |
| +--+ +--+ | Ipv4
| Counter1 Dropper1 Queue2| +--+ Fwd.
+---+ | +--+ +--->|A | +-+
| A|---+ | |------>|B | | |
------>| B|------------------------------>| | +--->|C |->| |->
| C|---+ +--+ | +->|D | | |
| D|-+ | | | +--+ +-+
+---+ | | +---+ Queue3| | Scheduler
Classifier1 | | | A|------------> +--+ | |
| +->| | | |--+ |
| | B|--+ +--+ +-------->| | |
| +---+ | | | | +--+ |
| Meter2 +->| |-+ |
| | | |
| +--+ Queue4 |
| Marker1 +--+ |
+---------------------------->| |----+
| |
+--+
(c) Another LFB topology as configured by the CE and
accepted by the FE
Figure 7. An example of configuring LFB topology.
Figure 7 shows an example where a QoS-enabled router has several
line cards that have a few ingress ports and egress ports, a
specialized classification chip, a network processor containing
codes for FE blocks like meter, marker, dropper, counter, queue,
scheduler and Ipv4 forwarder. Some of the LFB topology is already
fixed and has to remain static due to the physical layout of the
line cards. For example, all of the ingress ports might be hard-
wired into the classification chip so all packets must flow from
the ingress port into the classification engine. On the other
hand, the LFBs on the network processor and their execution order
are programmable. However, certain capacity limits and linkage
constraints could exist between these LFBs. Examples of the
capacity limits might be: 8 meters; 16 queues in one FE; the
scheduler can handle at most up to 16 queues; etc. The linkage
constraints might dictate that the classification engine may be
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followed by a meter, marker, 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 datapath before going into the egress ports must
be the IPv4 forwarder, etc.
Once the FE reports these capabilities and capacity limits to the
CE, it is now up to the CE to translate the QoS policy into a
desirable configuration for the FE. Figure 7(a) depicts the FE
capability while 7(b) and 7(c) depict two different topologies
that the CE may request the FE to configure. Note that both the
ingress and egress are omitted in (b) and (c) to simplify the
representation. The topology in 7(c) is considerably more complex
than 7(b) but both are feasible within the FE capabilities, and so
the FE should accept either configuration request from the CE.
4.
Model and Schema for LFB Classes
The main goal of the FE model is to provide an abstract, generic,
modular, implementation-independent representation of the FEs.
This is facilitated using the concept of LFBs, which are
instantiated from LFB classes. LFB classes and associated
definitions will be provided in a collection of XML documents. The
collection of these XML documents is called a LFB class library,
and each document is called an LFB class library document (or
library document, for short). Each of the library documents will
conform to the schema presented in this section. The root element
of the library document is the <LFBLibrary> element.
It is not expected that library documents will be exchanged
between FEs and CEs "over-the-wire". But the model will serve as
an important reference for the design and development of the CEs
(software) and FEs (mostly the software part). It will also serve
as a design input when specifying the ForCES protocol elements for
CE-FE communication.
4.1. Namespace
The LFBLibrary element and all of its sub-elements are defined in
the following namespace:
http://ietf.org/forces/1.0/lfbmodel
4.2. <LFBLibrary> Element
The <LFBLibrary> element serves as a root element of all library
documents. It contains one or more of the following main blocks:
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. <frameTypeDefs> for the frame declarations;
. <dataTypeDefs> for defining common data types;
. <metadataDefs> for defining metadata, and
. <LFBClassDefs> for defining LFB classes.
Each block is optional, that is, one library document may contain
only metadata definitions, another may contain only LFB class
definitions, yet another may contain all of the above.
In addition to the above main blocks, a library document can
import other library documents if it needs to refer to definitions
contained in the included document. This concept is similar to
the "#include" directive in C. Importing is expressed by the
<load> elements, which must precede all the above elements in the
document. For unique referencing, each LFBLibrary instance
document has a unique label defined in the "provide" attribute of
the LFBLibrary element.
The <LFBLibrary> element also includes an optional <description>
element, which can be used to provide textual description about
the library document.
The following is a skeleton of a library document:
<?xml version="1.0" encoding="UTF-8"?>
<LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
provides="this_library">
<description>
...
</description>
<!-- Loading external libraries (optional) -->
<load library="another_library"/>
...
<!-- FRAME TYPE DEFINITIONS (optional) -->
<frameTypeDefs>
...
</frameTypeDefs>
<!-- DATA TYPE DEFINITIONS (optional) -->
<dataTypeDefs>
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...
</dataTypeDefs>
<!-- METADATA DEFINITIONS (optional) -->
<metadataDefs>
...
</metadataDefs>
<!ùLFB CLASS DEFINITIONS (optional) -->
<LFBCLassDefs>
...
</LFBCLassDefs>
</LFBLibrary>
4.3. <load> Element
This element is used to refer to another LFB library document.
Similar to the "#include" directive in C, this makes the objects
(metadata types, data types, etc.) defined in the referred library
document available for referencing in the current document.
The load element must contain the label of the library document to
be included and may contain a URL to specify where the library can
be retrieved. The load element can be repeated unlimited times.
Three examples for the <load> elements:
<load library="a_library"/>
<load library="another_library" location="another_lib.xml"/>
<load library="yetanother_library"
location="http://www.petrimeat.com/forces/1.0/lfbmodel/lpm.xml"/>
4.4. <frameDefs> Element for Frame Type Declarations
Frame names are used in the LFB definition to define the types of
frames the LFB expects at its input port(s) and emits at its
output port(s). The <frameDefs> optional element in the library
document contains one or more <frameDef> elements, each declaring
one frame type.
Each frame definition contains a unique name (NMTOKEN) and a brief
synopsis. In addition, an optional detailed description may be
provided.
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Uniqueness of frame types must be ensured among frame types
defined in the same library document and in all directly or
indirectly included library documents.
The following example defines two frame types:
<frameDefs>
<frameDef>
<name>ipv4</name>
<synopsis>IPv4 packet</synopsis>
<description>
This frame type refers to an IPv4 packet.
</description>
</frameDef>
<frameDef>
<name>ipv6</name>
<synopsis>IPv6 packet</synopsis>
<description>
This frame type refers to an IPv6 packet.
</description>
</frameDef>
...
</frameDefs>
4.5. <dataTypeDefs> Element for Data Type Definitions
The (optional) <dataTypeDefs> element can be used to define
commonly used data types. It contains one or more <dataTypeDef>
elements, each defining a data type with a unique name. Such data
types can be used in several places in the library documents,
including:
. Defining other data types
. Defining metadata
. Defining attributes of LFB classes
This is similar to the concept of having a common header file for
shared data types.
Each <dataTypeDef> element contains a unique name (NMTOKEN), a
brief synopsis, an optional longer description, and a type
definition element. The name must be unique among all data types
defined in the same library document and in any directly or
indirectly included library documents. For example:
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<dataTypeDefs>
<dataTypeDef>
<name>ieeemacaddr</name>
<synopsis>48-bit IEEE MAC address</synopsis>
... type definition ...
</dataTypeDef>
<dataTypeDef>
<name>ipv4addr</name>
<synopsis>IPv4 address</synopsis>
... type definition ...
</dataTypeDef>
...
</dataTypeDefs>
There are two kinds of data types: atomic and compound. Atomic
data types are appropriate for single-value variables (e.g.
integer, ASCII string, byte array).
The following built-in atomic data types are provided, but
additional atomic data types can be defined with the <typeRef> and
<atomic> elements:
<name> Meaning
---- -------
char 8-bit signed integer
uchar 8-bit unsigned integer
int16 16-bit signed integer
uint16 16-bit unsigned integer
int32 32-bit signed integer
uint32 32-bit unsigned integer
int64 64-bit signed integer
uint64 64-bit unisgned integer
boolean A true / false value where
0 = false, 1 = true
string[N] ASCII null-terminated string with
buffer of N characters (string max
length is N-1)
string ASCII null-terminated string without
length limitation
byte[N] A byte array of N bytes
octetstring[N] A buffer of N octets, which may
contain fewer than N octets. Hence
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the encoded value will always have
a length.
float16 16-bit floating point number
float32 32-bit IEEE floating point number
float64 64-bit IEEE floating point number
These built-in data types can be readily used to define metadata
or LFB attributes, but can also be used as building blocks when
defining new data types. The boolean data type is defined here
because it is so common, even though it can be built by sub-
ranging the uchar data type.
Compound data types can build on atomic data types and other
compound data types. Compound data types can be defined in one of
four ways. They may be defined as an array of elements of some
compound or atomic data type. They may be a structure of named
elements of compound or atomic data types (ala C structures).
They may be a union of named elements of compound or atomic data
types (ala C unions). They may also be defined as augmentations
(explained below in 4.5.6) of existing compound data types.
Given that the FORCES protocol will be getting and setting
attribute values, all atomic data types used here must be able to
be conveyed in the FORCES protocol. Further, the FORCES protocol
will need a mechanism to convey compound data types. However, the
details of such representations are for the protocol document to
define, not the model document.
For the definition of the actual type in the <dataTypeDef>
element, the following elements are available: <typeRef>,
<atomic>, <array>, <struct>, and <union>.
The predefined type alias is somewhere between the atomic and
compound data types. It behaves like a structure, one element of
which has special behavior. Given that the special behavior is
tied to the other parts of the structure, the compound result is
treated as a predefined construct.
4.5.1. <typeRef> Element for Aliasing Existing Data Types
The <typeRef> element refers to an existing data type by its name.
The referred data type must be defined either in the same library
document, or in one of the included library documents. If the
referred data type is an atomic data type, the newly defined type
will also be regarded as atomic. If the referred data type is a
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compound type, the new type will also be compound. Some usage
examples follow:
<dataTypeDef>
<name>short</name>
<synopsis>Alias to int16</synopsis>
<typeRef>int16</typeRef>
</dataTypeDef>
<dataTypeDef>
<name>ieeemacaddr</name>
<synopsis>48-bit IEEE MAC address</synopsis>
<typeRef>byte[6]</typeRef>
</dataTypeDef>
4.5.2. <atomic> Element for Deriving New Atomic Types
The <atomic> element allows the definition of a new atomic type
from an existing atomic type, applying range restrictions and/or
providing special enumerated values. Note that the <atomic>
element can only use atomic types as base types, and its result is
always another atomic type.
For example, the following snippet defines a new "dscp" data type:
<dataTypeDef>
<name>dscp</name>
<synopsis>Diffserv code point.</synopsis>
<atomic>
<baseType>uchar</baseType>
<rangeRestriction>
<allowedRange min="0" max="63"/>
</rangeRestriction>
<specialValues>
<specialValue value="0">
<name>DSCP-BE</name>
<synopsis>Best Effort</synopsis>
</specialValue>
...
</specialValues>
</atomic>
</dataTypeDef>
4.5.3. <array> Element to Define Arrays
The <array> element can be used to create a new compound data type
as an array of a compound or an atomic data type. The type of the
array entry can be specified either by referring to an existing
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type (using the <typeRef> element) or defining an unnamed type
inside the <array> element using any of the <atomic>, <array>,
<struct>, or <union> elements.
The array can be "fixed-size" or "variable-size", which is
specified by the "type" attribute of the <array> element. The
default is "variable-size". For variable size arrays, an optional
"max-length" attribute specifies the maximum allowed length. This
attribute should be used to encode semantic limitations, not
implementation limitations. The latter should be handled by
capability attributes of LFB classes, and should never be included
in data type definitions. If the "max-length" attribute is not
provided, the array is regarded as of unlimited-size.
For fixed-size arrays, a "length" attribute must be provided 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 can 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:
. Any declared key can be used in the ForCES protocol to select
an element for operations (for details, see the protocol).
. In any instance of the array, each declared key must be
unique within that instance. No two elements of an array may
have the same values on all the fields which make up a key.
Each key is declared with a keyID for use in the protocol, where
the unique key is formed by combining one or more specified key
fields. To support the case where an array of an atomic type with
unique values can be referenced by those values, the key field
identifier may be "*" (i.e., the array entry is the key). If the
value type of the array is a structure or an array, then the key
is one or more fields, each identified by name. Since the field
may be an element of the structure, the element of an element of a
structure, or further nested, the field name is actually a
concatenated sequence of part identifiers, separated by decimal
points ("."). The syntax for key field identification is given
following the array examples.
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The following example shows the definition of a fixed size array
with a pre-defined data type as the array elements:
<dataTypeDef>
<name>dscp-mapping-table</name>
<synopsis>
A table of 64 DSCP values, used to re-map code space.
</synopsis>
<array type="fixed-size" length="64">
<typeRef>dscp</typeRef>
</array>
</dataTypeDef>
The following example defines a variable size array with an upper
limit on its size:
<dataTypeDef>
<name>mac-alias-table</name>
<synopsis>A table with up to 8 IEEE MAC addresses</synopsis>
<array type="variable-size" max-length="8">
<typeRef>ieeemacaddr</typeRef>
</array>
</dataTypeDef>
The following example shows the definition of an array with a
local (unnamed) type definition:
<dataTypeDef>
<name>classification-table</name>
<synopsis>
A table of classification rules and result opcodes.
</synopsis>
<array type="variable-size">
<struct>
<element elementID="1">
<name>rule</name>
<synopsis>The rule to match</synopsis>
<typeRef>classrule</typeRef>
</element>
<element elementID="2">
<name>opcode</name>
<synopsis>The result code</synopsis>
<typeRef>opcode</typeRef>
</element>
</struct>
</array>
</dataTypeDef>
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In the above example, each entry of the array is a <struct> of two
fields ("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 and
prefix length. This means that in any instance of this table, no
two entries can have the same IP address and prefix length.
<dataTypeDef>
<name>ipPrefixInfo_table</name>
<synopsis>
A table of information about known prefixes
</synopsis>
<array type="variable-size">
<struct>
<element elementID="1">
<name>address-prefix</name>
<synopsis>the prefix being described</synopsis>
<typeRef>ipv4Prefix</typeRef>
</element>
<element elementID="2">
<name>source</name>
<synopsis>where is this from</synopsis>
<typeRef>uint16</typeRef>
</element>
<element elementID="3">
<name>prefInfo</name>
<synopsis>the information we care about</synopsis>
<typeRef>hypothetical-info-type</typeRef>
</element>
</struct>
<key keyID="1">
<keyField> address-prefix.ipv4addr </keyField>
<keyField> address-prefix.prefixlen </keyField>
<keyField> source </keyField>
</key>
</array>
</dataTypeDef>
Note that the keyField elements could also have been simply
address-prefix and source, since all of the fields of address-
prefix are being used.
4.5.3.1 Key Field References
In order to use key declarations, one must refer to fields that
are potentially nested inside other fields in the array. If there
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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 is defined as 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 elements of the structure.
In the special case of declaring a key for an array containing an
atomic type, where that content is unique and is to be used as a
key, the value "*" can be used as the single key field identifier.
4.5.4. <struct> Element to Define Structures
A structure is comprised of a collection of data elements. Each
data element has a data type (either an atomic type or an existing
compound type) and is assigned a name unique within the scope of
the compound data type being defined. These serve the same
function as "struct" in C, etc.
The actual type of the field can be defined by referring to an
existing type (using the <typeDef> element), or can be a locally
defined (unnamed) type created by any of the <atomic>, <array>,
<struct>, or <union> elements.
A structure definition is a series of element declarations. Each
element carries an elementID for use by the ForCES protocol. In
addition, the element contains the name, a synopsis, an optional
description, an optional declaration that the element itself is
optional, and the typeRef declaration that specifies the element
type.
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For a dataTypeDef of a struct, the structure definition may be
inherited from, and augment, a previously defined structured type.
This is indicated by including the derivedFrom attribute on the
struct declaration.
The result of this construct is always regarded a compound type,
even when the <struct> contains only one field.
An example:
<dataTypeDef>
<name>ipv4prefix</name>
<synopsis>
IPv4 prefix defined by an address and a prefix length
</synopsis>
<struct>
<element elementID="1">
<name>address</name>
<synopsis>Address part</synopsis>
<typeRef>ipv4addr</typeRef>
</element>
<element elementID="2">
<name>prefixlen</name>
<synopsis>Prefix length part</synopsis>
<atomic>
<baseType>uchar</baseType>
<rangeRestriction>
<allowedRange min="0" max="32"/>
</rangeRestriction>
</atomic>
</element>
</struct>
</dataTypeDef>
4.5.5. <union> Element to Define Union Types
Similar to the union declaration in C, this construct allows the
definition of overlay types. Its format is identical to the
<struct> element.
The result of this construct is always regarded a compound type,
even when the union contains only one element.
4.5.6 <alias> Element
It is sometimes necessary to have an element in an LFB or
structure refer to information in other LFBs. The <alias>
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declaration creates the constructs for this. The content of an
<alias> element is a named type. It can be a base type of a
derived type. The actual value referenced by an alias is known as
its target. When a GET or SET operation references the alias
element, the value of the target is returned or replaced. Write
access to an alias element is permitted if write access to both
the alias and the target are permitted.
The target of an <alias> element is determined by its properties.
Like all elements, the properties include the support / read /
write permission for the alias. In addition, several fields in
the property elements define the target of the alias. These
fields 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
element. The type of the target element must match the declared
type of the alias. Details of the alias property structure are
contained in the section of this document on properties.
Note that the read / write property of the alias refers to the
value. The CE can only determine if it can write the target
selection properties of the alias by attempting such a write
operation. (Property elements do not themselves have properties.)
4.5.6. Augmentations
Compound types can also be defined as augmentations of existing
compound types. If the existing compound type is a structure,
augmentation may add new elements to the type. The type of an
existing element can only be replaced with an augmentation derived
from the current type, an existing element cannot be deleted. If
the existing compound type is an array, augmentation means
augmentation of the array element type.
One consequence of this is that augmentations are compatible with
the compound type from which they are derived. As such,
augmentations are useful in defining attributes for LFB subclasses
with backward compatibility. In addition to adding new attributes
to a class, the data type of an existing attribute may be replaced
by an augmentation of that attribute, and still meet the
compatibility rules for subclasses.
For example, consider a simple base LFB class A that has only one
attribute (attr1) of type X. One way to derive class A1 from A
can be by simply adding a second attribute (of any type). Another
way to derive a class A2 from A can be by replacing the original
attribute (attr1) in A of type X with one of type Y, where Y is an
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augmentation of X. Both classes A1 and A2 are backward compatible
with class A.
The syntax for augmentations is to include a derivedFrom element
in a structure definition, indicating what structure type is being
augmented. Element names and element IDs within the augmentation
must not be the same as those in the structure type being
augmented.
4.6. <metadataDefs> Element for Metadata Definitions
The (optional) <metadataDefs> element in the library document
contains one or more <metadataDef> elements. Each <metadataDef>
element defines a metadata.
Each <metadataDef> element contains a unique name (NMTOKEN).
Uniqueness is defined to be over all metadata defined in this
library document and in all directly or indirectly included
library documents. The <metadataDef> element also contains a brief
synopsis, an optional detailed description, and a compulsory type
definition information. Only atomic data types can be used as
value types for metadata.
Two forms of type definitions are allowed. The first form uses the
<typeRef> element to refer to an existing atomic data type defined
in the <dataTypeDefs> element of the same library document or in
one of the included library documents. The usage of the <typeRef>
element is identical to how it is used in the <dataTypeDef>
elements, except here it can only refer to atomic types.
[EDITOR: The latter restriction is not yet enforced by the XML
schema.]
The second form is an explicit type definition using the <atomic>
element. This element is used here in the same way as in the
<dataTypeDef> elements.
The following example shows both usages:
<metadataDefs>
<metadataDef>
<name>NEXTHOPID</name>
<synopsis>Refers to a Next Hop entry in NH LFB</synopsis>
<typeRef>int32</typeRef>
</metadataDef>
<metadataDef>
<name>CLASSID</name>
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<synopsis>
Result of classification (0 means no match).
</synopsis>
<atomic>
<baseType>int32</baseType>
<specialValues>
<specialValue value="0">
<name>NOMATCH</name>
<synopsis>
Classification didnÆt result in match.
</synopsis>
</specialValue>
</specialValues>
</atomic>
</metadataDef>
</metadataDefs>
4.7. <LFBClassDefs> Element for LFB Class Definitions
The (optional) <LFBClassDefs> element can be used to define one or
more LFB classes using <LFBClassDef> elements. Each <LFBClassDef>
element defines an LFB class and includes the following elements:
. <name> provides the symbolic name of the LFB class. Example:
"ipv4lpm"
. <synopsis> provides a short synopsis of the LFB class.
Example: "IPv4 Longest Prefix Match Lookup LFB"
. <version> is the version indicator
. <derivedFrom> is the inheritance indicator
. <inputPorts> lists the input ports and their specifications
. <outputPorts> lists the output ports and their specifications
. <attributes> defines the operational attributes of the LFB
. <capabilities> defines the capability attributes of the LFB
. <description> contains the operational specification of the
LFB
. The LFBClassID attribute of the LFBClassDef element defines
the ID for this class. These must be globally unique.
. <events> defines the events that can be generated by
instances of this LFB.
[EDITOR: LFB class names should be unique not only among classes
defined in this document and in all included documents, but also
unique across a large collection of libraries. Obviously some
global control is needed to ensure such uniqueness. This subject
requires further study. The uniqueness of the class IDs also
requires further study.]
Here is a skeleton of an example LFB class definition:
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<LFBClassDefs>
<LFBClassDef LFBClassID="12345">
<name>ipv4lpm</name>
<synopsis>IPv4 Longest Prefix Match Lookup LFB</synopsis>
<version>1.0</version>
<derivedFrom>baseclass</derivedFrom>
<inputPorts>
...
</inputPorts>
<outputPorts>
...
</outputPorts>
<attributes>
...
</attributes>
<capabilities>
...
</capabilities>
<description>
This LFB represents the IPv4 longest prefix match lookup
operation.
The modeled behavior is as follows:
Blah-blah-blah.
</description>
</LFBClassDef>
...
</LFBClassDefs>
The individual attributes and capabilities will have elementIDs
for use by the ForCES protocol. These parallel the elementIDs
used in structs, and are used the same way. Attribute and
capability elementIDs must be unique within the LFB class
definition.
Note that the <name>, <synopsis>, and <version> elements are
required, all other elements are optional in <LFBClassDef>.
However, when they are present, they must occur in the above
order.
4.7.1. <derivedFrom> Element to Express LFB Inheritance
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The optional <derivedFrom> element can be used to indicate that
this class is a derivative of some other class. The content of
this element must be the unique name (<name>) of another LFB
class. The referred LFB class must be defined in the same library
document or in one of the included library documents.
[EDITOR: The <derivedFrom> element will likely need to specify the
version of the ancestor, which is not included in the schema yet.
The process and rules of class derivation are still being
studied.]
It is assumed that the derived class is backwards compatible with
the base class.
4.7.2. <inputPorts> Element to Define LFB Inputs
The optional <inputPorts> element is used to define input ports.
An LFB class may have zero, one, or more inputs. If the LFB class
has no input ports, the <inputPorts> element must be omitted. The
<inputPorts> element can contain one or more <inputPort> elements,
one for each port or port-group. We assume that most LFBs will
have exactly one input. Multiple inputs with the same input type
are modeled as one input group. Input groups are defined the same
way as input ports by the <inputPort> element, differentiated only
by an optional "group" attribute.
Multiple inputs with different input types should be avoided if
possible (see discussion in Section 3.2.1). Some special LFBs
will have no inputs at all. For example, a packet generator LFB
does not need an input.
Single input ports and input port groups are both defined by the
<inputPort> element, they are differentiated by only an optional
"group" attribute.
The <inputPort> element contains the following elements:
. <name> provides the symbolic name of the input. Example: "in".
Note that this symbolic name must be unique only within the
scope of the LFB class.
. <synopsis> contains a brief description of the input. Example:
"Normal packet input".
. <expectation> lists all allowed frame formats. Example:
{"ipv4" and "ipv6"}. Note that this list should refer to names
specified in the <frameDefs> element of the same library
document or in any included library documents. The
<expectation> element can also provide a list of required
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metadata. Example: {"classid", "vifid"}. This list should
refer to names of metadata defined in the <metadataDefs>
element in the same library document or in any included library
documents. For each metadata, it must be specified whether the
metadata is required or optional. For each optional metadata,
a default value must be specified, which is used by the LFB if
the metadata is not provided with a packet.
In addition, the optional "group" attribute of the <inputPort>
element can specify if the port can behave as a port group, i.e.,
it is allowed to be instantiated. This is indicated by a "yes"
value (the default value is "no").
An example <inputPorts> element, defining two input ports, the
second one being an input port group:
<inputPorts>
<inputPort>
<name>in</name>
<synopsis>Normal input</synopsis>
<expectation>
<frameExpected>
<ref>ipv4</ref>
<ref>ipv6</ref>
</frameExpected>
<metadataExpected>
<ref>classid</ref>
<ref>vifid</ref>
<ref dependency="optional" defaultValue="0">vrfid</ref>
</metadataExpected>
</expectation>
</inputPort>
<inputPort group="yes">
... another input port ...
</inputPort>
</inputPorts>
For each <inputPort>, the frame type expectations are defined by
the <frameExpected> element using one or more <ref> elements (see
example above). When multiple frame types are listed, it means
that "one of these" frame types are expected. A packet of any
other frame type is regarded as incompatible with this input port
of the LFB class. The above example list two frames as expected
frame types: "ipv4" and "ipv6".
Metadata expectations are specified by the <metadataExpected>
element. In its simplest form, this element can contain a list of
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<ref> elements, each referring to a metadata. When multiple
instances of metadata are listed by <ref> elements, it means that
"all of these" metadata must be received with each packet (except
metadata that are marked as "optional" by the "dependency"
attribute of the corresponding <ref> element). For a metadata
that is specified "optional", a default value must be provided
using the "defaultValue" attribute. The above example lists three
metadata as expected metadata, two of which are mandatory
("classid" and "vifid"), and one being optional ("vrfid").
[EDITOR: How to express default values for byte[N] atomic types is
yet to be defined.]
The schema also allows for more complex definitions of metadata
expectations. For example, using the <one-of> element, a list of
metadata can be specified to express that at least one of the
specified metadata must be present with any packet. For example:
<metadataExpected>
<one-of>
<ref>prefixmask</ref>
<ref>prefixlen</ref>
</one-of>
</metadataExpected>
The above example specifies that either the "prefixmask" or the
"prefixlen" metadata must be provided with any packet.
The two forms can also be combined, as it is shown in the
following example:
<metadataExpected>
<ref>classid</ref>
<ref>vifid</ref>
<ref dependency="optional" defaultValue="0">vrfid</ref>
<one-of>
<ref>prefixmask</ref>
<ref>prefixlen</ref>
</one-of>
</metadataExpected>
Although the schema is constructed to allow even more complex
definitions of metadata expectations, we do not discuss those
here.
4.7.3. <outputPorts> Element to Define LFB Outputs
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The optional <outputPorts> element is used to define output
ports. An LFB class may have zero, one, or more outputs. If the
LFB class has no output ports, the <outputPorts> element must be
omitted. The <outputPorts> element can contain one or more
<outputPort> elements, one for each port or port-group. If there
are multiple outputs with the same output type, we model them as
an output port group. Some special LFBs may have no outputs at
all (e.g., Dropper).
Single output ports and output port groups are both defined by the
<outputPort> element; they are differentiated by only an optional
"group" attribute.
The <outputPort> element contains the following elements:
. <name> provides the symbolic name of the output. Example:
"out". Note that the symbolic name must be unique only within
the scope of the LFB class.
. <synopsis> contains a brief description of the output port.
Example: "Normal packet output".
. <product> lists the allowed frame formats. Example: {"ipv4",
"ipv6"}. Note that this list should refer to symbols specified
in the <frameDefs> element in the same library document or in
any included library documents. The <product> element may also
contain the list of emitted (generated) metadata. Example:
{"classid", "color"}. This list should refer to names of
metadata specified in the <metadataDefs> element in the same
library document or in any included library documents. For
each generated metadata, it should be specified whether the
metadata is always generated or generated only in certain
conditions. This information is important when assessing
compatibility between LFBs.
In addition, the optional "group" attribute of the <outputPort>
element can specify if the port can behave as a port group, i.e.,
it is allowed to be instantiated. This is indicated by a "yes"
value (the default value is "no").
The following example specifies two output ports, the second being
an output port group:
<outputPorts>
<outputPort>
<name>out</name>
<synopsis>Normal output</synopsis>
<product>
<frameProduced>
<ref>ipv4</ref>
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<ref>ipv4bis</ref>
</frameProduced>
<metadataProduced>
<ref>nhid</ref>
<ref>nhtabid</ref>
</metadataProduced>
</product>
</outputPort>
<outputPort group="yes">
<name>exc</name>
<synopsis>Exception output port group</synopsis>
<product>
<frameProduced>
<ref>ipv4</ref>
<ref>ipv4bis</ref>
</frameProduced>
<metadataProduced>
<ref availability="conditional">errorid</ref>
</metadataProduced>
</product>
</outputPort>
</outputPorts>
The types of frames and metadata the port produces are defined
inside the <product> element in each <outputPort>. Within the
<product> element, the list of frame types the port produces is
listed in the <frameProduced> element. When more than one frame
is listed, it means that "one of" these frames will be produced.
The list of metadata that is produced with each packet is listed
in the optional <metadataProduced> element of the <product>. In
its simplest form, this element can contain a list of <ref>
elements, each referring to a metadata type. The meaning of such
a list is that "all of" these metadata are provided with each
packet, except those that are listed with the optional
"availability" attribute set to "conditional". Similar to the
<metadataExpected> element of the <inputPort>, the
<metadataProduced> element supports more complex forms, which we
do not discuss here further.
4.7.4. <attributes> Element to Define LFB Operational Attributes
Operational parameters of the LFBs that must be visible to the CEs
are conceptualized in the model as the LFB attributes. These
include, for example, flags, single parameter arguments, complex
arguments, and tables. Note that the attributes here refer to
only those operational parameters of the LFBs that must be visible
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to the CEs. Other variables that are internal to LFB
implementation are not regarded as LFB attributes and hence are
not covered.
Some examples for LFB attributes are:
. Configurable flags and switches selecting between operational
modes of the LFB
. Number of inputs or outputs in a port group
. Metadata CONSUME vs. PROPAGATE mode selectors
. Various configurable lookup tables, including interface
tables, prefix tables, classification tables, DSCP mapping
tables, MAC address tables, etc.
. Packet and byte counters
. Various event counters
. Number of current inputs or outputs for each input or output
group
. Metadata CONSUME/PROPAGATE mode selector
There may be various access permission restrictions on what the CE
can do with an LFB attribute. The following categories may be
supported:
. No-access attributes. This is useful when multiple access
modes may be defined for a given attribute to allow some
flexibility for different implementations.
. Read-only attributes.
. Read-write attributes.
. Write-only attributes. This could be any configurable data
for which read capability is not provided to the CEs. (e.g.,
the security key information)
. Read-reset attributes. The CE can read and reset this
resource, but cannot set it to an arbitrary value. Example:
Counters.
. Firing-only attributes. A write attempt to this resource
will trigger some specific actions in the LFB, but the actual
value written is ignored.
The LFB class may define more than one possible access mode for a
given attribute (for example, "write-only" and "read-write"), in
which case it is left to the actual implementation to pick one of
the modes. In such cases, a corresponding capability attribute
must inform the CE about the access mode the actual LFB instance
supports (see next subsection on capability attributes).
The attributes of the LFB class are listed in the <attributes>
element. Each attribute is defined by an <attribute> element. An
<attribute> element contains the following elements:
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. <name> defines the name of the attribute. This name must be
unique among the attributes of the LFB class. Example:
"version".
. <synopsis> should provide a brief description of the purpose
of the attribute.
. <optional/> indicates that this attribute is optional.
. The data type of the attribute can be defined either via a
reference to a predefined data type or providing a local
definition of the type. The former is provided by using the
<typeRef> element, which must refer to the unique name of an
existing data type defined in the <dataTypeDefs> element in
the same library document or in any of the included library
documents. When the data type is defined locally (unnamed
type), one of the following elements can be used: <atomic>,
<array>, <struct>, and <union>. Their usage is identical to
how they are used inside <dataTypeDef> elements (see Section
4.5).
. The optional <defaultValue> element can specify a default
value for the attribute, which is applied when the LFB is
initialized or reset. [EDITOR: A convention to define
default values for compound data types and byte[N] atomic
types is yet to be defined.]
The attribute element also has a mandatory elementID attribute,
which is a numeric value used by the ForCES protocol.
In addition to the above elements, the <attribute> element
includes an optional "access" attribute, which can take any of the
following values or even a list of these values: "read-only",
"read-write", "write-only", "read-reset", and "trigger-only". The
default access mode is "read-write".
By reading the property information of an element the CE can
determine whether optional elements are supported and whether
elements defined as read-write can actually be written for a given
LFB instance.
The following example defines two attributes for an LFB:
<attributes>
<attribute access="read-only" elementID=ö1ö>
<name>foo</name>
<synopsis>number of things</synopsis>
<typeRef>uint32</typeRef>
</attribute>
<attribute access="read-write write-only" elementID=ö2ö>
<name>bar</name>
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<synopsis>number of this other thing</synopsis>
<atomic>
<baseType>uint32</baseType>
<rangeRestriction>
<allowedRange min="10" max="2000"/>
</rangeRestriction>
</atomic>
<defaultValue>10</defaultValue>
</attribute>
</attributes>
The first attribute ("foo") is a read-only 32-bit unsigned
integer, defined by referring to the built-in "uint32" atomic
type. The second attribute ("bar") is also an integer, but uses
the <atomic> element to provide additional range restrictions.
This attribute has two possible access modes, "read-write" or
"write-only". A default value of 10 is provided.
Note that not all attributes are likely to exist at all times in a
particular implementation. While the capabilities will frequently
indicate this non-existence, CEs may attempt to reference non-
existent or non-permitted attributes anyway. The FORCES protocol
mechanisms should include appropriate error indicators for this
case.
The mechanism defined above for non-supported attributes can also
apply to attempts to reference non-existent array elements or to
set read-only elements.
4.7.5. <capabilities> Element to Define LFB Capability Attributes
The LFB class specification will provide some flexibility for the
FE implementation regarding how the LFB class is implemented. For
example, the instance may have some limitations that are not
inherent from the class definition, but rather the result of some
implementation limitations. For example, an array attribute may
be defined in the class definition as "unlimited" size, but the
physical implementation may impose a hard limit on the size of the
array.
Such capability related information is expressed by the capability
attributes of the LFB class. The capability attributes are always
read-only attributes, and they are listed in a separate
<capabilities> element in the <LFBClassDef>. The <capabilities>
element contains one or more <capability> elements, each defining
one capability attribute. The format of the <capability> element
is almost the same as the <attribute> element, it differs in two
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aspects: it lacks the access mode attribute (because it is always
read-only), and it lacks the <defaultValue> element (because
default value is not applicable to read-only attributes).
Some examples of capability attributes:
. The version of the LFB class that this LFB instance complies
with;
. Supported optional features of the LFB class;
. Maximum number of configurable outputs for an output group;
. Metadata pass-through limitations of the LFB;
. Maximum size of configurable attribute tables;
. Additional range restriction on operational attributes;
. Supported access modes of certain attributes (if the access
mode of an operational attribute is specified as a list of
two or mode modes).
The following example lists two capability attributes:
<capabilities>
<capability elementID="3">
<name>version</name>
<synopsis>
LFB class version this instance is compliant with.
</synopsis>
<typeRef>version</typeRef>
</capability>
<capability elementID="4">
<name>limitBar</name>
<synopsis>
Maximum value of the "bar" attribute.
</synopsis>
<typeRef>uint16</typeRef>
</capability>
</capabilities>
4.7.6. <events> Element for LFB Notification Generation
The <events> element contains the information about the
occurrences for which instances of this LFB class can generate
notifications to the CE.
The <events> element includes a baseID attribute, so it is always
<events baseID=önumberö>. The value of the baseID is the starting
element for the path which identifies events. It must not be the
same as the elementID of any top level attribute or capability of
the LFB class. In a derived LFB (i.e. one with a <derivedFrom>
element) the baseID must not.
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[Editors note: It may make sense to instead require the baseID
attribute and require it to have the same value as the parent
class events baseID. Both choices leave room for errors not
covered by the XML Schema.]
The <events> element contains 0 or more <event> elements, each of
which declares a single event. The <event> element has an eventID
attribute giving the unique ID of the event. The element will
include:
. <eventTarget> element indicating which LFB field is tested to
generate the event;
. condition element indicating what condition on the field will
generate the event from a list of defined conditions;
. <eventReports> element indicating what values are to be
reported in the notification of the event.
The <eventTarget> element contains information identifying a field
in the LFB. Specifically, the <target> element contains one or
more <eventField> or <eventSubscript> elements. These elements
represent the textual equivalent of a path select a component of
the LFB. The <eventField> element contains the name of an element
of the LFB or struct. The first element in a <target> must be an
<eventField> element and must name a field in the LFB. The
following element must identify a valid field within the
containing context. If an <eventField> identifies an array, and
is not the last element in the target, then the next element MUST
be an <eventSubscript>. <eventSubscript> elements MUST occur only
after <eventField> names that identifies an array. An
<eventSubscript> may contain a numeric value to indicate that this
event applies to a specific element of the array. More commonly,
the event is being defined across all elements of the array. In
that case, <eventSubscript> will contain a name. The name in an
<eventSubscript> element is not a field name. It is a variable
name for use in the <report> elements of this LFB definition.
This name MUST be distinct from any field name that can validly
occur in the <eventReport> clause. Hence it SHOULD be distinct
from any field name used in the LFB or in structures used within
the LFB.
The <eventTarget> provides additional components for the path used
to reference the event. The path will be the baseID for events,
followed by the ID for the specific event, followed by a value for
each <eventSubscript> element in the <eventTarget>. This will
identify a specific occurrence of the event. So, for example, it
will appear in the event notification LFB. It is also used for
the SET-PROPERTY operation to subscribe to a specific event. A
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SET-PROPERTY of the subscription property (but not of any other
writeable properties) may sent be the CE with any prefix of the
path of the event. So, for an event defined on a table, a SET-
PROPERTY with a path of the baseID and the eventID will subscribe
the CE to all occurrences of that event on any element of the
table. This is particularly useful for the <eventCreated/> and
<eventDestroyed/> conditions. Events using those conditions will
generally be defined with a field / subscript sequence that
identifies an array, and that ends with an <eventSubscript>
element. Thus, the event notification will indicate which array
entry has been created or destroyed. A typical subscribe however
will subscribe for the array, not for a specific element, so it
will use a shorter path.
The condition element represents a condition that triggers a
notification. The list of conditions is:
. <eventCreated/> the target must be an array, ending with a
subscript indication. The event is generated when an entry
in the array is created. This occurs even if the entry is
created by CE direction.
. <eventDeleted/> the target must be an array, ending with a
subscript indication. The event is generated when an entry
in the array is destroyed. This occurs even if the entry is
destroyed by CE direction.
. <eventChanged/> the event is generated whenever the target
element changes in any way, subject to hysteresis suppression
for integer targets. The hysteresis suppression level is
part of the properties of the event.
. <eventGreaterThan/> the event is generated whenever the
target element becomes greater than the threshold, subject to
hysteresis suppression. The threshold and hysteresis
suppression are part of the properties of the event.
. <eventLessThan/> the event is generated whenever the target
element becomes less than the threshold, subject to
hysteresis suppression. The threshold and hysteresis
suppression are part of the properties of the event.
Numeric conditions will have hysteresis. The level of the
hysteresis is defined by a property of the event. This allows the
FE to notify the CE of the hysteresis applied, and if it chooses
the FE can allow the CE to modify the hysteresis. This applies to
<eventChanged/> for a numeric field, and to <eventGreaterThan/>
and <eventLessThan/>. The content of a <variance> element is a
numeric value. The FE is required to track the value of the
element and make sure that the condition has become untrue by at
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least the hysteresis from the event property. To be specific, if
the hysteresis is V, then
. For a <eventChanged/> condition, if the last notification was
for value X, then the <changed/> notification will not be
generated until the value reaches X +/- V.
. For a <eventGreaterThan/> condition with threshold T, once
the event has been generated at least once it will not be
generated again until the field first becomes less than or
equal to T û V, and then exceeds T.
. For a <eventLessThan/> condition with threshold T, once the
event has been generated at least once it will not be
generated again until the field first becomes greater than or
equal to T + V, and then becomes less than T.
This allows the FE to suppress floods of events resulting from
oscillation around a condition value. For FEs that do not support
flood suppression, the hysteresis property will be set to 0, and
the property will be read only.
The <eventReports> element of an <event> describes what
information is to be delivered by the FE along with the
notification of the occurance of the event. The <reports> element
contains one or more <eventReport> elements. Each <report>
element identifies a piece of data from the LFB which will be
reported. The notification carries as a DATARAW the data as if
the collection of <eventReport> elements has been defined in a
structure. Each <eventReport> element thus needs to identify a
field in the LFB. The syntax is exactly the same as used in the
<eventTarget> element, using <eventField> and <eventSubscript>
elements. <eventSubcripts> may contain integers. If they contain
names, they must be names from <eventSubscript> elements of the
<eventTarget>. The selection for the report will use the value
for that subscript that identifies that specific element
triggering the event. This can be used to reference the structure
/ field causing the event, or to reference related information in
parallel tables. This event reporting structure is designed to
allow the LFB designer to specify information that is likely not
known a priori by the CE and is likely needed by the CE to process
the event. While the structure allows for pointing at large
blocks of information (full arrays or complex structures) this is
not recommended. Also, the variable reference / subscripting in
reporting only captures a small portion of the kinds of related
information. Chaining through index fields stored in a table, for
example, is not supported. In general, the <eventReports>
mechanism is an optimization for cases that have been found to be
common.
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4.7.7. <description> Element for LFB Operational Specification
The <description> element of the <LFBClass> provides unstructured
text (in XML sense) to verbally describe what the LFB does.
4.8.Properties
Elements of LFBs have properties which are important to the CE.
The most important property is the existence / readability /
writeability of the element. Depending up the type of the
element, other information may be of importance.
The model provides the definition of the structure of property
information. There is a base class of property information. For
the array, alias, and event elements there are subclasses of
property information providing additional fields. This
information is accessed by the CE (and updated where applicable)
via the PL protocol. While some property information is
writeable, there is no mechanism currently provided for checking
the properties of a property element. Writeability can only be
checked by attempting to modify the value.
The basic property definition, along with the scalar for
accessibility is below. Note that this access permission
information is generally read-only.
<dataTypeDef>
<name>accessPermissionValues</name>
<synopsis>
The possible values of attribute access permission
</synopsis>
<atomic>
<baseType>uchar</baseType>
<specialValues>
<specialValue value="0">
<name>None</name>
<synopsis>Access is prohibited</synopsis>
</specialValue>
<specialValue value="1">
<name> Read-Only </name>
<synopsis>Access is read only</synopsis>
</specialValue>
<specialValue value="2">
<name>Write-Only</name>
<synopsis>
The attribute may be written, but not read
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</synopsis>
</specialValue>
<specialValue value="3">
<name>Read-Write</name>
<synopsis>
The attribute may be read or written
</synopsis>
</specialValue>
</specialValues>
</atomic>
</dataTypeDef>
<dataTypeDef>
<name>baseElementProperties</name>
<synopsis>basic properties, accessibility</synopsis>
<struct>
<element elementID="1">
<name>accessibility</name>
<synopsis>
does the element exist, and can it be read or written
</synopsis>
<typeRef>accessPermissionValues</typeRef>
</element>
</struct>
</dataTypeDef>
The properties for an array add a number of important pieces of
information. These properties are also read-only.
<dataTypeDef>
<name>arrayElementProperties</name>
<struct>
<derivedFrom>baseElementProperties</derivedFrom>
<element elementID=ö2ö>
<name>entryCount</name>
<synopsis>the number of entries in the array</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID=ö3ö>
<name>highestUsedSubscript</name>
<synopsis>the last used subscript in the array</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID=ö4ö>
<name>firstUnusedSubscript</name>
<synopsis>
The subscript of the first unused array element
</synopsis>
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<typeRef>uint32</typeRef>
</element>
</struct>
</dataTypeDef>
The properties for an event add three (usually) writeable fields.
One is the subscription field. 0 means no notification is
generated. Any non-zero value (typically 1 is used) means that a
notification is generated. The hysteresis field is used to
suppress generation of notifications for oscillations around a
condition value, and is described in the text for events. The
threshold field is used for the <eventGreaterThan/> and
<eventLessThan/> conditions. It indicates the value to compare
the event target against. Using the properties allows the CE to
set the level of interest. FEs which do not supporting setting
the threshold for events will make this field read-only.
<dataTypeDef>
<name>eventElementProperties</name>
<struct>
<derivedFrom>baseElementProperties</derivedFrom>
<element elementID=ö2ö>
<name>registration</name>
<synopsis>has the CE registered to be notified of this
event
</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID=ö3ö>
<name>hysteresis</name>
<synopsis>region to suppress event recurrence notices
</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID=ö4ö>
<name>threshold</name>
<synopsis> comparison value for level crossing events
</synopsis>
<typeRef>uint32</typeRef>
</element>
</struct>
<dataTypeDef>
The properties for an alias add three (usually) writeable fields.
These combine to identify the target element the subject alias
refers to.
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<dataTypeDef>
<name>aliasElementProperties</name>
<struct>
<derivedFrom>baseElementProperties</derivedFrom>
<element elementID=ö2ö>
<name>targetLFBClass</name>
<synopsis>the class ID of the alias target</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID=ö3ö>
<name>targetLFBInstance</name>
<synopsis>the instance ID of the alias target</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID=ö4ö>
<name>targetElementPath</name>
<synopsis>
The path to the element target, each 4 octets is read
as one path element, using the path construction in
the PL protocol.
</synopsis>
<typeRef>octetstring[128]</typeRef>
</element>
</struct>
</dataTypeDef>
4.9. XML Schema for LFB Class Library Documents
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns="http://ietf.org/forces/1.0/lfbmodel"
xmlns:lfb="http://ietf.org/forces/1.0/lfbmodel"
targetNamespace="http://ietf.org/forces/1.0/lfbmodel"
attributeFormDefault="unqualified"
elementFormDefault="qualified">
<xsd:annotation>
<xsd:documentation xml:lang="en">
Schema for Defining LFB Classes and associated types (frames,
data types for LFB attributes, and metadata).
</xsd:documentation>
</xsd:annotation>
<xsd:element name="description" type="xsd:string"/>
<xsd:element name="synopsis" type="xsd:string"/>
<!-- Document root element: LFBLibrary -->
<xsd:element name="LFBLibrary">
<xsd:complexType>
<xsd:sequence>
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<xsd:element ref="description" minOccurs="0"/>
<xsd:element name="load" type="loadType" minOccurs="0"
maxOccurs="unbounded"/>
<xsd:element name="frameDefs" type="frameDefsType"
minOccurs="0"/>
<xsd:element name="dataTypeDefs" type="dataTypeDefsType"
minOccurs="0"/>
<xsd:element name="metadataDefs" type="metadataDefsType"
minOccurs="0"/>
<xsd:element name="LFBClassDefs" type="LFBClassDefsType"
minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="provides" type="xsd:Name"
use="required"/>
</xsd:complexType>
<!-- Uniqueness constraints -->
<xsd:key name="frame">
<xsd:selector xpath="lfb:frameDefs/lfb:frameDef"/>
<xsd:field xpath="lfb:name"/>
</xsd:key>
<xsd:key name="dataType">
<xsd:selector xpath="lfb:dataTypeDefs/lfb:dataTypeDef"/>
<xsd:field xpath="lfb:name"/>
</xsd:key>
<xsd:key name="metadataDef">
<xsd:selector xpath="lfb:metadataDefs/lfb:metadataDef"/>
<xsd:field xpath="lfb:name"/>
</xsd:key>
<xsd:key name="LFBClassDef">
<xsd:selector xpath="lfb:LFBClassDefs/lfb:LFBClassDef"/>
<xsd:field xpath="lfb:name"/>
</xsd:key>
</xsd:element>
<xsd:complexType name="loadType">
<xsd:attribute name="library" type="xsd:Name" use="required"/>
<xsd:attribute name="location" type="xsd:anyURI"
use="optional"/>
</xsd:complexType>
<xsd:complexType name="frameDefsType">
<xsd:sequence>
<xsd:element name="frameDef" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element ref="description" minOccurs="0"/>
</xsd:sequence>
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</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="dataTypeDefsType">
<xsd:sequence>
<xsd:element name="dataTypeDef" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element ref="description" minOccurs="0"/>
<xsd:group ref="typeDeclarationGroup"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<!--
Predefined (built-in) atomic data-types are:
char, uchar, int16, uint16, int32, uint32, int64, uint64,
string[N], string, byte[N], boolean, octetstring[N]
float16, float32, float64
-->
<xsd:group name="typeDeclarationGroup">
<xsd:choice>
<xsd:element name="typeRef" type="typeRefNMTOKEN"/>
<xsd:element name="atomic" type="atomicType"/>
<xsd:element name="array" type="arrayType"/>
<xsd:element name="struct" type="structType"/>
<xsd:element name="union" type="structType"/>
<xsd:element name=öaliasö type="typeRefNMTOKEN"/>
</xsd:choice>
</xsd:group>
<xsd:simpleType name="typeRefNMTOKEN">
<xsd:restriction base="xsd:token">
<xsd:pattern value="\c+"/>
<xsd:pattern value="string\[\d+\]"/>
<xsd:pattern value="byte\[\d+\]"/>
<xsd:pattern value="octetstring\[\d+\]"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType name="atomicType">
<xsd:sequence>
<xsd:element name="baseType" type="typeRefNMTOKEN"/>
<xsd:element name="rangeRestriction"
type="rangeRestrictionType" minOccurs="0"/>
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<xsd:element name="specialValues" type="specialValuesType"
minOccurs="0"/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="rangeRestrictionType">
<xsd:sequence>
<xsd:element name="allowedRange" maxOccurs="unbounded">
<xsd:complexType>
<xsd:attribute name="min" type="xsd:integer"
use="required"/>
<xsd:attribute name="max" type="xsd:integer"
use="required"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="specialValuesType">
<xsd:sequence>
<xsd:element name="specialValue" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
</xsd:sequence>
<xsd:attribute name="value" type="xsd:token"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="arrayType">
<xsd:sequence>
<xsd:group ref="typeDeclarationGroup"/>
<xsd:element name="contentKey" minOccurs="0
maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="contentKeyField"
maxOccurs="unbounded"
type="xsd:string"/>
</xsd:sequence>
<xsd:attribute name="contentKeyID" use="required"
type="xsd:integer"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
<xsd:attribute name="type" use="optional"
default="variable-size">
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<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="fixed-size"/>
<xsd:enumeration value="variable-size"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
<xsd:attribute name="length" type="xsd:integer" use="optional"/>
<xsd:attribute name="maxLength" type="xsd:integer"
use="optional"/>
<!--declare keys to have unique IDs -->
<xsd:key name="contentKeyID">
<xsd:selector xpath="lfb:contentKey"/>
<xsd:field xpath="@contentKeyID"/>
</xsd:key>
</xsd:complexType>
<xsd:complexType name="structType">
<xsd:sequence>
<xsd:element name=öderivedFromö type=ötypeRefNMTOKENö
minOccurs=ö0ö/>
<xsd:element name="element" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element name="optional" minOccurs="0"/>
<xsd:group ref="typeDeclarationGroup"/>
</xsd:sequence>
<xsd:attribute name="elementID" use="required"
type="xsd:integer"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
<!-- key declaration to make elementIDs unique in a struct -->
<xsd:key name="structElementID">
<xsd:selector xpath="lfb:element"/>
<xsd:field xpath="@elementID"/>
</xsd:key>
</xsd:complexType>
<xsd:complexType name="metadataDefsType">
<xsd:sequence>
<xsd:element name="metadataDef" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element ref="description" minOccurs="0"/>
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<xsd:choice>
<xsd:element name="typeRef" type="typeRefNMTOKEN"/>
<xsd:element name="atomic" type="atomicType"/>
</xsd:choice>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="LFBClassDefsType">
<xsd:sequence>
<xsd:element name="LFBClassDef" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element name="version" type="versionType"/>
<xsd:element name="derivedFrom" type="xsd:NMTOKEN"
minOccurs="0"/>
<xsd:element name="inputPorts" type="inputPortsType"
minOccurs="0"/>
<xsd:element name="outputPorts" type="outputPortsType"
minOccurs="0"/>
<xsd:element name="attributes" type="LFBAttributesType"
minOccurs="0"/>
<xsd:element name="capabilities"
type="LFBCapabilitiesType" minOccurs="0"/>
<xsd:element name="events"
type="eventsType" minOccurs="0"/>
<xsd:element ref="description" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="LFBClassID" use="required"
type="xsd:integer"/>
</xsd:complexType>
<!-- Key constraint to ensure unique attribute names within
a class:
-->
<xsd:key name="attributes">
<xsd:selector xpath="lfb:attributes/lfb:attribute"/>
<xsd:field xpath="lfb:name"/>
</xsd:key>
<xsd:key name="capabilities">
<xsd:selector xpath="lfb:capabilities/lfb:capability"/>
<xsd:field xpath="lfb:name"/>
</xsd:key>
<!-- does the above ensure that attributes and capabilities
have different names?
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If so, the following is the elementID constraint -->
<xsd:key name="attributeIDs">
<xsd:selector xpath="lfb:attributes/lfb:attribute"/>
<xsd:field xpath="@elementID"/>
</xsd:key>
<xsd:key name="capabilityIDs">
<xsd:selector xpath="lfb:attributes/lfb:capability"/>
<xsd:field xpath="@elementID"/>
</xsd:key>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:simpleType name="versionType">
<xsd:restriction base="xsd:NMTOKEN">
<xsd:pattern value="[1-9][0-9]*\.([1-9][0-9]*|0)"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType name="inputPortsType">
<xsd:sequence>
<xsd:element name="inputPort" type="inputPortType"
maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="inputPortType">
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element name="expectation" type="portExpectationType"/>
<xsd:element ref="description" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="group" type="booleanType" use="optional"
default="no"/>
</xsd:complexType>
<xsd:complexType name="portExpectationType">
<xsd:sequence>
<xsd:element name="frameExpected" minOccurs="0">
<xsd:complexType>
<xsd:sequence>
<!-- ref must refer to a name of a defined frame type --
>
<xsd:element name="ref" type="xsd:string"
maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="metadataExpected" minOccurs="0">
<xsd:complexType>
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<xsd:choice maxOccurs="unbounded">
<!-- ref must refer to a name of a defined metadata -->
<xsd:element name="ref" type="metadataInputRefType"/>
<xsd:element name="one-of"
type="metadataInputChoiceType"/>
</xsd:choice>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="metadataInputChoiceType">
<xsd:choice minOccurs="2" maxOccurs="unbounded">
<!-- ref must refer to a name of a defined metadata -->
<xsd:element name="ref" type="xsd:NMTOKEN"/>
<xsd:element name="one-of" type="metadataInputChoiceType"/>
<xsd:element name="metadataSet" type="metadataInputSetType"/>
</xsd:choice>
</xsd:complexType>
<xsd:complexType name="metadataInputSetType">
<xsd:choice minOccurs="2" maxOccurs="unbounded">
<!-- ref must refer to a name of a defined metadata -->
<xsd:element name="ref" type="metadataInputRefType"/>
<xsd:element name="one-of" type="metadataInputChoiceType"/>
</xsd:choice>
</xsd:complexType>
<xsd:complexType name="metadataInputRefType">
<xsd:simpleContent>
<xsd:extension base="xsd:NMTOKEN">
<xsd:attribute name="dependency" use="optional"
default="required">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="required"/>
<xsd:enumeration value="optional"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
<xsd:attribute name="defaultValue" type="xsd:token"
use="optional"/>
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
<xsd:complexType name="outputPortsType">
<xsd:sequence>
<xsd:element name="outputPort" type="outputPortType"
maxOccurs="unbounded"/>
</xsd:sequence>
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</xsd:complexType>
<xsd:complexType name="outputPortType">
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element name="product" type="portProductType"/>
<xsd:element ref="description" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="group" type="booleanType" use="optional"
default="no"/>
</xsd:complexType>
<xsd:complexType name="portProductType">
<xsd:sequence>
<xsd:element name="frameProduced">
<xsd:complexType>
<xsd:sequence>
<!-- ref must refer to a name of a defined frame type --
>
<xsd:element name="ref" type="xsd:NMTOKEN"
maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="metadataProduced" minOccurs="0">
<xsd:complexType>
<xsd:choice maxOccurs="unbounded">
<!-- ref must refer to a name of a defined metadata -->
<xsd:element name="ref" type="metadataOutputRefType"/>
<xsd:element name="one-of"
type="metadataOutputChoiceType"/>
</xsd:choice>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="metadataOutputChoiceType">
<xsd:choice minOccurs="2" maxOccurs="unbounded">
<!-- ref must refer to a name of a defined metadata -->
<xsd:element name="ref" type="xsd:NMTOKEN"/>
<xsd:element name="one-of" type="metadataOutputChoiceType"/>
<xsd:element name="metadataSet" type="metadataOutputSetType"/>
</xsd:choice>
</xsd:complexType>
<xsd:complexType name="metadataOutputSetType">
<xsd:choice minOccurs="2" maxOccurs="unbounded">
<!-- ref must refer to a name of a defined metadata -->
<xsd:element name="ref" type="metadataOutputRefType"/>
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<xsd:element name="one-of" type="metadataOutputChoiceType"/>
</xsd:choice>
</xsd:complexType>
<xsd:complexType name="metadataOutputRefType">
<xsd:simpleContent>
<xsd:extension base="xsd:NMTOKEN">
<xsd:attribute name="availability" use="optional"
default="unconditional">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="unconditional"/>
<xsd:enumeration value="conditional"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
<xsd:complexType name="LFBAttributesType">
<xsd:sequence>
<xsd:element name="attribute" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element ref="description" minOccurs="0"/>
<xsd:element name="optional" minOccurs="0"/>
<xsd:group ref="typeDeclarationGroup"/>
<xsd:element name="defaultValue" type="xsd:token"
minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="access" use="optional"
default="read-write">
<xsd:simpleType>
<xsd:list itemType="accessModeType"/>
</xsd:simpleType>
</xsd:attribute>
<xsd:attribute name="elementID" use="required"
type="xsd:integer"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:simpleType name="accessModeType">
<xsd:restriction base="xsd:NMTOKEN">
<xsd:enumeration value="read-only"/>
<xsd:enumeration value="read-write"/>
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<xsd:enumeration value="write-only"/>
<xsd:enumeration value="read-reset"/>
<xsd:enumeration value="trigger-only"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType name="LFBCapabilitiesType">
<xsd:sequence>
<xsd:element name="capability" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element ref="description" minOccurs="0"/>
<xsd:element name="optional" minOccurs="0"/>
<xsd:group ref="typeDeclarationGroup"/>
</xsd:sequence>
<xsd:attribute name="elementID" use="required"
type="xsd:integer"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="eventsType">
<xsd:sequence>
<xsd:element name="event" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="name" type="xsd:NMTOKEN"/>
<xsd:element ref="synopsis"/>
<xsd:element name="eventTarget" type="eventPathType"/>
<xsd:element ref="eventCondition"/>
<xsd:element name="eventReports" type="eventReportsType"
minOccurs="0"/>
<xsd:element ref="description" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="eventID" use="required"
type="xsd:integer"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
<xsd:attribute name="baseID" type="xsd:integer"
use="optional"/>
</xsd:complexType>
<!-- the substitution group for the event conditions -->
<xsd:element name="eventCondition" abstract="true"/>
<xsd:element name="eventCreated"
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substitutionGroup="eventCondition"/>
<xsd:element name="eventDeleted"
substitutionGroup="eventCondition"/>
<xsd:element name="eventChanged"
substitutionGroup="eventCondition"/>
<xsd:element name="eventGreaterThan"
substitutionGroup="eventCondition"/>
<xsd:element name="eventLessThan"
substitutionGroup="eventCondition"/>
<xsd:complexType name="eventPathType">
<xsd:sequence>
<xsd:element ref="eventPathPart" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<!-- the substitution group for the event path parts -->
<xsd:element name="eventPathPart" type="xsd:string"
abstract="true"/>
<xsd:element name="eventField" type="xsd:string"
substitutionGroup="eventPathPart"/>
<xsd:element name="eventSubscript" type="xsd:string"
substitutionGroup="eventPathPart"/>
<xsd:complexType name="eventReportsType">
<xsd:sequence>
<xsd:element name="eventReport" type="eventPathType"
maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<xsd:simpleType name="booleanType">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="yes"/>
<xsd:enumeration value="no"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:schema>
5.
FE Attributes and Capabilities
A ForCES forwarding element handles traffic on behalf of a ForCES
control element. While the standards will describe the protocol
and mechanisms for this control, different implementations and
different instances will have different capabilities. The CE
needs to be able to determine what each instance it is responsible
for is actually capable of doing. As stated previously, this is
an approximation. The CE is expected to be prepared to cope with
errors in requests and variations in detail not captured by the
capabilities information about an FE.
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In addition to its capabilities, an FE will have attribute
information that can be used in understanding and controlling the
forwarding operations. Some of the attributes will be read only,
while others will also be writeable.
In order to make the FE attribute information easily accessible,
the information will be stored in an LFB. This LFB will have a
class, FEObject. The LFBClassID for this class is 1. Only one
instance of this class will ever be present, and the instance ID
of that instance in the protocol is 1. Thus, by referencing the
elements of class:1, instance:1 a CE can get all the information
about the FE. For model completeness, this LFB Class is described
in this section.
There will also be an FEProtocol LFB Class. LFBClassID 2 is
reserved for that class. There will be only one instance of that
class as well. Details of that class are defined in the ForCES
protocol document.
5.1. XML for FEObject Class definition
<?xml version="1.0" encoding="UTF-8"?>
<LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://ietf.org/forces/1.0/lfbmodel"
provides="FEObject">
<dataTypeDefs>
<dataTypeDef>
<name>LFBAdjacencyLimitType</name>
<synopsis>Describing the Adjacent LFB</synopsis>
<struct>
<element elementID="1">
<name>NeighborLFB</name>
<synopsis>ID for that LFB Class</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="2">
<name>ViaPorts</name>
<synopsis>
the ports on which we can connect
</synopsis>
<array type="variable-size">
<typeRef>String</typeRef>
</array>
</element>
</struct>
</dataTypeDef>
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<dataTypeDef>
<name>PortGroupLimitType</name>
<synopsis>
Limits on the number of ports in a given group
</synopsis>
<struct>
<element elementID="1">
<name>PortGroupName</name>
<synopsis>Group Name</synopsis>
<typeRef>String</typeRef>
</element>
<element elementID="2">
<name>MinPortCount</name>
<synopsis>Minimum Port Count</synopsis>
<optional/>
<typeRef>uint32</typeRef>
</element>
<element elementID="3">
<name>MaxPortCount</name>
<synopsis>Max Port Count</synopsis>
<optional/>
<typeRef>uint32</typeRef>
</element>
</struct>
</dataTypeDef>
<dataTypeDef>
<name>SupportedLFBType</name>
<synopsis>table entry for supported LFB</synopsis>
<struct>
<element elementID="1">
<name>LFBName</name>
<synopsis>
The name of a supported LFB Class
</synopsis>
<typeRef>string</typeRef>
</element>
<element elementID="2">
<name>LFBClassID</name>
<synopsis>the id of a supported LFB Class</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="3">
<name>LFBOccurrenceLimit</name>
<synopsis>
the upper limit of instances of LFBs of this class
</synopsis>
<optional/>
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<typeRef>uint32</typeRef>
</element>
<!-- For each port group, how many ports can exist -->
<element elementID="4">
<name>PortGroupLimits</name>
<synopsis>Table of Port Group Limits</synopsis>
<optional/>
<array type="variable-size">
<typeRef>PortGroupLimitType</typeRef>
</array>
</element>
<!-- for the named LFB Class, the LFB Classes it may follow -->
<element elementID="5">
<name>CanOccurAfters</name>
<synopsis>
List of LFB Classes that this LFB class can follow
</synopsis>
<optional/>
<array type="variable-size">
<typeRef>LFBAdjacencyLimitType</typeRef>
</array>
</element>
<!-- for the named LFB Class, the LFB Classes that may follow it
-->
<element elementID="6">
<name>CanOccurBefores</name>
<synopsis>
List of LFB Classes that can follow this LFB class
</synopsis>
<optional/>
<array type="variable-size">
<typeRef>LFBAdjacencyLimitType</typeRef>
</array>
</element>
</struct>
</dataTypeDef>
<dataTypeDef>
<name>FEStatusValues</name>
<synopsis>The possible values of status</synopsis>
<atomic>
<baseType>uchar</baseType>
<specialValues>
<specialValue value="0">
<name> AdminDisable </name>
<synopsis>
FE is administratively disabled
</synopsis>
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</specialValue>
<specialValue value="1">
<name>OperDisable</name>
<synopsis>FE is operatively disabled</synopsis>
</specialValue>
<specialValue value="2">
<name> Operenable </name>
<synopsis>FE is operating</synopsis>
</specialValue>
</specialValues>
</atomic>
</dataTypeDef>
<dataTypeDef>
<name>FEConfiguredNeighborType</name>
<synopsis>Details of the FE's Neighbor</synopsis>
<struct>
<element elementID="1">
<name>NeighborID</name>
<synopsis>Neighbors FEID</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="2">
<name>interfaceToNeighbor</name>
<synopsis>
FE's interface that connects to this neighbor
</synopsis>
<optional/>
<typeRef>String</typeRef>
</element>
<element elementID="3">
<name>neighborNetworkAddress</name>
<synopsis>The network layer address of the neighbor
Presumably, the network type can be
determined from the interface information
</synopsis>
<typeRef>OctetSting[16]</typeRef>
</element>
<element elementID="4">
<name>neighborMACAdddress</name>
<synopsis>the media access control address of
the neighbor. Again, it is presumed
the type can be determined
from the interface information
</synopsis>
<typeRef>octetstring[8]</typeRef>
</element>
</struct>
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</dataTypeDef>
<dataTypeDef>
<name>LFBSelectorType</name>
<synopsis>
Unique identification of a LFB class-instance
</synopsis>
<struct>
<element elementID="1">
<name>LFBClassID</name>
<synopsis>LFB Class Identifier</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="2">
<name>LFBInstanceID</name>
<synopsis>LFB Instance ID</synopsis>
<typeRef>uint32</typeRef>
</element>
</struct>
</dataTypeDef>
<dataTypeDef>
<name>LFBLinkType</name>
<synopsis>
Link between two LFB instances of topology
</synopsis>
<struct>
<element elementID="1">
<name>FromLFBID</name>
<synopsis>LFB src</synopsis>
<typeRef>LFBSelector</typeRef>
</element>
<element elementID="2">
<name>FromPortGroup</name>
<synopsis>src port group</synopsis>
<typeRef>String</typeRef>
</element>
<element elementID="3">
<name>FromPortIndex</name>
<synopsis>src port index</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="4">
<name>ToLFBID</name>
<synopsis>dst LFBID</synopsis>
<typeRef>LFBSelector</typeRef>
</element>
<element elementID="5">
<name>ToPortGroup</name>
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<synopsis>dst port group</synopsis>
<typeRef>String</typeRef>
</element>
<element elementID="6">
<name>ToPortIndex</name>
<synopsis>dst port index</synopsis>
<typeRef>uint32</typeRef>
</element>
</struct>
</dataTypeDef>
</dataTypeDefs>
<LFBClassDefs>
<LFBClassDef LFBClassID="1">
<name>FEObject</name>
<synopsis>Core LFB: FE Object</synopsis>
<version>1.0<version/>
<attributes>
<attribute access="read-write" elementID="1">
<name>LFBTopology</name>
<synopsis>the table of known Topologies</synopsis>
<array type="variable-size">
<typeRef>LFBLinkType</typeRef>
</array>
</attribute>
<attribute access="read-write" elementID="2">
<name>LFBSelectors</name>
<synopsis>
table of known active LFB classes and
instances
</synopsis>
<array type="variable-size">
<typeRef>LFBSelectorType</typeRef>
</array>
</attribute>
<attribute access="read-write" elementID="3">
<name>FEName</name>
<synopsis>name of this FE</synopsis>
<typeRef>string[40]</typeRef>
</attribute>
<attribute access="read-write" elementID="4">
<name>FEID</name>
<synopsis>ID of this FE</synopsis>
<typeRef>uint32</typeRef>
</attribute>
<attribute access="read-only" elementID="5">
<name>FEVendor</name>
<synopsis>vendor of this FE</synopsis>
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<typeRef>string[40]</typeRef>
</attribute>
<attribute access="read-only" elementID="6">
<name>FEModel</name>
<synopsis>model of this FE</synopsis>
<typeRef>string[40]</typeRef>
</attribute>
<attribute access="read-only" elementID="7">
<name>FEState</name>
<synopsis>model of this FE</synopsis>
<typeRef>FEStatusValues</typeRef>
</attribute>
<attribute access="read-write" elementID="8">
<name>FENeighbors</name>
<synopsis>table of known neighbors</synopsis>
<array type="variable-size">
<typeRef>FEConfiguredNeighborType</typeRef>
</array>
</attribute>
</attributes>
<capabilities>
<capability elementID="30">
<name>ModifiableLFBTopology</name>
<synopsis>
Whether Modifiable LFB is supported
</synopsis>
<optional/>
<typeRef>boolean</typeRef>
</capability>
<capability elementID="31">
<name>SupportedLFBs</name>
<synopsis>List of all supported LFBs</synopsis>
<optional/>
<array type="variable-size">
<typeRef>SupportedLFBType</typeRef>
</array>
</capability>
</capabilities>
</LFBClassDef>
</LFBClassDefs>
</LFBLibrary>
5.2. FE Capabilities
The FE Capability information is contained in the capabilities
element of the class definition. As described elsewhere,
capability information is always considered to be read-only.
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The currently defined capabilities are ModifiableLFBTopology and
SupportedLFBs. Information as to which attributes of the FE LFB
are supported is contained in the properties information for those
elements.
5.2.1. ModifiableLFBTopology
This element has a boolean value that indicates whether the LFB
topology of the FE may be changed by the CE. If the element is
absent, the default value is assumed to be true, and the CE
presumes the LFB topology may be changed. If the value is present
and set to false, the LFB topology of the FE is fixed. If the
topology is fixed, the LFBs supported clause may be omitted, and
the list of supported LFBs is inferred by the CE from the LFB
topology information. If the list of supported LFBs is provided
when ModifiableLFBTopology is false, the CanOccurBefore and
CanOccurAfter information should be omitted.
5.2.2. SupportedLFBs and SupportedLFBType
One capability that the FE should include is the list of supported
LFB classes. The SupportedLFBs element, is an array that contains
the information about each supported LFB Class. The array
structure type is defined as the SupportedLFBType dataTypeDef.
Each occurrence of the SupportedLFBs array element describes an
LFB class that the FE supports. In addition to indicating that
the FE supports the class, FEs with modifiable LFB topology should
include information about how LFBs of the specified class may be
connected to other LFBs. This information should describe which
LFB classes the specified LFB class may succeed or precede in the
LFB topology. The FE should include information as to which port
groups may be connected to the given adjacent LFB class. If port
group information is omitted, it is assumed that all port groups
may be used.
5.2.2.1. LFBName
This element has as its value the name of the LFB being described.
5.2.2.2. LFBOccurrenceLimit
This element, if present, indicates the largest number of
instances of this LFB class the FE can support. For FEs that do
not have the capability to create or destroy LFB instances, this
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can either be omitted or be the same as the number of LFB
instances of this class contained in the LFB list attribute.
5.2.2.3. PortGroupLimits and PortGroupLimitType
The PortGroupLimits element is an array of information about the
port groups supported by the LFB class. The structure of the port
group limit information is defined by the PortGroupLimitType
dataTypeDef.
Each PortGroupLimits array element contains information describing
a single port group of the LFB class. Each array element contains
the name of the port group in the PortGroupName element, the
fewest number of ports that can exist in the group in the
MinPortCount element, and the largest number of ports that can
exist in the group in the MaxPortCount element.
5.2.2.4.CanOccurAfters and LFBAdjacencyLimitType
The CanOccurAfters element is an array that contains the list of
LFBs the described class can occur after. The array elements are
defined in the LFBAdjacencyLimitType dataTypeDef.
The array elements describe a permissible positioning of the
described LFB class, referred to here as the SupportedLFB.
Specifically, each array element names an LFB that can
topologically precede that LFB class. That is, the SupportedLFB
can have an input port connected to an output port of an LFB that
appears in the CanOccurAfters array. The LFB class that the
SupportedLFB can follow is identified by the NeighborLFB element
of the LFBAdjacencyLimitType array element. If this neighbor can
only be connected to a specific set of input port groups, then the
viaPort element is included. This element occurs once for each
input port group of the SupportedLFB that can be connected to an
output port of the NeighborLFB.
[e.g., Within a SupportedLFBs element, each array element of the
CanOccurAfters array must have a unique NeighborLFB, and within
each array element each viaPort must represent a distinct and
valid input port group of the SupportedLFB. The LFB Class
definition schema does not yet support uniqueness declarations]
5.2.2.5. CanOccurBefores and LFBAdjacencyLimitType
The CanOccurBefores array holds the information about which LFB
classes can follow the described class. Structurally this element
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parallels CanOccurAfters, and uses the same type definition for
the array element.
The array elements list those LFB classes that the SupportedLFB
may precede in the topology. In this element, the
viaPort element of the array value represents the output port
group of the SupportedLFB that may be connected to the
NeighborLFB. As with CanOccurAfters, viaPort may occur multiple
times if multiple output ports may legitimately connect to the
given NeighborLFB class.
[And a similar set of uniqueness constraints apply to the
CanOccurBefore clauses, even though an LFB may occur both in
CanOccurAfter and CanOccurBefore.]
5.2.2.6. LFBClassCapabilities
This element contains capability information about the subject LFB
class whose structure and semantics are defined by the LFB class
definition.
[Note: Important Omissions]
However, this element does not appear in the definition, because
the author can not figure out how to write it.
5.3. FEAttributes
The attributes element is included if the class definition
contains the attributes of the FE that are not considered
"capabilities". Some of these attributes are writeable, and some
are read-only, which should be indicated by the capability
information.
[Editors note - At the moment, the set of attributes is woefully
incomplete.]
5.3.1. FEStatus
This attribute carries the overall state of the FE. For now, it
is restricted to the strings AdminDisable, OperDisable and
OperEnable.
5.3.2. LFBSelectors and LFBSelectorType
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The LFBSelectors element is an array of information about the LFBs
currently accessible via ForCES in the FE. The structure of the
LFB information is defined by the LFBSelectorType.
Each entry in the array describes a single LFB instance in the FE.
The array element contains the numeric class ID of the class of
the LFB instance and the numeric instance ID for this instance.
5.3.3. LFBTopology and LFBLinkType
The optional LFBTopology element contains information about each
inter-LFB link inside the FE, where each link is described in an
LFBLinkType element. The LFBLinkType element contains sufficient
information to identify precisely the end points of a link. The
FromLFBID and ToLFBID fields specify the LFB instances at each end
of the link, and must reference LFBs in the LFB instance table.
The FromPortGroup and ToPortGroup must identify output and input
port groups defined in the LFB classes of the LFB instances
identified by FromLFBID and ToLFBID. The FromPortIndex and
ToPortIndex fields select the elements from the port groups that
this link connects. All links are uniquely identified by the
FromLFBID, FromPortGroup, and FromPortIndex fields. Multiple
links may have the same ToLFBID, ToPortGroup, and ToPortIndex as
this model supports fan in of inter-LFB links but not fan out.
5.3.4. FENeighbors an FEConfiguredNeighborType
The FENeighbors element is an array of information about manually
configured adjacencies between this FE and other FEs. The content
of the array is defined by the FEConfiguredNeighborType element.
This array is intended to capture information that may be
configured on the FE and is needed by the CE, where one array
entry corresponds to each configured neighbor. Note that this
array is not intended to represent the results of any discovery
protocols, as those will have their own LFBs.
Similarly, the MAC address information in the table is intended to
be used in situations where neighbors are configured by MAC
address. Resolution of network layer to MAC address information
should be captured in ARP LFBs and not duplicated in this table.
Note that the same neighbor may be reached through multiple
interfaces or at multiple addresses. There is no uniqueness
requirement of any sort on occurrences of the FENeighbors element.
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Information about the intended forms of exchange with a given
neighbor is not captured here, only the adjacency information is
included.
5.3.4.1.NeighborID
This is the ID in some space meaningful to the CE for the
neighbor. If this table remains, we probably should add an FEID
from the same space as an attribute of the FE.
5.3.4.2.NeighborInterface
This identifies the interface through which the neighbor is
reached.
[Editors note: As the port structures become better defined, the
type for this should be filled in with the types necessary to
reference the various possible neighbor interfaces, include
physical interfaces, logical tunnels, virtual circuits, etc.]
5.3.4.3. NeighborNetworkAddress
Neighbor configuration is frequently done on the basis of a
network layer address. For neighbors configured in that fashion,
this is where that address is stored.
5.3.4.4.NeighborMacAddress
Neighbors are sometimes configured using MAC level addresses
(Ethernet MAC address, circuit identifiers, etc.) If such
addresses are used to configure the adjacency, then that
information is stored here. Note that over some ports such as
physical point to point links or virtual circuits considered as
individual interfaces, there is no need for either form of
address.
6.
Satisfying the Requirements on FE Model
This section describes how the proposed FE model meets the
requirements outlined in Section 5 of RFC 3654 [1]. The
requirements can be separated into general requirements (Sections
5, 5.1 - 5.4) and the specification of the minimal set of logical
functions that the FE model must support (Section 5.5).
The general requirement on the FE model is that it be able to
express the logical packet processing capability of the FE,
through both a capability and a state model. In addition, the FE
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model is expected to allow flexible implementations and be
extensible to allow defining new logical functions.
A major component of the proposed FE model is the Logical
Function Block (LFB) model. Each distinct logical function in an
FE is modeled as an LFB. Operational parameters of the LFB that
must be visible to the CE are conceptualized as LFB attributes.
These attributes express the capability of the FE and support
flexible implementations by allowing an FE to specify which
optional features are supported. The attributes also indicate
whether they are configurable by the CE for an LFB class.
Configurable attributes provide the CE some flexibility in
specifying the behavior of an LFB. When multiple LFBs belonging
to the same LFB class are instantiated on an FE, each of those
LFBs could be configured with different attribute settings. By
querying the settings of the attributes for an instantiated LFB,
the CE can determine the state of that LFB.
Instantiated LFBs are interconnected in a directed graph that
describes the ordering of the functions within an FE. This
directed graph is described by the topology model. The
combination of the attributes of the instantiated LFBs and the
topology describe the packet processing functions available on
the FE (current state).
Another key component of the FE model is the FE attributes. The
FE attributes are used mainly to describe the capabilities of the
FE, but they also convey information about the FE state.
The FE model also includes a definition of the minimal set of LFBs
that is required by Section 5.5 of RFC 3564[1]. The sections that
follow provide more detail on the specifics of each of those LFBs.
Note that the details of the LFBs are contained in a separate LFB
Class Library document. [EDITOR - need to add a reference to that
document].
6.1. Port Functions
The FE model can be used to define a Port LFB class and its
technology-specific subclasses to map the physical port of the
device to the LFB model with both static and configurable
attributes. The static attributes model the type of port, link
speed, etc. The configurable attributes model the addressing,
administrative status, etc.
6.2. Forwarding Functions
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Because forwarding function is one of the most common and
important functions in the forwarding plane, it requires special
attention in modeling to allow design flexibility, implementation
efficiency, modeling accuracy and configuration simplicity.
Toward that end, it is recommended that the core forwarding
function being modeled by the combination of two LFBs -- Longest
Prefix Match (LPM) classifier LFB and Next Hop LFB. Special header
writer LFB is also needed to take care of TTL decrement and
checksum etc.
6.3. QoS Functions
The LFB class library includes descriptions of the Meter, Queue ,
Scheduler, Counter and Dropper LFBs to support the QoS functions
in the forwarding path. The FE model can also be used to define
other useful QoS functions as needed. These LFBs allow the CE to
manipulate the attributes to model IntServ or DiffServ functions.
6.4. Generic Filtering Functions
Various combinations of Classifier, Redirector, Meter and Dropper
LFBs can be used to model a complex set of filtering functions.
6.5. Vendor Specific Functions
New LFB classes can be defined according to the LFB model as
described in Section 4 to support vendor specific functions. A
new LFB class can also be derived from an existing LFB class
through inheritance.
6.6.High-Touch Functions
High-touch functions are those that take action on the contents or
headers of a packet based on content other than what is found in
the IP header. Examples of such functions include NAT, ALG,
firewall, tunneling and L7 content recognition. It is not
practical to include all possible high-touch functions in the
initial LFB library due to the number and complexity. However, the
flexibility of the LFB model and the power of interconnection in
LFB topology should make it possible to model any high-touch
functions.
6.7. Security Functions
Security functions are not included in the initial LFB class
library. However, the FE model is flexible and powerful enough to
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model the types of encryption and/or decryption functions that an
FE supports and the associated attributes for such functions.
The IP Security Policy (IPSP) Working Group in the IETF has
started work in defining the IPSec Policy Information Base [8].
We will try to reuse as much of the work as possible.
6.8. Off-loaded Functions
In addition to the packet processing functions typically found on
the FEs, some logical functions may also be executed
asynchronously by some FEs, as directed by a finite-state machine
and triggered not only by packet events, but by timer events as
well. Examples of such functions include; finite-state machine
execution required by TCP termination or OSPF Hello processing
off-loaded from the CE. By defining LFBs for such functions, the
FE model is capable of expressing these asynchronous functions to
allow the CE to take advantage of such off-loaded functions on the
FEs.
6.9. IPFLOW/PSAMP Functions
RFC 3917 [9] defines an architecture for IP traffic flow
monitoring, measuring and exporting. The LFB model supports
statistics collection on the LFB by including statistical
attributes (Section 4.7.4) in the LFB class definitions; in
addition, special statistics collection LFBs such as meter LFBs
and counter LFBs can also be used to support accounting functions
in the FE.
[10] describes a framework to define a standard set of
capabilities for network elements to sample subsets of packets by
statistical and other methods. Time event generation, filter LFB,
and counter/meter LFB are the elements needed to support packet
filtering and sampling functions -- these elements can all be
supported in the FE model.
7.
Using the FE model in the ForCES Protocol
The actual model of the forwarding plane in a given NE is
something the CE must learn and control by communicating with the
FEs (or by other means). Most of this communication will happen
in the post-association phase using the ForCES protocol. The
following types of information must be exchanged between CEs and
FEs via the ForCES protocol:
1) FE topology query;
2) FE capability declaration;
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3) LFB topology (per FE) and configuration capabilities
query;
4) LFB capability declaration;
5) State query of LFB attributes;
6) Manipulation of LFB attributes;
7) LFB topology reconfiguration.
Items 1) through 5) are query exchanges, where the main flow of
information is from the FEs to the CEs. Items 1) through 4) are
typically queried by the CE(s) in the beginning of the post-
association (PA) phase, though they may be repeatedly queried at
any time in the PA phase. Item 5) (state query) will be used at
the beginning of the PA phase, and often frequently during the PA
phase (especially for the query of statistical counters).
Items 6) and 7) are "command" types of exchanges, where the main
flow of information is from the CEs to the FEs. Messages in Item
6) (the LFB re-configuration commands) are expected to be used
frequently. Item 7) (LFB topology re-configuration) is needed
only if dynamic LFB topologies are supported by the FEs and it is
expected to be used infrequently.
Among the seven types of payload information the ForCES protocol
carries between CEs and FEs, the FE model covers all of them
except item 1), which concerns the inter-FE topology. The FE
model focuses on the LFB and LFB topology within a single FE.
Since the information related to item 1) requires global
knowledge about all of the FEs and their inter-connection with
each other, this exchange is part of the ForCES base protocol
instead of the FE model.
The relationship between the FE model and the seven post-
association messages are visualized in Figure 9:
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+--------+
..........-->| CE |
/----\ . +--------+
\____/ FE Model . ^ |
| |................ (1),2 | | 6, 7
| | (off-line) . 3, 4, 5 | |
\____/ . | v
. +--------+
e.g. RFCs ..........-->| FE |
+--------+
Figure 9. Relationship between the FE model and the ForCES
protocol messages, where (1) is part of the ForCES base protocol,
and the rest are defined by the FE model.
The actual encoding of these messages is defined by the ForCES
protocol and beyond the scope of the FE model. Their discussion
is nevertheless important here for the following reasons:
. These PA model components have considerable impact on the FE
model. For example, some of the above information can be
represented as attributes of the LFBs, in which case such
attributes must be defined in the LFB classes.
. The understanding of the type of information that must be
exchanged between the FEs and CEs can help to select the
appropriate protocol format and the actual encoding method
(such as XML, TLVs).
. Understanding the frequency of these types of messages
should influence the selection of the protocol format
(efficiency considerations).
An important part of the FE model is the port the FE uses for its
message exchanges to and from the CE. In the case that a
dedicated port is used for CE-FE communication, we propose to use
a special port LFB, called the CE-FE Port LFB (a subclass of the
general Port LFB in Section 6.1), to model this dedicated CE-FE
port. The CE-FE Port LFB acts as both a source and sink for the
traffic from and to the CE. Sometimes the CE-FE traffic does not
have its own dedicated port, instead the data fabric is shared
for the data plane traffic and the CE-FE traffic. A special
processing LFB can be used to model the ForCES packet
encapsulation and decapsulation in such cases.
The remaining sub-sections of this section address each of the
seven message types.
7.1. FE Topology Query
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An FE may contain zero, one or more external ingress ports.
Similarly, an FE may contain zero, one or more external egress
ports. In other words, not every FE has to contain any external
ingress or egress interfaces. For example, Figure 10 shows two
cascading FEs. FE #1 contains one external ingress interface but
no external egress interface, while FE #2 contains one external
egress interface but no ingress interface. It is possible to
connect these two FEs together via their internal interfaces to
achieve the complete ingress-to-egress packet processing function.
This provides the flexibility to spread the functions across
multiple FEs and interconnect them together later for certain
applications.
While the inter-FE communication protocol is out of scope for
ForCES, it is up to the CE to query and understand how multiple
FEs are inter-connected to perform a complete ingress-egress
packet processing function, such as the one described in Figure
10. The inter-FE topology information may be provided by FEs, may
be hard-coded into CE, or may be provided by some other entity
(e.g., a bus manager) independent of the FEs. So while the ForCES
protocol supports FE topology query from FEs, it is optional for
the CE to use it, assuming the CE has other means to gather such
topology information.
+-----------------------------------------------------+
| +---------+ +------------+ +---------+ |
input| | | | | | output |
---+->| Ingress |-->|Header |-->|IPv4 |---------+--->+
| | port | |Decompressor| |Forwarder| FE | |
| +---------+ +------------+ +---------+ #1 | |
+-----------------------------------------------------+ V
|
+-----------------------<-----------------------------+
|
| +----------------------------------------+
V | +------------+ +----------+ |
| input | | | | output |
+->--+->|Header |-->| Egress |---------+-->
| |Compressor | | port | FE |
| +------------+ +----------+ #2 |
+----------------------------------------+
Figure 10. An example of two FEs connected together.
Once the inter-FE topology is discovered by the CE after this
query, it is assumed that the inter-FE topology remains static.
However, it is possible that an FE may go down during the NE
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operation, or a board may be inserted and a new FE activated, so
the inter-FE topology will be affected. It is up to the ForCES
protocol to provide a mechanism for the CE to detect such events
and deal with the change in FE topology. FE topology is outside
the scope of the FE model.
7.2. FE Capability Declarations
FEs will have many types of limitations. Some of the limitations
must be expressed to the CEs as part of the capability model. The
CEs must be able to query these capabilities on a per-FE basis.
Examples:
. Metadata passing capabilities of the FE. Understanding these
capabilities will help the CE to evaluate the feasibility of
LFB topologies, and hence to determine the availability of
certain services.
. Global resource query limitations (applicable to all LFBs of
the FE).
. LFB supported by the FE.
. LFB class instantiation limit.
. LFB topological limitations (linkage constraint, ordering
etc.)
7.3. LFB Topology and Topology Configurability Query
The ForCES protocol must provide the means for the CEs to discover
the current set of LFB instances in an FE and the interconnections
between the LFBs within the FE. In addition, sufficient
information should be available to determine whether the FE
supports any CE-initiated (dynamic) changes to the LFB topology,
and if so, determine the allowed topologies. Topology
configurability can also be considered as part of the FE
capability query as described in Section 9.3.
7.4. LFB Capability Declarations
LFB class specifications define a generic set of capabilities.
When an LFB instance is implemented (instantiated) on a vendor's
FE, some additional limitations may be introduced. Note that we
discuss only those limitations that are within the flexibility of
the LFB class specification. That is, the LFB instance will
remain compliant with the LFB class specification despite these
limitations. For example, certain features of an LFB class may be
optional, in which case it must be possible for the CE to
determine if an optional feature is supported by a given LFB
instance or not. Also, the LFB class definitions will probably
contain very few quantitative limits (e.g., size of tables), since
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these limits are typically imposed by the implementation.
Therefore, quantitative limitations should always be expressed by
capability arguments.
LFB instances in the model of a particular FE implementation will
possess limitations on the capabilities defined in the
corresponding LFB class. The LFB class specifications must define
a set of capability arguments, and the CE must be able to query
the actual capabilities of the LFB instance via querying the value
of such arguments. The capability query will typically happen
when the LFB is first detected by the CE. Capabilities need not
be re-queried in case of static limitations. In some cases,
however, some capabilities may change in time (e.g., as a result
of adding/removing other LFBs, or configuring certain attributes
of some other LFB when the LFBs share physical resources), in
which case additional mechanisms must be implemented to inform the
CE about the changes.
The following two broad types of limitations will exist:
. Qualitative restrictions. For example, a standardized multi-
field classifier LFB class may define a large number of
classification fields, but a given FE may support only a
subset of those fields.
. Quantitative restrictions, such as the maximum size of
tables, etc.
The capability parameters that can be queried on a given LFB class
will be part of the LFB class specification. The capability
parameters should be regarded as special attributes of the LFB.
The actual values of these arguments may be, therefore, obtained
using the same attribute query mechanisms as used for other LFB
attributes.
Capability attributes will typically be read-only arguments, but
in certain cases they may be configurable. For example, the size
of a lookup table may be limited by the hardware (read-only), in
other cases it may be configurable (read-write, within some hard
limits).
Assuming that capabilities will not change frequently, the
efficiency of the protocol/schema/encoding is of secondary
concern.
7.5. State Query of LFB Attributes
This feature must be provided by all FEs. The ForCES protocol and
the data schema/encoding conveyed by the protocol must together
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satisfy the following requirements to facilitate state query of
the LFB attributes:
. Must permit FE selection. This is primarily to refer to a
single FE, but referring to a group of (or all) FEs may
optional be supported.
. Must permit LFB instance selection. This is primarily to
refer to a single LFB instance of an FE, but optionally
addressing of a group of LFBs (or all) may be supported.
. Must support addressing of individual attribute of an LFB.
. Must provide efficient encoding and decoding of the
addressing info and the configured data.
. Must provide efficient data transmission of the attribute
state over the wire (to minimize communication load on the
CE-FE link).
7.6. LFB Attribute Manipulation
This is a place-holder for all operations that the CE will use to
populate, manipulate, and delete attributes of the LFB instances
on the FEs. These operations allow the CE to configure an
individual LFB instance.
The same set of requirements as described in Section 9.5 for
attribute query applies here for attribute manipulation as well.
Support for various levels of feedback from the FE to the CE
(e.g.,
request received, configuration completed), as well as multi-
attribute configuration transactions with atomic commit and
rollback, may be necessary in some circumstances.
(Editor's note: It remains an open issue as to whether or not
other methods are needed in addition to "get attribute" and "set
attribute" (such as multi-attribute transactions). If the answer
to that question is yes, it is not clear whether such methods
should be supported by the FE model itself or the ForCES
protocol.)
7.7. LFB Topology Re-configuration
Operations that will be needed to reconfigure LFB topology:
. Create a new instance of a given LFB class on a given FE.
. Connect a given output of LFB x to the given input of LFB y.
. Disconnect: remove a link between a given output of an LFB
and a given input of another LFB.
. Delete a given LFB (automatically removing all interconnects
to/from the LFB).
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8.
Example
This section contains an example LFB definition. While some
properties of LFBs are shown by the FE Object LFB, this endeavors
to show how a data plain LFB might be built. This example is a
fictional case of an interface supporting a coarse WDM optical
interface carrying Frame Relay traffic. The statistical
information (including error statistics) is omitted.)
<?xml version="1.0" encoding="UTF-8"?>
<LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://ietf.org/forces/1.0/lfbmodel"
provides="LaserFrameLFB">
<frameDefs>
<frameDef>
<name>FRFrame</name>
<synopsis>
A frame relay frame, with DLCI (without stuffing)
</synopsis>
</frameDef>
<frameDef>
<name>IPFrame</name>
<synopsis>An IP Packet</synopsis>
</frameDef>
</frameDefs>
<dataTypeDefs>
<dataTypeDef>
<name>frequencyInformationType</name>
<synopsis>Information about a single CWDM
frequency</synopsis>
<struct>
<element elementID="1">
<name>LaserFrequency</name>
<synopsis>encoded frequency (channel)</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="2">
<name>FrequencyState</name>
<synopsis>state of this frequency</synopsis>
<typeRef>PortStatusValues</typeRef>
</element>
<element elementID="3">
<name>LaserPower</name>
<synopsis>current observed power</synopsis>
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<typeRef>uint32</typeRef>
</element>
<element elementID="4">
<name>FrameRelayCircuits</name>
<synopsis>
Information about circuits on this frequency
</synopsis>
<array>
<typeRef>frameCircuitsType</typeRef>
</array>
</element>
</struct>
</dataTypeDef>
<dataTypeDef>
<name>frameCircuitsType</name>
<synopsis>
Information about a single Frame Relay circuit
</synopsis>
<struct>
<element elementID="1">
<name>DLCI</name>
<synopsis>DLCI of the circuit</synopsis>
<typeRef>uint32</typeRef>
</element>
<element elementID="2">
<name>CircuitStatus</name>
<synopsis>state of the circuit</synopsis>
<typeRef>PortStatusValues</typeRef>
</element>
<element elementID="3">
<name>isLMI</name>
<synopsis>is this the LMI circuit</synopsis>
<typeRef>boolean</typeRef>
</element>
<element elementID="4">
<name>associatedPort</name>
<synopsis>
which input/output port is associated with this circuit
</synopsis>
<typeRef>uint32</typeRef>
</element>
</struct>
</dataTypeDef>
<dataTypeDef>
<name>PortStatusValues</name>
<synopsis>
The possible values of status. Used for both
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administrative and operation status
</synopsis>
<atomic>
<baseType>uchar</baseType>
<specialValues>
<specialValue value="0">
<name>Disabled </name>
<synopsis>the component is disabled</synopsis>
</specialValue>
<specialValue value="1">
<name>Enable</name>
<synopsis>FE is operatively disabled</synopsis>
</specialValue>
</specialValues>
</atomic>
</dataTypeDef>
</dataTypeDefs>
<metadataDefs>
<metadataDef>
<name>DLCI</name>
<synopsis>The DLCI the frame arrived on</synopsis>
<typeRef>uint32</typeRef>
</metadataDef>
<metadataDef>
<name>LaserChannel</name>
<synopsis>The index of the laser channel</synopsis>
<typeRef>uint32</typeRef>
</metadataDef>
</metadataDefs>
<LFBClassDefs>
<LFBClassDef LFBClassID="-255">
<name>FrameLaserLFB</name>
<synopsis>Fictional LFB for Demonstartions</synopsis>
<version>1.0</version>
<inputPorts>
<inputPort group="yes">
<name>LMIfromFE</name>
<synopsis>
Ports for LMI traffic, for transmission
</synopsis>
<expectation>
<frameExpected>
<ref>FRFrame</ref>
</frameExpected>
<metadataExpected>
<ref>DLCI</ref>
<ref>LaserChannel</ref>
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</metadataExpected>
</expectation>
</inputPort>
<inputPort>
<name>DatafromFE</name>
<synopsis>Ports for data to be sent on
circuits</synopsis>
<expectation>
<frameExpected>
<ref>IPFrame</ref>
</frameExpected>
<metadataExpected>
<ref>DLCI</ref>
<ref>LaserChannel</ref>
</metadataExpected>
</expectation>
</inputPort>
</inputPorts>
<outputPorts>
<outputPort group="yes">
<name>LMItoFE</name>
<synopsis>Ports for LMI traffic for processing</synopsis>
<product>
<frameProduced>
<ref>FRFrame</ref>
</frameProduced>
<metadataProduced>
<ref>DLCI</ref>
<ref>LaserChannel</ref>
</metadataProduced>
</product>
</outputPort>
<outputPort group="yes">
<name>DatatoFE</name>
<synopsis>Ports for Data traffic for
processing</synopsis>
<product>
<frameProduced>
<ref>IPFrame</ref>
</frameProduced>
<metadataProduced>
<ref>DLCI</ref>
<ref>LaserChannel</ref>
</metadataProduced>
</product>
</outputPort>
</outputPorts>
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<attributes>
<attribute access="read-write" elementID="1">
<name>AdminPortState</name>
<synopsis>is this port allowed to function</synopsis>
<typeRef>PortStatusValues</typeRef>
</attribute>
<attribute access="read-write" elementID="2">
<name>FrequencyInformation</name>
<synopsis>
table of information per CWDM frequency
</synopsis>
<array type="variable-size">
<typeRef>frequencyInformationType</typeRef>
</array>
</attribute>
</attributes>
<capabilities>
<capability elementID="31">
<name>OperationalState</name>
<synopsis>
whether the port over all is operational
</synopsis>
<typeRef>PortStatusValues</typeRef>
</capability>
<capability elementID="32">
<name>MaximumFrequencies</name>
<synopsis>
how many laser frequencies are there
</synopsis>
<typeRef>uint16</typeRef>
</capability>
<capability elementID="33">
<name>MaxTotalCircuits</name>
<synopsis>
Total supportable Frame Relay Circuits,
across all laser frequencies
</synopsis>
<optional/>
<typeRef>uint32</typeRef>
</capability>
</capabilities>
<events baseID="61">
<event eventID="1">
<name>FrequencyState</name>
<synopsis>The state of a frequency has changed</synopsis>
<eventTarget>
<eventField>FrequencyInformation</eventField>
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<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>FrequencyState</eventField>
</eventTarget>
<eventChanged/>
<eventReports>
<!-- report the new state -->
<eventReport>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>FrequencyState</eventField>
</eventReport>
</eventReports>
</event>
<event eventID="2">
<name>CreatedFrequency</name>
<synopsis>A new frequency has appeared</synopsis>
<eventTarget>
<eventField>FrequencyInformation></eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
</eventTarget>
<eventCreated/>
<eventReports>
<eventReport>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>LaserFrequency</eventField>
</eventReport>
</eventReports>
</event>
<event eventID="3">
<name>DeletedFrequency</name>
<synopsis>
A frequency Table entry has been deleted
</synopsis>
<eventTarget>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
</eventTarget>
<eventDeleted/>
</event>
<event eventID="4">
<name>PowerProblem</name>
<synopsis>
There are problems with the laser power level
</synopsis>
<eventTarget>
<eventField>FrequencyInformation</eventField>
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<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>LaserPower</eventField>
</eventTarget>
<eventLessThan/>
<eventReports>
<eventReport>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>LaserPower</eventField>
</eventReport>
<eventReport>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>LaserFrequency</eventField>
</eventReport>
</eventReports>
</event>
<event eventID="5">
<name>FrameCircuitChanged</name>
<synopsis>
The state of an Fr circuit on a frequency has changed
</synopsis>
<eventTarget>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>FrameRelayCircuits</eventField>
<eventSubscript>FrameCircuitIndex</eventSubscript>
<eventField>CircuitStatus</eventField>
</eventTarget>
<eventChanged/>
<eventReports>
<eventReport>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>FrameRelayCircuits</eventField>
<eventSubscript>FrameCircuitIndex</eventSubscript>
<eventField>CircuitStatus</eventField>
</eventReport>
<eventReport>
<eventField>FrequencyInformation</eventField>
<eventSubscript>_FrequencyIndex_</eventSubscript>
<eventField>FrameRelayCircuits</eventField>
<eventSubscript>FrameCircuitIndex</eventSubscript>
<eventField>DLCI</eventField>
</eventReport>
</eventReports>
</event>
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</events>
</LFBClassDef>
</LFBClassDefs>
</LFBLibrary>
8.1.Data Handling
This LFB is designed to handle data packets coming in from or
going out to the external world. It is not a full port and lacks
many useful statistics, but it serves to show many of the relevant
behaviors.
Packets arriving without error from the physical interface come in
on a Frame Relay DLCI on a laser channel. These two values are
used by the LFB to look up the handling for the packet. If the
handling indicates that the packet is LMI, then the output index
is used to select an LFB port from the LMItoFE port group. The
packet is sent as a full Frame Relay frame (without any bit or
byte stuffing) on the selected port. The laser channel and DLCI
are sent as meta-data, even though the DLCI is also still in the
packet.
Good packets that arrive and are not LMI and have a frame relay
type indicator of IP are sent as IP packets on the port in the
DatatoFE port group, using the same index field from the table
based on the laser channel and DLCI. The channel and DLCI are
attached as meta-data for other use (classifiers, for example.)
The current definition does not specify what to do if the Frame
Relay type information is not IP.
Packets arriving on input ports arrive with the Laser Channel and
Frame Relay DLCI as meta-data. As such, a single input port could
have been used. With the structure that is defined (which
parallels the output structure), the selection of channel and DLCI
could be restricted by the arriving input port group (LMI vs data)
and port index. As an alternative LFB design, the structures
could require a 1-1 relationship between DLCI and LFB port, in
which case no meta-data would be needed. This would, however, be
quite complex and noisy. The intermediate level of structure here
allows parallelism between input and output, without requiring
excessive ports.
8.1.1. Setting up a DLCI
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When a CE chooses to establish a DLCI on a specific laser channel,
it sends a SET request directed to this LFB. The request might
look like:
T = SET-OPERATION
T = PATH-DATA
Path: flags = first-avail, length = 4, path = 2, channel, 4
DataRaw: DLCI, Enable(1), false, out-idx
This request would establish the DLCI as enabled, with traffic
going to a specific element of the output port group DatatoFE.
(The CE would ensure that output port is connected to the right
place before issuing this request.
The response to the operation would include the actual index
assigned to this Frame Relay circuit. This table is structured to
use separate internal indices and DLCIs. An alternative design
could have used the DLCI as index, trading off complexities.
One could also imagine that the FE has an LMI LFB. Such an LFB
would be connected to the LMItoFE and LMIfromFE port groups. It
would process LMI information. It might be the LFBÆs job to set
up the frame relay circuits. The LMI LFB would have an alias
entry that points to the Frame Relay circuits table it manages, so
that it can manipulate those entities.
8.1.2. Error Handling
The LFB will receive invalid packets over the wire. Many of these
will simply result in incrementing counters. The LFB designer
might also specify some error rate measures. This puts more work
on the FE, but allows for more meaningful alarms.
There may be some error conditions that should cause parts of the
packet to be sent to the CE. The error itself is not something
that can cause an event in the LFB. There are two ways this can
be handled.
One way is to define a specific field to count the error, and a
field in the LFB to hold the required portion of the packet. The
field could be defined to hold the portion of the packet from the
most recent error. One could then define an event that occurs
whenever the error count changes, and declare that reporting the
event includes the LFB field with the packet portion. For rare
but extremely critical errors, this is an effective solution. It
ensures reliable delivery of the notification. And it allows the
CE to control if it wants the notification. (Use of the event
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variance property would suppress multiple notifications. It would
suppress them even if they were many hours apart, so the CE is
unlikely to use that.)
Another approach is for the LFB to have a port that connects to a
redirect sink. The LFB would attach the laser channel, the DLCI,
and the error indication as meta-data, and ship the packet to the
CE.
Other aspects of error handling are discussed under events below.
8.2. LFB Attributes
This LFB is defined to have two top level attributes. One
reflects the administrative state of the LFB. This allows the CE
to disable the LFB completely.
The other attribute is the table of information about the laser
channels. It is a variable sized array. Each array entry
contains an identifier for the laser frequency this entry is
associated with, whether that frequency is operational, the power
of the laser at that frequency, and a table of information about
frame relay circuits on this frequency. There is no
administrative status since a CE can disable an entry simply by
removing it. (Frequency and laser power of a non-operational
channel are not particularly useful. Knowledge about what
frequencies can be supported would be a table in the capabilities
section.)
The Frame Relay circuit information contains the DLCI, the
operational circuit status, whether this circuit is to be treated
as carrying LMI information, and which port in the output port
group of the LFB traffic is to be sent to. As mentioned above,
the circuit index could, in some designs, be combined with the
DLCI.
8.3. Capabilities
The capability information for this LFB includes whether the
underlying interface is operational, how many frequencies are
supported, and how many total circuits, across all channels, are
permitted. The maximum number for a given laser channel can be
determined from the properties of the FrameRelayCircuits table. A
GET-Properties on path 2.channel.4 will give the CE the properties
of the array which include the number of entries used, the first
available entry, and the maximum number of entries permitted.
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8.4. Events
This LFB is defined to be able to generate several events that the
CE may be interested in. There are events to report changes in
operational state of frequencies, and the creation and deletion of
frequency entries. There is an event for changes in status of
individual frame relay circuits. So an event notification of
61.5.3.11 would indicate that there had been a circuit status
change on subscript 11 of the circuit table in subscript 3 of the
frequency table. The event report would include the new status of
the circuit and the DLCI of the circuit. Arguably, the DLCI is
redundant, since the CE presumably knows the DLCI based on the
circuit index. It is included here to show including two pieces
of information in an event report.
Another event shown is a laser power problem. This event is
generated whenever the laser falls below the specified threshold.
Thus, a CE can register for the event of laser power loss on all
circuits. It would do this as follows:
T = SET-Properties
Path-TLV: flags=0, length = 2, path = 61.4
Path-TLV: flags = property-field, length = 1, path = 2
Dataraw = 1 (register)
Path-TLV: flags = property-field, length = 1, path = 4
Dataraw = 15 (threshold)
This would set the registration for the event on all entries in
the table. It would also set the threshold for the event, causing
reporting if the power falls below 15. (Presumably, the CE knows
what the scale is for power, and has chosen 15 as a meaningful
problem level.)
If a laser oscillates in power near the 15 mark, one could get a
lot of notifications. (If it flips back and forth between 9 and
10, each flip down will generate an event.) Suppose that the CE
decides to suppress this oscillation somewhat on laser channel 5.
It can do this by setting the variance property on that event.
The request would look like:
T = SET-Properties
Path-TLV: flags=0, length = 3, path = 61.4.5
Path-TLV: flags = property-field, length = 1, path = 3
Dataraw = 2 (variance)
Setting the variance to 2 suppresses a lot of spurious
notifications. When the level first falls below 10, a
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notification is generated. If the power level increases to 10 or
11, and then falls back below 10, an event will not be generated.
The power has to recover to at least 12 and fall back below 10 to
generate another event. Once common cause of this form of
oscillation is when the actual value is right near the border. If
it is really 9.5, tiny changes might flip it back and forth
between 9 and 10. A variance level of 1 will suppress this sort
of condition. Many other events have oscillations that are
somewhat wider, so larger variance settings can be used with
those.
9.
Acknowledgments
Many of the colleagues in our companies and participants in the
ForCES mailing list have provided invaluable input into this work.
10.
Security Considerations
The FE model describes the representation and organization of data
sets and attributes in the FEs. The ForCES framework document [2]
provides a comprehensive security analysis for the overall ForCES
architecture. For example, the ForCES protocol entities must be
authenticated per the ForCES requirements before they can access
the information elements described in this document via ForCES.
Access to the information contained in the FE model is
accomplished via the ForCES protocol, which will be defined in
separate documents, and thus the security issues will be addressed
there.
11.
Normative References
[1] Khosravi, H. et al., "Requirements for Separation of IP
Control and Forwarding", RFC 3654, November 2003.
[2] Yang, L. et al., "Forwarding and Control Element Separation
(ForCES) Framework", RFC 3746, April 2004.
12.
Informative References
[3] Bernet, Y. et al., "An Informal Management Model for Diffserv
Routers", RFC 3290, May 2002.
[4] Chan, K. et al., "Differentiated Services Quality of Service
Policy Information Base", RFC 3317, March 2003.
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[5] Sahita, R. et al., "Framework Policy Information Base", RFC
3318, March 2003.
[6] Moore, B. et al., "Information Model for Describing Network
Device QoS Datapath Mechanisms", RFC 3670, January 2004.
[7] Snir, Y. et al., "Policy Framework QoS Information Model", RFC
3644, Nov 2003.
[8] Li, M. et al., "IPsec Policy Information Base", work in
progress, April 2004, <draft-ietf-ipsp-ipsecpib-10.txt>.
[9] Quittek, J. et Al., "Requirements for IP Flow Information
Export", RFC 3917, October 2004.
[10] Duffield, N., "A Framework for Packet Selection and
Reporting", work in progress, January 2005, <draft-ietf-psamp-
framework-10.txt>.
[11] Pras, A. and Schoenwaelder, J., RFC 3444 "On the Difference
between Information Models and Data Models", January 2003.
13.
Authors' Addresses
L. Lily Yang
Intel Corp.
Mail Stop: JF3-206
2111 NE 25th Avenue
Hillsboro, OR 97124, USA
Phone: +1 503 264 8813
Email: lily.l.yang@intel.com
Joel M. Halpern
Megisto Systems, Inc.
20251 Century Blvd.
Germantown, MD 20874-1162, USA
Phone: +1 301 444-1783
Email: jhalpern@megisto.com
Ram Gopal
Nokia Research Center
5, Wayside Road,
Burlington, MA 01803, USA
Phone: +1 781 993 3685
Email: ram.gopal@nokia.com
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Alan DeKok
Infoblox, Inc.
475 Potrero Ave,
Sunnyvale CA 94085
Phone:
Email: alan.dekok@infoblox.com
Zsolt Haraszti
Clovis Solutions
1310 Redwood Way, Suite B
Petaluma, CA 94954
Phone: 707-796-7110
Email: zsolt@clovissolutions.com
Steven Blake
Modular Networks
Phone: +1 919 434-1485
Email: slblake@modularnet.com
Ellen Deleganes
Intel Corp.
Mail Stop: CO5-156
15400 NW Greenbrier Parkway
Beaverton,OR 97006 USA
Phone: +1 503 677-4996
Email: ellen.m.deleganes@intel.com
14.
Intellectual Property Right
The authors are not aware of any intellectual property right
issues pertaining to this document.
15.
IANA consideration
A namespace is needed to uniquely identify the LFB type in the LFB
class library.
Frame type supported on input and output of LFB must also be
uniquely identified.
A set of metadata supported by the LFB model must also be uniquely
identified with names or IDs.
16.
Copyright Statement
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"Copyright (C) The Internet Society (2005). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights."
"This document and the information contained herein are provided
on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE."
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