draft-ietf-forces-model-02.txt   draft-ietf-forces-model-03.txt 
Internet Draft L. Yang Internet Draft L. Yang
Expiration: July 2004 Intel Corp. Expiration: July 2004 Intel Corp.
File: draft-ietf-forces-model-02.txt J. Halpern File: draft-ietf-forces-model-03.txt J. Halpern
Working Group: ForCES Megisto Systems Working Group: ForCES Megisto Systems
R. Gopal R. Gopal
Nokia Nokia
A. DeKok A. DeKok
IDT Inc. IDT Inc.
Z. Haraszti Z. Haraszti
S. Blake S. Blake
Ericsson Ericsson
E. Deleganes E. Deleganes
Intel Corp. Intel Corp.
February 2004
ForCES Forwarding Element Model ForCES Forwarding Element Model
draft-ietf-forces-model-02.txt draft-ietf-forces-model-03.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are all provisions of Section 10 of RFC2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), working documents of the Internet Engineering Task Force (IETF),
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Abstract Abstract
This document defines the forwarding element (FE) model used in the This document defines the forwarding element (FE) model used in the
Forwarding and Control Plane Separation (ForCES) protocol. The Forwarding and Control Element Separation (ForCES) protocol. The
model represents the capabilities, state and configuration of model represents the capabilities, state and configuration of
forwarding elements within the context of the ForCES protocol, so forwarding elements within the context of the ForCES protocol, so
that control elements (CEs) can control the FEs accordingly. More that control elements (CEs) can control the FEs accordingly. More
specifically, the model describes the logical functions that are specifically, the model describes the logical functions that are
present in an FE, what capabilities these functions support, and present in an FE, what capabilities these functions support, and
how these functions are or can be interconnected. This FE model is how these functions are or can be interconnected. This FE model is
intended to satisfy the model requirements specified in the ForCES intended to satisfy the model requirements specified in the ForCES
requirements draft [1]. A list of the basic logical functional requirements draft [1]. A list of the basic logical functional
blocks (LFBs) is also defined in the LFB class library to aid the blocks (LFBs) is also defined in the LFB class library to aid the
effort in defining individual LFBs. effort in defining individual LFBs.
Table of Contents Table of Contents
Abstract.........................................................1 Abstract.........................................................1
1. Definitions...................................................4 1. Definitions...................................................4
2. Introduction..................................................6 2. Introduction..................................................5
2.1. Requirements on the FE model.............................6 2.1. Requirements on the FE model.............................6
2.2. The FE Model in Relation to FE Implementations...........6 2.2. The FE Model in Relation to FE Implementations...........6
2.3. The FE Model in Relation to the ForCES Protocol..........7 2.3. The FE Model in Relation to the ForCES Protocol..........7
2.4. Modeling Language for FE Model...........................8 2.4. Modeling Language for the FE Model.......................7
2.5. Document Structure.......................................8 2.5. Document Structure.......................................8
3. FE Model Concepts.............................................8 3. FE Model Concepts.............................................8
3.1. State Model and Capability Model.........................9 3.1. FE Capability Model and State Model......................9
3.2. LFB (Logical Functional Block) Modeling.................11 3.2. LFB (Logical Functional Block) Modeling.................11
3.2.1. LFB Input and Input Group..........................14 3.2.1. LFB Outputs........................................13
3.2.2. LFB Output and Output Group........................15 3.2.2. LFB Inputs.........................................16
3.2.3. Packet Type........................................16 3.2.3. Packet Type........................................19
3.2.4. Metadata...........................................16 3.2.4. Metadata...........................................20
3.2.5. LFB Versioning.....................................22 3.2.5. LFB Versioning.....................................27
3.2.6. LFB Inheritance....................................23 3.2.6. LFB Inheritance....................................27
3.3. FE Datapath Modeling....................................24 3.3. FE Datapath Modeling....................................28
3.3.1. Alternative Approaches for Modeling FE Datapaths...24 3.3.1. Alternative Approaches for Modeling FE Datapaths...29
3.3.2. Configuring the LFB Topology.......................29 3.3.2. Configuring the LFB Topology.......................33
4. Model and Schema for LFB Classes.............................33 4. Model and Schema for LFB Classes.............................37
4.1. Namespace...............................................33 4.1. Namespace...............................................37
4.2. <LFBLibrary> Element....................................33 4.2. <LFBLibrary> Element....................................37
4.3. <load> Element..........................................35 4.3. <load> Element..........................................39
4.4. <frameDefs> Element for Frame Type Declarations.........35 4.4. <frameDefs> Element for Frame Type Declarations.........39
4.5. <dataTypeDefs> Element for Data Type Definitions........36 4.5. <dataTypeDefs> Element for Data Type Definitions........40
4.5.1. <typeRef> Element for Aliasing Existing Data Types.38 4.5.1. <typeRef> Element for Aliasing Existing Data Types.42
4.5.2. <atomic> Element for Deriving New Atomic Types.....39 4.5.2. <atomic> Element for Deriving New Atomic Types.....42
4.5.3. <array> Element to Define Arrays...................39 4.5.3. <array> Element to Define Arrays...................43
4.5.4. <struct> Element to Define Structures..............41 4.5.4. <struct> Element to Define Structures..............45
4.5.5. <union> Element to Define Union Types..............42 4.5.5. <union> Element to Define Union Types..............46
4.5.6. Augmentations......................................42 4.5.6. Augmentations......................................46
4.6. <metadataDefs> Element for Metadata Definitions.........43 4.6. <metadataDefs> Element for Metadata Definitions.........47
4.7. <LFBClassDefs> Element for LFB Class Definitions........44 4.7. <LFBClassDefs> Element for LFB Class Definitions........48
4.7.1. <derivedFrom> Element to Express LFB Inheritance...45 4.7.1. <derivedFrom> Element to Express LFB Inheritance...49
4.7.2. <inputPorts> Element to Define LFB Inputs..........46 4.7.2. <inputPorts> Element to Define LFB Inputs..........49
4.7.3. <outputPorts> Element to Define LFB Outputs........48 4.7.3. <outputPorts> Element to Define LFB Outputs........52
4.7.4. <attributes> Element to Define LFB Operational 4.7.4. <attributes> Element to Define LFB Operational
Attributes................................................50 Attributes................................................54
4.7.5. <capabilities> Element to Define LFB Capability 4.7.5. <capabilities> Element to Define LFB Capability
Attributes................................................53 Attributes................................................57
4.7.6. <description> Element for LFB Operational 4.7.6. <description> Element for LFB Operational
Specification.............................................54 Specification.............................................58
4.8. XML Schema for LFB Class Library Documents..............54 4.8. XML Schema for LFB Class Library Documents..............58
5. FE Attributes and Capabilities...............................63 5. FE Attributes and Capabilities...............................67
5.1. XML Schema for FE Attribute Documents...................64 5.1. XML Schema for FE Attribute Documents...................68
5.2. FEDocument..............................................68 5.2. FEDocument..............................................72
5.2.1. FECapabilities.....................................68 5.2.1. FECapabilities.....................................72
5.2.2. FEAttributes.......................................71 5.2.2. FEAttributes.......................................75
5.3. Sample FE Attribute Document............................73 5.3. Sample FE Attribute Document............................77
6. LFB Class Library............................................76 6. LFB Class Library............................................80
6.1. Port LFB................................................76 6.1. Port LFB................................................80
6.2. L2 Interface LFB........................................77 6.2. L2 Interface LFB........................................81
6.3. IP interface LFB........................................79 6.3. IP interface LFB........................................82
6.4. Classifier LFB..........................................80 6.4. Classifier LFB..........................................84
6.5. Next Hop LFB............................................81 6.5. Next Hop LFB............................................85
6.6. Rate Meter LFB..........................................83 6.6. Rate Meter LFB..........................................87
6.7. Redirector (de-MUX) LFB.................................84 6.7. Redirector (de-MUX) LFB.................................87
6.8. Packet Header Rewriter LFB..............................84 6.8. Packet Header Rewriter LFB..............................88
6.9. Counter LFB.............................................85 6.9. Counter LFB.............................................88
6.10. Dropper LFB............................................85 6.10. Dropper LFB............................................89
6.11. IPv4 Fragmenter LFB....................................86 6.11. IPv4 Fragmenter LFB....................................89
6.12. L2 Address Resolution LFB..............................86 6.12. L2 Address Resolution LFB..............................90
6.13. Queue LFB..............................................86 6.13. Queue LFB..............................................90
6.14. Scheduler LFB..........................................87 6.14. Scheduler LFB..........................................91
6.15. MPLS ILM/Decapsulation LFB.............................88 6.15. MPLS ILM/Decapsulation LFB.............................91
6.16. MPLS Encapsulation LFB.................................88 6.16. MPLS Encapsulation LFB.................................92
6.17. Tunnel Encapsulation/Decapsulation LFB.................88 6.17. Tunnel Encapsulation/Decapsulation LFB.................92
6.18. Replicator LFB.........................................89 6.18. Replicator LFB.........................................93
7. Satisfying the Requirements on FE Model......................89 7. Satisfying the Requirements on FE Model......................93
7.1. Port Functions..........................................90 7.1. Port Functions..........................................94
7.2. Forwarding Functions....................................90 7.2. Forwarding Functions....................................94
7.3. QoS Functions...........................................91 7.3. QoS Functions...........................................94
7.4. Generic Filtering Functions.............................91 7.4. Generic Filtering Functions.............................95
7.5. Vendor Specific Functions...............................91 7.5. Vendor Specific Functions...............................95
7.6. High-Touch Functions....................................91 7.6. High-Touch Functions....................................95
7.7. Security Functions......................................91 7.7. Security Functions......................................95
7.8. Off-loaded Functions....................................92 7.8. Off-loaded Functions....................................95
7.9. IPFLOW/PSAMP Functions..................................92 7.9. IPFLOW/PSAMP Functions..................................96
8. Using the FE model in the ForCES Protocol....................92 8. Using the FE model in the ForCES Protocol....................96
8.1. FE Topology Query.......................................94 8.1. FE Topology Query.......................................98
8.2. FE Capability Declarations..............................96 8.2. FE Capability Declarations..............................99
8.3. LFB Topology and Topology Configurability Query.........96 8.3. LFB Topology and Topology Configurability Query.........99
8.4. LFB Capability Declarations.............................96 8.4. LFB Capability Declarations............................100
8.5. State Query of LFB Attributes...........................97 8.5. State Query of LFB Attributes..........................101
8.6. LFB Attribute Manipulation..............................98 8.6. LFB Attribute Manipulation.............................101
8.7. LFB Topology Re-configuration...........................98 8.7. LFB Topology Re-configuration..........................102
9. Acknowledgments..............................................98 9. Acknowledgments.............................................102
10. Security Considerations.....................................99 10. Security Considerations....................................102
11. Normative References........................................99 11. Normative References.......................................102
12. Informative References......................................99 12. Informative References.....................................103
13. Authors' Addresses.........................................100 13. Authors' Addresses.........................................103
14. Intellectual Property Right................................101 14. Intellectual Property Right................................104
15. IANA consideration.........................................101 15. IANA consideration.........................................105
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC-2119]. this document are to be interpreted as described in [RFC-2119].
1. Definitions 1. Definitions
A set of terminology associated with the ForCES requirements is Terminology associated with the ForCES requirements is defined in
defined in [1] and is not copied here. The following list of [1] and is not copied here. The following list of terminology is
terminology is relevant to the FE model defined in this document. relevant to the FE model defined in this document.
FE Model -- The FE model is designed to model the logical FE Model -- The FE model is designed to model the logical
processing functions of an FE. The FE model proposed in this processing functions of an FE. The FE model proposed in this
document includes three components: the modeling of individual document includes three components: the modeling of individual
logical functional blocks (LFB model), the logical interconnection logical functional blocks (LFB model), the logical interconnection
between LFBs (LFB topology) and the FE level attributes, including between LFBs (LFB topology) and the FE level attributes, including
FE capabilities. The FE model provides the basis to define the FE capabilities. The FE model provides the basis to define the
information elements exchanged between the CE and the FE in the information elements exchanged between the CE and the FE in the
ForCES protocol. ForCES protocol.
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forwarding plane inside an FE. Note that more than one datapath forwarding plane inside an FE. Note that more than one datapath
can exist within an FE. can exist within an FE.
LFB (Logical Function Block) class (or type) -- A template LFB (Logical Function Block) class (or type) -- A template
representing a fine-grained, logically separable and well-defined representing a fine-grained, logically separable and well-defined
packet processing operation in the datapath. LFB classes are the packet processing operation in the datapath. LFB classes are the
basic building blocks of the FE model. basic building blocks of the FE model.
LFB (Logical Function Block) Instance -- As a packet flows through LFB (Logical Function Block) Instance -- As a packet flows through
an FE along a datapath, it flows through one or multiple LFB an FE along a datapath, it flows through one or multiple LFB
instances, with each implementing an instance of a certain LFB instances, where each LFB implements an instance of a specific LFB
class. There may be multiple instances of the same LFB in an FE's class. Multiple instances of the same LFB class can be present in
datapath. Note that we often refer to LFBs without distinguishing an FE's datapath. Note that we often refer to LFBs without
between LFB class and LFB instance when we believe the implied distinguishing between an LFB class and LFB instance when we
reference is obvious for the given context. believe the implied reference is obvious for the given context.
LFB Model -- The LFB model describes the content and structures in LFB Model -- The LFB model describes the content and structures in
an LFB, plus the associated data definition. There are four types an LFB, plus the associated data definition. Four types of
of information defined in the LFB model. The core part of the LFB information are defined in the LFB model. The core part of the LFB
model is the LFB class definitions; the other three types define model is the LFB class definitions; the other three types define
the associated data including common data types, supported frame the associated data including common data types, supported frame
formats and metadata. formats and metadata.
LFB Metadata -- Metadata is used to communicate per-packet state LFB Metadata -- Metadata is used to communicate per-packet state
from one LFB to another, but is not sent across the network. The from one LFB to another, but is not sent across the network. The
FE model defines how such metadata is identified, produced and FE model defines how such metadata is identified, produced and
consumed by the LFBs, but not how metadata is encoded within an consumed by the LFBs, but not how the per-packet state is
implementation. implemented within actual hardware.
LFB Attribute -- Operational parameters of the LFBs that must be LFB Attribute -- Operational parameters of the LFBs that must be
visible to the CEs are conceptualized in the FE model as the LFB visible to the CEs are conceptualized in the FE model as the LFB
attributes. The LFB attributes include, for example, flags, single attributes. The LFB attributes include: flags, single parameter
parameter arguments, complex arguments, and tables that the CE can arguments, complex arguments, and tables that the CE can read
read or/and write via the ForCES protocol. or/and write via the ForCES protocol.
LFB Topology -- Representation of how the LFB instances are LFB Topology -- A representation of the logical interconnection and
logically interconnected and placed along the datapath within one the placement of LFB instances along the datapath within one FE.
FE. Sometimes it is also called intra-FE topology, to be Sometimes this representation is called intra-FE topology, to be
distinguished from inter-FE topology. LFB topology is outside of distinguished from inter-FE topology. LFB topology is outside of
the LFB model, but is part of the FE model. the LFB model, but is part of the FE model.
FE Topology -- A representation of how the multiple FEs within a FE Topology -- A representation of how multiple FEs within a single
single NE are interconnected. Sometimes this is called inter-FE NE are interconnected. Sometimes this is called inter-FE topology,
topology, to be distinguished from intra-FE topology (i.e., LFB to be distinguished from intra-FE topology (i.e., LFB topology).
topology). An individual FE might not have the global knowledge of An individual FE might not have the global knowledge of the full FE
the full FE topology, but the local view of its connectivity with topology, but the local view of its connectivity with other FEs is
other FEs is considered to be part of the FE model. The FE considered to be part of the FE model. The FE topology is
topology is discovered by the ForCES base protocol or some other discovered by the ForCES base protocol or some other means.
means.
Inter-FE Topology -- See FE Topology. Inter-FE Topology -- See FE Topology.
Intra-FE Topology -- See LFB Topology. Intra-FE Topology -- See LFB Topology.
LFB class library -- A set of LFB classes that is identified as the LFB class library -- A set of LFB classes that has been identified
most common functions found in most FEs and hence should be defined as the most common functions found in most FEs and hence should be
first by the ForCES Working Group. defined first by the ForCES Working Group.
2. Introduction 2. Introduction
[2] specifies a framework by which control elements (CEs) can [2] specifies a framework by which control elements (CEs) can
configure and manage one or more separate forwarding elements (FEs) configure and manage one or more separate forwarding elements (FEs)
within a networking element (NE) using the ForCES protocol. The within a networking element (NE) using the ForCES protocol. The
ForCES architecture allows Forwarding Elements of varying ForCES architecture allows Forwarding Elements of varying
functionality to participate in a ForCES network element. The functionality to participate in a ForCES network element. The
implication of this varying functionality is that CEs can make only implication of this varying functionality is that CEs can make only
minimal assumptions about the functionality provided by FEs in an minimal assumptions about the functionality provided by FEs in an
NE. Before CEs can configure and control the forwarding behavior NE. Before CEs can configure and control the forwarding behavior
of FEs, CEs need to query and discover the capabilities and states of FEs, CEs need to query and discover the capabilities and states
of their FEs. [1] mandates that the capabilities, states and of their FEs. [1] mandates that the capabilities, states and
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within a networking element (NE) using the ForCES protocol. The within a networking element (NE) using the ForCES protocol. The
ForCES architecture allows Forwarding Elements of varying ForCES architecture allows Forwarding Elements of varying
functionality to participate in a ForCES network element. The functionality to participate in a ForCES network element. The
implication of this varying functionality is that CEs can make only implication of this varying functionality is that CEs can make only
minimal assumptions about the functionality provided by FEs in an minimal assumptions about the functionality provided by FEs in an
NE. Before CEs can configure and control the forwarding behavior NE. Before CEs can configure and control the forwarding behavior
of FEs, CEs need to query and discover the capabilities and states of FEs, CEs need to query and discover the capabilities and states
of their FEs. [1] mandates that the capabilities, states and of their FEs. [1] mandates that the capabilities, states and
configuration information be expressed in the form of an FE model. configuration information be expressed in the form of an FE model.
RFC 3444 [11] made the observation that information models (IMs) RFC 3444 [11] observed that information models (IMs) and data
and data models (DMs) are different because they serve different models (DMs) are different because they serve different purposes.
purposes. "The main purpose of an IM is to model managed objects "The main purpose of an IM is to model managed objects at a
at a conceptual level, independent of any specific implementations conceptual level, independent of any specific implementations or
or protocols used". "DMs, conversely, are defined at a lower level protocols used". "DMs, conversely, are defined at a lower level of
of abstraction and include many details. They are intended for abstraction and include many details. They are intended for
implementors and include protocol-specific constructs." Sometimes implementors and include protocol-specific constructs." Sometimes
it is difficult to draw a clear line between the two. The FE model it is difficult to draw a clear line between the two. The FE model
described in this document is first and foremost an information described in this document is primarily an information model, but
model, but it also includes some aspects of a data model, such as also includes some aspects of a data model, such as explicit
explicit definitions of the LFB class schema and FE schema. It is definitions of the LFB class schema and FE schema. It is expected
expected that this FE model will be used as the basis to define the that this FE model will be used as the basis to define the payload
payload for information exchange between the CE and FE in the for information exchange between the CE and FE in the ForCES
ForCES protocol. protocol.
2.1. Requirements on the FE model 2.1. Requirements on the FE model
[1] defines requirements, which must be satisfied by a ForCES FE [1] defines requirements that must be satisfied by a ForCES FE
model. To summarize, an FE model must define: model. To summarize, an FE model must define:
. Logically separable and distinct packet forwarding operations . Logically separable and distinct packet forwarding operations
in an FE datapath (logical functional blocks or LFBs); in an FE datapath (logical functional blocks or LFBs);
. The possible topological relationships (and hence the sequence . The possible topological relationships (and hence the sequence
of packet forwarding operations) between the various LFBs; of packet forwarding operations) between the various LFBs;
. The possible operational capabilities (e.g., capacity limits, . The possible operational capabilities (e.g., capacity limits,
constraints, optional features, granularity of configuration) constraints, optional features, granularity of configuration)
of each type of LFB; of each type of LFB;
. The possible configurable parameters (i.e., attributes) of . The possible configurable parameters (i.e., attributes) of
each type of LFB; each type of LFB;
. Metadata that may be exchanged between LFBs. . Metadata that may be exchanged between LFBs.
2.2. The FE Model in Relation to FE Implementations 2.2. The FE Model in Relation to FE Implementations
The FE model proposed here is based on an abstraction of distinct The FE model proposed here is based on an abstraction of distinct
logical functional blocks (LFBs), which are interconnected in a logical functional blocks (LFBs), which are interconnected in a
directed graph, and receive, process, modify, and transmit packets directed graph, and receive, process, modify, and transmit packets
along with metadata. Note that a real forwarding datapath along with metadata. The FE model should be designed such that
implementation should not be constrained by the model. On the different implementations of the forwarding datapath can be
contrary, the FE model should be designed such that different logically mapped onto the model with the functionality and sequence
implementations of the forwarding datapath can all be logically of operations correctly captured. However, the model itself does
mapped onto the model with the functionality and sequence of not directly address how a particular implementation maps to an LFB
operations correctly captured. However, the model itself does not topology. It is left to the forwarding plane vendors to define how
directly address the issue of how a particular implementation maps the FE functionality is represented using the FE model. Our goal
to an LFB topology. It is left to the forwarding plane vendors to is to design the FE model such that it is flexible enough to
define how the FE functionality is represented using the FE model. accommodate most common implementations.
Nevertheless, we do strive to design the FE model such that it is
flexible enough to accommodate most common implementations.
The LFB topology model for a particular datapath implementation The LFB topology model for a particular datapath implementation
MUST correctly capture the sequence of operations on the packet. MUST correctly capture the sequence of operations on the packet.
Metadata generation (by certain LFBs) must always precede any use Metadata generation (by certain LFBs) must always precede any use
of that metadata (by subsequent LFBs in the topology graph); this of that metadata (by subsequent LFBs in the topology graph); this
is required for logically consistent operation. Further, is required for logically consistent operation. Further,
modifications of packet fields that are subsequently used as inputs modification of packet fields that are subsequently used as inputs
for further processing must occur in the order specified in the for further processing must occur in the order specified in the
model for that particular implementation to ensure correctness. model for that particular implementation to ensure correctness.
2.3. The FE Model in Relation to the ForCES Protocol 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 The ForCES base protocol is used by the CEs and FEs to maintain the
communication channel between the CEs and FEs. The ForCES protocol communication channel between the CEs and FEs. The ForCES protocol
may be used to query and discover the inter-FE topology. The may be used to query and discover the inter-FE topology. The
details of a particular datapath implementation inside an FE, details of a particular datapath implementation inside an FE,
including the LFB topology, along with the operational capabilities including the LFB topology, along with the operational capabilities
and attributes of each individual LFB, are conveyed to the CE and attributes of each individual LFB, are conveyed to the CE
within information elements in the ForCES protocol. The model of within information elements in the ForCES protocol. The model of
an LFB class should define all of the information that would need an LFB class should define all of the information that needs to be
to be exchanged between an FE and a CE for the proper configuration exchanged between an FE and a CE for the proper configuration and
and management of that LFB. management of that LFB.
Definition of the various payloads of ForCES messages (irrespective Specifying the various payloads of the ForCES messages in a
of the transport protocol ultimately selected) cannot proceed in a systematic fashion is difficult without a formal definition of the
systematic fashion until a formal definition of the objects being objects being configured and managed (the FE and the LFBs within).
configured and managed (the FE and the LFBs within) is undertaken.
The FE Model document defines a set of classes and attributes for The FE Model document defines a set of classes and attributes for
describing and manipulating the state of the LFBs of an FE. These describing and manipulating the state of the LFBs within an FE.
class definitions themselves will generally not appear in the These class definitions themselves will generally not appear in the
ForCES protocol. Rather, ForCES protocol operations will reference ForCES protocol. Rather, ForCES protocol operations will reference
classes defined in this model, including relevant attributes (and classes defined in this model, including relevant attributes (and
operations if such are defined). the defined operations).
Section 8 provides more detailed discussion on how the FE model Section 8 provides more detailed discussion on how the FE model
should be used by the ForCES protocol. should be used by the ForCES protocol.
2.4. Modeling Language for FE Model 2.4. Modeling Language for the FE Model
Even though not absolutely required, it is beneficial to use a Even though not absolutely required, it is beneficial to use a
formal data modeling language to represent the conceptual FE model formal data modeling language to represent the conceptual FE model
described in this document and a full specification will be written described in this document. Use of a formal language can help to
using such a data modeling language. Using a formal language can enforce consistency and logical compatibility among LFBs. A full
help to enforce consistency and logical compatibility among LFBs. specification will be written using such a data modeling language.
In addition, the formal definition of the LFB classes has the The formal definition of the LFB classes has the potential to
potential to facilitate the eventual automation of some part of the facilitate the eventual automation of some part of the code
code generation process and the functional validation of arbitrary generation process and the functional validation of arbitrary LFB
LFB topologies. topologies.
Human readability was the most important factor considered when Human readability was the most important factor considered when
selecting the specification language. Encoding, decoding and selecting the specification language. Encoding, decoding and
transmission performance was not a selection factor for the transmission performance was not a selection factor for the
language because the encoding method for over the wire transport is language because the encoding method for over the wire transport is
an issue independent of the specification language chosen. It is an issue independent of the specification language chosen. It is
outside the scope of this document and up to the ForCES protocol to outside the scope of this document and up to the ForCES protocol to
define. define.
XML was chosen as the specification language in this document, XML was chosen as the specification language in this document,
because XML has the advantage of being both human and machine because XML has the advantage of being both human and machine
readable with widely available tools support. readable with widely available tools support.
2.5. Document Structure 2.5. Document Structure
Section 3 provides a conceptual overview of the FE model, laying Section 3 provides a conceptual overview of the FE model, laying
the foundation for the more detailed discussion and specifications the foundation for the more detailed discussion and specifications
in the sections that follow. Section 4 and 5 constitute the core of in the sections that follow. Section 4 and 5 constitute the core
the FE model, detailing the two major components in the FE model: of the FE model, detailing the two major components in the FE
LFB model and FE level attributes including capability and LFB model: LFB model and FE level attributes including capability and
topology. Section 6 presents a list of LFB classes in the LFB LFB topology. Section 6 presents a list of LFB classes in the LFB
class library that will be further specified in separate documents class library that will be further specified in separate documents
according to the FE model presented in Sections 4 and 5. Section 7 according to the FE model presented in Sections 4 and 5. Section 7
directly addresses the model requirements imposed by the ForCES directly addresses the model requirements imposed by the ForCES
requirement draft [1] while Section 8 explains how the FE model requirement draft [1] while Section 8 explains how the FE model
should be used in the ForCES protocol. should be used in the ForCES protocol.
3. FE Model Concepts 3. FE Model Concepts
Some of the important concepts used throughout this document are Some of the important concepts used throughout this document are
introduced in this section. Section 3.1 explains the difference introduced in this section. Section 3.1 explains the difference
between a state model and a capability model, and how the two can between a state model and a capability model, and how the two can
be combined in the FE model. Section 3.2 introduces the concept of be combined in the FE model. Section 3.2 introduces the concept of
LFBs (Logical Functional Blocks) as the basic functional building LFBs (Logical Functional Blocks) as the basic functional building
blocks in the FE model. Section 3.3 discusses the logical inter- blocks in the FE model. Section 3.3 discusses the logical inter-
connection and ordering between LFB instances within an FE, that connection and ordering between LFB instances within an FE, that
is, the LFB topology. is, the LFB topology.
The FE model proposed in this document is comprised of two major The FE model proposed in this document is comprised of two major
components: LFB model, and FE level attributes including FE components: LFB model and FE level attributes, including FE
capabilities and LFB topology. The LFB model provides the content capabilities and LFB topology. The LFB model provides the content
and data structures to define each individual LFB class. FE and data structures to define each individual LFB class. FE
attributes provide information at the FE level and the capabilities attributes provide information at the FE level particularly the
about what the FE can or cannot do at a coarse level. Part of the capabilities of the FE at a coarse level. Part of the FE level
FE level information is the LFB topology which expresses the information is the LFB topology, which expresses the logical inter-
logical inter-connection between the LFB instances along the connection between the LFB instances along the datapath(s) within
datapath(s) within the FE. Details on these components are the FE. Details of these components are described in Section 4 and
described in Section 4 and 5. The intention of this section is to 5. The intent of this section is to discuss these concepts at the
discuss these concepts at the high level and lay the foundation for high level and lay the foundation for the detailed description in
the detailed description in the following sections. the following sections.
3.1. State Model and Capability Model 3.1. FE Capability Model and State Model
The FE capability model describes the capabilities and capacities The ForCES FE model must describe both a capability and a state
of an FE in terms of variations of functions supported or model. The FE capability model describes the capabilities and
limitations contained. Conceptually, the FE capability model capacities of an FE by specifying the variation in functions
presents the many possible states allowed on an FE with capacity supported and any limitations. The FE state model describes the
information indicating certain quantitative limits or constraints. current state of the FE, that is, the instantaneous values or
For example, an FE capability model may describe the FE at a coarse operational behavior of the FE.
level such as:
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 which configurations are applicable to a particular
FE and which ones are not. 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 handle IPv4 and IPv6 forwarding;
. this FE can perform classification on the following fields: . this FE can perform classification on the following fields:
source IP address, destination IP address, source port number, source IP address, destination IP address, source port number,
destination port number, etc; destination port number, etc;
. this FE can perform metering; . this FE can perform metering;
. this FE can handle up to N queues (capacity); . this FE can handle up to N queues (capacity);
. this FE can add and remove encapsulating headers of types . this FE can add and remove encapsulating headers of types
including IPSec, GRE, L2TP. including IPSec, GRE, L2TP.
On the other hand, an FE state model describes the current state of While one could try and build an object model to fully represent
the FE, that is, the instantaneous values or operational behavior the FE capabilities, other efforts found this to be a significant
of the FE. The FE state model presents the snapshot view of the FE undertaking. The main difficulty arises in describing detailed
to the CE. For example, using an FE state model, an FE may be limits, such as the maximum number of classifiers, queues, buffer
described to its CE as the following: pools, and meters the FE can provide. We believe that a good
. on a given port the packets are classified using a given balance between simplicity and flexibility can be achieved for the
FE model by combining the 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].
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; classification filter;
. the given classifier results in packets being metered in a . the given classifier results in packets being metered in a
certain way, and then marked in a certain way; certain way, and then marked in a certain way;
. the packets coming from specific markers are delivered into a . the packets coming from specific markers are delivered into a
shared queue for handling, while other packets are delivered shared queue for handling, while other packets are delivered
to a different queue; to a different queue;
. a specific scheduler with specific behavior and parameters . a specific scheduler with specific behavior and parameters
will service these collected queues. will service these collected queues.
The information on the capabilities and capacities of the FE helps
the CE understand the flexibility and limitations of the FE
functions, so that the CE knows at a coarse level which
configurations are applicable to the FEs and which ones are not.
It gets more complicated for the capability model to cope with the
detailed limits, such as the maximum number of the following items:
classifiers, queues, buffer pools, and meters the FE can provide.
While one could try to build an object model to fully represent the
FE capabilities, other efforts have found this to be a significant
undertaking. A middle of the road approach is to define coarse-
grained capabilities and simple capacity measures. Then, if the CE
attempts to instruct the FE to set up some specific behavior it is
not capable of, the FE will return an error indicating the problem.
Examples of this approach include Framework Policy Information Base
(PIB) [RFC3318) and Differentiated Services QoS Policy Information
Base [4]. The capability reporting classes in the DiffServ and
Framework PIBs are all meant to allow the device to indicate some
general guidelines about what it can or cannot do, but do not
necessarily allow it to indicate every possible configuration that
it can or cannot support. If a device receives a configuration
that it cannot implement, it can reject that configuration by
responding with a failure report.
Figure 1 shows the concepts of FE state, capabilities and Figure 1 shows the concepts of FE state, capabilities and
configuration in the context of CE-FE communication via the ForCES configuration in the context of CE-FE communication via the ForCES
protocol. protocol.
+-------+ +-------+ +-------+ +-------+
| | FE capabilities: what it can/cannot do. | | | | FE capabilities: what it can/cannot do. | |
| |<-----------------------------------------| | | |<-----------------------------------------| |
| | | | | | | |
| CE | FE state: what it is now. | FE | | CE | FE state: what it is now. | FE |
| |<-----------------------------------------| | | |<-----------------------------------------| |
| | | | | | | |
| | FE configuration: what it should be. | | | | FE configuration: what it should be. | |
| |----------------------------------------->| | | |----------------------------------------->| |
+-------+ +-------+ +-------+ +-------+
Figure 1. Illustration of FE state, capabilities and configuration Figure 1. Illustration of FE state, capabilities and configuration
exchange in the context of CE-FE communication via ForCES. exchange in the context of CE-FE communication via ForCES.
The ForCES FE model must include both a state model and a The concepts relating to LFB, particularly capability at the LFB
capability model. We believe that a good balance between level, and LFB topology will be discussed in the rest of this
simplicity and flexibility can be achieved for the FE model by section.
combining the coarse level capability reporting with the error
reporting mechanism. Examples of similar approaches include
DiffServ PIB [4] and Framework PIB [5].
The concepts of LFB and LFB topology will be discussed in the rest
of this section. It will become clear that a capability model is
needed at both the FE level and LFB level.
Capability information at the LFB level is an integral part of the Capability information at the LFB level is an integral part of the
LFB model, and is modeled the same way as the other operational LFB model, and is modeled the same way as the other operational
parameters inside an LFB. For example, certain features of an LFB parameters inside an LFB. For example, certain features of an LFB
class may be optional, in which case it must be possible for the CE class may be optional, in which case it must be possible for the CE
to determine whether or not an optional feature is supported by a to determine whether or not an optional feature is supported by a
given LFB instance. Such capability information can be modeled as given LFB instance. Such capability information can be modeled as
a read-only attribute in the LFB instance, see Section 4.7.5 for a read-only attribute in the LFB instance, see Section 4.7.5 for
details. details.
Capability information at the FE level may describe the LFB classes Capability information at the FE level may describe the LFB classes
the FE can instantiate; the number of instances of each can be the FE can instantiate; the number of instances of each that can be
created; the topological (i.e., linkage) limitations between these created; the topological (i.e., linkage) limitations between these
LFB instances, etc. Section 5 defines the FE level attributes LFB instances, etc. Section 5 defines the FE level attributes
including capability information. including capability information.
Once the FE capability is described to the CE, the FE state Once the FE capability is described to the CE, the FE state
information can be represented by two levels. The first level is information can be represented by two levels. The first level is
the logically separable and distinctive packet processing the logically separable and distinctive packet processing
functions, and we call these individual functions Logical functions, called Logical Functional Blocks (LFBs). The second
Functional Blocks (LFBs). The second level of information is about level of information describes how these individual LFBs are
how these individual LFBs are ordered and placed along the datapath ordered and placed along the datapath to deliver a complete
to deliver a complete forwarding plane service. The forwarding plane service. The interconnection and ordering of the
interconnection and ordering of the LFBs is called LFB Topology. LFBs is called LFB Topology. Section 3.2 discusses high level
Section 3.2 discuss high level concepts around LFBs while Section concepts around LFBs, whereas Section 3.3 discusses LFB topology
3.3 discuss issues around LFB topology. issues.
3.2. LFB (Logical Functional Block) Modeling 3.2. LFB (Logical Functional Block) Modeling
Each LFB performs a well-defined action or computation on the Each LFB performs a well-defined action or computation on the
packets passing through it. Upon completion of such a function, packets passing through it. Upon completion of such a function,
either the packets are modified in certain ways (e.g., either the packets are modified in certain ways (e.g.,
decapsulator, marker), or some results are generated and stored, decapsulator, marker), or some results are generated and stored,
probably in the form of metadata (like a classifier). Each LFB often in the form of metadata (like a classifier). Each LFB
typically does one thing and one thing only. Classifiers, shapers, typically performs a single action. Classifiers, shapers, meters
meters are all examples of LFBs. Modeling LFBs at such a fine are all examples of such LFBs. Modeling LFBs at such a fine
granularity allows us to use a small number of LFBs to create the granularity allows us to use a small number of LFBs to express the
higher-order FE functions (such as an IPv4 forwarder) precisely, higher-order FE functions (such as an IPv4 forwarder) precisely,
which in turn can describe more complex networking functions and which in turn can describe more complex networking functions and
vendor implementations of software and hardware. Section 6 provides vendor implementations of software and hardware. Section 6
a list of useful LFBs with such granularity. provides a list of useful LFBs with such granularity.
An LFB has one or more inputs, each of which takes a packet P, and 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 optionally metadata M; and produces one or more outputs, each of
which carries a packet P', and optionally metadata M'. Metadata is which carries a packet P', and optionally metadata M'. Metadata is
data associated with the packet in the network processing device data associated with the packet in the network processing device
(router, switch, etc.) and passed from one LFB to the next, but not (router, switch, etc.) and is passed from one LFB to the next, but
sent across the network. It is most likely that there are multiple is not sent across the network. In general, multiple LFBs are
LFBs within one FE, as shown in Figure 2, and all the LFBs share contained in one FE, as shown in Figure 2, and all the LFBs share
the same ForCES protocol termination point that implements the the same ForCES protocol termination point that implements the
ForCES protocol logic and maintains the communication channel to ForCES protocol logic and maintains the communication channel to
and from the CE. and from the CE.
+-----------+ +-----------+
| CE | | CE |
+-----------+ +-----------+
^ ^
| Fp reference point | Fp reference point
| |
skipping to change at page 13, line 4 skipping to change at page 12, line 32
| | :LFB1 | | : LFB2 | | | | :LFB1 | | : LFB2 | |
| =====>| v |============>| v |======>...| | =====>| v |============>| v |======>...|
| Inputs| +----------+ |Outputs | +----------+ | | | Inputs| +----------+ |Outputs | +----------+ | |
| (P,M) | |Attributes| |(P',M') | |Attributes| |(P",M") | | (P,M) | |Attributes| |(P',M') | |Attributes| |(P",M") |
| | +----------+ | | +----------+ | | | | +----------+ | | +----------+ | |
| +--------------+ +--------------+ | | +--------------+ +--------------+ |
| | | |
+--------------------------------------------------------------+ +--------------------------------------------------------------+
Figure 2. Generic LFB Diagram Figure 2. Generic LFB Diagram
An LFB, as shown in Figure 2, has inputs, outputs and attributes An LFB, as shown in Figure 2, has inputs, outputs and attributes
that can be queried and manipulated by the CE indirectly via Fp that can be queried and manipulated by the CE indirectly via an Fp
reference point (defined in [2]) and the ForCES protocol reference point (defined in [2]) and the ForCES protocol
termination point. The horizontal axis is in the forwarding plane termination point. The horizontal axis is in the forwarding plane
for connecting the inputs and outputs of LFBs within the same FE. for connecting the inputs and outputs of LFBs within the same FE.
The vertical axis between the CE and the FE denotes the Fp The vertical axis between the CE and the FE denotes the Fp
reference point where bidirectional communication between the CE reference point where bidirectional communication between the CE
and FE happens: the CE to FE communication is for configuration, and FE occurs: the CE to FE communication is for configuration,
control and packet injection while FE to CE communication is used control and packet injection while FE to CE communication is used
for packet re-direction to the control plane, monitoring and for packet re-direction to the control plane, monitoring and
accounting information, errors, etc. Note that the interaction accounting information, errors, etc. Note that the interaction
between the CE and the LFB is only abstract and indirect. The between the CE and the LFB is only abstract and indirect. The
result of such interaction is for the CE to indirectly manipulate result of such an interaction is for the CE to indirectly
the attributes of the LFB instances. manipulate the attributes of the LFB instances.
A namespace is used to associate a unique name or ID with each LFB A namespace is used to associate a unique name or ID with each LFB
class. The namespace must be extensible so that new LFB class can class. The namespace must be extensible so that a new LFB class
also be added later to accommodate future innovation in the can also be added later to accommodate future innovation in the
forwarding plane. forwarding plane.
LFB operation must be specified in the model to allow the CE to LFB operation must be specified in the model to allow the CE to
understand the behavior of the forwarding datapath. For instance, understand the behavior of the forwarding datapath. For instance,
the CE must understand at what point in the datapath the IPv4 the CE must understand at what point in the datapath the IPv4
header TTL is decremented (i.e., it needs to know if a control header TTL is decremented (i.e., it needs to know if a control
packet could be delivered to the CE either before or after this packet could be delivered to the CE either before or after this
point in the datapath). In addition, the CE must understand where point in the datapath). In addition, the CE must understand where
and what type of header modifications (e.g., tunnel header append and what type of header modifications (e.g., tunnel header append
or strip) are performed by the FEs. Further, the CE must verify or strip) are performed by the FEs. Further, the CE must verify
that various LFBs along a datapath within an FE are compatible to that the various LFBs along a datapath within an FE are compatible
link together. to link together.
There is value to vendors if the operation of LFB classes can be There is value to vendors if the operation of LFB classes can be
expressed in sufficient detail so that physical devices expressed in sufficient detail so that physical devices
implementing different LFB functions can be integrated easily into implementing different LFB functions can be integrated easily into
an FE design. Therefore, a semi-formal specification is needed; an FE design. Therefore, a semi-formal specification is needed;
that is, a text description of the LFB operation (human readable), that is, a text description of the LFB operation (human readable),
but sufficiently specific and unambiguous to allow conformance but sufficiently specific and unambiguous to allow conformance
testing and efficient design (i.e., eliminate guess-work), so that testing and efficient design (i.e., eliminate guess-work), so that
interoperability between different CEs and FEs can be achieved. interoperability between different CEs and FEs can be achieved.
The LFB class model specifies information like: The LFB class model specifies information such as:
. number of inputs and outputs (and whether they are . number of inputs and outputs (and whether they are
configurable) configurable)
. metadata read/consumed from inputs; . metadata read/consumed from inputs;
. metadata produced at the outputs; . metadata produced at the outputs;
. packet type(s) accepted at the inputs and emitted at the . packet type(s) accepted at the inputs and emitted at the
outputs; outputs;
. packet content modifications (including encapsulation or . packet content modifications (including encapsulation or
decapsulation); decapsulation);
. packet routing criteria (when multiple outputs on an LFB are . packet routing criteria (when multiple outputs on an LFB are
present); present);
skipping to change at page 14, line 19 skipping to change at page 13, line 49
. packet flow ordering modifications; . packet flow ordering modifications;
. LFB capability information; . LFB capability information;
. LFB operational attributes, etc. . LFB operational attributes, etc.
Section 4 of this document provides a detailed discussion of the Section 4 of this document provides a detailed discussion of the
LFB model with a formal specification of LFB class schema. The LFB model with a formal specification of LFB class schema. The
rest of Section 3.2 only intends to provide a conceptual overview rest of Section 3.2 only intends to provide a conceptual overview
of some important issues in LFB modeling, without covering all the of some important issues in LFB modeling, without covering all the
specific details. specific details.
3.2.1. LFB Input and Input Group 3.2.1. LFB Outputs
An LFB output is a conceptual port on an LFB that can send
information to another LFB. The information is typically a packet
and its associated metadata, although in some cases it might
consist of only metadata, i.e., with no packet data.
An LFB input is a conceptual port of the LFB where the LFB can A single LFB output can be connected to only one LFB input. This
receive information from other LFBs. The information is typically a is required to make the packet flow through the LFB topology
packet (or frame in general) and associated metadata, although in unambiguously.
some cases it might consist of only metadata, i.e., with a Null-
packet.
It is inevitable that there will be LFB instances that will receive Some LFBs will have a single output, as depicted in Figure 3.a.
packets from more than one other LFB instances (fan-in). If these
fan-in links all carry the same type of information (packet type
and set of metadata) and require the same processing within the
LFB, then one input should be sufficient. If, however, the LFB
class can receive two or more very different types of input, and
the processing of these inputs are also very distinct, then that
may justify the definition of multiple inputs. But in these cases
splitting the LFB class into two LFB classes should always be
considered as an alternative. In intermediate cases, e.g., where
the inputs are somewhat different but they require very similar
processing, the shared input solution should be preferred. For
example, if an Ethernet framer LFB is capable of receiving IPv4 and
IPv6 packets, these can be served by the same LFB input.
Note that we assume the model allows for connecting more than one +---------------+ +-----------------+
LFB output to a single LFB input directly. There is no restriction | | | |
on the number of up-stream LFBs connecting their outputs to the | | | OUT +-->
same input of a single LFB instance. Note that the behavior of the ... OUT +--> ... |
system when multiple packets arrive at such an input simultaneously | | | EXCEPTIONOUT +-->
is not defined by the model. If such behavior needs to be | | | |
described, it can be done either by separating the single input to +---------------+ +-----------------+
become multiple inputs (one per output), or by inserting other
appropriate LFBs (such as Queues and possibly Schedulers) between
the multiple outputs and the single input.
If there are multiple inputs with the same input type, we model a. One output b. Two distinct outputs
them as an input group, that is, multiple instances of the same
input type. In general, an input group is useful to allow an LFB
to differentiate packet treatment based on where the packet came
from.
+----+ +----+ +---------------+ +-----------------+
|LFB1+---+ |LFB1+---+ | | | EXCEPTIONOUT +-->
+----+ | +---------+ +----+ | +-----------+ | OUT:1 +--> | |
+--->|in LFB3 | input / +--->|in:1 LFB3 | ... OUT:2 +--> ... OUT:1 +-->
+----+ | +---------+ group \ +--->|in:2 | | ... +... | OUT:2 +-->
|LFB2+---+ +----+ | +-----------+ | OUT:n +--> | ... +...
+----+ |LFB2+---+ +---------------+ | OUT:n +-->
+----+ +-----------------+
(a) without input group (b) with input group c. One output group d. One output and one output group
Figure 3. An example of using input group. Figure 3. Examples of LFBs with various output combinations.
Consider the following two cases in Figure 3(a) and (b). In Figure To accommodate a non-trivial LFB topology, multiple LFB outputs are
3(a), the output from two LFBs are directly connected into one needed so that an LFB class can fork the datapath. Two mechanisms
input of LFB3, assuming that it can be guaranteed that no two are provided for forking: multiple singleton outputs and output
packets arrive at the same time instance. If LFB3 must do groups (the two concepts can be also combined in the same LFB
something different based on the source of the packet (LFB1 or class).
LFB2), the only way to model that is to make LFB1 and LFB2 pass
some metadata with different values so that LFB3 can make the
differentiation based on that metadata. In Figure 3(b), that
differentiation can be elegantly expressed within LFB3 using the
input group concept where the instance id can server as the
differentiating key. For example, a scheduler LFB can potentially
use an input group consisting of a variable number of inputs to
differentiate the queues from which the packets are coming.
3.2.2. LFB Output and Output Group 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 is known when the LFB class is
defined. Additional singleton outputs cannot be created at LFB
instantiation time, nor can they be created on the fly after the
LFB is instantiated.
An LFB output is a conceptual port of the LFB that can send For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one
information to some other LFBs. The information is typically a output(OUT) to send those packets for which the LPM look-up was
packet (or frame in general) and associated metadata, although in successful (passing a META_ROUTEID as metadata); and have another
some cases it might emit only metadata, i.e., with a Null-packet. 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, plus they also carry different metadata. This concept assumes
that the number of distinct outputs is known when the LFB class is
defined. For each singleton output, the LFB class definition
defines what types of frames and metadata the output emits.
We assume that a single LFB output can be connected to only one LFB An output group, on the other hand, is used to model the case where
input (this is required to make the packet flow through the LFB a flow of seemingly similar packets with an identical set of
topology unambiguous). Therefore, to allow any non-trivial metadata needs to be split into multiple paths, and where the
topology, multiple outputs must be allowed for an LFB class. If number of such paths is not known when the LFB class is defined
there are multiple outputs with the same output type, we model them (i.e., because it is not an inherent property of the LFB class).
as output group, that is, multiple instances of the same output An output group consists of a number of outputs (called the output
type. For illustration of output group, consider the hypothetical instances of the group), all sharing the same frame and metadata
LFB in Figure 4. The LFB has two types of outputs, one of which emission definitions (see Figure 3.c). Each output instance can
can be instantiated to form an output group. connect to a different downstream LFB, just as if they were
separate singleton outputs. But the number of output instances can
be different between one instance of the LFB class and another.
The class definition may include a lower and/or an upper limit on
the number of output instances. In addition, for configurable FEs,
the FE capability information may include 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
| UNPROC +--> direct packets to one of N downstream paths based on one of the
| | metadata associated with each arriving packet. Such an LFB is
| PKTOUT:1 +--> \ fairly versatile and can be used in many different places in a
--> PKTIN PKTOUT:2 +--> | topology. For example, a redirector can be used to divide the data
| . + . | Output group 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
| PKTOUT:N +--> / the COLOR metadata (red, yellow, green; N=3), etc.
+------------------+
Figure 4. An example of an LFB with output group. 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.
Multiple outputs should mainly be used for functional separation Note that other LFBs may also use the output group concept to build
where the outputs are connected to very different types of LFBs. in similar adaptive forking capability. For example, a classifier
For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one LFB with one input and N outputs can be defined easily by using the
default output to send those packets for which look-up was output group concept. Alternatively, a classifier LFB with one
successful (passing a META_ROUTEID as metadata); and have another singleton output in combination with an explicit N-output re-
output for sending packets for which the look-up failed. The director LFB models the same processing behavior. The decision of
former output may be connected to a route handler LFB, while the whether to use the output group model for a certain LFB class is
latter can be connected to an ICMP response generator LFB or to a left to the LFB class designers.
packet handler LFB that passes the packet up to the CE.
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, i.e., 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 of the latter
two. 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
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, i.e., with no 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:
- Implicit multiplexing via a single input
- Explicit multiplexing via multiple singleton inputs
- Explicit multiplexing via a group of inputs (input group)
The above modes can be combined in the same LFB.
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 for more
than one LFB output to connect to the same input of an LFB. 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 this 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,
i.e., by defining how the LFB class handles the contending packets.
+--------------+ +------------------------+
| LFB X +---+ | |
+--------------+ | | |
| | |
+--------------+ v | |
| LFB Y +---+-->|input Meter LFB |
+--------------+ ^ | |
| | |
+--------------+ | | |
| LFB Z |---+ | |
+--------------+ +------------------------+
(a) An LFB connects with multiple upstream LFBs via a single input.
+--------------+ +------------------------+
| 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
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 the contention handling
must be modeled explicitly, but the number of inputs are not
inherent from the class (and hence 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 3.2.3. Packet Type
When LFB classes are defined, the input and output packet formats When LFB classes are defined, the input and output packet formats
(e.g., IPv4, IPv6, Ethernet, etc.) must be specified: these are the (e.g., IPv4, IPv6, Ethernet, etc.) must be specified: these are the
types of packets a given LFB input is capable of receiving and types of packets a given LFB input is capable of receiving and
processing, or a given LFB output is capable of producing. This processing, or a given LFB output is capable of producing. This
requires that distinct frame types be uniquely labeled with a requires that distinct packet types be uniquely labeled with a
symbolic name and/or ID. symbolic name and/or ID.
Note that each LFB has a set of packet types that it operates on, Note that each LFB has a set of packet types that it operates on,
but it does not care about whether the underlying implementation is but does not care about whether the underlying implementation is
passing a greater portion of the packets. For example, an IPv4 LFB passing a greater portion of the packets. For example, an IPv4 LFB
might only operate on IPv4 packets, but the underlying might only operate on IPv4 packets, but the underlying
implementation may or may not be stripping the L2 header before implementation may or may not be stripping the L2 header before
handing it over -- whether that is happening or not is opaque to handing it over -- whether that is happening or not is opaque to
the CE. the CE.
3.2.4. Metadata 3.2.4. Metadata
Metadata is the per-packet state that is passed from one LFB to Metadata is the per-packet state that is passed from one LFB to
another. The metadata is passed with the packet to assist with another. The metadata is passed with the packet to assist
further processing of that packet. The ForCES model must capture subsequent LFBs to process that packet. The ForCES model captures
how the per-packet state information is propagated from one LFB to how the per-packet state information is propagated from one LFB to
other LFBs. Practically, such metadata propagation can happen other LFBs. Practically, such metadata propagation can happen
within one FE, or cross the FE boundary between two interconnected within one FE, or cross the FE boundary between two interconnected
FEs. We believe that the same metadata model can be used for both FEs. We believe that the same metadata model can be used for both
situations, however, our focus here is for intra-FE metadata. situations, 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. Our 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 "explicit", which describes
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 "explicit"
metadata. In situations where the metadata is not physically
represented, it is called "implicit" metadata. This situation
occurs, for example, when a particular path through a network
device is intended to be traversed only by particular kinds of
packets, such as an IPv4 router. An implementation may not mark
every packet along this path as being of type "IPv4", but the
intention of the designers is that every packet is of that type.
This understanding can be thought of as metadata about the packet,
which is implicitly attached to the packet through the intent of
the designers.
In the ForCES model, we do NOT discuss or represent metadata
"internal" to vendor implementations of LFBs. Our focus is solely
on metadata "external" to the LFBs, and therefore visible in the
ForCES model. The metadata discussed within this model may, or may
not, be visible outside of the particular FE implementing the LFB
model. In this regard, the scope of the metadata within ForCES is
very narrowly defined.
Note also that while we define metadata within this model, it is
only a model. There is no requirement that vendor implementations
of ForCES use the exact metadata representations described in this
document. The only implementation requirement is that vendors
implement the ForCES protocol, not the model.
3.2.4.2. Metadata lifecycle within the ForCES model
Each metadata can be conveniently modeled as a <label, value> pair, Each metadata can be conveniently modeled as a <label, value> pair,
where the label identifies the type of information, (e.g., where the label identifies the type of information, (e.g.,
"color"), and its value holds the actual information (e.g., "red"). "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 The tag here is shown as a textual label, but it can be replaced or
associated with a unique numeric value (identifier). associated with a unique numeric value (identifier).
The metadata life-cycle is defined in this model using three types The metadata life-cycle is defined in this model using three types
of events: "write", "read" and "consume". The first "write" of events: "write", "read" and "consume". The first "write"
initializes the value of the metadata (implicitly creating and/or initializes the value of the metadata (implicitly creating and/or
initializing the metadata), and hence starts the life-cycle. The initializing the metadata), and hence starts the life-cycle. The
explicit "consume" event terminates the life-cycle. Within the explicit "consume" event terminates the life-cycle. Within the
life-cycle, that is, after a "write" event, but before the next life-cycle, that is, after a "write" event, but before the next
"consume" event, there can be an arbitrary number of "write" and "consume" event, there can be an arbitrary number of "write" and
"read" events. These "read" and "write" events can be mixed in an "read" events. These "read" and "write" events can be mixed in an
arbitrary order within the life-cycle. Outside of the life-cycle of arbitrary order within the life-cycle. Outside of the life-cycle
the metadata, that is, before the first "write" event, or between a of the metadata, that is, before the first "write" event, or
"consume" event and the next "write" event, the metadata should be between a "consume" event and the next "write" event, the metadata
regarded non-existent or non-initialized. Thus, reading a metadata should be regarded non-existent or non-initialized. Thus, reading
outside of its life-cycle is considered an error. a metadata outside of its life-cycle is considered an error.
To ensure inter-operability between LFBs, the LFB class To ensure inter-operability between LFBs, the LFB class
specification must define what metadata the LFB class "reads" or specification must define what metadata the LFB class "reads" or
"consumes" on its input(s) and what metadata it "produces" on its "consumes" on its input(s) and what metadata it "produces" on its
output(s). For maximum extensibility, this definition should not output(s). For maximum extensibility, this definition should
specify which LFBs the metadata is expected to come from for a neither specify which LFBs the metadata is expected to come from
consumer LFB, or which LFBs are expected to consume metadata for a for a consumer LFB, nor which LFBs are expected to consume metadata
producer LFB. for a given producer LFB.
While it is important to define the metadata types passing between While it is important to define the metadata types passing between
LFBs, it is not necessary to define the exact encoding mechanism LFBs, it is not appropriate to define the exact encoding mechanism
used by LFBs for that metadata. Different implementations are used by LFBs for that metadata. Different implementations are
allowed to use different encoding mechanisms for metadata. For allowed to use different encoding mechanisms for metadata. For
example, one implementation may store metadata in registers or example, one implementation may store metadata in registers or
shared memory, while another implementation may encode metadata in- shared memory, while another implementation may encode metadata in-
band as a preamble in the packets. band as a preamble in the packets.
At any link between two LFBs, the packet is marked with a finite At any link between two LFBs, the packet is marked with a finite
set of active metadata, where active means the metadata is within set of active metadata, where active means the metadata is within
its life-cycle. (i.e., the metadata has been properly initialized its life-cycle. (i.e., the metadata has been properly initialized
and has not been consumed yet.) There are two corollaries of this and has not been consumed yet.) There are two corollaries of this
model: model:
1. No uninitialized metadata exists in the model. 1. No uninitialized metadata exists in the model.
2. No more than one occurrence of each metadata tag can be 2. No more than one occurrence of each metadata tag can be
associated with a packet at any given time. associated with a packet at any given time.
3.2.4.1. LFB Operations on Metadata 3.2.4.3. LFB Operations on Metadata
When the packet is processed by an LFB (i.e., between the time it 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, is received and forwarded by the LFB), the LFB may perform read,
write and/or consume operations on any active metadata associated 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 with the packet. If the LFB is considered to be a black box, one
the following operations is performed on each active metadata. of the following operations is performed on each active metadata.
- IGNORE: ignores and forwards the metadata - IGNORE: ignores and forwards the metadata
- READ: reads and forwards the metadata - READ: reads and forwards the metadata
- READ/RE-WRITE: reads, over-writes and forwards the metadata - READ/RE-WRITE: reads, over-writes and forwards the metadata
- WRITE: writes and forwards the metadata - WRITE: writes and forwards the metadata
(can also be used to create new metadata) (can also be used to create new metadata)
- READ-AND-CONSUME: reads and consumes the metadata - READ-AND-CONSUME: reads and consumes the metadata
- CONSUME consumes metadata without reading - CONSUME consumes metadata without reading
The last two operations terminate the life-cycle of the metadata, The last two operations terminate the life-cycle of the metadata,
meaning that the metadata is not forwarded with the packet when the meaning that the metadata is not forwarded with the packet when the
packet is sent to the next LFB. packet is sent to the next LFB.
In our model, a new metadata is generated by an LFB when the LFB In our model, a new metadata is generated by an LFB when the LFB
applies a WRITE operation into a metadata type that was not present applies a WRITE operation into a metadata type that was not present
when the packet was received by the LFB. Such implicit creation may when the packet was received by the LFB. Such implicit creation
be unintentional by the LFB, that is, the LFB may apply the WRITE may be unintentional by the LFB, that is, the LFB may apply the
operation without knowing or caring if the given metadata existed WRITE operation without knowing or caring if the given metadata
or not. If it existed, the metadata gets over-written; if it did existed or not. If it existed, the metadata gets over-written; if
not exist, the metadata gets created. it did not exist, the metadata is created.
For source-type LFBs (i.e., an LFB that inserts packets into the For source-type LFBs (i.e., an LFB that inserts packets into the
model), WRITE is the only meaningful metadata operation. model), WRITE is the only meaningful metadata operation.
Sink-type LFBs (i.e., an LFB that removes the packet from the Sink-type LFBs (i.e., an LFB that removes the packet from the
model), may either READ-AND-CONSUME (read) or CONSUME (ignore) each model), may either READ-AND-CONSUME (read) or CONSUME (ignore) each
active metadata associated with the packet. active metadata associated with the packet.
3.2.4.2. Metadata Production and Consumption 3.2.4.4. Metadata Production and Consumption
For a given metadata on a given packet path, there must be at least 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 producer LFB that creates that metadata and should be at least
one consumer LFB that needs the metadata. In this model, the one consumer LFB that needs the metadata. In this model, the
producer and consumer LFBs of a metadata are not required to be producer and consumer LFBs of a metadata are not required to be
adjacent. There may be multiple consumers for the same metadata and adjacent. There may be multiple consumers for the same metadata
there may be multiple producers of the same metadata. When a packet and there may be multiple producers of the same metadata. When a
path involves multiple producers of the same metadata, then the packet path involves multiple producers of the same metadata, then
second, third, etc. producers overwrite that metadata value. subsequent producers overwrite that metadata value.
The metadata that is produced by an LFB is specified by the LFB 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 class definition on a per output port group basis. A producer may
always generate the metadata on the port group, or may generate it always generate the metadata on the port group, or may generate it
only under certain conditions. We call the former an only under certain conditions. We call the former an
"unconditional" metadata, whereas the latter is a "conditional" "unconditional" metadata, whereas the latter is a "conditional"
metadata. In the case of conditional metadata, it should be metadata. In the case of conditional metadata, it should be
possible to determine from the definition of the LFB when a possible to determine from the definition of the LFB when a
"conditional" metadata is produced. "conditional" metadata is produced.
The consumer behavior of an LFB, that is, the metadata that the LFB 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 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 a per input port group basis. An input port group may "require" a
given metadata, or may treat it as "optional" information. In the given metadata, or may treat it as "optional" information. In the
latter case, the LFB class definition must explicitly define what latter case, the LFB class definition must explicitly define what
happens if an optional metadata is not provided. One approach is to happens if an optional metadata is not provided. One approach is
specify a default value for each optional metadata, and assume that to specify a default value for each optional metadata, and assume
the default value is used if the metadata is not provided with the that the default value is used if the metadata is not provided with
packet. the packet.
When a consumer requires a given metadata, it has dependencies on When a consumer requires a given metadata, it has dependencies on
its up-stream LFBs. That is, the consumer LFB can only function if 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 there is at least one producer of that metadata and no intermediate
LFB consumes the metadata. LFB consumes the metadata.
The model should expose this inter-dependency. Furthermore, it The model should expose this inter-dependency. Furthermore, it
should be possible to take this inter-dependency into consideration should be possible to take this inter-dependency into consideration
when constructing LFB topologies, and also that the dependency can when constructing LFB topologies, and also that the dependency can
be verified when validating topologies. be verified when validating topologies.
For extensibility reasons, the LFB specification should define what For extensibility reasons, the LFB specification should define what
metadata the LFB requires without specifying which LFB(s) it expect metadata the LFB requires without specifying which LFB(s) it
a certain metadata to come from. Similarly, LFBs should specify expects a certain metadata to come from. Similarly, LFBs should
what metadata they produce without specifying which LFBs the specify what metadata they produce without specifying which LFBs
metadata is meant for. the metadata is meant for.
When specifying the metadata tags, some harmonization effort must When specifying the metadata tags, some harmonization effort must
be made so that the producer LFB class uses the same tag as its be made so that the producer LFB class uses the same tag as its
intended consumer(s), or vice versa. intended consumer(s), or vice versa.
3.2.4.3. Fixed, Variable and Configurable Tag 3.2.4.5. Fixed, Variable and Configurable Tag
When the produced metadata is defined for a given LFB class, most When the produced metadata is defined for a given LFB class, most
metadata will be specified with a fixed tag. For example, a Rate metadata will be specified with a fixed tag. For example, a Rate
Meter LFB will always produce the "Color" metadata. Meter LFB will always produce the "Color" metadata.
A small subset of LFBs need to have the capability to produce one A small subset of LFBs need to have the capability to produce one
or more of their metadata with tags that are not fixed in the LFB 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 class definition, but instead can be selected per LFB instance. An
example of such an LFB class is a Generic Classifier LFB. We call example of such an LFB class is a Generic Classifier LFB. We call
this variable tag metadata production. If an LFB produces metadata this variable tag metadata production. If an LFB produces metadata
with variable tag, a corresponding LFB attribute--called the tag with a variable tag, a corresponding LFB attribute--called the tag
selector--specifies the tag for each such metadata. This mechanism selector--specifies the tag for each such metadata. This mechanism
is to improve the versatility of certain multi-purpose LFB classes, is to improve the versatility of certain multi-purpose LFB classes,
since it allows the same LFB class be used in different topologies, since it allows the same LFB class be used in different topologies,
producing the right metadata tags according to the needs of the producing the right metadata tags according to the needs of the
topology. topology.
Depending on the capability of the FE, the tag selector can be a Depending on the capability of the FE, the tag selector can be a
read-only or a read-write attribute. In the former case, the tag read-only or a read-write attribute. In the former case, the tag
cannot be modified by the CE. In the latter case the tag can be cannot be modified by the CE. In the latter case the tag can be
configured by the CE, hence we call this "configurable tag metadata configured by the CE, hence we call this "configurable tag metadata
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the different metadata needs. Most LFB classes will specify their the different metadata needs. Most LFB classes will specify their
metadata needs using fixed metadata tags. For example, a Next Hop metadata needs using fixed metadata tags. For example, a Next Hop
LFB may always require a "NextHopId" metadata; but the Redirector LFB may always require a "NextHopId" metadata; but the Redirector
LFB may need to use a "ClassID" metadata in one instance, and a LFB may need to use a "ClassID" metadata in one instance, and a
"ProtocolType" metadata in another instance as a basis for "ProtocolType" metadata in another instance as a basis for
selecting the right output port. In this case, an LFB attribute is selecting the right output port. In this case, an LFB attribute is
used to provide the required metadata tag at run-time. This used to provide the required metadata tag at run-time. This
metadata tag selector attribute may be read-only or read-write, metadata tag selector attribute may be read-only or read-write,
depending on the capabilities of the LFB instance and the FE. depending on the capabilities of the LFB instance and the FE.
3.2.4.4. Metadata Usage Categories 3.2.4.6. Metadata Usage Categories
Depending on the role and usage of a metadata, various amount of Depending on the role and usage of a metadata, various amount of
encoding information must be provided when the metadata is defined, encoding information must be provided when the metadata is defined,
and some cases offer less flexibility in the value selection than and some cases offer less flexibility in the value selection than
others. others.
As far as usage of a metadata is concerned, three types of metadata There are three types of metadata related to metadata usage:
exist:
- Relational (or binding) metadata - Relational (or binding) metadata
- Enumerated metadata - Enumerated metadata
- Explicit/external value metadata - Explicit/external value metadata
The purpose of the relational metadata is to refer in one LFB The purpose of the relational metadata is to refer in one LFB
instance (producer LFB) to a "thing" in another downstream LFB instance (producer LFB) to a "thing" in another downstream LFB
instance (consumer LFB), where the "thing" is typically an entry in instance (consumer LFB), where the "thing" is typically an entry in
a table attribute of the consumer LFB. a table attribute of the consumer LFB.
For example, the Prefix Lookup LFB executes an LPM search using its For example, the Prefix Lookup LFB executes an LPM search using its
prefix table and resolves to a next-hop reference. This reference prefix table and resolves to a next-hop reference. This reference
needs to be passed as metadata by the Prefix Lookup LFB (producer) 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 to the Next Hop LFB (consumer), and must refer to a specific entry
in the next-hop table within the consumer. in the next-hop table within the consumer.
Expressing and propagating such binding relationship is probably Expressing and propagating such a binding relationship is probably
the most common usage of metadata. One or more objects in the the most common usage of metadata. One or more objects in the
producer LFB are related (bound) to a specific object in the producer LFB are related (bound) to a specific object in the
consumer LFB. Such a relation is established by the CE very consumer LFB. Such a relationship is established by the CE very
explicitly, i.e., by properly configuring the attributes in both explicitly, i.e., by properly configuring the attributes in both
LFBs. Available methods include the following: LFBs. Available methods include the following:
The binding may be expressed by tagging the involved objects in The binding may be expressed by tagging the involved objects in
both LFBs with the same unique (but otherwise arbitrary) both LFBs with the same unique (but otherwise arbitrary)
identifier. The value of the tag is explicitly configured (written identifier. The value of the tag is explicitly configured (written
by the CE) into both LFBs, and this value is also the value that by the CE) into both LFBs, and this value is also carried by the
the metadata carries between the LFBs. metadata between the LFBs.
Another way of setting up binding relations is to use a naturally Another way of setting up binding relations is to use a naturally
occurring unique identifier of the consumer's object (for example, occurring unique identifier of the consumer's object (for example,
the array index of a table entry) as a reference (and as a value of the array index of a table entry) as a reference (and as a value of
the metadata. In this case, the index is obtained (read) or the metadata). In this case, the index is either read or inferred
inferred by the CE by communicating with the consumer LFB. Once the by the CE by communicating with the consumer LFB. Once the CE
CE obtains the index, it needs to plug (write) it into the producer obtains the index, it needs to write it into the producer LFB to
LFB to establish the binding. establish the binding.
Important characteristics of the binding usage of metadata are: Important characteristics of the binding usage of metadata are:
- The value of the metadata shows up in the CE-FE communication for - The value of the metadata shows up in the CE-FE communication for
BOTH the consumer and the producer. That is, the metadata value BOTH the consumer and the producer. That is, the metadata value
must be carried over the ForCES protocol. Using the tagging 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 both LFBs. Using the other
technique, the value is WRITTEN to only the producer LFB and may be technique, the value is WRITTEN to only the producer LFB and may be
READ from the consumer LFB. READ from the consumer LFB.
- The actual value is irrelevant for the CE, the binding is simply - The metadata value is irrelevant to the CE, the binding is simply
expressed by using the SAME value at the consumer and producer expressed by using the SAME value at the consumer and producer
LFBs. LFBs.
- Hence the definition of the metadata does not have to include - Hence the definition of the metadata is not required to include
value assignments. The only exception is when some special value(s) value assignments. The only exception is when some special
of the metadata must be reserved to convey special events. Even value(s) of the metadata must be reserved to convey special events.
though these special cases must be defined with the metadata Even though these special cases must be defined with the metadata
specification, their encoded values can be selected arbitrarily. specification, their encoded values can be selected arbitrarily.
For example, for the Prefix Lookup LFB example, a special value may 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 reserved to signal the NO-MATCH case, and the value of zero may
be assigned for this purpose. be assigned for this purpose.
The second class of metadata is the enumerated type. An example is The second class of metadata is the enumerated type. An example is
the "Color" metadata that is produced by a Meter LFB and consumed the "Color" metadata that is produced by a Meter LFB. As the name
by some other LFBs. As the name suggests, enumerated metadata has a suggests, enumerated metadata has a relatively small number of
relatively small number of possible values, each with a very possible values, each with a very specific meaning. All of the
specific meaning. All of the possible cases must be enumerated when possible cases must be enumerated when defining this class of
defining this class of metadata. Although a value encoding must be metadata. Although a value encoding must be included in the
included in the specification, the actual values can be selected specification, the actual values can be selected arbitrarily (e.g.,
arbitrarily (e.g., <Red=0, Yellow=1, Green=2> and <Red=3, Yellow=2, <Red=0, Yellow=1, Green=2> and <Red=3, Yellow=2, Green 1> would be
Green 1> would be both valid encodings, what is important is that both valid encodings, what is important is that an encoding is
an encoding is specified). specified).
The value of the enumerated metadata may or may not be conveyed via The value of the enumerated metadata may or may not be conveyed via
the ForCES protocol between the CE and FE. the ForCES protocol between the CE and FE.
The third class of metadata is the explicit type. This refers to The third class of metadata is the explicit type. This refers to
cases where the value of the metadata is explicitly used by the cases where the value of the metadata is explicitly used by the
consumer LFB to change some packet header fields. In other words, consumer LFB to change some packet header fields. In other words,
its value has a direct and explicit impact on some field and will its value has a direct and explicit impact on some field and will
be visible externally when the packet leaves the NE. Examples are: be visible externally when the packet leaves the NE. Examples are:
TTL increment given to a Header Modifier LFB, and DSCP value for a TTL increment given to a Header Modifier LFB, and DSCP value for a
Remarker LFB. For explicit metadata, the value encoding must be Remarker LFB. For explicit metadata, the value encoding must be
explicitly provided in the metadata definition, where the values explicitly provided in the metadata definition, the values cannot
cannot be selected arbitrarily, but rather they should conform to be selected arbitrarily, but rather they should conform to what is
what is commonly expected. For example, a TTL increment metadata commonly expected. For example, a TTL increment metadata should be
should encode with zero for the no increment case, by one for the encoded as zero for the no increment case, one for the single
single increment case, etc. A DSCP metadata should use 0 to encode increment case, etc. A DSCP metadata should use 0 to encode
DSCP=0, 1 to encode DSCP=1, etc. DSCP=0, 1 to encode DSCP=1, etc.
3.2.5. LFB Versioning 3.2.5. LFB Versioning
LFB class versioning is a method to enable incremental evolution of LFB class versioning is a method to enable incremental evolution of
LFB classes. Unlike inheritance (discussed next in Section 3.2.6), LFB classes. In general, an FE is not allowed to contain an LFB
where it assumed that an FE datapath model containing an LFB instance for more than one version of a particular class.
instance of a particular class C could also simultaneously contain Inheritance (discussed next in Section 3.2.6) has special rules. If
an LFB instance of a class C' inherited from class C; with an FE datapath model containing an LFB instance of a particular
versioning, an FE would not be allowed to contain an LFB instance class C also simultaneously contains an LFB instance of a class C'
for more than one version of a particular class. inherited from class C; C could have a different version than C'.
LFB class versioning is supported by requiring a version string in LFB class versioning is supported by requiring a version string in
the class definition. CEs may support backwards compatibility the class definition. CEs may support backwards compatibility
between multiple versions of a particular LFB class, but FEs are between multiple versions of a particular LFB class, but FEs are
not allowed to support more than one single version of a particular not allowed to support more than one single version of a particular
class. class.
3.2.6. LFB Inheritance 3.2.6. LFB Inheritance
LFB class inheritance is supported in the FE model as a means of LFB class inheritance is supported in the FE model as a method to
defining new LFB classes. This also allows FE vendors to add define new LFB classes. This also allows FE vendors to add vendor-
vendor-specific extensions to standardized LFBs. An LFB class specific extensions to standardized LFBs. An LFB class
specification MUST specify the base class (with version number) it specification MUST specify the base class (with version number) it
inherits from (with the default being the base LFB class). inherits from (with the default being the base LFB class).
Multiple-inheritance is not allowed, though, to avoid the Multiple-inheritance is not allowed, though, to avoid the
unnecessary complexity. unnecessary complexity.
Inheritance should be used only when there is significant reuse of Inheritance should be used only when there is significant reuse of
the base LFB class definition. A separate LFB class should be the base LFB class definition. A separate LFB class should be
defined if there is not enough reuse between the derived and the defined if little or no reuse is possible between the derived and
base LFB class. the base LFB class.
An interesting issue related to class inheritance is backward An interesting issue related to class inheritance is backward
compatibility (between a descendant and an ancestor class). compatibility (between a descendant and an ancestor class).
Consider the following hypothetical scenario where there exists a Consider the following hypothetical scenario where a standardized
standardized LFB class "L1". Vendor A builds an FE that implements LFB class "L1" exists. Vendor A builds an FE that implements LFB
LFB "L1" and vendors B builds a CE that can recognize and operate "L1" and vendor B builds a CE that can recognize and operate on LFB
on LFB "L1". Suppose that a new LFB class, "L2", is defined based "L1". Suppose that a new LFB class, "L2", is defined based on the
on the existing "L1" class (for example, by extending its existing "L1" class (for example, by extending its capabilities in
capabilities in some incremental way). Lets first examine the FE some incremental way). Lets first examine the FE backward
backward compatibility issue by considering what would happen if compatibility issue by considering what would happen if vendor B
vendor B upgrades its FE from "L1" to "L2" while vendor C's CE is upgrades its FE from "L1" to "L2" while vendor C's CE is not
not changed. The old L1-based CE can interoperate with the new L2- changed. The old L1-based CE can interoperate with the new L2-
based FE if the derived LFB class "L2" is indeed backward based FE if the derived LFB class "L2" is indeed backward
compatible with the base class "L1". compatible with the base class "L1".
The reverse scenario is a much less problematic case, i.e., when CE 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 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 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 older LFB classes, this problem does not affect the model; hence we
will use the term "backward compatibility" to refer to the first will use the term "backward compatibility" to refer to the first
scenario concerning FE backward compatibility. scenario concerning FE backward compatibility.
Inheritance can be designed into the model with backward Backward compatibility can be designed into the inheritance model
compatibility support by constraining the LFB inheritance such that by constraining LFB inheritance to require the derived class be a
the derived class is always a functional superset of the base functional superset of the base class (i.e. the derived class can
class, i.e., the derived class can only grow on top of the base only add functions to the base class, but not remove functions).
class, but not shrink from it. Additionally, the following Additionally, the following mechanisms are required to support FE
mechanisms are required to support FE backward compatibility: backward compatibility:
1) When detecting an LFB instance of an LFB type that is 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 unknown to the CE, the CE MUST be able to query the base
class of such an LFB from the FE. class of such an LFB from the FE.
2) The LFB instance on the FE SHOULD support a backward 2) The LFB instance on the FE SHOULD support a backward
compatibility mode (meaning the LFB instance reverts itself compatibility mode (meaning the LFB instance reverts itself
back to the base class instance), and the CE SHOULD be able back to the base class instance), and the CE SHOULD be able
to configure the LFB to run in such mode. to configure the LFB to run in such a mode.
3.3. FE Datapath Modeling 3.3. FE Datapath Modeling
Packets coming into the FE from ingress ports generally flow Packets coming into the FE from ingress ports generally flow
through multiple LFBs before leaving out of the egress ports. How 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 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 packet (e.g., IPv4, IPv6 or MPLS), actual header values, time of
arrival, etc. The result of the operation of an LFB may have an arrival, etc. The result of the operation of an LFB may have an
impact on how the packet is to be treated in further (downstream) impact on how the packet is to be treated in further (downstream)
LFBs and this differentiation of packet treatment downstream can be LFBs and this differentiation of packet treatment downstream can be
conceptualized as having alternative datapaths in the FE. For conceptualized as having alternative datapaths in the FE. For
example, the result of a 6-tuple classification (performed by a example, the result of a 6-tuple classification (performed by a
classifier LFB) controls what rate meter is applied to the packet classifier LFB) could control which rate meter is applied to the
(by a rate meter LFB) in a later stage in the datapath. packet (by a rate meter LFB) in a later stage in the datapath.
LFB topology is a directed graph representation of the logical LFB topology is a directed graph representation of the logical
datapaths within an FE, with the nodes representing the LFB datapaths within an FE, with the nodes representing the LFB
instances and the directed link the packet flow direction from one instances and the directed link the packet flow direction from one
LFB to the next. Section 3.3.1 discusses how the FE datapaths can 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 be modeled as LFB topology; while Section 3.3.2 focuses on issues
around LFB topology reconfiguration. around LFB topology reconfiguration.
3.3.1. Alternative Approaches for Modeling FE Datapaths 3.3.1. Alternative Approaches for Modeling FE Datapaths
There are two basic ways to express the differentiation in packet There are two basic ways to express the differentiation in packet
treatment within an FE, one representing the datapath directly and treatment within an FE, one represents the datapath directly and
graphically (topological approach) and the other utilizing metadata graphically (topological approach) and the other utilizes metadata
(the encoded state approach). (the encoded state approach).
. Topological Approach . Topological Approach
Using this approach, differential packet treatment is expressed Using this approach, differential packet treatment is expressed
via actually splitting the LFB topology into alternative paths. by splitting the LFB topology into alternative paths. In other
In other words, if the result of an LFB must control how the words, if the result of an LFB must control how the packet is
packet is further processed, then such an LFB will have separate further processed, then such an LFB will have separate output
output ports (one for each alternative treatment) connected to ports (one for each alternative treatment) connected to separate
separate sub-graphs (each expressing the respective treatment sub-graphs (each expressing the respective treatment
downstream). downstream).
. Encoded State Approach . Encoded State Approach
An alternative way of expressing differential treatment is using An alternative way of expressing differential treatment is using
metadata. The result of the operation of an LFB can be encoded metadata. The result of the operation of an LFB can be encoded
in a metadata which is passed along with the packet to in a metadata, which is passed along with the packet to
downstream LFBs. A downstream LFB, in turn, can use the downstream LFBs. A downstream LFB, in turn, can use the
metadata (and its value, e.g., as an index into some table) to metadata (and its value, e.g., as an index into some table) to
decide how to treat the packet. decide how to treat the packet.
Theoretically, the two approaches can substitute for each other, so Theoretically, the two approaches can substitute for each other, so
one may consider using purely one (or the other) approach to one could consider using a single pure approach to describe all
describe all datapaths in an FE. However, neither model by itself datapaths in an FE. However, neither model by itself is very
is very useful for practically relevant cases. For a given FE with useful for all practically relevant cases. For a given FE with
certain logical datapaths, applying the two different modeling certain logical datapaths, applying the two different modeling
approaches would result in very different looking LFB topology approaches result in very different looking LFB topology graphs. A
graphs. A model using purely the topological approach may require model using only the topological approach may require a very large
a very large graph with many links (i.e., paths) and nodes (i.e., graph with many links (i.e., paths) and nodes (i.e., LFB instances)
LFB instances) to express all alternative datapaths. On the other to express all alternative datapaths. On the other hand, a model
hand, a model using purely the encoded state model would be using only the encoded state model would be restricted to a string
restricted to a string of LFBs, which would make it very of LFBs, which makes it unintuitive to describe different datapaths
unintuitive to describe very different datapaths (such as MPLS and (such as MPLS and IPv4). Therefore, a mix of these two approaches
IPv4). Therefore, a mix of these two approaches will likely be will likely be used for a practical model. In fact, as we
used for a practical model. In fact, as we illustrate it below, illustrate below, the two approaches can be mixed even within the
the two approaches can be mixed even within the same LFB. same LFB.
Using a simple example of a classifier with N classification Using a simple example of a classifier with N classification
outputs followed by some other LFBs, Figure 5(a) shows what the LFB outputs followed by other LFBs, Figure 5(a) shows what the LFB
topology looks like by using the purely topological approach. Each topology looks like by using the pure topological approach. Each
output from the classifier goes to one of the N LFBs followed and output from the classifier goes to one of the N LFBs where no
no metadata is needed here. The topological approach is simple, metadata is needed. The topological approach is simple,
straightforward and graphically intuitive. However, if N is large straightforward and graphically intuitive. However, if N is large
and the N nodes followed the classifier (LFB#1, LFB#2, ..., LFB#N) and N nodes following the classifier (LFB#1, LFB#2, ..., LFB#N) all
all belong to the same LFB type (for example, meter) but each with belong to the same LFB type (for example, meter), but each has its
its own independent attributes, the encoded state approach gives a own independent attributes, the encoded state approach gives a much
much simpler topology representation, as shown in Figure 5(b). The simpler topology representation, as shown in Figure 5(b). The
encoded state approach requires that a table of N rows of meter encoded state approach requires that a table of N rows of meter
attributes is provided in the Meter node itself, with each row attributes is provided in the Meter node itself, with each row
representing the attributes for one meter instance. A metadata M representing the attributes for one meter instance. A metadata M
is also needed to pass along with the packet P from the classifier 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 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 to find the corresponding row of the attributes that should be used
for any particular packet P. for any particular packet P.
Now what if all the N nodes (LFB#1, LFB#2, ..., LFB#N) are not of Now what if all the N nodes (LFB#1, LFB#2, ..., LFB#N) are not of
the same type? For example, if LFB#1 is a queue while the rest are 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? all meters, what is the best way to represent such datapaths?
While it is still possible to use either the pure topological While it is still possible to use either the pure topological
approach or the pure encoded state approach, the natural approach or the pure encoded state approach, the natural
combination of the two seems the best by representing the two combination of the two appears to be the best option. Figure 5(c)
different functional datapaths using topological approach while depicts two different functional datapaths using the topological
leaving the N-1 meter instances distinguished by metadata only, as approach while leaving the N-1 meter instances distinguished by
shown in Figure 5(c). metadata only, as shown in Figure 5(c).
+----------+ +----------+
P | LFB#1 | P | LFB#1 |
+--------->|(Attrib-1)| +--------->|(Attrib-1)|
+-------------+ | +----------+ +-------------+ | +----------+
| 1|------+ P +----------+ | 1|------+ P +----------+
| 2|---------------->| LFB#2 | | 2|---------------->| LFB#2 |
| classifier 3| |(Attrib-2)| | classifier 3| |(Attrib-2)|
| ...|... +----------+ | ...|... +----------+
| N|------+ ... | N|------+ ...
+-------------+ | P +----------+ +-------------+ | P +----------+
skipping to change at page 27, line 22 skipping to change at page 31, line 25
+-------------+ | ... | +-------------+ | ... |
| (Attrib-N) | | (Attrib-N) |
+-------------+ +-------------+
5(c) Using a combination of the two, if LFB#1, LFB#2, ..., and 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). LFB#N are of different types (e.g., queue and meter).
Figure 5. An example of how to model FE datapaths Figure 5. An example of how to model FE datapaths
From this example, we demonstrate that each approach has distinct From this example, we demonstrate that each approach has distinct
advantage for different situations. Using the encoded state advantages depending on the situation. Using the encoded state
approach, fewer connections are typically needed between a fan-out approach, fewer connections are typically needed between a fan-out
node and its next LFB instances of the same type, because each node and its next LFB instances of the same type, because each
packet carries metadata with it so that the following nodes can packet carries metadata the following nodes can interpret and hence
interpret and hence invoke a different packet treatment. For those invoke a different packet treatment. For those cases, a pure
cases, a pure topological approach forces one to build elaborate topological approach forces one to build elaborate graphs with many
graphs with a lot more connections and often results in an unwieldy more connections and often results in an unwieldy graph. On the
graph. On the other hand, a topological approach is intuitive and other hand, a topological approach is intuitive and most useful for
most useful for representing functionally very different datapaths. representing functionally different datapaths.
For complex topologies, a combination of the two is the most useful For complex topologies, a combination of the two is the most useful
and flexible. Here we provide a general design guideline as to and flexible. A general design guideline is provided to indicate
what approach is best used for what situation. The topological which approach is best used for a particular situation. The
approach should primarily be used when the packet datapath forks topological approach should primarily be used when the packet
into areas with distinct LFB classes (not just distinct datapath forks into areas with distinct LFB classes (not just
parameterizations of the same LFB classes), and when the fan-outs distinct parameterizations of the same LFB class), and when the
do not require changes (adding/removing LFB outputs) at all or fan-outs do not require changes (adding/removing LFB outputs) or
require only very infrequent changes. Configuration information require only very infrequent changes. Configuration information
that needs to change frequently should preferably be expressed by that needs to change frequently should be expressed by the internal
the internal attributes of one or more LFBs (and hence using the attributes of one or more LFBs (and hence using the encoded state
encoded state approach). approach).
+---------------------------------------------+ +---------------------------------------------+
| | | |
+----------+ V +----------+ +------+ | +----------+ V +----------+ +------+ |
| | | | |if IP-in-IP| | | | | | | |if IP-in-IP| | |
---->| ingress |->+----->|classifier|---------->|Decap.|---->---+ ---->| ingress |->+----->|classifier|---------->|Decap.|---->---+
| ports | | |----+ | | | ports | | |----+ | |
+----------+ +----------+ |others+------+ +----------+ +----------+ |others+------+
| |
V V
skipping to change at page 28, line 28 skipping to change at page 32, line 28
--->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|-> --->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|->
| ports | | |----+ | | | | | ports | | |----+ | | | |
+-------+ +-----------+ |others +------+ +-----------+ +-------+ +-----------+ |others +------+ +-----------+
| |
V V
(b) The LFB topology without the loop utilizing two (b) The LFB topology without the loop utilizing two
independent classifier instances. independent classifier instances.
Figure 6. An LFB topology example. Figure 6. An LFB topology example.
It is important to point out that the LFB topology here is the It is important to point out that the LFB topology described here
logical topology that the packets flow through, not the physical is the logical topology, not the physical topology (e.g. how the FE
topology as determined by how the FE hardware is laid out. hardware is actually laid out). Nevertheless, the actual
Nevertheless, the actual implementation may still influence how the implementation may still influence how the functionality is mapped
functionality should be mapped into the LFB topology. Figure 6 to the LFB topology. Figure 6 shows one simple FE example. In
shows one simple FE example. In this example, an IP-in-IP packet this example, an IP-in-IP packet from an IPSec application like VPN
from an IPSec application like VPN may go to the classifier first may go to the classifier first and have the classification done
and have the classification done based on the outer IP header; upon based on the outer IP header; upon being classified as an IP-in-IP
being classified as an IP-in-IP packet, the packet is then sent to packet, the packet is then sent to a decapsulator to strip off the
a decapsulator to strip off the outer IP header, followed by a outer IP header, followed by a classifier again to perform
classifier again to perform classification on the inner IP header. classification on the inner IP header. If the same classifier
If the same classifier hardware or software is used for both outer hardware or software is used for both outer and inner IP header
and inner IP header classification with the same set of filtering classification with the same set of filtering rules, a logical loop
rules, a logical loop is naturally present in the LFB topology, as is naturally present in the LFB topology, as shown in Figure 6(a).
shown in Figure 6(a). However, if the classification is However, if the classification is implemented by two different
implemented by two different pieces of hardware or software with pieces of hardware or software with different filters (i.e., one
different filters (i.e., one set of filters for outer IP header set of filters for outer IP header while another set for inner IP
while another set for inner IP header), then it is more natural to header), then it is more natural to model them as two different
model them as two different instances of classifier LFB, as shown instances of classifier LFB, as shown in Figure 6(b).
in Figure 6(b).
To distinguish multiple instances of the same LFB class, each LFB To distinguish multiple instances of the same LFB class, each LFB
instance has its own LFB instance ID. One way to encode the LFB instance has its own LFB instance ID. One way to encode the LFB
instance ID is to encode it as x.y where x is the LFB class ID instance ID is to encode it as x.y where x is the LFB class ID
while y is the instance ID within each LFB class. while y is the instance ID within each LFB class.
3.3.2. Configuring the LFB Topology 3.3.2. Configuring the LFB Topology
While there is little doubt that the individual LFB must be While there is little doubt that the individual LFB must be
configurable, the configurability question is more complicated for configurable, the configurability question is more complicated for
LFB topology. Since LFB topology is really the graphic LFB topology. Since LFB topology is really the graphic
representation of the datapaths within FE, configuring the LFB representation of the datapaths within an FE, configuring the LFB
topology means dynamically changing the datapaths including changes topology means dynamically changing the datapaths, including
to the LFBs along the datapaths on an FE, e.g., creating (i.e., changes to the LFBs along the datapaths on an FE (e.g., creating,
instantiating) or deleting LFBs, setting up or deleting instantiating or deleting LFBs), setting up or deleting
interconnections between outputs of upstream LFBs to inputs of interconnections between outputs of upstream LFBs to inputs of
downstream LFBs. downstream LFBs.
Why would the datapaths on an FE ever change dynamically? The Why would the datapaths on an FE ever change dynamically? The
datapaths on an FE is set up by the CE to provide certain data datapaths on an FE is set up by the CE to provide certain data
plane services (e.g., DiffServ, VPN, etc.) to the NE's customers. plane services (e.g., DiffServ, VPN, etc.) to the Network Element's
The purpose of reconfiguring the datapaths is to enable the CE to (NE) customers. The purpose of reconfiguring the datapaths is to
customize the services the NE is delivering at run time. The CE enable the CE to customize the services the NE is delivering at run
needs to change the datapaths when the service requirements change, time. The CE needs to change the datapaths when the service
e.g., when adding a new customer, or when an existing customer requirements change (e.g., when adding a new customer, or when an
changes their service. However, note that not all datapath changes existing customer changes their service). However, note that not
result in changes in the LFB topology graph, and that is determined all datapath changes result in changes in the LFB topology graph.
by the approach we use to map the datapaths into LFB topology. As Changes in the graph are dependent on the approach used to map the
discussed in 3.3.1, the topological approach and encoded state datapaths into LFB topology. As discussed in 3.3.1, the
approach can result in very different looking LFB topologies for topological approach and encoded state approach can result in very
the same datapaths. In general, an LFB topology based on a pure different looking LFB topologies for the same datapaths. In
topological approach is likely to experience more frequent topology general, an LFB topology based on a pure topological approach is
reconfiguration than one based on an encoded state approach. likely to experience more frequent topology reconfiguration than
However, even an LFB topology based entirely on an encoded state one based on an encoded state approach. However, even an LFB
approach may have to change the topology at times, for example, to topology based entirely on an encoded state approach may have to
totally bypass some LFBs or insert new LFBs. Since a mix of these change the topology at times, for example, to bypass some LFBs or
two approaches is used to model the datapaths, LFB topology insert new LFBs. Since a mix of these two approaches is used to
reconfiguration is considered an important aspect of the FE model. 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 We want to point out that allowing a configurable LFB topology in
the FE model does not mandate that all FEs must have such the FE model does not mandate that all FEs must have this
capability. Even if an FE supports configurable LFB topology, it capability. Even if an FE supports configurable LFB topology, it
is expected that there will be FE-specific limitations on what can is expected there will be FE-specific limitations on what can
actually be configured. Performance-optimized hardware actually be configured. Performance-optimized hardware
implementation may have zero or very limited configurability, while implementations may have zero or very limited configurability,
FE implementations running on network processors may provide more while FE implementations running on network processors may provide
flexibility and configurability. It is entirely up to the FE more flexibility and configurability. It is entirely up to the FE
designers to decide whether or not the FE actually implements such designers to decide whether or not the FE actually implements
reconfiguration and how much. Whether it is a simple runtime reconfiguration and if so, how much. Whether it is a simple
switch to enable or disable (i.e., bypass) certain LFBs, or more runtime switch to enable or disable (i.e., bypass) certain LFBs, or
flexible software reconfiguration is all implementation detail more flexible software reconfiguration is all implementation detail
internal to the FE and outside of the scope of FE model. In either 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 case, the CE(s) must be able to learn the FE's configuration
capabilities. Therefore, the FE model must provide a mechanism for capabilities. Therefore, the FE model must provide a mechanism for
describing the LFB topology configuration capabilities of an FE. describing the LFB topology configuration capabilities of an FE.
These capabilities may include (see Section 5 for full details): These capabilities may include (see Section 5 for full details):
. What LFB classes can the FE instantiate? . What LFB classes can the FE instantiate
. How many instances of the same LFB class can be created? . Maximum number of instance of the same LFB class that can be
. What are the topological limitations? For example: created
o How many instances of the same class or any class can be . Any topological limitations, For example:
created on any given branch of the graph? 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 o Ordering restrictions on LFBs (e.g., any instance of LFB
class A must be always downstream of any instance of LFB class A must be always downstream of any instance of LFB
class B). class B).
Even if the CE is allowed to configure LFB topology for an FE, how Note that even when the CE is allowed to configure LFB topology for
can the CE interpret an arbitrary LFB topology (presented to the CE the FE, the CE is not expected to be able to interpret an arbitrary
by the FE) and know what to do with it? In other words, how does LFB topology and determine which specific service or application
the CE know the mapping between an LFB topology and a particular NE (e.g. VPN, DiffServ, etc.) is supported by the FE. However, once
service or application (e.g., VPN, DiffServ, etc.)? We argue that the CE understands the coarse capability of an FE, it is the
first of all, it is unlikely that an FE can support any arbitrary responsibility of the CE to configure the LFB topology to implement
LFB topology; secondly, once the CE understands the coarse the network service the NE is supposed to provide. Thus, the
capability of an FE, it is up to the CE to configure the LFB mapping the CE has to understand is from the high level NE service
topology according to the network service the NE is supposed to to a specific LFB topology, not the other way around. The CE is not
provide. So the more important mapping that the CE has to expected to have the ultimate intelligence to translate any high
understand is from the high level NE service to a specific LFB level service policy into the configuration data for the FEs.
topology, not the other way around. Do we expect the CE has the However, it is conceivable that within a given network service
ultimate intelligence to translate any high level service policy domain (such as DiffServ), a certain amount of intelligence can be
into the configuration data for the FEs? No, but it is conceivable programmed into the CE to give the CE a general understanding of
that within a given network service domain (like DiffServ), a the LFBs involved to allow the translation from a high level
certain amount of intelligence can be programmed into the CE such service policy to the low level FE configuration to be done
that the CE has a general understanding of the LFBs involved and so automatically. Note that this is considered an implementation
the translation from a high level service policy to the low level issue internal to the control plane and outside the scope of the FE
FE configuration can be done automatically. In any event, this is model. Therefore, it is not discussed any further in this draft.
considered an implementation issue internal to the control plane
and outside the scope of the FE model. Therefore, it is not
discussed any further in this draft.
+----------+ +-----------+ +----------+ +-----------+
---->| Ingress |---->|classifier |--------------+ ---->| Ingress |---->|classifier |--------------+
| | |chip | | | | |chip | |
+----------+ +-----------+ | +----------+ +-----------+ |
v v
+-------------------------------------------+ +-------------------------------------------+
+--------+ | Network Processor | +--------+ | Network Processor |
<----| Egress | | +------+ +------+ +-------+ | <----| Egress | | +------+ +------+ +-------+ |
+--------+ | |Meter | |Marker| |Dropper| | +--------+ | |Meter | |Marker| |Dropper| |
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accepted by the FE accepted by the FE
Figure 7. An example of configuring LFB topology. Figure 7. An example of configuring LFB topology.
Figure 7 shows an example where a QoS-enabled router has several Figure 7 shows an example where a QoS-enabled router has several
line cards that have a few ingress ports and egress ports, a line cards that have a few ingress ports and egress ports, a
specialized classification chip, a network processor containing specialized classification chip, a network processor containing
codes for FE blocks like meter, marker, dropper, counter, queue, codes for FE blocks like meter, marker, dropper, counter, queue,
scheduler and Ipv4 forwarder. Some of the LFB topology is already scheduler and Ipv4 forwarder. Some of the LFB topology is already
fixed and has to remain static due to the physical layout of the fixed and has to remain static due to the physical layout of the
line cards. For example, all the ingress ports might be already line cards. For example, all the ingress ports might be hard-wired
hard wired into the classification chip and so all packets must into the classification chip and so all packets must flow from the
follow from the ingress port into the classification engine. On ingress port into the classification engine. On the other hand,
the other hand, the LFBs on the network processor and their the LFBs on the network processor and their execution order are
execution order are programmable, even though there might exist programmable. However, certain capacity limits and linkage
certain capacity limits and linkage constraints between these LFBs. constraints could exist between these LFBs. Examples of the
Examples of the capacity limits might be: there can be no more than capacity limits might be: 8 meters; 16 queues in one FE; the
8 meters; there can be no more than 16 queues in one FE; the
scheduler can handle at most up to 16 queues; etc. The linkage scheduler can handle at most up to 16 queues; etc. The linkage
constraints might dictate that classification engine may be constraints might dictate that the classification engine may be
followed by a meter, marker, dropper, counter, queue or IPv4 followed by a meter, marker, dropper, counter, queue or IPv4
forwarder, but not scheduler; queues can only be followed by a forwarder, but not a scheduler; queues can only be followed by a
scheduler; a scheduler must be followed by the IPv4 forwarder; the scheduler; a scheduler must be followed by the IPv4 forwarder; the
last LFB in the datapath before going into the egress ports must be last LFB in the datapath before going into the egress ports must be
the IPv4 forwarder, etc. the IPv4 forwarder, etc.
Once the FE reports such capability and capacity to the CE, it is Once the FE reports these capabilities and capacity limits to the
now up to the CE to translate the QoS policy into the desirable CE, it is now up to the CE to translate the QoS policy into a
configuration for the FE. Figure 7(a) depicts the FE capability desirable configuration for the FE. Figure 7(a) depicts the FE
while 7(b) and 7(c) depict two different topologies that the FE capability while 7(b) and 7(c) depict two different topologies that
might be asked to configure to. Note that both the ingress and the FE might be asked to configure to. Note that both the ingress
egress are omitted in (b) and (c) for simple representation. The and egress are omitted in (b) and (c) to simplify the
topology in 7(c) is considerably more complex than 7(b) but both representation. The topology in 7(c) is considerably more complex
are feasible within the FE capabilities, and so the FE should than 7(b) but both are feasible within the FE capabilities, and so
accept either configuration request from the CE. the FE should accept either configuration request from the CE.
4. Model and Schema for LFB Classes 4. Model and Schema for LFB Classes
The main goal of the FE model is to provide an abstract, generic, The main goal of the FE model is to provide an abstract, generic,
modular, implementation-independent representation of the FEs. This modular, implementation-independent representation of the FEs.
is facilitated using the concept of LFBs which are instantiated This is facilitated using the concept of LFBs, which are
from LFB classes. LFB classes and associated definitions will be instantiated from LFB classes. LFB classes and associated
provided in a collection of XML documents. The collection of these definitions will be provided in a collection of XML documents. The
XML documents is called a LFB class library, and each document is collection of these XML documents is called a LFB class library,
called an LFB class library document (or library document, for and each document is called an LFB class library document (or
short). Each of the library documents will conform to the schema library document, for short). Each of the library documents will
presented in this section. The root element of the library document conform to the schema presented in this section. The root element
is the <LFBLibrary> element. of the library document is the <LFBLibrary> element.
It is not expected that library documents will be exchanged between It is not expected that library documents will be exchanged between
FEs and CEs "over-the-wire". But the model will serve as an FEs and CEs "over-the-wire". But the model will serve as an
important reference for the design and development of the CEs important reference for the design and development of the CEs
(software) and FEs (mostly the software part). It will also serve (software) and FEs (mostly the software part). It will also serve
as a design input when specifying the ForCES protocol elements for as a design input when specifying the ForCES protocol elements for
CE-FE communication. CE-FE communication.
4.1. Namespace 4.1. Namespace
skipping to change at page 34, line 11 skipping to change at page 38, line 11
The <LFBLibrary> element serves as a root element of all library The <LFBLibrary> element serves as a root element of all library
documents. It contains one or more of the following main blocks: documents. It contains one or more of the following main blocks:
. <frameTypeDefs> for the frame declarations; . <frameTypeDefs> for the frame declarations;
. <dataTypeDefs> for defining common data types; . <dataTypeDefs> for defining common data types;
. <metadataDefs> for defining metadata, and . <metadataDefs> for defining metadata, and
. <LFBClassDefs> for defining LFB classes. . <LFBClassDefs> for defining LFB classes.
Each block is optional, that is, one library may contain only Each block is optional, that is, one library may contain only
metadata defintions, another may contain only LFB class metadata definitions, another may contain only LFB class
definitions, yet another may contain all of the above. definitions, yet another may contain all of the above.
In addition to the above main blocks, a library document can import In addition to the above main blocks, a library document can import
other library documents if it needs to refer to definitions other library documents if it needs to refer to definitions
contained in the included document. This concept is similar to the contained in the included document. This concept is similar to the
"#include" directive in C. Importing is expressed by the <load> "#include" directive in C. Importing is expressed by the <load>
elements, which must precede all the above elements in the elements, which must precede all the above elements in the
document. For unique referencing, each LFBLibrary instance document document. For unique referencing, each LFBLibrary instance
has a unique label defined in the "provide" attribute of the document has a unique label defined in the "provide" attribute of
LFBLibrary element. the LFBLibrary element.
The <LFBLibrary> element also includes an optional <description> The <LFBLibrary> element also includes an optional <description>
element, which can be used to provide textual description about the element, which can be used to provide textual description about the
library. library.
Following is a skeleton of a library document: The following is a skeleton of a library document:
<?xml version="1.0" encoding="UTF-8"?> <?xml version="1.0" encoding="UTF-8"?>
<LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel" <LFBLibrary xmlns="http://ietf.org/forces/1.0/lfbmodel"
provides="this_library"> provides="this_library">
<description> <description>
... ...
</description> </description>
<!-- Loading external libraries (optional) --> <!-- Loading external libraries (optional) -->
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... ...
<!-- FRAME TYPE DEFINITIONS (optional) --> <!-- FRAME TYPE DEFINITIONS (optional) -->
<frameTypeDefs> <frameTypeDefs>
... ...
</frameTypeDefs> </frameTypeDefs>
<!-- DATA TYPE DEFINITIONS (optional) --> <!-- DATA TYPE DEFINITIONS (optional) -->
<dataTypeDefs> <dataTypeDefs>
... ...
</dataTypeDefs> </dataTypeDefs>
<!-- METADATA DEFINITIONS (optional) --> <!-- METADATA DEFINITIONS (optional) -->
<metadataDefs> <metadataDefs>
... ...
</metadataDefs> </metadataDefs>
<!LFB CLASS DEFINITIONS (optional) --> <!LFB CLASS DEFINITIONS (optional) -->
<LFBCLassDefs> <LFBCLassDefs>
... ...
</LFBCLassDefs> </LFBCLassDefs>
</LFBLibrary> </LFBLibrary>
4.3. <load> Element 4.3. <load> Element
This element is used to refer to another LFB library document. This element is used to refer to another LFB library document.
Similar to the "include" directive in C, this makes the objects Similar to the "include" directive in C, this makes the objects
(metadata types, data types, etc.) defined in the referred library (metadata types, data types, etc.) defined in the referred library
available for referencing in the current document. available for referencing in the current document.
The load element must contain the label of the library to be The load element must contain the label of the library to be
included and may contain a URL to specify where the library can be included and may contain a URL to specify where the library can be
retrieved. The load element can be repeated unlimited times. Three retrieved. The load element can be repeated unlimited times.
examples for the <load> elements: Three examples for the <load> elements:
<load library="a_library"/> <load library="a_library"/>
<load library="another_library" location="another_lib.xml"/> <load library="another_library" location="another_lib.xml"/>
<load library="yetanother_library" <load library="yetanother_library"
location="http://www.petrimeat.com/forces/1.0/lfbmodel/lpm.xml"/> location="http://www.petrimeat.com/forces/1.0/lfbmodel/lpm.xml"/>
4.4. <frameDefs> Element for Frame Type Declarations 4.4. <frameDefs> Element for Frame Type Declarations
Frame names are used in the LFB definition to define what types of 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 frames the LFB expects at its input port(s) and emits at its output
port(s). The <frameDefs> optional element in the library document port(s). The <frameDefs> optional element in the library document
contains one or more <frameDef> elements, each declaring one frame contains one or more <frameDef> elements, each declaring one frame
type. type.
Each frame definition contains a unique name (NMTOKEN) and a brief Each frame definition contains a unique name (NMTOKEN) and a brief
synopsis. In addition, an optional detailed description may be synopsis. In addition, an optional detailed description may be
provided. provided.
Uniqueness of frame types must be ensured among frame types defined Uniqueness of frame types must be ensured among frame types defined
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including: including:
. Defining other data types . Defining other data types
. Defining metadata . Defining metadata
. Defining attributes of LFB classes . Defining attributes of LFB classes
This is similar to the concept of having a common header file for This is similar to the concept of having a common header file for
shared data types. shared data types.
Each <dataTypeDef> element contains a unique name (NMTOKEN), a Each <dataTypeDef> element contains a unique name (NMTOKEN), a
brief brief synopsis, an optional longer description, and a type
synopsis, an optional longer description, and a type definition definition element. The name must be unique among all data types
element. The name must be unique among all data types defined in defined in the same library document and in any directly or
the same library document and in any directly or indirectly indirectly included library documents. For example:
included library documents. For example:
<dataTypeDefs> <dataTypeDefs>
<dataTypeDef> <dataTypeDef>
<name>ieeemacaddr</name> <name>ieeemacaddr</name>
<synopsis>48-bit IEEE MAC address</synopsis> <synopsis>48-bit IEEE MAC address</synopsis>
... type definition ... ... type definition ...
</dataTypeDef> </dataTypeDef>
<dataTypeDef> <dataTypeDef>
<name>ipv4addr</name> <name>ipv4addr</name>
<synopsis>IPv4 address</synopsis> <synopsis>IPv4 address</synopsis>
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<dataTypeDef> <dataTypeDef>
<name>short</name> <name>short</name>
<synopsis>Alias to int16</synopsis> <synopsis>Alias to int16</synopsis>
<typeRef>int16</typeRef> <typeRef>int16</typeRef>
</dataTypeDef> </dataTypeDef>
<dataTypeDef> <dataTypeDef>
<name><name>ieeemacaddr</name> <name><name>ieeemacaddr</name>
<synopsis>48-bit IEEE MAC address</synopsis> <synopsis>48-bit IEEE MAC address</synopsis>
<typeRef>byte[6]</typeRef> <typeRef>byte[6]</typeRef>
</dataTypeDef> </dataTypeDef>
4.5.2. <atomic> Element for Deriving New Atomic Types
4.5.2. <atomic> Element for Deriving New Atomic Types
The <atomic> element allows the definition of a new atomic type The <atomic> element allows the definition of a new atomic type
from an existing atomic type, applying range restrictions and/or from an existing atomic type, applying range restrictions and/or
providing special enumerated values. Note that the <atomic> providing special enumerated values. Note that the <atomic>
element can only use atomic types as base types, and its result is element can only use atomic types as base types, and its result is
always another atomic type. always another atomic type.
For example, the following snippet defines a new "dscp" data type: For example, the following snippet defines a new "dscp" data type:
<dataTypeDef> <dataTypeDef>
<name>dscp</name> <name>dscp</name>
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The <array> element can be used to create a new compound data type 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 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 array entry can be specified either by referring to an existing
type (using the <typeRef> element) or defining an unnamed type type (using the <typeRef> element) or defining an unnamed type
inside the <array> element using any of the <atomic>, <array>, inside the <array> element using any of the <atomic>, <array>,
<struct>, or <union> elements. <struct>, or <union> elements.
The array can be "fixed-size" or "variable-size", which is The array can be "fixed-size" or "variable-size", which is
specified by the "type" attribute of the <array> element. The specified by the "type" attribute of the <array> element. The
default is "variable-size". For variable size arrays an optional default is "variable-size". For variable size arrays, an optional
"max-length" attribute can specify the maximum allowed length. This "max-length" attribute can specify the maximum allowed length. This
attribute should be used to encode semantic limitations, and not attribute should be used to encode semantic limitations, and not
implementation limitations. The latter should be handled by implementation limitations. The latter should be handled by
capability attributes of LFB classes, and should never be included capability attributes of LFB classes, and should never be included
in data type definitions. If the "max-length" attribute is not in data type definitions. If the "max-length" attribute is not
provided, the array is regarded as of unlimited-size. provided, the array is regarded as of unlimited-size.
For fixed-size arrays a "length" attribute must be provided which For fixed-size arrays, a "length" attribute must be provided that
specifies the constant size of the array. specifies the constant size of the array.
The result of this construct is always a compound type, even if the The result of this construct is always a compound type, even if the
array has a fixed size of 1. array has a fixed size of 1.
Arrays can only be subscripted by integers, and will be presumed to Arrays can only be subscripted by integers, and will be presumed to
start with index 0. start with index 0.
The following example shows the definition of a fixed size array The following example shows the definition of a fixed size array
with pre-defined data type as array elements: with pre-defined data type as array elements:
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</dataTypeDef> </dataTypeDef>
In the above example each entry of the array is a <struct> of two In the above example each entry of the array is a <struct> of two
fileds ("rule" and "opcode"). fileds ("rule" and "opcode").
4.5.4. <struct> Element to Define Structures 4.5.4. <struct> Element to Define Structures
A structure is comprised of a collection of data elements. Each A structure is comprised of a collection of data elements. Each
data element has a data type (either an atomic type or an existing 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 compound type) and is assigned a name unique within the scope of
the compound data type being defined. These serve the same function the compound data type being defined. These serve the same
as "struct" in C, etc. function as "struct" in C, etc.
The actual type of the field can be defined by referring to an The actual type of the field can be defined by referring to an
existing type (using the <typeDef> element), or can be a locally existing type (using the <typeDef> element), or can be a locally
defined (unnamed) type created by any of the <atomic>, <array>, defined (unnamed) type created by any of the <atomic>, <array>,
<struct>, or <union> elements. <struct>, or <union> elements.
The result of this construct is always regarded a compound type, The result of this construct is always regarded a compound type,
even if the <struct> contains only one field. even if the <struct> contains only one field.
An example: An example:
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definition of overlay types. Its format is identical to the definition of overlay types. Its format is identical to the
<struct> element. <struct> element.
The result of this construct is always regarded a compound type, The result of this construct is always regarded a compound type,
even if the union contains only one element. even if the union contains only one element.
4.5.6. Augmentations 4.5.6. Augmentations
Compound types can also be defined as augmentations of existing Compound types can also be defined as augmentations of existing
compound types. If the existing compound type is a structure, compound types. If the existing compound type is a structure,
augmentation may add new elements to the type. They may replace augmentation may add new elements to the type. The type of an
the type of an existing element with an augmentation derived from existing element can only be replaced with an augmentation derived
the current type. They may not delete an existing element, nor may from the current type, an existing element cannot be deleted. If
they replace the type of an existing element with one that is not the existing compound type is an array, augmentation means
an augmentation of the type that the element has in the basis for augmentation of the array element type.
the augmentation. If the existing compound type is an array,
augmentation means augmentation of the array element type.
One consequence of this is that augmentations are compatible with One consequence of this is that augmentations are compatible with
the compound type from which they are derived. As such, the compound type from which they are derived. As such,
augmentations are useful in defining attributes for LFB subclasses augmentations are useful in defining attributes for LFB subclasses
with backward compatibility. In addition to adding new attributes with backward compatibility. In addition to adding new attributes
to a class, the data type of an existing attribute may be replaced to a class, the data type of an existing attribute may be replaced
by an augmentation of that attribute, and still meet the by an augmentation of that attribute, and still meet the
compatibility rules for subclasses. compatibility rules for subclasses.
For example, consider a simple base LFB class A that has only one For example, consider a simple base LFB class A that has only one
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<metadataDefs> <metadataDefs>
<metadataDef> <metadataDef>
<name>NEXTHOPID</name> <name>NEXTHOPID</name>
<synopsis>Refers to a Next Hop entry in NH LFB</synopsis> <synopsis>Refers to a Next Hop entry in NH LFB</synopsis>
<typeRef>int32</typeRef> <typeRef>int32</typeRef>
</metadataDef> </metadataDef>
<metadataDef> <metadataDef>
<name>CLASSID</name> <name>CLASSID</name>
<synopsis> <synopsis>
Result of classification (0 means no match). Result of classification (0 means no match).
</synopsis> </synopsis>
<atomic> <atomic>
<baseType>int32</baseType> <baseType>int32</baseType>
<specialValues> <specialValues>
<specialValue value="0"> <specialValue value="0">
<name>NOMATCH</name> <name>NOMATCH</name>
<synopsis> <synopsis>
Classification didnt result in match. Classification didnt result in match.
</synopsis> </synopsis>
</specialValue> </specialValue>
</specialValues> </specialValues>
</atomic> </atomic>
</metadataDef> </metadataDef>
</metadataDefs> </metadataDefs>
4.7. <LFBClassDefs> Element for LFB Class Definitions 4.7. <LFBClassDefs> Element for LFB Class Definitions
The (optional) <LFBClassDefs> element can be used to define one or The (optional) <LFBClassDefs> element can be used to define one or
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This LFB represents the IPv4 longest prefix match lookup This LFB represents the IPv4 longest prefix match lookup
operation. operation.
The modeled behavior is as follows: The modeled behavior is as follows:
Blah-blah-blah. Blah-blah-blah.
</description> </description>
</LFBClassDef> </LFBClassDef>
... ...
</LFBClassDefs> </LFBClassDefs>
Except the <name>, <synopsis>, and <version> elements, all other Note that the <name>, <synopsis>, and <version> elements, all other
elements are optional in <LFBClassDef>, though when they are elements are optional in <LFBClassDef>. However, when they are
present, they must occur in the above order. present, they must occur in the above order.
4.7.1. <derivedFrom> Element to Express LFB Inheritance 4.7.1. <derivedFrom> Element to Express LFB Inheritance
The optional <derivedFrom> element can be used to indicate that The optional <derivedFrom> element can be used to indicate that
this class is a derivative of some other class. The content of this this class is a derivative of some other class. The content of
element must be the unique name (<name>) of another LFB class. The this element must be the unique name (<name>) of another LFB class.
referred LFB class must be defined in the same library document or The referred LFB class must be defined in the same library document
in one of the included library documents. or in one of the included library documents.
[EDITOR: The <derivedFrom> element will likely need to specify the [EDITOR: The <derivedFrom> element will likely need to specify the
version of the ancestor, which is not included in the schema yet. version of the ancestor, which is not included in the schema yet.
The process and rules of class derivation are still being studied.] The process and rules of class derivation are still being studied.]
It is assumed that the derived class is backwards compatible with It is assumed that the derived class is backwards compatible with
the base class. the base class.
4.7.2. <inputPorts> Element to Define LFB Inputs 4.7.2. <inputPorts> Element to Define LFB Inputs
The optional <inputPorts> element is used to define input ports. An The optional <inputPorts> element is used to define input ports.
LFB class may have zero, one, or more inputs. If the LFB class has An LFB class may have zero, one, or more inputs. If the LFB class
no input ports, the <inputPorts> elements must be omitted. The has no input ports, the <inputPorts> element must be omitted. The
<inputPorts> element can contain one or more <inputPort> elements, <inputPorts> element can contain one or more <inputPort> elements,
one for each port or port-group. We assume that most LFBs will have one for each port or port-group. We assume that most LFBs will
exactly one input. Multiple inputs with the same input type are have exactly one input. Multiple inputs with the same input type
modeled as one input group. Input groups are defined the same way are modeled as one input group. Input groups are defined the same
as input ports by the <inputPort> element, differentiated only by way as input ports by the <inputPort> element, differentiated only
an optional "group" attribute. by an optional "group" attribute.
Multiple inputs with different input types should be avoided if Multiple inputs with different input types should be avoided if
possible (see discussion in Section 3.2.1). Some special LFBs will 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 have no inputs at all. For example, a packet generator LFB does
need an input. not need an input.
Single input ports and input port groups are both defined by the Single input ports and input port groups are both defined by the
<inputPort> element, they are differentiated by only an optional <inputPort> element, they are differentiated by only an optional
"group" attribute. "group" attribute.
The <inputPort> element contains the following elements: The <inputPort> element contains the following elements:
. <name> provides the symbolic name of the input. Example: "in". . <name> provides the symbolic name of the input. Example: "in".
Note that this symbolic name must be unique only within the Note that this symbolic name must be unique only within the
scope of the LFB class. scope of the LFB class.
. <synopsis> contains a brief description of the input. Example: . <synopsis> contains a brief description of the input. Example:
"Normal packet input". "Normal packet input".
. <expectation> lists all allowed frame formats. Example: {"ipv4" . <expectation> lists all allowed frame formats. Example: {"ipv4"
and "ipv6"}. Note that this list should refer to names and "ipv6"}. Note that this list should refer to names
specified in the <frameDefs> element of the same library specified in the <frameDefs> element of the same library
document or in any included library documents. The <expectation> document or in any included library documents. The
element can also provide a list of required metadata. Example: <expectation> element can also provide a list of required
{"classid", "vifid"}. This list should refer to names of metadata. Example: {"classid", "vifid"}. This list should
metadata defined in the <metadataDefs> element in the same refer to names of metadata defined in the <metadataDefs> element
library document or in any included library documents. For each in the same library document or in any included library
metadata it must be specified whether the metadata is required documents. For each metadata, it must be specified whether the
or optional. For each optional metadata a default value must be metadata is required or optional. For each optional metadata, a
specified, which is used by the LFB if the metadata is not default value must be specified, which is used by the LFB if the
provided with a packet. metadata is not provided with a packet.
In addition, the optional "group" attribute of the <inputPort> In addition, the optional "group" attribute of the <inputPort>
element can specify if the port can behave as a port group, i.e., 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" it is allowed to be instantiated. This is indicated by a "yes"
value (the default value is "no"). value (the default value is "no").
An example <inputPorts> element, defining two input ports, the An example <inputPorts> element, defining two input ports, the
second one being an input port group: second one being an input port group:
<inputPorts> <inputPorts>
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</metadataExpected> </metadataExpected>
</expectation> </expectation>
</inputPort> </inputPort>
<inputPort group="yes"> <inputPort group="yes">
... another input port ... ... another input port ...
</inputPort> </inputPort>
</inputPorts> </inputPorts>
For each <inputPort>, the frame type expectations are defined by For each <inputPort>, the frame type expectations are defined by
the <frameExpected> element using one or more <ref> elements (see the <frameExpected> element using one or more <ref> elements (see
example above). When multiple frame types are listed, it means that example above). When multiple frame types are listed, it means
"one of these" frame types are expected. A packet of any other that "one of these" frame types are expected. A packet of any
frame type is regarded as incompatible with this input port of the other frame type is regarded as incompatible with this input port
LFB class. The above example list two frames as expected frame of the LFB class. The above example list two frames as expected
types: "ipv4" and "ipv6". frame types: "ipv4" and "ipv6".
Metadata expectations are specified by the <metadataExpected> Metadata expectations are specified by the <metadataExpected>
element. In its simplest form this element can contain a list of element. In its simplest form, this element can contain a list of
<ref> elements, each referring to a metadata. When multiple <ref> elements, each referring to a metadata. When multiple
instances of metadata are listed by <ref> elements, it means that instances of metadata are listed by <ref> elements, it means that
"all of these" metadata must be received with each packet (except "all of these" metadata must be received with each packet (except
metadata that are marked as "optional" by the "dependency" metadata that are marked as "optional" by the "dependency"
attribute of the corresponding <ref> element). For a metadata that attribute of the corresponding <ref> element). For a metadata that
is specified "optional", a default value must be provided using the is specified "optional", a default value must be provided using the
"defaultValue" attribute. The above example lists three metadata as "defaultValue" attribute. The above example lists three metadata
expected metadata, two of which are mandatory ("classid" and as expected metadata, two of which are mandatory ("classid" and
"vifid"), and one being optional ("vrfid"). "vifid"), and one being optional ("vrfid").
[EDITOR: How to express default values for byte[N] atomic types is [EDITOR: How to express default values for byte[N] atomic types is
yet to be defined.] yet to be defined.]
The schema also allows for more complex definitions of metadata The schema also allows for more complex definitions of metadata
expectations. For example, using the <one-of> element, a list of expectations. For example, using the <one-of> element, a list of
metadata can be specified to express that at least one of the metadata can be specified to express that at least one of the
specified metadata must be present with any packet. For example: specified metadata must be present with any packet. For example:
<metadataExpected> <metadataExpected>
<one-of> <one-of>
<ref>prefixmask</ref> <ref>prefixmask</ref>
<ref>prefixlen</ref> <ref>prefixlen</ref>
</one-of> </one-of>
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</one-of> </one-of>
</metadataExpected> </metadataExpected>
Although the schema is constructed to allow even more complex Although the schema is constructed to allow even more complex
definition of metadata expectations, we do not discuss these here. definition of metadata expectations, we do not discuss these here.
4.7.3. <outputPorts> Element to Define LFB Outputs 4.7.3. <outputPorts> Element to Define LFB Outputs
The optional <outputPorts> element is used to define output ports. The optional <outputPorts> element is used to define output ports.
An LFB class may have zero, one, or more outputs. If the LFB class 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 has no output ports, the <outputPorts> element must be omitted.
<outputPorts> element can contain one or more <outputPort> The <outputPorts> element can contain one or more <outputPort>
elements, one for each port or port-group. If there are multiple 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 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., group. Some special LFBs may have no outputs at all (e.g.,
Dropper). Dropper).
Single output ports and output port groups are both defined by the Single output ports and output port groups are both defined by the
<outputPort> element, they are differentiated by only an optional <outputPort> element; they are differentiated by only an optional
"group" attribute. "group" attribute.
The <outputPort> element contains the following elements: The <outputPort> element contains the following elements:
. <name> provides the symbolic name of the output. Example: "out". . <name> provides the symbolic name of the output. Example:
Note that the symbolic name must be unique only within the scope "out". Note that the symbolic name must be unique only within
of the LFB class. the scope of the LFB class.
. <synopsis> contains a brief description of the output port. . <synopsis> contains a brief description of the output port.
Example: "Normal packet output". Example: "Normal packet output".
. <product> lists the allowed frame formats. Example: {"ipv4", . <product> lists the allowed frame formats. Example: {"ipv4",
"ipv6"}. Note that this list should refer to symbols specified "ipv6"}. Note that this list should refer to symbols specified
in the <frameDefs> element in the same library document or in in the <frameDefs> element in the same library document or in
any included library documents. The <product> element may also any included library documents. The <product> element may also
contain the list of emitted (generated) metadata. Example: contain the list of emitted (generated) metadata. Example:
{"classid", "color"}. This list should refer to names of {"classid", "color"}. This list should refer to names of
metadata specified in the <metadataDefs> element in the same metadata specified in the <metadataDefs> element in the same
library document or in any included library documents. For each library document or in any included library documents. For each
generated metadata, it should be specified whether the metadata generated metadata, it should be specified whether the metadata
is always generated or generated only in certain conditions. is always generated or generated only in certain conditions.
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<ref>ipv4</ref> <ref>ipv4</ref>
<ref>ipv4bis</ref> <ref>ipv4bis</ref>
</frameProduced> </frameProduced>
<metadataProduced> <metadataProduced>
<ref availability="conditional">errorid</ref> <ref availability="conditional">errorid</ref>
</metadataProduced> </metadataProduced>
</product> </product>
</outputPort> </outputPort>
</outputPorts> </outputPorts>
What types of frames and metadata the port produces are defined The types of frames and metadata the port produces are defined
inside the <product> element in each <outputPort>. Within the inside the <product> element in each <outputPort>. Within the
<product> element, the list of frame types the port produces is <product> element, the list of frame types the port produces is
listed in the <frameProduced> element. When more than one frame is listed in the <frameProduced> element. When more than one frame is
listed, it means that "one of" these frames will be produced. 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 list of metadata that is produced with each packet is listed in
the optional <metadataProduced> element of the <product>. In its the optional <metadataProduced> element of the <product>. In its
simplest form, this element can contain a list of <ref> elements, simplest form, this element can contain a list of <ref> elements,
each referring to a metadata type. The meaning of such a list is each referring to a metadata type. The meaning of such a list is
that "all of" these metadata are provided with each packet, except that "all of" these metadata are provided with each packet, except
those that are listed with the optional "availability" attribute those that are listed with the optional "availability" attribute
set to "conditional." Similar to the <metadataExpected> element of set to "conditional". Similar to the <metadataExpected> element of
the <inputPort>, the <metadataProduced> element supports more the <inputPort>, the <metadataProduced> element supports more
complex forms, which we do not discuss here further. complex forms, which we do not discuss here further.
4.7.4. <attributes> Element to Define LFB Operational Attributes 4.7.4. <attributes> Element to Define LFB Operational Attributes
Operational parameters of the LFBs that must be visible to the CEs Operational parameters of the LFBs that must be visible to the CEs
are conceptualized in the model as the LFB attributes. These are conceptualized in the model as the LFB attributes. These
include, for example, flags, single parameter arguments, complex include, for example, flags, single parameter arguments, complex
arguments, and tables. Note that the attributes here refer to only arguments, and tables. Note that the attributes here refer to only
those operational parameters of the LFBs that must be visible to those operational parameters of the LFBs that must be visible to
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case. case.
The mechanism defined above for non-supported attributes can also The mechanism defined above for non-supported attributes can also
apply to attempts to reference non-existent array elements or to apply to attempts to reference non-existent array elements or to
set read-only elements. set read-only elements.
4.7.5. <capabilities> Element to Define LFB Capability Attributes 4.7.5. <capabilities> Element to Define LFB Capability Attributes
The LFB class specification will provide some flexibility for the The LFB class specification will provide some flexibility for the
FE implementation regarding how the LFB class is implemented. For FE implementation regarding how the LFB class is implemented. For
example the class may define some features optional, in which case example, the class may define some optional features, in which case
the actual implementation may or may not provide the given feature. the actual implementation may or may not provide the given feature.
In these cases the CE must be able to query the LFB instance about In these cases the CE must be able to query the LFB instance about
the availability of the feature. In addition, the instance may have the availability of the feature. In addition, the instance may
some limitations that are not inherent from the class definition, have some limitations that are not inherent from the class
but rather the result of some implementation limitations. For definition, but rather the result of some implementation
example, an array attribute may be defined in the class definition limitations. For example, an array attribute may be defined in the
as "unlimited" size, but the physical implementation may impose a class definition as "unlimited" size, but the physical
hard limit on the size of the array. implementation may impose a hard limit on the size of the array.
Such capability related information is expressed by the capability Such capability related information is expressed by the capability
attributes of the LFB class. The capability attributes are always attributes of the LFB class. The capability attributes are always
read-only attributes, and they are listed in a separate read-only attributes, and they are listed in a separate
<capabilities> element in the <LFBClassDef>. The <capabilities> <capabilities> element in the <LFBClassDef>. The <capabilities>
element contains one or more <capability> elements, each defining element contains one or more <capability> elements, each defining
one capability attribute. The format of the <capability> element is one capability attribute. The format of the <capability> element
almost the same as the <attribute> element, it differs in two is almost the same as the <attribute> element, it differs in two
aspects: it lacks the access mode attribute (because it is always aspects: it lacks the access mode attribute (because it is always
read-only), and it lacks the <defaultValue> element (because read-only), and it lacks the <defaultValue> element (because
default value is not applicable to read-only attributes). default value is not applicable to read-only attributes).
Some examples of capability attributes: Some examples of capability attributes:
. The version of the LFB class that this LFB instance complies . The version of the LFB class that this LFB instance complies
with; with;
. Supported optional features of the LFB class; . Supported optional features of the LFB class;
. Maximum number of configurable outputs for an output group; . Maximum number of configurable outputs for an output group;
. Metadata pass-through limitations of the LFB; . Metadata pass-through limitations of the LFB;
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<xsd:element name="version" type="versionType"/> <xsd:element name="version" type="versionType"/>
<xsd:element name="derivedFrom" type="xsd:NMTOKEN" <xsd:element name="derivedFrom" type="xsd:NMTOKEN"
minOccurs="0"/> minOccurs="0"/>
<xsd:element name="inputPorts" type="inputPortsType" <xsd:element name="inputPorts" type="inputPortsType"
minOccurs="0"/> minOccurs="0"/>
<xsd:element name="outputPorts" type="outputPortsType" <xsd:element name="outputPorts" type="outputPortsType"
minOccurs="0"/> minOccurs="0"/>
<xsd:element name="attributes" type="LFBAttributesType" <xsd:element name="attributes" type="LFBAttributesType"
minOccurs="0"/> minOccurs="0"/>
<xsd:element name="capabilities" <xsd:element name="capabilities"
type="LFBCapabilitiesType" type="LFBCapabilitiesType" minOccurs="0"/>
minOccurs="0"/>
<xsd:element ref="description" minOccurs="0"/> <xsd:element ref="description" minOccurs="0"/>
</xsd:sequence> </xsd:sequence>
</xsd:complexType> </xsd:complexType>
<!-- Key constraint to ensure unique attribute names within <!-- Key constraint to ensure unique attribute names within
a class: a class:
--> -->
<xsd:key name="attributes"> <xsd:key name="attributes">
<xsd:selector xpath="lfb:attributes/lfb:attribute"/> <xsd:selector xpath="lfb:attributes/lfb:attribute"/>
<xsd:field xpath="lfb:name"/> <xsd:field xpath="lfb:name"/>
</xsd:key> </xsd:key>
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support for some capability and/or attribute information. support for some capability and/or attribute information.
If a protocol using binary encoding of this information is adopted If a protocol using binary encoding of this information is adopted
by the ForCES working group, then each relevant element defined in by the ForCES working group, then each relevant element defined in
the schema will have a "ProtocolEncoding" attribute added, with a the schema will have a "ProtocolEncoding" attribute added, with a
"Fixed" value providing the value that is used in the protocol for "Fixed" value providing the value that is used in the protocol for
that element, so that the XML and the on the wire protocol can be that element, so that the XML and the on the wire protocol can be
correlated. correlated.
5.2.1. FECapabilities 5.2.1. FECapabilities
This element, which if it occurs must occur only once, contains all
This element, if it occurs, must occur only once and contains all
the capability related information about the FE. Capability the capability related information about the FE. Capability
information is always considered to be read-only. information is always considered to be read-only.
The currently defined elements allowed within the FECapabilities The currently defined elements allowed within the FECapabilities
element are ModifiableLFBTopology, LFBsSupported, element are ModifiableLFBTopology, LFBsSupported,
WriteableAttributes and ReadableAttributes. WriteableAttributes and ReadableAttributes.
5.2.1.1. ModifiableLFBTopology 5.2.1.1. ModifiableLFBTopology
This element has a boolean value. This element indicates whether This element has a boolean value. This element indicates whether
the LFB topology of the FE may be changed by the CE. If the 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 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 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. In present and set to false, the LFB topology of the FE is fixed. In
that case, the LFBs supported clause may be omitted, and the list that case, the LFBs supported clause may be omitted, and the list
of supported LFBs is inferred by the CE from the LFB topology of supported LFBs is inferred by the CE from the LFB topology
information. If the list of supported LFBs is provided when information. If the list of supported LFBs is provided when
ModifiableLFBTopology is false, the CanOccurBefore and ModifiableLFBTopology is false, the CanOccurBefore and
CanOccurAfter information should be omitted. CanOccurAfter information should be omitted.
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This element, if present, indicates the largest number of instances 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 of this LFB class the FE can support. For FEs that do not have the
capability to create or destroy LFB instances, this can either be capability to create or destroy LFB instances, this can either be
omitted or be the same as the number of LFB instances of this class omitted or be the same as the number of LFB instances of this class
contained in the LFB list attribute. contained in the LFB list attribute.
5.2.1.2.3. PortGroupLimits and PortGroupLimit 5.2.1.2.3. PortGroupLimits and PortGroupLimit
The PortGroupLimits element is the wrapper to hold information The PortGroupLimits element is the wrapper to hold information
about the port groups supported by the LFB class. It holds multiple about the port groups supported by the LFB class. It holds
occurrences of the PortGroupLimit element. multiple occurrences of the PortGroupLimit element.
Each occurrence of the PortGroupLimit element contains the port Each occurrence of the PortGroupLimit element contains the port
occurrence information for a single port group of the LFB class. occurrence information for a single port group of the LFB class.
Each occurrence has the name of the port group in the PortGroupName Each occurrence has the name of the port group in the PortGroupName
element, the fewest number of ports that can exist in the group in element, the fewest number of ports that can exist in the group in
the MinPortCount element, and the largest number of ports that can the MinPortCount element, and the largest number of ports that can
exist in the group in the MaxPortCount element. exist in the group in the MaxPortCount element.
5.2.1.2.4.CanOccurAfters and CanOccurAfter 5.2.1.2.4.CanOccurAfters and CanOccurAfter
The CanOccurAfters element is a wrapper to hold the multiple The CanOccurAfters element is a wrapper to hold the multiple
occurrences of the CanOccurAfter permissible placement information. occurrences of the CanOccurAfter permissible placement information.
The CanOccurAfter element describes a permissible positioning of The CanOccurAfter element describes a permissible positioning of
the SupportedLFB. Specifically, it names an LFB that can the SupportedLFB. Specifically, it names an LFB that can
topologically precede the SupportedLFB. That is, the SupportedLFB topologically precede the SupportedLFB. That is, the SupportedLFB
can have an input port connected to an output port of the LFB that can have an input port connected to an output port of the LFB that
it CanOccurAfter. The LFB class that the SupportedLFB can follow is it CanOccurAfter. The LFB class that the SupportedLFB can follow
identified by the NeighborLFB element of the CanOccurAfter element. is identified by the NeighborLFB element of the CanOccurAfter
If this neighbor can only be connected to a specific set of input element. If this neighbor can only be connected to a specific set
port groups, then the viaPort element is included. This element of input port groups, then the viaPort element is included. This
occurs once for each input port group of the SupportedLFB that can element occurs once for each input port group of the SupportedLFB
be connected to an output port of the NeighborLFB. that can be connected to an output port of the NeighborLFB.
[e.g., Within a SupportedLFB element, each CanOccurAfter element [e.g., Within a SupportedLFB element, each CanOccurAfter element
must have a unique NeighborLFB, and within each CanOccurAfter must have a unique NeighborLFB, and within each CanOccurAfter
element each viaPort must represent a unique and valid input port element each viaPort must represent a unique and valid input port
group of the SupportedLFB. The "unique" clauses for this have not group of the SupportedLFB. The "unique" clauses for this have not
yet been added to the schema.] yet been added to the schema.]
5.2.1.2.5. CanOccurBefores and CanOccurBefore 5.2.1.2.5. CanOccurBefores and CanOccurBefore
The CanOccurBefores element is a wrapper to hold the multiple The CanOccurBefores element is a wrapper to hold the multiple
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The CanOccurBefore element similarly lists those LFB classes that The CanOccurBefore element similarly lists those LFB classes that
the SupportedLFB may precede in the topology. In this element, the the SupportedLFB may precede in the topology. In this element, the
viaPort element represents the output port group of the viaPort element represents the output port group of the
SupportedLFB that may be connected to the NeighborLFB. As with SupportedLFB that may be connected to the NeighborLFB. As with
CanOccurAfter, viaPort may occur multiple times if multiple output CanOccurAfter, viaPort may occur multiple times if multiple output
ports may legitimately connect to the given NeighborLFB class. ports may legitimately connect to the given NeighborLFB class.
[And a similar set of uniqueness constraints apply to the [And a similar set of uniqueness constraints apply to the
CanOccurBefore clauses, even though an LFB may occur both in CanOccurBefore clauses, even though an LFB may occur both in
CanOccurAfter and CanOccurBefore.] CanOccurAfter and CanOccurBefore.]
5.2.1.2.6. LFBClassCapabilities 5.2.1.2.6. LFBClassCapabilities
This element contains capability information about the subject LFB This element contains capability information about the subject LFB
class whose structure and semantics are defined by the LFB class class whose structure and semantics are defined by the LFB class
definition. definition.
5.2.1.3. SupportedAttributes 5.2.1.3. SupportedAttributes
This element serves as a wrapper to hold the information about This element serves as a wrapper to hold the information about
attributed related capabilities. Specifically, attributes should be attributed related capabilities. Specifically, attributes should
described in this element if: be described in this element if:
a) they are optional elements in the standard and are supported a) they are optional elements in the standard and are supported
by the FE, or by the FE, or
b) the standard allows for a range of access permissions (for b) the standard allows for a range of access permissions (for
example, read-only or read-write). example, read-only or read-write).
Each attribute so described is contained in the SupportedAttributes Each attribute so described is contained in the SupportedAttributes
element. That element contains an AttributeName element whose value element. That element contains an AttributeName element whose
is the name of the element being described and an AccessModes value is the name of the element being described and an AccessModes
element, whose value is the list of permissions. element, whose value is the list of permissions.
5.2.2. FEAttributes 5.2.2. FEAttributes
The FEAttributes element contains the attributes of the FE that are The FEAttributes element contains the attributes of the FE that are
not considered "capabilities". Some of these attributes are not considered "capabilities". Some of these attributes are
writeable, and some are read-only, which should be indicated by the writeable, and some are read-only, which should be indicated by the
capability information. At the moment, the set of attributes is capability information. At the moment, the set of attributes is
woefully incomplete. Each attribute is identified by a unique woefully incomplete. Each attribute is identified by a unique
element tag, and the value of the element is the value of the element tag, and the value of the element is the value of the
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class this instance has, and an LFBInstanceID indicating the ID class this instance has, and an LFBInstanceID indicating the ID
used for referring to this instance. For now, the ID uses the used for referring to this instance. For now, the ID uses the
NMTOKEN construction. Further protocol work is likely to replace NMTOKEN construction. Further protocol work is likely to replace
this with a range restricted integer. this with a range restricted integer.
5.2.2.3. LFBTopology and LFBLink 5.2.2.3. LFBTopology and LFBLink
This optional element contains the information about each inter-LFB This optional element contains the information about each inter-LFB
link inside the FE. Each link is described in an LFBLink element. link inside the FE. Each link is described in an LFBLink element.
This element contains sufficient information to identify precisely This element contains sufficient information to identify precisely
the end points of a link. The FromLFBID and ToLFBID fields indicate the end points of a link. The FromLFBID and ToLFBID fields
the LFB instances at each end of the link, and must reference LFBs indicate the LFB instances at each end of the link, and must
in the LFB instance table. The FromPortGroup and ToPortGroup must reference LFBs in the LFB instance table. The FromPortGroup and
identify output and input port groups defined in the LFB classes of ToPortGroup must identify output and input port groups defined in
the LFB instances identified by the FromLFBID and ToLFBID. The the LFB classes of the LFB instances identified by the FromLFBID
FromPortIndex and ToPortIndex fields select the elements from the and ToLFBID. The FromPortIndex and ToPortIndex fields select the
port groups that this link connects. All links are uniquely elements from the port groups that this link connects. All links
identified by the FromLFBID, FromPortGroup, and FromPortIndex are uniquely identified by the FromLFBID, FromPortGroup, and
fields. Multiple links may have the same ToLFBID, ToPortGroup, and FromPortIndex fields. Multiple links may have the same ToLFBID,
ToPortIndex as this model supports fan in of inter-LFB links but ToPortGroup, and ToPortIndex as this model supports fan in of
not fan out. inter-LFB links but not fan out.
5.2.2.4. FEConfiguredNeighbors an FEConfiguredNeighbor 5.2.2.4. FEConfiguredNeighbors an FEConfiguredNeighbor
The FEConfiguredNeighbors element is a wrapper to hold the The FEConfiguredNeighbors element is a wrapper to hold the
configuration information that one or more FEConfiguredNeighbor configuration information that one or more FEConfiguredNeighbor
elements convey about the configured FE topology. elements convey about the configured FE topology.
The FEConfiguredNeighbor element occurs once for each configured FE The FEConfiguredNeighbor element occurs once for each configured FE
neighbor the FE knows about. It should not be filled in based on neighbor the FE knows about. It should not be filled in based on
FE level protocol operations. In general, neighbor discovery FE level protocol operations. In general, neighbor discovery
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specific LFB sub-classes will be derived. Hence, the base classes specific LFB sub-classes will be derived. Hence, the base classes
may not be used directly in a particular FE's model, but the sub- may not be used directly in a particular FE's model, but the sub-
classes (yet to be defined) could be. This initial list attempts classes (yet to be defined) could be. This initial list attempts
to describe LFB classes at the expected level of granularity. This to describe LFB classes at the expected level of granularity. This
list is neither exhaustive nor sufficiently detailed. list is neither exhaustive nor sufficiently detailed.
Several working groups in the IETF have already done some relevant Several working groups in the IETF have already done some relevant
work in modeling the provisioning policy data for some of the work in modeling the provisioning policy data for some of the
functions we are interested in, for example, the DiffServ functions we are interested in, for example, the DiffServ
(Differentiated Services) PIB [4] and IPSec PIB [8]. Whenever (Differentiated Services) PIB [4] and IPSec PIB [8]. Whenever
possible, we have tried to reuse the work done elsewhere instead of possible, we have tried to reuse the work done elsewhere.
reinventing the wheel.
6.1. Port LFB 6.1. Port LFB
A Port LFB is used to model physical I/O ports on the FE. It is A Port LFB is used to model physical I/O ports on the FE. It is
both a source of data "received" by the FE and a sink of data both a source of data "received" by the FE and a sink of data
"transmitted" by the FE. The Port LFB contains a number of static "transmitted" by the FE. The Port LFB contains a number of static
attributes, which may include, but are not limited to, the attributes, which may include, but are not limited to, the
following items: following items:
. the number of physical ports on this LFB . the number of physical ports on this LFB
. physical port type . physical port type
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interface identifier. When receiving frames from an adjacent interface identifier. When receiving frames from an adjacent
upstream LFB, the frame is accompanied by two items of metadata: upstream LFB, the frame is accompanied by two items of metadata:
frame length and outgoing port identifier. frame length and outgoing port identifier.
Statistics are not maintained by the Port LFB; statistics Statistics are not maintained by the Port LFB; statistics
associated with a particular port may be maintained by an L2 associated with a particular port may be maintained by an L2
interface LFB (see Section 6.2). interface LFB (see Section 6.2).
6.2. L2 Interface LFB 6.2. L2 Interface LFB
The L2 Interface LFB models an L2 protocol termination. The L2 The L2 Interface LFB models L2 protocol termination. The L2
Interface LFB performs two sets of functions: decapsulation and Interface LFB performs two sets of functions: decapsulation and
demultiplexing as needed on the receive side of an FE, and demultiplexing as needed on the receive side of an FE, and
encapsulation and multiplexing as needed on the transmit side. encapsulation and multiplexing as needed on the transmit side.
Hence the LFB has two distinct sets of inputs and outputs tailored Hence the LFB has two distinct sets of inputs and outputs tailored
for these separate functions. The L2 Interface LFB is not modeled for these separate functions. The L2 Interface LFB is not modeled
as two separate (receive/transmit) LFBs because there are shared as two separate (receive/transmit) LFBs because there are shared
attributes between the decapsulation and encapsulation functions. attributes between the decapsulation and encapsulation functions.
On the decapsulation input(s), the LFB accepts an L2 protocol On the decapsulation input(s), the LFB accepts an L2 protocol
specific frame, along with frame length and L2 interface metadata. specific frame, along with frame length and L2 interface metadata.
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. L2 or L3 interface metadata for next-layer packet . L2 or L3 interface metadata for next-layer packet
. LFB output port. . LFB output port.
The LFB may support multiple decapsulation output ports within two The LFB may support multiple decapsulation output ports within two
output groups; one for normal forwarding, and one for exception output groups; one for normal forwarding, and one for exception
packets. The LFB emits the decapsulated packet along with the packets. The LFB emits the decapsulated packet along with the
modified frame length metadata, an L2 or L3 protocol type metadata, modified frame length metadata, an L2 or L3 protocol type metadata,
and an L2 or L3 interface metadata. and an L2 or L3 interface metadata.
On the encapsulation input(s), the LFB accepts a packet along with On the encapsulation input(s), the LFB accepts a packet along with
frame length, protocol type, and L2 interface metadata. The L2 frame length, protocol type, and L2 interface metadata. The L2
interface metadata is used to select an L2 interface attribute interface metadata is used to select an L2 interface attribute,
which supports a number of additional attributes, including: which supports a number of additional attributes, including:
. L2-specific transmit counters (byte, packet) . L2-specific transmit counters (byte, packet)
. counting mode (may be taken from receive counters mode) . counting mode (may be taken from receive counters mode)
. L2 or L3 interface metadata for next-layer frame (we assume . L2 or L3 interface metadata for next-layer frame (we assume
that L2 that L2 protocols could be layered on top of an L3 protocol;
. protocols could be layered on top of an L3 protocol; e.g., e.g., L2TP or PWE3), or port metadata.
L2TP or . LFB output port
. PWE3), or port metadata.
. LFB output port.
The LFB encapsulates the packet using the appropriate L2 The LFB encapsulates the packet using the appropriate L2
header/trailer and protocol type information (calculating header/trailer and protocol type information (calculating
checksums/CRCs as necessary), and provides the frame to the next checksums/CRCs as necessary), and provides the frame to the next
LFB along with incremented frame length metadata, updated protocol LFB along with incremented frame length metadata, updated protocol
type metadata, and updated interface (or port) metadata, on a type metadata, and updated interface (or port) metadata, on a
configurable LFB encapsulation output. configurable LFB encapsulation output.
As in the case of the Port LFB, technology specific variants of the As in the case of the Port LFB, technology specific variants of the
L2 interface LFB will be sub-classes of the L2 Interface LFB. L2 interface LFB will be sub-classes of the L2 Interface LFB.
Example sub-classes include: Example sub-classes include:
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address attribute. Note that each technology specific sub-class address attribute. Note that each technology specific sub-class
may require additional metadata. For example, the Ethernet/802.1Q may require additional metadata. For example, the Ethernet/802.1Q
Interface LFB requires an outgoing MAC destination address to Interface LFB requires an outgoing MAC destination address to
generate an outgoing Ethernet header. generate an outgoing Ethernet header.
The L2 interface management function is separated into a distinct The L2 interface management function is separated into a distinct
LFB from the Port LFB because L2 encapsulations can be nested LFB from the Port LFB because L2 encapsulations can be nested
within frames; e.g., PPP-over-Ethernet-over-ATM AAL5 (PPPoEoA). within frames; e.g., PPP-over-Ethernet-over-ATM AAL5 (PPPoEoA).
6.3. IP interface LFB 6.3. IP interface LFB
The IP Interface LFB models a container for IP interface-specific The IP Interface LFB models a container for IP interface-specific
attributes. These may include: attributes. These may include:
. IP protocols supported (IPv4 and/or IPv6) . IP protocols supported (IPv4 and/or IPv6)
. IP MTU . IP MTU
. interface MIB counters . interface MIB counters
. table metadata for associated forwarding tables (LPM, . table metadata for associated forwarding tables (LPM,
multicast) multicast)
. table metadata for associated classification tables. . table metadata for associated classification tables.
The IP Interface LFB also performs basic protocol-specific packet The IP Interface LFB also performs basic protocol-specific packet
heade validation functions (e.g., IP version, IPv4 header length, header validation functions (e.g., IP version, IPv4 header length,
IPv4 header checksum, MTU, TTL=0, etc.). The IP Interface LFB IPv4 header checksum, MTU, TTL=0, etc.). The IP Interface LFB
class supports three different L3 protocols: IPv4, IPv6, and MPLS, class supports three different L3 protocols: IPv4, IPv6, and MPLS,
although individual LFB instances might support a subset of these although individual LFB instances might support a subset of these
protocols, configurable on each interface attribute. protocols, configurable on each interface attribute.
As with the L2 Interface LFB, the IP Interface LFB supports two As with the L2 Interface LFB, the IP Interface LFB supports two
modes of operation: one needed on the receive side of an FE, and modes of operation: one needed on the receive side of an FE, and
one on the transmit side, using separate sets of LFB inputs and one on the transmit side, using separate sets of LFB inputs and
outputs. In the first mode of operation (for FE receive outputs. In the first mode of operation (for FE receive
processing), the IP Interface LFB accepts IP packets along with processing), the IP Interface LFB accepts IP packets along with
frame length, L3 protocol type, and interface metadata (possibly frame length, L3 protocol type, and interface metadata (possibly
including additional metadata items such as L2-derived class including additional metadata items such as L2-derived class
metadata). The interface metadata is used to select an interface metadata). The interface metadata is used to select an interface
attribute, and the protocol type is checked against the protocols attribute, and the protocol type is checked against the protocols
supported for this interface. Error checks are applied, including supported for this interface. Error checks are applied, including
whether the particular protocol type is supported on this whether the particular protocol type is supported on this
interface, and if no errors occur, the appropriate counters are interface, and if no errors occur, the appropriate counters are
incremented and the protocol type is used to select the outgoing incremented and the protocol type is used to select the outgoing
LFB output from a set dedicated to the first mode of operation. The LFB output from a set dedicated to the first mode of operation.
IP header protocol type/next header field may also be used to The IP header protocol type/next header field may also be used to
select an LFB output; for example, IPv4 packets with AH header may select an LFB output; for example, IPv4 packets with AH header may
be directed to a particular next LFB, or IPv6 packets with Hop-by- be directed to a particular next LFB, or IPv6 packets with Hop-by-
Hop Options. If errors do occur, the appropriate error counters Hop Options. If errors do occur, the appropriate error counters
are incremented, and the error type is used to select a specific are incremented, and the error type is used to select a specific
exception LFB output. exception LFB output.
In the second mode of operation (for FE transmit processing), the In the second mode of operation (for FE transmit processing), the
IP Interface LFB accepts an IP packet along with frame length, IP Interface LFB accepts an IP packet along with frame length,
protocol type, and interface metadata. Again, the interface protocol type, and interface metadata. Again, the interface
metadata is used to select an interface attribute. The interface metadata is used to select an interface attribute. The interface
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protocol operation, a separate LFB may be defined (e.g., IP protocol operation, a separate LFB may be defined (e.g., IP
Interface LFB, which performs header verification). Interface LFB, which performs header verification).
Several common applications need to classify packets using a Several common applications need to classify packets using a
particular mathematical operation (e.g., longest prefix match (LPM) particular mathematical operation (e.g., longest prefix match (LPM)
or ternary match) against a fixed set of fields in a packet's or ternary match) against a fixed set of fields in a packet's
header plus metadata, or an easily recognized part of the packet header plus metadata, or an easily recognized part of the packet
payload. Two example applications are classification for payload. Two example applications are classification for
Differentiated Services or for security processing. Typically the Differentiated Services or for security processing. Typically the
packet is evaluated against a potentially large set of rules packet is evaluated against a potentially large set of rules
(called "filters") which are processed in a particular order to (called "filters"), which are processed in a particular order to
ensure a deterministic result. This sort of classification ensure a deterministic result. This sort of classification
functionalit is modeled by the Classifier LFB. functionality is modeled by the Classifier LFB.
The Classifier LFB accepts an input packet and metadata, and The Classifier LFB accepts an input packet and metadata, and
produces the unmodified packet along with a class metadata, which produces the unmodified packet along with a class metadata, which
may be used to map the packet to a particular LFB output. may be used to map the packet to a particular LFB output.
The Classifier LFB supports multiple classifier attributes. Each The Classifier LFB supports multiple classifier attributes. Each
classifier is parameterized by one or more filters. Classification classifier is parameterized by one or more filters. Classification
is performed by selecting the classifier to use on a particular is performed by selecting the classifier to use on a particular
packet (e.g., by metadata lookup on a configurable metadata item), packet (e.g., by metadata lookup on a configurable metadata item),
and by evaluating the selected contents of the accepted packet and by evaluating the selected contents of the accepted packet
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RPF check. No additional metadata is produced for the latter, but RPF check. No additional metadata is produced for the latter, but
for the former, the following metadata may be produced: for the former, the following metadata may be produced:
. outgoing interface(s) . outgoing interface(s)
. next hop IP address(es) . next hop IP address(es)
. TTL decrement value (if TTL decrement is not performed by the . TTL decrement value (if TTL decrement is not performed by the
Next Hop LFB) Next Hop LFB)
An alternative mode of operation produces index metadata instead of An alternative mode of operation produces index metadata instead of
outgoing interface and next hop IP address metadata. This index outgoing interface and next hop IP address metadata. This index
metadata is used to access a cache of the outgoing interface and metadata is used to access a cache of the outgoing interface and
next hop IP address that may be stored on the egress FE (this next hop IP address that may be stored on the egress FE (this
permits more efficient communication across the Fi interface). permits more efficient communication across the FE interface).
This index metadata can also be used as input metadata to a MPLS This index metadata can also be used as input metadata to a MPLS
Encapsulation LFB. Encapsulation LFB.
The Next Hop LFB supports an exception output port group. The Next Hop LFB supports an exception output port group.
Exception conditions include: Exception conditions include:
. RPF test failed . RPF test failed
. No route to host . No route to host
. No route to network . No route to network
. Packet too big . Packet too big
. TTL expired . TTL expired
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state maintained by the attribute. A color metadata is associated state maintained by the attribute. A color metadata is associated
with the packet in accordance with the metering algorithm used. with the packet in accordance with the metering algorithm used.
The color metadata is optionally emitted with the packet, or used The color metadata is optionally emitted with the packet, or used
to map the packet to a particular LFB output. Color-aware metering to map the packet to a particular LFB output. Color-aware metering
algorithms use color metadata if provided with the packet (e.g., by algorithms use color metadata if provided with the packet (e.g., by
a Classifier LFB), or assume a default color value. a Classifier LFB), or assume a default color value.
The Rate Meter LFB supports a number of static attributes, The Rate Meter LFB supports a number of static attributes,
including: including:
. supported metering algorithms . supported metering algorithms
. maximum number of meter attributes. . maximum number of meter attributes
The Rate Meter LFB supports a number of configurable attributes, The Rate Meter LFB supports a number of configurable attributes,
including: including:
. number of LFB inputs . number of LFB inputs
. number of LFB outputs . number of LFB outputs
. mapping of LFB input to meter attribute (when mapped . mapping of LFB input to meter attribute (when mapped
statically) statically)
. metadata item to select for mapping to meter attribute . metadata item to select for mapping to meter attribute
. mapping of metadata value to meter attribute . mapping of metadata value to meter attribute
. default meter attribute (when not mapped statically or via . default meter attribute (when not mapped statically or via
correct correct
. metadata) . metadata)
. per-attribute metering algorithm . per-attribute metering algorithm
. per-attribute metering paramters, including: . per-attribute metering parameters, including:
. minimum rate . minimum rate
. maximum rate . maximum rate
. burst size . burst size
. color metadata enable . color metadata enable
. mapping of packet color to LFB output. . mapping of packet color to LFB output
A Rate Meter LFB can be used to implement a policing function, by A Rate Meter LFB can be used to implement a policing function, by
connecting a LFB output directly to a Dropper LFB, and mapping non- connecting a LFB output directly to a Dropper LFB, and mapping non-
conforming (e.g., "red") traffic to that output. conforming (e.g., "red") traffic to that output.
6.7. Redirector (de-MUX) LFB 6.7. Redirector (de-MUX) LFB
The Redirector LFB is used to select between alternative datapaths The Redirector LFB is used to select between alternative datapaths
based on the value of some metadata item. The Redirector LFB based on the value of some metadata item. The Redirector LFB
accepts an input packet P, and uses associated metadata item M to accepts an input packet P, and uses associated metadata item M to
demultiplex that packet onto one of N outputs; e.g., unicast demultiplex that packet onto one of N outputs; e.g., unicast
forwarding, multicast, or broadcast. Configurable attributes forwarding, multicast, or broadcast. Configurable attributes
include: include:
. number of LFB output ports (N) . number of LFB output ports (N)
. metadata item to demultiplex on (M) . metadata item to demultiplex on (M)
. mapping of metadata value to output port . mapping of metadata value to output port
. default output port (for un-matched input metadata values). . default output port (for un-matched input metadata values).
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including: including:
. number of LFB inputs . number of LFB inputs
. mapping of LFB input to count attribute (when mapped . mapping of LFB input to count attribute (when mapped
statically) statically)
. metadata item to select for mapping to count attribute . metadata item to select for mapping to count attribute
. mapping of metadata value to count attribute . mapping of metadata value to count attribute
. default count attribute (when not mapped statically or via . default count attribute (when not mapped statically or via
correct correct
. metadata) . metadata)
. counting mode per-attribute . counting mode per-attribute
. logging mode per-attribute. . logging mode per-attribute
The Counter LFB does not perform any time-dependent counting. The The Counter LFB does not perform any time-dependent counting. The
time at which a count is made may, however, be logged as part of time at which a count is made may, however, be logged as part of
the count attribute. the count attribute.
Other LFBs may maintain internal statistics (e.g., interface LFBs). Other LFBs may maintain internal statistics (e.g., interface LFBs).
The Counter LFB is especially useful for maintain counts associated The Counter LFB is especially useful to maintain counts associated
with QoS policy. with QoS policy.
6.10. Dropper LFB 6.10. Dropper LFB
A Dropper LFB has one input, and no outputs. It discards all A Dropper LFB has one input, and no outputs. It discards all
packets that it accepts without any modification or examination of packets that it accepts without any modification or examination of
those packets. those packets.
The purpose of a Dropper LFB is to allow the description of "sinks" The purpose of a Dropper LFB is to allow the description of "sinks"
within the model, where those sinks do not result in the packet within the model, where those sinks do not result in the packet
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corrected frame length metadata. corrected frame length metadata.
The source of the outgoing interface MTU is TBD. The IPv4 The source of the outgoing interface MTU is TBD. The IPv4
fragmentation function is not incorporated into the IP Interface fragmentation function is not incorporated into the IP Interface
LFB because forwarding implementations may include additional LFB because forwarding implementations may include additional
forwarding functions between fragmentation and final output forwarding functions between fragmentation and final output
interface processing. interface processing.
6.12. L2 Address Resolution LFB 6.12. L2 Address Resolution LFB
The L2 Address Resolution LFB is used to map an next hop IP address The L2 Address Resolution LFB is used to map a next hop IP address
into an L2 address. The LFB accepts packets with output L2 into an L2 address. The LFB accepts packets with output L2
interface and next hop IP address metadata, and produces the packet interface and next hop IP address metadata, and produces the packet
along with the correct L2 destination address. The L2 Address along with the correct L2 destination address. The L2 Address
Resolution LFB maintains multiple address resolution table Resolution LFB maintains multiple address resolution table
attributes accessed by the output L2 interface metadata. Each attributes accessed by the output L2 interface metadata. Each
table attribute maintains a set of configurable L2 address table attribute maintains a set of configurable L2 address
attributes, accessed by the next hop IP address. attributes, accessed by the next hop IP address.
The L2 Address Resolution LFB has a normal output group which The L2 Address Resolution LFB has a normal output group, which
produces the L2 destination address metadata, as well as an produces the L2 destination address metadata as well as an
exception output. This exception output can be used to divert the exception output. This exception output can be used to divert the
packet to another LFB (e.g., an ARP/ND Protocol LFB, or a Port LFB packet to another LFB (e.g., an ARP/ND Protocol LFB, or a Port LFB
used to reach the CE) for address resolution. used to reach the CE) for address resolution.
6.13. Queue LFB 6.13. Queue LFB
The Queue LFB is used to represent queueing points in the packet The Queue LFB is used to represent queueing points in the packet
datapath. It is always used in combination with one or more datapath. It is always used in combination with one or more
Scheduler LFBs. The Queue LFB manages one or more FIFO packet Scheduler LFBs. The Queue LFB manages one or more FIFO packet
queues as configurable attributes. The Queue LFB provides one or queues as configurable attributes. The Queue LFB provides one or
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datapath. It is always used in combination with one or more datapath. It is always used in combination with one or more
Scheduler LFBs. The Queue LFB manages one or more FIFO packet Scheduler LFBs. The Queue LFB manages one or more FIFO packet
queues as configurable attributes. The Queue LFB provides one or queues as configurable attributes. The Queue LFB provides one or
more LFB inputs, and packets are mapped from LFB inputs to queues, more LFB inputs, and packets are mapped from LFB inputs to queues,
either statically, or via queue metadata. Each queue attribute is either statically, or via queue metadata. Each queue attribute is
mapped one-to-one with a scheduling input on a downstream Scheduler mapped one-to-one with a scheduling input on a downstream Scheduler
LFB. The Queue LFB provides one or more LFB outputs, along with LFB. The Queue LFB provides one or more LFB outputs, along with
optional scheduling input metadata. optional scheduling input metadata.
Additional per-queue configurable attributes include the following: Additional per-queue configurable attributes include the following:
. maximum depth discard behavior (tail drop/head drop/Active . maximum depth discard behavior (tail drop/head drop/Active
Queue Management (AQM)) Queue Management (AQM))
. AQM parameters (specific to the AQM algorithm; e.g., RED) . AQM parameters (specific to the AQM algorithm; e.g., RED)
. Explicit Congestion Notification (ECN) enable. . Explicit Congestion Notification (ECN) enable
Packets are provided to the Queue LFB along with a packet length Packets are provided to the Queue LFB along with a packet length
metadata and an optional queue metadata. Because the Queue LFB can metadata and an optional queue metadata. Because the Queue LFB can
model sophisticated AQM mechanisms such as per-color marking model sophisticated AQM mechanisms such as per-color marking
thresholds (e.g., Weighted RED), packets may also be accompanied thresholds (e.g., Weighted RED), packets may also be accompanied
with color metadata. with color metadata.
If ECN is enabled on a queue serving IP packets, then the IP packet If ECN is enabled on a queue serving IP packets, then the IP packet
header is modified if congestion is marked. A protocol type header is modified if congestion is marked. A protocol type
metadata must accompany the packet to indicate the packet protocol metadata must accompany the packet to indicate the packet protocol
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Packets are provided to the Scheduler LFB along with a packet Packets are provided to the Scheduler LFB along with a packet
length metadata and an optional scheduling input metadata. length metadata and an optional scheduling input metadata.
Configurable attributes include: Configurable attributes include:
. number of logical scheduler inputs . number of logical scheduler inputs
. number of LFB inputs . number of LFB inputs
. mapping of LFB input to scheduler input . mapping of LFB input to scheduler input
. scheduling algorithm . scheduling algorithm
. per-input scheduling parameters, including: . per-input scheduling parameters, including:
. priority . priority
. minimum service rate . minimum service rate
. maximum service rate . maximum service rate
. burst duration (at maximum service rate). . burst duration (at maximum service rate)
Hierarchical scheduling configurations can be created by cascading Hierarchical scheduling configurations can be created by cascading
two or more Scheduler LFBs. two or more Scheduler LFBs.
6.15. MPLS ILM/Decapsulation LFB 6.15. MPLS ILM/Decapsulation LFB
The MPLS Incoming Label Map (ILM)/Decapsulation LFB accepts MPLS- The MPLS Incoming Label Map (ILM)/Decapsulation LFB accepts MPLS-
encapsulated packets, examines (and possibly removes) the top-most encapsulated packets, examines (and possibly removes) the top-most
label, and emits the packet on one output within an output group, label, and emits the packet on one output within an output group,
along with configurable index and class metadata. The configurable along with configurable index and class metadata. The configurable
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extensible to allow defining new logical functions. extensible to allow defining new logical functions.
A major component of the proposed FE model is the Logical Function A major component of the proposed FE model is the Logical Function
Block (LFB) model. Each distinct logical function in an FE is Block (LFB) model. Each distinct logical function in an FE is
modeled as an LFB. Operational parameters of the LFB that must be modeled as an LFB. Operational parameters of the LFB that must be
visible to the CE are conceptualized as LFB attributes. These visible to the CE are conceptualized as LFB attributes. These
attributes support flexible implementations by allowing an FE to attributes support flexible implementations by allowing an FE to
specify supported optional features and to indicate which specify supported optional features and to indicate which
attributes are configurable by the CE for an LFB class (e.g., attributes are configurable by the CE for an LFB class (e.g.,
express the capability of the FE). Configurable attributes also express the capability of the FE). Configurable attributes also
provide the CE some flexibility in specifying the behavior of a provide the CE some flexibility in specifying the behavior of an
LFB. When multiple LFBs belonging to the same LFB class are LFB. When multiple LFBs belonging to the same LFB class are
instantiated on an FE, each of those LFBs could be configured with instantiated on an FE, each of those LFBs could be configured with
different attribute settings. By querying the settings of the different attribute settings. By querying the settings of the
attributes for an instantiated LFB, one can determine the state of attributes for an instantiated LFB, one can determine the state of
that LFB. that LFB.
Instantiated LFBs are interconnected in a directed graph that Instantiated LFBs are interconnected in a directed graph that
describes the ordering of the functions within an FE. This describes the ordering of the functions within an FE. This
directed graph is described by the topology model. The combination directed graph is described by the topology model. The
of the attributes of the instantiated LFBs and the topology combination of the attributes of the instantiated LFBs and the
describe the packet processing functions available on the FE topology describe the packet processing functions available on the
(current state). FE (current state).
Another key component of the FE model is the FE attributes. The FE Another key component of the FE model is the FE attributes. The FE
attributes are used mainly to describe the capabilities of the FE, attributes are used mainly to describe the capabilities of the FE,
but they also convey information about the FE state. but they also convey information about the FE state.
The FE model also includes a definition of the minimal set of LFBs The FE model also includes a definition of the minimal set of LFBs
that is required by Section 5.5 of [1]. The sections that follow that is required by Section 5.5 of [1]. The sections that follow
provide more detail on the specifics of each of those LFBs. provide more detail on the specifics of each of those LFBs.
7.1. Port Functions 7.1. Port Functions
The FE model can be used to define a Port LFB class and its The FE model can be used to define a Port LFB class and its
technology-specific subclasses (see Section 6.1) to map the technology-specific subclasses (see Section 6.1) to map the
physical port of the device to the LFB model with both static and physical port of the device to the LFB model with both static and
configurable attributes. The static attributes model the type of configurable attributes. The static attributes model the type of
port, link speed etc. The configurable attributes model the port, link speed, etc. The configurable attributes model the
addressing, administrative status etc. addressing, administrative status etc.
7.2. Forwarding Functions 7.2. Forwarding Functions
Because forwarding function is one of the most common and important Because forwarding function is one of the most common and important
functions in the forwarding plane, it requires special attention in functions in the forwarding plane, it requires special attention in
modeling to allow design flexibility, implementation efficiency, modeling to allow design flexibility, implementation efficiency,
modeling accuracy and configuration simplicity. Toward that end, modeling accuracy and configuration simplicity. Toward that end,
it is recommended that the core forwarding function being modeled it is recommended that the core forwarding function being modeled
by the combination of two LFBs -- Longest Prefix Match (LPM) by the combination of two LFBs -- Longest Prefix Match (LPM)
classifier LFB (see Section 6.4) and Next Hop LFB (see Section classifier LFB (see Section 6.4) and Next Hop LFB (see Section
6.5). Special header writer LFB (see Section 6.8) is also needed 6.5). Special header writer LFB (see Section 6.8) is also needed
to take care of TTL decrement and checksum etc. to take care of TTL decrement and checksum etc.
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by the combination of two LFBs -- Longest Prefix Match (LPM) by the combination of two LFBs -- Longest Prefix Match (LPM)
classifier LFB (see Section 6.4) and Next Hop LFB (see Section classifier LFB (see Section 6.4) and Next Hop LFB (see Section
6.5). Special header writer LFB (see Section 6.8) is also needed 6.5). Special header writer LFB (see Section 6.8) is also needed
to take care of TTL decrement and checksum etc. to take care of TTL decrement and checksum etc.
7.3. QoS Functions 7.3. QoS Functions
The LFB class library already includes descriptions of the Meter The LFB class library already includes descriptions of the Meter
(Section 6.6.), Queue (Section 6.13), Scheduler (Section 6.14), (Section 6.6.), Queue (Section 6.13), Scheduler (Section 6.14),
Counter (Section 6.9) and Dropper (Section 6.10) LFBs to support Counter (Section 6.9) and Dropper (Section 6.10) LFBs to support
the QoS functions in the forwarding path. FE model can also be the QoS functions in the forwarding path. The FE model can also be
used to define other useful QoS functions as needed. These LFBs used to define other useful QoS functions as needed. These LFBs
allow the CE to manipulate the attributes to model IntServ or allow the CE to manipulate the attributes to model IntServ or
DiffServ functions. DiffServ functions.
7.4. Generic Filtering Functions 7.4. Generic Filtering Functions
Various combinations of Classifier (Section 6.4), Redirector Various combinations of Classifier (Section 6.4), Redirector
(Section 6.7), Meter (Section 6.6.) and Dropper (Section 6.10) LFBs (Section 6.7), Meter (Section 6.6.) and Dropper (Section 6.10) LFBs
can model a complex set of filtering functions. can model a complex set of filtering functions.
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headers of a packet based on content other than what is found in headers of a packet based on content other than what is found in
the IP header. Examples of such functions include NAT, ALG, the IP header. Examples of such functions include NAT, ALG,
firewall, tunneling and L7 content recognition. It is not firewall, tunneling and L7 content recognition. It is not
practical to include all possible high touch functions in the practical to include all possible high touch functions in the
initial LFB library in Section 6 due to the number and complexity. initial LFB library in Section 6 due to the number and complexity.
However, the flexibility of the LFB model and the power of However, the flexibility of the LFB model and the power of
interconnection in LFB topology should make it possible to model interconnection in LFB topology should make it possible to model
any high-touch functions. any high-touch functions.
7.7. Security Functions 7.7. Security Functions
Security functions are not included in the initial LFB class Security functions are not included in the initial LFB class
library. However, the FE model is flexible and powerful enough to library. However, the FE model is flexible and powerful enough to
model the types of encryption and/or decryption functions that an model the types of encryption and/or decryption functions that an
FE supports and the associated attributes for such functions. FE supports and the associated attributes for such functions.
The IP Security Policy (IPSP) Working Group in the IETF has started The IP Security Policy (IPSP) Working Group in the IETF has started
work in defining the IPSec Policy Information Base [8]. We should work in defining the IPSec Policy Information Base [8]. We will
try to reuse the work as much as we can. try to reuse as much of the work as possible.
7.8. Off-loaded Functions 7.8. Off-loaded Functions
In addition to the packet processing functions that are typical to In addition to the packet processing functions that are typical to
find on the FEs, some logical functions may also be executed find on the FEs, some logical functions may also be executed
asynchronously by some FEs, according to a certain finite-state asynchronously by some FEs, according to a certain finite-state
machine, triggered not only by packet events, but by timer events machine, triggered not only by packet events, but by timer events
as well. Examples of such functions include finite-state machine as well. Examples of such functions include finite-state machine
execution required by TCP termination or OSPF Hello processing off- execution required by TCP termination or OSPF Hello processing off-
loaded from the CE. By defining LFBs for such functions, the FE loaded from the CE. By defining LFBs for such functions, the FE
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and other methods. Time event generation, filter LFB, and and other methods. Time event generation, filter LFB, and
counter/meter LFB are the elements needed to support packet counter/meter LFB are the elements needed to support packet
filtering and sampling functions -- these elements can all be filtering and sampling functions -- these elements can all be
supported in the FE model. supported in the FE model.
8. Using the FE model in the ForCES Protocol 8. Using the FE model in the ForCES Protocol
The actual model of the forwarding plane in a given NE is something 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 the CE must learn and control by communicating with the FEs (or by
other means). Most of this communication will happen in the post- other means). Most of this communication will happen in the post-
association phase using the ForCES protocol. The following types of association phase using the ForCES protocol. The following types
information must be exchanged between CEs and FEs via the ForCES of information must be exchanged between CEs and FEs via the ForCES
protocol: protocol:
1) FE topology query; 1) FE topology query;
2) FE capability declaration; 2) FE capability declaration;
3) LFB topology (per FE) and configuration capabilities query; 3) LFB topology (per FE) and configuration capabilities query;
4) LFB capability declaration; 4) LFB capability declaration;
5) State query of LFB attributes; 5) State query of LFB attributes;
6) Manipulation of LFB attributes; 6) Manipulation of LFB attributes;
7) LFB topology reconfiguration. 7) LFB topology reconfiguration.
Items 1) through 5) are query exchanges, where the main flow of 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 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- typically queried by the CE(s) in the beginning of the post-
association (PA) phase, though they may be repeatedly queried at 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 any time in the PA phase. Item 5) (state query) will be used at
beginning of the PA phase, and often frequently during the PA phase the beginning of the PA phase, and often frequently during the PA
(especially for the query of statistical counters). phase (especially for the query of statistical counters).
Items 6) and 7) are "command" types of exchanges, where the main 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) flow of information is from the CEs to the FEs. Messages in Item
(the LFB re-configuration commands) are expected to be used 6) (the LFB re-configuration commands) are expected to be used
frequently. Item 7) (LFB topology re-configuration) is needed only frequently. Item 7) (LFB topology re-configuration) is needed only
if dynamic LFB topologies are supported by the FEs and it is if dynamic LFB topologies are supported by the FEs and it is
expected to be used infrequently. expected to be used infrequently.
Among the seven types of payload information the ForCES protocol Among the seven types of payload information the ForCES protocol
carries between CEs and FEs, the FE model covers all of them except carries between CEs and FEs, the FE model covers all of them except
item 1), which concerns the inter-FE topology. The FE model item 1), which concerns the inter-FE topology. The FE model
focuses on the LFB and LFB topology within a single FE. Since the focuses on the LFB and LFB topology within a single FE. Since the
information related to item 1) requires global knowledge about all information related to item 1) requires global knowledge about all
of the FEs and their inter-connection with each other, this of the FEs and their inter-connection with each other, this
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typically imposed by the implementation. Therefore, quantitative typically imposed by the implementation. Therefore, quantitative
limitations should always be expressed by capability arguments. limitations should always be expressed by capability arguments.
LFB instances in the model of a particular FE implementation will LFB instances in the model of a particular FE implementation will
possess limitations on the capabilities defined in the possess limitations on the capabilities defined in the
corresponding LFB class. The LFB class specifications must define corresponding LFB class. The LFB class specifications must define
a set of capability arguments, and the CE must be able to query the 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 actual capabilities of the LFB instance via querying the value of
such arguments. The capability query will typically happen when such arguments. The capability query will typically happen when
the LFB is first detected by the CE. Capabilities need not be re- the LFB is first detected by the CE. Capabilities need not be re-
queried in case of static limitations. In some cases, however, some queried in case of static limitations. In some cases, however,
capabilities may change in time (e.g., as a result of some capabilities may change in time (e.g., as a result of
adding/removing other LFBs, or configuring certain attributes of adding/removing other LFBs, or configuring certain attributes of
some other LFB when the LFBs share physical resources), in which some other LFB when the LFBs share physical resources), in which
case additional mechanisms must be implemented to inform the CE case additional mechanisms must be implemented to inform the CE
about the changes. about the changes.
The following two broad types of limitations will exist: The following two broad types of limitations will exist:
. Qualitative restrictions. For example, a standardized multi- . Qualitative restrictions. For example, a standardized multi-
field classifier LFB class may define a large number of field classifier LFB class may define a large number of
classification fields, but a given FE may support only a classification fields, but a given FE may support only a
subset of those fields. subset of those fields.
. Quantitative restrictions, such as the maximum size of tables, . Quantitative restrictions, such as the maximum size of tables,
etc. etc.
The capability parameters that can be queried on a given LFB class The capability parameters that can be queried on a given LFB class
will be part of the LFB class specification. The capability will be part of the LFB class specification. The capability
parameters should be regarded as special attributes of the LFB. The parameters should be regarded as special attributes of the LFB.
actual values of these arguments may be, therefore, obtained using The actual values of these arguments may be, therefore, obtained
the same attribute query mechanisms as used for other LFB using the same attribute query mechanisms as used for other LFB
attributes. attributes.
Capability attributes will typically be read-only arguments, but in Capability attributes will typically be read-only arguments, but in
certain cases they may be configurable. For example, the size of a certain cases they may be configurable. For example, the size of a
lookup table may be limited by the hardware (read-only), in other lookup table may be limited by the hardware (read-only), in other
cases it may be configurable (read-write, within some hard limits). cases it may be configurable (read-write, within some hard limits).
Assuming that capabilities will not change frequently, the Assuming that capabilities will not change frequently, the
efficiency of the protocol/schema/encoding is of secondary concern. efficiency of the protocol/schema/encoding is of secondary concern.
8.5. State Query of LFB Attributes 8.5. State Query of LFB Attributes
This feature must be provided by all FEs. The ForCES protocol and This feature must be provided by all FEs. The ForCES protocol and
the data schema/encoding conveyed by the protocol must together the data schema/encoding conveyed by the protocol must together
satisfy the following requirements to facilitate state query of the satisfy the following requirements to facilitate state query of the
LFB attributes: LFB attributes:
. Must permit FE selection. This is primarily to refer to a . Must permit FE selection. This is primarily to refer to a
single FE, but referring to a group of (or all) FEs may single FE, but referring to a group of (or all) FEs may
optional be supported. optional be supported.
. Must permit LFB instance selection. This is primarily to refer . Must permit LFB instance selection. This is primarily to
to a single LFB instance of an FE, but optionally addressing refer to a single LFB instance of an FE, but optionally
of a group of LFBs (or all) may be supported. addressing of a group of LFBs (or all) may be supported.
. Must support addressing of individual attribute of an LFB. . Must support addressing of individual attribute of an LFB.
. Must provide efficient encoding and decoding of the addressing . Must provide efficient encoding and decoding of the addressing
info and the configured data. info and the configured data.
. Must provide efficient data transmission of the attribute . Must provide efficient data transmission of the attribute
state over the wire (to minimize communication load on the CE- state over the wire (to minimize communication load on the CE-
FE link). FE link).
8.6. LFB Attribute Manipulation 8.6. LFB Attribute Manipulation
This is a place-holder for all operations that the CE will use to This is a place-holder for all operations that the CE will use to
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10. Security Considerations 10. Security Considerations
The FE model describes the representation and organization of data The FE model describes the representation and organization of data
sets and attributes in the FEs. ForCES framework document [2] sets and attributes in the FEs. ForCES framework document [2]
provides a comprehensive security analysis for the overall ForCES provides a comprehensive security analysis for the overall ForCES
architecture. For example, the ForCES protocol entities must be architecture. For example, the ForCES protocol entities must be
authenticated per the ForCES requirements before they can access authenticated per the ForCES requirements before they can access
the information elements described in this document via ForCES. the information elements described in this document via ForCES.
The access to the information contained in the FE model is The access to the information contained in the FE model is
accomplished via the ForCES protocol which will be defined in accomplished via the ForCES protocol, which will be defined in
separate documents and so the security issues will be addressed separate documents, and so the security issues will be addressed
there. there.
11. Normative References 11. Normative References
[1] Khosravi, H. et al., "Requirements for Separation of IP Control [1] Khosravi, H. et al., "Requirements for Separation of IP Control
and Forwarding", RFC 3654, November 2003. and Forwarding", RFC 3654, November 2003.
[2] Yang, L. et al., "Forwarding and Control Element Separation [2] Yang, L. et al., "Forwarding and Control Element Separation
(ForCES) Framework", work in progress, November 2003, <draft-ietf- (ForCES) Framework", work in progress, November 2003, <draft-ietf-
forces-framework-13.txt>. forces-framework-13.txt>.
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Email: ram.gopal@nokia.com Email: ram.gopal@nokia.com
Alan DeKok Alan DeKok
IDT Inc. IDT Inc.
1575 Carling Ave. 1575 Carling Ave.
Ottawa, ON K1G 0T3, Canada Ottawa, ON K1G 0T3, Canada
Phone: +1 613 724 6004 ext. 231 Phone: +1 613 724 6004 ext. 231
Email: alan.dekok@idt.com Email: alan.dekok@idt.com
Zsolt Haraszti Zsolt Haraszti
Ericsson Modular Networks
920 Main Campus Dr, St. 500 First Flight Venture Center
Raleigh, NC 27606, USA 2 Davis Drive
Phone: +1 919 472 9949 PO Box 12076
Email: zsolt.haraszti@ericsson.com Research Triangle Park, NC 27709, USA
Phone: +1 919 765 0027 x2017
Email: zsolt@modularnet.com
Steven Blake Steven Blake
Ericsson Modular Networks
920 Main Campus Dr, St. 500 First Flight Venture Center
Raleigh, NC 27606, USA 2 Davis Drive
Phone: +1 919 472 9913 PO Box 12076
Email: steven.blake@ericsson.com Research Triangle Park, NC 27709, USA
Phone: +1 919 765 0027 x2016
Email: slblake@modularnet.com
Ellen Deleganes Ellen Deleganes
Intel Corp. Intel Corp.
Mail Stop: JF3-206 Mail Stop: JF3-206
2111 NE 25th Avenue 2111 NE 25th Avenue
Hillsboro, OR 97124, USA Hillsboro, OR 97124, USA
Phone: +1 503 712 4173 Phone: +1 503 712 4173
Email: ellen.m.deleganes@intel.com Email: ellen.m.deleganes@intel.com
14. Intellectual Property Right 14. Intellectual Property Right
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