Working Group: ForCES J. Halpern Internet-Draft Self Intended status: Standards Track J. Hadi Salim Expires: April 10, 2009 Znyx Networks October 7, 2008 ForCES Forwarding Element Model draft-ietf-forces-model-16.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 10, 2009. Comments are solicited and should be addressed to the working group's mailing list at forces@peach.ease.lsoft.com and/or the author(s). Abstract This document defines the forwarding element (FE) model used in the Forwarding and Control Element Separation (ForCES) protocol [2]. The model represents the capabilities, state and configuration of forwarding elements within the context of the ForCES protocol, so that control elements (CEs) can control the FEs accordingly. More specifically, the model describes the logical functions that are present in an FE, what capabilities these functions support, and how these functions are or can be interconnected. This FE model is Halpern & Hadi Salim Expires April 10, 2009 [Page 1] Internet-Draft ForCES FE Model October 2008 intended to satisfy the model requirements specified in the ForCES requirements document, RFC3654 [6]. Table of Contents 1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Requirements on the FE model . . . . . . . . . . . . . . 7 2.2. The FE Model in Relation to FE Implementations . . . . . 8 2.3. The FE Model in Relation to the ForCES Protocol . . . . . 8 2.4. Modeling Language for the FE Model . . . . . . . . . . . 9 2.5. Document Structure . . . . . . . . . . . . . . . . . . . 10 3. ForCES Model Concepts . . . . . . . . . . . . . . . . . . . . 10 3.1. ForCES Capability Model and State Model . . . . . . . . . 11 3.1.1. FE Capability Model and State Model . . . . . . . . . 12 3.1.2. Relating LFB and FE Capability and State Model . . . 13 3.2. Logical Functional Block (LFB) Modeling . . . . . . . . . 14 3.2.1. LFB Outputs . . . . . . . . . . . . . . . . . . . . . 17 3.2.2. LFB Inputs . . . . . . . . . . . . . . . . . . . . . 20 3.2.3. Packet Type . . . . . . . . . . . . . . . . . . . . . 23 3.2.4. Metadata . . . . . . . . . . . . . . . . . . . . . . 24 3.2.5. LFB Events . . . . . . . . . . . . . . . . . . . . . 26 3.2.6. Component Properties . . . . . . . . . . . . . . . . 28 3.2.7. LFB Versioning . . . . . . . . . . . . . . . . . . . 28 3.2.8. LFB Inheritance . . . . . . . . . . . . . . . . . . . 29 3.3. ForCES Model Addressing . . . . . . . . . . . . . . . . . 30 3.3.1. Addressing LFB Components: Paths and Keys . . . . . . 31 3.4. FE Datapath Modeling . . . . . . . . . . . . . . . . . . 32 3.4.1. Alternative Approaches for Modeling FE Datapaths . . 32 3.4.2. Configuring the LFB Topology . . . . . . . . . . . . 36 4. Model and Schema for LFB Classes . . . . . . . . . . . . . . 40 4.1. Namespace . . . . . . . . . . . . . . . . . . . . . . . . 41 4.2. Element . . . . . . . . . . . . . . . . . . 41 4.3. Element . . . . . . . . . . . . . . . . . . . . . 43 4.4. Element for Frame Type Declarations . . . . . 44 4.5. Element for Data Type Definitions . . . . 44 4.5.1. Element for Renaming Existing Data Types . 48 4.5.2. Element for Deriving New Atomic Types . . . 48 4.5.3. Element to Define Arrays . . . . . . . . . . 49 4.5.4. Element to Define Structures . . . . . . . . 53 4.5.5. Element to Define Union Types . . . . . . . . 55 4.5.6. Element . . . . . . . . . . . . . . . . . . . 55 4.5.7. Augmentations . . . . . . . . . . . . . . . . . . . . 56 4.6. Element for Metadata Definitions . . . . . 57 4.7. Element for LFB Class Definitions . . . . 58 4.7.1. Element to Express LFB Inheritance . . 61 4.7.2. Element to Define LFB Inputs . . . . . . 61 Halpern & Hadi Salim Expires April 10, 2009 [Page 2] Internet-Draft ForCES FE Model October 2008 4.7.3. Element to Define LFB Outputs . . . . . 64 4.7.4. Element to Define LFB Operational Components . . . . . . . . . . . . . . . . . . . . . 66 4.7.5. Element to Define LFB Capability Components . . . . . . . . . . . . . . . . . . . . . 69 4.7.6. Element for LFB Notification Generation . . 70 4.7.7. Element for LFB Operational Specification . . . . . . . . . . . . . . . . . . . . 77 4.8. Properties . . . . . . . . . . . . . . . . . . . . . . . 77 4.8.1. Basic Properties . . . . . . . . . . . . . . . . . . 78 4.8.2. Array Properties . . . . . . . . . . . . . . . . . . 80 4.8.3. String Properties . . . . . . . . . . . . . . . . . . 80 4.8.4. Octetstring Properties . . . . . . . . . . . . . . . 81 4.8.5. Event Properties . . . . . . . . . . . . . . . . . . 82 4.8.6. Alias Properties . . . . . . . . . . . . . . . . . . 85 4.9. XML Schema for LFB Class Library Documents . . . . . . . 86 5. FE Components and Capabilities . . . . . . . . . . . . . . . 97 5.1. XML for FEObject Class definition . . . . . . . . . . . . 98 5.2. FE Capabilities . . . . . . . . . . . . . . . . . . . . . 104 5.2.1. ModifiableLFBTopology . . . . . . . . . . . . . . . . 105 5.2.2. SupportedLFBs and SupportedLFBType . . . . . . . . . 105 5.3. FE Components . . . . . . . . . . . . . . . . . . . . . . 108 5.3.1. FEState . . . . . . . . . . . . . . . . . . . . . . . 108 5.3.2. LFBSelectors and LFBSelectorType . . . . . . . . . . 108 5.3.3. LFBTopology and LFBLinkType . . . . . . . . . . . . . 109 5.3.4. FENeighbors and FEConfiguredNeighborType . . . . . . 109 6. Satisfying the Requirements on FE Model . . . . . . . . . . . 110 7. Using the FE model in the ForCES Protocol . . . . . . . . . . 111 7.1. FE Topology Query . . . . . . . . . . . . . . . . . . . . 113 7.2. FE Capability Declarations . . . . . . . . . . . . . . . 114 7.3. LFB Topology and Topology Configurability Query . . . . . 114 7.4. LFB Capability Declarations . . . . . . . . . . . . . . . 114 7.5. State Query of LFB Components . . . . . . . . . . . . . . 116 7.6. LFB Component Manipulation . . . . . . . . . . . . . . . 116 7.7. LFB Topology Re-configuration . . . . . . . . . . . . . . 116 8. Example LFB Definition . . . . . . . . . . . . . . . . . . . 117 8.1. Data Handling . . . . . . . . . . . . . . . . . . . . . . 124 8.1.1. Setting up a DLCI . . . . . . . . . . . . . . . . . . 125 8.1.2. Error Handling . . . . . . . . . . . . . . . . . . . 125 8.2. LFB Components . . . . . . . . . . . . . . . . . . . . . 126 8.3. Capabilities . . . . . . . . . . . . . . . . . . . . . . 126 8.4. Events . . . . . . . . . . . . . . . . . . . . . . . . . 127 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 128 9.1. URN Namespace Registration . . . . . . . . . . . . . . . 128 9.2. LFB Class Names and LFB Class Identifiers . . . . . . . . 128 10. Authors Emeritus . . . . . . . . . . . . . . . . . . . . . . 129 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 130 12. Security Considerations . . . . . . . . . . . . . . . . . . . 130 Halpern & Hadi Salim Expires April 10, 2009 [Page 3] Internet-Draft ForCES FE Model October 2008 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 130 13.1. Normative References . . . . . . . . . . . . . . . . . . 130 13.2. Informative References . . . . . . . . . . . . . . . . . 131 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 131 Intellectual Property and Copyright Statements . . . . . . . . . 132 Halpern & Hadi Salim Expires April 10, 2009 [Page 4] Internet-Draft ForCES FE Model October 2008 1. Definitions The use of compliance terminology (MUST, SHOULD, MAY, MUST NOT) is used in accordance with RFC2119 [1]. Such terminology is used in describing the required behavior of ForCES forwarding elements or control elements in supporting or manipulating information described in this model. Terminology associated with the ForCES requirements is defined in RFC3654 [6] and is not copied here. The following list of terminology relevant to the FE model is defined in this section. FE Model -- The FE model is designed to model the logical processing functions of an FE. The FE model proposed in this document includes three components: the modeling of individual logical functional blocks (LFB model), the logical interconnection between LFBs (LFB topology) and the FE level attributes, including FE capabilities. The FE model provides the basis to define the information elements exchanged between the CE and the FE in the ForCES Protocol [2]. Datapath -- A conceptual path taken by packets within the forwarding plane inside an FE. Note that more than one datapath can exist within an FE. LFB (Logical Functional Block) Class (or type) -- A template that represents a fine-grained, logically separable aspect of FE processing. Most LFBs relate to packet processing in the data path. LFB classes are the basic building blocks of the FE model. LFB Instance -- As a packet flows through an FE along a datapath, it flows through one or multiple LFB instances, where each LFB is an instance of a specific LFB class. Multiple instances of the same LFB class can be present in an FE's datapath. Note that we often refer to LFBs without distinguishing between an LFB class and LFB instance when we believe the implied reference is obvious for the given context. LFB Model -- The LFB model describes the content and structures in an LFB, plus the associated data definition. XML is used to provide a formal definition of the necessary structures for the modeling. Four types of information are defined in the LFB model. The core part of the LFB model is the LFB class definitions; the other three types of information define constructs associated with and used by the class definition. These are reusable data types, supported frame (packet) formats, and metadata. Element -- Element is generally used in this document in accordance with the XML usage of the term. It refers to an XML tagged part of Halpern & Hadi Salim Expires April 10, 2009 [Page 5] Internet-Draft ForCES FE Model October 2008 an XML document. For a precise definition, please see the full set of XML specifications from the W3C. This term is included in this list for completeness because the ForCES formal model uses XML. Attribute -- Attribute is used in the ForCES formal modelling in accordance with standard XML usage of the term. i.e, to provide attribute information include in an XML tag. LFB Metadata -- Metadata is used to communicate per-packet state from one LFB to another, but is not sent across the network. The FE model defines how such metadata is identified, produced and consumed by the LFBs, but not how the per-packet state is implemented within actual hardware. Metadata is sent between the FE and the CE on redirect packets. ForCES Component -- a ForCES Component is a well-defined, uniquely identifiable and addressable ForCES model building block. A component has a 32-bit ID, name, type and an optional synopsis description. These are often referred to simply as components. LFB Component -- A ForCES component that defines the Operational parameters of the LFBs that must be visible to the CEs. Structure Component -- A ForCES component that is part of a complex data structure to be used in LFB data definitions. The individual parts which make up a structured set of data are referred to as Structure Components. These can themselves be of any valid data type, including tables and structures. Property -- ForCES components have properties associated with them, such as readability. Other examples include lengths for variable sized components. These properties are acessed by the CE for reading (or, where appropriate, writing.) Details on the ForCES properties are found in section 4.8. LFB Topology -- A representation of the logical interconnection and the placement of LFB instances along the datapath within one FE. Sometimes this representation is called intra-FE topology, to be distinguished from inter-FE topology. LFB topology is outside of the LFB model, but is part of the FE model. FE Topology -- A representation of how multiple FEs within a single NE (Network Element) are interconnected. Sometimes this is called inter-FE topology, to be distinguished from intra-FE topology (i.e., LFB topology). An individual FE might not have the global knowledge of the full FE topology, but the local view of its connectivity with other FEs is considered to be part of the FE model. The FE topology is discovered by the ForCES base protocol or by some other means. Halpern & Hadi Salim Expires April 10, 2009 [Page 6] Internet-Draft ForCES FE Model October 2008 Inter-FE Topology -- See FE Topology. Intra-FE Topology -- See LFB Topology. LFB class library -- A set of LFB classes that has been identified as the most common functions found in most FEs and hence should be defined first by the ForCES Working Group. 2. Introduction RFC3746 [7] specifies a framework by which control elements (CEs) can configure and manage one or more separate forwarding elements (FEs) within a networking element (NE) using the ForCES protocol. The ForCES architecture allows Forwarding Elements of varying functionality to participate in a ForCES network element. The implication of this varying functionality is that CEs can make only minimal assumptions about the functionality provided by FEs in an NE. Before CEs can configure and control the forwarding behavior of FEs, CEs need to query and discover the capabilities and states of their FEs. RFC3654 [6] mandates that the capabilities, states and configuration information be expressed in the form of an FE model. RFC3444 [10] observed that information models (IMs) and data models (DMs) are different because they serve different purposes. "The main purpose of an IM is to model managed objects at a conceptual level, independent of any specific implementations or protocols used". "DMs, conversely, are defined at a lower level of abstraction and include many details. They are intended for implementors and include protocol-specific constructs." Sometimes it is difficult to draw a clear line between the two. The FE model described in this document is primarily an information model, but also includes some aspects of a data model, such as explicit definitions of the LFB class schema and FE schema. It is expected that this FE model will be used as the basis to define the payload for information exchange between the CE and FE in the ForCES protocol. 2.1. Requirements on the FE model RFC3654 [6]defines requirements that must be satisfied by a ForCES FE model. To summarize, an FE model must define: o Logically separable and distinct packet forwarding operations in an FE datapath (logical functional blocks or LFBs); o The possible topological relationships (and hence the sequence of packet forwarding operations) between the various LFBs; Halpern & Hadi Salim Expires April 10, 2009 [Page 7] Internet-Draft ForCES FE Model October 2008 o The possible operational capabilities (e.g., capacity limits, constraints, optional features, granularity of configuration) of each type of LFB; o The possible configurable parameters (e.g., components) of each type of LFB; o Metadata that may be exchanged between LFBs. 2.2. The FE Model in Relation to FE Implementations The FE model proposed here is based on an abstraction using distinct logical functional blocks (LFBs), which are interconnected in a directed graph, and receive, process, modify, and transmit packets along with metadata. The FE model is designed, and any defined LFB classes should be designed, such that different implementations of the forwarding datapath can be logically mapped onto the model with the functionality and sequence of operations correctly captured. However, the model is not intended to directly address how a particular implementation maps to an LFB topology. It is left to the forwarding plane vendors to define how the FE functionality is represented using the FE model. Our goal is to design the FE model such that it is flexible enough to accommodate most common implementations. The LFB topology model for a particular datapath implementation must correctly capture the sequence of operations on the packet. Metadata generation by certain LFBs MUST always precede any use of that metadata by subsequent LFBs in the topology graph; this is required for logically consistent operation. Further, modification of packet fields that are subsequently used as inputs for further processing MUST occur in the order specified in the model for that particular implementation to ensure correctness. 2.3. The FE Model in Relation to the ForCES Protocol The ForCES base Protocol [2] is used by the CEs and FEs to maintain the communication channel between the CEs and FEs. The ForCES protocol may be used to query and discover the intra-FE topology. The details of a particular datapath implementation inside an FE, including the LFB topology, along with the operational capabilities and attributes of each individual LFB, are conveyed to the CE within information elements in the ForCES protocol. The model of an LFB class should define all of the information that needs to be exchanged between an FE and a CE for the proper configuration and management of that LFB. Specifying the various payloads of the ForCES messages in a Halpern & Hadi Salim Expires April 10, 2009 [Page 8] Internet-Draft ForCES FE Model October 2008 systematic fashion is difficult without a formal definition of the objects being configured and managed (the FE and the LFBs within). The FE Model document defines a set of classes and components for describing and manipulating the state of the LFBs within an FE. These class definitions themselves will generally not appear in the ForCES protocol. Rather, ForCES protocol operations will reference classes defined in this model, including relevant components and the defined operations. Section 7 provides more detailed discussion on how the FE model should be used by the ForCES protocol. 2.4. Modeling Language for the FE Model Even though not absolutely required, it is beneficial to use a formal data modeling language to represent the conceptual FE model described in this document. Use of a formal language can help to enforce consistency and logical compatibility among LFBs. A full specification will be written using such a data modeling language. The formal definition of the LFB classes may facilitate the eventual automation of some of the code generation process and the functional validation of arbitrary LFB topologies. These class definitions form the LFB Library. Documents which describe LFB Classes are therefore referred to as LFB Library documents. Human readability was the most important factor considered when selecting the specification language, whereas encoding, decoding and transmission performance was not a selection factor. The encoding method for over the wire transport is not dependent on the specification language chosen and is outside the scope of this document and up to the ForCES protocol to define. XML is chosen as the specification language in this document, because XML has the advantage of being both human and machine readable with widely available tools support. This document uses an XML Schema to define the structure of the LFB Library documents, as defined in [11] and [4] and [5]. While these LFB Class definitions are not sent in the ForCES protocol, these definitions comply with the recommendations in RFC3470 [11] on the use of XML in IETF protocols. By useing XML Schema to define the structure for the LFB Library documents, we have a very clear set of syntactic restrictions to go with the desired semantic descriptions and restrictions covered in this document. As a corrolary to that, if it is determined that a change in the syntax is needed then a new schema will be required. This would be identified by a different URN to identify the namespace for such a new schema. Halpern & Hadi Salim Expires April 10, 2009 [Page 9] Internet-Draft ForCES FE Model October 2008 2.5. Document Structure Section 3 provides a conceptual overview of the FE model, laying the foundation for the more detailed discussion and specifications in the sections that follow. Section 4 and Section 5 constitute the core of the FE model, detailing the two major aspects of the FE model: a general LFB model and a definition of the FE Object LFB, with its components, including FE capabilities and LFB topology information. Section 6 directly addresses the model requirements imposed by the ForCES requirements defined in RFC3654 [6] while Section 7 explains how the FE model should be used in the ForCES protocol. 3. ForCES Model Concepts Some of the important ForCES concepts used throughout this document are introduced in this section. These include the capability and state abstraction, the FE and LFB model construction, and the unique addressing of the different model structures. Details of these aspects are described in Section 4 and Section 5. The intent of this section is to discuss these concepts at the high level and lay the foundation for the detailed description in the following sections. The ForCES FE model includes both a capability and a state abstraction. o The FE/LFB capability model describes the capabilities and capacities of an FE/LFB by specifying the variation in functions supported and any limitations. Capacity describes the limits of specific components (an example would be a table size limit). o The state model describes the current state of the FE/LFB, that is, the instantaneous values or operational behavior of the FE/ LFB. Section 3.1 explains the difference between a capability model and a state model, and describes how the two can be combined in the FE model. The ForCES model construction laid out in this document allows an FE to provide information about its structure for operation. This can be thought of as FE level information and information about the individual instances of LFBs provided by the FE. o The ForCES model includes the constructions for defining the class of logical function blocks (LFBS) that an FE may support. These classes are defined in this and other documents. The definition of such a class provides the information content for monitoring Halpern & Hadi Salim Expires April 10, 2009 [Page 10] Internet-Draft ForCES FE Model October 2008 and controlling instances of the LFB class for ForCES purposes. Each LFB model class formally defines the operational LFB components, LFB capabilities, and LFB events. Essentially, Section 3.2 introduces the concept of LFBs as the basic functional building blocks in the ForCES model. o The FE model also provides the construction necessary to monitor and control the FE as a whole for ForCES purposes. For consistency of operation and simplicity, this information is represented as an LFB, the FE Object LFB class and a singular LFB instance of that class, defined using the LFB model. The FE Object class defines the components to provide information at the FE level, particularly the capabilities of the FE at a coarse level, i.e., not all possible capabilities nor all details about the capabilities of the FE. Part of the FE level information is the LFB topology, which expresses the logical inter-connection between the LFB instances along the datapath(s) within the FE. Section 3.3 discusses the LFB topology. The FE Object also includes information about what LFB classes the FE can support. The ForCES model allows for unique identification of the different constructs it defines. This includes identification of the LFB classes, and of LFB instances within those classes, as well as identification of components within those instances. The ForCES Protocol [2] encapsulates target address(es) to eventually get to a fine-grained entity being referenced by the CE. The addressing hierarchy is broken into the following: o An FE is uniquely identified by a 32 bit FEID. o Each Class of LFB is uniquely identified by a 32 bit LFB ClassID. The LFB ClassIDs are global within the Network Element and may be issued by IANA. o Within an FE, there can be multiple instances of each LFB class. Each LFB Class instance is identified by a 32 bit identifier which is unique within a particular LFB class on that FE. o All the components within an LFB instance are further defined using 32 bit identifiers. Refer to Section 3.3 for more details on addressing. 3.1. ForCES Capability Model and State Model Capability and state modelling applies to both the FE and LFB abstraction. Halpern & Hadi Salim Expires April 10, 2009 [Page 11] Internet-Draft ForCES FE Model October 2008 Figure 1 shows the concepts of FE state, capabilities and configuration in the context of CE-FE communication via the ForCES protocol. +-------+ +-------+ | | FE capabilities: what it can/cannot do. | | | |<-----------------------------------------| | | | | | | CE | FE state: what it is now. | FE | | |<-----------------------------------------| | | | | | | | FE configuration: what it should be. | | | |----------------------------------------->| | +-------+ +-------+ Figure 1: Illustration of FE capabilities, state and configuration exchange in the context of CE-FE communication via ForCES. 3.1.1. FE Capability Model and State Model Conceptually, the FE capability model tells the CE which states are allowed on an FE, with capacity information indicating certain quantitative limits or constraints. Thus, the CE has general knowledge about configurations that are applicable to a particular FE. 3.1.1.1. FE Capability Model The FE capability model may be used to describe an FE at a coarse level. For example, an FE might be defined as follows: o the FE can handle IPv4 and IPv6 forwarding; o the FE can perform classification based on the following fields: source IP address, destination IP address, source port number, destination port number, etc.; o the FE can perform metering; o the FE can handle up to N queues (capacity); o the FE can add and remove encapsulating headers of types including IPsec, GRE, L2TP. While one could try to build an object model to fully represent the FE capabilities, other efforts found this approach to be a significant undertaking. The main difficulty arises in describing Halpern & Hadi Salim Expires April 10, 2009 [Page 12] Internet-Draft ForCES FE Model October 2008 detailed limits, such as the maximum number of classifiers, queues, buffer pools, and meters that the FE can provide. We believe that a good balance between simplicity and flexibility can be achieved for the FE model by combining coarse level capability reporting with an error reporting mechanism. That is, if the CE attempts to instruct the FE to set up some specific behavior it cannot support, the FE will return an error indicating the problem. Examples of similar approaches include DiffServ PIB RFC3317 [8] and Framework PIB RFC3318 [9]. 3.1.1.2. FE State Model The FE state model presents the snapshot view of the FE to the CE. For example, using an FE state model, an FE might be described to its corresponding CE as the following: o on a given port, the packets are classified using a given classification filter; o the given classifier results in packets being metered in a certain way and then marked in a certain way; o the packets coming from specific markers are delivered into a shared queue for handling, while other packets are delivered to a different queue; o a specific scheduler with specific behavior and parameters will service these collected queues. 3.1.1.3. LFB Capability and State Model Both LFB Capability and State information are defined formally using the LFB modelling XML schema. Capability information at the LFB level is an integral part of the LFB model and provides for powerful semantics. For example, when certain features of an LFB class are optional, the CE needs to be able to determine whether those optional features are supported by a given LFB instance. The schema for the definition of LFB classes provides a means for identifying such components. State information is defined formally using LFB component constructs. 3.1.2. Relating LFB and FE Capability and State Model Capability information at the FE level describes the LFB classes that the FE can instantiate, the number of instances of each that can be created, the topological (linkage) limitations between these LFB Halpern & Hadi Salim Expires April 10, 2009 [Page 13] Internet-Draft ForCES FE Model October 2008 instances, etc. Section 5 defines the FE level components including capability information. Since all information is represented as LFBs, this is provided by a single instance of the FE Object LFB Class. By using a single instance with a known LFB Class and a known instance identification, the ForCES protocol can allow a CE to access this information whenever it needs to, including while the CE is establishing the control of the FE. Once the FE capability is described to the CE, the FE state information can be represented at two levels. The first level is the logically separable and distinct packet processing functions, called LFBs. The second level of information describes how these individual LFBs are ordered and placed along the datapath to deliver a complete forwarding plane service. The interconnection and ordering of the LFBs is called LFB Topology. Section 3.2 discusses high level concepts around LFBs, whereas Section 3.3 discusses LFB topology issues. This topology information is represented as components of the FE Object LFB instance, to allow the CE to fetch and manipulate this. 3.2. Logical Functional Block (LFB) Modeling Each LFB performs a well-defined action or computation on the packets passing through it. Upon completion of its prescribed function, either the packets are modified in certain ways (e.g., decapsulator, marker), or some results are generated and stored, often in the form of metadata (e.g., classifier). Each LFB typically performs a single action. Classifiers, shapers and meters are all examples of such LFBs. Modeling LFBs at such a fine granularity allows us to use a small number of LFBs to express the higher-order FE functions (such as an IPv4 forwarder) precisely, which in turn can describe more complex networking functions and vendor implementations of software and hardware. These fine grained LFBs will be defined in detail in one or more documents to be published separately, using the material in this model. It is also the case that LFBs may exist in order to provide a set of components for control of FE operation by the CE (i.e., a locus of control), without tying that control to specific packets or specific parts of the data path. An example of such an LFB is the FE Object which provides the CE with information about the FE as a whole, and allows the FE to control some aspects of the FE, such as the datapath itself. Such LFBs will not have the packet oriented properties described in this section. In general, multiple LFBs are contained in one FE, as shown in Figure 2, and all the LFBs share the same ForCES protocol (Fp) termination point that implements the ForCES protocol logic and Halpern & Hadi Salim Expires April 10, 2009 [Page 14] Internet-Draft ForCES FE Model October 2008 maintains the communication channel to and from the CE. +-----------+ | CE | +-----------+ ^ | Fp reference point | +--------------------------|-----------------------------------+ | FE | | | v | | +----------------------------------------------------------+ | | | ForCES protocol | | | | termination point | | | +----------------------------------------------------------+ | | ^ ^ | | : : Internal control | | : : | | +---:----------+ +---:----------| | | | :LFB1 | | : LFB2 | | | =====>| v |============>| v |======>...| | Inputs| +----------+ |Outputs | +----------+ | | | (P,M) | |Components| |(P',M') | |Components| |(P",M") | | | +----------+ | | +----------+ | | | +--------------+ +--------------+ | | | +--------------------------------------------------------------+ Figure 2: Generic LFB Diagram An LFB, as shown in Figure 2, may have inputs, outputs and components that can be queried and manipulated by the CE via an Fp reference point (defined in RFC3746 [7]) and the ForCES protocol termination point. The horizontal axis is in the forwarding plane for connecting the inputs and outputs of LFBs within the same FE. P (with marks to indicate modification) indicates a data packet, while M (with marks to indicate modification) indicates the metadata associated with a packet. The vertical axis between the CE and the FE denotes the Fp reference point where bidirectional communication between the CE and FE occurs: the CE to FE communication is for configuration, control, and packet injection, while FE to CE communication is used for packet redirection to the control plane, reporting of monitoring and accounting information, reporting of errors, etc. Note that the interaction between the CE and the LFB is only abstract and indirect. The result of such an interaction is for the CE to manipulate the components of the LFB instances. Halpern & Hadi Salim Expires April 10, 2009 [Page 15] Internet-Draft ForCES FE Model October 2008 An LFB can have one or more inputs. Each input takes a pair of a packet and its associated metadata. Depending upon the LFB input port definition, the packet or the metadata may be allowed to be empty (or equivalently to not be provided.) When input arrives at an LFB, either the packet or its associated metadata must be non-empty or there is effectively no input. (LFB operation generally may be triggered by input arrival, by timers, or by other system state. It is only in the case where the goal is to have input drive operation that the input must be non-empty.) The LFB processes the input, and produces one or more outputs, each of which is a pair of a packet and its associated metadata. Again, depending upon the LFB output port definition, either the packet or the metadata may be allowed to be empty (or equivalently to be absent.) Metadata attached to packets on output may be metadata that was received, or may be information about the packet processing that may be used by later LFBs in the FEs packet processing. A namespace is used to associate a unique name and ID with each LFB class. The namespace MUST be extensible so that a new LFB class can be added later to accommodate future innovation in the forwarding plane. LFB operation is specified in the model to allow the CE to understand the behavior of the forwarding datapath. For instance, the CE needs to understand at what point in the datapath the IPv4 header TTL is decremented by the FE. That is, the CE needs to know if a control packet could be delivered to it either before or after this point in the datapath. In addition, the CE needs to understand where and what type of header modifications (e.g., tunnel header append or strip) are performed by the FEs. Further, the CE works to verify that the various LFBs along a datapath within an FE are compatible to link together. Connecting incompatible LFB instances will produce a non- working data path. So the model is designed to provide sufficient information for the CE to make this determination. Selecting the right granularity for describing the functions of the LFBs is an important aspect of this model. There is value to vendors if the operation of LFB classes can be expressed in sufficient detail so that physical devices implementing different LFB functions can be integrated easily into an FE design. However, the model, and the associated library of LFBs, must not be so detailed and so specific as to significantly constrain implementations. Therefore, a semi- formal specification is needed; that is, a text description of the LFB operation (human readable), but sufficiently specific and unambiguous to allow conformance testing and efficient design, so that interoperability between different CEs and FEs can be achieved. Halpern & Hadi Salim Expires April 10, 2009 [Page 16] Internet-Draft ForCES FE Model October 2008 The LFB class model specifies information such as: o number of inputs and outputs (and whether they are configurable) o metadata read/consumed from inputs; o metadata produced at the outputs; o packet type(s) accepted at the inputs and emitted at the outputs; o packet content modifications (including encapsulation or decapsulation); o packet routing criteria (when multiple outputs on an LFB are present); o packet timing modifications; o packet flow ordering modifications; o LFB capability information components; o events that can be detected by the LFB, with notification to the CE; o LFB operational components; o etc. Section 4 of this document provides a detailed discussion of the LFB model with a formal specification of LFB class schema. The rest of Section 3.2 only intends to provide a conceptual overview of some important issues in LFB modeling, without covering all the specific details. 3.2.1. LFB Outputs An LFB output is a conceptual port on an LFB that can send information to another LFB. The information sent on that port is a pair of a packet and associated metadata, one of which may be empty. (If both were empty, there would be no output.) A single LFB output can be connected to only one LFB input. This is required to make the packet flow through the LFB topology unambiguous. Some LFBs will have a single output, as depicted in Figure 3.a. Halpern & Hadi Salim Expires April 10, 2009 [Page 17] Internet-Draft ForCES FE Model October 2008 +---------------+ +-----------------+ | | | | | | | OUT +--> ... OUT +--> ... | | | | EXCEPTIONOUT +--> | | | | +---------------+ +-----------------+ a. One output b. Two distinct outputs +---------------+ +-----------------+ | | | EXCEPTIONOUT +--> | OUT:1 +--> | | ... OUT:2 +--> ... OUT:1 +--> | ... +... | OUT:2 +--> | OUT:n +--> | ... +... +---------------+ | OUT:n +--> +-----------------+ c. One output group d. One output and one output group Figure 3: Examples of LFBs with various output combinations. To accommodate a non-trivial LFB topology, multiple LFB outputs are needed so that an LFB class can fork the datapath. Two mechanisms are provided for forking: multiple singleton outputs and output groups, which can be combined in the same LFB class. Multiple separate singleton outputs are defined in an LFB class to model a pre-determined number of semantically different outputs. That is, the LFB class definition MUST include the number of outputs, implying the number of outputs is known when the LFB class is defined. Additional singleton outputs cannot be created at LFB instantiation time, nor can they be created on the fly after the LFB is instantiated. For example, an IPv4 LPM (Longest-Prefix-Matching) LFB may have one output (OUT) to send those packets for which the LPM look-up was successful, passing a META_ROUTEID as metadata; and have another output (EXCEPTIONOUT) for sending exception packets when the LPM look-up failed. This example is depicted in Figure 3.b. Packets emitted by these two outputs not only require different downstream treatment, but they are a result of two different conditions in the LFB and each output carries different metadata. This concept assumes the number of distinct outputs is known when the LFB class is defined. For each singleton output, the LFB class definition defines Halpern & Hadi Salim Expires April 10, 2009 [Page 18] Internet-Draft ForCES FE Model October 2008 the types of frames (packets) and metadata the output emits. An output group, on the other hand, is used to model the case where a flow of similar packets with an identical set of permitted metadata needs to be split into multiple paths. In this case, the number of such paths is not known when the LFB class is defined because it is not an inherent property of the LFB class. An output group consists of a number of outputs, called the output instances of the group, where all output instances share the same frame (packet) and metadata emission definitions (see Figure 3.c). Each output instance can connect to a different downstream LFB, just as if they were separate singleton outputs, but the number of output instances can differ between LFB instances of the same LFB class. The class definition may include a lower and/or an upper limit on the number of outputs. In addition, for configurable FEs, the FE capability information may define further limits on the number of instances in specific output groups for certain LFBs. The actual number of output instances in a group is an component of the LFB instance, which is read-only for static topologies, and read-write for dynamic topologies. The output instances in a group are numbered sequentially, from 0 to N-1, and are addressable from within the LFB. To use Output Port groups, the LFB has to have a built-in mechanism to select one specific output instance for each packet. This mechanism is described in the textual definition of the class and is typically configurable via some attributes of the LFB. For example, consider a redirector LFB, whose sole purpose is to direct packets to one of N downstream paths based on one of the metadata associated with each arriving packet. Such an LFB is fairly versatile and can be used in many different places in a topology. For example, given LFBs which record the type of packet in a FRAMETYPE metadatum, or a packet rate class in a COLOR metadatum, one may uses these metadata for branching. A redirector can be used to divide the data path into an IPv4 and an IPv6 path based on a FRAMETYPE metadatum (N=2), or to fork into rate specific paths after metering using the COLOR metadatum (red, yellow, green; N=3), etc. Using an output group in the above LFB class provides the desired flexibility to adapt each instance of this class to the required operation. The metadata to be used as a selector for the output instance is a property of the LFB. For each packet, the value of the specified metadata may be used as a direct index to the output instance. Alternatively, the LFB may have a configurable selector table that maps a metadatum value to output instance. Note that other LFBs may also use the output group concept to build in similar adaptive forking capability. For example, a classifier LFB with one input and N outputs can be defined easily by using the Halpern & Hadi Salim Expires April 10, 2009 [Page 19] Internet-Draft ForCES FE Model October 2008 output group concept. Alternatively, a classifier LFB with one singleton output in combination with an explicit N-output re- director LFB models the same processing behavior. The decision of whether to use the output group model for a certain LFB class is left to the LFB class designers. The model allows the output group to be combined with other singleton output(s) in the same class, as demonstrated in Figure 3.d. The LFB here has two types of outputs, OUT, for normal packet output, and EXCEPTIONOUT for packets that triggered some exception. The normal OUT has multiple instances, thus, it is an output group. In summary, the LFB class may define one output, multiple singleton outputs, one or more output groups, or a combination thereof. Multiple singleton outputs should be used when the LFB must provide for forking the datapath and at least one of the following conditions hold: o the number of downstream directions is inherent from the definition of the class and hence fixed; o the frame type and set of permitted metadata emitted on any of the outputs are different from what is emitted on the other outputs (i.e., they cannot share their frametype and permitted metadata definitions). An output group is appropriate when the LFB must provide for forking the datapath and at least one of the following conditions hold: o the number of downstream directions is not known when the LFB class is defined; o the frame type and set of metadata emitted on these outputs are sufficiently similar or, ideally, identical, such they can share the same output definition. 3.2.2. LFB Inputs An LFB input is a conceptual port on an LFB on which the LFB can receive information from other LFBs. The information is typically a pair of a packet and its associated metadata. Either the packet, or the metadata, may for some LFBs and some situations be empty. They can not both be empty, as then there is no input. For LFB instances that receive packets from more than one other LFB instance (fan-in) there are three ways to model fan-in, all supported by the LFB model and can all be combined in the same LFB: Halpern & Hadi Salim Expires April 10, 2009 [Page 20] Internet-Draft ForCES FE Model October 2008 o Implicit multiplexing via a single input o Explicit multiplexing via multiple singleton inputs o Explicit multiplexing via a group of inputs (input group) The simplest form of multiplexing uses a singleton input (Figure 4.a). Most LFBs will have only one singleton input. Multiplexing into a single input is possible because the model allows more than one LFB output to connect to the same LFB input. This property applies to any LFB input without any special provisions in the LFB class. Multiplexing into a single input is applicable when the packets from the upstream LFBs are similar in frametype and accompanying metadata, and require similar processing. Note that this model does not address how potential contention is handled when multiple packets arrive simultaneously. If contention handling needs to be explicitly modeled, one of the other two modeling solutions must be used. The second method to model fan-in uses individually defined singleton inputs (Figure 4.b). This model is meant for situations where the LFB needs to handle distinct types of packet streams, requiring input-specific handling inside the LFB, and where the number of such distinct cases is known when the LFB class is defined. For example, an LFB which can perform both Layer 2 decapsulation (to Layer 3) and Layer 3 encapsulation (to Layer 2) may have two inputs, one for receiving Layer 2 frames for decapsulation, and one for receiving Layer 3 frames for encapsulation. This LFB type expects different frames (L2 vs. L3) at its inputs, each with different sets of metadata, and would thus apply different processing on frames arriving at these inputs. This model is capable of explicitly addressing packet contention by defining how the LFB class handles the contending packets. Halpern & Hadi Salim Expires April 10, 2009 [Page 21] Internet-Draft ForCES FE Model October 2008 +--------------+ +------------------------+ | 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. Halpern & Hadi Salim Expires April 10, 2009 [Page 22] Internet-Draft ForCES FE Model October 2008 Figure 4: Examples of LFBs with various input combinations. The third method to model fan-in uses the concept of an input group. The concept is similar to the output group introduced in the previous section and is depicted in Figure 4.c. An input group consists of a number of input instances, all sharing the properties (same frame and metadata expectations). The input instances are numbered from 0 to N-1. From the outside, these inputs appear as normal inputs, i.e., any compatible upstream LFB can connect its output to one of these inputs. When a packet is presented to the LFB at a particular input instance, the index of the input where the packet arrived is known to the LFB and this information may be used in the internal processing. For example, the input index can be used as a table selector, or as an explicit precedence selector to resolve contention. As with output groups, the number of input instances in an input group is not defined in the LFB class. However, the class definition may include restrictions on the range of possible values. In addition, if an FE supports configurable topologies, it may impose further limitations on the number of instances for particular port group(s) of a particular LFB class. Within these limitations, different instances of the same class may have a different number of input instances. The number of actual input instances in the group is a component defined in the LFB class, which is read-only for static topologies, and is read-write for configurable topologies. As an example for the input group, consider the Scheduler LFB depicted in Figure 4.c. Such an LFB receives packets from a number of Queue LFBs via a number of input instances, and uses the input index information to control contention resolution and scheduling. In summary, the LFB class may define one input, multiple singleton inputs, one or more input groups, or a combination thereof. Any input allows for implicit multiplexing of similar packet streams via connecting multiple outputs to the same input. Explicit multiple singleton inputs are useful when either the contention handling must be handled explicitly, or when the LFB class must receive and process a known number of distinct types of packet streams. An input group is suitable when contention handling must be modeled explicitly, but the number of inputs is not inherent from the class (and hence is not known when the class is defined), or when it is critical for LFB operation to know exactly on which input the packet was received. 3.2.3. Packet Type When LFB classes are defined, the input and output packet formats (e.g., IPv4, IPv6, Ethernet, etc.) MUST be specified. These are the types of packets that a given LFB input is capable of receiving and processing, or that a given LFB output is capable of producing. This Halpern & Hadi Salim Expires April 10, 2009 [Page 23] Internet-Draft ForCES FE Model October 2008 model requires that distinct packet types be uniquely labeled with a symbolic name and/or ID. Note that each LFB has a set of packet types that it operates on, but does not care whether the underlying implementation is passing a greater portion of the packets. For example, an IPv4 LFB might only operate on IPv4 packets, but the underlying implementation may or may not be stripping the L2 header before handing it over. Whether such processing is happening or not is opaque to the CE. 3.2.4. Metadata Metadata is state that is passed from one LFB to another alongside a packet. The metadata passed with the packet assists subsequent LFBs to process that packet. The ForCES model defines metadata as precise atomic definitions in the form of label, value pairs. The ForCES model provides to the authors of LFB classes a way to formally define how to achieve metadata creation, modification, reading, as well as consumption (deletion). Inter-FE metadata, i.e, metadata crossing FEs, while it is likely to be semantically similar to this metadata, is out of scope for this document. Section 4 has informal details on metadata. 3.2.4.1. Metadata Lifecycle Within the ForCES Model Each metadatum is modeled as a pair, where the label identifies the type of information, (e.g., "color"), and its value holds the actual information (e.g., "red"). The label here is shown as a textual label, but for protocol processing it is associated with a unique numeric value (identifier). To ensure inter-operability between LFBs, the LFB class specification must define what metadata the LFB class "reads" or "consumes" on its input(s) and what metadata it "produces" on its output(s). For maximum extensibility, this definition should neither specify which LFBs the metadata is expected to come from for a consumer LFB, nor which LFBs are expected to consume metadata for a given producer LFB. 3.2.4.2. Metadata Production and Consumption For a given metadatum on a given packet path, there MUST be at least one producer LFB that creates that metadatum and SHOULD be at least Halpern & Hadi Salim Expires April 10, 2009 [Page 24] Internet-Draft ForCES FE Model October 2008 one consumer LFB that needs that metadatum. In the ForCES model, the producer and consumer LFBs of a metadatum are not required to be adjacent. In addition, there may be multiple producers and consumers for the same metadatum. When a packet path involves multiple producers of the same metadatum, then subsequent producers overwrite that metadatum value. The metadata that is produced by an LFB is specified by the LFB class definition on a per-output-port-group basis. A producer may always generate the metadata on the port group, or may generate it only under certain conditions. We call the former "unconditional" metadata, whereas the latter is a "conditional" metadata. For example, deep packet inspection LFB might produce several pieces of metadata about the packet. The first metadatum might be the IP protocol (TCP, UDP, SCTP, ...) being carried, and two additional metadata items might be the source and destination port number. These additional metadata items are conditional on the value of the first metadatum (IP carried protocol) as they are only produced for protocols which use port numbers. In the case of conditional metadata, it should be possible to determine from the definition of the LFB when "conditional" metadata is produced. The consumer behavior of an LFB, that is, the metadata that the LFB needs for its operation, is defined in the LFB class definition on a per-input- port-group basis. An input port group may "require" a given metadatum, or may treat it as "optional" information. In the latter case, the LFB class definition MUST explicitly define what happens if any optional metadata is not provided. One approach is to specify a default value for each optional metadatum, and assume that the default value is used for any metadata which is not provided with the packet. When specifying the metadata tags, some harmonization effort must be made so that the producer LFB class uses the same tag as its intended consumer(s). 3.2.4.3. LFB Operations on Metadata When the packet is processed by an LFB (i.e., between the time it is received and forwarded by the LFB), the LFB may perform read, write, and/or consume operations on any active metadata associated with the packet. If the LFB is considered to be a black box, one of the following operations is performed on each active metadatum. * IGNORE: ignores and forwards the metadatum * READ: reads and forwards the metadatum Halpern & Hadi Salim Expires April 10, 2009 [Page 25] Internet-Draft ForCES FE Model October 2008 * READ/RE-WRITE: reads, over-writes and forwards the metadatum * WRITE: writes and forwards the metadatum (can also be used to create new metadata) * READ-AND-CONSUME: reads and consumes the metadatum * CONSUME consumes metadatum without reading The last two operations terminate the life-cycle of the metadatum, meaning that the metadatum is not forwarded with the packet when the packet is sent to the next LFB. In the ForCES model, a new metadatum is generated by an LFB when the LFB applies a WRITE operation to a metadatum type that was not present when the packet was received by the LFB. Such implicit creation may be unintentional by the LFB, that is, the LFB may apply the WRITE operation without knowing or caring if the given metadatum existed or not. If it existed, the metadatum gets over-written; if it did not exist, the metadatum is created. For LFBs that insert packets into the model, WRITE is the only meaningful metadata operation. For LFBs that remove the packet from the model, they may either READ- AND-CONSUME (read) or CONSUME (ignore) each active metadatum associated with the packet. 3.2.5. LFB Events During operation, various conditions may occur that can be detected by LFBs. Examples range from link failure or restart to timer expiration in special purpose LFBs. The CE may wish to be notified of the occurrence of such events. The description of how such messages are sent, and their format, is part of the Forwarding and Control Element Separation (ForCES) protocol [2] document. Indicating how such conditions are understood is part of the job of this model. Events are declared in the LFB class definition. The LFB event declaration constitutes: o a unique 32 bit identifier. o An LFB component which is used to trigger the event. This entity is known as the event target. Halpern & Hadi Salim Expires April 10, 2009 [Page 26] Internet-Draft ForCES FE Model October 2008 o A condition that will happen to the event target that will result in a generation of an event to the CE. Examples of a condition include something getting created, deleted, config change, etc. o What should be reported to the CE by the FE if the declared condition is met. The declaration of an event within an LFB class essentially defines what part of the LFB component(s) need to be monitored for events, what condition on the LFB monitored LFB component an FE should detect to trigger such an event, and what to report to the CE when the event is triggered. While events may be declared by the LFB class definition, runtime activity is controlled using built-in event properties using LFB component Properties (discussed in Section 3.2.6). A CE subscribes to the events on an LFB class instance by setting an event property for subscription. Each event has a subscription property which is by default off. A CE wishing to receive a specific event needs to turn on the subscription property at runtime. Event properties also provide semantics for runtime event filtering. A CE may set an event property to further suppress events to which it has already subscribed. The LFB model defines such filters to include threshold values, hysteresis, time intervals, number of events, etc. The contents of reports with events are designed to allow for the common, closely related information that the CE can be strongly expected to need to react to the event. It is not intended to carry information that the CE already has, nor large volumes of information, nor information related in complex fashions. From a conceptual point of view, at runtime, event processing is split into: 1. detection of something happening to the (declared during LFB class definition) event target. Processing the next step happens if the CE subscribed (at runtime) to the event. 2. checking of the (declared during LFB class definition) condition on the LFB event target. If the condition is met, proceed with the next step. 3. checking (runtime set) event filters if they exist to see if the event should be reported or suppressed. If the event is to be reported proceed to the next step. Halpern & Hadi Salim Expires April 10, 2009 [Page 27] Internet-Draft ForCES FE Model October 2008 4. Submitting of the declared report to the CE. Section 4.7.6 discusses events in more details. 3.2.6. Component Properties LFBs and structures are made up of Components, containing the information that the CE needs to see and/or change about the functioning of the LFB. These Components, as described in detail in Section 4.7, may be basic values, complex structures (containing multiple Components themselves, each of which can be values, structures, or tables), or tables (which contain values, structures or tables). Components may be defined such that their appearence in LFB instances is optional. Components may be readable or writable at the discretion of the FE implementation. The CE needs to know these properties. Additionally, certain kinds of Components (arrays / tables, aliases, and events) have additional property information that the CE may need to read or write. This model defines the structure of the property information for all defined data types. Section 4.8 describes properties in more details. 3.2.7. LFB Versioning LFB class versioning is a method to enable incremental evolution of LFB classes. In general, an FE is not allowed to contain an LFB instance for more than one version of a particular class. Inheritance (discussed next in Section 3.2.8) has special rules. If an FE datapath model containing an LFB instance of a particular class C also simultaneously contains an LFB instance of a class C' inherited from class C; C could have a different version than C'. LFB class versioning is supported by requiring a version string in the class definition. CEs may support multiple versions of a particular LFB class to provide backward compatibility, but FEs MUST NOT support more than one version of a particular class. Versioning is not restricted to making backwards compatible changes. It is specifically expected to be used to make changes that cannot be represented by inheritance. Often this will be to correct errors, and hence may not be backwards compatible. It may also be used to remove components which are not considered useful (particularly if they were previously mandatory, and hence were an implementation impediment.) Halpern & Hadi Salim Expires April 10, 2009 [Page 28] Internet-Draft ForCES FE Model October 2008 3.2.8. LFB Inheritance LFB class inheritance is supported in the FE model as a method to define new LFB classes. This also allows FE vendors to add vendor- specific extensions to standardized LFBs. An LFB class specification MUST specify the base class and version number it inherits from (the default is the base LFB class). Multiple inheritance is not allowed, however, to avoid unnecessary complexity. Inheritance should be used only when there is significant reuse of the base LFB class definition. A separate LFB class should be defined if little or no reuse is possible between the derived and the base LFB class. An interesting issue related to class inheritance is backward compatibility between a descendant and an ancestor class. Consider the following hypothetical scenario where a standardized LFB class "L1" exists. Vendor A builds an FE that implements LFB "L1" and vendor B builds a CE that can recognize and operate on LFB "L1". Suppose that a new LFB class, "L2", is defined based on the existing "L1" class by extending its capabilities incrementally. Let us examine the FE backward compatibility issue by considering what would happen if vendor B upgrades its FE from "L1" to "L2" and vendor C's CE is not changed. The old L1-based CE can interoperate with the new L2-based FE if the derived LFB class "L2" is indeed backward compatible with the base class "L1". The reverse scenario is a much less problematic case, i.e., when CE vendor B upgrades to the new LFB class "L2", but the FE is not upgraded. Note that as long as the CE is capable of working with older LFB classes, this problem does not affect the model; hence we will use the term "backward compatibility" to refer to the first scenario concerning FE backward compatibility. Backward compatibility can be designed into the inheritance model by constraining LFB inheritance to require the derived class be a functional superset of the base class (i.e. the derived class can only add functions to the base class, but not remove functions). Additionally, the following mechanisms are required to support FE backward compatibility: 1. When detecting an LFB instance of an LFB type that is unknown to the CE, the CE MUST be able to query the base class of such an LFB from the FE. 2. The LFB instance on the FE SHOULD support a backward compatibility mode (meaning the LFB instance reverts itself back to the base class instance), and the CE SHOULD be able to Halpern & Hadi Salim Expires April 10, 2009 [Page 29] Internet-Draft ForCES FE Model October 2008 configure the LFB to run in such a mode. 3.3. ForCES Model Addressing Figure 5 demonstrates the abstraction of the different ForCES model entities. The ForCES protocol provides the mechanism to uniquely identify any of the LFB Class instance components. FE Address = FE01 +--------------------------------------------------------------+ | | | +--------------+ +--------------+ | | | LFB ClassID 1| |LFB ClassID 91| | | | InstanceID 3 |============>|InstanceID 3 |======>... | | | +----------+ | | +----------+ | | | | |Components| | | |Components| | | | | +----------+ | | +----------+ | | | +--------------+ +--------------+ | | | +--------------------------------------------------------------+ Figure 5: FE Entity Hierarchy At the top of the addressing hierachy is the FE identifier. In the example above, the 32-bit FE identifier is illustrated with the mnemonic FE01. The next 32-bit entity selector is the LFB ClassID. In the illustration above, two LFB classes with identifiers 1 and 91 are demonstrated. The example above further illustrates one instance of each of the two classes. The scope of the 32-bit LFB class instance identifier is valid only within the LFB class. To emphasize that point, each of class 1 and 91 has an instance of 3. Using the described addressing scheme, a message could be sent to address FE01, LFB ClassID 1, LFB InstanceID 3, utilizing the ForCES protocol. However, to be effective, such a message would have to target entities within an LFB. These entities could be carrying state, capability, etc. These are further illustrated in Figure 6 below. Halpern & Hadi Salim Expires April 10, 2009 [Page 30] Internet-Draft ForCES FE Model October 2008 LFB Class ID 1,InstanceID 3 Components +-------------------------------------+ | | | LFB ComponentID 1 | | +----------------------+ | | | | | | +----------------------+ | | | | LFB ComponentID 31 | | +----------------------+ | | | | | | +----------------------+ | | | | LFB ComponentID 51 | | +----------------------+ | | | LFB ComponentID 89 | | | | +-----------------+ | | | | | | | | | | +-----------------+ | | | +----------------------+ | | | | | +-------------------------------------+ Figure 6: LFB Hierarchy Figure 6 zooms into the components carried by LFB Class ID 1, LFB InstanceID 3 from Figure 5. The example shows three components with 32-bit component identifiers 1, 31, and 51. LFB ComponentID 51 is a complex structure encapsulating within it an entity with LFB ComponentID 89. LFB ComponentID 89 could be a complex structure itself but is restricted in the example for the sake of clarity. 3.3.1. Addressing LFB Components: Paths and Keys As mentioned above, LFB components could be complex structures, such as a table, or even more complex structures such as a table whose cells are further tables, etc. The ForCES model XML schema (Section 4) allows for uniquely identifying anything with such complexity, utilizing the concept of dot-annotated static paths and content addressing of paths as derived from keys. As an example, if the LFB Component 51 were a structure, then the path to LFB ComponentID 89 above will be 51.89. LFB ComponentID 51 might represent a table (an array). In that case, to select the LFB Component with ID 89 from within the 7th entry of Halpern & Hadi Salim Expires April 10, 2009 [Page 31] Internet-Draft ForCES FE Model October 2008 the table, one would use the path 51.7.89. In addition to supporting explicit table element selection by including an index in the dotted path, the model supports identifying table elements by their contents. This is referred to as using keys, or key indexing. So, as a further example, if ComponentID 51 was a table which was key index-able, then a key describing content could also be passed by the CE, along with path 51 to select the table, and followed by the path 89 to select the table structure element, which upon computation by the FE would resolve to the LFB ComponentID 89 within the specified table entry. 3.4. FE Datapath Modeling Packets coming into the FE from ingress ports generally flow through one or more LFBs before leaving out of the egress ports. How an FE treats a packet depends on many factors, such as type of the packet (e.g., IPv4, IPv6, or MPLS), header values, time of arrival, etc. The result of LFB processing may have an impact on how the packet is to be treated in downstream LFBs. This differentiation of packet treatment downstream can be conceptualized as having alternative datapaths in the FE. For example, the result of a 6-tuple classification performed by a classifier LFB could control which rate meter is applied to the packet by a rate meter LFB in a later stage in the datapath. LFB topology is a directed graph representation of the logical datapaths within an FE, with the nodes representing the LFB instances and the directed link depicting the packet flow direction from one LFB to the next. Section 3.4.1 discusses how the FE datapaths can be modeled as LFB topology; while Section 3.4.2 focuses on issues related to LFB topology reconfiguration. 3.4.1. Alternative Approaches for Modeling FE Datapaths There are two basic ways to express the differentiation in packet treatment within an FE, one represents the datapath directly and graphically (topological approach) and the other utilizes metadata (the encoded state approach). o Topological Approach Using this approach, differential packet treatment is expressed by splitting the LFB topology into alternative paths. In other words, if the result of an LFB operation controls how the packet is further processed, then such an LFB will have separate output ports, one for each alternative treatment, connected to separate sub-graphs, each expressing the respective treatment downstream. Halpern & Hadi Salim Expires April 10, 2009 [Page 32] Internet-Draft ForCES FE Model October 2008 o Encoded State Approach An alternate way of expressing differential treatment is by using metadata. The result of the operation of an LFB can be encoded in a metadatum, which is passed along with the packet to downstream LFBs. A downstream LFB, in turn, can use the metadata and its value (e.g., as an index into some table) to determine how to treat the packet. Theoretically, either approach could substitute for the other, so one could consider using a single pure approach to describe all datapaths in an FE. However, neither model by itself results in the best representation for all practically relevant cases. For a given FE with certain logical datapaths, applying the two different modeling approaches will result in very different looking LFB topology graphs. A model using only the topological approach may require a very large graph with many links or paths, and nodes (i.e., LFB instances) to express all alternative datapaths. On the other hand, a model using only the encoded state model would be restricted to a string of LFBs, which is not an intuitive way to describe different datapaths (such as MPLS and IPv4). Therefore, a mix of these two approaches will likely be used for a practical model. In fact, as we illustrate below, the two approaches can be mixed even within the same LFB. Using a simple example of a classifier with N classification outputs followed by other LFBs, Figure 7.a shows what the LFB topology looks like when using the pure topological approach. Each output from the classifier goes to one of the N LFBs where no metadata is needed. The topological approach is simple, straightforward and graphically intuitive. However, if N is large and the N nodes following the classifier (LFB#1, LFB#2, ..., LFB#N) all belong to the same LFB type (e.g., meter), but each has its own independent components, the encoded state approach gives a much simpler topology representation, as shown in Figure 7.b. The encoded state approach requires that a table of N rows of meter components is provided in the Meter node itself, with each row representing the attributes for one meter instance. A metadatum M is also needed to pass along with the packet P from the classifier to the meter, so that the meter can use M as a look-up key (index) to find the corresponding row of the attributes that should be used for any particular packet P. What if those N nodes (LFB#1, LFB#2, ..., LFB#N) are not of the same type? For example, if LFB#1 is a queue while the rest are all meters, what is the best way to represent such datapaths? While it is still possible to use either the pure topological approach or the pure encoded state approach, the natural combination of the two appears to be the best option. Figure 7.c depicts two different functional datapaths using the topological approach while leaving the N-1 meter instances distinguished by metadata only, as shown in Halpern & Hadi Salim Expires April 10, 2009 [Page 33] Internet-Draft ForCES FE Model October 2008 Figure 7.c. +----------+ P | LFB#1 | +--------->|(Compon-1)| +-------------+ | +----------+ | 1|------+ P +----------+ | 2|---------------->| LFB#2 | | classifier 3| |(Compon-2)| | ...|... +----------+ | N|------+ ... +-------------+ | P +----------+ +--------->| LFB#N | |(Compon-N)| +----------+ (a) Using pure topological approach +-------------+ +-------------+ | 1| | Meter | | 2| (P, M) | (Compon-1) | | 3|---------------->| (Compon-2) | | ...| | ... | | N| | (Compon-N) | +-------------+ +-------------+ (b) Using pure encoded state approach to represent the LFB topology in 5(a), if LFB#1, LFB#2, ..., and LFB#N are of the same type (e.g., meter). +-------------+ +-------------+ (P, M) | queue | | 1|------------->| (Compon-1) | | 2| +-------------+ | 3| (P, M) +-------------+ | ...|------------->| Meter | | N| | (Compon-2) | +-------------+ | ... | | (Compon-N) | +-------------+ (c) Using a combination of the two, if LFB#1, LFB#2, ..., and LFB#N are of different types (e.g., queue and meter). Figure 7: An example of how to model FE datapaths From this example, we demonstrate that each approach has a distinct Halpern & Hadi Salim Expires April 10, 2009 [Page 34] Internet-Draft ForCES FE Model October 2008 advantage depending on the situation. Using the encoded state approach, fewer connections are typically needed between a fan-out node and its next LFB instances of the same type because each packet carries metadata the following nodes can interpret and hence invoke a different packet treatment. For those cases, a pure topological approach forces one to build elaborate graphs with many more connections and often results in an unwieldy graph. On the other hand, a topological approach is the most intuitive for representing functionally different datapaths. For complex topologies, a combination of the two is the most flexible. A general design guideline is provided to indicate which approach is best used for a particular situation. The topological approach should primarily be used when the packet datapath forks to distinct LFB classes (not just distinct parameterizations of the same LFB class), and when the fan-outs do not require changes, such as adding/removing LFB outputs, or require only very infrequent changes. Configuration information that needs to change frequently should be expressed by using the internal attributes of one or more LFBs (and hence using the encoded state approach). +---------------------------------------------+ | | +----------+ V +----------+ +------+ | | | | | |if IP-in-IP| | | ---->| ingress |->+----->|classifier|---------->|Decap.|---->---+ | ports | | |---+ | | +----------+ +----------+ |others +------+ | V (a) The LFB topology with a logical loop +-------+ +-----------+ +------+ +-----------+ | | | |if IP-in-IP | | | | --->|ingress|-->|classifier1|----------->|Decap.|-->+classifier2|-> | ports | | |----+ | | | | +-------+ +-----------+ |others +------+ +-----------+ | V (b)The LFB topology without the loop utilizing two independent classifier instances. Figure 8: An LFB topology example. Halpern & Hadi Salim Expires April 10, 2009 [Page 35] Internet-Draft ForCES FE Model October 2008 It is important to point out that the LFB topology described here is the logical topology, not the physical topology of how the FE hardware is actually laid out. Nevertheless, the actual implementation may still influence how the functionality is mapped to the LFB topology. Figure 8 shows one simple FE example. In this example, an IP-in-IP packet from an IPSec application like VPN may go to the classifier first and have the classification done based on the outer IP header; upon being classified as an IP-in-IP packet, the packet is then sent to a decapsulator to strip off the outer IP header, followed by a classifier again to perform classification on the inner IP header. If the same classifier hardware or software is used for both outer and inner IP header classification with the same set of filtering rules, a logical loop is naturally present in the LFB topology, as shown in Figure 8.a. However, if the classification is implemented by two different pieces of hardware or software with different filters (i.e., one set of filters for the outer IP header and another set for the inner IP header), then it is more natural to model them as two different instances of classifier LFB, as shown in Figure 8.b. 3.4.2. Configuring the LFB Topology While there is little doubt that an individual LFB must be configurable, the configurability question is more complicated for LFB topology. Since the LFB topology is really the graphic representation of the datapaths within an FE, configuring the LFB topology means dynamically changing the datapaths, including changing the LFBs along the datapaths on an FE (e.g., creating/instantiating, updating or deleting LFBs) and setting up or deleting interconnections between outputs of upstream LFBs to inputs of downstream LFBs. Why would the datapaths on an FE ever change dynamically? The datapaths on an FE are set up by the CE to provide certain data plane services (e.g., DiffServ, VPN, etc.) to the Network Element's (NE) customers. The purpose of reconfiguring the datapaths is to enable the CE to customize the services the NE is delivering at run time. The CE needs to change the datapaths when the service requirements change, such as adding a new customer or when an existing customer changes their service. However, note that not all datapath changes result in changes in the LFB topology graph. Changes in the graph are dependent on the approach used to map the datapaths into LFB topology. As discussed in Section 3.4.1, the topological approach and encoded state approach can result in very different looking LFB topologies for the same datapaths. In general, an LFB topology based on a pure topological approach is likely to experience more frequent topology reconfiguration than one based on an encoded state approach. However, even an LFB topology based entirely on an encoded state Halpern & Hadi Salim Expires April 10, 2009 [Page 36] Internet-Draft ForCES FE Model October 2008 approach may have to change the topology at times, for example, to bypass some LFBs or insert new LFBs. Since a mix of these two approaches is used to model the datapaths, LFB topology reconfiguration is considered an important aspect of the FE model. We want to point out that allowing a configurable LFB topology in the FE model does not mandate that all FEs are required to have this capability. Even if an FE supports configurable LFB topology, the FE may impose limitations on what can actually be configured. Performance-optimized hardware implementations may have zero or very limited configurability, while FE implementations running on network processors may provide more flexibility and configurability. It is entirely up to the FE designers to decide whether or not the FE actually implements reconfiguration and if so, how much. Whether a simple runtime switch is used to enable or disable (i.e., bypass) certain LFBs, or more flexible software reconfiguration is used, is an implementation detail internal to the FE and outside of the scope of FE model. In either case, the CE(s) MUST be able to learn the FE's configuration capabilities. Therefore, the FE model MUST provide a mechanism for describing the LFB topology configuration capabilities of an FE. These capabilities may include (see Section 5 for full details): o Which LFB classes the FE can instantiate o The maximum number of instances of the same LFB class that can be created o Any topological limitations, for example: * The maximum number of instances of the same class or any class that can be created on any given branch of the graph * Ordering restrictions on LFBs (e.g., any instance of LFB class A must be always downstream of any instance of LFB class B). The CE needs some programming help in order to cope with the range of complexity. In other words, even when the CE is allowed to configure LFB topology for the FE, the CE is not expected to be able to interpret an arbitrary LFB topology and determine which specific service or application (e.g. VPN, DiffServ, etc.) is supported by the FE. However, once the CE understands the coarse capability of an FE, the CE MUST configure the LFB topology to implement the network service the NE is supposed to provide. Thus, the mapping the CE has to understand is from the high level NE service to a specific LFB topology, not the other way around. The CE is not expected to have the ultimate intelligence to translate any high level service policy into the configuration data for the FEs. However, it is conceivable Halpern & Hadi Salim Expires April 10, 2009 [Page 37] Internet-Draft ForCES FE Model October 2008 that within a given network service domain, a certain amount of intelligence can be programmed into the CE to give the CE a general understanding of the LFBs involved to allow the translation from a high level service policy to the low level FE configuration to be done automatically. Note that this is considered an implementation issue internal to the control plane and outside the scope of the FE model. Therefore, it is not discussed any further in this draft. +----------+ +-----------+ ---->| Ingress |---->|classifier |--------------+ | | |chip | | +----------+ +-----------+ | v +-------------------------------------------+ +--------+ | Network Processor | <----| Egress | | +------+ +------+ +-------+ | +--------+ | |Meter | |Marker| |Dropper| | ^ | +------+ +------+ +-------+ | | | | +----------+-------+ | | | | | +---------+ +---------+ +------+ +---------+ | | |Forwarder|<------|Scheduler|<--|Queue | |Counter | | | +---------+ +---------+ +------+ +---------+ | +--------------------------------------------------------------+ Figure 9: The Capability of an FE as reported to the CE Figure 9 shows an example where a QoS-enabled router has several line cards that have a few ingress ports and egress ports, a specialized classification chip, and a network processor containing codes for FE blocks like meter, marker, dropper, counter, queue, scheduler, and IPv4 forwarder. Some of the LFB topology is already fixed and has to remain static due to the physical layout of the line cards. For example, all of the ingress ports might be hardwired into the classification chip so all packets flow from the ingress port into the classification engine. On the other hand, the LFBs on the network processor and their execution order are programmable. However, certain capacity limits and linkage constraints could exist between these LFBs. Examples of the capacity limits might be: o 8 meters o 16 queues in one FE o the scheduler can handle at most up to 16 queues Halpern & Hadi Salim Expires April 10, 2009 [Page 38] Internet-Draft ForCES FE Model October 2008 o The linkage constraints might dictate that: * the classification engine may be followed by: + a meter + marker + dropper + counter + queue or IPv4 forwarder, but not a scheduler * queues can only be followed by a scheduler * a scheduler must be followed by the IPv4 forwarder * the last LFB in the datapath before going into the egress ports must be the IPv4 forwarder +-----+ +-------+ +---+ | A|--->|Queue1 |--------------------->| | ------>| | +-------+ | | +---+ | | | | | | | | +-------+ +-------+ | | | | | B|--->|Meter1 |----->|Queue2 |------>| |->| | | | | | +-------+ | | | | | | | |--+ | | | | +-----+ +-------+ | +-------+ | | +---+ classifier +-->|Dropper| | | IPv4 +-------+ +---+ Fwd. Scheduler Figure 10: An LFB topology as configured by the CE and accepted by the FE Once the FE reports these capabilities and capacity limits to the CE, it is now up to the CE to translate the QoS policy into a desirable configuration for the FE. Figure 9 depicts the FE capability while Figure 10 and Figure 11 depict two different topologies that the CE may request the FE to configure. Note that Figure 11 is not fully drawn, as inter-LFB links are included to suggest potential complexity, without drawing in the endpoints of all such links. Halpern & Hadi Salim Expires April 10, 2009 [Page 39] Internet-Draft ForCES FE Model October 2008 Queue1 +---+ +--+ | A|------------------->| |--+ +->| | | | | | | B|--+ +--+ +--+ +--+ | | +---+ | | | | | | | Meter1 +->| |-->| | | | | | | | | | +--+ +--+ | Ipv4 | Counter1 Dropper1 Queue2| +--+ Fwd. +---+ | +--+ +--->|A | +-+ | A|---+ | |------>|B | | | ------>| B|------------------------------>| | +-->|C |->| |-> | C|---+ +--+ | +>|D | | | | D|-+ | | | +--+ +-+ +---+ | | +---+ Queue3 | |Scheduler Classifier1 | | | A|------------> +--+ | | | +->| | | |-+ | | | B|--+ +--+ +-------->| | | | +---+ | | | | +--+ | | Meter2 +->| |-+ | | | | | | +--+ Queue4 | | Marker1 +--+ | +---------------------------->| |---+ | | +--+ Figure 11: Another LFB topology as configured by the CE and accepted by the FE Note that both the ingress and egress are omitted in Figure 10 and Figure 11 to simplify the representation. The topology in Figure 11 is considerably more complex than Figure 10 but both are feasible within the FE capabilities, and so the FE should accept either configuration request from the CE. 4. Model and Schema for LFB Classes The main goal of the FE model is to provide an abstract, generic, modular, implementation-independent representation of the FEs. This is facilitated using the concept of LFBs, which are instantiated from LFB classes. LFB classes and associated definitions will be provided in a collection of XML documents. The collection of these XML documents is called a LFB class library, and each document is called an LFB class library document (or library document, for short). Each Halpern & Hadi Salim Expires April 10, 2009 [Page 40] Internet-Draft ForCES FE Model October 2008 of the library documents MUST conform to the schema presented in this section. The schema here, and the rules for confoming to the schema are those defined by the W3C in the definitions of XML schema in XML Schema [4] and XML Schema DataTypes [5]. The root element of the library document is the element. It is not expected that library documents will be exchanged between FEs and CEs "over-the-wire". But the model will serve as an important reference for the design and development of the CEs (software) and FEs (mostly the software part). It will also serve as a design input when specifying the ForCES protocol elements for CE-FE communication. The following sections describe the portions of an LFBLibrary XML Document. The descriptions primarily provide the necessary semantic information to understand the meaning and uses of the XML elements. The XML Schema below provides the final definition on what elements are permitted, and their base syntax. Unfortunately, due to the limitations of english and XML, there are constraints described in the semantic sections which are not fully captured in the XML Schema, so both sets of information need to be used to build a compliant library document. 4.1. Namespace A namespace is needed to uniquely identify the LFB type in the LFB class library. The reference to the namespace definition is contained in Section 9, IANA Considerations. 4.2. Element The element serves as a root element of all library documents. A library document contains a sequence of top level elements. The following is a list of all the elements which can occur directly in the element. If they occur, they must occur in the order listed. o providing a text description of the purpose of the library document. o for loading information from other library documents. o for the frame declarations; o for defining common data types; o for defining metadata, and Halpern & Hadi Salim Expires April 10, 2009 [Page 41] Internet-Draft ForCES FE Model October 2008 o for defining LFB classes. Each element is optional. One library document may contain only metadata definitions, another may contain only LFB class definitions, yet another may contain all of the above. A library document can import other library documents if it needs to refer to definitions contained in the included document. This concept is similar to the "#include" directive in C. Importing is expressed by the use of elements, which must precede all the above elements in the document. For unique referencing, each LFBLibrary instance document has a unique label defined in the "provide" attribute of the LFBLibrary element. Note that what this performs is a ForCES inclusion, not an XML inclusion. The semantic content of the library referenced by the element is included, not the xml content. Also, in terms of the conceptual processing of elements, the total set of documents loaded are considered to form a single document for processing. A given document is included in this set only once, even if it is referenced by elements several times, even from several different files. As the processing of LFBLibrary information is not order dependent, the order for processing loaded elements is up to the implementor, as long as the total effect is as if all of the information from all the files were available for referencing when needed. Note that such computer processing of ForCES model library documents may be helpful for various implementations, but is not required to define the libraries, or for the actual operation of the protocol itself. The following is a skeleton of a library document: Halpern & Hadi Salim Expires April 10, 2009 [Page 42] Internet-Draft ForCES FE Model October 2008 ... ... ... ... 4.3. Element This element is used to refer to another LFB library document. Similar to the "#include" directive in C, this makes the objects (metadata types, data types, etc.) defined in the referred library document available for referencing in the current document. The load element MUST contain the label of the library document to be included and MAY contain a URL to specify where the library can be retrieved. The load element can be repeated unlimited times. Three examples for the elements: Halpern & Hadi Salim Expires April 10, 2009 [Page 43] Internet-Draft ForCES FE Model October 2008 4.4. Element for Frame Type Declarations Frame names are used in the LFB definition to define the types of frames the LFB expects at its input port(s) and emits at its output port(s). The optional element in the library document contains one or more elements, each declaring one frame type. Each frame definition MUST contain a unique name (NMTOKEN) and a brief synopsis. In addition, an optional detailed description MAY be provided. Uniqueness of frame types MUST be ensured among frame types defined in the same library document and in all directly or indirectly included library documents. The following example defines two frame types: ipv4 IPv4 packet This frame type refers to an IPv4 packet. ipv6 IPv6 packet This frame type refers to an IPv6 packet. ... 4.5. Element for Data Type Definitions The (optional) element can be used to define commonly used data types. It contains one or more elements, each defining a data type with a unique name. Such data types can be used in several places in the library documents, including: Halpern & Hadi Salim Expires April 10, 2009 [Page 44] Internet-Draft ForCES FE Model October 2008 o Defining other data types o Defining components of LFB classes This is similar to the concept of having a common header file for shared data types. Each element MUST contain a unique name (NMTOKEN), a brief synopsis, and a type definition element. The name MUST be unique among all data types defined in the same library document and in any directly or indirectly included library documents. The element MAY also include an optional longer description, For example: ieeemacaddr 48-bit IEEE MAC address ... type definition ... ipv4addr IPv4 address ... type definition ... ... There are two kinds of data types: atomic and compound. Atomic data types are appropriate for single-value variables (e.g. integer, string, byte array). The following built-in atomic data types are provided, but additional atomic data types can be defined with the and elements: Halpern & Hadi Salim Expires April 10, 2009 [Page 45] Internet-Draft ForCES FE Model October 2008 Meaning ---- ------- char 8-bit signed integer uchar 8-bit unsigned integer int16 16-bit signed integer uint16 16-bit unsigned integer int32 32-bit signed integer uint32 32-bit unsigned integer int64 64-bit signed integer uint64 64-bit unsigned integer boolean A true / false value where 0 = false, 1 = true string[N] A UTF-8 string represented in at most N Octets. string A UTF-8 string without a configured storage length limit. byte[N] A byte array of N bytes octetstring[N] A buffer of N octets, which MAY contain fewer than N octets. Hence the encoded value will always have a length. float16 16-bit floating point number float32 32-bit IEEE floating point number float64 64-bit IEEE floating point number These built-in data types can be readily used to define metadata or LFB attributes, but can also be used as building blocks when defining new data types. The boolean data type is defined here because it is so common, even though it can be built by sub-ranging the uchar data type, as defined under atomic types (Section 4.5.2). Compound data types can build on atomic data types and other compound data types. Compound data types can be defined in one of four ways. They may be defined as an array of components of some compound or atomic data type. They may be a structure of named components of compound or atomic data types (c.f. C structures). They may be a union of named components of compound or atomic data types (c.f. C unions). They may also be defined as augmentations (explained in Section 4.5.7) of existing compound data types. Given that the ForCES protocol will be getting and setting component values, all atomic data types used here must be able to be conveyed in the ForCES protocol. Further, the ForCES protocol will need a mechanism to convey compound data types. However, the details of such representations are for the ForCES Protocol [2] document to define, not the model document. Strings and octetstrings must be conveyed by the protocol with their length, as they are not delimited, the value does not itself include the length, and these Halpern & Hadi Salim Expires April 10, 2009 [Page 46] Internet-Draft ForCES FE Model October 2008 items are variable length. With regard to strings, this model defines a small set of restrictions and definitions on how they are structured. String and octetstring length limits can be specified in the LFB Class definitions. The component properties for string and octetstring components also contain actual lengths and length limits. This duplication of limits is to allow for implementations with smaller limits than the maximum limits specified in the LFB Class definition. In all cases, these lengths are specified in octets, not in characters. In terms of protocol operation, as long as the specified length is within the FE's supported capabilities, the FE stores the contents of a string exactly as provided by the CE, and returns those contents when requested. No canonicalization, transformations, or equivalences are performed by the FE. Components of type string (or string[n]) MAY be used to hold identifiers for correlation with components in other LFBs. In such cases, an exact octet for octet match is used. No equivalences are used by the FE or CE in performing that matching. The ForCES Protocol [2] does not perform or require validation of the content of UTF-8 strings. However, UTF-8 strings SHOULD be encoded in the shortest form to avoid potential security issues described in [12]. Any entity displaying such strings is expected to perform its own validation (for example for correct multi-byte characters, and for ensuring that the string does not end in the middle of a multi-byte sequence.) Specific LFB class definitions MAY restrict the valid contents of a string as suited to the particular usage (for example, a component that holds a DNS name would be restricted to hold only octets valid in such a name.) FEs should validate the contents of SET requests for such restricted components at the time the set is performed, just as range checks for range limited components are performed. The ForCES protocol behavior defines the normative processing for requests using that protocol. For the definition of the actual type in the element, the following elements are available: , , , , and . The predefined type alias is somewhere between the atomic and compound data types. Alias is used to allow a component inside an LFB to be an indirect reference to another component inside the same or a different LFB class or instance. The alias component behaves like a structure, one component of which has special behavior. Given that the special behavior is tied to the other parts of the structure, the compound result is treated as a predefined construct. Halpern & Hadi Salim Expires April 10, 2009 [Page 47] Internet-Draft ForCES FE Model October 2008 4.5.1. Element for Renaming Existing Data Types The element refers to an existing data type by its name. The referred data type MUST be defined either in the same library document, or in one of the included library documents. If the referred data type is an atomic data type, the newly defined type will also be regarded as atomic. If the referred data type is a compound type, the new type will also be compound. Some usage examples follow: short Alias to int16 int16 ieeemacaddr 48-bit IEEE MAC address byte[6] 4.5.2. Element for Deriving New Atomic Types The element allows the definition of a new atomic type from an existing atomic type, applying range restrictions and/or providing special enumerated values. Note that the element can only use atomic types as base types, and its result MUST be another atomic type. For example, the following snippet defines a new "dscp" data type: dscp Diffserv code point. uchar DSCP-BE Best Effort ... Halpern & Hadi Salim Expires April 10, 2009 [Page 48] Internet-Draft ForCES FE Model October 2008 4.5.3. Element to Define Arrays The element can be used to create a new compound data type as an array of a compound or an atomic data type. Depending upon context, this document, and others, refer to such arrays as tables or arrays interchangeably, without semantic or syntactic implication. The type of the array entry can be specified either by referring to an existing type (using the element) or defining an unnamed type inside the element using any of the , , , or elements. The array can be "fixed-size" or "variable-size", which is specified by the "type" attribute of the element. The default is "variable-size". For variable size arrays, an optional "maxlength" attribute specifies the maximum allowed length. This attribute should be used to encode semantic limitations, not implementation limitations. The latter (support for implementation constraints) should be handled by capability components of LFB classes, and should never be included as the maxlength in a data type array which is regarded as being of unlimited size. For fixed-size arrays, a "length" attribute MUST be provided that specifies the constant size of the array. The result of this construct is always a compound type, even if the array has a fixed size of 1. Arrays MUST only be subscripted by integers, and will be presumed to start with index 0. In addition to their subscripts, arrays MAY be declared to have content keys. Such a declaration has several effects: o Any declared key can be used in the ForCES protocol to select a component for operations (for details, see the ForCES Protocol [2]). o In any instance of the array, each declared key MUST be unique within that instance. That is, no two components of an array may have the same values on all the fields which make up a key. Each key is declared with a keyID for use in the ForCES Protocol [2], where the unique key is formed by combining one or more specified key fields. To support the case where an array of an atomic type with unique values can be referenced by those values, the key field identifier MAY be "*" (i.e., the array entry is the key). If the value type of the array is a structure or an array, then the key is one or more components of the value type, each identified by name. Halpern & Hadi Salim Expires April 10, 2009 [Page 49] Internet-Draft ForCES FE Model October 2008 Since the field MAY be a component of the contained structure, a component of a component of a structure, or further nested, the field name is actually a concatenated sequence of component identifiers, separated by decimal points ("."). The syntax for key field identification is given following the array examples. The following example shows the definition of a fixed size array with a pre-defined data type as the array content type: Halpern & Hadi Salim Expires April 10, 2009 [Page 50] Internet-Draft ForCES FE Model October 2008 dscp-mapping-table A table of 64 DSCP values, used to re-map code space. dscp The following example defines a variable size array with an upper limit on its size: mac-alias-table A table with up to 8 IEEE MAC addresses ieeemacaddr The following example shows the definition of an array with a local (unnamed) content type definition: classification-table A table of classification rules and result opcodes. rule The rule to match classrule opcode The result code opcode In the above example, each entry of the array is a of two components ("rule" and "opcode"). Halpern & Hadi Salim Expires April 10, 2009 [Page 51] Internet-Draft ForCES FE Model October 2008 The following example shows a table of IP Prefix information that can be accessed by a multi-field content key on the IP Address, prefix length, and information source. This means that in any instance of this table, no two entries can have the same IP address, prefix length, and information source. ipPrefixInfo_table A table of information about known prefixes address-prefix the prefix being described ipv4Prefix source the protocol or process providing this information uint16 prefInfo the information we care about hypothetical-info-type address-prefix.ipv4addr address-prefix.prefixlen source Note that the keyField elements could also have been simply address- prefix and source, since all of the fields of address-prefix are being used. 4.5.3.1. Key Field References In order to use key declarations, one must refer to components that are potentially nested inside other components in the array. If there are nested arrays, one might even use an array element as a key Halpern & Hadi Salim Expires April 10, 2009 [Page 52] Internet-Draft ForCES FE Model October 2008 (but great care would be needed to ensure uniqueness.) The key is the combination of the values of each field declared in a keyField element. Therefore, the value of a keyField element MUST be a concatenated Sequence of field identifiers, separated by a "." (period) character. Whitespace is permitted and ignored. A valid string for a single field identifier within a keyField depends upon the current context. Initially, in an array key declaration, the context is the type of the array. Progressively, the context is whatever type is selected by the field identifiers processed so far in the current key field declaration. When the current context is an array, (e.g., when declaring a key for an array whose content is an array) then the only valid value for the field identifier is an explicit number. When the current context is a structure, the valid values for the field identifiers are the names of the components of the structure. In the special case of declaring a key for an array containing an atomic type, where that content is unique and is to be used as a key, the value "*" MUST be used as the single key field identifier. In reference array or structure elements, it is possible to construct keyFields that do not exist. keyField references SHOULD never reference optional structure components. For references to array elements, care must be taken to ensure that the necessary array elements exist when creating or modifying the overall array element. Failure to do so will result in FEs returning errors on the creation attempt. 4.5.4. Element to Define Structures A structure is comprised of a collection of data components. Each data components has a data type (either an atomic type or an existing compound type) and is assigned a name unique within the scope of the compound data type being defined. These serve the same function as "struct" in C, etc. These components are defined using elements. A element MAY contain an optional derivation indication, a element. The structure definition MUST contain a sequence of one or more elements. The actual type of the component can be defined by referring to an existing type (using the element), or can be a locally defined (unnamed) type created by any of the , , , or elements. Halpern & Hadi Salim Expires April 10, 2009 [Page 53] Internet-Draft ForCES FE Model October 2008 The element MUST include a componentID attribute. This provides the numeric ID for this component, for use by the protocol. The MUST contain a component name and a synopsis. It MAY contain a element giving a textual description of the component. The definition MAY also include a element, which indicates that the component being defined is optional. The definition MUST contain elements to define the data type of the component, as described above. For a dataTypeDef of a struct, the structure definition MAY be inherited from, and augment, a previously defined structured type. This is indicated by including the optional derivedFrom attribute in the struct declaration before the definition of the augmenting or replacing components. The augmentation (Section 4.5.7) section describes how this is done in more detail. The componentID attribute for different components in a structure (or in an LFB) MUST be distinct. They do not need to be in order, nor do they need to be sequential. For clarity of human readability, and ease of maintanence, it is usual to define at least sequential sets of values. But this is for human ease, not a model or protocol requirement. Halpern & Hadi Salim Expires April 10, 2009 [Page 54] Internet-Draft ForCES FE Model October 2008 The result of this construct is always a compound type, even when the contains only one field. An example: ipv4prefix