The intended audience of this document is system architects, solution architects, technology evaluators, technology decision makers, business strategists and business investment decision makers interested in developing IIoT systems.

The Industrial Internet of Things (IIoT) landscape today is a confusing mix of proprietary connectivity technologies and standards. Some are general purpose, some are mostly appropriate for enterprise applications, and others are optimized for a narrow set of domain-specific use cases in vertically integrated systems. This confusion hinders the ability to share data that IIoT systems need. Clarity on how to share data across disparate systems unlocks the potential of a global IIoT marketplace. The IICF provides a comprehensive treatment of connectivity as a means of building interoperable IIoT systems. The IICF defines the minimum expectation: an IIoT connectivity framework provides syntactic interoperability between IIoT components and subsystems. Syntactic interoperability is the unambiguous exchange of structured data between participants. Higher layers of interoperability (e.g., semantic interoperability) increase integration automation; they continue to be an active area of research. However, the IICF clarifies that syntactic interoperability is the essential building block for interoperable systems and thus critical for unleashing the potential of IIoT. The IICF crystallizes the layers, functions, and considerations of the IIoT connectivity stack. It includes a deep assessment template to evaluate any connectivity technology and place it correctly on the stack. It then catalogs common IIoT standards, evaluating them against the template. The IICF thus establishes a starting point for accelerating connectivity technology selection. The criteria and analysis help guide the selection of core connectivity standards.

Specifically, the IICF addresses the following questions:

• What is the role of connectivity in an IIoT architecture?
• What connectivity layers does an IIoT system need, and what are each layer's core functions, considerations and trade-offs?
• How can communication extend from a generic IIoT design to participants using a domain-specific connectivity technology?
• What must core connectivity standards provide?
• How to categorize and evaluate a given connectivity technology?
• How to assess suitability of a connectivity technology against system requirements?
• How to choose the right core connectivity standard for a problem domain?

The IICF defines the role of a connectivity framework as providing syntactic interoperability for communicating between disparate IIoT systems and components developed by different parties at different times. The IICF clarifies the layers of the IIoT connectivity stack. Specifically, for the first time the framework layer is being identified as an essential element of the IIoT connectivity puzzle. The IICF defines its core functions and typical considerations. The IICF accepts the reality that multiple connectivity technologies will abound in IIoT systems, and defines a reference architecture for opening up data otherwise locked in a plethora of domain-specific connectivity technologies. Data from domain-specific technologies are shared using gateways to one of a few core connectivity standards. These standards can provide syntactic interoperability without compromising the fidelity of the functional and non-functional aspects of the domain-specific technology. The IICF establishes the minimum criteria to qualify as an IIoT core connectivity standard. Rather than building many bridges between many domain-specific standards, each core connectivity standard need only connect to the other core connectivity standards through standardized core gateways. This architecture provides cross-industry wide connectivity without requiring an unlimited number of core gateways. The IICF defines an assessment template for evaluating and categorizing any connectivity technology and determine its suitability for the system at hand. The assessment template is a worksheet that can be used by IIoT architects to evaluate any connectivity technology from an IIoT perspective, and determine its suitability for a set of system requirements. The IICF recognizes that a useful IIoT connectivity stack will involve the most appropriate 'core' connectivity standards for each of the layers of the connectivity stack. Rather than taking a prescriptive approach, the IICF offers a practical toolkit for IIoT connectivity technology selection. It also provides a base set of in-depth assessments for relevant IIoT connectivity framework and transports available today for IIoT projects, and identifies potential core connectivity standards.

For the first time, the IICF firmly establishes syntactic interoperability as the foundation offered by the connectivity stack. It defines the core functions, typical considerations of the framework layer and provides an assessment template worksheet as a tool for technology selection. It defines a reference architecture to incorporate data from any domain-specific connectivity technology via core connectivity standards, and establishes the criteria defining the minimum exceptions. It provides a catalog of common IIoT connectivity standards, and shows how the potential core connectivity standards fare against the criteria. The IICF aims for broad applicability across the IIoT by providing IIoT architects a comprehensive tool that will help determine the most suitable connectivity technologies and core connectivity standards for their system. The IICF paves the way for IIoT adoption by enabling a marketplace for gateways and core gateways, creating more choices for IIoT system developers, which will help accelerate IIoT development. The overarching goal is to help IIoT architects unlock data in isolated systems and enable data sharing and interoperability between previously closed components and subsystems (e.g. brownfield systems and applications) and to accelerate the development of new applications (e.g. greenfield systems and applications) within and across industries.

The IICF is organized into 9 chapters and 10 annexes. Chapter 1 provides and introductory overview. Chapters 2 to 6 cover the reference model, including the role of connectivity, the connectivity stack layers, the connectivity reference architecture, core standards criteria, and the assessment template worksheet. Annexes A to F provide detailed assessments for three connectivity framework standards and three connectivity transport standards relevant to IIoT. Chapters 7 categorizes the relevant IIoT connectivity standards assessed in annexes A to F by applying the reference model established in chapters 1-2. Chapter 8 evaluates the connectivity standards from the point of view of the core standards criteria. Chapter 9 concludes by providing suggestions for applying the IICF guidance to specific functional domains. Annexes G to J provide the revision history, acronyms, glossary, and references.

The Industrial Internet Reference Architecture (IIRA) is a standards-based architectural template and methodology enabling Industrial Internet of Things system architects to design their own systems based on a common framework and concepts. It identifies Connectivity as one of the key crosscutting functions. The Industrial Internet Security Framework (IISF) is a comprehensive resource for understanding Industrial Internet of Things (IIoT) security considerations. It defines functional building blocks for addressing security concerns, provides implementation guidance and practical techniques for IIoT security. It provides guidance on how to protect the communication and connectivity functions. The Industrial Internet Connectivity Framework (IICF) is a comprehensive resource for understanding Industrial Internet of Things (IIoT) connectivity considerations. It builds on the foundation established by the IIRA and IISF by elaborating on the connectivity aspects of building IIoT systems. It is a keystone document in a series of IIC technical publications. All extend from the top-level Industrial Internet Reference Architecture.

The IICF is a collaboration of members of the Industry IoT Consortium. This publication reflects thousands of hours of knowledge and experiences from connectivity experts, collected, researched and evaluated for the benefit of all IIoT system deployments. Contributors dedicated their valuable time and expertise in authoring, editing and other ways. For a list of editors, primary authors and contributing organizations, see the IICF landing page.

The IICF guidance can be used in a number of ways:

• The assessment template is a 4-page worksheet that one can fill out for any connectivity standard or technology. It can be used to place a technology correctly on the IIoT connectivity stack, and understand the gaps.
• As one would expect, filling out this worksheet for a connectivity technology is a laborious process requiring deep technical expertise in the technology and its usage. The IICF offers a starter set of completed assessment template worksheets for relevant IIoT connectivity frameworks and transport standards.
• Using the assessments, and applying the criteria for core standards, the IICF identifies potential core connectivity standards, and highlights the non-overlapping system aspects targeted by them.
• The assessment template worksheet can also be used to establish an IIoT system's connectivity requirements, identify gaps with respect to a particular connectivity technology, and also to identify the core connectivity standards most aligned with the system's needs.

When developing IIoT solutions, systems developers need to assess and make selection of various technologies pertinent to the problem domain. Systems developers can use the architectural qualities, core functions and architectural considerations that have been identified by the IICF that are relevant to their domain.

When determining which connectivity technologies to include in their products, technology providers can use the information contained within the IICF in making their technology assessments. Likewise, technology providers can use the IICF to determine gaps that might exist in their technology offerings.

SDOs that develop connectivity standards can use the architectural qualities, core functions and architectural considerations as inputs and requirements to their standard specifications. For SDOs that develop specifications for connectivity architectures, the SDOs can express their architectures in the context of the IICF in order to see if there are gaps or elements that need to be considered for deployment of their standards in IIoT deployments.

Platform providers can use the architectural qualities, core functions and architectural considerations of the IICF to determine gaps that might exist in their technology offerings.

The IICF is a reference architecture that describes the elements necessary for connectivity frameworks that are deployed within the IIoT. Industrial connectivity standards that exist or are in development today can use the IICF to determine where in the connectivity landscape they can be utilized in IIoT solutions. In addition, these standards can use the IICF to determine what features are needed to be successfully deployed in IIoT solutions. As the framework, core functions, and considerations described in the IICF are applied to industrial Internet testbeds and use cases, IIoT connectivity standards will be tested and gaps in those standards will be identified. The Industry IoT Consortium exposes the existence of those gaps to the appropriate standards organizations in the form of recommendations. Based upon these recommendations and the reported testing, it is our hope that the standards organizations will address those gaps. The IICF is not a standard and does not strive to become a standard. The IICF is a living document reflecting collaboration of cross-industry expertise and the actual testing of these concepts, recommendations and practices. It is our intent to be a resource for standards organizations and to positively influence the development of IIoT connectivity standards. In this way, we will drive progress toward interoperable and trustworthy IIoT systems.

The IICF describes the architectural qualities, core functions and architectural considerations that are applicable to the IT and OT domains. When assessing a technology in a particular domain, the core functions of that are important to that domain will take a priority over core functions that are less relevant in that setting. Since even within a particular domain, the connectivity functions needed by solutions differ, the priority assigned to a core function will differ by solution.

Syntactic interoperability is the unambiguous exchange of structured data between participants. The structured data exchange is the essential building block for creating interoperable systems and unleashing the potential of IIoT. Higher layers of interoperability (e.g., semantic interoperability) continue to be an active area of research. Syntactic interoperability is the foundation required to support intelligent and autonomous systems that operate in the physical-world. The connectivity framework layer in the IICF provides syntactic interoperability. The minimum expectation of an IIoT connectivity stack is the connectivity infrastructure should be able to handle sharing of structured data types between participants.

Consider the scenario where a participant wants to share a datatype containing a string and two numbers. The other participants may be on different platforms and may be built at different times by different people in different places using different programming languages. Syntactic interoperability ensures that the datatype is interpreted unambiguously by all the parties. That requires the connectivity infrastructure to maintain sufficient context, including for example, the string representation (e.g. ASCII or Unicode), the number representation (e.g. integer or floating point), the field sizes and names. It also requires that the connectivity infrastructure encode the data on the wire in an unambiguous manner such that it is decoded correctly by the connectivity infrastructure independently of the implementation, platform or the programming language. The connectivity framework layer in the IICF is responsible for providing the unambiguous structured data exchange, regardless of the platform or the programming language.

In the syntactic interoperability example above, the connectivity framework ensures that the datatype containing a string and two numbers is shared unambiguously. However, the connectivity infrastructure is not required to provide any additional context on how those members are to be interpreted. For example, does the string represent a room in a building, and the numbers represent the temperature and pressure? Or, does the string represent the address of a parking lot and the numbers represent the location of a car in that parking lot? What are the units and ranges of the fields? Semantic interoperability infrastructure provides that additional context beyond syntax so that structured data can be meaningfully interpreted. It is currently an active area of research, and beyond the scope of the connectivity framework. The IICF suggests the 'Distributed Data Interoperability and Management' layer as the place in the reference architecture for providing the support needed for semantic interoperability.

The connectivity literature recognizes two prominent connectivity stack models that were established long before IIoT: the OSI 7-layer model and the Internet 4-layer model. The layers above the network layer have evolved rapidly in the last decade and are not as widely recognized or understood. The IICF clarifies the connectivity stack from an IIoT perspective, and introduces a new 'connectivity framework layer' responsible for providing syntactic interoperability, essential to constructing IIoT systems. The IICF provides a comprehensive treatment of the core functions and typical considerations for the framework layer. It also delineates the transport layer, which sits below the framework layer, and clearly defines the transport layer core functions and typical considerations in the context of IIoT.

A 'core' connectivity standard is one that can allow endpoints of another connectivity to communicate through it without compromising the fidelity of the data or the quality of service. To share data from a proprietary or a domain-specific connectivity technology, one only need to build a gateway to a core connectivity standard. Once the data is available over a core connectivity standard, it can be shared with endpoints on any other connectivity technology that has a gateway to that core connectivity standard. Core connectivity standards are essential to enable integration of legacy and emerging connectivity technologies into a stable and long-term IIoT system architecture strategy. A useful IIoT connectivity stack will involve the most appropriate 'core' connectivity standards for each of the layers of the connectivity stack. The IICF establishes the criteria for a core standards at the connectivity framework layer, and evaluates relevant IIoT connectivity frameworks against those criteria. For the transport layer, UDP and TCP are identified as the prominent core connectivity standards. For the network layer, the internet protocol (IP) is identified as the prominent core connectivity standard. The IICF allows for multiple core connectivity standards.

A 'core gateway' refers to a gateway between two core connectivity standards. Core gateways ensure that once the data is shared via a core connectivity standard, it can be available at any other endpoint that is connected via a gateway to another core connectivity standard. Core gateways are the key to building a horizontal interoperable IIoT architecture that spans across industries. One of the criterion to qualify as a core connectivity standard is to have a standardized gateway to all the other connectivity standards at that level of interoperability.

The IICF describes a logical connectivity architecture, and makes no assumption on where the connectivity endpoints are deployed. Thus, endpoints may be deployed on the edge, at a plant, in the fog, in the cloud, or in a continuum as described by the layered databus architecture in the IIRA implementation viewpoint. The deployment will vary per system unique requirements. The IICF does not impose any deployment constraints or requirements.

After much consideration, we deliberately chose to focus this document on the connectivity stack above the network layer since those layers have evolved rapidly in the last decade and are not as widely recognized or understood in the context of IIoT. In particular, the newly introduced 'connectivity framework layer', while critical for building IIoT systems, is the least understood. Thus, the document has focused on connectivity standards at the framework and transport layers of the connectivity stack. The process of selecting the relevant connectivity standards was driven by working with the IIC's extensive membership. The process also resulted in assessment template worksheets for several framework standards and transport standards. The catalog of assessment template worksheets is meant to be a starter set to accelerate IIoT projects. While it focuses on a few relevant standards, it provides a much deeper in-depth technical analysis for each of them from an IIoT perspective. Thus, rather than taking a prescriptive approach, we chose to provide a toolkit that could be used by IIoT architects to do their own connectivity technology evaluation and selection. The assessment template can be used to asses a specific technology, standard, or IIoT project connectivity requirements. While we have built up a starter set of the assessments, we hope that community will use the template to contribute additional assessments as they use the assessment template worksheet for their IIoT projects.