Integrating RFID with IP Host Identities

1. Introduction

The “Internet of Things” semantically means ‘‘a world-wide network of interconnected objects uniquely addressable, based on standard communication protocols” [1]. The vision describes a world that enables physical objects to act as nodes in a networked physical world [2]. The terms ‘‘Internet of Things” can be attributed to The Auto-ID Labs, a world-wide network of academic research laboratories in the field of networked RFID and emerging sensing technologies [2]. Together with EPCglobal^®, these institutions have been architecting the Internet of Thing since their establishment. Their focus has primarily been on the development of the Electronic Product Code™ (EPC) to support the wide-spread use of RFID in modern, global trading networks, and to create an industry-driven set of global standards for the EPCglobal Network.

EPCglobal Network was created for “traditional” low-cost tags [3]. The main functionality of the EPCglobal Network is to provide data assigned to a specific tag, so that each RFID read event can be stored in a database and applications can be built on this data. Since tags were not originally considered to carry or compute additional data, the EPCglobal Network does not traditionally provide a mechanism to address remote tags from networked applications.

The data flow in these networks works from tags via readers to a couple of networked servers. Passive, low-cost RFID tags are widely available and the EPCglobal Network was defined to support open-loop supply chain applications. Basically, this is accomplished by allowing servers to communicate over the Internet. Although RFID technology is quite accepted in closed-loop applications, the evolution towards open-loop systems using the EPCglobal Network with distributed databases did not take place as predicted due to problems in the access control layer of such systems.

RFID-sensor networks are an emerging part of the Internet of Things [5]. These devices combine sensing capabilities with an RFID interface that allow the retrieval of sensed data. In fact, they can cooperate with RFID systems to better track the status of things e.g., their location, temperature, movements, etc. A sensor-enabled RFID tag (also known as sensor-tags) is an RFID tag which contains one or more sensors to monitor some physical parameter (e.g., temperature) but also contains the same identification function as a “normal” RFID tag does. This kind of sensor tag may fall into class 2, class 3 or class 4 in EPCglobal's tag classification [3]. A fully passive, class 2 sensor-tag can measure physical parameters, i.e., use sensors, only when powered by a reader. In contrast, class 3 tags are battery assisted. They can work independently of the reader and can be suitable for RFID-sensor networks.

In this chapter, we will discuss different ways to achieve the Internet of Things vision by internetworking passive RFID tags over IPv6. The chapter is organized as follows: Section 2 presents related works and discusses the novelty of the work presented here. Section 3 introduces the key technologies for the convergence of RFID and Internet namespaces and to provide an address mapping needed to internetwork passive RFID tags. In Section 4, some common examples of RFID usage are given and discussed in the context of globally networked tags. Subsequently, Section 5 introduces a testbed built to study the interconnection of passive RFID tags over IPv6. The different strategies that can be used for integrating RFID with IPv6 are discussed in Section 6 and this discussion is followed by mobility considerations in Section 7. Finally, Section 8 concludes the discussion and outlines anticipated future work in this area.

2. Related works

Most objects in our surrounding are not equipped with microprocessors and hence cannot attach to a computer network. However, these objects can be equipped with passive, low-cost RFID tags either as tags integrated or adhesively stuck to the object and hereby provide a mean of communications. Dominikus et al. [14] has suggested a way to integrate passive RFID systems into the Internet of Things, by using readers that function as IPv6 routers. In their work, an IPv6 addressing scheme that map tag IDs to network addresses was defined. Furthermore, the mobility problem, which arises when tags physically moves around, was investigated and the use of Mobile IPv6 (MIPv6) to cope with tag mobility was suggested. In contrast to the work presented by Dominikus et al. [14], this chapter opens the discussion on the proper formatting of the IPv6 addressing by introducing cryptographic hashing techniques as well as the possibility of separating identity and location information when forming an IPv6 address. The use of hashing techniques to construct an IPv6 address from an EPC, as opposed by using a compressed EPC format [14], eases practical implementations and allows the use of the same mapping scheme for all EPC types.

An alternative approach is to provide the tags themselves with the IPv6 protocol stack, making them able to use IPv6 communication over the Internet whenever close to a reader. This requires several changes to the design of existing tags. In this case, the tags do all the work themselves and need a separate power source. A solution where the tags are modified to hold the IPv6 stack on them is discussed by Rahman et al. [4]. The tags EPC, which is its identity, would then be made into a part of the tags IPv6 address due to the design of the tags proposed. This makes these tags too expensive for integration into the Internet of Things since the price of the tags could easily exceed the value of the “things” themselves.

Barish et al. [13], describes a somewhat similar setup than the one proposed here. In their approach, a global address manager is used to keep track of tags. The basic idea is that an application sends the EPC to a global server along with the IP address that the tag has been associated with. When a corresponding node wants to communicate with the tagged object, it contacts the last known address. If the tag is in the field of the reader the connection is established and communication can begin. If the tag is not present at the location the request is redirected to the global address server that returns the tag’s present address or just redirects the request to the correct address. In contrast to the proposed solution by Barish et al. [13], the approach described here does not include extra nodes in the network to construct network addresses but adds functionality to the RFID readers residing at the network edge.

Xu et al. [25] proposed a general address mapping scheme based on a proprietary protocol named General Identity Protocol (GIP). The scheme takes all existing RFID systems into account, and allows heterogeneous RFID systems to interwork over the Internet. This is accomplished by mapping RFID tag identifiers to IPv6 addresses, constructing a GIP message with details of the RFID systems in use, and finally encapsulating the message in IPv6 and routing the packet over the Internet. This chapter describes a solution that minimizes the need for control protocols.

3. Enabling technologies

There are a couple of ways to interconnect objects by using RFID with IPv6 [6]. One solution would be to give the tags the ability to communicate via the Internet. The communication can be both reader-initiated and tag-initiated. The latter requires specific tags that require electrical and processing power to be available in the tag such as e.g., EPC class 3 tags. Most of the computational work takes place in the tags, i.e., the tag is reachable and visible as an IPv6 connected host as long as it is within the electric field of a reader.

Passive RFID tags, such as EPC class 2 tags, do not have the possibility to power a network protocol stack and therefore a network address cannot be directly assigned to the tag’s microchip. However, the passive tag can be represented by virtual interfaces residing in the reader interrogating the tag.

3.1. Radio Frequency Identification (RFID)

RFID systems are composed of one or more readers and several electronic tags. Tags are characterized by a unique identifier that takes the form as a binary number. They are applied to objects and even persons or animals as implants. From a physical point of view, an RFID tag is a small microchip attached to an antenna that is used for both receiving the reader signal and transmitting the tag ID. The dimensions of each tag can be very small with tag dimensions down to 0.05 mm x 0.05 mm with a thickness of 0.005 mm [7]. There are more than 60 tag manufacturers world-wide [8].

RFID tags will act as electronic identification for physical objects to which they are linked. In the Internet of Things, all objects, virtual as well as physical, are interconnected and reachable via for example IPv6 in combination with RFID technology [6]. Essentially, the tag connects to physical objects that we want to authenticate and track when they come in contact with readers. A reader can read or modify tag’s information. The back-end database keeps information related to different tags/readers.

For reader-initiated communication the reader triggers the tag’s transmission by generating an appropriate signal, which represents a query for the possible presence of tags in the surrounding area and for the reception of their identification codes (IDs).

Active tags come with a power source that can drive a microprocessor (or microcontroller). Furthermore, it allows a stronger electromagnetic field to be generated in response to an incoming RFID air protocol message and larger read distances can be achieved. More advanced active tags or sensor-tags may run additional software and can be equipped with communication software such as an IP protocol stack [9].

In contrast, passive tags rely on the incoming electromagnetic field from the reader to power the circuit and to deliver power to drive the response to an interrogating request. These devices do not run communication software and cannot be actively involved in a protocol message exchange. To communicate with these devices there is a need for software agents to act on their behalf.

RFID tags can only be “online” when they are in the electric field of a reader field. For high velocity applications, where tags only remain certain seconds in a reader field, the proposed approach of networking these tags is not applicable.

3.2. RFID namespaces

Essentially, RFID comes with two namespace to be used with RFID applications: the EPC addresses and the Object Name Service (ONS). A namespace can represent objects as well as concepts and may be generalized as a container for a set of identifiers (names). The EPC is an identifier based on the standards established by EPCglobal^® [10]. It is designed to allow the automatic identification of objects anywhere. EPC defines three layers of identity: the pure identity, the encoding layer identity and the physical realization of an encoding. The EPC tag data standard [10] identify how existing coding systems such as the GS1 family codes for serialized human readable representations e.g. GTIN, GCN, SSCC, GRAI, GIAI, GSRM, GDTI and a small number of other identities should be embedded within the EPC.

A canonical representation of an EPC is the pure identity Uniform Resource Indicator (URI) representation, which is intended for communicating and storing EPC in information systems, databases and applications. The purpose is to insulate EPCs from knowledge about the physical nature of the tag, so that although 64-bit tags may differ from 96-bit tags in the choice of binary header values and in the number of bits allocated to each element or field within the EPC, the pure identity URI format does not require the information system to know about these details. Hence, the pure identity URI can be regarded as a pure identifier [10]. Tags are identified by URIs such as e.g., urn:epc:id:sgtin:0523141.000024.120 that comprise both tag number and associated coding scheme.

Encoding is the process of translating the pure identity EPC into a specific instantiation incorporated into tags for a specific purpose. During the encoding process the URI information is translated into a binary encoding that is stored in the tag. Subsequently, translating between the different levels of representation can be accomplished in a consistent way.

Figure 1 shows the structure of the data layout of an EPC code for the Global Trade Item Number (GTIN) and Serialized Global Trade Item Number 96-bit (SGTIN-96) tag.

Figure 1.

Tag data layout example.

EPC generation 1 standards, i.e., class 0 and class 1 tags, use a Cyclic Redundancy Check (CRC) to verify data integrity and a password as a “kill code” to disable the tags. The password must never be transmitted under any circumstances [3]. The Item reference identifies a class of objects to be tagged and it allows the grouping of items. The Serial number identifies an instance of a particular item. Company prefixes (also known as General Manager Numbers) point to the organization responsible for the subsequent partition. Finally, Indicator digits are used to specify length, type, structure, version, and generation of the EPC. This latter part is further used to guarantee uniqueness in the EPC namespace. For the GTIN encoding a Check digit is used.

Since the EPC is the only required data stored on a tag, it must be used as a “pointer” to find additional data about an object to which it attaches. This additional data should be stored on a server connected to the enterprise network or to the Internet. The server is identified via a look-up system which is called ONS.

ONS acts as a directory service for organizations wishing to look up product numbers (also known as EPC numbers) on the Internet. The ONS is operated as part of the EPCglobal Network. It is based on the well-known DNS service and it realizes the link between EPC numbers and EPC Information Services (EPCIS) as illustrated in Figure 2. When an RFID reader reads a tag, the EPC is passed to a middleware which then looks the EPC up either on the local machine, or enquires ONS through the Internet.

Figure 2.

The Object Name System (ONS). Adapted from [11].

The ONS resolution process takes the EPC code and returns network location(s) where information resides, i.e., the EPCIS server, which typically holds web pages with information about tags. The Physical Markup Language (PML), based on XML technology, is intended to be the standard in which information about tags should be written.

In contrast, the DNS of the Internet will handle many more requests in the future. Therefore, enterprises will likely maintain ONS servers locally, which will store information for quick retrieval. Hence, a manufacturer may store ONS data from its current suppliers on its own network, rather than pulling the information off a Web site every time a shipment arrives at the assembly plant.

3.3. Internet namespaces

There are two principal namespaces in use in the Internet: IP addresses and domain names. Domain names provide hierarchically assigned names for some computing platforms and some services. Each level in the hierarchy is delegated from the level above. Email, Hypertext Transfer Protocol (HTTP), and Session Initiation Protocol (SIP) addresses all reference domain names to mention its most wide-spread use.

On the network layer, IP addresses are used. IPv6 was introduced in the 90’ies due to the foreseen lack of globally unique IPv4 addresses, resulting in a protocol specification released in 1998 [17]. The IPv6 address is a 128-bit address that takes the form of a 64-bit network prefix appended by a 64-bit host suffix/interface identifier. The network prefix is used for routing purpose and determines the location of the host in the Internet. The host itself is identified by an interface ID. Figure 3 shows the IPv6 address format and gives an example on how a 96-bit EPC can be mapped to a network address.

Figure 3.

IPv6 address format compared to a 96-bit General Identifier (GID-96) EPC format. An ‘X’ indicates a grouping of 4 bits.

It can be observed form the figure that not all 96 bits of the EPC can be fitted within the host suffix/interface identifier of an IPv6 address. Because of this deficiency, the implementers of an RFID-to-IPv6 mapping scheme is faced with a number of design options. These options basically govern the strategy for the mapping and are the subject of our discussion in Section 6.

3.4. Cryptographically generated addresses

Cryptographically Generated Addresses (CGAs) are IPv6 addresses for which the host suffix/interface identifier is generated by computing a cryptographic one-way hash function from a binary input such as e.g., a public key [27]. CGAs are intended to be globally unique in a statistical sense but these may not necessarily be routable addresses at the IP layer [12].

The Overlay Routable Cryptographic Hash Identifiers (ORCHID) is a new, experimental class of identifiers based on CGAs. ORCHIDs have an IPv6-like address format and can be used with existing applications built on IPv6 [12]. These identifiers are intended to be used as pure endpoint identifiers for applications and Application Programming Interfaces (APIs) and not as identifiers for network location. This is in contrast to the IPv6 address that uses the 64-bit network prefix as locator [17].

While ORCHIDs use public cryptographic keys as input bit strings, it is possible to use the binary EPC encoding instead. The algorithm to generate an ORCHID in an RFID context is outlined below [12]. The algorithm takes a bitstring and some context identifier as inputs and produces an ORCHID output that is formatted as an IPv6 address.

$$Input\mathrm{ }:=anybitstring$$E1

$$HashInput\mathrm{ }:=\mathrm{ }ContextID\mathrm{ }|\mathrm{ }Input$$E2

$$Hash\mathrm{ } :=\mathrm{ } Hash\_function\left(\mathrm{ }HashInput\mathrm{ }\right)$$E3

$$ORCHID\mathrm{ } :=\mathrm{ } Prefix\mathrm{ }\left|\mathrm{ }Encod{e}_n\right(\mathrm{ }Hash\mathrm{ })$$E4

Concatenation of bitstrings is denoted '|'. The Input is a bitstring that is unique within a given context. The Context ID is a randomly generated value defining the expected usage context for the particular ORCHID and the hash function to be used for generation of ORCHID in this context. The purpose of a context ID is to be able to differentiate between various experiments that share the ORCHID namespace. The Hash_function is a one-way hash function to be used to generate ORCHIDs such as SHA1 [23] or MD5 [24]. SHA1 and MD5 produce a 160-bit and a 128-bit output, respectively. Encode_n is a function to extract an n-bit-long bitstring from its argument. Finally, Prefix is an IPv6 network prefix.

To construct a CGA an input bitstring and context identifier are concatenated to form an input datum, which is then fed to the cryptographic hash function. The result of the hash function is processed by an encoding function, resulting in an n-bit-long output. This value is prepended with the network prefix resulting in a 128-bit-long bitstring identifier that can be used for programming with the IPv6 API.

To create a CGA namespace for RFID tags the EPC of a tag and the network prefix assigned to the reader that interrogates the tag are used as input. Furthermore, an Encode₆₄ function is used to extract 64 bits from the hash. A key advantage of using hash values over the actual raw host identity resulting from the EPC is its fixed length. This makes protocol implementations easier and it alleviates the management of packet sizes. However, a claimed drawback is that CGAs work one-way, meaning that it is not possible directly to create the original identity from the hash.

A CGA can be globally unique or globally unique in a statistical sense. That is, the probability of the same CGA being used to refer to different entities in the Internet must be sufficiently low so that it can be ignored for all practical purposes. Even though CGA collisions are expected to be extremely rare, collisions may still happen since it is possible that two different input bitstrings within the same context may map to the same CGA. A second type of collision can happen if two input bitstrings, used in different contexts, map to the same CGA. In this case, the main confusion is about which context to use. In order to preserve a low enough probability of collisions, it is required that applications ensure that distinct input bitstrings are either unique or statistically unique within a given context. By adhering to the EPCglobal standards, this requirement is fulfilled.

3.5. Host identities and host identity protocol

A host identity is an abstract concept assigned to a computing identity platform. In this section, we will generalize this concept to cover thin compact platforms that can be equipped with RFID tags. The discussion starts by introducing the host identities and the host identity protocol [15][16].

The Host Identity Protocol (HIP) supports an architecture that decouples the transport layer (TCP, UDP, etc.) from the internetworking layer (IPv4 and IPv6) by using public/private key pairs, instead of IP addresses, as host identities [15][16]. The public keys are typically, but not necessarily, self-generated. HIP introduces a new Host Identity (HI) namespace, based on these public keys, from which end-point identifiers are taken. Host identifiers are used to bind to higher layer protocols instead of binding to IP addresses. A key benefit of this approach is that it is compatible with existing APIs such as the socket API. HIP uses existing IP addressing and forwarding for locators and packet delivery, respectively.

Figure 4 illustrates the difference between binding of the logical entities service and end-points to an IP address (left side of figure). The service typically binds to the IP stack via the socket API. By using the host identity abstraction of the HIP architecture, the service and the end-point bind to the host identity whereas the location is still anchored with the IP address.

Figure 4.

Illustration of the difference between the bindings of the logical entities. Adapted from [15].

There are two main representations of the host identity, the full Host Identifier (HI) and the Host Identity Tag (HIT). The HI is a public key and directly represents the identity. The HIT is the operational representation of a host. It has a 128-bit long representation and is used in the HIP payloads to index the corresponding state of the end hosts. By introducing an identity concept at the network layer, where every host is represented by an asymmetric key pair consisting of a public and private key, it turns IP addresses into pure locators.

The proposed HI namespace fills an important gap between the IP and DNS namespaces. A public key is used as the HIP Host Identity (HI), while the private key serves as proof of ownership of the public key. To seamlessly integrate HIP with protocols above the network layer, a 128-bit cryptographic hash of the HI, i.e., the HIT, was introduced to fit the IPv6 address space. The HIT is a statistically unique flat identifier. When HIP is used, the transport layer binds to HITs. In this process it becomes unaware of the IP addresses that are used for routing.

To be able to setup communication between peers that use HI, a light-weighted protocol exchange called the HIP Base Exchange has been specified. In Figure 5, the HIP Base Exchange is adapted to an RFID setup. The setup is somewhat similar to the one presented by Urien et al. [11].

Since the deployed tags are passive, there is a need for a proxy to act on behalf of the tags in the protocol exchange. The role of this proxy will be explained further in Section 5.

Figure 5.

HIP Base Exchange adapted to a RFID communication scenario. Adapted from [15].

The HIP Base Exchange is a four-way handshake between two hosts wanting to initiate communication (see Figure 4). The Initiate 1 packet is the first packet sent in the handshake. It is an unencrypted and unsigned packet, meaning that the Initiator would like to talk HIP with the Responder. The HIP packet contains the HIT of the Initiator (HIT_I) and the Responder (HIT_R). The responder’s IP address can be derived from the DNS. Respond 1 is sent as a reply to the Initiate 1 packet. Besides the HIT_I-HIT_R identity pair, it contains a cryptographic puzzle challenge, and Diffie-Hellman parameters (DH_R) for the Diffie-Hellman key agreement. The Diffie–Hellman key exchange method allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure communication channel. Subsequently, the secret key can be used for encryption and integrity protection of the communication channel. The purpose of the HIP puzzle mechanism is to protect the Responder from denial-of-service attacks. The Initiate 2 packet returns the corresponding Diffie-Hellman parameter (DH_I) to the Responder. It carries an encoded solution to the puzzle. Upon reception of the Initiate 2 packet, the responder can now generate the keying material, and it is capable of using it in encryption and integrity protection algorithms. The Response 2 packet completes the HIP Base Exchange. After the Base Exchange, there is no longer difference between the Initiator and Responder and data can securely be exchanged between the communicating peers.

4. Use cases and application examples

RFID applications are numerous and far reaching [8]. The most interesting and widely used applications include those for security and access control, supply chain management, and the tracking of important objects and personnel. This section outlines a number of commonly encountered use cases for RFID technology, and discusses these in the context of networked RFID tags.

4.1. Access control

Access control systems are an important part of the security of government buildings, companies, schools, residences and private areas and RFID technology has been widely adopted in access control systems. These systems often use RFID identification cards based on the IEC/ISO 14443 [18], IEC/ISO 15693 [19], or IEC/ISO 18000 standards [20]. The identification cards work much like a traditional key for unlocking doors or otherwise granting access. However, RFID technology does not provide authentication to the holder of the RFID card (or tag). Any unauthorized people holding an authorized RFID card could get access to secured area. Therefore, RFID technology should be combined with other means of identification such as e.g., face recognition to strengthen the security of the access control system.

By associating a passive RFID tag such as a key card with a globally unique IPv6 address we will be able to use access control and security policy mechanisms with Internet technologies to provide the desired access control applications. In this scenario a door locking mechanism would be connected over the Internet resulting in a more open system architecture.

4.2. Supply chain management

Most supply chain applications involve the concept of inventory tracking. An example of a proposed use of RFID is to ensure safety in the supply chain [21].

To illustrate the potential of using network RFID tags with supply chain applications an example taken from the Tag Data Standard v1.6 issue 2 [10]. The example text is quoted below:

“… a shipment arriving on a pallet may consist of a number of cases tagged with SGTIN identifiers and a returnable pallet identified by a GRAI identifier but also carrying an SSCC identifier to identify the shipment as a whole. If a portal reader at a dock door simply returns a number of binary EPCs, it is helpful to have translation software which can automatically detect which binary values correspond to which coding scheme, rather than requiring that the coding scheme and inbound representation are specified in addition to the input value.”

Each of the cases tagged will be given a unique IPv6 address when they enter the electric field of a reader. This process involves the extracting of the essential bitstring of the SGTIN identifier for each case. Likewise, the returnable pallet and the shipment as a whole will be given IPv6 addresses that can be built based on the GRAI and the SSCC, respectively. By using the assigned IPv6 unicast addresses it is possible to establish communication to individual cases as well as the pallet. However, it may be of less interest to address individual cases at this point in the supply chain but rather to address the ensemble of cases. By introducing multicasting at the network layer it can be possible to communicate with groups of cases on the pallet.

4.3. Object/asset tracking

Because moving objects can easily carry RFID tags, a common use is to track the movement of people and the information associated with them. By associating a particular tag’s EPC with a global network address the task of tracking the object/asset become equivalent to locating a mobile host in the network. In general, this is a key challenge in mobility research and several solutions have been proposed [22][26], and this will be the subject of our discussion in Section 7. Another interesting use case can be applied to sensor-tags. When these sensor-tags connect to a network sensor data can be retrieved from the tag.

5. Networked RFID testbed

To study the internetworking of objects with passive RFID tags, a simple testbed has been built. The approach makes use of an RFID reader and an application that works as a proxy for the tags we wish to communicate with. The proxy is capable of making a virtual representation of the passive RFID tag on the Internet by creating a Virtual Network Interface (VNI) with an IPv6 address that can be attributed to each tag that comes within the electric field of a reader. Hence, the tags do not terminate IPv6 traffic directly but merely communicate with an entity which represents the tag (physical object) that we wish to communicate with.

The approach taken is software-oriented. The application runs as standalone but it can be embedded on the reader or it can be run on a computer local to the reader. The application receives the EPC of a tag attached to a physical object via the reader. The application then creates an IPv6 address from one of the mentioned methods. Hereafter, it is possible to route IP traffic to the particular Internet end-point. This will in effect make the application act as a proxy that for example can keep the most recently read tags “online”. The solution gives a one-to-one mapping of physical objects to the virtual representations that are needed to communicate over the Internet.

Figure 6 illustrates the system implemented. In practice, the application host has a predetermined number of Virtual Network Interfaces (VNIs) installed. These interfaces work as the online virtual representation of the tag swiped at the reader. In other words, this is the interface the outside world can contact. In the testbed, the network interfaces are virtualized in a way similar to a loopback interface [17]. As the system works as a testbed the database is merely there as a logging service. In the future, it is planned to use the database as foundation for a local ONS. The corresponding node is there to illustrate possible communication over the Internet.

Figure 6.

Simple setup to give RFID tags virtual identification on the Internet. A RedBee RFID Reader v1.1 is used. The application is built on the Microsoft^®.NET connection software.

Figure 7 shows a state machine diagram for a single VNI resulting from a tag swiped in an access control application.

When the tag is swiped at the reader, the application host creates an IPv6 address by combining the network prefix configured at the reader with the tags identity as illustrated by the Example in Table 1. In the initial state, the software is waiting for a TagSwipe event to occur. Subsequently, the interface is put online with the address constructed, and it is kept alive as long the expiration time is greater than 0 (zero) seconds.

Tags are only reachable while they are within reader range. This makes it hard to communicate with the real tag, simply because it is only reachable for a short duration of time. When the tag’s attachment to the network is virtualized it is possible to set up an expiration value. This value effectively serves as the time the tags virtual representation on the network can be reached.

The tag identity together with the constructed IPv6 address and a timestamp is stored on the database. Table 1 shows an example of the steps taken to construct an IPv6 address from an EM4100 tag ID.

Figure 7.

State machine for the virtual interface resulting from a TagSwipe event at the reader.

Tag identity (Example with EM4100 tag)	5 decimal numbers (40 bits)	127 0 58 207 19
Binary ID representation with left-zero-padding	64 bits	0000 0000 0000 0000 0000 0000 0111 1111 0000 0000 0011 1010 1100 1111 0001 0011
Converted to hexadecimal	4 groups of 4 hex. digits	0000 007f 003a cf13
Network prefix of RFID reader (example)	4 groups of 4 hex. digits	2001:16d8:dd92:aaaa::/64
Unicast IPv6 address associated with the tag	8 groups of 4 hex. digits	2001:16d8:dd92:aaaa::007f:003a:cf13/128

Table 1.

Example of IPv6 network address construction from on EM4100 tag ID.

The application has no visual interface and all configurations must be done in software. For example, it is possible to use more than one virtual interface to represent the tags online. These interfaces need to be preinstalled, as already mentioned, and some parameters in the application need to be configured. Hereafter, it is possible to make use of at least 5 virtual interfaces.

Although focus is on the assignment of IPv6 unicast addresses, tags can also be assigned to become member of multicast groups thereby facilitating one-to-many communication. As an example an application may want to address all tags at a particular reader. Likewise, readers can become members of multicast groups hereby enabling communication to all readers in the multicast group e.g., a particular logical area. Details on how to operate an RFID in IPv6 multicast networks are beyond the scope of this chapter.

6. Strategies for interworking IPv6 with RFID tags

In this Section we will discuss different methods of mapping between an RFID namespace and an Internet namespace.

6.1. Address mappings

The most simple approach to find IPv6 addresses for tags is the mapping of the tag ID to an IPv6 address, i.e., the bits from the ID are used to form the IP address [14]. As different passive RFID standards exist there is no common ID structure for tags. Even within standards, there are different types of IDs with different structures. This means, that a general concept to map tag IDs to IPv6 addresses will not work.

Table 2 shows a list of some commonly encountered passive tags and their ID formats.

IC type	Frequency band	Memory	Standards compliance
EM4100 series	LF (125 KHz)	64 bits	EM4100. Proprietary standard issued by EM Microelectronics [28].
EM4450/4550	LF (125 KHz)	1024 bits	EM4450/4550. Proprietary standard issued by EM Microelectronics [29].
NXP Hitag family (Hitag 1, Hitag 2, Hitag S, Hitag μ)	LF (125 KHz)	256 bit to 2048 bit	IEC/ISO 18000-2 (Hitag μ) [20].
NXP Mifare family (Ultralight/ MF1S20/MF1S50/MF1S70/ DESFire EV1)	HF (13,56 MHz)	64 bytes to 4096 bytes; 2K/4K/8K	ISO 14443A [18].
LEGIC Advant family (ATC128, ATC256, ATC1024, ATC2048, ATC4096)	HF (13,56 MHz)	128 bytes to 4096 bytes	IEC/ISO 15693 [19] (ATC128, ATC256, ATC1024), IEC/ISO 14443 A [18] (ATC 2048, ATC 4096).
NXP UCode HSL	UHF (868 MHz or 915 MHz)	2048 bit	ISO18000-4 and 18000-6B [20].
NXP UCode EPC Gen2	UHF (868 MHz or 915 MHz)	512 bit	EPCglobal class 1 gen2 and ISO 18000-6C [20].

Table 2.

Common passive RFID tag and their characteristics.

An EPC with the length of 64 bits maps well in the IPv6 address format and can result in globally unique addresses. With longer EPCs it is impossible to map the EPC directly into the IPv6 address space and here specialized functions are needed. One solution would be to simply hash the longer EPC's into a length of 64 bits and then use the direct mapping method again. The hashing technique used to derive identifiers was described in Section 3.4, when the CGA namespace was introduced. Another method would be to identify if there are some bits in the longer EPC's that can be removed without affecting the uniqueness property of the tags.

A key benefit of the proposed solution is that there is no need to change the design of existing RFID technology with its EPC namespace conventions. The application can be installed on a computer connected to the reader, and then all objects with RFID tags that pass this reader will put the objects online and thereby giving them the ability to communicate over the Internet as long as the tag is within range of a reader.

Table 3.

Overview of strategies for mapping Tag ID codes to IPv6 network addresses.

Table 3 outlines the different strategies for mapping of tag IDs to IPv6 addresses. Essentially, these divide into methods that work with tags of 64-bit identification or less and tags that use more that 64-bit for identification.

7. Mobility considerations

One of the largest challenges for a dynamic, networked system lies within the mobility support of the network. In the case described here, we consider a system of fixed readers that are connected in a common network infrastructure. Mobility arises when tags are moved between readers. Readers will be wired or wireless and they will have different communication ranges according to their MAC technology. Moreover, they will forward the read tag IDs to the server through the common network infrastructure.

When a tag moves from one reader to another, the network prefix will change but the host suffix/interface ID will still match the tag's EPC. The tag will in effect change its network address every time it passes a new reader. Hence, the challenge is to effectively keep track of tags when the address changes this rapidly.

There are basically two distinct ways to solve the mobility problem. One is a centralized approach, such as mobile IPv6 [30], where a central server, i.e., the home agent, is used to keep track of the mobile hosts that move around in the world. The mobile IPv6 architecture relies on the concept of a home agent and a care-of address. The method is based on some software on the network layer that can send messages to the home agent making sure that the home agent is holding an updated address list at all times. Initially, traffic destined to the mobile host is routed to the home network and subsequently tunneled to the foreign network that the host is visiting. Fortunately, IPv6 supports mechanisms to circumvent the triangular routing problem that arises in this setup [30].

Dominikus et al. [14], proposed to use mobile IPv6 to handle the mobility of IPv6-enabled tags. In their approach, the care-of address refers to the subnet of the RFID reader, where the tag is currently present. Whilst the care-of address is a globally unique address assigned to the host, i.e., the tag visiting a foreign network, the home agent address is specific to the enterprise using the issued tags.

Alternatively, mobility support can be obtained in a more distributed way by separating location and identity information. This can be achieved by using the HIP approach [22]. In this approach, there is a need to compute the routable IPv6 address from the given non-routable HIT the host has been given.

HIP allows consenting hosts to securely establish and maintain shared IP-layer state, allowing separation of the identifier and locator roles of IP addresses, thereby enabling continuity of communications across IP address changes. A consequence of such a decoupling is that new solutions to network-layer mobility and host multi-homing are possible [22].

8. Conclusions

Metcalfe’s law states that the value of a telecommunications network is proportional to the square of the number of connected users of the system. When the law is applied to a network of objects on the scale predicted by the vision of the Internet of Things it is clear that a single, open architecture for networking physical objects is much more valuable than small scale and fragmented alternatives.

RFID plays an increasingly important in our daily life from management of goods, e-tickets, healthcare, transports, even the identity cards are embedded with RFID tags. In this chapter, we have sketched methods on how to use RFID technology to connect “things” over the Internet by using IPv6. This includes a discussion on the different strategies for mapping of tag IDs to globally unique IPv6 addresses.

For tags with large identification numbers (more than 64 bits) it is proposed to use cryptographic techniques to extract the 64 bits and use these to create a host suffix that is statistically unique.

A testbed used to experiment with the internetworking of low-cost, passive RFID tags to the Internet has been presented. Since these tags do not have electrical and processing power to run an IP protocol stack a virtual network interface (VNI) concept has been introduced. Proxies can be deployed on the edge of the Internet to act on behalf of these passive tags in a protocol message exchange.

To solve the mobility problem, two approaches have been discussed: one being the mobile IPv6 approach and the other being the HIP approach. Both have strengths and both have weaknesses. Mobile IPv6 will need some software to make the connection between the tags and the home agent. The HIP approach needs some computation to take place in order to be able to construct routable IPv6 addresses. Both approaches imply changes to be made to the Internet, as we know it today, before it is possible to effectively achieve the desired results.

Most RFID applications today include mobility as an essential part of their value creation. Therefore, future research in this area must focus on mobility aspects of the Internet of Things.