1.1.1 Why 6LoWPAN?
There are a huge range of applications which could benefit from a Wireless Embedded Internet approach. Today these applications are implemented using a wide range of proprietary technologies which are difficult to integrate into larger networks and with Internet-based services. The benefits of using Internet protocols in these applications, and thus integrating them with the Internet of Things include [RFC4919]:
- IP-based devices can be connected easily to other IP networks without the need for translation gateways or proxies.
- IP networks allow the use of existing network infrastructure.
- IP-based technologies have existed for decades, are very well known, and have been proven to work and scale. The socket API (application programming interface) is one of the most well-known and widely used APIs in the world.
- IP technology is specified in an open and free way, with standards processes and documents available to anyone. The result is that IP technology encourages innovation and is better understood by a wider audience.
- Tools for managing, commissioning and diagnosing IP-based networks already exist (although many management protocols need optimization for direct use with 6LoWPAN Nodes as we will discuss in Chapter 5).
Until now only powerful embedded devices and networks have been able to participate natively with the Internet. Direct communication with traditional IP networks requires many Internet protocols, often requiring an operating system to deal with the complexity and maintainability. Traditional Internet protocols are demanding for embedded devices for the following reasons:
- Security: IPv6 includes optional support for IP Security (IPsec) [RFC4301] authentication and encryption, and web services typically make use of secure sockets or transport layer security mechanisms. These techniques may be too complex, especially for simple embedded devices.
- Web services: Internet services today rely on web-services, mainly using the transmission control protocol (TCP), HTTP, SOAP and XML with complex transaction patterns.
- Management: Management with the simple network management protocol (SNMP) and web-services is often inefficient and complex.
- Frame size: Current Internet protocols require links with sufficient frame length (minimum of 1280 bytes for IPv6), and heavy application protocols require substantial bandwidth.
These requirements have in practice limited the Internet of Things to devices with a powerful processor, an operating system with a full TCP/IP stack, and an IP-capable communication link. Typical embedded Internet devices today include industrial devices with Ethernet interfaces, M2M gateways with cellular modems, and advanced smart phones. A large majority of embedded applications involve limited devices, with low-power wireless and wired network communications. Wireless embedded devices and networks are particularly challenging for Internet protocols:
- Power and duty-cycle: Battery-powered wireless devices need to keep low duty cycles (the percentage of time active). The basic assumption of IP is that a device is always connected.
- Multicast: Wireless embedded radio technologies, such as IEEE 802.15.4, do not typically support multicast, and flooding in such a network is wasteful of power and bandwidth. Multicast is crucial to the operation of many IPv6 features.
- Mesh topologies: The applications of wireless embedded radio technology typically benefit from multihop mesh networking to achieve the required coverage and cost efficiency. Current IP routing solutions may not easily be applicable to such networks (discussed at length in Chapter 4).
- Bandwidth and frame size: Low-power wireless embedded radio technology usually has limited bandwidth (on the order of 20–250 kbit/s) and frame size (on the order of 40– 200 bytes). In mesh topologies, bandwidth further decreases as the channel is shared and is quickly reduced by multihop forwarding. The IEEE 802.15.4 standard has a 127- byte frame size, with layer-2 payload sizes as low as 72 bytes. The minimum frame size for standard IPv6 is 1280 bytes [RFC2460], thus requiring fragmentation.
- Reliability: Standard Internet protocols are not optimized for low-power wireless networks. For example, TCP is not able to distinguish between packets dropped because of congestion or packets lost on wireless links. Further unreliability occurs in wireless embedded networks because of node failure, energy exhaustion and sleep duty cycles.
The IETF 6LoWPAN working group [6LoWPAN] was created to tackle these problems, and to specifically enable IPv6 to be used with wireless embedded devices and networks. Features of the IPv6 design such as a simple header structure, and its hierarchical addressing model, made it ideal for use in wireless embedded networks with 6LoWPAN.
Additionally, by creating a dedicated group of standards for these networks, the minimum requirements for implementing a lightweight IPv6 stack with 6LoWPAN could be aligned with the most minimal devices. Finally by designing a version of Neighbor Discovery (ND) specifically for 6LoWPAN, the particular characteristics of low-power wireless mesh networks could be taken into account.
The result of 6LoWPAN is the efficient extension of IPv6 into the wireless embedded domain, thus enabling end-to-end IP networking and features for a wide range of embedded applications. Refer to [RFC4919] for the detailed assumptions, problem statement and goals of early 6LoWPAN standardization. Although 6LoWPAN was targeted originally at IEEE 802.15.4 radio standards and assumed layer-2 mesh forwarding [RFC4944], it was later generalized for all similar link technologies, with additional support for IP routing in [ID-6lowpan-hc, ID-6lowpan-nd].
Coming up in Part 2: 6LoWPAN history, applications and its relation to other trends.
Printed with permission from John Wiley & Sons, Ltd. Copyright 2009. "6LoWPAN: The Wireless Embedded Internet" by Zach Shelby and Carsten Bormann, ISBN: 978-0-470-74799-5. For more information about this title and other similar books, please visit John Wiley & Sons.
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