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Understanding 6LoWPAN: The wireless embedded Internet (Part 4)

Posted: 03 Oct 2011     Print Version  Bookmark and Share

Keywords:Wireless Embedded Internet  IPv6  RFC2464 

IP addressing with 6LoWPAN works just like in any IPv6 network, and is similar to addressing over Ethernet networks as defined by [RFC2464]. IPv6 addresses are usually formed automatically from the prefix of the LoWPAN and the link-layer address of the wireless interfaces. The difference in a LoWPAN is with the way low-power wireless technologies support link-layer addressing; a direct mapping between the link-layer address and the IPv6 address is used for achieving compression. This will be explained in another section.

Low-power wireless radio links typically make use of flat link-layer addressing for all devices, and support both unique long addresses (e.g. EUI-64) and configurable short addresses (usually 8–16 bits in length). The IEEE 802.15.4 standard, for example, supports unique EUI-64 addresses carried in all radio chips, along with configurable 16bit short addresses. These networks by nature also support broadcast (address 0xFFFF in IEEE 802.15.4), but do not support native multi-cast.

IPv6 addresses are 128 bits in length, and (in the cases relevant here) consist of a 64bit prefix part and a 64bit interface identifier (IID) [RFC4291]. Stateless address autoconfiguration (SAA) [RFC4862] is used to form the IPv6 interface identifier from the link-layer address of the wireless interface as per [RFC4944]. For simplicity and compression, 6LoWPAN networks assume that the IID has a direct mapping to the link layer address, therefore avoiding the need for address resolution. The IPv6 prefix is acquired through Neighbour Discovery Router Advertisement (RA) messages [ID-6lowpan-nd] as on a normal IPv6 link.

The construction of IPv6 addresses in 6LoWPAN from known prefix information and known link-layer addresses, is what allows a high header compression ratio. A reference for IPv6, including the IPv6 addressing model, is provided in Appendix A.

Header format
The main functionality of 6LoWPAN is in its LoWPAN adaptation layer, which allows for the compression of IPv6 and following headers such as UDP along with fragmentation and mesh addressing features. 6LoWPAN headers are defined in [RFC4944] which has been later improved and extended by [ID-6lowpan-hc]. 6LoWPAN compression is stateless, and thus very simple and reliable. It relies on shared information known by all nodes from their participation in that LoWPAN, and the hierarchical IPv6 address space which allows IPv6 addresses to be elided completely most of the time.

6LoWPAN header compression

Figure 1: Here's an example of 6LoWPAN header compression (L = LoWPAN header).

The LoWPAN header consists of a dispatch value identifying the type of header, followed by an IPv6 header compression byte indicating which fields are compressed, and then any in-line IPv6 fields. If, for example, UDP or IPv6 extension headers follow IPv6, then these headers may also be compressed using what is called next-header compression [ID-6lowpan-hc]. An example of 6LoWPAN compression is given in figure 1. In the upper packet a one-byte LoWPAN dispatch value is included to indicate full IPv6 over IEEE 802.15.4.

Figure 2 gives an example of 6LoWPAN/UDP in its simplest form (equivalent to the lower packet in figure 1), with a dispatch value and IPv6 header compression (LOWPAN_IPHC) as per [ID-6lowpan-hc] (2B), all IPv6 fields compressed, then followed by a UDP next-header compression byte (LOWPAN_NHC) with compressed source and destination port fields and the UDP checksum (4B). Therefore in the likely best case the 6LoWPAN/UDP header is just 6B in length.

 compressed headers

Figure 2: 6LoWPAN/UDP compressed headers (6B).

By comparison a standard IPv6/UDP header is 48B in length as shown in figure 3. Considering that in the worst case IEEE 802.15.4 has only 72B of payload available after link-layer headers, compression is important. The 6LoWPAN format and features are described in another section.

 IPv6/UDP headers

Figure 3: Standard IPv6/UDP headers (48B).

Bootstrapping, mesh topologies
Applications of 6LoWPAN most often involve completely autonomous devices and networks, which must autoconfigure themselves without human intervention. Bootstrapping first needs to be performed by the link layer, in order to enable basic communication between nodes within radio range. Basic link layer configuration usually involves the channel setting, default security key and address settings. Once the link layer is functioning and single-hop communications between devices is possible, 6LoWPAN Neighbour Discovery [ID-6lowpan-nd] is used to bootstrap the whole LoWPAN.

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