30. 6LoWPAN: Transmission of IPv6 Packets over IEEE 802.15.4 Networks

This chapter describes the implementation of ns-3 model for the compression of IPv6 packets over IEEE 802.15.4-Based Networks as specified by RFC 4944 (“Transmission of IPv6 Packets over IEEE 802.15.4 Networks”) and RFC 6282 (“Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks”).

30.1. Model Description

The source code for the sixlowpan module lives in the directory src/sixlowpan.

30.1.1. Design

The model design does not follow strictly the standard from an architectural standpoint, as it does extend it beyond the original scope by supporting also other kinds of networks.

Other than that, the module strictly follows RFC 4944 and RFC 6282, with the exception that HC2 encoding is not supported, as it has been superseded by IPHC and NHC compression type (RFC 6282).

IPHC sateful (context-based) compression is supported but, since RFC 6775 (“Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)”) is not yet implemented, it is necessary to add the context to the nodes manually.

This is possible though the SixLowPanHelper::AddContext function. Mind that installing different contexts in different nodes will lead to decompression failures. NetDevice

The whole module is developed as a transparent NetDevice, which can act as a proxy between IPv6 and any NetDevice (the module has been successfully tested with PointToPointNedevice, CsmaNetDevice and LrWpanNetDevice).

For this reason, the module implements a virtual NetDevice, and all the calls are passed without modifications to the underlying NetDevice. The only important difference is in GetMtu behaviour. It will always return at least 1280 bytes, as is the minimum IPv6 MTU.

The module does provide some attributes and some tracesources. The attributes are:

  • Rfc6282 (boolean, default true), used to activate HC1 (RFC 4944) or IPHC (RFC 6282) compression.
  • OmitUdpChecksum (boolean, default true), used to activate UDP checksum compression in IPHC.
  • FragmentReassemblyListSize (integer, default 0), indicating the number of packets that can be reassembled at the same time. If the limit is reached, the oldest packet is discarded. Zero means infinite.
  • FragmentExpirationTimeout (Time, default 60 seconds), being the timeout to wait for further fragments before discarding a partial packet.
  • CompressionThreshold (unsigned 32 bits integer, default 0), minimum compressed payload size.
  • ForceEtherType (boolean, default false).
  • EtherType (unsigned 16 bits integer, default 0xFFFF), to force a particular L2 EtherType.
  • UseMeshUnder (boolean, default false), it enables mesh-under flood routing.
  • MeshUnderRadius (unsigned 8 bits integer, default 10), the maximum number of hops that a packet will be forwarded.
  • MeshCacheLength (unsigned 16 bits integer, default 10), the length of the cache for each source.
  • MeshUnderJitter (ns3::UniformRandomVariable[Min=0.0|Max=10.0]), the jitter in ms a node uses to forward mesh-under packets - used to prevent collisions.

The CompressionThreshold attribute is similar to Contiki’s SICSLOWPAN_CONF_MIN_MAC_PAYLOAD option. If a compressed packet size is less than the threshold, the uncompressed version is used (plus one byte for the correct dispatch header). This option is useful when a MAC requires a minimum frame size (e.g., ContikiMAC) and the compression would violate the requirement.

The last two attributes are needed to use the module with a NetDevice other than 802.15.4, as neither IANA or IEEE did reserve an EtherType for 6LoWPAN. As a consequence there might be a conflict with the L2 multiplexer/demultiplexer which is based on EtherType. The default value is 0xFFFF, which is reserved by IEEE (see [IANA802] and [Ethertype]). The default module behaviour is to not change the EtherType, however this would not work with any NetDevice actually understanding and using the EtherType.

Note that the ForceEtherType parameter have also a direct effect on the MAC address kind the module is expecting to handle: * ForceEtherType true: Mac48Address (Ethernet, WiFi, etc.). * ForceEtherType false: Mac16Address or Mac64Address (IEEE 802.15.4).

Note that using 6LoWPAN over any NetDevice other than 802.15.4 will produce valid .pcap files, but they will not be correctly dissected by Wireshark. The reason lies on the fact that 6LoWPAN was really meant to be used only over 802.15.4, so Wireshark dissectors will not even try to decode 6LoWPAN headers on top of protocols other than 802.15.4.

The Trace sources are:

  • Tx - exposing packet (including 6LoWPAN header), SixLoWPanNetDevice Ptr, interface index.
  • Rx - exposing packet (including 6LoWPAN header), SixLoWPanNetDevice Ptr, interface index.
  • Drop - exposing DropReason, packet (including 6LoWPAN header), SixLoWPanNetDevice Ptr, interface index.

The Tx and Rx traces are called as soon as a packet is received or sent. The Drop trace is invoked when a packet (or a fragment) is discarded. Mesh-Under routing

The module provides a very simple mesh-under routing [Shelby], implemented as a flooding (a mesh-under routing protocol is a routing system implemented below IP).

This functionality can be activated through the UseMeshUnder attribute and fine-tuned using the MeshUnderRadius and MeshUnderJitter attributes.

Note that flooding in a PAN generates a lot of overhead, which is often not wanted. Moreover, when using the mesh-under facility, ALL the packets are sent without acknowledgment because, at lower level, they are sent to a broadcast address.

At node level, each packet is re-broadcasted if its BC0 Sequence Number is not in the cache of the recently seen packets. The cache length (by default 10) can be changed through the MeshCacheLength attribute.

30.1.2. Scope and Limitations Context-based compression

IPHC sateful (context-based) compression is supported but, since RFC 6775 (“Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)”) is not yet implemented, it is necessary to add the context to the nodes manually. 6LoWPAM-ND

Future versions of this module will support RFC 6775, however no timeframe is guaranteed. Mesh-under routing

It would be a good idea to improve the mesh-under flooding by providing the following:

  • Adaptive hop-limit calculation,
  • Adaptive forwarding jitter,
  • Use of direct (non mesh) transmission for packets directed to 1-hop neighbors. Mixing compression types in a PAN

The IPv6/MAC addressing scheme defined in RFC 6282 and RFC 4944 is different. One adds the PanId in the pseudo-MAC address (4944) and the other doesn’t (6282).

The expected use cases (confirmed by the RFC editor) is to never have a mixed environment where part of the nodes are using HC1 and part IPHC because this would lead to confusion on what the IPv6 address of a node is.

Due to this, the nodes configured to use IPHC will drop the packets compressed with HC1 and vice-versa. The drop is logged in the drop trace as DROP_DISALLOWED_COMPRESSION. Using 6LoWPAN with IPv4 (or other L3 protocols)

As the name implies, 6LoWPAN can handle only IPv6 packets. Any other protocol will be discarded. Moreover, 6LoWPAN assumes that the network is uniform, as is all the devices connected by the same same channel are using 6LoWPAN. Mixed environments are not supported by the standard. The reason is simple: 802.15.4 frame doesn’t have a “protocol” field. As a consequence, there is no demultiplexing at MAC layer and the protocol carried by L2 frames must be known in advance.

In the ns-3 implementation it is possible, but not advisable, to violate this requirement if the underlying NetDevice is capable of discriminating different protocols. As an example, CsmaNetDevice can carry IPv4 and 6LoWPAN at the same time. However, this configuration has not been tested.

30.1.3. References

[IANA802]IANA, assigned IEEE 802 numbers: http://www.iana.org/assignments/ieee-802-numbers/ieee-802-numbers.xml
[Ethertype]IEEE Ethertype numbers: http://standards.ieee.org/develop/regauth/ethertype/eth.txt
  1. Shelby and C. Bormann, 6LoWPAN: The Wireless Embedded Internet. Wiley, 2011. [Online]. Available: https://books.google.it/books?id=3Nm7ZCxscMQC

30.2. Usage

30.2.1. Enabling sixlowpan

Add sixlowpan to the list of modules built with ns-3.

30.2.2. Helper

The helper is patterned after other device helpers.

30.2.3. Examples

The following example can be found in src/sixlowpan/examples/:

  • example-sixlowpan.cc: A simple example showing end-to-end data transfer.

In particular, the example enables a very simplified end-to-end data transfer scenario, with a CSMA network forced to carry 6LoWPAN compressed packets.

30.2.4. Tests

The test provided checks the connection between two UDP clients and the correctness of the received packets.

30.3. Validation

The model has been validated against WireShark, checking whatever the packets are correctly interpreted and validated.