6LoWPAN Paging Dispatch

The design of Low Power and Lossy Networks (LLNs) is generally focused on saving energy, which often is a very constrained resource. Other constraints, such as memory capacity and duty cycle restrictions on LLN devices, usually derive from that primary concern. Energy is often available only from primary batteries that are expected to last for years, or is scavenged from the environment in very limited amounts. Any protocol that is intended for use in LLNs must be designed with a primary focus on saving energy, which is a strict requirement. Controlling the amount of data transmission is one possible means of saving energy. In a number of LLN standards, the frame size is limited to much smaller values than the IPv6 maximum transmission unit (MTU) of 1280 bytes. In particular, an LLN that relies on the classical Physical Layer (PHY) of IEEE 802.15.4 is limited to 127 bytes per frame. The need to compress IPv6 packets over IEEE 802.15.4 led to the 6LoWPAN Header Compression work (6LoWPAN-HC). As more and more protocols need to be compressed, the encoding capabilities of the original dispatch defined in the 6lo adaptation layer framework (,) becomes saturated. This specification introduces a new context switch mechanism for 6LoWPAN compression, expressed in terms of Pages and signaled by a new Paging Dispatch mechanism.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . The Terminology used in this document is consistent with and incorporates that described in Terms Used in Routing for Low-Power and Lossy Networks and Terminology for Constrained-Node Networks .

This draft adapts 6LoWPAN while maintaining backward compatibility with IPv6 over IEEE 802.15.4 by introducing a concept of a "parsing context" in the 6LoWPAN parser, a context being identified by a Page Number. This specification defines 16 Pages. Pages are delimited in a 6LoWPAN packet by a Paging Dispatch value that indicates the next current Page. The Page Number is encoded in a Paging Dispatch with the Value Bit Pattern of 1111xxxx where xxxx is the Page Number, 0 to 15, as described in :

Values of the Dispatch byte defined in are considered as belonging to the Page 0 parsing context, which is the default and does not need to be signaled explicitly at the beginning of a 6LoWPAN packet. This ensures backward compatibility with existing implementations of 6LoWPAN. The Dispatch bits defined in Page 0 by are free to be reused in Pages 1 to 15. This specification allocates some values in Page 1 in and leaves the rest open for future allocations. Note: This specification does not use the Escape Dispatch, which extends Page 0 to more values, but rather allocates another Dispatch Bit Pattern (1111xxxx) for a new Paging Dispatch, that is present in all Pages, including Page 0 and Pages defined in future specifications, to indicate the next parsing context represented by its Page Number. The rationale for avoiding that approach is that there can be multiple occurrences of a new header indexed by this specification in a single frame and the overhead on an octet each time for the Escape Dispatch would be prohibitive. A Page (say Page N) is is said to be active once the Page N Paging Dispatch is parsed, and as long as no other Paging Dispatch is parsed.

This specification defines some special properties for Page 1, detailed below: The Dispatch bits defined for LOWPAN_IPHC by the Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks are defined with the same values in Page 1 so there is no need to switch context from Page 1 to Page 0 to decode a packet that is encoded per . Mesh Headers represent Layer-2 information and are processed before any Layer-3 information that is encoded in Page 1. If a 6LoWPAN packet requires a Mesh header, the Mesh Header MUST always be placed in the packet before the first Page 1 Paging Dispatch, if any. For the same reason, Fragment Headers as defined in MUST always be placed in the packet before the first Page 1 Paging Dispatch, if any. The NALP Dispatch Bit Pattern as defined in is only defined for the first octet in the packet. Switching back to Page 0 for NALP inside a 6LoWPAN packet does not make sense. As a result, there is no need so far for restoring the Page 0 parsing context after a context was switched to Page 1, so the value for the Page 0 Paging Dispatch of 11110000 may not actually occur in those packets that adhere to 6LoWPAN specifications available at the time of writing this specification.

The security considerations of and apply.

This document allocates 16 values from the Dispatch type field registry that was created for . The allocated values are from 11 110000 through 11 111111 and represent Page Numbers 0 through 15 as discussed in this document.

This document creates 15 new IANA registries for the Per-Page Dispatch type fields, indexed by Page Number, 1 to 15. Each Registry corresponds to a bit-field of one octet. Future assignments in these registries are to be coordinated via IANA under the policy of "Specification Required" . It is expected that this policy will allow for other (non-IETF) organizations to more easily obtain assignments. These registries extend the Dispatch type field registry that was created for , which is considered as the registry for Page 0. As described above, this document allocates in the registry associated to Page 1 the Per-Page Dispatch type field values that are allocated in the Dispatch type field for LOWPAN_IPHC by . Those values are from 01 100000 through 01 111111 and they have the same definition in Page 1 as they do in Page 0, meaning that the registries for Page 0 and Page 1 are an exact overlap in this range.

The authors wish to thank Tom Phinney, Thomas Watteyne, Tengfei Chang, Martin Turon, James Woodyatt, Samita Chakrabarti, Jonathan Hui, Gabriel Montenegro and Ralph Droms for constructive reviews to the design in the 6lo Working Group.