6LoWPAN Paging DispatchCisco SystemsBuilding D - Regus45 Allee des OrmesBP1200MOUGINS - Sophia Antipolis06254FRANCE+33 4 97 23 26 34pthubert@cisco.comARM Ltd.110 Fulbourn RoadCambridgeCB1 9NJUKrobert.cragie@gridmerge.com
Internet
6loThis specification updates RFC 4944 to introduce a new context switch
mechanism for 6LoWPAN compression, expressed in terms of Pages and signaled
by a new 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 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 extends the Dispatch type field registry that was created for
and updated by the , by adding
a new column called "Page".
This document defines 16 Pages, "Page 0" to "Page 15".
The content of the incumbent registry is assigned to "Page 0".
This document also places in the registry associated to Page 1 the
Dispatch type field values that are allocated for LOWPAN_IPHC by
. These values range from 01 100000 through
01 111111 and have the same definition in Page 1 as they do in Page 0;
as a result, the registry entries for Page 0 and Page 1 are an exact
overlap in this range.
The resulting registry may be represented as a table as follow (partial):
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.
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.IEEE std. 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area NetworksIEEE standard for Information Technology