A compression mechanism for the RPL option
draft-thubert-6lo-rpl-nhc-00
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| Last updated | 2014-08-28 | ||
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draft-thubert-6lo-rpl-nhc-00
6lo P. Thubert, Ed.
Internet-Draft Cisco
Updates: 6282 (if approved) August 28, 2014
Intended status: Standards Track
Expires: February 27, 2015
A compression mechanism for the RPL option
draft-thubert-6lo-rpl-nhc-00
Abstract
This document proposes a compression mechanism for the RPL option.
This operation saves up to 48 bits in each frame compared to RFC
6553.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 27, 2015.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. On Wasted Resources . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Updating RFC 6282 . . . . . . . . . . . . . . . . . . . . . . 4
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5. New RPL Next Header Compression . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The emergence of radio technology enabled a large variety of new
types of devices to be interconnected, at a very low marginal cost
compared to wire, at any range from Near Field to interplanetary
distances, and in circumstances where wiring would be less than
practical, for instance rotating devices.
In particular, IEEE802.14.5 [IEEE802154] that is chartered to specify
PHY and MAC layers for radio Lowpower Lossy Networks (LLNs), defined
the TimeSlotted Channel Hopping [I-D.ietf-6tisch-tsch] (TSCH) mode of
operation as part of the IEEE802.15.4e MAC specification in order to
address Time Sensitive applications.
The 6TiSCH architecture [I-D.ietf-6tisch-architecture] specifies
the operation IPv6 over TSCH wireless networks attached and
synchronized by backbone routers.
With 6TiSCH, the route Computation may be achieved in a centralized
fashion by a Path Computation Element (PCE), in a distributed fashion
using the Routing Protocol for Low Power and Lossy Networks
[RFC6550] (RPL), or in a mixed mode.
6TiSCH was created to simplify the adoption of IETF technology by
other Standard Defining Organizations (SDOs), in particular in the
Industrial Automation space, which already relies on variations of
IEEE802.15.4e TSCH for Wireless Sensor Networking.
ISA100.11a [ISA100.11a] (now IEC62734) is an example of such
industrial WSN standard, using IEEE802.15.4e over the classical
IEEE802.14.5 PHY. In that case, after security is applied, roughly 80
octets are available per frame for IP and Payload. In order to 1)
avoid fragmentation and 2) conserve energy, the SDO will scrutinize
any bit in the frame and reject any waste.
The challenge to obtain the adoption of IPv6 in the original standard
was really to save any possible bit in the frames, including the UDP
checksum which was an interesting discussion on its own. This work
was actually one of the roots for the 6LoWPAN Header Compression
[RFC6282] work, which goes down to the individual bits to save space
in the frames for actual data, and allowed ISA100.11a to adopt IPv6.
In order to get an SDO such as ISA100 to adopt RPL and 6TiSCH, it is
mandatory to maintain the same degree to requirement and maximize the
compression of all possible protocol information, and in particular
the overhead that RPL imposes on all packets.
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2. On Wasted Resources
The design of Lowpower Lossy Networks is generally focussed on saving
energy, which is the most constrained resource of all. The other
constraints, such as the memory capacity and the duty cycling of the
LLN devices, derive from that primary concern. Energy is typically
available from batteries that are expected to last for years, or
scavenged from the environment in very limited quantities. Any
protocol that is intended for use in LLNs must be designed with the
primary concern of saving energy as a strict requirement.
The Routing Protocol for Low Power and Lossy Networks (RPL)
[RFC6550] specification defines a generic Distance Vector protocol
that is indeed designed for very low energy consumption and adapted
to a variety of LLNs. RPL forms Destination Oriented Directed
Acyclic Graphs (DODAGs) which root often acts as the Border Router to
connect the RPL domain to the Internet. The root is responsible to
select the RPL Instance that is used to forward a packet coming from
the Internet into the RPL domain and set the related RPL information
in the packets.
A classical RPL implementation will use the RPL Option for Carrying
RPL Information in Data-Plane Datagrams [RFC6553] to tag a packet
with the Instance ID and other information that RPL requires for its
operation within the RPL domain. In particular, the Rank, which is
the scalar metric computed by an specialized Objective Function such
as [RFC6552], is modified at each hop and allows to validate that the
packet progresses in the expected direction each upwards or downwards
in along the DODAG.
------+--------- ^
| Internet |
| | Native IPv6
+-----+ |
| | Border Router (RPL Root) ^ | ^
| | | | |
+-----+ | | | IPv6 +
| | | | HbH
o o o o | | | headers
o o o o o o o o o | | |
o o o o o o o o o o | | |
o o o o o o o o o | | |
o o o o o o o o v v v
o o o o
LLN
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With [RFC6553], the RPL option is encoded as 6 Octets; it must be
placed in a Hop-by-Hop header that represents 2 additional octets for
a total of 8. In order to limit its range to the inside the RPL
domain, the Hop-by-Hop header must be added to (or removed from)
packets that cross the border of the RPL domain. For reasons such as
the capability to send ICMP errors back to the source, this operation
involves an extra IP-in-IP encapsulation inside the RPL domain for
all the packets which path is not contained within the RPL domain.
The 8-octets overhead is detrimental to the LLN operation, in
particular with regards to bandwidth and battery constraints. These
octets may cause a containing frame to grow above maximum frame size,
leading to Layer 2 or 6LoWPAN [RFC4944] fragmentation, which in turn
cause even more energy spending and issues discussed in the LLN
Fragment Forwarding and Recovery [I-D.thubert-6lo-forwarding-
fragments].
Considering that, in the classical IEEE802.14.5 PHY that is used by
6TiSCH, roughly 80 octets are available per frame after security is
applied, and any additional transmitted bit weights in the energy
consumption and drains the batteries.
For timing reasons, [RFC6282] failed to provide an adapted
compression for the RPL option so the cost in current implementations
can not be alleviated in any fashion. This document provides thus
the much-needed efficient compression of the RPL option as a logical
extension to [RFC6282].
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The Terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks' [RFC7102] and [RFC6550].
4. Updating RFC 6282
This specification proposes a new 6LoWPAN Next Header Compression
(NHC) for the RPL option [RFC6553], called RPL_NHC, to be placed in
an [RFC6282]-compressed packet.
It updates [RFC6282] in that the necessary property of encoding
headers using LOWPAN_NHC becomes that the immediately preceding
header must be encoded using either LOWPAN_IPHC, RPL_NHC or
LOWPAN_NHC.
Additionally, the necessary property of encoding headers using
RPL_NHC is that the immediately preceding header must be encoded
using either LOWPAN_IPHC or LOWPAN_NHC.
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5. New RPL Next Header Compression
[RFC6550] section 11.2 specifies the RPL information as a set of
fields that are to be placed into the packets for the purpose of
Instance Identification, as well as Loop Avoidance and Detection.
Those fields include an 'O', an 'R' and an 'F' bits, a 8-bit
RPLInstanceID, which is in fact an encoded structure, and a 16-bit
SenderRank.
The SenderRank is the result of the DAGRank operation on the rank of
the sender, here the DAGRank operation is defined in section 3.5.1
as:
DAGRank(rank) = floor(rank/MinHopRankIncrease)
If MinHopRankIncrease is set to a multiple of 256, it appears that
the least significant 8 bits of the SenderRank will be all zeroes and
can be elided, in which case the SenderRank can be compressed into
one octet.
[RFC6553] defines an encoding for the RPL information as a RPL option
located in a Hop-by-hop header. The RPL_NHC provides a compressed
form for that the RPL information and is constructed as follows:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 0 | I | K | O | R | F |NH |
+---+---+---+---+---+---+---+---+
The RPL_NHC is immediately followed by the RPLInstanceID, unless it
is elided, and then the SenderRank, which is either compressed into
one octet or fully inlined as the whole 2 octets. Bits in the
RPL_NHC indicate whether the RPLInstanceID is elided and/or the
SenderRank is compressed:
O, R, and F bits The O, R, and F bits are defined in [RFC6550]
section 11.2.
NH: 1-bit. The Next Header (NH) bit is defined in [RFC6282]
section 4.2, and it is set to indicate that the next header is
encoded using LOWPAN_NHC
I: 1-bit. If it is set, the Instance ID is elided and the
RPLInstanceID is the Global RPLInstanceID 0. If it is not set,
the octet immediately following the RPL_NHC contains the
RPLInstanceID as specified in [RFC6550] section 5.1.
K: 1-bit. If it is set, the SenderRank is be compressed into one
octet, and the lowest significant octet is elided. If it is
not set, the SenderRank, is fully inlined as 2 octets.
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In the following case, the RPLInstanceID is the Global RPLInstanceID
0, and the MinHopRankIncrease is a multiple of 256 so the least
significant octet is all zeroes and can be elided:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|1|1|O|R|F|N| SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the following case, the RPLInstanceID is the Global RPLInstanceID
0, but both octets of the SenderRank are significant so it can not be
compressed:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|1|0|O|R|F|N| SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the following case, the RPLInstanceID is not the Global
RPLInstanceID 0, and the MinHopRankIncrease is a multiple of 256:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|1|O|R|F|N| RPLInstanceID | SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the following case, the RPLInstanceID is not the Global
RPLInstanceID 0, and both octets of the SenderRank are significant:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|O|R|F|N| RPLInstanceID | SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Depending on the RPLInstanceID and the MinHopRankIncrease, the
proposed format Squeezes the RPL information in 16 to 32 bits, which
compares to 64 bits when using a Hop-by-hop option with the RPL
option as specified in [RFC6553].
6. Security Considerations
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Using a compressed format as opposed to the full inline RPL option is
logically equivalent and does not create an opening for a new threat
when compared to [RFC6553].
7. IANA Considerations
This document updates IANA registry for the LOWPAN_NHC defined in
[RFC6282] and assigns the previously unassigned:
10IOKRFN: RPL Information [this]
Capital letters in bit positions represent class-specific bit
assignments. N indicates whether or not additional LOWPAN_NHC
encodings follow, as defined in Section 4.2. IOKRF represents
variables specific to RPL Information compression defined in Section
5.
8. Acknowledgements
The author wishes to thank Laurent Toutain and Carsten Bormann for
suggesting this work .
9. References
9.1. Normative References
[IEEE802154]
IEEE standard for Information Technology, "IEEE std.
802.15.4, Part. 15.4: Wireless Medium Access Control
(MAC) and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", June 2011.
[ISA100.11a]
ISA, "ISA100, Wireless Systems for Automation", May 2008,
< http://www.isa.org/Community/
SP100WirelessSystemsforAutomation>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP. and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
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[RFC6552] Thubert, P., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)", RFC
6552, March 2012.
[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553, March
2012.
9.2. Informative References
[I-D.ietf-6tisch-architecture]
Thubert, P., Watteyne, T. and R. Assimiti, "An
Architecture for IPv6 over the TSCH mode of IEEE
802.15.4e", Internet-Draft draft-ietf-6tisch-
architecture-01, February 2014.
[I-D.ietf-6tisch-tsch]
Watteyne, T., Palattella, M. and L. Grieco, "Using
IEEE802.15.4e TSCH in an LLN context: Overview, Problem
Statement and Goals", Internet-Draft draft-ietf-6tisch-
tsch-00, November 2013.
[I-D.thubert-6lo-forwarding-fragments]
Thubert, P. and J. Hui, "LLN Fragment Forwarding and
Recovery", Internet-Draft draft-thubert-6lo-forwarding-
fragments-01, February 2014.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, January 2014.
Author's Address
Pascal Thubert, editor
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis, 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
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