A RPL DODAG Configuration Option for the 6LoWPAN Routing HeaderCisco Systems, IncBuilding D45 Allee des Ormes - BP1200 MOUGINS - Sophia Antipolis06254FRANCE+33 497 23 26 34pthubert@cisco.comCisco Systems, IncXinsi BuildingNo. 926 Yi Shan Rd SHANGHAI 200233CHINAliz3@cisco.com
Routing Area
ROLLDraft
This document updates RFC 8138 by defining a bit in the RPL DODAG
Configuration Option to indicate whether compression is used within the
RPL Instance, and specify the behavior of RFC 8138-capable nodes
when the bit is set and reset.
Introduction
The packet compression technique defined in can
only be activated in a RPL network when all the
nodes support it. Otherwise, if acting as a leaf, a node that does not
support the compression would fail to communicate; if acting as a router
it would drop the compressed packets and black-hole a portion of the
network.
The original idea was to use a flag day but that proved impractical in a
number of situations such as a large metering network that is used in
production and incurs financial losses when interrupted.
This specification is designed for the scenario where a live network is
upgraded to support . During the migration, the
compression should remain inactive, until all nodes are upgraded.
This document complements and dedicates a flag in
the RPL DODAG Configuration Option to indicate whether the
compression should be used within the RPL DODAG.
The setting of this new flag is controlled by the Root and propagates as
is in the whole network as part of the normal RPL signaling.
The flag is cleared to maintain the compression inactive during
the migration phase. When the migration is complete (e.g., as known by
network management and/or inventory), the flag is set and the compression
is globally activated in the whole DODAG.
TerminologyReferences
The terminology used in this document is consistent with and incorporates
that described in "Terms Used in Routing for Low-Power
and Lossy Networks (LLNs)".
Other terms in use in LLNs are found in
"Terminology for Constrained-Node Networks".
"RPL", the "RPL Packet Information" (RPI), and "RPL Instance" (indexed by a
RPLInstanceID) are defined in "RPL: IPv6 Routing
Protocol for Low-Power and Lossy Networks". The RPI is the abstract
information that RPL defines to be placed in data packets, e.g., as the RPL
Option within the IPv6 Hop-By-Hop Header.
By extension the term "RPI" is often used to refer to the RPL Option itself.
The DODAG Information Solicitation (DIS), Destination Advertisement Object
(DAO) and DODAG Information Object (DIO) messages are also specified in
.
This document uses the terms RPL-Unaware Leaf (RUL) and RPL-Aware Leaf
(RAL) consistently with
"Using RPI Option Type, Routing Header for Source Routes and IPv6-in-IPv6 encapsulation in the RPL Data Plane".
The term RPL-Aware Node (RAN) refers to a node that is either
a RAL or a RPL Router. A RAN manages the reachability of its addresses and
prefixes by injecting them in RPL by itself. In contrast, a RUL leverages
"Registration Extensions for IPv6 over
Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery"
to obtain reachability services from its parent router(s)
as specified in
"Routing for RPL Leaves".
Glossary This document often uses the following acronyms:
6LoWPAN:
IPv6 over Low-Power Wireless Personal Area Network
6LoRH:
6LoWPAN Routing Header
DIO:
DODAG Information Object (a RPL message)
DODAG:
Destination-Oriented Directed Acyclic Graph
LLN:
Low-Power and Lossy Network
RPL:
IPv6 Routing Protocol for Low-Power and Lossy Networks
OF:
RPL Objective Function
OCP:
RPL Objective Code Point
MOP:
RPL Mode of Operation
RPI:
RPL Packet Information
RAL:
RPL-Aware Leaf
RAN:
RPL-Aware Node
RUL:
RPL-Unaware Leaf
SRH:
Source Routing Header
Requirements Language
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 BCP 14
when, and only when, they
appear in all capitals, as shown here.
The RPL DODAG Configuration Option
The DODAG Configuration Option is defined in Section 6.7.6 of .
The RPL DODAG Configuration Option is typically placed in
a DODAG Information Object (DIO) message. The DIO message propagates down the
DODAG to form and then maintain its structure. The DODAG Configuration Option
is copied unmodified from parents to children.
As shown in , the DODAG Configuration Option was
designed with 4 bit positions reserved for future use as Flags.
This specification defines a new flag "Enable RFC8138 Compression" (T).
The "T" flag is set to turn-on the use of the compression of RPL artifacts
with within the DODAG. The new "T" flag is encoded
in position 2 of the reserved Flags field in the RPL
DODAG Configuration Option, and set to 0 in legacy implementations as
specified in Section 6.7.6 of .
states, when referring to the DODAG Configuration
Option, that "Nodes other than the DODAG Root MUST
NOT modify this information when propagating the DODAG Configuration option".
Therefore, a legacy parent propagates the "T" flag as set by the Root
whether it supports this specification or not. So when the "T" flag is set,
it is transparently flooded to all the nodes in the DODAG.
Section 6.3.1 of defines a 3-bit Mode of Operation
(MOP) in the DIO Base Object. This specification applies to MOP values 0 to 6.
For a MOP value of 7, the compression MUST be used by default regardless of
the setting of the âTâ flag.
Updating RFC 8138
A node SHOULD generate packets in the compressed form using
if and only if the "T" flag
is set. This behavior can be overridden by configuration or network
management. Overriding may be needed e.g., to turn on the compression in a
network where all nodes support but the Root does
not support this specification and cannot set the "T" flag, or to disable it
locally in case of a problem.
The decision to use is made by the originator of
the packet depending on its capabilities and its knowledge of the state of
the "T" flag.
A router encapsulating a packet is the originator of the resulting
packet and is responsible for compressing the outer headers with
, but it MUST leave the encapsulated packet as is.
An external target is not
expected to support . In most cases, packets to and
from an external target are tunneled back and forth between the border router
(referred to as 6LR) that serves the external target and the Root, regardless
of the MOP used in the RPL DODAG.
The inner packet is typically not compressed with ,
so for outgoing packets, the border router just needs to decapsulate the
(compressed) outer header and forward the (uncompressed) inner packet towards
the external target.
A router MUST uncompress a packet that is to be forwarded to an external
target. Otherwise, the router MUST forward the packet in the form that the
source used, either compressed or uncompressed.
A RUL is both a leaf and an
external target. A RUL does not participate in RPL and
depends on the parent router to obtain connectivity. In the case of a RUL,
forwarding towards an external target actually means delivering the packet.
Transition Scenarios
A node that supports but not this specification
can only be used in an homogeneous network.
Enabling the compression without a turn-on signaling
method requires a "flag day"; by which time all nodes must be upgraded, and
at which point the network can be rebooted with the
compression turned on.
The intent for this specification is to perform a migration once and for all
without the need for a flag day. In particular it is not the intention to
undo the setting of the "T" flag. Though it is possible to roll back
(see ), adding nodes that do not support
after a roll back may be problematic if the roll
back did not fully complete.
Coexistence
A node that supports this specification can operate in a network with the
compression turned on or off with the "T" flag set
accordingly and in a network in transition from off to on or on to off
(see ).
A node that does not support can interoperate with
nodes that do in a network with compression turned
off. If the compression is turned on, all the RPL-Aware Nodes are expected
to be able to handle compressed packets in the compressed form. A node that
cannot do so may remain connected to the network as a RUL as described in
.
Inconsistent State While Migrating
When the "T" flag is turned on by the Root, the
information slowly percolates through the DODAG as the DIO gets propagated.
Some nodes will see the flag and start sourcing packets in the compressed
form while other nodes in the same RPL DODAG are still not aware of it.
In non-storing mode, the Root will start using
with a Source Routing Header 6LoRH (SRH-6LoRH)
that routes all the way to the parent router or to the leaf.
To ensure that a packet is forwarded across the RPL DODAG in the form in
which it was generated, it is required that all the RPL nodes support
at the time of the switch.
Setting the "T" flag is ultimately the responsibility of the Network
Administrator. The expectation is that the network management or upgrading
tools in place enable the Network Administrator to know when all the nodes
that may join a DODAG were migrated. In the case of a RPL instance with
multiple Roots, all nodes that participate to the RPL Instance may
potentially join any DODAG.
The network MUST be operated with the "T" flag reset until all nodes in the
RPL Instance are upgraded to support this specification.
Rolling Back
When turning compression off in the network, the
Network Administrator MUST wait until all nodes have converged to the
"T" flag reset before allowing nodes that do not support the compression in
the network. To that effect, whether the compression is active in a node
SHOULD be exposed the node's management interface.
It is RECOMMENDED to only deploy nodes that support
in a network where the compression is turned on. A node that does not support
MUST only be used as a RUL.
IANA Considerations
IANA is requested to assign a new option flag from the Registry for the
"DODAG Configuration Option Flags" that was created for
as follows:
New DODAG Configuration Option Flag
Bit Number
Capability Description
Reference
2 (suggested)
Turn on RFC8138 Compression (T)
THIS RFC
Security Considerations
First of all, it is worth noting that with , every
node in the LLN that is RPL-aware can inject any RPL-based attack in the
network.
A trust model is REQUIRED in an effort to exclude rogue nodes from
participating to the RPL and the 6LoWPAN signaling, as well as from the data
packet exchange. This trust model could at a minimum be based on a Layer-2
Secure joining and the Link-Layer security. This is a generic RPL and 6LoWPAN
requirement, see Req5.1 in Appendix of .
Setting the "T" flag before all routers are upgraded may cause a loss
of packets. The new bit is protected as the rest of the configuration so
this is just one of the many attacks that can happen if an attacker manages
to inject a corrupted configuration.
Setting and resetting the "T" flag may create inconsistencies in the network
but as long as all nodes are upgraded to support
they will be able to forward both forms. The source is responsible
for selecting whether the packet is compressed or not, and all routers must
use the format that the source selected. So the result of an inconsistency
is merely that both forms will be present in the network, at an additional
cost of bandwidth for packets in the uncompressed form.
An attacker in the middle of the network may reset the "T" flag to cause
extra energy spending in its subDAG. Conversely it may set the "T" flag, so
that nodes located downstream would compress when that it is not desired,
potentially resulting in the loss of packets. In a tree structure, the
attacker would be in position to drop the packets from and to the attacked
nodes. So the attacks above would be more complex and more visible than
simply dropping selected packets. The downstream node may have other
parents and see both settings, which could raise attention.
Acknowledgments
The authors wish to thank Meral Shirazipour,
Nagendra Kumar Nainar, Stewart Bryant, Carles Gomez, Alvaro Retana,
Dominique Barthel and Rahul Jadhav for their in-depth reviews and
constructive suggestions.
Also many thanks to Michael Richardson for being always helpful and responsive
when need comes.
Normative ReferencesInformative References