Configuration option for RFC 8138Cisco 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 complements RFC 8138 and dedicates a bit in the RPL
configuration option defined in RFC 6550 to indicate whether RFC 8138
compression is used within the RPL Instance.
Introduction
The transition of a RPL network to activate the
compression defined in can only be done when all
routers in the network support it. Otherwise, a non-capable node acting as a router
would drop the compressed packets and black-hole its subDAG. In a mixed
case with both RFC8138-capable and non-capable nodes, the compression
may be turned on only if all the non-capable nodes act as Hosts and
their RPL parents handle the compression/decompression for them.
This document complements and dedicates a flag in the RPL
configuration option to indicate whether compression should be
used within the RPL Instance. The setting of this new flag is controlled by the
Root and propagates as is in the whole network. When the bit is not set,
source nodes that support should refrain from using the
compression unless the information is superseded by configuration.
With RPL, a leaf is an IPv6 Host, which implies that leaves do not forward packets.
This specification provides scenarios that force a non-capable RPL-Aware Node
(RAN) to become a leaf.
The parent router must know, e.g., by configuration, or leveraging
"RPL Capabilities",
when a leaf does not support the compression defined in
. This is implicitly the case for a RPL-Unaware
Leaf (RUL) but is not known for a RPL-Aware Leaf (RAL). The parent router
must uncompress the packets before delivering them to a non-capable leaf
and it must compress the traffic from the leaf.
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), "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
BCP 14
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.
Updating RFC 6550
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 a RPL Instance. If a RPL Instance has
multiple Roots then they must be coordinated to use the same setting.
RPL defines a Configuration Option that is registered to IANA in section
20.14. of . The "T" flag is encoded in one of the
reserved control bits in the RPL Configuration Option. The bit position of
the "T" flag is indicated in .
Section 6.3.1.
of defines a 3-bit Mode of Operation (MOP) in the
DIO Base Object. The new "T" flag is defined only for MOP value between 0 to
6. For a MOP value of 7 or above, the flag MAY indicate something different
and MUST NOT be interpreted as "Enable RFC8138 Compression" unless the
specification of the MOP indicates to do so.
Updating RFC 8138
A node that supports this specification MUST source packets in the compressed
form using if and only if the "T" flag
is set. This behaviour can be overridden by the configuration of the
node in order to cope with intermediate implementations of the Root that
support but not this specification and cannot set
the "T" flag.
The decision of using is made by the originator of
the packet depending on its capabilities and its knowledge of the state of
the "T" flag.
A router that encapsulates a packet is the originator of the resulting packet
and decides whether to compress the outer headers as indicated above.
An external target is not
expected to support .
An intermediate router MUST forward the packet in the form that the source
used, either compressed or uncompressed, unless it is forwarding to an
external target or delivering to a leaf that is not known to support
, in which cases it MUST uncompress the packet.
Transition Scenarios
A node that supports but not this specification
can only be used in an homogeneous network. Enabling the
compression requires a "flag day"; all nodes must
be upgraded, and then the network can be rebooted with the
compression turned on.
A node that supports this specification can work in a network with
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, the node cannot forward compressed packets and
therefore it cannot act as a router. It may remain connected to that network
as a leaf, generates uncompressed packets, and can receive
packets if they are delivered by the parent router in the uncompressed form.
Unless this is known by other means, the node SHOULD join as a RUL as an
indication that its parent router needs to uncompress the packets before
delivering.
states that "Nodes other than the DODAG Root MUST
NOT modify this information when propagating the DODAG Configuration option".
Therefore, even 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 RPL Instance.
Sections 8.5 and 9.2 of also suggests that a
RAN may only attach to a DODAG as a leaf when it does
not support the Mode of Operation of a RPL Instance, the Objective Function
(OF) as indicated by the Objective Code Point (OCP) or some other parameters
in the configuration option.
This specification reiterates that a RAN that is configured to operate in a
RPL Instance but does not support a value for a known parameter that is
mandatory for routing, such as the OCP, MUST NOT operate as a router but MAY
still join as a leaf. Note that a legacy RAN will not recognize when a
reserved field is used and will not turn to a leaf when the "T" flag is set.
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, and 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 is not fully complete (see caveats in ).
Inconsistent State While Migrating
When the "T" flag is turned on in the configuration option 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 Instance are still not aware of it.
Conversely, 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 Instance in the form in
which it was generated, it is required that all the routers support
at the time of the switch, and that all nodes that
do not support only operate as leaves.
Setting the "T" flag is ultimately the responsibility of the network
administrator. In a case of upgrading a network to turn the compression on,
the network SHOULD be operated with the "T" flag reset until all targeted
nodes are upgraded to support this specification.
and provide possible transition scenarios where this can
be enforced.
Single RPL Instance Scenario
In a Single RPL Instance Scenario, nodes that support are configured with a new OCP, that may use the same OF operation or a variation of it,
while nodes that do not support are not, but are configured
to join an unknown OCP.
The Root migrates to the new OCP before it sets the "T" flag, so that nodes
that do not support are all attached as leaves when
the "T" flag is eventually set.
The parent router - which supports - compresses the
packets originated from the leaf and uncompresses the packets going to the
leaf. This may be done on the fly by the parent of a non-capable RAL, or as
part of the tunneling operation between the parent and the Root,
if the leaf behaves as a RUL. This is described in section 7, 8, and 9 of
.
Note that though tunneling from the Root to the parent is the generic case
for RULs, on paper it is possible for the Root to avoid it for the traffic
that it originates. The Root SHOULD always use tunneling to the parent of a
RUL, even for its own packets, unless it knows that the leaf supports
.
This scenario presents a number of caveats:
The method consumes an extra OCP. It also forces nodes that do not support
to operate as RULs, unless there is a method
to let the parent router know that it must uncompress the packet for this
RAL.
If the RPL implementation of a node does not turn it to a leaf when the OCP is changed to
an unknown one, then the node may be stalled.
If the only possible parents of a node are nodes that do not support ,
then that node will loose all its parent at the time of the migration and it
will be stalled until a parent is deployed with the new capability.
Double RPL Instances ScenarioAn alternative to the Single RPL Instance Scenario is to deploy an additional
RPL Instance for the nodes that support .
The two RPL Instances operate independently as specified
in . The preexisting RPL Instance does not use
, whereas the new RPL Instance does. This is signaled
by the "T" flag which is only set in the configuration option in DIO messages
in the new RPL Instance.
Nodes that support participate in both Instances but favor the new
RPL Instance for the traffic that they source.
By contrast, nodes that only support the uncompressed format would
either not be configured for the new RPL Instance, or would be configured to
join it as leaves only.
This method eliminates the risks of nodes being stalled that are described in
but requires implementations to support at least two RPL
Instances and demands management capabilities to introduce new RPL Instances
and deprecate old ones.
Rolling Back
After downgrading a network to turn the compression
off, the administrator SHOULD make sure that all nodes have converged to the
"T" flag reset before allowing nodes that do not support the compression in
the network (see caveats in ).
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 leaf.
IANA Considerations
This specification updates 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
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 MUST be put in place so that rogue nodes are excluded
from participating to the RPL and the 6LowpAN signaling, and from the data
packet exchange. This trust model could be at a minimum 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 some 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.
AcknowledgmentsThe authors wish to thank Dominique Barthel and Rahul Jadhav for their in-depth reviews and constructive suggestions.
Normative ReferencesInformative References