Configuration option for RFC 8138
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Routing Area
ROLL
Draft
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.
The transition to in a network can only be
done when all nodes support the specification. In a mixed case with both
RFC8138-capable and non-capable nodes, the compression should be turned
off.
This document complements RFC 8138 and dedicates a bit in the RPL
configuration option to indicate whether RFC 8138 compression should be
used within the RPL instance. When the bit is not set, source nodes that
support RFC 8138 should refrain from using the compression unless the
information is superseded by configuration.
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.
RPL defines a configuration option that is registered to
IANA in section 20.14. of . This specification
defines a new flag "Enable RFC8138 Compression" (T) that is encoded in
one of the reserved control bits in the option. The new flag is set to turn
on the use of the compression of RPL artifacts with RFC 8138.
This document specifies controls that enable and disable the use of the
compression in a RPL Instance. Arguably, this could
have been done in itself.
A node that supports this specification SHOULD source packets in the
compressed form using if the new "T" flag is set in
the RPL configuration option from its parents. Failure to do so will result
in larger packets, yields higher risks of loss and may cause a fragmentation.
A node that supports this specification SHOULD refrain from sourcing
packets in the compressed form using if the "T" flag
is reset. This behavior can be overridden by a 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 RFC 8138 to compress a packet is made at the source
depending on its capabilities and its knowledge of the state of the "T" flag.
A router MUST forward the packet in the form that the source used, either
compressed or uncompressed. A router that encapsulates a packet is the
source of the resulting packet and the rules above apply to it in that case.
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 in that network.
states that "Nodes other than the DODAG root MUST
NOT modify this information when propagating the DODAG Configuration option".
In other words, the configuration option is a way for the root to configure
the LLN nodes but it cannot be used by a parent to advertise its capabilities
down the DODAG. It results whether a parent supports RFC 8138 is not known
by the child with the current level of specifications, and a child cannot
favor a parent based on a particular support.
Sections 8.5 and 9.2 of also suggests that a
RPL-aware node may attach to a DODAG as a leaf node only, e.g., when a
node 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.
But the node is also free to refrain from joining
an Instance when a parameter is not suitable. This means that changing the
OCP in a DODAG can be used to force nodes that do not support a particular
feature to join as leaf only. This specification reiterates that a
node that is configured to operate in an Instance but does not support a
value for a known parameter that is mandatory for routing MUST NOT operate
as a router but MAY still joins as a leaf. Note that a legacy node will not
recognize when a reserved field is now used and will not turn to a leaf
when that happens.
A node that supports but not this specification
can only be used in an homogeneous network and an upgrade requires a
"flag day" where all nodes are updated and then the network is rebooted
with implicitely RFC 8138 compression turned on with the "T" flag set on.
A node that supports this specification can work in a network with RFC 8138
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 a
node that supports this specification in a network with RFC 8138 compression
turned off. But it cannot forward compressed packets and therefore it cannot
act as a router in a network with RFC 8138 compression turned on. It may
remain connected to that network as a leaf and generate uncompressed packets
as long as imcoming packets are decapsulated by the parent and delivered in
uncompressed form.
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 ).
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 instance are still not aware of it.
Conversely, in non-storing mode, the root will start using RFC 8138 with a
SRH-6LoRH that routes all the way to the last router or possibly to the leaf,
if the leaf supports RFC 8138.
This is why it is required that all the routers in the Instance support
at the time of the switch, and 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.
In a single instance scenario, nodes that support RFC 8138 are configured
with a new OCP, that may use the same OF operation or a variation of it. when
it finally sets the "T" flag, the root also migrates to the new OCP. As a
result, nodes that do not support RFC 8138 join as leaves and do not forward
packets anymore. The leaves generate packets without compression. The parents
- which supports RFC 8138 - may encapsulate the packets using RFC 8138 if
needed. The other way around, the root encapsulates packets to the leaves all
the way to the parent, which decapsulates and distribute the uncompresses
inner packet to the leaf, as illustrated in Section 4.3 of
This scenario presents a number of caveats:
The method consumes an extra OCP. It also requires a means to signal the
capabilities of the leaf, e.g., using
"RPL Mode of Operation extension".
If an implementation does not move to a leaf mode 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 RFC 8138,
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.
Nodes that only support RFC8138 for forwarding may not parse the RPI in
native form. If such nodes are present, the parent needs to encapsulate with
RFC8138.
An alternate to the Single Instance Scenario is to deploy an additional
Instance for the nodes that support .
The two instances operate as ships-in-the-night as specified in
. The preexisting Instance that does not use
, whereas the new Instance does. This is signaled
by the "T" flag which is only set in the configuration option in DIO messages
in the new Instance.
The legacy nodes would not be configured to participate to the second
instance, and islands that are only connected via legacy nodes would not be
reachable over the second instance.
Nodes that support RFC 8138 participate to both Instances but favor the new
Instance for the traffic that they source.
On the other hand, nodes that only support the uncompressed format would
either not be configured for the new 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 Instances and
deprecate old ones.
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 ). This also requires a means
to signal the current state of the setting of the logic that controls the
compression in the node, also using .
This specification updates the "Registry for the DODAG Configuration Option
Flags" that was created for as follows:
Bit Number
Meaning
Defining Spec
2 (suggested)
Turn on RFC8138 Compression (T)
This
No specific threat was identified with this specification.