Internet DRAFT - draft-thubert-roll-turnon-rfc8138
draft-thubert-roll-turnon-rfc8138
ROLL P. Thubert, Ed.
Internet-Draft L. Zhao
Updates: 6550,8138 (if approved) Cisco Systems
Intended status: Standards Track July 8, 2019
Expires: January 9, 2020
Configuration option for RFC 8138
draft-thubert-roll-turnon-rfc8138-03
Abstract
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.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . . 2
4. Updating RFC 8138 . . . . . . . . . . . . . . . . . . . . . . 3
5. Transition Scenarios . . . . . . . . . . . . . . . . . . . . 3
5.1. Inconsistent State While Migrating . . . . . . . . . . . 4
5.2. Single Instance Scenario . . . . . . . . . . . . . . . . 5
5.3. Double Instance Scenario . . . . . . . . . . . . . . . . 5
5.4. Rolling Back . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The transition to [RFC8138] 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unless the information is superseded by configuration.
2. 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 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Updating RFC 6550
RPL defines a configuration option that is registered to IANA in
section 20.14. of [RFC6550]. 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.
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4. Updating RFC 8138
This document specifies controls that enable and disable the use of
the [RFC8138] compression in a RPL Instance. Arguably, this could
have been done in [RFC8138] itself.
A node that supports this specification SHOULD source packets in the
compressed form using [RFC8138] 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 [RFC8138] 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 [RFC8138] 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.
5. Transition Scenarios
It is RECOMMENDED to only deploy nodes that support [RFC8138] in a
network where the compression is turned on. A node that does not
support [RFC8138] MUST only be used as a leaf in that network.
[RFC6550] 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 [RFC6550] 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
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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 [RFC8138] 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 Section 5.1).
A node that does not support [RFC8138] 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 Section 5.4), adding nodes that do not
support [RFC8138] after a roll back may be problematic if the roll
back is not fully complete (see caveats in Section 5.2).
5.1. 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 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 [RFC8138] at the time of the switch, and all nodes that do
not support [RFC8138] only operate as leaves.
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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. Section 5.2 and Section 5.3 provide possible
transition scenarios where this can be enforced.
5.2. Single Instance Scenario
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
[I-D.ietf-roll-useofrplinfo]
This scenario presents a number of caveats:
o 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" [I-D.rahul-roll-mop-ext].
o 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.
o 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.
o 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.
5.3. Double Instance Scenario
An alternate to the Single Instance Scenario is to deploy an
additional Instance for the nodes that support [RFC8138]. The two
instances operate as ships-in-the-night as specified in [RFC6550].
The preexisting Instance that does not use [RFC8138], 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.
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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 Section 5.2 but requires implementations to support at
least two RPL Instances and demands management capabilities to
introduce new Instances and deprecate old ones.
5.4. Rolling Back
After downgrading a network to turn the [RFC8138] 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 Section 5.2). 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
[I-D.rahul-roll-mop-ext].
6. IANA Considerations
This specification updates the "Registry for the DODAG Configuration
Option Flags" that was created for [RFC6550] as follows:
+---------------+---------------------------------+----------------+
| Bit Number | Meaning | Defining Spec |
+---------------+---------------------------------+----------------+
| 2 (suggested) | Turn on RFC8138 Compression (T) | This |
+---------------+---------------------------------+----------------+
Table 1: New DODAG Configuration Option Flag
7. Security Considerations
No specific threat was identified with this specification.
8. Acknowledgments
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9. References
9.1. Normative References
[I-D.ietf-roll-useofrplinfo]
Robles, I., Richardson, M., and P. Thubert, "Using RPL
Option Type, Routing Header for Source Routes and IPv6-in-
IPv6 encapsulation in the RPL Data Plane", draft-ietf-
roll-useofrplinfo-31 (work in progress), July 2019.
[I-D.rahul-roll-mop-ext]
Jadhav, R. and P. Thubert, "RPL Mode of Operation
extension", draft-rahul-roll-mop-ext-01 (work in
progress), June 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., 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,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>.
Authors' Addresses
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Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Li Zhao
Cisco Systems, Inc
Xinsi Building
No. 926 Yi Shan Rd
SHANGHAI 200233
CHINA
Email: liz3@cisco.com
Thubert & Zhao Expires January 9, 2020 [Page 8]
