Configuration option for RFC 8138
draft-ietf-roll-turnon-rfc8138-03
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (roll WG) | |
|---|---|---|---|
| Authors | Pascal Thubert , Li Zhao | ||
| Last updated | 2020-01-22 (Latest revision 2019-12-12) | ||
| Replaces | draft-thubert-roll-turnon-rfc8138 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-roll-turnon-rfc8138-03
ROLL P. Thubert, Ed.
Internet-Draft L. Zhao
Updates: 6550, 8138 (if approved) Cisco Systems
Intended status: Standards Track 22 January 2020
Expires: 25 July 2020
Configuration option for RFC 8138
draft-ietf-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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 July 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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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. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . . 2
4. Updating RFC 8138 . . . . . . . . . . . . . . . . . . . . . . 3
5. Transition Scenarios . . . . . . . . . . . . . . . . . . . . 3
5.1. Inconsistent State While Migrating . . . . . . . . . . . 5
5.2. Single RPL Instance Scenario . . . . . . . . . . . . . . 5
5.3. Double RPL Instances Scenario . . . . . . . . . . . . . . 6
5.4. Rolling Back . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . 7
10. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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. The bit
position of the "T" flag is indicated in Section 6.
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Section 6.3.1. of [RFC6550] 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.
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 behaviour 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
A node that supports [RFC8138] but not this specification can only be
used in a homogeneous network and an upgrade requires a "flag day"
where all nodes are updated and then the network is rebooted with
implicitly 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
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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. The leaf can receive packets
if they are delivered by the parent 6LR in the uncompressed form.
This requires a knowledge by the 6LR that the leaf does not support
RFC 8138. A RPL-Unaware-Leaf (RUL) [USEofRPLinfo] is an external
target and by default is not expected to support RFC 8138.
[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. A parent propagates the
"T" flag as set whether it supports RFC 8138 or not. The setting of
the "T" flag can thus not be used as an indication of the support by
the sender, and a child cannot favor a parent based on it.
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. [USEofRPLinfo]
indicates that the node may also join as a RUL, in which case it
refrains from participating to RPL and depends on the 6LR to ensure
connectivity regardless on the way the RPL network is operated.
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 a RPL 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 join 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.
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).
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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 RPL
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 RPL Instance
support [RFC8138] at the time of the switch, and all nodes that do
not support [RFC8138] 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. Section 5.2 and Section 5.3 provide possible
transition scenarios where this can be enforced.
5.2. Single RPL Instance Scenario
In a Single RPL 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. The root sets the "T" flag at the time it 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 uncompressed inner
packet to the leaf.
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" [MOP-EXT].
* 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.
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* 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 RPL Instances Scenario
An alternate to the Single RPL Instance Scenario is to deploy an
additional RPL Instance for the nodes that support [RFC8138]. The
two RPL Instances operate independently as specified in [RFC6550].
The preexisting RPL Instance that does not use [RFC8138], 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 RFC 8138 participate to both Instances but favor
the new RPL 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 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 Section 5.2 but requires implementations to support at
least two RPL Instances and demands management capabilities to
introduce new RPL 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).
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.
6. IANA Considerations
This specification updates the Registry for the "DODAG Configuration
Option Flags" that was created for [RFC6550] as follows:
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+------------+---------------------------------+-----------+
| Bit Number | Capability Description | Reference |
+============+=================================+===========+
| 2 | Turn on RFC8138 Compression (T) | THIS RFC |
+------------+---------------------------------+-----------+
Table 1: New DODAG Configuration Option Flag
7. Security Considerations
Turning the "T" flag on 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.
Turning the "T" flag on and off may create inconsistencies in the
network but as long as all nodes are upgraded to RFC 8138 support
they will be able to forward both forms. The draft insists that 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.
8. Acknowledgments
9. Normative References
[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>.
[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>.
[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>.
[USEofRPLinfo]
Robles, I., Richardson, M., and P. Thubert, "Using RPI
Option Type, Routing Header for Source Routes and IPv6-in-
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IPv6 encapsulation in the RPL Data Plane", Work in
Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-34,
20 January 2020, <https://tools.ietf.org/html/draft-ietf-
roll-useofrplinfo-34>.
10. 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>.
[MOP-EXT] Jadhav, R., Thubert, P., and M. Richardson, "Mode of
Operation extension and Capabilities", Work in Progress,
Internet-Draft, draft-ietf-roll-mopex-cap-01, 2 November
2019, <https://tools.ietf.org/html/draft-ietf-roll-mopex-
cap-01>.
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 MOUGINS - Sophia Antipolis
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
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