ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                   L. Zhao
Updates: 6550, 8138 (if approved)                          Cisco Systems
Intended status: Standards Track                        12 December 2019
Expires: 14 June 2020

                   Configuration option for RFC 8138


   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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   This Internet-Draft will expire on 14 June 2020.

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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  . . . . . . . . . . . . . . . .   6
     5.4.  Rolling Back  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   10. Informative References  . . . . . . . . . . . . . . . . . . .   7
   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   unless the information is superseded by configuration.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",
   "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

   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 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

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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 as long as imcoming packets
   are decapsulated by the parent and delivered in uncompressed form.

   [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
   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.

   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

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   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.

   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.

   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.

   *  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.

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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.

   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).

   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:

       | Bit Number | Capability Description          | Reference |
       | 2          | Turn on RFC8138 Compression (T) | THIS RFC  |

               Table 1: New DODAG Configuration Option Flag

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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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

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, <>.

   [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

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              2019, <

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D, 45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis

   Phone: +33 497 23 26 34

   Li Zhao
   Cisco Systems, Inc
   Xinsi Building, No. 926 Yi Shan Rd


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