ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                   L. Zhao
Updates: 6550, 8138 (if approved)                          Cisco Systems
Intended status: Standards Track                         24 January 2020
Expires: 27 July 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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 27 July 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Updating RFC 8138 . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Transition Scenarios  . . . . . . . . . . . . . . . . . . . .   4
     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 of a RPL [RFC6550] network to activate the compression
   defined in [RFC8138] can only be done when all routers in the network
   support it.  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 leaves and their
   RPL parents handle the compression/decompression on their behalf.

   This document complements RFC 8138 and dedicates a flag in the RPL
   configuration option to indicate whether RFC 8138 compression should
   be used within the RPL Instance.  The setting of 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 RFC 8138 should
   refrain from using the compression unless the information is
   superseded by configuration.

   This specification provides scenarios that force a legacy node to
   become a RPL-Aware-Leaf (RAL).  In that case, the 6LR must be aware
   by means out of scope that it must uncompress the packets before
   delivering to the RAL.

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.

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3.  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 [RFC8138] 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 [RFC6550].  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.

   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

   A node that supports this specification MUST source packets in the
   compressed form using [RFC8138] if and only if the "T" flag is set.
   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"

   The decision of using [RFC8138] 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
   [USEofRPLinfo] is not expected to support [RFC8138].  An intermediate
   router MUST forward the packet in the form that the source used,
   either compressed or uncompressed, unless it is either forwarding to
   an external target or delivering to a leaf that is not known to
   support RFC 8138, in which cases it MUST uncompress the packet.

   A RPL-Unaware Leaf (RUL) [UNAWARE-LEAVES] is both a leaf and an
   external target.  A RUL does not participate to RPL and depends on
   the 6LR to ensure its connectivity.  Packets from/to a RUL are
   tunneled back and forth to the Root regardless of the MOP used in the
   RPL Instance.  A node that supports this specification but does not
   support [RFC8138] SHOULD join as a RUL to ensure that the 6LR is
   aware it needs to uncompress the packets before delivering.

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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 nodes
   that do in a network with RFC 8138 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, in which case it generates uncompressed
   packets and can receive packets if they are delivered by the parent
   6LR in the uncompressed form.

   [RFC6550] 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 [RFC6550] also suggests that a RPL-aware node
   may only attach to a DODAG as a leaf node when the 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.

   Per the above, 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
   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 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

   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

   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 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

   The authors wish to thank Rahul Jadhav for his in-depth review and
   constructive suggestions.

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,

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              Robles, I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", Work in
              Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-34,
              20 January 2020, <

              Thubert, P. and M. Richardson, "Routing for RPL Leaves",
              Work in Progress, Internet-Draft, draft-ietf-roll-unaware-
              leaves-08, 16 December 2019, <

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
              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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