ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                   L. Zhao
Updates: 8138 (if approved)                                Cisco Systems
Intended status: Standards Track                           5 August 2020
Expires: 6 February 2021

    A RPL DODAG Configuration Option for the 6LoWPAN Routing Header


   This document updates RFC 8138 by defining a bit in the RPL DODAG
   Configuration Option to indicate whether compression is used within
   the RPL Instance, and specify the behavior of RFC 8138-capable nodes
   when the bit is set and reset.

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 6 February 2021.

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  References  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  The RPL DODAG Configuration Option  . . . . . . . . . . . . .   4
   4.  Updating RFC 8138 . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Transition Scenarios  . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Coexistence . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Inconsistent State While Migrating  . . . . . . . . . . .   6
     5.3.  Rolling Back  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   10. Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The packet compression technique defined in [RFC8138] can only be
   activated in a RPL [RFC6550] network when all the nodes support it.
   Otherwise, a non-capable node acting as leaf-only would fail to
   communicate, and acting as a router it would drop the compressed
   packets and black-hole a portion of the network.

   The original idea was to use a flag day but that proved impractical
   in a number of situations such as a large metering network that is
   used in production and incurs financial losses when interrupted.

   This specification is designed for the scenario where a live network
   is upgraded to support [RFC8138].  During the migration, the
   compression should remain inactive, until all nodes are upgraded.
   This document complements [RFC8138] and dedicates a flag in the RPL
   DODAG Configuration Option to indicate whether the [RFC8138]
   compression should be used within the RPL DODAG.

   The setting of this new flag is controlled by the Root and propagates
   as is in the whole network as part of the normal RPL signaling.

   The idea is to use the flag to maintain the compression inactive
   during the migration phase.  When the migration is complete (e.g., as
   known by network management and/or inventory), the flag is set and
   the compression is globally activated in the whole DODAG.

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2.  Terminology

2.1.  References

   The terminology used in this document is consistent with and
   incorporates that described in "Terms Used in Routing for Low-Power
   and Lossy Networks (LLNs)" [RFC7102].  Other terms in use in LLNs are
   found in "Terminology for Constrained-Node Networks" [RFC7228].

   "RPL", the "RPL Packet Information" (RPI), and "RPL Instance"
   (indexed by a RPLInstanceID) are defined in "RPL: IPv6 Routing
   Protocol for Low-Power and Lossy Networks" [RFC6550].  The RPI is the
   abstract information that RPL defines to be placed in data packets,
   e.g., as the RPL Option [RFC6553] within the IPv6 Hop-By-Hop Header.
   By extension the term "RPI" is often used to refer to the RPL Option
   itself.  The DODAG Information Solicitation (DIS), Destination
   Advertisement Object (DAO) and DODAG Information Object (DIO)
   messages are also specified in [RFC6550].

   This document uses the terms RPL-Unaware Leaf (RUL) and RPL-Aware
   Leaf (RAL) consistently with "Using RPI Option Type, Routing Header
   for Source Routes and IPv6-in-IPv6 encapsulation in the RPL Data
   Plane" [USEofRPLinfo].  The term RPL-Aware Node (RAN) refers to a
   node that is either a RAL or a RPL Router.  A RAN manages the
   reachability of its addresses and prefixes by injecting them in RPL
   by itself.  In contrast, a RUL leverages "Registration Extensions for
   IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor
   Discovery" [RFC8505] to obtain reachability services from its parent
   router(s) as specified in "Routing for RPL Leaves" [UNAWARE-LEAVES].

2.2.  Glossary

   This document often uses the following acronyms:

   6LoWPAN:  IPv6 over Low-Power Wireless Personal Area Network
   6LoRH:  6LoWPAN Routing Header
   DIO:  DODAG Information Object (a RPL message)
   DODAG:  Destination-Oriented Directed Acyclic Graph
   LLN:  Low-Power and Lossy Network
   RPL:  IPv6 Routing Protocol for Low-Power and Lossy Networks
   OF:  RPL Objective Function
   OCP:  RPL Objective Code Point
   MOP:  RPL Mode of Operation
   RPI:  RPL Packet Information
   RAL:  RPL-Aware Leaf
   RAN:  RPL-Aware Node
   RUL:  RPL-Unaware Leaf
   SRH:  Source Routing Header

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2.3.  Requirements Language

   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.  The RPL DODAG Configuration Option

   The DODAG Configuration Option is defined in Section 6.7.6 of

   The RPL DODAG Configuration Option is typically placed in a DODAG
   Information Object (DIO) message.  The DIO message propagates down
   the DODAG to form and then maintain its structure.  The DODAG
   Configuration Option is copied unmodified from parents to children.

   As shown in Figure 1, the DODAG Configuration Option was designed
   with 4 bit positions reserved for future use as Flags.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Type = 0x04 |Opt Length = 14| Flags |A|       ...           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     +
     |                               ...                             |

            Figure 1: DODAG Configuration Option (Partial View)

   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 the DODAG.  The new "T" flag is
   encoded in the Flags field in the RPL DODAG Configuration Option.
   The suggested bit position of the "T" flag is indicated in Section 6.

   [RFC6550] states, when referring to the DODAG Configuration Option,
   that "Nodes other than the DODAG Root MUST NOT modify this
   information when propagating the DODAG Configuration option".
   Therefore, 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 DODAG.

   Section 6.3.1 of [RFC6550] defines a 3-bit Mode of Operation (MOP) in
   the DIO Base Object.  This specification applies to MOP values 0 to
   6.  For a MOP value of 7, the compression MUST be used by default
   regardless of the setting of the "T" flag.

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4.  Updating RFC 8138

   A node SHOULD source packets in the compressed form using [RFC8138]
   if and only if the "T" flag is set.  This behaviour can be overridden
   by e.g., configuration or network management.  Overriding may be
   needed e.g., to cope with a legacy implementation of the Root that
   supports [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 is responsible to compress the
   outer headers with [RFC8138], but it MUST leave the encapsulated
   packet as is.

   An external target [USEofRPLinfo] is not expected to support
   [RFC8138].  In most cases, packets from and to an external target are
   tunneled back and forth between the border router (referred to as
   6LR) that serves the external target and the Root, regardless of the
   MOP used in the RPL DODAG.  The inner packet is typically not
   compressed with [RFC8138], so for outgoing packets, the border router
   just needs to decapsulate the (compressed) outer header and forward
   the (uncompressed) inner packet towards the external target.

   A router MUST uncompress a packet that is to be forwarded to an
   external target.  Otherwise, the router MUST forward the packet in
   the form that the source used, either compressed or uncompressed.

   A RUL [UNAWARE-LEAVES] is both a leaf and an external target.  A RUL
   does not participate in RPL and depends on the parent router to
   obtain connectivity.  In the case of a RUL, forwarding towards an
   external target actually means delivering the packet.

5.  Transition Scenarios

   A node that supports [RFC8138] but not this specification can only be
   used in an homogeneous network.  Enabling the [RFC8138] compression
   without a turn-on signaling requires a "flag day"; all nodes must be
   upgraded, and then the network can be rebooted with the [RFC8138]
   compression turned on.

   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.  Though it is possible
   to roll back (see Section 5.3), adding nodes that do not support
   [RFC8138] after a roll back may be problematic if the roll back did
   not fully complete.

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5.1.  Coexistence

   A node that supports this specification can operate in a network with
   the [RFC8138] 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 5.2).

   A node that does not support [RFC8138] can interoperate with nodes
   that do in a network with [RFC8138] compression turned off.  If the
   compression is turned on, all the RPL-Aware Nodes are expected to be
   able to handle compressed packets in the compressed form.  A node
   that cannot do so may remain connected to the network as a RUL, but
   how the node is modified to turn into a RUL is out of scope.

5.2.  Inconsistent State While Migrating

   When the "T" flag is turned on 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 DODAG are still not aware of it.
   In non-storing mode, the Root will start using [RFC8138] with a
   Source Routing Header 6LoRH (SRH-6LoRH) that routes all the way to
   the parent router or to the leaf.

   To ensure that a packet is forwarded across the RPL DODAG in the form
   in which it was generated, it is required that all the RPL nodes
   support [RFC8138] at the time of the switch.

   Setting the "T" flag is ultimately the responsibility of the Network
   Administrator.  The expectation is that the network management or
   upgrading tools in place enable the Network Administrator to know
   when all the nodes that may join a DODAG were migrated.  In the case
   of a RPL instance with multiple Roots, all nodes that participate to
   the RPL Instance may potentially join any DODAG.  The network MUST be
   operated with the "T" flag reset until all nodes in the RPL Instance
   are upgraded to support this specification.

5.3.  Rolling Back

   When turning [RFC8138] compression off in the network, the Network
   Administrator MUST wait until all nodes have converged to the "T"
   flag reset before allowing nodes that do not support the compression
   in the network.

   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 RUL.

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6.  IANA Considerations

   IANA is requested to assign a new option flag from the Registry for
   the "DODAG Configuration Option Flags" that was created for [RFC6550]
   as follows:

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

                Table 1: New DODAG Configuration Option Flag

7.  Security Considerations

   First of all, it is worth noting that with [RFC6550], every node in
   the LLN that is RPL-aware can inject any RPL-based attack in the
   network.  A trust model has to be put in place in an effort to
   exclude rogue nodes from participating to the RPL and the 6LoWPAN
   signaling, as well as from the data packet exchange.  This trust
   model could be at a minimum based on a Layer-2 Secure joining and the
   Link-Layer security.  This is a generic RPL and 6LoWPAN requirement,
   see Req5.1 in Appendix of [RFC8505].

   Setting the "T" flag before all 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 [RFC8138] support
   they will be able to forward both forms.  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.

   An attacker in the middle of the network may reset the "T" flag to
   cause extra energy spending in its subDAG.  Conversely it may set the
   "T" flag, so that nodes located downstream would compress when that
   it is not desired, potentially resulting in the loss of packets.  In
   a tree structure, the attacker would be in position to drop the
   packets from and to the attacked nodes.  So the attacks above would
   be more complex and more visible than simply dropping selected
   packets.  The downstream node may have other parents and see both
   settings, which could raise attention.

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8.  Acknowledgments

   The authors wish to thank Carles Gomez, Alvaro Retana, Dominique
   Barthel and Rahul Jadhav for their in-depth reviews and constructive

   Also many thanks to Michael Richardson for being always helpful and
   responsive when need comes.

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,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <>.

   [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, <>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,

              Thubert, P. and M. Richardson, "Routing for RPL Leaves",
              Work in Progress, Internet-Draft, draft-ietf-roll-unaware-
              leaves-18, 12 June 2020, <

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10.  Informative References

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

              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-40,
              25 June 2020, <

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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