ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                   L. Zhao
Updates: 8138 (if approved)                                Cisco Systems
Intended status: Standards Track                        18 December 2020
Expires: 21 June 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 unset.

Status of This Memo

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   This Internet-Draft will expire on 21 June 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.  Extending RFC 6550  . . . . . . . . . . . . . . . . . . . . .   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 design of Low Power and Lossy Networks (LLNs) is generally
   focused on saving energy, which is the most constrained resource of
   all.  The routing optimizations in the "Routing Protocol for Low
   Power and Lossy Networks" [RFC6550] (RPL) such as routing along a
   Destination-Oriented Directed Acyclic Graph (DODAG) to a Root Node
   and the associated routing header compression and forwarding
   technique specified in [RFC8138] derive from that primary concern.

   Enabling [RFC8138] on a running network requires a Flag Day where the
   network is upgraded and rebooted.  Otherwise, if acting as a Leaf, a
   node that does not support the compression would fail to communicate;
   if acting as a router it would drop the compressed packets and black-
   hole a portion of the network.  This specification enables a hot
   upgrade where a live network is migrated.  During the migration, the
   compression remains inactive, until all nodes are upgraded.

   This document complements [RFC8138] and signals whether it should be
   used within a RPL DODAG with a new flag in the RPL DODAG
   Configuration Option.  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 flag is cleared 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
   SubDAG:  A DODAG rooted at a node which is a child of that node and a
      subset of a larger DAG
   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.  Extending RFC 6550

   The DODAG Configuration Option is defined in Section 6.7.6 of
   [RFC6550].  Its purpose is extended to distribute configuration
   information affecting the construction and maintenance of the DODAG,
   as well as operational parameters for RPL on the DODAG, through the
   DODAG.  As shown in Figure 1, the Option was originally 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| | |T| |A|       ...           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     +
                                     <- Flags ->

            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 [RFC8138] within the
   DODAG.  The "T" flag is encoded in position 2 of the reserved Flags
   in the DODAG Configuration Option (counting from bit 0 as the most
   significant bit) and set to 0 in legacy implementations as specified
   respectively in Sections 20.14 and 6.7.6 of [RFC6550].

   Section 4.3 of [USEofRPLinfo] updates [RFC6550] to indicate that the
   definition of the Flags applies to Mode of Operation (MOP) values
   zero (0) to six (6) only.  For a MOP value of 7, [RFC8138] MUST be
   used on Links where 6LoWPAN Header Compression [RFC6282] applies and
   MUST NOT be used otherwise.

   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.
   [RFC6550] states 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, and when the "T" flag is set, it is transparently
   flooded to all the nodes in the DODAG.

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4.  Updating RFC 8138

   A node SHOULD generate packets in the compressed form using [RFC8138]
   if and only if the "T" flag is set.  This behavior can be overridden
   by configuration or network management.  Overriding may be needed
   e.g., to turn on the compression in a network where all nodes support
   [RFC8138] but the Root does not support this specification and cannot
   set the "T" flag, or to disable it locally in case of a problem.

   The decision to use [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 encapsulating a packet is the originator of the
   resulting packet and is responsible for compressing 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 to and from 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 a homogeneous network.  Enabling the [RFC8138] compression
   without a turn-on signaling method requires a "flag day"; by which
   time all nodes must be upgraded, and at which point 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), the roll back operation SHOULD be
   complete before the network operator adds nodes that do not support

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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 as
   described in [UNAWARE-LEAVES].

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 unset 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 unset before allowing nodes that do not support the compression
   in the network.  To that effect, whether the compression is active in
   a node SHOULD be exposed the node's management interface.

   Nodes that do not support [RFC8138] SHOULD NOT be deployed in a
   network where the compression is turned on.  If that is done, the
   node can only operate as a RUL.

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6.  IANA Considerations

   This specification updates the Registry that was created for
   [RFC6550] as the registry for "DODAG Configuration Option Flags" and
   updated as the registry for "DODAG Configuration Option Flags for MOP
   0..6" by [USEofRPLinfo], by allocating one new Flag as follows:

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

                Table 1: New DODAG Configuration Option Flag

   IANA is requested to add [this document] as a reference for MOP 7 in
   the RPL Mode of Operation registry.

7.  Security Considerations

   It is worth noting that in RPL [RFC6550], every node in the LLN that
   is RPL-aware and has access to the RPL domain can inject any RPL-
   based attack in the network, more in [RFC7416].  This document
   applies typically to an existing deployment and does not change its
   security requirements and operations.  It is assumed that the
   security mechanisms as defined for RPL are followed.

   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 unsetting 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.

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   An attacker may unset the "T" flag to force additional energy
   consumption of child or descendant nodes 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.

8.  Acknowledgments

   The authors wish to thank Murray Kucherawy, Meral Shirazipour, Barry
   Leiba, Tirumaleswar Reddy, Nagendra Kumar Nainar, Stewart Bryant,
   Carles Gomez, Eric Vyncke, Roman Danyliw, and especially Benjamin
   Kaduk, Alvaro Retana, Dominique Barthel and Rahul Jadhav for their
   in-depth reviews and constructive suggestions.

   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, <>.

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   [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-27, 17 December 2020, <

10.  Informative References

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [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,

   [RFC7416]  Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
              and M. Richardson, Ed., "A Security Threat Analysis for
              the Routing Protocol for Low-Power and Lossy Networks
              (RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,

              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-42,
              12 November 2020, <

Authors' Addresses

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   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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