Internet Engineering Task Force (IETF)                   P. Thubert, Ed.
Request for Comments: 9035                                       L. Zhao
Updates: 8138                                              Cisco Systems
Category: Standards Track                                     April 2021
ISSN: 2070-1721


       A Routing Protocol for Low-Power and Lossy Networks (RPL)
Destination-Oriented Directed Acyclic Graph (DODAG) Configuration Option
                     for the 6LoWPAN Routing Header

Abstract

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9035.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  Related Documents
     2.2.  Glossary
     2.3.  Requirements Language
   3.  Extending RFC 6550
   4.  Updating RFC 8138
   5.  Transition Scenarios
     5.1.  Coexistence
     5.2.  Inconsistent State While Migrating
     5.3.  Rolling Back
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgments

   Authors' Addresses

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 "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550], 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 compression per [RFC8138] 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, 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 ensure that compression remains 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
   compression is globally activated in the whole DODAG.

2.  Terminology

2.1.  Related Documents

   The terminology used in this document is consistent with, and
   incorporates the terms provided in, "Terms Used in Routing for
   Low-Power and Lossy Networks" [RFC7102].  Other terms in use as
   related to LLNs are found in "Terminology for Constrained-Node
   Networks" [RFC7228].

   "RPL", "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" [RFC9008].  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 (Routing Protocol for
   Low-Power and Lossy Networks) Leaves" [RFC9010].

2.2.  Glossary

   This document often uses the following abbreviations:

   6LoRH:  6LoWPAN Routing Header
   6LoWPAN:  IPv6 over Low-Power Wireless Personal Area Network
   DIO:  DODAG Information Object (a RPL message)
   DODAG:  Destination-Oriented Directed Acyclic Graph
   LLN:  Low-Power and Lossy Network
   MOP:  RPL Mode of Operation
   RAL:  RPL-Aware Leaf
   RAN:  RPL-Aware Node
   RPI:  RPL Packet Information
   RPL:  IPv6 Routing Protocol for Low-Power and Lossy Networks
   RUL:  RPL-Unaware Leaf
   SRH:  Source Routing Header
   Sub-DODAG:  The sub-DODAG of a node is a DODAG rooted at that node
      that is a subset of a main DODAG the node belongs to.  It is
      formed by the other nodes in the main DODAG whose paths to the
      main DODAG root pass through that node.

2.3.  Requirements Language

   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.  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.  The DODAG Configuration option was originally designed with
   four 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 Compression per RFC
   8138 (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 in Sections 20.14 and 6.7.6 of [RFC6550], respectively.

   Section 4.1.2 of [RFC9008] 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 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.

4.  Updating RFC 8138

   A node SHOULD generate packets in 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 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 per [RFC8138], but it MUST NOT perform compression on the
   encapsulated packet.

   An external target [RFC9008] 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 a 6LoWPAN
   Router (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 per [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 border router that forwards a packet to an external target MUST
   uncompress the packet first.  In all other cases, a router MUST
   forward a packet in the form that the source used, either compressed
   or uncompressed.

   A RUL [RFC9010] 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 compression per [RFC8138]
   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 6LoRH compression [RFC8138] turned on.

   The intent of this specification is to perform a migration once and
   for all, without the need for a flag day.  In particular, the intent
   is not to undo the setting of the 'T' flag.  Though it is possible to
   roll back (see Section 5.3), the rollback operation SHOULD be
   complete before the network operator adds nodes that do not support
   [RFC8138].

5.1.  Coexistence

   A node that supports this specification can operate in a network with
   6LoRH compression [RFC8138] 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 6LoRH compression [RFC8138] turned off.  If
   compression is turned on, all the RANs are expected to be able to
   handle packets in compressed form.  A node that cannot do so may
   remain connected to the network as a RUL as described in [RFC9010].

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 compressed form,
   while other nodes in the same RPL DODAG will still not be 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 in
   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 6LoRH compression [RFC8138] off in the network, the
   network administrator MUST wait until each node has its 'T' flag
   unset before allowing nodes that do not support compression in the
   network.  Information regarding whether compression is active in a
   node SHOULD be exposed in the node's management interface.

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

6.  IANA Considerations

   This specification updates the "DODAG Configuration Option Flags for
   MOP 0..6" registry [RFC9008] (formerly the "DODAG Configuration
   Option Flags" registry, which was created for [RFC6550]), by
   allocating one new flag as follows:

     +------------+-------------------------------------+-----------+
     | Bit Number | Capability Description              | Reference |
     +------------+-------------------------------------+-----------+
     | 2          | Enable Compression per RFC 8138 (T) | RFC 9035  |
     +------------+-------------------------------------+-----------+

               Table 1: New DODAG Configuration Option Flag

   IANA has added 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; see [RFC7416] for details.  This
   document typically applies 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 benefits from the same protection as the
   rest of the information in the DODAG Configuration option that
   transports it.  Touching the new bit is just one of the many attacks
   that can happen if an attacker manages to inject a corrupted
   configuration option in the network.

   Setting and unsetting the 'T' flag may create inconsistencies in the
   network, but as long as all nodes are upgraded to provide support for
   [RFC8138], 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 uncompressed form.

   An attacker may unset the 'T' flag to force additional energy
   consumption of child or descendant nodes in its sub-DODAG.
   Conversely, it may set the 'T' flag so that nodes located downstream
   would compress packets even when compression is not desired,
   potentially causing packet loss.  In a tree structure, the attacker
   would be in a position to drop the packets from and to the attacked
   nodes.  So, the attacks mentioned above would be more complex and
   more visible than simply dropping selected packets.  The downstream
   node may have other parents and see the bit with both settings; such
   a situation may be detected, and an alert may be triggered.

8.  References

8.1.  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>.

   [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>.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <https://www.rfc-editor.org/info/rfc7102>.

   [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>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc8505>.

   [RFC9010]  Thubert, P., Ed. and M. Richardson, "Routing for RPL
              (Routing Protocol for Low-Power and Lossy Networks)
              Leaves", RFC 9010, DOI 10.17487/RFC9010, April 2021,
              <https://www.rfc-editor.org/info/rfc9010>.

8.2.  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,
              <https://www.rfc-editor.org/info/rfc6282>.

   [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,
              <https://www.rfc-editor.org/info/rfc6553>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [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,
              <https://www.rfc-editor.org/info/rfc7416>.

   [RFC9008]  Robles, M.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", RFC 9008,
              DOI 10.17487/RFC9008, April 2021,
              <https://www.rfc-editor.org/info/rfc9008>.

Acknowledgments

   The authors wish to thank Murray Kucherawy, Meral Shirazipour, Barry
   Leiba, Tirumaleswar Reddy, Nagendra Kumar Nainar, Stewart Bryant,
   Carles Gomez, Éric 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 always being helpful and
   responsive when the need arises.

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