Internet Engineering Task Force (IETF)                  R.A. Jadhav, Ed.
Request for Comments: 9009                                        Huawei
Category: Standards Track                                     P. Thubert
ISSN: 2070-1721                                                    Cisco
                                                              R.N. Sahoo
                                                                  Z. Cao
                                                                  Huawei
                                                              April 2021


                      Efficient Route Invalidation

Abstract

   This document explains the problems associated with the use of No-
   Path Destination Advertisement Object (NPDAO) messaging in RFC 6550
   and also discusses the requirements for an optimized route
   invalidation messaging scheme.  Further, this document specifies a
   new proactive route invalidation message called the "Destination
   Cleanup Object" (DCO), which fulfills requirements for optimized
   route invalidation messaging.

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/rfc9009.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language and Terminology
     1.2.  RPL NPDAO Messaging
     1.3.  Why Is NPDAO Messaging Important?
   2.  Problems with the RPL NPDAO Messaging
     2.1.  Lost NPDAO Due to Link Break to the Previous Parent
     2.2.  Invalidating Routes of Dependent Nodes
     2.3.  Possible Route Downtime Caused by Asynchronous Operation of
           the NPDAO and DAO
   3.  Requirements for NPDAO Optimization
     3.1.  Req. #1: Remove Messaging Dependency on the Link to the
           Previous Parent
     3.2.  Req. #2: Route Invalidation for Dependent Nodes at the
           Parent Switching Node
     3.3.  Req. #3: Route Invalidation Should Not Impact Data Traffic
   4.  Changes to RPL Signaling
     4.1.  Change in RPL Route Invalidation Semantics
     4.2.  Transit Information Option Changes
     4.3.  Destination Cleanup Object (DCO)
       4.3.1.  Secure DCO
       4.3.2.  DCO Options
       4.3.3.  Path Sequence in the DCO
       4.3.4.  Destination Cleanup Option Acknowledgment (DCO-ACK)
       4.3.5.  Secure DCO-ACK
     4.4.  DCO Base Rules
     4.5.  Unsolicited DCO
     4.6.  Other Considerations
       4.6.1.  Invalidation of Dependent Nodes
       4.6.2.  NPDAO and DCO in the Same Network
       4.6.3.  Considerations for DCO Retries
       4.6.4.  DCO with Multiple Preferred Parents
   5.  IANA Considerations
     5.1.  New Registry for the Destination Cleanup Object (DCO) Flags
     5.2.  New Registry for the Destination Cleanup Object (DCO)
           Acknowledgment Flags
     5.3.  RPL Rejection Status Values
   6.  Security Considerations
   7.  Normative References
   Appendix A.  Example Messaging
     A.1.  Example DCO Messaging
     A.2.  Example DCO Messaging with Multiple Preferred Parents
   Acknowledgments
   Authors' Addresses

1.  Introduction

   RPL (the Routing Protocol for Low-Power and Lossy Networks) as
   defined in [RFC6550] specifies a proactive distance-vector-based
   routing scheme.  RPL has optional messaging in the form of DAO
   (Destination Advertisement Object) messages, which the 6LBR (6LoWPAN
   Border Router) and 6LR (6LoWPAN Router) can use to learn a route
   towards the downstream nodes. ("6LoWPAN" stands for "IPv6 over Low-
   Power Wireless Personal Area Network".)  In Storing mode, DAO
   messages would result in routing entries being created on all
   intermediate 6LRs from a node's parent all the way towards the 6LBR.

   RPL allows the use of No-Path DAO (NPDAO) messaging to invalidate a
   routing path corresponding to the given target, thus releasing
   resources utilized on that path.  An NPDAO is a DAO message with a
   route lifetime of zero.  It originates at the target node and always
   flows upstream towards the 6LBR.  This document explains the problems
   associated with the use of NPDAO messaging in [RFC6550] and also
   discusses the requirements for an optimized route invalidation
   messaging scheme.  Further, this document specifies a new proactive
   route invalidation message called the "Destination Cleanup Object"
   (DCO), which fulfills requirements for optimized route invalidation
   messaging.

   This document only caters to RPL's Storing Mode of Operation (MOP).
   The Non-Storing MOP does not require the use of an NPDAO for route
   invalidation, since routing entries are not maintained on 6LRs.

1.1.  Requirements Language and Terminology

   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.

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550].

   Low-Power and Lossy Network (LLN):
      A network in which both the routers and their interconnects are
      constrained.  LLN routers typically operate with constraints on
      processing power, memory, and energy (battery power).  Their
      interconnects are characterized by high loss rates, low data
      rates, and instability.

   6LoWPAN Router (6LR):
      An intermediate router that is able to send and receive Router
      Advertisements (RAs) and Router Solicitations (RSs) as well as
      forward and route IPv6 packets.

   Directed Acyclic Graph (DAG):
      A directed graph having the property that all edges are oriented
      in such a way that no cycles exist.

   Destination-Oriented DAG (DODAG):
      A DAG rooted at a single destination, i.e., at a single DAG root
      with no outgoing edges.

   6LoWPAN Border Router (6LBR):
      A border router that is a DODAG root and is the edge node for
      traffic flowing in and out of the 6LoWPAN.

   Destination Advertisement Object (DAO):
      DAO messaging allows downstream routes to the nodes to be
      established.

   DODAG Information Object (DIO):
      DIO messaging allows upstream routes to the 6LBR to be
      established.  DIO messaging is initiated at the DAO root.

   Common ancestor node:
      A 6LR/6LBR node that is the first common node between two paths of
      a target node.

   No-Path DAO (NPDAO):
      A DAO message that has a target with a lifetime of 0.  Used for
      the purpose of route invalidation.

   Destination Cleanup Object (DCO):
      A new RPL control message code defined by this document.  DCO
      messaging improves proactive route invalidation in RPL.

   Regular DAO:
      A DAO message with a non-zero lifetime.  Routing adjacencies are
      created or updated based on this message.

   Target node:
      The node switching its parent whose routing adjacencies are
      updated (created/removed).

1.2.  RPL NPDAO Messaging

   RPL uses NPDAO messaging in Storing mode so that the node changing
   its routing adjacencies can invalidate the previous route.  This is
   needed so that nodes along the previous path can release any
   resources (such as the routing entry) they maintain on behalf of the
   target node.

   Throughout this document, we will refer to the topology shown in
   Figure 1:

                                  (6LBR)
                                    |
                                    |
                                    |
                                   (A)
                                   / \
                                  /   \
                                 /     \
                               (G)     (H)
                                |       |
                                |       |
                                |       |
                               (B)     (C)
                                 \      ;
                                  \    ;
                                   \  ;
                                    (D)
                                    / \
                                   /   \
                                  /     \
                                (E)     (F)

                         Figure 1: Sample Topology

   Node D is connected via preferred parent B.  D has an alternate path
   via C towards the 6LBR.  Node A is the common ancestor for D for
   paths through B-G and C-H.  When D switches from B to C, RPL allows
   sending an NPDAO to B and a regular DAO to C.

1.3.  Why Is NPDAO Messaging Important?

   Resources in LLN nodes are typically constrained.  There is limited
   memory available, and routing entry records are one of the primary
   elements occupying dynamic memory in the nodes.  Route invalidation
   helps 6LR nodes to decide which routing entries can be discarded for
   better use of the limited resources.  Thus, it becomes necessary to
   have an efficient route invalidation mechanism.  Also note that a
   single parent switch may result in a "subtree" switching from one
   parent to another.  Thus, the route invalidation needs to be done on
   behalf of the subtree and not the switching node alone.  In the above
   example, when Node D switches its parent, route updates need to be
   done for the routing table entries of C, H, A, G, and B with
   destinations D, E, and F.  Without efficient route invalidation, a
   6LR may have to hold a lot of stale route entries.

2.  Problems with the RPL NPDAO Messaging

2.1.  Lost NPDAO Due to Link Break to the Previous Parent

   When a node switches its parent, the NPDAO is to be sent to its
   previous parent and a regular DAO to its new parent.  In cases where
   the node switches its parent because of transient or permanent parent
   link/node failure, the NPDAO message may not be received by the
   parent.

2.2.  Invalidating Routes of Dependent Nodes

   RPL does not specify how route invalidation will work for dependent
   nodes in the switching node subDAG, resulting in stale routing
   entries of the dependent nodes.  The only way for a 6LR to invalidate
   the route entries for dependent nodes would be to use route lifetime
   expiry, which could be substantially high for LLNs.

   In the example topology, when Node D switches its parent, Node D
   generates an NPDAO on its own behalf.  There is no NPDAO generated by
   the dependent child Nodes E and F, through the previous path via D to
   B and G, resulting in stale entries on Nodes B and G for Nodes E and
   F.

2.3.  Possible Route Downtime Caused by Asynchronous Operation of the
      NPDAO and DAO

   A switching node may generate both an NPDAO and a DAO via two
   different paths at almost the same time.  It is possible that the
   NPDAO may invalidate the previous route and the regular DAO sent via
   the new path gets lost on the way.  This may result in route
   downtime, impacting downward traffic for the switching node.

   In the example topology, say that Node D switches from parent B to C.
   An NPDAO sent via the previous route may invalidate the previous
   route, whereas there is no way to determine whether the new DAO has
   successfully updated the route entries on the new path.

3.  Requirements for NPDAO Optimization

3.1.  Req. #1: Remove Messaging Dependency on the Link to the Previous
      Parent

   When the switching node sends the NPDAO message to the previous
   parent, it is normal that the link to the previous parent is prone to
   failure (that's why the node decided to switch).  Therefore, it is
   required that the route invalidation does not depend on the previous
   link, which is prone to failure.  The previous link referred to here
   represents the link between the node and its previous parent (from
   which the node is now disassociating).

3.2.  Req. #2: Route Invalidation for Dependent Nodes at the Parent
      Switching Node

   It should be possible to do route invalidation for dependent nodes
   rooted at the switching node.

3.3.  Req. #3: Route Invalidation Should Not Impact Data Traffic

   While sending the NPDAO and DAO messages, it is possible that the
   NPDAO successfully invalidates the previous path, while the newly
   sent DAO gets lost (new path not set up successfully).  This will
   result in downstream unreachability to the node switching paths.
   Therefore, it is desirable that the route invalidation is
   synchronized with the DAO to avoid the risk of route downtime.

4.  Changes to RPL Signaling

4.1.  Change in RPL Route Invalidation Semantics

   As described in Section 1.2, the NPDAO originates at the node
   changing to a new parent and traverses upstream towards the root.  In
   order to solve the problems discussed in Section 2, this document
   adds a new proactive route invalidation message called the
   "Destination Cleanup Object" (DCO), which originates at a common
   ancestor node and flows downstream the old path.  The common ancestor
   node generates a DCO when removing a next hop to a target -- for
   instance, as a delayed response to receiving a regular DAO from
   another child node with a Path Sequence for the target that is the
   same or newer, in which case the DCO transmission is canceled.

   The 6LRs in the path for the DCO take such action as route
   invalidation based on the DCO information and subsequently send
   another DCO with the same information downstream to the next hop(s).
   This operation is similar to how the DAOs are handled on intermediate
   6LRs in the Storing MOP [RFC6550].  Just like the DAO in the Storing
   MOP, the DCO is sent using link-local unicast source and destination
   IPv6 addresses.  Unlike the DAO, which always travels upstream, the
   DCO always travels downstream.

   In Figure 1, when child Node D decides to switch the path from parent
   B to parent C, it sends a regular DAO to Node C with reachability
   information containing the address of D as the target and an
   incremented Path Sequence.  Node C will update the routing table
   based on the reachability information in the DAO and will in turn
   generate another DAO with the same reachability information and
   forward it to H.  Node H recursively follows the same procedure as
   Node C and forwards it to Node A.  When Node A receives the regular
   DAO, it finds that it already has a routing table entry on behalf of
   the Target Address of Node D.  It finds, however, that the next-hop
   information for reaching Node D has changed, i.e., Node D has decided
   to change the paths.  In this case, Node A, which is the common
   ancestor node for Node D along the two paths (previous and new), can
   generate a DCO that traverses the network downwards over the old path
   to the target.  Node A handles normal DAO forwarding to the 6LBR as
   required by [RFC6550].

4.2.  Transit Information Option Changes

   Every RPL message is divided into base message fields and additional
   options, as described in Section 6 of [RFC6550].  The base fields
   apply to the message as a whole, and options are appended to add
   message-specific / use-case-specific attributes.  As an example, a
   DAO message may be attributed by one or more "RPL Target" options
   that specify that the reachability information is for the given
   targets.  Similarly, a Transit Information option may be associated
   with a set of RPL Target options.

   This document specifies a change in the Transit Information option to
   contain the "Invalidate previous route" (I) flag.  This 'I' flag
   signals the common ancestor node to generate a DCO on behalf of the
   target node with a RPL Status of 195, indicating that the address has
   moved.  The 'I' flag is carried in the Transit Information option,
   which augments the reachability information for a given set of one or
   more RPL Targets.  A Transit Information option with the 'I' flag set
   should be carried in the DAO message when route invalidation is
   sought for the corresponding target or targets.

   Value 195 represents the 'U' and 'A' bits in RPL Status, to be set as
   per Figure 6 of [RFC9010], with the lower 6 bits set to the 6LoWPAN
   Neighbor Discovery (ND) Extended Address Registration Option (EARO)
   Status value of 3 indicating 'Moved' as per Table 1 of [RFC8505].

      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 = 0x06 | Option Length |E|I|  Flags    | Path Control  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Path Sequence | Path Lifetime |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 2: Updated Transit Information Option (New 'I' Flag Added)

   I (Invalidate previous route) flag:  The 'I' flag is set by the
      target node to indicate to the common ancestor node that it wishes
      to invalidate any previous route between the two paths.

   [RFC6550] allows the parent address to be sent in the Transit
   Information option, depending on the MOP.  In the case of the Storing
   MOP, the field is usually not needed.  In the case of a DCO, the
   Parent Address field MUST NOT be included.

   Upon receiving a DAO message with a Transit Information option that
   has the 'I' flag set, and as a delayed response removing a routing
   adjacency to the target indicated in the Transit Information option,
   the common ancestor node SHOULD generate a DCO message to the next
   hop associated to that adjacency.  The 'I' flag is intended to give
   the target node control over its own route invalidation, serving as a
   signal to request DCO generation.

4.3.  Destination Cleanup Object (DCO)

   A new ICMPv6 RPL control message code is defined by this
   specification and is referred to as the "Destination Cleanup Object"
   (DCO), which is used for proactive cleanup of state and routing
   information held on behalf of the target node by 6LRs.  The DCO
   message always traverses downstream and cleans up route information
   and other state information associated with the given target.  The
   format of the DCO message is shown in Figure 3.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | RPLInstanceID |K|D|   Flags   |   RPL Status  | DCOSequence   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                      DODAGID (optional)                       +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Option(s)...
     +-+-+-+-+-+-+-+-+

                         Figure 3: DCO Base Object

   RPLInstanceID:  8-bit field indicating the topology instance
      associated with the DODAG, as learned from the DIO.

   K:  The 'K' flag indicates that the recipient of a DCO message is
      expected to send a DCO-ACK back.  If the DCO-ACK is not received
      even after setting the 'K' flag, an implementation may retry the
      DCO at a later time.  The number of retries is implementation and
      deployment dependent and is expected to be kept similar to the
      number of DAO retries [RFC6550].  Section 4.6.3 specifies the
      considerations for DCO retries.  A node receiving a DCO message
      without the 'K' flag set MAY respond with a DCO-ACK, especially to
      report an error condition.  An example error condition could be
      that the node sending the DCO-ACK does not find the routing entry
      for the indicated target.  When the sender does not set the 'K'
      flag, it is an indication that the sender does not expect a
      response, and the sender SHOULD NOT retry the DCO.

   D:  The 'D' flag indicates that the DODAGID field is present.  This
      flag MUST be set when a local RPLInstanceID is used.

   Flags:  The 6 bits remaining unused in the Flags field are reserved
      for future use.  These bits MUST be initialized to zero by the
      sender and MUST be ignored by the receiver.

   RPL Status:  As defined in [RFC6550] and updated in [RFC9010].  The
      root or common parent that generates a DCO is authoritative for
      setting the status information, and the information is unchanged
      as propagated down the DODAG.  This document does not specify a
      differentiated action based on the RPL Status.

   DCOSequence:  8-bit field incremented at each unique DCO message from
      a node and echoed in the DCO-ACK message.  The initial DCOSequence
      can be chosen randomly by the node.  Section 4.4 explains the
      handling of the DCOSequence.

   DODAGID (optional):  128-bit unsigned integer set by a DODAG root
      that uniquely identifies a DODAG.  This field MUST be present when
      the 'D' flag is set and MUST NOT be present if the 'D' flag is not
      set.  The DODAGID is used when a local RPLInstanceID is in use, in
      order to identify the DODAGID that is associated with the
      RPLInstanceID.

4.3.1.  Secure DCO

   A Secure DCO message follows the format shown in [RFC6550], Figure 7,
   where the base message format is the DCO message shown in Figure 3 of
   this document.

4.3.2.  DCO Options

   The DCO message MUST carry at least one RPL Target and the Transit
   Information option and MAY carry other valid options.  This
   specification allows for the DCO message to carry the following
   options:

   0x00  Pad1
   0x01  PadN
   0x05  RPL Target
   0x06  Transit Information
   0x09  RPL Target Descriptor

   Section 6.7 of [RFC6550] defines all the above-mentioned options.
   The DCO carries a RPL Target option and an associated Transit
   Information option with a lifetime of 0x00000000 to indicate a loss
   of reachability to that target.

4.3.3.  Path Sequence in the DCO

   A DCO message includes a Transit Information option for each
   invalidated path.  The value of the Path Sequence counter in the
   Transit Information option allows identification of the freshness of
   the DCO message versus the newest known to the 6LRs along the path
   being removed.  If the DCO is generated by a common parent in
   response to a DAO message, then the Transit Information option in the
   DCO MUST use the value of the Path Sequence as found in the newest
   Transit Information option that was received for that target by the
   common parent.  If a 6LR down the path receives a DCO with a Path
   Sequence that is not newer than the Path Sequence as known from a
   Transit Information option in a DAO message, then the 6LR MUST NOT
   remove its current routing state, and it MUST NOT forward the DCO
   down a path where it is not newer.  If the DCO is newer, the 6LR may
   retain a temporary state to ensure that a DAO that is received later
   with a Transit Information option with an older sequence number is
   ignored.  A Transit Information option in a DAO message that is as
   new as or newer than that in a DCO wins, meaning that the path
   indicated in the DAO is installed and the DAO is propagated.  When
   the DCO is propagated upon a DCO from an upstream parent, the Path
   Sequence MUST be copied from the received DCO.

4.3.4.  Destination Cleanup Option Acknowledgment (DCO-ACK)

   The DCO-ACK message SHOULD be sent as a unicast packet by a DCO
   recipient in response to a unicast DCO message with the 'K' flag set.
   If the 'K' flag is not set, then the receiver of the DCO message MAY
   send a DCO-ACK, especially to report an error condition.  The format
   of the DCO-ACK message is shown in Figure 4.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | RPLInstanceID |D|   Flags     |  DCOSequence  | DCO-ACK Status|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                      DODAGID (optional)                       +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: DCO-ACK Base Object

   RPLInstanceID:  8-bit field indicating the topology instance
      associated with the DODAG, as learned from the DIO.

   D:  The 'D' flag indicates that the DODAGID field is present.  This
      flag MUST be set when a local RPLInstanceID is used.

   Flags:  7-bit unused field.  The field MUST be initialized to zero by
      the sender and MUST be ignored by the receiver.

   DCOSequence:  8-bit field.  The DCOSequence in the DCO-ACK is copied
      from the DCOSequence received in the DCO message.

   DCO-ACK Status:  Indicates completion status.  The DCO-ACK Status
      field is defined based on Figure 6 of [RFC9010] defining the RPL
      Status Format.  A StatusValue of 0 along with the 'U' bit set to 0
      indicates Success / Unqualified acceptance as per Figure 6 of
      [RFC9010].  A StatusValue of 1 with the 'U' bit set to 1 indicates
      'No routing entry' as defined in Section 5.3 of this document.

   DODAGID (optional):  128-bit unsigned integer set by a DODAG root
      that uniquely identifies a DODAG.  This field MUST be present when
      the 'D' flag is set and MUST NOT be present when the 'D' flag is
      not set.  The DODAGID is used when a local RPLInstanceID is in
      use, in order to identify the DODAGID that is associated with the
      RPLInstanceID.

4.3.5.  Secure DCO-ACK

   A Secure DCO-ACK message follows the format shown in [RFC6550],
   Figure 7, where the base message format is the DCO-ACK message shown
   in Figure 4 of this document.

4.4.  DCO Base Rules

   1.  If a node sends a DCO message with newer or different information
       than the prior DCO message transmission, it MUST increment the
       DCOSequence field by at least one.  A DCO message transmission
       that is identical to the prior DCO message transmission MAY
       increment the DCOSequence field.  The DCOSequence counter follows
       the sequence counter operation as defined in Section 7.2 of
       [RFC6550].

   2.  The RPLInstanceID and DODAGID fields of a DCO message MUST have
       the same values as those contained in the DAO message in response
       to which the DCO is generated on the common ancestor node.

   3.  A node MAY set the 'K' flag in a unicast DCO message to solicit a
       unicast DCO-ACK in response, in order to confirm the attempt.

   4.  A node receiving a unicast DCO message with the 'K' flag set
       SHOULD respond with a DCO-ACK.  A node receiving a DCO message
       without the 'K' flag set MAY respond with a DCO-ACK, especially
       to report an error condition.

   5.  A node receiving a unicast DCO message MUST verify the stored
       Path Sequence in context to the given target.  If the stored Path
       Sequence is as new as or newer than the Path Sequence received in
       the DCO, then the DCO MUST be dropped.

   6.  A node that sets the 'K' flag in a unicast DCO message but does
       not receive a DCO-ACK in response MAY reschedule the DCO message
       transmission for another attempt, up until an implementation-
       specific number of retries.

   7.  A node receiving a unicast DCO message with its own address in
       the RPL Target option MUST strip off that Target option.  If this
       Target option is the only one in the DCO message, then the DCO
       message MUST be dropped.

   The scope of DCOSequence values is unique to the node that generates
   them.

4.5.  Unsolicited DCO

   A 6LR may generate an unsolicited DCO to unilaterally clean up the
   path on behalf of the target entry.  The 6LR has all the state
   information, namely, the Target Address and the Path Sequence,
   required for generating a DCO in its routing table.  The conditions
   under which a 6LR may generate an unsolicited DCO are beyond the
   scope of this document, but possible reasons could be as follows:

   1.  On route expiry of an entry, a 6LR may decide to graciously clean
       up the entry by initiating a DCO.

   2.  A 6LR needs to entertain higher-priority entries in case the
       routing table is full, thus resulting in eviction of an existing
       routing entry.  In this case, the eviction can be handled
       graciously by using a DCO.

   A DCO that is generated asynchronously to a DAO message and is meant
   to discard all state along the path regardless of the Path Sequence
   MUST use a Path Sequence value of 240 (see Section 7.2 of [RFC6550]).
   This value allows the DCO to win against any established DAO path but
   to lose against a DAO path that is being installed.  Note that if an
   ancestor initiates a unilateral path cleanup on an established path
   using a DCO with a Path Sequence value of 240, the DCO will
   eventually reach the target node, which will thus be informed of the
   path invalidation.

4.6.  Other Considerations

4.6.1.  Invalidation of Dependent Nodes

   The RPL specification [RFC6550] does not provide a mechanism for
   route invalidation for dependent nodes.  This document allows the
   invalidation of dependent nodes.  Dependent nodes will generate their
   respective DAOs to update their paths, and the previous route
   invalidation for those nodes should work in a manner similar to what
   is described for a switching node.  The dependent node may set the
   'I' flag in the Transit Information option as part of a regular DAO
   so as to request invalidation of the previous route from the common
   ancestor node.

   Dependent nodes do not have any indication regarding whether any of
   their parents have in turn decided to switch their parent.  Thus, for
   route invalidation, the dependent nodes may choose to always set the
   'I' flag in all their DAO messages' Transit Information options.
   Note that setting the 'I' flag is not counterproductive even if there
   is no previous route to be invalidated.

4.6.2.  NPDAO and DCO in the Same Network

   The NPDAO mechanism provided in [RFC6550] can still be used in the
   same network where a DCO is used.  NPDAO messaging can be used, for
   example, on route lifetime expiry of the target or when the node
   simply decides to gracefully terminate the RPL session on graceful
   node shutdown.  Moreover, a deployment can have a mix of nodes
   supporting the DCO and the existing NPDAO mechanism.  It is also
   possible that the same node supports both NPDAO and DCO signaling for
   route invalidation.

   Section 9.8 of [RFC6550] states, "When a node removes a node from its
   DAO parent set, it SHOULD send a No-Path DAO message (Section 6.4.3)
   to that removed DAO parent to invalidate the existing route."  This
   document introduces an alternative and more optimized way to perform
   route invalidation, but it also allows existing NPDAO messaging to
   work.  Thus, an implementation has two choices to make when a route
   invalidation is to be initiated:

   1.  Use an NPDAO to invalidate the previous route, and send a regular
       DAO on the new path.

   2.  Send a regular DAO on the new path with the 'I' flag set in the
       Transit Information option such that the common ancestor node
       initiates the DCO message downstream to invalidate the previous
       route.

   This document recommends using option 2, for the reasons specified in
   Section 3 of this document.

   This document assumes that all the 6LRs in the network support this
   specification.  If there are 6LR nodes that do not support this
   document that are in the path of the DCO message transmission, then
   the route invalidation for the corresponding targets (targets that
   are in the DCO message) may not work or may work partially.
   Alternatively, a node could generate an NPDAO if it does not receive
   a DCO with itself as the target within a specified time limit.  The
   specified time limit is deployment specific and depends upon the
   maximum depth of the network and per-hop average latency.  Note that
   sending an NPDAO and a DCO for the same operation would not result in
   unwanted side effects because the acceptability of an NPDAO or a DCO
   depends upon the Path Sequence freshness.

4.6.3.  Considerations for DCO Retries

   A DCO message could be retried by a sender if it sets the 'K' flag
   and does not receive a DCO-ACK.  The DCO retry time could be
   dependent on the maximum depth of the network and average per-hop
   latency.  This could range from 2 seconds to 120 seconds, depending
   on the deployment.  If the latency limits are not known, an
   implementation MUST NOT retry more than once in 3 seconds and MUST
   NOT retry more than three times.

   The number of retries could also be set depending on how critical the
   route invalidation could be for the deployment and the link-layer
   retry configuration.  For networks supporting only Multi-Point to
   Point (MP2P) and Point-to-Multipoint (P2MP) flows, such as in
   Advanced Metering Infrastructure (AMI) and telemetry applications,
   the 6LRs may not be very keen to invalidate routes, unless they are
   highly memory constrained.  For home and building automation networks
   that may have substantial P2P traffic, the 6LRs might be keen to
   invalidate efficiently because it may additionally impact forwarding
   efficiency.

4.6.4.  DCO with Multiple Preferred Parents

   [RFC6550] allows a node to select multiple preferred parents for
   route establishment.  Section 9.2.1 of [RFC6550] specifies, "All DAOs
   generated at the same time for the same target MUST be sent with the
   same Path Sequence in the Transit Information."  Subsequently, when
   route invalidation has to be initiated, an NPDAO, which can be
   initiated with an updated Path Sequence to all the parent nodes
   through which the route is to be invalidated, can be used; see
   [RFC6550].

   With a DCO, the target node itself does not initiate the route
   invalidation; this is left to the common ancestor node.  A common
   ancestor node when it discovers an updated DAO from a new next hop,
   it initiates a DCO.  It is recommended that an implementation
   initiate a DCO after a time period (DelayDCO) such that the common
   ancestor node may receive updated DAOs from all possible next hops.
   This will help to reduce DCO control overhead, i.e., the common
   ancestor can wait for updated DAOs from all possible directions
   before initiating a DCO for route invalidation.  After timeout, the
   DCO needs to be generated for all the next hops for which the route
   invalidation needs to be done.

   This document recommends using a DelayDCO timer value of 1 second.
   This value is inspired by the default DelayDAO timer value of 1
   second [RFC6550].  Here, the hypothesis is that the DAOs from all
   possible parent sets would be received on the common ancestor within
   this time period.

   It is still possible that a DCO is generated before all the updated
   DAOs from all the paths are received.  In this case, the ancestor
   node would start the invalidation procedure for paths from which the
   updated DAO is not received.  The DCO generated in this case would
   start invalidating the segments along these paths on which the
   updated DAOs are not received.  But once the DAO reaches these
   segments, the routing state would be updated along these segments;
   this should not lead to any inconsistent routing states.

   Note that there is no requirement for synchronization between a DCO
   and DAOs.  The DelayDCO timer simply ensures that DCO control
   overhead can be reduced and is only needed when the network contains
   nodes using multiple preferred parents.

5.  IANA Considerations

   IANA has allocated codes for the DCO and DCO-ACK messages from the
   "RPL Control Codes" registry.

   +======+===========================================+===============+
   | Code |                Description                |   Reference   |
   +======+===========================================+===============+
   | 0x07 |         Destination Cleanup Object        | This document |
   +------+-------------------------------------------+---------------+
   | 0x08 | Destination Cleanup Object Acknowledgment | This document |
   +------+-------------------------------------------+---------------+
   | 0x87 |     Secure Destination Cleanup Object     | This document |
   +------+-------------------------------------------+---------------+
   | 0x88 |     Secure Destination Cleanup Object     | This document |
   |      |               Acknowledgment              |               |
   +------+-------------------------------------------+---------------+

             Table 1: New Codes for DCO and DCO-ACK Messages

   IANA has allocated bit 1 from the "Transit Information Option Flags"
   registry for the 'I' flag (Invalidate previous route; see
   Section 4.2).

5.1.  New Registry for the Destination Cleanup Object (DCO) Flags

   IANA has created a registry for the 8-bit Destination Cleanup Object
   (DCO) Flags field.  The "Destination Cleanup Object (DCO) Flags"
   registry is located in the "Routing Protocol for Low Power and Lossy
   Networks (RPL)" registry.

   New bit numbers may be allocated only by IETF Review [RFC8126].  Each
   bit is tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following bits are currently defined:

       +============+==============================+===============+
       | Bit number |         Description          |   Reference   |
       +============+==============================+===============+
       |     0      |     DCO-ACK request (K)      | This document |
       +------------+------------------------------+---------------+
       |     1      | DODAGID field is present (D) | This document |
       +------------+------------------------------+---------------+

                          Table 2: DCO Base Flags

5.2.  New Registry for the Destination Cleanup Object (DCO)
      Acknowledgment Flags

   IANA has created a registry for the 8-bit Destination Cleanup Object
   (DCO) Acknowledgment Flags field.  The "Destination Cleanup Object
   (DCO) Acknowledgment Flags" registry is located in the "Routing
   Protocol for Low Power and Lossy Networks (RPL)" registry.

   New bit numbers may be allocated only by IETF Review [RFC8126].  Each
   bit is tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following bit is currently defined:

       +============+==============================+===============+
       | Bit number |         Description          |   Reference   |
       +============+==============================+===============+
       |     0      | DODAGID field is present (D) | This document |
       +------------+------------------------------+---------------+

                         Table 3: DCO-ACK Base Flag

5.3.  RPL Rejection Status Values

   This document adds a new status value to the "RPL Rejection Status"
   subregistry initially created per Section 12.6 of [RFC9010].

               +=======+==================+===============+
               | Value |     Meaning      |   Reference   |
               +=======+==================+===============+
               |   1   | No routing entry | This document |
               +-------+------------------+---------------+

                Table 4: Rejection Value of the RPL Status

6.  Security Considerations

   This document introduces the ability for a common ancestor node to
   invalidate a route on behalf of the target node.  The common ancestor
   node could be directed to do so by the target node, using the 'I'
   flag in a DCO's Transit Information option.  However, the common
   ancestor node is in a position to unilaterally initiate the route
   invalidation, since it possesses all the required state information,
   namely, the Target Address and the corresponding Path Sequence.
   Thus, a rogue common ancestor node could initiate such an
   invalidation and impact the traffic to the target node.

   The DCO carries a RPL Status value, which is informative.  New Status
   values may be created over time, and a node will ignore an unknown
   Status value.  This enables the RPL Status field to be used as a
   cover channel.  But the channel only works once, since the message
   destroys its own medium, i.e., the existing route that it is
   removing.

   This document also introduces an 'I' flag, which is set by the target
   node and used by the ancestor node to initiate a DCO if the ancestor
   sees an update in the routing adjacency.  However, this flag could be
   spoofed by a malicious 6LR in the path and can cause invalidation of
   an existing active path.  Note that invalidation will work only if
   the Path Sequence condition is also met for the target for which the
   invalidation is attempted.  Having said that, such a malicious 6LR
   may spoof a DAO on behalf of the (sub) child with the 'I' flag set
   and can cause route invalidation on behalf of the (sub) child node.
   Note that by using existing mechanisms offered by [RFC6550], a
   malicious 6LR might also spoof a DAO with a lifetime of zero or
   otherwise cause denial of service by dropping traffic entirely, so
   the new mechanism described in this document does not present a
   substantially increased risk of disruption.

   This document assumes that the security mechanisms as defined in
   [RFC6550] are followed, which means that the common ancestor node and
   all the 6LRs are part of the RPL network because they have the
   required credentials.  A non-secure RPL network needs to take into
   consideration the risks highlighted in this section as well as those
   highlighted in [RFC6550].

   All RPL messages support a secure version of messages; this allows
   integrity protection using either a Message Authentication Code (MAC)
   or a signature.  Optionally, secured RPL messages also have
   encryption protection for confidentiality.

   This document adds new messages (DCO and DCO-ACK) that are
   syntactically similar to existing RPL messages such as DAO and DAO-
   ACK.  Secure versions of DCO and DCO-ACK messages are added in a way
   that is similar to the technique used for other RPL messages (such as
   DAO and DAO-ACK).

   RPL supports three security modes, as mentioned in Section 10.1 of
   [RFC6550]:

   Unsecured:  In this mode, it is expected that the RPL control
      messages are secured by other security mechanisms, such as link-
      layer security.  In this mode, the RPL control messages, including
      DCO and DCO-ACK messages, do not have Security sections.  Also
      note that unsecured mode does not imply that all messages are sent
      without any protection.

   Preinstalled:  In this mode, RPL uses secure messages.  Thus, secure
      versions of DCO and DCO-ACK messages MUST be used in this mode.

   Authenticated:  In this mode, RPL uses secure messages.  Thus, secure
      versions of DCO and DCO-ACK messages MUST be used in this mode.

7.  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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

Appendix A.  Example Messaging

A.1.  Example DCO Messaging

   In this example, Node D (Figure 1) switches its parent from B to C.
   This example assumes that Node D has already established its own
   route via Node B-G-A-6LBR using pathseq=x.  The example uses DAO and
   DCO messaging conventions and specifies only the required parameters
   to explain the example, namely, the parameter 'tgt', which stands for
   "Target option"; the value of this parameter specifies the address of
   the target node.  The parameter 'pathseq' specifies the Path Sequence
   value carried in the Transit Information option, and the parameter
   'I_flag' specifies the 'I' flag in the Transit Information option.
   The sequence of actions is as follows:

   1.  Node D switches its parent from Node B to Node C.

   2.  D sends a regular DAO(tgt=D,pathseq=x+1,I_flag=1) in the updated
       path to C.

   3.  C checks for a routing entry on behalf of D; since it cannot find
       an entry on behalf of D, it creates a new routing entry and
       forwards the reachability information of the target D to H in a
       DAO(tgt=D,pathseq=x+1,I_flag=1).

   4.  Similar to C, Node H checks for a routing entry on behalf of D,
       cannot find an entry, and hence creates a new routing entry and
       forwards the reachability information of the target D to A in a
       DAO(tgt=D,pathseq=x+1,I_flag=1).

   5.  Node A receives the DAO(tgt=D,pathseq=x+1,I_flag=1) and checks
       for a routing entry on behalf of D.  It finds a routing entry but
       checks that the next hop for target D is different (i.e., Node
       G).  Node A checks the I_flag and generates the
       DCO(tgt=D,pathseq=x+1) to the previous next hop for target D,
       which is G.  Subsequently, Node A updates the routing entry and
       forwards the reachability information of target D upstream using
       the DAO(tgt=D,pathseq=x+1,I_flag=1).

   6.  Node G receives the DCO(tgt=D,pathseq=x+1).  It checks to see if
       the received Path Sequence is later than the stored Path
       Sequence.  If it is later, Node G invalidates the routing entry
       of target D and forwards the (un)reachability information
       downstream to B in the DCO(tgt=D,pathseq=x+1).

   7.  Similarly, B processes the DCO(tgt=D,pathseq=x+1) by invalidating
       the routing entry of target D and forwards the (un)reachability
       information downstream to D.

   8.  D ignores the DCO(tgt=D,pathseq=x+1), since the target is itself.

   9.  The propagation of the DCO will stop at any node where the node
       does not have routing information associated with the target.  If
       cached routing information is present and the cached Path
       Sequence is higher than the value in the DCO, then the DCO is
       dropped.

A.2.  Example DCO Messaging with Multiple Preferred Parents

   As shown in Figure 5, node (N41) selects multiple preferred parents
   (N32) and (N33).  The sequence of actions is listed below the figure.

                                   (6LBR)
                                     |
                                     |
                                     |
                                   (N11)
                                    / \
                                   /   \
                                  /     \
                               (N21)   (N22)
                                 /      / \
                                /      /   \
                               /      /     \
                            (N31)  (N32)  (N33)
                                :    |    /
                                 :   |   /
                                  :  |  /
                                   (N41)

                        Figure 5: Sample Topology 2

   1.   (N41) sends a DAO(tgt=N41,PS=x,I_flag=1) to (N32) and (N33).
        Here, 'I_flag' refers to the Invalidation flag, and 'PS' refers
        to the Path Sequence in the Transit Information option.

   2.   (N32) sends the DAO(tgt=N41,PS=x,I_flag=1) to (N22).  (N33) also
        sends the DAO(tgt=N41,PS=x,I_flag=1) to (N22).  (N22) learns
        multiple routes for the same destination (N41) through multiple
        next hops.  (N22) may receive the DAOs from (N32) and (N33) in
        any order with the I_flag set.  The implementation should use
        the DelayDCO timer to wait to initiate the DCO.  If (N22)
        receives an updated DAO from all the paths, then the DCO need
        not be initiated in this case.  Thus, the routing table at N22
        should contain (Dst,NextHop,PS): { (N41,N32,x), (N41,N33,x) }.

   3.   (N22) sends the DAO(tgt=N41,PS=x,I_flag=1) to (N11).

   4.   (N11) sends the DAO(tgt=N41,PS=x,I_flag=1) to (6LBR).  Thus, the
        complete path is established.

   5.   (N41) decides to change the preferred parent set from
        { N32, N33 } to { N31, N32 }.

   6.   (N41) sends the DAO(tgt=N41,PS=x+1,I_flag=1) to (N32).  (N41)
        sends the DAO(tgt=N41,PS=x+1,I_flag=1) to (N31).

   7.   (N32) sends the DAO(tgt=N41,PS=x+1,I_flag=1) to (N22).  (N22)
        has multiple routes to destination (N41).  It sees that a new
        Path Sequence for Target=N41 is received and thus waits for a
        predetermined time period (the DelayDCO time period) to
        invalidate another route { (N41),(N33),x }.  After the time
        period, (N22) sends the DCO(tgt=N41,PS=x+1) to (N33).  Also
        (N22) sends the regular DAO(tgt=N41,PS=x+1,I_flag=1) to (N11).

   8.   (N33) receives the DCO(tgt=N41,PS=x+1).  The received Path
        Sequence is the latest and thus invalidates the entry associated
        with the target (N41).  (N33) then sends the DCO(tgt=N41,PS=x+1)
        to (N41).  (N41) sees itself as the target and drops the DCO.

   9.   From Step 6 above, (N31) receives the
        DAO(tgt=N41,PS=x+1,I_flag=1).  It creates a routing entry and
        sends the DAO(tgt=N41,PS=x+1,I_flag=1) to (N21).  Similarly,
        (N21) receives the DAO and subsequently sends the
        DAO(tgt=N41,PS=x+1,I_flag=1) to (N11).

   10.  (N11) receives the DAO(tgt=N41,PS=x+1,I_flag=1) from (N21).  It
        waits for the DelayDCO timer, since it has multiple routes to
        (N41).  (N41) will receive the DAO(tgt=N41,PS=x+1,I_flag=1) from
        (N22) from Step 7 above.  Thus, (N11) has received the regular
        DAO(tgt=N41,PS=x+1,I_flag=1) from all paths and thus does not
        initiate the DCO.

   11.  (N11) forwards the DAO(tgt=N41,PS=x+1,I_flag=1) to (6LBR), and
        the full path is established.

Acknowledgments

   Many thanks to Alvaro Retana, Cenk Gundogan, Simon Duquennoy,
   Georgios Papadopoulos, and Peter van der Stok for their review and
   comments.  Alvaro Retana helped shape this document's final version
   with critical review comments.

Authors' Addresses

   Rahul Arvind Jadhav (editor)
   Huawei
   Whitefield
   Kundalahalli Village
   Bangalore 560037
   Karnataka
   India

   Phone: +91-080-49160700
   Email: rahul.ietf@gmail.com


   Pascal Thubert
   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


   Rabi Narayan Sahoo
   Huawei
   Whitefield
   Kundalahalli Village
   Bangalore 560037
   Karnataka
   India

   Phone: +91-080-49160700
   Email: rabinarayans0828@gmail.com


   Zhen Cao
   Huawei
   W Chang'an Ave
   Beijing
   China

   Email: zhencao.ietf@gmail.com