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Versions: (draft-jadhav-roll-efficient-npdao) 00 01 02 03 04 05 06 07 08 09

ROLL                                                      R. Jadhav, Ed.
Internet-Draft                                                    Huawei
Intended status: Standards Track                              P. Thubert
Expires: April 17, 2019                                            Cisco
                                                                R. Sahoo
                                                                  Z. Cao
                                                                  Huawei
                                                        October 14, 2018


                      Efficient Route Invalidation
                   draft-ietf-roll-efficient-npdao-09

Abstract

   This document describes the problems associated with NPDAO messaging
   used in RPL for route invalidation and signaling changes to improve
   route invalidation efficiency.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 17, 2019.

Copyright Notice

   Copyright (c) 2018 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language and Terminology . . . . . . . . . .   3
     1.2.  Current NPDAO messaging . . . . . . . . . . . . . . . . .   4
     1.3.  Why NPDAO is important? . . . . . . . . . . . . . . . . .   5
   2.  Problems with current         NPDAO messaging . . . . . . . .   5
     2.1.  Lost NPDAO due to link break to the previous parent . . .   5
     2.2.  Invalidate routes of dependent nodes  . . . . . . . . . .   5
     2.3.  Possible route downtime caused by async operation of
           NPDAO and DAO . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements for the NPDAO Optimization . . . . . . . . . . .   6
     3.1.  Req#1: Remove messaging dependency on link to the
           previous             parent . . . . . . . . . . . . . . .   6
     3.2.  Req#2: Dependent nodes route invalidation on parent
           switching . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Req#3: Route invalidation should not impact data traffic    6
   4.  Proposed changes to RPL signaling . . . . . . . . . . . . . .   6
     4.1.  Change in RPL route invalidation semantics  . . . . . . .   6
     4.2.  Transit Information Option changes  . . . . . . . . . . .   7
     4.3.  Destination Cleanup Object (DCO)  . . . . . . . . . . . .   8
       4.3.1.  Secure DCO  . . . . . . . . . . . . . . . . . . . . .  10
       4.3.2.  DCO Options . . . . . . . . . . . . . . . . . . . . .  10
       4.3.3.  Path Sequence number in the DCO . . . . . . . . . . .  10
       4.3.4.  Destination Cleanup Option Acknowledgement (DCO-ACK)   10
       4.3.5.  Secure DCO-ACK  . . . . . . . . . . . . . . . . . . .  11
     4.4.  Other considerations  . . . . . . . . . . . . . . . . . .  12
       4.4.1.  Dependent Nodes invalidation  . . . . . . . . . . . .  12
       4.4.2.  NPDAO and DCO in the same network . . . . . . . . . .  12
       4.4.3.  DCO with multiple preferred parents . . . . . . . . .  12
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Appendix A.  Example Messaging  . . . . . . . . . . . . . . . . .  14
     A.1.  Example DCO Messaging . . . . . . . . . . . . . . . . . .  14
     A.2.  Example DCO Messaging with multiple preferred parents . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   RPL [RFC6550] (Routing Protocol for Low power and lossy networks)
   specifies a proactive distance-vector based routing scheme.  RPL has
   an optional messaging in the form of DAO (Destination Advertisement
   Object) messages using which the 6LBR (6Lo Border Router) and 6LR



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   (6Lo Router) can learn route towards the downstream nodes.  In
   storing mode, DAO messages would result in routing entries been
   created on all intermediate 6LRs from the node's parent all the way
   towards the 6LBR.

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

   The document only caters to the RPL's storing mode of operation
   (MOP).  The non-storing MOP does not require use of 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   6LR: 6LoWPAN Router.  This is an intermediate 6lowpan router which
   allows traffic routing through itself in a multihop 6lo network.

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

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

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

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

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

   Common Ancestor node: 6LR/6LBR node which is the first common node
   between two paths of a target node.



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   NPDAO: No-Path DAO.  A DAO message which has target with lifetime 0.

   DCO: Destination Cleanup Object, A new RPL control message type
   defined by this draft.  DCO messaging improves proactive route
   invalidation in RPL.

   Regular DAO: A DAO message with non-zero lifetime.

   LLN: Low Power and Lossy Networks.

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

   This document also uses terminology described in [RFC6550].

1.2.  Current NPDAO messaging

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

   For the rest of this document consider the following topology:

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

                         Figure 1: Sample topology



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   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 NPDAO to (B) and regular DAO to (C).

1.3.  Why NPDAO is important?

   Nodes in LLNs may be resource 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 entries could be discarded to better achieve
   resource utilization.  Thus it becomes necessary to have efficient
   route invalidation mechanism.  Also note that a single parent switch
   may result in a "sub-tree" switching from one parent to another.
   Thus the route invalidation needs to be done on behalf of the sub-
   tree and not the switching node alone.  In the above example, when
   Node (D) switches parent, the route updates needs to be done for the
   routing tables entries of (C),(H),(A),(G), and (B) with destination
   (D),(E) and (F).  Without efficient route invalidation, a 6LR may
   have to hold a lot of stale route entries.

2.  Problems with current 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 then the NPDAO message is bound to fail.

2.2.  Invalidate routes of dependent nodes

   RPL does not specify how route invalidation will work for dependent
   nodes rooted at switching node, resulting in stale routing entries of
   the dependent nodes.  The only way for 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 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).








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2.3.  Possible route downtime caused by async operation of NPDAO and DAO

   A switching node may generate both an NPDAO and DAO via two different
   paths at almost the same time.  There is a possibility that an NPDAO
   generated 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, consider Node (D) switches from parent (B)
   to (C).  An NPDAO sent via 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 the NPDAO Optimization

3.1.  Req#1: Remove messaging dependency on 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 (thats 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 here
   represents the link between the node and its previous parent (from
   whom the node is now disassociating).

3.2.  Req#2: Dependent nodes route invalidation on parent switching

   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.  Proposed changes to RPL signaling

4.1.  Change in RPL route invalidation semantics

   As described in Section 1.2, the NPDAO originates at the node
   switching the parent and traverses upstream towards the root.  In
   order to solve the problems as mentioned in Section 2, the draft adds
   new pro-active route invalidation message called as "Destination
   Cleanup Object" (DCO) that originates at a common ancestor node



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   between the new and old path.  The common ancestor node generates a
   DCO in response to the change in the next-hop on receiving a regular
   DAO with updated path sequence for the target.

   In Figure 1, when node D decides to switch the path from B to C, it
   sends a regular DAO to node C with reachability information
   containing target as address of D and a incremented path sequence
   number.  Node C will update the routing table based on the
   reachability information in DAO and in turn generate another DAO with
   the same reachability information and forward it to H.  Node H also
   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. the 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), should generate a DCO which traverses
   downwards in the network.

4.2.  Transit Information Option changes

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

   The draft proposes a change in Transit Information option to contain
   "Invalidate previous route" (I) bit.  This I-bit signals the common
   ancestor node to generate a DCO on behalf of the target node.  The
   I-bit is carried in the transit information option which augments the
   reachability information for a given set of RPL Target(s).  Transit
   information option should be carried in the DAO message with I-bit
   set in case route invalidation is sought for the correspondig
   target(s).














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     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 |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     +                                                               +
     |                                                               |
     +                        Parent Address*                        +
     |                                                               |
     +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 2: Updated Transit Information Option (New I flag added)

   I (Invalidate previous route) bit: 1 bit flag.  The 'I' flag is set
   by the target node to indicate that it wishes to invalidate the
   previous route by a common ancestor node between the two paths.

   The common ancestor node SHOULD generate a DCO message in response to
   this I-bit when it sees that the routing adjacencies have changed for
   the target.  I-bit governs the ownership of the DCO message in a way
   that the target node is still in control of its own route
   invalidation.

4.3.  Destination Cleanup Object (DCO)

   A new ICMPv6 RPL control message type is defined by this
   specification called as "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.















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     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   |   Reserved    | 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 is expected to send a
   DCO-ACK back.  If the DCO-ACK is not received even after setting the
   'K', an implementation may choose to retry the DCO at a later time.
   The number of retries are implementation and deployment dependent.
   This document recommends using retries similar to what will be set
   for DAO-ACK handling.

   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.

   Reserved: 8-bit unused field.  The field MUST be initialized to zero
   by the sender and MUST be ignored by the receiver.

   DCOSequence: 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.

   DODAGID (optional): 128-bit unsigned integer set by a DODAG root that
   uniquely identifies a DODAG.  This field is only present when the 'D'
   flag is set.  This field is typically only present when a local
   RPLInstanceID is in use, in order to identify the DODAGID that is
   associated with the RPLInstanceID.  When a global RPLInstanceID is in
   use, this field need not be present.  Unassigned bits of the DCO Base



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   are reserved.  They MUST be set to zero on transmission and MUST be
   ignored on reception.

4.3.1.  Secure DCO

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

4.3.2.  DCO Options

   The DCO message MAY carry 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

   The DCO carries a 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 number in the DCO

   A DCO message may contain a Path Sequence in the transit information
   option to identify the freshness of the DCO message.  The Path
   Sequence in the DCO MUST use the same Path Sequence number present in
   the regular DAO message when the DCO is generated in response to DAO
   message.  The DAO and DCO path sequence are picked from the same
   sequence number set.  Thus if a DCO is received by a 6LR and
   subsequently a DAO is received with old seqeunce number, then the DAO
   should be ignored.

4.3.4.  Destination Cleanup Option Acknowledgement (DCO-ACK)

   The DCO-ACK message may be sent as a unicast packet by a DCO
   recipient in response to a unicast DCO message.













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     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|  Reserved   |  DCOSequence  |    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.

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

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

   Status: Indicates the completion.  Status 0 is defined as unqualified
   acceptance in this specification.  The remaining status values are
   reserved as rejection codes.

   DODAGID (optional): 128-bit unsigned integer set by a DODAG root that
   uniquely identifies a DODAG.  This field is only present when the 'D'
   flag is set.  This field is typically only present when a local
   RPLInstanceID is in use, in order to identify the DODAGID that is
   associated with the RPLInstanceID.  When a global RPLInstanceID is in
   use, this field need not be present.  Unassigned bits of the DCO-Ack
   Base are reserved.  They MUST be set to zero on transmission and MUST
   be ignored on reception.

4.3.5.  Secure DCO-ACK

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





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4.4.  Other considerations

4.4.1.  Dependent Nodes invalidation

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

4.4.2.  NPDAO and DCO in the same network

   Even with the changed semantics, the current NPDAO mechanism in
   [RFC6550] can still be used, for example, when the route lifetime
   expiry of the target happens 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 proposed
   DCO and the existing NPDAO mechanism.

4.4.3.  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".  Thus a DAO message
   with the same path sequence MUST be sent to all the parents.
   Subsequently when route invalidation has to be initiated, RPL
   mentions that an NPDAO must be initiated with updated path sequence
   to all the routes to be invalidated.

   With DCO, the Target node itself does not initiate the route
   invalidation and it 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.  With multiple preferred parents, this handling
   does not change.  But in this case it is recommended that an
   implementation initiates a DCO after a time period 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.  The time period for
   initiating a DCO could be based on the depth of the network.  After
   timeout, the DCO needs to be generated for all the next-hops for whom
   the route invalidation needs to be done.





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5.  Acknowledgements

   Many thanks to Cenk Gundogan, Simon Duquennoy, Georgios
   Papadopoulous, Peter Van Der Stok for their review and comments.

6.  IANA Considerations

   IANA is requested to allocate new ICMPv6 RPL control codes in RPL
   [RFC6550] for DCO and DCO-ACK messages.

   +------+---------------------------------------------+--------------+
   | Code |                 Description                 |  Reference   |
   +------+---------------------------------------------+--------------+
   | 0x04 |          Destination Cleanup Object         |     This     |
   |      |                                             |   document   |
   | 0x05 |  Destination Cleanup Object Acknowledgement |     This     |
   |      |                                             |   document   |
   | 0x84 |      Secure Destination Cleanup Object      |     This     |
   |      |                                             |   document   |
   | 0x85 |      Secure Destination Cleanup Object      |     This     |
   |      |               Acknowledgement               |   document   |
   +------+---------------------------------------------+--------------+

   IANA is requested to allocate bit 18 in the Transit Information
   Option defined in RPL [RFC6550] section 6.7.8 for Invalidate route
   'I' flag.

7.  Security Considerations

   All RPL messages support a secure version of messages which allows
   integrity protection using either a MAC or a signature.  Optionally,
   secured RPL messages also have encryption protection for
   confidentiality.

   The document adds new messages (DCO, DCO-ACK) which are syntactically
   similar to existing RPL messages such as DAO, DAO-ACK.  Secure
   versions of DCO and DCO-ACK are added similar to other RPL messages
   (such as DAO, DAO-ACK).

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

   1.  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, DCO-ACK, do not have Security sections.
   2.  Preinstalled: In this mode, RPL uses secure messages.  Thus
       secure versions of DCO, DCO-ACK MUST be used in this mode.



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

8.  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>.

Appendix A.  Example Messaging

A.1.  Example DCO Messaging

   In Figure 1, node (D) switches its parent from (B) to (C).  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 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.
   4.  Similar to C, node H checks for 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 H in a
       DAO.
   5.  Node A receives the DAO, and checks for routing entry on behalf
       of D.  It finds a routing entry but checks that the next hop for
       target D is now changed.  Node A checks the I_flag and generates
       DCO(tgt=D,pathseq=pathseq(DAO)) to previous next hop for target D
       which is G.  Subsequently, A updates the routing entry and
       forwards the reachability information of target D upstream
       DAO(tgt=D,pathseq=x+1,I_flag=x) (the I_flag carries no
       significance henceforth).
   6.  Node G receives the DCO and invalidates routing entry of target D
       and forwards the (un)reachability information downstream to B.
   7.  Similarly, B processes the DCO by invalidating the routing entry
       of target D and forwards the (un)reachability information
       downstream to D.



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   8.  D ignores the DCO since the target is itself.
   9.  The propagation of the DCO will stop at any node where the node
       does not have an routing information associated with the target.
       If the routing information is present and the pathseq associated
       is not older, then still the DCO is dropped.

A.2.  Example DCO Messaging with multiple preferred parents

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

                        Figure 5: Sample topology 2

   In Figure 5, node (N41) selects multiple preferred parents (N32) and
   (N33).  The sequence of actions is as follows:

   1.  (N41) sends DAO(tgt=N41,PS=x,I_flag=1) to (N32) and (N33).  Here
       I_flag refers to the Invalidation flag and PS refers to Path
       Sequence in Transit Information option.
   2.  (N32) sends DAO(tgt=N41,PS=x,I_flag=1) to (N22).  (N33) also
       sends DAO(tgt=N41,PS=x,I_flag=1) to (N22).  (N22) learns multiple
       routes for the same destination (N41) through multiple next-hops.
       The route table at N22 should contain (Dst,NextHop,PS): {
       (N41,N32,x), (N41,N33,x) }.
   3.  (N22) sends DAO(tgt=N41,PS=x,I_flag=1) to (N11).
   4.  (N11) sends DAO(tgt=N41,PS=x,I_flag=1) to (6LBR).  Thus the
       complete path is established.
   5.  (N41) decides to change preferred parent set from { N32, N33 } to
       { N31, N32 }.
   6.  (N41) sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N32).  (N41) sends
       DAO(tgt=N41,PS=x+1,I_flag=1) to (N31).
   7.  (N32) sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N22).  (N22) has
       multiple routes to destination (N41).  It sees that a new path



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       sequence for Target=N41 is received and thus it waits for pre-
       determined time period to invalidate another route
       {(N41),(N33),x}. After time period, (N22) sends
       DCO(tgt=N41,PS=x+1) to (N33).

Authors' Addresses

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

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


   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254
   France

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com


   Rabi Narayan Sahoo
   Huawei
   Kundalahalli Village, Whitefield,
   Bangalore, Karnataka  560037
   India

   Phone: +91-080-49160700
   Email: rabinarayans@huawei.com


   Zhen Cao
   Huawei
   W Chang'an Ave
   Beijing
   China

   Email: zhencao.ietf@gmail.com






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