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Versions: (draft-jadhav-roll-efficient-npdao) 00

ROLL                                                      R. Jadhav, Ed.
Internet-Draft                                                  R. Sahoo
Intended status: Standards Track                                  Z. Cao
Expires: December 28, 2017                                   Huawei Tech
                                                           June 26, 2017


                       No-Path DAO modifications
                   draft-ietf-roll-efficient-npdao-00

Abstract

   This document describes the problems associated with the use of No-
   Path DAO messaging in RPL and a 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 http://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 December 28, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language and Terminology . . . . . . . . . .   3
     1.2.  Current No-Path DAO messaging . . . . . . . . . . . . . .   3
     1.3.  Cases when No-Path DAO may be used  . . . . . . . . . . .   4
     1.4.  Why No-Path DAO is important? . . . . . . . . . . . . . .   5
   2.  Problems with current  No-Path DAO messaging  . . . . . . . .   5
     2.1.  Lost NP-DAO due to link break to the previous parent  . .   5
     2.2.  Invalidate routes to dependent nodes of the switching
           node  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Route downtime caused by asynchronous operation of
           NPDAO and DAO . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements for the No-Path DAO Optimization . . . . . . . .   6
     3.1.  Req#1: Tolerant to the link failures to the previous
           parents . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Req#2: Dependent nodes route invalidation on parent
           switching . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Req#3: No impact on traffic while NP-DAO operation in
           progress  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Proposed changes to NPDAO signaling . . . . . . . . . . . . .   7
     4.1.  Change in NPDAO semantics . . . . . . . . . . . . . . . .   7
     4.2.  DAO message format changes  . . . . . . . . . . . . . . .   7
       4.2.1.  Path Sequence number in the reverse NPDAO . . . . . .   9
     4.3.  Example messaging . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Other considerations  . . . . . . . . . . . . . . . . . .  10
       4.4.1.  Dependent Nodes invalidation  . . . . . . . . . . . .  10
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Additional Stuff . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   RPL [RFC6550] specifies a proactive distance-vector based routing
   scheme.  The specification has an optional messaging in the form of
   DAO messages using which the 6LBR can learn route towards any of the
   nodes.  In storing mode, DAO messages would result in routing entries
   been created on all intermediate hops 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 No-Path DAO is a DAO message with



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   route lifetime of zero, signaling route invalidation for the given
   target.  This document explains the problems associated with the
   current use of NPDAO messaging and also discusses the requirements
   for an optimized No-Path DAO messaging scheme.  The signalling change
   specified fulfills all mentioned requirements of an optimized NPDAO
   messaging.

   6TiSCH architecture [I-D.ietf-6tisch-architecture] leverages RPL and
   specifies use of non-storing and storing MOP for its routing
   operation.  Thus an improvement in NPDAO messaging will help optimize
   6TiSCH based networks.

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].

   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.

   Common Ancestor node: 6LR node which is the first common node on the
   old and new path for the child node.

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

   Reverse NPDAO: A No-Path DAO message which traverses downstream in
   the network.

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

   This document also uses terminology described in [RFC6550].

1.2.  Current No-Path DAO messaging

   RPL introduced No-Path DAO messaging in the storing mode so that the
   node switching its current parent can inform its parents and
   ancestors to invalidate the existing route.  Subsequently parents or
   ancestors would release any resources (such as the routing entry) it
   maintains on behalf of target node.  The NPDAO message always
   traverses the RPL tree in upward direction, originating at the target
   node itself.

   For the rest of this document consider the following topology:






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                                   (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 BR.  Node (A) is the common ancestor for (D)
   for paths through (B)-(G) and (C)-(H).  When (D) switches from (B) to
   (C), [RFC6550] suggests sending No-Path DAO to (B) and regular DAO to
   (C).

1.3.  Cases when No-Path DAO may be used

   There are following cases in which a node switches its parent and may
   employ No-Path DAO messaging:

   Case I: Current parent becomes unavailable because of transient or
   permanent link or parent node failure.

   Case II: The node finds a better parent node i.e. the metrics of
   another parent is better than its current parent.

   Case III: The node switches to a new parent whom it "thinks" has a
   better metric but does not in reality.

   The usual steps of operation when the node switches the parent is
   that the node sends a No-Path DAO message via its current parent to
   invalidate its current route and subsequently it tries to establish a
   new routing path by sending a new DAO via its new parent.



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1.4.  Why No-Path DAO is important?

   Nodes in LLNs may be resource constrained.  There is limited memory
   available and routing entry records are the 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 in case of contention.  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 invalidation
   needs to be done for (D), (E) and (F).  Thus without efficient route
   invalidation, a 6LR may have to hold a lot of unwanted route entries.

2.  Problems with current No-Path DAO messaging

2.1.  Lost NP-DAO due to link break to the previous parent

   When a node switches its parent, the NPDAO is to be sent via its
   previous parent and a regular DAO via 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.  RPL
   assumes communication link with the previous parent for No-Path DAO
   messaging.

   RPL allows use of route lifetime to remove unwanted routes in case
   the routes could not be refreshed.  But route lifetimes in case of
   LLNs could be substantially high and thus the route entries would be
   stuck for long.

2.2.  Invalidate routes to dependent nodes of the switching node

   No-path DAO is sent by the node who has switched the parent but it
   does not work for the dependent child nodes below it.  The
   specification does not specify how route invalidation will work for
   sub-childs, resulting in stale routing entries on behalf of the sub-
   childs on the previous route.  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.  Post switching, Node (D) transmits
   a DIO with incremented DTSN so that child nodes, node (E) and (F),
   generate DAOs to trigger route update on the new path for themselves.
   There is no NPDAO generated by these child nodes through the previous
   path resulting in stale entries on nodes (B) and (G) for nodes (E)
   and (F).



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2.3.  Route downtime caused by asynchronous 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 thus impacting downward traffic for the switching node.  In
   the example topology, consider Node (D) switches from parent (B) to
   (C) because the metrics of the path via (C) are better.  Note that
   the previous path via (B) may still be available (albeit at
   relatively bad metrics).  An NPDAO sent from previous route may
   invalidate the existing route whereas there is no way to determine
   whether the new DAO has successfully updated the route entries on the
   new path.

   An implementation technique to avoid this problem is to further delay
   the route invalidation by a fixed time interval after receiving an
   NPDAO, considering the time taken for the new path to be established.
   Coming up with such a time interval is tricky since the new route may
   also not be available and it may subsequently require more parent
   switches to establish a new path.

3.  Requirements for the No-Path DAO Optimization

3.1.  Req#1: Tolerant to the link failures to the previous parents

   When the switching node send the NP-DAO message to the previous
   parent, it is normal that the link to the previous parent is prone to
   failure.  Therefore, it is required that the NP-DAO message MUST be
   tolerant to the link failure during the switching.

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

   While switching the parent node and sending NP-DAO message, it is
   required that the routing entries to the dependent nodes of the
   switching node will be updated accordingly on the previous parents
   and other relevant upstream nodes.

3.3.  Req#3: No impact on traffic while NP-DAO operation in progress

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





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4.  Proposed changes to NPDAO signaling

4.1.  Change in NPDAO semantics

   As described in Section 1.2, currently 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
   proposes to change the way NPDAO originates and traverses the
   network.  The proposed NPDAO originates at a common ancestor node
   between the new and old path.  The trigger for the common ancestor
   node to generate this NPDAO is the change in the next hop for the
   node on reception of an update message in the form of regular DAO for
   the target.

   In the 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), may generate an NPDAO which traverses
   downwards in the network.  The document in the subsequent section
   will explain the message format changes to handle this downward flow
   of NPDAO.

4.2.  DAO message format 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 specifies 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 DAO message to contain "Invalidate
   previous route" (I) bit.  This I-bit which is carried in regular DAO
   message, signals the common ancestor node to generate a downstream
   NPDAO on behalf of the target node.  The I-bit is carried in the
   transit container option which augments the reachability information
   for a given set of RPL 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 NPDAO thus generated by the common ancestor node needs to
   traverse downstream.  An additional flag called as "Reverse NPDAO"
   (R) is added in the base DAO object to signal this change.

      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|R| Flags   |   Reserved    | DAOSequence   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                            DODAGID*                           +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Option(s)...
     +-+-+-+-+-+-+-+-+

           Figure 3: Updated DAO base object (New R flag added)

   R (Reverse DAO) bit: 1 bit flag.  The 'R' flag is used to signal that
   the DAO traverses downwards.





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4.2.1.  Path Sequence number in the reverse NPDAO

   Every DAO message may contain a Path Sequence in the transit
   information option to identify the freshness of the DAO message.  The
   Path Sequence in the downward NPDAO generated by common ancestor
   should use the same Path Sequence number present in the regular DAO
   message.

4.3.  Example 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
       downstream NPDAO(tgt=D,pathseq=x+1,R_flag=1) 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 downstream NPDAO and invalidates routing
       entry of target D and then checks the reverse (R) flag and
       forwards the (un)reachability information downstream to B.

   7.  Similarly, B processes the downstream NPDAO by invalidating the
       routing entry of target D and then checks the reverse (R) flag
       and forwards the (un)reachability information downstream to D.

   8.  D ignores the downstream NPDAO since the target is itself.





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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 section describes approaches for invalidating routes of
   dependent nodes if the implementation chooses to solve this problem.
   The common ancestor node realizes that the paths for dependent nodes
   have changed (based on next hop change) when it receives a regular
   DAO on behalf of the dependent nodes.  Thus dependent nodes route
   invalidation can be handled in the same way as the switching node.
   Note that there is no way that dependent nodes can set the I_flag in
   the DAO message selectively since they are unaware that their parent/
   grand parent node is switching paths.  There are two ways to handle
   dependent node route invalidation:

   1.  One way to resolve is that the common ancestor does not depend
       upon the I_flag to generate the reverse NPDAO.  The only factor
       it makes the decision will be based on next_hop change for an
       existing target to generate the NPDAO.  Thus when the switching
       nodes and all the below dependent nodes advertise a regular DAO,
       the common ancestor node will detect a change in next hop and
       generate NPDAO for the same target as in the regular DAO.

   2.  Another way is that the nodes always set the I_flag whenever they
       send regular DAO.  Thus common ancestor will first check whether
       I_flag is set and then check whether the next_hop has changed and
       subsequently trigger NPDAO if required.

   This document recommends the approach in point 2.  The advantage with
   I_flag is that the generation of downstream NPDAO is still controlled
   by the target node and thus is still in control of its own routing
   state.

5.  Acknowledgements

   We would like to thank Cenk Gundogan, Simon Duquennoy and Pascal
   Thubert for their review and insightful comments.

6.  IANA Considerations

   IANA is requested to allocate bit 11 in the DAO base object defined
   in RPL [RFC6550] section 6.4 for reverse 'R' NPDAO flag.






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   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

   This draft does not add any new messages but extends existing
   messaging.  The seucrity considerations applicable to DAO messaging
   in RPL is also applicable here.

8.  References

8.1.  Normative References

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-11 (work
              in progress), January 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6550>.

8.2.  Informative References

   [CONTIKI]  Thingsquare, "Contiki: The Open Source OS for IoT", 2012,
              <http://www.contiki-os.org>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

Appendix A.  Additional Stuff

   This becomes an Appendix.







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Authors' Addresses

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

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


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

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


   Zhen Cao
   Huawei Tech
   W Chang'an Ave
   Beijing  560037
   China

   Email: zhencao.ietf@gmail.com






















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