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ROLL Working Group                                             M. Robles
Internet-Draft                                                  Ericsson
Intended status: Informational                             M. Richardson
Expires: October 7, 2016                                             SSW
                                                              P. Thubert
                                                                   Cisco
                                                           April 5, 2016


              When to use RFC 6553, 6554 and IPv6-in-IPv6
                    draft-ietf-roll-useofrplinfo-04

Abstract

   This document looks at different data flows through LLN networks
   where RPL is used to establish routing.  The document enumerates the
   cases where RFC 6553, RFC 6554 and IPv6-in-IPv6 encapsulation is
   required.  This analysis provides the basis on which to design
   efficient compression of these headers.

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 October 7, 2016.

Copyright Notice

   Copyright (c) 2016 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Requirements Language . . . . . . . . . . . .   3
   3.  Sample/reference topology . . . . . . . . . . . . . . . . . .   3
   4.  Use cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Storing mode  . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Example of Flow from RPL-aware-leaf to root . . . . . . .   9
     5.2.  Example of Flow from root to RPL-aware-leaf . . . . . . .   9
     5.3.  Example of Flow from root to not-RPL-aware-leaf . . . . .  10
     5.4.  Example of Flow from not-RPL-aware-leaf to root . . . . .  11
     5.5.  Example of Flow from RPL-aware-leaf to Internet . . . . .  11
     5.6.  Example of Flow from Internet to RPL-aware-leaf . . . . .  12
     5.7.  Example of Flow from not-RPL-aware-leaf to Internet . . .  12
     5.8.  Example of Flow from Internet to non-RPL-aware-leaf . . .  13
     5.9.  Example of Flow from RPL-aware-leaf to RPL-aware-leaf . .  14
     5.10. Example of Flow from RPL-aware-leaf to non-RPL-aware-leaf  15
     5.11. Example of Flow from not-RPL-aware-leaf to RPL-aware-leaf  17
     5.12. Example of Flow from not-RPL-aware-leaf to not-RPL-aware-
           leaf  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Non Storing mode  . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Example of Flow from RPL-aware-leaf to root . . . . . . .  19
     6.2.  Example of Flow from root to RPL-aware-leaf . . . . . . .  20
     6.3.  Example of Flow from root to not-RPL-aware-leaf . . . . .  20
     6.4.  Example of Flow from not-RPL-aware-leaf to root . . . . .  21
     6.5.  Example of Flow from RPL-aware-leaf to Internet . . . . .  22
     6.6.  Example of Flow from Internet to RPL-aware-leaf . . . . .  22
     6.7.  Example of Flow from not-RPL-aware-leaf to Internet . . .  23
     6.8.  Example of Flow from Internet to non-RPL-aware-leaf . . .  23
     6.9.  Example of Flow from RPL-aware-leaf to RPL-aware-leaf . .  24
     6.10. Example of Flow from RPL-aware-leaf to not-RPL-aware-leaf  25
     6.11. Example of Flow from not-RPL-aware-leaf to RPL-aware-leaf  26
     6.12. Example of Flow from not-RPL-aware-leaf to not-RPL-aware-
           leaf  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Observations about the problem  . . . . . . . . . . . . . . .  27
     7.1.  Storing mode  . . . . . . . . . . . . . . . . . . . . . .  27
     7.2.  Non-Storing mode  . . . . . . . . . . . . . . . . . . . .  28
   8.  6LoRH Compression cases . . . . . . . . . . . . . . . . . . .  28
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  29
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  29
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     12.2.  Informative References . . . . . . . . . . . . . . . . .  30



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   RPL [RFC6550] is a routing protocol for constrained networks.  RFC
   6553 [RFC6553] defines the "RPL option" (RPI), carried within the
   IPv6 Hop-by-Hop header to quickly identify inconsistencies (loops) in
   the routing topology.  RFC 6554 [RFC6554] defines the "RPL Source
   Route Header" (RH3), an IPv6 Extension Header to deliver datagrams
   within a RPL routing domain, particularly in non-storing mode.

   These various items are referred to as RPL artifacts, and they are
   seen on all of the data-plane traffic that occurs in RPL routed
   networks; they do not in general appear on the RPL control plane
   traffic at all which is mostly hop-by-hop traffic (one exception
   being DAO messages in non-storing mode).

   It has become clear from attempts to do multi-vendor
   interoperability, and from a desire to compress as many of the above
   artifacts as possible that not all implementors agree when artifacts
   are necessary, or when they can be safely omitted, or removed.

   An interim meeting went through the 24 cases defined here to discover
   if there were any shortcuts, and this document is the result of that
   discussion.  This document should not be defining anything new, but
   it may clarify what is correct and incorrect behaviour.

   The related document A Routing Header Dispatch for 6LoWPAN (6LoRH)
   [I-D.ietf-roll-routing-dispatch] defines a method to compress RPL
   Option information and Routing Header type 3 (RFC6554) and an
   efficient IP-in-IP technique.  Uses cases proposed for the
   [Second6TischPlugtest] involving 6loRH.

2.  Terminology and Requirements Language

   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].

   Terminology defined in [RFC7102]

3.  Sample/reference topology

   A RPL network is composed of a 6LBR (6LoWPAN Border Router), Backbone
   Router (6BBR), 6LR (6LoWPAN Router) and 6LN (6LoWPAN Node) as leaf
   logically organized in a DODAG structure (Destination Oriented
   Directed Acyclic Graph).




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   RPL defines the RPL Control messages (control plane ), a new ICMPv6
   message with Type 155.  DIS, DIO and DAO messages are all RPL Control
   messages but with different Code values.

   RPL supports two modes of Downward traffic: in storing mode, it is
   fully stateful or an in non-storing, it is fully source routed.  A
   RPL Instance is either fully storing or fully non-storing, i.e. a RPL
   Instance with a combination of storing and non-storing nodes is not
   supported with the current specifications.

   +--------------+
   | Upper Layers |
   |              |
   +--------------+
   |   RPL        |
   |              |
   +--------------+
   |   ICMPv6     |
   |              |
   +--------------+
   |   IPv6       |
   |              |
   +--------------+
   |   6LoWPAN    |
   |              |
   +--------------+
   |   PHY-MAC    |
   |              |
   +--------------+



                           Figure 1: RPL Stack.


















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                                    +---------+
                                +---+Internet |
                                |   +---------+
                                |
                           +----+--+
                           |DODAG  |  node:01
                 +---------+Root   +----------+
                 |         |6LBR   |          |
                 |         +----+--+          |
                 |              |             |
                 |              |             |
                ...            ...           ...
                 |              |             |
           +-----+-+         +--+---+      +--+---+
           |6LR    |         |      |      |      |
     +-----+       |         |      |      |      |
     |     |   11  |         |   12 |      |   13 +------+
     |     +-----+-+         +-+----+      +-+----+      |
     |           |             |             |           |
     |           |             |             |           |
     | 21        | 22          | 23          | 24        | 25
   +-+---+     +-+---+      +--+--+       +- --+     +---+-+
   |Leaf |     |     |      |     |       |Leaf|     |Leaf |
   | 6LR |     |     |      |     |       | 6LN|     | 6LR |
   +-----+     +-----+      +-----+       +----+     +-----+


                    Figure 2: A reference RPL Topology.

   The numbers in or above the nodes are there so that they may be
   referenced in subsequent sections.  The leaf marked 6LN (24) is a
   device which does not speak RPL at all, but uses Router-
   Advertisements, 6LowPAN DAR/DAC and efficient-ND only to participate
   in the network.

   This document is in part motivated by the work that is ongoing at the
   6TiSCH working group.  The 6TiSCH architecture
   [I-D.ietf-6tisch-architecture] draft explains the network
   architecture of a 6TiSCH network.  This architecture is used for the
   remainder of this document.

   The scope of the 6TiSCH Architecture is a Backbone Link that
   federates multiple LLNs (mesh) as a single IPv6 Multi-Link Subnet.
   Each LLN in the subnet is anchored at a Backbone Router (6BBR).  The
   Backbone Routers interconnect the LLNs over the Backbone Link and
   emulate that the LLN nodes are present on the Backbone thus creating
   a so-called: Multi-Link Subnet.  An LLN node can move freely from an
   LLN anchored at a Backbone Router to another LLN anchored at the same



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   or a different Backbone Router inside the Multi-Link Subnet and
   conserve its addresses.


                  |
               +-----+
               |     | Border Router to the RPL domain
               |     |  (may be a RPL virtual root)
               +-----+
                  |
                  |          Backbone
            +-------------------+-------------------+
            |                   |                   |
         +-----+             +-----+             +-----+
         |     | Backbone    |     | Backbone    |     | Backbone
         |     | router      |     | router      |     | router
         +|---|+             +-|||-+             +-[_]-+
          |   | PCI-exp       / | \ USB             | Ethernet
         ( ) ( )            ( )( )( )     (6LBR == RPL DODAG root)
        o o   o  o       o o   o  o  o            o  o   o
        o o   o o  o    o  o   o  o  o  o     o   o  o  o   o
       o  o o  o o       o   o  o  o  o     6LR == RPL router) o o
       o   o  o  o          o    o  o             z
       o   o o               o  o   o       (6LoWPAN Host)

       <----------------------- RPL Instance ------------------------>


                     Figure 3: RPL domain architecture

4.  Use cases

   In data plane context a combination of RFC6553, RFC6554 and IPv6-in-
   IPv6 encapsulation is going to be analyzed for the following traffic
   flows:

      RPL-aware-leaf to root

      root to RPL-aware-leaf

      not-RPL-aware-leaf to root

      root to not-RPL-aware-leaf

      RPL-aware-leaf to Internet

      Internet to RPL-aware-leaf




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      not-RPL-aware-leaf to Internet

      Internet to not-RPL-aware-leaf

      RPL-aware-leaf to RPL-aware-leaf

      RPL-aware-leaf to not-RPL-aware-leaf

      not-RPL-aware-leaf to RPL-aware-leaf

      not-RPL-aware-leaf to not-RPL-aware-leaf

   This document assumes a rule that a Header cannot be inserted or
   removed on the fly inside an IPv6 packet that is being routed.  This
   is a fundamental precept of the IPv6 architecture as outlined in
   [RFC2460] is that Extensions may not be added or removed except by
   the sender or the receiver.

   A second important thing is that packets with a Hop-by-Hop option
   which are marked with option type 01 ([RFC2460] section 4.2) must be
   discarded if received by a host or router which does not understand
   that option.  This means that in general, any packet that leaves the
   RPL domain of an LLN (or leaves the LLN entirely) is likely to be
   discarded if it still contains an [RFC6553] RPL Option Header known
   as the RPI.

   The combination of these two rules means that the arrangement of
   headers must be done so that traffic intended to exit the RPL domain
   can have the RPI option removed prior to leaving the RPL domain.

   An intermediate router that needs to add a header must encapsulate
   the packet in an (additional) outer IP header where the new header
   can be placed.

   This also means that a Header can only be removed by an intermediate
   router if it is placed in an encapsulating IPv6 Header, and in that
   case, the whole encapsulating header must be removed - a replacement
   may be added.  Further, an intermediate router can only remove such
   an outer header if that outer header has the router as the
   destination!

   Both RPI and RH3 headers may be modified by routers on the path of
   the packet without the need to add to remove an encapsulating header.
   Both headers were designed with this modification in mind, and both
   the RPL RH and the RPL option are marked mutable but recoverable, so
   an IPsec AH security header can be applied across these headers, but
   it may not secure all the values in those headers.




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   RPI should be present in every single RPL data packet.  There is one
   exception in non-storing mode: when a packet is going down from the
   route.  In a downward non-storing mode, the entire route is written,
   so there can be no loops by construction, nor any confusion about
   which forwarding table to use.  There may be cases (such as in
   6tisch) where the instanceID may still be needed to pick an
   appropriate priority or channel at each hop.

   The applicability for storing (RPL-SN) and non-Storing (RPL-NSN)
   modes for the previous cases is showed as follows:

   In tables, the term "RPL aware leaf" is has been shortened to "Raf",
   and "not-RPL aware leaf" has been shortened to "~Raf" to make the
   table fit in available space.

   The earlier examples are more complete to make sure that the process
   is clear, while later examples are more consise.

5.  Storing mode

   This table summarizes what headers are needed in the following
   scenarios, and indicates the IPIP header must be inserted on a hop-
   by-hop basis, and when it can target the destination node directly.
   There are three possible situations: hop-by-hop necessary (indicated
   by "hop"), or destination address possible (indicated by "dst").  In
   all cases hop by hop can be used.  In cases where no IPIP header is
   needed, the column is left blank.

         +--------------+-------+-------+-----------+-----------+
         | Use Case     | RPI   | RH3   | IP-in-IP  | IPIP dst  |
         +--------------+-------+-------+-----------+-----------+
         | Raf to root  | Yes   | No    | No        | --        |
         | root to Raf  | Yes   | No    | No        | --        |
         | root to ~Raf | Yes   | No    | Yes       | hop       |
         | ~Raf to root | Yes   | No    | Yes       | root      |
         | Raf to Int   | Yes   | No    | Yes       | root      |
         | Int to Raf   | Yes   | No    | Yes       | raf       |
         | ~Raf to Int  | Yes   | No    | Yes       | root      |
         | Int to ~Raf  | Yes   | No    | Yes       | hop       |
         | Raf to Raf   | Yes   | No    | No        | --        |
         | Raf to ~Raf  | Yes   | No    | Yes       | hop       |
         | ~Raf to Raf  | Yes   | No    | Yes       | dst       |
         | ~Raf to ~Raf | Yes   | No    | Yes       | hop       |
         +--------------+-------+-------+-----------+-----------+

        Table 1: Headers needed in Storing mode: RPI, RH3, IP-in-IP
                               encapsulation




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5.1.  Example of Flow from RPL-aware-leaf to root

   As states in Section 16.2 of [RFC6550]  a RPL-aware-leaf node does
   not generally issue DIO messages; a leaf node accepts DIO messages
   from upstream.  (When the inconsistency in routing occurs, a leaf
   node will generate a DIO with an infinite rank, to fix it).  It may
   issue DAO and DIS messages though it generally ignores DAO and DIS
   messages.

   In storing mode, it is suitable to use RFC 6553 (RPI) to send RPL
   Information instanceID and rank information.

   In this case the flow comprises:

   RPL-aware-leaf (6LN) --> 6LR --> 6LR,... --> root (6LBR)

   Note: In this document 6LRs, 6LBR are always full-fledge RPL routers,
   and are the RPL root node.

   The 6LN inserts the RPI header, and send the packet to 6LR which
   decrement the rank in RPI and send the packet up.  When the packet
   arrives to 6LBR, the RPI is removed and the packet is processed.

   The RPI header can be removed by the 6LBR because the packet is
   addressed to the 6LBR.  The 6LN must know that it is communicating
   with the 6LBR in order to be able to make use of this scenario.  The
   6LN can know the address of the 6LBR because it knows the address of
   the root via the DODAGID in the DIO messages.

                 +-------------------+-----+------+------+
                 | Header            | 6LN | 6LR  | 6LBR |
                 +-------------------+-----+------+------+
                 | Inserted headers  | RPI | --   | --   |
                 | Removed headers   | --  | --   | RPI  |
                 | Re-added headers  | --  | --   | --   |
                 | Modified headers  | --  | RPI  | --   |
                 | Untouched headers | --  | --   | --   |
                 +-------------------+-----+------+------+

    Storing: Summary of the use of headers from RPL-aware-leaf to root

5.2.  Example of Flow from root to RPL-aware-leaf

   In this case the flow comprises:

   root (6LBR)--> 6LR --> RPL-aware-leaf (6LN)





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   In this case the 6LBR insert RPI header and send the packet down, the
   6LR is going to increment the rank in RPI (examines instanceID for
   multiple tables), the packet is processed in 6LN and RPI removed.

   No IPIP header is required.

                +-------------------+------+-------+------+
                | Header            | 6LBR | 6LR   | 6LN  |
                +-------------------+------+-------+------+
                | Inserted headers  | RPI  | --    | --   |
                | Removed headers   | --   | --    | RPI  |
                | Re-added headers  | --   | --    | --   |
                | Modified headers  | --   | RPI   | --   |
                | Untouched headers | --   | --    | --   |
                +-------------------+------+-------+------+

    Storing: Summary of the use of headers from root to RPL-aware-leaf

5.3.  Example of Flow from root to not-RPL-aware-leaf

   In this case the flow comprises:

   root (6LBR)--> 6LR --> not-RPL-aware-leaf (6LN)

   It includes IPv6-in-IPv6 encapsulation to transmit information not
   related with the RPL domain.  In the 6LBR the RPI header is inserted
   into an IPv6-in-IPv6 header addressed to the last 6LR, which removes
   the header before pass the packet to the IPv6 node.

   The question in this scenario is how the root knows how to address
   the IPv6-in-IPv6 header.  It can not know that the destination isn't
   RPL aware, so it must insert an IPv6 that can be removed on the last
   RPL aware node.  Since the root can not know in a storing network
   where the last RPL aware node is, the IPv6-in-IPv6 header must added
   hop-by-hop along the path from root to leaf.

   An alternative option is to add an attribute in the RPL Target Option
   to indicate that the target is not RPL aware: future work may explore
   this possibility.












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           +-------------------+-----------+-----------+------+
           | Header            | 6LBR      | 6LR       | IPv6 |
           +-------------------+-----------+-----------+------+
           | Inserted headers  | IPIP(RPI) | --        | --   |
           | Removed headers   | --        | IPIP(RPI) | --   |
           | Re-added headers  | --        | --        | --   |
           | Modified headers  | --        | --        | --   |
           | Untouched headers | --        | --        | --   |
           +-------------------+-----------+-----------+------+

    Storing: Summary of the use of headers from root to not-RPL-aware-
                                   leaf

5.4.  Example of Flow from not-RPL-aware-leaf to root

   In this case the flow comprises:

   not-RPL-aware-leaf (6LN) --> 6LR --> root (6LBR)

   When the packet arrives from IPv6 node to 6LR, the 6LR will insert an
   RPI header, encapsuladed in a IPv6-in-IPv6 header.  The IPv6-in-IPv6
   header can be addressed to the next hop, or to the root.  The root
   removes the header and process the packet.

           +-------------------+------+------------+-----------+
           | Header            | IPv6 | 6LR        | 6LBR      |
           +-------------------+------+------------+-----------+
           | Inserted headers  | --   | IPIP(RPI)  | --        |
           | Removed headers   | --   | --         | IPIP(RPI) |
           | Re-added headers  | --   | --         | --        |
           | Modified headers  | --   | --         | --        |
           | Untouched headers | --   | --         | --        |
           +-------------------+------+------------+-----------+

     Storing: Summary of the use of headers from not-RPL-aware-leaf to
                                   root

5.5.  Example of Flow from RPL-aware-leaf to Internet

   RPL information from RFC 6553 should not go out to Internet as it
   will cause the packet to be discarded at the first non-RPI aware
   router.  The 6LBR must be able to take this information out before
   sending the packet upwards to the Internet.  This requires the RPI
   header be placed in an IPIP header that the root can remove.

   In this case the flow comprises:

   RPL-aware-leaf (6LN) --> 6LR --> root (6LBR) --> Internet



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   The 6LN will insert the RPI in a IPv6-in-IPv6 in a outer header,
   which may be addressed to the 6LBR (root), or alternatively, it could
   be addressed hop-by-hop.

      +-------------------+-----------+------+-----------+----------+
      | Header            | 6LN       | 6LR  | 6LBR      | Internet |
      +-------------------+-----------+------+-----------+----------+
      | Inserted headers  | IPIP(RPI) | --   | --        | --       |
      | Removed headers   | --        | --   | IPIP(RPI) | --       |
      | Re-added headers  | --        | --   | --        | --       |
      | Modified headers  | --        | RPI  | --        | --       |
      | Untouched headers | --        | --   | --        | --       |
      +-------------------+-----------+------+-----------+----------+

       Storing: Summary of the use of headers from RPL-aware-leaf to
                                 Internet

5.6.  Example of Flow from Internet to RPL-aware-leaf

   In this case the flow comprises:

   Internet --> root (6LBR) --> 6LR --> RPL-aware-leaf (6LN)

   When the packet arrives from Internet to 6LBR the RPI header is added
   in a outer IPv6-in-IPv6 header and send to 6LR, which modifies the
   rank in the RPI.  When the packet arrives 6LN the RPI header is
   removed and the packet processed.

     +-------------------+----------+------------+------+------------+
     | Header            | Internet | 6LBR       | 6LR  | 6LN        |
     +-------------------+----------+------------+------+------------+
     | Inserted headers  | --       | IPIP(RPI)  | --   | --         |
     | Removed headers   | --       | --         | --   | IPIP(RPI)  |
     | Re-added headers  | --       | --         | --   | --         |
     | Modified headers  | --       | --         | RPI  | --         |
     | Untouched headers | --       | --         | --   | --         |
     +-------------------+----------+------------+------+------------+

    Storing: Summary of the use of headers from Internet to RPL-aware-
                                   leaf

5.7.  Example of Flow from not-RPL-aware-leaf to Internet

   In this case the flow comprises:

   not-RPL-aware-leaf (6LN) --> 6LR --> root (6LBR) --> Internet





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   The 6LR node will add an IPIP(RPI) header addressed either to the
   root, or hop-by-hop such that the root can remove the RPI header
   before passing upwards.

   The originating node will ideally leave the IPv6 flow label as zero
   so that it can be better compressed through the LLN, and the 6LBR
   will set the flow label to a non-zero value when sending to the
   Internet.

     +-------------------+------+------------+------------+----------+
     | Header            | 6LN  | 6LR        | 6LBR       | Internet |
     +-------------------+------+------------+------------+----------+
     | Inserted headers  | --   | IPIP(RPI)  | --         | --       |
     | Removed headers   | --   | --         | IPIP(RPI)  | --       |
     | Re-added headers  | --   | --         | --         | --       |
     | Modified headers  | --   | --         | --         | --       |
     | Untouched headers | --   | --         | --         | --       |
     +-------------------+------+------------+------------+----------+

     Storing: Summary of the use of headers from not-RPL-aware-leaf to
                                 Internet

5.8.  Example of Flow from Internet to non-RPL-aware-leaf

   In this case the flow comprises:

   Internet --> root (6LBR) --> 6LR --> not-RPL-aware-leaf (6LN)

   The 6LBR will have to add an RPI header within an IPIP header.  The
   IPIP will need to be addressed hop-by-hop along the path as in
   storing mode, the 6LBR has no idea if the 6LN is RPL aware or not,
   nor what the closest attached 6LR node is.

   The 6LBR MAY set the flow label on the inner IPIP header to zero in
   order to aid in compression, as the packet will not emerge again from
   the LLN.















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     +-------------------+----------+------------+------------+------+
     | Header            | Internet | 6LBR       | 6LR        | IPv6 |
     +-------------------+----------+------------+------------+------+
     | Inserted headers  | --       | IPIP(RPI)  | --         | --   |
     | Removed headers   | --       | --         | IPIP(RPI)  | --   |
     | Re-added headers  | --       | --         | --         | --   |
     | Modified headers  | --       | --         | --         | --   |
     | Untouched headers | --       | --         | --         | --   |
     +-------------------+----------+------------+------------+------+

     Storing: Summary of the use of headers from Internet to non-RPL-
                                aware-leaf

5.9.  Example of Flow from RPL-aware-leaf to RPL-aware-leaf

   In [RFC6550] RPL allows a simple one-hop optimization for both
   storing and non-storing networks.  A node may send a packet destined
   to a one-hop neighbor directly to that node.  Section 9 in [RFC6550].

   In this case the flow comprises:

   6LN --> 6LR --> common parent (6LR) --> 6LR --> 6LN

   This case is assumed in the same RPL Domain.  In the common parent,
   the direction of RPI is changed (from increasing to decreasing the
   rank).

   While the 6LR nodes will update the RPI, no node needs to add or
   remove the RPI, so no IPIP headers are necessary.  The ability to do
   this depends upon the sending know that the destination is: a) inside
   the LLN, and b) RPL capable.

   The sender can determine if the destination is inside the LLN by
   looking if the destination address is matched by the DIO's PIO
   option.  This check may be modified by the use of backbone routers,
   but in this case it is assumed that the backbone routers are RPL
   capable and so can process the RPI header correctly.

   The other check, that the destination is RPL capable is not currently
   discernible by the sender.  This information is necessary to
   distinguish this test case from Section 5.10.










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   +-------------+-------+---------------+---------------+-----+-------+
   | Header      | 6LN   | 6LR           | 6LR (common   | 6LR | 6LN   |
   |             | src   |               | parent)       |     | dst   |
   +-------------+-------+---------------+---------------+-----+-------+
   | Inserted    | RPI   | --            | --            | --  | --    |
   | headers     |       |               |               |     |       |
   | Removed     | --    | --            | --            | --  | RPI   |
   | headers     |       |               |               |     |       |
   | Re-added    | --    | --            | --            | --  | --    |
   | headers     |       |               |               |     |       |
   | Modified    | --    | RPI           | RPI           | --  | --    |
   | headers     |       | (decreasing   | (increasing   |     |       |
   |             |       | rank)         | rank)         |     |       |
   | Untouched   | --    | --            | --            | --  | --    |
   | headers     |       |               |               |     |       |
   +-------------+-------+---------------+---------------+-----+-------+

     Storing: Summary of the use of headers for RPL-aware-leaf to RPL-
                                aware-leaf

5.10.  Example of Flow from RPL-aware-leaf to non-RPL-aware-leaf

   In this case the flow comprises:

   6LN --> 6LR --> common parent (6LR) --> 6LR --> not-RPL-aware 6LN

   The sender, being aware out of band, that the receiver is not RPL
   aware, sends adds an RPI header inside an IPIP header.  The IPIP
   header needs to be addressed on a hop-by-hop basis so that the last
   6LR can remove the RPI header.





















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                                ,---.
                               /     \
                              (  6LR2 ) IP3,RPI,IP,ULP
                             ,-"      .
                          ,-"   `---'  `.
                        ,'               `.
              ,---.  ,-"                   `,---.
             /     +"                      /     \
            ( 6LR1  )   Remove the IP3,RPI(  6LR3 )
             \     /                       \     /
              /---'                         `---'|
             /    IP2,RPI,IP,ULP                 \
            /                                     |
           /                                      \
      ,---+-.                                      |
     /       \                                  +--+----+
    (  6LN    )                                 |       |
     \       /                                  |  IPv6 |  IP,ULP
      `-----'                                   |       |
           IP1,RPI,IP,ULP                       +-------+



                Figure 4: Solution IPv6-in-IPv6 in each hop

   Alternatively, if the definition of the Option Type field of RPL
   Option '01' were changed so that it isn't a "discard if not
   recognized", then no IPIP header would be necessary.  This change is
   an incompatible on-the-wire change and would require some kind of
   flag day, possibly a change that is done simultaenously with an
   updated 6LoRH compress.




















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   +-----------+-----------+-----------+------------+-----------+------+
   | Header    | 6LN       | 6LR       | 6LR        | 6LR       | IPv6 |
   |           |           |           | (common    |           |      |
   |           |           |           | parent)    |           |      |
   +-----------+-----------+-----------+------------+-----------+------+
   | Inserted  | IPIP(RPI) | --        | --         | --        | --   |
   | headers   |           |           |            |           |      |
   | Removed   | --        | --        | --         | IPIP(RPI) | --   |
   | headers   |           |           |            |           |      |
   | Re-added  | --        | --        | --         | --        | --   |
   | headers   |           |           |            |           |      |
   | Modified  | --        | IPIP(RPI) | IPIP(RPI)  | --        | --   |
   | headers   |           |           |            |           |      |
   | Untouched | --        | --        | --         | --        | --   |
   | headers   |           |           |            |           |      |
   +-----------+-----------+-----------+------------+-----------+------+

    Storing: Summary of the use of headers from RPL-aware-leaf to not-
                              RPL-aware-leaf

5.11.  Example of Flow from not-RPL-aware-leaf to RPL-aware-leaf

   In this case the flow comprises:

   not-RPL-aware 6LN --> 6LR --> common parent (6LR) --> 6LR --> 6LN

   The 6LR receives the packet from the the IPv6 node and inserts and
   the RPI header encapsulated in IPv6-in-IPv6 header.  The IPIP header
   could be addresses to the 6LN if the destination is known to the RPL
   aware, otherwise must send the packet using a hop-by-hop IPIP header.
   Similar considerations apply from section Section 5.10.




















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   +-----------+------+-----------+------------+-----------+-----------+
   | Header    | IPv6 | 6LR       | common     | 6LR       | 6LN       |
   |           |      |           | parent     |           |           |
   |           |      |           | (6LR)      |           |           |
   +-----------+------+-----------+------------+-----------+-----------+
   | Inserted  | --   | IPIP(RPI) | --         | --        | --        |
   | headers   |      |           |            |           |           |
   | Removed   | --   | --        | --         | --        | IPIP(RPI) |
   | headers   |      |           |            |           |           |
   | Re-added  | --   | --        | --         | --        | --        |
   | headers   |      |           |            |           |           |
   | Modified  | --   | --        | IPIP(RPI)  | IPIP(RPI) | --        |
   | headers   |      |           |            |           |           |
   | Untouched | --   | --        | --         | --        | --        |
   | headers   |      |           |            |           |           |
   +-----------+------+-----------+------------+-----------+-----------+

     Storing: Summary of the use of headers from not-RPL-aware-leaf to
                              RPL-aware-leaf

5.12.  Example of Flow from not-RPL-aware-leaf to not-RPL-aware-leaf

   In this case the flow comprises:

   not-RPL-aware 6LN (IPv6 node)--> 6LR --> root (6LBR) --> 6LR --> not-
   RPL-aware 6LN (IPv6 node)

   This flow combines the problems of the two previous sections.  There
   is no choice at the first 6LR: it must insert an RPI, and to do that
   it must add an IPIP header.  That IPIP header must be addressed on a
   hop-by-hop basis.




















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   +-------------+--------+-----------+-----------+-----------+--------+
   | Header      | IPv6   | 6LR       | 6LR       | 6LR       | IPv6   |
   |             | src    |           | (common   |           | dst    |
   |             |        |           | parent)   |           |        |
   +-------------+--------+-----------+-----------+-----------+--------+
   | Inserted    | --     | IPIP(RPI) | --        | --        | --     |
   | headers     |        |           |           |           |        |
   | Removed     | --     | --        | --        | IPIP(RPI) | --     |
   | headers     |        |           |           |           |        |
   | Re-added    | --     | --        | --        | --        | --     |
   | headers     |        |           |           |           |        |
   | Modified    | --     | --        | --        | --        | --     |
   | headers     |        |           |           |           |        |
   | Untouched   | --     | --        | --        | --        | --     |
   | headers     |        |           |           |           |        |
   +-------------+--------+-----------+-----------+-----------+--------+

     Storing: Summary of the use of headers from not-RPL-aware-leaf to
                            not-RPL-aware-leaf

6.  Non Storing mode

            +--------------+------+------+-------+-----------+
            | Use Case     | RPI  | RH3  | IPIP  | IPIP dst  |
            +--------------+------+------+-------+-----------+
            | Raf to root  | Yes  | No   | No    | --        |
            | root to Raf  | Yes  | Yes  | No    | --        |
            | root to ~Raf | No   | Yes  | Yes   | 6LR       |
            | ~Raf to root | Yes  | No   | Yes   | root      |
            | Raf to Int   | Yes  | No   | Yes   | root      |
            | Int to Raf   | opt  | Yes  | Yes   | dst       |
            | ~Raf to Int  | Yes  | No   | Yes   | root      |
            | Int to ~Raf  | opt  | Yes  | Yes   | 6LR       |
            | Raf to Raf   | Yes  | Yes  | Yes   | root/dst  |
            | Raf to ~Raf  | Yes  | Yes  | Yes   | root/6LR  |
            | ~Raf to Raf  | Yes  | Yes  | Yes   | root/6LN  |
            | ~Raf to ~Raf | Yes  | Yes  | Yes   | root/6LR  |
            +--------------+------+------+-------+-----------+

      Table 2: Headers needed in Non-Storing mode: RPI, RH3, IP-in-IP
                               encapsulation

6.1.  Example of Flow from RPL-aware-leaf to root

   In non-storing mode the leaf node uses default routing to send
   traffic to the root.  The RPI header must be included to avoid/detect
   loops.




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   RPL-aware-leaf (6LN) --> 6LR --> root (6LBR)

   This situation is the same case as storing mode.

                 +-------------------+-----+-----+------+
                 | Header            | 6LN | 6LR | 6LBR |
                 +-------------------+-----+-----+------+
                 | Inserted headers  | RPI | --  | --   |
                 | Removed headers   | --  | --  | RPI  |
                 | Re-added headers  | --  | --  | RPI  |
                 | Modified headers  | --  | --  | --   |
                 | Untouched headers | --  | --  | --   |
                 +-------------------+-----+-----+------+

     Non Storing: Summary of the use of headers from RPL-aware-leaf to
                                   root

6.2.  Example of Flow from root to RPL-aware-leaf

   In this case the flow comprises:

   root (6LBR)--> 6LR --> RPL-aware-leaf (6LN)

   The 6LBR will insert an RH3, and may optionally insert an RPI header.
   No IPIP header is necessary as the traffic originates with an RPL
   aware node.

         +-------------------+-----------------+------+----------+
         | Header            | 6LBR            | 6LR  | 6LN      |
         +-------------------+-----------------+------+----------+
         | Inserted headers  | (opt: RPI), RH3 | --   | --       |
         | Removed headers   | --              | --   | RH3,RPI  |
         | Re-added headers  | --              | --   | --       |
         | Modified headers  | --              | RH3  | --       |
         | Untouched headers | --              | --   | --       |
         +-------------------+-----------------+------+----------+

    Non Storing: Summary of the use of headers from root to RPL-aware-
                                   leaf

6.3.  Example of Flow from root to not-RPL-aware-leaf

   In this case the flow comprises:

   root (6LBR)--> 6LR --> not-RPL-aware-leaf (IPv6 node)

   In 6LBR the RH3 is added, and modified in 6LR where it is fully
   consumed, but left there.  If the RPI is left present, the IPv6 node



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   which does not understand it will drop it, therefore the RPI should
   be removed before reaching the IPv6-only node.  To permit removal, an
   IPIP header (hop-by-hop) or addressed to the last 6LR is necessary.
   Due the complete knowledge of the topology at the root, the 6LBR is
   able to address the IPIP header to the last 6LR.

   Omitting the RPI entirely is therefore a better solution, as no IPIP
   header is necessary.

                 +-------------------+------+-----+------+
                 | Header            | 6LBR | 6LR | IPv6 |
                 +-------------------+------+-----+------+
                 | Inserted headers  | RH3  | --  | --   |
                 | Removed headers   | --   | --  | --   |
                 | Re-added headers  | --   | --  | --   |
                 | Modified headers  | --   | RH3 | --   |
                 | Untouched headers | --   | --  | --   |
                 +-------------------+------+-----+------+

     Non Storing: Summary of the use of headers from root to not-RPL-
                                aware-leaf

6.4.  Example of Flow from not-RPL-aware-leaf to root

   In this case the flow comprises:

   IPv6-node --> 6LR1 --> 6LR2 --> root (6LBR)

   In this case the RPI is added by the first 6LR, encapsulated in an
   IPIP header, and is not modified in the followings 6LRs.  The RPI and
   entire packet is consumed by the root.

       +-------------------+------+------------+------+------------+
       | Header            | IPv6 | 6LR1       | 6LR2 | 6LBR       |
       +-------------------+------+------------+------+------------+
       | Inserted headers  | --   | IPIP(RPI)  | --   | --         |
       | Removed headers   | --   | --         | --   | IPIP(RPI)  |
       | Re-added headers  | --   | --         | --   | --         |
       | Modified headers  | --   | --         | --   | --         |
       | Untouched headers | --   | IPIP(RPI)  | --   | --         |
       +-------------------+------+------------+------+------------+

   Non Storing: Summary of the use of headers from not-RPL-aware-leaf to
                                   root







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6.5.  Example of Flow from RPL-aware-leaf to Internet

   In this case the flow comprises:

   RPL-aware-leaf (6LN) --> 6LR --> root (6LBR) --> Internet

   This case requires that the RPI be added, but remoted by the 6LBR.
   The 6LN must therefore add the RPI inside an IPIP header, addressed
   to the root.  This case is identical to storing-mode case.

   The IPv6 flow label should be set to zero to aid in compression, and
   the 6LBR will set it to a non-zero value when sending towards the
   Internet.

     +-------------------+-----------+------+------------+----------+
     | Header            | 6LN       | 6LR  | 6LBR       | Internet |
     +-------------------+-----------+------+------------+----------+
     | Inserted headers  | IPIP(RPI) | --   | --         | --       |
     | Removed headers   | --        | --   | IPIP(RPI)  | --       |
     | Re-added headers  | --        | --   | --         | --       |
     | Modified headers  | --        | --   | --         | --       |
     | Untouched headers | --        | RPI  | --         | --       |
     +-------------------+-----------+------+------------+----------+

     Non Storing: Summary of the use of headers from RPL-aware-leaf to
                                 Internet

6.6.  Example of Flow from Internet to RPL-aware-leaf

   In this case the flow comprises:

   Internet --> root (6LBR) --> 6LR --> RPL-aware-leaf (6LN)

   The 6LBR must add an RH3 header.  As the 6LBR will know the path and
   address of the target not, it can address the IPIP header to that
   node.  The 6LBR will zero the flow label upon entry in order to aid
   compression.

   The RPI may be added or not.












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   +----------------+----------+--------------------+------------+-----+
   | Header         | Internet | 6LBR               | 6LR        | 6LN |
   +----------------+----------+--------------------+------------+-----+
   | Inserted       | --       | IPIP(RH3,opt:RPI)  | --         | --  |
   | headers        |          |                    |            |     |
   | Removed        | --       | --                 | IPIP(RH3)  | --  |
   | headers        |          |                    |            |     |
   | Re-added       | --       | --                 | --         | --  |
   | headers        |          |                    |            |     |
   | Modified       | --       | --                 | IPIP(RH3)  | --  |
   | headers        |          |                    |            |     |
   | Untouched      | --       | --                 | --         | --  |
   | headers        |          |                    |            |     |
   +----------------+----------+--------------------+------------+-----+

     Non Storing: Summary of the use of headers from Internet to RPL-
                                aware-leaf

6.7.  Example of Flow from not-RPL-aware-leaf to Internet

   In this case the flow comprises:

   not-RPL-aware-leaf (6LN) --> 6LR --> root (6LBR) --> Internet

   In this case the flow label is recommended to be zero in the IPv6
   node.  As RPL headers are added in the IPv6 node, the first 6LN will
   add an RPI header inside a new IPIP header.  The IPIP header will be
   addressed to the root.  This case is identical to the storing-mode
   case.

     +-------------------+------+-----------+------------+----------+
     | Header            | IPv6 | 6LR       | 6LBR       | Internet |
     +-------------------+------+-----------+------------+----------+
     | Inserted headers  | --   | IPIP(RPI) | --         | --       |
     | Removed headers   | --   | --        | IPIP(RPI)  | --       |
     | Re-added headers  | --   | --        | --         | --       |
     | Modified headers  | --   | --        | --         | --       |
     | Untouched headers | --   | --        | --         | --       |
     +-------------------+------+-----------+------------+----------+

   Non Storing: Summary of the use of headers from not-RPL-aware-leaf to
                                 Internet

6.8.  Example of Flow from Internet to non-RPL-aware-leaf

   In this case the flow comprises:

   Internet --> root (6LBR) --> 6LR --> not-RPL-aware-leaf (6LN)



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   The 6LBR must add an RH3 header inside an IPIP header.  The 6LBR will
   know the path, and will recognize that the final node is not an RPL
   capable node as it will have received the connectivity DAO from the
   nearest 6LR.  The 6LBR can therefore make the IPIP header destination
   be the last 6LR.  The 6LBR will zero the flow label upon entry in
   order to aid compression.

   +--------------+----------+-------------------+--------------+------+
   | Header       | Internet | 6LBR              | 6LR          | IPv6 |
   +--------------+----------+-------------------+--------------+------+
   | Inserted     | --       | IPIP(RH3,opt:RPI) | --           | --   |
   | headers      |          |                   |              |      |
   | Removed      | --       | --                | IPIP(RH3,    | --   |
   | headers      |          |                   | RPI)         |      |
   | Re-added     | --       | --                | --           | --   |
   | headers      |          |                   |              |      |
   | Modified     | --       | --                | --           | --   |
   | headers      |          |                   |              |      |
   | Untouched    | --       | --                | --           | --   |
   | headers      |          |                   |              |      |
   +--------------+----------+-------------------+--------------+------+

    NonStoring: Summary of the use of headers from Internet to non-RPL-
                                aware-leaf

6.9.  Example of Flow from RPL-aware-leaf to RPL-aware-leaf

   In this case the flow comprises:

   6LN --> 6LR --> root (6LBR) --> 6LR --> 6LN

   This case involves only nodes in same RPL Domain.  The originating
   node will add an RPI header to the original packet, and send the
   packet upwards.

   The originating node could put the RPI into an IPIP header addressed
   to the root, so that the 6LBR can remove that header.

   The 6LBR will need to insert an RH3 header, which requires that it
   add an IPIP header.  It may be able to remove the RPI if it was
   contained in an IPIP header addressed to it.  Otherwise, there may be
   an RPI header buried inside the inner IP header, which should get
   ignored.

   Networks that use the RPL P2P extension [RFC6997] are essentially
   non-storing DODAGs and fall into this scenario.





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   +----------------+-----------+----------------+-----+---------------+
   | Header         | 6LN src   | 6LBR           | 6LR | 6LN dst       |
   +----------------+-----------+----------------+-----+---------------+
   | Inserted       | IPIP(RPI) | IPIP(RH3 to    | --  | --            |
   | headers        |           | 6LN,RPI)       |     |               |
   | Removed        | --        | --             | --  | IPIP(RH3,RPI) |
   | headers        |           |                |     |               |
   | Re-added       | --        | --             | --  | --            |
   | headers        |           |                |     |               |
   | Modified       | --        | --             | --  | --            |
   | headers        |           |                |     |               |
   | Untouched      | --        | --             | --  | --            |
   | headers        |           |                |     |               |
   +----------------+-----------+----------------+-----+---------------+

   Non Storing: Summary of the use of headers for RPL-aware-leaf to RPL-
                                aware-leaf

6.10.  Example of Flow from RPL-aware-leaf to not-RPL-aware-leaf

   In this case the flow comprises:

   6LN --> 6LR --> root (6LBR) --> 6LR --> not-RPL-aware 6LN

   As in the previous case, the 6LN will insert an RPI header which MUST
   be in an IPIP header addressed to the root so that the 6LBR can
   remove this RPI.  The 6LBR will then insert an RH3 inside a new IPIP
   header addressed to the 6LN above the destination node.

   +---------------+-----------+---------------+----------------+------+
   | Header        | 6LN       | 6LBR          | 6LR            | IPv6 |
   +---------------+-----------+---------------+----------------+------+
   | Inserted      | IPIP(RPI) | IPIP(RH3, opt | --             | --   |
   | headers       |           | RPI)          |                |      |
   | Removed       | --        | IPIP(RPI)     | IPIP(RH3, opt  | --   |
   | headers       |           |               | RPI)           |      |
   | Re-added      | --        | --            | --             | --   |
   | headers       |           |               |                |      |
   | Modified      | --        | --            | --             | --   |
   | headers       |           |               |                |      |
   | Untouched     | --        | --            | --             | --   |
   | headers       |           |               |                |      |
   +---------------+-----------+---------------+----------------+------+

     Non Storing: Summary of the use of headers from RPL-aware-leaf to
                            not-RPL-aware-leaf





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6.11.  Example of Flow from not-RPL-aware-leaf to RPL-aware-leaf

   In this case the flow comprises:

   not-RPL-aware 6LN --> 6LR --> root (6LBR) --> 6LR --> 6LN

   This scenario is mostly identical to the previous one.  The RPI is
   added by the first 6LR inside an IPIP header addressed to the root.
   The 6LBR will remove this RPI, and add it's own IPIP header
   containing an RH3 header.

    +-------------------+------+------------+-----------+------------+
    | Header            | IPv6 | 6LR        | 6LBR      | 6LN        |
    +-------------------+------+------------+-----------+------------+
    | Inserted headers  | --   | IPIP(RPI)  | IPIP(RH3) | --         |
    | Removed headers   | --   | IPIP(RPI)  | --        | IPIP(RH3)  |
    | Re-added headers  | --   | --         | --        | --         |
    | Modified headers  | --   | --         | --        | --         |
    | Untouched headers | --   | --         | --        | --         |
    +-------------------+------+------------+-----------+------------+

   Non Storing: Summary of the use of headers from not-RPL-aware-leaf to
                              RPL-aware-leaf

6.12.  Example of Flow from not-RPL-aware-leaf to not-RPL-aware-leaf

   In this case the flow comprises:

   not-RPL-aware 6LN --> 6LR --> root (6LBR) --> 6LR --> not-RPL-aware
   6LN

   This scenario is the combination of the previous two cases.



















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   +--------------+------+-----------+-----------+--------------+------+
   | Header       | IPv6 | 6LR       | 6LBR      | 6LR          | IPv6 |
   +--------------+------+-----------+-----------+--------------+------+
   | Inserted     | --   | IPIP(RPI) | IPIP(RH3) | --           | --   |
   | headers      |      |           |           |              |      |
   | Removed      | --   | --        | IPIP(RPI) | IPIP(RH3,    | --   |
   | headers      |      |           |           | opt RPI)     |      |
   | Re-added     | --   | --        | --        | --           | --   |
   | headers      |      |           |           |              |      |
   | Modified     | --   | --        | --        | --           | --   |
   | headers      |      |           |           |              |      |
   | Untouched    | --   | --        | --        | --           | --   |
   | headers      |      |           |           |              |      |
   +--------------+------+-----------+-----------+--------------+------+

   Non Storing: Summary of the use of headers from not-RPL-aware-leaf to
                            not-RPL-aware-leaf

7.  Observations about the problem

7.1.  Storing mode

   In the completely general storing case, which includes not-RPL aware
   leaf nodes, it is not possible for a sending node to know if the
   destination is RPL aware, and therefore it must always use hop-by-hop
   IPIP encapsulation, and it can never omit the IPIP encapsulation.
   See table Table 1

   The simplest fully general stiaution for storing mode is to always
   put in hop-by-hop IPIP headers.  [I-D.ietf-roll-routing-dispatch]
   shows that this hop-by-hop IPIP header can be compressed down to
   {TBD} bytes.

   There are potential significant advantages to having a single code
   path that always processes IPIP headers with no options.

   If all RPL aware nodes can be told/configured that there are no non-
   RPL aware leaf nodes, then the only case where an IPIP header is
   needed is when communicating outside the LLN.  The 6LBR knows well
   when the communication is from the outside, and the 6LN can tell by
   comparing the destination address to the prefix provided in the PIO.
   If it is known that there are no communications outside the RPL
   domain (noting that the RPL domain may well extend to outside the
   LLN), then RPI headers can be included in all packets, and IPIP
   headers are *never* needed.  This may be significantly advantageous
   in relatively closed systems such as in building or industrial
   automation.  Again, there are advantages to having a single code
   path.



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   In order to support the above two cases with full generality, the
   different situations (always do IPIP vs never use IPIP) should be
   signaled in the RPL protocol itself.

7.2.  Non-Storing mode

   This the non-storing case, dealing with non-RPL aware leaf nodes is
   much easier as the 6LBR (DODAG root) has complete knowledge about the
   connectivity of all nodes, and all traffic flows through the root
   node.

   The 6LBR can recognize non-RPL aware leaf nodes because it will
   receive a DAO about that node from the 6LN immediately above that
   node.  This means that the non-storing mode case can avoid ever using
   hop-by-hop IPIP headers.

   It is unclear what it would mean for an RH3 header to be present in a
   hop-by-hop IPIP header.  The receiving node ought to consume the IPIP
   header, and therefore consume the RH3 as well, and then attempt to
   send the packet again.  But intermediate 6LN nodes would not know how
   to forward the packet, so the RH3 would need to be retained.  This is
   a new kind of IPv6 packet processing.  Therefore it may be that on
   the outbound leg of non-storing RPL networks, that hop-by-hop IPIP
   header can NOT be used.

   [I-D.ietf-roll-routing-dispatch] shows how the destination=root, and
   destination=6LN IPIP header can be compressed down to {TBD} bytes.

   Unlike in the storing mode case, there are no need for all nodes to
   know about the existence of non-RPL aware nodes.  Only the 6LBR needs
   to change when there are non-RPL aware nodes.  Further, in the non-
   storing case, the 6LBR is informed by the DAOs when there are non-RPL
   aware nodes.

8.  6LoRH Compression cases

   The [I-D.ietf-roll-routing-dispatch] proposes a compression method
   for RPI, RH3 and IPv6-in-IPv6.

   In Storing Mode, for the examples of Flow from RPL-aware-leaf to non-
   RPL-aware-leaf and non-RPL-aware-leaf to non-RPL-aware-leaf comprise
   an IP-in-IP and RPI compression headers.  The type of this case is
   critical since IP-in-IP is encapsulating a RPI header.








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   +--+-----+---+--------------+-----------+-------------+-------------+
   |1 | 0|0 |TSE| 6LoRH Type 6 | Hop Limit | RPI - 6LoRH | LOWPAN IPHC |
   +--+-----+---+--------------+-----------+-------------+-------------+


                    Figure 5: Critical IP-in-IP (RPI).

9.  IANA Considerations

   There are no IANA considerations related to this document.

10.  Security Considerations

   The security considerations covering of [RFC6553] and [RFC6554] apply
   when the packets get into RPL Domain.

11.  Acknowledgments

   This work is partially funded by the FP7 Marie Curie Initial Training
   Network (ITN) METRICS project (grant agreement No.  607728).

   The authors would like to acknowledge the review, feedback, and
   comments of Thomas Watteyne, Xavier Vilajosana, Robert Cragie and
   Simon Duquennoy.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [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>.








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   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <http://www.rfc-editor.org/info/rfc6553>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <http://www.rfc-editor.org/info/rfc6554>.

12.2.  Informative 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-09 (work
              in progress), November 2015.

   [I-D.ietf-roll-routing-dispatch]
              Thubert, P., Bormann, C., Toutain, L., and R. Cragie,
              "6LoWPAN Routing Header", draft-ietf-roll-routing-
              dispatch-00 (work in progress), March 2016.

   [RFC6997]  Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and
              J. Martocci, "Reactive Discovery of Point-to-Point Routes
              in Low-Power and Lossy Networks", RFC 6997,
              DOI 10.17487/RFC6997, August 2013,
              <http://www.rfc-editor.org/info/rfc6997>.

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

   [Second6TischPlugtest]
              "2nd 6Tisch Plugtest", <http://www.ietf.org/mail-
              archive/web/6tisch/current/pdfgDMQcdCkRz.pdf>.

Authors' Addresses

   Maria Ines Robles
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: maria.ines.robles@ericsson.com




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   Michael C. Richardson
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa, ON  K1Z 5V7
   CA

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/


   Pascal Thubert
   Cisco Systems, Inc
   Village d'Entreprises Green Side 400, Avenue de Roumanille
   Batiment T3, Biot - Sophia Antipolis    06410
   France

   Email: pthubert@cisco.com


































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