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ROLL Working Group                                             M. Robles
Internet-Draft                                                     Aalto
Updates: 6553, 6550, 8138 (if approved)                    M. Richardson
Intended status: Standards Track                                     SSW
Expires: November 17, 2019                                    P. Thubert
                                                                   Cisco
                                                            May 16, 2019


Using RPL Option Type, Routing Header for Source Routes and IPv6-in-IPv6
                  encapsulation in the RPL Data Plane
                    draft-ietf-roll-useofrplinfo-27

Abstract

   This document looks at different data flows through LLN (Low-Power
   and Lossy Networks) where RPL (IPv6 Routing Protocol for Low-Power
   and Lossy Networks) is used to establish routing.  The document
   enumerates the cases where RFC 6553 (RPL Option Type), RFC 6554
   (Routing Header for Source Routes) and IPv6-in-IPv6 encapsulation is
   required in data plane.  This analysis provides the basis on which to
   design efficient compression of these headers.  This document updates
   RFC 6553 adding a change to the RPL Option Type.  Additionally, this
   document updates RFC 6550 defining a flag in the DIO Configuration
   Option to indicate about this change and updates RFC8138 as well to
   consider the new Option Type when the RPL Option is decompressed.

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 November 17, 2019.








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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology and Requirements Language . . . . . . . . . . . .   5
   3.  Updates to RFC6553, RFC6550 and RFC8138 . . . . . . . . . . .   6
     3.1.  Updates to RFC6553: Indicating the new RPI value. . . . .   6
     3.2.  Updates to RFC6550: Indicating the new RPI in the
           DODAG Configuration Option Flag.  . . . . . . . . . . . .  10
     3.3.  Updates to RFC8138: Indicating the way to decompress with
           the new RPI value.  . . . . . . . . . . . . . . . . . . .  11
   4.  Sample/reference topology . . . . . . . . . . . . . . . . . .  11
   5.  Use cases . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Storing mode  . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Storing Mode: Interaction between Leaf and Root . . . . .  17
       6.1.1.  SM: Example of Flow from RPL-aware-leaf to root . . .  17
       6.1.2.  SM: Example of Flow from root to RPL-aware-leaf . . .  19
       6.1.3.  SM: Example of Flow from root to not-RPL-aware-leaf .  19
       6.1.4.  SM: Example of Flow from not-RPL-aware-leaf to root .  20
     6.2.  Storing Mode: Interaction between Leaf and Internet.  . .  21
       6.2.1.  SM: Example of Flow from RPL-aware-leaf to Internet .  21
       6.2.2.  SM: Example of Flow from Internet to RPL-aware-leaf .  22
       6.2.3.  SM: Example of Flow from not-RPL-aware-leaf to
               Internet  . . . . . . . . . . . . . . . . . . . . . .  23
       6.2.4.  SM: Example of Flow from Internet to not-RPL-aware-
               leaf. . . . . . . . . . . . . . . . . . . . . . . . .  24
     6.3.  Storing Mode: Interaction between Leaf and Leaf . . . . .  25
       6.3.1.  SM: Example of Flow from RPL-aware-leaf to RPL-aware-
               leaf  . . . . . . . . . . . . . . . . . . . . . . . .  25
       6.3.2.  SM: Example of Flow from RPL-aware-leaf to not-RPL-
               aware-leaf  . . . . . . . . . . . . . . . . . . . . .  26
       6.3.3.  SM: Example of Flow from not-RPL-aware-leaf to RPL-
               aware-leaf  . . . . . . . . . . . . . . . . . . . . .  27



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       6.3.4.  SM: Example of Flow from not-RPL-aware-leaf to not-
               RPL-aware-leaf  . . . . . . . . . . . . . . . . . . .  29
   7.  Non Storing mode  . . . . . . . . . . . . . . . . . . . . . .  30
     7.1.  Non-Storing Mode: Interaction between Leaf and Root . . .  31
       7.1.1.  Non-SM: Example of Flow from RPL-aware-leaf to root .  32
       7.1.2.  Non-SM: Example of Flow from root to RPL-aware-leaf .  32
       7.1.3.  Non-SM: Example of Flow from root to not-RPL-aware-
               leaf  . . . . . . . . . . . . . . . . . . . . . . . .  33
       7.1.4.  Non-SM: Example of Flow from not-RPL-aware-leaf to
               root  . . . . . . . . . . . . . . . . . . . . . . . .  34
     7.2.  Non-Storing Mode: Interaction between Leaf and Internet .  35
       7.2.1.  Non-SM: Example of Flow from RPL-aware-leaf to
               Internet  . . . . . . . . . . . . . . . . . . . . . .  35
       7.2.2.  Non-SM: Example of Flow from Internet to RPL-aware-
               leaf  . . . . . . . . . . . . . . . . . . . . . . . .  36
       7.2.3.  Non-SM: Example of Flow from not-RPL-aware-leaf to
               Internet  . . . . . . . . . . . . . . . . . . . . . .  37
       7.2.4.  Non-SM: Example of Flow from Internet to not-RPL-
               aware-leaf  . . . . . . . . . . . . . . . . . . . . .  38
     7.3.  Non-Storing Mode: Interaction between Leafs . . . . . . .  39
       7.3.1.  Non-SM: Example of Flow from RPL-aware-leaf to RPL-
               aware-leaf  . . . . . . . . . . . . . . . . . . . . .  39
       7.3.2.  Non-SM: Example of Flow from RPL-aware-leaf to not-
               RPL-aware-leaf  . . . . . . . . . . . . . . . . . . .  41
       7.3.3.  Non-SM: Example of Flow from not-RPL-aware-leaf to
               RPL-aware-leaf  . . . . . . . . . . . . . . . . . . .  42
       7.3.4.  Non-SM: Example of Flow from not-RPL-aware-leaf to
               not-RPL-aware-leaf  . . . . . . . . . . . . . . . . .  43
   8.  Operational Considerations of supporting
       not-RPL-aware-leaves  . . . . . . . . . . . . . . . . . . . .  44
   9.  Operational considerations of introducing 0x23  . . . . . . .  45
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  47
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  50
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     13.2.  Informative References . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

1.  Introduction

   RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks)
   [RFC6550] is a routing protocol for constrained networks.  RFC 6553
   [RFC6553] defines the "RPL option" (RPL Packet Information or 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




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   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 implementers agree when artifacts
   are necessary, or when they can be safely omitted, or removed.

   The ROLL WG analysized how [RFC2460] rules apply to storing and non-
   storing use of RPL.  The result was 24 data plane use cases.  They
   are exhaustively outlined here in order to be completely unambiguous.
   During the processing of this document, new rules were published as
   [RFC8200], and this document was updated to reflect the normative
   changes in that document.

   This document updates RFC6553, changing the RPI option value to make
   RFC8200 routers ignore this option by default.

   A Routing Header Dispatch for 6LoWPAN (6LoRH)([RFC8138]) defines a
   mechanism for compressing RPL Option information and Routing Header
   type 3 (RH3) [RFC6554], as well as an efficient IPv6-in-IPv6
   technique.

   Since some of the uses cases here described, use IPv6-in-IPv6
   encapsulation.  It MUST take in consideration, when encapsulation is
   applied, the RFC6040 [RFC6040], which defines how the explicit
   congestion notification (ECN) field of the IP header should be
   constructed on entry to and exit from any IPV6-in-IPV6 tunnel.
   Additionally, it is recommended the reading of
   [I-D.ietf-intarea-tunnels].

1.1.  Overview

   The rest of the document is organized as follows: Section 2 describes
   the used terminology.  Section 3 describes the updates to RFC6553,
   RFC6550 and RFC 8138.  Section 4 provides the reference topology used
   for the uses cases.  Section 5 describes the uses cases included.
   Section 6 describes the storing mode cases and section 7 the non-
   storing mode cases.  Section 8 describes the operational
   considerations of supporting not-RPL-aware-leaves.  Section 9 depicts
   operational considerations for the proposed change on RPL Option




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   type, section 10 the IANA considerations and then section 11
   describes the security aspects.

2.  Terminology and Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119], [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Terminology defined in [RFC7102] applies to this document: LBR, LLN,
   RPL, RPL Domain and ROLL.

   RPL-node: A device which implements RPL, thus the device is RPL-
   aware.  Please note that the device can be found inside the LLN or
   outside LLN.  In this document a RPL-node which is a leaf of a
   (Destination Oriented Directed Acyclic Graph) DODAG is called RPL-
   aware-leaf (Raf).

   RPL-not-capable: A device which does not implement RPL, thus the
   device is not-RPL-aware.  Please note that the device can be found
   inside the LLN.  In this document a not-RPL-aware node which is a
   leaf of a DODAG is called not-RPL-aware-leaf (~Raf).

   6LN: [RFC6775] defines it as: "A 6LoWPAN node is any host or router
   participating in a LoWPAN.  This term is used when referring to
   situations in which either a host or router can play the role
   described.".  In this document, a 6LN acts as a leaf.

   6LR: [RFC6775] defines it as:" An intermediate router in the LoWPAN
   that is able to send and receive Router Advertisements (RAs) and
   Router Solicitations (RSs) as well as forward and route IPv6 packets.
   6LoWPAN routers are present only in route-over topologies."

   6LBR: [RFC6775] defines it as:"A border router located at the
   junction of separate 6LoWPAN networks or between a 6LoWPAN network
   and another IP network.  There may be one or more 6LBRs at the
   6LoWPAN network boundary.  A 6LBR is the responsible authority for
   IPv6 prefix propagation for the 6LoWPAN network it is serving.  An
   isolated LoWPAN also contains a 6LBR in the network, which provides
   the prefix(es) for the isolated network."

   Flag Day: A transition that involves having a network with different
   values of RPL Option Type.  Thus the network does not work correctly.

   Hop-by-hop re-encapsulation: The term "hop-by-hop re-encapsulation"
   header refers to adding a header that originates from a node to an



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   adjacent node, using the addresses (usually the GUA or ULA, but could
   use the link-local addresses) of each node.  If the packet must
   traverse multiple hops, then it must be decapsulated at each hop, and
   then re-encapsulated again in a similar fashion.

   RPL defines the RPL Control messages (control plane), a new ICMPv6
   [RFC4443] message with Type 155.  DIS (DODAG Information
   Solicitation), DIO (DODAG Information Object) and DAO (Destination
   Advertisement Object) messages are all RPL Control messages but with
   different Code values.  A RPL Stack is shown in Figure 1.

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

                           Figure 1: RPL Stack.

   RPL supports two modes of Downward traffic: in storing mode (RPL-SM),
   it is fully stateful; in non-storing mode (RPL-NSM), 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 at the
   time of writing this document.

3.  Updates to RFC6553, RFC6550 and RFC8138

3.1.  Updates to RFC6553: Indicating the new RPI value.

   This modification is required to be able to send, for example, IPv6
   packets from a RPL-aware-leaf to a not-RPL-aware node through
   Internet (see Section 6.2.1), without requiring IPv6-in-IPv6
   encapsulation.



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   [RFC6553] (Section 6, Page 7) states as shown in Figure 2, that in
   the Option Type field of the RPL Option header, the two high order
   bits must be set to '01' and the third bit is equal to '1'.  The
   first two bits indicate that the IPv6 node must discard the packet if
   it doesn't recognize the option type, and the third bit indicates
   that the Option Data may change in route.  The remaining bits serve
   as the option type.


          Hex Value     Binary Value
                        act  chg  rest     Description        Reference
          ---------     ---  ---  -------  -----------------  ----------
            0x63         01    1   00011   RPL Option         [RFC6553]


                   Figure 2: Option Type in RPL Option.

   This document illustrates that is is not always possible to know for
   sure at the source that a packet will only travel within the RPL
   domain or may leave it.

   At the time [RFC6553] was published, leaking a Hop-by-Hop header in
   the outer IPv6 header chain could potentially impact core routers in
   the internet.  So at that time, it was decided to encapsulate any
   packet with a RPL option using IPv6-in-IPv6 in all cases where it was
   unclear whether the packet would remain within the RPL domain.  In
   the exception case where a packet would still leak, the Option Type
   would ensure that the first router in the Internet that does not
   recognize the option would drop the packet and protect the rest of
   the network.

   Even with [RFC8138] that compresses the IP-in-IP header, this
   approach yields extra bytes in a packet which means consuming more
   energy, more bandwidth, incurring higher chances of loss and possibly
   causing a fragmentation at the 6LoWPAN level.  This impacts the daily
   operation of constrained devices for a case that generally does not
   happen and would not heavily impact the core anyway.

   While intention was and remains that the Hop-by-Hop header with a RPL
   option should be confined within the RPL domain, this specification
   modifies this behavior in order to reduce the dependency on IP-in-IP
   and protect the constrained devices.  Section 4 of [RFC8200]
   clarifies the behaviour of routers in the Internet as follows: "it is
   now expected that nodes along a packet's delivery path only examine
   and process the Hop-by-Hop Options header if explicitly configured to
   do so".  This means that while it should be avoided, the impact on
   the Internet of leaking a Hop-by-Hop header is acceptable.




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   When unclear about the travel of a packet, it becomes preferable for
   a source not to encapsulate, accepting the fact that the packet may
   leave the RPL domain on its way to its destination.  In that event,
   the packet should reach its destination and should not be discarded
   by the first node that does not recognize the RPL option.  But with
   the current value of the Option Type, if a node in the Internet is
   configured to process the Hop-by-Hop header, and if such node
   encounters an option with the first two bits set to 01 and conforms
   to [RFC8200], it will drop the packet.  Host systems should do the
   same, irrespective of the configuration.

   Thus, this document updates the Option Type field to (Figure 3): the
   two high order bits MUST be set to '00' and the third bit is equal to
   '1'.  The first two bits indicate that the IPv6 node MUST skip over
   this option and continue processing the header ([RFC8200]
   Section 4.2) if it doesn't recognize the option type, and the third
   bit continues to be set to indicate that the Option Data may change
   en route.  The remaining bits serve as the option type and remain as
   0x3.  This ensures that a packet that leaves the RPL domain of an LLN
   (or that leaves the LLN entirely) will not be discarded when it
   contains the [RFC6553] RPL Hop-by-Hop option known as RPI.

   With the new Option Type, if an IPv6 (intermediate) node (RPL-not-
   capable) receives a packet with an RPL Option, it should ignore the
   Hop-by-Hop RPL option (skip over this option and continue processing
   the header).  This is relevant, as it was mentioned previously, in
   the case that there is a flow from RPL-aware-leaf to Internet (see
   Section 6.2.1).

   This is a significant update to [RFC6553].


          Hex Value     Binary Value
                        act  chg  rest     Description        Reference
          ---------     ---  ---  -------  -----------------  ----------
            0x23         00    1   00011   RPL Option         [RFCXXXX]


               Figure 3: Revised Option Type in RPL Option.

   Without the signaling described below, this change would otherwise
   create a flag day for existing networks which are currently using
   0x63 as the RPI value.  A move to 0x23 will not be understood by
   those networks.  It is suggested that implementations accept both
   0x63 and 0x23 when processing.

   When forwarding packets, implementations SHOULD use the same value as
   it was received (This is required because, RPI type code can not be



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   changed by [RFC8200]).  It allows to the network to be incrementally
   upgraded, and for the DODAG root to know which parts of the network
   are upgraded.

   When originating new packets, implementations SHOULD have an option
   to determine which value to originate with, this option is controlled
   by the DIO option described below.

   A network which is switching from straight 6lowpan compression
   mechanism to those described in [RFC8138] will experience a flag day
   in the data compression anyway, and if possible this change can be
   deployed at the same time.

   The change of RPI option type from 0x63 to 0x23, makes all [RFC8200]
   Section 4.2 compliant nodes tolerant of the RPL artifacts.  There is
   therefore no longer a necessity to remove the artifacts when sending
   traffic to the Internet.  This change clarifies when to use an IPv6-
   in-IPv6 header, and how to address them: The Hop-by-Hop Options
   Header containing the RPI option MUST always be added when 6LRs
   originate packets (without IPv6-in-IPv6 headers), and IPv6-in-IPv6
   headers MUST always be added when a 6LR find that it needs to insert
   a Hop-by-Hop Options Header containing the RPI option.  The IPv6-in-
   IPv6 header is to be addressed to the RPL root when on the way up,
   and to the end-host when on the way down.

   Non-constrained uses of RPL are not in scope of this document, and
   applicability statements for those uses may provide different advice,
   E.g.  [I-D.ietf-anima-autonomic-control-plane].

   In the non-storing case, dealing with not-RPL aware leaf nodes is
   much easier as the 6LBR (DODAG root) has complete knowledge about the
   connectivity of all DODAG nodes, and all traffic flows through the
   root node.

   The 6LBR can recognize not-RPL aware leaf nodes because it will
   receive a DAO about that node from the 6LR immediately above that
   not-RPL aware node.  This means that the non-storing mode case can
   avoid ever using hop-by-hop re-encapsulation headers for traffic
   originating from the root to the leafs.

   The non-storing mode case does not require the type change from 0x63
   to 0x23, as the root can always create the right packet.  The type
   change does not adversely affect the non-storing case.








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3.2.  Updates to RFC6550: Indicating the new RPI in the DODAG
      Configuration Option Flag.

   In order to avoid a Flag Day caused by lack of interoperation between
   new RPI (0x23) and old RPI (0x63) nodes, this section defines a flag
   in the DIO Configuration Option, to indicate when then new RPI value
   can be safely used.  This means, the flag is going to indicate the
   type of RPI that the network is using.  Thus, when a node join to a
   network will know which value to use.  With this, RPL-capable nodes
   know if it is safe to use 0x23 when creating a new RPI.  A node that
   forwards a packet with an RPI MUST not modify the option type of the
   RPI.

   This is done via a DODAG Configuration Option flag which will
   propagate through the network.  If the flag is received with a value
   zero (which is the default), then new nodes will remain in RFC6553
   Compatible Mode; originating traffic with the old-RPI (0x63) value.

   As stated in [RFC6550] the DODAG Configuration option is present in
   DIO messages.  The DODAG Configuration option distributes
   configuration information.  It is generally static, and does not
   change within the DODAG.  This information is configured at the DODAG
   root and distributed throughout the DODAG with the DODAG
   Configuration option.  Nodes other than the DODAG root do not modify
   this information when propagating the DODAG Configuration option.

   The DODAG Configuration Option has a Flag field which is modified by
   this document.  Currently, the DODAG Configuration Option in
   [RFC6550] states: "the unused bits MUST be initialize to zero by the
   sender and MUST be ignored by the receiver".

   Bit number three of the flag field in the DODAG Configuration option
   is to be used as shown in Figure 4 :


                +------------+-----------------+---------------+
                | Bit number |   Description   |   Reference   |
                +------------+-----------------+---------------+
                |      3     | RPI 0x23 enable | This document |
                +------------+-----------------+---------------+


    Figure 4: DODAG Configuration Option Flag to indicate the RPI-flag-
                                   day.

   In case of rebooting, the node (6LN or 6LR) does not remember if the
   flag is set, so DIO messages would be set with the flag unset until a
   DIO is received with the flag set.



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3.3.  Updates to RFC8138: Indicating the way to decompress with the new
      RPI value.

   This modification is required to be able to decompress the RPL RPI
   option with the new value (0x23).

   RPI-6LoRH header provides a compressed form for the RPL RPI [RFC8138]
   in section 6.  A node that is decompressing this header MUST
   decompress using the RPL RPI option type that is currently active:
   that is, a choice between 0x23 (new) and 0x63 (old).  The node will
   know which to use based upon the presence of the flag in the DODAG
   Configuration Option defined in Section 3.2.  E.g.  If the network is
   in 0x23 mode (by DIO option), then it should be decompressed to 0x23.

   [RFC8138] section 7 documents how to compress the IPv6-in-IPv6
   header.

   There are potential significant advantages to having a single code
   path that always processes IPv6-in-IPv6 headers with no conditional
   branches.

   In Storing Mode, for the examples of Flow from RPL-aware-leaf to not-
   RPL-aware-leaf and not-RPL-aware-leaf to not-RPL-aware-leaf comprise
   an IPv6-in-IPv6 and RPI compression headers.  The use of the IPv6-in-
   IPv6 header is MANDATORY in this case, and it SHOULD be compressed
   with [RFC8138] section 7.  As exemplification of compressing the RPI,
   section A.1 of [RFC8138] illustrates the case in Storing mode where
   the packet is received from the Internet, then the root encapsulates
   the packet to insert the RPI.  The result is shown in Figure 5.


 +-+ ... -+-+-...-+-+-- ... -+-+-+-+- ... -+-+ ... -+-+-+ ... -+-+-+...
 |11110001| RPI-  | IP-in-IP | NH=1        |11110CPP| Compressed | UDP
 |Page 1  | 6LoRH |  6LoRH   | LOWPAN_IPHC | UDP    | UDP header | Payld
 +-+ ... -+-+-...-+-+-- ... -+-+-+-+- ... -+-+ ... -+-+-+ ... -+-+-+...


            Figure 5: RPI Inserted by the Root in Storing Mode

4.  Sample/reference topology

   A RPL network in general 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.

   Figure 6 shows the reference RPL Topology for this document.  The
   letters above the nodes are there so that they may be referenced in
   subsequent sections.  In the figure, 6LR represents a full router



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   node.  The 6LN is a RPL aware router, or host (as a leaf).
   Additionally, for simplification purposes, it is supposed that the
   6LBR has direct access to Internet, thus the 6BBR is not present in
   the figure.

   The 6LN leaves (Raf) marked as (F, H and I) are RPL nodes with no
   children hosts.

   The leafs marked as ~Raf (G and J) are devices which do not speak RPL
   at all (not-RPL-aware), but uses Router-Advertisements, 6LowPAN DAR/
   DAC and efficient-ND only to participate in the network [RFC6775].
   In the document these leafs (G and J) are also referred to as an IPv6
   node.

   The 6LBR ("A") in the figure is the root of the Global DODAG.




































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                     +------------+
                     |  INTERNET  ----------+
                     |            |         |
                     +------------+         |
                                            |
                                            |
                                            |
                                          A |
                                      +-------+
                                      |6LBR   |
                          +-----------|(root) |-------+
                          |           +-------+       |
                          |                           |
                          |                           |
                          |                           |
                          |                           |
                          | B                         |C
                      +---|---+                   +---|---+
                      |  6LR  |                   |  6LR  |
            +---------|       |--+             +---       ---+
            |         +-------+  |             |  +-------+  |
            |                    |             |             |
            |                    |             |             |
            |                    |             |             |
            |                    |             |             |
            | D                  |  E          |             |
          +-|-----+          +---|---+         |             |
          |  6LR  |          |  6LR  |         |             |
          |       |    +------       |         |             |
          +---|---+    |     +---|---+         |             |
              |        |         |             |             |
              |        |         +--+          |             |
              |        |            |          |             |
              |        |            |          |             |
              |        |            |        I |          J  |
           F  |        | G          | H        |             |
        +-----+-+    +-|-----+  +---|--+   +---|---+     +---|---+
        |  Raf  |    | ~Raf  |  | Raf  |   |  Raf  |     | ~Raf  |
        |  6LN  |    |  6LN  |  | 6LN  |   |  6LN  |     |  6LN  |
        +-------+    +-------+  +------+   +-------+     +-------+


                    Figure 6: A reference RPL Topology.








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5.  Use cases

   In the data plane a combination of RFC6553, RFC6554 and IPv6-in-IPv6
   encapsulation are going to be analyzed for a number of representative
   traffic flows.

   This document assumes that the LLN is using the no-drop RPI option
   (0x23).

   The uses cases describe the communication between RPL-aware-nodes,
   with the root (6LBR), and with Internet.  This document also
   describes the communication between nodes acting as leaves that do
   not understand RPL (~Raf nodes), but are part of the LLN.  (e.g.
   Section 6.1.4 Flow from not-RPL-aware-leaf to root) This document
   depicts as well the communication inside of the LLN when it has the
   final destination addressed outside of the LLN e.g. with destination
   to Internet.  For example, Section 6.2.3 Flow from not-RPL-aware-leaf
   to Internet

   The uses cases comprise as follow:

   Interaction between Leaf and Root:

      RPL-aware-leaf(Raf) to root

      root to RPL-aware-leaf(Raf)

      not-RPL-aware-leaf(~Raf) to root

      root to not-RPL-aware-leaf(~Raf)

   Interaction between Leaf and Internet:

      RPL-aware-leaf(Raf) to Internet

      Internet to RPL-aware-leaf(Raf)

      not-RPL-aware-leaf(~Raf) to Internet

      Internet to not-RPL-aware-leaf(~Raf)

   Interaction between Leafs:

      RPL-aware-leaf(Raf) to RPL-aware-leaf(Raf) (storing and non-
      storing)

      RPL-aware-leaf(Raf) to not-RPL-aware-leaf(~Raf) (non-storing)




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      not-RPL-aware-leaf(~Raf) to RPL-aware-leaf(Raf) (storing and non-
      storing)

      not-RPL-aware-leaf(~Raf) to not-RPL-aware-leaf(~Raf) (non-storing)

   This document is consistent with the 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 [RFC8200].  Extensions headers may not be added or
   removed except by the sender or the receiver.

   However, unlike [RFC6553], the Hop-by-Hop Option Header used for the
   RPI artifact has the first two bits set to '00'.  This means that the
   RPI artifact will be ignored when received by a host or router that
   does not understand that option ( Section 4.2 [RFC8200]).

   This means that when the no-drop RPI option code 0x23 is used, a
   packet that leaves the RPL domain of an LLN (or that leaves the LLN
   entirely) will not be discarded when it contains the [RFC6553] RPL
   Hop-by-Hop option known as RPI.  Thus, the RPI Hop-by-Hop option is
   left in place even if the end host does not understand it.

   NOTE: No clear attack has been described when the RPI information is
   released to the Internet.  At a minimum, it is clear that the RPI
   option would waste some network bandwidth when it escapes.  This is
   traded off against the savings in the LLN by not having to
   encapsulate the packet in order to remove the artifact.  Please check
   the Security Considerations sections Section 11 for further details.

   As the rank information in the RPI artifact is changed at each hop,
   it will typically be zero when it arrives at the DODAG root.  The
   DODAG root MUST force it to zero when passing the packet out to the
   Internet.  The Internet will therefore not see any SenderRank
   information.

   Despite being legal to leave the RPI artifact in place, an
   intermediate router that needs to add an extension header (e.g.  RH3
   or RPI Option) MUST still encapsulate the packet in an (additional)
   outer IP header.  The new header is placed after this new outer IP
   header.

   A corollary is that an RH3 or RPI Option can only be removed by an
   intermediate router if it is placed in an encapsulating IPv6 Header,
   which is addressed TO the intermediate router.  When it does so, the
   whole encapsulating header must be removed.  (A replacement may be
   added).  This sometimes can result in outer IP headers being
   addressed to the next hop router using link-local address.




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

   RPI MUST be present in every single RPL data packet.

   Prior to [RFC8138], there was significant interest in removing the
   RPI for downward flows in non-storing mode.  The exception covered a
   very small number of cases, and causes significant interoperability
   challenges, yet costed significant code and testing complexity.  The
   ability to compress the RPI down to three bytes or less removes much
   of the pressure to optimize this any further
   [I-D.ietf-anima-autonomic-control-plane].

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

6.  Storing mode

   In storing mode (fully stateful), the sender can determine if the
   destination is inside the LLN by looking if the destination address
   is matched by the DIO's Prefix Information Option (PIO) option.

   The following table (Figure 7) itemizes which headers are needed in
   each of the following scenarios.  It indicates if the IPv6-in-IPv6
   header that is added, must be addressed to the final destination (the
   Raf node that is the target(tgt)), to the "root" or if a hop-by-hop
   header must be added (indicated by "hop").

   In cases where no IPv6-in-IPv6 header is needed, the column states as
   "No".  If the IPv6-in-IPv6 header is needed is a "must".

   In all cases the RPI headers are needed, since it identifies
   inconsistencies (loops) in the routing topology.  In all cases the
   RH3 is not needed because it is not used in storing mode.

   In each case, 6LR_i is the intermediate router from source to
   destination.  "1 <= i <= n", n is the number of routers (6LR) that
   the packet goes through from source (6LN) to destination.

   The leaf can be a router 6LR or a host, both indicated as 6LN.  The
   root refers to the 6LBR (see Figure 6).





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  +---------------------+--------------+------------+------------------+
  | Interaction between |   Use Case   |IPv6-in-IPv6| IPv6-in-IPv6 dst |
  +---------------------+--------------+------------+------------------+
  |                     |  Raf to root |    No      |         No       |
  +                     +--------------+------------+------------------+
  |     Leaf - Root     |  root to Raf |    No      |        No        |
  +                     +--------------+------------+------------------+
  |                     | root to ~Raf |    No      |        No        |
  +                     +--------------+------------+------------------+
  |                     | ~Raf to root |   must     |        root      |
  +---------------------+--------------+------------+------------------+
  |                     |  Raf to Int  |    No      |        No        |
  +                     +--------------+------------+------------------+
  |   Leaf - Internet   |  Int to Raf  |   must     |      Raf (tgt)   |
  +                     +--------------+------------+------------------+
  |                     |  ~Raf to Int |   must     |        root      |
  +                     +--------------+------------+------------------+
  |                     |  Int to ~Raf |   must     |       hop        |
  +---------------------+--------------+------------+------------------+
  |                     |  Raf to Raf  |    No      |        No        |
  +                     +--------------+------------+------------------+
  |                     |  Raf to ~Raf |    No      |        No        |
  +     Leaf - Leaf     +--------------+------------+------------------+
  |                     |  ~Raf to Raf |   must     |     Raf (tgt)    |
  +                     +--------------+------------+------------------+
  |                     | ~Raf to ~Raf |   must     |       hop        |
  +---------------------+--------------+------------+------------------+

      Figure 7: Table of IPv6-in-IPv6 encapsulation in Storing mode.

6.1.  Storing Mode: Interaction between Leaf and Root

   In this section is described the communication flow in storing mode
   (SM) between,

      RPL-aware-leaf to root

      root to RPL-aware-leaf

      not-RPL-aware-leaf to root

      root to not-RPL-aware-leaf

6.1.1.  SM: Example of Flow from RPL-aware-leaf to root

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




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   As stated 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 this case the flow comprises:

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

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node A root(6LBR)

   As it was mentioned in this document 6LRs, 6LBR are always full-
   fledged RPL routers.

   The 6LN (Node F) inserts the RPI header, and sends the packet to 6LR
   (Node E) which decrements the rank in RPI and sends the packet up.
   When the packet arrives at 6LBR (Node A), the RPI is removed and the
   packet is processed.

   No IPv6-in-IPv6 header is required.

   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 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.

   The Table 1 summarizes what headers are needed for this use case.

            +-------------------+---------+-------+----------+
            | Header            | 6LN src | 6LR_i | 6LBR dst |
            +-------------------+---------+-------+----------+
            | Inserted headers  | RPI     | --    | --       |
            | Removed headers   | --      | --    | RPI      |
            | Re-added headers  | --      | --    | --       |
            | Modified headers  | --      | RPI   | --       |
            | Untouched headers | --      | --    | --       |
            +-------------------+---------+-------+----------+

   Table 1: Storing mode: Summary of the use of headers from RPL-aware-
                               leaf to root







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6.1.2.  SM: Example of Flow from root to RPL-aware-leaf

   In this case the flow comprises:

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

   For example, a communication flow could be: Node A root(6LBR) -->
   Node B --> Node D --> Node F

   In this case the 6LBR inserts RPI header and sends the packet down,
   the 6LR is going to increment the rank in RPI (it examines the
   instanceID to identify the right forwarding table), the packet is
   processed in the 6LN and the RPI removed.

   No IPv6-in-IPv6 header is required.

   The Table 2 summarizes what headers are needed for this use case.

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

     Table 2: Storing mode: Summary of the use of headers from root to
                              RPL-aware-leaf

6.1.3.  SM: Example of Flow from root to not-RPL-aware-leaf

   In this case the flow comprises:

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

   For example, a communication flow could be: Node A root(6LBR) -->
   Node B --> Node E --> Node G

   As the RPI extension can be ignored by the not-RPL-aware leaf, this
   situation is identical to the previous scenario.

   The Table 3 summarizes what headers are needed for this use case.







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         +-------------------+----------+-------+----------------+
         | Header            | 6LBR src | 6LR_i | IPv6 dst node  |
         +-------------------+----------+-------+----------------+
         | Inserted headers  | RPI      | --    | --             |
         | Removed headers   | --       | --    | --             |
         | Re-added headers  | --       | --    | --             |
         | Modified headers  | --       | RPI   | --             |
         | Untouched headers | --       | --    | RPI (Ignored)  |
         +-------------------+----------+-------+----------------+

     Table 3: Storing mode: Summary of the use of headers from root to
                            not-RPL-aware-leaf

6.1.4.  SM: Example of Flow from not-RPL-aware-leaf to root

   In this case the flow comprises:

   not-RPL-aware-leaf (IPv6) --> 6LR_1 --> 6LR_i --> root (6LBR)

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A root(6LBR)

   When the packet arrives from IPv6 node (Node G) to 6LR_1 (Node E),
   the 6LR_1 will insert a RPI header, encapsulated in a IPv6-in-IPv6
   header.  The IPv6-in-IPv6 header can be addressed to the next hop
   (Node B), or to the root (Node A).  The root removes the header and
   processes the packet.

   The Figure 8 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.





















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+-----------+------+--------------+-----------------+--------------------+
|   Header  | IPv6 |     6LR_1    |      6LR_i      |       6LBR dst     |
|           | src  |              |                 |                    |
|           | node |              |                 |                    |
+-----------+------+--------------+-----------------+--------------------+
|  Inserted |  --  | IP6-IP6(RPI) | IP6-IP6(RPI)[1] |         --         |
|  headers  |      |              |                 |                    |
+-----------+------+--------------+-----------------+--------------------+
|  Removed  |  --  |      --      |        --       | IP6-IP6(RPI)[1][2] |
|  headers  |      |              |                 |                    |
+-----------+------+--------------+-----------------+--------------------+
|  Re-added |  --  |      --      | IP6-IP6(RPI)[1] |         --         |
|  headers  |      |              |                 |                    |
+-----------+------+--------------+-----------------+--------------------+
|  Modified |  --  |      --      | IP6-IP6(RPI)[2] |         --         |
|  headers  |      |              |                 |                    |
+-----------+------+--------------+-----------------+--------------------+
| Untouched |  --  |      --      |        --       |         --         |
|  headers  |      |              |                 |                    |
+-----------+------+--------------+-----------------+--------------------+

    Figure 8: Storing mode: Summary of the use of headers from not-RPL-
      aware-leaf to root.  [[1] Case where the IPv6-in-IPv6 header is
    addressed to the next hop (Node B). [2] Case where the IPv6-in-IPv6
                 header is addressed to the root (Node A).

6.2.  Storing Mode: Interaction between Leaf and Internet.

   In this section is described the communication flow in storing mode
   (SM) between,

      RPL-aware-leaf to Internet

      Internet to RPL-aware-leaf

      not-RPL-aware-leaf to Internet

      Internet to not-RPL-aware-leaf

6.2.1.  SM: Example of Flow from RPL-aware-leaf to Internet

   RPL information from RFC 6553 may go out to Internet as it will be
   ignored by nodes which have not been configured to be RPI aware.

   In this case the flow comprises:

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




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   For example, the communication flow could be: Node F --> Node D -->
   Node B --> Node A root(6LBR) --> Internet

   No IPv6-in-IPv6 header is required.

   Note: In this use case it is used a node as leaf, but this use case
   can be also applicable to any RPL-node type (e.g. 6LR)

   The Table 4 summarizes what headers are needed for this use case.

      +-------------------+---------+-------+------+----------------+
      | Header            | 6LN src | 6LR_i | 6LBR | Internet dst   |
      +-------------------+---------+-------+------+----------------+
      | Inserted headers  | RPI     | --    | --   | --             |
      | Removed headers   | --      | --    | --   | --             |
      | Re-added headers  | --      | --    | --   | --             |
      | Modified headers  | --      | RPI   | --   | --             |
      | Untouched headers | --      | --    | RPI  | RPI (Ignored)  |
      +-------------------+---------+-------+------+----------------+

   Table 4: Storing mode: Summary of the use of headers from RPL-aware-
                             leaf to Internet

6.2.2.  SM: Example of Flow from Internet to RPL-aware-leaf

   In this case the flow comprises:

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

   For example, a communication flow could be: Internet --> Node A
   root(6LBR) --> Node B --> Node D --> Node F

   When the packet arrives from Internet to 6LBR the RPI header is added
   in a outer IPv6-in-IPv6 header (with the IPv6-in-IPv6 destination
   address set to the 6LR) and sent to 6LR, which modifies the rank in
   the RPI.  When the packet arrives at 6LN the RPI header is removed
   and the packet processed.

   The Figure 9 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.











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+-----------+----------+--------------+--------------+------------------+
|   Header  | Internet |     6LBR     |     6LR_i    |     6LN dst      |
|           | src      |              |              |                  |
+-----------+----------+--------------+--------------+------------------+
|  Inserted |    --    | IP6-IP6(RPI) |      --      |         --       |
|  headers  |          |              |              |                  |
+-----------+----------+--------------+--------------+------------------+
|  Removed  |    --    |      --      |      --      |   IP6-IP6(RPI)   |
|  headers  |          |              |              |                  |
+-----------+----------+--------------+--------------+------------------+
|  Re-added |    --    |      --      |      --      |         --       |
|  headers  |          |              |              |                  |
+-----------+----------+--------------+--------------+------------------+
|  Modified |    --    |      --      | IP6-IP6(RPI) |         --       |
|  headers  |          |              |              |                  |
+-----------+----------+--------------+--------------+------------------+
| Untouched |    --    |      --      |      --      |         --       |
|  headers  |          |              |              |                  |
+-----------+----------+--------------+--------------+------------------+

    Figure 9: Storing mode: Summary of the use of headers from Internet
                            to RPL-aware-leaf.

6.2.3.  SM: Example of Flow from not-RPL-aware-leaf to Internet

   In this case the flow comprises:

   not-RPL-aware-leaf (IPv6) --> 6LR_1 --> 6LR_i -->root (6LBR) -->
   Internet

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A root(6LBR) --> Internet

   The 6LR_1 (i=1) node will add an IPv6-in-IPv6(RPI) header addressed
   either to the root, or hop-by-hop such that the root can remove the
   RPI header before passing upwards.  The IPv6-in-IPv6 addressed to the
   root cause less processing overhead.  On the other hand, with hop-by-
   hop the intermediate routers can check the routing tables for a
   better routing path, thus it could be more efficient and faster.
   Implementation should decide which approach to take.

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

   The Figure 10 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.



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  +---------+-------+------------+--------------+-------------+--------+
  |  Header | IPv6  |   6LR_1    |     6LR_i    |     6LBR    |Internet|
  |         |  src  |            |  [i=2,...,n] |             |  dst   |
  |         | node  |            |              |             |        |
  +---------+-------+------------+--------------+-------------+--------+
  | Inserted|   --  |IP6-IP6(RPI)| IP6-IP6(RPI) |      --     |    --  |
  | headers |       |            |      [2]     |             |        |
  +---------+-------+------------+--------------+-------------+--------+
  | Removed |   --  |    --      | IP6-IP6(RPI) | IP6-IP6(RPI)|    --  |
  | headers |       |            |      [2]     |    [1][2]   |        |
  +---------+-------+------------+--------------+-------------+--------+
  | Re-added|   --  |    --      |      --      |      --     |    --  |
  | headers |       |            |              |             |        |
  +---------+-------+------------+--------------+-------------+--------+
  | Modified|   --  |    --      | IP6-IP6(RPI) |      --     |    --  |
  | headers |       |            |      [1]     |             |        |
  +---------+-------+------------+--------------+-------------+--------+
  |Untouched|   --  |    --      |      --      |      --     |    --  |
  | headers |       |            |              |             |        |
  +---------+-------+------------+--------------+-------------+--------+

   Figure 10: Storing mode: Summary of the use of headers from not-RPL-
     aware-leaf to Internet.  [1] Case when packet is addressed to the
         root.  [2] Case when the packet is addressed hop-by-hop.

6.2.4.  SM: Example of Flow from Internet to not-RPL-aware-leaf.

   In this case the flow comprises:

   Internet --> root (6LBR) --> 6LR_i --> not-RPL-aware-leaf (IPv6)

   For example, a communication flow could be: Internet --> Node A
   root(6LBR) --> Node B --> Node E --> Node G

   The 6LBR will have to add an RPI header within an IPv6-in-IPv6
   header.  The IPv6-in-IPv6 is addressed hop-by-hop.

   The final node should be able to remove one or more IPv6-in-IPv6
   headers which are all addressed to it.  The final node does not
   process the RPI, the node ignores the RPI.  Furhter details about
   this are mentioned in [I-D.thubert-roll-unaware-leaves], which
   specifies RPL routing for a 6LN acting as a plain host and not aware
   of RPL.

   The 6LBR may set the flow label on the inner IPv6-in-IPv6 header to
   zero in order to aid in compression.





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   The Figure 11 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.

   +-----------+----------+--------------+--------------+--------------+
   |   Header  | Internet |     6LBR     |     6LR_i    |IPv6 dst node |
   |           |   src    |              |              |              |
   +-----------+----------+--------------+--------------+--------------+
   |  Inserted |    --    | IP6-IP6(RPI) |      --      |      --      |
   |  headers  |          |              |              |              |
   +-----------+----------+--------------+--------------+--------------+
   |  Removed  |    --    |      --      |              |  IP6-IP6(RPI)|
   |  headers  |          |              |              |  RPI Ignored |
   +-----------+----------+--------------+--------------+--------------+
   |  Re-added |    --    |      --      |      --      |      --      |
   |  headers  |          |              |              |              |
   +-----------+----------+--------------+--------------+--------------+
   |  Modified |    --    |      --      | IP6-IP6(RPI) |      --      |
   |  headers  |          |              |              |              |
   +-----------+----------+--------------+--------------+--------------+
   | Untouched |    --    |      --      |      --      |      --      |
   |  headers  |          |              |              |              |
   +-----------+----------+--------------+--------------+--------------+

   Figure 11: Storing mode: Summary of the use of headers from Internet
                          to not-RPL-aware-leaf.

6.3.  Storing Mode: Interaction between Leaf and Leaf

   In this section is described the communication flow in storing mode
   (SM) between,

      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

6.3.1.  SM: 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.  See section 9 in
   [RFC6550].

   When the nodes are not directly connected, then in storing mode, the
   flow comprises:



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   6LN --> 6LR_ia --> common parent (6LR_x) --> 6LR_id --> 6LN

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node E --> Node H

   6LR_ia (Node D) is the intermediate router from source to the common
   parent (6LR_x) (Node B) In this case, "1 <= ia <= n", n is the number
   of routers (6LR) that the packet goes through from 6LN (Node F) to
   the common parent (6LR_x).

   6LR_id (Node E) is the intermediate router from the common parent
   (6LR_x) (Node B) to destination 6LN (Node H).  In this case, "1 <= id
   <= m", m is the number of routers (6LR) that the packet goes through
   from the common parent (6LR_x) to destination 6LN.

   It is assumed that the two nodes are in the same RPL Domain (that
   they share the same DODAG root).  At the common parent (Node B), 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 IPv6-in-IPv6 headers are necessary.

   The Table 5 summarizes what headers are needed for this use case.

   +---------------+--------+--------+---------------+--------+--------+
   | Header        | 6LN    | 6LR_ia | 6LR_x (common | 6LR_id | 6LN    |
   |               | src    |        | parent)       |        | dst    |
   +---------------+--------+--------+---------------+--------+--------+
   | Inserted      | RPI    | --     | --            | --     | --     |
   | headers       |        |        |               |        |        |
   | Removed       | --     | --     | --            | --     | RPI    |
   | headers       |        |        |               |        |        |
   | Re-added      | --     | --     | --            | --     | --     |
   | headers       |        |        |               |        |        |
   | Modified      | --     | RPI    | RPI           | RPI    | --     |
   | headers       |        |        |               |        |        |
   | Untouched     | --     | --     | --            | --     | --     |
   | headers       |        |        |               |        |        |
   +---------------+--------+--------+---------------+--------+--------+

    Table 5: Storing mode: Summary of the use of headers for RPL-aware-
                          leaf to RPL-aware-leaf

6.3.2.  SM: Example of Flow from RPL-aware-leaf to not-RPL-aware-leaf

   In this case the flow comprises:





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   6LN --> 6LR_ia --> common parent (6LR_x) --> 6LR_id --> not-RPL-aware
   6LN (IPv6)

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node E --> Node G

   6LR_ia is the intermediate router from source (6LN) to the common
   parent (6LR_x) In this case, "1 <= ia <= n", n is the number of
   routers (6LR) that the packet goes through from 6LN to the common
   parent (6LR_x).

   6LR_id (Node E) is the intermediate router from the common parent
   (6LR_x) (Node B) to destination not-RPL-aware 6LN (IPv6) (Node G).
   In this case, "1 <= id <= m", m is the number of routers (6LR) that
   the packet goes through from the common parent (6LR_x) to destination
   6LN.

   This situation is identical to the previous situation Section 6.3.1

   The Table 6 summarizes what headers are needed for this use case.

   +-----------+------+--------+---------------+--------+--------------+
   | Header    | 6LN  | 6LR_ia | 6LR_x(common  | 6LR_id | IPv6 dst     |
   |           | src  |        | parent)       |        | node         |
   +-----------+------+--------+---------------+--------+--------------+
   | Inserted  | RPI  | --     | --            | --     | --           |
   | headers   |      |        |               |        |              |
   | Removed   | --   | --     | --            | --     | --           |
   | headers   |      |        |               |        |              |
   | Re-added  | --   | --     | --            | --     | --           |
   | headers   |      |        |               |        |              |
   | Modified  | --   | RPI    | RPI           | RPI    | --           |
   | headers   |      |        |               |        |              |
   | Untouched | --   | --     | --            | --     | RPI(Ignored) |
   | headers   |      |        |               |        |              |
   +-----------+------+--------+---------------+--------+--------------+

    Table 6: Storing mode: Summary of the use of headers for RPL-aware-
                        leaf to not-RPL-aware-leaf

6.3.3.  SM: Example of Flow from not-RPL-aware-leaf to RPL-aware-leaf

   In this case the flow comprises:

   not-RPL-aware 6LN (IPv6) --> 6LR_ia --> common parent (6LR_x) -->
   6LR_id --> 6LN





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   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node D --> Node F

   6LR_ia (Node E) is the intermediate router from source (not-RPL-aware
   6LN (IPv6)) (Node G) to the common parent (6LR_x) (Node B).  In this
   case, "1 <= ia <= n", n is the number of routers (6LR) that the
   packet ges through from source to the common parent.

   6LR_id (Node D) is the intermediate router from the common parent
   (6LR_x) (Node B) to destination 6LN (Node F).  In this case, "1 <= id
   <= m", m is the number of routers (6LR) that the packet goes through
   from the common parent (6LR_x) to destination 6LN.

   The 6LR_ia (ia=1) (Node E) receives the packet from the the IPv6 node
   (Node G) and inserts and the RPI header encapsulated in IPv6-in-IPv6
   header.  The IPv6-in-IPv6 header is addressed to the destination 6LN
   (Node F).

   The Figure 12 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.

+----------+-----+--------------+--------------+--------------+------------+
|  Header  |IPv6 |    6LR_ia    |    Common    |    6LR_id    |    6LN     |
|          |src  |              |    Parent    |              |    dst     |
|          |node |              |    (6LRx)    |              |            |
+----------+-----+--------------+--------------+--------------+------------+
| Inserted |  -- | IP6-IP6(RPI) |      --      |      --      |    --      |
| headers  |     |              |              |              |            |
+----------+-----+--------------+--------------+--------------+------------+
| Removed  |  -- |      --      |      --      |      --      |IP6-IP6(RPI)|
| headers  |     |              |              |              |            |
+----------+-----+--------------+--------------+--------------+------------+
| Re-added |  -- |      --      |      --      |      --      |    --      |
| headers  |     |              |              |              |            |
+----------+-----+--------------+--------------+--------------+------------+
| Modified |  -- |      --      | IP6-IP6(RPI) | IP6-IP6(RPI) |    --      |
| headers  |     |              |              |              |            |
+----------+-----+--------------+--------------+--------------+------------+
|Untouched |  -- |      --      |      --      |      --      |    --      |
| headers  |     |              |              |              |            |
+----------+-----+--------------+--------------+--------------+------------+

   Figure 12: Storing mode: Summary of the use of headers from not-RPL-
                       aware-leaf to RPL-aware-leaf.







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6.3.4.  SM: Example of Flow from not-RPL-aware-leaf to not-RPL-aware-
        leaf

   In this case the flow comprises:

   not-RPL-aware 6LN (IPv6 src)--> 6LR_1--> 6LR_ia --> 6LBR --> 6LR_id
   --> not-RPL-aware 6LN (IPv6 dst)

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A (root) --> Node C --> Node J

   Internal nodes 6LR_ia (e.g: Node E or Node B) is the intermediate
   router from the not-RPL-aware source (Node G) to the root (6LBR)
   (Node A).  In this case, "1 < ia <= n", n is the number of routers
   (6LR) that the packet goes through from IPv6 src to the root.

   6LR_id (C) is the intermediate router from the root (Node A) to the
   destination Node J.  In this case, "1 <= id <= m", m is the number of
   routers (6LR) that the packet goes through from the root to
   destination (IPv6 dst).

   The RPI is ignored at the IPv6 dst node.

   The 6LR_1 (Node E) receives the packet from the the IPv6 node (Node
   G) and inserts the RPI header (RPI), encapsulated in an IPv6-in-IPv6
   header.  The IPv6-in-IPv6 header is addressed hop-by-hop.

   The Figure 13 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.






















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   +---------+------+-------+-------+---------+-------+-------+
   |  Header | IPv6 | 6LR_1 | 6LR_ia|   6LBR  |6LR_id | IPv6  |
   |         |  src |       |       |         |       | dst   |
   |         | node |       |       |         |       | node  |
   +---------+------+-------+-------+---------+-------+-------+
   | Inserted|  --  |IP6-IP6|  --   |         |  --   |   --  |
   | headers |      | (RPI )|       |         |       |       |
   |         |      |       |       |         |       |       |
   +---------+------+-------+-------+---------+-------+-------+
   | Removed |  --  |  --   |  --   |         |  --   |IP6-IP6|
   | headers |      |       |       |         |       |(RPI)  |
   |         |      |       |       |         |       |  RPI  |
   |         |      |       |       |         |       |Ignored|
   +---------+------+-------+-------+---------+-------+-------+
   | Re-added|  --  |  --   |  --   |    --   |  --   |   --  |
   | headers |      |       |       |         |       |       |
   +---------+------+-------+-------+---------+-------+-------+
   | Modified|  --  |  --   |IP6-IP6| IP6-IP6 |IP6-IP6|  --   |
   | headers |      |       | (RPI) | (RPI)   | (RPI) |       |
   |         |      |       |       |         |       |       |
   +---------+------+-------+-------+---------+-------+-------+
   |Untouched|  --  |  --   |  --   |    --   |  --   |    -- |
   | headers |      |       |       |         |       |       |
   +---------+------+-------+-------+---------+-------+-------+

   Figure 13: Storing mode: Summary of the use of headers from not-RPL-
                     aware-leaf to not-RPL-aware-leaf

7.  Non Storing mode

   In Non Storing Mode (Non SM) (fully source routed), the 6LBR (DODAG
   root) has complete knowledge about the connectivity of all DODAG
   nodes, and all traffic flows through the root node.  Thus, there is
   no need for all nodes to know about the existence of not-RPL aware
   nodes.  Only the 6LBR needs to act if compensation is necessary for
   not-RPL aware receivers.

   The following table (Figure 14) summarizes what headers are needed in
   the following scenarios, and indicates when the RPI, RH3 and IPv6-in-
   IPv6 header are to be inserted.  It depicts the target destination
   address possible (indicated by "Raf"), to a 6LR (parent of a 6LN) or
   to the root.  In cases where no IPv6-in-IPv6 header is needed, the
   column states as "No".  There is no expectation on RPL that RPI can
   be omitted, because it is needed for routing, quality of service and
   compression.  This specification expects that is always a RPI
   Present.





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   The leaf can be a router 6LR or a host, both indicated as 6LN
   (Figure 3).  In the Figure the (1) indicates a 6tisch case [RFC8180],
   where the RPI header may still be needed for the instanceID to be
   available for priority/channel selection at each hop.

+-----------------+--------------+-----+-----+------------+------------+
|   Interaction   |   Use Case   | RPI | RH3 |IPv6-in-IPv6|IPv6-in-IPv6|
|      between    |              |     |     |            |     dst    |
+-----------------+--------------+-----+-----+------------+------------+
|                 |  Raf to root | Yes | No  |    No      |    No      |
+                 +--------------+-----+-----+------------+------------+
|   Leaf - Root   |  root to Raf | Yes | Yes |    No      |    No      |
+                 +--------------+-----+-----+------------+------------+
|                 | root to ~Raf | Yes | Yes |   must     |    6LR     |
|                 |              | (1) |     |            |            |
+                 +--------------+-----+-----+------------+------------+
|                 | ~Raf to root | Yes | No  |   must     |   root     |
+-----------------+--------------+-----+-----+------------+------------+
|                 |  Raf to Int  | Yes | No  |   No       |    No      |
+                 +--------------+-----+-----+------------+------------+
| Leaf - Internet |  Int to Raf  | Yes | Yes |   must     |    Raf     |
+                 +--------------+-----+-----+------------+------------+
|                 |  ~Raf to Int | Yes | No  |   must     |    root    |
+                 +--------------+-----+-----+------------+------------+
|                 |  Int to ~Raf | Yes | Yes |   must     |     6LR    |
+-----------------+--------------+-----+-----+------------+------------+
|                 |  Raf to Raf  | Yes | Yes |   must     |  root/Raf  |
+                 +--------------+-----+-----+------------+------------+
|                 |  Raf to ~Raf | Yes | Yes |   must     |  root/6LR  |
+   Leaf - Leaf   +--------------+-----+-----+------------+------------+
|                 |  ~Raf to Raf | Yes | Yes |   must     |  root/Raf  |
+                 +--------------+-----+-----+------------+------------+
|                 | ~Raf to ~Raf | Yes | Yes |   must     |  root/6LR  |
+-----------------+--------------+-----+-----+------------+------------+

   Figure 14: Table that shows headers needed in Non-Storing mode: RPI,
                     RH3, IPv6-in-IPv6 encapsulation.

7.1.  Non-Storing Mode: Interaction between Leaf and Root

   In this section is described the communication flow in Non Storing
   Mode (Non-SM) between,

      RPL-aware-leaf to root

      root to RPL-aware-leaf

      not-RPL-aware-leaf to root



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      root to not-RPL-aware-leaf

7.1.1.  Non-SM: 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 since it
   contains the rank information, which is used to avoid/detect loops.

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

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node A (root)

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 <= i <= n", n is the number of routers (6LR) that the packet
   goes through from source (6LN) to destination (6LBR).

   This situation is the same case as storing mode.

   The Table 7 summarizes what headers are needed for this use case.

            +-------------------+---------+-------+----------+
            | Header            | 6LN src | 6LR_i | 6LBR dst |
            +-------------------+---------+-------+----------+
            | Inserted headers  | RPI     | --    | --       |
            | Removed headers   | --      | --    | RPI      |
            | Re-added headers  | --      | --    | --       |
            | Modified headers  | --      | RPI   | --       |
            | Untouched headers | --      | --    | --       |
            +-------------------+---------+-------+----------+

    Table 7: Non Storing mode: Summary of the use of headers from RPL-
                            aware-leaf to root

7.1.2.  Non-SM: Example of Flow from root to RPL-aware-leaf

   In this case the flow comprises:

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

   For example, a communication flow could be: Node A (root) --> Node B
   --> Node D --> Node F

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 <= i <= n", n is the number of routers (6LR) that the packet
   goes through from source (6LBR) to destination (6LN).





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   The 6LBR inserts an RH3, and a RPI header.  No IPv6-in-IPv6 header is
   necessary as the traffic originates with an RPL aware node, the 6LBR.
   The destination is known to be RPL-aware because the root knows the
   whole topology in non-storing mode.

   The Table 8 summarizes what headers are needed for this use case.

         +-------------------+----------+-----------+-----------+
         | Header            | 6LBR src | 6LR_i     | 6LN dst   |
         +-------------------+----------+-----------+-----------+
         | Inserted headers  | RPI, RH3 | --        | --        |
         | Removed headers   | --       | --        | RH3, RPI  |
         | Re-added headers  | --       | --        | --        |
         | Modified headers  | --       | RPI, RH3  | --        |
         | Untouched headers | --       | --        | --        |
         +-------------------+----------+-----------+-----------+

   Table 8: Non Storing mode: Summary of the use of headers from root to
                              RPL-aware-leaf

7.1.3.  Non-SM: Example of Flow from root to not-RPL-aware-leaf

   In this case the flow comprises:

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

   For example, a communication flow could be: Node A (root) --> Node B
   --> Node E --> Node G

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 <= i <= n", n is the number of routers (6LR) that the packet
   goes through from source (6LBR) to destination (IPv6).

   In 6LBR the RH3 is added, it is modified at each intermediate 6LR
   (6LR_1 and so on) and it is fully consumed in the last 6LR (6LR_n),
   but left there.  As the RPI is added, then the IPv6 node which does
   not understand the RPI, will ignore it (following RFC8200), thus
   encapsulation is not necessary.

   The Figure 15 depicts the table that summarizes what headers are
   needed for this use case.










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   +-----------+----------+--------------+----------------+----------+
   |   Header  |   6LBR   |     6LR_i    |      6LR_n     |   IPv6   |
   |           |          | i=(1,..,n-1) |                |    dst   |
   |           |          |              |                |   node   |
   +-----------+----------+--------------+----------------+----------+
   |  Inserted | RPI, RH3 |      --      |       --       |    --    |
   |  headers  |          |              |                |          |
   +-----------+----------+--------------+----------------+----------+
   |  Removed  |    --    |      --      |                |    --    |
   |  headers  |          |              |                |          |
   +-----------+----------+--------------+----------------+----------+
   |  Re-added |    --    |      --      |       --       |    --    |
   |  headers  |          |              |                |          |
   +-----------+----------+--------------+----------------+----------+
   |  Modified |    --    |   RPI, RH3   |      RPI,      |    --    |
   |  headers  |          |              | RH3(consumed)  |          |
   +-----------+----------+--------------+----------------+----------+
   | Untouched |    --    |      --      |       --       | RPI, RH3 |
   |  headers  |          |              |                |  (both   |
   |           |          |              |                | ignored) |
   +-----------+----------+--------------+----------------+----------+

   Figure 15: Non Storing mode: Summary of the use of headers from root
                           to not-RPL-aware-leaf

7.1.4.  Non-SM: Example of Flow from not-RPL-aware-leaf to root

   In this case the flow comprises:

   not-RPL-aware-leaf (IPv6) --> 6LR_1 --> 6LR_i --> root (6LBR)

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A (root)

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 < i <= n", n is the number of routers (6LR) that the packet
   goes through from source (IPv6) to destination (6LBR).  For example,
   6LR_1 (i=1) is the router that receives the packets from the IPv6
   node.

   In this case the RPI is added by the first 6LR (6LR1) (Node E),
   encapsulated in an IPv6-in-IPv6 header, and is modified in the
   following 6LRs.  The RPI and entire packet is consumed by the root.

   The Figure 16 shows the table that summarizes what headers are needed
   for this use case.





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+----------+------+-------------------+------------------+-----------------+
|  Header  | IPv6 |       6LR_1       |       6LR_i      |     6LBR dst    |
|          | src  |                   |                  |                 |
|          | node |                   |                  |                 |
+----------+------+-------------------+------------------+-----------------+
| Inserted |  --  | IPv6-in-IPv6(RPI) |         --       |        --       |
| headers  |      |                   |                  |                 |
+----------+------+-------------------+------------------+-----------------+
| Removed  |  --  |         --        |         --       |IPv6-in-IPv6(RPI)|
| headers  |      |                   |                  |                 |
+----------+------+-------------------+------------------+-----------------+
| Re-added |  --  |         --        |         --       |        --       |
| headers  |      |                   |                  |                 |
+----------+------+-------------------+------------------+-----------------+
| Modified |  --  |         --        | IPv6-in-IPv6(RPI)|        --       |
| headers  |      |                   |                  |                 |
+----------+------+-------------------+------------------+-----------------+
|Untouched |  --  |         --        |         --       |        --       |
| headers  |      |                   |                  |                 |
+----------+------+-------------------+------------------+-----------------+

   Figure 16: Non Storing mode: Summary of the use of headers from not-
                          RPL-aware-leaf to root

7.2.  Non-Storing Mode: Interaction between Leaf and Internet

   This section will describe the communication flow in Non Storing Mode
   (Non-SM) between:

      RPL-aware-leaf to Internet

      Internet to RPL-aware-leaf

      not-RPL-aware-leaf to Internet

      Internet to not-RPL-aware-leaf

7.2.1.  Non-SM: Example of Flow from RPL-aware-leaf to Internet

   In this case the flow comprises:

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

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node A --> Internet






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   6LR_i is the intermediate router from source to destination.  In this
   case, "1 <= i <= n", n is the number of routers (6LR) that the packet
   goes through from source (6LN) to 6LBR.

   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.

   The Table 9 summarizes what headers are needed for this use case.

      +-------------------+---------+-------+------+----------------+
      | Header            | 6LN src | 6LR_i | 6LBR | Internet dst   |
      +-------------------+---------+-------+------+----------------+
      | Inserted headers  | RPI     | --    | --   | --             |
      | Removed headers   | --      | --    | --   | --             |
      | Re-added headers  | --      | --    | --   | --             |
      | Modified headers  | --      | RPI   | --   | --             |
      | Untouched headers | --      | --    | RPI  | RPI (Ignored)  |
      +-------------------+---------+-------+------+----------------+

    Table 9: Non Storing mode: Summary of the use of headers from RPL-
                          aware-leaf to Internet

7.2.2.  Non-SM: Example of Flow from Internet to RPL-aware-leaf

   In this case the flow comprises:

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

   For example, a communication flow could be: Internet --> Node A
   (root) --> Node B --> Node D --> Node F

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 <= i <= n", n is the number of routers (6LR) that the packet
   goes through from 6LBR to destination(6LN).

   The 6LBR must add an RH3 header.  As the 6LBR will know the path and
   address of the target node, it can address the IPv6-in-IPv6 header to
   that node.  The 6LBR will zero the flow label upon entry in order to
   aid compression.

   The Table 10 summarizes what headers are needed for this use case.







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   +-----------+----------+--------------+--------------+--------------+
   | Header    | Internet | 6LBR         | 6LR_i        | 6LN src      |
   |           | dst      |              |              |              |
   +-----------+----------+--------------+--------------+--------------+
   | Inserted  | --       | IPv6-in-IPv6 | --           | --           |
   | headers   |          | (RH3,RPI)    |              |              |
   | Removed   | --       | --           | --           | IPv6-in-IPv6 |
   | headers   |          |              |              | (RH3,RPI)    |
   | Re-added  | --       | --           | --           | --           |
   | headers   |          |              |              |              |
   | Modified  | --       | --           | IPv6-in-IPv6 | --           |
   | headers   |          |              | (RH3,RPI)    |              |
   | Untouched | --       | --           | --           | --           |
   | headers   |          |              |              |              |
   +-----------+----------+--------------+--------------+--------------+

      Table 10: Non Storing mode: Summary of the use of headers from
                        Internet to RPL-aware-leaf

7.2.3.  Non-SM: Example of Flow from not-RPL-aware-leaf to Internet

   In this case the flow comprises:

   not-RPL-aware-leaf (IPv6) --> 6LR_1 --> 6LR_i -->root (6LBR) -->
   Internet

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A --> Internet

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 < i <= n", n is the number of routers (6LR) that the packet
   goes through from source(IPv6) to 6LBR. e.g 6LR_1 (i=1).

   In this case the flow label is recommended to be zero in the IPv6
   node.  As RPL headers are added in the IPv6 node packet, the first
   6LR (6LR_1) will add a RPI header inside a new IPv6-in-IPv6 header.
   The IPv6-in-IPv6 header will be addressed to the root.  This case is
   identical to the storing-mode case (see Section 6.2.3).

   The Figure 17 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.










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+-----------+------+--------------+--------------+--------------+----------+
|   Header  | IPv6 |     6LR_1    |     6LR_i    |     6LBR     | Internet |
|           | src  |              |  [i=2,..,n]  |              |   dst    |
|           | node |              |              |              |          |
+-----------+------+--------------+--------------+--------------+----------+
|  Inserted |  --  | IP6-IP6(RPI) |      --      |      --      |    --    |
|  headers  |      |              |              |              |          |
+-----------+------+--------------+--------------+--------------+----------+
|  Removed  |  --  |      --      |      --      | IP6-IP6(RPI) |    --    |
|  headers  |      |              |              |              |          |
+-----------+------+--------------+--------------+--------------+----------+
|  Re-added |  --  |      --      |      --      |      --      |    --    |
|  headers  |      |              |              |              |          |
+-----------+------+--------------+--------------+--------------+----------+
|  Modified |  --  |      --      | IP6-IP6(RPI) |      --      |    --    |
|  headers  |      |              |              |              |          |
+-----------+------+--------------+--------------+--------------+----------+
| Untouched |  --  |      --      |      --      |      --      |    --    |
|  headers  |      |              |              |              |          |
+-----------+------+--------------+--------------+--------------+----------+

   Figure 17: Non Storing mode: Summary of the use of headers from not-
                        RPL-aware-leaf to Internet

7.2.4.  Non-SM: Example of Flow from Internet to not-RPL-aware-leaf

   In this case the flow comprises:

   Internet --> root (6LBR) --> 6LR_i --> not-RPL-aware-leaf (IPv6)

   For example, a communication flow could be: Internet --> Node A
   (root) --> Node B --> Node E --> Node G

   6LR_i is the intermediate router from source to destination.  In this
   case, "1 < i <= n", n is the number of routers (6LR) that the packet
   goes through from 6LBR to not-RPL-aware-leaf (IPv6).

   The 6LBR must add an RH3 header inside an IPv6-in-IPv6 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 IPv6-in-IPv6
   header destination be the last 6LR.  The 6LBR will set to zero the
   flow label upon entry in order to aid compression.

   The Figure 18 shows the table that summarizes what headers are needed
   for this use case.





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+-----------+--------+--------------+--------------+--------------+------+
|   Header  |Internet|     6LBR     |     6LR_1    |     6lR_i    | IPv6 |
|           |  src   |              |              |  (i=2,...,n) | dst  |
|           |        |              |              |              | node |
+-----------+--------+--------------+--------------+--------------+------+
|  Inserted |   --   | IPv6-in-IPv6 |      --      |      --      |  --  |
|  headers  |        |   (RH3,RPI)  |              |              |      |
+-----------+--------+--------------+--------------+--------------+------+
|  Removed  |   --   |      --      |      --      | IPv6-in-IPv6 |  --  |
|  headers  |        |              |              | (RH3,RPI)[1] |      |
+-----------+--------+--------------+--------------+--------------+------+
|  Re-added |   --   |      --      |      --      |      --      |  --  |
|  headers  |        |              |              |              |      |
+-----------+--------+--------------+--------------+--------------+------+
|  Modified |   --   |      --      | IPv6-in-IPv6 | IPv6-in-IPv6 |  --  |
|  headers  |        |              |   (RH3,RPI)  |   (RH3,RPI)  |      |
+-----------+--------+--------------+--------------+--------------+------+
| Untouched |   --   |      --      |      --      |      --      |  --  |
|  headers  |        |              |              |              |      |
+-----------+--------+--------------+--------------+--------------+------+

      Figure 18: Non-Storing mode: Summary of the use of headers from
   Internet to not-RPL-aware-leaf [1] The last 6LR before the IPv6 node.

7.3.  Non-Storing Mode: Interaction between Leafs

   In this section is described the communication flow in Non Storing
   Mode (Non-SM) between,

      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

7.3.1.  Non-SM: Example of Flow from RPL-aware-leaf to RPL-aware-leaf

   In this case the flow comprises:

   6LN src --> 6LR_ia --> root (6LBR) --> 6LR_id --> 6LN dst

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node A (root) --> Node B --> Node E --> Node H






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   6LR_ia is the intermediate router from source to the root In this
   case, "1 <= ia <= n", n is the number of routers (6LR) that the
   packet goes through from 6LN to the root.

   6LR_id is the intermediate router from the root to the destination.
   In this case, "1 <= ia <= m", m is the number of the intermediate
   routers (6LR).

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

   The originating node must put the RPI into an IPv6-in-IPv6 header
   addressed to the root, so that the 6LBR can remove that header.  If
   it does not, then additional resources are wasted on the way down to
   carry the useless RPI option.

   The 6LBR will need to insert an RH3 header, which requires that it
   add an IPv6-in-IPv6 header.  It should be able to remove the RPI, as
   it was contained in an IPv6-in-IPv6 header addressed to it.
   Otherwise, there may be a 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 or scenario
   Section 7.1.2, with the originating node acting as 6LBR.

   The Figure 19 shows the table that summarizes what headers are needed
   for this use case.






















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+---------+------------+------------+------------+------------+------------+
|  Header |    6LN     |   6LR_ia   |    6LBR    |   6LR_id   |     6LN    |
|         |    src     |            |            |            |     dst    |
+---------+------------+------------+------------+------------+------------+
| Inserted|IPv6-in-IPv6|            |IPv6-in-IPv6|    --      |     --     |
| headers |   (RPI1)   |            |(RH3-> 6LN, |            |            |
|         |            |            |   RPI2)    |            |            |
+---------+------------+------------+------------+------------+------------+
| Removed |    --      |    --      |IPv6-in-IPv6|    --      |IPv6-in-IPv6|
| headers |            |            |   (RPI1)   |            |   (RH3,    |
|         |            |            |            |            |   RPI2)    |
+---------+------------+------------+------------+------------+------------+
| Re-added|    --      |    --      |    --      |    --      |    --      |
| headers |            |            |            |            |            |
+---------+------------+------------+------------+------------+------------+
| Modified|    --      |IPv6-in-IPv |    --      |IPv6-in-IPv6|    --      |
| headers |            |   (RPI1)   |            |   (RPI2)   |            |
+---------+------------+------------+------------+------------+------------+
|Untouched|    --      |    --      |    --      |    --      |    --      |
| headers |            |            |            |            |            |
+---------+------------+------------+------------+------------+------------+

    Figure 19: Non Storing mode: Summary of the use of headers for RPL-
                       aware-leaf to RPL-aware-leaf

7.3.2.  Non-SM: Example of Flow from RPL-aware-leaf to not-RPL-aware-
        leaf

   In this case the flow comprises:

   6LN --> 6LR_ia --> root (6LBR) --> 6LR_id --> not-RPL-aware (IPv6)

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node A (root) --> Node B --> Node E --> Node G

   6LR_ia is the intermediate router from source to the root In this
   case, "1 <= ia <= n", n is the number of intermediate routers (6LR)

   6LR_id is the intermediate router from the root to the destination.
   In this case, "1 <= ia <= m", m is the number of the intermediate
   routers (6LRs).

   As in the previous case, the 6LN will insert a RPI (RPI_1) header
   which must be in an IPv6-in-IPv6 header addressed to the root so that
   the 6LBR can remove this RPI.  The 6LBR will then insert an RH3
   inside a new IPv6-in-IPv6 header addressed to the 6LR_id.





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   The Figure 20 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.

  +-----------+---------+---------+---------+---------+---------+------+
  |   Header  |   6LN   |  6LR_ia |   6LBR  |  6LR_id |  6LR_m  | IPv6 |
  |           |   src   |         |         |         |         | dst  |
  |           |         |         |         |         |         | node |
  +-----------+---------+---------+---------+---------+---------+------+
  |  Inserted | IP6-IP6 |         | IP6-IP6 |    --   |    --   |  --  |
  |  headers  |  (RPI1) |         |  (RH3,  |         |         |      |
  |           |         |         |   RPI2) |         |         |      |
  +-----------+---------+---------+---------+---------+---------+------+
  |  Removed  |    --   |    --   | IP6-IP6 |    --   | IP6-IP6 |  --  |
  |  headers  |         |         |  (RPI1) |         |  (RH3,  |      |
  |           |         |         |         |         |   RPI2) |      |
  +-----------+---------+---------+---------+---------+---------+------+
  |  Re-added |    --   |    --   |    --   |    --   |    --   |  --  |
  |  headers  |         |         |         |         |         |      |
  +-----------+---------+---------+---------+---------+---------+------+
  |  Modified |    --   | IP6-IP6 |    --   | IP6-IP6 |         |  --  |
  |  headers  |         |  (RPI1) |         |  (RH3,  |         |      |
  |           |         |         |         |   RPI2) |         |      |
  +-----------+---------+---------+---------+---------+---------+------+
  | Untouched |    --   |    --   |    --   |    --   |    --   |  --  |
  |  headers  |         |         |         |         |         |      |
  +-----------+---------+---------+---------+---------+---------+------+

   Figure 20: Non Storing mode: Summary of the use of headers from RPL-
                     aware-leaf to not-RPL-aware-leaf.

7.3.3.  Non-SM: Example of Flow from not-RPL-aware-leaf to RPL-aware-
        leaf

   In this case the flow comprises:

   not-RPL-aware 6LN (IPv6) --> 6LR_1 --> 6LR_ia --> root (6LBR) -->
   6LR_id --> 6LN

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A (root) --> Node B --> Node E --> Node H

   6LR_ia is the intermediate router from source to the root.  In this
   case, "1 <= ia <= n", n is the number of intermediate routers (6LR)

   6LR_id is the intermediate router from the root to the destination.
   In this case, "1 <= ia <= m", m is the number of the intermediate
   routers (6LR).




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   This scenario is mostly identical to the previous one.  The RPI is
   added by the first 6LR (6LR_1) inside an IPv6-in-IPv6 header
   addressed to the root.  The 6LBR will remove this RPI, and add it's
   own IPv6-in-IPv6 header containing an RH3 header and an RPI (RPI_2).

   The Figure 21 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.

  +-----------+------+---------+---------+---------+---------+---------+
  |   Header  | IPv6 |  6LR_1  |  6LR_ia |   6LBR  |  6LR_id |   6LN   |
  |           |  src |         |         |         |         |   dst   |
  |           | node |         |         |         |         |         |
  +-----------+------+---------+---------+---------+---------+---------+
  |  Inserted |  --  | IP6-IP6 |    --   | IP6-IP6 |    --   |    --   |
  |  headers  |      |  (RPI1) |         |  (RH3,  |         |         |
  |           |      |         |         |   RPI2) |         |         |
  +-----------+------+---------+---------+---------+---------+---------+
  |  Removed  |  --  |         |    --   | IP6-IP6 |    --   | IP6-IP6 |
  |  headers  |      |         |         |  (RPI1) |         |  (RH3,  |
  |           |      |         |         |         |         |   RPI2) |
  +-----------+------+---------+---------+---------+---------+---------+
  |  Re-added |  --  |         |    --   |    --   |    --   |    --   |
  |  headers  |      |         |         |         |         |         |
  +-----------+------+---------+---------+---------+---------+---------+
  |  Modified |  --  |         | IP6-IP6 |    --   | IP6-IP6 |    --   |
  |  headers  |      |         |  (RPI1) |         |  (RH3,  |         |
  |           |      |         |         |         |   RPI2) |         |
  +-----------+------+---------+---------+---------+---------+---------+
  | Untouched |  --  |         |    --   |    --   |    --   |    --   |
  |  headers  |      |         |         |         |         |         |
  +-----------+------+---------+---------+---------+---------+---------+

   Figure 21: Non Storing mode: Summary of the use of headers from not-
                     RPL-aware-leaf to RPL-aware-leaf.

7.3.4.  Non-SM: Example of Flow from not-RPL-aware-leaf to not-RPL-
        aware-leaf

   In this case the flow comprises:

   not-RPL-aware 6LN (IPv6 src) --> 6LR_1 --> 6LR_ia --> root (6LBR) -->
   6LR_id --> not-RPL-aware (IPv6 dst)

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A (root) --> Node C --> Node J

   6LR_ia is the intermediate router from source to the root.  In this
   case, "1 <= ia <= n", n is the number of intermediate routers (6LR)



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   6LR_id is the intermediate router from the root to the destination.
   In this case, "1 <= ia <= m", m is the number of the intermediate
   routers (6LR).

   This scenario is the combination of the previous two cases.

   The Figure 22 shows the table that summarizes what headers are needed
   for this use case.  In the figure, IP6-IP6 refers to IPv6-in-IPv6.

   +---------+------+-------+-------+---------+-------+---------+------+
   |  Header | IPv6 | 6LR_1 | 6LR_ia|   6LBR  |6LR_id |  6LR_m  | IPv6 |
   |         |  src |       |       |         |       |         |  dst |
   |         | node |       |       |         |       |         | node |
   +---------+------+-------+-------+---------+-------+---------+------+
   | Inserted|  --  |IP6-IP6|  --   | IP6-IP6 |  --   |    --   |  --  |
   | headers |      | (RPI1)|       |  (RH3,  |       |         |      |
   |         |      |       |       |   RPI2) |       |         |      |
   +---------+------+-------+-------+---------+-------+---------+------+
   | Removed |  --  |  --   |  --   | IP6-IP6 |  --   | IP6-IP6 |  --  |
   | headers |      |       |       |  (RPI1) |       |  (RH3,  |      |
   |         |      |       |       |         |       |   RPI2) |      |
   +---------+------+-------+-------+---------+-------+---------+------+
   | Re-added|  --  |  --   |  --   |    --   |  --   |    --   |  --  |
   | headers |      |       |       |         |       |         |      |
   +---------+------+-------+-------+---------+-------+---------+------+
   | Modified|  --  |  --   |IP6-IP6|    --   |IP6-IP6|    --   |  --  |
   | headers |      |       | (RPI1)|         | (RH3, |         |      |
   |         |      |       |       |         |  RPI2)|         |      |
   +---------+------+-------+-------+---------+-------+---------+------+
   |Untouched|  --  |  --   |  --   |    --   |  --   |    --   |  --  |
   | headers |      |       |       |         |       |         |      |
   +---------+------+-------+-------+---------+-------+---------+------+

   Figure 22: Non Storing mode: Summary of the use of headers from not-
                   RPL-aware-leaf to not-RPL-aware-leaf

8.  Operational Considerations of supporting not-RPL-aware-leaves

   Roughly half of the situations described in this document involve
   leaf ("host") nodes that do not speak RPL.  These nodes fall into two
   further categories: ones that drop a packet that have RPI or RH3
   headers, and ones that continue to process a packet that has RPI and/
   or RH3 headers.

   [RFC8200] provides for new rules that suggest that nodes that have
   not been configured (explicitly) to examine Hop-by-Hop headers,
   should ignore those headers, and continue processing the packet.
   Despite this, and despite the switch from 0x63 to 0x23, there may be



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   hosts that are pre-RFC8200, or simply intolerant.  Those hosts will
   drop packets that continue to have RPL artifacts in them.  In
   general, such hosts can not be easily supported in RPL LLNs.

   There are some specific cases where it is possible to remove the RPL
   artifacts prior to forwarding the packet to the leaf host.  The
   critical thing is that the artifacts have been inserted by the RPL
   root inside an IPv6-in-IPv6 header, and that the header has been
   addressed to the 6LR immediately prior to the leaf node.  In that
   case, in the process of removing the IPv6-in-IPv6 header, the
   artifacts can also be removed.

   The above case occurs whenever traffic originates from the outside
   the LLN (the "Internet" cases above), and non-storing mode is used.
   In non-storing mode, the RPL root knows the exact topology (as it
   must be create the RH3 header), and therefore knows what the 6LR
   prior to the leaf.  For example, in Figure 5, node E is the 6LR prior
   to the leaf node G, or node C is the 6LR prior to the leaf node J.

   Traffic originating from the RPL root (such as when the data
   collection system is co-located on the RPL root), does not require an
   IPv6-in-IPv6 header (in either mode), as the packet is originating at
   the root, and the root can insert the RPI and RH3 headers directly
   into the packet, as it is formed.  Such a packet is slightly smaller,
   but only can be sent to nodes (whether RPL aware or not), that will
   tolerate the RPL artifacts.

   An operator that finds itself with a lot of traffic from the RPL root
   to RPL-not-aware-leaves, will have to do IPv6-in-IPv6 encapsulation
   if the leaf is not tolerant of the RPL artifacts.  Such an operator
   could otherwise omit this unnecessary header if it was certain of the
   properties of the leaf.

   As storing mode can not know the final path of the traffic,
   intolerant (that drop packets with RPL artifacts) leaf nodes can not
   be supported.

9.  Operational considerations of introducing 0x23

   This section describes the operational considerations of introducing
   the new RPI value of 0x23.

   During bootstrapping the node gets the DIO with the information of
   RPL Option Type, indicating the new RPI in the DODAG Configuration
   Option Flag.  The DODAG root is in charge to configure the current
   network to the new value, through DIO messages and when all the nodes
   are set with the new value.  The DODAG should change to a new DODAG
   version.  In case of rebooting, the node does not remember the RPL



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   Option Type.  Thus, the DIO is sent with a flag indicating the new
   RPI value.

   The DODAG Configuration option is contained in a RPL DIO message,
   which contains a unique DTSN counter.  The leaf nodes respond to this
   message with DAO messages containing the same DTSN.  This is a normal
   part of RPL routing; the RPL root therefore knows when the updated
   DODAG Configuration Option has been seen by all nodes.

   Before the migration happens, all the RPL-aware nodes should support
   both values .  The migration procedure it is triggered when the DIO
   is sent with the flag indicating the new RPI value.  Namely, it
   remains at 0x63 until it is sure that the network is capable of 0x23,
   then it abruptly change to 0x23.  This options allows to send packets
   to not-RPL nodes, which should ignore the option and continue
   processing the packets.

   In case that a node join to a network that only process 0x63, it
   would produce a flag day as was mentioned previously.  Indicating the
   new RPI in the DODAG Configuration Option Flag is a way to avoid the
   flag day in a network.  It is recommended that a network process both
   options to enable interoperability.

10.  IANA Considerations

   This document updates the registration made in [RFC6553] Destination
   Options and Hop-by-Hop Options registry from 0x63 to 0x23 as shown in
   Figure 23.

   [RFCXXXX] represents this document.


  Hex Value  Binary Value
             act  chg  rest     Description              Reference
  ---------  ---  ---  -------  -----------------       ----------
   0x23      00    1   00011   RPL Option                [RFCXXXX]
   0x63      01    1   00011   RPL Option(DEPRECATED) [RFC6553][RFCXXXX]


                   Figure 23: Option Type in RPL Option.

   DODAG Configuration option is updated as follows (Figure 24):









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                 +------------+-----------------+---------------+
                 | Bit number |   Description   |   Reference   |
                 +------------+-----------------+---------------+
                 |      3     | RPI 0x23 enable | This document |
                 +------------+-----------------+---------------+


   Figure 24: DODAG Configuration Option Flag to indicate the RPI-flag-
                                   day.

11.  Security Considerations

   The security considerations covered in [RFC6553] and [RFC6554] apply
   when the packets are in the RPL Domain.

   The IPv6-in-IPv6 mechanism described in this document is much more
   limited than the general mechanism described in [RFC2473].  The
   willingness of each node in the LLN to decapsulate packets and
   forward them could be exploited by nodes to disguise the origin of an
   attack.

   While a typical LLN may be a very poor origin for attack traffic (as
   the networks tend to be very slow, and the nodes often have very low
   duty cycles) given enough nodes, they could still have a significant
   impact, particularly if the target of the attack is targeting another
   LLN.  Additionally, some uses of RPL involve large backbone ISP scale
   equipment [I-D.ietf-anima-autonomic-control-plane], which may be
   equipped with multiple 100Gb/s interfaces.

   Blocking or careful filtering of IPv6-in-IPv6 traffic entering the
   LLN as described above will make sure that any attack that is mounted
   must originate from compromised nodes within the LLN.  The use of
   BCP38 [BCP38] filtering at the RPL root on egress traffic will both
   alert the operator to the existence of the attack, as well as drop
   the attack traffic.  As the RPL network is typically numbered from a
   single prefix, which is itself assigned by RPL, BCP38 filtering
   involves a single prefix comparison and should be trivial to
   automatically configure.

   There are some scenarios where IPv6-in-IPv6 traffic should be allowed
   to pass through the RPL root, such as the IPv6-in-IPv6 mediated
   communications between a new Pledge and the Join Registrar/
   Coordinator (JRC) when using [I-D.ietf-anima-bootstrapping-keyinfra]
   and [I-D.ietf-6tisch-dtsecurity-secure-join].  This is the case for
   the RPL root to do careful filtering: it occurs only when the Join
   Coordinator is not co-located inside the RPL root.





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   With the above precautions, an attack using IPv6-in-IPv6 tunnels can
   only be by a node within the LLN on another node within the LLN.
   Such an attack could, of course, be done directly.  An attack of this
   kind is meaningful only if the source addresses are either fake or if
   the point is to amplify return traffic.  Such an attack, could also
   be done without the use of IPv6-in-IPv6 headers using forged source
   addresses.  If the attack requires bi-directional communication, then
   IPv6-in-IPv6 provides no advantages.

   Whenever IPv6-in-IPv6 headers are being proposed, there is a concern
   about creating security issues.  In the security section of
   [RFC2473], it was suggested that tunnel entry and exit points can be
   secured, via "Use IPsec".  This recommendation is not practical for
   RPL networks.  [RFC5406] goes into some detail on what additional
   details would be needed in order to "Use IPsec".  Use of ESP would
   prevent RFC8183 compression (compression must occur before
   encryption), and RFC8183 compression is lossy in a way that prevents
   use of AH.  These are minor issues.  The major issue is how to
   establish trust enough such that IKEv2 could be used.  This would
   require a system of certificates to be present in every single node,
   including any Internet nodes that might need to communicate with the
   LLN.  Thus, "Use IPsec" requires a global PKI in the general case.

   More significantly, the use of IPsec tunnels to protect the IPv6-in-
   IPv6 headers would in the general case scale with the square of the
   number of nodes.  This is a lot of resource for a constrained nodes
   on a constrained network.  In the end, the IPsec tunnels would be
   providing only BCP38-like origin authentication!  Just doing BCP38
   origin filtering at the entry and exit of the LLN provides a similar
   level amount of security without all the scaling and trust problems
   of using IPsec as RFC2473 suggested.  IPsec is not recommended.

   An LLN with hostile nodes within it would not be protected against
   impersonation with the LLN by entry/exit filtering.

   The RH3 header usage described here can be abused in equivalent ways
   (to disguise the origin of traffic and attack other nodes) with an
   IPv6-in-IPv6 header to add the needed RH3 header.  As such, the
   attacker's RH3 header will not be seen by the network until it
   reaches the end host, which will decapsulate it.  An end-host should
   be suspicious about a RH3 header which has additional hops which have
   not yet been processed, and SHOULD ignore such a second RH3 header.

   In addition, the LLN will likely use [RFC8138] to compress the IPv6-
   in-IPv6 and RH3 headers.  As such, the compressor at the RPL-root
   will see the second RH3 header and MAY choose to discard the packet
   if the RH3 header has not been completely consumed.  A consumed
   (inert) RH3 header could be present in a packet that flows from one



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   LLN, crosses the Internet, and enters another LLN.  As per the
   discussion in this document, such headers do not need to be removed.
   However, there is no case described in this document where an RH3 is
   inserted in a non-storing network on traffic that is leaving the LLN,
   but this document should not preclude such a future innovation.  It
   should just be noted that an incoming RH3 must be fully consumed, or
   very carefully inspected.

   The RPI header, if permitted to enter the LLN, could be used by an
   attacker to change the priority of a packet by selecting a different
   RPLInstanceID, perhaps one with a higher energy cost, for instance.
   It could also be that not all nodes are reachable in an LLN using the
   default instanceID, but a change of instanceID would permit an
   attacker to bypass such filtering.  Like the RH3, a RPI header is to
   be inserted by the RPL root on traffic entering the LLN by first
   inserting an IPv6-in-IPv6 header.  The attacker's RPI header
   therefore will not be seen by the network.  Upon reaching the
   destination node the RPI header has no further meaning and is just
   skipped; the presence of a second RPI header will have no meaning to
   the end node as the packet has already been identified as being at
   it's final destination.

   The RH3 and RPI headers could be abused by an attacker inside of the
   network to route packets on non-obvious ways, perhaps eluding
   observation.  This usage is in fact part of [RFC6997] and can not be
   restricted at all.  This is a feature, not a bug.

   [RFC7416] deals with many other threats to LLNs not directly related
   to the use of IPv6-in-IPv6 headers, and this document does not change
   that analysis.

   Nodes within the LLN can use the IPv6-in-IPv6 mechanism to mount an
   attack on another part of the LLN, while disguising the origin of the
   attack.  The mechanism can even be abused to make it appear that the
   attack is coming from outside the LLN, and unless countered, this
   could be used to mount a Distributed Denial Of Service attack upon
   nodes elsewhere in the Internet.  See [DDOS-KREBS] for an example of
   such attacks already seen in the real world.

   If an attack comes from inside of LLN, it can be alleviated with SAVI
   (Source Address Validation Improvement) using [RFC8505] with
   [I-D.ietf-6lo-ap-nd].  The attacker will not be able to source
   traffic with an address that is not registered, and the registration
   process checks for topological correctness.  Notice that there is an
   L2 authentication in most of the cases.  If an attack comes from
   outside LLN IPv6-in- IPv6 can be used to hide inner routing headers,
   but by construction, the RH3 can typically only address nodes within




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   the LLN.  That is, a RH3 with a CmprI less than 8 , should be
   considered an attack (see RFC6554, section 3).

   Nodes outside of the LLN will need to pass IPv6-in-IPv6 traffic
   through the RPL root to perform this attack.  To counter, the RPL
   root SHOULD either restrict ingress of IPv6-in-IPv6 packets (the
   simpler solution), or it SHOULD walk the IP header extension chain
   until it can inspect the upper-layer-payload as described in
   [RFC7045].  In particular, the RPL root SHOULD do [BCP38] processing
   on the source addresses of all IP headers that it examines in both
   directions.

   Note: there are some situations where a prefix will spread across
   multiple LLNs via mechanisms such as the one described in
   [I-D.ietf-6lo-backbone-router].  In this case the BCP38 filtering
   needs to take this into account, either by exchanging detailed
   routing information on each LLN, or by moving the BCP38 filtering
   further towards the Internet, so that the details of the multiple
   LLNs do not matter.

12.  Acknowledgments

   This work is done thanks to the grant by the Stand.ICT project.

   A special BIG thanks to C.  M.  Heard for the help with the
   Section 3.  Much of the redaction in that section is based on his
   comments.

   Additionally, the authors would like to acknowledge the review,
   feedback, and comments of (alphabetical order): Robert Cragie, Simon
   Duquennoy, Ralph Droms, Cenk Guendogan, Rahul Jadhav, Matthias
   Kovatsch, Peter van der Stok, Xavier Vilajosana and Thomas Watteyne.

13.  References

13.1.  Normative References

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/bcp38>.

   [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>.





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   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/info/rfc6040>.

   [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>.

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

   [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,
              <https://www.rfc-editor.org/info/rfc6554>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <https://www.rfc-editor.org/info/rfc7045>.

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

13.2.  Informative References








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   [DDOS-KREBS]
              Goodin, D., "Record-breaking DDoS reportedly delivered by
              >145k hacked cameras", September 2016,
              <http://arstechnica.com/security/2016/09/botnet-of-145k-
              cameras-reportedly-deliver-internets-biggest-ddos-ever/>.

   [I-D.ietf-6lo-ap-nd]
              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", draft-ietf-6lo-ap-nd-12 (work in
              progress), April 2019.

   [I-D.ietf-6lo-backbone-router]
              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", draft-ietf-6lo-backbone-router-11 (work
              in progress), February 2019.

   [I-D.ietf-6tisch-dtsecurity-secure-join]
              Richardson, M., "6tisch Secure Join protocol", draft-ietf-
              6tisch-dtsecurity-secure-join-01 (work in progress),
              February 2017.

   [I-D.ietf-anima-autonomic-control-plane]
              Eckert, T., Behringer, M., and S. Bjarnason, "An Autonomic
              Control Plane (ACP)", draft-ietf-anima-autonomic-control-
              plane-19 (work in progress), March 2019.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-20 (work in progress), May 2019.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-09 (work in
              progress), July 2018.

   [I-D.thubert-roll-unaware-leaves]
              Thubert, P., "Routing for RPL Leaves", draft-thubert-roll-
              unaware-leaves-07 (work in progress), April 2019.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.






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   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC5406]  Bellovin, S., "Guidelines for Specifying the Use of IPsec
              Version 2", BCP 146, RFC 5406, DOI 10.17487/RFC5406,
              February 2009, <https://www.rfc-editor.org/info/rfc5406>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [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,
              <https://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, <https://www.rfc-editor.org/info/rfc7102>.

   [RFC7416]  Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
              and M. Richardson, Ed., "A Security Threat Analysis for
              the Routing Protocol for Low-Power and Lossy Networks
              (RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,
              <https://www.rfc-editor.org/info/rfc7416>.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.





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Authors' Addresses

   Maria Ines Robles
   Aalto University
   Otaniemi
   Espoo  02150
   Finland

   Email: mariainesrobles@gmail.com


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

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


   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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