ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                   L. Zhao
Updates: 6550, 8138 (if approved)                          Cisco Systems
Intended status: Standards Track                           24 March 2020
Expires: 25 September 2020

                   Configuration option for RFC 8138


   This document complements RFC 8138 and dedicates a bit in the RPL
   configuration option defined in RFC 6550 to indicate whether RFC 8138
   compression is used within the RPL Instance.

Status of This Memo

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

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   This Internet-Draft will expire on 25 September 2020.

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  References  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Updating RFC 8138 . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Transition Scenarios  . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Inconsistent State While Migrating  . . . . . . . . . . .   6
     5.2.  Single RPL Instance Scenario  . . . . . . . . . . . . . .   6
     5.3.  Double RPL Instances Scenario . . . . . . . . . . . . . .   7
     5.4.  Rolling Back  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   10. Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The transition of a RPL [RFC6550] network to activate the compression
   defined in [RFC8138] can only be done when all routers in the network
   support it.  Otherwise, a non-capable node acting as a router would
   drop the compressed packets and black-hole its subDAG.  In a mixed
   case with both RFC8138-capable and non-capable nodes, the compression
   may be turned on only if all the non-capable nodes act as Hosts and
   their RPL parents handle the compression/decompression for them.

   This document complements [RFC8138] and dedicates a flag in the RPL
   configuration option to indicate whether [RFC8138] compression should
   be used within the RPL Instance.  The setting of this new flag is
   controlled by the Root and propagates as is in the whole network.
   When the bit is not set, source nodes that support [RFC8138] should
   refrain from using the compression unless the information is
   superseded by configuration.

   With RPL, a leaf is an IPv6 Host, which implies that leaves do not
   forward packets.  This specification provides scenarios that force a
   non-capable RPL-Aware Node (RAN) to become a leaf.  The parent router
   must know, e.g., by configuration, or leveraging "RPL Capabilities"
   [CAPABILITIES], when a leaf does not support the compression defined
   in [RFC8138].  This is implicitly the case for a RPL-Unaware Leaf
   (RUL) but is not known for a RPL-Aware Leaf (RAL).  The parent router
   must uncompress the packets before delivering them to a non-capable
   leaf and it must compress the traffic from the leaf.

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2.  Terminology

2.1.  References

   The Terminology used in this document is consistent with and
   incorporates that described in "Terms Used in Routing for Low-Power
   and Lossy Networks (LLNs)" [RFC7102].  Other terms in use in LLNs are
   found in "Terminology for Constrained-Node Networks" [RFC7228].

   "RPL", the "RPL Packet Information" (RPI), "RPL Instance" (indexed by
   a RPLInstanceID) are defined in "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550].  The RPI is the abstract
   information that RPL defines to be placed in data packets, e.g., as
   the RPL Option [RFC6553] within the IPv6 Hop-By-Hop Header.  By
   extension the term "RPI" is often used to refer to the RPL Option
   itself.  The DODAG Information Solicitation (DIS), Destination
   Advertisement Object (DAO) and DODAG Information Object (DIO)
   messages are also specified in [RFC6550].

   This document uses the terms RPL-Unaware Leaf (RUL) and RPL Aware
   Leaf (RAL) consistently with "Using RPI Option Type, Routing Header
   for Source Routes and IPv6-in-IPv6 encapsulation in the RPL Data
   Plane" [USEofRPLinfo].  The term RPL-Aware Node (RAN) refers to a
   node that is either a RAL or a RPL Router.  A RAN manages the
   reachability of its addresses and prefixes by injecting them in RPL
   by itself.  In contrast, a RUL leverages "Registration Extensions for
   IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor
   Discovery" [RFC8505] to obtain reachability services from its parent
   router(s) as specified in "Routing for RPL Leaves" [UNAWARE-LEAVES].

2.2.  Glossary

   This document often uses the following acronyms:

   6LoWPAN:  IPv6 over Low-Power Wireless Personal Area Network
   6LoRH:  6LoWPAN Routing Header
   DIO:  DODAG Information Object (a RPL message)
   DODAG:  Destination-Oriented Directed Acyclic Graph
   LLN:  Low-Power and Lossy Network
   RPL:  IPv6 Routing Protocol for Low-Power and Lossy Networks
   OF:  RPL Objective Function
   OCP:  RPL Objective Code Point
   MOP:  RPL Mode of Operation
   RPI:  RPL Packet Information
   RAL:  RPL-Aware Leaf
   RAN:  RPL-Aware Node
   RUL:  RPL-Unaware Leaf
   SRH:  Source Routing Header

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2.3.  BCP 14

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

3.  Updating RFC 6550

   This specification defines a new flag "Enable RFC8138 Compression"
   (T).  The "T" flag is set to turn on the use of the compression of
   RPL artifacts with [RFC8138] within a RPL Instance.  If a RPL
   Instance has multiple Roots then they must be coordinated to use the
   same setting.

   RPL defines a Configuration Option that is registered to IANA in
   section 20.14. of [RFC6550].  The "T" flag is encoded in one of the
   reserved control bits in the RPL Configuration Option.  The bit
   position of the "T" flag is indicated in Section 6.

   Section 6.3.1.  of [RFC6550] defines a 3-bit Mode of Operation (MOP)
   in the DIO Base Object.  The new "T" flag is defined only for MOP
   value between 0 to 6.  For a MOP value of 7 or above, the flag MAY
   indicate something different and MUST NOT be interpreted as "Enable
   RFC8138 Compression" unless the specification of the MOP indicates to
   do so.

4.  Updating RFC 8138

   A node that supports this specification MUST source packets in the
   compressed form using [RFC8138] if and only if the "T" flag is set.
   This behaviour can be overridden by the configuration of the node in
   order to cope with intermediate implementations of the Root that
   support [RFC8138] but not this specification and cannot set the "T"

   The decision of using [RFC8138] is made by the originator of the
   packet depending on its capabilities and its knowledge of the state
   of the "T" flag.  A router that encapsulates a packet is the
   originator of the resulting packet and decides whether to compress
   the outer headers as indicated above.  An external target
   [USEofRPLinfo] is not expected to support [RFC8138].  An intermediate
   router MUST forward the packet in the form that the source used,
   either compressed or uncompressed, unless it is forwarding to an
   external target or delivering to a leaf that is not known to support
   [RFC8138], in which cases it MUST uncompress the packet.

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5.  Transition Scenarios

   A node that supports [RFC8138] but not this specification can only be
   used in an homogeneous network.  Enabling the [RFC8138] compression
   requires a "flag day"; all nodes must be upgraded, and then the
   network can be rebooted with the [RFC8138] compression turned on.

   A node that supports this specification can work in a network with
   [RFC8138] compression turned on or off with the "T" flag set
   accordingly and in a network in transition from off to on or on to
   off (see Section 5.1).

   A node that does not support [RFC8138] can interoperate with nodes
   that do in a network with [RFC8138] compression turned off.  If the
   compression is turned on, the node cannot forward compressed packets
   and therefore it cannot act as a router.  It may remain connected to
   that network as a leaf, generates uncompressed packets, and can
   receive packets if they are delivered by the parent router in the
   uncompressed form.  Unless this is known by other means, the node
   SHOULD join as a RUL as an indication that its parent router needs to
   uncompress the packets before delivering.

   [RFC6550] states that "Nodes other than the DODAG Root MUST NOT
   modify this information when propagating the DODAG Configuration
   option".  Therefore, even a legacy parent propagates the "T" flag as
   set by the Root whether it supports this specification or not.  So
   when the "T" flag is set, it is transparently flooded to all the
   nodes in the RPL Instance.

   Sections 8.5 and 9.2 of [RFC6550] also suggests that a RAN may only
   attach to a DODAG as a leaf when it does not support the Mode of
   Operation of a RPL Instance, the Objective Function (OF) as indicated
   by the Objective Code Point (OCP) or some other parameters in the
   configuration option.

   This specification reiterates that a RAN that is configured to
   operate in a RPL Instance but does not support a value for a known
   parameter that is mandatory for routing, such as the OCP, MUST NOT
   operate as a router but MAY still join as a leaf.  Note that a legacy
   RAN will not recognize when a reserved field is used and will not
   turn to a leaf when the "T" flag is set.

   The intent for this specification is to perform a migration once and
   for all without the need for a flag day.  In particular it is not the
   intention to undo the setting of the "T" flag, and though it is
   possible to roll back (see Section 5.4), adding nodes that do not
   support [RFC8138] after a roll back may be problematic if the roll
   back is not fully complete (see caveats in Section 5.2).

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5.1.  Inconsistent State While Migrating

   When the "T" flag is turned on in the configuration option by the
   Root, the information slowly percolates through the DODAG as the DIO
   gets propagated.

   Some nodes will see the flag and start sourcing packets in the
   compressed form while other nodes in the same RPL Instance are still
   not aware of it.  Conversely, in non-storing mode, the Root will
   start using [RFC8138] with a Source Routing Header 6LoRH (SRH-6LoRH)
   that routes all the way to the parent router or to the leaf.

   To ensure that a packet is forwarded across the RPL Instance in the
   form in which it was generated, it is required that all the routers
   support [RFC8138] at the time of the switch, and that all nodes that
   do not support [RFC8138] only operate as leaves.

   Setting the "T" flag is ultimately the responsibility of the network
   administrator.  In a case of upgrading a network to turn the
   compression on, the network SHOULD be operated with the "T" flag
   reset until all targeted nodes are upgraded to support this
   specification.  Section 5.2 and Section 5.3 provide possible
   transition scenarios where this can be enforced.

5.2.  Single RPL Instance Scenario

   In a Single RPL Instance Scenario, nodes that support [RFC8138] are
   configured with a new OCP, that may use the same OF operation or a
   variation of it, while nodes that do not support [RFC8138] are not,
   but are configured to join an unknown OCP.

   The Root migrates to the new OCP before it sets the "T" flag, so that
   nodes that do not support [RFC8138] are all attached as leaves when
   the "T" flag is eventually set.

   The parent router - which supports [RFC8138] - compresses the packets
   originated from the leaf and uncompresses the packets going to the
   leaf.  This may be done on the fly by the parent of a non-capable
   RAL, or as part of the tunneling operation between the parent and the
   Root, if the leaf behaves as a RUL.  This is described in section 7,
   8, and 9 of [USEofRPLinfo].

   Note that though tunneling from the Root to the parent is the generic
   case for RULs, on paper it is possible for the Root to avoid it for
   the traffic that it originates.  The Root SHOULD always use tunneling
   to the parent of a RUL, even for its own packets, unless it knows
   that the leaf supports [RFC8138].

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   This scenario presents a number of caveats:

   *  The method consumes an extra OCP.  It also forces nodes that do
      not support [RFC8138] to operate as RULs, unless there is a method
      to let the parent router know that it must uncompress the packet
      for this RAL.

   *  If the RPL implementation of a node does not turn it to a leaf
      when the OCP is changed to an unknown one, then the node may be

   *  If the only possible parents of a node are nodes that do not
      support [RFC8138], then that node will loose all its parent at the
      time of the migration and it will be stalled until a parent is
      deployed with the new capability.

5.3.  Double RPL Instances Scenario

   An alternative to the Single RPL Instance Scenario is to deploy an
   additional RPL Instance for the nodes that support [RFC8138].

   The two RPL Instances operate independently as specified in
   [RFC6550].  The preexisting RPL Instance does not use [RFC8138],
   whereas the new RPL Instance does.  This is signaled by the "T" flag
   which is only set in the configuration option in DIO messages in the
   new RPL Instance.

   Nodes that support [RFC8138] participate in both Instances but favor
   the new RPL Instance for the traffic that they source.  By contrast,
   nodes that only support the uncompressed format would either not be
   configured for the new RPL Instance, or would be configured to join
   it as leaves only.

   This method eliminates the risks of nodes being stalled that are
   described in Section 5.2 but requires implementations to support at
   least two RPL Instances and demands management capabilities to
   introduce new RPL Instances and deprecate old ones.

5.4.  Rolling Back

   After downgrading a network to turn the [RFC8138] compression off,
   the administrator SHOULD make sure that all nodes have converged to
   the "T" flag reset before allowing nodes that do not support the
   compression in the network (see caveats in Section 5.2).

   It is RECOMMENDED to only deploy nodes that support [RFC8138] in a
   network where the compression is turned on.  A node that does not
   support [RFC8138] MUST only be used as a leaf.

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6.  IANA Considerations

   This specification updates the Registry for the "DODAG Configuration
   Option Flags" that was created for [RFC6550] as follows:

       | Bit Number | Capability Description          | Reference |
       | 2          | Turn on RFC8138 Compression (T) | THIS RFC  |

               Table 1: New DODAG Configuration Option Flag

7.  Security Considerations

   First of all, it is worth noting that with [RFC6550], every node in
   the LLN that is RPL-aware can inject any RPL-based attack in the
   network.  A trust model MUST be put in place so that rogue nodes are
   excluded from participating to the RPL and the 6LowpAN signaling, and
   from the data packet exchange.  This trust model could be at a
   minimum based on a Layer-2 Secure joining and the Link-Layer
   security.  This is a generic RPL and 6LoWPAN requirement, see Req5.1
   in Appendix of [RFC8505].

   Setting the "T" flag before some routers are upgraded may cause a
   loss of packets.  The new bit is protected as the rest of the
   configuration so this is just one of the many attacks that can happen
   if an attacker manages to inject a corrupted configuration.

   Setting and resetting the "T" flag may create inconsistencies in the
   network but as long as all nodes are upgraded to [RFC8138] support
   they will be able to forward both forms.  The source is responsible
   for selecting whether the packet is compressed or not, and all
   routers must use the format that the source selected.  So the result
   of an inconsistency is merely that both forms will be present in the
   network, at an additional cost of bandwidth for packets in the
   uncompressed form.

8.  Acknowledgments

   The authors wish to thank Dominique Barthel and Rahul Jadhav for
   their in-depth reviews and constructive suggestions.

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,

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              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

              Robles, I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", Work in
              Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-38,
              23 March 2020, <

              Thubert, P. and M. Richardson, "Routing for RPL Leaves",
              Work in Progress, Internet-Draft, draft-ietf-roll-unaware-
              leaves-13, 17 March 2020, <

10.  Informative References

   [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,

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network

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              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <>.

   [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,

              Jadhav, R., Thubert, P., Richardson, M., and R. Sahoo,
              "RPL Capabilities", Work in Progress, Internet-Draft,
              draft-ietf-roll-capabilities-02, 11 March 2020,

Authors' Addresses

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

   Phone: +33 497 23 26 34

   Li Zhao
   Cisco Systems, Inc
   Xinsi Building
   No. 926 Yi Shan Rd


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