The Routing Protocol for Low-Power and Lossy Networks (RPL) Option for Carrying RPL Information in Data-Plane Datagrams

Internet Engineering Task Force (IETF)                            J. Hui
Request for Comments: 6553                                   JP. Vasseur
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                               March 2012


   The Routing Protocol for Low-Power and Lossy Networks (RPL) Option
          for Carrying RPL Information in Data-Plane Datagrams

Abstract

   The Routing Protocol for Low-Power and Lossy Networks (RPL) includes
   routing information in data-plane datagrams to quickly identify
   inconsistencies in the routing topology.  This document describes the
   RPL Option for use among RPL routers to include such routing
   information.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6553.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. Overview ........................................................3
   3. Format of the RPL Option ........................................3
   4. RPL Router Behavior .............................................5
   5. Security Considerations .........................................6
      5.1. DAG Inconsistency Attacks ..................................6
      5.2. Destination Advertisement Object (DAO)
           Inconsistency Attacks ......................................7
   6. IANA Considerations .............................................7
   7. Acknowledgements ................................................8
   8. References ......................................................8
      8.1. Normative References .......................................8
      8.2. Informative References .....................................8

1.  Introduction

   RPL is a distance vector IPv6 routing protocol designed for Low-Power
   and Lossy Networks (LLNs) [RFC6550].  Such networks are typically
   constrained in energy and/or channel capacity.  To conserve precious
   resources, a routing protocol must generate control traffic
   sparingly.  However, this is at odds with the need to quickly
   propagate any new routing information to resolve routing
   inconsistencies quickly.

   To help minimize resource consumption, RPL uses a slow proactive
   process to construct and maintain a routing topology but a reactive
   and dynamic process to resolving routing inconsistencies.  In the
   steady state, RPL maintains the routing topology using a low-rate
   beaconing process.  However, when RPL detects inconsistencies that
   may prevent proper datagram delivery, RPL temporarily increases the
   beacon rate to quickly resolve those inconsistencies.  This dynamic
   rate control operation is governed by the use of dynamic timers also
   referred to as "Trickle" timers and defined in [RFC6206].  In
   contrast to other routing protocols (e.g., OSPF [RFC2328]), RPL
   detects routing inconsistencies using data-path verification, by
   including routing information within the datagram itself.  In doing
   so, repair mechanisms operate only as needed, allowing the control
   and data planes to operate on similar time scales.  The main
   motivation for data-path verification in LLNs is that control-plane
   traffic should be carefully bounded with respect to the data traffic.
   Intuitively, there is no need to solve routing issues (which may be
   temporary) in the absence of data traffic.

   RPL constructs a Directed Acyclic Graph (DAG) that attempts to
   minimize path costs to the DAG root according to a set of metrics and
   Objective Functions.  There are circumstances where loops may occur,
   and RPL is designed to use a data-path loop detection method.  This
   is one of the known requirements of RPL, and other data-path usage
   might be defined in the future.

   To that end, this document defines a new IPv6 option, called the RPL
   Option, to be carried within the IPv6 Hop-by-Hop header.  The RPL
   Option is only for use between RPL routers participating in the same
   RPL Instance.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Overview

   The RPL Option provides a mechanism to include routing information
   with each datagram that a router forwards.  When receiving datagrams
   that include routing information, RPL routers process the routing
   information to help maintain the routing topology.

   Every RPL router along a packet's delivery path processes and updates
   the RPL Option.  If the received packet does not already contain a
   RPL Option, the RPL router must insert a RPL Option before forwarding
   it to another RPL router.  This document also specifies the use of
   IPv6-in-IPv6 tunneling [RFC2473] when attaching a RPL option to a
   packet.  Use of tunneling ensures that the original packet remains
   unmodified and that ICMP errors return to the RPL Option source
   rather than the source of the original packet.

3.  Format of the RPL Option

   The RPL Option is carried in an IPv6 Hop-by-Hop Options header,
   immediately following the IPv6 header.  This option has an alignment
   requirement of 2n.  The option has the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |  Option Type  |  Opt Data Len |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |O|R|F|0|0|0|0|0| RPLInstanceID |          SenderRank           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         (sub-TLVs)                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 1: RPL Option

   Option Type:  0x63

   Opt Data Len:  8-bit field indicating the length of the option, in
         octets, excluding the Option Type and Opt Data Len fields.

   Down 'O':  1-bit flag as defined in Section 11.2 of [RFC6550].  The
         processing SHALL follow the rules described in Section 11.2 of
         [RFC6550].

   Rank-Error 'R':  1-bit flag as defined in Section 11.2 of [RFC6550].
         The processing SHALL follow the rules described in Section 11.2
         of [RFC6550].

   Forwarding-Error 'F':  1-bit flag as defined in Section 11.2 of
         [RFC6550].  The processing SHALL follow the rules described in
         Section 11.2 of [RFC6550].

   RPLInstanceID:  8-bit field as defined in Section 11.2 of [RFC6550].
         The processing SHALL follow the rules described in Section 11.2
         of [RFC6550].

   SenderRank:  16-bit field as defined in Section 11.2 of [RFC6550].
         The processing SHALL follow the rules described in Section 11.2
         of [RFC6550].

   The two high order bits of the Option Type MUST be set to '01' and
   the third bit is equal to '1'.  With these bits, according to
   [RFC2460], nodes that do not understand this option on a received
   packet MUST discard the packet.  Also, according to [RFC2460], the
   values within the RPL Option are expected to change en route.  The
   RPL Option Data Length is variable.

   The action taken by using the RPL Option and the potential set of
   sub-TLVs carried within the RPL Option MUST be specified by the RFC
   of the protocol that uses that option.  No sub-TLVs are defined in
   this document.  A RPL device MUST skip over any unrecognized sub-TLVs
   and attempt to process any additional sub-TLVs that may appear after.

4.  RPL Router Behavior

   Datagrams sent between RPL routers MUST include a RPL Option or RPL
   Source Route Header ([RFC6554]) and MAY include both.  A datagram
   including a Source Routing Header (SRH) does not need to include a
   RPL Option since both the source and intermediate routers ensure that
   the SRH does not contain loops.

   When the router is the source of the original packet and the
   destination is known to be within the same RPL Instance, the router
   SHOULD include the RPL Option directly within the original packet.
   Otherwise, routers MUST use IPv6-in-IPv6 tunneling [RFC2473] and
   place the RPL Option in the tunnel header.  Using IPv6-in-IPv6
   tunneling ensures that the delivered datagram remains unmodified and
   that ICMPv6 errors generated by a RPL Option are sent back to the
   router that generated the RPL Option.

   A RPL router chooses the next RPL router that should process the
   original packet as the tunnel exit-point.  In some cases, the tunnel
   exit-point will be the final RPL router along a path towards the
   original packet's destination, and the original packet will only
   traverse a single tunnel.  One example is when the final destination
   or the destination's attachment router is known to be within the same
   RPL Instance.

   In other cases, the tunnel exit-point will not be the final RPL
   router along a path and the original packet may traverse multiple
   tunnels to reach the destination.  One example is when a RPL router
   is simply forwarding a packet to one of its Destination-Oriented DAG
   (DODAG) parents.  In this case, the RPL router sets the tunnel exit-
   point to a DODAG parent.  When forwarding the original packet hop-by-
   hop, the RPL router only makes a determination on the next hop
   towards the destination.

   A RPL router receiving an IPv6-in-IPv6 packet destined to it
   processes the tunnel packet as described in Section 3 of [RFC2473].
   Before IPv6 decapsulation, the RPL router MUST process the RPL
   Option, if one exists.  After IPv6 decapsulation, if the router
   determines that it should forward the original packet to another RPL
   router, it MUST encapsulate the packet again using IPv6-in-IPv6
   tunneling to include the RPL Option.  Fields within the RPL Option
   that do not change hop-by-hop MUST remain the same as those received
   from the prior tunnel.

   RPL routers are responsible for ensuring that a RPL Option is only
   used between RPL routers:

   1.  For datagrams destined to a RPL router, the router processes the
       packet in the usual way.  For instance, if the RPL Option was
       included using tunneled mode and the RPL router serves as the
       tunnel endpoint, the router removes the outer IPv6 header, at the
       same time removing the RPL Option as well.

   2.  Datagrams destined elsewhere within the same RPL Instance are
       forwarded to the correct interface.

   3.  Datagrams destined to nodes outside the RPL Instance are dropped
       if the outermost IPv6 header contains a RPL Option not generated
       by the RPL router forwarding the datagram.

   To avoid fragmentation, it is desirable to employ MTU sizes that
   allow for the header expansion (i.e., at least 1280 + 40 (outer IP
   header) + RPL_OPTION_MAX_SIZE), where RPL_OPTION_MAX_SIZE is the
   maximum RPL Option header size for a given RPL network.  To take
   advantage of this, however, the communicating endpoints need to be
   aware of the MTU along the path (i.e., through Path MTU Discovery).
   Unfortunately, the larger MTU size may not be available on all links
   (e.g., 1280 octets on IPv6 Low-Power Wireless Personal Area Network
   (6LoWPAN) links).  However, it is expected that much of the traffic
   on these types of networks consists of much smaller messages than the
   MTU, so performance degradation through fragmentation would be
   limited.

5.  Security Considerations

   The RPL Option assists RPL routers in detecting routing
   inconsistencies.  The RPL message security mechanisms defined in
   [RFC6550] do not apply to the RPL Option.

5.1.  DAG Inconsistency Attacks

   Using the Down 'O' flag and SenderRank field, an attacker can cause
   RPL routers to believe that a DAG inconsistency exists within the RPL
   Instance identified by the RPLInstanceID field.  This attack would
   cause a RPL router to reset its DODAG Information Object (DIO)
   Trickle timer and begin transmitting DIO messages more frequently.

   In order to avoid any unacceptable impact on network operations, an
   implementation MAY limit the rate of Trickle timer resets caused by
   receiving a RPL Option to no greater than MAX_RPL_OPTION_RANK_ERRORS
   per hour.  A RECOMMENDED value for MAX_RPL_OPTION_RANK_ERRORS is 20.

5.2.  Destination Advertisement Object (DAO) Inconsistency Attacks

   In Storing mode, RPL routers maintain Downward routing state.  Under
   normal operation, the RPL Option assists RPL routers in cleaning up
   stale Downward routing state by using the Forwarding-Error 'F' flag
   to indicate that a datagram could not be delivered by a child and is
   being sent back to try a different child.  Using this flag, an
   attacker can cause a RPL router to discard Downward routing state.

   In order to avoid any unacceptable impact on network operations, an
   implementation MAY limit the rate of discarding Downward routing
   state caused by receiving a RPL Option to no greater than
   MAX_RPL_OPTION_FORWARD_ERRORS per hour.  A RECOMMENDED value for
   MAX_RPL_OPTION_FORWARD_ERRORS is 20.

   In Non-Storing mode, only the Low-Power and Lossy Network Border
   Router (LBR) maintains Downward routing state.  Because RPL routers
   do not maintain Downward routing state, the RPL Option cannot be used
   to mount such attacks.

6.  IANA Considerations

   IANA has assigned a new value in the Destination Options and Hop-by-
   Hop Options registry.  The value is as follows:

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

   As specified in [RFC2460], 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 en route.
   The remaining bits serve as the option type.

   IANA has created a registry called RPL-option-TLV, for the sub-TLVs
   carried in the RPL Option header.  New codes may be allocated only by
   IETF Review [RFC5226].  The type field is an 8-bit field whose value
   be between 0 and 255, inclusive.

7.  Acknowledgements

   The authors thank Jari Arkko, Ralph Droms, Adrian Farrel, Stephen
   Farrell, Richard Kelsey, Suresh Krishnan, Vishwas Manral, Erik
   Nordmark, Pascal Thubert, Sean Turner, and Tim Winter, for their
   comments and suggestions that helped shape this document.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

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

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

   [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, March 2012.

8.2.  Informative References

   [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,
              March 2012.

Authors' Addresses

   Jonathan W. Hui
   Cisco Systems
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Phone: +408 424 1547
   EMail: jonhui@cisco.com


   JP. Vasseur
   Cisco Systems
   11, Rue Camille Desmoulins
   Issy Les Moulineaux  92782
   France

   EMail: jpv@cisco.com