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Keywords: [--------], LLN, LLNs, Trickle
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.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hui & Vasseur Standards Track [Page 1]
RFC 6553 RPL Option March 2012
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.
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RFC 6553 RPL Option March 2012
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:
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RFC 6553 RPL Option March 2012
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.
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RFC 6553 RPL Option March 2012
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
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RFC 6553 RPL Option March 2012
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.
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RFC 6553 RPL Option March 2012
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.
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RFC 6553 RPL Option March 2012
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.
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RFC 6553 RPL Option 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
Hui & Vasseur Standards Track [Page 9]