ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track October 17, 2011
Expires: April 19, 2012
RPL adaptation for asymmetrical links
draft-thubert-roll-asymlink-00
Abstract
The Routing Protocol for Low Power and Lossy Networks defines a
generic Distance Vector protocol for Low Power and Lossy Networks,
many of which exhibit strongly asymmetrical characteristics. This
draft proposes an extension for that optimizes RPL operations whereby
upwards and downwards direction-optimized RPL instances are
associated.
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].
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 19, 2012.
Copyright Notice
Copyright (c) 2011 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The asymmetrical link problem . . . . . . . . . . . . . . . . . 4
4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . . 4
5. Modified DODAG Information Object (DIO) . . . . . . . . . . . . 5
6. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Backward compatibility . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The IETF ROLL Working Group has defined application-specific routing
requirements for a Low Power and Lossy Network (LLN) routing
protocol, specified in [RFC5548], [RFC5673], [RFC5826], and
[RFC5867], many of which explicitly or implicitly refer to links with
asymmetrical properties.
Upon those requirements, the Routing Protocol for Low Power and Lossy
Network [I-D.ietf-roll-rpl] was designed as a platform that can be
extended by further specifications or guidances, by adding new
metrics, Objective Functions, or additional options.
RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs)
within instances of the protocol. Each instance is associated with
an Objective Function that is designed to solve the problem that is
addressed by that instance.
In one hand, RPL requires bidirectional links for the control, but on
the other, there is no requirement that the properties of a link are
the same in both directions. In fact, such a symmetry is rarely
present in LLNs, whether links are based on radios or power-line.
Some initial implementations require that the quality of both
directions of a link is evaluated as very good so that the link can
be used for control and data in both directions. This eliminates
asymmetrical links that are very good in one direction, but only good
enough for scarce activity in the other direction.
In practice, a DAG that is built to optimize upwards traffic is
generally not congruent with a DAG that is built to optimize
downwards traffic. This is why this specification is designed to
enable asymmetrical routing DAGs that are bound together to get the
maximum benefits of all bidirectional links.
2. Terminology
The terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks' [I-D.ietf-roll-terminology] and [I-D.ietf-roll-rpl].
The term upwards qualifies a link, a DODAG or an instance that is
optimal for sending traffic in the general direction of the root,
though may be usable but suboptimal for traffic coming form the
direction of the root. The term downwards qualifies the same words
for the opposite direction.
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The term parenting applied to instances refers to the directional
association of two instances. The graph formed by parented instances
must be a DAG. Traffic may be transferred from an instance onto a
parent instance under specified circumstances.
3. The asymmetrical link problem
4. Solution Overview
With the core RPL specification, [I-D.ietf-roll-rpl] each instance is
a separate routing topology, and packets must be forwarded within the
same topology / same instance. One direct consequence of that design
choice is that a topology must be very good for both upwards and
downwards traffic; otherwise, traffic between two nodes in the
instance may suffer.
A simple approach to address bidirectional but asymmetrical links
with RPL is to construct two DAGs, one for upwards traffic and one
for downwards traffic, each DAG a separate instance, and then bind
the two together. In order to benefit from both instances for a same
packet, this solution extends RPL to allow traffic to be transferred
from one instance to the next.
It can be noted at this point that with [I-D.ietf-roll-rpl], traffic
that goes down does not generally go back up again, whereas P2P
traffic within a DODAG might go up to a common parent and then down
to the destination. In terms of instance relationship, this means
that when an upwards and a downwards instance are bound together,
traffic from the former may be transferred to the latter, but not the
other way around. In other words, there is an order, a parent-child
relationship, between the two instances.
Additionally, if there is no next-hop for a packet going down within
the instance, then with [I-D.ietf-roll-rpl] the packet must be
dropped. In order to limit that risk, it is tempting though
inefficient to lower the constraints that are applied to build the
topology. It can be more efficient to actually keep the constraints
as they should be, but, instead, enable a less constrained, more
spanning, fall-back topology into which traffic can be transferred.
For that reason, this solution allows for more complex instance
relationships than plain child-parent associations. In order to
avoids loops which could be created when transferring packets from
one instance to the next, this solution requires that the instances
be themselves organized as a superior Directed Acyclic Graph, and
enforce that inter-DAG transfers occur only within that superior
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super-DAG of DAG instances.
5. Modified DODAG Information Object (DIO)
The DODAG Information Object [I-D.ietf-roll-rpl] carries information
that allows a node to discover a RPL Instance, learn its
configuration parameters, select a DODAG parent set, and maintain the
DODAG. This specification defines a new flag bit to indicate that
the DAG is directional.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |Version Number | Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|D| MOP | Prf | DTSN | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 1: The DIO Base Object
Directional (D): The Directional (D) flag is set to indicate that
the instance is intended for directional operation, and reset
otherwise. When it is set, a MOP of 0 indicates the upwards
direction whereas any other value specified in
[I-D.ietf-roll-rpl] indicates downwards. All other values of
MOP will be considered downwards unless explicitly specified
otherwise.
6. Operations
This specification allows an organization of Instances as follows:
Instances MUST be organized as a Directed Acyclic Graph. This
information MUST be commissioned into the devices so they know
both which instances they should participate in, and which
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direction of transfer is allowed between instances.
A spanning instance using OF0 [I-D.ietf-roll-of0] MAY be used as
root in that instance DAG.
This specification defines a new bit in the RPL [I-D.ietf-roll-rpl]
DODAG Information Object (DIO) with the Directional (D) flag that
indicates a directional operation for a given instance. An
implementation that does not support that new bit will not be able to
propagate it.
In case of a directional operation,
The direction is indicated by the MOP field, a MOP of 0 means
upwards and otherwise is downwards.
Links are still REQUIRED to allow bidirectional operations
Only the metrics that correspond to the DAG direction are used for
the parent selection.
An upward instance SHOULD install routes that lead to the root and
beyond - typically the default route.
A downwards instance MAY ONLY install more specific routes that
are injected by nodes in the DODAG through the DAO process.
P2P operations are achieved by associating a child upwards
instance with a parent downwards instance.
A packet MUST NOT be transferred from a parent instance to a child
instance.
A packet MAY be transferred from a child instance to its parent
instance if and only if the child instance does not provide a
route to the destination, or the parent instance provides a more
specific route (longer match) to the destination.
Transferring from an upwards instance to a downwards instance if
generally desirable. Other forms of transfers are generally not
desirable. Policies MAY be put in place to ovreride that general
guidance.
7. Backward compatibility
An OF is generally designed to compute a Rank of a directional link
in a fashion that is diffent from a bidirectional link, and in
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particular will not use the same metrics and thusobtain different
ranks for a same situation. For that reason, it is important that
the OF is aware that an instance is supposed to define a directional
DODAG, and it is RECOMMENDED that only devices that support
directional DODAGs are allowed in a directional instance.
It might happen that for some purposes like higher availability, an
implementation that does not support directional links is
administratively allowed to join a directional DODAG. In that case,
the extension of the DODAG that starts at that device will not be
directional, but the instance will still be functional.
In that case, it might also happen that a device that supports
directional DODAGs per this specification sees candidate neighbors
that expose the Directional flag and some others that do not. An OF
that supports directional links SHOULD favor directional links over
non directional links, in a fashion that is to be specified with the
OF. In the case of OF0 [I-D.ietf-roll-of0], the 'D' flag should be
accounted for before the computation of item 8 in the "Selection Of
The Preferred Parent" section 4.2.1., that is before Ranks and be
calculated and compared.
8. IANA Considerations
This specification requires that a bit in DIO be assigned to indicate
directional link operations as specified in section
9. Security Considerations
Security Considerations for this proposal are to be developed in
accordance with recommendations laid out in, for example,
[I-D.tsao-roll-security-framework].
10. Acknowledgements
The author wishes to recognize Richard Kelsey, JP Vasseur, Tom
Phinney, Robert Assimiti, Don Sturek and Yoav Ben-Yehezkel for their
various contributions.
11. References
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11.1. Normative References
[I-D.ietf-roll-rpl]
Winter, T., Thubert, P., Brandt, A., Clausen, T., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., and J.
Vasseur, "RPL: IPv6 Routing Protocol for Low power and
Lossy Networks", draft-ietf-roll-rpl-19 (work in
progress), March 2011.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-06 (work in
progress), September 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.ietf-roll-of0]
Thubert, P., "RPL Objective Function Zero",
draft-ietf-roll-of0-20 (work in progress), September 2011.
[I-D.tsao-roll-security-framework]
Tsao, T., Alexander, R., Daza, V., and A. Lozano, "A
Security Framework for Routing over Low Power and Lossy
Networks", draft-tsao-roll-security-framework-02 (work in
progress), March 2010.
[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
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Author's Address
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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