Internet Engineering Task Force (IETF) P. Levis

Internet Engineering Task Force (IETF) P. Levis
Request for Comments: 6206 Stanford University
Category: Standards Track T. Clausen
ISSN: 2070-1721 LIX, Ecole Polytechnique
J. Hui
Arch Rock Corporation
O. Gnawali
Stanford University
J. Ko
Johns Hopkins University
March 2011

The Trickle Algorithm

Abstract

The Trickle algorithm allows nodes in a lossy shared medium (e.g.,
low-power and lossy networks) to exchange information in a highly
robust, energy efficient, simple, and scalable manner. Dynamically
adjusting transmission windows allows Trickle to spread new
information on the scale of link-layer transmission times while
sending only a few messages per hour when information does not
change. A simple suppression mechanism and transmission point
selection allow Trickle's communication rate to scale logarithmically
with density. This document describes the Trickle algorithm and
considerations in its use.

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/rfc6206.

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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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................3
3. Trickle Algorithm Overview ......................................3
4. Trickle Algorithm ...............................................5
4.1. Parameters and Variables ...................................5
4.2. Algorithm Description ......................................5
5. Using Trickle ...................................................6
6. Operational Considerations ......................................7
6.1. Mismatched Redundancy Constants ............................7
6.2. Mismatched Imin ............................................7
6.3. Mismatched Imax ............................................8
6.4. Mismatched Definitions .....................................8
6.5. Specifying the Constant k ..................................8
6.6. Relationship between k and Imin ............................8
6.7. Tweaks and Improvements to Trickle .........................9
6.8. Uses of Trickle ............................................9
7. Acknowledgements ...............................................10
8. Security Considerations ........................................10
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................11

1. Introduction

The Trickle algorithm establishes a density-aware local communication
primitive with an underlying consistency model that guides when a
node transmits. When a node's data does not agree with its
neighbors, that node communicates quickly to resolve the
inconsistency (e.g., in milliseconds). When nodes agree, they slow
their communication rate exponentially, such that nodes send packets
very infrequently (e.g., a few packets per hour). Instead of

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flooding a network with packets, the algorithm controls the send rate
so each node hears a small trickle of packets, just enough to stay
consistent. Furthermore, by relying only on local communication
(e.g., broadcast or local multicast), Trickle handles network
re-population; is robust to network transience, loss, and
disconnection; is simple to implement; and requires very little
state. Current implementations use 4-11 bytes of RAM and are
50-200 lines of C code [Levis08].

While Trickle was originally designed for reprogramming protocols
(where the data is the code of the program being updated), experience
has shown it to be a powerful mechanism that can be applied to a wide
range of protocol design problems, including control traffic timing,
multicast propagation, and route discovery. This flexibility stems
from being able to define, on a case-by-case basis, what constitutes
"agreement" or an "inconsistency"; Section 6.8 presents a few
examples of how the algorithm can be used.

This document describes the Trickle algorithm and provides guidelines
for its use. It also states requirements for protocol specifications
that use Trickle. This document does not provide results regarding
Trickle's performance or behavior, nor does it explain the
algorithm's design in detail: interested readers should refer to
[Levis04] and [Levis08].

2. Terminology

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
RFC 2119 [RFC2119].

Additionally, this document introduces the following terminology:

Trickle communication rate: the sum of the number of messages sent
or received by the Trickle algorithm in an interval.

Trickle transmission rate: the sum of the number of messages sent by
the Trickle algorithm in an interval.

3. Trickle Algorithm Overview

Trickle's basic primitive is simple: every so often, a node transmits
data unless it hears a few other transmissions whose data suggest its
own transmission is redundant. Examples of such data include routing
state, software update versions, and the last heard multicast packet.
This primitive allows Trickle to scale to thousand-fold variations in
network density, quickly propagate updates, distribute transmission

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load evenly, be robust to transient disconnections, handle network
re-populations, and impose a very low maintenance overhead: one
example use, routing beacons in the Collection Tree Protocol (CTP)
[Gnawali09], requires sending on the order of a few packets per hour,
yet CTP can respond to topology changes in milliseconds.

Trickle sends all messages to a local communication address. The
exact address used can depend on the underlying IP protocol as well
as how the higher-layer protocol uses Trickle. In IPv6, for example,
it can be the link-local multicast address or another local multicast
address, while in IPv4 it can be the broadcast address
(255.255.255.255).

There are two possible results to a Trickle message: either every
node that hears the message finds that the message data is consistent
with its own state, or a recipient detects an inconsistency.
Detection can be the result of either an out-of-date node hearing
something new, or an updated node hearing something old. As long as
every node communicates somehow -- either receives or transmits --
some node will detect the need for an update.

For example, consider a simple case where "up to date" is defined by
version numbers (e.g., network configuration). If node A transmits
that it has version V, but B has version V+1, then B knows that A
needs an update. Similarly, if B transmits that it has version V+1,
A knows that it needs an update. If B broadcasts or multicasts
updates, then all of its neighbors can receive them without having to
advertise their need. Some of these recipients might not have even
heard A's transmission. In this example, it does not matter who
first transmits -- A or B; the inconsistency will be detected in
either case.

The fact that Trickle communication can be either transmission or
reception enables the Trickle algorithm to operate in sparse as well
as dense networks. A single, disconnected node must transmit at the
Trickle communication rate. In a lossless, single-hop network of
size n, the Trickle communication rate at each node equals the sum of
the Trickle transmission rates across all nodes. The Trickle
algorithm balances the load in such a scenario, as each node's
Trickle transmission rate is 1/nth of the Trickle communication rate.
Sparser networks require more transmissions per node, but the
utilization of a given broadcast domain (e.g., radio channel over
space, shared medium) will not increase. This is an important
property in wireless networks and other shared media, where the
channel is a valuable shared resource. Additionally, reducing
transmissions in dense networks conserves system energy.

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4. Trickle Algorithm

This section describes the Trickle algorithm.

4.1. Parameters and Variables

A Trickle timer runs for a defined interval and has three
configuration parameters: the minimum interval size Imin, the maximum
interval size Imax, and a redundancy constant k:

o The minimum interval size, Imin, is defined in units of time
(e.g., milliseconds, seconds). For example, a protocol might
define the minimum interval as 100 milliseconds.

o The maximum interval size, Imax, is described as a number of
doublings of the minimum interval size (the base-2 log(max/min)).
For example, a protocol might define Imax as 16. If the minimum
interval is 100 ms, then the amount of time specified by Imax is
100 ms * 65,536, i.e., 6,553.6 seconds or approximately
109 minutes.

o The redundancy constant, k, is a natural number (an integer
greater than zero).

In addition to these three parameters, Trickle maintains three
variables:

o I, the current interval size,

o t, a time within the current interval, and

o c, a counter.

4.2. Algorithm Description

The Trickle algorithm has six rules:

1. When the algorithm starts execution, it sets I to a value in the
range of [Imin, Imax] -- that is, greater than or equal to Imin
and less than or equal to Imax. The algorithm then begins the
first interval.

2. When an interval begins, Trickle resets c to 0 and sets t to a
random point in the interval, taken from the range [I/2, I), that
is, values greater than or equal to I/2 and less than I. The
interval ends at I.

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3. Whenever Trickle hears a transmission that is "consistent", it
increments the counter c.

4. At time t, Trickle transmits if and only if the counter c is less
than the redundancy constant k.

5. When the interval I expires, Trickle doubles the interval length.
If this new interval length would be longer than the time
specified by Imax, Trickle sets the interval length I to be the
time specified by Imax.

6. If Trickle hears a transmission that is "inconsistent" and I is
greater than Imin, it resets the Trickle timer. To reset the
timer, Trickle sets I to Imin and starts a new interval as in
step 2. If I is equal to Imin when Trickle hears an
"inconsistent" transmission, Trickle does nothing. Trickle can
also reset its timer in response to external "events".

The terms "consistent", "inconsistent", and "events" are in quotes
because their meaning depends on how a protocol uses Trickle.

The only time the Trickle algorithm transmits is at step 4 of the
above algorithm. This means there is an inherent delay between
detecting an inconsistency (shrinking I to Imin) and responding to
that inconsistency (transmitting at time t in the new interval).
This is intentional. Immediately responding to detecting an
inconsistency can cause a broadcast storm, where many nodes respond
at once and in a synchronized fashion. By making responses follow
the Trickle algorithm (with the minimal interval size), a protocol
can benefit from Trickle's suppression mechanism and scale across a
huge range of node densities.

5. Using Trickle

A protocol specification that uses Trickle MUST specify:

o Default values for Imin, Imax, and k. Because link layers can
vary widely in their properties, the default value of Imin SHOULD
be specified in terms of the worst-case latency of a link-layer
transmission. For example, a specification should say "the
default value of Imin is 4 times the worst-case link-layer
latency" and should not say "the default value of Imin is
500 milliseconds". Worst-case latency is approximately the time
until the first link-layer transmission of the frame, assuming an
idle channel (does not include backoff, virtual carrier sense,
etc.).

o What constitutes a "consistent" transmission.

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o What constitutes an "inconsistent" transmission.

o What "events", if any -- besides inconsistent transmissions --
reset the Trickle timer.

o What information a node transmits in Trickle messages.

o What actions outside the algorithm the protocol takes, if any,
when it detects an inconsistency.

6. Operational Considerations

It is RECOMMENDED that a protocol that uses Trickle include
mechanisms to inform nodes of configuration parameters at runtime.
However, it is not always possible to do so. In the cases where
different nodes have different configuration parameters, Trickle may
have unintended behaviors. This section outlines some of those
behaviors and operational considerations as educational exercises.

6.1. Mismatched Redundancy Constants

If nodes do not agree on the redundancy constant k, then nodes with
higher values of k will transmit more often than nodes with lower
values of k. In some cases, this increased load can be independent
of the density. For example, consider a network where all nodes but
one have k=1, and this one node has k=2. The different node can end
up transmitting on every interval: it is maintaining a Trickle
communication rate of 2 with only itself. Hence, the danger of
mismatched k values is uneven transmission load that can deplete the
energy of some nodes in a low-power network.

6.2. Mismatched Imin

If nodes do not agree on Imin, then some nodes, on hearing
inconsistent messages, will transmit sooner than others. These
faster nodes will have their intervals grow to a size similar to that
of the slower nodes within a single slow interval time, but in that
period may suppress the slower nodes. However, such suppression will
end after the first slow interval, when the nodes generally agree on
the interval size. Hence, mismatched Imin values are usually not a
significant concern. Note that mismatched Imin values and matching
Imax doubling constants will lead to mismatched maximum interval
lengths.

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6.3. Mismatched Imax

If nodes do not agree on Imax, then this can cause long-term problems
with transmission load. Nodes with small Imax values will transmit
faster, suppressing those with larger Imax values. The nodes with
larger Imax values, always suppressed, will never transmit. In the
base case, when the network is consistent, this can cause long-term
inequities in energy cost.

6.4. Mismatched Definitions

If nodes do not agree on what constitutes a consistent or
inconsistent transmission, then Trickle may fail to operate properly.
For example, if a receiver thinks a transmission is consistent, but
the transmitter (if in the receiver's situation) would have thought
it inconsistent, then the receiver will not respond properly and
inform the transmitter. This can lead the network to not reach a
consistent state. For this reason, unlike the configuration
constants k, Imin, and Imax, consistency definitions MUST be clearly
stated in the protocol and SHOULD NOT be configured at runtime.

6.5. Specifying the Constant k

There are some edge cases where a protocol may wish to use Trickle
with its suppression disabled (k is set to infinity). In general,
this approach is highly dangerous and it is NOT RECOMMENDED.
Disabling suppression means that every node will always send on every
interval; this can lead to congestion in dense networks. This
approach is especially dangerous if many nodes reset their intervals
at the same time. In general, it is much more desirable to set k to
a high value (e.g., 5 or 10) than infinity. Typical values for k
are 1-5: these achieve a good balance between redundancy and low cost
[Levis08].

Nevertheless, there are situations where a protocol may wish to turn
off Trickle suppression. Because k is a natural number
(Section 4.1), k=0 has no useful meaning. If a protocol allows k to
be dynamically configured, a value of 0 remains unused. For ease of
debugging and packet inspection, having the parameter describe k-1
rather than k can be confusing. Instead, it is RECOMMENDED that
protocols that require turning off suppression reserve k=0 to mean
k=infinity.

6.6. Relationship between k and Imin

Finally, a protocol SHOULD set k and Imin such that Imin is at least
two to three times as long as it takes to transmit k packets.
Otherwise, if more than k nodes reset their intervals to Imin, the

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resulting communication will lead to congestion and significant
packet loss. Experimental results have shown that packet losses from
congestion reduce Trickle's efficiency [Levis04].

6.7. Tweaks and Improvements to Trickle

Trickle is based on a small number of simple, tightly integrated
mechanisms that are highly robust to challenging network
environments. In our experiences using Trickle, attempts to tweak
its behavior are typically not worth the cost. As written, the
algorithm is already highly efficient: further reductions in
transmissions or response time come at the cost of failures in edge
cases. Based on our experiences, we urge protocol designers to
suppress the instinct to tweak or improve Trickle without a great
deal of experimental evidence that the change does not violate its
assumptions and break the algorithm in edge cases.

With this warning in mind, Trickle is far from perfect. For example,
Trickle suppression typically leads sparser nodes to transmit more
than denser ones; it is far from the optimal computation of a minimum
cover. However, in dynamic network environments such as wireless and
low-power, lossy networks, the coordination needed to compute the
optimal set of transmissions is typically much greater than the
benefits it provides. One of the benefits of Trickle is that it is
so simple to implement and requires so little state yet operates so
efficiently. Efforts to improve it should be weighed against the
cost of increased complexity.

6.8. Uses of Trickle

The Trickle algorithm has been used in a variety of protocols, in
operational as well as academic settings. Giving a brief overview of
some of these uses provides useful examples of how and when it can be
used. These examples should not be considered exhaustive.

Reliable flooding/dissemination: A protocol uses Trickle to
periodically advertise the most recent data it has received,
typically through a version number. An inconsistency occurs when a
node hears a newer version number or receives new data. A
consistency occurs when a node hears an older or equal version
number. When hearing an older version number, rather than reset its
own Trickle timer, the node sends an update. Nodes with old version
numbers that receive the update will then reset their own timers,
leading to fast propagation of the new data. Examples of this use
include multicast [Hui08a], network configuration [Lin08] [Dang09],
and installing new application programs [Hui04] [Levis04].

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Routing control traffic: A protocol uses Trickle to control when it
sends beacons that contain routing state. An inconsistency occurs
when the routing topology changes in a way that could lead to loops
or significant stretch: examples include when the routing layer
detects a routing loop or when a node's routing cost changes
significantly. Consistency occurs when the routing topology is
operating well and is delivering packets successfully. Using the
Trickle algorithm in this way allows a routing protocol to react very
quickly to problems (Imin is small) but send very few beacons when
the topology is stable. Examples of this use include the IPv6
routing protocol for low-power and lossy networks (RPL) [RPL], CTP
[Gnawali09], and some current commercial IPv6 routing layers
[Hui08b].

7. Acknowledgements

The authors would like to acknowledge the guidance and input provided
by the ROLL chairs, David Culler and JP Vasseur.

The authors would also like to acknowledge the helpful comments of
Yoav Ben-Yehezkel, Alexandru Petrescu, and Ulrich Herberg, which
greatly improved the document.

8. Security Considerations

As it is an algorithm, Trickle itself does not have any specific
security considerations. However, two security concerns can arise
when Trickle is used in a protocol. The first is that an adversary
can force nodes to send many more packets than needed by forcing
Trickle timer resets. In low-power networks, this increase in
traffic can harm system lifetime. The second concern is that an
adversary can prevent nodes from reaching consistency.

Protocols can prevent adversarial Trickle resets by carefully
selecting what can cause a reset and protecting these events and
messages with proper security mechanisms. For example, if a node can
reset nearby Trickle timers by sending a certain packet, this packet
should be authenticated such that an adversary cannot forge one.

An adversary can possibly prevent nodes from reaching consistency by
suppressing transmissions with "consistent" messages. For example,
imagine node A detects an inconsistency and resets its Trickle timer.
If an adversary can prevent A from sending messages that inform
nearby nodes of the inconsistency in order to repair it, then A may
remain inconsistent indefinitely. Depending on the security model of
the network, authenticated messages or a transitive notion of
consistency can prevent this problem. For example, let us suppose an
adversary wishes to suppress A from notifying neighbors of an

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inconsistency. To do so, it must send messages that are consistent
with A. These messages are by definition inconsistent with those of
A's neighbors. Correspondingly, an adversary cannot simultaneously
prevent A from notifying neighbors and not notify the neighbors
itself (recall that Trickle operates on shared, broadcast media).
Note that this means Trickle should filter unicast messages.

9. References

9.1. Normative References

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

9.2. Informative References

[Dang09] Dang, T., Bulusu, N., Feng, W., and S. Park, "DHV: A Code
Consistency Maintenance Protocol for Multi-hop Wireless
Networks", Wireless Sensor Networks: 6th European
Conference Proceedings EWSN 2009 Cork, February 2009,
<http://portal.acm.org/citation.cfm?id=1506781>.

[Gnawali09]
Gnawali, O., Fonseca, R., Jamieson, K., Moss, D., and P.
Levis, "Collection Tree Protocol", Proceedings of the 7th
ACM Conference on Embedded Networked Sensor
Systems, SenSys 2009, November 2009,
<http://portal.acm.org/citation.cfm?id=1644038.1644040>.

[Hui04] Hui, J. and D. Culler, "The dynamic behavior of a data
dissemination protocol for network programming at scale",
Proceedings of the 2nd ACM Conference on Embedded
Networked Sensor Systems, SenSys 2004, November 2004,
<http://portal.acm.org/citation.cfm?id=1031506>.

[Hui08a] Hui, J., "An Extended Internet Architecture for Low-Power
Wireless Networks - Design and Implementation", UC
Berkeley Technical Report EECS-2008-116, September 2008,
<http://www.eecs.berkeley.edu/Pubs/>.

[Hui08b] Hui, J. and D. Culler, "IP is dead, long live IP for
wireless sensor networks", Proceedings of the 6th ACM
Conference on Embedded Networked Sensor Systems, SenSys
2008, November 2008,
<http://portal.acm.org/citation.cfm?id=1460412.1460415>.

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[Levis04] Levis, P., Patel, N., Culler, D., and S. Shenker,
"Trickle: A Self-Regulating Algorithm for Code Propagation
and Maintenance in Wireless Sensor Networks", Proceedings
of the First USENIX/ACM Symposium on Networked Systems
Design and Implementation, NSDI 2004, March 2004,
<http://portal.acm.org/citation.cfm?id=1251177>.

[Levis08] Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S.,
Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A.
Woo, "The Emergence of a Networking Primitive in Wireless
Sensor Networks", Communications of the ACM, Vol. 51 No.
7, July 2008,
<http://portal.acm.org/citation.cfm?id=1364804>.

[Lin08] Lin, K. and P. Levis, "Data Discovery and Dissemination
with DIP", Proceedings of the 7th international conference
on Information processing in sensor networks, IPSN 2008,
April 2008,
<http://portal.acm.org/citation.cfm?id=1371607.1372753>.

[RPL] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Clausen,
T., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik,
R., and JP. Vasseur, "RPL: IPv6 Routing Protocol for Low
power and Lossy Networks", Work in Progress, March 2011.

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Authors' Addresses

Philip Levis
Stanford University
358 Gates Hall
Stanford, CA 94305
USA

Phone: +1 650 725 9064
EMail: [email protected]

Thomas Heide Clausen
LIX, Ecole Polytechnique

Phone: +33 6 6058 9349
EMail: [email protected]

Jonathan Hui
Arch Rock Corporation
501 2nd St., Suite 410
San Francisco, CA 94107
USA

EMail: [email protected]

Omprakash Gnawali
Stanford University
S255 Clark Center, 318 Campus Drive
Stanford, CA 94305
USA

Phone: +1 650 725 6086
EMail: [email protected]

JeongGil Ko
Johns Hopkins University
3400 N. Charles St., 224 New Engineering Building
Baltimore, MD 21218
USA

Phone: +1 410 516 4312
EMail: [email protected]

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