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Versions: (draft-clausen-manet-jitter) 00 01
02 03 04 RFC 5148
Mobile Ad hoc Networking (MANET) T. Clausen
Internet-Draft LIX, Ecole Polytechnique, France
Intended status: Informational C. Dearlove
Expires: December 31, 2007 BAE Systems Advanced Technology
Centre
B. Adamson
Naval Research Laboratory
June 29, 2007
Jitter considerations in MANETs
draft-ietf-manet-jitter-01
Status of this Memo
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Copyright (C) The IETF Trust (2007).
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Abstract
This document provides recommendations for jittering (randomly
modifying timing) of control traffic transmissions in MANET routing
protocols to reduce the probability of packet collisions.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. Protocol Overview and Functioning . . . . . . . . . . . . . . 6
5. Jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Periodic Message Generation . . . . . . . . . . . . . . . 7
5.2. Externally Triggered Message Generation . . . . . . . . . 8
5.3. Message Forwarding . . . . . . . . . . . . . . . . . . . . 9
5.4. Maximum Jitter Determination . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
In a wireless network, simultaneous packet transmission by nearby
nodes is undesirable as, depending on the medium access control and
other lower layer mechanisms, the interference between these
transmissions may cause at best increased delay, and at worst
complete packet loss.
The problems of simultaneous packet transmissions are amplified if
any of the following features are present in a protocol:
Regularly scheduled messages - If two nodes generate packets
containing regularly scheduled messages of the same type at the
same time, and if, as is typical, they are using the same message
interval, all further transmissions of these messages will thus
also be at the same time.
Event-triggered messages - If nodes respond to changes in their
circumstances, in particular changes in their neighborhood, with
an immediate message generation and transmission, then two nearby
nodes which respond to the same change will transmit messages
simultaneously.
Schedule reset - When a node sends an event-triggered message of a
type which is usually regularly scheduled, then there is no
apparent reason why it should not restart its corresponding
message schedule. This may result in nodes responding to the same
change also sending future messages simultaneously.
Forwarding - If nodes forward messages they receive from other
nodes, then nearby nodes will commonly receive and forward the
same message. If forwarding is performed immediately then the
resulting packet transmissions may interfere with each other.
A possible solution to these problems is to employ jitter, a
deliberate random variation in timing. This document discusses
applying jitter to packet transmissions, with the purpose of avoiding
collisions, with particular reference to the features listed above.
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2. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [1].
Additionally, this document uses the following terminology:
Node - A MANET router which implements a message sending protocol.
MANET interface - A network device participating in a MANET. A node
may have one or more MANET interfaces.
Message - An entity carrying protocol information intended for
exchange between nodes. Messages are transmitted over MANET
interfaces embedded in packets.
Packet - An entity embedding zero or more messages for transmission
over a MANET interface of the node.
Transmission - A packet being sent over a MANET interface of the
node. A transmission can be due to either a message being
generated or a message being forwarded.
Generation - Creation of a new message for transmission over one or
more MANET interfaces of the node. Typically, a node will
generate messages based on a message schedule (periodic or
otherwise) or as a response to changes in circumstances.
Forwarding - Retransmission of a received message over one or more
MANET interfaces of the node.
Collision - A specific instance of interference, where two or more
nodes transmit a packet at the same time and within the same
signal space (at the same frequency and/or encoding) such that
another, closely located, node which should receive and decode
these packets instead fails to do so, and loses one or more of the
packets.
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3. Applicability Statement
The mechanisms described in this document are applicable to any MANET
protocol in which simultaneous transmissions by different nodes are
undesirable and which contains mechanisms, such as periodic message
transmission, triggered message transmission, or message forwarding,
which either make the simultaneous transmission more likely, or cause
it to be repeated when it occurs. This particularly applies to
protocols using broadcast transmissions in wireless networks, where
proactive MANET routing protocols such as [2] employ scheduled
messages, where reactive MANET routing protocols such as [3] employ
event-triggered messages, and where both employ message forwarding.
These mechanisms are intended for application where the underlying
medium access control and lower layers do not provide effective
mechanisms to avoid such collisions. Where these layers do provide
effective mechanisms, the approach of this document is not needed.
The approach described in this document uses random variations in
timing to achieve a reduction in collisions. Alternatives using, for
example, pseudo-random variation based on node identity, may be
considered, but are not discussed by this document.
Any protocol based on [4] and using the message forwarding mechanism
facilitated by that structure is a particular candidate for
application of at least some of these mechanisms.
The document has been generalized from the jitter mechanism used in
the proactive MANET routing protocol OLSR (The Optimized Link State
Routing Protocol) [2].
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4. Protocol Overview and Functioning
This document does not specify a protocol, nor does it mandate
specific node or protocol behavior. Rather, it outlines mechanisms
for message transmission (and retransmission) applicable in MANET
routing protocols and other protocols employing a periodic or
triggered message schedule and running over wireless interfaces where
simultaneous transmissions from two (or more) adjacent nodes causes
delays, packet losses and other problems. Any protocol using jitter
as outlined here must specify its precise usage insofar as is
necessary for interoperability.
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5. Jitter
In order to prevent nodes in a MANET from simultaneous transmission,
whilst retaining the MANET characteristic of maximum node autonomy, a
randomization of the transmission time of packets by nodes, known as
jitter, MAY be employed. Three jitter mechanisms, which target
different aspects of this problem, MAY be employed, with the aim of
reducing the likelihood of simultaneous transmission, and, if it
occurs, preventing it from continuing.
Three cases exist:
o Periodic message generation;
o Externally triggered message generation;
o Message forwarding.
Each of these cases uses a parameter, denoted MAXJITTER, for the
maximum timing variation that it introduces. If more than one of
these cases is used by a protocol, it MAY use the same or a different
value of MAXJITTER for each case. It also MAY use the same or
different values of MAXJITTER according to message type, and under
different circumstances - in particular if other parameters (such as
message interval) vary.
Issues relating to the value of MAXJITTER are considered in
Section 5.4.
5.1. Periodic Message Generation
When a node generates a message periodically, two successive messages
will be separated by a well-defined interval, denoted
MESSAGE_INTERVAL. A node MAY maintain more than one such interval,
e.g. for different message types or in different circumstances (such
as backing off transmissions to avoid congestion). Jitter MAY be
applied by reducing this delay by a random amount, so that the delay
between consecutive transmissions of messages of the same type is
equal to (MESSAGE_INTERVAL - jitter), where jitter is the random
value.
Subtraction of the random value from the message interval ensures
that the message interval never exceeds MESSAGE_INTERVAL, and does
not adversely affect timeouts or other mechanisms which may be based
on message late arrival or failure to arrive. By basing the message
transmission time on the previous transmission time, rather than by
jittering a fixed clock, nodes can become completely desynchronized,
which minimizes their probability of repeated collisions. This is
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particularly useful when combined with externally triggered message
generation and rescheduling.
The jitter value SHOULD be taken from a uniform distribution between
zero and MAXJITTER.
Note that a node will know its own MESSAGE_INTERVAL value and can
readily ensure that any MAXJITTER value used satisfies the conditions
in Section 5.4.
5.2. Externally Triggered Message Generation
An internal or external condition or event MAY trigger message
generation by a node. Depending upon the protocol, this condition
MAY trigger generation of a single message, initiation of a new
periodic message schedule, or rescheduling of existing periodic
messaging. Collision between externally triggered messages is made
more likely if more than one node is likely to respond to the same
event. To reduce this likelihood, an externally triggered message
MAY be jittered by delaying it by a random duration; an internally
triggered message MAY also be so jittered if appropriate. This delay
SHOULD be generated uniformly in an interval between zero and
MAXJITTER. If periodically transmitted messages are rescheduled,
then this SHOULD be based on this delayed time, with subsequent
messages treated as described in Section 5.1.
When messages are triggered, whether or not they are also
periodically transmitted, a protocol MAY impose a minimum interval
between messages of the same type, denoted MESSAGE_MIN_INTERVAL. It
is however appropriate to also allow this interval to be reduced by
jitter, so that when a message is transmitted the next message is
allowed after a time (MESSAGE_MIN_INTERVAL - jitter), where jitter
SHOULD be generated uniformly in an interval between zero and
MAXJITTER (using a value of MAXJITTER appropriate to periodic message
transmission). This is because otherwise, when external triggers are
more frequent than MESSAGE_MIN_INTERVAL, it takes the role of
MESSAGE_INTERVAL and the arguments applying to jittering of the
latter also apply to the former. This also permits
MESSAGE_MIN_INTERVAL to equal MESSAGE_INTERVAL even when jitter is
used.
When a triggered message is delayed by jitter, the node MAY also
postpone generation of the triggered message. If a node is then
triggered to generate a message of the same type while waiting, it
can generate a single message. If however the node generates a
message when it is triggered, and then receives a another trigger
while waiting to send that message then the appropriate action to
take is protocol specific (typically to discard the earlier message
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or to transmit both, possibly modifying timing to maintain message
order).
5.3. Message Forwarding
When a node forwards a message, it may be jittered by delaying it by
a random duration. This delay SHOULD be generated uniformly in an
interval between zero and MAXJITTER.
Unlike the cases of periodically generated and externally triggered
messages, a node is not automatically aware of the message
originator's value of MESSAGE_INTERVAL, which is required to select a
value of MAXJITTER which is known to be valid. This may require
prior agreement as to the value (or minimum value) of
MESSAGE_INTERVAL, may be by inclusion in the message of
MESSAGE_INTERVAL (the time until the next relevant message, rather
than the time since the last message) or be by any other protocol
specific mechanism, which may include estimation of the value of
MESSAGE_INTERVAL based on received message times.
For several possible reasons (differing parameters, message
rescheduling, extreme random values) a node may receive a message
while still waiting to forward an earlier message of the same type
originating from the same node. This is possible without jitter, but
may occur more often with it. The appropriate action to take is
protocol specific (typically to discard the earlier message or to
forward both, possible modifying timing to maintain message order).
In many cases, including [2] and protocols using the full
functionality of [4], messages are transmitted hop by hop in
potentially multi-message packets, and some or all of those messages
may need to be forwarded. For efficiency this should be in a single
packet, and hence the forwarding jitter of all messages received in a
single packet should be the same. (This also requires that a single
value of MAXJITTER is used in this case.) For this to have the
intended uniform distribution it is necessary to choose a single
random jitter for all messages. It is not appropriate to give each
message a random jitter and then to use the smallest of these jitter
values, as that produces a jitter with a non-uniform distribution and
a reduced mean value.
In addition, the protocol may permit messages received in different
packets to be combined, possibly also with locally generated messages
(periodically generated or triggered). However in this case the
purpose of the jitter will be accomplished by choosing any of the
independently scheduled times for these events as the single
forwarding time; this may have to be the earliest time to achieve all
constraints. This is because without combining messages, a
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transmission was due at this time anyway.
5.4. Maximum Jitter Determination
In considering how the maximum jitter (one or more instances of
parameter MAXJITTER) may be determined, the following points may be
noted:
o While jitter may resolve the problem of simultaneous
transmissions, the timing changes (in particular the delays) it
introduces will otherwise typically have a negative impact on a
well-designed protocol. Thus MAXJITTER should always be
minimized, subject to acceptably achieving its intent.
o When messages are periodically generated, all of the following
that are relevant apply to each instance of MAXJITTER:
* it MUST NOT be negative;
* it MUST NOT be greater than MESSAGE_INTERVAL/2;
* it SHOULD be significantly less than MESSAGE_INTERVAL;
* it MUST NOT be greater than MESSAGE_MIN_INTERVAL;
* it SHOULD NOT be greater than MESSAGE_MIN_INTERVAL/2.
o As well as the decision as to whether to use jitter being
dependent on the medium access control and lower layers, the
selection of the MAXJITTER parameter should be appropriate to
those mechanisms.
o As jitter is intended to reduce collisions, greater jitter, i.e.
an increased value of MAXJITTER, is appropriate when the chance of
collisions is greater. This is particularly the case with
increased node density, where node density should be considered
relative to (the square of) the interference range rather than
useful signal range.
o The choice of MAXJITTER used when forwarding messages may also
take into account the expected number of times that the message
may be sequentially forwarded, up to the network diameter in hops.
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6. IANA Considerations
This document presents no IANA considerations.
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7. Security Considerations
This document does not specify any security considerations.
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8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
8.2. Informative References
[2] Clausen, T. and P. Jacquet, "The Optimized Link State Routing
Protocol", RFC 3626, October 2003.
[3] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-Demand
Distance Vector (AODV) Routing", RFC 3561, July 2003.
[4] Clausen, T., Dearlove, C., Dean, J., and C. Adjih, "Generalized
MANET Packet/Message Format", Work In
Progress draft-ietf-manet-packetbb-07.txt, June 2007.
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Appendix A. Acknowledgements
The authors would like to acknowledge the MANET working group and the
OLSRv2 Design team, in particular Joe Macker and Justin Dean (both
NRL), for their contributions and discussions in developing and
testing the concepts retained in this document, and Alan Cullen (BAE
Systems) for his careful review of this specification. OLSRv1, as
specified in [2], introduced the concept of jitter on control
traffic, which was tested thoroughly by Gitte Hansen and Lars
Christensen (then, both Aalborg University).
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Authors' Addresses
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
Email: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/
Christopher Dearlove
BAE Systems Advanced Technology Centre
Phone: +44 1245 242194
Email: chris.dearlove@baesystems.com
URI: http://www.baesystems.com/
Brian Adamson
Naval Research Laboratory
Email: adamson@itd.nrl.navy.mil
URI: http://www.nrl.navy.mil/
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