One document matched: draft-clausen-manet-jitter-00.txt
Mobile Ad hoc Networking (MANET) T. Clausen
Internet-Draft LIX, Ecole Polytechnique, France
Expires: August 6, 2007 C. Dearlove
BAE Systems Advanced Technology
Centre
February 2, 2007
Jitter considerations in MANETs
draft-clausen-manet-jitter-00
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Abstract
This document describes considerations regarding jittering of control
traffic transmissions in MANET routing protocols.
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 . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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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 over a received message over one or more
MANET interfaces of the node.
Collision - A specific instance of interference, where two nodes
both transmit a packet at the same time. A third node receives
these two transmissions, and experiences a "collision", causing it
to lose either or both transmissions.
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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.
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. Note that while jitter may resolve the
problem of simultaneous transmissions, the delays it introduces will
otherwise only have a negative impact on a well-designed protocol.
Thus jitter parameters should always be minimized, subject to their
acceptably achieving their intent. 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.
5.1. Periodic message generation
When a node generates a message periodically, two successive messages
will be separated by a well-defined interval, denoted here
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 a 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 the nominal message interval,
and does not adversely affect timeouts or other mechanisms which may
be based on message late arrival or failure to arrive. Note that 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
collisions.
It is appropriate and convenient for the jitter value to be taken
from a uniform distribution between zero and a maximum value, denoted
here MAXJITTER. MAXJITTER must be significantly less than the
current value of MESSAGE_INTERVAL. MAXJITTER may be a single fixed
parameter (in which case it must be significantly less than all
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values of MESSAGE_INTERVAL) or be based on MESSAGE_INTERVAL (for
example it may be a fixed proportion of MESSAGE_INTERVAL).
Note that a node will know its own MESSAGE_INTERVAL value and can
readily ensure that any MAXJITTER value used is appropriate.
5.2. Externally triggered message generation
When a node responds to an externally triggered change in
circumstances which is likely to also affect other nodes by
generating a message, that message may be jittered by delaying it by
a random duration. If this message is of a type which is
periodically transmitted then it may be appropriate to restart its
schedule of these messages, this should be based on this delayed
time. In some cases there may be a minimum interval between such
messages, in this case it may be appropriate to jitter that minimum
interval time.
The normal delay on a triggered message may be generated uniformly in
an interval between zero and a maximum delay, denoted here MAXJITTER.
Selection of MAXJITTER will be protocol specific. In some cases the
delay may be fixed, or fixed according to the message type. In the
case of a regularly scheduled message, at an interval denoted here
MESSAGE_INTERVAL, MAXJITTER must be significantly less than
MESSAGE_INTERVAL. This may require prior agreement as to the value
(or minimum value) of MESSAGE_INTERVAL, be by inclusion of
MESSAGE_INTERVAL (the time until the next relevant message, rather
than the time since the last) in the message, or use any other
protocol specific mechanism.
5.3. Message forwarding
When a node forwards a message it may be jittered by delaying it by a
random time. The normal delay on a triggered message may be
generated uniformly in an interval between zero and a maximum delay,
denoted here MAXJITTER. The value of MAXJITTER will be protocol
specific and may in some cases be fixed, possibly by message type.
However in the case of a regularly scheduled message, at an interval
denoted here MESSAGE_INTERVAL, MAXJITTER must be significantly less
than MESSAGE_INTERVAL. 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. The choice of MAXJITTER may also
take into account the expected number of times that the message may
be forwarded.
For several possible reasons (differing parameters, message
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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. For this to have the intended
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 using the smallest of these jitter values, as that produces
a jitter with a reduced mean value.
In addition, the protocol may permit messages received in different
packets to be combined, possibly also with locally generated messages
(scheduled 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 transmission was due at this
time anyway.
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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., Dean, J., Dearlove, C., and C. Adjih, "Generalized
MANET Packet/Message Format", Work In
Progress draft-ietf-manet-packetbb-03.txt, January 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 Brian Adamsson and Justin Dean
(both NRL), for their contributions and discussions in developing and
testing the concepts retained in this document. OLSRv1, as specified
in [2] introduced the concept of jitter on control traffic in OLSR,
which was tested throughly in the context of OLSRv1 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.lix.polytechnique.fr/Labo/Thomas.Clausen/
Christopher M. Dearlove
BAE Systems Advanced Technology Centre
Phone: +44 1245 242194
Email: chris.dearlove@baesystems.com
URI: http://www.baesystems.com/ocs/sharedservices/atc/
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