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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: October 22, 2007                BAE Systems Advanced Technology
                                                                  Centre
                                                              B. Adamson
                                               Naval Research Laboratory
                                                          April 20, 2007


                    Jitter considerations in MANETs
                       draft-ietf-manet-jitter-00

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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., Dean, J., Dearlove, C., and C. Adjih, "Generalized
        MANET Packet/Message Format", Work In
        Progress draft-ietf-manet-packetbb-04.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 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 M. 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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