





Network Working Group                                         T. Clausen
Request for Comments: 5148              LIX, Ecole Polytechnique, France
Category: Informational                                      C. Dearlove
                                  BAE Systems Advanced Technology Centre
                                                              B. Adamson
                                          U.S. Naval Research Laboratory
                                                           February 2008


        Jitter Considerations in Mobile Ad Hoc Networks (MANETs)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document provides recommendations for jittering (randomly
   modifying timing) of control traffic transmissions in Mobile Ad hoc
   NETwork (MANET) routing protocols to reduce the probability of
   transmission collisions.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Applicability Statement .........................................4
   4. Protocol Overview and Functioning ...............................4
   5. Jitter ..........................................................5
      5.1. Periodic Message Generation ................................5
      5.2. Externally Triggered Message Generation ....................6
      5.3. Message Forwarding .........................................7
      5.4. Maximum Jitter Determination ...............................8
   6. Security Considerations .........................................9
   7. References .....................................................10
      7.1. Normative References ......................................10
      7.2. Informative References ....................................10
   Appendix A. Acknowledgements ......................................11











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1.  Introduction

   In a wireless network, simultaneous packet transmission by nearby
   nodes is often undesirable.  This is because any resulting collision
   between these packets may cause a receiving node to fail to receive
   some or all of these packets.  This is a physical problem, which
   occurs before packets can be inserted into the receiver queue.
   Depending on the characteristics of the medium access control and
   other lower layer mechanisms, in particular whether retransmission of
   unacknowledged packets is supported, this may cause at best increased
   delay, and at worst complete packet loss.  In some instances, these
   problems can be solved in these lower layers, but in other instances,
   some help at the network and higher layers is necessary.

   This document considers the case when that help is required, and
   provides recommendations for using jitter (randomly varying timing)
   to provide it.  It is possible that the techniques described here
   could be implemented either by IP protocols designed for wireless
   networks or in conjunction with lower-layer mechanisms.

   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.  Note that the following mechanisms may
      make this a likely occurrence.

   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 that respond to the same change will transmit messages
      simultaneously.

   Schedule reset - When a node sends an event-triggered message of a
      type that 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.





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   A possible solution to these problems is to employ jitter, a
   deliberate random variation in timing.  Such jitter is employed in
   e.g., [2], [3], and [4], in which transmission intervals for
   regularly scheduled messages are reduced by a small, bounded and
   random amount in order to desynchronize transmitters and thereby
   avoid overloading the transmission medium as well as receivers.  This
   document discusses and provides recommendations for applying jitter
   to control packet transmissions in Mobile Ad hoc NETworks (MANETs),
   with the purpose of avoiding collisions, with particular reference to
   the features listed above.

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 that 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 (rather than a received and
      forwarded 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 (whether modified
      or unchanged) 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




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      another, closely located, node that should receive and decode
      these packets instead fails to do so, and loses one or more of the
      packets.

3.  Applicability Statement

   The mechanisms described in this document are applicable to the
   control messages of any MANET protocol in which simultaneous
   transmissions by different nodes are undesirable, and that contains
   mechanisms, such as periodic control message transmission, triggered
   control message transmission, or control message forwarding, which
   either make a simultaneous transmission more likely, or cause one to
   be repeated when it occurs.  This particularly applies to protocols
   using broadcast transmissions in wireless networks, where proactive
   MANET routing protocols such as [5] employ scheduled messages, where
   reactive MANET routing protocols such as [6] 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 recommendations of this document are 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 [7] 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) [5].

4.  Protocol Overview and Functioning

   This document provides recommendations for message transmission (and
   retransmission) that may be used by MANET routing protocols.  It may
   also be used by other protocols that employ a periodic or triggered
   message schedule running over wireless interfaces.  Using such
   simultaneous message transmissions from two (or more) adjacent nodes
   may cause 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, SHOULD be employed.  Three jitter mechanisms, which target
   different aspects of this problem, SHOULD 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.

   For the first of these cases, jitter is used to reduce the interval
   between successive message transmission by a random amount; for the
   latter two cases, jitter is used to delay a message being generated
   or forwarded by a random amount.

   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 SHOULD 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



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   not adversely affect timeouts or other mechanisms that 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
   particularly useful when combined with externally triggered message
   generation and rescheduling.

   The jitter value SHOULD be generated uniformly in an interval 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 (including, but not
   limited to, an acknowledgement 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 SHOULD 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.  In
   the case that such an interval is not required, MESSAGE_MIN_INTERVAL
   is considered to be zero.  When MESSAGE_MIN_INTERVAL is non-zero, it
   is however appropriate to also allow this interval to be reduced by
   jitter.  Thus, when a message is transmitted, the next message is
   allowed after a time (MESSAGE_MIN_INTERVAL - jitter).  This jitter
   SHOULD be generated uniformly in an interval between zero and
   MAXJITTER (using a value of MAXJITTER appropriate to periodic message
   transmission).

   It might appear counterintuitive to have a defined
   MESSAGE_MIN_INTERVAL, yet allow this to be reduced by jittering.  For
   periodic messages, setting MESSAGE_INTERVAL, MAXJITTER and
   MESSAGE_MIN_INTERVAL such that (MESSAGE_INTERVAL-MAXJITTER) >



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   MESSAGE_MIN_INTERVAL would ensure at least MESSAGE_MIN_INTERVAL would
   elapse between two subsequent message transmissions.  In a highly
   dynamic network with triggered messages, however, external
   circumstances might be such that external triggers are more frequent
   than MESSAGE_MIN_INTERVAL, effectively making MESSAGE_MIN_INTERVAL
   take the role of MESSAGE_INTERVAL as the "default" interval at which
   messages are transmitted.  Thus, in order to avoid synchronization in
   this highly dynamic case, jittering SHOULD be applied to
   MESSAGE_MIN_INTERVAL.  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
   or to transmit both, possibly modifying timing to maintain message
   order).

5.3.  Message Forwarding

   When a node forwards a message, it SHOULD 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 that 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, possibly modifying timing to maintain message order).

   In many cases, including [5] and protocols using the full
   functionality of [7], messages are transmitted hop-by-hop in



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   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 control messages received in
   different packets to be combined, possibly also with locally
   generated control messages (periodically generated or triggered), as
   supported by [7].  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 would be 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 NOT be greater than MESSAGE_INTERVAL/4.

   o  If MESSAGE_MIN_INTERVAL > 0, then:

         *  MAXJITTER MUST NOT be greater than MESSAGE_MIN_INTERVAL;

         *  MAXJITTER SHOULD NOT be greater than MESSAGE_MIN_INTERVAL/2.




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   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.  For example, MAXJITTER should be significantly
      greater than (e.g., an order of magnitude greater than) any medium
      access control frame period.

   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, which is significant 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,
      so that the maximum accumulated delay is bounded.

6.  Security Considerations

   This document provides recommendations for mechanisms to be used in
   protocols; full security considerations are to be provided by those
   protocols, rather than in this document.

   It may however be noted that introduction of random timing by these
   recommendations may provide some security advantage to such a
   protocol in that it makes the prediction of transmission times, and
   thereby intentional interference with a protocol functioning through
   selectively scheduling jamming transmissions to coincide with
   protocol message transmissions, more difficult.





















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7.  References

7.1.  Normative References

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

7.2.  Informative References

   [2]  Moy, J., "OSPF Database Overflow", RFC 1765, March 1995.

   [3]  Marlow, D., "Host Group Extensions for CLNP Multicasting", RFC
        1768, March 1995.

   [4]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
        Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [5]  Clausen, T., Ed., and P. Jacquet, Ed., "Optimized Link State
        Routing Protocol (OLSR)", RFC 3626, October 2003.

   [6]  Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-Demand
        Distance Vector (AODV) Routing", RFC 3561, July 2003.

   [7]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih, "Generalized
        MANET Packet/Message Format", Work in Progress.


























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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 [5], introduced the concept of jitter on control
   traffic, which was tested thoroughly by Gitte Hansen and Lars
   Christensen (then, both Aalborg University).

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
   U.S. Naval Research Laboratory

   Phone: +1 202 404 1194
   EMail: adamson@itd.nrl.navy.mil
   URI:   http://www.nrl.navy.mil/
















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