Network Working Group                                         N. Khademi
Internet-Draft                                                  M. Welzl
Intended status: Experimental                         University of Oslo
Expires: September 21, 2018                                  G. Armitage
                                      Swinburne University of Technology
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                          March 20, 2018

                 TCP Alternative Backoff with ECN (ABE)


   Active Queue Management (AQM) mechanisms allow for burst tolerance
   while enforcing short queues to minimise the time that packets spend
   enqueued at a bottleneck.  This can cause noticeable performance
   degradation for TCP connections traversing such a bottleneck,
   especially if there are only a few flows or their bandwidth-delay-
   product is large.  An Explicit Congestion Notification (ECN) signal
   indicates that an AQM mechanism is used at the bottleneck, and
   therefore the bottleneck network queue is likely to be short.  This
   document therefore proposes an update to RFC3168, which changes the
   TCP sender-side ECN reaction in congestion avoidance to reduce the
   Congestion Window (cwnd) by a smaller amount than the congestion
   control algorithm's reaction to inferred packet loss.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 21, 2018.

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Copyright Notice

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Why Use ECN to Vary the Degree of Backoff?  . . . . . . .   4
     4.2.  Focus on ECN as Defined in RFC3168  . . . . . . . . . . .   5
     4.3.  Choice of ABE Multiplier  . . . . . . . . . . . . . . . .   5
   5.  ABE Deployment Requirements . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Revision Information  . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Explicit Congestion Notification (ECN) [RFC3168] makes it possible
   for an Active Queue Management (AQM) mechanism to signal the presence
   of incipient congestion without incurring packet loss.  This lets the
   network deliver some packets to an application that would have been
   dropped if the application or transport did not support ECN.  This
   packet loss reduction is the most obvious benefit of ECN, but it is
   often relatively modest.  Other benefits of deploying ECN have been
   documented in RFC8087 [RFC8087].

   The rules for ECN were originally written to be very conservative,
   and required the congestion control algorithms of ECN-Capable

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   transport protocols to treat ECN congestion signals exactly the same
   as they would treat an inferred packet loss [RFC3168].

   Research has demonstrated the benefits of reducing network delays
   that are caused by interaction of loss-based TCP congestion control
   and excessive buffering [BUFFERBLOAT].  This has led to the creation
   of new AQM mechanisms like PIE [RFC8033] and CoDel
   [CODEL2012][RFC8289], which prevent bloated queues that are common
   with unmanaged and excessively large buffers deployed across the
   Internet [BUFFERBLOAT].

   The AQM mechanisms mentioned above aim to keep a sustained queue
   short while tolerating transient (short-term) packet bursts.
   However, currently used loss-based congestion control mechanisms
   cannot always utilise a bottleneck link well where there are short
   queues.  For example, a TCP sender must be able to store at least an
   end-to-end bandwidth-delay product (BDP) worth of data at the
   bottleneck buffer if it is to maintain full path utilisation in the
   face of loss-induced reduction of cwnd [RFC5681], which effectively
   doubles the amount of data that can be in flight, the maximum round-
   trip time (RTT) experience, and the path's effective RTT using the
   network path.

   Modern AQM mechanisms can use ECN to signal the early signs of
   impending queue buildup long before a tail-drop queue would be forced
   to resort to dropping packets.  It is therefore appropriate for the
   transport protocol congestion control algorithm to have a more
   measured response when an early-warning signal of congestion is
   received in the form of an ECN CE-marked packet.  Recognizing these
   changes in modern AQM practices, more recent rules have relaxed the
   strict requirement that ECN signals be treated identically to
   inferred packet loss [RFC8311].  Following these newer, more flexible
   rules, this document defines a new sender-side-only congestion
   control response, called "ABE" (Alternative Backoff with ECN).  ABE
   improves TCP's average throughput when routers use AQM controlled
   buffers that allow for short queues only.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Specification

   This specification updates the congestion control algorithm of an
   ECN-Capable TCP transport protocol by changing the TCP sender
   response to feedback from the TCP receiver that indicates reception

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   of a CE-marked packet, i.e., receipt of a packet with the ECN-Echo
   flag (defined in [RFC3168]) set.

   It updates the following text in section 6.1.2 of the ECN
   specification [RFC3168] :

      The indication of congestion should be treated just as a
      congestion loss in non-ECN-Capable TCP.  That is, the TCP source
      halves the congestion window "cwnd" and reduces the slow start
      threshold "ssthresh".

   Replacing this with:

      Receipt of a packet with the ECN-Echo flag SHOULD trigger the TCP
      source to set the slow start threshold (ssthresh) to 0.8 times the
      FlightSize, with a lower bound of 2 * SMSS applied to the result.
      As in [RFC5681], the TCP sender also reduces the cwnd value to no
      more than the new ssthresh value.  RFC 3168 section 6.1.2 provides
      guidance on setting a cwnd less than 2 * SMSS.

4.  Discussion

   Much of the technical background to ABE can be found in a research
   paper [ABE2017].  This paper used a mix of experiments, theory and
   simulations with NewReno [RFC5681] and CUBIC [RFC8312] to evaluate
   the technique.  The technique was shown to present "...significant
   performance gains in lightly-multiplexed [few concurrent flows]
   scenarios, without losing the delay-reduction benefits of deploying
   CoDel or PIE".  The performance improvement is achieved when reacting
   to ECN-Echo in congestion avoidance (when ssthresh > cwnd) by
   multiplying cwnd and ssthresh with a value in the range [0.7,0.85].
   Applying ABE when cwnd <= ssthresh is not currently recommended, but
   may benefit from additional attention, experimentation and

4.1.  Why Use ECN to Vary the Degree of Backoff?

   AQM mechanisms such as CoDel [RFC8289] and PIE [RFC8033] set a delay
   target in routers and use congestion notifications to constrain the
   queuing delays experienced by packets, rather than in response to
   impending or actual bottleneck buffer exhaustion.  With current
   default delay targets, CoDel and PIE both effectively emulate a
   bottleneck with a short queue (section II, [ABE2017]) while also
   allowing short traffic bursts into the queue.  This provides
   acceptable performance for TCP connections over a path with a low
   BDP, or in highly multiplexed scenarios (many concurrent transport
   flows).  However, in a lightly-multiplexed case over a path with a

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   large BDP, conventional TCP backoff leads to gaps in packet
   transmission and under-utilisation of the path.

   Instead of discarding packets, an AQM mechanism is allowed to mark
   ECN-Capable packets with an ECN CE-mark.  The reception of a CE-mark
   feedback not only indicates congestion on the network path, it also
   indicates that an AQM mechanism exists at the bottleneck along the
   path, and hence the CE-mark likely came from a bottleneck with a
   controlled short queue.  Reacting differently to an ECN-signalled
   congestion than to an inferred packet loss can then yield the benefit
   of a reduced back-off when queues are short.  Using ECN can also be
   advantageous for several other reasons [RFC8087].

   The idea of reacting differently to inferred packet loss and
   detection of an ECN-signalled congestion pre-dates this document.
   For example, previous research proposed using ECN CE-marked feedback
   to modify TCP congestion control behaviour via a larger
   multiplicative decrease factor in conjunction with a smaller additive
   increase factor [ICC2002].  The goal of this former work was to
   operate across AQM bottlenecks using Random Early Detection (RED)
   that were not necessarily configured to emulate a short queue (The
   current usage of RED as an Internet AQM method is limited [RFC7567]).

4.2.  Focus on ECN as Defined in RFC3168

   Some transport protocol mechanisms rely on ECN semantics that differ
   from the original ECN definition [RFC3168].  For instance, Accurate
   ECN [I-D.ietf-tcpm-accurate-ecn] permits more frequent and detailed
   feedback.  Use of such mechanisms (including Accurate ECN, Datacenter
   TCP (DCTCP) [RFC8257], or Congestion Exposure (ConEx) [RFC7713]) is
   out of scope for this document.  This specification focuses on ECN as
   defined in [RFC3168].

4.3.  Choice of ABE Multiplier

   ABE decouples the reaction of a TCP sender to inferred packet loss
   and ECN-signalled congestion in the congestion avoidance phase.  To
   achieve this, ABE uses a different scaling factor in Equation 4 in
   Section 3.1 of [RFC5681].  The description respectively uses
   beta_{loss} and beta_{ecn} to refer to the multiplicative decrease
   factors applied in response to inferred packet loss, and in response
   to a receiver indicating ECN-signalled congestion.  For non-ECN-
   enabled TCP connections, only beta_{loss} applies.

   In other words, in response to inferred packet loss:

      ssthresh = max (FlightSize * beta_{loss}, 2 * SMSS)

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   and in response to an indication of an ECN-signalled congestion:

      ssthresh = max (FlightSize * beta_{ecn}, 2 * SMSS)


      cwnd = ssthresh

      (If ssthresh == 2 * SMSS, RFC 3168 section 6.1.2 provides guidance
      on setting a cwnd lower than 2 * SMSS.)

   where FlightSize is the amount of outstanding data in the network,
   upper-bounded by the smaller of the sender's cwnd and the receiver's
   advertised window (rwnd) [RFC5681].  The higher the values of
   beta_{loss} and beta_{ecn}, the less aggressive the response of any
   individual backoff event.

   The appropriate choice for beta_{loss} and beta_{ecn} values is a
   balancing act between path utilisation and draining the bottleneck
   queue.  More aggressive backoff (smaller beta_*) risks underutilising
   the path, while less aggressive backoff (larger beta_*) can result in
   slower draining of the bottleneck queue.

   The Internet has already been running with at least two different
   beta_{loss} values for several years: the standard value is 0.5
   [RFC5681], and the Linux implementation of CUBIC [RFC8312] has used a
   multiplier of 0.7 since kernel version 2.6.25 released in 2008.  ABE
   proposes no change to beta_{loss} used by current TCP

   The recommendation in Section 3 in this document corresponds to a
   value of beta_{ecn}=0.8.  This recommended beta_{ecn} value is only
   applicable for the standard TCP congestion control [RFC5681].  The
   selection of beta_{ecn} enables tuning the response of a TCP
   connection to shallow AQM marking thresholds.  beta_{loss}
   characterizes the response of a congestion control algorithm to
   packet loss, i.e., exhaustion of buffers (of unknown depth).
   Different values for beta_{loss} have been suggested for TCP
   congestion control algorithms.  Consequently, beta_{ecn} is likely to
   be an algorithm-specific parameter rather than a constant multiple of
   the algorithm's existing beta_{loss}.

   A range of tests (section IV, [ABE2017]) with NewReno and CUBIC over
   CoDel and PIE in lightly-multiplexed scenarios have explored this
   choice of parameter.  The results of these tests indicate that CUBIC
   connections benefit from beta_{ecn} of 0.85 (cf.  beta_{loss} = 0.7),

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   and NewReno connections see improvements with beta_{ecn} in the range
   0.7 to 0.85 (cf. beta_{loss} = 0.5).

5.  ABE Deployment Requirements

   This update is a sender-side only change.  Like other changes to
   congestion control algorithms, it does not require any change to the
   TCP receiver or to network devices.  It does not require any ABE-
   specific changes in routers or the use of Accurate ECN feedback
   [I-D.ietf-tcpm-accurate-ecn] by a receiver.

   RFC3168 states that the congestion control response to an ECN-
   signalled congestion is the same as the response to a dropped packet
   [RFC3168].  [RFC8311] updates this specification to allow systems to
   provide a different behaviour when they experience ECN-signalled
   congestion rather than packet loss.  The present specification
   defines such an experiment and has thus been assigned an Experimental
   status before being proposed as a Standards-Track update.

   The purpose of the Internet experiment is to collect experience with
   deployment of ABE, and confirm the safety in deployed networks using
   this update to TCP congestion control.

   When used with bottlenecks that do not support ECN-marking the
   specification does not modify the transport protocol.

   To evaluate the benefit, this experiment therefore requires support
   in AQM routers for ECN-marking of packets carrying the ECN-Capable
   Transport, ECT(0), codepoint [RFC3168].

   If the method is only deployed by some senders, and not by others,
   the senders that use this method can gain some advantage, possibly at
   the expense of other flows that do not use this updated method.
   Because this advantage applies only to ECN-marked packets and not to
   packet loss indications, an ECN-Capable bottleneck will still fall
   back to dropping packets if an TCP sender using ABE is too
   aggressive, and the result is no different than if the TCP sender was
   using traditional loss-based congestion control.

   A TCP sender reacts to loss or ECN marks only once per round-trip
   time.  Hence, if a sender would first be notified of an ECN mark and
   then learn about loss in the same round-trip, it would only react to
   the first notification (ECN) but not to the second (loss).  RFC3168
   specified a reaction to ECN that was equal to the reaction to loss

   ABE also responds to congestion once per RTT, and therefore it does
   not respond to further loss within the same RTT, since ABE has

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   already reduced the congestion window.  If congestion persists after
   such reduction, ABE continues to reduce the congestion window in each
   consecutive RTT.  This consecutive reduction can protect the network
   against long-standing unfairness in the case of AQM algorithms that
   do not keep a small average queue length.

   The result of this Internet experiment ought to include an
   investigation of the implications of experiencing an ECN-CE mark
   followed by loss within the same RTT.  At the end of the experiment,
   this will be reported to the TCPM WG (or IESG).

6.  Acknowledgements

   Authors N.  Khademi, M.  Welzl and G.  Fairhurst were part-funded by
   the European Community under its Seventh Framework Programme through
   the Reducing Internet Transport Latency (RITE) project (ICT-317700).
   The views expressed are solely those of the authors.

   The authors would like to thank Stuart Cheshire for many suggestions
   when revising the draft, and the following people for their
   contributions to [ABE2017]: Chamil Kulatunga, David Ros, Stein
   Gjessing, Sebastian Zander.  Thanks also to (in alphabetical order)
   Roland Bless, Bob Briscoe, David Black, Markku Kojo, John Leslie,
   Lawrence Stewart, Dave Taht and the TCPM working group for providing
   valuable feedback on this document.

   The authors would finally like to thank everyone who provided
   feedback on the congestion control behaviour specified in this update
   received from the IRTF Internet Congestion Control Research Group

7.  IANA Considerations


   This document includes no request to IANA.

8.  Implementation Status

   ABE is implemented as a patch for Linux and FreeBSD.  It is meant for
   research and available for download from This code was used to
   produce the test results that are reported in [ABE2017].  The FreeBSD
   code has been committed to the mainline kernel on March 19, 2018

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9.  Security Considerations

   The described method is a sender-side only transport change, and does
   not change the protocol messages exchanged.  The security
   considerations for ECN [RFC3168] therefore still apply.

   This is a change to TCP congestion control with ECN that will
   typically lead to a change in the capacity achieved when flows share
   a network bottleneck.  This could result in some flows receiving more
   than their fair share of capacity.  Similar unfairness in the way
   that capacity is shared is also exhibited by other congestion control
   mechanisms that have been in use in the Internet for many years
   (e.g., CUBIC [RFC8312]).  Unfairness may also be a result of other
   factors, including the round trip time experienced by a flow.  ABE
   applies only when ECN-marked packets are received, not when packets
   are lost, hence use of ABE cannot lead to congestion collapse.

10.  Revision Information


   -07.  Addressed comments following WGLC.

   o  Updated Reference citations

   o  Removed paragraph containing a wrong statement related to timeout
      in section 4.1.

   o  Discuss what happens when cwnd <= ssthresh

   o  Added text on Concern about lower bound of 2*SMSS

   -06.  Addressed Michael Scharf's comments.

   -05.  Refined the description of the experiment based on feedback at
   IETF-100.  Incorporated comments from David Black.

   -04.  Incorporates review comments from Lawrence Stewart and the
   remaining comments from Roland Bless.  References are updated.

   -03.  Several review comments from Roland Bless are addressed.
   Consistent terminology and equations.  Clarification on the scope of
   recommended beta_{ecn} value.

   -02.  Corrected the equations in Section 4.3.  Updated the
   affiliations.  Lower bound for cwnd is defined.  A recommendation for
   window-based transport protocols is changed to cover all transport
   protocols that implement a congestion control reduction to an ECN

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   congestion signal.  Added text about ABE's FreeBSD mainline kernel
   status including a reference to the FreeBSD code review page.
   References are updated.

   -01.  Text improved, mainly incorporating comments from Stuart
   Cheshire.  The reference to a technical report has been updated to a
   published version of the tests [ABE2017].  Used "AQM Mechanism"
   throughout in place of other alternatives, and more consistent use of
   technical language and clarification on the intended purpose of the
   experiments required by EXP status.  There was no change to the
   technical content.

   -00. draft-ietf-tcpm-alternativebackoff-ecn-00 replaces draft-
   khademi-tcpm-alternativebackoff-ecn-01.  Text describing the nature
   of the experiment was added.

   Individual draft -01.  This I-D now refers to draft-black-tsvwg-ecn-
   experimentation-02, which replaces draft-khademi-tsvwg-ecn-
   response-00 to make a broader update to RFC3168 for the sake of
   allowing experiments.  As a result, some of the motivating and
   discussing text that was moved from draft-khademi-alternativebackoff-
   ecn-03 to draft-khademi-tsvwg-ecn-response-00 has now been re-
   inserted here.

   Individual draft -00. draft-khademi-tsvwg-ecn-response-00 and draft-
   khademi-tcpm-alternativebackoff-ecn-00 replace draft-khademi-
   alternativebackoff-ecn-03, following discussion in the TSVWG and TCPM
   working groups.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

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   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,

11.2.  Informative References

              "ABE patch review in FreeBSD",

   [ABE2017]  Khademi, N., Armitage, G., Welzl, M., Fairhurst, G.,
              Zander, S., and D. Ros, "Alternative Backoff: Achieving
              Low Latency and High Throughput with ECN and AQM", IFIP
              NETWORKING 2017, Stockholm, Sweden, June 2017.

              Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
              the Internet", November 2011.

              Nichols, K. and V. Jacobson, "Controlling Queue Delay",
              July 2012, <>.

              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-06 (work in progress), March 2018.

   [ICC2002]  Kwon, M. and S. Fahmy, "TCP Increase/Decrease Behavior
              with Explicit Congestion Notification (ECN)", IEEE
              ICC 2002, New York, New York, USA, May 2002,

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   [RFC7713]  Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,

   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,

   [RFC8289]  Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
              Iyengar, Ed., "Controlled Delay Active Queue Management",
              RFC 8289, DOI 10.17487/RFC8289, January 2018,

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,

Authors' Addresses

   Naeem Khademi
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316


   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316


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   Grenville Armitage
   Internet For Things (I4T) Research Group
   Swinburne University of Technology
   PO Box 218
   John Street, Hawthorn
   Victoria  3122


   Godred Fairhurst
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen  AB24 3UE


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