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Versions: (draft-bormann-core-cocoa) 00 01 02 03

CoRE Working Group                                            C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                                A. Betzler
Expires: August 25, 2018                                  Fundacio i2CAT
                                                                C. Gomez
                                                             I. Demirkol
                     Universitat Politecnica de Catalunya/Fundacio i2CAT
                                                       February 21, 2018


                CoAP Simple Congestion Control/Advanced
                        draft-ietf-core-cocoa-03

Abstract

   CoAP, the Constrained Application Protocol, needs to be implemented
   in such a way that it does not cause persistent congestion on the
   network it uses.  The CoRE CoAP specification defines basic behavior
   that exhibits low risk of congestion with minimal implementation
   requirements.  It also leaves room for combining the base
   specification with advanced congestion control mechanisms with higher
   performance.

   This specification defines more advanced, but still simple CoRE
   Congestion Control mechanisms, called CoCoA.  The core of these
   mechanisms is a Retransmission TimeOut (RTO) algorithm that makes use
   of Round-Trip Time (RTT) estimates, in contrast with how the RTO is
   determined as per the base CoAP specification (RFC 7252).  The
   mechanisms defined in this document have relatively low complexity,
   yet they improve the default CoAP RTO algorithm.  The design of the
   mechanisms in this specification has made use of input from
   simulations and experiments in real networks.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on August 25, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Area of Applicability . . . . . . . . . . . . . . . . . . . .   4
   4.  Advanced CoAP Congestion Control: RTO Estimation  . . . . . .   5
     4.1.  Blind RTO Estimate  . . . . . . . . . . . . . . . . . . .   6
     4.2.  Measurement-based RTO Estimate  . . . . . . . . . . . . .   6
       4.2.1.  Differences with the algorithm of RFC 6298  . . . . .   7
       4.2.2.  Discussion  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Lifetime, Aging . . . . . . . . . . . . . . . . . . . . .   8
   5.  Advanced CoAP Congestion Control: Non-Confirmables  . . . . .   9
     5.1.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Supporting evidence  . . . . . . . . . . . . . . . .  11
     A.1.  Older versions of the draft and improvement . . . . . . .  12
     A.2.  References  . . . . . . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Pseudocode . . . . . . . . . . . . . . . . . . . . .  13
     B.1.  Updating the RTO estimator  . . . . . . . . . . . . . . .  13
     B.2.  RTO aging . . . . . . . . . . . . . . . . . . . . . . . .  14
     B.3.  Variable Backoff Factor . . . . . . . . . . . . . . . . .  14
   Appendix C.  Examples . . . . . . . . . . . . . . . . . . . . . .  15
     C.1.  Example A.1: weak RTTs  . . . . . . . . . . . . . . . . .  15
     C.2.  Example A.2: VBF and aging  . . . . . . . . . . . . . . .  15
     C.3.  Example B: VBF and aging  . . . . . . . . . . . . . . . .  16
   Appendix D.  Analysis: difference between strong and weak



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                estimators . . . . . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   CoAP, the Constrained Application Protocol, needs to be implemented
   in such a way that it does not cause persistent congestion on the
   network it uses.  The CoRE CoAP specification defines basic behavior
   that exhibits low risk of congestion with minimal implementation
   requirements.  It also leaves room for combining the base
   specification with advanced congestion control mechanisms with higher
   performance.

   The present specification defines such an advanced CoRE Congestion
   Control mechanism, with the goal of improving performance while
   retaining safety as well as the simplicity that is appropriate for
   constrained devices.  Hence, we are calling this mechanism Simple
   Congestion Control/Advanced, or CoCoA for short.

   CoCoA calculates the retransmission time-out (RTO) based on RTT
   estimations with and without loss.  By taking retransmissions (in a
   potentially lossy network) into account when estimating the RTT, this
   algorithm reacts to congestion with a lower sending rate.  For non-
   confirmable packets, it also limits the sending rate to 1/RTO;
   assuming that the RTO estimation in CoCoA works as expected, RTO
   should be slightly greater than the RTT, thus CoCoA would be more
   conservative than the original specification in [RFC7641].

   In the Internet, congestion control is typically implemented in a way
   that it can be introduced or upgraded unilaterally.  Still, a new
   congestion control scheme must not be introduced lightly.  To ensure
   that the new scheme is not posing a danger to the network,
   considerable work has been done on simulations and experiments in
   real networks.  Some of this work will be mentioned in "Discussion"
   subsections in the following sections; an overview is given in
   Appendix A.  Extended rationale for this specification can also be
   found in the historical Internet-Drafts
   [I-D.bormann-core-congestion-control] and
   [I-D.eggert-core-congestion-control], as well as in the minutes of
   the IETF 84 CoRE WG meetings.

1.1.  Terminology

   This specification uses terms from [RFC7252].  In addition, it
   defines the following terminology:





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   Initiator:  The endpoint that sends the message that initiates an
      exchange.  E.g., the party that sends a confirmable message, or a
      non-confirmable message (see Section 4.3 of [RFC7252]) conveying a
      request.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The term "byte", abbreviated by "B", is used in its now customary
   sense as a synonym for "octet".

2.  Context

   In the definition of the CoAP protocol [RFC7252], an approach was
   taken that includes a very simple basic scheme (lock-step with the
   number of parallel exchanges usually limited to 1) in the base
   specification together with performance-enhancing advanced
   mechanisms.

   The present specification is based on the approved text in the
   [RFC7252] base specification.  It is making use of the text that
   permits advanced congestion control mechanisms and allows them to
   change protocol parameters, including NSTART and the binary
   exponential backoff mechanism.  Note that Section 4.8 of [RFC7252]
   limits the leeway that implementations have in changing the CoRE
   protocol parameters.

   The present specification also assumes that, outside of exchanges,
   non-confirmable messages can only be used at a limited rate without
   an advanced congestion control mechanism (this is mainly relevant for
   [RFC7641]).  It is also intended to address the [RFC8085] guideline
   about combining congestion control state for a destination; and to
   clarify its meaning for CoAP using the definition of an endpoint.

   The present specification does not address multicast or dithering
   beyond basic retransmission dithering.

3.  Area of Applicability

   The present algorithm is intended to be generally applicable.  The
   objective is to be "better" than default CoAP congestion control in a
   number of characteristics, including achievable goodput for a given
   offered load, latency, and recovery from bursts, while providing more
   predictable stress to the network and the same level of safety from
   catastrophic congestion.  The algorithm defined in this document is



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   intended to adapt to the current characteristics of any underlying
   network, and therefore is well suited for a wide range of network
   conditions, in terms of bandwidth, latency, load, loss rate,
   topology, etc.  In particular, CoCoA has been found to perform well
   in scenarios with latencies ranging from the order of milliseconds to
   peaks of dozens of seconds, as well as in single-hop and multihop
   topologies.  Link technologies used in existing evaluation work
   comprise IEEE 802.15.4, GPRS, UMTS and Wi-Fi (see Appendix A).  CoCoA
   is also expected to work suitably across the general Internet.  The
   algorithm does require three state variables per scope plus the state
   needed to do RTT measurements, so it may not be applicable to the
   most constrained devices (say, class 1 as per [RFC7228]).

   The scope of each instance of the algorithm in the current set of
   evaluations has been the five-tuple, i.e., CoAP + endpoint (transport
   address) for Initiator and Responder.  Potential applicability to
   larger scopes needs to be examined.

4.  Advanced CoAP Congestion Control: RTO Estimation

   For an initiator that plans to make multiple requests to one
   destination endpoint, it may be worthwhile to make RTT measurements
   in order to compute a more appropriate RTO than the default initial
   timeout of 2 to 3 s.  In particular, a wide spectrum of RTT values is
   expected in different types of networks where CoAP is used.  Those
   RTTs range from several orders of magnitude below the default initial
   timeout to values larger than the default.  The algorithm defined in
   this document is based on the algorithm for RTO estimation defined in
   [RFC6298], with appropriately extended default/base values, as
   proposed in Section 4.2.1.  Note that such a mechanism must, during
   idle periods, decay RTO estimates that are shorter or longer than the
   default RTO estimate back to the default RTO estimate, until fresh
   measurements become available again, as proposed in Section 4.3.

   RTT variability challenges RTO estimation.  In TCP, delayed ACKs
   contribute to RTT variability, since this option adds a delay of up
   to 500 ms (typically, 200 ms) before an ACK is sent by a receiving
   TCP endpoint.  However, one important consideration not relevant for
   TCP is the fact that a CoAP round-trip may include application
   processing time, which may be hard to predict, and may differ between
   different resources available at the same endpoint.  Also, for
   communications with networks of constrained devices that apply radio
   duty cycling, large and variable round-trip times are likely to be
   observed.  Servers will only trigger their early ACKs (with a non-
   piggybacked response to be sent later) based on the default timers,
   e.g. after 1 s.  A client that has arrived at a RTO estimate shorter
   than 1 s SHOULD therefore use a larger backoff factor for
   retransmissions to avoid expending all of its retransmissions



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   (MAX_RETRANSMIT, see Section 4.2 of [RFC7252], normally 4) in the
   default interval of 2 to 3 s.  The approach chosen for a mechanism
   with variable backoff factors is presented in Section 4.2.1.

   It may also be worthwhile to perform RTT estimation not just based on
   information measured from a single destination endpoint, but also
   based on entire hosts (IP addresses) and/or complete prefixes (e.g.,
   maintain an RTT estimate for a whole /64).  The exact way this can be
   used to reduce the amount of state in an initiator is for further
   study.

4.1.  Blind RTO Estimate

   The initial RTO estimate for an endpoint is set to 2 seconds (the
   initial RTO estimate is used as the initial value for both E_weak_
   and E_strong_ below).

   If only the initial RTO estimate is available, the RTO estimate for
   each of up to NSTART exchanges started in parallel is set to 2 s
   times the number of parallel exchanges, e.g. if two exchanges are
   already running, the initial RTO estimate for an additional exchange
   is 6 seconds.

4.2.  Measurement-based RTO Estimate

   The RTO estimator runs two copies of the algorithm defined in
   [RFC6298], using the same variables and calculations to estimate the
   RTO, with the differences introduced in Section 4.2.1: One copy for
   exchanges that complete on initial transmissions (the "strong
   estimator", E_strong_), and one copy for exchanges that have run into
   retransmissions, where only the first two retransmissions are
   considered (the "weak estimator", E_weak_).  For the latter, there is
   some ambiguity whether a response is based on the initial
   transmission or the retransmissions.  For the purposes of the weak
   estimator, the time from the initial transmission counts.  Responses
   obtained after the third retransmission are not used to update an
   estimator.

   The overall RTO estimate is an exponentially weighted moving average
   computed of the strong and the weak estimator, which is evolved after
   each contribution to the weak estimator (1) or to the strong
   estimator (2), from the estimator (either the weak or strong
   estimator) that made the most recent contribution:

   RTO := w_weak   * E_weak_   + (1 - w_weak)   * RTO       (1)

   RTO := w_strong * E_strong_ + (1 - w_strong) * RTO       (2)




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   (Splitting this update into the two cases avoids making the
   contribution of the weak estimator too big in naturally lossy
   networks.)

   The default values for the corresponding weights, w_weak and
   w_strong, are 0.25 and 0.5, respectively.  These values have been
   found to offer good performance in evaluations (see Appendix A).
   Pseudocode and examples for the overall RTO estimate presented are
   available in Appendix B.1 and Appendix C.1.

4.2.1.  Differences with the algorithm of RFC 6298

   This subsection presents three differences of the algorithm defined
   in this document with the one defined in [RFC6298].  The first two
   recommend new parameter settings.  The third one is the variable
   backoff factor (VBF), which replaces RFC6298's simple exponential
   backoff that always multiplies the RTO by a factor of 2 when the RTO
   timer expires.

   The initial value for each of the two RTO estimators is 2 s.

   For the weak estimator, the factor K (the RTT variance multiplier) is
   set to 1 instead of 4.  This is necessary to avoid a strong increase
   of the RTO in the case that the RTTVAR value is very large, which may
   be the case if a weak RTT measurement is obtained after one or more
   retransmissions.

   In order to avoid that exchanges with small initial RTOs (i.e.  RTO
   estimate lower than 1 s) use up all retransmissions in a short
   interval of time, the RTO for a retransmission is multiplied by 3 for
   each retransmission as long as the RTO is less than 1 s.

   On the other hand, to avoid exchanges with large initial RTOs (i.e.,
   RTO estimate greater than 3 s) not being able to carry out all
   retransmissions within MAX_TRANSMIT_WAIT (normally 93 s), the RTO is
   multiplied only by 1.5 when RTO is greater than 3 s.

   Pseudocode for the variable backoff factor is in Appendix B.3.

   The binary exponential backoff is truncated at 32 seconds.  Similar
   to the way retransmissions are handled in the base specification,
   they are dithered between 1 x RTO and ACK_RANDOM_FACTOR x RTO.

4.2.2.  Discussion

   In contrast to [RFC6298], this algorithm attempts to make use of
   ambiguous information from retransmissions.  This is motivated by the
   high non-congestion loss rates expected in constrained node networks,



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   and the need to update the RTO estimators even in the presence of
   loss.  This approach appears to contravene the mandate in
   Section 3.1.1 of [RFC8085] that "latency samples MUST NOT be derived
   from ambiguous transactions".  However, those samples are not simply
   combined into the strong estimator, but are used to correct the
   limited knowledge that can be gained from the strong RTT measurements
   by employing an additional weak estimator.  In fact, the weak
   estimator allows to better update the RTO estimator when mostly weak
   RTTs are available, either due to the lossy nature of links or due to
   congestion-induced losses.  In the presence of the latter, and
   compared to a strong-only estimator (w_weak=0), spurious timeouts are
   avoided and the rate of retries is reduced, which allows to decrease
   congestion.  Evidence that has been collected from experiments
   appears to support that the overall effect of using this data in the
   way described is beneficial (Appendix A).

   Some evaluation has been done on earlier versions of this
   specification [Betzler2013].  A more recent (and more comprehensive)
   reference is [Betzler2015].

4.3.  Lifetime, Aging

   The state of the RTO estimators for an endpoint SHOULD be kept as
   long as possible.  If other state is kept for the endpoint (such as a
   DTLS connection), it is very strongly RECOMMENDED to keep the RTO
   state alive at least as long as this other state.  In the absence of
   such other state, the RTO state SHOULD be kept at least long enough
   to avoid frequent returns to inappropriate initial values.  For the
   default parameter set of Section 4.8 of [RFC7252], it is strongly
   RECOMMENDED to keep it for at least 255 s.

   If an estimator has a value that is lower than 1 s, and it is left
   without further update for 16 times its current value, the RTO
   estimate is doubled.  If an estimator has a value that is higher than
   3 s, and it is left without further update for 4 times its current
   value, the RTO estimate is set to be

      RTO := 1 s + (0.5 * RTO)

   (Note that, instead of running a timer, it is possible to implement
   these RTO aging calculations cumulatively at the time the estimator
   is used next.)

   Pseudocode and examples for the aging mechanism presented are
   available in Appendix B.2 and in Appendix C.2.






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5.  Advanced CoAP Congestion Control: Non-Confirmables

   A CoAP endpoint MUST NOT send non-confirmables to another CoAP
   endpoint at a rate higher than defined by this document.  Independent
   of any congestion control mechanisms, a CoAP endpoint can always send
   non-confirmables if their rate does not exceed 1 B/s.

   Non-confirmables that form part of exchanges are governed by the
   rules for exchanges.

   Non-confirmables outside exchanges (e.g., [RFC7641] notifications
   sent as non-confirmables) are governed by the following rules:

   1.  Of any 16 consecutive messages towards this endpoint that aren't
       responses or acknowledgments, at least 2 of the messages must be
       confirmable.

   2.  An RTO as specified in Section 4 must be used for confirmable
       messages.

   3.  The packet rate of non-confirmable messages cannot exceed 1/RTO,
       where RTO is the overall RTO estimator value at the time the non-
       confirmable packet is sent.

5.1.  Discussion

   The mechanism defined above for non-confirmables is relatively
   conservative.  More advanced versions of this algorithm could run a
   TFRC-style Loss Event Rate calculator [RFC5348] and apply the TCP
   equation to achieve a higher rate than 1/RTO.

   [RFC7641], Section 4.5.1, specifies that the rate of Non-Confirmables
   SHOULD NOT exceed 1/RTT on average, if the server can maintain an RTT
   estimate for a client.  CoCoA limits the packet rate of Non-
   Confirmables in this situation to 1/RTO.  Assuming that the RTO
   estimation in CoCoA works as expected, RTO[k] should be slightly
   greater than the RTT[k], thus CoCoA would be more conservative.  The
   expectation therefore is that complying with the NON rate set by
   CoCoA leads to complying with [RFC7641].

6.  IANA Considerations

   This document makes no requirements on IANA.  (This section to be
   removed by RFC editor.)







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7.  Security Considerations

   The security considerations of, e.g., [RFC5681], [RFC2914], and
   [RFC8085] apply.  Some issues are already discussed in the security
   considerations of [RFC7252].

   If a malicious node manages to prevent the delivery of some packets,
   a consequence will be an RTO increase, which will further reduce
   network performance.  Note that this type of attack is not specific
   for CoCoA (and not even specific for CoAP), and many congestion
   control algorithms increase the RTO upon packet loss detection.
   While it is hard to prevent radio jamming, some mitigation for other
   forms of this type of attack is provided by network access control
   techniques.  Also, the weak estimator in CoCoA increases the chances
   of obtaining RTT measurements in the presence of heavy packet losses,
   allowing to keep the RTO updated, which in turn allows recovery from
   a jamming attack in reasonable time.

8.  References

8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.



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8.2.  Informative References

   [Betzler2013]
              Betzler, A., Gomez, C., Demirkol, I., and J. Paradells,
              "Congestion control in reliable CoAP communication",
              ACM MSWIM'13 p. 365-372, DOI 10.1145/2507924.2507954,
              2013.

   [Betzler2015]
              Betzler, A., Gomez, C., Demirkol, I., and J. Paradells,
              "CoCoA+: an Advanced Congestion Control Mechanism for
              CoAP", Ad Hoc Networks Vol. 33 pp. 126-139,
              DOI 10.1016/j.adhoc.2015.04.007, October 2015.

   [I-D.bormann-core-congestion-control]
              Bormann, C. and K. Hartke, "Congestion Control Principles
              for CoAP", draft-bormann-core-congestion-control-02 (work
              in progress), July 2012.

   [I-D.eggert-core-congestion-control]
              Eggert, L., "Congestion Control for the Constrained
              Application Protocol (CoAP)", draft-eggert-core-
              congestion-control-01 (work in progress), January 2011.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,
              <https://www.rfc-editor.org/info/rfc5348>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

Appendix A.  Supporting evidence

   (Editor's note: The references local to this appendix may need to be
   merged with those from the specification proper, depending on the
   discretion of the RFC editor.)



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   CoCoA has been evaluated by means of simulation and experimentation
   in diverse scenarios comprising different link layer technologies,
   network topologies, traffic patterns and device classes.  The main
   overall evaluation result is that CoCoA consistently delivers a
   performance which is better than, or at least similar to, that of
   default CoAP congestion control.  While the latter is insensitive to
   network conditions, CoCoA is adaptive and makes good use of RTT
   samples.

   It has been shown over real GPRS and IEEE 802.15.4 mesh network
   testbeds that in these settings, in comparison to default CoAP, CoCoA
   increases throughput and reduces the time it takes for a network to
   process traffic bursts, while not sacrificing fairness.  In contrast,
   other RTT-sensitive approaches such as Linux-RTO or Peak-Hopper-RTO
   may be too simple or do not adapt well to IoT scenarios,
   underperforming default CoAP under certain conditions [1].  On the
   other hand, CoCoA has been found to reduce latency in GPRS and WiFi
   setups, compared with default CoAP [2].

   CoCoA performance has also been evaluated for non-confirmable traffic
   over emulated GPRS/UMTS links and over a real IEEE 802.15.4 mesh
   testbed.  Results show that since CoCoA is adaptive, it yields better
   packet delivery ratio than default CoAP (which does not apply
   congestion control to non-confirmable messages) or Observe (which
   introduces congestion control that is not adaptive to network
   conditions) [3, 4].

A.1.  Older versions of the draft and improvement

   CoCoA has evolved since its initial draft version.  Its core has
   remained mostly stable since draft-bormann-core-cocoa-02.  The
   evolution of CoCoA has been driven by research work.  This process,
   including evaluations of early versions of CoCoA, as well as
   improvement proposals that were finally incorporated in CoCoA, is
   reflected in published works [5-10].

A.2.  References

   [1] A.  Betzler, C.  Gomez, I.  Demirkol, J.  Paradells, "CoAP
   congestion control for the Internet of Things", IEEE Communications
   Magazine, July 2016.

   [2] F.  Zheng, B.  Fu, Z.  Cao, "CoAP Latency Evaluation", draft-
   zheng-core-coap-lantency-evaluation-00, 2016 (work in progress).

   [3] A.  Betzler, C.  Gomez, I.  Demirkol, "Evaluation of Advanced
   Congestion Control Mechanisms for Unreliable CoAP Communications",
   PE-WASUN, Cancun, Mexico, 2015.



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   [4] A.  Betzler, J.  Isern, C.  Gomez, I.  Demirkol, J.  Paradells,
   "Experimental Evaluation of Congestion Control for CoAP
   Communications without End-to-End Reliability", Ad Hoc Networks,
   Volume 52, 1 December 2016, Pages 183-194.

   [5] A.  Betzler, C.  Gomez, I.  Demirkol, J.  Paradells, "Congestion
   Control in Reliable CoAP Communication", 16th ACM International
   Conference on Modeling, Analysis and Simulation of Wireless and
   Mobile Systems (MSWIM'13), Barcelona, Spain, Nov. 2013.

   [6] A.  Betzler, C.  Gomez, I.  Demirkol, M.  Kovatsch, "Congestion
   Control for CoAP cloud services", 8th International Workshop on
   Service-Oriented Cyber-Physical Systems in Converging Networked
   Environments (SOCNE) 2014, Barcelona, Spain, Sept. 2014.

   [7] A.  Betzler, C.  Gomez, I.  Demirkol, J.  Paradells, "CoCoA+: an
   advanced congestion control mechanism for CoAP", Ad Hoc Networks
   journal, 2015.

   [8] Bhalerao, Rahul, Sridhar Srinivasa Subramanian, and Joseph
   Pasquale.  "An analysis and improvement of congestion control in the
   CoAP Internet-of-Things protocol." 2016 13th IEEE Annual Consumer
   Communications & Networking Conference (CCNC).  IEEE, 2016.

   [9] I Jaervinen, L Daniel, M Kojo, "Experimental evaluation of
   alternative congestion control algorithms for Constrained Application
   Protocol (CoAP)", IEEE 2nd World Forum on Internet of Things (WF-
   IoT), 2015.

   [10] Balandina, Ekaterina, Yevgeni Koucheryavy, and Andrei Gurtov.
   "Computing the retransmission timeout in coap."  Internet of Things,
   Smart Spaces, and Next Generation Networking.  Springer Berlin
   Heidelberg, 2013. 352-362.

Appendix B.  Pseudocode

B.1.  Updating the RTO estimator














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   // Default values
   ALPHA = 0.125 // RFC 6298
   BETA = 0.25 // RFC 6298
   W_STRONG = 0.5
   W_WEAK = 0.25

   updateRTO(retransmissions, RTT) {
     if (retransmissions == 0) {
       RTTVAR_strong = (1 - BETA) * RTTVAR_strong
                     + BETA * (RTT_strong - RTT);
       RTT_strong  = (1 - ALPHA) * RTT_strong + ALPHA * RTT;
       E_strong = RTT_strong  + 4 * RTTVAR_strong;
       RTO = W_STRONG * E_strong + (1 - W_STRONG) * RTO;
     } else if (retransmissions <= 2) {
       RTTVAR_weak = (1 - BETA) * RTTVAR_weak
                   + BETA * (RTT_weak - RTT);
       RTT_weak  = (1 - ALPHA) * RTT_weak + ALPHA * RTT;
       E_weak = RTT_weak  + 1 * RTTVAR_weak;
       RTO = W_WEAK * E_weak + (1 - W_WEAK) * RTO
     }
   }

B.2.  RTO aging

   checkAging() {
     clock_time difference = getCurrentTime() - lastUpdatedTime;

     if ((RTO < 1s) && (difference > (16 * RTO))) {
       RTO = 2 * RTO;
       lastUpdatedTime = getCurrentTime();
     } else if ((RTO > 3s) && (difference > (4 * RTO))) {
       RTO = 1s + 0.5 * RTO;
       lastUpdatedTime = getCurrentTime();
     }
   }

B.3.  Variable Backoff Factor

   backOffRTO() {
     if (RTO < 1s) {
       RTO = RTO * 3;
     } else if (RTO > 3s) {
       RTO = RTO * 1.5;
     } else {
       RTO = RTO * 2;
     }
   }




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Appendix C.  Examples

C.1.  Example A.1: weak RTTs

   A large network of sensor nodes that report periodical measurements
   is operating normally, without congestion.  The nodes transmit their
   sensor readings via CON messages every 20 s in an asynchronous way
   towards a server located behind a gateway, obtaining strong RTT
   measurements (RTT 1.1 s, RTTVAR 0.1 s) that lead to the calculation
   of an RTO of 1.5 s (in average) in each node.  In this mode of
   operation, no aging is applied, since the RTO is refreshed before the
   aging mechanism applies.

   Suddenly, upon detection of a global event, the majority of sensor
   nodes start transmitting at a higher rate (every 5 s) to increase the
   resolution of the acquired data, which creates heavy congestion that
   leads to packet losses and an important increase of real RTT between
   the nodes and the server (RTT 2 s, RTTVAR 1 s).  Due to the packet
   losses and spurious retransmissions (which can fuel congestion even
   more), many nodes are not able to update their RTO via strong RTT
   measurements, but they are able to obtain weak RTT measurements.  A
   node with an initial RTO of 1.5 s would run into a retransmission,
   before obtaining an ACK (given the RTT of 2 s and that the ACK is not
   lost).

   This weak RTT measurement would increase the overall RTO of the node
   to 1.875 s (RTO = 0.25 * 3 s + 0.75 * 1.5 s).  Following the same
   calculus (and RTT/RTTVAR values), after obtaining another weak RTT,
   the RTO would increase to 2.156 s.  At this point, the benefits of
   the weak RTT measurements are twofold:

   1.  Further spurious retransmissions are avoided as the RTO has
       increased above the real RTT.

   2.  The increase of RTOs across the whole network reduces the rate
       with which retransmissions are generated, decreasing the network
       congestion (which leads to an RTT and packet loss decrease).

C.2.  Example A.2: VBF and aging

   Assuming that the frequency of message generation is even higher
   (every 3 s) and the real RTT would further increase due to
   congestion, the RTO at some point would increase to 4 s.  Since now
   the RTO is above 3 s, no longer a binary backoff is used to avoid the
   RTO growing too much in case of retransmissions.  As the generation
   of data from the nodes ceases at some point (the network returns to a
   normal state), the aging mechanism would reduce the RTO automatically




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   (with an RTO of 4 s, after 16 s the RTO would be shifted to 3 s
   before a new RTT is measured).

C.3.  Example B: VBF and aging

   A network of nodes connected over 4G with an Internet service is
   calculating very small RTO values (0.3 s) and the nodes are
   transmitting CON messages every 1 s.  Suddenly, the connection
   quality gets worse and the nodes switch to a more stable, yet slower
   connection via GPRS.  As a result of this change, the nodes run into
   retransmissions, as the real RTT has increased above the calculated
   RTO.

   Since the RTO is below 1 s, the Variable Backoff Factor increases the
   backoff values quickly to avoid spurious retransmissions (0.9 s first
   retry, 2.7 s second retry, etc.).  Further, if due to the packet
   losses and increased delays in the network no new RTT measurements
   are obtained, the aging mechanism automatically increases the RTO
   (doubling it) after 3.8 s (16 * 0.3 s) to adapt better to the sudden
   changes of network conditions.  Without the Variable Backoff Factor
   and the aging mechanism, the number of spurious retransmissions would
   be much higher and the RTO would be corrected more slowly.

Appendix D.  Analysis: difference between strong and weak estimators

   This section analyzes the difference between the strong and weak RTO
   estimators.  If there is no congestion, assume a static RTT of R'.
   Then, E_strong_can be expressed as:

      E_strong_ = R' + G,

   since RTTVAR is reduced constantly by RTTVAR = RTTVAR * 3/4
   (according to [RFC6298], and SRTT=R'), G would be dominant term in
   the max(G, K * RTTVAR) expression in the long run.

   For the weak estimator: assume that the RTO setting converges to
   E_strong_ calculated above in the long run.  If there is a packet
   loss, and an RTT is obtained for the first retransmission, then the
   weak RTT sample obtained by the weak estimator is:

      RW' = R'+ G + R'

   Therefore, E_weak_ can be expressed as:

      E_weak_ = RW' + max(G, RW'/2) = 3 * R'






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Acknowledgements

   The first document to examine CoAP congestion control issues in
   detail was [I-D.eggert-core-congestion-control], to which this draft
   owes a lot.

   Michael Scharf did a review of CoAP congestion control issues that
   asked a lot of good questions.  Several Transport Area
   representatives made further significant inputs this discussion
   during IETF84, including Lars Eggert, Michael Scharf, and David
   Black.  Andrew McGregor, Eric Rescorla, Richard Kelsey, Ed Beroset,
   Jari Arkko, Zach Shelby, Matthias Kovatsch and many others provided
   very useful additions.  Further reviews by Michael Scharf and Ingemar
   Johansson led to further improvements, including some more discussion
   in the appendices.

   Authors from Universitat Politecnica de Catalunya have been supported
   in part by the Spanish Government's Ministerio de Economia y
   Competitividad through projects TEC2009-11453, TEC2012-32531,
   TEC2016-79988-P and FEDER.

   Carles Gomez has been funded in part by the Spanish Government
   (Ministerio de Educacion, Cultura y Deporte) through the Jose
   Castillejo grant CAS15/00336.  His contribution to this work has been
   carried out in part during his stay as a visiting scholar at the
   Computer Laboratory of the University of Cambridge, in collaboration
   with Prof. Jon Crowcroft.

Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org


   August Betzler
   Fundacio i2CAT
   Mobile and Wireless Internet Group
   C/ del Gran Capita, 2
   Barcelona  08034
   Spain

   Email: august.betzler@i2cat.net



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   Carles Gomez
   Universitat Politecnica de Catalunya/Fundacio i2CAT
   Escola d'Enginyeria de Telecomunicacio i Aeroespacial
   de Castelldefels
   C/Esteve Terradas, 7
   Castelldefels  08860
   Spain

   Phone: +34-93-413-7206
   Email: carlesgo@entel.upc.edu


   Ilker Demirkol
   Universitat Politecnica de Catalunya/Fundacio i2CAT
   Departament d'Enginyeria Telematica
   C/Jordi Girona, 1-3
   Barcelona  08034
   Spain

   Email: ilker.demirkol@entel.upc.edu































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