Fast-Slow Retransmission Timeout and Congestion Control Algorithm for CoAP
draft-jarvinen-core-fasor-00

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CoRE Working Group                                           I. Jarvinen
Internet-Draft                                                   M. Kojo
Intended status: Experimental                               I. Raitahila
Expires: January 4, 2019                          University of Helsinki
                                                                  Z. Cao
                                                                  Huawei
                                                            July 3, 2018

 Fast-Slow Retransmission Timeout and Congestion Control Algorithm for
                                  CoAP
                      draft-jarvinen-core-fasor-00

Abstract

   This document specifies an alternative retransmission timeout and
   congestion control back off algorithm for the CoAP protocol, called
   Fast-Slow RTO (FASOR).

   The algorithm specified in this document employs an appropriate and
   large enough back off of Retransmission Timeout (RTO) as the major
   congestion control mechanism to allow acquiring unambiguous RTT
   samples with high probability and to prevent building a persistent
   queue when retransmitting.  The algorithm also aims to retransmit
   quickly using an accurately managed retransmission timeout when link-
   errors are occuring, basing RTO calculation on unambiguous round-trip
   time (RTT) samples.

Status of This Memo

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   This Internet-Draft will expire on January 4, 2019.

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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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problems with Existing CoAP Congestion Control Algorithms . .   3
   4.  FASOR Algorithm . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Computing Normal RTO (FastRTO)  . . . . . . . . . . . . .   4
     4.2.  Slow RTO  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Retransmission Timeout Back Off Logic . . . . . . . . . .   6
       4.3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . .   6
       4.3.2.  Retransmission State Machine  . . . . . . . . . . . .   7
     4.4.  Retransmission Count Option . . . . . . . . . . . . . . .   9
     4.5.  Alternatives for Exchanging Retransmission Count
           Information . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   CoAP senders use retransmission timeout (RTO) to infer losses that
   have occurred in the network.  For such a heuristic to be correct,
   the RTT estimate used for calculating the retransmission timeout must
   match to the real end-to-end path characteristics.  Otherwise,
   unnecessary retransmission may occur.  Both default RTO mechanism for
   CoAP [RFC7252] and CoCoA [I-D.ietf-core-cocoa] have issues in dealing
   with unnecessary retransmissions and in the worst-case the situation
   can persist causing congestion collapse [JRCK18].

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