Fast-Slow Retransmission Timeout and Congestion Control Algorithm for CoAP
draft-ietf-core-fasor-00
CoRE Working Group I. Jarvinen
Internet-Draft M. Kojo
Intended status: Experimental I. Raitahila
Expires: September 3, 2020 University of Helsinki
Z. Cao
Huawei
March 2, 2020
Fast-Slow Retransmission Timeout and Congestion Control Algorithm for
CoAP
draft-ietf-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 September 3, 2020.
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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
Appendix A. Pseudocode for Basic FASOR without Dithering . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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 [JRCK18a].
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