Fast-Slow Retransmission Timeout and Congestion Control Algorithm for CoAP

Document Type Replaced Internet-Draft (candidate for core WG)
Authors Ilpo Järvinen  , Markku Kojo  , Iivo Raitahila  , Zhen Cao 
Last updated 2020-01-07 (latest revision 2019-07-06)
Replaced by draft-ietf-core-fasor
Stream IETF
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Expired & archived
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Stream WG state Call For Adoption By WG Issued
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IESG IESG state Replaced by draft-ietf-core-fasor
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


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.


Ilpo Järvinen (
Markku Kojo (
Iivo Raitahila (
Zhen Cao (

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)