Status CoAP Simple Congestion Control/Advanced

Author: Jaime Jiménez

Tentative Timeline

• draft summary
• IESG Submission @due(27-12-14 15:28)

Summary draft-ietf-core-cocoa-02

CoCOA provides a better congestion control mechanism than default CoAP in terms of goodput, latency and performance given network stress conditions. Such conditions would occur when an event would force the network to increase their the transmission rate per second while still avoiding fatal congestion.

It does require three state variables per scope plus the state needed to do Round-Trip Time (RTT) measurements, so it may not be applicable to the most constrained devices.

To better understand it, it is useful to read example C.1. first, which is pretty much self-explanatory; A network with a certain Retransmission TimeOut (RTO) suffers from an event that forces the endpoints to report more frequently. Therefore the RTO is recalculated based on the RTT estimation and the endpoints are adjusted.

The overall RTO estimate is an exponentially weighted moving average computed of a weak (1) and a strong (2) estimator:

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

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

The RTO estimators have a lifetime of at least 255s, after some time without updates the RTO estimate is recalculated.

Shepherd Writeup

Summary

• Document Shepherd: Jaime Jiménez, jaime.jimenez@ericsson.com
• Area Director: Alexey Melnikov, aamelnikov@fastmail.fm

The CoAP 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.

The document is intended as Informational.

Review and Consensus

The document has gone through multiple expert reviews and has been discussed on multiple IETF meetings. Before the last IETF the WGLC was completed.

Intellectual Property

Each author has stated that they do not have direct, personal knowledge of any IPR related to this document. I am not aware of any IPR discussion about this document on the CoRE WG.

Other Points

There are RFC Editor comments that need to be edited out. This document makes no requirements on IANA.

Checklist

• Does the shepherd stand behind the document and think the document is ready for publication?
• Is the correct RFC type indicated in the title page header?
• Is the abstract both brief and sufficient, and does it stand alone as a brief summary?
• Is the intent of the document accurately and adequately explained in the introduction?
• Have all required formal reviews (MIB Doctor, Media Type, URI, etc.) been requested and/or completed?
• Has the shepherd performed automated checks – idnits (see http://www.ietf.org/tools/idnits/ and the Internet-Drafts Checklist), checks of BNF rules, XML code and schemas, MIB definitions, and so on – and determined that the document passes the tests?
• Has each author stated that their direct, personal knowledge of any IPR related to this document has already been disclosed, in conformance with BCPs 78 and 79?
• Have all references within this document been identified as either normative or informative, and does the shepherd agree with how they have been classified?
• Are all normative references made to documents that are ready for advancement and are otherwise in a clear state?
• If publication of this document changes the status of any existing RFCs, are those RFCs listed on the title page header, and are the changes listed in the abstract and discussed (explained, not just mentioned) in the introduction? Does not apply

• If this is a bis document, have all of the errata been considered? Does not apply

• IANA Considerations: There are no requirements on IANA

  • Are the IANA Considerations clear and complete? Remember that IANA have to understand unambiguously what’s being requested, so they can perform the required actions.
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  • For any new registries that this document creates, has the working group actively chosen the allocation procedures and policies and discussed the alternatives?
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