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"Too Many Requests" Response Code for the Constrained Application Protocol

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8516.
Author Ari Keränen
Last updated 2020-09-10 (Latest revision 2018-11-06)
Replaces draft-keranen-core-too-many-reqs
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Carsten Bormann
Shepherd write-up Show Last changed 2018-08-15
IESG IESG state RFC 8516 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alexey Melnikov
Send notices to Carsten Bormann <>
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
Network Working Group                                         A. Keranen
Internet-Draft                                                  Ericsson
Intended status: Standards Track                        November 7, 2018
Expires: May 11, 2019

Too Many Requests Response Code for the Constrained Application Protocol


   A Constrained Application Protocol (CoAP) server can experience
   temporary overload because one or more clients are sending requests
   to the server at a higher rate than the server is capable or willing
   to handle.  This document defines a new CoAP Response Code for a
   server to indicate that a client should reduce the rate of requests.

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

   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."

   This Internet-Draft will expire on May 11, 2019.

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
   ( 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  CoAP Server Behavior  . . . . . . . . . . . . . . . . . . . .   3
   4.  CoAP Client Behavior  . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] Response Codes
   are used by a CoAP server to indicate the result of the attempt to
   understand and satisfy a request sent by a client.

   CoAP Response Codes are similar to the HTTP [RFC7230] Status Codes
   and many codes are shared with similar semantics by both CoAP and
   HTTP.  HTTP has the code "429" registered for "Too Many Requests"
   [RFC6585].  This document registers a CoAP Response Code "4.29" for
   similar purpose and uses the Max-Age option (see Section 5.10.5 of
   [RFC7252]) to indicate a back-off period after which a client can try
   the request again.

   While a server may not be able to respond to one kind of request, it
   may be able to respond to a request of different kind, even from the
   same client.  Therefore the back-off period applies only to similar
   requests.  For the purpose of this response code, a request is
   similar if it has the same method and Request-URI.  Also if a client
   is sending a sequence of requests that are part of the same series
   (e.g., a set of measurements to be processed by the server) they can
   be considered similar even if request URIs may be different.  Because
   request similarity is context-dependent, it is up to the application
   logic to decide how the similarity of the requests should be

   The 4.29 code is similar to the 5.03 "Service Unavailable" [RFC7252]
   code in a way that the 5.03 code can also be used by a server to
   signal an overload situation.  The 5.03 code also uses the Max-Age
   option to indicate the time after which a client can retry.  However
   the 4.29 code indicates that the too-frequent requests from the
   requesting client are the reason for the overload.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   Readers should also be familiar with the terms and concepts discussed
   in [RFC7252].

3.  CoAP Server Behavior

   If a CoAP server is unable to serve a client that is sending CoAP
   request messages more often than the server is capable or willing to
   handle, the server SHOULD respond to the request(s) with the Response
   Code 4.29, "Too Many Requests".  The Max-Age option is used to
   indicate the number of seconds after which the server assumes it is
   OK for the client to retry the request.

   An action result payload (see Section 5.5.1 of [RFC7252]) can be sent
   by the server to give more guidance to the client, e.g., about the
   details of the overload situation.

   The 4.29 Response Code is only returned to the client(s) sending
   requests too frequently; if other clients are sending requests that
   cannot be served due to server overload, the 5.03 Response Code is
   more appropriate.

   If a client repeats a request that was answered with 4.29 before Max-
   Age time has passed, it is possible that the client sent multiple
   requests before receiving the first answer or that the client did not
   recognize the Response Code.  To slow down clients that do not
   recognize the 4.29 code, the server MAY respond with a more generic
   error code (e.g., 5.03).  The server SHOULD rate-limit 4.29 replies
   taking into account its usual load shedding policies.  However, any
   such method that adds per-client state to the server may be
   counterproductive to reducing load.

4.  CoAP Client Behavior

   If a client receives the 4.29 Response Code from a CoAP server to a
   request, it SHOULD NOT send a similar request to the server before
   the time indicated in the Max-Age option has passed.  If the 4.29
   response does not contain a Max-Age option, the default value (60
   seconds, as defined in Section 5.10.5 of [RFC7252]) is assumed.

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   Note that a client may receive a 4.29 Response Code already on a
   first request to a server.  This can happen, for example, if there is
   a proxy on the path and the server replies based on the load from
   multiple clients aggregated by the proxy, or if a client has
   restarted recently and does not remember its recent requests.

   A client should not rely on a server being able to send the 4.29
   Response Code in an overload situation because an overloaded server
   may not be able to reply at all to some requests.

5.  Security Considerations

   Security considerations of [RFC7252] apply also to this Response

   Replying to CoAP requests with a Response Code consumes resources
   from a server.  For a server under attack it may be more appropriate
   to simply drop requests without responding at all.  However, dropping
   requests is likely to cause also well-behaving clients to simply
   retry the requests.

   As with any other CoAP reply, a client should trust this Response
   Code only to extent it trusts the underlying security mechanisms
   (e.g., DTLS [RFC6347]) for authentication and freshness.  If a CoAP
   reply with the Too Many Requests Response Code is not authenticated
   and integrity protected, an attacker can attempt to spoof a reply and
   make the client wait for an extended period of time before trying

   If the Response Code is sent without encryption, it may leak
   information about the server overload situation and client traffic

6.  IANA Considerations

   IANA is requested to register the following Response Code in the
   "CoRE Parameters Registry", "CoAP Response Codes" sub-registry:

   o  Response Code: 4.29

   o  Description: Too Many Requests

   o  Reference: [[This document]]

   IANA is requested to add this document as an additional reference for
   the Max-Age option in the "CoAP Option Numbers" sub-registry.

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7.  Acknowledgements

   This Response Code definition was originally part of the "Publish-
   Subscribe Broker for CoAP" document [I-D.ietf-core-coap-pubsub].
   Author would like to thank Abhijan Bhattacharyya, Carsten Bormann,
   Daniel Migault, Gyorgy Rethy, Jana Iyengar, Jim Schaad, Klaus Hartke,
   Mohit Sethi, and Sandor Katona for their contributions and reviews.

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,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

8.2.  Informative References

              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-ietf-core-coap-pubsub-05 (work in
              progress), July 2018.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

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Author's Address

   Ari Keranen


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