Internet Engineering Task Force (IETF)                        A. Keranen
Request for Comments: 8516                                      Ericsson
Category: Standards Track                                   January 2019
ISSN: 2070-1721

                 "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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019

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.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   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 an 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 purposes 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 a 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 are different.
   Because request similarity is context-dependent, it is up to the
   application logic to decide how the similarity of the requests should
   be evaluated.

   The 4.29 code is similar to the 5.03 "Service Unavailable" [RFC7252]
   code in 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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RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019

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., 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 the 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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RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019

   Note that a client may receive a 4.29 response code 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 to this response code

   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 also likely to cause well-behaving clients to simply
   retry the requests.

   As with any other CoAP reply, a client should trust this response
   code only to the extent that 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 again.

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

6.  IANA Considerations

   IANA has registered the following response code in the "CoAP Response
   Codes" subregistry within the "Constrained RESTful Environments
   (CoRE) Parameters" registry:

   o  Response Code: 4.29

   o  Description: Too Many Requests

   o  Reference: RFC 8516

   IANA has added this document as an additional reference for the
   Max-Age option in the "CoAP Option Numbers" subregistry.

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RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019

7.  References

7.1.  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, <>.

7.2.  Informative References

              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", Work in Progress, draft-ietf-core-coap-pubsub-06,
              January 2019.

   [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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RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019


   This response code definition was originally part of the "Publish-
   Subscribe Broker for CoAP" document [CoAP-BROKER].  The 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.

Author's Address

   Ari Keranen
   Hirsalantie 11
   02420 Jorvas


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