    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

    Abstract

       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
       https://www.rfc-editor.org/info/rfc8516.

    Copyright Notice

       Copyright (c) 2019 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
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       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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    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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    2.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "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
       also.

       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
       patterns.

    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,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
                  Application Protocol (CoAP)", RFC 7252,
                  DOI 10.17487/RFC7252, June 2014,
                  <https://www.rfc-editor.org/info/rfc7252>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

    7.2.  Informative References

       [CoAP-BROKER]
                  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, <https://www.rfc-editor.org/info/rfc6347>.

       [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
                  Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
                  <https://www.rfc-editor.org/info/rfc6585>.

       [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,
                  <https://www.rfc-editor.org/info/rfc7230>.

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

    Acknowledgements

       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
       Ericsson
       Hirsalantie 11
       02420 Jorvas
       Finland

       Email: ari.keranen@ericsson.com

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