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Internet Engineering Task Force (IETF)                        A. KeranenRequest for Comments: 8516                                      EricssonCategory: Standards Track                                   January 2019ISSN: 2070-1721                 "Too Many Requests" Response Code for                  the Constrained Application ProtocolAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) 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.Keranen                      Standards Track                    [Page 1]

RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .33.  CoAP Server Behavior  . . . . . . . . . . . . . . . . . . . .34.  CoAP Client Behavior  . . . . . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . .46.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .47.  References  . . . . . . . . . . . . . . . . . . . . . . . . .57.1.  Normative References  . . . . . . . . . . . . . . . . . .57.2.  Informative References  . . . . . . . . . . . . . . . . .5   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .61.  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 (seeSection 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.Keranen                      Standards Track                    [Page 2]

RFC 8516       "Too Many Requests" Response Code for CoAP   January 20192.  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 inBCP 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 (seeSection 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 inSection 5.10.5 of [RFC7252]) is assumed.Keranen                      Standards Track                    [Page 3]

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.Keranen                      Standards Track                    [Page 4]

RFC 8516       "Too Many Requests" Response Code for CoAP   January 20197.  References7.1.  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 inRFC2119 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>.Keranen                      Standards Track                    [Page 5]

RFC 8516       "Too Many Requests" Response Code for CoAP   January 2019Acknowledgements   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.comKeranen                      Standards Track                    [Page 6]