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rfc:rfc8516

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
 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 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.

Keranen Standards Track [Page 2] RFC 8516 "Too Many Requests" Response Code for CoAP January 2019

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.

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 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>.

Keranen Standards Track [Page 5] 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

Keranen Standards Track [Page 6]

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