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RateLimit header fields for HTTP
draft-ietf-httpapi-ratelimit-headers-08

Document Type Active Internet-Draft (httpapi WG)
Authors Roberto Polli , Alex Martínez Ruiz , Darrel Miller
Last updated 2024-10-07
Replaces draft-polli-ratelimit-headers
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draft-ietf-httpapi-ratelimit-headers-08
HTTPAPI                                                         R. Polli
Internet-Draft                         Team Digitale, Italian Government
Intended status: Standards Track                             A. Martinez
Expires: 10 April 2025                                           Red Hat
                                                               D. Miller
                                                               Microsoft
                                                          7 October 2024

                    RateLimit header fields for HTTP
                draft-ietf-httpapi-ratelimit-headers-08

Abstract

   This document defines the RateLimit-Policy and RateLimit HTTP header
   fields for servers to advertise their quota policies and the current
   service limits, thereby allowing clients to avoid being throttled.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit-
   headers/.

   Discussion of this document takes place on the HTTPAPI Working Group
   mailing list (mailto:httpapi@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/httpapi/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/httpapi/.  Working Group
   information can be found at https://datatracker.ietf.org/wg/httpapi/
   about/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-httpapi/ratelimit-headers.

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 https://datatracker.ietf.org/drafts/current/.

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   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 10 April 2025.

Copyright Notice

   Copyright (c) 2024 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  RateLimit-Policy Field  . . . . . . . . . . . . . . . . . . .   6
     3.1.  Quota Policy Item . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  Quota Parameter . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  Quota Unit Parameter  . . . . . . . . . . . . . . . .   6
       3.1.3.  Window Parameter  . . . . . . . . . . . . . . . . . .   7
       3.1.4.  Partition Key Parameter . . . . . . . . . . . . . . .   7
     3.2.  RateLimit Policy Field Examples . . . . . . . . . . . . .   7
   4.  RateLimit Field . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Service Limit Item  . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  Remaining Parameter . . . . . . . . . . . . . . . . .   8
       4.1.2.  Reset Parameter . . . . . . . . . . . . . . . . . . .   8
       4.1.3.  Partition Key Parameter . . . . . . . . . . . . . . .   9
     4.2.  RateLimit Field Examples  . . . . . . . . . . . . . . . .   9
   5.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Generating Partition Keys . . . . . . . . . . . . . . . .  10
     5.2.  Performance Considerations  . . . . . . . . . . . . . . .  10
   6.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Consuming Partition Keys  . . . . . . . . . . . . . . . .  12
     6.2.  Intermediaries  . . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13

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     7.1.  Throttling does not prevent clients from issuing
           requests  . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  Information disclosure  . . . . . . . . . . . . . . . . .  13
     7.3.  Remaining quota units are not granted requests  . . . . .  14
     7.4.  Reliability of the reset parameter  . . . . . . . . . . .  14
     7.5.  Resource exhaustion . . . . . . . . . . . . . . . . . . .  14
       7.5.1.  Denial of Service . . . . . . . . . . . . . . . . . .  15
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  RateLimit quota unit registry . . . . . . . . . . . . . .  16
       9.1.1.  Registration Template . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Rate-limiting and quotas . . . . . . . . . . . . . .  18
     A.1.  Interoperability issues . . . . . . . . . . . . . . . . .  19
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  19
     B.1.  Responses without defining policies . . . . . . . . . . .  19
       B.1.1.  Throttling information in responses . . . . . . . . .  20
       B.1.2.  Multiple policies in response . . . . . . . . . . . .  20
       B.1.3.  Use for limiting concurrency  . . . . . . . . . . . .  21
       B.1.4.  Use in throttled responses  . . . . . . . . . . . . .  22
     B.2.  Responses with defined policies . . . . . . . . . . . . .  23
       B.2.1.  Throttling window specified via parameter . . . . . .  23
       B.2.2.  Dynamic limits with parameterized windows . . . . . .  23
       B.2.3.  Dynamic limits for pushing back and slowing down  . .  24
     B.3.  Dynamic limits for pushing back with Retry-After and slow
           down  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
       B.3.1.  Missing Remaining information . . . . . . . . . . . .  25
       B.3.2.  Use with multiple windows . . . . . . . . . . . . . .  26
   FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   RateLimit header fields currently used on the web . . . . . . . .  29
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  31
   Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     Since draft-ietf-httpapi-ratelimit-headers-07 . . . . . . . . .  31
     Since draft-ietf-httpapi-ratelimit-headers-03 . . . . . . . . .  31
     Since draft-ietf-httpapi-ratelimit-headers-02 . . . . . . . . .  31
     Since draft-ietf-httpapi-ratelimit-headers-01 . . . . . . . . .  32
     Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   Rate limiting of HTTP clients has become a widespread practice,
   especially for HTTP APIs.  Typically, servers who do so limit the
   number of acceptable requests in a given time window (e.g. 10
   requests per second).  See Appendix A for further information on the
   current usage of rate limiting in HTTP.

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   Currently, there is no standard way for servers to communicate quotas
   so that clients can throttle their requests to prevent errors.  This
   document defines a set of standard HTTP header fields to enable rate
   limiting:

   *  RateLimit-Policy: a quota policy, defined by the server, that
      client HTTP requests will consume.

   *  RateLimit: the currently remaining quota available for a specific
      policy.

   These fields enable establishing complex rate limiting policies,
   including using multiple and variable time windows and dynamic
   quotas, and implementing concurrency limits.

1.1.  Goals

   The goals of this document are:

   Interoperability:  Standardize the names and semantics of rate-limit
      headers to ease their enforcement and adoption.

   Resiliency:  Improve resiliency of HTTP infrastructure by providing
      clients with information useful to throttle their requests and
      prevent 4xx or 5xx responses.

   Documentation:  Simplify API documentation by eliminating the need to
      include detailed quota limits and related fields in API
      documentation.

   The following features are out of the scope of this document:

   Authorization:  RateLimit header fields are not meant to support
      authorization or other kinds of access controls.

   Response status code:  RateLimit header fields may be returned in
      both successful (see Section 15.3 of [HTTP]) and non-successful
      responses.  This specification does not cover whether non
      Successful responses count on quota usage, nor does it mandates
      any correlation between the RateLimit values and the returned
      status code.

   Throttling algorithm:  This specification does not mandate a specific
      throttling algorithm.  The values published in the fields,
      including the window size, can be statically or dynamically
      evaluated.

   Service Level Agreement:  Conveyed quota hints do not imply any

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      service guarantee.  Server is free to throttle respectful clients
      under certain circumstances.

1.2.  Notational Conventions

   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.

   The term Origin is to be interpreted as described in Section 7
   of[WEB-ORIGIN].

   This document uses the terms List, Item and Integer from Section 3 of
   [STRUCTURED-FIELDS] to specify syntax and parsing, along with the
   concept of "bare item".

2.  Terminology

   Quota:  A quota is an allocation of capacity used by a resource
      server to limit client requests.  That capacity is measured in
      quota units and may be reallocated at the end of a time window.

   Quota Unit:  A quota unit is the unit of measure used to measure the
      activity of a client.

   Quota Partition:  A quota partition is a division of a server's
      capacity across different clients, users and owned resources.

   Time Window:  A time window indicates a period of time associated to
      the allocated quota.

   Quota Policy:  A quota policy is implemented by the server to
      regulate the activity within a specified quota partition,
      quantified in quota units, over a defined time window.  This
      activity is restricted to a predefined limit, known as the quota.
      Quota policies can be advertised by servers, but they are not
      required to be, and more than one quota policy can affect a given
      request from a client to a server.

   Service Limit:  A service limit is the currently remaining quota from
      a specific quota policy and, if defined, the remaining time before
      quota is reallocated.

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3.  RateLimit-Policy Field

   The "RateLimit-Policy" response header field is a non-empty List of
   Quota Policy Items (Section 3.1).  The Item value MUST be a String.
   Its value is informative.  The field value is expected to remain
   consistent over a the lifetime of a connection.  It is this
   characteristic that differentiates it from the RateLimit (Section 4)
   field that contains information that may change on every request.

      RateLimit-Policy: "burst";q=100;w=60,"daily";q=1000;w=86400

3.1.  Quota Policy Item

   A quota policy Item contains an identifier for the policy and a set
   of parameters that contain information about a server's capacity
   allocation for the policy.

   The following parameters are defined:

   q:  The REQUIRED "q" parameter indicates the quota allocated by this
      policy measured in quota units.

   qu:  The OPTIONAL "qu" parameter value conveys the quota units
      associated to the "q" parameter.  The default quota unit is
      "requests".

   w:  The OPTIONAL "w" parameter value conveys a time window.

   pk:  The OPTIONAL "pk" parameter value conveys the partition key
      associated to the corresponding request.

   Other parameters are allowed and can be regarded as comments.

   Implementation- or service-specific parameters SHOULD be prefixed
   parameters with a vendor identifier, e.g. acme-policy, acme-burst.

3.1.1.  Quota Parameter

   The "q" parameter value MUST be a non-negative Integer.  The value
   indicates the quota allocated for client activity (measured in quota
   units) for a given quota partition.

3.1.2.  Quota Unit Parameter

   The "qu" parameter value conveys the quota units applicable to the
   quota (Section 3.1.1).  The value MUST be a String.  Allowed values
   are listed in the RateLimit Quota Units registry (Section 9.1).  This
   specification defines three quota units:

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   requests:  This value indicates the quota is based on the number of
      requests processed by the resource server.  Whether a specific
      request actually consumes a quota unit is implementation-specific.

   content-bytes:  This value indicates the quota is based on the number
      of content bytes processed by the resource server.

   concurrent-requests:  This value indicates the quota is based on the
      number of concurrent requests processed by the resource server.

3.1.3.  Window Parameter

   The "w" parameter value conveys a time window applicable to the quota
   (Section 3.1.1).  The time window MUST be a non-negative Integer
   value expressing an interval in seconds, similar to the "delay-
   seconds" rule defined in Section 10.2.3 of [HTTP].  Sub-second
   precision is not supported.

3.1.4.  Partition Key Parameter

   The "pk" parameter value conveys the partition key associated to the
   request.  The value MUST be a Byte Sequence.  Servers MAY use the
   partition key to divide server capacity across different clients and
   resources.  Quotas are allocated per partition key.

3.2.  RateLimit Policy Field Examples

   This field MAY convey the time window associated with the quota, as
   shown in this example:

      RateLimit-Policy: "default";q=100;w=10

   These examples show multiple policies being returned:

      RateLimit-Policy: "permin";q=50;w=60,"perhr";q=1000;w=3600

   The following example shows a policy with a partition key:

      RateLimit-Policy: "peruser";q=100;w=60;pk=:cHsdsRa894==:

   The following example shows a policy with a partition key and a quota
   unit:

   RateLimit-Policy: "peruser";q=65535;qu="bytes";w=10;pk=:sdfjLJUOUH==:

   This field cannot appear in a trailer section.

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4.  RateLimit Field

   A server uses the "RateLimit" response header field to communicate
   the service limit for a quota policy for a particular partition key.

   The field is expressed as List of Service Limit Items (Section 4.1).

      RateLimit: "default";r=50;t=30

4.1.  Service Limit Item

   Each service limit Item identifies the quota policy (Section 3.1)
   associated with the request and contains parameters with information
   about the current service limit.

   The following parameters are defined in this specification:

   r:  This parameter value conveys the remaining quota units for the
      identified policy (Section 4.1.1).

   t:  This OPTIONAL parameter value conveys the time window reset time
      for the identified policy (Section 4.1.2).

   pk:  The OPTIONAL "pk" parameter value conveys the partition key
      associated to the corresponding request.

   This field cannot appear in a trailer section.  Other parameters are
   allowed and can be regarded as comments.

   Implementation- or service-specific parameters SHOULD be prefixed
   parameters with a vendor identifier, e.g. acme-policy, acme-burst.

4.1.1.  Remaining Parameter

   The "r" parameter indicates the remaining quota units for the
   identified policy (Section 4.1.1).

   It is a non-negative Integer expressed in quota units.  Clients MUST
   NOT assume that a positive remaining value is a guarantee that
   further requests will be served.  When remaining parameter value is
   low, it indicates that the server may soon throttle the client (see
   Section 5).

4.1.2.  Reset Parameter

   The "t" parameter indicates the number of seconds until the quota
   associated with the quota policy resets.

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   It is a non-negative Integer compatible with the delay-seconds rule,
   because:

   *  it does not rely on clock synchronization and is resilient to
      clock adjustment and clock skew between client and server (see
      Section 5.6.7 of [HTTP]);

   *  it mitigates the risk related to thundering herd when too many
      clients are serviced with the same timestamp.

   The client MUST NOT assume that all its service limit will be reset
   at the moment indicated by the reset parameter.  The server MAY
   arbitrarily alter the reset parameter value between subsequent
   requests; for example, in case of resource saturation or to implement
   sliding window policies.

4.1.3.  Partition Key Parameter

   The "pk" parameter value conveys the partition key associated to the
   request.  The value MUST be a Byte Sequence.  Servers MAY use the
   partition key to divide server capacity across different clients and
   resources.  Quotas are allocated per partition key.

4.2.  RateLimit Field Examples

   This example shows a RateLimit field with a remaining quota of 50
   units and a time window reset in 30 seconds:

      RateLimit: "default";r=50;t=30

   This example shows a remaining quota of 999 requests for a partition
   key that has no time window reset:

      RateLimit: "default";r=999;pk=:dHJpYWwxMjEzMjM=:

   This example shows a 300MB remaining quota for an application in the
   next 60 seconds:

      RateLimit: "default";r=300000000;t=60;pk=:QXBwLTk5OQ==:

5.  Server Behavior

   A server MAY return RateLimit header fields independently of the
   response status code.  This includes on throttled responses.  This
   document does not mandate any correlation between the RateLimit
   header field values and the returned status code.

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   Servers should be careful when returning RateLimit header fields in
   redirection responses (i.e., responses with 3xx status codes) because
   a low remaining parameter value could prevent the client from issuing
   requests.  For example, given the RateLimit header fields below, a
   client could decide to wait 10 seconds before following the
   "Location" header field (see Section 10.2.2 of [HTTP]), because the
   remaining parameter value is 0.

   HTTP/1.1 301 Moved Permanently
   Location: /foo/123
   RateLimit: "problemPolicy";r=0;t=10

   If a response contains both the Retry-After and the RateLimit header
   fields, the reset parameter value SHOULD reference the same point in
   time as the Retry-After field value.

   A service using RateLimit header fields MUST NOT convey values
   exposing an unwanted volume of requests and SHOULD implement
   mechanisms to cap the ratio between the remaining and the reset
   parameter values (see Section 7.5); this is especially important when
   a quota policy uses a large time window.

   Under certain conditions, a server MAY artificially lower RateLimit
   header field values between subsequent requests, e.g. to respond to
   Denial of Service attacks or in case of resource saturation.

5.1.  Generating Partition Keys

   Servers MAY choose to return partition keys that distinguish between
   quota allocated to different consumers or different resources.  There
   are a wide range of strategies for partitioning server capacity,
   including per user, per application, per HTTP method, per resource,
   or some combination of those values.  The server SHOULD document how
   the partition key is generated so that clients can predict the key
   value for a future request and determine if there is sufficient quota
   remaining to execute the request.  Servers should avoid returning
   partition keys that contain sensitive information.  Servers SHOULD
   only use information that is present in the request to generate the
   partition key.

5.2.  Performance Considerations

   Servers are not required to return RateLimit header fields in every
   response, and clients need to take this into account.  For example,
   an implementer concerned with performance might provide RateLimit
   header fields only when a given quota is close to exhaustion.

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   Implementers concerned with response fields' size, might take into
   account their ratio with respect to the content length, or use
   header-compression HTTP features such as [HPACK].

6.  Client Behavior

   The RateLimit header fields can be used by clients to determine
   whether the associated request respected the server's quota policy,
   and as an indication of whether subsequent requests will.  However,
   the server might apply other criteria when servicing future requests,
   and so the quota policy may not completely reflect whether requests
   will succeed.

   For example, a successful response with the following fields:

      RateLimit: "default";r=1;t=7

   does not guarantee that the next request will be successful.
   Servers' behavior may be subject to other conditions.

   A client is responsible for ensuring that RateLimit header field
   values returned cause reasonable client behavior with respect to
   throughput and latency (see Section 7.5 and Section 7.5.1).

   A client receiving RateLimit header fields MUST NOT assume that
   future responses will contain the same RateLimit header fields, or
   any RateLimit header fields at all.

   Malformed RateLimit header fields MUST be ignored.

   A client SHOULD NOT exceed the quota units conveyed by the remaining
   parameter before the time window expressed in the reset parameter.

   The value of the reset parameter is generated at response time: a
   client aware of a significant network latency MAY behave accordingly
   and use other information (e.g. the "Date" response header field, or
   otherwise gathered metrics) to better estimate the reset parameter
   moment intended by the server.

   The details provided in the RateLimit-Policy header field are
   informative and MAY be ignored.

   If a response contains both the RateLimit and Retry-After fields, the
   Retry-After field MUST take precedence and the reset parameter MAY be
   ignored.

   This specification does not mandate a specific throttling behavior
   and implementers can adopt their preferred policies, including:

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   *  slowing down or pre-emptively back-off their request rate when
      approaching quota limits;

   *  consuming all the quota according to the exposed limits and then
      wait.

6.1.  Consuming Partition Keys

   Partition keys are useful for a client if it is likely that single
   client will make requests that consume different quota allocations.
   E.g. a client making requests on behalf of different users or for
   different resources that have independent quota allocations.

   If a server documents the partition key generation algorithm, clients
   MAY generate a partition key for a future request.  Using this key,
   and comparing to the key returned by the server, the client can
   determine if there is sufficient quota remaining to execute the
   request.

   For cases where the partition key generation algorithm of a server is
   unknown, clients MAY use heuristics to guess if a future request will
   be successful based on its similarity to previous requests.

6.2.  Intermediaries

   This section documents the considerations advised in Section 16.3.2
   of [HTTP].

   An intermediary that is not part of the originating service
   infrastructure and is not aware of the quota policy semantic used by
   the Origin Server SHOULD NOT alter the RateLimit header fields'
   values in such a way as to communicate a more permissive quota
   policy; this includes removing the RateLimit header fields.

   An intermediary MAY alter the RateLimit header fields in such a way
   as to communicate a more restrictive quota policy when:

   *  it is aware of the quota unit semantic used by the Origin Server;

   *  it implements this specification and enforces a quota policy which
      is more restrictive than the one conveyed in the fields.

   An intermediary SHOULD forward a request even when presuming that it
   might not be serviced; the service returning the RateLimit header
   fields is the sole responsible of enforcing the communicated quota
   policy, and it is always free to service incoming requests.

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   This specification does not mandate any behavior on intermediaries
   respect to retries, nor requires that intermediaries have any role in
   respecting quota policies.  For example, it is legitimate for a proxy
   to retransmit a request without notifying the client, and thus
   consuming quota units.

   Privacy considerations (Section 8) provide further guidance on
   intermediaries.

6.3.  Caching

   [HTTP-CACHING] defines how responses can be stored and reused for
   subsequent requests, including those with RateLimit header fields.
   Because the information in RateLimit header fields on a cached
   response may not be current, they SHOULD be ignored on responses that
   come from cache (i.e., those with a positive current_age; see
   Section 4.2.3 of [HTTP-CACHING]).

7.  Security Considerations

7.1.  Throttling does not prevent clients from issuing requests

   This specification does not prevent clients from making requests.
   Servers should always implement mechanisms to prevent resource
   exhaustion.

7.2.  Information disclosure

   Servers should not disclose to untrusted parties operational capacity
   information that can be used to saturate its infrastructural
   resources.

   While this specification does not mandate whether non-successful
   responses consume quota, if error responses (such as 401
   (Unauthorized) and 403 (Forbidden)) count against quota, a malicious
   client could probe the endpoint to get traffic information of another
   user.

   As intermediaries might retransmit requests and consume quota units
   without prior knowledge of the user agent, RateLimit header fields
   might reveal the existence of an intermediary to the user agent.

   Where partition keys contain identifying information, either of the
   client application or the user, servers should be aware of the
   potential for impersonation and apply the appropriate security
   mechanisms.

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7.3.  Remaining quota units are not granted requests

   RateLimit header fields convey hints from the server to the clients
   in order to help them avoid being throttled out.

   Clients MUST NOT consider the quota returned in the remaining
   parameter (Section 4.1.1) as a service level agreement.

   In case of resource saturation, the server MAY artificially lower the
   returned values or not serve the request regardless of the advertised
   quotas.

7.4.  Reliability of the reset parameter

   Consider that quota might not be restored after the moment referenced
   by the reset parameter (Section 4.1.2), and the reset parameter value
   may not be constant.

   Subsequent requests might return a higher reset parameter value to
   limit concurrency or implement dynamic or adaptive throttling
   policies.

7.5.  Resource exhaustion

   When returning reset values, servers must be aware that many
   throttled clients may come back at the very moment specified.

   This is true for Retry-After too.

   For example, if the quota resets every day at 18:00:00 and your
   server returns the reset parameter accordingly

      Date: Tue, 15 Nov 1994 18:00:00 GMT
      RateLimit: "daily";r=1;t=36400

   there's a high probability that all clients will show up at 18:00:00.

   This could be mitigated by adding some jitter to the reset value.

   Resource exhaustion issues can be associated with quota policies
   using a large time window, because a user agent by chance or on
   purpose might consume most of its quota units in a significantly
   shorter interval.

   This behavior can be even triggered by the provided RateLimit header
   fields.  The following example describes a service with an unconsumed
   quota policy of 10000 quota units per 1000 seconds.

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   RateLimit-Policy: "somepolicy";q=10000;w=1000
   RateLimit: "somepolicy";r=10000;t=10

   A client implementing a simple ratio between remaining parameter and
   reset parameter could infer an average throughput of 1000 quota units
   per second, while the quota parameter conveys a quota-policy with an
   average of 10 quota units per second.  If the service cannot handle
   such load, it should return either a lower remaining parameter value
   or an higher reset parameter value.  Moreover, complementing large
   time window quota policies with a short time window one mitigates
   those risks.

7.5.1.  Denial of Service

   RateLimit header fields may contain unexpected values by chance or on
   purpose.  For example, an excessively high remaining parameter value
   may be:

   *  used by a malicious intermediary to trigger a Denial of Service
      attack or consume client resources boosting its requests;

   *  passed by a misconfigured server;

   or a high reset parameter value could inhibit clients to contact the
   server (e.g. similarly to receiving "Retry-after: 1000000").

   To mitigate this risk, clients can set thresholds that they consider
   reasonable in terms of quota units, time window, concurrent requests
   or throughput, and define a consistent behavior when the RateLimit
   exceed those thresholds.  For example this means capping the maximum
   number of request per second, or implementing retries when the reset
   parameter exceeds ten minutes.

   The considerations above are not limited to RateLimit header fields,
   but apply to all fields affecting how clients behave in subsequent
   requests (e.g.  Retry-After).

8.  Privacy Considerations

   Clients that act upon a request to rate limit are potentially re-
   identifiable (see Section 5.2.1 of [PRIVACY]) because they react to
   information that might only be given to them.  Note that this might
   apply to other fields too (e.g.  Retry-After).

   Since rate limiting is usually implemented in contexts where clients
   are either identified or profiled (e.g. assigning different quota
   units to different users), this is rarely a concern.

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   Privacy enhancing infrastructures using RateLimit header fields can
   define specific techniques to mitigate the risks of re-
   identification.

9.  IANA Considerations

   IANA is requested to update one registry and create one new registry.

   Please add the following entries to the "Hypertext Transfer Protocol
   (HTTP) Field Name Registry" registry ([HTTP]):

         +==================+===========+=======================+
         | Field Name       | Status    | Specification         |
         +==================+===========+=======================+
         | RateLimit        | permanent | Section 4 of RFC nnnn |
         +------------------+-----------+-----------------------+
         | RateLimit-Policy | permanent | Section 3 of RFC nnnn |
         +------------------+-----------+-----------------------+

                                 Table 1

9.1.  RateLimit quota unit registry

   This specification establishes the registry "Hypertext Transfer
   Protocol (HTTP) RateLimit Quota Units" registry to be located at
   https://www.iana.org/assignments/http-ratelimit-quota-units.
   Registration is done on the advice of a Designated Expert, appointed
   by the IESG or their delegate.  All entries are Specification
   Required ([IANA], Section 4.6).

   The registry has the following initial content:

                   +===============+===========+=======+
                   | Quota Unit    | Reference | Notes |
                   +===============+===========+=======+
                   | request       | RFC nnnn  |       |
                   +---------------+-----------+-------+
                   | content-bytes | RFC nnnn  |       |
                   +---------------+-----------+-------+

                                  Table 2

9.1.1.  Registration Template

   The registration template for the RateLimit Quota Units registry is
   as follows:

   *  Quota Unit: The name of the quota unit.

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   *  Reference: A reference to the document that specifies the quota
      unit.

   *  Notes: Any additional notes about the quota unit.

10.  References

10.1.  Normative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [IANA]     Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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/rfc/rfc2119>.

   [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/rfc/rfc8174>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

   [WEB-ORIGIN]
              Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/rfc/rfc6454>.

10.2.  Informative References

   [HPACK]    Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7541>.

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   [HTTP-CACHING]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9111>.

   [PRIVACY]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/rfc/rfc6973>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/rfc/rfc3339>.

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

   [UNIX]     The Open Group, "The Single UNIX Specification, Version 2
              - 6 Vol Set for UNIX 98", February 1997.

Appendix A.  Rate-limiting and quotas

   Servers use quota mechanisms to avoid systems overload, to ensure an
   equitable distribution of computational resources or to enforce other
   policies - e.g. monetization.

   A basic quota mechanism limits the number of acceptable requests in a
   given time window, e.g. 10 requests per second.

   When quota is exceeded, servers usually do not serve the request
   replying instead with a 4xx HTTP status code (e.g. 429 or 403) or
   adopt more aggressive policies like dropping connections.

   Quotas may be enforced on different basis (e.g. per user, per IP, per
   geographic area, ..) and at different levels.  For example, an user
   may be allowed to issue:

   *  10 requests per second;

   *  limited to 60 requests per minute;

   *  limited to 1000 requests per hour.

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   Moreover system metrics, statistics and heuristics can be used to
   implement more complex policies, where the number of acceptable
   requests and the time window are computed dynamically.

   To help clients throttling their requests, servers may expose the
   counters used to evaluate quota policies via HTTP header fields.

   Those response headers may be added by HTTP intermediaries such as
   API gateways and reverse proxies.

   On the web we can find many different rate-limit headers, usually
   containing the number of allowed requests in a given time window, and
   when the window is reset.

   The common choice is to return three headers containing:

   *  the maximum number of allowed requests in the time window;

   *  the number of remaining requests in the current window;

   *  the time remaining in the current window expressed in seconds or
      as a timestamp;

A.1.  Interoperability issues

   A major interoperability issue in throttling is the lack of standard
   headers, because:

   *  each implementation associates different semantics to the same
      header field names;

   *  header field names proliferates.

   User agents interfacing with different servers may thus need to
   process different headers, or the very same application interface
   that sits behind different reverse proxies may reply with different
   throttling headers.

Appendix B.  Examples

B.1.  Responses without defining policies

   Some servers may not expose the policy limits in the RateLimit-Policy
   header field.  Clients can still use the RateLimit header field to
   throttle their requests.

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B.1.1.  Throttling information in responses

   The client exhausted its quota for the next 50 seconds.  The limit
   and time-window is communicated out-of-band.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit: "default";r=0;t=50

   {"hello": "world"}

   Since the field values are not necessarily correlated with the
   response status code, a subsequent request is not required to fail.
   The example below shows that the server decided to serve the request
   even if remaining parameter value is 0.  Another server, or the same
   server under other load conditions, could have decided to throttle
   the request instead.

   Request:

   GET /items/456 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit: "default";r=0;t=48

   {"still": "successful"}

B.1.2.  Multiple policies in response

   The server uses two different policies to limit the client's
   requests:

   *  5000 daily quota units;

   *  1000 hourly quota units.

   The client consumed 4900 quota units in the first 14 hours.

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   Despite the next hourly limit of 1000 quota units, the closest limit
   to reach is the daily one.

   The server then exposes the RateLimit header fields to inform the
   client that:

   *  it has only 100 quota units left in the daily quota and the window
      will reset in 10 hours;

   The server MAY choose to omit returning the hourly policy as it uses
   the same quota units as the daily policy and the daily policy is the
   one that is closest to being exhausted.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit: "dayLimit";r=100;t=36000

   {"hello": "world"}

B.1.3.  Use for limiting concurrency

   RateLimit header fields may be used to limit concurrency, advertising
   limits that are lower than the usual ones in case of saturation, thus
   increasing availability.

   The server adopted a basic policy of 100 quota units per minute, and
   in case of resource exhaustion adapts the returned values reducing
   both limit and remaining parameter values.

   After 2 seconds the client consumed 40 quota units

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

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   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Policy: "basic";q=100;w=60
   RateLimit: "basic";r=60;t=58

   {"elapsed": 2, "issued": 40}

   At the subsequent request - due to resource exhaustion - the server
   advertises only r=20.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Policy: "basic";q=100;w=60
   RateLimit: "basic";r=20;t=56

   {"elapsed": 4, "issued": 41}

B.1.4.  Use in throttled responses

   A client exhausted its quota and the server throttles it sending
   Retry-After.

   In this example, the values of Retry-After and RateLimit header field
   reference the same moment, but this is not a requirement.

   The 429 (Too Many Request) HTTP status code is just used as an
   example.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

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   HTTP/1.1 429 Too Many Requests
   Content-Type: application/json
   Date: Mon, 05 Aug 2019 09:27:00 GMT
   Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
   RateLimit: "default";r=0;t=5

   {
   "title": "Too Many Requests",
   "status": 429,
   "detail": "You have exceeded your quota"
   }

B.2.  Responses with defined policies

B.2.1.  Throttling window specified via parameter

   The client has 99 quota units left for the next 50 seconds.  The time
   window is communicated by the w parameter, so we know the throughput
   is 100 quota units per minute.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit: "fixedwindow";r=99;t=50
   RateLimit-Policy: "fixedwindow";q=100;w=60
   {"hello": "world"}

B.2.2.  Dynamic limits with parameterized windows

   The policy conveyed by the RateLimit header field states that the
   server accepts 100 quota units per minute.

   To avoid resource exhaustion, the server artificially lowers the
   actual limits returned in the throttling headers.

   The remaining parameter then advertises only 9 quota units for the
   next 50 seconds to slow down the client.

   Note that the server could have lowered even the other values in the
   RateLimit header field: this specification does not mandate any
   relation between the field values contained in subsequent responses.

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   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Policy: "dynamic";q=100;w=60
   RateLimit: "dynamic";r=9;t=50

   {
     "status": 200,
     "detail": "Just slow down without waiting."
   }

B.2.3.  Dynamic limits for pushing back and slowing down

   Continuing the previous example, let's say the client waits 10
   seconds and performs a new request which, due to resource exhaustion,
   the server rejects and pushes back, advertising r=0 for the next 20
   seconds.

   The server advertises a smaller window with a lower limit to slow
   down the client for the rest of its original window after the 20
   seconds elapse.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 429 Too Many Requests
   Content-Type: application/json
   RateLimit-Policy: "dynamic";q=15;w=20
   RateLimit: "dynamic";r=0;t=20

   {
     "status": 429,
     "detail": "Wait 20 seconds, then slow down!"
   }

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B.3.  Dynamic limits for pushing back with Retry-After and slow down

   Alternatively, given the same context where the previous example
   starts, we can convey the same information to the client via Retry-
   After, with the advantage that the server can now specify the
   policy's nominal limit and window that will apply after the reset,
   e.g. assuming the resource exhaustion is likely to be gone by then,
   so the advertised policy does not need to be adjusted, yet we managed
   to stop requests for a while and slow down the rest of the current
   window.

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 429 Too Many Requests
   Content-Type: application/json
   Retry-After: 20
   RateLimit-Policy: "dynamic";q=100;w=60
   RateLimit: "dynamic";r=15;t=40

   {
     "status": 429,
     "detail": "Wait 20 seconds, then slow down!"
   }

   Note that in this last response the client is expected to honor
   Retry-After and perform no requests for the specified amount of time,
   whereas the previous example would not force the client to stop
   requests before the reset time is elapsed, as it would still be free
   to query again the server even if it is likely to have the request
   rejected.

B.3.1.  Missing Remaining information

   The server does not expose remaining values (for example, because the
   underlying counters are not available).  Instead, it resets the limit
   counter every second.

   It communicates to the client the limit of 10 quota units per second
   always returning the limit and reset parameters.

   Request:

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   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Policy: quota;q=100;w=1
   RateLimit: quota;t=1

   {"first": "request"}

   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Policy: quota;q=10
   RateLimit: quota;t=1

   {"second": "request"}

B.3.2.  Use with multiple windows

   This is a standardized way of describing the policy detailed in
   Appendix B.1.2:

   *  5000 daily quota units;

   *  1000 hourly quota units.

   The client consumed 4900 quota units in the first 14 hours.

   Despite the next hourly limit of 1000 quota units, the closest limit
   to reach is the daily one.

   The server then exposes the RateLimit header fields to inform the
   client that:

   *  it has only 100 quota units left;

   *  the window will reset in 10 hours;

   *  the expiring-limit is 5000.

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   Request:

   GET /items/123 HTTP/1.1
   Host: api.example

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   RateLimit-Policy: "hour";q=1000;w=3600, "day";q=5000;w=86400
   RateLimit: "day";r=100;t=36000

   {"hello": "world"}

FAQ

   This section is to be removed before publishing as an RFC.

   1.  Why defining standard fields for throttling?

       To simplify enforcement of throttling policies and enable clients
       to constraint their requests to avoid being throttled.

   2.  Can I use RateLimit header fields in throttled responses (eg with
       status code 429)?

       Yes, you can.

   3.  Are those specs tied to RFC 6585?

       No.  [RFC6585] defines the 429 status code and we use it just as
       an example of a throttled request, that could instead use even
       403 or whatever status code.

   4.  Why is the partition key necessary?

       Without a partition key, a server can only effectively only have
       one scope (aka partition), which is impractical for most
       services, or it needs to communicate the scopes out-of-band.
       This prevents the development of generic connector code that can
       be used to prevent requests from being throttled.  Many APIs rely
       on API keys, user identity or client identity to allocate quota.
       As soon as a single client processes requests for more than one
       partition, the client needs to know the corresponding partition
       key to properly track requests against allocated quota.

   5.  Why using delay-seconds instead of a UNIX Timestamp?  Why not
       using subsecond precision?

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       Using delay-seconds aligns with Retry-After, which is returned in
       similar contexts, eg on 429 responses.

       Timestamps require a clock synchronization protocol (see
       Section 5.6.7 of [HTTP]).  This may be problematic (e.g. clock
       adjustment, clock skew, failure of hardcoded clock
       synchronization servers, IoT devices, ..).  Moreover timestamps
       may not be monotonically increasing due to clock adjustment.  See
       Another NTP client failure story
       (https://community.ntppool.org/t/another-ntp-client-failure-
       story/1014/)

       We did not use subsecond precision because:

       *  that is more subject to system clock correction like the one
          implemented via the adjtimex() Linux system call;

       *  response-time latency may not make it worth.  A brief
          discussion on the subject is on the httpwg ml
          (https://lists.w3.org/Archives/Public/ietf-http-
          wg/2019JulSep/0202.html)

       *  almost all rate-limit headers implementations do not use it.

   6.  Shouldn't I limit concurrency instead of request rate?

       You can use this specification to limit concurrency at the HTTP
       level (see {#use-for-limiting-concurrency}) and help clients to
       shape their requests avoiding being throttled out.

       A problematic way to limit concurrency is connection dropping,
       especially when connections are multiplexed (e.g.  HTTP/2)
       because this results in unserviced client requests, which is
       something we want to avoid.

       A semantic way to limit concurrency is to return 503 + Retry-
       After in case of resource saturation (e.g. thrashing, connection
       queues too long, Service Level Objectives not meet, ..).
       Saturation conditions can be either dynamic or static: all this
       is out of the scope for the current document.

   7.  Do a positive value of remaining paramter imply any service
       guarantee for my future requests to be served?

       No.  FAQ integrated in Section 4.1.1.

   8.  Is the quota-policy definition too complex?

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       You can always return the simplest form

   RateLimit:"default";r=50;t=60

   The policy key clearly connects the current usage status of a policy
   to the defined limits.  So for the following field:

   RateLimit-Policy: "sliding";q=100;w=60;burst=1000
   RateLimit-Policy: "fixed";q=5000;w=3600;burst=0
   RateLimit: "sliding";r=50;t=44

   the value "sliding" identifies the policy being reported.

   1.  Can intermediaries alter RateLimit header fields?

       Generally, they should not because it might result in unserviced
       requests.  There are reasonable use cases for intermediaries
       mangling RateLimit header fields though, e.g. when they enforce
       stricter quota-policies, or when they are an active component of
       the service.  In those case we will consider them as part of the
       originating infrastructure.

   2.  Why the w parameter is just informative?  Could it be used by a
       client to determine the request rate?

       A non-informative w parameter might be fine in an environment
       where clients and servers are tightly coupled.  Conveying
       policies with this detail on a large scale would be very complex
       and implementations would be likely not interoperable.  We thus
       decided to leave w as an informational parameter and only rely on
       the limit, remaining and reset parameters for defining the
       throttling behavior.

   3.  Can I use RateLimit fields in trailers?  Servers usually
       establish whether the request is in-quota before creating a
       response, so the RateLimit field values should be already
       available in that moment.  Supporting trailers has the only
       advantage that allows to provide more up-to-date information to
       the client in case of slow responses.  However, this complicates
       client implementations with respect to combining fields from
       headers and accounting for intermediaries that drop trailers.
       Since there are no current implementations that use trailers, we
       decided to leave this as a future-work.

RateLimit header fields currently used on the web

   This section is to be removed before publishing as an RFC.

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   Commonly used header field names are:

   *  X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset;

   There are variants too, where the window is specified in the header
   field name, eg:

   *  x-ratelimit-limit-minute, x-ratelimit-limit-hour, x-ratelimit-
      limit-day

   *  x-ratelimit-remaining-minute, x-ratelimit-remaining-hour, x-
      ratelimit-remaining-day

   Here are some interoperability issues:

   *  X-RateLimit-Remaining references different values, depending on
      the implementation:

      -  seconds remaining to the window expiration

      -  milliseconds remaining to the window expiration

      -  seconds since UTC, in UNIX Timestamp [UNIX]

      -  a datetime, either IMF-fixdate [HTTP] or [RFC3339]

   *  different headers, with the same semantic, are used by different
      implementers:

      -  X-RateLimit-Limit and X-Rate-Limit-Limit

      -  X-RateLimit-Remaining and X-Rate-Limit-Remaining

      -  X-RateLimit-Reset and X-Rate-Limit-Reset

   The semantic of RateLimit depends on the windowing algorithm.  A
   sliding window policy for example, may result in having a remaining
   parameter value related to the ratio between the current and the
   maximum throughput. e.g.

   RateLimit-Policy: "sliding";q=12;w=1
   ; using 50% of throughput, that is 6 units/s
   RateLimit: "sliding";q=12;r=6;t=1

   If this is the case, the optimal solution is to achieve

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   RateLimit-Policy: "sliding";q=12;w=1
   ; using 100% of throughput, that is 12 units/s
   RateLimit: "sliding";q=12;r=1;t=1

   At this point you should stop increasing your request rate.

Acknowledgements

   Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro
   Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark
   Nottingham for being the initial contributors of these
   specifications.  Kudos to the first community implementers: Aapo
   Talvensaari, Nathan Friedly and Sanyam Dogra.

   In addition to the people above, this document owes a lot to the
   extensive discussion in the HTTPAPI workgroup, including Rich Salz,
   and Julian Reschke.

Changes

   This section is to be removed before publishing as an RFC.

Since draft-ietf-httpapi-ratelimit-headers-07

   This section is to be removed before publishing as an RFC.

   *  Refactored both fields to lists of Items that identify policy and
      use parameters

   *  Added quota unit parameter

   *  Added partition key parameter

Since draft-ietf-httpapi-ratelimit-headers-03

   This section is to be removed before publishing as an RFC.

   *  Split policy informatiom in RateLimit-Policy #81

Since draft-ietf-httpapi-ratelimit-headers-02

   This section is to be removed before publishing as an RFC.

   *  Address throttling scope #83

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Since draft-ietf-httpapi-ratelimit-headers-01

   This section is to be removed before publishing as an RFC.

   *  Update IANA considerations #60

   *  Use Structured fields #58

   *  Reorganize document #67

Since draft-ietf-httpapi-ratelimit-headers-00

   This section is to be removed before publishing as an RFC.

   *  Use I-D.httpbis-semantics, which includes referencing delay-
      seconds instead of delta-seconds. #5

Authors' Addresses

   Roberto Polli
   Team Digitale, Italian Government
   Italy
   Email: robipolli@gmail.com

   Alejandro Martinez Ruiz
   Red Hat
   Email: alex@flawedcode.org

   Darrel Miller
   Microsoft
   Email: darrel@tavis.ca

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