RateLimit Header Fields for HTTP
draft-ietf-httpapi-ratelimit-headers-01

Document Type Active Internet-Draft (httpapi WG)
Authors Roberto Polli  , Alex Martínez 
Last updated 2021-05-11
Replaces draft-polli-ratelimit-headers
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HTTPAPI                                                         R. Polli
Internet-Draft                         Team Digitale, Italian Government
Intended status: Standards Track                             A. Martinez
Expires: 12 November 2021                                        Red Hat
                                                             11 May 2021

                    RateLimit Header Fields for HTTP
                draft-ietf-httpapi-ratelimit-headers-01

Abstract

   This document defines the RateLimit-Limit, RateLimit-Remaining,
   RateLimit-Reset fields for HTTP, thus allowing servers to publish
   current service limits and clients to shape their request policy and
   avoid being throttled out.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (httpapi@ietf.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-httpapi-wg/
   (https://lists.w3.org/Archives/Public/ietf-httpapi-wg/).

   The source code and issues list for this draft can be found at
   https://github.com/ietf-wg-httpapi/ratelimit-headers
   (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/.

   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 12 November 2021.

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Copyright Notice

   Copyright (c) 2021 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Rate-limiting and quotas  . . . . . . . . . . . . . . . .   3
     1.2.  Current landscape of rate-limiting headers  . . . . . . .   4
       1.2.1.  Interoperability issues . . . . . . . . . . . . . . .   4
     1.3.  This proposal . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Notational Conventions  . . . . . . . . . . . . . . . . .   6
   2.  Expressing rate-limit policies  . . . . . . . . . . . . . . .   6
     2.1.  Time window . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Service limit . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Quota policy  . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Header Specifications . . . . . . . . . . . . . . . . . . . .   8
     3.1.  RateLimit-Limit . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  RateLimit-Remaining . . . . . . . . . . . . . . . . . . .   9
     3.3.  RateLimit-Reset . . . . . . . . . . . . . . . . . . . . .   9
   4.  Providing RateLimit fields  . . . . . . . . . . . . . . . . .  10
     4.1.  Performance considerations  . . . . . . . . . . . . . . .  11
   5.  Intermediaries  . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Caching . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Receiving RateLimit fields  . . . . . . . . . . . . . . . . .  12
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Unparameterized responses . . . . . . . . . . . . . . . .  13
       8.1.1.  Throttling information in responses . . . . . . . . .  13
       8.1.2.  Use in conjunction with custom fields . . . . . . . .  14
       8.1.3.  Use for limiting concurrency  . . . . . . . . . . . .  15
       8.1.4.  Use in throttled responses  . . . . . . . . . . . . .  16
     8.2.  Parameterized responses . . . . . . . . . . . . . . . . .  17
       8.2.1.  Throttling window specified via parameter . . . . . .  17
       8.2.2.  Dynamic limits with parameterized windows . . . . . .  17
       8.2.3.  Dynamic limits for pushing back and slowing down  . .  18
     8.3.  Dynamic limits for pushing back with Retry-After and slow
           down  . . . . . . . . . . . . . . . . . . . . . . . . . .  19

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       8.3.1.  Missing Remaining information . . . . . . . . . . . .  20
       8.3.2.  Use with multiple windows . . . . . . . . . . . . . .  20
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
     9.1.  Throttling does not prevent clients from issuing
           requests  . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.2.  Information disclosure  . . . . . . . . . . . . . . . . .  21
     9.3.  Remaining quota-units are not granted requests  . . . . .  22
     9.4.  Reliability of RateLimit-Reset  . . . . . . . . . . . . .  22
     9.5.  Resource exhaustion . . . . . . . . . . . . . . . . . . .  22
     9.6.  Denial of Service . . . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  RateLimit-Limit Field Registration . . . . . . . . . . .  23
     10.2.  RateLimit-Remaining Field Registration . . . . . . . . .  24
     10.3.  RateLimit-Reset Field Registration . . . . . . . . . . .  24
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  25
   Appendix B.  FAQ  . . . . . . . . . . . . . . . . . . . . . . . .  25
   RateLimit fields currently used on the web  . . . . . . . . . . .  29
   Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   The widespreading of HTTP as a distributed computation protocol
   requires an explicit way of communicating service status and usage
   quotas.

   This was partially addressed by the "Retry-After" header field
   defined in [SEMANTICS] to be returned in "429 Too Many Requests" (see
   [STATUS429]) or "503 Service Unavailable" responses.

   Still, there is not a standard way to communicate service quotas so
   that the client can throttle its requests and prevent 4xx or 5xx
   responses.

1.1.  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 - eg. monetization.

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

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

   Quotas may be enforced on different basis (eg. 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 request per minute;

   *  limited to 1000 request per hour.

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

1.2.  Current landscape of rate-limiting headers

   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;

1.2.1.  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.

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   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.

1.3.  This proposal

   This proposal defines syntax and semantics for the following fields:

   *  "RateLimit-Limit": containing the requests quota in the time
      window;

   *  "RateLimit-Remaining": containing the remaining requests quota in
      the current window;

   *  "RateLimit-Reset": containing the time remaining in the current
      window, specified in seconds.

   The behavior of "RateLimit-Reset" is compatible with the "delay-
   seconds" notation of "Retry-After".

   The fields definition allows to describe complex policies, including
   the ones using multiple and variable time windows and dynamic quotas,
   or implementing concurrency limits.

1.4.  Goals

   The goals of this proposal are:

   1.  Standardizing the names and semantics of rate-limit headers to
       ease their enforcement and adoption;

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

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

   The goals do not include:

   Authorization:  The rate-limit fields described here are not meant to
      support authorization or other kinds of access controls.

   Throttling scope:  This specification does not cover the throttling
      scope, that may be the given resource-target, its parent path or
      the whole Origin (see Section 7 of [RFC6454]).

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

   Throttling policy:  This specification does not mandate a specific
      throttling policy.  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
      service guarantee.  Server is free to throttle respectful clients
      under certain circumstances.

1.5.  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.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405] along with the "#rule" extension defined in
   Section 5.6.1 of [SEMANTICS].

   The term Origin is to be interpreted as described in Section 7 of
   [RFC6454].

   The "delay-seconds" rule is defined in Section 10.2.4 of [SEMANTICS].

2.  Expressing rate-limit policies

2.1.  Time window

   Rate limit policies limit the number of acceptable requests in a
   given time window.

   A time window is expressed in seconds, using the following syntax:

   time-window = delay-seconds

   Subsecond precision is not supported.

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2.2.  Service limit

   The service-limit is a value associated to the maximum number of
   requests that the server is willing to accept from one or more
   clients on a given basis (originating IP, authenticated user,
   geographical, ..) during a "time-window" as defined in Section 2.1.

   The "service-limit" is expressed in "quota-units" and has the
   following syntax:

      service-limit = quota-units
      quota-units = 1*DIGIT

   The "service-limit" SHOULD match the maximum number of acceptable
   requests.

   The "service-limit" MAY differ from the total number of acceptable
   requests when weight mechanisms, bursts, or other server policies are
   implemented.

   If the "service-limit" does not match the maximum number of
   acceptable requests the relation with that SHOULD be communicated
   out-of-band.

   Example: A server could

   *  count once requests like "/books/{id}"

   *  count twice search requests like "/books?author=Camilleri"

   so that we have the following counters

   GET /books/123                  ; service-limit=4, remaining: 3, status=200
   GET /books?author=Camilleri     ; service-limit=4, remaining: 1, status=200
   GET /books?author=Eco           ; service-limit=4, remaining: 0, status=429

2.3.  Quota policy

   This specification allows describing a quota policy with the
   following syntax:

      quota-policy = service-limit; "w" "=" time-window
                     *( OWS ";" OWS quota-comment)
      quota-comment = token "=" (token / quoted-string)

   quota-policy parameters like "w" and quota-comment tokens MUST NOT
   occur multiple times within the same quota-policy.

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   An example policy of 100 quota-units per minute.

      100;w=60

   The definition of a quota-policy does not imply any specific
   distribution of quota-units over time.  Such service specific details
   can be conveyed in the "quota-comments".

   Two examples of providing further details via custom parameters in
   "quota-comments".

      100;w=60;comment="fixed window"
      12;w=1;burst=1000;policy="leaky bucket"

3.  Header Specifications

   The following "RateLimit" response fields are defined

3.1.  RateLimit-Limit

   The "RateLimit-Limit" response field indicates the "service-limit"
   associated to the client in the current "time-window".

   If the client exceeds that limit, it MAY not be served.

   The field value is

      RateLimit-Limit = expiring-limit [, 1#quota-policy ]
      expiring-limit = service-limit

   The "expiring-limit" value MUST be set to the "service-limit" that is
   closer to reach its limit.

   The "quota-policy" is defined in Section 2.3, and its values are
   informative.

      RateLimit-Limit: 100

   A "time-window" associated to "expiring-limit" can be communicated
   via an optional "quota-policy" value, like shown in the following
   example

      RateLimit-Limit: 100, 100;w=10

   If the "expiring-limit" is not associated to a "time-window", the
   "time-window" MUST either be:

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   *  inferred by the value of "RateLimit-Reset" at the moment of the
      reset, or

   *  communicated out-of-band (eg. in the documentation).

   Policies using multiple quota limits MAY be returned using multiple
   "quota-policy" items, like shown in the following two examples:

      RateLimit-Limit: 10, 10;w=1, 50;w=60, 1000;w=3600, 5000;w=86400
      RateLimit-Limit: 10, 10;w=1;burst=1000, 1000;w=3600

   This field MUST NOT occur multiple times and can be sent in a trailer
   section.

3.2.  RateLimit-Remaining

   The "RateLimit-Remaining" response field indicates the remaining
   "quota-units" defined in Section 2.2 associated to the client.

   The field value is

      RateLimit-Remaining = quota-units

   This field MUST NOT occur multiple times and can be sent in a trailer
   section.

   Clients MUST NOT assume that a positive "RateLimit-Remaining" value
   is a guarantee that further requests will be served.

   A low "RateLimit-Remaining" value is like a yellow traffic-light for
   either the number of requests issued in the "time-window" or the
   request throughput: the red light may arrive suddenly (see
   Section 4).

   One example of "RateLimit-Remaining" use is below.

      RateLimit-Remaining: 50

3.3.  RateLimit-Reset

   The "RateLimit-Reset" response field indicates either

   *  the number of seconds until the quota resets.

   The field value is

      RateLimit-Reset = delay-seconds

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   The delay-seconds format is used 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 [SEMANTICS]);

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

   This field MUST NOT occur multiple times and can be sent in a trailer
   section.

   An example of "RateLimit-Reset" use is below.

      RateLimit-Reset: 50

   The client MUST NOT assume that all its "service-limit" will be
   restored after the moment referenced by "RateLimit-Reset".  The
   server MAY arbitrarily alter the "RateLimit-Reset" value between
   subsequent requests eg. in case of resource saturation or to
   implement sliding window policies.

4.  Providing RateLimit fields

   A server MAY use one or more "RateLimit" response fields defined in
   this document to communicate its quota policies.

   The returned values refers to the metrics used to evaluate if the
   current request respects the quota policy and MAY not apply to
   subsequent requests.

   Example: a successful response with the following fields

      RateLimit-Limit: 10
      RateLimit-Remaining: 1
      RateLimit-Reset: 7

   does not guarantee that the next request will be successful.  Server
   metrics may be subject to other conditions like the one shown in the
   example from Section 2.2.

   A server MAY return "RateLimit" response fields independently of the
   response status code.  This includes throttled responses.

   This document does not mandate any correlation between the
   "RateLimit" values and the returned status code.

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   Servers should be careful in returning "RateLimit" fields in
   redirection responses (eg. 3xx status codes) because a low
   "RateLimit-Remaining" value could limit the client from issuing
   requests.  For example, given the rate limiting fields below, a
   client could decide to wait 10 seconds before following the
   "Location" header, because "RateLimit-Remaining" is 0.

   HTTP/1.1 301 Moved Permanently
   Location: /foo/123
   RateLimit-Remaining: 0
   RateLimit-Limit: 10
   RateLimit-Reset: 10

   If a response contains both the "Retry-After" and the "RateLimit-
   Reset" fields, the value of "RateLimit-Reset" SHOULD reference the
   same point in time as "Retry-After".

   When using a policy involving more than one "time-window", the server
   MUST reply with the "RateLimit" fields related to the window with the
   lower "RateLimit-Remaining" values.

   A service returning "RateLimit" fields MUST NOT convey values
   exposing an unwanted volume of requests and SHOULD implement
   mechanisms to cap the ratio between "RateLimit-Remaining" and
   "RateLimit-Reset" (see Section 9.5); this is especially important
   when quota-policies use a large "time-window".

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

   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.  Nonetheless servers MAY decide to send the
   "RateLimit" fields in a trailer section.

4.1.  Performance considerations

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

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

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5.  Intermediaries

   This section documents the considerations advised in Section 16.3.3
   of [SEMANTICS].

   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 fields' values in
   such a way as to communicate a more permissive quota-policy; this
   includes removing the RateLimit fields.

   An intermediary MAY alter the RateLimit 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 fields is
   the sole responsible of enforcing the communicated quota-policy, and
   it is always free to service incoming requests.

   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.

6.  Caching

   As is the ordinary case for HTTP caching ([RFC7234]), a response with
   RateLimit fields might be cached and re-used for subsequent requests.
   A cached "RateLimit" response does not modify quota counters but
   could contain stale information.  Clients interested in determining
   the freshness of the "RateLimit" fields could rely on fields such as
   "Date" and on the "time-window" of a "quota-policy".

7.  Receiving RateLimit fields

   A client MUST process the received "RateLimit" fields.

   A client MUST validate the values received in the "RateLimit" fields
   before using them and check if there are significant discrepancies
   with the expected ones.  This includes a "RateLimit-Reset" moment too
   far in the future or a "service-limit" too high.

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   A client receiving "RateLimit" fields MUST NOT assume that subsequent
   responses contain the same "RateLimit" fields, or any "RateLimit"
   fields at all.

   Malformed "RateLimit" fields MAY be ignored.

   A client SHOULD NOT exceed the "quota-units" expressed in "RateLimit-
   Remaining" before the "time-window" expressed in "RateLimit-Reset".

   A client MAY still probe the server if the "RateLimit-Reset" is
   considered too high.

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

   The "quota-policy" values and comments provided in "RateLimit-Limit"
   are informative and MAY be ignored.

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

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

   *  slowing down or preemptively backoff their request rate when
      approaching quota limits;

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

8.  Examples

8.1.  Unparameterized responses

8.1.1.  Throttling information in responses

   The client exhausted its service-limit for the next 50 seconds.  The
   "time-window" is communicated out-of-band or inferred by the field
   values.

   Request:

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

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   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Limit: 100
   Ratelimit-Remaining: 0
   Ratelimit-Reset: 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 "RateLimit-Remaining" 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-Limit: 100
   Ratelimit-Remaining: 0
   Ratelimit-Reset: 48

   {"still": "successful"}

8.1.2.  Use in conjunction with custom fields

   The server uses two custom fields, namely "acme-RateLimit-DayLimit"
   and "acme-RateLimit-HourLimit" to expose the following policy:

   *  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-*" fields to inform the client
   that:

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   *  it has only 100 quota-units left;

   *  the window will reset in 10 hours.

   Request:

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

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   acme-RateLimit-DayLimit: 5000
   acme-RateLimit-HourLimit: 1000
   RateLimit-Limit: 5000
   RateLimit-Remaining: 100
   RateLimit-Reset: 36000

   {"hello": "world"}

8.1.3.  Use for limiting concurrency

   Throttling 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 "RateLimit-Limit" and "RateLimit-Remaining".

   After 2 seconds the client consumed 40 quota-units

   Request:

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

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Limit: 100
   RateLimit-Remaining: 60
   RateLimit-Reset: 58

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

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   At the subsequent request - due to resource exhaustion - the server
   advertises only "RateLimit-Remaining: 20".

   Request:

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

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Limit: 100
   RateLimit-Remaining: 20
   RateLimit-Reset: 56

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

8.1.4.  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-Reset"
   reference the same moment, but this is not a requirement.

   The "429 Too Many Requests" 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-Reset: 5
   RateLimit-Limit: 100
   Ratelimit-Remaining: 0

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

8.2.  Parameterized responses

8.2.1.  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-Limit: 100, 100;w=60
   Ratelimit-Remaining: 99
   Ratelimit-Reset: 50

   {"hello": "world"}

8.2.2.  Dynamic limits with parameterized windows

   The policy conveyed by "RateLimit-Limit" 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 "RateLimit-Remaining" then advertises only 9 quota-units for the
   next 50 seconds to slow down the client.

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

   Request:

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

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Limit: 10, 100;w=60
   Ratelimit-Remaining: 9
   Ratelimit-Reset: 50

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

8.2.3.  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 "RateLimit-Remaining:
   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:

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   HTTP/1.1 429 Too Many Requests
   Content-Type: application/json
   RateLimit-Limit: 0, 15;w=20
   Ratelimit-Remaining: 0
   Ratelimit-Reset: 20

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

8.3.  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,
   ie. 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-Limit: 15, 100;w=60
   Ratelimit-Remaining: 15
   Ratelimit-Reset: 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.

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8.3.1.  Missing Remaining information

   The server does not expose "RateLimit-Remaining" values, but resets
   the limit counter every second.

   It communicates to the client the limit of 10 quota-units per second
   always returning the couple "RateLimit-Limit" and "RateLimit-Reset".

   Request:

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

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Limit: 10
   Ratelimit-Reset: 1

   {"first": "request"}

   Request:

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

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   RateLimit-Limit: 10
   Ratelimit-Reset: 1

   {"second": "request"}

8.3.2.  Use with multiple windows

   This is a standardized way of describing the policy detailed in
   Section 8.1.2:

   *  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" 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.

   Request:

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

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   RateLimit-Limit: 5000, 1000;w=3600, 5000;w=86400
   RateLimit-Remaining: 100
   RateLimit-Reset: 36000

   {"hello": "world"}

9.  Security Considerations

9.1.  Throttling does not prevent clients from issuing requests

   This specification does not prevent clients to make over-quota
   requests.

   Servers should always implement mechanisms to prevent resource
   exhaustion.

9.2.  Information disclosure

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

   While this specification does not mandate whether non 2xx responses
   consume quota, if 401 and 403 responses count on quota a malicious
   client could probe the endpoint to get traffic information of another
   user.

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   As intermediaries might retransmit requests and consume quota-units
   without prior knowledge of the User Agent, RateLimit fields might
   reveal the existence of an intermediary to the User Agent.

9.3.  Remaining quota-units are not granted requests

   "RateLimit-*" fields convey hints from the server to the clients in
   order to avoid being throttled out.

   Clients MUST NOT consider the "quota-units" returned in "RateLimit-
   Remaining" as a service level agreement.

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

9.4.  Reliability of RateLimit-Reset

   Consider that "service-limit" may not be restored after the moment
   referenced by "RateLimit-Reset", and the "RateLimit-Reset" value
   should not be considered fixed nor constant.

   Subsequent requests may return an higher "RateLimit-Reset" value to
   limit concurrency or implement dynamic or adaptive throttling
   policies.

9.5.  Resource exhaustion

   When returning "RateLimit-Reset" you 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 "RateLimit-Reset" accordingly

      Date: Tue, 15 Nov 1994 08:00:00 GMT
      RateLimit-Reset: 36000

   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 field-value.

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

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

   RateLimit-Limit: 10000, 10000;w=1000
   RateLimit-Remaining: 10000
   RateLimit-Reset: 10

   A client implementing a simple ratio between "RateLimit-Remaining"
   and "RateLimit-Reset" could infer an average throughput of 1000
   quota-units per second, while "RateLimit-Limit" 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 "RateLimit-
   Remaining" value or an higher "RateLimit-Reset" value.  Moreover,
   complementing large "time-window" quota-policies with a short "time-
   window" one mitigates those risks.

9.6.  Denial of Service

   "RateLimit" fields may assume unexpected values by chance or purpose.
   For example, an excessively high "RateLimit-Remaining" 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 an high "RateLimit-Reset" value could inhibit clients to contact
   the server.

   Clients MUST validate the received values to mitigate those risks.

10.  IANA Considerations

10.1.  RateLimit-Limit Field Registration

   This section registers the "RateLimit-Limit" field in the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" registry ([SEMANTICS]).

   Field name: "RateLimit-Limit"

   Status: permanent

   Specification document(s): Section 3.1 of this document

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10.2.  RateLimit-Remaining Field Registration

   This section registers the "RateLimit-Remaining" field in the
   "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry
   ([SEMANTICS]).

   Field name: "RateLimit-Remaining"

   Status: permanent

   Specification document(s): Section 3.2 of this document

10.3.  RateLimit-Reset Field Registration

   This section registers the "RateLimit-Reset" field in the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" registry ([SEMANTICS]).

   Field name: "RateLimit-Reset"

   Status: permanent

   Specification document(s): Section 3.3 of this document

11.  References

11.1.  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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

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

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [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>.

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   [SEMANTICS]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-15, 30 March 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              semantics-15.txt>.

11.2.  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/info/rfc7541>.

   [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/info/rfc3339>.

   [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>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [STATUS429]
              Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              RFC 6525, DOI 10.17487/RFC6525, February 2012,
              <https://www.rfc-editor.org/info/rfc6525>.

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

Appendix A.  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 implementors: Aapo
   Talvensaari, Nathan Friedly and Sanyam Dogra.

Appendix B.  FAQ

   1.  Why defining standard fields for throttling?

       To simplify enforcement of throttling policies.

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   2.  Can I use RateLimit-* 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.  The goal of this specification is
       to standardize the name and semantic of three ratelimit fields
       widely used on the internet.  Stricter relations with status
       codes or error response payloads would impose behaviors to all
       the existing implementations making the adoption more complex.

   4.  Why don't pass the throttling scope as a parameter?

       After a discussion on a similar thread
       (https://github.com/httpwg/http-core/pull/317#issuecomment-
       585868767) we will probably add a new "RateLimit-Scope" field to
       this spec.

       I'm open to suggestions: comment on this issue
       (https://github.com/ioggstream/draft-polli-ratelimit-headers/
       issues/70)

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

       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 [SEMANTICS]).  This may be problematic (eg.
       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;

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       *  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.  Why not support multiple quota remaining?

       While this might be of some value, my experience suggests that
       overly-complex quota implementations results in lower
       effectiveness of this policy.  This spec allows the client to
       easily focusing on RateLimit-Remaining and RateLimit-Reset.

   7.  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 (eg.  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 (eg. 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.

   8.  Do a positive value of "RateLimit-Remaining" imply any service
       guarantee for my future requests to be served?

       No.  FAQ integrated in Section 3.2.

   9.  Is the quota-policy definition Section 2.3 too complex?

       You can always return the simplest form of the 3 fields

   RateLimit-Limit: 100
   RateLimit-Remaining: 50
   RateLimit-Reset: 60

   The key runtime value is the first element of the list: "expiring-
   limit", the others "quota-policy" are informative.  So for the
   following field:

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   RateLimit-Limit: 100, 100;w=60;burst=1000;comment="sliding window", 5000;w=3600;burst=0;comment="fixed window"

   the key value is the one referencing the lowest limit: "100"

   1.  Can we use shorter names?  Why don't put everything in one field?

   The most common syntax we found on the web is "X-RateLimit-*" and
   when starting this I-D we opted for it
   (https://github.com/ioggstream/draft-polli-ratelimit-headers/
   issues/34#issuecomment-519366481)

   The basic form of those fields is easily parseable, even by
   implementors procesing responses using technologies like dynamic
   interpreter with limited syntax.

   Using a single field complicates parsing and takes a significantly
   different approach from the existing ones: this can limit adoption.

   1.  Why don't mention connections?

       Beware of the term "connection": &#65532; &#65532; - it is just
       _one_ possible saturation cause.  Once you go that path &#65532;
       you will expose other infrastructural details (bandwidth, CPU, ..
       see Section 9.2) &#65532; and complicate client compliance;
       &#65532; - it is an infrastructural detail defined in terms of
       server and network &#65532; rather than the consumed service.
       This specification protects the services first, and then the
       infrastructures through client cooperation (see Section 9.1).
       &#65532; &#65532; RateLimit fields enable sending _on the same
       connection_ different limit values &#65532; on each response,
       depending on the policy scope (eg. per-user, per-custom-key, ..)
       &#65532;

   2.  Can intermediaries alter RateLimit fields?

       Generally, they should not because it might result in unserviced
       requests.  There are reasonable use cases for intermediaries
       mangling RateLimit 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.

   3.  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

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       and implementations would be likely not interoperable.  We thus
       decided to leave "w" as an informational parameter and only rely
       on "RateLimit-Limit", "RateLimit-Remaining" and "RateLimit-Reset"
       for defining the throttling behavior.

RateLimit fields currently used on the web

   _RFC Editor: Please remove this section before publication._

   Commonly used header field names are:

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

   *  "X-Rate-Limit-Limit", "X-Rate-Limit-Remaining", "X-Rate-Limit-
      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" [SEMANTICS] 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

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   The semantic of RateLimit-Remaining depends on the windowing
   algorithm.  A sliding window policy for example may result in having
   a "RateLimit-Remaining" value related to the ratio between the
   current and the maximum throughput.  Eg.

   RateLimit-Limit: 12, 12;w=1
   RateLimit-Remaining: 6          ; using 50% of throughput, that is 6 units/s
   RateLimit-Reset: 1

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

   RateLimit-Limit: 12, 12;w=1
   RateLimit-Remaining: 1          ; using 100% of throughput, that is 12 units/s
   RateLimit-Reset: 1

   At this point you should stop increasing your request rate.

Changes

   _RFC Editor: Please remove this section before publication._

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

   *  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: amr@redhat.com

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