Network Working Group R. Polli
Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track September 05, 2019
Expires: March 8, 2020

RateLimit Header Fields for HTTP


This document defines the RateLimit-Limit, RateLimit-Remaining, RateLimit-Reset header fields for HTTP, thus allowing servers to publish current request quotas 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 (, which is archived at

The source code and issues list for this draft can be found at

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Table of Contents

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 with the Retry-After header field defined in [RFC7231] to be returned in 429 Too Many Requests 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.

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:

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:

1.2.1. Interoperability issues

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

Client applications 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 header fields:

The behavior of RateLimit-Reset is compatible with the one of Retry-After.

The preferred syntax for RateLimit-Reset is the seconds notation respect to the timestamp one.

The header 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 semantic of rate-limit headers;
  2. Improve resiliency of HTTP infrastructures simplifying the enforcement and the adoption of rate-limit headers;
  3. Simplify API documentation avoiding expliciting rate-limit header fields semantic in documentation.

The goals do not include:

The rate-limit headers 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 [RFC6454] section 7.
Response status code:
The rate-limit headers may be returned in both Successful and non Successful responses. This specification does not cover whether non Successful responses count on quota usage.
Throttling policy:
This specification does not mandate a specific throttling policy. The values published in the headers, 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] and [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 7 of [RFC7230].

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

The “delta-seconds” rule is defined in [RFC7234] section 1.2.1.

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 = delta-seconds

Subsecond precision is not supported.

2.2. Request quota

The request-quota 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 request-quota is expressed in quota-units and has the following syntax:

request-quota = quota-units
quota-units = 1*DIGIT

The request-quota SHOULD match the maximum number of acceptable requests.

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

If the request-quota does not match the maximum number of acceptable requests the relation with that SHOULD be communicated out-of-bound.

Example: A server could

so that we have the following counters

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

2.3. Quota policy

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

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

An example policy of 100 quota-units per minute.


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

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

3. Header Specifications

The following RateLimit response header fields are defined

3.1. RateLimit-Limit

The RateLimit-Limit response header field indicates the request-quota associated to the client in the current time-window.

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

The header value is

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

The expiring-limit value MUST be set to the request-quota 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;window=10

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

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

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

3.2. RateLimit-Remaining

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

The header syntax is:

RateLimit-Remaining = quota-units

Clients MUST NOT assume that a positive RateLimit-Remaining value is a guarantee of being served.

A low RateLimit-Remaining value is like a yellow traffic-light: the red light may arrive suddenly.

One example of RateLimit-Remaining use is below.

   RateLimit-Remaining: 50

3.3. RateLimit-Reset

The RateLimit-Reset response header field indicates either

The header value is:

RateLimit-Reset = delta-seconds / IMF-fixdate

The IMF-fixdate format is defined in [RFC7231] appendix D.

The RateLimit-Reset value:

The IMF-fixdate format is NOT RECOMMENDED.

The preferred format is the delta-seconds one, because:

Two examples of RateLimit-Reset use are below.

   RateLimit-Reset: 50                              ; preferred delta-seconds notation
   RateLimit-Reset: Tue, 15 Nov 1994 08:12:31 GMT   ; IMF-fixdate notation

The client MUST NOT give for granted that all its request-quota 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 headers

A server MAY use one or more RateLimit response header 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 header 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 header fields independently of the response status code. This includes throttled responses.

If a response contains both the Retry-After and the RateLimit-Reset header fields, the value of RateLimit-Reset MUST be consistent with the one of Retry-After.

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

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

5. Receiving RateLimit headers

A client MUST process the received RateLimit headers.

A client MUST validate the values received in the RateLimit headers 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 request-quota too high.

Malformed RateLimit headers 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 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 header fields, the Retry-After header field MUST take precedence and the RateLimit-Reset header field MAY be ignored.

6. Examples

6.1. Unparameterized responses

6.1.1. Throttling informations in responses

The client exhausted its request-quota for the next 50 seconds. The time-window is communicated out-of-bound or inferred by the header values.


  GET /items/123


  HTTP/1.1 200 Ok
  Content-Type: application/json
  RateLimit-Limit: 100
  Ratelimit-Remaining: 0
  Ratelimit-Reset: 50

  {"hello": "world"}

6.1.2. Use in conjunction with custom headers

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

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-* headers to inform the client that:


  GET /items/123


  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"}

6.1.3. Use for limiting concurrency

Throttling headers 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


  GET /items/123


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

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

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


  GET /items/123


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

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

6.1.4. Use in throttled responses

A client exhausted its quota and the server throttles the request sending the Retry-After response header field.

The values of Retry-After and RateLimit-Reset are consistent as they reference the same moment.

The 429 Too Many Requests HTTP status code is just used as an example.


  GET /items/123


  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"

6.2. Parameterized responses

6.2.1. Throttling window specified via parameter

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


  GET /items/123


  HTTP/1.1 200 Ok
  Content-Type: application/json
  RateLimit-Limit: 100, 100;window=60
  Ratelimit-Remaining: 99
  Ratelimit-Reset: 50

  {"hello": "world"}

6.2.2. Dynamic limits with parameterized windows

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

Due to resource exhaustion, the server artificially lowers the actual limits returned in the throttling headers.

The current policy advertises then only 9 quota-units in the next 50 seconds.

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


  GET /items/123


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

  {"hello": "world"}

6.2.3. Missing Remaining informations

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.


  GET /items/123


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

  {"first": "request"}

  GET /items/123


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

  {"second": "request"}

6.2.4. Use with multiple windows

This is a standardized way of describing the policy detailed in Section 6.1.2:

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 headers to inform the client that:


  GET /items/123


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

  {"hello": "world"}

7. Security Considerations

7.1. 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.

7.2. Information disclosure

Servers should not disclose operational capacity informations 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 informations of another user .

7.3. Remaining quota-units are not granted requests

RateLimit-* headers 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.

7.4. Reliability of RateLimit-Reset

Consider that request-quota 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.

7.5. Resource exhaustion and clock skew

Implementers returning RateLimit-Reset must be aware that many throttled clients may come back at the very moment specified. For example, when returning

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

there’s a high probability that all clients will show up at 08:00:00.

This could be mitigated adding some jitter to the header value.

7.6. Denial of Service

RateLimit header fields may assume unexpected values by chance or purpose. For example, an excessively high RateLimit-Remaining value may be:

or an high RateLimit-Reset value could inhibit clients to contact the server.

Clients MUST validate the received values to mitigate those risks.

8. IANA Considerations

8.1. RateLimit-Limit Header Field Registration

This section registers the RateLimit-Limit header field in the “Permanent Message Header Field Names” registry ([RFC3864]).

Header field name: RateLimit-Limit

Applicable protocol: http

Status: standard

Author/Change controller: IETF

Specification document(s): Section 3.1 of this document

8.2. RateLimit-Remaining Header Field Registration

This section registers the RateLimit-Remaining header field in the “Permanent Message Header Field Names” registry ([RFC3864]).

Header field name: RateLimit-Remaining

Applicable protocol: http

Status: standard

Author/Change controller: IETF

Specification document(s): Section 3.2 of this document

8.3. RateLimit-Reset Header Field Registration

This section registers the RateLimit-Reset header field in the “Permanent Message Header Field Names” registry ([RFC3864]).

Header field name: RateLimit-Reset

Applicable protocol: http

Status: standard

Author/Change controller: IETF

Specification document(s): Section 3.3 of this document

9. References

9.1. 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.
[RFC3864] Klyne, G., Nottingham, M. and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014.
[RFC7234] Fielding, R., Nottingham, M. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[UNIX] The Open Group, ., "The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98", February 1997.

9.2. Informative References

[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012.

Appendix A. Change Log


Appendix B. Acknowledgements

Thanks to Willi Schoenborn, Alessandro Ranellucci, Erik Wilde and Mark Nottingham for being the initial contributors of this specifications.

Appendix C. RateLimit headers currently used on the web


Commonly used header field names are:

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

Here are some interoperability issues:

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;window=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;window=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.

Appendix D. FAQ

  1. Why defining standard headers for throttling?

    To simplify enforcement of throttling policies.
  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.
  4. Why don’t pass the trottling scope as a parameter?

    I’m open to suggestions. File an issue if you think it’s worth ;).
  5. Why using delta-seconds instead of UNIX Timestamp? Why IMF-fixdate is NOT RECOMMENDED? Why not using subsecond precision?

    Using delta-seconds permits to align with Retry-After, which is returned in similar contexts, eg on 429 responses.

    delta-seconds as defined in [RFC7234] section 1.2.1 clarifies some parsing rules too.

    As explained in [RFC7231] section using IMF-fixdate requires a clock synchronization protocol. This may be problematic (eg. clock skew, failure of hardcoded clock synchronization servers, IoT devices, ..). See Another NTP client failure story

    We did not use subsecond precision because almost all rate-limit headers implementations do not use it. Conveyed values are subject to response-time latency. A brief discussion on the subject is on the httpwg ml
  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 do both. The goal of this spec is to provide guidance for clients in shaping their requests without being throttled out.

    Limiting concurrency results in unserviced client requests, which is something we want to avoid.

    A standard 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, ..).

    Dynamically lowering the values returned by the rate-limit headers, and returning retry-after along with them can improve availability.

    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. The returned values were used to decide whether to serve or not the current request and do not imply any guarantee that future requests will be successful.

    Instead they help to understand when future requests will probably be throttled. A low value for RateLimit-Remaining should be intepreted as a yellow traffic-light for either the number of requests issued in the time-window or the request throughput.
  9. Is the quota-policy definition Section 2.3 too complex?

    You can always return the simplest form of the 3 headers

    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 header:

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

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

Author's Address

Roberto Polli Team Digitale, Italian Government EMail: