Internet-Draft RateLimit header fields for HTTP April 2026
Polli, et al. Expires 22 October 2026 [Page]
Workgroup:
HTTPAPI
Internet-Draft:
draft-ietf-httpapi-ratelimit-headers-latest
Published:
Intended Status:
Standards Track
Expires:
Authors:
R. Polli
Team Digitale, Italian Government
A. Martinez
Red Hat
D. Miller
Microsoft

RateLimit header fields for HTTP

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 shape their requests to avoid exceeding server-imposed limits. It also defines problem types that servers can use to inform clients about quota-related conditions.

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/.

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 22 October 2026.

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Table of Contents

1. Introduction

Rate limiting of HTTP [HTTP] clients is a widespread practice. Servers use quota mechanisms to avoid overload, to ensure an equitable distribution of computational resources, or to enforce other policies such as monetization. Typically, servers limit the number of acceptable requests in a given time window (e.g. 10 requests per second). When a quota is exceeded, servers usually respond 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 bases (e.g. per user, per IP, per geographic area) and at different levels. To help clients pace their requests, servers can expose quota information via HTTP header fields. This document defines a set of standard HTTP header fields to enable servers to communicate rate limiting information to clients:

These fields enable servers to communicate complex rate limiting policies, including using multiple and variable time windows and dynamic quotas, and implementing concurrency limits. This information helps improve the resiliency of HTTP infrastructure by providing clients with the information needed to throttle their requests and avoid 4xx or 5xx responses.

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 mandate 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 service guarantee. Server is free to throttle clients that adhere to the server’s recommended limits under certain circumstances.

1.1. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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

The term "problem type" in this document is to be interpreted as described in [PROBLEM].

Quota:

A quota is an allocation of capacity used by a server to limit client requests. That capacity is measured in quota units that the client may consume within a time window.

Quota Unit:

A quota unit is the unit of measurement 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 a server to regulate 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. Servers MAY advertise quota policies. More than one quota policy can affect a given request.

Service Limit:

A service limit is the currently available quota under a specific quota policy and, if defined, the effective time window within which the client can use no more than the available quota.

2. RateLimit-Policy Field

The "RateLimit-Policy" response header field is a List of Strings. Each list item identifies a Quota Policy (Section 2.1).

This field MUST NOT appear in a trailer section.

The "RateLimit-Policy" field enables clients to pace their requests based on policy information provided by the server. The field value SHOULD remain consistent over a sequence of HTTP responses, as policy information is not expected to change frequently. Situations where throttling constraints are highly dynamic are better served using the RateLimit field (Section 3) that communicates the latest service information a client can react to. Both fields can be communicated by the server when appropriate.

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

2.1. Quota Policy Item

A Quota Policy is identified by a String. A set of Parameters describe mandatory and optional details of the policy.

The following parameters are defined:

q:

The REQUIRED "q" parameter indicates the quota allocated by this policy measured in quota units (Section 2.1.1).

qu:

The OPTIONAL "qu" parameter value conveys the quota units associated to the "q" parameter (Section 2.1.2). The default quota unit is "requests".

w:

The OPTIONAL "w" parameter value conveys a time window (Section 2.1.3).

pk:

The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request (Section 2.1.4).

Unknown parameters MUST be ignored by recipients. A recipient that receives a RateLimit-Policy field with a parameter value that does not conform to the syntax defined in this section SHOULD ignore the malformed list item.

2.1.1. 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.

2.1.2. Quota Unit Parameter

The "qu" parameter value conveys the quota units applicable to the quota (Section 2.1.1). The value MUST be a String. Allowed values are listed in the RateLimit Quota Units registry (Section 11.3). This specification defines three quota units:

requests:

This value indicates the quota is based on the number of requests processed by the 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 server.

concurrent-requests:

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

2.1.3. Window Parameter

The "w" parameter value conveys a time window applicable to the quota (Section 2.1.1). The time window MUST be a non-negative, non-zero, 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.

2.1.4. 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. RateLimit Field

The "RateLimit" response header field is a List of Strings. Each list item identifies the current service limit for a quota policy.

This field MUST NOT appear in a trailer section.

A server uses this field to communicate the remaining quota for a specific policy at the time the response is generated.

   RateLimit: "default";a=50;w=30

3.1. Service Limit Item

Each Service Limit Item identifies the quota policy (Section 2.1) associated with the request and contains parameters with information about the current service limit.

The following parameters are defined in this specification:

a:

This REQUIRED parameter value conveys the available quota under the identified policy (Section 3.1.1).

w:

This OPTIONAL parameter value conveys the effective window under the identified policy: the time within which the client can use no more than the available quota (Section 3.1.2).

pk:

The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request (Section 3.1.3).

c:

The OPTIONAL "c" parameter value conveys the cost of the corresponding request in quota units (Section 3.1.4).

Unknown parameters MUST be ignored by recipients.

3.1.1. Available Quota Parameter

The "a" parameter indicates the available quota under the identified policy.

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

3.1.2. Effective Window Parameter

The "w" parameter indicates effective window under the identified policy: the number of seconds within which the client can use no more than the available quota.

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 fully restored after the time indicated by the effective window parameter. The server MAY arbitrarily alter the available quota and effective window between subsequent requests; for example, in case of resource saturation or to implement sliding window policies.

3.1.3. 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.

3.1.4. Cost Parameter

The "c" parameter indicates the cost of the corresponding request in quota units. It is a non-negative Integer. When present, it enables a client to estimate how many similar requests it can make within the available quota without relying on the difference between successive available quota values, which may be unreliable when requests are made concurrently or when the server adjusts quota between responses.

4. RateLimit-Partition Field

The "RateLimit-Partition" response header field is a List of Strings. Each list item identifies a quota policy and declares the dimensions used to partition quota for that policy.

This field MUST NOT appear in a trailer section.

   RateLimit-Partition: "api";user_id;method

4.1. Partition Dimension Item

Each item in the RateLimit-Partition field is a String that matches the policy identifier used in the corresponding RateLimit-Policy (Section 2) and RateLimit (Section 3) fields. The item's Parameters declare the partition dimensions.

Each parameter key names a partition dimension registered in the RateLimit Partition Key Dimensions registry (Section 11.7).

A dimension parameter with a Boolean value of true (e.g. ;user_id) indicates that the dimension varies across partitions — the server uses the request's value for that dimension when constructing the partition key.

A dimension parameter with a non-Boolean value (e.g. ;method=GET) indicates that the policy applies only to requests where that dimension matches the specified value. This enables a client to determine whether a policy is relevant to a given request before it is sent.

Unknown dimension parameters SHOULD be ignored by recipients when processing the field, but a client that does not recognize or cannot compute the value of any dimension MUST treat the partition key for that policy as opaque and MUST NOT attempt to construct partition keys for prediction purposes.

4.2. Partition Key Construction

When a server provides both the RateLimit-Partition field and a "pk" parameter in the RateLimit field, the "pk" Byte Sequence MUST be constructed as follows:

  1. Collect the dimension parameters from the RateLimit-Partition item. For dimensions with a Boolean value of true, the value is the request's value for that dimension. For dimensions with a non-Boolean value, the value is the parameter value itself.

  2. Sort the dimensions by parameter key in lexicographic order.

  3. For each dimension in sorted order, encode the value as a UTF-8 byte string.

  4. Concatenate the encoded values in order, separated by the byte 0x1F (Unit Separator, US).

  5. The resulting byte string is the partition key Byte Sequence.

Dimension values MUST NOT contain the byte 0x1F. If a dimension value would naturally contain this byte, the server MUST define an escaping or encoding mechanism in the dimension's registry entry.

For example, if the server declares:

   RateLimit-Partition: "api";user_id;method

and the request is made by user "alice" using the GET method, the partition key is constructed by sorting the dimensions alphabetically (method, user_id), then concatenating "GET", 0x1F, and "alice". The RateLimit field conveys this as a Byte Sequence:

   RateLimit: "api";a=99;w=60;pk=:R0VUH2FsaWNl:

A server might also declare a policy that applies only to a specific method:

   RateLimit-Partition: "reads";user_id;method=GET

A client receiving this knows the "reads" policy only applies to GET requests. For non-GET requests, this policy is not relevant and can be ignored.

A client that knows its own values for the declared dimensions can construct the expected partition key for a future request and look it up in its local quota tracking state, enabling proactive traffic shaping without requiring API-specific knowledge.

If a server provides a "pk" parameter without a corresponding RateLimit-Partition field item, the structure of the partition key is not defined by this specification and any interpretation of its structure MUST be communicated out of band. Clients MAY use heuristics to estimate partition membership based on observed partition key values and request similarity.

5. Server Behavior

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

Servers SHOULD avoid returning RateLimit header fields in redirection responses (i.e., responses with 3xx status codes) because a low available quota could prevent the client from following the redirect. 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 available quota is 0.

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

If a response contains both the Retry-After and the RateLimit header fields, the Retry-After field value SHOULD NOT reference a point in time earlier than the end of the effective window.

Servers using RateLimit header fields SHOULD implement mechanisms to cap the ratio between the available quota and the effective window values (see Section 9.5); this is especially important when a quota policy uses a large time window, as a large available quota relative to a small effective window could inadvertently encourage a burst of requests.

Under certain conditions, a server MAY reduce the available quota or increase the effective window between subsequent requests, e.g. to respond to Denial of Service attacks or in case of resource saturation.

5.1. 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.

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. Quota Partitions

Servers commonly enforce different quota allocations depending on attributes of the request — for example, per authenticated user, per client application, per HTTP method, or per resource. When a single client makes requests that span multiple such allocations, the client needs to track quota for each allocation independently in order to shape its traffic effectively.

A key goal of partition keys is to enable generic client-side traffic shaping without requiring API-specific knowledge. Historically, client-side rate limiting code has relied on out-of-band documentation to understand how a server partitions its quota — for example, by reading API documentation that explains rate limits are per-user or per-endpoint. This prevents the development of generic HTTP client libraries that can automatically shape traffic for any API.

Partition keys address this by allowing the server to communicate the dimensions along which quota is partitioned, using attributes that the client can observe from its own requests. When a client can determine which partition a future request will fall into, it can proactively check whether sufficient quota is available before sending the request, rather than waiting to be throttled.

The RateLimit-Partition field (Section 4) enables servers to declare the named dimensions that make up the partition key for each policy. When this field is provided, clients can construct partition keys deterministically from their own request attributes and predict which quota partition a future request will consume.

For example, a server that allocates separate quotas per user and HTTP method might declare:

   RateLimit-Partition: "api";user_id;method

Partition keys returned by the server in RateLimit header fields allow a single client to track the quota of multiple partitions independently. This is useful when a single client makes requests that consume different quota allocations, for example when acting on behalf of different users or accessing different resources that have independent quotas.

Servers SHOULD construct partition keys using only information that is present in the request. Servers SHOULD avoid returning partition keys that contain sensitive information. 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.

For cases where the server does not provide a RateLimit-Partition field, or where a client does not recognize all declared dimensions, clients MAY use heuristics to estimate whether a future request will be successful based on its similarity to previous requests.

7. 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 be successful. 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";a=1;w=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 9.5 and Section 9.5.1).

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 available quota within the time expressed in the effective window parameter.

The value of the effective window 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 schedule requests.

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 effective window MAY be ignored.

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

7.1. 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 semantics 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 that is aware of the quota unit semantics used by the origin server MAY alter the RateLimit header fields to communicate a different quota policy, for example when the intermediary enforces its own quota policy or when it can absorb additional load through caching or request collapsing.

An intermediary SHOULD forward a request even when presuming that it might not be serviced; the service returning the RateLimit header fields is solely responsible for enforcing the communicated quota policy, and MAY still choose to service incoming requests.

This specification does not mandate any behavior on intermediaries with respect to retries, nor does it require 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 10) provide further guidance on intermediaries.

8. Problem Types

8.1. Quota Exceeded

This section defines the "https://iana.org/assignments/http-problem-types#quota-exceeded" problem type. A server MAY use this problem type if it wants to communicate to the client that the requests sent by the client exceed one or more Quota Policies. This problem type defines the extension member "violated-policies" as an array of strings, whose value is the names of policies where the quota was exceeded.

HTTP/1.1 429 Too Many Requests
Content-Type: application/problem+json

{
  "type": "https://iana.org/assignments/http-problem-types#quota-exceeded",
  "title": "Request cannot be satisfied as assigned quota has been exceeded",
  "violated-policies": ["daily","bandwidth"]
}

8.2. Temporary Reduced Capacity

This section defines the "https://iana.org/assignments/http-problem-types#temporary-reduced-capacity" problem type. A server MAY use this problem type if it wants to communicate that the client’s requests currently cannot be satisfied due to a temporary reduction in server capacity. The server MAY choose to include a RateLimit-Policy field indicating the new temporarily lower quota. This problem type defines the extension member "violated-policies" as an array of strings, whose value is the names of policies where the quota was exceeded.

HTTP/1.1 503 Service Unavailable
Content-Type: application/problem+json

{
  "type": "https://iana.org/assignments/http-problem-types#temporary-reduced-capacity",
  "title": "Request cannot be satisfied due to temporary server capacity constraints",
  "violated-policies": ["hourly"]
}

8.3. Abnormal Usage Detected

This section defines the "https://iana.org/assignments/http-problem-types#abnormal-usage-detected" problem type. A server MAY use this problem type to communicate to the client that it has detected a pattern of requests that suggest unintentional or malicious behaviour on the part of the client. This problem type defines the extension member "violated-policies" as an array of strings, whose value is the names of policies where the quota was exceeded.

HTTP/1.1 429 Too Many Requests
Content-Type: application/problem+json

{
  "type": "https://iana.org/assignments/http-problem-types#abnormal-usage-detected",
  "title": "Request not satisifed due to detection of abnormal request pattern",
  "violated-policies": ["hourly"]
}

8.4. 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]).

9. Security Considerations

9.1. 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.

9.2. 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.

Partition keys may contain identifying information; see Section 6 for guidance on constructing partition keys.

9.3. Available 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 available quota parameter (Section 3.1.1) 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.

9.4. Variability of the effective window

The effective window parameter (Section 3.1.2) does not imply anything about when the server will increase the available quota. The effective window does not necessarily end at a fixed point in time.

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

9.5. Resource exhaustion

When returning effective window 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 effective window accordingly

   Date: Tue, 15 Nov 1994 18:00:00 GMT
   RateLimit: "daily";a=1;w=86400

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 effective window.

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.

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

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

9.5.1. Denial of Service

RateLimit header fields may contain unexpected values by chance or on purpose. For example, an excessively high available quota 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 large effective window 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 effective window 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).

10. 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.

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

11. IANA Considerations

IANA is requested to update two registries and create five new registries.

11.1. Update HTTP Field Name Registry

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

Table 1
Field Name Structured Type Status Specification
RateLimit List permanent Section 3 of RFC nnnn
RateLimit-Policy List permanent Section 2 of RFC nnnn
RateLimit-Partition List permanent Section 4 of RFC nnnn

11.2. Update HTTP Problem Type registry

IANA is asked to register the following entries in the "HTTP Problem Types" registry at https://www.iana.org/assignments/http-problem-types.

11.2.1. Registration of "quota-exceeded" Problem Type

Type URI: https://iana.org/assignments/http-problem-types#quota-exceeded

Title: Quota Exceeded

Recommended HTTP status code: 429

Reference: Section 8.1 of this document

11.2.2. Registration of "temporary-reduced-capacity" Problem Type

Type URI: https://iana.org/assignments/http-problem-types#temporary-reduced-capacity

Title: Temporary Reduced Capacity

Recommended HTTP status code: 503

Reference: Section 8.2 of this document

11.2.3. Registration of "abnormal-usage-detected" Problem Type

Type URI: https://iana.org/assignments/http-problem-types#abnormal-usage-detected

Title: Abnormal Usage Detected

Recommended HTTP status code: 429

Reference: Section 8.3 of this document

11.3. 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:

Table 2
Quota Unit Reference Notes
request RFC nnnn  
content-bytes RFC nnnn  
concurrent-requests RFC nnnn  

11.3.1. Registration Template

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

  • Quota Unit: The name of the quota unit.

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

  • Notes: Any additional notes about the quota unit.

11.4. RateLimit-Policy parameters registry

This specification establishes the registry "Hypertext Transfer Protocol (HTTP) RateLimit-Policy Parameters" to be located at https://www.iana.org/assignments/http-ratelimit-policy-parameters. 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).

Each registered parameter defines a named parameter that may appear on items in the RateLimit-Policy header field. Parameters not registered in this registry SHOULD use a vendor prefix (e.g. acme-burst) to avoid collisions.

The registry has the following initial content:

Table 3
Parameter Description Reference
q Quota allocated by the policy RFC nnnn
qu Quota units for the policy RFC nnnn
w Time window for the policy RFC nnnn
pk Partition key RFC nnnn

11.4.1. Registration Template

The registration template for the RateLimit-Policy Parameters registry is as follows:

  • Parameter: The parameter key. The key MUST conform to the syntax for Structured Fields parameter keys.

  • Description: A description of the parameter's semantics.

  • Reference: A reference to the document that specifies the parameter.

11.5. RateLimit parameters registry

This specification establishes the registry "Hypertext Transfer Protocol (HTTP) RateLimit Parameters" to be located at https://www.iana.org/assignments/http-ratelimit-parameters. 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).

Each registered parameter defines a named parameter that may appear on items in the RateLimit header field. Parameters not registered in this registry SHOULD use a vendor prefix (e.g. acme-burst) to avoid collisions.

The registry has the following initial content:

Table 4
Parameter Description Reference
a Available quota RFC nnnn
w Effective window RFC nnnn
pk Partition key RFC nnnn
c Cost of the request in quota units RFC nnnn

11.5.1. Registration Template

The registration template for the RateLimit Parameters registry is as follows:

  • Parameter: The parameter key. The key MUST conform to the syntax for Structured Fields parameter keys.

  • Description: A description of the parameter's semantics.

  • Reference: A reference to the document that specifies the parameter.

11.6. RateLimit-Partition parameters registry

This specification establishes the registry "Hypertext Transfer Protocol (HTTP) RateLimit-Partition Parameters" to be located at https://www.iana.org/assignments/http-ratelimit-partition-parameters. 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).

Each registered parameter defines a named parameter that may appear on items in the RateLimit-Partition header field. Partition dimension parameters are registered separately in the RateLimit Partition Key Dimensions registry (Section 11.7). Other parameters not registered in this registry SHOULD use a vendor prefix to avoid collisions.

The registry has no initial content beyond the partition key dimensions defined in Section 11.7.

11.6.1. Registration Template

The registration template for the RateLimit-Partition Parameters registry is as follows:

  • Parameter: The parameter key. The key MUST conform to the syntax for Structured Fields parameter keys.

  • Description: A description of the parameter's semantics.

  • Reference: A reference to the document that specifies the parameter.

11.7. RateLimit partition key dimensions registry

This specification establishes the registry "Hypertext Transfer Protocol (HTTP) RateLimit Partition Key Dimensions" registry to be located at https://www.iana.org/assignments/http-ratelimit-partition-key-dimensions. 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).

Each registered dimension defines a named attribute of an HTTP request that a server may use to partition quota allocations. For a dimension to be useful for client-side traffic shaping, the client MUST be able to determine the dimension value from information available before sending the request.

The registry has the following initial content:

Table 5
Dimension Description Reference
user_id The identifier of the user on whose behalf the request is made, as known to the server. RFC nnnn
client_id The identifier of the client application making the request, as known to the server. RFC nnnn
method The HTTP method of the request, encoded as its canonical uppercase form (e.g. "GET", "POST"). RFC nnnn

11.7.1. Registration Template

The registration template for the RateLimit Partition Key Dimensions registry is as follows:

  • Dimension: The name of the partition key dimension. The name MUST conform to the syntax for Structured Fields Tokens, starting with a lowercase letter.

  • Description: A description of the dimension, including the request attribute it corresponds to and any canonicalization or normalization rules for the value.

  • Reference: A reference to the document that specifies the dimension.

  • Notes: Any additional notes about the dimension.

12. References

12.1. Normative References

[HTTP]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <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, , <https://www.rfc-editor.org/rfc/rfc8126>.
[PROBLEM]
Nottingham, M., Wilde, E., and S. Dalal, "Problem Details for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, , <https://www.rfc-editor.org/rfc/rfc9457>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <https://www.rfc-editor.org/rfc/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", RFC 9651, DOI 10.17487/RFC9651, , <https://www.rfc-editor.org/rfc/rfc9651>.

12.2. Informative References

[HPACK]
Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541, , <https://www.rfc-editor.org/rfc/rfc7541>.
[HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", STD 98, RFC 9111, DOI 10.17487/RFC9111, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc3339>.
[RFC6585]
Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, , <https://www.rfc-editor.org/rfc/rfc6585>.
[UNIX]
The Open Group, "The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98", .

Appendix A. Examples

A.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.

A.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";a=0;w=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 the available quota 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";a=0;w=48

{"still": "successful"}

A.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.

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 end 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";a=100;w=36000

{"hello": "world"}

A.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 available quota and effective window values.

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-Policy: "basic";q=100;w=60
RateLimit: "basic";a=60;w=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";a=20;w=56

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

A.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:

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

{
"type": "https://iana.org/assignments/http-problem-types#quota-exceeded"
"title": "Too Many Requests",
"status": 429,
"violated-policies": ["default"]
}

A.2. Responses with defined policies

A.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";a=99;w=50
RateLimit-Policy: "fixedwindow";q=100;w=60
{"hello": "world"}

A.2.2. Dynamic limits with parameterized windows

The policy conveyed by the RateLimit-Policy 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 RateLimit header field 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.

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";a=9;w=50


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

A.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";a=0;w=20

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

A.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 retry time, 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";a=15;w=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 end of the effective window, as it would still be free to query again the server even if it is likely to have the request rejected.

A.3.1. Use with multiple windows

This is a standardized way of describing the policy detailed in Appendix A.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 effective window is 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-Policy: "hour";q=1000;w=3600, "day";q=5000;w=86400
RateLimit: "day";a=100;w=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 (e.g. 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 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?

    Using delay-seconds aligns with Retry-After, which is returned in similar contexts, e.g. 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, etc.). Moreover timestamps may not be monotonically increasing due to clock adjustment. See Another NTP client failure story

    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

    • 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 client requests not being handled, 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, etc.). 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 parameter imply any service guarantee for my future requests to be served?

    No. FAQ integrated in Section 3.1.1.

  8. Is the quota-policy definition too complex?

    You can always return the simplest form

RateLimit:"default";a=50;w=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";a=50;w=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 requests not being handled. 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 cases 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 likely be 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 it 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.

Commonly used header field names are:

There are variants too, where the window is specified in the header field name, e.g.:

Here are some interoperability issues:

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;a=6;w=1

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

RateLimit-Policy: "sliding";q=12;w=1
; using 100% of throughput, that is 12 units/s
RateLimit: "sliding";q=12;a=1;w=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-08

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

  • Added Problem Types

  • Clarified when to use RateLimit-Policy vs RateLimit fields

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

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
Alejandro Martinez Ruiz
Red Hat
Darrel Miller
Microsoft