From 10657fe1107f382abb4c47cb24657a36a5b8aa1f Mon Sep 17 00:00:00 2001 From: Darrel Miller Date: Sun, 29 Sep 2024 15:45:17 -0400 Subject: [PATCH] Revert "Added linting fixes" This reverts commit e29b364c985a4427c40b5ab5276cbb2403b9b217. --- .gitignore | 1 + draft-ietf-httpapi-ratelimit-headers.html | 2948 +++++++++++++++++++++ draft-ietf-httpapi-ratelimit-headers.md | 17 +- draft-ietf-httpapi-ratelimit-headers.md~ | 1169 ++++++++ draft-ietf-httpapi-ratelimit-headers.txt | 1435 ++++++++++ 5 files changed, 5561 insertions(+), 9 deletions(-) create mode 100644 .gitignore create mode 100644 draft-ietf-httpapi-ratelimit-headers.html create mode 100644 draft-ietf-httpapi-ratelimit-headers.md~ create mode 100644 draft-ietf-httpapi-ratelimit-headers.txt diff --git a/.gitignore b/.gitignore new file mode 100644 index 0000000..7951405 --- /dev/null +++ b/.gitignore @@ -0,0 +1 @@ +lib \ No newline at end of file diff --git a/draft-ietf-httpapi-ratelimit-headers.html b/draft-ietf-httpapi-ratelimit-headers.html new file mode 100644 index 0000000..60ed7d9 --- /dev/null +++ b/draft-ietf-httpapi-ratelimit-headers.html @@ -0,0 +1,2948 @@ + + + + + + +RateLimit header fields for HTTP + + + + + + + + + + + + + + + + + + + + + + + + + +
Internet-DraftRateLimit header fields for HTTPSeptember 2024
Polli, et al.Expires 2 April 2025[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 service policy limits and the current limits, thereby allowing clients to avoid being throttled.

+
+
+

+About This Document +

+

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

+

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

+

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

+

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

+
+
+
+

+Status of This Memo +

+

+ This Internet-Draft is submitted in full conformance with the + provisions of BCP 78 and BCP 79.

+

+ Internet-Drafts are working documents of the Internet Engineering Task + Force (IETF). Note that other groups may also distribute working + documents as Internet-Drafts. The list of current Internet-Drafts is + at https://datatracker.ietf.org/drafts/current/.

+

+ 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 2 April 2025.

+
+
+ +
+
+ ▲

+Table of Contents +

+ +
+
+
+
+

+1. Introduction +

+

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

+

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

+
    +
  • +

    RateLimit: to convey +the server's current limit of quota units available to the client in the policy time window, +the remaining quota units in the current window, +and the time remaining in the current window, specified in seconds, and

    +
    • +
  • +

    RateLimit-Policy: the service policy limits.

    +
    • +
+

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

+

The behavior of the RateLimit header field is compatible with the delay-seconds notation of Retry-After.

+
+
+

+1.1. Goals +

+

The goals of this document are:

+
+
Interoperability:
+
+

Standardize the names and semantics of rate-limit headers +to ease their enforcement and adoption;

+
+
+
Resiliency:
+
+

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

+
+
+
Documentation:
+
+

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

+
+
+
+

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

+
+
Authorization:
+
+

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

+
+
+
Response status code:
+
+

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

+
+
+
Throttling algorithm:
+
+

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

+
+
+
Service Level Agreement:
+
+

Conveyed quota hints do not imply any service guarantee. +Server is free to throttle respectful clients under certain circumstances.

+
+
+
+
+
+
+
+

+1.2. Notational Conventions +

+

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

+

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

+

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

+
+
+
+
+
+
+

+2. Terminology +

+
+
+

+2.1. Quota +

+

A quota is an allocation of capacity to enable a server to limit client requests. That capacity is counted in quota units and may be reallocated at the end of a time window Section 2.4.

+
+
+
+
+

+2.2. Quota Unit +

+

A quota unit is the unit of measure used to count the activity of a client.

+
+
+
+
+

+2.3. Quota Partition +

+

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

+
+
+
+
+

+2.4. Time Window +

+

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

+

The time window is a non-negative Integer value expressing an interval in seconds, similar to the "delay-seconds" rule defined in Section 10.2.3 of [HTTP]. Sub-second precision is not supported.

+
+
+
+
+

+2.5. Quota Policy +

+

A quota policy is maintained by a server to limit the activity (counted in quota units (Section 2.2)) of a given quota partition (Section 2.3) over a period of time (known as the time window (Section 2.4)) to a specified amount known as the Section 2.1.

+

Quota policies can be advertised by servers (see Section 3), but they are not required to be, and more than one quota policy can affect a given request from a client to a server.

+
+
+
+
+

+2.6. Service Limit +

+

A service limit is the current limit of the amount of activity that a server will allow based on the remaining quota for a particular quota partition within the time-window, if defined.

+
+
+
+
+
+
+

+3. RateLimit-Policy Field +

+

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

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

+3.1. Quota Policy Item +

+

A quota policy Item contains information about a server's capacity allocation for a quota partition associated with the request.

+

The following parameters are defined in this specification:

+
+
q:
+
+

The REQUIRED "q" parameter indicates the quota allocated. (Section 3.1.1)

+
+
+
qu:
+
+

The OPTIONAL "qu" parameter value conveys the quota units associated to the "q" parameter. The default quota unit is "request". (Section 3.1.2)

+
+
+
w:
+
+

The OPTIONAL "w" parameter value conveys a time "window" (Section 2.4). (Section 3.1.3)

+
+
+
pk:
+
+

The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request. Section 3.1.4

+
+
+
+

Other parameters are allowed and can be regarded as comments.

+

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

+
+
+

+3.1.1. Quota Parameter +

+

The "q" parameter uses a non-negative integer value to indicate the quota allocated for client activity (counted in quota units) for a given quota partition (Section 2.6).

+
+
+
+
+

+3.1.2. Quota Unit Parameter +

+

The "qu" parameter value conveys the quota units associated to the "q" parameter.

+
+
+
+
+

+3.1.3. Window Parameter +

+

The "w" parameter value conveys a time "window" in seconds. (Section 2.4).

+
+
+
+
+

+3.1.4. Partition Key Parameter +

+

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

+
+
+
+
+
+
+

+3.2. RateLimit Policy Field Examples +

+

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

+
+
+   RateLimit-Policy: default;l=100;w=10
+
+
+

These examples show multiple policies being returned:

+
+
+   RateLimit-Policy: permin;l=50;w=60,perhr;l=1000;w=3600,perday;l=5000;w=86400
+
+
+

The following example shows a policy with a partition key:

+
+
+   RateLimit-Policy: peruser;l=100;w=60;pk=user123
+
+
+

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

+
+
+   RateLimit-Policy: peruser;l=65535;w=10;pk=user123;qu=bytes
+
+
+

This field cannot appear in a trailer section.

+
+
+
+
+
+
+

+4. RateLimit Field +

+

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

+

The field is expressed as List of Section 4.1.

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

+4.1. Service Limit Item +

+

Each service limit item in identifies the quota policy associated with the request and

+

The following parameters are defined in this specification:

+
+
r:
+
+

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

+
+
+
t:
+
+

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

+
+
+
pk:
+
+

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

+
+
+
+

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

+

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

+
+
+

+4.1.1. Remaining Parameter +

+

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

+

It is a non-negative Integer expressed in quota units (Section 2.2). +Clients MUST NOT assume that a positive remaining value is a guarantee that further requests will be served. +When remaining parameter value is low, it indicates that the server may soon throttle the client (see Section 5).

+
+
+
+
+

+4.1.2. Reset Parameter +

+

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

+

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

+
    +
  • +

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

    +
    • +
  • +

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

    +
    • +
+

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

+
+
+
+
+

+4.1.3. Partition Key Parameter +

+

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

+
+
+
+
+
+
+

+4.2. RateLimit Field Examples +

+

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

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

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

+
+
+   RateLimit: default;r=999;pk=trial-121323
+
+
+

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

+
+
+   RateLimit: default;r=300000000;pk=App-999;t=60;qu=bytes
+
+
+
+
+
+
+
+
+

+5. Server Behavior +

+

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

+

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

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

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

+

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

+

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

+
+
+

+5.1. 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. Client Behavior +

+

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

+

For example, a successful response with the following fields:

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

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

+

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

+

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

+

Malformed RateLimit header fields MUST be ignored.

+

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

+

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

+

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

+

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

+

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

+
    +
  • +

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

    +
    • +
  • +

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

    +
    • +
+
+
+

+6.1. Intermediaries +

+

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

+

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

+

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

+
    +
  • +

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

    +
    • +
  • +

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

    +
    • +
+

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

+

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

+

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

+
+
+
+
+

+6.2. Caching +

+

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

+
+
+
+
+
+
+

+7. Security Considerations +

+
+
+

+7.1. Throttling does not prevent clients from issuing requests +

+

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

+
+
+
+
+

+7.2. Information disclosure +

+

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

+

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

+

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

+

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

+
+
+
+
+

+7.3. Remaining quota units are not granted requests +

+

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

+

Clients MUST NOT consider the quota units (Section 2.6) returned in remaining keyword as a service level agreement.

+

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

+
+
+
+
+

+7.4. Reliability of the reset keyword +

+

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

+

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

+
+
+
+
+

+7.5. Resource exhaustion +

+

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

+

This is true for Retry-After too.

+

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

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

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

+

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

+

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

+

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

+
+
+RateLimit-Policy: somepolicy;l=10000;w=1000
+RateLimit: somepolicy;r=10000;t=10
+
+
+

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

+
+
+

+7.5.1. Denial of Service +

+

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

+
    +
  • +

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

    +
    • +
  • +

    passed by a misconfigured server;

    +
    • +
+

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

+

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

+

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

+
+
+
+
+
+
+
+
+

+8. Privacy Considerations +

+

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

+

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

+

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

+
+
+
+
+

+9. IANA Considerations +

+

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

+

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

+ + + + + + + + + + + + + + + + + + + + + +
Table 1
Field NameStatusSpecification
RateLimitpermanent + Section 4 of RFC nnnn
RateLimit-Policypermanent + Section 3 of RFC nnnn
+
+
+
+

+10. References +

+
+
+

+10.1. Normative References +

+
+
[HTTP]
+
+Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <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>.
+
+
[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 8941, DOI 10.17487/RFC8941, , <https://www.rfc-editor.org/rfc/rfc8941>.
+
+
[WEB-ORIGIN]
+
+Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, , <https://www.rfc-editor.org/rfc/rfc6454>.
+
+
+
+
+
+
+

+10.2. Informative References +

+
+
[HPACK]
+
+Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541, , <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. Rate-limiting and quotas +

+

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

+

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

+

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

+

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

+
    +
  • +

    10 requests per second;

    +
    • +
  • +

    limited to 60 requests per minute;

    +
    • +
  • +

    limited to 1000 requests per hour.

    +
    • +
+

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

+

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

+

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

+

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

+

The common choice is to return three headers containing:

+
    +
  • +

    the maximum number of allowed requests in the time window;

    +
    • +
  • +

    the number of remaining requests in the current window;

    +
    • +
  • +

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

    +
    • +
+
+
+

+A.1. Interoperability issues +

+

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

+
    +
  • +

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

    +
    • +
  • +

    header field names proliferates.

    +
    • +
+

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

+
+
+
+
+
+
+

+Appendix B. Examples +

+
+
+

+B.1. Responses without defining policies +

+

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

+
+
+

+B.1.1. Throttling information in responses +

+

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

+

Request:

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

Response:

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

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

+

Request:

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

Response:

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

+B.1.2. Multiple policies in response +

+

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

+
    +
  • +

    5000 daily quota units;

    +
    • +
  • +

    1000 hourly quota units.

    +
    • +
+

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

+

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

+

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

+
    +
  • +

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

    +
    • +
+

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

+

Request:

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

Response:

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

+B.1.3. Use for limiting concurrency +

+

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

+

The server adopted a basic policy of 100 quota units per minute, +and in case of resource exhaustion adapts the returned values +reducing both limit and remaining keyword 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;l=100;w=60
+RateLimit: basic;r=60;t=58
+
+{"elapsed": 2, "issued": 40}
+
+
+

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

+

Request:

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

Response:

+
+
+HTTP/1.1 200 Ok
+Content-Type: application/json
+RateLimit-Policy: basic;l=100;w=60
+RateLimit: basic;r=20;t=56
+
+{"elapsed": 4, "issued": 41}
+
+
+
+
+
+
+

+B.1.4. Use in throttled responses +

+

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

+

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

+

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

+

Request:

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

Response:

+
+
+HTTP/1.1 429 Too Many Requests
+Content-Type: application/json
+Date: Mon, 05 Aug 2019 09:27:00 GMT
+Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
+RateLimit: default;r=0;t=5
+
+{
+"title": "Too Many Requests",
+"status": 429,
+"detail": "You have exceeded your quota"
+}
+
+
+
+
+
+
+
+
+

+B.2. Responses with defined policies +

+
+
+

+B.2.1. Throttling window specified via parameter +

+

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

+

Request:

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

Response:

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

+B.2.2. Dynamic limits with parameterized windows +

+

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

+

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

+

The remaining keyword 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;l=100;w=60
+RateLimit: dynamic;r=9;t=50
+
+
+{
+  "status": 200,
+  "detail": "Just slow down without waiting."
+}
+
+
+
+
+
+
+

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

+

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

+

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

+

Request:

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

Response:

+
+
+HTTP/1.1 429 Too Many Requests
+Content-Type: application/json
+RateLimit-Policy: dynamic;l=15;w=20
+RateLimit: dynamic;r=0;t=20
+
+{
+  "status": 429,
+  "detail": "Wait 20 seconds, then slow down!"
+}
+
+
+
+
+
+
+
+
+

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

+

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

+

Request:

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

Response:

+
+
+HTTP/1.1 429 Too Many Requests
+Content-Type: application/json
+Retry-After: 20
+RateLimit-Policy: dynamic;l=100;w=60
+RateLimit: dynamic;r=15;t=40
+
+{
+  "status": 429,
+  "detail": "Wait 20 seconds, then slow down!"
+}
+
+
+

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

+
+
+

+B.3.1. Missing Remaining information +

+

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

+

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

+

Request:

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

Response:

+
+
+HTTP/1.1 200 Ok
+Content-Type: application/json
+RateLimit-Policy: quota;l=100;w=1
+RateLimit: quota;t=1
+
+{"first": "request"}
+
+
+

Request:

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

Response:

+
+
+HTTP/1.1 200 Ok
+Content-Type: application/json
+RateLimit-Policy: quota;l=10
+RateLimit: quota;t=1
+
+{"second": "request"}
+
+
+
+
+
+
+

+B.3.2. Use with multiple windows +

+

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

+
    +
  • +

    5000 daily quota units;

    +
    • +
  • +

    1000 hourly quota units.

    +
    • +
+

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

+

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

+

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

+
    +
  • +

    it has only 100 quota units left;

    +
    • +
  • +

    the window will reset in 10 hours;

    +
    • +
  • +

    the expiring-limit is 5000.

    +
    • +
+

Request:

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

Response:

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

+FAQ +

+

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

+
    +
  1. +

    Why defining standard fields for throttling?

    +

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

    +
    2. +
  2. +

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

    +

    +Yes, you can.

    +
    3. +
  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. +
  4. +

    Why is the partition key necessary?

    +

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

    +
    5. +
  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 [HTTP]). +This may be problematic (e.g. clock adjustment, clock skew, failure of hardcoded clock synchronization servers, +IoT devices, ..). +Moreover timestamps may not be monotonically increasing due to clock adjustment. +See Another NTP client failure story

    +

    +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. +
  6. +

    Shouldn't I limit concurrency instead of request rate?

    +

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

    +

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

    +

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

    +
    7. +
  7. +

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

    +

    +No. FAQ integrated in Section 4.1.1.

    +
    8. +
  8. +

    Is the quota-policy definition Section 2.5 too complex?

    +

    +You can always return the simplest form

    +
    9. +
+
+
+RateLimit:default;r=50;t=60
+
+
+

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

+
+
+RateLimit-Policy: sliding;l=100;w=60;burst=1000;comment="sliding window", fixed;l=5000;w=3600;burst=0;comment="fixed window"
+RateLimit: sliding;r=50;t=44
+
+
+

the value "sliding" identifies the policy being reported.

+
    +
  1. +

    Can intermediaries alter RateLimit header fields?

    +

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

    +
    2. +
  2. +

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

    +

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

    +
    3. +
  3. +

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

    +
    4. +
+
+
+
+
+

+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:

+
    +
  • +

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

    +
    • +
+

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

+
    +
  • +

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

    +
    • +
  • +

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

    +
    • +
+

Here are some interoperability issues:

+
    +
  • +

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

    +
      +
    • +

      seconds remaining to the window expiration

      +
      • +
    • +

      milliseconds remaining to the window expiration

      +
      • +
    • +

      seconds since UTC, in UNIX Timestamp [UNIX]

      +
      • +
    • +

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

      +
      • +
    +
    • +
  • +

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

    +
      +
    • +

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

      +
      • +
    • +

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

      +
      • +
    • +

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

      +
      • +
    +
    • +
+

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

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

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

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

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, +Darrel Miller +and Julian Reschke.

+
+
+
+
+

+Changes +

+

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

+
+
+

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

+

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

+
    +
  • +

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

    +
    • +
  • +

    Added quota unit parameter

    +
    • +
  • +

    Added partition key parameter

    +
    • +
+
+
+
+
+

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

+

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

+
    +
  • +

    Split policy informatiom in RateLimit-Policy #81

    +
    • +
+
+
+
+
+

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

+

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

+
    +
  • +

    Address throttling scope #83

    +
    • +
+
+
+
+
+

+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
+ +
+
+
+ + + diff --git a/draft-ietf-httpapi-ratelimit-headers.md b/draft-ietf-httpapi-ratelimit-headers.md index 2294126..2fcf76d 100644 --- a/draft-ietf-httpapi-ratelimit-headers.md +++ b/draft-ietf-httpapi-ratelimit-headers.md @@ -147,7 +147,7 @@ The time window is a non-negative Integer value expressing an interval in second ## Quota Policy {#quota-policy} -A quota policy is maintained by a server to limit the activity (counted in [quota units](#quota-units)) of a given [quota partition](#quota-partition) over a period of time (known as the [time window](#time-window)) to a specified amount known as the [quota](#quota). +A quota policy is maintained by a server to limit the activity (counted in [quota units](#quota-unit)) of a given [quota partition](#quota-partition) over a period of time (known as the [time window](#time-window)) to a specified amount known as the [quota](#quota). Quota policies can be advertised by servers (see {{ratelimit-policy-field}}), but they are not required to be, and more than one quota policy can affect a given request from a client to a server. @@ -211,7 +211,7 @@ This field MAY convey the time window associated with the expiring-limit, as sho These examples show multiple policies being returned: ~~~ - RateLimit-Policy: permin;l=50;w=60, perhr;l=1000;w=3600, perday;l=5000;w=86400 + RateLimit-Policy: permin;l=50;w=60,perhr;l=1000;w=3600,perday;l=5000;w=86400 ~~~ The following example shows a policy with a partition key: @@ -240,7 +240,7 @@ The field is expressed as List of {{servicelimit-item}}. ## Service Limit Item {#servicelimit-item} -Each service limit item in identifies the quota policy associated with the request and +Each service limit item in identifies the quota policy associated with the request and The following parameters are defined in this specification: @@ -251,7 +251,7 @@ The following parameters are defined in this specification: : This OPTIONAL parameter value conveys the time window reset time for the identified policy ({{ratelimit-reset-parameter}}). pk: - : The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request. + : The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request. This field cannot appear in a trailer section. Other parameters are allowed and can be regarded as comments. @@ -262,7 +262,7 @@ Implementation- or service-specific parameters SHOULD be prefixed parameters wit The "r" parameter indicates the remaining quota units for the identified policy ({{ratelimit-remaining-parameter}}). -It is a non-negative Integer expressed in [quota units](#quota-units). +It is a non-negative Integer expressed in [quota units](#quota-unit). Clients MUST NOT assume that a positive remaining value is a guarantee that further requests will be served. When remaining parameter value is low, it indicates that the server may soon throttle the client (see {{providing-ratelimit-fields}}). @@ -329,7 +329,6 @@ Servers are not required to return RateLimit header fields in every response, an 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=RFC7541}}. - # Client Behavior {#receiving-fields} The RateLimit header fields can be used by clients to determine whether the associated request respected the server's quota policy, and as an indication of whether subsequent requests will. However, the server might apply other criteria when servicing future requests, and so the quota policy may not completely reflect whether requests will succeed. @@ -614,7 +613,7 @@ RateLimit: default;r=0;t=48 {"still": "successful"} ~~~ -### Multiple policies in response +### Multiple policies in response {#use-with-multiple-policies} The server uses two different policies to limit the client's requests: @@ -923,7 +922,7 @@ RateLimit: quota;t=1 ### Use with multiple windows This is a standardized way of describing the policy -detailed in {{use-with-custom-fields}}: +detailed in {{use-with-multiple-policies}}: - 5000 daily quota units; - 1000 hourly quota units. @@ -973,7 +972,7 @@ RateLimit: day;r=100;t=36000 No. {{?RFC6585}} defines the `429` status code and we use it just as an example of a throttled request, that could instead use even `403` or whatever status code. - + 4. Why is the partition key necessary? Without a partition key, a server can only effectively only have one scope (aka partition), which is impractical for most services, or it needs to communicate the scopes out-of-band. diff --git a/draft-ietf-httpapi-ratelimit-headers.md~ b/draft-ietf-httpapi-ratelimit-headers.md~ new file mode 100644 index 0000000..2fcf76d --- /dev/null +++ b/draft-ietf-httpapi-ratelimit-headers.md~ @@ -0,0 +1,1169 @@ +--- +title: RateLimit header fields for HTTP +abbrev: +docname: draft-ietf-httpapi-ratelimit-headers-latest +category: std + +ipr: trust200902 +area: Applications and Real-Time +workgroup: HTTPAPI +keyword: Internet-Draft + +stand_alone: yes +pi: [toc, tocindent, sortrefs, symrefs, strict, compact, comments, inline, docmapping] + +venue: + group: HTTPAPI + type: Working Group + home: https://datatracker.ietf.org/wg/httpapi/about/ + mail: httpapi@ietf.org + arch: https://mailarchive.ietf.org/arch/browse/httpapi/ + repo: https://github.com/ietf-wg-httpapi/ratelimit-headers + +author: + - + ins: R. Polli + name: Roberto Polli + org: Team Digitale, Italian Government + email: robipolli@gmail.com + country: Italy + - + ins: A. Martinez + name: Alejandro Martinez Ruiz + org: Red Hat + email: alex@flawedcode.org + - + ins: D. Miller + name: Darrel Miller + org: Microsoft + email: darrel@tavis.ca + +entity: + SELF: "RFC nnnn" + +normative: + IANA: RFC8126 + HTTP: RFC9110 + +informative: + PRIVACY: RFC6973 + UNIX: + title: The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98 + author: + name: The Open Group + ins: The Open Group + date: 1997-02 +--- abstract + +This document defines the RateLimit-Policy and RateLimit HTTP header fields for servers to advertise their service policy limits and the current limits, thereby allowing clients to avoid being throttled. + +--- middle + +# Introduction + +Rate limiting of HTTP clients has become a widespread practice, especially for HTTP APIs. Typically, servers who do so limit the number of acceptable requests in a given time window (e.g. 10 requests per second). See {{rate-limiting}} for further information on the current usage of rate limiting in HTTP. + +Currently, there is no standard way for servers to communicate quotas so that clients can throttle their requests to prevent errors. This document defines a set of standard HTTP header fields to enable rate limiting: + +- RateLimit: to convey + the server's current limit of quota units available to the client in the policy time window, + the remaining quota units in the current window, + and the time remaining in the current window, specified in seconds, and +- RateLimit-Policy: the service policy limits. + +These fields enable establishing complex rate limiting policies, including using multiple and variable time windows and dynamic quotas, and implementing concurrency limits. + +The behavior of the RateLimit header field is compatible with the delay-seconds notation of Retry-After. + +## Goals {#goals} + +The goals of this document are: + + Interoperability: + : Standardize the names and semantics of rate-limit headers + to ease their enforcement and adoption; + + Resiliency: + : Improve resiliency of HTTP infrastructure by + providing clients with information useful + to throttle their requests and + prevent 4xx or 5xx responses; + + Documentation: + : Simplify API documentation by eliminating the need + to include detailed quota limits + and related fields in API documentation. + +The following features are out of the scope of this document: + + Authorization: + : RateLimit header fields are not meant to support + authorization or other kinds of access controls. + + Response status code: + : RateLimit header fields may be returned in both + successful (see {{Section 15.3 of HTTP}}) and non-successful responses. + This specification does not cover whether non Successful + responses count on quota usage, + nor does it mandates any correlation between the RateLimit values + and the returned status code. + + Throttling algorithm: + : This specification does not mandate a specific throttling algorithm. + The values published in the fields, including the window size, + can be statically or dynamically evaluated. + + Service Level Agreement: + : Conveyed quota hints do not imply any service guarantee. + Server is free to throttle respectful clients under certain circumstances. + +## Notational Conventions + +{::boilerplate bcp14} + +The term Origin is to be interpreted as described in Section 7 of {{!WEB-ORIGIN=RFC6454}}. + +This document uses the terms List, Item and Integer from {{Section 3 of !STRUCTURED-FIELDS=RFC8941}} to specify syntax and parsing, along with the concept of "bare item". + +# Terminology + +## Quota {#quota} + +A quota is an allocation of capacity to enable a server to limit client requests. That capacity is counted in quota units and may be reallocated at the end of a time window {{time-window}}. + +## Quota Unit {#quota-unit} + +A quota unit is the unit of measure used to count the activity of a client. + +## Quota Partition {#quota-partition} + +A quota partition is a division of a server's capacity across different clients, users and owned resources. + +## Time Window {#time-window} + +A time window indicates a period of time associated to the allocated quota. + +The time window is a non-negative Integer value expressing an interval in seconds, similar to the "delay-seconds" rule defined in {{Section 10.2.3 of HTTP}}. Sub-second precision is not supported. + +## Quota Policy {#quota-policy} + +A quota policy is maintained by a server to limit the activity (counted in [quota units](#quota-unit)) of a given [quota partition](#quota-partition) over a period of time (known as the [time window](#time-window)) to a specified amount known as the [quota](#quota). + +Quota policies can be advertised by servers (see {{ratelimit-policy-field}}), but they are not required to be, and more than one quota policy can affect a given request from a client to a server. + +## Service Limit {#service-limit} + +A service limit is the current limit of the amount of activity that a server will allow based on the remaining quota for a particular quota partition within the time-window, if defined. + +# RateLimit-Policy Field {#ratelimit-policy-field} + +The "RateLimit-Policy" response header field is a non-empty List of {{quotapolicy-item}}. Its value is informative. The values are expected to remain consistent over a the lifetime of a connection. It is this characteristic that differentiates it from the [RateLimit](#ratelimit-field) that contains values that may change on every request. + +~~~ + RateLimit-Policy: burst;q=100;w=60,daily;q=1000;w=86400 +~~~ + +## Quota Policy Item {#quotapolicy-item} +A quota policy Item contains information about a server's capacity allocation for a quota partition associated with the request. + +The following parameters are defined in this specification: + + q: + : The REQUIRED "q" parameter indicates the quota allocated. ({{ratelimitpolicy-quota}}) + + qu: + : The OPTIONAL "qu" parameter value conveys the quota units associated to the "q" parameter. The default quota unit is "request". ({{ratelimitpolicy-quotaunit}}) + + w: + : The OPTIONAL "w" parameter value conveys a time "window" ({{time-window}}). ({{ratelimitpolicy-window}}) + + pk: + : The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request. {{ratelimitpolicy-partitionkey}} + +Other parameters are allowed and can be regarded as comments. + +Implementation- or service-specific parameters SHOULD be prefixed parameters with a vendor identifier, e.g. `acme-policy`, `acme-burst`. + +### Quota Parameter {#ratelimitpolicy-quota} + + The "q" parameter uses a non-negative integer value to indicate the quota allocated for client activity (counted in quota units) for a given quota partition ({{service-limit}}). + +### Quota Unit Parameter {#ratelimitpolicy-quotaunit} + +The "qu" parameter value conveys the quota units associated to the "q" parameter. + +### Window Parameter {#ratelimitpolicy-window} + +The "w" parameter value conveys a time "window" in seconds. ({{time-window}}). + +### Partition Key Parameter {#ratelimitpolicy-partitionkey} + +The "pk" parameter value conveys the partition key associated to the request. Servers MAY use the partition key to divide server capacity across different clients and resources. Quotas are allocated per partition key. + +## RateLimit Policy Field Examples + +This field MAY convey the time window associated with the expiring-limit, as shown in this example: + +~~~ + RateLimit-Policy: default;l=100;w=10 +~~~ + +These examples show multiple policies being returned: + +~~~ + RateLimit-Policy: permin;l=50;w=60,perhr;l=1000;w=3600,perday;l=5000;w=86400 +~~~ + +The following example shows a policy with a partition key: + +~~~ + RateLimit-Policy: peruser;l=100;w=60;pk=user123 +~~~ + +The following example shows a policy with a partition key and a quota unit: + +~~~ + RateLimit-Policy: peruser;l=65535;w=10;pk=user123;qu=bytes +~~~ + +This field cannot appear in a trailer section. + +# RateLimit Field {#ratelimit-field} + +A server uses the "RateLimit" response header field to communicate the service limit for a quota policy for a particular partition key. + +The field is expressed as List of {{servicelimit-item}}. + +~~~ + RateLimit: default;r=50;t=30 +~~~ + +## Service Limit Item {#servicelimit-item} + +Each service limit item in identifies the quota policy associated with the request and + +The following parameters are defined in this specification: + + r: + : This parameter value conveys the remaining quota units for the identified policy ({{ratelimit-remaining-parameter}}). + + t: + : This OPTIONAL parameter value conveys the time window reset time for the identified policy ({{ratelimit-reset-parameter}}). + + pk: + : The OPTIONAL "pk" parameter value conveys the partition key associated to the corresponding request. + +This field cannot appear in a trailer section. Other parameters are allowed and can be regarded as comments. + +Implementation- or service-specific parameters SHOULD be prefixed parameters with a vendor identifier, e.g. `acme-policy`, `acme-burst`. + + +### Remaining Parameter {#ratelimit-remaining-parameter} + +The "r" parameter indicates the remaining quota units for the identified policy ({{ratelimit-remaining-parameter}}). + +It is a non-negative Integer expressed in [quota units](#quota-unit). +Clients MUST NOT assume that a positive remaining value is a guarantee that further requests will be served. +When remaining parameter value is low, it indicates that the server may soon throttle the client (see {{providing-ratelimit-fields}}). + +### Reset Parameter {#ratelimit-reset-parameter} + +The "t" parameter indicates the number of seconds until the quota associated with the quota policy resets. + +It is a non-negative Integer compatible with the delay-seconds rule, because: + +- it does not rely on clock synchronization and is resilient to clock adjustment + and clock skew between client and server (see {{Section 5.6.7 of HTTP}}); +- it mitigates the risk related to thundering herd when too many clients are serviced with the same timestamp. + +The client MUST NOT assume that all its service limit will be reset at the moment indicated by the reset keyword. The server MAY arbitrarily alter the reset parameter value between subsequent requests; for example, in case of resource saturation or to implement sliding window policies. + +### Partition Key Parameter {#ratelimit-partitionkey} + +The "pk" parameter value conveys the partition key associated to the request. Servers MAY use the partition key to divide server capacity across different clients and resources. Quotas are allocated per partition key. + + +## RateLimit Field Examples + +This example shows a RateLimit field with a remaining quota of 50 units and a time window reset in 30 seconds: + +~~~ + RateLimit: default;r=50;t=30 +~~~ + +This example shows a remaining quota of 999 requests for a partition key that has no time window reset: + +~~~ + RateLimit: default;r=999;pk=trial-121323 +~~~ + +This example shows a 300MB remaining quota for an application in the next 60 seconds: + +~~~ + RateLimit: default;r=300000000;pk=App-999;t=60;qu=bytes +~~~ + + +# Server Behavior {#providing-ratelimit-fields} + +A server MAY return RateLimit header fields independently of the response status code. This includes on throttled responses. This document does not mandate any correlation between the RateLimit header field values and the returned status code. + +Servers should be careful when returning RateLimit header fields in redirection responses (i.e., responses with 3xx status codes) because a low remaining keyword value could prevent the client from issuing requests. For example, given the RateLimit header fields below, a client could decide to wait 10 seconds before following the "Location" header field (see {{Section 10.2.2 of HTTP}}), because the remaining keyword value is 0. + +~~~ http-message +HTTP/1.1 301 Moved Permanently +Location: /foo/123 +RateLimit: problemPolicy;r=0, t=10 + +~~~ + +If a response contains both the Retry-After and the RateLimit header fields, the reset keyword value SHOULD reference the same point in time as the Retry-After field value. + +A service using RateLimit header fields MUST NOT convey values exposing an unwanted volume of requests and SHOULD implement mechanisms to cap the ratio between the remaining and the reset keyword values (see {{sec-resource-exhaustion}}); this is especially important when a quota policy uses a large time window. + +Under certain conditions, a server MAY artificially lower RateLimit header field values between subsequent requests, e.g. to respond to Denial of Service attacks or in case of resource saturation. + +## 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=RFC7541}}. + +# Client Behavior {#receiving-fields} + +The RateLimit header fields can be used by clients to determine whether the associated request respected the server's quota policy, and as an indication of whether subsequent requests will. However, the server might apply other criteria when servicing future requests, and so the quota policy may not completely reflect whether requests will succeed. + +For example, a successful response with the following fields: + +~~~ + RateLimit: default;r=1;t=7 +~~~ + +does not guarantee that the next request will be successful. Servers' behavior may be subject to other conditions. + +A client is responsible for ensuring that RateLimit header field values returned +cause reasonable client behavior with respect to throughput and latency +(see {{sec-resource-exhaustion}} and {{sec-dos}}). + +A client receiving RateLimit header fields MUST NOT assume that future responses will contain the same RateLimit header fields, or any RateLimit header fields at all. + +Malformed RateLimit header fields MUST be ignored. + +A client SHOULD NOT exceed the quota units conveyed by the remaining keyword before the time window expressed in the reset keyword. + +The value of the reset keyword is generated at response time: a client aware of a significant network latency MAY behave accordingly and use other information (e.g. the "Date" response header field, or otherwise gathered metrics) to better estimate the reset keyword moment intended by the server. + +The details provided in the RateLimit-Policy header field are informative and MAY be ignored. + +If a response contains both the RateLimit and Retry-After fields, the Retry-After field MUST take precedence and the reset keyword MAY be ignored. + +This specification does not mandate a specific throttling behavior and implementers can adopt their preferred policies, including: + +- slowing down or pre-emptively back-off their request rate when + approaching quota limits; +- consuming all the quota according to the exposed limits and then wait. + +## Intermediaries {#intermediaries} + +This section documents the considerations advised in {{Section 16.3.2 of HTTP}}. + +An intermediary that is not part of the originating service infrastructure and is not aware of the quota policy semantic used by the Origin Server SHOULD NOT alter the RateLimit header fields' values in such a way as to communicate a more permissive quota policy; this includes removing the RateLimit header fields. + +An intermediary MAY alter the RateLimit header fields in such a way as to communicate a more restrictive quota policy when: + +- it is aware of the quota unit semantic used by the Origin Server; +- it implements this specification and enforces a quota policy which + is more restrictive than the one conveyed in the fields. + +An intermediary SHOULD forward a request even when presuming that it might not be serviced; the service returning the RateLimit header fields is the sole responsible of enforcing the communicated quota policy, and it is always free to service incoming requests. + +This specification does not mandate any behavior on intermediaries respect to retries, nor requires that intermediaries have any role in respecting quota policies. For example, it is legitimate for a proxy to retransmit a request without notifying the client, and thus consuming quota units. + +[Privacy considerations](#privacy) provide further guidance on intermediaries. + +## Caching + +{{?HTTP-CACHING=RFC9111}} 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}}). + +# Security Considerations + +## Throttling does not prevent clients from issuing requests {#sec-throttling-does-not-prevent} + +This specification does not prevent clients from making requests. +Servers should always implement mechanisms to prevent resource exhaustion. + +## Information disclosure {#sec-information-disclosure} + +Servers should not disclose to untrusted parties operational capacity information +that can be used to saturate its infrastructural resources. + +While this specification does not mandate whether non-successful responses consume quota, +if error responses (such as 401 (Unauthorized) and 403 (Forbidden)) count against quota, +a malicious client could probe the endpoint to get traffic information of another user. + +As intermediaries might retransmit requests and consume +quota units without prior knowledge of the user agent, +RateLimit header fields might reveal the existence of an intermediary +to the user agent. + +Where partition keys contain identifying information, either of the client application or the user, servers should be aware of the potential for impersonation and apply the appropriate security mechanisms. + +## Remaining quota units are not granted requests {#sec-remaining-not-granted} + +RateLimit header fields convey hints from the server +to the clients in order to help them avoid being throttled out. + +Clients MUST NOT consider the [quota units](#service-limit) returned in remaining keyword 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. + +## Reliability of the reset keyword {#sec-reset-reliability} + +Consider that quota might not be restored after the moment referenced by the [reset keyword](#ratelimit-reset-parameter), +and the reset parameter value may not be constant. + +Subsequent requests might return a higher reset parameter value +to limit concurrency or implement dynamic or adaptive throttling policies. + +## Resource exhaustion {#sec-resource-exhaustion} + +When returning reset values, servers must be aware that +many throttled clients may come back at the very moment specified. + +This is true for Retry-After too. + +For example, if the quota resets every day at `18:00:00` +and your server returns the reset parameter accordingly + +~~~ + Date: Tue, 15 Nov 1994 18:00:00 GMT + RateLimit: daily;r=1;t=36400 +~~~ + +there's a high probability that all clients will show up at `18:00:00`. + +This could be mitigated by adding some jitter to the reset value. + +Resource exhaustion issues can be associated with quota policies using a +large time window, because a user agent by chance or on purpose +might consume most of its quota units in a significantly shorter interval. + +This behavior can be even triggered by the provided RateLimit header fields. +The following example describes a service +with an unconsumed quota policy of 10000 quota units per 1000 seconds. + +~~~ +RateLimit-Policy: somepolicy;l=10000;w=1000 +RateLimit: somepolicy;r=10000;t=10 +~~~ + +A client implementing a simple ratio between remaining keyword and reset keyword could infer an average throughput of 1000 quota units per second, while the limit keyword conveys a quota-policy with an average of 10 quota units per second. +If the service cannot handle such load, it should return either a lower remaining keyword value or an higher reset keyword value. +Moreover, complementing large time window quota policies with a short time window one mitigates those risks. + + +### Denial of Service {#sec-dos} + +RateLimit header fields may contain unexpected values by chance or on purpose. +For example, an excessively high remaining keyword value may be: + +- used by a malicious intermediary to trigger a Denial of Service attack + or consume client resources boosting its requests; +- passed by a misconfigured server; + +or a high reset keyword value could inhibit clients to contact the server (e.g. similarly to receiving "Retry-after: 1000000"). + +To mitigate this risk, clients can set thresholds that they consider reasonable in terms of quota units, time window, concurrent requests or throughput, and define a consistent behavior when the RateLimit exceed those thresholds. +For example this means capping the maximum number of request per second, or implementing retries when the reset keyword 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). + + +# Privacy Considerations {#privacy} + +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. + +# IANA Considerations + +IANA is requested to update one registry and create one new registry. + +Please add the following entries to the +"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry ({{HTTP}}): + +|---------------------|-----------|---------------| +| Field Name | Status | Specification | +|---------------------|-----------|---------------| +| RateLimit | permanent | {{ratelimit-field}} of {{&SELF}} | +| RateLimit-Policy | permanent | {{ratelimit-policy-field}} of {{&SELF}} | +|---------------------|-----------|---------------| + + +--- back + +# Rate-limiting and quotas {#rate-limiting} + +Servers use quota mechanisms to avoid systems overload, to ensure an equitable distribution of computational resources or to enforce other policies - e.g. monetization. + +A basic quota mechanism limits the number of acceptable requests in a given time window, e.g. 10 requests per second. + +When quota is exceeded, servers usually do not serve the request replying instead with a 4xx HTTP status code (e.g. 429 or 403) or adopt more aggressive policies like dropping connections. + +Quotas may be enforced on different basis (e.g. per user, per IP, per geographic area, ..) and at different levels. For example, an user may be allowed to issue: + +- 10 requests per second; +- limited to 60 requests per minute; +- limited to 1000 requests per hour. + +Moreover system metrics, statistics and heuristics can be used to implement more complex policies, +where the number of acceptable requests and the time window are computed dynamically. + +To help clients throttling their requests, +servers may expose the counters used to evaluate quota policies via HTTP header fields. + +Those response headers may be added by HTTP intermediaries such as API gateways and reverse proxies. + +On the web we can find many different rate-limit headers, +usually containing the number of allowed requests in a given time window, and when the window is reset. + +The common choice is to return three headers containing: + +- the maximum number of allowed requests in the time window; +- the number of remaining requests in the current window; +- the time remaining in the current window expressed in seconds or + as a timestamp; + +## Interoperability issues + +A major interoperability issue in throttling is the lack of standard headers, because: + +- each implementation associates different semantics to the + same header field names; +- header field names proliferates. + +User agents interfacing with different servers may thus need to process different headers, +or the very same application interface that sits behind different reverse proxies +may reply with different throttling headers. + +# Examples + +## 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. + +### 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: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit: default;r=0;t=50 + +{"hello": "world"} +~~~ + +Since the field values are not necessarily correlated with +the response status code, +a subsequent request is not required to fail. +The example below shows that the server decided to serve the request +even if remaining keyword value is 0. +Another server, or the same server under other load conditions, could have decided to throttle the request instead. + +Request: + +~~~ http-message +GET /items/456 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit: default;r=0;t=48 + +{"still": "successful"} +~~~ + +### Multiple policies in response {#use-with-multiple-policies} + +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 reset in 10 hours; + +The server MAY choose to omit returning the hourly policy as it uses the same quota units as the daily policy and the daily policy is the one that is closest to being exhausted. + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit: dayLimit;r=100;t=36000 + +{"hello": "world"} +~~~ + +### Use for limiting concurrency {#use-for-limiting-concurrency} + +RateLimit header fields may be used to limit concurrency, +advertising limits that are lower than the usual ones +in case of saturation, thus increasing availability. + +The server adopted a basic policy of 100 quota units per minute, +and in case of resource exhaustion adapts the returned values +reducing both limit and remaining keyword values. + +After 2 seconds the client consumed 40 quota units + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit-Policy: basic;l=100;w=60 +RateLimit: basic;r=60;t=58 + +{"elapsed": 2, "issued": 40} +~~~ + +At the subsequent request - due to resource exhaustion - +the server advertises only `r=20`. + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit-Policy: basic;l=100;w=60 +RateLimit: basic;r=20;t=56 + +{"elapsed": 4, "issued": 41} +~~~ + +### 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: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 429 Too Many Requests +Content-Type: application/json +Date: Mon, 05 Aug 2019 09:27:00 GMT +Retry-After: Mon, 05 Aug 2019 09:27:05 GMT +RateLimit: default;r=0;t=5 + +{ +"title": "Too Many Requests", +"status": 429, +"detail": "You have exceeded your quota" +} +~~~ + +## Responses with defined policies + +### 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: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit: fixedwindow;r=99;t=50 +RateLimit-Policy: fixedwindow;l=100;w=60 +{"hello": "world"} +~~~ + + +### Dynamic limits with parameterized windows + +The policy conveyed by the RateLimit header field states that +the server accepts 100 quota units per minute. + +To avoid resource exhaustion, the server artificially lowers +the actual limits returned in the throttling headers. + +The remaining keyword 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: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit-Policy: dynamic;l=100;w=60 +RateLimit: dynamic;r=9;t=50 + + +{ + "status": 200, + "detail": "Just slow down without waiting." +} +~~~ + +### 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: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 429 Too Many Requests +Content-Type: application/json +RateLimit-Policy: dynamic;l=15;w=20 +RateLimit: dynamic;r=0;t=20 + +{ + "status": 429, + "detail": "Wait 20 seconds, then slow down!" +} +~~~ + +## Dynamic limits for pushing back with Retry-After and slow down + +Alternatively, given the same context where the previous example starts, we +can convey the same information to the client via Retry-After, with +the advantage that the server can now specify the policy's nominal limit and +window that will apply after the reset, e.g. assuming the resource exhaustion +is likely to be gone by then, so the advertised policy does not need to be +adjusted, yet we managed to stop requests for a while and slow down the rest of +the current window. + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 429 Too Many Requests +Content-Type: application/json +Retry-After: 20 +RateLimit-Policy: dynamic;l=100;w=60 +RateLimit: dynamic;r=15;t=40 + +{ + "status": 429, + "detail": "Wait 20 seconds, then slow down!" +} +~~~ + +Note that in this last response the client is expected to honor +Retry-After and perform no requests for the specified amount of +time, whereas the previous example would not force the client to stop +requests before the reset time is elapsed, as it would still be free to +query again the server even if it is likely to have the request rejected. + +### Missing Remaining information + +The server does not expose remaining values +(for example, because the underlying counters are not available). +Instead, it resets the limit counter every second. + +It communicates to the client the limit of 10 quota units per second +always returning the limit and reset keywords. + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit-Policy: quota;l=100;w=1 +RateLimit: quota;t=1 + +{"first": "request"} +~~~ + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 Ok +Content-Type: application/json +RateLimit-Policy: quota;l=10 +RateLimit: quota;t=1 + +{"second": "request"} +~~~ + +### Use with multiple windows + +This is a standardized way of describing the policy +detailed in {{use-with-multiple-policies}}: + +- 5000 daily quota units; +- 1000 hourly quota units. + +The client consumed 4900 quota units in the first 14 hours. + +Despite the next hourly limit of 1000 quota units, the closest limit +to reach is the daily one. + +The server then exposes the RateLimit header fields to inform the client that: + +- it has only 100 quota units left; +- the window will reset in 10 hours; +- the expiring-limit is 5000. + +Request: + +~~~ http-message +GET /items/123 HTTP/1.1 +Host: api.example + +~~~ + +Response: + +~~~ http-message +HTTP/1.1 200 OK +Content-Type: application/json +RateLimit-Policy: hour;l=1000;w=3600, day;l=5000;w=86400 +RateLimit: day;r=100;t=36000 + +{"hello": "world"} +~~~ + +# FAQ +{:numbered="false" removeinrfc="true"} + +1. Why defining standard fields for throttling? + + To simplify enforcement of throttling policies and enable clients to constraint their requests to avoid being throttled. + +2. Can I use RateLimit header fields in throttled responses (eg with status code 429)? + + Yes, you can. + +3. Are those specs tied to RFC 6585? + + No. {{?RFC6585}} defines the `429` status code and we use it just as an example of a throttled request, + that could instead use even `403` or whatever status code. + +4. Why is the partition key necessary? + + Without a partition key, a server can only effectively only have one scope (aka partition), which is impractical for most services, or it needs to communicate the scopes out-of-band. + This prevents the development of generic connector code that can be used to prevent requests from being throttled. + Many APIs rely on API keys, user identity or client identity to allocate quota. + As soon as a single client processes requests for more than one partition, the client needs to know the corresponding partition key to properly track requests against allocated quota. + +5. Why using delay-seconds instead of a UNIX Timestamp? + Why not using subsecond precision? + + Using delay-seconds aligns with Retry-After, which is returned in similar contexts, + eg on 429 responses. + + Timestamps require a clock synchronization protocol + (see {{Section 5.6.7 of HTTP}}). + This may be problematic (e.g. clock adjustment, clock skew, failure of hardcoded clock synchronization servers, + IoT devices, ..). + Moreover timestamps may not be monotonically increasing due to clock adjustment. + See [Another NTP client failure story](https://community.ntppool.org/t/another-ntp-client-failure-story/1014/) + + We did not use subsecond precision because: + + - that is more subject to system clock correction + like the one implemented via the adjtimex() Linux system call; + - response-time latency may not make it worth. A brief discussion on the subject is + on the [httpwg ml](https://lists.w3.org/Archives/Public/ietf-http-wg/2019JulSep/0202.html) + - almost all rate-limit headers implementations do not use it. + +6. Shouldn't I limit concurrency instead of request rate? + + You can use this specification to limit concurrency + at the HTTP level (see {#use-for-limiting-concurrency}) + and help clients to shape their requests avoiding being throttled out. + + A problematic way to limit concurrency is connection dropping, + especially when connections are multiplexed (e.g. HTTP/2) + because this results in unserviced client requests, + which is something we want to avoid. + + A semantic way to limit concurrency is to return 503 + Retry-After + in case of resource saturation (e.g. thrashing, connection queues too long, + Service Level Objectives not meet, ..). + Saturation conditions can be either dynamic or static: all this is out of + the scope for the current document. + +7. Do a positive value of remaining paramter imply any service guarantee for my + future requests to be served? + + No. FAQ integrated in {{ratelimit-remaining-parameter}}. + +8. Is the quota-policy definition {{quota-policy}} too complex? + + You can always return the simplest form + +~~~ +RateLimit:default;r=50;t=60 +~~~ + + The policy key clearly connects the current usage status of a policy to the defined limits. + So for the following field: + +~~~ +RateLimit-Policy: sliding;l=100;w=60;burst=1000;comment="sliding window", fixed;l=5000;w=3600;burst=0;comment="fixed window" +RateLimit: sliding;r=50;t=44 +~~~ + + the value "sliding" identifies the policy being reported. + +9. Can intermediaries alter RateLimit header fields? + + Generally, they should not because it might result in unserviced requests. + There are reasonable use cases for intermediaries mangling RateLimit header fields though, + e.g. when they enforce stricter quota-policies, + or when they are an active component of the service. + In those case we will consider them as part of the originating infrastructure. + +10. Why the `w` parameter is just informative? + Could it be used by a client to determine the request rate? + + A non-informative `w` parameter might be fine in an environment + where clients and servers are tightly coupled. Conveying policies + with this detail on a large scale would be very complex and implementations + would be likely not interoperable. We thus decided to leave `w` as + an informational parameter and only rely on the limit, remaining and reset keywords + for defining the throttling + behavior. + +11. Can I use RateLimit fields in trailers? + Servers usually establish whether the request is in-quota before creating a response, so the RateLimit field values should be already available in that moment. + Supporting trailers has the only advantage that allows to provide more up-to-date information to the client in case of slow responses. + However, this complicates client implementations with respect to combining fields from headers and accounting for intermediaries that drop trailers. + Since there are no current implementations that use trailers, we decided to leave this as a future-work. + +# RateLimit header fields currently used on the web +{:numbered="false" removeinrfc="true"} + +Commonly used header field names are: + +- `X-RateLimit-Limit`, + `X-RateLimit-Remaining`, + `X-RateLimit-Reset`; + +There are variants too, where the window is specified +in the header field name, eg: + +- `x-ratelimit-limit-minute`, `x-ratelimit-limit-hour`, `x-ratelimit-limit-day` +- `x-ratelimit-remaining-minute`, `x-ratelimit-remaining-hour`, `x-ratelimit-remaining-day` + +Here are some interoperability issues: + +- `X-RateLimit-Remaining` references different values, depending on the implementation: + + * seconds remaining to the window expiration + * milliseconds remaining to the window expiration + * seconds since UTC, in UNIX Timestamp [UNIX] + * a datetime, either `IMF-fixdate` {{HTTP}} or {{?RFC3339}} + +- different headers, with the same semantic, are used by different implementers: + + * X-RateLimit-Limit and X-Rate-Limit-Limit + * X-RateLimit-Remaining and X-Rate-Limit-Remaining + * X-RateLimit-Reset and X-Rate-Limit-Reset + +The semantic of RateLimit depends on the windowing algorithm. +A sliding window policy for example, may result in having a remaining keyword value related to the ratio between the current and the maximum throughput. +e.g. + +~~~ +RateLimit-Policy: sliding;l=12;w=1 +RateLimit: sliding;l=12;r=6;t=1 ; using 50% of throughput, that is 6 units/s + +~~~ + +If this is the case, the optimal solution is to achieve + +~~~ +RateLimit-Policy: sliding;l=12;w=1 +RateLimit: sliding;l=12;r=1;t=1 ; using 100% of throughput, that is 12 units/s +~~~ + +At this point you should stop increasing your request rate. + +# Acknowledgements +{:numbered="false"} + +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, +Darrel Miller +and Julian Reschke. + +# Changes +{:numbered="false" removeinrfc="true"} + +## Since draft-ietf-httpapi-ratelimit-headers-07 +{:numbered="false" removeinrfc="true"} + +* 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 +{:numbered="false" removeinrfc="true"} + +* Split policy informatiom in RateLimit-Policy #81 + + +## Since draft-ietf-httpapi-ratelimit-headers-02 +{:numbered="false" removeinrfc="true"} + +* Address throttling scope #83 + +## Since draft-ietf-httpapi-ratelimit-headers-01 +{:numbered="false" removeinrfc="true"} + +* Update IANA considerations #60 +* Use Structured fields #58 +* Reorganize document #67 + +## Since draft-ietf-httpapi-ratelimit-headers-00 +{:numbered="false" removeinrfc="true"} + +* Use I-D.httpbis-semantics, which includes referencing delay-seconds + instead of delta-seconds. #5 diff --git a/draft-ietf-httpapi-ratelimit-headers.txt b/draft-ietf-httpapi-ratelimit-headers.txt new file mode 100644 index 0000000..1deaa44 --- /dev/null +++ b/draft-ietf-httpapi-ratelimit-headers.txt @@ -0,0 +1,1435 @@ + + + + +HTTPAPI R. Polli +Internet-Draft Team Digitale, Italian Government +Intended status: Standards Track A. Martinez +Expires: 2 April 2025 Red Hat + D. Miller + Microsoft + 29 September 2024 + + + RateLimit header fields for HTTP + draft-ietf-httpapi-ratelimit-headers-latest + +Abstract + + This document defines the RateLimit-Policy and RateLimit HTTP header + fields for servers to advertise their service policy limits and the + current limits, thereby allowing clients to avoid being throttled. + +About This Document + + This note is to be removed before publishing as an RFC. + + Status information for this document may be found at + https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit- + headers/. + + Discussion of this document takes place on the HTTPAPI Working Group + mailing list (mailto:httpapi@ietf.org), which is archived at + https://mailarchive.ietf.org/arch/browse/httpapi/. Subscribe at + https://www.ietf.org/mailman/listinfo/httpapi/. Working Group + information can be found at https://datatracker.ietf.org/wg/httpapi/ + about/. + + Source for this draft and an issue tracker can be found at + https://github.com/ietf-wg-httpapi/ratelimit-headers. + +Status of This Memo + + This Internet-Draft is submitted in full conformance with the + provisions of BCP 78 and BCP 79. + + Internet-Drafts are working documents of the Internet Engineering + Task Force (IETF). Note that other groups may also distribute + working documents as Internet-Drafts. The list of current Internet- + Drafts is at https://datatracker.ietf.org/drafts/current/. + + 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 2 April 2025. + +Copyright Notice + + Copyright (c) 2024 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents (https://trustee.ietf.org/ + license-info) in effect on the date of publication of this document. + Please review these documents carefully, as they describe your rights + and restrictions with respect to this document. Code Components + extracted from this document must include Revised BSD License text as + described in Section 4.e of the Trust Legal Provisions and are + provided without warranty as described in the Revised BSD License. + +Table of Contents + + 1. Introduction + 1.1. Goals + 1.2. Notational Conventions + 2. Terminology + 2.1. Quota + 2.2. Quota Unit + 2.3. Quota Partition + 2.4. Time Window + 2.5. Quota Policy + 2.6. Service Limit + 3. RateLimit-Policy Field + 3.1. Quota Policy Item + 3.1.1. Quota Parameter + 3.1.2. Quota Unit Parameter + 3.1.3. Window Parameter + 3.1.4. Partition Key Parameter + 3.2. RateLimit Policy Field Examples + 4. RateLimit Field + 4.1. Service Limit Item + 4.1.1. Remaining Parameter + 4.1.2. Reset Parameter + 4.1.3. Partition Key Parameter + 4.2. RateLimit Field Examples + 5. Server Behavior + 5.1. Performance Considerations + 6. Client Behavior + 6.1. Intermediaries + 6.2. Caching + 7. Security Considerations + 7.1. Throttling does not prevent clients from issuing requests + 7.2. Information disclosure + 7.3. Remaining quota units are not granted requests + 7.4. Reliability of the reset keyword + 7.5. Resource exhaustion + 7.5.1. Denial of Service + 8. Privacy Considerations + 9. IANA Considerations + 10. References + 10.1. Normative References + 10.2. Informative References + Appendix A. Rate-limiting and quotas + A.1. Interoperability issues + Appendix B. Examples + B.1. Responses without defining policies + B.1.1. Throttling information in responses + B.1.2. Multiple policies in response + B.1.3. Use for limiting concurrency + B.1.4. Use in throttled responses + B.2. Responses with defined policies + B.2.1. Throttling window specified via parameter + B.2.2. Dynamic limits with parameterized windows + B.2.3. Dynamic limits for pushing back and slowing down + B.3. Dynamic limits for pushing back with Retry-After and slow + down + B.3.1. Missing Remaining information + B.3.2. Use with multiple windows + FAQ + RateLimit header fields currently used on the web + Acknowledgements + Changes + Since draft-ietf-httpapi-ratelimit-headers-07 + Since draft-ietf-httpapi-ratelimit-headers-03 + Since draft-ietf-httpapi-ratelimit-headers-02 + Since draft-ietf-httpapi-ratelimit-headers-01 + Since draft-ietf-httpapi-ratelimit-headers-00 + Authors' Addresses + +1. Introduction + + Rate limiting of HTTP clients has become a widespread practice, + especially for HTTP APIs. Typically, servers who do so limit the + number of acceptable requests in a given time window (e.g. 10 + requests per second). See Appendix A for further information on the + current usage of rate limiting in HTTP. + + Currently, there is no standard way for servers to communicate quotas + so that clients can throttle their requests to prevent errors. This + document defines a set of standard HTTP header fields to enable rate + limiting: + + * RateLimit: to convey the server's current limit of quota units + available to the client in the policy time window, the remaining + quota units in the current window, and the time remaining in the + current window, specified in seconds, and + + * RateLimit-Policy: the service policy limits. + + These fields enable establishing complex rate limiting policies, + including using multiple and variable time windows and dynamic + quotas, and implementing concurrency limits. + + The behavior of the RateLimit header field is compatible with the + delay-seconds notation of Retry-After. + +1.1. Goals + + The goals of this document are: + + Interoperability: Standardize the names and semantics of rate-limit + headers to ease their enforcement and adoption; + + Resiliency: Improve resiliency of HTTP infrastructure by providing + clients with information useful to throttle their requests and + prevent 4xx or 5xx responses; + + Documentation: Simplify API documentation by eliminating the need to + include detailed quota limits and related fields in API + documentation. + + The following features are out of the scope of this document: + + Authorization: RateLimit header fields are not meant to support + authorization or other kinds of access controls. + + Response status code: RateLimit header fields may be returned in + both successful (see Section 15.3 of [HTTP]) and non-successful + responses. This specification does not cover whether non + Successful responses count on quota usage, nor does it mandates + any correlation between the RateLimit values and the returned + status code. + + Throttling algorithm: This specification does not mandate a specific + throttling algorithm. The values published in the fields, + including the window size, can be statically or dynamically + evaluated. + + Service Level Agreement: Conveyed quota hints do not imply any + service guarantee. Server is free to throttle respectful clients + under certain circumstances. + +1.2. Notational Conventions + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and + "OPTIONAL" in this document are to be interpreted as described in + BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all + capitals, as shown here. + + The term Origin is to be interpreted as described in Section 7 of + [WEB-ORIGIN]. + + This document uses the terms List, Item and Integer from Section 3 of + [STRUCTURED-FIELDS] to specify syntax and parsing, along with the + concept of "bare item". + +2. Terminology + +2.1. Quota + + A quota is an allocation of capacity to enable a server to limit + client requests. That capacity is counted in quota units and may be + reallocated at the end of a time window Section 2.4. + +2.2. Quota Unit + + A quota unit is the unit of measure used to count the activity of a + client. + +2.3. Quota Partition + + A quota partition is a division of a server's capacity across + different clients, users and owned resources. + +2.4. Time Window + + A time window indicates a period of time associated to the allocated + quota. + + The time window is a non-negative Integer value expressing an + interval in seconds, similar to the "delay-seconds" rule defined in + Section 10.2.3 of [HTTP]. Sub-second precision is not supported. + +2.5. Quota Policy + + A quota policy is maintained by a server to limit the activity + (counted in quota units (Section 2.2)) of a given quota partition + (Section 2.3) over a period of time (known as the time window + (Section 2.4)) to a specified amount known as the Section 2.1. + + Quota policies can be advertised by servers (see Section 3), but they + are not required to be, and more than one quota policy can affect a + given request from a client to a server. + +2.6. Service Limit + + A service limit is the current limit of the amount of activity that a + server will allow based on the remaining quota for a particular quota + partition within the time-window, if defined. + +3. RateLimit-Policy Field + + The "RateLimit-Policy" response header field is a non-empty List of + Section 3.1. Its value is informative. The values are expected to + remain consistent over a the lifetime of a connection. It is this + characteristic that differentiates it from the RateLimit (Section 4) + that contains values that may change on every request. + + RateLimit-Policy: burst;q=100;w=60,daily;q=1000;w=86400 + +3.1. Quota Policy Item + + A quota policy Item contains information about a server's capacity + allocation for a quota partition associated with the request. + + The following parameters are defined in this specification: + + q: The REQUIRED "q" parameter indicates the quota allocated. + (Section 3.1.1) + + qu: The OPTIONAL "qu" parameter value conveys the quota units + associated to the "q" parameter. The default quota unit is + "request". (Section 3.1.2) + + w: The OPTIONAL "w" parameter value conveys a time "window" + (Section 2.4). (Section 3.1.3) + + pk: The OPTIONAL "pk" parameter value conveys the partition key + associated to the corresponding request. Section 3.1.4 + + Other parameters are allowed and can be regarded as comments. + + Implementation- or service-specific parameters SHOULD be prefixed + parameters with a vendor identifier, e.g. acme-policy, acme-burst. + +3.1.1. Quota Parameter + + The "q" parameter uses a non-negative integer value to indicate the + quota allocated for client activity (counted in quota units) for a + given quota partition (Section 2.6). + +3.1.2. Quota Unit Parameter + + The "qu" parameter value conveys the quota units associated to the + "q" parameter. + +3.1.3. Window Parameter + + The "w" parameter value conveys a time "window" in seconds. + (Section 2.4). + +3.1.4. Partition Key Parameter + + The "pk" parameter value conveys the partition key associated to the + request. Servers MAY use the partition key to divide server capacity + across different clients and resources. Quotas are allocated per + partition key. + +3.2. RateLimit Policy Field Examples + + This field MAY convey the time window associated with the expiring- + limit, as shown in this example: + + RateLimit-Policy: default;l=100;w=10 + + These examples show multiple policies being returned: + + RateLimit-Policy: permin;l=50;w=60,perhr;l=1000;w=3600,perday;l=5000;w=86400 + + The following example shows a policy with a partition key: + + RateLimit-Policy: peruser;l=100;w=60;pk=user123 + + The following example shows a policy with a partition key and a quota + unit: + + RateLimit-Policy: peruser;l=65535;w=10;pk=user123;qu=bytes + + This field cannot appear in a trailer section. + +4. RateLimit Field + + A server uses the "RateLimit" response header field to communicate + the service limit for a quota policy for a particular partition key. + + The field is expressed as List of Section 4.1. + + RateLimit: default;r=50;t=30 + +4.1. Service Limit Item + + Each service limit item in identifies the quota policy associated + with the request and + + The following parameters are defined in this specification: + + r: This parameter value conveys the remaining quota units for the + identified policy (Section 4.1.1). + + t: This OPTIONAL parameter value conveys the time window reset time + for the identified policy (Section 4.1.2). + + pk: The OPTIONAL "pk" parameter value conveys the partition key + associated to the corresponding request. + + This field cannot appear in a trailer section. Other parameters are + allowed and can be regarded as comments. + + Implementation- or service-specific parameters SHOULD be prefixed + parameters with a vendor identifier, e.g. acme-policy, acme-burst. + +4.1.1. Remaining Parameter + + The "r" parameter indicates the remaining quota units for the + identified policy (Section 4.1.1). + + It is a non-negative Integer expressed in quota units (Section 2.2). + Clients MUST NOT assume that a positive remaining value is a + guarantee that further requests will be served. When remaining + parameter value is low, it indicates that the server may soon + throttle the client (see Section 5). + +4.1.2. Reset Parameter + + The "t" parameter indicates the number of seconds until the quota + associated with the quota policy resets. + + It is a non-negative Integer compatible with the delay-seconds rule, + because: + + * it does not rely on clock synchronization and is resilient to + clock adjustment and clock skew between client and server (see + Section 5.6.7 of [HTTP]); + + * it mitigates the risk related to thundering herd when too many + clients are serviced with the same timestamp. + + The client MUST NOT assume that all its service limit will be reset + at the moment indicated by the reset keyword. The server MAY + arbitrarily alter the reset parameter value between subsequent + requests; for example, in case of resource saturation or to implement + sliding window policies. + +4.1.3. Partition Key Parameter + + The "pk" parameter value conveys the partition key associated to the + request. Servers MAY use the partition key to divide server capacity + across different clients and resources. Quotas are allocated per + partition key. + +4.2. RateLimit Field Examples + + This example shows a RateLimit field with a remaining quota of 50 + units and a time window reset in 30 seconds: + + RateLimit: default;r=50;t=30 + + This example shows a remaining quota of 999 requests for a partition + key that has no time window reset: + + RateLimit: default;r=999;pk=trial-121323 + + This example shows a 300MB remaining quota for an application in the + next 60 seconds: + + RateLimit: default;r=300000000;pk=App-999;t=60;qu=bytes + +5. Server Behavior + + A server MAY return RateLimit header fields independently of the + response status code. This includes on throttled responses. This + document does not mandate any correlation between the RateLimit + header field values and the returned status code. + + Servers should be careful when returning RateLimit header fields in + redirection responses (i.e., responses with 3xx status codes) because + a low remaining keyword value could prevent the client from issuing + requests. For example, given the RateLimit header fields below, a + client could decide to wait 10 seconds before following the + "Location" header field (see Section 10.2.2 of [HTTP]), because the + remaining keyword value is 0. + + HTTP/1.1 301 Moved Permanently + Location: /foo/123 + RateLimit: problemPolicy;r=0, t=10 + + If a response contains both the Retry-After and the RateLimit header + fields, the reset keyword value SHOULD reference the same point in + time as the Retry-After field value. + + A service using RateLimit header fields MUST NOT convey values + exposing an unwanted volume of requests and SHOULD implement + mechanisms to cap the ratio between the remaining and the reset + keyword values (see Section 7.5); this is especially important when a + quota policy uses a large time window. + + Under certain conditions, a server MAY artificially lower RateLimit + header field values between subsequent requests, e.g. to respond to + Denial of Service attacks or in case of resource saturation. + +5.1. 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. Client Behavior + + The RateLimit header fields can be used by clients to determine + whether the associated request respected the server's quota policy, + and as an indication of whether subsequent requests will. However, + the server might apply other criteria when servicing future requests, + and so the quota policy may not completely reflect whether requests + will succeed. + + For example, a successful response with the following fields: + + RateLimit: default;r=1;t=7 + + does not guarantee that the next request will be successful. + Servers' behavior may be subject to other conditions. + + A client is responsible for ensuring that RateLimit header field + values returned cause reasonable client behavior with respect to + throughput and latency (see Section 7.5 and Section 7.5.1). + + A client receiving RateLimit header fields MUST NOT assume that + future responses will contain the same RateLimit header fields, or + any RateLimit header fields at all. + + Malformed RateLimit header fields MUST be ignored. + + A client SHOULD NOT exceed the quota units conveyed by the remaining + keyword before the time window expressed in the reset keyword. + + The value of the reset keyword is generated at response time: a + client aware of a significant network latency MAY behave accordingly + and use other information (e.g. the "Date" response header field, or + otherwise gathered metrics) to better estimate the reset keyword + moment intended by the server. + + The details provided in the RateLimit-Policy header field are + informative and MAY be ignored. + + If a response contains both the RateLimit and Retry-After fields, the + Retry-After field MUST take precedence and the reset keyword MAY be + ignored. + + This specification does not mandate a specific throttling behavior + and implementers can adopt their preferred policies, including: + + * slowing down or pre-emptively back-off their request rate when + approaching quota limits; + + * consuming all the quota according to the exposed limits and then + wait. + +6.1. Intermediaries + + This section documents the considerations advised in Section 16.3.2 + of [HTTP]. + + An intermediary that is not part of the originating service + infrastructure and is not aware of the quota policy semantic used by + the Origin Server SHOULD NOT alter the RateLimit header fields' + values in such a way as to communicate a more permissive quota + policy; this includes removing the RateLimit header fields. + + An intermediary MAY alter the RateLimit header fields in such a way + as to communicate a more restrictive quota policy when: + + * it is aware of the quota unit semantic used by the Origin Server; + + * it implements this specification and enforces a quota policy which + is more restrictive than the one conveyed in the fields. + + An intermediary SHOULD forward a request even when presuming that it + might not be serviced; the service returning the RateLimit header + fields is the sole responsible of enforcing the communicated quota + policy, and it is always free to service incoming requests. + + This specification does not mandate any behavior on intermediaries + respect to retries, nor requires that intermediaries have any role in + respecting quota policies. For example, it is legitimate for a proxy + to retransmit a request without notifying the client, and thus + consuming quota units. + + Privacy considerations (Section 8) provide further guidance on + intermediaries. + +6.2. Caching + + [HTTP-CACHING] defines how responses can be stored and reused for + subsequent requests, including those with RateLimit header fields. + Because the information in RateLimit header fields on a cached + response may not be current, they SHOULD be ignored on responses that + come from cache (i.e., those with a positive current_age; see + Section 4.2.3 of [HTTP-CACHING]). + +7. Security Considerations + +7.1. Throttling does not prevent clients from issuing requests + + This specification does not prevent clients from making requests. + Servers should always implement mechanisms to prevent resource + exhaustion. + +7.2. Information disclosure + + Servers should not disclose to untrusted parties operational capacity + information that can be used to saturate its infrastructural + resources. + + While this specification does not mandate whether non-successful + responses consume quota, if error responses (such as 401 + (Unauthorized) and 403 (Forbidden)) count against quota, a malicious + client could probe the endpoint to get traffic information of another + user. + + As intermediaries might retransmit requests and consume quota units + without prior knowledge of the user agent, RateLimit header fields + might reveal the existence of an intermediary to the user agent. + + Where partition keys contain identifying information, either of the + client application or the user, servers should be aware of the + potential for impersonation and apply the appropriate security + mechanisms. + +7.3. Remaining quota units are not granted requests + + RateLimit header fields convey hints from the server to the clients + in order to help them avoid being throttled out. + + Clients MUST NOT consider the quota units (Section 2.6) returned in + remaining keyword as a service level agreement. + + In case of resource saturation, the server MAY artificially lower the + returned values or not serve the request regardless of the advertised + quotas. + +7.4. Reliability of the reset keyword + + Consider that quota might not be restored after the moment referenced + by the reset keyword (Section 4.1.2), and the reset parameter value + may not be constant. + + Subsequent requests might return a higher reset parameter value to + limit concurrency or implement dynamic or adaptive throttling + policies. + +7.5. Resource exhaustion + + When returning reset values, servers must be aware that many + throttled clients may come back at the very moment specified. + + This is true for Retry-After too. + + For example, if the quota resets every day at 18:00:00 and your + server returns the reset parameter accordingly + + Date: Tue, 15 Nov 1994 18:00:00 GMT + RateLimit: daily;r=1;t=36400 + + there's a high probability that all clients will show up at 18:00:00. + + This could be mitigated by adding some jitter to the reset value. + + Resource exhaustion issues can be associated with quota policies + using a large time window, because a user agent by chance or on + purpose might consume most of its quota units in a significantly + shorter interval. + + This behavior can be even triggered by the provided RateLimit header + fields. The following example describes a service with an unconsumed + quota policy of 10000 quota units per 1000 seconds. + + RateLimit-Policy: somepolicy;l=10000;w=1000 + RateLimit: somepolicy;r=10000;t=10 + + A client implementing a simple ratio between remaining keyword and + reset keyword could infer an average throughput of 1000 quota units + per second, while the limit keyword conveys a quota-policy with an + average of 10 quota units per second. If the service cannot handle + such load, it should return either a lower remaining keyword value or + an higher reset keyword value. Moreover, complementing large time + window quota policies with a short time window one mitigates those + risks. + +7.5.1. Denial of Service + + RateLimit header fields may contain unexpected values by chance or on + purpose. For example, an excessively high remaining keyword value + may be: + + * used by a malicious intermediary to trigger a Denial of Service + attack or consume client resources boosting its requests; + + * passed by a misconfigured server; + + or a high reset keyword value could inhibit clients to contact the + server (e.g. similarly to receiving "Retry-after: 1000000"). + + To mitigate this risk, clients can set thresholds that they consider + reasonable in terms of quota units, time window, concurrent requests + or throughput, and define a consistent behavior when the RateLimit + exceed those thresholds. For example this means capping the maximum + number of request per second, or implementing retries when the reset + keyword exceeds ten minutes. + + The considerations above are not limited to RateLimit header fields, + but apply to all fields affecting how clients behave in subsequent + requests (e.g. Retry-After). + +8. Privacy Considerations + + Clients that act upon a request to rate limit are potentially re- + identifiable (see Section 5.2.1 of [PRIVACY]) because they react to + information that might only be given to them. Note that this might + apply to other fields too (e.g. Retry-After). + + Since rate limiting is usually implemented in contexts where clients + are either identified or profiled (e.g. assigning different quota + units to different users), this is rarely a concern. + + Privacy enhancing infrastructures using RateLimit header fields can + define specific techniques to mitigate the risks of re- + identification. + +9. IANA Considerations + + IANA is requested to update one registry and create one new registry. + + Please add the following entries to the "Hypertext Transfer Protocol + (HTTP) Field Name Registry" registry ([HTTP]): + + +==================+===========+=======================+ + | Field Name | Status | Specification | + +==================+===========+=======================+ + | RateLimit | permanent | Section 4 of RFC nnnn | + +------------------+-----------+-----------------------+ + | RateLimit-Policy | permanent | Section 3 of RFC nnnn | + +------------------+-----------+-----------------------+ + + Table 1 + +10. References + +10.1. Normative References + + [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, + Ed., "HTTP Semantics", STD 97, RFC 9110, + DOI 10.17487/RFC9110, June 2022, + . + + [IANA] Cotton, M., Leiba, B., and T. Narten, "Guidelines for + Writing an IANA Considerations Section in RFCs", BCP 26, + RFC 8126, DOI 10.17487/RFC8126, June 2017, + . + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, + . + + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC + 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, + May 2017, . + + [STRUCTURED-FIELDS] + Nottingham, M. and P. Kamp, "Structured Field Values for + HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, + . + + [WEB-ORIGIN] + Barth, A., "The Web Origin Concept", RFC 6454, + DOI 10.17487/RFC6454, December 2011, + . + +10.2. Informative References + + [HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for + HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, + . + + [HTTP-CACHING] + Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, + Ed., "HTTP Caching", STD 98, RFC 9111, + DOI 10.17487/RFC9111, June 2022, + . + + [PRIVACY] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., + Morris, J., Hansen, M., and R. Smith, "Privacy + Considerations for Internet Protocols", RFC 6973, + DOI 10.17487/RFC6973, July 2013, + . + + [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, + . + + [UNIX] The Open Group, "The Single UNIX Specification, Version 2 + - 6 Vol Set for UNIX 98", February 1997. + +Appendix A. Rate-limiting and quotas + + Servers use quota mechanisms to avoid systems overload, to ensure an + equitable distribution of computational resources or to enforce other + policies - e.g. monetization. + + A basic quota mechanism limits the number of acceptable requests in a + given time window, e.g. 10 requests per second. + + When quota is exceeded, servers usually do not serve the request + replying instead with a 4xx HTTP status code (e.g. 429 or 403) or + adopt more aggressive policies like dropping connections. + + Quotas may be enforced on different basis (e.g. per user, per IP, per + geographic area, ..) and at different levels. For example, an user + may be allowed to issue: + + * 10 requests per second; + + * limited to 60 requests per minute; + + * limited to 1000 requests per hour. + + Moreover system metrics, statistics and heuristics can be used to + implement more complex policies, where the number of acceptable + requests and the time window are computed dynamically. + + To help clients throttling their requests, servers may expose the + counters used to evaluate quota policies via HTTP header fields. + + Those response headers may be added by HTTP intermediaries such as + API gateways and reverse proxies. + + On the web we can find many different rate-limit headers, usually + containing the number of allowed requests in a given time window, and + when the window is reset. + + The common choice is to return three headers containing: + + * the maximum number of allowed requests in the time window; + + * the number of remaining requests in the current window; + + * the time remaining in the current window expressed in seconds or + as a timestamp; + +A.1. Interoperability issues + + A major interoperability issue in throttling is the lack of standard + headers, because: + + * each implementation associates different semantics to the same + header field names; + + * header field names proliferates. + + User agents interfacing with different servers may thus need to + process different headers, or the very same application interface + that sits behind different reverse proxies may reply with different + throttling headers. + +Appendix B. Examples + +B.1. Responses without defining policies + + Some servers may not expose the policy limits in the RateLimit-Policy + header field. Clients can still use the RateLimit header field to + throttle their requests. + +B.1.1. Throttling information in responses + + The client exhausted its quota for the next 50 seconds. The limit + and time-window is communicated out-of-band. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit: default;r=0;t=50 + + {"hello": "world"} + + Since the field values are not necessarily correlated with the + response status code, a subsequent request is not required to fail. + The example below shows that the server decided to serve the request + even if remaining keyword value is 0. Another server, or the same + server under other load conditions, could have decided to throttle + the request instead. + + Request: + + GET /items/456 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit: default;r=0;t=48 + + {"still": "successful"} + +B.1.2. Multiple policies in response + + The server uses two different policies to limit the client's + requests: + + * 5000 daily quota units; + + * 1000 hourly quota units. + + The client consumed 4900 quota units in the first 14 hours. + + Despite the next hourly limit of 1000 quota units, the closest limit + to reach is the daily one. + + The server then exposes the RateLimit header fields to inform the + client that: + + * it has only 100 quota units left in the daily quota and the window + will reset in 10 hours; + + The server MAY choose to omit returning the hourly policy as it uses + the same quota units as the daily policy and the daily policy is the + one that is closest to being exhausted. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit: dayLimit;r=100;t=36000 + + {"hello": "world"} + +B.1.3. Use for limiting concurrency + + RateLimit header fields may be used to limit concurrency, advertising + limits that are lower than the usual ones in case of saturation, thus + increasing availability. + + The server adopted a basic policy of 100 quota units per minute, and + in case of resource exhaustion adapts the returned values reducing + both limit and remaining keyword 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;l=100;w=60 + RateLimit: basic;r=60;t=58 + + {"elapsed": 2, "issued": 40} + + At the subsequent request - due to resource exhaustion - the server + advertises only r=20. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit-Policy: basic;l=100;w=60 + RateLimit: basic;r=20;t=56 + + {"elapsed": 4, "issued": 41} + +B.1.4. Use in throttled responses + + A client exhausted its quota and the server throttles it sending + Retry-After. + + In this example, the values of Retry-After and RateLimit header field + reference the same moment, but this is not a requirement. + + The 429 (Too Many Request) HTTP status code is just used as an + example. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 429 Too Many Requests + Content-Type: application/json + Date: Mon, 05 Aug 2019 09:27:00 GMT + Retry-After: Mon, 05 Aug 2019 09:27:05 GMT + RateLimit: default;r=0;t=5 + + { + "title": "Too Many Requests", + "status": 429, + "detail": "You have exceeded your quota" + } + +B.2. Responses with defined policies + +B.2.1. Throttling window specified via parameter + + The client has 99 quota units left for the next 50 seconds. The time + window is communicated by the w parameter, so we know the throughput + is 100 quota units per minute. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit: fixedwindow;r=99;t=50 + RateLimit-Policy: fixedwindow;l=100;w=60 + {"hello": "world"} + +B.2.2. Dynamic limits with parameterized windows + + The policy conveyed by the RateLimit header field states that the + server accepts 100 quota units per minute. + + To avoid resource exhaustion, the server artificially lowers the + actual limits returned in the throttling headers. + + The remaining keyword 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;l=100;w=60 + RateLimit: dynamic;r=9;t=50 + + + { + "status": 200, + "detail": "Just slow down without waiting." + } + +B.2.3. Dynamic limits for pushing back and slowing down + + Continuing the previous example, let's say the client waits 10 + seconds and performs a new request which, due to resource exhaustion, + the server rejects and pushes back, advertising r=0 for the next 20 + seconds. + + The server advertises a smaller window with a lower limit to slow + down the client for the rest of its original window after the 20 + seconds elapse. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 429 Too Many Requests + Content-Type: application/json + RateLimit-Policy: dynamic;l=15;w=20 + RateLimit: dynamic;r=0;t=20 + + { + "status": 429, + "detail": "Wait 20 seconds, then slow down!" + } + +B.3. Dynamic limits for pushing back with Retry-After and slow down + + Alternatively, given the same context where the previous example + starts, we can convey the same information to the client via Retry- + After, with the advantage that the server can now specify the + policy's nominal limit and window that will apply after the reset, + e.g. assuming the resource exhaustion is likely to be gone by then, + so the advertised policy does not need to be adjusted, yet we managed + to stop requests for a while and slow down the rest of the current + window. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 429 Too Many Requests + Content-Type: application/json + Retry-After: 20 + RateLimit-Policy: dynamic;l=100;w=60 + RateLimit: dynamic;r=15;t=40 + + { + "status": 429, + "detail": "Wait 20 seconds, then slow down!" + } + + Note that in this last response the client is expected to honor + Retry-After and perform no requests for the specified amount of time, + whereas the previous example would not force the client to stop + requests before the reset time is elapsed, as it would still be free + to query again the server even if it is likely to have the request + rejected. + +B.3.1. Missing Remaining information + + The server does not expose remaining values (for example, because the + underlying counters are not available). Instead, it resets the limit + counter every second. + + It communicates to the client the limit of 10 quota units per second + always returning the limit and reset keywords. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit-Policy: quota;l=100;w=1 + RateLimit: quota;t=1 + + {"first": "request"} + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 Ok + Content-Type: application/json + RateLimit-Policy: quota;l=10 + RateLimit: quota;t=1 + + {"second": "request"} + +B.3.2. Use with multiple windows + + This is a standardized way of describing the policy detailed in + Appendix B.1.2: + + * 5000 daily quota units; + + * 1000 hourly quota units. + + The client consumed 4900 quota units in the first 14 hours. + + Despite the next hourly limit of 1000 quota units, the closest limit + to reach is the daily one. + + The server then exposes the RateLimit header fields to inform the + client that: + + * it has only 100 quota units left; + + * the window will reset in 10 hours; + + * the expiring-limit is 5000. + + Request: + + GET /items/123 HTTP/1.1 + Host: api.example + + Response: + + HTTP/1.1 200 OK + Content-Type: application/json + RateLimit-Policy: hour;l=1000;w=3600, day;l=5000;w=86400 + RateLimit: day;r=100;t=36000 + + {"hello": "world"} + +FAQ + + This section is to be removed before publishing as an RFC. + + 1. Why defining standard fields for throttling? + + To simplify enforcement of throttling policies and enable clients + to constraint their requests to avoid being throttled. + + 2. Can I use RateLimit header fields in throttled responses (eg with + status code 429)? + + Yes, you can. + + 3. Are those specs tied to RFC 6585? + + No. [RFC6585] defines the 429 status code and we use it just as + an example of a throttled request, that could instead use even + 403 or whatever status code. + + 4. Why is the partition key necessary? + + Without a partition key, a server can only effectively only have + one scope (aka partition), which is impractical for most + services, or it needs to communicate the scopes out-of-band. + This prevents the development of generic connector code that can + be used to prevent requests from being throttled. Many APIs rely + on API keys, user identity or client identity to allocate quota. + As soon as a single client processes requests for more than one + partition, the client needs to know the corresponding partition + key to properly track requests against allocated quota. + + 5. Why using delay-seconds instead of a UNIX Timestamp? Why not + using subsecond precision? + + Using delay-seconds aligns with Retry-After, which is returned in + similar contexts, eg on 429 responses. + + Timestamps require a clock synchronization protocol (see + Section 5.6.7 of [HTTP]). This may be problematic (e.g. clock + adjustment, clock skew, failure of hardcoded clock + synchronization servers, IoT devices, ..). Moreover timestamps + may not be monotonically increasing due to clock adjustment. See + Another NTP client failure story + (https://community.ntppool.org/t/another-ntp-client-failure- + story/1014/) + + We did not use subsecond precision because: + + * that is more subject to system clock correction like the one + implemented via the adjtimex() Linux system call; + + * response-time latency may not make it worth. A brief + discussion on the subject is on the httpwg ml + (https://lists.w3.org/Archives/Public/ietf-http- + wg/2019JulSep/0202.html) + + * almost all rate-limit headers implementations do not use it. + + 6. Shouldn't I limit concurrency instead of request rate? + + You can use this specification to limit concurrency at the HTTP + level (see {#use-for-limiting-concurrency}) and help clients to + shape their requests avoiding being throttled out. + + A problematic way to limit concurrency is connection dropping, + especially when connections are multiplexed (e.g. HTTP/2) + because this results in unserviced client requests, which is + something we want to avoid. + + A semantic way to limit concurrency is to return 503 + Retry- + After in case of resource saturation (e.g. thrashing, connection + queues too long, Service Level Objectives not meet, ..). + Saturation conditions can be either dynamic or static: all this + is out of the scope for the current document. + + 7. Do a positive value of remaining paramter imply any service + guarantee for my future requests to be served? + + No. FAQ integrated in Section 4.1.1. + + 8. Is the quota-policy definition Section 2.5 too complex? + + You can always return the simplest form + + RateLimit:default;r=50;t=60 + + The policy key clearly connects the current usage status of a policy + to the defined limits. So for the following field: + +RateLimit-Policy: sliding;l=100;w=60;burst=1000;comment="sliding window", fixed;l=5000;w=3600;burst=0;comment="fixed window" +RateLimit: sliding;r=50;t=44 + + the value "sliding" identifies the policy being reported. + + 1. Can intermediaries alter RateLimit header fields? + + Generally, they should not because it might result in unserviced + requests. There are reasonable use cases for intermediaries + mangling RateLimit header fields though, e.g. when they enforce + stricter quota-policies, or when they are an active component of + the service. In those case we will consider them as part of the + originating infrastructure. + + 2. Why the w parameter is just informative? Could it be used by a + client to determine the request rate? + + A non-informative w parameter might be fine in an environment + where clients and servers are tightly coupled. Conveying + policies with this detail on a large scale would be very complex + and implementations would be likely not interoperable. We thus + decided to leave w as an informational parameter and only rely on + the limit, remaining and reset keywords for defining the + throttling behavior. + + 3. Can I use RateLimit fields in trailers? Servers usually + establish whether the request is in-quota before creating a + response, so the RateLimit field values should be already + available in that moment. Supporting trailers has the only + advantage that allows to provide more up-to-date information to + the client in case of slow responses. However, this complicates + client implementations with respect to combining fields from + headers and accounting for intermediaries that drop trailers. + Since there are no current implementations that use trailers, we + decided to leave this as a future-work. + +RateLimit header fields currently used on the web + + This section is to be removed before publishing as an RFC. + + Commonly used header field names are: + + * X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset; + + There are variants too, where the window is specified in the header + field name, eg: + + * x-ratelimit-limit-minute, x-ratelimit-limit-hour, x-ratelimit- + limit-day + + * x-ratelimit-remaining-minute, x-ratelimit-remaining-hour, x- + ratelimit-remaining-day + + Here are some interoperability issues: + + * X-RateLimit-Remaining references different values, depending on + the implementation: + + - seconds remaining to the window expiration + + - milliseconds remaining to the window expiration + + - seconds since UTC, in UNIX Timestamp [UNIX] + + - a datetime, either IMF-fixdate [HTTP] or [RFC3339] + + * different headers, with the same semantic, are used by different + implementers: + + - X-RateLimit-Limit and X-Rate-Limit-Limit + + - X-RateLimit-Remaining and X-Rate-Limit-Remaining + + - X-RateLimit-Reset and X-Rate-Limit-Reset + + The semantic of RateLimit depends on the windowing algorithm. A + sliding window policy for example, may result in having a remaining + keyword value related to the ratio between the current and the + maximum throughput. e.g. + +RateLimit-Policy: sliding;l=12;w=1 +RateLimit: sliding;l=12;r=6;t=1 ; using 50% of throughput, that is 6 units/s + + If this is the case, the optimal solution is to achieve + +RateLimit-Policy: sliding;l=12;w=1 +RateLimit: sliding;l=12;r=1;t=1 ; using 100% of throughput, that is 12 units/s + + 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, + Darrel Miller and Julian Reschke. + +Changes + + This section is to be removed before publishing as an RFC. + +Since draft-ietf-httpapi-ratelimit-headers-07 + + This section is to be removed before publishing as an RFC. + + * Refactored both fields to lists of Items that identify policy and + use parameters + + * Added quota unit parameter + + * Added partition key parameter + +Since draft-ietf-httpapi-ratelimit-headers-03 + + This section is to be removed before publishing as an RFC. + + * Split policy informatiom in RateLimit-Policy #81 + +Since draft-ietf-httpapi-ratelimit-headers-02 + + This section is to be removed before publishing as an RFC. + + * Address throttling scope #83 + +Since draft-ietf-httpapi-ratelimit-headers-01 + + This section is to be removed before publishing as an RFC. + + * Update IANA considerations #60 + + * Use Structured fields #58 + + * Reorganize document #67 + +Since draft-ietf-httpapi-ratelimit-headers-00 + + This section is to be removed before publishing as an RFC. + + * Use I-D.httpbis-semantics, which includes referencing delay- + seconds instead of delta-seconds. #5 + +Authors' Addresses + + Roberto Polli + Team Digitale, Italian Government + Italy + Email: robipolli@gmail.com + + + Alejandro Martinez Ruiz + Red Hat + Email: alex@flawedcode.org + + + Darrel Miller + Microsoft + Email: darrel@tavis.ca